Insight of the metal-ligand interaction in f elements complexes by paramagnetic NMR spectroscopy

Abstract : The magnetic properties of Ln$^{III}$ and An$^{III}$ complexes formed with dipicolinate ligands have been studied by NMR spectroscopy. To know precisely the geometry of these complexes, a crystallographic study by single-crystal X-Ray Diffraction (XRD) and by Extended X-Ray Absorption Fine Structure (EXAFS) in solution was performed. Several separation methods of paramagnetic shifts observed on the NMR spectra were applied to these complexes. Methods using several nuclei of the dipicolinate ligands revealed an abrupt change in the geometry of the complexes and a metal-ligand interaction in the middle of the lanthanide series. The study of paramagnetic shifts with temperature demonstrated that higher order terms in the dipolar and contact contributions are required especially for the lightest Ln$^{III}$ cations and almost all studied An$^{III}$. The Bleaney's parameters $_a$ and C$_a^D$ relating to the contact and dipolar terms respectively were deduced from experimental data and compared to ab-initio calculations. Quite good agreement is found for $_a$ and C$_a^D$ temperature dependences. However C$_a^D$ values obtained from cation magnetic anisotropy calculations lead to some differences with Bleaney equations defined forLn$^{III}$. Other parameters such the crystal field parameter and the hyperfine constants Fi obtained from experimental data with [An(ethyl-DPA)$_3$]$^{3-}$ complexes are at odds with assumptions underlying Bleaney's theory.