Sandwich-type lanthanide(III) complexes of a tetraiminodiphenol macrocyclic ligand derived from 4-methyl-2,6-diformylphenol and 1,3-diaminopropane: structure, NMR and luminescence

Abstract

The macrocyclic ligand salt [H₂LH₂](ClO₄)₂, derived from the [2+2] condensation process involving 4-methyl-2,6-diformylphenol and 1,3-diaminopropane, has been used to synthesize the sandwich-type lanthanide(III) complexes [Ln(LH₂)₂](ClO₄)₃•2H₂O 16, Ln = La–Eu, except Pm), [Ln(LH₂)₂](ClO₄)₃•CH₃CN 7 13, Ln = Gd–Lu, except Er) and [Ln(LH₂)(LH)](BPh₄)₂ ( 14– 16, Ln = La, Nd and Sm). The X-ray crystal structures of the compounds 1, 7, 12, 13, 14 and 15 have been determined. In the isostructural [Ln(LH₂)₂](ClO₄)₃•CH₃CN complexes of Gd (7), Yb (12) and Lu (13), the two macrocyclic ligands are considerably folded, albeit to different extents, and not symmetrically bound to the metal center. In these compounds, the metal center is coordinated with two imine nitrogen atoms and two phenolate oxygen atoms of each of the ligands (LH₂), whose two other uncoordinated imine nitrogens are protonated and intramolecularly hydrogen-bonded with the phenolate oxygens. The structures determined for the La (14) and Nd (15) complexes of [Ln(LH₂)(LH)](BPh₄)₂ are found to be identical. In these complexes, the ligand LH contains one N–H⋯O hydrogen-bonded unit, as against two in the ligand LH₂. The [Ln(N₂O₂)₂] coordination polyhedron in all the cases is a distorted square antiprism, although the extent of distortion in 7, 12 and 13 is less compared to that in 14 and 15. ¹H NMR spectra of the [Ln(LH₂)₂]³⁺complexes in solution indicate a C₂ symmetry. The ¹H and ¹³C NMR signals of the Eu^III complex 6 have been assigned on the basis of COSY, DEPT, HMQC and NOESY spectra. The photoluminescence spectra of the Sm(III) (5) and Eu(III) (6) complexes have been studied in a methanol–ethanol (1:4) glassy matrix at 77 K. The luminescence intensity of the hypersensitive ⁵D₀ → ⁷F₂ transition in 6 increases with the addition of triethylamine due to the formation of [Eu(LH₂)(LH)]²⁺ and [Eu(LH)₂]⁺ species, which in turn partially attenuate the quenching effect of the N–H⋯O oscillators on the emitting states of the metal ion.