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)(2)UI (1), which is then used to obtain the uranium(IV) and uranium(III) dihalides (Cp-iPr4)(2)(UX2)-X-IV (2-X) and [cation]-[(Cp-iPr4)(2)(UX2)-X-III] (3-X, [cation](+) = [Cp*Co-2](+), [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)(2)U(mu-Cl)(2)Cs](n) (4-Cl) and [(Cp-iPr4)(2)U(mu-CN)(2)Na(OEt2)(2)](n) (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the base-free metallocenium cation salt [(Cp-iPr4)(2)U][B(C6F5)(4)] (5) and its DME adduct [(Cp-iPr4)(2)U(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)(2)UX2] 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.