Models of the iron-only hydrogenase: Reactions of [Fe2(CO)6(μ-pdt)] with small bite-angle diphosphines yielding bridge and chelate diphosphine complexes [Fe2(CO)4(diphosphine)(μ-pdt)]

Reactions of [Fe2(CO)6(μ-pdt)] (1) (pdt = SCH2CH2CH2S) and small bite-angle diphosphines have been studied. A range of products can be formed being dependent upon the nature of the diphosphine and reaction conditions. With bis(diphenylphosphino)methane (dppm), thermolysis in toluene leads to the formation of a mixture of bridge and chelate isomers [Fe2(CO)4(μ-dppm)(μ-pdt)] (2) and [Fe2(CO)4(κ2-dppm)(μ-pdt)] (3), respectively. Both have been crystallographically characterised, 3 being a rare example of a chelating dppm ligand in a first row binuclear system. At room temperature in MeCN with added Me3NO · 2H2O, the monodentate complex [Fe2(CO)5(κ1-dppm)(μ-pdt)] (4) is initially formed. Warming 4 to 100 °C leads the slow conversion to 2, while oxidation (on alumina) gives [Fe2(CO)5(κ1-dppmO)(μ-pdt)] (5). With bis(dicyclohexylphosphino)methane (dcpm), heating in toluene cleanly affords [Fe2(CO)4(μ-dcpm)(μ-pdt)] (6). With Me3NO · 2H2O in MeCN the reaction is not clean as the phosphine is oxidised but monodentate [Fe2(CO)5(κ1-dcpm)(μ-pdt)] (7) can be seen spectroscopically. With 1,2-bis(diphenylphosphino)benzene (dppb) and cis-1,2-bis(diphenylphosphino)ethene (dppv) the chelate complexes [Fe2(CO)4(κ2-dppb)(μ-pdt)] (8) and [Fe2(CO)4(κ2-dppv)(μ-pdt)] (9), respectively are the final products under all conditions, although a small amount of [Fe2(CO)5(κ2-dppvO)(μ-pdt)] (10) was also isolated. Protonation of 2 with HBF4 affords a cation with poor stability while with the more basic diiron centre in 6 readily forms the stable bridging-hydride complex [(μ-H)Fe2(CO)4(μ-dcpm)(μ-pdt)][BF4] (11) which has been crystallographically characterised.