Theoretical investigation of the reactivity of bispentamethylcyclopentadienyl uranium(IV) bisthiolate complexes with the heteroallene molecules CS2 and CO2

The insertion reactions of the bispentamethylcyclopentadienyl bisthiolate uranium(IV) complexes [U(Cp*)2(SR)2] (Cp* = η–C5Me5; R = Me (1), tBu (2), iPr (3), Ph (4)) with CO2 or CS2 were investigated using Density Functional Theory (DFT), solvent effects being taken into account using the SMD continuum solvation model. The optimized geometries of the bisthiolate compounds computed at the DFT/B3PW91 level are in good agreement with available X–ray experimental data. The energy profiles of their reactions with CO2 and CS2 were determined. The formation of the products can be explained by a unique reaction mechanism involving an uranium(IV)–bridged heteroallene transition state. The CO2 insertion reactions exhibit lower activation barriers than those of CS2 insertion in accordance with the experiments showing that the CO2 insertion reactions are faster. As expected, compound 2 (R = tBu) was found to be the most difficult to undergo the insertion reaction because of steric hindrance. The geometrical parameters of the CS2 insertion derivative [U(Cp*)2(StBu)(S2CStBu)] (5) and the mixed insertion complex [U(Cp*)2(O2CStBu)(S2CStBu)] (6) obtained after treatment of 5 with CO2 are consistent with those determined by X-ray diffraction. The performed orbital analysis reveal the respective role of the actinide 7s, 6d and 5f orbitals, whereas the Wiberg Bond Indices (WBI) afford a good explanation of the structural variations during the insertion reactions. Finally, the Natural Population Analyses account for the different charge transfers occurring during the insertion processes.