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 functionalised 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, the synthetic strategy to construct functionalized oligo(1,2,4-triazoles) have 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. Two identical halves of the dinuclear molecules of 1 and 2 are bridged by two deprotonated alkoxy groups from the tri- and tetradentate ligands, respectively. In complex 3, the pending hydroxymethyl group of (HL3)- stays 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 behavior 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 (λex = 310–311 nm), the ligands in solution give rise to strong broad bands centred at near 444 nm with Stokes shifts of ~133 nm. Photoluminescence quenching observed for 1–3 (1 × 10-5 М, DMSO) as compared to the free ligands was rationalized with use of DFT calculations performed with ORCA 6.0.1 program package.