Unraveling the Catalytic Role of Metal- and Halide-Free Phosphonium Systems in CO2 Transformation

Abstract

Abstract The cycloaddition of CO2 to epoxides to form cyclic carbonates represents an attractive route toward sustainable chemical transformations. Herein, the catalytic performance of a halide- and metal-free catalyst, tetramethylphosphonium acetate (TMPA), is investigated using density functional theory (DFT). Benchmark studies employing 13 DFT functionals, including dispersion-corrected and nondispersion-corrected methods, identified B3LYP-D3 as providing the closest agreement with experimental reactivity. Using this framework, we systematically examined the effects of dispersion, solvation, and counterion substitution on the rate-determining activation step. Implicit consideration of excess propylene oxide (PO) as the reaction medium dramatically lowers the activation barrier by ∼16 kcal/mol, underscoring the critical role of solvent-assisted activation. In contrast, replacing the acetate counterion with iodide transforms the reaction from a kinetically accessible and balanced catalytic cycle into a kinetically hindered pathway, despite enhanced stabilization of the final cyclic carbonate product. These findings establish four key mechanistic principles for the rational design of efficient metal- and halide-free catalysts for CO2 conversion: (i) excess reactants can play a critical solvent-like role and must be explicitly considered, (ii) the identity of the counterion in ionic catalysts strongly governs catalytic performance, (iii) directional noncovalent interactions are essential for efficient transition-state organization, and (iv) optimal catalysts achieve balanced intermediate stabilization together with strong transition-state stabilization to maintain a kinetically accessible catalytic cycle.