Cooperative Heterobimetallic CO₂ Activation Involving a Mononuclear Aluminum(II) Intermediate

Molecular chemistry of aluminum most commonly involves Al^III ions due to their noble gas electronic configurations. In contrast, the chemistry of Al^II ions is underexplored and may contain undiscovered reaction manifolds. Here, we report the CO₂ activation chemistry of a transient Al^II intermediate supported by a chelating, dianionic ligand and investigate the electronic structure details and reaction mechanisms required to access this reactivity. We found that a heterobinuclear complex, (NON)Al-FeCp(CO)₂ (1), undergoes Al-Fe bond homolysis at ambient conditions to reveal the [(NON)Al]^•/[CpFe(CO)₂]^• radical pair in situ. The presence of predominantly Al-centered spin density (i.e., an Al^II ion) within this radical pair was established by quantum-chemical calculations and with experiments in which radical scavengers (TEMPO, benzophenone) induce Al-Fe bond homolysis. Exposure of 1 to CO₂ atmosphere resulted in insertion of CO₂ into the Al-Fe bond. This net 2-electron CO₂ reduction process was computationally modeled using density functional theory and direct dynamics simulations, revealing that reduction involves two 1-electron steps and, thus, depends on stabilization of high-energy [CO₂]^•- by coordination to aluminum. This mechanism for CO₂ activation is unexpected given the canonical predisposition of CO₂ for multielectron reduction processes and demonstrates the possibility of discovering new reaction profiles using earth-abundant elements in unusual oxidation states.