Bio-inspired hydrogenase models: mixed-valence triion complexes as proton reduction catalysts

Triiron Complexes Featuring Azadiphosphine Related to the Active Site of [FeFe]-Hydrogenases: Their Redox Behavior and Protonation

The design of iron clusters featuring a bimetallic core and several protonation sites in the second coordination sphere of the metal centers is important for modeling the activity of polymetallic active sites such as the H-cluster of [FeFe]-hydrogenases. For this purpose, the syntheses of complexes [Fe3(CO)5(κ2-PPh2NR2)(μ-pdt)2] (R = Ph (1), Bn (2)) and [Fe3(CO)5(κ2-PPh2NR2)(μ-adtBn)(μ-pdt)] (R = Ph (3), Bn (4)) were carried out by reacting hexacarbonyl precursors [Fe2(CO)6(µ-xdt)] (xdt = pdt (propanedithiolate), adtBn (azadithiolate) with mononuclear complexes [Fe(κ2-pdt)(CO)2(κ2-PPh2NR2)] (PPh2NR2 = (PPhCH2NRCH2)2, R = Ph, Bn) in order to introduce amine functions, through well-known PPh2NR2 diphosphine, into the vicinity of the triiron core. The investigation of the reactivity of these triiron species towards the proton (in the presence of CF3SO3H) and the influence of the pendant amines on the redox properties of these complexes were explored using spectroscopic and electrochemical methods. The protonation sites in such triiron clusters and their relationships were identified. The orientation of the first and second protonation processes depends on the arrangement of the second coordination sphere. The similarities and differences, due to the extended metal nuclearity, with their dinuclear counterparts [Fe2(CO)4(κ2-PPh2NR2)(μ-pdt)], were highlighted.

Mixed valence triiron complexes from the conjugation of [FeIFeI] and [FeII] complexes via intermolecular carbyne/alkyne coupling

We present a new synthetic strategy for obtaining mixed-valence triiron complexes where the metal centers are bridged by a novel, highly functionalized hydrocarbyl ligand. The alkynyl-vinyliminium complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCH)CHCNMe2}]CF3SO3 (X = 4-C6H4, [2a1]CF3SO3; X = (CH2)3, [2a2]CF3SO3) were synthesized in almost quantitative yields from the aminocarbyne precursor [Fe2Cp2(CO)2(μ-CO){μ-CNMe2}]CF3SO3, [1a]CF3SO3, and the di-alkynes HCC-X-CCH. Then, the ferracycle [Fe(Cp)(CO){C(NMe2)CHC(4-C6H4CCH)C(O)}], 4a1, was produced in 47% yield from the cleavage of [2a1]CF3SO3 promoted by pyrrolidine. Subsequent reactions of the acetonitrile adducts [Fe2Cp2(CO)(μ-CO)(NCMe){μ-CNMe(R)}]CF3SO3 (R = Me, [1aACN]CF3SO3; R = Xyl, [1bACN]CF3SO3) ([FeIFeI]) with 4a1 ([FeII]) at room temperature resulted in the formation of [FeIFeIFeII] complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCHC(NMe2)FeCp(CO)CO)CHCNMe(R)}]CF3SO3 (R = Me, [5a1]CF3SO3; R = Xyl, [5b1]CF3SO3) in yields ranging from 56% to 64%. The new products were characterized by IR and multinuclear NMR spectroscopy, and the structures of [2a2]CF3SO3 and 4a1 were confirmed by single crystal X-ray diffraction. Cyclic voltammetry and spectroelectrochemical studies on [5a1]+ have revealed that reduction and oxidation events occur almost independently at the [FeIFeI] and [FeII] units, respectively. This observation underscores a minimal electronic interaction between the two fragments within the triiron complex. Accordingly, DFT studies pointed out that the HOMO and LUMO orbitals are predominantly localized in the two distinct compartments of [5a1]+.

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.

Electrocatalytic proton-reduction behaviour of telluride-capped triiron clusters: tuning of overpotentials and stabilization of redox states relative to lighter chalcogenide analogues

Electrocatalytic proton reduction catalyzed by [Fe₃(CO)₉(μ₃-Te)₂], and a number of its phosphine and diphosphine derivatives, has been studied.

Electrocatalytic proton reduction catalysed by the low-valent tetrairon-oxo cluster [Fe4(CO)10(κ2-dppn)(μ4-O)]2− [dppn = 1,1′-bis(diphenylphosphino)naphthalene]

[Fe₄(CO)₁₀(κ²-dppn)(μ₄-O)]²⁻ reduces protons and DFT calculations support the sequential formation of hydride and dihydrogen ligands at the unique iron centre.

