Thiolate‐Bridged Iron–Nickel Models for the Active Site of [NiFe] Hydrogenase

Assessment of heavy metals and their human health risks in selected spices from South Africa

The concerns of food safety are rising in developing countries such as South Africa as a result of heavy metal contamination of culinary herbs and spices. Spices and herbs are used for therapeutic purposes as well as flavoring and coloring food. Heavy metals in spices represent significant health risks due to their high toxicity in high quantities. A total of 20 spices samples were purchased from different registered shops, for heavy metals analysis. The samples were prepared, digested, and analyzed with an inductively coupled plasma mass spectrometer (ICP-MS). To ensure the method's accuracy, Polish Certified Reference Mixed Polish Herbs (INCT-MPH-2) from the Food and Drugs Control Center, Poland, was analyzed. The concentrations of Fe (32.22 ± 1.22-131.1 ± 3.26 mg/kg), As (ND to 0.12 ± 0.04 mg/kg), Cr (0.08 ± 0.01-3.2 ± 0.09 mg/kg), Pb (N.D - 0.21 ± 0.02 mg/kg) and Cd (ND to 0.14 ± 0.08 mg/kg) mg/kg were measured. The results revealed that the concentrations of Cr in all spice samples tested were much higher than the Maximum permissible limit (MPL) values. All spices in this study had THQ and HI values less than one, indicating that consumers will experience no potential health hazards from consuming specific metals through spices. However, continual scrutiny should be maintained.

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 Dithiolato and Hydrido Bridged (CO/CN)Fe-Ni Complex with Unprotected CN: A Model for the [Ni-R] State of the [Ni-Fe] Hydrogenase Active Site

A dithiolate/hydride bridged Fe-Ni complex, [(CN)(CO)₂Fe^II(μ-pdt)(μ-H)Ni^II(CN)(PCy₃)]^- (2, pdt = propane-1,3-dithiolate) has been synthesized by the reaction of [(CN)₂(CO)₂Fe^II(pdt)]^2- with [Ni^II(Cl)(H)(PCy₃)₂] as a synthetic analogue of the Ni-R state of the active site of the [Ni-Fe] hydrogenase. X-ray crystallography of this model complex suggests that the hydride unsymmetrically binds to Ni and Fe similar to natural [Ni-Fe] hydrogenases.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates

Described are the syntheses of several Ni(μ-SR)₂Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS₃)^- and Ni(NS₃)^- (PS₃^3- = tris(phenyl-2-thiolato)phosphine, NS₃^3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C₆F₅)₂. The following new nickel precursor complexes were characterized: PPh₄[Ni(NS₃)] and the dimeric imino/amino-phosphine complexes [NiCl₂(PCH═N^An)]₂ and [NiCl₂(PCH₂NH^An)]₂ (P = Ph₂PC₆H₄-2-). The iron(II) reagents include [CpFe(CO)₂(thf)]BF₄, [Cp*Fe(CO)(MeCN)₂]BF₄, FeI₂(CO)₄, FeCl₂(diphos)(CO)₂, and Fe(pdt)(CO)₂(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS₃), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF₄, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)₃]BAr^F₄, [(PCH═N^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH₂NH^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH═N^An)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF₄, [(dppv)(CO)Fe(μ-pdt)]₂Ni, {H[(dppv)(CO)Fe(μ-pdt)]₂Ni]}BF₄, and (C₆F₅)₂Ni(μ-pdt)Fe(CO)₂(dppv) (DIPAMP = (CH₂P(C₆H₄-2-OMe)₂)₂; BAr^F₄^- = [B(C₆H₃-3,5-(CF₃)₂]₄^-)) Within the context of Ni-(SR)₂-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]₂Ni}⁺, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)₄ site, as supported by density functional theory calculations.

Homogeneous catalytic reduction of polyoxometalate by hydrogen gas with a hydrogenase model complex

Homogeneous catalytic reduction of polyoxometalates by hydrogen gas with a hydrogenase model complex was investigated for the first time.

