Alkyne-Induced Facile C–C Bond Formation of Two η2‑Alkynes on Dinuclear Tantalum Bis(alkyne) Complexes To Give Dinuclear Tantalacyclopentadienes

The dinuclear tantalum–alkyne complexes [(η2-RCCR)­TaCl2]2(μ-OMe)2(μ-thf) (2a, R = Et; 2b, R = nPr) were synthesized by treating the mononuclear tantalum–alkyne complexes (η2-RCCR)­TaCl3(dme) (1a, R = Et; 1b, R = nPr) with 1 equiv of NaOMe in THF. We found that adding a catalytic amount (20 mol %) of 3-hexyne to 2a induced the spontaneous formation of Ta2Cl4(OMe)2(μ-C4Et4)­(thf) (4a). Similarly, Ta2Cl4(OMe)2(μ-C4nPr4)­(thf) (4b) was obtained by treatment of 2b with a catalytic amount (20 mol %) of 4-octyne. Reaction of 4a,b with 4-dimethylaminopyridine gave 4-dimethylaminopyridine-coordinated complexes 6a,b, whose structures were elucidated by the X-ray structure of 6a. We conducted a control experiment in which 10 equiv of 4-octyne was added to 2a to give Ta2Cl4(OMe)2(μ-C4-2,3-nPr2-4,5-Et2)­(thf) (7) in 90% yield, indicating that free 4-octyne reacted with the tantalacyclopropene moiety of 2a to form a dissymmetric tantalacyclopentadiene, followed by the release of 3-hexyne. The catalytic activity of 4a–6a for [2 + 2 + 2] cyclotrimerization of 3-hexyne was examined, and we found that their activities were in the order 5a > 4a ≫ 6a.