A Metallocene Bis(phosphoranocarbene) of Uranium and a Probe of Its Reactivity with Alcohols.

The addition of 2 equiv of the phosphaylide HC═PPh to the dimethyl uranium metallocene CpUMe (Cp = η-CMe) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) CpU[C(H)PPh] () in good yield. Characterization of by X-ray crystallographic analysis reveals two short uranium-carbon bonds, ranging from 2.301(5) to 2.322(5) Å, consistent with the presence of U═C carbene-type bonds. Monitoring the reaction by NMR spectroscopy suggests that it proceeds through the intermediate formation of the methyl carbene complex CpUC(H)PPh (); however, prolonged heating of these solutions leads to the -cyclometalated carbene species CpU{κ-[C(H)PPh(CH)]} () via intramolecular C-H activation. Rapid conversion from to occurs within hours upon heating its toluene solutions to 100 °C. Preliminary reactivity studies of show that it readily reacts with alcohols, such as HODipp (Dipp = 2,6-diisopropylphenyl) and HOC(CF), to give the mixed carbene alkoxide compounds CpUC(H)PPh (R = Dipp (), C(CF) ()). In one case, the reaction of with HODipp in the presence of adventitious water led to the formation of a few crystals of the terminal U(IV) oxo complex, [PhPCH][Cp*U(O)(ODipp)] (). The isolation of marks the first instance of an unchelated, heteroatom-stabilized bis(carbene) complex of uranium that also provides an entryway to the synthesis of its monocarbene derivatives through protonolysis.