Exploring the Role of Neutral 4‑Amino-1,2,4-triazole in the Formation of Hexanuclear f‑Element Hydrolysis Products

Our recent observations of an unexpected Ce­(III) hydrolysis product from the reaction of 4-amino-1,2,4-triazole (4-NH2-1,2,4-Triaz) with CeCl3·7H2O, [Ce6(μ3-O)4(μ3-OH)2(μ3-Cl)2(Cl)6(μ2-4-NH2-1,2,4-Triaz)12]·7H2O, the first high-nuclearity lanthanide complex where all Ln atoms are connected pairwise through 12 N-donor ligands or 12 neutral bridging ligands of any type, prompted us to explore the utility of this ligand in trapping additional f-element examples. Reactions of LnCl3·6H2O (Ln = Nd, Eu, Ho) with a large excess of 4-NH2-1,2,4-Triaz (20 equiv) and with the addition of small amounts of water to help solubilize the metal salts led to the isolation of the unique hydrolysis products [Nd6(μ3-OH)8Cl6(μ2-4-NH2-1,2,4-Triaz)12]­[Cl4]·2H2O, [Eu6(μ6-Cl)0.23(μ3-O0.77)4(μ3-O)2.6(μ3-Cl)0.4Cl6(μ2-4-NH2-1,2,4-Triaz)12], and [Ho6(μ6-Cl)0.21(μ3-O0.79)4(μ3-OH)2Cl6(μ2-4-NH2-1,2,4-Triaz)12]­[Cl]3.4. We also report a Ce­(III) analogue prepared in glassware contaminated with Pb­(OAc)2, namely, [Ce6(μ3-OH)8(BrPbBr5)­(μ2-4-NH2-1,2,4-Triaz)11.5(OH2)6]­[Pb0.84Br4.2]­[Br]3.8·2­(4-NH2-1,2,4-Triaz)·3.6H2O. The Nd­(III) complex is the structurally most ordered with a clear [Nd6(μ3-OH)8] cluster core, while the Eu­(III) and Ho­(III) compounds contain partial occupancy of a μ6 position and thus result in an incomplete Ln6O9 cluster core formation. The crystallographic results suggest that the 4-NH2-1,2,4-Triaz ligand brings Ln­(III) ions together, followed by the formation of an Ln6O8 or Ln6O9 core with whatever remaining anions or ligands can be incorporated. Given the complexity of the hydrolysis products of nuclear waste, we expect to continue to find a myriad of closely related complex structures of these types for the f-elements.