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^iPr4) ligand to UI₃(1,4-dioxane)_1.5 results in the formation of the bent metallocene uranium(III) complex (Cp^iPr4)₂UI (1), which is then used to obtain the uranium(IV) and uranium(III) dihalides (Cp^iPr4)₂U^IVX₂ (2-X) and [cation][(Cp^iPr4)₂U^IIIX₂] (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 [(Cp^iPr4)₂U(μ-Cl)₂Cs] (4-Cl) and [(Cp^iPr4)₂U(μ-CN)₂Na(OEt₂)₂] (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the "base-free" metallocenium cation salt [(Cp^iPr4)₂U][B(C₆F₅)₄] (5) and its DME adduct [(Cp^iPr4)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 [(Cp^iPr4)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.