Two [Ln4] molecular rings folded as compact tetrahedra

Abstract

The design of photoactive ligands may provide access to multifunctional molecular nanomagnets. We report here the design and synthesis of a new multitopic chelating ligand, 1,2-bis-(5-(N′-(pyridine-2-yl-methylene)-carbohydrazide)-3-methyl-thien-3-yl)-cyclopentene (H₂L), bearing a photoswitchable spacer and demonstrate that it undergoes reversible photoisomerization in solution. In the presence of the base NaH, H₂L reacts with LnCl₃ salts (Ln = Dy, Tb) to form the supramolecular assemblies H₂O@[Ln₄L₄Cl₄(H₂O)₄] (Ln = Dy, 1; Tb, 2). The single crystal X-ray diffraction data of 1 and 2 reveal that both compounds consist of highly symmetric molecular rings efficiently folded through intramolecular weak interactions in the form of tetrahedra. This arrangement imparts a conformation to the photochromic spacer of the ligand which inhibits its photoswitching activity. Despite the low symmetry of the coordination geometry around the Ln ions, the Dy analog exhibits slow relaxation of the magnetization. The relaxation rate under various conditions has been investigated, allowing us to elucidate the mechanisms involved in this relaxation.