Update on the investigation of the ligand configuration in Th(IV) N-donor ligand complexes

For the separation of trivalent actinides and lanthanides highly selective ligands are required. Soft N-donor ligands such as bis(triazinyl)pyridines (BTPs) are known for their high selectivity.1, 2 The molecular origin of the selectivity still is a topic of fundamental scientific interest.

To probe the effect of the cation’s nuclear charge on the bonding properties in actinide N-donor complexes, we extended our studies to tetravalent actinides. Here, we report a NMR spectroscopic and theoretical investigation on the complexation of Th(IV) with nPr-BTP. For the first time we observed the formation of different complex species for [Th(nPr-BTP)3](OTf)4 depending on the solvent used. The higher charge compared to An(III) and Ln(III) results in a stronger complex-solvent interaction which leads to slightly different ligand configurations in the complex.3 The ligand configuration highly depends on the solvent’s ability to actively form H-bonds. This correlation was found by preparing [Th(nPr-BTP)3](OTf)4 in a range of deuterated organic solvents as well as in binary solvent mixtures.

The figure above shows the results of the 15N NMR data analysis of [Th(nPr-BTP)3]4+ in THF-d8 (lower spectrum) and methanol-d4 (upper spectrum). ... mehrIn THF-d8, the symmetrical organization of the side chains results in a minimal repulsion between the three BTP ligands. This complex structure was also described for Am(III) and Ln(III) BTP complexes.3 In methanol-d4, the symmetry is lifted due to a strong complex-solvent interaction. The side chains of one 1,2,4-triazinyl ring are organized in a way that prevents a fast solvent exchange resulting in twice as many signals in 1H, 13C and 15N spectra as observed in THF-d8 or for the Am(III) and Ln(III) complexes. The experimental findings are supported by complementary calculations on the density functional theory level probing the effect of the torsion of triazine rings and nPr side chains. A possible torsion of an entire triazine ring as seen for the solvated nPr-BTP complex can be excluded, the calculations show that it is energetically not feasible for the Th complex system.

Summarizing, the results provide valuable information on the bonding mode and bonding differences in tetravalent actinide N-donor complexes compared to the trivalent lanthanide and actinide ions. Furthermore, this study serves as a valuable reference for future NMR studies of paramagnetic tetravalent actinides U(IV), Pu(IV), and Np(IV).

References
1. Panak, P. J.; Geist, A. Chem. Rev., 2013, 113, 1199-1236.
2. Geist A.; Panak, P. J. Solvent Extr. Exch., 2021, 39, 128-151.
3. Adam, C. et al. Dalton Trans., 2013, 42, 14068-14074.