Evidence for the Formation of Singly Bonded Dimers During the Reductive Electrochemistry of Methanofullerenes

Abstract

Four methanofullerene derivatives, with phosphonate or sulfone groups attached to a C(60) core through a Bingel procedure, were synthesized to probe their stability upon electrolytic reduction. Derivatives 1 and 2 are the most stable upon electroreduction and do not exhibit retro-cyclopropanation reactions until more than three electrons per C(60) derivative are transferred. The cyclopropane ring is then removed and C(60)(>CH(2))(n) (n=1-3) products result from reactions of the trianion of C(60) with the solvent, CH(2)Cl(2). The situation with diphosphonate 3 or phosphonatecarboxylate 4 is dramatically different. For 3, quantitative retro-cyclopropanation occurs when 2.8 e(-) per molecule are transferred. In the case of 4, when more than two electrons per molecule are transferred, there is evidence of the reversible formation of a very stable intermediate, which is oxidized at a potential 500 mV more positive than the first fullerene-based reduction of the parent compound. Electrolysis of a simple C(70)-Bingel monoadduct (5) also exhibits the formation of a similar intermediate. On the basis of cyclic voltammetry, ESR spectroscopy, and MALDI analysis of products, the intermediate observed during the electrolysis of compounds 4 and 5 is assigned to a dimeric structure.

Reaction of [70]Fullerene with Tetraethyl Methylenediphosphonate or Diethyl (Cyanomethyl)phosphonate Revisited

“…[8] Instead, a doublet peak at δ = 17.68 ppm with 1 J CP = 185.8 Hz appeared in the 13 C NMR spectrum of product 3 and should be assigned as the methine carbon. The observed upfield shift (14.55-15.65 ppm) for the bridgehead carbon of product 3 relative to that of the corresponding C 60 analogue (δ = 32.23 [8] and 33.33 [10] ppm) is reasonable because the bridgehead carbon of compound 5 (δ = 25.11 ppm [6k] ) has approximately the same upfield shift compared to that of its C 60 counterpart (δ = 39.38 [6k] and 39.84 [10] ppm). It can be clearly seen that four sp 2 -carbon atoms of the C 70 cage at δ = 146.61 ( 3 J CP = 4.3 Hz), 143.00 ( 3 J CP = 5.6 Hz), 139.96 ( 3 J CP = 4.6 Hz), and 137.23 ( 3 J CP = 5.2 Hz) ppm are split by the phosphorus atom.…”

Electrochemically Induced Retro‐Cyclopropanation Reactions

“…[22] The nature of these intermediates has been recently determined and will be the subject of discussion in section 6. [23]  60 Additional applications of the retro-Bingel reaction, acting in concert with the isomerization reaction, started to emerge immediately after the discovery of these electrochemical processes. CPE studies of seven different isolated and characterized isomers of tris[bis(ethoxycarbonyl)methano]- [60]fullerene demonstrated that after discharging two electrons per molecule in CH 2 Cl 2 , [24] a competition between retro-Bingel and isomerization reactions takes place, with preferential loss of one or two addends.…”

“…These values were in reasonable agreement with the reported ionic complex of a singly bonded (C 70 Ϫ ) 2 dimer with cyclotriveratrylene [84] and other solid structures previously reported. [23] Additionally, the ESR spectra appeared to be dimer-concentration dependent. These observations were in agreement with the existence of an equilibrium between two C 70 adduct radical anions, if the negative charge was localized in the fullerene core or in the adduct.…”

“…[23] Figure 19. Phosphonate or sulfone methanofullerenes studied by reductive electrochemistry Controlled-potential electrolyses performed after each reduction step of 41 showed that a chemical reaction occurred Ϫ /molecule were transferred, and unexpected changes in the cyclic voltammogram were observed.…”

“…Methanofullerenes C 60 Ͼ(CH 2 ) n (n ϭ 1Ϫ3) were recovered after reoxidation. [23] Komatsu et al [75] recently reported the isolation of a dimer formed by coupling of monomer radicals with diethoxyphosphorylmethyl groups, and were able to obtain an X-ray crystal structure of it. Other dimeric fullerene derivatives have been generated in different ways [76Ϫ82] and similar observations to the ones obtained for 41 -oxidation of a fullerene derivative monoanion followed by a rapid recombination reaction to a neutral singly bonded dimer with 1-octynyl groups on each C 60 cage -have been reported.…”