Add time:08/16/2019         Source:sciencedirect.com

Discrete Zn(II) and Cu(II) complexes have been synthesized by using N-hydroxy-N,N′-diarylformamidine ligands: N-hydroxy-N,N′-bis(2,6-diisopropylphenyl)formamidine (L1), N-hydroxy-N,N′-bis(2,6-dimethyl)formamidine (L2), N-hydroxy-N-(2-methoxyphenyl)-N′-(2,6-diisopropylphenyl)formamidine (L3), and N-hydroxy-N-(2-methoxyphenyl)-N′-(2,6-dimethylphenyl)formamidine (L4). Reaction of ligands L1–L4 with either ZnOAc2·2H2O or CuOAc2·2H2O in aqueous ethanol gave mononuclear Zn(II) or Cu(II) complexes [Zn-(L1)2] (1), [Zn-(L2)2] (2), [Zn-(L3)2] (3) and [Zn-(L4)2] (4), or [Cu-(L1)2] (5), [Cu-(L2)2] (6), [Cu-(L3)2] (7) and [Cu-(L4)2] (8), respectively, with high yields of up to 84%. All the complexes were characterized by elemental analysis, IR, NMR and mass spectroscopy. The molecular structures of complexes 3 and 7 were determined by single crystal X-ray diffraction analyses. The Zn(II) center in complex 3 exhibited a distorted tetrahedral geometry while in complex 7, the Cu(II) center had a square planar geometry with near C2 symmetry. In both structures, the coordination sites are occupied by imino N- and hydroxyl O-donor atoms from the chelating ligands. All complexes showed catalytic activity and exhibited well-controlled living polymerization process in ring-opening polymerization of ε-caprolactone and l-lactide in the presence of benzyl alcohol co-initiator. The molecular weights were found to be low ranging from 1855 to 3999 Da for polycaprolactone (PCL) and up to 1720 Da for polylactic acid (PLA). The Zn(II) catalysts were found to be more active than Cu(II) catalysts with complex 2 (kapp = 0.1751 h−1) being the most active.

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