Carbonate Formation from CO2 via Oxo versus Oxalate Pathway: Theoretical Investigations into the Mechanism of Uranium-Mediated Carbonate Formation

We report theoretical investigations of the reaction of [(((ArO)-Ar-Me)(3)mes)U] with CO2 in order to support previously reported experimental data. Experimentally, the reaction in toluene leads to the immediate formation of the bridging carbonate complex [{(((ArO)-Ar-Me)(3)mes)U-IV}(2)(mu-eta(2):eta(2)-CO3)] at room temperature. DFT calculations show that the preferred reaction pathway is a three-step mechanism: first, the formation of a dinuclear CO2 complex, followed by concomitant release of CO, forming the corresponding bridging mu-oxo species. The final step involves insertion of a CO2 molecule into a U-O bond, forming the carbonate product. Calculations reveal this three-step process to be thermodynamically favorable and kinetically accessible. An alternate pathway that proceeds through an oxalate dinuclear complex is also explored. Although the oxalate complex is calculated to be the thermodynamic product of the reaction, a high activation barrier prevents its formation.