DFT Study on Fe(IV)-Peroxo Formation and H Atom Transfer Triggered O2 Activation by NiFe Complex

The mechanism for dioxygen activation using the biomimetic model complex of [NiFe]-hydrogenase, [NiLFe­(η5-C5Me5)]+ [L = N,N′-diethyl-3,7-diazanonane-1,9-dithiolato] was established using density functional theory (DFT) and artificial force-induced reaction (AFIR) methods. Our computational results suggest that O2 binds to the FeII center in an end-on fashion and forms a high-valent iron complex, NiFe–peroxo (NiIIFeIV(η2-O2)), which has been experimentally observed. The O–O bond cleavage occurs in the presence of borohydride (BH4–) through hydrogen atom transfer (HAT). Once the HAT occurs, the generated BH3 radical anion (BH3•–) binds to the terminal oxygen of NiFe–OOH, giving rise to BH3OH– and NiIIFeIVO. The second HAT from BH4– to the oxygen of NiIIFeIVO leads to BH3OH– and Fe-reduced NiIIFeII complex. Importantly, the dioxygen activation is triggered by HAT, not by proton transfer or hydride transfer. The O2 is activated by the Fe center, and the oxidation state of Fe varies during the process, while the oxidation state of Ni is conserved. These mechanistic insights into O2 activation are essential in understanding the formation of the inactive state and reactivation process in hydrogenase.