Schiff-base molecules and COFs as metal-free catalysts or silver supports for carboxylation of alkynes with CO2

Abstract

Carboxylation of terminal alkynes with CO₂ to produce propiolic acids is an atom economical and high-value route for CO₂ fixation and utilization, but the conversion under mild conditions needs transition metal catalysts. In this article, we demonstrated for the first time the transition-metal-free organocatalysts for the reaction. The efficient catalysts are Schiff bases derived from 1,3,5-triformylphloroglucinol (Tp), either homogeneous (discrete molecules) or heterogeneous (covalent organic frameworks, COFs). The key catalytic sites are phenoxo and imine groups, which activate CO₂ through phenoxo–CO₂ complexation and also activate the C(sp)–H bond through bifurcate C–H⋯N_imine and C–H⋯O_phenoxo hydrogen bonds. The 2,2′-bipyridyl sites in the COF also contribute to the catalytic performance. The COF catalyst is less active than the molecular one but has the advantages of heterogeneous catalysis. Higher performance was also demonstrated by combining silver nanoparticles (AgNPs) with the intrinsically catalytic COF. This work opens up the potential of developing transition-metal-free catalysts for the CO₂ conversion reaction and demonstrates the new prospects of COFs as tailorable platforms for heterogeneous catalysis.