Exploring the conformational space of cobalt(III)-salen catalyst for CO2 /epoxide copolymerization: Effect of quaternary ammonium salts on preference of alternative isomers

Abstract

Model catalysts for CO₂ /epoxide copolymerization based on Co(III) complexes were studied, with focus on the preference of their alternative isomers, cisβ vs. trans. The systems range from model structures without the co-catalyst, as derived from crystallographic data, to complex models with two (CH₂ )₄ N⁺ R₃ co-catalyst chains (R = Me, Bu) grafted onto a Co(III)-salcy core. To explore the conformational space of the latter complexes, a computational protocol was developed, combining a systematic model-building approach with static and molecular dynamics calculations, and multilevel energy assessment (PM7 and DFT). Results demonstrate an influence of the co-catalyst on the relative stability of the isomers. The cisβ isomer is preferred for complexes without N⁺ -chains and the cisβ ↔ trans isomerization is feasible. Five-coordinate species and open-shell electronic configurations are energetically disfavored. The cisβ preference decreases with the introduction and enlargement of (CH₂ )₄ N⁺ R₃ : both isomers can be populated for R = Me, while the trans isomer is visibly preferred for R = Bu. © 2018 Wiley Periodicals, Inc.

Keywords: CO2/epoxide copolymerization; Co(III)-salen complexes; bifunctional Co(III) catalysts; cisβ/trans complexes; conformational space.