The Roles of Chalcogenides in O2 Protection of NiFe Complexes Related to the [NiFe]- and [NiFeSe]-Hydrogenases Active Sites
Abstract
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. The work in my dissertation laid the studies of O₂ reactions with the Nickel/Iron complexes with features of the active sites of [NiFeS]- and [NiFeSe]-H₂ase. We have observed the oxygen uptake and removal in Ni(µ-EPhX)(µS’N₂)Fe (E = S or Se, SN₂= Me-diazacycloheptane-CH₂CH₂S, Fe = (η⁵ -C₅H₅)FeII(CO)) complexes, and the O₂ reactivity can be controlled by electron density on chalcogenides.
Firstly, a biomimetic study for S/Se oxygenation in Ni(µ-EPh)(µ-SN₂)Fe is described. Mono- and di-oxygenates (major and minor species, respectively) of the chalcogens result from exposure of the heterobimetallics to O₂; one was isolated and structurally characterized to have Ni-O-SePh-Fe-S connectivity within a 5-membered ring. A compositionally analogous mono-oxy species was implicated by ν(CO) IR spectroscopy to be the corresponding Ni-O-SPh-Fe-S complex; treatment with O-abstraction agents such as P(o-tolyl)₃ or PMe₃ remediated the O damage.
In attempts to identify the controlling features of S-site oxygen uptake, related Ni(µ-EPhX)(µ-S’N₂)Fe 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. 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. 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.
To improve the “oxygen tolerance” of the NiSeFe complexes, steric hindrance and electronic effects in the system were further explored and discussed. The SePh group was replaced by a less sterically encumbered group, SeMe; however, it resulted in less yield of oxygenates. Comparisons were also made between CpFe(CO) (Cp = η⁵ -C₅H₅) and Cp*Fe(CO) (Cp* = η⁵ -C₅Me₅); the latter has higher electron density and resulted in better yield of oxygenates.
Citation
Yang, Xuemei (2020). The Roles of Chalcogenides in O2 Protection of NiFe Complexes Related to the [NiFe]- and [NiFeSe]-Hydrogenases Active Sites. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /192805.