Cooperative Iron-Oxygen-Copper Catalysis in the Reduction of Benzaldehyde under Water-Gas Shift Reaction Conditions

Two Fe-Cu heterobimetallic complexes have been synthesized from the reactions of the (cyclopentadienone)iron complexes {2,5-(EMe3)(2)-3,4-(CH2)(4)(eta(4)-C4C=O)}Fe(CO)(3) (E = Si, 1a; E = C, lb) with (IPr)CuOH (Pr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene). X-ray crystallographic studies show that these complexes adopt a structure featuring a bridging hydride which can be described by the formula {2,5-(EMe3)(2)-3,4-(CH2)(4)(eta(4)-C4C=O)}(CO)(2)Fe(mu-H)Cu(IPr) (E = Si, 4a'; E = C, 4b'). When they are dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of 4a' or 4b' and the terminal iron hydride 12,5-(EMe3)(2)-3,4(CH3)(4)-(eta(5)-C4COCuIPr)}Fe(CO)(2)H (E = Si, 4a; E = C, 4b). The solution structure for the Me3Si derivative is dominated by 4a. Both 4a and 4b/4b' react with HCO2H to form a monometallic iron hydride and (IPr)CuOCHO. They also undergo displacement of (IPr)CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas shift conditions; 4a exhibits higher activity than 4b/4b'. Control experiments with monometallic species establish the cooperativity between a bifunctional iron fragment and a copper fragment during the catalytic reaction. A mechanistic investigation including stoichiometric reactions, various control experiments, and labeling studies has led to the proposal of two different catalytic cycles.