Computational Comparative Mechanistic Study of C−E (E=C,N,O,S) Coupling Reactions through CO2Activation Mediated by Uranium(III) Complexes

A DFT mechanistic study is undertaken on the functionalization of CO2 to form C-C, C-N, C-S, and C-O bonds promoted by trivalent uranium complexes (Tp*)(2)UR [Tp*=hydrotris(3,5-dimethylpyrazolyl)-borate ligand, R= -C equivalent to CPh (Cpda-CC), -C equivalent to CSiMe3 (Cpda-CSi), -NHPh (Cpda-N), -SPh (Cpda-S), and -OPh (Cpda-O)]. These model systems are similar in view of their two-step reaction mechanisms, that is, the insertion of CO2 into the U-E (E=C, N, O, S) bond to form a [U-kappa(1)-O2C] intermediate, followed by the reorientation of the carboxylate group to coordinate with the U atom in the kappa(2) manner (Cpdb-X, X=CC, CSi, N, S, O). However, the free energy barriers to the rate-determining steps are substantially different, increasing in the order Cpda-S