Structure and Reactivity of η2(3e)-Nitrile and η2(3e)-Alkyne Complexes of Niobium

Abstract

Nitrile adducts of the type Tp*Nb(CO)(PhC⋮CMe)(RC⋮Ν) (Tp* = hydridotris(3,5-dimethylpyrazolyl)borate; R = Me (2a), Et (2b), PhCH2 (2c), 4-MeOC6H5 (2d), Ph (2e), 4-CF3C6H5 (2f)) have been prepared in fair yields by CO displacement from Tp*Nb(CO)2(PhC⋮CMe) (1) and fully characterized. 13C NMR spectroscopy (δ 180−190 for the nitrile and alkyne contact carbons) and X-ray crystallography (for 2b) indicate an η2-coordination and an unprecedented formally three-electron-donor (3e) behavior for both the alkyne and the nitrile. Electrochemical studies suggest that an equilibrium between this η2(3e)-nitrile/η2(3e)-alkyne form and an otherwise undetected η1(2e)-nitrile/η2(4e)-alkyne form exists in solution. PhC⋮N is thermally displaced from 2e by PMe2Ph or PhC⋮CMe to give Tp*Nb(CO)(PhC⋮CMe)(PMe2Ph) or Tp*Nb(CO)(PhC⋮CMe)2 (3a), respectively. 3a does not react with PMe2Ph. Protonation of 2b with HBF4 induces nitrile/alkyne coupling to give the niobacycle Tp*NbF(CPhCMeCPhNH), via the isolated η2-iminoacyl alkyne cation [Tp*Nb(CO)(η2-CPhNH)(PhC⋮CMe)][BF4], characterized by NMR spectroscopy at 243 K. Protonation of 3a with HBF4 leads to the niobacycle Tp*NbF[C(Ph)C(Me)C(CMeCPhH)O] (X-ray structure), in which CO is included in the alkyne coupling reaction. The intermediate [Tp*Nb(CO)(CMeCPhH)(PhC⋮CMe)][BF4] has been characterized by NMR at 193 K. A deshielded Cα (δ 205), a shielded Cβ (δ 97), and a reduced 1JCH(114 Hz) suggest that a β-CH agostic interaction stabilizes this vinyl alkyne cation. Protonation of the new phosphinoalkyne derivative Tp*Nb(CO)(PhC⋮CMe)(PPh2C⋮CPh) affords the stable phosphonium complex {Tp*Nb(CO)[Ph2P(H)C⋮CPh](PhC⋮CMe)}[BF4], in which the two alkyne ligands act as 3e donors.