Mechanism and Selectivity of the Bingel–Hirsch Reaction of TiSc ₂ N@C ₈₀ : A Comprehensive Density Functional Theory Study

The Bingel–Hirsch (BH) reaction is a key strategy for functionalizing fullerenes and endohedral metallofullerenes, typically affording cycloadducts with reduced reactivity toward metallofullerenes. Unexpectedly, a recent experiment on TiSc2N@C80 revealed single-bond BH adducts with markedly enhanced reactivity. However, the detailed mechanism and precise adduct structures remain unclear due to limited experimental evidence. Here, we report a comprehensive density functional theory investigation of the BH reaction between TiSc2N@C80 and diethyl bromomalonate. We show that single-bond addition pathways have substantially lower free-energy barriers than cycloaddition pathways, supporting experimental observations. Moreover, single-bond addition at pentagon–hexagon–hexagon ([566]) junctions is both thermodynamically and kinetically more favorable than at hexagon–hexagon–hexagon ([666]) sites, suggesting that the experimental products correspond to [566] adducts. This regioselectivity originates from the higher relative energies of [566] intermediates, which render them more reactive toward subsequent transformations. Remarkably, the single-bond adducts feature the most reduced fullerene cage (oxidation state −7) synthesized to date. These findings clarify the origin of the BH reactivity and selectivity of TiSc2N@C80 and offer valuable guidance for future functionalization of other paramagnetic metallofullerenes.