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.