Kinetics of acid-catalysed dissociation of tetraazamacrocyclic ligand complexes of copper(II)

Abstract

The dissociation kinetics of copper(II) complexes of 1,4,7,10-tetraazacyclododecane ([12]aneN4), 1,4,7,10-tetraazacyclotridecane ([13]aneN4), 1,4,8,12-tetraazacyclopentadecane ([15]aneN4) and 1,5,9,13-tetraazacyclohexadecane ([16]aneN4), have been studied spectrophotometrically in 0.015–2.50 mol dm–3 HNO3at 25.0 ± 0.1 °C and I= 5.0 mol dm–3(HNO3+ NaNO3). The rate law for the dissociation reaction of [Cu([12]aneN4)]2+ is rate =(k1[H+]2+k2[H+])[complex] with k1=(5.80 ± 0.24)× 10–4 dm6 mol–2 s–1 and k2=(1.19 ± 0.54)× 10–4 dm3 mol–1 s–1. The rate law for the reaction of [Cu([13]aneN4)]2+ is rate =k3[H+][complex] with k3=(7.99 ± 0.07)× 10–4 dm3 mol–1 s–1. The rate law for the reactions of [Cu([15]aneN4)]2+ and [Cu([16]aneN4)]2+ is rate =k4[H+][complex]/(1 +k5[H+]); for the reaction of [Cu([15]aneN4)]2+, k4=(1.42 ± 0.34)× 10–2 dm3 mol–1 s–1 and k5=(9.91 ± 1.65)× 10–1 dm3 mol–1; for the reaction of [Cu([16]aneN4)]2+, k4= 3.27 ± 1.37 dm3 mol–1 s–1 and k5= 6.96 ± 2.04 dm3 mol–1. The possible mechanisms for these reactions, the factors influencing the rates, and the factors affecting the relative importance of the solvation pathway and the protonation pathway are discussed.