Lanthanoid metal ions, Ln³⁺, can exchange with, and mimic the function of, Ca²⁺ ions in the human body. Lanthanum carbonate is administered as chewable tablets under the tradename of Fosrenol^® to patients with particularly high levels of phosphate ions in the blood. However, there are significant gastrointestinal side-effects, and more soluble forms of lanthanide-containing drugs are being sought. Recent research in this field has investigated the reactions of Ln(NO₃)₃·6H₂O with Hma (defined below) in the presence of NaOH or Et₃N.

(a) Why are ligands with O-donor atoms chosen to bind Ln³⁺? 

(b) What is the reason for adding base to the reaction mixture? 

(c) The highest mass peaks in the electrospray ionization mass spectra of the complexes formed with La³⁺ and Eu³⁺ are at m/z 537 and 551, respectively. How do these peaks arise? 

(d) Elemental analytical data for the Eu³⁺ complex are C 39.65, H 3.14%. What can you deduce from these data? Suggest a structure for the europium(III) complex. 

(e) Complexes were formed with La³⁺, Eu³⁺, Gd³⁺, Tb³⁺ and Yb³⁺, but ¹H NMR spectroscopic data were only reported for the La³⁺ complex. Suggest a reason for this.

(f) The Ln³⁺ complexes were subjected to hydroxyapatite-binding studies. Comment on the reasons for carrying out such investigations.

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0 Hma OH