Discrete Fulleride Anions and Fullerenium Cations

UC Riverside UC Riverside Previously Published Works Title Discrete fulleride anions and fullerenium cations. Permalink https://escholarship.org/uc/item/60b5m71z Journal Chemical reviews, 100(3) ISSN 0009-2665 Authors Reed, CA Bolskar, RD Publication Date 2000-03-01 DOI 10.1021/cr980017o Peer reviewed eScholarship.org Powered by the California Digital Library University of California Chem. Rev. 2000, 100, 1075−1120 1075 Discrete Fulleride Anions and Fullerenium Cations Christopher A. Reed* and Robert D. Bolskar Department of Chemistry, University of CaliforniasRiverside, Riverside, California 92521-0403 Received June 22, 1999 Contents iii. Anisotropy 1100 iv. Problem of the Sharp Signals 1101 I. Introduction, Scope, and Nomenclature 1075 v. Origins of Sharp Signals 1102 II. Electrochemistry 1078 vi. The C O Impurity Postulate 1103 A. Reductive Voltammetry 1078 120 vii. The Dimer Postulate 1104 B. Oxidative Voltammetry 1079 2- C. Features of the C60 EPR Spectrum 1106 III. Synthesis 1079 3- D. Features of the C60 EPR Spectrum 1107 A. Chemical Reduction of Fullerenes to 1079 4- 5- Fullerides E. Features of C60 and C60 EPR Spectra 1107 i. Metals as Reducing Agents 1080 F. EPR Spectra of Higher Fullerides 1107 ii. Coordination and Organometallic 1080 G. EPR Spectra of Fullerenium Cations 1108 Compounds as Reducing Agents X. Chemical Reactivity 1108 iii. Organic/Other Reducing Agents 1081 A. Introduction 1108 B. Electrosynthesis of Fullerides 1081 B. Fulleride Basicity 1109 C. Chemical Oxidation of Fullerenes to 1081 C. Fulleride Nucleophilicity/Electron Transfer 1109 Fullerenium Cations D. Fullerides as Intermediates 1110 IV. Electronic (NIR) Spectroscopy 1082 E. Fullerides as Catalysts 1111 A. Introduction 1082 F. Fullerides as Ligands 1111 n- B. C60 Fullerides 1082 G. Fullerenium Cations 1112 C. C70 and Higher Fullerenes 1085 XI. Conclusions and Future Directions 1113 D. Fullerenium Cations 1086 XII. Acknowledgments 1114 E. Diffuse Interstellar Bands 1086 XIII. References 1114 V. Vibrational Spectroscopy 1087 A. Infrared Spectroscopy 1087 B. Raman Spectroscopy 1087 I. Introduction, Scope, and Nomenclature VI. X-ray Crystallography 1088 The electron-accepting ability of C60, the archetypal A. Introduction 1088 fullerene, is its most characteristic chemical property. 2- B. [PPN]2[C60] and Related C60 Structures 1088 It is a natural consequence of electronic structure and - C. C60 Structures 1090 was anticipated in early molecular orbital calcula- 3- 1 D. C60 Structures 1091 tions which place a low-lying unoccupied t1u level 2-4 E. Comparison of Discrete and Extended 1091 about 2 eV above the hu HOMO: Structures VII. Magnetic Susceptibility and Spin States 1092 VIII. NMR Spectroscopy 1093 A. Introduction 1093 B. 13C NMR Data in Solution 1094 C. Interpretation of Solution NMR Data 1095 D. 13C NMR Data in the Solid State 1096 E. Knight Shift in A3C60 1098 F. 3He NMR of Endohedral Fullerides 1098 G. 13C NMR of Derivatized Fullerenes 1099 Early in the gas-phase investigations of fullerenes, H. 13C NMR of Fullerenium Cations 1099 the electron affinity of C60 was measured and found IX. EPR Spectroscopy 1099 to be high (2.69 eV).5-7 When the macroscopic era of A. Introduction 1099 C60 chemistry began in 1990, this property was soon - B. Features of the C60 Spectrum 1099 found to translate into the solution phase.8 In a i. The Low g Value 1100 rather remarkable cyclic voltammogram (see Figure ii. Temperature Dependence of the Line 1100 1), the reversible stepwise addition of up to six 9,10 Width (∆Hpp) electrons was soon demonstrated electrochemically. 10.1021/cr980017o CCC: $35.00 © 2000 American Chemical Society Published on Web 02/16/2000 1076 Chemical Reviews, 2000, Vol. 100, No. 3 Reed and Bolskar Chris Reed received his education in his native land at The University of Auckland, New Zealand. His Ph.D. studies were on iridium chemistry with Warren Roper F.R.S. His postdoctoral work was on picket-fence porphyrins with Jim Collman at Stanford University. He joined the faculty at the University of Southern California in 1973, becoming Professor in 1982. Figure 1. Cathodic cyclic and differential pulse voltam- Since 1998 he has been Distinguished Professor of Chemistry at the mograms of C60 in CH3CN/toluene at -10 °C showing University of California, Riverside. His research interests include reactive 6- successive reversible reductions to C60 . (Reprinted with cation and superacid chemistry with inert carborane counterions, ionic permission from ref 9. Copyright 1992 American Chemical liquids, bioinorganic chemistry, spin-coupling phenomena, fullerene Society.) chemistry, and new carbon- or porphyrin-based materials. He is a Sloan Fellow, a Dreyfus Teacher−Scholar Awardee, and a Guggenheim Fellow. but tangential contact at