Ethylene elimination upon addition of acids to the μ-ethenyl complex [Fe2(CO)4(μ-η2-HC=CH2)(μ-PCy2)(μ-dppm)]: Synthesis of halide- and carboxylate-bridged diiron carbonyl complexes

Article abstract of DOI:10.1016/S0022-328X(96)06331-0

Addition of acids to the μ-ethenyl complex [Fe₂(CO)₄(μ-η²-HC=CH₂)(μ-PCy₂)(μ-dppm)] 1 results in selective proton transfer to the α-carbon and loss of ethylene. When the anion is coordinating, subsequent attack at the diiron centre provides a convenient synthesis of halide- and carboxylate-bridged diiron complexes [Fe₂(CO)₄(μ-X)(μ-PCy₂)(μ-dppm)] 2a-c (X = F, Cl, Br) and [Fe₂(CO)₄(μ-η²-O₂CY)(μ-PCy₂)(μ-dppm)] 3a-e (Y = H, CF₃, CCl₃, CBr₃, CO₂H) respectively. When it is poorly coordinating, scavenging of two carbonyls affords the hexacarbonyl complexes [Fe₂(CO)₆(μ-PCy₂)(μ-dppm)][Z] 4 (Z = F, BF₄, PF₆, 0.5SO₄), the yield of which is increased under a carbon-monoxide atmosphere. In order to elucidate mechanistic details the reactions have been monitored by IR, ¹H and ³¹P NMR spectroscopies. These reveal the formation of an intermediate, spectroscopically characterised as the vinyl-hydride [HFe₂(CO)₄(μ-η²-HC=CH₂)(μ-PCy₂)(μ-dppm)][BF₄] 5, which is stable in the presence of ethylene and propene. Crystallographic studies have been carried out on [Fe₂(CO)₄(μ-Cl)(μ-PCy₂)(μ-dppm)] 2b, [Fe₂(CO)₄(μ-Br)(μ-PCy₂)(μ-dppm)] 2c and [Fe₂(CO)₄(μ-η²-O₂CH)(μ-PCy₂)(μ-dppm)] 3a. All contain a short iron-iron vector bridged symmetrically by diphosphine and phosphido moieties with a relative trans orientation, and the halide or carboxylate anion which lies cis to both the latter.