It is generally assumed that astatide (At(-) ) is the predominant astatine species in basic aqueous media. This assumption is questioned in non-complexing and non-reductive aqueous solutions by means of high-pressure anion-exchange chromatography. Contrary to what is usually believed, astatide is found to be a minor species at pH=11. A different species, which also bears a single negative charge, becomes predominant when the pH is increased beyond 7. Using competition experiments, an equilibrium constant value of 10(-6.9) has been determined for the formation of this species from AtO(OH) with the exchange of one proton. The identification of this species, AtO(OH)2 (-) , is achieved through relativistic quantum mechanical calculations, which rule out the significant formation of the AtO2 (-) species, while leading to a hydrolysis constant of AtO(OH) in excellent agreement with experiment when the AtO(OH)2 (-) species is considered. Beyond the completion of the Pourbaix diagram of astatine, this new information is of interest for the development of (211) At radiolabeling protocols.

Bispidine (3,nonane) provides a rigid and preorganized scaffold that is particularly interesting for the stable and inert complexation of metal ions, especially for their application in medical imaging. In this study, we present the synthesis of two bispidine ligands with N-methanephosphonate (H 4 L 1 ) and N-methanecarboxylate (H 3 L 2 ) substituents as well as the physico-chemical properties of the corresponding Mn 2+ and Zn 2+ complexes. The two complexes [Mn(L 1 )] 2− and [Mn(L 2 )] − have relatively moderate thermodynamic stability constants according to potentiometric titration data. However, they both display an exceptional kinetic inertness, as assessed by transmetallation experiments in the presence of 50 equiv excess of Zn 2+ , showing only ∼40 and 20% of dissociation for [Mn(L 1 )] 2− and [Mn(L 2 )] − , respectively, after 150 days at pH 6 and 37 °C. Proton relaxivities amount to r 1 = 4.31 mM −1 s −1 ([Mn(L 1 )] 2− ) and 3.64 mM −1 s −1 ([Mn(L 2 )] − ) at 20 MHz, 25 °C, and are remarkable for Mn 2+ complexes with one inner-sphere water molecule (q = 1); they are comparable to that of the commercial contrast agent [Gd(DOTA)(H 2 O)] − . The presence of one inner-sphere water molecule and an associative water exchange mechanism was confirmed by temperature-dependent transverse 17 O relaxation rate measurements, which yielded k ex 298 = 0.12 × 10 7 and 5.5 × 10 7 s −1 for the water exchange rate of the phosphonate and the carboxylate complex, respectively. In addition, radiolabeling experiments with 52 Mn were also performed with H 2 (L 1 ) 2− showing excellent radiolabeling properties and quantitative complexation at pH 7 in 15 min at room temperature as well as excellent stability of the complex in various biological media over 24 h.

We propose quantitative assessment of zinc by combining nuclear and MR imaging. We use a cocktail of a Gd3+-complex providing a Zn2+-dependent MRI response and its 165Er3+ analogue allowing for concentration assessment. 165Er is readily obtained in a cyclotron and purified, which is indispensable for successful quantification of metal ions.