Coordinative Unsaturation versus Oxophilicity: <i>ansa</i> Dimethylsilyl Oxo-Bridged Uranium Metallocenes. Synthesis, Structures, and Unexpected Catalytic Activity with Alkynes and Silanes

The tetrachloride salt of uranium reacts with 1 equiv of the lithium ligand Li2[(C5Me4)2SiMe2] in DME to form the complex [η5-(C5Me4)2SiMe2]UCl2·2LiCl·2DME (1), which undergoes a rapid hydrolysis either in DME with equimolar amounts of water to give the coordinatively unsaturated bridged monooxide and monochloride uranium lithium salt complex { [η5-(C5Me4)2SiMe2]UCl}2(μ-O)(μ-Cl)·Li(DME)3·DME (2) or in toluene to yield the dimeric bridged monochloride monooxide lithium salt complex [{[η5-(C5Me4)2SiMe2]UCl}2(μ-O)(μ-Cl)·Li·1/2DME]2 (3). Alkylation of either complex 2 or 3 with BuLi in DME gives the monobridged dibutyl complex {[η5-(C5Me4)2SiMe2]UBu}2(μ-O) (4). Complex 2 was characterized by solid state X-ray analysis. Complex 4 was found to be an active catalyst for the disproportionation metathesis of TMSCCH and the cross-metathesis of TMSCCH or TMSCCTMS with various terminal alkynes. To shed some light on the possible mechanistic scenario, different alkynes, TMSCCH, TMSCCTMS, or TMSCCPri, were reacted in the presence of complex 4 with PhSiH3, producing a myriad of products in addition to the unexpected TMSH and SiH4 (caution), indicating the cleavage of the trimethylsilyl group from the alkyne and the formation of a uranium-silyl intermediate. Complex 4 was also found to be an active catalyst for the phenyl cleavage metathesis of PhSiH3 to form Ph2SiH2 and Ph3SiH. In C6D6 solutions, the C−D activation of the aromatic ring takes place, forming C6D5SiH3. Plausible mechanisms for all the catalytic processes are presented.