Fullerenes

Chemistry and Reactions

Andreas Hirsch, Michael Brettreich

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Andreas HirschMichael BrettreichFullerenes

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Fullerenes

Chemistry and Reactions

Andreas Hirsch, Michael Brettreich

Authors:

Prof. Dr. Andreas HirschDr. Michael Brettreich

Department of Organic ChemistryFriedrich Alexander Universityof Erlangen-NurembergHenkestrasse 4291054 ErlangenGermany

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V

Foreword

Ten years ago Prof. Andreas Hirsch, then finishing his “Habilitation” (an advancedjunior research academic rank used in some European countries) in Prof. Hanack’slaboratories in Tübingen, wrote THE seminal book about fullerene chemistry: “TheChemistry of the Fullerenes”. This book, in our group, has many yellowed pages andan abundance of fingerprints, attesting to its outstanding usefulness as a referencebook, or, better, a manual. The book has nine chapters covered in 203 pages,including an index.

In the intervening decade, though it would appear to some chemists in the worldthat the field of fullerene science had “matured”, the current version of the bookproves otherwise. In “Fullerenes – Chemistry and Reactions” Professor Hirsch hasexpanded the original 9 chapters to 14. The new chapters cover halogenation(Chap. 9), regiochemistry (Chap. 10), cluster modified fullerenes (Chap. 11),heterofullerenes (Chap. 12) and higher fullerenes (Chap. 13). The book is now 419pages long, over double the number of the original book! The last three chaptersgive a strong impression that the field of fullerene chemistry is still vibrant andreplete with challenges. Upon perusing its content, it is obvious that this book willbe equally, or more, useful than its antecedent.

The statement I made on the back cover of “The Chemistry of the Fullerenes” appliestoday to “Fullerenes – Chemistry and Reactions” with only very minor changes thattook place with time:

“Though the synthetic fullerenes have been with us for only three years, thescientific articles based on them number in the thousands. It is thereforeimportant and necessary to have a source of information which summarizesthe most important and fundamental aspects of the organic and organometallicchemistry of the fullerenes. Dr. Hirsch is already well known for his importantcontributions to the field and is uniquely qualified to write what will becomethe primary source of information for the practicing organic and organo-metallic chemist.This book is logically arranged and information is easy to retrieve. The stylelends itself to effortless reading and to learning more about the chemicalproperties of a family of molecules that constitute new building blocks fornovel architectures in the still expanding universe of synthetic chemistry.This book will not only be found in university libraries but also on book-shelves of chemists interested in the art and science of structure and propertymanipulation by synthesis. Dr. Hirsch’s “The Chemistry of the Fullerenes” is a

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genuine, single author book. It is the first, and so far the best, monographin the field. It stands out because its quality surpasses that of other multi-author compendia, that preceded it.”

September 2004 Fred Wudl

Foreword

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VII

Preface

It has now been more the ten years after the The Chemistry of the Fullerenes waspublished in 1994 by Thieme. This book became a classic in the field and was themajor source of information summarizing the fundamental aspects of organic andorganometallic chemistry of fullerenes. In the meantime many new achievementsin fullerene chemistry were accomplished. For example, unprecedented exohedraladdition reactions have been discovered and the knowledge how to design functio-nalized mono- and multiple adducts in a selective manner has continuouslyincreased. New types of fullerene derivatives such as heterofullerenes and clustermodified fullerenes capable of entrapping small guest molecules have been madeavailable for the first time. Thanks to the developments in preparative fullerenechemistry the performance of highly sophisticated derivatives was steadily improvedleading to systems with remarkable biological and materials properties. Fullerenechemistry continues to excite. So it came as no surprise that the discovery of thefullerenes by Curl, Kroto and Smalley was rewarded with the Noble prize in 1996.

Within the last years I was repeatedly asked by friends and colleagues: When willyou present the second edition? Of course the demand for an updated book wasobvious. However, I hesitated to take action because it was clear that preparing anupdate means more or less writing a new book. Then about two years ago afterWiley-VCH took over the Chemistry monographs of Thieme I had a nice conversa-tion with Elke Maase from Wiley-VCH who finally convinced me to write the secondedition. I could not resist any longer. But I needed help. Due to the many newachievements in the field there was no doubt to me that updating the field offullerene chemistry in a reasonable time is only possible if I would find a co-author.I was very happy that I could convince Michael Brettreich to participate in thisexciting project and become the co-author. Michael Brettreichs accomplishmentsin synthetic and supramolecular fullerene chemistry are outstanding and he wasthe best person I could think of to work with me on the book.

Just by comparing the number of publications on fullerenes (5500 until 1994and 21500 between 1994 and now) it was immediately clear that the whole literatureeven that focussing on fullerene chemistry cannot be covered in a single monograph.We had to carefully select and condense the material, refer to the many excellentreviews of specific subjects such as chemistry of higher fullerenes and supra-molecular chemistry of fullerenes.

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VIII

Also fascinating developments in various fields of applications such as opto-electronic-, materials- and medical applications are just barely touched. Our mainintention was to put the focus on molecular fullerene chemistry.

