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

Abstract

The crystal structures of the hydrated salts of [Gd.DOTAM]3+ and its more hydrophobic derivative [Gd.]3+, bearing 4 alpha-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, 33.3x10(4) s-1 (298 K), respectively. For [Gd.]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 1x10(4) s-1) for chloride, bromide, iodide, acetate, triflate and nitrate salts, notwithstanding the different nature and extent of hydration found in the crystalline lattice.

Design Principles and Theory of Paramagnetic Fluorine‐Labelled Lanthanide Complexes as Probes for 19F Magnetic Resonance: A Proof‐of‐Concept Study

“…They exhibit a markedly different temperature dependence (Figure 10) associated with a very long water exchange lifetime (t m % 100 ms) that is independent of the nature of X (X = Cl, Br, I, OAc, NO 3 ). [49] The long…”

“…If water exchange is slow (t m > T 1m ), then the measured relaxivity may tend to a lower limit in which only the second-sphere and outer-sphere terms contribute. [17,[49][50][51] In the case of [Gd(L 1 )A C H T U N G T R E N N U N G (OH 2 )]Cl 3 , the limited water solubility of this complex precluded accurate measurements of the temperature dependence of the transverse relaxation rate of the 17 O water signal. For each of the other complexes examined, variable-temperature 17 O NMR measurements were undertaken with 19 to 32 mm solutions of the complex in 17 O-enriched water and the data were analysed by using standard Swift-Connick methodology [52] to give estimates of the water exchange lifetime,…”

Definition of the Labile Capping Bond Effect in Lanthanide Complexes

“…[1] On the contrary, the inclusion of hydrophobic units around the water binding site minimizes hydrogen-bonding between the coordinated water and the second coordination sphere, which results in slower water exchange rates. [16,19] The wider and more extensive number of studies have focused on monoaqua nine-coordinated complexes. Many of these are FULL PAPER derivatives of dota in which the water molecule occupies the apical position, capping the upper square face of the antiprism.…”

Synthesis and Relaxometric Properties of Gadolinium(III) Complexes of New Triazine‐Based Polydentate Ligands

“…The relaxivity is constant in the pH range 4 to 9, whereas it increases both at low pH and in the basic region. The observed behavior is reminiscent of a similar trend found in the The above qualitative considerations can find support in the determination and analysis of the magnetic-field dependence of the relaxivity, the so-called nuclear magnetic relaxation dispersion (NMRD) profile and in the temperature dependence of the 17 O-NMR transverse relaxation rate that provides direct information on the kinetic parameters of the H 2 O exchange [15]. The NMRD profiles, measured at 298 and 310 K and at neutral pH are characterized by the typical shape and amplitude of monoaquo, low-molecular-mass complexes, featuring a single dispersion around 4 -6 MHz (Fig.…”

“…It is well known that in general, the NMRD profiles do not allow extracting accurate information on the parameters that characterize the H 2 O exchange. These are conveniently evaluated through the association of variable-temperature 17 O-NMR study in a fairly concentrated solution of the paramagnetic complex. The R 2p data, measured at 9.4 T in an 10.9 mm aqueous solution of [Gd(L1)], show an increase when the temperature increases in the range 298 -340 K (Fig.…”