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

Abstract

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-η2O2CY)(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.