Formation of Uranium(IV) Oxide Clusters from Uranocene [U(η⁸-C₈H₈)₂] and Uranyl [UO₂X₂] Compounds

Uranocene [U(η8-C8H8)2] reacted in refluxing pyridine with 2 equiv of the uranyl(VI) compound [UO2(OTf)2] or [UO2I2(THF)3] (OTf = O3SCF3; THF = tetrahydrofuran) or 1 equiv of the uranyl(V) complex [UO2(py)2.3K(OTf)2] to afford the hexanuclear uranium(IV) oxide cluster [U6O8(OTf)8(py)8] (1) or [U6O8I8(py)10] (3). Complexes 1 and 3 were easily isolated in good yield because they were deposited as microcrystalline powders with the release of free cyclooctatetraene as a unique byproduct. A similar reaction in THF gave [U6O8(OTf)8(THF)8] (2), whose isolation was impeded by polymerization of the solvent. With complexes [U(η8-C8H8)2] and [UO2(OTf)2] in a molar ratio of 4:3 or in the presence of excess uranocene, only crystals of the mono(η8-C8H8) aggregate [U6O8(η8-C8H8)(OTf)6(py)8] (4) were obtained. The crystal structures of 1·2py, 2·2THF, 3·py, and 4·py were determined. Whereas uranocene and uranyl(VI) compounds are generally viewed as the most robust uranium compounds, they were found to fuse together into hexanuclear assemblages. These studies revealed the unusual four-electron reductive capacity of a uranium(IV) complex, in particular uranocene, which, through the loss of its two C8H8 ligands, induces the activation and reduction of the strong UO bonds of the uranyl moiety.