Spectroscopic Investigation of the Covalence in An(III) Complexes with Picolindiamides

N-donor and O-donor ligands form strong, ninefold coordinated complexes with trivalent actinide and lanthanide ions.$^{[1, 2]}$ These excellent coordination properties make them suitable for the application as extractants and are interesting systems to study from a fundamental scientific perspective. Extraction studies have shown that N-donor ligands (e.g. nPr-BTP) are suitable for the selective separation of An(III) from Ln(III).$^{[3, 4]}$ The selectivity stems from a different metal-ligand interaction and thus leads to significant differences in complex stability between the Ln(III) and An(III) complexes.$^{[5-7]}$ In O-donor complexes (e.g. TODGA), stability constants and extraction properties are quite similar.$^{[8, 9]}$ In conclusion, the An(III)-O and Ln(III)-O binding properties are expected to be equivalent.

Considering this, the question arises how the metal-ligand interaction changes if structural features of N- and O-donor ligands are combined. For that reason, we present a spectroscopic study of the complexation of Ln(III) and Am(III) with the N,O-donor ligand N,N,N',N'-tetraethyl-2,6-carboxamidopyridine (Et- Pic). By using 13C and 15N NMR spectroscopy, we have found slight differences in the Ln(III)-N and Am(III)-N interaction. ... mehrIn contrast, the Ln(III)-O and Am(III)-O bond in the [M(Et-Pic)$_3$]$^{3+}$ (M = Ln, Am) complex shows similar binding properties. In addition, we have observed significant differences in complex stability constants by using time-resolved laser fluorescence spectroscopy (TRLFS). TRLFS results show that Cm(III) forms a stronger 1:3 complex by one order of magnitude compared to the respective Eu(III) complex. Therefore, Et-Pic shows a minor selectivity towards An(III) ions. The data suggests that the observed selectivity descends solely from an increased partial covalent interaction in the An(III)-N bond.

References
1. P. J. Panak and A. Geist, Chem. Rev., 2013, 113, 1199-1236.
2. A. Geist and P. J. Panak, Solvent Extraction and Ion Exchange, 2021, 39, 128-151.
3. Z. Kolarik, U. Müllich and F. Gassner, Solvent Extr. Ion Exch., 1999, 17, 1155-1170.
4. N. L. Banik, M. A. Denecke, A. Geist, G. Modolo, P. J. Panak and J. Rothe, Inorg. Chem. Commun., 2013, 29, 172-174.
5. M. A. Denecke, A. Rossberg, P. J. Panak, M. Weigl, B. Schimmelpfennig and A. Geist, Inorg. Chem., 2005, 44, 8418-8425.
6. C. Adam, P. Kaden, B. B. Beele, U. Müllich, S. Trumm, A. Geist, P. J. Panak and M. A. Denecke, Dalton Trans., 2013, 42, 14068-14074.
7. C. Adam, B. B. Beele, A. Geist, U. Müllich, P. Kaden and P. J. Panak, Chem. Sci., 2015, 6, 1548-1561.
8. A. Geist, U. Müllich, D. Magnusson, P. Kaden, G. Modolo, A. Wilden and T. Zevaco, Solvent Extr. Ion Exch., 2012, 30, 433-444.
9. A. Wilden, G. Modolo, S. Lange, F. Sadowski, B. B. Beele, A. Skerencak-Frech, P. J. Panak, M. Iqbal, W. Verboom, A. Geist and D. Bosbach, Solvent Extr. Ion Exch., 2014, 32, 119-137.