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)ethyl)imino)ethyl)phenol, and L 3 -H = 2-(1-((2-(dimethylamino)ethyl)imino)ethyl)-5-methylphenol) with Cu(OAc)2·H2O (1 equiv) in refluxing ethanol afforded five-coordinate mono- or bimetallic copper complexes ([(L 1 )Cu(OAc)­(H2O)] (1); [(L 2 )Cu(OAc)­(H2O)] (2); [(L 3 )2Cu2­(OAc)2] (3)) in high yields. Dinuclear copper acetate analogue [(L 1 )2Cu2(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 )2Cu3(OAc)4] (5), was obtained on utilizing 2-(1-((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.