Controlling O 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 O are guided by nature's design of hydrogenase active sites. The affinity of oxygen for sulfur and selenium, in [NiFeS]- and [NiFeSe]-Hase, 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 (μ-E)(μ-S') (E = S or Se, = (η-CH)Fe(CO)) complexes were electronically tuned by the para-substituent on μ-EPhX (X = CF, Cl, H, OMe, NMe) 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, (μ-E)(μ-S'), to a five-membered ring Ni-O-E-Fe-S', where an O atom inserts between E and Ni. In the E = S, X = NMe case, the two-oxygen uptake complex was isolated and characterized as the sulfinato species with the second O of the OS 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. Density functional theory computational results support the observed remote effects on the NiFe core reactivity; the more electron-rich sulfurs are more O responsive in the S series; the selenium analogues were even more reactive with O. Mass spectral analysis of the sulfinato products using a mixture of O/O suggests a concerted mechanism in O 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]-Hase (Horch, M.; Lauterbach, L.; et al. , , 2555-2564).