Elucidating the electronic origins of a halide-dependent inversion of magnetic anisotropy in tetravalent uranium amidinate complexes.

Recent synthesis and characterization of tetravalent U and Np complexes [AnIVX((S)-PEBA)3] and [AnIVX(iPr2BA)3] (X = F, Cl) bearing the (S,S)-N,N'-bis(1-phenylethyl)benzamidinate ((S)-PEBA) and the N,N'-bis(isopropyl)benzamidinate (iPr2BA) ligands, respectively, have revealed a characteristic inversion of paramagnetic 1H NMR shifts for both [UF(L)3] representatives compared to the [UCl(L)3] compounds. The same effect is not observed for [NpF(L)3] and [NpCl(L)3], with currently no satisfactory explanation for these observations. Using straightforward CASSCF-SOC-NEVPT2 calculations and point-group-optimized geometries, in combination with the Kuprov model approach [G. T. P. Charnock and I. Kuprov, Phys. Chem. Chem. Phys., 2014, 16] for the calculation of pseudo-contact shifts (PCS), the experimentally derived 1H paramagnetic shifts could be reproduced qualitatively and in most cases also quantitatively. The good agreement for most signals allowed for a first evaluation of the contribution of Fermi-contact shifts (FCS) for more strongly deviating data points, which could be supported by unrestricted DFT spin densities and spin populations. To finally gain a deeper understanding as to where the differing magnetic anisotropy for the [UF(L)3] compounds stems from, the magnetic sublevels of the ground atomic multiplet |J,mJ〉 were investigated for all complexes. These revealed that only for the [UF(L)3] compounds a high |mJ〉 (pseudo) doublet showing sufficient isolation is stabilized in the ground state, whose high axiality in the end enables the realization of a prolate χ tensor responsible for the inverse PCS field.