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 [Fe-2(CO)(6)(mu-pdt)] (pdt = SCH2CH2CH2S) and diphosphines, Ph2P(CH2)(n)PPh2 (n = 2-4) and trans-Ph2PCH=CHPPh2, have been carried out under different conditions. For all, at room temperature in MeCN with added Me3NO center dot 2H(2)O the diphosphine-linked complexes [{Fe-2(CO)(5)(mu-pdt)}(2)(mu,K-1,K-1-diphosphine)] result. For trans-Ph2PCH=CHPPh2 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)(mu-diphosphine)(mu-pdt)] and [Fe-2(CO)(4)(kappa(2)-diphosphine)(mu-pdt)], respectively, while with dppb the bridged complex [Fe-2(CO)(4)(mu-dppb)(mu-pdt)] is the only product. In MeCN at 60-70 degrees C (with added Me3NO center dot 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)(mu-dppe) (mu-pdt)], [Fe-2(CO)(4)(kappa(2)-dppp)(mu-pdt)] and [Fe-2(CO)(4)(mu-dppb)(mu-pdt)], have been crystallographically characterised and are compared to the previously reported dppm (n = 1) complexes [Fe-2(CO)(4)(mu-dppm)(mu-pdt)] and [Fe-2(CO)(4)(kappa(2)-dppm)(mu-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)(kappa(2)-dppp)(mu-pdt)] and [Fe-2(CO)(4)(kappa(2)-dppm)(mu-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 center dot Et2O to [Fe-2(CO)(4)(mu-dppe)(mu-pdt)] affords a bridging hydride complex with poor stability, while in contrast with [Fe-2(CO)(4)(mu-dppb)(mu-pdt)] the stable hydride [(mu-H)Fe-2(CO)(4)(mu-dppb)(mu-pdt)][BF4] results. This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb. With [Fe-2(CO)(4)(kappa(2)-dppp)(mu-pdt)] the bridging hydride complex [(mu-H)Fe-2(CO)(4)(kappa(2)-dppp) (mu-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 basalapical 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)(kappa(2)-dppe)(mu-pdt)].