A covalent triazine framework-based heterogenized Al–Co bimetallic catalyst for the ring-expansion carbonylation of epoxide to β-lactone

b-Lactones are important intermediates for ring-opening polymerization to form poly(b-hydroxyalkanoates), a naturally occurring biodegradable and thermoplastic polyester. Many synthetic methods are available for preparing b-lactones, among which catalytic ring-opening carbonylation of epoxides has received considerable attention because of its ability to produce enantiopure b-lactones. Over the past several decades, it has been reported that Co2(CO)8 precursors catalyze the carbonylation of epoxides to lactones. Particularly, Coates et al. employed homogenous bimetallic Lewis acid–base salts [(salph) M(THF)2][Co(CO)4] (salph 1⁄4 N,N0-o-phenylene bis(3,5-di-tertbutylsalicylideneimine), M 1⁄4 Al, Cr), as catalysts for the conversion of epoxides to lactones, which are the best catalysts to date for epoxide carbonylation. However, difficulty in product separation and poor catalyst recycling may signicantly limit their practical utilization. To overcome these drawbacks, heterogenization of the active sites of previously reported homogenous catalysts on the solid support, guarantying high catalytic efficiency with facile separation, is highly desired. In recent years, covalent triazine frameworks (CTFs) have attracted much attention as heterogeneous catalytic supports, because they have a high surface area, stability, large pore volumes and structural tunability. In particular, bipyridine (bpy)-based CTF (bpy-CTF, represented as 1), which contains bipyridine ligand motifs in pore walls of the framework as shown in Scheme S1 (ESI†), has potential to form complexes with N–N coordination environment. Hence, we hypothesized that immobilization of an {Al(OTf)2}[Co(CO)4] (OTf 1⁄4 triuoromethane sulfonate) motif on the bpy-CTF system, as