Synthesis, characterization and electrochemical behavior of some N-heterocyclic carbene-containing active site models of [FeFe]-hydrogenases

Insights into Triazolylidene Ligands Behaviour at a Di-Iron Site Related to [FeFe]-Hydrogenases

The behaviour of triazolylidene ligands coordinated at a {Fe₂(CO)₅(µ-dithiolate)} core related to the active site of [FeFe]-hydrogenases have been considered to determine whether such carbenes may act as redox electron-reservoirs, with innocent or non-innocent properties. A novel complex featuring a mesoionic carbene (MIC) [Fe₂(CO)₅(Pmpt)(µ-pdt)] (1; Pmpt = 1-phenyl-3-methyl-4-phenyl-1,2,3-triazol-5-ylidene; pdt = propanedithiolate) was synthesized and characterized by IR, ¹H, ¹³C{¹H} NMR spectroscopies, elemental analyses, X-ray diffraction, and cyclic voltammetry. Comparison with the spectroscopic characteristics of its analogue [Fe₂(CO)₅(Pmbt)(µ-pdt)] (2; Pmbt = 1-phenyl-3-methyl-4-butyl-1,2,3-triazol-5-ylidene) showed the effect of the replacement of a n-butyl by a phenyl group in the 1,2,3-triazole heterocycle. A DFT study was performed to rationalize the electronic behaviour of 1, 2 upon the transfer of two electrons and showed that such carbenes do not behave as redox ligands. With highly perfluorinated carbenes, electronic communication between the di-iron site and the triazole cycle is still limited, suggesting low redox properties of MIC ligands used in this study. Finally, although the catalytic performances of 2 towards proton reduction are weak, the protonation process after a two-electron reduction of 2 was examined by DFT and revealed that the protonation process is favoured by S-protonation but the stabilized diprotonated intermediate featuring a {Fe-H⋯H-S} interaction does not facilitate the release of H₂ and may explain low efficiency towards HER (Hydrogen Evolution Reaction).

Mechanism of Diiron Hydrogenase Complexes Controlled by Nature of Bridging Dithiolate Ligand

Bio-inorganic complexes inspired by hydrogenase enzymes are designed to catalyze the hydrogen evolution reaction (HER). A series of new diiron hydrogenase mimic complexes with one or two terminal tris(4-methoxyphenyl)phosphine and different μ-bridging dithiolate ligands and show catalytic activity towards electrochemical proton reduction in the presence of weak and strong acids. A series of propane- and benzene-dithiolato-bridged complexes was synthesized, crystallized, and characterized by various spectroscopic techniques and quantum chemical calculations. Their electrochemical properties as well as the detailed reaction mechanisms of the HER are elucidated by density functional theory (DFT) methods. The nature of the μ-bridging dithiolate is critically controlling the reaction and performance of the HER of the complexes. In contrast, terminal phosphine ligands have no significant effects on redox activities and mechanism. Mono- or di-substituted propane-dithiolate complexes afford a sequential reduction (electrochemical; E) and protonation (chemical; C) mechanism (ECEC), while the μ-benzene dithiolate complexes follow a different reaction mechanism and are more efficient HER catalysts.

[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.

Dinuclear Gold(I) Complexes Bearing N,N'-Allyl-Bridged Bisimidazolylidene Ligands

A novel N,N'-allyl-bridged bisimidazolium salt and a novel dinuclear Ag(I) and a Au(I) NHC complex are reported. Both metallacyclic complexes have a twisted structural shape due to the rigid allylic system and form two different isomers relating to the position of the double bonds. The allyl-group shows photoisomerisation, but no reactivity towards bases for the additional coordination of Pd(II).

Synthesis, characterization, electrochemical properties and catalytic reactivity of N-heterocyclic carbene-containing diiron complexes

Four new [Fe–Fe]–NHC complexes were synthesized and used as highly selective homogeneous catalysts for the direct hydroxylation of benzene to phenol.

Synthesis and characterization of diiron dithiolate complexes containing a quinoxaline bridge

A potential model complex for the hydrogenase active site, [Fe(2){(μ-CH(2)S)(2)R}(CO)(6)] (1) (R = quinoxaline), was synthesized by condensation of [(μ-LiS)(2)Fe(2)(CO)(6)] with 2,3-bis(bromomethyl)quinoxaline. Reactions of 1 with bis(diphenylphosphino)methane (dppm) under a range of conditions yielded substituted complexes [Fe(2){(μ-CH(2)S)(2)R}(CO)(5)(dppm)] (2), [Fe(2){(μ-CH(2)S)(2)R}(CO)(4)(k(2)-dppm)] (3) and [Fe(2){(μ-CH(2)S)(2)R}(CO)(4)(μ-dppm)] (4). X-ray crystallography confirms that in 2, the dppm is terminally bonded to an iron atom via one phosphorus atom, whereas in 3, it acts as a chelating ligand to coordinate to an iron center in a dibasal-substituted manner. In 4, the dppm bridges the two iron atoms in a cis basal/basal fashion with one phosphorus bonded to each iron atom. Treatment of 1 with various tertiary phosphines at room temperature in acetonitrile (MeCN) generates a range of mono-substituted products [Fe(2){(μ-CH(2)S)(2)R}(CO)(5)L] (5, L = PEt(3); 6, PMe(3); 7, PPh(3); 8, Me(2)PPh). With Bu(t)NC, mono- and di-substituted [Fe(2){(μ-CH(2)S)(2)R}(CO)(5)(Bu(t)NC)] (9) and [Fe(2){(μ-CH(2)S)(2)R}(CO)(4)(Bu(t)NC)(2)] (10) complexes are generated. All the complexes were characterized by elemental analysis, IR, MS and NMR spectroscopy. IR and NMR spectroscopic studies suggest that addition of excess HBF(4)·OEt(2) acid to 1-4 led to the protonation of quinoxaline nitrogen atoms. In contrast, 5-10 were not stable in acidic media. Electrochemistry of 1-4 was investigated in the acetonitrile medium (0.1 M Bu(4)NPF(6)). The electrochemical instability of the reduced ligand, quinoxaline, and the reduced forms of these complexes revealed from the electrochemical studies suggests that they do not provide ideal models of the hydrogenase active site.

3,3-Dimethyl-1,1-(propane-1,3-diyl)diimidazol-1-ium tetrabromidocadmate(II)

The title compound, (C₁₁H₁₈N₄)[CdBr₄], was prepared by an anion exchange. The dihedral angle between the two planar imidazolium rings in the cation is 74.4 (4)°. The crystal packing is stabilized by weak inter­molecular C—H⋯Br hydrogen bonds between the cation and the tetrahedral anion, building up a three-dimensionnal network.