LAUSR

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, in [NiFeS]-H2ase, and selenium, in [NiFeSe]-H2ase, yields oxygenated chalcogens under aerobic conditions, and delays irreversible oxygen damage at the metals by maintenance of the NiFe core structure of active sites. In attempts 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 the phenyl ring (X = CF3, Cl, H, OMe, NMe2) and compared in aspects of communication between Ni and Fe, redox potentials, and chemical reactivities. Studies of S- and Se-oxygenation by O2 found both single and double O-atom uptake at the chalcogens that led to the conversion of the 4-membered core, Ni(µ-EPhX)(µ-S'N2)Fe, to a 5-membered Ni-O-E-Fe-S' arrangement, where an O atom is inserted between E and Ni. In the E = S and X = NMe2 case, the 2-oxygen uptake complex was isolated and structurally characterized as the sulfinato species with the second O of the O2SPh-NMe2 unit pointing out of the 5-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, indicating the importance of remote effects on the NiFe core reactivity. Consistent with DFT computational results the more electron-rich sulfurs were more O2 responsive in the SPhX series; and the selenium analogues were even more reactive with O2. Mass spectral analysis of the sulfinato products from O2 addition in a crossover experiment using a mixture of 18O2/ 16O2 suggests a concerted mechanism in O-atom addition. Deoxygenation, by reduction or chemical abstraction reagents, occurs for the mono-oxygen addition complexes, while the 2-O, sulfinato, analogues are inert to O-abstraction reagents. The abstraction of oxygen from the mono-O, sulfenato species, is related to oxygen removal in soluble, NAD+-reducing [NiFe]- H2ase (Horch, M.; Lauterbach, L.; Mroginski, M. A.; Hildebrandt, P.; Lenz, O.; Zebger, I. J. Am. Chem. Soc. 2015, 137, 2555-2564.).