Characterization of the "Absent" Vanadium Oxo V(=O){N(SiMe3)2}3, Imido V(=NSiMe3){N(SiMe3)2}3, and Imido-Siloxy V(=NSiMe3)(OSiMe3){N(SiMe3)2}2 Complexes Derived from V{N(SiMe3)2}3 and Kinetic Study of the Spontaneous Conversion of the Oxo Complex into Its Imido-Siloxy Isomer

The synthesis and characterization of V(=O){N(SiMe3)(2)}(3) (1), V(=NSiMe3){N(SiMe3)(2)}(3) (2), and V(=NSiMe3)(OSiMe3){N(SiMe3)(2)}(2) (3) are described. Prior attempts to synthesize the vanadium(V) oxo complex 1 via salt metathesis of VOCl3 with the lithium or sodium silylamide salt had yielded either the putative rearranged species V(=NSiMe3)(OSiMe3){N(SiMe3)(2)}(2) (3) or the oxo-bridged, dimetallic {(mu-O)(2)V-2[N(SiMe3)(2)](4)}. We now show that complex 1 is available by treatment of the vanadium(III) tris(silylamide) V{N(SiMe3)(2)}(3) with iodosylbenzene. The imido complex 2 was obtained by treatment of V{N(SiMe3)(2)}(3) with trimethylsilyl azide. Sublimation of 1 formed complex 3, which was determined to be V(=NSiMe3)(OSiMe3){N(SiMe3)(2)}(2), on the basis of infrared, electronic, and H-1 and V-51 NMR spectroscopies. Crystallographic disorder precluded a complete structural characterization of 3, although a four-coordinate V atom, as well as severely disordered ligands, were apparent. Comparison of the vibrational spectra of 1 and 2 allowed an unambiguous assignment of the V-O (995 cm(-1)) and V-N-imide (1060 cm(-1)) stretching bands. The vibrational spectrum of complex 3 displayed strong absorbances at 1090 and 945 cm(-1), indicative of its metal imide and metal siloxide moieties. The H-1 NMR spectrum of 1 in deuterated benzene showed overlapping signals for the ligand protons proximal and distal to the oxo moiety at 0.52 and 0.38 ppm. The H-1 NMR spectrum of 2 in deuterated methylene chloride displayed distinct signals for the imido (0.41 ppm) and amido (0.35 ppm) protons, whereas H-1 NMR spectroscopy of 3 showed three signals in an intensity ratio consistent with the formula V(=NSiMe3)(OSiMe3){N(SiMe3)(2)}(2). V-51 NMR spectra of 1-3 revealed singlet resonances at -119 ppm (1), -24 ppm (2), and -279 ppm (3). The electronic spectra of 1-3 displayed single absorbances in the charge transfer region, consistent with their d(0) electron configurations. Kinetic studies of the spontaneous conversion of complex 1 to 3 were used to determine the rate constants (ca. 0.0002 s(-1) (63 degrees C), 0.0006 s(-1) (73 degrees C), 0.002 s(-1) (83 degrees C)) and activation energy (ca. 20 kcal/mol) of this first-order process.