The reactivity of bridging hydride ligands at the di-iron centre. Synthesis and reactivity towards alkynes of [Fe 2(CO) 6(μ-H)(μ-CO)(μ-dppm)][BF 4]. X-ray crystal structure of [Fe 2(CO) 6(μ-PhCCH 2)(μ-dppm)][BF 4

Abstract

Addition of HBF 4·Et 2O to [Fe 2(CO) 6(μ-CO)(μ-dppm) ( 1) results in rapid and quantitative proton addition across the di-iron vector to yield [Fe 2(CO) 6(μ-H)(μ-CO)(μ-dppm)][BF 4] ( 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 σ - π vinyl complexes. With ethyne the previously characterised μ-ethenyl complex [Fe 2(CO) 6(μ-HCCH 2)(μ-dppm)][BF 4] ( 3) is formed in high yield. Addition of phenylacetylene occurs with high regioselectivity to give predominantly the 1-phenylvinyl isomer [Fe 2(CO) 6(μ-PhCCH 2)(μ -dppm)][BF 4] ( 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 [Fe 2(CO) 6 (μ-PhCCH(Ph)(μ-dppm)][BF 4] ( 5) with different relative orientations of vinyl and diphosphine ligands.