Synthesis, Structure, and Theoretical Calculations on NpO₂Br₄²

The actinide-halogen complexes (AnO₂X₄^2-, X = Cl, Br, and I) are the simplest and most representative compounds for studying the bonding nature of actinides with ligands. In this work, we attempted to synthesize the crystals of NpO₂X₄^2- (X = Cl, Br, and I). The crystals of NpO₂Cl₄^2- and NpO₂Br₄^2- were successfully synthesized, in which the structure of NpO₂Br₄^2- was obtained for the first time. The crystal of NpO₂I₄^2- could not be obtained due to the rapid reduction of Np(VI) to Np(V) by I^-. The molecular structures of NpO₂Cl₄^2- and NpO₂Br₄^2- were characterized by single-crystal X-ray diffraction and infrared, Raman, and UV-Vis-NIR absorption spectroscopy. The complexes of NpO₂X₄^2- (X = Cl, Br, and I) were also investigated by density functional theory calculations, and the calculated vibration frequencies and absorption features were comparable to the experimental results. Both the experimental results and theoretical calculations demonstrate the strengthened Np-O bonds and the weakened Np-X bonds across the NpO₂X₄^2- series; however, the population analysis on the frontier molecular orbitals (MOs) of NpO₂X₄^2- indicates a slight reduction in the Np-O bonding covalency and an enhancement in the Np-X bonding covalency from NpO₂Cl₄^2- to NpO₂I₄^2-. Results in this work have enriched the crystal database of the AnO₂X₄^2- family and provided insights into the bonding nature in the actinide complexes with soft- and hard-donor ligands.