Compared to The Chemistry of the Fullerenes of 1994 the content as determinedfrom the number of pages was more than doubled. The predecessor served as agood basis for Chapters 1 to 8 which are The Parent Fullerenes, Reduction, NucleophilicAdditions, Cycloadditions, Hydrogenation, Radical Additions, Transition Metal ComplexFormation as well as Oxidation and Reactions with Electrophiles. However, anenormous amount of new material needed to be included making some of thesechapters, especially Chapter 3: Nucleophilic Additions, Chapter 4: Cycloadditions andChapter 7: Transition Metal Complex Formation to appear in new light. Moreover,four completely new chapters needed to be added because of recent ground breakingdevelopments. These are Chapter 10: Regiochemistry of Multiple Additions, Chapter 11:Cluster Modified Fullerenes, Chapter 12: Heterofullerenes and Chapter 13: Chemistryof Higher fullerenes. Moreover Halogenation which was part of Chapter 8: Oxidationand Reactions of Electrophiles, is now presented independently in Chapter 9, becauseof very fruitful accomplishments in this area. The final Chapter: Principles andPerspectives of Fullerene Chemistry (now Chapter 14) was also heavily rearrangedand extended, for example, by the comparison of exohedral- and endohedralreactivity as well as detailed analysis of structural aspects and aromaticity offullerenes. Finally, the entire artwork, especially the drawing of the molecules inschemes and figures was improved considerably. As a consequence, what we presentnow is indeed a new book, which is also expressed by changing the title to Fullerenes– Chemistry and Reactions.

The authors would like to acknowledge a number of persons, who were veryimportant to us during the process of writing the book. We thank Christian Huband Dr. Otto Vostrowsky for their help with preparing formulas, schemes and otherart work. We also thank Prof. Dr. Olga Boltalina, Dr. Otto Vostrowsky, Dr. NicosChronakis and Irene Brettreich for proof reading of the manuscript. It was a majorrelief that Erna Erhardt continuously assisted us in organizing the referencing andin the preparation of tables. The beautiful cover picture was designed by AdrianJung. Thank you very much. From Wiley-VCH we thank Elke Maase for the excellentcollaboration. We also thank her and many colleagues from the fullerene communityfor motivating us to write this book. Our specials thanks go to our wives and familiesfor their enormous patience.

Erlangen Andreas HirschAugust 2004

Preface

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IX

Preface of “The Chemistry of the Fullerenes”by Andreas Hirsch (1994)

“Buckminsterfullerene: Is it a real thing?” This question was in our heads thatevening in November 1990, when Fred Wudl came into the lab and showed us thefirst 50 mg sample of C60. Although at that time there was evidence for thegeometrical structures of this soccer ball shaped molecule and his bigger brotherthe “American football” C70, no one knew what the chemical and physical propertiesof these fascinating molecular allotropes of carbon would be. On that same nightwe started to work on C60 and three reversible one electron reductions of the carbonsphere were found – one more than already detected by the Rice group. We were allenthusiastic and Fred projected possible chemical transformations and proposedremarkable electronic properties of fulleride salts – a prediction that shortlythereafter became reality by the discovery of the superconductivity of K3C60 in theBell labs. It was one of the greatest opportunities in my scientific life, that Fredasked me to participate in the ball game and to investigate organic fullerenechemistry. He also encouraged me to carry on this work in Germany, once I hadfinished my postdoctoral time in his group in Santa Barbara. The pioneeringresearch of these early days is still a basis of my present work. Fullerene chemistry,which is unique in many respects, has meanwhile exploded. In a very short periodof time a huge number of chemical transformations of the “real thing” C60 andoutstanding properties of fullerene derivatives have been discovered. Manyprinciples of fullerene chemistry are understood. The fullerenes are now anestablished compound class in organic chemistry.

It is therefore the right time to give a first comprehensive overview of fullerenechemistry, which is the aim of this book. This summary addresses chemists, materialscientists and a broad readership in industry and the scientific community. Thenumber of publications in this field meanwhile gains such dimensions that fornonspecialists it is very difficult to obtain a facile access to the topics of interest. Inthis book, which contains the complete important literature, the reader will find allaspects of fullerene chemistry as well as the properties of fullerene derivatives.After a short description of the discovery of the fullerenes all methods of theproduction and isolation of the parent fullerenes and endohedrals are discussed indetail (Chapter 1). In this first chapter the mechanism of the fullerene formation,the physical properties, for example the molecular structure, the thermodynamic,electronic and spectroscopic properties as well as solubilities are also summarized.This knowledge is necessary to understand the chemical behavior of the fullerenes.

