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

The addition of 2 equiv of the phosphaylide H₂C═PPh₃ to the dimethyl uranium metallocene Cp*₂UMe₂ (Cp* = η⁵-C₅Me₅) in toluene with gentle heating at 40 °C generates the phosphorano-stabilized bis(carbene) Cp*₂U[C(H)PPh₃]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*₂U[C(H)PPh₃](Me) (1); however, prolonged heating of these solutions leads to the ortho-cyclometalated carbene species Cp*₂U{κ²-[C(H)PPh₂(C₆H₄)]} (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(CF₃)₃, to give the mixed carbene alkoxide compounds Cp*₂U[C(H)PPh₃](OR) (R = Dipp (4), C(CF₃)₃ (5)). 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, [Ph₃PCH₃][Cp*₂U(O)(ODipp)] (3). 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.