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

Abstract

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.

Controlling O2 Reactivity in Synthetic Analogues of [NiFeS]- and [NiFeSe]-Hydrogenase Active Sites

“…Crystallography finds only minimal changes in the coordination sphere of the bimetallic complex; the NiFe core is maintained with marginal differences in the Ni···Fe distances even though the -E PhX bridging ligand has been expanded into an Ni–O–S–Fe or Ni–O–Se–Fe unit. Supported by earlier DFT computations, we surmise that the rigidity of the tridentate N 2 S “pincer”-type ligand guides production of E-oxygenates at the more mobile, monodentate, bridging E PhX ligand site. Consistent with this conclusion are results from the Ogo group using NiN 2 S 2 (with N 2 S 2 as a fixed tetradentate binding site for Ni II ) as metalloligand to Cp*Fe II , bearing an open site on iron. − Under O 2 , such Ni–Fe complexes yield isolable Fe IV (peroxo) species, with O 2 2– side-on bound to Fe in [NiN 2 S 2 –Fe­(O 2 )­Cp*] + rather than any of the S-oxygenates displayed in Figure . The oxidation states of Ni and Fe in the product oxygenates of our study remain at Ni II and Fe II for both the selenium and the sulfur derivatives. However, we note that low temperature (0 °C) monitors of the O 2 reactions with the Ni–Fe containing the μ-S PhNMe2 bridging ligand found a buildup of a transient (but long-lived) EPR-active species as the reaction proceeded; a signal at g avg ≈ 2.09 is assigned to Ni III , while one at g = 4.19 is likely Fe III , see Supporting Information (Figures S60–S62).…”

“…Supported by earlier DFT computations, we surmise that the rigidity of the tridentate N 2 S “pincer”-type ligand guides production of E-oxygenates at the more mobile, monodentate, bridging E PhX ligand site. Consistent with this conclusion are results from the Ogo group using NiN 2 S 2 (with N 2 S 2 as a fixed tetradentate binding site for Ni II ) as metalloligand to Cp*Fe II , bearing an open site on iron. − Under O 2 , such Ni–Fe complexes yield isolable Fe IV (peroxo) species, with O 2 2– side-on bound to Fe in [NiN 2 S 2 –Fe­(O 2 )­Cp*] + rather than any of the S-oxygenates displayed in Figure .…”

[NiFe], [FeFe], and [Fe] hydrogenase models from isomers

“…The BP86 functional has been used to model biomimetic NiFe complexes (26)(27)(28)(29). Furthermore, in our previously studied analogical NiFe complexes, the BP86 functional correctly described the ground-state spin multiplicity, while two hybrid functionals, B3LYP-D3 (D3-corrected Becke three-parameter Lee-Yang-Parr functional) (23,30) and TPSSh (Tao-Perdew-Staroverov-Scuseria) (31), failed to predict (32,33). Thus, the choice of BP86 functional is appropriate.…”

Oxygen uptake in complexes related to [NiFeS]- and [NiFeSe]-hydrogenase active sites

“…Ogo, et al isolated a high valent iron( iv ) peroxo complex on reacting solvent-coordinated complexes [Ni II LFe II (RCN)(η 5 -C 5 Me 5 )] + with O 2 12–14. However, no oxygenated sulfur species were reported from their system.…”