Role of Ligand-Bound CO2 in the Hydrogenation of CO2 to Formate with a (PNP)Mn Catalyst

Herein, we describe the catalytic hydrogenation of CO2 to formate with (PNP)­Mn–H (PNP = 2,6-bis­(di-tert-butylphosphinomethyl)­pyridine; Mn = Mn­(CO)2). Contrary to the established mechanism for CO2 hydrogenation, mechanistic studies indicate that CO2 does not insert into the Mn–H bond of (PNP)­Mn–H to give the formate species, (PNP)­Mn–OCHO. The lack of reactivity is confirmed by thermochemical studies that show that (PNP)­Mn–H is not sufficiently hydridic to reduce CO2. Deprotonation of the hydride to give [(*PNP)­Mn–H]– (* indicates the deprotonated ligand) enhances the hydricity by ∼17 kcal·mol–1 and hence should be sufficiently hydridic to hydrogenate CO2. This reactivity is not observed, and CO2 instead binds to the backbone to generate another anionic hydride species [(CO2-PNP)­Mn–H]. The formate is lost only from this species, through hydride transfer to an external CO2. These findings are unexpected because substrate binding to the backbone of catalysts that can undergo metal–ligand cooperativity (MLC) is thought to be detrimental to catalysis; this work suggests that alternative mechanisms should be considered. The enhanced hydricity observed upon deprotonation may be broadly applicable to systems capable of undergoing MLC. Moreover, this work shows an example of how thermochemical analysis can be used to advance mechanistic understanding in (de)­hydrogenation catalysis.