Researchers in this field have investigated photochemical and electrochemical CO 2 reduction to CO or formate, even to methanol, using transition metal electrodes, metal complexes, semiconductors, and also organic molecules, and the details of these achievements can be found in many reviews recently published. 14−31 While recent progress in this field is quite remarkable in photochemical CO 2 reduction using semiconductors or heterogeneous systems, most experiments have not been confirmed using labeled CO 2 (i.e., 13 CO 2 ) and H 2 O (i.e., H 2 18Figure 1. Z-Scheme for photocatalytic CO 2 reduction coupled to methanol oxidation. Reproduced with permission from ref 46.

This review summarizes research on the electrochemical and photochemical reduction of CO 2 using a variety of iron and cobalt porphyrins, phthalocyanines and related complexes. Metalloporphyrins and metallophthalocyanines are visible light absorbers with extremely large extinction coefficients. However, yields of photochemically-generated active catalysts for CO 2 reduction are typically low owing to the requirement of a second photoinduced electron. This requirement is not relevant to the case of electrochemical CO 2 reduction. Recent progress on efficient and stable electrochemical systems includes the use of FeTPP catalysts that have prepositioned phenyl OH groups in their second coordination spheres. This has led to remarkable progress in carrying out coupled proton-electron transfer reactions for CO 2 reduction. Such ground-breaking research has to be continued in order to produce renewable fuels in an economically feasible manner.

Proton responsive ligands offer control of catalytic reactions through modulation of pH-dependent properties, second coordination sphere stabilization of transition states, or by providing a local proton source for multiproton, multielectron reactions. Two fac-[Re(I)(α-diimine)(CO)3Cl] complexes with α-diimine = 4,4'- (or 6,6'-) dihydroxy-2,2'-bipyridine (4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts for the reduction of carbon dioxide. Consecutive electrochemical reduction of these complexes yields species identical to those obtained by chemical deprotonation. An energetically feasible mechanism for reductive deprotonation is proposed in which the bpy anion is doubly protonated followed by loss of H2 and 2H(+). Cyclic voltammetry reveals a two-electron, three-wave system owing to competing EEC and ECE pathways. The chemical step of the ECE pathway might be attributed to the reductive deprotonation but cannot be distinguished from chloride dissociation. The rate obtained by digital simulation is approximately 8 s(-1). Under CO2, these competing reactions generate a two-slope catalytic waveform with onset potential of -1.65 V vs Ag/AgCl. Reduction of CO2 to CO by the [Re(I)(4DHBP-2H(+))(CO)3](-) suggests the interaction of CO2 with the deprotonated species or a third reduction followed by catalysis. Conversely, the reduced form of [Re(6DHBP)(CO)3Cl] converts CO2 to CO with a single turnover.

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Gold(I) bis(acetylide) complexes [PPN][AuR(2)] (1-3) where PPN = bis(triphenylphosphine)iminium) and R = ethisterone (1); 1-ethynylcyclopentanol (2); 1-ethynylcyclohexanol (3) have been prepared. The reaction of 1 with [Cu(MeCN)(4)][PF(6)] in a 1:1 or 3:2 ratio provides the octanuclear complex [Au(4)Cu(4)(ethisterone)(8)] (4) or pentanuclear complex [PPN][Au(3)Cu(2)(ethisterone)(6)] (5). Complexes 2 and 3 react with [Cu(MeCN)(4)][PF(6)] to form only pentanuclear Au(I)/Cu(I) complexes [PPN][Au(3)Cu(2)(1-ethynylcyclopentanol)(6)] (6) and [PPN][Au(3)Cu(2)(1-ethynylcyclohexanol)(6)] (7). X-ray crystallographic studies of 1-3 reveal nontraditional hydrogen bonding between hydroxyl groups and the acetylide units of adjacent molecules. Complexes 6 and 7 each form polymorphs in which the structures (6 a,b and 7 a,b,c) differ by Au...Au, Au...Cu, and Cu-C distances. The polymorphs exhibit different emission energies with colors ranging from blue to yellow in the solid state. In solution, pentanuclear clusters 5-7 emit with lambda(max) = 570-580 nm and Phi = 0.05-0.19. Complex 4 emits at 496 nm in CH(2)Cl(2) with a quantum yield of 0.65. Complex 5 exists in equilibrium with 1 and 4 in the presence of methanol, ethanol, ethyl acetate, or water. This equilibrium has been probed by X-ray crystallography, NMR spectroscopy, and luminescence experiments. DFT calculations have been performed to analyze the orbitals involved in the electronic transitions of 4, 6, and 7.