Bis-salicylaldiminato complexes of zinc. Examination of the catalyzed Epoxide/CO2 copolymerization

A series of salicylaldimine ligands of the general formula ((NRC7H5-x)-C-2( R-1)(x)OH) [x = 1 or 2; R-1 = Me, (BU)-B-t, Cl, OMe: R-2 = 2 6-(Pr2C6H3)-Pr -1, or 3,5-(CF3)(2)C6H3] have been synthesized and characterized via H-1 an d C-13 NMR, elemental analysis, and X-ray crystallography. The concomitant series of zinc bis(salicylaldiminato) complexes of the general formula ((NR C7H5-x)-C-2(R-1)(x)O)(2)Zn have been synthesized and characterized in the s olid state by X-ray crystallography. All complexes crystallized as four coo rdinate monomers with distorted tetrahedral geometry about the zinc center. The O-Zn-O angles range between 105 and 112.5 degrees, and the N-Zn-N bond angles were more obtuse spanning the range 122.9-128.9 degrees The only de viation from distorted tetrahedral geometry occurred when R-2 = 3,5-(CF3)(2 )C6H3 which crystallized as a distorted trigonal bipyramidal dimeric specie s with O-ax-Zn-O-ax bond angles of 165.00(15)degrees. The equatorial angles approach 120 degrees except for the N-eq-Zn-N-eq angle of 110.54(16)degree s which is attributed to the strain of the bridging ligands. The zinc bis(s alicylaldiminato) complexes showed varying activities as catalyst precursor s for the copolymerization of CO2 and cyclohexene oxide. Activation is prop osed to occur via CO, insertion in the phenolic Zn-O bond with simultaneous ring-opening resulting in a site for epoxide binding. The difference in ac tivity has been ascribed to the different steric/electronic effects provide d by the R-1 and R-2 substituents an the various steps of the copolymerizat ion mechanism, The activity of the zinc bis(salicyaldiminato) catalyst prec ursors (< 16 g<bullet>poly/m/g . Zn/hr) were similar to the activities of t he previously reported zinc phenoxide complexes for this reaction: however, unlike; the zinc phenoxide catalysts, the zinc bis(salicylaldiminato) comp lexes produced poly(cyclohexane carbonate) with greater than 99% carbonate linkages.