Comprehensive reaction mechanisms at and near the Ni-Fe active sites of [NiFe] hydrogenases

[NiFe] hydrogenase (H2ase) catalyzes the oxidation of dihydrogen to two protons and two electrons and/or its reverse reaction. For this simple reaction, the enzyme has developed a sophisticated but intricate mechanism with heterolytic cleavage of dihydrogen (or a combination of a hydride and a proton), where its Ni-Fe active site exhibits various redox states. Recently, thermodynamic parameters of the acid-base equilibrium for activation-inactivation, a new intermediate in the catalytic reaction, and new crystal structures of [NiFe] H2ases have been reported, providing significant insights into the activation-inactivation and catalytic reaction mechanisms of [NiFe] H2ases. This Perspective provides an overview of the reaction mechanisms of [NiFe] H2ases based on these new findings.

Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN^- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N₂ S₂)Fe(CO)₂(CN)₂], [Ni( S₄)Fe(CO)₂(CN)₂], and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)₂(CN)₂} unit. X-ray crystallographic studies on [Ni( N₂ S₃)Fe(CO)₂(CN)₂], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc⁺/Fc and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] possesses a reversible oxidation process at 0.17 V vs Fc⁺/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a Ni^IIIFe^II formulation for [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺. The singly occupied molecular orbital (SOMO) in [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ is based on Ni 3d_z² and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a Ni^IIIFe^II formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N₂ S₃)Fe(CO)₂(CN)₂] and its [Ni( N₂ S₃)] precursor, together with calculations on the oxidized [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ and [Ni( N₂ S₃)]⁺ forms suggests that the binding of the {Fe(CO)(CN)₂} unit to the {Ni(CysS)₄} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Light-Driven Hydrogen Evolution by Nickel-Substituted Rubredoxin

An enzymatic system for light-driven hydrogen generation has been developed through covalent attachment of a ruthenium chromophore to nickel-substituted rubredoxin (NiRd). The photoinduced activity of the hybrid enzyme is significantly greater than that of a two-component system and is strongly dependent on the position of the ruthenium phototrigger relative to the active site, indicating a role for intramolecular electron transfer in catalysis. Steady-state and time-resolved emission spectra reveal a pathway for rapid, direct quenching of the ruthenium excited state by nickel, but low overall turnover numbers suggest initial electron transfer is not the rate-limiting step. This approach is ideally suited for detailed mechanistic investigations of catalysis by NiRd and other molecular systems, with implications for generation of solar fuels.

Tris(2-mercaptoimidazolyl)hydroborato Cadmium Thiolate Complexes, [TmBut]CdSAr: Thiolate Exchange at Cadmium in a Sulfur-Rich Coordination Environment

A series of cadmium thiolate compounds that feature a sulfur-rich coordination environment, namely [TmBut]CdSAr, have been synthesized by the reactions of [TmBut]CdMe with ArSH (Ar = C6H4-4-F, C6H4-4-But, C6H4-4-OMe, and C6H4-3-OMe). In addition, the pyridine-2-thiolate and pyridine-2-selenolate derivatives, [TmBut]CdSPy and [TmBut]CdSePy have been obtained via the respective reactions of [TmBut]CdMe with pyridine-2-thione and pyridine-2-selone. The molecular structures of [TmBut]CdSAr and [TmBut]CdEPy (E = S or Se) have been determined by X-ray diffraction and demonstrate that, in each case, the [CdS4] motif is distorted tetrahedral and approaches a trigonal monopyramidal geometry in which the thiolate ligand adopts an equatorial position; [TmBut]CdSPy and [TmBut]CdSePy, however, exhibit an additional long-range interaction with the pyridyl nitrogen atoms. The ability of the thiolate ligands to participate in exchange was probed by 1H and 19F nuclear magnetic resonance (NMR) spectroscopic studies of the reactions of [TmBut]CdSC6H4-4-F with ArSH (Ar = C6H4-4-But or C6H4-4-OMe), which demonstrate that (i) exchange is facile and (ii) coordination of thiolate to cadmium is most favored for the p-fluorophenyl derivative. Furthermore, a two-dimensional EXSY experiment involving [TmBut]CdSC6H4-4-F and 4-fluorothiophenol demonstrates that degenerate thiolate ligand exchange is also facile on the NMR time scale.

