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

Abstract

Strategies for limiting, or reversing, the degradation of airsensitive, base metal catalysts for the hydrogen evolution/oxidation reaction on contact with adventitious O 2 are guided by nature′s design of hydrogenase active sites. The affinity of oxygen for sulfur and selenium, in [NiFeS]-and [NiFeSe]-H 2 ase, 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 = (η 5 -C 5 H 5 )Fe II (CO)) complexes were electronically tuned by the parasubstituent on μ-EPhX (X = CF 3 , Cl, H, OMe, NMe 2 ) 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 = NMe 2 case, the two-oxygen uptake complex was isolated and characterized as the sulfinato species with the second O of the O 2 S 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 O 2 responsive in the S PhX series; the selenium analogues were even more reactive with O 2 . Mass spectral analysis of the sulfinato products using a mixture of 18 O 2 / 16 O 2 suggests a concerted mechanism in O 2 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]-H 2 ase (

The roles of chalcogenides in O2 protection of H2ase active sites

“…That is, the 2-oxy sulnate more readily forms and is likely to remain in this form. 50 Computations also agree the observation that the bridging-S that is tethered to the N of the diazacycle are disfavored to become oxygenated. A computational cleavage of the tether results in a 15 kcal mole À1 lowering of the oxygenated sulfur, and a preference for it over the S of the PhS.…”

“…32,33 An isotopic labeling/mass spectroscopy experiment analyzing products from a mixture of 32 O 2 / 36 O 2 found the two oxygens on À S(O) 2 C 6 H 4 NH 2 are from the same O 2 molecule, consistent with a concerted dioxygen addition reaction. 50 If analogous to the concerted addition of 32 O 2 / 36 O 2 earlier studied, this result suggests O 2 binding at Ni as precursor to its transfer to the chalcogen. However the nuclearity of the complex that takes up the O 2 is as unknown as is the fate of the second oxygen in the 1-oxy species.…”

“…The simple dinickel buttery complex precursor may be split by various nucleophiles including carbenes, imidazoles, thiolates, and selenoates. [49][50][51] With the chalcogenates, entry to NiFe bimetallics is possible making use of the (h5-C 5 H 5 ) Fe(CO)(NCCH 3 ) 2 + receiver species outlined in Fig. 6.…”

“…The 5-membered rings in the selenium and sulfur forms are largely the same as the additional oxygen in the latter projects outside of the 5-membered ring. 49,50 Notably the 2-oxy species and the 1-oxy sulfur species have been detected by mass spectroscopy but are not isolated sufficiently pure for XRD determinations. Interesting features in these reactions and products include (a) the more rapid reaction of O 2 with the Se as compared to the S analogues; (b) oxygen partitioning in S vs.…”

“…49 While various ratios of 1-and 2-oxy species are found in the oxygenation reactions of most À E-Ph-X derivatives in this family of complexes, the thiolate of greater basicity, -SC 6 H 4 NH 2 , shows selectivity for the 2-oxy species. 50 Such a result provided opportunity to explore one aspect of the mechanism of O 2 addition via isotope distribution reactions, as presaged in O 2 addition studies of the monomeric NiN 2 S 2 species. 32,33 An isotopic labeling/mass spectroscopy experiment analyzing products from a mixture of 32 O 2 / 36 O 2 found the two oxygens on À S(O) 2 C 6 H 4 NH 2 are from the same O 2 molecule, consistent with a concerted dioxygen addition reaction.…”