Green and selective cycloaddition of CO2 to oxiranes over Schiff base CuII complexes as catalysts

The present work reports the green catalytic activity of homogeneous Schiff base common and mixed ligand CuII complexes for the synthesis of cyclic carbonates by cycloaddition of CO2 to oxiranes under optimized reaction conditions to get the TON of 1.98 x 104. Both selectivity and conversion of cyclic carbonate were 99% achieved at 100 degrees C, 1 atmospheric pressure of CO2, and a TBAB co-catalyst, in 12 h without solvent. [Cu(L1)2(H2O)2]<middle dot>2H2O (1), [Cu(L2)2(H2O)2]<middle dot>2H2O (2), and [Cu(L1L3)]<middle dot>3H2O (3) were used as catalysts; [Cu(L1L3)]<middle dot>3H2O (3) showed promising results for CO2 fixation to oxiranes for the production of cyclic carbonates, supposedly due to mixed ligands and square planar geometry of the complex. All three complexes were synthesized in 5 h by the conventional method using methanol and characterized by NMR, FT-IR, ESI mass, UV-visible, ESR, , TGA, microelemental analysis, and magnetic susceptibility. The FT-IR and 1H NMR spectroscopic data suggest bidentate coordinating ligand via ON donors. The electronic transitions observed were 2B1g -> 2A1g (nu 1),2B1g -> 2B2g (nu 2), and 2B1g -> 2Eg (nu 3), attributed to the tetragonally elongated octahedral geometry (distorted octahedral) around CuII for 1 and 2, while 2B1g -> 2B2g (nu 1), 2B1g -> 2A1g (nu 2), and 2B1g -> 2Eg (nu 3), from the square planar geometry around CuII for 3, supported by g values from ESR graphs, thermogravimetric weight loss of lattice and coordinated water molecules.