Synthesis and characterization of niobium(V) and tantalum(V) derivatives with diamido ligands.: Molecular structure of {4,5-Me2-0-C6H2(NSiMe3)2}2NbCl 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(trimethylsilyl)-o-dimethylphenylene diamide o-C6H2Me2(NSiMe3)(2) in Et2O-DME at -78 degreesC leads to a tantalum(V) imide 3. Monosubstitution with diamides 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 diffraction. The metal is pentacoordinated for I bearing two bidentate o-phenylenediamide ligands and a chlorine in the apical position of a distorted square-tetragonal pyramid. The salient features are a disymmetrical coordination mode of the diamide ligand with two quite different Nb-N distances namely 1.981(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 imido ligand [Me2C6H2N(SiMe2Cl)(NSiMe3-xClx)](2-) (x = 1 or 0) which was generated by N-Si bond cleavage and Si-Me/Si-Cl scrambling. Tantalum is six-coordinate, linked to the bulky imido ligand, three chlorine and a bidentate dimethoxyethane. The surrounding of the metal is distorted with a Ta = N bond distance of 1.706(5) Angstrom and a large Ta-N(2)-C(2) angle [170.7(5)degrees]. The thermal behavior as well as the reactivity as soluble a-olefin polymerization catalyst were evaluated for 1. (C) 2001 Elsevier Science Ltd. All rights reserved.