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

Bimetallic H2 Addition and Intramolecular Caryl-H Activation Mediated by an Iron-Zinc Hydride

Heteronuclear Fe(μ-H)Zn hydride Cp*Fe(1,2-Cy2PC6H4)HZnEt (3) undergoes reversible intramolecular Caryl-H reductive elimination through coupling of the cyclometalated phosphinoaryl ligand and the hydride, giving rise to a formal Fe(0)-Zn(II) species. Addition of CO intercepts this equilibrium, affording Cp*(Cy2PPh)(CO)Fe-ZnEt that features a dative Fe-Zn bond. Significantly, this system achieves bimetallic H2 addition, as demonstrated by the transformation of the monohydride Fe(μ-H)Zn to a deuterated dihydride Fe-(μ-D)2-Zn upon reaction with D2.

Functionalized nickel(II)-iron(II) dithiolates as biomimetic models of [NiFe]-H2ases

To develop the structural and functional modeling chemistry of [NiFe]-H₂ases, a series of new biomimetics for the active site of [NiFe]-H₂ases have been prepared by various synthetic methods. Treatment of the mononuclear Ni complex (pnp)NiCl₂ (pnp = (Ph₂PCH₂)₂NPh) with (dppv)Fe(CO)₂(pdt) (dppv = 1,2-(Ph₂P)₂C₂H₂, pdt = 1,3-propanedithiolate) and KPF₆ gave the dicarbonyl complex [(pnp)Ni(pdt)Fe(CO)₂(dppv)](PF₆)₂ ([1](PF₆)₂). Further treatment of [1](PF₆)₂ and [(dppe)Ni(pdt)Fe(CO)₂(dppv)](BF₄)₂ (dppe = 1,2-(Ph₂P)₂C₂H₄) with the decarbonylation agent Me₃NO and pyridine afforded the novel sp³ C-Fe bond-containing complexes [(pnp)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]PF₆ ([2]PF₆) and [(dppe)Ni(SCH₂CH₂CHS)Fe(CO)(dppv)]BF₄ ([3]BF₄). More interestingly, the first t-carboxylato complexes [(pnp)Ni(pdt)Fe(CO)(t-O₂CR)(dppv)]PF₆ ([4]PF₆, R = H; [5]PF₆, R = Me; [6]PF₆, R = Ph) could be prepared by reactions of [1]PF₆ with the corresponding carboxylic acids RCO₂H in the presence of Me₃NO, whereas further reactions of [4]PF₆-[6]PF₆ with aqueous HPF₆ and 1.5 MPa H₂ gave rise to the μ-hydride complex [(pnp)Ni(pdt)Fe(CO)(μ-H)(dppv)]PF₆ ([7]PF₆). Except for H₂ activation by t-carboxylato complexes [4]PF₆-[6]PF₆ to give a μ-hydride complex ([7]PF₆), the sp³ C-Fe bond-containing complex [2]PF₆ was found to be a catalyst for proton reduction to H₂ under CV conditions. Furthermore, the chemical reactivity of the μ-hydride complex [7]PF₆ displayed in the e^- transfer reaction with FcPF₆ in the presence of CO, the H₂ evolution reaction with the protonic acid HCl, and the H^- transfer reaction with N-methylacridinium hexafluorophosphate ([NMA]PF₆) was systematically studied. As a result, a series of the expected products such as H₂, ferrocene, the dicarbonyl complex [1](PF₆)₂, the μ-chloro complex [(pnp)Ni(pdt)Fe(CO)(μ-Cl)(dppv)]PF₆ ([8]PF₆), the t-MeCN-coordinated complex [(pnp)Ni(pdt)Fe(CO)(t-MeCN)(dppv)](PF₆)₂ ([9](PF₆)₂) and the H^- transfer product AcrH₂ were produced. While all the newly prepared model complexes were structurally characterized by spectroscopic methods, the molecular structures of some of their representatives were confirmed by X-ray crystallography.

