Models of the iron-only hydrogenase: Synthesis and protonation of bridge and chelate complexes [Fe2(CO)4{Ph2P(CH2)nPPh2}(μ-pdt)] (n = 2–4) – evidence for a terminal hydride intermediate

Reactions of [Fe2(CO)6(μ-pdt)] (pdt = SCH2CH2CH2S) and diphosphines, Ph2P(CH2)nPPh2 (n = 2–4) and trans-Ph2PCHCHPPh2, have been carried out under different conditions. For all, at room temperature in MeCN with added Me3NO·2H2O the diphosphine-linked complexes [{Fe2(CO)5(μ-pdt)}2(μ,κ1,κ1-diphosphine)] result. For trans-Ph2PCHCHPPh2 this is the only product under all conditions. It has been crystallographically characterised revealing a C2 symmetric structure with apical substitution at the diiron centres. In refluxing toluene, reactions with dppe and dppp lead to the formation of a mixture of diphosphine-bridged and chelate isomers [Fe2(CO)4(μ-diphosphine)(μ-pdt)] and [Fe2(CO)4(κ2-diphosphine)(μ-pdt)], respectively, while with dppb the bridged complex [Fe2(CO)4(μ-dppb)(μ-pdt)] is the only product. In MeCN at 60–70 °C (with added Me3NO·2H2O) similar products result although the ratios differ providing evidence for the conversion of chelate to bridge isomers. Three complexes, [Fe2(CO)4(μ-dppe)(μ-pdt)], [Fe2(CO)4(κ2-dppp)(μ-pdt)] and [Fe2(CO)4(μ-dppb)(μ-pdt)], have been crystallographically characterised and are compared to the previously reported dppm (n = 1) complexes [Fe2(CO)4(μ-dppm)(μ-pdt)] and [Fe2(CO)4(κ2-dppm)(μ-pdt)]. Diphosphine-bridged complexes are structurally superficially similar although significant differences are noted in some key bond lengths and angles, while chelate complexes [Fe2(CO)4(κ2-dppp)(μ-pdt)] and [Fe2(CO)4(κ2-dppm)(μ-pdt)] differ in adopting basal–apical and dibasal coordination geometries, respectively, in the solid state. A number of protonation studies have been carried out. Addition of HBF4·Et2O to [Fe2(CO)4(μ-dppe)(μ-pdt)] affords a bridging hydride complex with poor stability, while in contrast with [Fe2(CO)4(μ-dppb)(μ-pdt)] the stable hydride [(μ-H)Fe2(CO)4(μ-dppb)(μ-pdt)][BF4] results. This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb. With [Fe2(CO)4(κ2-dppp)(μ-pdt)] the bridging hydride complex [(μ-H)Fe2(CO)4(κ2-dppp)(μ-pdt)][BF4] is the final product, in which the diphosphine occupies two basal sites. Monitoring by NMR at low temperature shows the initial formation of a terminal hydride, which rapidly rearranges to a bridged isomer in which the diphosphine adopts a basal–apical geometry and this in turn rearranges in a slower process to the dibasal isomer. This behavior is similar to that recently communicated for [Fe2(CO)4(κ2-dppe)(μ-pdt)]. [S. Ezzaher, J.-F. Capon, F. Gloaguen, F.Y. Pétillon, P. Schollhammer, J. Talarmin, R. Pichon, N. Kervarec, Inorg. Chem. 46 (2007) 3426–3428.]