| Abstract
| - Ruthenium-catalyzed hydrogenation of carbon dioxide to formic acid was theoretically investigatedwith DFT and MP4(SDQ) methods, where a real catalyst, cis-Ru(H)2(PMe3)3, was employed in calculationsand compared with a model catalyst, cis-Ru(H)2(PH3)3. Significant differences between the real and modelsystems are observed in CO2 insertion into the Ru(II)−H bond, isomerization of a ruthenium(II) η1-formateintermediate, and metathesis of the η1-formate intermediate with a dihydrogen molecule. All these reactionsmore easily occur in the real system than in the model system. The differences are interpreted in termsthat PMe3 is more donating than PH3 and the trans-influence of PMe3 is stronger than that of PH3. Therate-determining step is the CO2 insertion into the Ru(II)−H bond. Its ΔG°⧧ value is 16.8 (6.8) kcal/mol,where the value without parentheses is calculated with the MP4(SDQ) method and that in parentheses iscalculated with the DFT method. Because this insertion is considerably endothermic, the coordination ofthe dihydrogen molecule with the ruthenium(II)-η1-formate intermediate must necessarily occur to suppressthe deinsertion. This means that the reaction rate increases with increase in the pressure of dihydrogenmolecule, which is consistent with the experimental results. Solvent effects were investigated with the DPCMmethod. The activation barrier and reaction energy of the CO2 insertion reaction moderately decrease inthe order gas phase > n-heptane > THF, while the activation barrier of the metathesis considerably increasesin the order gas phase < n-heptane < THF. Thus, a polar solvent should be used because the insertionreaction is the rate-determining step.
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