The role of halide ions in the carboxylative cyclization of propargyl alcohol with CO2 on CuX-deposited UiO-67(bpy) MOFs (X = F, Cl, Br, and I): a DFT study

Abstract

Theoretical investigation of the carboxylative cyclization of propargyl alcohol (PA) with CO₂ on CuX-deposited UiO-67(bpy) MOFs (X = F, Cl, Br, and I) in collaboration with a DBU base was carried out using the M06-L//ONIOM (M06-L:UFF) method. Two modes of PA adsorption on the deposited CuX were proposed: C-bound (via the alkyne bond) and O-bound (via the OH group). The former mode is proposed as the initial step of the catalytic process. Except for the deposited CuF system, the former mode is more stable than the latter mode by about 4.5 kcal mol⁻¹. A strong hydrogen-bonding complex between the OH of the PA molecule and CuF makes the latter mode as strong as the former mode. The catalytic process is proposed to take place via three consecutive steps: OH/CO₂ activation, cyclization, and deprotonation. The electronic effect from halide ions plays a role in the acidity of the Cu cation and the withdrawing of electrons from the UiO-67(bpy) support. The greater back-bonding from the filled Cu d orbital, in the case of the fluoride ligand, enhances the basicity of the OH group of the PA molecule. This results in the increase of the activation energy during the OH/CO₂ activation as compared to that of the other systems. For the second step, the height of the activation energy for the cyclization process is proportional to the acidity of Cu in the systems. The activation energies of the last step in all systems are almost identical. The second step of the reaction is the rate-determining step (RDS) of the reaction. The activation energy of the RDS is predicted to be 13.6 kcal mol⁻¹ (CuF), 11.7 kcal mol⁻¹ (CuCl), 11.4 kcal mol⁻¹ (CuBr), and 10.9 kcal mol⁻¹ (CuI), respectively.