THE REACTIVITY OF BRIDGING HYDRIDE LIGANDS AT THE DIIRON CENTER - SYNTHESIS AND REACTIVITY TOWARDS ALKYNES OF [FE2(CO)6(MU-H)(MU-CO)-DPPM)][BF4] - X-RAY CRYSTAL-STRUCTURE OF [FE2(CO)6(MU-PHC=CH2(MU-DPPM)][BF4]

Addition of HBF4.Et2O to [Fe2(CO)6(mu-CO)(mu-dppm) (1) results in rapid and quantitative proton addition across the di-iron vector to yield [Fe2(CO)6(mu-H)(mu-CO)(mu-dppm)][BF4] (2). Unlike 1, in which all carbonyls are fluxional even at low temperatures, those in 2 are all static at room temperature, a feature that is attributed to the blocking of the previously vacant bridging site by the hydride ligand. The hydride in 2 is acidic. Thus upon addition of water, amines, or phosphines rapid and quantitative regeneration of 1 occurs. In contrast, alkyne insertion into the di-iron hydride function is rapid, and yields sigma-pi vinyl complexes. With ethyne the previously characterised mu-ethenyl complex [Fe2(CO)6(mu-HC=CH2)(mu-dppm)][BF4] (3) is formed in high yield. Addition of phenylacetylene occurs with high regioselectivity to give predominantly the 1-phenylvinyl isomer [Fe2(CO)6(mu-PhC=CH2)(mu-dppm)][BF4] (4a), which has been characterised crystallographically, and shown to contain a trans arrangement of vinyl and diphosphine ligands. Insertion of diphenyacetylene gives two products, which are tentatively characterised as isomers of [Fe2(CO)6(mu-PhC=CH(Ph)}(mu-dppm)][BF4] (5) with different relative orientations of vinyl and diphosphine ligands.