Electronic interactions between metal and ligand are the origin of additional NMR chemical shifts observed on nuclei of the ligand in paramagnetic metal-organic complexes. The major two contributors to these paramagnetic chemical 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 mathematical separation of paramagnetic contributions in complexes relies on the availability of a suitable diamagnetic reference 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 as well.[1,2]
Comparing isostructural diamagnetic complexes of both f-element series of tetravalent metal ions (Ce(IV) and Th(IV)) allows for an estimation of additional influences to the chemical shifts and the effect of contributions usually omitted by commonly used mathematical theories.
To assess the chemical bonding situation via the influences on NMR chemical shifts (via FCS) we started to study paramagnetic metal-organic complexes of the tetravalent actinides (An(IV)). With increasing number of unpaired electrons throughout the 5 f-series additional effects to the observed chemical shift are expected. Structural properties of the complexes as derived from PCS contributions can be compared to single crystal X-ray diffraction structures. Herein we report the first results of investigations of N- and N,O-donor ligand complexes of the An(IV) series.
1 C. Adam, P. Kaden, B. B. Beele, U. Müllich, S. Trumm, A. Geist, P. J. Panak, M. A. Denecke, Dalton Trans., 42, 14068-14074 (2013).
2. C. Adam, B. B. Beele, A. Geist, U. Müllich, P. Kaden, P. J. Panak, Chemical Science, 6, 1548-1561 (2015).