Conversion of CO2 into cyclic carbonates in the presence of metal complexes as catalysts

The sterically hindered salicylaldimine ligands N,N'-(1,5-diaminonaphthalene)-3,5-bu(2)(t)-salicylaldimine (L-1), N,N'-(2,7-diaminofluaren)-3,5-bu(2)(t)-salicylaldimine (L-2) and N,N'-(1,8-diaminonaphthaline)-3,5-bu(2)(t)-salicylaldimine (L-3) have been synthesised by the condensation of 1,5-diaminonaphthalene, 2,7-diaminofluarene, and 1,8-diaminonaphthaline with 3,5-di-tert-butylsalicylaldehyde, respectively. Dinuclear M(II) complexes of L-1 and L-2 and mononuclear M(II) complexes of L-3 have been prepared using Cu(II), Ni(II), Co(II), and Mn(II) salts and characterised. The synthesised sterically-hindered, salen-type complexes were tested as catalysts for the formation of cyclic carbonates from CO2 and liquid epoxides (propylene oxide, epichlorohydrine, 1,2-epoxybutane and styrene oxide), which served as both reactant and solvent. Ligand structure and the type of metal centre have a marked influence on the catalytic activity. Mn(II) complexes showed the highest catalytic activity.