Synthesis and characterization of niobium(V) and tantalum(V) derivatives with diamido ligands. Molecular structure of {4,5-Me-2-0-C6H2(NSiMe3)(2)}(2)NbCl and of a tantalum imide

The reactions between lithium bis(trimethyl)phenylenediamide and niobium or tantalum pentachlorides at low temperature afforded [{4,5-Me-2-C6H2(NSiMe3 )(2)}(2)MCl] (M = Nb (1), Ta (2)) independently of the stoichiometry used, 1:1 or 2:1. The reaction between tantalum pentachloride and N,N ' -bis(trim ethylsilyl)-o-dimethylphenylene diamide o-C6H2Me2(NSiMe3)(2) in Et2O-DME at -78 degreesC leads to a tantalum(V) imide 3. Monosubstitution with diamide s was achieved on tantalum with the more rigid 1,8-diamido-naphtalene. The various compounds were characterized by elemental analysis, FT-IR and NMR. Compounds I and 3 were characterized also by single crystal X-ray diffracti on. The metal is pentacoordinated for I bearing two bidentate o-phenylenedi amide ligands and a chlorine in the apical position of a distorted square-t etragonal pyramid. The salient features are a disymmetrical coordination mo de of the diamide ligand with two quite different Nb-N distances namely 1.9 81(2) and 2.053(3) A, as well as short Nb-C distances with the phenyl ring [2.495(3) and 2.471(3) Angstrom]. The diamide ligand is thus coordinated in a eta (4)-fashion in the solid state. The salient feature of the structure of 3 is the presence in the metal coordination sphere of the functional im ido ligand [Me2C6H2N(SiMe2Cl)(NSiMe3-xClx)](2-) (x = 1 or 0) which was gene rated by N-Si bond cleavage and Si-Me/Si-Cl scrambling. Tantalum is six-coo rdinate, linked to the bulky imido ligand, three chlorine and a bidentate d imethoxyethane. The surrounding of the metal is distorted with a Ta = N bon d distance of 1.706(5) Angstrom and a large Ta-N(2)-C(2) angle [170.7(5)deg rees]. The thermal behavior as well as the reactivity as soluble a-olefin p olymerization catalyst were evaluated for 1. (C) 2001 Elsevier Science Ltd. All rights reserved.