Kinetics of replacement of aminocarboxylates by macrocyclic polyamines in copper(II) complexes

Abstract

The rate constants have been determined for the reaction of macrocyclic polyamine ligands and their protonated forms with [Cu(ida)], [Cu(nta)]^–, [Cu(hedta)]^–, and [Cu(edta)]^2–[ida = iminodiacetate, nta = nitrilotriacetate, hedta =N-(2-hydroxyethyl)ethylenediamine-NN′N′-triacetate, and edta = ethylenediaminetetra-acetate]. The macrocyclic ligands include 1,4,7,10-tetra-azacyclotridecane, 1,4,8,12-tetra-azacyclopentadecane, and 1,4,7,10,13- penta-azacyclopentadecane. These ligand-displacement reactions demonstrate the effects of protonation of macrocyclic polyamines having different ring sizes and denticities. The formation of the macrocyclic complex from [Cu(ida)] or [Cu(nta)] occurs as fast as (or sometimes even faster) from Cu²⁺(aq), despite the fact that the former reactions require the replacement of strongly co-ordinated aminocarboxylate ligands. The reaction rates for the unprotonated or protonated macrocycles are inversely dependent on the stability constants of the aminocarboxylate complexes, suggesting that the rate-determining step is rupture of the ida segment of an aminocarboxylate from copper. Compared with the same replacement reactions with corresponding linear polyamines, the rate constants are consistently 10³—10⁴ times smaller, indicating a close parallel in the two reaction mechanisms.