Synthesis and Electrochemistry of Electronegative Spiroannelated Methanofullerenes:  Theoretical Underpinning of the Electronic Effect of Addends and a Reductive Cyclopropane Ring-Opening Reaction

Abstract

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 C60. 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 C60, resulting in a less negative first-reduction potentials compared to C60. 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 C60. 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 C60. 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.

Calculations of the geometries, electronic structures and nonlinear second-order optical susceptibilities of spiroannelated quinone-type methanofullerenes

“…This value is in good agreement with both the values of 1.521 A Ê reported using Knight's PM3 method [11] (shown in Table 2) and the values of 1.512 A Ê reported for a (diphenylmethano) fullerene [26]. The C 57 ±C 61 ±C 60 angle predicted for 1a (60.8°) is very close to that obtained for the latter (61.6°).…”

“…The quinone ring perpendicularly bisects the C 57 ±C 60 bond and forms a methanofullerene structure with the fullerene cage. Compound 1a thus shows C 2v symmetry, in accord with the 15 peaks observed [11,12] for the fullerene moiety in the 13 C NMR spectrum, indicating the existence of two planes of symmetry. The symmetry of 1a is consistent with Knight's calculation using the PM3 method [12].…”

Electrochemically Induced Retro‐Cyclopropanation Reactions

“…[36] These compounds exhibit irreversible electrochemistry, most probably resulting from the cleavage of one of the cyclopropane bonds connecting the addend to C 60 upon reduction. [37,38] These observations, coupled with those of the retro-Bingel reaction, motivated the study of the CPE behavior of compounds 26Ϫ31 (Figure 8). …”

Synthesis, Spectroscopic and Nonlinear Optical Properties of Multiple [60]Fullerene–Oligo(p‐phenylene ethynylene) Hybrids

“…In this case, the fullerene p-electrons may possibly communicate with the alkynyl p-electrons in a through-space p-orbital overlapping mechanism, namely "periconjugation." [42,[69][70][71] It is likely that such structural features would give rise to enhanced electronic interactions between fullerenes and conjugated functionalities, thereby providing a uniformly shaped molecular building block (tecton) for the construction of various molecular machinery and nano-architectures, as well as supramolecular assemblies.…”