Ionothermal synthesis of octahedral lanthanoid coordination networks exhibiting slow magnetization relaxation and efficient photoluminescence

Abstract

An ionothermal reaction of lanthanoid salts with tetraethyl-p-xylenediphosphonate (tepxdp) in ionic liquids, such as choline chloride and malonic acid, resulted in the formation of three novel lanthanoid–organic coordination networks with the formula [Ln(H₂pxdp)_1.5]_n {Ln = Tb (1), Dy (2) and Ho(3) and H₄pxdp = p-xylenediphosphonic acid}. The structures, photoluminescence and magnetic properties of the three compounds were investigated in detail. Single crystal X-ray diffraction analysis revealed that the three compounds are isostructural and the Ln³⁺ ions show an unusual six-coordinate environment with the {LnO₆} octahedron. In these compounds, each {PO₃C} tetrahedron is corner-shared with two {LnO₆} octahedra and each {LnO₆} octahedron is corner-shared with six {PO₃C} tetrahedra, thus forming an inorganic layer in the crystallographic ab plane. The inorganic layers are further connected by a phenyl group, leading to a three-dimensional framework. Compound 1 exhibits the strong and characteristic emission of Tb^III with an impressive quantum yield of 46.2%. Detailed magnetic analysis demonstrated that compound 2 displays a slow magnetic relaxation of magnetization with multiple relaxation mechanisms. The anisotropic energy barrier and the pre-exponential factor τ₀ are 51.2 K and 3.9 × 10⁻⁷ s, respectively, in the presence of a direct-current field of 500 Oe. This work demonstrates a successful strategy to isolate octahedrally coordinated lanthanoid complexes through ionothermal synthesis to exhibit the single-ion-magnet-like behaviour and photoluminescence properties.