Alkylation and Insertion Reactions in Dichloro Azatantalacyclopropane Complexes. X-ray Crystal Structures of [TaCpCl₂{C(Ph)CHCMe₂NAr-κ²C,N}] (Cp = η⁵-C₅Me₅, η⁵-C₅H₄SiMe₃; Ar = 2,6-Me₂C₆H₃)^§

Dichloro azatantalacyclopropane complexes [TaCpCl₂(CMe₂NAr-κ²C,N)] (Ar = 2,6-Me₂C₆H₃; Cp = η⁵-C₅Me₅, 1; η⁵-C₅H₄SiMe₃, 2) can be obtained by treating TaCpCl₂Me₂ with 1 equiv of 2,6-dimethylphenylisocyanide. Alkylation of 1 with an excess of MgCl(CH₂SiMe₃) leads to [TaCp*Cl(CH₂SiMe₃)(CMe₂NAr-κ²C,N)] (Cp* = η⁵-C₅Me₅; Ar = 2,6-Me₂C₆H₃, 3), which in solution slowly decomposes to give [TaCp*Cl{C(Me)CH₂}(NAr)] (Cp* = η⁵-C₅Me₅; Ar = 2,6-Me₂C₆H₃, 4) with elimination of SiMe₄. The same reaction with other alkylating reagents takes place with the formation of 4, although we have not observed similar intermediate monoalkyl derivatives. The complexes 1 and 2 react with ethylene to give the dichloro azatantalacyclopentane compounds [TaCpCl₂(CH₂CH₂CMe₂NAr-κ²C,N)] (Ar = 2, 6-Me₂C₆H₃; Cp = η⁵-C₅Me₅, 5; η⁵-C₅H₄SiMe₃, 6). However, benzene-d₆ solutions of 5 and 6 decompose at room temperature to give a dichloro(imido) tantalum complex [TaCpCl₂(NAr)] (Ar = 2,6-Me₂C₆H₃, Cp = η⁵-C₅Me₅, 7;η⁵-C₅H₄SiMe₃, 8) with elimination of 2-methyl-2-butene (Me₂CCH−Me) and 3-methyl-1-butene (H₂CCH−CHMe₂) (decomposition of 5) and only 2-methyl-2-butene (decomposition of 6). The complex [TaCp*Cl₂{CH₂CH(Me)CMe₂NAr- κ²C,N}] (Cp* = η⁵-C₅Me₅; Ar = 2,6-Me₂C₆H₃, 9) was detected by NMR spectroscopy after treatment of 1 with propylene in dichloromethane-d₂. The analogous reaction of 1 or 2 with alkynes RC⋮CH (R = H, Ph, SiMe₃) leads to the formation of the dichloro azatantalacyclopentene complexes [TaCpCl₂(CRCHCMe₂NAr-κ²C,N)] (Ar = 2,6-Me₂C₆H₃; Cp = η⁵-C₅Me₅, R = H, 10; Ph, 11; SiMe₃, 12; Cp = η⁵-C₅H₄SiMe₃, R = H, 13; Ph, 14; SiMe₃, 15), the process being regioselective when monosubstituted alkynes RC⋮CH (R = Ph, SiMe₃) were used. All the new compounds were studied by IR and NMR spectroscopy, and the molecular structures of complexes 11 and 14 were determined by X-ray diffraction methods.