Ethylene elimination upon addition of acids to the mu-ethenyl complex [Fe-2(CO)(4)(mu-eta(2)-HC=CH2)(mu-PCy(2))(mu-dppm)]: Synthesis of halide- and carboxylate-bridged diiron carbonyl complexes

Addition of acids to the mu-ethenyl complex [Fe-2(CO)(4)(mu-eta(2)-HC=CH2)(mu-PCy(2))(mu-dppm)] 1 results in selective proton transfer to the alpha-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 [Fe-2(CO)(4)(mu-X)(mu-PCy(2))(mu-dppm)] 2a-c (X = F, Cl, Br) and [Fe-2(CO)(4)(mu-eta(2)- O2CY)(mu-PCy(2))(mu-dppm)] 3a-e (Y = H, CF3, CCl3, CBr3, CO2H) respectively. When it is poorly coordinating, scavenging of two carbonyls affords the hexacarbonyl complexes [Fe-2(CO)(6)(mu-PCy(2))(mu-dppm)][Z] 4 (Z = F, BF4, PF6, 0.5SO(4)), the yield of which is increased under a carbon-monoxide atmosphere. In order to elucidate mechanistic details the reactions have been monitored by TR, H-1 and P-31 NMR spectroscopies. These reveal the formation of an intermediate, spectroscopically characterised as the vinyl-hydride [HFe2(CO)(4)(mu-eta(2)-HC=CH2)(mu-PCy(2))(mu-dppm)][BF4] 5, which is stable in the presence of ethylene and propene. Crystallographic studies have been carried out on [Fe-2(CO)(4)(mu-Cl)(mu-PCy(2))(mu-dppm)] 2b, [Fe-2(CO)(4)(mu-Br)(mu-PCy(2))(mu-dppm)] 2c and [Fe-2(CO)(4)(mu- eta(2)-O2CH)(mu-PCy(2))(mu-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.