A Comparison Of Two Uranium Oxo Dimers Featuring Diamond Core Geometry
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Barker, Tiffany
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Abstract
Two bis-(µ-O)2 uranium dimers [(μ-O)2(UV(NPC)3)2] (A, NPC [NP(tBu[pyrr]2]-) and [(μ-O)2(UIV(NPC)3)2 [K(THF)x]2, (B) were synthesized and structurally characterized by single-crystal X-ray diffraction (SC-XRD), spectroscopically analyzed via nuclear magnetic resonance (NMR), Fourier-transfer infrared (FTIR), and ultraviolet-visible/near-infrared (UV-Vis/NIR) spectroscopies, and electrochemically analyzed via cyclic voltammetry (CV). Both compounds feature a diamond-shaped inner core geometry that, while common for transition metal dimers, is less prevalent in uranium chemistry. All bond lengths and angles for the diamond cores of A and B are in good agreement with reported data for previously published diamond-core uranium complexes. The UV-Vis spectrum of A exhibited a broad charge transfer feature at ~300 nm that tails into the visible region with ε ≈ 29000 M-1∙cm-1. Compound B showed also showed a charge transfer feature at 310 nm that tailed into the visible region that was comparatively weaker with ε ≈ 8686 M-1∙ cm-1. B showed several f→f transitions in the near-IR spectrum as well, with ε ranging from ~45 M-1∙cm-1 to ~76 M-1∙cm-1. The voltammogram of A exhibited two reversible redox events at E1/2 = -1.15 V, and -0.47 V vs. Fc/Fc+ and a quasireversible event at -2.4 V vs. Fc/Fc+. The difference between these two potentials was used to estimate the comproportionation constant, Kc, giving an approximate value of 3.13 x 1011, suggesting strong electronic communication between the central uranium atoms. Cyclic voltammetry of B was drastically different with one quasireversible event at E1/2 = -1.25 V and one irreversible event at +0.26 V vs. Fc/Fc+. Compound B was also electrochemically unstable and decomposed rapidly during scanning. Magnetic measurements for A displayed antiferromagnetic behavior with a Néel temperature of 40 K, a moderate value compared to other similar compounds who typically have Néel temperatures of ~15 – 70 K. Compound B exhibited paramagnetic behavior, and neither compound showed any evidence of magnetic hysteresis.
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2023-12-18
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