Addition of Methyl Iodide Produces a Cationic Ta Complex

The nucleophilic properties of the imido nitrogen in 3 were demonstrated by its reaction with MeI. The product 5 is highly insoluble in nonpolar solvents (Et₂O, benzene) but is soluble in CH₂Cl₂. This, and the downfield shifts for the MeNTa group in the ¹H (3.39 ppm) and ¹³C (53.4 ppm) NMR spectra, suggested that 5 might be cationic, which was confirmed by X-ray crystallography. The Ta-I distance of 5.311(1) Å, and the undistorted three-legged piano stool geometry about the metal center confirmed that the iodide is not coordinated to Ta. We attribute this ionic structure to steric crowding about Ta, which prevents coordination of the anion, and to the possibility for stabilization of the cation by donation of pi-electron density from the amido nitrogens.

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