Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications

Terpyridine-based Materials By Ulrich S. Schubert, Andreas Winter, and George R. Newkome ffi 27 J l 2011 9 48 33 Rel...

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Terpyridine-based Materials

By Ulrich S. Schubert, Andreas Winter, and George R. Newkome

ffi

27 J l 2011 9 48 33

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27 J l 2011 9 48 33

Ulrich S. Schubert, Andreas Winter, and George R. Newkome

Terpyridine-based Materials For Catalytic, Optoelectronic and Life Science Applications

WILEY-VCH Verlag GmbH & Co. KGaA

ffi

27 J l 2011 9 48 33

The Authors Prof. Dr. Ulrich S. Schubert Friedrich-Schiller-University Jena Laboratory of Organic and Macromolecular Chemistry (IOMC) and Jena Center for Soft Matter (JCSM) Humboldtstr. 10 07743 Jena Germany Dr. Andreas Winter Friedrich-Schiller University Jena Laboratory of Organic and Macromolecular Chemistry (IOMC) and Jena Center for Soft Matter (JCSM) Humboldtstraße 10 07743 Jena Germany Prof. Dr. George R. Newkome The University of Akron Departments of Polymer Science and Chemistry & The Maurice Morton Institute of Polymer Science Akron, OH, 44325-4717 USA

& All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at ohttp://dnb.d-nb.deW. & 2011 Wiley-VCH Verlag & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Cover Design Formgeber, Eppelheim Typesetting MPS Limited, a Macmillan Company, Chennai Printing and Binding Fabulous Printers Pte Ltd, Singapore Printed in Singapore Printed on acid-free paper Print ISBN: ePDF ISBN: ePub ISBN: Mobi ISBN: oBook ISBN:

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978-3-527-33038-6 978-3-527-63964-9 978-3-527-63963-2 978-3-527-63965-6 978-3-527-63962-5

|V

Contents Preface ix List of Abbreviations

xi

1

1

Introduction

2

Synthesis, Properties, and Applications of Functionalized 2,2u:6u,2v-Terpyridines 13

2.1 2.2 2.2.1 2.2.2 2.3 2.3.1 2.3.2 2.4

Introduction 13 Basic Synthetic Strategies 13 Ring-Assembly Methodologies 14 Cross-Coupling Procedures 18 Synthesis and Properties of 2,2u:6u,2v-Terpyridine Derivatives 19 4u-Substituted 2,2u:6u,2v-Terpyridinoxy Derivatives 19 Miscellaneous 4u-Substituted 2,2u:6u,2v-Terpyridine Derivatives 24 2,2u:6u,2v-Terpyridines Symmetrically Substituted on the Outer Pyridine Rings 28 Ziessel-Type 2,2u:6u,2v-Terpyridines 31 ¨hnke-Type 2,2u:6u,2v-Terpyridines 38 Kro Miscellaneous Terpyridine-Analogous Compounds 49 Rigid U- and S-Shaped Terpyridines 49 Five-Membered N-Heterocycles Replacing the Outer Pyridine Rings 51 The Swedish Concept: Expanded Bite Angles in Tridentate Ligands 53

2.5 2.6 2.7 2.7.1 2.7.2 2.7.3

3

Chemistry and Properties of Terpyridine Transition Metal Ion Complexes 65

3.1 3.2 3.3

Introduction 65 Basic Synthetic Strategies and Characterization Tools 66 RuII and OsII Complexes 73

Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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VI

| Contents 3.3.1 3.3.2 3.3.2.1 3.3.2.2 3.3.2.3 3.3.2.4

Synthesis of RuII and OsII Bis(terpyridine) Complexes 73 RuII Ions and Terpyridine Ligands – A Happy Marriage? 75

Photophysical Properties 75 Mononuclear RuII Bis(terpyridine) Complexes 76 Oligonuclear Complexes Containing RuII/OsII Bis(terpyridine) Units 89 Dendritic and Star-Shaped Systems Containing RuII Bis(terpyridine) Units 102

3.4 3.5

Iridium(III) Complexes with Terpyridine Ligands Platinum(II) Mono(terpyridine) Complexes 115

4

Metallo-Supramolecular Architectures Based on Terpyridine Complexes 129

4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.7.1 4.7.2

Introduction 129 Terpyridine-Containing Metallo-Macrocycles The HETTAP Concept 148 Racks and Grids 154 Helicates 171 Rotaxanes and Catenanes 177 Miscellaneous Structures 182 Cyclodextrin Derivatives 182 Other Assemblies 185

5

p-Conjugated Polymers Incorporating Terpyridine Metal Complexes 199

5.1 5.2 5.3

Introduction 199 Metallo-Supramolecular Polymerization 200 Metallopolymers Based on p-Conjugated Bis(terpyridine)s 204 Polymerization by Transition Metal Ion Coordination 204 Self-Assembly of Metallopolymers 212 Chiral Metallopolymers 219 Non-Classical Metallopolymers 220 Polymerization Using the ‘‘Complex First’’ Method 224 Main-Chain Metallopolymers Based on Terpyridine-Functionalized p-Conjugated Polymers 229

5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 5.4

107

130

6

Functional Polymers Incorporating Terpyridine-Metal Complexes 241

6.1 6.2 6.2.1 6.2.2

Introduction 241 Polymers with Terpyridine Units in the Side-Chain 242 Materials Based on Flexible Organic Polymers 242 Materials Based on p-Conjugated Polymers 257

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Contents

6.3 6.3.1 6.3.2 6.3.3 6.3.4 6.3.4.1 6.3.4.2 6.3.4.3 6.3.5 6.3.5.1 6.3.5.2 6.3.6

Polymers with Terpyridines within the Polymer Backbone 262 Polymers from Organic Small-Molecule Building Blocks 263 Chain-Extended Polymers from Polymeric Building Blocks 269 Monotopic Macroligands by End-Group Functionalization 272 Functional Terpyridine-Containing Initiators 277

7

Terpyridine Metal Complexes and their Biomedical Relevance 319

7.1 7.2 7.2.1 7.2.1.1 7.2.1.2 7.2.1.3

Introduction 319 Terpyridine Metal Complexes with Biological Activity 320 Intercalation and Cytotoxicity 320

7.2.2

Biolabeling

8

Terpyridines and Nanostructures 399

8.1 8.2 8.3 8.4

Introduction 399 Terpyridines and Surface Chemistry 401 Terpyridines and Inorganic Nanomaterials 420 Terpyridines and Nano-Structured TiO2: Photovoltaic Applications 431 Organopolymeric Resins, Beads, and Nanoparticles 447

8.5

Initiation of Ionic Polymerization Reactions 277 Initiation of Controlled Radical Polymerization Reactions Post-Polymerization Functionalization 288

281

Mononuclear Metallo-supramolecular Polymers 291

Supramolecular A-[M]-A Homopolymers Supramolecular Block Copolymers 294

291

Oligonuclear Metallo-Supramolecular Copolymers

308

Terpyridine Complexes with d8 Late Transition Metal Ions 320 Terpyridine Complexes with Heavy d6 Transition Metal Ions 350 Terpyridine Complexes with Miscellaneous Transition Metal Ions 364 376

9

Catalytic Applications of Terpyridines and Their Transition Metal Complexes 459

9.1 9.2 9.3 9.4 9.4.1 9.4.2

Introduction 459 (Asymmetric) Catalysts in Organic Reactions 460 Electrocatalytic Oxidation and Reduction Processes 476 Photocatalytic Processes 480 Light-Driven Hydrogen Formation 483 Molecular Terpyridine-Based Catalysts for Water Oxidation 488

10

Concluding Remarks

507

Index 509

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| VII

| ix

Preface Over the past few decades, supramolecular chemistry, that is, the self-assembly of molecules into complex architectures based on weak secondary interactions (e.g., metal-to-ligand coordination or hydrogen bonding), has evolved from a primary scientific field into daily-life applications. In particular, the combination of transition metal ions with N-heteroaromatics, as ligands, has rapidly moved to the center of attention in laboratories around the world. As one of the most prominent representatives of this family, terpyridine was discovered in 1931 and has been shown to lend itself to the construction of specific, stable metal complexes with unique properties that can easily be tuned by the choice of the metal ion and/or the structural modification. An overview of the syntheses and early applications of the parent and substituted 2,20 :60 ,200 -terpyridines was given in the book Modern Terpyridine Chemistry in 2006. The emerging applications in the fields of polymer science, optoelectronic devices, medicinal chemistry, nanotechnology, and molecular catalysis prompted us to review the utilization of terpyridine-based materials in more detail and with respect to current applications. We have attempted to compile the key examples in each field to assist and help future researchers in this arena; many excellent examples are available and support the rationale for continued exploitation of this family of heterocyclic compounds, but we must admit that not all of them could be chosen due to space constraints. Therefore, we apologize in advance to those authors whose work has not been incorporated. The authors would be, as always, most grateful to know of any errors, which may have crept into the manuscript despite the multiple proofreading by many of our colleagues. We also thank our spouses, relatives, and friends for their patience and assistance in completing this work. Jena and Akron, July 2011

Ulrich S. Schubert Andreas Winter George R. Newkome


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| xi

List of Abbreviations A2780 A-498 A-549 AAS acac ACI AFM AIBN Alq3 AP APCE ATRP AUC BA BCP BEL-7420 BGC-823 BINOL bip bip-OH bmim-PF6 BNCT BODIPY BPG bpm bpp bpy bpz BSA BTB

human ovarian carcinoma cells human kidney carcinoma cells human lung carcinoma cells atomic absorption spectroscopy acetylacetonate average current intensity atomic force microscopy 2,20 -azobisiso-butyronitrile tris(8-hydroxyquinoline)aluminum aminopentanol absorbed photon-to-current efficiency atom-transfer radical polymerization analytical ultracentrifugation butyl acrylate bond critical point human hepato carcinoma cells human gastric gland carcinoma cells 1,10 -binaphth-2-ol 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridin-4-ol 1-butyl-3-methylimidazolium hexafluorophosphate boron neutron capture therapy boron-dipyrromethene basal plane pyrolytic graphite 2,20 -bipyrimidine 1,4-bis(2,6-di(1H-pyrazol-1-yl)pyridin-4-yl)benzene 2,20 -bipyridine 2,20 -bipyrazine bovine serum albumin bipyridine-terpyridine-bipyridine


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xii

| List of Abbreviations btp

btpyan bzimpy C13 CALB DAC CCAAC CCD19Lu CD CDI CFS CH1 CIE CITS C^N^N CNT COD CRP cryo-TEM CS CSF ct CT CTA CV CVP Cyst Cyt-c CzMA dba DCA DEDTC DFT DHP diad DLS dmbpy DMF or dmf DMPO DMSO or dmso DNA DOSY DP

1,4-bis(2,6-bis(1-butyl-1H-1,2,3-triazol-4-yl)pyridin-4-yl)-benzene or 4,40 -di(tert-butyl)-2,20 -bipyridine or 2,6-bis(1H-1,2,3-triazol-4yl)pyridine(s) 1,8-bis(2,20 :60 ,2“-terpyridin-4-yl)anthracene 2,6-bis(benzimidazol-2-yl)pyridine human ovarian carcinoma cells lipase B from Candida antarctica deoxycholic acid CuI-catalyzed alkyne-azide cycloaddition (reaction) human normal pulmonary cell circular dichroism N,N0 -carbonyldiimidazole competitive fluorescence spectroscopy human larynx and pharynx cancer Commision International d’Eclairage current imaging tunneling spectroscopy mono-cyclometalating tridentate (ligand) carbon nanotube cycloocta-1,5-diene controlled radical polymerization cryogenic transition electron microscopy charge-separated competitive fluorescence spectroscopy calf thymus charge-transfer chain-transfer agent cyclic voltammetry chemical vapor deposition cystine cytochrome-c 2-(N-carbazolyl)ethyl methacrylate dibenzylideneacetone deoxycholic acid diethyldithiocarbamate density functional theory di(hexadecyl)phosphate or 1,4-dihydropyridine di(iso-propylazo)dicarboxylate dynamic light scattering 4,40 -dimethyl-2,20 -bipyridine N,N-dimethylformamide 5,5-dimethyl-1-pyrroline-N-oxide dimethylsulfoxide deoxyribonuleic acid diffusion-ordered spectroscopy (NMR) degree-of-polymerization

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List of Abbreviations

D-P-A dpp dppene dppf DPV dpbq dpp dppf dppt dppz dppzp DS DSB DSC DSSC DTC DTE E0-0 EDA EDOT EDTA EHMO EI EF EL emim-I EPMA EPR EPT ESI ET EthBr EVSA-T EWG Fc FF FID FS FTICR FTO FWHM Gly GMP GR

donor-photosensitizer-acceptor (array) 2,4-di(pyridin-2-yl)pyrazolate or 2,9-diphenyl-1,10phenanthroline cis-1,2-bis(diphenylphosphino)ethylene 1,10 -bis(diphenylphosphino)ferrocene differential pulse voltammetry 8,80 -diphenyl-3,30 -biisoquinoline 2,9-diphenyl-1,10-phenanthroline 1,10 -bis(diphenylphosphino)ferrocene 5,6-diphenyl-3-(phenanthrolin-2-yl)-1,2,4-triazine dipyrido[3,2-a:20 ,30 -c]phenazine 60 -(200 -pyridyl)dipyrido[3,2-a:20 ,30 -c]phenazine degree-of-substitution double-strand break differential scanning calorimetry dye-sensitized solar cell dithiocarbamate dithienylethene zero-zero spectroscopic energy ethyl diazoacetate 3,4-ethylenedioxythienyl ethylenediaminetetraacetic acid ¨ckel molecular orbital extended Hu electron ionization (mass spectrometry) (luminescence) enhancement factor electroluminescence 1-ethyl-3-methylimidazolium iodide electron probe microanalysis (spectrum) electron paramagnetic resonance (spectroscopy) electron/proton-transfer electrospray ionization electron-transfer ethidium bromide human breast cancer cells electron-withdrawing group ferrocene fill factor fluorescent intercalator displacement Fremy’s salt (potassium nitrosodisulfonate) Fourier-transform ion cyclotron resonance fluorine-doped tin oxide full width at half maximum glycine guanosine 50 -monophosphate glutathione reductase

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| xiii

xiv

| List of Abbreviations GS GSH H226 HBC HCT-116, -15 HEEDTA HeLa HepG2 HET HETPHEN HETTAP HL-60 HOMO HOPG HS HSA HT-29 HTelo hTrx hTrxR HWE Isc IC50 ICP IDA IET IGROV IL ILCT IPCE IRE ITC ITE ITO J KB KB K0 KSV L L1210 LAS LB LBL LC

ground state glutathione human non-small lung cancer cells hexa-peri-hexabenzocoronene human colon adenocarcinoma cells sodium salt of N-hydroxyethylethylenediamine triacetic acid cervical cancer cells from Henrietta Lacks human liver carcinoma cell helix-extension (parameter) heteroleptic phenanthroline (complexation) heteroleptic terpyridine and phenanthroline (complexation) human promyelocytic leukemia cells highest occupied molecular orbital highly-ordered pyrolytic graphite high-spin (state) human serum albumin human colon adenocarcinoma cells human telomeric (DNA sequence) thioredoxin thioredoxin reductase Horner-Wadsworth-Emmons (condensation reaction) short circuit current half maximal inhibitory concentration inductively coupled plasma (mass spectrometry) interdigitated (microelectrode) array interfacial electron transfer human ovarian carcinoma cells intraligand intraligand charge-transfer incident photon-to-current conversion efficiency iron regulatory element isothermal titration calorimetry interfacial electron transfer indium tin oxide current density human epidermoid cancer cells binding constant ion-free binding constant Stern-Volmer constant luminance murine leukemia cell light absorption sensitizer Langmuir-Blodgett layer-by-layer ligand-centered or liquid crystalline

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LCST LD LDA LF LHE LIESST LLCT L/L0 LS LUMO M19 MEL MALDI-TOF MC MCF-7 mCPBA MeCN MeCys MEF MEMS MEPE mes MF MLCT MLLCT MMA MMM MMNVM Mn MO MPEG MRI MS MV2+ MW MWNT NCI-H460 NDR NEM NHE NLO NMP NMR N^N N^N^N

lower critical solution temperature linear dichroism (spectroscopy) lithium di(iso-propyl)amide lactoferrin light harvesting efficiency light-induced excited-state spin-trapping ligand-to-ligand charge-transfer relative contour length low-spin (state) lowest unoccupied molecular orbital human melanoma cells matrix-assisted laser desorption/ionization time-of-flight (mass spectrometry) metal-centered human breast cancer cells m-chloroperbenzoic acid acetonitrile S-methylcysteine metal-enhanced fluorescence micro-electro-mechanical systems metallo-supramolecular polyelectrolytes 2,4,6-trimethylphenyl (mesityl) melamine formaldehyde metal-to-ligand charge-transfer metal-ligand-to-ligand charge-transfer methyl methacrylate molecular monolayer memory (device) molecular monolayer non-volatile memory (devices) number-average molar mass molecular orbital poly(ethylene oxide) monomethyl ether magnetic resonance imaging mass spectrometry 1,10 -dimethyl-4,40 -bipyridinium, methyl viologen molecular wire multi-walled carbon nanotubes human lung carcinoma cells negative differential resistance N-ethylmorpholine normal hydrogen electrode non-linear optics nitroxide-mediated polymerization or N-methylpyrrolidone nuclear magnetic resonance (spectroscopy) bidentate N-heteroaromatic ligand tridentate (ligand)

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| xv

xvi

| List of Abbreviations ODN ODT OEGMA OHT OL OLED OPE OPV ORTEP OSC P2VP/P4VP P-388 P3HT PAA PAC pbpy PC-3 PCBM PCD PCET PCL PDA PDI PDMAA PDMS PEB PEC PEG PEI PEDOT PEtOx PET PFDS phen phi PI pia PL PLA PLED PMDETA PMMA PNIPAM POM PPFS

oligodeoxynucleotide n-octadecanethiol oligo(ethylene oxide) methacrylate oligohexylthiophene optical limiter organic light-emitting diode oligo(p-phenylene-ethylene) oligo(phenylenevinylidene) Oak Ridge thermal-ellipsoid plot organic solar cells poly(2-vinylpyridine)/poly(4-vinylpyridine) human leukemia cell lines poly(3-hexylthiophene) poly(acrylic acid) polyelectrolyte-amphiphile complex 6-phenyl-2,20 -bipyridine human prostate adenocarcinoma cells [6,6]-phenyl-C61-methyl butyrate poly(chloromethylstyrene-co-divinylbenzene) proton-coupled electron-transfer poly(e-caprolactone) photo-diode array perylene diimide poly(N,N-dimethyl-acrylamide) poly(dimethylsiloxane) poly(ethylene-co-butylene) photoelectrochemical poly(ethylene glycol) poly(ethylene imine) poly(3,4-ethylenedioxythiophene) poly(2-ethyloxazoline) photoinduced electron-transfer poly(ferrocenyldimethylsilane) 1,10-phenanthroline 9,10-phenanthrenequinone diimine polyisoprene photoinduced absorption photoluminescence poly(L-lactide) polymer light-emitting diode N,N,N,N“,N“-pentamethyldiethylenetriamine poly(methyl methacrylate) poly(N-isopropylacrylamide) polarized optical microscopy or polyoxometalate poly(pentafluorostyrene)

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PPG PPV PS PSC PSCA psl PSS PTFMS PTHF PUR PV PVC PVD pydic QCM-D QCR QGY-TR50 QQN QTAIM qu RAFT Rh RNA ROESY ROMP ROP RPTEC SAM SAML SANS SAXS SC SCE SEC SEM SF-268 SGC-7901 SHE SHG SIMS SK-MEL-2 SKOV-3 SmC SMM

poly(propylene glycol) poly(p-phenylenevinylidene) poly(styrene) polymer solar cell potential-step chronoamperometry postsynthetic labeling poly(styrene sulfonate) poly(4-trifluorometylstyrene) poly(tetrahydrofuran) polyurethane photovoltaic poly(vinylchloride) physical vapor deposition pyridine-2,6-dicarboxylic acid quartz crystal microbalance with dissipation (monitoring) quartz crystal resonator human hepatocellular carcinoma cell line quasi-quadratic network quantum theory of atoms in molecules quinoline reversible addition-fragmentation chain-transfer polymerization hydrodynamic radius ribonucleic acid rotating-frame Overhauser effect spectroscopy ring-opening metathesis polymerization ring-opening polymerization human normal cell line self-assembled monolayer self-assembled multilayer small angle neutron scattering small angel X-ray scattering spin-crossover standard calomel electrode size exclusion chromatography scanning electron microscopy glioblastoma cell lines human gastric carcinoma cells standard hydrogen electrode second-harmonic generation secondary ion mass spectrometry human skin melanoma cells human ovary adenocarcinoma cells smectic C (phase) single molecule magnet

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| xvii

SPAN SPG SPM SPS SSB STM SUNE-1 SWNT T taz TBAP TBT TD TEA TEG TEM TEMPO TEOA TEOS TFA Tg TGA THF THz TIPNO Tm TMP TP+ tppz tptz tpy TR tpy-PO(OH)2 tpy-SPG ttpy TWIM tppz UCST UPy Voc WAXD WIDR WOLED WRER XAS

sulfonated polyaniline schizophyllan scanning probe microscopy surface plasmon spectroscopy or solid-phase synthesis single-strand break scanning tunneling microscopy human nasopharyngeal carcinoma cells single-walled carbon nanotube temperature 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl-1,2,4-triazole tetra(n-butyl)ammonium hexafluorophosphate terpyridine-bipyridine-terpyridine time-dependent triethylamine tetra(ethylene glycol) transmission electron microscopy 2,2,6,6-tetramethylpiperidinyl-1-oxyl triethanolamine tetraethoxysilane trifluoroacetic acid glass transition temperature thermal gravimetric analysis tetrahydrofuran terahertz (spectroscopy) 2,2,5-trimethyl-4-phenyl-3-azahexane nitroxide melting tempoerature 2,2,6,6-tetramethylpiperidine 2,4,6-triarylpyridium 2,3,5,6-tetra(pyridin-2yl)pyrazine tris(pyridin-2-yl)triazine 2,20 :60 ,200 -terpyridine trypanothione reductase 2,20 :60 ,2“-terpyridin-40 -yl-phosphonic acid terpyridine-modified schizophyllan 40 -tolyl-2,20 :60 ,200 -terpyridine travelling wave ion mobility (MS) 2,3,5,6-tetrakis(pyridin-2-yl)pyrazine upper critical solution temperature ureidopyrimidinone open circuit voltage wide angle powder X-ray diffraction human colon carcinoma cells white organic light-emitting diode write-multiple read-erase-multiple read X-ray absorption spectroscopy

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XPS XRD e Z er fPL fPV tn xM 2VP 4VP

X-ray photoelectron spectroscopy X-ray diffraction elipticy overall power conversion efficiency or cell efficiency relative permittivity value photoluminescence quantum yield photovoltaic quantum yield electron lifetimes molar susceptibility 2-vinylpyridine 4-vinylpyridine

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| xix

|1

1

Introduction

In 1987, J.-M. Lehn, C.J. Pedersen, and D.J. Cram were honored with the Nobel Prize in Chemistry for their work on selective host–guest chemistry [1–3]. Since then, supramolecular chemistry has evolved into one of the most active fields within today’s research community. This concept has been delineated by Lehn [4]: “supramolecular chemistry may be defined as ‘chemistry beyond the molecule’ and is based on organized entities of higher complexity that result from the association of two or more chemical species held together by intermolecular forces.” Self-recognition and self-assembly processes represent the basic operational components underpinning supramolecular chemistry in which interactions are mainly non-covalent in nature (e.g., van der Waals, hydrogen bonding, ionic or coordinative interactions). In general, these interactions are weaker and usually reversible when compared to traditional covalent bonds. Nature itself represents the ultimate benchmarks for the design of artificial supramolecular processes. Inter- and intramolecular non-covalent interactions are of major importance for most biological processes such as highly selective catalytic reactions and information storage [5]; different non-covalent interactions are present in proteins, giving them their specific structures. DNA represents one of the most famous natural examples, where selfrecognition of the complementary base-pairs by hydrogen bonding leads to the self-assembly of the double helix. Starting with the development and design of crown ethers, spherands, and cryptands, modern supramolecular chemistry depicts the creation of well-defined structures via self-assembly processes [6] (similar to the well-known systems found in Nature [7]). One of the most important interactions applied in supramolecular chemistry is metal-to-ligand coordination. In this arena, chelate complexes derived from N-heteroaromatic ligands, in particular based on 2,2u-bipyridine, 1,10-phenanthroline, and 2,2u:6u,2v-terpyridine (Figure 1.1), have become an ever-expanding synthetic and structural frontier. Bipyridine has been known since 1888 when Blau first reported the formation of a bipyridine–iron complex [8]. One year later, Blau also synthesized and analyzed Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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2

| 1 Introduction 6 3' 4'

N

5' 6'

bpy

5

5

N 3

4

6

3'

7

4 3

8 2

N

N

9

6

N

5

5'

N

N 3

4

phen

4'

3''

tpy

6'' 5''

4''

Figure 1.1 General structures of 2,2u-bipyridine (bpy), 1,10-phenanthroline (phen), and 2,2u:6u,2v-terpyridine (tpy).

bipyridine by dry distillation of copper picolinate [9]. Since this parent molecule consists of two identical parts, no directed coupling procedure is required for its construction. Therefore, unsubstituted and symmetrically substituted, in particular 4,4u-functionalized, bipyridines are readily accessible in good yields by simple coupling procedures [10, 11]. Apart from this, their transition metal (in particular RuII) complexes [12–14] feature interesting photochemical properties, making them ideal candidates for solar energy conversion, for example, in photovoltaic devices [15–23] and light-emitting electrochemical cells [24–28]. The chemistry of 2,2u:6u,2v-terpyridines (often referred to as simply terpyridine or tpy; the other structural isomers are duly noted but not considered further herein) is much younger than that of 2,2u-bipyridines. About 80 years ago, terpyridine was isolated for the first time by Morgan and Burstall by a process in which pyridine was heated (340 1C) in the presence of anhydrous FeCl3 in an autoclave (50 atm) for 36 h [29, 30]; the parent terpyridine was isolated along with a myriad of other N-containing products. It was subsequently discovered that the addition of FeII ions to a solution of diverse terpyridines gave rise to a purple color indicative of metal complex formation. Since this pioneering work, the chemistry of terpyridine remained merely a curiosity for nearly 60 years, at which point its unique properties were incorporated into the construction of supramolecular assemblies. Terpyridines and their structural analogs have gained much interest in the last two decades as functional templates in the fields of supramolecular and coordination chemistry as well as in materials science [31–38]. This is expressed by the enormous number of scientific publications and patents dealing with the synthesis, properties, and applications of terpyridine-containing systems (March 2011: about 5950 hits in SciFindert, Figure 1.2). The terpyridine unit contains three nitrogen atoms and can, therefore, act as a tridentate ligand [39, 40]. The rich coordination chemistry and high binding affinity towards various interesting transition as well as rare earth metal ions, in concert with the resulting redox and photophysical properties, have given rise to diverse metallo-supramolecular architectures and a multitude of potential applications. Owing to their distinct photophysical, electrochemical, catalytic, and magnetic properties, terpyridines and their complexes have been studied regarding a wide range of potential applications covering light-into-electricity conversion [16, 41–60], light-emitting electrochemical cells (LECs) [61, 62], (electro)luminescent systems (e.g., organic light-emitting diodes) [63–68], and nonlinear optical devices [69–78]. Moreover, ditopic and dendritic terpyridine ligands may form

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1 Introduction

polymetallic species, which can then be utilized as luminescent or electrochemical sensors [79–134]. Besides these objectives, their biomedical and pharmaceutical utilizations (e.g., as DNA binding or antitumor active agents) are currently rapidly growing fields of research [79, 135–146]. Furthermore, the catalytic activity of terpyridines and their transition metal complexes has been employed to enhance various (asymmetric) organic transformations [147–149]: carbon–carbon single bond formation [150], etherification [151], oxidation of alcohols or ethers [152, 153], cyclopropanation [154, 155], epoxidation [156], CuI-catalyzed alkyne-azide cycloaddition (CCAAC) [157], hydrosilylation [158], and controlled radical polymerization to name only a few [159]. Additionally, RuII bis(terpyridine) complexes have also been used for the photocatalytic splitting of water [160–162]. Well-designed supramolecular (co)polymer architectures have been realized, based on the metal-terpyridine connectivity, opening up avenues to smart “selfhealing” materials with the opportunity of switching the physical and/or chemical properties of materials depending on parameters such as pH value or temperature [36, 38, 163–173]. Finally, the self-assembly of terpyridine complexes onto nanostructures (e.g., based on gold, silver, CdS, TiO2, carbon nanotubes) [174–178] as well as surfaces (e.g., glass, indium tin oxide, gold, graphite) [179–187] is considered in this context. The diversity of applications related to terpyridines and their metal complexes calls for a high structural variability of the basic 2,2u:6u,2v-terpyridine subunit. In particular, terpyridine designs featuring p-conjugated substituents, commonly 450 400

Number of publications

350 300 250 200 150 100 50 0 1950

1960

1970

1980

1990

2000

2010

Publication year Figure 1.2 Histogram of the number of publications containing the term “terpyridine” using SciFindert (the search was performed on 1st March 2011).

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|3

4

| 1 Introduction R

R'

N R

R'

R

X

n

N 1

N

N R

R

n

N

N

R

2

N

N N

3

N

R

N

N R

4

¨ Figure 1.3 Chemical structures of 4u-functionalized (1, X ¼ O, N or S), a Krohnke-type (2), a rigid U-shaped (3, n ¼ 0, 1 or 2), and a Ziessel-type terpyridine (4).

attached in 4u-position, are of increasing interest. Figure 1.3 depicts the general schematic structures of four widely used types of terpyridines. Terpyridines (1) can be considered as “workhorses” in the field of metallo-supramolecular chemistry – a multitude of terpyridine-functionalized polymers has been derived from this structural motif [36, 38, 166, 171, 188, 189]. By far, most conjugated terpyridine¨hnke-motif, which containing systems used today are based on the so-called Kro features a functionalized phenyl moiety at the 4u-position of the terpyridine unit (2) [37]. Their rigid U-shaped counterparts 3 have – mainly due to synthetic limitations – been employed less frequently [190] but offer entree to a more rigid configuration. The Ziessel-type terpyridines 4, where p-conjugation is extended via ethynyl-based systems, have been studied in particular with respect to electrontransfer processes [187, 191]. In view of the notable importance of 2,2u:6u,2v-terpyridines and their metal complexes in current research, we herein focus on architectures containing these types of ligand and their corresponding metal complexes. The earlier book Modern Terpyridine Chemistry aimed mainly to summarize the syntheses, chemistry, and properties of functional terpyridine architectures: complexes, supramolecular polymers, 3D-structures, and surfaces [192]. Owing to the fast development of terpyridine-based materials, this book presents a detailed look beyond the basic concepts of syntheses and properties to applications with relevance to various aspects of human life. Therefore, this book consists of different topics related to “terpyridine-based materials,” each of which is discussed in an individual chapter. Chapter 2 summarizes the known synthetic strategies leading to different terpyridines. Since terpyridines of types 1–4 currently represent the most valuable derivatives, emphasis is laid on the discussion of the various routes of their syntheses. In this context, their properties, in particular their photophysical behavior, is also evaluated. Chapter 3 describes the preparation and properties of mononuclear terpyridine metal complexes. Emphasis will be on bis(terpyridine) complexes of RuII, OsII, IrIII, and PtII ions as well as their photophysical and electrochemical properties. Moreover, oligonuclear complexes, such as dyads and triads, are included. In particular, architectures based on RuII ions are featured in which combinations with other transition metal ions could, for example, potentially lead to “molecular switches” opening up avenues to the construction of nanodevices.

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References

Chapter 4 features more advanced supramolecular aggregates composed of terpyridine-metal subunits: macrocycles, grids, helicates, or rotaxanes. Such materials are of interest for the understanding of supramolecular aggregation into 2D and 3D architectures. Furthermore, applications as either “molecular machines” or optoelectronic devices have been envisioned. The combination of p-conjugated bis(terpyridine)s with transition metal ions affords high molar mass p-conjugated metallopolymers; in these materials, the properties of conventional conjugated polymers and terpyridine complexes are merged (Chapter 5). Polymer light-emitting diodes (PLEDs) or polymer solar cells (PSCs) are the most prominent targets of research in this emerging field. Polymeric architectures containing terpyridine systems with various architectures, from side-chain-functionalized polymers to main-chain metallopolymers, are summarized in Chapter 6. The incorporation of terpyridine complexes into polymer architectures enables the synthesis of advanced multiblock copolymers (that, for instance, can form micelles or phase-separate in the bulk) or polymerbound photoactive metal complexes for optoelectronic applications. Chapter 7 summarizes terpyridine metal complexes that have recently found application in the fields of biochemistry and pharmacy. In particular, PtII mono (terpyridine)s are potential cytotoxic agents that could be potential replacements for the traditional PtII-based drugs (e.g., cisplatin, carboplatin). Oxidative DNA cleavage, induced by various types of terpyridine complexes, is another major field in the biomedical arena. Photoluminescent complexes can be attached to biomolecules and, therewith, be utilized as labeling agents in pharmaceutical applications. The covalent binding of terpyridines to surfaces has led to the development of molecular wires. Fast energy-transfer processes along these wires point to potential applications in organic electronics. Besides their attachment to surfaces, the binding of terpyridine ligands (or their complexes) to organic as well as inorganic nanomaterials will, then, be considered in Chapter 8. Chapter 9 describes applications of terpyridines and their complexes in the fields of organometallic catalysis. Terpyridine ligands (and their complexes) have been used as homogeneous or heterogeneous catalysts in various types of (asymmetric) organic reactions; important contributions will be summarized. Utilization of photoactive terpyridine complexes in energy-transfer reactions will be considered with respect to “artificial photosynthesis” and photocatalytic water splitting reactions. Finally, Chapter 10 provides a few concluding remarks.

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115 Yang, Q.-Z., Wu, L.-Z., Wu, Z.-X., Zhang, L.-P., and Tung, C.-H. (2004) Chin. J. Chem., 22, 1–3. 116 Ulrich, G. and Ziessel, R. (2004) J. Org. Chem., 69, 2070–2083. 117 Goze, C., Ulrich, G., Charbonniere, L., Cesario, M., Prange, T., and Ziessel, R. (2003) Chem. Eur. J., 9, 3748–3755. 118 Ziessel, R. (1999) J. Inclusion Phenom. Macrocycl. Chem., 23, 369–379. 119 Baxter, P.N.W. (2003) Chem. Eur. J., 9, 5011–5022. 120 del Pilar Taboada Sotomayor, M., Tescardello Dias, I.L., de Oliviera Neto, N., and Kubota, L.T. (2003) Anal. Chim. Acta, 494, 199–205. 121 Loiseau, F., Passalacqua, R., Campagna, S., Polson, M.I.J., Fang, Y.Q., and Hanan, G.S. (2002) Photochem. Photobiol. Sci., 1, 982–990. 122 Salimi, A., Pourbeyram, S., and Amini, M.K. (2002) Analyst, 127, 1649–1656. 123 Zhang, Y., Murphy, C.B., and Jones, W.E. Jr (2002) Macromolecules, 35, 630–636. 124 Jiang, B.-W., Zhang, Y., Sahay, S., Chatterjee, S., and Jones, W.E. Jr (1999) Proc. SPIE Int. Soc. Opt. Eng., 3856, 212. 125 Aiet-Haddou, H., Wiskur, S.L., Lynch, V.M., and Anslyn, E.V. (2001) J. Am. Chem. Soc., 123, 11296–11297. 126 Kimura, M., Hamakawa, T., Hanabusa, K., Shirai, H., and Kobayashi, N. (2001) Inorg. Chem., 40, 4775–4779. ´z, D.J., Bernhard, S., Storrier, G.D., 127 Dia ã, H.D. (2001) J. Phys. Chem. and Abrun B, 105, 8746–8754. 128 Padilla-Tosta, M.E., Lloris, J.M., ń ˜ez, R., Pardo, T., Martıńez-Ma Sancenon, F., Soto, J., and Marcos, M. D. (2001) Eur. J. Inorg. Chem., 1221–1226. 129 Padilla-Tosta, M.E., Lloris, J.M., ń ˜ez, R., Marcos, M.D., Martıńez-Ma Miranda, M.A., Pardo, T., Sancenon, F., and Soto, J. (2001) Eur. J. Inorg. Chem., 1475–1482. 130 Padilla-Tosta, M.E., Lloris, J.M., ń ˜ez, R., Benito, A., Soto, Martıńez-Ma J., Pardo, T., Miranda, M.A., and Marcos, M.D. (2000) Eur. J. Inorg. Chem., 741–748.

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| 1 Introduction 131 Padilla-Tosta, M.E., Lloris, J.M., ´˜ Martıńez-Ma nez, R., Pardo, T., Miranda, M.A., and Marcos, M.D. (2000) Inorg. Chem. Commun., 3, 45–48. ¨ter, 132 Fabre, B., Lehmann, U., and Schlu A.D. (2001) Electrochim. Acta, 46, 2855–2861. 133 Goodall, W. and Williams, J.A.G. (2000) J. Chem. Soc., Dalton Trans., 2893–2895. 134 Murtaza, Z. and Lakowicz, J.R. (1999) Proc. SPIE Int. Soc. Opt. Eng., 3602, 309. 135 Peterson, J.R., Smith, T.A., and Thordarson, P. (2007) Chem. Commun., 1899–1901. 136 Peterson, J.R., Smith, T.A., and Thordarson, P. (2010) Org. Biomol. Chem., 8, 151–162. 137 Piao, X.-J., Zou, Y., Wu, J.-C., Li, C.-Y., and Yi, T. (2009) Org. Lett., 11, 3818–3821. 138 Bertrand, H., Sombard, S., Monchaud, D., Talbot, E., Guedin, A., Mergny, J.-L., Grunert, R., Bednarski, P.J., and Teulade-Fichou, M.-P. (2009) Org. Biomol. Chem., 7, 2864–2871. 139 Eryazici, I., Moorefield, C.N., and Newkome, G.R. (2008) Chem. Rev., 108, 1834–1895. 140 Jung, Y.-W. and Lippard, S.J. (2007) Chem. Rev., 107, 1387–1407. 141 Kelland, J. (2007) Nat. Rev. Cancer, 7, 573–584. 142 Lo, Y.-C., Ko, T.-P., Su, W.-C., Su, T.L., and Wang, A.H.-J. (2009) J. Inorg. Biochem., 103, 1082–1092. 143 Wu, P., Wong, E.L.-M., Ma, D.-K., Tong, G.S.-M., Ng, K.-M., and Che, C.M. (2009) Chem. Eur. J., 15, 3652–3656. 144 Gao, Y.-H., Wu, J.-Y., Li, Y.-M., Sun, P.-P., Zhou, H.-P., Yang, J.-X., Zhang, S.-Y., Jin, B.-K., and Tian, Y.-P. (2009) J. Am. Chem. Soc., 131, 5208–5213. 145 Anthonysamy, A., Balasubramaniam, S., Shanmugaiah, V., and Mathivanan, N. (2008) Dalton Trans., 2136–2143. 146 Lo, K.K.-W., Chung, C.-K., Ng, D.C.-M., and Zhu, N. (2002) New J. Chem., 26, 81–88. 147 Dannacher, J. (2006) J. Mol. Catal. A: Chem., 251, 159–176.

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148 Kwong, H.-L., Yeung, H.-L., Yeung, C.T., Lee, W.-S., Lee, C.-S., and Wong, W.-T. (2007) Coord. Chem. Rev., 251, 2188–2222. ¨gerlein, W., Dreisbach, C., Hugl, 149 Ma H., Tse, M.-K., Klawonn, M., Bhor, S., and Beller, M. (2007) Catal. Today, 121, 140–150. 150 Liu, P., Zhou, C.-Y., Xiang, S., and Che, C.-M. (2010) Chem. Commun., 46, 2739–2741. 151 Gnanamgari, D., Leung, C.-H., Schley, N.D., Hilton, S.T., and Crabtree, R.H. (2008) Org. Biomol. Chem., 6, 4442– 4445. 152 Chen, X., Liu, Q., Sun, H.-B., Yu, X.Q., and Pu, L. (2010) Tetrahedron Lett., 51, 2345–2347. 153 Kamijo, S., Amaoka, Y., and Inoue, M. (2010) Chem. Asian J., 5, 486–489. 154 Yeung, C.-T., Lee, W.-S., Tsang, C.-S., Yiu, S.-M., Wong, W.-T., Wong, W.-Y., and Kwong, H.-L. (2010) Polyhedron, 29, 1497–1507. 155 Yeung, C.-T., Teng, P.-F., Yeung, H.-L., Wong, W.-T., and Kwong, H.-L. (2007) Org. Biomol. Chem., 5, 3859–3864. 156 Liu, P., Wong, E.L.-M., Yuen, A.W.-H., and Che, C.-M. (2008) Org. Lett., 10, 3275–3278. 157 Suzuka, T., Ooshiro, K., and Kina, K. (2010) Heterocycles, 81, 601–610. ¨ zdemir, I., Ko ÿtepe, S., 158 Seckin, T., O ¨rbu ¨z, N. (2009) J. Inorg. and Gu Organomet. Polym. Mater., 19, 143–151. 159 Tang, H., Rodosz, M., and Shen, Y. (2009) AiChE J., 55, 737–746. 160 Chen, Z.-F., Concepcion, J.J., Jurss, J. W., and Meyer, T.J. (2009) J. Am. Chem. Soc., 113, 15580–15581. 161 Concepcion, J.J., Jurss, J.W., Norris, M. R., Chen, Z.-F., Templeton, J.L., and Meyer, T.J. (2010) Inorg. Chem., 49, 1277–1279. 162 Wasylenko, D.J., Ganesamoorthy, C., Koivisto, B.D., Henderson, M.A., and Berlinguette, C.P. (2010) Inorg. Chem., 49, 2202–2209. 163 Hager, M.D., Greil, P., Leyens, C., van der Zwaag, S., and Schubert, U.S. (2011) Adv. Mater., 5424–5430. 164 Hofmeier, H. and Schubert, U.S. (2004) Chem. Soc. Rev., 33, 373–399.

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References 165 Mansfeld, U., Hager, M.D., Hoogenboom, R., Ott, C., Winter, A., and Schubert, U.S. (2009) Chem. Commun., 3386–3388. 166 Lohmeijer, B.G.G., and Schubert, U.S. (2005) J. Polym. Sci., Part A: Polym. Chem., 43, 6331–6344. 167 Gohy, J.-F., Ott, C., Hoeppener, S., and Schubert, U.S. (2009) Chem. Commun., 6038–6040. 168 Chiper, M., Fournier, D., Hoogenboom, R., and Schubert, U.S. (2008) Macromol. Rapid Commun., 29, 1640–1647. 169 Landsmann, S., Winter, A., Chiper, M., Fustin, C.-A., Hoeppener, S., and Schubert, U.S. (2008) Macromol. Chem. Phys., 209, 1666–1672. 170 Mugemana, C., Guillet, P., Hoeppener, S., Schubert, U.S., Fustin, C.-A., and Gohy, J.-F. (2010) Chem. Commun., 46, 1296–1298. 171 Fustin, C.-A., Guillet, P., Schubert, U. S., and Gohy, J.-F. (2007) Adv. Mater., 19, 1665–1673. 172 Hofmeier, H. and Schubert, U.S. (2003) Macromol. Chem. Phys., 204, 1391–1397. 173 Ott, C., Hoogenboom, R., and Schubert, U.S. (2008) Chem. Commun., 3516–3518. 174 Chan, Y.-T., Li, S., Moorefield, C.N., Wang, P., Shreiner, C.D., and Newkome, G.R. (2010) Chem. Eur. J., 16, 4164–4168. 175 Duffort, V., Thouvenot, R., Afonso, C., Izzet, G., and Proust, A. (2009) Chem. Commun., 6062–6064. 176 Kubo, W., Nagao, M., Otsuka, Y., Homma, T., and Miyata, H. (2009) Langmuir, 25, 13340–13343. 177 Pan, Y.-X., Tong, B., Shi, J.-B., Zhao, W., Shen, J.-B., Zhi, J., and Dong, Y.-P. (2010) J. Phys. Chem. C, 114, 8040–8047. 178 Gao, Y.-H., Wu, J.-Y., Zhao, Q., Zheng, L.-X., Zhou, H.-P., Zhang, S.-Y., Yang, J.-X., and Tian, Y.-P. (2009) New J. Chem., 33, 607–611.

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179 Smith, H.L., Usala, R.L., McQueen, E. W., and Goldsmith, J.I. (2010) Langmuir, 26, 3342–3349. 180 Alvaro, M., Aprile, C., Ferrer, B., Sastre, F., and Garcia, H. (2009) Dalton Trans., 7437–7444. 181 Haensch, C., Chiper, M., Ulbricht, C., Winter, A., Hoeppener, S., and Schubert, U.S. (2008) Langmuir, 24, 12981–12985. 182 Tuccitto, N., Delfanti, I., Torrisi, V., Scandola, F., Chioboli, C., Stepanenko, ¨rthner, F., and Licciardello, A. V., Wu (2009) Phys. Chem. Chem. Phys., 11, 4033–4038. 183 Tuccitto, N., Ferri, V., Cazazzini, M., Quici, S., Zhavnerko, G., Liciardello, A., and Rampi, M.A. (2009) Nat. Mater., 8, 41–46. 184 Tuccitto, N., Torissi, V., Cavazzini, M., Morotti, T., Puntoriero, F., Quici, S., Campagna, S., and Liciardello, A. (2007) ChemPhysChem, 8, 227–230. 185 Utsuno, M., Toshimitsu, F., Kume, S., and Nishihara, H. (2008) Macromol. Symp., 270, 153–160. 186 Auditore, A., Tuccitto, N., Marzanni, G., Quici, S., Puntoriero, F., Campagna, S., and Liciardello, A. (2003) Chem. Commun., 2494–2495. 187 Kurita, T., Nishimori, Y., Toshimitsu, F., Muratsugu, S., Kume, S., and Nishihara, H. (2010) J. Am. Chem. Soc., 132, 4524–4525. 188 Whittle, G.R., Hager, M.D., Schubert, U.S., and Manners, I., Nat. Mater., 10, 176–188. 189 Chiper, M., Hoogenboom, R., and Schubert, U.S. (2009) Macromol. Rapid Commun., 30, 565–578. ¨rke, U., 190 Sielemann, D., Winter, A., Flo and Risch, N. (2004) Org. Biomol. Chem., 2, 863–868. 191 Benniston, A.C., Harriman, A., and Li, P.-Y. (2010) J. Am. Chem. Soc., 132, 26–27. 192 Schubert, U.S., Hofmeier, H., and Newkome, G.R. (2006) Modern Terpyridine Chemistry, Wiley-VCH Verlag GmbH, Weinheim.

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| 13

2

Synthesis, Properties, and Applications of Functionalized 2,20 :60 ,200 -Terpyridines*

2.1 Introduction

In view of the wide range of research and potential applications of 2,20 :60 ,200 -terpyridines, an easily accessible “pool” of different functionalized building blocks is mandated. For this purpose, highly efficient routes to these ligands are as essential as their well-defined derivatization at every ring-position. Functional groups may be introduced directly during their construction or by various substitution/interconversion reactions. While publications concerning the chemistry of terpyridine complexes have continued to increase, comparably few preparation procedures of functionalized 2,20 :60 ,200 -terpyridine derivatives have been reported. In 1997, Thompson [1] reviewed the historical syntheses of the simple terpyridine ligands and, in 2003, Fallahpour [2] as well as Heller and Schubert [3] reviewed the 40 substituted terpyridines. This chapter presents both innovative new synthetic strategies and an up-to-date overview of the reported conventional approaches leading to 2,20 :60 ,200 -terpyridine derivatives. New developments in the preparation of ditopic and star-shaped terpyridines will also be evaluated. In addition, the (photophysical) properties as well as potential applications of terpyridines, in particular of p-conjugated species, are also included.

2.2 Basic Synthetic Strategies

The first synthesis of 2,20 :60 ,200 -terpyridine (tpy) was reported in 1932 by Morgan and Burstall, who isolated tpy in poor yield as a by-product of the oxidative condensation of pyridine with FeIII chloride [4]. Since then, diverse preparations of basic terpyridine and the introduction of various substituents, suited for further

*Parts of this chapter are reproduced from Chem. Soc. Rev. 40 (2011) 1459–1511 by permission of The Royal Society of Chemistry. Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

02

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14

| 2 Synthesis, Properties, and Applications of Functionalized 2,2 :6 ,2 -Terpyridines 0

synthesis of the central ring via ring-assembly

N

00

synthesis via cross-coupling of pyridine derivatives

N

N

0

N

N

N

(a)

(b)

Figure 2.1 The two methods for synthesizing tpy ligands: ring-assembly (a) and crosscoupling reactions (b).

functional-group-interconversions, have been published. In general, there are two basic synthetic approaches to terpyridines – ring-assembly and cross-coupling methodologies (Figure 2.1). Ring-assembly is still a common and general strategy, in particular for the synthesis of 40 -aryl-substituted terpyridines, the so-called Kr€ohnke-type terpyridines. Owing to their multiplicity and efficiency, modern Pd0catalyzed cross-coupling procedures have recently become a highly versatile alternative to the rather traditional ring-closure processes. 2.2.1 Ring-Assembly Methodologies

Over the last couple of decades, various new terpyridine ring-assembly strategies have been developed, basically relying on the traditional Hantzsch-type [5] and Tschitschibabin-type [6] syntheses of pyridine derivatives. Scheme 2.1 depicts the routes that are frequently used today. The most common ring-assembly O R1

O R'

N

+

N

1

N

route a

NH4OAc R2

N

3

R'

X

2

R' O

O R1

NH4OAc

N R2

N R1

N

N R2

4

route b

SMe O R

SMe N

OK

SMe +

NH4OAc R'

5

OK

O NMe2 N

route c

N

+ N

6

N R

N

N R'

NH4OAc route d

N

N 7

N

Scheme 2.1 General ring-assembly methods: Kr€ ohnke condensation (routes a and b), Potts’ methodology (route c), and Jameson’s protocol (route d).

02

27 J l 2011 15 38 20

2.2 Basic Synthetic Strategies

methodology of substituted terpyridines 4 is still the well-known Kr€ ohnke condensation (route a), involving the synthesis of N-heteropyridinium salts 2 (X ¼ Br or I) via an Ortoleva–King reaction [7] and subsequent ammonia condensation with an enone 1 [prepared in an aldol reaction from 2-acetylpyridine and a (hetero)aromatic aldehyde] [8, 9]; alternatively, 1,5-diketones 3 can be prepared (in an aldol–Michael cascade reaction), followed by ring-closure with an appropriate N-source (route b) [10–12]. The Kr€ ohnke protocol enables the synthesis of both symmetrical (i.e., R1 ¼ R2) and unsymmetrical terpyridines (i.e., R1 6¼ R2) in moderate to good yields. The versatility of this multistep condensation protocol prompted Fritz Kr€ohnke to opine that “every pyridine motif that you compose in the comfort of your house can now be synthesized in the laboratory the next day right away” [13]. However, some considerable limitations of the method have to be addressed: the substituent R0 must be (hetero)aromatic and sensitive functional groups are not tolerated under the reaction conditions. Therefore, various more efficient and environmentally friendlier variants of the original reaction protocol have been recently developed, including the utilization of microwave irradiation (to decrease the reaction times) [14–16], the use of “green solvents” [e.g., poly(ethylene glycol) (PEG)] [17, 18], ionic liquids [19], and solvent-free conditions [20– 22] or aqueous ammonia (avoiding the excess of ammonium acetate, as a nitrogen source) [23]. As a result, a wide range of functional groups (i.e., R1, R2, and R0 ) is tolerated under these reaction conditions and the Kr€ ohnke-type terpyridines 4 have become the most frequently used building blocks, in particular in the field of pconjugated terpyridines (Chapter 5). In this context, some alternative approaches for the synthesis of terpyridines via ring-assembly of the central pyridine ring need to be considered: the a-oxoketene dithioacetal methodology (5), as introduced by Potts [8] (route c), or the Jameson method [24], involving the condensation of a N,N-dimethylaminenone (6) with 2acetylpyridinenolate (route d), have been utilized (Scheme 2.1). However, limitations of these with respect to accessible structural diversity make them less attractive than Kr€ohnke’s approaches (a and b). Nonetheless, the latter is still the most efficient methodology for synthesizing the parent 2,20 :60 ,200 -terpyridine (7). Furthermore, various methodologies to obtain tpy derivatives with a high degree of functionalization on the outer pyridine rings have also been established. For instance, a multistep procedure towards symmetric 5,500 -disubstituted terpyridines was reported by Adrian et al. [25]. Remarkably high yields (73–93%) were reported, but the relatively long reaction sequence and low variability of the substituents prevented a broader application of this protocol. The pyrolysis of hydrazonium salts (at about 200 1C), a reaction widely used for the synthesis of substituted pyridine derivatives, was applied for the preparation of 6,600 -dimethyl-2,20 :60 ,200 terpyridine (in 47% yield), as the exclusive example [26]. The regioselective cyclocondensation of carboxamidrazones (8) with a-pyridylglyoxal (9) in aqueous ethanol gave 3,5-di(pyridin-2-yl)[1,2,4]triazines (10) that underwent an inverse-type Diels–Alder reaction in ortho-dichlorobenzene, as solvent, with either norbornadiene or ethynyltri(n-butyl)tin to form terpyridines 11 and 12, respectively, in good yields (Scheme 2.2) [27].

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27 J l 2011 15 38 20

| 15

16


N R

N

NH2 R aq. EtOH

+

N R

N

O O

00

NH2

8

9

0

N

N N

Sn(n-Bu)3

10 Sn(n-Bu)3

R

N

N 12

Scheme 2.2 Sauer’s methodology involving an inverse-type Diels–Alder reaction.

More recently, an effective and simple twofold Kr€ ohnke-type synthesis of highly substituted symmetric terpyridines from 2,6-diacetylpyridine (13) has been described by Sasaki et al. [28] (Scheme 2.3a), where bis(pyridinium) iodide 14 (obtained from 13 in 85% yield) is subsequently reacted with an a,b-unsaturated aldehyde (15), at 80 1C for 4 h in DMF, in the presence of ammonium acetate to give symmetric terpyridine 16 (yields ranged from 47% to 91%). Since 4-functionalized 2,6-diacetylpyridines (19), key substrates for the Kr€ ohnke methodology, have become available [29], Sasaki’s procedure could also be extend towards 40 functionalized derivatives 20 [30]. For this purpose, 19 was prepared in 36% overall yield from 4-hydroxypyridine-2,6-dicarboxylic acid (17, “chelidamic acid”) by converting it first into a 40 -substituted 2,6-bis(chlorocarbonyl)pyridine (18), followed by treatment with Meldrum’s acid (2,2-dimethyl-1,3-dioxan-4,6-dione) and, subsequently, hydrolysis with aqueous acetic acid enabling a twofold decarboxylation (Scheme 2.3b). The same starting materials (i.e., 13 and 19) were also reacted with electron-deficient 1-methyl-3,5-dinitro-2-pyridone (21), in the presence of ammonia, to give 5,500 -dinitro-2,20 :60 ,200 -terpyridines 22 in about 70% yield (Scheme 2.3c). Different types of terpyridine derivatives, the more rigid and conformationally constricted terpyridines 25 (so-called U-shaped terpyridines), were investigated by the Thummel and Risch groups. However, the Kr€ ohnke-type condensation cannot be used for the synthesis of these annulated derivatives; thus, different approaches were established that rely on the increased reactivity of activated carbonyl derivatives, as starting materials. Thummel et al. reacted the enamines of cyclic a-pyridyl ketones (23; n ¼ 0, 1, or 2) with (hetero)aromatic aldehydes to obtain the 1,5diketone derivatives 24 (as structural analogous to 3; see also Scheme 2.1); in a second step, the central pyridine ring of 25 was assembled by ring-closure with NH4OAc, as a N-source, and subsequent oxidation under the reaction conditions (35–65% yield) (Scheme 2.4) [31–38]. Alternatively, ternary iminium salts 27 [R ¼ H, (hetero)aryl, isopropyl, or tert-butyl] can be utilized for the reaction with imines 26 (n ¼ 0, 1, or 2; generated in situ from the corresponding ketone and NH4OAc) [39–42]. In this case, a mixture of U-shaped terpyridines 25 and their Sshaped regioisomers (28) is formed; however, under the optimized reaction conditions, both isomers can be synthesized selectively (up to 95% chemoselectivity) [43, 44].

02

27 J l 2011 15 38 21

N

11

N

N

N

N

| 17

2.2 Basic Synthetic Strategies R1

O R2 15 NH4OAc, DMF

(a) I2, pyridine

N

N

N

N O

O

13

(b)

N

14

OH

O

C

O

17

R1

R1

R1

N

R2

16

R1

O

OH

HO

R2

2I

O

N

N

Cl

N

N

O

O 18 1 R = OEt or Cl

O

O

19

R1 R2

N

R3

R2

N

N

R3

20 R4

R4

R

(c) O2N 13 or 19

NO2

+

N

NH4OAc

N

21

N

N

O

R = H,OEt or Cl

22

O2N

NO2

Scheme 2.3 Variants of the Kr€ ohnke-type condensation according to Sasaki (a) and Fallahpour (b and c).

Thummel's methodology (a)

O O R 1,4-dioxane, reflux,12 h

N N

R

O

R

n

N O

n

NH4OAc

N

n

N

n

23

n

N 24

N 25

Risch's methodology (b)

NH N

R' + Cl

n

R

N

n

n

and/ or

N

R

n

26

N

R'

NH4OAc,DMSO, 80 °C,2 h

N

27

25

N N n

N

28

Scheme 2.4 Synthesis of rigid terpyridines 25 according to the groups of Thummel (a) and Risch (b).

02

27 J l 2011 15 38 21

R

18


0

00

Compared to Kr€ohnke-type terpyridines, the U-shaped structures are, by far, less utilized in the literature, mainly due to the easy access to cyclic ketone precursors and the rather low tolerance towards the presence of functional groups under the reaction conditions. The protocols for the preparation of either 4 or 25 are exclusively suited for the particular class of terpyridines. A universal condensation method giving access to both classes has yet to be reported. 2.2.2 Cross-Coupling Procedures

In the last two decades, directed cross-coupling procedures, as appropriate tools for the construction of the (un)functionalized tpy core, have become a focus of preparative chemists. Traditional methodologies, such as the cross-coupling of organosulfur compounds [45] or lithiopyridines with CuCl2 [46], suffer from poor conversions and, in some cases, low directionality. Moreover, the syntheses often involve harsh reaction conditions and, therefore, many functional groups are not tolerated. In contrast, the modern Pd0-catalyzed cross-coupling reactions combine the desired efficiency and simplicity with controllable substitution possibilities; consequently the pioneering research in this field was honored very recently with the Nobel Prize in Chemistry [47]. The Suzuki- [48], Negishi- [49], and Stille crosscoupling reactions [50] are all based on a Pd0/PdII catalytic cycle and, in particular, the latter has become a popular route towards terpyridines, for several reasons: (i) its universal building block principle, (ii) multigram product accessibility, and (iii) well-directed functionalization at almost every desired position [51]. 2,20 :60 ,200 -Terpyridines, functionalized at the central and/or the outer pyridine rings (33), can be obtained in high to excellent yields utilizing appropriate 2,6dihalopyridines 29, as central building blocks, which can be reacted with 2trialkylstannylpyridines 30 and a Pd0-catalyst in toluene under reflux for at least 24 h (Scheme 2.5, route a) [52–59]. Alternatively, synthesis via the Stille

R1

R1 R2

+ X

N X 29 (X = Cl, Br or I)

(alkyl)3Sn

N

N 31

(alkyl)3Sn

30

route

R2

+ Sn(alkyl)3

Br

N 32

b route

a

cat. Pd(PPh3)4 or Pd(PPh3)2Cl2,

R1 5 H, alkyl, NO2, COOEt R2 5 H, alkyl

toluene, reflux, 2-24 h R1

N R2

N

N R2

33

Scheme 2.5 Synthesis of substituted 2,20 :60 ,200 -terpyridines via Stille cross-coupling reactions.

02

27 J l 2011 15 38 21

2.3 Synthesis and Properties of 2,20 :60 ,200 -Terpyridine Derivatives

cross-coupling procedure can also be conducted by utilizing 2,6-bis(trimethylstannyl)pyridines 31, as a central ring, and coupling them with the corresponding 2-bromopyridines 32 (Scheme 2.5, route b) [60, 61]. Other Pd-catalyzed, cross-coupling procedures have not yet been used for the construction of terpyridines themselves, but seem to be appropriate methods: for instance, Negishi cross-coupling was used for the synthesis of substituted 2,20 bipyridines [62] or 2,6-di(pyridin-2-yl)benzenes (the mono-cyclometalating N4C4N-analogous to the N4N4N-type terpyridine ligands, see also Chapters 3.3.2.2 and 3.4) [63] in excellent yields. The Suzuki cross-coupling reaction of boronic acids was applied successfully in the synthesis of 2,6-di(quinolin-8-yl)pyridine (dqp) derivatives – a family of ligands featuring an expanded bite angle compared to 2,20 :60 ,200 -terpyridines (for their utilization, as ligands, in the coordination sphere of transition metal ions; see Chapter 3.3.2.2) [64]. Though utilization of boronic acids, as coupling partners, avoids the occurrence of toxic byproducts (as in the Stille reaction), the instability and facile proto-deboronation of pyridin-2-yl boronic acids [65] might limit potential applications of the Suzuki reaction in this field.


The derivatization of terpyridines may occur on either the outer pyridine rings or central ring. Since the underlying tpy core is chemically robust, appropriate substituents, suitable for subsequent interconversion of functional groups, need to be introduced during the synthesis of the terpyridine via ring-assembly or cross-coupling procedures (see previous sections). Since the entire field of functionalized terpyridines cannot be detailed within the scope of this chapter, general methods of functionalization as well as selected examples of substituted 2,20 :60 ,200 -terpyridines – with respect to their properties and potential applications – will be highlighted. For detailed overviews on particular types of functionalized terpyridines (e.g., 40 -functionalized, p-conjugated) the reader is referred to review articles [2, 3]. 2.3.1 40 -Substituted 2,20 :60 ,200 -Terpyridinoxy Derivatives

The most common and applicable strategy for functionalization of the central pyridine ring is the formation of 40 -terpyridinoxy derivatives. To date, structure 36 represents the dominant substitution pattern due to convenient accessibility via nucleophilic aromatic substitution of 40 -chloro-2,20 :60 ,200 -terpyridine (35) [66], which has now became commercially available, by any primary alcohol and their analogs (alternatively, also the 40 -bromo- [67] or 40 -triflato-counterparts [68] may be used) or SN2-type nucleophilic substitution of the alcoholates of 40 -hydroxyterpyridines [i.e., the “enol” tautomer of 2,6-di(pyridin-2-yl)pyridin-4(1H)-one (34) [66]). Scheme 2.6 presents an overview of these routes.

02

27 J l 2011 15 38 21

| 19

20


0

00

O O

O

acetone

2

OEt

NaH, THF

N

O

O

NH4OAc N N

N

N

N H

34

POCl3, PCl5

r

R-B OR

Cl R-OH

N

N

36

N

N

N

N 35

Scheme 2.6 Synthetic approaches towards 40 -terpyridinoxy derivatives 36 via nucleophilic substitution reactions.

Over the last two decades, various groups have reported structurally simple 40 terpyridinoxy derivatives, including linear or branched aliphatic chains [69–72] as well as aliphatic chains with terminal hydroxy- [73–78], carboxy- [76, 79, 80], thio[75, 81, 82], amino- [83–86], primary or secondary amine groups [87], and halogen atoms [82]. In general, these compounds were prepared in high yields (60–90%) from 35 with an alcohol and a suspension of base (KOH or NaH) in a polar, nonprotic solvent (e.g., DMSO or DMF). The same concept was applied, in particular by the Schubert group, for the end-group modification of hydroxy-terminated polymers (for details, see Chapter 6). The terminal functional groups enable the incorporation of terpyridine ligands into more advanced architectures, such as polymers. Esterification of the terminal –OH of 37 by reaction with the respective acid chloride gave the corresponding acrylates and methacrylates 38a/b, as monomers for radical (co)polymerizations (Scheme 2.7, see also Chapter 5.2.1) [88]. Besides this, the NH2-functionalized derivatives were also utilized in macromolecular chemistry in which the functionalization of preformed polymers by amidation of the pendant CO2H-moieties along the polymer backbone (a “grafting-onto” strategy) afforded side-chainfunctionalized macroligands, in high yields (see also Chapter 6.2.1) [89, 90]. In related work, a terminal –NH2 unit could be converted efficiently into an R

R O

HO

Cl O NEt3, CH2Cl2, 0° C, 2 h

n O

O n O

N

N

N N

37 n 2-4

N

38a: "acrylate"(R H) 38b: "methacrylate"(R CH3) n 2-4 N

Scheme 2.7 Synthesis of acrylate- and methacrylate-functionalized terpyridine monomers 38.

02

27 J l 2011 15 38 21


isocyanate group using di(tert-butyl) tricarbonate as reagent [91]. The reactive building block was subsequently used for the functionalization of polymers bearing hydroxy groups at the chain end or as pendant substituents [91, 92]. The combination of terpyridine units, as binding site for transition metal ions, and fullerene derivatives, widely used as electron–acceptors in bulk-heterojunction solar cells (BHSCs) [93], was investigated by the Schubert and Meijer groups [73, 94]. Amidation of 39 with the C60-derivative 40 was carried out with thionyl chloride, as reagent, to afford the C60-functionalized terpyridine derivative 41 in moderate yield (47%, Scheme 2.8) [73]. Supramolecular donor–acceptor arrays were obtained by the directed self-assembly of 41 and a terpyridine-terminated oligo(p-phenylenevinylene) (OPV) substrate with RuII ions; photo-induced absorption (PIA) measurements indicated the formation of a charge-separated state with a lifetime (t) of 100 ms [94]. Various types of (aza-)crown-ether derivatives were attached to the tpy core via ether linkages [95, 96]; these multitopic ligands were proposed to function as luminescence sensors [97] or photoactive nucleoside diphosphate cleavage agents [98, 99]. Furthermore, substrates either with terminal –CH¼CH2 or –CCH moieties were synthesized [100–102]; in these cases, the alternative “pyridone route” was found to be more applicable. The –HC¼CH2 units could, for instance, be utilized for the modification with a terminal trimethoxysilane group by reaction with an appropriate thiol derivative (irradiation with l ¼ 365 nm for 20 min, the “thiol-ene” reaction) [101, 102]. The co-hydrolysis of 42 with tetraethoxysilane (TEOS), in the presence of rare earth metal ions (i.e., LnIII, Figure 2.2a), yielded the hybrid material 43 as well-defined particles that exhibited characteristic emissions of, for example, EuIII and TbIII ions by intramolecular energy transfer from the triplet state energy of the tpy moiety to the resonant emissive energy level of the central LnIII ion; Figure 2.2b shows the scanning electron microscopy (SEM) image of the EuIII-containing material. Related to this, Cho et al. utilized similar hybrid gels for the adsorption of organic dyes, in the presence or absence of transition metal ions (e.g., ZnII and CuII) [103] and as immobilized support for the Pd0-catalyzed Heck cross-coupling reaction [85]. N O N

NH2 39 i) SOCl2, benzene ii) CH2Cl2

N +

O

N O

O

47% Cl

N

41 N

40

Scheme 2.8 Synthesis of fullerene-functionalized terpyridine 41.

02

27 J l 2011 15 38 22

NH

| 21

22


(a)

0

00

(b)

N O

S

Si(OMe)3

42

N N

Si(OEt)4, Ln(NO2)3*6H2O DMF/H2O O

N O2N O2N

N Ln NO2

N

S

10 µm

SiO2

43 (LnIII EuIII or TbIII)

Figure 2.2 (a) Synthesis of the luminescent hybrid material 43; (b) SEM image of nanoparticles 43 (LnIII ¼ EuIII) [101]. Figure reproduced with kind permission; r 2007 Elsevier B.V.

Further sophisticated residues, such as a spin-labeled derivative [R ¼ 2,2,6,6tetramethylpiperidin-1-oxyl (TEMPO)], for potential application as electron paramagnetic resonance (EPR) probe [104], or a carbaborane cluster (i.e., 1-substituted closo-1,2-C2B10H11), as source of boron in boron neutron capture therapy (BNCT, for examples see also Chapter 7.2.1.1) [105], were also introduced via the routes outlined in Scheme 2.6. Similarly, the attachment of carbohydrate molecules – either directly or with a spacer linkage – was achieved by reacting bromo- or iodofunctionalized mono- and disaccharides with 34 under basic conditions [106, 107]. The metal-free ligands were readily recognized and hydrolyzed by the enzyme glucosidase, whereas the corresponding bis-complexes were not; thus, potential applications in metal-activated drug delivery systems were envisioned [106]. The Mitsunobu reaction, a mild variant of the Williamson-type etherification [employing triphenylphosphine (PPh3) and di(isopropylazo)dicarboxylate (DIAD) as coupling reagents], was utilized by Hovinen et al. to link a dibenzyl-protected nucleoside (i.e., 3,30 -O-Urd) to the terpyridine moiety [108]. A similar approach was followed by the Inoue group for modification of oligonucleotides with terpyridine units [109]. An important example in the field of metallo-supramolecular polymers was reported by the Schubert group: the TIPNO-functionalized terpyridine 44 (TIPNO: 2,2,5-trimethyl-4-phenyl-3-azahexane nitroxide [110–112]) was prepared in moderate yield (61%) according to Scheme 2.9 and was utilized, as initiator, in the controlled and “living” nitroxide-mediated polymerization (NMP) of a broad range of monomers (for a detailed overview over this topic, see Chapter 6.3.4) [113]. The reaction of 36 with 30 -hydroxymethyl-2,20 :50 ,200 -terthiophene under basic conditions in DMSO afforded the terthiophene-functionalized terpyridine 45 (Figure 2.3a) that was utilized by Zanardi et al. for electropolymerization onto

02

27 J l 2011 15 38 22


34

N

O O

K2CO3, DMF

+

61% O

N

N 44

N N

"tpy-TIPNO"

Cl

Scheme 2.9 Synthesis of tpy-TIPNO (44), as NMP initiator.

indium tin oxide (ITO) electrodes [114]. The electrochemical potentiodynamic polymerization [within the limits of 0.35 and þ 0.80 V, in CH2Cl2 with tetra(nbutyl)ammonium hexafluorophosphate (TBAP), as electrolyte, Figure 2.3b] gave a homogeneous orange film. Spectrophotometric analysis at 0.35 V revealed the presence of a single absorption band (labs ¼ 450 nm) that was assigned to the pp* transition band of the polythiophene backbone. The reaction of dihydroxy compounds (e.g., alkan-a,o-diols, cyclohexan-1,2diols) with 40 -chloro-2,20 :60 ,200 -terpyridine (35) gave access to flexible telechelic bis(terpyridine) ligands (in moderate to high yields using KOH/DMSO as reaction medium) that could be self-assembled with various transition metal ions into macrocycles and/or flexible metallopolymers, depending on the reaction conditions (for more details on these topics see Chapters 4.2 and 5.3, respectively) [115]. Moreover, various a,o-dihydroxy-functionalized polymers – such as PEG [(PEG: poly(ethylene glycol)] and PTHF [PTHF: poly(tetrahydrofuran)] – could efficiently be converted into telechelic macroligands according to Scheme 2.6 [116, 117]. These types of materials as well as their “chain-extended” polymers (obtained by self-assembly with transition metal ions) are discussed in Chapter 6.3.2.

(a)

(b) S

3.5E-05

S

3.0E-05 2.5E-05 O

45 N

N

I(A)

S

N

4.0E-05

2.0E-05 1.5E-05 1.0E-05 5.0E-05 0.0E-05 5.0E-05 1.0E-05 0.40

0.20

0.00

0.20

0.40

0.60

0.80

E (V)

Figure 2.3 (a) Terthiophene-functionalized terpyridine 45; (b) electrochemical potentiodynamic polymerization of 45 [5 103 M in CH2Cl2/TBAP (0.1 M), as solvent; ten subsequent scans, scan rate of 0.05 V s1] [114]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

02

27 J l 2011 15 38 22

| 23

24


O

H2NC(CH2CH2CH2OH)3 + 3 eq. 35 KOH, DMSO, 60 °C, 20 h

00

O

Cl

O

O

Cl

Cl

O

O

Cl

O H2NC-CH2CH2CH2O 3 46

0

O

O O

NEt3, 25 °C, 3 d C

O

O

N H

4 47

= 2,2':6', 2''-terpyridin-4'-yl

O

Scheme 2.10 Synthesis of a dendritic terpyridine architecture.

Beyond such ditopic bis(terpyridine) derivatives, the utilization of 35, as key building block, in the preparation of metallo-supramolecular dendritic (i.e., oligotopic) architectures was pioneered by the Newkome and Constable groups [118]. The step-wise construction of the dendron 47 involved the synthesis of tris(terpyridine) 46 [by nucleophilic substitution reaction of 4-amino-4-(3-hydroxypropyl) heptane-l,7-diol and 35] and subsequent amidation reaction with a star-shaped acid chloride (Scheme 2.10). This initial second-generation-dendrimer 47 was used for the coordination of RuII ions, yielding a dodecanuclear metallodendrimer; the homologous third-generation species, containing 36 terpyridine moieties on the outer sphere, was also shown [79]. Similar approaches were followed later, for ã [119], Constable [120, 121], Lin [122, 123], and Newkome instance, by the Abrun groups [80, 124–126] for the construction of metallo-dendritic architectures with the [M(tpy)2]2 þ units (MII ¼ transition metal ion) in the core, as non-core connectors, or at the surface. The broad field of (terpyridine-containing) metallodendrimers with respect to their synthesis and applications was summarized by Newkome et al. and the reader is referred to these reviews for further details [127–129]. 2.3.2 Miscellaneous 40 -Substituted 2,20 :60 ,200 -Terpyridine Derivatives

Among the functional groups directly linked to the terpyridine unit in 40 -position, the –NO2 group is of significant relevance – due to its pronounced electronwithdrawing character [Hammett parameter (s) of 0.81] and the versatility for functional-group-interconversions (as typical for aromatic nitro compounds, see Scheme 2.11). The synthesis of such nitro-substituted terpyridines via Stille crosscoupling reaction is generally carried out according to Scheme 2.5. The –NO2 moiety can be converted into –NH2 by reduction, for example, with hydrazine hydrate and palladium on charcoal, as catalyst [57]. Fallahpour et al. showed that the highly electron-rich –NH2 group [Hammett parameter (s) of 0.57] can be substituted by a halogen in Schiemann- (i.e., F) or Sandmeyer-type reactions (i.e., Br, I) [2]. Utilizing milder reaction conditions for the reduction of –NO2

02

27 J l 2011 15 38 22


tpy

NH2

tpy

X

(X F,Br or I)

tpy

N3

tpy

N tpy

NO2

tpy

N

(X = F,Br or I) 0

Scheme 2.11 4 -Nitro-functionalized terpyridines, as substrates for functional-groupinterconversion (“tpy” denotes any substituted 2,20 :60 ,200 -terpyridine motif) .

(e.g., NaBH4 or SnCl2 2H2O, as reductants) gave access to (E)-1,2-bis(terpyridin40 -yl)diazenes in moderate yields [130]. The central –N¼N– unit exhibited photoinduced cis/trans-isomerization (the lifetime of the trans-isomer was about 40 min) and the resulting dinuclear RuII/OsII complexes showed intramolecular energy-transfer processes (when the diazo group was reduced to the corresponding radical anion; see also Chapter 3.3.2.3). With respect of applications, as reactant, in CuI-catalyzed alkyne-azide [2 þ 3]cycloaddition (CCAAC) reactions, 40 -azido-2,20 :60 ,200 -terpyridine (48) has recently gained considerable interest: the so-called “click reaction” enables the functionalization of terpyridine derivatives by reaction with terminal alkynes, yielding (1H1,2,3-triazol-1-yl)-substituted terpyridines 49. Besides the substitution of conventional leaving groups at the 40 -position of the terpyridine system (i.e., methylsulfonato, triflate, or mesylate) by azide [2], the nitro group can also be replaced by N3 , as a good nucleophile, in good yield (DMF, as solvent, 150 1C, about 70% yield, Scheme 2.12) [59]. The broad scope of the CCAAC reaction was shown by the Schubert group by the reaction of various types of ethynyl compounds with 48 in the presence of CuI ions, as catalyst: ethynyl-functionalized small organic molecules as well as polymers were reacted with 48 in high yields (>69%, Scheme 2.12) [131]. In related work, Constable et al. reported on the CCAAC reaction of FeII bis-complexes of 48 with small organic molecules [132]. Moreover, the thermal reaction of 48 with electronpoor alkynes (e.g., dimethyl acetylenedicarboxylate) gave access to 40 -(2,3-disubstituted 2H-azirin-2-yl)-2,20 :60 ,200 -terpyridines 50 [133]. Under basic conditions, the photo-induced generation of a singlet triplet nitrene, as a reactive species, was observed that underwent electrocyclic ring-expansion, via several intermediates, into the 3,5-disubstituted 1H-1,4-diazepin-7(6H)-one system 51 (Scheme 2.12) [133]. The nucleophilic substitution with 35 with various aromatic amines was conducted under solvent-free conditions (i.e., in the melt, at >200 1C). According to this protocol, amine-bridged mono-, bis-, tris-, and tetrakis(terpyridine)s 52–55 were obtained in moderate to high yields (up to 80%, Scheme 2.13) [134]. Moreover, the reaction of 35 or 40 -bromo-2,20 :60 ,200 -terpyridine (56) with 40 -amino-2,20 :60 ,200 -terpyridine (57) gave access to bis(2,20 :60 ,200 -terpyridin-40 -yl)amine (58) [2, 135]. The Pd0-catalyzed cross-coupling reaction of aliphatic as well as aromatic amines and 35 (i.e., the Buchwald–Hartwig reaction) was investigated by

02

27 J l 2011 15 38 23

| 25

26


0

00

N3

|N| hν

N

N

N

N

N

N

48

EWG

R "click reaction"

EWG

basic conditions, MeOH

R EWG

N N N

N EWG

N

N

N

N

R = CH2OH,C6H5, C6H4CHO or poly(2-ethyl-oxazoline)

N

49 0

0

0

O

N

N

N

50

H N

N

51

00

Scheme 2.12 4 -Azido-2,2 :6 ,2 -terpyridine (48), as substrate for functional-group modification (EWG: electron-withdrawing group).

NHAr ∆

35 Ar-NH2

N

N

N 52

H N

H N

H N

53

H N

21

NH HN

HN

NH

N

N

N

55 HN

35 or 56

∆

57

H N 58

Scheme 2.13 Synthesis of amine bridged oligo-terpyridines 52–55 and 58 via nucleophilic aromatic substitution reactions.

02

27 J l 2011 15 38 23


Johansson [136, 137]. In particular, cyclic secondary amines (e.g., pyrrolidine or piperazine) were coupled to 35 in high yields (about 95%) utilizing Pd(dba)2/2dicyclohexylphosphino-20 ,60 -dimethoxybiphenyl, as catalyst, and potassium tertbutoxide, as base (dba: dibenzylideneacetone). This metal-mediated reaction was significantly more efficient than the conventional nucleophilic substitution reaction described by Wieprecht et al. [138, 139]. Similarly, dialkyl 2,20 :60 ,200 -terpyridine-40 -phosphonates 59a/b (Figure 2.4) were prepared from 57 by treatment with HPO(OR)2 in the presence of Pd(PPh3)4 as catalyst (R ¼ alkyl) [140, 141]; the alkyl ester could be cleaved under acidic conditions to give the corresponding phosphonic acid 59c [142], a versatile ligand for the robust anchoring of RuII complexes to semiconducting electrodes (e.g., TiO2 or ITO) for photovoltaic applications (for details on this topic see Chapter 8.4). 40 (Diphenylphosphino)-2,20 :60 ,200 -terpyridine (60) was obtained from 35 in an aromatic SN reaction using Li(PPh2), as nucleophile (Figure 2.4) [143]. Aspley and Williams synthesized the 40 -(boronate ester)-substituted terpyridine ligand 61 (Figure 2.4), as a reactant for subsequent Suzuki cross-coupling reactions, via a Pd0-catalyzed Miyaura cross-coupling reaction [144], from which the reaction of 57, bis(neopentylglycolato)diboron, potassium acetate, and Pd (dppf)2Cl2 [dppf: 1,10 -bis(diphenylphosphino)ferrocene], as catalyst, afforded 61 in moderate (68%) yield. Finally, the carboalkoxylation of 40 -triflato-substituted terpyridine 62 with CO in n-butanol and tri(n-butyl)amine represents one further example of Pd0-catalyzed functional-group transformation [n-butyl 2,20 :60 ,200 -terpyridine-40 -carboxylate (63) was obtained in 76% yield, Figure 2.4] [68]. The family of 40 -alkyl-functionalized terpyridines 65 was derived from 40 -methyl2,20 :60 ,200 -terpyridine (64); the methyl group was initially deprotonated with lithium di(isopropyl)amide (LDA), followed by treatment with the corresponding alkyl bromide (Scheme 2.14) [76]. Moreover, deprotonation of 64 with LDA, substitution with PPh2Cl and subsequent oxidation with NaIO4 gave access to 66 [145], which was applied by Pickaert and Ziessel as substrate for the Wittig– Horner coupling with carotene bis-aldehydes to form a series of carotene-bridged

PO(OR)2 O

P N

B

O

N

N

N

59a (R Me) 59b (R Et) 59c (R H)

N

N

N

60

61

SO2CF3 N

N

COOC4H9 N

N

N

N

62

N

63 0

0

Figure 2.4 Miscellaneous 4 -functionalized 2,2 :6 ,200 -terpyridines.

02

0

27 J l 2011 15 38 23

N

| 27

28


0

00

Ph2OP

N

N

N

N

N

N

64

66 i) LDA ii) Br-(CH2)n-R

i) LDA ii) carotene bisaldehyde

(CH2)nR

N N

N

N

N

N n

65

N

n 2-4 R CH3,CH2OH, CHO, CH2NH2 or CH2Br

N

67a (n 1) 67b (n 2)

n N

Scheme 2.14 40 -Methyl-2,20 :60 ,200 -terpyridine as building block for the functionalization of 2,20 :60 ,200 -terpyridines at the 40 -position.

bis(terpyridine)s (67) with five, seven, or nine double bonds in an all-trans configuration was obtained (representatively, 67a/b are depicted in Scheme 2.14). These ditopic ligands exhibited a pronounced bathochromic shift of their lowest energy UV–vis absorption band with increasing number of conjugated double bonds (about 20 nm per incremental unit); the authors envisioned the application of the conjugated bis(terpyridine)s, as building blocks for metallo-supramolecular molecular wires – for the coordination chemistry of such conjugated bis(terpyridine) ligands see Chapters 3.3.2.3 and 5. 2.4 2,20 :60 ,200 -Terpyridines Symmetrically Substituted on the Outer Pyridine Rings

Heller and Schubert significantly improved the yield of 6,600 -dimethyl-2,20 :60 ,200 terpyridine to 43% by applying the Stille cross-coupling reaction of 2,6-dibromopyridine and 2-tri(n-butyl)stannyl-6-methylpyridine (see also Scheme 2.5) [54]. Alternatively, the Constable group utilized the Kr€ ohnke-type ring-assembly strategy with different Mannich salts and N-acylpyridinium bromides for the same purpose; various symmetrically 4,400 - and 6,600 -disubstituted as well as 4,6,400 ,600 tetrasubstituted terpyridines were obtained in moderate to good yields (56–71%) [146]. These two approaches – the construction of the terminally substituted terpyridine core by either Stille cross-coupling or ring-assembly methods – resemble the synthetic pathways towards terpyridine derivatives bearing simple alkyl- or phenyl-based substituents on the outer pyridine rings [2, 3]. Besides these general examples, protocols for the introduction of more sophisticated functional groups have been established. In this respect, 2,20 :60 ,200 terpyridine-1,100 -dioxides (68) were found to be highly versatile substrates. The

02

27 J l 2011 15 38 23

2.4 2,20 :60 ,200 -Terpyridines Symmetrically Substituted on the Outer Pyridine Rings

oxidation of 2,20 :60 ,200 -terpyridines with two equivalents of meta-chloroperbenzoic acid (mCPBA) selectively yielded the di-N-oxides in high yields (>80%) [147]. Galaup et al. applied the so-called Reissert–Henze reaction (i.e., treatment of 68 with Me3SiCN and, subsequently, benzoyl chloride) for the preparation of the 6,600 dinitrile 69; the CN-groups were converted into the corresponding carboxylic acid or ester as well as hydroxymethyl or bromomethyl moieties via standard organic transformations (Scheme 2.15) [148]. Reduction of the nitrile with NaBH4 yielded the 6,600 -bis(aminomethyl) derivatives 70, which were tetraalkylated by reaction with ethyl bromoacetate; subsequent hydrolysis of the esters gave the corresponding tetraacids 71. These ligands with increased denticity were used, as chelates, for the coordination of lanthanide ions (e.g., EuIII or TbIII) [149]; the highly luminescent and stable complexes were introduced, as labeling agents, into biomolecules, such as proteins, and could be used, as tracers, in immunological binding assays (for a more detailed view on these applications see Chapter 7.2.2). A similar strategy for extending the denticity of terpyridine-based ligands was followed by Andreiadis et al. [150] in which thermal cycloaddition of the cyanosubstituents of 69 with sodium azide yielded the pentadentate ligands 72 (Scheme 2.16). The luminescence of lanthanide ions in the visible to near-IR regions was efficiently sensitized by these ligands. Remarkably high quantum yields (FPL) were observed; in particular, the values for the NdIII complexes of 72 were among the highest reported so far in the literature (FPL ¼ 0.10.3%). The Sauer methodology (Scheme 2.2) was utilized by Kozhevnikov et al. for the synthesis of terpyridine derivatives with liquid crystalline (LC) behavior [151]. The condensation of an anisyl-based hydrazone with pyridine-2,6-dicarbaldehyde gave

R O N

N

O N

68 i) Me3SiCN ii) C6H5COCl R

R NaBH4 NC

N

N

N

CN

N

H 2N

N

69

N

NH2

70 i) BrCH2COOEt, LDA ii) hydrolysis

R

HOOC

N

N

N

N

71

HOOC

N

COOH COOH

Scheme 2.15 Synthesis of tetraacid-functionalized terpyridine 71 (R denotes a substituent).

02

27 J l 2011 15 38 23

| 29

30


0

00

R R= NC

N

N

N

S

Br

CN

N

Br

S S

69

C8H17

green, red, NIR

NaN3, NH4Cl, DMF, 120 °C

UV-vis

R N

N NH N

N

N

N

HN N N N

LnIII

72

Scheme 2.16 Terpyridine-based pentadentate ligands 72 according to Andreiadis et al. [150]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

the (1,2,4-triazine)-derivative 73. The subsequent inverse-type Diels–Alder reaction with norbornene or 1-morpholino-cyclopentene gave terpyridines 74 and 75, respectively; the long alkyl chains in 76 and 77 (8, 10, 12, 14, or 16 C-atoms) were introduced by hydrolysis of the methyl ether and esterification with the corresponding n-alkoxybenzoic acid (Scheme 2.17). The mesomorphism of 76 and 77

MeO

OMe N

73

N

N N

N

N N

N

MeO

OMe

N

N

MeO

N 74

N

N

N

75

OR

N

OMe

N

RO

O

N

RO

O

76

OR

N

N

N

77

R OCnH2n1 (n 8-16, even numbers only)

Scheme 2.17 Terpyridines 76 and 77, which exhibit liquid crystalline behavior [151].

02

27 J l 2011 15 38 23

2.5 Ziessel-Type 2,20 :60 ,200 -Terpyridines

(a)

(b)

(c)

Figure 2.5 Optical microscopy images of 77 (n ¼ 16) for the N-to-SmC transition: nematic phase (a), phase transition (b), and SmC phase (c) [151]. Figure reproduced with kind permission; r 2007 The Royal Society of Chemistry.

was investigated utilizing polarized optical microscopy and differential scanning calorimetry (DSC). In particular, the annulated terpyridines 77 showed nematic and smectic C phases, being characteristic of rod-like materials. Optical microscopy revealed the absence of chiral domains in the N phase and both broken-fan and schlieren textures in the SmC phase (Figure 2.5). Moreover, the steric demands of the pentalene led to a considerable destabilization of the crystal phases and mesophases: 77 having a C12-chain melted to the nematic phase at 139 1C and cleared at 175 1C; whereas, the analogous terpyridine 76 melted to the B2 phase at 245 1C and cleared at 253 1C. For details on the fundamentals of liquid crystallinity, the reader is referred to Chandrasekhar’s book [152]. The Ziessel group also reported on liquid crystalline terpyridines [153] in which the phasmidic tails (i.e., long alkyl chains bound to the ends of a rod-like core) attached in 40 - or 6,600 position of the terpyridine via imine moieties were responsible for the mesomorphism of both the metal-free ligands and the dinuclear CuI complexes. Further examples of terpyridines with LC properties are discussed separately in the context of the Ziessel-type terpyridines (Section 2.5).


The so-called Ziessel-type 2,20 :60 ,200 -terpyridines (78) feature an ethynyl-substituent in the 40 -position, as their structural characteristic. The Pd0-catalyzed Sonogashira cross-coupling reaction of 40 -bromo- (56) [67] or 40 -triflato-2,20 :60 ,200 -terpyridine (62) [68] with terminal alkyne derivatives has a broad scope, since the isolated yields are usually moderate to good and most functional groups are compatible with the reaction protocol (Scheme 2.18). In addition, the CuI-catalyzed Glaser reaction represents an appropriate tool for the dimerization of terpyridines bearing a terminal alkyne unit. The Ziessel group pioneered the field of 40 -ethynyl-functionalized terpyridines (78) by synthesizing diverse mono- and bis(terpyridine)s; owing to the extended p-conjugated system, outstanding optoelectronic properties (e.g., room temperature luminescence of their RuII bis-complexes) were achieved. The synthesis, properties, and potential applications of 40 -ethynyl-2,20 :60 ,200 -terpyridines were reviewed by Ziessel in 1999 [154] and, very recently, by Wild et al.

02

27 J l 2011 15 38 24

| 31

32


0

00

N

R' R N

N

N

"cat. Pd0"

N

R

56 (R' Br) or 62 (R' OTf)

78

N

Scheme 2.18 Synthesis of Ziessel-type terpyridines 78 via Sonogashira cross-coupling reaction.

[155]. Within the scope of this section, selected examples of mono- and bis(terpyridine)s of the Ziessel-type will be highlighted; the mono- and dinuclear RuII and OsII complexes of these ligands are evaluated separately in Chapter 3.3.2. The parent compound, 40 -ethynyl-2,20 :60 ,200 -terpyridine (79), was obtained in very high yield (>85%) according to Scheme 2.19a by ethynylation of 62 with trimethylsilylacetylene or 2-methylbut-3-yn-2-ol under mild conditions and subsequent deprotection with KF or KOH, respectively [156, 157]. Aiming for pconjugated carbon-rich bridges to support long-range electronic (or magnetic) interactions (“polyyne”-type), terpyridines with two (80) and three (81) –CC– units were prepared, in moderate to low yields, utilizing the CuI-catalyzed Cadiot– Chodkiewicz homologation (Scheme 2.19b) [158]. The authors proposed that such a stepwise oligomerization should be feasible until solubility and/or inertness of the intermediates constrains the reaction (as already observed in the second sequence in which 80 was converted into 81 in about 20% yield). The Glaser homocoupling of mono(terpyridine)s 79–81 afforded oligoyne-bridged bis(terpyridine)s 82–84 with even numbers of triple bonds in the spacer (i.e., butadiyne, octatetrayne, and dodecahexayne, respectively; Scheme 2.19c). Owing to the stiff, rod-like structure of 82–84, a significant decrease of the solubility with increasing spacer length became apparent. 1,2-Bis(2,20 :60 ,200 -terpyridin-40 -yl)acetylene (85), as the shortest homolog having one triple bond, as spacer, was prepared by directed cross-coupling reaction of the orthogonally functionalized building blocks 62 and 79 (82% yield) [67]. Hissler and Ziessel also reported the coupling of 79 and 80 to trans-[Pt(nBu3P)2Cl2] using CuI, as catalyst, and isopropylamine, as base. Thereby, bis(terpyridine) ligands with a heavy metal atom in the oligoyne-bridge, as a high-energy barrier to through-bond electronic coupling, were obtained in high yields (78– 84%) [159]. Recently, materials containing –CC–PtL2–CC– units (L denotes a phosphine ligand) have gained considerable interest in the development of new types of organometallic photovoltaics [160–162]. Besides this, particularly the groups of Ziessel and Harriman synthesized a wide range of bis(terpyridine)s, as ditopic ligands, for the construction of photoactive molecular wires, by coupling 79 with various types of dibromo-functionalized aromatics (e.g., phenyl, naphthalene, and anthracene) as well as heteroaromatics (e.g., 2,20 -bipyridine, 1,10phenanthroline, 2,20 -bipyrimidine, 3-dodecylthiophene) [163–167]. These rigid

02

27 J l 2011 15 38 24


(a)

N 98% N

TMS Pd(PPh3)2Cl2, CuI

TMS N

KF N

97%

N

89%

62

N KOH

OH Pd(PPh3)4

79

N

N 98%

OH N

(b) N

N

a,b 79 52%

a,b N

N a: Et3Si

N 21% 80

81

N

Br, CuCl, NH2OH*HCl, n-PrNH2, THF b: NaOH, MeOH

(c) N

N

N

N

nH

CuCl2, CuCl, DMF, O2

N

N

67-73%

79-81 (n 13)

n N

82-84 (n 13)

N

N

Scheme 2.19 Synthesis of ethynyl-functionalized mono(terpyridine)s 79–81 (a and b) and oligoyne-bridged bis(terpyridine)s 82–84 (c).

p-conjugated bis(terpyridine)s were applied, as ligands, for the preparation of homometallic dyads (containing RuII ions) or heterometallic triads (containing RuII and ZnII/FeII ions). The photophysical properties of these di- and trinuclear complexes are discussed in Chapter 3.3.2.3. As a considerable drawback, the increase of the conjugation length of such rigid ligands is accompanied by a gradual decrease in solubility. Taking this into account, Khatyr and Ziessel developed a synthetic approach for the design of wellsoluble bis(terpyridine) ligands 86 without losing the effectiveness of electronic conductivity by an iterative strategy based on Pd0-promoted cross-coupling reactions (Scheme 2.20) [168, 169]. The intermediates 87–92 were readily prepared by Pd0-mediated coupling procedures, consisting of several protection, deprotection and cross-coupling steps. The terminal alkyne and halide derivatives (87–90) were reacted with 62 or 79; using a Pd0-catalyst in the presence of a base to gain access to 86a–d, notably, the utilization of a catalytic amount of CuI was not reported. Bis

02

27 J l 2011 15 38 24

| 33

34


0

00

C12H25O 87 N C12H25O

n=

C12H25O

N 88

C12H25O

OC12H25

C12H25O

OC12H25

80

n=

%

2, 7 0%

C12H25O

N

Br 89

OC12H25

Pd (PPh3)4, n-PrNH2

N

n 3, 55%

OC12H25

N C12H25O

N

n

79

Br OC12H25

OC12H25 86a-e

% 4, 27

3 90

N

N n

N C12H25O

N C12H25O

2 eq. 79

Br OC12H25

1,

79

Br

N

2 eq. 62

OC12H25

OC12H25

n

5,

%

57

N C12H25O

C12H25O

C12H25O

C12H25O

Br

2

Br OC12H25

91

OC12H25

OC12H25

N 92

N

N OC12H25

2 eq. 92

CuCl, O2

OC12H25

N

OC12H25

N

N

74% N

C12H25O

93

C12H25O

N

Scheme 2.20 Synthesis of soluble bis(terpyridine)s 86 and 93 via an iterative Sonogashira cross-coupling approach [168, 169].

(terpyridine) 86e was obtained from trimeric dibromide 91with two equivalents of building block 92. Additionally, the oxidative Glaser homocoupling of mono(terpyridine) 92 yielded bis(terpyridine) 93 (Scheme 2.20). Electronic communication along the molecular axis of 86 and 93 was confirmed by UV–vis absorption spectroscopy, where the lowest energy absorption transition shifted progressively towards higher wavelengths with increasing number of repeating units (Table 2.1). Additionally, this behavior mirrored the increasing degree of p-conjugation. Within this series, bis(terpyridine) 86e showed the most pronounced bathochromic shift (labs ¼ 440 nm), due to a total number of 78 p-electrons contributing to the conjugated system. The same stands for the emission behavior of 86 and 93, for which an excited triplet state could be excluded by the fact that no change in the steady-state emission occurred for oxygen-degassed solutions of the respective bis(terpyridine). The large Stokes shifts (1450–2990 cm1) revealed the charge-transfer characteristics of the excited state (Table 2.1) [169]. Both 86 and 93 have been used in the preparation of RuII- and RuII/OsII-dyads, to study the

02

27 J l 2011 15 38 24

N

2.5 Ziessel-Type 2,20 :60 ,200 -Terpyridines Table 2.1

Photophysical properties of bis(terpyridine)s 86 and 93 [168, 169].a

Bis(terpyridine)

kabs (nm)

kPL (nm)

Stokes shift (cm1)

UPL

86a 86b 86c 86d 86e 93

385 408 420 425 440 406

435 445 460 465 470 445

2990 2040 2070 2020 1450 2160

0.25 0.30 0.39 0.30 0.29 0.42

a

| 35

Photophysical properties in dilute solution at room temperature.

intramolecular electron and triplet-energy transfer processes along the molecular axis (Chapter 3.3.2.3). In related work, the groups of Lin [170–172] and Sun [173] synthesized bis(terpyridine)s 94 and 95, which have fluorene-based p-conjugated spacer units bearing alkyl-chains or carbazole pendants at the 9-position of the fluorene core (Figure 2.6). In the case of 95, additional phenyl units with electron-donating or -accepting units were incorporated. In all cases, Sonogashira cross-coupling of dibromo-functionalized spacer units with 62 was performed to give 94 in moderate (about 40%) and 95 in high (72–89%) yields. The photophysical properties of 94

N

N 94a: R n-alkyl

N

R N

N

94b: R

N

R

94c: R

N

O O R

R

N C3H7

N

C3H7

R

N

R

N

N

N O N

Ph P

S

N S

N

N N

96

N

Figure 2.6 Fluorene- and dithieno[3,2-b:20 ,30 -d]phosphole-containing Ziessel-type bis(terpyridine)s (94–96).

02

27 J l 2011 15 38 25

N N

95a: R H 95b: R Me 95c: R OMe 95d: R F

36


0

00

and 95 were investigated in dilute solutions as well as solid state. The sterically demanding carbazole unit in the side-chain of 94c had no significant impact on the photophysical behavior. In contrast, bis(terpyridine) 95c bearing electron-rich methoxy groups featured a bathochromic shift in absorption of about 30 nm when compared to its electron-poorer counterparts (95a/d). This behavior was attributed to the charge-transfer (CT) characteristics of the longest wavelength absorption band. Both 94 and 95 exhibited intense violet to blue emission with quantum yields (FPL) up to 0.29 in dilute solutions. Thin film investigations on 95 revealed similar results for the absorption behavior, but a strong redshift in emission (about 80 nm) was attributed to aggregation of the linear chromophores in the solid state. Furthermore, the group of Baumgartner recently reported the incorporation of dithieno[3,2-b:20 ,30 -d]phospholes, as conjugated spacer, via a Sonogashira crosscoupling reaction (96, Figure 2.6) [174]. Upon bromination of P-phenyl-dithieno[3,2-b:20 ,30 -d]phosphole simultaneous oxidation of the phosphorus center was observed; this aromatic dibromide was subsequently reacted under Sonogashira conditions with 79. The photophysical properties of 96 (labs ¼ 433 nm, lPL ¼ 503 nm, FPL ¼ 0.79) were supported by theoretical calculations at the B3LYP/6-31G* level, predicting a HOMO–LUMO energy gap of 2.87 eV. The influence of pH on the photophysical properties was also investigated; the addition of acetic acid led only to a negligible bathochromic shift in the emission. This behavior was explained by the more electron-withdrawing nature of the dithieno[3,2-b:20 ,30 -d] phosphole spacer, preventing charge-transfer processes with the electron-poor, protonated terpyridine moieties [174]. Brombosz et al. utilized the Sonogashira cross-coupling procedure for the synthesis of bis(terpyridine) 97 in low yield (Figure 2.7) [175]. The X-shaped bis(terpyridine) 97 was subsequently utilized for photoluminescence sensing experiments with main group and transition metal ions as well as inorganic anions, where high sensitivity towards various transition metal ions was observed; for example, for ZnII ions the emission color was shifted from blue to orange. Furthermore, the ZnII complex of 97 served as an efficient sensor for fluoride ions compared to other inorganic anions (Figure 2.7). Not only bis(terpyridine)s of the Ziessel-type but also a wide range of mono(terpyridine)s have been described in the literature. Among others, fused aromatic systems functionalized with an ethynyl group have been coupled by a Sonogashira reaction to the 2,20 :60 ,200 -terpyridine core. For instance, pyrene was linked via its 1position to 40 -ethynyl-2,20 :60 ,200 -terpyridine (98, 95% yield, Figure 2.8); the resulting RuII bis(terpyridine) complex [Ru(98)2]2 þ featured a remarkably long excitedstate lifetime of 580 ns (Chapter 3.3.2.2) [176]. The disk-like hexa-peri-hexabenzocoronene (HBC) unit represents the largest fused aromatic system that has been coupled to terpyridines up to now [177]. In the case of 99 (Figure 2.8), 79 was coupled to an iodine-functionalized HBC in a Sonogashira cross-coupling reaction (the HBC unit itself was prepared by a Diels–Alder [4 þ 2]-cycloaddition reaction of 1-tert-butyl-4-[(4-iodophenyl)ethynyl]benzene and 2,3,4,5-tetrakis(4-tert-butylphenyl)cyclopenta-2,4-dienone followed by oxidative cyclodehydrogenation). Within the family of HBC derivatives [178], 99 (and its 40 -phenyl-2,20 :60 ,200 -terpyridine

02

27 J l 2011 15 38 31


Li+

K+

Cs+

TI+

Hg2+

Cd2+

Ca2+

Ag+

Mn2+

Cu2+

Mg2+

Er3+

Sn2+

Zn2+

In3+

XF

XF Zn2+

F-

CI-

Br

I-

NO3-

PO43-

Na+

N

A

N

N

N 97

N

N

C

(b) (a)

Figure 2.7 (a) The X-shaped bis(terpyridine) 97. (b) The sensing experiments with 97 according to Brombosz et al. [175]: the top two rows show the emission color of 97 in an acetone–H2O mixture in the presence of various metal ions; the bottom row depicts the emission color of 97 (denoted as XF), the ZnII complex of 97 (denoted as XF Zn2 þ ), and the change in emission in the presence of various anions. Figure reproduced with kind permission; r 2007 American Chemical Society.

homolog) are the first examples with pendant polypyridyl ligands and open the way for the marriage of the optoelectronic properties of the “superaromatic” HBC substituent with those of metal ion bis(terpyridine) complexes. A central 4-methyl-3,5-diacylaminophenyl moiety equipped with two lateral aromatic rings – each bearing three n-dodecyl chains – was linked to the 2,20 :60 ,200 terpyridine core through a linear acetylene unit (100, Figure 2.9a) [179]. In contrast to analogous compounds where the linkage was realized via nonlinear, polar ester or amide bonds, the non-polar, linear linker forced 100 into a columnar mesophase organized in a rectangular lattice of p2gg symmetry. Molecular modeling studies suggested dimerization of two non-discotic molecules via H-bonding (Figure 2.9b) and p–p stacking in a head-to-tail fashion to form columns. Analysis with thermogravimetry (TGA), DSC, and polarized optical microscopy revealed three reversible transitions of 100 in which, above 200 1C, the compound appeared

N

N 98 N

| 37

99 N

N

N

Figure 2.8 Ziessel-type mono(terpyridine)s 98 and 99 with fused aromatic substituents.

02

27 J l 2011 15 38 31

38


(a)

OC12H25 OC12H25

OC12H25 C12H25O H N

C12H25O

H N

O

0

00

(c)

OC12H25 O

100 N

N

N

(b)

Figure 2.9 (a and b) Terpyridine 100 and molecular model of the dimer (100)2; (c) terpyridine 100 viewed by optical microscopy under crossed polarizers (symbolized by the cross in the corner of the picture upon cooling) at 193 1C (top), 165 1C (middle), and 133 1C (bottom) [179]. Figure reproduced with kind permission; r 2006 Wiley-VCH.

fluid and isotropic between crossed polarizers; upon cooling, the isotropic liquid became birefringent and a texture characteristic of a columnar phase with pseudofan shapes was observed. Between 174 and 150 1C, fluting appeared on the large oriented domain; finally, below 150 1C, cracks indicative of a crystalline material were observed [the crystallinity of the low-temperature-phase was confirmed by Xray diffraction (XRD) analysis]. Various oligotopic terpyridine derivatives were reported by the Schmittel group in which the Sonogashira cross-coupling of 4-iodophenyl-substituted mono(terpyridine)s with 1,3,5-triethynylbenzene or hexaethynylbenzene, respectively, gave access to star-shaped tris- and hexakis(terpyridine)s in moderate to good yields [180, 181]. These ligands were utilized, as building blocks, for the construction of metallo-supramolecular architectures according to the so-called HETTAP concept (Chapter 4.3).

2.6 Kr€ ohnke-Type 2,20 :60 ,200 -Terpyridines

40 -Aryl- and 40 -heteroaryl-2,20 :60 ,200 -terpyridines can be easily prepared by ringassembly methodologies according to Kr€ ohnke (Scheme 2.1 routes a/b). By

02

27 J l 2011 15 38 32

2.6 Kr€ohnke-Type 2,20 :60 ,200 -Terpyridines

applying the original reaction protocol [9] or one of the modern variants [17, 18, 20, 23], a large selection of Kr€ohnke-type terpyridines has been reported to date (about 1500 structures were registered in SciFinderTM by March 15th 2011). In the following, these terpyridines will be discussed based on selected examples; thereby, the focus will particularly be on mono- and bis(terpyridine)s, as templates for the preparation of mono- and dinuclear RuII/OsII complexes (Chapter 3.3.2) as well as metallopolymers (Chapter 5) [155]. Notably, tritopic ligands were also obtained following a threefold Kr€ohnke-type condensation reaction [182, 183] when, in 1992, Constable and Cargill Thompson reported on the synthesis of the rigid 1,3,5-tris(2,20 :60 ,200 -terpyridin-40 -yl)benzene in moderate yield (about 25%). 40 -Phenyl-2,20 :60 ,200 -terpyridines 101 with reactive functional groups on the phenyl ring are of utmost importance for the construction of more advanced architectures (e.g., extended p-conjugated systems), for instance via condensation or metal-mediated cross-coupling reactions. Table 2.2 summarizes the most relevant building blocks in this respect. The displacement of a 4-fluoro-substituent in perfluorinated phenyl derivatives by a nucleophile under mild conditions is a well-established concept in organic [229] and macromolecular chemistry [230]. Owing its efficiency, this substitution is

Table 2.2

Important Kr€ ohnke-type mono(terpyridine) building blocks 101.a R 101

N

N

N

Mono(terpyridine)

R

References

101a 101b 101c 101d 101e 101f 101g 101h 101i 101j 101k 101l 101m 101n

4–OH 4–Br 4–I 4–CCH 4–CH¼CH2 4–B(OR)2 4–CH2OH 4–CH2Br 4–CHO 4–CH2PO(OEt)2 4–CH2PPh3 þ Br 4–CH2CN 4–NH2 4–Sn(n-Bu)3

[23, 184, 185] [14, 17, 186–192] [193–195] [196–201] [187, 202–204] [205–207] [17, 208] [185, 197, 209] [197, 210–216] [186, 197, 217, 218] [200, 219–222] [223] [224–228] [186]

a

In this overview, only selected examples are included.

02

27 J l 2011 15 38 33

| 39

40


0

00

sometimes considered as a type of “click reaction” according to the criteria defined by Sharpless [231]. Constable et al. synthesized 40 -(pentafluorophenyl)-2,20 :60 ,200 terpyridine (102) via step-wise Kr€ohnke-type condensation (38% yield) [232]. As a proof-of-concept, the 4-position of the pentafluorophenyl unit was subsequently attacked by various O-nucleophiles (i.e., alcohols, sugars). These examples indicate the high potential of 102 to serve, as a reactive building block, in the synthesis of more advanced terpyridine-based architectures. The substituted as well as non-substituted furan-, thiophene-, and pyrrole-substituted terpyridine derivatives 103–105 were obtained from 2-acetylpyridine and the corresponding heteroaromatic carbaldehydes in moderate yields (up to 50%, Figure 2.10a) [233–235]. Terpyridines 103 (X ¼ O or S; R1/R2 ¼ H) and 104 exhibited topoisomerase I inhibition and antitumor cytotoxicity against various types of cancer cell lines (Chapter 7.2.1.3) [233]. Moreover, the oligothiophenesubstituted terpyridines 103a (X ¼ S, R1/R2 ¼ H) and 105 were utilized, as ligands, for the formation of homoleptic RuII and OsII bis-complexes; the electropolymerization thereof into p-conjugated metallopolymeric films was investigated by Hjelm et al. (Chapter 5.3.5) [236]. A furan-2-yl substituent at the 40 -position (e. g., 103b with X ¼ O and R1/R2 ¼ H) can be utilized, as precursor, for the synthesis of 2,20 :60 ,200 -terpyridin-40 -yl-carboxylic acids, such as 106 [237]: oxidative degradation of the furan ring was carried out in good yields (about 78%) with KMnO4, as oxidant, in an aqueous solution (Figure 2.10b). In Kr€ohnke-type terpyridines, the phenyl and central pyridine rings are twisted on average about 20–301 due to the unfavorable interaction between the adjacent protons (Figure 2.11a) [238]. The molecules are non-planar with respect to the terpyridyl moiety and the phenyl ring and mismatches between the ground and

(a)

R1

R2

H X

X

S n

N

N

N

N

N

N

103 X O or S 1 2 R , R H or Br

N

104 X O or S

N

N

105 n 2 or 3

(b) O

HO

i) KMnO4,H2O

O

ii) Na2S2O3, H2O N

N

N

78%

N

103b

N

N

106 0

0

00

Figure 2.10 (a) Heteroaryl-substituted 2,2 :6 ,2 -terpyridines 103–105; (b) oxidative degradation of 40 -(furan-2-yl)-2,20 :60 ,200 -terpyridines into 2,20 :60 ,200 -terpyridin-40 -yl-carboxylic acids.

02

27 J l 2011 15 38 33


(a)

R

R

H H N

N

H H N

N

H

N

HN

N

107

R

R

NH2

N

N

HCl N

N

N

Kröhnke-type terpyridine

(b)

N

H

N

X

N

NaOMe, MeOH

N

20-85%

108

107

N

N

N

Figure 2.11 (a) Non-planar Kr€ ohnke-type terpyridines and planar 40 -(pyrimidin-2-yl)-2,20 :60 ,200 terpyridines (107); (b) synthesis of 107.

excited state geometries arise; as a consequence, the effect of increased p-delocalization is minimized – being counterproductive for the improvement of the photophysical properties (Chapter 3.3.2) [239, 240]. However, it was shown by Presselt et al. by means of Bader’s quantum theory of atoms in molecules (QTAIM) [241] that lateral substituents in the para-position of the attached phenyl ring, such as vinyl or ethynyl moieties, or coordination to transition metal ions (e.g., ZnII or RuII) can enhance the delocalization expressed by the parameter elipticity (e) in the bond critical point (BCP), that is, the phenyl–pyridine bond [202, 242, 243]. Fang et al. could significantly reduce the torsion by replacing the phenyl-ring by a pyrimidin-2-yl one (107, Figure 2.11a) [244]; the almost flat molecular structure of 107, enabling good electronic p-conjugation, was confirmed by X-ray single-crystal structure analysis (for the properties of the [Ru(107)2]2 þ complexes, see Chapter 3.3.2). Since 2-formyl pyrimidines – required for the synthesis via a Kr€ohnke-type condensation – are difficult to access, the preparation of 107 followed a different route, as outlined in Figure 2.11b: 40 -amidine-2,20 :60 ,200 -terpyridine hydrochloride (108) was reacted with vinamidium salts, under basic conditions in methanol, to give 107 in moderate to good yields (up to 85%). The incorporation of Kr€ohnke-type terpyridines into highly conjugated architectures is currently a highly active field of research. The directed Pd0-catalyzed cross-coupling or Wittig and Horner–Wadsworth–Emmons (HWE) condensation reactions of appropriately functionalized terpyridines (e.g., 101b–f and 101i–k, respectively) with orthogonally p-conjugated building blocks represent the most widely applied strategies in this respect. In particular, the synthesis and photophysical properties of linear and angular bis- as well as star-shaped

02

27 J l 2011 15 38 33

| 41

42


0

00

and dendritic oligo(terpyridine)s were reviewed recently by Schubert and coworkers [155]. Though the Stille cross-coupling reaction has gained considerable interest for the synthesis of highly functionalized mono(terpyridine) derivatives (Scheme 2.5) [53–55, 60], it is still only of minor importance for the construction of more advanced p-conjugated bis(terpyridine)s [3, 52–55, 155]. Variants of the Kumada and Yamamoto cross-coupling reaction were also utilized in the past for the Nicatalyzed homo- or hetero-coupling of mono(terpyridine)s. In 1990, Constable and Ward reported the synthesis of bis(2,20 :60 ,200 -terpyridin-40 -yl) 109 applying a NiII catalyst (Figure 2.12) [66]. The Kumada approach was applied by the Constable [66] and Sauvage groups [245, 246] to gain access to the homologous 1,4-bis(2,20 :60 ,200 terpyridin-40 -yl)benzene (110, Figure 2.12; the same compound was also prepared by a Kr€ohnke-type condensation of terephthaldehyde with 2-acetylpyridine [17]). However, by far the most relevant cross-coupling procedure for the connection of terpyridines via C–C single bonds to p-conjugated backbones is the Suzuki– Miyaura reaction [191, 192, 247–251] in which an aromatic dibromide is reacted with the terpyridine moiety bearing a boronic acid (or its ester), or vice versa, using Pd0/PdII catalysts. In 1999, the group of Rehahn reported the synthesis of a bis(terpyridine) ligand bearing a terphenyl unit with solubilizing n-hexyl side-chains, as p-conjugated spacer, by Suzuki–Miyaura cross-coupling [252]. The power of the method in comparison to conventional ring-assembly methods was demonstrated by the fact that the product could be obtained in almost quantitative yield after recrystallization. Moreover, the Kurth group developed an efficient synthesis protocol by employing either a Suzuki-type cross-coupling dimerization or a

N

N

N

N

N

N

109 N

N

R1

N R

2

N R1

111 (18 examples) spacer

N

N 116

N

N

N

N

spacer: none, 1,4-phenyl, 1,3-phenyl, 2,6-pyridyl, 2,5-thienyl, 5,5'-bithienyl R1, R2: H, OMe or CN (a)

N

N R2 (b)

Figure 2.12 (a) Bis(terpyridine)s synthesized via Suzuki–Miyaura cross-coupling reaction; (b) emission colors of selected bis(terpyridine)s (111) with no (left), a 2,5-thienyl (middle), and a 5,50 -bithienyl spacer (right) [248]. Figure reproduced with kind permission; r 2008 Elsevier B.V.

02

27 J l 2011 15 38 34


| 43

two-step coupling reaction sequence – including a Miyaura boronic ester formation – for the synthesis of peripheral symmetrically and asymmetrically substituted bis(terpyridine) derivatives with aromatic and heteroaromatic spacer groups 111 (Figure 2.12) [191, 192, 247–250]. The Kurth groups investigated the optoelectronic properties of 111. The electronic nature of the substituents R1/R2 influenced the photophysical behavior – the electron-donating substituents (e.g., –OMe) increased the extinction coefficient and quantum yield [192, 248]. The emission maximum was significantly redshifted by about 70 nm when the phenyl moiety, as spacer, was replaced by a thienyl one (Figure 2.12); this behavior was attributed to a strong push–pull effect between the electron-rich thiophene-containing bridge and electron-poor terpyridine units [192]. Zhong et al. reported the synthesis of mono 112a/b and bis(terpyridine) 113 featuring a photochromic dithienylethene (DTE) bridge by Suzuki–Miyaura crosscoupling (Figure 2.13a) [253, 254]. Such ligands are of particular interest for applications in the fields of controlled electron transfer and luminescence alteration due to the possibility of “switching” the molecule between an open and closed

(a) N

N S

S

N

R

S

N

S

N 112a: R CI 112b: R Ph 112c: R 4-(H25C12O)-Ph

N

113 N

N

(c) 0.9

Absorance

(b)

N

S S open form hν

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

closed form open form

250

350

450

550

650

Wavelength (nm) S

S

closed form

Figure 2.13 Dithienylethene-bridged terpyridines 112 and 113; (b) reversible photochemical open–close cycle; (c) UV–vis absorption spectra of 113 in the open (solid line) and closed (dashed line) forms [254]. Figure reproduced with kind permission; r 2007 American Chemical Society.

02

27 J l 2011 15 38 34

750

44


0

00

form, simply by irradiation at specific wavelengths (Figure 2.13b) [254]. It was shown that irradiation at 320 nm of a colorless solution of the open form of 113 for a few minutes led to a color change to purple, resulting in a broad absorption band at 570 nm; this characteristic behavior was ascribed to the more extended p-conjugated system in the closed form than in the open state, with the higher degree of electron delocalization resulting in a redshift of the longest absorption wavelength band from 340 to 570 nm (Figure 2.13c) [253–255]. Online 1H NMR spectroscopy experiments revealed a quantitative and reversible open-to-close conversion [254]. Recently, Piao et al. reported the application of 112c, as fluorescent probe, in bioimaging [188], where due to its low cytotoxicity it could be utilized, as detector, for biological processes such as metal-ion transmembrane transport. The Pd0/PdII-catalyzed Heck reaction enables the connection of terpyridine moieties to p-conjugated residues via –C¼C– bonds [256]: in general, a terminal alkene is reacted with an aryl halide, resulting in the thermodynamically more stable (E)-alkene. However, this approach has been less frequently applied to terpyridine chemistry than the Suzuki–Miyaura reaction. For instance, Mikroyannidis et al. utilized 40 -(4-vinylphenyl)-2,20 :60 ,200 -terpyridine (101e), as substrate, for the PdII-catalyzed cross-coupling reaction with 101b [yielding the trans-stilbene-bridged bis(terpyridine) 114] as well as some other dibromofunctionalized building blocks (to give 115a/b in moderate yields of 46–61%, Figure 2.14) [187]. The latter were introduced, as emissive materials, into double-layer organic light-emitting devices (OLEDs); devices of the configuration ITO/PEDOT/bis(terpyridine)/Ca/Al were fabricated and the electroluminescence properties were investigated [PEDOT: poly(3,4-ethylenedioxythiophene)]. Turn-on voltages of 8–10 V were required and, at a driving voltage of 14 V, a maximum luminance (L) of 63 cd m2 and a current density (J) of 688 mA cm2 were obtained for 115a. In addition to the Pd0/PdII mediated cross-coupling procedures, various groups applied the Wittig and HWE condensation reactions – both established synthetic tools in p-conjugated polymer chemistry – to connect p-conjugated systems with the terpyridine moieties via –C¼C– double bonds [257, 258]. In particular, the

N

N 114

N

spacer

N 115a

N

N 115b

C6H13

C6H13 OC12H25

C12H25O

Figure 2.14 Bis(terpyridine)s 114 and 115 synthesized via Heck cross-coupling reaction.

02

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Schubert group followed this approach by linking the terpyridine moieties with pconjugated building blocks, which have been widely used in the field of poly(pphenylene-ethynylene)/poly(p-phenylenevinylene) (PPE/PPV) type polymers. A diverse library of bis(terpyridine) architectures (116) was obtained by HWE condensation in high yields (above 80%, Scheme 2.21). For this purpose, the key building block 101j was combined with aromatic dialdehyde derivatives (the utilized aromatic dialdehydes were mainly based on 1,4-dialkoxybenzene and thiophene units, thus possessing electron-rich character) [197, 259, 260]. The high (E)selectivity of the HWE condensation was proven by 1H NMR spectroscopy. Taking all this into account, the HWE reaction was found to be more efficient for the attachment of p-conjugated spacer units to the terpyridine moiety compared to the Pd0-catalyzed methods. Independently, Yuan et al. also reported the synthesis of bis(terpyridine)s by Wittig reaction of 101k [261]. The investigation of the photophysical properties of 116 revealed a distinct redshift of the absorption maxima together with a strong increase in the molar extinction coefficient with increasing p-conjugation length. The emission maxima of 116 covered nearly the whole blue range (444–500 nm) and quantum yields of up to 85% were obtained. Additionally, the emission data of spin-coated films were compared to those of dilute solutions. Owing to strong pp interaction in the solid state, a redshift of the emission maxima (~30 nm) together with a decrease in the quantum yields (~25%) were observed. Winter et al. also synthesized the aldehyde-functionalized p-conjugated mono(terpyridine) 117 by Sonogashira cross-coupling reaction of 101d [197, 262]; 117 was subsequently utilized for the preparation of mono(terpyridine) 118, the isomeric set of bis(terpyridine)s 119, and star-shaped oligo(terpyridine)s 120 and 121 (reaction with the appropriate benzylic phosphonates with potassium tert-butylate, as base; Figure 2.15) [260]. Since 118 and para-119 were considered as building blocks for photoactive complexes [262–264] and metallopolymers [265] (see Chapters 3.3.2 and 5.3, respectively), the ultrafast photoinduced dynamics of these ligands were investigated by femtosecond time-resolved absorption spectroscopy

O 101j or 101k

spacer

O 79-91%

KOtBu, THF, reflux

N

N

N

N

spacer

N

116 (15 examples)

N

Scheme 2.21 Synthesis of bis(terpyridine)s 116 via Wittig or Horner–Wadsworth–Emmons (HWE) condensation reaction.

02

27 J l 2011 15 38 34

| 45

46


0

00

R

O

118

OC8H17 R

C8H17O R 117

para-119: 1,4-bis ortho-119: 1,2-bis meta-119: 1,3-bis

R

N OC8H17 120

N

N

N

R

R

R

R

R: N C8H17O

121 R

N

R

Figure 2.15 Aldehyde-functionalized p-conjugated mono(terpyridine) 117 and mono-, bis-, tris-, and tetrakis(terpyridine)s 118–121, respectively.

as well as time-resolved fluorescence and Raman measurements [266, 267]. Ultrafast intramolecular charge-transfer in concert with excited-state-planarization of the molecule was concluded from these studies. Moreover, 120 and 121 were utilized, as rigid cores, for the preparation of amphiphilic rigid-rod metallosupramolecular stars that self-assembled into micelles in aqueous media (Chapter 6.3.6) [268, 269]. Additionally, mono(terpyridine) 117 was applied in a Knoevenagel-type condensation with benzyl cyanide derivatives; both the absorption and emission maxima of the synthesized terpyridines experienced a significant bathochromic shift when compared to the corresponding terpyridines without electron-withdrawing CN-substituents [223]. The efficient delocalization between the terpyridine units and their p-conjugated substituents is a prerequisite for potential applications in various types of optoelectronic devices (e.g., OLEDS or photovoltaics). As pointed out above, the tpy-

NMe2 n

N Bu2

NnBu2

122a N

N

N

122b N

N

N

122c N

N

Figure 2.16 Mono(terpyridine)s (122) with nonlinear optical behavior.

02

27 J l 2011 15 38 35

N


phenyl linkages, due to the torsion of about 20–301, do in general not meet this requirement. However, in the field on nonlinear optical (NLO) devices such nonplanarity might even be beneficial. Tessore et al. investigated several push–pullsubstituted terpyridines (Figure 2.16) as well as their mono-complexes with ZnII, RuII, and IrIII ions with respect to their second-order response (b1.34: quadratic hyperpolarizability working with an incident non-resonant 1.34 mm wavelength) [219]. For this purpose, electric-field-induced second harmonic generation and solvatochromic methods were utilized. In comparison to 4-substituted pyridines (e.g., 4–Me2N–C5H4N or 4–NC–C5H4N), terpyridines 122 (coordinated or metalfree) featured a significantly higher second-order NLO response (b1.34 ¼ 22 95 1030 esu; 4-substituted pyridines gave values of b1.34 o 0.5 1030 esu). Apparently, the non-planarity of the chromophore strongly enhanced the quadratic hyperpolarizability. Moreover, the value (enhancement by a factor of 3–5) and the sign of the second-order NLO response could be tuned via the nature of the coordinated transition metal ion (i.e., soft or hard metal centers). Most reported p-conjugated bis(terpyridine)s feature spacer units with electrondonating properties [155]. Recently, the Schubert group reported the synthesis of a series of bis(terpyridine)s 123 (Table 2.3) with electron-acceptor containing linkages via Sonogashira cross-coupling reactions of 101d (Scheme 2.22) [270] where [2,1,3] benzothiadiazole, terephthalates, thieno[3,4-b]pyrazine, quinoxaline, and nitrilecontaining systems were combined with terpyridine moieties (Scheme 2.22). Detailed investigation of their steady-state photophysical properties were carried out to elaborate the influence of the electron-acceptor spacers on the p-conjugated system. Within the series, the thieno[3,4-b]pyrazine-bridged bis(terpyridine) 123e exhibited the most pronounced bathochromic shift in absorption and emission, resulting also in the smallest optical energy band gap. This behavior was explained by strong electronic interaction of the lateral electron-rich p-dialkoxybenzene substituents with the electron-poor thieno[3,4-b]pyrazine part, resulting in an intramolecular charge-transfer. All bis(terpyridine)s featured blue to red emissions

Table 2.3

Selected photophysical properties of electron-acceptor bis(terpyridine)s 123 [270].a

Bis(terpyridine)

kabs (nm)

kPL (nm)

UPL

123a 123b 123c 123d 123e 123f 123g 123h

423 373 395 401 498 433 427 478

503 424 443 484, 503(s)b 587 498 490 570

0.79 0.63 0.68 0.37 0.69 0.65 0.57 0.58

a

Photophysical properties in dilute solution at room temperature. s: shoulder.

b

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27 J l 2011 15 38 35

| 47

48


spacer

2eq. 101d + Br

00

N

Pd(PPh3)4 CuI NEt3, THF

Br

0

N

spacer

N

N

123 N

N OC8H17 C8H17O

O N

S

C8H17

OC8H17 N

N

OC8H17

NC

C8H17 N

N

C8H17O

O

123a

123b

C N 123d

123c

N

123e S

C8H17O C8H17O

N

O

C8H17O

S

O

O

O

OC8H17

C8H17O

123f

O

C8H17O

N

C8H17O

C8H17O

O N

S

OC8H17

C8H17O

O

OC6H13

C6H13O

OC8H17

O

N

123g

OC6H13

C6H13O

123h

Scheme 2.22 Synthesis of bis(terpyridine)s 123 bearing electron-acceptor-type p-conjugated spacers [270].

C10H21O

OC10H21

C10H21O

OC10H21

101d Pd(PPh3)2Cl2/CuI THF/iPr2NH

I2/HIO3 I N

N B

F

87%

I N

N B

F

F

F

C10H21O

N

OC10H21

N

N

N N 124

N

F

N B

F

Scheme 2.23 Synthesis of BODIPY-bridged bis(terpyridine) 124.

02

27 J l 2011 15 38 35

N

2.7 Miscellaneous Terpyridine-Analogous Compounds

| 49

with high quantum yields (FPL) in the range of 0.37–0.79. These ditopic building blocks were utilized, as monomers, for the synthesis of ZnII metallo-homopolymers as well as random copolymers (Chapter 5.3) [270]. Finally, Bozdemir et al. reported the synthesis of the boron-dipyrromethane (BODIPY) containing bis(terpyridine) 124 by applying Sonogashira cross-coupling [271]. These BODIPY dyes have recently been studied extensively, as fluorescent materials, for energy transfer cascades, in artificial light-harvesting systems, or as sensitizers for dye-sensitized solar cells (for a recent example see Reference [272] and references cited therein). Starting from 3,5-bis(decyloxy)benzaldehyde and 2,4dimethylpyrrole, the green-emitting BODIPY unit was obtained in moderate yields (Scheme 2.23). After iodination and subsequent cross-coupling with 101d under the usual Sonogashira-conditions, bis(terpyridine) 124 was obtained in high yields (87%, Scheme 2.23). The longest wavelength absorption band revealed a large bathochromic shift (labs ¼ 577 nm); 124 showed bright orange to red emission (labs ¼ 608 nm, FPL ¼ 0.47).

2.7 Miscellaneous Terpyridine-Analogous Compounds 2.7.1 Rigid U- and S-Shaped Terpyridines

In contrast to acyclic ketones (e.g., 2-acetylpyridine) that can be readily applied, as reagents, in Kr€ohnke-type condensations, cyclic ketones generally do not react under these conditions. To overcome this limitation, the groups of Thummel [31, 33] and Risch [42, 43] developed two approaches – both based on the concept of the enhanced reactivity of activated carbonyl compounds (i.e., enamines, imines, and iminium salts) – for the application of such ketones, in particular of 6,7-dihydroquinolin-8(5H)-one, for the synthesis of rigid U-shaped terpyridines (see also Scheme 2.4). Non-enolizable aldehydes can be quantitatively converted into ternary iminium salts according to B€ohme’s protocol [273]. The intrinsic high sensitivity of such iminium salts with respect to hydrolysis could be reduced using (hetero)aromatic aldehydes and morpholine, as secondary amine [43]. A broad range of ternary

R

"step-wise" DMSO

N

N N N 125 U-shaped terpyridine

O

O

N

Cl

R R (hetero)aryl, iso-propyl, tert-butyl,H

"one-pot" CHCl3

N N N R 126 S-shaped terpyridine

Scheme 2.24 Synthesis of U- (125) and S-shaped terpyridines (126) according to Risch and coworkers [43].

02

27 J l 2011 15 38 35

50


0

00

iminium salts, derived from substituted (hetero)aromatic aldehydes, was reacted in a multistep, one-pot Mannich reaction to selectively yield U-shaped mono(terpyridine)s 125 in moderate to good yields (44–77%, Scheme 2.24) [43]; under appropriate reaction conditions, the reaction pathway could be changed, thus affording exclusively the S-shaped isomers (up to 90% yield, chemoselectivity >95%). In the U-shaped structure 125, the outer pyridine rings are fixed in an all-cis-configuration by annulation (in contrast, 2,20 :60 ,200 -terpyridines favor an alltrans-configuration with respect to the pyridine-N-atoms). Accordingly, homoleptic and heteroleptic bis-complexes with RuII ions [41, 44] as well as monocomplexes with PtII ions [43] are accessible using 125, as tridentate ligand. The S-shaped terpyridines 126 resemble a phenanthroline-type motif with an additional annulated pyridine ring rather than a terpyridine structure. Thus, the coordination behavior of 126 is typical of bidentate ligands and various heteroleptic RuII, OsII, RhIII, and IrIII complexes have been synthesized [41, 274]. Moreover, bis- and tris(iminium salt)s were introduced by Winter et al. to extend the class of rigid U-shaped terpyridines to ditopic and tritopic derivatives, respectively [44]. Various fully p-conjugated derivatives (e.g., oligo phenylene and thiophene units) could be utilized and the bis(U-terpyridine)s 127 were obtained in moderate yields (26–63%), depending on the nature of the iminium salt (Scheme 2.25a). Poor solubility of 127, due to the absence of any solubilizing substituents,

(a) R2N

4 eq.

spacer NR2

2Cl

NH4OAc, DMSO, 120 °C, 12 h N

127b

N spacer

N

26-63% O

127a

N

N

N N

127

127c

127d S

127e

(b)

O

127f

127g

Cl N

2 eq.

NH4OAc, DMSO, 120 °C, 12 h N O

N

127h

N 128

N N N

N

31% 129 N

N

Scheme 2.25 Synthesis of bis(U-terpyridine)s 127 and of the heteroditopic ligand 129.

02

27 J l 2011 15 38 36

N

2.7 Miscellaneous Terpyridine-Analogous Compounds R R

R

R 130

131

R

N N

N or

N N

N N

N N

N

132

N

N

Figure 2.17 Symmetrical and unsymmetrical ditopic terpyridine derivatives 130–132 synthesized via Suzuki–Miyaura cross-coupling.

was reported as disadvantageous. The ditopic ligands were coordinated to RuII ions, yielding heteroleptic dinuclear complexes. Recently, the terpyridine-functionalized iminium salt 128 was applied by Hummel et al. for the synthesis of an unsymmetrical bis(terpyridine) 129, which consists of one U-shaped and one 2,20 :60 ,200 -terpyridine, bridged by a phenyl moiety (Scheme 2.25b) [206]. The same group also synthesized linear and angular rigid bis(U-terpyridine)s 130 as well as their S-shaped counterparts 131 by utilizing the Suzuki–Miyaura cross-coupling reaction (43–99% yield, Figure 2.17) [275]; the unsymmetrical derivative 132, consisting of a rigid S-shaped and a 2,20 :60 ,200 -terpyridine unit, was obtained from the reaction of (4-bromophenyl)-substituted S-terpyridine and 101f (R ¼ H) under the same reaction conditions (62% yield) [206]. 2.7.2 Five-Membered N-Heterocycles Replacing the Outer Pyridine Rings

The replacement of the outer pyridine rings of the general 2,20 :60 ,200 -terpyridine motif by, in particular, a five-membered N-heterocycle was reported by various groups. These structural modifications were conducted to refine the binding strength of the resulting tridentate ligand and/or to fine-tune the optoelectronic properties of the derived complexes. In the following, some selected examples are highlighted. The Rowan group investigated the tridentate ligand 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridin-4-ol (133, BIP-OH), as terpyridine-analog, where both outer pyridine rings were replaced by 1-methyl-1H-benzo[d]imidazoles [276]. The ability of BIP to bind transition metal ions in a 2 : 1 ratio (i.e., giving complexes with distorted octahedral geometry) was shown by Piguet et al. [277]; the larger lanthanide ions were even able to coordinate three BIP molecules. In analogy to the reactivity of 34 (see also Scheme 2.6), 133 was reacted with a,o-diiodo-functionalized oligo(ethylene glycol) in a Williamson-type etherification to yield the

02

27 J l 2011 15 38 41

| 51

52


(a) I

OH 2 eq.

