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 [L1-H = 2,4-di-tert-butyl-6-(((2-(dimethylamino)­ethyl)­imino)­methyl)­phenol, L2-H = 2-(((2-(dimethylamino)­ethyl)­imino)­methyl)-4,6-bis­(2-phenylpropan-2-yl)­phenol,
L3-H = 2-(((2-(dimethylamino)­ethyl)­imino)­methyl)­phenol]
with Ni­(OAc)2·4H2O in refluxing ethanol
afforded mono- or bimetallic nickel complexes {[(L1)­Ni­(OAc)]
(1); (L2)­Ni­(OAc)] (2); (L3)2Ni2(OAc)2­(H2O)] (3)}. Alcohol-solvated trimetallic nickel acetate
complexes {[(L3)2Ni3(OAc)4­(MeOH)2] (4); (L3)2Ni3(OAc)4­(EtOH)2] (5)} could be generated from the reaction of L3-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 L4-H to L6-H [L4-H = 2-(((2-(dimethylamino)­ethyl)­imino)­methyl)-5-methoxyphenol,
L5-H = 2-(((2-(dimethylamino)­ethyl)­imino)­methyl)-4-methoxy-phenol,
L6-H = 2-(((2-(dimethylamino)­ethyl)­imino)­(phenyl)­methyl)­phenol]
produced, respectively, the alcohol-free trinuclear nickel complexes
{[(L4)2Ni3(OAc)4] (7); [(L5)2Ni3(OAc)4] (8); [(L6)2Ni3(OAc)4] (9)} with the same ratio in refluxing EtOH
under the atmospheric environment. Interestingly, recrystallization
of [(L3)2Ni3(OAc)4­(MeOH)]
(4) or [(L3)2Ni3(OAc)4­(EtOH)] (5) in the mixed solvent of CH2Cl2/hexane gives [(L3)2Ni3(OAc)4] (6), which is isostructural
with analogues 7–9. All bi- and trimetallic
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).