π-Electron ring-current effects in multiple adducts of ³He@C₆₀ and ³He@C₇₀: A ³He NMR study

Bis- to hexakisadducts of C₆₀ (1-6) and mono- to tetrakisadducts of C₇₀ (7-13) containing ³He atoms (endohedral helium complexes) were prepared and studied by ³He NMR spectroscopy to determine the influence of degree of functionalization and addition pattern on the chemical shift of the ³He atom. In the series of C₆₀ derivatives, which included the previously measured 6-6 monoadduct ³He@C₆₁H₂₊ the ³He resonance was shifted considerably upfield relative to that of ³He@C₆₀ (δ = -6.36) up to the bisadduct 1 (δ= -11.45). The resonances of the higher adducts 3-6, however, were shifted only slightly further up-field and all appeared in a rather narrow spectral range between δ = 11.84 and -12.26. The absence of further substantial up field shifts was rationalized in terms of the compensation of deshielding due to the functionalization-induced decrease in the diamagnetic π-electron ring currents that extend around the fullerene sphere by the shielding that results from the weaker pentagonal-ring paramagnetic currents and the increased number of localized benzenoid substructures in 3-6. In contrast, all ³He resonances of the C₇₀ adducts are shifted downfield relative to the signal of ³He@C₇₀ (δ = -28.81). A monotonic relationship exists between the chemical shift and the degree of functionalization, whereby the ³He chemical shifts steadily decrease from monoadduct 7 (δ= -27.53) to the isomeric tetrakisadducts 12 (δ= -21.09) and 13 (δ= -20.68). This deshielding was explained by the reduction of the strong diamagnetic ring currents extending along the polar corannulene perimeters as a result of the functionalization at α- and β-type polar bonds in 7-13. In both series, the addition pattern was also found to have a distinct influence on the ³He chemical shift.