Reactive alkyne complexes of tantalum and their metallacyclization chemistry: Models for alkyne cyclotrimerization by the early transition metals

Article abstract of DOI:10.1021/om00099a031

The reduction of Ta(DIPP)₃Cl₂ (DIPP = 2,6-diisopropylphenoxide) in the presence of bulky alkynes RC≡CR′ (R = R′ = Ph; R = Me₃Si, R′ = Me) provides the alkyne adducts (DIPP)₃Ta(RC≡CR′) in high yield. Unlike all previously known tantalum alkyne complexes, (DIPP)₃Ta(PhC≡CPh) readily undergoes metallacyclization reactions with smaller alkynes RC≡CR′ (R = R′ = Me or Et) and terminal alkynes RC≡CR′ (R = CMe₃, SiMe₃, or Ph; R′ = H) to form the tantalacyclopentadienes (DIPP)₃Ta(CPh=CPhCR′=CR). The molecular structure of the related metallacyclic complex (DIPP)₃Ta(CEt=CEtCEt=CEt) has been determined. This compound crystallizes in the monoclinic space group P2₁/n [No. 14] with a = 14.340 (15) ?, b = 16.322 (22) ?, c = 19.929 (11) ?, β = 94.05 (7)°, V = 4655.9 ?³, and ρ(calcd) = 1.25 g cm^-3 for mol wt 877.05 and Z = 4. Structure solution and refinement included 3191 reflections with F_o² > 3.0σ(F_o²) of 9088 total (8245 unique) reflections measured for final discrepancy indices of R_F = 4.6% and R_wF = 4.5%. The molecular structure reveals a severely crowded coordination sphere, which is consistent with the fact that alkyne cyclotrimerization does not proceed beyond this point. By using the less crowded precursor Ta(DIPP)₂Cl₃ and decreasing the alkyne size from Me₃SiC≡CSiMe₃ to PhC≡CPh to MeC≡ CMe, successively higher coordinated alkyne cyclooligomers (C₂, C₄, and C₆ compounds, respectively) can be isolated.