N

133 (BIP-OH)

I

N

N N

N N

N

00

3

K2CO3, DMSO, 90 °C, 12 h

N N

O

O

0

O

O

4

54%

N

N

O

N N

134

N

N

(b) N

N

OC8H17 N 135

N N

N

N

N

N C8H17O

N

N

N

N

N

N

N N

136 N

N

N

Figure 2.18 Ditopic BIP-type ligands 134–136, according to the Rowan group.

flexible bis-BIP ligand 134 (Figure 2.18a). Subsequently, 134 was utilized for the preparation of multistimuli-responsive metallopolymers (Chapter 5.3). Furthermore, the same group described the synthesis of analogous structures to bis(terpyridine)s reported by the group of Ziessel [67, 278]. In this case, 4-ethynylfunctionalized BIP was reacted with 1,4-diiodo-2,5-bis(octyloxy)benzene under Sonogashira cross-coupling conditions to yield 135 (82%); 136 was obtained by dimerization of 4-ethynyl-BIP in the presence of oxygen under Glaser conditions (Figure 2.18b) [279–282]. The CCAAC reaction (“click” reaction) was not only applied for the functionalization of 2,20 :60 ,200 -terpyridines in 40 -position (Scheme 2.12), but also for the synthesis of 2,6-bis(1H-1,2,3-triazol-4-yl)pyridines (137, BTP-type), as a new type of tridentate ligand. For this purpose, 2,6-diethynylpyridine was reacted with an azide derivative (R ¼ aryl or alkyl) in good to high yields (up to 95%); solubilizing groups or functionalities could be easily introduced on the two outer 1H-1,2,4triazole or at the 4-position of the central pyridine ring of the ligand (Figure 2.19a). The groups of Flood [283], Schubert [284], and Hecht [285] showed that BTP ligands could be coordinated to transition (e.g., FeII or RuII ions) as well as rare earth metal ions (e.g., EuIII ions), giving rise to complexes featuring rich photophysical properties and to advanced metallo-supramolecular architectures. Chandrasekhar [286] as well as Schulze et al. [287] reported on ditopic BTP-based ligands 138–140 that were synthesized, in good overall yields, by multistep syntheses involving Pd0-catalyzed cross-coupling and HWE condensation reactions (Scheme 2.19b).

02

27 J l 2011 15 38 41

2.7 Miscellaneous Terpyridine-Analogous Compounds

| 53

R2 R2

(a)

+

CuI catalyst

N3R1

N N N R1

N 1

R : alkyl or aryl R2: H,Br, aryl, alkoxy

N

N

N N R1

137 BIP ligand

(b) R

N

N

N

N

N

R

N

N

N

R

R

N

N 138

N N

N N

N

N

N

N

N

R

R

N

N

139

N

N

N N

N

N

N

R

N N

N 140 R

N

N

N

N

N

R

N

Figure 2.19 BTP-based ligands 137–140 obtained via the CCAAC reaction.

2.7.3 The Swedish Concept: Expanded Bite Angles in Tridentate Ligands

Recently, Hammarstr€om and Johansson summarized their research targeting improvement of the photophysical properties of RuII bis-tridentate complexes [64]. In RuII bis(terpyridine) complexes, the distorted octahedral geometry – due to unfavorable bite angles – gives rise to a rather weak ligand field and, therewith, low-lying and thermally accessible metal-centered (3MC) states (for more details on this topic, see Chapter 3.3.2). A more octahedral coordination in bis-tridentate complexes could be achieved by expanding the bite angle – a stronger ligand field and a lower rate of population of the 3MC state were the resultant benefits. Various types of tridentate ligands with expanded bite angles, based on the basic tpy motif, have been reported in recent years. The first example in this respect was reported in 2004 in which the tpy motif was modified by inserting a sp3-carbon between the central and one outer pyridine ring (Scheme 2.26a) [288]. The -CH2-group of 142 could subsequently be alkylated using LDA, as base, and methyl iodide [289]; alternatively, hydroxy-substituents were introduced at the sp3-center by choosing 6-lithiated 2,20 -bipyridine and 2acetylpyridine as reagents [290].

02

27 J l 2011 15 38 42

N

R

N

N R

N

N

R

54


0

00

N

(a)

N

N

N

Et2O

N

Li

N

CN

NH2NH2, NaOH, ethyleneglycol O

38%

141

N

95%

142

N

N

(b) O

N

pyrrolidine, EtOH

N

K2Cr2O7, H2SO4

N NOH

68% NOH

N

N O

53%

143

N

N

N NH2 O KOH, EtOH

N

79%

N 144

O

N N

Scheme 2.26 Synthesis of terpyridine-like ligands 141–144, according to the groups of Hammarstr€ om and Thummel.

Carbonyl-functionalized derivatives, such as 141 or the phenanthroline-derived ¨nder reaction, Scheme 2.26b) [291], were also ligand 143 (synthesized via Friedla employed, as ligands, in the coordination sphere of RuII ions. A similar Fried¨nder reaction of 8-aminoquinoline-7-carbaldehyde and 1-(quinolin-8-yl)ethanone la afforded the phenanthroline-quinoline ligand 144 (Scheme 2.26b) [292, 293]. All these terpyridine-like compounds were found to be versatile ligands for the complexation of RuII and PtII ions. The established protocols for the preparation of 2,20 :60 ,200 -terpyridines – Pd0catalyzed cross-coupling of pyridine derivatives or ring-assembly methodologies – were applied in the synthesis of 2,6-di(quinolin-8-yl)pyridines (145, dqp). The Kr€ohnke-type condensation of 1-(quinolin-8-yl)ethanone and aromatic aldehydes according to Wang and Hanan proceeded in low to moderate yields (23–35%, Scheme 2.27 route a) [294]. Moreover, the microwave-assisted Suzuki cross-

R O

N

N

R

B(OH)2

O KOH, aq. NH3, EtOH

a

b

R N

23-35%, R aryl

N N 145

X

N

Pd(dba)2, S-Phos., K2CO3, MeCN/H2O

70-80% R H, CO2Et, NO2, OH, OMe or CH2OH

Scheme 2.27 Synthesis of dqp ligands 145 via ring-assembly or Suzuki cross-coupling reaction.

02

27 J l 2011 15 38 42

X

References

coupling reaction of quinolin-8-yl boronic acid and 4-substituted 2,6-dihalopyridines was carried out in high yields (145, 70–80%, Scheme 2.27 route b). The remarkable properties of the homoleptic and heteroleptic RuII bis-complexes – strong 1MLCT transitions in the visible part of the spectrum and 3MLCT excited-state luminescent lifetimes of up to a few ms – are discussed further in Chapter 3.3.2.

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3

Chemistry and Properties of Terpyridine Transition Metal Ion Complexes

3.1 Introduction

In the 1930s, Morgan and Burstall reported the first complex possessing both a transition metal ion with a terpyridine ligand, specifically [Fe(tpy)2]2 þ (tpy ¼ 2,20 : 60 ,200 -terpyridine) [1, 2]. Since then, a tremendous variety of complexes in which terpyridine ligands were coordinated to main group, transition as well as lanthanide/actinide metal ions have been published – from mononuclear small species to polynuclear macromolecules. Figure 3.1 depicts the distribution of these scientific reports over a wide range of metals.

Li 11

Be 0

Na 23

Mg 4

k 7

Ca 7

Sc 2

Ti 2

V 20

Cr 41

Mn 140

Fe 445

Co 373

Ni 188

Cu 409

Zn 211

Ga 6

Rb 5

Sr 2

Y 9

Zr 1

Nb 2

Mo 18

Tc 10

Ru Rh 1448 47

Pd 82

Ag 51

Cd 55

In 15

Sn 30

Cs 4

Ba 4

Hf 0

Ta 1

W 7

Re 42

Os 209

Ir 68

Pt 399

Au 22

Hg 24

TI 7

Pb 21

Bi 6

Fr 0

Ra 0 La 30

Ca 13

Pr 24

Nd 41

Pm 1

Sm 27

Eu 128

Gd 34

Tb 49

Dy 28

Ho 17

Er 37

Ac 0

Th 0

Pa 0

U 16

Np 2

Pu 2

Am 2

Cm 2

AI 3

Tm 13

Yb 37

Figure 3.1 Periodic table of elements (only the metals are shown). The number for each element denotes the number of scientific papers dealing with the respective terpyridine complexes as determined by SciFinderTM (3890 publications in total; search performed on 31st December 2010). Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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Lu 15

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| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes COOH

HOOC

COOH

N N 1

SCN

Ru

N

N

N

NCS NCS

Pt

2

Cl

N Cl

Figure 3.2 Mono(terpyridine) complexes 1 and 2.

Although the complexes of the early transition or actinide metal ions [e.g., ScIII, Ti , TaIV, or U(O)2VI] might be considered as chemical oddities, their counterparts based on the late transition metal ions (and EuIII) have been extensively investigated. In particular, complexes of ions with d6 (e.g., FeII, RuII, and OsII), d8 (e.g., NiII and PtII), or d10 electron configuration (e.g., ZnII) are in the limelight of current research. The coordination geometries vary depending on the nature of the utilized metal ion; a (distorted) octahedral configuration is by far the most common motif (either two tridentate terpyridine ligands or one terpyridine and additional ancillary ligands are coordinated to the metal center); trigonal-bipyramidal (e.g., CuII ions) and square-planar species (e.g., PdII, PtII, or AuIII ions) are known. Moreover, rare earth metal ions can – due to their f-orbitals – establish up to nine coordinative bonds and, therefore, even bind three tridentate ligands. In 1997, Moore et al. tabulated the coordination geometries of the d-block elements and their ions, quantitatively addressing various oxidation states, coordination numbers, and coordination geometries of particular elements and ions. As one starts to examine the complexation of diverse elements with terpyridine and related ligands, their tables are a good starting point to appreciate the supramolecular interactions [3]. The stability and rich photophysical/electrochemical properties of many terpyridine complexes evolved into applications in the emerging fields of catalysis, optoelectronics, and life-sciences. Here, the “black dye” 1, widely used, as a sensitizer, in dye-sensitized solar cells (DSSCs) [4], and the PtII mono(terpyridine) complex 2, a potent anti-tumor drug [5], are possibly the most prominent representatives (Figure 3.2). However, a complete evaluation of terpyridine complexes is far beyond the scope of this chapter, thus only the most important types of materials will be considered herein, specifically complexes based on RuII/OsII ions. A short overview on systems containing IrIII or PtII ions will also be given. IV

3.2 Basic Synthetic Strategies and Characterization Tools

Bis(terpyridine) metal complexes of the general type [M(tpy)2]X2 (where the counterion X is, commonly, Cl, ClO4 , or PF6 ) have long been known [6–8]. A major structural characteristic of these complexes is the strength of the metal-toligand coordinative bond. With many transition metal ions, in their low oxidation

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3.2 Basic Synthetic Strategies and Characterization Tools Stability constants [9] and binding enthalpies [10] of various [M(tpy)2]2 þ complexes. Table 3.1

Metal ion

Log K1a

Log K2a

DH1 (kJ mol1)b

MnII FeII CoII NiII CuII ZnII

4.4 7.1 8.4 10.7 N.a. 6.7

N.a.c 13.8 9.9 11.1 N.a. 5.2

N.a. 79.9 61.5 66.9 92.2 60.7

a

Stability constants were determined by a stopped-flow procedure in water. Binding enthalpies were determined by ITC in MeCN. c N.a.: not available. b

states, a bis(terpyridine) complex is formed possessing a pseudo-octahedral coordination geometry at the metal center. The inherent stability of such complexes can be explained by the strong metal–ligand d–p* back-donation as well as by the dynamic chelate effect. According to the Irving-Williams series – predicting the order of stability for complexes of divalent first row transition metal ions (i.e., MnII o FeII o CoII o NiII o CuII W ZnII) – the complexes exhibit different stabilities depending on the specific metal ion, as expressed in the stability constants (K values), where K1 represents the mono(terpyridine) complex (1 : 1 ratio of ligand and metal ion) and K2 the bis(terpyridine) complex (2 : 1 ratio). The kinetics of complexation was investigated by a stopped-flow method, from which a pseudo-first order behavior was observed in all cases (Table 3.1) [9]. Applying isothermal titration calorimetry (ITC) permitted access to thermodynamic parameters, such as the binding enthalpies of the complexes [M(tpy)2]2 þ [10]. The stability of the metal-to-ligand coordinative bond is a crucial issue when solution-based characterization tools, such as size exclusion chromatography (SEC), are utilized. The stability constant of a complex can be solvent- and temperature-depended, thus K also depends on concentration. If the binding constant is too small, affording kinetically labile complexes, dissociation of such complexes will occur under the SEC conditions. Only in those cases when K is high enough (for so-called kinetically inert complexes) SEC can be applied to characterize the metallo-supramolecular materials; this applies to the bis(terpyridine) complexes of NiII, RuII, OsII, and IrIII ions [11, 12]. Terpyridine ligands, as well as their transition metal ion complexes, were also studied regarding their thermal stability utilizing thermogravimetric analysis (TGA) in which, when compared to the metalfree ligand, the metal complexes revealed a significantly increased thermal stability, as concluded from the 5%-onset of weight loss [13]. To synthesize complexes of the general [M(tpy-R)2]2 þ type (where R denotes any substituent), the ligand is commonly treated with a specific metal ion (e.g., ZnII, CoII, CuII, NiII, FeII) in a 2 : 1 metal-to-ligand ratio (Scheme 3.1a); purification involves exchange of the counterions to guarantee solubility in organic solvents,

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| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes (a) one-step procedure MII

2 R

(b) directed two-step procedure MII

R

MIII

R

R

R

R N

MIII

MIII = RuIII, OsIII, RhIII, IrIII

MII FeII, NiII, ZnII, ...

N

R

R

MII

R

MIII

MIII

N

II R M II Ru or OsII

or R

MIII RhIII or IrIII

Scheme 3.1 One-step and directed two-step strategies towards homoleptic (a) or heteroleptic bis(terpyridine) complexes (b).

such as acetone or MeCN, and subsequent recrystallization. Addition of metal salts to a mixture of two different terpyridine derivatives gives rise to a statistical mixture of homoleptic as well as heteroleptic complexes. Thus, one has to apply a directed two-step procedure when selectively aiming for heteroleptic complexes. Owing to the special coordination chemistry of their MIII/MII ions, Ru and Os are well-suited for this strategy; in these cases, the MIII mono(terpyridine) complex can be isolated and subsequently reduced in situ to the corresponding MII species, Scheme 3.1b; for further details, see Section 3.3 [6]. Moreover, heteroleptic RhIII and IrIII bis(terpyridine) complexes can also be obtained (Section 3.4) [14–16]. Recently, the Gohy group showed that kinetically stable heteroleptic NiII and CoIII bis(terpyridine) complexes can be obtained via a directed two-step synthesis [17]. The UV–vis absorption spectra of terpyridine metal complexes [M(tpy)2]2 þ show a pronounced bathochromic shift of the ligand-centered (LC) absorption bands for all types of metal ions (Figure 3.3a). With the FeII and RuII bis(terpyridine) complexes, a characteristic metal-to-ligand charge-transfer (MLCT) band can be observed. This absorption lies in the visible region and is responsible for the intense color of the complexes (i.e., purple for FeII and red for RuII). For the FeII complexes, a distinct metal-centered (MC) band can be detected, whereas for the RuII species, only a shoulder could be detected. Various mass spectrometric (MS) techniques have been applied for the characterization of bis(terpyridine) complexes. In particular, the soft techniques [i.e., electrospray ionization (ESI) and matrix-assisted laser desorption/ionization timeof-flight (MALDI TOF) MS] are well-suited tools for their analysis [18, 19]. ESI MS allows the detection of multiply charged ions, does not alter the connectivity of the complexes (through disassembly–reassembly processes), and causes very little fragmentation [18]. A representative ESI MS of a dinuclear RuII bis(terpyridine) complex (3) is depicted in Figure 3.3b. In contrast, bis(terpyridine) complexes were found to be highly susceptible to the conditions applied during the MALDI process in which, at higher laser intensities, fragmentation of the complex cation occurred; the degree of fragmentation was dependant on the applied laser energy and, thus, the binding strength of the complexes could be estimated [19]. In addition, matrix

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(a) Lc 0.4 tpy absorption (a.u.)

[Zn(tpy)2]2 [Co(tpy)2]2 [Fe(tpy)2]2 [Ru(tpy)2]2 0.2 MLCT

MC 0.0 200

300

400

500

600

700

wavelength (nm)

(b) 34

N

366.1

N

N N

N

Ru2 O

N

3

N

N

N

536.7

877.1

O

4ⴙ

32

N N

4PF6

33

N Ru2

3ⴙ

3 364

600

366

368

533

536

539

1899.2 1800 m/z

1200

Figure 3.3 (a) UV–vis absorption spectra of various [M(tpy)2]2 þ complexes (MII ¼ ZnII, CoII, FeII, and RuII) compared to the free tpy ligand (all spectra were measured in MeCN). (b) ESI MS of dinuclear complex 3 (the numbers on top of the peaks give the charge state and the m/z value of the most abundant isotope); inset: experimental (top) and calculated (bottom) isotope patterns of the quadruply and triply charged quasi-molecular ions of 3 [18]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

adducts could be observed for many bis(terpyridine) metal ion complexes and adducts of the cations from the dopant salt (i.e., NaI or KI) could also be detected. All species detectable by MALDI TOF MS are singly, positively charged – this phenomenon can be explained by a plume of charged particles emitted in the desorption

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| 69

70

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes process that also contains electrons. Subsequently, a partial neutralization occurs from which the singly-charged species survives the longest; therefore, only these ions can be detected (the surviving higher-charged species have such a low intensity that they are generally not observable) [20]. The use of ESI and MALDI MS to analyze structures possessing tpy-[M]-tpy connectivity can easily generate numerous differently charged complexes as well as fragments, which can offer obstacles to structural verification. Drift-cell ion mobility spectroscopy MS or traveling wave ion mobility mass spectrometry (TWIM MS) [21–24] offer a new solution to this problem since they afford dispersion based on the shape and charge state. With this insight, fragments and intact-assemblies of the same mass-to-charge ratio as well as isomeric architectures of the same self-assembly can be easily deconvoluted [25–30]. For diamagnetic complexes, 1H NMR spectroscopy can be utilized, as an analytical tool. In most cases, the signals of the 6,600 -protons of the coordinated ligand exhibit a characteristic shift compared to the metal-free ligand (Figure 3.4a). In an octahedral complex, the ligands are oriented perpendicular to each other; thus, the 6,600 -protons in bis(terpyridine) complexes are located above the ring-plane of

(a)

N

N N

6,6‘‘-protons N

N Ru2+ N

N N

N

2 PF6

9.0

(b)

8.9

8.8

8.7

8.6

(c)

8.5

8.4

8.3

8.2

complex: CH as neighbor

8.1

8.0

7.9

7.8

7.7

7.6

7.5

7.4

7.3

7.2

7.1

7.0

(ppm)

free ligand: N as neighbor

Figure 3.4 (a) Aromatic region of the 1H NMR spectra of tpy (top) and [Ru(tpy)2]2 þ (bottom) revealing the shifts of the signals upon complexation (both spectra were measured in d3-MeCN); (b and c) the critical steric interactions in the complex as well as in the metalfree ligand.

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| 71

the central pyridine ring of the adjacent ligand, inducing an observed up-field shift of these unique protons (Figure 3.4b; also the signals for the other protons reveal less dramatic shifts, when compared to the free ligand). Besides the influence of the metal-to-ligand coordination, the chemical environment is different in that in the free ligand the nitrogen-atoms possess an all-anti orientation (as shown by many X-ray single-crystal analyses), but in the complexes the all-syn orientation is needed to enable the tridentate N-coordination (Figure 3.4c). Despite their paramagnetic nature, the CoII bis(terpyridine) complexes exhibit an interesting 1H NMR spectroscopic behavior in which the spectra typically appear well-resolved and the chemical shifts are remarkably influenced by the paramagnetic character of the CoII core. Owing to hyperfine interactions of the unpaired electrons of the CoII ions with the ligand protons, pronounced low-filed shifts of the signals can be observed (the so-called “Knight shifts” [31]), while maintaining their well-resolved sharp shapes [32]. The Constable group reported that the electronic state of the CoII ions, that is, high-spin or low-spin state, can be distinguished by the chemical shifts of the NMR signals [33], in that the lowest resonance can generally be found at about 100 ppm for low-spin complexes and about 250 ppm for high-spin complexes. There are stable exceptions from this particular behavior, as observed for ligands establishing a weak ligand field, for example, 40 -bromo-2,20 :60 ,200 -terpyridine [34]. Figure 3.5 depicts a typical 1H NMR spectrum of a homoleptic low-spin CoII bis(terpyridine) complex (4) [35].

OC7H15

N N

N Co2

N

N

8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 (ppm)

N 4

2 PF6

OC7H15

110

100

90

80

70

60

50

40

30

Figure 3.5 Knight-shifted 1H NMR spectrum of the CoII bis(terpyridine) complex 4 (300 MHz, d3-MeCN) [35].

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20

10

(ppm)

72

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes N

(a)

N

N

R1

i) Cu(OTf)2 ii) NH4PF6

R1

N N

R1

Cu2+ N N

R2

R1

N

5

N

R2

N

2 PF6

R2

R2

F4

(b)

F6 F5 P1

F2 F3

F1 C44

C42 N41

C45 C35

C36

C46

N31

C54

C53

C43 C52

C55

N51

C56 C12

C13

Cu C34

C33

C32

N11 C14 N21

C17 C16

C22

C26

C23

C15

C25 C24

C27

Figure 3.6 (a) Synthesis of the heteroleptic CuII complexes 5; (b) representation of the X-ray single-crystal structure of 5a (R1 ¼ H, R2 ¼ Me; H-atoms and solvent molecules omitted for clarity) [36]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

The octahedral geometry is, by far, the most common motif in the coordination chemistry of terpyridines and transition metal ions. A different type of structure was obtained when CuII ions were treated with equimolar amounts of a tridentate terpyridine and a bidentate bipyridine ligand, whereby such heteroleptic CuII complexes could be prepared in a one-step procedure (Figure 3.6a) [37, 38]. According to X-ray single-crystal analysis, the CuII centers were pentacoordinated in a square-pyramidal geometry with the sixth coordination site being occupied by a loosely bound PF6 counterion (Figure 3.6b). This synthetic approach was, for example, utilized by the Lehn group for the synthesis of helical trinuclear complexes (see also Chapter 4.5) [39]. Similar mixed-ligand complexes of ZnII ions (i.e., composed of terpyridine and phenanthroline ligands in the trigonal-bipyramidal coordination sphere at the metal center) were introduced by Schmittel and coworkers in the context of the HETTAP concept (HETTAP: heteroleptic terpyridine and phenanthroline complexation; see also Chapter 4.3) [40].

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3.3 RuII and OsII Complexes

| 73

3.3 RuII and OsII Complexes 3.3.1 Synthesis of RuII and OsII Bis(terpyridine) Complexes

The coordination of two terpyridine ligands to a RuII center can be conducted in a simple two-step sequence (Scheme 3.1 and Scheme 3.2) in which the RuIII intermediate is isolated (but not characterized) and subsequently reacted with a second equivalent of either the same or a different terpyridine ligand under reductive conditions to generate the desired homoleptic or heteroleptic RuII bis(terpyridine) complex, respectively [6–7]. First, RuCl3 xH2O is added to either a MeOH or EtOH solution of the initial terpyridine ligand. The resulting RuIII mono(terpyridine) complex is generally insoluble and, in most cases, can be simply filtered but, notably, due to its paramagnetic nature NMR spectroscopy as characterization is hardly be utilized. This intermediate is subsequently suspended with the second ligand in an aliphatic alcohol, containing catalytic amounts of N-ethylmorpholine (NEM) [42], and refluxed for several hours. Recently, this reaction time could be significantly decreased by applying microwave irradiation (i.e., 15–60 min at 120 1C using specialized microwave synthesizers) [43, 44]. In both cases, the solvent can also be the reducing agent for the RuIII-into-RuII conversion. To activate the RuIII precursor complex, an equimolar amount of AgBF4 can be added to a suspension of the initial complex in DMF or acetone to remove the chloride ions [41]; the vacant coordination sites are then occupied by the weakly binding solvent molecules, thus affording a soluble activated species, in stark contrast to the original [(R1-tpy)RuCl3] complex. After filtration of the AgCl precipitate, the intermediate is reacted without isolation/purification with the second terpyridine ligand (R2-tpy) to give the RuII bis(terpyridine) complex (typically, the yields are in the 50–90% range). However, the formation of statistical mixtures of complexes (i.e., homoleptic and heteroleptic) via ligand exchange processes has been reported in a very few cases, despite the highly selective and directed nature of the protocol [45]. In the two-step

R2 N N 1

R

N

RuCl3∗xH2O, EtOH, reflux, 12 h

N

Cl N R u Cl Cl N

1

R

N

EtOH, cat. NEM, reflux, 1-12 h

N N 1

R

NH4PF6, EtOH, roomtemp., 1 h

N

N 2

N

Ru N

N

R2

N 2PF6

II

Scheme 3.2 Synthesis of heteroleptic Ru bis(terpyridine) complexes (NEM: Nethylmorpholine) [7, 41].

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74

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes C3 C2 C4 C1

C5 C7 C6

N1 C10

C11 Ru

C8 N2

C9

O1

6a

Figure 3.7 X-Ray single-crystal structure of cis-[(tpy)Ru(dmso)Cl2] (6a, R1 ¼ H, the H-atoms are omitted for clarity) [48]. Figure reproduced with kind permission; r 2004 American Chemical Society.

procedure to heteroleptic products, traces of unreacted RuCl3 can also lead to unwanted mixtures of homoleptic materials. The mildest preparation of RuII bis(terpyridine) complexes has been found with Ru(dmso)4Cl2 [46], as the precursor complex. By means of this RuII reagent, terpyridines possessing sensitive functional groups (e.g., ethynyl moieties [47, 48]) or terpyridine-functionalized polymers [49] can be coordinated to the metal center under mild conditions. In the first step, the key building block, cis-[(R1-tpy)Ru(dmso)Cl2] (6, see Figure 3.7 for a representative X-ray single-crystal structure) is prepared by reaction of Ru(dmso)4Cl2 with R1-tpy at 80 1C in CHCl3 [48]; subsequently, the second ligand is coordinated to 6 in MeOH (using AgI salts, as activating agents). Homoleptic RuII bis(terpyridine) complexes can also be obtained via the RuCl3/ NEM-method – either in a two-step procedure by applying the same ligand twice or in a one-pot reaction (using either conventional or microwave-assisted heating [50]). Owing to its high reactivity, the availability of Ru(dmso)4Cl2, as starting material, should also be considered [51]. An alternative procedure was described by Kelch and Rehahn [52] in which RuCl3 xH2O was dechlorinated with AgBF4 in acetone to give the [Ru(acetone)6]3 þ complex, possessing six weakly bound solvent ligands that were subsequently replaced by the terpyridine ligands under reductive conditions. Notably, [Ru(tpy)2]2 þ can be reduced by electrocrystallization to yield neutral complexes [53]. The X-ray single-crystal structures of both [Ru(tpy)2]2 þ and [Ru(tpy)2]0 complexes were determined; the latter neutral complex did not possess any external counter ions as concluded from the increased number of voids in the

03

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unit cell. The observed species actually existed not as a Ru0 but as a RuII bearing two terpyridine radical anions as the internal counter ions. Moreover, a directed two-step protocol was reported by Meyer and coworkers for the synthesis of the mixed-ligand aquo-complexes [(tpy)(bpy)M(H2O)]2 þ (M ¼ RuII or OsII, bpy ¼ 2,20 -bipyridine) [54]. The loosely bound H2O molecules could be substituted by stronger ligands; thus, reaction with terminal alkynes gave access to a family of acetylide complexes that were weakly emissive at room temperature (lPL ¼ 748–786 nm). Addition of MeOH to the triple bond yielded carbene complexes that were emissive only at low temperatures [assigned to dp(RuII)-p* (oligopyridyl) 3MLCT transitions; lPL ¼ 597–615 nm at 77 K] and were unstable with respect to photolysis (reductive elimination of an aldehyde derivative) [55]. Several structurally related RuII complexes were also reported in the literature: [(tpy)(bpy)(py)Ru]2 þ (py ¼ pyridine derivatives) [56], [(tpy)(bpy)Ru(L)] þ (L ¼ 5substituted tetrazolates) [57], [(tpy)(phen)Ru(MeCN)]2 þ (phen ¼ 1,10-phenanthroline) [58], and [(tpy)(acac)Ru(L)] (acac ¼ acetylacetonate, L ¼ cyanamide anion) [59], to name just a few. Homoleptic and heteroleptic OsII bis(terpyridine) complexes can be prepared according to the protocols available for their RuII analogs – with the exception that ligand exchange with Os(dmso)4Cl2 and N-heteroaromatic ligands has not yet been described, presumably due to the strong binding of the dmso molecules to the OsII center [46]. The most common one-step synthesis starting from OsCl3 xH2O is depicted in Scheme 3.1 [6, 44, 60, 61]. 3.3.2 RuII Ions and Terpyridine Ligands – A Happy Marriage?

Considering the magnitude of literature dealing with RuII mono- and bis(terpyridine) complexes (Figure 3.1), the question has to be raised why this particular combination has had such a remarkable scientific impact. Certainly, the kinetically inert character of the RuII-terpyridine coordinative bonds is a key feature that gave rise to the development of advanced metallo-supramolecular polymers; in this respect, the [Ru(tpy)2]2 þ units can serve as stable linkage between two different polymer chains (see Chapter 6 for a detailed overview on this topic). Moreover, the catalytic activity of diverse RuII bis(terpyridine) complexes has been applied to the fields of light-into-energy conversion (Chapter 8.4) or artificial photosynthesis (Chapter 9.3). However, with respect to optoelectronic applications, complexes of the general type [Ru(N4N)3]2 þ (N4N: bidentate N-heteroaromatic ligand, for example, bpy or phen) are often superior. Is the combination of RuII ions and terpyridine ligands a happy marriage? To answer this question, the photophysical properties of RuII bis(terpyridine) complexes [in comparison to their RuII tris(bipyridine) counterparts] have to be evaluated first. 3.3.2.1 Photophysical Properties In general, RuII oligopyridine complexes possess rich photophysical properties that can be readily tuned via ligand modification [62–66]. Combined with the facile

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| 75

76

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes synthetic strategies, such complexes have become important targets for potential applications in artificial photosynthesis [64, 65] or molecular electronics [67, 68]. Most commonly, RuII tris-bidentate complexes, such as the [Ru(bpy)3]2 þ prototype, are applied for these purposes and exhibit intense UV–vis absorption bands that can be assigned to LC and MLCT transitions. The molecular excitation in any of its absorption bands rapidly populates a luminescent 3MLCT excited-state that decays to the ground state within about 850 ns [69]. However, complexes of the type [Ru(bpy)3]2 þ can exist as a mixture of D and L isomers (whereas, asymmetric bidentate ligands form fac- and mer-isomers); this isomer problem can become even more complicated when oligonuclear complexes bearing unsymmetrical ligands are considered. As summarized by Sauvage et al., RuII bis(terpyridine) complexes offer structural advantages over their RuII tris(bipyridine) counterparts [6, 70] in that complexes of the general type [Ru(tpy)2]2 þ are achiral and possess higher degrees of symmetry (D2d vs. D3 for Ru(bpy)3]2 þ ) and, thus, they are predestinated for the construction of linear arrays. Photophysically, the ground state properties of [Ru(tpy)2]2 þ are similar to those of [Ru(bpy)3]2 þ ; however, the excited-state properties of the former are rather poor, since at room temperature the 3MLCT excited-state lifetime of [Ru(tpy)2]2 þ is of the order of 0.25 ns [71], leading to inefficient electron/energy transfer processes to neighboring quenchers [72–74]. The steric strain resulting from the tridentate coordination reduces the ligand field strength of terpyridines relative to bidentate bipyridines [53], therefore the low-lying metal-centered (3MC), non-emissive states are thermally accessible. At low temperatures by dissolution in a rigid glassy matrix at 77 K, this path becomes less favored and luminescence can be observed [75]. 3.3.2.2 Mononuclear RuII Bis(terpyridine) Complexes To combine the favorable structural properties of RuII bis(terpyridine) complexes with prolonged excited-state lifetimes, various terpyridine derivatives and terpyridinelike structures for RuII bis-tridentate complexes possessing improved photophysical properties have been prepared over the last two decades [76–78]. For RuII oligopyridine complexes, radiative as well as non-radiative decay pathways from the 3MLCT excited-state to ground state contribute to the lifetime (t) according to Eq. (3.1). At room temperature, the excited-state decay mainly occurs by thermal population of 3MC states (kact: activated rate constant), whereas direct radiative (kr) and non-radiative (knr) decays to the ground state (the latter governed by the energy gap law) dominate at lower temperatures.

t¼

1 ¼ A expfDE=ðR TÞ ðkr þ knr þ kact Þkact

(3.1)

The synthetic strategies utilized for terpyridine modification can be summarized as follows: (i) methods involving changes in the coordination sphere (i.e., structural changes of the tpy skeleton) and (ii) methods involving substitution on the tpy core (i.e., maintaining the basic tpy scaffold). Both approaches aim to enhance the 3MLCT lifetime in RuII bis(terpyridine) complexes, though following different

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| 77


(a)

(b)

eⴚ

Energy

eⴚ

1

MLCT 3

MC

3

k act nr

3

(a)

MLCT

hν

MLCT

knr

P

D

MC

3

A

(b) ∆E

hν

(c)

kr

D: electron donor P: photosensitizer A: electron acceptor

∗ D-P-A kcs

ⴙ ⴚ

D-P-A

kcs’

ⴙ

kcr kcr’ D-P-A

Figure 3.8 (a) The two strategies (a and b) to increase the 3MLCT excited-state lifetime in RuII bis(terpyridine) complexes (kr: rate constant of the radiative decay, knr: rate constant of the non-radiative decay, kact: activated rate constant) [76]; (b) linear arrangement in a D–P–A triad, based on RuII bis-tridentate complexes; (c) processes involved in the photoinduced charge separation (cs: primary charge separation, cr: primary charge recombination; cs0 : secondary charge separation, cr0 : final charge recombination) [66]. Figure reproduced with kind permission; r 2010 Elsevier B.V. and 2007 Springer Verlag, respectively.

mechanisms: decreasing the 3MLCT energy (a), increasing the 3MC energy (b), or combining both to increase the energy gap DE (thus, minimizing kact). Figure 3.8a shows a representation of the energy scheme [76]. In particular, research has focused on (a) by introducing lateral substituents or other heterocycles resulting in a significant impact on the 3MLCT energy; however, lowering the 3MLCT excitedstate (therewith increasing knr according to the energy gap) might be (partially) compensated by an increase of the p-electron delocalization in the 3MLCT excited-state. As a result of a small displacement between the excited-state and ground-state molecular structures, a smaller knr value might be obtained [79, 80]. Conversely, the second strategy (b) theoretically leaves the 3MLCT excited-state energy unaffected, with the 3MC state being selectively destabilized by a strong ligand field. Alternatively, the luminescence lifetime could be increased via additional organic chromophores with low-lying pp* triplet energy levels that repopulate the luminescent 3MLCT excited-state [81, 82]. The Johansson group reported various approaches to prolong the 3MLCT lifetimes; their particular tridentate ligands all featured expanded tpy cores [76]. When coordinated to RuII centers, the ligand’s bite angles were increased and its steric strain was reduced. As a result, the RuII bis-tridentate complexes exhibited remarkable lifetimes, in the ms-regime, and were successfully applied

03

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ⴚ

D-P-A

1/τ

hν

78

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes N O

O

N

N

N

N N Ru2 N

N

N

N

N

Ru2

N Ru2

N

N

N N

7

8

O

N N

O

9

II

Figure 3.9 Ru bis-tridentate complexes 7–9, based on expanded terpyridine-type ligands (counterions omitted for clarity) [76].

Table 3.2

Selected photophysical properties of complexes 7–9 at room temperature.

7 8 9 [Ru(bpy)3]2 þ [Ru(tpy)2]2 þ

kabs (nm)a

kPL (nm)b

s (ns)b

UPLb

Reference

477 562, 522 491 450 476

655 608 700 630 629

15.0 3300 3000 1150 0.25

1 103 0.3 0.02 0.089 –

[76, 83] [76, 84] [76, 85] [69] [6]

a

MeCN, as solvent; only the MLCT band of lowest energy is listed. Deoxygenated EtOH–MeOH, as solvent.

b

in electron-donor–photosensitizer–electron-acceptor assemblies (D–P–A arrays, Figure 3.8b) for efficient vectorial photoinduced charge separation [76]. Figure 3.9 shows three typical homoleptic representatives, bearing a bipyridyl-pyridine ligand bridged by –CH2– (7) [83], an oligopyridine ligand bridged by –CO– (8) [84], and a diquinolinyl-pyridine ligand (9) [85]. The photophysical data – in comparison to [Ru(bpy)3]2 þ and [Ru(tpy)2]2 þ , as references – are summarized in Table 3.2. In particular, complex 9 is one of the best candidates in the family of RuII bis-tridentate complexes with respect to the photophysical properties: at room temperature, 3 MLCT emission at 700 nm, a lifetime (t) of 3.0 ms, and a quantum yield (FPL) of 0.02 were observed; the emission was blue-shifted to 673 nm at 77 K (t ¼ 8.5 ms, FPL ¼ 0.06). The excellent photophysical behavior (at room temperature) was ascribed to an almost ideal octahedral coordination geometry around the RuII center. A detailed overview concerning the manipulation of the photophysical properties of RuII complexes employing expanded terpyridine-type ligands is presented in a recent review [76]. Maestri et al. and Wang et al. showed that simple electron-donating (e.g., –Cl, –OH, –OEt, –NMe2) or electron-withdrawing substituents (e.g., –SO2Me, –CN) at the 40 -position of the terpyridine are suited for the manipulation of the MLCT states of the resulting RuII bis(terpyridine) complexes 10 relative to their MC states

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3.3 RuII and OsII Complexes R1

N N

N Ru2

N

10a: R1, R2 Cl 10b: R1, R2 Ph 10c: R1, R2 SO2Me 10d: R1, R2 CN

11a: R1, R2 = thiophen-2-yl 11b: R1, R2 = 2,2'-bithiophen-5-yl 11c: R1, R2 = furan-2-yl 11d: R1, R2 = pyrrol-2-yl

10e: R1 H, R2 SO2Me 10f: R1 = H, R2 CN

11e: R1 = H, R2 = thiophen-2-yl

N N

R2

Figure 3.10 RuII bis(terpyridine) complexes 10 and 11 (counterions omitted for clarity) [42, 86].

Table 3.3

Selected photophysical properties of RuII bis(terpyridine) complexes 10.a

[Ru(tpy)2]2 þ 10a 10b 10c 10d 10e 10f a

kabs (nm)

kPL (nm)

s (ns)

Reference

476 480 487 486 490 482 480

629 653 715 666 680 679 701

0.25 0.2 1.0 25.0 50.0 36.0 75.0

[71] [42] [42] [42] [86] [42] [86]

All measurements were performed in deoxygenated MeCN at room temperature.

[42, 86]. All homoleptic and heteroleptic complexes 10 (Figure 3.10) displayed strong luminescence in a rigid matrix at 77 K (t ¼ 1–10 ms); the energy of the emission maximum was redshifted for both the electron-accepting and -donating substituents when compared to that of [Ru(tpy)2]2 þ . At room temperature, the electron-accepting substituents increased FPL and t; whereas, the electrondonating substituents showed an opposite effect. Correlations of the electrochemical redox potentials (i.e., the Hammett s parameter) and the energy of the luminescent level showed that electron-accepting substituents were stabilizing the LC p*-orbital to a greater extent than the MC state. Thus, the excited-states were, in essence, populated longer (10c: t ¼ 25.0 ns, 10d: t ¼ 50 ns) when compared to [Ru(tpy)2]2 þ or 10a (0.25 and 0.2 ns, respectively, see Table 3.3). For heteroleptic complexes (e.g., 10e/f), the excited-state lifetime could be prolonged; thus, in the presence of strongly electron-donating ligands, the lowest 3MLCT state was located on the electron-poor ligand and, overall, higher energy MC states were retained. As a result, excited-state lifetimes of up to 75 ns at room temperature were observed for 10f [86]. Various five-membered heteroaromatic substituents (e.g., thiophene, furan, pyrrole) at the 40 -position increased the p-conjugation in complexes 11

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| 79

80

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes 12a: [(tpy)Ru(L1)]3 12b: [Ru(L1)2]4 12c: [(ttpy)Ru(L2)]3 12d: [Ru(L2)2]4 12e: [(ttpy)Ru(L3)]3 13a: [(ttpy)Os(L2)]3 13b: [(ttpy)Os(L3)]3

N N

R

N

N

N

N R

N

L1

N

L2: R H L3: R CH3

Figure 3.11 TP þ -functionalized RuII and OsII bis(terpyridine) complexes 12 and 13 (TP þ : 2,4,6-triarylpyridinium, ttpy: 40 -tolyl-2,20 :60 ,200 -terpyridine; counterions omitted for clarity) [73, 90–92].

(Figure 3.10), thereby stabilizing the 3MLCT state and increasing the energy gap to the non-emissive MC state [87–89]. For instance, UV–vis absorption (labs) and emission maxima (lPL) of 498 and 664 nm, respectively, and an excited-state lifetime of about 9 ns were observed for 11a. Laine et al. investigated a series of TP þ -functionalized terpyridine and 40 phenylterpyridine RuII and OsII complexes (12 and 13, TP þ : 2,4,6-triarylpyridinium, ttpy: 40 -tolyl-2,20 :60 ,200 -terpyridine, Figure 3.11) [73, 90–92]. Enhanced room temperature luminescence was observed (e.g., 12a: lPL ¼ 670 nm, t ¼ 55 ns; 12b: lPL ¼ 644 nm, t ¼ 27 ns) [73]. This behavior was ascribed to an inductive, through-bond electronic substituent effect increasing the energy gap between the 3 MLCT and 3MC states. The homologous complexes incorporating an additional phenyl spacer between the tpy and acceptor moieties (12c/d) only gave a slight improvement of t in the sub-ns-regime, reflecting the reduced influence of the TP þ unit onto the coordinated metal center [73]. Time-resolved, ultrafast spectroscopy experiments on 12c and 13a gave evidence for an excited-state planarization of the sterically less constrained derivatives; in contrast, the conformationally locked complexes 12e and 13b did not show this equilibration behavior [91, 92]. These observations indicated that excited-state relaxation and thermal structure fluctuations of these molecules determined both the rate constant of the decay processes and decay mechanism (Figure 3.8c) in which the photo-induced generation of charge-separated (CS) states can be controlled via the degree of conformational freedom. Another example, in which a pyridinium derivative (as viologen-type acceptor) was connected to a terpyridine unit, is depicted in Figure 3.12. This ligand (L4) was combined with donor-type ligands (ttpy, L5 and L6) to yield heteroleptic RuII as well as OsII bis(terpyridine) D–P–A triads (14 and 15). Collin et al. reported that the charge-separated D þ –P–A state (see Figure 3.8c) was not realized in the triad 14b; however, for 14c such a species could be inferred since a strong luminescence quenching was observed and transient absorption spectroscopy showed that the charge separation was followed by a very fast charge recombination reaction (t o 100 ns) [72]. The OsII complex 15a [93] exhibited a decrease of the 3MLCT lifetime in deoxygenated MeCN (t ¼ 0.72 ns; [Os(ttpy)2]2 þ : t ¼ 240 ns). This shortening was attributed to oxidative quenching of the 3MLCT excited-state,

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27 J l 2011 15 42 31


N

N N

L5: N

R

S

N

N L4

N

N

OCH3

ttpy: R CH3

N 14a: [(L4)Ru(ttpy)]4 14b: [(L4)Ru(L5)]4 14c: [(L4)Ru(L6)]4

15a: [(L4)Os(ttpy)]4 15b: [(L4)Os(L5)]4 15c: [(L4)Os(L6)]4

L6:

N

OCH3

Figure 3.12 RuII and OsII bis(terpyridine) complexes 14 and 15, as potential D–P–A triads (counterions omitted for clarity) [72, 93].

which is in contrast to the endergonic triplet–triplet energy transfer (DGEnT ¼ þ 1.3 eV), the intramolecular electron-transfer is exergonic by DGET ¼ 0.55 eV [94]. Since the relaxation of the charge-separated state in 15a was faster than its formation, a significant population of this state could not be reached [95]. For neither complexes 15b nor 15c was a fully charge-separated state observed via transient absorption spectroscopy [96]; it was proposed that a poor electronic coupling between the donor (L5) and OsII center in 15b did not allow efficient electron-transfer and the charge-separated state in 15c was believed to be too shortlived. ohnke-type), the phenyl and In 40 -phenyl-substituted terpyridines (so-called Kr€ central pyridine rings are twisted about 20–301 due to unfavorable interactions between the adjacent (hetero)aryl protons [97]. The non-planarity between the terpyridine moiety and aromatic substituent leads to a mismatch between the ground and excited-state geometries, thus minimizing the overall p-electron delocalization. As a result, the photophysical properties of the derived RuII bis(terpyridine)s are, in general, rather poor; for instance, the 3MLCT lifetime of [Ru(ttpy)2]2 þ is only slightly prolonged (t ¼ 1.0 ns) when compared to [Ru(tpy)2]2 þ (see also Table 3.3). The increase of t cannot simply be rationalized via the electron-donating or -accepting ability according to the Hammett equation, since the s values for H and phenyl are essentially identical. The aromatic substituent can conjugate with the low-lying p* level of the tpy unit, thereby extending the p-conjugation and stabilizing the 3MLCT excited-states relative to the nonemissive 3MC states [78]. Fang et al. reported that this effect is pronounced in systems where the pendant ring favors a coplanar arrangement with the terpyridine unit through intramolecular H-bonding interactions as observed, for example, in 40 -(pyrimidin-2-yl)-2,20 :60 ,200 terpyridines (Figure 3.13a) [98, 99]. The almost flat molecular structure of both L7 and the derived RuII bis(terpyridine) complexes 16 was confirmed by X-ray single-crystal analysis (Figure 3.13b). The coplanar arrangement of the rings helped

03

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| 81

82

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes R

R

H H

N

H H

N

H N

N

N

L7a: R = H L7b: R = C6H5 L7c: R = CN L7d: R = C6F5

N H N

16: [(tpy)Ru(L7)]2+ N

N

L7e: R = N

(a)

Kröhnke-type terpyridine

L7

N1 N4 N2 N5

L7b

N3

N8

N1 N4 N7

N2 N3

N5

N6

16b:

(b)

[(tpy)Ru(L7b)2]2+

Figure 3.13 (a) Non-planar Kr€ ohnke-type terpyridines and planar (pyrimidin-2-yl)-substituted terpyridines L7; (b) representations (two orthogonal views) of the X-ray single-crystal structures of ligand L7b and complex 16b (H-atoms and counterions omitted for clarity) [99]. Figure reproduced with kind permission; r 2007 American Chemical Society.

to lower the energy of the p* orbitals due to improved p-conjugation; as a result of the extended delocalization in the 3MLCT excited-state, the emission energies of 16 were significantly lower than those of [Ru(tpy)2]2 þ or [Ru(tpy-Ph)2]2 þ and the excited-state lifetime was increased (Table 3.4). Owing to the strong electron-

Table 3.4

Selected photophysical properties of RuII bis(terpyridine) complexes 16.a

16a 16b 16c 16d 16e [Ru(tpy-Ph)2]2 þ a

kabs (nm)

kPL (nm)

s (ns)

104UPL

Reference

486 489 495 488 489 487

675 680 713 689 690 715

8 15 200 36 43 1

2.0 1.8 8.9 7.5 3.8 0.4

[98, [98, [98, [98, [98] [42]

All measurements were performed in deoxygenated MeCN at room temperature.

03

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99] 99] 99] 99]


withdrawing nature of the cyano-substituent in complex 16c, t was prolonged even up to 200 ns. Further planarization of the ligands could be achieved by introducing substituents at the 5-position of the pyrimidine ring that could form hydrogen bonds so that the distortion observed between the pyrimidine and phenyl rings of 16b (Figure 3.13b) could be overcome by intramolecular F H or N H interactions in 16d and 16e, respectively. Consequently, the excited-state lifetime of these complexes could be increased by a factor of 2.5 to 3. Similar results were obtained when the central pyridine ring was replaced by 1,3,5-triazine [100]; the strongly p-accepting triazine moiety stabilized the 1MLCT and 3MLCT excited-states. Furthermore, intramolecular H-bonding to adjacent hydrogen atoms (i.e., in ortho-position of substituents in 3-position) forced the ligand into a coplanar arrangement, thereby further stabilizing the 3MLCT state. The heteroleptic RuII bis-complexes, with tpy as the second ligand, were found to emit a wavelength of 732–754 nm with lifetimes of 8–15 ns, depending on the substituents. In contrast, the heteroleptic complexes of 1,5-bis(pyridin-2-yl)triazine had significantly shorter lifetimes due to the reduced energies of the MC and MLCT states; thermal access of the MC state from the 3MLCT excited-state was facile. A wide range of substituted Kr€ohnke-type terpyridines have been utilized, as ligands, for the construction of homoleptic as well as heteroleptic RuII bis(terpyridine) complexes [101]; however, their photophysical properties did not significantly differ from those of [Ru(ttpy)2]2 þ . Thus, simple substituents, either of electron-donating or -withdrawing nature on the phenyl ring, are hardly suited for manipulation of the photophysical properties mainly due to their non-planar orientation. Notably, unsaturated substituents, such as vinyl or ethynyl groups, can enhance delocalization. Presselt et al. investigated a series of heteroleptic RuII bis(terpyridine) complexes by means of Bader’s quantum theory of atoms in molecules (QTAIM) [102]; an increase of the parameter elipticity (e) in the bond critical point (BCP), that is, the phenyl–pyridine bond, could be observed for these substituents [103]. It was concluded that extended p-conjugated substituents attached to the phenyl-tpy core at the 4-position of the phenyl ring can improve the photophysics of the corresponding RuII (or OsII) bis(terpyridine) complexes. Since ultrafast excited-state planarization was reported for terpyridine L8 (Figure 3.14a) [104, 105], the system was investigated further by Siebert et al., who studied the homoleptic RuII and OsII bis(terpyridine) complexes 17a/b by means of temperature-dependent luminescence experiments and femtosecond time-resolved transient absorption spectroscopy [44, 106]. Dual luminescence was observed for both complexes; this finding was rationalized on the basis of photoinduced excitedstate planarization of L8, which strongly affected the electronic properties and reduced the coupling to the MLCT excited-states. Moreover, the extended p-system of the ligand stabilized the 3MLCT excited-state against non-radiative decay back to the electronic ground state – an effect that was supported by the stabilizing influence of LC triplet states. As a net effect, dual emission from the 3MLCT excited-state (phosphorescence at lPL ¼ 640 for 17a and lPL ¼ 725 nm for 17b) and the S1 excited-state of the ligand itself (fluorescence at lPL ¼ 450 nm) occurred [44].

03

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| 83

84

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes The luminescence lifetimes were 36.5 and 136 ns for the RuII and OsII bis-complex, respectively (the quantitative Jablonski scheme of 17a is depicted in Figure 3.14b). The excited-state lifetime of 17a was significantly increased in comparison to [Ru(ttpy)2]2 þ (t ¼ 1 ns). In contrast, the OsII bis-complex featured a rather shortlived excited state relative to [Os(tpy)2]2 þ (t ¼ 270 ns) [107] and other examples ń et al. [60], due to a reduced barrier between (t E 220 ns) reported by Alema the 3MLCT and higher lying MLCT states, which are prone to non-radiative decay [106]. In the early 1990s, the research groups of Ziessel and Harriman elaborated that the attachment of electron-withdrawing ethynyl derivatives in the tpy 40 -position can significantly prolong the excited-state lifetimes by lowering the energy of the

(a)

N OC8H17 N C8H17O

L8

N

17a : [Ru(L8)2]2+

17b : [Os(L8)2]2+

(b) S1 hot

cooling 1.9 ps planarization 23 ps

S1 cool

IC < 100% ISC

1

3

MLCT hot

0

ISC

IC E = 0.07 eV 3

MLCT *

3

LCCT

IC E >> 0.4 eV

excitation (500 nm)

phosphorescence = 36.5 ns

cooling 1.9 ps planarization 23 ps 3 MLCT cool T1 cool

ISC

1

internal conversion (IC) E = 0.22 eV

Energy (eV)

excitation (400 nm)

2

fluorescence = 1.64 ns, = 1∗104 %

MLCT hot

HS

dd

ISC

3

S0 cool

Figure 3.14 (a) RuII and OsII bis(terpyridine) complexes 17a/b (counterions omitted for clarity); (b) Jablonski scheme of 17a (solid lines: energy levels with defined energetic positions; dashed lines: excited-states, the energy of which can only be indirectly inferred or depends on excitation wavelength; for details see Reference [106]). Figure reproduced with kind permission; r 2011 The Royal Society of Chemistry.

03

27 J l 2011 15 42 32

3.3 RuII and OsII Complexes (a) N

Ru2+

N N

N

N N

Ru2+

N

N

N

N 18

N

N 19

(b) 2.5106

Emission Intensity

2.010

increasing temperature

6

1.5106 1.0106 5.0105 0.0 600

650

700

750

Wavelength (nm)

Figure 3.15 (a) RuII bis(terpyridine) complexes 18 and 19 (counterions omitted for clarity); (b) temperature-dependent emission spectra of 18 (room temperature to 80 K) [109]. Figure reproduced with kind permission; r 2004 American Chemical Society.

MLCT excited states and, consequently, decoupling the MC states [108]. Besides various types of oligonuclear arrays (Section 3.3.2.3), the mononuclear RuII bis(terpyridine) complexes were also investigated. For instance, the RuII bis-complex 18 (Figure 3.15a) showed weak luminescence at room temperature (lPL ¼ 680 nm, t E 44 ns); however, with decreasing temperature, a pronounced increase of emission could be observed (Figure 3.15b) [109]. Comparison of the photophysical behavior with that of the unsubstituted complex [Ru(tpy)2]2 þ led to the conclusion that the enhanced delocalization reduced the electron-vibrational coupling to the MC state, which is responsible for radiationless deactivation. Subsequently, the photophysical properties of various structurally related compounds by replacing the phenyl ring with other (hetero)aromatic residues were reported [110–113]. Of these compounds, the pyrene-substituted complex 19 is noteworthy, since at room temperature the excited-state lifetime was 580 ns. The triplet state of the lateral ethynyl-pyrene moiety was higher in energy than the 3MLCT excited-state associated with the RuII complex; the prolonged excited-state lifetime was attributed to a high degree of pp* orbital mixing between these two states [111]. A similar complex, where the terpyridine and ethynyl-pyrene moieties were separated by an alkylated thiophene-ethynyl spacer, showed an intra-ligand charge-transfer state (3ILCT) state as the lowest energy emitting state at room temperature with an intrinsic excited-state lifetime of about 2.5 ms [114].

03

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| 85

86

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes A different strategy in manipulating the photophysical properties of RuII bis(terpyridine) complexes that has recently gained considerable interest is often referred to as the “bichromophoric strategy” [115] in which the excited-state lifetime of a mononuclear complex can be prolonged by incorporating additional chromophores with their excited-states being isoenergetic to the 3MLCT state of the RuII complex. In such systems, the components have to be electronically independent of each other, thus maintaining the individual properties of the chromophores. The anthracene-substituted homoleptic RuII bis(terpyridine) complex 20a was non-emissive at room temperature, as the lower lying T1 excitedstate of the anthracene fully quenched the luminescence (Figure 3.16) [116]. In contrast, the corresponding OsII bis(terpyridine) complex 20b was emitting (comparable to [Os(tpy)2]2 þ , lPL ¼ 728 nm), since the 3MLCT excited-state of the OsII complex was lower in energy than the T1 excited-state of anthracene. However, an efficient bichromophoric system was obtained when the anthracenesubstituted 40 -(pyrimidin-2-yl)-terpyridine ligand L7f (R ¼ anthracen-9-yl) was used (see also Figure 3.13) [81]. For complex 21, the T1 state of the anthracene unit and the 3MLCT state associated with the complexed RuII center were almost isoenergetic. Since a perpendicular arrangement of the anthracene unit relative to the planar pyrimidine-terpyridine system was favored, the individual energetics of both components were retained, giving rise to a bichromophoric effect. Complex 21 showed a bi-exponential decay of the luminescence (lPL ¼ 675 nm) due to direct emission from the 3MCLT excited-state (t ¼ 5.8 ns), followed by a longer-lived emission after the reversible population of the T1 state (t ¼ 1806 ns).

N

N N

N N

N

N

M2 N

Ru2 N

N

N

N

N

N 20a (MII RuII) 20b (MII OsII)

N

N

21

Figure 3.16 Anthracene-substituted complexes 20 and 21 (counterions omitted for clarity) [116].

03

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N N N N N N N L9 R R R alkyl or aryl 22a: [Ru(L9)2]2 22b: [(tpy)Ru(L9)]2

N N

N

N N

N

N

N

L10

N

N

N

N

L11

23: [(tpy)Ru(L10)]

24: [(tpy)Ru(L11)]2

Figure 3.17 RuII complexes bearing bis-triazole-pyridine (22 and 23) and carbene-type ligands (24) (counterions omitted for clarity).

Replacing one or two of the outer pyridine rings of the basic tpy core with alternative heterocycles generates new tridentate ligands. For instance, triazole units can be used for this purpose – thus Schulze et al. synthesized a series of homoleptic and heteroleptic RuII bis-complexes 22a/b, based on 1,6-bis(1H-1,2,3triazol-4-yl)pyridine ligands (L9) (Figure 3.17) [117]. However, only the heteroleptic complexes 22b, with tpy being the second ligand, showed emission at low temperature (in comparison to [Ru(tpy)2]2 þ , the emission maximum was blue-shifted by about 20 nm); the homoleptic complex 22a was non-emissive, even at 77 K. The poor photophysical properties of 22a/b were attributed to a destabilization of the 3 MLCT excited-state by the strong p-acceptors L9, thus decreasing the energy gap towards the 3MC state. More promising results were obtained when the anionic, strong s-donor type ligand L10 was utilized [118]; in the heteroleptic complex 23 (Figure 3.17) the ground state was destabilized, thereby lowering the energy of the 3 MCLT excited state. Consequently, emission at rather low energy (lPL at about 700 nm) and a prolonged excited-state lifetime of about 77 ns were observed. Protonation of the anionic 1,2,4-triazole rings resulted in a quenching of the excited-state, since the protonated rings reduced the s-donating ability and, thereby, reversed the process. Moreover, carbene-type ligands (e.g., L11) are extremely good s-donors that strongly bind to the metal centers, thereby increasing the energy of the MC states. For carbene complexes, such as 24 (Figure 3.17), the 1MLCT states were excited at wavelengths between 340 and 380 nm, thus demonstrating a significant blue-shift when compared to [Ru(tpy)2]2 þ (lexc ¼ 475 nm) [119]. The excited-state lifetimes (at lPL ¼ 532 nm) were remarkably enhanced for the Br salts in aqueous solutions (t ¼ 3100 ns) when compared to the PF6 salts in MeCN (t ¼ 820 ns). Recently, bis-methylation of L9 (R ¼ aryl) at both N3-positions of the triazole ring gave access to a new type of abnormal or mesoionic carbene-type ligand [120]. The heteroleptic RuII bis-complex, with tpy as second ligand, exhibited intense room temperature emission with high quantum yields (FPL ¼ 0.044) that were close to those reported for [Ru(bpy)3]2 þ (see Table 3.2). Furthermore, the emission showed a slow and monoexponential decay and, thus, arose from a single, phosphorescent triplet state; the excited-state lifetime of 633 ns could almost compete with [Ru(bpy)3]2 þ and was 2500 times longer than for [Ru(tpy)2]2 þ . Finally, cyclometalating ligands possessing improved s-donating properties in the coordination sphere of RuII ions are addressed. The two simplest

03

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| 87

88


N

N N

L12 25a: [(ttpy)Ru(L12)] 25b: [(ttpy)Os(L12)]

N

R L14 27: [(tpy)Ru(L14)]

N

N N

L13 26a: [(ttpy)Ru(L13)] 26b: [(ttpy)Os(L13)]

L15 28: [(tpy)Ru(L15)]

Figure 3.18 Cyclometalating ligands (L12–L15) and their mono-cyclometalated RuII/OsII complexes 25–28 (counterions omitted for clarity).

Selected photophysical properties of mono-cyclometalated RuII and OsII complexes

Table 3.5

a

25 and 26.

25a 25b 26a [Ru(ttpy)2]2 þ [Os[ttpy)2]2 þ

kabs (nm)

kPL (nm)

s (ns)

UPL

504, 550 503, 537, 765 523 490 490, 667

784 824 808 640 734

4.5 –b 60 0.95 220

4.5 5.4 5.0 3.2 2.0

Reference

105 106 106 105 102

[126] [126] [123] [127] [96]

a

All spectra were measured in deoxygenated n-butyronitrile at room temperature. Not detectable.

b

representatives are ligands L12 and L13, where either the central [121, 122] or one outer pyridine ring [123], respectively, is replaced by a phenyl moiety (Figure 3.18). The photophysical properties of the resulting heteroleptic complexes 25 and 26 revealed the impact of the cyclometalating ligand on both the UV–vis absorption and emission behavior. For example, the mono-cyclometalated RuII complex 26a exhibited a 1MLCT band at 523 nm; the efficient s-donation destabilized the ground state, thus lowering the energy of the 1MLCT state and, consequently, the 3MLCT state. The emission maximum was redshifted by about 25 nm in comparison to the mono-cyclometalated regioisomer 25a and the excited-state lifetime was prolonged by a factor of 13 (t ¼ 60 ns) (Table 3.5) [123]. More rigid cyclometalating ligands (L14 and L15) were also utilized by the Sauvage and Thummel groups (Figure 3.18) [124, 125]. In these cases, two competing factors influenced the photophysical properties of the corresponding RuII complexes (27 and 28), one of which increased steric strain, due to the rigidity of the ligands, lowered the energy of the MC states; conversely, the enhanced

03

27 J l 2011 15 42 33


s-donating ability of the ligands contributed to a stabilization of the MLCT states (while destabilizing the MC states). The anticipated prolonged excited-state lifetimes were, however, only observed in very few cases, specifically when additional aryl substituents stabilized the interligand pp interactions. 3.3.2.3 Oligonuclear Complexes Containing RuII/OsII Bis(terpyridine) Units Many systems containing two or more [M(tpy)2]2 þ units, linked by different spacers, have been synthesized and homonuclear (i.e., one type of metal ions) as well as heteronuclear species (i.e., different metal ions) are well documented (Figure 3.19) [6, 77, 78, 95]. If a [Ru(tpy)2]2 þ complex is connected to [Os(tpy)2]2 þ by means of a p-conjugated linkage, excitation of the RuII center can result in energy- and/or electron-transfer to the OsII center. These processes make such oligonuclear arrays interesting examples of potential “molecular wires.” Most arrays are based on rigid bis(terpyridine) ligands, thus the resulting complexes possess rod-like structures. The photophysical properties, in particular the excited-state lifetimes, depend on the degree of electronic communication in the oligonuclear system. This extent of electronic communication has been studied as a function of distance between the chromophoric sites in the rod-like-type molecules; in general, the efficiency of electronic communication strongly depends on the nature of the bridging ligand [128]. Considering potential applications in light-harvesting devices, electrontransfer needs to be efficient over long distances [95]. However, a certain degree of localization is required to retain a multicomponent supramolecular system with individual redox and excited-state processes associated with each individual component. However, if the excited-states are highly localized, the overall photophysical properties of the oligonuclear system resemble those of the single mononuclear components. This effect was reported for a series of dinuclear RuII bis(terpyridine) complexes 29–32 (Figure 3.20a) [129, 130]. Extensive electronic coupling was observed for complex 29, where the RuII centers were very close to each other. As a result, UV–vis absorption occurred at significantly lower energy (labs ¼ 650 nm); 29 showed room temperature emission at lPL ¼ 826 nm with an intrinsic excited-state lifetime of 100 ns. The low energy emission resulted in a shorter excited-state lifetime than one should expect due to a direct relaxation back to the ground state, according to the energy gap law [78]. According to the

N R

N

N M

N

2

N N

spacer

N

N

N 2

M N

N

R

N

MII RuII or OsII

Figure 3.19 Generalized representation of a dinuclear bis(terpyridine) complex (counterions omitted for clarity).

03

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| 89

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes (a) N N

N Ru

N

N

2+

N

N 2+

N

Ru

N

N

N

N 29

tppz ligand

N N

N Ru

2+

N N

N

N Ru

2+

N

n

N

N

N

N

30 (n 0) 31 (n 1) 32 (n 2)

(b)

109

108 [Ru(ttpy)2]2+

1/ (S–1)

90

31

107

30

106

105 4

6

8

10

1/T 1000 (K–1) Figure 3.20 (a) Dinuclear RuII bis(terpyridine) complexes 29–32 [tppz: 2,3,5,6-tetra(pyridin2-yl)pyrazine, counterions omitted for clarity]; (b) plot of 1/t versus 1000/T for dinuclear complexes 30 and 31 and [Ru(ttpy)2]2 þ , as in Reference [130]. Figure reproduced with kind permission; r 1997 American Chemical Society.

Robin–Day classification, 29 was considered to be of type-III because of its large comproportionation constant (Kc ¼ 105) [131]. The 1MLCT absorption bands of complexes 30 and 31 were located at 520 and 499 nm, respectively. The directlylinked complex 30 featured an enhanced delocalization leading to an emission maximum at 720 nm (at room temperature), an excited-state lifetime of 570 ns, and improved quantum yield. Though being electronically more localized than 29, two cathodic peaks could be resolved for complex 30 by cyclic voltammetry (CV). The incorporation of phenyl- or biphenyl-spacers (31 and 32) reduced the extent of electronic coupling (Figure 3.20b).

03

27 J l 2011 15 42 33


N 1

R

N

N Ru

2

N N

N

N Os

2

N

n N

N

N

N

R2

33a (R1, R2 tolyl; n 0) 33b (R1, R2 tolyl; n 1) 33c (R1, R2 tolyl; n 2) 33d (R1 SO2Me, R2 H, n 0) 33e (R1 SO2Me, R2 H, n 1) 33f (R1 SO2Me, R2 tolyl, n 1)

Figure 3.21 Heterometallic dinuclear complexes 33a–f (counterions omitted for clarity) [133].

Based on the same ligand set, various dinuclear heterometallic complexes were prepared. The complex [(ttpy)Ru(tppz)IrCl]2 þ emitted at 810 nm (t ¼ 22 ns) from a RuII-based 3MLCT excited-state; efficient energy transfer from the IrIII to RuII center could be observed [132]. The series of dinuclear species [(ttpy)Ru(tpy-Phntpy)Os(ttpy)]4 þ (33a–c, n ¼ 0, 1 or 2) were investigated by Barigelletti et al. [127, 133] (Figure 3.21), Here, a strong electronic coupling of the metal centers in a system without phenyl-spacer (33a, n ¼ 0) was significantly reduced upon insertion of a phenyl group (33b, n ¼ 1); the next homologue, bearing the biphenylspacer (33c, n ¼ 2), showed a further, albeit small, reduction of the interaction. Overall, the energy transfer from the RuII to the OsII center was highly efficient and occurred with a high rate constant (k W 5 1010 s1), even for the biphenyl bridged system with a RuII-to-OsII distance of almost 20 A . Further modification and fine-tuning of the photophysical properties of the RuII-OsII arrays could be achieved by introducing an electron-accepting substituent onto the terminal ligands (R1 ¼ –SO2Me, 33d–f) [133]. Electrochemical experiments revealed both strong metal–metal as well as ligand–ligand interactions; photophysical studies showed a very efficient energy transfer mechanism that could be attributed to an electron-exchange mechanism. In related work, a series of dinuclear complexes [(ttpy)Ru(tpy-Phn-tpy)Rh(ttpy)]5 þ was investigated [134] in which energy transfer from the excited RhIII to RuII was observed for all three systems (n ¼ 0, 1, and 2); in contrast, electrontransfer from RuII to RhIII was only found in the case of direct linkage of complexes (n ¼ 0). The analogous dyads, based on the couple RuII/CoIII, also showed electron-transfer processes from the RuII side to CoIII centers [135]. The mononuclear complexes [(ttpy)Ru(tpy-Phn-typ)]2 þ (34) were utilized for the reversible modulation of their luminescence behavior. For instance, the nonemissive complexes became luminescent after protonation, thus giving access to a switchable luminescent pH-sensor [136]. Moreover, the highly luminescent trinuclear rod-like complexes 35 were obtained when ZnII ions were added to solutions of 34 (Figure 3.22) [137]. A linear increase of the emission intensity up to a molar RuII/ZnII-ratio of 1 : 2 became evident; the luminescence enhancement factor (EF) was larger than 10. However, due to the kinetically labile character of the central [Zn(typ)2]2 þ unit, these systems also gave rise to “switchable” emitters. Collin et al. reported on the porphyrin-containing arrays 36a/b and 37 (Figure 3.23) [138]. In triads 36a/b, the emission of the central porphyrin moiety was fully quenched by the outer RuII (36a) or RhIII bis(terpyridine) complexes (36b) [139].

03

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| 91

92

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes 8

6 N

N

N Ru

2

N

l/a.u.

N

N

4

n

N

34

N N

addition II of Zn

2

0

ZnII

600

800 λ (nm)

N

N

N

Ru2 N

N

N

N

Zn2 N

N n

35

N

N Ru2 N

N n

N

N

N

N

N

Figure 3.22 Synthesis of heterometallic trinuclear complex 35. The increase in luminescence, as a function of added ZnII ions, is also depicted (counterions omitted for clarity) [137]. Figure reproduced with kind permission; r 1998 The Royal Society of Chemistry.

N

N N

N

N

M

Zn n

N

N

N

N

N

N

N M

N

n

N

N

N

36a (M RuII) 36b (M RhIII)

N

N

N Zn

N

N 2

N

Ru

N N

N N

N

N Au N N

37

Figure 3.23 Porphyrin-containing triads 36 and 37 (counterions omitted for clarity) [139, 140].

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The quenching was attributed to an intramolecular electron-transfer from the excited singlet state of the porphyrin to the metal centers. For the directly coupled systems (n ¼ 0), both the forward and reverse electron-transfer steps were extremely fast. The insertion of a phenyl ring between the components (n ¼ 1) had a significant effect on the dynamics of electron-transfer, in particular with respect to charge recombination. The lifetime of the charge-separated state of conjugate 36a (n ¼ 1) was, however, prolonged to t ¼ 2 ns. In triad 37, the central [Ru(tpy)2]2 þ was flanked by two different porphyrin moieties [140]. Excitation of the ZnII-porphyrin unit resulted in an electron-transfer to the RuII center, which was followed by a secondary electrontransfer to the appended AuIII-porphyrin; thus, electron-transfer occurred via two consecutive steps over a total porphyrin center-to-center distance of about 30 A . The rate constants for the individual steps were determined by laser photolysis studies [84], which found that the primary electron-transfer step, leading to the reduction of the RuII center (DG1 ¼ 0.25 eV), required 50 ps and was essentially quantitative. The second electron-transfer, resulting in the reduction of the distant AuIIIporphyrin, occurred at a rate constant of 6 108 s1. This process (DG1 ¼ 0.6 eV) was in competition with the reverse electron-transfer to restore the ground state. The subsequent charge-transfer between the terminal metal-porphyrin subunits occurred with a rate constant of k ¼ 3 107 s1 to regenerate the initial system. The lifetime of the ultimate charge-separated state was 33 ns and the system retained about 60% (1.2 eV) of a photonic input of 2.1 eV. The direct attachment of ethynyl groups to the 40 -position of the tpy system in the dinuclear arrays 38 significantly prolonged the excited-state lifetimes by up to a factor of 3000 when compared to the parent complex [Ru(tpy)2]2 þ (e.g., 38a: t ¼ 565 ns, 35b: t ¼ 720 ns) (Figure 3.24) [108]. The unexpected low quantum yield for emission that is lower than for structurally similar mononuclear complexes [141] was attributed to the presence of more than one 3MLCT state in this complex allowing access of the lowest energy triplet state from a higher one, associated with a dpp* (terminal) tpy transition (the lower energy state having significantly more “triplet” character than the higher energy one). An equilibrium was believed to exist between these two MLCT states, which led to the population of the lower MLCT state – a process restricted by spin rules, which will compete with a direct emission from the higher energy state with greater “singlet” character. Polymerization of 38 into molecular films, containing the RuII centers dispersed along the conjugated backbone, by reductive electrolysis has been reported; however, due to their low solubility, their photophysical properties were not evaluated [142]. Apart from prolonging the excited-state lifetime (i.e., complexes 38), ultrafast energy transfer processes were shown to occur along the molecular axis in mixedmetal systems 39 [143]. The triplet energy-transfer from the RuII to the appended OsII center was quantitative within 20 ps at room temperature with rate constants of k ¼ 7.1 1010 s1 (39a) and k ¼ 5.0 1010 s1 (39b). The through-bond energytransfer (exclusively via a Dexter-type mechanism) showed a remarkably small attenuation factor (b) of the order of 0.17 A 1; thus the ethynyl-bridge was a highly suited linkage for through-bond electron-transfer processes in comparison to other (e.g., phenyl-based) linkages. Similar to the trinuclear RuII/ZnII array 35

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| 93

94

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes N N

N Ru

N

2

N

N

n

N

N

N M

N

MII RuII 38b: n 2 38c: n 2

38a: n 1

2

N

N

38d: n = 4

M Os 39a: n 1 39b: n 2 II

N

N Ru2

N

II

N

N N

40a: MII ZnII

M2

N

N

N

N

40b: MII FeII

N N

Ru2

N

N

N

N

N

N

40c: MII CoII

Figure 3.24 Dinuclear and trinuclear complexes 38–40 (counterions omitted for clarity) [144].

N

N Ru2 N

N N

N

N

OC12H25 n

M2

N

C12H25O

N

N

N

N 41 (MII RuII, n 1-5) 42 (MII OsII, n 1-5)

Figure 3.25 Dinuclear complexes 41 and 42 (counterions omitted for clarity) [145, 146].

(Figure 3.22), the trinuclear mixed-metal complexes 40 were also synthesized (Figure 3.24) [142, 144]. To increase the solubility of the dinuclear complexes and, moreover, of the targeted oligo-/polynuclear species, ligands possessing alternating ethynyl and 2,5dialkoxybenzene units were synthesized [145, 146]. The corresponding dinuclear RuII complexes 41 showed a shift of the 1MLCT absorption band to lower energies with increasing spacer length (Figure 3.25). However, the excited-state properties were found to be independent of the number of ethynyl/dialkylbenzene units; the complexes were emissive (lPL ¼ 692–695 nm) with the excited-state lifetimes being associated with ethynyl-substituted RuII bis(terpyridine) complexes (t ¼ 125–140 ns). Thus, the lowest energy excited-state was located on a terminal ligand. Apparently, the dialkoxybenzene rings disrupted the electronic coupling along the molecular axis and raised the triplet energy of the connector. Accordingly, the mixed-metal complexes 42 showed a similar behavior in which no energy-transfer

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N N

N Ru

2

N

N

N

N

N N

N 43

Ru2 N

N N

Ru

N

N

N

N

N Ru2 N

N

N

N 2

N

N

44a (n 1) 44b (n 2) 44c (n 4)

N N

N Ru

2

N N

N

N

N Ru2 N

N N

45

N

Figure 3.26 Dinuclear RuII bis(terpyridine) complexes 43–45 (counterions omitted for clarity).

from the RuII to the OsII center along the molecular axis could be confirmed spectroscopically [146]. A set of homometallic RuII dyads with different central aromatic units was studied by El-ghayoury et al. with respect to their photophysical behavior (Figure 3.26). The phenyl unit of 43 acted as partial insulator, thus inhibiting the throughbond electron exchange [147]. The excited-state lifetime of 43 was shortened in comparison to the dialkoxybenzene derivatives 41. Replacing the phenyl moiety by a naphthalen-1,4-yl moiety (44a) led to a modest redshift of the emission maximum and a significant prolongation of the excited-state lifetime (t ¼ 475 ns). Similar effects were observed for 45, featuring a pyren-1,6-yl unit, as the central chromophore, and were attributed to increased electron delocalization over an extended p* orbital at the triplet level [148]. Furthermore, Benniston et al. reported homologues to 44a containing either two (44b) or four ethynyl-naphthalen-1,4-yl units (44c) within their conjugated backbone [149]; the shift of the emission maxima of 44b/c to lower energies indicated an increased electronic coupling between RuII centers. The electronic coupling of the terminal metal centers in dinuclear arrays can be controlled via the degree of conjugation of the bis(terpyridine) ligand. The Harriman group showed that a short dialkoxy strap (46a, n ¼ 1), attached to the central

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| 95

96

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes 4,40 -biphenyl unit, restricted the rotational freedom and favored a coplanar conformation, thus enhancing the delocalization and electronic communication (Figure 3.27) [150, 151]. The RuII dyad 46a exhibited an excited-state lifetime of 25 ns at room temperature. Increasing the strap’s length (n ¼ 2–5) led to an increased flexibility of the biphenyl part, and thus to a decrease of the degree of conjugation. The mixed-metal trinuclear array 47 with a RuII-to-RuII distance of about 50 A exhibited highly efficient energy-transfer from the terminal RuII units to the OsII core at all temperatures (Figure 3.27) [152]. In a glassy matrix at very low temperatures (below 160 K), F€orster-type energy-transfer processes dominated that were mixed with Dexter-type electron exchange at higher temperatures in the solid state – involving long-range “super-exchange” interactions between the metal centers. In solution, a charge-transfer state localized on the bridge was populated, as an intermediate species; subsequently, energy was transferred from this state (possibly via a second charge-transfer state) to the central OsII acceptor-complex. In the all previous examples, the spacer’s rigidity was maintained by conjugated systems (e.g., aromatic or ethynyl linkages). A different type of bridge was reported by Barigelletti et al. in which a saturated bicyclic hydrocarbon was utilized, as a linker, in the mixed-metal RuII-OsII dyad 48 (Figure 3.27) [153]. The central bicyclo[2.2.2]octane moiety functioned as an efficient insulator preventing the electronic communication between the metal centers; thus, the electrochemistry of 48 resembled that of their isolated parent ethynyl-substituted complexes. At room temperature in solution, no energy-transfer was found; the 3MLCT excited-state lifetime of the RuII bis(terpyridine) unit was too short to allow any energy- and/or electron-transfer processes (t ¼ 1.1 ns). However, at low temperatures (77 K) in a rigid matrix, the lifetime was long enough (t ¼ 10.5 ms) to allow an energy-transfer to the OsII center with a rate constant of 4.4 106 s1. Constable and coworkers used thiophene, as a spacer, in the bis(terpyridine) ligand L16. In contrast to [Ru(tpy-R)2]2 þ (R ¼ thiophen-2-yl, lPL ¼ 664 nm, FPL ¼ 0.9 104), the di- and trinuclear RuII complexes (49 and 50) showed enhanced emission at 736 nm (FPL ¼ 6.8 104) with excited-state lifetimes of about 335 ns (Figure 3.28) [87]. Intermetal electronic communication in the dyad/triad stabilized a cluster of luminescent 3MLCT states leading to a larger energy gap and reduced probability of non-radiative deactivation. Moreover, the di-and trinuclear mixed-metal RuII-OsII complexes 51 and 52 were investigated with respect to intramolecular energy and electron-transfer processes [154]. The mixed-metal complexes 51 and 52 showed directed end-to-end or periphery-to-center RuII-toOsII energy-transfer, respectively. In both cases, strong OsII-based emission at about 800 nm with excited-state lifetimes of 120 ns was observed. A structurally related system (L17), where the terminal terpyridine moieties and central alkylated thiophene were separated by ethynyl groups, was used for the preparation of the dinuclear RuII complex 53 and the mixed-metal RuII-ZnII triad 54 [155]. Although the thiophene unit acted as an insulator, preventing full delocalization, enhanced room temperature luminescence was found when compared to the phenyl-containing analogue 43, due to an improved stabilization of the triplet state (53: lPL ¼ 705 nm, t ¼ 320 ns).

03

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27 J l 2011 15 42 35

N

N

Ru2

N

N

N

N

N

N

Ru N

2

N

N

Ru2

N

N

N

O O

(CH2)n

48

47

46a-e (n 1-5)

O O

N N

N

N

Os2 N

N

N N

N

N

N

N

N

Ru2 N

N

Os

N

2

N N

O O N

N

N

N

Ru2 N

N

Figure 3.27 Di- and trinuclear complexes 46–48 bearing a “strapped” biphenyl (46 and 47) or a bicyclo[2.2.2]octane bridge (48) (counterions omitted for clarity).

N

N

N

N


| 97

98


N S

N

49: [(tpy)Ru(L16)Ru(tpy)]4 50: [(tpy)Ru(L16)Ru(L16)Ru(tpy)]6 51: [(tpy)Ru(L16)Os(tpy)]4 52: [(tpy)Ru(L16)Os(L16)Ru(tpy)]6

N

L16 N

N C12H25

N

N S

53: [(tpy)Ru(L17)Ru(tpy)]4 54: [(tpy)Ru(L17)Zn(L17)Ru(tpy)]6

N

N L17

N

N

Figure 3.28 Thiophene-containing bis(terpyridine) ligands L16 and L17 as well as the corresponding di- and trinuclear complexes 49–54 (counterions omitted for clarity).

C4H9

(a) N N Ru2+ N

C4H9 N

S n

N

N

55a-e (n 1–5) N

N 2+

Ru N

N

N

N

(b)

55e (n 5)

Figure 3.29 (a) Dinuclear RuII complexes 55 (counterions omitted for clarity); (b) representation of the structure of 55e (n ¼ 5) according to molecular modeling studies (HyperChemTM) [156]. Figure reproduced with kind permission; r 2003 American Chemical Society.

A series of bis(terpyridine) ligands was synthesized, where 3,4-dibutylthiophene units were incorporated into the ethynyl-terpyridine backbones [156]. The photophysical properties of the corresponding dinuclear RuII bis-complexes (55), as a function of the number of thiophene groups (n ¼ 1–5), were investigated (Figure 3.29) [157]. The 1MLCT absorption bands were located between 498 and 512 nm, depending on the number of thiophene units. The lowest energy absorption

03

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N N

N Ru

2

N spacer

N

N

N Ru

2

N

N

| 99 N

N

N N

N

Ru

2

N

N

Ru

2

N N

N

N

N

N

N

N 60

N spacer

N

N

N

N

= 56

N

58 N

57

N

N

59

N

Figure 3.30 Dinuclear complexes 56–60 bearing N-heteroaromatic groups in the spacer (counterions omitted for clarity).

(labs ¼ 512 nm) was observed for the complex with the shortest spacer (n ¼ 1); small shifts to higher energy occurred as the number of thiophene fragments increased. The longer systems also showed weaker luminescence, mainly due to decreased electronic communication between the RuII centers and interplay of the close-lying 3MLCT excited-states. Dinuclear complexes with potential coordination sites (e.g., bipyridine, phenanthroline, and 2,20 -bipyrimidine), as part of the conjugated backbone of the bis(terpyridine) ligands, have been synthesized. Ziessel et al. showed that the attempted preparation of dyads 56–58 by reacting the bis(terpyridine) ligands with a (tpy)RuCl3 precursor complex was not successful; instead, the Pd0-catalyzed Sonogashira cross-coupling reaction of preformed [(tpy)Ru(tpy-Br)]2 þ with appropriate diethynyl-functionalized building blocks gave the desired complexes 56–58 (Figure 3.30) [48]. The photophysical properties of the dinuclear complexes 56 and 57 were similar to those reported for the phenyl-containing system 43. Thus, only a marginal degree of mixing between the p* orbitals localized on the heteroaromatic spacer and those on the flanking ethynyl-terpyridine moieties could be concluded [158]. Dyad 59 without the ethynyl groups showed a significantly reduced excited-state lifetime when compared to 56 (56: t ¼ 100 ns, 59: t ¼ 5 ns) despite having a similar emission energy [159]. The 2,20 -bipyrimidine derivative 58 showed a slight redshift of the emission (lPL ¼ 705 nm) along with a reduced lifetime (t ¼ 60 ns) [160]. Finally, the dinuclear complex 60, based on a 2,20 ,60 ,6“tetra(pyridin-2-yl)-4,40 -bipyrimidine (Figure 3.30), [161] emitted at remarkably low energy in the near-IR region (lPL ¼ 819 nm) with an excited-state lifetime of 420 ns at room temperature. The excited-state was unusually long-lived for such a low energy emission, due to a combination of factors such as the acceptor orbitals of the emissive 3MLCT state being lowered in energy due to the ligand’s additional N-atom and a second contributor to stabilization of the 3MLCT state results from the favorable coplanar interaction of the bridging ligand in the ground state via intramolecular H-bonding between the non-coordinated N-atoms of one

03

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N

N

100

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes pyrimidine ring and the adjacent H-atom at the 5-positions. These two effects in concert reduced the efficiency of the MC deactivation pathway. Hissler et al. used 56 as a luminescence sensor for ions that were coordinated to the vacant 2,20 -bipyridine moiety [158]. Alkali metal ions (e.g., LiI, NaI, KI) had hardly any influence on the luminescence properties, whereas protonation gave a slight increase in quantum yield. Upon protonation, the energy of the lowest unoccupied molecular orbital (LUMO) localized on the 2,20 -bipyridine unit was lowered, thereby increasing the degree of interaction with the p* orbitals located on the ethynyl-substituted [Ru(tpy)2]2 þ moiety; in return, the degree of electron delocalization along the bis(terpyridine) ligand was marginally increased. Strongly binding transition metal ions (e.g., AgI or HgII) caused a dramatic quenching of the luminescence in which the photoinduced electron-transfer from [Ru(tpy)2]2þ to these easily reducible metal centers represented an efficient deactivation pathway of the 3MLCT excited state. A similar effect was observed upon methylation of both vacant N-atoms of the bridge in which the luminescence was quenched due to light-induced electron-transfer from the terminal RuII chromophores to the central viologen-type unit. In contrast, weakly binding transition metal ions (e.g., ZnII, CdII, BaII) increased the luminescence by a factor of 1.8–2.5. CV experiments revealed an enhanced electronic interaction between the peripheral RuII chromophores upon coordination of ZnII ions and the bridge of the bis(terpyridine) ligand. Trinuclear mixed-metal RuII–IrIII complexes, such as 61, were reported recently by Cavazzini et al. (Figure 3.31a) [162]. In these cases, bis-cyclometalated IrIII centers were coordinated to angular dinuclear RuII complexes bearing 2,20 -bipyridine units in their backbone. The luminescence of the bis-cyclometalated IrIII center (occurring at 540 nm in the absence of terminal RuII centers) was efficiently quenched by photoinduced energy-transfer to the lower-lying 3MLCT states of the RuII bis(terpyridine) units (Figure 3.31b). The 1,1w-bis(2,20 :60 ,200 -terpyridin-4-yl)-1,10 -biferrocene ligand L18 was utilized by Dong et al. for the preparation of redox-active oligonuclear RuII complexes 62–64 (Figure 3.32a, see also Chapter 8.2 for a potential application of this system in nanotechnology) [163]. M€ossbauer spectroscopy indicated that the FeII oxidation state was fully retained in 62–64 (Figure 3.32b); CV experiments showed peaks at 1.35 and 0.4–0.9 V corresponding to the RuII/RuIII and the FeII/FeIII redox couples, respectively; the ligand-based redox steps were observed between 1.2 and 1.4 V. In particular, the variations observed for the FeII/FeIII oxidation potentials indicated electronic interaction between the ferrocene-containing spacer and RuII centers. The redshift of the 1MLCT band to labs ¼ 570 nm for the oligonuclear complexes was also attributed to a strong electronic coupling within the array. The T-shaped molecular device c-65, which consists of a “switchable” spiropyran moiety annulated to the conjugated bridge of the bis(terpyridine) ligand, was reported by Amini et al. (Figure 3.33a) [164]. The spiropyran unit could be opened to the merocyanine form (o-65) by irradiation with UV light (lirr W 350 nm) (Figure 3.33b). At room temperature, a very fast recyclization of o-65 into c-65 occurred as shown by the instantaneous disappearance of the blue color (o-65:

03

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(a) N N

N Ru2+

N

N N

F N N

F F

Ir+

61

N N F

N N

N Ru2+

N

N N

(b)

E (eV)

Iridium

Ruthenium

3

MLCT

3

MLCT

GS Figure 3.31 (a) Trinuclear mixed-metal RuII-IrIII complex 61 (counterions omitted for clarity); (b) proposed energy transfer processes involved in the quenching of the IrIII-based luminescence of 61 [162]. Figure reproduced with kind permission; r 2009 American Chemical Society.

labs ¼ 665 nm). Below 290 K, the thermal cyclization was slow, thus allowing a determination of the rate constant and activation energy for this ring-closure by UV–vis absorption spectroscopy (k ¼ 9 103 s1, Ea ¼ 91 kJ mol1). Molecular modeling studies of c-65 suggested that the MLCT state, involving the LUMO, spanned over the entire conjugated bis(terpyridine) ligand; whereas in o-65, an alternative pathway to the side chain was opened (Figure 3.33c).

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| 101

102

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes

(a)

(b) N

0.0 N

T%

Fe N

N Fe L18

2.0 4.0

62

N

6.0 4+

N

–4

62: [(Fc-tpy)Ru(L18)Ru(typ-Fc)]

–3

–2

–1

0

1

2

3

4

–1 2 0 1 Velocity (mm/sec)

3

4

63: [(Fc-Fc-tpy)Ru(L18)Ru(typ-Fc-Fc)]4+ 64: [(Fc-tpy)Ru(L18)Ru(L18)Ru(typ-Fc)]6+

0.0

T%

2.0 4.0

L18

6.0 –4

–3

–2

Figure 3.32 (a) 1,10 -Biferrocene-based ligand L18 and the corresponding oligonuclear RuII complexes 62–64 (Fc ¼ ferrocene) (counterions omitted for clarity); (b) M€ ossbauer spectra of L18 (bottom) and 62 (top) [163]. Figure reproduced with kind permission; r 2004 American Chemical Society.

Diazo groups were used as linkers between the terminal tpy sites (L19) [165]. In the neutral state, neither the dinuclear RuII (66) nor OsII analogue (67) was luminescent (Figure 3.34). It was speculated that the quenching of the excitedstate resulted from numerous contributions: internal conversion/relaxation, according to the energy gap law; isomerization/twisting at the central –N¼N– bond; population of the L19-localized triplet state from the 3MLCT excited-state, followed by relaxation to the ground state; and interaction of the diazo linkage with solvent facilitating non-radiative decay to the ground state. However, upon reduction of the diazo group (to the radical anion), 67 became emissive at room temperature (lPL ¼ 777 nm), whereas 66 showed emission only at low temperatures (lPL ¼ 610 nm). The reduced mixed-metal complex 68 also showed a strong emission at 775 nm via an efficient energy-transfer from the RuII to OsII center. The attachment of coumarin moieties, as donor substituents, on the RuII part (69) further enhanced the energy-transfer from the radical anion to the OsII center (68: 40% energy-transfer, 69: 70% energy-transfer) [166]. 3.3.2.4 Dendritic and Star-Shaped Systems Containing RuII Bis(terpyridine) Units Within the large field of star-shaped and dendritic architectures, metal-containing systems are of particular interest with respect to potential applications in the fields

03

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| 103

(a) N N N

OC16H33

N R u 2+

N

N N

O

N

N R u 2+

N

N

N

N c-65

(b)

closed f orm: c-65 R

open f orm: o-65

h 1

R

e h 2 O

A1

D

A1

D

N

O

e A2

A2

(c)

Figure 3.33 (a) T-shaped molecular device c-65 (counterions omitted for clarity); (b) representation of the reversible light-induced switching between c-65 and o-65; (c) representation of the location of LUMOs of c-65 (left) and o-65 (right) [164]. Figure reproduced with kind permission; r 2003 Elsevier B.V.

of catalysis, molecular carriers, and light-harvesting devices [167–171]. In general, there are two approaches for the construction of dendritic arrays: the convergent and divergent routes. The former involves the synthesis from the periphery to a central point; however, the latter is based on the sequential addition of small molecular (monomeric) building blocks to a branching center. The first terpyridine-containing dendrimer was reported in 1993 by the Newkome and Constable groups [172]. In 1999, a detailed review on metallodendrimers and dendrimers containing terpyridine ligands was published, dealing with both structural and synthetic aspects [173]. A remarkably large, fourth-generation dendrimer featuring in total 64 [Ru(tpy)2]2 þ centers, synthesized via peptide coupling reactions, was reported by Storrier et al. [174]; therein all RuII centers were located on the periphery of the

03

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N

104

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes (a) N N

N

N

N N L19

N

N

66: [(tpy)Ru(L19)Ru(tpy)]4+ 67: [(tpy)Os(L19)Os(tpy)]4+ 68: [(tpy)Ru(L19)Os(tpy)]4+ 69: [(coumarin-tpy)Ru(L19)Os(tpy)]4+

(b)

Emission

Excitation

N N

Ru2+

N

N

N N

N N

Os2+ N

N N

N

N

N

68

Energy transfer

Figure 3.34 (a) Diazo-bis(terpyridine) L19 and the corresponding dinuclear complexes 66–69 (counterions omitted for clarity); (b) representation of the energy-transfer in the radical anion of 68 [165].

organic dendrimer. To use metallodendrimers in light-harvesting applications, efficient energy- and/or electron-transfer is a necessary prerequisite and, thus, long distances along with insulating organic groups would hardly favor such dendritic architectures for such applications. Consequently, placing the RuII bis(terpyridine) complex into the branching core would appear to be a more promising approach. Accordingly, various types of photoactive dendritic architectures bearing RuII tris-bidentate complexes [e.g., with bipyridine or 2,3-bis(pyridin-2-yl) pyrazine derivatives, as ligands] in the core, arms, as well as in the periphery have been prepared [175]. The most straightforward examples for terpyridine-containing dendrimers, the first-generation species, were synthesized based on star-shaped terpyridines, such as tris(terpyridine)s L20–L27 (Figure 3.35) [162, 176–180]. The trinuclear RuII complexes were obtained by reaction of the star-shaped ligands with (R-tpy)RuCl3 precursor complexes (e.g., R ¼ H, OH, OEt, Cl, Ph, NMe2, SMe, or SO2Me). As a general trend, the 1MLCT transition was observed at 487–507 nm, thus being redshifted compared to mononuclear RuII bis(terpyridine) complexes, in essence due to steric crowding around the aromatic core and a reduced degree of conjugation [181].

03

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3.3 RuII and OsII Complexes N N

N

| 105

N N

N

n

N

N N

n

n

N

N N

G

L20: n 0 L21: n 1 L22: n 2

C6H13 C6H13

C6H13 C6H13

N N

N L 23

N

G

G

N

N

L24

L25

C6H13 C6H13

=

N

N

L26 S

G

L27

Figure 3.35 Selected star-shaped terpyridine ligands L20–L27, as templates, for firstgeneration dendrimers.

The Constable group reported the synthesis of first- and second-generation dendrimers (e.g., 70 and 71) via nucleophilic substitution (i.e., Williamson-type etherification) reaction of orthogonally functionalized mononuclear RuII bis(terpyridine) complexes (Figure 3.36) [182]. The complexes showed 1MLCT absorption bands between 480 and 485 nm; overall, the UV–vis absorption spectra appeared as superimpositions of the incorporated mononuclear complexes, indicating very poor electronic coupling between RuII centers resulting from the insulating character of the ether linkages. The complexes 70 and 71 were found to emit from their lowest lying 3MLCT excited-state at (70: lPL ¼ 640 nm, 71: lPL ¼ 662 nm) with very short lifetimes. The mixed-metal species 72 and 73 showed a temperaturedependent energy transfer from the RuII to OsII centers. At low temperature, the energy-transfer was efficient and the typical OsII-centered emission was observed; whereas at room temperature, the RuII centers were efficiently deactivated and, thus, no energy-transfer was observed. In particular, the tetratopic ligand tetrakis(2,20 :60 ,200 -terpyridin-40 -oxymethyl) methane was found to be a versatile building block for the construction of metallodendrimers [183]. By following a divergent approach, the RuII centers were

03

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N

03

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N

N

N

Ru

N

2

N

N

N

N

O

Y

N

M

N 2

N

N

O

N

N 2

N

N

X

X

N

N

II

70: M Ru 72: MII OsII II

X

N

II

Ru2

N

N

N

N

Ru2

N

71: M Ru 73: MII OsII II

X H or Cl; Y H or thiophen-2-yl

N

Ru

N

N

N

N

N

O

N

N

O

N

M2

N

O

N

N

O

N

N

N

N

N

Ru2

N

N

Ru2

N

Figure 3.36 V-shaped (70 and 72) and X-shaped (71 and 73) oligonuclear complexes (counterions omitted for clarity) [182].

X

N

N

N

N

X

X

106


3.4 Iridium(III) Complexes with Terpyridine Ligands

added stepwise and the second-generation dendrimer featured 16 RuII bis(terpyridine) units with overall 32 positive charges. The same pentaerythritol-based ligand was applied in the synthesis of chiral RuII dendrimers with lateral BINOLsubstituents (BINOL: 1,10 -binaphth-2-ol) [184]. All of these metallodendrimers were non-emissive at room temperature and the UV–vis absorption spectra resembled the mononuclear complexes, indicating very little electronic coupling between metal centers [185].


In 1990, the first IrIII bis(terpyridine) complex was reported by Ayala et al., who prepared [Ir(tpy)2]3 þ in a fusion reaction that was utilized earlier in the synthesis of [Ir(bpy)3]3 þ [186]. In this initial report, the authors addressed the harsh reaction conditions and exhausting purification of the material, which are the major drawbacks limiting research in this field, arising from the kinetically inert character associated with the low-spin d6 electronic configuration of the IrIII center. Moreover, the directed synthesis of heteroleptic IrIII bis(terpyridine) complexes [e. g., of two different 40 -(2,20 :60 ,2“-terpyridinyl) ligands] appears not to be straightforward. Although these ligands react under similar conditions with RuIII or OsIII ions (Scheme 3.1b), the Ir(R-tpy)Cl3 mono-complexes (R ¼ aryl) are rarely isolated in high purity and significant scrambling of the ligands has often been observed during the subsequent coordination with a second terpyridine ligand, giving rise to mixtures of both hetero- and homoleptic complexes [16]. Thus, the accessibility to the mono- and bis(terpyridine) complexes of RuII and OsII (see the previous section) as well as of PtII ions (Section 3.5) favored their use as photoactive materials. But why did the combination of IrIII ions with terpyridine ligands re-attract considerable scientific interest in recent years? In many cases, the photoluminescence of IrIII bis(terpyridine) complexes has a low sensitivity towards quenching by dissolved oxygen and, thus, such systems have become attractive candidates for potential applications as luminescent sensors or in directed energy/ electron-transfer processes [16, 187]. [Ir(tpy)2]3 þ is photoluminescent in solution under ambient conditions, displaying a green-blue emission upon excitation in the near-UV [186, 188]. As depicted in Figure 3.37a, the emission profile is highly structured, with a vibrational spacing of about 1400 cm1, typical of coupling with aromatic –C¼C– vibrations and characteristic for emission from excited-states that are mainly LC in nature (with relatively little involvement of the metal). Density functional theory (DFT) calculations confirmed the LC nature of the excited-state [16]. The IrIII ion barely contributes to the highest occupied molecular orbital, which was mainly ligand-based as shown in the contour plots of the HOMO and LUMO of [Ir(tpy)2]3 þ depicted in Figure 3.37b. This behavior is contrary to that found for the corresponding RuII bis(terpyridine) complexes: for [Ru(tpy)2]2 þ the lowest-lying

03

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| 107

108

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes excited-state is of 3MLCT character and lower in energy by about 0.6 eV compared to [Ir(tpy)2]3 þ [130]. This property can be attributed to the different oxidation states of the d6 metal ions. The formal RuII/RuIII oxidation is relatively facile, the IrIII/ IrIV oxidation potential is at least þ 2.4 V vs. SCE (SCE: standard calomel electrode), lifting the MLCT states to energies higher than those of the 3LC states. In degassed MeCN at room temperature, the excited-state lifetime and quantum yield of [Ir(tpy)2]3 þ are 1.2 ms and 0.03, respectively [188]. Placing aryl-substituents in 40 -position of the tpy significantly influences the luminescence properties of the resulting IrIII bis(terpyridine) complex. For instance, complex [Ir(ttpy)2]3 þ displayed a redshifted, less structured spectrum (Figure 3.37a), a prolonged excited-state lifetime (t ¼ 9.5 ms), and an emission that was more sensitive to quenching by oxygen [188]. Since the heteroleptic complex [(tpy)Ir(ttpy)]3 þ exhibited similar behavior, it can be concluded that the lowestenergy excited-state was strongly associated with the aryl-modified ligand. Differences in both position and shape of the emission spectrum might be related to effects of the pendent aryl group in which an extended degree of conjugation in the pp* excited state and the presence of various conformers of different torsion angles contribute to a less well-resolved spectra; for example, in the complex [Ir(tpy-mes)2]3þ (mes ¼ 2,4,6-trimethylphenyl), rotation of the bulky substituent was inhibited and yielded luminescence data essentially identical to those of [Ir(tpy)2]3þ [189]. In contrast to simple aryl substituents, the introduction of pyridinecontaining groups at the 40 -position of the terpyridine did not significantly alter the luminescence properties of the homoleptic IrIII bis(terpyridine) complexes 74 (Figure 3.38a) in terms of profile and excited-state lifetime, presumably due to low p-conjugation between the N-heteroaromatic substituent and the tpy moiety [190]. However, the heteroleptic complexes 75 (Figure 3.38a) showed broader, less

(a)

(b) [Ir(tpy)2]3

100

Intensity (a.u)

[Ir(ttpy)2]3

[Ir(tpy)2]3

Home

LUMO

0 450

500

550

600

650

700

λ (nm) Figure 3.37 (a) Photoluminescence spectra of [Ir(tpy-R)2]3 þ in degassed MeCN at room temperature (R ¼ H: solid line; R ¼ CH3: dashed line); (b) representation of the DFT calculated HOMO and LUMO of [Ir(tpy)2]3 þ [16]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

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(a)

R

N

N

R =

N

L28

L29

N N N

74a: [Ir(L28)2]3+ 74b: [Ir(L29)2]3+ 74c: [Ir(L30)2]3+ 74d: [Ir(L31)2]3+ 75a:[(ttpy)Ir(L28)]3+ 75b:[(ttpy)Ir(L29)]3+ 75c:[(ttpy)Ir(L30)]3+ 75d:[(ttpy)Ir(L31)]3+

L30

N

L31

100

100

80

90 80

60

70

40

60 20 50

Emission intensity (open shapes)

Emission intensity (filled shapes)

(b)

0 0

2

4

6

8

10

pH value Figure 3.38 (a) IrIII bis(terpyridine) complexes 74 and 75 (counterions omitted for clarity); (b) pH-dependency of the emission intensity (lPL ¼ 510 nm) of 75a (filled squares), 75b (filled circles), 75c (filled triangles) and 75d (open circles) [190]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

structured, and more redshifted spectra than [Ir(ttpy)2]3 þ (t ¼ 6 ms), implying that Kasha’s law of emission from the excited-state of lowest energy (localized on the ttpy ligand) was exclusively obeyed [191]. The ability of complexes 75 to function as luminescent pH sensors was investigated by the Williams’ group [190]. Complex 75a displayed a quenching of the emission intensity by a factor of eight upon protonation in aqueous solution (Figure 3.38b); t was reduced in a similar fashion from 4.7 ms (at pH 7) to 480 ns (pH 2). Moreover, the pH range over which a response was observed could be controlled by the ligand’s structure; for instance, the isomeric complex 75b showed a comparable response but shifted by about two units to lower pH values due to electrostatic effect caused by the proximity of the site of protonation to the IrIII center. The response of 75c was located within the neutral pH regime; however, changes in the emission intensities appeared less pronounced. The decrease of the

03

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| 109

110

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes emission intensities and excited-state lifetimes could be attributed to a lowering of the energies of the MLCT excited-states upon protonation (in analogy to similar RuII bis(terpyridine) complexes [192]), thus increasing the charge-transfer character of the emissive state. The degree of conjugation in the excited-state could be increased for terpyridine ligands with extended chromophores. The biphenyl-substituted complex [(tpy)Ir(tpy-R)]3 þ (R ¼ biphenyl) showed a redshift of the emission to about 560 nm; the excited-state lifetimes of this complex was remarkably long in deoxygenated solution (t W 100 ms) [189]. The long-lived emission, mainly of pp* character, was efficiently quenched by O2 (t ¼ 3.6 ms in air-equilibrated aqueous solution). Thus, the complex could be used as new type of sensitizer of 1Dg singlet oxygen (1O2): under ambient conditions, the value of F(1O2) was 0.95 (70.05) [193]. Furthermore, Haider et al. observed efficient energy-transfer from the same complex to a [Ru(bpy)3]2 þ moiety within a supramolecular array (Chapter 4.7) [194] in which the RuII center was attached to a b-cyclodextrin unit via one bpy ligand and the IrIII complex was bound to the cyclodextrin, thus bringing both metal centers into close proximity, thereby facilitating the energy-transfer. A similar covalent mixed-metal dyad (76) was prepared by Arm and Williams via Suzuki cross-coupling of orthogonally functionalized IrIII bis(terpyridine) and RuII tris(bipyridine) complexes (Figure 3.39) [195]. Excitation of 76 led exclusively to emission characteristic of the [Ru(bpy)3]2 þ center (lPL ¼ 630 nm) with no luminescence detectable in the higher-energy region (i.e., ~530 nm) as would be expected for [Ir(tpy)2]3 þ . Quenching of the IrIII-based emission was attributed to rapid intramolecular energy-transfer from the IrIII to RuII center. Related to this, Quici and coworkers studied a dyad consisting of [Ir(tpy)2]3 þ and [Ru(tpy)2]2 þ units that were linked via meta-substituted phenyl rings [196]. The meta-linkage reduced the electronic coupling of the metal centers and the emission from the IrIII center was observed at room temperature, whereas at low temperature the excited-state lifetime of the IrIII donor was sufficiently long to allow quantitative energy-transfer to the RuII acceptor. Electron-rich substituents on the tpy moiety significantly influenced the photophysical properties of the resulting homoleptic IrIII bis-complexes [197]: complexes 77 (Figure 3.40) were red in color (labs E 500 nm) and their emission maxima were remarkably redshifted (77a: lPL ¼ 754 nm with tailing into the nearIR regime). These changes in photophysical behavior were assigned to the generation of an ILCT excited-state of low energy, in which the electron-rich substituents acted as donor and the [Ir(tpy)2]3 þ as the acceptor. This assumption was supported by the electrochemical data obtained for 77d, which revealed relatively accessible reduction and oxidation potentials associated with the [Ir(tpy)2]3 þ and pyrenyl units, respectively; moreover, complex 77d displayed a pronounced negative solvatochromism in the UV–vis absorption spectrum (i.e., a bathochromic shift of 2300 cm1 when changing solvent from MeCN to CH2Cl2), which is consistent with a large change in dipole moment between the ground and excited-state as well as with a charge-transfer axis lying along the C2 axis. Finally, the low energy absorption band of 77c disappeared upon protonation; this

03

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(a) N N

N Ir

N

3+

N N

N

N

N Ru

N

2+

N

N

76

Energy transf er

(b)

Emission intensity

ε 103 (M1.cm1)

80

60

40

20

0 300

400

500 600 λ (nm)

700

800

Figure 3.39 (a) Mixed-metal IrIII-RuII complex 76 (counterions omitted for clarity); (b) UV–vis absorption and emission spectra of 76 (solid line) in comparison to the IrIII bis(terpyridine) (dotted line) and RuII tris(bipyridine) (dashed line) building blocks [195]. Figure reproduced with kind permission; r 2006 The Royal Society of Chemistry.

N R

Ir3

N N

77a:

NMe2

77c:

77b:

NPh2

77d:

N N

NMe2

R

N

Figure 3.40 IrIII bis(terpyridine) complexes 77 (counterions omitted for clarity).

reversible effect was attributed to considerable destabilization of the ILCT state upon protonation of the –NMe2 group. In contrast to the luminescence of, for example, 77a, which is easily quenched by water, 77b remained luminescent in the near-IR even in aqueous solution. This behavior was believed to arise from the different basicity of the amine substituents – the enhanced ease of establishing H-

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| 111

112


N

N

N Zn

N

N N

N 3

Ir N

N

N Au N N

N

N 78

MeO N HN

N Ir3

N

N

N

O

N

N

O

O

NH

N O

O O

N 79

MeO

Figure 3.41 Donor–[Ir(tpy)2]–acceptor arrays 78 and 79 (counterions omitted for clarity) [198–201].

bonding interactions with the solvent promoted a non-radiative depopulation of the excited state. The [Ru(tpy)2]2 þ unit was widely applied, as photo-/electroactive template, for the assembly of electron-donor and -acceptor molecules to generate long-lived chargeseparated (CS) excited-states (a key step in artificial photosynthesis, that is, lightinto-energy conversion) [188]. The significantly higher energy, longer lifetime, and pronounced photo-oxidizing nature of the [Ir(tpy)2]3 þ prompted Sauvage and coworkers to evaluate the potential of such donor–[Ir(tpy)2]–acceptor systems [95]; two different types of such arrays (78 and 79) are depicted in Figure 3.41 [198–201]. The porphyrin-based 78 (for the analogous RuII array, see Figure 3.23) featured an ultrafast two-step electron transfer from the donor (i.e., ZnII-porphyrin) to acceptor (i.e., AuIII-porphyrin) upon excitation (t o 20 ps) in toluene, generating the chargeseparated state with very high efficiency (t ¼ 450 ns). However, the second step of the process became thermodynamically unfavorable in more polar solvents (e.g., CH2Cl2), resulting in a fast decay to the ground state [198–200]. A much longer-lived CS state, with a lifetime of about 120 ms in solution at room temperature, was obtained using a triphenylamine-type donor and a naphthalene-diimide-based acceptor (79) [201]; however, the yield for forming the CS state was comparably poor due to competitive charge recombination processes. Various types of tridentate mono- and bis-cyclometalating ligands (the prototypes of these ligands are depicted in Figure 3.42) have also been combined with terpyridine ligands in the coordination sphere of IrIII ions. In general, IrIII biscomplexes bearing any combination of terpyridines, 1,3-di(pyridin-2-yl)benzenes

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3.4 Iridium(III) Complexes with Terpyridine Ligands mono-cyclometalating

N

N

N

| 113

bis-cyclometalating

N

N

L12

L13

L32

(N^C^N)-type

(N^N^C)-type

(C^N^C)-type

Figure 3.42 Mono- and bis-cyclometalating tridentate ligands L12, L13, and L32.

(L12), 6-phenyl-2,20 -bipyridine (L13), and 2,6-diphenylpyridine (L32) are isostructural and differ only in the relative number of pyridine and cyclometalating phenyl rings. Thus, investigating these types of complexes gives insight into the influence of cyclometalation on the photophysical properties. As summarized by Williams et al. [16], a general trend from the characteristic LC luminescence of IrIII bis(terpyridine) complexes to predominantly MLCT states in tris-cyclometalated IrIII bis-complexes (not discussed within the scope of this book) can be observed. Between these two extremes, one will find both the monocyclometalated species (i.e., [(tpy)Ir(N4C4N)]2 þ or [(tpy)Ir(N4N4C)]2 þ ) exhibiting excited states with a high degree of ligand-to-ligand charge-transfer (LLCT) character and bis-cyclometalated complexes (i.e., [(tpy)Ir(C4N4C)] þ ) with excited states of mixed LLCT/MLCT character. Figure 3.43 gives a schematic overview of the effect of cyclometalation on the ligand- and metal-based orbitals.

[(tpy)Ir(N^C^N)]3 Energy [(tpy)Ir(N^C^N)]3

πB

*

[(N^N^C)Ir(N^C^N)]3

[(N^C^N)Ir(N^N^C)]3

πB*

[(tpy)Ir(tpy)]3

πA*

πB* πA*

πB

n d πB πA

πA

d Figure 3.43 Simplified energy level scheme, showing the influence of cyclometalation on the frontier orbitals in IrIII bis-tridentate complexes [16]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

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MLCT

ILCT/MLCT)

ILCT/MLCT)

ILCT

LC

ILCT

πA *

d πB πA

114

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes In IrIII bis(terpyridine) complexes, the d-orbitals of the metal ion are low enough in energy that the HOMO is ligand-based (Figure 3.37), resulting in a LC pp* emissive state. If electron-rich substituents are incorporated (e.g., complex 77a), the localization of the HOMO is shifted to another part of the ligand, thus n-p* (or p-p*) ILCT states arise. The replacement of one pyridine ring by a cyclometalating phenyl moiety has two considerable consequences [16]: a destabilization of the metal d-orbital and, simultaneously, an increase of the energy of the corresponding ligand’s p- and p*-orbitals. This substitution interferes with the molecular orbitals (MOs) of the non-cyclometalating ligand (i.e., terpyridine) to a minor extent – thus switching of the relative localization of the HOMO and LUMO ligand may occur, leading to a cyclometalating ligand - non-cyclometalating ligand LLCT emissive state (the DFT calculated HOMO and LUMO of 80 are depicted in Figure 3.44a [202]). However, the low contribution of metal character to the excited-state results in poor coupling, a low rate constant (kr), and hence a weak emission.

(a)

N N

N Ir

2+

N

N 80

HOMO

LUMO

1.0

N

N Ir

2+

N N

N Ir

N

Emission intensity

(b)

2+

n

N

N

N

N

81a (n = 0) 81b (n = 1) 81c (n = 2)

81a 0.5

81b/c 0.0 500

600

700

800

λ (nm)

(c) N 1

R

+

Ir

N N

N

2

R

2

82a (R1 = Br, R = 4-Me-C6H4) 1 2 82b (R = CO Et, R = H) 2

Figure 3.44 (a) Mono-cyclometalated IrIII bis-complex 80 (left) and the DFT calculated HOMO and LUMO of 80 (right) [202]; (b) representation of the dinuclear complexes 81 (left and the corresponding emission spectra at room temperature (right) [203]; (c) representation of the bis-cyclometalated IrIII complexes 82. In all cases, the counterions are omitted for clarity. Figure reproduced with kind permission; r 2006 American Chemical Society.

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3.5 Platinum(II) Mono(terpyridine) Complexes

For instance, the mono-cyclometalated complex 80 displayed very weak emission (lPL ¼ 502 nm, FPL o 102), even at 77 K [202]. Of the series of linear “backto-back” dinuclear IrIII bis-complexes 81, only the one with the shortest spacer (81a, n ¼ 0) was luminescent at room temperature (FPL ¼ 3 103), although with a very short lifetime (t ¼ 6.4 ns) (Figure 3.44b) [203]. This behavior was attributed to a stabilization of the LUMO, localized on the non-cyclometalating bis(terpyridine) ligand, due to increased delocalization and the electrostatic influence of the proximity of the IrIII centers. CV measurements of 81b/c revealed a shift of the first reduction potential to more negative values compared to 81a, indicating the disrupted conjugation due to the interposed phenyl units. However, 81c was luminescent at 77 K with the emission spectrum resembling that of the metal-free bis(terpyridine) ligand (t ¼ 49 ms); under these conditions, the luminescence originated from a LC state localized on the bridging ligand. The introduction of a second cyclometalation via the same ligand (i.e., axialsymmetrical complexes of the [(tpy)Ir(C4N4C)] þ type with a trans-arrangement of the C-atoms) repeats the shift in energy levels. As a consequence, the energy of the metal d-orbital becomes comparable to that of the p-orbital of the bis-cyclometalating ligand and a heavily mixed LLCT/MLCT excited-state arises. The additional metal character promotes the radiative rate constant (kr), leading to a more intense emission than for the mono-cyclometalated systems. The bis-cyclometalated IrIII complexes 82a/b (Figure 3.44c) exhibited emission maxima at 690 and 707 nm, respectively, with excited-state lifetimes of 1.7 ms and kr values of ~104 s1 [16, 204, 205]. Thus, for complexes [(N4C4N)Ir(N4N4C)] þ where cyclometalation occurs via two different ligands (leading to a cis-orientation of the Catoms), one should expect an analogous rise in the metal-based orbitals, but none of the ligands is destabilized to the same extent as in the C4N4C system. Consequently, the metal center contributes a higher extent to the HOMO, giving the excited state a more distinct MLCT character, and kr is an order of magnitude higher than for the [(N4N4N)Ir(C4N4C)] þ isomers.


The square-planar mono(terpyridine) complexes of the late d8 transition metal ions (i.e., PtII, PdII, and AuIII) were reviewed by Newkome and coworkers with respect to synthesis, (photo)physical properties, and potential applications [5]. In general, PtII mono(terpyridine) complexes can be expected to exhibit luminescence, since their square-planar geometry encourages D2d distortions, promoting a radiationless decay. The emission properties of such complexes in solution were first reported by Aldridge et al. [206]; in contrast to the parent complex [(tpy)PtCl]Cl, complexes [(tpy)Pt(L)] þ showed broad, structureless emission at 621 (L ¼ OH), 588 (L ¼ CS), and 654 nm (L ¼ MeO) that were assigned to arise from 3 MLCT excited-states. Various types of functional groups on the terpyridine moiety as well as different ancillary ligands (L) were utilized to fine-tune the luminescence

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| 115

116

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes R N R

N N

Pt

1

N

Pt

N

R2

N

Cl 83a (R = NMe2) 83b (R = pyren-1-yl)

84a (R1 H, R2 C6H5) 84b (R1 H, R2 4-Cl-C6H4) 84c (R1 H, R2 4-Me-C6H4) 84d (R1 H, R2 4-NO2-C6H4) 84e (R1 4-Me-C6H4, R2 CH2OH) 84f (R1 4-Me-C6H4, R2 CH2CH2CH3)

Figure 3.45 Luminescent PtII mono(terpyridine) complexes 83 and 84 (counterions omitted for clarity).

properties [207]. For instance, complexes of 40 -substituted terpyridines (i.e., [(40 -Rtpy)PtCl] þ , 83) were highly emissive with excited-state lifetimes in the ms-regime in non-coordinating solvents (Figure 3.45). More important, highly photoluminescent PtII mono(terpyridine) complexes with ethynyl-based co-ligands (84) have been introduced [208–210], thereby combining the properties of (poly)platinynes with those of terpyridine complexes [211]. The UV–vis absorption spectra of these complexes displayed unique MLCT bands mixed with alkyne-to-terpyridine charge-transfer (LLCT) bands in the range 410– 480 nm [212]; the emission spectra revealed 3MLCT/3LLCT bands between 550 and 670 nm. The highest values for t and FPL were reported for complexes 84e/f in deoxygenated CH2Cl2 (84a: t ¼ 14.6 ms and FPL ¼ 0.30, 84b: t ¼ 10.3 ms and FPL ¼ 0.25). The electron-donating alkynyl ligands increased the energy gap between the 3d-d and 3MLCT excited states and, thus, the radiationless decay of the 3MLCT state, mediated by a low-lying 3d-d state, became less preferred [209]. Even though the ethynyl-substituted PtII complexes were luminescent at room temperature, both lateral substituents on either of the ligands as well as the nature of the solvent (i.e., coordinating vs. non-coordinating) had a dramatic effect on the excited-state lifetimes and quantum yields. Consequently, luminescent and colored PtII mono(terpyridine) complexes, being sensitive to their environment (e.g., concentration, solvent, acidity, and counter-ions), were employed as sensors for pH value, (metal) ions, and solvents. The reversible responsive behavior of the complexes also suggested their potential use as molecular switches. Yam et al. reported the solvent-responsive complex 85 (Figure 3.46a) [213] for which increasing the content of diethyl ether in either an acetone or a MeCN solution induced a significant color change (Figure 3.46b). The UV–vis absorption spectra revealed a decrease in intensity in the MLCT band (labs ¼ 415 nm) and the evolution of a new band at lower energy (labs ¼ 615 nm), in which the intensity was dramatically enhanced upon increasing the diethyl ether composition (Figure 3.46c, left). Owing to the insolubility of 85 in the latter solvent, aggregation into oligomeric species was assumed (corresponding to the

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(a)

| 117

2.0

6

N

N

Absorbance

Pt+

N

85

(b)

5

1.5

4

1.0

3 2

0.5 1

0.0

0

500 600 700 800 300 400 500 600 700 Wavelength/nm Figure 3.46 (a) PtII mono(terpyridine) complex 85 (counterion omitted for clarity); (b) representation of solutions of 85 in MeCN–Et2O mixtures displaying color changes with increasing Et2O fraction (from left to right: 64%, 68%, 72%, 74%, 76%, 78%, and 80% Et2O, respectively); (c) UV–vis absorption (left) and emission spectra (right) of those solutions; the darker blue solutions exhibited stronger emission [213]. Figure reproduced with kind permission; r 2002 American Chemical Society.

new MMLCT absorption band at about 600 nm and 3MMLCT emission band at about 785 nm (Figure 3.46c, right). The nicotinamide-functionalized complex 86 featured reversible vapochromic behavior [214] as upon exposure to MeOH vapors a color change from red to orange, in concert with a blue-shift of the emission band, could be observed (Figure 3.47). The red form (packed as a pseudo-linear, chain-extended structure in the single crystal) showed a solid-state emission band at 660 nm, which was assigned to the 3MMLCT excited state; however, the orange form (packed as a chain-like structure with a zigzag conformation of the PtII centers and having MeOH molecules in the structure) displayed its emission maximum at 630 nm – attributed to a 3MLCT state, since it had only weak d–d and pp interactions. The emission band of the red form could be shifted to higher energy upon exposure to MeOH, whereas upon heating in vacuo the emission band shifted back to its initial position (Figure 3.32c). This reversible vapochromic response cycle could be repeated with no indication of chemical decomposition of 86. A reversible, pH-dependent color change in complexes 87 (Figure 3.48) upon the consecutive addition of p-toluenesulfonic acid and NEt3 was shown by Wong et al. [215]. Complex 87a exhibited a low-energy absorption band at 546 nm (acetyleneto-terpyridine LLCT transition with some MLCT character) and a MLCT band at 412 nm. Upon protonation, the intensity of its LLCT band was significantly decreased and the intensity of the MLCT band was increased, with a clear isosbestic point at 460 nm, indicative of complete conversion of 87a into the corresponding protonated forms. Moreover, all protonated complexes 87 showed a new emission band at about 600 nm, which was attributed to the 3MLCT excited-state. Similar reversible absorption and emission behavior was observed by Yang et al. for complexes 88 [216].

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Emission Intensity (A.U.)

(c)


(a)

(b) O N

H2N

N Pt+ Cl

N N

86

Normalization emission intensity

118

1.2 orange form red form

1 0.8 0.6 0.4 0.2 0 560

610

660 710 Wavelength (nm)

760

(c)

MeOH (g) - MeOH (g)

Figure 3.47 (a) PtII mono(terpyridine) complex 86 (counterion omitted for clarity); (b) solidstate emission of 86 in the presence (orange form) and absence of MeOH (red form); (c) Xray single-crystal structures of the nearly linear red form (left) and the zigzag stacking in the orange form of 86 (right) [214]. Figure reproduced with kind permission; r 2004 American Chemical Society.

N

N Pt+

N

NR2

R1

N

Pt+

N

R2

N 87a (R H) 87b (R CH3) 87c (R CH2CH2OCH3)

88a (R1 CH3, R2 NMe2) 88b (R1 NMe2, R2 H) 88c (R1, R2 NMe2)

Figure 3.48 pH-responsive PtII mono(terpyridine) complexes 87 and 88 (counterions omitted for clarity).

To end this section, some oligonuclear PtII mono(terpyridine) complexes are considered (for a more detailed overview, see Reference [5]). The luminescent, dinuclear oligoacetylene-bridged rods 89 (n ¼ 1, 2, or 4) were reported by Yam et al. (for solubility reasons, the terminal tpy ligands were modified with tert-butyl groups) (Figure 3.49a) [217]. X-Ray single-crystal analysis revealed Pt Pt dis tances of 5.16 (n ¼ 1), 7.71 (n ¼ 2) and 12.83 A (n ¼ 4). The absorption spectra of 89 were dominated by low-energy MLCT bands (mixed with some LLCT character) and high-energy IL pp* bands, attributed to the acetylene and terpyridine ligands, respectively. The emission spectra of 89 exhibited strong luminescence in the range lPL ¼ 550–625 nm, which was assigned to the dominant 3MLCT excited state mixed with contributions from the 3LLCT/3IL states; upon increasing the

03

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N

N Pt

N

n

N

Pt+

N N

N Pt

N

N

Pt2 N

N

N

89 (n 1,2 or 4)

N 90

Figure 3.49 Dinuclear PtII mono(terpyridine) complexes 89 and 90 (counterions omitted for clarity).

degree of conjugation (i.e., the length of the bridge) the 3IL character was enhanced at the cost of the 3LLCT state. In a different example, the same group utilized 4-ethynylpyridine, as supramolecular linker, in the dinuclear complex 90 (Pt Pt distance of 9.4 A ) (Figure 3.49b) [218]. The different chemical environII ment of the Pt centers could be demonstrated by CV experiments in which four quasi-reversible redox couples, two at about 0.82 to 0.93 and two at 1.33 to 1.42 V versus SCE, were attributed to successive one-electron reduction steps on each of the PtII mono(terpyridine) subunits. Besides these homometallic species, various types of mixed-metal complexes containing the PtII mono(terpyridine) moiety are also known. The intramolecular electron processes in dyads 91 containing a porphyrin unit linked to the PtII center via an ethynylphenyl bridge were investigated by the Odobel group (Figure 3.50) [219]; a rapid electron-transfer occurred on a ps-timescale from an excited-state of the porphyrin unit to the PtII centers. The quenching of the porphyrin-based fluorescence via an ultrafast charge recombination overcame the possible CS state of the dyads. CV measurements gave evidence for weak interactions between the metal centers. According to detailed photophysical studies (i.e., steady-state, timeresolved, and femtosecond transient absorption spectroscopy), the rate constants (kr) of the electron-transfer within the dyads were consistent with Marcus’ theory (i.e., electron-transfer occurred through the p-conjugated phenyl-bridge) [220]. Lam et al. reported a set of mixed-metal molecular rods (92), where a PtII mono(terpyridine) unit and a ReI(CO)3 mono-bidentate were linked via a conjugated a,o-diethynyl derivative (Figure 3.50) [221]. The UV–vis absorption spectra exhibited a low energy band at about 404–486 nm, which was attributed to a mixing of MLCT bands (dpPt-p*tpy and dpRe-p*bpy) and the pethynyl-p*tpy LLCT band; upon excitation at lexc W 380 nm, intense orange-red emission bands were observed for dyads 92 (lPL ¼ 570–580 nm) with excited-state lifetimes of ¨ckel 0.52–0.94 ms. Additional electrochemical studies as well as extended Hu molecular orbital (EHMO) calculations revealed that low energy emissions originating from the 3MLCT excited-state (dpPt-p*tpy) were actually mixed with either the pethynyl-p*tpy LLCT state or, unexpectedly, an MLLCT excited-state (i.e., pCC– phenyl–CC–Re-p*tpy).

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| 119

120

| 3 Chemistry and Properties of Terpyridine Transition Metal Ion Complexes R2 N

R1

N Pt

N

N M

N

N

N R2

R

92a: R tert-butyl, n 1, N^N 2,2'-bipyridine 92b: R tert-butyl, n 1, N^N 4,4'-di(tert-butyl)-2,2'-bipyridine 92c: R tert-butyl, n 1, N^N 4,4'-bis(trifluoromethyl)-2,2'-bipyridine 92d: R tert-butyl, n 1, N^N 5-nitro-1,10-phenanthroline 92e: R H, n 1, N^N 4, 4'-di(tert-butyl)-2,2'-bipyridine 92f: R tert-butyl, n 0, N^N 4, 4'-di(tert-butyl)-2,2'-bipyridine

N R

91a: R1 PO(OEt)2, MII ZnII 91b: R1 H, MII ZnII 91c: R1 OC7H15, MII ZnII 91d: R1 PO(OEt)2, MII MgII 91e: R1 H,MII MgII 91f: R1 OC7H15, MII MgII in all cases: R2 3, 5-di(tert-butyl)phenyl

N

Pt

N N Re CO n OC CO

N R

R1

R1 N R1

N Pt

N

R2

N

N M2

N N

N R2

N

N

Pt N N

R1

93a: R1 tert-butyl, MII FeII, R2 nothing 93b: R1 tert-butyl, MII ZnII, R2 nothing OC4H9 93c: R1 H, MII FeII, R2 R1 C4H9O

R1

Figure 3.50 Dyads and triads 91–93 based on PtII mono(terpyridine) complexes (counterions omitted for clarity).

Finally, the first triads (93) with square-planar mono(terpyridine) units and a central octahedral bis(terpyridine) complex (of either FeII or ZnII ions) were reported by Ziessel and coworkers [222]. To guarantee sufficient solubility, tert-butyl groups on the outer terpyridine ligands (93a/b) or a 1,4-dialkoxybenzene acetylene part (93c) were used (Figure 3.50). The UV–vis absorption spectra of the FeII-containing triads 90a/c displayed a 1MLCT band at about 580 nm arising from the central [Fe(tpy)2]2 þ unit and 1MLCT bands mixed with some LLCT character (labs ¼ 425 nm) that were assigned to the PtII mono(terpyridine) moieties. The ZnII-containing triad 93b only showed the PtII-based 1MLCT band. Electrochemical investigations of 93a/c revealed the successive reduction of four terpyridine moieties connected to FeII and PtII centers at 1.27 and 1.40 V as well as at 0.96 and 1.54 V, respectively.

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4

Metallo-Supramolecular Architectures Based on Terpyridine Complexes

4.1 Introduction

The preceding chapter described the synthesis and properties of terpyridine complexes with transition metal ions. Besides these linear assemblies from monoto oligonuclear species, two- and three-dimensional architectures have moved into the focus of current interest. In general, such advanced supramolecular assemblies are formed by the self-assembly of oligo(terpyridine) ligands with appropriate transition metal ions, following the rules of thermodynamics. This field of research was pioneered by Lehn [1] and a palate of colorful architectures has been reported: cycles, racks, grids, helicates, rotaxanes, and catenanes. These advanced metallo-supramolecular structures can be discussed from two different viewpoints: a solely scientific perspective or an application-based direction. Controlling the architecture, as determined by the structure and stability of the supramolecules, based on the interplay of ligands and metal ions under specific conditions, is ´ski hexagonal mainly of scientific interest. As representative examples, the “Sierpin gasket,” “techno-mimetic spoke wheel,” or “molecular machines” synthesized by the groups of Newkome [2], Schmittel [3], and Sauvage [4], respectively, are named. The challenge of shrinking devices down to the nanometer scale by controlling the design of supramolecular materials is essential; over recent decades, many molecular-scale components have been designed featuring properties related to organic electronics and nanotechnology: light-emitting devices, organic solar cells, organic thin film transistors, sensors, nanowires, and supramolecular machines (e.g., switches and motors) [5–12]. This chapter summarizes well-defined supramolecular architectures. In particular, the focus will be on macrocyclic as well as grid-like systems, but also important contributions with respect to other assemblies will be included: helicates and interlocked species (e.g., catenanes and rotaxanes).


