Insertion of CO and CNR into Tantalum−Methyl Bonds of Imido(pentamethylcyclopentadienyl)tantalum Complexes. X-ray Crystal Structures of [TaCp*(NR)Me{η²-C(Me)NR}] and [TaCp*Cl(O){η²-C(Me)NR}] (R = 2,6-Me₂C₆H₃)

Abstract

[TaCp*Cl2{N(2,6-Me2C6H3)}] reacts with 2 equiv of LiMe at −78 °C to give the imido dimethyl complex TaCp*Me2{N(2,6-Me2C6H3)} (1) in almost quantitative yield, whereas the monomethyl imido derivatives [TaCp*MeX{N(2,6-Me2C6H3)}] (X = Cl (2), OC6H3Me2 (3)) can be prepared by addition of HX (X = Cl, OC6H3Me2) to toluene solutions of [TaCp*Me(NR){NR(CMeCMe2)}] (R = 2,6-Me2C6H3) with elimination of the corresponding imine RNCMeCHMe2. Complex 1 reacts with CO to give the dinuclear ene diolate complex [TaCp*{N(2,6-Me2C6H3)Me}]2{μ-η2-OC(Me)C(Me)O} (5) via intermolecular coupling between two acyl carbon atoms of the intermediate η2-acyl complex TaCp*Me{N(2,6-Me2C6H3)}{η2-C(Me)O} (4). However, when the same reaction was carried out with the chloro imido methyl derivative 2, the unexpected oxo η2-iminoacyl complex [TaCp*Cl(O){η2-C(Me)N(2,6-Me2C6H3)}] (7) was obtained via the barely stable intermediate η2-acyl complex [TaCp*Cl{N(2,6-Me2C6H3)}{η2-C(Me)O}] (6). The analogous reaction of [TaCp*Cl2Me2] with CO leads to the formation of the dichloro oxotantalacyclopropane complex [TaCp*Cl2(η2-CMe2O)] (8), but the acyl intermediate species was not observed. Similarly, the azatantalacyclopropane complexes [TaCp*XMe(η2CMe2-NR)] react with 1 equiv of CO to form the stable enolate derivatives [TaCp*X(NR){OC(Me)CMe2}] (X = Cl (9), Me (10); R = 2,6-Me2C6H3). The stable 18-electron imido η2-iminoacyl derivatives [TaCp*X(NR){η2-C(Me)NR}] (X = Me (11), Cl (12), OC(Me)CMe2 (13); R = 2,6-Me2C6H3) are formed when the isocyanide RNC (1 equiv) is added to toluene solutions of the imido complexes 1, 2, and 10, respectively. All compounds were characterized by IR and NMR (1H and 13C), and the molecular structures of 7 and 11 were studied by X-ray diffraction methods.