Synthesis, Structures and Chemical Reactivity of Dithiolato-Bridged Ni-Fe Complexes as Biomimetics for the Active Site of [NiFe]-Hydrogenases

To develop the structural and functional modeling chemistry of [NiFe]-H2ases, we have carried out a study regarding the synthesis, structural characterization and reactivity of a new series of [NiFe]-H2ase model complexes. Thus, treatment of diphosphine dppb-chelated Ni complex (dppb)NiCl2 (dppb = 1,2-(Ph2P)2C6H4) with (dppv)Fe(CO)2(pdt) (dppv = 1,2-(Ph2P)2C2H2, pdt = 1,3-propanedithiolate) and NaBF4 gave dicarbonyl complex [(dppb)Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([A](BF4)2). Further treatment of [A](BF4)2 with Me3NO and Bu4NCN or KSCN afforded t-cyanido and t-isothiocyanato complexes [(dppb)Ni(pdt)Fe(CO)(t-R)(dppv)]BF4 ([1]BF4, R = CN; [2]BF4, R = NCS), respectively. While azadiphosphine MeN(CH2PPh2)2-chelated t-hydride complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)(t-H)(dppv)]BF4 ([3]BF4) was prepared by treatment of dicarbonyl complex [MeN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([B](BF4)2) with Me3NO and 1.5 MPa of H2, treatment of dicarbonyl complex [B](BF4)2 with Me3NO (without H2) in pyridine resulted in formation of a novel monocarbonyl complex [MeN(CH2PPh2)2Ni(SCHCH2CH2S)Fe(CO)(dppv)]BF4 ([4]BF4) via the unexpected sp3 C-H bond activation reaction. Furthermore, azadiphosphine PhN(CH2PPh2)2-chelated µ-mercapto complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-SH)(dppv)]BF4 ([5]BF4) was prepared by treatment of dicarbonyl complex [PhN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppv)](BF4)2 ([C](BF4)2) with Me3NO and H2S gas, whereas treatment of azadiphosphine Ph2CHN(CH2PPh2)2-chelated dicarbonyl complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)2(dppe)](BF4)2 ([D](BF4)2, dppe = 1,2-(Ph2P)2C2H4) with Me3NO⋅2H2O gave rise to µ-hydroxo complex [Ph2CHN(CH2PPh2)2Ni(pdt)Fe(CO)(µ-OH)(dppe)]BF4 ([6]BF4). All the possible pathways for formation of the new model complexes are briefly discussed, and their structures were fully characterized by various spectroscopic techniques and for six of them by X-ray crystallography.

Di-, tri- and tetraphosphine-substituted Fe/Se carbonyls: Synthesis, Characterization and electrochemical properties

The active sites of [FeFe]-hydrogenase promoted by Fe/E (E=S, Se) clusters have attracted considerable interest due to their significance for understanding the interconversion of hydrogen with protons and electrons. As...

[FeFe]-Hydrogenases: maturation and reactivity of enzymatic systems and overview of biomimetic models

[FeFe]-hydrogenases recieved increasing interest in the last decades. This review summarises important findings regarding their enzymatic reactivity as well as inorganic models applied as electro- and photochemical catalysts.

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.

The group 9 cyclopentadienylmetal cis-ethylenedithiolates as metallodithiolene ligands in metal carbonyl chemistry: analogies to benzene metal carbonyl complexes

The species CpMS₂C₂H₂ (M = Co, Rh, Ir) are aromatic systems that can function as pentahapto six-electron metallodithiolene ligands through all five atoms of their MS₂C₂ rings as in the lowest energy heterobimetallic complexes CpMS₂C₂H₂·Cr(CO)_n (n = 3, 2) and CpMS₂C₂H₂·Fe(CO)₂.

Biomimetic catalytic oxidative coupling of thiols using thiolate-bridged dinuclear metal complexes containing iron in water under mild conditions

A green approach to disulfides via aerobic oxidative coupling of thiols was developed with a thiolate-bridged heteronuclear complex in water.

