Oxygen uptake in complexes related to [NiFeS]- and [NiFeSe]-hydrogenase active sites

Protein-based models offer mechanistic insight into complex nickel metalloenzymes

There are ten nickel enzymes found across biological systems, each with a distinct active site and reactivity that spans reductive, oxidative, and redox-neutral processes. We focus on the reductive enzymes, which catalyze reactions that are highly germane to the modern-day climate crisis: [NiFe] hydrogenase, carbon monoxide dehydrogenase, acetyl coenzyme A synthase, and methyl coenzyme M reductase. The current mechanistic understanding of each enzyme system is reviewed along with existing knowledge gaps, which are addressed through the development of protein-derived models, as described here. This opinion is intended to highlight the advantages of using robust protein scaffolds for modeling multiscale contributions to reactivity and inspire the development of novel artificial metalloenzymes for other small molecule transformations.

Ethereal Hydroperoxides: Powerful Reagents for S-Oxygenation of Bridging Thiophenolate Functions

S-Oxygenation of thiophenolate bridges by ethereal hydroperoxides was studied. [Ni^II₂L^S(PhCO₂)]⁺ (1), where L^S = macrocyclic aminethiolate supporting ligand, is S-oxygenated readily in a mixed methanol/acetonitrile solution with ether/dioxygen at room temperature in the presence of daylight. The reactions were found to depend strongly on the choice of the ether. Uptake of two O atoms occurs in dioxane to give a mixed thiolate/sulfinate complex [Ni^II₂L^SO₂(PhCO₂)]⁺ (2) containing the rare five-membered Ni(μ_1,1-S)(μ_1,2-OS)Ni core. In tetrahydrofuran, four O atoms are taken up by 1 to generate the bis(sulfinate) species [Ni^II₂L^SO₄(PhCO₂)]⁺ (3). A mono-S-oxygenated sulfenate intermediate can be detected by electrospray ionization mass spectrometry. The oxygenation reactions proceed in high yields without complex disintegration and invariably provide μ_1,2-bridging sulfinates as established by spectroscopy (IR and UV/vis), X-ray crystallography, and accompanying density functional theory calculations. The oxygenation of the S atoms has a strong impact on the electronic structures of the nickel complexes. The monosulfinate complex 2 has an S = 2 ground state resulting from moderate ferromagnetic exchange coupling interactions (J = +15.7 cm^-1; H = -2JS₁S₂), while an antiferromagnetic exchange interaction in 3 shows the presence of a ground state with spin S = 0 (J = -0.56 cm^-1).

Self-Assembled Nickel-4 Supramolecular Squares and Assays for HER Electrocatalysts Derived Therefrom

Solid-state structures find a self-assembled tetrameric nickel cage with carboxylate linkages, [Ni(N₂S'O)I(CH₃CN)]₄ ([Ni-I]₄⁰), resulting from sulfur acetylation by sodium iodoacetate of an [NiN₂S]₂²⁺ dimer in acetonitrile. Various synthetic routes to the tetramer, best described from XRD as a molecular square, were discovered to generate the hexacoordinate nickel units ligated by N₂S_thioether, iodide, and two carboxylate oxygens, one of which is the bridge from the adjacent nickel unit in [Ni-I]₄⁰. Removal of the four iodides by silver ion precipitation yields an analogous species but with an additional vacant coordination site, [Ni-Solv]⁺, a cation but with coordinated solvent molecules. This also recrystallizes as the tetramer [Ni-Solv]₄⁴⁺. In solution, dissociation into the (presumed) monomer occurs, with coordinating solvents occupying the vacant site [Ni(N₂S'O)I(solv)]⁰, ([Ni-I]⁰). Hydrodynamic radii determined from ¹H DOSY NMR data suggest that monomeric units are present as well in CD₂Cl₂. Evans method magnetism values are consistent with triplet spin states in polar solvents; however, in CD₂Cl₂ solutions no paramagnetism is evident. The abilities of [Ni-I]₄⁰ and [Ni-Solv]₄⁴⁺ to serve as sources of electrocatalysts, or precatalysts, for the hydrogen evolution reaction (HER) were explored. Cyclic voltammetry responses and bulk coulometry with gas chromatographic analysis demonstrated that a stronger acid, trifluoroacetic acid, as a proton source resulted in H₂ production from both electroprecatalysts; however, electrocatalysis developed primarily from uncharacterized deposits on the electrode. With acetic acid as a proton source, the major contribution to the HER is from homogeneous electrocatalysis. Overpotentials of 490 mV were obtained for both the solution-phase [Ni-I]⁰ and [Ni-Solv]⁺. While the electrocatalyst derived from [Ni-Solv]⁺ has a substantially higher TOF (102 s^-1) than [Ni-I]⁰ (19 s^-1), it has a shorter catalytically active lifespan (4 h) in comparison to [Ni-I]⁰ (>18 h).