Dithiolato- and halogenido-bridged nickel–iron complexes related to the active site of [NiFe]-H2ases: preparation, structures, and electrocatalytic H2 production

A new series of [NiFe]-H₂ase mimics (5a,b–7a,b) has been prepared and structurally characterized; particularly, they have been found to be pre-catalysts for H₂ production from Cl₂CHCO₂H under CV conditions.

A platinum(ii)–palladium(ii)–nickel(ii) heterotrimetallic coordination polymer showing a cooperative effect on catalytic hydrogen evolution

Stepwise construction of a 1D heterotrimetallic coordination polymer containing all three group 10 metal ions and its heterogeneous catalytic activity for electrochemical hydrogen evolution are reported.

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.

Hydrogenase Enzymes and Their Synthetic Models: The Role of Metal Hydrides

Hydrogenase enzymes efficiently process H2 and protons at organometallic FeFe, NiFe, or Fe active sites. Synthetic modeling of the many H2ase states has provided insight into H2ase structure and mechanism, as well as afforded catalysts for the H2 energy vector. Particularly important are hydride-bearing states, with synthetic hydride analogues now known for each hydrogenase class. These hydrides are typically prepared by protonation of low-valent cores. Examples of FeFe and NiFe hydrides derived from H2 have also been prepared. Such chemistry is more developed than mimicry of the redox-inactive monoFe enzyme, although functional models of the latter are now emerging. Advances in physical and theoretical characterization of H2ase enzymes and synthetic models have proven key to the study of hydrides in particular, and will guide modeling efforts toward more robust and active species optimized for practical applications.

Nickel-centred proton reduction catalysis in a model of [NiFe] hydrogenase

Hydrogen production through water splitting is one of the most promising solutions for the storage of renewable energy. [NiFe] hydrogenases are organometallic enzymes containing nickel and iron centres that catalyse hydrogen evolution with performances that rival those of platinum. These enzymes provide inspiration for the design of new molecular catalysts that do not require precious metals. However, all heterodinuclear NiFe models reported so far do not reproduce the Ni-centred reactivity found at the active site of [NiFe] hydrogenases. Here, we report a structural and functional NiFe mimic that displays reactivity at the Ni site. This is shown by the detection of two catalytic intermediates that reproduce structural and electronic features of the Ni-L and Ni-R states of the enzyme during catalytic turnover. Under electrocatalytic conditions, this mimic displays high rates for H₂ evolution (second-order rate constant of 2.5 × 10⁴ M^-1 s^-1; turnover frequency of 250 s^-1 at 10 mM H⁺ concentration) from mildly acidic solutions.

Models of the Ni-L and Ni-SIa States of the [NiFe]-Hydrogenase Active Site

A new class of synthetic models for the active site of [NiFe]-hydrogenases are described. The Ni(I/II)(SCys)2 and Fe(II)(CN)2CO sites are represented with (RC5H4)Ni(I/II) and Fe(II)(diphos)(CO) modules, where diphos = 1,2-C2H4(PPh2)2(dppe) or cis-1,2-C2H2(PPh2)2(dppv). The two bridging thiolate ligands are represented by CH2(CH2S)2(2-) (pdt(2-)), Me2C(CH2S)2(2-) (Me2pdt(2-)), and (C6H5S)2(2-). The reaction of Fe(pdt)(CO)2(dppe) and [(C5H5)3Ni2]BF4 affords [(C5H5)Ni(pdt)Fe(dppe)(CO)]BF4 ([1a]BF4). Monocarbonyl [1a]BF4 features an S = 0 Ni(II)Fe(II) center with five-coordinated iron, as proposed for the Ni-SIa state of the enzyme. One-electron reduction of [1a](+) affords the S = 1/2 derivative [1a](0), which, according to density functional theory (DFT) calculations and electron paramagnetic resonance and Mössbauer spectroscopies, is best described as a Ni(I)Fe(II) compound. The Ni(I)Fe(II) assignment matches that for the Ni-L state in [NiFe]-hydrogenase, unlike recently reported Ni(II)Fe(I)-based models. Compound [1a](0) reacts with strong acids to liberate 0.5 equiv of H2 and regenerate [1a](+), indicating that H2 evolution is catalyzed by [1a](0). DFT calculations were used to investigate the pathway for H2 evolution and revealed that the mechanism can proceed through two isomers of [1a](0) that differ in the stereochemistry of the Fe(dppe)CO center. Calculations suggest that protonation of [1a](0) (both isomers) affords Ni(III)-H-Fe(II) intermediates, which represent mimics of the Ni-C state of the enzyme.

