Slow Magnetic Relaxation and Luminescent Properties of Mononuclear Lanthanide-Substituted Keggin-Type Polyoxotungstates with Compartmental Organic Ligands

Abstract

The reaction of mid to late lanthanide ions with the <i>N</i>,<i>N</i>'-dimethyl-<i>N</i>,<i>N</i>'-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylene-diamine organic ligand and monolacunary Keggin type [α-SiW₁₁O₃₉]^8- anion affords a series of isostructural compounds, namely, K₅[Ln^III(α-SiW₁₁O₃₉)(C₂₀H₂₂Br₂N₂O₄)]·14H₂O (<b>1</b>-Ln, Ln = Sm to Lu). The molecular structure of these sandwich-type complexes is formed by the Ln^III ion in a biaugmented trigonal prismatic geometry, which occupies the external O₄ site of the organic ligand and the vacant site of the lacunary polyoxometalate (POM) unit. The empty N₂O₂ coordination site of the organic ligand allows its unprecedented folding, which displays a relative perpendicular arrangement of aromatic groups. Weak Br···Br and π-π interactions established between adjacent molecular units govern the crystal packing, which results in the formation of assemblies containing six hybrid species assembled in a chairlike conformation. <b>1</b>-Gd and <b>1</b>-Yb display slow relaxation of the magnetization after the application of an external magnetic field with maxima in the out-of-phase magnetic susceptibility plots below ∼5-6 K, which is ascribed to the presence of various relaxation mechanisms. Moreover, photoluminescent emission is sensitized for <b>1</b>-Sm and <b>1</b>-Eu in the visible region and <b>1</b>-Er and <b>1</b>-Yb in the NIR. In contrast, the quenching of metal-centered luminescence in the <b>1</b>-Tb derivative has been attributed to the out-of-pocket coordination mode of the lanthanide center within the POM fragment. It is demonstrated that the <b>1</b>-Yb dual magneto-luminescent material represents the first lanthanide-containing POM reported to date with simultaneous slow magnetic relaxation and NIR emission. Solution stability of the hybrid molecular species in water is also confirmed by ESI-mass spectrometry experiments carried out for <b>1</b>-Tb and <b>1</b>-Tm.