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

Abstract

Abstract Reactions of [Fe 2 (CO) 6 (μ-pdt)] (pdt = SCH 2 CH 2 CH 2 S) and diphosphines, Ph 2 P(CH 2 ) n PPh 2 ( n = 2–4) and trans -Ph 2 PCH CHPPh 2 , have been carried out under different conditions. For all, at room temperature in MeCN with added Me 3 NO·2H 2 O the diphosphine-linked complexes [{Fe 2 (CO) 5 (μ-pdt)} 2 (μ,κ 1 ,κ 1 -diphosphine)] result. For trans -Ph 2 PCH CHPPh 2 this is the only product under all conditions. It has been crystallographically characterised revealing a C 2 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 [Fe 2 (CO) 4 (μ-diphosphine)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -diphosphine)(μ-pdt)], respectively, while with dppb the bridged complex [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] is the only product. In MeCN at 60–70 °C (with added Me 3 NO·2H 2 O) similar products result although the ratios differ providing evidence for the conversion of chelate to bridge isomers. Three complexes, [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)], [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)], have been crystallographically characterised and are compared to the previously reported dppm ( n = 1) complexes [Fe 2 (CO) 4 (μ-dppm)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -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 [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] and [Fe 2 (CO) 4 (κ 2 -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 HBF 4 ·Et 2 O to [Fe 2 (CO) 4 (μ-dppe)(μ-pdt)] affords a bridging hydride complex with poor stability, while in contrast with [Fe 2 (CO) 4 (μ-dppb)(μ-pdt)] the stable hydride [(μ-H)Fe 2 (CO) 4 (μ-dppb)(μ-pdt)][BF 4 ] results. This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb. With [Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)] the bridging hydride complex [(μ-H)Fe 2 (CO) 4 (κ 2 -dppp)(μ-pdt)][BF 4 ] 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 [Fe 2 (CO) 4 (κ 2 -dppe)(μ-pdt)]. [S. Ezzaher, J.-F. Capon, F. Gloaguen, F.Y. Petillon, P. Schollhammer, J. Talarmin, R. Pichon, N. Kervarec, Inorg. Chem. 46 (2007) 3426–3428.]