An Insight into the Aromaticity of Fullerene Anions:  Experimental Evidence for Diamagnetic Ring Currents in the Five-Membered Rings of C₆₀^6- and C₇₀^6-

Abstract

Reduction of the two "closed" [6,6] methanofullerenes, [6,6]C(61)H(2) (1) and [6,6]C(71)H(2) (5), to the corresponding hexaanions with lithium metal causes the bridgehead-bridgehead bonds to open, at least partially, and this change gives rise to diamagnetic ring currents in the resulting homoconjugated six-membered rings (6-MRs). These new ring currents shield the overlying hydrogen atoms on the methylene bridge and induce upfield shifts of 1.60 and 0.11 ppm in their (1)H NMR resonances, respectively. Analogous reduction of the already "open" [5,6]methanofullerenes, [5,6]C(61)H(2) (2) and [5,6]C(71)H(2) (3 and 4), only slightly enhances the shielding of the hydrogen atoms over the homoconjugated 6-MRs (upfield shifts of 0.13, 0.68, and 0.14 ppm, respectively) but leads to exceptionally strong diamagnetic ring currents in the homoconjugated five- membered rings (5-MRs), as evidenced by dramatic shielding of the hydrogen atoms situated over them (upfield shift of 5.01, 6.78, and 1.63 ppm, respectively). The strongest shielding is seen for the hydrogen atom sitting over the 5-MR at the pole of C(71)H(2)(6)(-) (delta = -0.255 ppm) indicating that the excess charge density is concentrated at the poles.

An Insight into the Local Aromaticities of Polycyclic Aromatic Hydrocarbons and Fullerenes

“…For these systems it has been found that five-membered rings have nonaromatic or slightly antiaromatic character, while six-membered rings display meaningful local aromaticities with some exceptions. [39,40] Finally, the local aromaticities of higher fullerenes and nanotubes have recently been studied by B¸hl [37] and by van Lier and coworkers, [41] respectively. [38] The local aromatic character of C 60 and C 70 has previously been studied by use of HOMA [6] and NICS [37] indices.…”

Addition of Carbene to the Equator of C70 To Produce the Most Stable C71H2 Isomer: 2 aH‐2(12)a‐Homo(C70‐D5h(6))[5,6]fullerene

“…The synthetic chemistry of C 70 is centered on the areas near the poles, [1, 2] as these areas have the highest curvature and hence high bond strain. [7,[9][10][11] Computations show that the equatorial six-membered ring of C 70 has the largest diamagnetic ring current, [11,12] but no solid experimental evidence exists. The equatorial region of C 70 , on the other hand, has little curvature and hence lower bond strain.…”

“…[7,[9][10][11] Computations show that the equatorial six-membered ring of C 70 has the largest diamagnetic ring current, [11,12] but no solid experimental evidence exists. [7,[9][10][11] Computations show that the equatorial six-membered ring of C 70 has the largest diamagnetic ring current, [11,12] but no solid experimental evidence exists.…”

Reduction

“…The paramagnetic or diamagnetic ring-current does not effect the 13 C chemical shift, but it does effect the shift of 13 C or 1 H attached to exohedral addends [42] or the shift of endohedral 3 He [43][44][45][46][47]. The intensity of this shift depends on the temperature, the charge, the physical condition (solid or solute) and the spin state of the anion salt or intercalated fulleride.…”