Structural properties of the chelating agent 2,6-bis(1-(3-hydroxypropyl)-1,2,3-triazol-4-yl)pyridine: a combined XRD and DFT structural study

Abstract

The conformational isomerism of the chelating agent 2,6-bis(1-(3-hydroxypropyl)-1,2,3-triazol-4-yl)pyridine (PTD), exploited in fuel reprocessing in spent nuclear waste, has been studied by single crystal X-ray diffraction analysis in combination with an extensive DFT conformational investigation. In the solid-state, the elucidated crystal structure (i.e., not yet published) shows that by thermal treatment (DSC) no other phases are observed upon crystallization from the melt, indicating that the conformation observed by X-ray data is rather stable. Mapping of intermolecular and intramolecular noncovalent interactions has been used to elucidate the unusual arrangement of the asymmetric unit. Considerations relating to the stability of different conformational isomers in aqueous and non-aqueous solutions are also presented. The accurate structural description reported here might open various research topics such as the potential of PTD to act as an outer sphere ligand in the formation of second sphere coordination complexes and their interconversion by mechanochemical means.

Scheme 1. Molecular structure of 2,6-bis(1-(3-hydroxypropyl)-1,2,3-triazol-4-yl)pyridine (PTD) showing the torsion angles among the triazole and pyridine rings.

Fig. 1. Experimental (a) and simulated (b) powder XRD pattern of PTD.

Fig. 2. ORTEP representation of the SC-XRD structure of PTD.

Fig. 3. Map of the intermolecular interactions of one reference PTD molecule with its neighbours. Colour-coding was used to distinguish ‘islands’ representing strong hydrogen bonds (purple), π⋯π (orange), CH⋯π interactions (cyan), and weak CH⋯O and CH⋯N interactions (green).

Fig. 4. Different details of PTD crystal structure. Bright colours are used as visual aid to distinguish between superimposed molecules. Chemical detail is retained where necessary. Dashed purple lines are used to represent hydrogen bonds. (a) View of a stacked dimer along the b axis. (b) View of a stack of hydrogen-bonded PTD units along the c axis. (c) View of the linking region between neighbouring stacks (see caption of Fig. 2 for color-coding).

Fig. 5. Representation of interlinked stacks within ac (a) and bc (b) crystallographic planes. Bright colours are used as visual aid to distinguish between adjacent lamellae. Dashed purple lines are used to represent strong hydrogen bonds. For clarity, one PTD dimer is highlighted in green.

Fig. 6. Torsional potential energy surface for a PTD congener with truncated side chains. Conformers A through D are signaled by circles, squares, diamonds, and triangles, respectively. Labels and corresponding relative electronic energies (kJ mol⁻¹) are reported only once per symmetry-related minimum. Optimized geometries and dihedral angles are reported for quick reference.