Copper(II) carboxylates with 2-aminopyridine. Synthesis, characterization and a study of the dimer–monomer equilibrium in acetonitrile solutions by VIS-spectroscopic and microcalorimetric titrations

Abstract

A series of copper(II) carboxylates with 2-aminopyridine was prepared. In all cases typical dinuclear complexes with a paddle-wheel type structure, Cu₂X₄L₂, as well as mononuclear complexes of type CuX₂L₂ were obtained (X = hexanoate, heptanoate, octanoate or nonanoate, L = 2-aminopyridine). The structures of monoclinic Cu₂(O₂CC₇H₁₅)₄(C₅H₆N₂)₂, 1, and Cu(O₂CC₇H₁₅)₂(C₅H₆N₂)₂, 2, were determined by X-ray diffraction. VIS-absorption spectroscopic and isothermal calorimetric titrations, were employed to investigate the dimer–monomer equilibrium in acetonitrile solutions. The results were analysed on the basis of the mass-action model to obtain the equilibrium constants of monomer formation (K_M ≈ 10⁴ L mol⁻¹). The thermodynamic analysis shows that for all studied systems monomer formation is an enthalpy-driven process (ΔH_M° ≈  −14 kJ mol⁻¹). The observed exothermic ΔH_M° values result most likely from the attractive coordinative interactions between Cu²⁺ ion and 2-aminopyridine.