New boronate esters were prepared and used as catalysts for the synthesis of cyclic carbonates from CO ₂ and epoxides without solvent.

Three neutral salen ligands (L_1-3) with three cationic charged salen ligands (L_1a-3a) and the corresponding neutral boron [(L_1-3)BPh₂] and cationic charged boron [(L_1a-3a)BPh₂] compounds have been successfully synthesized and characterized under ambient conditions. All the newly synthesized salen ligands and their boron–phenyl complexes were characterized by NMR (¹H, ¹³C, ³¹P, and ¹¹B) spectroscopy, FT-IR spectroscopy, UV-Vis spectroscopy, TGA-DTA, LC-MS spectroscopy, and melting point and elemental analysis. Following full characterization, the study of spectroscopic properties of these novel salen ligands and their boron–phenyl complexes was done in detail. Then, the neutral boron [(L_1-3)BPh₂] and the corresponding cationic charged boron [(L_1a-3a)BPh₂] complexes were evaluated as catalysts and show good catalytic activity for the coupling of CO₂ and epoxides to obtain cyclic carbonates under suitable conditions (1.6 MPa CO₂, 100 °C, 2 h) without organic solvent. It was clearly observed that the triphenylphosphonium group bound on the salen ligands of cationic charged boron catalysts had a significant effect on the catalytic activity. Among the catalysts investigated, the cationic charged boron complex [L_2aBPh₂] and DMAP showed the best catalytic activity (96.4%) and selectivity (99.9%) for the coupling of CO₂ and ECH to obtain ECHC.