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Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes

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journal contribution
posted on 2019-11-26, 19:06 authored by Michael A. Boreen, Daniel J. Lussier, Brighton A. Skeel, Trevor D. Lohrey, Fabian A. Watt, David K. Shuh, Jeffrey R. Long, Stephan Hohloch, John Arnold
Addition of the potassium salt of the bulky tetra­(isopropyl)­cyclopentadienyl (CpiPr4) ligand to UI3(1,4-dioxane)1.5 results in the formation of the bent metallocene uranium­(III) complex (CpiPr4)2UI (1), which is then used to obtain the uranium­(IV) and uranium­(III) dihalides (CpiPr4)2UIVX2 (2-X) and [cation]­[(CpiPr4)2UIIIX2] (3-X, [cation]+ = [Cp*2Co]+, [Et4N]+, or [Me4N]+) as mononuclear, donor-free complexes, for X = F, Cl, Br, and I. Interestingly, reaction of 1 with chloride and cyanide salts of alkali metal ions leads to isolation of the chloride- and cyanide-bridged coordination solids [(CpiPr4)2U­(μ-Cl)2Cs]n (4-Cl) and [(CpiPr4)2U­(μ-CN)2Na­(OEt2)2]n (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the “base-free” metallocenium cation salt [(CpiPr4)2U]­[B­(C6F5)4] (5) and its DME adduct [(CpiPr4)2U­(DME)]­[B­(C6F5)4] (5-DME). Solid-state structures of all of the compounds, determined by X-ray crystallography, facilitate a detailed analysis of the effect of changing oxidation state or halide ligand on the molecular structure. NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV–visible spectroscopy studies of 2-X and 3-X further reveal that the difluoride species in both series exhibit properties that differ significantly from trends observed among the other dihalides, such as a substantial negative shift in the potential of the [(CpiPr4)2UX2] uranium­(III/IV) redox couple. Magnetic characterization of 1 and 5 reveals that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism.

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