Theoretical Studies On CO₂/Eepoxide Co-polymerization Catalyzed By Binuclear Metal Complexes

Carbon dioxide is a kind of raw materials of chemical industry which is rich, nonflammable, low-cost and non-toxic. The methods to converting carbon dioxide into chemicals effectively have received extensive attention. It's notable that copolymerizing epoxides and CO2 to biodegradable polycarbonate or cyclic carbonate is one of the most reasonable ways to utilize carbon dioxide. A series of new types of binuclear complexes have been synthesized and reported by prof. Williams group, which have shown good activity and selectivity for the polymerization of carbon dioxide and epoxides. Given that different metal complexes performing discrepant properties in the catalytic processes, it is worthy of investigating the structure of active species and the reasons for yielding different products. Density functional theory (DFT) studies have been carried out on the mechanism of copolymerization reaction in this thesis. The key transition states and intermediates of the reaction were computationally analyzed by bond length, natural charge and bond order. The results revealed intermetallic cooperation and the origin of the activity discrepancy as well as product selectivity during the reaction. The detailed results in this paper are described as follows:1. In the catalytic system of binuclear iron complex (LFe2Cl4, H2L= tetraaminodiphenol macrocycle anion), the rate-determining step of copolymerization is the ring-opening process of epoxide; as the lowest energy pathway showing, the ring-opening process alternatively takes place between the two metal centers, while inserting of carbon dioxide occurs on a single iron center, which adopted a different cooperative manner from that both ring-opening and carbon dioxide insertion alternatively occur between two metal centers; after adding the co-catalyst bis(triphenylphosphine)iminium chloride (PPN[C1]), copolymerization terminates, the cis/trans selectivity for generating cyclic carbonate correlates with the charge distribution on the carbon and oxygen atoms and overlap manner of p orbit of C-O bond;2. In the catalytic system of heterodinuclear Zn-Mg complex (LZn/Mg(OAc)2, H2L tetraaminodiphenol macrocycle anion), the epoxide coordinates to zinc center at the initial stage of the reaction to form a stable complex, then carbonate connecting to the metal magnesium initiates nucleophilic attack, reaction site transforms in one catalytic cycle between the two metal centers; as bond order displaying, an epoxide coordinates with zinc center is easier, and the weaker Mg-O and Mg-Br interaction accelerates nucleophilic attack on coordinated epoxide, which makes the copolymerization activity of heterodinuclear complex higher than corresponding homodinuclear complexes.