Coordination equilibrium— a clue for fast water exchange on potential magnetic resonance imaging contrast agents?

A temperature‐dependent UV–visible spectrophotometric study on [Eu(DO3A)(H 2 O) n ] proved the presence of a hydration equilibrium ( n  = 1, 2), strongly shifted towards the bisaqua species [DO3A = 1,4,7‐tris(carboxymethyl)‐1,4,7,10‐tetraazacyclododecane]. The thermodynamic parameters and the reaction volume were determined for the equilibrium [Eu(DO3A)(H 2 O)] + H 2 O ⇌ [Eu(DO3A)(H 2 O 2 )] and the same results were extra‐polated for the Gd(III) analogue (Δ H ° = −12.6 kJ mol −1 , Δ S ° = −25.2 J mol −1 K −1 , K Eu 298  = 7.7 and Δ V ° = −7.5 cm 3 mol −1 ). The variable‐temperature 17 O NMR data on [Gd(DO3A)(H 2 O) n ] were analysed by two approaches: (i) with the Swift–Connick equations (two‐site exchange) and (ii) with the Kubo–Sack formalism (three‐site exchange). The comparison of the results obtained by the two different analyses show that, despite the crude approximation of treating the system as a two‐site exchange problem, the Swift–Connick method gives a correct value for the water exchange rate. Based on previous observations on the relationship between inner sphere structure and water exchange rate, one can expect higher rates for complexes with a hydration equilibrium. Indeed, the water exchange on [Gd(DO3A)(H 2 O) n ] is about twice as fast as on [Gd(DOTA)(H 2 O)] − ( k ex 298  = 11 × 10 6 and 4.8 × 10 6 s −1 , respectively), although it is still much slower than that on [Gd(H 2 O) 8 ] 3+ ( k ex 298  = 804 × 10 6 s −1 ). The limited gain in the water exchange rate is explained in terms of a rigid inner sphere structure introduced by the macrocyclic ligand which makes difficult the transition from the reactant to the transition state, and consequently, results in a slower exchange as compared to the Gd 3+ aqua ion. The activation parameters of the water exchange are $\Delta H_{\rm ex}^{\neq} = 33.6\,\hbox{kJ mol}^{-1}$ and $\Delta S_{\rm ex}^{\neq} = +2.4\,\hbox{J mol}^{-1} \hbox{K}^{-1}$ , and the mechanism is proposed to be dissociatively activated. Copyright © 1999 John Wiley & Sons, Ltd.