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

C-Cl Bond Activation at Rotated vs Unrotated Dinuclear Site Related to [FeFe]-Hydrogenases

The novel dinuclear complex related to the [FeFe]-hydrogenases active site, [Fe2(μ-pdt)(κ2-dmpe)2(CO)2] (1), is highly reactive toward chlorinated compounds CHxCl4-x (x = 1, 2) affording selectively terminal or bridging chloro diiron isomers through a C-Cl bond activation. DFT calculations suggest a cooperative mechanism involving a formal concerted regioselective chloronium transfer depending on the unrotated or rotated conformation of two isomers of 1.

Unravelling the Mechanistic Pathway of the Hydrogen Evolution Reaction Driven by a Cobalt Catalyst

A cobalt complex bearing a κ-N3 P2 ligand is presented (1+ or CoI (L), where L is (1E,1'E)-1,1'-(pyridine-2,6-diyl)bis(N-(3-(diphenylphosphanyl)propyl)ethan-1-imine). Complex 1+ is stable under air at oxidation state CoI thanks to the π-acceptor character of the phosphine groups. Electrochemical behavior of 1+ reveals a two-electron CoI /CoIII oxidation process and an additional one-electron reduction, which leads to an enhancement in the current due to hydrogen evolution reaction (HER) at Eonset =-1.6 V vs Fc/Fc+ . In the presence of 1 equiv of bis(trifluoromethane)sulfonimide, 1+ forms the cobalt hydride derivative CoIII (L)-H (22+ ), which has been fully characterized. Further addition of 1 equiv of CoCp*2 (Cp* is pentamethylcyclopentadienyl) affords the reduced CoII (L)-H (2+ ) species, which rapidly forms hydrogen and regenerates the initial CoI (L) (1+ ). The spectroscopic characterization of catalytic intermediates together with DFT calculations support an unusual bimolecular homolytic mechanism in the catalytic HER with 1+ .

Deuteration mechanistic studies of hydrogenase mimics

The role of deuterium in disentangling key steps of the mechanisms of H₂ activation by mimics of hydrogenases is presented. These studies have allowed to a better understanding of the mode of action of the natural enzymes and their mimics.

[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.

Triiron clusters derived from dinuclear complexes related to the active site of [Fe–Fe] hydrogenases: steric effect of the dithiolate bridge on redox properties, a DFT analysis

CV and DFT calculations reveal that electrochemical behaviours of triiron clusters [Fe₃(CO)₅(κ₂-dppe)(μ-pdt^R2)(μ-pdt)] depend on the nature of the dithiolate bridge.

Mixed-Valent Diiron μ-Carbyne, μ-Hydride Complexes: Implications for Nitrogenase

Binding of N₂ by the FeMo-cofactor of nitrogenase is believed to occur after transfer of 4 e^- and 4 H⁺ equivalents to the active site. Although pulse EPR studies indicate the presence of two Fe-(μ-H)-Fe moieties, the structural and electronic features of this mixed valent intermediate remain poorly understood. Toward an improved understanding of this bioorganometallic cluster, we report herein that diiron μ-carbyne complex (P₆ArC)Fe₂(μ-H) can be oxidized and reduced, allowing for the first time spectral characterization of two EPR-active Fe(μ-C)(μ-H)Fe model complexes linked by a 2 e^- transfer which bear some resemblance to a pair of E_n and E_n+2 states of nitrogenase. Both species populate S = 1/2 states at low temperatures, and the influence of valence (de)localization on the spectroscopic signature of the μ-hydride ligand was evaluated by pulse EPR studies. Compared to analogous data for the {Fe₂(μ-H)}₂ state of FeMoco (E₄(4H)), the data and analysis presented herein suggest that the hydride ligands in E₄(4H) bridge isovalent (most probably Fe^III) metal centers. Although electron transfer involves metal-localized orbitals, investigations of [(P₆ArC)Fe₂(μ-H)]⁺¹ and [(P₆ArC)Fe₂(μ-H)]^-1 by pulse EPR revealed that redox chemistry induces significant changes in Fe-C covalency (-50% upon 2 e^- reduction), a conclusion further supported by X-ray absorption spectroscopy, ⁵⁷Fe Mössbauer studies, and DFT calculations. Combined, our studies demonstrate that changes in covalency buffer against the accumulation of excess charge density on the metals by partially redistributing it to the bridging carbon, thereby facilitating multielectron transformations.

