[Fe–Fe] hydrogenase models: Iron(I)-carbonyl clusters coupled to alpha- and para-toluenethiolate ligands

Hydroxyl-Decorated Diiron Complex as a [FeFe]-Hydrogenase Active Site Model Complex: Light-Driven Photocatalytic Activity and Heterogenization on Ethylene-Bridged Periodic Mesoporous Organosilica

A biomimetic model complex of the [FeFe]-hydrogenase active site (FeFeOH) with an ethylene bridge and a pendant hydroxyl group has been synthesized, characterized and evaluated as catalyst for the light-driven hydrogen production. The interaction of the hydroxyl group present in the complex with 3-isocyanopropyltriethoxysilane provided a carbamate triethoxysilane bearing a diiron dithiolate complex (NCOFeFe), thus becoming a potentially promising candidate for anchoring on heterogeneous supports. As a proof of concept, the NCOFeFe precursor was anchored by a grafting procedure into a periodic mesoporous organosilica with ethane bridges (EthanePMO@NCOFeFe). Both molecular and heterogenized complexes were tested as catalysts for light-driven hydrogen generation in aqueous solutions. The photocatalytic conditions were optimized for the homogenous complex by varying the reaction time, pH, amount of the catalyst or photosensitizer, photon flux, and the type of light source (light-emitting diode (LED) and Xe lamp). It was shown that the molecular FeFeOH diiron complex achieved a decent turnover number (TON) of 70 after 6 h, while NCOFeFe and EthanePMO@NCOFeFe had slightly lower activities showing TONs of 37 and 5 at 6 h, respectively.

Biomimetics of [FeFe]-hydrogenases incorporating redox-active ligands: Synthesis, redox and spectroelectrochemistry of diiron-dithiolate complexes with ferrocenyl-diphosphines as Fe4S4 surrogates

[FeFe]-ase biomimics containing a redox-active ferrocenyl diphosphine have been prepared and their ability to reduce protons and oxidise H2 studied, including 1,1’-bis(diphenylphosphino)ferrocene (dppf) complexes Fe2(CO)4(-dppf)(-S(CH2)nS) (n = 2, edt; n...

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.

Intramolecular stabilization of a catalytic [FeFe]-hydrogenase mimic investigated by experiment and theory

The mono-substituted complex [Fe2(CO)5(μ-naphthalene-2-thiolate)2(P(PhOMe-p)3)] was prepared taking after the structural principles from both [NiFe] and [FeFe]-hydrogenase enzymes. Crystal structures are reported for this complex and the all carbonyl analogue. The bridging naphthalene thiolates resemble μ-bridging cysteine amino acids. One of the naphthyl moieties forms π-π stacking interactions with the terminal bulky phosphine ligand in the crystal structure and in calculations. This interaction stabilizes the reduced and protonated forms during electrocatalytic proton reduction in the presence of acetic acid and hinders the rotation of the phosphine ligand. The intramolecular π-π stabilization, the electrochemistry and the mechanism of the hydrogen evolution reaction were investigated using computational approaches.

Crystal and electronic structure of a hexacarbonyldiiron cluster tethered to naphthalene-2-thiolate ligands

The structure of the previously reported complex bis(μ-naphthalene-2-thiolato-κ²S:S)bis(tricarbonyliron)(Fe-Fe), [Fe₂(C₁₀H₇S)₂(CO)₆], has been characterized by X-ray diffraction. In the solid state, the dinuclear complex adopts a butterfly-like shape, with an equatorial-axial spatial orientation of the naphthalene groups covalently coupled to the [S₂Fe₂(CO)₆] unit. The asymmetric unit contains three independent [(μ-naphthalene-2-thiolato)₂Fe₂(CO)₆] molecules. These molecules show intermolecular π-π stacking interactions between the naphthalene rings, which was confirmed by Hirshfield surface analysis. The electronic spectrum of the complex recorded in acetonitrile shows a band centered at 350 nm (ℇ = 4.6 × 10³ M^-1 cm^-1) and tailing into the visible region. This absorption can be attributed to a π→π* electronic transition within the naphthalene moiety and a metal-based d→d transition.

Direct synthesis of phenol by novel [FeFe]-hydrogenase model complexes as catalysts of benzene hydroxylation with H2O2

Compared the catalytic performance of complexes 1–3, the complex 2 has the highest phenol yield (24.6%) and phenol selectivity (92%), which has the highest electron densities of the catalytically active sites.