RESUMEN
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-(dimethylamino)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)2·4H2O in refluxing ethanol afforded mono- or bimetallic nickel complexes {[(L(1))Ni(OAc)] (1); (L(2))Ni(OAc)] (2); (L(3))2Ni2(OAc)2(H2O)] (3)}. Alcohol-solvated trimetallic nickel acetate complexes {[(L(3))2Ni3(OAc)4(MeOH)2] (4); (L(3))2Ni3(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 [L(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))2Ni3(OAc)4] (7); [(L(5))2Ni3(OAc)4] (8); [(L(6))2Ni3(OAc)4] (9)} with the same ratio in refluxing EtOH under the atmospheric environment. Interestingly, recrystallization of [(L(3))2Ni3(OAc)4(MeOH)] (4) or [(L(3))2Ni3(OAc)4(EtOH)] (5) in the mixed solvent of CH2Cl2/hexane gives [(L(3))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).
RESUMEN
A series of bimetallic penta-coordinated copper complexes [Ln2Cu2(OAc)2] (1, 3-7), a mononuclear tetra-coordinated copper complex [LnCu(OAc)] (8 and 9), and a penta-coordinated copper complex [L2Cu(OAc)(H2O)] (10) were prepared by the reaction of Cu(OAc)2·H2O with a variety of NNO-tridentate Schiff-base ligands (L1-H-L9-H) in refluxing 95% ethanol, respectively. However, a dinuclear copper complex [(L2)2Cu2(OAc)2] (2) can be obtained from the treatment of L2-H with a stoichiometric amount of anhydrous Cu(OAc)2 in refluxing absolute EtOH under a dry nitrogen atmosphere. All of these copper complexes are active for the alternating copolymerization of cyclohexene oxide and cyclic anhydride, affording polyesters with moderate polydispersity. In particular, dinuclear copper complexes 1 and 2 performed satisfactorily to produce polyesters with controllable molecular weights and high ester linkages. This is the first example of well-defined copper acetate catalysts active for the copolymerization of cyclohexene oxide-phthalic anhydride or cyclohexene oxide-succinic anhydride which may be advantageous in terms of obviating the use of co-catalysts and low cost as well as an effective copolymerization for the formation of biodegradable polyesters in a controlled fashion.