Manganese PNN pincer complex for hydrogenation of carbamate and urea derivatives: Reaction mechanism examined by DFT calculations

A comprehensive computational study is reported here on the mechanism of the reaction through which a Mn(I) pincer catalyst is able to assist the hydrogenation process of carbamate and urea derivatives, chosen as representative linker units of polyurethanes. Four substrate models, carbamate and urea derivatives, have been examined in order to evaluate their influence on the hydrogenation mechanism of polyurethanes to generate useful alcohols and amines organic building blocks. The outcomes of the computational exploration highlight a different behavior of the catalyst in assisting hydrogenation of carbamate or carbamide derivatives in the different steps of the whole reaction mechanism. Indeed, for the first hydrogenation step, the viability of a ligand-assisted mechanism has been found only for carbamide derivatives and involves a reversible structural change of the pincer ligand that can partially detach from the metal to favor the coordination of the molecule to be processed. The carbamate derivatives, instead, go through the formation of an intermediate in which the substrate interacts with the complex without binding. The mechanism of second hydrogenation is common to all the examined cases and requires the exploitation of the hemilabile nature of the PNN ligand. From the calculated energies put into play it clearly appears the great ability of the Mn(I) complex in catalyzing the hydrogenation of both carbamate and carbamide derivatives.