Niobium- and tantalum-benzamidinato complexes with trimethylphosphine, imido, or η-cyclopentadienyl derivatives

The mononuclear benzamidinato compounds, [M{(2,4,6-tri-Pr-i)C6H2C(NSiMe3)(2)}Cl-4] (M = Nb (1) or Ta (2*)), [Nb{(2,4,6-tri-Pr-i)C6H2C(NSiMe3)(2)}Cl-4 (PMe3)] (4*), [M{(2,4,6-tri-Pr-i)C6H2C(NSiMe3)(2)}((NBu)-Bu-t)((H2NBu)-Bu-t)Cl-2] (M = Nb (5), M = Ta (6)), [Nb{(2,4,6-tri-Pr-i)C6H2C(NSiMe3)(2)}((NBu)-Bu-t)(L)Cl-2] (L = THF (7), L = pyridine (8)), and [M{(2,4,6-tri-Pr-i)C6H2C(NSiMe3)(2)}(eta-C5H5)Cl-3] (M = Nb (9), M = Ta (10*), and a nitrile tantalum complex, [Ta{(2,4,6-tri-Pr-i)C6H2CN}Cl-5] (3*), obtained in a side- reaction have been prepared:* indicates the crystal structures have been determined. Variable temperature NMR studies show there is hindered rotation of the aryl group around C-ipso-C bonds for compounds 5 and 6; Delta G double dagger = 55.2 kJ mol(-1) at 268 K for 5. Hindered rotation about the N-Si bonds for compounds 9 and 10 occurs with the Delta G double dagger = 51.0 kJ mol(-1) at 273 K and 40.6 kJ mol(-1) at 213 K for 10. The compounds 1, 5 and 9 are very poor pre- catalysts for ethene polymerisation.