Iron Catalysts Supported by a PNP Ligand with an Additional Hemilabile Donor for CO₂ Hydrogenation

Iron complexes supported by pincer ligands of the type R’N(CH2CH2PR2)2 (RPNR’P) are highly active catalysts for a range of hydrogenation and dehydrogenation reactions but are prone to catalyst deactivation. Here, the synthesis of (MeOCH2CH2)N(CH2CH2PiPr2)2 (iPrPNOMeP), a RPNR’P-type ligand containing a pendant hemilabile ether donor, is described and the preparation and characterization of a series of iron complexes ligated by iPrPNOMeP is reported. It is demonstrated that iron hydride species supported by iPrPNOMeP give comparable turnover numbers for catalytic CO2 hydrogenation to formate as the state-of-the-art iron system ligated by MeN(CH2CH2PiPr2)2 (iPrPNMeP). Further, iPrPNOMeP ligated iron catalysts show improved catalytic lifetimes compared to iPrPNMeP-supported systems, albeit with lower turnover frequencies. Experimental and theoretical studies indicate that the pendant hemilabile ether donor aids in stabilizing the catalyst by binding to the iron center but impedes catalytic turnover by forming an off-cycle cationic complex, which is the catalyst resting state. Overall, this work provides an unusual example of a pincer ligand with an additional hemilabile donor and demonstrates that the incorporation of an additional donor is a viable strategy for optimizing the catalytic performance of systems supported by RPNR’P ligands.