Metal-organic complexes of tetravalent actinides with soft-donor ligands investigated by paramagnetic NMR spectroscopy

In NMR spectra of paramagnetic metal-organic complexes electronic interactions are the origin of additional NMR chemical shifts observed on resonances of nuclei of the ligand. The major two contributors to these hyperfine shifts are Fermi-contact shifts (FCS) and pseudo-contact shifts (PCS). FCS are due to delocalisation of unpaired electron density in molecular orbitals involving both metal and ligand orbitals and thus report on the bond properties. PCS are originating from distance- and angle-dependent dipolar coupling of electron spins through space and are therefore bearing structural information.
The paramagnetic contributions can be mathematical separated provided that a suitable diamagnetic reference is available (to subtract non-paramagnetic contributions). For the trivalent actinides no diamagnetic reference in the same series is available in milligram scale. 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.[1,2]
Comparing studies of isostructural diamagnetic complexes of both f-element series of tetravalent metal ions (Ce(IV) and Th(IV)) allow for an estimation of additional influences to the chemical shifts and the effect of contributions usually omitted by commonly used mathematical theories.
We started to study paramagnetic metal-organic complexes of the tetravalent actinides (An(IV)). Throughout the 5f-series additional effects to the observed chemical shift are expected with increasing number of unpaired electrons. Assessing the chemical bonding situation is possible via the influences on NMR chemical shifts (via FCS) and structural properties of the complexes (via PCS) can be compared to SC-XRD structures. Herein we report the first results of investigations of N- and N,O-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).