NMR investigations of paramagnetic effects in metal-organic complexes of trivalent and tetravalent actinides with soft-donor ligands


NMR investigations of paramagnetic effects in metal-organic complexes of trivalent and tetravalent actinides with soft-donor ligands

Radoske, T.; Adam, C.; Schöne, S.; Patzschke, M.; März, J.; Kaden, P.

When NMR spectroscopy is applied to paramagnetic metal-organic complexes additional chemical shifts are observed on nuclei of the ligands that originate from electronic interactions between metal and ligand. The major two contributors to these paramagnetic chemical shifts are either due to delocalisation of unpaired electron density in molecular orbitals involving both metal and ligand orbitals (Fermi contact shift, FCS), or due to distance- and angle-dependent dipolar coupling of electron spins through space (pseudo contact shift, PCS). However, mathematical models for the treatment of paramagnetic chemical shifts are not yet applicable to actinide compounds.
Covalence is assumed to be the reason for some soft-donor ligands selectivity for the complexation of trivalent actinides over lanthanide ions. This long-kept notion was recently substantiated by evaluation of paramagnetic chemical shifts of respective Am(III) complexes1,2. The mathematical separation of contributions in complexes of the trivalent actinides, however, is hampered by the lack of a reliable diamagnetic reference in the actinide series. Furthermore, all available theories behind mathematical disentangling of contributions to the paramagnetic chemical shift, even for the lanthanide series, omit the influence of spin-orbit effects that might have a sizeable contribution as well.
To assess the chemical bonding situation via the influences on paramagnetic chemical shifts we started to study metal-organic complexes of tetravalent actinides (An(IV)) with soft-donor ligands with Th(IV) as diamagnetic reference. With increasing number of unpaired electrons throughout the series additional effects to the observed chemical shift are expected. Herein we report the first results of investigations of N-donor ligand complexes of the An(IV) series.
References
1. C. Adam, P. Kaden, B. B. Beele, U. Müllich, S. Trumm, A. Geist, P. J. Panak, M. A. Denecke, “Evidence for covalence in a N-donor complex of americium(III)”, Dalton Trans., 42, 14068-14074 (2013).
2. C. Adam, B. B. Beele, A. Geist, U. Müllich, P. Kaden, P. J. Panak, “NMR and TRLFS studies of Ln(III) and An(III) C5-BPP complexes”, Chemical Science, 6, 1548-1561 (2015).

Keywords: NMR; paramagnetic; diamagnetic reference; covalence; Fermi contact; pseudo-contact; U(IV); Th(IV); Am(III)

  • Lecture (Conference)
    Actinides 2017, 09.-14.07.2017, Sendai, Japan

Permalink: https://www.hzdr.de/publications/Publ-25860
Publ.-Id: 25860