Spectroscopic study of the hydration equilibria and water exchange dynamics of lanthanide(III) complexes of 1,7-bis(carboxymethyl)-1,4,7,10- tetraazacyclododecane (DO2A)

The hydration state of a series of [Ln(DO2A)(H₂O)(n)]⁺ complexes in aqueous solution at pH = 6.4-7.0 was studied by measuring the lanthanide- induced ¹⁷O shifts (LIS) of water [Ln includes elements from Ce to Yb; DO2A = 1,7-bis(carboxymethyl)-1,4,7,10-tetraazacyclododecane]. Their contact contribution, obtained from Reilley plots, indicated a decrease in the inner- sphere water coordination number of the [Ln(DO2A)(H₂O)(n)]⁺ complexes from n = 3 (Ce-Eu), to n = 2 (Tb-Yb). A temperature-dependent UV/Vis absorption study of the 578-582 nm ⁷F₀ → ⁵D₀ transition band of [Eu(DO2A)(H₂O)(n)]⁺ in aqueous solution showed that this complex is present in an equilibrium between eight- and nine-coordinate species with n = 2 and n = 3, respectively. The hydration equilibrium parameters (2 ⇆ 3), K_{2- 3}²⁹⁸ = 4.0 ± 0.2, ΔH_2-3⁰ = -12.1 ±1 kJ mol^-1 and ΔS_2-3/⁰ = -28.9 ± 3 J mol^-1 K^-1 correspond to an average hydration number of 2.65-2.85 in the temperature range 273-363 K. A variable temperature, multiple field ¹⁷O NMR study combined with direct EPR measurements of the transverse electronic relaxation rates has been used to obtain the parameters characterizing water exchange, rotation and electronic relaxation, all influencing the proton relaxivity of [Gd(DO2A)(H₂O)_2-3]⁺. The small increase in the water exchange rate of [Gd(DO2A)(H₂O)_2-3]⁺ (k(ex)²⁹⁸ = (10 ± 5) x 10⁶s^-1) relative to that of [Gd(DOTA)(H₂O)]^- (4.8 x 10⁶ s^-1) is a consequence of an unfavorable interplay of charge and hydration equilibria. The value of τ(R)²⁹⁸ = 40 ± 1 ps is short, and the electronic relaxation rate (1/T(2e) ≃ 1.2 x 10¹⁰ s^-1) is fast relative to [Gd(DOTA)(H₂O)]^- (1.3-2.4 x 10⁹ s^-1 for B = 0.34 T). These parameters negate to some extent the expected increase in proton relaxivity of the [Gd(DO2A)(H₂O)_2-3]⁺ complex.