Hydrolysis and Aggregation of Uranyl Ions in Triazole-Based Ligand Systems

Abstract Reactions of aqueous uranyl salt solutions in a low-melting cocrystal of 4-NH2-1,2,4-triazole (4-NH2-1,2,4-Triaz) and 1,2,3-triazole (1,2,3-Triaz), serving simultaneously as solvent and ligand source, or ultimately in neat 4-NH2-1,2,4-Triaz, afforded a diverse series of anhydrous coordination compounds and hydrolysis-derived uranyl assemblies of increasing structural complexity. Here we report the crystallographic analysis of these products, including three anhydrous coordination complexes [UO2(NO3)2(1,2,3-Triaz)2] (C1), [UO2(NO3)2(4-NH2-1,2,4-Triaz)2] (C2), [UO2Cl2(4-NH2-1,2,4-Triaz)3] (C3); a hydrolyzed one dimensional coordination polymer [(UO2)(μ2-OH)(μ2-4-NH2-1,2,4-Triaz)(4-NH2-1,2,4-Triaz)]n[ClO4]n (1DCP); tetramers: [(UO2)4(μ3-O)2(μ2-OH)2(μ2-4-NH2-1,2,4-Triaz)2(4-NH2-1,2,4-Triaz)4(OH2)2][ClO4]2 (T1), [(UO2)4(μ3-O)2(μ2-OH)2Cl2(μ2-4-NH2-1,2,4-Triaz)2(4-NH2-1,2,4-Triaz)2(OH2)2]·2H2O (T2), [(UO2)4(μ3-O)2(μ2-Cl)4(4-NH2-1,2,4-Triaz)6] (T3), including a polymer of tetramers [HC1im]2n[(UO2)4(μ2-Cl)6(μ3-O)2(1,2,4-Triaz)2]n (T4CP); and hexamers [(UO2)6(μ3-O)4(μ2-OH)2(μ2-4-NH2-1,2,4-Triaz)4(4-NH2-1,2,4-Triaz)4(OH2)2][ClO4]2 (H1), [(UO2)6(μ3-O)4(μ2-OH)2(μ2-4-NH2-1,2,4-Triaz)4(4-NH2-1,2,4-Triaz)4(OH2)2][ClO4]2·2H2O (H2). These results demonstrate that 4-NH2-1,2,4-Triaz is able to stabilize both discrete, non-hydrolyzed uranyl coordination complexes and hydrolysis-derived oxo/hydroxo-bridged uranyl assemblies ranging from simple mononuclear complexes to one-dimensional coordination polymers and oligonuclear clusters, highlighting the ability of the triazole ligand environment to accommodate and potentially direct the progressive hydrolysis and condensation chemistry of U(VI).