SYNTHESIS, CHARACTERIZATION, AND REACTIONS OF CARBON-DIOXIDE BRIDGED IRON-RHENIUM COMPLEXES

A new mu(2)-eta(2)-CO2-bridged complex, CpFe(CO)(PPh(3))(CO2)Re(CO)(4)[P(OEt)(3)] (3), has been characterized. Thermolysis of 3 in benzene at 60 degrees C occurs with loss of CO from the rhenium center and provides a mu(2)-eta(3)-CO2-bridged complex, 4a. Compound 4a has been characterized by X-ray crystallography (space group P2(1)/n, a = 10.229(6) Angstrom, b = 13.737(9) Angstrom, c = 25.25(2) Angstrom, beta = 99.20(7)degrees, Z = 4, rho(calc) = 1.69 g cm(-3), and R = 0.038). The structure shows that the rhenium atom has facial geometry and that the phosphorus ligands on iron and rhenium are anti; additionally, the plane of the CO2 Ligand bisects the angle defined by the phosphorus, iron, and carbonyl carbon atoms. Exhaustive thermolysis of 3, in solution or in the solid state, leads to a different mu(2)-eta(3)-CO2-bridged complex, 5b, in which the PPh(3) and P(OEt)(3) ligands have exchanged metal centers; 5b retains facial geometry at rhenium. Attempted thermolysis of 4a in the solid state under similar conditions resulted in no reaction. Studies of the thermolysis reactions of 3, in solution, under milder conditions revealed the presence of intermediate compounds, 4b and 5a, which isomerized to 5b as the final product. Compounds 4b and 5a also have facial geometry at rhenium. Partial thermolysis of 4a in solution, in the presence of CO, led to mixtures containing 4b, 5a, and 5b. Reaction of 4a with P(OEt)(3) led to a new mu(2)-eta(2)-CO2-bridged complex, 6; reaction of 4a with CO at room temperature afforded partial conversion to 3. Thermolysis pathways are proposed in which O-Re bond breaking in the mu(2)-eta(3) complexes is followed by ligand capture (CO or P(OEt)(3)), facial to facial rearrangement, or phosphorus ligand exchange between the two metal centers. The thermolysis reactions clearly demonstrate the robust nature of the CO2 bridging ligands in this series of compounds.