Pnictogen-Bonding Catalysis: Copolymerization of CO₂ and Epoxides on Antimony(V) Platforms

The copolymerization of CO₂ and epoxides to access polycarbonates represents a promising strategy for CO₂ utilization and for the production of useful polymers. Aiming to explore alternative transition-metal-free approaches that support this chemistry, we have investigated a series of triaryl-catecholatostiboranes as pnictogen-bonding platforms for the copolymerization of CO₂ and cyclohexene oxide (CHO). Our survey of these antimony species has identified motifs that promote this polymerization reaction efficiently, provided that bis-(triphenylphosphine)-iminium chloride is administered as an activator. By coupling these polymerization studies with a careful assessment of the structure, electronic attributes and Lewis acidity of the catecholatostiboranes, this work shows that high activity is generally observed with the weakest pnictogen-bond donors or Lewis acids investigated. Mechanistic studies, which indicate that the polymerization reaction is first order in stiborane, reveal a nonlinear dependence on the CO₂ pressure. This nonlinear dependence could be satisfactorily modeled based on a pre-equilibrium process involving the reversible insertion of the gaseous monomer into the growing chain. Altogether these findings greatly expand the reach of pnictogen bond catalysis while also providing an entry for the use of heavy group 15 elements as competent platforms for CO₂ utilization.