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| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes 4.2 Terpyridine-Containing Metallo-Macrocycles

In general, macrocyclic assemblies as well as coordination polymers can be obtained by complexation of telechelic terpyridine ligands with transition metal ions. As shown by the groups of Schubert [13–15], Constable [16–19], and Kurth [20], flexible bis(terpyridine) building blocks tend to form both linear assemblies (i.e., oligomers or polymers) as well as various size macrocycles. Thus, the exclusive formation of macrocyclic products from such ligands is not necessarily straightforward. Despite these restrictions, bis(terpyridine)s with different flexible groups were successfully employed in the construction of rings; however, mixtures of rings and coordination polymers were obtained in most cases. Therefore, high dilution conditions had to be applied to drive the ring–chain equilibrium to favor cyclization [21]. Furthermore, purification of the product mixture, for example, generally by column chromatography, was necessary. Structurally, the simplest architecture, that is, mononuclear macrocycles of the so-called [1 þ 1]-type, can only be formed if the spacer combines both sufficient flexibility and appropriate length. When the terpyridines were connected via their 5-positions, the connecting points of the resulting complexes were in close proximity; therefore, even short spacers allowed intramolecular cyclization. Priimov et al. utilized building blocks where the terminal terpyridines were linked via their 5-positions: 1,3-bis[(2,20 :60 ,200 -terpyridin-5-yl)methylsulfanyl]propane (1a) and 1,4-bis[(2,20 :60 ,200 -terpyridin-5-yl)methylsulfanyl]butane (1b) (Figure 4.1a) [22]. According to molecular modeling, the length of these linkers should be optimal for the formation of mononuclear cycles (i.e., lower strain energy for 1a/b in

(a) N

N

N

N S

(b)

N

N

S n

1a (n 1) 1b (n 2) 1c (n 3)

Figure 4.1 (a) Dithiol-bridged bis(terpyridine)s 1; (b) representation of the X-ray crystal structure of [Ni(1a)]2 þ (left) and [Ni(1b)]2 þ (right) (H-atoms, counterions, and solvent molecules omitted for clarity) [22]. Figure reproduced with kind permission; r 2000 The Royal Society of Chemistry.

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4.2 Terpyridine-Containing Metallo-Macrocycles

| 131

comparison to 1c). Indeed, the addition of either FeII or NiII ions resulted in an intramolecular cyclization, as verified by X-ray single-crystal structure analysis (Figure 4.1b). In the macrocycle, the terpyridines were almost orthogonal to each other (the angles were 851 and 891, respectively, for [Ni(1a)]2 þ and [Ni(1b)]2 þ ), indicating the low degree of distortion of the coordination geometry due to cyclization. Similarly, 1,4,7-tris[(2,20 :60 ,200 -terpyridin-5-yl)methyl]-1,4,7-triazanonane was used for the complexation with EuIII ions via nine coordinative bonds, generating a room temperature luminescent complex involving all three terpyridine groups [23]. With 40 -functionalized terpyridines, the functionality is attached on opposite faces of the resultant octahedral bis(terpyridine) complex; thus, either a much longer spacer or special geometry is required to generate the desired mononuclear macrocycles. For instance, Constable et al. prepared a ligand in which two terpyridine moieties were linked via tri(ethylene glycol) spacers to a rigid, central 5,50 -bis(phenyl)bipyridine unit [24]. The flexible chains protrude at a 1201 angle from the rigid central moiety; thus, the terpyridine groups were able to coordinate the metal ions intramolecularly. Owing to the molecular structure of 2 (Figure 4.2), the bipyridine unit was still accessible for further complexation to construct extended supramolecular systems. A macrocycle containing a 2,7-di(ethylene glycol)naphthalene linker was also prepared (3, Figure 4.1); after coordination with FeII ions, the naphthalene unit was conformationally locked on one side in the cleft between the terpyridine moieties, resulting in a chiral structure [25]. The axial

N O

O

O

O

O

N O

O O

O

N Fe2

N N

Fe2

2 PF6 N

O

N

N

O N

O

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N

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N

2

O

N

O

O

O N

O

N N N

O

O

O

O

N N

N

O

O N

O

N

N N

4

O

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O

Figure 4.2 Mononuclear metallo-macrocycles 2 [24] and 3 [25] as well as the multifunctional bis(terpyridine) ligand 4 [28].

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3

132

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes chirality in this conformationally locked macrocycle was investigated further in the presence of chiral bulky anions by means of 1H NMR spectroscopy as well as circular dichroism (CD) measurements [26]. A more advanced hairpin double helicate was obtained by replacing the terpyridine moieties with 2,20 :60 ,20000 :60000 ,200000 :6000000 ,2000000 -quinquepyridines and subsequent assembly of the telechelic ligand with CuII ions [27]. A different type of mononuclear ring structure was derived from multifunctional ligand 4 (Figure 4.2): an extended polytopic ligand containing two terpyridine moieties – linked via azacrown ether moieties to a central phenanthroline unit – was complexed with ZnII ions to give the mononuclear complex [Zn(4)]2 þ [28]. A heterometallic [2 þ 2]-macrocycle was obtained by a step-wise assembly of 40 (2-propyn-1-oxy)-2,20 :60 ,200 -terpyridine in which the reaction with trans-PtI2(PEt3)2 regioselectively afforded the platinyne 5 (Figure 4.3a) with a trans-configuration of the PEt3-moieties that upon treatment with FeII ions cyclized to give a [2 þ 2]macrocycle; the coupling constant of 1JP–Pt ¼ 2353 Hz, as determined by 195Pt NMR spectroscopy, confirmed the trans-arrangement [17]. Moreover, the X-ray single-crystal structure analysis revealed the anticipated structure with two halves of the structure being related by an inversion center; an almost ideal coplanar orientation of the four metal centers was observed with non-bonding Fe Fe and Pt Pt distances of 10.5 and 13.6 A , respectively, and a Fe Pt distance of 8.4 A (see Figure 4.3b for a molecular model of the [2 þ 2]-macrocycle). The group of Constable described various types of supramolecular [2 þ 2]macrocycles, utilizing oligo(ethylene glycol)s that were a,o-functionalized with terpyridine units. Their step-wise complexation with RuII ions afforded both homoleptic as well as heteroleptic binuclear macrocycles [18]. Moreover, a family of macrocycles was described in which the 40 -position of the terpyridines was linked by flexible tri(ethylene glycol) chains [19]. Since the applied linker was too short to enable the formation of a [1 þ 1]-macrocycle, only [3 þ 3]- and [4 þ 4]species were obtained, accompanied by some polymeric species. To minimize the formation of the linear structures, the self-assembly was conducted at a low

(a)

(b)

[Fe(tpy)]2+

N N

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PEt3 Pt PEt3

O

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N

N

5

PtII

N

Figure 4.3 (a) Platinyne-bis(terpyridine) 5; (b) space-filling model of [Fe2(5)2]4 þ (H-atoms and ethyl-groups omitted for clarity) [17]. Figure reproduced with kind permission; r 2005 The Royal Society of Chemistry.

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concentration of about 0.4 mol l1; the individual cyclic species were isolated by column chromatography and their composition was confirmed by electrospray ionization (ESI) MS and 1H NMR. A similar approach was followed by Schubert et al. when a n-hexyl-spacer linked the terpyridine moieties: tri- as well as tetranuclear macrocycles containing FeII ions were isolated and identified by matrixassisted laser desorption/ionization time-of-flight (MALDI-TOF) MS [14]. Furthermore, when a long hexadecyl spacer between terpyridines was utilized its treatment with FeII ions gave (presumably cyclic) aggregates up to a composition of [9 þ 9]; however, the [2 þ 2]-macrocycle was the major product [15]. Moreover, a viologen-type bis(terpyridine), obtained by selective N-alkylation of the pyridinylsubstituent of 40 -(pyridin-4-yl)-2,20 :60 ,200 -terpyridine with 4,40 -bis(bromomethyl) biphenyl, was self-assembled with FeII ions to give the box-like [2 þ 2]-macrocycle 6a (Figure 4.4) [29, 30]. The structural analogue 6b containing RuII ions was prepared via an orthogonal approach in which the RuII bis-complex of 40 -(pyridin4-yl)-2,20 :60 ,200 -terpyridine was initially prepared, then cyclization was achieved by subsequent N-alkylation with 4,40 -bis(bromomethyl)biphenyl [31]. The spiro-metallodendrimers 7 and 8, based on a pentaerythritol core, were reported by Newkome and coworkers. A single terpyridine unit was incorporated on the surface of each dendron of a four-directional dendrimer; subsequent complexation with either FeII or RuII ions led exclusively to intramolecular cyclization of the spiro-type [32]. According to molecular modeling studies, 7 had an open structure with the metal centers separated by 25.76 A and a distance of the central carbon atom to the metal centers of 13.38 A ; an overall diameter of 34.5 A was estimated (Figure 4.5). The terpyridine-containing penta-amine macrocycle 9, connected at the 6,600 position, was constructed by Bazzicalupi et al. (Figure 4.6a) [33]; owing to a fixed geometry with the metal-binding site pointing inside-the-ring, conventional

N

N

N N

N N

N

M2 N

N N

N M2

N

N N

8 BF4 N

6a (MII FeII) 6b (MII RuII)

N

Figure 4.4 Structure of [2 þ 2]-macrocycle 6 [29].

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27 J l 2011 15 45 53

| 133

134

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes H OO O N

O N N

N M N

O O

HN

O

N

O

N

O

O H O O N O O

N

N M N

O

O

N N

O NH

O

O O

O

O

N

N

7

OO O

OO O

N

O

O

O

N O OO H O O

N

O NH

O

O N O H OO O HN O

N M

O O

N

OO

O

O O

HN O O O HN O OO O O HN O O O O O

O N

O NH

O HN O O H N O O O

O

O H O N

O

HN

O HN

O

O O

O OO

O

O O O O O

NH O

O

O NH

NH O O O O H N O O O O

O NH

O O OO

O

O O OO NH O O N H

O O N O H O O

O N N

OO

O

O NH

H N O

N M N

N N

O

8

Figure 4.5 Spiro-metallodendrimers 7 and 8 (M ¼ FeII or RuII) along with a space-filling representation of the molecular model of 7 [32]. Figure reproduced with kind permission; r 2002 The Royal Society of Chemistry.

bis(terpyridine) complexes could not be formed. Thus, hexacoordinated metal complexes were generated based on two ligands and two transition metal ions (e.g., ZnII and CdII ions) with a m-hydroxy or a m-bromide bridge, respectively [see Figure 4.6b/c for the X-ray crystal structures of [Zn(9-H)]2(m-OH)5 þ and [Cd(9-H)]2(m-Br)5 þ ]. Within these dimeric structures, strong p–p stacking interactions were present. Under mild acidic conditions, the secondary amine groups were selectively protonated; protonation of the terpyridine nitrogen atoms occurred only ã utilized such with strong acids [34, 35]. More recently, the group of Garcıá-Espan “terpyridinophane” macroligands for novel CO2 fixation and activation [36]. In an analogous system, a crown ether was attached to a chiral terpyridine; amino acid derivatives were shown to enantioselectively bind, as guests, to this macrocyclic host. The resulting fluorescence quenching – of the metal-free ligand as well as the ZnII (mono)terpyridine complex – made this macrocycle a useful sensor for amino acids [37, 38]. Other ethereal bridged terpyridines were also reported, for example, a 5,500 -bridged terpyridine as part of a 34-membered ring encompassing a bisphenol-A moiety; the subsequent addition of PdII ions and

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(a)

N

HN

H N

N

HN

H N

| 135

N HN 9

(c)

(b) N2 N1

N10 N11

N3

N14

N6

N2 N1

N6 N7

N15 N7

Zn1

N5

N4

N8

N16

N13

N12

N8 N5

N4

N14

Cd2

Cd1

Zn2 O1

N10

N9

Br1

N16

N15 N11 N12

Figure 4.6 (a) Macrocyclic terpyridine 9; (b) representation of the X-ray single-crystal structure of [Zn(9-H)]2(m-OH)5 þ ; (c) representation of the X-ray single-crystal structure of [Cd(9-H)]2(m-Br)5 þ [33]. In both representations, the H-atoms, solvent molecules, and counterions are omitted for clarity. Figure reproduced with kind permission; r 2004 American Chemical Society.

2,6-bis[oligo(ethylene oxide)]pyridine afforded a pseudo-rotaxane, based on a square-planar complex [39]. The self-assembly of this ligand and a rigid-linear p-conjugated phenanthroline derivative with ZnII or CuII ions gave a supramolecular rotaxane with a trigonal-bipyramidal arrangement of the five pyridine units around the metal centers (see also Section 4.6) [40]. In contrast to their rigid-linear counterparts (forming metallo-supramolecular polymers by self-assembly with transition metal ions, see also Chapter 5), rigid bis(terpyridine)s with angular geometries can be utilized to construct metallomacrocycles; in many cases, due to a given geometry, the size of the rings is predetermined. For entropic reasons, small cyclic assemblies are generally preferred over polymers at low to medium concentrations. Newkome et al. reported formation of a series of linear oligomers containing 2–6 bis(terpyridine) units, based on angular bis(terpyridine) 10 and RuII ions (Figure 4.7) [41]. The individual oligomers with metal-free end groups could be isolated by fractionation and characterized. In all cases, the cyclic combination of these oligomers was driven by the laws of thermodynamics, giving selectively hexagonal metallo-macrocycle [Ru(10)]6(PF6)12. Sonogashira cross-coupling of 2,20 :60 ,200 -terpyridin-40 -yl triflate or 40 -ethynyl2,20 :60 ,200 -terpyridine was applied by Ziessel and coworkers for the synthesis of various types of linear and angular bis(terpyridine) derivatives [42]. The initial report on welldefined small conjugated metallo-macrocycles dates back to 1996 when the same group reported on the self-assembly of an angular bis(terpyridine) with FeII ions [43]. The 601 angle of the two ethynyl-terpyridines, linked by a phenanthroline moiety, favored the formation of the trinuclear triangle 11 (Figure 4.8); however, the less stable tetranuclear square was also observed. A more compact metallo-supramolecular triangle was reported by Newkome et al. (Figure 4.8); in addition to obtaining

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N13

136

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes N

N

N

N

n

10 N

N

linear oligomers (n 2-6)

(b)

RuII

[6 6]-metallo-macrocycle

(c)

(a)

Figure 4.7 Angular bis(terpyridine) 10 (a) and linear oligomers (b) as well as the hexagonal macrocycle (c) obtained by self-assembly of 10 with RuII ions [41].

N N N

N N

N

Fe2 N

N

N

N M2

N

N N

N

N N

N

N

N

N

2

N

N

N

N

N

N N

N N

N N

N M2

N

N

Fe2 N

N M2

Fe

N N

N N

N N

11

12 II II II II II M Ru , Fe or Ru /Fe (2:1 ratio)

Figure 4.8 Homo- and heterometallic triangular [3 þ 3]-macrocycles 11 and 12 [43, 44].

the homometallic triangle by the one-step macrocyclization of 1,2-bis(2,20 :60 ,200 terpyridin-4-ylethynyl)benzene with either FeII or RuII ions, the formation of its heterometallic analogue via a directed step-wise process was also shown [44]. The most prominent and fascinating examples of exclusively bis(terpyridine)based metallo-macrocycles with designed dimensionality and nuclearity were, however, published in recent years by Newkome and coworkers [2, 45–62]. As the most common structural feature, the metal-binding sites were attached at the 1and 3-positions of a phenyl ring [e.g., bis(terpyridine)s 13–17, Figure 4.9] and, thus, the terpyridine units formed an angle of 1201 with respect to each other, which is exactly the internal angle of a planar hexagon. The formation of a stable pentameric by-product was observed but in low yield. Structural variants of this general motif included the incorporation of 3,6-bis(2,20 :60 ,200 -terpyridin-40 -yl)-9alkyl-carbazole (18) [50] or 4,40 -bis(2,20 :60 ,200 -terpyridin-40 -yl)triphenylamine (19),

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| 137

R R n

13 R CH3, Br, OBn, CH2OH or tBu

14a (n 0) 14b (n 1)

15 R H, CH3, tBu, OC12H25

C12H25O

16

17

N

C6H13 N

N N N

18

19

Figure 4.9 Family of angular bis(terpyridine) ligands 13–19, as building blocks for hexa- and pentagonal metallo-macrocycles [45].

[53, 55] as monomers (Figure 4.9), each possessing a directionality of the terminal terpyridine moieties favoring pentamer formation. For the carbazole derivative, the angle between the terpyridinyl moieties was contracted to about 1051 and, thus, the formation of pentagonal macrocycles was favored. Various hexagonal homometallo-macrocycles, based on RuII, FeII, ZnII, or CdII ions, were obtained by self-assembly of angular p-conjugated bis(terpyridine)s. The formation of the RuII-containing macrocycles required relatively harsh reaction conditions, involving, in some cases, a step-wise assembly of the building blocks; the isolated yields were reported to be in the range 30–50%. In contrast, the selfassembly of angular bis(terpyridine) ligands with FeII, ZnII, or CdII ions into macrocycles generally proceeded under much milder conditions and the products were formed in very good to almost quantitative yields. For the synthesis ¨hnke-type condensation of the angular bis(terpyridine) ligands, either the Kro [63, 64] or the Pd0-catalyzed Sonogashira cross-coupling reaction [65, 66] was used (see also Chapter 2). In addition to homometallo-macrocycles, some metallo-macrocycles based on both RuII and FeII ions were also described by Newkome and coworkers [48]. The synthesis of these heterometallo-macrocycles was enabled following a so-called semi-self-assembly approach. Applying this strategy to preformed building blocks, such as the dinuclear RuII bis(terpyridine) complex [(13)Ru(13)Ru(13)]Cl4 (with R ¼ Me), self-assembly into macrocycles was achieved by the addition of FeII ions. As a result of this directed synthesis, a series of hexameric metallo-macrocycles of the general formula [(13)6Fe6nRun](PF6)12 (R ¼ Me; n ¼ 3–5) was obtained (see Scheme 4.1 for the synthesis of [(13)6Fe6nRun](PF6)12 with n ¼ 4).

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138

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes R

N

N

N

N

N

N R

i) 1eq. FeCl2*4H2O, MeOH, 12 h, reflux ii) NH4PF6, MeOH [(13)Ru(13)Ru(13)]Cl4 (R=CH3)

N

N

N

N N

R

12PF6

N N

N

N

N

2

Fe 85%

N Ru2

Ru2

N

N

N

(R = CH3)

N

R

N N

Ru

N N

N N N

N Fe2

[(13)6Fe2Ru4](PF6)12

R

N

N 2

2

Ru N

N

N

N N

R

Scheme 4.1 Synthesis of the heterometallo-macrocycle [(13)6Fe2Ru4](PF6)12 according to Newkome et al. [48].

Within this class of metallo-supramolecular macrocycles, the non-branched ´ski hexagonal gasket” – is fractal structure 21 – a molecular equivalent of a “Sierpin the most advanced example [2]. Fractal constructs are based on the incorporation of identical motifs that repeat at differing size scales. Thus, 21 was created based on repeating hexameric architectures incorporated with increasing dimensions at successive higher levels or generations. Based on the fractal definition by the ´sk [67] and the collection of equilateral triPolish mathematician Wac"aw Sierpin ´ski gasket” by Mandelbrot [68], this non-dendritic angles termed the “Sierpin metallo-macrocycle contained 24 bis(terpyridine) and 12 tris(terpyridine) building blocks (a total number of 84 terpyridine moieties) bound together by 36 RuII and six FeII ions. The synthesis involved the semi-self-assembly of 13 (R ¼ Me) and 20 (in a 4 : 2 ratio) with RuII ions to yield the outer, small hexagons, followed by their self-assembly with FeII to the fractal gasket 21 in 35% yield (Figure 4.10). In addition to NMR, ESI MS was found to be a highly versatile tool for the characterization of metallo-macrocycles; in particular, for systems containing the highly stable RuII bis(terpyridine) connectivity no fragmentation of the supramolecular assemblies was observed under the applied measurement conditions [54]. Recently, travelling wave ion mobility (TWIM) mass spectrometry was utilized for the characterization of metallo-macrocycles [60]. In general, TWIM MS enables the two-dimensional gas-phase separation and complete deconvolution of the isotopic patterns of ions with the same m/z value. Ions of different charges and

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R

N

N

N

N

N N

N

N N

N

N N

13 (R CH3)

21

N N

20

N

Figure 4.10 The Sierpin´ski hexagonal gasket 21 and its building blocks 13 (R ¼ Me) and 20 Figure redrawn according to Reference [2].

(a)

5.13 1 amu

1 amu

[6L+6Cd]4

[3L3Cd]2 10.30

6.45

2.57

5.13

7.70 10.26 Drift Time (ms)

12.83

15.39

17.96

(b) 17.5 Å

37.5 Å

10 nm

Figure 4.11 (a) Representation of a two-dimensional ESI-TWIM MS plot of [(13)6Cd6][(NO3)12 (with R ¼ tBu) for m/z ¼ 1188 (measured at a traveling wave height of 9.3 V and a velocity of 380 m s1). Ion mobility separation gave signals at 10.30, 6.45, and 5.13 ms, corresponding to linear [(13)3Cd3]2 þ , linear [(13)6Cd6]4 þ , and cyclic [(13)6Cd6]4 þ , respectively. The observed isotopic patterns (shown) matched closely those calculated for the mentioned compositions [60]. (b) TEM image of a metallo-macrocycle built-up from 13 (R ¼ CH2OH) and FeII ions (magnification: 200 000 ) showing individual, regular hexagons and a molecular model for comparison [47]. Figure reproduced with kind permission; r 2009 American Chemical Society and 2002 Wiley-VCH.

ions with cyclic and linear shapes are separated, based on their drift time in the ion mobility device – thus self-assembled supramolecular materials can be investigated with this technique. Figure 4.11a depicts the ESI-TWIM MS of [(13)6Cd6](NO3)12 (with R ¼ tBu).

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| 139

140

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes X-Ray photoelectron spectroscopy (XPS) was used, as reliable tool, to quantify the atomic ratio of transition metal ions within the heterometallo-macrocycles [2]. Furthermore, single-molecule imaging techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning tunneling microscopy (STM), were utilized to visualize the nanoscopic hexagonal macrocycles [2, 47]. Figure 4.11b shows the TEM image of a hexagonal homometallomacrocycle containing FeII ions. The TEM analysis gave evidence of the size and shape of the macrocycle, which was in good agreement with the value determined from molecular modeling studies. Newkome and coworkers have described a reversible, assembly–disassembly procedure using a metallo-macrocycle containing twelve terpyridine groups enclosed within a 114-membered macrocyclic framework [51]. The self-assembly of the alkene-functionalized angular bis(terpyridine) 22 (Figure 4.12a) with FeII ions yielded the cyclic [(22)6Fe6]12 þ hexamer. The peripheral alkene groups were connected by metathesis reaction using the Grubbs’ catalyst to yield an imbedded metallocycle 23 (Figure 4.12b). Treatment with base (i.e., K2CO3 in DMF) quantitatively displaced the metal centers to afford a cyclic array of bound bis(terpyridine) ligands; with the reintroduction of FeII ions, the hexameric supramolecular motif was readily regenerated (Figure 4.12c). Further proof-of-structure included the selective reduction of the double bonds with Raney-Ni and hydrazine as well as the Pd0-catalyzed cleavage of the terpyridine moieties from the macrocycle. Though the previous example indicates the potential of a so-called “nanofabrication” process with respect to molecular imprinting [69], the metallo-macrocycles, based on octahedral bis(terpyridine) complexes, still appear to be mainly of scientific interest and applications looking beyond their fascinating structures have hardly been investigated. However, based on their electro-optical properties, the implementation of the materials in, for example, dye-sensitized solar cells (DSSCs) [50, 53, 61] or organic light-emitting diodes (OLEDs) [50, 56, 57] has been envisioned. Referring to these, a few examples shall be briefly highlighted. The fabrication of DSSCs of the general configuration ITO:TiO2/metallo-macrocycle: KI–I2 electrolyte/graphite showed moderate to good performances for different types of macrocycles; the best results were obtained for the pentameric system [(18)5Ru5](PF6)10, which showed a short circuit current (Isc) of 0.3 mA cm2, open circuit voltage (Voc) of 290 mV, fill factor (FF) of 26.3%, and photoconversion efficiency (Z) of 1.53% [50]. Furthermore, the ionic coordination of a water-soluble hexameric metallo-macrocycle containing RuII ions and oxidized single-wall carbon nanotubes (oxi-SWNT) furnished a composite with high potential for utilization in the fields of photovoltaics or electronic nanodevices (Figure 4.13a) [58]. The formation of composite materials, based on multiple ion-pair association between the positively charged metallo-macrocycle and the negatively charged oxiSWNTs, was visualized by TEM and AFM measurements (Figure 4.13b and c). A different type of composite material, containing a hexagonal RuII metallomacrocycle as well as a dendrimer, was also reported. In this case, multiple ionpairing produced nanofibers with diameters of about 550 nm and an average length of 22.5 mm (according to TEM measurements) [70].

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22 N

O

metal

base

N

O

O

C6H13CH CHC6H13

N

(b)

O

O

O

O

O

N

N

O

N

Fe2

N

N

N

N

N

N

N

N

Fe2

N

N

Fe2

N

N

N

N

N

O

O

O

O

O

N

N

N

N

N

N

Fe2

N

N

Fe2

N

N

N

N

N

N

N

Fe2

N

O

N

N

O

O

O

O

O

Figure 4.12 (a) Alkene-functionalized angular bis(terpyridine) 22; (b) the imbedded hexameric metallo-macrocycle within a macromolecular superstructure (23); and (c) the reversible assembly–disassembly process. Figure redrawn according to Reference [51].

(c)

(a)

O


| 141

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes (a)

(b)

12 PF6-

O

O ONa n

O

n

- nNaPF6

oxi-SWNT Ru (II)

(c)

Figure 4.13 (a) Multi-ion pair association of a water-soluble, metallo-macrocycle with oxidized single-wall carbon nanotubes (oxi-SWNT); (b) TEM image of the composite material on a carbon-covered metal grid (the scale bar is 20 nm); (c) AFM height image of the composite material on a mica substrate (scan area: 6.4 6.4 mm, z-scale: 0–50 nm) [58]. Figure reproduced with kind permission; r 2007 The Royal Society of Chemistry.

2.3 2.7

2.9 4.1

PEDOT

4.7 ITO

ZnMC: BCzVBi

BCP

AIq3

5.2 5.8

5.5

LiF/AI

EL intensity (A.U.)

0.10

Energy(eV)

142

0.18 0.16 0.14 0.12

6.1

0.00 400

(a)

500


800

(b) Figure 4.14 (a) Schematic diagram of the energy levels for the OLED device components [PEDOT: poly(3,4-ethylenedioxythiophene), Zn-MC: ZnII metallo-macrocycle, BCzVBi: 4,40 -[bis (9-ethyl-3-carbazovinylene)]-1,10 -biphenyl, BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, Alq3: tris(8-hydroxyquinoline)aluminum]; (b) electroluminescence spectra of the OLED device at different current densities (green: 10 mA cm2, blue: 50 mA cm2, red: 100 mA cm2) [56, 57]. Figure reproduced with kind permission; r 2006 Wiley-VCH.

A ZnII-based hexagonal metallo-macrocycle was also introduced, as emissive species, into an electroluminescent (EL) device [56]. Figure 4.14 summarizes the device configuration and the energy levels of the individual components. At a bias voltage of 6.9 V, green emission with a peak maximum at 515 nm was observed [Commission International d’Eclairage (CIE) coordinates: x ¼ 0.28, y ¼ 0.48]. The optimized device showed a maximum efficiency of 0.16 cd A1 and a turn-on voltage of about 4 V; the maximum luminance of 39 cd m2 was obtained under a driving voltage of 7.6 V. Recently, femtosecond time-resolved absorption spectroscopy was used to investigate the excited-state charge-transfer dynamics in trinuclear hexagonal metallo-macrocycles containing FeII ions [71]. It was shown that the lowest

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| 143

metal-to-ligand charge-transfer (3MLCT) state had an excited-state lifetime longer than 5 ns, indicating the possibility of light-induced excited-state spin-trapping (LIESST) – a critical characteristic with respect to potential photomagnetic applications [72]. Figure 4.15 depicts a typical Jablonski diagram, indicating the involved energy conversion steps and their timescale. Notably, shape-persistent metal-free hexagonal macrocycles have been prepared where the terpyridine units were linearly incorporated into the framework. The parent compound 18[(2,6)-pyridino-6-coronand-6] (24) or simplified “cyclosexipyridine” was reported by Newkome and Lee in 1983 (Figure 4.16) [73]. This compound might be considered as a “terpyridine dimer” where the two units were linked in a head-to-head fashion via their 6,600 -positions. The non-planarity of the six connected pyridine rings, presumably due to crowding of the adjacent lone Nelectron pairs, was concluded from computational studies: a gas-phase lowest energy D3 conformation with alternating “up” and “down” nitrogen lone electron

hv1 LC

L1

hv2 M1

e-

e-

hv1

e-

e-

~ 100 fs

1MLCT

M2

~ 9 ps 3MLCT

hv2 > 5 ns

L2

L3

GS

M3 (a)

(b)

Figure 4.15 Energy- and charge-transfer scheme (a) and excited-state energy relaxation diagram (b) of a trinuclear hexagonal metallo-macrocycle containing FeII ions [GS: ground state; hn1: excitation energy for ligand-centered (LC) p–p*-state; hn2: energy for metal-to-ligand charge-transfer (MLCT)] [71]. Figure reproduced with kind permission; r 2010 American Chemical Society.

N N N

N N

N

N

N

N N

C6H13OH2C

N

C6H13OH2C

N

N

CH2OC6H13

N

CH2OC6H13

N

N

N

sexipyridine 24

hexaazakekulene 25

N 26

Figure 4.16 Shape-persistent metal-free hexagonal macrocycles 24–26.

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144

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes pairs was determined (this structure was found to be 75.8 kJ mol1 lower in energy than a flat D6h symmetrical molecule). The same symmetry was estimated for the complexes with either NaI or KI ions or template cations, such as H3O þ or NH4þ , in which a lower C3 symmetry was predicted [74]. Moreover, symmetrically di- and trisubstituted derivatives were reported by the groups of Toner [75] and Potvin [76]. Heterokekulenes – the heteroaromatic counterparts of kekulene – are classic examples of hexameric molecules composed of fused, cyclized benzenoid rings. The D6h-symmetrical dodecahydro-19,20,21,22,23,24-hexaazakekulene (25), an ideal template for the complexation of small ions (due to the six coplanar nitrogen atoms), was synthesized by Staab et al. as a first example of a hexaazakekulene [77]. The intrinsic low solubility of such materials was overcome by Bell and Firestone, who introduced solubilizing n-butyl groups at the 4-position of alternating pyridine rings. This soluble hexaazakekulene derivative was found to be a good host for the binding of alkaline earth metal ions [78, 79]. The first ¨ter and cowextension of the basic sexipyridine structure was reported by Schlu orkers [80]. Applying preparative size exclusion chromatography (SEC), the cyclic product 26 could be separated from linear oligomeric and polymeric species. Furthermore, diverse shape-persistent macrocycles incorporating one or two terpyridine units, bridged by phenylacetylene moieties, have also been prepared [81]. Transition metal ions with d6 (e.g., FeII, RuII) or d10 (e.g., ZnII, CdII) electron configuration preferentially form complexes of octahedral geometry with two terpyridine ligands. In addition, terpyridines are also known to form stable squareplanar complexes with late d8 transition metal ions (e.g., PdII, PtII, AuIII) and a monodentate ancillary ligand; however, few examples have been incorporated into advanced macrocyclic or 3D-architectures based exclusively on square-planar terpyridine complexes. The group of Bosnich reported rectangular and trigonal-prismatic assemblies containing PdII or PtII ions; these supramolecular materials were utilized as selective molecular recognition centers for various molecular guests [82–92]. For this purpose, the molecular clefts 27–29 were utilized for the self-assembly with linear bidentate (30) and trigonal-planar tridentate ligands (31) as shown in Figure 4.17. The kinetically more labile and thermodynamically less stable PdII complexes 27 and 29 readily formed the supramolecular structure at ambient temperature within hours, whereas the PtII complex 28 required higher temperatures and longer reaction times [92]. Figure 4.18a depicts the X-ray single-crystal structure of the tetranuclear complex [(28)2Pt4(30a)2](PF6)4. The rectangular structure displayed a meso-conformation with one spacer in the (R,R)- and the other in the (S,S)-configuration. The pair of 4,40 -bipyridyl linkers (30a) were buckled, nearly perpendicular to the coplanar PtII (mono)terpyridine moieties and, thus, almost parallel to each other within the same cleft (a distance of 6.9 A ). The closest distance between two 4,40 -bipyridines was 1.9 A , allowing appropriate guests to intercalate within the resultant void region. These supramolecular self-assemblies were utilized as molecular recognition centers for various types of small planar (aromatic) molecules. Binding of such guests occurred in the molecular cavities, in particular, via p–p interactions with aromatic groups or d–d orbital interactions with guest

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N

30a

N

N

N

N

N

N

31a

N

N

30b

N

M

31b

N

N

N

M

N

L

N

L

N

2PF6

N

29

M–L

M–L

27–29

N

M

M

rectangle

R

R

2 PF6

NC-CH3

N

P d NC-CH3

N

N

M

M

(30)

1 eq. (31)

N

1 eq. R

N N Pd

R

R

M

M M

trigonal prism

M

M

M

[(x)12)

Figure 4.17 Ditopic and tritopic building blocks, utilized by Bosnich and coworkers, for the construction of rectangular and trigonal-prismatic assemblies [82–92]. Figure reproduced from [93] with kind permission; r 2005 Wiley-VCH.

N

27 (MII PdII, L CH3CN) 28 (MII PtII, L acetone)

N

N


| 145

146

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes

(a)

N(8A)

N(4A) N(3A) Pt(1A) N(2A) N(9)

N(5A)

N(1A)

N(6) Pt(2) N(5)

Pt(2A) N(6A)

N(1) Pt(1) N(3) N(4)

N(8)

(b) 0.3

0.2

∆δ(ppm)

∆δ(ppm)

0.25

0.15 0.1 0.05 0 1

2

3

N(9A)

N(2)

N(7)

0

N(7A)

4 5 6 7 [9-MA]/[Rectangle]

8

9

10

0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00

Trigonal prism: [(27)3(31a)2]6( ) [(27)3(31b)2]6( ) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 [9-MA]/[Trigonal Prisms]

Figure 4.18 (a) Representation of the X-ray single-crystal structure of the supramolecular rectangle [(27)2Pt4(30a)2]4 þ (H-atoms, counterions, and the terminal part of the cleft omitted for clarity) [92]; (b) representation of the determination of host–guest stoichiometry by 1H NMR titration experiments [83, 87]. Figure reproduced with kind permission; r 2005 WileyVCH, 2001 Thieme Publishing Group, and 2003 The Royal Society of Chemistry, respectively.

metal centers [93]. The intercalation of small molecules could be followed by UV–vis and 1H NMR titration experiments. For instance, four 9-methylanthracene (9-MA) molecules, as guests, were found to bind to the rectangle [(27)2Pt4(30a)2]4 þ [83]. With increased cavity size, additional guest molecules could be hosted; thus, the trigonal-prismatic structures [(27)3Pt3(31a)2]4 þ and [(27)3Pt3(31b)2]4 þ were able to accommodate six and seven 9-MA molecules, respectively [87]. Recently, bis(terpyridine) 13 (with R ¼ tBu) and 4,40 -bipyridine (30a) were assembled with PdII ions into a hexameric dodecanuclear metallo-macrocycle. Formation of a cyclic structure of the formula [{(13)Pd2(30a)}6](BF4)24 was confirmed by ESI-TWIM MS analysis [94]. A different type of supramolecular array was reported by Hui et al., in which an array containing six PtII ions (33) was prepared via the reaction of the face-to-face dinuclear PtII acetylene complex 32 with [(tpy)Pt(MeCN)]2 þ in a 1 : 4 ratio (Scheme 4.2) [95]. The X-ray single-crystal analysis of 32 revealed two PtII centers bridged by two diphenylphosphino ligands forming an eight-membered-ring in a face-to-face arrangement with a distance of 3.28 A , suggesting a possible Pt Pt interaction. The single-crystal structure of 33 displayed a short Pt Pt core distance (3.18 A ), which was attributed to the decreased electron density of the PtII centers in the core upon formation of four peripheral complexes with PtII (mono)terpyridine moieties. The two adjacent PtII (mono)terpyridine units remained parallel (interplanar distance of 3.67 A ),

04

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PPh2

Ph2P N

Pt

N

N

N

Pt

N PPh2

Ph2P

32

N

N Pt2 NCCH3

2 CF3SO3

33

Scheme 4.2 Synthesis of the hexanuclear PtII complex 33 [95]. Figure reproduced with kind permission; r 2002 American Chemical Society.

N N

N

N M

2

N

N [Re(CO)5]Br

N

N 2 PF6

34 (MII FeII, RuII or OsII)

Scheme 4.3 Self-assembly of bis(terpyridine) complexes 34 with [Re(CO)5]Br into a supramolecular square [96, 97]. Figure reproduced with kind permission; r 2000 American Chemical Society.

suggesting weak p–p interactions; however, the Pt Pt distances of 5.08 A between these moieties did not show any Pt Pt interaction [95]. Large molecular squares were constructed involving a two-step, self-assembly approach [96, 97]. The bis-complexes of 40 -(pyridin-4-yl)-2,20 :60 ,200 -terpyridine with FeII, RuII, or OsII ions (34) [98] were employed in a self-assembly reaction with [Pd(dppf)(H2O)2](OTf)2 [dppf ¼ 1,10 -bis(diphenylphosphino)ferrocene] or [Re(CO)5Br] (Scheme 4.3). According to molecular modeling studies, the average size of the

04

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| 147

148


supramolecular squares was about 22 22 A 2. Electrochemical studies revealed multi-electron redox processes (indicating negligible electronic communication between individual chromophores); luminescence was exclusively observed for the ReI/OsII-containing square. The emission was attributed to an OsII-based 3MLCT transition. Owing to a more efficient non-radiative decay, luminescence of the square exhibited a lower energy emission maximum (at 748 nm) with a shorter lifetime and lower quantum yield than the corresponding OsII bis(terpyridine) complex 34. 4.3 The HETTAP Concept

The directed complexation of terpyridine ligands with numerous transition metal ions (i.e., RuII, OsII, IrIII, and – to a minor extent – NiII or CoIII) into heteroleptic octahedral complexes is an established tool in metallo-supramolecular chemistry. Furthermore, in particular RuII, OsII, and IrIII ions have also been widely used to obtain heteroleptic complexes with three bidentate ligands (e.g., bipyridines or phenanthrolines). Heteroleptic complexes of tetrahedral geometry containing, for instance, CuI ions and two of these bidentate ligands are, however, difficult to prepare and, in general, the self-assembly yields a statistical mixture of both homoleptic and heteroleptic complexes. In recent years, Schmittel et al. established two general concepts for the directed self-assembly of phenanthroline-type ligands with transition metal ions into heteroleptic complexes: the HETPHEN [99] and HETTAP [100] concepts. The heteroleptic phenanthroline (HETPHEN) complexation approach was utilized for the construction of various types of supramolecular architectures (i.e., ladders, boxes, grids, racks, and baskets) (see Reference [101] and references cited therein). For this purpose, phenanthrolines with sterically demanding substituents at the 2- and 9-position (e.g., bulky aryl moieties) were designed to avoid selfassembly with many transition metal ions (e.g., CuI, AgI, and ZnII). Thus, in combination with non-sterically hindered phenanthrolines, merely heteroleptic bis (phenanthroline) complexes were formed. To extend this approach, terpyridine ligands (not possessing substantial steric demand) were applied to the formation of trigonal bipyramidal complexes in combination with a bulky phenanthroline ligand (HETTAP: heteroleptic terpyridine and phenanthroline complexation) [100]. According to this concept, nanoscaled ladders (L) were obtained by the assembly of bis(terpyridine) 35 with bis(phenanthroline)s 36a,b in the presence of CuI, ZnII, or HgII ions (Figure 4.19a/b) [100]. Formation of supramolecular architectures was confirmed by NMR, ESI MS, and X-ray single-crystal analysis (Figure 4.19c); moreover utilizing analytical ultracentrifugation (AUC), single species with a molar mass of 5020 7 500 g mol1 (for the nanoladder [(36a)2Zn2(35)2](CF3SO3)8, L1) were observed in velocity sedimentation experiments. The dynamic nature of assembly was proven by metal- and ligand-exchange experiments, since due to their higher binding constants in these heteroleptic complexes the CuI ions of [(36a)2Cu2(35)2](CF3SO3)8 (log K ¼ 4.3) could be exchanged by ZnII (log K ¼ 6.5) as well as HgII ions (log K ¼ 8.0). This process was easily monitored by changes in fluorescence (only the ZnII-containing ladder was emissive). Moreover, mixing of

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4.3 The HETTAP Concept

| 149

(a) N

(b)

N

N

8+ 2x

N

4x Zn(II)

2x 35

36 N

L1 or L2

N

35

L1: [(36a)2Zn4(35)2](OTf)8 L2: [(36b)2Zn4(35)2](OTf)8 (c) N

N

N R R

X

N

R X

R

3.4 nm

36a (R CH3, X Br) 36b (R H, X OH) 36c (R H, X OC6H12Fc with Fc = ferrocene) 36d (R H, X CH3)

2.7 nm

L1

Figure 4.19 (a) Ditopic ligands 35 and 36 utilized for HETTAP complexation; (b) synthesis of nanoladders L1 and L2; (c) stick (left) and space-filling representation (right) of the X-ray single-crystal structure of L1 (hydrogen atoms, counterions, and solvent molecules omitted for clarity) [100]. Figure reproduced with kind permission; r 2005 American Chemical Society.

L1 and L2 in solution instantaneously gave a statistical mixture of L1, L2, and the mixed-ligand species [(36a)(36b)Zn2(35)2](CF3SO3)8 (L3). Similar nanoladders, where the ferrocenyl moieties were attached via a short flexible linker to the bis (phenanthroline) ligands (36c, see Figure 4.19a), were also reported [102]. The ferrocene units were located inside the voids of L, which are in close proximity to each other, thus influencing the redox behavior of the FeII centers (revealed by a pronounced anodic shift of the redox potentials). Clip-shaped aggregates were obtained by assembling the flexible bis(phenanthroline) 37 with a rigid bis(terpyridine) (35 or 38) (Figure 4.20a and b) [103]. The formation of molecular clips was concluded from NMR and ESI MS. The ability of these clips to serve as host for other metal ions (see Figure 4.20b for a molecular model of the host–guest structure) was investigated by UV–vis absorption and emission spectroscopy. The addition of main group metal ions (i.e., NaI, KI, CaII, BaII, PbII) did not induce any structural change in the clip architecture, whereas HgII ions slowly replaced the ZnII ions in [(37)Zn2(35)]4 þ to yield an isostructural HgII-clip (due to the much more locked and strained architecture, this exchange was not observed for [(37)Zn2(38)]4 þ ). Cation exchange was accompanied by a full quenching of the emission due to electron transfer from the chromophore to the HgII centers; in contrast, the divalent metal ions (e.g., PbII) enhanced the emission by almost 50% (Figure 4.20c), giving evidence for the formation of the anticipated host–guest structures [103]. The self-assembly of C3v-symmetric tris(phenanthroline) 39 with bis(terpyridine)s 35, 38, or 40 and CuI ions yielded the nanoprisms NP1–NP3 (Figures 4.21 and

04

27 J l 2011 15 46 0

04

27 J l 2011 15 46 1

4

N

[(37)Zn2(35)]

N

37

N

N

N

O

Clips

+ ZnII, Mn

37 and 35 or 38

O

O

4

[(37)Zn2(38)]

38

O

O O

N

N

N

O

(c)

0 300

100

200

300

400

500

O

N

400

N

Wavelength/nm

500

600

[(37)Zn2(35)]4+ + HgII

[(37)Zn2(35)]4+

[(37)Zn2(35)]4+ + PbII

Figure 4.20 (a) Ditopic bis(phenanthroline) and bis(terpyridine) building blocks 37 and 38; (b) molecular structure of the clips as well as of host–guest complexes (HyperChemTM was utilized to generate the model); (c) emission spectra of the clip [(37)Zn2(35)]4 þ in the presence of other metal ions (i.e., PbII and HgII) [103]. Figure reproduced with kind permission; r 2006 American Chemical Society.

(b)

(a)

Emission intensity (a.u.)

150



N

Br

N N

N N

N

N Zn

N

N

N

39

N N

40

N Br

| 151

N

Br

N

N

N

N

N

N

N

N

41

N 43

N

N N

N

42

N N

N

Figure 4.21 Di- and tritopic building blocks 39–43 for HETTAP self-assembly into nanoprismatic structures [104].

4.22a) [104]. In contrast to the stable ladder- or clip-structures, the nanoprisms were in equilibrium with various (undefined) oligomeric species. However, for NP3 containing the ZnII-porphyrin-bis(terpyridine) 40, structural stabilization could be achieved by utilizing templates of suitable size, such as tris(pyridine) 41 or C60 (see Figure 4.22c for a molecular model of complex NP3 41). Strong interaction between the tris(pyridine) template and porphyrin units of NP3 provided an additional driving force that shifted the equilibrium completely towards the desired structures; for steric reasons, tris(terpyridine) 42 could not be placed into a nanoprism. Owing to constriction of the prismatic structure, the C60 template was strongly bound to all three porphyrin moieties, as confirmed by ESI MS, cyclic voltammetry (CV), and differential pulse voltammetry (DPV, Figure 4.22d) [104]. Similar hollow and filled nanoprisms were also constructed from tris(terpyridine) 43 with bis(phenanthroline)s 36b or 36c, respectively, and ZnII ions [105]. Moreover, fabrication of heterometallic triangles and trapezoids via self-sorting processes was investigated recently [106–108]. Besides the step-wise construction of isosceles triangles (containing CuI as well as ZnII ions) [106], the selective onepot self-assembly into triangles and trapezoids reveals the power of the HETPHEN and HETTAP concepts. Optimization of the interplay of steric and electronic effects, p–p interactions, as well as metal–ligand specificities paved the way to the

04

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N

152

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes (c)

(a) 2(39) + 3 (35, 38 or 40) + 6 CuI

NP1-NP3 (b)

1.35 nm N

0.5 6n m

N

0.2

Zn Zn m 2n

0.3 5n m

0.2

Zn m 8n

0.63 nm

0.78 nm

n Zn

Zn

41

C60 12

(d)

10 8 I/107A

Zn

1.09 nm

reduction waves of C60

6 4 4 0

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 -0.0 -0.5 -1.0 -1.5 E/V

Figure 4.22 (a) HETTAP self-assembly into nanoprisms NP1–NP3; (b) idealized Zn–Zn distances in NP3 with 41 (left) or C60 as template (right); (c) representation of a molecular model (HyperChemTM was utilized to generate the model) of the [NP3 42] complex (atoms/ ligand are colored in cyan (C), blue (N), yellow (Br), red (Cu), white (Zn), and green (42) ligand; hydrogen atoms omitted for clarity); (d) differential pulse voltammogram of NP3 (black) and [NP3 C60] (red) measured in MeCN–THF (1 : 1 ratio) using ferrocene as internal standard [104]. Figure reproduced with kind permission; r 2008 American Chemical Society.

synthesis of a dynamic trapezoid that contained two different hetero-ditopic and a homo-ditopic ligands (44–46, Figure 4.23a/b) in combination with two types of transition metal ions (i.e., CuI and ZnII) [107]. In addition, the self-assembly of three appropriate hetero-ditopic ligands with CuI and ZnII ions gave a fivecomponent triangle [108]. The structure of both architectures (see Figure 4.23c for a molecular trapezoid model) could be confirmed by 1H NMR, ESI MS, and DPV. A “spoked wheel”-type architecture was obtained from the D6h-symmetric hexakis (terpyridine) 47 and bis(phenanthroline) 36d (Figure 4.24) [3]. For this purpose, 47, utilized as the molecular spoke set, and three equivalents of 36d, as the rim elements, were self-assembled with CuI ions, following the HETTAP concept (see Reference [109] for further approaches towards this type of technomimetic construction). Formation of the “spoke set with rim elements” assembly could be confirmed by NMR and UV–vis absorption spectroscopy as well as ESI MS.

04

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N

(a) N N

N

44

N OR'

R

N

N

R

R

N

N

R

R

R

R'O R

R 45 (R OMe, R' C10H21)

N

46

N

N

Br

N

Br

(b)

ZnII CuI

Isosceles Trapezold

dzn-Cu 1.6 nm

(c)

dZn-Zn 1.7 nm

dCu-Cu 1.3 nm

dzn-Cu 1.6 nm

Figure 4.23 (a) The (hetero-)ditopic ligands 44–46; (b) self-assembly of 44–46 with CuI and ZnII ions into the isosceles trapezoid; (c) representation of a molecular model (HyperChemTM was utilized to generate the model) of the trapezoid [107]. Figure reproduced with kind permission; r 2009 American Chemical Society.

04

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| 153

154


(a)

spoke set (b)

spoke set with rim elements (c)

R 47

R

R

R

R R

[(47)Cu6(36d)3](PF6)6

spoked wheel

CuI N

47 + 36d

R= N N

Diameter ~ 3.9 nm

Perimeter ~ 12.3 nm

Figure 4.24 (a) Design of a technomimetic spoked wheel; (b) hexakis(terpyridine) 47; (c) self-assembly of 47 and 36d with CuI ions into a “spoke set with rim elements” architecture; a molecular model of the structure is also shown (HyperChemTM was utilized to generate the model) [3]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

According to computational studies, the diameter and perimeter of the structure were 3.9 and 12.3 nm, respectively; however, the final synthetic step, that is, macrocyclization of the coordinated rim elements into a full rim (e.g., by either oxidative coupling of alkyne or metathesis of alkene moieties), has not yet been achieved [3]. 4.4 Racks and Grids

The formation of metallo-supramolecular grid-like architectures relies on directed coordination, based on both the coordination geometry of transition metal ions as well as the structure of the ligand’s binding sites. In general, a perpendicular arrangement of the ligand planes at each metal center is a prerequisite. If such a coordination environment around a metal center is given, linear and rigid extension of the ligand system – from mono- to multi-topicity – will directly give a grid-like two-dimensional coordination network with regularly arrayed metal ions [1, 110– 112]. According to this general concept, such supramolecular arrays can be prepared by a careful pre-arrangement of the subunits utilizing any combination of metal ions and organic ligands possessing compatible coordination features. These requirements are met by ligands containing either bidentate or tridentate binding sites in combination with transition metal ions, allowing tetrahedral or octahedral (and in some cases bipyramidal) coordination geometry, respectively (Figure 4.25a) [110]. Two-dimensional, grid-like coordination arrays are characterized by a highly ordered 2D-arrangement of an exact number of metal ions. Various types of square [n n] and rectangular [n m] grids (with n, m r 4) have been reported for different transition metal ions (with tetrahedral octahedral or square-bipyramidal coordination geometry). Therefore, ligands with n coordination sites are capable of

04

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4.4 Racks and Grids

(a) (i)

(b) (ii) [2 2]

[3 3]

[4 4]

Tetrahedral coordination geometry [2 3]

Octahedral coordination geometry

[2 [42]]

[4 [22]]

Figure 4.25 (a) Perpendicular arrangement of a ligand L around a metal center M: (i) [ML2] units are formed with monotopic ligands (n ¼ 1); (ii) grid-like arrays [M4L4] are formed with multitopic ligands (here n ¼ 2). (b) The different types of grid-type architectures: squares [n n], rectangles [n m], and incomplete architectures [p [n m]] [110]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

forming homoleptic [n n] metallo-supramolecular grids, composed of 2n organic ligands and n2 transition metal ions in an overall stoichiometry of [(Mn)nL2n] [110, 113]. Moreover, mixtures of different ligands with unequal numbers of coordination sites n and m will – in addition to square arrays – yield rectangular structures [Mn mLn þ m] with a total of (n þ m) organic ligands and (n m) transition metal ions. For incomplete grid-like coordination compounds, the available coordination sites are not occupied quantitatively, thus generating several (p) subsets of [n n] or [n m] arrays within the same coordination compound (Figure 4.25b) [110]. In all cases, chelating ligands are applied, as building blocks, since their increased pre-organization and enhanced metal-binding affinity result in cooperative effects during the self-assembly process. Furthermore, most ligands feature rigid aromatic ring systems (i.e., mostly pyridine moieties), which yield kinetically labile intermediates as well as thermodynamically stable products with many MI, MII, and (very few) MIII ions. The rigidity of these aromatic ring systems and their ability to participate in p–p interactions are further stabilizing factors for the formation of grids. Though, energetically, formation of an ordered grid-type motif always competes with the generation of other types of structures (e.g., coordination oligomers and polymers), the defined grid-like array is favored enthalpically (maximizing the coordination site occupancy) [114] as well as entropically (giving rise to the largest number of discrete entities). Out of the diversity of ligands utilized for the construction of such grid-like assemblies, only terpyridine-type derivatives (see Figure 4.26 for some representative examples) will be discussed in detail within the scope of this section. In particular, appropriately substituted pyrazine and pyrimidine derivatives 48–50 (so-called “fused terpyridines”) are often used as building blocks and enable the formation of both grid- and rack-type architectures. In the absence of metal ions, the ligands adopt a transoid conformation of the pyridine N-atoms relative to the C–C single bond linking the pyridine rings. Conversion of the all-trans-conformer into the energetically disfavored all-cisoid

04

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| 155

156


N 48

R2

R2

N

n N

N N

N

N

N R3

N

N

N R3

1

N

N 49

N N

N

N

R 50a (n 1; R1,R1,R1 H) 50b (n 1; R1 9-anthryl; R2,R3=H) 50c (n 2; R1, R2,R3 H) 50d (n 3; R1,R2,R3 H) 50e (n 1; R1,R2 H; R3 4-MeO-Ph) 50f (n 1; R1 Ph; R2,R3 H) 50g (n 1; R1,R3 H; R2 COOMe) 50h (n 1; R1 CH3; R2,R3 H) 50i (n 1; R1 4-Me2N-Ph; R2,R3 H) 50j (n 1; R1 Ph,R2 H; R3 CH3) 50k (n 1; R1,R3 H; R2 SCH2CH2CH3) 50l (n 1; R3 CH3; R1,R2 H)

Figure 4.26 Terpyridine-type ligands 48–50 that are commonly employed in the self-assembly of grid-like architectures.

counterpart in the presence of transition metal ions occurs at the cost of considerable conformational energy, which has to be overcompensated by the interaction energy resulting from metal ion coordination. For instance, the rotation from an all-transoid to all-cisoid form of 50a costs about 100 kJ mol1 per ligand (i.e., 400 kJ mol1 in total for the formation of the [2 2] grid complex), and energy costs escalate to the range of 1500 kJ mol1 when self-assembling 50d with PbII ions into a [4 4] grid [110]. The selective preparation of rack-type architectures – often referred to as “halfgrids” in the literature – requires either the incomplete formation of a grid (by using an imbalanced stoichiometry) or the directed self-assembly with, for example, RuII ions (allowing the formation of heteroleptic complexes) or ZnII ions (by applying the HETPHEN concept [100, 101]). Following the latter, Lehn and coworkers reported rack-type architectures based on ligands 50a–d (for the synthesis of ligands 50a–d see Reference [115]). The ligands were synthesized by Stille cross-coupling reaction of 6-stannylated bipyridine with 4,6-dichloropyrimidines and subsequently coordinated to [Ru(tpy)Cl3] under reductive conditions, yielding the di- and trinuclear racks [(50a/b)Ru2(tpy)2](PF6)4 (Figure 4.27a) and [(50c)Ru3(tpy)3](PF6)6, respectively [116]. X-Ray single-crystal analysis revealed that the ligand axis was bent due to the distorted octahedral geometry for each complex center. However, the bend was reduced in the case of the anthracene-containing rack [(50b)Ru2(tpy)2]4 þ due to steric repulsion between the central aromatic system and the juxtaposed terpyridine units; the anthryl group was sandwiched between the neighboring terpyridines (Figure 4.27b). The unusual green color of the complexes was rationalized by the nature of the bridging ligands and metal–metal interactions. UV–vis absorption spectroscopy

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4.4 Racks and Grids

(a)

(b) N N

N6

N

Ru2 N

N

N4 N3

N 2

Ru N

Ru2

[(50a)Ru2(tpy)2](PF6)4 [(50b)Ru2(tpy)2](PF6)4

R1

N

N5

N

N

N

Ru1

N

N2 N1

[(50b)Ru2(tpy)2]4

Figure 4.27 (a) Dinuclear racks obtained from ligands 50a/b; (b) ball–stick (left) and spacefilling (right) representation of the solid-state structure [(50b)Ru2(tpy)2]4 þ (H-atoms and counterions omitted) [116]. Figure reproduced with kind permission; r 1995 Wiley-VCH.

exhibited three MLCT absorption bands due to splitting of the p* level. Different substituents R1 (e.g., Me or Ph) were introduced to induce different bending angles for the bridging ligand and convergence angles of the coordinated terpyridine moieties; a simple Me-group was found to be the ideal option to align the geometry [117]. Photophysical investigations revealed an emission originating from the anthracene-containing rack [(50b)Ru2(tpy)2](PF6)4 in the infrared region [118]. Comparison of the emission spectra and lifetimes with those obtained for the individual subunits showed that, in the metallo-supramolecular racks, excitation energy flows with unitary efficiency to the lowest excited state, regardless of which chromophoric subunit was excited. Luminescence could be observed at room temperature as well as at 77 K in a frozen matrix. In the trinuclear rack [(50c)Ru3(tpy)3](PF6)6, energy-transfer from the central complex to the peripheral ones could also be observed [119]. More recently, multinuclear rack-type architectures in which up to six RuII (mono)terpyridine units were coordinated to the extended hydrazone-based ligands 51–54 (Figure 4.28a) were also prepared and the electrochemical properties determined by DPV [120, 121]. All racks exhibited multiple oxidation processes and the first two peaks were assigned to a split oxidation of the two peripheral metal centers (Figure 4.28b). This splitting of the oxidation peak allowed the estimation of the attenuation term b, as a measurable parameter for electronic interaction within a material. Though the estimated b-value for the series was lower (0.23 A 1) than for oligophenylene derivatives (b ¼ 0.5 A 1) [122] or metallo supramolecular RuII/OsII arrays (b ¼ 0.32 A 1) [123], a “molecular wire”-type behavior could be observed (see Reference [124] for a recent review on this topic). Similar to the early contributions by Lehn and coworkers, the group of Siegel utilized 50d for the complexation with various RuII (mono)terpyridine complexes to synthesize the so-called “Borromean rings” [125]. Furthermore, the more rigid phenanthroline-type ligands 55–57 were applied by Brown et al. for the formation of

04

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| 157

158


(a)

(b)

N

N

N N

N N

N N

N

N N

N

N N

N

N

N N

N

N

N

N

N

N

N

N

N

N

N

N

N

N n

51

[(51)Ru2(tpy)2]4+ N

N

N

∗

∗

N

∗

52

∗

53 (n 1) 54 (n 2)

[(52)Ru3(tpy)3]6+

∗ ∗

[(53)Ru4(tpy)4]8+ 0.90

1.35 E (V)

1.80

Figure 4.28 (a) Multitopic hydrazone-based tridentate ligands 51–54; (b) differential pulse voltammograms (DPVs) of the RuII racks (asterisks indicate the splitting of the oxidation peak of the peripheral RuII centers) [120]. Figure reproduced with kind permission; r 2007 Wiley-VCH.

racks with [Ru(tpy)Cl3] (Figure 4.29a) [126]. X-Ray single-crystal analysis as well as UV–vis absorption spectroscopy revealed no significant difference in either structure or photophysical behavior (Figure 4.29b). Recently, Constable et al. showed that CuII ions can also be utilized in the formation of metallo-supramolecular racks [127]. Since CuII ions prefer a trigonal-bipyramidal coordination geometry, the assembly of 3,6-di (pyridin-2-yl)pyridazine with 2,20 :60 ,200 -terpyridine in the presence of these ions (under microwave irradiation) selectively gave the dinuclear rack; notably, neither the CuII bis(terpyridine) complex nor a [2 2] CuII-grid were observed. In 1992, Youinou et al. reported the first example of a [2 2] grid based on four ditopic 3,6-bis(pyridin-2-yl)pyridazine ligands and four CuI ions [128]. In the following, this approach was extended, in particular by Lehn et al., towards [2 2] grids based on ditopic tridentate (terpyridine-type) ligands 50 and late first- and second-row transition metal ions (e.g., MnII, FeII, CoII, NiII, CuII, ZnII, CdII) as well as some main group metal ions (e.g., PdII) (Figure 4.30) [129–132]. A broad range of (transition) metal ions that could be utilized as lattice points in the grid-type architectures gave rise to a wide variety of optical, electrochemical, photophysical, and magnetic properties. Moreover, structural modifications of 50 (i.e., using different substituents R1, R2, and R3) enabled further fine-tuning of the

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4.4 Racks and Grids

(a)

(b) N

N N

55

N

C2

N25N19

N N

C18

N31

N

N1

Ru N

56

N15 C16

N N

N

C3 C4

C14 N12

C13

C11

C17

C10

N

C9

C5 C6

C8

C7

]4

[(57)Ru2(tpy)2

57

N

N

N

N

N

N

Figure 4.29 (a) Phenanthroline-based ditopic tridentate ligands 55–57; (b) ball–stick representation of the X-ray single-crystal structure of the rack [(57)Ru2(tpy)2]4 þ (H-atoms omitted for clarity) [126]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

properties. In this respect, the special redox behavior of the grids [(50)4Co4](BF4)8 is noteworthy [133, 134]. CV measurements of the grid containing ligands 50a/f revealed nine well-resolved single-electron reduction steps (DMF, as solvent, room temperature), utilizing 50g (bearing electron-withdrawing ester moieties) increased the number of reversible reduction steps up to 12 (Figure 4.31a). Similar grids containing FeII, RuII, OsII, ZnII, or MnII ions did not show this behavior; these materials were more sensitive to decomposition and exhibited complex CV spectra due to irreversible reduction processes. Spectroelectrochemical experiments showed that reduction exclusively took place on the coordinated organic ligands, independent of the metal ions involved (i.e., reduction of CoII, ZnII, or FeII centers could not be detected within the accessible potential range) [see Figure 4.31b for [(50g)4Co4](BF4)8]. The high regularity in the disposition of the reduction levels for the CoII-containing grids indicated the ability of the CoII centers to allow electronic interactions between the reduced ligands and points to possible applications of the arrays as multilevel supramolecular electronic devices. X-Ray single-crystal structures have shown that anions might be located inside cage-like architectures in the solid-state; however, this is by far not a requirement for the stability of [2 2] grids (for instance, see Figure 4.30b). In the case of the [(50a)4Pb4](OTf)8 array, a direct coordinative role of the anions was observed [130] in which one triflate anion was directly coordinated to each PbII center to fulfill the coordinative requirements of the large PbII ions to favor square-antiprismatic coordination geometry (Figure 4.30c). Though the average Pb–Pb distance was roughly the same as observed for other [2 2] grids, the PbII–N distances were – as

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| 159

160

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes (a) R2

R2

MI N

N

N N

R3

50

[2 2] grid

N

N

R3

R

(c)

(b)

PbII

Trif late anion [(50f )4Zn4]8+

Figure 4.30 (a) Self-assembly of 50 with divalent metal ions MII into a [2 2] grid; (b) ball– stick representation (side view) of the X-ray single-crystal structures of the [2 2] grid of ligand 50f with ZnII ions (H-atoms and counterions omitted for clarity) [129]; (c) ball–stick representation (top view) of the [2 2] grid of ligand 50a with PbII ions; one triflate anion is coordinated to each PbII center (H-atoms and the other counterions omitted for clarity) [130]. Figure reproduced with kind permission; r 1999 Wiley-VCH.

(a)

(b)

0

0

10 20 30

5

0.6 Absorption (a.u.)

I (µm)

10

Deconvolution (a.u.)

5

0.4

0.2 0

0.0 2.5

2.0

1.5

1.0 E (V)

0.5

0.0

400

600

4 3 2

7 6 5

800

λ (nm)

Figure 4.31 (a) CV spectrum (bold line) and the semi-derivative deconvolution (thin line) of [(50g)4Co4]8 þ [DMF containing 0.1 M (n-Bu4N)(PF6) as solvent, room temperature, potentials given vs. ferrocene]; (b) time-resolved UV–vis absorption spectra during the step-wise reduction of [(50f)4Co4]8 þ [DMF containing 0.1 M (n-Bu4N)(PF6) as solvent; the first six reduction steps are shown] [133]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

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1000

4.4 Racks and Grids

a result of the different coordination behavior – significantly longer (PbII–Npyridine: 2.58 A , PbII–Npyrimidine: 2.77 A ) than for the related grid containing CoII ions (CoII–Npyridine: 2.10 A , CoII–Npyrimidine: 2.23 A [135]). The application of metallo-supramolecular materials with slowly relaxing magnetic moments, as potential magnetic information storage devices, has been envisioned. These so-called “single-molecule magnets” (SMMs) combine both macroscopic (e.g., magnetism) and quantum regime properties (e.g., quantum tunneling) within monodisperse molecular entities [136]. The first (and possibly still most prominent) example exhibiting SMM-like behavior was a spin-cluster referred to as “Mn12” in the literature [137, 138]; later, other structures such as “Fe8,” “Fe19,” or “Mn4” were reported [139–141]. Targeting this application, the spin-bearing [2 2] grid [(50h)4Co4]8 þ was investigated, as a model system, to gain insight into magnetic interactions in discrete metallo-supramolecular entities [142, 143]. However, only weak antiferromagnetic intramolecular exchange coupling could be observed by studying the magnetic momentum in the solid state (microcrystals and powder) as well as in solution (the same also stood for [(50h)4M4]8 þ with MII ¼ MnII, NiII, or CuII). This behavior was rationalized by the nature of ligand 50, in which the pyrimidine unit was able to mediate antiferromagnetic but not ferromagnetic exchange coupling. The weak magnitude of the coupling parameter J (for neighboring metal centers) could be attributed to the long M M distances of around 6.5 A [110]. Moreover, the spin-crossover (SC) phenomenon between the low-spin (LS) and high-spin (HS) states of FeII ions represents one further possibility to enable molecular memory effects. In general, such SC systems possess a unique simultaneity of possible “write” (e.g., temperature, pressure, light) and “read” (e.g., magnetic, optical) parameters [144]. SC was also observed for [2 2] grids containing FeII ions: the internal spin-states of the FeII ions in [(50f)4Fe4](ClO4)8 could be switched between the diamagnetic LS and paramagnetic HS state by applying external stimuli (e.g., temperature, pressure, light) on the macroscopic samples (Figure 4.32a) [145]. The substituent R1 on the pyrimidine ring of 50 strongly influenced the SC properties of [(50)4Fe4]8 þ ; thus substituents R1 favoring a relatively strong ligand field (e.g., R1 ¼ H or OH) did not enable SC from the diamagnetic LS; however, grids bearing substituents R1 that influenced the ligand field by steric and/or electronic effects (e.g., R1 ¼ Me or Ph) exhibit a temperature triggered SC [146, 147]. The thermomagnetic switching could be investigated in solution (by 1H NMR and UV–vis spectroscopy) and in the solid state (by X-ray ¨ssbauer crystal structure analysis, magnetic susceptibility measurements, and Mo spectroscopy) (Figure 4.32b). For all magnetically active FeII arrays, gradual and incomplete SC without hysteresis was a typical feature. Intramolecular cooperativity between the four FeII centers was observed and the intermolecular interaction between the [2 2] grids could be improved by linking individual grids via hydrogen bonding, thus increasing the HS fraction over the whole temperature range [147]. Introduction of substituents in the 5-position of the peripheral pyridine rings of 50 (i.e., R3 6¼ H) was investigated by Schubert and coworkers [148]. The grid

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| 161

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes (b)

(a)

[(50h)4Fe4]8 [(50f)4Fe4]8

ll

4 Fe (HS)

2.5 χmT/4 / cm3mol1K

162

[(50i)4Fe4]8

2.0 1.5 1.0 0.5

T 9

0.0 0

50

100

150 T/K

200

250

300

4 Fell (LS)

Figure 4.32 (a) The (temperature-induced) gradual switching between the spin states in [(50)4Fe4]8 þ ; (b) magnetic susceptibility curves of grids with ligands 50f/h/i [146]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

R

R

R

S

Figure 4.33 Formation of a chiral heterobimetallic [2 2] grid by self-assembly of two cornertype homochiral precursors (R þ R) or (S þ S) [150].

[(50j)4Co4](PF6)8 could be detected in an unfragmentated form by MALDI-TOF MS [149]. Apart from the homometallic [2 2] grids, which were obtained by straightforward self-assembly of stoichiometric amounts of ditopic tridentate ligands with transition metal ions, heterometallic [2 2] grid structures were also reported [150, 151]. The controlled introduction of two different types of transition metal ions requires a step-wise protocol involving stereochemical features: for instance, the homochiral assembly of the (R) or (S) corner-type intermediates (Figure 4.33). This self-assembly can be considered as a “Coupe de Roi”-process, where an achiral object is divided into two identical homochiral components [150]. A [2 2] grid containing two different metal ions might exist in the anti- or syntopoisomer. In general, one cannot predict which form will be generated by simply mixing together the components (e.g., ligands 50, MII, and M0 II in a 2 : 1 : 1 ratio).

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4.4 Racks and Grids

| 163

However, the anti-topoisomer (i.e., with diagonally located identical MII ions) can be obtained selectively from kinetically inert, homochiral precursor complexes (e.g., [(50a)2M]2 þ with MII ¼ RuII or OsII) [150]. These precursor complexes contain a vacant coordination site, which was then combined with a second type of metal ions (typically of lower binding strength) to yield the heterometallic [2 2] grids, as the thermodynamic products. Interestingly, the combination of homochiral fragments always leads to a C2-symmetric array in a process featuring spontaneous chiral discrimination (Figure 4.33). Besides the homometallic RuII and OsII species, a library of symmetric heterobimetallic [2 2] grids [(50k)4M2M0 2](PF6)8 was obtained (MII ¼ RuII or OsII; M0 II ¼ FeII, CoII, or NiII). The photophysical and electrochemical properties were investigated; for all metallo-supramolecular grids, a complex redox behavior was found in CV and DPV measurements (see Figure 4.34 for the CV spectra of [(50k)4Ru2Fe2](PF6)8); the single oxidation and reduction processes could be assigned to specific grid positions. Moreover, a grid containing three different transition metal ions could be obtained by self-assembling an equimolar mixture of [(50k)2Ru]2 þ and [(50k)2Os]2 þ with FeII ions; however, a full separation of the heterotrimetallic [(50k)4RuOsFe2] grid from the heterobimetallic species was not achieved [150]. As noted above, heterobimetallic [2 2] grids of C2-symmetry are chiral, but due to a lack of asymmetric discrimination both enantiomers are formed (the expected 1 : 1 ratio could be determined by X-ray crystal analysis) [145]. The ditopic tridentate ligand 48 was utilized by Bark et al. for the self-assembly with ZnII ions into a [2 2] grid of D4-symmetry (the antiparallel orientation of the two bipyridine units attached at the 2,5-position forces the ligand to coordinate one metal ion from above and the second from below the plane) [152]. Thus, in contrast to the previous examples by Lehn and coworkers, the supramolecular grid-type array of 48 was molecularly chiral despite being a racemic mixture. Consequently, the enantiomerically pure pinene-derived ligand 49 yielded [(49)4Zn4]4 þ in high diastereomeric purity (as concluded from 1H NMR and X-ray single-crystal structure

(a)

(b)

I/A Fc

1

2

3

4

5

6

c

7

(c) 1

5

a

I/A

a

c

4

7

b

Fc

3

0.80 0.40 0.00 0.40 0.80 1.20 1.60 E/V

b

6

2

c

1.80

1.40

1.00

Figure 4.34 Electrochemical redox properties of the grid [(50k)Ru2Fe2]8 þ (CV curves were measured in MeCN, potentials are given versus SCE, ferrocene was used as internal reference). The reduction (a) and oxidation processes (c) can be assigned to specific sites of the supramolecular array (b) [151]. Figure reproduced with kind permission; r 2003 WileyVCH.

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0.60

0.20

E/V

164

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes analysis). This was the first example of a stereoselective synthesis of a metallosupramolecular grid (later, the same group reported further chiral ditopic tridentate ligands derived from myrtenal, pinocarvone, and car-3(10)-en-2-one [153]). Architectures of higher order were obtained when hydrogen-bonding donorand/or acceptor-sites were incorporated into the ligand structure; monotopic (58 and 59) as well as ditopic (60 and 61) ligands can be utilized. Owing to the presence of directing self-complementary hydrogen-bonding arrays, the [2 2] grids assembled into chessboard-type “grid of grids” arrangements (Figure 4.35). For instance, the self-complementary aminopyrimidine units of monotopic ligand 58 formed an infinite 2D grid-like structure of {[Co(58)2](PF6)2}n in the solid state [154]. Double hydrogen-bonds between the peripheral amino groups and non-coordinating pyrimidine N-atoms were observed (NH H distances of 2.918 –3.018 A ). The overall structure was of a sinusoidal arrangement of the complexes,

(a) H2N

H N

N N

N

N

58

H2N

H N

N H

N

N

N

N

N H

N N

N H

N

H

60

NH2

N N

N

N

N

N

N

C5H11

N

N N

H2N

59

N N

H N H

H N H

N N

N N

N

N N H

H H2N

N N

N

N N

61

C5H11 [2 2] grid with periphal H-donor groups

(b) 4 4

4 4

[2 2] grid with periphal H-acceptor groups

Infinite 2D “grid of grids“ array

Figure 4.35 (a) Mono- and ditopic ligands modified with hydrogen-bonding units; (b) twostep hierarchical self-assembly into an infinite 2D “grid of grids” pattern [110]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

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NH2

4.4 Racks and Grids

which are interwoven into a 2D network (Figure 4.36a). Changing the counterions from PF6 to BF4 lead to a partial break-up of the second-order grid leaving about one quarter of the H-bonding sites unsaturated (Figure 4.36b). Only half of the hydrogen-bonds were established for [Zn(59)2](PF6)2, mainly due to crystal packing effects caused by the triflate anions and lateral alkyl chains; only one of two orthogonal ligands had H-bonds to the neighboring ligand and, thus, a linear supramolecular polymer rather than a “grid of grids” was obtained (Figure 4.36c). Extension of this methodology towards [2 2] grids utilizing ligands 60 and 61 gave 2D-arrays exhibiting dual levels of supramolecular organization. {[Co4(61)4](BF4)8}n was investigated by X-ray single-crystal analysis [155]. The formation of “grid of grids” architectures could not be observed; hydrogen-bonding was only directed in one dimension, leading to the formation of infinite 1D-chains of grids. Neighboring complexes without H-bonding interactions showed weak ligand p-stacking. Thus, {[Co4(60)4](BF4)8}n generated only a partially hydrogen-bonded network due to the strong competition between H-bonding and p–p stacking interactions. A different type of supramolecular organization of higher order was shown by Ruben et al. in which pyridine-substituted ligands 62 and 63 were self-assembled with FeII ions into the corresponding [2 2] grids (Figure 4.37) showing the typical SC behavior as discussed above (see also Figure 4.32) [156]. A gradual increase of the molar susceptibility (wM) and absence of any hysteresis in the (wMT)/4 vs. T plot indicated weak cooperative interactions during the SC process. At room temperature, (wMT)/4 values of 3.2 cm3 K mol1 for [(62)4Fe4](BF4)8 and 1.2 cm3 K mol1 for [(63)4Fe4](BF4)8 were observed, corresponding to four and two highspin FeII ions per grid, respectively. In a second step, LaIII and AgI ions were selfassembled with the [2 2] grids via their vacant pyridine moieties. The LaIII ions formed a columnar 1D-superstructure with [(62)4Fe4](BF4)8 and the AgI gave a wall-like 2D-layer with [(63)4Fe4](BF4)8 (Figure 4.37). The magnetic properties of the superstructures differed from the individual grids: in the 1D-columns of {{[Fe4(62)4]Ln}(ClO4)11}n, the LS state was stabilized (only incomplete SC at room temperature), whereas, due to enhanced steric hindrance within the 2D-network {{[Fe4(63)4]Ag4}(BF4)12}n, the spin-distribution of [(63)4Fe4](BF4)8 was retained (i.e., two FeII ions in the HS state) almost independent of temperature [156].

{[Co(58)2](PF6)2}n

{[Co(58)2](BF4)2}n

{[Zn(59)2](OTf)2}n

(a)

(b)

(c)

Figure 4.36 Space-filling representation of the X-ray single-crystal structures of (a) {[Co(58)2](PF6)2}n, (b) {[Co(58)2](BF4)2}n, and (c) {[Zn(59)2](OTf)2}n (H-atoms and counterions omitted for clarity) [154]. Figure reproduced with kind permission; r 2000 Wiley-VCH.

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| 165

166


N

N 62

N

N

N

N N

N

N

N

N 63

N N

N

N

LaIII

FeII

1D column

[Fe4(62)4](BF4)8

62

FeII

{{[Fe4(62)4]Ln}(ClO4)11}n

AgI 2D wall

63

[Fe4(63)4](BF4)8

{{[Fe4(63)4]Ag4}(BF4)12}n

Figure 4.37 Step-wise hierarchical self-assembly of metallo-supramolecular architectures. In the first step, ligands 62 and 63 form [2 2] grids with FeII ions. Subsequently, these grids were assembled into columnar 1D- or wall-like 2D-superstructures with LaIII or AgI ions, respectively [156]. Figure reproduced with kind permission; r 2005 Wiley-VCH.

Assembly of metallo-supramolecular grids into highly ordered structures on surfaces was investigated in particular by scanning probe techniques (i.e., AFM and STM). For instance, highly stable monolayers of the [2 2] grids [Co4(50)4](BF4)8 were prepared on highly-ordered pyrolytic graphite (HOPG) surfaces by drop-casting from dilute acetone solutions; almost defect-free areas of up to 0.5 mm2 were obtained [157]. This process of spontaneously growth of ordered structures might be considered as a 2D-crystallization. STM imaging revealed that the orientation of the grids relative to the surface plane was controlled by the substitution pattern of 50; for ligand 50l, a perpendicular orientation of the grids with respect to the surface was found, whereas ligand 50h formed flat tiles giving a super-array of [2 2] grids (Figure 4.38a/b). For the latter, an orthogonal 2.5 nm 2.4 nm periodicity of the arrangement was determined by STM measurements. When an intense 500 mV pulse was applied to the STM tip (commonly operated at below 50 mV for imaging), a single grid could be removed from the monolayer, resulting in a square hole in the film, showing the dimensions of a single grid-like molecule (Figure 4.38c).

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4.4 Racks and Grids

19 nm 19 nm

(a)

(b)

| 167

(c)

1nm

Å

13 .5 Å

26

20 nm 20 nm 2.5 nm

2.4 nm

10 nm 10 nm

Figure 4.38 (a) STM height image of [Co4(50l)4](BF4)8 drop-casted onto HOPG. The small picture shows an STM image of the underlying graphite lattice. The tentative interpretation of the vertical arrangement of the grids on the surface is shown below. (b) STM height image of [Co4(50h)4](BF4)8 drop-casted onto HOPG. The tentative interpretation of the horizontal arrangement of the grids on the surface is shown below. (c) STM height image of a hole in the monolayer produced by potential-induced removal of a single [Co4(50h)4]8 þ molecule with the STM tip [157]. Figure reproduced with kind permission; r 1999 Wiley-VCH.

(a)

(b)

(c)

100 nm Figure 4.39 (a) 3D-Representation of a STM topography image of a line of [Co4(50f)4](BF4)8 grids attached to the monocrystalline steps of HPOG; (b) experimental topography map of a single [Co4(50f)4](BF4)8 molecule; (c) experimental CITS map of [Co4(50f)4](BF4)8 at 0.942 V (the arrows indicate shaper maxima in the tunneling current) [112]. Figure reproduced with kind permission; r 2006 The Royal Society of Chemistry.

The migration rate of the hole was about 200 times slower than for a monolayer of cycloalkanes on a HOPG surface, reflecting the strong adsorption of the metallogrids to the graphite surface [158]. In contrast, [2 2] grids of CoII ions with ligands having lateral substituents R2 [e.g., thiopropyl (50k) or pyridine units (62, 63)] were adsorbed more weakly onto the HOPG surface and exhibited higher mobility than the corresponding parent [Co4(50a)4]8 þ grid [159]. Ruben et al. investigated the properties of [Co4(50f)4](BF4)8 after deposition by drop-casting onto a HOPG substrate. Three different types of molecular arrangements could be visualized by STM measurements: 2D arrays, 1D lines (Figure 4.39a), and isolated, free standing units of the [2 2] grid (Figure 4.39b) [112]. At very

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168

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes low surface coverage, a well-ordered distribution of isolated entities could be observed. This demonstrated the sensitivity of the adsorbate–substrate interactions for these systems; thus, thermal mobility becomes a considerable issue at room temperature [160]. The grids could be ordered along the monocrystalline steps of the HOPG surface into 1D lines (Figure 4.39a). The STM topography image of an isolated [Co4(50f)4](BF4)8 molecule appeared as a bright uniform spot with a crosssection of about 1.7 nm surrounded by a diminished defined halo (attributed to the BF4 anions and solvent molecules). The overall size of the spot corresponds well with the molecular size estimated by X-ray single-crystal structure analysis (approximately 1.65 nm in the square) [129]. To gain further insight into the submolecular structure, current imaging tunneling spectroscopy (CITS) was applied to the film of the grid on the HOPG substrate [112]. The CITS current map of [Co4(50f)4](BF4)8 exhibited a square array of four bright spots representing sharp local peaks in the tunneling current (Figure 4.39c). The distance between these maxima was about 7 A , corresponding to the distance between neighboring CoII ions [129]. Furthermore, density functional theory (DFT) calculations of the electron density distribution for the two different energy levels confirmed the experimental CITS data, that is, all four peaks were exclusively caused by the CoII ions (Figure 4.40). Moreover, thin films of the [2 2] grids containing CoII ions on gold surfaces were prepared and subsequently imaged by in situ electrochemical deposition STM. The obtained films were poorly conductive (o106 S cm1); however, doping with an excess of CdII ions enhanced the conductivity by four orders of magnitude to approximately 102 S cm1, which was attributed to additional electronic states in the material’s insulating bandgap. This increase was large, but

(a)

DFT Theory

2.5 nm 1.0 V

0.55 V

(b)

Experiment 5 nA

2.5 nm 0.68 V

1.03 V

Figure 4.40 (a) 3D-representation of the DFT-calculated electron density maps for two different energy levels; (b) representation of the central section of the corresponding experimental CITS maps (see also Figure 4.39) [112]. Figure reproduced with kind permission; r 2006 The Royal Society of Chemistry.

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4.4 Racks and Grids

the conductivity was still low when compared to metallic conductors such as copper (106–108 S cm1) [161]. When processed into thin films, grids containing CoII ions could serve as components for metallo-supramolecular polymers, providing new properties for the polymeric network [162, 163]. Lehn and coworkers also extended their research to larger grid-like assemblies, namely, regular [3 3] and [4 4] species utilizing tri- and tetratopic terpyridinetype ligands (e.g., 50c/d). Tritopic ligands 50c and 50m–o (Figure 4.41a) were selfassembled with PbII ions as well as various transition metal ions (i.e., ZnII, CoII, CuII, FeII, HgII) [130, 164]. Depending on the particular ligand, metal ion, counterion, solvent, and reaction conditions, different grid architectures were obtained: complete [3 3] and incomplete [3 2] grids. Under appropriate conditions, the saturated coordination and, therewith, the formation of complete grids containing six ligands and nine metal centers could only be achieved for 50c (with ZnII) and 50m (with HgII and PbII). In all other cases, incomplete grids, in most cases consisting of five ligands and six metal ions, were predominantly formed (the coordination sites of the middle row remained vacant). This deviation from the general concept of “maximum coordination site occupancy” could be explained by the enhanced ligand distortion in which metal ions of small ionic radii were not able to form the complete grid, thus the incomplete [3 2] architecture appeared

(a)

R2

R2

N N

R2

N N

N R1

N N

N

N

50c (R1, R2 H) 50m (R1 H, R2 SCH2CH2CH3) 50n (R1 Me, R2 H) 50o (R1 Ph, R2 H)

R1

(b)

[Co6(50o)5]12 Figure 4.41 (a) Tritopic ligands 50; (b) the two different structures of the incomplete [3 2] grid obtained by self-assembly of 50o with CoII ions [the phenyl moiety (R1) is omitted for clarity] [164]. Figure reproduced with kind permission; r 2002 Wiley-VCH.

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| 169

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2200 2000 1800 1600 1400 1200 1000 ppm

PbII/50p= 2:1

N

N

N

N

n-PrS

N

N 50p

N

n-PrS

2200 2000 1800 1600 1400 1200 1000 ppm

PbII/50p = 3:1

N N

N N

n-PrS

n-PrS

(b)

N

2200 2000 1800 1600 1400 1200 1000 ppm

PbII/50p = 4:1

(c)

Figure 4.42 Regular [4 4] grid (a), double-T shaped [2 2] grid (b), and double-cross-shaped structure (c) formed by self-assembly of 50p with PbII ions. The corresponding space-filling representations of the X-ray single-crystal structures (if available) and the 207Pb NMR spectra are depicted in the middle and bottom row, respectively [132]. Figure reproduced with kind permission; r 2003 American Chemical Society.

(a)

170


4.5 Helicates

to be the most thermodynamically stable product. In contrast, large metal ions (i.e., HgII and PbII) caused less ligand strain and the formation of the complete [3 3] grids could be confirmed by ESI MS [165]. X-Ray single-crystal analysis of the incomplete [3 2] grid of CoII ions with 50o revealed the presence of two differently arranged structures [164]; the first included three parallel ligands with a distance of 6.7 A , which were twisted into a transoid conformation around the central pyrimidine–pyridine–pyrimidine C–C bonds (Figure 4.41b, left). The central binding site of each ligand was unoccupied. The second structure was composed of two units markedly warped, whose N-atoms were fully coordinated and in a cisoid conformation (Figure 4.41b, right), in which the CoII ions were oriented into two rows of three ions with a Co–Co separation of 6.5 A . The through-space distance between two CoII centers was 13.8 A . The largest regular square grid obtained from terpyridine-type ligands was reported by Garcia et al. [130] in which eight equivalents of tetratopic ligand 50p and 16 PbII ions were assembled into a [4 4] grid, based on the remarkable amount of 96 overall coordinative bonds. The dynamic nature of the supramolecular architecture was shown by 1H NMR titration experiments [132] in which, by gradually increasing the PbII : 50p ratio, the [4 4] grid (Figure 4.42a) was converted into a double-cross-shaped structure (Figure 4.42c) via an intermediate double-T shaped [2 2] grid structure (Figure 4.42b). NMR was used to investigate the self-assembly processes in solution; the 207Pb NMR spectra (Figure 4.42, bottom line) revealed four different PbII centers for the [4 4] grid and two different PbII centers for both other species. The X-ray single-crystal analysis of the [4 4] grid showed the coordination of triflate counterions and water molecules to the supramolecular structure (Figure 4.42, middle row). The eight 50p molecules were arranged into two perpendicularly disposed sets of four outer and four inner ligands, resulting in a set of four [2 2] sub-grids rather than a regular [4 4] grid. Each square of PbII ions was linked on three sides by bridging triflate anions, the remaining coordination sites at the PbII ions were occupied by non-bridging triflate anions and water molecules. The use of 2,4,6-tris(pyrimidin-2-yl)-1,3,5-triazine (64), representing three fused terpyridine moieties, drove the self-assembly process to give 3D-architectures; when PbII ions were added to mixtures of 64 and 50k or 64 and 50n, cylindrical, cage-like structures were obtained [166]. The instantaneous formation of 36 and 54 coordinative Pb–N bonds for [Pb6(64)2(50k)3]12 þ and [Pb9(64)3(50n)3]18 þ , respectively, demonstrated the power and selectivity of the self-assembly process. The cage-like structures were confirmed by 1H NMR, ESI MS, and X-ray singlecrystal analysis (in the case of [Pb6(64)2(50k)3]12 þ , Figure 4.43).

4.5 Helicates

The self-assembly of polytopic ligand strands with transition metal ions into helical metallo-supramolecular architectures represents a further self-organization process.

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| 171

172


(b)

N

N

N

N

N N

N N

64

N

[Pb6(64)2(50k)3]12 Figure 4.43 (a) Star-shaped tritopic ligand 64; (b) ball–stick representations (left: side view; right: view along the vertical axis) of the cage-like structure of [Pb6(64)2(50k)3]12 þ (H-atoms, solvent molecules, and counterions omitted for clarity) [166]. Figure reproduced with kind permission; r 1999 Wiley-VCH.

In particular, metal-containing double helicates have been of interest since their initial description [167–170]. In the pioneering work, Constable and coworkers reported the synthesis and characterization of double helical dinuclear complexes of first row transition or rare earth metal ions with the linear 2,20 :60 ,200 :600 ,2w-quaterpyridine (65) [171, 172], 2,20 :60 ,200 :600 ,2w:6w,20000 -quinquepyridine (66) [173–175], and 2,20 :60 ,200 :600 ,2w:6w,20000 :60000 ,200000 -sexipyridine (67) [176] (Figure 4.44a). The asymmetric quaterpyridine ligands 65a–c were utilized by the same group to introduce directionality into the self-assembly process; however, due to the steric demand of the tert-butyl groups, helicates of the type [Cu2(65c)2]2 þ were exclusively formed in the less sterically hindered head-to-tail (HT) fashion (Figure 4.44b) (no or only poor directionality was observed for 65a and 65b, respectively) [177–179]. The X-ray single-crystal structure of the double helicate obtained from septipyridine 68 with ZnII ions revealed a high preference for a head-to-head orientation of the two ligands (each ZnII center was sixfold coordinated and both ligand strands were identical in a head-to-head arrangement with the terminal pyridine ring of each being uncoordinated, Figure 4.44c) [180]. The formation and redox behavior of double helicates, based on various alkylthio-substituted oligopyridines and monovalent (i.e., CuI) as well as divalent transition metal ions (i.e., CuII, NiII, CoII, FeII), were investigated by the group of ã [181–185]. For instance, the 40 ,400 ,40000 ,400000 -tetra(alkylthio)septipyridine 69 Abrun was treated with CuII or CoII ions to generate double-helices involving two ligand moieties and two metal ions (Figure 4.45, right-hand side) [184]. A mixture of CuII/CuI ions (1 : 2 ratio) yielded a trinuclear helix with the CuII ions being tetracoordinated (i.e., bipyridine motif on the ligand) (Figure 4.45, middle). With CuI ions only, a tetranuclear complex was formed (here, the terminal pyridine rings acted as monodentate ligands) (Figure 4.45, left-hand side). With the analogous sexipyridine 70, oxidative switching between a trinuclear CuI and a dinuclear CuII species could be achieved (Figure 4.46) [182]. A trinuclear heteroduplex helicate could be obtained by the self-assembly of one flexible tris(terpyridine) (71) and one flexible tris(bipyridine) ligand strand (72) with CuII ions. X-Ray single-crystal structure analysis of [Cu3(71)(72)]6 þ revealed three pentacoordinated CuII centers with the outer two [Cu(bpy)(tpy)]2 þ units in

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4.5 Helicates

| 173

(a)

N

N

N

N

N

N

65

N

N

N

66

N

N

N

N

N

N

67

R2

(b)

N (c) N

N

N

N

N

N

N

N

N

68 R1

R

R

65a (R1 Me, R2 H) 65b (R1 Me, R2 SCH3) 2 65c (R1 tBu, R H)

head (H)

tail (T) H H

H

N(1)

N(2)

Zn(1)

N(3)

H T

N(4) Zn(2)

T

T

H

T

HH-

N(5)

T

N(6)

HT-

N(7)

isomer

65a: 1:1 65b: 3:2 65c: 1:0

Uncoordinated terminal pyridine rings

Figure 4.44 (a) Oligopyridines, as building blocks for helicates; (b) directional self-assembly of 65a–c into double helicates [177, 178]; (c) disubstituted septipyridine 68 and ball–stick representation of the X-ray single-crystal structure of the head-to-head helicate [Zn2(67)2]4 þ (H-atoms and counterions omitted for clarity) [180]. Figure reproduced with kind permission; r 1996 The Royal Society of Chemistry.)

trigonal-bipyramidal and the central one in square-pyramidal complex geometry (Figure 4.47) [186]. Moreover, the complementary flexible ligand strands BTB (bipyridine–terpyridine–bipyridine) and TBT (terpyridine–bipyridine–terpyridine) gave, upon addition of CuII ions, the trinuclear double helicate [(BTB)Cu3(TBT)]6 þ ; self-assembly of the BTB ligand with a mixture of FeII and CuI ions (1 : 2 ratio) resulted in the selective assembly into the heterodimetallic helicate [Cu(BTB)Fe(BTB)Cu]4 þ with the FeII ions being hexacoordinated by the central terpyridine units and the outer CuI ions forming a tetrahedron with two bipyridine sites [187] (for a more detailed insight into the self-recognition behavior of BTB, TBT, and related flexible ligands, see References [188, 189]). A different approach to heterodimetallic helicates was followed by Constable et al. [190]: equimolar

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N

174

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes SCH3

N

N

N

N

N

N

N

N

N

SCH3 N

N

N

N

N

69 SCH2CH2CH3

CuI (under N2)

SCH2CH2CH3

CuII or CoII

CuI (in air)

N N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N

N N

N

N

N N

N N

N

N

N

N N

N

= Cu(II)/Cu(I)

Figure 4.45 Redox-switching of the self-assembly process of septipyridine 69 into helicates [184]. Figure reproduced with kind permission; r 1993 American Chemical Society.

amounts of dinuclear homometallic double helicates, formed by a di(alkylthio)substituted quaterpyridine with either CuII or NiII ions, were redistributed into a heterometallic double helicate with one pentacoordinated CuII and one hexacoordinated NiII ion per molecule. Different types of ligand strands were reported by Rice et al. in which the incorporation of coordinating thiazole moieties into the terpyridine-based ligand structure enabled the formation of di- and trinuclear double helicates with CuII, NiII, CoII, as well as CdII ions [191]. The N-oxides of ligands (73 and 74) could also be used, as ligands, for the formation of double helicates with CdII ions [192]. The solid-state structures of the helicates revealed the ability of N-oxide units to partition the ligands into two separate binding domains with a [Cd(m-O2)Cd] moiety, as the central part of the helicate structure (Figure 4.48). The regiochemical assembly of asymmetric ligand strands into double helicates could be controlled via bulky substituents (see Figure 4.44). The helicates formed were chiral, but due to the absence of any stereochemical discrimination both the P- and the M-enantiomer were equally generated. The diastereoselective formation of double helicates with CuI ions was achieved by introducing a bulky, chiral substituent at one end of a terpyridine ligand strand [193]. Thus, the head-to-tail

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4.5 Helicates

| 175

SCH2CH2CH3

N

N

N

N

N

N

N

N

N 70

N

N

N

SCH2CH2CH3

CuI (under N2)

N

N

N

N

N

N

N

N

N

CuII

N

N

N

N

N

N

Oxidation Reduction

N

N

N

N

N

N N

N

N

Figure 4.46 Reversible redox-state-dependent switching of the self-assembly process of sexipyridine 70 into helicates [182]. Figure reproduced with kind permission; r 1993 American Chemical Society.

N N

N

71

N

N

N

N

N

71

N

72

N

N N

3

N 72

O N

O N

Figure 4.47 Self-assembly of ligand strands 71 and 72 with CuII ions into a heteroduplex helicate. Figure redrawn according to Reference [186].

orientation of either the (S)- or (R)-isomer of the ligands within the helicate led to the diastereoselective formation of the enantiopure (S,S)-M and (R,R)-P helicates, respectively [according to molecular modeling studies, the (R,R)-M and (S,S)-P enantiomers were energetically unfavored by 10 kcal mol1]. Other chiral terpyridine and quaterpyridine ligands were utilized for the diastereoselective selfassembly with tetrahedral metal centers (i.e., AgI and CuI ions) into enantiopure homometallic as well as heterodimetallic double helicates [194–197] (see Figure 4.49 for a representative example). Chiroptical measurements could show that, in

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CuII

176


S

S N

N

N

N

N O

N 73 N

S

S N

N

N

N O

N 74

(b)

(c)

Figure 4.48 (a) Ligand N-oxides 73 and 74; (b) ball–stick representation of the X-ray singlecrystal structure of the double helicate [Cd2(73)2](ClO4)3 þ ; (c) ball–stick representation of the X-ray single-crystal structure of the double helicate [Cd2(74)2]4 þ [192]. For both solid-state structures, the H-atoms, counterions, and solvent molecules are omitted for clarity. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

N

N

N

N

CuI

75

M-helicate

P-helicate

Figure 4.49 Chiral quaterpyridine 75 and the self-assembly with CuI ions into M- or Phelicates.

all cases, the helix, and not the ligand-based chirality, was the major contributor to the ellipticity (i.e., the CD response). Overall, the self-assembly of terpyridine-containing ligands with transition metal ions into (double) helicates is mainly of scientific interest; however, the application of chiral helicates in the field of asymmetric catalysis is noteworthy (Chapter 9). The group of Kwong recently introduced such metallo-supramolecular assemblies, as catalysts, for enantioselective allylic substitution or asymmetric cyclopropanations [198–200].

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4.6 Rotaxanes and Catenanes

| 177


Rotaxanes are defined as mechanically-interlocked molecular structure consisting of a dumbbell-shaped molecule that is threaded through a macrocycle; two (or more) interlocked macrocycles are referred to as catenanes. The first report on a supramolecular rotaxane-type architecture dates back to the late 1960s [201]. Terpyridine-containing rotaxanes were introduced by Sauvage et al. almost 30 years later [202] and their research is still at the forefront in this structural arena [4]; for a more in-depth view of interlocked supramolecular architectures, the reader is referred to recent reviews on this topic [203–209]. In an early contribution, CuII ions and 2,9-diphenylphenanthroline 76 (incorporated into a crown ether ring structure) were assembled into a CuII mono(phenanthroline) complex (for steric reasons, a bis-complex was not formed). The terpyridine–phenanthroline–terpyridine strand 77 was threaded into a macrocyclic ring to give a tetrahedral CuII bis(phenanthroline) complex. Subsequently, the catenane was formed by addition of Ru(dmso)4Cl2 (intramolecular complexation with RuII ions) [210]; alternatively, the rotaxane was obtained by reaction of Ru(tpy)Cl3. The CuII ions were removed by treatment with KCN to yield the CuII-free catenane or rotaxane, respectively [202, 211] (Figure 4.50). The catenane could be remetalated with ZnII or AgI ions. Photophysical investigations of all systems with

CuII

77

76 RuCl3

Ru(dmso)4Cl2 77

76 N

N

N

N

catenane O

O

O

O

roxatene

O

RuII

O O

O

N

N

N

KCN

N

N

N

Figure 4.50 Phenanthroline-based ligands 76 and 77 and their step-wise self-assembly into a catenane and rotaxane [211]. Figure reproduced with kind permission; r 1997 American Chemical Society.

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KCN

178

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes these compounds revealed electron- and energy-transfer processes depending on the metal ion used. The ligand-centered luminescence was quenched in case of CuII-free catenane; the opposite effect occurred for the CuII catenane. The latter system was envisioned as a possible “molecular switch” [212]. The phenomenon of “molecular motion” was investigated by Sauvage et al. For this purpose, two different ligand sites (i.e., one phenanthroline and one terpyridine) were incorporated into the threads and rotaxanes were subsequently formed by either inter- or intramolecular self-assembly with phenanthroline-containing crown ether systems in the presence of transition metal ions. Molecular motion, that is migration of the macrocycle from one binding site to another along the thread, was found to depend on various parameters. For instance, the high stability of two different coordination environments for CuI and CuII ions was utilized; in the CuI state, two phenanthroline units (one of the macrocycle, one of the thread) coordinated to the metal ion in a tetrahedral geometry, whereas in the CuII state (after electro- or photochemical oxidation) the phenanthroline of the cycle and the terpyridine of the thread formed a five-coordinate geometry around the metal center (Figure 4.51a) [213]. Replacing the sterically demanding 2,9-diphenylphenanthroline on the thread with a 8,80 -diphenyl-3,30 -biisoquinoline (dpbq) enhanced the mobility of the macrocycle when the oxidation state of the Cu ion was changed electrochemically [214]. Recently, a similar rotaxane featuring three different chelating groups (i.e., phenanthroline, bipyridine, and terpyridine) in the axis was reported; fast gliding of the surrounding macrocycle (containing a dpbq unit), induced by the CuI/CuII ion couple, was observed [215]. Besides gliding of the macrocycle along the axis, rotation of the macrocycle around a phenanthroline-containing axis could also be achieved by incorporating both the phenanthroline and terpyridine unit into the same ring (Figure 4.51b) [216]. In this initial example, the rotation was relatively slow with the re-organization from the pentacoordinated CuII to the tetracoordinated CuI state being the rate-determining

(a)

(b)

CuI state

CuI state

CuII state Oxidation & ”rotation“

Oxidation & ”gliding“

CuII state phenanthroline

terpyridine

bulky stopper

Figure 4.51 Two types of oxidation-induced “molecular motion” in phenanthroline– terpyridine rotaxanes: (a) gliding and (b) rotation [213]. Figure reproduced with kind permission; r 1999 American Chemical Society.

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step (i.e., CuI - CuII: t ¼ 50 ms; CuII - CuI: few minutes); the process could be accelerated remarkably by replacing the phenanthroline unit on the thread by the more flexible bipyridine (CuI- CuII: t ¼ 2 ms; CuII- CuI: t ¼ 200 ms) [217–221]. Thus, a strong dependence of the rotation rate on the thread’s structure could be concluded; rigidity and steric hindrance lead to slow-moving molecular machines [222]. The molecules dynamics (i.e., gliding and rotation) were easily followed by Xray absorption spectroscopic (XAS) techniques, allowing monitoring of the different coordination geometries for the Cu ions in their different oxidation states [223]. Weber et al. showed that the rotaxanes could be bound via thiol-groups to gold surfaces, yielding a self-assembled monolayer of fixed rotaxanes (here, the Au surface acted as bulky stopper at one end of the thread) [224]. The multifunctional 78 (Figure 4.52a), consisting of a phenanthroline moiety within a crown ether-like macrocycle and another phenanthroline bound to a terpyridine moiety (bearing a bulky substituent in its 500 -position) at the 5-position, was prepared and, subsequently, self-assembled with CuI ions to yield a bisrotaxane (see Figure 4.52b for the solid-state structure) [225, 226]. Replacement of the CuI ions by ZnII ions generated a switch based on a change from a tetra- to pentacoordinated coordination geometry, involving the terpyridine. As a result, the (a)

O O

O O

N N O

N

O

O O

N

N

N N

78

O 78 =

phenanthroline terpyridine

(b)

(c) ZnII CuI Extended

Contracted

Figure 4.52 (a) Multifunctional ligand 78; (b) space-filling representation of the X-ray crystal structure of the bis-rotaxane [Cu2(78)2]2 þ (H-atoms omitted for clarity); (c) reversible extension–contraction process [226]. Figure reproduced with kind permission; r 2002 Wiley-VCH.

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| 179

180

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes bis-rotaxane was contracted (Figure 4.52c). The authors compared this fully reversible extension and contraction process to the mode of action of muscles (for more information on the concept of “molecular machines,” in particular on artificial muscles, see Reference [227]). Rotaxanes with terpyridine units at the thread’s termini have also appeared in the literature. These molecules were initially designed to investigate electrontransfer processes along wire-type molecules that were embedded in macrocyclic structures [228]. The group of Loeb synthesized an extended terpyridine ligand with cationic pyridinium units in the backbone to act as an axle in a supramolecular array using crown ethers [229]. The rotaxane macroligand was obtained after the self-assembly process and subsequent functionalization of the axle with a bulky end-group. Unsubstituted 24-crown-8 as well as their dibenzo- and dinaphthyl-analogues were employed; Figure 4.53 depicts the rotaxane containing the dinaphthyl crown ether. Formation of the FeII bis(terpyridine) complexes was monitored by UV–vis absorption spectroscopy and the MLCT transition of the rotaxane dimer was found to be strongly dependent on the nature of the macrocycle; the aryl-substituted crown ethers significantly stabilized the MLCT by p–p interactions with the (partially) conjugated terpyridine thread. The heteroleptic RuII bis(terpyridine) complexes of the terpyridine rotaxane were also investigated [230]. Emission in the near-infrared regime at room temperature (lPL ¼ 800–850 nm) and a relatively long-lived 3MLCT excited-state (t B 20 ns) could be observed.

(a)

(b) N(2) N(3)

N O N

N 79

O O

N

O N

O

O

N(1)

O

N

O(4) O(5)

O

O(6) O(7)

O(3) O(1)

O(8)

N(6)

Figure 4.53 (a) Terpyridine-rotaxane 79; (b) ball–stick (left) and space-filling (right) representation of the X-ray single-crystal structure of 79 (H-atoms, counterions, and solvent molecules omitted) [229]. Figure reproduced with kind permission; r 2003 The Royal Society of Chemistry.

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| 181

These changes in the photophysical behavior in comparison to the parent complex [Ru(tpy)2]2 þ were attributed to the strong electron acceptor (i.e., the pyridinium substituents) in the rotaxane ligand. Molecular motion – comparable to the previously discussed gliding and rotation in rotaxanes – was also observed for chemically interlocked catenanes: Livoreil et al. showed that the interlocked rings 76 and 80 could be selectively organized in the presence of CuI/CuII ions (Figure 4.54) [231]. The catenane’s molecular motion, that is, reversible interconversion of the tetra- and pentacoordinated metal centers, could be induced chemically, electrochemically, or photochemically by changing the oxidation state of the Cu ion. A molecular knot, a special type of interlocked molecules, was prepared by a metathesis reaction of a dinuclear double helicate containing FeII ions [232]. Thus, ditopic ligand 81 was self-assembled into the double helicate [Fe2(81)2]4 þ and, subsequently, treated with Grubbs’ catalyst to yield the trefoil knot (finally, the double bonds were hydrogenated) (Figure 4.55). The knotted structure was confirmed by 2D ROESY experiments (ROESY: rotating-frame Overhauser effect spectroscopy), which showed a long-range interaction between the central protons of the terpyridine units (i.e., 30 -, 40 -, and 50 -position) with protons of the chain; this type of interaction could only be explained in the case of a knot. However, the same approach could not be applied for the synthesis of a catenane starting from the FeII bis(terpyridine) complex [Fe(82)2]2 þ [233]: instead, an unexpected 8shaped molecule with a twisted core was identified by 1H NMR and X-ray singlecrystal analysis (Figure 4.56). Owing to insufficient “interwinding” of the chains, the cyclization occurred laterally and not “beyond” the metal. The interweaving of three individual macrocycles (i.e., without any covalent or concatenate linkage of these) is referred to as “Borromean links” [203]; Figure 4.57a depicts the three basic types of structures. Siegel et al. reported a synthetic approach towards the “orthogonal rings” motif, based on the connection of two macrocycles via formation of two heteroleptic RuII bis(terpyridine) complexes (Figure 4.57b) [234]. The vacant bipyridine sites of 83 served, as template, for the

N

N

N

76

N

80

O O

Oxidation

O

O O

Reduction

CuI state

O O

O

N

N

CuII state (b)

N

(a)

Figure 4.54 (a) Macrocycles 76 and 80 as building blocks for catenanes; (b) reversible molecular motion of the catenane in the presence of CuI/CuII ions (see also Figure 4.51) [231]. Figure reproduced with kind permission; r 1997 American Chemical Society.

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182


O

N

N

4

N

N

O

N

4

81

O

O

O

O

O

O

O

O N N O

N Fe N

N

N

N

N

N Fe N

O N N

i) metathesis

O

O

O

O

N

Fe N

O

N

N N

O

N

N

N

N

Fe N

N N O

O

O O

Z

O

O

ii) hydrogenation O

O

O

O

O

O O

Double helicate

O

Z

O

O

Trefoil knot

Figure 4.55 Bis(terpyridine) 81; the double helicate [Fe2(81)2]4 þ was converted into a trefoil knot by metathesis reaction (Z ¼ CH¼CH) and subsequent hydrogenation (Z ¼ CH2–CH2) [232]. Figure reproduced with kind permission; r 1999 American Chemical Society.

coordination of the third orthogonal ring. The X-ray single-crystal structure of 83 revealed the orthogonal orientation of the two rings linked via two [Ru(tpy)2]2 þ connectivities (Figure 4.57c); the largest dimension of the bimacrocyclic complex was roughly 29 A (the Ru Ru distance was 16.4 A ). The outer, more flexible ring adopted a chair-like configuration. According to the solid-state structure, the dimensions of the third ring (to be threaded inside of the inner and outside of the outer ring) must be at least 20 A . 4.7 Miscellaneous Structures 4.7.1 Cyclodextrin Derivatives

Cyclodextrins are water-soluble molecules with a well-defined hydrophobic cavity. Thus, they have been widely applied in supramolecular chemistry, as host molecules, for binding of photoactive guests [1]. In this respect, the major advantage of the cyclodextrin approach is that light-induced processes can be observed between two photoactive units held together via non-covalent interactions in aqueous solutions [235]. The Pikramenou group synthesized mono(terpyridine) permethylated b-cyclodextrin (84), as a ligand for transition or rare earth metal ions (Figure 4.58); the addition of FeII, RuII, and EuIII ions gave the homoleptic cyclodextrin-complexes [Fe(84)2]2 þ , [Ru(84)2]2 þ , and [Eu(84)3)]3 þ , respectively [236, 237]. 40 -Tolyl-2,20 :60 ,200 terpyridine (ttpy) was used, as an additional ligand, for the synthesis of the heteroleptic RuII bis(terpyridine) complex [(ttpy)Ru(84)]2 þ , which was emissive at room temperature (lPL ¼ 640 nm, FPL ¼ 4.1 105, and tPL ¼ 1.9 ns). The potential of

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4.7 Miscellaneous Structures

(a) O

O

O

N

N

N

O

O

O 82

O

(b)

O O

O

O O N

N

N

N

N

N

N

N

O

O

O

O O

O

N

N

metathesis

N

O

O

O

N

O O

O

O

O

O

[Fe(82)2]2+

O O

(c)

O2

C18

C35 C36

C17 C16

C19

O1

O3

C14 C15

C20

C13

C21

C52 C39 C38 C40

C34 C33

N5

N6

N3 N4 Fe

C12 C23

C28 O6

O4 O4A C24

C25

C10

O5 C27

C24A

C26

N2

C9 C8

C7

O10 C54

C2

N1 C6

C50

O11 C41

O7

C42

C22

C51

C49

O12

C48 C3 O9a

O9

C5

C47

C4 C45

C46

Figure 4.56 (a) Dialkenyl-functionalized terpyridine 82; (b) ring-closure metathesis yielding the 8-shaped molecule; (c) ORTEP representation of the X-ray single-crystal structure of the 8-shaped molecule (H-atoms, counterions, and solvent molecules omitted for clarity) [233]. Figure reproduced with kind permission; r 2000 American Chemical Society.

complexes [Ru(84)2]2 þ and [(ttpy)Ru(84)]2 þ to act, as optical sensors, for the hydrophobic binding of guests into the cyclodextrin cavity was shown by photoluminescence titration experiments with quinone derivatives; quenching of the emission was attributed to an intermolecular electron transfer from the RuII center to the quinone guest molecules [237]. Moreover, the heteroleptic OsII bis(terpyridine) complex 85a was used as a guest molecule. After oxidation of the OsII center, an intramolecular electron transfer from RuII to OsIII was observed. Owing to the short

04

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| 183

184


(a)

Venn rings

Chain rings

Orthogonal rings

(c)

(b) N N

N Ru2+

O

N

O

N N

O

O O

O N

N

N

N

O

O

O

O

83

N O

N

N Ru2+

N

O N

4 PF6

N

Figure 4.57 (a) The three types of “Borromean links;” (b) the bicyclic metallo-supramolecular assembly 83; (c) space-filling representation of the X-ray single-crystal structure of 83 (H-atoms, counterions, and solvent molecules omitted for clarity) [234]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

emission lifetime of the RuII complex, a RuII-to-OsII energy transfer could not be detected. Recently, the same group showed that energy transfer from monoanthracenyl permethylated b-cyclodextrin to 85a (located as a guest in the cyclodextrin cavity) occurred [238]. Intramolecular metal-to-metal energy transfer could be achieved utilizing complexes with longer emission lifetimes. Thus, the RuII tris(bipyridine) complex [Ru(86)3]2 þ bearing three cyclodextrin units was synthesized [239]. Utilizing 85a, as a guest, vectorial energy transfer from the RuII core to OsII periphery was observed (Figure 4.59). Replacement of two b-cyclodextrin units with the smaller a-cyclodextrin enabled a selectivity of supramolecular binding in which the bulky adamantyl group of 85d could only be placed into the b-cyclodextrin cavity; subsequently, the a-cyclodextrin pockets were filled with octanoic acid molecules that were endfunctionalized with anthracene units. Upon excitation of the lateral anthracene moieties, a vectorial energy transfer cascade occurred with rates of 1.8 1010 s1

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| 185

(OCH3)14

O

OH

(OCH3)6

N

O

84

7

HO

R

M2

N

OH

N N

N

N

85a (MII OsII, R biphenyl) 85b (MII IrIII, R biphenyl) 85c (MII CoII, R biphenyl) 85d (MII OsII, R adamantyl)

N

N

β-Cyclodextrin

N

hν

N

N O

Ru2 N

N

N N

N

N Os

2

N

N N

N

Electron transfer

Figure 4.58 Terpyridine-cyclodextrin 84, the heteroleptic bis(terpyridine) complexes 85, and the RuII-OsII dyad [237].

(generating the excited state at the central RuII center) and 0.9 109 s1 (population of the lowest excited state on the OsII center) [240]. The direction of the vectorial energy transfer could also be reversed. In the case of IrIII bis(terpyridine) complex 85b, as guest, energy was transferred from the periphery to the RuII core. This type of process is of relevance for the development of light-harvesting devices (e.g., for artificial photosynthesis or water oxidation applications) [239]. Surface-active monolayers of cyclodextrin-containing complexes were also reported [241]. Heteroleptic RuII bis(terpyridine) complexes of 84 and a thiolfunctionalized terpyridine were prepared and anchored to indium tin oxide (ITO) surfaces. Supramolecular binding to the cyclodextrin units pointing outside the monolayers was investigated by electrochemical measurements. Moreover, pyridine-difunctionalized cyclodextrin molecules were self-assembled into a monolayer on a platinum substrate (the defects could be back-filled with nonanethiol) [242]. The inclusion of the CoII bis(terpyridine) complex 85c into the cyclodextrin pockets was monitored by CV measurements; the assembly of terpyridines and their transition metal complexes on surfaces is discussed in more detail in Chapter 8. 4.7.2 Other Assemblies

Cavitands are cyclic aromatic arrays that are commonly utilized, as templates, in supramolecular chemistry [243]. Mattay et al. functionalized the upper rim of

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186

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes hν

Energy transfer

R

N N (OCH3)14

N Ru

2

O

N N

N

N

N Os

2

N

N N

N

R O

N

N

(OCH3)6

R

86 Energy transfer

R

hν N

N Ru2

N

O

N

Ir3

N N

N

N N

N N

N

R R

Figure 4.59 Bipyridine-cyclodextrin 86 and the energy transfer in the tetranuclear RuII-OsII and RuII-IrIII arrays [239].

resorc[4]arene with terpyridine units via a Suzuki cross-coupling reaction [244]. The tetrakis(terpyridine)-cavitand 87 (Figure 4.60) was self-assembled with ZnII ions into a spheroidal cage (consisting of six molecules 87 and 12 ZnII ions) that could be characterized by ESI MS, small angle X-ray scattering (SAXS), and diffusion NMR. A diameter of about 4 nm was estimated by the latter technique, corresponding to the diameter of 4.6 nm, predicted by molecular modeling (Figure 4.60). The supramolecular cage was considered to be a new type of template for host–guest interactions; the diameter of the largest sphere that would fit into the cage structure was calculated to be about 3.0 nm, which corresponds to a volume of about 13.7 nm3; between the bridging units, holes with a minimal diameter of 7.7 A should allow solvent molecules to enter and leave the capsule. Calixarenes are a different type of macrocycle that have been studied as materials for supramolecular host–guest interactions [245]. In contrast to the rather conformationally rigid resorcarenes, calixarenes are characterized by enhanced flexibility; they can be prepared with various ring sizes with diverse functional groups attached either at the lower or upper rim. Terpyridine-calixarenes have been reported in the literature, attached at both faces of the macrocycle. Molard and Parrot-Lopez synthesized a calix[4]arene derivative where the lower rim was decorated with four terpyridine units (attached via flexible spacers). Coordination

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(a) R O

R O

OO

R OO

R

N

O R=

H

H

H

N

H N

87

(b)

0.77 nm

4.6 nm

Figure 4.60 (a) Terpyridine-functionalized resorc[4]arene 87; (b) representation of a molecular model of the supramolecular cage [Zn12(87)6]24 þ [244]. Figure reproduced with kind permission; r 2008 Elsevier B.V.

of transition metal ions to the macrocyclic ligand 88 was investigated by UV–vis titration experiments (Figure 4.61). Intramolecular complexation with NiII, CuII, and CoII ions into supramolecular assemblies of the type [M2(88)]4 þ was observed [246]. Functionalization of the upper rim yielded the C2v-symmetric cone-shaped derivative 89 [247]. Self-assembly of 89 with ZnII ions gave various supramolecular architectures, including octahedral cages. Owing to the enhanced flexibility of 89 in comparison to 87, the self-assembly process was poorly controlled and strongly depended on the experimental conditions (e.g., concentrations of the ligand and metal ions, polarity of solvent, counterions, etc.). A metallo-supramolecular “zipper” was obtained from the self-assembly of 6,600 di(pyridin-4-yl)-2,20 :60 ,200 -terpyridine (90) with CoII or PbII ions (Figure 4.62a) [248]. The quinquepyridine 90 acted as a self-complementary molecular cleft and the formation of the zipper-like structure in the solid state was driven by both metal-to-ligand coordination as well as p–p stacking (Figure 4.62b). The metal ions were coordinated to both the terpyridine and pyridine, and the two vacant coordination sites were occupied by either solvent (i.e., MeCN) or trifluorosulfonate counterions (Figure 4.62c).

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| 187

188

| 4 Metallo-Supramolecular Architectures Based on Terpyridine Complexes (a) N HN R= N

O O ROC

O

O

ROC

ROC

R

R

88

O

N

COR N

R

R

N

R= O

O

89

O

O

N

(b)

03 02

01

04

N12

N6

N5

N4

N7

N10

N1

N11

N3 N2 N8 N9

Figure 4.61 (a) Calix[4]arenes functionalized with four terpyridine ligands at the lower (88) and upper rim (89) [246, 247]; (b) ball–stick representation of the X-ray single-crystal structure of 89 (H-atoms and solvent molecules omitted for clarity) [247]. Figure reproduced with kind permission; r 2009 Elsevier B.V.

A heteroleptic RuII complex with phenanthroline, terpyridine, and benzonitrile ligands (91) was designed by Schofield et al. [249]. The latter ligand was covalently attached to the terpyridine moiety and, therefore, was able to coordinate to the RuII ion in a “back-biting” manner. This scorpionate-type complex reversibly released the benzonitrile in water, yielding the aquo-complex 92 through photochemical induction (Figure 4.63).

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(a)

| 189

(c)

N

N N

N

N

90

(b)

Figure 4.62 (a) Quinquepyridine 90; (b) space-filling representation of the X-ray single-crystal structure of the supramolecular zipper (H-atoms, counterions, and solvent molecules omitted for clarity); (c) stick representation of the X-ray single-crystal structure of the dimer {[Co2(90)2]4 þ (MeCN)4} (H-atoms and counterions omitted for clarity) [248]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

(a)

N1 N5

N2 N3 Ru N4 N6 C42

(b) N N

N Ru

O

N

2+

N N

N hν, H2O

N

∆, -H2O

H2O

O

O

N N

O O

N Ru2+

91

92

Figure 4.63 (a) Ball–stick representation of the X-ray single-crystal structure of the scorpionate-complex 91; (b) reversible switching between the “back-biting” complex 91 and the aquo-complex 92 [249]. Figure reproduced with kind permission; r 2003 The Royal Society of Chemistry.

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O

N

190

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239 Haider, J.M., Williams, R.M., De Cola, L., and Pikramenou, Z. (2003) Angew. Chem., Int. Ed., 42, 1830–1833. 240 Faiz, J.A., Williams, R.M., Pereira Silva, M.J.J., De Cola, L., and Pikramenou, Z. (2006) J. Am. Chem. Soc., 128, 4520–4521. 241 Pereira Silva, M.J.J., Bertoncello, P., Daskalakis, N.N., Spencer, N., Kariuki, B.M., Unwin, P.R., and Pikramenou, Z. (2007) Supramol. Chem., 19, 115–127. 242 Mallon, C.T., Forster, R.J., McNally, A., Campagnoli, E., Pikramenou, Z., and Keyes, T.E. (2007) Langmuir, 23, 6997–7002. 243 Cram, D.J. (1983) Science, 219, 1177–1183. ¨der, T., Brodbeck, R., Letzel, M. 244 Schro C., Mix, A., Schnatwinkel, B., Tonigold, M., Volkmer, D., and Mattay, J. (2008) Tetrahedron Lett., 49, 5939–5942. 245 Gutsche, C.D. (2008) Calixarenes: An Introduction, 2nd edn, RSC Publishing, Cambridge. 246 Molard, Y. and Parrot-Lopez, H. (2002) Tetrahedron Lett., 43, 6355–6358. ¨tter, 247 Liu, J.-M., Tonigold, M., Bredenko ¨der, T., Mattay, J., and B., Schro Volkmer, D. (2009) Tetrahedron Lett., 50, 1303–1306. 248 Barboiu, M., Petit, E., and Vaughan, G. (2004) Chem. Eur. J., 10, 2263–2270. 249 Schofield, E.R., Collin, J.-P., Gruber, N., and Sauvage, J.-P. (2003) Chem. Commun., 188–189.

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p-Conjugated Polymers Incorporating Terpyridine Metal Complexes*

5.1 Introduction

Synthetic organic polymers play, due to the enormous variation of their physical and chemical properties as well as wide-spread commercial application, an important role in modern society and research. In particular, in the last decade – despite the well-known, cheap and widely-used thermoplastics and elastomers – the focus has shifted to more high-value materials [1–8]. Metal ions also play a pivotal role in modern and materials science; for example, with respect to electronic applications [9–13]; furthermore, metal centers play a central role in the function of biological macromolecules. Thus, combining these two, organic polymers and metal ions, turned out to be a fruitful avenue to new functional materials. The successful free radical polymerization of vinyl ferrocene more than 50 years ago [14] was the starting point for the field of metallopolymers; thereafter a broad range of different metallopolymers have been synthesized and investigated [7, 10, 13, 15–25]. These early efforts made metallopolymers one of the fastest developing fields in macromolecular chemistry. In traditional polymers, the monomers are linked by covalent bonds and it is their nature as well as the molar mass that define the resultant polymer’s physical properties. The interest in metallopolymers is mainly attributed to their special properties, which represent a unique combination of the physical, electronic, and optical properties of the organic component as well as the respective properties of the incorporated metal center(s). For this reason, p-conjugated oligomers and polymers, one of the younger polymer classes that was discovered almost 40 years ago [26], are highly interesting building blocks for combination with metal complexes. These (semiconducting) polymers feature outstanding electrical and optical properties; owing to these properties they are also considered as synthetic metals [27]. They have found applications in many fields, such as polymer light-emitting

*Parts of this chapter are reproduced from Chem. Soc. Rev. 40 (2011) 1459–1511 by permission of The Royal Society of Chemistry. Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes diodes (PLEDs) [1] and organic solar cells [2, 28]. Importantly – in particular, when considering p-conjugated polymers – the introduction of metal complexes can also lead to undesired properties, thus requiring a chemical decoupling of the metal complex from the conjugated oligomer/polymer. The fruitful combination of transition metal complexes and p-conjugated polymers has led to many applications – for instance, OLEDs [9, 25, 29], photovoltaic devices [30], as well as in photonic crystals – depending on the respective polymer [11, 13, 31]. In the following, we define metallo-supramolecular polymers according to the concepts of macromolecular chemistry and introduce the basic synthetic strategies commonly used for the preparation of terpyridine-containing metallopolymers: the polymerization by complexation and the so-called “complex first” method. The broad field of potential applications – from OLED devices, organometallic solar cells, and luminescent sensors to nanotechnology – will be highlighted.

5.2 Metallo-Supramolecular Polymerization

The definition of metallopolymers, that is, all organic polymers that contain metal ions, is as diverse and sometimes complex as the structures of the polymers themselves. In general, metal complex moieties can be attached to the polymer as part of the main-chain or as pendant group (i.e., in the side-chain). According to Ciardelli et al., one has to distinguish between three different types of metal-containing polymers (Figure 5.1) [32]. In type-I polymers, the metal ions/complexes are attached to the polymer at the side-chain or as an end-group by (i) electrostatic interaction, (ii) coordinative bonds, and/or (iii) covalent bonding. Depending on the distance between the polymer and complexes either no or only weak interactions between both can be observed. Type-II polymers possess the metal as part of their main-chain, by either (i) covalent linkage or (ii) metal-to-ligand coordination. Direct coupling of the metal to the conjugated backbone leads to an increased influence of the metal complex on the properties of the entire conjugated system. In type-III polymers, the metal ions are embedded into the polymer matrix by only physical interaction(s). The driving force for forming such polymers is the negative free energy as a result of the chelate effect of the metal binding units within the polymer. In this chapter, the focus is on terpyridines and other structurally-related tridentate ligands, as complexing agents for transition metal ions, and thus only type-Ib and type-IIb are considered [33]. p-Conjugated polymers are of great interest due to their diverse applications; therefore, we will concentrate on metallopolymers consisting of conjugated oligomers/polymers, which are connected by transition metal complexes. In addition, we will also include conjugated polymers of type-Ib. The emerging family of non-conjugated (i.e., conventional) polymers with terpyridine moieties either as pendant groups or as an end-group is discussed separately in Chapter 6. To synthesize such polymers, three main routes are commonly used. A complex-containing monomer can be polymerized, resulting in a polymer bearing a

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Type I Metal ions/complexes bound to a chain of the polymer (a) Metal ions attached to the polymer by electrostatic attraction (b) Metal complex linked to the polymer by coordination

(c) Metal complex covalently linked to the polymer as pendant or end group

Type II Metal complexes as part of a polymer chain (a) Metal complexes as part of the main chain by covalent linkage

(b) Metal ligand coordination complex as part of the main chain

Type III Metal ions/complexes physically interact with the polymer chain

Figure 5.1 General architectures of metallopolymers [32, 33]. Figure reproduced with kind permission; r 2011 The Royal Society of Chemistry.

metal complex in the side-chain. If the complex is stable enough under the polymerization conditions, one will have neither uncomplexed ligands nor free metal ions within the polymer (Figure 5.2, route-Ib1). Alternatively, a suitable polymer is decorated with a metal complex bearing a functionality that can bind to the polymer backbone (route-Ib2); using this procedure, no uncomplexed ligands or metal ions will be present in the final polymer. Depending on the coupling reaction, the conversion of the linker groups may be incomplete, thus the open sites can be further utilized to synthesize polymers bearing different amounts of metal complexes. Polymers, possessing a ligand bound to the backbone, can be complexed either by appropriate metal ions, leading to crosslinking, or by a metal complex, resulting in polymers bearing a side-chain metal complex (type-Ib3). Using this route, in contrast to route-Ib1 and route-Ib2, uncomplexed ligands can still be present in the final polymer. Depending on the complex’s stability and final application, one has to choose a suitable route for an individual polymer (for nonconjugated polymers with terpyridine units in the side-chain, see Chapter 6.2).

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| 201

202

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes Ib1 (Co)polymerizing complex monomers

Ib2 Decoration of the polymer with complex

Ib3 Complexation at the polymer

IIb1 Ring opening polymerization

IIb2 Polycondensation of complex monomers

IIb3 Polymerization by complexation

Figure 5.2 Different approaches for the synthesis of metallopolymers (type-I and type-II) containing transition metal complexes [33]. Figure reproduced with kind permission; r 2011 The Royal Society of Chemistry.

Focus is herein placed on metallopolymers bearing metal complexes in the main-chain. Three routes can be applied to synthesize such polymers. Figure 5.2: route-IIb1 is based on cyclic monomers that can be opened to generate the linear polymer. This route is very popular, in particular, for the synthesis of metallocenophanes [19]. In route-IIb2, the p-conjugated system connecting the metal complexes is formed upon polymerization. Thus, the metal complexes have to be stable under the polymerization conditions. In principle, the established polymerization techniques towards conjugated polymers (e.g., electropolymerization, Pd0-catalyzed cross-coupling reactions) can be utilized. The molar masses of polymers synthesized according to this route can be determined via the Carothers’ equation [34]. Both an exact stoichiometry and high yield of the coupling reaction are necessary to obtain high molar mass polymers; however, this technique is not as versatile when compared to route-IIb3. In the latter route, the step-growth

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10000 Degree of Polymerization

1M

1000

100

0.05 M

10

1

101

102

103

104

105

106

107

Ka Figure 5.3 Theoretical plot of the degree of polymerization (DP) versus association constant Ka (mol1) at two different concentrations according to an isodesmic self-assembly model [39]. Figure reproduced with kind permission; r 2001 American Chemical Society.

reaction of conjugated oligomer/polymers bearing two coordination sites is realized by the appropriate metal ion [for the polymerization of telechelic nonconjugated bis(terpyridine)s according to route-IIb3, see Chapters 6.3.1 and 6.3.2]. Owing to the attractive experimental protocol, that is, the telechelic conjugated ligand and particular metal salt are reacted in a suitable solvent, this method is frequently applied. However, the complexation constant must be high enough to form polymers with high molar masses [18]. There are different non-covalent interactions available for the synthesis of coordination polymers: hydrogen bonding [20, 35–39], p–p-stacking [36–39], van der Waals interactions [36–39], and transition metal ion coordination [7, 15, 16, 18 –21, 36, 37, 40]. Furthermore, combinations of these interactions within one supramolecular material, for example, hydrogen bonding and metal coordination, have been reported [41–45]. The main difference between conventional, covalent polymers and supramolecular polymers is the dependence of the average degree of polymerization (DP) on the strength of the end-group interaction [46] and, therewith, on different variables, such as solvent, temperature, and concentration. For reversible coordination polymers Eq. (5.1) and Figure 5.3 can be applied to estimate the DP [36, 39, 47]: 1

DP ðK ½MÞ2

(5.1)

where K is the binding constant, [M] is the monomer concentration. To obtain polymers with a high DP, a high binding constant between the repeating units is required. In analogy to covalent condensation polymers, the

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| 203

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| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes chain length of supramolecular polymers can be varied by addition of monofunctional “chain stoppers” [48]. The major advantage of coordination polymers, compared to their classical counterparts, is the possibility to produce tailor-made materials, utilizing knowledge of binding constants and, consequently, the manner of coordination. For intermolecular interactions, a detailed investigation of both the thermodynamics and kinetics of the metal complexes is necessary to choose a suitable metal–ligand combination for the respective application. The thermodynamic properties, expressed by the binding constants, determine whether the complex will be stable, whereas the kinetic properties are referred to by the terms inert or labile and can be expressed, for instance, in the half-life of the respective metal–ligand pair [18]. For various applications the ideal combination is, therefore, a system that provides a complex of high thermodynamic stability combined with high kinetic lability. This combination ensures the construction of a high molar mass coordination polymer, improved by the inherent advantages of reversible supramolecular systems, specifically, self-healing; the incorporation of additional moieties with identical ligand units produces, for example, copolymers and switching between the polymeric, oligomeric, and monomeric states. Furthermore, external stimuli can be applied to the reversible polymers to initiate switching from the monomeric state to the polymeric state or between different self-assembled architectures, such as macrocycles and polymers. Some of these systems can be repeatedly switched, in contrast to classical covalent polymers, which are formed irreversibly [20, 36, 49–53].

5.3 Metallopolymers Based on p-Conjugated Bis(terpyridine)s 5.3.1 Polymerization by Transition Metal Ion Coordination

Various N-heterocyclic ligands (e.g., 2,20 -bipyridine, 1,10-phenanthroline, and 2,20 :60 ,200 -terpyridine) have attracted considerable interest, as supramolecular templates, due to their high binding affinity towards many transition metal ions. Among these, 2,20 :60 ,200 -terpyridine is ideally suited for the construction of coordination polymers [54, 55]. Prevention of D/L-chirality in comparison to related metal complexes with 2,20 -bipyridine ligands by formation of distorted octahedral complexes as well as the easy access to 40 -substituted terpyridines enables the efficient coupling to stiff conjugated spacer molecules and, therewith, offers a possibility to synthesize linear metallopolymers [54–58]. The most commonly used method for the synthesis of such terpyridine-based metallopolymers is the complexation of a linear ditopic ligand by addition of stoichiometric amounts of transition metal ions (Figure 5.4). In view of potential optoelectronic applications, a large variety of spacer units has been introduced for this purpose. In this context, the ability of rare earth metal ions to form 3D-networks by complexation of ditopic tridentate ligands has to be mentioned. Participation of

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5.3 Metallopolymers Based on p-Conjugated Bis(terpyridine)s

x x1 Figure 5.4 Formation of a linear coordination polymer using a ditopic ligand and transition metal ion.