Protonation and electrochemical properties of a bisphosphide diiron hexacarbonyl complex bearing amino groups on the phosphide bridge

A bisphosphide-bridged diiron hexacarbonyl complex 3 with NEt2 groups on the phosphide bridge was synthesized to examine a new proton relay system from the NEt2 group to the bridging hydride between the two iron centers. As a precursor of the bridging moiety, peri-Et2NP-PNEt2-bridged naphthylene 5 was synthesized by the reaction of 1,8-dilithionaphthylene with two equivalents of Cl2PNEt2 followed by reductive P-P bond formation by magnesium. The reaction of the diphosphine ligand 5 with Fe2(CO)9 gave the diiron hexacarbonyl complex 3, in which the P-P bond of the ligand was cleaved to form the bisphosphide-bridge. The molecular structure of 3 indicated that the trigonal plane of the NEt2 group was forced to face the Fe-Fe bond to avoid steric congestion with the naphthylene group linking the two phosphide groups. The NEt2 group could be protonated by p-toluenesulfonic acid. Density functional theory (DFT) calculations confirmed that the proton of the N(H)Et2 group adopted a position close to the bridging hydride. The DFT results for the ferrocene analogue 1, in which the 1,8-naphthylene group of 3 was replaced with the 1,1'-ferrocenylene group, also revealed that the most stable orientation of the protonated NHEt2 group was that in the protonated 3. As a result, electrochemical proton reduction reactions using complexes 1 and 3 proceeded with similar catalytic efficiencies. Unfortunately, the catalytic efficiencies (CEs) of these complexes were much lower than those of the complexes with a proton relay system of the terminal hydrogen, indicating that the reactive properties of the bridging hydride in the present proton relay system cannot exceed those of the terminal hydride.

Reactivity toward Unsaturated Small Molecules of Thiolate-Bridged Diiron Hydride Complexes

In the presence of 1 equiv of ^tBuNC, the homolytic cleavage of the Fe^III-H bond in the diiron terminal hydride complex [Cp*Fe( t-H)(μ-η²:η⁴-bdt)FeCp*][BF₄] (1[BF₄]) smoothly took place to release 1/2 H₂, followed by binding of a ^tBuNC group to the unsaturated Fe^II center. Interestingly, upon exposure of 1[BF₄] to 1 atm of acetylene, the isomerization process of the hydride ligand from the terminal to bridging coordination site was unaffected. Upon treatment of the diiron hydride bridged complex 2[BF₄] with acetylene at 30 °C, two Fe^III-H bonds were broken, and then an acetylene molecule was coordinated to the diiron centers in a novel μ-η²:η² side-on fashion. In the above reaction system, the hydride ligands whether terminal or bridging all play a role as the electron donor for the reduction of the diiron centers from Fe^IIIFe^III to Fe^IIIFe^II. These reaction patterns are reminiscent of the vital E₄ state responsible for N₂ binding and H₂ liberation in the catalytic cycle of nitrogenase, which contains two {Fe-H-Fe} motifs as electron reservoirs for the reduction of the iron centers. Differently, when treating 1[BF₄] with TMSN₃, the terminal hydride ligand was inserted into the azide group to give a diiron amide complex 4[BF₄] in moderate yield.

Reactant or reagent? Oxidation of H2 at electronically distinct nickel-thiolate sites [Ni2(μ-SR)2]+ and [Ni–SR]+

The chemical bond between a Lewis-acidic metal and a Brønsted/Lewis-basic sulphur donor provides M–S structures with functional properties that are relevant for a variety of processes such as the heterolytic cleavage of H₂.

Mechanistic Insight into Electrocatalytic H2 Production by [Fe2(CN){μ-CN(Me)2}(μ-CO)(CO)(Cp)2]: Effects of Dithiolate Replacement in [FeFe] Hydrogenase Models

DFT has been used to investigate viable mechanisms of the hydrogen evolution reaction (HER) electrocatalyzed by [Fe₂(CN){μ-CN(Me)₂}(μ-CO)(CO)(Cp)₂] (1) in AcOH. Molecular details underlying the proposed ECEC electrochemical sequence have been studied, and the key functionalities of CN^- and amino-carbyne ligands have been elucidated. After the first reduction, CN^- works as a relay for the first proton from AcOH to the carbyne, with this ligand serving as the main electron acceptor for both reduction steps. After the second reduction, a second protonation occurs at CN^- that forms a Fe(CNH) moiety: i.e., the acidic source for the H₂ generation. The hydride (formally 2e/H⁺), necessary to the heterocoupling with H⁺ is thus provided by the μ-CN(Me)₂ ligand and not by Fe centers, as occurs in typical L₆Fe₂S₂ derivatives modeling the hydrogenase active site. It is remarkable, in this regard, that CN^- plays a role more subtle than that previously expected (increasing electron density at Fe atoms). In addition, the role of AcOH in shuttling protons from CN^- to CN(Me)₂ is highlighted. The incompetence for the HER of the related species [Fe₂{μ-CN(Me)₂}(μ-CO)(CO)₂(Cp)₂]⁺ (2⁺) has been investigated and attributed to the loss of proton responsiveness caused by CN^- replacement with CO. In the context of hydrogenase mimicry, an implication of this study is that the dithiolate strap, normally present in all synthetic models, can be removed from the Fe₂ core without loss of HER, but the redox and acid-base processes underlying turnover switch from a metal-based to a ligand-based chemistry. The versatile nature of the carbyne, once incorporated in the Fe₂ scaffold, could be exploited to develop more active and robust catalysts for the HER.

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.