New insights into the chemistry of di- and trimetallic iron dithiolene derivatives. Structural, Mössbauer, magnetic, electrochemical and theoretical studies

Reaction of Fe3(CO)12 with 1,2-dithiolene HSC6H2Cl2SH affords a mixture of complexes [Fe2(CO)6(μ-SC6H2Cl2S)] 1, [Fe2(SC6H2Cl2S)4] 2 and [Fe3(CO)7(μ3-SC6H2Cl2S)2] 3. In the course of the reaction the trimetallic cluster 3 is first formed and then converted into the known dinuclear compound 1 to afford finally the neutral diiron tetrakis(dithiolato) derivative 2. Compounds 2 and 3 have been studied by Mössbauer spectroscopy, X-ray crystallography and theoretical calculations. In compound 2 the metal atoms are in an intermediate-spin Fe(III) state (S(Fe) = 3/2) and each metal is bonded to a bridging dithiolene ligand and a non-bridging thienyl radical (S = 1/2). Magnetic measurements show a strong antiferromagnetic coupling in complex 2. Cyclic voltammetry experiments show that the mixed valence trinuclear cluster 3 undergoes a fully reversible one electron reduction. Additionally, compound 3 behaves as an electrocatalyst in the reduction process of protons to hydrogen.

Hepta-carbonyl-bis-(μ-propane-1,3-di-thiol-ato)triiron(I,II)(2 Fe-Fe)

The trinuclear title compound, [Fe3(C3H6S2)2(CO)7], is a mixed-valent Fe(I)/Fe(II) complex and crystallizes with two mol-ecules of similar configuration in the asymmetric unit. The three Fe atoms in each mol-ecule display a bent arrangement [Fe-Fe-Fe = 156.22 (4) and 157.06 (3)°]. Both outer Fe(I) atoms are six-coordinated in a distorted ocahedral coordination geometry defined by the bridging Fe(II) atom, three carbonyl C atoms and two bridging S atoms. The coordination number of the central Fe(II) atom is seven and includes bonding to the two outer Fe(I) atoms, four bridging S atoms and one carbonyl C atom. The resulting coordination polyhedron might be described as a highly distorted monocapped trigonal prism. In the crystal packing, the mol-ecules exhibit a chain-like arrangement parallel to [100] and [001], and the resulting layers are stacked along [010]. The cohesion of the structure is dominated by van der Waals inter-actions.

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(μ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x (PPh3) x (μ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] as Proton Reduction Catalysts

The mixed-valence triiron complexes [Fe3(CO)7-x (PPh3) x (μ-edt)2] (x = 0-2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(μ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)-Fe(II)-Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at -0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between -1.47 V for [Fe3(CO)7(μ-edt)2] and around -1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(μ-edt)], [Fe3(CO)7(μ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.

ATP-independent substrate reduction by nitrogenase P-cluster variant

The P-cluster of nitrogenase is largely known for its function to mediate electron transfer to the active cofactor site during catalysis. Here, we show that a P-cluster variant (designated P*-cluster), which consists of paired [Fe(4)S(4)]-like clusters, can catalyze ATP-independent substrate reduction in the presence of a strong reductant, europium(II) diethylenetriaminepentaacetate [Eu(II)-DTPA]. The observation of a decrease of activity in the rank ΔnifH, ΔnifBΔnifZ, and ΔnifB MoFe protein, which corresponds to a decrease of the amount of P*-clusters in these cofactor-deficient proteins, firmly establishes P*-cluster as a catalytically active metal center in Eu(II)-DTPA-driven reactions. More excitingly, the fact that P*-cluster is not only capable of catalyzing the two-electron reduction of proton, acetylene, ethylene, and hydrazine, but also capable of reducing cyanide, carbon monoxide, and carbon dioxide to alkanes and alkenes, points to a possibility of developing biomimetic catalysts for hydrocarbon production under ambient conditions.

Triosmium Clusters Containing 2-Mercaptobenzothiazolate Ligands

Reaction of the labile cluster [Os3(CO)10(NCMe)2] with 2,2′-benzothiazyl disulfide leads to the isolation of four products, namely the known hydride complex [Os3(CO)10(µ-H)(µ- S2NC7H4)], [Os3(CO)10(µ-S2NC7H4)2] (1) in which both thiolate ligands act as three-electron donor ligands and span a single osmium-osmium vector and two isomers of [Os3(CO)9(µ-S2NC7H4)(µ3-η2-S2NC7H4)] (2, 3) in which one thiolate caps a face of the triosmium cluster via secondary nitrogen coordination. In a separate experiment thermolysis of 1 in n-heptane affords only 2. Cluster 1 contains two 2-mercaptobenzothiazolate ligands in a µ-η1 mode of bonding, while 2 and 3 are isomers differing in the relative disposition of the two 2-mercaptobenzothiazolate ligands.