N

N

N

N N N

O

O

O

O

O

O

N N

1

N

N

Figure 5.5 Ditopic bip-type ligand 1 [49].

their f-orbitals enables lanthanide ions to establish up to nine coordinative bonds and, thus, bind up to three tridentate ligands [59–63]. The multiresponsive behavior of 3D-metallopolymers based on the bip-type ligand 1 (bip: 2,6-bis (1-methyl-1H-benzo[d]imidazol-2-yl)pyridine, Figure 5.5) and various transition and rare earth metal ions are detailed in Chapter 6 [20, 49–51]. The synthesis and properties of different types of p-conjugated bis(terpyridine) architectures are summarized in Chapter 2; most of these ligands were also used for the synthesis of coordination polymers, incorporating different transition metal ions. In general, the ligand is dissolved in a polar, non-coordinating solvent to ensure solubility and high molar masses of the resulting polymer. Subsequently, the transition metal salt is added either at once or in small portions. After an appropriate reaction time, anion exchange is carried out in most cases to instill solubility in a specific organic solvent. Following this protocol, RuII-, ZnII-, FeII-, NiII-, CoII-, and CdII-containing metallopolymers were obtained. Selection of the metal ion depends on the targeted potential application of the resultant metallopolymer. For instance, metals forming kinetically inert complexes (e.g., RuII, NiII, FeII, and CoII ions) will quench the emission of the conjugated spacer unit by metal-to-ligand charge-transfer (MLCT, radiationless deactivation caused by the low metal-centered (3MC) state) [64], whereas the complexes with labile ZnII ions do not show MLCT absorption and, therefore, the corresponding metallopolymers remain emissive. In 1992, the group of Constable introduced the general concept of terpyridinylfunctionalized telechelics [i.e., bis(terpyridine)s 2 and 3], which can form coordination polymers upon addition of metal ions (Figure 5.6) [65–67]. In addition, the structural prototype for all bis(terpyridine) motifs – the ditopic

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| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes N

N N

N

N

2 N

N

3

N

4

N N

N N

N N

N N

N

N

Figure 5.6 Basic bis(terpyridine) motifs, as templates for rigid-linear metallopolymers.

2,3,5,6-tetrakis-(pyridin-2-yl)pyrazine (tppz, 4, Figure 5.6) [68] – was utilized by the same group to prepare coordination polymers; the self-assembly of 4 with CoII ions led to a metallopolymer featuring three different CoII environments in the solid state, as shown by X-ray diffraction [69]. However, the first soluble and welldefined coordination polymer of 4 with RuII ions was reported by Fantacci et al., almost a decade later [70]. Metallopolymers based on RuII ions and terpyridine ligands possess very promising photophysical and electrochemical properties, since they have potential to facilitate the charge carrier generation (i.e., the formation of electrons and holes in semiconducting materials). Such metal complexes usually exhibit a reversible RuII/RuIII or some ligand-centered redox process [64]. In addition, when incorporated into a polymer, a RuII complex will influence the optical as well as the electronic properties of the metallopolymer due to its characteristic MLCT transition around 500 nm, thus extending the material’s absorption range. Therefore, such RuII coordination polymers were used, as a photoactive layer, in solar cells [71, 72]. Vellis et al. used three different linear ditopic ligands and a tritopic ligand for complexation. Reaction with Zn(OAc)2 at room temperature or RuCl3 in a EtOH–THF mixture at reflux lead to linear polymers (5b/c and 6b/c, respectively) and 3D networks (7 and 8, Figure 5.7) [71]. The ruthenium polymers were then used, as an additional layer, in a P3HT/PCBM bulk-heterojunction solar cell (BHSC), resulting in efficiencies of 0.33–0.71% (P3HT: poly(3-hexylthiophene), PCBM: [6,6]-phenyl-C61-methyl butyrate). Utilizing in both cases the same ligands (Figure 5.7) [72], Wild et al. synthesized two ZnII- (5a and 9) and two RuIIcontaining p-conjugated metallopolymers (6a and 10), which were identified by size-exclusion chromatography (SEC), analytical ultracentrifugation (AUC), and viscosimetry as high molar mass materials. Homogeneous films of these polymers could be produced by spin coating; however, the introduction of the RuII metallopolymers, blended with PCBM, as photoactive layer in bulk-heterojunction solar cells, resulted in very low efficiencies due to morphology problems caused by the different solubility behavior of the PCBM derivative and 6a or 10. Beside polymer solar cells (PSCs), (supramolecular) polymer light-emitting devices (PLEDs) represent a further highly active field of research [1, 9, 18]. In this respect, the combination of ZnII or CdII ions with conjugated bis(terpyridine)s

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4n PF6

6n PF6

N

C8H17

9: M ZnII 10: M RuII

N

C8H17

7: M Zn 8: M RuII

II

N N

N M N

N

N

N

N

N

N

N

M

N

M N

N

3 n

N

N

N

n

C12H25O b

C8H17 C8H17

OC12H25

spacer-unit

Figure 5.7 RuII and ZnII coordination polymers according to Vellis et al. [71] and Wild et al. [72].

N

2n PF6

5a-c: M ZnII 6a-c: M RuII

spacer-unit

N

a

c C6H13 C6H13

C8H17 C8H17

N N

N M

N

N N n


| 207

208

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes N

N spacer-unit

spacer-unit

N m

Cd2 N

N

C8H17 C8H17

n 2n PF6

N

N

11

12a (m 1) 12b (m 2) 12c (m 3)

Figure 5.8 Metallopolymers containing CdII ions [73].

appears to be well-suited, since – contrary to materials containing RuII or FeII ions – the incorporation of metal ions with a d10 electron configuration will not quench the emission of the conjugated linker in the metallopolymer. In general, depending on the electronic nature of the spacer, the absorption and emission maximum are only slightly shifted. Thus, the emission color of the so-produced metallopolymers can easily be tuned by simply varying the conjugated spacer. Chen et al. presented a series of CdII coordination polymers (11 and 12), synthesized by self-assembly polymerization of bis(terpyridine) ligands, featuring triphenylamine and oligofluorene spacers, respectively (Figure 5.8) [73]. As a considerable drawback, CdII salts are expensive and known to be cancerogenic, since CdII can replace ZnII in many biological systems. Thus, the low cost and environmentally friendly ZnII is more often used for the synthesis of room temperature, emissive terpyridine-based metallopolymers. Since terpyridine complexes with ZnII ion are kinetically labile, the mixing of different monomers will result in a random copolymer, enabling the coverage of the whole visible spectrum. The assumption of reversible self-assembly and disassembly of ZnII-bis(terpyridine) coordination polymers upon addition of an excess of ZnII ions was supported by diffusion-ordered NMR spectroscopy (DOSY NMR) as well as fluorescence anisotropy measurements [74]. Both methods are sensitive to the diffusion coefficients of the analyte and resulted in a significantly decreased diffusion coefficient for a ZnII polymer (1 : 1 ratio of ligand and ZnII) compared to the fragments obtained by exceeding the ideal 1 : 1 ratio. Owing to the complexes’ lability, it is difficult to determine the DP of the metallopolymers. The common methods, such as SEC, AUC, or matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, are not applicable for these particular materials; however, end-group analysis using 1 H NMR spectroscopy can be applied to estimate their DP. Atomic force microscopy (AFM) investigations by Stepanenko et al. revealed the deposition of rigid metallopolymers with average chain lengths of up to 35 repeating units from solution onto a mica surface [75]. Table 5.1 provides a representative overview of the photophysical and electroluminescence data of the large variety of synthesized systems possessing a general structure {[Zn(bis(terpyridine))](PF6)2n}. Contrary to metallopolymers with RuII, FeII, or OsII ions, all ZnII polymers summarized here were – due to their lability – exclusively synthesized via the coordination route (Figure 5.2, route-IIb3). In

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5.3 Metallopolymers Based on p-Conjugated Bis(terpyridine)s Table 5.1

Optical properties of p-conjugated ZnII coordination polymers (N.a. means not

available).

N

N

spacer-unit

2

Zn

N N

2n PF6

N

n

N

Metallopolymer

kabs (nm)

kPL (nm)

kEL (nm)

UPL

13 14 15 16 17 18 19 20 21 22

414a 429a 405d 403a 403a 400d 376d/400e 370g/369e 576i 594k

459a,b 488a,b 511b,d 470a,b/568e 468a,b 586b,d 418d/513e 449g/528e 602i N.a.

N.a. N.a. N.a. 600 N.a. N.a. 551 553 N.a. N.a.

0.15a,c 0.12a,c 0.31c,d 0.36a,c/0.15c,e 0.52a,c 0.42c,d 0.23d,f 0.33f,g/0.32e,h 0.49i,j 0.61k

Reference [29] [29] [79] [29] [29] [79] [77] [76] [81] [74]

CHCl3–DMF (2 : 1 ratio, 106 M, room temp.). Excited at labs. c Absolute values. d DMF (106 M, room temp.). e Measured in thin film. f Coumarin-1 in EtOH (FPL ¼ 0.90) was used, as reference. g DMF (105 M, room temp.). h 9,10-Diphenylanthracene doped in PMMA was used, as reference. i CHCl3–MeOH (4 : 1 ratio, 2.5 105 M, room temp.). j Sulforhodamine-101 hydrate in EtOH (FPL ¼ 0.90) was used, as reference. k CHCl3–MeOH (3 : 2 ratio, 105 M, room temp.). a b

particular, the groups of Lin [76–78] and Schubert [29, 72, 79, 80] presented highly diverse libraries of supramolecular polymers by variation of the conjugated spacer units (representative examples 13–20 are depicted in Figure 5.9). The resultant materials showed absorption maxima in the range 350–450 nm; introduction of electron-accepting moieties (e.g., 15 and 18) enabled a redshift of the photoluminescence maxima to almost 600 nm [79]. The introduction of BODIPY (21, BODIPY: boron-dipyrromethene) [81, 82] or perylene-bisimide spacer units (22) [74, 75] similarly pushed the absorption maximum of the resultant polymer up to around 600 nm (Figure 5.9). As noted above, random ZnII copolymers can be synthesized by simply mixing the respective ligand monomers in an appropriate ratio. After addition of ZnII ions, the monomers will then assemble in a quasi-statistical manner. According to Lin et al., the stepwise-directed assembly of two bis(terpyridine)s with ZnII ions

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| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes

spacer-unit

C8H17O OC6H13

OC6H13

OC8H17

OC6H13 N

13

C6H13O

C6H13O

C6H13O

S

OC6H13 18

OC6H13 14

C6H13O C6H13O

C8H17O

15

C8H17O

O

O

N OC8H17

S

N

19 C6H13

F

O F

OC8H17

F OC8H17

16

C6H13

O

C18H37O

C10H21O

20

OC10H21

OC6H13

17

N

N

C6H13O

B F F

C6H13 C6H13

F

R R O

O

N

N

O

N

O R R

21

22a (R tBuPhO) 22b (R tOcPhO)

Figure 5.9 Selected examples of ZnII metallopolymers (see also Table 5.1).

resulted in a metallo-copolymer with alternating ligands [77]; however, due to the ¨tter et al. syncomplex liability, a quasi-statistical order results. Recently, Schlu thesized the statistical ZnII coordination polymer 9 (i.e., {[Zn(23)0.5(24)0.5](PF6)2}n), where an energy-transfer between the electron-rich “donor” (23) and electron-deficient “acceptor” ligand (24) could be observed [79]. Contrary to the behavior of the simply mixed monomers [bis(terpyridine)s 23 and 24] (Figure 5.10), excitation at the longest absorption band wavelength of the metallopolymer resulted in a distinct domination of the emission attributed to the acceptor moiety, whereas only a shoulder confirmed the emission originating from the donor component. Both decreasing and increasing the excitation wavelength led to a further reduction of the donor-based emission. The simple mixture of 23 and 24 revealed no indication for such energy-transfer; thus, the corresponding ZnIIbased statistical copolymer proved that the central ZnII bis(terpyridine) moieties can play a crucial role in such a transfer process. Supramolecular polymers based on ZnII ions with tunable and predictable photophysical properties are not only easy to synthesize but can be processed in a straightforward manner from solution (e.g., by spin coating or inkjet printing

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0.0

0.2

0.4

0.6

0.8

350

N

1.0

N

400

23

0.0 400

24

450

500 (b)

600

N

N

N

N

550

N

600

mixtures of 23 and 24 1:1 ratio, λex 370 nm 2:1 ratio, λex 370 nm 1:2 ratio, λex 370 nm

C 8H 17

(a)

550

0.2

0.4

0.6

0.8

1.0

1.2

1.4

N

N

C 8H 17

λ ( nm)

500

N

N

N

λ ( nm)

400

9; λex 310 nm 9; λex 330 nm 9; λex 370 nm 9; λex 390 nm 9; λex 400 nm acceptor monomer (24) donor monomer (23)

C 8H 17 C 8H 17

N

Normalized PL

Figure 5.10 (a) Normalized photoluminescence (PL) spectra of {[Zn(23)0.5(24)0.5](PF6)2}n (9) at different excitation wavelengths compared to monomers 23 and 24; (b) normalized PL spectra at different acceptor (24) to donor (23) ratios [79]. Figure reproduced with kind permission; r 2010 American Chemical Society.

Normalized PL

N


| 211

212

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes techniques); thus, they can be considered as emissive materials in light-emitting devices. The first examples of PLED devices using such ZnII coordination polymers were constructed by Che et al. [83], followed by others [29, 76–78]. To achieve good processability, adequate solubility in organic solvents is a critical requirement. Most known systems still exhibit limited solubility due to their pronounced rigid-linear architecture, even in the presence of long or branched alkyl chains. A significant improvement was shown by Winter et al., who introduced poly(e-caprolactone) chains via SnII-catalyzed ring-opening polymerization of ecaprolactone with o-hydroxyalkyl-functionalized bis(terpyridine)s 25, as initiator, on the polymer’s backbone to enhance both its processability and film-forming ability (Figure 5.11) [80]. The macroligands 26 were utilized for self-assembly with ZnII ions into metallopolymers 27 and its photophysical properties were investigated in both the liquid (dilute solution) and solid state (thin films on glass slides). 5.3.2 Self-assembly of Metallopolymers

Besides directly depositing and investigating the metallopolymers via spin coating from a DMF solution onto surfaces, the Kurth group used the coordination polymers for the self-assembly of advanced layer architectures. These systems were then successfully applied as either electrochromic devices or molecular switches [21, 84–86]. To gain further insight into the interplay of electrostatic interactions between polyelectrolyte polymers and their counterions, detailed investigations were conducted using two different RuII metallopolymers, based on oligophenylene-bridged bis(terpyridine)s [87]. Utilizing the spin-carrying dianion Fremy’s salt (FS, potassium nitrosodisulfonate), Hinderberger et al. demonstrated by continuous-wave as well as double-electron electron paramagnetic resonance (EPR) spectroscopy that the spatial distribution of the counterions reflects the charges along the polymer chain of 28 [87]. Furthermore, the counterion cloud around bis(terpyridine)-coordinated transition metal centers could be shown to have an almost Gaussian radial distribution with a width of about 0.5 nm (Figure 5.12). Owing to the high charge density, the solubility of metallo-supramolecular polyelectrolytes (MEPEs) critically depends on the counterions. Since most bis(terpyridine) ligands are hydrophobic, water-solubility mediating counterions, such as chlorine or acetate, have to be used to obtain the desired water-soluble MEPEs. In particular, the group of Kurth has been active in this field, synthesizing MEPEs 29 with bis(terpyridine) 3 and FeII, CoII, or NiII ions, as well as carrying out detailed characterization of the resulting polymers, then self-assembling them by making use of their pronounced polyelectrolyte character (Figure 5.13) [15, 16, 18, 60, 88, 89, 96–99]. This same group developed a reliable method to obtain MEPEs in a highly reproducible way by conductometric titration of the metal ions to the ligand, affording an exact 1 : 1 stoichiometry, a prerequisite for a high DP [100]. After spin

05

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05 2

2 PF6

N

N

OR 27a: R1 CH3, R2(CH2)6O[CO(CH2)5O]nH 1 1 27b: R R (CH2)6O[CO(CH2)5O]nH

R1O

2

OR

25a: R1 CH3, R2 (CH2)6OH 25b: R1 R2 (CH2)6OH

R1O

N

N N

N Zn N

2

N N

cat. Sn(II), neat, 120 °C, 4 h

M/I 100

n

OR

2

26a

27a

26a

26a: R1 CH3, R2 (CH2)6O[CO(CH2)5O]nH 26b: R1 R2 (CH2)6O[CO(CH2)5O]nH

R1O

i) Zn(OAc)2, DMF, 80 ºC, 12 h ii) NH4PF6

N

N

N

N

N

N

27a

Figure 5.11 Synthesis of metallopolymers via ring-opening polymerization and subsequent self-assembly with ZnII ions. Images of the dilute solutions as well as thin solid films of macroligand 26a and metallopolymer 27a, upon irradiation with UV-light, are also shown [80].

N

N

N

O

O


27 J l 2011 15 50 47

| 213

214

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes (a) OR

N

Ru2

N RO R (CH2CH2O)3CH3

N

N

O3S

N

N

n

2n PF6

28

N O

SO3 FS

(b)

2.35 nm

2.35 nm

Figure 5.12 (a) RuII metallopolymer 28 and Fremy’s salt (FS). (b) Possible localized binding of FS ions to RuII centers in 28; an end-capped coordination dimer that was geometry-optimized by force-field calculations. The distinct distances between FS anions found from double-electron EPR data analysis (about 2.4 nm and about 4.7 nm) agreed with the distances between RuII centers separated by one or two organic spacers, respectively. Additionally, a schematic plot of the most probable attachment sites for FS ions (gray area) is shown [87]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

coating of 29 (M ¼ FeII or NiII), the shape and size of the supramolecular metallopolymers could be determined by AFM and small-angle neutron scattering (SANS); the chains obtained from rigid monomer 3 revealed an estimated length of up to 500 nm, corresponding to a DP of ~370 or a molar mass of ~200 000 g mol1 (Figure 5.14a–c). For comparison, the flexible 1,3-bis[40 -oxa(2,20 :60 ,200 -terpyridin4-yl)]propane ligand was also utilized, resulting in smaller, discrete assemblies (30), presumably rings with three to four repeating units (Figure 5.14d). The positive charges of MEPEs 29 could further be used for their deposition on negatively charged surfaces, such as poly(styrene sulfonate) (PSS). The alternating layers of PSS and MEPE on a poly(ethylene imine) (PEI) modified quartz substrate (Figure 5.13) were characterized by UV–vis absorption spectroscopy, microgravimetry, cyclic voltammetry (CV), and permeability measurements as well as surface plasmon spectroscopy (SPS) [90, 96, 97]. In related work, Stepanenko et al. performed an electrostatic layer-to-layer self-assembly using ZnII coordination polymers, such as 22 and PSS [75]. UV–vis spectroscopy clearly indicated a linear increase of all complex bands after each double layer addition (Figure 5.15a). AFM measurements revealed that the surface roughness of a PEI/PSS multilayer is about four times larger than that of a PSS/22 multilayer film (Figure 5.15b). After

05

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N

N N

N

MX2

N

N

N M

2

N

n

M = FeII, NiII or CoII N

| 215

3

N

N 2n X

N

Electrostatic self-assembly

29

PSS DHP

PEI coated substate Core-shell particles and hollow shells

Layer by layer self assembly

Polyelectrolyte amphiphile complexes

Figure 5.13 Hierarchical self-assembly of terpyridine-based coordination polymers. Figure redrawn according to publications by Kurth et al. [21, 88–95]).

deposition of twelve polymer layers, the root-mean-square value of the surface roughness was determined to be 1.2, 9.2, and 2.1 nm for a plain quartz substrate, PEI/PSS film, and PSS/22 multilayer film, respectively. The same step-wise protocol for assembly could also be applied with polystyrene (PS) latex particles, as template for the formation of core–shell particles (Figure 5.13) [88, 89]. For this purpose, PS lattices (diameters of about 70 nm) were coated with 15 alternating layers of 29 (M ¼ FeII) and low molar mass PSS to avoid particle bridging. Closer observations of the coated particles’ surface revealed a different surface texture when compared to uncoated particles, suggesting the presence of polyelectrolyte multilayers on the surface (Figure 5.16a and b). Moreover, the (29/PSS)-coated PS lattices displayed an uneven contour (Figure 5.16c and d) that was attributed to a rigid-linear conformation of the FeII-MEPE polymer. Weakly crosslinked melamine formaldehyde (MF) particles (diameter of about 1.7 mm), as a decomposable colloid template, were also applied. Five alternating layers of 29 and PSS (Mn ¼ 70 000 g mol1) were assembled onto the particles, followed by subsequent decomposition of the template under basic conditions. The so-obtained hollow capsules with an average shell thickness of 10 nm were investigated by transmission electron microscopy (TEM) and AFM (Figure 5.16c and d).

05

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| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes µm

3

µm

3

2

2

1

1

(b)

(a) 0

1

2

µm

0 3

0

1

3

2

µm

0 3 3 µm

10.0 nm

µm

216

5.0 nm

2

2 0.0 nm

1

1

(c) 0

(d) 1

2

µm

0 3

N O 30

0

N

2

µm

0 3

N Fe2

N

1

N

N

O

n

2n CH3COO

Figure 5.14 AFM images of spin-coated solutions of 29 (M ¼ FeII) on mica substrates for [FeII]/[3] ¼ (a) 0.60, (b) 0.85, and (c) 0.98 and (d) 30. The corresponding average lengths of the MEPE rods were (a) 150, (b) 320, and (c) 460 nm. The uncorrected average diameter of the dots observed for 30 (d) is 6 nm. The rod diameter was about 1.5 nm [100]. Figure reproduced with kind permission; r 2010 American Chemical Society.

Counterion exchange of 29 by amphiphilic anions, such as di(hexadecyl)phosphate (DHP), furnished so-called polyelectrolyte-amphiphile complexes (PACs) (Figure 5.13) [91–95]. The PACs were soluble in organic solvents, spreading at the air–water interface, and the resulting Langmuir monolayer could be transferred onto a solid support, employing the Langmuir–Blodgett (LB) technique [101]. In addition, the PACs could be aligned by co-adsorbing them with higher alkanes on graphite surfaces (Figure 5.17a and b) [92]. Another interesting feature of these PACs is the possible spin-crossover phenomenon [91, 95, 102, 103]. In general, FeII complexes can adopt both a low-spin (LS) 1A1g as well as high-spin (HS) 5T2g conformation in an octahedral ligand field. It is known that 2,20 :60 ,200 -terpyridines introduce a strong ligand field; therefore, the metal ions will be forced into a LS

05

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0.4

@330 nm @480 nm @590 nm @700 nm

0.14

Abs. (a.u.)

0.12

Abs. (a.u.)

0.3

0.10 0.08 0.06 0.04

0.2

0.02 0.00

0

2

4 6 8 10 Number of Layers

0.1

0.0

300

450

600 λ (nm)

750

12

900

(a)

(b)

Figure 5.15 (a) UV–vis spectra of self-assembled alternating multilayers of a [quartz/PEI/ (PSS/22)12] configuration. Inset: absorbance at the absorption maxima at 330 (’), 480 (○), 590 (~), and 700 nm (), as a function of the number of layers of 22; the arrows indicate the change of absorbance with increasing number of layers. (b) Height AFM image of a film after deposition of coordination polymers; the scale bar is 1 mm; z data scale is 25 nm. Inset: an AFM angle view image [75]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

(c)

(a)

60 nm

500 nm

(d)

(b)

60 nm

500 nm

Figure 5.16 TEM images of (a) uncoated 70 nm diameter PS latex particles, (b) the same particles coated with [(PSS/29)7/PSS], (c) [(PSS/29)2/PSS]-coated 1.7 mm diameter MF (melamine formaldehyde) particles, and (d) hollow shells obtained after decomposition of the colloidal template [88, 89]. Figure reproduced with kind permission; r 1999 American Chemical Society.

05

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| 217

218


(a)

(d)

N N

N N Fe2 N

N

n

31

(b)

N N

N N

2n BF4

(e)

xT [emu k mo[1]

3.0

100 nm

2.5 2.0 1.5 1.0

T1/2 = 323 K

0.5

(c)

∆T1/2 = 10 K

0.0

T

0

50

100

150

200

250

300

350

400

Temperature [K] eg

t2g Figure 5.17 (a) Nanostructures (not scaled) of PAC-29 on a graphite surface. The alkane (thick gray rods) is adsorbed epitaxially on the lattice of the basal plane of the underlying graphite surface; this alkane monolayer served as template for orienting the rigid PAC rods. (b) AFM image of PAC-29 adsorbed in the presence of C32H66 on the basal plane of graphite [92]. (c) Upon heating PAC-29, the alkyl chains of the mesophase melt cause a distortion of the metal ion coordination geometry. The unfavorable coordination of terpyridines around the FeII cations induced a lowering of the energy gap between the d-orbital subset giving rise to a reversible transition from a diamagnetic low-spin state (left) to a paramagnetic high-spin state (right) [95]. (d) Metallopolymer 31, containing a bpp-type ligand. (e) Temperature-dependent magnetic measurements of 31 between 4.5 and 300 K (k cooling mode; m heating mode) [104]. Figure reproduced with kind permission; r 2002 Wiley-VCH, 2005 American Chemical Society and 2007 The Royal Society of Chemistry, respectively.

state. For FeII ions, the terpyridine coordination polymers are diamagnetic in that all electrons are paired. At low temperature, PAC-29 densely assembled in both LB films as well as a crystalline state; therefore, it did not affect the strong crystal field around the FeII ions. Upon heating the multilayer, the hexadecyl chains melted, resulting in a distortion of the metal ion coordination geometry. Consequently, the crystal field splitting of the d-orbital subsets decreased, resulting in a reversible

05

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5.3 Metallopolymers Based on p-Conjugated Bis(terpyridine)s (a)

C4H9

N

32a (n 0) 32b (n 1) 32c (n 2) C4H9

(b) C10H21

N

N

N N

N

N

N

N

C4H9

{[Eu(32b)3](NO3)3}n

N

n

N

N

N

N N

N

| 219

N

N

N

N

C4H9

C10H21

spacer-unit 33

N

spacer-unit

N 34

C10H21

N

N

N

N

N

N

C10H21

Figure 5.18 Btb-type ligands, as a building block, for rigid-linear metallopolymers and 3D metallo-networks. The solid-state photoluminescence of {[Eu(32b)3(NO3)3]}n is also depicted (scale bar: 10 mm) in part (a) [105]. Figure reproduced with kind permission; r 2010 American Chemical Society.

spin-transition from a diamagnetic LS to a paramagnetic HS state (Figure 5.17c) [95, 103]. Chandrasekar et al. showed that bis(terpyridine) analogs, such as 1,4-bis[2,6-di (1H-pyrazol-1-yl)pyridin-4-yl]benzene (bpp-type) [104] or 1,4-bis[2,6-bis(1-butyl-1H1,2,3-triazol-4-yl)pyridin-4-yl]benzene (btp-type) [105], were suitable ditopic ligands for the synthesis of metallopolymers. For instance, addition of one equivalent of FeII to a BPP-type ligand led to the rigid-linear coordination polymer 31 (Figure 5.17d), which exhibited a reversible spin-transition at 323 K with a 10 K ¨ssbauer spectroscopy studies hysteresis loop, as shown by magnetic and Mo (Figure 5.17e). The self-assembly of btb-ligands 32, using EuIII ions, resulted in a highly luminescent 3D-polymer (Figure 5.18a) [105]. The structurally related telechelics 33 and 34 were synthesized by Schulze et al. and subsequently polymerized using [Ru(acetone)6](BF4)3, leading to linear, rigid coordination polymers (Figure 5.18b) [106]. Beside classical uncharged bis(terpyridine)s, viologen-like [107] structures were also utilized in the synthesis of metallopolymers 35 using, for instance, CoII, CdII, ZnII, and FeII ions [85, 86]. Following a layer-by-layer self-assembly approach, a multilayer architecture of the configuration [(PSS/35)40] (M ¼ CoII) on indium tin oxide (ITO) electrodes was successfully used in an electrochromic device (Figure 5.19) [86]. 5.3.3 Chiral Metallopolymers

ã group reported the synthesis of RuII metallopolymers bearing chiral The Abrun pinene moieties annelated with outer terpyridine rings [108]. Bridging two of these

05

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220

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes E 1.1V

0.20 0.18 35 H2 N C

H2 C N

M2+

N N

4nX II

II

II

N N

n

Abs. 643 nm

0.16 N

N

0.14 0.12 0.10 0.08

II

M Co , Cd , Zn , Fe

0.06

E 0.0V

0.04 0 (a)

2

4

6 t(min)

8

10

12

(b)

Figure 5.19 (a) Metallopolymers 35 containing a viologen-type bis(terpyridine) ligand; (b) absorbance at 643 nm of a (PSS/35)40-coated ITO electrode during subsequent doublepotential steps between 0 and 21.1 V (versus Ag/AgCl electrode) [86]. Figure reproduced with kind permission; r 2005 The Royal Society of Chemistry.

modified terpyridines via their 40 -positions with a 4,40 -biphenyl spacer and subsequent self-assembly of the rigid bis(terpyridine) with RuII ions resulted in metallopolymer 36; however, bridging via one pinene unit using a p-xylene spacer gave the flexible polymer 37a (Figure 5.20a). The polymerization of both enantiomers of the same ligand with FeII ions was monitored by UV–vis spectroscopy. As expected, the circular dichroism (CD) spectra of both enantiomers of polymer 37b showed mirror images (Figure 5.20b). Scanning tunneling microscopy (STM) of thin films on highly oriented pyrolytic graphite (HOPG) confirmed the two different helical arrangements [i.e., right-handed (M) and left-handed (P)] that were used to estimate the DP to be 40–60 metal centers per chain (Figure 5.20c) [109]. Another chiral bis(terpyridine) building block was produced by Kimura et al.: upon complexation of either the (R)- or (S)-enantiomer of a BINOL-type bis(terpyridine) with FeII ions the MLCT band in the UV–vis spectrum indicated the formation of metallopolymer 38 [110]. A DP of about 50 was estimated applying size-exclusion chromatography; CD studies of 38 confirmed the chiral nature of the polymer (Figure 5.21a and b). Furthermore, complexation with FeII ions and introducing chiral tetra(ethylene glycol) units at the 6-position of both terpyridine units resulted in optically active supramolecular metallopolymers 39 (Figure 5.21a) [111]. The change in CD intensities as a function of added FeII ions confirmed formation of optically active coordination polymers. 5.3.4 Non-Classical Metallopolymers

For most terpyridine-based metallopolymers, a classical metal complex with two terpyridine units coordinated to the metal centers represents the connective unit between various types of p-conjugated spacers. However, besides these

05

27 J l 2011 15 50 50


(a)

N

N 2

N

Ru

N

N N

M

N

N

n

2n PF6

2n PF6

N

N

N

N

37a (M RuII) 37b (M FeII)

36

n

(c)

(b) 60

∆ε/M1cm1

40 20 0 20

mirror

40 300

400

500

600

700

Wavelength/nm

Figure 5.20 (a) Chiral pinene-derived metallopolymers; (b) CD spectra of ( þ )-37b (solid line) and ()-37b (dashed line) measured in CHCl3 (10 mM); (c) STM images of films of ( þ )37b (left) and ()-37b (right) on a HOPG surface showing left- and right-handed helices, respectively [109]. Figure reproduced with kind permission; r 2001 American Chemical Society.

“conventional” bis(terpyridine) coordination polymers, the group of Rehahn also synthesized a rod-like polymeric material (42) containing mono-cyclometalated heteroleptic RuII complexes (Scheme 5.1) in which p-conjugated bis(2-phenylbipyridine)s were used, as (co)monomer. In the first step of the polymer synthesis, bis(terpyridine) 40 was treated with two equivalents of ruthenium(III) chloride. The resultant dinuclear “metallomonomer” was subsequently converted with equimolar amounts of 41 into metallopolymer 42. The homogeneous constitution of this soluble polymer was confirmed; DPs >20 were estimated (1H NMR). Han et al. performed a systematic variation of both the spacer unit (i.e., one or two phenyl rings) and electronic nature of the terpyridine moiety by introducing electron-withdrawing bromine or electron-donating methoxy substituents at the 6-position. After polymerization with FeII, RuII, and CoII ions, respectively, these authors investigated the optical properties of the resulting polymers 43–45 (Figure 5.22a) [113–116]. Studying the electrochromic behavior revealed that introduction of the methoxy group noticeably enhanced the switching stability as well as reversibility of the resulting MEPEs, with an observed absorption loss of about 20% after 4000 redox cycles (Figure 5.22b), and also reduced the switching potential of the derived MEPEs [113]. Furthermore, the emission properties of the

05

27 J l 2011 15 50 50

| 221

222

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes (b)

(a) N O

N

N Fe

2

N

n

O N 38

N

2n PF6

R N

N

m

Fe2

N

m

N n

N

N

2n PF6 R R

O

O

O

O

(R)38 right-handed (P) helix

39a (m 0) 39b (m 1)

O

(S)38 left-handed (M) helix

Figure 5.21 (a) Chiral FeII-containing metallopolymers 38 and 39 [110, 111]; (b) representation of the two different helical orientations of the two enantiomers of 38 [110]. Figure reproduced with kind permission; r 1999 American Chemical Society.

N

N C6H13

N

N C6H13

N

40

N N

N C6H13

1. RuCl3, n-butanol 2. AgBF4, acetone 3. 41, DMA

C6H13

N

C6H13

2n BF4

N

N N

N

C6H13

N C6H13

C6H13

Ru2 N

N

N 41

N Ru2 N

n

N

42

Scheme 5.1 Synthesis of metallopolymer 42 containing mono-cyclometalated RuII complexes [112].

RuII-containing polymers (44) were investigated in detail at ambient and 77 K temperatures [116]. Moreover, the influence of a methyl-group, as an electrondonating substituent, at the 6-position was also investigated; again RuII-, FeII-, and CoII-containing polymers were synthesized and characterized by UV–vis absorption and emission spectroscopy [117]. Thereby, it was shown that the

05

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| 223

(a) MII FeII, X OAc R N M2+

N N 2n X

43a (R H, m 1) 44a (R H, m 1) 43b (R H, m 2) 44b (R H, m 2) 43c (R H, m 3) 44c (R OMe, m 1) 43d (R OMe, m 1) 44d (R OMe, m 2) 43e (R OMe, m 2) 44e (R Br, m 1) 43f (R Br, m 1) 44f (R Me, m 1) 43g (R Me, m 1) 44g (R Me, m 2) 43h (R Me, m 2) 43i (R tetra(ethylene glycol), m 2) 43j (R tetra(ethylene glycol), m 3)

N N

n

m

MII RuII, X Cl

N R

MII CoII, X OAc 45a (R H, m 1) 45b (R H, m 2) 45c (R OMe, m 1) 45d (R OMe, m 2) 45e (R Br, m 1) 45f (R Me, m 1) 45g (R Me, m 2)

(b)

Abs.

0.01

0

20

40

60

80 100 Time (s)

120

140

160

(c)

Figure 5.22 (a) Metallopolymers 43–45 bearing functional groups at the 6-position of the terpyridine moieties; (b) changes of the MLCT absorbance in a thin film of 44d at potentials of þ 1.5 and 0.0 V versus Ag/AgCl electrode with a 4 s delay between switching [the first 40 (blue), middle 40 (black) and last 40 (red) of 4000 redox cycles are shown]; (c) representation of the colors of 43a/b/d/e/f (0.25 mM, MeOH) (left), 44a–e [0.05 mM, MeOH/H2O (4 : 1 ratio)] (middle), and 45a–e (0.5 mM, MeOH) (right) [113]. Figure reproduced with kind permission; r 2008 American Chemical Society.

luminescence of the resulting RuII polymers was quenched due to its introduction. With FeII polymers, the extinction coefficients of the methyl-substituted polymers 43g/h were much lower than for the unsubstituted counterparts (43a/b). Thus, the methyl substituent prevented the MLCT due to steric hindrance. Additionally, Pal et al. showed how the introduction of tetra(ethylene glycol) (TEG) chains at the ortho-position of the peripheral pyridine ring led to enhanced fluorescence of the corresponding FeII metallo-supramolecular polyelectrolytes (43i/j). Metallopolymer 43j (FPL ¼ 0.11) displayed a retention of quantum yield that is nearly threefold higher than that of the unsubstituted analog (43c; FPL ¼ 0.04) [118]. This effect was

05

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224

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes attributed to a charge-transfer occurring within the monomer between the electron-rich TEG chain and the metal-coordinated, electron-deficient terpyridine moiety. Furthermore, the TEG chains could effectively shield the metallopolymers and, consequently, the fluorescence quantum yields were enhanced. The color of the metallopolymers, containing FeII, RuII, or CoII ions, is depicted in Figure 5.22c. Contrary to color tuning by using different p-conjugated spacers (e.g., see Table 5.1), in this case attachment of substituents to one pyridine ring and variation of the transition metal ions resulted in materials covering a large part of the color spectrum. The previously discussed examples were all based on monodisperse, well-defined, p-conjugated bis(terpyridine) ligands; however, metallopolymers derived from polydisperse, p-conjugated telechelic macroligands are rare. Weder et al. utilized their bip-type macroligands, as prepolymer, for the supramolecular polymerization by complexation with ZnII and FeII ions (Figure 5.23) [59, 119]. The self-assembly of 46 with FeII ions led to drastic changes in optical properties in which additional absorption – the MLCT band of the FeII complex – occurred and the fluorescence was completely quenched, in contrast to that of the ZnII analogues. In the case of the macroligand with additional aliphatic hexamethylene spacers, only slight changes of the optical properties upon complexation with FeII ions (47) were observed due to the electronic decoupling of the p-conjugated system of the oligomer from the metal complex site [120]. 5.3.5 Polymerization Using the “Complex First” Method

In previous sections, the focus was on classical metallopolymers, that is, they were synthesized by coordination utilizing a telechelic bis(terpyridine) ligand and a transition metal ion (Figure 5.2, route-IIb3). Since the binding constant of metal bis(terpyridine) complexes is very high for some transitions metal ions (e.g., RuII,

N

C8H17O

N N

2n PF6

1

2

OR

1

2

n

N N

N

OR

M2

N

1

RO

N N

N O(CH2CH2)2O

m

RO 47 M Fe or Zn 1 2 R C8H17, R 2-EtHex

N

1

OR

O(CH2CH2)3O

M2

N

m 46 M Fe or Zn

N N

N

OC8H17

RO

N 2n PF6

N

Figure 5.23 Metallopolymers based on polydisperse bip-type ligands [59, 119, 120].

05

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N N N

n


OsII, FeII) [18, 54] the orthogonal “complex first” procedure (Figure 5.2, route-IIb2) was applied for the construction of rigid-linear metallopolymers. In 1999, Kelch and Rehahn compared these two general synthetic strategies, that is, route-IIb2 and route-IIb3 (Scheme 5.2) [121, 122]. Applying the polymerization by coordination, bis(terpyridine) 40 was connected with RuII ions. For this purpose, RuCl3 xH2O was activated with AgBF4 in acetone, to which 40 was added and metallopolymer 48 was obtained by in situ reduction of RuIII to RuII ions. The DP was estimated by 1H NMR spectroscopic end-group analysis to be higher than 30, corresponding to a molar mass (Mn) of W 36 000 g mol1. The intrinsic viscosity (Z) was determined to be about 300 ml g1, which was comparable to the values observed for “classical” poly(p-phenylene)s. A strong polyelectrolyte effect – due to a positively charged backbone structure – was also found, but could be suppressed by performing the viscosity measurements in the presence of screening salts. Theoretically, route-IIb2 should lead to the same metallopolymer; however, a polymer could not be obtained when the RuII bis-complex

N

N C6H13

N

N

N

[Ru(acetone)6](BF4)3

C6H13

40

N

route-IIb3

C6H13

N

N Ru2

N

N n

48

N

C6H13

route-IIb3: X Cl route-IIb2: X BF4

N

2n X

route-IIb2

N Br

N 49

Ru N

C6H13

N 2

N

Br

(HO)2B C6H13

N 2 BF4

B(OH)2 50

Scheme 5.2 Two orthogonal synthetic strategies towards metallopolymer 48.

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| 225

226

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes 49 was reacted with bis-boronic acid 50. Owing to many side reactions, only oligomeric species were obtained. Storrier et al. synthesized amide- and imide-linked FeII polymers and also compared both routes (Scheme 5.3) [123, 124]. Both approaches – the conversion

2X N

N H2N

M2

N

N

NH2

N

N

51a (M FeII, X BF4) 51b (M RuII, X PF6)

route-IIb2

N

N spacer-unit

M

N N

2n X {[Fe(52)](BF4)2]}n (M FeII, X BF4) {[Ru(53)](PF6)2]}n (M RuII, X PF6)

2

N

n

N

route-IIb3 N

N

spacer-unit

N

52

N

spacer-unit

O

53b

CF3

N O

N 53e

O

N O O O

N

N O

O

N

O

O

F 3C

O

O 52c, 53a O

53c

N

O O

N H

52b

O

N

N O

O

O

O

O

H N

O

52a O

N

HN

NH

O

N

O N O

N 53d

O O S O 53f

O

O N O

Scheme 5.3 Synthesis of metallopolymers bearing amido- and imido-linkages.

05

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of the FeII complex 51a into metallopolymers {[Fe(52)](BF4)2}n (by reaction with the appropriate carboxylic acid derivatives) as well as the self-assembly by coordination using ditopic ligands 52a/b – resulted in metallopolymers with comparable NMR spectra. Since neither mass spectrometry nor SEC analysis could be applied to determine the DP, end-group analysis (1H NMR) was used. For this purpose, the polymerization of 52c was quenched with an excess of acetyl chloride and a DP of 17 could be calculated. Polyimides {[Ru(53)](PF6)2}n were synthesized by Chan et al. by polymerizing RuII complex 51b with different aromatic dianhydride monomers [125]. The first step of the polymerization involved addition of the amino group to an acid anhydride to afford the corresponding polyamic acid. The imidization was completed by heating the polyamic acid solution in a pyridine–acetic anhydride solution. Electron- and hole-carrier mobilities within an order of magnitude of 104 cm2 V1 s1 suggested that the electron-withdrawing diimide moieties played a substantial role in the charge transport processes of RuII polyimides. Emission from the metal complexes and charge-transfer states were observed. The polyimides also exhibited electroluminescent behavior (emission maxima around 650 nm) when the polymer was fabricated, as the emitting layer, in a PLED device. The external quantum efficiency and maximum luminance of these devices were found to be 0.1% and 120 cd m2, respectively. The same research group also prepared the RuII metallopolymers 57 via routeIIb2 by applying the Heck cross-coupling of diiodo-functionalized RuII complex 54 and 1,4-divinylbenzene (55) (Figure 5.24a) [126, 127]. Utilizing 1,4-dibromo-2,5-di(hexyloxy)benzene (56), as co-monomer, and performing the polymerization at different monomer ratios enabled the formation of metallopolymers with varying amounts of terpyridine complexes within the polymer chain. As a consequence, the macroscopic properties of the materials, for example, the solubility, could be tuned [126]. For metallopolymer 57 of components 54 and 55 (i.e., n ¼ 0), a molar mass of Mn ¼ 8900 g mol1 and a polydispersity index (PDI) of 1.5 according to SEC analysis were determined. The inherent viscosity was measured to be 0.35 dl g1 (in DMF at 30 1C). Furthermore, end-group analysis (1H NMR) revealed the signal of the vinyl end-group to be less than 3%, indicating a high DP [127]. Using a layer-bylayer (LBL) processing technique up to 60 bilayers of 57 and a sulfonated polyaniline (SPAN) were deposited. The particular deposition conditions (e.g., pH, electrolyte, and thermal annealing of multilayer films) had a significant impact on the film surface morphologies, which were investigated by AFM. Photovoltaic cells were fabricated by sandwiching the multilayer films between ITO and Al electrodes (Figure 5.24b); upon illumination with AM 1.5 simulated solar light, power conversion efficiencies of the order of 103% were achieved (Figure 5.24c). The maximum incident photon-to-electron conversion efficiency (IPCE) of these devices was found to be about 2% at 510 nm, which was consistent with the absorption maximum of the incorporated [Ru(tpy)2]2 þ moieties. Thus, the photosensitization process could be attributed to electronic excitation of the ruthenium complex. Electropolymerization, as synthetic tool, was also applied for the route-IIb2 preparation of metallopolymers with the complex as part of the main-chain as well as the side-chain (Figure 5.2a). Hjelm et al. synthesized 40 -functionalized RuII and

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| 227

228


(a) N

N I

Ru

N 2 PF6

C6H13O 2+

N

N

N

I

Br

+

+

54

Br

55

56

OC6H13

Pd(OAc)2, N(n-C4H9)3, DMF

N

N OC6H13

2+

Ru

N n

2 PF6

57

N

N

n p

N

C6H13O

(c) 102

current (µA cm2)

(b)

101 100

AM 1.5 dark

101 102 103 0.5

0

0.5 Voltage (V)

1

Figure 5.24 (a) Synthesis of RuII metallopolymers 57 by Heck cross-coupling reaction; (b) cross-section TEM image of a [57/SPAN] multilayer deposited on a silicon substrate; (c) current–voltage characteristics for the device [ITO/(57/SPAN)20/Al] under dark conditions and illumination with simulated AM 1.5 solar light (intensity 100 mW cm2) [127]. Figure reproduced with kind permission; r 2006 American Chemical Society.

OsII bis-complexes and reported their subsequent polymerization (Scheme 5.4a) [128–131]. The complexes were equipped with mono-, di-, or trithiophene units at the 40 -position of the terpyridines (RuII: 58, OsII: 59), which could subsequently be polymerized electrochemically. Continuous voltammetric cycling in degassed dichloromethane or MeCN solutions (containing 0.1 M [(n-C4H9)4N][BF4], 0.5–1 mM of complex and about 5% BF3 OEt2) resulted in the growth of the metal oxidation wave (0.89 V for RuII/RuIII and 0.48 V for OsII/OsIII) and a collapse of the ligand-based oxidation wave. The linear increase of the peak currents with the scan number indicated accumulation of the redox-active polymer (60 and 61) onto

05

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5.4 Main-Chain Metallopolymers Based on Terpyridine-Functionalized p-Conjugated Polymers

| 229

(a) N

2 PF6

N M

N S

m

N

S

m N

N

II

58a (M Ru , m 1) 58b (M RuII, m 2) 59a (M OsII, m 1) 59b (M OsII, m 2) 59c (M OsII, m 3)

N

N

electropolymerization 2n e-, 2n H

S

S II

60a (M Ru , m 1) 60b (M RuII, m 2) 61a (M OsII, m 1) 61b (M OsII, m 2) 61c (M OsII, m 3)

N

M

N

m

n

N

N

2n BF4

O

O O

O

(b) O

O

O S 62

N N

N Ru2

N N N

N N

O electropolymerization 2n e-, 2n H

S

N N

N Ru

N

2 PF6

N N

S

N N

2n PF6

63

Scheme 5.4 Electropolymerization of thiophene-substituted RuII and OsII bis-complexes.

the electrode surface (Figure 5.25a). From semi-empirical calculations of the excited state according to Zerner’s model of “intermediate neglect of differential overlap” (ZINDO/S), it was concluded that – due to the generation of a positive charge at the C5thienyl-position – this pattern is favored for coupling to give linear metallopolymers [128]. Upon electropolymerization, the metallopolymers were deposited at the electrodes as insoluble materials and, thus, the DP could not be determined. Despite this limitation, absorption spectroscopy, SEM, and CV could be applied to characterize both the monomers and resulting polymer films. Investigation of the polymer films of 60b and 61b, on interdigitated platinum microelectrode arrays (Pt-IDAs) revealed that different morphologies were obtained. Thus, nucleation and growth dynamics of the materials differed significantly (Figure 5.25b). Holliday et al. synthesized a 2,6-di(1H-pyrazol-1-yl)pyridine derivative, which was substituted with 3,4-ethylenedioxy-thienyl (EDOT) groups at the 4,400 -positions (Scheme 5.4b) [132]. The heteroleptic RuII terpyridine complex 62 was synthesized and electropolymerization was carried out in a 0.01 mM CH2Cl2 solution. For this purpose, continuous cycling between 1.25 and 1.25 V, at a scan rate of 100 mV s1, was performed. XPS analysis, UV–vis absorption, and UV–vis-NIR spectroelectrochemistry revealed that the polymer possesses the expected structure and is highly conjugated. 5.4 Main-Chain Metallopolymers Based on Terpyridine-Functionalized p-Conjugated Polymers

Besides dye-sensitized systems, combining conjugated polymer-based organic solar cells [2] as well as so-called hybrid cells [133] is the focus of considerable research effort. Among others, incorporation of transition metal ions – such as

05

27 J l 2011 15 50 53

S

n

05 1

(b)

Figure 5.25 (a) Cyclic voltammogram recorded at 50 mV s with a Pt electrode (0.5 mM 58a in CH2Cl2 containing catalytic amounts of BF3 OEt2); the linear increase of the peak currents with scan number indicates the accumulation of the redox active polymer 60a onto the electrode surface [128]. (b) SEM images of polymer-coated Pt-IDAs (5 mm digit width; 5 mm interdigit gaps) used for conductivity measurements [131]. Figure reproduced with kind permission; r 2002 The Royal Society of Chemistry and 2005 American Chemical Society, respectively.

(a)

230


27 J l 2011 15 50 53


| 231

RuII, OsII, or ReI – via complexation into p-conjugated polymers is a common approach in the field of hybrid materials [17]. In particular, Krebs et al. reported on the synthesis of p-conjugated oligomers [134, 135] and polymers [136, 137] of poly(p-phenylenevinylidene)-type (PPV) end-capped with a terpyridine moiety and also on their application, as building blocks, for construction of photovoltaic devices. Synthesis of (monodisperse) oligomers followed a stepwise and unidirectional approach [138], that is, the Horner–Wadsworth– Emmons (HWE) condensation and hydrolytic deprotection in turn. Scheme 5.5 depicts the stepwise preparation of a terpyridine-functionalized oligo(phenylenevinylidene) (OPV) trimer (66). The acetal-protected monomer 65 was utilized, as key substrate, and reacted with terpyridine 64 by HWE condensation to give (overall 43%) 66. The main advantages of this step-wise protocol are (i) the length of the pconjugated system can be easily controlled via the number of reaction sequences and (ii) a reactive end-group (i.e., aldehyde moiety) for further modifications is maintained. A similar approach was followed by El-ghayoury et al. where an alkyne-terminated OPV derivative was reacted with 40 -bromo-2,20 :60 ,200 -terpyridine under Sonogashira cross-coupling conditions to yield 67 (Scheme 5.5) [139]. The philosophy of a stepwise and unidirectional synthesis was developed further by Hagemann et al., resulting in a so-called “all-in-one molecule” (68) for organic solar cell applications (Figure 5.26) [134]. The central part of 68, which was prepared in a complex multistep organic synthesis, consisted of a dyad of a ZnII porphyrin and bis(terpyridine) RuII complex. Pendant arms of an oligo(hexylthiophene) (OHT) and an OPV were added to function both as antennae for light harvesting as well as donor and acceptor groups for attraction of photoinduced

N

N C3H7O O N (EtO)2OP

i) KOtBu, THF ii) deprotection

CHO

C3H7O

OC3H7

O OC3H7

N

64

O OC3H7

C12H25O

N

C3H7O

43% over two cycles

PO(OEt)2

64

N

i) KOtBu, THF ii) deprotection

C3H7O O 65

N

87%

O

OC3H7

66

N

OR

C12H25O

OR

N

C12H25O 67

RO

N RO N

Scheme 5.5 Synthesis of terpyridine-functionalized OPV-trimers following a stepwise, unidirectional protocol.

05

27 J l 2011 15 50 53

N

3 O

05

27 J l 2011 15 50 53

C6H13

S

C6H13

S

4 S

C12H25

N

N Zn N

N

C12H25

charge separation

electron transfer

light harvesting

S

N N

N

N N

Ru

Ru

Ru

N

2+

Ru

e-

11nm

2X

Zn

Zn

Zn

68

C3H7O

OC3H7

hν

C3H7O

OC3H7

S

Figure 5.26 Multidomain 68 (a) and its function in a photovoltaic device (b). (Figure redrawn according to Reference [131]).

(b)

Me2N

(a)

C3H7O OC3H7

NC

COOEt

232



| 233

charges (Figure 5.26); however, very poor performance, in terms of converting light into energy, was observed for 68 when fabricated into a bulk-heterojunction PV device of the common geometry [ITO/PEDOT:PSS/68/Al] (about 1000 times less efficient than a device based on a 3-hexylthiophene oligomer). This was mainly due to a poor morphology as well as non-radiative energy conversion by the ZnII porphyrin moiety. Since charge separation could theoretically also arise through the MLCT transition, followed by migration of the electron and hole into different pendant p-conjugated arms, the porphyrin moiety might be obsolete. Consequently, the donor- and acceptor-type terpyridine macroligands 71 and 72 were utilized by Duprez et al. for the preparation of two homopolymers (73 and 74) and copolymer 75 by selective complexation with RuII ions in moderate yields (Scheme 5.6) [136]. For the preparation of the PPV-type macroligands (71 and 72) bearing a terpyridine ligand at one chain end, a directed Heck cross-coupling reaction of 40 -(4bromophenyl)-2,20 :60 ,200 -terpyridine (69) and monomer 70 was applied to obtain donor- (OF-tpy, 71, Mn ¼ 1700 g mol1, PDI ¼ 1.18) and acceptor-type (CNOF-tpy, 72, Mn ¼ 2800 g mol1, PDI ¼ 1.51) polymers (Scheme 5.6). The metallopolymers were characterized by 1H NMR, UV–vis absorption, and photoluminescence spectroscopy. The materials were fabricated into two different types of devices – a polymer solar cell and a dye-sensitized solar cell (DSSC). Concerning the first type, very low efficiencies were found in all cases (below 0.01%). The poor film-forming ability resulted in low device resistance, open circuit voltage (Voc), and short circuit current (Isc). Sublimation of a layer of C60 on top of a spin-coated film of 73 helped to improve slightly the photovoltaic performance. The best results were obtained on employing a device geometry of [ITO/PEDOT:PSS/73/Al] with an output power of 0.86 mW cm2 and a fill factor of 25.8%. Despite the absence of specific anchoring groups (e.g., carboxylate or phosphonate), absorption on TiO2 films was observed and

69/70 1:20 Pd2(dba)3, (tBu3PH) BF4, N-methyldicyclohexylamine, DMF, reflux, 24 h

Br

N

R N

+ 69

N

C8H17 Br

C8H17 70a (R H) 70b (R CN)

N

N

N

R 71 (OF-tpy, R H) 72 (CNOF-tpy, R CN)

Br n C8H17

C8H17 Br

C8H17

N

R 73(R R' H) 74(R R' CN) 75(R H, R' CN)

Ru N

2 BF4

N

N

n

2

N

R'

N

Scheme 5.6 Synthesis of tpy-functionalized PPVs (71 and 72) by directed Heck crosscoupling polymerization, and the metallopolymers 73–75.

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C8H17 Br n

C8H17

C8H17

234

| 5 p-Conjugated Polymers Incorporating Terpyridine Metal Complexes improved performances were achieved when the polymers were applied in DSSCs. The best performance was found when [Co(btp)3](ClO4)2 [btp: 4,40 -di(tert-butyl)-2,20 bipyridine] was used, as electrolyte, which allows polymer-based dye-sensitization without the drawback of polymer degradation (as often observed for iodine, as electrolyte). For the copolymer 75, promising I–V characteristics were recorded [Isc ¼ 0.71 mA cm2, Voc ¼ 315 mV, and maximum power (Pmax) of 0.22 mW]. The Krebs group also reported the synthesis of a poly(3-hexylthiophene) (P3HT) that was end-functionalized with a terpyridine ligand. The polymerization could not be conducted using uncomplexed terpyridine, as the chain-terminating terminating agent, and, thus, an appropriately functionalized heteroleptic RuII complex had to be utilized [137]. A P3HT chain, prepared in a Stille cross-coupling polymerization, was added to the zwitterionic RuII complex 76, affording a three domain metallopolymer (77), featuring anchoring, charge injection, and lightharvesting/energy transfer domains (Figure 5.27a). A DP of about 12 (Mn ¼ 2100 g

(a) C6H13

N

S

Br C6H13

N

S

C6H13

Br

C6H13 S

n

C6H13

O

N

N

S S

P

(PPh3)2PdCl2, DMSO, DMF, 75C, 24 h

SnMe3

C6H13

N

O

76

Br

O

Ru2+

N

S

N

N

N Ru

2+

N

77

(b)

N

OO P O

N

PEC cell Pt electrode Nanoporous TiO2 (4.5 µm) SurlynTM film (50 µm) Compact anatase layer (0.1 µm) F-doped ITO (3 mm)

Figure 5.27 (a) The three-domain metallopolymer 77; (b) general configuration of a photoelectrochemical cell (PEC). Figure redrawn according to Reference [137].

05

27 J l 2011 15 50 54

References

mol1) was estimated by SEC and 1H NMR spectroscopy. When fabricated into an all-polymer PV device, only very poor efficiencies were observed; however, the incorporation of 76 or 77 into PEC solar cells (Figure 5.27b) revealed a potential for such materials to serve as dyes in PV devices. For 77, the I–V characteristics were Isc ¼ 0.456 mA cm2, Voc ¼ 555 mV, and Pmax ¼ 0.055 mW.

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115 Han, F.-S., Higuchi, M., Akasaka, Y., Otsuka, Y., and Kurth, D.G. (2008) Thin Solid Films, 516, 2469–2473. 116 Han, F.-S., Higuchi, M., Ikeda, T., Negishi, Y., Tsukuda, T., and Kurth, D.G. (2008) J. Mater. Chem., 18, 4555–4560. 117 Li, J.-H. and Higuchi, M. (2010) J. Inorg. Organomet. Polym. Mater., 20, 10–18. 118 Pal, R.R., Higuchi, M., Negishi, Y., Tsukuda, T., and Kurth, D.G. (2010) Polym. J., 42, 336–341. 119 Burnworth, M., Knapton, D., Rowan, S. J., and Weder, C. (2007) J. Inorg. Organomet. Polym., 17, 91–103. 120 Burnworth, M., Mendez, J.D., Schroeter, M., Rowan, S.J., and Weder, C. (2008) Macromolecules, 41, 2157–2163. 121 Kelch, S. and Rehahn, M. (1999) Macromolecules, 32, 5818–5828. 122 Kelch, S. and Rehahn, M. (1999) Chem. Commun., 1123–1124. 123 Storrier, G.D. and Colbran, S.B. (1996) J. Chem. Soc., Dalton Trans., 2185. 124 Storrier, G.D., Colbran, S.B., and Craig, D.C. (1997) J. Chem. Soc., Dalton Trans., 3011–3028. 125 Ng, W.-Y., Gong, X., and Chan, W.-K. (1999) Chem. Mater., 11, 1165–1170. 126 Ng, W.-Y. and Chan, W.-K. (1997) Adv. Mater., 9, 716–719. 127 Man, K.Y.-K., Wong, H.-L., Chan, W.K., Djurisˇic, A.B., Beach, E., and Rozeveld, S. (2006) Langmuir, 22, 3368–3375. 128 Hjelm, J., Constable, E.C., Figgemeier, E., Hagfeldt, A., Handel, R.W., Housecroft, C.E., Mukhtar, E., and Schofield, E.R. (2002) Chem. Commun., 284–285. 129 Hjelm, J., Handel, R.W., and Hagfeldt, A. (2004) Electrochem. Commun., 6, 193–200. 130 Hjelm, J., Handel, R.W., Hagfeldt, A., Constable, E.C., Housecroft, C.E., and Forster, R.J. (2003) J. Phys. Chem. B, 107, 10431–10439. 131 Hjelm, J., Handel, R.W., Hagfeldt, A., Constable, E.C., Housecroft, C.E., and Forster, R.J. (2005) Inorg. Chem., 44, 1073–1081.

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6

Functional Polymers Incorporating Terpyridine-Metal Complexes

6.1 Introduction

In 1987, the Nobel Prize in Chemistry was awarded to Lehn, Pedersen, and Cram for their work on crown ethers and host–guest interactions [1]. Since then, supramolecular chemistry has evolved into a highly active field in modern polymer and materials research, owing to its versatility and extensive potential. Today, many different types of interactions are utilized for the self-assembly of (macro)molecules into advanced supramolecular architectures – hydrogen bonding, metal-to-ligand coordination, p–p- and ionic interactions, donor–acceptor association, and hydrophilic–hydrophobic as well as van der Waals forces [2]. In general, these supramolecular interactions feature different specificities (e.g., interaction stabilities, directionality, and reversibility), enabling the synthesis of large and complex structures with diverse functions that are interesting for many useful fields. By using a relatively large, “classical” polymer content in a resulting supramolecular material, it has been possible to produce – on a large scale – materials that combine the new and interesting features and structures of supramolecular species with the desired (macroscopic) polymeric properties. Owing to their polymeric nature, these materials can be, for example, processed into thin films by spin coating and inkjet printing techniques or used in-bulk, thus allowing technical processing via traditional polymer engineering methodologies. By altering the polymer portion, the macroscopic properties of the material can be dramatically changed, for example from rigid to elastic, hydrophobic to hydrophilic, or liquid to solid. Besides the well-known H-bonding systems [3–6], metal-to-ligand coordination is a key concept in supra-macromolecular chemistry. Chelate complexes of polypyridines, in particular those of transition metal ions with 2,20 -bipyridines and 2,20 :60 ,200 -terpyridines, have attracted special attention due to their outstanding electro-optical properties (see also Chapter 3). The origin of this research can be traced back to the early 1970s when scientists introduced 2,20 -bipyridine units into large poly(ethylene glycolic) macromolecules [“poly(crown ether)s”], polymers, Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes thin films, and membranes. The chemistry of 2,20 :60 ,200 terpyridines is – relatively speaking – much younger than that of their 2,20 -bipyridine counterparts (2,20 bipyridine itself was discovered by Blau in the late 1880s [7, 8], while the first report on 2,20 :60 ,200 terpyridine was made a half century later [9, 10]). The same stands for terpyridine-containing dendrimers and then polymers, in which only in more recent years has the first example appeared [11]. This delayed inclusion is probably based on the synthetic availability of the key functionalized terpyridines, which generally required more steps to prepare than the related 2,20 -bipyridine counterparts. Polymers containing bipyridine and phenanthroline moieties (as well as structurally related analogs) will not be considered herein, but overviews on macromolecules bearing either phenanthrolines [12] or bipyridines [11, 13] are available. Furthermore, various reviews have dealt with metal-containing polymers in general [12, 14–22]. The theory of metallo-supramolecular chemistry and the synthetic approaches towards terpyridine-containing polymers have been detailed in Chapter 5. According to Ciardelli et al., there are three different types of metal-containing polymers [23]; within the scope of this chapter, polymers of types Ib and IIb will be evaluated (see also Figure 5.1). Section 6.2 discusses macromolecules of the type-Ib, that is, bearing terpyridine units in the side-chain. Historically, these materials were the first reported terpyridine-containing polymers and were synthesized in a facile fashion by polymerizing monomers that were functionalized with appropriate terpyridine moieties. In an overview of dendronized polymers, Frauenrath has addressed the different basic strategies [24]. Section 6.3 describes polymers with terpyridine units in the main chain (type-IIb). By complexing mono- or bisfunctionalized prepolymers, various architectures (e.g., chain-extended linear or block copolymers) could be obtained [25, 26]. Such supramolecular metallopolymers exhibit a complex aggregation behavior in solution and diverse structured nanomaterials can be derived. Biopolymers (e.g., enzymes, peptides or DNA/RNA) covalently functionalized with terpyridine complexes represent an emerging field with respect to possible applications in cancer therapy or biolabeling. Thus, the supramolecular chemistry of terpyridine metal complexes with biosystems is considered in Chapter 7. 6.2 Polymers with Terpyridine Units in the Side-Chain 6.2.1 Materials Based on Flexible Organic Polymers

In 1988, Potts and Usifer incorporated terpyridinyl units, as side-chains, into a polymer backbone using a free-radical homopolymerization process of either 4- or 40 -vinyl-2,20 :60 ,200 -terpyridine (Scheme 6.1) or the copolymerization of 40 -vinylterpyridine with styrene [27]. The resultant polymers were white powders with molar masses up to 60 000 g mol1 [Mw determined by size exclusion chromatography (SEC) with polystyrene (PS) standards] and polydispersity indices (PDIs) in the

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6.2 Polymers with Terpyridine Units in the Side-Chain

N

N

N

N

N

N

AIBN, 60 °C

n

n

N

N

N

N

N

N

Scheme 6.1 Polymerization of vinyl-functionalized 2,20 :60 ,200 -terpyridines.

n

m

O

O

O O 1

⫽

N

2

m

n

O

5 m:n ⫽ 94.3:5.7

3 m:n ⫽ 98.1:1.9

N

N

O

O

n

m

m

n

OO

O

4 m:n ⫽ 98.5:1.5

O

m

HO

6 m:n ⫽ 94.6:5.4

O

O

n

O

7 m:n ⫽ 94.9:5.1

Figure 6.1 Schematic representation of copolymers with a pendant terpyridine unit in the side-chain.

range 2–44. The addition of transition metal ions to these polymers generated insoluble metallopolymer complex networks. The uncomplexed polymers could be recovered by using hot, concentrated hydrochloric acid. Though attempts to homopolymerize the corresponding heteroleptic complexes of the type [(tpy)M(40 vinyl-tpy)](PF6)2 failed, copolymers with styrene were readily formed from complexes with CoII and RuII ions [28]. In the early 1990s, Hanabusa et al. published a series of macromolecules based on either 40 -[4-(2-acryloyloxyethoxy)phenyl]-2,20 :60 ,200 -terpyridine (1) or 40 -(4-styryl)2,20 :60 ,200 -terpyridine (2), as monomers, for free-radical homo- and copolymerization (Figure 6.1) [29, 30]. The terpyridine derivative 1 was copolymerized with styrene or methyl methacrylate (MMA), as co-monomers, producing polymers with a molar mass of Mn ¼ 6400 g mol1 for the styrene copolymer (3) and Mn ¼ 15 000 g mol1 for that with MMA (4). Furthermore, monomer 2 was copolymerized with styrene, vinyl acetate, and acrylic acid to yield the desired

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| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes copolymers 5–7, respectively. While the homopolymers of 1 and 2 were insoluble, all copolymers were soluble due to their low (up to about 5%) terpyridine content. In all cases, the corresponding complexes were formed upon addition of CoII, FeII, NiII, and CuII ions. Similar random copolymers with pendant terpyridine units in the side-chain (in free as well as complexed forms), prepared by free-radical polymerization, were also reported by the groups of Tew [31] and Schubert [32–35]. In recent years, the focus has shifted towards more precise polymer architectures [i.e., defined (block co)polymers, narrow PDI values] by employing diverse controlled or living polymerization techniques. To achieve this goal, two general synthetic approaches can be utilized: a post-polymerization modification of functionalized polymers with appropriate terpyridine derivatives or a direct controlled polymerization of terpyridine-containing monomers. The latter was implemented in recent years to obtain macromolecules containing metal complexes. The RuII-catalyzed ring-opening metathesis polymerization (ROMP) of norbornene derivatives – bearing RuII complexes of bipyridine as well as phenanthroline, cyclometalated IrIII complexes or PdII pincer systems – was widely applied by Sleiman [36–38] and Weck [39–45] to obtain polymers with transition metal complexes in their side-chains. In particular, the various controlled radical polymerization (CRP) techniques – including atom-transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP), and reversible addition-fragmentation chain-transfer (RAFT) polymerization – are versatile tools to control the architectures (i.e., molar mass, PDI value, and composition of the polymer backbone) and exhibit a good tolerance to a broad range of functionalities. Thus, Aamer and Tew investigated if these CRP techniques could also be applied to synthesize (block co)polymers starting from terpyridine-functionalized monomers [46, 47]. Their unsuccessful attempts to apply ATRP were rationalized by deactivation of the CuI polymerization catalyst by complexation; Fraser et al. had shown earlier that ATRP of FeII tris(bipyridine) complexes can be applied to achieve well-defined (co)polymers functionalized with transition metal ion complexes [48–50]. However, Tzanetos et al. described the homopolymerization of an uncomplexed terpyridine-containing monomer (8) under ATRP conditions with CuBr/2,20 bipyridine (bpy), as the catalytic system (Figure 6.2) [51]. Kinetic studies revealed a good control over the polymerization and that polymer 9 was obtained with molar masses in the range Mn ¼ 1900–6100 g mol1 (according to SEC) as well as narrow PDI values (1.14–1.38), depending on the reaction conditions (e.g., monomer-toinitiator ratio, amount of catalyst/base, and reaction time). Moreover, a telechelic bifunctional initiator, containing a green light-emitting di(styryl)anthracene unit, was used for the preparation of a triblock copolymer. A RuIII-terpyridine monocomplex could be grafted effectively onto the polymers, giving soluble side-chain RuII metallopolymers that were investigated by UV–vis absorption spectroscopy. Tew and Aamer reacted vinylbenzamide monomer 10 under NMP conditions in the presence of an alkoxyamine initiator [52], which is known to control the polymerization of styrene monomers. The random, block-random, and block-wise

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6.2 Polymers with Terpyridine Units in the Side-Chain R

(a)

CuBr, bpy Ph2O, 110 °C

O R

Br

R⫽

Br n

⫹

O

Bn O N Bn O

N

or

N

N

8

N

N

9

H3COOC O H3COOC

(b)

3.0

Ln ([M]0/[M])

2.5 2.0 1.5 1.0 0.5 0.0 0

25

50

75 100 125 150 175 200 225 250 270 Time (min)

Figure 6.2 (a) Homopolymerization of 8 under ATRP conditions; (b) representation of the ln([M]0/[M]) versus time plot for the polymerization of 8 in Et2O at 110 1C ([8]0/[initiator]0/[CuBr]0/[bpy]0 molar ratio ¼ 10 : 1 : 2 : 2) [51]. Figure reproduced with kind permission; r 2005 John Wiley & Sons, Inc.

incorporation of styrene and 10 into a polymer was performed by bulk polymerization (Figure 6.3). A slightly increased reactivity of 10 compared to styrene was concluded from kinetic studies of the formation of random polymer 11. Block architecture 12, with the terpyridine units being confined to the second block, could be obtained starting from a polystyrene-based NMP macroinitiator. The increase in molar mass could be followed by SEC, a distinct shift of the SEC trace was observed (Figure 6.3). Lastly, these authors synthesized a block copolymer from the same macroinitiator and 10; however, block copolymer 13, with a presumably high content of terpyridine units, could not be characterized by

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246

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes O

O N

10, styrene, bulk, 125 °C

N

n

m

random copolymer (11) CONHR

n

10, styrene, cat. acetic anhydride, bulk, 125 °C

O N

O N

n

p q

m

CONHR block-random copolymer (12) macroinitiator O N n

m

block copolymer (13) CONHR O N H

1.1

O 10

12 Mn ⫽ 67.2 kDa Mw ⫽ 93.1 kDa PDI ⫽ 1.40

1.0

N

0.9 N

Detector response (A.U.)

n

10, cat. acetic anhydride, bulk, 125 °C

O N

N

R

0.8

macroinitiator Mn ⫽ 44.3 kDa Mw ⫽ 50.5 kDa PDI ⫽ 1.13

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 ⫺0.1 12

13

14

15

16

17

18

19

20

21

Retention time (min)

Figure 6.3 Synthesis of a random (11), a block-random (12), and a block copolymer (13) by polymerization of vinylbenzamide monomer 10. SEC traces (THF, as solvent) of the macroinitiator (red curve) and block-random copolymer 12 (blue curve) are also shown [47]. Figure reproduced with kind permission; r 2005 Elsevier B.V.

SEC due to the strong interactions with the stationary phase [47]. The intrinsic viscosity of the dilute polymer solutions (11 and 12) increased significantly upon addition of CuII ions, indicating the formation of a highly crosslinked supramolecular gel by intra- as well as intermolecular complexation of the CuII. Reversibility of gelation could be proven by adding PMDETA (PMDETA ¼ N,N,N,N00 ,N00 pentamethyldiethylenetriamine), a strong and chelating base, which led to an immediate decrease of the viscosity of the polymer solutions.

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22


cat. AIBN, benzene, 80 °C

S S

CN

+

O

n

O

MMA

10, MMA cat. AIBN, benzene, 80 °C

S NC

O

O

macroinitiator

O n

NC O

O

S

m

p q

O

CONHR

Scheme 6.2 RAFT polymerization of MMA and 10 [46].

Since NMP is not the best choice for the polymerization of MMA, the same authors explored the RAFT polymerization to obtain a random copolymer 14 of monomer 10 and MMA [46] (Scheme 6.2). Following the same approach as discussed above, first a macroinitiator was generated by RAFT homopolymerization of MMA. Subsequently, the second, random block was grown by addition of 10 and MMA (1 : 9 ratio). Elemental analysis and 1H NMR spectroscopy revealed a content of terpyridines of only 2.5 mol-%, indicating that monomer 10 had a lower tendency to be incorporated into the polymer under RAFT conditions [46]. O’Reilly et al. prepared the block-random copolymer 15 under NMP conditions, similar to the work of Tew et al. [53]. A terpyridine content of 6.3 mol-% was determined by 1H NMR spectroscopy. The conversion of 15 into the amphiphilic copolymer 16, with retention of terpyridine content, was performed using trifluoroacetic acid (Figure 6.4a). Subsequently, micelles composed of 16 were formed in THF–water. Crosslinking of the hydrophilic core into nanoparticles, using amidation chemistry, was achieved under mild conditions (Figure 6.4b). Investigation by dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed the formation of micelles and nanoparticles (Figure 6.4c), respectively, with narrow size distributions. Owing to the crosslinking throughout the shell layer, only a single phase transition corresponding to the polystyrene-type core could be observed by differential scanning calorimetry (DSC); in contrast, two phase transitions occurred for the precursor micelles. Upon addition of RuIII or FeII ions, supramolecular loading of the nanoparticles by complexation of the terpyridine moieties was achieved. UV–vis absorption spectroscopy revealed the successful complexation within the cores as shown by the characteristic metal-to-ligand chargetransfer (MLCT) bands at 390 nm (for RuII) and 550 nm (for FeII). For these hybrid materials, no alteration of particle size and shape was detected [53]. Supramolecular gels are of particular interest with respect to their potential applications as rheology modifiers, stimuli-responsive materials (e.g., sensor or actuators), self-healing systems, and drug carriers [18]. In contrast to materials where H-bonding is employed, as supramolecular linkages [3], supramolecular gels based on metal-to-ligand interactions are less well known. Schubert et al. studied the changes in viscosity through stepwise addition of FeII or ZnII ions to a solution of various poly(methyl methacrylate)s (PMMAs). The formation of large aggregates of different sizes, as a function of terpyridine content in the polymer as

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| 247

O

S

block-random copolymer (14)

248

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes

(a)

(c) O N n

O R

m

p q

15 (R ⫽ tBu)

O

CF3COOH, CH2Cl2, room temp.

O

16 (R ⫽ H) N

N

100 nm

N

(b)

(i)

(ii)

16

Figure 6.4 (a) Block-random copolymers 15 and 16; (b) micellization of 16 in THF–water (i) and the formation of nanoparticles by crosslinking with an aliphatic diamine (ii); (c) TEM image of the nanoparticles (stained with phosphotungstic acid, drop-deposited onto a carbon-coated Cu-grid) [53]. Figure reproduced with kind permission; r 2008 American Chemical Society.

well as concentration, was shown (Figure 6.5a) [35]. Moreover, the type of MII ions also played an important role; for instance, the addition of ZnII ions led to crosslinked metallopolymers of lower viscosity than those derived from FeII. After an over-titration, the relative viscosity eventually dropped due to the reversibility of the formation of ZnII bis-complexes leading to the favored ZnII monocomplexes. Furthermore, random terpolymers 17 with different ratios of terpyridine binding sites (enabling supramolecular crosslinking) and epoxide functionalities, suited for covalent crosslinking, were studied by Hofmeier and Schubert (Figure 6.6) [34]. Supramolecular crosslinking with FeII, CoII, and ZnII ions was combined with covalent crosslinking initiated by Lewis acids (e.g., AlCl3). The swelling behavior as a factor of the degree of crosslinking was investigated. The authors compared a covalently crosslinked gel (m ¼ 0) and a material featuring both types of crosslinking (m,p 6¼ 0) to a gel, based only on supramolecular interactions (p ¼ 0). Therefore, the gels were weighed in the dry as well as swollen state (CHCl3 as solvent). The Q factor was calculated, according to Eq. (6.1); the value 1/Q is equivalent to the degree of crosslinking [54]. For the non-covalently crosslinked gels (with FeII or CoII ions) 1/Q was in the range 0.04–0.08, whereas a value of about 0.30 was obtained for the covalently-linked material. For a gel featuring both

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(a)

MII

MII dilution (for MII ⫽ ZnII)

MII ⫽ FeII or ZnII

Relative viscosity

(b)

2,6 2,5 2,4 2,3 2,2 2,1 2,0 1,9 1,8 1,7 1,6 1,5 1,4 1,3 1,2

10% tpy, FeII

5% tpy, FeII

10% tpy, ZnII 0,0

0,2

0,4

0,6

0,8

Equivalents

1,0 MII

1,2

1,4

1,6

ions

Figure 6.5 (a) Reversible self-assembly of terpyridine-functionalized PMMAs into supramolecular gels; (b) changes of the relative viscosity as a function of added MII ions (MII ¼ FeII or ZnII) [35]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

types of crosslinking, a 1/Q of almost 1.00 was determined. Further approaches for the covalent crosslinking in addition to the supramolecular interactions within random copolymers were also reported, including photochemical crosslinking of olefinic groups, thermal curing of hydroxyl groups with bis-isocyanate derivatives, and Lewis-acid-mediated ring-opening of oxetanes [55, 56]:

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| 249

250

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes m

O

n

O O

OO

p

O O

17

O

N

N

N

(a)

(b)

Figure 6.6 Random terpolymer 17, featuring two orthogonal functionalities for crosslinking (a) and image of a supramolecular gel obtained from 17 and CoII ions (b) [34]. Figure reproduced with kind permission; r 2003 European Polymer Federation.

Q¼

ða bÞ b

(6.1)

where a is the weight of swollen gel; b: is the weight of unswollen gel. Post-polymerization functionalization represents a versatile alternative to obtain (co)polymers with pendant terpyridine units in the side-chains and has been widely applied. The main advantage of this approach is the possibility to overcome limitations in (controlled) polymerization reactions when uncomplexed terpyridine monomers were utilized [18, 57]. A so-called prepolymer, bearing reactive groups on the backbone or in the side-chain, is prepared by controlled polymerization techniques. In general, these polymerizations can be well controlled with respect to, for instance, molar mass, PDI, composition, or architecture. Since the so-called “active ester” monomer (N-methacryloxysuccinimide, 18) can be polymerized under various CRP conditions without any visible decomposition, the functional group can be utilized to graft molecules onto the polymer in high yields and under mild conditions, while keeping the initial polymer structure intact [57]. The Tew group transferred this strategy from the life sciences to the field of supramolecular and, in particular, to terpyridine chemistry [47, 57–61]. A general ATRP protocol was established that allowed the synthesis of a broad range of polymers (homopolymers as well as block and random ones) from 18 and methacrylate [methyl, n-butyl, and poly(ethylene glycol) methyl ether] as well as styrene co-monomers [57]. In all cases, favorable control of the molar mass and relatively narrow PDI values were reported. The subsequent transformation with amino-functionalized terpyridine 19 derivatives was achieved in DMSO in the presence of triethylamine, as base, at 60 1C (see Scheme 6.3 for 20, as a representative example).

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EtO2C O

O

O N

O

Br O O N

O

O

"active ester" monomer (18)

n

m

19, NEt3, DMSO, 60 °C, 2 h

O

EtO2C

| 251 Br

n

m

O

O

NH

O O 20

N H2N

O

O

19

N N

N

N

Scheme 6.3 Post-polymerization functionalization of polymers by transformation of the “active ester” moiety.

(a) EtO C 2

(b)

Br n

m

O

O O

NH

21a

(c) O

Ln3+

O2N NO2

21c

blend of 21a and 21b

(d)

heating to 50 ⴗC cooling to roomtemp.

N N

21b

N NO 2

21a (Ln ⫽ Eu ) 21b (LnIII ⫽ TbIII) 21c (LnIII ⫽ EuIII and TbIII, 1:1 ratio) 21d (LnIII ⫽ DyIII, EuIII and TbIII) III

III

Figure 6.7 (a) Metallopolymers 21; (b) emission colors of 21a, 21b, a blend (1 : 1 ratio) of 21a/21b, and 21c (from left to right) in the solid state; (c) thermochromism of 21c in the solid state [58]. Figure reproduced with kind permission; r 2005 American Chemical Society. (d) Emission of white light from 21d in solution (left) as well as in the solid state (right) [59] Figure reproduced with kind permission; r 2007 John Wiley & Sons, Inc..

The coordination of lanthanide ions to terpyridine ligands of polymer 20 yielded metal-functionalized polymers 21a/b (Figure 6.7a) that exhibit bright pink or green emission in the solid state in the case of EuIII and TbIII ions, respectively [57, 58]. Metallopolymer 21c with an equimolar ratio of EuIII and TbIII ions showed a yellow emission (Figure 6.7b). In contrast, when polymers 21a and 21b were blended into a thin film, a green emission occurred, since TbIII ions are more emissive. Titration experiments could show that a yellow emission only occurs at an exact 1 : 1 ratio of

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21c

21d

N

252

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes both lanthanide ions. This observation could be attributed to the formation of a EuIII/TbIII alloy (or a weakly associated bimetallic complex), due to a spatial arrangement along the polymer backbone. Furthermore, an interesting thermochromism of 21c could be observed (Figure 6.7c). Upon heating the thin film at 50 1C, a change of emission color from yellow to orange/pink (similar to the emission color of 21a) occurred that supported the dissociation of the alloy into individual EuIII and TbIII complexes along the polymer chain. Whereas emission of the TbIII centers was quenched at 50 1C, the EuIII centers remained emissive. Cooling the film to room temperature regenerated the alloy and the emission color change back to yellow [61]. According to the RGB concept, to generate white light-emitting diodes (WOLEDs) [61] the coordination of DyIII ions (blue emission) to polymer 20, in addition to EuIII (red emission) and TbIII ions (green emission), should – at a certain ion ratio – lead to a white light-emitting material. At a LnIII ion ratio of 1 : 1 : 1, in solution, white emission with quantum yields (FPL) of about 0.05 was observed with coordinates of x ¼ 0.3124 and y ¼ 0.3295, which are very close to those defined by the Commission International d0 Eclairage (CIE) for “ideal” white light (x ¼ 0.3127 and y ¼ 0.3291). Owing to the absence of complex–solvent interactions in the solid state (i.e., drop-casted films on glass slides), a DyIII : EuIII : TbIII-ratio of 1.8 : 1 : 1 had to be adopted to achieve almost “ideal” white emission (x ¼ 0.3112 and y ¼ 0.3281) [59]. Those authors could furthermore show that, by varying the lanthanide ion ratio, the emission color in the solid state could be tuned over almost the entire visible regime. In a second approach, Tew et al. also utilized the polymerization of styrene and tert-butyl acrylate under ATRP conditions to obtain block copolymer 22a with good control over the molar masses and narrow PDI values below 1.10 [60]. Quantitative cleavage of the ester units could be achieved under mild conditions with trifluoroacetic acid (TFA) in dichloromethane; the efficiency of deprotection was monitored by 1H NMR as well as FT-IR spectroscopy. The subsequent amidation reaction of 22b with terpyridine 19 was performed with 1.5–1.2 equivalents of 19 per acid group to guarantee full conversion (Scheme 6.4). The homoleptic as well

n

m

O

Br

19, DCC, HOBT, DMF, room temp., 48 h

m

OH

22b

n

i) Ir(tpy)Cl3, ethylene glycol, 20 min, 180 °C

NH

ii) NH4PF6

Br

O 23

Br

q n

NH O

NH

O

O N

n

O

O

Br

p

24

CF3COOH, CH2Cl2, room temp. 24 h m

m

N

N

O N

N N

N

N Ir3+

O

N

N N

22a

3p PF6

Scheme 6.4 Synthesis of metallopolymers with IrIII bis(terpyridine) complexes in the sidechain [60].

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N


as heteroleptic bis-complexes of terpyridine ligands with IrIII ions have been investigated and shown to exhibit intense emission and long-lived excited states [62]. These properties, in combination with the kinetically inert character of the complexes [63, 64], make them promising candidates in applications such as luminescent labels in proteins [65] or anion sensors [66] and as building blocks for linear multicomponent arrays within donor–acceptor-systems in photoinduced charge separation processes [67–71]. Therefore, the incorporation of [Ir(tpy)2]3 þ into (block co)polymer architectures was investigated, applying the synthetic protocol established by Collin et al. [68]. The Ir(tpy)Cl3 precursor was added to the terpyridine-functionalized block copolymer 23 to yield metallopolymer 24. Since the formation of IrIII bis-complexes does not generally proceed in high yields, the amide block of the copolymer was only randomly decorated with IrIII complexes (a degree of functionalization of 35% was calculated) and, therefore, the overall composition of polymer 24 is of a block-random type [60]. Photophysical measurements revealed that the polymer backbone had a negligible influence on the absorption behavior, but significantly redshifted the photoluminescence maximum originating from the IrIII centers. Owing to the polyelectrolyte character of copolymer 24, monomodal aggregation in various solvents with different dielectric constants was observed. Beside modification of (co)polymers with small metal ion complexes at the end of the side-chains, the grafting of larger molecules, in particular of other polymers, by applying selective coordination chemistry has been reported. The previous example shows that coordination of transition metal ions to binding sites on the polymer chain does not necessarily occur quantitatively, in particular not when kinetically inert complexes of the type [M(tpy)2]3 þ have to be formed (M ¼ RuII, OsII, or IrIII). Thus, an alternative strategy was reported by Aamer et al. in which functionalized RuII bis-complexes were utilized [61, 72]. In analogy to the strategy depicted in Scheme 6.4, the amine-functionalized heteroleptic RuII complex 27 was grafted onto homopolymer 25, block copolymer 22b, and four-arm star polymer 26, using standard peptide chemistry (Scheme 6.5). In all cases, a high degree of grafting (>95%) could be concluded from 1H NMR and FT-IR spectroscopy. Assuming negligible degradation of the initial polymers, for block copolymer 29 and star polymer 30 molar masses of Mn ¼ 92 400 g mol1 (PDI ¼ 1.07) and Mn ¼ 299 000 g mol1 (PDI ¼ 1.12), respectively, could be calculated. The hierarchical self-assembly of supramolecular polymers 28–30 into advanced “structure-within-structure” morphologies was investigated. Owing to an increase in persistence length generated by the metal complexes, homopolymer 28 could be considered as a rigid-rod molecule with the rigidity originating from the charged nature of the RuII bis(terpyridine) moieties, leading to so-called “polyelectrolyte-surfactant-like” complexes. According to the theory of such systems [73], in the bulk 28 can self-assemble into hexagonal columnar rods with a long period of 5.1 nm [72]. Small-angle X-ray scattering (SAXS) experiments gave evidence that the morphology was also retained in block copolymer 29; the long period of 5.7 nm corresponded to (poorly oriented) hexagonally-packed rods

06

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| 253

254


O Br

MeO

MeO

n

25

O

28

OH

Br

O

OH

26

n 4

m

O

Br

NHC5H10OR

n

m

O 27

N

Br

Br

NHC5H10OR 29

R=

O O

+

H2N

22b

O

DCC, HOBT, DMF, room temp., 48 h

OR

n

m

n

Ru2+

N

O

N O N

OH N

O

30

N

n 4

m

OC16H23

Br

NHC5H10OR

2 PF6

Scheme 6.5 Synthesis of a homo-, block co-, and four-arm star polymers (28–30, respectively) with RuII bis(terpyridine) complexes in the side-chains via post-polymerization functionalization [61, 72].

(b) 8⫻104 7⫻104 6⫻104

Intensity (a.u.)

(a)

5⫻104 4⫻104 3⫻104 2⫻104 221

1⫻104

321 113

0 10

20

30

313 223

40

50

531 442 542 603 60

70

2theta (°) Figure 6.8 (a) Oriented SAXS pattern leading to the characterization of the morphology of block copolymer 29 (top) and schematic illustration of the hierarchical ordered “cylinders in a sea of rods” as proposed for 29 (bottom); (b) WAXD pattern data for a model copolymer without C16 chains (blue curve), block copolymer 29 (red curve), star-shaped copolymer 30 (yellow curve), and homopolymer 28 (green curve) [61]. Figure reproduced with kind permission; r 2008 American Chemical Society.

and, as a result of their microphase separation, alternating polystyrene and terpyridine-containing blocks showed a periodicity of 38 nm [61]. The overall morphology was described as “cylinders in a sea of rods” (Figure 6.8a). However, star-shaped polymer 30 exhibited no “structure-within-structure,” only the

06

29 J l 2011 18 45 41


expected microphase separation of the two blocks could be confirmed by SAXS. In chloroform, lyotropic liquid crystalline (LC) behavior could be observed by polarized optical microscopy (POM); applying wide-angle powder X-ray diffraction (WAXD), the chains were found to crystallize in the solid state, independently of the nature of the polymer (Figure 6.8b). The traditional approach of selective RuIII/RuII coordination chemistry was followed by Schubert and Hofmeier to synthesize supramolecular graft copolymers, based on the [Ru(tpy)2]2 þ connectivity [33]. For this purpose, poly(ethylene glycol) (PEG) and poly(L-lactide) chains, each end-group functionalized with a terpyridine unit [74, 75], were reacted with RuCl3 xH2O to give the corresponding RuIII mono-complexes (32 and 33) (Figure 6.9). The grafting onto random copolymer 31 (Mn ¼ 3700 g mol1, m/n ratio of 2 : 98) was subsequently achieved, in high yields, under reducing conditions (i.e., EtOH, cat. N-ethylmorpholine). When comparing the thermal properties of the grafted copolymers to those of the starting material, DSC measurements showed significant differences, in particular in the glass transition temperatures (Tgs), demonstrating the influence of the grafting process. For instance, copolymer 31 showed a Tg of 93 1C, whereas for the graft copolymer 34 the Tg was observed

m

O

O O

O

n

O

O

O

O

OH

O

68

87

O N

N O

31

N

32 N

N

Ru

Cl

Cl

N

N

N

Cl

33 Cl

Ru Cl

N Cl

31 33

32

35

34

Figure 6.9 Synthesis of graft copolymers 34 and 35 via selective RuIII/RuII coordination chemistry [33]. Figure reproduced with kind permission; r 2002 Wiley-VCH.

06

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| 255

256

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes SH n

Cl

OH n

60 °C, 24 h

n

Cl

N

R

O t

N

37

36a (R = NH2)

N O

O

HO

35

PVC

36b, SnII cat., room temp., 12 h

m

S

Cl

m

S

O

O

BuO O O OtBu room temp., 15 min

36b (R = NCO)

O

N

N

N

Scheme 6.6 Synthesis of terpyridine-functionalized PVC [76].

at 11 1C [a blend of 31 with poly(ethylene oxide) had a Tg of 17 1C and a melting temperature (Tm) of 55 1C]. Not only laboratory-made polymers but also industrial polymers, such as poly(vinyl chloride) (PVC), were utilized for decoration with terpyridine ligands. Meier et al. modified PVC with (2-mercaptophenyl)methanol, obtaining the randomly hydroxyl-functionalized copolymer 35 (Scheme 6.6); due to the low reactivity of the chlorine-groups a degree of substitution of only about 4% could be reached (n/m ratio of 96 : 4) [76]. The side-chains were added to the polymer backbone in high yield and under mild conditions via a SnII-catalyzed reaction with isocyanatoalkyl-substituted terpyridine 36b. The grafting of oligo(ethylene glycol) chains as well as the supramolecular crosslinking with various transition metal ions (e.g., FeII, CoII, NiII, ZnII) was also studied. Furthermore, the SnII-catalyzed polycondensation of 5,500 -bis(hydroxymethyl)2,20 :60 ,200 -terpyridine with a commercial bis-isocyanate-functionalized polyurethane (PUR) prepolymer was investigated by Schubert et al. The obtained AB multiblock copolymer was characterized by SEC showing a high molar mass (Mn ¼ 10 500 g mol–1) and a PDI of around 1.5. Subsequent treatment of a solution of the block copolymer with CoII ions in chloroform revealed a red–brown coloration. In contrast to the highly viscous free polymer, the crosslinked metallopolymer showed rubber-like properties [77]. A convergent approach to polymers functionalized with RuII complexes in their side-chains was utilized by Cho et al. [78]. Living anionic polymerization was applied to synthesize ABA triblock copolymers consisting of 2-(N-carbazolyl)ethyl methacrylate (CzMA), as outer blocks, and 2-vinylpyridine (2VP), as the inner block. The size of CzMA blocks was kept constant (about 20 units) and the length of the central part was varied (from 0 to 20 2VP units). Subsequently, the heteroleptic RuII complex [Ru(tpy)(dmbpy)Cl] (dmbpy ¼ 4,40 -dimethyl-2,20 -bipyridine) was coordinated to the pyridine moieties of the polymer (Figure 6.10a). Fast

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6.2 Polymers with Terpyridine Units in the Side-Chain (a)

(c) 40 O N

N RuII(tpy)(dmbpy)

O

O N

2m Cl 38 Ag Alq3 TAZ Ru-coordinated polymer PEDOT ITO Glass substrate

30

30

20

20

10

10

0

0 0

5

10

15

20

25

Voltage (V)

Figure 6.10 (a) Triblock copolymer 38; (b) PLED device configuration [ITO: indium tin oxide; PEDOT: poly(3,4-ethylenedioxythiophene); TAZ: 3-(4-biphenylyl)-4-phenyl-5-tert-butylphenyl1,2,4-triazole; Alq3: tris(8-quinolinolato)aluminum]; (c) current–voltage–luminance characteristics of the device [78]. Figure reproduced with kind permission; r 2002 Wiley-VCH.

inter- and intramolecular energy-transfer from the carbazole-units, acting as donor sites, to the RuII centers was evidenced by UV–vis absorption and emission experiments. Polymers 38 with a high RuII content (i.e., 14–20 units per polymer chain) were fabricated into a polymer light-emitting diode (PLED) of the configuration [ITO/PEDOT/38/TAZ/Alq3/Ag] (Figure 6.10b). At a turn-on voltage of 17 V, electroluminescence with a maximum at around 450 nm was observed; the maximum luminance was 32 cd m2 (Figure 6.10c). 6.2.2 Materials Based on p-Conjugated Polymers

An enormous diversity of p-conjugated polymers targeting applications, for example, in the fields of organic light-emitting diodes (OLEDs) or organic solar cells (OSCs), has been published within the last few decades [79–82]. For the synthesis of such materials, polycondensation reactions are widely applied; among others, the efficient Pd0-catalyzed cross-coupling reactions are pivotal to modern polymer research. Different functional groups, conjugated and non-conjugated, have been introduced as lateral side-chains by polymerization of respectively functionalized monomers or via post-polymerization modifications (e.g., grafting methods). In addition, transition metal binding sites, such as bipyridine or terpyridine moieties, can be introduced efficiently, as side, chains, into polymeric materials using these approaches [62, 79]. In particular, two types of conjugated polymers with grafted terpyridines have been reported, where (i) conjugated system extends over the entire molecule, including the terpyridine side chain, or (ii) conjugation into the side chains is interrupted by heteroatoms or alkyl spacers [18]. For the synthesis of fully conjugated polymers 39–47 (Figure 6.11), different polycondensations, such as Pd0-catalyzed cross-coupling reactions (i.e., Sonogashira

06

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30

Current (mA)

O

40

n

m

Luminance (Cd/m2)

n

(b)

| 257

258

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes OC6H13

C6H13O N

n

X n

C6H13O

40a (X ⫽ C(n-C6H13)2, m ⫽ 0) 40b (X ⫽ C(n-C6H13)2, m ⫽ 1) 40c (X ⫽ N(C2H5), m ⫽ 1)

39

n

S OC6H13

41

RO S

m n

S

OR

42a (R ⫽ C6H13) 42b (R ⫽ C12H25)

N

=

N

n

N

C6H13

C6H13

43

n

Spacer

Spacer

=

OR

C8H17

C8H17

C8H17

C8H17

RO

44 (R ⫽ C6H13) 45 (R ⫽ C12H25)

46

47

Figure 6.11 p-Conjugated polymers with pendant terpyridine moieties.

[83, 84], Suzuki [84], Heck [84], and Buchwald–Hartwig [85]) and the Wittig [86] as well as the Horner–Wadsworth–Emmons condensation [87] were applied. Highly soluble and fluorescent polymers were obtained in 45–93% yields. The number-average molar masses (Mn) and PDI values were determined by SEC, 1H NMR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (Table 6.1). In no case, however, was there carried out a full characterization of the polymers, that is, of their thermal and mechanical properties as well as their processability, film-forming behavior, or solid-state properties. Thus, characterization only focused on the photophysical and electrochemical properties in solution (Table 6.1). The photophysical data were in full accordance with those reported for similar p-conjugated polymers [79]. In detail, UV–vis absorption bands in the range of 300–450 nm were assigned to p–p* transitions of the polymer backbone, while additional bands around 300 nm correspond to p–p* transitions of the terpyridine substituents. Bright photoluminescence (lPL ¼ 400–520 nm, FPL up to 0.55) originated from the p-conjugated structures, with only little interference of the terpyridine arms. In the presence of various transition or rare earth metal ions, quenching of the photoluminescence (e.g., FeII, NiII, CoII, CuII) and a pronounced ionochromism (e.g., FeII, CoII, CuII, PdII, EuIII) was observed and the polymers, therefore, might be of interest for applications as sensitive and specific fluorometric chemosensors. The quenching of the photoluminescence of polymers 40c and 42b, depending on the nature of the transition metal ions, is depicted in Figure 6.12 [83, 88].

06

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| 259

Table 6.1 Molar masses, PDI values, and photophysical properties of side-chain terpyridinefunctionalized p-conjugated polymers [83–87].

Polymer 39 40a 40b 40c 41 42a 42b 44 45 46 47 48 49

Mn (g mol1)

PDIa

kabs (nm)b

kPL (nm)b

UPLb,c

4000d 4000e 20 950d 18 700d 5500f 5500f 170 000e 28 700e 31 300e 25 200e 26 500e 1200d 1600d

N.a. N.a. 1.25a 1.06a N.a. N.a. 1.40d 1.48d 1.62d 1.60d 1.90d 1.09d 1.23d

450 407 384 355 328 430 454 370 370 367 350 436 425

524 510 50 456 430 420 508 403, 425 403, 424 401, 426 404, 427 About 475 About 475

N.a. 0.55 0.62 0.13 0.02 0.05 0.40 0.25 0.16 0.37 0.41 0.33 0.21

PDI ¼ Mw/Mn. Photophysical properties in solution at room temperature. c Relative quantum efficiencies. d Determined by SEC (polystyrene calibration). e Determined by MALDI-TOF MS. f Determined by 1H NMR end-group analysis. a

450

40c CdII CrVI MnII NiII

500 550 600 Wavelength/nm (a)

42b MnII Fluorescence Intensity (a.u.)

Fluorescence Intensity (a.u.)

b

650

AlIII CoII Fe II ZnII NiII CuII Fe III

400

450

500

600 550 Wavelength/nm (b)

650

Figure 6.12 Relative fluorescence response of polymers 40c (b) and 42b (a) upon addition of different transition metal ions (all spectra measured in THF) [83, 88]. Figure reproduced with kind permission; r 2002 American Chemical Society and 2010 John Wiley & Sons, Inc., respectively.

06

29 J l 2011 18 45 44

700

260

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes N N C6H13

Zn(OAc)2

n

C6H13

n

C6H13

C6H13

40a

N N N

N

N

Zn2⫹

N

N N

N

Toluene

{[Zn(40a)](OAc)2}n

THF

Toluene C6H13 N

THF

C6H13 n

Figure 6.13 Supramolecular crosslinking of p-conjugated polymer 40a with ZnII ions. Emission images of the polymer and supramolecular gel in nonpolar as well as polar solvents upon irradiation with UV light are shown [85]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

Addition of transition metal ions to solutions of 39–47 will lead to crosslinking of the materials via intermolecular complexation. Rabindranath et al. investigated this behavior and observed the formation of a supramolecular gel from 40a and ZnII ions by crosslinking. The emission color was significantly redshifted upon addition of ZnII ions and the supramolecular gel precipitated from nonpolar solvents (Figure 6.13) [85]; however, detailed insight into the structure, properties, and potential applications of these supramolecular gel-like materials is not yet available. Beside these materials, where the p-conjugated system is expanded into the terpyridine-functionalized side-chains, few examples are known where terpyridine side-chains are attached to the p-conjugated backbone via non-conjugated spacer moieties [83, 89]. Cheng et al. utilized a Suzuki cross-coupling reaction to synthesize low molar mass poly(dialkoxyphenylene-thiophene) (48) and poly(dialkylfluorene-thiophene) (49) with one and two pendant terpyridine side chains per monomer unit, respectively (Table 6.1) [89]. Aiming for photovoltaic applications, these moieties were subsequently used for the preparation of trithiocyanato RuII terpyridine complexes, which are known to be versatile sensitizers in dye-sensitized solar cells [90]. Metallopolymers 50 and 51, obtained from 48 and 49 in high yields, respectively (Figure 6.14a), showed a very broad absorption, enabling the utilization of solar light in the near-IR region. Bulk-heterojunction photovoltaic (PV) cells of the simple configuration [ITO/(50 or 51)/C60/Al] were fabricated (Figure 6.14b). In comparison to a heterojunction PV device from ionic polythiophene and C60 [91], higher short circuit current (Isc) values (2.58 versus 0.97 mA cm2) were observed, but the efficiency (B0.12%) was limited by a lower short circuit voltage

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6.2 Polymers with Terpyridine Units in the Side-Chain (a)

OCH3 S

S

n

n O 48 or 50

48 or 49

i) RuCl3, DMF ii) NaSCN, DMF iii) (n-C4H9)4NBr

R⫽

49 or 51

O

R

R

N

N

N N

N Ru2⫹ SCN NCS NCS

O

R

(b)

(n-C4H9)4N

(c)

e⫺ AI C60 Polymer ITO

50 or 51 R⫽

N O

| 261

R

Current (mA/cm2)

4 3 2

e⫺

1 0 ⫺0.1

hv

⫺1

0

0.1

0.2 0.3 Voltage (V)

⫺2 ⫺3 ⫺4

50 (illum.) 50 (dark) 51 (illum.) 51 (dark)

Figure 6.14 (a) Synthesis of conjugated polymers functionalized with RuII tris-thiocyanato complexes; (b) bulk-heterojunction photovoltaic device configuration; (c) current–voltage characteristics of the device for polymers 50/51 under dark (black symbols) and illumination with simulated AM 1.5 solar light (100 mW cm2) [89]. Figure reproduced with kind permission; r 2008 John Wiley & Sons, Inc.

(Vsc ¼ 120 mV). This was attributed to the relatively high HOMO levels of 50/51 resulting in a smaller gap between the donor HOMO and acceptor LUMO. No improvement in performance could be achieved by introducing an additional layer of PEDOT:PSS [PSS: poly(styrene sulfonate)]; thus, optimization of the device, by both variation of the conjugated system and use of different electron-accepting layers, is still required. Polymerization using Heck cross-coupling conditions was conducted with 1,4divinylbenzene, 1,4-dialkoxy-2,5-diiodobenzene, and an diiodo-functionalized terpyridine-RuII complex, as monomers [92]. The advantages of this earlier approach, compared to the work by Cheng et al. [89], are: (i) the content of the RuII complex in the polymer can be easily tuned via the monomers’ ratio and (ii) complete coordination of all terpyridine sites by RuII ions is guaranteed by performing the complexation prior to polymerization. Poly(p-phenylenevinylene)s (PPVs) 52 with different m/n ratios exhibited yellow photo- and electroluminescence at room

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0.4

262


OC12H25

H3CO

I +

+

I

I

I OC12H25

R=

OR

C5H10O

Pd(OAc)2, (o-MeC6H5)3P, DMF, 100 °C, 24 h H3CO C12H25O

n

OR m

Ru

N N

2 PF6

N

N

N N

52

OC12H25

Scheme 6.7 Synthesis of PPVs containing RuII complexes in the side-chain by a Heck crosscoupling polymerization [92].

temperature (Scheme 6.7). Photoconductivity measurements on spin-coated samples on ITO at 490 nm resulted in photoconductivities of the order of 1012 O1 cm1. Single-layer PLEDs produced from the polymer (configuration: [ITO/52/Al]) gave a turn-on voltage of 5 V (rectification greater than 103 at 15 V) and a maximum luminance of 360 cd m2.

6.3 Polymers with Terpyridines within the Polymer Backbone

Over the last decade, the combined properties of conventional polymers with those of [tpy-M-tpy0 ]2 þ complexes have become of increasing interest [18–21]. There are mainly two different chemical approaches to introduce terpyridines (and their transition metal ion complexes) into the backbone of polymeric systems [93, 94]: (i) by functionalizing modified polymers with terpyridine ligands or (ii) by using a functionalized terpyridine, as an initiator (the convergent approach starting from uncomplexed terpyridine). These main approaches also apply to the corresponding metal bis(terpyridine) complexes (divergent approach in which the metallopolymers are formed starting from the complex). By having a functionalized polymer with free terpyridine ligands and subsequently forming different combinations of bis-complexes with different metals, a rich variety of metallo-superstructures is possible. In recent years, the combination of rigid, conjugated bis(terpyridine)s with transition metal ions into p-conjugated metallopolymers has evolved; these types of materials are discussed separately in Chapter 5. In general, polymeric materials bearing terpyridine moieties in their main-chain can be either mono-functionalized or of telechelic nature, that is, possessing two or more terpyridines per chain. Having terpyridine units at both ends of each chain allows access to linearly extended chains containing metal “linkers”. Scheme 6.8 depicts the general concept of these chain-extended polymers.

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bis-terpyridine

chain-extended polymer ll

ll

ll

ll

ll

ll

= Fe , Zn , Ni , Co , Cd , Ru , ... Scheme 6.8 Formation of chain-extended polymers via self-assembly of telechelic bis(terpyridine) ligands with transition metal ions.

Constable introduced this approach in 1995, noting that metallo-supramolecular principles (see also Chapter 5.2) are suited to prepare oligomers as well as polymers with pre-coded properties via coordination to metal ions [95]. The addition of octahedral coordinating transition metal ions to bis(terpyridine)s leads to a spontaneous self-assembly, following the rules of polyaddition. In general, formation of such supramolecular assemblies can be reversed, for example, by changing the pH value [96] or by applying electrochemical [97] or thermal changes [98]. The formation of the terpyridine metal complexes (and, thus, the noncovalent coordination polymers) can easily be monitored by various techniques, such as 1H NMR and UV–vis spectroscopy as well as titration microcalorimetry or viscosimetry experiments. 6.3.1 Polymers from Organic Small-Molecule Building Blocks

As pointed out above, the broad field of rigid-rod metallopolymers derived from pconjugated bis(terpyridine)s is considered as topic on its own (Chapter 5). Thus, only telechelic systems that have a flexible spacer between the terpyridine sites are covered in this section. Some 15 years ago, Colbran et al. reported a series of bis(terpyridine)s 53 and chain-extended polymers derived thereof [99, 100]. Starting from 40 -(4-aminophenyl)-2,20 :60 ,200 -terpyridine, metallopolymers {[Fe(53)](BF4)2}n were prepared by two different synthetic routes: (a) coupling of two ligands with a difunctional organic reagent (e.g., pyromellitic anhydride, terephthaloyl chloride, or adipoyl chloride), and then treating the new telechelic ligand with metal ions, or (b) preforming bis-complexes of the same terpyridine (54), followed by treatment with the same difunctional organic reagents (Scheme 6.9). Following the second approach, the polymerization was quenched with excess acetyl chloride and a molar mass (Mn) of 18 000 g mol1, corresponding to a degree of polymerization (DP) of 17, was calculated based on the number of end groups (1H NMR). Similar results were obtained for the polymers prepared via the first route. The intrinsic limitations in characterizing these materials are evident from the early contributions where neither mass spectrometry nor SEC can be applied for the estimation of the molar masses. This crucial issue with respect to chainextended polymers has been addressed, in particular, by the groups of Schubert ¨rthner [21, 101]. In general, the DP of such coordination polymers depends and Wu on the solvent- and temperature-dependent binding constant of the respective

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| 263

264

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes 2 BF4 N

N H2N

Fe2+

N

N

NH2

N

N

54 route b

N spacerunit 2n BF4

N M2+

N N

N n

N

{[Fe(53)](BF4)2]}n

spacerunit O

route a

HN

NH N N N

53a

N spacerunit 53

O

O

O

H N O

N H

N

53b

O 53c

O N O

N N

Scheme 6.9 Synthesis of metallopolymers via two different approaches [99, 100].

metal complex and, therewith, on the concentration. For instance, applying SEC for characterizing coordination polymers is not suitable if the binding constant is too small; in these cases, fragmentation (i.e., disassembly of the coordination polymer) will occur under the SEC conditions. If the binding constant is high enough [e.g., for RuII, OsII, or IrIII bis(terpyridine) complexes], SEC can be applied to characterize the metallo-supramolecular materials [102, 103]. The role of the solvent with respect to the preparation of high molar mass assemblies was first addressed by Rehahn [12] in which organic solvents (e.g., MeCN) can compete during synthesis with the chelating ligand by “catching” the available metal ions. The solvent can also influence the stability of the resulting metallo-supramolecular structures, in particular in the case of weak complexes [21]. The formation of coordination polymers follows a polycondensation mechanism, thus a high DP is only achievable at high monomer concentrations [6]. Moreover, a high concentration can influence the architecture of the final material, which can be either cyclic or linear. When aiming for high molar mass materials, the stoichiometry of the components has to be precisely 1 : 1. Furthermore, the structure of the telechelic ligand is of relevance for the architecture of the final metallo-supramolecular product(s). The design of the spacer dictates the properties and the cyclic or linear suprastructures. For instance, utilizing rigid-linear bis(terpyridine)s prevents intramolecular cyclization into macrocycles, whereas for rigid angular bis(terpyridine)s macrocyclic assembly is favored (see also Chapter 4.2) [104]. In general, the synthesis of small-molecule telechelics involves nucleophilic substitution reactions of 40 -chloro-2,20 :60 ,200 -terpyridine [93]. A large variety of

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| 265

functional groups may be utilized for this purpose: alcohols, amines, and thiols to name but a few (see Chapter 2.3.1) [105]. The approach can be also applied for aand/or o-functionalized polymers [106], which will be considered in detail later. Taking all this into account, bis(terpyridine)s with flexible spacers will selfassemble into either cycles or linear chains, depending on the reaction conditions [107]. Various types of macrocyclic assemblies have been reported [22], including mononuclear [1 þ 1]-species [108, 109], dinuclear [2 þ 2]- [110], and oligonuclear [3 þ 3]- or [4 þ 4]-species [107, 111]. The particular field of terpyridine-based macrocycles is discussed in more detail in Chapter 4. The groups of Constable and Schubert reported on the formation of coordination polymers 55 and 56, respectively, by self-assembly of a telechelic bis(terpyridine) with FeII ions (Figure 6.15). In the first case, two main products were isolated, which could be identified by electrospray mass spectrometry as macrocyclic assemblies. A smaller fraction that could not be eluted from the silica column was believed to be the targeted metallopolymer 55 [111]. 1H NMR spectroscopy and UV– vis titration as well as viscosimetry experiments gave evidence for the formation of 56; however, refluxing the polymer in MeOH–CHCl3 for 48 h also resulted in the entropy-driven formation of various types of macrocycles [112, 113]. The equilibrium between linear and cyclic structures can be avoided by utilizing RuII, instead of FeII, ions due to the kinetically inert character of the RuII bis(terpyridine) complexes. Thus, the self-assembly of telechelic bis(terpyridine) ligands with RuII ions will lead to supramolecular architectures that can be investigated by 1H NMR, viscosimetry measurements, analytical ultracentrifugation (AUC), and, most important, SEC [102, 114–116]. Meier et al. optimized the conditions for the polymerization of 57 with RuCl3 xH2O under reducing conditions (Scheme 6.10) [117]. The highest conversion (80–85% after 5 h) was achieved when a mixture of n-butanol –N-ethylmorpholine (NEM) was employed as reducing medium. Increasing the monomer concentration (up to 0.2 mmol ml1) enabled the formation of high molar mass polymers at the cost of the low molar mass species (i.e., cycles and oligomers). In the presence of PEG-macroligand 58 (Mn ¼ 3000 g mol1), as chain-stopper (10 mol-%), the formation of macrocycles could be suppressed almost completely and the A(Bn)A triblock copolymer 59 was obtained after anion exchange. The SEC traces of 59 and (for comparison) of [Ru(58)2](PF6)2 are depicted in Figure 6.16a, indicating the higher molar mass of 59 and the expected broad PDI (B1.6). However, a reliable estimation of the molar mass could not be performed due to the unavailability of an appropriate calibration [e.g., PS and PMMA calibrations

N

N Fe

N N

2⫹

N

N

N N

O 2n X

O

O

O

Fe

N

n

N

55 2n X

Figure 6.15 FeII metallopolymers with flexible spacers [111–113].

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2⫹

N

N

O

O m

56a (m ⫽ 3) 56b (m ⫽ 8)

n

266

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes N

N

N

O

O

RuCl3, cat. NEM/BuOH DMA, 130 °C

⫹

N

n

8

57 N

N

mixtures of linear chains and cycles

N i) RuCl3, cat. NEM/BuOH, DMA, 130 °C, 5 h ii) NH4PF6, MeOH, 40 °C, 1 h

N H3Cn(OH2CH2C)O

N

Ru

N

O

O

8

N

59

N

N m

O(CH2CH2O)nCH3 58

N

N

N 2⫹

N

N Ru

N

2⫹

N

O(CH2CH2O)nCH3

N

(2m⫹2) PF6 A(B n)A block copolymer

n

Scheme 6.10 Synthesis of metallo-supramolecular materials by self-assembly of bis(terpyridine) 57 with RuII ions [117].

Normalized RI signal

(a) 1.0

(b)

(c)

0.8 0.6 0.4 0.2 0.0 100 nm

15

18 Time/min

21

Figure 6.16 (a) SEC traces of A(Bn)A triblock copolymer 59 (dashed line) in comparison to model complex [Ru(58)2](PF6)2 (solid line) (DMF containing 5 mM NH4PF6, as eluent); (b) micelle formation of 59; (c) TEM image of micelles of 59 (without staining) [117]. Figure reproduced with kind permission; r 2006 American Chemical Society.

overestimated the Mn value, whereas a PEG calibration underestimated the molar mass of the (Bn)-block]. Since the outer PEG-block is water soluble and the central metal-rich (Bn)-block is hydrophobic, the amphiphilic material’s ability to form micelles was also investigated (Figure 6.16b). Micellization occurred upon slow

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| 267

addition of water to a solution of 59 in acetone or DMSO, as unselective solvent. After dialysis against water, the micelles were characterized by DLS, resonant mode scanning probe microscopy (SPM) and TEM (Figure 6.16c). According to these measurements, the total size (core and corona) of the hydrated micelles was estimated to be about 70 nm [117]. Since the scope of the SEC characterization could recently be extended to further MII bis(terpyridine) complexes of macroligand 58 (MII ¼ NiII, CoII, FeII) [118], the above protocol was utilized to synthesize A(Bn)A triblock copolymers 60a–c containing these transition metal ions (Figure 6.17a) [119]. In all cases, the particular

(a) N H3Cn(OH2CH2C)O

M2⫹

N

N

N N

O

O 8

N

M2⫹

N m

N

N

N

O(CH2CH2O)nCH3

N

60a (MII ⫽ NiII) 60b (MII ⫽ FeII) 60c (MII ⫽ CoII)

N

(2m⫹2) PF6

(b)

100 nm

60c

60b

60a

100 nm

100 nm

(c) Normalized RI signal [a.u.]

100

58 [Ni(58)2](PF6)2 60a (10 mol-% 58) 60a (20 mol-% 58)

80 60 40 20 0 10 8 9 Elution volume [mL]

7

11

Figure 6.17 (a) Metallo-supramolecular triblock copolymers 60; (b) cryo-TEM pictures of micelles of A(Bn)A triblock copolymers 60 in acetone–water; (c) SEC elution curves (RI detector) for PEG-macroligand 58, the [Ni(58)2](PF6)2 model complex, and A(Bn)A triblock copolymer 60a (with 10 and 20 mol-% of 58) (DMF containing 5 mM NH4PF6, as eluent) [119]. Figure reproduced with kind permission; r 2008 American Chemical Society.

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268

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes reaction conditions as well as the characterization tools had to be individually optimized. The micellization in acetone–water of the amphiphilic copolymers was investigated and spherical objects with radii of 10–14 nm could be visualized applying cryo-TEM (Figure 6.17b); moreover, for the NiII triblock copolymer 60a, the amount of the added end-capper was varied, allowing it to influence the length of the (Bn)-block, which could then be confirmed by SEC. If rare earth metal ions are utilized for complexation of ditopic tridentate ligands, 3D-networks will be formed. Owing to their f-orbitals, these metal ions can establish up to nine coordinative bonds and, therefore, bind up to three tridentate ligands [120–124]. A remarkable example taking advantage of this special complexation behavior was published by Rowan et al. [17, 125–127] where ditopic bip-type ligand 61 [bip ¼ 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine] with a flexible pentaethylene glycol spacer was utilized (Figure 6.18). The synthesis of metallopolymers, based on these ligands with 97% CoII or ZnII and 3% LaIII or EuIII, produced gel-like materials. All four possible combinations were realized, leading to thermoresponsive polymers – upon heating the gel became fluid again. Furthermore, the polymers were found to be thixotropic, meaning the gels became fluid upon shaking. The intense luminescence of the gel with the couple

(a) N

N

N

N ⫽ N

O

N

61

O

O

O

O

O

N N

⫽ transition metal ion

N

N

⫽ lanthanide metal ion

(b)

Heat ca. 100 ⬚C Cool to RT

(c)

Shake Rest

61 with CoII/LaIII swollen with MeCN

61 with ZnII/LaIII swollen with MeCN

Figure 6.18 (a) Metallo-supramolecular polymerization of ditopic bip ligand 61 with transition and lanthanide metal ions; (b) thermoresponsive nature of CoII/LaIII; (c) mechanoresponsive nature of a thixotropic ZnII/LaIII system [125]. Figure reproduced with kind permission; r 2003 American Chemical Society.

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ZnII/EuIII could be “switched off” either by heating or adding a dilute solution of formic acid, which bound to the EuIII centers and quenched the emission. The materials reported by Rowan et al. are excellent examples of multiresponsive metallo-supramolecular polymers. The entire field of chain-extended polymers, in particular based on flexible (i.e., non-conjugated) telechelic terpyridine-type ligands, with respect to synthesis and potential applications was reviewed recently by Chiper et al. [21]. 6.3.2 Chain-Extended Polymers from Polymeric Building Blocks

The previous section described the construction of supramolecular metallopolymers from well-defined (i.e., monodisperse) organic monomers; however, these compounds were characterized by a high charge density and, in general, their mechanical properties were not of the best quality. It has been a target of modern research to combine the properties of known polymers with the features of supramolecular entities. One way to reach this goal is the utilization of polymeric telechelics in which polymers of rather low molar mass, a,o-functionalized with terpyridines, can be used as building blocks for various polymeric superstructures such as chain-extended polymers or block copolymers. In analogy to bis(terpyridine)s with flexible small-molecule spacers, the nucleophilic aromatic substitution of 40 -chloro-2,20 :60 ,200 -terpyridine (62) [128, 129] represents the most general approach. Hydroxyl- or thiol-functionalized molecules gave the corresponding oxo- or thioethers, respectively, in high yields. Commonly, the reaction conditions utilize KOH, as base, in DMSO. As a first example, a-carboxy-oterpyridin-40 -yl-functionalized poly(oxytetramethylene) was reported [130]. Starting from this, Schubert et al. utilized various a,o-dihydroxyl-functionalized polymers with different macroscopic properties (e.g., hydrophilic or hydrophobic) to obtain the telechelic bis(terpyridine)s 63–66 (Scheme 6.11) [106, 129, 131, 132]. In particular, commercial poly(ethylene glycol) (PEG) was utilized for the synthesis of water-soluble bis(terpyridine)s 63 with different lengths of the PEGchains (e.g., n ¼ 43 or 179) [102, 106, 114, 132]. The telechelic polymers were all characterized by 1H NMR, SEC, and MALDI-TOF MS, and were subsequently used for the self-assembly with FeII or RuII ions into chain-extended polymers. Concerning the latter, initially the activated [Ru(acetone)6](PF6)2 precursor complex was employed to guarantee high conversion. For the chain-extended RuII polymer 67 (Figure 6.19a), a DP of 15 was estimated from viscosimetry experiments, corresponding to a molar mass (Mn) of around 123 000 g mol1 [114]. The high Mn of the supramolecular material could be confirmed by AUC experiments [115]. AFM imaging revealed a lamellar-like ordering of the chain-extended polymer in the solid state (Figure 6.19b). Later, the synthetic protocol was further optimized to allow the direct synthesis of 67 with high conversion from 63b and RuCl3 xH2O under reducing conditions [132]. SEC experiments showed a broad, but monomodal distribution of 67 (PDI E 2.30) in accordance with the mechanism of the metallo-supramolecular polycondensations (Figure 6.19c).

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| 269

270


N

N

N

Cl N

+

OH

N

O

N

62

polymer

O

N

63-66 N

m

n

n

poly(ethylene glycol) (PEG) 63a (n = 43) 63b (n = 179 O

N

O

O

=

O 10

n

poly(ethylene-co-butylene) 64 (n + m = 70)

O

10

30

66a

poly(tetrahydrofuran) (PTHF) 65a (n = 8) 65b (n = 40) 65c (n = 89) O

14

24

14

66b

poly(ethylene glycol)-b-poly(propylene glycol) (PluronicsTM)

Scheme 6.11 Synthesis of the telechelic bis(terpyridine)s 63–66 from a,o-dihydroxyfunctionalized polymers.

(a)

(b) N O

O

N 2⫹

N

Ru

N

n

180

2n PF6

N

N 67

(c) Normalized RI signal [a.u.]

polymer

KOH, DMSO

polymer

HO

1.0 0.8 0.6 0.4 0.2 0.0 12

14 16 18 Elution time [min]

20

Figure 6.19 (a) Chain-extended polymer 67; (b) AFM phase image of 67 [114]. (c) SEC traces of 67 (red curve) and macroligand 63b (dashed curve) (DMF containing 5 mM NH4PF6, as eluent) [132]. Figure reproduced with kind permission; r 2003 Wiley-VCH and 2008 European Polymer Federation, respectively.

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| 271

Bis(terpyridine) 64 features a hydrophobic poly(ethylene-co-butylene) linkage (Mn ¼ 4500 g mol1, PDI ¼ 1.19). The prepolymer was prepared by anionic polymerization of butadiene, with a protected hydroxyl-group in the initiator molecule, and subsequent end-capping of the polymer with ethylene oxide. Hydrogenation of the double bonds and deprotection yielded the a,o-dihydroxyl-functionalized building block that was treated with 62 according to Scheme 6.11. 1H NMR spectroscopy confirmed the high degree of end-group functionality (1.92 7 0.03) [74]. Owing to its very low Tg and Tm values, poly(tetrahydrofuran) (PTHF) is a frequently used soft segment in thermoplastics and crosslinked elastomers [133]. The combination of these properties with those of transition metal ion complexes of terpyridines would expand the supramolecular toolbox towards new types of functional materials. The functionalization of commercial PTHF with terpyridine ligands on both ends of the polymer was first reported by Schubert et al. in 2000 (polymers 65a/b) [106]. The cationic ring-opening polymerization of THF and subsequent end-group functionalization was elaborated in detail by Goethals et al. [134, 135]. Following this protocol, Chiper et al. performed the bulk polymerization of THF using triflic anhydride, as initiator, at ambient temperature and water, as the terminator. Post-polymerization with 62 was achieved as depicted in Scheme 6.11 (with KOtBu, as base, and THF, as solvent) [131]. SEC (Figure 6.20a), UV–vis titration, and MALDI-TOF MS were applied to characterize the telechelic PTHF 65c: m/z differences of 72 between the single peaks in the mass spectrum corresponded to one THF repeat unit. Moreover, end-group analysis confirmed the presence of the terpyridine moieties at the end of the polymer chains. Triblock copolymers, consisting of hydrophilic PEG and hydrophobic poly(propylene glycol) (PPG) blocks, of the order of either PEG-b-PPG-b-PEG or PPGb-PEG-b-PPG, represent an important class of temperature-responsive copolymers and are referred to as PluronicsTM [136]. Aiming for new types of multiresponsive supramolecular polymers, low molar mass dihydroxy-functionalized prepolymers (known to possess a low degree of defect structures) were utilized for the functionalization of terpyridines [131]. The bis(terpyridine)s 66a and 66b were

1.0

0.6 0.4 0.2

1.0 Normalized RI signal [a.u.]

Normalized RI signal [a.u.]

Normalized RI signal [a.u.]

1.0 0.8

0.8 0.6 0.4 0.2 0.0

0.0 6

7

9 10 8 Elution volume [ml]

11

12

0.8 0.6 0.4 0.2 0.0

16

(a)

17


21

22

16

17


(b)

Figure 6.20 SEC elution curves for the telechelic bis(terpyridine)s 65c (a), 66a (b), and 66b (c) (dotted curves); for comparison, the corresponding prepolymers are also depicted (solid curves). In all cases, CHCl3 was used, as eluent [131]. Figure reproduced with kind permission; r 2010 Elsevier B.V.

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(c)

21

272

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes characterized by 1H NMR and SEC; shoulders in the SEC traces (Figure 6.20b and c) were attributed to defect structures due to transetherification under the harsh basic conditions. 6.3.3 Monotopic Macroligands by End-Group Functionalization

When considering a polymer that is functionalized on one side with a terpyridine moiety, one might utilize either a terpyridine-functionalized initiator or an appropriate terpyridine-containing end-capping agent or follow – in analogy to the examples discussed in Section 6.3.2 – a post-polymerization modification approach. Indeed, the latter strategy has been widely applied and 40 -chloro-2,20 :60 ,200 -terpyridine (62) is again the reactant of choice. Polymers covering a broad range of materials properties have been o-functionalized with hydroxyl-groups and, subsequently, transferred into the corresponding monotopic macroligands. These are key substrates for the construction of advanced polymeric structures, based on metallo-supramolecular linkages. Among others, A-[M]-A homopolymers and A-[M]-B diblock copolymers as well as triblock copolymers, in combination with ditopic (macro)ligands, of the type A-[M]-B-[M]-A are accessible, where 40 -substituted-2,20 :60 ,200 -terpyridine is denoted as “-[” (Figure 6.21). Lohmeijer and Schubert demonstrated this way of connecting polymer chains as “playing LEGO with macromolecules” [93, 137]. Before addressing these supramolecular architectures in more detail, the monotopic macroligands behind them will be evaluated. The example of choice is poly(ethylene glycol) monomethyl ether (MPEG) that has been functionalized with a terpyridine moiety (58) [74]. Today, this macroligand is possibly the most widely utilized and best characterized monotopic polymeric terpyridine derivative; furthermore, the characteristics related to the PEG chain (e.g., hydrophilicity, biocompatibility, and softness) can easily be introduced into metallo-supramolecular architectures. The synthetic protocol, which was initially developed for oligo(ethylene glycol) [106, 130], was successfully applied to yield MPEG-tpy 58a–d of different molar masses [Mn ¼ 3000, 5200, 10 000, and 16 500 g mol1, respectively]. Nucleophilic substitution of 62 with commercial o-hydroxyl-functionalized MPEGs under basic conditions yielded 58a–d (Figure 6.22a); the monotopic

A-[M]-A A-[M]-B-[M]-A A-[M]-B A-B-[M]-C Figure 6.21 Different metallo-supramolecular architectures based on monotopic terpyridinemacroligands. Figure redrawn according to References [93, 137].

06

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6.3 Polymers with Terpyridines within the Polymer Backbone N

(a)

O

O

N

| 273

(c)

O

O

n

58a (n ⫽ 70) 58b (n ⫽ 120) 58c (n ⫽ 225) 58d (n ⫽ 375)

N

(b) ––– 58a - - - 58b ····· 58c –· – · 58d

16

18

20

22

3000

t / min

4000

5000 m/z

6000

Figure 6.22 (a) MPEG-terpyridines 58; (b) SEC traces of 58 [CHCl3–NEt3–iso-PrOH (94 : 4 : 2 ratio) as eluent]; (c) MALDI-TOF MS of poly(ethylene oxide) with a DP of 120 (top) and MPEG-tpy 58b (bottom) [116]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

macroligands were characterized by 1H NMR, SEC, and MALDI-TOF MS [116]. An optimized eluent for SEC was introduced (CHCl3–NEt3–iso-PrOH in a 94 : 4 : 2 ratio), reducing the column interactions of the terpyridine-functionalized polymer and leading to more reliable Mn and PDI values; however, for low molar masses, SEC still underestimates Mn. Moreover, MALDI-TOF MS was found to be a highly versatile tool for the end-group analysis of such polymers. Comparing the mass spectra of each MPEG-OH to that of the respective macroligands 58, a clear shift of m/z ¼ 232 for each single signal showed the successful and complete end-group modification (Figure 6.22c). Similar strategies were applied to attach terpyridine to other o-hydroxyl-functionalized polymers by nucleophilic substitutions. However, the synthesis had to be optimized in every single case due to the properties of particular polymers (e.g., solubility, reactivity). Commercial hydroxyl-functionalized polystyrene (DP ¼ 20 or 104) and poly(ethylene-co-butylene) (PEB) were reacted with 62 in THF and potassium tert-butoxide, as base, to yield macroligands 68 and 69, respectively (Figure 6.23) [74]. Furthermore, polystyrene-block-poly(2-vinylpyridine) and polybutadiene-block-poly(ethylene glycol) were utilized to yield terpyridine-functionalized diblock copolymers 70 and 71, respectively [138]. In all four cases, living anionic polymerization was used for the preparation of the initial polymers [139– 142], followed by end-group functionalization with OH groups, with good control over the molar masses and narrow PDI values. Methoxy-poly(tetrahydrofuran) (MPTHF) was synthesized by bulk polymerization, initiated by methyl triflate and terminated by water (Mn ¼ 4400 g mol1, PDI ¼ 1.17). Subsequently, nucleophilic substitution to yield 72 was achieved in THF utilizing KOtBu as base (with DMSO–KOH decomposition of the polymer backbone was observed) [131]. Poly(2oxazoline)s are a polymer class for which numerous potential applications have been envisioned [143–145]. Thus, it was desirable to combine the materials’ properties of such polymers with those of supramolecular entities. The living

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7000


274

cationic ring-opening polymerization of 2-ethyl oxazoline was initiated by methyl tosylate; after quenching the polymerization with water, the desired hydroxylfunctionalized poly(2-ethyloxazoline) (PEtOx) was obtained (Mn ¼ 5000 g mol1, PDI ¼ 1.05) [146]. To avoid depolymerization during the substitution reaction with 62, for the synthesis of 73, the same protocol as for 72 was applied [131]. The last polymer class that has been combined with terpyridines by a straightforward nucleophilic substitution of 62 was poly(dimethylsiloxane)s (PDMSs). The synthesis of macroligands, such as 74, was targeted due to the appealing properties of PDMSs: low Tg values, high thermal and oxidative stability, UV–vis resistance, good electrical properties, and high permeability to many gases, as well as low surface energy and hydrophobicity [147]. However, the reaction of mono-hydroxylfunctionalized PDMS with 62 in DMSO yielded PDMS-terpyridine 74 in only low yields (notably, changing solvent and/or base did not improve the yield data) [148]. Figure 6.23 gives an overview of the monotopic terpyridine-macroligands that were

N O

n

68a (n ⫽ 20)

n

68b (n ⫽ 104)

O

m

32

69 (n ⫹ m ⫽ 70)

n

m

O 48

72 ⫽

N O

50

N

Si N

O

n

m

n

76 (m ⫹ n ⫽ 90) S 30

O

O

77

Fe H 12

R⫽

H N

Si

p

75 (m ⫹ n ⫹ p ⫽ 135)

75-77

NC

O 68

R⫽ m

HN

Si

74

R⫽

N H

O

N

73

O RO

O

O 60

O

71 (n ⫹ m ⫽ 68)

O

O

70

O O

13

O 78

Figure 6.23 Monotopic terpyridine-functionalized macroligands 68–78.

