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Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and Cobalt Complexes: A Computational Design of Base Metal Catalysts

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journal contribution
posted on 05.08.2011, 00:00 by Xinzheng Yang
The reaction mechanisms for hydrogenation of carbon dioxide catalyzed by PNP-ligated (PNP = 2,6-bis(di-iso-propylphosphinomethyl)pyridine) metal pincer complexes, (PNP)IrH3 (1-Ir), trans-(PNP)Fe(H)2CO (1-Fe) and (PNP)CoH3 (1-Co), were studied computationally by using the density functional theory (DFT). 1-Ir is a recently reported high efficiency catalyst for the formation of formic acid from H2 and CO2. 1-Fe and 1-Co are computationally designed low-cost base metal complexes for catalytic CO2 reduction. For the formation of formic acid from H2 and CO2 catalyzed by 1-Ir, 1-Fe, and 1-Co, the reaction pathways with direct H2 cleavage by OH without the participation of the PNP ligand are about 20 kcal mol–1 more favorable than a previously postulated H2 cleavage mechanism that involves the aromatization and dearomatization of the pyridine ring in the PNP ligand. This finding reveals the essential role of the base, OH, in the catalytic CO2 reduction cycle and suggests that the incorporation of strong bases and unsaturated ligands may be critical for new catalyst design in the area of hydrogen activation and low energy proton transfers. The calculated overall enthalpy barriers for the formation of formic acid from H2 and CO2 catalyzed by 1-Ir, 1-Fe, and 1-Co are 18.6, 21.9, and 22.6 kcal mol–1, respectively. Such low barriers explain the observed unprecedented high catalytic acitivity of 1-Ir and indicate that 1-Fe and 1-Co can be considered as promising low-cost catalyst candidates for fast hydrogenation of CO2.