Theoretical Studies On Reactions Catalyzed By Transition-metal Complexes

Homogeneous reactions catalyzed by transition-metal complexes(TMCs) have attracted great interest not only for their application in industrial and synthetic processes but also for being a model of hetergeneous reactions. Recent progress in computational chemistry has shown that many important chemical and physical properties of the species involved in these reactions can be predicted by computational techniques. This ability is especially important in those cases where experimental results are difficult to obtain. Numerous theoretical studies have been carried out to understand and verify the mechanisms of catalytic reactions catalyzed by TMCs, explain and illustrate experimental results, find and explore the relationship between the reactivity&selectivity and the structures of the TMCs, and even design the catalysts(also have many limitations). Here, this paper focuses on theoretical studies on three kinds of homogeneous catalytic reactions by means of quantum chemistry method, the conclusions are listed as follows:1. In the study on the hydroformylation, all structural geometries of reactant, intermediates, transition states and product are optimized at HF/LANL2DZ level under the effective core potential (ECP) approximation. The potential energy profile for some elementary reactions of hydroformylation catalyzed by Co2(CO)8 and Co2(CO)6(PH3)2 are calculated respectively. The transition states are further confirmed by having one and only one imaginary vibrational frequency. The results indicate that: (l)In two possible reaction paths of carbonyl insertion, the reaction path via ethyl migration is more energetically favourable than that via carbonyl migration. That Co-CO bond is difficult to break is in that the carbonyl group is a good receptor and is easy to form d feedback bond. The more the positive charge of the carbon atom of ethyl group in transition state (one that forms three-member ring transition state with the carbon atom of carbonyl group and transition-metal atom) is, the faster the rate of carbonyl insertion is. It is in agreement with the experimental results. (2) The activation energies of olefin insertion, carbonyl insertion, H2 oxidative addition and aldehyde reductive elimination fundamental steps catalyzed by HCo(CO)3, are 2.11, 16.22, 33.03 and 10.71 kcal/mol, respectively. The H2 oxidative addition step is predicted to be the rate-determining step in thecase due to a high endothermicity. (3) The activation energies for carbonyl insertion, H2 oxidative addition and reductive elimination catalyzed by HCo(CO)2(PH3) are 15.48x 32.03 and 10.39 kJ/mol, respectively. The results shows that H2 oxidative addition, play a more important role than carbonyl insertion and reductive elimination in the cycle of olefin hydroformylation. Because of the experimental fact that the hydroformylation rate catalyzed by HCo(CO)3 is faster than that catalyzed by HCo(CO)2(PH3) , in this paper, the n intermediates of two reaction were not available in the optimization, and the transition state of olefin insertion for the latter was not available either, this paper predicts that the rate-determining step for the former is H2 oxidative addition, but that for the latter is olefin insertion due to steric effect.2. The isomerization of trans- and cm- [Rh(CO)2I2] species are discussed at first. And the whole catalytic cycle of the carbonylation catalyzed by Rh complex are theoretically studied. All structural geometries of reactant, intermediates, transition states and product are optimized at HF/LANL2DZ level under the ECP approximation. The potential energy profile for elementary reactions of carbonylation are calculated respectively. The transition states are further confirmed by having one and only one imaginary vibrational frequency. The results indicate that: (1) The trans- [Rh(CO)2I2] species are more stable than cis- one. The isomerization is likely to carry out via a dissocitative mechanism more than via associative mechanism. (2) The carbonyl stretching frequencies of intermediates are calculated and have a good...