Cooperative CO2 activation involving a mononuclear aluminum(II) intermediate

An important aspect of sustainable chemistry research is the discovery of novel chemical mechanisms by which otherwise inert molecules become activated toward useful transformations by earth’s most abundant elements. Compounds involving the most abundant metal on earth, aluminum, most commonly involve AlIII ions due to their noble gas electronic configurations. Although compounds with AlI ions have also been studied, the chemistry of AlII ions is nearly unknown and may contain undiscovered reaction manifolds. Here, we report the CO2 activation chemistry of an AlII complex 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 reversible 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 AlII ion) within this radical pair was established using experiments with radical scavengers as well as electronic structure calculations. 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 quantum chemical calculations 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 multi-electron reduction processes and demonstrates the possibility of discovering new reaction profiles of earth- abundant elements in unusual oxidation states.