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(η4-C4CO)}­Fe­(CO)3 (E = Si, 1a; E = C, 1b) with (IPr)­CuOH (IPr = 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(η4-C4CO)}­(CO)2Fe­(μ-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 {2,5-(EMe3)2-3,4-(CH2)4(η5-C4COCuIPr)}­Fe­(CO)2H (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.