Theoretical Study On Several Transition-metal Complexes Catalyzed Reactions Of Carbon Dioxide With Epoxides

Catalytic transformation of carbon dioxide into chemical products has attracted intense attention for the carbon source in industry and environmental problems. One of the promising methodologies in chemical CO₂ fixation is the cycloaddition between carbon dioxide with epoxides to afford the five-membered cyclic carbonates. A remarkable variety of catalysts, especially non-oxidative and low valent oxidative transition-metal complexes, have been continuously explored for the CO₂/epoxides coupling reactions over the past decades. However, due to reaction intermediates are difficultly detected by experimental methods, there generally appears to be a lack of detailed mechanistic studies till now. Alternatively, the quantum chemistry means are effective to provide important information about property of catalyst at the molecular level. Obtaining desired products with high selectivity and yield by modifying and designing catalyst is still a challenging and promising project.The purpose of this work is to clarify the detailed mechanism of the CO₂/epoxides coupling reactions catalyzed by some transition-metal complexes, with the hopes of shedding light on some of the important differences in reaction pathways. Furthermore, our studies reveal the relationship between the structure and reactivity of catalysts, and hence offer theoretical guidance to the development of more powerful catalysts. By means of the B3LYP density functional method, the detailed mechanism of the experimentally observed formation of five-membered cyclic carbonates in the presence of NCCH2Cu, (Ph₃P)₂Ni, and Re(CO)₅Br complexes. The main contents and results are as follows.1. Mechanism of NCCH2Cu-catalyzed coupling between CO₂ with propylene oxideThe NCCH2Cu-catalyzed synthesis of cyclic carbonates is stepwise and considered to include two processes. In process 1, CO₂ insertion into the Cu(I)-C bond of copper(I) cyanomethyl affords activated carbon dioxide carriers. The formation of copper(I) cyanoacetate actually proceeds via six steps. The first step is the formation of complex 5 by the directly side-on attack of carbon dioxide to species 1; secondly, the side-on-coordination carbon dioxide complex 5 is transformed into the more stable end-on-coordination carbon dioxide complex 4; the third step is the production of species 2 by the release of CO₂ from 4; the fourth step is the attack of CO₂ to 2 affording five-membered cyclic intermediate 6; the following step corresponds to the torsion of copper of intermediate 6 leading to intermediate 7; the last step is a tautomerization step from 7 to 8. The increasing energy from the reactants (NCCH₂Cu + CO₂) to the activated carbon dioxide carriers (6, 7, and 8) implies that the formation of 6 (7 or 8) is an unspontaneous and endothermic process. In process 2, O-coordination of propylene oxide molecule to the electrophilic copper center of carriers 6 (7 or 8) occurs. From the calculated barrier heights and reaction energies, it is concluded that the best activated carbon dioxide carrier is species 8 and path 3 is more favored. Kinetically, the ring-opening of epoxide to copper atom forming the four-membered ring intermediates 14 (path 2) and 22 (path 3) is more favored than that to OCO₂ atom to form the eight-membered ring species 9 (path 1). Thermodynamically, the total energy of 8a is slightly lower than that of 7a. In addition, natural bond orbital (NBO) analysis results show that the copper atom serves as an orbital or charge reservoir in the overall reaction. The eight-membered ring intermediate oxidation transformation in process 2 effectively demonstrates the cooperativity of CH2CN moiety with the center copper. These results could explain satisfactorily the reported experimental observations.2. Mechanism of Ni(0)-catalyzed coupling between CO₂ with epoxyethaneThe favorable reaction pathway of the Ni(PH₃)₂-mediated coupling reaction between CO₂ and epoxyethane proceeds via the following elementary steps: (a) epoxide coordination and oxidative addition, (b) carbon dioxide insertion, and (c) reductive elimination of cyclic carbonate. The calculated activation energy barrier of the formation of six-membered nickelacycle through the approach of epoxyethane on (H₃P)₂Ni(CO₂) is in the vicinity of 32 kcal/mol higher than the approach of CO₂ on (H₃P)₂Ni[O(CH₂)₂]. Thus, the oxidative addition of epoxide to (H₃P)₂Ni leading to oxametallacyclobutane 2 takes place first during the process of obtaining cyclic carbonate. Species 2, which is kinetically and thermodynamically stable in benzene solution, is a crucial intermediate along the reaction path. There exists the thermodynamic equilibrium between bisphosphine intermediate 2 and monophosphine intermediate 14 under experimental conditions. An incoming CO₂ molecule is favored to approach 14, and then produce six-membered ring species 15. From 15, the reductive elimination of cyclic carbonate occurs via the three-center transition state structure. The last step rather than CO₂ insertion is the controlling step in the overall cyclic mechanism. Our results agree perfectly with the previous experimental findings and support the validity of the proposed mechanism. Additional experimental work aimed at trapping proposed crucial intermediates (2, 4a, and 15) is expected to be developed in future.3. Mechanism of Re(I)-catalyzed coupling between CO₂ with chloromethyloxiraneThe calculated bond dissociation energies indicate that the real active catalyst is the unsaturated complex Re(CO)4Br via the loss of equatorial CO rather than free radical species (Re(CO)5 or Br radicals). The preferred mechanism promoted by Re(CO)4Br for the production of cyclic carbonates can be divided into three main stages involving epoxide coordination and oxidative addition, carbon dioxide insertion, and reductive elimination of cyclic carbonate. Firstly, the chloromethyloxirane is activated by Re(I) center, leading to the reactive oxametallacyclobutane 2b. Due to the conformation of intermediate 2b has no vacant site in the axial position of Re(CO)4Br fragment, so the CO dissociation from 2b to 3 is an essential step, which facilitates CO₂ coordination and insertion leading to metallacarbonate 6. From species 6, the cyclic carbonate reductive elimination occurs via a three-center transition state structure. Since the ring-opening of epoxide from 1d to 2b and the CO₂ multistep insertion from 2b to 6 have close activation energies, each of them can be the rate-determining step with variation of the reaction conditions (temperature and pressure). The key intermediates in the whole catalytic cycle are predicted to be labile, so no reaction intermediate can be observed or captured by experimental method to date.