Electron-Rich, Diiron Bis(monothiolato) Carbonyls: C-S Bond Homolysis in a Mixed Valence Diiron Dithiolate

The synthesis and redox properties are presented for the electron-rich bis(monothiolate)s Fe₂(SR)₂(CO)₂(dppv)₂ for R = Me ([1]⁰), Ph ([2]⁰), CH₂Ph ([3]⁰). Whereas related derivatives adopt C₂-symmetric Fe₂(CO)₂P₄ cores, [1]⁰-[3]⁰ have C_s symmetry resulting from the unsymmetrical steric properties of the axial vs equatorial R groups. Complexes [1]⁰-[3]⁰ undergo 1e^- oxidation upon treatment with ferrocenium salts to give the mixed valence cations [Fe₂(SR)₂(CO)₂(dppv)₂]⁺. As established crystallographically, [3]⁺ adopts a rotated structure, characteristic of related mixed valence diiron complexes. Unlike [1]⁺ and [2]⁺ and many other [Fe₂(SR)₂L₆]⁺ derivatives, [3]⁺ undergoes C-S bond homolysis, affording the diferrous sulfido-thiolate [Fe₂(SCH₂Ph)(S)(CO)₂(dppv)₂]⁺ ([4]⁺). According to X-ray crystallography, the first coordination spheres of [3]⁺ and [4]⁺ are similar, but the Fe-sulfido bonds are short in [4]⁺. The conversion of [3]⁺ to [4]⁺ follows first-order kinetics, with k = 2.3 × 10^-6 s^-1 (30 °C). When the conversion is conducted in THF, the organic products are toluene and dibenzyl. In the presence of TEMPO, the conversion of [3]⁺ to [4]⁺ is accelerated about 10×, the main organic product being TEMPO-CH₂Ph. DFT calculations predict that the homolysis of a C-S bond is exergonic for [Fe₂(SCH₂Ph)₂(CO)₂(PR₃)₄]⁺ but endergonic for the neutral complex as well as less substituted cations. The unsaturated character of [4]⁺ is indicated by its double carbonylation to give [Fe₂(SCH₂Ph)(S)(CO)₄(dppv)₂]⁺ ([5]⁺), which adopts a bioctahedral structure.

Sterically Stabilized Terminal Hydride of a Diiron Dithiolate

The kinetically robust hydride [t-HFe₂(Me₂pdt)(CO)₂(dppv)₂]⁺ ([t-H1]⁺) (Me₂pdt^2- = Me₂C(CH₂S^-)₂; dppv = cis-1,2-C₂H₂(PPh₂)₂) and related derivatives were prepared with ⁵⁷Fe enrichment for characterization by NMR, FT-IR, and NRVS. The experimental results were rationalized using DFT molecular modeling and spectral simulations. The spectroscopic analysis was aimed at supporting assignments of Fe-H vibrational spectra as they relate to recent measurements on [FeFe]-hydrogenase enzymes. The combination of bulky Me₂pdt^2- and dppv ligands stabilizes the terminal hydride with respect to its isomerization to the 5-16 kcal/mol more stable bridging hydride ([μ-H1]⁺) with t_1/2(313.3 K) = 19.3 min. In agreement with the nOe experiments, the calculations predict that one methyl group in [t-H1]⁺ interacts with the hydride with a computed CH···HFe distance of 1.7 Å. Although [t-H⁵⁷1]⁺ exhibits multiple NRVS features in the 720-800 cm^-1 region containing the bending Fe-H modes, the deuterated [t-D⁵⁷1]⁺ sample exhibits a unique Fe-D/CO band at ∼600 cm^-1. In contrast, the NRVS spectra for [μ-H⁵⁷1]⁺ exhibit weaker bands near 670-700 cm^-1 produced by the Fe-H-Fe wagging modes coupled to Me₂pdt^2- and dppv motions.

