SYNTHESIS AND ELECTROCHEMISTRY OF ELECTRONEGATIVE SPIROANNELATED METHANOFULLERENES - THEORETICAL UNDERPINNING OF THE ELECTRONIC EFFECT OF ADDENDS AND A REDUCTIVE CYCLOPROPANE RING-OPENING REACTION

Spiroannelated methanofullerenes bearing quinone-type addends includin g TCNQ and DCNQI analogues (3a-c, 6a,b, 8, 10, and 11) have been prepa red, and their structural and electronic properties have been characte rized by both experimental techniques and quantum-chemical calculation s. The spiro[2,5-cyclohexadienone-4,61'-methanofullerene] derivatives (3a-c), the spiro[10-anthrone-9,61'-methanofullerene] (8), and the TCN Q-and DCNQI-type derivatives (10 and 11) were isolated as [6,6] adduct s. The spiro[cyclohexanone-4,61'-methanofullerene] (6) was however obt ained as a mixture of [5,6] and [6,6] isomers. The novel methanofuller enes, with the only exception of 6, show irreversible cyclic voltammog rams with additional reduction peaks. The conjugated cyclohexadienone derivatives 3 exhibit better acceptor abilities than the parent C-60. Semiempirical PM3 calculations show that the addend lies perpendicular to the transanular bond in 3, while it folds down and adopts a butter fly shaped structure for compounds 8, 10, and 11, For compounds 3, per iconjugative interactions transmit the inductive effect of the addend and produce a small stabilization of the orbitals of C-60, resulting i n a less negative first-reduction potentials compared to C-60. For com pounds 8, 10, and 11, the folding of the addend prevents periconjugati ve effects. Theoretical calculations performed on 3a(.-) and 3a(2-) at the semiempirical (PM3), density functional (B3P86/3-21G), and ab ini tio (HF/6-31G) levels indicate that the attachment of the first elect ron causes the homolytic cleavage of one of the bonds connecting the a ddend to C-60. The resulting open-cyclopropane structure is stabilized by the aromaticity of the phenoxyl radical structure presented by the addend. The second electron enters in the addend forming the phenoxyl anion. This ring opening is supported by ESR measurements and explain s the irreversible electrochemical behavior of compounds 3. The noncon jugated nature of the cyclohexanone ring in 6 determines that reductio n takes place via a closed-cyclopropane structure with an electrochemi cal behavior similar to that observed for C-60. Compounds 8, 10, and 1 1 are proposed to undergo reduction via an open-cyclopropane structure now obtained after the attachment of the second electron which produc es the heterolytic opening of the cyclopropane ring. The lack of plana rity shifts the reduction of the addend to more negative potentials.