Bis-salicylaldiminato complexes of zinc. Examination of the catalyzed Epoxide/CO2 copolymerization
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A series of salicylaldimine ligands of the general formula (NR2C7H5-x(R1)xOH) [x = 1 or 2; R1 = Me, tBu, Cl, OMe; R2 = 2,6-iPr2C6H3, or 3,5-(CF3)2C6H3] have been synthesized and characterized via 1H and 13C NMR, elemental analysis, and X-ray crystallography. The concomitant series of zinc bis(salicylaldiminato) complexes of the general formula (NR2C7H5-x(R1)xO) 2Zn 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 R2 = 3,5-(CF3)2C6H3 which crystallized as a distorted trigonal bipyramidal dimeric species with Oax-Zn-Oax bond angles of 165.00(15). The equatorial angles approach 120 except for the Neq-Zn-Neq 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 CO2 and cyclohexene oxide. Activation is proposed to occur via CO2 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 R1 and R2 substituents on the various steps of the copolymerization mechanism. The activity of the zinc bis(salicyaldiminato) catalyst precursors (<16 gpolym/gZn/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.