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(eta(5)-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 O-2 binds to the Fe-II center in an end-on fashion and forms a high-valent iron complex, ((NiFeIV)-Fe-II(eta(2)-O-2) NiFe-peroxo O-2)), 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 BH 3 radical anion (BH3 center dot-) binds to the terminal oxygen of NiFe-OOH, giving rise to BH3OH- and (NiFeIV)-Fe-II=O. The second HAT from BH4- to the oxygen of (NiFeIV)-Fe-II=O leads to BH3OH- and Fe-reduced complex. Importantly, the dioxygen activation is triggered by HAT, not by proton transfer or hydride transfer. The O-2 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 O-2 activation are essential in understanding the formation of the inactive state and reactivation process in hydrogenase.