Oxo and imido/imido exchange and C–H activation reactions based on pentamethylcylopentadienyl imido tantalum complexes

Reactions of [TaCp*Cl4] with two, three and four equivalents of LiNHtBu give the halo- and amido-imido complexes [TaCp*Cl2(NtBu)] (1a), [TaCp*Cl(NtBu)(NHtBu)] (2) and [TaCp*(NtBu)(NHtBu)2] (3), respectively. The related complex [TaCp*Cl2{N(2,6-Me2C6H3)}] (1b) is prepared by a similar reaction using two equivalents of Li[NH(2,6-Me2C6H3)]. Complex 3 can be transformed into 2 and further into 1a by reaction with SiClMe3. Complex 1a reacts with CNtBu to give the 18-electron adduct [TaCp*Cl2(NtBu)(CNtBu)] (4) whereas addition of excess CN(2,6-Me2C6H3) results in reductive elimination of the carbodiimide tBuNCN(2,6-Me2C6H3) (5) to give [TaCp*Cl2{CN(2,6-Me2C6H3)}3]. However complex 1b does not react with any of the isocyanide ligands. Both complexes 1a and 1b react with PhCHO undergoing imido/oxo exchange to give the imines PhCHNR (R=tBu, 2,6-Me2C6H3 (6)) and dimeric [TaCp*Cl2(O)]2 or trimeric [(TaCp*Cl)3(μ2-Cl)(μ2-O)3(μ3-O)] oxo-complexes, whereas only 1a reacts with CO2, PhCHNR′ (R′=Ph, Me) and (2,6-Me2C6H3)NCNtBu producing tBuNCO, PhCHNtBu and tBuNCNtBu, respectively and the corresponding oxo or imido tantalum derivative. None of the complexes reacts with CO or NCR (R=Me, Ph). The complex [TaCp*Me(NtBu)(NHtBu)] activates C–H bonds when heated in benzene and toluene affording [TaCp*Ph(NtBu)(NHtBu)] (7) and a mixture of [TaCp*(m-MeC6H4)(NtBu)(NHtBu)] 8a and [TaCp*(p-MeC6H4)(NtBu)(NHtBu)] (8b). All of the reported organic compounds and tantalum complexes were characterized by 1H- and 13C-NMR spectroscopy.