A Diferrous Dithiolate as a Model of the Elusive H_ox^inact State of the [FeFe] Hydrogenases: An Electrochemical and Theoretical Dissection of Its Redox Chemistry

The reduction of the Fe­(II)­Fe­(II) complex [Fe₂(CO)₂{P­(OMe)₃}₂(κ²-I_Me-CH₂-I_Me)­(μ-CO)­(μ-pdt)]²⁺ (<b>2P²⁺</b>; pdt = S­(CH₂)₃S), which is a synthetic model of the H cluster of the [FeFe] hydrogenases in its inactive state, has been investigated electrochemically and theoretically (by density functional theory, DFT) in order to determine the mechanisms, intermediates, and products of the related processes. The electrochemical reduction of <b>2P²⁺</b> occurs according to an ECE-type reaction where the intervening chemical step is the loss of one P­(OMe)₃ ligand. This outcome, which is based on cyclic voltammetric experiments, is strongly supported by DFT calculations that provide additional information on the intermediates and the energetics of the reactions involved. The electrochemical reoxidation of the neutral product of the reduction follows an EEC process where the chemical step is the binding of P­(OMe)₃ to a dicationic intermediate. DFT calculations reveal that this intermediate has an unusual geometry wherein one of the two C–H bonds of a side methylene from the pdt group forms an agostic interaction with one Fe center. This interaction is crucial to stabilize the 32e⁻ diferrous center and concomitantly to preserve Fe­(II) from binding of weakly coordinating species. Nonetheless, it could be displaced by a relatively stronger electron donor such as H₂, which could be relevant for the design of new oxidation catalysts

Electrochemical and Theoretical Studies of the Impact of the Chelating Ligand on the Reactivity of [Fe₂(CO)₄(κ²-LL)(μ-pdt)]⁺ Complexes with Different Substrates (LL = I_Me-CH₂-I_Me, dppe; I_Me = 1-Methylimidazol-2-ylidene)

The reactivity of [Fe₂(CO)₄(κ²-LL)­(μ-pdt)]⁺ complexes (pdt = S­(CH₂)₃S, propanedithiolate) with different substrates L′ (L′ = CO, MeCN, P­(OMe)₃) was investigated electrochemically in order to assess the influence of the chelating ligand κ²-LL (LL = I_Me-CH₂-I_Me (<b>1</b>^<b>+</b>), dppe (<b>2</b>^<b>+</b>); I_Me = 1-methylimidazol-2-ylidene). This latter ligand is effectively shown to affect the reactivity of the cations in different ways: when L′ = CO, the adduct [Fe₂(CO)₄(μ-CO)­(κ²-dppe)­(μ-pdt)]⁺ (<b>2-CO</b>^<b>+</b>) was clearly observed by cyclic voltammetry, whereas [Fe₂(CO)₄(μ-CO)­(κ²-I_Me-CH₂-I_Me)­(μ-pdt)]⁺ (<b>1-CO</b>^<b>+</b>) was not detected, although DFT calculations show that the energies of the products and the activation barriers to their formation are similar. When L′ = MeCN, the adducts <b>X-MeCN</b>^<b>+</b> with <b>X</b> = <b>1</b>, <b>2</b> are both observed, but the formation is easier when LL = dppe. Finally, the reaction of [Fe₂(CO)₄(κ²-I_Me-CH₂-I_Me)­(μ-pdt)]⁺ (<b>1</b>^<b>+</b>) with P­(OMe)₃ produces the disubstituted dication [Fe₂(CO)₂{P­(OMe)₃}₂(κ²-I_Me-CH₂-I_Me)­(μ-CO)­(μ-pdt)]²⁺ (<b>4</b>^<b>2+</b>) via a disproportionation reaction, while previous studies demonstrated that monocationic derivatives were obtained when LL = dppe. Complex <b>4</b>[PF₆]₂ was fully characterized, and its X-ray crystal structure confirms the presence of a carbonyl ligand in a bridging position, which did not exist in the related P­(OMe)₃-substituted κ²-dppe cations

Acid-Base Control of Hemilabile Proton-Responsive Protecting Devices in Dimolybdenum, Thiolate-Bridged Complexes

Dimolybdenum thiolate-bridged complexes [Mo₂Cp₂(μ-SMe)₂(μ-SCH₂CH₂E)] (E = O (<b>2</b>) or NH (<b>4</b>)) with a proton-dependent protecting device have been synthesized by reaction of [Mo₂Cp₂(μ-SMe)₂(μ-Cl)₂] (<b>1</b>) with SCH₂CH₂EH. The reactivity of the resultant quadruply bridged complexes with acid was investigated in the absence and in the presence of a potential ligand (N₂, MeCN, RNC). While the protonation of complexes <b>2</b> and <b>4</b> under N₂ in dichloromethane produced only the oxidized derivatives instead of the desired diazenido compound, ligand binding was observed in MeCN or in the presence of RNC (R = <i>t</i>-Bu, Xyl). Whereas acetonitrile loss from [Mo₂Cp₂(μ-SMe)₂(μ-SCH₂CH₂OH)­(MeCN)₂]⁺ (<b>8</b>^<b>+</b>) prevented the isolation and characterization of this species, the <i>t</i>-BuNC analogue (<b>6</b>^<b>+</b>) could be characterized by an X-ray crystal structure. The electrochemistry of <b>2</b> and <b>2</b>^<b>+</b> was investigated in CH₂Cl₂ and in MeCN, both in the absence and in the presence of acid. While the addition of HBF₄·Et₂O to a dichloromethane solution of <b>2</b> only produced <b>2</b>^<b>+</b> (and presumably H₂), <b>8</b>^<b>+</b> was the major product of the protonation in MeCN

