MII-meso-tetrakis(2,3-dichlorophenyl)porphyrins as catalysts for CO2 cycloaddition reaction under mild conditions: kinetic and theoretical investigation

A series of new porphyrins with ZnII, CoII, and CuII (1-3) as metal centers are reported. These complexes were evaluated in the CO2 cycloaddition reaction with epoxides under mild conditions (30-80°C and 1 bar of CO2 pressure) and solvent-free conditions; in optimal conditions, they presented high conversions (>99 %) and selectively produced styrene cyclic carbonate. To comprehend the mechanism of the reaction, synchronously, a kinetic approach was carried out using the 1/TBAB system and only TBAB to evaluate the activation energy (Ea) for catalyzed and uncatalyzed reactions. The results support the synergism between the catalyst and the cocatalyst decreasing the Ea in 20.48 kJ mol−1. In addition, the thermodynamics parameters (ΔG‡, ΔH‡, ΔS‡) were also determined using Eyring´s equation. Moreover, a unique example of a dormant species for catalyst 3 bearing two DMAP axially coordinated to CoII was determined by X-ray diffractometry. Theoretical studies also supported the experimental data; the catalytic cycle has been revealed at the molecular level and pointed out the best epoxide activation for the ZnII complex.