Synthesis and characterization of a family of thioether-dithiolate-bridged heteronuclear iron complexes

The thioether-dithiolate-bridged heterotrinuclear complexes [Cp*Fe(μ-1_k³SSS':2_k²SS-tpdt)M(μ-2_k²SS:3_k³SSS'-tpdt)FeCp*][PF₆]₂ (Cp* = η⁵-C₅Me₅; tpdt = S(CH₂CH₂S)₂; 2, M = Co; 3, M = Ni; 4, M = Pd) have been prepared by a reaction of [Cp*Fe(η³-tpdt)] (1) with complexes CoCl₂, NiCl₂(PPh₃)₂, and PdCl₂(PPh₃)₂, respectively. Similarly, treatment of complex 1 with CuCl(PPh₃) or AgPF₆ afforded two heterotrinuclear complexes, [Cp*Fe(μ-1_k³SSS':2_k²SS-tpdt)M(μ-2_k²SS:3_k³SSS'-tpdt)FeCp*][PF₆] (5, M = Cu; 6, M = Ag), while reaction of 1 with the complex AuCl(PPh₃) gave a heterobinuclear complex, [Cp*Fe(μ-1_k³SSS':2_k¹S-tpdt)Au(PPh₃)][PF₆] (7). These complexes have been spectroscopically and crystallographically characterized. An X-ray diffraction analysis showed that complexes 2, 3, 5, and 6 feature a heterometal center binding four sulfur atoms of two tpdt ligands with a cis orientation. However, in the Pd-containing complex 4, two tpdt ligands are arranged in a trans configuration. The μ_eff data and EPR results indicate that complexes 2, 4, 5, 6, and 7 are paramagnetic and only complex 3 is diamagnetic. Electrochemical experiments on these heteronuclear clusters were performed at room temperature. Discrepancy of the redox couples in the CV plots of these complexes indicates different one-electron transfer processes.

Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]⁰, Cp*^- = Me₅C₅^-, pdt^2- = CH₂(CH₂S^-)₂, dppe = Ph₂PC₂H₄PPh₂) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]⁺. As established by X-ray crystallography, [1(t-H)]⁰ features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]⁰ does not isomerize to the bridging hydride [1(μ-H)]⁰. Oxidation of [1(t-H)]⁰ gives [1(t-H)]⁺, which was also characterized crystallographically as its BF₄^- salt. Density functional theory (DFT) calculations indicate that [1(t-H)]⁺ is best described as containing an Cp*Fe^III center. In solution, [1(t-H)]⁺ isomerizes to [1(μ-H)]⁺, as anticipated by DFT. Reduction of [1(μ-H)]⁺ by Cp₂Co afforded the diferrous bridging hydride [1(μ-H)]⁰. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]^+/0 and [1(μ-H)]^+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]⁰ and [1(μ-H)]⁰ are close in free energy. Protonation of [1(t-H)]⁰ in MeCN solution affords H₂ even with weak acids via hydride transfer. In contrast, protonation of [1(μ-H)]⁰ yields 0.5 equiv of H₂ by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert.

Synthetic Models for Nickel-Iron Hydrogenase Featuring Redox-Active Ligands

The nickel-iron hydrogenase enzymes efficiently and reversibly interconvert protons, electrons, and dihydrogen. These redox proteins feature iron-sulfur clusters that relay electrons to and from their active sites. Reported here are synthetic models for nickel-iron hydrogenase featuring redox-active auxiliaries that mimic the iron-sulfur cofactors. The complexes prepared are NiII(μ-H)FeIIFeII species of formula [(diphosphine)Ni(dithiolate)(μ-H)Fe(CO)2(ferrocenylphosphine)]+ or NiIIFeIFeII complexes [(diphosphine)Ni(dithiolate)Fe(CO)2(ferrocenylphosphine)]+ (diphosphine = Ph2P(CH2)2PPh2 or Cy2P(CH2)2PCy2; dithiolate = -S(CH2)3S-; ferrocenylphosphine = diphenylphosphinoferrocene, diphenylphosphinomethyl(nonamethylferrocene) or 1,1'-bis(diphenylphosphino)ferrocene). The hydride species is a catalyst for hydrogen evolution, while the latter hydride-free complexes can exist in four redox states - a feature made possible by the incorporation of the ferrocenyl groups. Mixed-valent complexes of 1,1'-bis(diphenylphosphino)ferrocene have one of the phosphine groups unbound, with these species representing advanced structural models with both a redox-active moiety (the ferrocene group) and a potential proton relay (the free phosphine) proximal to a nickel-iron dithiolate.

