The metal-sulphur active sites of hydrogenases catalyse hydrogen evolution or uptake at rapid rates. Understanding the structure and function of these active sites--through mechanistic studies of hydrogenases, synthetic assemblies and in silico models--will help guide the design of new materials for hydrogen production or uptake. Here we report the assembly of the iron-sulphur framework of the active site of iron-only hydrogenase (the H-cluster), and show that it functions as an electrocatalyst for proton reduction. Through linking of a di-iron subsite to a {4Fe4S} cluster, we achieve the first synthesis of a metallosulphur cluster core involved in small-molecule catalysis. In addition to advancing our understanding of the natural biological system, the availability of an active, free-standing analogue of the H-cluster may enable us to develop useful electrocatalytic materials for application in, for example, reversible hydrogen fuel cells. (Platinum is currently the preferred electrocatalyst for such applications, but is expensive, limited in availability and, in the long term, unsustainable.).

No abstract

The reaction of Fe 2 (S 2 C 2 H 4)(CO) 6 with cis-Ph 2 PCH=CHPPh 2 (dppv) yields Fe 2 (S 2 C 2 H 4) (CO) 4 (dppv), 1(CO) 4 , wherein the dppv ligand is chelated to a single iron center. NMR analysis indicates that in 1(CO) 4 , the dppv ligand spans axial and basal coordination sites. In addition to the axial-basal isomer, the 1,3-propanedithiolate and azadithiolate derivatives exist as dibasal isomers. Density functional theory (DFT) calculations indicate that the axial-basal isomer is destabilized by nonbonding interactions between the dppv and the central NH or CH 2 of the larger dithiolates. The Fe(CO) 3 subunit in 1(CO) 4 undergoes substitution with PMe 3 and cyanide to afford 1(CO) 3 (PMe 3) and (Et 4 N)[1(CN)(CO) 3 ], respectively. Kinetic studies show that 1(CO) 4 reacts faster with donor ligands than does its parent Fe 2 (S 2 C 2 H 4)(CO) 6. The rate of reaction of 1(CO) 4 with PMe 3 was first order in each reactant, k = 3.1 × 10 − 4 M −1 s −1. The activation parameters for this substitution reaction, ΔH ‡ = 5.8(5) kcal/mol and ΔS ‡ = −48(2) cal/deg•mol, indicate an associative pathway. DFT calculations suggest that, relative to Fe 2 (S 2 C 2 H 4)(CO) 6 , the enhanced electrophilicity of 1(CO) 4 arises from the stabilization of a "rotated" transition state, which is favored by the unsymmetrically disposed donor ligands. Oxidation of MeCN solutions of 1(CO) 3 (PMe 3) with Cp 2 FePF 6 yielded [Fe 2 (S 2 C 2 H 4)(μ-CO)(CO) 2 (dppv)(PMe 3)(NCMe)](PF 6) 2. Reaction of this compound with PMe 3 yielded [Fe 2 (S 2 C 2 H 4)(μ-CO)(CO)(dppv)(PMe 3) 2 (NCMe)](PF 6) 2 .

Nature is a valuable source of inspiration in the design of catalysts, and various approaches are used to elucidate the mechanism of hydrogenases, the enzymes that oxidize or produce H2. In FeFe hydrogenases, H2 oxidation occurs at the H-cluster, and catalysis involves H2 binding on the vacant coordination site of an iron centre. Here, we show that the reversible oxidative inactivation of this enzyme results from the binding of H2 to coordination positions that are normally blocked by intrinsic CO ligands. This flexibility of the coordination sphere around the reactive iron centre confers on the enzyme the ability to avoid harmful reactions under oxidizing conditions, including exposure to O2. The versatile chemistry of the diiron cluster in the natural system might inspire the design of novel synthetic catalysts for H2 oxidation.