Andre E. Merbach

Water exchange on metal ions: experiments and simulations

Abstract The water exchange reaction between coordination shells around metal ions in aqueous solution is a fundamental reaction in understanding the reactivity of these ions in chemical and biological systems. The results reviewed in this paper demonstrate the complementary of experimental studies and computer simulations or quantum chemical calculations performed on such systems. Due to the large range of exchange rate constants, a variety of experimental and different computer techniques have to be applied. Very fast exchange reactions between first and second coordination shell and between second shell and bulk solvent can be simulated by classical molecular dynamics technique. Reaction pathways for water exchange on metal ions with a less labile first coordination shell can be followed by calculation of clearly defined transition states. Success and shortcomings of the techniques are discussed by means of recent publications.

Superparamagnetic gadonanotubes are high-performance MRI contrast agents

We report the nanoscale loading and confinement of aquated Gd3+n-ion clusters within ultra-short single-walled carbon nanotubes (US-tubes); these Gd3+n@US-tube species are linear superparamagnetic molecular magnets with Magnetic Resonance Imaging (MRI) efficacies 40 to 90 times larger than any Gd3+-based contrast agent (CA) in current clinical use.

Relaxivity of MRI contrast agents

In the recent years Magnetic Resonance Imaging has evolved into one of the most powerful diagnostic techniques in medicine, in part thanks to the application of suitable contrast agents. The design of new, more efficient MRI contrast media requires the complete understanding of all factors and mechanisms that influence proton relaxivity, hence efficiency of Gd(III) complexes. In this chapter we give an overview of our current knowledge in this field by shortly surveying theory and citing the most illustrative examples. We discuss each of the underlying factors, including the possible ways of their determination. Most recent developments in the field of electron spin relaxation and outer sphere relaxivity have also been reviewed. The last part is devoted to the comparison of the isoelectronic Eu(II) and Gd(III) complexes, which can give insight into the relaxation mechanisms of paramagnetic lanthanides in general, and thus help design novel Gd(III) based agents.

The hydration of Dy3+ and Yb3+ in aqueous solution: A neutron scattering first order difference study

The coordination of water molecules around the metal ion in acidified 1 m DyCl3, 1.0 m Dy(ClO4)3, 0.3 m Dy(ClO4)3, and 1.0 m Yb(ClO4)3 solutions in D2O have been determined by the neutron diffraction difference technique. A coordination number of eight is found for the two ions, which depends neither on the counterion, nor on the concentration. The near metal–oxygen and metal–deuterium distances are, respectively, 2.39±0.02 and 3.03±0.02 A for dysprosium and 2.33±0.02 and 2.98±0.02 A for ytterbium. A general discussion concerning the hydration of the lanthanide ions is given.

Use of high pressure kinetic studies in determining inorganic substitution mechanisms

High pressure kinetic studies are particularly useful for . the determination of the activation node of simple inorganic substitu— tion processes. Applications to solvent exchange and complex formation reactions of tetrahedral and octahedral species in aqueous and non— aqueous solvents are presented. The gradual substitution mechanism changeover revealed by variable pressure studies for both di— and trivalent hexasolvates along the first row transition metal ions is discussed. Some methodological and technical aspects of high pressure high resolution multinuclear magnetic resonance applied to solvent exchange studies are treated.

High Relaxivity for Monomeric Gd(DOTA)-Based MRI Contrast Agents, Thanks to Micellar Self-Organization

A new amphiphilic GdIIIchelate, which is capable of forming micelles in aqueous solution (see diagram), has been synthesized. Due to this self-aggregation, the compound has a long rotational correlation time and, consequently, has high proton relaxivities that thus far have only been obtained with macromolecular complexes.

