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.