Five new mononuclear complexes formed by the polynucleating ligand 2-[1-(3,5-dimethyl)pyrazolyl]-2-hydroxyimino-N'-[1-(2pyridyl)ethylidene]acetohydrazide (HL): [Ni(L)(HL)]ClO 4 •2CH 3 OH (1), [Ni(L) 2 •CH 3 OH (2), [Zn(L)(HL)]ClO 4 •2CH 3 OH (3), [Zn(L) 2 ]•CH 3 OH (4) and [Cu(L) 2 ]•CH 3 OH (5) were synthesized and characterized by elemental analysis, mass-spectrometry, IR-spectroscopy and X-ray analysis. The complexes reveal distorted octahedral N 4 O 2 coordination arrangement formed by both protonated and deprotonated (1, 3) or two deprotonated ligand molecules (2, 4, 5). The presence of non-coordinated oxime and pyrazole groups resulted in the formation of extensive systems of hydrogen bonds in the crystal packing of 1-5. Potentiometric titrations, ESI-MS and spectrophotometric studies of complex formation in MeOH/H 2 O solutions indicated the presence of mono-and polynuclear complexes with Cu(II), Ni(II) and Zn(II) ions. The solution studies carried out for an excess of Cu(II) over HL ligand (2:1 and 3:2 molar ratio) indicated also the formation of polynuclear Cu 3 L 2 H x species, with an involvement of additional nitrogen donors in copper coordination.In binuclear complex[Cu 2 (L) 2 (DMF) 2 ](ClO 4) 2 •DMF(6) obtained in solid state,the Cu(II) coordination, analogous to the one in 1-5, was supported by pyrazole N atom.
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