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prepared by nucleophilic substitution of mono-hydroxyl-terminated polymers and 40 -chloro-2,20 :60 ,200 -terpyridine (62). The knowledge gained in characterizing terpyridine-containing macroligands by SEC and MALDI-TOF MS was utilized to confirm the end-group functionalization of the polymers with terpyridine units [74, 103, 131, 138, 148]. Though nucleophilic substitution reactions are commonly used, their limitations have to be addressed: the approach is limited to non-reactive polymer backbones. Lohmeijer et al. showed that commercial polymers, such as polybutadiene and polyisoprene (due to crosslinking of the polyene structure) or metalcontaining polymers {e.g., poly(ferrocenyldimethylsilane), PFDS [149]}, could not be treated with 62. Thus, an alternative protocol was developed in which an isocyanate-functionalized terpyridine and various hydroxyl-terminated polymers could be reacted under mild conditions (i.e., SnII catalyst, room temperature) to afford new macroligands with a stable urethane linkage. This isocyanate route enabled the synthesis of terpyridine-modified polyisoprene and polybutadiene (75 and 76, respectively) as well as poly(ferrocenyldimethylsilane) (77) [150, 151]. In recent years, other mild procedures for the coupling of OH- or NH2-functionalized supramolecular entities to hydroxyl-terminated polymers have been reported (e.g., the Mitsunobu reaction [152] or a CDI-mediated amidation, CDI ¼ N,N0 carbonyldiimidazole [153]). The latter was applied by Chiper et al. for the endgroup functionalization of temperature-responsive poly(N-isopropylacrylamide) (PNIPAM). RAFT polymerization and subsequent cleavage of the RAFT agent by aminolysis yielded PNIPAM (Mn ¼ 4500 g mol1, PDI ¼ 1.20). The SH group was activated with CDI and reacted with 36a to yield macroligand 78 [154]. All previous examples of macroligands 68–78 bear the risk that the terpyridine unit (e.g., ether or urethane linkage) might be broken under harsh conditions, for example, upon coordination with RuII or OsII ions. Anionic polymerization represents an alternative route to functionalized polymers, where the terpyridine moiety is attached to the polymer backbone via C–C single bonds, thus increasing its chemical/thermal stability. Under stringent reaction conditions (i.e., a moisture- and oxygen-free atmosphere), the anionic polymerization can be performed, at short reaction times, enabling control over the molar mass and yielding low PDI values. It has been found that the high reactivity of the “living” polymeric organolithium chain-ends can be restrained by reaction with 1,1-diphenylethylene [155]; this intermediate functionalization step promotes efficient chain-end modification and avoids undesired chain-coupling reactions (due to steric hindrance). According to this, Schubert et al. optimized the reaction conditions for the synthesis of 79, utilizing a high-throughput experimentation approach that employed an automated synthesizer and fast characterization tools (Figure 6.24). It was shown that the molar masses of macroligands 79 could be easily tuned, keeping the PDI values well below 1.20. Furthermore, almost quantitative endgroup functionalization, as proven by UV–vis titration experiments as well as MALDI-TOF MS, was achieved (Figure 6.24) [156]. The versatility of this protocol was further exploited and copolymers of styrene and diphenylethylene were prepared. The interplay of reactivity and steric

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| 275

276

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes 1.1 eq.

sec-BuLi, cyclohexane, 50 ⬚C, 30 min

n

50 ⬚C, 0.5 min

Li

n

m/z ⫽ 3485

2000

3000

4000 m/z

5000

room temp., 15 h

N

n

79

N

6000

7000

Figure 6.24 Synthesis of terpyridine-terminated polystyrene via anionic polymerization. A representative MALDI-TOF MS of 79 (Mn ¼ 2700 g mol-1, n ¼ 29) is also shown [156]. Figure reproduced with kind permission; r 2005 American Chemical Society.

N

4

length/nm

1000

Li

62

N

expected mass of 79 (n ⫽ 29): 3485 g·mol⫺1

n

copolymer

2

PS

1 0

N

n

3 80

N

copolymer PS

Kuhn length A

diameter d

Figure 6.25 Comparison of the conformational parameters of 80 (“copolymer”) with those of 79 (“PS”) [158]. Figure reproduced with kind permission; r 2009 John Wiley & Sons, Inc.

hindrance of the two monomers enables their incorporation into polymers in almost perfect alternating order. Such copolymers exhibit improved long-term service temperatures due to the stiffening of the polymer main-chain (when compared to conventional polystyrene) and are referred to as “superpolystyrene” [157]. A series of well-defined terpyridine-terminated poly(styrene-alt-diphenylethylene) copolymers (80) was prepared and characterized by 1H NMR, SEC (Mn ¼ 110016 900 g mol1, PDI ¼ 1.111.23), UV–vis titration experiments, and MALDI-TOF MS (Figure 6.25) [158]. Furthermore, AUC was applied, as an additional analytical tool, to obtain the absolute molar masses of the copolymers. The conformational parameters (i.e., Kuhn length, A, and hydrodynamic diameter,

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| 277

d) were determined and revealed a higher stiffness of the materials due to the bulky phenyl groups and restricted mobility of the macromolecules (Figure 6.25) [158, 159]. However, the synthesis of terpyridine-macroligands via anionic polymerization and subsequent quenching with 62 was limited to styrene-type monomers. Utilization of other monomers, such as isoprene, for the anionic polymerization of homopolymers or block copolymers (in combination with the couple styrene/ diphenylethylene) resulted in low degrees of end-group functionalization after reaction with 62 [160, 161]. 6.3.4 Functional Terpyridine-Containing Initiators 6.3.4.1 Initiation of Ionic Polymerization Reactions Having dealt with macroligands in which terpyridine units have been introduced by end-group functionalization methods, the utilization of terpyridine-containing initiators for various types of polymerizations will be considered. In general, two types of initiators have been applied for this purpose: the convergent approach utilizes uncomplexed ligands, as initiators, affording polymers with potential metal binding sites; the divergent route starts from metallo-supramolecular initiators and, consequently, supramolecular metallopolymer architectures are directly accessible [94]. Already in the late 1990s, transition metal ion complexes of bromomethylfunctionalized 2,20 -bipyridines were used, as initiator, for the cationic ring-opening polymerization of 2-oxazolines [162–164]. The living character of the polymerization and improved control over the molar mass could be confirmed by 1H NMR and SEC measurements. Moreover, further metal binding sites (e.g., terpyridines) could be introduced at the other chain-end by quenching the polymerization with an appropriate terminating-agent; for instance, bis-functional ligands, such as 81, could be prepared this way (Figure 6.26) [94]. Since the polymerization with uncomplexed ligands proceeded only with poor initiator efficiency and lower control over the molar mass [162], an indirect route to metal-free poly(2-oxazoline)macroligands was established in which the decomplexation under acidic or basic conditions could be performed in high yields for bipyridine-macroligand

N

N O N

N N

R1

O N 2

R

N n

m

R1 N

O

O

N

N

m 2

O

Cu

O

N

N R

metallo-supramolecular initiator poly(2-oxazoline)-chain (two blocks)

terpyridine-functionalized terminating-agent

Figure 6.26 Poly(2-oxazoline)-macroligand with two different metal binding sites (R1, R2 ¼ different side chains; the counterions are omitted) [94].

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N

n 2

N 81

278

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes complexes of FeII, CoII, or CuI ions [94]. Since RuII tris(bipyridine)-macroligand complexes were inert even at low pH values, cleavage of the side-chains of poly(2ethyloxazoline)s could be realized without breaking the RuII tris(bipyridine) core. As a result, metallo-supramolecular poly(ethylene imine)s (PEIs) for potential applications, for example, in gene delivery, could be obtained [165]. Beside bromomethyl-functionalized bipyridines, similar terpyridines (82 [166] and 83 [167]) were coordinated to FeII ions and the derived complexes were subsequently utilized as initiator for the cationic ring-opening polymerization of 2ethyloxazoline. The macroligand bis-complexes 84 and 85 were obtained after chain-termination with piperidine (Scheme 6.12). The linear relationship between Mn and the monomer/initiator ([M]/[I]) ratio revealed the living character of the polymerization. Both initiators [Fe(L)2](PF6)2 (L ¼ 82 or 83) offered good control over the molar masses and the isolated polymers revealed low PDI values (according to SEC) [168, 169]. Further blocks of poly(2-oxazoline)s could be grown using the living chain ends by sequential addition of the monomers; for instance, an amphiphilic poly(2-ethyloxazoline)-block-poly(2-phenyloxazoline) could be realized. Polymerization with 83, as initiator, was also investigated; for [M]/[I]-ratios o50, monomodal molar mass distributions were observed by SEC (PDI values of around 1.12). Thus, uncomplexed 83 can also be utilized as initiator for the cationic ring-opening polymerization of 2-oxazolines – at least when potential sidereactions, such as the addition of cationic species to the pyridine rings, were suppressed by keeping the polymerization times short [169]. Figure 6.27 depicts a molecular model (MAC Spartan PlusTM, MM2 level) of the supramolecular metallopolymer [Fe(84)2](PF6)2. The star-shaped architecture with the FeII bis(terpyridine) unit, as core, could be decomplexed, for example, under acidic or basic conditions, giving the linear-flexible macroligand 84. The high degree of reversibility of this assembly–disassembly process was about 95% as shown by repeated UV–vis titration experiments [168].

i) FeSO4, MeOH, room temp. 3 h ii) NH4PF6

O i) [Fe(L)2](PF6)2 L = 82 or 83

82 or 83

N

CH3CN, 80 °C, 24 h

[Fe(L)2](PF6)2 L = 84 or 85

ii) piperidine, MeCN, 4 h R

N

N

N

R

R

84 (R =

N

N O

83 (R = Br)

N

82 (R = Br) )

n

85 (R =

N N

R2

N

N O

n

R2

Scheme 6.12 Synthesis of FeII bis(terpyridine)-PEtOx complexes by cationic ring-opening polymerization.

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)


aacid or base FeII ions 84 [Fe(84)2](PF6)2 Figure 6.27 Representation of the molecular models (MAC Spartan PlusTM, MM2 level) of [Fe(84)2](PF6)2 and 84 [168]. Figure reproduced with kind permission; r 1999 Springer Verlag.

The controlled coordinative ring-opening polymerization (ROP) of cyclic esters (e.g., L-lactide and e-caprolactone) [170] resembles another type of polymerization in which supramolecular initiators have been utilized. In general, alcohols (socalled “initiators”) together with a metal compound [e.g., Al(OR)3, AlEt3, Sn(Oct)2] are used, as the catalytic system, and yielded well-defined polymers with adjustable molar mass (determined by the stoichiometry) and low PDI values. The resulting polyester chains are biocompatible and biodegradable, with applications in medicine and tissue engineering; combination with chelating ligands (and their transition metal complexes) and utilization of such materials in catalysis or as new types of diagnostic tools has been envisioned [94]. This work was pioneered on bipyridine-based systems by Fraser [171] and Schubert [172–174], who synthesized polyester-macroligands from hydroxymethyl-functionalized bipyridines and FeII or RuII tris(bipyridine) complexes. Various hydroxyl-functionalized terpyridine derivatives (86–89) were prepared and applied as initiator for the ROP of L-lactide and e-caprolactone [25, 75, 175, 176]. Tin(II) octanoate was utilized, as catalyst, for the preparation of macroligands 90–92 and 89a, respectively, by bulk polymerization (Scheme 6.13). In all cases, good control over the molar masses (Mn values close to the theoretical ones) and narrow PDIs were observed. The living character of the polymerizations was confirmed by SEC measurements; 1H NMR revealed a high degree of end-group functionalization with terpyridine units (>95%); thus, initiation of the polymerization by impurities (e.g., water) could be excluded. Transition metal ion bis-complexes with terpyridine ligands bearing viologentype substituents were reported to show a different UV–vis absorption behavior than those of conventional 40 -aryl-2,20 :60 ,200 -terpyridines; FeII bis-complexes of the latter type are typically purple in color, whereas similar complexes with the former are blue. This shift of the absorption maximum can be explained by changes in the electronic properties of the terpyridine ligand due to the positive charge and low energy of the lowest unoccupied molecular orbital (LUMO) of the overall ligand system. The same behavior was observed for macroligands 92; upon UV–vis

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| 279

280

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes RO

OR OR

O N RO

N

N

N

86 (R = H) 90a (R = poly(ε-caprolactone) 90b (R = poly(L-lactide) O

O

PF6

O

N

N

N

N

87 (R = H) 91a (R = poly(ε-caprolactone) 91b (R = poly(L-lactide) O

ε-caprolactone (I)

88 (R = H) 92a (R = poly(ε-caprolactone) 92b (R = poly(L-lactide)

N

N

89 (R = H) 89a (R = poly(ε-caprolactone)

poly(ε-caprolactone): O L-lactide (II)

O

nH

O poly(L-lactide):

O

O

H n

O

I or II cat. SnII, 110 °C, 3 h 86-89

N

O

O O

N

N

90a/b or 91a/b or 92a/b or 89a

Scheme 6.13 Synthesis of poly(L-lactide)- and poly(e-caprolactone)-macroligands.

titration with FeII ions, the characteristic MLCT band at 600 nm corresponding to the deep-blue color of the polymer solution became visible [176]. Circular dichroism (CD) spectroscopy confirmed the optical purity of the poly(Llactide)-macroligand 90b as well as its FeII bis-complex: the characteristic positive dichroic band at l ¼ 210 nm (assigned to n - p* absorptions of the ester moieties) was observed. Thus, racemization during the polymerization process could be excluded; however, no evidence was provided for the existence of secondary structures (e.g., a-helices) in either solution or solid state [75]. Polyester-macroligands 89–92 all have a hydroxyl-group at their chain-end, which could be utilized for further functionalization. One example is the initiation of a ROP yielding a terpyridine-substituted diblock copolymer. Macroligand 90a was utilized to initiate the ROP of L-lactide; the increase in molar mass could be followed by SEC (however, broadening of the SEC traces indicated that the second polymerization proceeded with a loss of control) [169]. A by-far more promising approach towards high molar mass supramolecular polymers was reported by Hofmeier et al. [175], in which the chain-end of macroligand 89a was modified with an ureido-pyrimidinone (UPy) moiety to generate 93 (Figure 6.28). This self-complementary quadruple H-bonding array enabled the combination of two orthogonal supramolecular entities in one molecule. A wide range of supramolecular polymers based on the dimerization of UPy [association constant (Ka) of 6 107 mol1 in CHCl3] can be found in the literature [3–6]; the response of the non-covalent polymers to concentration, temperature, and solvent conditions has been reported. The expected dimerization of 93 in nonpolar solvents could be confirmed by 1H NMR, where signals corresponding to the H-bonded protons of 93 93 were observed between 10.2 and 13.1 ppm

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N

O O N

O

O O

29

O N H

N H

H N H

N

N N

O

93 N

UPy-side: quadruple hydrogen bonding

tpy-side: metal-to-ligand complexation

solvent conditions temperature

metal ions

Figure 6.28 Macroligand 93 featuring two orthogonal supramolecular binding sites. The reversible self-assembly via H-bonding and metal ion complexation into supramolecular polymers is depicted below. Insert: image of a spin-coated film of {[Fe(93 93)](PF6)2} [175]. Figure reproduced with kind permission; r 2005 American Chemical Society.

(in CDCl3). The subsequent addition of ZnII or FeII ions furnished high molar mass chain-extended polymers (Figure 6.28) as concluded from viscosimetry measurements. Concentration-dependent viscosimetry measurements revealed an exponential dependence of the viscosity on the concentration – typical behavior for reversible supramolecular polymers. Good film-forming behavior of the metalcontaining materials was observed (Figure 6.28). Rheometry confirmed the improved properties of {[Fe(93 93)](PF6)2}n in comparison to 93. Thus, the combination of the properties of both supramolecular systems can be considered as beneficial for the development of “switchable” functional materials. 6.3.4.2 Initiation of Controlled Radical Polymerization Reactions Substantial progress in the field of polymer chemistry has mainly been achieved in the last two decades by the development of the family of “living” CRP techniques – ATRP, RAFT, and NMP. Section 6.2.1 detailed the application of these three CRPs with respect to polymers with terpyridine units in the side-chain. Beside the previously discussed terpyridine-containing monomers, initiators for all types of CRPs have also been developed in recent years.

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| 281

282

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes Among the three different CRP techniques, ATRP is the most widely explored. In general, a CuI species supported by an amine ligand is applied as catalytic system. In particular, either 2,20 -bipyridine or PMDETA is the more commonly used amine in ATRP [177–179]. Thus, utilizing functionalized bipyridines, also as initiators for the polymerization of monomers, such as MMA or tert-butyl acrylate (tBA), under ATRP conditions was a consequent step towards new functional materials [50, 180, 181]. Terpyridines have also been employed, as ancillary ligands, to stabilize the active CuI species in the catalytic cycle of ATRP [182]; however, the low solubility of the CuI mono(terpyridine) complexes resulted in significantly lower reaction rates in comparison to bipyridine, as ligand [183, 184]. To the best of our knowledge, Pefkianakis et al. were the first to apply an uncomplexed terpyridine (83) as an initiator for ATRP [185]. The CuI-catalyzed homopolymerization of a vinyloxadiazole monomer was performed in diphenyl ether at 110 1C (Figure 6.29a). To overcome the low reaction rates, PMDETA or bipyridine had to be added, as a supporting ligand. The resultant polymers (94) were soluble in a wide range of solvents (e.g., CHCl3, THF, DMF) and molar

N

(a)

N

Br O N

⫹

n

Br

N

CuBr, PMDETA or bpy, Ph2O, 110 ⬚C, 18 h

n

N 94

83

N

N N vinyloxadiazole

O

O

N N

O

(b) N Br

N

n

N OC12H25

2+

Ru N

N

O

N 2 PF6

95

OC12H25

O

O

N N

Figure 6.29 (a) Synthesis of macroligand 94 by terpyridine-initiated ATRP; (b) the metallosemiconducting material 95.

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| 283

masses (Mn) in the range of 2000–43 000 g mol1 (PDI ¼ 1.18–1.38) were determined by SEC. Subsequent complexation with a RuIII mono(terpyridine) complex yielded the metallo-supramolecular material 95 (Figure 6.29b). Photophysical investigations on 95 revealed an efficient energy-transfer process, in that after excitation at around 300 nm the absorption maximum of the organic part resulted in intense red-light emission (centered at around 700 nm), while the characteristic blue emission of the oxadiazole units at 360 nm was not detected. The RAFT process is the youngest member of the CRP family and was discovered in the late 1990s [186]. Since then, numerous scientific contributions have dealt with this metal-free CRP method – utilizing very different types of monomers as well as introducing new sulfur-containing chain-transfer agents (CTAs). The first CTAs containing chelating ligands (i.e., bipyridines) were reported by Chen et al. [187] as well as Zhou and Harruna [188, 189]. A similar terpyridine-functionalized CTA (96) [190] and the thiocarbonate derivative 98 have been reported [191]. Thioester 96 was applied in the RAFT polymerization of styrene and NIPAM (Nisopropylacrylamide), respectively (Scheme 6.14). For both monomers, a linear increase of ln([M]0/[M]) with the polymerization time (pseudo-first-order kinetics) was observed; the molar masses of 97a and 97b according to SEC increased linearly with conversion (PDI values o 1.20). The good agreement of the Mn values as determined by 1H NMR with the theoretical ones indicated that most of the polymer chains were functionalized at the chain-end by a terpyridine group derived from the CTA [190]. A similar performance in terms of control over the molar mass (i.e., linear increase of Mn, narrow PDI values) was observed for the telechelic thiocarbonate 98 [191]. Utilizing styrene and n-butyl acrylate (nBA), homopolymers 99a and 99b with various chain lengths were prepared (Figure 6.30a). Moreover, 99a was applied, as a macroinitiator, for the chain-extension with nBA. The second monomer was inserted into the middle of the PS block, yielding the triblock copolymer tpy-PS-b-nBA-b-PS-tpy (100a). Accordingly, tpy-nBA-b-PS-b-nBA-tpy (100b) was obtained by chain extension of 99b with styrene. In both cases, a shift of the SEC trace – with a narrow, monomodal distribution – towards shorter elution volume indicated the formation of the block copolymer (Figure 6.30b). NMP has attracted great interest in polymer science due to its simplicity since, in most cases, only the addition of a suitable alkoxyamine is required. Furthermore, the protocol is metal-free and well-compatible with a broad range of monomers

S

R

n

R cat. AIBN, dioxane, 75 °C

S

N

N

N

n

N

96 N

97a: R = N

Scheme 6.14 RAFT polymerization of styrene (R ¼ phenyl) and NIPAM [R ¼ CONHCH(CH3)2] using a terpyridine-functionalized chain-transfer agent [190].

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S S

97b: R = HN

O

284

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes S

(a) S

S 98

i

ii C4H9O

S nS

O

⫽ S

O

S

nS

n

S

OC4H9 n

99a

99b N

ii

C4H9O

O

S

O

N

OC4H9

S mS 100a

n

N

i

C4H9O n

m

O

S n

O

S mS 100b

m

15 20 Time (min)

25

OC4H9 n

(b) 99b 100b

99a 100a

5

10

15 20 Time (min)

25

5

10

30

Figure 6.30 (a) Polymerization of styrene and nBA using the telechelic CTA 98 [(i): styrene, 110 1C, 7 h; (ii) nBA, cat. AIBN, toluene, 60 1C, 9.5 h]; (b) SEC traces of the chain-extension utilizing 99a (left) and 99b (right) as macro-CTA (THF, as eluent) [191]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

[192]. In general, the control and “living” character of this polymerization can be correlated to the structure of the alkoxyamine initiator. Thus, significant efforts have been made to prepare an initiator showing a high performance in the polymerization. Hawker et al. developed the chloromethyl-functionalized TIPNO-based (TIPNO: 2,2,5-trimethyl-4-phenyl-3-azahexane nitroxide) system 101, as universal unimolecular initiator, that was suitable for the polymerization of diverse monomers: styrenes, acrylates, acrylamides, dienes, and vinylpyridines [52, 193, 194]. Reacting this initiator with 2,6-bis(pyridin-2-yl)-4-pyridone (102) [128, 129] enabled Lohmeijer et al. to generate the first high-performance NMP initiator bearing a supramolecular binding site (“tpy-TIPNO,” 103) (Scheme 6.15) [195]. To date, 103 is the most powerful terpyridine-functionalized initiator in CRPs and has been utilized for polymerizing a wide range of monomers with high conversion, good control over molar mass, and narrow PDI (mostly below 1.30). Table 6.2 gives an overview of the polymers that have been prepared from 103 [137, 195–200]. 1H NMR spectroscopic end-group analysis, MALDI-TOF MS, and UV–vis titration experiments revealed the high degree of end-group functionalization with the terpyridine

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unit. However, auto-initiation and auto-polymerization limited both the degree of end-group functionality and reachable PDI values when aiming for high molar mass polymers 104 [195]. Generally speaking, the propagation rates of acrylates and

N O HN 102

N +

N

O Cl

O

N

N

O

K2CO3, DMF 61%

N

101 N

103 (tpy-TIPNO)

Scheme 6.15 Synthesis of the NMP initiator “tpy-TIPNO” (103) [195].

Table 6.2

Homopolymers prepared by NMP utilizing 103 as initiator.

N Polymer

N

O

O

104-115

N N

Structure number

Monomer

Polymer

Reference

104 105 106 107 108 109 110 111 112 113 114 115

Styrene n-Butyl acrylate tert-Butyl acrylate Methyl acrylate 2-Ethylhexyl acrylate N,N-Dimethylacrylamide N-iso-Propylacrylamide Isoprene 2-Vinylpyridine 4-Vinylpyridine 4-Trifluoromethylstyrene Pentafluorostyrene

tpy-PSn-TIPNOa,b tpy-PnBAn-TIPNOb,c tpy-PtBAn-TIPNOd tpy-PMA60-TIPNOd tpy-P2EHA118-TIPNOd tpy-PDMAA36-TIPNOd tpy-PNIPAM29-TIPNOd tpy-PI-TIPNOb,e tpy-P2VP-TIPNOb,f tpy-P4VP-TIPNOg tpy-PTFMS42-TIPNOd tpy-PPFS30-TIPNOd

[195, 197] [137, 196] [196–198] [202] [202] [137] [202] [137] [137] [137] [200] [199]

Mn ¼ 4700–55 600 g mol1; PDI ¼ 1.08–1.30. SEC was applied to determine Mn and PDI values. c Mn ¼ 4500–10 100 g mol1; PDI ¼ 1.28–1.30. d DP determined by 1H NMR spectroscopic end-group analysis. e Mn ¼ 4700 g mol1; PDI ¼ 1.23. f Mn ¼ 5100 g mol1; PDI ¼ 1.27. g Mn ¼ 4000 g mol1; PDI ¼ 1.13. a b

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| 285

286

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes acrylamides are much higher compared to styrene, leading to rather high PDI values when using TIPNO [201]. An improved control over the polymerization generally can be gained by adding free nitroxide radicals [52]; an excess of nitroxide radicals can slow down the polymerization and enable the preparation of well-defined homopolymers (105–110) [196, 202]. The polymerization of isoprene proceeded with very slow conversion and rather high PDI values were obtained for vinylpyridines. Thus, anionic polymerization and subsequent end-group modification were found to be the superior method when aiming for terpyridine-functionalized polyisoprene (PI) or poly(2-vinylpyridine) (see also Section 6.3.3) [137]. In recent years, fluorinated polymers have attracted much attention due to their potential applications in microelectronic devices or as antifouling/antifogging agents; their characteristics include high thermal stability, chemical resistance, excellent mechanical properties (at extreme temperatures), superior weatherability, and low flammability [203]. Two types of fluorinated styrene-derivatives could also be utilized, as monomers, for NMP, yielding polymers 114 and 115. For both monomers, the polymerization proceeded with excellent control over the molar mass and PDI values below 1.15 [199, 200]. The high end-group fidelity of the polymers was applied to chain-extension, specifically di- and triblock copolymers with tunable molar masses as well as monomodal and narrow molar mass distributions. However, some requirements have to be considered – the polymerizations need to be stopped before reaching 80% conversion, otherwise there is a significant probability of forming dead polymer chains by irreversible termination. A second prerequisite that is difficult to fulfill due to the reactivity differences of various monomers is the fast reinitiation with respect to the propagation according to the polymerization conditions of the first block. For instance, a polystyrene macroinitiator (e.g., 104) will not efficiently reinitiate the polymerization of nBA as it would the reverse (i.e., reinitiation of 105 with styrene) [192]. Also in this case, the addition of free nitroxide radicals can help to overcome this limitation by suppressing the high propagation rates of acrylates (i.e., shifting the equilibrium towards the dormant species) [204]. As a proof of concept, Schubert et al. used the macroinitiators 104–106 to prepare a library of diblock copolymers of the general type B-b-A-tpy (116–122) by reinitiation with styrene, acrylates, isoprene, and 2-vinylpyridine. Though a slight broadening of the molar mass distributions could be observed, well-defined diblock copolymers with a high degree of end-group functionalization could be obtained [196]. In the presence of free nitroxide radicals, the polymerization of tBA initiated by 104 (n ¼ 160) was well-controlled; the clean chain-extension, as a function of polymerization time as monitored by SEC- is shown in Figure 6.31 (in the absence of free nitroxide radical, poor control was observed: Mn ¼ 153 300 g mol1, PDI W 2) [196]. Diblock copolymers tpy-PS50-b-PPFS80-TIPNO (123) and tpy-PS50-b-PTFMS34TIPNO (124) could be obtained utilizing macroinitiator 104 (n ¼ 50). In these cases, no additional nitroxide radicals were required due to the similar reactivity of both fluorinated monomers and styrene. Reinitiation of 104 to yield 123 and 124 could be monitored by SEC; the SEC traces were shifted to shorter elution time and no broadening of the polydispersity index was observed. Similarly, the

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preparation of 125 and 126 – having the inverse order of the two blocks compared to 123 and 124, respectively – were synthesized applying the same protocol (Figure 6.31) [199, 200]. Table 6.3 lists the entire range of diblock copolymers 116–126 that has been prepared from 103 via sequential NMP. Macroligands 116–119 containing a hard (PS) and a soft block (PtBA or PMA) are important building blocks with respect to phase separating supramolecular materials [26, 205]. Moreover, amphiphilic copolymers are accessible when the acrylate ester moieties are cleaved; accordingly, the PtBA blocks could be converted into poly(acrylic acid) (PAA) blocks by treatment with TFA

1.2

]-PS160-TIPNO ]-PS160-b-PtBA-TIPNO (1 h) ]-PS160-b-PtBA-TIPNO (4 h) ]-PS160-b-PtBA-TIPNO (18 h) ]-PS160-b-PtBA-TIPNO (21 h)

0.8



1.0

0.6 0.4 0.2

]-PTFMS42-TIPNO (114) ]-PTFMS42-b-PS76 -TIPNO (126)

0.8

0.4

0.0

0.0 15

16

17

18

19

20

21

22

12

23

14

16

18

Elution volume/ml

Elution volume/ml

(a)

(b)

20

Figure 6.31 (a) SEC traces of the re-initiation of tpy-PS160-TIPNO (104) with tBA; the increase of molar mass with polymerization time (from 0 to 21 h) is shown (DMF with 5 mM NH4PF6 as eluent) [196]; (b) SEC traces of homopolymer tpy-PTFMS42-TIPNO (114) and diblock copolymer tpy-PTFMS42-b-PS76-TIPNO (126) [200]. Figure reproduced with kind permission; r 2006 Wiley-VCH and 2009 The Royal Society of Chemistry, respectively. Table 6.3

Diblock copolymers synthesized via sequential NMP.

Structure number

Diblock copolymers

Mn (g mol1)a

Mn (g mol1)b

Reference

116 117 118 119 120 121 122 123 124 125 126

tpy-PS160-b-PtBA90-TIPNO tpy-PS35-b-PMA54-TIPNO tpy-PnBA80-b-PS100-TIPNO tpy-PtBA85-b-PS230-TIPNO tpy-PtBA40-b-PI22-TIPNO tpy-PS70-b-PI65-TIPNO tpy-PS85-b-P2VP130-TIPNO tpy-PS50-b-PPFS80-TIPNO tpy-PS50-b-PTFMS34-TIPNO tpy-PPFS30-b-PS73-TIPNO tpy-PTFMS42-b-PS76-TIPNO

28 000 38 700 21 000 32 800 7 100 12 500 23 100 20 700 11 700 14 000 15 700

32 900 (1.17) 31 900 (1.23) 18 700 (1.21) 30 400 (1.13) 9300 (1.20) 13 900 (1.23) 27 500 (1.16) 11 300 (1.23) 10 200 (1.17) 9900 (1.22) 13 100 (1.18)

[196] [196] [196] [196] [196] [196] [196] [202] [202] [199] [200]

a

Mn values determined by 1H NMR end-group analysis. Mn values determined by SEC; the corresponding PDI value is given in brackets.

b

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| 287

288

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes in dichloromethane (see also Scheme 6.4) [202]. Other functional blocks feature the possibility of crosslinking (PI), are water-soluble (PDMAA), and pH- (P2VP) or temperature-responsive (PNIPAM or PTFMS). Thus, diblock copolymers with tunable properties over a broad range are accessible via the sequential NMP procedure. In an extension of this protocol, triblock copolymers tpy-A-b-B-b-C were also synthesized by reinitiating diblock copolymer 117 with pentafluorostyrene and 4-trifluoromethylstyrene, respectively. Though the SEC traces revealed a slight tailing on the lower molar mass side (indicating that not all macroinitiator chains initiated the polymerization), acceptable molar mass distributions were still observed for polymers 127 (tpy-PS35-b-PMA54-b-PPFS97-TIPNO, PDI ¼ 1.28) and 128 (tpy-PS35-b-PMA54-b-PTFMS20-TIPNO, PDI ¼ 1.33) [202]. Hofmeier et al. combined two of the most common supramolecular entities, namely, terpyridine ligands and quadruple H-bonding arrays, within a single macroligand [25, 175]. For this purpose a step-wise procedure was applied that involved synthesis of a terpyridine macroligand by ring-opening polymerization of e-caprolactone and subsequent end-group modification with a functionalized ureido-pyrimidone derivative (see also Figure 6.28). The utilization of two orthogonal supramolecular binding sites gave access to switchable materials of high molar mass. A more direct route towards such polymers was published recently by Mansfeld et al. [206], in which the highly efficient tpy-TIPNO initiator (103) was modified with a lateral UPy-substituent. Thus, a heterotelechelic initiator (129) was prepared, merging the possibility of orthogonal supramolecular interaction with the power of the nitroxide-mediated polymerization. Figure 6.32 depicts the initiator 129 and its application in the polymerization of styrene under NMP conditions. Characterization of tpy-PSn-TIPNO-UPy (130, n ¼ 40, 60, or 120) by 1H NMR, UV–vis titration experiments as well as SEC revealed the high degree of end-group functionalization and the controlled character of the polymerization (Mn ¼ 6800–11 400 g mol1, PDI ¼ 1.071.16, Figure 6.32). 6.3.4.3 Post-Polymerization Functionalization Post-polymerization reactions are versatile tools for the functionalization of welldefined polymers and have been applied to various types of terpyridine-containing polymers. The cleavage of ester side-chains with TFA, resulting in free carboxylic acid moieties, has been addressed previously (e.g., see Scheme 6.3) [60, 202]. In addition, the end-group modification of poly(e-caprolactone)-macroligand 89a with an UPy-group (see Figure 6.28) has to be mentioned in this context [25, 175]; the combination of two orthogonal supramolecular entities (i.e., terpyridine ligand and H-bonding array) is of particular relevance for the preparation of multiresponsive, switchable, and reversible (“self-healing”) materials [207]. Functionalized maleimides have been reported as versatile substrates for the end-group modification of polymers bearing the TIPNO moiety [208]. The strategy is based on the controlled mono-addition of maleimide derivatives to the alkoxyamine chain-end, followed by irreversible elimination of the mediating nitroxide radical. Lohmeijer adopted the approach that reacted tpy-PS-TIPNO 104 (Mn ¼ 7700 g mol1) with terpyridine-substituted maleimide 131 (Scheme 6.16).

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29 J l 2011 18 45 51

N

N

N

O

N

O

N

(a)

130 tpy-PSn-TIPNO-UPy (n ⫽ 40, 60 or 120)

nO

129 tpy-TIPNO-UPy

O

N

N H

O

O N H

anisole, 123 ⬚C, 6 h

n

O H N 3

H

N

N

H N

N

O

H

H N 3

O

H N

N

O

O

Normalized RI signal 0

0.2

0.4

0.6

0.8

1.0

14

15

16

(b)

17 18 19 Elution volume (ml)

tpy-PS120-TIPNO-UPy (130c)

tpy-PS60-TIPNO-UPy (130b)

tpy-PS40-TIPNO-UPy (130a)

tpy-TIPNO-UPy (129)

20

21

22

Figure 6.32 (b) Synthesis of the heterodifunctional initiator 129 and the polymerization of styrene initiated therewith; (b) the corresponding SEC traces [206]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

N

N

O


| 289

290


N

131 N

n O

N

N

O N 104 tpy-PSn-TIPNO −1 (Mn = 7700 g mol , PDI = 1.07)

N

O

N

O

O

n

N

O N

N

tert.-butylbenzene, 100 °C (2 h) & 125 °C (4 h)

O

N N

132 (Mn = 7800 g mol−1, PDI = 1.08)

O

N N

N

Scheme 6.16 Synthesis of unsymmetric bis(terpyridine) building blocks.

NH2-PEG75-OH

115

133

5-aminopentanol

L-lactide, cat. SnII

134

135

Scheme 6.17 Modification of macroligand 115 by grafting-onto/grafting-from methods [199]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

Full conversion was observed (>95% according to 1H NMR) and the resulting polymeric bis(terpyridine) 132 was investigated by SEC, confirming that chain coupling could be almost fully suppressed. Mass spectrometric end-group analysis confirmed the polymer’s telechelic nature [195]. Various graft-onto as well as graft-from strategies were devised by Ott et al. [199], in particular making use of the ability of pentafluorophenyl derivatives to undergo nucleophilic substitution reactions at their labile para-positions (e.g., by primary amines), a concept that just recently introduced in polymer science [209]. For this purpose, homopolymer 115 and diblock copolymer 125 were utilized, as key building blocks. The substitution of 115 with amino-functionalized PEG75 (Mn ¼ 3400 g mol1) was carried out in N-methylpyrrolidone (NMP) under microwave irradiation (95 1C, 20 min) to generate the graft copolymer tpy-PPFS30g-(PEG75)10-TIPNO (133) (Scheme 6.17). SEC traces of the purified polymer showed a clear chain-extension, indicating the attachment of several PEG chains (Mn ¼ 27 700 g mol1, PDI ¼ 1.12). The number of attached PEG chains was determined (1H NMR). The scope of the concept was extended by reacting 115 with 5-aminopentanol (AP); the incorporation of hydroxyl groups allowed further

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| 291

5-aminopentanol

O

Br

Br NEt3 OEGMA475, CuBr/PMDETA 138


1.2

136

125

125 136 137 138

0.8

0.4

137 0.0 14 (a)

16 18 Elution volume / ml (b)

Figure 6.33 (a) Synthesis of the “graft-on-graft” architecture 138; (b) SEC traces of polymers 125 and 136–138 involved in the reaction sequence [199]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

modification, exploiting the organic and polymer chemist’s toolbox. Graft polymer 134 (tpy-PPFS30-g-AP9-TIPNO) was subsequently used, as macroinitiator, for the SnII-catalyzed controlled ring-opening polymerization of L-lactide (100 1C, 5 h) (Scheme 6.17). Investigation by SEC showed an increase of the molar mass in time with narrow PDI values; for the final graft polymer 135 [tpy-PPFS30-g-AP(PLA11)9TIPNO] a Mn of 27 200 g mol1 (PDI ¼ 1.12) was determined. 1H NMR data confirmed that on average nine AP units had been grafted onto 115 and that all hydroxyl groups initiated the ROP of L-lactide with a DP of 11 [199]. Moreover, a block “graft-on-graft” architecture could be realized by the same group by combination of two CRP techniques: NMP and ATRP (Figure 6.33) [199]. For this purpose, diblock copolymer 125 was reacted with AP to yield polymer 136 (tpy-PPFS30-g-AP7-b-PS73-TIPNO). According to 1H NMR spectroscopy, seven AP units were inserted per chain. Esterification with 2-bromoisobutyryl bromide yielded the ATRP initiator 137 [tpy-PPFS30-g-(AP-Br)7-b-PS73-TIPNO]. In the next step, CRP of oligo(ethylene oxide) methacrylate (OEGMA475) was carried out in toluene (CuBr/PMDETA, as the catalytic system, 75 1C, 5 h). A monomodal SEC trace was obtained for 138 (tpy-PPFS30-g-AP[(OEGMA475)10]7-b-PS73-TIPNO), indicating that the polymerization proceeded in a highly controlled fashion. As shown by 1H NMR, ten OEGMA arms were grown from each side-chain when assuming a uniform distribution. Figure 6.33 shows the SEC traces of the polymers involved in this sequence [199]. 6.3.5 Mononuclear Metallo-Supramolecular Polymers 6.3.5.1 Supramolecular A-[M]-A Homopolymers The self-assembly of 58 with transition metal ions into supramolecular A-[M]-A homopolymers of the type [M(58)2]X2 can be regarded as a model reaction, since many basic lessons of the supramolecular “playing LEGO” methodology [93] can be learned from this particular complexation. Macroligand 58a was coordinated to

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20

292

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes various transition metal ions (i.e., RuII, FeII, CoII, NiII, CuII, ZnII, and CdII) affording water-soluble polymers that were characterized by SEC, MALDI-TOF MS, and UV–vis spectroscopy [116]. Furthermore, 1H NMR could be applied for diamagnetic complexes (i.e., RuII, FeII, ZnII, and CdII). Owing to the octahedral complex geometry, the ligand protons in 6,600 -position are located above the central ring of the second terpyridine ligand; thus, distinct shifts – depending on the particular metal ion – can be observed [210]. In this respect, the paramagnetic [Co(58a)2](PF6)2 complex showed an interesting behavior, that is, the so-called Knight shift, induced by a specific coupling of the electronic and nuclear spin in which the proton signals were shifted about 110 ppm relative to TMS (see also Chapter 3.2) [211]. Polymer-complexes [M(58a)2](PF6)2 were investigated with respect to their pH sensitivity; it was found that the complexes containing FeII, CoII, ZnII, or CdII ions exhibited decomplexation at high and low pH (13 and 1, respectively). Moreover, [Cu(58a)2]2 þ disassociated after keeping the solution at a low pH for several days. Only the RuII- and NiII-containing systems showed complete inertness to changes in pH ranging from 0 to 14, even after several days [116]. Thus, the [M(tpy)2]2 þ linkage can be reversed by adjusting the pH value in the case of selective transition metal ions. It could be shown further that [Ru(tpy)2]2 þ complexes can be opened under harsh conditions, for example, by adding HEEDTA, as a strong competitive ligand (HEEDTA: sodium salt of N-(hydroxyethyl)ethylenediamine triacetic acid) [197, 212], or applying redox chemistry [oxidizing RuII to RuIII with Ce(SO4)2] [213]. Concerning the characterization of these systems by MS techniques, only in the cases of CoII and RuII ions, which are among the most stable, could the unfragmented polymeric bis(terpyridine) complex as well as the free ligand 58a be detected by MALDI-TOF MS [214]. Increasing the laser intensity led to an increased 58a : [M(58a)2]2 þ ratio; this was not unexpected due to the known partial dissociation of the complex, which was observed upon excitation. Relative binding strength under MALDI-TOF MS conditions could be concluded from these experiments. A detailed investigation of macroligands 58a and 68a as well as complexes Ru(58a)Cl3 and [Ru(L)2]2(PF6)2 (L ¼ 58a, 68a) was conducted to elaborate a reliable SEC system that is suited also for metallo-supramolecular assemblies by suppressing all non-specific interaction of the charged polymer with the column material. Meier et al. could show that DMF, as eluent, containing 5.5 mM NH4PF6, gave the best results and enabled the separation of metal-containing polymers only by size (thus, excluding all chromatographic effects) [103]. Figure 6.34a depicts the SEC traces of the investigated polymers. Coupling an in-line photo-diode array (PDA) detector to the SEC system afforded further insights into the integrity of the supramolecular polymers (from the MLCT bands of the [Ru(tpy)2]2 þ moieties at 490 nm) as well as their purity. More recently, Chiper et al. could show that the stability of complexes [M(58a)2](PF6)2 during SEC measurements decreases in the order NiII W CoII W FeII due to the differences in the binding strength [118]. Therefore, supramolecular architectures, based on the [Ni(tpy)2]2 þ connectivity, can also be well characterized by SEC (1H NMR cannot be applied for these paramagnetic materials). AUC characterization of [Fe(58a)2](PF6)2 and [Ru(L)2](PF6)2 (L ¼ 58c and 67a) in solution, using sedimentation equilibrium and velocity measurements, was

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Normalized c(Mn) distribution


1.0 Ru(58a)CI3 Ru(68a)2](PF6)2 [(58a)Ru(68a)](PF6)2 [Ru(58a)2](PF6)2

0.8 0.6 0.4 0.2 0.0 16

18

20

1.00

[Ru(68a)2](PF6)2 c(Mn) from equilibrium c(Mn) from velocity

0.75 0.50 0.25 0.00 8000

22

16000

Elution volume/ml

Mn/g.mol−1

(a)

(b)

24000

Figure 6.34 (a) SEC traces of various metallo-supramolecular polymers (DMF with 5.5 mM NH4PF6, as eluent) [103]; (b) normalized c(Mn) distributions according to AUC measurements of [Ru(68a)2](PF6)2 [216]. Figure reproduced with kind permission; r 2003 Wiley-VCH and 2006 Springer Verlag, respectively.

performed. Schubert et al. could show that these techniques – originally developed for proteins as well as nucleic acids and applied later for synthetic polymers – can also be used for supramolecular polymers containing [M(tpy)2]2 þ linkages [215, 216]. No dissociation of the polymers could be observed during the measurements and – in addition to other parameters such as the average friction ratio (f/f0), the sedimentation coefficient (Sn), or the average diffusion coefficient (D) – the molar mass (Mn) can be determined (Figure 6.34b). Thus, four independent characterization techniques are available to determine the Mn values: SEC, 1H NMR, MALDI-TOF MS, and AUC. Among these, AUC has been considered to be more accurate and able to overcome limitations such as dissociation or fragmentation (in case of SEC and MALDI-TOF MS, respectively) [217]. Furthermore, for a series of A-[M]-A homopolymers {i.e., [M(58a)2](PF6)2 and [M(67a)2](PF6)2}, independence of the electrophoretic mobilities from the nature of the transition metal ion was observed. On the other hand, the method was found to be a useful tool for the molar mass distribution characterization of supramolecular polymers based on just one set of reference materials [218]. In general, A-[M]-A homopolymers resemble versatile model compounds and have been used in particular for reference purposes (e.g., in SEC or AUC measurements) in investigating more advanced metallo-supramolecular architectures (see also the following sections) [119, 190, 219, 220]. However, potential applications of these materials have scarcely been noted. As an exception, the supramolecular polymer [M(77)2]X2 (M ¼ FeII or ZnII; X ¼ PF6 , OAc or C1) was studied by Chiper et al. with respect to its thermoresponsive behavior [154]. Thermosensitive polymers feature an interesting physical behavior and undergo a transition from soluble to collapsed precipitated chains upon heating in aqueous solution; the precipitation temperature is called the lower critical solution temperature (LCST). In general, this phase transition phenomenon is reversible and the LCST was

06

| 293

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294


Transmittance [%]

100 PNIPAM tpy-PIPAM (78) [Fe(78)2](PF6)2 [Fe(78)2](OAc)2 [Fe(78)2]Cl2

80 60

NC

N O

30

HN

40

H N

S O

O

N 78 N

PNIPAM

20 0 20

25

30 Temperature [⬚C]

35

40

Figure 6.35 Effect of the temperature on the transmittance of PNIPAM, tpy-PNIPAM (78), and [Fe(78)2]X2 with different counterions (Cl, AcO, and PF6 ) in water (5 mg ml1). Image of a vial with 78 below the LCST (top left) and above the LCST (bottom left) as well as with [Fe(78)2]X2 below the LCST (top right) and above the LCST (bottom right) [154]. Figure reproduced with kind permission; r 2008 Wiley-VCH.

reported to be sensitive to pH, type and concentration of salts, and utilized solvents. PNIPAM is an extensively studied representative for these kind of polymers [221] and, therefore, was end-group functionalized with a terpyridine moiety (see also Figure 6.23) to combine the LCST behavior with supramolecular interactions. For [M(78)2]X2 the cloud point (i.e., the LCST temperature) depended on the nature of the transition metal ion (attributed to the ion’s radius) as well on the counterion (increase with the hydrophilicity: Cl W AcO W PF6 ) (Figure 6.35). 6.3.5.2 Supramolecular Block Copolymers The self-assembly of terpyridine-based macroligands A-[ with transition metal ions gives rise to homopolymers of the general type A-[M]-A. As pointed out in the previous section, this approach can, in theory, be applied under appropriate conditions to all transition metal ions forming octahedral complexes with two terpyridine ligands. However, this protocol is limited to the preparation of A-[M]-A homopolymers in which the self-assembly of two different macroligands will always lead to statistical mixtures of both homopolymers (A-[M]-A and B-[M]-B) and diblock copolymer A-[M]-B (Figure 6.36). Thus, a directed route was established making use of the special RuIII/RuII coordination chemistry [74, 212]. This step-wise procedure involves the selective formation of a A-[RuCl3 complex by reaction of RuCl3 xH2O with a monotopic

(a)

A-[M]-A homopolymer

(b)

A-[M]-B diblock copolymer

Figure 6.36 General structure of a metallo-supramolecular homopolymer (a) as well as a diblock copolymer (b).

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macroligand. These RuIII mono(terpyridine) complexes can be characterized by SEC and UV–vis absorption spectroscopy (MLCT absorption band at about 400 nm) (Figure 6.37) [222]. 1H NMR spectroscopy can be applied as an indirect tool; the signals corresponding to the terpyridine protons will disappear due to the paramagnetic nature of the RuIII ion. In the second step, the mono-complex and a second macroligand are then dissolved in an alcohol containing catalytic amounts of NEM (to in situ reduce RuIII to RuII). After anion exchange, the supramolecular diblock copolymers A-[Ru]-B can be obtained. The formation of the polymeric RuII bis(terpyridine) can be followed by SEC (under the optimized conditions, see Section 6.3.5.1) and UV–vis absorption spectroscopy (the MLCT band is shifted to 490 nm). The harsh reaction conditions for the step-wise protocol – required for the reduction of RuIII – can be avoided by utilizing a Ru(dmso)4Cl2 precursor complex. According to Ziessel et al., RuII mono-complexes A-[Ru(dmso)Cl2 are formed at a 1 : 1 stoichiometry [223]. Activation with AgI salts (to abstract the chloride ligands) and subsequent coordination of a second macroligand enabled the synthesis of the diblock copolymers A-[Ru]-B under mild conditions and in high yields [219]. Directed routes towards heteroleptic OsII and IrIII bis(terpyridine) complexes were also reported in the literature, but have not been applied yet for terpyridine macroligands. Recently, also A-[M]-B diblock copolymers with NiII or CoIII ions were reported by Gohy and coworkers [224]. In a step-wise procedure 104 (tpy-PS240-TIPNO) was coordinated to NiII and CoII ions, respectively, in DMF, which is a good solvent for such types of polymeric ligands. A second ligand (58c, tpy-PEG230) was added in situ to the mono-complex TIPNO-PS240-[M(dmf)3Cl2] to yield the diblock copolymers (TIPNO-PS240-[M]-PEG230) after anion exchange with NH4PF6 (Scheme 6.18). Owing to stability under the measurement conditions, the NiII

(a)

(b)

tpy-PEG230 (58c) tpy-PS240-TIPNO (104) [(104)Ni(58c)](PF6)2 [(104)Co(58c)](PF6)3

a.u.

MLCT band of (tpy)Rulll

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 30 0 m n /

0 5

40

50

10

15

n

25

mi

0

60

20

e/

0

7 30 00

0

tim

wa

th

ng

le ve

20

25

30 35 40 Retention time / min

45

Figure 6.37 (a) SEC trace (PDA detector) of (58a)RuCl3; the characteristic MLCT absorption band at 400 nm is visualized [222]. (b) SEC traces of CoIII- and NiII-based diblock copolymers as well as of the parent terpyridine macroligands [224]. For all SEC measurements: DMF with 5 mM NH4PF6 as eluent. Figure reproduced with kind permission; r 2007 Springer Verlag and 2010 The Royal Society of Chemistry, respectively.

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| 295

296


N

240 O

O

N

MCl2, DMF, room temp., 1 min (MII = CoII or NiII)

N N

104 (tpy-PS240-TIPNO)

240 O

N DMF

M2+

DMF DMF tpy-PEG230 (58c), DMF, room temp., 10 eq. AgNO3, NH4PF6 [(104)Co(58c)](PF6)3

N

O N [(104)M(dmf)3]Cl2 N 2 Cl tpy-PEG230 (58c), DMF, room temp., NH4PF6

diblock copolymers TIPNO-PS240-[M]-PEG230

[(104)Ni(58c)](PF6)2

Scheme 6.18 Synthesis of A-[M]-B diblock copolymers containing CoIII or NiII ions via directed step-wise self-assembly [224].

diblock copolymer could be characterized by SEC. In contrast, the corresponding unstable CoII diblock copolymer had to be oxidized with AgNO3 to generate a kinetically inert CoIII bis(terpyridine) complex. For both materials, no shoulders were observed in the respective SEC traces, indicating the successful preparation of supramolecular diblock copolymers. In general, the lessons learned in characterizing A-[M]-A homopolymers can be applied also to their diblock copolymer counterparts: SEC, 1H NMR, MALDI-TOF MS, and AUC are today the main tools for the estimation of molar masses and confirmation of the self-assembly process into the targeted supramolecular polymers. Combining terpyridine-functionalized macroligands with different chemical, physical, or mechanical properties by transition metal ion coordination gives new types of functional materials that, for example, form micelles in selective solvents (due to the amphilicity of the blocks) or show microphase separation in the solid state. For clarity, the preparation of these metallo-supramolecular block copolymers as well as their basic characterization will not be detailed; charges, counterions, or lateral substituents, such as the TIPNO moiety, are also omitted in most examples. Owing to the often observed thermodynamic incompatibility between different polymer blocks, phase separation is a characteristics of block copolymers. Since the blocks are connected to each other (in the case of metallo-supramolecular polymers via metal complexes), this microphase separation is spatially limited and results in self-assembled structures whose characteristic sizes are of the order of a few times the radius of gyration of the constituent block and, thus, in about the 10–100 nm range [225]. Furthermore, these structures tend to be regularly distributed throughout the bulk material, giving rise to long-range ordering and the formation of structures including cubic arrays of spheres or hexagonally packed cylinders. As a result, nanostructured materials with size and morphology depending on the degree of incompatibility between the different blocks and the relative volume fraction and length of the blocks are obtained [225]. By dissolving a block copolymer in a selective solvent [i.e., a good solvent for one of the block(s) and a poor solvent for the other block(s)], microphase separation will lead to micelle formation. Thus, block copolymer micelles contain a micellar core,

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consisting of the insoluble block(s), which is surrounded by a micellar corona formed by the soluble block(s) [26, 205, 226]. Such micellar systems have attracted significant scientific interest due to their numerous applications in various fields ranging from drug delivery to templates for nanoparticles [227]. Owing to the differences between individual blocks with respect to solubility and immiscibility in the solid state, PEG-[Ru]-PS diblock copolymers are the by far the most intensively investigated examples. Schubert et al. pioneered this field by studying the micellization of PS20-[Ru]-PEG70 in water (addition of water to a DMF solution of the diblock copolymer, followed by dialysis against water) [228, 229]. Owing to its hydrophobic nature, the PS block consists of the core of the micelle with the water-soluble PEG chains forming the corona; the charged [Ru(tpy)2]2 þ moieties are located on the interface between the core and corona (Figure 6.38a). DLS of the opalescent solution revealed the presence of two distributions: firstly, a monomodal one was assigned to the expected spherical micelles with a hydrodynamic diameter (Dh) of 65 7 4 nm and, secondly, a polydisperse population

(a)

(b) PS core [Ru(tpy)2]2+ units PEG corona

100 nm 40.0 nm (c)

(d)

150 20.0 nm 100 0.0 nm 50

0 0

50

100

150

nm

Figure 6.38 (a) Micellar structure of the PEG70-[Ru]-PS20 diblock copolymer in aqueous medium; (b) TEM image at high magnification of an individual micelle in water (without staining); (c) AFM height image of an individual micelle in water [228]; (d) cryo-TEM-image of PS20-[Ru]-PEO70 micelles (arrows indicate an individual micelle as well as a small cluster of micelles) [230]. Figure reproduced with kind permission; r 2002 American Chemical Society and, 2004 Springer Verlag, respectively.

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| 297

298

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes originated from the assembly of individual micelles into larger aggregates (Dh ¼ 202 7 37 nm). This turned out to be a common feature for these types of metallo-supramolecular micelles (“classical” covalent diblock copolymers of similar compositions did not show this aggregation behavior). Moreover, individual micelles could be visualized by (cryo-)TEM as well as AFM measurements (Figure 6.38b–d) [228, 230]. The micelle preparation was optimized and, thus, the micellar aggregation could be almost totally suppressed in that slow and regular addition of water to the initial DMF solution of the PS20-[Ru]-PEG70 diblock copolymer (via a syringe pump) resulted in a well-defined micellar solution with almost no aggregation of micelles [231]. Though it could be expected that the unimer-micelle–aggregate equilibrium for metallo-supramolecular block copolymers is continuously disturbed and re-established during the measurements, AUC was successfully applied to investigate micelles of the same diblock copolymer system. This method permitted the determination of the weight fractions of unimers, micelles, and other species (e.g., aggregates of micelles). An average molar mass of 318 000 g mol1 was found, corresponding to an aggregation number of 53 PS20-[Ru]-PEG70 chains per micelle [231, 232]. The solution behavior as well as melt properties of PS20-[Ru]-PEG70 were studied. SAXS measurements revealed that microphase separation occurred (this was not observed for the covalent diblock copolymer PS22-b-PEG70). With PF6 as counterion, a spherical morphology was observed, not the predicted lamellar structure according to the composition of the copolymer [233]. Aggregation of the [Ru(tpy)2]2 þ complexes with the counterions led to spheres with a radius of about 1.5 nm surrounded by a polymer shell (outer radius of about 2.4 nm). Exchanging the counterion with the bulkier BPh4 yielded a highly ordered lamellar melt with a periodicity of 11.9 nm (after annealing at 55 1C for 40 h) [234]. The SAXS measurements indicated that the lamellar structure was composed of segregated domains of the PS- and PEG-blocks with the [Ru(tpy)2](BPh4)2 units confined to the lamellar interfaces. Thus, PS20-[Ru]-PEG70 behaved as a quasi-ABC-triblock copolymer, whereas the metal-ligand complex (and the associated counterions) acts as a middle-block, strongly incompatible with the two outer blocks. The order– disorder temperature of the system was found to be around 70 1C. This transition was reversible upon cooling; however, more time was required to achieve the lamellar morphology when increasing the annealing temperature. Figure 6.39 depicts the morphologies obtained for both types of counterions. Detailed investigation of the system PS20-[Ru]-PEG70 – in a selective solvent as well as in the bulk – indicated an influence of the supramolecular linkage on the microphase separation between the two blocks. To gain further insight into this behavior, compositions other than a PS/PEG ratio of 20 : 70 were investigated. Applying the directed step-wise protocol, a (4 4)-library of PSx-[Ru]-PEGy diblock copolymers was synthesized and the morphology of thin spin-casted films was investigated by AFM (Figure 6.40) [235]. Using this combinatorial approach, a wide range of morphologies with tunable domain size could be obtained from a small number of terpyridine-functionalized polymer blocks. For instance, block

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(a)

| 299

(b)

PS PEO [Ru(tpy)2]2+ PF6−

PS PEO [Ru(tpy)2]2+ BPh4−

L/2 L

Figure 6.39 Melt morphology of PS20-[Ru]-PEG70 with either PF6 (a) or BPh4 (b) counterions [233, 234]. Figure reproduced with kind permission; r 2003 and 2005, respectively, American Chemical Society.

PS20-[

PS70-[

PS200-[

PS240-[

PEG70-[

PEG125-[

PEG225-[

PEG225-[

Figure 6.40 AFM phase images of the block copolymer library PSx-[Ru]-PEGy (thin spincasted films, no annealing). In each case, the upper number denotes the molar mass, the lower numbers correspond to the volume fractions (PS-block/[Ru(tpy)2](PF6)2 unit/PEG-block). Scale bar represents 100 nm [235]. Figure reproduced with kind permission; r 2004 The Royal Society of Chemistry.

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300

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes copolymers 3, 4, 8, and 13 were expected to give rise to a spherical morphology in the bulk and, indeed, spherical features could be observed (however, 13 showed only crystalline PEO-microdomains). For block copolymers 2, 7, 9, 11, and 12 the predicted cylinders could be visualized. Notably, the PEO microdomains were always oriented perpendicular to the film surface and fully penetrated the film (the phenomenon of spontaneous cylinder arrangement has been discussed in detail by Russell et al. [236]). Phase separation into cylindrical microdomains was further utilized by Fustin et al. in the preparation of nanoporous films [213]. For this purpose, the PS375-[Ru]PEG225 diblock copolymer was spin-coated from a non-selective solvent onto silicon. In accord with the previously discussed model, cylindrical microdomains that are perpendicular to the substrate were formed without further treatment (Figure 6.41a). Subsequently, the metallo-supramolecular linkages were opened by oxidation of RuII to RuIII (with CeIV, as oxidant, at pH 1) to generate the monocomplex. Thus, the minor block was released and nanopores were created. Removal of the PEO-block was confirmed by AFM (Figure 6.41b), X-ray reflectivity, and X-ray photoelectron spectroscopy (XPS) (Figure 6.41c). Later, it was also shown that the ordering of the cylindrical domains could be significantly improved by annealing with polar solvent vapor (e.g., THF) [237]. Another 13-member library of the type PSx-[Ru]-PEGy was studied with respect to micellization behavior in water [238]. AFM and TEM characterization (see Figure 6.42a for the diblock copolymer with x ¼ 200 and y ¼ 375) of the micelles revealed that the core size does not scale linearly with the DP of the PS-block, as expected from the theory of classical covalent copolymers. For AFM measurements, the micelles were deposited on a silicon wafer. Since the imaging was performed in the dry state, the flexible PEO-corona chains were expected to be oriented flat on the surface and, therefore, the height of the micelles directly corresponded to the size of the core (Figure 6.42b). For TEM imaging, a selective staining of the PS with RuO4 allowed the visualization of the micellar core. Only two core sizes were observed: one of about 10 nm for DP values of 70 and below and one around 20 nm for a DP of 200 and above. Diblock copolymers having a DP in the region between 70 and 200 exhibited two populations. This unusual behavior was attributed to electrostatic repulsions between the charged [Ru(tpy)2]2 þ complexes that make up the junction of the two blocks, which strongly affected the self-assembly behavior; this was demonstrated by preparing micelles in the presence of salt to screen the repulsions. In this case, the metallosupramolecular diblock copolymers behaved as their covalent counterparts, with the core diameter scaling linearly with DP3/5. Figure 6.42c shows the evolution of the core diameter for micelles prepared with and without salt [238]. The influence of polyelectrolyte counterions on the micellization behavior of the PS20-[Ru]-PEG70 diblock copolymer was investigated by Guillet et al. [239]. Exchange of the PF6 counterion by PSS, as polyanion, yielded flexible amphiphilic brushes with a random-coil conformation in DMF (Figure 6.43). The characteristic size of these metallo-supramolecular structures could be tuned by varying the length of the PSS chains and the [Ru(tpy)2]2 þ ] : sulfonate molar ratio.

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40.0 °

40.0 °

(a)

20.0 °

20.0 °

0.0 °

0.0 °

(b)

(c)

Intensity (a.u.)

PS375-[Ru]-PEG225 film nanoporous film

PS

PEG

294

292

286 290 288 Binding Energy (eV)

284

282

280

Figure 6.41 (a) AFM phase image of a spin-coated film (75 nm thick) of PS375-[Ru]-PEG225 diblock copolymer on silicon (showing the cylindrical PEG microdomains orthogonal to the substrate); (b) AFM phase image of the same film after creation of the nanopores by treatment with aqueous Ce(SO4)2 at pH 1; (c) XPS (C1s) spectra of the metallosupramolecular film before (solid line) and after treatment with the Ce(SO4)2 solution (dashed line) [213]. Figure reproduced with kind permission; r 2005 Wiley-VCH.

Subsequently, micellization was induced by the addition of water to the initial amphiphilic brushes, followed by a dialysis step against water to completely remove DMF. The formation of micelles was confirmed by DLS and (cryo-)TEM measurements. For the shorter PSS chains (i.e., DP up to 607), well-defined spherical micelles were formed, whereas longer PSS chains triggered a transition to worm-like morphologies (Figure 6.43). Though an effect on the core–corona interfacial energy could not be excluded, the origin of the morphological transition was attributed to steric constraints introduced by the polymeric counterion.

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| 301

302


(a)

(b)

h ~ Dcore (c) Dc water Dc 1 M NaCI

25

Dc

20 15 10 5 5

200 nm

10

15

20

25

30

DP3/5

Figure 6.42 (a) AFM phase image (top, 1 1 mm2) and TEM image (bottom) of micelles prepared from the PS200-[Ru]-PEG375 diblock copolymer in water. (b) A micelle as envisioned by AFM. (c) Relationship between the AFM-measured core diameter (Dc) of the micelles and the DP3/5 of the PS-block (for micelles prepared in pure water and in 1 M NaCl). The solid line represents the linear regression obtained from the data in 1 M NaCl. The dashed lines, evidencing the presence of only two sizes, are only a guide for the eyes [238]. Figure reproduced with kind permission; r 2006 American Chemical Society.

amphiphilic brush PS20-[Ru]-PEG70 × PSS× formation of micelles in water

PS20-[Ru]-PEG70 × PSS607

PS20-[Ru]-PEG70 × PSS4005

Figure 6.43 Amphiphilic metallo-supramolecular brush that formed micelles in water. Depending on the length of the attached PSS chain, spherical (left) or worm-like (right) micelles were formed. The TEM images confirmed the shape of the micelles in both cases [239]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

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Beside intensively studied diblock copolymers of the type PS-[Ru]-PEG, systems like PDMS-[Ru]-PEG or PEB-[Ru]-PEG were also examined with respect to their micellization behavior [148, 212, 240]. For instance, the PEB70-[Ru]-PEG70 diblock copolymer formed spherical micelles (Dh ¼ 36 7 1 nm) as well as clusters of micelles (Dh ¼ 115 7 2 nm) in aqueous medium. The ratio between the two populations could be shifted to the first one by dilution or increasing the temperature. In contrast, for PS20-[Ru]-PEG70 the glassy nature of the PS block led to “frozen” micelles that could not undergo this reversible aggregation of second order. Moreover, the supramolecular linkage could be cleaved by addition of HEEDTA, yielding smaller particles (Dh ¼ 13 7 4 nm, according to DLS and AFM), which were assigned to the PEB cores (electrostatically stabilized by the remaining RuII ions) [240]. Metallo-supramolecular micelles of a different geometry were observed for PFDS12-[Ru]-PEG70 (Figure 6.44a). This diblock copolymer was synthesized via directed step-wise self-assembly of macroligands 58a and 77 [151]. Crystallization of the organometallic PFDS blocks occurred during the micellization process (as evidenced by DSC), giving rise to the formation of cylindrical, rod-like micelles that were further characterized by DLS, AFM, and TEM (Figure 6.44a). The micelles were found to be small and constant in diameter, but long and polydisperse in length. Ultrasonic treatment resulted in shortening of the cylindrical micelles, while their diameter was unaffected. Since the PFDS cylindrical cores of these micelles are known to be useful for charge transport and are precursors to ferromagnetic nanostructures [15, 241, 242], these water-soluble substances were believed to be promising candidates for applications in nanotechnology. Zhou and Harruna reported the synthesis of an amphiphilic diblock copolymer by self-assembly of the ligands 97a and 97b with RuII ions (see also Scheme 6.14) [190]. The introduction of the thermoresponsive PNIPAM-block with LCST behavior is the first example for a temperature-responsive metallo-supramolecular block copolymer. AFM measurements revealed the formation of spherical aggregates on a solid support (Figure 6.44b). Figure 6.4 depicts the formation of crosslinked nanoparticles with terpyridines located in the hydrophobic core [53]. In a similar approach, O’Reilly et al. reported the synthesis of functional nanocages [197]. First, micelles of the diblock copolymer PS120-[Ru]-PAA135 were formed in water and, subsequently, crosslinking of the PAA core was achieved by amidation with 2,20 -(ethylenedioxy)-bis(ethylamine). Using HEEDTA, as decomplexation agent, enabled the hollowing out of the nanoparticles. The shell crosslinked micelles were dialyzed into a THF–water (2 : 1 ratio) mixture to fully remove the uncomplexed PS chains from the core (Figure 6.45). The hollow nature of the nanocages was confirmed by sequestration experiments with a hydrophobic dye (i.e., Red Nile). The response of the nanocages to changes of the pH value was also investigated; their size between about 90 and 240 nm could be changed reversibly by varying the pH value between 5 and 9. Another pH-responsive metallo-supramolecular material was reported by Gohy et al. [138]: applying the directed step-wise complexation of terpyridine macroligands 58a and 69 with RuII ions yielded the triblock copolymer PS32-b-P2VP13-[Ru]-PEG70.

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| 303

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H

S

Fe

S

O

Si

N H

2 PF6

PFDS12-[Ru]-PEG70

m

NH

12

O

N N

N

N

N

Ru2+

2 PF6

O

N

N

N

Ru2+

N N

PNIPAM-[Ru]-PS

N

N

n

S S

O(CH2CH2O)70CH3

50 nm

Figure 6.44 (a) PFDS12-[Ru]-PEG70 diblock copolymer (left) and TEM image of cylindrical micelles of the diblock copolymer in water (right) [151]; (b) PNIPAM[Ru]-PS diblock copolymer (left) and AFM image of the diblock copolymer drop-casted onto a solid support [190]. Figure reproduced with kind permission; r 2004 Wiley-VCH and 2005 American Chemical Society, respectively.

(b)

(a)

304



PS120-[Ru]-PAA135

(i)

(ii)

(iii)

Figure 6.45 Formation of hollow nanostructures from diblock copolymer PS120-[Ru]-PAA135: (i) the copolymer is self-assembled into micelles in water, (ii) the PAA corona is crosslinked, and (iii) the RuII bis(terpyridine) complexes are opened and the released PS blocks are removed by dialysis in THF–water mixtures. The corresponding TEM images, after staining with phosphotungstic acid, deposition onto a carbon-coated Cu-grid, and drying, are also depicted (scale bar is 100 nm) [197]. Figure reproduced with kind permission; r 2008 American Chemical Society.

Then, core–shell–corona micelles consisting of a PS core, a P2VP shell, and a PEG corona were prepared. Owing to the ability of protonation/deprotonation of the P2VP shell, the response to changes in the pH value could be observed. The initial Dh was 81 nm at acidic pH values; after neutralization, the size of the micelles was Dh ¼ 63 nm. In agreement with the pKa value of P2VP, a sharp transition between both regimes was apparent at about pH 5.5. The pH response of these micelles could, for instance, be utilized for the encapsulation/release of active species that were reversibly trapped in the P2VP shell. Hillmyer et al. showed recently that micelles consisting of a phase-separated core can selectively store and release different hydrophobes [243]. Furthermore, the P2VP could be applied as a new type of nanoreactor, for example, for the preparation of metal nanocapsules [244]. The previous example represents a typical amphiphilic ABC triblock copolymer [245]. The so-called multicompartment micelles are formed when only one segment is soluble in the solvent and, additionally, phase separation of the two nonsoluble blocks in the micellar core occurs. A different type of micelle is obtained if two of the three polymer blocks are soluble in the solvent, in which case the formation of micelles is observed featuring a mixed-arm soluble corona. Regarding the first case, the hydrophobic microdomains of multicompartment micelles must substantially differ from each other; this can be achieved when hydrocarbon and fluorocarbon blocks are combined within one copolymer. Ott et al. showed that the triblock copolymer PS76-b-PTFMS42-[Ru]-PEG70 fulfilled these requirements (Figure 6.46) [200]. The amphiphilic nature of the copolymer was used to prepare micelles in various alcohols, as solvent. The solubility of the central fluorinated block could be tuned depending on the polarity of the alcohol and, consequently, micelles of various morphologies were obtained. All basic micellar morphologies – spherical micelles, worm-like micelles, and vesicles – were formed from the copolymer only by changing the solvent used. Moreover, the thermoresponsive (i.e., upper critical solution temperature, UCST) behavior of the

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| 305

306

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes PS76-b-PTFMS42-[Ru]-PEG70

(a)

[Ru] in methanol & ethanol at RT

in propanol & butanol at RT

[Ru]

[Ru]

solvophilic

solvophobic

solvophobic

solvophilic

vesicles & hollow tubes

micelles & short worms (b)

0.5 μm

0.5 μm

0.5 μm

0.5 μm

1 μm

0.5 μm

Figure 6.46 (a) Self-assembly of triblock copolymer PS76-b-PTFMS42-[Ru]-PEG70 in different solvents; (b) unstained TEM images of the triblock copolymer in different solvents [200]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

fluorinated block allowed a reversible control over the size and morphology of the self-assembled objects. Extension of the concept towards metallo-supramolecular tetrablock copolymers was reported recently by Gohy et al. [246]: copolymer PTFMS20-b-PtBA25-b-PS35-[Ru]-PEG70 formed multicompartment micelles. In ethanol, micelles with a PS core and a corona containing all three other blocks were observed. In iso-propanol, larger multicompartment micelles containing discrete PS and PTFMS nanodomains linked by PtBA chains and further stabilized by the PEG blocks were obtained at temperatures below 45 1C. Above 50 1C (i.e., above the UCST of the PTFMS block), those micelles reversibly reorganized into micelles identical in size and shape to the ones observed in ethanol. A different micellar architecture can be obtained when an A-b-B-tpy diblock copolymer – containing a hydrophobic block A and a hydrophilic block B – is selfassembled prior to the addition of transition metal ions. These micelles are functionalized with terpyridine moieties at the outside. Considering a highly diluted solution of micelles, the addition of metal ions will result in intra-micellar complexation. Conversely, the addition of metal ions to more concentrated solutions of these micelles will lead to inter-micellar complexation, thus generating “micellar gels.” Though such micelles might be of interest with respect to the formation of nano-objects with “sticky end-groups” through metal-to-ligand

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(a)

(b) 1/2 eq.

(c) 1 eq.

200 nm

PEO125

Dialysis against water

Figure 6.47 (a) TEM image (unstained) of the micelles of diblock copolymer PS47-b-PtBA55-tpy in EtOH; (b) schematic representation of the formation of flower-like micelles by addition of 0.5 equiv. of a transition metal ion; (c) schematic representation of the grafting of PEG125-[ onto micelles of PS47-b-PtBA55-tpy containing 1 equiv. of FeII ions [247]. Figure reproduced with kind permission; r 2008 The Royal Society of Chemistry.

complexation, they have been barely reported in the literature. Guillet et al. investigated how the size of such micelles can be tuned by changing the conformation of the coronal chains, utilizing metallo-supramolecular interactions [247]. Diblock copolymer PS47-b-PtBA55-tpy was used for the formation of welldefined micelles in ethanol [hydrodynamic radius (Rh) of 22 nm] that consisted of a PS core and a PtBA corona with the terpyridine ligands located at the end of the PtBA chains (Figure 6.47a). Subsequently, transition metal ions (i.e., ZnII, NiII, and FeII) were added to the micelles to induce the formation of complexes with the terpyridine terminal groups. At a metal : terpyridine ratio of 1 : 2, the formation of bis(terpyridine) complexes in the micellar corona occurred (Figure 6.47b). Owing to the looping of the coronal chains, highly stable flower-like micelles were obtained and the Rh increased significantly. The stability of the loops in the presence of higher amounts of the transition metal ions was studied intensively. In summary, the experimental findings mirrored the general stability constants of the respective bis(terpyridine) complexes: NiII formed the most stable complexes, then FeII, and ZnII the least stable. Additionally, the initial core–shell assembly could be extended to a core–shell– corona by complexing a terpyridine-macroligand to the PS47-b-PtBA55-tpy micelles. Thus, PEO125-tpy (58b) was added to the micelles previously loaded with one equivalent of FeII ions, yielding PS47-b-PtBA55-[Fe]-PEO125 micelles (Figure 6.47c). To confirm the grafting of the PEG chains onto the initial micelles, the metallosupramolecular triblock copolymer was transferred into water. Stable micelles consisting of a PS core, a PtBA shell, and a PEG corona were obtained [247]. Micelles of diblock copolymer PS80-b-PtBA200-tpy could be self-assembled by ZnII, NiII, or FeII ions, yielding “micellar gels” by inter-micellar linking (Figure 6.48a). In this case, a significantly higher concentration of the micelles was used to compete against the intra-micellar complexation. Rheological measurements, namely, determination of the storage and loss moduli (G0 and G00 , respectively), were applied to characterize the mechanical properties of the gels (Figure 6.48b

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| 307

308


(a) 2nd level of assembly

1st level of assembly

Metal ion

(b)

(c) 103

101

102 102 γ0 G’ G”

G’ G’’

100

ω = 10 s blank

Zn2+

Ni2+

−1

101 0

Fe2+

T = 20°C ω = 1 S−1 1000

2000 t/s

3000

101

4000

Figure 6.48 (a) Hierarchical self-assembly of PS80-b-PtBA200-tpy diblock copolymer micelles into a “micellar gel;” (b) comparison of the storage modulus (G0 ) and loss modulus (G00 ) at o ¼ 10 s1 between the initial micelles and those charged with 0.5 equiv. of various transition metal ions; (c) evolution of G0 and G00 with time following two successive pulses of high deformation (full lines), recorded on a “micellar gel” based on FeII ions at a copolymer concentration of 14% [198]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

and c). The gels were found to exhibit a stimuli responsive behavior; thus, the network could be broken either chemically or mechanically. In the latter case, an almost instantaneous recovery of the initial properties was observed upon stopping the mechanical solicitation. This reversible and tunable mechanical response (i.e., the dependency on the nature of the transition metal) could lead to new selfhealing materials. For example, a large amplitude deformation could be used to heal cracks, utilizing the spontaneous recovery of the network structure [198]. 6.3.6 Oligonuclear Metallo-Supramolecular Copolymers

Oligonuclear metallo-supramolecular block copolymers can be considered as a transition from mononuclear A-[M]-A homopolymers or A-[B]-diblock copolymers to the multinuclear chain-extended polymers, for example, of the general type A-[M]-(B-[M]-)nB-[M]-A (see also Chapter 5 and Section 6.3.2). In contrast to these two extreme poles, which have been widely discussed in the literature (see the previous sections), oligonuclear block copolymers have only just been introduced.

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G’, G”/Pa

102

γ0 /%

G’, G” [Pa]

103


| 309

This might be mainly attributed to the required directed step-wise assembly of macroligands, followed by the purification procedure, both of which are more demanding than, for example, for A-[M]-B diblock copolymers. ¨hwald et al. combined the divergent step-wise assembly of 1,4-bis(2,20 :60 ,200 Mo terpyridin-40 -yl)benzene and RuII ions with a convergent step to prepare a linear rod consisting of seven RuII centers [248]. Although the subunits are not polymers, this example shows both the scope and limitation of the general protocol whereby linear assemblies can be constructed with reasonable control over the molar mass (in the particular case leading to monodisperse materials) and, conversely, the yield decreased significantly with increasing number of assembly steps. Dinuclear A-[M]-B-[M]-A block copolymers have been prepared from RuII ions, telechelic terpyridine-macroligands (e.g., 63b or 64), and styrenic blocks (PS or SPS), but are rare [150, 202]. Chiper et al. reported a series of A-[Ru]-B-[Ru]-A triblock copolymers where the middle block tpy-B-tpy was a short rigid, p-conjugated bis(terpyridine) ligand (139a–d, Figure 6.49 [167]). PEG44-tpy was utilized

OC8H17

(a)

OC8H17

139a

C8H17O

139b

C8H17O N OC18H37

S 139c

⫽

S

C18H37O

N

N OC18H37

139d

OC18H37

C18H37O

C18H37O

(b) PEG44-[Ru]-(139a)-[Ru]-PEG44

PEG44-[Ru]-(139c)-[Ru]-PEG44

PEG44-[Ru]-(139b)-[Ru]-PEG44

PEG44-[Ru]-(139d)-[Ru]-PEG44

Figure 6.49 (a) p-Conjugated bis(terpyridine)s 139 used, as building block, for the synthesis of coil-rod-coil triblock copolymers PEG44-[Ru]-(139)-[Ru]-PEG44 [167]; (b) cryo-TEM images of the micelles formed by PEG44-[Ru]-(139)-[Ru]-PEG44 in water (scale bar is 200 nm) [219]. Figure reproduced with kind permission; r 2008 American Chemical Society.

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310

| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes as the flexible, outer block A-tpy and the directed self-assembly into coil-rod-coil triblock copolymers was achieved with Ru(dmso)4Cl2, as precursor complex [219]. Owing to the amphiphilic nature of the triblock copolymers, the formation of spherical micelles was observed in aqueous solution. As also shown by DLS and (cryo)-TEM measurements (Figure 6.49), the typical clustering into larger aggregates was observed for all four representatives. In contrast to block copolymers composed of coiled blocks, the self-assembly of coil-rod-coil block copolymers is not only determined by the phase separation of the incompatible blocks but is also known to be driven by other processes, such as the aggregation of the rod-like segments into (liquid) crystalline domains [249]. However, no evidence for a LC behavior of the micelles was found, presumably due to the preparation protocol. ¨tter et al. applied AUC to investigate the composition of the More recently, Schlu micelles formed by such coil-rod-coil triblock copolymers [250]: various fractions of micelles/clusters could be identified, with the number of molecules ranging from five (micellar radius of about 3 nm) to about 5000 (average radius of 30 nm).

(a) N = C8H17O

OC8H17

N

C8H17O

N C8H17O

C8H17O

C8H17O

OC8H17

OC8H17 C8H17O OC8H17

OC8H17

C8H17O

141

OC8H17

140 OC8H17

(b)

((c)

Figure 6.50 (a) Tris- (140) and tetrakis(terpyridine)s (141) used, as rigid core, for three- and four-arm metallo-supramolecular copolymers, respectively [251]; (b) a vesicle formed from the three-arm block copolymer (PEG44-[Ru]-)3(140) in acetone; (c) cryo-TEM image of spherical micelles formed from the four-arm block copolymer (PEG44-[Ru]-)4(141) [220]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

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| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes 167 Winter, A., Egbe, D.A.M., and Schubert, U.S. (2007) Org. Lett., 9, 2345–2348. 168 Schubert, U.S., Eschbaumer, C., Nuyken, O., and Hochwimmer, G. (1999) J. Inclusion Phenom., 35, 23–34. 169 Heller, M. and Schubert, U.S. (2001) Macromol. Symp., 177, 87–96. 170 Kricheldorf, H.R., Kreiser-Saunders, I., ¨rgens, C., and Wolter, D. (1996) Ju Macromol. Symp., 103, 85–102. 171 Corbin, P.S., Webb, M.P., McAlvin, J.E., and Fraser, C.L. (2001) Biomacromolecules, 2, 223–232. 172 Schubert, U.S. and Hochwimmer, G. (2001) Macromol. Rapid Commun., 22, 274–280. 173 Marin, V., Holder, E., Hoogenboom, R., and Schubert, U.S. (2004) J. Polym. Sci., Part A: Polym. Chem., 42, 4153–4160. 174 Holder, E., Marin, V., Alexeev, A.S., and Schubert, U.S. (2005) J. Polym. Sci., Part A: Polym. Chem., 43, 2765–2776. 175 Hofmeier, H., Hoogenboom, R., Wouters, M.E.L., and Schubert, U.S. (2005) J. Am. Chem. Soc., 127, 2913–2921. 176 Winter, A. and Schubert, U.S. (2007) Macromol. Rapid Commun., 208, 1956–1964. 177 (2009) Controlled/Living Radical Polymerization: Progress in ATRP (ed. K. Matyjaszewski), Oxford University Press, Washington, D.C. 178 Matyjaszewski, K. and Xia, J.-H. (2001) Chem. Rev., 101, 2921–2990. 179 Kamigaito, M., Ando, T., and Sawamoto, M. (2001) Chem. Rev., 101, 3689–3746. 180 Smith, A.P. and Fraser, C.L. (2003) Macromolecules, 36, 2654–2660. 181 Viau, L., Even, M., Maury, O., Haddleton, D.M., and Le Bozec, H. (2003) Macromol. Rapid Commun., 24, 630–635. 182 Kickelbick, G. and Matyjaszewski, K. (1999) Macromol. Rapid Commun., 20, 341–346. 183 Pintauer, T. and Matyjaszewski, K. (2005) Coord. Chem. Rev., 249, 1155–1184.

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184 Tang, W., Kwak, Y., Braunecker, W., Tasarevsky, N.V., Coote, M.L., and Matyjaszewski, K. (2008) J. Am. Chem. Soc., 130, 10702–10713. 185 Pefkianakis, E.K., Tzanetos, N.P., Chochos, C.L., Andreopoulou, A.K., and Kallitsis, J.K. (2009) J. Polym. Sci., Part A: Polym. Chem., 47, 1939–1952. 186 (2008) Handbook of RAFT Polymerization (ed. C. Barner-Kowollik), Wiley-VCH Verlag GmbH, Weinheim. 187 Chen, M., Ghiggino, K.P., Launikonis, A., Mau, A.W.H., Rizzardo, E., Sasse, W.H.F., Tang, S.H., and Wilson, G.J. (2003) J. Mater. Chem., 13, 2696–2700. 188 Zhou, G.-C. and Harruna, I.I. (2004) Macromolecules, 37, 7132–7139. 189 Zhou, G.-C., Harruna, I.I., and Ingram, C.W. (2005) Polymer, 46, 10672–10677. 190 Zhou, G.-C. and Harruna, I.I. (2005) Macromolecules, 38, 4114–4123. 191 Zhang, L.-W., Zhang, Y.-H., and Chen, Y.-M. (2006) Eur. Polym. J., 42, 2398–2406. 192 Hawker, C.J., Bosman, A.W., and Harth, E. (2001) Chem. Rev., 101, 3661–3688. 193 Bosman, A.W., Vestberg, R., Heumann, A., Frechet, J.M.J., and Hawker, C.J. (2003) J. Am. Chem. Soc., 125, 715–728. 194 Benoit, D., Harth, E., Fox, P., Waymouth, R.M., and Hawker, C.J. (2000) Macromolecules, 33, 363–370. 195 Lohmeijer, B.G.G. and Schubert, U.S. (2004) J. Polym. Sci., Part A: Polym. Chem., 42, 4016–4027. 196 Ott, C., Lohmeijer, B.G.G., Wouters, D., and Schubert, U.S. (2006) Macromol. Chem. Phys., 207, 1439–1449. 197 Ievins, A.D., Moughton, A.O., and O’Reilly, R.K. (2008) Macromolecules, 41, 3571–3578. 198 Guillet, P., Mugemana, C., Stadler, F.J., Schubert, U.S., Fustin, C.-A., Bailly, C., and Gohy, J.-F. (2009) Soft Matter, 5, 3409–3411. 199 Ott, C., Hoogenboom, R. and Schubert, U.S. (2008) Chem. Commun., 3516–3518. 200 Ott, C., Hoogenboom, R., Hoeppener, S., Wouters, D., Gohy, J.-F., and Schubert, U.S. (2009) Soft Matter, 5, 84–91.

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215 Tziatzios, C., Precup, A.A., Lohmeijer, ¨rger, L., Schubert, U.S., and B.G.G., Bo Schubert, D. (2004) Prog. Colloid Polym. Sci., 127, 54–60. 216 Ras- a, M., Tziatzios, C., Lohmeijer, B.G.G., Schubert, D., and Schubert, U. S. (2006) Prog. Colloid Polym. Sci., 131, 165–171. 217 Schubert, D., Tziatzios, C., Schuck, P., and Schubert, U.S. (1999) Chem. Eur. J., 5, 1377–1383. 218 Oudhoff, K.A., Schoenmakers, P.J., and Kok, W.T. (2004) Chromatographia, 60, 475–480. 219 Chiper, M., Winter, A., Hoogenboom, R., Egbe, D.A.M., Wouters, D., Hoeppener, S., Fustin, C.-A., Gohy, J.-F., and Schubert, U.S. (2008) Macromolecules, 41, 8823–8831. 220 Gohy, J.-F., Chiper, M., Guillet, P., Fustin, C.-A., Hoeppener, S., Winter, A., Hoogenboom, R., and Schubert, U. S. (2009) Soft Matter, 5, 2954–2961. 221 Fournier, D., Hoogenboom, R., Thijs, H.M.L., Paulus, R.M., and Schubert, U. S. (2007) Macromolecules, 40, 915–920. 222 Ott, C., Wouters, D., Thijs, H.M.L., and Schubert, U.S. (2007) J. Inorg. Organomet. Polym. Mater., 17, 241–249. 223 Ziessel, R., Grosshenny, V., Hissler, M., and Stroh, C. (2004) Inorg. Chem., 43, 4262–4271. 224 Mugemana, C., Guillet, P., Hoeppener, S., Schubert, U.S., Fustin, C.-A., and Gohy, J.-F. (2010) Chem. Commun., 46, 1296–1298. 225 Hamley, I.W. (1998) The Physics of Block Copolymers, Oxford Science Publications, Oxford. 226 Gohy, J.-F. (2005) Adv. Polym. Sci., 190, 65–136. 227 Riess, G. (2003) Prog. Polym. Sci., 28, 1107–1170. 228 Gohy, J.-F., Lohmeijer, B.G.G., and Schubert, U.S. (2002) Macromolecules, 35, 4560–4563. 229 Gohy, J.-F., Lohmeijer, B.G.G., Varshney, S.K., and Schubert, U.S. (2002) Macromolecules, 35, 7427–7435. 230 Regev, O., Gohy, J.-F., Lohmeijer, B.G.G., Varshney, S.K., Hubert, D.H. W., Frederik, P.M., and Schubert, U.S. (2004) Colloid. Polym. Sci., 282, 407–411.

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| 6 Functional Polymers Incorporating Terpyridine-Metal Complexes 231 Mayer, G., Vogel, V., Lohmeijer, B.G.G., Gohy, J.-F., van den Broek, J.A., Haase, W., Schubert, U.S., and Schubert, D. (2004) J. Polym. Sci., Part A: Polym. Chem., 2, 4458–4465. 232 Vogel, V., Gohy, J.-F., Lohmeijer, B.G. G., van den Broek, J.A., Haase, W., Schubert, U.S., and Schubert, D. (2003) J. Polym. Sci., Part A: Polym. Chem., 41, 3159–3168. 233 Al-Hussein, M., Lohmeijer, B.G.G., Schubert, U.S., and de Jeu, W.H. (2003) Macromolecules, 36, 9281–9284. 234 Al-Hussein, M., de Jeu, W.H., Lohmeijer, B.G.G., and Schubert, U.S. (2005) Macromolecules, 38, 2832–2836. 235 Lohmeijer, B.G.G., Wouters, D., Yin, Z.-H., and Schubert, U.S. (2004) Chem. Commun., 2886–2887. 236 Lin, Z., Kim, D.-H., Boosahda, L., Stone, D., LaRose, L., and Russell, T.P. (2002) Adv. Mater., 14, 1373–1376. 237 Fustin, C.-A., Guillet, P., Misner, M.J., Russell, T.P., Schubert, U.S., and Gohy, J.-F. (2008) J. Polym. Sci., Part A: Polym. Chem., 46, 4719–4724. 238 Guillet, P., Fustin, C.-A., Lohmeijer, B. G.G., Schubert, U.S., and Gohy, J.-F. (2006) Macromolecules, 39, 5484–5488. 239 Guillet, P., Fustin, C.-A., Wouters, D., Hoeppener, S., Schubert, U.S., and Gohy, J.-F. (2009) Soft Matter, 5, 1460–1465.

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240 Gohy, J.-F., Lohmeijer, B.G.G., and Schubert, U.S. (2002) Macromol. Rapid Commun., 23, 555–560. 241 Eloi, J.-C., Chabanne, L., Whittle, G.R., and Manners, I. (2008) Mater. Today, 11, 28–36. 242 Whittle, G.R. and Manners, I. (2007) Adv. Mater., 19, 3439–3468. 243 Lodge, T.P., Rasdal, A., Li, Z.-B., and Hillmyer, M.A. (2005) J. Am. Chem. Soc., 127, 17608–17609. 244 Gohy, J.-F., Willet, N., Varshney, S.K., ˆme, R. (2001) Zhang, J.-X., and Jero Angew. Chem., Int. Ed., 40, 3214–3216. 245 Fustin, C.-A., Abetz, V., and Gohy, J.-F. (2005) Eur. Phys. J. E, 16, 291–302. 246 Gohy, J.-F., Ott, C., Hoeppener, S., and Schubert, U.S. (2009) Chem. Commun., 6038–6040. 247 Guillet, P., Fustin, C.-A., Mugemana, C., Ott, C., Schubert, U.S., and Gohy, J.-F. (2008) Soft Matter, 4, 2278–2282. 248 Janini, T.E., Fattore, J.L., and Mohler, D.L. (1999) J. Organomet. Chem., 578, 260–263. 249 Klok, H.-A. and Lecommandoux, S. (2001) Adv. Mater., 13, 1217–1229. ¨tter, F., Pavlov, G.M., Gohy, J.-F., 250 Schlu Winter, A., Wild, A., Hager, M.D., Hoeppener, S., and Schubert, U.S. (2011) J. Polym. Sci., Part A: Polym. Chem., 49, 1396–1408. 251 Winter, A., Friebe, C., Hager, M.D., and Schubert, U.S. (2009) Eur. J. Org. Chem., 801–809.

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7

Terpyridine Metal Complexes and their Biomedical Relevance*

7.1 Introduction

Over the last two decades there has been increasing interest in transition metal complexes that can bind and/or interact with biomolecules at specific locations, for example, as DNA/RNA intercalators or inhibitors for certain enzymes [1–5]. In particular, cisplatin is still one of the most powerful chemotherapeutical agents in this respect [6, 7]. Intercalators are, per definition, “small molecules that contain a planar aromatic heterocyclic functionality, which can insert and stack between the base pairs of double-helical DNA” [8]. Lippard and coworkers were the first to establish square-planar PtII complexes, containing a heteroaromatic ligand, as DNA metallo-intercalators [9]. Barton’s group later extended the scope of metallointercalation to three dimensions by utilizing octahedral complexes, therewith enabling the targeting of specific DNA sites by matching the shape, symmetry, and functionalities of the metal complex to that of the DNA target [10]. By taking advantage of the photophysical, photochemical, and redox properties of metallointercalators, sensitive spectroscopic and reactive probes of DNA have been developed [2]. In this respect, complexes of RuII, OsII, or RhIII with three bidentate ligands [e.g., derivatives of 2,20 -bipyridine (bpy) or 1,10-phenanthroline (phen)] have been widely used. Tridentate chelating ligands, such as 2,20 :60 ,200 -terpyridines, can also lead to potent intercalators. In 1978, Lippard proposed that PtII mono(terpyridine) complexes might represent new types of antitumor agents due to their ability to efficiently intercalate with DNA [11]. Reedijk et al. could show that Ru(tpy)Cl3 strongly interacted with DNA and had activity against L1210 leukemia cells comparable to that of cisplatin [12]. Since then, the scope of metallo-intercalators, based on terpyridine metal complexes, has emerged. The first part of this chapter summarizes various examples for this type of intercalator as well as their mode of action. *

Parts of this chapter are reproduced from Chem. Rev. 108 (2008) 1834–1895 and Curr. Top. Med. Chem. 11 (2011) in press by permission of the American Chemical Society and Bentham Science Publishers, respectively.


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| 7 Terpyridine Metal Complexes and their Biomedical Relevance Furthermore, the labeling of small biomolecules or biopolymers by covalent linkage to luminescent terpyridine metal complexes (e.g., with RuII, PtII, or IrIII ions) has become a versatile tool in modern bioanalytical research and diagnostics [13–15]. Such materials will be evaluated in the second part of this chapter.

7.2 Terpyridine Metal Complexes with Biological Activity 7.2.1 Intercalation and Cytotoxicity

There are several ways in which a molecule can interact with DNA: (i) covalent binding, (ii) electrostatic interaction, or (iii) intercalation. Intercalation occurs when a substrate of appropriate size and chemical nature fits itself in-between the DNA’s base pairs. In general, such molecules are polycyclic, aromatic, and planar. DNA intercalators are, in particular, employed in chemotherapeutic treatment to inhibit DNA replication in rapidly growing cancer cells, such as in Hodgkin’s lymphoma, Wilms’ tumor, Ewing’s sarcoma, or rhabdomyosarcoma [16]. To enable a molecule to intercalate between base pairs, the DNA must dynamically open a space between its base pairs by unwinding, which induces local structural changes to the DNA strand, such as lengthening of the DNA strand or twisting of the base pairs. An increase of the melting temperature and changes in the extent of supercoiling in closed circular duplex DNA occur and this is often reflected by reduced sedimentation coefficients [17]. Moreover, these structural modifications can lead to functional changes, often to the inhibition of transcription, replication, and DNA repair processes, which makes intercalators potent mutagens. For these reasons, DNA intercalators are often highly carcinogenic. Intercalation of small molecules into the DNA by stacking between its base pairs was suggested by Lerman in 1961 to explain the high binding affinity of planar dyes to DNA [8]. If a dye is intercalated into DNA, the UV–vis spectra will be shifted and there can be an induced circular dichroism (CD) effect. The proposed mechanism for intercalation is as follows: in aqueous isotonic solution, the cationic intercalator is attracted electrostatically by the polyanionic DNA. Subsequently, the NaI and/or MgII ions that always surround DNA (to balance its charge) will be substituted and a weak electrostatic bond with the outer surface of the DNA will be established, then the intercalator can slide into the hydrophobic region between the base pairs and away from the hydrophilic outer environment surrounding the DNA [16]. 7.2.1.1 Terpyridine Complexes with d8 Late Transition Metal Ions 7.2.1.1.1 Metallo-Intercalators Based on Mono(terpyridine) d8 Metal Complexes In the 1970s, the ability for d8 late transition metal ions (i.e., PtII, PdII, AuIII) to form stable square-planar complexes with terpyridine ligands was discovered. Among these, complexes containing PtII ions were extensively studied due to their

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7.2 Terpyridine Metal Complexes with Biological Activity

| 321

unique luminescence properties [18–20]. Consequently, potential applications in the fields of chemosensing of solvents [21–23] and metal ions [24] or photocatalysis [25, 26] were envisioned. Beside these, in recent years, the biological activity of PtII mono-complexes has attracted widespread interest [13]. As early as the 1970s, Lippard et al. suggested an intercalation behavior for flat, square-planar PtII mono(terpyridine) complexes, which are quite comparable to small molecule dyes [9, 11]. A library of such PtII complex (e.g., 1) was prepared and their ability to intercalate was confirmed (Figure 7.1a). Covalent binding of the complex to the DNA could be suppressed due to the chemically inert Pt–S bond. The UV–vis absorption spectra of 1 with increasing amounts of ct-DNA (ct: calf thymus) revealed remarkable changes, such as a strong decrease in the peak intensity (hypochromicity) and redshift of the bands at about 550 nm with (c)

N N HO

r/CF ( 104 M1 )

(a)

N

P t+

N

S

1

N

P t+

N

S

2

30 1

20

10

0 (b) 0.10

0.10

0.06 0.04

C

1

D

2 3

E

5

0.06 0.04

4

0.02 0 300

2

r/c ( 104 )

Absorbance

0.08 B

0.02

400

500

600

700

0.2

40

A

0.08

0.1

0

Wavelength/nm

30

20

10

0

0.2

0.4

0.6

r

Figure 7.1 (a) PtII mono(terpyridine) complexes 1 and 2 (counterions omitted for clarity). (b) UV–vis absorption spectra of 1 upon addition of various amounts of ct-DNA in a 1 mM Na3PO4 buffer with 3 mM NaCl at pH 6.8. In curves A–E, the concentration of 1 was 6.97 mM and the DNA concentrations were (A) 0, (B) 17, (C) 34, (D) 146, and (E) 303 mM; in curves 1–5, the concentration of 1 was 70.4 mM and the DNA concentrations were (1) 0, (2) 97.7, (3) 189, (4) 356, and (5) 700 mM [9]. (c) Scatchard plots for the binding of 1 (in a buffer of ion strength I ¼ 0.003; top) and 2 (in a buffer with I ¼ 0.01; bottom) to ct-DNA [28]. Figure reproduced with kind permission of the author., Figure reproduced with kind permission; r 1987 Portland Press.

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0.3

322

| 7 Terpyridine Metal Complexes and their Biomedical Relevance well-defined isosbestic points (Figure 7.1b). A strong dependency of these shifts based on the concentration of the intercalator, the DNA, and buffer solution as well as its ionic strength (I ¼ [Mþ]) was observed. In general, redshifts of the absorption bands can be attributed to the intercalation binding mode, whereas the pronounced hypochromicity corresponds to an electronic interaction between bound molecules and the DNA [17]. A so-called Scatchard analysis (i.e., r/c vs. r diagram; with r ¼ [bound-1]/[DNA] and c ¼ [free-1]) was carried out [27]; at large rvalues, a concave upward curve was observed (Figure 7.1c) [9, 28]. Two different binding modes were concluded from these results: a strong intercalative mode and a weaker non-intercalative secondary interaction [29]. Similar results were obtained for complex 2 and were further supported by the analysis of the ct-DNA contour length ratio (L/L0) in the presence (L) and absence (L0) of 2, where a linear increase in helix extension was observed up to about r ¼ 0.2, followed by a strong decrease of the helix extension suggesting a non-intercalative binding component. The binding constants (KB) of intercalators 1 and 2 into DNA at buffers of low ionic strength had to be estimated by extrapolating the data of the Scatchard plot to the y-axis, since the data did not fit to the model introduced by McGhee and von Hippel [30]; however, by increasing the buffer’s ionic strength, the secondary nonintercalative interaction of 1 and 2, respectively, with ct-DNA was suppressed. Thus, the UV–vis titration data fit the McGhee–von Hippel equation and KB could be determined. Table 7.1 summarizes the data for various PtII mono(terpyridine) complexes (1–13, see Figure 7.2) as well as for one PdII mono(terpyridine) complex (14) [9, 29, 32, 33, 39]. Most of the complexes 1–14 bound to DNA exclusively via intercalation. Since 11, 13, and 14 contained labile ancillary ligands (i.e., Cl or OH), covalent binding as well as intercalation to the DNA were observed. The linear dependence between the observed KB of 2 and the ionic strength of the medium (I ¼ [Mþ]) in a logarithmic plot was explained by the polyelectrolyte theory, according to Eq. (7.1) [40]. KB was considered to be free of ion concentration effects and calculated by extrapolating the plot of log(KB) versus log ([Mþ]). The determined ion-free binding constants (K0) of 1 (3.5 103 M1, Figure 7.3a) [41] and 2 (4.1 103 M1) [31] were about 20 times lower than that of ethidium bromide, as reference (7.4 104 M1) [42]: logðKB Þ ¼ k logð½Mþ Þ þ logðK0 Þ

(7.1)

where K0 ¼ binding constant of the intercalator to DNA at [Mþ] ¼ 1 mol l1; k ¼ constant. To accommodate an intercalator, DNA must partly unwind, which induces an increase in the DNA’s relative contour length (L/L0) leading to its eventual stiffening [29] and an increased viscosity [43]. For instance, the specific viscosity (Zsp) of a DNA solution increased upon the addition of 1, reaching its saturation point at r ¼ 0.23 (Figure 7.3b) [9]. The L/L0 of DNA was determined by viscosity experiments [41, 44] and similar behavior in viscosity and counter length was

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7.2 Terpyridine Metal Complexes with Biological Activity Table 7.1 DNA binding constants (KB) of intercalators 1–14 and their effect on the melting temperature (DTm) of ct-DNA.

Complex

KB (M1)

I ¼ [Mþ]

DTm (1C)

Reference

1

1.2 7 0.2 105a,b 5.1 7 0.2105c,d 3.9 7 0.3 105c,e 4.5 7 0.3 105f,g 1.3 7 0.2 104f,g 8.4 7 0.5 103f,g 4.3 7 0.5 105c,e 0.5 7 0.1 105c,e 1.0 7 0.2 105c,e 3.5 7 0.3 104f,h 3.0 7 0.3 103f,h 5.3 7 0.5 104f,h 1.8 7 0.2 107i,d 4.9 7 0.3 107c,d N.a. 9.8 7 1.3 103f,h 1.3 7 0.2 105c,d N.a. 7.0 7 0.6 104a,k 1.9 7 0.2 105c,d

0.003 0.1 0.2 0.1

5.0b 3.4d 2.5e N.a.

[9]

0.2 0.2 0.2 0.15 0.15 0.15 0.1 0.1 0.003 0.15 0.2 0.001 N.a. 0.1

N.a. N.a. N.a. 12.2h 12.3h 6.0

[32] [32] [32] [33] [33] [33, 34]

3.0b N.a. 5.0d 7.8j N.a. N.a.

[35] [36] [32, 34] [37] [38] [32]

2

3 4 5 6 7 8

9 10 11 12 13 14

[31]

a

Calculated by Scatchard analysis of the UV–vis absorption data. 1 mM phosphate buffer with 3 mM NaCl at pH 6.8. c Calculated by Scatchard analysis of the fluorescence study. d 50 mM TrisTM HCl buffer with 0.1 M NaCl at pH 7.5. e 50 mM TrisTM HCl buffer with 0.2 M NaCl at pH 7.5. f Calculated by McGhee–von Hippel analysis of the UV–vis absorption data. g 2 mM Hepes buffer with 0.1 M KF at pH 7. h 1 mM phosphate buffer with 0.15 M NaNO3 at pH 7. i Calculated by analysis of the CD spectra. j 45 mM TrisTM buffer with 1 mM Na2H2EDTA at pH 7.5. k 50 mM EPPS buffer at pH 9. b

observed for structurally related PtII mono(terpyridine) complexes, as intercalators to DNA [9, 29, 31, 33, 34, 41]. As a more straightforward technique, a quartz crystal resonator (QCR) was introduced to directly allow the determination of the binding mode of molecules to DNA by measuring the increase in viscosity [45]. As a further result of the unwinding of DNA due to intercalation, an increase in the melting temperature (Tm) can be observed, which refers to a transition of the double-stranded to a single-stranded DNA by thermally breaking the hydrogen bonds. This thermal denaturation process was easily monitored by changes in the UV–vis absorption at 260 nm (Figure 7.4). For PtII complexes 6–8, thermal denaturation followed a two-step mechanism; the part of the DNA that was not bound to the intercalator melted first, followed by a second transition at higher

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| 323

324

| 7 Terpyridine Metal Complexes and their Biomedical Relevance R2

N N

M

N

1

R

MII complex 3 4

R1 (M = Pt, R2 = H) + NH3

S

O

S

MII complex

M

9

Pt

10

Pt

11

Pt

Cl

H

O

5

S

COOH

R1 S

R2 H

N

+ NH3

6

N

12

Pt

Me

H

7

N

13

Pt

OH

H

8

N

14

Pd

Cl

H

Figure 7.2 Metallo-intercalators 3–14 (counterions omitted for clarity).

temperature caused by the melting of the DNA-intercalator portion [33, 34]. In contrast, full denaturation of the DNA with intercalators 1 and 11 occurred in a single step [9, 33, 34, 41]. Table 7.1 lists the differences in melting temperature (DTm) of ct-DNA upon the addition of various intercalators. CD measurements were applied, as an additional tool, to confirm intercalation of the PtII mono(terpyridine) complexes into DNA: in the range 300–500 nm, signals were attributed to an induced CD effect [9, 33–35]. For instance, the CD spectra of 6 and 7 showed positive bands between 300 and 400 nm in the presence of ct-DNA (Figure 7.5). Furthermore, McCoubrey et al. used CD titration experiments of intercalator 8 to determine the binding constant KB to DNA [34]. The initial equilibrium binding constant was calculated to be KB ¼ 2 107 M1, suggesting that the size of the binding site was about that of four base pairs; increasing the ratio of PtII complex revealed a second binding mode and KB of 1 106 M1, suggesting a binding site of only two base pairs. Competitive fluorescence spectroscopy (CFS) was carried out with ethidium bromide (EthBr), as the reference material. The Scatchard [27] analysis for binding of EthBr to DNA in the presence of an increasing amount of PtII mono(terpyridine) intercalators exhibited two different effects [9, 32]. The Scatchard plot using 1 vs. EthBr with DNA showed: (i) the expected competitive inhibition between

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(a)

Log(K5)

5

4 Log(K0) 3.5

−1

0

−2

Log([M+]) (b)

(c) 0.35 1.2 0.30

ηsp

L/L0 0.25

1.1

0.20 1.0 0.15

0

0.1

0.2 r

0.3

0.4

0.05

0.10 r

0.15

0.20

Figure 7.3 (a) Plot of log(KB) vs. log([Mþ]) for 2 and ct-DNA [31] (b) specific viscosity of DNA with intercalator 1 (1 mM phosphate buffer with I ¼ 0.003 at pH 6.8) [9]. (c) Relative counter length (L/L0) of DNA as a function of r ¼ [bound-1]/[DNA] (50 mM TrisTM buffer with I ¼ 0.2 at pH 7.5) [41]. Figure reproduced with kind permission; r 1987 Portland Press., Figure reproduced with kind permission of the author., Figure reproduced with kind permission; r 1979 American Chemical Society.

EthBr and 1, which was characterized by a decrease in the slope due to the presence of an increasing amount of the complex with no change in the intercept at the x-axis (Figure 7.6a) [9]; in contrast, the Scatchard plot of 11 vs. 1 in the presence of DNA revealed two different features resulting from a competitive inhibition between EthBr and 11 (Figure 7.6b, lines 1–3) as well as (ii) a noncompetitive inhibition, which is illustrated by changes in both the slope and intercept of the x-axis for [DNA] : [11] o2 (Figure 7.6b, lines 4–5). In both cases, the competitive inhibition of EthBr was attributed to intercalation and the noncompetitive feature of 11 was caused by the abstraction of the labile Cl ligand

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| 325

326

| 7 Terpyridine Metal Complexes and their Biomedical Relevance 1.3

Relative Absorbance (260 nm)

Relative Absorbance (270 nm)

1.50

A 1.2

B C

1.1

1 30

40

50

60 70 80 Temperature (° C) (a)

90

100

1.40 1.30

D E

1.20 1.10 1.00

70

80 90 100 Temperature (° C) (b)

Figure 7.4 (a) Thermal denaturation curves of 400 mM of ct-DNA (A), with 20 mM of 11 (B), and 8 (C) [34]. (b) Thermal denaturation curves of 85 mM of ct-DNA (D) and with 3.5 mM of 1 (E) [9]. Figure reproduced with kind permission; r 1996 Elsevier B.V., Figure reproduced with kind permission of the author.

10

10 6

7 5 Milidegrees

Milidegrees

5 0 −5

0 −5

−10 300

325

350 375 Wavelength (nm) (a)

400

−10 300

325

350 375 Wavelength (nm) (b)

Figure 7.5 CD spectrum of (a) 6 and (b) 7 (50–80 mM) in the presence of a ten-fold excess of DNA at 25 1C (1 mM phosphate buffer with I ¼ 0.0015 at pH 7.0) [33]. Figure reproduced with kind permission; r 1999 American Chemical Society.

from the intercalator leading to a covalent binding to the DNA. The KB values of 1 and 11 (in the range of [DNA] : [11] >2) to DNA were calculated from the CFS studies [9]; moreover, the binding constant of 11 to ct-DNA (KB ¼ 4.95 7 0.30 107 M1) was about 250 times larger than the binding constant of 1 to ct-DNA [34]. Closed-circular DNA was further utilized to prove the intercalative binding mode of the PtII mono(terpyridine) complexes; Figure 7.7a depicts the different topologies of circular DNA. It was predicted that the total winding of the a-strands resulting from normal b-turns and superhelical t-turns must remain constant in

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400

7.2 Terpyridine Metal Complexes with Biological Activity (b) 60

40

3

2

2 1

r/cF ( × 10−3 M−1)

r/cF ( × 10−3 M−1)

(a) 60

4 5 20

0

0

0.04

0.08

0.12 r

0.16

0.20

0.24

40

4

1

3

5 20

0

0

0.04

0.08

0.12 r

0.16

0.20

0.24

Figure 7.6 (a) Scatchard analysis for the binding of EthBr ([EthBr] ¼ 4.9–12 mM) to ct-DNA ([DNA] ¼ 3.5 mM) in a buffer with I ¼ 0.2 at pH 7.5 (line 1) and with a decreasing [DNA] : [1] ratio: 4.5 (line 2), 1.8 (line 3), 0.90 (line 4), and 0.45 (line 5) [9]; (b) Scatchard analysis for the binding of EthBr ([EthBr] ¼ 5.2–20 mM) to ct-DNA ([DNA] ¼ 5.8 mM) in a buffer with I ¼ 0.1 at pH 7.5 (line 1) and with a decreasing [DNA] : [11] ratio: 5.2 (line 2), 2.6 (line 3), 1.0 (line 4), and 0.52 (line 5) [11]. Figure reproduced with kind permission; r 1978 American Chemical Society.

(a)

(b) I

II

I 3.4 Å

10.2 Å = [Pt(tpy)(R)]+

I0

II

Figure 7.7 (a) Representation of the different topologies of closed-circular DNA with several superhelical turns (form I), no superhelical turns (I0), and nicked (II) [11]. (b) The double helix DNA (I) and neighbor-exclusion binding of the intercalator (dark areas) to the DNA (II) [46]. Figure reproduced with kind permission; r 1978 American Chemical Society, Figure reproduced with kind permission of the author.

the absence of backbone chain scission (i.e., nicking): a ¼ b þ t [47]. As observed for EthBr [48], the unwinding of closed-circular DNA (form I or I0) due to intercalation of 1 was detected by its velocity sedimentation behavior; 1 did not affect the band sedimentation of the nicked DNA (form II), which is not subject to the topological constraint [9, 11, 32]. The helix unwinding angles were calculated by viscosity titration of closed circular DNA to be 17.51 [29]. So far, general analytical tools to prove the intercalation of the PtII mono(terpyridine) complexes into DNA have been introduced; however, stereochemical

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| 327

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| 7 Terpyridine Metal Complexes and their Biomedical Relevance details to understand the real effect of the intercalation on the DNA backbone geometry cannot be derived from these studies, in particular with respect to the pucker of the deoxyribose ring. Crothers introduced a “neighbor exclusion model” that proposed that, upon saturation, every third interbase pair site will possess a bound intercalator (Figure 7.7b) [49]. The electron-rich intercalator 1 was utilized, as a labeling agent, to investigate the X-ray fiber diffraction pattern of ct-DNA; patterns strongly supported this model in which electron-rich PtII centers are evenly distributed (distance of 10.2 A ) along the backbone of DNA, corresponding to intercalation in every third interbase pair site [46]. The single-crystal X-ray structure analysis of 1 with a DNA fragment (deoxyCpG, Figure 7.8a) gave a clear picture of how the intercalation occurred by modifying the DNA backbone conformation [50]. In the crystal lattice, two cations of 1 formed a neutral complex with a dimer of deoxy-CpG, which was formed via triple hydrogen bonding of paired guanine and cytosine bases, as in the native double helical DNA. Complex 1 was located between two base pairs of the DNA fragment (the view down the a-axis is depicted in Figure 7.8b). The conformation of the deoxyribose moiety at the 30 -end of the DNA fragment possessed a C20 -endo pucker, which is the normal pucker conformation found in B-DNA.

(a)

(b)

NH2 N

HO O

N

3 5

O

C2 endo

C3 endo

O OH O P O O O

O N N

NH N

C3 endo

NH2

C2 endo deoxy-CpG

5 3

OH OH

(c)

Figure 7.8 (a) Deoxycytidinyl-(30 ,50 )-deoxyguanosine (deoxy-CpG); (b) X-ray crystal structure of [deoxy-CpG:1] complex looking down the a-axis; (c) X-ray structure of the same complex looking down the b-axis; the top base pairs are drawn in shaded solid black, 1 in the center is stippled, while the bottom base pair is unshaded [50]. Figure reproduced with kind permission (H-atoms omitted for clarity); r 1978 Nature Publishing Group.

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The deoxyribose unit at the 50 -end of the DNA fragment, however, was a C30 -endo pucker, a modified conformation that is generally found in double helical ribonucleic acid (RNA), not B-DNA. This conformational modification of the deoxyribose ring was concluded from X-ray fiber diffraction pattern, model building, and Fourier transform studies [46]. An unwinding angle of about 231 for the double helical deoxy-CpG dimer with 1 was found to be very similar to the one observed for the unwinding of ct-DNA with 1 (22.61 from X-ray fiber diffraction). A representation of the X-ray crystal structure along the b-axis clearly shows the conformational differences in the deoxyribose ring at the 30 - and 50 -end of the double helical fragment (Figure 7.8c) [50]. The guanine and cytosine bases exhibited extensive p–p interactions with 1; in particular, the O6 atoms of the guanine subunits were positioned virtually above and below the PtII center, which was sepa rated by 3.4 A from each oxygen. The intercalation of 1 into different types of DNA of various guanine-cytosine (G-C) contents was investigated by means of the binding affinity parameter (s), which was obtained by extrapolating the Scatchard plot to its y-axis [42]. Furthermore, the relative binding affinity (e) was calculated from the ratio of the binding affinities of two different types of DNA. Table 7.2 summarizes these two binding parameters. The linear relationships between different G-C contents of DNAs and binding affinities (s) of 1 expressed the preference of the PtII complex to selectively intercalate between the G-C DNA base pair (Figure 7.9). The specific binding could be explained by the stabilization of the PtII center that was sandwiched between two guanine-O6 atoms (Figure 7.8c) [50]. The relative binding affinity (e) of 2 between Micrococcus lysodeikticus (G-C content of 72%) and Clostridium perfringens DNA (G-C content of 30%) was calculated to be 2.4, displaying the same behavior as 1 (e ¼ 2.62) [31, 42]. Owing to the pioneering work by Lippard and his coworkers, the intercalation of PtII mono(terpyridine) complexes into DNA is today well-understood and, since then, new generations of such metallo-intercalators have been developed. Various PtII mono(terpyridine) complexes were reported by Rendina et al., in which 10B-containing closo-dodecarborane cages were linked to the metal center via

Table 7.2 Binding parameters of intercalator 1 with different DNAs (varying in their G-C content) [42].a

DNA

G-C content (%) KB 104 (M1) (K0 104 (M1))

Clostridium perfringens Escherichia coli Calf thymus Micrococcus luteus

30 42 51 72

5.9 7 0.6 (2.2 7 0.2) 8.5 7 0.4 (3.2 7 0.2) 10 7 2 (4.2 7 0.5) 10 7 2 (4.2 7 0.5)

s 104 (M1) eobsb (ecalc.c)

0.89 1.24 1.55 2.34

1.0 1.39 (1.40) 1.71 (1.70) 2.62 (2.40)

For all measurements: 50 mM TrisTM HCl buffer with 0.1 M NaCl at pH 7.5. Binding affinity (s) of respective DNA divided by binding affinity (s) of C. perfringens DNA. c G-C mole-fraction of respective DNA divided by G-C mole-fraction of C. perfringens DNA. a b

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| 329

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| 7 Terpyridine Metal Complexes and their Biomedical Relevance

σ (×10−3 M−1)

30

20

10

0

20

40 60 80 G-C content (%)

100

Figure 7.9 Binding affinity (s) dependence of 1 in DNA on various G-C contents [42]. Figure reproduced with kind permission; r 1979 American Chemical Society.

OAc O

AcO AcO

OAc O

OAc

AcO

15 (R tert-Bu)

O

X

OAc AcO 19 OAc (R tert-Bu)

X NHAc

AcO AcO

O OAc

OAc AcO OAc O AcO 17 (R t ert-Bu)

OAc

AcO AcO

O OAc

R

Pt X

AcO

N

X

AcO O

X

OAc O OAc

11 (R H, X Cl)

O

20 OAc (R H)

X

O OAc

AcHN O

R

N N

X

O

AcO O

X

X

16 (R tert-Bu)

O

AcO

R O

22 (R H)

AcO 21 (R H)

18 (R tert-Bu)

Figure 7.10 The (glycosylated) PtII mono(terpyridine) complexes 15–22 (counterions omitted for clarity) [56].

monothiolate bridges [51–54]. The molecular design suggested coupling the DNA intercalation and boron neutron capture therapy (BNCT) to treat cancer cells (the natural abundance of 10B is about 20%) [55]. UV–vis titration experiments of these complexes with an increasing amount of DNA showed the expected bathochromic shifts and hypochromicity as evidence for intercalation [53]; however, the titration data did not fit the Scatchard plot and, therefore, binding constants (KB) could not be determined. At concentrations higher than 13 mM, aggregation of the complexes leads to considerable deviations from Beer’s law. Ma et al. performed DNA binding studies with water-soluble, glycosylated acetylide and arylacetylide PtII mono(terpyridine) complexes 15–22 (Figure 7.10),

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OAc OAc


as possible antitumor drugs and potential luminescent probes, via binding to glycosylated biomolecules [56]. By Scatchard analysis of the UV–vis absorption data, the binding constants of 20–22 to ct-DNA were determined to be KB ¼ 4.8 105 (20), 3.7 105 (21), and 6.9 105 M1 (22). The typical hypochromic and bathochromic shifts, observed in UV–vis absorption titration experiments of 20 and 22 with DNA, suggested intercalation; however, gel mobility shift assay studies for 15 possessing bulky tert-butyl groups, as well as 21, indicated that 15 bound exclusively electrostatically to the DNA minor groove, whereas 21 bound both electrostatically and intercalatively. The ability of terpyridine metal ion complexes to intercalate into G-quadruplex DNA was recently shown by Teulade-Fichou et al. [10, 57]. The coordination geometry at the metal center governed the discrimination of G-quadruplex from double helix DNA upon intercalation. In general, square-planar PtII as well as square-pyramidal CuII mono(terpyridine) complexes preferred interaction with G-quadruplex DNA, whereas trigonal-bipyramidal ZnII or octahedral RuII mono(terpyridine) complexes interacted exclusively with double helical DNA (Figure 7.11). This behavior was unambiguously attributed to steric hindrance; therefore, p–p interaction with the external G-quartets should be favored in the former cases and impeded for the latter [10]. Furthermore, complexes 11 and 23 were utilized for the intercalation into loops of the G-quadruplex of the human telomeric (HTelo) sequence [57]. CD spectroscopy (Figure 7.12b), a fluorescent intercalator displacement (FID) assay [58], and FRET-melting analysis [59] were carried out to confirm the intercalation of the complexes into DNA. In particular, the latter two experiments demonstrated that the extension of the aromatic surface of the terpyridine ligand significantly increased the G-quadruplex affinity. Complex 23 preferentially interacted with the lower tetrad, which is surrounded by a diagonal loop; thus a supporting role of the structural element might be speculated. This interaction leads to subsequent

N

G-quadruplex DNA

RN N

R N

double helix DNA N

R N

Square-planar (e.g.PtII)

Square-pyramidal (e.g.CuII)

RN

N R N R

N

R RN

N

Trigonal-pyramidal (e.g.ZnII)

Octahedral (e.g.RuII)

Figure 7.11 Different types of coordination geometries and their preferences to bind to Gquadruplex DNA (left) and double helix DNA (right), respectively [10]. Figure reproduced with kind permission; r 2007 The Royal Society of Chemistry.

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| 331

332


(a)

10

(b)

N

N N 23

Pt Cl

CD (mdeg)

5

N

N

24

Pt Cl

N

0 250 5

270

290

10 15

double helix

G15 G14

A735G4 11

G4

G18 G613GG G713GG

Wavelength (nm)

G-quadruplex

G20

G3

G8 G9

G21

24 G22

A1335G4 23

(c)

guanine

thymine

adenine

cytosine

Figure 7.12 (a) PtII mono(terpyridine) complexes 23 and 24 (counterions omitted for clarity). (b) CD analysis of the interaction of 23 with G-quadruplex DNA (22AG, 3 mM) upon addition of increasing amounts of 23 [from 0 (black curve) to 5 equivalents (violet curve)] in sodium cacodylate buffer (10 mM) and 100 mM NaCl. The black and red arrows indicate the starting quadruplex and the final complex, respectively. (c) The [35G4/13GG] DNA system. Red arrows indicate the potential binding sites for PtII complexes. The preferred mode of platination for complexes 11 and 23 is also depicted [57]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

coordination to the most accessible position, that is, A13. In contrast, 11 with low tetrad-interacting ability coordinated preferentially to A7 (Figure 7.12c). This selectivity was proposed to be governed mainly by the predominant binding of 11 via stacking on external tetrads, which allows the trapping of the adenine in the vicinity. This study also demonstrates that the interaction efficiency of the PtII mono(terpyridine) complexes is highly dependent on the accessibility of the PtII center; the PtII complex 24 with an extended dibenzoterpyridine ligand did not coordinate to the quadruplex due to effective shielding of the PtII ion [57]. Selective intercalation of PtII mono(terpyridine) complexes 25–27 (Figure 7.13) into quadruplex DNA was also targeted by Vilar et al. [60]. Utilizing the FID assay, it was shown that strong interaction to quadruplex DNA in HTelo and the oncogene c-myc occurred; however, the selectivity for coordination to quadruplex DNA versus intercalation into double helix DNA was low. Though the terminal aminofunctionalities improved the solubility of the complexes in water and, therewith,

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310

7.2 Terpyridine Metal Complexes with Biological Activity OR

R

N N

N

Pt Cl

N

N

R= N

25

N

26

N 28

Au Cl 2

Au Cl n

N

29: R H, n 2 30: R 2-hydroxy-naphthalen-1-yl, n2 31: R NHCH2CH2SO3–, n 1 32: R PPh3, n 3

O N

N

N

27

Figure 7.13 PtII (25–27) and AuIII mono(terpyridine) complexes (28–32) (counterions omitted for clarity).

the interaction with the DNA, a blocking of the possibility to intercalate into double helix DNA (due to the flexible substituents) could not be achieved. Not only PtII mono(terpyridine) complexes but also their isoelectric AuIII counterparts 28–32 have been utilized as metallo-intercalators to DNA (Figure 7.13) [61, 62]. For 28, a binding constant to DNA (KB) of 5 103 M1 was determined, which is two orders of magnitude lower than for its PtII-analog 11 (KB E 1.3 105) [61]. Ultradialysis experiments indicated the possible electrostatic binding of 28 to ct-DNA as well as intercalation. More evidence for the intercalation arose from an induced CD effect of 28 with DNA and an increase of the melting temperature of ctDNA in its presence (DTm ¼ 12.4 1C). The effects of bulky substituents (29 and 30) and the overall charge (31 and 32) on the binding properties of the AuIII complexes to DNA were also studied [62]. Intercalation of 29–32 into DNA was mirrored by hypochromicity and bathochromic shifts in UV–vis titration experiments, an induced CD effect, and competitive fluorescence spectroscopy (CFS) measurements with EthBr, as reference. Apparently, smaller substituents R favored the replacement of EthBr via intercalation; the decreased fluorescence intensity of EthBr with DNA gave the order 29 Z 31 W 32 W 30. The intensity decrease in CD spectral shifts in the presence of increasing amounts of AuIII complex, however, follows the order 32 W 29 W 31 W 30, suggesting that increasing the overall charge favors binding. The UV–vis absorption and CD spectra of 29–32 gave similar results, reflecting both steric and electrostatic effects of the chemical groups on the binding, whereas CFS mainly illustrated the effect on intercalation. Furthermore, in vitro DNA binding studies of AuIII complexes 29 and 32 were performed by incubation with human epithelical kidney cells (293T). After 12 h of incubation, the DNA was isolated from the cells and a concentration of 207.8 mg ml1 was determined. Inductively coupled plasma (ICP) mass spectrometry revealed that the isolated DNA contained 18.9 ng ml1 of 29 and 24.9 ng ml1 of 32, corresponding to an intercalation degree of one AuIII ion per 6400 and 4900 nucleotides, respectively.

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| 333

334

| 7 Terpyridine Metal Complexes and their Biomedical Relevance N

N N

N N

N

Pt S

N

(CH2)n S 2 Pt N N

Pt S

S N

N

N

2

Pt

S

N

N

N 33-39 (n 4-10)

N

Pt S

Pt

2 N

N 40

41

Figure 7.14 Dinuclear PtII mono(terpyridine) complexes 33–41, as potential bis-intercalators for DNA (counterions omitted for clarity). Table 7.3

Binding parameters of mono-intercalator 2 and bis-intercalators 33–39.

Entry

HETa

L/L0 (A )

Unwinding angle

KB 104 (M1)b

eobsc

2 33 34 35 36 37 38 39

0.60 0.73 1.12 1.13 1.14 0.83 1.13 0.92

2.0 2.5 3.8 3.8 3.9 2.8 3.8 3.1

17.51 31.11 31.71 36.01 32.01 23.41 25.21 22.91

0.84 3.0 N.a. 19 N.a. N.a. N.a. N.a.

2.4 1.1 1.4 1.3 1.7 1.2 1.4 1.2

a

HET: helix-extension parameter. Measured in a 2 mM Hepes/KOH buffer with 0.5 M KF at pH 7.0. c Relative binding affinity of the complexes between Micrococcus lysodeikticus (G-C content of 72%) and Clostridium perfringens DNA (G-C content of 30%). b

A series of dinuclear DNA intercalators 33–41 was synthesized by linking two PtII mono(terpyridine) units either through a,o-dithiols of the type HS-(CH2)n-SH (n ¼ 4–10) or phenylenedi(methanethiol)s (Figure 7.14) [29, 31, 35]. Intercalation of these dinuclear complexes was shown by bathochromic shifts and hypochromicity in UV–vis titration experiments, induced CD effects, and increased viscosity as well as melting temperatures for ct-DNA. Moreover, helix-extension parameters (HETs) were calculated for 33–39 from the plot of the relative contour length (L/L0) versus the complex/nucleotide (C/P) or the binding ratio (r) [29]. The unwinding angles were determined by viscosity titration measurements with closed-circular DNA. Comparing the HET values and unwinding angles of bis-intercalators 33–39 to those of mono-intercalator 2 suggested that 34, 35, and 38 showed mainly bis-intercalation, whereas 36 and 37 displayed a mixture of both mono- and bisintercalation (Table 7.3). The obtained data did not allow a definitive assignment of

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[q]/104 degcm2moI1

(a)

| 335

(b) 40

Bisintercalation of 40

41

Mono-intercalation of 41 S S

200

240

280

320 200

240

280

320

Wavelength (nm)

S S

S S

= [Pt(tpy)]+

Figure 7.15 CD spectra (a) and proposed binding modes (b) for dinuclear PtII complexes 40 and 41 with ct-DNA [35]. Figure reproduced with kind permission; r 2003 Elsevier B.V.

the binding mode for 33 and 39. Binding constants were calculated only for 2, 33, and 35 [31]. The relative binding affinity (e) for the dinuclear complexes revealed a reduced preference for binding to the G-C base pair of DNA in comparison to 2. The xylene-bridged dinuclear PtII complexes 40 and 41 were also utilized, as bisintercalators [35], and the CD spectra displayed a normal induced effect for 41 and its mononuclear counterpart 9 (Figure 7.15a). Thus, intercalation of only one PtII mono(terpyridine) side of this dinuclear complex was concluded (Figure 7.15b). In contrast, a negative Cotton effect was observed for 40, suggesting that the DNA was distorted (presumably by bis-intercalation as depicted in Figure 7.15b). The first hetero-dinuclear complexes containing both a square-planar PtII mono(terpyridine) and an octahedral RuII bis(terpyridine) complex, connected through a flexible diethylene glycol ether linker, was reported by van der Schilden et al. [63]. X-Ray single-crystal structure analysis of 42 revealed an intermolecular stacking between the PtII mono(terpyridine) moieties despite the bulky [Ru(tpy)2]2þ groups; the PtII centers were infinitely stacked in a head-to-tail fashion with alternating short and long Pt Pt distances (3.49 and 6.7 A ) (Figure 7.16) Complex 42 intercalated and covalently bound to DNA (as shown by binding studies to 9-ethylguanidine), even though the complex contained the large [Ru(tpy)2]2þ tail that could also electrostatically bind to DNA. Several related hetero-dinuclear complexes 43–45 with a square-planar PtII mono(terpyridine) moiety connected to a second planar half-sandwich IrIII-dppz moiety through short peptide linkers were synthesized as potential bisintercalators (Figure 7.17) [64, 65]. Dipyrido[3,2-a:20 ,30 -c]phenazine (dppz) is a well-known intercalator that is widely used, for example, as a ligand in RuII metallo-intercalators [2]. The UV–vis absorption spectra of 43 (short peptide linker) as well as 44 and 45 (longer peptide linkers) in the presence of DNA revealed that only mono-intercalation to DNA occurred selectively from the IrIII-dppz end; the hypochromic and bathochromic shifts at 382 nm corresponded to the dppz and not the terpyridine [64]. Binding constants (KB) of 3.3 106 M1 (43) and 1.4 106 M1 (44 and 45) were calculated by UV–vis titration experiments at

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336

| 7 Terpyridine Metal Complexes and their Biomedical Relevance (a)

3 N N

N Ru

N

O O

N

N

42

N O

N P t Cl N

(b) N111 N141

N221

CI1 PI1

N121

N231

N211 Ru1 N151

N131 O3 O2

O1

N161

Ru Pt

Pt Ru

Figure 7.16 (a) A RuII-PtII mixed-metal dinuclear complex (counterions omitted for clarity); (b) top: representation of the X-ray single-crystal structure of 42 (H-atoms, counterions, and solvent molecules omitted for clarity); bottom: representation of the packing of 42 in the crystal where alternating short and long Pt. . .Pt distances could be observed [63]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

382 nm, which were in agreement with the non-cooperative, non-specific binding model, according to Bard [66], Smith [67], and their coworkers. Ligand 46 was reported to self-assemble into the hairpin-like structure 47 by wrapping itself around a lanthanide(III) metal ion (i.e., NdIII, EuIII, or LaIII) and connecting to two equivalents of 11 via its thiol ends (Scheme 7.1) [68]. The trinuclear complex 47 in which LnIII ¼ NdIII showed the characteristic NIR emissions at 1060 and 1340 nm for NdIII ions with an excited-state lifetime of 670 ns. Upon excitation at 515 nm, the relative quantum yield of 47 remained constant

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4

N N

N Ir

N

N Ir

N

S

O

4

N

N

S

N

O

HN

N

HN NH2

Pt

NH NH2

N R1/R2

HOOC

HOOC

3

N

O R 44: R1, R2, R3 CH3 45: R1 CH2-Ph, R2 H, R3 H

43

Pt

N

2

N

Figure 7.17 IrIII-PtII mixed–metal dinuclear complexes 43–45 (counterions omitted for clarity) [64, 65].

HS

SH NH O HOOC

N

N

HN

N

46

N

+ 2 Cl Pt

O COOH

N

N 11

COOH

+ LnCl3

LnIIIion

(tpy)Pt-unit

47

Scheme 7.1 Synthesis of the LnIII-PtII mixed-metal trinuclear complex 47 (LnIII ¼ NdIII, EuIII, or LaIII; counterions omitted for clarity) [68]. Figure reproduced with kind permission; r 2003 American Chemical Society.

after intercalation into ct-DNA. Thus, the LnIII unit could be utilized as a luminescent “reporter” group, with the LnIII being encapsulated by the aminocarboxylate binding site of 46, which did not interact with DNA. Intercalation of 47 into ct-DNA was further investigated by flow linear dichroism (LD) spectroscopy [69], revealing that the complex bis-intercalated into DNA with a binding constant of KB ¼ 9.5 106 M1. Owing to the high degree of intercalation, a pronounced stiffening of the DNA structure was concluded from the LD experiments [68]. Williams et al. used artificial oligopeptides that were functionalized with pyridine moieties, as designated binding sites for the connection to [(tpy)Pt]2þ units. According to their approach, oligonuclear PtII mono(terpyridine) complexes 48–52 were obtained (Figure 7.18a) [71, 72]. The authors assumed that the four tethered

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| 337

338


(b) N N

Pt2+ N N

48

2 ClO4

O

O N H

12-bp ds-DNA

O N

48 (n = 1) 49 (n = 4) 50 (n = 5) 51 (n = 6) 52 (n = 10)

n

N H

NH2 O

Figure 7.18 (a) Oligopeptides with tethered PtII mono(terpyridine) complexes (48–52); (b) space filled molecular model of 48 (left) and the DNA fragment [70]. Figure reproduced with kind permission; r 2005 American Chemical Society.

