Modeling the Active Site of [NiFe] Hydrogenases and the [NiFeu] Subsite of the C-Cluster of Carbon Monoxide Dehydrogenases: Low-Spin Iron(II) Versus High-Spin Iron(II)

Molecular Transition Metal Oxide Electrocatalysts for the Reversible Carbon Dioxide-Carbon Monoxide Transformation

Carbon monoxide dehydrogenase (CODH) enzymes are active for the reversible CO oxidation-CO₂ reduction reaction and are of interest in the context of CO₂ abatement and carbon-neutral solar fuels. Bioinspired by the active-site composition of the CODHs, polyoxometalates triply substituted with first-row transition metals were modularly synthesized. The polyanions, in short, {SiM₃ W₉ } and {SiM'₂ M''W₉ }, M, M', M''=Cu^II , Ni^II , Fe^III are shown to be electrocatalysts for reversible CO oxidation-CO₂ reduction. A catalytic Tafel plot showed that {SiCu₃ W₉ } was the most reactive for CO₂ reduction, and electrolysis reactions yielded significant amounts of CO with 98 % faradaic efficiency. In contrast, Fe-Ni compounds such as {SiFeNi₂ W₉ } preferably catalyzed the oxidation of CO to CO₂ similar to what is observed for the [NiFe]-CODH enzyme. Compositional control of the heterometal complexes, now and in the future, leads to control of reactivity and selectivity for CO₂ electrocatalytic reduction.

Bimetallic nickel-cobalt hydrides in H2 activation and catalytic proton reduction

The synergism of the electronic properties of nickel and cobalt enables bimetallic NiCo complexes to process H2. The nickel-cobalt hydride [(dppe)Ni(pdt)(H)CoCp*]+ ([1H]+ ) arising from protonation of the reduced state 1 was found to be an efficient electrocatalyst for H2 evolution with Cl2CHCOOH, and the oxidized [Ni(ii)Co(iii)]2+ form is capable of activating H2 to produce [1H]+ . The features of stereodynamics, acid-base properties, redox chemistry and reactivity of these bimetallic NiCo complexes in processing H2 are potentially related to the active site of [NiFe]-H2ases.

Complexes of MN2S2·Fe(η5-C5R5)(CO) as platform for exploring cooperative heterobimetallic effects in HER electrocatalysis

The control of aggregation at sulfur by metallodithiolates (MN₂S₂) has made them prime candidates as building blocks for the synthesis of biomimetics of various bimetallic enzyme active sites, with reactivity consequences implicating redox control by both metal centers. Recent studies of MN₂S₂ (M = Ni²⁺, Fe(NO)²⁺) bound to [(η⁵-C₅H₅)Fe(CO)]⁺ as electrocatalysts for proton reduction, the hydrogen evolution reaction, demonstrated reduction-induced hemi-lability of the bridging cis-dithiolates as a key step in the electrochemical proton reduction process (Ding, et al., J. Am. Chem. Soc., 2016, 138, 12920-12927). The MN₂S₂·Fe(η⁵-C₅R₅)(CO) platform offers numerous possibilities for tuning the electronic character of the M(μ-S₂)Fe core. As well as modifying M within the metallodithiolate ligand, replacing H by CH₃ at the η⁵-C₅R₅ moiety increases the electron density at the Fe center, which might facilitate the reductive Fe-S bond cleavage. Although release of a free thiolate in these hemi-labile ligands creates a needed internal pendant base, this benefit might be countered by the increase in over-potential for addition of the first electron. Herein we report the preparation and characterization of four bimetallic aggregates with the (η⁵-C₅R₅)Fe(CO) (R = H, CH₃; Fe' or Fe*', respectively) or the dicarbonyl (η⁵-C₅R₅)Fe(CO)₂ scaffold (R = H, CH₃; Fe'' or Fe*'', respectively) bound to redox active MN₂S₂ ligands (M = Ni²⁺, Co(NO)²⁺; N₂S₂ = bismercaptoethane diazacycloheptane) Co-Fe*', Ni-Fe*', Co-Fe' and Co-Fe*'' complexes. The bidentate complexes were found to be electrocatalysts for proton reduction, although at high over-potential, especially for the derivatives of the electron-rich (η⁵-C₅(CH₃)₅)Fe(CO)⁺. The turnover (TON) and turnover frequencies (TOF) were determined and found to be comparable to the previously reported MN₂S₂·Fe(η⁵-C₅H₅)(CO)⁺ analogues.

