Models of the iron-only hydrogenase:Synthesis and protonation of bridge and chelate complexes[Fe2(CO)4{Ph2P(CH2)_nPPh2}(mu-pdt)](n=2-4)-evidence for a terminal hydride intermediate

摘要

Reactions of[Fe2(CO)6(mu-pdt)](pdt=SCH2CH2CH2S)and diphosphines,Ph2P(CH2)_nPPh2(n=2-4)and trans Ph2PCH=CHPPh2,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(mu-pdt)}2(mu,k1,k1-diphosphine)]result.For trans-Ph2PCH=CHPPh2 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(mu-diphosphine)(mu-pdt)]and[Fe2(CO)4(k2-diphosphine)(mu-pdt)],respectively,while with dppb the bridged complex[Fe2(CO)4(mu-dppb)(mu-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(mu-dppe)(mu-pdt)],[Fe2(CO)4(k2-dppp)(mu-pdt)]and[Fe2(CO)4(mu-dppb)(mu-pdt)],have been crystallographically characterised and are compared to the previously reported dppm(n=1)complexes[Fe2(CO)4(mu-dppm)(mu-pdt)]and[Fe2(CO)4(k2-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[Fe2(CO)4(k2-dppp)(mu-pdt)]and[Fe2(CO)4(k2-dppm)(mu-pdt)]differ in adopting basal-apical and dibasal co-ordination geometries,respectively,in the solid state.A number of protonation studies have been carried out.Addition of HBF4·Et2O to[Fe2(CO)4(mu-dppe)(mu-pdt)]affords a bridging hydride complex with poor stability,while in contrast with[Fe2(CO)4(mu-dppb)(mu-pdt)]the stable hydride[(mu-H)Fe2(CO)4(mu-dppb)(mu-pdt)][BF4]results.This difference is partially ascribed to the greater flexibility of the diphosphine backbone in dppb.With[Fe2(CO)4(k2-dppp)(mu-pdt)]the bridging hydride complex[(mu-H)Fe2(CO)4(k2-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 basal-apical geometry and this in turn rearranges in a slower process to the dibasal isomer.This behavior is similar to that recently commu-nicated for[Fe2(CO)4(k2-dppe)(mu-pdt)].