Iron Catalysts\nSupported by a PNP Ligand with an Additional\nHemilabile Donor for CO<sub>2</sub> Hydrogenation

Iron\ncomplexes supported by pincer ligands of the type R’N(CH<sub>2</sub>CH<sub>2</sub>PR<sub>2</sub>)<sub>2</sub> (<sup>R</sup>PN<sup>R’</sup>P) are highly active catalysts for a range of hydrogenation\nand dehydrogenation reactions but are prone to catalyst deactivation.\nHere, the synthesis of (MeOCH<sub>2</sub>CH<sub>2</sub>)N(CH<sub>2</sub>CH<sub>2</sub>P<sup>i</sup>Pr<sub>2</sub>)<sub>2</sub> (<sup>iPr</sup>PN<sup>OMe</sup>P), a <sup>R</sup>PN<sup>R’</sup>P-type ligand\ncontaining a pendant hemilabile ether donor, is described and the\npreparation and characterization of a series of iron complexes ligated\nby <sup>iPr</sup>PN<sup>OMe</sup>P is reported. It is demonstrated\nthat iron hydride species supported by <sup>iPr</sup>PN<sup>OMe</sup>P give comparable turnover numbers for catalytic CO<sub>2</sub> hydrogenation\nto formate as the state-of-the-art iron system ligated by MeN(CH<sub>2</sub>CH<sub>2</sub>P<sup>i</sup>Pr<sub>2</sub>)<sub>2</sub> (<sup>iPr</sup>PN<sup>Me</sup>P). Further, <sup>iPr</sup>PN<sup>OMe</sup>P ligated iron catalysts show improved catalytic lifetimes compared\nto <sup>iPr</sup>PN<sup>Me</sup>P-supported systems, albeit with lower\nturnover frequencies. Experimental and theoretical studies indicate\nthat the pendant hemilabile ether donor aids in stabilizing the catalyst\nby binding to the iron center but impedes catalytic turnover by forming\nan off-cycle cationic complex, which is the catalyst resting state.\nOverall, this work provides an unusual example of a pincer ligand\nwith an additional hemilabile donor and demonstrates that the incorporation\nof an additional donor is a viable strategy for optimizing the catalytic\nperformance of systems supported by <sup>R</sup>PN<sup>R’</sup>P ligands.