Kinetics of ligand-displacement reactions of copper(II) complexes of deprotonated linear and macrocyclic dioxotetra-amines. Comparative studies with glycylglycylglycine and glycylglycylhistidine

Abstract

The kinetics of displacement of doubly deprotonated linear (X¹) and 13–15-membered macrocyclic dioxotetra-amines (X²–X⁴), compared with glycylglycylglycine (X⁵) and glycylglycylhistidine (X⁶), from their copper(II) complexes (all commonly expressed as [CuH_–2X]) have been studied with ligands of various degrees of nucleophilicity including a linear tetra-amine (L¹), macrocyclic tetra-amines (L² and L³), or ethylenediaminetetra-acetate (edta) in borate (pH 8–9.5) or lutidine buffers (pH 6–7.5). The proposed mechanisms for X¹–X⁴ are analogous to that reported for X⁶, where proton assistance is needed to aid cleavage of the non-terminal copper–imide bonds and provide open sites for nucleophilic attack. The effects of cyclization of the X ligands and ring size on the relative stability of the [CuH_–2X] complexes are well manifested in the ligand-replacement kinetics. Thus, the 14-membered X³ which forms the most stable complex among X¹–X⁶ is the most inert to the substitution reactions.