Abstract
The preference of open chain of growing macromolecule vs. possible cyclic form was examined for the bifunctional cobalt(III)-salen catalyst for the copolymerization of CO2 with epoxides. A variety of possible isomers was considered (resulting from trans/cis-β salen arrangement, different mutual orientation of quaternary ammonium-chains, and possible binding modes). To explore the conformational space, a combined approach was applied, utilizing semiempirical (PM7) MD and the DFT calculations. The preference of the open and cyclic macromolecules attached to the metal center was compared with the corresponding results for isolated model macromolecules, and the systems built of the macromolecule interacting with the tetra-butyl ammonium cation. Result shows that the cyclic structures are strongly preferred for isolated ions, with relatively low cyclization barriers. In the field of positive point charge, the open structures are strongly preferred. For the ions interacting with tetrabutyl ammonium cation, the cyclic structures are preferred, due to delocalization of the positive charge in the cation. For the complexes involving model and “real” Co(III)-salen catalysts, the open structures are strongly preferred. The possible cyclization by dissociation of alkoxide and its transfer to the neighborhood of quaternary ammonium cation is characterized by high activation barriers. Further, the transfer of alkoxide from the metal center to the cation is less likely than the transfer of carbonate, since the metal-alkoxide bond-energy energy is much stronger than energy of metal-carbonate bonding, as shown by ETS-NOCV results. The conclusions are in qualitative agreement with experimental data showing high selectivity towards copolymer formation in the copolymerization processes catalyzed by bifunctional Co(III) salen-complexes.Graphical abstract
Highlights
The main goal of the present account is to compare the energetic preference of open -chain of growing macromolecule vs possible cyclic form, resulting from the alkoxide back-biting in the process of CO2 copolymerization with oxiranes (ethylene oxide and propylene oxide) catalyzed by the bifunctional, cobalt(III) complexes with salen-type ligands tethered by two quaternary ammonium salts (Fig. 1a)
The results indicate that a preference of the trans or cis-β isomers is strongly influenced by the presence and the composition of quaternary ammonium salt attached to the salen-core; the trans/cis-β isomerization can proceed with relatively low barriers [25]
It includes two main propagation steps: (i) epoxide opening by nucleophilic attack of the carbonateheaded growing macromolecule attached to the metal center, to give alkoxide-ended system; (ii) CO2 attachment leading to carbonate species, extended by CH2-CHR-O-COO− group compared to initial carbonate system
Summary
The main goal of the present account is to compare the energetic preference of open -chain of growing macromolecule vs possible cyclic form, resulting from the alkoxide back-biting in the process of CO2 copolymerization with oxiranes (ethylene oxide and propylene oxide) catalyzed by the bifunctional, cobalt(III) complexes with salen-type ligands tethered by two quaternary ammonium salts (Fig. 1a). For the systems with the model Co(III)-salen catalyst without N+-chains, the prefixes 2t- and 2c- will be used, for the trans and cis-β isomers, respectively; examples for propylene oxide (methylene-ring-opening, -PO1) are shown in Fig. 5b (2t-P1-PO1 vs 2t-P1C-PO1), and 5c (2c-P1-PO1 vs 2cP1C-PO1).
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