(E)-2-(2-(2-hydroxyphenyl)hydrazono)-1-phenylbutane-1,3-dione: Tautomery and coordination to copper(II)

(E)-2-(2-(2-hydroxyphenyl)hydrazono)-1-phenylbutane-1,3-dione (H2L) was synthesized by azocoupling of diazonium salt of 2-hydroxyaniline with 1-phenylbutane-1,3-dione and characterized by IR, 1H and 13C NMR spectroscopies and X-ray diffraction analysis. In solution, H2L exists as a mixture of the enol-azo and hydrazone tautomeric forms and a decrease of temperature and of solvent polarity shifts the tautomeric balance to the hydrazone form. In the solid state, H2L crystallizes from ethanol–water in the monohydrate hydrazone form, as shown by X-ray analysis. The dissociation constants of H2L (pK1 = 5.98 ± 0.04, pK2 = 9.72 ± 0.03) and the stability constants of its copper(II) complex (log β1 = 11.01 ± 0.07, log β2 = 20.19 ± 0.08) were determined by the potentiometric method in aqueous-ethanol solution. The copper(II) complex [Cu2(μ-L)2]n was isolated in the solid state and found by X-rays to be a coordination polymer of a binuclear core with a distorted square pyramidal metal coordination geometry.