Tetrahalide Complexes of the [U(NR)2]2+ Ion: Synthesis, Theory, and Chlorine K‑Edge X‑ray Absorption Spectroscopy

Synthetic routes to salts containing uranium bis-imido tetrahalide anions [U­(NR)2X4]2– (X = Cl–, Br–) and non-coordinating NEt4+ and PPh4+ countercations are reported. In general, these compounds can be prepared from U(NR)2I2(THF)x (x = 2 and R = tBu, Ph; x = 3 and R = Me) upon addition of excess halide. In addition to providing stable coordination complexes with Cl–, the [U­(NMe)2]2+ cation also reacts with Br– to form stable [NEt4]2[U­(NMe)2Br4] complexes. These materials were used as a platform to compare electronic structure and bonding in [U­(NR)2]2+ with [UO2]2+. Specifically, Cl K-edge X-ray absorption spectroscopy (XAS) and both ground-state and time-dependent hybrid density functional theory (DFT and TDDFT) were used to probe U–Cl bonding interactions in [PPh4]2[U­(NtBu)2Cl4] and [PPh4]2[UO2Cl4]. The DFT and XAS results show the total amount of Cl 3p character mixed with the U 5f orbitals was roughly 7–10% per U–Cl bond for both compounds, which shows that moving from oxo to imido has little effect on orbital mixing between the U 5f and equatorial Cl 3p orbitals. The results are presented in the context of recent Cl K-edge XAS and DFT studies on other hexavalent uranium chloride systems with fewer oxo or imido ligands.