Ascendancy of Nitrogen Heterocycles in the Computationally Designed Mn(I)PNN Pincer Catalysts on the Hydrogenation of Carbon Dioxide to Methanol.

Abstract

The development of sustainable catalysts to get methanol from CO2 under milder conditions and without any additives is still considered an arduous task. In many instances, transition-metal-catalyzed carbon dioxide to formic acid formation is more facile than methanol formation. This article provides comprehensive density functional theoretic investigations of six new Mn(I)PNN complexes, which are designed to perform CO2 to methanol conversion under milder reaction conditions. All these six catalysts have similar structural features except at terminal nitrogen, -N (1), where adenine-inspired nitrogen heterocycles containing pyridine and pyrimidine moieties are attached to instill an electron withdrawing effect on the central metal and thus to facilitate dihydrogen polarization during the catalyst regeneration. All these computationally modeled Mn(I)PNN complexes demonstrate the promising catalytic activity to get methanol through cascade catalytic cycles at 298.15 K. The metal-ligand cooperative (MLC) as well as noncooperative (NC) pathways are investigated for each catalytic cycle. The NC pathway is the preferred pathway for formic acid and formaldehyde formation, whereas methanol formation proceeds through only the MLC pathway. Different nitrogen heterocycles attached to the -N (1) terminal manifested a considerable amount of impact on the Gibbs free energies, overall activation energies, and computed turnover frequencies (TOFs). Among all the catalysts, SPCAT02 provides excellent TOFs for HCO2H (500 151 h-1), HCHO (11 912 h-1), and CH3OH (2 372 400 h-1) formation at 50 °C. SPCAT04 is found to be a better catalyst for the selective formation of formic acid formation at room temperature than the rest of the catalysts. The computed TOF results are found reliable upon comparison with experimentally established catalysts. To establish the structure-activity relationship, the activation strain model and Fukui function calculations are performed on all the catalysts. Both these studies provide complementary results. The present study revealed a very important finding that a more electrophilic metal center could facilitate the CO2 hydrogenation reaction robustly. All computationally designed catalysts could be cheaper and better alternatives to convert CO2 to methanol under mild reaction conditions in an aqueous medium.