New zinc/tetradentate N4 ligand complexes: Efficient catalysts for solvent- free preparation of cyclic carbonates by CO2/epoxide coupling

Tetra-nitrogenated Zn-Schiff-base complexes have been synthetized and studied by X-ray diffraction. The prepared Zn-N-4-ligands complexes could not follow the 18-electron rule. This property has allowed the complexes to be efficient in the coordination and activation of the substrates. The zinc complex of N,N'-bis(2-p-tosylaminobenzylidene)-1,2-diaminecyclohexane, Zn(Cy-Ts) combined to tetra-butylammonium iodide (TBAI) in a ratio of 1:1, or to triethylamine, catalyzed efficiently the CO2/epoxides coupling. These catalytic reactions were carried out without solvent at 120 degrees C under CO2 pressure. Cyclic carbonates were selectively obtained with high yield up to 99% (TON up to 4900). Two plausible pathways of CO2/epoxide coupling mechanism were proposed without halide intervention, where both reactants are activated by the same Zn metal center.