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.

Mechanism of H2 Production by Models for the [NiFe]-Hydrogenases: Role of Reduced Hydrides

The intermediacy of a reduced nickel-iron hydride in hydrogen evolution catalyzed by Ni-Fe complexes was verified experimentally and computationally. In addition to catalyzing hydrogen evolution, the highly basic and bulky (dppv)Ni(μ-pdt)Fe(CO)(dppv) ([1](0); dppv = cis-C2H2(PPh2)2) and its hydride derivatives have yielded to detailed characterization in terms of spectroscopy, bonding, and reactivity. The protonation of [1](0) initially produces unsym-[H1](+), which converts by a first-order pathway to sym-[H1](+). These species have C1 (unsym) and Cs (sym) symmetries, respectively, depending on the stereochemistry of the octahedral Fe site. Both experimental and computational studies show that [H1](+) protonates at sulfur. The S = 1/2 hydride [H1](0) was generated by reduction of [H1](+) with Cp*2Co. Density functional theory (DFT) calculations indicate that [H1](0) is best described as a Ni(I)-Fe(II) derivative with significant spin density on Ni and some delocalization on S and Fe. EPR spectroscopy reveals both kinetic and thermodynamic isomers of [H1](0). Whereas [H1](+) does not evolve H2 upon protonation, treatment of [H1](0) with acids gives H2. The redox state of the "remote" metal (Ni) modulates the hydridic character of the Fe(II)-H center. As supported by DFT calculations, H2 evolution proceeds either directly from [H1](0) and external acid or from protonation of the Fe-H bond in [H1](0) to give a labile dihydrogen complex. Stoichiometric tests indicate that protonation-induced hydrogen evolution from [H1](0) initially produces [1](+), which is reduced by [H1](0). Our results reconcile the required reductive activation of a metal hydride and the resistance of metal hydrides toward reduction. This dichotomy is resolved by reduction of the remote (non-hydride) metal of the bimetallic unit.

Synthesis of Diiron(I) Dithiolato Carbonyl Complexes

Virtually all organosulfur compounds react with Fe(0) carbonyls to give the title complexes. These reactions are reviewed in light of major advances over the past few decades, spurred by interest in Fe2(μ-SR)2(CO)x centers at the active sites of the [FeFe]-hydrogenase enzymes. The most useful synthetic route to Fe2(μ-SR)2(CO)6 involves the reaction of thiols with Fe2(CO)9 and Fe3(CO)12. Such reactions can proceed via mono-, di-, and triiron intermediates. The reactivity of Fe(0) carbonyls toward thiols is highly chemoselective, and the resulting dithiolato complexes are fairly rugged. Thus, many complexes tolerate further synthetic elaboration directed at the organic substituents. A second major route involves alkylation of Fe2(μ-S2)(CO)6, Fe2(μ-SH)2(CO)6, and Li2Fe2(μ-S)2(CO)6. This approach is especially useful for azadithiolates Fe2[(μ-SCH2)2NR](CO)6. Elaborate complexes arise via addition of the FeSH group to electrophilic alkenes, alkynes, and carbonyls. Although the first example of Fe2(μ-SR)2(CO)6 was prepared from ferrous reagents, ferrous compounds are infrequently used, although the Fe(II)(SR)2 + Fe(0) condensation reaction is promising. Almost invariably low-yielding, the reaction of Fe3(CO)12, S8, and a variety of unsaturated substrates results in C-H activation, affording otherwise inaccessible derivatives. Thiones and related C═S-containing reagents are highly reactive toward Fe(0), often giving complexes derived from substituted methanedithiolates and C-H activation.

