Polymorphism of isotypic series of homoleptic 1,2,3-triazolate MOFs [Ln(Tz*)₃] containing the heavy lanthanides Gd–Lu: from open to dense frameworks of varying dimensionality

Abstract

Homoleptic MOF structures of the formulas ²_∞[Ln(Tz*)₃] and ³_∞[Ln(Tz*)₃], (Tz*)^– = 1,2,3-triazolate anion, C₂H₂N₃^–, Ln = Gd-Lu, were obtained by the reaction of the lanthanide metals with the aromatic heterocyclic amine 1H-1,2,3-triazole. The isotypic series of compounds are real polymorphs as they have identical constitution. The 2-dimensional MOF is formed at lower temperatures whereas formation of the 3-dimensional MOF requires higher temperatures, in some cases solvothermal reaction conditions. We therefore address them as low-temperature α-form and high-temperature β-form. The thermodynamically stable β-form exhibits high thermal stabilities regarding compounds that contain three neighboring nitrogen atoms in one organic ring, ranging up to 380 °C. Transition from α to β-form is accompanied by transformation from open to dense frameworks as only the α-form contains structural voids. This goes along with both an increase of dimensionality of the linkage from 2D to 3D and an increase of the density of the structures. The α-form adopts the AlCl₃ structure, the β-form can be identified with a (10,3)-b net. Although no single crystals could be obtained for β-³_∞[Ln(Tz*)₃], structure solution and reasonable refinement were successful from X-ray powder data. The crystal structure of α-²_∞[Ln(Tz*)₃] was previously solved from single crystal data.