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X

The description of the chemistry of the fullerenes themselves is organized accordingto the type of chemical transformation, starting from the reduction (Chapter 2),nucleophilic additions (Chapter 3), cycloadditions (Chapter 4), hydrogenation(Chapter 5), radical additions (Chapter 6), transition metal complex formation(Chapter 7) through oxidation and reactions with electrophiles (Chapter 8). Mostof the examples presented in these chapters are reactions with C60, since only verylittle work has been published on C70 and the higher fullerenes. It is the aim toprovide an understanding of the basic characteristics of fullerene chemistry. Thisis achieved by a comparative description of both experimental and theoreticalinvestigations in each of these chapters. It is also emphasized in each chapterwherever a reaction type leads to a fullerene derivative with a potential for practicalapplication. In the last chapter (Chapter 9) the emerging principles of fullerenechemistry, such as reactivity and regiochemistry, are evaluated and summarized.The fullerene chemistry is still in an early stage of development. For syntheticchemists a lot of challenging work remains to be done. A prediction of the futuredirections of fullerene chemistry is therefore also given in Chapter 9 and finally,since from the beginning of the fullerene era many practical uses have beenproposed, perspectives for applications are evaluated.

Writing this book prevented me for some time from carrying out practical workwith my own hands. Nevertheless, since I have had co-workers like Thomas Grösser,Iris Lamparth, Andreas Skiebe and Antonio Soi, the experimental work on fullerenesin my lab has proceeded, even with much success. I am also indebted to myco-workers for their assistance in preparing figures and illustrations presented inthis book. I thank Dr. L. R. Subramanian for reading the entire manuscript and foruseful suggestions.

I thank Dr. J. P. Richmond for the excellent co-operation, which enabled the fastrealization of this book project.

I am very grateful to my teacher Prof. Dr. Dr. h. c. M. Hanack, who has beensupporting me for many years and who provided the starting conditions for writingthis book.

It was Prof. Dr. F. Wudl who introduced me to the art of fullerene chemistry andwho inspired me to continue with this work, for which I want to thank him verymuch. On reviewing the manuscript he came up with cogent comments andsuggestions.

My special thanks go to my wife Almut, who, despite the fact that for severalmonths she was very often deprived of the company of her husband, respondedwith warmth and understanding.

Tübingen, May 1994 Andreas Hirsch

Preface of “The Chemistry of the Fullerenes” by Andreas Hirsch (1994)

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XI

Contents

Foreword V

Preface VII

Preface of “The Chemistry of the Fullerenes” by Andreas Hirsch (1994) IX

Abbreviations XVII

1 Parent Fullerenes 11.1 Fullerenes: Molecular Allotropes of Carbon 11.2 Discovery of the Fullerenes 41.3 Fullerene Production 61.3.1 Fullerene Generation by Vaporization of Graphite 61.3.1.1 Resistive Heating of Graphite 61.3.1.2 Arc Heating of Graphite 91.3.1.3 Solar Generators 101.3.1.4 Inductive Heating of Graphite and Other Carbon Sources 101.3.2 Fullerene Synthesis in Combustion 111.3.3 Formation of Fullerenes by Pyrolysis of Hydrocarbons 111.3.4 Generation of Endohedral Fullerenes 121.3.5 Total Synthesis Approaches 171.3.6 Formation Process 191.4 Separation and Purification 241.5 Properties 291.5.1 Structures 291.5.2 Physical and Spectroscopic Properties 33

References 39

2 Reduction 492.1 Introduction 492.2 Fulleride Anions 492.3 Reductive Electrosynthesis 552.3.1 Electrocrystallization 552.3.2 Electrophilic Additions to Fulleride Anions 572.4 Reduction with Metals 582.4.1 Alkali Metal Fullerides 58

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XII Contents

2.4.1.1 Generation in Solution and Quenching Experiments 582.4.1.2 Synthesis and Properties of Alkali Metal Fulleride Solids 592.4.2 Alkaline Earth Metal Fullerides 632.4.3 Reduction with Mercury 632.5 Reduction with Organic Donor Molecules 64

References 67

3 Nucleophilic Additions 733.1 Introduction 733.2 Addition of Carbon Nucleophiles 733.2.1 Hydroalkylation and Hydroarylation of C60 and C70 733.2.2 Cyclopropanation of C60 and C70 803.2.3 Addition of Cyanide 863.3 Addition of Amines 873.4 Addition of Hydroxide and Alkoxides 913.5 Addition of Phosphorus Nucleophiles 923.6 Addition of Silicon and Germanium Nucleophiles 933.7 Addition of Macromolecular Nucleophiles – Fullerene Polymers 93

References 96

4 Cycloadditions 1014.1 Introduction 1014.2 [4+2] Cycloadditions 1014.3 [3+2] Cycloadditions 1194.3.1 Addition of Diazomethanes, Diazoacetates and Diazoamides 1194.3.2 Addition of Azides 1344.3.3 Addition of Trimethylenemethanes 1384.3.4 Addition of Azomethine Ylides 1414.3.5 Addition of Nitrile Oxides and Nitrile Imines 1514.3.6 Addition of Sulfinimides and Thiocarbonyl Ylides 1534.3.7 Addition of Carbonyl Ylides 1554.3.8 Addition of Nitrile Ylides and Isonitriles 1564.3.9 Addition of Disiliranes 1574.4 [2+2] Cycloadditions 1584.4.1 Addition of Benzyne 1584.4.2 Addition of Enones 1594.4.3 Addition of Electron-rich Alkynes and Alkenes 1614.4.4 Addition of Ketenes and Ketene Acetals 1644.4.5 Addition of Quadricyclane 1664.4.6 Photodimerization of C60 1664.5 [2+1] Cycloadditions 1684.5.1 Addition of Carbenes 1684.5.2 Addition of Nitrenes 1704.5.3 Addition of Silylenes 172

