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

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posted on 13.09.2019, 13:34 by Xuemei Yang, Lindy C. Elrod, Trung Le, Valeria S. Vega, Haley Naumann, Yohannes Rezenom, Joseph H. Reibenspies, Michael B. Hall, Marcetta Y. Darensbourg
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-EPhX)­(μ-S′N2)Fe (E = S or Se, Fe = (η5-C5H5)­FeII(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-EPhX)­(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 O2SPh‑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 EPhX. 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 SPhX 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).