Synthesis and characterization of niobium(V) and tantalum(V) derivatives with diamido ligands. Molecular structure of {4,5-Me 2- o-C 6H 2(NSiMe 3) 2} 2NbCl and of a tantalum imide

Abstract

The reactions between lithium bis(trimethyl)phenylenediamide and niobium or tantalum pentachlorides at low temperature afforded [{4,5-Me 2-C 6H 2(NSiMe 3) 2} 2MCl] (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-C 6H 2Me 2(NSiMe 3) 2 in Et 2O–DME at −78 °C 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 1 and 3 were characterized also by single crystal X-ray diffraction. The metal is pentacoordinated for 1 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) Å, as well as short Nb–C distances with the phenyl ring [2.495(3) and 2.471(3) Å]. The diamide ligand is thus coordinated in a η 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 [Me 2C 6H 2N(SiMe 2Cl)(NSiMe 3− x Cl x )] 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) Å and a large Ta–N(2)–C(2) angle [170.7(5)°]. The thermal behavior as well as the reactivity as soluble α-olefin polymerization catalyst were evaluated for 1.