Tetrahalide Complexes
of the [U(NR)<sub>2</sub>]<sup>2+</sup> Ion: Synthesis, Theory, and
Chlorine K‑Edge X‑ray
Absorption Spectroscopy
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Abstract
Synthetic routes to salts containing uranium bis-imido
tetrahalide
anions [U(NR)<sub>2</sub>X<sub>4</sub>]<sup>2–</sup> (X = Cl<sup>–</sup>, Br<sup>–</sup>) and non-coordinating NEt<sub>4</sub><sup>+</sup> and PPh<sub>4</sub><sup>+</sup> countercations
are reported. In general, these compounds can be prepared from U(NR)<sub>2</sub>I<sub>2</sub>(THF)<sub><i>x</i></sub> (<i>x</i> = 2 and R = <sup><i>t</i></sup>Bu,
Ph; <i>x</i> = 3 and R = Me) upon addition of excess halide.
In addition to providing stable coordination complexes with Cl<sup>–</sup>, the [U(NMe)<sub>2</sub>]<sup>2+</sup> cation also
reacts with Br<sup>–</sup> to form stable [NEt<sub>4</sub>]<sub>2</sub>[U(NMe)<sub>2</sub>Br<sub>4</sub>] complexes. These materials
were used as a platform to compare electronic structure and bonding
in [U(NR)<sub>2</sub>]<sup>2+</sup> with [UO<sub>2</sub>]<sup>2+</sup>. 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 [PPh<sub>4</sub>]<sub>2</sub>[U(N<sup><i>t</i></sup>Bu)<sub>2</sub>Cl<sub>4</sub>] and [PPh<sub>4</sub>]<sub>2</sub>[UO<sub>2</sub>Cl<sub>4</sub>]. 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