17O nuclear magnetic resonance in aqueous solutions of Cu2+ : The combined effect of Jahn–Teller inversion and solvent exchange on relaxation rates

The dynamics of water molecules in Cu2+ aqueous solutions were investigated by 17O NMR at several temperatures and magnetic fields. A theory is developed for scalar relaxation in the presence of Jahn–Teller inversion of the aquaion and water exchange between the aquaion and the bulk, for a model where the inversion is much faster than the exchange and the scalar coupling is negligible for axially bound water molecules. It is shown that this model is consistent with the experimental results, while the standard expressions, ignoring the influence of the Jahn–Teller inversion, are clearly inconsistent with the observed longitudinal relaxation rates. The inversion process is characterized by τ 298i=(5.1±0.6)×10−12 s and Ei=3.5±1.5 kJ mol−1. The water exchange process is characterized by k298ex=(4.4±0.1)×109 s−1, ΔH‡=11.5±0.3 kJ mol−1 and ΔS‡=−21.8±0.9 J K−1 mol−1.

Recent developments in solvation and dynamics of the lanthanide(III) ions

This paper presents recent developments concerning the behaviour of the trivalent lanthanide ions in solution. Counterion complexation, coordinntion numbers and kinetic properties are discussed. Special interest is devoted to the results obtained in aqueous and N,N-dimethylformamide solutions, where the solvent exchange reactions have been characterized at variable temperature and pressure. The coordination properties, solvent exchange rates and mechanisms are discussed in terms of electrostatic and steric factors.

Coordination equilibria and water exchange kinetics of lanthanide(III) propylenediaminetetraacetates and other magnetic resonance imaging related complexes

Abstract The UV-Vis absorption spectra of aqueous solutions of Eu 3+ complexes with the hexadentate polyaminocarboxylate ligands EDTA 4− , CDTA 4− , HDTA 3− and PDTA 4− were measured as a function of temperature and pressure in the frequency region corresponding to the 7 F 0 → 5 D 0 transition of Eu 3+ . The results can be explained in terms of equilibria between nine-coordinate and eight-coordinate species where the eight-coordinate species contain one less inner sphere water molecule than the nine-coordinate species. The thermodynamic parameters, including the reaction volume, for these equilibria were determined. 17 O NMR transverse relaxation rates and chemical shifts were measured for aqueous solutions of the eight-coordinate complexes [Ln(PDTA)(H 2 O) 2 ] − (LnTb, Dy, Er, Tm, Yb) and [Er(EDTA)(H 2 O) 2 ] − as a function of temperature, pressure and magnetic field. The results were analysed in terms of the water exchange kinetics on the complexes. The water exchange rate on [Ln(PDTA)(H 2 O) 2 ] − decreases dramatically with decreasing ionic radius across the lanthanide series from k ex 298 = (2.4±0.1)×10 7 s −1 for LnTb to k ex 298 = (2.8±0.3)×10 5 s −1 for LnYb. The activation volumes show that this is accompanied by a change of exchange mechanism from associatively activated for LnTb ( ΔV # = −7.6±0.3 cm 3 mol −1 ) to dissociatively activated for LnYb ( ΔV # = +7.4±0.8 cm 3 mol −1 ). Water exchange on [Er(EDTA)(H 2 O) 2 ] − ( k ex 298 = (9.8±1.9)×10 6 s −1 ) is more than an order of magnitude faster than on [Er(PDTA)(H 2 O) 2 ] − ( k ex 298 = (5.6±0.5)×10 5 s −1 ). These kinetic results can be interpreted in terms of the equilibria measured by UV-Vis spectrophotometry. The implications of these observations for the design of new MRI contrast agents are discussed.

Water exchange on magnesium(II) in aqueous solution: a variable temperature and pressure 17O NMR study†

Fast water exchange rate constants on diamagnetic metal ions can be measured using either a paramagnetic chemical shift agent or a paramagnetic relaxation agent. An approximate analytical equation was developed for the chemical shift agent method, valid in the limit of rapid exchange. The limits of validity of these methods are discussed. These were applied to the study of water exchange on [Mg(H2O)]62+ over a large temperature range (252–350 K) and up to a pressure of 200 MPa by 17O NMR, leading to the following kinetic parameters: kMg298=6.7±0.2×105 s‐1; ΔHMg‡=49.1±0.7 kJ mol‐1; ΔSMg‡=+31.1±2.2 J K‐1; ΔVMg‡=+6.7±0.7 cm3 mol‐1. From these data, it is concluded that this process has an Id mechanism with strong d character, without excluding a limiting D mechanism.