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

Abstract

The mononuclear benzamidinato compounds, [M{(2,4,6-tri-ⁱPr)C₆H₂C(NSiMe₃)₂}Cl₄] (M = Nb (1) or Ta (2*)), [Nb{(2,4,6-tri-ⁱPr)C₆H₂C(NSiMe₃)₂}Cl₄(PMe₃)] (4*), [M{(2,4,6-tri-ⁱPr)C₆H₂C(NSiMe₃)₂}(N^tBu)(H₂N^tBu)Cl₂] (M = Nb (5), M = Ta (6)), [Nb{(2,4,6-tri-ⁱPr)C₆H₂C(NSiMe₃)₂}(N^tBu)(L)Cl₂] (L = THF (7), L = pyridine (8)), and [M{(2,4,6-tri-ⁱPr)C₆H₂C(NSiMe₃)₂}(η-C₅H₅)Cl₃] (M = Nb (9), M = Ta (10*), and a nitrile tantalum complex, [Ta{(2,4,6-tri-ⁱPr)C₆H₂CN}Cl₅] (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; ΔG^‡ = 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 ΔG^‡ = 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.