Bis-Salicylaldiminato Complexes of Zinc. Examination of the Catalyzed Epoxide/CO2 Copolymerization

A series of salicylaldimine ligands of the general formula (NR 2 C 7 H 5 - x (R 1 ) x OH) [ x = 1 or 2; R 1 = Me, t Bu, Cl, OMe; R 2 = 2,6- i Pr 2 C 6 H 3, or 3,5-(CF 3 ) 2 C 6 H 3 ] have been synthesized and characterized via 1 H and 13 C NMR, elemental analysis, and X-ray crystallography. The concomitant series of zinc bis(salicylaldiminato) complexes of the general formula have been synthesized and characterized in the solid state by X-ray crystallography. All complexes crystallized as four coordinate monomers with distorted tetrahedral geometry about the zinc center. The O−Zn−O angles range between 105 and 112.5°, and the N−Zn−N bond angles were more obtuse spanning the range 122.9−128.9°. The only deviation from distorted tetrahedral geometry occurred when R 2 = 3,5-(CF 3 ) 2 C 6 H 3 which crystallized as a distorted trigonal bipyramidal dimeric species with O ax −Zn−O ax bond angles of 165.00(15)°. The equatorial angles approach 120° except for the N eq −Zn−N eq angle of 110.54(16)° which is attributed to the strain of the bridging ligands. The zinc bis(salicylaldiminato) complexes showed varying activities as catalyst precursors for the copolymerization of CO 2 and cyclohexene oxide. Activation is proposed to occur via CO 2 insertion in the phenolic Zn−O bond with simultaneous ring-opening resulting in a site for epoxide binding. The difference in activity has been ascribed to the different steric/electronic effects provided by the R 1 and R 2 substituents on the various steps of the copolymerization mechanism. The activity of the zinc bis(salicyaldiminato) catalyst precursors (<16 g·polym/g·Zn/hr) were similar to the activities of the previously reported zinc phenoxide complexes for this reaction; however, unlike the zinc phenoxide catalysts, the zinc bis(salicylaldiminato) complexes produced poly(cyclohexane carbonate) with greater than 99% carbonate linkages.