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 [Fe-2(CO)(6)(mu-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 [Fe-2(CO)(4)(mu-dppm)(mu-pdt)] (2) and [Fe-2(CO)(4)(kappa(2)-dppm)(mu-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 center dot 2H(2)O, the monodentate complex [Fe-2(CO)(5)(kappa(1)-dppm)(mu-pdt)] (4) is initially formed. Warming 4 to 100 degrees C leads the slow conversion to 2, while oxidation (on alumina) gives [Fe-2(CO)(5)(kappa(1)-dppmO)(mu-pdt)] (5). With bis(dicyclohexylphosphino)methane (dcpm), heating in toluene cleanly affords [Fe-2(CO)(4)(mu-dcpm)(mu-pdt)] (6). With Me3NO center dot 2H(2)O in MeCN the reaction is not clean as the phosphine is oxidised but monodentate [Fe-2(CO)(5)(kappa(1)-dcpm)(mu-pdt)] (7) can be seen spectroscopically. With 1,2-bis(diphenylphosphino) benzene (dppb) and cis-1,2-bis(diphenylphosphino)ethene (dppv) the chelate complexes [Fe-2(CO)(4)(kappa(2)-dppb)(mu-pdt)] (8) and [Fe-2(CO)(4)(kappa(2)-dppv)(mu-pdt)] (9), respectively are the final products under all conditions, although a small amount of [Fe-2(CO)(5)(kappa(2)-dppvO)(mu-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 [(mu-H)Fe-2(CO)(4)(mu-dcpm)(mu-pdt)][BF4] (11) which has been crystallographically characterised. (c) 2007 Elsevier B.V. All rights reserved.