A matrix of heterobimetallic complexes for interrogation of hydrogen evolution reaction electrocatalysts

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 -S δ-···H+··· δ-S- arrangement.

Iron(II) Complexes of a Hemilabile SNS Amido Ligand: Synthesis, Characterization, and Reactivity

We report an easily prepared bis(thioether) amine ligand, S^MeN^HS^Me, along with the synthesis, characterization, and reactivity of the paramagnetic iron(II) bis(amido) complex, [Fe(κ³-S^MeNS^Me)₂] (1). Binding of the two different thioethers to Fe generates both five- and six-membered rings with Fe-S bonds in the five-membered rings (av 2.54 Å) being significantly shorter than those in the six-membered rings (av 2.71 Å), suggesting hemilability of the latter thioethers. Consistent with this hypothesis, magnetic circular dichroism (MCD) and computational (TD-DFT) studies indicate that 1 in solution contains a five-coordinate component [Fe(κ³-S^MeNS^Me)(κ²-S^MeNS^Me)] (2). This ligand hemilability was demonstrated further by reactivity studies of 1 with 2,2'-bipyridine, 1,2-bis(dimethylphosphino)ethane, and 2,6-dimethylphenyl isonitrile to afford iron(II) complexes [L₂Fe(κ²-S^MeNS^Me)₂] (3-5). Addition of a Brønsted acid, HNTf₂, to 1 produces the paramagnetic, iron(II) amine-amido cation, [Fe(κ³-S^MeNS^Me)(κ³-S^MeN^HS^Me)](NTf₂) (6; Tf = SO₂CF₃). Cation 6 readily undergoes amine ligand substitution by triphos, affording the 16e^- complex [Fe(κ²-S^MeNS^Me)(κ³-triphos)](NTf₂) (7; triphos = bis(2-diphenylphosphinoethyl)phenylphosphine). These complexes are characterized by elemental analysis; ¹H NMR, Mössbauer, IR, and UV-vis spectroscopy; and single-crystal X-ray diffraction. Preliminary results of amine-borane dehydrogenation catalysis show complex 7 to be a selective and particularly robust precatalyst.

Quantum chemical approaches to [NiFe] hydrogenase

The mechanism by which [NiFe] hydrogenase catalyses the oxidation of molecular hydrogen is a significant yet challenging topic in bioinorganic chemistry. With far-reaching applications in renewable energy and carbon mitigation, significant effort has been invested in the study of these complexes. In particular, computational approaches offer a unique perspective on how this enzyme functions at an electronic and atomistic level. In this article, we discuss state-of-the art quantum chemical methods and how they have helped deepen our comprehension of [NiFe] hydrogenase. We outline the key strategies that can be used to compute the (i) geometry, (ii) electronic structure, (iii) thermodynamics and (iv) kinetic properties associated with the enzymatic activity of [NiFe] hydrogenase and other bioinorganic complexes.

Comparisons of MN2S2vs. bipyridine as redox-active ligands to manganese and rhenium in (L–L)M′(CO)3Cl complexes

Electronic communication was established for a heterobimetallic complex which upon reduction at one metal center modulates ligand loss and subsequent electron uptake at the second metal.

Synthetic [NiFe] models with a fluxional CO ligand

A [NiFe] complex [(dppe)Ni(pdt)FeCp*(CO)]BF₄ was characterized as two isomers, and their interconversions were established by thermal process and electrochemistry.

Nickel–ruthenium-based complexes as biomimetic models of [NiFe] and [NiFeSe] hydrogenases for dihydrogen evolution

Electrocatalytic proton reduction was studied using nickel–ruthenium complexes that were developed as models for [NiFe] and [NiFeSe] hydrogenases.

Heteronuclear assembly of Ni–Cu dithiolato complexes: synthesis, structures, and reactivity studies

A mild route was discovered to synthesize heterometallic [Ni^IICu^I] complexes featuring square-planar Ni(ii) and distorted tetrahedral Cu(i).