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

Strategies for limiting, or reversing, the degradation of air-sensitive, base metal catalysts for the hydrogen evolution/oxidation reaction on contact with adventitious O₂ are guided by nature's design of hydrogenase active sites. The affinity of oxygen for sulfur and selenium, in [NiFeS]- and [NiFeSe]-H₂ase, yields oxygenated chalcogens under aerobic conditions, and delays irreversible oxygen damage at the metals by maintaining the NiFe core structures. To identify the controlling features of S-site oxygen uptake, related Ni(μ-E_PhX)(μ-S'_N2)Fe (E = S or Se, Fe = (η⁵-C₅H₅)Fe^II(CO)) complexes were electronically tuned by the para-substituent on μ-EPhX (X = CF₃, Cl, H, OMe, NMe₂) and compared in aspects of communication between Ni and Fe. Both single and double O atom uptake at the chalcogens led to the conversion of the four-membered ring core, Ni(μ-E_PhX)(μ-S'_N2)Fe, to a five-membered ring Ni-O-E-Fe-S', where an O atom inserts between E and Ni. In the E = S, X = NMe₂ case, the two-oxygen uptake complex was isolated and characterized as the sulfinato species with the second O of the O₂S_Ph-NMe2 unit pointing out of the five-membered Ni-O-S-Fe-S' ring. Qualitative rates of reaction and ratios of oxygen-uptake products correlate with Hammett parameters of the X substituent on E_PhX. Density functional theory computational results support the observed remote effects on the NiFe core reactivity; the more electron-rich sulfurs are more O₂ responsive in the S_PhX series; the selenium analogues were even more reactive with O₂. Mass spectral analysis of the sulfinato products using a mixture of ¹⁸O₂/¹⁶O₂ suggests a concerted mechanism in O₂ addition. Deoxygenation, by reduction or O atom abstraction reagents, occurs for the 1-O addition complexes, while the 2-O, sulfinato, analogues are inert. The abstraction of oxygen from the 1-O, sulfenato species, is related to oxygen repair in soluble, NAD⁺-reducing [NiFe]-H₂ase (Horch, M.; Lauterbach, L.; et al. J. Am. Chem. Soc. 2015, 137, 2555-2564).