References 172

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XIIIContents

5 Hydrogenation 1855.1 Introduction 1855.2 Oligohydrofullerenes C60Hn and C70Hn (n = 2–12) 1865.2.1 Hydrogenation via Hydroboration and Hydrozirconation 1865.2.2 Reduction with Reducing Metals (Zn/Cu) 1885.2.3 Hydrogenation with Hydrazine and with Organic Reducing Agents 1915.2.4 Theoretical Investigations 1915.3 Polyhydrofullerenes C60Hn and C70Hn (n = 14–60) 1975.3.1 Birch–Hückel Reduction 1975.3.2 Reduction with Zn/HCl 1985.3.3 Transfer Hydrogenation of C60 and C70 1995.3.4 Reduction with Molecular Hydrogen 2025.3.5 Theoretical Investigations 203

References 208

6 Radical Additions 2136.1 Introduction 2136.2 ESR Investigations of Radical Additions 2136.2.1 Addition of Single Radicals 2136.2.2 Multiple Radical Additions 2206.3 Addition of Tertiary Amines 2236.4 Photochemical Reaction with Silanes 2256.5 Metalation of C60 with Metal-centered Radicals 2276.6 Addition of bis(Trifluoromethyl)nitroxide 228

References 228

7 Transition Metal Complex Formation 2317.1 Introduction 2317.2 (η2-C60) Transition Metal Complexes 2317.3 Multinuclear Complexes of C60 2417.4 Hydrometalation Reactions 2457.5 Organometallic Polymers of C60 246

References 248

8 Oxidation and Reactions with Electrophiles 2518.1 Introduction 2518.2 Electrochemical Oxidation of C60 and C70 2518.3 Oxygenation 2538.4 Osmylation 2578.5 Reactions with Strong Oxidizing Reagents and Acids 2608.6 Reactions with Lewis Acids and Fullerylation of Aromatics and

Chloroalkanes 263References 264

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XIV Contents

9 Halogenation 2679.1 Introduction 2679.2 Fluorination 2679.2.1 Direct Fluorination with F2 2699.2.2 Fluorination with Noble Gas Fluorides and Halogen Fluorides 2719.2.3 Reactions with Metal Fluorides 2719.2.4 Reactions of Fluorofullerenes 2769.3 Chlorination 2799.3.1 Synthesis and Properties of Chlorofullerenes 2799.3.2 Reactions of Chlorofullerenes 2809.4 Bromination 2829.5 Reaction with Iodine 284

References 285

10 Regiochemistry of Multiple Additions 28910.1 Introduction 28910.2 Addition of Segregated Addends – The Inherent Regioselectivity 28910.2.1 Subsequent Cycloadditions to [6,6]-double Bonds 29110.2.2 Adducts with an Inherently Chiral Addition Pattern 30210.2.3 Vinylamine Mode 30610.2.4 Cyclopentadiene Mode 30710.3 Concepts for Regio- and Stereoselective Multiple Functionalization

of C60 31010.3.1 Template Mediation Approaches 31010.3.1.1 Th-Symmetrical Hexakisadducts 31010.3.1.2 Mixed Hexakisadducts 31410.3.2 Topochemically Controlled Solid-state Reactions 32510.3.3 Tether-directed Remote Functionalization of C60 32610.3.3.1 Higher Adducts with the Addends Bound in Octahedral Sites 32610.3.3.2 Multiple Cyclopropanation of C60 by Tethered Malonates 32910.3.3.3 Highly Regioselective Cyclopropanation of C60 with

Cyclo-[n]-alkylmalonates 33410.3.3.4 Double Diels–Alder Tethers 33810.3.3.5 Other Bisfunctional Tethers 340

References 341

11 Cluster Modified Fullerenes 34511.1 Introduction 34511.2 Cluster Opened Fullerene Derivatives 34511.2.1 “Fulleroids”: Bridged Adducts with Open Transannular Bonds 34511.2.2 Ring-enlargement Reactions of Bisfulleroids 35011.2.3 Cluster Opened Lactams, Ketolactams and Lactones 35311.3 Quasi-fullerenes 35511.4 Outlook 356

References 357

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XV

12 Heterofullerenes 35912.1 Introduction 35912.2 Synthesis of Nitrogen Heterofullerenes from Exohedral Imino Adducts

of C60 and C70 36012.2.1 Synthesis of bis(Aza[60]fullerenyl) (C59N)2 36012.2.2 Synthesis of bis(Aza[70]fullerenyl) (C69N)2 36312.3 Chemistry of Azafullerenes 36612.4 Outlook 371

References 373

13 Chemistry of Higher Fullerenes 37513.1 Introduction 37513.2 Exohedral Reactivity Principles 37513.3 Adducts of C70 37713.3.1 Monoadducts 37713.3.2 Multiple Adducts 37813.4 Adducts of C76, C78 and C84 380

