Binuclear organometallic iron compounds as structural and functional models for the active site of iron-only hydrogenase

The active site of Fe-only hydrogenase contains a binuclear iron center with two bridging thiolates and diatomic ligands CO and CN, (μ-pdt)[Fe(CO) ₂(CN)][Fe(CO)(CN)S_cys] (pdt = −SCH ₂CH₂CH₂S−). Using PMe ₃ as a mimic of the CN ligand in Fe-only hydrogenase, the binuclear FeIFeI compound (μ-pdt)[Fe(CO)₂(PMe ₃)]₂ was synthesized from the reaction of PMe₃ with (μ-pdt)[Fe(CO)₃]₂. The two Fe centers exist as edge-bridged bi-square pyramids with two bridging thiolates and an Fe-Fe bond distance of 2.55 Å. The Fe₂S₂ butterfly core is therefore a good structural analogue of the [Fe]H₂ase active site. The Fe-Fe bond in (μ-pdt)[Fe(CO)₂(PMe₃)] ₂ reacts with HCl to produce the binuclear FeIIFe II compound {lcub}(μ-H)(μ-pdt)[Fe(CO)₂(PMe₃)] ₂{rcub}+[PF₆]−, which was characterized as a face-bridged bioctahedron with (μ-H)(μ-SRS) bridges and an Fe-Fe distance of 2.58 Å. The 3-carbon S to S link enforces a syn′ orientation at the dithiolate bridge. The coordinated phosphines are directed into basal positions (ba/ba, transoid) on the edge-bridged square pyramids.; The {lcub}(μ-H)(μ-pdt)[Fe(CO)₂(PMe₃)]₂{rcub} +[PF6]− compound undergoes H/D exchange at the μ-H in the presence of D₂O or D₂ gas. Studies of the gaseous reaction were performed in medium pressure NMR tubes, using 2H NMR as monitor. H₂/D₂ mixtures showed HD formation as catalyzed by the active-site mimics. Such H/D exchange reactions have been used as assays of hydrogenase activity, indicating that compound {lcub}(μ-H)(μ-pdt)[Fe(CO) ₂(PMe₃)]₂{rcub}+[PF6]− and analogues (similar binuclear FeII(μ-H)Fe II complexes with different bridges, μ-SCH₂CH₂ S, μ-o-SCH₂C₆H₄CH₂S, μ-SCH ₂CH₂PPh₂ and μ-SEt) serve as functional models for the heterolytic H₂ activation in Fe-only hydrogenase. Under photolysis, the H/D exchange reaction between D₂ and small alkenes (ethylene, propene and 1-butene), but not bulkier alkenes such as 2-butene or cyclohexene, can also be catalyzed by {lcub}(μ-H)(μ-pdt)[Fe(CO)₂ (PMe₃)]₂{rcub}+[PF₆] −. The two-iron organometallics, inspired from active sites of biological catalysts, appear to function as do expensive noble metals which perform 2-electron redox processes.