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

Homoleptic MOF structures of the formulas (2)(infinity)[Ln(Tz*)(3)] and (3)(infinity)[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 H-1-1,2,3-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 alpha-form and high-temperature beta-form. The thermodynamically stable beta-form exhibits high thermal stabilities regarding compounds that contain three neighboring nitrogen atoms in one organic ring, ranging up to 380 degrees C. Transition from alpha- to beta-form is accompanied by transformation from open to dense frameworks as only the alpha-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 alpha-form adopts the AlCl3 structure, the beta-form can be identified with a (10,3)-b net. Although no single crystals could be obtained for beta-(3)(infinity)[Ln(Tz*)(3)], structure solution and reasonable refinement were successful from Xray powder data. The crystal structure of alpha-(2)(infinity)[Ln(Tz*)(3)] was previously solved from single crystal data.