On the role of the counter-ion in defining water structure and dynamics: Order, structure and dynamics in hydrophilic and hydrophobic gadolinium salt complexes

The crystal structures of the hydrated salts of [Gd.DOTAM]³⁺ and its more hydrophobic derivative [Gd.2]³⁺, bearing 4 α-phenylethyl groups, (both Gd and Yb salts) are reported and compared. The nature of the anion determines the degree of ordering in the lattice and the extent of hydration. These effects are correlated with the results of ¹⁷O and ¹H NMR measurements of water exchange dynamics in solution. With [Gd.DOTAM]³⁺, structural ordering or the extent of hydration in the hydrated lattice follows the sequence Cl^- > Br^- > I^- and this order also defines the water exchange rate in solution: 7.3, 19.5, 33.3 × 10⁴ s^-1 (298 K), respectively. For [Gd.2]³⁺ salts, the measured relaxivity is determined purely by the outer sphere term and the water exchange rate at 298 K is very similar (typically 1 × 10⁴ s^-1) for chloride, bromide, iodide, acetate, triflate and nitrate salts, notwithstanding the different nature and extent of hydration found in the crystalline lattice.