Bis‐ through Tetrakis‐Adducts of C₆₀ by Reversible Tether‐Directed Remote Functionalization and systematic investigation of the changes in fullerene properties as a function of degree, pattern, and nature of functionalization

Abstract

By the tether-directed remote functionalization method, a series of bis-to hexakis-adducts of C,,, i.e., 1-7 (Fig. I ) , had previously been prepared with high regioselectivity. An efficient method for the removal of the tether-reactive-group conjugate was now developed and its utility demonstrated in the regioselective synthesis of bis-to tetrakis(methano)fullerenes (= di-to tetracyclopropafullerenes-C,,-Z,,) 9-11 starting from 4, 5, and 7, respectively (Schemes 2 , 4 , and 5). This versatile protocol consists of a '0, ene reaction with the two cyclohexene rings in the starting materials, reduction of the formed mixture of isomeric allylic hydroperoxides to the corresponding alcohols, acid-promoted elimination of H,O to cyclohexa-I ,3-dienes, Diels-Alder addition of dimethyl acetylenedicarboxylate, retro-Diels-Alder addition, and, ultimately, transesterification. In the series 9-11, all methano moieties are attached along an equatorial belt of the fullerene. Starting from C,,-symmetrical tetrakis-adduct 15, transesterification with dodecan-1-01 or octan-1-01 yielded the octaesters 16 and 17, respectively, as noncrystalline fullerene derivatives (Scheme 3). The X-ray crystal structure of a CHCI, solvate of 11 (Fig. 3) showed that the residual conjugated n-chromophore of the C-sphere is reduced to two tetrabenzopyracylene substructures connected by four biphenyl-type bonds (Fig. 5). In the eight six-membered rings surrounding the two pyracylene (= cyclopentlfg]acenaphthylene) moieties, 6-6 and 6-5 bond-length alteration (0.05 A) was reduced by ca. 0.01 8, as compared to the free C,, skeleton (0.06 A) (Fig. I). The crystal packing (Fig. 6 ) revealed short contacts between C1-atoms of the solvent molecule and sp2-and sp3-C-atoms of the C-sphere, as well as short contacts between C1-atoms and 0-atoms of the EtOOC groups attached to the methano moieties of 11. The physical properties and chemical reactivity of compounds 1 -11 were comprehensively investigated as a function of degree and pattern of addition and the nature of the addends. Methods applied to this study were UV/VIS (Figs. 7 -I f ) , IR, and NMR spectroscopy (Table 2), cyclic (CV) and steady-state (SSV) voltammetry (Table I ) , calculations of the energies of the lowest unoccupied molecular orbitals (LUMOs) and electron affinities (Figs. I2 and f3), and evaluation of chemical reactivity in competition experiments. It was found that the properties of the fullerene derivatives were not only affected by the degree and pattern of addition but also, in a remarkable way, by the nature of the addends (methano vs. but-2-ene-1,4-diyl) anellated to the C-sphere. Attachment of multiple methano moieties along an equatorial belt as in the series 8-11 induces only a small perturbation of the original fullerene n-chromophore. In general, with increasing attenuation of the conjugated fullerene n-chromophore, the optical (HOMO-LUMO) gap in the UVjVIS spectrum is shifted to higher energy, the number of reversible one-electron reductions decreases, and the first reduction p...

Basic hydrolysis of fullerene-based esters: a tiny step away from nucleophilic addition to fullerene

“…Namely, it is characteristic for T h -symmetric hexakis adducts that due to the reduced conjugation (there are eight isolated benzenoid rings left on the fullerene cage) they demonstrate only minimal absorption in the region of visible light, tailing until 460 nm. 20 Moreover, comparison of spectra of various T h -symmetric hexakis adducts, presented by Diederich 20 and by Lamparth, 21,23, shows a common feature of such spectra: a distinctive local absorption minimum at ≈ 260 nm surrounded by two pronounced absorption peaks, and a shoulder in the region between 320 and 340 nm. Such UV/Vis spectra are typical for all hexakis adducts because of the same conjugation left on the C 60 skeleton, and are almost independent on the type of the addend.…”

Photophysical Properties of Three Methanofullerene Derivatives

“…[17±19] Complex functional group effects on the spectra have been reported by Cardullo et al [24] for multigrafted C 60 with cyclopropyl-and cyclohexyl-fused functionalization.…”

Discrete Fulleride Anions and Fullerenium Cations

“…88 In keeping with their delocalized charges, the first oxidation potentials of C 60 , C 70 , C 76 , and C 78 follow the same trend as their first ionization potentials (7.61, 7.58, 7.10, and 7.05 eV, respectively). 89,90 In contrast to reduction, the oxidation of fullerenes typically becomes easier upon derivatization, 59,80,91,92 particularly with bis(silyl) derivatization, 93 but no cations have been characterized.…”