posted on 2019-11-26, 19:06authored byMichael
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.