An electron spin resonance study on the interactions between bis-(pentane-2,4-dionato)copper(II) and some heterocycles. Part 2. The 1 : 1 adducts with aza-aromatic compounds and azacyclohexanes

Abstract

E.s.r. spectra of a series of Cu(acac)₂ adducts of aza-aromatic compounds and azacyclohexanes (L) were measured. The 1 : 1 adduct Cu(acac)₂L is formed almost exclusively in chloroform solution, L occupying an apical site. Their g_∥ and |A_∥| values were determined in order to investigate the steric and electronic effects on co-ordination. When the frontal steric hindrance of the base is not large, the |A_∥| values of Cu(acac)₂L are linearly related to the pK_a values of L. Since the nitrogen atoms in 2,6-dimethylpyridine, 2- and 8-methylquinoline, acridine, and N-methylazacyclohexanes have considerable frontal steric hindrance, remarkable deviations from the straight line are observed for these compounds. The presence of two 1 : 1 adducts is shown for N-methylmorpholine, the O-adduct being more favourable than the N-adduct because of steric hindrance.