α-, β-, and δ-Hydrogen Abstraction in the Thermolysis of Paramagnetic Vanadium(III) Dialkyl Complexes

Electron deficient paramagnetic vanadium(III) diakyls CpV(CH2CMe2R)2(PMe3) (14 electron, R = Me (2), Ph (3)) and CpV[CH(SiMe3)2]2 (12 electron, 4) have been synthesized. At ambient temperature 2 decomposes through α-hydrogen abstraction to produce, in the presence of dmpe (1,2-bis(dimethylphosphino)ethane), the first vanadium alkylidene CpV(CHCMe3)dmpe (6), which has been structurally characterized. In contrast, 3 decomposes in the presence of excess PMe3 through orthometalation of the aryl substituent to give the metallacycle complex CpV(σ2-CH2CMe2C6H4)(PMe3)2 (7). In the absence of excess PMe3 the (μ-alkyl)2(μ-aryl)2 dimer [CpV(μ2-CH2CMe2C6H4)]2 (8) is formed, with a short (2.313(2) A) metal-metal distance. Reaction of CpV(Me)Cl(PMe3)2 (9) with n-BuLi produces the 1-butene complex CpV(η2-CH=CHEt)(PMe3)2 (10) through β-hydrogen abstraction and reductive elimination. Reactivity of the alkylidene complex 6 includes the formation of the first imido complex of trivalent vanadium, CpV[=NC(CMe3)=C(CMe3)H](dmpe) (12), with V=N = 1.707(2) A and a Z-configuration around the C-C bond, in the reaction with t-BuCN. Crystallographic data for 6: P21/a, a = 13.699(2) A, b = 8.966(1) A, c = 15.386(2) A, β = 101.87(1)°, Z = 4. For 7: P1, a = 9.671(2) A, b = 14.261(4) A, c = 8.528(2) A, α = 94.89(2)°, β = 100.55(2)°, γ = 77.78(2)°, Z = 2. For 8: P21/c, a = 9.812(9) A, b = 14.019(10) A, c = 17.407(10) A, β = 96.76(6)°, Z = 4. For 12: P1, a = 9.147(1) A, b = 11.005(1) A, c = 13.489(1) A, α = 71.95(1)°, β = 89.70(1)°, γ = 69.56(1)°, Z = 2.