Investigation of the Ligand Configuration in Th(IV) N-Donor Complexes

For the separation of trivalent actinides and lanthanides highly selective ligands are required. Soft N-donor ligands like bis(triazinyl)pyridines (BTPs) are known for their high separation factors. Though several investigations on the interaction of actinides with these ligands are available, the molecular origin of the selectivity remains largely unclear and a topic of fundamental scientific interest. NMR spectroscopy is a valuable tool to determine interactions between metal ions and donor ligands. For actinides and lanthanides, the interactions are dominated by electrostatic interaction. However, due to the overlap of frontier orbitals, the interaction is also partially covalent, resulting in a change of electron density distribution in the ligand. This corresponds directly to the change of the local magnetic field and is therefore precisely observable by the chemical shift. It was found that the fraction of covalency in the metal nitrogen bond in Am(III) complexes is significantly higher than in similar weak paramagnetic lanthanide complexes [1].
In the present work, we performed NMR investigations on diamagnetic Th(IV) complexes with nPr-BTP and iPr-BTP. ... mehrFor the first time, we observed different ligand configurations for [Th(IV)(BTP)3](OTf)4 depending on the solvent that was used. Compared to Ln(III)/Am(III) the higher charge results in a stronger complex-solvent interaction which leads to a cis/trans isomerisation of the 1,2,4-triazinyl rings depending on the solvent polarity. In polar aprotic solvents, the nitrogen atoms in 2-position of both 1,2,4-triazinyl rings interact with the metal ion (cis), resulting in exactly the same number of signals in the NMR spectra as observed for Ln(III)/Am(III) complexes. In contrast, NMR spectra taken in polar protic solvents show twice as many signals indicating that one 1,2,4-triazinyl ring flips and binds via the nitrogen in 4-position (trans).
These results provide valuable information on the bonding mode and bonding differences in lanthanide/actinide N-donor complexes, which are expected to be the driving force of the ligands’ selectivity.
[1] Adam et al. Dalton Trans. 2013, 42, 14068.