Dinuclear and Trinuclear Nickel Complexes as Effective Catalysts for Alternating Copolymerization on Carbon Dioxide and Cyclohexene Oxide

A series of novel nickel complexes 1-9 supported by NNO-tridentate Schiff-base derivatives have been synthesized and characterized. Treatment of the pro-ligands [L-1-H = 2,4-di-tert-butyl-6-(((2-(dirnethylamino)ethyl)imino)methyl)phenol, L-2-H = 2-(((2-(dimethylamino)ethyl) imino) methyl)-4,6-bis(2-phenylpropan-2-yl)-phenol, L-3-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)phenol] with Ni(OAc)(2)center dot 4H(2)O in refluxing ethanol afforded mono- or bimetallic nickel complexes {[(L-1)Ni(OAc)] (1); (L-2)Ni(OAc)] (2); (L-3)(2)Ni-2(OAc)(2)-(H2O)} (3)}. Alcohol-solvated trimetallic nickel acetate complexes {[(L-3)(2)Ni-3(OAc)(4)(MeOH)(2)] (4); (L-3)2Ni(3)(OAc)(4)(EtOH)(2)] (5)} could be generated from the reaction of L-3-H and anhydrous nickel(II) acetate with a ratio of 2:3 in refluxing anhydrous MeOH or EtOH. The reaction of nickel acetate tetrahydrate and L-4-H to L-6-H [1,4-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-5-methoxyphenol, L-5-H = 2-(((2-(dimethylamino)ethyl)imino)methyl)-4-methoxy-phenol, L-6-H = 2-(((2-(dimethylamino)ethyl)imino)(phenyl)methyl)phenol] produced, respectively, the alcohol-free trinuclear nickel complexes {[(L-4)(2)Ni-3(OAc)(4)] (7); [(L-5)(2)Ni-3(OAc)(4)] (8); [(L-6)(2)Ni-3(OAc)(4)] (9)} with the same ratio in refluxing EtOH under the atmospheric environment. Interestingly, recrystallization of [(L-3)(2)Ni-3(OAc)(4)(MeOH)] (4) or [(L-3)2Ni(3)(OAc)(4)(EtOH)] (5) in the mixed solvent of CH2Cl2/hexane gives [(L-3)(2)Ni-3(OAc4] (6), which is isostructural with analogues 7-9. All bi- and trirnetallic nickel complexes exhibit efficient activity and good selectivity for copolymerization of CO2 with cyclohexene oxide, resulting in copolymers with a high alternating microstructure possessing, >= 99% carbonate-linkage content. This is the first example to apply well-defined trinuclear nickel complexes as efficient catalysts for the production of perfectly alternating poly(cyclohexene carbonate).