Five-coordinate Schiff base complexes of gallium. Potential catalysts for the copolymerization of carbon dioxide and epoxides

We report the synthesis of the five-coordinate Schiff-base complexes, (cyc)salenGaCl, (cyc)salenGa(N-3), (et)salenGaCl, and (et)salenGa(N-3) ((cyc)salenH(2) = N,N'-bis-(3,5-di-tert-butylsalicylidene)-1,2-cyclohexyldiimine and (et)salenH(2) = N,N'-bis(3,5-di-tert-butylsaticylidene)-1,2-ethylenediimine). The four complexes exhibit distorted square pyramidal geometry with the salen ligand forming the basal plane and the chloride or azide in the apical position. Copolymerization studies were conducted with cyclohexene oxide and carbon dioxide utilizing these derivatives as potential catalysts. In the presence of a Lewis-base co-catalyst, N-methylimidazole or phosphine, formation of polymer is not observed. In the absence of N-methylimidiazole, low molecular weight homopolymer is observed when the chloride species is the catalyst. This is in contrast to studies conducted with (cyc)salenCr(III)Cl, (et)salenCr(III)Cl, or azide derivatives thereof, which produced high-molecular-weight polycyclohexylcarbonate with CO2 incorporation >99% in the presence of N-methylimidazole or phosphines. (C) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.