References 380

14 Principles and Perspectives of Fullerene Chemistry 38314.1 Introduction 38314.2 Reactivity 38314.2.1 Exohedral Reactivity 38314.2.2 Endohedral Reactivity 39014.3 Regiochemistry of Addition Reactions 39314.3.1 Bond Length Alternation – Preferred Additions to [6,6]-Double Bonds 39314.3.2 1,2-Additions with Preferred e- and cis-1 Modes: the trans-1 Effect 39914.3.3 Vinyl Amine Mode 40014.3.4 Cyclopentadiene Mode 40114.3.5 Further Modes 40114.4 Aromaticity of Fullerenes 40114.4.1 Structural Criteria 40214.4.2 Energetic Criteria 40314.4.3 Reactivity Criteria 40314.4.4 Magnetic Criteria 40314.4.5 2(N + 1)2-Rule for Spherical Aromaticity 40514.5 Seven Principles of Fullerene Chemistry: a Short Summary 40614.6 The Future of Fullerene Chemistry 40714.7 Fullerenes as Building Blocks for Molecular Engineering

(Nanotechnology) and Practical Applications 409References 412

Subject Index 417

Contents

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Abbreviations

a.u. arbitrary unitsAIBN 2,2′-azobisisobutyronitrilALS amyotrophic lateral sclerosisAPCI atmospheric pressure chemical ionizationATP adenosintriphosphateBN boron nitrideBP biphenylBtOH 1H-benzotriazolCD circular dichroismCI chemical ionizationCSA camphor sulfonic acidCT charge transferCV cyclic voltammetryCVM chemical vapor modificationDABCO 1,4-diazabicyclo[2.2.2]octaneDBU 1,8-diazabicyclo[5.4.0]undec-7-eneDCC N,N′-dicyclohexylcarbodiimideDCI desorptive chemical ionizationDDQ 2,3-dichloro-5,6-dicyanobenzoquinoneDFT density functional theoryDIBAL-H diisobutylaluminium-hydrideDIOP 2,3-O-isopropylidene-2,3-dihydroxy-

1,4-bis(diphenylphosphanyl)butaneDMA 9,10-dimethylanthraceneDMAD dimethylacetylenedicarboxylateDMAP 4-(dimethylamino)pyridineDMB dimethoxybenzeneDMF dimethylformamideDMSO dimethylsulfoxideDNA desoxyribonucleid acidDOS density of statesDPIF 1,3-diphenylisobenzofuranedppb 1,2-bis(diphenylphosphino)benzeneDPPC dipalmitoylphosphatidylcholine

XVII

X Abbreviations

dppe 1,2-bis(diphenylphosphino)ethanedppf 1,1′-bis(diphenylphosphino)-ferroceneDSC digital scanning calorimetryEDCI N′-(3′-dimethylaminopropyl)-N-ethylcarbodiimide

hydrochlorideEI electron impactEPR electron paramagnetic resonanceESCA electron spectroscopy for chemical analysisESR electron spin resonanceFAB fast atom bombardmentFD field desorptionFT-ICR fourier transform ion cyclotron resonanceFVP flash-vacuum pyrolysisGPC gel permeation chromatographyHETCOR heteronuclear chemical shift correlationHIV human immunodeficiency virusHMPA hexamethyl phosphoric acidHOMO highest occupied molecular orbitalHPLC high pressure liquid chromatographyIC ion chromatographyICR ion cyclotron resonanceINADEQUATE incredible natural abundance double quantum transition

experimentIPR isolated pentagon ruleIR infraredITO indium-tin-oxideLB Langmuir-BlodgettLDA Lithium diisopropylamideLESR light-induced ESR measurementLUMO lowest unoccupied molecular orbitalMALDI matrix-assisted laser desorption ionizationMCPBA m-chloroperoxybenzoic acidMEM methoxy ethoxy methylMO molecular orbitalMRI magnetic resonance imagingMS mass spectrometryNCS N-chlorosuccinimideNICS nucleus-independent chemical shiftNIR near infraredNMA+ N-methylacridinium-hexafluorophosphateNMR nuclear magnetic resonanceODCB ortho-dichlorobenzeneOL optical limitingPAH polycyclic aromatic hydrocarbonPCBA [6,6]-phenyl-C61-butyric acid

VIII

XIAbbreviations

PCC pyridinium chlorochromatePET photoinduced electron transferPOAV π orbital axis vectorPOM polarizing optical microscopyPPV poly-para-phenylen-vinylenePVK poly(N-vinylcarbazole)QCM quartz crystal microbalanceRE resonance energyRETOF reflectron time of flightSAM self-assembled monolayerSEC size-exclusion chromatographySET single electron transferSWNT single-walled carbon nanotubeTBA tetrabutylammoniumTCE trichloroethyleneTCNE tetracyanoethyleneTCNQ tetracyano-p-quinodimethaneTDAE tetrakis(dimethylamino)ethyleneTEMPO 2,2,6,6-tetramethylpiperidin-1-yloxylTFA trifluoroacetic acidTGA thermal gravimetric analysisTHA tetrahexylammoniumTHF tetrahydrofuranTMEDA N,N,N′,N′-tetramethylene diamineTMM trimethylenemethaneTOF time of flightTosOH 4-toluenesulfonic acidTPP tetraphenylporphyrinTPP+ triphenylpyriliumtetrafluoroborateTTF tetrathiafulvaleneUHV-STM ultra-high vacuum scanning tunneling microscopyUV ultravioletUV/Vis ultraviolet/visibleVB valence bondVis visibleXPS X-ray photoelectron spectroscopy

