Exploring the catalytic performance of ligand-functionalized Cu-BTC paddlewheels in carboxylative cyclization of propargyl alcohol with CO2: DFT and SISSO insights

The M06-L functional with the 6-31G(d,p) and SDD ECP basis sets, was used to investigate the structure and electronic properties of defective linker-coordinated paddlewheel complexes (Cu-BTC(L1–L4)) in the catalytic conversion of propargyl alcohol (PA) and CO₂ into cyclic carbonate. Two catalytic processes are proposed based on the different PA adsorption modes at the Cu center. The reaction proceeds via adsorption by the hydroxyl group in two sequential steps: PA/CO₂ activation and cyclization. This pathway is proposed as the dominant process in the Cu-BTC and Cu-BTC(L1–L3) systems. However, only Cu-BTC(L3) and Cu-BTC(L4), which exhibit stronger electron back-donation compared to the other systems, effectively promote the catalytic process via PA adsorption through its alkyne bond. In this latter mode, the reaction proceeds through three consecutive steps: PA/CO₂ activation, ring closure, and H-transfer. Compared to pristine Cu-BTC, Cu-BTC(L3) and Cu-BTC(L4) are proposed as more efficient catalysts for the carboxylative cycloaddition of CO₂ with PA. The rate-determining step for the reaction on these two systems is the PA/CO₂ activation via the latter mechanism. This step has an activation free energy of 16.7 kcal/mol and 15.0 kcal/mol for the Cu-BTC(L3) and Cu-BTC(L4). The SISSO model reveals the role of the Cu center in activating PA and stabilizing the generated intermediate, thereby lowering the activation free energy for PA/CO₂ activation.