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 Int ); 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 Dipp ), C­(CF3)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, [Ph3PCH3]­[Cp*2U­(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.