X

1

1Parent Fullerenes

1.1Fullerenes: Molecular Allotropes of Carbon

For synthetic chemists, who are interested in the transformation of known and thecreation of new matter, elemental carbon as starting material once played a minorrole. This situation changed dramatically when the family of carbon allotropesconsisting of the classical forms graphite and diamond became enriched by thefullerenes. In contrast to graphite and diamond, with extended solid state structures,fullerenes are spherical molecules and are soluble in various organic solvents, animportant requirement for chemical manipulations.

Fullerenes are built up of fused pentagons and hexagons. The pentagons, absentin graphite, provide curvature. The smallest stable, and also the most abundantfullerene, obtained by usual preparation methods is the Ih-symmetrical Buck-minsterfullerene C60 (Figure 1.1). Buckminsterfullerene has the shape of a soccerball. The next stable homologue is C70 (Figure 1.2) followed by the higher fullerenesC74, C76, C78, C80, C82, C84, and so on. The building principle of the fullerenes is aconsequence of the Euler theorem, which says that for the closure of each sphericalnetwork of n hexagons, 12 pentagons are required, with the exception of n = 1.

Compared to small two-dimensional molecules, for example the planar benzene,the structures of these three-dimensional systems are aesthetically appealing. Thebeauty and the unprecedented spherical architecture of these molecular cagesimmediately attracted the attention of many scientists. Indeed, Buckminster-fullerene C60 rapidly became one of the most intensively investigated molecules.For synthetic chemists the challenge arose to synthesize exohedrally modifiedderivatives, in which the properties of fullerenes can be combined with those ofother classes of materials. The following initial questions concerned the derivatiza-tion of fullerenes: What kind of reactivity do the fullerenes have? Do they behavelike a three-dimensional “superbenzene”? What are the structures of exohedralfullerene derivatives and how stable are they?

The IUPAC method of naming Buckminsterfullerene given below is too lengthyand complicated for general use [1]:

Hentriacontacyclo[29.29.0.0.2,14.03,12.04,59.05,10.06,58.07,55.08,53.09,21.011,20.013,18.015,30

.016,28.017,25.019,24.022,52.023,50.026,49.027,47.029,45.032,44.033,60.034,57.035,43.036,56.037,41

.038,54.039,51.040,48.042,46]hexaconta-1,3,5(10),6,8,11,13(18),14,16,19,21,23,25,27,

2 1 Parent Fullerenes

29(45),30,32,(44),33,35(43),36,38(54),39(51),40(48),41,46,49,52,55,57,59-triaconta-ene.

Furthermore, the enormous number of derivatives, including the multitude ofpossible regioisomers, available by chemical modifications requires the introductionof a simple nomenclature. According to the latest recommendation, the icosahedralBuckminsterfullerene C60 was named as (C60-Ih)[5,6]fullerene and its higherhomologue C70 as (C70-D5h)[5,6]fullerene [2, 3]. The parenthetical prefix gives thenumber of C-atoms and the point group symbol; the numbers in brackets indicatethe ring sizes in the fullerenes. Fullerenes involving rings other then pentagonsand hexagons are conceptually possible (quasi-fullerenes [4]). The identification ofa well defined and preferably contiguous helical numbering pathway is the basisfor the numbering of C-atoms within a fullerene. Such a numbering system isimportant for the unambiguous description of the multitude of possible regio-isomeric derivatives formed by exohedral addition reactions. A set of rules for theatom numbering in fullerenes has been adopted [2, 3]. The leading rule (Fu-3.1.1) is:

Figure 1.1 Schematic representations of C60. (A) ball and stick model,(B) space filling model, (C) VB formula, (D) Schlegel diagram withnumbering of the C-atoms (according to [4]).

31.1 Fullerenes: Molecular Allotropes of Carbon

Proper rotation axes (Cn) are examined in sequence from the highest-order tothe lowest-order axis, until at least one contiguous helical pathway is found thatbegins in a ring through which a proper rotation axis passes, at the end a bondbisected by a proper rotation axis, or at an atom through which a proper rotationaxis passes. Numbering begins at the end of such a contiguous helical pathway,and the corresponding axis is called the “reference axis”.

This system allows also for the indication of the absolute configuration ofinherently chiral fullerenes by introducing the stereodescriptors (f,sC) and (f,sA)(“f” = fullerene; “s” = systematic numbering; “C” = clockwise; “A” = anti-clockwise).

