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The NiFe hydrogenase biomimetics are protected from oxygen invaders by sulfur and selenium castle guards., A biomimetic study for S/Se oxygenation in Ni(μ-EPh)(μ-SN2)Fe, (E = S or Se; SN2 = Me-diazacycloheptane-CH2CH2S); Fe = (η5-C5H5)FeII(CO) complexes related to the oxygen-damaged active sites of [NiFeS]/[NiFeSe]-H2ases 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. Computational studies (DFT) found that the lowest energy isomers of mono-oxygen derivatives of Ni(μ-EPh)(μ-SN2)Fe complexes were those with O attachment to Ni rather than Fe, a result consonant with experimental findings, but at odds with oxygenates found in oxygen-damaged [NiFeS]/[NiFeSe]-H2ase structures. A computer-generated model based on substituting –SMe for the N-CH2CH2S– sulfur donor of the N2S suggested that constraint within the chelate hindered O-atom uptake at that sulfur site.

Nitrosyls as electron reservoirs guide protons to favorable sites in bimetallic HER catalysts., Experimental and computational studies address key questions in a structure–function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN2S2 or [(NO)Fe]N2S2 as donors to (η5-C5H5)Fe(CO)+ or [Fe(NO)2]+/0 generate a series of four bimetallics, gradually “softened” by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe–S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S–H+···–H–Fe units of the “harder” bimetallic arrangements with more localized electron density, while softer units convert H–···H–via reductive elimination from two Fe–H deriving from the highly delocalized, doubly reduced [Fe2(NO)3]– derivative. Computational studies also account for the inactivity of a Ni2Fe complex resulting from entanglement of added H+ in a pinched –Sδ–···H+···δ–S– arrangement.

Concepts from organometallic chemistry are used to define possibilities of cyanide as a docking unit for bioassembly processes., Developing from certain catalytic processes required for ancient life forms, the H2 processing enzymes [NiFe]- and [FeFe]-hydrogenase (H2ase) have active sites that are organometallic in composition, possessing carbon monoxide and cyanide as ligands. Simple synthetic analogues of the 2Fe portion of the active site of [FeFe]-H2ase have been shown to dock into the empty carrier (maturation) protein, apo-Hyd-F, via the bridging ability of a terminal cyanide ligand from a low valent FeIFeI unit to the iron of a 4Fe4S cluster of Hyd-F, with spectral evidence indicating CN isomerization during the coupling process (Berggren, et al., Nature, 2013, 499, 66–70). To probe the requirements for such cyanide couplings, we have prepared and characterized four cyanide-bridged analogues of 3-Fe systems with features related to the organoiron moiety within the loaded HydF protein. As in classical organometallic chemistry, the orientation of the CN bridge in the biomimetics is determined by the precursor reagents; no cyanide flipping or linkage isomerization was observed. Density functional theory computations evaluated the energetics of cyanide isomerization in such [FeFe]–CN–Fe ⇌ [FeFe]–NC–Fe units, and found excessively high barriers account for the failure to observe the alternative isomers. These results highlight roles for cyanide as an unusual ligand in biology that may stabilize low spin iron in [FeFe]-hydrogenase, and can act as a bridge connecting multi-iron units during bioassembly of the active site.

[This corrects the article DOI: 10.1039/D3SC01546G.]., (This journal is © The Royal Society of Chemistry.)