Direct Evidence for a [4+2] Cycloaddition Mechanism of Alkynes to Tantallacyclopentadiene on Dinuclear Tantalum Complexes as a Model of Alkyne Cyclotrimerization

A dinuclear tantalum complex, [Ta2Cl6(-C4Et4)] (2), bearing a tantallacyclopentadiene moiety, was synthesized by treating [((2)-EtCCEt)TaCl3(DME)] (1) with AlCl3. Complex 2 and its Lewis base adducts, [Ta2Cl6(-C4Et4)L] (L=THF (3a), pyridine (3b), THT (3c)), served as more active catalysts for cyclotrimerization of internal alkynes than 1. During the reaction of 3a with 3-hexyne, we isolated [Ta2Cl4(-(4):(4)-C6Et6)(-(2):(2)-EtCCEt)] (4), sandwiched by a two-electron reduced -(4):(4)-hexaethylbenzene and a -(2):(2)-3-hexyne ligand, as a product of an intermolecular cyclization between the metallacyclopentadiene moiety and 3-hexyne. The formation of arene complexes [Ta2Cl4(-(4):(4)-C6Et4Me2)(-(2):(2)-Me3SiCCSiMe3)] (7b) and [Ta2Cl4(-(4):(4)-C6Et4RH)(-(2):(2)-Me3SiCCSiMe3)] (R=nBu (8a), p-tolyl (8b)) by treating [Ta2Cl4(-C4Et4)(-(2):(2)-Me3SiCCSiMe3)] (6) with 2-butyne, 1-hexyne, and p-tolylacetylene without any isomers, at room temperature or low temperature were key for clarifying the [4+2] cycloaddition mechanism because of the restricted rotation behavior of the two-electron reduced arene ligands without dissociation from the dinuclear tantalum center.