Trivalent Uranium Complex As a Catalyst to Promote the Functionalization of Carbon Dioxide and Carbon Disulfide: A Computational Mechanistic Study

We report our recent DFT mechanistic study on the functionalization of CO2 and CS2 promoted by a trivalent uranium complex (Tp*)(2)UCH2Ph. In the calculations, the uranium atom is described by a quasi-relativistic 5f-in-core ECP basis set (LPP) developed for the trivalent uranium cation, which was qualified by the calculations with a quasi-relativistic small core ECP basis set (SPP) for uranium. According to our calculations, the functionalization proceeds in a stepwise manner, and the CO2 or CS2 does not interact with the central uranium atom to form a stable complex prior to the reaction due to the steric hindrance from the bulky ligands but directly cleaves the U-C (benzyl) bond by forming a C-C covalent bond. The released coordination site of uranium is concomitantly occupied by one chalcogen atom of the incoming molecule and gives an intermediate with the uranium atom interacting with the functionalized CO2 or CS2 in an eta(1) fasion. This step is followed by a reorientation of the (dithio)carboxylate side chain of the newly formed PhCH2CE2- (E = O, S) ligand to give the corresponding product. Energetically, the first step is characterized as the rate-determining step with a barrier of 9.5 (CO2) or 25.0 (CS2) kcal/mol, and during the reaction, the chalcogen atoms are reduced, while the methylene of the benzyl group is oxidized. Comparison of the results from SPP and LPP calculations indicates that our calculations qualify the use of an LPP treatment of the uranium atom toward a reasonable description of the model systems in the present study.