PtII complexes on the backbone of the peptide chain, as in 49, which resembled nucleic acids on the DNA sugar-phosphate backbone, would increase the binding affinity. Consequently, a double-stranded (ds) DNA fragment consisting of 12 base pairs (bp) was chosen, as a model, for the binding studies; its sequence was 50 CGT-GAC-CAG-CTG-30 (G-C content of 75%, Figure 7.18b) [70]. The high G-C content was chosen to improve the hybridization efficiency and circumvent hairpin formation. The binding constants (KB) were calculated by isothermal titration microcalorimetry (ITC), which revealed that two tetrapeptides 48 bound to each 12-bp ds-DNA with a KB of 1.7 106 M1 and possessing 0.67 PtII centers per base pair. For comparison, ITC binding studies of 8 with the same DNA fragment gave a KB of 2.5 104 M1 and 0.16 PtII centers atoms per base pair. Thus, the binding affinity of 48 to the DNA was significantly higher compared to the mononuclear complex 8. Moreover, the CD spectra as well as thermal denaturation experiments of 48 with DNA confirmed the formation of a (48)2:(12-bp ds-DNA) adduct by an increase in the melting temperature from 60 to 85 1C (i.e., DTm ¼ 25 1C) [70], which is about twice the effect that the mono-intercalators 6–8 had on the melting behavior of ct-DNA (DTm E 12 1C, see also Table 7.1) [33]. 7.2.1.1.2 Covalent Binding to Small-Molecule Biomolecules Intercalation of PtII mono(terpyridine) complexes into DNA represents an attractive route towards new types of antitumor drugs, comparable to the wellknown cisplatin [73–75]. Beside the high potential with respect to biomedical applications, the limitations of PtII complexes, as drugs, are evident: administering the metallo-drug by injection or infusion could lead to undesired reactions of the PtII center with sulfur-containing biomolecules in the blood by formation of a stable PtII–S bond prior to reaching the targeted DNA, to intercalate or to covalently react with the guanine base to interrupt its functions [76–78]. To gain more insight into the reactivity and mechanisms of interaction for some of the biomolecules with PtII-containing drugs, various sulfur- and

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nitrogen-containing amino acids, short peptides, small biomolecules, nucleic acids, ribonucleosides, and ribonucleotides were investigated for their covalent binding abilities and kinetics with [(tpy)PtCl]þ (11) and [(tpy)Pt(H2O)]2þ (53). In these studies, the square-planar PtII mono(terpyridine) complexes were chosen, as model compounds, for two reasons: (i) they contain labile leaving groups (e.g., Cl or H2O) and (ii) have lower pKa values than similar complexes of tridentate N4N4N-ligands with PtII ions (e.g., 53: pKa ¼ 4.5) [79–81]. According to Kostic´ and coworkers [79–82], only three chemical moieties in all of the proteinogenic amino acids were found to react with 11: the thiol in cysteine (Cys), the imidazole in histidine (His), and the guanidine in arginine (Arg). Binding of 11 to biomolecules containing these amino acids was confirmed by NMR (1H and 195Pt), UV– vis absorption spectroscopy [new metal-to-ligand charge-transfer (MLCT) bands between 300 and 350 nm could be observed], and mass spectrometry. Kinetic studies showed that biomolecules containing thiol functionalities [i.e., Cys and the Glu-Cys-Gly tripeptide glutathione (GSH)] reacted under similar conditions about 300 times faster than the imidazole-containing His, His-His, or Gly-His-Gly (Gly: glycine) [81]. Treatment of an equimolar mixture of GSH and Gly-His-Gly with 11 revealed that the [(tpy)Pt]2þ moiety was exclusively bound to the thiol-group of GSH. The PtII mono(terpyridine) complex 54 (i.e., His-bound) reacted by nucleophilic substitution with the Cys (56) to yield complex 55 (Scheme 7.2) [79]. The PdII mono(terpyridine) complex 14 also was shown to strongly bound to the thiol in Cys [83]. The reaction kinetics and rate constants of 11 and 14 with Cys, GSH, and D-penicillamine were reported [84, 85]. In general, due to their size and the lower pKa values, thiols were much more reactive towards 11 than imidazole. Appleton et al. suggested that the PtII mono(terpyridine) complexes could bind to both positions of the imidazole moiety in His and N-acetyl-His: N1 (major) and N3 (minor) [86]. Various guanidine-containing biomolecules (e.g., methylguanidine, Arg, and N-acetyl-Arg; pKa values of 13.5, 12.5, and 12.5, respectively) were reacted with 11 under relatively harsh conditions (i.e., high temperatures and/or mildly basic). As an exception, canavanine (pKa ¼ 7) could be reacted with 11 already under neutral conditions [82]. Both Arg and canavanine with 11 formed yellow mono- (57 and 58) as well as red dinuclear complexes (59 and 60), respectively (Figure 7.19a). X-Ray singlecrystal analysis of 60 revealed that the interplanar distance between the two [(tpy) Pt]2þ moieties, linked via a guanidinyl group, was about 2.8 A , suggesting that the d-d

N N

NH2

Pt N N

54

NH

COOH

HS

N

56 NH2

Pt S

N

COOH HN

NH2 COOH

NH2 NH

COOH

N

55

Scheme 7.2 An SN-reaction of 54 with cysteine (56) giving 55 (counterions omitted for clarity) [79].

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| 339

340

| 7 Terpyridine Metal Complexes and their Biomedical Relevance N Pt

N

NH

3

N N N

Pt

2

N H2N

(a)

NH2

2.8Å N

NH NH X

57 (X = C) 58 (X = O)

N NH2

N

COOH

P t NH X 59 (X = C) 60 (X = O)

NH2 COOH

(b)

60

Figure 7.19 (a) Mono- and dinuclear PtII mono(terpyridine) complexes with Arg (X ¼ C, 57 and 59) and canavanine (X ¼ O, 58 and 60) as ligands (counterions omitted for clarity); (b) X-ray single-crystal structure of 60 (H-atoms and counterions omitted for clarity) [82]. Figure reproduced with kind permission; r 1990 American Chemical Society.

orbital interaction between the PtII centers as well as p–p interaction between the terpyridine systems contributed to the low energy absorption bands (Figure 7.19b). Substrates containing a R2S-unit [e.g., methionine (Met) and its derivatives (i.e., amides and esters), cystine (Cyst), S-methylcysteine (MeCys), oxidized glutathione or the tetrapeptide Trp-Met-Asp-Phe] did not exhibit any reactivity towards complexes 11 and 14, even under harsh conditions (i.e., 100 1C, tenfold excess of substrate) [80, 83]. Some sulfur-containing biomolecules (e.g., thiourea, diethyldithiocarbamate (DEDTC), thiosulfate, Cys, GSH, and penicillamine) showed a high tendency to react with 52 (under neutral conditions) in the presence of some nucleosides [76, 78, 87]. Figure 7.20 presents an overview of reactive and unreactive amino acids and short peptides with respect to the PtII and PdII mono(terpyridine) complexes 11 and 14. The reaction kinetics and rate constants (k1) of thiourea, Cys, GSH, and PCA were determined under acidic conditions (pH 1) to keep the nucleophiles fully protonated to circumvent reactions in the N- or O-position [78]. The order of reactivity (i.e., PCA o Cys o GSH o thiourea) towards [(tpy)Pt(H2O)]2þ (53) was confirmed by comparing their pseudo-first-order rate constants (kobs in s1) [76, 87]. Finally, guanine-containing nucleosides and nucleotides 61–64 (Figure 7.21) were attached to the [Pt(tpy)]2þ moiety via covalent binding at the N7-position of the guanine base, which was proven by X-ray single-crystal analysis (see Figure 7.21 for the structure of [(tpy)Pt(63)]2þ) as well as by detailed mass spectrometric analysis [76, 88]. In contrast, adenosine (65) and cytidine (66) were covalently bound to the PtII mono(terpyridine) complex both mono- and difunctionally in N1,N6- and N3,N4-positions, respectively, which was proven by NMR (1H and 195Pt) and mass spectrometry [88,89]. Additionally, the X-ray crystal structures of mono- and dinuclear containing PtII and PdII mono(terpyridine) complexes with N-methylcytosine gave further evidence that the binding sites of 66 were indeed the N3,N4-positions [91]. What are the lessons learned from these considerations concerning the covalent binding of PtII mono(terpyridine) complexes to small biomolecules?

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| 341

Reactive biomolecules S

S N H2 N

NH2

S diethyldithiocarbamate (DEDTC)

thiourea(tu)

NH2 HOOC

HN N

2

O

HN

H N

COOH

NH2

HN

histidine-histidine (His-His)

HOOC COOH

NH2

histidine-lysine (His-Lys)

N H

COOH

NH N glycine-histidine-glycine (Gly-His-Gly)

histidine (His)

HN

imidazole (im) O

H N O

H2N

O

N

penicillamine (PCA)

H2N N

N

NH COOH

NH2

NH2 cysteine (Cys)

COOH

H N

COOH

HS

HN

H2N

N

COOH

HS

O

N H O SH gluthathione (GSH, γ-Glu-Cys-Gly)

HN

O S S O O thiosulfate (sts)

X

H N

NH

NH2

NH arginine(X = C,Arg) canavanine(X = O,Can)

H2N NHR guanidine(R = H, gua) methylguanidin (R = Me,MeGua)

Unreactive biomolecules S H2N NH2 NH2 S

COOH S-methyl-cysteine (MeCys)

S

COOH

HOOC

S

S

Cystine (Cyst)

H N

N H

O

N COOH H COOH tryptine-methionine-asparagine-phenylalanine (Trp-Met-Asp-Phe)

HN NH2

NH2 methionine (Met)

COOH

O

O

Figure 7.20 Reactive and unreactive biomolecules with respect to [(tpy)PtCl]þ (11) and [(tpy) PdCl]þ (14) [76, 78–82, 87].

In double-stranded DNA, the N1,N6- and N3,N4-positions of adenosine and cytidine are involved in H-bonding and, therefore, PtII mono(terpyridine) complexes can be attached to the guanine base at its N7-position. Moreover, nucleoside ligands can easily be replaced by the more nucleophilic thiol-containing molecules (e.g., thiourea, DEDTC, GSH, Cys, sts) and a subsequent replacement by ribonucleotides cannot occur [76]. In general, the reactivity of the square-planar complexes [(tpy)PtCl)]þ (11) and [(tpy)Pt(H2O)]2þ (53) with biomolecules could be attributed to the p-electron accepting effect of the PtII center and electronic communication between the pyridine rings of the terpyridine system [92, 93]. 7.2.1.1.3 Cytotoxicity The intercalation of PtII mono-complexes into the DNA and their (covalent) binding to biomolecules (e.g., peptides and enzymes) has been studied intensively. Of these two types of interaction, intercalation with DNA or enzymes will induce morphology deformations and, consequently, lead to a malfunction of these biomolecules and eventually to cell destruction. Various types of planar dyes – such as

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342

| 7 Terpyridine Metal Complexes and their Biomedical Relevance O N

RO O

7

NH

N

RO

N

O

61 (R = H) 62 (R = PO3H)

OH

OH

N

NH2

1

N

N

NH N

HO

NH2

63 (R = H) 64 (R = PO3H)

C4 C3

C8 C9

C6 C5 N2

C2

X O P O O

C10 C11

N

HO

N

O 3 1

N 67 (X = O) 68 (X = S)

X O P O O

NH O

N

OH OH

C1 O1 C19 N7

C17

C18

C25

N6 N8 [(tpy)Pt(63)]2+

O5

N5 O2 C21 C24 C22 03

C23 04

Figure 7.21 Nucleosides and nucleotides 61–70 as binding partners for [(tpy)M]2þ moieties (M ¼ Pt or Pd) [76, 80, 81, 87–90]. The crystal structure of the complex [(tpy)Pt(63)]2þ is also shown [76]. Figure reproduced with kind permission; r 2004 The Royal Society of Chemistry.

dactinomycin, adriamycin, ellipticine, bleomycin, and their analogues – that can intercalate into DNA were clinically used as antitumor and antiprotozoal drugs [94]. Besides these solely organic intercalators, various square-planar PtII, PdII, and AuIII metallo-intercalators were also investigated in vitro and in vivo, as antitumor and antiprotozoal drugs. Chemotherapeutic Agents In 1985, McFadyen et al. reported the first detailed cytotoxicity study of various PtII mono(terpyridine) complexes (11, 71, and 72, see Figure 7.22) against L1210 murine leukemia cells in both culture and mice [95]. To determine their cytotoxicity, L1210 cells were incubated with these complexes at 37 1C; after two days, cells were counted on a Coulter counter. The IC50 (IC50: half maximal inhibitory concentration) value, which is the concentration required to inhibit the growth of cells by 50%, was determined by plotting the cell growth as a percentage of control versus the drug concentration. For complexes 71 and 72, the IC50 values against L1210 lines were in the range of 4–32 mM; however, 11 exhibited an IC50 of 450 mM against L1210, suggesting the possible covalent binding to other biomolecules prior to reaching the targeted DNA. Moreover, 11 showed enhanced cytotoxicity against MCF-7 breast cancer epithelial cells (IC50 of 25 mM in vitro) when compared to L1210, but it was not as good as cisplatin, which

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66

OH OH

69 (X = O) 70 (X = S)

C12 C13

O

N

O

NH2

N O

N N

2

N3

1

65

OH

OH OH

N1 Pt1 N3 C14 C15 N4 C20 C16

N

O

6

C7

4NH

6

O 7

1

O


N

N R

N

Pt S

N 71

N

N

N R1

N

Pt N

N 2

Pt

N

N

L n

N R1

N

Pt

S R2

N

n

R2

73a (R1 H, R2 H) 73b (R1 H, R2 4-Me) 73c (R1 H, R2 4-Br) 73d (R1 H, R2 4-MeCO) 73e (R1 H, R2 4-NMe2) 73f (R1 H, R2 2-F) 73g (R1 H, R2 3-F) 73h (R1 H, R2 CH2OH)

74a (R H, L CH3CN, n 2) 74b (R H, L thiazole, n 2) 74c (R H, L imidazole, n 1) 74d (R H, L NH3, n 2) 74e (R Cl, L NH3, n 2) 74f (R Cl, L OH, n 1) 74g (R 4-Br-phenyl, L NH3, n2)

N R

Pt S

72a (R H) 72b (R 2-OCH3) 72c (R 3-OCH3) 72d (R 4-OCH3) 72e (R 4-NO2)

72f (R 4-F) 72g (R 4-Cl) 72h (R 4-Br) 72i (R 4-CH3) 72j (R NH3)

73i (R2 4-Me, R1 F) 73j (R2 4-Me,R1 Cl) 73k (R2 4-Me, R1 Br) 73l (R2 4-Me, R1 MeO) 73m (R2 4-Me, R1 N(CH2CH2OH)2) 73n (R2 4-Me, R1 NMe(CH2CH2OH)) 73o (R2 4-Me, R1 NH(CH2CH2OH)) 73p (R2 4-Me, R1 NH2) 73q (R2 4-Me, R1 NHNH2) 73r (R2 4-Me, R1 NMeNH2) 73s (R2 4-Me, R1 4-Me-phenyl) 73t (R2 4-Me, R1 4-Br-phenyl) 73u (R2 4-Me, R1 pyridin-2-yl)

75a (R1 Cl, R2 CH2CH2OH) 75b (R1 EtO, R2 CH2CH2OH) 75c (R1 EtO, L imidazole, n 1) 75d (R1 Cl, R2 pyridin-4-yl) 75e (R1 Cl, R2 pyridin-2-yl) 75f (R1 = Cl, R2 = pyrimidin-2-yl)

Figure 7.22 Various PtII mono(terpyridine) complexes (71–75) as potential antitumor agents (counterions omitted for clarity) [95–98].

has an IC50 of 5.6 mM in vitro against MCF-7 [99]. The potential antitumor complex 72a (IC50 of 4 mM in vitro) was also investigated in vivo against L1210 in mice; however, it lacked any antitumor activity [95]. Surprisingly, some of the free terpyridine ligands displayed cytotoxicity with IC50 values of around 2 mM against L1210 – even higher than the corresponding PtII complexes – suggesting that a free terpyridine ligand may either induce metal-deficient states or form metal complexes in the media that can then inhibit cell growth. The mono- (2) and dinuclear intercalators (33–39), all having flexible thioalkyl chains, displayed in vitro cytotoxicity against L1210 cells with IC50 values in the range 4–14 mM, suggesting that cytotoxicity is independent of the intercalator character [100]. Moreover, these complexes produced extensive cell lysis,

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| 343

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| 7 Terpyridine Metal Complexes and their Biomedical Relevance N

R

1

N

N spacer

Pt N N

N Pt

N

N

R

N

H N

H N O

N

R1 H or Cl

R

N

76

4

spacer 1

NH3 Pt N NH3

R

O 4

N

N

N

Pt

Pt

N

N

R1

N

N

N 77a (R H) 77b (R CH3)

R1

Figure 7.23 Dinuclear PtII mono(terpyridine) complexes 76 and 77 according to Lowe et al. (counterions omitted for clarity) [97].

suggesting that they might be only effective on the cell membrane but did not reach the cell nucleus to intercalate into DNA. Owing to the high intercalative binding affinity [35] and the ability for covalent binding to DNA [88, 89] of PtII mono(terpyridine) complex [(tpy)Pt(pyridine)]2þ (6) and its derivatives, Lowe and coworkers investigated their cytotoxic properties against parasites [96] and cancer cells [97]. Thus, various mononuclear (73, Figure 7.22) as well as dinuclear (76 and 77, Figure 7.23) PtII complexes were investigated for their potential to be antiprotozoal and antitumor agents; their results were compared to the common cisplatin and carboplatin drugs. Similar mononuclear PtII complexes with U-shaped terpyridine ligands were also reported, but intercalation or cytotoxicity studies thereof are unavailable [101]. In particular, several mono- and dinuclear complexes 74–76 were investigated for their in vitro cytotoxicity against five human ovarian carcinoma cell lines [97]: CH1, cisplatin-resistant CH1cisR, doxorubicin-resistant CH1doxR, A2780, and cisplatin-resistant A2780cisR cell lines; the SKOV-3 cell line was also included, since it is one of the most resistant to known Pt drugs. The cells were incubated with the metallo-drugs for four days and, subsequently, the IC50 values were calculated (Table 7.4). The most effective complexes against human ovarian carcinoma cells in vitro were found to be the dinuclear PtII species with short and rigid spacers (e.g., 76 with a trans-vinyl or butadiyne linkage), which were slightly more effective than cisplatin against cisplatin-resistant lines (i.e., CH1cisR and A2780cisR). In contrast, the dinuclear complex 77a with a flexible linker showed relatively low cytotoxicity when compared to its rigid counterparts 76 (Table 7.4). Various mononuclear complexes, such as 73c, exhibited a significant cytotoxicity against human ovarian carcinoma cells; however, complexes with bulky and electron-donating substitutes on the terpyridine (e.g., 73n) showed a considerably reduced antitumor activity.

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7.2 Terpyridine Metal Complexes with Biological Activity IC50 values (mM, after 4 days) for the in vitro growth inhibition of human ovarian cell lines by mono- and dinuclear PtII mono(terpyridine) complexes.

Table 7.4

Complex

CH1

CH1cisR

RFa

CH1doxR

RFa

A2780

A2780cisR

RFa

SKOV-3

Cisplatin Carboplatin 11 73c 73n 76ab 76bc 76cd 77a

0.4 6.2 6.6 2.1 W100 1.35 0.73 0.55 48

1.2 14 6.4 2.1 W100 0.63 0.73 0.81 42

3.0 2.3 1 1 N.a. 0.46 1 1.5 0.9

0.5 6.0 3.75 0.85 17.5 5.1 0.44 0.42 40

1.2 1.0 0.6 0.41 N.a. 3.8 0.6 0.8 0.8

0.53 35 49 5.8 40 1.6 2 13.5 19

8.8 W100 41 6.7 W100 2.4 1.8 20.5 40

16.6 N.a. 0.8 1.16 N.a. 1.5 0.9 1.5 2.1

2.25 W100 19.5 9.2 W100 1.3 1.7 1.7 9.8

a

Resistance factor: IC50 of the resistant line divided by IC50 of parent line. Spacer: trans-vinyl, R1 ¼ H. c Spacer: butadiyne, R1 ¼ H. d Spacer: Phenyl-Pt(NH3)2-Phenyl, R1 ¼ Cl. b

Table 7.5 IC50 values (mM) of carborane-containing complexes 78–84 against selected cancer cell lines [52, 54].

Complex

L1210

L1210cisR

2008

C13cisR

Cisplatin 78 79 80b 80d 82 83 84a 84b

0.5 N.a. N.a. 1.6 N.a. 0.9 7.4 24.5 26.5

6.9 N.a. N.a. 0.9 N.a. 0.8 10 5.3 7.0

0.6 4.6 26 1.7 5.3 N.a. N.a. N.a. N.a.

2.1 5.1 21 2.1 4.1 N.a. N.a. N.a. N.a.

Bulky substituents were believed to prevent intercalation [97]. Further, the structurally simple complexes 11, 53, and 74d were less cytotoxic than other mononuclear PtII complexes due to their covalent binding affinity towards other biomolecules (Section 7.2.1.1.2). The carborane cage-containing mononuclear (78–81) and dinuclear complexes (82–84) were investigated against L1210 murine leukemia cell line, its cisplatin resistant variant L1210cisR, the 2008 human ovarian cell line, and its cisplatin resistant variant C13cisR (Figure 7.24) [52, 54]. The mononuclear 80b displayed a significant in vitro cytotoxicity against these cell lines when compared to cisplatin (Table 7.5). Moreover, 84 displayed remarkable cytotoxicity in vitro against L1210 and L1210cisR (Table 7.5) [52]. The poor cytotoxicity of all other dinuclear

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| 345

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| 7 Terpyridine Metal Complexes and their Biomedical Relevance H

H N S

N S 1,7-carborane 78

Pt N

N

n

H Pt

N

S

N

N

Pt N

1,2-carborane 80a (n 0) 80c (n 2) 80b (n 1) 80d (n 3)

1,12-carborane 79

N

OH OH N

O

N Pt

N

N

S S

Pt

N

N N

N Pt N

N

1,2-carborane-glycerol 81

S

S

Pt

N

N 82 N S

Pt

N

N

N

83 N

Pt N

S n

84a (n 1) 84b (n 3)

n S

N Pt N

Figure 7.24 Carborane-containing mononuclear (78–81) and dinuclear (82–84) PtII complexes [52–54].

complexes and 79 was attributed to their low solubility under the physiological conditions [52, 54]. Since various common planar dyes (e.g., EthBr, acriflavine, ellipticine, and bleomycine) exhibited activity against Typanosoma and Leishmania parasites, Lowe and coworkers also studied the antiprotozoal activity of their mono- and dinuclear complexes 72–77 (Figures 7.22 and 7.23) against Leishmania donovani, Typanosoma cruzi, and Typanosoma brucei, which are the causes for leishmaniasis, Chagas disease, and sleeping sickness, respectively. The reported inhibition percentages of selected complexes against L. donovani and T. cruzi parasites in vitro are summarized in Table 7.6; notably at all concentrations (30 to 1 mM), 100% inhibition against Trypanosoma brucei parasites was observed [96]. The first generation of PtII mono(terpyridine) complexes, as drugs for parasites, revealed that complexes 53 and 74d were effective against L. donovani and T. cruzi, whereas 73b/c, and 74e performed better against T. brucei [96]. The second-generation PtII

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N

7.2 Terpyridine Metal Complexes with Biological Activity Table 7.6

Percent inhibition of selected complexes in vitro against parasites [96].a L. donovani

Complex

11b 53b 74db 73ab 73jc 73kc 74ed 74gd

T. cruzi

T. brucei

30 lM

10 lM

3 lM

1 lM

30 lM

10 lM

3 lM

1 lM

30 lM to 1 lM

99.1 T/100 96.1 99.5 93.3 100 T/100 T/100

94.9 T/100 91.7 92.2 89.5 92.8 T/100 T/100

22.0 96.5 27.5 0 47.9 5.4 99 T/100

0 2.0 5.0 0 2.5 0 99 100

99.3 23.8 100 27.2 100 T/þ T/100 T/100

99.0 0 100 0 97.0 T/þ T/100 T/100

76.0 0 72.3 0 73.5 T/þ T/100 T/100

52.0 0 0.5 0 59.7 T/0 50.8 64.5

100 100 100 100 100 100 100 100

a T/100: toxic to macrophages, 100% inhibition; T/þ: toxic to macrophages, parasites still present. b First-generation complexes. c Second-generation complexes. d Third-generation complexes.

mono(terpyridine) drugs were more effective than the earlier examples against these types of parasites. Among these, the complexes 73s/t, 73j/s, and 73j/k/s were the most effective against L. donovani, T. cruzi, and T. brucei, respectively. Thirdgeneration PtII mono(terpyridine) drugs were designed by considering the former results and gave the best inhibition percentages so far in vitro against these parasites. For instance, 74e/g displayed outstanding antiprotozoal activities (Table 7.6). Against L. donovani these complexes were even more effective than that from the first or second generation; for T. cruzi they displayed comparable toxicity and for T. brucei they caused complete inhibition at concentrations of >0.003 mM. Kinetic and spectroscopic studies revealed that complexes 73 and 74 irreversibly bound to the Cys-52 residue of the trypanothione reductase (TR) enzyme of T. cruzi and eventually inhibited its function, which significantly contributed to their antiprotozoal activities in addition to the effect caused by intercalation into DNA [98]. In contrast to the parasite enzyme, most PtII mono(terpyridine) complexes interacted reversibly with human glutathione reductase (GR), which is similar in structure and function to the TR enzyme. Moreover, an irreversible inhibitor (85, with an 9-aminoacridine dye attached to the terpyridine system at the 40 -position; Figure 7.25) displayed the same inhibition behavior as 73 and 74 against T. cruzi TR and human GR [102]. The human thioredoxin system, containing the 12-kDa protein thioredoxin (hTrx) and the seleno-enzyme thioredoxin reductase (hTrxR), was involved in thiol-mediated antioxidant defense and redox regulatory processes, including transcriptional control, DNA synthesis, and apoptosis, thus, supporting cell proliferation. Many tumor cells have been reported to possess increased Trx and TrxR and they can release the TrxR enzyme to stimulate autocrine cell growth.

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| 347

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| 7 Terpyridine Metal Complexes and their Biomedical Relevance Cl

N HN

N (CH2)n H X 2 NO3

O

N

Pt

85a (X S, n 2) 85b (X O, n 2) 85c (X O, n 4) 85d (X none , n 6)

S

N

OH

Cl

Cl N

N N

Pt

N

N N

N

86

S N

Pt S

N

Pt

S

H N

N

Pt

N

N 87

N

88

N II

Figure 7.25 Pt mono(terpyridine) complexes 85–88 (counterions omitted for clarity) [102, 103].

Table 7.7

IC50 (mM) values of 87 and 88 against glioblastoma cell lines [103, 104].

Complex

87 88

Tumor cell line NCH37

NCH87

NCH89

HNO97

HNO199

C6

10.5 5.7

7.4 3.9

2.5 2.5

5.5 4.8

9.2 6.2

3.5 N.a.

Thus, inhibition of TrxR could selectively induce death of fast growing cancer cells. Becker et al. reported on the effective inhibition of the hTrxR enzyme of complexes 86–88 (Figure 7.25), either by their reversible competitive or irreversible tight-binding to the enzyme structure [103]. Effective in vitro cytotoxicity with remarkable IC50 values against the glioblastoma cell lines NCH37, NCH87, NCH89, HNO97, HNO199, and C6 was observed (Table 7.7) [103, 104]. The effects of the potent hTrxR inhibitors 86 and 87 on glioblastoma in rat models were reported [104]. Triple intravenous application of 25–35 mg kg–1 of these PtII mono(terpyridine) drugs resulted in a significant decrease in tumor growth, as determined by magnetic resonance imaging (MRI, Figure 7.26). Applying a low-dose therapy (i.e., 15 mg kg1 of 86, 25 mg kg1 of 87), a reduction in tumor growth of 22% was observed; at high doses (i.e., 25 mg kg1 of 86 and 35 mg kg1 of 87) the tumor growth was reduced by 36% and 40%, respectively. Ma et al. investigated glycosylated acetylide- and arylacetylide-complexes 15–22 (Figure 7.10) with respect to their cytotoxicity against five human carcinoma cell lines (HeLa, HepG2, SF-268, NCI-H460, MCF-7) and normal kidney cells (293) [56]. Table 7.8 summarizes the IC50 values of 15–22 and cisplatin (for reference).

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SS

SS

SS

(a)

(b)

(c)

Figure 7.26 Volumetric MRI scans on day 15 presenting tumor growth in untreated animals (a), early therapy with 15 mg kg–1 of 86 (b) and late therapy with 35 mg kg–1 of 87 (c). Early therapy: treatment at days 4, 8, and 12 after tumor inoculation; late therapy: treatment at days 9, 11, and 13 after tumor inoculation. For all images: the dark arrow indicates the sphenodial sinus (SS) and the arrow heads delineate the tumor region [104]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

IC50 (mM) values of various glycosylated PtII mono-complexes and cisplatin against various human carcinoma cells and normal 293 cells [56]. Table 7.8

Complex

15 17 18 19 21 22 Cisplatin

Human carcinoma cells

Cell-293

HeLa

HepG2

SF-268

NCT-H460

MCF-7

0.1 2.0 0.09 0.2 0.2 2.7 11.6

0.1 1.7 0.1 0.1 0.2 3.0 20.6

0.06 1.3 0.08 0.1 0.1 2.1 15.6

0.1 2.8 0.1 0.2 0.2 2.5 25.1

0.08 1.9 0.1 0.2 0.1 3.4 19.1

0.5 10.5 0.3 0.9 0.5 4.6 W100

In particular, complexes 15, 17–19, and 21–22 showed significant cytotoxicity against these human carcinoma cells and 15 displayed remarkable cytotoxicity that is about 100 times more effective than clinically proven cisplatin drugs. Moreover, 15 and 19 were found to have higher cytotoxicity against cancer cells than normal 293 human kidney cells. The utilization of AuIII mono(terpyridine) complexes, as cytotoxic agents, has been investigated to a lesser extent than their PtII counterparts. In the only detailed study available, complexes 28–32 (Figure 7.13) displayed in vitro cytotoxicity against various human cancer cell lines (A2780, cisplatin-resistant A2780cisR, A549, SGC-7901, HeLa, HCT-116, BEL-7402, HL-60, and P-388) [61, 62]. The IC50 values of 28 in vitro against A2780 and A2780cisR were calculated to be 0.2 and 0.37 mM, respectively, which were more effective than cisplatin (1.22 and 14.16 mM, respectively) [61]. Complex 32 exhibited the highest cytotoxicity by inhibiting 80% of the cell growth in A-549, HeLa, and HCT-116 due to its high solubility under

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| 349

350

| 7 Terpyridine Metal Complexes and their Biomedical Relevance physiological conditions; moreover, some of the free terpyridine ligands displayed strong cytotoxicity and – in some cases – even higher than their corresponding AuIII complexes [61, 62]. Radiotherapeutic Agents Biomolecules can be damaged by the photoabsorption of X-rays specifically designed to ionize the molecules by forming Auger electrons [105]. The Auger process, which generates electrons and a charged-center from electron-emitting radionuclides (e.g., 125I, 131I, and 32P), can induce cleavage of chemical bonds in their neighborhood by direct ionization or charge recombination [106]. This process, which can cause cell death in vivo and single-strand breaks (SSBs) as well as double-strand breaks (DSBs) of DNA in vitro, was used as a radiotherapeutic tool to kill leukemia and thyroid tumors. Le Sech et al. introduced heavy PtII ions to circular plasmid DNA by intercalation and/or covalent binding of the [(tpy)PtCl]þ unit (11) to allow the use of energetic Xrays (soft g-rays, 11 keV) on DNA [107]. It was suggested that the absorption of photons from these soft g-rays in the LIII inner shell of a PtII center, which was bound to circular plasmid DNA (dry sample), induced both SSBs and DSBs of the DNA. This process was detected by fluorescence spectroscopy after submitting the sample to agarose gel electrophoresis. Later, SSBs and DSBs of the DNA, containing 11, were spectroscopically enhanced by tuning the experimental procedures [105]. The PtII-bound circular DNA that – when irradiated with X-rays in aqueous solution – increased SSBs and DSBs due to the formation of free radicals from water, which could be a possible application in hadrontherapy [106]. Moreover, fast He2þ ion irradiation of circular plasmid DNA, which contained 11, caused the SSBs and DSBs of the DNA. This experiment displayed similar results to that using X-ray irradiation [108]. 7.2.1.2 Terpyridine Complexes with Heavy d6 Transition Metal Ions 7.2.1.2.1 DNA Binding and Oxidative DNA Cleavage As detailed in the first part of this chapter, derivatives of 2,20 :60 ,200 -terpyridine have frequently been used, as chelating ligands, in the design of metallo-intercalators and chemotherapeutic agents based on d8 transition metal ions [13]. Various d6 transition metal ions (e.g., RuII, OsII, or RhIII) have also been utilized for the development of metallo-intercalators. In particular, these metallo-intercalators are octahedral complexes of the general type [ML3]nþ, where Mnþ represents a d6 transition metal ion and L a bidentate ligand (e.g., 2,20 -bipyridine, 1,10-phenanthroline, or dppz) [2]. Such complexes exhibit rich photophysical, photochemical, and redox properties that can be applied further, for example, for the design of sensitive spectroscopic or reactive probes for DNA. Concerning the reaction of the metallo-intercalators with DNA, three main types of reactions have to be distinguished: direct oxidative cleavage (i.e., reaction with the sugar), hydrolytic strand cleavage, and oxidative reaction with the DNA bases [2]. Complexes of terpyridines (as well as analogues tridentate ligands) with d6 transition metal ions have been less well studied concerning their potential biomedical applications. Compared to square-planar complexes, the ability of octahedral complexes containing the terpyridine unit to intercalate into DNA is

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(a)

[(tpy)Ru(bpy)(HO)]2 H e

[(tpy)Ru(bpy)(H2O)]2+ E1/2 0.49 V

89a

[(tpy)Ru(bpy)(O)]2 H e

[(tpy)Ru(bpy)(HO)]2+ E1/2 0.62 V

(b)

5'

RO3PO 3' RO3PO

| 351

90a 5'

O

RO3PO

B

O

5'

O

3' RO3PO

B

O

RO3PO

O

3'

RO3PO

O

5'

O

RO3PO

Scheme 7.3 (a) Electrochemical synthesis of the [RuO]IV complex 90a (counterions omitted for clarity) [116]; (b) proposed mechanism for oxidative cleavage of the sugar moieties of DNA [112].

significantly reduced for steric reasons. For instance, lengthening of the ct-DNA structure – characteristic evidence for successful intercalation – by the [(tpy)Ru(bpy)(H2O)]2þ (bpy: 2,20 -bipyridine, 89a) complex could not be observed in a viscosimetry assay [109, 110]. The binding ability of RuII mono(terpyridine) complexes to DNA could either be improved by attaching an acridine moiety to one of the ligands [111] or by simply replacing the bpy ligand by a dppz group, which is itself a good intercalator [112]. Viscosimetry, topoisomerase inhibition, and AFM experiments revealed unwinding and lengthening of the DNA structure by [(tpy)Ru(dppz)(H2O)]2þ (89b) as expected for classical intercalation; the lengthening and non-electrostatic binding affinity were both similar to that of EthBr. The ability of the RuII oxo-complex 90a to oxidize organic hydrocarbons and alcohols via hydride transfer was reported by Meyer and coworkers in the early 1980s [113, 114]. Thorp et al. showed that 90a could also be used as efficient oxidizing agent for DNA [115–118]. Such [RuO]IV mono(terpyridine) complexes could be prepared electrochemically from the corresponding aquo-complexes (Scheme 7.3). It was further shown that these complexes cleave DNA by guanine oxidation, presumably via an “inner-sphere” mechanism and by sugar oxidation at the 10 -position [117]; the cleavage selectivity is a function of electrostatic recognition [119], chemical reactivity of the oxidized functionality [120], and solvent accessibility of the oxidized function in the biopolymer [121]. Understanding the mechanism of DNA oxidation by transition metal complexes is of importance for gaining insight into the processes involved in chemotherapy [122, 123], biological DNA damage and mutagenesis [124], and nucleotide metabolism [117]. Utilizing [RuO]IV mono(terpyridine) complexes for this purpose was found to be a straightforward technique to follow the time course of the reaction [125], since the UV–vis absorption spectra of the oxidized [RuO]IV and the reduced RuII species are distinct. Thorp et al. studied the DNA oxidation by [RuO]IV metallo-intercalators [(tpy)Ru (L)(O)]2þ (90) [112]. For this purpose, a random-coil DNA oligomer was oxidized by

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352

| 7 Terpyridine Metal Complexes and their Biomedical Relevance Relative cleavage intensities at individual sites for single-stranded (ss) and doublestranded (ds) forms of d[50 -A1T2A3C4G5C6A7A8G9G10G11C12A13T14].a

Table 7.9

C4

G5

C6

A7

A8

G9

G10

G11

C12

A13

ss ds

[(tpy)Ru(bpy)O]2þ (90a) [121] 20 90 20 6 1 130 20 9

4 20

50 70

100 100

70 60

10 30

10 30

ss ds

[(tpy)Ru(dppz)(O)]2þ (90b) [112] 120 140 100 20 30 6 o3 o1

10 o1

30 4

100 100

100 8

20 o1

20 o1

a

Cleavage intensities were obtained from high-resolution electrophoresis measurements. All intensities were recorded using 30 mM of complex 90 and are relative to G10 for each case.

90b (L ¼ dppz) and the results were compared to those obtained for 90a. Since both complexes have almost identical electronic properties, differences in the oxidation behavior could be attributed to additional intercalation arising from the dppz ligand. The single- as well as double-stranded DNA oligomer was cleaved nonspecifically by 90a and the same stands for 90b with respect to the single-stranded oligonucleotide; however, due to intercalation into the DNA structure, a remarkable selectivity of 90b for the cleavage of the strand at a single site was observed (Table 7.9). As a second target, tRNA was chosen, since the structure has been well-documented and its conformation can be controlled in a well-defined way by changing the salt concentration. Owing to the different conformations (i.e., native and semidenaturated form), different cleavage patterns upon oxidation became evident. Folded tRNA could not host any intercalator; therefore, the pattern was similar for both complexes (90a and 90b) in which cleavage occurred mainly at guanidine and adenine sites in the D and anticodon loops. The semi-denaturated tRNA produced a larger number of cleavages with 90b compared to 90a, indicating that the helices in the unfolded secondary structure can accommodate the octahedral complex. Additionally, the authors examined the cleavage pattern of 90a with and without PtII mono(terpyridine) complex 1, as intercalator, thus decoupling the effects of intercalative recognition and oxidation process. When the folded tRNA was oxidized by 90a, no changes in the cleavage pattern could be determined in the presence of 1 confirming the preclusion of intercalation in this case. Owing to intercalation with 1, the structure of semi-denaturated tRNA was significantly altered and additional sites became accessible to the oxidant 90a, leading to a higher degree of cleavage. Therefore, the additional cleavage sites observed for 90b in the oxidation of semi-denaturated tRNA could be attributed to a combination of both intercalative recognition and structural perturbation resulting from this intercalative binding [112]. The iron regulatory elements (IREs) are a family of mRNA regulatory structures that control mRNA function and – among others – the synthesis of ferritin and the transferrin receptor. IREs feature the longest conserved nucleotide sequence

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known in RNA and the elements were reported to be specific to contemporary animals. The secondary structure is a hairpin and, in ferritin mRNA, the IRE interacts further with a base-paired flanking region (FL). Two different cleavage reagents, the photocleavage agent [Rh(phen)2(phi)]3þ (phen: 1,10-phenanthroline; phi: 9,10-phenanthrenequinone diimine) and the thermal agent [(tpy)Ru(bpy) (O)]2þ (90a), were used and the cleavage of IRE and FL was found to be distinct for each reagent [126]. Different substructures in the IRE and FL could be identified and had different functional effects, as shown by mutation. For instance, the RhIII agent recognized a site in the FL that modulated negative translational control but not binding the regulatory system (IRP) while 90a identified a site in the hairpin loop at G14 that affected IRP binding (Figure 7.27). In earlier studies with more classical reagents, G14 exhibited no special reactivity: the cleavage of G14 by RNase T1 and S1 was predicted from the secondary structure for the IRE [127, 128]. The specificity of the substructure at G14 was emphasized by the fact that the mutation IL-2 (increasing the size of the hairpin loop) changed the structure recognized by 90a. In contrast, mutations in the FL (FL2, FL2R) or base substitutions in the hairpin loop (HL-1) had no effect on the cleavage of G14 by 90a. The two substructures, detected by probing the mRNA with complexes [Rh(phen)2(phi)]3þ and 90a, have different functional effects, indicating that the ferritin IRE and FL can be thought of as a composite of at least two structure– function subdomains [126]; subdomains with specific structural features have previously been observed, for example, in the group I intron of Tetrahymena [129]. The subdomain with sites recognized by the photocleavage RhIII agent had regulatory activity for negative control as well as a conserved sequence, though it was [(tpy)Ru(bpy)(O)]2

GU A G C U AU AU CG UA UA C C G U UG CG UA UA GC A C GC AU U GC AU U C U U CG UA CG AU GC

[Ru(phen)2(phi)]3

Figure 7.27 Secondary structure of IRE and specific binding sites for [(tpy)Ru(bpy)(O)]2þ (90a) and [Rh(phen)2(phi)]3þ (counterions omitted for clarity) [126]. Figure reproduced with kind permission; r 2006 Elsevier B.V.

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| 353

354

| 7 Terpyridine Metal Complexes and their Biomedical Relevance located in the FL (outside the binding site for the IRP) [130]. In contrast, the IRE domain with a site recognized by 90a had regulatory activity for both negative and positive controls, had a conserved sequence common to other members of the IRE family, and was within the binding site for the IRP [130]. Though organic substrates can be oxidized efficiently by the strong oxidant dioxo-ruthenium(IV) complexes (e.g., tetrahydrofuran to g-butyrolactone) [131], a release of base could not be detected from the reaction of ct-DNA with trans-[(tpy)Ru(H2O)(O)2]2þ. According to the UV–vis absorption spectra, initially a [RuO]IV species was formed that interacted with the DNA, but slowly dimerized into an inactive oxo-bridged dimer. However, treatment of supercoiled FX174 plasmid with the dioxo-complex resulted in the conversion of the supercoiled structure I into the nicked circular form II (Figure 7.7a), as shown by electrophoresis measurements. These results indicate that designated DNA-oxidizing agents of similar structure can differ dramatically in the kinetics and mechanisms of DNA interaction: the oxo-complex [(tpy)Ru(bpy)(O)]2þ is a powerful DNA-oxidizer, whereas oxidation of DNA by the dioxo-complex [(tpy)Ru(bpy)(O)2]2þ is inhibited [132]. The competition for binding by redox-inactive divalent cations and chemical deactivation by redox-active divalent cations are important issues in studying chemical nucleases (i.e., DNA cleavage agents). Therefore, the reaction of various divalent metal ions with the oxidant 90a in the presence of DNA was investigated by gel electrophoresis and UV–vis absorption spectroscopy [133]. The reaction of MnII with 90a produced a transient intermediate with a new absorption at 531 nm and a second-order rate constant of 1500 M1 s1. This rate constant was 100times higher than that of the reaction with guanosine 50 -mono-phosphate (GMP), suggesting that MnII ions were kinetically efficient inhibitors of DNA oxidation. For comparison, a second-order rate constant of 1.1 M1 s1 was measured for the reduction of [(tpy)Ru(bpy)(OH)]2þ 89a by MnII ions; this relationship of rate constants for [RuO]IV and [Ru(OH)]III by the same substrate is characteristic for this pair of oxidants (Table 7.10). Plasmid gel electrophoresis demonstrated that MnII was also a powerful inhibitor for the conversion of the supercoiled form I of FX174 plasmid DNA into the circular form II (Figure 7.28b). Divalent cations,

Table 7.10

Rate constants for oxidation reactions with a [RuO]IV and a [Ru(OH)]III complex

[133]. Complex

Substrate

k (M1 s1)

[(tpy)Ru(bpy)(O)]2þ (90a)

GMPa MnII dCMPb iso-Propanol MnII iso-Propanol

9 [120] 1,500 0.031[120] 0.067 [114] 1.1 1.1 104 [114]

[(tpy)Ru(bpy)(OH)]2þ GMP: guanosine 50 -mono-phosphate. dCMP: 2-cytosine-50 -mono-phosphate.

a b

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(a)

(b)

3.0

1

2

3

4

5

6

7

NiII 2.0 II(I0II0)I II0(III)I 1.0

MgII

G18 G16

MnII 0.0 0

100

200

300

400

500

[MII] (µM)

Figure 7.28 (a) Representation of the integrated intensities of supercoiled (I) and circular (II) form of FX174 plasmid after cleavage by 90a in the presence of NiII, MgII, and MnII ions, respectively. Intensities are given as the fraction of circular DNA [ ¼ II/(IþII)] over the fraction of form II at zero MII concentration [ ¼ II0/(I0þII0)]. (b) Autoradiogram of the polyacrylamide sequencing gel showing the effect of MnII ions on the cleavage of d[50 TTCAACAG16TG18TTTG22AA] by 25 mM 90a (lane 1: DNA control; lanes 2 and 3: 90a; lanes 4– 7: 90a with 5, 10, 15, and 20 mM MnII ions, respectively) [133]. Figure reproduced with kind permission; r 1998 American Chemical Society.

such as MgII and NiII, which do not react with the oxidant but compete for electrostatic binding to the biopolymer, did not inhibit the plasmid cleavage (MgII ions were completely inactive, NiII showed an effect that was assigned to conformational changes due to strong electrostatic binding) (Figure 7.28a). In high-resolution electrophoresis experiments, the extent of quenching of oxidation by MnII in the DNA 16-mer d[50 -TTCAACAG16TG18TTTG22AA] and the RNA 30-mer r[50 -GUUCUUG7CUUCAACG16UG18UUUG22AACG26G27AAC] was dependent on the oligomer structure, where cleavage of residues in the hairpin loop was inhibited most efficiently. In contrast, quenching by MgII, NiII, and CoII ions was much less efficient and occurred only in the double-stranded regions. The selectivity of inhibition by MnII ions could be attributed to differential rates of deactivation of the bound oxidant 90a. The binding of [(tpy)Ru(bpy)(L)]2þ complexes (L ¼ labile ligand, e.g., Cl or H2O) to proteins, such as cytochrome c (cyt-c), was found to preferentially occur at the imidazole N-atoms of histidine residues [134]. To gain more insight into this binding behavior, Yang et al. synthesized a set of model complexes [(tpy)Ru(bpy)(L)](PF6)2 (91), where the heteroaromatic ligand L mimicked the binding to the protein (Figure 7.29a). The complexes with imidazole and 4-methylimidazole ligands (91a and 91b) exhibited photoluminescence with a maximum at 662 and 667 nm, respectively (Figure 7.29b). Since similar binding to the His sites of the protein could be assumed, this emission behavior could be applied to investigating the [(tpy)Ru(bpy)]2þ–protein adduct. The coupling of the [(tpy)Ru(bpy)]2þ fragment to cytochrome c (Yeast iso-1) starting from aquo-complex 89a was carried out at 35 1C and pH 7.0. The emission properties of the adduct were in good agreement with those found for 91a/b (Figure 7.29b and c). The location where the [(tpy)Ru(bpy)]2þ fragment was coupled to the protein was identified by digestion of the adduct with trypsin (cutting the C-terminal to Lys and Arg) and subsequent

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| 355

356

| 7 Terpyridine Metal Complexes and their Biomedical Relevance (b) 1.2

(a)

1 N N

91a (L = imidazole) 91b (L = 4-methyl-imidazole) 91c (L = 2-methyl-imidazole) 91d (L = benzimidazole) 91e (L = 4,5-diphenyl-imidazole) 91f (L = indazole) 91g (L = pyrazole) 91h (L = 3-methyl-pyrazole)

N Ru2+

N

L N 91

91a 91b 91c

0.8 0.6 0.4 0.2 0 570

670

770

Wavelength/nm (c)

(d)

20000

Emission intensity/cps

His38

16000 His31

12000 His44

8000

His23

4000 0 600

650

700

750

800

Wavelength/nm

Figure 7.29 (a) RuII complexes 91 (counterions omitted for clarity); (b) photoluminescence spectra of complexes 91a–c in water (2 105 mol l1, lexc ¼ 475 nm); (c) photoluminescence spectrum of [(tpy)Ru(bpy)(cyt-c)]2þ in water (lexc ¼ 475 nm); (d) molecular model of [(tpy)Ru (bpy)]2þ coupled to His44 of cyt-c; the imidazole ligand of 91a was superimposed with the His side-chain of cyt-c using the program Insight-IITM (AccelrysTM) [134]. Figure reproduced with kind permission; r 2005 The Royal Society of Chemistry.

analysis by HPLC coupled to high-resolution mass spectrometry. Figure 7.29d depicts a molecular model showing the binding of the RuII complex to the protein at His44. Amino acids, as ligands in transition metal complexes, have attracted scientific interest due to their ability to combine the biological role of amino acids with the properties of the metal complex (e.g., DNA binding) [135–137]. The binding of amino acids (by acting as bidentate N4O ligand) to RuII mono(terpyridine) (92) as well as mono[tris(pyridin-2-yl)triazine] (tptz) complexes (93) was reported by Kumar et al. (Figure 7.30) [138]. As proven by gel mobility shift assays, the complexes 92b/e and 93b/d exhibited DNA binding behavior and acted as mild DNA topoisomerase II (of the filarial parasite Setari cervi) inhibitors (10–40%). Moreover, the complexes also inhibited heme polymerase activity of the malaria parasite

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7.2 Terpyridine Metal Complexes with Biological Activity (a)

(b) 1

N N N N

3

4

5

6

N N

N

N

Ru2+ Ph3P

2

N Ru2+

O

Ph3P

N

O N 93

92 92/93a (NÔ glycine) 92/93b (NÔ leucine) 92/93c (NÔ isoleucine) 92/93d (NÔ valine) 92/93e (NÔ tyrocine) 92/93f (NÔ proline) 92g (NÔ phenylalanine)

Figure 7.30 (a) RuII mono-tpy (92) and mono-tptz complexes (93) with amino acids (counterions omitted for clarity); (b) gel mobility shift assay of S. cervi topoisomerase II by complexes 92b/e and 93b/d (0.2 mg): lane 1: supercoiled pBR322 DNA (0.25 mg) alone; lane 2: pBR322 with S. cervi Topo II; lanes 3–6: pBR322 with S. cervi Top II and 93b, 93d, 92b, and 92e, respectively (from left to right) [138]. Figure reproduced with kind permission; r 2009 Elsevier B.V.

R R 94a N N

94d

N Ru2

N

94b N

N 94 R

94e 94c

Figure 7.31 Homoleptic RuII bis(terpyridine) complexes 94 (counterions omitted for clarity) [139].

Plasmodium yoelii lysate that was studied. Inhibition in the range 50–60% was determined for complexes 92b/e and 93b/d; however, [Ru(tptz)Cl2(PPh3)] showed 94% inhibition. This significant decrease in inhibition efficiency could be attributed to steric hindrance due to the bulky amino acids coordinated to the RuII centers [138]. The previous examples for RuII complexes that showed binding affinity to DNA or proteins exclusively contained one terpyridine ligand and further (ancillary) mono- and bidentate ligands. Patel et al. reported a series of homoleptic RuII bis (terpyridine) complexes 94 in which planar aromatic substituents were attached at the 40 -position of the terpyridine unit (Figure 7.31) [139]. The non-covalent

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| 357

358

| 7 Terpyridine Metal Complexes and their Biomedical Relevance interaction of these RuII bis(terpyridine) complexes with ct-DNA was investigated by UV–vis titration, CD, and flow LD experiments. Intercalation of the aryl tail into the DNA at low complex concentrations could be observed. The similarity in flow LD shape for each of the complexes when bound to the DNA indicated similar binding modes. All complexes, except for biphenyl derivative 94a, have their z-axis approximately parallel to the DNA bases. At low metal complex concentration, UV –vis absorption spectra exhibited a characteristic redshift and stiffening of the DNA was observed. The naphthyl derivative 94b was bound both by intercalation as well as by a non-intercalative mode, even at very low metal complex concentrations. At higher concentrations, when exceeding a DNA : 94 ratio of 7 : 1 (for 94c/e) or 4 : 1 (for 94d), aggregation of the complexes by p–p stacking was observed (exciton bands in the CD spectra). The pronounced z-polarized transitions of 94a were oriented in a manner consistent with its z-axis laying more parallel than perpendicular to the helix axis; therefore, a groove binding mode was ascertained rather than intercalation. Though the X-ray single-crystal structure of 94a indicated the presence of a supramolecular polymer formed by intermolecular p–p interactions of the biphenyl-groups, there was no evidence for poly[(G-C)]2- or ct-DNA facilitating such a structure, since no exciton CD signal could be observed. However, with poly[(A-T)]2-DNA, an exciton CD signal was detected. Thus, 94a was stacked along the DNA structure in the latter case (the minor groove is less sterically hindered), whereas in the other cases, individual molecules bind, probably to the major groove [139]. The surface area for intercalative binding to DNA was relatively small in complexes 94. According to Pyle and Barton, the binding affinity could be increased by increasing the overall surface area of the metallo-intercalator [140]; however, if the increased part is non-planar relative to the parent ligand, the binding affinity – even the binding mode – might be changed [141]. Chao et al. reported a set of heteroleptic RuII bis-complexes 95 containing a 2,20 :60 ,200 -terpyridine and an unsymmetrical 3-(phenanthrolin-2-yl)-1,2,4-triazine ligand (Figure 7.32a) [142]. UV–vis titration experiments showed the characteristic hypochromism of the MLCT absorption band of 9.4% (95a) and above 20% (95b/c), respectively, and the binding constants (KB) were calculated using the MLCT absorption. According to these data, the interaction with DNA decreased in the order 95c W 95b W 95a; this trend could also be concluded from competitive binding experiments using EthBr. Moreover, the Stern–Volmer constants (KSV) were determined by fluorescence quenching experiments. Table 7.11 summarizes the UV–vis absorption data and DNA binding parameters. Viscosimetry was applied, as an additional tool, to investigate the DNA binding behavior of 95. For 95b/c, a similar increase in viscosity compared to the good RuII metallo-intercalator [Ru(bpy)2(dppz)]2þ could be observed (Figure 7.32b). However, 95a exhibited no effect on the DNA viscosity at low [95a] : [DNA] ratios, at higher ratios the DNA viscosity even decreased. Thus, complexes 95 bound to DNA in two different modes: 95a by a partial, non-classical intercalation and 95b/c by classical intercalation. This assumption could be supported by X-ray single-crystal analysis of the complexes. In contrast to the coplanar ligands in 95b/c, the dppt ligand of 95a was hindered from planarity due to the

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| 359

UV–vis absorption data and DNA binding constants of complexes 95 and 96

Table 7.11

[142, 143]. Complex

kabs (nm)a

Hypochroism (%)

KB (M1)

KSV (M1)

95a 95b 95c 96a 96b

452 485 506 482 486

9.4 22.5 28.1 8.0 27.6

2.49 104 9.51 104 1.62 105 1.6 103 3.2 104

4.89 27.28 30.47 N.a. N.a.

a

[(tpy)Ru(L)]2+

95

(c)

(b)

1.3

1.24

N

N

1.20

N

1.16

1.2

95a (L dppt)

N N N

N N

95b (L pta)

1.12

(η/η0)1/3

N N

(η/η0)1/3

(a)

UV–vis absorption wavelength of the MLCT band.

1.08 1.04 1.00

1.1 1.0 0.9

0.96 N

N N

N

0.92 0.00

0.02 0.04 0.06 0.08 0.10 [Ru]/[DNA]

0.8 0.00

0.05

0.10 0.15 [Ru]/[DNA]

N 95c (L ptp)

Figure 7.32 (a) RuII complexes 95; (b) effect of increasing the [95] : [DNA] ratio on the relative viscosities of ct-DNA at 30 1C (95a:’; 95b: ; 95c: ~; [Ru(bpy)2(dppz)]2þ: ) [142]; (c) effect of increasing the [96] : [DNA] ratio on the relative viscosities of ct-DNA at 28 1C (96a: ; 96b: ’; [Ru(bpy)2(dppz)]2þ: ~; [Ru(bpy)3]2þ: ) [143]. Figure reproduced with kind permission; r 2002 and 2003, respectively, Elsevier B.V.

8

8

phenyl-substituents being rotated away from the 1,2,4-triazine ring (37.21 and 48.31). Therefore, in this case, only partial intercalation, by prizing one side of a DNA base pair stack away, could occur and the resulting static bend or kink in the helix would explain the decrease in DNA viscosity. Similar DNA binding behavior was observed for complexes 96 (Figure 7.33). The optical experiments (Table 7.11) as well as the viscosimetry measurements (Figure 7.32c) suggested that 96a bound to DNA via an electrostatic interaction, whereas 96b could partially intercalate via the naphthyl moiety into DNA. In contrast to 95b/c, a shielded surface for 96a/b, due to the compact ligand geometries, prevented the complexes from deeply intercalating into the DNA and, therefore, relatively low KB values were observed [143].

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0.20

360

| 7 Terpyridine Metal Complexes and their Biomedical Relevance H N

N N

N

N

96a (L phbi) 96b (L phni) 97 (L dppzp)

N phni

phbi

[(tpy)Ru(L)](ClO4)2

H N

N

N N N N

N

dppzp

Figure 7.33 RuII complexes 96 and 97 [143].

[(tpy)Ru(L)]2+

[(phen)2Ru(L)]2+

[Re(CO)3(CH3CN)(L)]2+

98a (L L1) 98b (L L2) 98c (L L3) 98d (L L4) 98e (L L5)

99a (L L1) 98b (L L2) 98c (L L4) 98d (L L3)

100a (L L1) 100b (L L2) 100c (L L4)

N

R

N

N

L1 (R = Ph) L2 (R = pyridin-4-yl)

N

N N

N N

N N

N N

L3

N N

L4

N

N

N

N

N

N N L5

Figure 7.34 Heteroleptic RuII and ReI complexes for DNA binding studies (counterions omitted for clarity) [145].

Very recently, 60 -(200 -pyridyl)dipyrido[3,2-a:20 ,30 -c]phenazine (dppzp) was utilized by Stewart et al., as a tridentate ligand, for the synthesis of the heteroleptic RuII complex 97 (Figure 7.33). The complex was applied as a DNA “light switch:” in an aqueous buffer solution no emission signal could be detected, but 97 became emissive (lex ¼ 698 nm), with a remarkable excited-state lifetime of 25 ns, as a result of intercalation into double-helical DNA. The intercalation of 97 via its dppzp system into DNA shielded the N-atoms of the phenazine moiety from a solvent-assisted quenching mechanism [144]. Metcalfe et al. investigated a series of RuII complexes containing terpyridine (or structurally related) ligands; in complexes 98, the ligands L were used as tridentate chelates, while coordination in 99 occurred in a bidentate mode (Figure 7.34). Though, in general, high binding affinities were observed (comparable to those of other metallo-intercalators), interaction with DNA via a groove binding mode was concluded from UV–vis absorption and emission as well as viscosimetry experiments. For instance, no lengthening of the DNA structure, as would have been expected for intercalation, could be observed. Similar results were also obtained for the ReI complexes 100 (Figure 7.34). The binding affinity was dependent on the

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N


geometry of the metal complex and decreased in the order 100a W 99a W 98a (for ligand L1). Thus, the steric as well as electronic properties of the complexes influenced the interaction with the DNA structure. Complexes containing the ligands L1 or L2 exhibited significantly higher binding affinities than those containing the more electron-deficient ligands L3–L5. Therefore, a less sterically demanding ligand set in combination with extended ligands (that are relatively electron rich) should be a prerequisite for a high DNA binding affinity of a transition metal ion complex [145]. As a last example for the interaction of RuII complexes with DNA, the homoleptic complexes 101 reported by Sathayaraj et al. should be noted (Figure 7.35a) [146]. For 101a, pronounced bathochromic shifts and hypochromism were observed in UV–vis titration experiments and a KB of 4.3 103 M1 was calculated from the spectroscopic data; according to this, 101a was bound to ct-DNA intercalatively. In contrast, 101b showed a dual mode of binding to DNA in which, at lower concentrations, 101b behaved as a DNA groove binder: whereas at higher concentrations intercalation could also be observed. The changes in the melting behavior of ct-DNA in the presence of the complexes supported this assumption in that the good intercalator 101a increased Tm by almost 8 1C (101b: DTm ¼ 4.4 1C) (Figure 7.35b). Both complexes were found to be agents for the photocleavage of DNA (upon irradiation a 440 nm). However, the DNA cleavage performance of 101b was significantly lower than that of 101a; only at high concentrations could sufficient cleavage rates be achieved (Figure 7.35c). Since the cleavage was not suppressed by adding histidine, as a singlet-oxygen quencher, the photoexcited state of the complex was responsible for the observed DNA cleavage [146]. 7.2.1.2.2 Cytotoxicity The cytotoxic activity of d8 transition metal complexes, in particular of PtII mono(terpyridine) complexes (Section 7.2.1.1.3), against several human tumor cell lines is well-established [13]. In addition, various types of RuII-containing (potential) antitumor drugs are known from the literature [147–152]. The focus shifted towards these types of metallo-drugs, since the higher coordination number of RuII ions in comparison to PtII ions provides additional sites, which could theoretically be used to fine-tune the properties of the complex by influencing how the complex interacts with DNA. Moreover, the different redox behavior can play an important role in the transport mechanism of the drug in the body as well as in the interaction between the drug and several different biologically relevant proteins [153]. The kinetics of ligand-substitution reactions and/or the solubility of the compounds in water are further issues to be considered. In addition, the particular ruthenium chemistry allows for photodynamic therapeutic approaches [154–158]. In the past, RuII-containing drugs have been synthesized employing various types of ligands: amines, imines, DMSO, oligopyridines, and arenes to name only a few [153, 159, 160]. It has been concluded from the diversity of active structures that the mechanism of action may differ for different types of complexes [147]. The cytotoxicity of mer-(tpy)RuCl3 was already reported in 1995 by Reedijk and coworkers [12, 161]. Besides the ability to interstrand-wise crosslink DNA, the

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| 361

| 7 Terpyridine Metal Complexes and their Biomedical Relevance (a) [Ru(L)2]2+

N

N

NH

NH

101a (L = L1) 101b (L = L2) N

N

N

N

(b)

N

N

L1

L2

1.70 1.65 Absorbance at 260 nm

362

1.60 1.55 1.50 1.45 1.40 1.35 1.30 40

50

70 60 Temperature C

80

90

(c)

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Figure 7.35 (a) Homoleptic RuII bis(terpyridine) complexes 101 (counterions omitted for clarity); (b) melting curves of ct-DNA (100 mM) at 260 nm in the absence and presence of complexes (DNA alone: ~; with 101a (20 mM): ’ with 101b (20 mM): ); (c) cleavage of supercoiled pUC18 DNA by 101a (top) and 101b (bottom) when incubated for 1 h and followed by irradiation at 440 nm for 30 min (lanes 1 and 9: DNA alone; lanes 2–7: DNA with 4, 8, 12, 16, 20, and 24 mM of 101a, respectively; lane 8: same as lane 7 but with additional histidine; lanes 10–15: DNA with 4, 8, 12, 16, 20, and 24 mM of 101b; lane 16: same as lane 15 but with additional histidine [146]. Figure reproduced with kind permission; r 2010 Elsevier B.V.

in vitro activity against L1210 cancer cell lines was also studied: the IC50 value of the RuII complex was around 8 mM and, thus, the complex was less active than cisplatin (IC50 of 1.3 7 0.4 mM) but much more potent than carboplatin (IC50 of 47 7 9 mM).

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7.2 Terpyridine Metal Complexes with Biological Activity {[(tpy)RuL1]-H2N(CH2)6NH2-[L1Ru(tpy)]}4 [(tpy)RuL1L2]n 2 102a (X,Y N, L Cl , n 1) 103 (X,Y N) 102b (X,Y N, L2 H2O, n 2) 102c (X,Y N, L2 CH3CN, n 2) Y 102d (X N, Y CH, L2 Cl, n 1) X N 102e (X,Y CH, L2 Cl, n 1) N L1

Figure 7.36 Mononuclear RuII complexes 102 and dinuclear complex 103 (counterions omitted for clarity). [151].

Table 7.12

IC50 (mM) values of complexes 102 and 103 against selected cell lines [151].a

Cell line

102a

102b

102c

102d

102e

103

Cisplatin

A498 EVSA-T H226 IGROV M19 MEL MCF7 WIDR A2780 A2780cisR A498 EVSA-T

W96 7 17 W96 25 13 66 23 25 W96 7

W81 6 17 44 26 18 50 11 30 W81 6

W82 6 26 78 30 21 73 31 28 W82 6

39 11 34 65 15 30 51 19 42 39 11

N.a. N.a. N.a. N.a. N.a. N.a. N.a. W100 62 N.a. N.a.

W40 17 28 W40 33 W40 W40 33 28 W40 17

2 1 2 0.2 3 2 2 6 25 2 1

a

A498, EVSA-T, H226, IGOV, M19 MEL, MCF7 and WIDR were treated for 5 days; A2780, A2780cisR, L1210/0 and L1210/2 were treated for 48 h.

A more recent generation of RuII complexes 102 focused on the combination of the parent [(tpy)Ru(L1)(L2)]2þ motif with a bidentate ligand L1 (i.e., arylazopyridine or aryliminopyridine) as well as a labile ligand L2 (i.e., Cl, H2O; MeCN) (Figure 7.36) [162–164]. The coordination of 102a–e to 9-ethylguanidine, as a model base for DNA, was confirmed by 1H NMR and MS (the dinuclear complex 103 did not yield a coordination product with the base). The coordination to ct-DNA was subsequently studied by means of UV–vis absorption, CD, and flow LD spectroscopy. For all complexes, including 103, binding to DNA was observed. However, kinking or coiling of the DNA structure as reported for various PtII mono(terpyridine) complexes could not be detected (see also Section 7.2.1.1.1). A detailed in vitro cytotoxicity study against a variety of human cancer cell lines was carried out (according to the anticancer screening panel of the National Cancer Institute, USA): WIDR (colon cancer), IGROV (ovarian cancer), M19 MEL (melanoma), A498 (renal cancer), and H226 (non-small lung cancer). Also screened were two types of human breast cancer cell lines (MCF7 and EVSA-T), the cisplatin-sensitive (L1210/0) and cisplatin-resistant mouse leukemia cells (L1210/2), and the human ovarian carcinoma cells (A2780 and the cisplatin-resistant A2780cisR) (Table 7.12)

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| 363

364

| 7 Terpyridine Metal Complexes and their Biomedical Relevance [151]. In general, good activity against the EVSA-T cell line and moderate activity against H226, M19 MEL, and MCF7 cell lines was observed – independent of the nature of the ligands L1 and L2. Since 102e showed poor (or no) activity against the cell lines tested, the diazo-group as structural element might be of relevance. With respect to A2780, 102b gave the best results, but all RuII metallo-drugs were less effective than cisplatin. The two resistant cell lines were affected by complexes 102 to a similar extent as by cisplatin and the corresponding non-resistant cell line. Thus, a mechanism of action different from that of cisplatin was assumed. Although various types of RhIII [165] and IrIII [166–169] complexes have been utilized in recent years, as enzyme inhibitors, and their cytotoxicity studies have been reported, IrIII mono- or bis(terpyridine) complexes targeting these applications have rarely been described in the literature [165, 168]. The moderate to low yields in the preparation, for example, of IrIII mono(terpyridine) complexes [i.e., Ir(tpy)X3 with X ¼ Cl or Br], under harsh reaction conditions needs to be overcome to make such complexes more readily accessible for biomedical applications [170]. 7.2.1.3 Terpyridine Complexes with Miscellaneous Transition Metal Ions Before detailing terpyridine complexes containing other transition metal ions, it has to be noted in this context that metal-free terpyridine ligands also showed biological activity. In continuation of previous work, where Zhao et al. showed that terthiophenes, as bioisosteres of terpyridines, exhibited a reasonable inhibition of protein kinase C activity as well as antitumor activity against various human tumor cell lines, various 40 -phenyl-, 40 -furyl- and 40 -thiophenyl-substituted terpyridine derivatives 104–106 (Figure 7.37) were prepared and their topoisomerase I inhibitory activity as well as their antitumor cytotoxicity were evaluated [171]. For this purpose, different human tumor cell lines [A-498 (kidney carcinoma), PC-3 (prostate adenocarcinoma), HT-29 (colon adenocarcinoma), A-549 (lung carcinoma), HCT-15 (colon adenocarcinoma), SKOV-3 (ovary adenocarcinoma), and SK-MEL-2 (malignant melanoma)] and a human normal cell line (RPTEC) were screened. The terpyridines with a “normal” alignment of the pyridine rings (104) showed a higher cytotoxicity than their “abnormal” counterparts (105 and 106). The parent 2,20 :60 ,200 -terpyridine (tpy) and its derivatives 104 displayed better cytotoxicity against A-498, PC-3, and HT-29 cancer cell lines than the reference R

N

N

R

N

N

R

N

104

105

R= b

N 106

O a

N

N

N

S O

c

d

e

S

Figure 7.37 A series of 40 -(hetero)aryl-substituted 2,20 :60 ,20 - (104), 2,20 :60 ,300 - (105) and 2,20 :60 ,400 -terpyridines (106) with biological activity [171].

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material (i.e., doxorubicin). In contrast, the poorly cytotoxic terpyridines 105 and 106 exhibited strong topoisomerase I inhibition, whereas the highly cytotoxic substrates 104 did not. Thus, inhibition of topoisomerase I appeared not to be the primary mechanism for the antitumor activity of metal-free terpyridine derivatives. When considering terpyridine complexes with biomedical relevance, despite those of the previously discussed d6 (i.e., RuII or IrIII) or d8 transition metal ions (i.e., PtII, PdII and AuIII), several interesting candidates can easily by reduced to a handful of examples: complexes containing either first-row transition metal ions or rare earth metal ions (e.g., LaIII complexes of terpyridine-based chelates) were utilized for the sequence-specific cleavage of RNA [172]. Among the d10 transition metal ions, CdII and HgII ions exhibit pronounced toxicity and are, therefore, hardly relevant for biomedical applications. In contrast, zinc is one of the most important trace metals in the human body, and ZnII ions are known to be vital for growth and development. They also play a major role in the regulation of the metabolism of cells, are cytoprotective, and suppress the apoptotic pathways [173]. In general, nitrogen-containing chelating ligands, in addition to ancillary ligands (enabling membrane permeability), are the key structural features in biologically active ZnII complexes [174]. Therefore, the study of the pharmacological properties of ZnII complexes, with respect to their antitumor activity, is of growing interest [175–182]. Jiang et al. reported on the first ZnII bis(terpyridine) complexes (107, Figure 7.38) with in vitro cytotoxic activity against various human tumor cell lines [179]. Efficient intercalation into supercoiled pUC19 DNA by 107a was concluded from CD experiments. In contrast, 107b did not intercalate, thus the lateral adenine-substituent governed the disturbance of the DNA base pair stacking. Similarly, the nuclease activity of 107a was higher than for 107b; cleavage of supercoiled pUC19 DNA into the nicked (23%) and linear form (8%) was observed at a concentration of 60 mM for 107a and 107b converted the DNA into the nicked form only at concentrations above 80 mM. The in vitro cytotoxicity against four human cancer cell lines (HeLa, MCF-7, HepG2, and PC-3) was investigated and compared to cisplatin as well as to both metal-free ligands (Table 7.13). Both complexes showed potent activity against all cancer cell lines that was about ten-times higher than the corresponding values for cisplatin. Complex 107a was significantly more active than its metal-free terpyridine ligand, indicating the impact of metallization. For 107b, the metal-free ligand gave similar IC50 values, thus reflecting the impact of the ligand. In addition, the Table 7.13

L1 L2 107a 107b Cisplatin

IC50 values (mM) of complexes 107 and their metal-free ligands [179]. MCF-7

HePG2

PC-3

HeLa

18.4 1.71 1.96 1.81 14.49

N.a. N.a. 0.68 0.43 2.45

6.23 1.33 0.64 0.76 8.18

N.a. N.a. 4.20 1.53 10.47

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| 365

366


[Zn(L)2](PF6)2

R

NH2

107a (L L1) 107b (L = L2)

N L1: R

N

N N

108a: {[(ph-typ)Zn](SO4)}2 108b: [(ph-typ)Zn(NO3)](NO3) 108c: [(ph-typ)Zn]Cl2 108d: [(ph-typ)Zn](OAc)2 N

N

N

N

L2: R H

N ph-tpy

Figure 7.38 ZnII bis(terpyridine) complexes 107 [179] and ZnII mono(terpyridine) complexes 108 [180].

order of activity was inverted: 107b was more cytotoxic than 107a (though 107a was the better intercalator and DNA cleaving agent). Therefore, different mechanisms for the DNA binding/damage on the one hand and cytotoxicity on the other hand were concluded [179]. The antitumor activity of ZnII mono(terpyridine) complexes 108 (Figure 7.38) was studied by Ma et al. [180]. No counterion dependency on the in vitro activity against human cancer cell lines HL-60, Bel-7420, BGC-823, and KB was observed. High cytotoxicity was observed in all cases with IC50 values lower than 1.0 mM; thus, complexes 108 were more active than cisplatin against these cell lines. However, the current bench-mark in cytotoxicity reported for ZnII complexes with a tetraazamacrocyclic ligand (35.6% growth inhibition in vitro against P388 cancer cell lines at a concentration of 0.1 mM) has not yet been reached by terpyridinecontaining ZnII complexes [183]. Copper is also a biologically relevant element and many enzymes whose activity depends on CuII ions have been identified. The metabolic conversions catalyzed by most of these enzymes are oxidative. Because of their biological relevance numerous CuII complexes have been synthesized, in particular for the purpose of application as chemical nuclease agents [184–186]. The mode of action of CuII complexes in this respect is well understood: DNA can be cleaved by generation of hydrogen-abstracting activated oxygen species [187] and RNA scission may occur via phosphodiester transesterification [188]. The group of Sigman showed that a mixture of 1,10-phenanthroline, CuII ions, and a thiol can depolymerize poly[d(A–T)] in solution under aerobic conditions [189]. Following this, numerous CuII oligopyridine complexes, most of which contain 1,10-phenanthroline or its derivatives, as ligands, [190–196] have been synthesized and utilized as artificial nucleases. The complexes have been applied as foot-printing agents of both proteins and DNA [197], probes of the dimensions of the minor groove of the double-stranded structure [198], and identifiers of transcription start sites [199]. In contrast to bidentate ligands in the coordination sphere of CuII ions, their tridentate counterparts have been investigated, so far, only to a minor extent [200, 201]. Nair and coworkers claimed that CuII complexes of tridentate ligands should be potent foot-printing agents due to their weaker binding affinity to DNA [200]. Using a 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) ligand various fundamental conclusions could be drawn: UV–vis titration experiments showed the

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N


characteristic features (bathochromic shifts and hypochromicity), but intercalation of [Cu(bzimpy)Cl]Cl into DNA occurred only with a moderate binding constant [KB ¼ 1.8 7 0.2 104 M1] compared to other common (metallo-)intercalators; similar results were obtained from thermal denaturation studies (the melting temperature of ct-DNA was increased by about 5 1C) and viscosimetry experiments. Most important, DNA cleavage experiments were conducted: in the presence of H2O2 (as the most common substrate) no hydroxy radicals were produced and, thus, cleavage of plasmid DNA was detected. However, utilizing either ascorbic acid or glutathione, as co-reductants, efficient cleavage of the DNA was confirmed by gel electrophoresis measurements. Superoxide radicals, generated by the mechanisms depicted in Scheme 7.4, were proposed to be the active species in DNA cleavage. Additionally, electrochemical measurements were conducted to elaborate the relative binding affinity of [Cu(bzimpy)Cl]þ and [Cu(bzimpy)]þ to DNA, according to Eq. (7.2) [66]. The Kþ/K2þ value of 1.59 indicated that the CuI species was a more potent binder to DNA [200]. Similar results were reported by Wang et al., who applied dppt and pta, as unsymmetrical ligands, for the coordination of CuII ions (see Figure 7.32a for the structures of the dppt and pta ligands) [201]. In accordance with previous results on the related RuII complexes [142], complex [Cu(pta)Cl]Cl, having the more planar chelating ligand, showed the higher DNA cleavage activity: Eb0 Ef0 ¼ 0:0519 logðK þ =K2þ Þ

(7.2)

where Eb0 and Ef0 are the formal potentials of the redox couple in the DNA bound and free form, respectively; Kþ/K2þ is the ratio of binding constant of the CuI and of the CuII form to DNA. With respect to terpyridines, as ligands, two different types of CuII complexes have been reported: CuII mono(terpyridine) as well as CuII bis(terpyridine) complexes. Three CuII bis(terpyridine) complexes 109 (Figure 7.39a) were synthesized and investigated for their DNA binding and cleaving activity [202, 203]. Interaction with ct-DNA was studied by UV–vis absorption and CD titration experiments. Enhancement of the CD signal with increasing amounts of 109 indicated complex intercalation into DNA; moderate binding constants were estimated from these results (109a: KB ¼ 8.4 7 0.2 103 M1; 109b: KB ¼ 10.9 7 0.2 104 M1; 109c: KB ¼ 4.26 7 0.2 103 M1). Addition of 109 to ct-DNA stabilized the

[Cu(L)Cl] e

[Cu(L)Cl]

[Cu(L)Cl] O2

[Cu(L)Cl] O 2

L is a tridentate ligand Scheme 7.4 Formation of superoxide radicals, catalyzed by CuII complexes bearing a tridentate ligand [200].

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| 367

368


(a)

(c) [Cu(L)2](ClO4)2

[Cu(L)Cl]Cl

109a (L = L1 with R = CH3) 109b (L = L1 with R = Br) 109c (L = L2) 110a (L = L1 with R = CH3) 110b (L = L2)

60000 40000

HN

N

N

N

N

N

N

N

L1

Intensity (a.u)

R

20000 0 20000 40000

L2

60000 (b) 1

2

3

4

5

6

7

3400

3520 3440 3480 Magnetic Field (G)

8 Form II Form III Form I

Figure 7.39 (a) CuII bis- and mono(terpyridine) complexes 109 and 110. (b) Cleavage of pBR322 DNA by 109c in the presence of H2O2: DNA (120 ng) was incubated with 109c for 90 min in a TrisTM buffer at pH 7.5; lane 1: DNA control; lane 2: DNA þ 109c (50 mM) alone; lane 3: DNA þ H2O2 (200 mM) alone; lane 4: DNA þ 109c (25 mM) þ H2O2 (300 mM); lane 5: DNA þ 109c (25 mM) þ H2O2 (400 mM); lane 6: DNA þ 109c (50 mM) þ H2O2 (300 mM); lane 7: DNA þ 109c (50 mM) þ H2O2 (400 mM); and lane 8: DNA þ 109c (50 mM) þ H2O2 (400 mM) þ EtOH (1 M). (c) EPR spectrum of the reaction mixture containing 109c (50 mM), H2O2 (125 mM), and DMPO (200 mM) [203]. Figure reproduced with kind permission; r 2005 Elsevier B.V.

double-helical structure, as revealed by an increase of the melting temperature by about 5 1C in each case. The viscosity (Z) of DNA experienced a steady increase with increasing concentration of 109, which is characteristic for an intercalative binding mode, whereas other non-classical binding modes leading to bending or kinking of the DNA would produce a decrease of Z. DNA cleavage experiments with 109 as metallo-nucleases were conducted on plasmid pBR322 DNA and monitored by gel electrophoresis. The naturally occurring supercoiled form (form I) gave rise to an open circular relaxed form (when nicked: form II) and resulted in the linear form upon further cleavage (form III). When subjected to gel electrophoresis, relatively fast migration could be observed for I, whereas II migrated slowly (the migration speed of III was between I and II (Figure 7.39b). In the presence of H2O2, complexes 109 efficiently cleaved DNA; Figure 7.39b depicts the cleavage pattern observed at different concentrations of nuclease 109c and H2O2. In general, the nuclease activity of CuII complexes in the presence of peroxide has been attributed to the participation of hydroxyl radicals [204]. The addition of ethanol to the reaction mixture before electrophoresis was found to suppress the

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3560


DNA cleavage (Figure 7.39b, lane 8), giving evidence for the involvement of hydroxyl radicals in the observed nuclease activity of 109c in the presence of peroxide. The formation of hydroxyl radicals could be confirmed by spin-trap experiments. The electron paramagnetic resonance (EPR) spectra of 109c in the presence of H2O2 and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), as spin-trapping agent, gave a four-line spectrum (aN ¼ 14.85 G and aH ¼ 14.85 G) that was attributed to the [DMPO(HO )] adduct (Figure 7.39c), thus proving the presence of hydroxy radicals in the DNA cleavage promoted by 109c in the presence of H2O2 [203]. A binuclear complex, containing both a CuII mono(terpyridine) and CuII monobipyridine unit (linked by a flexible amido spacer), exhibited a highly active cleavage of ribonucleotides with a remarkable selectivity for adenine (A) bases [205]. The relative rate (krel) to selectively cleave a ApA dimer was 12–87 times higher than for other nucleobase (homo)dimers. The relative cleavage rate of a stoichiometric mixture of [Cu(tpy)(H2O)]2þ and [Cu(bpy)(H2O)]2þ was about ten-times lower. Thus, the covalent linkage of the two metal centers resulted in a chemoselective cleavage of ribonucleotides. The group of Nair also reported CuII mono(terpyridine) complexes 110, using the same ligands as for their CuII bis(terpyridine) complexes 109 (Figure 7.39a) [206]. UV–vis absorption titration, viscosimetry, and CD experiments revealed an intercalative binding mode to ct-DNA for 110. The binding constants were of the same order of magnitude as those found for the analogous bis-complexes 109a/c (110a: 5.6 7 0.2 104 M1 and 110b: 1.4 7 0.2 104 M1). Computational studies at the B3LYP/6-31G* level showed that the aromatic p-electron cloud was more uniformly distributed in the case of 110a (Figure 7.40a), which explained the better stacking interactions with the DNA bases and, hence, the observed higher KB value. Gel electrophoresis experiments showed that both complexes cleaved plasmid DNA in the presence of ascorbic acid, as co-reductant (Figure 7.40b). The higher cleavage efficiency of complex 110a in comparison to 110b (in accordance with the observed intercalation behavior) could be visualized from a semi-logarithmic plot of the reaction kinetics (assuming a pseudo-first-order process) (Figure 7.40c); the rate constants (k) of 4.32 104 s1 (110a) and 3.14 104 s1 (110b) mirrored the better nuclease activity of 110a relative to 110b. An alternate strategy to utilizing complexes of transition metal ions with terpyridine ligands, as anticancer agents, is the usage of metallocene derivatives, for example, titanocene dichloride, ferrocifen, and half-sandwich (arene)ruthenium complexes [207–212]. The lipophilic nature of the ferrocene moiety could improve cell permeability and stability in biological aqueous medium; furthermore, its reversible redox property makes it suitable for designing ferrocene-based bioorganometallic species for therapeutic applications [208, 213–215]. For example, the ferrocenyl moiety in ferrocifen – a ferrocene-substituted tamoxifen – significantly enhanced the anticancer activity of tamoxifen [208]. Ferrocene itself is not cytotoxic and does not show significant antitumor activity; however, the ferrocenium cation, the one-electron oxidized product of ferrocene, is known to exhibit cytotoxic activity [216, 217]. Osella and coworkers have reported the cytotoxicity of ferrocenium cationic salts showing oxidative DNA damage [216]. Since the ferrocenium

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| 369

370


(a) (c)

4.2

L1 with R CH3

(b)

1

2

3

4

In (% SC DNA)

4.0

L2 5

6

7

8

3.8 110b

3.6 3.4 3.2

9

110a Form II Form III Form I

3.0 500 1000 1500 2000 2500 3000 3500 4000 Time [s]

Figure 7.40 (a) Total electron density map of ligands L1 (R ¼ CH3) and L2 as obtained by DFT calculations at the B3LYP/6-31G* level. (b) Cleavage of pBR322 DNA by complexes 110a/b in the presence of ascorbic acid: DNA (250 ng) was incubated with 110a/b for 60 min in TrisTM buffer at pH 7.5; lane 1: DNA control; lane 2: DNA þ 110a (75 mM) alone; lane 3: DNA þ ascorbic acid (150 mM); lane 4: DNA þ 110a (25 mM) þ ascorbic acid (75 mM); lane 5: DNA þ 110b (25 mM) þ ascorbic acid (75 mM); lane 6: DNA þ 110a (50 mM) þ ascorbic acid (75 mM); lane 7: DNA þ 110b (50 mM) þ ascorbic acid (75 mM); lane 8: DNA þ 110a (75 mM) þ ascorbic acid (150 mM); and lane 9: DNA þ 110b (75 mM) þ ascorbic acid (150 mM). (c) Semi-logarithmic plot of the content of native (supercoiled, SC) DNA versus time at a complex concentration of 75 mM [206]. Figure reproduced with kind permission; r 2007 Wiley-VCH.

cation is known to degrade in biological aqueous medium, the conjunction of an aromatic system could stabilize the ferrocenium cation. Ferrocenium cations can, for instance, be generated by photoactivation in visible light by CuII complexes conjugated to the ferrocene moiety. Maity et al. functionalized ferrocene with a terpyridine binding site for CuII ions [218]. Additionally, planar phenanthroline-based intercalating ligands were coordinated to the CuII center (111a–c) to improve the binding to double-stranded DNA (Figure 7.41a). For comparison, complex 112 was also prepared. The complexes 111 and 112 bind to ct-DNA with KB values in the range of 1.4 104 to 5.6 105 M1 with an order 112 ~ 111c W 111b W 111a. Thermal denaturation as well as changes in viscosimetry were investigated and suggested groove binding and/or partial intercalation into the DNA. The melting temperature of ct-DNA increased by 7–10 1C (according to their KB values); the determined DTm data were of the same order of magnitude as for structurally related RuII complexes [109] but lower than for EthBr (DTm ¼ 14 1C). Complexes 111 and 112 showed the expected chemical nuclease activity in the presence of 3-mercaptopropionic acid and H2O2 (Figure 7.41b). Moreover, 111b/c and 112 efficiently cleaved plasmid DNA upon irradiation with visible light by forming hydroxyl radicals (Figure 7.41c). In these experiments, the ferrocene-containing derivative 111c was more efficient than 112, due to a cooperative effect of the two metal centers. The substantial role of hydroxyl radicals was evidenced by control experiments in the presence of various radical scavengers (e.g., DMSO, NaN3, KI). Photocytotoxicity studies with HeLa cancer cells also revealed a higher activity of 111c than of 112; IC50 values of 3.7 and 6.1 mM, respectively, were determined. (Figure 7.41d) [218].

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| 371

(a) N

N

N

N

N

N

N

N

N

N

N

N

N

phen

111a (N^N = phen) 111b (N^N= dpq) 111c( N^N = dppz)

dpq

112 (N^N = dppz)

N N N

(d)

Cu2+

100

N

N

(b) 5

1

9

58

2

77

3

92

4

5

99

12

6

52

7

8

70

9

84 % NC NC SC 10 Lane

(c) 6

21 27 68 92

61 31

85 99

1

2

6

8

3

4

5

7

dppz

72 26 65 78 56 % NC NC SC 9 10 11 12 13 14 Lane

Cell viability (%)

N

N

Fe

N =

Cu2+

N

80 60 111c (light) 112 (light) 111c (dark) 112 (dark)

40 20 0 0.1

1 10 Concentration (µM)

Figure 7.41 (a) CuII mono(terpyridine) complexes 111 and 112 (counterions omitted for clarity). (b) Gel electrophoresis diagram showing the chemical nuclease activity (formation of the nicked (NC) form in %) of 111 and 112 (10 mM) using supercoiled pUC19 DNA (0.2 mg, 30 mM base pairs) in the presence of 3-mercaptopropionic acid (MPA, 0.5 mM) and H2O2 (0.25 mM); lane 1: DNA control; lane 2: DNA þ MPA; lane 3: DNA þ 111a þ MPA; lane 4: DNA þ 111b þ MPA; lane 5: DNA þ 111c þ MPA; lane 6: DNA þ 112 þ MPA; lane 7: DNA þ H2O2; lane 8: DNA þ 111a þ H2O2; lane 9: DNA þ 111b þ H2O2; and lane 10: DNA þ 111c þ H2O2. (c) Gel electrophoresis diagram showing the light-induced DNA cleavage activity (formation of the NC form in %) of 111 and 112 using supercoiled pUC19 DNA (0.2 mg, 30 mM base pairs) and a photo-exposure time of 2 h; lane 1: DNA control, @ 458 nm; lane 2: DNA þ 40 -ferrocenyl-2,20 :60 ,200 -terpyridine (15 mM), @ 458 nm; lane 3: DNA þ 111a, @ 458 nm; lane 4: DNA þ 111b, @ 458 nm; lane 5: DNA þ 111c, @ 458 nm; lane 6: DNA þ 112, @ 458 nm; lane 7: DNA þ 111a, @ 568 nm; lane 8: DNA þ 111b, 568 nm; lane 9: DNA þ 111c, @ 568 nm); lane 10: DNA þ 112, @ 568 nm; lane 11: DNA þ 111a, @ 647 nm; lane 12: DNA þ 111b, @ 647 nm); lane 13: DNA þ 111c, @ 647 nm; and lane 14: DNA þ 112, @ 647 nm. (d) Cytotoxicity of 111c and 112 against HeLa cancer cells upon 3 h incubation in the dark followed by irradiation with visible light (400–700 nm). The darktreated and photo-exposed cells are shown by black and lighter symbols, respectively [218]. Figure reproduced with kind permission; r 2010 American Chemical Society.

Besides the hetero-dinuclear complex 42, containing a square-planar PtII mono(terpyridine) and an octahedral RuII bis(terpyridine) complex [63], the series of heteronuclear complexes 113–115 (Figure 7.42), based on the RuII/CuII couple, were also reported by the group of Reedijk [219]. The authors investigated the synergistic effect between the RuII bis(terpyridine) complex (as DNA-targeting agent) and CuII mono(terpyridine) complex. The charged RuII complex was

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100

372

| 7 Terpyridine Metal Complexes and their Biomedical Relevance N N

Ru N

113

N 2

N N

O

N

O

O

N

N Cl2Cu

N N

N

N

O

O

O

115

N

Ru2 N

N

114

N

N CuCl2 N

N O

O

N

N Cl2Cu

O

N

Ru N

O

O

O

N

CuCl2

N

N

N

N

2

N

O

O

O

N

CuCl2

2

N

N

Figure 7.42 Heteronuclear complexes 113–115 (counterions omitted for clarity) [219].

chosen to direct the CuII centers to the DNA, where subsequently oxidative cleavage of the DNA should occur. In contrast to complexes containing FeII, CoII, NiII, or MnII ions, cleavage of supercoiled circular FX174 DNA by CuII-containing complexes 113–115 using 3-mercaptopropionic acid (at pH 7.2), as reductant, could be observed. The following order of cleavage activity was found: 115 W 114 W 113 c [Cu(tpy)Cl]Cl. The increased activity of 115 and 114 with respect to 113 was attributed to the presence of two active CuII centers. For 115, a large fraction of linear DNA was observed even at micromolar concentrations; presumably, the relatively high positive charge of þ4 for 115 led to an increased electrostatic interaction with DNA (113 and 114 were only doubly charged), which was beneficial for the subsequent cleavage by the CuII mono(terpyridine) complexes. Substitution-inert RuII oligopyridinyl complexes are known to strongly bind to DNA via electrostatic interactions as well as by surface binding or partial intercalation [2]. It might be expected that charged but passive (in terms of their ability to cleave DNA) units supporting electrostatic interaction with DNA will also result in improved nuclease activity. The high potential of FeII complexes to perform, as efficient anticancer agents, is emphasized by the iron-containing glycopeptide bleomycin (currently in use, as antitumor agent, in chemotherapy), causing oxidative DNA damage in the presence of O2 and H2O2 [220–222]. However, complexes of FeII ions with terpyridine ligands showing either cytotoxic behavior and/or the ability to cleave DNA have not been reported; this might be attributed to the low stability, for example, of FeII

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7.2 Terpyridine Metal Complexes with Biological Activity NCCH3 2+ 2+

NCCH3

R N

N

Fe N N

R N

Fe

N

N

N

N

HO

N NCCH3

116a (R = H) 116b (R = CH3)

OH

117

Figure 7.43 FeII complexes 116 and 117 with pentadentate oligopyridine ligands [223].

Cytotoxicity values (IC50) of the FeII complexes 116 and 117 against selected human carcinoma and normal cell lines [223].

Table 7.14

Complex

116a 116b 117 Bleomycin Cisplatin

IC50 (lM) after 48 h HeLa

MCF-7

SUNE-1

HepG2

Hep3B

QGY-TR50

CCD-19Lu

4.00 7.74 1.17 1.00 20.05

12.20 68.50 27.40 W100 78.80

2.77 11.20 6.01 N.a. 14.20

0.74 0.13 0.93 5.40 32.70

0.13 0.24 13.50 N.a. 11.82

0.67 N.a. 6.75 N.a. 31.01

11.90 17.70 40.70 N.a. 117.30

mono(terpyridine) complexes, under physiological conditions. Wong et al. showed that by increasing the denticity of the chelating ligand stable FeII complexes (116 and 117) could be obtained (Figure 7.43) [223]. Complexes 116 bearing 2,20 :60 ,200 :600 ,2w:6w,20000 -quinquepyridine ligands were found to cleave supercoiled pCR21 DNA under physiological conditions (pH 7.5, 37 1C) without any additional reductant, conversely, no cleavage was observed for 117. The cleavage rate (50% of the DNA was cleaved after 10 s on agarose gel) was lower compared to bleomycin (100% cleavage under the same conditions). The cytotoxicity of 116 and 117 was tested in vitro against various types of human cancer cell lines, including some drug resistant variants (Table 7.14 summarizes the observed IC50 values). All complexes showed acute cytotoxic effects (within 48 h) against HeLa and HepG2 cell lines when compared to the clinically used anti-neoplastic drug bleomycin. Moreover, 116 and 117 were more potent than cisplatin in this respect; 116a was five-times more potent than cisplatin towards HeLa and SUNE-1 cell lines, whereas 117 was 18- and 2-times more potent, respectively. The FeII complexes appeared to be less toxic towards MCF-7 (human breast carcinoma) with IC50 values in the range 10–70 mM. Among the selected carcinoma cell lines, 116 and 117 were more potent (IC50 ¼ 0.1–14 mM) against hepatocellular carcinoma cell lines. In particular, 116a exhibited excellent cytotoxic effects on the Taxol-resistant

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| 373

374

| 7 Terpyridine Metal Complexes and their Biomedical Relevance N 118a: [Co(L1)2](ClO4)2 118b: [Co(L1)2](ClO4)3 119a: [Co(L2)2](ClO4)2 119b: [Co(L2)2](ClO4)3

HN

N

N

N

L1

N

N

N

N

L2

Figure 7.44 CoII and CoIII bis(terpyridine) complexes 118 and 119.

human hepatocellular carcinoma cell line QGY-TR50 (117 was ten-times less active) [223]. Cobalt-containing complexes, due to their application as potential hypoxia-activated prodrugs [224–226] and DNA cleavage properties that are comparable to the previously discussed RuII systems, have been reported [227, 228]. Indumathy et al. investigated homoleptic complexes of terpyridines with CoII as well as CoIII ions (118 and 119, Figure 7.44) [229, 230]. UV–vis titration and competitive binding experiments as well as viscosimetry measurements revealed intercalative binding of 118 in both oxidation states to ct-DNA [229]. The binding constants (KB) of 1.97 7 0.15 104 M1 (118a) and 2.7 7 0.20 104 M1 (118b) indicated that electrostatic interaction with the DNA also played an important role in the binding behavior of 118. Electrochemical studies according to Eq. (7.2) yielded a ratio of binding constants of 0.82, which was close to that derived from optical investigations (0.73). The studies on DNA cleavage (plasmid pBR322 DNA) showed that CoIII bis(terpyridine) complex 118b had the ability to photocatalytically (irradiation at 350 nm) cleave supercoiled DNA into the nicked form. In contrast to 118b, where about 90% of the DNA was converted into the nicked form, 118a did not exhibit any DNA cleaving activity. Further experiments in the presence of sodium azide, as a scavenger for singlet oxygen, were also conducted. Since the activity of 118b upon irradiation was not reduced, any role for singlet oxygen in the mechanism for the photocleavage could be excluded. Thus, the photoexcited state of the CoIII complex was responsible for the observed DNA cleavage; similar observations have been made in the CoIII-mediated photocleavage of proteins [231]. As concluded from optical investigations, as well as viscosimetry measurements, complexes 119 revealed similar intercalative behavior to that of 118; owing to the higher charge, a higher KB value was observed for CoIII bis(terpyridine) complex 119b (119a: 5.07 7 0.12 103 M1, 119b: 7.46 7 0.16 103 M1) [230]. Complex 119b exhibited efficient nuclease activity in the presence of ascorbic acid and H2O2, whereas 119a brought about DNA cleavage in the presence of hydrogen peroxide, as a co-reagent. This different behavior could be rationalized by the mechanism proposed by Fenton in which 119b, bound to DNA through outer sphere, was reduced to 119a by ascorbic acid, which in turn reacted with H2O2 to generate the reactive hydroxyl radicals. The freely diffusible hydroxyl subsequently then cleaved the DNA. Consequently, 119a was able to cleave DNA even in the absence of ascorbic acid, as co-reagent. The addition of DMSO, as hydroxyl radical scavenger, inhibited cleavage activity.

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7.2 Terpyridine Metal Complexes with Biological Activity O O

O V

N

O O N

N

VO(O2)2V complex

O O

O V hν

| 375

O

N

1O 2

N N

VO(O2)III complex

O

V

N

N

O

VO3V complex

Scheme 7.5 Photolytic generation of singlet oxygen (1O2) from a [VO(O2)2(tpy)]þ complex [232].

Finally, two examples for light transition metal ion complexes with terpyridines that showed nuclease activity are considered: CrIII bis(terpyridine) and [VO(O2)2]V mono(terpyridine) complexes [232–234]. Since peroxides play a crucial role in the oxidative cleavage of DNA and RNA, for example, by RuII, CuII, or CoII/III oligopyridine complexes, Kwong et al. raised the question: can transition metal ion complexes with peroxide ligands also act as efficient nuclease agents [232]? Several VV peroxo-complexes have been reported to exhibit antitumor activity [235] and Hiort et al. showed that [VO(O2)2]V complexes with phenanthroline-type ligands can photochemically cleave DNA [236]. Various [VO(O2)2]V complexes, including [VO(O2)2(tpy)(H2O)](ClO4), were investigated with respect to their ability to photocleave DNA [232]. The authors proposed the formation of singlet oxygen by photolysis (irradiation at 365 nm) of the complexes according to the mechanism depicted in Scheme 7.5; a VIII intermediate was postulated as a result of the photooxidation of the peroxo ligand and, subsequently, the chelating ligand was cleaved upon oxidation of the VIII center. However, an alternative mechanism involving the localization of electrons on the oligopyridine ligand could also be speculated. Exposure of DNA to singlet oxygen (1O2) is known to lead to oxidative base damage and strand breaks; in the given examples, [VO(O2)2(tpy)(H2O)]þ cleaved 23% of the DNA, which was significantly less than its bipyridine or phenanthroline counterparts (50–100% cleavage) [232]. Vaidyanathan and Nair used the same ligands L1 for their CuII bis(terpyridine) complexes 109 (see Figure 7.39a) also for the synthesis of CrIII bis(terpyridine) complexes 120a (R ¼ CH3) and 120b (R ¼ Br) [233, 234]. Moderate binding (120a: KB ¼ 3.1 7 0.2 103 M1, 120b: KB ¼ 1.25 7 0.2 104 M1) to ct-DNA was determined from UV–vis absorption titration and thermal denaturation experiments (increase in Tm of about 4 1C). The CrIII complexes were bound to DNA as well as to all four mononucleotides (i.e., dGMP, dAMP, dCMP, and dTMP); as a result, the photoluminescence intensity of 120b was fully quenched (only DNA, dGMP, and dAMP quenched the emission of 120a). The excited-state potentials of the complexes were estimated to be 1.65 V (120a) and 1.85 V (120b) vs. the standard hydrogen electrode (SHE); thus, 120b appeared to be the more potent photooxidant. The ability of 120 to act as photochemical nucleases was monitored by gel electrophoresis of plasmid DNA; supercoiled, closed-circular DNA was converted into an open-circular form, induced by a single-strand nick. The guanine base is the most easily oxidizable nucleobases and, therefore, the formation of a radical cation of guanine by electron-transfer to the excited-state of 120 was assumed to

07

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N

376

| 7 Terpyridine Metal Complexes and their Biomedical Relevance play a key role in the photochemical damage of DNA by CrIII bis(terpyridine) complexes [234]. 7.2.2 Biolabeling

In the first part of this chapter, we showed that the design of transition metal ion complexes that can bind and/or react at specific positions of biomolecules has attracted considerable interest. Moreover, the flexibility in coordination geometry and the rich photophysical as well as electrochemical properties of such complexes can also be applied to the covalent linkage to, for instance, nucleoside phosphoramidites for solid-state DNA synthesis or to other biological substrates (e.g., nucleic acids, peptides, and proteins) for a wide range of mechanistic and analytical investigations (see Reference [237] and references cited therein). In the following, we summarize examples where terpyridine-containing transition metal ion complexes were utilized in this respect. Ossipov et al. utilized the carboxy-functionalized complex [(tpy)Ru(dppzCOOH)(MeCN)]2þ (121, Figure 7.45a) to conjugate a RuII center to an

COOH

(a) N N

N 2+

N

Ru N

N N 121

NCCH3

(b)

Me C N

G

N N

G 2+

OH2 2+

hν, H2O

N Ru N N N N

N N

– CH3CN

N N

Ru N N

N

(c) 5'

5'

O

O

O 3'

O

O L

O

NH2

O 3'

N

O O

O

O Ru

N H

N

N

N Ru2+

N

N

N

Figure 7.45 RuII complex 121 (a), the photo-initiated crosslinking to yield the ODNDNA duplex (b), and the binding site and the tethered RuII complex (c) (see also Table 7.15; counterions omitted for clarity) [238]. Figure reproduced with kind permission; r 2002 American Chemical Society.

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N


oligodeoxynucleotide (ODN) strand either by solid-phase synthesis (sps) or postsynthetic labeling (psl) in high yields [238]. Table 7.15 summarizes the RuIIlabeled ODN-conjugates that were synthesized. A duplex with a complementary target DNA strand was formed; this ODNDNA duplex was significantly more stable than a complex-free ODNDNA duplex due to the partial intercalation of the dppz moiety. The reactive aquo-complex [(tpy)Ru(dppz–ODN)(H2O)]2þ was generated in situ and was photolytically crosslinked the duplex DNA strands by reaction with the G residue of the opposite strand (Figure 7.45b). The highest yield (34%) of the photochemically crosslinked product was obtained when the ODN strand was functionalized at both ends (i.e., 30 - and 50 -position) with the RuII complexes. ODNs carrying one RuII center were less efficient in crosslinking and the observed yield followed the order “30 -(RuII-complex)-ODN” W “50 -(RuII-complex)-ODN” W “middle-(RuII-complex)-ODN.” The cross-coupling efficiency decreased as the stabilization of the resulting duplex increased: for the most rigidly-packed structure [i.e., the duplex with the “middle-(RuII-complex)-ODN”], the flexibility of the RuII center was reduced and partially prevented the reaction with the opposite strand. The crosslinking yield was determined by gel electrophoresis, followed by mass spectrometric analysis. Another example of the useful incorporation of a terpyridine complex into an oligonucleotide was shown by Daniher and Bashkin [239]. A 17-mer oligonucleotide probe containing a serinol-functionalized terpyridine unit (122, Figure 7.46a), which can be utilized as a building block in DNA sequencing, was designed to target a 159-mer fragment of the HIV gag gene mRNA (Figure 7.46b). Experiments showed that upon complexation of the terpyridine unit with CuII ions, the target mRNA was specifically cleaved after forming a duplex. Five different 17-mer DNA probe sequences 123 were prepared, via automated DNA

Table 7.15

Synthesis of RuII-labeled ODN-conjugates and their target complementary oligo-

DNA [238]. Substrate

Modification

Sequencea

Yield (%) spsb

ODN

Natural 50 -Modified 30 -Modified Middle 50 -Modified

oligo-DNA

50 -CTTACCAATC-30 50 -L-pCTTACCAATC-30 50 -Ru-pCTTACCAATC-30 50 -CTTACCAATCp-L-30 50 -CTTACCAATCp-Ru-30 50 -CTTACp-L-pCAATC-30 50 -CTTACp-Ru-pCAATC-30 50 -L-p-CTTACCAATCp-L-30 50 -Ru-pCTTACCAATCp-Ru-50 50 -TGATTGGTAAG-30

pslc

67 56 46 59 69 87

5 28

64 34 87

a L and Ru represent the binding site and the tethered RuII complex, respectively (see Figure 7.45c). b sps: solid-phase synthesis. c psl: postsynthetic labeling.

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| 377

378


H N

O O N

N

OH

OH

N 122

(b) 5’-1(775)GGAGAA6 AAUAAAAUAG46 GACAUAAGAC86 AGACCGGUUC126 CAG159(933)-3’

AUUUAUAAAA16 UAAGAAUGUA56 AAGGACCAAA96 UAUAAAACUC136

GAUGGAUAAU26 UAGCCCUACC66 GGAACCUUUA106 UAAGAGCCGA146

CCUGGGAUUA36 CAGCAUUCUG76 GAGACUAUGU116 GCAAGCUUCA156

(c) 123a: 123b: 123c: 123d: 123e:

5’-CTACATAGTCTCTAAAG-3’ 5’-XTACATAGTCTCTAAAG-3’ 5’-CTACAXAGTCTCTAAAG-3’ 5’-CTACATAGXCTCTAAAG-3’ 5’-CTACATAGTCXCTAAAG-3’

X: location of 122 within the chain

Figure 7.46 (a) Serino-functionalized terpyridine 122; (b) sequence of a 159-mer fragment of the HIV gag gene mRNA (the 17-mer recognition unit is underlined); (c) sequence of the 17mer DNA probes 123 [239].

synthesis, with 122 included at a different position of each probe. Gel electrophoresis confirmed the specific cleavage of the mRNA, according to the specific position of the serinol-terpyridine in the probe. The best cleavage performance was observed for probe 123e, showing a cleavage rate of 84% after 72 h at 45 1C. Stewart and McLaughlin have described the connection of terpyridine ligands via tri(ethylene glycol)-spacers to two complementary 20-mer DNA fragments (A and B) (Figure 7.47a). Complexation with RuII ions gave the DNA-containing complexes A-[Ru]-A and B-[Ru]-B that can be considered as metallo-supramolecular homopolymers (see also Chapter 6.3.2.1). The mixing of these two complexes resulted in long linear arrays through self-assembly (i.e., hybridization of the DNA). The length of these “chain-extended” DNA-polymers could be adjusted by varying the stoichiometric ratio of the complexes, according to the rules of polyaddition reactions [240]. In a related work, Choi et al. prepared three “DNA diblock copolymers” by self-assembly with FeII ions (the diblock copolymers were separated from the homopolymers by preparative gel electrophoresis). Mixing of A[Fe]-B0 , B-[Fe]-C0 , and C-[Fe]-A0 (A/A0 , B/B0 , and C/C0 represent complementary strands) in equimolar amounts yielded a triangular arrangement of doublestranded DNA [241]. The attachment of self-complementary DNA sequences to the 6,600 -position of a terpyridine resulted in a hairpin-mimic; notably, hairpin structures of DNA and RNA are common structural motifs in nucleic acids [242]. The terpyridinecontaining hairpin-mimic was thermodynamically more stable than conventional examples (based on dA4 or dT4 loops). Various transition metal ions were coordinated to the terpyridine moiety [owing to the geometrical restriction, only

07

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N

(a)

20-merDNAs: A: 5‘-R-ACG CCG CTA TTA TCG CCG CA-3‘ B: 5‘-R-TGC GCG GAT Aat AGC GCG GT -3‘

R= O(H2CH2CO)3H2C

N

N

(b)

A-[Ru-]-A B-[Ru-]-B hybridization A{-[Ru]-AB}n-[Ru]-B

Figure 7.47 Representation of 20-mer DNA fragments end-functionalized with a terpyridine site (a) and self-assembly with RuII into A-[Ru]-A and B-[Ru]-B homopolymers as well as chainextended A{-[Ru]-AB}n-[Ru]-B polymers (b) [240].

Ph

Ph N N

N

NH

N

Mll

N

N

NH

M2

N H

N H

Tm 70.8 C (Pdll) Tm 64.3 C (Cull) Tm 61.0 C (Znll) Tm 58.5 C (Nill) Tm 58.0 C (Coll)

Tm 83.5 C EDTA

Figure 7.48 Reversible switching between a metal-free hairpin-mimic (left) and the complexed one (right); the melting temperatures of both species are also listed [243]. Figure reproduced with kind permission; r 2004 Wiley-VCH.

mono(terpyridine) complexes were formed], which strongly influenced the melting behavior of these species; the melting temperature decreased in the order PdII W CuII W ZnII W NiII ~ CoII and was in all cases lower than for the metal-free hairpin-mimic (Figure 7.48). Thus, structural changes in the hairpin-mimic upon complexation could be concluded. Moreover, the coordination could be reversibly switched by using EDTA, as competing ligand [243]. In related work, two ligands – a terpyridine and 2,9-diphenyl-1,10-phenanthroline (dpp) – were attached site-specifically to DNA strands by using automated

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| 379

380


M

tpy2:ab

M

M•tpy2:ab

dpp2:ab

M

M•dpp2:ab

tpy:dpp:ab

M•tpy:dpp:ab

Figure 7.49 Transition metal ion coordination to three different DNA–ligand environments [244]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

Thermal denaturation temperatures of duplex DNA strands with different DNA– ligand environments [244].

Table 7.16

Binding motif

Metal-free

AgI

CuI

CuII

ZnII

CoII

FeII

tpy:tpy dpp:dpp tpy:dpp

69 52 64

66 64 63

N.a.a 80 63

66 N.a.a 69

64 63 64

74 57 60

83 54 63b

a

Unstable species. FeII ion preferred intermolecular binding by forming FeII bis(terpyridine) complexes.

b

procedures [244]. Assembly of the structures through DNA hybridization afforded double-stranded DNA helixes with three different types of binding sites for transition metal ions: tpy:tpy, dpp:dpp, and tpy:dpp, respectively, which were in close proximity to the DNA base stack (Figure 7.49). As shown by thermal denaturation experiments, specific transition metal ions displayed a strong preference for a single coordination environment; thus, FeII and CoII showed a high preference for binding to tpy:tpy, CuI preferred dpp:dpp, and CuII preferred the mixed-ligand environment (tpy:dppy). Owing to the strongly bound metal ions, a remarkable stabilization of the DNA structures (revealed by an increase of the melting temperature), in particular for the FeII bis(terpyridine) case, could be observed (Table 7.16). Electron-transfer (ET) rates in azurin from Pseudomonas aeruginosa were investigated by Gray et al. [245]. This protein is widely studied as a representative for electron-transfer proteins, in particular with respect to the redox processes of the coordinated CuII ion (see references cited in Reference [245]). Labeling of azurin in His83-position with [(tpy)Ru(L)]2þ (L ¼ bpy or phen) allowed one to determine the time constants for the reorganization energy of the CuI - RuIII process; the values observed in the single crystals (0.6–0.8 eV) were roughly the

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same as those measured in solution, indicating very similar protein structures in both states. Both an enzyme (lipase B from Candida antarctica, CALB) and a protein (bovine serum albumin, BSA) were functionalized with a terpyridine ligand by reaction of the free thiol groups, which were obtained after reduction of exposed disulfide bonds with a maleimide-functionalized terpyridine derivative [246, 247]. The biomolecules were self-assembled by adding FeII ions. Analytical ultracentrifugation (AUC) experiments revealed the formation of monomeric, dimeric, and tetrameric metallo-proteins as well as larger aggregates. As shown by CD measurements, the content of helical structures did not decrease at low concentrations (o50 mM); the addition of 50% ethanol led to the formation of solely monomeric species, although the helical character was not disrupted. Full denaturation (i.e., dissociation and unfolding) was observed at temperatures above 90 1C. Figure 7.50 summarizes the proposed equilibria of the aggregation behavior of the metalcontaining BSA derivative. A class of bio-surfactants consisting of a protein or an enzyme linked to a hydrophobic organic polymer is referred to as “giant amphiphiles” in the literature (Figure 7.51a) [248, 252]. Velonia et al. conjugated polystyrene (PS) that was endfunctionalized with a terpyridine ligand with terpyridine-bearing BSA or CALB, applying the special coordination chemistry of RuII ions [250, 251]; the metalcontaining giant amphiphiles were found to aggregate in a similar way to that of molecular amphiphiles. The superstructures formed by this modular approach differed significantly from those formed by direct coupling of the polymer to the protein surface (Figure 7.51b). Later studies showed that cytochrome c could also be functionalized with terpyridine [253] as well as RuII bis(terpyridine) complexes [254, 255]. The bioconjugates could be obtained in high purity and were suggested to be possible candidates for application in bioelectronic devices, where redoxactive protein-based bioconjugates play a crucial role. Schubert and coworkers reported terpyridine derivatives in which a biotin moiety (vitamin H) was attached at the 40 -position via a short alkyl spacer (124) or a

∆C, ∆T

∆C, ∆T

EtOH

EtOH

∆C, ∆T

EtOH

EtOH

Figure 7.50 Equilibrium of different aggregates of the metal-containing BSA derivative [247]. Figure reproduced with kind permission; r 2004 CSIRO Publishing.

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| 381

382


(b)

CALB-PS CALB-[Ru]-PS

Figure 7.51 (a) Representation of a computer-generated model of a giant amphiphile (lipase-PS, the degree of polymerization of the PS block was 40, molecular volume ¼ 25 nm3, Mn ¼ 40 000 g mol1) [248]; (b) transmission microscopy images (TEM) of a CALB-PS giant amphiphile [249] and its metallo-supramolecular analogue (the proposed tubular structure is also shown) [250, 251]. Figure reproduced with kind permission; r 2006 Springer Verlag.

long poly(ethylene glycol) chain (PEG75, 125) (Figure 7.52a) [256]. Biotin itself is a well-known binding unit for proteins, for example, avidin (see Figure 7.52b for a representation of the biotin–avidin complex) or streptavidin. Terpyridines 124 and 125 represent strong non-covalent building blocks for both biology and synthetic supramolecular chemistry. Various types of applications were envisioned by the authors, indicating the potentially broad field of utilization: immobilized biosensors (e.g., on flat surfaces, nanoparticles, or polymer beads), giant amphiphiles (by replacing the hydrophilic PEG by a hydrophobic PS chain), or micelles (as nanoreactors or carriers for drug delivery). The group of Lo reported a series of isocyanate-functionalized heteroleptic IrIII bis(terpyridine) complexes 126 (Figure 7.53) [237]; intense and long-lived emission in degassed as well as aerated MeCN solutions at 298 K and in low temperature glass was observed. In general, emission of such complexes originated from a predominant intraligand (3IL) excited state mixed with some 3MLCT character [170]. The isocyanate-group can be used for the reaction with primary amines of biomolecules to form stable thiourea moieties and, in the present case, for the labeling of human serum albumin (HSA) and BSA with 126a. From the UV–vis absorption spectra, a degree of labeling of 2.7 and 1.4 was determined for 126aHAS and 126a-BSA, respectively. Both bioconjugates exhibited intense and long lived emission at lPL ¼ 530 nm in aqueous buffers under ambient conditions; inefficient quenching of the emission by oxygen was rationalized by the shielding of the complex by the protein amino acid residues. Owing to the relatively long emission lifetimes of the proteins in aerated aqueous buffer, complexes such as 126 could be utilized in time-resolved bio-assays. The covalent modification of amino acid residues has been an important tool for structural, spectroscopic, and mechanistic studies of proteins [79, 81, 82]. Proteins labeled with transition metal ions have been utilized in X-ray crystallography and electron microscopy as well as in NMR relaxation and EPR spectroscopy

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7.2 Terpyridine Metal Complexes with Biological Activity (a)

O HN H

N

NH H

H N

S O

O

N

124 N

O HN H

N

NH H

H N

S O

O

75

O

O

N

125 N

(b)

Figure 7.52 (a) Biotin-modified terpyridines 124 and 125; (b) representation of the computer-generated model of the biotin–avidin complex [256]. Figure reproduced with kind permission; r 2004 American Chemical Society.

N R

N

N Ir

N

3

N

NCS

N 3 PF6

126a (R = H) 126b (R = C6H5) 126c (R = 4-CH3-C6H4) 126d (R = 4-Cl-C6H4)

Figure 7.53 IrIII bis(terpyridine) complexes 126 [237].

experiments [257]. In comparison to terpyridine complexes of other transition metal ions, the application of square-planar PtII mono(terpyridine) complexes, as labeling agents for biomolecules, is a relatively old field of research; the selective reactivity of [(tpy)PtCl]þ (11) towards the amino acids cysteine, histidine, and arginine was utilized by Kostic´ in the late 1980s [80, 81]. Cytochrome-c proteins from horse and tuna heart, Candida krusei, and baker’s yeast were chosen for labeling studies, since they contain those amino acids that can most easily be reacted with 11. Various earlier studies on the protein structure of cytochrome c revealed that the accessible positions of the reactive amino acids

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| 383

384

| 7 Terpyridine Metal Complexes and their Biomedical Relevance Positions of accessible amino acids that are exposed on the surface of cytochrome c proteins and could react with 11.

Table 7.17

Cytochrome c from

Amino acid and its location

Reference

Horse heart Tuna heart Candida krusei Bakers’ yeast (iso-1 form)

His26,His33, Arg91 His26, Arg91 His33, His39, Arg91 His33, His39, Arg91, Cys102

[258] [259–264] [265] [266, 267]

were located on the outer-sphere of the protein structure; cytochrome c protein isolated from bakers’ yeast (iso-1 form) contains all three reactive amino acids (i.e., Cys, His, and Arg) exposed to the surface of the protein; however, cytochrome c from tuna or horse heart as well as from Candida krusei only contains histidine and arginine on the surface (Table 7.17) [258–267]. All cytochrome c proteins were incubated with equimolar amounts of 11 at 25 1C for 24 h (0.1 M acetate buffer at pH 5.0) [80, 81]. Under these conditions, only the Cys and His residues reacted with 11; however, the Arg91 residues in cytochrome c proteins required longer incubation times and additional heating in a buffer at pH 7.0. The products were separated by cation exchange chromatography and the resulting tagged fractions were investigated by UV–vis absorption and NMR (1H and 195Pt) as well as electrospray ionization (ESI) MS. Cytochrome-c from horse heart was mainly labeled with [Pt(tpy)]2þ at His33 in the hydrophilic outer-sphere (50% relative yield); the His26 residue – located in a hydrophobic pocket of the protein – was only labeled to a minor extent (about 5% relative yield) [258]. Since cytochrome c from tuna only possesses the less reactive His26 (Table 7.17), the degree of labeling was low (10% relative yield) [259–264]. The UV–vis absorption spectrum of [Pt(tpy)]2þ-tagged cytochrome c from horse heart revealed two unique bands at 328 and 342 nm corresponding to the MLCT bands of the [(tpy)Pt(His)]2þ complexes, which displayed different ratios of the extinction coefficients, depending on the position – and therewith on the environment – to which the PtII center was attached (His33: e342/e328 ¼ 1.51, His26: e342/e328 ¼ 1.15). Thus, 11 could be utilized as a sensitive protein probe. Among the cytochrome c proteins, only that from bakers’ yeast contained a Cys residue at the 102-position (i.e., near the carbonyl terminus) [266, 267]. This residue was expected to react with 11 since it was proven that thiol-containing biomolecules are much more reactive than Ncontaining analogues [81]. However, the yeast protein (iso-1 form) was mainly labeled at the His33 and His39 residues (overall 40% relative yield), and not at the Cys102 residue. The cysteine moiety was found to be located deep in the protein’s hydrophobic region [266, 267], thus being almost inaccessible for complexation [81]. The C. krusei protein, a structural analogue to the yeast protein lacking the free Cys moiety [265], was labeled at His33 and His39 (30% relative yield in each case) [81]. The Arg91 residues, which were barely exposed at the surface in either horse or tuna proteins [258–267], were labeled with 10% yield by [Pt(tpy)(Cl)]þ under forcing conditions [79]. Therefore, the non-invasive labeling agent

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[Pt(tpy)(Cl)]þ (11) could only be attached to reactive groups on the protein’s surface [81]. Moreover, it could be shown by UV–vis absorption, 1H NMR, and EPR spectroscopy as well as cyclic and pulse voltammetry that the protein’s morphology was not disturbed upon labeling with PtII mono(terpyridine) units [80]. Brothers and Kostic´ also reported a reversible, non-invasive modification of serine proteases enzymes (i.e., a-chymotrypsin and a-lytic proteases) at their His57 and His40 positions in the former and His57 in the latter. Though labeling the His57 residue of these enzymes altered their catalytic triad site (Ser195, His57, and Asp102), the [Pt(tpy)Cl]þ-tagged enzymes still featured esterase and amidase activity, suggesting that the PtII mono(terpyridine) labels for these enzymes were non-invasive [268]. Strothkamp and Lippard reported the exclusive labeling of the alternating copolymer of the nucleosides adenine and uracil possessing the phosphorothioate backbone, poly(S-A-U), with a PtII mono(terpyridine) complex, which was attached to the sulfur of a phosphorothioate group. No degradation or loss of sulfur from the polymer could be observed upon coordination to the PtII center. It was proposed that complexes 11 or 53 ([(tpy)Pt(H2O)]2þ) could be labeling agents for the sequencing of nucleic acids by electron microscopy, since the PtII complex reacted exclusively with phosphorothioate groups that were incorporated into the RNA or DNA backbone adjacent to a specific base (Figure 7.54) [90]. Lowe and coworkers designed a family of dinuclear PtII mono(terpyridine) complexes 127 (Figure 7.55), which could intercalate into two DNA duplexes in A

U

A

U

A

U

O

X

O

X

O

X

P

P

P

P

P

P

O

O

O

O

O

O

poly(A-U): O poly(s-A-A): S

Figure 7.54 Alternating copolymers of nucleosides adenine and uracil with phosphate and phosphorothioate backbone [90]. Figure reproduced with kind permission of the author.

spacer N N3

N

Pt N

=

N spacer

N 4 or 6

127

N

Pt

N

N3 N

N

NH3 Pt N NH3 N

NH3 Pt N NH3

Figure 7.55 Azido-functionalized dinuclear PtII mono(terpyridine) complexes 127 (counterions omitted for clarity) [269].

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| 385

386

| 7 Terpyridine Metal Complexes and their Biomedical Relevance N N

N

Pt N

R

N

Pt

Cl

N

TfO 128a (R NCS) 128b (R NHCOOCH2I)

HO

129

Figure 7.56 Reactive PtII mono(terpyridine) complexes 128 [270] and the estrogen-modified PtII mono(terpyridine) complex 129 (counterions omitted for clarity) [271].

close spacial proximity to each other, in order to study the topology of DNA. The azido-groups were introduced at the 40 -positions of the terpyridines to enable further labeling of the sites of intercalation and the linker was designed to be susceptible to cleavage by thiols and cyanides, a requirement for 2D electrophoresis to identify actual the sites of intercalation [269]. A more recent generation of luminescent biolabeling agents was designed by introducing a reactive isothiocyanate- (128a) or iodoacetimide-group (128b) on the acetylene co-ligand of the complexes (Figure 7.56) [270]. Specifically, HSA, which is the most abundant plasma protein with many physiological functions, was successfully labeled with complex 128a and 128b from its amino- and thiol-functionalized residues, respectively, forming stable thiourea and thioether linkages. These PtII-tagged HSAs displayed induced low-energy MLCT/LLCT absorption and 3MLCT/3LLCT emission bands at about 470 and 630 nm, respectively. The emission bands of the labeled HSAs differed significantly from those of 128, proving successful labeling. The AuIII mono(terpyridine) complex 28 was also tried as a label for BSA; however, progressive reduction of the AuIII center and decomposition of the complex were observed [272]. The estrogen-containing PtII mono(terpyridine) complex 129 (Figure 7.56) was designed to facilitate the cellular delivery of the metallo-intercalator to cells with an estrogen receptor [271]. The single-crystal X-ray structure of 129 revealed an extended chain-like stacking through p–p and unusual Pt–p packing without any Pt Pt interactions. Complex 129 was successfully bound to the estrogen receptors in MCF-7 cell lines as well as to HSA and BSA, which are steroidtransporting proteins and were covalently attached to the guanine bases in DNA and 12-mer base pair DNA fragments. The binding of 129 to these biomolecules was characterized by a competitive radiometric binding assay, CD spectroscopy, Fourier-transform ion cyclotron resonance (FTICR), and ESI MS. Lanthanide chelates, due to their unique luminescence properties (long emission lifetimes, large Stokes’ shifts, narrow emission bands, and negligible concentration quenching), have gained much attention with respect to their applications in a wide variety of bioanalytical assays in diagnostics, research, drug discovery, as sensing tools, and in bio-imaging (see Reference [273] and references

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7.2 Terpyridine Metal Complexes with Biological Activity O I

NH

OMe NCS

N N N

Eu

N 3

COO COO

N

N N

COONa

COO 130 Eu-TMT-Isothiocyanate (GE Healthcare)

N

Ln3 COO

N

LnIII = EuIII, SmIII or TbIII N

COONa

COO

COO 131 Ln-W8164 (Perkin-Elmer Life & Anal. Sciences)

Figure 7.57 Commercially available LnIII mono(terpyridine) complexes 130 and 131 [282].

cited therein). Among others, aryl-substituted terpyridine units were utilized, as the energy-absorbing/-donating group, and coordinated to lanthanide (LnIII) centers as the emitting ions (e.g., EuIII, SmIII, or TbIII); the stabilizing carboxy moieties and reactive groups for coupling the chelate to biomolecules (e.g., via iodoacetamido or the isothiocyanato groups) were added [274, 275]. Such systems became commercially available (130 and 131, Figure 7.57) and have been routinely used in various types of time-resolved fluorometry (or quenching) based assays [276–286]. For instance, Poupart et al. synthesized an EuIII chelate containing the wellknown nonadentate 6,600 -bis(aminomethyl)-40 -phenyl-2,20 :60 ,200 -terpyridine chelating unit and an aminopropargyl linker that could be applied to attach reactive functionalities (e.g., an iodoacetamido or aldehyde group) [282]. The linker was found to be a versatile tool for solution-phase peptide labeling. Furthermore, the use of a non-standard thiol protecting group that was removable under mild reducing conditions for temporarily masking a lysine residue enabled the development of an efficient method for producing dual fluorescently-tagged peptides (Figure 7.58). This labeling strategy was complementary to the conventional solidphase methods [283, 287–289], since the experimental conditions were compatible with a wide range of fluorophores showing only poor or moderate chemical stability. The photophysical properties (i.e., luminescence lifetime and quantum yield) of the free chelate, however, differed from those found in the bioconjugate (due to changes of the chemical environment). Nishioka et al. synthesized the EuIII chelates 132 and 133 (Figure 7.59), which could be easily attached to the streptavidin protein and a DNA fragment in aqueous solution (using a weakly basic carbonate buffer). In contrast to the previously

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| 387


388

SO3-

-OOC N -OOC

N Eu3 N

O

N

HN

HO3S

N

-OOC N

N

O O

HN NaOOC

S Ac-Cys-Asp-Glu-Val-Asp-Lys-NH

Figure 7.58 A short dual fluorescently-labeled peptide [282].

Cl N Cl

N N

NH

NH

O

O

O N O

N N N

Eu3

N N

COOCOO-

O

N N

132

COONa

N

COO-

Eu3

N

COOCOO-

N 133

COONa COO-

Figure 7.59 Europium(III) chelates for the labeling of biomolecules [284].

reported LnIII chelates, the luminescence intensity was not dependant on the buffer and did not change when attached to the biomolecule. Moreover, higher solubility in water-based buffers, easier attachment to biomolecules, longer luminescence lifetimes, and longer excitation wavelength range (which is of relevance for applications in laser-induced luminescence measurements) could be achieved [284]. More recently, the synthesis of photoluminescent LnIII chelates that allow selective solution phase labeling of bioactive molecules using phosphate elimination, native chemical ligation, and “click-chemistry” was also described [290].

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286 Hashino, K., Ikawa, K., Ito, M., Hosoya, C., Nishioka, T., Makiuchi, M., and Matsumoto, K. (2007) Anal. Biochem., 364, 89–91. 287 Kruger, R.G., Dostal, P., and McCafferty, D.G. (2002) Chem. Commun., 2092–2093.

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Terpyridines and Nanostructures*

8.1 Introduction

Nanotechnology is one of the key topics in current science and technology that is devoted to the manipulation of matter on an atomic or molecular scale. The structures commonly have sizes between 1 and 100 nm in at least one dimension and modern nanoscience deals with the development of devices within that regime [1, 2]. Research in this field was inspired by the talk entitled “There’s plenty of room at the bottom,” given in 1959 by visionary Richard Feynman, who realized that the ability to produce miniature scaled devices has several advantages [3]. The most straightforward advantage is that they can be economical, since small devices make better use of available resources. Secondly, small devices with completely new properties are accessible – quantum mechanical effects play a role in nanometer-sized devices. Feynman proposed the fabrication of synthetic devices that operate at the nanometer scale; such fabrication of “small machines” could be realized starting from a small machine that, in turn, could produce even smaller machines that, in turn, . . . until finally devices are produced on an atom-by-atom scale. Devices operating in the nanoregime are expected to provide several breakthrough applications, such as more powerful computers and increased data storage due to more efficient and smaller components [4]. Moreover, molecular control over the assembly process would permit the fabrication of extremely strong or highly conductive materials due to the absence of defects (the properties of materials critically depend on the degree of ordering of their molecular building blocks). The broad range of potential applications of nanoscaled devices covers the fields of solar cells, (molecular) electronics, biosensors, and medicine [5–11]. In general, there are two different approaches towards the fabrication of functional nanostructures: the bottom-up and top-down strategies (Figure 8.1). The bottom-up approach utilizes small and rather simple building blocks that will self-assemble into larger, more complex nanostructures. For these techniques, (bio)chemists are inspired by Mother Nature, who uses a large variety of covalent and non-covalent *Parts of this chapter are reproduced from Adv. Mater. 23 (2011) DOI: 10.1002/adma.200101251 by permission of Wiley-VCH Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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| 8 Terpyridines and Nanostructures (bio)chemistry: engineering up

mm

mm

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Å

physics: engineering down Figure 8.1 The two key strategies for the fabrication of functional nanostructures: bottom-up and top-down [12]. Figure reproduced with kind permission of the author.

(i.e., supramolecular) interaction mechanisms. Indeed, nature itself is full of functional devices in which each cell in our bodies consists of a complicated interplay of thousands of “small machines”: proteins, enzymes, and DNA. The field of supramolecular chemistry has emerged, specifically applying non-covalent interactions like hydrogen bonding, p–p stacking, ionic, and metal-to-ligand coordination as well as van der Waals forces to assemble small molecules into well-defined complex architectures by (directed) recognition processes [13–16]. Since these interactions are generally weaker than covalent bonds but still dynamic, a certain degree of reversibility is retained. Over the last few decades, a broad range of functional materials, based on the concepts of supramolecular chemistry has been developed. The second approach, top-down fabrication, scales down existing tools and techniques to create smaller devices. Typical examples of these products are the so-called micro-electromechanical systems (MEMS) and products of the silicon semiconductor industry. The top-down approach utilizes the large tool-box of available modes of construction (i.e., deposition and removal of materials) and imaging methods. Deposition techniques include chemical vapor deposition (CVP) [17], physical vapor deposition (PVD) [18], electro-deposition [19], epitaxy, oxidation, and casting. Patterning of surfaces is performed by inkjet printing or lithographic processes, such as UV lithography as well as electron- and ion-beam patterning; however, these techniques often require the formation of suitable masks. Additionally, various types of etching methods have been introduced to develop the patterned substrates. Among others, redox-active complexes with terpyridine ligands have been bound to surfaces – covalently as well as non-covalently – to yield highly-ordered mono- or multilayer arrays [20, 21]. The potential application of such modified surfaces in the field of molecular electronics is the main driving force for this research. The modification of inorganic or organometallic nanoparticles is another intersection of modern terpyridine chemistry and nanotechnology [22]. Terpyridine ligands have been utilized for two main purposes: stabilization of nanostructures (i.e., non-covalent interactions with the particles’ surfaces via the pyridine moieties) [23] and functionalization of nanostructures (i.e., covalent attachment of terpyridines to nanoparticles via lateral substituents) [24]. Terpyridine chemistry meets nanotechnology – the literature dealing with this emerging topic will be summarized in this chapter. The contributions concerning nanometer-sized structures derived from terpyridine-containing polymers (e.g., micelles, thin solid films, phase-separated areas in the bulk) are discussed in Chapters 5 and 6.

