CO2 Hydrogenation to Methanol Catalyzed by Mn-SNS Complex: Role of fac / mer Isomerism on Catalytic Reactivities

Abstract

We present a DFT investigation of the indirect hydrogenation of CO2 to methanol catalyzed by a Mn–SNS complex (Mn-fac) in the presence of morpholine as a co-catalyst. Formation of the active hydride species [MnH–NH] (1) proceeds with a kinetic preference for 1-mer, while interconversion enables access to 1-fac under the reaction conditions. Mechanistic analysis reveals complementary roles of the two hydride species, with 1-fac promoting CO2 hydrogenation to formic acid and 1-mer facilitating the subsequent reduction of N-formylmorpholine to methanol via a kinetically favorable pathway involving C–N bond hydrogenation followed by C═O bond hydrogenation. This pathway exhibits a lower energetic span for 1-mer (δE = 35.0 (32.3) kcal/mol) than for 1-fac (δE = 42.3 (38.6) kcal/mol). The overall reactivity is primarily governed by heterolytic H2 activation and is strongly influenced by the ligand trans to H2, where increasing π-acceptor character destabilizes the H2-coordinated intermediates and leads to higher activation barriers. These findings establish the cooperative roles of 1-fac/1-mer hydride species and underscore the importance of the trans ligand in governing the catalytic performance in Mn–SNS systems for CO2 hydrogenation.