Highly efficient electrocatalytic proton-reduction by coordinatively and electronically unsaturated Fe(CO)(κ2-dppn)(κ2-tdt)

Coordinatively and electronically unsaturated square-pyramidal Fe(CO)(κ2-dppn)(κ2-tdt) (2) is shown to be amongst the most efficient proton-reduction catalysts reported to date. It is formed from the reaction of Fe2(CO)6(μ-tdt) (tdt = 3,4-toluenedithiolate) with 1,8-bis(diphenylphosphino)naphthalene (dppn) in presence of Me3NO∙2H2O affording Fe2(CO)4(κ2-dppn)(μ-tdt) (1) as the major product, together with smaller but reproducible amounts of 2. Both have been characterized by single crystal X-ray diffraction. The electrochemistry of 2 is solvent dependent but in both CH2Cl2 and in a 1:1 mixture of CH2Cl2/MeCN it shows a reversible reduction at E1/2 = –1.54 V and E1/2 = –1.68 V respectively. While 2 degrades in the presence of the strong acid HBF4∙2H2O it is catalytically active for proton-reduction using CF3CO2H. Catalysis occurs at the first reduction potential and it displays an impressive icat/ip ratio of 33 after addition of 20 equivalents CF3CO2H. It is amongst the most efficient molecular proton-reduction catalysts reported to date.