Preparation and characterisation of an equatorial para-adduct of (PhCH2)HC70 from the reaction of C702− with benzyl bromide and H2O: addition effects in the polar and equatorial regions of C70

Abstract

Benzyl(hydro)[70]fullerene regioisomers with the addends in both the equatorial and polar regions of C(70) have been prepared via the reaction of dianionic C(70) with benzyl bromide and H(2)O. HRMS, UV-vis, (1)H, (13)C, HMQC (heteronuclear multiple quantum coherence) and HMBC (heteronuclear multiple bond coherence) NMR characterisations have shown that the addition in the equatorial region of C(70) affords a new (PhCH(2))HC(70) regioisomer with para-positioned addends across a six-membered ring, which is different from the "polar" regioisomers where the addends have an ortho-addition pattern. (1)H NMR characterisations have shown a much stronger shielding effect for the addends in the equatorial region with respect to the counterparts in the polar region of C(70), while cyclic voltammetry study has shown a surprising positive shift for the first reduction potential of the equatorial regioisomer with respect to those of the polar regioisomer and pristine C(70), suggesting that the equatorial region of C(70) is rather electropositive than electronegative. D(2)O experiment has shown a significant difference of the deuterated product distribution between the equatorial and polar regioisomers, which can be justified by the different acidity of the (PhCH(2))HC(70) regioisomers. Computational calculations have been carried out to rationalize the formation of the C(70)HR regioisomers.

Reductive Activation of C70 Equatorial Carbons and Structurally Characterized C70 δ-Adduct with Closed [5,6]-Ring Fusion

“…The 1 H NMR spectra of the β- and δ-isomers (Figures S8 and S12a) exhibit broad peaks corresponding to the methylene and phenyl protons, where partial resonances arising from the methylene protons are incidentally overlapped. An increased shielding effect is shown for both the phenyl and methylene protons as the o -xylene moves from the polar to the equatorial region (Figures S4, S8, and S12), which is likely associated with the strong equatorial ring currents in C 70 and consistent with previous work on C 70 derivatives. ,, The 13 C NMR spectra of the regioisomers (Figures S5, S9, and S13) are consistent with the structural assignment with the exhibition of two resonances for the sp 3 C 70 carbons of the α-isomer and one resonance for the sp 3 C 70 carbons of the β- and δ-isomers. The UV–vis spectra of the α-, β-, and δ-isomers (Figures S3, S7, and S11) show absorptions at 334, 395, 459, 535, and 662 nm; 322, 362, 396, 434, 584, and 697 nm; and 325, 374, 396, 511, and 646 nm, respectively, in agreement with the reported spectral feature for the regioisomers…”

“…The result is different from that of the typical fullerene derivatives, where the derivatives are usually less electron-deficient due to the cleavage of the electron-withdrawing fullerene π-conjugation. 37 Notably, C 70 equatorial para adducts exhibit similar anodic shift, 27,38 suggesting that the unusual strong electron deficiency is likely related to the involvement of the d-carbons in the addition. As shown by the single crystal structure of the δ-isomer, the bond delocalization at the equatorial region is considerably disrupted by the closed [5,6]-addition to the δ-bond, whereas it is well retained when the addition occurs at the relatively far away β-(Figure 4) and α-bond (refer to cif in ref 15), indicating that the unusual electron deficiency of the δ-isomer is likely related to the disruption of the conjugated electronic structure in the equatorial region, which, however, should be electron-donating rather than electron-withdrawing as observed in C 60 and the polar region of C 70 .…”

“…Previous work on C 70 functionalization with anionic ,,,− or cationic C 70 species has shown the preferential formation of equatorial para adducts (addition at d , d -carbons across the equator), suggesting that the inert d -type of carbon atoms are likely activated in charged C 70 species. However, such reactions are limited to the para additions only, and no work on cycloaddition reactions has been reported.…”

A faux hawk fullerene with PCBM-like properties

“…We also expected 1 – to undergo loss of the H atom to form [ 1 – H] – , as it did in the LT-PES experiment discussed above and in the 1 + CoCp 2 reaction discussed below. Furthermore, hydrofullerenes such as 1,9-C 60 H 2 , 45 , 46 1,9-C 60 (CH 2 C 6 H 5 )H, 49 and isomers of C 70 (CH 2 C 6 H 5 )H 50 are known to undergo observable H-atom loss upon one-electron reduction unless the CV scan speed is extremely high or the solution is cooled to a low temperature. Nevertheless, our expectations notwithstanding, and in the absence of additional electrochemical experiments, the redox potentials for 1 listed in Table 2 are correctly assigned.…”

Fullerenes

“…37 Novel (PhCH 2 )HC 70 regioisomers with the addends in both the equatorial and polar regions of C 70 have been prepared via the reaction of C 70 2with benzyl bromide and H 2 O. Computational calculations have been carried out to rationalise the formation of the C 70 HR regioisomers. 38 Two exohedral C 70 derivatives, C 70 (C 14 H 10 ) and C 70 (C 5 H 6 ) have been chromatographically separated from flame soot of the acetylene-benzene combustion and characterised by various spectroscopic techniques in combination with theoretical simulation. 39 2.1.3 Hydrogenation.…”