Synthesis, crystal structures and NMR and luminescence spectra of lanthanide complexes of 1,4,7,10-tetraazacyclododecane with N-methylene(phenyl)phosphinic acid pendant arms†

Abstract

Complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayltetramethylenetetrakis(phenylphosphinic acid) H₄L¹ with yttrium and a number of lanthanides were synthesized and the crystal structures of Li[LaL¹(H₂O)]·10H₂O and Li[CeL¹(H₂O)]·10H₂O determined. The complexes are isostructural with RSRS configurations at the phosphorus atoms. The ligand is co-ordinated by four nitrogen atoms in a plane and by four phosphinate oxygen atoms in a parallel plane. The planes form a twisted square prismatic co-ordination sphere. A molecule of water capping the O₄ plane completes the co-ordination sphere of both ions. Solution properties of the complexes were investigated by ¹H and ³¹P NMR. In solution six possible isomers with different configurations on phosphorus atoms are in equilibrium. The most abundant shows the RRRS configuration. A dynamic behavior similar to lanthanide complexes of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid is not observed. The NMR as well as luminescence measurements show that no water molecule is co-ordinated directly to the lanthanide ions in solution. The ions Li⁺and Na⁺ form stable ion pairs with the complexes in methanol-d₄ solution as confirmed by the lanthanide induced shift of ⁷Li and ²³Na resonances in the presence of paramagnetic lanthanide ions. The ion pairs are not stable in aqueous solution. The alkali metal ion is located close to fourfold magnetic axes of the complexes above the oxygen atoms and between the phenyl rings. Luminescence spectra of [LnL¹]^–, Ln = Eu or Tb, indicate low symmetry of the species and co-ordination number 8.