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8.2 Terpyridines and Surface Chemistry

Various (asymmetric) organic transformations have been found to be catalyzed by terpyridine ligands or their transition metal complexes. In most cases, homogeneous catalysts are employed that have to be removed after the reaction and cannot, in general, be recycled afterwards. The immobilization of terpyridine ligands enables the synthesis of new heterogeneous catalysts that can be separated easily from the reaction mixture and (potentially) be reused. The functionalization of organic beads and resins on their outer shell with terpyridine ligands will be considered in the last part of this chapter.


In recent years, considerable attention has been focused on surface modification by forming highly ordered organic films of few nanometers to several hundred nanometers thickness [25–28]. The most straightforward and general approach to such ultrathin films is the immersion of a substrate in a dilute solution (commonly in the mM regime) of the organic compound at ambient conditions. Such resultant unimolecular organic films are referred to as self-assembled monolayers (SAMs) in the literature. SAM formation provides easy access to surface functionalization with organic (aliphatic and aromatic) molecules containing suitable functional groups: thiols, nitriles, carboxylates, amines, and silanes to name only a few. Various types of metallic (e.g., Au, Cu, Ag, Pd, Pt, Hg) as well as semiconducting surfaces [e.g., C, Si, GaAs, indium tin oxide (ITO)] have been utilized, as substrates, for the growth of SAMs [29]. These types of SAM-modified surfaces are highly useful for investigating fundamental phenomena on artificially designed nanostructures: distance-dependent electron-transfer processes [30], the mechanism of single electron transfer [31], and the observation of molecular events (e.g., coulomb staircases) [32]. Moreover, monolayers on metallic or semiconducting surfaces are also of relevance for several applications, such as (bio)chemical sensing [33, 34], control of surface properties (e.g., wettability and friction) [35], corrosion protection [36], patterning [37, 38], semiconductor passivation [39], or nonlinear optics [40]. Molecular engineering is an interdisciplinary area where supramolecular systems, which are capable of electronic operations like switching, gating, rectification, or amplification, have to be developed. For this purpose, the synthesis of suitable building blocks with novel and potentially useful electronic properties is a key objective. Besides the organization of molecules in two dimensions, progress in nanotechnology calls for functional building blocks capable of extending self-assembly towards 3D-nanoarchitectures. In general, the self-assembly process is a simple way to organize about 1013 molecules per cm2 and, hence, is predestined to reach these targets [41]. In most cases, SAMs based on aromatic molecules are more stable than their aliphatic counterparts. Furthermore, conjugated aromatic groups provide better electronic conduction properties – not only between the assembled compounds and substrate (that are useful for possible applications in molecular electronics [42]) but also when

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| 8 Terpyridines and Nanostructures perpendicular to the surface plane (i.e., between the assembled tail-groups via p–p interactions) [43]. In this respect, terpyridine ligands are highly attractive tailgroups to be incorporated into SAMs since they are redox-active species and can coordinate to a broad range of (transition) metal ions, either reversibly (e.g., ZnII, alkaline, and alkaline earth metal ions) or irreversibly in the form of inert complexes (e.g., RuII and OsII ions) [22]. These complexes can also be photo- and redoxactive and, thus, add important photophysical and electrochemical properties to the organized assemblies. A crucial issue concerning the self-assembly of large, bulky molecules into densely-packed monolayers was addressed by Auditore et al. [44]. For instance, to overcome the formation of liquid-like regions within the SAMs due to steric hindrance, compounds can be utilized in which their bulky tails are separated from the surface-bound heads by long alkyl chains. Alternatively, mixed-compound monolayers can be then prepared when shorter molecules, lacking the bulky tail-groups, filled the spaces between the bulky molecules. Following the latter strategy, 40 -(4-sulfanylphenyl)-2,20 :60 ,200 -terpyridine (1) and sulfanylbenzene (2) were co-assembled onto gold surfaces (Figure 8.2a). Over a wide range of stoichiometric ratios of 1 and 2 in solution, an almost constant ratio of 1 and 2 within the SAM was observed by X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), and contact angle measurements. In all experiments, constant contact angles of water (47 7 51) were found. In Figure 8.2b, the molar fraction (w) of 1 at the Au surface, as determined by XPS, is plotted against the corresponding molar ratio (x) in solution. Over a wide range of molar ratios (i.e., 0.05 o x(1) o 0.95) the surface composition changed only marginally around a value of w ¼ 0.5 – for an ideal mixture a large deviation from the linear behavior should be expected where one kind of molecule can be replaced by another without changing the film structure and stability; however, in the present case, the presence of even a small amount of one component (i.e., 1 or 2) in the pure solution of the other species drove the system to the formation of a surface layer where 1 and 2 were “intercalated” in an approximately 1 : 1 ratio [44]. The coordination of these surface-bound terpyridine moieties to RuII ions was later shown [45]. Almost quantitative loading of the terpyridine sites by reaction with Ru(tpy)Cl3 was confirmed by XPS. Formation of molecular wires standing upright on a gold surface was realized via a bottom-up approach in which metallo-supramolecular self-assembled multilayers (SAMLs) were grown from the terpyridine-functionalized Au surface (Figure 8.2) by successive, stepwise coordination reactions with PDIm-bridged bis(terpyridine) 3 and FeII ions (PDIm ¼ perylene diimide, Figure 8.3a) [46]. The PDIm bridge in 3 was chosen due to its rich photophysical and electrochemical properties, where intense luminescence, light fastness, and n-type semiconductor behavior were reported for this type of p-conjugated material [47]. Functionalization of the dye with supramolecular binding sites enable its self-assembly into pstacks, hydrogen-bonded networks, and metallo-supramolecular cycles as well as polymers (see also Chapter 5) [48, 49]. Furthermore, well-defined SAMs have been achieved on various surfaces; multilayer assemblies were obtained on quartz

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Figure 8.2 (a) Thiol-functionalized compounds 1 and 2 and their self-assembly into monolayers on gold surfaces [45]; (b) plot of the molar fraction (w) of 1 in the SAM, obtained from XPS intensities, as a function of the composition of the starting solution, x(1) ¼ [1]/([1] þ [2]) [44]. Figure reproduced with kind permission; r 2007 Wiley-VCH and 2003 The Royal Society of Chemistry, respectively.

substrates by a layer-by-layer approach [50]. The step-wise growth of nanowire-type multilayers was investigated by SIMS and UV–vis absorption spectroscopy (using a semi-transparent gold foil, as substrate) (Figure 8.3b). Atomic force microscopy (AFM) measurements of patterned surfaces, such as a gold surface covered with octadecanethiol (ODT) lines by microcontact printing (the ODT-free areas were subsequently covered with a SAM of 1 and 2), confirmed the step-wise growth of the film. According to this applied layer-by-layer assembly process, a linear increase in height, as a function of the (nominal) number of layers, was observed (Figure 8.3c); an average layer thickness of B2.7 nm could be estimated from a height histogram. Time-resolved spectroscopy on a 28-layered substrate revealed ultrafast (i.e., sub-ps) transfer of the energy absorbed by the multilayer to the gold surface. The same group also self-assembled bis(terpyridine) 4 with FeII or CoII ions into supramolecular nanowires perpendicular to the terpyridine-functionalized Au surface [51]. The conductance of nanowires of different chain lengths (i.e., 15–40 nm long, estimated by UV–vis absorption spectroscopy and AFM) was investigated utilizing a hanging mercury-drop electrode (Figure 8.4a). As depicted in Figure 8.4b, 14 nm-long nanowires containing CoII ions exhibited a conductance that was still similar to that determined for the “anchoring platform” (i.e., the initial 1 nm thick SAM). In the case of FeII ions, a linear decrease in conductance with increasing chain length occurred; however, the decrease was not as pronounced as

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Figure 8.3 (a) Step-wise self-assembly of bis(terpyridine) 3 with FeII ions onto a terpyridinefunctionalized SAM on a gold substrate; (b) optical density (A) as a function of the nominal number of layers; (c) height difference between the initial height of the SAM and the selfassembled multilayers as a function of the nominal number of layers [46]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

for surface-bound polyphenylenes, as reference materials. Thus, the conductance of these nanowires could be correlated to the nature of the incorporated transition metal ion. Consequently, incorporating different metal centers during the step-bystep self-assembly process should permit tuning of the conductance along the supramolecular chain. The authors concluded that the combination of the electrical properties with the remarkable mechanical robustness makes these materials highly attractive candidates for applications in molecular and/or organic electronics [51].

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Au Figure 8.4 (a) Pictorial representation of bis(terpyridine) 4 and of the interface of the Au-[Fe (4)]n2n þ –Hg junction; (b) current density (log J) vs. length of the FeII- and CoII-based molecular wires (MWs); filled square: current value measured for junctions incorporating the 1 nm-thick “anchoring platform;” filled inverted triangle: data for junctions incorporating polyphenylene-based wires [51]. Figure reproduced with kind permission; r 2009 Nature Publishing Group.

The selective binding of lactoferrin (LF, Figure 8.5a), an antimicrobial and antiviral glycoprotein component of mammalian milk, to terpyridine-functionalized surfaces was reported by Tuccitto et al. [52]. This protein is known to possess two specific iron-binding sites, each involving four amino acid residues [53]. To achieve efficient binding, a patterned terpyridine-functionalized surface was loaded with FeII ions and, subsequently, LF was adsorbed selectively onto the patterned surface; notably, LF did not assemble on the area covered with the 11-sulfanylundecan-1-ol matrix. SIMS time-of-flight measurements [54] as well as utilizing a quartz crystal microbalance with dissipation monitoring (QCM-D) enabled monitoring of the adsorption process. In particular, the latter technique allowed to follow the in situ kinetics of surface adsorption from the liquid phase, simultaneously providing information on the viscoelastic properties of the adsorbed layer. Both the frequency and dissipation curves, as a function of adsorption time onto an unpatterned SAM of FeII (mono)terpyridine complexes, are depicted in Figure 8.5b and c (for comparison, adsorption onto the metal-free mixed-SAM of 1 and 2 as well as onto a SAM of 11-sulfanylundecan-1-ol are also

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0.2 0.0

Figure 8.5 (a) 3D-representation of lactoferrin; QCM-D monitoring [frequency (b) and dissipation (c) curves] of the LF adsorption onto the FeII-containing, metal-free, and matrix SAM [52]. Figure reproduced with kind permission; r 2007 The Royal Society of Chemistry.

shown) [52]. Fast binding of LF to the FeII-containing SAM could be observed; an uptake of 450 ng cm2 was calculated from the frequency shift, corresponding to 3 1012 molecules per cm2 (Figure 8.5b) The dissipation curves (Figure 8.5c), which depend upon the viscoelastic properties of the films, provided a sensitive tool to measure the relative binding strength of LF to the three investigated surfaces; the behavior observed for LF adsorbed onto the matrix corresponds to the formation of very rigid elastic films (i.e., the small amount of adsorbed LF did not significantly influence the rigidity of the combined LF-SAM layer). In addition, the system where LF was adsorbed onto the FeII-containing SAM exhibited a remarkably higher rigidity; moreover, the adsorbed mass was five-times higher than the adsorption on the matrix, revealing that LF was strongly bound to the FeII-containing layer. The stability and reversibility of supramolecular binding of different surfacebound terpyridine complexes (i.e., using IrIII, FeII, and ZnII ions) were investigated by Haensch et al. [55]. In this case, the CuI-catalyzed alkyne-azide cycloaddition (CCAAC; the so-called “click reaction”) was applied by these authors to attach terpyridine complexes efficiently to the azide-functionalized SAMs under mild conditions [28] (for further examples of terpyridines functionalized with surfaceanchoring groups, see Reference [56]). In particular, the kinetically labile ZnII bis (terpyridine) complexes, which are efficiently re-opened under acidic conditions, were established as a new type of supramolecular “protecting group chemistry.” The binding behavior of phosphonate-functionalized terpyridine (5) to metaloxide-modified surfaces was studied by Spampinato et al. [57]. For this purpose, a ZrIV phosphate monolayer on a SiO2 substrate was prepared and served as a model substrate for any oxide surface capable of being hydroxylated. Owing to the ambient binding ability of 5, an unselective anchoring to the ZrIV ions was observed (Figure 8.6, route A); therefore, a “protecting group” approach (Figure 8.6, route B) was followed in which the ZnII bis(terpyridine) complex [Zn(5)2]2 þ was prepared and binding to the ZrIV ions occurred via the phosphonate groups.

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∆D (106)

406


N

| 407

route A 5

N

PO(OH)2

and/or

5 N

HCI [Zn(5)2]2 route B

Figure 8.6 Anchoring of terpyridine 5 to a ZrIV phosphate monolayer (routes A and B) [57]. Figure reproduced with kind permission; r 2010 American Chemical Society.

N N

N N

N N

S N N

N

N 6

S

7 N SH

N N

N

N O

O

N

N

N Fe

N

8

N

N

9

Figure 8.7 Terpyridine building blocks 6–9 [58].

Decomplexation was subsequently achieved under aqueous acidic conditions. The different modes of binding could be confirmed by XPS and SIMS time-of-flight measurements. Nishihara’s group utilized a different functional group for the binding of terpyridine ligands to gold surfaces in which the dithiol-derivative 6 was attached to Au-covered mica plates under mild conditions (CHCl3, room temperature); the thiolderivatives 1 and 7 were also used for the formation of terpyridine-functionalized Au surfaces [58, 59] (Figures 8.7 and 8.8a). Subsequently, the step-by-step self-assembly of FeII ions with bis(terpyridine)s 4 and 8 (one to five cycles) was performed and the

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N

408


(a) N

N

N

N

N

N

S

S

S

ket 7.5

n

7

N N

N

(III)

(II)

Fe

N

N

(I)

(b)

N

N

N F e 2+

6.5

N N

e–

Inket

N

6 5.5 5 4.5

N N

N F e 2+

N

0

20

N

40 60 Wire length/Å

80

100

N

Figure 8.8 (a) Terpyridine-modified Au surfaces I–III. (b) Left: energy transfer from the surface to the terminal ferrocene moiety. Right: plots of ln(ket) versus the length of the nanowires (I: blue; II: green; III: red); the dashed lines were obtained by least-squares fitting for each case and the solid lines were obtained by assuming the same slope for I–III [59]. Figure reproduced with kind permission; r 2010 American Chemical Society.

ferrocene-substituted terpyridine 9 was applied as an end-capping ligand (Figure 8.7). The degree of surface coverage (i.e., the number G of ferrocene moieties at the end of the films, as a benchmark for the number of surface-active terpyridine units) was determined by cyclic voltammetry (CV); for instance, G values of 1.7 1010, 1.4 1010, and 6.0 1011 mol cm2 were estimated for the assembly of 4 onto I, II, and III, respectively [59]. The smaller G value for III was rationalized by the increased steric demand of the terpyridine 7. The rate constants (ket) for electron transfer between the electrode and terminal ferrocene units of these nanowires were measured by potential-step chronoamperometry (PSCA). No dependence of the

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electron-transfer rate on the length of the nanowire could be observed; however, the nature of the anchoring group clearly influenced ket and a decrease in the rate constant in the order I W III c II was concluded from the PSCA measurements (Figure 8.8b). Moreover, dendritic nanowires were obtained when the star-shaped tris(terpyridine) ligand 10 was used for the step-by-step, self-assembly with FeII ions onto the terpyridine-functionalized Au surface I (Figure 8.9a and b) [60]. At low surface coverage, scanning transmission microscopy (STM) measurements revealed an even distribution of nanodots of uniform size and shape – for the linear nanowires, based on bis(terpyridine) 4, near close-packing into circular domains (outside diameter of 6 nm) could be observed, indicating a p-stacking of the molecular chains [61]. Extensive PSCA experiments were carried out and the current–time characteristics suggested a through-bond electron transport involving two kinetic factors (“electronhopping mechanism”) in which electron transfer between the nearest redox site and electrode (k1 in s1) as well as electron transfer between neighboring redox centers along the molecular wire (k2 in cm2 mol1 s1); a diffusion motion model, as an alternative mechanism, was excluded. The intrinsic kinetic parameters for the dendritic nanowires were k10 ¼ 100 s1 and k20 ¼ 4.1 1011 cm2 mol1 s1. Kubo et al. utilized the in situ generated diazonium salt derived from 40 -(4aminophenyl)-2,20 :60 ,200 -terpyridine (11) to covalently attach the terpyridines to carbon plates (Figure 8.10) [62]. Owing to the high reactivity of the diazonium salt, unspecific binding of 11 to the surface was observed. The subsequent step-wise assembly of bis(terpyridine) 4 with either FeII or CoII ions resulted in multilayered supramolecular films on the surface. The formation of these metallo-supramolecular assemblies was confirmed by XPS, AFM, and CV. In contrast to the work of Nishimori (see Figure 8.9), p-conjugation of the film into the surface could be retained and, thus, significantly faster electron transfer properties were observed

(b)

(a)

N

N

N 10

N

N

N

N N

N

Au surface

Figure 8.9 Pictorial representations of (a) tris(terpyridine) 10; (b) self-assembly of 10 with FeII ions onto a gold surface [60]. Figure reproduced with kind permission; r 2007 WileyVCH.

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| 409

410

| 8 Terpyridines and Nanostructures N H2N

NaNO2, HCl

N

carbon substrate

11 N

N

N N

N N

N

N

N N

N

N

N

N

N

N

N

N

Figure 8.10 Covalent attachment of amino-functionalized terpyridine 11 to a carbon substrate [62].

in this case (ket ¼ 3000 s1). This turnover rate was close to the highest value (ket ¼ 5000 s1) so far reported [63] and suggested the usefulness of the composite, as redox mediator in (bio)electrochemical applications [62]. A different type of molecular wire was reported by Ng et al. [64] in which thiol-functionalized oligo(phenylene-ethylene)s (OPEs) that were end-capped with a pyridine moiety were assembled onto a gold surface. Subsequently, [Ru(tpy)(bpy)]2 þ or cyclometalated [Pt(pbpy)] þ (pbpy: 6-phenyl-2,20 -bipyridine) were coordinated onto the organic SAM via its terminal pyridine units. Current– voltage (I–V) studies revealed strong electronic coupling between the transition metal center and organic wire; in both cases, conductance was higher than for the original organic chain. The presence of metal complexes in the hybrid molecular wire introduced distinctive negative differential resistance (NDR) effects. The adsorption of heteroleptic complexes 12 (MII ¼ RuII or OsII) via their uncomplexed lateral pyridine moieties onto a platinum surface was shown by Figgemeier et al. (Figure 8.11a) [65]. Here, immersing Pt foils into an aqueous acetone solution of 12 for 1 h resulted in a spontaneous adsorption into monolayers that were investigated by STM as well as electrochemical techniques. The STM images revealed the formation of a hexagonal array of 12a with an average distance between adjacent spots of about 2.9 nm; the individual spots had radii of about 5.5 A , indicating a rather loose packing behavior [corresponding to a surface coverage (G) of 2.2 1011 mol cm2] (Figure 8.11c). The CV of 12 showed that the peak currents increased linearly with the scan rate, which is indicative of monolayer formation on the Pt electrode (Figure 8.11b). Compared to the theoretical full-width at half-maximum (DEp½) potential of 90.6 V (calculated for a one-electron processes), much lower values for DEp½ were obtained for the RuII and OsII

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N


(b)

(a)

| 411

2.0102 1.5102

N

N M

N

2

N

1.0102

N

5.0102

N 12a (M Ru ) 12b (MII OsII) II

II

(c)

IP/Am2

N

0.0 5.0101 1.0102 1.5102 2.0102 2.5102

0

500

1000

1500

Scan rate/V s1

Figure 8.11 (a) Terpyridine complexes 12; (b) peak current density as a function of scan rate for a monolayer of 12a on a platinum microelectrode; (c) STM image (14 45 nm2) of a monolayer of 12a on a Pt(100) surface [65]. Figure reproduced with kind permission; r 2003 American Chemical Society.

bis(terpyridine) complex monolayers (150–160 mV). This also indicated the formation of only loosely packed monolayers. The saturation surface coverage derived from the charge under the oxidation and reduction peaks gave a G of 2.5 7 0.2 1011 mol cm2, which was in good agreement with the value determined from STM imaging. Very robust SAMs of RuII or OsII bis(terpyridine) complexes on polished gold electrodes were reported by Draper et al. [66]. The heteroleptic piperazinefunctionalized complexes 13 assembled onto the surface by the in situ formation of a dithiocarbamate (DTC) anchoring group at the terminal piperazine N-atom by reaction with CS2 (Figure 8.12a and b). An average surface coverage of 5.5 mol cm2 was calculated from CV data. Moreover, an electrochemical investigation revealed that the monolayers showed excellent reversible redox behavior and exceptional stability, where only 10% of the surface-bound complexes were decomposed after 700 CV cycles up to 1.1 V (the SAMs also resisted stripping agents that usually destroy thiol-based SAMs on Au surfaces). The remarkable stability of these SAMs was attributed to a strong bidentate attachment to the gold surface of the DTC tether and favorable low oxidation potentials of the complexes (13a: 0.62 V, 13b: 0.35 V, Figure 8.12c), which resulted from the electron-rich piperazine substituent on one of the terpyridine ligands. The electrochemically determined electron-transfer rates were higher than for similar complexes/nanowires bound to surfaces via conventional thiol groups (RuII complex 13a: ket ¼ 351 s1; OsII complex 13b: ket ¼ 248 s1). The covalent binding of thiol-functionalized CoII bis(terpyridine) complexes to platinum microelectrodes was reported by Campagnoli et al. [67]. The SAM of these complexes showed reversible and well-defined behavior in CV experiments when switching between the CoII and CoIII oxidation state. The entire adsorption

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2000

2500

| 8 Terpyridines and Nanostructures (a)

(b) PF6 2 PF6

N N

N N

N M2+

N M2+

N

N

N N

N N

N

MII RuII or OsII

N

13a (MII RuII) 13b (MII OsII)

N N H

(c)

S

S

8105 4105

Current (A)

412

0 4105 8105 0

0.2

0.4

0.6

0.8

0.1

0.2

Potential (V) Figure 8.12 (a) Piperazine-functionalized bis(terpyridine) complexes 13a/b; (b) surfacebound complexes; (c) cyclic voltammograms showing the MII/MIII oxidation processes for SAMs of 13a (black line) and 13b (gray line) vs. Fc/Fc þ [recorded at room temperature at a scan rate of 8 V s1 in MeCN with 0.1 M (n-Bu4N)(PF6), as supporting electrolyte, using a Pt wire counter electrode and SCE reference electrode] [66]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

process was irreversible, leading to a maximum surface coverage of G ¼ 6.3 7 0.3 1011 mol cm2. The rate of SAM formation appeared to be controlled not by mass transport or interfacial binding but rather by surface diffusion of the complex in which the surface diffusion coefficient was 5.5 7 1.1 1 07 cm2 s1, indicating that, prior to formation of an equilibrated monolayer, the adsorbates had significant surface mobility. The [Co(tpy)2]2 þ linkage was utilized later by Tang et al. for the preparation of a molecular transistor [68, 69] where two SAMs of 1 were self-assembled onto Pt surfaces by the so-called self-aligned lithography; the nanosized gap of 3 nm between the surfaces was determined by scanning electron microscopy (SEM) (Figure 8.13a and b). Subsequently, the electric circuit was closed by complexation of the terpyridine units with CoII ions and a

08

27 J l 2011 16 15 24


(a)

(b) N

S

N

N

N

N

S

N

Pt

Pt 3 nm

(c) electrode

electrode

Figure 8.13 (a) Two SAMs of 1; prepared by self-aligned lithography; (b) STM image of the nanogap between the Pt microelectrodes; (c) formation of the electric circuit by self-assembly with CoII ions (green ellipsoid: CoII ion) [69]. Figure reproduced with kind permission; r 2007 Wiley-VCH.

conductance of 1.3 104 to 1.6 106 e2 h1 was estimated (assuming one to 80 molecular bridges, Figure 8.13c). Density functional theory (DFT) calculations could show that only two CoII mono(terpyridine) complexes, bridged by the acetate counterions, can effectively span the nanogap [70]. This m-acetate-bridged complex was found to be both more flexible and more conductive than the alternative structure based on a conventional CoII bis(terpyridine) complex. Though the latter complex is the more stable structure in a non-confined environment (i.e., in solution), it was less effective in connecting the leads of the fabricated gap and, thus, less likely to result in a conductive device. Molecular monolayer memory (MMM) devices are one further example where terpyridine complexes have been applied in molecular electronics. In general, for the fabrication of voltage-driven molecular monolayer non-volatile memory devices (MMNVMs), the design of redox-active molecular memory SAMs becomes a critical factor, in particular for the development of a voltage-driven MMNVM that requires direct contact measurement of the memory effect through a molecular monolayer between the bottom and top electrodes [71]. Lee’s group demonstrated by STM that RuII bis(terpyridine) complexes without alkyl chains featured a voltage-driven molecular switch in the solid-state molecular junction [72]; however, the direct fabrication of such complexes into MMM devices resulted in electrical shortcuts. This effect could be reduced remarkably by utilizing RuII bis(terpyridine) complexes bearing thiol-terminated alkyl chains (such as 14) [73]. Figure 8.14a depicts a typical MMNVM device set-up; the alkyl chains prevented penetration of both the poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS) layer and the top metal electrode into the SAMs, resulting in a high

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| 413

414


(a) Au

C13H26SH

PEDOT:PSS SiNx

Au

o

N

Ti

Sio2/Si(100)

N

2PF6

N Ru2+

N

o S

=

14

N

PEDOT

n n

PSS

N

SO3H

C13H26SH

(c) V (V)

(b)

300

1st run 2nd run 3rd run

1.0 0.5

0

0.0 V/V

0.5

// µA

2

1

0 V/V

400

600

800

(d)

400 6002.0

600

200

t/s

200

300

0 1 1.5 0

1st run 2nd run 3rd run 4th run 5th run 6th run 7th run 1.5

I (µA)

600

1500 1000 500 0 500 1000 1500

// µA

900

// µA

1.5 1

1.5 V/V

1

1.0

2

200 210 220 230 240 250 260 50

OFF State ON State

100

150 200 Cycle

250

300

Figure 8.14 (a) A MMNVM device incorporating RuII complex 14. (b) Hysteretic I–V characteristics of the device [Au/14/PEDOT:PSS/Au)]; its I–V characteristics were recorded by scanning the applied voltage from 0 to þ 2 V and then to 2 V followed by a reverse scan from 2 to þ 2 V (top inset: control experiment using PEDOT:PSS without RuII-containing SAMs between the top and bottom electrodes; bottom inset: magnification of the hysteretic I–V-curve). (c) WRER cycles of the MMNVM device containing 14 for a rewritable data storage application (the writing, reading, erasing, and reading voltages were 1.5, 1, þ 1.5, and þ 1 V, respectively). (d) Current in the ON and OFF states as a function of the number of WRER cycles (300 cycles were run under inert conditions) [73]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

device performance (up to 81%) and reasonably short retention times (Figure 8.14b). However, the performance of this supramolecular device was poor when compared to the state-of-the-art Si technology. Write–multiple read–erase–multiple read (WRER) pulse cycles were repeatedly operated at low driving voltages with reasonable ON/OFF ratios (Figure 8.14c and d). Moreover, the device experienced no significant degradation after several hundred pulse cycles. In the same context, an earlier contribution by Park et al. has to be considered [74] in which homoleptic (alkyl)thiol-functionalized CoII bis(terpyridine) complexes were placed into a gap of a 200 nm wide gold wire; the gap was derived from

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| 415

electromigration by ramping to large voltages at cryogenic temperatures. During the process, some of the complexes, which were initially bound to the gold wire before breakage, migrate to the 1–2 nm gap. Utilizing complexes with different thiol-to-thiol distances by varying the length of the thiol-functionalized alkyl chain exhibited different physical effects on a molecular level. For instance, the “longer” complex possessing a C5-chain showed behavior typical of a molecular singleelectron transistor [75] with the gate being the degenerately doped Si substrate. For the “shorter” complex with no alkyl chain, a pronounced coupling between the transition metal ion and electrons was observed, thus leading to Kondo-assisted tunneling [76], which could be described as the formation of a bound state between a local spin in the CoII center and the conduction electrons in the electrodes, leading to a conductance enhancement at low biases. Schubert and Gaub et al. investigated the binding strength of RuII bis(terpyridine) complexes at the molecular level using single-molecule force spectroscopy [77]. For this purpose, an unsymmetric a,o-difunctionalized poly(ethylene glycol) (PEG) unit was attached to an amino-functionalized surface via its carboxylic acid residue and, subsequently, the lateral terpyridine moiety was treated with RuCl3 xH2O, yielding a surface-bound RuIII mono(terpyridine) complex; the same metal-free ligand was also attached onto an AFM tip. Upon bringing the tip into close proximity of the surface, a RuII bis(terpyridine) complex was formed (Figure 8.15a). Force extension curves were measured upon pulling back the tip until the rupture occurred (the experiments were performed under mild conditions in water–DMSO at room temperature to avoid breaking the PEG chains). A binding force of about 95 pN at a force loading of 1 nN s1 was detected (Figure 8.15b); the histogram of the bond rupture forces showed two weaker peaks at 171 and 253 pN that were attributed to the simultaneous

(a)

Amino-functionalized AFM tip

(b)

40

RuIII

Counts

30 RuII

20 10 0 0

100

200 300 Rupture force (pN)

Figure 8.15 (a) Schematic representation of the experimental set-up; (b) histogram of the bond rupture forces at a pulling velocity of 118 nm s1 [77]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

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400

416


(a) N

OR

N

N

(b)

N

N

15a(R = n-C18H37) 15b(R = n-C12H25)

N

OR 16a (R = n-C18H37) 16b (R = n-C8H17)

(c)

Figure 8.16 (a) Alkoxy-substituted terpyridines 15 and 16. (b) STM image of 15b adsorbed onto HOPG (an intersection of three different domains is shown; the scale bar is 25 nm). (c) High-resolution STM images of 15b (left) and 15a (right); the molecular model has been superimposed for comparison (scale bar: 2 and 2.5 nm, respectively) [79]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

rupture of two or three complexes, respectively, by comparison to superimposed theoretical force vs. extension curves (calculated using Monte Carlo simulations). The authors proposed that this analytical tool might be applicable to determining the binding strength of metallo-supramolecular entities on a molecular level. Besides the previous examples where the terpyridine units were covalently attached perpendicular to the surface, a parallel-to-the-surface self-assembly has been investigated where the interaction of the organic molecules with the bulk substrates occurs via weak van der Waals as well as p–p interactions. Both the Schubert and Constable groups showed that alkoxy-functionalized terpyridines 15 and 16 (Figure 8.16a) can be assembled into highly ordered architectures on graphite surfaces [78–80]. STM was utilized to study the adsorbed terpyridines at the solid–liquid interface of highly ordered pyrolytic graphite (HOPG); well-ordered 2D arrays of 15 were observed on HOPG in 1-phenyloctane (Figure 8.16b). Large (>500 nm), well-defined lamellar domains could be visualized. High-resolution images were obtained, revealing the alignment of the terpyridines in long uniform double rows with their alkyl chains packing in an alternating zipper-like fashion; the polar head groups were oriented head-to-head with a repeating distance estimated to be about 3.5 nm. In all cases, the observed sizes and shapes of the molecules corresponded exactly to their modeled geometries and solid-state structures (if available) (Figure 8.16c). Golubkov et al. published the synthesis of terpyridine 17 featuring a p-conjugated hydrophobic PDIm chromophore and a hydrophilic PEG chain (Figure 8.17a) [81, 82]. The self-assembly of 17 in a water–THF mixture into long, ribbon-like

08

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(a)

O

O

N

| 417

O

O

N

n

O

N

O

N

17 N

(c)

(b)

1.8 nm 7 nm 4 nm 1.8 nm PEG-PDI (17) 4 nm

fiber edge 1.9 nm fiber face

8 nm

1.9 nm one segment

100 nm

8 nm

Figure 8.17 (a) PEG-PDIm terpyridine 17. (b) Cryo-TEM image of 17 in a water–THF mixture (9 : 1 ratio); right inset: enlarged image of a segmented fiber (scale bar is 40 nm); left inset: enlarged image of a fiber twist (scale bar is 20 nm); the white arrows point to twisting regions of ribbon-like fibers. (c) Suggested structure of the fibers of 17; one fiber segment corresponding to the segment observed in cryo-TEM and two aligned fibers are presented; the given dimensions are based on molecular modeling studies [82]. Figure reproduced with kind permission; r 2009 Wiley-VCH.

fibers by p–p stacking of the PDIm moiety could be evidenced by cryogenic transition electron microscopy (cryo-TEM); however, the length of the fibers could not be estimated, since end-groups could not be identified. Most of the fibers appeared to extend over the entire cryo-TEM image and, thus, might have reached the length of several microns. The aligned, tightly packed fibers exhibited a fiberto-fiber spacing of 7.1 7 0.8 nm (Figure 8.17b) that was assigned to a high-contrast ordered aromatic core (responsible for fiber images in cryo-TEM) and low-contrast solvated PEG chains (interfiber area). The individual fibers showed an uncommon segmented structure (see the inset in Figure 8.17b). The uniform segment periodicity of 1.9 nm corresponded to the PDIm dimensions. A possible structure of the fibers, based on molecular modeling studies, is depicted in Figure 8.17c. Moreover, 17 was utilized, as a template, for self-assembly processes that were regulated through transition metal ion coordination. It was shown that a range of supramolecular nanostructures could be obtained by simply varying the coordinated metal ion: tubular fibers for PdII, vesicles of 26 nm diameter on average for PtII, or nanoplatelets with dimensions of 60 40 nm2 for AgI ions.

08

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418


(a)

(b) N N

N

(c)

N N

N

N

N 18

Figure 8.18 (a) Molecular structure of 2,40 -BTP (18); (b) STM image (20 20 nm2) of the QQN structure formed by 18 on Ag(111)/Ru(0001) upon applying the vacuum deposition technique; (c) space filling model of the windmill-like building block of the QQN motif [83]. Figure reproduced with kind permission; r 2007 American Chemical Society.

The adsorption of 2-phenyl-4,6-bis[6-(pyridin-2-yl)-4-(pyridin-4-yl)pyridin-2-yl]pyrimidine (2,40 -BTP, 18, Figure 8.18a) onto weakly (i.e., HOPG) or more strongly [i.e., Au(111) and (111)-oriented Ag films on Ru(0001)] interacting surfaces was reported by Hoster et al. [83]. Vapor deposition as well as deposition from solution was utilized to obtain the same type of adlayer structure, that is, a quasi-quadratic network (QQN) motif. The structures were investigated by high-resolution STM imaging and intermolecular interactions, such as C–H N-type hydrogen bonds and C–H H–C interactions between adjacent molecules, were found to control the self-assembly processes (for a high-resolution STM image of 18 adsorbed onto Ag(111)/Ru(0001), see Figure 8.18b). The experimentally observed general structure agreed both qualitatively and quantitatively with predictions from molecular modeling studies (Figure 8.18c). Slight deviations from the predicted ordering were attributed to the specific substrate–adsorbate interactions. The fact that similar adlayer structures were obtained on HOPG either under ultrahigh vacuum conditions (i.e., at the solid–gas interface) or in trichlorobenzene (i.e., at the solid–liquid interface) indicated that the intermolecular interactions were not severely affected by the solvent. ¨ger’s group studied the adsorption and self-organization of ferroceneTra functionalized terpyridine 19 (Figure 8.19a) on gold substrates by means of in situ optical second-harmonic generation (SHG), a sensitive technique that allows one to investigate the orientation of molecules with respect to the surface and can also be applied to follow the ordering within films in real time [84]. The SHG data were compared to ellipsometric measurements and revealed that adsorption led to the formation of monolayer films at the interface on a time scale of several minutes (the adsorption process followed diffusion-limited Langmuir kinetics). Subsequently, a second process occurred on a much longer time scale, that is, the selfassembly of ordered redox-active ferrocenyl-nanostructures. The same terpyridine derivative 19 was also assembled onto HOPG surfaces [85] in which highly ordered and densely packed SAMs were obtained (Figure 8.19b). The growth of gold nanoparticles onto the SAM, following the Volmer–Weber growth mode, was monitored by in situ reflection spectroscopy and STM imaging under ambient

08

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8.3 Terpyridines and Inorganic Nanomaterials

(a)

| 419

(b)

N

N Fe 19

N

(c)

(d)

240 nm

5 nm

100 nm

5 nm

120 nm

50 nm

0 nm

0 nm 0 nm 0 nm

120 nm

240 nm

0 nm 0 nm

50 nm

Figure 8.19 (a) Ferrocene-functionalized terpyridine 19; (b) STM image of the topography of the SAM of 19 on HOPG along with a pair of molecular models (the dimensions of the models were obtained from X-ray single-crystal structure analysis); the red and blue models represent molecules with downwards and upwards oriented ferrocene units, respectively; the scale bar is 4 nm; (c) STM image of Au nanoparticles on bare HOPG (coverage of 0.45 1016 atoms cm–2; (d) STM image of Au nanoparticles on a SAM of 19 on HOPG (coverage of 0.46 1016 atoms cm–2) [85]. Figure reproduced with kind permission; r 2009 Springer Verlag.

conditions. In comparison to bare graphite, the SAM reduced the diffusion length of the Au nanoparticles and increased the sticking coefficient. As a consequence, nucleation was increased, leading to the growth of densely packed Au nanoparticles with diameters in the nanometer range (Figure 8.19c and d). Furthermore, well-developed localized surface plasmon polariton resonances were observed for the nanoparticles on the redox-active SAMs, which is an important criterion for potential applications in plasmon-electric devices [86].


From the fabrication of terpyridine-functionalized surfaces detailed in the previous section, attention is drawn in the following section to the interactions of terpyridines with various nanostructures, based on gold, silver, CdS, TiO2, or carbon.

08

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100 nm

420

| 8 Terpyridines and Nanostructures In 2002, Rotello et al. showed that gold nanoparticles can be modified with thioalkyl-functionalized terpyridines in which the length of the alkyl chain was varied between C4 and C11 [87]. The subsequent addition of FeII, ZnII, AgI, or CuI ions led to aggregation of the terpyridine-decorated nanoparticles. The concept was followed later by Dong et al., who formed heteroleptic RuII bis(terpyridine) complexes (bearing lateral ferrocene units) on the shell of the nanoparticles [88, 89]. The same group reported the preparation of a stabilizing monolayer of dithiol-functionalized RuII bis(terpyridine) complexes on Au nanoparticles (Figure 8.20) [90]. TEM imaging revealed that spherical, uniform nanosized structures of 4.7 7 0.3 and 4.3 7 0.2 nm in toluene and tetra(n-octyl)ammonium bromide, respectively, were generated. Adsorption of the RuII-modified nanoparticles on an Au(111) metal surface was imaged using AFM. The nanoparticle layer exhibited a distinct surface morphology in which irregular-shaped domains with dimensions from 20 to 60 nm and root mean square heights of 2.401 nm were observed. The morphology of the layers was not, however, homogeneous, partly due to the presence of 3D clusters and the coexistence of various orientations. The co-chemisorption of thiol-functionalized terpyridine 21 and 10-(1H-pyrrol-1yl)decane-1-thiol onto Au nanoparticles was investigated by Fujihara et al. [91], in

(a) N

N

O(CH2)8SH 20 HS

N

HS

HS

SH

HS

Au nanoparticle

SH HS

Au(111) surface

SH

[Ru(20)2](PF6)2 HS

HS

SH HS

SH SH

HS

SH

SH HS

SH

(b)

SH

HS

SH

HS

HS

HS

SH

(c)

Figure 8.20 (a) Attachment of [Ru(20)2](PF6)2 to Au nanoparticles and subsequent assembly onto a gold surface; (b) TEM image of the Au nanoparticles with a SAM of [Ru(20)2](PF6)2 (scale bar is 10 nm); (c) topographic AFM image (1.0 1.0 mm2) of the covalent attachment of the RuII-modified nanoparticles onto an Au(111) surface [90]. Figure reproduced with kind permission; r 2007 Elsevier B.V.

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which 21 was assembled onto the nanoparticles via a two-phase reaction. The binding of 21 to the surface of the particle was confirmed by 1H NMR and UV–vis absorption (plasmon resonance band at 505 nm). Complexation of the pendant terpyridine units with Ru(tpy)Cl3 led to surface-bound heteroleptic RuII complexes that were characterized by 1H NMR and UV–vis as well as CV. The shift of the UV ndash;vis plasmon resonance (to 510 nm) and the results of the TEM imaging revealed an increase in particle size after complexation. Particles of an average size of 5.5 7 1.0 nm were observed, whereas the initial particle size was about 2.0 nm (Figure 8.21b, top/middle). A ligand-exchange reaction with the

N

S(H2C)8O

N

N

10 nm 21

N S(H2C)8O

N Ru2

N N

N

N

10 nm 2 µA S(H2C)10

N

N S(H2C)8O

N

N Ru

2

N

N

N

(a) 1.0 0.5 E / V vs. Ag/Ag

0

(b) Figure 8.21 (a) Au nanoparticles modified with terpyridine ligands, RuII bis(terpyridine) complexes, and RuII bis(terpyridine) complexes as well as pyrrole (top to bottom); (b) TEM images of Au nanoparticles before (top) and after (middle) complexation with RuII ions; (bottom): oxidative electropolymerization of the nanoparticles by repeated potential scans on a glassy carbon electrode (scan rate 100 mV s1) [91]. Figure reproduced with kind permission; r 2005 The Royal Society of Chemistry.

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| 421

422

| 8 Terpyridines and Nanostructures thiol-functionalized pyrrole derivative gave the heterodifunctional Au nanoparticles bearing both redox-active RuII complexes as well as (electro)polymerizable groups. The electropolymerization was carried out by CV (Figure 8.21b, bottom); poly(pyrrole gold nanoparticle)s bearing RuII complexes were instantaneously electrodeposited onto the electrode (irreversible oxidation wave at E ¼ þ 1.16 V, increase in the redox-peak currents at repeated scans). Such RuII nanocomposite materials are of interest with respect to possible applications as sensor devices and in catalysis or nanoelectronics. Distinctly different 3D assemblies of Au nanoparticles were obtained based on either a weak or strong coordination behavior of the involved metal ions [92]. The reduction of KAuCl4 with NaBH4 in the presence of 40 -(4-phenylmethanethiol)2,20 :60 ,200 -terpyridine (22) gave terpyridine-functionalized Au nanoparticles, which spontaneously assembled in situ into large 3D aggregates via weak coordination between the alkali metal ions and terpyridine ligands attached to the particles (Figure 8.22, left); disassembly into individual nanoparticles was achieved by adding DMF. Transition metal ions featuring strong coordination with terpyridines (e.g., CoII ions) caused them to reassemble into highly dispersed spherical 3D nanostructures (Figure 8.22, right). Wide- and small-angle X-ray diffraction (XRD) revealed that the assemblies were formed from small individual Au nanoparticles (average diameter of about 1.6 nm), which is consistent with the TEM results. According to thermal analysis, the functionalized particles contained 19.9 wt-% of terpyridine ligands. The controlled assembly and disassembly processes were accompanied by distinct shifts in the surface plasmon resonance. The particular parameters influencing the aggregation behavior of terpyridine-

N

N S

Au

N

M

Au

N

S N

n l io eta tion m a li in lka ord = A k Co M ea W

N

22

St

ro M ng Co Co( or II) din ati on Highly disperesed 3D Spherical Assmbly

Large 3D Assembly

Co2 Transformation

Figure 8.22 Self-assembly of terpyridine functionalized Au nanoparticles, as a function of the coordination strength of the metal ion (left: weak coordination, right: strong coordination) [92]. Figure reproduced with kind permission; r 2006 The Royal Society of Chemistry.

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functionalized Au nanoparticles (via thiol groups) were addressed by Montalti et al. [93]. At a low degree of surface-coverage with terpyridine units, the Au nanoparticles did not alter their stability; only when a critical degree of functionalization was reached did aggregation occur. Apparently, the binding of the lateral thiol groups to the gold was strong and nearly quantitative even at very low concentrations; in contrast, a much weaker interaction with the terpyridine moieties only became relevant at high degrees of surface-coverage, that is, when several units cooperated in interlinking the nanoparticles. This interlinking could be suppressed efficiently by the addition of an excess of ZnII ions by forming ZnII mono(terpyridine) complexes on the nanoparticles. Thus, the stabilization of individual Au nanoparticles in solution could be achieved by a metallo-supramolecular chemistry approach. The formation of metallo-supramolecular 2D networks of Au nanoparticles was published by the Hobara group [94]. Terpyridines 23 and 24 (Figure 8.23a) were adsorbed via their thiol functionalities onto the surface of Au nanoparticles with an average diameter of 4.7 7 1.1 nm according to TEM. The Langmuir–Blodgett (LB) technique was used to prepare a monolayer of nanoparticles on the water surface. The monolayer was transferred onto an oxide surface of a Si/SiO2 substrate on which interdigitated Au electrode patterns with a Cr adhesion layer underneath had been formed (the SEM images of the LB film are depicted in Figure 8.23c). The gate oxide (SiO2) was 150 nm long; the gate length and width of the device were 50 mm and 8.8 mm, respectively. The addition of FeII ions induced the formation of a 2D network based on FeII bis(terpyridine) complexes interlinking adjacent nanoparticles. The conductivity of the network (Figure 8.23b) containing the p-conjugated terpyridine 23 was four to five orders of magnitude higher when compared to networks containing the non-conjugated ligands 24. The conductivity of the molecular device was modulated by the gate voltage only when a conjugated derivative 23 was used. The FeII bis(terpyridine) complexes of ligands 24 were also anchored to Au nanoparticles (diameter of 3–5 nm) [95]. Upon laser flash excitation, a long-lived charge separation occurred, in both conventional organic solvents (e.g., MeCN) and ionic liquids (1-butyl-3-methylimidazolium hexafluorophosphate, bmim-PF6). The nature of the transient species depended on the laser excitation wavelength: 308 nm for [Fe(tpy)2]2 þ and Au or 532 nm for solvated electrons. In control experiments, no transients on a microsecond timescale were observed in MeCN upon excitation of unfunctionalized gold nanoparticles. The FeII-containing nanoparticles were subsequently utilized, as an integrated photocatalyst (i.e., containing all the components, for efficient water splitting in a single unit: a lightharvesting dye and colloidal Au, as catalyst). The photogenerated hydrogen (irradiation with visible light of l W 400 nm) was collected in an inverted burette and analyzed by gas chromatography. The total H2 volume produced in 100 min was about three times lower than for a reference system based on colloidal Pt and [Ru(bpy)3]2 þ [additionally containing methyl viologen (MV2 þ ), as electron relay] [96]; however, the initial photogenerated H2 rate (up to 20 min) was similar for both systems (Au vs. Pt). Taking a cost–benefit analysis into account, the

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| 423

424


(a) C6H13

N

S S

S

SH

N

C6H13

N

N

O(CH2)nSH

23

C6H13 N

(b)

N

24 n 3, 6 or 9

N

N

N

S

M N

source

N

S

N

Au nanoparticle

linker molecule

SiO2

drain

N++ Si

Au (c)

Figure 8.23 (a) Thiol-functionalized terpyridines 23 and 24; (b) representation of the device set-up containing the supramolecular 2D network of Au nanoparticles; (c) SEM images of a Au nanoparticle array formed by the LB technique (left: top view; right: angled cross-sectional view) [94]. Figure reproduced with kind permission; r 2007 Wiley-VCH.

integrated system reported by Alvaro et al. might be of interest for future commercial implementation: although the Fe/Au couple showed only a moderate performance, its relative price compared to Ru/Pt is significantly lower [95]. Homoleptic bis-complexes of the disulfide-functionalized terpyridine 25 with either FeII or CdII ions were employed for the self-assembly of Au nanorods (Figure 8.24a and b) [24]. The predominately end-to-end assembly (besides some other structures) was confirmed by XPS, UV–vis, and TEM measurements (Figure 8.24c). For the array based on FeII ions, disassembly was achieved by treatment with an aqueous NaOH solution. The directed reassembly of the terpyridine-functionalized nanorods with FeII ions into linear arrays, however, failed; instead of inter-nanorod complexation, a selective intra-nanorod complexation occurred. The addition of an excess of CdII ions to the CdII-based chains gave Au nanorods decorated with CdII

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gate


(a)

(c)

N

O

N

(ii)

(i)

N S

H N

S

O

25

170 nm

230 nm (iii)

(b)

| 425

(iv)

m

Au nanorod [Fe(25)2]2

80 nm (v)

30 nm (vi)

m

NaOH 30 nm

Figure 8.24 (a) Disulfide-functionalized terpyridine 25; (b) reversible self-assembly of [Fe (25)2]2 þ and Au nanorods; (c) TEM images of the native Au nanorods (i) and of the assemblies with [Fe(25)2]2 þ [(ii) low magnification, (iii) linear end-to-end connectivity, (iv) a branched structure, (v) end-to-end as well as end-to-wall connectivity, and (vi) a cyclic structure] [24]. Figure reproduced with kind permission; r 2010 Wiley-VCH.

mono(terpyridine) complexes; these structures could be reassembled into linear end-to-end arrays with metal-free terpyridine functionalized Au nanorods. Thus, utilizing the proper choice of transition metal ions enabled the construction of advanced multicomponent supramolecular nanoarchitectures. Magnetic Fe3O4 nanoparticles were coated with a very thin layer of gold by citrate reduction of AuIII in an aqueous solution [97]. Subsequently, the citrate molecules stabilizing the Au layer could be partially replaced by a substitution reaction with a disulfide-bridged bis(terpyridine) ligand. The paramagnetic behavior, photophysical properties, and TEM data confirmed that the Fe3O4 nanoparticles were coated with a thin layer of Au atoms. The latter technique was used to determine the size of the water-soluble Fe3O4@Au-tpy nanoparticles (average diameter of 30 nm); their composition (i.e., the core–shell architecture) was investigated with electron diffraction analysis. The authors proposed potential applications of the terpyridine-functionalized Au-coated magnetic nanoparticles in the fields of catalysis as well as analytical and biomedical sensor technology. Silver nanoparticles that were surface-functionalized by terpyridine moieties were prepared by the Tian group by reduction of AgI ions with NaBH4 in an EtOH solution in the presence of 26 as stabilizing agent (Figure 8.25a) [23]. TEM imaging of the 26-functionalized Ag nanoparticles (after a reaction time of 60 min) showed nearly spherical structures possessing a size distribution of 8–14 nm (Figure 8.25b).

08

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40 nm

426


(a) N

OCH2CH2OCH2CH2OCH3 OCH2CH2OCH2CH2OCH3

N 26

OCH2CH2OCH2CH2OCH3

N

(b)

(c)

1. Toluene 2. CH2CI2 3. Ethyl acetate 1 23

4 5 6

4. Ethanol

PL

5. DMF 6. DMSO

400

450

500 550 600 Wavelength /nm

650

700

750

Figure 8.25 (a) Terpyridine 26; (b) TEM Image of the Ag nanocrystals that were surfacefunctionalized with 26; (c) photoluminescence spectra of 26-capped Ag nanocrystals in several organic solvents of differing polarity [23]. Figure reproduced with kind permission; r 2009 The Royal Society of Chemistry.

The X-ray diffraction patterns indicated that the particles were face-centered cubic Ag (nano)crystals; the loading of these nanocrystals with 26 was about 16 wt-% as determined by thermal gravimetry analysis (TGA). According to XPS and 1H NMR, binding of 26 to the Ag surface occurred exclusively via the terpyridine moieties. The fluorescence of dyes can be enhanced when the dye is attached to the surface of metal nanoparticles; this phenomenon is referred to as metal-enhanced fluorescence (MEF) [98], in which an electromagnetic field is believed to exist near the metal core, causing a MEF through coupling with the chromophore. Among others, silver is a common metal used for MEF due to its strong plasmon resonances and low absorption loss in the visible region, and this photophysical property can be utilized, for instance, to develop next-generation biological and chemical sensors [99]. In the present case, 26 solubilized the Ag nanocrystals and solvent-resolved fluorescence occurred in organic solvents (Figure 8.25c). Owing to MEF, these materials exhibited broad full-width at half-maximum (FWHM, about 100 nm) and a decreased average fluorescence lifetime [23]. A similar approach was followed by Gao et al., who bound a fullerene-substituted terpyridine derivative to palladium nanoparticles via its pyridine moieties (named “C60tpy@Pd”) [100]. This work aimed to synthesize a new type of optical limiter (OL). In general, OL materials feature strong attenuation of intense laser

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| 427

beams, while retaining high transmittance for low-intensity ambient light levels; the applications are evident: protection of human eyes and optical sensors against intense laser threat [101]. Notably, fullerenes, metallo-phthalocyanines, or metal clusters have been extensively investigated with respect to their strong OL performances. Nanomaterials – such as carbon nanotubes or metal and semiconductor nanoparticles – have recently emerged as new classes of OL materials. Gao et al. investigated the OL effects of C60tpy@Pd under irradiation with 8 ns laser pulses at 532 nm [100]. Although the mechanism of the OL responses of nanomaterials is still not fully understood, the OL effects are believed to arise mainly from nonlinear absorption and nonlinear scattering. The C60typ@Pd exhibited a stronger nonlinear absorption and nonlinear scattering compared to C60. In particular, this nonlinear scattering was attributed to the Pd nanoparticles; the OL performance was improved by the large light-scattering centers, induced by excited-state absorption from the C60tpy units. The scattering centers were ascribed to vaporization or fragmentation of the Pd nanoparticles. Both the nonlinear absorption and nonlinear scattering were responsible for the strong OL in C60tpy@Pd; these cooperative nonlinear effects produced good OL materials. Various transition metal nanoparticles were formed on the side-walls of singlewalled carbon nanotubes (SWNTs) via non-covalent functionalization with 2,20 :60 ,200 -terpyridine (Figure 8.26a) [102]. According to UV–vis spectroscopy and thermal analysis, binding occurred via nitrogen-mediated interactions rather than

adsorption of tpy

(a)

SWNT (c)

E [V(NHE)]

4.0

0.5

4.5

0

5.0 5.5

Mn+tpy@SWNT

tpy@SWNT

Work function (V) 0

0.5 SWNT ∆Eg ≈ 0.3 eV

(b)

Mn+

1.0

Zn2 2e

Zn E0 0.762 V

Sn2 2e An E0 0.138 V Ru3 2e Ru2 E0 0.2487 V Cu2 2e Cu E0 0.3419 V Ru2 2e Ru E0 0.455 V

(iv)

(v)

6.0

Figure 8.26 (a) Representation of the adsorption of tpy onto SWNTs via a non-covalent interaction and subsequent formation of transition metal nanoparticles. (b) AFM height images: bare SWNTs (i), tpy-coated SWNTs (ii), and Cu nanoparticles formed on tpy@SWNT (iii) (for all images: the scale bar is 100 nm); height profiles of bare SWNTs (iv) and Cu nanoparticles on tpy@SWNT (v). (c) Correlated energy diagram of work function and standard electrochemical reduction potential; the standard reduction potentials of the employed transition metal ions show higher energies than the bandgap energy of SWNTs [102]. Figure reproduced with kind permission; r 2005 Wiley-VCH.

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6 nm

428

| 8 Terpyridines and Nanostructures through p–p interactions. Immersing glass slides containing tpy@SWNTs into solutions of RuCl3, CuCl2, ZnCl2, and SnCl2 instantaneously afforded the respective nanoparticles on the side-walls of the SWNTs in high yields. AFM height measurements revealed that the nanoparticles’ average diameters were 3.2, 2.7, 3.0, and 6.3 nm for Ru, Cu, Zn, and Sn, respectively (Figure 8.26b). The standard reduction potentials of the employed transition metal ions all had negative or small positive values while the Fermi energy level of semiconducting SWNTs is about þ 0.5 V (versus NHE, Figure 8.26c). Thus, electron transfer from a SWNT to a transition metal ion should be thermodynamically unfavorable; to overcome the energy barrier prohibiting the spontaneous nanoparticle formation on the SWNTs, a suitable anchoring group would be required that can attract both the transition metal ions and SWNTs. In the present case, the terpyridine ligands played this crucial role, notably, in the absence of tpy no nanoparticle formation could be observed. Conductance and XPS measurements showed that the Fermi level of the SWNTs was elevated after ligand adsorption to the nanotube’s surface. Covalent attachment of RuII bis(terpyridine) complexes to carbon nanotube (CNTs) – either as a “monomeric” complex or polymer chain bearing the complexes in their side chains – was reported by Stefopoulos et al. [103]. For this purpose, both SWNT and multi-walled carbon nanotubes (MWNTs) were applied and modified with RuII complexes following either a “grafting to” or “grafting from” approach. The latter utilized the atom-transfer radical polymerization (ATRP) reaction of CNTs that were functionalized a priori with an ATRP initiator in which the conversion was >94% according to thermal analysis. Subsequently, a vinyl-functionalized terpyridine 27, as monomer, was polymerized in the presence of CuBr and PMDETA, as the catalytic system (PMDETA: N,N,N,N00 ,N00 -pentamethyldiethylenetriamine). Subsequently, the terpyridine units of the side chains were complexed by reaction with Ru(28)Cl3 under reducing conditions (Figure 8.27a). The final hybrid CNT-metallopolymer (Figure 8.27b) was investigated by TGA and UV–vis absorption spectroscopy. The successful coordination of the RuII ions to the polymer-bound terpyridine sites was estimated by the characteristic metal-to-ligand charge-transfer (MLCT) absorption band at 495 nm (Figure 8.27c). Analysis of the TGA data gave further evidence for the incorporation of the RuII centers into the material. The structural differences between the CNT-metallopolymer and a SWNT bearing a “monomeric” RuII bis(terpyridine) complex were studied by resonant Raman scattering measurements [104]. In general, changes in the transition energies and widths of resonance windows were observed due to the chemical modification of the SWNTs. The pronounced redshift of the nanotube radial breathing mode for the polymeric sample was attributed to wrapping of the polymer chain around the nanotube. Shi, Dong, and their coworkers followed a similar approach to that of Kubo et al. (see Reference [62] and Figure 8.10) by utilizing benzenediazonium tetrafluoroborate (29), as a reactive intermediate, for the photo-induced covalent linking of terpyridine units to the surface of various types of substrates (i.e., MWNTs, quartz wafers, ITO glass, and silicon) [105]. The solubility of the MWNTs modified by terpyridine groups was significantly enhanced in common organic solvents; TGA

08

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| 429

(a) N

N OC12H25

N

N

O 27

28

N

N

(b)

OC12H25

(c) H N

O Br n

O

absorbance (a.u.)

O

2n PF6

N N

N Ru2

N

[(tpy)Ru(28)](PF6)2 (SWNT-tpy)Ru(28)(PF6)2 SWNT-poly([(tpy)Ru(28)](PF6)2)n bare SWNT

N N

300 C12H25O

OC12H25

400

500 600 wavelength (nm)

700

Figure 8.27 (a) Terpyridine building blocks 27 and 28; (b) representation of the CNTmetallopolymer as new type of nano-structured composite material; (c) UV–vis absorption spectra (in DMF) of modified SWNTs as well as of reference materials [bare SWNT and a RuII bis(terpyridine) complex] [103]. Figure reproduced with kind permission; r 2009 John Wiley & Sons, Inc.

and high-resolution TEM confirmed the nanotubes’ functionalization. Compared to bare MWNTs, an ultrathin and flat film was formed on the side-wall of the nanotubes; the film was about 2.3 nm thick, which is about twice the axial length of the tpy molecule; thus, the terpyridine-containing film was most likely composed of dimeric species (Figure 8.28a). By applying the layer-by-layer (LBL) self-assembly protocol, the terminal terpyridine groups on the MWNTs were used, as binding sites, for the RuII ions to generate supramolecular multilayer films of tpy-MWNTs and RuII ions on terpyridine-modified substrates (Figure 8.28a). The progressive selfassembly was studied by UV–vis spectroscopy; a linear increase of the absorbance with an increasing number of bilayers indicated the regular growth of the multilayer film. The SEM image showed a high coverage of tpy-MWCNTs bound to a terpyridine-modified silicon wafer (Figure 8.28b). Under illumination, the selfassembled films on ITO exhibited an effective photoinduced charge-transfer. As the number of bilayers was increased, the photocurrent increased and reached its maximum value of 65 nA cm2 at six bilayers; the assembly of further layers lead to a current drop due to the increase in the cell resistance (Figure 8.28c). To end this section, a different type of hybrid material is mentioned briefly, that is, early transition metal oxygen cluster anions, named polyoxometalates (POMs),

08

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800

430


(a) RuII & tpy-MWNTs inturns

substrate N

N

N

or

N

N

N

= tpy-MWNT

N N

(c)

average current intensity (nA)

(b)

N

70 60 50 40 30 20 0

2

2

6

number of bilayers

8

10

Figure 8.28 (a) Representative formation of the layer-by-layer, self-assembled films of RuII ions and tpy-MTNTs on a terpyridine-functionalized surface; (b) SEM image (scale bar: 5 mm) of five bilayers of RuII/tpy-MTNT on a tpy-modified Si surface; a detailed image is also shown (tenfold magnification); (c) average current intensity (ACI) as a function of the number of bilayers [105]. Figure reproduced with kind permission; r 2010 American Chemical Society.

which form a remarkable class of well-defined nanoclusters with an enormous diversity of structures and properties [106]. In recent years, they have attracted considerable interest due to their wide range of properties and applications in many fields, such as analytical chemistry, catalysis, materials science, and medicine. For instance, POMs can reversibly accept several electrons and, therefore, may act as electron reservoirs with redox tuning properties. As a result, POMs are attractive candidates for the development of photochemical devices; however, their electrostatic or covalent attachment to a light-harvesting antenna is sensible, since POMs are themselves photoactive, but only in the UV part of the solar spectrum. To overcome this limitation, Duffort et al. functionalized a bis(4-iodophenyl)silanemodified [PW11O37]7 nanocluster with two terpyridine moieties via a Sonogashira cross-coupling reaction [107]. As expected, the redox behavior of the POM was not

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8.4 Terpyridines and Nano-structured TiO2: Photovoltaic Applications

affected by the chemical modification on the periphery. The terpyridine-functionalized POM represents a new type of hybrid material that might be utilized – with complexes possessing appropriate transition metal ions – in photovoltaic or molecular memory applications. 8.4 Terpyridines and Nano-Structured TiO2: Photovoltaic Applications

The dye-sensitized solar cell (DSSC) is a type of low-cost solar collector belonging to the group of thin film organic solar cells [108]. The basic operating principle of such a photovoltaic device is based on a key process in which optical absorption and charge separation take place via the association of a sensitizer, as the lightabsorbing material, with a wide band gap semiconductor material of nanocrys¨tzel and O’Regan in 1991 talline morphology [109]. DSSCs were invented by Gra ¨tzel cells [110]. and are commonly referred to as Gra Figure 8.29a depicts a pictorial representation of a DSSC. The mesoporous oxide layer, generally composed of nanometer-sized particles that have been sintered together to enable electronic conduction, is deposited onto a conducting glass slide. Today, the most commonly utilized semiconducting material is TiO2 (“anatase”), although ZnO or Nb2O5 have been investigated, as alternatives. A monolayer of the charge-transfer dye is adsorbed on top of the nanocrystalline layer. Upon photoexcitation, an electron of the dye is injected into the conduction band of the oxide. Subsequently, the electronic state of the dye is restored by electron donation from an electrolyte (i.e., an organic solvent containing a redox system, such as an I/I3 couple); regeneration of the sensitizer by I prevents back-donation of the conduction band electron to the oxidized dye. The iodide itself is regenerated by

Conducting glass Dye TiO2

(a)

E vs NHE (V)

0.5

Injection

(b) Electrolyte S∗ Maximum voltage

0 hv 0.5 1.0

Cathode

Red

Ox Mediator Interception Diffusion S/S

Figure 8.29 (a) Representation of the principle of a dye-sensitized solar cell (DSSC) – the energy levels are also shown; (b) SEM image of a nanocrystalline TiO2 film typically used in DSSCs [109]. Figure reproduced with kind permission; r 2003 Elsevier B.V.

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| 431

432

| 8 Terpyridines and Nanostructures reduction of triiodide at the counter-electrode (for a detailed investigation of these processes utilizing resonance micro-Raman spectrophotoelectrochemistry, see Reference [111]). The electric circuit is completed via electron migration through an external load. Overall, the voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the solid and the redox potential of the electrolyte. Moreover, a DSSC generates electric power from light without suffering any permanent chemical transformation. Figure 8.29b shows an SEM image of a typical TiO2 film on a conducting glass slide (deposited by screen printing). In general, the film is between 5 and 20 mm thick (TiO2 mass of about 1–4 mg cm2). The porosity of the nanoporous structure varies from 50% to 60%, with an average pore size of 15 nm. The favored prevailing structures of the anatase nanoparticles are square-bipyramidal, pseudocubic, and stab-like; according to high-resolution TEM, the (101)-face is mostly exposed, followed by (100) and (001) surface orientations [109]. Various types of molecules have been employed, as sensitizers: organic dyes, quantum dots, and transition metal complexes; however, the best photovoltaic performance in terms of both conversion yield and long-term stability has been ¨tzel’s group reported for oligopyridyl complexes of RuII and OsII ions. In 1993, Gra introduced cis-RuL2(NCS)2 (29; L ¼ 2,20 -bipyridyl-4,40 -dicarboxylic acid), which became the benchmark for a heterogeneous charge-transfer sensitizer for mesoporous solar cells for almost a decade [112]. The fully protonated 29 showed UV–vis absorption maxima at 518 and 380 nm (extinction coefficients of 1.3 and 1.33 104 M1 cm1, respectively); photoluminescence occurred at 750 nm with a lifetime of 60 ns. The optical transition was of MLCT character in which excitation of the dye involved electron transfer from the RuII center to the p*-orbital of the surface-anchoring carboxylated bipyridyl ligand from where it was released within femto- to picoseconds into the conduction band of TiO2, generating electric charges with almost unit quantum yield [113]. The “black dye” [RuL(NCS)3](n-Bu4N)3 (30, L ¼ 2,20 :60 ,200 -terpyridyl-4,40 400 -tricarboxylate) exhibited a remarkable redshift in absorption when compared to other complexes of the [Ru(tpy)2]2 þ type [114–116]. A photovoltaic cell containing an adsorbed monolayer of 30 on TiO2-covered glass in conjunction with the redox electrolyte LiI/LiI3 in di(n-propyl) carbonate, as solvent, showed an incident photonto-current conversion efficiency (IPCE) of 80% over a broad region with an integral photocurrent density (iph) of 20.5 mA cm2 [117, 118]. Figure 8.30 compares sensitizers 29 and 30 with respect to their IPCE as a function of the excitation wavelength. Both supramolecular dyes exhibited very high IPCE values in the visible range; however, the response of “black dye” 30 extended 100 nm further into the IR regime than that of 29 (the photocurrent onset was close to 920 nm, i.e., near the optimal threshold for single junction converters). Taking into account the reflection and absorption losses in the conducting glass, the IPCE was practically quantitative over the entire visible spectrum. Under standard AM 1.5 sunlight (air mass 1.5 sunlight is the spectrum of sunlight that has been filtered by passing through 1.5 times the thicknesses of the earth’s atmosphere), an open circuit voltage (Voc) of 0.72 V was observed, affording an overall power conversion efficiency (Z) of 10.4%.

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| 433

HOOC

N OOC 29

NCS Ru2

N N

NCS

N

OOC

COOH

OOC

N HOOC

Ru2

N N

30

NC S NCS

NCS

Incident Photon to Current Efficiency

80

30 60

29 40

bare TiO2

20

0 400

600

3 (n-C4H9)4N

OOC

800 Wavelength [nm] (b)

(a)

Figure 8.30 (a) RuII-based sensitizers 29 and 30; (b) comparison of the incident photon-tocurrent efficiency of sensitizers 29 and 30 (compared to bare TiO2) [117]. Figure reproduced with kind permission; r 2001 American Chemical Society.

The combination of both dyes, 29 and 30, within a DSSC resulted in an increased photocurrent and IPCE [119]. This multiple-dye system showed a short circuit current (Isc) of 10.2 mA cm2 and a cell efficiency (Z) of 2.8 while broadening the cell’s spectral sensitivity. In control experiments, the individual dyes yielded Isc values of 6 and 5 mA cm2 (with cell efficiencies of 1.7 and 1.2) for 29 and 30, respectively. Notably, so-called “tandem cells” containing both of these dyes – connected either in parallel or series – featured improved spectral responses, higher photocurrent (increase by almost 20%), and higher conversion efficiencies than the corresponding single cells [120]. A Pt-mesh sheet was introduced, as a cathode, to efficiently separate the two half-cells [121]. The absorption and crystallization behavior of 30 [122] as well as the interfacial electron-transfer dynamics [123] in this “black dye”-based DSSC were also investigated. XPS was applied to determine the average degree of surface-coverage (i.e., bound dye molecules per Ti atoms of the surface); the stoichiometric Ru : Ti ratio of about 1 : 100 corresponded to a monomolecular coating [124]. DFT calculations gave further insight into the mechanism of electron injection into TiO2 through the excited-state(s) and electron acceptance from the redox system [125]. The excited electron, which was localized on the terpyridine ligand, could efficiently be injected into the oxide; the generated hole, which was found to be localized on the S-atom of NCS ligand, was then filled with an electron from the redox systems. According to photoelectron spectroscopy, a mixed binding mode of 30 to the TiO2 film was concluded [126]. The O1s spectra revealed a similar binding to the TiO2 surface of 29 and 30 via their carboxylate moieties; however, the S2p spectra suggested a partial interaction (fraction of about 15%) of 30 with the oxide surface via

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1000

434

| 8 Terpyridines and Nanostructures the NCS ligands (for comparison, a fraction of 30% was estimated for 29). A small fraction of the n-Bu4N þ cations was also found to be co-adsorbed onto the surface. The electron injection process of the 30/TiO2 system was studied by means of transient absorption spectroscopy [127, 128]; under ambient conditions, electron injection occurred in the ps time range. The efficiency of electron injection could be increased by immersion of the film in MeCN owing to stabilization of the photo-excited-state by solvent molecules. The enhancement was due to the opening of an additional pathway for electron injection occurring slowly in the 100 ps time range. The latter could be suppressed by the addition of 4-tert-butylpyridine, which is known to improve the performance of solar cell devices owing to an increased energy level of the conduction band. Thus, electron injection may occur from two distinct energy levels: (i) an ultrafast injection from a vibronically nonthermalized singlet state and (ii) a slower injection from a relaxed excited tripletstate in the sensitizer aggregates. Treatment of the nanoporous TiO2 film with HCl increased the adsorption of 30; increased surface loading with “black dye,” which could be monitored by UV–vis absorption spectroscopy (Figure 8.31a), reduced the dark current and increased the resistance at the TiO2/dye/electrolyte interface [129, 130]. These changes in concert contributed to an overall efficiency improvement in which an energy conversion efficiency of 10.5% was achieved. Comparison of the IPCE curves of a DSSC based on 30 with and without acidic treatment is shown in Figure 8.31b. Furthermore, enhanced adsorption of 30 on nanoporous TiO2 under pressurized CO2 conditions was reported (the adsorption time was 10–100-times faster than in conventional solution-based processes); the performance of the fabricated DSSC device [i.e., Isc, Voc, and fill factor (FF)] was also improved [131]. Intensity-modulated photovoltage spectroscopy gave detailed insight into the impact of surface protonation on device performance [132] – upon protonation the conduction band

0.8

0.7 untreated HCI treated

untreated HCI treated

0.7 0.6

0.4 0.3

0.5

Current / mA cm2

0.5 IPCE

Absorbance

0.6

0.4 0.3

0.2

0.2

0.1

0.1

0.0

0.0 400

500

600

800

700

300

400

20 15 10 5 0 0.0 0.2 0.4 0.6 0.8 Voltage / V

500

600

700

Wavelength / nm

Wavelength / nm

(a)

(b)

800

900

Figure 8.31 (a) UV–vis absorption spectra for the dye-loaded untreated (dotted line) and HCl treated (solid line) TiO2 films; a bare TiO2 film was used as a reference. (b) IPCE curves for DSSCs with scattering TiO2 films (32 mm); inset: the corresponding I–V curves [129]. Figure reproduced with kind permission; r 2005 American Chemical Society.

08

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1000


| 435

edge of TiO2 was positively shifted by 28 mV. Simultaneously, the charge recombination rate constant was slowed by a factor of 5, corresponding to an enhancement of Voc by 50 mV. The spectroscopically determined net increase in Voc, as a result of both conduction band edge movement and suppressed charge recombination, was in good agreement with the experimentally observed enhancement of Voc under illumination of the device with AM 1.5 sunlight. The conventional technique for generating nanocrystalline TiO2 films (hydrolysis of TiIV isopropoxide with aqueous HNO3) was modified by the Kusama group [133] where hydrolysis from TiCl4 with ammonium carbonate yielded nanocrystalline TiO2 particles, which had a pure anatase crystal structure (for the XRD data, see Figure 8.32a). Moreover, smaller surface areas, larger size distributions, and mean sizes, when compared to nanoparticles prepared from Ti(OCHMe)2)4/ HNO3, were observed (Figure 8.32b). The 30-adsorbed TiO2 photoelectrode synthesized via the new approach featured a more thorough surface coverage of 30; the Isc and IPCE values of the DSSC were higher and, thus, the solar energy conversion efficiency (Z) was increased by 16% under AM 1.5 sunlight (Figure 8.32c). Apparently, pure anatase crystallites, the reduced surface area, and the more complete surface coverage of the sensitizing dye enhanced the absorbed

(a)

(b)

(c)

Current / mA/cm2

15

Brookite

10

20

30

40 50 2θ/ degree

60

70

80

12 9 6 3 0

0

0.1

0.2

0.3 0.4 Potential / V

0.5

Figure 8.32 (a) XRD patterns of nanocrystalline TiO2 prepared from TiCl4/(NH4)2CO3 (top) and Ti(OCHMe2)4/HNO3 (bottom); (b) SEM image of nanocrystalline TiO2 prepared from TiCl4/(NH4)2CO3 (scale bar is 100 nm); (c) I–V characteristics of DSSCs containing “blackdye” 30, as sensitizer [black curve: TiO2 prepared from TiCl4; gray curve: Ti(OCHMe2)4 precursor] [133]. Figure reproduced with kind permission; r 2007 Elsevier B.V.

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0.6

0.7

436

| 8 Terpyridines and Nanostructures photon-to-current efficiency (APCE) and the light-harvesting efficiency (LHE), which, in turn, improved the IPCE, Isc, and Z values. The role of the electrolyte on DSSC device performance was addressed by Hara et al. and others [134–136]. The MLCT absorption band of 30 in solution and adsorbed on a TiO2 film was redshifted in the presence of LiI cations; this shift was attributed to the formation of a –CO2Li-type dye or interaction between the dyes and intercalated/adsorbed LiI cations on the TiO2 surface. An organic liquid electrolyte, composed of the ionic liquid 1-ethyl-3-methylimidazolium iodide (EMImI, 1.5 M), iodine (0.05 M), and MeCN, was applied and a high solar-energyto-electricity conversion efficiency of 9.2% was observed under AM 1.5 irradiation (100 mW cm2). Applying a LBL fabrication technique, polyelectrolyte multilayer composites [i.e., linear poly(ethylene imine) (PEI) or PEG in combination with poly(acrylic acid) (PAA)] were used, as templates, for the construction of a porous TiO2 network, which could be sensitized by 30 [137]. Moreover, these ion-conducting polymer composites (conductivity of 105 and 104 S cm2, respectively) filled the porous inorganic framework, resulting in a high interfacial contact area that had a beneficial impact on device performance. Acetonitrile is commonly considered as the best solvent for solar cell performance. This assumption was quantified by Katoh et al. by comparison to other solvents with relative permittivity values (er) in the range 35–65 [i.e., 3-methoxypropionitrile, g-butyrolactone, and di(n-propyl) carbonate] [138]. For this purpose, a DSSC containing 30, as sensitizer, was fabricated and an electron-injection efficiency for the air-dried film was estimated to be 0.4 using a time-resolved microwave conductivity technique. Indeed, MeCN, as solvent, showed the highest efficiency in DSSC devices (about 0.65); for the other solvents, lower values (0.5– 0.6) were determined. The enhancement of device performance in the presence of MeCN was attributed to strong interactions of the RuII center of 30 and the –CN groups of the solvent. The effects of deoxycholic acid (DCA), as a co-adsorbate, and guanidinium thiocyanate, as an electrolyte additive, on the photovoltaic performance of DSSCs based on 30 were investigated [139]. The presence of DCA (up to 2 mM) increased both the photovoltage and photocurrent of the device (the photocurrent decreased at higher concentration). Imaging with STM revealed that the lateral distribution of the adsorbed dye molecules was sensitive to the presence of DCA [140]; the fraction of single, isolated molecules of 30 increased at the cost of larger aggregates of dye molecules. The addition of guanidinium thiocyanate into the electrolyte enhanced the performance of the DSSCs (Isc ¼ 8.20 mA cm2, Voc ¼ 0.755 V); adsorption of the guanidinium cations subsequent to the adsorption of 30 facilitated the self-assembly of dye molecules, thus either the dark current was reduced or the conduction band edge of the TiO2 was shifted positively [139]. A DSSC containing two different types of dyes, as sensitizers – the “black dye” 30 as well as the organic dye 31 – yielded a high external quantum efficiency (Figure 8.33) [141]. A mixture of these dyes was adsorbed onto the TiO2 electrode from solution without the dyes interfering with each other during the electrontransfer processes (i.e., no energy transfer from the organic dye to the RuII

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| 437

1.0 30 & 31 30 31

0.8

IPCE[-]

COOH CN N 31

0.6 0.4 0.2 0.0 300

400

500

600 700 800 Wavelength [nm]

Figure 8.33 IPCE curves of the DSSCs containing dyes 30 and/or 31 [141]. Figure reproduced with kind permission; r 2009 American Institute of Physics.

complex was observed). The multiple dye system achieved a power conversion efficiency of about 11.0%; the IPCE curves revealed that the two dyes together performed better than a single dye alone (i.e., either 30 or 31). A similar approach was followed by the Park group, who sequentially adsorbed dyes on a single TiO2 electrode [142]. These authors utilized a mild protocol for positioning of three different types of dyes of different absorption ranges (29, 30, and 32, Figure 8.34a) in a mesoporous TiO2 film by mimicking the concept of the stationary phase and mobile phase in column chromatography; a polystyrene-filled mesoporous TiO2 film was used as the stationary phase and a Brønsted-basecontaining polymer solution was developed, as a mobile phase, for the selective desorption of the adsorbed dye. By controlling desorption and adsorption depths, the organic dyes 32, 29, and 30 (from bottom to top) were vertically aligned within a TiO2 film (Figure 8.34a). The alignment could be confirmed by electron probe micro-analysis [for the 2D-Ru-EPMA (electron probe microanalysis) spectrum after the adsorption of all three dyes see Figure 8.34a]. The triple-dye-layer DSSC exhibited a broad-band IPCE feature (Figure 8.34b) composed of each narrow IPCE obtained from the respective single-dye cells. Structural changes of the basic “black dye” motif 30 to improve the efficiency of DSSCs have been reported by various research groups. Based on DFT molecular orbital calculations, Aiga and Tada predicted that substitution of all NCS ligands by 4-sulfanylpyridine molecules should lead to both a higher electron-transfer rate from the redox system to the oxidized dye as well as a higher absorption efficiency of solar light compared to 30, while still retaining the high electron-transfer rate from the photooxidized state to TiO2. However, an experimental proof of this theoretical study has not yet been published [143]. The RuIII tris(thiocyanato) complex 33 exhibited better light-harvesting behavior and higher absorbance in the near-IR region than “black dye” 30 (Table 8.1) [144]. The DSSC containing this dye

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900

1000

438


(a)

29 S O

32 400

5 µm

CN COOH

Ru EPMA

substrate

600 800 1000 Wavelength (nm)

30 29 32 30 (7 µm)

(b) 60 IPCE (%)

TiO2 film

Absorbance (a.u.)

30

40 29 (9 µm)

20

0

32 (9 µm)

400


1000

Figure 8.34 (a) Left: UV–vis absorption spectra of dyes 32, 29, and 30 (from bottom to top); middle: pictorial representation of the vertical alignment of the three dyes after adsorption onto the TiO2 electrode; right: 2D-Ru-EPMA spectrum of the TiO2 film after sequential adsorption of all three dyes (the bright regions indicate a high concentration of RuII ions). (b) IPCE curves of the triple-dye-layered DSSC in comparison to the respective single-layered DSSCs [142]. Figure reproduced with kind permission; r 2009 Nature Publishing Group.

showed 35% IPCE at 900 nm; this is the highest IPCE value reported so far for RuII complexes in the near-IR region. Different homo- as well as heteroleptic RuII bis(terpyridine) complexes have also been investigated, as photo-active materials, for dye-sensitized solar cell applications. For this purpose, the basic terpyridine motif was modified by introducing surface-anchoring moieties (e.g., carboxylic, phosphonic, or boronic acid residues) or by extending the p-conjugated system. Moreover, oligonuclear assemblies have been reported in the literature. ˆte et al. investigated the charge-separation on In an early contribution, Bonho donor-functionalized sensitizers 34a/b that were adsorbed onto nanocrystalline TiO2 (Figure 8.35a) [145, 146]. With 34a, the observed charge separation was similar to the unfunctionalized derivative 34c. Spectroelectrochemistry, resonance Raman spectroscopy, and laser flash photolysis indicated that very fast recombination reactions were limiting the charge separation. In contrast, 34b, possessing an electronically-decoupled donor substituent, exhibited an improved photoinduced charge-separation. This increased efficiency was attributed to the electron

08

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8.4 Terpyridines and Nano-structured TiO2: Photovoltaic Applications Photovoltaic performances of DSSCs sensitized with “black dye” 30 and 33 under irradiation in limited wavelength regions [144.].

Table 8.1

HOOC

COOH

N N SCN

Ru

N 33

NCS NCS

Dye

Wavelength (nm)

Isc (mA cm2)

Voc (V)

FF

gsun (%)a

30

W720b W750b W800b W720b W750b W800b

4.0 2.4 0.8 6.0 4.6 2.6

0.46 0.44 0.41 0.29 0.28 0.26

0.69 0.70 0.70 0.54 0.55 0.56

1.30 0.76 0.21 0.95 0.71 0.38

33

a

Overall solar-light-to-electricity conversion. Wavelength of the irradiated light through cut-off filters.

b

in the excited-state being localized more on the ligand bound to the semiconductor surface; the resonance Raman spectrum of 34b resembled that obtained for [(HO)2OP-tpy]RuCl2, as reference (Figure 8.35b). To elucidate the limiting effect of electron-transfer processes on the photovoltaic performance of the system 34b/TiO2, the efficiency of the entire charge separation process was measured in a mixed monolayer of 34 and sodium 3-(4-phenylphenoxy)propyl-1-phosphonate (35). The number of electrons that were delivered by one molecule in the electrical circuit per 100 absorbed photons was obtained by dividing the IPCE (at a wavelength of 550 nm) where no saturation occurred (absorption o1) by the fraction of the absorbed light. This photovoltaic quantum yield (FPV) increased remarkably with surface dilution; this finding was attributed to the concomitant reduction of recombination due to lateral electron transfer, when the dye molecules were moved away from each other (Table 8.2) [145]. Wang et al. reported the RuIII mono(terpyridine) complex 36 as a “black dye”analog material (Figure 8.36) [147]. Broader absorption in the visible region, when compared to 30, was observed for [(36)Ru(NCS)3]. The dye was adsorbed on nanocrystalline TiO2 and gave an IPCE value of about 90% at the maximum absorption wavelength (IPCE in the near-IR region was about 20%). The overall energy conversion efficiency (Z) of a DSSC containing [(36)Ru(NCS)3] as sensitizer was 2.9% [irradiation with white light (78.0 mW cm2)]; the photovoltaic characteristics were: Isc ¼ 6.1 mA cm2, Voc ¼ 0.58 V, and FF ¼ 0.62. The lower efficiency in comparison to 30 (Z ¼ 0.68) was attributed to a weaker interaction of the terpyridine ligand (via its single carboxy group) with the TiO2. The same mode of surface binding was utilized by Figgemeier et al. [148] in which two heteroleptic RuII bis(terpyridine) complexes (37a/b) were investigated to elucidate the influence of lateral substituents (i.e., H versus 2-thiophenyl) on the

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| 439

08

27 J l 2011 16 15 39

N

N

34a

N

OCH3

OCH3

N

N

PO3 2

N

Ru2+

N

R

H2O3P 35

O

O

34b

N

OCH3

OCH3

(b)

0

5

10

15

20

1100

1200

1100

1200

1300

1400

1259 1341

1100

1300

1400

1400

1600

1600

1610 1566

1537

1500

1476

1609

1600

1562

1531

1500

1473

1500

1561

1529 1601 1475

Raman shift (cm1)

1200

1170 1245 1142

1350 (H2O3P-tpy)RuCl2 1292 1273

1000

1400

1357

1300

1020 1093 1167 1256 1049

34b

1000

1020 1246 1343 1047 1285 1166 1091

34a

1000

0

1

2

3

4

0

2

4

6

8

1700

1700

1700

Figure 8.35 (a) RuII bis(terpyridine) complexes 34 and phosphonic acid 35; (b) resonance Raman spectra of 34a (top, laser excitation @ 530.9 nm), 34b (middle, laser excitation @ 482.5 nm), and [(HO)2OP-tpy]RuCl2 (bottom, laser excitation @ 530.9 nm] [145]. Figure reproduced with kind permission; r 1999 American Chemical Society.

R H (34c)

R

(a)

Intensity (103 cps)

440


8.4 Terpyridines and Nano-structured TiO2: Photovoltaic Applications Table 8.2

| 441

Photovoltaic quantum yield (FPV) at 550 nm of the system 34b/35/TiO2 [145].

xa

Absorption (%)b

IPC (%)c

UPV (%)

1.00 0.80 0.65 0.50 0.20

77 44 30 24 20

37 32 24 24 19

48 73 81 100 96

7 7 7 7 7

2 2 6 8 9

Molar fraction of 34b and 35 on the surface. Light absorption due to 34b at 550 nm (average of three points of the electrode). c Electrolyte: 1 M LiI, 0.01 M LiI3 in propylene carbonate. a b

R R

COOH 36 R

N N

N

R H (38a)

Ru NCS

SCN

N N

R H (37a) S

N

38b N

C8H17

N

S

2 PF6

Ru2+

37b

C8H17

S

N

N

S

N Ru2+

R

N

S

N

NCS

38c

PO3H2

N N 2 PF6

R

N N

N

S PO3H2

2+

Ru N CO2H

39

N N 2 PF6 R

Figure 8.36 A series of RuIII- or RuII-containing sensitizers (36–39).

efficiency of DSSCs containing these dyes (Figure 8.36). Owing to the 2-thiophenyl moiety in 37b, the lowest unoccupied molecular orbital (LUMO) and lowest-lying triplet excited-state were shifted from the surface-bound to remote ligand; thus, the efficiency of electron injection into the TiO2 was decreased and a poor IPCE value of about 2.5% was obtained (37a yielded an IPCE of about 20%). Similar results were obtained when the bithiophene- and terthiophene-substituted RuII

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S

442

| 8 Terpyridines and Nanostructures dyes (38b/c) [149] or complexes with phenolic as well as carotenoic residues [150] were attached via their phosphonate groups to nanocrystalline TiO2. The DSSCs of 38b/c, containing an additional hole-conducting layer of poly(n-octylthiophene), were characterized and compared to a device based on 38a. The poor electron injection efficiency of 38b/c led to a low overall photovoltaic performance when compared to 38a (38a: Z ¼ 0.16; 38b: Z ¼ 0.08; 38c: Z ¼ 0.09). Thus, localization of the LUMO on the surface-bound and not on the remote/ancillary ligand appeared to be a prerequisite for designing new materials for dye-sensitized solar cells. Consequently, a terpyridine containing a 40 -(thiophen-5-yl-2-phosphonic acid) substituent was incorporated in the heteroleptic complex 39 (Figure 8.36) [151]. When introduced, as sensitizer, into a photovoltaic cell, the photoconversion efficiency – compared to 38a – was significantly improved (39: Z ¼ 0.41%; 38a: Z ¼ 0.17%); according to IR transient spectroscopy, the electron injection rate from the excited MLCT state through the thiophene unit was unrestricted. Nanocrystalline SnO2 (Figure 8.37a) was investigated, as an alternative electrode material for DSSCs based on RuII dyes [152]. The RuII bis(terpyridine) complex 40 was bound to the SnO2 surface via its phosphonate functionality (Figure 8.37b) and the I–V-characteristics as well as device performance were determined; efficient short circuit photocurrents together with low overall energy conversion efficiencies were observed (IPCE ¼ 0.16%). The structural and electronic properties of TiO2 nanoparticles sensitized with various RuII bis(terpyridine) complexes were investigated theoretically using DFT calculations combined with time-dependent (TD) DFT calculations [153]. In particular, the effects of carboxylic and phosphonic acid anchor groups, as well as of a phenylene group used to separate the anchor from the terpyridine unit, on the optical properties of the dyes and electronic interactions in the dye-sensitized TiO2 nanoparticles were studied. Comparison of the calculated electronic coupling strengths suggested that both the nature of the anchor group and presence of phenylene spacer were capable of significantly influencing the electron-transfer

(b)

80.000 nM

(a)

N I

N Ru2

N N

N

O O P O

N SnO2 40

0.2 0.4 0.6 0.8

µM

Figure 8.37 (a) 3D AFM height image of the nanocrystalline SnO2 film; (b) surface-bound RuII dye 40 [152]. Figure reproduced with kind permission; r 2005 Elsevier B.V.

08

27 J l 2011 16 15 40


rates across the dye–semiconductor interface. According to DFT calculations, the phosphonic acid anchor decreased the interfacial electronic coupling by a factor of 6.4, whereas insertion of phenylene groups lowered the coupling by a factor of about 2.6. Comparing the strongest coupling case (i.e., carboxylic acid anchor and no spacer group) to the weakest one (i.e., phosphonic acid anchor and a phenylene spacer) indicated a multiplicative combination of the effects of the anchor and spacer groups; the electronic coupling decreases by a factor of 16.6, which is identical to the value obtained by multiplying both individual factors (i.e., 2.6 and 6.4). The strong electronic coupling resulted in estimated electron injection times that are very fast; the injection times determined for the carboxylic acid, as an anchor group, were twice as fast as those for the phosphonic anchor group. Besides surface binding via carboxylate or phosphonate groups, the ability of boronic acids to adsorb onto TiO2 was also used [154]; however, to achieve a sufficient surface coverage and, therewith a reasonable photocurrent efficiency, at least two boronic acid moieties per dye molecule were required. The Balasubramanian group showed that strong binding to nanocrystalline TiO2 could be achieved via lateral methacryloxy groups [155]. For this purpose, the homoleptic RuII bis(terpyridine) complex 41 was synthesized and adsorbed onto the FTO electrode material from solution (FTO: fluorine-doped tin oxide) (Figure 8.38). The conversion efficiency of the resulting DSSC was Z ¼ 3.6%, which is comparable to the value reported for 29 (Z ¼ 4.0% [156]). This relatively low photocurrent was attributed to the absence of an absorption maximum at higher wavelength and the formation of high density traps on the surface of the film by the high molar mass dye anchored to the TiO2, which limited the collection of electron–hole pairs. However, a remarkable long-term stability of the DSSC was observed in which about 70% of the initial efficiency was obtained even after 25 days under direct atmospheric contact without sealing. The utilization of RuII bis(terpyridine) complexes based on terpyridine-functionalized polymers, as sensitizers in DSSCs, is evaluated separately in Chapters 5 and 6 (for instance, see References [157–159]).

(a)

41: [Ru(L)2](PF6)2

(b)

L

N O O N

N

Figure 8.38 (a) RuII complex 41; (b) SEM image of 41 and TiO2 coated onto a FTO glass slide (scale bar is 1 mm) [155]. Figure reproduced with kind permission; r 2007 IOP Publishing.

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27 J l 2011 16 15 40

| 443

444

| 8 Terpyridines and Nanostructures Islam et al. introduced b-diketone derivatives, as ancillary ligands, in RuII mono (terpyridine) complexes 42 (Figure 8.39a) [160]. The exchange of two thiocyanate ligands of 30 by a ditopic diketonate moiety resulted in an enhanced light absorption above 700 nm. Under comparable conditions, 42a/b and 30 showed similar photocurrent action spectra; in accordance with the electronic spectra, all dyes 42 showed higher IPCE values than 30 in the 720–900 nm region. By comparing the photocurrents of 42a/b with and without DCA, as co-adsorbent, it was shown that the long alkyl chain of 42b efficiently prevented surface aggregation of the sensitizer. The aryl-substituted complexes 42c exhibited intense visible light absorption with MLCT bands above 600 nm [161, 162]. DFT calculations of the fluorine-substituted complex 42c revealed that the HOMO was localized on the NCS ligand and the LUMO on the terpyridine moiety, which was anchored to the TiO2 nanoparticles. Utilizing 42c, as sensitizers, gave IPCE values above 80% in the whole visible range extending up to 950 nm. Under standard AM 1.5 irradiation (100 mW cm2), a DSSC containing the fluorine-functionalized complex 42c yielded I–V-characteristics of Isc ¼ 19.1 mA cm2, Voc ¼ 0.66 V, and FF ¼ 0.72, corresponding to an overall conversion efficiency of 9.1%.

R

(a)

(b) R

N N

N

N

Ru2+ O F3C

O R

NCS

N

42a : R CH3 42b : R C16H33 42c : R phenyl (4-H, Cl or F)

N

N Ru2+

N R

N 2+

N

Ru

N N

N

N

N Zn

N N

N

43a : R = 4-Me-phenyl 43b : R = 4-(OC10H21)-phenyl

N 2+

N

Ru

N

R

N

N

N

N

8PF6

(c) N N

Ru2+

N

N N

N

R

Figure 8.39 (a) Diketonato complexes 42; (b) tetranuclear RuII ZnII-porphyrin complexes 43; (c) TEM image of the self-assembled nanowires of the discotic ZnII-free porphyrin complex [163]. Figure reproduced with kind permission; r 2007 The Royal Society of Chemistry.

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Tetranuclear RuII bis(terpyridine) ZnII-porphyrin complexes 43 were reported by the Newkome group (Figure 8.39b) [163]. The tetrakis(terpyridine)-porphyrin cores were prepared by condensation reactions and used for the subsequent stepwise complexation with RuII and ZnII ions. The discotic ZnII-free complexes were found to self-assemble by p–p stacking of the porphyrin units into nanowires that were about 7 nm wide (matching the diameter of the complex) and 0.3–0.5 mm long (Figure 8.39c). The photovoltaic results suggested that the peripheral long aliphatic chains of 43b promoted a decrease in recombination of the photogenerated electrons. The electron lifetimes (tn) for the DSSCs containing dyes 43a/b were obtained from electrochemical impedance spectroscopy experiments under polychromatic illumination (2.2 mW cm2). Lifetimes of 5.14 and 0.13 ms were determined for the systems 43b/TiO2 and 43a/TiO2, respectively; thus, electron recombination was almost 40 times slower for 43b/TiO2. Further types of dendritic, oligonuclear RuII bis(terpyridine) complexes were also investigated, as potential sensitizers in DSSCs; poor photovoltaic performances were obtained, mainly due to insufficient binding of the dye to the nanocrystalline TiO2 film [164]. Mono-cyclometalated RuII complexes of the general formula [(C4N4N)Ru (N4N4N)] þ were utilized recently, as a new type of sensitizer, in DSSCs (C4N4N: mono-cyclometalating tridentate ligand; N4N4N: tridentate ligand) [165]. In comparison to analogous RuII bis(terpyridine) complexes (e.g., 44), their mono-cyclometalated counterparts exhibit a broader and redshifted visible absorption behavior. For instance, complexes 45a/b were fabricated into DSSCs (with g-butyrolactone, as electrolyte) and yielded high photocurrents upon irradiation with AM 1.5 sunlight (45b: ISC ¼ 12.0 mA cm2; 29: ISC ¼ 11.5 mA cm2). A significant enhancement of the maximum IPCE was observed for the dyes 45a/b (44a: 55% at 530 nm; 45b: 70% at 550 nm) (Figure 8.40); the higher IPCE for 45b was attributed to a more efficient interaction of the sensitizer with TiO2. In contrast, the mono-cyclometalated RuII complex 46 with a N4C4N ligand gave very low efficiencies (IPCE value of 8% at 500 nm) [166]. According to time-dependent DFT calculations, complexes 45 with the C4N4N ligand possessed an excitedstate located on the cyclometalated ligand, allowing efficient charge injection; in contrast, complex 46 with the N4C4N had its isolated excited-state located on the remote terpyridine ligand and, therefore, was incapable of efficiently injecting charges into the TiO2 conduction band. Thus, the nature of the covalent Ru–C bond directed the electronic properties of the dye and, therewith, determined the efficiency of the DSSCs containing these materials. To end this section, the interaction of a different type of transition metal ion complexes with nanocrystalline surfaces is considered, specifically MnII mono (terpyridine) complexes. Coordination compounds of MnII ions have widely been investigated as effective homogeneous catalysts for oxidation reactions [167–169], including the biomimetic m-dioxo bridged MnII complexes based on terpyridine ligands that can be utilized for water splitting [170] or the regio- and stereoselective alkane hydroxylation [171] (such application of MnII complexes in the field of catalysis will be detailed in Chapter 9). In general, the activation of such catalysts typically requires sacrificial electron scavengers (e.g., Oxones or H2O2) that oxidize the MnII

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| 445

446


N

Ru2+

N

44

N

N

COOH

N

0.8

N PF6 R2

0.6

N

45a : R COOH, R H 45b : R1, R2 COOH

N Ru2+

N

2

R1

N

IPCE (%)

1

0.4

N

0.2

PF6

N

46

N Ru2+

N

0.0

400

COOH


700

800

(b) N

N PF6 (a)

Figure 8.40 (a) (Mono-cyclometalated) RuII complexes 44–46; (b) IPCE curves of 29 (black), 44 (blue), 45a (red), 45b (green), and 46 (magenta) in a DSSC with TiO2, as the electrode material, and using 0.5 M LiI and 0.05 M I2 in g-butyrolactone as the electrolyte [166]. Figure reproduced with kind permission; r 2010 American Chemical Society.

ions to a sufficiently high valent state to enable it to react with H2O or aliphatic hydrocarbons, yielding the oxidized products, such as O2, alcohols, and epoxides. The Crabtree group addressed the question of whether the oxidation state of MnII complexes can be activated by photoexcitation and interfacial electron transfer (IET) into semiconducting TiO2 nanoparticles, avoiding the need to consume any primary oxidant and also circumventing generation of the associated waste materials [172]. The MnII complex 47a photosensitized TiO2 nanoparticles to visible light absorption (Figure 8.41a); efficient charge separation and reversible photochemistry were observed by terahertz (THz) spectroscopy when 47a was bound to TiO2, either in colloidal thin films or in aqueous suspensions, even at higher oxidation states generated by photoexcitation and IET. Their UV–vis absorption behavior and the IET into the nanoparticles were simulated to gain further insight into the electron-injection mechanism at the molecular level. In analogy to other sensitized TiO2 materials, the observed stability of the surface-bound MnII complex was attributed to the highly efficient charge-separation processes triggered by photoexcitation, including photooxidation of the surface-bound complex, and ultrafast IET into the conduction band of TiO2. Regeneration of the initial MnII complex by electron–hole recombination was limited by the underlying trapping/detrapping dynamics of the photoinjected electrons within the nanoparticles. Electron paramagnetic resonance (EPR) studies suggested that these processes occurred on the order of 20 s at 6 K (the

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8.5 Organopolymeric Resins, Beads, and Nanoparticles

R

N

N

N OH L1: R

bare TiO2 catechol /TiO2 47a/TiO2

absorption (arb units)

47a : [Mn(L1)(H2O)3]2 47b : [Mn(L2)(H2O)3]2

OH

L2: R

O

O

NH

OH

(a)

400

600 wavelength (nm) (b)

II

Figure 8.41 (a) Mn mono(terpyridine) complexes 47;. (b) UV–vis absorption spectra of bare TiO2 (black), catechol/TiO2 (red), and 47a/TiO2 (blue) (solid curves: experimental spectra; dashed curves: simulated spectra) [172]. Figure reproduced with kind permission; r 2007 American Chemical Society.

half-time for the regeneration of the MnII species was about 23 s); the THz experiments suggested a time frame of 500 ps to 1 ms at room temperature. The acetylacetonate (acac) group was used, as an alternative anchor, for binding MnII complex (47b) to the TiO2 surface [173]. The acac moiety is well known for forming stable coordination complexes with titanium and other metal ions in various oxidation states [174]. In particular, coordination compounds of TiIV have been investigated in the context of precursors for TiO2 synthesis [175] and are commonly stable towards hydrolysis over a wide range of pH values; acac also forms stable adducts with TiIII ions [176] and may be expected to be robust during IET. McNamara et al. showed that the acac linkage in 47b/TiO2 was more stable to aqueous and oxidative conditions than the classic carboxylate anchors in DSSCs (e.g., “black dye” 30) [173]. Thus, binding a sensitizer to nano-structured TiO2 via acac might be promising for the development for DSSCs that are not sensitive to humidity.


Compared to other fields of research involving terpyridines and their (transition) metal ion complexes, their immobilization onto polymeric resins, beads, or nanoparticles has been investigated to a limited extent [177]. The main target of research in this direction is the application of such materials to heterogeneous catalysis. Yoo et al. reported the functionalization of poly(chloromethylstyrene-codivinylbenzene) (PCD) with 40 -(4-hydroxyphenyl)-2,20 :60 ,200 -terpyridine (48, Scheme 8.1) [178]. The subsequent addition of an excess of Fe(ClO4)3 gave, at low concentrations, the FeIII mono(terpyridine) complexes. The formation of intramolecular FeIII bis(terpyridine) complexes was unlikely, since the terpyridine ligands

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| 447

448

| 8 Terpyridines and Nanostructures OH N PCD

Cl

K2CO3, DMF

N

N

N

N

PCD

O

N

48

Scheme 8.1 Synthesis of terpyridine-functionalized PCD resins [179].

∗

n

∗

O O 49 exo:endo 1:2

N N

N

N

N

poly(49) N

Figure 8.42 Terpyridine 49 and the ROMP-product poly(49) [180].

were attached to the resin in a rather sterically confined, rigid manner. This material was utilized, as an efficient Lewis-acidic catalyst, for the nucleophilic opening of different epoxides with methanol or water; in many cases, very high conversions were observed even after short reaction times at room temperature (for instance, cyclohexene oxide was opened by methanol to >99% within 2 h). ¨ll et al. investigated the possibility of developing a polymer-supported catalyst Kro for (heterogeneous) ATRP [180]. For this purpose, linear homopolymers, as well as grafted and coated silica- and poly(styrene-co-divinylbenzene)-based polymer supports, were prepared from the norbornene-substituted terpyridine 49 via ringopening metathesis polymerization (ROMP) (Figure 8.42). Subsequently, monomer 49 and the polymers were loaded with CuI, CuII, FeII, or HgII ions and were utilized for the ATRP of styrene. Under optimized conditions, the homopolymer poly(49), coated onto silica and loaded with CuI ions, afforded poly(styrene) (PS) in 30% yield and with reasonable polydispersity indices (PDIs) within 2 h at 70 1C (Mn ¼ 30 600 g mol1, PDI ¼ 1.57). In the isolated polymer, a remaining metal content of o100 ng g1 was determined by atomic absorption spectroscopy (AAS). The loading of different transition metal ions (i.e., FeII, CoII, CuII, RuIII, and NiII) onto terpyridine-functionalized, TentaGelTM microbeads {i.e., crosslinked poly[styrene-g-poly(ethylene glycol)], d ¼ 20 mm} was investigated by Schubert et al. (Figure 8.43a) [181]. UV–vis absorption spectroscopy of suspensions of the beads revealed typical absorption bands for bis(terpyridine) complexes (e.g., the MLCT band at labs ¼ 490 nm for beads loaded with FeII ions); complexation also became apparent from the resultant coloration of the material (Figure 8.43b).

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(a)

Cl N

O

N

N

N N

DMSO, 60 C, 2 d O

OH

O N

(b)

(c)

20 µm

10 µm

10 µm

Figure 8.43 (a) Synthesis of terpyridine-functionalized TentaGelTM; (b) image of tpyfunctionalized TentaGelTM beads after loading with FeII ions [the purple color is characteristic for FeII bis(terpyridine) complexes]; inset: optical microscopy image of these beads; (c) SEM images of the tpy-functionalized beads before (left) and after loading with CoII ions (right) (the scale bar is 10 mm) [181]. Figure reproduced with kind permission; r 2003 Wiley-VCH.

Furthermore, the degree of loading could be quantified by AAS – the loading rates were found to be in accordance with quantitative mono- and bis-complexation of the terpyridine moieties on the microbeads. SEM imaging showed a smoothing of the surface after loading with metal ions (Figure 8.43c). In terms of characterizing terpyridines moieties attached to solid PS supports, Heinze et al. utilized electron ionization (EI) mass spectrometry to detect the attached terpyridine units [182]. Crosslinked polystyrene was first modified with a silyl-ether linker, which was then reacted with 48. Dry samples of the resulting material were ground to a fine powder before introduction into the EI MS. This method permitted detection of fragments from the support, which could be distinguished from unbound (i.e., physically adsorbed) material – a discrimination that cannot be obtained by applying, for instance, elemental analysis. In the previous examples, well-defined (crosslinked) polymers were functionalized with terpyridine ligands and, subsequently, loaded with transition metal ions. An alternative approach is to prepare well-defined beads or nanoparticles from polymers already containing terpyridine complexes. Nanoprecipitation was developed as a general route to preparing polymeric nanoparticles under mild

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| 449

450

| 8 Terpyridines and Nanostructures conditions. In general, this technique is based on the self-assembly of polymers in solution by displacement of a water-miscible solvent (e.g., acetone, DMF, or THF) against the non-solvent water; nanoprecipitation has been applied to various types of polymers, including polysaccharide derivatives (e.g., dextran) [183]. Wild et al. functionalized dextran with 6-(2,20 :60 ,200 -terpyridin-40 -yloxy)hexanoic acid (50) [two degrees of substitution (DS): 0.23 and 0.41] and subsequently applied nanoprecipitation to self-assemble the hydrophobic terpyridine-functionalized dextran 51 into well-defined spherical particles (d ¼ 960 nm, PDIparticle ¼ 0.551, Figure 8.44) [184]. The particles were capable of coordinating transition metal ions; the addition of FeII ions to a suspension of the particles resulted in a color change of the milky suspensions into light purple. Upon loading with FeII ions, the particles did not significantly alter their size and shape (d ¼ 1100 nm; PDIparticle ¼ 0.424). In related work, the Rehahn group showed the successful modification of cellulose by partial substitution of the acetate protecting groups by 40 -(4-bromomethylphenyl)2,20 :60 ,200 -terpyridine and the complexation of this terpyridine-functionalized cellulose derivative, using an activated RuIII mono(terpyridine) precursor complex [185]. It was also reported how the composite of a terpyridine-modified schizophyllan (tpy-SPG) and SWNTs yielded sheet-like morphologies in the presence of FeII ions [186]. According to TEM, morphologies were created by FeII bis(terpyridine) complexes crosslinking the tpy-SPG/SWNT components. In general, b-1,3-glucans, such as schizophyllan, are known to adopt a triple-stranded helical structure in nature, which dissociates (i.e., denatures) into single chains

(a)

(b) O O

RO RO O

O

O

OH

O

R H or tpy DS 0.28 DS 0.49 O

N

N 50

1 µm

O

N

N

N

N 51

200 nm

Figure 8.44 (a) Terpyridine 50 and terpyridine-functionalized dextran 51; (b) SEM images of samples of 51 prepared by nanoprecipitation before (top) and after the addition of FeII ions (bottom) [184]. Figure reproduced with kind permission; r 2010 Wiley-VCH.

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References

container (tpy-SPG) cargo (SWNT)

rail (F-actin) wheel (myosin)

Figure 8.45 Pictorial representation of the concept of the container transportation system for nanomaterials [188]. Figure reproduced with kind permission; r 2010 Wiley-VCH.

upon dissolution in DMSO; the single chains can recover (i.e., renatures) their original triple-stranded helical motif when DMSO is exchanged for water [187]. The ability of b-1,3-glucans to act, as hosts, that helically wrap nanomaterials (e.g., CNTs, conjugated polymers, DNA, and gold nanoparticles) permits these nanomaterials to be dissolved in water through the denature–renature process. These authors applied this behavior to develop a container transportation system [188]: the “cargo” (SWNT) was packed into the “container” (tpy-SPG) and transported on the “rail” (actin filament, F-actin) by “wheels” (myosin) (Figure 8.45). This artificial system was inspired by a container transportation system, based on the motion of vesicles in biological cells.

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9

Catalytic Applications of Terpyridines and Their Transition Metal Complexes*

9.1 Introduction

This last chapter summarizes applications of terpyridines as well as their transition metal complexes in the broad field of catalysis. In particular, two types of molecular catalysis have to be considered: organometallic catalysis [i.e., the (asymmetric) catalysis of organic transformations] [1] and metal-catalyzed oxidation or reduction reactions (i.e., artificial photosynthesis) [2, 3]. In 1980, Meyer and coworkers described the electrochemical oxidation of organic compounds catalyzed by various types of mixed-ligand RuII complexes [4]; the concept was extended later by the Che group towards the chemical oxidation of organic substrates utilizing tert-butyl peroxide, as oxidant [5]. Other examples of catalytic activity of such RuII complexes deal with C–C bond formation by electrocatalytic reduction of CO2 [6] and – most important – with the chemical water oxidation [7] (in a similar fashion, complexes of CoII or RhIII ions have also been reported [8, 9]). Besides RuII as the transition metal ion, complexes containing MnII, FeII, and NiII ions have been widely used; the oxidation potential of MnII mono(terpyridine) complexes was successfully applied to the catalytic formation of O2 from water (according to the photosystem-II model) [2, 10] or as a bleaching agent in household applications [11, 12]; and imino-functionalized FeII mono(terpyridine) complexes were used for the oligomerization of ethylene [13]. An untypical NiI mono(terpyridine) complex was shown to function, as a catalyst, in cross-coupling reactions [14]; the corresponding NiII species were catalytically active in the (co)-polymerization of styrene and norbornene [15]. The photocatalytic activity of luminescent square-planar PtII mono(terpyridine) complexes was utilized, for instance, in the reduction of water (i.e., photocatalytic H2 formation) [16]. Moreover, diverse studies have dealt with terpyridine complexes of AgI [17], ReI [18], PdII [19], CdII [20], CrIII [21], AuI [22], ScIII [23], ZnII [24], or OsII ions [25] for various catalytic purposes.

*Parts of this chapter are reproduced from ChemCatChem 3 (2011) DOI: 10.1002/cctc.201100118 by permission of Wiley-VCH Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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460

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes R

R N

N

R

N

O

O

N

N

N

R

O 1a: R H 1b: R benzyl 1c: R Me 1d: R n-butyl 1e: R iso-propyl

2a: R H 2b: R benzyl 2c: R Me 2d: R n-butyl 2e: R iso-propyl O

N

N

N

N

O N

N

O 2f

1f

Figure 9.1 Chiral terpyridines 1a–f and their Lewis base-type tri-N-oxides 2a–f.

From this summary, it appears that the concept “terpyridines and catalysis” is highly diverse – both in terms of involved structures and targeted applications. The first part of this chapter highlights some applications of terpyridine complexes as catalysts in organic reactions (including polymerization reactions), while the second part evaluates the electro- or photochemically induced processes, in particular the splitting of water, focusing on complexes based on RuII, MnII, and PtII ions.

9.2 (Asymmetric) Catalysts in Organic Reactions

Before considering the terpyridine complexes with respect to their catalytic activity, it has to be noted that metal-free systems have also been utilized as (asymmetric) catalysts in organic reactions: the chiral terpyridine tri-N-oxides 2 (Figure 9.1) were prepared from their parent terpyridines 1 [26] by oxidation with m-chloroperbenzoic acid (mCPBA) under mild conditions (i.e., 12 h, room temperature) and were utilized, as asymmetric Lewis bases, in the catalytic allylation of aldehydes [27] (for further examples where chiral oligopyridine N-oxides were applied in organocatalysis, see Reference [1]). As a model reaction, the allylation of benzaldehyde by allyltrichlorosilane with 10 mol-% of 2 was investigated (Scheme 9.1). Good to high isolated yields of the allylation product (85–97%) with moderate to ¨nig’s base, 0 1C, good enantioselectivities (34–74% ee) were observed (CH2Cl2, Hu 3 h, see Table 9.1). Catalyst 2b gave the best enantioselectivity (74% ee), whereas 2c

O R

SiCl3

2 (10 mol-%), CH2Cl2, NEt2iPr, 0 C, 3 h R aryl or alkyl

OH R

Scheme 9.1 Allylation reaction catalyzed by a tri-N-oxide 2 [27].

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9.2 (Asymmetric) Catalysts in Organic Reactions Table 9.1

| 461

Allylation of aldehydes using allyltrichlorosilane and 2, as catalyst [27].

R

Catalyst

Yield (%)a

% eeb

Configurationc

Phenyl Phenyl Phenyl Phenyl Phenyl 4-MeO-phenyl 4-NO2-phenyl n-Octyl

2b 2c 2d 2e 2f 2b 2b 2b

89 97 87 85 85 94 91 80

74 64 67 44 34 65 86 20

(R) (R) (R) (R) (R) (R) (R) (S)

a

Isolated yield. Determined by chiral HPLC analysis. c Absolute configurations were assigned by comparing the retention time to those of known compounds. b

was most efficient at giving (97%) the homoallylic alcohol. The comparably poor performance of 2e and 2f (44% ee and 34% ee, respectively) was attributed to steric demand of the chiral groups preventing a good coordination between the catalyst and allyltrichlorosilane. The scope of this reaction (2a, as catalyst, and allyltrichlorosilane, as reagent) was further studied using different substrates, including aromatic and aliphatic aldehydes; electron-donating substituents at the 4-position of benzaldehyde (e.g., –OMe) exhibited a negative impact on the enantioselectivity, while electron-withdrawing groups (e.g., –NO2) remarkably improved the enantioselectivity to 86% ee (with 91% isolated yield, Table 9.1). The same group also utilized CuII complexes of the chiral terpyridine ligands 1, as catalysts, in the asymmetric cyclopropanation of styrene derivatives with ethyl diazoacetate (EDA) according to Scheme 9.2 [26]. The catalysts were generated

Cu(OTf)2 (2 mol-%), 1 (2.2 mol-%), CH2Cl2, room temperature, 16 h

(a) O H

+

COOEt

OEt N2

O

R 1

R

R3

+

H

COOEt trans-isomer

cis-isomer

(b) 2

+

OEt

[Re(1)(CO)3](OTf) (2 mol-%), CH2Cl2, 50 C, 4 h R2 R1

N2

COOEt R3 COOEt

+

R1

R3

R2

COOEt

trans-isomer cis-isomer cyclopropanation

EtOOC +

and/or EtOOC

Scheme 9.2 Asymmetric cyclopropanation catalyzed by [Cu(1)]2 þ (a) [26] and [Re(1)(CO)3] þ complexes (b) [28, 29].

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COOEt

coupling

462

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes in situ from 1 and CuII triflate; all systems [Cu(1)]2 þ were found to be active catalysts and the cyclopropyl esters were isolated in very high yields (87–98%). The best results were achieved with ligand 1d, giving the trans-isomer (90% ee) and cisisomer (94% ee) in a 70 : 30 ratio (96% isolated yield). It was further shown that variation of the diazo ester structure influenced the trans/cis ratio: bulky ester moieties (e.g., tert-butyl diazoacetate) enhanced the diastereoselectivity of the reaction (i.e., increased the trans/cis ratio); however, the enantioselectivity for both the trans- and cis-isomer was decreased [30]. The relative reaction rates for the transformation of substituted styrene derivatives with EDA were studied: the reaction was enhanced by electron-donating groups and retarded by electronwithdrawing groups. In comparison, the RuII mono(terpyridine) complexes [Ru(1) Cl2] – generated in situ from 1 and [Ru(p-cymene)2Cl2] – provided efficient catalysis in terms of yields, but gave negligible enantioselectivities [31]. The corresponding RhIII mono(terpyridine) complexes [Rh(1)](OTf)3 (–OTf: triflate) revealed a trans : cis ratio of cyclopropane derivatives that was influenced via the steric demand of the R-group of 1a–e (Figure 9.1) [31], in which the bulkier substituents (1b/e) gave a high preference for the trans-isomer (about 70% for 1e) but the isolated yields (53 –75%) and the observed enantioselectivities (up to 59% ee) were only moderate. The more flexible terpyridine derivatives 3–6 were used by Chelucci and coworkers for the same purpose (Figure 9.2) [32, 33]. The 6,6-dimethylnorpinen-2-yl substituent of 3 and 4 was unable to offer any convincing enantioselectivity (only up to 5% ee was observed) [32]; this low selectivity of the CuII-catalyzed cyclopropanation was attributed to the conformational mobility of the substituent, thus minimizing the steric interaction between the stereogenic centers of the ligands and the substrate (which was required for the formation of an enantioselective transition state). In contrast to the CuII-catalyzed cyclopropanation using the chiral ligands 1f, 5, or 6, the corresponding RhIII complexes gave significantly better results [33]. For instance, an enantiomeric excess of 54% and 64% could be obtained for the trans- and cis-isomer, respectively, when [Rh(6)]3 þ was used as catalyst. Notably, the absolute configuration of the trans- (1R,2R) and cis-isomer (1R,2S), generally observed in the case of CuII-based catalysts, was inverted [trans: (1S,2S); cis: (1S,2R)] when changing to RhIII-based catalysts.

N

N

N

N

N

N

5

3

N

N

N

N

4

N 6

Figure 9.2 Chiral terpyridine ligands 3–6.

09

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N


| 463

In a more recent contribution, the ReI(CO)3 mono(terpyridine) complexes of 1a/c were reported by Yeung et al. [28, 29]; high chemoselectivity, good regioselectivity, and moderate enantioselectivity were observed (Scheme 9.2b). The selectivity for cyclopropanation versus the competing coupling reaction was high (up to 34), in particular when sterically demanding alkenes were utilized (e.g., prop-1-en2-ylbenzene or 1,1-diphenylethylene). The trans : cis ratio for cyclopropanation was about 30 : 70, with maximum enantioselectivities of 62% ee (trans-isomer) and 73% ee (cis-isomer) (utilizing ligand 1c) [29]. Two additional examples, where chiral terpyridine ligands were used in asymmetric catalysis, are the hydrosilylation [34] and allylic substitution reactions [35]. The metal-catalyzed addition of a R3Si–H unit to a C¼O bond (followed by hydrolysis of the resulting silyl ether) is formally the reduction of a carbonyl group to the corresponding alcohol. Hydrosilylation of acetophenone with diphenylsilane is commonly chosen as model reaction in this respect (Scheme 9.3a) [36]. The chiral ligands 3 and 4 with C1- and C2-symmetry, respectively, were reacted with [Rh(cod)Cl]2 (COD ¼ cycloocta-1,5-diene) to generate in situ the active RhI catalyst. In both cases, low conversion and low reaction rates were observed; the enantioselectivity did not exceed 14% ee. The same ligands were applied in Pd0-catalyzed allylic substitution, commonly using [Pd(Z3-C3H5)Cl]2, as the pro-catalyst, and dimethyl sodiomalonate, as the nucleophile [37]. Chelucci et al. reported moderate catalytic activity (48 h and 7 h for total conversion) and enantioselectivity (38% and 40% ee) for ligands 3 and 4 [35]. Epoxidation of a wide range of branched aliphatic and aromatic alkenes was achieved by using [(tpy)Ru(pydic)] as catalyst (tpy: 2,20 :60 ,200 -terpyridine; pydic: pyridine-2,6-dicarboxylate) [38–40]. Initially, Nishiyama and Motoyama used bis (acetoxy)iodobenzene, as strong oxidant, in MeOH–water mixtures at reflux [41]. Following a much milder and environmentally more friendly epoxidation protocol (i.e., very low catalyst loading and H2O2, as oxidant, at room temperature), highly substituted epoxides could be obtained in good to excellent yields (in many cases >95%) (Scheme 9.4a). For instance, the tetrasubstituted C¼C bond of (3-methylbut-2-en-2-yl)benzene could be epoxidized with full conversion (96% isolated yield). The performance of this catalyst was comparable to those reported for similar RuII catalysts with 2,6-bis(4,5-dihydrooxazol-2-yl)pyridine or

(a) O + H2SiPh2

3 or 4 (2.4 mol-%), [Rh(cod)Cl]2 (0.5 mol-%), CH2Cl2, room temp.

OSiHPh2 *

(b)

3 or 4 (10 mol-%), [Pd(η3-C3H5)Cl]2 (2.5 mol-%), CH2Cl2, room temp.

OCOCH3 + NaCH(COOCH3)2

CH(COOCH3)2 *

Scheme 9.3 Metal-catalyzed hydrosilylation (a) and allylic substitution reaction (b).

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464

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes

(a)

R3 R4

R1

[(tpy)Ru(pydic)] (0.5 mol-%), 30% aq. H2O2 (300 mol-%), t AmOH, 12 h, room temp.

R3 O R4 R1

(27 examples)

R2 (b)

R2

[Mn(tpy)](OTf )2 (1 to 0.1 mol-%), MeCN, 5 min, room temp. + CH3COOOH

R

O

R

R = n-alkyl 7 (2mol-%), 1.5 eq 35% aq. H2O2, CH3COOOH (20 mol-%), MeCN, 3 min, 0 C

(c) R2 R1 R3

(10 examples)

N N R2 O R1

Cl R3

7

Fe Cl

N

Cl O N

Fe

Cl N

N

Scheme 9.4 Epoxidation of substituted alkenes using a heteroleptic RuII complex (a), a MnII mono(terpyridine) complex (b), or a dinuclear FeII complex (c).

2,6-bis(5,6-dihydro-4H-1,3-oxazin-2-yl)pyridine ligands [39]. In an asymmetric variant, with [(1)Ru(pydic)] as chiral catalyst, (2-methylprop-1-enyl)benzene, as model substrate, could be epoxidized in high yield and moderate enantioselectivities (up to 54% ee). A different catalytic system, generated in situ from Mn(OTf)2 and tpy, exhibited high efficiency in the epoxidation of terminal aliphatic alkenes with peracetic acid (Scheme 9.4b) [42, 43]. The reaction times were remarkably short (5 min) and high yields (>80%) of the epoxide were obtained, even at low catalyst loadings (down to 0.1 mol.%, corresponding to a turnover of about 460). Moreover, the epoxidation of 1,1- and 1,2-disubstituted olefins could be achieved in high yields (>99% and 65%, respectively) under mild conditions using the dioxygen-bridged dinuclear MnII complex [(ttpy)Mn(H2O)O]2 (ttpy: 40 -tolyl2,20 :60 200 -terpyridine) and tetrabutylammonium Oxones, as oxidant [44]. A highly robust and efficient catalyst for the epoxidation of olefins was introduced by Che and coworkers [45]. The homoleptic FeII bis(terpyridine) complex [Fe(8a)2]Cl2 in combination with Oxones, as oxidant, oxidized a wide range of alkene derivatives to the corresponding epoxides in high yields and diastereoselectivities. For instance, aryl alkenes (either terminal or internal) were oxidized almost quantitatively without formation of the 1,2-diol; cis-stilbene was oxidized in 96% yield and 96% diastereoselectivity (cis : trans ratio of 27 : 1). The oxidation of cyclohexene afforded exclusively the epoxide without either alcohol or ketone being formed as by-product. Furthermore, various a,b-unsaturated alkenes were converted into the corresponding epoxides quantitatively within 2 h; steroids of the (pregn-4-en-3-one)-type were selectively oxidized to their epoxide in an a : b ratio of 3 : 1 (Scheme 9.5). A recyclable terpyridine-based catalyst was provided in which the PEG-functionalized ligand 8b [PEG: poly(ethylene glycol), Mn ¼ 7500 g mol1]

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9.2 (Asymmetric) Catalysts in Organic Reactions R

[Fe(8a)2]2(5 mol-%), 1.3eq NH4HCO3, 4 eq. oxone®, MeCN/H2O (2:1), 2 h, roomtemp.

R N

N

N

H H

Cl Cl 8a: R Cl 8b: R OPEG-OCH3

R H H

H O

O e.g. R OH

O

Scheme 9.5 Epoxidation of an a,b-unsaturated steroid catalyzed by a homoleptic FeII bis (terpyridine) complex [45].

could be isolated from the reaction mixtures and reused as its FeII bis(terpyridine) complex in subsequent epoxidations without any significant loss of activity over five runs. A chiral hexatopic sexipyridine ligand was used in the formation of the dinuclear FeII complex 7 [46]. The oxidizing couple 7/H2O2 was found to be a highly versatile catalytic system for the epoxidation of styrene derivatives, featuring a short reaction time of 3 min, high chemoselectivities (i.e., carbonyl or alcohol by-products were observed in very few cases), high to excellent yields, and moderate enantioselectivities (Scheme 9.4c, Table 9.2). In comparison, various chiral terpyridine ligands (such as 1 and 3–6) showed poor catalytic activity in the epoxidation of stilbenes with H2O2 [47]. Since the aziridination reaction is mechanistically related to olefin epoxidations, similar catalytic systems have been employed in these two reactions; thus, the FeII bis(terpyridine) complex [Fe(8a)2]2 þ has to be considered (see also Scheme 9.5) [45]. Liu et al. showed that both aziridination of alkenes and amidation of

Table 9.2

Selected examples for the asymmetric epoxidation of styrene derivatives catalyzed

by 7 [46]. R (Scheme 9.4c)

Conversion (%)a

Selectivity (%)a

Yield (%)a

% eeb

R1 ¼ C6H5 R2, R3 ¼ H R1 ¼ 4-MeO-C6H4 R2, R3 ¼ H R1 ¼ 4-Cl-C6H4 R2, R3 ¼ H R1, R2 ¼ C6H5 R3 ¼ H R1 ¼ C6H5 R2 ¼ H, R3 ¼ CH3

100

98

95

43 (R)

100

100

100

15 (R)

95

90

90

42 (R)

56

57

52

–

62

100

100

40 (1S,2R)

a

Determined by gas chromatography. Determined by chiral HPLC; the absolute configurations were determined by comparing the elution time with those of samples with known configuration.

b

09

| 465

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H α:β 3:1

466

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes sulfamate esters proceeded in high yields when this particular catalyst was used. Styrenes as well as terminal and cyclic alkenes were transformed into the corresponding N-substituted aziridines by reaction with a nitrene-transfer reagent according to Scheme 9.6. Moreover, unsaturated sulfonamides and sulfamate esters were intramolecularly cyclized and amidated, respectively, in very high chemoselectivity (>95%) and high yields (>85%); in both cases, the nitrenetransfer agent was generated in situ by reaction of the –SO2NH2 moiety with PhI(OAc)2. The relative rates of epoxidation and aziridination of para-substituted styrenes (e.g., –OMe, –Me, –H, –Cl, –Br, –CF3) were examined; with electron-rich substituents, the reaction was accelerated whereas electron-deficient substituents retarded the reaction. However, the reaction rates were only influenced to a minor extent by the electronic nature of the para-substituent when compared to other systems (e.g., using peracids or peroxides [48, 49]), indicating that less charged benzylic intermediates were involved in the reaction. He and coworkers reported two different types of aziridination catalysts, based on terpyridine complexes. Reaction of the ligand 4,40 ,400 -tri(tert-butyl)-2,20 :60 ,200 terpyridine (9) with AgNO3 afforded the dinuclear species [Ag2(9)2(NO3)](NO3) as the catalytically active species; the structure of the complex was confirmed by X-ray single-crystal analysis (Figure 9.3) [17]. A wide range of aliphatic and aromatic alkenes was reacted with this catalyst in the presence of PhI¼NTs, as the nitrenetransfer agent (Ts: para-toluenesulfonyl) to give the corresponding aziridines in good to high yields (up to 91%). An even more powerful catalyst was obtained with AuI ions [22]: the complex [Au(9)](OTf) efficiently mediated the aziridination of substituted styrenes with ortho-nitrosulfonamide (NsNH2) and PhI(OAc)2, as oxidant; even some cyclic alkenes (e.g., cyclooctene and norbornene) were

(a) O I

1

R

+

R2 R1: aryl or alkyl R2: H or alkyl

(b) SO2NH2

N

S

R3 O

[Fe(8a)2]2+ (5 mol-%), MeCN, 40 C, 12 h (5 examples)

R1 NSO2R3 R2

R3: C6H5 or 4-Me-C6H4

[Fe(8a)2]2+ (5 mol-%), 1.5 eq. PhI(OAc)2 MeCN, 40 C, 12 h

O

S

O N

(5 more examples)

(c)

[Fe(8a)2]2+ (5mol-%), 1.5 eq. PhI(OAc)2, 2.3 eq. MgO MeCN, 40 C,12 h OSO2NH2

(2 more examples)

H O N S O O

Scheme 9.6 The (intramolecular) aziridination of alkenes (a and b) and the intramolecular amidation of sulfamate esters (c) catalyzed by [Fe(8a)2]2 þ [45].

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N7 N5 Ag2 N4 N6 Ag1

N3

N

N

N Ag+

Ag

N

O N O O

N2

N

N

N1

NO3

(b)

(a)

Figure 9.3 (a) Dinuclear AgI complex [Ag2(9)2(NO3)](NO3); (b) representation of the X-ray single-crystal structure of the dinuclear AgI complex (hydrogen atoms and counterions omitted for clarity) [17]. Figure reproduced with kind permission; r 2003 American Chemical Society.

aziridinated in moderate yields, but the related AgI complex was inactive. As an additional feature, the catalyst was active in the carbene-insertion to benzene; good chemoselectivity with respect to the competing coupling reaction was observed but the yield of the reaction was rather low (Scheme 9.7). The allylation of carbonyl compounds, which affords synthetically useful homoallylic alcohols, has been a subject of extensive investigations (for a summary of different synthetic protocols, see Reference [50]). Aoyama et al. reported that the allylation of aromatic and aliphatic aldehydes as well as activated ketones occurred in very high yields (up to 99%) with allyltrimethoxysilane, as the allylation reagent, and [Cd(tpy)F2], as catalyst [20] (Scheme 9.8, Table 9.3). With respect to the toxicity of cadmium compounds, it is worth mentioning that the catalyst could be fully recovered (by extraction into water) and, subsequently, reused without significant loss of its catalytic activity. [Au(9)](OTf ) (3 mol-%), 1.2 eq. PhI(OAc)2, molecular sieves MeCN, 50 C, 12 h

(a) SO2NH2

R2 R

1

+

R1

(13 examples)

NO2

R2

1

R : various subsituents R2: H, C6H5, Me or COOEt

O2N O N S O

COOEt

(b) [Au(9)](OTf) (3 mol-%), MeCN, 50 C, 12 h

O + N 2

EtOOC

OEt

EtOOC + 25%

and/or EtOOC

0%

COOEt

Scheme 9.7 Aziridination of substituted styrenes (a) and carbene-insertion (b) catalyzed by [Au(9)](OTf) [22].

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| 467

468

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes O R1

R2

[Cd(tpy)F2] (5 mol-%), THF/H2O (9:1), 30 C, 9 h

Si(OMe)3

(8 examples)

1.5 eq

R1 R2

OH

Scheme 9.8 Allylation of carbonyl compounds catalyzed by [Cd(tpy)F2] [20].

A notable RuII catalyst (10) for the transfer hydrogenation of ketones was introduced by Kelson and Phengsy [51]. Complex 10, containing two monodentate kN-bound 2-pyridonato ligands, an uncommon saturated hard donor coordination motif (for the X-ray single-crystal structure, see Figure 9.4a), was found to catalyze in high yields (>95%) the reduction of ketones by 2-propanol and even of bulky ketones (e.g., adamantan-2-one). In the reduction of acetophenone, high total turnover numbers (>1000) were observed with little or no indication of catalyst degradation. The catalyst exhibited a high chemoselectivity: the C¼C bond of alkenes (e.g., cyclohexene or styrene) were not reduced, neither did an excess of cyclohexene hinder the reduction of acetophenone, thus making 10 a promising catalyst for applications in selective carbonyl reduction. A structurally simpler catalyst for the transfer hydrogenation of ketones was reported by the Beller group [52] in which either [Fe3(CO)12] or FeCl2 – as a source for FeII ions – and tpy/triphenylphosphine (PPh3) were utilized for the in situ generation of a FeII mono(terpyridine) complex (Scheme 9.9). In general, the reduction of the ketones occurred in high to excellent yields; however, replaceable substituents almost fully retarded the hydrogenation due to poisoning of the catalyst (e.g., R1 ¼ aryl, R2 ¼ CH2Cl). Detailed investigation, utilizing deuterated 2propanol, gave insight into the mechanistic pathway of the hydrogen transfer; thus, the reduction was believed to follow a “monohydride mechanism” rather than a “dihydride mechanism” (Scheme 9.9b). An aldol-type condensation of benzaldehydes with methyl isocyanato-acetate was catalyzed by the square-planar PdII complex [(tpy)Pd(NCMe)](BF4)2 (Scheme 9.10a) Table 9.3 Selected examples for the allylation of carbonyl derivatives catalyzed by [Cd(tpy)F2] (Scheme 9.8) [20].

