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

The reaction of mid to late lanthanide ions with the N , N '-dimethyl- N , N '-bis(2-hydroxy-3-formyl-5-bromobenzyl)ethylene-diamine organic ligand and monolacunary Keggin type [α-SiW 11 O 39 ] 8- anion affords a series of isostructural compounds, namely, K 5 [Ln III (α-SiW 11 O 39 )(C 20 H 22 Br 2 N 2 O 4 )]·14H 2 O ( 1 -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 O 4 site of the organic ligand and the vacant site of the lacunary polyoxometalate (POM) unit. The empty N 2 O 2 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. 1 -Gd and 1 -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 1 -Sm and 1 -Eu in the visible region and 1 -Er and 1 -Yb in the NIR. In contrast, the quenching of metal-centered luminescence in the 1 -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 1 -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 1 -Tb and 1 -Tm.