Copper(II) complexes of hydrolysis products of the anticancer bis(3,5-dioxopiperazin-1-yl)alkanes. Displacement of co-ordinated carboxylate ligands by deprotonated amide groups in basic solution

The copper(II) complexes of the diacid diamide OCCH(HNOCCH)NCH CHRN(CHCONH)-CHCO (R = H, L; Me, L) which have in-plane 2N 2 O ligands at pH 6 undergo amide group deprotonation in basic solution to give CuN(amine)N(amide) chromophores resulting from the displacement of the in-plane carboxylate ligands by the superior s-donating, deprotonated, amide nitrogens. The concentration pK values for the deprotonations at 25° C, I = 0.1 mol dm are 9.19 ± 0.02 and 9.87 ± 0.02 for [CuL], and 8.60 ± 0.02 and 9.81 ± 0.02 for [CuL]. The unexpectedly large differences in the first pK values is attributed to a steric interaction between the methyl substituent and an axially co-ordinated CHCONH group, in the complex of L which is relieved as a result of the first ionisation step.