Bis- through tetrakis-adducts of C-60 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

By the tether-directed remote functionalization method, a series of bis- to hexakis-adducts of C-60, 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-60-I-h) 9-11 starting from 4, 5, and 7, respectively (Schemes 2, 4, and 5). This versatile protocol consists of a O-1(2) 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 H2O to cyclohexa-1,3-dienes, Diels-Alder, addition of dimethyl acetylenedicarboxylate, retro-Diels-Alder, addition, and, ultimately, transesterification. In the series 9-11, ail methano moieties are attached along an equatorial belt of the fullerene. Starting from C-2 upsilon-symmetrical tetrakis-adduct 15, transesterification with dodecan-1-ol or octan-1-ol yielded the octaesters 16 and 17, respectively, as noncrystalline fullerene derivatives (Scheme 3). The X-ray crystal structure of a CHCl3 solvate of 11 (Fig. 3) showed that the residual conjugated pi-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(= cyclopent[fg]acenaphthylene) moieties, 6-6 and 6-5 bond-length alteration (0.05 Angstrom) was reduced by ctr. 0.01 Angstrom as compared to the free C-60 skeleton (0.06 Angstrom) (Fig. 4). The crystal packing (Fig. 6) revealed short contacts between Cl-atoms of the solvent molecule and sp(2)- and sp(3)-C-atoms of the C-sphere, as well as short contacts between Cl-atoms and O-atoms of the EtOOC groups attached to the methane 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. 7-11), IR, and NMR spectroscopy (Table 2), cyclic (CV) and steady-state (SSV) voltammetry (Table 1), calculations of the energies of the lowest unoccupied molecular orbitals (LUMOs) and electron affinities (Figs. 12 and 13), 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 (methane vs. but-2-ene-1,4-diyl) anellated to the C-sphere. Attachment of multiple methane moieties along an equatorial belt as in the series 8-11 induces only a small perturbation of the original fullerene pi-chromophore. In general, with increasing attenuation of the conjugated fullerene pi-chromophore, the optical (HOMO-LUMO) gap in the UV/VIS spectrum is shifted to higher energy, the number of reversible one-electron reductions decreases, and the first reduction potential becomes increasingly negative, the computed LUMO energy increases and the electron affinity decreases, and the reactivity of the fullerene towards nucleophiles and carbenes and as dienophile in cycloadditions decreases.