Lewis Acid-Base Adducts of Sc2C2@C3v(8)-C82/N-Heterocyclic Carbene: Toward Isomerically Pure Metallofullerene Derivatives.

The addition of a bulky N-heterocyclic carbene (NHC) to Sc2C2@C3v(8)-C82 affords two monoadducts (2a and 2b) quantitatively and regioselectively, representing the first examples of Lewis acid-base pairs of metal carbide cluster fullerenes. 2b is likely a kinetically favorable labile product that cannot be isolated from the solution. The crystallographic results of 2a unambiguously demonstrate that one polarized C-C single bond is formed between the normal carbene site C2N of the NHC and a specific [5,6,6]-carbon atom out of 17 types of nonequivalent cage carbon atoms of Sc2C2@C3v(8)-C82. Theoretical calculations demonstrate that the high regioselectivity, the unexpected addition pattern, and the quantitative formation of monoadducts are synergistic results from the cage geometry and electron distribution on the cage.