A monoaqua zinc complex. Unique acid dissociation behaviour

Abstract

Using the 1 : 2 condensate of benzil dihydrazone and 2-acetylpyridine as the ligand L, two complexes of zinc, [ZnL(CH₃COO)]PF₆ (1) and [ZnL(H₂O)ClO₄]ClO₄·H₂O (2), are synthesised from Zn(CH₃COO)₂·2H₂O and Zn(ClO₄)₂·6H₂O, respectively. From X-ray crystallography, both the complexes are found to be single helical with the metal in distorted octahedral N₄O₂ environment. In 1, the two oxygen atoms come from the bidentate acetate while 2 is a monoaqua complex with a perchlorate anion bound to the metal in monodentate fashion. The perchlorate in 2 is not at all weakly bound [Zn–O(perchlorate) 2.256(4) Å]. Still in acetonitrile solution, the coordinated perchlorate ion dissociates upon deprotonation [reaction (i)]. This is supported by gas phase AM1 calculations. From solution conductivity, the pK_a value of reaction (i) is determined as 5.71 (±0.19). It is estimated that had 2 been soluble in water, it would have a pK_a value of ∼2 in water. The cation [ZnL(OH)]⁺ generated in reaction (i) could not be isolated in the solid state. From AM1 calculations, it is found to have a distorted tetrahedral ZnN₃O core. Complex 2 is ineffective in bringing about hydration of CO₂.