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

The addition of 2 equiv of the phosphaylide H ₂ C═PPh ₃ to the dimethyl uranium metallocene Cp* ₂ UMe ₂ (Cp* = η ⁵ -C ₅ Me ₅ ) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) Cp* ₂ U[C(H)PPh ₃ ] ₂ ( 1 ) in good yield. Characterization of 1 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 Cp* ₂ U[C(H)PPh ₃ ](Me) ( 1 ^Int ); however, prolonged heating of these solutions leads to the ortho -cyclometalated carbene species Cp* ₂ U{κ ² -[C(H)PPh ₂ (C ₆ H ₄ )]} ( 2 ) via intramolecular C-H activation. Rapid conversion from 1 to 2 occurs within hours upon heating its toluene solutions to 100 °C. Preliminary reactivity studies of 1 show that it readily reacts with alcohols, such as HODipp (Dipp = 2,6-diisopropylphenyl) and HOC(CF ₃ ) ₃ , to give the mixed carbene alkoxide compounds Cp* ₂ U[C(H)PPh ₃ ](OR) (R = Dipp ( 4 ^Dipp ), C(CF ₃ ) ₃ ( 5 ^CF3 )). In one case, the reaction of 1 with HODipp in the presence of adventitious water led to the formation of a few crystals of the terminal U(IV) oxo complex, [Ph ₃ PCH ₃ ][Cp* ₂ U(O)(ODipp)] ( 3 ^oxo ). The isolation of 1 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.