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

Abstract

The synthesis and characterization of V(O){N(SiMe 3 ) 2 } 3 (1), V(NSiMe 3 ){N(SiMe 3 ) 2 } 3 (2), and V(NSiMe 3 )(OSiMe 3 ){N(SiMe 3 ) 2 } 2 (3) are described. Prior attempts to synthesize the vanadium(V) oxo complex 1 via salt metathesis of VOCl 3 with the lithium or sodium silylamide salt had yielded either the putative rearranged species V(NSiMe 3 )(OSiMe 3 ){N(SiMe 3 ) 2 } 2 (3) or the oxo-bridged, dimetallic {(μ-O) 2 V 2 [N(SiMe 3 ) 2 ] 4 }. We now show that complex 1 is available by treatment of the vanadium(III) tris(silylamide) V{N(SiMe 3 ) 2 } 3 with iodosylbenzene. The imido complex 2 was obtained by treatment of V{N(SiMe 3 ) 2 } 3 with trimethylsilyl azide. Sublimation of 1 formed complex 3, which was determined to be V(NSiMe 3 )(OSiMe 3 ){N(SiMe 3 ) 2 } 2 , on the basis of infrared, electronic, and 1 H and 51 V 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 1 H 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 1 H NMR spectrum of 2 in deuterated methylene chloride displayed distinct signals for the imido (0.41 ppm) and amido (0.35 ppm) protons, whereas 1 H NMR spectroscopy of 3 showed three signals in an intensity ratio consistent with the formula V(NSiMe 3 )(OSiMe 3 ){N(SiMe 3 ) 2 } 2 . 51 V 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 °C), 0.0006 s −1 (73 °C), 0.002 s −1 (83 °C)) and activation energy (ca. 20 kcal/mol) of this first-order process.

Hydrides, Halides, and Polymers: Some Unexpected Intermediates on the Routes to First-Row Transition Metal M{N(SiMe3)2}n (n = 2, 3) Complexes

“…A coordination chemistry was initiated for the titanium and vanadium tris­(silylamide) complexes (previously thought not possible due to steric crowding) to give M­{N­(SiMe 3 ) 2 } 3 L 2 (L = a nitrile or isocyanide ligand) complexes . Additionally, the isolation of a new vanadium­(V) oxy silylamide complex, e.g., V­(=O)­{N­(SiMe 3 ) 2 } 3 , that was absent from the literature despite several attempts to isolate it, and the synthesis of several unusual metal hydride complexes that are available by reduction of M­{N­(SiMe 3 ) 2 } 3 complexes were described …”

Reductions of M{N(SiMe3)2}3 (M = V, Cr, Fe): Terminal and Bridging Low-Valent First-Row Transition Metal Hydrido Complexes and “Metallo-Transamination”

“…These were first reported in the early 1960s by Bürger and Wannagat , and later studied extensively by Bradley and co-workers. − Currently, three-coordinate transition metal M­{N­(SiMe 3 ) 2 } 3 complexes are known for the first row metals from scandium through cobalt, − ,− several lanthanides, ,− as well as plutonium and uranium. , Our recent finding that the complex V­{N­(SiMe 3 ) 2 } 3 had been initially mischaracterized in the 1970s ,, led us to reexamine first-row transition metal M­{N­(SiMe 3 ) 2 } 3 complexes. Advances in techniques for the handling and characterization of such complexes has allowed the development of the coordination chemistry for the titanium and vanadium tris­(silylamide) complexes, as well as the isolation of several high-valent vanadium complexes that were unknown despite earlier efforts to synthesize them. − As described in these reports, attempts to extend this chemistry to other first-row transition metal M­{N­(SiMe 3 ) 2 } 3 complexes (M = Cr, Mn, Fe, Co) were largely unsuccessful. However, isolation of the chromium­(II) complex [K­(18-crown-6)­(Et 2 O) 2 ]­[Cr­{N­(SiMe 3 ) 2 } 3 ] by reduction of Cr­{N­(SiMe 3 ) 2 } 3 suggested that the later metal tris­(silylamide) complexes may be useful synthons for lower valent species …”

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co)

“…Most prominent examples among the bis­(trimethylsilyl)­amido complexes are two coordinate complexes such as [Fe­{N­(SiMe 3 )­(Dipp)} 2 ] (dipp = C 6 H 3 -2,6- i Pr 2 ). , But also three coordinate iron­(II/III), cobalt­(II), and nickel­(I) complexes, i.e., [M­{N­(SiMe 3 ) 3 } 2 L] (M = Fe, Co; L = ligand), − [Fe­{N­(SiMe 3 ) 3 } 3 ], , and [Ni­(N­(SiMe 3 ) 2 )­(PPh 3 ) 2 ] and four-coordinate chromium­(II) complexes [Cr­(N­(SiMe 3 ) 2 ) 2 (L) 2 ] (L = py, thf) attracted some attention . One further important aspect is studies of the coordination behavior of these coordinatively unsaturated compounds, a situation which also enables them to activate small molecules. − Finally, basic aspects like optimization of existing synthetic protocols including the synthesis of new compounds are still an actual and common interest like evidenced by some recent papers. − …”