Rationally assembled nonanuclear lanthanide clusters: Dy9 displays slow relaxation of magnetization and Tb9 serves as luminescent sensor for Fe3+, CrO42− and Cr2O72−

Abstract

The utilization of a Schiff base ligand 2-((2-hydroxy-4-methoxy-benzylideneamino)methyl)phenol (H₂L) afforded five nonanuclear lanthanide(III) clusters of formula [Ln₉L₈(μ₃-OH)₈(μ₅-O)₂(DMF)₈][H₃O]·nH₂O (Ln = Eu, n = 1 (1[H₃O]·H₂O); Ln = Gd, n = 4 (2[H₃O]·4H₂O); Ln = Tb, n = 3 (3[H₃O]·3H₂O); Ln = Dy, n = 6 (4[H₃O]·6H₂O); Ln = Ho, n = 2 (5[H₃O]·2H₂O)). Single-crystal X-ray diffraction analysis revealed that all of the complexes were isomorphous and had a diabolo-like topology. The intriguing Ln₉ core was constructed by two square pyramidal units that shared a vertex metal centre. Variable-temperature dc magnetic susceptibility studies indicated weak antiferromagnetic interactions for 2[H₃O]·4H₂O, 3[H₃O]·3H₂O, 4[H₃O]·6H₂O and 5[H₃O]·2H₂O. The Dy^III₉ cluster displayed a slow relaxation of magnetization. Fluorescence measurements revealed an intense green photoluminescence for the Tb^III₉ compound. The luminescence tests showed that the Tb^III₉ analogue could perform as a highly sensitive, selective and recyclable luminescence sensing material for Fe³⁺, CrO₄²⁻ and Cr₂O₇²⁻. In this work, the H₂L ligand was derived by finely modifying the previously designed compounds (((2-hydroxy-3-methoxybenzyl)imino)methyl)-6-ethoxyphenol and (((2-hydroxy-3-methoxybenzyl)imino)methyl)-6-methoxyphenol, from which the Ln₂ and Ln₄ clusters were successfully prepared and reported. Noticeably, the decisive decrease in the steric hindrance of the predesigned ligands resulted in the expansion of the nuclearity. This work demonstrated the synthetic potential for assembling the large clusters by rationally decreasing the steric hindrance of the analogous ligands.