Bis (pyrazolyl) pyridine Cu (II) and Zn (II)complexes : syntheses, molecular structures and polymerization reactions of ε-Caprolactone and lactides.
Reactions of 2,6-bis((3,5-dimethyl-1H-pyrazol-1-ylmethyl)pyridine (L1) and 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine (L2) ligands with Zn(II) and Cu(II), acetates and carboxylates (benzoates, 2-chlorobenzoate, & 2-bromobenzoate) in a single pot afforded seven Zn(II) and Cu(II) carboxylate complexes. The complexes [Zn(L1)(C6H5COO)2] (1), [Zn(L1)(2-Br-C6H4COO)2] (2), [Cu(L1)(C6H5COO)2] (3), [Cu(L1)(2-Br-C6H4COO)2] (4), [Zn(L2)(C6H5COO)2] (5), Zn(L2)(2-Cl-C6H4COO)2 (6) and Cu(L2)(2-Cl-C6H4COO)2 (7) were obtained in excellent yields. On the other hand, reactions of 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine (L2) with Zn(II) and Cu(II) acetates afforded complexes Zn(L2)(OAc)2 (8) and Cu(L2)(OAc)2 (9) in moderate to good yields. The complexes were characterized by employing a combination of 1H NMR and IR spectroscopies, microanalyses, mass spectrometry and single crystal X-ray crystallography for 1, 2 and 5-9 complexes. Complexes 1-6 and 8-9 were employed as catalysts/initiators in the ring-opening polymerization (ROP) of -caprolacone ( -CL) and they produced active initiators at 110 oC. Compounds 1 and 8 were screened in the ROP of lactide and were found to be active. Detailed kinetics and mechanistic studies were performed for polymerization reaction of -CL using complexes 1-8 and revealed that the reactions follow pseudo-first-order in regards to monomer and second order overall rates. Polymerization reactions using these complexes proceeded in a controlled manner to afford polymers with moderate molecular weights and narrow polydispersities (PDIs). The polymerization reactions occurred through coordination insertion mechanism.