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 -O2CCH2(H2NOCCH2)NCH2CHRN(CH2CONH2)CH2CO2- (R = H, L3; Me, L4) which have in-plane 2N(amine,) 2 O(carboxylate) ligands at pH 6 undergo amide group deprotonation in basic solution to give CuN(amine)2N- (amide)2 chromophores resulting from the displacement of the in-plane carboxylate ligands by the superior sigma-donating, deprotonated, amide nitrogens. The concentration pK(a) values for the deprotonations at 25-degrees-C, l = 0.1 mol dm-3 are 9.19 +/- 0.02 and 9.87 +/- 0.02 for [CuL3], and 8.60 +/- 0.02 and 9.81 +/- 0.02 for [CuL4]. The unexpectedly large differences in the first pK(a) values is attributed to a steric interaction between the methyl substituent and an axially co-ordinated CH2CONH2 group, in the complex of L4 which is relieved as a result of the first ionisation step.