A Cationic Imido Complex of Permethyltantalocene: H2 and Carbon−Hydrogen Bond Activation, [2 + 2] Cycloaddition Reactions, and an Unusual Reaction with Carbon Dioxide That Affords Coordinated Isocyanate

Treatment of the imido hydride complex Cp* 2 Ta( NCMe 3 )H (Cp* = (η 5 -C 5 Me 5 )) with [Ph 3 C][B(C 6 F 5 ) 4 ] in tetrahydrofuran solution yields the cationic imido complex [Cp* 2 Ta( NCMe 3 )(THF)][B(C 6 F 5 ) 4 ] ( 1 ). Cation 1 reacts cleanly with H 2 to yield [Cp* 2 Ta(NHCMe 3 )H][B(C 6 F 5 ) 4 ]. Carbon−hydrogen bond-activation reactions are observed with propyne or phenylacetylene to afford [Cp* 2 Ta(NHCMe 3 )(C≡CR)][B(C 6 F 5 ) 4 ] (R = CH 3, C 6 H 5 ). In the reaction with propyne, an initial mixture of the [2 + 2] cycloaddition product and C−H activation product is thermally driven to the C−H activation product. The heterolytic cleavage reactions for 1 may be rationalized in terms of the presence of both electrophilic and nucleophilic sites of reactivity in the same molecule. Intramolecular activation of a carbon−hydrogen bond of a Cp* methyl group to ultimately C−H activation of the carbon−hydrogen bonds of methane. An unusual reaction occurs with carbon dioxide: dealkylation of the imido group is observed, liberating isobutylene and yielding the isocyanate complex [Cp* 2 Ta(OH)(NCO)][B(C 6 F 5 ) 4 ]. Complex 1 reacts with HCl to afford [Cp* 2 Ta(NHCMe 3 )Cl][B(C 6 F 5 ) 4 ] and decomposes cleanly in methylene chloride solution to give [Cp* 2 Ta(NHCMe 3 )Cl][B(C 6 F 5 ) 4 ] by formal HCl abstraction. X-ray crystal structure determinations and [Cp* 2 Ta(OH)(NCO)][B(C 6 F 5 ) 4 ] are reported.