Synthesis and structures of soluble magnesium and zinc carboxylates containing intramolecular NH⋯O hydrogen bonds in nonpolar solvents

Abstract

Readily soluble magnesium and zinc carboxylates [M(Ln)₂(H₂O)₄] (MLn₂) (M = Mg, Zn; L1 = O₂C-2-Ar₃CCONH-6-n-BuCONHC₆H₃; L2 = O₂C-2-Ar₃CCONHC₆H₄; Ar = 4-t-BuC₆H₄) containing intramolecular NH⋯O hydrogen bonds in nonpolar solvents were synthesized and their molecular structures were determined by X-ray analysis. The complexes were crystallized in the trans or cis configuration. The M–O bond distances were dependent on the mode of the hydrogen bonds. ¹H NMR spectral measurements revealed a fast trans–cis isomerization of MLn₂ in CDCl₃, which was converted into a unique fac-[M(Ln)₃(H₂O)₃]⁻ (fac-[MLn₃]⁻) by the addition of equimolar [Ln]⁻. The theoretical calculations supported the existence of the facial configuration. The coordinated water molecules of MLn₂ were detected by ¹H NMR spectroscopy and the acidity was estimated in the order of ML1₂ > ML2₂. Calcium afforded only the dinuclear complex, [Ca₂(L1)₂(H₂O)₅(1,4-dioxane)] (Ca₂L1₄), which showed a low hydrolytic activity.