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

The asymmetric macrocyclic compartmental ligand H 2 L, 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 -(CH 3 OH)] (Ln = La-Nd, Sm-Lu) where the lanthanide(III) ion resides in the N 3 O 2 Schiff base coordination site and the sodium ion is located in the O 3 O 2 crown-like cavity. The new heterodinuclear complexes [LnCa(L)(Cl) 2 (CH30H)(H20)]Cl (Ln = Tb, Dy, Tm, Yb) were prepared and characterised by single-crystal X-ray diffraction, IR, 1 H and 1 3 C 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)-(H 2 O)].Cl.2H 2 O has been determined. The complex is monoclinic, space group P2 1 /c, with a = 10.033(2), b = 11.702(2), c = 27.796(6) A and β = 105.51(3)°. The ytterbium ion is seven-coordinate within the N 3 O 2 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 3 O 2 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 2 3 Na NMR spectroscopy, the transmetallation reaction involving the coordinated Na + ion in the complexes [LnNa(L)(Cl) 2 (CH 3 OH)] (Ln = Tb, Dy, Tm and Yb) and Li + , K + and Ca 2 + . The affinity constants follow the general order K C a >> K L i ≥ K K . Quantitative kinetic analyses were performed on the complex [TmNa(L)(Cl) 2 -(CH 3 OH)] using variable temperature 2 3 Na NMR spectroscopy in CD 3 OD. It was ascertained that the exchange rate Na b o u n d /Na f r e e is independent of the presence of increasing amounts of water in the methanolic solution.