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 Oyl 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 Oyl···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.