Dissociation kinetics of macrocyclic trivalent lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (DO2A)

Abstract

The [H⁺]-catalyzed dissociation rate constants of several trivalent lanthanide (Ln) complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (LnDO2A⁺, Ln = La, Pr, Eu, Er and Lu) have been determined in two pH ranges: 3.73–5.11 and 1.75–2.65 at four different temperatures (19–41.0 °C) in aqueous media at a constant ionic strength of 0.1 mol dm⁻³ (LiClO₄). For the study in the higher pH range, i.e. pH 3.73–5.11, copper(II) ion was used as the scavenger for the free ligand DO2A in acetate/acetic acid buffer medium. The rates of Ln(III) complex dissociation have been found to be independent of [Cu²⁺] and all the Ln(III) complexes studied show [H⁺]-dependence at low acid concentrations but become [H⁺]-independent at high acid concentrations. Influence of the acetate ion content in the buffer on the dissociation rate has also been investigated and all the complexes exhibit a first-order dependence on [Acetate]. The dissociation reactions follow the rate law: k_obs = k_Ac[Acetate] + K′k_lim[H⁺]/(1 + K′[H⁺]) where k_AC is the dissociation rate constant for the [Acetate]-dependent pathway, k_lim is the limiting rate constant, and K′ is the equilibrium constant for the reaction LnDO2A⁺ + H⁺ ⇔ LnDO2AH²⁺. In the lower pH range, i.e. pH 1.75–2.65, the dye indicator, cresol red, was used to monitor the dissociation rate, and all the Ln(III) complexes also show [H⁺]-dependence dissociation pathways but without the rate saturation observed at higher pH range. The dissociation reactions follow the simple rate law: k_obs = k_H[H⁺], where k_H is the dissociation rate constant for the pathway involving monoprotonated species. The absence of an [H⁺]-independent pathway in both pH ranges indicates that LnDO2A⁺ complexes are kinetically rather inert. The obtained k_AC values follow the order: LaDO2A⁺ > PrDO2A⁺ > EuDO2A⁺ > ErDO2A⁺ > LuDO2A⁺, whereas the k_lim and k_H values follow the order: LaDO2A⁺ > PrDO2A⁺ > ErDO2A⁺ > EuDO2A⁺ > LuDO2A⁺, mostly consistent with their thermodynamic stability order, i.e. the more thermodynamically stable the more kinetically inert. In both pH ranges, activation parameters, ΔH*, ΔS* and ΔG*, for both acetate-dependent and proton-catalyzed dissociation pathways have been obtained for most of the La(III), Pr(III), Eu(III), Er(III) and Lu(III) complexes, from the temperature dependence measurements of the rate constants in the 19–41 °C range. An isokinetic (linear) relationship is found between ΔH* and ΔS* values, which supports a common reaction mechanism.