Synthesis of lanthanide(III) complexes of a 20-membered hexaaza macrocycle

Abstract

Lanthanide(III) and yttrium(III) complexes of the 20-membered hexaaza Schiff-base macrocycle 6,20-dimethyl-2,10,16,24,29,31-hexaazapentacyclo[23.3.1.1^4,8.1^11,15.1^18,22]dotriaconta-1(29),2,4,6,8(30),9,11,13,15(31),16,18,20,22(32),23,25,27-hexadecaene-30,32-diol (H₂L³) have been synthesised by metal-template condensation of 2,6-diformyl-4-methylphenol and 2,6-diaminopyridine: [Pr(H₂L³)(NO₃)₂(H₂O)₃]NO₃·3H₂O, [Ln(HL³)(NO₃)(H₂O)₃]NO₃·4H₂O (Ln = La³⁺, Nd³⁺, Sm³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺, Er³⁺ or Tm³⁺), [Eu(HL³)(NO₃)(H₂O)₂]NO₃·5H₂O and [Y(HL³)(NO₃)(H₂O)₃]NO₃·5H₂O. The yield of the complexes vary from 93 to 75%(for La³⁺–Er³⁺ and Y³⁺) to 50%(for Tm³⁺). The diminution in the ionic radii of the lanthanide(III) cations does not affect their template potential. The metal ions are bound to the macrocycle and to one bidentate chelating nitrate ion (two for Pr³⁺) and to two or three water molecules. The complexes are stable in air in the solid state and are inert to release of metal ions in dimethylformamide. The exocyclic ligands are substitution labile and undergo anion metathesis when treated with ClO₄^– or SCN^–. The complexes are thermally stable and the macrocycle remains intact with the metal ions up to 338 °C. The essential requisite for synthesis of the complexes of these metal ions in the same ligand framework seems to be the flexibility of the latter. The template potential of the lanthanide(III) cations of varying sizes in the H₂L³ assembly is due to the flexibility of the macrocycle to adapt to the geometrical requirements of the metal ions and to the steric demands of the exocyclic ligands.