Enhancement of Two Types of CO₂ Conversion by Regulating Functional Thiophene Groups within Zn-MOF

Sustainable conversion of epoxides and propargylic amines using CO₂ could produce valuable chemical products. Efficient conversion generally requires harsh conditions such as noble-metal catalysts, cocatalysts, and toxic solvents, thereby underscoring the crucial need for environmentally friendly non-noble-metal metal-organic framework (MOF) catalysts. In this study, we designed a novel zinc-based metal-organic framework (MOF) with a 5-fold interpenetrating diamond framework, specifically {[Zn(DMTDC)(bpy)]·H₂O} (Zn-MOF-2), where H₂DMTDC represents 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid and bpy denotes 4,4'-bipyridine. Zn-MOF-2 serves as a bifunctional heterogeneous catalyst to promote the cyclization reaction of epoxides and propargylamine with CO₂ under mild conditions. The isolated yields of the resulting cyclic carbonates and oxazolidinones were 92% and 93%, respectively. Notably, the catalyst maintained good catalytic activity after five catalytic cycles in both types of CO₂ conversion. Control experiments confirmed the effective catalytic activity of the Zn²⁺ Lewis acid sites in Zn-MOF-2. The isostructural CPO-5 assembled from 4,4'-biphenyldicarboxylic acid has also been demonstrated to catalyze the cycloaddition of epoxides and CO₂ but with an inferior performance in catalyzing the carboxyl cyclization of propargylic amines. This result shows that the thiophene ring in the ligand plays a pronounced role in the catalytic performance. The research will enhance the development of effective MOF catalysts for the conversion of CO₂.