Rational Synthesis of the Carbonyl(perthiolato)diiron [Fe2(S3CPh2)(CO)6] and Related Complexes

The photochemical reaction of Fe2(S2)(CO)6 and Ph2CS affords the perthiolate Fe2(S3CPh2)(CO)6 (1) in good yield. As confirmed crystallographically, 1 contains a previously elusive perthiolate ligand. The related reaction of Fe2(S2)(CO)5-(PPh3) and Ph2CS gave Fe2(S3CPh2)(CO)5(PPh3). Although Fe3S2(CO)9 and Ph2CS failed to efficiently give Fe2(S2CPh2)(CO)6, this compound could be prepared by desulfurization of 1 using PPh3.

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.

Structural characterization and proton reduction electrocatalysis of thiolate-bridged bimetallic (CoCo and CoFe) complexes

Using an assembly method, dinuclear CoCo and CoFe complexes supported by a bdt ligand, [Cp*Co(μ–η²:η²-bdt)(μ-I)CoCp*][PF₆] (1[PF6], Cp* = η⁵-C₅Me₅, bdt = benzene-1,2-dithiolate), and [Cp*Co(μ–η²:η⁴-bdt)FeCp′][PF₆] (3[PF6], Cp′ = η⁵-C₅Me₄H) were synthesized in high yields.

Synthesis and vibrational spectroscopy of (57)Fe-labeled models of [NiFe] hydrogenase: first direct observation of a nickel-iron interaction

Isotopically labelled Ni⁵⁷Fe models of the [NiFe] hydrogenase active site have been prepared and studied with nuclear resonant vibrational spectroscopy, enabling direct characterization of metal–metal bonding.

A new route to iron carbonyls has enabled synthesis of ⁵⁷Fe-labeled [NiFe] hydrogenase mimic (OC)₃⁵⁷Fe(pdt)Ni(dppe). Its study by nuclear resonance vibrational spectroscopy revealed Ni–⁵⁷Fe vibrations, as confirmed by calculations. The modes are absent for [(OC)₃⁵⁷Fe(pdt)Ni(dppe)]⁺, which lacks Ni–⁵⁷Fe bonding, underscoring the utility of the analyses in identifying metal–metal interactions.

New tetracobalt cluster compounds for electrocatalytic proton reduction: syntheses, structures, and reactivity

Reaction of Co2(CO)8 and 1,3-propanedithiol in a 1:1 molar ratio in toluene affords a novel tetracobalt complex, [(μ2-pdt)2(μ3-S)Co4(CO)6] (pdt = -SCH2CH2CH2S-, 1), which possesses some of the structural features of the active site of [FeFe]-hydrogenase. Carbonyl displacement reaction of complex 1 in the presence of mono- or diphosphine ligands leads to the formation of [(μ2-pdt)2(μ3-S)Co4(CO)5(PCy3)] (2) and [(μ2-pdt)2(μ3-S)Co4(CO)4(L)] [L = Ph2PCH=CHPPh2, 3; Ph2PCH2N(Ph)CH2PPh2, 4; Ph2PCH2N(iPr)CH2PPh2, 5]. Complexes 1-5 have been fully characterized by spectroscopy and single-crystal X-ray diffraction studies. Cyclic voltammetry has revealed that complexes 1-5 show a reversible first reduction wave and are active for electrocatalytic proton reduction in the presence of CF3COOH. Protonation reactions have been monitored by (31)P and (1)H NMR and infrared spectroscopies, which revealed the formation of different protonated species. The mono-reduced species of 1-5 have been spectroscopically characterized by EPR and spectro-electro-infrared techniques.

A new FeMo complex as a model of heterobimetallic assemblies in natural systems: Mössbauer and density functional theory investigations

The design of the new FeMo heterobimetallic species [FeMo(CO)5(κ(2)-dppe)(μ-pdt)] is reported. Mössbauer spectroscopy and density functional theory calculations give deep insight into the electronic and structural properties of this compound.