Synthesis and reactivity of new mono- and dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [Ta(η5-C5H4SiMe3)Cl2{double bond; length as m-dashNC6Me4-4-(N(SiMe3)2)}]

The reaction of [1,4-{SiMe3(H)N}2C6Me4] (1) with 2 equivalents of LiBun followed by the addition of SiMe3Cl gave the diamine compound [1,4-{(SiMe3)2N}2C6Me4] (2). [Ta(η5-C5H4SiMe3)Cl4] reacts with 2, in a 2:1 stoichiometric ratio, to initially yield a mixture of the dinuclear, [{Ta(η5-C5H4SiMe3)Cl2}2(μ-1,4-NC6Me4N)] (3), and mononuclear, [Ta(η5-C5H4SiMe3)Cl2{double bond; length as m-dashNC6Me4-4-(N(SiMe3)2)}] (4), imido complexes. 3 can be obtained exclusively by submitting the reaction mixture to repeated cycles of evacuation, to remove volatiles, followed by addition of solvent and subsequent heating. The mononuclear imido complex 4 was isolated from the reaction of [Ta(η5-C5H4SiMe3)Cl4] with 2 in a 1:1 stoichiometric ratio. The molecular structure of 4 was determined by X-ray diffraction studies. [TaCl3(CH3CN)2{double bond; length as m-dashNC6Me4-4-(N(SiMe3)2)}] (5) has been prepared by the reaction of one molar equivalent of TaCl5 with 2 in a CH3CN/CH2Cl2 solvent mixture. The synthesis of the niobium complexes, [{Nb(η5-C5H4SiMe3)Cl2}2(μ-1,4-NC6Me4N)] (6) and [Nb(η5-C5H4SiMe3)Cl2{double bond; length as m-dashNC6Me4-4-(N(SiMe3)2)}] (7), was achieved in a similar manner to their tantalum analogues. The reactivity of 7 towards nucleophilic reagents, namely lithium benzamidinate, lithium (trimethylsilyl)cyclopentadienyl or lithium dimethylamide, has been studied and the following compounds prepared:[Nb(η5-C5H4SiMe3)RCl{double bond; length as m-dashNC6Me4-4-(N(SiMe3)2)}] (R = η5-C5H4SiMe3 (8), PhC(NSiMe3)2 (9), NMe2 (10)). In an attempt to form the hetero bimetallic complex, [{Nb(η5-C5H4SiMe3)Cl2}(μ-1,4-NC6Me4N){Ta(η5-C5H4SiMe3)Cl2}] (11), the reaction of 7 with [Ta(η5-C5H4SiMe3)Cl4] has been studied. Analysis of the reaction products showed that 11 may exist in equilibrium with the homo bimetallic complexes 3 and 6.