Uranyl Oxo Silylation Promoted by Silsesquioxane Coordination

The reaction of [UO₂(N(SiMe₃)₂)₂(THF)₂] with 1 equiv of Cy₇Si₇O₉(OH)₃ in THF affords [U(OSiMe₃)₃(Cy₇Si₇O₁₂)] (1) as orange plates in 24% isolated yield. Its X-ray crystal structure reveals three silylated O_yl ligands, confirming the unprecedented conversion of the uranyl ion to a U(VI) silyloxide. We propose that the formation of 1 proceeds through a transient uranyl silsesquioxide intermediate, [{Cy₇Si₇O₁₁(OH)}UO₂], which undergoes rapid oxo silylation by HN(SiMe₃)₂, followed by silyloxy ligand scrambling, to form 1 and the U(VI) bis(silsesquioxane) complex, [U(Cy₇Si₇O₁₂)₂] (3), among other products. The formation of 3 was confirmed by its independent synthesis and comparison of its ²⁹Si{¹H} NMR spectrum with that of the in situ reaction mixture. In contrast to the reaction in THF, the reaction of [UO₂(N(SiMe₃)₂)₂(THF)₂] with Cy₇Si₇O₉(OH)₃ in hexanes, followed by recrystallization from Et₂O/MeCN, results in the formation of the uranyl cluster, [(UO₂)₃(Cy₇Si₇O₁₂)₂(Et₂O)(MeCN)₂] (2), as yellow rods in 42% isolated yield. Complex 2 features two O_yl···U dative interactions, but in contrast to 1, none of its three uranyl fragments are silylated. Overall, the conversion of [UO₂(N(SiMe₃)₂)₂(THF)₂] to 1 and 3 is likely promoted by the strong electron donor ability of the silsesquioxane ligand and suggests that the actinide coordination chemistry of mineral surface mimics, such as silsesquioxane, is a fruitful arena for the discovery of new reactivity.