R1

R2

C6H5

H

4-MeO-C6H4 4-O2N-C6H4 2-HO-C6H4 C6H5-CH2CH2 4-O2N-C6H4 C6H5

H H H H CH3 CH3

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Yield (%) 92 (1st run) 87 (2nd run) 90 (3rd run) 81 99 99 72 86 32


| 469

C3

(a)

(b) C2

C4 C5 C17

C18

C1

N1

02 C13

C14 C15

C16

N Ru 10

C25

C24

OH2 N O

N3

N5 Ru1

C21

C23

N

N

C20 N4

C22

N

O C12

C19 C11 H1a 01 H1b

N2

C10

R

1

03

OH

C9

1

R

C7

trans-[(tpy)Ru(κN-NC5H4O)2(H2O)]

10 (0.5 mol-%), NaOH (10 mol-%), 2-propanol, reflux, 0.5 to 3 h

(5 examples)

C6

O

OH

+ R2

2

+

R

C8

Figure 9.4 (a) Representation of the RuII complex 10, together with its solid-state structure (hydrogen atoms omitted for clarity); (b) transfer hydrogenation of ketones catalyzed by 10 [51]. Figure reproduced with kind permission; r 2000 The Royal Society of Chemistry.)

[19]. The reaction products – 4,5-disubstituted oxazolines – were obtained in high yield (90%), as a mixture of cis/trans-isomers in an almost 1 : 1 ratio. The rearrangement of disubstituted oxaziridines, themselves derived from the Na2WO4catalyzed oxidation of the corresponding imines, into amides was investigated by

(a) O R

(b)

1

+

OH

2

R

[Fe3(CO)12]1/3/tpy/PPh3 (1 mol-%), 2-propanol, Na 2-propanolate, 100 C, 7 h

OH 1

R

(10 examples)

+

2

O

R

pathway A: “monohydride mechanism”

H

O

O

OH

- acetone

*H

H*

[M-H*] pathway B: “dihydride mechanism”

O

H

O

OH

- acetone

*H

H*

[H-M-H*]

OH* +

50%

H 50%

Scheme 9.9 (a) Reduction of ketones using a three-component FeII catalyst; (b) two possible mechanisms for the hydrogen transfer [52].

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O

470

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes [(tpy)Pd(MeCN)]2+ (1 mol-%), Hünig base (10 mol-%), CH2Cl2, reflux, 1 h

(a) O +

CN

+ O

(b) N

R

Na2WO4 (2 mol-%), 35% aq. H2O2, MeCN, 10 h, room temp.

2

COOEt

COOEt

COOEt

N

R

O

O

R2

N

O

11 (2.5 mol-%), MeCN, reflux, 12 h

R1

1

N

R1

(8 examples)

N H

R2

R1: aryl, R2: alkyl

[(tpy)Ru(PPh3)Cl2] (1 mol-%), toluene, reflux, 8 to 10 h

(c) N

OH

(7 examples)

R

O R

O NH2

[(tpy)Ru(PPh3)Cl2] (1 mol-%), NH2OH*HCl, NaHCO3 toluene, reflux, 17 h

O

(8 examples)

R

R: (hetero) aryl, alkyl

R

NH2

R: (hetero) aryl, alkyl

Scheme 9.10 Aldol-type condensation reaction catalyzed by a PdII mono(terpyridine) complex [19] (a), the rearrangement of oxaziridines into amides [53] (b), and the RuIIcatalyzed oxime/aldehyde-to-amide conversion [54] (c).

Crabtree and coworkers [53]. Under ambient conditions, the dinuclear MnII complex [(ptpy)2Mn2(m-O)2(H2O)2](ClO4)3 (11, ptpy: 40 -phenyl-2,20 :60 ,200 -terpyridine) showed the best performance in terms of isolated yield of amide (>74%) in comparison to other catalysts, based on PdII, RhI, or IrI ions (Scheme 9.10b). The same group reported that the RuII mono(terpyridine) complex [(tpy)Ru (PPh3)Cl2] could be a highly efficient catalyst for converting aldoximes into amides in isolated yields of 83–90% [54]. Moreover, aldehydes could be converted into amides in a one-pot process using NH2OH HCl and NaHCO3, as additives, for the in situ generation of the oxime (Scheme 9.10c). This approach appeared to be favorable when compared to other protocols for the direct aldehyde-into-amide conversion using toxic reagents and/or harsh reaction conditions. Besides the application of [(tpy)Ru(PPh3)Cl2], as catalyst, in the formation of amides, the same complex was utilized to accelerate the alcohol cross-coupling reaction between a primary and secondary benzylic alcohol according to Scheme 9.11 [55]. In general, high conversion of the starting materials (typically >90%),

OH R

1

OH

a or b

R2

OH R

1

R

2

O

R

1

R2

a: [(tpy)Ru(PPh3)Cl2] (1 mol-%), 1 eq. KOH, toluene, reflux, 1 to 7 h b: [(tpy)IrCl3] (1 mol-%), 0.2 eq. KOH, neat, 120 C, 0.5 to 3 h Scheme 9.11 Alcohol cross-coupling catalyzed by a RuII or an IrIII mono(terpyridine) complex [55].

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| 471

Selected examples for the alcohol cross-coupling reaction catalyzed by RuII or IrIII mono(terpyridine) complexes (Scheme 9.11) [55]. Table 9.4

R1

R2

Catalyst

Conversion (%)a

Yield (%)

Alcohol : ketoneb

C6H5 C6H5 C6H5 C6H5 C6H5 C6H5

C6H5 4-Cl-C6H4 n-Pr C6H5 4-Cl-C6H4 n-Pr

[(tpy)Ru(PPh3)Cl2] [(tpy)Ru(PPh3)Cl2] [(tpy)Ru(PPh3)Cl2] [(tpy)IrCl3] [(tpy)IrCl3] [(tpy)IrCl3]

94 82 95 99 95 74

65 56 84 95 65 95

100 : 0 100 : 0 90 : 10 93 : 7 96 : 4 93 : 7

a

Consumption of the 21 alcohol was determined by gas chromatography. Determined by 1H NMR spectroscopy.

b

moderate to high yields (56–85%), and excellent chemoselectivity with respect to the competing ketone formation (>90%) were observed (Table 9.4); however, for aliphatic primary alcohols, there was a somewhat slower conversion and ketone formation could be detected. The isostructural [(tpy)IrCl3] complex was more efficient in terms of yield (65–95%), but the chemoselectivity was slightly decreased. Both catalysts, though, were much more active than the previously utilized RuII complexes with cyclopentadienyl or p-cymene ligands. The linear co-dimerization of 2-norbornenes with acrylic derivatives was reported by the Mitsudo group [56]. The catalytic system of Zn/[(tpy)RuCl3] offered regio- and stereoselectivities for a wide range of substrates (i.e., acrylic esters and amides) in alcoholic solvents, from which the exo-regioisomers were obtained in good yields (>50%) with high stereoselectivity (trans : cis ratios of up to 40 : 1) (Figure 9.5a). [(tpy)RuCl3] (1 mol-%), Zn (10 mol-%), alcohol, reflux, 1 to 24 h

(a)

+

exo-trans-isomer

exo-cis-isomer

COOR

COOR

(11 examples)

(b)

COOR

(c) N(1) C(1)

R X

+ BrZn

R: alkyl; X: Br or I N(2)

[Ni] (5 mol-%) THF, 23 h, room temp. (5 examples)

R

[Ni]: [(tpy)Ni(CH3)] or [(tpy)NiI]2

C(2)

Figure 9.5 (a) Linear co-dimerization of alkenes catalyzed by [Ru(tpy)Cl3] [56]; (b) representation of the X-ray single-crystal structure of [(tpy)Ni(Me)], indicating the head-to head packing with an average Ni. . .Ni distance of 3.18 A [14]; (c) Negishi-type cross-coupling reaction catalyzed by a NiI or NiII mono(terpyridine) complex [14, 57]. Figure reproduced with kind permission; r 2004 American Chemical Society.

09

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472

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes Both an uncommon NiI-alkyl and a cationic NiII mono(terpyridine) complexes were utilized by Vicic and coworkers for cross-coupling reactions of alkyl electrophiles under Negishi-type conditions (the X-ray single-crystal structure of the NiI complex [(tpy)Ni(Me)] is depicted in Figure 9.5b) [14, 57]. In the latter case, the active catalyst was generated in situ from Ni(cod)2 and 4,40 ,400 -tri(tert-butyl)terpyridine. The cross-coupling of 1-iodo-3-phenylpropane and n-pentylzinc bromide preceded under mild reaction conditions (THF, as solvent, at room temperature) in very high yield (98%) (Figure 9.5c). As an extension, Smith and Fu recently established a protocol for the cross-coupling of secondary ZnII-nucleophiles with secondary (mainly propargylic) electrophiles at room temperature using NiCl2 glyme/tpy (10 mol.%), as catalyst [58]. The catalytic ring-opening of aliphatic and aromatic epoxides was shown by Lee and coworkers, who utilized a polymer-bound FeIII mono(terpyridine) complex [59]. This polymer-bound complex was prepared by functionalization of poly(chloromethylstyrene-co-divinylbenzene) (PCD) via a SN2 reaction with 40 -(4hydroxyphenyl)-2,20 :60 ,200 -terpyridine and subsequent complexation with FeIII ions (see also Chapter 8.5). Both the ring-opening with water (i.e., hydrolysis) as well as with MeOH (i.e., alcoholysis) occurred under mild conditions and with very high conversion (about >90%). Screening various alcohols, as nucleophiles, indicated that the regiochemistry of the ring-opening catalyzed by [PCD-tpy)Fe(H2O)3]3 þ was controlled by the electronic nature of the substrate rather than by steric factors. Related to this, the Suzuka group utilized a polymer-bound CuI mono(terpyridine) complex for a CuI-catalyzed alkyne-azide cycloaddition (CCAAC) reaction [60]. The terpyridine unit was attached to the hydrophilic end of an amphiphilic polystyrene-b-poly(ethylene glycol) diblock copolymer via a sulfamide linkage. Terminal as well as internal alkynes reacted in a 1,3-dipolar cycloaddition with substituted azides in up to 87% yield and excellent regioselectivity (i.e., terminal alkynes selectively yielded 1,4-disubstituted 1H-1,2,3-triazoles). The catalyst could be recovered and reused several times without any loss of catalytic activity. The enantioselective aminolysis of meso-epoxides could be performed using a chiral ScIII mono(terpyridine) complex generated in situ from Sc(OTf)3 and terpyridine 12 (Scheme 9.12) [23]. The aminolysis of cis-stilbene oxide with aniline afforded the corresponding amino-alcohol in moderate yield and enantioselectivity

OH

OH N

N

N 12

Sc(OTf)3(10 mol-%), 12(12 mol-%), CH2Cl2, 24 h, roomtemp.

O R

R

+ H2N

(2 examples)

HO

HN

R

R

Scheme 9.12 Asymmetric aminolysis of meso-epoxides catalyzed by Sc(OTf)3/12 [23].

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(up to 60% and 51% ee, respectively); in the case of cyclohexene oxide, the ringopening with aniline was more efficient (93%), but at the cost of selectivity (only 6% ee). The neutral RuII complex [(tpy)Ru(pydic)] is not only a versatile catalyst for the epoxidation of alkenes (see Scheme 9.4a) [38–40] but also a powerful catalyst for the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively, with H2O2, as the oxidant [61]. Utilizing the optimized protocol (i.e., alcohol : peroxide ratio of 1 : 2.5, 0.01 mol-% catalyst loading, no additional solvent or additive, room temperature, for 1 h), a wide range of benzylic and cyclic alcohols could be oxidized in high to excellent yields (>80%); however, the protocol could not be applied for aliphatic alcohols (no conversion was detected). The remarkable performance of the catalyst was revealed by the high productivity (turnover numbers >10 000) and activity (turnover frequency up to 14 800 h1). The same complex was also reported to catalyze the oxidation of naphthalenes to the corresponding 1,4-naphthoquinones [62]. Compared to conventional protocols for this type of reaction, where often stoichiometric amounts of toxic metal salts (e.g., CrIV, MnIII, or CeIV salts), strongly acidic conditions, and high concentrations of H2O2 were required, the procedure reported by Beller et al. was more environmentally friendly: 2 mol-% [(tpy)Ru(pydic)], 1–2.5 mol-% phase-transfer catalyst, 7 equiv. H2O2 (30% in water), 40 1C, 1–18 h. In particular, alkyl-substituted and electron-poor naphthalenes were obtained in high yields. The catalytic dehydrogenation of alcohols is of relevance with respect to future applications, for example, efficiently producing molecular hydrogen from renewable resources. Beller’s group showed that the combination of [Ru(p-cymene)Cl2]2 and an amino compound is well suited for producing H2 from 2-propanol at 90 1C under basic conditions [63]. Strongly chelating ligands, such as terpyridine derivatives, were, however, less efficient for this purpose than either 2-(dimethylamino)ethanol or N,N-dimethylaniline. A different type of catalytic system, based on a dinuclear m-(O)2-bridged MnIII/ MnIV mono(terpyridine) catalyst (for initial reports of the catalytic activity of such complexes, see References [2, 64]), was introduced by Crabtree, Brudvig, and their coworkers [65]. Molecular recognition through H-bonding in concert with C–H activation resulted in a highly efficient regioselective functionalization of sp3hybridized C-centers, remote from the –CO2H recognition group. The catalyst contained a (tpy)(H2O)MnIII(m-O)2MnIV(H2O)(tpy) reactive center and ligand 13, based on Kemp’s triacid, that directed a –CO2H group to anchor the carboxylic acid group of the substrate and, thus, mediated the oxidative selectivity (Figure 9.6a). Ibuprofen, as a model substrate, could be oxidized regioselectively under mild conditions using tetrabutylammonium Oxones, as the oxidant (71% yield, 97% chemoselectivity, 710 turnovers) (Figure 9.6b). In contrast, the 40 -phenyl-2,20 :60 ,200 terpyridine ligand did not offer any remote recognition center and, therefore, a significant decrease of the chemoselectivity was observed, when the oxidation was catalyzed by [(ptpy)(H2O)MnIII(m-O)2MnIV(H2O)(ptpy)]3 þ [66]. The structurally simpler MnII mono(terpyridine) activated H2O2 and catalyzed the oxidation of some organic dyes in aqueous alkaline solution (the catalyst

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| 473

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| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes O ibuprofen

OH O

(a)

H

O

O H

O

oxidizable C-H close to the active site

N

HOOC O

Mn(µ-O)2Mn N

N O

13 N

Regioselective product

(b) [(13)(H2O)Mn(µ-O)2Mn(H2O)(13)] (0.1 mol-%), 5 eq. tetrabutylammonium oxone®, MeCN, 2 h, room temp.

O

OH O

OH O O Alternative product

Figure 9.6 (a) Transition state pre-determining the regioselectivity of the C–H oxidation; (b) regioselective oxidation of ibuprofen catalyzed by the dinuclear MnIII/MnIV mono(terpyridine) complex. [65]. Figure reproduced with kind permission; r 2006 Science.

activity was pH dependent with a maximum at a pH of 10) [11]. In particular, the introduction of electron-rich substituents at the 4,40 ,400 -positions of the terpyridine (e.g., hydroxy or amino groups) significantly enhanced the catalytic activity when compared to the unsubstituted complex [(tpy)MnCl2]. Recently, mechanistic insight into the oxidative degradation of organic dyes by H2O2, catalyzed by various MnII salts/complexes, was given by van Eldik and coworkers [67]; electron paramagnetic resonance (EPR) measurements indicated the in situ formation of a catalytically active MnIV=O species. Wieprecht et al. investigated the usage of such catalysts in laundry bleach applications [12]. For this purpose, tea-stained cotton fabrics were treated at 40 and 25 1C under typical washing conditions. The performance of the MnII mono(terpyridine) complexes with electron-rich substituents was similar to that of tetraacetylethylenediamine (TAED), the most commonly used activator, in terms of bleaching and dye damage. As a drawback, the

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disproportionation of H2O2 into O2 and H2O was also catalyzed by MnII mono(terpyridine) complexes and, thereby, drastically increased the consumption of the bleaching agent. Besides the previously detailed broad range of applications in organometallic catalysis, terpyridine complexes have also been utilized as catalysts in the field of polymer synthesis. In particular, the CuI-catalyzed atom-transfer radical polymerization (ATRP) has to be named in this context. Matyjaszewski et al. showed that the catalyst’s activity strongly depended on the nature of the ligands defining the coordination sphere around the metal center. The best results in terms of reaction rate and control over the molar mass were reported with oligodentate amine ligands, such as N,N,N0 ,N00 ,N00 -pentamethyldiethylenetriamine (PMDETA) and 2,20 -bipyridine [68, 69]. The bulk polymerization of styrene or methyl acrylate was uncontrolled when utilizing terpyridine in a 1 : 1 ratio with CuI ions, as catalyst. The poor performance of this catalytic system was attributed to the low solubility of the in situ generated CuII mono(terpyridine) complex, thus leading to considerable deactivation rates. Better control over the polymerization was achieved when the more soluble 4,40 ,400 -tris(5-nonyl)-substituted derivative was employed: a linear increase of the molar mass with time as well as low polydispersity indices (PDIs) were observed (PDI ¼ Mw/Mn o 1.2) [70]. The application of ATRP and other controlled radical polymerization techniques with respect to terpyridine-functionalized initiators or catalysts has been detailed in Chapter 6. The Yasuda group investigated the potential of terpyridine complexes of FeII and FeIII ions in the polymerization of conjugated dienes (i.e., 1,3-butadiene and isoprene) [71]. Isoprene was polymerized by [Fe(tpy)2](FeCl4)2 in the presence of modified methylalumoxane (MMAO was previously reported to efficiently support the polymerization of alkenes catalyzed by FeII ions [72]) at 25 1C to afford high molar mass polymers (Mn ¼ 14.9–77.7 104 g mol1) with PDI values of 1.31–2.74. The microstructure of these polymers was determined by 1H NMR, revealing a mixture of 1,2-, 3,4-, and cis-1,4-polyisoprene in a ratio of 36 : 50 : 14. The FeIII mono(terpyridine) complex [(tpy)FeCl3] exhibited high catalytic activity, affording high molar mass polymers composed of 1,2-, 3,4-, and cis-1,4-polyisoprene in a ratio of 27 : 61 : 12. The 3,4-polyisoprene content was further enriched by utilizing a more soluble 4,40 ,400 -tri(tert-butyl)-substituted terpyridine as ligand, with which the relative amount of 3,4-polyisoprene reached >82% (at reaction temperatures of 0–25 1C). Moreover, a higher activity of [(tBu3-tpy)FeCl3] (k ¼ 14 000 g mol1 h1) in comparison to both [Fe(tpy)2](FeCl4)2 and [(tpy)FeCl3] (k ¼ 6000 g mol1 h1) at the early stage of the polymerization was apparent. The same complexes also catalyzed the polymerization of 1,3-butadiene at 25 1C (the isolated yields approached quantitative). Polymerization with [Fe(tpy)2](FeCl4)2 furnished a mixture of cis-1,4-, trans-1,4-, and 1,2-polybutadiene in a ratio of 20 : 51 : 29, whereas [(tpy)FeCl3] predominantly produced the trans-1,4-rich polymer with broad PDI values of 3.09–3.75. Applying [(tBu3-tpy)FeCl3] as catalyst, very high molar masses (Mn ¼ 62.1–96.5 104) and low polydispersity indices

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| 475

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| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes were obtained, The resulting polymers were composed of a mixture of cis-1,4-, trans-1,4- and 1,2-polybutadiene in a ratio of 28 : 20 : 52. Density functional theory (DFT) calculations gave insight into the polymerization mechanism of 1,3-butadiene catalyzed by [(tpy)FeCl3] [73]. Accordingly, p-allyl-insertion was involved in the chain growth with a [(Z3-RC3H4)Fe(tpy)(Z2-C4H6)] þ complex being the active catalyst (RC3H4: growing polybutadiene chain); the alternative s-allyl-insertion pathway was found to be inoperable.

9.3 Electrocatalytic Oxidation and Reduction Processes

In 1980, Moyer, Thompson, and Meyer described the concept of electrocatalytic oxidation of alcohols, aldehydes, and unsaturated hydrocarbons at 25–50 1C using the redox system [(tpy)(bpy)Ru(H2O)]2 þ /[(tpy)(bpy)RuO]2 þ , as catalyst (Scheme 9.13a) [4]. This catalytic system was significantly more stable, which was attributed to the strong chelate effect of tpy, than the previously reported one based on cis[(bpy)2(py)Ru(H2O)]2 þ , for which slow decomposition via abstraction of the labile pyridine ligand led to reduced activity in the electrocatalytic oxidation [74]. The oxidation reactions were carried out in conventional three-electrode electrolysis cells with the organic substrates being dissolved (or suspended) in an aqueous buffer (pH 7); the applied potentials of 0.6–0.8 V were sufficient to cause oxidation of the RuII state to RuIV. A simplified view of the catalytic cycle is depicted in Scheme 9.13b, in which an organic substrate (S) undergoes a net two-electron oxidation; the balancing chemical reaction in the second electrode compartment is the reduction of H þ to ½H2. In this process, either water acts as the primary

(a)

+0.61 V

[(tpy)(bpy)RuO]2+

[(tpy)(bpy)Ru(OH)]2+

RuIV state

+0.48 V

[(tpy)(bpy)Ru(H2O)]2+ RuII state

RuIII state

(b) [(tpy)(bpy)RuO]2+

[(tpy)(bpy)Ru(H2O)]2+ + S=O

+ S + H2O

2H+

(c) S + H2O

H2

catalyst, electrolysis S=O + H2

or

catalyst, electrolysis SH2

S + H2

Scheme 9.13 (a) Redox systems RuIII/RuIV and RuII/RuIII in aqueous medium [at pH 7, the potentials are given vs. a standard calomel electrode (SCE)]; (b) oxidation of an organic substrate (denoted as “S”) by a RuIV complex; (c) two possible pathways of the oxidation process: oxidation by water (left) or oxidative dehydrogenation (right) [4].

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oxidant in the electrochemically driven oxidation of S to S¼O (Scheme 9.13c, left) or the organic compound is dehydrogenated oxidatively (Scheme 9.13c, right) [4]. Using this electrocatalytic approach, Ph3P was converted into Ph3PO, 2-propanol and EtOH were oxidized to acetone and acetate, respectively, and acetaldehyde was oxidized to acetate. Moreover, toluene was directly oxidized to the benzoate anion and para-xylene yielded the terephthalate dianion – in both cases without the appearance of intermediates. Ethylene, as the simplest unsaturated hydrocarbon, did not react; however, cyclohexene was oxidized first to 2-cyclohexen-1-one and, subsequently, to para-benzoquinone, which decomposed in solution. Overall, the catalyst exhibited a high robustness (>75% recoverable after 100 catalytic cycles). Controlled electrooxidation, that is, selective oxidation of primary alcohols to the corresponding aldehydes, was achieved by monitoring the current (I) as function of time (t) during the oxidation process; from these I–t-curves, the relative reactivity of various primary alcohols could be concluded in a semi-quantitative manner [75]. Thus, the least reactive primary aliphatic alcohols could be selectively oxidized to the corresponding aldehydes, due to the reaction’s long half-life (t½). The more reactive allylic or benzylic alcohols gave mixtures of the corresponding aldehydes and carboxylates; the product distribution was shifted to the former by carefully controlling the number of coulombs that passed through the reaction cell. Similarly, oxidation of cyclic alkenes could be controlled; the uncontrolled electrooxidation of, for instance, cyclohexene gives para-benzoquinone [4], whereas careful control of the coulombs passed through the solution enabled the electrochemical synthesis of 2-cyclohexen-1-one in moderate yield (about 50%) [76]. Furthermore, [(tpy)(bpy)Ru(H2O)]2 þ was also utilized for the selective oxidation of diols [77]. Thus, 1,2-butandiol and 1,3-butandiol were selectively oxidized to 1-hydroxybutan-2-one and 1-hydroxybutan-3-one, respectively, when the coulombs corresponding to a two-electron process were applied. Oxidation of 1,4-butandiol gave g-butyrolactone, as the exclusive product, presumably via an initial twoelectron oxidation of one hydroxy group to the corresponding aldehyde, followed by intramolecular formation of the hemiacetal and electrochemical oxidation to the lactone. The mechanism for the oxidation of benzaldehydes, as model substrates, to their corresponding carboxylic acids was investigated by Seok and Meyer [78]. In comparison, [(tpy)(bpy)RuO]2 þ was a stronger two-electron oxidant (0.08 V) than cis-[(bpy)2(py)RuO]2 þ . Kinetic studies (i.e., applying a stopped-flow monitoring technique) revealed that the oxidation of benzaldehyde was first order in both benzaldehyde and [(tpy)(bpy)RuO]2 þ ; in MeCN at 25 1C, the rate constant for oxidation by [(tpy)(bpy)RuO]2 þ was a factor of about 3 greater than that for oxidation by cis-[(bpy)2(py)RuO]2 þ (3.67 vs. 1.05 M1 s1). A moderate PhCHO/ PhCDO kinetic isotope effect of kCH/kCD ¼ 5.7 7 0.4 was observed and the mechanism of PhCHO oxidation was, by analogy to the oxidation by cis-[(bpy)2(py)RuO]2 þ , presumably based on a H-atom transfer (HAT) mechanism as well. To facilitate the separation and catalyst recycling, the RuII complexes have been immobilized onto electrodes by a polymer coating, via either incorporation into a polymer matrix [79–81] or covalent binding to a polymer [82–89]. The former

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| 477

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| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes approach, for example, utilizing graphite electrodes coated with NafionTM as host for [(tpy)(bpy)Ru(H2O)]2 þ , gave promising results in terms of activity in the oxidation of benzaldehyde, but the catalyst was deactivated after about 150 turnovers [80]. In general, covalent binding of the catalyst to the electrode, for example, via ethereal linkages, significantly improved the stability and performance of the electrooxidation catalyst. Enhanced current efficiencies and improved chemoselectivities (i.e., a low tendency to over-oxidation of primary alcohols to carboxylic acids) were the main benefits of this strategy. For example, Geneste and Moinet reported the selective electrooxidation of primary and secondary benzylic alcohols in high yields to give the corresponding aldehydes and ketones, respectively, catalyzed by [(tpy)(phen)Ru(H2O)]2 þ , which was bound to a carbon felt electrode [90]. In a different approach, the phosphonate-functionalized RuII mono(terpyridine) complex [(H2O3P-tpy)Ru(H2O)3]2 þ was anchored onto glass/ITO or glass/ITO/ TiO2 electrodes (ITO: indium tin oxide) [91]. After attachment to the metal oxide surface via a phosphonate linkage, the RuII centers were oxidized – either chemically or electrochemically – to the highly reactive RuVI-dioxo species trans[(H2O3P-tpy)Ru(O)2(H2O)]2 þ , which remained attached to the surface. The surface-bound RuVI complex reacted with benzylic alcohols via a similar mechanistic pathway as proposed in solution [92–97]; the RuVI state was reduced stepwise in two two-electron transfer processes, via a RuIV to a RuII state. Reduction of the RuVI center was accompanied by insertion into the C–H bond of benzyl alcohol, as the rate-limiting step, to yield a coordinated aldehyde hydrate, which was subsequently solvolyzed to the aldehyde (Figure 9.7a). The surface-bound RuVI complex acted as an electrooxidation catalyst, which passed through about 130 two-electron turnovers before its deactivation. Overall, the redox-properties of the involved

(a) H H

OH

insertion & four-electron reduction

RuVI=O

RuII-O

H

solvolysis by MeCN

OH

- H2O

H O RuII-MeCN

+0.67 V

(b) [(tpy)Ru(O)2(H2O)]2+ RuVI state

+1.03 V

[(tpy)RuO(H2O)2]2+

+0.87 V

[(tpy)Ru(H2O)3]3+ RuIII state

RuIV state

+0.47 V

[(tpy)Ru(H2O)3]2+ RuII state

+0.85 V

Figure 9.7 (a) Proposed mechanism of the oxidation of benzyl alcohol catalyzed by a RuVIdioxo complex [91]; (b) redox systems RuVI/RuIV, RuIV/RuIII, and RuIII/RuIII in aqueous medium (pH 1, the potentials are given vs. SCE) [98].

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RuVI/RuIV, RuIV/RuIII, and RuIII/RuII couples were reported to be strongly pH depended (for the redox potentials vs. SCE at pH 1, see Figure 9.7b) and featured highly defined regions of stability [98]. Mono(terpyridine) complexes of RuII ions have not only been utilized, as catalysts, in electrooxidations but also in electroreductions. The electroreduction of CO2, catalyzed by [(tpy)Ru(dppene)Cl] þ [dppene: cis-l,2-bis(diphenylphosphino)ethylene], was investigated by the Meyer group [6]. The traditional electrochemical reduction of CO2, a process of technological relevance to the development of fuel cells, requires large negative overvoltages in the range of 2.2 to 2.0 V, depending on the solvent; applying transition metal ion complexes, as homogeneous catalysts, the reduction potential could be shifted to more positive values, making the electroreduction of CO2 more applicable [50]. At a reduction potential of 1.40 V, a distinct two-electron transfer exclusively yielded CO (Scheme 9.14). Tetra(n-butyl)ammonium hexafluorophosphate [TBAP, (n-Bu)4N(PF6)] was used, as both co-electrolyte and base, undergoing Hofmann degradation. Under the applied conditions, the formation of formate anion was not detected. In contrast, RhIII mono(terpyridine) complexes, such as [(tpy)RhCl3], catalyzed the electroreductive formation of formate at potentials in the range 1.35 to 1.45 V with high efficiency (about 82%), but with low turnover numbers of 7–11 [99]. As an alternative to complexes containing second row transition metal ions (i.e., RuII or RhIII), homoleptic bis(terpyridine) complexes of first row transition metal ions also exhibited catalytic activity in the CO2 reduction process. In particular, when the complexes were immobilized on the electrode, good catalytic activity – when compared to homogeneous catalysis by [Co(tpy)2]2 þ in solution – was observed. Guadalupe et al. reported the electropolymerization of [Co(tpyvinyl)2]2 þ [100] and attributed the increased efficiency in the catalysis of the electroreduction of CO2 to a more rigid environment within the polymer matrix and enhanced charge-transport properties [8]. However, a solvent dependence was observed in which the electrode-bound catalysts only showed high activity in polar weakly-coordinating solvents (e.g., DMF). Electrodes with a catalyst surface coverage of about 108 mol cm2 were applied for the reduction in a CO2saturated DMF solution containing TBAP, as co-electrolyte, at 0.90 V, that is, at a more positive potential than for the RuII complex [101]. Based on the production of formate, a catalyst turnover number of >500 was estimated; chronocoulometry was conducted to determine the current efficiency of the system. The amount of charge passed during the electrolysis was compared to the amount of charge consumed in the production of formate; the current efficiency of electropolymerized films was close to 100%. Similarly, the corresponding electrode-anchored FeII and NiII

(n-Bu)4N 2 CO2 2 e

(n-Bu)3N CH2 CHEt CO HCO3

Scheme 9.14 Reduction of CO2 in a two-electron process.

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| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes Table 9.5 Electroreduction of CO2 to formaldehyde, catalyzed by electrode-bound bis (terpyridine) complexes.a

Catalystb

Charge (coulombs)

Amount H2CO (in eq. coulombs)c

Turnoverd

Yield (%)

[Co(L)2]2 þ [Fe(L)2]2 þ [Cr(L)2]2 þ

2.31 2.88 1.10

0.90 0.80 0.96

11 000 15 000 6 100

39 28 87

CO2-saturated aqueous NaClO4 solution (0.1 M), applied voltage: 1.10 V. Catalyst was immobilized on a glassy carbon electrode by electropolymerization; L: 4-vinyl2,20 :60 ,200 -terpyridine. c Charge corresponding to the number of moles of formaldehyde detected. d Estimated from the amount of charge consumed and the surface coverage. a b

bis(terpyridine) complexes catalyzed the electroreduction of CO2 at potentials well below 1.0 V [102, 103]. In aqueous solution, the selective electroreduction of CO2 to formaldehyde, catalyzed by electrode-bound bis(terpyridine) complexes of some first row transition metal ions, occurred at about 1.10 V (Table 9.5) [104]. Within the series, the CrII species exhibited the highest current efficiency (about 87%) for the reduction of CO2 to formaldehyde. The catalytic activity of complexes containing first row transition metal ions (e.g., FeII, CoII, CrII) was higher then for those containing second/third row transition metal ions (i.e., RuII and OsII), which was attributed to the different nature of redox (i.e., metal-based vs. ligand-based) processes. Coordinating counterions, for example, hydrogen phosphate (HPO42), were found to completely inhibit the catalysis of the electroreduction. In the electroreduction of oxygen, a strong dependency of the product formed on the surface coverage of the electrode and, therewith, on the ability of O2 to permeate to the active metal centers became apparent [101]; for CoII bis(terpyridine) complexes, at low surface coverage (G o 2.5 109 mol cm2, about 25 monolayers), the two-electron reduction of O2 to peroxide (reduction potential of 0.50 V) was dominant, whereas at higher surface coverage the four-electron reduction to water (reduction potential of 0.8 V) became more important.

9.4 Photocatalytic Processes

Any photochemical or photophysical process starts with the absorption of a photon by a molecule. Thereby, an excited-state of the molecule (M*) will be formed that is higher in energy than the ground state species (M). As depicted in Figure 9.8, the excited-state species M* will, subsequently, follow some type of deactivation pathway: (i) loss of the original molecule (i.e., photochemical

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Photochemical reaction products h·ν

kr

Luminescence

M + h·ν kL

M

M* kd

M + heat

Radiationless deactivation

+Q

M (+ products)

Quenching process Figure 9.8 Possible deactivation channels of excited-states (M: molecule, Q: quencher). Figure redrawn according to Reference [105].

reaction), (ii) emission of light (i.e., luminescence), (iii) loss of excess energy via radiationless deactivation, or (iv) interactions with another species present in the environment (i.e., quenching) [105]. Each of these decay processes is characterized by its own rate constant and the excited-state is characterized by its lifetime. If the lifetime of the excited-state is sufficiently long, the excited molecule has a high probability of reacting with another molecule, such as Q. Then, a specific interaction might occur, where kinetics have revealed that only excited-states with lifetimes W 109 s have any chance of becoming involved in such interaction. With respect to transition metal ion complexes, only the lowest spin-forbidden, metal-toligand charge-transfer (3MLCT) state meets this requirement [106, 107]. Besides catalytic deactivation, energy- [107–109] and electron-transfer processes [106, 108, 109] are of particular importance; the latter process involves either the oxidation or reduction of the excited-state (Scheme 9.15). Electron- and energy-transfer processes are important, since they can be utilized to quench an excited-state, avoiding its undesired luminescence via intramolecular deactivation. In addition, these processes can sensitize other species, for instance, causing chemical changes or luminescence from species that do not absorb light themselves. The kinetics of energy- and electron-transfer processes are discussed in detail elsewhere [106, 110]; however, a molecule in its excited-state can exhibit different properties when compared to its ground state. The ability of an excited-state to participate in energy-transfer is related to its zero-zero spectroscopic energy (E00). The thermodynamic parameters of relevance in electron transfer are the oxidation and reduction potentials of the A*/A þ and A*/A redox couples. Owing to its higher energy, an excited-state is both a stronger reductant and stronger oxidant than the corresponding ground state; the redox potentials for the excited-state couples can be calculated, to a first approximation, from the potentials of the

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| 481

482

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes kchem

(a) kdiff M* + Q

M*…Q

ke kcat

1/t

C+D

Chemical reaction

A + B*

Energy transfer

A+B

Catalytic deactivation

M (+ products)

(b)

A* + B

oxidative electron-transfer

reductive electron-transfer

A + B

A + B

Scheme 9.15 Various bimolecular processes in the presence of a quencher [105].

ground-state redox couples [E(A þ /A) and E(A/A)] and E00, according to Eq. (9.1): ðaÞ oxidation potential :

ðaÞ reduction potential :

0 1 0 1 þ A Aþ E @ A ¼ E @ A E 00 A A 0 1 0 1 A A E @ A ¼ E @ A þ E 00 A A

(9.1)

Complexes of d6 transition metal ions with oligopyridine ligands have widely been used, as mediators, in photochemical processes [107, 109] and their general role is briefly detailed here. Many conceivable reactions do not occur because the reactants are not able to absorb light and/or the excited-state, responsible for a specific photochemical reaction, possesses too short a lifetime. For instance, a chemical reaction of the type A þ B - A þ þ B that cannot occur because of its endergonic nature could upon excitation with light become thermodynamically allowed (Figure 9.9). If neither A nor B are able to absorb the exciting light, the reaction will still not occur. This limitation can be overcome by utilizing a so-called light absorption sensitizer (LAS) of specific electrospectroscopic properties; thus, a LAS has to absorb light resulting in an excited-state and this excited-state must be able to oxidize (or reduce) one of the reactants. Finally, if the reduced (or oxidized) LAS is able to reduce (or oxidize) a second reactant, then the redox cycle is completed by regenerating the LAS. For an ideal LAS, some requirements can be proposed: (i) reversible redox behavior; (ii) suitable ground- and excited-state potentials; (iii) stability towards thermal and photochemical decomposition, absorption as high as possible in an appropriate spectral region; (iv) a small energy gap between the excited-states; (v) high quantum yield of the reactive excited-state; (vi) a proper lifetime of the

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LAS

ABh·ν

AB

reductive route B

oxidative route A

A LAS

LAS

B

A

A LAS

B

B LAS

LAS*

h·ν

LAS*

h·ν

Figure 9.9 Photosensitized electron transfer process via light absorption.

reactive excited-state; and (vii) a high energy content of the reactive excited-state [105]. By far the most important applications of LASs are in the fields of artificial photosynthesis, water splitting into molecular hydrogen and oxygen, and water reduction of CO2 to methanol or hydrocarbons – the key challenges for a sustainable energy future (Scheme 9.16) [111, 112]. Notably, the water oxidation to O2 is an essential half-reaction in both cases. 9.4.1 Light-Driven Hydrogen Formation

The light-driven generation of hydrogen catalyzed by transition metal ion complexes is a highly robust field of research in which molecular hydrogen, obtained from renewable solar energy at low cost, is the ideal fuel that upon combustion produces exclusively water. To reach this goal, various types of homogeneous as well as heterogeneous catalytic systems have been developed [113, 114]. In particular, [Ru(bpy)3]2 þ and its derivatives have widely been used as LASs in photoinduced H2-production systems [114–121]. The splitting of water into hydrogen and oxygen is endergonic by 1.23 V vs. SHE (at pH 7 in the dark, Scheme 9.17a) [122]. In the presence of [Ru(bpy)3]2 þ , as LAS at pH 7, the relevant reaction steps become exergonic; however, reaction (2) is, in practice, too slow to compete with the excited-state deactivation [105]. Thus, a reducing agent is added

(a)

2 H2O 4hν

(b) 2 H2O CO2 8hν

O2 + 2 H2 2 O2 + CH4

Scheme 9.16 Photo-induced water splitting (a) and reduction of CO2 (b).

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| 483

484

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes (a) (1)

1/2 O2 + H2

∆G = +237 kJ·mol–1

2 [(bpy)3Ru]3+ + H2

∆G = –85 kJ·mol–1

2 [(bpy)3Ru]3+ + H2O

2 [(bpy)3Ru]2+ + 1/2 O2 + 2 H+

∆G = –86 kJ·mol–1

[(bpy)3Ru]*2+ + MV2+

[(bpy)3Ru]3+ + MV+

H2O

(2) 2 [(bpy)3Ru]*2+ + 2 H+ (3)

(b)

[(bpy)3Ru]2+ + Red+

[(bpy)3Ru]3+ + Red 2 MV+ + 2 H2O

cat.

2 MV2+ + H2 + 2 OH–

Scheme 9.17 Reduction of water (a) in the absence and (b) in the presence of an electronmediator, a reductant, and a multi-electron redox catalyst.

as a sacrificial electron-donor to trap the [Ru(bpy)3]3 þ produced in the quenching reaction (Scheme 9.17b). Additionally, methyl viologen (1,10 -dimethyl-4,40 -bipyridinium, MV2 þ ), as an electron-mediator, was found to be applicable to transport electrons to the multi-electron redox catalyst (e.g., colloidal noble metals), which combines two protons and electrons to give molecular hydrogen. Most commonly, triethylamine (TEA), triethanolamine (TEOA) or ethylenediaminetetraacetic acid (EDTA) are utilized, as reducing agents [123–125]. Besides the established RuII tris(bipyridine) and, more recently, bis-cyclometalated IrIII complexes [126] as photoactive materials for the intermolecular electrontransfer, various photocatalytic multicomponent systems based on intramolecular electron-transfer processes have been developed [113, 127–129]. In contrast to these examples, where the RuII tris-bidentate complexes were the most common structural motif in the LAS part, the analogous systems based on RuII bis(terpyridine) complexes, such as the dyads consisting of a bio-inspired [FeFe]-hydrogenase moiety (14) or a coordinated PdII and RhIII center (15a and 15b), respectively, were found to inactivate the production of molecular hydrogen, mainly due to insufficient electronic coupling of the metal centers (Figure 9.10) [130–133]. The electrochemical and photochemical reduction of water to yield molecular hydrogen via [Pt(tpy)Cl] þ was reported by Abe et al. [134]. In an electrochemical process, the basal plane pyrolytic graphite (BPG) or ITO electro-coated NafionTM membrane, incorporating the PtII mono(terpyridine) complex, was analyzed by potentiometric electrolysis at an applied potential of 0.95 V (vs. Ag/AgCl) in water at pH 5.9; moreover, cyclic voltammetry (CV), UV–vis absorption as well as X-ray photoelectron spectroscopy (XPS) studies confirmed the reduction process. For the photochemical process, a multicomponent system consisting of [Pt(tpy)Cl] þ , as the active catalyst, [Ru(bpy)3]2 þ , as the LAS, MV2 þ , as an acceptor, and EDTA, as sacrificial donor, was utilized to reduce H þ to H2. The reduction mechanism was not fully understood; however, it was suggested that the methyl viologen radical cation (MV þ ) was involved in the process, since the reduction potential of H þ /H2 (0.54 V vs. Ag/AgCl at pH 5.9) was slightly lower than

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CO N N

N Ru

2

N

N

S

Fe

S

Fe

N

N

14

CO CO CO CO

CO

Cl

Cl N

Pd

N N

Br

N

Ru

2

N N

N

Cl Ru

N N

15a

N

15b

N

N

N

N

N

Cl Rh Cl N 2

Figure 9.10 RuII bis(terpyridine) complexes 14 and 15a and 15b, as catalysts for the photochemical reduction of water.

MV2 þ /MV þ (0.64 V vs. Ag/AgCl); this radical was observed throughout the reaction. Recently, Okazaki et al. showed that the same complex could also function as both a photosensitizer and H2-evolving catalyst in water; any contribution from possible contamination by colloidal platinum was ruled out [135]. The rate of H2 formation was dependent on the concentration of [Pt(tpy)Cl] þ , suggesting that a bimolecular pathway would determine the overall kinetics of the photoinduced H2 formation. A long-lived emissive 3MMLCT excited-state of a dimeric {[Pt(tpy)Cl] þ }2 species was attributed to be the efficient photosensitizer in the observed process. However, the mechanism leading to the formation of molecular hydrogen could not yet be clarified. In a more defined array, the PtII mono(terpyridine) acetylene complex 16 was used as the LAS for the photocatalytic generation of H2 [MV2 þ , TEOA, and colloidal Pt (particle size of 5–7 nm) served as the acceptor, donor, and catalyst, respectively] [136]. Both MV2 þ and TEOA efficiently quenched the photoluminescence of 16 (lPL ¼ 500–800 nm) via an oxidative and reductive process, respectively (Scheme 9.18). A degassed solution of 16 in MeCN with MV2 þ was colorless after irradiation, suggesting a rapid and efficient back-electron-transfer from MV þ to MV2 þ upon quenching. However, when both quenchers were mixed with 16, a deep blue colored solution was generated, indicative of the for mation of the MV þ radical via reductive quenching, followed by an oxidative decomposition of TEOA and second electron-transfer from MV þ (yielding glycoaldehyde and diethanolamine). The addition of colloidal Pt particles to the 16/ MV2 þ /TEOA mixture caused an electron transfer from MV þ to the colloidal catalyst and, subsequently, proton reduction at the Pt surface generated H2 from water. The efficiency of the system was dependent on the pH value; notably, the highest yield (3.5 106 mol-H2) was observed at pH 7 after irradiation times of up to 4 h. Later, the performance of the device was enhanced by replacing MV2 þ with the 2,20 -bipyridine derivative 17 (Scheme 9.18) as electron-mediator [137],

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| 485

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes 16*

16 16* MV2

16 MV

16* TEOA

16 TEOA

16 MV2

16 MV

16 MV

16 MV2

16 TEOA

16 TEOA

TEOA

N N

MV2 2 MV + H2O

Pt-colloid H2 + 2

OH

HOCH2CHO + NH(CH2CH2OH)2

MV

+ 2 MV

Pt+ N

16

TEOA

N N 17

2

Scheme 9.18 Reduction of H2O catalyzed by PtII mono(terpyridine) complex 16 [136].

with which 800 turnovers of H2 (i.e., 67% yield, based on TEOA as sacrificial electron-donor) after 20 h of photolysis with lexcW410 nm could be achieved. Cobaloxime complexes have been found to be highly efficient electrocatalysts for hydrogen evolution [138–140] and were introduced by Lehn and coworkers, as catalysts, for photochemical hydrogen production with [Ru(bpy)3]2 þ , as LAS [117]. The Eisenberg group combined PtII complex 16, as photosensitizer, with cobaloxime 18 (Figure 9.11a), as catalyst, and TEOA, as sacrificial donor, to give a highly efficient three-component system (Figure 9.11b) [141, 142]. Upon increasing the concentration of TEOA by 2.4 104-fold relative to that of 16, turnover numbers of 1000 (based on 16) and 28 (based on 18), respectively, were achieved after an irradiation period of 10 h (lexc ¼ 380 nm) in a MeCN–water solution at pH 8.5. It was further shown that the system was homogeneous and that

(a)

(b) 200 Cl H O O N N Co N N O O H N 18

H2 production (turnovers)

486

150

100

50

0

3

4

5

6

7

8

9

10

11

12

13

14

pH Value

Figure 9.11 (a) Cobaloxime complex 18; (b) H2 production as a function of the pH value (16 as LAS, 18 as catalyst, TEOA as electron-donor, 5 h of irradiation at 410 nm) [142]. Figure reproduced with kind permission; r 2008 American Chemical Society.

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| 487

catalysis did not originate from the in situ generated Co particles, since control experiments with colloidal cobalt particles (instead of 18 as catalysts) did not show any catalytic activity upon photolysis. A remarkable photocatalytic oxidation of Hantzsch-type 1,4-dihydropyridines (DHPs) as well as their 4-alkyl and 4-aryl derivatives was reported by Zhang et al. [143]. The oxidation, catalyzed by PtII mono(terpyridine) complexes, such as 16, 19, or [Pt(ttpy)Cl] þ , gave the corresponding pyridines and H2 in quantitative yields with high catalytic turnover numbers (Scheme 9.19). Surprisingly, the 4-(isopropyl)- and 4-benzyl-substituted DHPs experienced dealkylation, thus, 4-unsubstituted pyridine derivatives and the corresponding alkane were isolated. According to a mechanistic study by Narayana-Prabhu and Schmehl [144], applying transient absorption spectroscopy of PtII mono(terpyridine) acetylene complexes with various quenchers (e.g., NEt3, N-methylphenothiazine, DHP) in degassed MeCN, a one-electron reduced PtII intermediate was postulated; however, it was previously suggested that DHP behaves rather as a H-atom donor in its photooxidation [143]. Similarly, 1,3,4-triaryldihydropyrroles were photochemically oxidized to the corresponding pyrrole derivatives in the presence of PtII complex 19, as catalyst [145]. The one-pot reaction proceeded in a homogeneous solution with a large catalytic turnover number upon irradiation with visible light. Moreover, catalyst 19 could be separated from the reaction mixture and reused up to five times without losing its catalytic activity. A spectroscopic study and product analysis gave evidence for an initial photoinduced electron-transfer (PET), followed by a protoncoupling process, finally leading to the formation of 1,3,4-triarylpyrroles and molecular hydrogen.

N H3CO

N Pt N

H

R

EtOOC

COOEt

PtII complex hν

19

R or H COOEt

EtOOC

(9 examples)

N H

H2 (R H, alkyl, or aryl) or R-H (for R -CH(CH3)2 or -CH2Ph)

N R2 N R1

R2 N

PtII complex hν

(5 examples)

R1

R1

R1

H2

R1 aryl R2 aryl or thiophen-3-yl

Scheme 9.19 Photocatalytic oxidation of dihydropyridines and 1,3,4-triaryldihydropyrroles by PtII mono(terpyridine) complexes [143, 145].

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488

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes 9.4.2 Molecular Terpyridine-Based Catalysts for Water Oxidation

When considering efficient catalysts for homogeneous water oxidation, several typical design elements have to be named: (i) a suitable redox-active metal center, (ii) an oxidatively stable coordination environment, and (iii) at least one coordination site being occupied by a labile ligand (e.g., solvent, halide) [2, 3, 146–154]. It was a long-held paradigm that multiple metal centers would be required to facilitate the multiple proton-coupled electron-transfer (PCET) steps [155] associated with water splitting [156]. However, it has been shown recently that mononuclear complexes of, in particular, RuII, MnII, or IrIII ions can mediate the formation of molecular oxygen from water (see References [157–160]). Compared to oligonuclear complexes, their mononuclear counterparts feature some important advantages: they are, in general, easier to synthesize and study with respect to the mechanism of homogeneously catalyzed water oxidation [152, 161]. The mechanism for water oxidation, based on PCET, has recently been summarized by Meyer and coworkers [3]; the potential of the one-electron oxidation of water to hydroxyl radicals is 2.4 V at pH 7. A mechanism involving a one-electron transfer is too slow to be of interest, mainly due to the high barriers that are required. The build-up of multiple oxidative equivalents at lower potentials at single catalyst sites or clusters is regarded as a prerequisite for water oxidation at reasonable rates [148]. However, potentials between adjacent one-electron couples increase by about 0.5–1.0 V – attributed to increased charges in the higher oxidation state couple. In contrast, the step-wise oxidation of the RuII complex cis-[Ru(bpy)2(py)(H2O)]2 þ to [Ru(bpy)2(py)(O)]2 þ is much easier, since there is no charge build-up between the redox couples (Figure 9.12a) [74, 162]. In this process, overall two electrons and two protons are lost, thus being half of the requirement for water oxidation (Figure 9.12b). The loss of a proton in the RuIII/RuII redox couple is triggered by increased acidity of the coordinated water in the RuIII species; in the RuIV species, the high oxidation state is stabilized via a Ru¼O bond [148]. Moreover, the oxidation has a PCET kinetic component [155] in which the initial electron transfer, followed by proton transfer from RuIV¼OH3 þ , is mechanistically viable, but electron transfer occurs only at potentials >1.6 V (Figure 9.12c) and, thus, oxidation in solution (or at electrodes) would be slow [163]. This limitation can by circumvented by a fast and direct electron/proton-transfer (EPT) without any intermediate states. This particular reactivity was first observed in the comproportionation reaction between cis-[Ru(bpy)2(py)(H2O)]2 þ and cis-[Ru(bpy)2(py)(O)]2 þ (H2O/D2O kinetic isotope effect is 16.1, Figure 9.12d). Following an EPT pathway (Figure 9.12e), electron transfer occurs between metal-centered dp orbitals and a proton is transferred from a sOH orbital of the aqua ligand to a lone pair on the oxo group [163, 164]. Together, PCET and EPT are essential for water oxidation and represent the mechanistic basis for one-electron activation of multi-electron catalysis. The first example of a molecular homogeneous catalyst for water oxidation was the dinuclear complex cis,cis-{O[Ru(H2O)(bpy)2]2}4 þ , the so-called “blue dimer”

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(a) +0.78 V

[Ru(bpy)2(py)(O)]2+

[Ru(bpy)2(py)(OH)]2+

RuIV state

(b)

[Ru(bpy)2(py) (H2O)]2+ RuII state

RuIII state 4e–, 4H+

2 H2O

+0.67 V

(c)

O2

< 0.45 V

[RuIV=O]2+

78 V

pKa < ⴚ6

> 1.6 V

[RuIV=OH]3+

(d)

[RuIIIO]+

< 0.

pKa > 14

[RuIIIOH]2+

k = 2.1 × 105 M1·s1 2+

[Ru(bpy)2(py)(O)]

+ [Ru(bpy)2(py)(H2O)]

2 [Ru(bpy)2(py)(OH)]2+

2+

k = 3.0 × 103 M1·s1

∆G0 = 2.5 kcal·mol1

(e) RuIV=O2+ + RuIIOH22+ e

[RuIV=O···H2ORuII]4+

[RuIV=O···H2ORuII]4+

[RuIIIOH···HORuIII]4+

H+

Figure 9.12 (a) Redox systems RuIII/RuIV and RuII/RuIII in aqueous medium (at pH 7, the potentials are given vs. SCE) [74]; (b) oxidation of water; (c) pKa–potential diagram for the oxidation of [Ru(bpy)2(py)(OH)]2 þ to [Ru(bpy)2(py)(O)]2 þ (at pH 7 vs. SEC) [155]; (d) comproportionation reaction; (e) concerted electron/proton transfer (EPT) process [163, 164].

[165, 166] in which an efficient generation of molecular oxygen with rates of up to 4.2 103 s1, for an average of 13 turnovers, was observed with CeIV, as the sacrificial oxidant (for the proposed mechanism, see References [3, 148]). The structural analog to this “blue dimer”, based on two RuIII mono(terpyridine) centers, was reported by the Meyer group [167]. The dimeric complex {O[Ru(C2O4)(tpy)]2(H2O)8} (C2O42: oxalate; the X-ray single-crystal structure is depicted in Figure 9.13a) gave {O[Ru(H2O)2(tpy)]2}4 þ , as the catalytically active species, under strongly acidic conditions. The difference in the coordination sphere of the RuIII centers – the tpy-containing system contained two water molecules per RuIII center rather than one – led to significant changes in the thermodynamics: at pH 7 the “blue dimer” was stabilized towards oxidation of RuIIIORuIII to RuIVORuIII by 0.14 V (and towards further oxidation to RuIVORuIV by W 0.43 V). This remarkable stabilization of the RuIVORuIII state toward further oxidation (or destabilization of RuIVORuIV) resulted in a lower oxidation potential to RuVORuIV and caused the instability of RuIVORuIV with respect to disproportionation. However, for the {O[Ru(H2O)2(tpy)]2}4 þ dimer a progressive increase in the redox potential for the one-electron RuIVORuIII/RuIIIORuIII and RuIVORuIV/RuIVORuIII couples was determined by pH-dependent CV measurements (Figure 9.13b), from which

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| 489

490

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes O24

O14

(a) C4 O23

C3

O22

C36

C42

C43

C35 C66

Ru2

Ru1 N31 C65

N41

O11 C32

C64

C46

C63 C52

C33

N11 N2

N51 C63

C56

O13

C34

C12 C13

N61

C44

CI

O1

O21

C45

C2

O12

C14

C26

C16 C22

C25

C15

C53

C23

C55 C54

(b) 0.91 V RuVORuV

1.43V

RuVORuIV

1.13V

RuIVORuIV

0.66 V

RuIVORuIII

0.41 V

RuIIIORuIII

0.90 V

Figure 9.13 (a) Representation of the X-ray single-crystal structure of the {O[Ru(C2O4)(tpy)]2} dimer complex (H-atoms and solvent molecules omitted for clarity); (b) redox systems of the dimeric complex in aqueous medium (pH 7, potentials vs. SCE) [167]. Figure reproduced with kind permission; r 1998 American Chemical Society.

RuIVORuIV appeared to be a stable oxidation state, that is, stable against disproportionation and incapable of oxidizing water to O2. Moreover, over-oxidation by CeIV, via a RuVORuV state, afforded the RuVI species trans-[Ru(tpy)(H2O)(O)2]2 þ . Thus, addition and oxidative cleavage prevented this complex from being a catalyst for water oxidation. In contrast to the poor performance of the oxo-bridged dimer, dinuclear RuIII mono(terpyridine) complexes containing chelating amine-based ligands, as the bridging unit, exhibited good catalytic activity with respect to water oxidation with CeIV, as the oxidant [168]. For instance, [(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)]3 þ [dpp: 2,4-di(pyridin-2-yl)pyrazolate] showed both a faster rate (1.4 102 s1) and higher turnover numbers (18.6) than similar oxo-bridged dimers. The increased rate and stability were attributed to the chelating nature of the bridge that slowed the catalyst decomposition by an oxidatively induced coordination of anions from the reaction mixture and, at the same time, promoted water oxidation by placing the two RuIII centers in close proximity. As pointed out above, mononuclear complexes with appropriate electrochemical properties were recognized recently as efficient catalysts for the formation of O2 from water (for a detailed study on the electrochemistry of various types

09

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of redox-active complexes see Reference [169]). According to the established mechanism for water oxidation by the “blue dimer” [148] and DFT calculations by Yang and Baik [170, 171], formation of the O–O bond, as the essential step, occurred at a single metal site. Concepcion et al. reported two RuII mono(terpyridine) complexes [Ru(tpy)(L)(H2O)]2 þ (20a/b) with a 2,20 -bipyrimidine (bpm) and a 2,20 -bipyrazine (bpz) ligand, respectively (Figure 9.14a) [159]. Both complexes were very effective single-site catalysts with CeIV, as oxidant, undergoing several hundreds of turnovers without decomposition. Figure 9.14b depicts the proposed mechanism for water oxidation, catalyzed by 20a/b. The catalytic cycle involved the three-electron-oxidized RuV¼O species. For [Ru(tpy)(L)(O)]3 þ , from a thermodynamic point of view, the use of this higher oxidation state was advantageous by DG1 ¼ 0.90 eV for water oxidation compared to DG1 ¼ þ 0.04 eV for the oxidation by RuIV. Furthermore, high energy peroxido intermediates were postulated to play a key role in the oxidation of H2O to H2O2 by [Ru(tpy)(L)(O)]3 þ . Similarly, complexes 20d/e with bidentate O- and P-based chelating ligands were active as water oxidation catalyst, though they were less stable than their counterparts with N-heteroaromatic ligands (20a–c) [172]. Complex 20c was reported to be a versatile catalyst for water oxidation (k ¼ 5.1 104 s1, 320 turnovers) [160]. Placing electron-withdrawing substituent on the bpy ligand reduced the catalytic activity, although the number of turnovers increased at the same time (i.e., the catalyst was stabilized with respect to decomposition via displacement of the bpy ligand); the introduction of electron-donating moieties had an inverse effect [173]. The analogous OsII complexes, such as [Os(tpy)(bpy)(H2O)]2 þ , were also investigated, as potential catalysts, for water oxidation. In general, the redox potentials for the OsIII/OsII and all higher oxidation state couples are 0.3–0.4 V lower than for the Ru system [174, 175]. Taking into account the overvoltage limitations arising from the pH-independent MV¼O/MIV¼O couples, the

(a)

(b)

[Ru(tpy)(L)(H2O)]2+

N

N

20a: L = L1 20b: L = L2 20c: L = L3 20d: L = L4 20e: L = L5 N N

N N L1: bpm

N L2: bpz

N

O

N L3: bpy Ph2P

OH

H2O [RuIVOO]2+

N Ce3 H+

2Ce3 2H+ 2Ce4

[Ru(tpy)(bpm)(H2O)]2+ 20a

Ce4 [RuIIIOOH]2+ H2O

L4: acac PPh2

[RuIIOH2]2+

O2

[RuIVO]2+ Ce4

[RuVO]3+

H+

L5: dppene

Figure 9.14 (a) Single-site RuII complexes 20a/b; (b) postulated mechanism of water oxidation catalyzed by 20 Figure redrawn according to Reference [159].

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Ce3

| 491

492

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes OsV¼O species is accessible at relatively low potentials. Initial studies by the Meyer group indicate a high possibility of such complexes as catalysts in the electro- and photochemical oxidation of water [3]. Inspired by the concept of photosystem-II, various di- and trinuclear mixedvalence Mn complexes, as molecular water oxidation catalysts, have appeared in the literature [176]. Limburg et al. showed that [(tpy)(H2O)Mn(O)2Mn(H2O)(tpy)]3 þ catalyzed the water oxidation when sodium hypochlorite (NaOCl) or potassium Oxones (KHSO5) was used as a two-electron donor oxidant [2, 64]. The proposed mechanism in homogeneous solution involved an intermediate that could exchange with water and, subsequently, react with either water or the oxidant. The authors proposed a formally MnIV(O)2MnV intermediate and that formation of the O–O bond involved attack of the solvent or oxidant on the oxo ligand [177]. Recent computational studies on the PCET revealed a strong dependency of the redox potential of the MnIV(O)2MnIV intermediate on both the pH value and presence of Lewis bases [178]. Even though oxygen evolution from water could be observed (k ¼ 1.6–2.7 103 s1), the maximum number of turnovers was poor (r4) [64]. Utilizing CeIV, as oxidant, an evolution of O2 could be observed, but with a single turnover; apparently, the low pH regime – required for the use of CeIV – caused complete deactivation of the catalyst by oxidation to permanganate [179]. In contrast, [(tpy)(H2O)Mn(m-O)2Mn(H2O)(tpy)]3 þ was found to be catalytically active, using CeIV, when absorbed within the pores of clay minerals (e.g., kaolinite or montmorillonite, Figure 9.15) [180–182]. The significant improvement in catalytic activity in the heterogeneous environment (up to 13.5 turnovers) was attributed to the clay’s stabilizing effects, which prevented the dimer from dissociating into monomers (and subsequent oxidation to MnO4). The rate dependence of O2-evolution on the square of catalyst concentration within the clay suggested that dimer–dimer interactions might be involved in the water oxidation. With respect to the design of water-splitting photoelectrochemical cells (PECs) [183], the electrical coupling of the molecular catalyst on a conductive surface is a requirement. Thus, the efficient removal and collection of electrons during the water oxidation is facilitated; moreover, separation of the gaseous products needs to be guaranteed. To reach this goal, three approaches have been followed [122]: (i) immobilization of molecular catalysts by adsorption, (ii) incorporation of the catalyst into a polymer, and (iii) covalent binding of the catalyst to the surface. Immobilization via drop-casting of the dinuclear RuII complex 21 (Figure 9.16) onto ITO, as an electrode, gave surface-active layers of the molecular catalyst [154]. Electrochemical oxidation of water, with a remarkable turnover number (TON) of 33 500, was achieved at an applied potential of 1.7 V (vs. Ag/AgCl, pH 4.0). This TON was one of the highest values reported to date for a molecular catalyst; however, after 40 h of operation, the catalyst was fully desorbed from the electrode. DFT calculations gave insight into the mechanism of water oxidation [184]; the RuII oxidation state was retained with the “non-innocent” quinone ligands and water molecules involved in redox processes along the catalytic cycle. [Ru2(btpyan)(qu1.5)2(O2)]0 [btpyan: 1,8-bis(2,20 :60 ,200 -terpyridin-4-yl)anthracene; qu: quinoline] is a key intermediate and the most reduced catalyst species formed by

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(a)

Homogeneous aq. solution N N

N OH2

O Mn Mn O N N

H2O

N

CeIVoxidant Adsorption onto clay Catalytic O 2 evolution from water

[(typ)(H2O)Mn(µ-O)2Mn(H2O)(tpy)]3 (b)

Formaton of MnO4-

1.2 Catalyst in aq. solution Blank aq. solution Catalyst on clay Blank clay

O2 evolved/µmol

1.0 0.8 0.6 0.4 0.2

(a) (b) (d)

0.0 0

2

4 Time/h

6

8

Figure 9.15 Reactivity of [(tpy)(H2O)Mn(m-O)2Mn(H2O)(tpy)]3 þ with CeIV in a homogeneous and heterogeneous environment; (b) time courses of the amount of O2 evolved under various reaction conditions (CeIV as the oxidant) [180]. Figure reproduced with kind permission; r 2004 American Chemical Society.

removal of all four protons before the four-electron oxidation; it was identified by applying computational studies. Alternatively, a supporting matrix that holds the catalysts at the electrode surface and limits its dissociation into the electrolyte might be used; among others, cationconducting membranes, such as NafionTM, are the most common polymers in this respect. NafionTM is a perfluorinated hydrophobic polymer with ionizable

2

N

N O t

Bu

Ru N

N OH

N

O

Ru N

HO

O

t

Bu

O t

t

Bu

Bu

21 (N^N^N:tpy)

II

Figure 9.16 Dinuclear Ru complex 21 as catalyst for water oxidation [154].

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| 493

494

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes hydrophilic head groups (i.e., sulfonic acid) [185]. It can be cast onto electrode surfaces to form membranes, where the sulfonic acid or sulfonate groups (as the Na þ salts) generate hydrophilic channels about 20 nm in diameter; these channels are permeable to protons and other cations but not anions. This particular property of NafionTM has been utilized in various applications, such as proton exchange membranes for various types of fuel cells, electrochromic devices, and electrolyzers for H2 generation. To immobilize a molecular catalyst, a solution of NafionTM is deposited onto a suitable electrode surface and allowed to dry. Subsequently, the resulting membrane is doped with the desired catalyst, for instance, by cation exchange from solution; doping can be monitored by UV–vis absorption spectrophotometry. The NafionTM-modified electrodes are then immersed in an appropriate electrolyte for electrochemical studies. Owing to the cation exchange properties of NafionTM, this technique is best suited either to catalysts that are cationic or to catalysts generated from cationic precursors. In particular, PtII mono(terpyridine) complexes have been combined with NafionTM for photocatalytic oxidation purposes. When NafionTM is swollen in water or MeOH, the structure of NafionTM resembles that of an inverse micelle in which the hydrated polar head groups are clustered within H2O-containing pockets (about 40 A in diameter) that are interconnected with each other by short channels within the perfluorocarbon matrix. This H2O-swollen NafionTM is known for its ability to host high concentrations of aromatic hydrocarbons and organic dyes; moreover, the concentration of O2 in this NafionTM is about ten-times higher than in organic solvents [185]. Zhang et al. incorporated photoluminescent complex 19 (Scheme 9.19) into a NafionTM membrane, via hydrophobic and electrostatic interactions, and utilized this system, as a photosensitizer, to generate transient singlet oxygen (1O2) for the oxidation of alkenes in aqueous or organic solutions [186]. Detection of 1O2 generation by the 19/NafionTM system was enabled by its immersion in O2-saturated MeOH; 2,2,6,6-tetramethylpiperidine (TMP), as a radical scavenger, was added and the mixture was irradiated with visible light (l W 450 nm) for 100 s. The formation of 1O2 was rationalized by an energy transfer between molecular O2 and the triplet excited-state of 19 [for the photophysical properties of PtII mono(terpyridine) complexes, see Chapter 3.5]. The stable free radical nitroxide (TMPO, 2,2,6,6-tetramethlypiperidine N-oxide), as the reaction product, was detected by EPR spectroscopy (Figure 9.17) [186]. Three substrates – 7-dehydrocholesterol, (R)-pinene, and cyclopentadiene – were oxidized by 1O2, generated by the 19/NafionTM system in aqueous and organic solutions [186]. In MeOH, the cholesterol derivative was converted into a peroxide in 95% yield based on the consumption of the starting material (Scheme 9.20a). The low conversion of about 20% was rationalized by solvent quenching of 1 O2, which reduced significantly the quantum yield. In deuterated MeOH (or water) the conversion of the starting material was increased to about 95%. (R)Pinene was oxidized to the peroxide in CH2Cl2 with D2O-swollen 19/NafionTM in 90% yield; the subsequent reduction with NaHSO3 afforded the corresponding alcohol in quantitative yield (Scheme 9.20b). Moreover, cyclopentadiene

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(a)

TMP

+

N H

1

19/Nafion™ hν O2 TMPO

N O

(b) 8

after irradiation at λ > 450 nm for 100 s

10 G 6 4 2

in the dark 0 0

2

4

6

8

Figure 9.17 (a) Formation of TMPO by reaction of TMP with transient singlet oxygen; (b) EPR spectrum of the TMPO radical generated by irradiation of the oxygen-saturated solution of TMP in MeOH containing 19/Nafion (l W 450 nm, 100 s) [186]. Figure reproduced with kind permission; r 2003 American Chemical Society.

underwent a [4 þ 2]-cycloaddition with 1O2, under the same reaction conditions, to give the epidioxide in quantitative yield based on the consumption of cyclopentadiene; addition of thiourea yielded the cis-1,3-diol with high diastereoselectivity (>95%) (Scheme 9.20c).

(a) main product

O2

1

O2

HO

solvent HO traces HO

OOH

(b) 1

O2

CH2Cl2

NaHSO3 OOH

OH

CH2Cl2

(c) 1

O2

CH2Cl2

thiourea O O

CH2Cl2

HO OH

Scheme 9.20 Oxidation of various alkenes by 1O2 generated by the 19/NafionTM system [186].

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| 495

496

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes 1

N 1

R

OH R

2

O2, MeCN,

1-6 h, 25 °C (10 examples)

R1 H, alkyloraryl R2 alkyl, aryl

O O N

R1 R

O R1

OH

2

R2

HNO2

R1-R2 cyclic

Scheme 9.21 Photocleavage of oximes catalyzed by 19/NafionTM [187].

The system 19/Nafion was also found to be a versatile catalyst for the photocleavage of oxime moieties, affording the corresponding carbonyl derivatives in good to excellent yields (57–94%, Scheme 9.21). The authors proposed that oxime deprotection occurred through an 1O2 mechanism in which the oximes undergo a [2 þ 2]-cycloaddition with 1O2 to form unstable dioxetane intermediates that decompose under the reaction condition to generate the carbonyl group. Formation of the nitrite by-product was detected using acidic ferrous sulfate. A direct electron-transfer mechanism, as an alternative pathway to the deprotection, was not conclusive, since none of the oximes used in the study could quench the strong 3 MLCT-based photoluminescence of 19 at about 620 nm in degassed MeCN at 25 1C. The same group also immobilized their photocatalysts by synthesizing a styrenefunctionalized PtII mono(terpyridine) complex, as co-monomer, that was copolymerized with styrene in a 1 : 100 ratio under free radical polymerization conditions [i.e., 2,20 -azobis(isobutyronitrile) (AIBN), DMF, 75 1C, 24 h]. The resulting copolymer 22 (Mn ¼ 5000 g mol1; PDI ¼ 2.92, Figure 9.18) retained the photophysical

*

m

n

*

O N

O N 22

Pt+ N

N O N Pt+ N

N H 23

Si

O O O

SiO2

Figure 9.18 Polymer- and silica-bound PtII mono(terpyridine) complexes 22 and 23, respectively.

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properties of the corresponding PtII model complex and, more importantly, exhibited a comparable quantum yield for 1O2 generation. The polymer-based PtII complex photosensitized the oxidation reactions (Scheme 9.20) with high efficiency (>95% conversion of the starting materials), could be reused without loss of reactivity (by precipitation from MeOH), and was well compatible with various solvents (e.g., CH2Cl2, CHCl3, EtOAc, DMF). Similarly, the catalyst could be bound to SiO2 particles by co-hydrolysis of a triethoxysilyl-functionalized PtII mono(terpyridine) complex and tetraethyl orthosilicate in a sol–gel process (23, Figure 9.18). With respect to the covalent attachment of photo-active terpyridine complexes to polymers, another recent example is highlighted in which the Llobet group reported the deposition of conducting polypyrrole polymers for the immobilization of the molecular water oxidation catalyst [(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)]3 þ on the electrode’s surface [188]. For this purpose, the tpy ligand was modified at the 40 -position with a [4-(pyrrol-1-yl)methylphenyl] substituent. Anodic polymerization of 24 gave the corresponding polypyrrole (poly-24), firmly anchoring the complex to the conductive fluorine-doped tin oxide (FTO) or vitreous carbon sponge (VCS) surface; the average surface coverage with active catalyst sites was G ¼ 1.0– 3.1 109 mol cm2. Electrocatalytic water oxidation, with CeIV, as the oxidant, at an applied potential of 1.22 V (vs. Ag/AgCl) yielded TONs of up to 120 (the photocatalytic system generated TONs of about 20). However, compared to the light-driven homogeneous catalysis in solution [168], a decrease in reaction rate by a factor of ten was observed, indicating that diffusion processes within the solid matrix were the reaction’s rate-determining step. As an advantage to photocatalysis by [(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)]3 þ in solution, the surface-bound catalyst could be easily recycled and reused (though with slightly reduced activity) (Figure 9.19). In dye-sensitized solar cells (DSSCs), charge-transfer to the electrode surface is realized via an O-linkage between the light-absorbing dye (i.e., the sensitizer) and the nano-structured semiconducting film (e.g., TiO2, typically 2–20 mm thick) that has been deposited onto the electrode surface [189]. Most commonly, the RuII bisbidentate or mono-tridentate complexes bear at least one carboxylate or phosphonate group that strongly interacts with the TiO2 surface (for details, see Chapter 8.4 and citations therein). If this concept of binding is applied to the development of photoelectrochemical water oxidation devices, one has to be aware of two limitations [122] in particular: (i) the CO2–TiO2 linkages are labile and can undergo hydrolysis in aqueous environments (i.e., the dye will be detached from the surface); and (ii) the RuII complexes that were found to be good DSSCs materials are, at the same time, typically poor water oxidation catalysts. The successful covalent anchoring of a functional molecular water oxidation catalyst to an electrode surface was reported by Liu et al. [190]. The dinuclear RuIII mono(terpyridine) complex {O[Ru(H2O)2(tpy-PO(OH)2)]2}4 þ [tpy-PO(OH)2: 2,20 :60 ,200 -terpyridin-40 -yl-phosphonic acid] was tethered to a range of metal oxide electrode surfaces: ITO as well as 10–20 nm thick films of TiO2, ZrO2, or SnO2 nanoparticles on ITO (Figure 9.20a). The surface coverage (G) was

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| 497

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes (a)

3 N

N

N

N N N Ru H O Ru 2 N N OH2 N N

N

N 24

(b)

2.0104 1.2104 4.0105

I (A)

498

4.0105 1.2104 2.0104

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

U (V)

Figure 9.19 (a) Pyrrole-functionalized dimeric RuII complex that was electropolymerized onto electrode surfaces; (b) growth of a FTO/poly-24 film over 30 repetitive scans between 0.0 and 1.2 V vs. SCE, obtained using a 0.2 M solution of 24 in CH2Cl2 [0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6); scan rate: 0.05 V s1; FTO electrode area: 1.0 1.0 cm2] [188]. Figure reproduced with kind permission; r 2008 Wiley-VCH.

(a)

(b)

2 N

H2O N

OH2

O Ru2+

N

OH2

OH2 N

N

N

Ru2+

N

N

OH2 Ru N N N

N

N

N

O P OH O

Ru

H2O

N

HO P O O

N

O

metal oxide electrode

O

rutile TiO2

Figure 9.20 Dimeric Ru mono(terpyridine) complex anchored to a range of semiconducting oxide surfaces via phosphonate groups (a) or a carboxylic acid group (b) [190, 191].

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7.0 1011 mol cm2 on bare ITO and 5 108 to 1.2 107 mol cm2 for the nanoparticles on ITO. Water oxidation was achieved by stepping the applied potential to either 1.15 or 1.32 V at pH 6 (thereby forming the active RuIVORuV or RuVORuV species, respectively); the O2 evolution was monitored using an oxygenselective electrode. Maximum turnovers of three molecules O2 per catalyst molecule were observed, with the catalytic current decaying to about 10% of the initial value after one hour of operation. The catalytic deactivation was attributed to an oxidative cleavage mechanism analogous to that observed for similar complexes in homogenous solution [167]. Similarly, dimeric [(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)]3 þ was anchored to a rutile surface via a CH2CO2H-substituent that was attached to the dpp ligand (at the 4-position of the central 1H-pyrazole ring, Figure 9.20b) [191]. The hybrid material catalytically oxidized water to molecular oxygen in a heterogeneous manner using CeIV, as oxidant; however, the generation of O2 was accompanied by the formation of CO2 as well as considerable leaching of the Ru catalyst. The direct coupling of a water oxidation catalysts to a light-absorbing antennae, within a single photoelectrochemical cell [183], potentially offers significant advantages in terms of cost, but only when the device efficiencies could be significantly improved. In recent years, various types of molecular as well as nonmolecular catalysts sensitized by light absorbing complexes and/or organic dyes have been reported [122]. For instance, the light-driven charge-transfer between a MnII mono(terpyridine) complex and TiO2 particles was achieved using a catechol moiety as anchor (Figure 9.21a) [192]. Rapid charge-transfer (about 300 fs) upon excitation was observed. High stability of the catechol linkage was found in an aqueous environment, even under oxidative conditions. However, recombination of the excited electron with the photo-oxidized MnIII center was very fast (500 ps to 1 ms at room temperature, according to EPR measurements), resulting in a high

(a) O

TiO2 O

N N

OH2

Mn2+ OH2 N

OH2

(b)

N O

HN

O

O

N

TiO2

OH2

Mn2+ OH2 N

OH2

Figure 9.21 Mononuclear MnII complexes anchored to TiO2 nanoparticles via a catechol (a) or an acac moiety (b) [192, 193].

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| 499

500

| 9 Catalytic Applications of Terpyridines and Their Transition Metal Complexes probability of electron back-transfer after excitation and, consequently, yielding a low injection efficiency. Alternatively, the mononuclear MnII complex could be attached to TiO2 via an acetylacetonate (acac) unit (Figure 9.21b) [193]. These mononuclear MnII complexes do not function as catalysts for water oxidation; however, the robust, but flexible, acac-system could be dimerized by oxidation with permanganate to form a mixed-valence MnIII(O)2MnIV surface-bound species (structurally reminiscent of the dinuclear water oxidation catalyst [(tpy)(H2O)Mn(m-O)2Mn(H2O)(tpy)]3 þ , see Figure 9.15a). Photo-oxidation by visible light to the MnIV(O)2MnIV state was confirmed by EPR measurements. The ability of surface-bound dinuclear MnIII/IV complexes to act as catalyst in water oxidation was reported by Li et al. [194]. According to DFT calculations, the chemisorption of [(tpy)(H2O)Mn(m-O)2Mn(H2O)(tpy)]3 þ onto the TiO2 surface occurred via an oxo bridge, formed upon substitution of a water ligand by the surface. On well-crystallized TiO2 nanoparticles, a MnIV tetramer (formed by oxidative dimerization of the dinuclear complex) was postulated to be catalytically active in the oxidation of water using CeIV, as the primary oxidant, in which O2 evolution was observed with turnovers greater than one (0.04 mmol O2 after 1000 s). In contrast, chemisorption of the dinuclear MnIII/IV complex on near-amorphous TiO2 did not generate an active catalyst for water oxidation.

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Concluding Remarks

In 1932, Morgan and Burstall isolated for the first time 2,20 :60 ,200 -terpyridine, in poor yield, as a by-product of the oxidative condensation of pyridine with FeIII chloride – a chemical oddity in its time. The last few decades have seen its rapid inclusion in both supramolecular chemistry, that is, the construction of advanced architectures based non non-covalent interactions, and coordination chemistry, that is, the utilization of transition metal ion complexes in various fields (e.g., optoelectronic devices, biomedicine, or catalysis). In this respect, 2,20 :60 ,200 -terpyridine and its derivatives have become one of the key molecular building blocks in the construction of materials that bridge diverse areas of application. Today, modern organic and macromolecular chemistry offer the synthetic tools to prepare almost any conceivable terpyridine motif – from small molecules to functionalized polymers. In addition, the ability of terpyridine ligands to coordinate a broad range of transition metal ions permits entree to the synthesis of tailor-made terpyridine ligands and complexes. Numerous polymeric materials have been derived from monodisperse and polydisperse building blocks by transition metal ion coordination, thereby combining the properties of the polymers (e.g., film-forming ability, flexibility) with those of the metal complexes (e.g., redox chemistry, photophysical behavior). In particular, p-conjugated telechelic bis(terpyridine)s have been used to generate linear metallopolymers that are potential candidates for commercialization, for example, in photo-active materials or optoelectronic devices. However, the poor solubility of these metallo-supramolecular assemblies needs to be addressed in order to achieve a suitable processability and enhanced film-forming ability with respect to future applications. Besides this, the directed assembly of terpyridinefunctionalized polymers has allowed the linkage of polymer chains by terpyridinemetal connectivity; for instance, hydrophilic [e.g., poly(ethylene glycol)] and hydrophobic chains (e.g., polystyrene) can be combined into amphiphilic block copolymers. Such metallo-supramolecular polymers exhibit a complex aggregation behavior in solution and access to diverse structured nanomaterials (e.g., micelles, vesicles). Many complexes of terpyridines with transition metal ions have been reported, but very few have made the next step, that is, from scientific research to common Terpyridine-based Materials: For Catalytic, Optoelectronic and Life Science Applications, First Edition. Ulrich S. Schubert, Andreas Winter, and George R. Newkome. r 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Published 2011 by WILEY-VCH Verlag GmbH & Co. KGaA

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| 10 Concluding Remarks applications. Here, two representatives in particular have to be named: the “black dye” and PtII mono(terpyridine) complexes. The former, a RuII mono(terpyridine) complex, is widely applied, as sensitizer, in dye-sensitized solar cells. The latter are powerful metallodrugs possessing remarkably high cytotoxicity against various types of cancer cell lines, but the rather low selectivity of their mode of action remains as a limiting factor. The utilization of the photocatalytic properties of RuII complexes with respect to applications in artificial photosynthesis (“water splitting”) is a further noteworthy achievement. In what direction will modern terpyridine chemistry evolve? For sure, research in this field will move away from structure-targeted synthesis to application-related investigations. This trend may be envisioned, based on a few examples: In contrast to conventional organic macromolecules, (metallo-)supramolecular polymers feature one interesting characteristic: reversibility-in-bonding. This unique ability can be utilized by adjusting the properties of such “living systems” by external stimuli. This dynamic nature allows adaptation of supramolecular architectures in biological environments or, in particular, as self-healing materials – a buzzword in modern materials science. In view of organometallic catalysis, catalytic systems, based on terpyridine, have reached high efficiencies in terms of conversion and chemoselectivity. However, their stereochemical performance is still rather poor, in particular when compared to the many powerful (organo)catalysts available. Thus, reliable asymmetric protocols need to be established if terpyridine-based catalysts are to be a competitive alternative in modern organometallic catalysis. Light-into-energy conversion is a highly active field of research and terpyridine complexes could be versatile candidates to catalyze this process, if their photophysical properties can be further improved. The same holds for the “orthogonal” application: energy-into-light conversion, that is, making efficient and stable materials, as emitters, in light-emitting devices (e.g., polymer light-emitting diodes); parameters, such as long-term stability and increased efficiency, will still need to addressed for these applications to become commercially viable. The 3D fabrication of nano-sized molecular devices on (semiconducting) surfaces via selfassembly of terpyridine ligands and transition metal ions is a comparably young field of research. Promising results have already been obtained and reported; there is little doubt that they will continue to stimulate the engineering of tomorrow’s molecular wires, switches, rectifiers, gate structures, solar collectors, and nanomedical devices. The future of this ever-expanding, terpyridine family looks bright and has a long life expectancy.

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Index a 40 -[4-(2-acryloyloxyethoxy)phenyl]-2,20 :60 ,200 terpyridine 243 actin filament (F-actin) 451 AgI complex – dinuclear 467 alcohol cross-coupling 470 aldol-type condensation 468ff. allylation reaction 460ff. allylic substitution reaction 463 amidation – intramolecular 466 aminolysis – asymmetric 472 40 -(4-aminophenyl)-2,20 :60 ,200 -terpyridine 410 antitumor agent 341ff. 40 -aryl-2,20 :60 ,200 -terpyridine 38ff. atom-transfer radical polymerization (ATRP) 244ff., 282 – CuI-catalyzed 475 Au nanoparticle 421f. – terpyridine ligand 422 Au surface 402ff. – terpyridine-modified 408f. AuIII mono(terpyridine) complex 333, 349, 386 40 -azido-2,20 :60 ,200 -terpyridine 25f. aziridination 466f. azurin 380 b para-benzoquinone 477 BINOL (1,10 -binaphth-2-ol) 107 – BINOL-type bis(terpyridine) 220 biolabeling 376 biomolecule – covalent binding to small-molecule 338 – labeling 388 biotin moiety 381

BIP (2,6-bis(1-methyl-1H-benzo[d]imidazol2-yl)pyridine) 205 – 4-ethynyl functionalized 52 – ditopic 52, 205, 268 – metallopolymer 224 – polydisperse 224 2,20 -bipyrazine (bpz) ligand 491 2,20 -bipyridine (bpy) 2, 282, 475 – cis-[(bpy)2(py)Ru(H2O)]2þ 476 – cis-[(bpy)2(py)RuO]2þ 477 bipyridine-cyclodextrin 186 2,20 -bipyrimidine (bpm) 491 6,600 -bis(aminomethyl)-40 -phenyl-2,20 :60 ,200 terpyridine chelating unit 387 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) 366 1,4-bis[2,6-bis(1-butyl-1H-1,2,3-triazol-4-yl) pyridin-4-yl]benzene (BTP-type) 219 bis-cyclometalating tridentate ligand 113 1,4-bis[2,6-di(1H-pyrazol-1-yl)pyridin-4-yl] benzene (BPP-type) 219 2,6-bis(5,6-dihydro-4H-1,3-oxazin-2-yl) pyridine 463 2,6-bis(4,5-dihydrooxazol-2-yl)pyridine 463 5,500 -bis(hydroxymethyl)-2,20 :60 ,200 terpyridine 256 bis-intercalator 334 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl) pyridin-4-ol (BIP-OH) 51 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl) pyridine, see BIP bis(phenanthroline) 150ff. 3,6-bis(pyridin-2-yl)pyridazine ligand 158 2,6-bis(pyridin-2-yl)-4-pyridone 284 1,5-bis(pyridin-2-yl)triazine 83 bis-rotaxane 179 1,2-bis(2,20 :60 ,200 -terpyridin-40 -yl)acetylene 32 bis(2,20 :60 ,200 -terpyridin-4-yl)amine 25


BINDEX

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510

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Index 1,4-bis(2,20 :60 ,200 -terpyridin-40 -yl)benzene 42 (E)-1,2-bis(terpyridin-4-yl)diazene 25 1,4-bis[(2,20 :60 ,200 -terpyridin-5-yl) methylsulfanyl]butane 130 1,3-bis[(2,20 :60 ,200 -terpyridin-5-yl) methylsulfanyl]propane 130 bis(terpyridine) 42ff., 149f., 182, 289 – alkene-functionalized angular 141 – amine-bridged 25 – angular 136ff. – BINOL-type 220 – boron-dipyrromethane (BODIPY)bridged 48f. – dithiol-bridged 130 – electron-acceptor-type p-conjugated spacer 48 – motif 206 – oligophenylene-bridged 212 – oligoyne-bridged 33 – photophysical property 35, 47 – p-conjugated 47, 204, 309 – piperazine-functionalized 412 – soluble 34 – step-wise self-assembly 404 – trans-stilbene-bridged 44 – telechelic 270 – thieno[3,4-b]pyrazine-bridged 47 – viologen-type 133, 220 – X-shaped 37 bis(terpyridine) complex – dinuclear 89 – heteroleptic 185 – self-assembly 147 bis(terpyridine) ligand 32 – angular 137 – heteroleptic 68 – homoleptic 68 – multifunctional 131 – telechelic 224 – thiophene-containing 98 1,1000 -bis(2,20 :60 ,200 -terpyridin-4-yl)-1,10 biferrocene 100 3,6-bis(2,20 :60 ,200 -terpyridin-40 -yl)-9-alkylcarbazole 136 1,4-bis(2,20 :60 ,200 -terpyridin-40 -yl)benzene 309 4,40 -bis(2,20 :60 ,200 -terpyridin-40 -yl) triphenylamine 136 2,6-bis(1H-1,2,3-triazol-4-yl)pyridine (BTP-type) 52 bis(U-terpyridine) 50f. black dye 66, 432ff. bleomycin – iron-containing glycopeptide 372

BINDEX

block copolymer – A-[M]-(B-[M]-)nB-[M]-A type 308 – dinuclear A-[M]-B-[M]-A 309 – oligonuclear metallo-supramolecular 308 blue dimer 489ff. BODIPY (boron-dipyrromethene) 209 boron neutron capture therapy (BNCT) 330 Borromean links (Borromean rings) 157, 181ff. bpy, see 2,20 -bipyridine 40 -(4-bromomethylphenyl)-2,20 :60 ,200 terpyridine 450 40 -(4-bromophenyl)-2,20 :60 ,200 -terpyridine 233 BTB (bipyridine–terpyridine–bipyridine) 173 – BTP-based ligand 52 – BTB-type ligand 219 n-butyl acrylate (nBA) 283f. 1-butyl-3-methylimidazolium hexafluorophosphate (bmim-PF6) 423 c C60tpy@Pd 426 C^N^N (mono-cyclometalating tridentate ligand) 445 [(C^N^N)Ru(N^N^N)]þ 445 cadmium 174 – CdII 292, 365 – CdII coordination polymer 208 – CdII mono(terpyridine) complex 425 – [Cd(tpy)F2] 467f. cage-like structure 172 calix[4]arene – derivative 186 – functionalized with four terpyridine ligands 188 e-caprolactone 279 2-(N-carbazolyl)ethyl methacrylate (CzMA) 256 carbene-insertion 467 catalyst in organic reaction – asymmetric 460 catenane 177, 181 – reversible molecular motion 181 CCAAC (CuI-catalyzed alkyne-azide [2þ3]cycloaddition) reaction (“click” reaction) 25, 52f., 406, 472 chain-transfer agent (CTA) 283 – telechelic 284 chemotherapeutic agent 342 40 -chloro-2,20 :60 ,200 -terpyridine 264ff. chromium – CrIII bis(terpyridine) complex 375 circular dichroism (CD) effect 320 co-dimerization – linear 471

27 J l 2011 9 29 9

Index cobaloxime complex 486 cobalt – CoII bis(terpyridine) complex 71, 185, 374, 412ff. – CoIII bis(terpyridine) complex 374 – CoII ion 244, 267, 292, 374, 403 – CoIII ion 374 – CoII mono(terpyridine) complex 413 – [Co(btp)3](ClO4)2 (btp: 4,40 -di(tert-butyl)-2,20 bipyridine) 234 – [Co(tpy)2]2þ 479 – [Co(tpy-vinyl)2]2þ 479 – CoII-containing grid 159 – grid 163ff. – homoleptic (alkyl)thiol-functionalized CoII bis(terpyridine) complex 415 – infinite 2D grid-like structure 164 – thiol-functionalized CoII bis(terpyridine) complex 412 competitive fluorescence spectroscopy (CFS) 324 container transportation system 451 coordination polymer 203 – terpyridine-based 215 copper 174f. – CCAAC 25, 52f., 406, 472 – CuII bis(terpyridine) complex 367ff. – CuII complex 366 – CuI ion 178ff. – CuII ion 177ff., 244, 292, 366ff., 380 – CuII mono(terpyridine) 367ff. – [Cu(bpy)(H2O)]2þ 369 – [Cu(bpy)(tpy)]2þ 172 – [Cu(BTB)Fe(BTB)Cu]4þ 173 – [Cu(bzimpy)]þ 367 – [Cu(bzimpy)Cl]þ 367 – [Cu(pty)Cl]Cl 367 – [Cu(tpy)(H2O)]2þ 369 – polymer-bound CuI mono(terpyridine) complex 472 cross-coupling procedures 18ff CRP (controlled radical polymerization) 244, 281 current imaging tunnelling spectroscopy (CITS) 168 cyclodextrin 182ff. cyclodextrin-complex – homoleptic 182 cyclometalation 113 cyclopropanation – asymmetric 461 cyclosexipyridine 143 cytochrome c protein 384 cytotoxicity 341f., 361, 373

BINDEX

d dendrimer – first-generation 105 dendritic terpyridine architecture 24 deoxycholic acid (DCA) 436 deoxycytidinyl-(30 ,50 )-deoxyguanosine (deoxy-CpG) 328 dextran – terpyridine-functionalized 450 di(hexadecyl)phosphate (DHP) 216 2,6-di(quinolin-8-yl)pyridine 54 dialkyl 2,20 :60 ,200 -terpyridine-40 phosphonate 27 diazo-bis(terpyridine) 104 diblock copolymer 286f., 303 – A-[M]-A 294 – A-[M]-B 272, 294ff. – B-b-A-tpy 286 – B-[M]-B 294 – PEG-[Ru]-PS 297 dihydride mechanism 468f. 1,4-dihydropyridine (DHP) 487 6,600 -dimethyl-2,20 :60 ,200 -terpyridine 15, 28 5,500 -dinitro-2,20 :60 ,200 -terpyridine 16 dinuclear complex 94ff. – bicyclo[2.2.2]octane bridge 97 – N-heteroaromatic groups in the spacer 99 – heterometallic 91 – strapped biphenyl 97 dinuclear rack 157 2,9-diphenyl-1,10-phenanthroline (dpp) 379 1,1-diphenylethylene 463 2,9-diphenylphenanthroline 177 40 -(diphenylphosphino)-2,20 :60 ,200 terpyridine 27 dipyrido[3,2-a:20 ,30 -c]phenazine (dppz) 335 40 -(2,3-disubstituted 2H-azirin-2-yl)-2,20 :60 ,200 terpyridine 25 dithienylethene (DTE) 43 – photochromic 43 DNA – A-[Fe]-B0 (A/A0 , and B/B0 represent complementary strands) 378 – A-[Ru]-A 378 – B-[Fe]-C0 (B/B0 , and C/C0 represent complementary strands) 378 – B-[Ru]-B 378 – C-[Fe]-A0 (A/A0 , and C/C0 represent complementary strands) 378 – damage 350f. – double helix 327ff. – double-strand break (DSB) 350 – G-quadruplex 331f. – ODN DNA duplex 377

27 J l 2011 9 29 9

| 511

512

|

Index DNA (continued) – oxidative cleavage 350f., 375 – single-strand break (SSB) 350 – topology 327 DNA binding 350ff. DNA diblock copolymer 378 DNA intercalation 320ff. – PtII mono(terpyridine) complex 321ff. DNA intercalator 320ff. dodecahydro-19,20,21,22,23,24hexaazakekulene 144 donor–[Ir(tpy)2]–acceptor array 112 double-cross-shaped structure 170 dye-sensitized solar cell (DSSC) 233f., 431ff., 497 dysprosium – DyIII ion 252 e electron hopping mechanism 409 electron/proton-transfer (EPT) 488 electroreduction 480 – CO2 480 – oxygen 480 energy and charge transfer 143 epoxidation 464f. – asymmetric epoxidation of styrene derivative 465 meso-epoxide 472 ethidium bromide (EthBr) 324 40 -ethynyl-2,20 :60 ,200 -terpyridine 31f., 135 europium – EuIII 269 – EuIII chelate 387f. – EuIII ion 182, 251f. excited-state energy relaxation diagram 143 f Fe – A-[Fe]-B0 (A/A0 , and B/B0 represent complementary DNA strands) 378 – B-[Fe]-C0 (B/B0 , and C/C0 represent complementary DNA strands) 378 – bio-inspired [FeFe]-hydrogenase moiety 484 – C-[Fe]-A0 (A/A0 , and C/C0 represent complementary DNA strands) 378 – dinuclear FeII complex 464 – FeII catalyst 469 – FeII complex 373, 464 – FeII ion 181f., 220, 244, 267ff., 292, 378, 403

BINDEX

– FeII bis(terpyridine) complex 180f., 380, 423, 464f. – FeII bis(terpyridine) unit 278 – FeII bis(terpyridine)-PEtOx complex 278 – FeII mono(terpyridine) complex 468 – FeIII mono(terpyridine) complex 472ff. – FeII tris(bipyridine) complex 244, 279 – [Fe(tpy)2](FeCl4)2 475 – Fe3O4@Au-tpy nanoparticle 425 – grid 163ff. – homoleptic FeII bis(terpyridine) complex 464 – metallopolymer 222, 265 – polymer-bound FeIII mono(terpyridine) complex 472 ferrocene – functionalized 370 ferrocenium cation 370 ferrocifen 369 flanking region (FL) 353 Fremy’s salt (FS, potassium nitrosodisulfonate) 212ff. FTO (fluorine-doped tin oxide) 443, 497 40 -(furan-2-yl)- 2,20 :60 ,200 -terpyridine 40 fused terpyridines 155 g gel (metallo-supramolecular) 248 giant amphiphiles 381 b-1,3-glucan 451 glutathione (GSH, Glu-Cys-Gly tripeptide) 339ff. gold, see Au graft copolymer 255, 289 graft-on-graft architecture 291 grid 154ff. – grid of grids arrangement 164 – grid-like architecture 156 – grid-like array 155 [2 2] grid 158ff. – chiral heterobimetallic 162 – double-T shaped 170f. – with periphal H-acceptor groups 164 – with periphal H-donor groups 164 [4 4] grid 170f. guanine-cytosine (G-C) DNA base pair 329 h halfgrid 156 head-to-head helicate 173 Heck cross-coupling reaction 44 HEEDTA (sodium salt of N-(hydroxyethyl) ethylenediamine triacetic acid) 292

27 J l 2011 9 29 9

Index helicate 171 – dinuclear double helicate 181 – heteroduplex 175 40 -heteroaryl-2,20 :60 ,200 -terpyridine 38ff. heteroditopic ligand 50 heterometallo-macrocycle 137f. HETPHEN (heteroleptic phenanthroline) complexation approach 148 HETTAP (heteroleptic terpyridine and phenanthroline) complexation concept 148 hexaazakekulene 143 hexakis(terpyridine) 152 – star-shaped 38 HgII ion 365 highly ordered pyrolytic graphite (HOPG) 416ff. homopolymer – A-[M]-A 272, 291ff. – metallo-supramolecular 294 – supramolecular 291 – tpy-TIPNO 285 Horner–Wadsworth–Emmons (HWE) condensation 41ff. hydrosilylation – metal-catalyzed 463 40 -(4-hydroxyphenyl)-2,20 :60 ,200 terpyridine 448, 472 i ibuprofen – regioselective oxidation 474 initiation – ionic polymerization reaction 277 initiator – functional terpyridine-containing 277 intercalation 320ff. – PtII mono(terpyridine) complex 321ff. interfacial electron transfer (IET) 446 intra-ligand charge-transfer (3ILCT) state 85 ionic liquid 423 ionochromism 258 IrIII bis-complex – mono-cyclometalated 114 IrIII bis(terpyridine) complex 107ff., 185, 383 – metallopolymer 252 IrIII complex – bis-cyclometalated 484 – terpyridine ligand 107 IrIII mono(terpyridine) complex 364, 470ff. IrIII-PtII mixed–metal dinuclear complex 337 IrIII-RuII complex 100f., 111 [Ir(tpy-mes)2]3þ (mes: 2,4,6trimethylphenyl) 108 iron, see Fe

BINDEX

iron regulatory element (IRE) 352 IRP (iron regulatory protein) 353f. isoprene 475 Irving-Williams series 67 j Jameson’s protocol 14 k ¨hnke condensation 14f., 40 Kro – variant 17 ¨hnke-motif 4 Kro ¨hnke-type mono(terpyridine) 39 Kro ¨hnke-type 2,20 :60 ,200 -terpyridine 38, 82 Kro – non-planar 41, 82 – planar (pyrimidin-2-yl)-substituted 82 l L-lactide 279 lactoferrin (LF) 405 labeling – biomolecule 388 lanthanide(III) – LnIII chelate 388 – LnIII mono(terpyridine) complex 387 – LnIII-PtII mixed-metal trinuclear complex 337 – metal ion 336 layer-by-layer (LBL) self-assembly 429f. Leishmania parasite 346f. LiI cation 436 ligand-to-ligand charge-transfer (LLCT) 113 light absorption sensitizer (LAS) 482 light-driven hydrogen formation 483 light-induced excited-state spin-trapping (LIESST) 143 lower critical solution temperature (LCST) 293f. m [M(tpy)2]2þ 292f. macrocycle 181 – shape-persistent metal-free hexagonal 143 [2þ2]-macrocycle 133 – heterometallic 132 [3þ3]-macrocycle – homo- and heterometallic triangular 136 macroligand 281 – ditopic 272 – monotopic 272ff. – monotopic terpyridine-functionalized 274 manganese, see Mn maximum coordination site occupancy – deviation 169

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| 513

514

|

Index melamine formaldehyde (MF) particle 215 metal-to-ligand coordination 1 metallo-dendritic architecture 24 metallo-intercalator 324ff. – mono(terpyridine) d8 metal complex 320 metallo-macrocycle – hexameric 141 – hexameric dodecanuclear 146 – mononuclear 131 – oxidized single-wall carbon nanotube (oxi-SWNT) 140ff. – terpyridine-containing 130 – trinuclear hexagonal metallo-macrocycle containing FeII ions 143 [6þ6]-metallo-macrocycle 136 3D metallo-network 219 metallo-semiconducting material 282 metallo-supramolecular architecture 272 – helical 171 – self-assembly 166 – terpyridine complex 129ff. metallo-supramolecular assembly – bicyclic 184 metallo-supramolecular brush – amphiphilic 302 metallo-supramolecular homopolymer 294 metallo-supramolecular material – self-assembly of bis(terpyridine) 266 metallo-supramolecular micelle 298 metallo-supramolecular polyelectrolyte (MEPE) 212 metallo-supramolecular polymer 22 – mononuclear 291 metallo-supramolecular polymerization 200 metallo-supramolecular triblock copolymer 267 metallo-supramolecular zipper 187 metallomonomer 221 metallopolymer 201ff., 219ff. – amido- and imido-linkages 226 – CdII ion 208 – chiral 219ff. – color 224, 251 – FeII ion 222 – IrIII bis(terpyridine) complexes in the side-chain 252 – non-classical 220 – p-conjugated bis(terpyridine) 204 – polydisperse BIP-type ligand 224 – RuII ion 206 – self-assembly 212 – synthesis 202, 213ff., 225f., 264 – terpyridine-functionalized p-conjugated polymer 229

BINDEX

– type 201 – ZnII ion 208ff. methoxy-poly(tetrahydrofuran) (MPTHF) 273 methyl methacrylate (MMA) 247 – RAFT polymerization 247 9-methylanthracene (9-MA) 146 micellar gel 308 micelle 297ff. – core–shell–corona 305 – PS47-b-PtBA55-[Fe]-PEO125 307 – PS47-b-PtBA55-tpy 307 – PS80-b-PtBA200-tpy 307 microbead 449 – terpyridine-functionalized TentaGelTM 449 3 MLCT (metal-to-ligand charge-transfer) excited-state 76ff., 481 – lifetime 76f. 3 MMLCT 485 MMLCT absorption 117 Mn – complex 492 – dinuclear m-(O)2-bridged MnIII/MnIV mono (terpyridine) catalyst 473 – Mn4 161 – Mn12 161 – MnIII(O)2MnIV 500 – MnIV(O)2MnIV 500 – [(ptpy)(H2O)MnIII(m-O)2MnIV(H2O) (ptpy)]3þ 473 – [(tpy)(H2O)Mn(m-O)2Mn(H2O)(tpy)]3þ 492 MnII complex – dinuclear 470 – dioxygen-bridged dinuclear 464 – mononuclear 499 MnII mono(terpyridine) complex 445ff., 464ff., 499 modified methylalumoxane (MMAO) 475 molecular monolayer memory (MMM) device 413f. molecular monolayer non-volatile memory device (MMNVM) 413f. molecular motion – oxidation-induced 178 molecular switch 178 molecular wire-type behavior 157 mono(terpyridine) – aldehyde-functionalized 45f. – amine-bridged 25 – ethynyl-functionalized 33 – nonlinear optical behavior 46

27 J l 2011 9 29 9

Index mono-terpyridine complex 66 – metallo-intercalator based on mono (terpyridine) d8 metal complex 320 mono[tris(pyridin-2-yl)triazine] (tptz) complex 356 monocyclometalated species 113 monocyclometalating tridentate ligand 113 monohydride mechanism 468f. multi-ion pair association 142 multi-walled carbon nanotube (MWNT) 428f. myosin 451 n N^C^N ligand 445 [(N^C^N)Ir(N^N^C)]þ 115 N^N^N-ligand (tridentate ligand) 339, 445 [(N^N^N)Ir(C^N^C)]þ 115 NafionTM 493f. nanocluster – bis(4-iodophenyl)silane-modified [PW11O37]7 431 nanomaterial – inorganic 420 nanoprism 149ff. – self-assembly 152 nanostructure 399ff. n BA (n-butyl acrylate) 283f. neighbor exclusion model 328 nickel – grid 163ff. – NiII ion 244, 267, 292 – NiII mono(terpyridine) complex 472 – [Ni(tpy)2]2þ connectivity 292 – [(tpy)Ni(Me)] 471f. NIPAM (N-isopropylacrylamide) 283 nitroxide-mediated polymerization (NMP) 244ff., 285 norbornene derivative 244 nucleoside 342 nucleotide 342

– middle-(RuII-complex)-ODN 377 – ODN DNA duplex 377 – 30 -(RuII-complex)-ODN 377 – 50 -(RuII-complex)-ODN 377 – RuII-labeled ODN-conjugate 377 organic light-emitting diode (OLED) 140ff., 200, 257 organic solar cell (OSC) 257 organopolymeric resin 447 OsII – grid 163 OsII bis-complex – thiophene-substituted 229 OsII bis(terpyridine) complex 81ff., 411 – anthracene-substituted 86 – heteroleptic 75, 183 – homoleptic 75ff. – synthesis 68ff. – TPþ-functionalized (TPþ: 2,4,6triarylpyridinium) 80 OsII bis(terpyridine) unit – oligonuclear complex 89 OsII complex 228, 491 – monocyclometalated 88 – photophysical property 88 OsIII mono(terpyridine) complex 68 oxidation – alkene 495 – electrocatalytic 476 – molecular terpyridine-based catalysts for water oxidation 488 – photocatalytic 487 – regioselective oxidation of ibuprofen 474 oxidation potential 482 oxime – photocleavage 496 oxygen – electroreduction 480 – singlet 375, 495f. p palladium – PdII mono(terpyridine) complex 340f., 470 – [(tpy)Pd(NCMe)](BF4)2 468 PCBM ([6,6]-phenyl-C61-methyl butyrate) 206 PDIm (perylene diimide) 402, 417 PEG (poly(ethylene glycol)) 255, 265ff., 464 – amino-functionalized PEG75 289 – PDMS-[Ru]-PEG 303 – PEB-[Ru]-PEG 303 – PEB70-[Ru]-PEG70 303 – PEG-b-PPG-b-PEG 271 – PEG-PDIm terpyridine 417 – (PEG44-[Ru]-)3 310f.

o oligo(ethylene oxide) methacrylate (OEGMA475) 291 oligo(hexylthiophene) (OHT) 231 oligo(phenylene-ethylene) (OPE) – thiol-functionalized 410 oligo(phenylenevinylidene) (OPV) trimer 231 – terpyridine-functionalized 231 oligonuclear complex – V-shaped 106 – X-shaped 106 oligodeoxynucleotide (ODN) 377

BINDEX

27 J l 2011 9 29 9

| 515

516

|

Index palladium (continued) – (PEG44-[Ru]-)4 310f. – PEG-[Ru]-PS 297 – PFDS12-[Ru]-PEG70 303f. – PPG-b-PEG-b-PPG 271 – PS22-b-PEG70 298 – PS76-b-PTFMS42-[Ru]-PEG70 306 – PS32-b-P2VP13-[Ru]-PEG70 303 – PS20-[Ru]-PEG70 297ff. – PSx-[Ru]-PEGy 298ff. – PTFMS20-b-PtBA25-b-PS35-[Ru]-PEG70 306 penta-amine macrocycle – terpyridine-containing 133 pentadentate ligand – terpyridine-based 30 40 -(pentafluorophenyl)-2,20 :60 ,200 terpyridine 40 PEO125-tpy 307 3-(phenanthrolin-2-yl)-1,2,4-triazine ligand 358 1,10-phenanthroline (phen) 319 phenanthroline-based ligand 177 phenanthroline–terpyridine rotaxane 178 2-phenyl-4,6-bis[6-(pyridin-2-yl)-4-(pyridin-4-yl) pyridin-2-yl]pyrimidine (2,40 -BTP) 417 40 -phenyl-2,20 :60 ,200 -terpyridine (ptpy) 39, 473 – [(ptpy)2Mn2(m-O)2(H2O)2](ClO4)3 470 40 -(4-phenylmethanethiol)-2,20 :60 ,200 terpyridine 421 photocatalytic process 480 photochemical open–close cycle – reversible 43 photochromic dithienylethene (DTE) 43 photocleavage – oxime 496 photoelectrochemical cell (PEC) 234 – water-splitting 492 photoinduced electron-transfer (PET) 487 photosensitized electron transfer process 483 photovoltaic application 431 photovoltaic (PV) cell – bulk-heterojunction 260 photovoltaic device 232 photovoltaic quantum yield 439ff. platinum, see Pt platinyne-bis(terpyridine) 132 PMDETA (N,N,N,N00 ,N00 pentamethyldiethylenetriamine) 246, 282, 428, 475 poly(acrylic acid) (PAA) 287, 436 – PS120-[Ru]-PAA135 303ff. poly(e-caprolactone)-macroligand 280ff. poly(chloromethylstyrene-codivinylbenzene) (PCD) 448, 472

BINDEX

– [PCD-tpy)Fe(H2O)3]3þ 472 – terpyridine-functionalized PCD resin 448 poly(dialkoxyphenylene-thiophene) 260 poly(dialkylfluorene-thiophene) 260 poly(dimethylsiloxane) (PDMS) 274 – PDMS-[Ru]-PEG 303 poly(ethylene glycol), see PEG poly(ethylene imine) (PEI) 214, 436 – metallo-supramolecular 278 poly(ethylene oxide) monomethyl ether (MPEG) 272 poly(ethylene-co-butylene) (PEB) 273 – PEB-[Ru]-PEG 303 – PEB70-[Ru]-PEG70 303 poly(3,4-ethylenedioxythiophene)–poly (styrenesulfonate) (PEDOT:PSS) layer 414 poly(2-ethyloxazoline) (PEtOx) – hydroxyl-functionalized 274 poly(2-ethyloxazoline)-block-poly (2-phenyloxazoline) 278 poly(ferrocenyldimethylsilane) (PFDS) 275 – PFDS12-[Ru]-PEG70 303f. poly(N-isopropylacrylamide) (PNIPAM) 275, 294 poly(L-lactide)-macroligand 280 poly(methyl methacrylate) (PMMA) 247 poly(n-octylthiophene) 442 poly(2-oxazoline)-macroligand 277 poly(oxytetramethylene) – a-carboxy-o-terpyridin-40 -ylfunctionalized 269 poly(p-phenylenevinylene) (PPV) 261 poly(p-phenylenevinylidene)-type (PPV) polymer 231 – tpy-functionalized 233 poly(propylene glycol) (PPG) 271 poly(styrene sulfonate) (PSS) 214 poly(styrene-alt-diphenylethylene) copolymer – terpyridine-terminated 276 poly(tetrahydrofuran) (PTHF) 271 poly(p-trifluoromethylstyrene) (PTFMS) 288 – PS76-b-PTFMS42-[Ru]-PEG70 306 – PTFMS20-b-PtBA25-b-PS35-[Ru]-PEG70 306 poly(vinyl chloride) (PVC) 256 – terpyridine-functionalized 256 poly(2-vinylpyridine) 286 polybutadiene – terpyridine-modified 275 polybutadiene-block-poly(ethylene glycol) 273 polyelectrolyte-amphiphile complex (PAC) 216 polyelectrolyte-surfactant-like complex 253 polyimide 227

27 J l 2011 9 29 10

Index polyisoprene (PI) – terpyridine-modified 275, 286 polymer – a,o-dihydroxy-functionalized 270 – block copolymer 254 – chain-extended polymers from polymeric building blocks 269 – functional polymers incorporating terpyridine-metal complexes 241ff. – metal ions/complexes 201 – p-conjugated 199ff., 257ff. – terpyridine metal complex 199ff. – terpyridine units in the side-chain 242 – terpyridine within the polymer backbone 262 – terpyridine-functionalized 259 polymer light-emitting diode (PLED) 199, 257 polymer light-emitting device (PLED) – supramolecular 206 polymer solar cell (PSC) 206 polymerization 243ff. – anionic 276 – complex first method 224 – ionic 277 – radical 281 – RAFT (reversible addition-fragmentation chain-transfer) 244ff. – transition metal ion coordination 204 polyoxometalate (POM) 430 polystyrene (PS) – latex particle 215 – PEG-[Ru]-PS 297 – polystyrene-block-poly(2-vinylpyridine) 273 – PS47-b-PtBA55-[Fe]-PEO125 307 – PS47-b-PtBA55-tpy 307 – PS80-b-PtBA200-tpy 307f. – PS22-b-PEG70 298 – PS76-b-PTFMS42-[Ru]-PEG70 306 – PS32-b-P2VP13-[Ru]-PEG70 303 – PS120-[Ru]-PAA135 303ff. – PS20-[Ru]-PEG70 297ff. – PSx-[Ru]-PEGy 298ff. – PTFMS20-b-PtBA25-b-PS35-[Ru]-PEG70 306 post-polymerization functionalization 250ff., 288 Potts’ methodology 14f. protecting group approach 406 proton-coupled electron-transfer (PCET) 488 PtII complex – cyclometalated [Pt(pbpy)]þ (pbpy: 6-phenyl2,20 -bipyridine) 410 – hexanuclear 147

BINDEX

– IrIII-PtII mixed–metal dinuclear complex 337 – LnIII-PtII mixed-metal trinuclear complex 337 – [Pt(tpy)]2þ 384 – [Pt(tpy)(Cl)]þ 384 – [(tpy)Pt(H2O)]2þ 385 – [(tpy)Pt(His)]2þ complex 384 PtII mono(terpyridine) complex 66, 115ff., 321ff., 333ff., 348, 383ff., 486ff. – antitumor agent 343 – azido-functionalized dinuclear 385 – carborane-containing 346 – dinuclear 119, 334ff. – DNA intercalation 321ff. – dyad 120 – estrogen-modified 386 – glycosylated 330, 349 – labeling 385 – luminescent 116 – mononuclear 345f. – pH-responsive 118 – polymer-bound 496 – silica-bound 496 – styrene functionalized 496 – triad 120 [Pt(tpy)Cl]þ 484f. [Pt(ttpy)Cl]þ 487 40 -(pyridin-4-yl)- 2,20 :60 ,200 -terpyridine 133 60 -(200 -pyridyl)dipyrido[3,2-a:20 ,30 -c]phenazine (dppzp) 360 40 -(pyrimidin-2-yl)-2,20 :60 ,200 -terpyridine 81 – planar 41 10-(1H-pyrrol-1-yl)decane-1-thiol 421 q quenching 481 quinquepyridine 187ff. 2,20 :60 ,200 :600 ,2000 :6000 ,20000 -quinquepyridine 172 – ligand 373 – 2,20 :60 ,200 :600 ,2000 -quaterpyridine 172, 176 r rack 154 – anthracene-containing 157 – rack-type architecture 156 radiotherapeutic agent 350 RAFT (reversible addition-fragmentation chain-transfer) polymerization 247 – methyl methacrylate (MMA) 247 – NIPAM 283 – styrene 283 ReI(CO)3 mono(terpyridine) complex 463 [Re(CO)5]Br 147

27 J l 2011 9 29 10

| 517

518

|

Index redox system 476, 489 reduction – CO2 479 – electrocatalytic 476 – ketone 469 – water 484 reduction potential 482 resorc[4]arene – terpyridine-functionalized 187 rhodium – RhIII complex 364 – RhIII mono(terpyridine) complex 462, 479 – [Rh(COD)Cl]2 (COD: cycloocta-1,5diene) 463 – [Rh(phen)2(phi)]3þ (phen: 1,10phenanthroline; phi: 9,10phenanthrenequinone diimine) 353 ring-assembly strategy 14 ring-opening metathesis polymerization (ROMP) – RuII-catalyzed 244 ring-opening polymerization (ROP) – cationic 278 – coordinative 279 Risch’s methodology 17 rotaxane 177f. Ru (ruthenium) complex – {O[Ru(H2O)2(tpy-PO(OH)2)]2}4þ [tpy-PO (OH)2: 2,20 :60 ,200 -terpyridin-40 -ylphosphonic acid] 497 – [(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)]3þ 499 RuII 292 – grid 163 RuII bis(terpyridine) complex 36, 78ff., 137, 180ff., 254, 371ff., 411ff., 428, 440ff., 484f. – anthracene-substituted 86 – dendritic, oligonuclear 445 – dinuclear 95ff. – dithiol-functionalized 420 – DNA intercalation 335 – heteroleptic 73, 420, 438 – homoleptic 74ff., 357ff., 438 – monomeric 428 – mononuclear 76 – photophysical property 75ff. – synthesis 73f. – TPþ-functionalized (TPþ: 2,4,6triarylpyridinium) 80 RuII bis(terpyridine) unit – dendritic system 102 – oligonuclear complex 89 – star-shaped system 102

BINDEX

RuII complex – A-[Ru]-A (A, B are complementary DNA strands) 378 – B-[Ru]-B (A, B are complementary DNA strands) 378 – bis-triazole-pyridine ligand 87 – carbene-type ligand 87 – diiodo-functionalized 227 – dinuclear 363, 493 – heteroleptic 188, 221, 256, 464 – monocyclometalated 88, 221, 445f. – mononuclear 363 – neutral 473 – photophysical property 88 – pyrrole-functionalized dimeric 498 – 30 -(RuII-complex)-ODN 377 – 50 -(RuII-complex)-ODN 377 – [Ru(tpy)(bpy)]2þ 410 – [(tpy)Ru(L)]2þ (L: bpy or phen) 380 – trans-[(tpy)Ru(kN-NC5H4O)2(H2O)] 469 RuII coordination polymer 207 RuII ion 182, 269 – metallopolymer 206, 212ff., 244 RuII mono(terpyridine) complex 157, 357, 444, 462ff. RuIII mono(terpyridine) complex 68, 439 RuII mono(tptz) complex 357 RuII oligopyridinyl complex 372 RuII polyimide 227 RuII tris(bipyridine) complex 184, 279, 484 RuII tris-thiocyanato complex 261 RuIII tris-thiocyanato complex 438 RuII ZnII-porphyrin complex – tetranuclear 444 RuII-IrIII array 186 RuII-OsII array 186 RuII-OsII dyad 185 RuIII/RuII coordination chemistry – selective 255 RuII/RuIII redox system 476, 478, 489 RuIII/RuIV redox system 476, 489 RuIV/RuIII redox system 478 RuVI/RuIV redox system 478 RuIII-terpyridine monocomplex 244 RuIIIORuIII 489 RuIVORuIII 489 RuIVORuIV 489f. RuVORuV 490Ru(acetone)6](BF4)3 219 [Ru(bpy)3]2þ 424, 483f. cis-[Ru(bpy)2(py)(H2O)]2þ 488 [Ru(bpy)2(py)(O)]2þ 488 [Ru2(btpyan) (qu1.5¯)(O2¯)]0 [btpyan: 1,8-bis (2,20 :60 ,200 -terpyridin-4-yl)anthracene; qu: quinoline] 492

27 J l 2011 9 29 10

Index [Ru(p-cymene)Cl2]2 473 Ru(dmso)4Cl2 177 cis-RuL2(NCS)2 (L: 2,20 -bipyridyl-4,40 dicarboxylic acid) 432 [RuL(NCS)3](n-Bu4N)3 (L: 2,20 :60 ,200 terpyridyl-4,40 400 -tricarboxylate) 432 [RuO]IV complex 351ff. [RuO]IV mono(terpyridine) complex 351 [Ru(OH)]III complex 354 [Ru(tptz)Cl2(PPh3)] 357 [Ru(tpy)2]2þ 181, 227, 255, 292ff. [Ru(tpy)2](BPh4)2 unit 298 [Ru(tpy)Cl3] 471 [Ru(tpy)(dmbpy)Cl] (dmbpy: 4,40 -dimethyl2,20 -bipyridine) 256 [Ru2(tpy)2](PF6)4 157 [Ru3(tpy)3](PF6)6 157 ruthenium polymer 206 – A-[Ru]-B 296 – A-[Ru]-B-[Ru]-A triblock copolymer 309 – PDMS-[Ru]-PEG 303 – PEB-[Ru]-PEG 303 – PEB70-[Ru]-PEG70 303 – (PEG44-[Ru]-)3 310f. – (PEG44-[Ru]-)4 310f. – PEG-[Ru]-PS 297 – PFDS12-[Ru]-PEG70 303f. – PS76-b-PTFMS42-[Ru]-PEG70 306 – PS120-[Ru]-PAA135 303ff. – PS20-[Ru]-PEG70 297ff. – PSx-[Ru]-PEGy 298ff. – PTFMS20-b-PtBA25-b-PS35-[Ru]-PEG70 306 s Sauer methodology 16 scandium – chiral ScIII mono(terpyridine) complex 472 schizophyllan 451 – terpyridine-modified (tpy-SPG) 451 scorpionate-complex 189 self-assembled monolayer (SAM) 401ff., 419 self-assembly 152ff. – bis(terpyridine) 266 – chiral heterobimetallic [2 2] grid 162 – directional 173 – FeII ion 378 – hierarchical 215, 253 – metallo-supramolecular architecture 166 – metallo-supramolecular material 266 – metallopolymer 212ff. – reversible redox-state-dependent switching of sexipyridine 175 – redox-switching of septipyridine 174 – step-wise 296, 404

BINDEX

– supramolecular polymer 253 – terpyridine functionalized Au nanoparticle 423 – terpyridine-functionalized PMMA 249 self-healing material 288 semi-self-assembly approach 137 sensitizer 445 – RuII-based 433ff. – RuIII-containing 441 septipyridine – disubstituted 173 – redox-switching of the self-assembly process 174 2,20 :60 ,200 :600 ,2000 :6000 ,20000 :60000 ,200000 sexipyridine 172 ´ski hexagonal gasket 138f. Sierpin silver, see Ag single-molecule magnet (SMM) 161 single-walled carbon nanotube (SWNT) 427f., 451 SnO2 – nanocrystalline 442 Sonogashira cross-coupling approach 35f. – iterative 34 spiro-metallodendrimer 133f. spoke set with rim elements 152ff. spoked wheel-type architecture 152ff. steroid – a-unsaturated 465 structure-within-structure morphology 253 styrene 283f. – asymmetric epoxidation of styrene derivative 465 – aziridination 466 – fluorinated derivative 286 – para-substituted 466 40 -(4-styryl)-2,20 :60 ,200 -terpyridine 243 sulfanylbenzene 402 40 -(4-sulfanylphenyl)-2,20 :60 ,200 terpyridine 402 11-sulfanylundecan-1-ol matrix 405 superoxide radical 367 superpolystyrene 276 supramolecular gel 260 – reversible self-assembly of terpyridinefunctionalized PMMA 249 supramolecular protecting group chemistry 406 Suzuki–Miyaura cross-coupling reaction 42ff. t tamoxifen – ferrocene-substituted 369

27 J l 2011 9 29 10

| 519

520

|

Index TBT (terpyridine–bipyridine– terpyridine) 173 terbium – TbIII ion 251f. terpolymer – random 250 2,20 :60 ,200 -terpyridin-40 -yl triflate 135 2,20 :60 ,200 -terpyridin-40 -yl-carboxylic acid 40 terpyridine 2ff., 426 – acrylate-functionalized monomer 20 – alkoxy-substituted 416 – 40 -alkyl-functionalized 27 – amino-functionalized 410 – anchoring 407 – biotin-modified 383 – catalytic application 459ff. – chiral 460 – dialkenyl-functionalized 183 – disulfide-functionalized 425 – dithienylethene-bridged 43 – ferrocene-functionalized 419 – fullerene-functionalized 21 – 40 -functionalized 4, 27 – fused 155 – [(H2O3P-tpy)Ru(O)2(H2O)]2þ 478 – inorganic nanomaterial 420 – liquid crystalline (LC) behaviour 29f. – macrocyclic terpyridine 135 – methacrylate-functionalized monomer 20 – molecular terpyridine-based catalyst for water oxidation 488 – nano-structured TiO2 431 – 40 -nitro-functionalized 25 – {O[Ru(C2O4)(tpy)]2(H2O)8} (C2O42-: oxalate) 489 – PS47-b-PtBA55-tpy 307 – PS80-b-PtBA200-tpy 307f. – rigid U-shaped 4, 49 – [(40 -R-tpy)PtCl]þ 116 – S-shaped 49 – serino-functionalized 378 – star-shaped 104 – terthiophene-functionalized 23 – tetraacid-functionalized 29 – thioalkyl-functionalized 420 – thiol-functionalized 424 – TIPNO-functionalized (tpy-TIPNO, TIPNO: 2,2,5-trimethyl-4-phenyl-3-azahexane nitroxide) 22f., 284 – [(tpy)(bpy)Ru(H2O)]2þ 477 – [(tpy)(bpy)Ru(H2O)]2þ/[(tpy)(bpy) RuO]2þ 476 – [(tpy)(bpy)RuO]2þ 477 – [(tpy)(H2O)Mn(O)2Mn(H2O)(tpy)]3þ 492

BINDEX

[(tpy)(H2O)Ru(dpp)Ru(H2O)(tpy)] 3þ 497 [(tpy)Ir(C^N^C)]þ 113ff. [(tpy)Ir(N^C^N)]2þ 113 [(tpy)Ir(N^N^C)]2þ 113 [(tpy)Ir(tpy-R)]3þ (R:biphenyl) 110 [tpy-M-tpy0 ]2þ complex 262 tpy-nBA-b-PS-b-nBA-tpy (nBA: n-butyl acrylate) 283 – [(tpy)(phen)Ru(H2O)]2þ 478 – tpy-PNIPAM 294 – tpy-PPFS30-g-AP7-b-PS73-TIPNO 291 – tpy-PPFS30-g-AP[(OEGMA475)10]7-b-PS73TIPNO 291 – tpy-PPFS30-g-AP(PLA11)9-TIPNO 291 – tpy-PPFS30-g-AP9-TIPNO 291 – tpy-PPFS30-g-(PEG75)10-TIPNO 289 – tpy-PS-b-nBA-b-PS-tpy 283 – tpy-PS35-b-PMA54-b-PPFS97-TIPNO 288 – tpy-PS35-b-PMA54-PTFMS20-TIPNO 288 – tpy-PS50-b-PPFS80-TIPNO 286 – tpy-PS50-b-PTFMS34-TIPNO 286 – tpy-PS160-TIPNO 287 – tpy-PS-TIPNO 288 – tpy-PSn-TIPNO-UPy 288 – [(tpy)Pt(H2O)]2þ 385 – [(tpy)Pt(His)]2þ complex 384 – tpy-PTFMS42-TIPNO 287 – tpy-PTFMS42-b-PS76-TIPNO 287 – [(tpy)Ru(bpy)(H2O)]2þ (bpy: 2,20 bipyridine) 351 – [(tpy)Ru(bpy)(L)]2þ complex (L: labile ligand) 355 – [(tpy)Ru(bpy)(L)](PF6)2 355 – [(tpy)Ru(bpy)(O)]2þ 353f. – [(tpy)Ru(bpy)(OH)]2þ 354 – [(tpy)Ru(bpy)]2þ–protein adduct 355 – mer-(tpy)RuCl3 361 – [(tpy)Ru(dppene)Cl]þ (dppene: cis-l,2-bis (diphenylphosphino)ethylene) 479 – [(tpy)Ru(dppz-COOH)(MeCN)]2þ 376 – [(tpy)Ru(dppz)(H2O)]2þ 351 – [(tpy)Ru(dppz–ODN)(H2O)]2þ 377 – [(tpy)Ru(L)]2þ (L: bpy or phen) 380 – [(tpy)Ru(PPh3)Cl2] 470 – [(tpy)Ru(pydic)] (pydic: pyridine-2,6dicarboxylate) 463ff. – 4,40 ,400 -tri(tert-butyl)-substituted 475 – (1H-1,2,3-triazol-1-yl)-substituted 25 2,20 :60 ,20 -terpyridine – 40 -(hetero)aryl-substituted 364 2,20 :60 ,200 -terpyridine (tpy) 2, 216, 358 – five-membered N-heterocycles replacing the outer pyridine rings 51 – 40 -functionalized 27 – – – – – – –

27 J l 2011 9 29 10

Index – synthesis 13ff. – symmetrically substituted on the outer pyridine rings 28 – vinyl-functionalized 243 2,20 :60 ,300 -terpyridine 364 2,20 :60 ,400 -terpyridine 364 terpyridine complex 411 – d8 late transition metal ion 320 – heavy d6 transition metal ion 350 – transition metal ion 364, 459ff. terpyridine derivative 19ff. – fullerene-substituted 427 – hydroxyl-functionalized 279 – liquid crystalline (LC) behaviour 29f. – 40 -substituted 24 – symmetrical and unsymmetrical ditopic 51 terpyridine ligand – 40 -(boronate ester)-substituted 27 – chiral 460 – star-shaped 105 – tri- and tetratopic 169 terpyridine macroligand – monotopic 272 terpyridine metal complex 319ff. – biological activity 320 – functional polymer 241ff. – p-conjugated polymer 199ff. terpyridine rotaxane 180 terpyridine transition metal ion complex 65ff. – catalytic application 459ff. – basic synthetic strategy 66ff. – binding enthalpy 67 – characterization tool 66ff. – stability constant 67 terpyridine-calixarenes 186 terpyridine-cyclodextrin 185 terpyridinophane macroligand 134 tetrabutylammonium Oxones 464 tetrakis(2,20 :60 ,200 -terpyridin-40 -oxymethyl) methane 105 tetrakis(terpyridine) 46, 310 – amine-bridged 25 tetrakis(terpyridine)-porphyrin 445 thermochromism 251 thieno[3,4-b]pyrazine-bridged bis (terpyridine) 47 40 -(thiophen-5-yl-2-phosphonic acid) 442 Thummel’s methodology 17 tin, see Sn TiO2 431ff., 447 – dye-sensitized nanoparticle 443 – nano-structured 431

BINDEX

TIPNO (2,2,5-trimethyl-4-phenyl-3-azahexane nitroxide) 284, 296 – TIPNO-PS240-[M(dmf)3Cl2] 295 – TIPNO-PS240-[M]-PEG230 295 TMPO (2,2,6,6-tetramethlypiperidine N-oxide) 494 40 -tolyl-2,20 :60 ,200 -terpyridine (ttpy) – [(ttpy)Mn(H2O)O]2 464 – [(ttpy)Ru(tppz)IrCl]2þ 91 – [(ttpy)Ru(tpy-Phn-typ)]2þ 91 – [(ttpy)Ru(tpy-Phn-tpy)Os(ttpy)]4þ 91 – [(ttpy)Ru(tpy-Phn-tpy)Rh(ttpy)]5þ 91 tpy, see terpyridine trefoil knot 182 4,40 ,400 -tri(tert-butyl)-2,20 :60 ,200 terpyridine 466 tri-N-oxide 460 triad – porphyrin-containing 92 1,3,4-triaryldihydropyrrole 487 triblock copolymer 283, 303 – A-[M]-B-[M]-A 272 – A-[Ru]-B-[Ru]-A 309 – coil-rod-coil 309f. – metallo-supramolecular 267 tridentate ligand – bis-cyclometalating 113 – C^N^N (mono-cyclometalating tridentate ligand) 445 – expanded bite angle 53 – monocyclometalating 113 – multitopic hydrazone-based 158 – N^N^N-ligand 339, 445 – phenanthroline-based ditopic 159 trinuclear complex 94ff. – heterometallic 92 – mixed-metal RuII–IrIII 100f. – strapped biphenyl 97 tris(phenanthroline) 149 tris(pyridine) 151 2,4,6-tris(pyrimidin-2-yl)-1,3,5-triazine 171 1,3,5-tris(2,20 :60 ,200 -terpyridin-40 -yl) benzene 39 1,4,7-tris[(2,20 :60 ,200 -terpyridin-5-yl)methyl]1,4,7-triazanonane 131 tris(terpyridine) 46, 104, 151, 310, 409 – amine-bridged 25 – star-shaped 38 trithiocyanato RuII terpyridine complex 260 tritopic ligand 169 – star-shaped 172 ttpy, see 40 -tolyl-2,20 :60 ,200 -terpyridine Typanosoma parasite 346f.

27 J l 2011 9 29 10

| 521

522

|

Index u ureido-pyrimidinone (UPy) moiety 280 v vanadium – [VO(O2)2]V complex 375 – [VO(O2)2]V mono(terpyridine) complex 375 – [VO(O2)2(tpy)]þ complex 375 – [VO(O2)2(tpy)(H2O)]þ 375 40 -vinyl-2,20 :60 ,200 -terpyridine 242 vinylbenzamide monomer – polymerization 246 viologen-like structure 219 vitamin H 381 Volmer–Weber growth mode 419 w water – oxidation 488 – reduction 484 white light-emitting diode (WOLED) 252 Wittig condensation 41ff.

BINDEX

z Ziessel-type bis(terpyridine) – dithieno[3,2-b:20 ,30 -d]phospholecontaining 35 – fluorene-containing 35 Ziessel-type mono(terpyridine) 37 Ziessel-type terpyridine 4 Ziessel-type 2,20 :60 ,200 -terpyridine 31 zinc – p-conjugated ZnII coordination polymer 209 – ZnIIion 248, 292 – ZnII bis(terpyridine) complex 366, 406 – ZnII coordination polymer 207 – ZnII copolymer 209 – ZnII metallopolymer 210 – ZnII mono(terpyridine) complex 366, 422 – ZnII-bis(terpyridine) coordination polymer 208 – {[Zn(bis(terpyridine))](PF6)2n} 208 – ZnII-porphyrin-bis(terpyridine) 151 zirconium – anchoring of terpyridine to a ZrIV phosphate monolayer 407

27 J l 2011 9 29 10

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