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.