near van der Waals separa- tion is sufficient to develop the appropriate electronic band structures. Under these circumstances, the electronic properties of the individual ions must be largely retained, suggesting that the electronic structure of discrete fulleride ions is the preferred point of departure for understanding their extended properties.17 This is one of the major reasons for studying isolated fulleride ions in detail. An example of where discrete fulleride chemistry meets materials science is in the determination of an appropriate value for the Knight shift in A3C60 metals. There are quite divergent opinions on how to partition the observed solid-state 13C NMR shifts in A3C60 compounds (185-196 ppm) into an intrinsic chemical shift and a Knight shift (i.e., the contribu- Robert Bolskar received his B.S. degree in Chemistry from Carroll College tion of the conduction electrons relative to an insu- in Waukesha, WI, in 1992. He earned his Ph.D. degree at the University lating state). Knight shifts as high as 61 ppm18 and of Southern California in Los Angeles under the direction of Christopher as low as 6 ppm19 have been suggested. The effect of Reed in 1997, investigating the synthetic redox chemistry of fullerene 3- anions and cations. In 1998 he moved to the University of California, unpaired electrons on the chemical shift in the C60 Riverside, as Managing Research Associate. He is currently Senior ion, present in both the conducting and the insulating Chemist at TDA Research in Wheat Ridge, CO. His research interests phases, has been largely neglected in the discussion. include the chemistry and physics of fullerene compounds, supramolecular Determining the correct value has significant rami- chemistry, strongly oxidizing systems, and electrophilic cations. fications for the calculated density of states at the 20 Fermi level, N(EF), in A3C60 conductors. n- However, the best known demonstration of the elec- An unanticipated property of discrete C60 ful- tronegative nature of C60 is the solid-state intercala- lerides is their propensity to disobey Hund’s rule. For tion of electropositive metals producing a large family example, the 3-fold degeneracy of the t1u LUMO leads of MxC60 salts called fullerides. The most remarkable to the expectation of a spin quartet ground state for ) 3- of these are the superconducting phases, A3C60 (A C60 : alkali metal cation).11 The extended properties of these systems have been extensively reviewed else- where12-15 but are part of this review when connec- n- tions can be made to the discrete C60 ions. Another 3- early development was the discovery of a unique ful- This is, however, not observed. C60 has a spin leride phase which displays ferromagnetism: [TDAE]- doublet ground state even though there is no require- 16 [C60] (TDAE ) tetrakis(dimethylamino)ethane). ment for a Jahn-Teller lowering of symmetry.21 2- Solid-state superconductivity and ferromagnetism There is a similar counterintuitive result with C60 in [60]-fulleride materials arises from cooperative whose electronic structure has been somewhat con- interactions between electrons on adjacent bucky- troversial. Resolving the conflicting interpretations balls. The ball-to-ball contact is relatively meager, of data and theory for this ion is one of the motiva- Discrete Fulleride Anions and Fullerenium Cations Chemical Reviews, 2000, Vol. 100, No. 3 1077 tions for compiling this review. Another is to assess for continued comprehensive development of fullerene the current status of the long-standing problem chemistry with an emphasis on the charged redox regarding the origin of the extra signals seen in the states of these novel curved-carbon structures. Donor- EPR spectra of fullerides. Of the six explanations that acceptor properties of fullerene and fulleride materi- have been offered to explain the origin of the sharp als have recently been reviewed.48 - signal (or “spike”) in the EPR spectrum of C60 , only two remain viable. Much of the early work is open to The literature on fullerenes is widely spread across reinterpretation, and we now conclude that C-C the disciplines of material science, physics, and n- n- bonded dimers of fullerides (C120O or C120 ) offer chemistry in all its branches, adding to the challenge the most compelling explanation for the extra signals. of writing a review that is both critical and compre- 22 23 Dimeric and polymeric AC60 phases containing hensive. Compounding this is the proliferation of C-C bonded fullerides have become quite well char- publications associated with conferences which, apart acterized in recent years. from sometimes undesirable duplication, may intro- Related to the idea of C-C bonded dimers is the duce data and interpretations that have not had the very

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