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

Abstract

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)_<i>x</i> (<i>x</i> = 2 and R = ^<i>t</i>Bu, Ph; <i>x</i> = 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(N^<i>t</i>Bu)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