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

The organo-diimido tantalum complexes [{Ta(L2)Cl3}2(mu-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 alkylcontaining complexes [{M(L)R3}2(mu-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(mu-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(mu-1,4NC6H4N)](12a) was prepared by addition of the corresponding Grignard reagent to a suspension of [{Nb(MeCN)2Cl3}2(mu-1,4-NC6H4N)] in THF. The reaction of complexes [{M(MeCN)2Cl3}2(mu-1,n-NC6H4N)](M = Ta, Nb; n= 3, 4) with Li[C5H4SiMe3] was carried out and afforded the tetrakis-cyclopentadienyl-diimido complexes [{M(5-C5H4SiMe3)2Cl}2(mu-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(mu-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(mu-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.