Thermodynamic Study on Adduct Formation of Copper(II) β-Diketonates with Heterocyclic Bases

Abstract

Abstract The stability constants and thermodynamic functions for 1:1 adduct formation of unidentate heterocyclic bases with copper(II) β-diketonates were determined in non-aqueous solvents by the spectrophotometric method. The main factors controlling the stability of the adducts were investigated and the bases were classified into two groups in adduct formation from the plots of the stability constants against pKH values of the bases: steric hindered and unhindered bases. Large negative entropy changes were observed for pyridine, β-picoline, and γ-picoline; these were ascribed to the π-bond character of the adduct bonds. A solvent effect was observed; the order of increasing adduct stability was as follows: acetone <butyl acetate <4-methyl-2-pentanone <chloroform <nitrobenzene <toluene \simeq benzene <chlorobenzene. This order agreed with the decreasing order of the solvent-chelate interaction and of the solvent-base interaction. In all cases, the adducts were stabilized by large exothermic enthalpy changes. Moreover, the introduction of a CF3 group into β-diketones also brought about a considerable entropy change which promoted the stabilization of the chelate. The effect of the other terminal groups was not clearly observed.