Approaches to the Chemical, Electrochemical, and Photochemical Activation of Carbon Dioxide by Transition Metal Complexes

The activation of CO2 by chemical, electrochemical, and photochemical means is discussed. Binuclear transition metal complexes mediate oxygen atom transfers from CO2 by three distinct chemical pathways: (i) deoxygenation of CO2, (ii) multiple bond metathesis, and (iii) disproportionation. The complex Ir2 (μ-CNR)2(CNR)2(dmpm),(dmpm = bis(dimethylphosphino)methane) undergoes double cycloaddition of CO2 to its μ-CNR ligands. A subsequent reaction produces the bis(carbamoyl) complex [Ir2(μ-CO)(μ-H)(CONHR)2(CNR)2(dmpm)2]Cl. Isotope labelling studies show that the μ-CO ligand results from net deoxygenation of CO2. In contrast, the binuclear nickel complex Ni2(μ-CNMe)(CNMe)2(dppm)2 (dppm = bis-(diphenylphosphino)methane) reacts with liquid CO2 to give the tricarbonyl complex Ni2(μ-CO)(CO)2(dppm)2. Isotope labelling indicates that the carbonyl ligands are not derived from CO2 deoxygenation, but from C/CO triple bond metathesis. The reaction of CO2 with the Ir(0) complex Ir2(CO)3(dmpm)2 leads to CO2 disproportionation by formation of the carbonate, Ir2(CO3)(CO)2(dmpm)2, and tetracarbonyl, Ir2(CO)4(dmpm)2, complexes. The complex Ir2(CO3)(CO)2 (dmpm)2 undergoes reversible O-atom transfers from its carbonate ligand. The electrochemical activation of CO2 by the binuclear Ni2(μ-CNMe)(CNMe)2(dppm)2 and trinuclear [Ni3(μ-CNMe)(μ-I)(dppm)3][PF6] species is described. The triangular nickel complex [Ni3(μ3-CNMe)(μ3-I)(dppm)3][PF6] is an electrocatalyst for the reduction of CO2. The cluster exhibits a reversible single electron reduction at E0(+/0) = −1.09 V vs. Ag/AgCl. In the presence of CO2, the cluster reduces CO2 by an EC' electrochemical mechanism. The reduction products correspond to the disproportionation and H-atom abstraction products of CO2*−, with a partitioning ratio of 10:1. Isotope labelling studies with 13CO2 indicate that 13CO2*− disproportionation produces 13CO and 13CO32−. Studies of the photochemical activation of CO2 by Ni2(μ-CNMe)(CNMe)2(dppm)2 are described. The bimolecular photochemical addition of CO2 to the complex was examined by laser transient absorbance spectroscopy. Photolysis at 355nm in the presence of CO2 (1 atm) leads to cycloaddition of CO2 to the μ-CNMe ligand and the complex Ni2(μ-CN(Me)C(O)O)(CNMe)2(dppm)2 with Φ355=0.05. The triplet excited state of Ni,(μ-CNMe)(CNMe)2(dppm)2 was determined to react with CO2 with the bimolecular reaction rate constant k = 1 × 104 M−1 s−1. Bridging ligand substituent effects and solvent dependence of the lowest energy electronic absorption spectral bands of the series of complexes, Ni2(μ-L)(CNMe)2(dppm)2, L = CNMe, CNC6H5, CN-p-C6H4Cl. and CN-p-C6H4Me, confirm the assignment of di-metal to bridging ligand charge transfer (M2μ-LCT). This assignment is supported by results of extended Huckel calculations which indicate a LUMO of predominantly μ-isocyanide π* character. A systematic study of the nature of the lowest excited states of related d10–d10 binuclear complexes of the type Ni2(μ-L)(CNMe)2(dppm)2, where L = CNMe(Ph)+, CNMe2+, CNMe(C5H11)+, CNMe(H)+, CNMe(CH2C6H5)+, and NO+ reveals dramatic differences in the lowest excited states of the three classes of complexes: μ-isocyanide, μ-aminocarbyne, and μ-nitrosyl. Spectroscopic and extended Huckel MO studies confirm that the μ-isocyanide complexes are characterized by di-metal to bridging ligand charge transfer (M2 μ-LCT) excited states. However, the μ-aminocarbyne and μ-nitrosyl complexes exhibit bridging ligand to metal charge transfer (μ-LM2) and intraligand (IL) lowest excited states, respectively.