Synthesis, structure, and reactivity of three-coordinate vanadium(III) chalcogenolates and vanadium(V) chalcogenide chalcogenolates

Reaction between [(Me(3)Si)(2)N]V-2(Br)(THF) and (THF)(2)LiER (E = Se,Te; R = Si(SiMe(3))(3), SiPh(3)) afforded the three-coordinate [(Me(3)Si)(2)N](2)VER complexes in 60-80% yield. The -SeSi(SiMe(3))(3) complex and both of the -TeR derivatives were characterized crystallographically, and their X-ray structures are presented for comparative purposes. The structures feature a three-coordinate vanadium center with agostic interactions between two C-H groups of the amido trimethylsilyl ligands at the vacant apical coordination sites of vanadium. Reaction of [(Me(3)Si)(2)N]V-2(Br)(THF) with (DME)LiSeC(SiMe(3))(3) (DME = 1,2-dimethoxyethane) afforded the vanadium(IV) dimer {[(Me(3)Si)(2)N]V-2(mu-Se)}(2) in moderate yield. X-ray crystallography confirmed a dimeric structure in the solid state with a long V-V distance of 3.044(1) Angstrom. The three-coordinate compounds were oxidized by styrene oxide, propylene sulfide, and Ph(3)PSe or Se to yield the oxo(-), sulfide(-), and selenide-vanadium(V) complexes, respectively; these derivatives were characterized by H-1, C-13{H-1}, and V-51 NMR spectroscopy. [(Me(3)Si)(2)N]V-2(E')[SeSi(SiMe(3))(3)] (E' = O, S, Se) and [(Me(3)Si)(2)N]V-2(O)[TeSi(SiMe(3))(3)] were isolated as deeply colored, crystalline solids and [(Me(3)Si)(2)N]V-2(Se)[SeSi(SiMe(3))(3)] was characterized crystallographically. Styrene oxide also reacted with [(Me(3)Si)(2)N]V-2(Br)(THF) affording [(Me(3)Si)(2)N]V-2(O)(Br) as Orange crystals in 70% yield. In general, the vanadium(V) selenolates are more stable than the tellurolates, the latter being susceptible to reduction. For example, [(Me(3)Si)(2)N]V-2(E)[TeSi(SiMe(3))(3)] (E = S, Se) cleanly decomposes in solution affording Te-2[Si(SiMe(3))(3)](2) and {[(Me(3)Si)(2)N]V-2(mu-E)}(2). The compounds [(Me(3)Si)(2)N]V-2(O)(X) (X = Br, SeSi(SiMe(3))(3)) thermally rearrange to the siloxide-imido species, [(Me(3)Si)(2)N](Me(3)SiN)V(OSiMe(3))(X). The isomerization occurs more readily when X = Br, where kinetic data demonstrated the reaction to be first order in starting material, implicating an intramolecular 1,3-trimethylsilyl migration from N to O. Activation parameters (Delta H-double dagger = 26 +/- 1 kcal mol(-1) and Delta S-double dagger = -0.60 +/- 2 eu) are in the range of those determined for the related thermal rearrangement of beta-ketosilanes to siloxyalkenes.