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

Abstract

Reactions of [TaCp*Cl4] with two, three and four equivalents of LiNHt Bu give the halo- and amido-imido complexes [TaCp*Cl2(Nt Bu)] (1a), [TaCp*Cl(Nt Bu)(NHt Bu)] (2) and [TaCp*(Nt Bu)(NHt Bu)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 CNt Bu to give the 18-electron adduct [TaCp*Cl2(Nt Bu)(CNt Bu)] (4) whereas addition of excess CN(2,6-Me2C6H3) results in reductive elimination of the carbodiimide t BuN 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=t Bu, 2,6-Me2C6H3 (6)) and dimeric [TaCp*Cl2(O)]2 or trimeric [(TaCp*Cl)3(m2-Cl)(m2-O)3(m3-O)] oxo-complexes, whereas only 1a reacts with CO2, PhCH NR% (R%=Ph, Me) and (2,6-Me2C6H3)N C Nt Bu producing t BuN CO, PhCH Nt Bu and t BuN C Nt Bu, 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(Nt Bu)(NHt Bu)] activates C–H bonds when heated in benzene and toluene affording [TaCp*Ph(Nt Bu)(NHt Bu)] (7) and a mixture of [TaCp*(m-MeC6H4)(Nt Bu)(NHt Bu)] 8a and [TaCp*(pMeC6H4)(Nt Bu)(NHt Bu)] (8b). All of the reported organic compounds and tantalum complexes were characterized by 1 H- and 13C-NMR spectroscopy