Electrochemical and Theoretical Studies of the Impact of the Chelating Ligand on the Reactivity of [Fe₂(CO)₄(κ²-LL)(μ-pdt)]⁺ Complexes with Different Substrates (LL = I_Me-CH₂-I_Me, dppe; I_Me = 1-Methylimidazol-2-ylidene)

The reactivity of [Fe₂(CO)₄(κ²-LL)­(μ-pdt)]⁺ complexes (pdt = S­(CH₂)₃S, propanedithiolate) with different substrates L′ (L′ = CO, MeCN, P­(OMe)₃) was investigated electrochemically in order to assess the influence of the chelating ligand κ²-LL (LL = I_Me-CH₂-I_Me (<b>1</b>^<b>+</b>), dppe (<b>2</b>^<b>+</b>); I_Me = 1-methylimidazol-2-ylidene). This latter ligand is effectively shown to affect the reactivity of the cations in different ways: when L′ = CO, the adduct [Fe₂(CO)₄(μ-CO)­(κ²-dppe)­(μ-pdt)]⁺ (<b>2-CO</b>^<b>+</b>) was clearly observed by cyclic voltammetry, whereas [Fe₂(CO)₄(μ-CO)­(κ²-I_Me-CH₂-I_Me)­(μ-pdt)]⁺ (<b>1-CO</b>^<b>+</b>) was not detected, although DFT calculations show that the energies of the products and the activation barriers to their formation are similar. When L′ = MeCN, the adducts <b>X-MeCN</b>^<b>+</b> with <b>X</b> = <b>1</b>, <b>2</b> are both observed, but the formation is easier when LL = dppe. Finally, the reaction of [Fe₂(CO)₄(κ²-I_Me-CH₂-I_Me)­(μ-pdt)]⁺ (<b>1</b>^<b>+</b>) with P­(OMe)₃ produces the disubstituted dication [Fe₂(CO)₂{P­(OMe)₃}₂(κ²-I_Me-CH₂-I_Me)­(μ-CO)­(μ-pdt)]²⁺ (<b>4</b>^<b>2+</b>) via a disproportionation reaction, while previous studies demonstrated that monocationic derivatives were obtained when LL = dppe. Complex <b>4</b>[PF₆]₂ was fully characterized, and its X-ray crystal structure confirms the presence of a carbonyl ligand in a bridging position, which did not exist in the related P­(OMe)₃-substituted κ²-dppe cations

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

A novel reaction pathway to synthesize the linear trinuclear clusters [Fe₃(CO)₅(κ²-diphosphine)­(μ-dithiolate)₂] via the direct reaction of the dinuclear hexacarbonyl precursor [Fe₂(CO)₆(μ-dithiolate)] with the mononuclear species [Fe­(CO)₂(κ²-diphosphine)­(κ²-dithiolate)] has been developed with diphosphine (dppe) and dithiolate (pdt = propanedithiolate) (<b>1</b>) or adt^Bn ({SCH₂}₂NBn = azadithiolate) (<b>2</b>). A crystallographic study was carried out on <b>2</b> and Mössbauer spectroscopy, and DFT calculations have been used to describe the electronic and structural properties of <b>1</b>. The electrochemical properties of <b>1</b> in the absence and in the presence of a weak acid have been the subject of a preliminary investigation

New Systematic Route to Mixed-Valence Triiron Clusters Derived from Dinuclear Models of the Active Site of [Fe–Fe]-Hydrogenases

A novel reaction pathway to synthesize the linear trinuclear clusters [Fe₃(CO)₅(κ²-diphosphine)­(μ-dithiolate)₂] via the direct reaction of the dinuclear hexacarbonyl precursor [Fe₂(CO)₆(μ-dithiolate)] with the mononuclear species [Fe­(CO)₂(κ²-diphosphine)­(κ²-dithiolate)] has been developed with diphosphine (dppe) and dithiolate (pdt = propanedithiolate) (<b>1</b>) or adt^Bn ({SCH₂}₂NBn = azadithiolate) (<b>2</b>). A crystallographic study was carried out on <b>2</b> and Mössbauer spectroscopy, and DFT calculations have been used to describe the electronic and structural properties of <b>1</b>. The electrochemical properties of <b>1</b> in the absence and in the presence of a weak acid have been the subject of a preliminary investigation

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(κ²-dppe)­(μ-pdt)] is reported. Mössbauer spectroscopy and density functional theory calculations give deep insight into the electronic and structural properties of this compound