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

2011-08-05T00:00:00Z (GMT) by Xinzheng Yang
The reaction mechanisms for hydrogenation of carbon dioxide catalyzed by PNP-ligated (PNP = 2,6-bis(di-<i>iso</i>-propylphosphinomethyl)pyridine) metal pincer complexes, (PNP)IrH<sub>3</sub> (<b>1-Ir</b>), <i>trans-</i>(PNP)Fe(H)<sub>2</sub>CO (<b>1-Fe</b>) and (PNP)CoH<sub>3</sub> (<b>1-Co</b>), were studied computationally by using the density functional theory (DFT). <b>1-Ir</b> is a recently reported high efficiency catalyst for the formation of formic acid from H<sub>2</sub> and CO<sub>2</sub>. <b>1-Fe</b> and <b>1-Co</b> are computationally designed low-cost base metal complexes for catalytic CO<sub>2</sub> reduction. For the formation of formic acid from H<sub>2</sub> and CO<sub>2</sub> catalyzed by <b>1-Ir</b>, <b>1-Fe</b>, and <b>1-Co</b>, the reaction pathways with direct H<sub>2</sub> cleavage by OH<sup>–</sup> without the participation of the PNP ligand are about 20 kcal mol<sup>–1</sup> more favorable than a previously postulated H<sub>2</sub> 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<sup>–</sup>, in the catalytic CO<sub>2</sub> 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 H<sub>2</sub> and CO<sub>2</sub> catalyzed by <b>1-Ir</b>, <b>1-Fe</b>, and <b>1-Co</b> are 18.6, 21.9, and 22.6 kcal mol<sup>–1</sup>, respectively. Such low barriers explain the observed unprecedented high catalytic acitivity of <b>1-Ir</b> and indicate that <b>1-Fe</b> and <b>1-Co</b> can be considered as promising low-cost catalyst candidates for fast hydrogenation of CO<sub>2</sub>.