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). ... mehrBy using 13C and 15N NMR spectroscopy, we have found slight differences in the Ln(III)-N and Am(III)-N interaction. In 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.

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