Synthetic and Mechanistic Investigations of Trimethylsilyl-Substituted Triamidoamine Complexes of Tantalum That Contain Metal−Ligand Multiple Bonds

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Abstract

[N3N]TaPPh ([N3N]3- = [(Me3SiNCH2CH2)3N]3-) reacts with excess lithium metal in tetrahydrofuran to give “[N3N]TaPLi”, as judged by NMR studies and by reactions with RX at −35 °C to afford the phosphinidene complexes [N3N]TaPR (R = Me, n-Bu, SiMe3, SiMe2Ph). [N3N]TaCl2 reacts with 2 equiv of LiN(H)R (R = H, CMe3, Ph) to produce 1 equiv of RNH2 and imido complexes [N3N]TaNR and with 2 equiv of benzylmagnesium chloride or ((trimethylsilyl)methyl)lithium to afford the alkylidene complexes [N3N]TaCHR (R = Ph or SiMe3). The ethylene complex [N3N]Ta(C2H4) is formed quantitatively upon addition of 2 equiv of ethylmagnesium chloride to [N3N]TaCl2. [N3N]Ta(C2H4) decomposes in a first-order manner in solution over a period of days at room temperature to give a complex in which a C−N bond in the TREN backbone has been cleaved. Alkylation of [N3N]TaCl2 with 2 equiv of RCH2CH2MgX (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3; X = Cl or Br) produces a mixture of alkylidene and products derived from decomposition of the incipient olefin complex. When R = t-Bu, only an alkylidene complex is formed as a consequence of a sterically disfavored β abstraction process. [N3N]TaCl2 reacts with 2 equiv of vinylmagnesium bromide to afford white crystalline [N3N]Ta(C2H2). An analogous benzyne complex can be prepared by refluxing [N3N]TaCl2 with 2 equiv of phenyllithium in toluene. [N3N]Ta(C2H4) reacts with a catalytic amount of phenylphosphine to afford [N3N]TaCHMe, while reactions with ammonia, aniline, or pentafluoroaniline yield [N3N]TaNR complexes. In contrast, excess Me3SiAsH2 reacts with [N3N]Ta(C2H4) to afford [N3N]TaCHMe first, and then what is proposed to be [N3N]TaAsSiMe3. [N3N]Ta(C2H4) reacts with dihydrogen to give [N3N]Ta(H)(C2H5) reversibly. [N3N]Ta(C6H4) reacts with ArNH2 (Ar = Ph, C6F5) to give [N3N]TaNAr complexes, but [N3N]Ta(C2H2) is relatively unreactive. X-ray structures of [N3N]Ta(Me)Et and [N3N]Ta(C2H2) are included.

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