Metalloligands based on Robson-type amino-thiophenolato macrocycles for assembly of heterotrimetallic complexes

The ability of macrocyclic Co and Ni aminothiolate complexes to act as metalloligands towards cuprate ions was established. Adduct formation is enabled by a thiolate-to-Cu⁺ charge transfer (CT) interaction giving stable heterotrimetallics with magnetic properties.

Thiolate Coordination vs C-S Bond Cleavage of Thiolates in Dinickel(II) Complexes

A detailed study for the synthesis of dinickel(II)-thiolate and dinickel(II)-hydrosulfide complexes and the complete characterization of the relevant intermediates involved in the C-S bond cleavage of thiolates are presented. Hydrated Ni(II) salts mediate the hydrolytic C-S bond cleavage of thiolates (NaSR/RSH; R = Me, Et, ⁿBu, ^tBu), albeit inefficiently, to yield a mixture of a dinickel(II)-hydrosulfide complex, [Ni₂(BPMP)(μ-SH)(DMF)₂]²⁺ (1), and the corresponding dinickel(II)-thiolate complexes, such as [Ni₂(BPMP)(μ-SEt)(ClO₄)]¹⁺ (2) (HBPMP is 2,6-bis[[bis(2-pyridylmethyl)amino]methyl]-4-methylphenol). A systematic study for the reactivity of thiolates with Ni(II) was therefore pursued which finally yielded 1 as a pure product which has been characterized in comparison with the dinickel(II)-dichloride complex, [Ni₂(BPMP)(Cl)₂(MeOH)₂]¹⁺ (3). While the reaction of thiolates with anhydrous Ni(OTf)₂ in dry conditions could only yield [Ni₂(BPMP)(OTf)₂]¹⁺ (5) instead of the expected dinickel(II)-thiolate compound, the C-S bond cleavage could be suppressed by the use of a chelating thiol, such as PhCOSH, to yield [Ni₂(BPMP)(SCOPh)₂]¹⁺ (6). Finally, with the suitable choice of a monodentate thiol, a dinickel(II)-monothiolate complex, [Ni₂(BPMP)(SPh)(DMF)(MeOH)(H₂O)]²⁺ (7), was isolated as a pure product within 1 h of reaction, which after a longer time of reaction yielded 1 and PhOH. Complex 7 may thus be regarded as the intermediate that precedes the C-S bond cleavage and is generated by the reaction of a thiolate with an initially formed dinickel(II)-solvento complex, [Ni₂(BPMP)(MeOH)₂(H₂O)₂]³⁺(4). Selected dinickel(II) complexes were explored further for the scope of substitution reactions, and the results include the isolation of a dinickel(II)-bis(thiolate) complex, [Ni₂(BPMP)(μ-SPh)₂]¹⁺ (8).

Biochemical and artificial pathways for the reduction of carbon dioxide, nitrite and the competing proton reduction: effect of 2nd sphere interactions in catalysis

Reduction of oxides and oxoanions of carbon and nitrogen are of great contemporary importance as they are crucial for a sustainable environment.

A NiRhS fuel cell catalyst - lessons from hydrogenase

We present a novel fuel cell heterogeneous catalyst based on rhodium, nickel and sulfur with power densities 5-28% that of platinum. The NiRhS heterogeneous catalyst was developed via a homogeneous model complex of the [NiFe]hydrogenases (H₂ases) and can act as both the cathode and anode of a fuel cell.

