Selectivity of asymmetric macrocyclic compartmental lanthanide(III) complexes towards alkali and alkaline-earth metal ions

The asymmetric macrocyclic compartmental ligand H2L, derived from the [1+1] condensation of 3,3?-(3-oxapentane-1,5-diyldioxy)bis(2- hydroxybenzaldehyde) with 1,5-diamine-3-azamethylpentane, forms the complexes [LnNa(L)(Cl)2-(CH3OH)] (Ln = La-Nd, Sm-Lu) where the lanthanide(III) ion resides in the N3O2 Schiff base coordination site and the sodium ion is located in the O3O 2 crown-like cavity. The new heterodinuclear complexes [LnCa(L)(Cl)2(CH3OH)(H2O)]Cl (Ln = Tb, Dy, Tm, Yb) were prepared and characterised by single-crystal X-ray diffraction, IR, 1H and 13C NMR spectroscopy and their homogeneity and stoichiometry (Ln:Ca:Cl = 1:1:3 molar ratio) were ascertained by SEM-EDS analysis. The single-crystal X-ray structure of [YbCa(L)(Cl2)(EtOH)- (H2O)]·Cl·2H2O has been determined. The complex is monoclinic, space group P21/c, with a = 10.033(2), b = 11.702(2), c = 27.796(6) Å and ? = 105.51(3)°. The ytterbium ion is seven-coordinate within the N3O2 site and is in a pentagonal bipyramidal environment and bonded, in the axial positions, to two chloride ions. The calcium ion is seven-coordinate within the O 3O2 site and is bonded to the ethyl alcohol oxygen and to one water molecule. The significant contact between the calcium ion and the chloride ions increases the coordination number of the metal ion to eight in a square antiprismatic environment. The selectivity of the etheric site towards alkali and alkaline-earth metal ions has been quantitatively assessed by evaluating, with 23Na NMR spectroscopy, the transmetallation reaction involving the coordinated Na+ ion in the complexes [LnNa(L)(Cl) 2 (CH3OH)] (Ln = Tb, Dy, Tm and Yb) and Li+, K+ and Ca2+. The affinity constants follow the general order KCa >> KLi >= KK. Quantitative kinetic analyses were performed on the complex [TmNa(L)(Cl)2-(CH 3OH)] using variable temperature 23Na NMR spectroscopy in CD3OD. It was ascertained that the exchange rate Na bound/Nafree is independent of the presence of increasing amounts of water in the methanolic solution.