Improving productivity and stability for CO2 hydrogenation by using pincer-ligated Mn complexes with hemilabile ligands

Abstract

Pincer ligands are widely used to support transition-metal catalysts, but first-row systems often deactivate too quickly for widespread use. Here, pincer ligands bearing an additional hemilabile donor were designed to stabilize Mn catalysts for CO2 hydrogenation to formate. Ligands of the type (RCH2CH2)N(CH2CH2PiPr2)2 (iPrPNRP; R = OMe, CH2OMe, NMe2, PiPr2) were synthesized and coordinated to Mn to form complexes of the type (iPrPNRP)Mn(CO)2H. Their performance was compared with that of {MeN(CH2CH2PiPr2)2}Mn(CO)2H, which lacks a pendant donor. The pendant donor identity and arm length strongly influenced catalytic activity, but all pendant-arm-containing systems extended catalyst lifetime. Notably, (iPrPNCH2OMeP)Mn(CO)2H achieved turnover frequencies of 158,000 h−1 and turnover numbers of 838,000, exceeding most state-of-the-art catalysts. Mechanistic studies revealed that hemilabile coordination enhances stability by avoiding deactivation, although excessive binding can lead to inactive states. This work demonstrates that hemilabile donors dramatically improve pincer-based Mn catalysts and provides design principles for broader applications.