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 including TCNQ and DCNQI analogues (3a-c, 6a,b, 8, 10, and 11) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The spiro[2,5-cyclohexadienone-4,61'-methanofullerene] derivatives (3a-c), the spiro[10-anthrone-9,61'-methanofullerene] (8), and the TCNQ-and DCNQI-type derivatives (10 and 11) were isolated as [6,6] adducts. The spiro[cyclohexanone-4,61'-methanofullerene] (6) was however obtained as a mixture of [5,6] and [6,6] isomers. The novel methanofullerenes, with the only exception of 6, show irreversible cyclic voltammograms 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 butterfly shaped structure for compounds 8, 10, and 11, For compounds 3, periconjugative interactions transmit the inductive effect of the addend and produce a small stabilization of the orbitals of C-60, resulting in a less negative first-reduction potentials compared to C-60. For compounds 8, 10, and 11, the folding of the addend prevents periconjugative effects. Theoretical calculations performed on 3a(.-) and 3a(2-) at the semiempirical (PM3), density functional (B3P86/3-21G), and ab initio (HF/6-31G*) levels indicate that the attachment of the first electron causes the homolytic cleavage of one of the bonds connecting the addend 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 explains the irreversible electrochemical behavior of compounds 3. The nonconjugated nature of the cyclohexanone ring in 6 determines that reduction takes place via a closed-cyclopropane structure with an electrochemical behavior similar to that observed for C-60. Compounds 8, 10, and 11 are proposed to undergo reduction via an open-cyclopropane structure now obtained after the attachment of the second electron which produces the heterolytic opening of the cyclopropane ring. The lack of planarity shifts the reduction of the addend to more negative potentials.