Synthesis and structural characterisation of new organo-diimido tantalum and niobium complexes

Abstract

The organo-diimido tantalum complexes [{Ta(L₂)Cl₃}₂(μ-1,n-NC₆H₄N)], L = MeCN (1) and 4-^tBupy (2), have been prepared by two alternative experimental methods: (1) the initial formation of the corresponding TaCl₅·L adduct, L = 4-^tBupy or CH₃CN, followed by addition of the appropriate phenylenediamine, or (2) the initial formation of the corresponding organo-diimido species and subsequent addition of the ligand L. These complexes, as well as some related niobium complexes, were alkylated using the appropriate Grignard reagent to afford the alkyl-containing complexes [{M(L)R₃}₂(μ-1,n-NC₆H₄N)] (4, 5, 6, 7, 8, 9) (M = Nb, Ta; n = 3, 4; L = MeCN, R = CH₂SiMe₃, CH₂CMe₃, CH₂CMe₂Ph). These complexes can be prepared by treating the corresponding chloro-complexes with dialkylmagnesium reagents MgR₂(THF)₂. Niobium complexes containing THF ligands, i.e. [{Nb(THF)R₃}₂(μ-1,3-NC₆H₄N)] (10, 11) (R = CH₂SiMe₃, CH₂CMe₃), were isolated by using THF as the solvent and also from the acetonitrile-containing complexes 8b and 9b by simply adding THF. The complex [{Nb(CH₂CMe₂Ph)₃}₂(μ-1,4-NC₆H₄N)] (12a) was prepared by addition of the corresponding Grignard reagent to a suspension of [{Nb(MeCN)₂Cl₃}₂(μ-1,4-NC₆H₄N)] in THF. The reaction of complexes [{M(MeCN)₂Cl₃}₂(μ-1,n-NC₆H₄N)] (M = Ta, Nb; n = 3, 4) with Li[C₅H₄SiMe₃] was carried out and afforded the tetrakis-cyclopentadienyl-diimido complexes [{M(η⁵-C₅H₄SiMe₃)₂Cl}₂(μ-1,n-NC₆H₄N)] [M = Ta, n = 4 (13a); M = Ta, n = 3 (13b); M = Nb, n = 4 (14a); M = Nb, n = 3 (14b)]. The structure of 13a has been solved by X-ray diffraction. Finally, we have performed the reactions between the diimido-containing compounds [{M(MeCN)₂Cl₃}₂(μ-1,n-NC₆H₄N)] (M = Ta, Nb; n = 3, 4) and the lithium benzamidinate Li[PhC(NSiMe₃)₂] to afford the benzamidinate-containing complexes [{M[PhC(NSiMe₃)₂]₂Cl}₂(μ-1,n-NC₆H₄N)] [M = Ta, n = 4 (15a); M = Ta, n = 3 (15b); M = Nb, n = 4 (16a)]. The structures of the different families of compounds were determined by spectroscopic methods.