Novel hydroxyl-functionalized 1,2,4-triazole-based oligomers: synthetic approach, photoluminescence and coordination behaviour as uranyl chelators

Abstract

The management and recycling of accumulated depleted uranium may benefit from the development of novel organic heterocyclic systems that can be used as ligands for uranyl complexes in advanced materials. Although functionalized 1,2,4-triazoles have shown promise as ligand systems for a variety of transition metals, little is known about their coordination behaviour towards uranyl and the characteristics of the resultant complexes. In the present study, a synthetic strategy to construct functionalized oligo(1,2,4-triazoles) has been developed to prepare mono- (H₂L¹), bis- (H₂L²) and tris(1,2,4-triazole)-based oligomers (H₂L³) bearing hydroxymethyl and hydroxyphenyl substituents. Uranyl complexes [UO₂(L¹)(CH₃OH)]₂·2CH₃OH (1), [UO₂(L²)]₂·4DMF (2) and [UO₂(HL³)(CH₃COO)]·CH₃OH·H₂O (3) were easily synthesized by combining UO₂(CH₃COO)₂·2H₂O with the ligand under suitable conditions. The identical halves of the dinuclear complexes 1 and 2 are bridged by two deprotonated alkoxy groups from the tri- and tetradentate ligands, respectively. In complex 3, the pendant hydroxymethyl group of (HL³)⁻ remains protonated, impeding dimerization of the molecule. Extensive N/O–H⋯O and C–H⋯N/O hydrogen bonding consolidates supramolecular 3D network structures of the uranyl complexes in the solid state. The behaviour of the ligands and complexes in DMSO solution was investigated using ¹H NMR and ¹H DOSY NMR spectroscopy, complemented by DFT calculations. Under excitation with UV light (λ_ex = 310–311 nm), the ligands in solution give rise to strong broad bands centred near 444 nm with Stokes shifts of ∼133 nm. The photoluminescence quenching observed for 1–3 (1 × 10⁻⁵ M, DMSO) as compared to the free ligands was rationalized with the use of DFT calculations performed with the ORCA 6.0.1 program package.