Theoretical study on co2 reduction mediated by ru-pincer complexes: an implication towards efficient catalyst design

Ankur K. Guha

Carbon dioxide is not only found to be economical and renewable carbon source but also serves as important C1 building block for many chemicals. Besides contributing in the sustainable development of chemical industry these products like formic acid or methanol also behave as potential hydrogen storage agents. As such, hydrogenation of CO2 has grabbed immense interest of researchers and its efficient conversion has turned out to be a subject of numerous studies. Few noble first row transition metals viz. Fe and Co have been used in homogenous systems to catalyse the reaction, however recent reports also include metals like Rh, Ir, Ru. Nozaki and co workers introduced an Ir based PNP complex for CO2 hydrogenation which involved a complex mechanism in which ligand dearomatization was a major step. Later on, theoretical reports of Yang and Ahlquist confirmed that base assisted mechanism was energetically more favourable. The use of Ru-PNP and PNN complexes in recent times is worth mentioning. These complexes reversibly bind CO2 forming 1,3- CO2 adducts. Huff and Sanford used PNN pincer ligands and reported a TOF of 2200 h-1. Filonenko et. al showed that metal-ligand cooperation in Ru-PNP complexes lowers the efficiency of the catalytic reaction. Although there exist a few theoretical reports on the mechanism of CO2 hydrogenation to formates, but a detailed investigation with thorough discussions on various steps generating different intermediates is rare. This has prompted us to design new Ru-catalysts by modifying the PNP framework of the ligand (1a, 1b and 1c) and study its effect on the energetics of the reaction. In addition, DFT calculations are performed to illustrate the estimate effect of replacing the substituents at P-atom with electron donating (Me) and electron withdrawing groups (F).