Structural variation within uraniumVI heterocyclic carboxylates: Solid and solution states studies

The solution chemistry and solid-state structures of two new uraniumVI metal–organic compounds, (tataH)2[(UO2)2(pdtc)2(μ-OH)2]·2H2O (1) and (AcrH)2[(UO2)(pydc)2] (2), [where tata = 1,3,5-triazine-2,4,6-triamine, Acr = acridine, pydc = pyridine-2,6-dicarboxylate and pdtc = pyridine-2,6-bis(monothiocarboxylate)] have been investigated. These compounds were obtained via proton-transfer methodology by reacting UO22+ with the preformed proton-transfer systems (tataH)2(pdtc) and (AcrH)2(pydc), respectively, in aqueous solution, after evaporation of the solvent. Our results show that noncovalent interactions such as hydrogen bonds, sulfur···sulfur, and π···π interactions, when acting cooperatively, are driving forces for the selection of different structures. The protonation constants of Acr, tata, pdtcH2, and pydcH2, the building blocks of the proton-transfer systems (tataH)2(pdtc) and (AcrH)2(pydc), and the stability constants of these systems were determined by potentiometric studies in a dioxane/H2O (1:1 v/v) mixture. The stoichiometry and the formation stability constants of complexes formed on reacting pydcH2/Acr or pdtcH2/tata 1:1 molar mixtures with the UO22+ ion in dioxane/H2O (1:1 v/v) were investigated by potentiometric pH titration methods. The stoichiometries of the complex species formed in solution were compared with those observed for the crystalline metal ion complexes 1 and 2.