Reactivation of sulfide-protected [FeFe] hydrogenase in a redox-active hydrogel

[FeFe] hydrogenases are highly active hydrogen conversion catalysts but are notoriously sensitive to oxidative damage. Redox hydrogels have been used for protecting hydrogenases from both high potential inactivation and oxygen inactivation under turnover conditions. However, [FeFe] hydrogenase containing redox hydrogels must be fabricated under strict anoxic conditions. Sulfide coordination at the active center of the [FeFe] hydrogenase from Desulfovibrio desulfuricans protects this enzyme from oxygen in an inactive state, which can be reactivated upon reduction. Here, we show that this oxygen-stable inactive form of the hydrogenase can be reactivated in a redox hydrogel enabling practical use of this highly O₂ sensitive enzyme without the need for anoxic conditions.

The roles of chalcogenides in O2 protection of H2ase active sites

At some point, all HER (Hydrogen Evolution Reaction) catalysts, important in sustainable H2O splitting technology, will encounter O2 and O2-damage. The [NiFeSe]-H2ases and some of the [NiFeS]-H2ases, biocatalysts for reversible H2 production from protons and electrons, are exemplars of oxygen tolerant HER catalysts in nature. In the hydrogenase active sites oxygen damage may be extensive (irreversible) as it is for the [FeFe]-H2ase or moderate (reversible) for the [NiFe]-H2ases. The affinity of oxygen for sulfur, in [NiFeS]-H2ase, and selenium, in [NiFeSe]-H2ase, yielding oxygenated chalcogens results in maintenance of the core NiFe unit, and myriad observable but inactive states, which can be reductively repaired. In contrast, the [FeFe]-H2ase active site has less possibilities for chalcogen-oxygen uptake and a greater chance for O2-attack on iron. Exposure to O2 typically leads to irreversible damage. Despite the evidence of S/Se-oxygenation in the active sites of hydrogenases, there are limited reported synthetic models. This perspective will give an overview of the studies of O2 reactions with the hydrogenases and biomimetics with focus on our recent studies that compare sulfur and selenium containing synthetic analogues of the [NiFe]-H2ase active sites.

Probing the Structure of [NiFeSe] Hydrogenase with QM/MM Computations

The geometry and vibrational behavior of selenocysteine [NiFeSe] hydrogenase isolated from Desulfovibrio vulgaris Hildenborough have been investigated using a hybrid quantum mechanical (QM)/ molecular mechanical (MM) approach. Structural models have been built based on the three conformers identified in the recent crystal structure resolved at 1.3 Å from X-ray crystallography. In the models, a diamagnetic Ni2+ atom was modeled in combination with both Fe2+ and Fe3+ to investigate the effect of iron oxidation on geometry and vibrational frequency of the nonproteic ligands, CO and CN-, coordinated to the Fe atom. Overall, the QM/MM optimized geometries are in good agreement with the experimentally resolved geometries. Analysis of computed vibrational frequencies, in comparison with experimental Fourier-transform infrared (FTIR) frequencies, suggests that a mixture of conformers as well as Fe2+ and Fe3+ oxidation states may be responsible for the acquired vibrational spectra.

Two heterodinuclear NiFe-based sulfenate complexes mimicking an S-oxygenated intermediate of an O2-tolerant [NiFe]-H2ase: synthesis, structures, and reactivity

Two biomimetic models for an O₂-tolerant [NiFe]-H₂ase are successfully prepared by reactions of sulfenate complex 2 with Fe₂(CO)₉ and CpFe(CO)₂BF₄.

Complete Protection of O2-Sensitive Catalysts in Thin Films

Energy conversion schemes involving dihydrogen hold great potential for meeting sustainable energy needs, but widespread implementation cannot proceed without solutions that mitigate the cost of rare metal catalysts and the O2 instability of biological and bioinspired replacements. Recently, thick films (>100 μm) of redox polymers were shown to prevent O2 catalyst damage but also resulted in unnecessary catalyst load and mass transport limitations. Here we apply novel homogeneous thin films (down to 3 μm) that provide protection from O2 while achieving highly efficient catalyst utilization. Our empirical data are explained by modeling, demonstrating that resistance to O2 inactivation can be obtained for nonlimiting periods of time when the optimal thickness for catalyst utilization and current generation is achieved, even when using highly fragile catalysts such as the enzyme hydrogenase. We show that different protection mechanisms operate depending on the matrix dimensions and the intrinsic catalyst properties and can be integrated together synergistically to achieve stable H2 oxidation currents in the presence of O2, potentially enabling a plethora of practical applications for bioinspired catalysts under harsh oxidative conditions.