Tetraaza macrocyclic complexes of Cu(II), Co(III) and Ni(II) derived from template condensation of 2,6-pyridinedicarbohydrazide and 2,6-diacetylpyridine: Stability investigation and coloring properties on plastics

Abstract

Completely conjugated 14-membered ring hexaaza ligand complexes of Cu(II), Co(III) and Ni(II) were synthesized by the template cyclo-condensation reaction of 2,6-diacetylpyridine and 2,6-pyridinedicarbohydrazide: [Cu(HL)Cl]·1.35H₂O (1), [Cu(HL)Br]·CH₃OH (2), [Cu(L)(H₂O)]·4H₂O (3), [Co(HL)Cl₂]·CH₃OH (4), (H₃O)[Co(H₂L)Cl₂][Co(HL)Cl₂]Cl₂·H₂O (5), [Co(HL)(SCN)₂]·CH₃OH (6), [Ni(L)]·3H₂O (7) and (H₃O)[Ni(L)]Cl·H₂O (8). The structure of all complexes derived from this novel diprotic macrocyclic ligand H₂L (5,7-dimethyl-3,4,8,9-tetraaza-1,6(2,6)-dipyridinacyclodecaphane-4,7-diene-2,10-dione) were determined by single crystal X-ray diffraction method. The tetraaza 14-membered conjugated macrocycle is N₄-tetracoordinated to metal, adopts an essentially planar configuration and forms four fused metallocycles rings, two five-membered and two six-membered chelates. The structures of the studied complexes reveal three isomers out of three possible: with a 5-6-5-6 order of alternation of metallocycles for 1–6, the 5-5-6-6 isomer in 8 and the coexistence of two different isomers with 5-5-6-6 and 6-6-5-5 alternation of metallocycles in 7. Based on the protonation state of the macrocyclic ligand, four forms of the ligand have been registered: bis-deprotonated L²⁻ (compounds 3, 7 and 8), monodeprotonated HL⁻ (1, 2 and 4), neutral H₂L (5) and for compound 6 tautomeric (amide → imidic acid) HL⁻ form. The organisation of crystal structures 1–8 depends on the nature of the metal and the composition of the solvents and outer-sphere ions, but shows clear common features for the same type of metal. The hydrolytic study demonstrated a high stability of the Co(III) and Ni(II) complexes in acidic solutions and the most thermally stable are the complexes 3 and 7 with the limit of 350 °C for Ni(II) complex. Testing of the coloring properties of the [Ni(L)] (8) complex on plastics indicates a high coloring efficiency, especially for polyamide polymers.