In another nomenclature recommendation it was suggested that fullerenes benamed in the same way as annulenes, for which the number of C-atoms is indicatedin square brackets in front of the word [4]. For fullerenes the number of C-atoms isaccompanied by the point group symmetry and by the number of the isomer (usingcapital Roman) in cases were there are more than one. This is especially importantfor higher fullerenes. Thus, for Buckminsterfullerene the full description is

Figure 1.2 Schematic representations of C70. (A) ball and stick model,(B) space filling model, (C) VB formula, (D) Schlegel diagram withnumbering of the C-atoms (according to [4]).

4 1 Parent Fullerenes

[60-Ih]fullerene and for C70 (Figure 1.2) [70-D5h]fullerene. In most cases furthersimplification to [60]fullerene and [70]fullerene or even C60 and C70 is made, sincethere are no other stable isomers of these fullerenes. An alternative numbering ofC-atoms to that pointed out above [2, 3] is also based on a contiguous spiral fashionbut numbers the bond of highest reactivity as 1,2 (Figure 1.1) [4]. For [60]fullerenethese are the bonds at the junction of two hexagons ([6,6]-bonds). Since the chemistryof [70]fullerene has many similarities to that of [60]fullerene, it is advantageous ifthe numbering scheme for [70]fullerene parallels that of [60]fullerenes, which isindeed possible (Figure 1.2) [4].

Valence bond (VB) formulas or Schlegel diagrams are useful for simple schematicrepresentations of fullerenes and their derivatives. VB formulas are mostly usedfor parent fullerenes or for derivatives with a few modifications of the cage structureonly. A Schlegel diagram shows each C-atom of the fullerene, which is flattenedout in two dimensions. This model is suitable for considering polyadducts, forexample, polyhydrofullerenes.

The main type of chemical fullerene derivatizations are addition reactions.Regardless of the relatively many possible reaction sites, addition reactions show aremarkable regioselectivity, especially when the number of addends is small. Thisis another fulfilled requirement, which makes these molecular spheres excitingobjects for synthetic chemists.

1.2Discovery of the Fullerenes

In 1966 Deadalus alias D.E.H. Jones considered the possibility of making largehollow carbon cages, structures now called giant fullerenes [5, 6]. This suggestionelicited no reaction from the scientific community. Four years later, in 1970,simulated by the synthesis of the bowl shaped corannulene 1 [7], Osawa firstproposed the spherical Ih-symmetric football structure for the C60 molecule (2)[8, 9]. During his efforts to find new three-dimensional superaromatic π-systems,he recognized corannulene to be a part of the football framework. Subsequently,some theoretical papers of other groups appeared, in which inter alia Hückelcalculations on C60 were reported [10–13].

In 1984 it was observed that, upon laser vaporization of graphite, large carbon-only clusters Cn with n = 30–190 can be produced [14]. The mass distribution ofthese clusters was determined by time-of-flight mass spectrometry. Only ions with

5

even numbers of carbon atoms were observable in the spectra of large carbon clusters(n ≥ 30). Although C60 and C70 were among these clusters, their identity was notrecognized. The breakthrough in the experimental discovery of the fullerenes camein 1985 [15] when Kroto visited the Rice University in Houston. Here, Smalley andco-workers developed a technique [16] for studying refractory clusters by massspectrometry, generated in a plasma by focusing a pulsed laser on a solid, in thiscase graphite. Kroto and Smalley’s original goal was to simulate the conditionsunder which carbon nucleates in the atmospheres of red giant stars. Indeed, thecluster beam studies showed that the formation of species such as the cyanopolyynesHC7N and HC9N, which have been detected in space [17, 18], can be simulated bylaboratory experiments [19]. These studies found that, under specific clusteringconditions, the 720 mass peak attributed to C60, and to a lesser extent the peakattributed to C70, exhibits a pronounced intensity in the spectra (Figure 1.3).Conditions could be found for which the mass spectra were completely dominatedby the C60 signal. Kroto and Smalley immediately drew the right conclusion ofthese experimental findings. The extra stability of C60 is due to its spherical structure,which is that of a truncated icosahedron with Ih symmetry [15]. This molecule wasnamed after the architect Buckminster Fuller, whose geodesic domes obey similarbuilding principles. Retrospectively, the enhanced intensity of the peak of C70, whichis also a stable fullerene, became understandable as well. Although Buckminster-fullerene (C60) was discovered, a method for its synthesis in macroscopic amountswas needed.

This second breakthrough in fullerene research was achieved by Krätschmerand Huffman [20]. Their intention was to produce laboratory analogues of interstellardust by vaporization of graphite rods in a helium atmosphere [21]. They observedthat, upon choosing the right helium pressure, the IR-spectrum of the soot,generated by the graphite vaporization, shows four sharp stronger absorptions,

Figure 1.3 Time-of-flight mass spectrum of carbon clustersproduced by laser vaporization of graphite under the optimumconditions for observation of a dominant C60 signal [15].

