The solution thermodynamic stability of desferrioxamine B (DFO) with Zr(IV)

Desferrioxamine B (DFO, [H₄L]⁺, ligand) is currently the preferred chelator for ⁸⁹Zr(IV), however the biological studies suggest that it releases the metal ion in vivo. Herein, we present the solution thermodynamics of complexes formed between Zr(IV) and this hexadentate chelating agent, the data surprisingly not yet available in the literature. Several techniques including electrospray ionization mass spectrometry (ESI-MS), potentiometry, UV-Vis spectroscopy and isothermal titration calorimetry (ITC) were used to determine the stoichiometry and thermodynamic stability of complexes formed in solution over pH range 1-11, overcoming all the difficulties with the characterisation of the aqueous solution chemistry of Zr(IV) complexes, like strong hydrolysis and lack of spectral information. A model containing only mononuclear complexes, i.e. [ZrHL]²⁺ [ZrL]⁺, [ZrLH_-1] throughout the entire measured pH range is proposed. The stability constants and pM (Zr(IV)) value determined for Zr(IV)-DFO system, place DFO among good Zr(IV) chelators, however the formation of 6-coordinate unsaturated complexes (i.e. with coordination sphere of 8-coordinate Zr(IV) completed by water molecules), together with the susceptibility of coordinated water molecule to deprotonation, are suggested to be the reason of in vivo instability of ⁸⁹Zr(IV)-DFO complexes.

Platonic Relationships in Metal Phosphonate Chemistry: Ionic Metal Phosphonates

Phosphonate ligands demonstrate strong affinity for metal ions. However, there are several cases where the phosphonate is found non-coordinated to the metal ion. Such compounds could be characterized as salts, since the interactions involved are ionic and hydrogen bonding. In this paper we explore a number of such examples, using divalent metal ions (Mg2+, Ca2+, Sr2+ and Ni2+) and the phosphonic acids: p-aminobenzylphosphonic acid (H2PABPA), tetramethylenediamine-tetrakis(methylenephosphonic acid) (H8TDTMP), and 1,2-ethylenediphosphonic acid (H4EDPA). The compounds isolated and structurally characterized are [Mg(H2O)6]·[HPABPA]2·6H2O, [Ca(H2O)8]·[HPABPA]2, [Sr(H2O)8]·[HPABPA]2, [Mg(H2O)6]·[H6TDTMP], and [Ni(H2O)6]·[H2EDPA]·H2O. Also, the coordination polymer {[Ni(4,4’-bpy)(H2O)4]·[H2EDPA]·H2O}n was synthesized and characterized, which contains a bridging 4,4’-bipyridine (4,4’-bpy) ligand forming an infinite chain with the Ni2+ cations. All these compounds contain the phosphonate anion as the counterion to charge balance the cationic charge originating from the metal cation.

Aminobisphosphonates based on cyclohexane backbone as coordinating agents for metal ions. Thermodynamic, spectroscopic and biological studies

Complex formation equilibria of calcium, magnesium, copper and nickel with amino-bisphosphonic ligands are described, together with a speciation study along with calorimetric outcome and cytotoxicity characteristics.