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

The activation of CO 2 by chemical, electrochemical, and photochemical means is discussed. Binuclear transition metal complexes mediate oxygen atom transfers from CO 2 by three distinct chemical pathways: (i) deoxygenation of CO 2 , (ii) multiple bond metathesis, and (iii) disproportionation. The complex Ir 2 (μ‐CNR) 2 (CNR) 2 (dmpm),(dmpm = bis(dimethylphosphino)methane) undergoes double cycloaddition of CO 2 to its μ‐CNR ligands. A subsequent reaction produces the bis(carbamoyl) complex [Ir 2 (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 CO 2 . In contrast, the binuclear nickel complex Ni 2 (μ‐CNMe)(CNMe) 2 (dppm) 2 (dppm = bis‐(diphenylphosphino)methane) reacts with liquid CO 2 to give the tricarbonyl complex Ni 2 (μ‐CO)(CO) 2 (dppm) 2 . Isotope labelling indicates that the carbonyl ligands are not derived from CO 2 deoxygenation, but from C/CO triple bond metathesis. The reaction of CO 2 with the Ir(0) complex Ir 2 (CO) 3 (dmpm) 2 leads to CO 2 disproportionation by formation of the carbonate, Ir 2 (CO 3 )(CO) 2 (dmpm) 2 , and tetracarbonyl, Ir 2 (CO) 4 (dmpm) 2 , complexes. The complex Ir 2 (CO 3 )(CO) 2 (dmpm) 2 undergoes reversible O‐atom transfers from its carbonate ligand. The electrochemical activation of CO 2 by the binuclear Ni 2 (μ‐CNMe)(CNMe) 2 (dppm) 2 and trinuclear [Ni 3 (μ‐CNMe)(μ‐I)(dppm) 3 ][PF 6 ] species is described. The triangular nickel complex [Ni 3 (μ 3 ‐CNMe)(μ 3 ‐I)(dppm) 3 ][PF 6 ] is an electrocatalyst for the reduction of CO 2 . The cluster exhibits a reversible single electron reduction at E 0 ( +/0 ) = −1.09 V vs. Ag/AgCl. In the presence of CO 2 , the cluster reduces CO 2 by an EC' electrochemical mechanism. The reduction products correspond to the disproportionation and H‐atom abstraction products of CO 2 *− , with a partitioning ratio of 10:1. Isotope labelling studies with 13 CO 2 indicate that 13 CO 2 *− disproportionation produces 13 CO and 13 CO 3 2− . Studies of the photochemical activation of CO 2 by Ni 2 (μ‐CNMe)(CNMe) 2 (dppm) 2 are described. The bimolecular photochemical addition of CO 2 to the complex was examined by laser transient absorbance spectroscopy. Photolysis at 355nm in the presence of CO 2 (1 atm) leads to cycloaddition of CO 2 to the μ‐CNMe ligand and the complex Ni 2 (μ‐CN(Me)C(O)O)(CNMe) 2 (dppm) 2 with Φ 355 =0.05. The triplet excited state of Ni,(μ‐CNMe)(CNMe) 2 (dppm) 2 was determined to react with CO 2 with the bimolecular reaction rate constant k = 1 × 10 4 M −1 s −1 . Bridging ligand substituent effects and solvent dependence of the lowest energy electronic absorption spectral bands of the series of complexes, Ni 2 (μ‐L)(CNMe) 2 (dppm) 2 , L = CNMe, CNC 6 H 5 , CN‐ p ‐C 6 H 4 Cl. and CN‐ p ‐C 6 H 4 Me, confirm the assignment of di‐metal to bridging ligand charge transfer (M 2 →μ‐LCT). This assignment is supported by results of extended Hückel calculations which indicate a LUMO of predominantly μ‐isocyanide π* character. A systematic study of the nature of the lowest excited states of related d 10 –d 10 binuclear complexes of the type Ni 2 (μ‐L)(CNMe) 2 (dppm) 2 , where L = CNMe(Ph) + , CNMe 2 + , CNMe(C 5 H 11 ) + , CNMe(H) + , CNMe(CH 2 C 6 H 5 ) + , 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 Hückel MO studies confirm that the μ‐isocyanide complexes are characterized by di‐metal to bridging ligand charge transfer (M 2 → μ‐LCT) excited states. However, the μ‐aminocarbyne and μ‐nitrosyl complexes exhibit bridging ligand to metal charge transfer (μ‐L→M 2 ) and intraligand (IL) lowest excited states, respectively.