International audienceThis work was directed at studying the capability of structurally defined, strongly Lewis-acidic metal centers to effect catalytic reductive fixation of the small molecule substrate CO2. Exposing solutions or solid samples of the ion pair [Cp*2Sc][HB(C6F5)3] 1CIP, in which the highly electrophilic decamethyl-scandocene cation and [HB(C6F5)]− as a potentially reactive source of hydride equivalents are associated, to CO2 selectively produces ion pair [Cp*2Sc][HCO2B(C6F5)3] 2CIP. The results of solution and solid state structural analysis of 2CIP imply ionic association of [Cp*2Sc]+ and [HCO2B(C6F5)3]− rather than B(C6F5)3-adduct formation to neutral Cp*2Sc-formate. In the presence of B(C6F5)3 co-catalyst and excess triethylsilane, the formation of 2CIP from 1CIP initiates the catalytic deoxygenative hydrosilation of CO2 to CH4. The roles of ion pairs 1 and 2, borane co-catalyst, and silane in the catalytic reaction were studied mechanistically by NMR spectroscopy. Intermediately formed 3,3,7,7-tetraethyl-3,7-disila-4,6-dioxanonane product was found to exert an accelerating effect on the overall reaction rate by promoting [HCO2B(C6F5)3]− dissociation to give 2SIP through formation of separated ion pairs [Cp*2Sc(κ2-(Et3SiO)2CH2)][HCO2{B(C6F5)3}n], n = 1, 2. DFT calculations show that the formation 2CIP from the reaction of 1CIP with CO2 is exoergic and without significant energy barriers. This work lays the basis for future studies of reactive ion pairs of this kind in the context of small molecule chemistry

We report theoretical investigations of the reaction of [(( Me ArO) 3 mes)U] with CO 2 in order to support previously reported experimental data. Experimentally, the reaction in toluene leads to the immediate formation of the bridging carbonate complex [{(( Me ArO) 3 mes)U IV } 2 (μ-η 2 :η 2 -CO 3 )] at room temperature. DFT calculations show that the preferred reaction pathway is a three-step mechanism: first, the formation of a dinuclear CO 2 complex, followed by concomitant release of CO, forming the corresponding bridging μ-oxo species. The final step involves insertion of a CO 2 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.