| The rational utilization of carbon dioxide is of great significanceto relieve the energy crisis problem and the greenhouse effect. Inrecent years, how to active carbon dioxide to expand its applicationrange has attracted more and more attention. The application ofmetal complex is high-profile in many aspects such as organicsynthesis, the activation of bond and catalysis. Therefore, it is veryimportant and meaningful to research carbon dioxide activation bymetal complex. In this article, the reaction mechanisms of carbondioxide by (PNN)Ru(H)(CO) and (NNS)Ru(H)(CO) complexes werecomprehensively investigated by means of density functionaltheory method. B3LYP method with combined basis set,6-311G (d, p)for all nonmetal atoms and Def2-SVP for Ru atom, was employed inall the calculations. The results are as follows:1) The mechanism of CO2activation by (PNN)Ru(H)(CO)complex.Geometries of all the stationary points were fully optimized without any constrains and were further proved by vibrationalanalysis (Gaussian03). For inclusion of salvation effect, single pointenergy calculations were performed with Gaussian09programpackage, using the integral equation formalism polarizablecontinuum model (IEF-PCM) with radius and the static relationshiptaken from SMD model. In addition, the empirical dispersioncorrection recommended by Grimme was added to the B3LYPenergies. According to the calculation results, we put forward areasonable reaction mechanism: Firstly, the tautomerizationbetween1and2accomplishes through two hydrogen migrationsteps. Because of their high barriers (49.95and29.03kcal/mol,respectively), at room temperature,1is hard to be transformed into2. The rate-determining step is TS1/3. Secondly, the barriers of thedirect [1,3] addition reaction of carbon dioxide to complex1and2to form product4and5are low (9.83and6.66kcal/mol,respectively), while the decomposition barriers of4and5are7.98and19.38kcal/mol, respectively. Moreover, product5is locatedlower than4. To sum up, at room temperature,4is the main product.But in case of the successfully tautomerization between1and2,compound5which is more stable in thermodynamics, will be themain product. Thirdly, since the relative energies of the reverse [1,3] addition between1and2to form6and7are located higher than TS1/4and TS2/5, these reaction channels are thermodynamically andkinetically unfavorable. These results are also explained well by thefrontier molecular orbital analysis of the carbon dioxide additionreaction. In conclusion, the theoretical calculation results areconsistent with the experimental results.2) The mechanism of CO2activation by (NNS)Ru(H)(CO)complex.Used the calculation method and results of the mechanism ofCO2activation by (PNN)Ru(H)(CO) for reference, we have made asimple discussion about the mechanism of CO2activation by(NNS)Ru(H)(CO): compound NNS-1and NNS-2can react with carbondioxide to generate NNS-4and NNS-5to overcome the barrier of22.15kcal/mol and9.62kcal/mol, respectively. The maximumbarrier of compound NNS-1into NNS-2is40.52kcal/mol andcompound NNS-2into NNS-1has to overcome the highest barrier of41.38kcal/mol. Therefore, when compound NNS-1reacts withcarbon dioxide, compound NNS-1will be more likely to reactdirectly with carbon dioxide to produce NNS-4. This channel iskinetically favorable. On the other hand, the relative energy ofcompound NNS-5is lower than NNS-4by8.47kcal/mol. Socompound NNS-5is the thermodynamic stability product. With theraise of the temperature, the reaction path way to generate compound NNS-5,4â†'1â†'3â†'2â†'5, may occur. When compound NNS-2reacts with carbon dioxide, no matter in the aspect of dynamics orthermodynamics, compound NNS-5will be the main product of thereaction. Because the energy barrier of the generation of NNS-5from compound NNS-2is smaller than that of NNS-4and thecompound NNS-5is more stable than NNS-4. |
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