[6,6]‐Open and [6,6]‐Closed Isomers of C₇₀(CF₂): Synthesis, Electrochemical and Quantum Chemical Investigation

Abstract

Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.

Regioselective Synthesis of [6,6]‐Open and [5,6]‐Closed C70(CF3)8[CH2] Methanofullerenes with Rapid [6,6]‐to‐[5,6] Phototransformation

“…That evidenced, in agreement with the structure of the proposed 3a or 3b, lack of direct bonding between the [22,27,49] vs. 56-72 ppm in the closed methanofullerenes [21,22,29,48] ). [50,48,19] To make a choice between 3a and 3b, we involved GIAO DFT calculations of the NMR chemical shifts (see Table S3 in the Supporting Information for predicted NMR shifts of the complete isomer list). [50,48,19] To make a choice between 3a and 3b, we involved GIAO DFT calculations of the NMR chemical shifts (see Table S3 in the Supporting Information for predicted NMR shifts of the complete isomer list).…”

“…Similar to other derivatives of C s -C 70 (CF 3 ) 8 that retain idealized mirror symmetry, the 19 F NMR spectrum consists of four multiplets A-D of equal intensity due to four pairs of equivalent CF 3 groups within the -δ F range of 61-67 ppm ( Figure 4c). [21,22,48] Lack of crystalline material of compound 3, the initially formed isomer of C 70 (CF 3 ) 8 [CH 2 ], restricted its identification to general chemical considerations and interpretation of its NMR spectroscopic data with the aid quantum chemical calculations ( Table 1). The 1 H NMR spectrum features a pair of coupled doublet signals of equal intensity at δ H = 2.2 and 4.8 ppm, 3 J H,H = 6.9 Hz (Figure 4d), thus reflecting different magnetic shielding of the geminal CH 2 hydrogen atoms situated over a hexagon (H h ) and over a pentagon (H p ).…”

Stepwise Regioselective Hydrogenation of cis‐2‐C60(CF2)2 Homofullerene with [6,6]‐Open/Closed Valence Tautomerism

“…Clearly, opening of both CF 2 moieties at the same time would increase steric strain in the pentagonal C 5 ring situated between them, and the NMR data are likely to reflect dynamic averaging of their alternate opening, whereas in the crystalline state asymmetry can be induced by the crystal field. Indeed, δ C chemical shifts of the bridgehead carbon atoms of cis ‐2‐C 60 (CF 2 ) 2 are between the typical ranges for methanofullerene and homofullerene ( δ C =68–80 and 98–119 ppm, respectively), and 19 F chemical shifts are, likewise, between those of [6,6]‐open and [6,6]‐closed difluoromethanofullerenes ( δ F =112–118 and 124–128 ppm, respectively) …”

“…Difluoromethanofullerenes demonstrate unusually flat potential energy surface (PES) cross‐sections along the C−C cleavage coordinate, the equilibrium C⋅⋅⋅C distance being highly sensitive to charge state and external fields . The latter properties suggest the applicability of such flexible moieties at fullerene cages as molecular switches with electrochemically or electrostatically controlled reversible “open”/“closed” transitions of the bond in question ,…”

“…To date, a several of difluoromethylenated fullerenes have been isolated and structurally characterized, including [6,6]‐open C 60 (CF 2 ), [6,6]‐open and [6,6]‐closed isomers of C 70 (CF 2 ), cis ‐2‐C 60 (CF 2 ) 2 , and C s ‐C 70 (CF 3 ) 8 (CF 2 ) . Insertion of CF 2 into the fullerene cage increases the extent of LUMO localization at the bridgehead sp 2 carbon atoms and thus favors nucleophilic attacks at those positions.…”