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) (<b>2a</b>, R = Et; <b>2b</b>, R = ^<i>n</i>Pr) were synthesized by treating the mononuclear tantalum–alkyne complexes (η²-RCCR)­TaCl₃(dme) (<b>1a</b>, R = Et; <b>1b</b>, R = ^<i>n</i>Pr) with 1 equiv of NaOMe in THF. We found that adding a catalytic amount (20 mol %) of 3-hexyne to <b>2a</b> induced the spontaneous formation of Ta₂Cl₄(OMe)₂(μ-C₄Et₄)­(thf) (<b>4a</b>). Similarly, Ta₂Cl₄(OMe)₂(μ-C₄^<i>n</i>Pr₄)­(thf) (<b>4b</b>) was obtained by treatment of <b>2b</b> with a catalytic amount (20 mol %) of 4-octyne. Reaction of <b>4a</b>,<b>b</b> with 4-dimethylaminopyridine gave 4-dimethylaminopyridine-coordinated complexes <b>6a</b>,<b>b</b>, whose structures were elucidated by the X-ray structure of <b>6a</b>. We conducted a control experiment in which 10 equiv of 4-octyne was added to <b>2a</b> to give Ta₂Cl₄(OMe)₂(μ-C₄-2,3-^<i>n</i>Pr₂-4,5-Et₂)­(thf) (<b>7</b>) in 90% yield, indicating that free 4-octyne reacted with the tantalacyclopropene moiety of <b>2a</b> to form a dissymmetric tantalacyclopentadiene, followed by the release of 3-hexyne. The catalytic activity of <b>4a</b>–<b>6a</b> for [2 + 2 + 2] cyclotrimerization of 3-hexyne was examined, and we found that their activities were in the order <b>5a</b> > <b>4a</b> ≫ <b>6a