1.2 Discovery of the Fullerenes

6 1 Parent Fullerenes

together with those of the continuum of regular soot (Figure 1.4) [22]. Theseabsorptions were close to the positions predicted by theory for Buckminsterfullerene[23]. The fullerenes were then isolated from the soot by sublimation or extractionwith benzene. This allowed the verification of their identity by spectroscopic andcrystallographic methods as well as by control experiments with 13C-enrichedmaterial. Along with Buckminsterfullerene C60, higher homologues are alsoobtained by this technique. Fullerenes were then available for the scientificcommunity.

1.3Fullerene Production

1.3.1Fullerene Generation by Vaporization of Graphite

1.3.1.1 Resistive Heating of GraphiteMacroscopic quantities of fullerenes were first generated by resistive heating ofgraphite [20]. This method is based on the technique for the production ofamorphous carbon films in a vacuum evaporator [24]. The apparatus (Figure 1.5)that Krätschmer and Fostiropoulos used for the first production of fullerenesconsisted of a bell jar as recipient, connected to a pump system and a gas inlet. Inthe interior of the recipient two graphite rods are kept in contact by a soft spring.Thereby, one graphite rod is sharpened to a conical point, whereas the end of theother is flat. The graphite rods are connected to copper electrodes.

Figure 1.4 IR-spectra of soot particles produced by evaporationof graphite under different helium quenching gas pressures. Theoccurrence of the four additional sharp peaks at elevated heliumpressures turned out to originate from [60-Ih]fullerene (C60) [20].

7

To produce soot, the apparatus is repeatedly evacuated and purged with heliumand finally filled with about 140 mbar of helium. After applying a voltage, the electriccurrent passing through the rods dissipates most of its Ohmic power heating atthe narrow point of contact. This leads to a bright glowing in this area at 2500–3000 °C. Simultaneously, smoke develops at the contact zone, being transportedaway by convection and collected on the cooler areas (bell jar and smoke catcher) ofthe apparatus. The evaporation of the graphite is most efficient at the sharpenedend of the rod. After the reaction is over, fullerenes are extracted from the soot, forexample with toluene, in about 10–15% yield.

Modifications of this type of fullerene reactor are gravity feed generators [25–27].The advantage of these generators is their simple construction principle. This,together with their low costs, makes them attractive for synthetic chemists.A schematic representation of such a simple benchtop reactor, developed by Wudl[27] is given in Figure 1.6. A thin graphite rod (3 mm), guided by a copper sleeve,with a sharpened tip is placed on a thick rod (12 mm). A commercially available arcwelder serves as power supply. After applying a current (AC or DC) of about 40–60A, only the material of the thin rod evaporates, whereupon it slips downward, guidedby the copper sleeve that keeps the electrical contact. After a few minutes the rod isconsumed to the point that it can not any longer make contact with the 12 mm rod.The power is then shut off. Based on evaporated graphite, fullerene yields of5–10% are obtained [27, 28].

Figure 1.5 Fullerene generatororiginally used by Krätschmer [20].

Figure 1.6 Simple benchtop reactor developedby Wudl [27]. Helium supply and connection toa vacuum system (A), Pyrex bell jar (B), graphiterod (3 mm) (C), graphite rod (12 mm) (D),copper electrode (E), manometer (F).

1.3 Fullerene Production

8 1 Parent Fullerenes

The buffer gas cools the plasma by collisions with the carbon vapor. The gas hasto be inert, to prevent reactions with smaller carbon clusters or atoms, initiallyformed by the evaporation. Using N2 dramatically reduces the yield of fullerenes,presumably due to nitrogen atoms, formed in the hot zone of the generator, reactingwith the carbon fragments [28]. The highest yields of fullerenes are obtained ifhelium is used as buffer gas. Also, the concentration of the buffer gas is important(Figure 1.7), with maximum yields obtained between 140 and 160 mbar [28]. Witha very low buffer gas pressure the carbon radicals diffuse far from the hot zone andthe clusters continue to grow in an area that is too cool to allow an annealing tospherical carbon molecules. Conversely, if the pressure of the buffer gas is toohigh, a very high concentration of carbon radical results in the hot reaction zone.This leads to a fast growth of particles far beyond 60 C-atoms and the annealingprocess to fullerenes cannot compete [29].

During these resistive heating procedures the formation of slag, depositing on thethicker graphite rod, can be observed after some time of evaporation. As long as thisvapor-deposited boundary layer remains between the two electrodes in a sufficientlythick and resistive form, the electrical power continues to be dissipated just in thissmall zone, and carbon vaporization from the end of the thin graphite rod proceedsefficiently [30]. Thus, the formation of such a resistive layer may be an importantrequirement for the continuation of smoke production. In the beginning of thereaction this was guaranteed by the sharpened thin graphite rod (heat dissipationin this small resistive zone). For graphite rods, with diameters of 6 mm or greater,the resistive layer does not remain sufficiently resistive and the entire length of thegraphite rod eventually begins to glow. This causes inefficient evaporation of carbonfrom the center of the rod. Therefore, only comparatively thin graphite rods can beused for efficient fullerene production by the resistive heating technique.

Figure 1.7 Dependence of the fullerene yield on the helium gas pressure in the fullerene generator.