Hydrotris(mesitylpyrazol-1-yl)borate uranium(IV) compounds: synthesis, structure, and ligand isomerization

Reaction of UCl4 with one equivalent of the thallium salt of [HB(3-Mspz)(3) ](-) (=Tp(Ms), Ms=mesityl) afforded UCl(3)Tp(Ms*) 1, due to isomerization o f the Tp(Ms) ligand to give [HB(3-Mspz)(2)(5-Mspz)](-) (=Tp(Ms*)). Crystals of UCl(3)Tp(Ms) 2 were obtained fortuitously in one particular instance, b ut a real synthetic route to this compound was not achieved. 1 adds THF to afford UCl(3)Tp(Ms*). THF 3. Derivatization of 1 allowed the synthesis of U Cl2[N(SiMe3)(2)]Tp(Ms*) 4 and UCl2[C6H4CH2NMe2-o]Tp(Ms*) 5. The X-ray analy sis of 1, 2 and 4 revealed that the uranium centre is in an octahedral conf iguration, while in 5 the uranium is seven-coordinated by an additional U-N donor bond due to the chelating nature of the hydrocarbyl ligand. Nitriles and isocyanides do not insert into the U-C bond of 5, but reaction with ac etone leads to formation of the uranium aldolate UCl2[eta (2)-OC(Me-2)(2)CH 2C(O)Me]Tp(Ms*) 6.