Atomic partitioning of M-H2 bonds in [NiFe] hydrogenase--a test case of concurrent binding

The possibility of simultaneous addition of η(2)-H2 to both the metals (Ni and Fe) in the active site of the as isolated state of the enzyme (Ni-SI) is examined here by an atom-by-atom electronic energy partitioning based on the QTAIM method. Results show that the 4LS state prefers H2 removal than addition. Destabilization of the atomic basins of the thiolate bridges and decrease of the electrophilicity of the Fe and Ni, resulting in poor back donation to the CO ligand, are the bottlenecks that hamper dihydrogen activation simultaneously. The study helps to understand why such states are seldom accessed in the activation of dihydrogen. Moreover, Ni has been found to be the natural choice for the dihydrogen binding.

Ferrous Carbonyl Dithiolates as Precursors to FeFe, FeCo, and FeMn Carbonyl Dithiolates

Reported are complexes of the formula Fe(dithiolate)(CO)2(diphos) and their use to prepare homo- and heterobimetallic dithiolato derivatives. The starting iron dithiolates were prepared by a one-pot reaction of FeCl2 and CO with chelating diphosphines and dithiolates, where dithiolate = S2(CH2)22- (edt2-), S2(CH2)32- (pdt2-), S2(CH2)2(C(CH3)2)2- (Me2pdt2-) and diphos = cis-C2H2(PPh2)2 (dppv), C2H4(PPh2)2 (dppe), C6H4(PPh2)2 (dppbz), C2H4[P(C6H11)2]2 (dcpe). The incorporation of 57Fe into such building block complexes commenced with the conversion of 57Fe into 57Fe2I4( i PrOH)4, which then was treated with K2pdt, CO, and dppe to give 57Fe(pdt)(CO)2(dppe). NMR and IR analyses show that these complexes exist as mixtures of all-cis and trans-CO isomers, edt2- favoring the former and pdt2- the latter. Treatment of Fe(dithiolate)(CO)2(diphos) with the Fe(0) reagent (benzylideneacetone)Fe(CO)3 gave Fe2(dithiolate)(CO)4(diphos), thereby defining a route from simple ferrous salts to models for hydrogenase active sites. Extending the building block route to heterobimetallic complexes, treatment of Fe(pdt)(CO)2(dppe) with [(acenaphthene)Mn(CO)3]+ gave [(CO)3Mn(pdt)Fe(CO)2(dppe)]+ ([3d(CO)]+). Reduction of [3d(CO)]+ with BH4- gave the Cs -symmetric μ-hydride (CO)3Mn(pdt)(H)Fe(CO)(dppe) (H3d). Complex H3d is reversibly protonated by strong acids, the proposed site of protonation being sulfur. Treatment of Fe(dithiolate)(CO)2(diphos) with CpCoI2(CO) followed by reduction by Cp2Co affords CpCo(dithiolate)Fe(CO)(diphos) (4), which can also be prepared from Fe(dithiolate)(CO)2(diphos) and CpCo(CO)2. Like the electronically related (CO)3Fe(pdt)Fe(CO)(diphos), these complexes undergo protonation to afford the μ-hydrido complexes [CpCo(dithiolate)HFe(CO)(diphos)]+. Low-temperature NMR studies indicate that Co is the kinetic site of protonation.