Structurally Diverse Copper Complexes Bearing NNO-Tridentate Schiff-Base Derivatives as Efficient Catalysts for Copolymerization of Carbon Dioxide and Cyclohexene Oxide

Structurally diverse copper acetate complexes based on NNO-tridentate Schiff-base ligands were synthesized and characterized as mono-, di-, and trinuclear complexes with respect to varied ancillary ligands. Treatment of the ligand precursors (L-1-H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-4-methylphenol, L-2-H = 4-chloro-2-(1-((2-(dimethylamino)ethypimino)ethyl)phenol, and L-3-H = 2-(1((2-(dimethylamino)ethyl)imino)ethyl)-5-methylphenol) with Cu(OAc)(2)center dot H2O (1 equiv) in refluxing ethanol afforded fivecoordinate mono- or bimetallic copper complexes ([(L-1)Cu-(OAc)(H2O)] (1); [(L-2)Cu(OAc)(H2O)] (2); [(L-3)(2)Cu2-(OAc)(2)] (3)) in high yields. Dinuclear copper acetate analogue [(L-1)(2)Cu-2(OAc)(2)] (4) resulted from treatment of L-1-H as the ligand precursor in refluxing anhydrous MeOH with equimolar proportions of metal acetate salt under a dry nitrogen atmosphere. However, a trinuclear complex, [(L-4)(2)Cu-3(OAc)(4)] (5), was obtained on utilizing 2-(14(2-(dimethylamino)ethyl)imino)ethyl)-5-methoxyphenol (L-4-H) as the proligand under the same synthetic route of 1-3; this complex was also synthesized in the reaction of L-4-H and copper(II) acetate monohydrate in the ratio of 2:3, giving a quantitative yield. All complexes are active catalysts for copolymerization of cyclohexene oxide (CHO) and CO2 without cocatalysts. In particular, dinuclear Cu complex 3 performed satisfactorily to produce polycarbonates with controllable molecular weights and high carbonate linkages. These copper complexes are the first examples that are effective for both CO2/CHO copolymerization and formation of polymers in a controlled fashion.