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

Addition of the potassium salt of the bulky tetra(isopropyl)cyclopentadienyl (Cp) ligand to UI(1,4-dioxane) results in the formation of the bent metallocene uranium(III) complex (Cp)UI (), which is then used to obtain the uranium(IV) and uranium(III) dihalides (Cp)UX () and [cation][(Cp)UX] (, [cation] = [Cp*Co], [EtN], or [MeN]) as mononuclear, donor-free complexes, for X = F, Cl, Br, and I. Interestingly, reaction of with chloride and cyanide salts of alkali metal ions leads to isolation of the chloride- and cyanide-bridged coordination solids [(Cp)U(μ-Cl)Cs] () and [(Cp)U(μ-CN)Na(OEt)] (). Abstraction of the iodide ligand from further enables isolation of the "base-free" metallocenium cation salt [(Cp)U][B(CF)] () and its DME adduct [(Cp)U(DME)][B(CF)] (). 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 and 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 [(Cp)UX] uranium(III/IV) redox couple. Magnetic characterization of and reveals that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism.