Formation of (Alkenylphosphonio)phenylruthenium Complexes from Diphenylacetylene and a [CpRu(dppm)] Cation: Experimental Evidence for the Equilibrium between η¹-Disubstituted Vinylidene and η²-Internal Alkyne

The reaction of diphenylacetylene at a cationic ruthenium complex with a dppm (Ph2PCH2PPh2) ligand, [CpRu­(dppm)]+, has been studied to reveal essentially for the first time the existence of an equilibrium between an η2-internal alkyne and η1-disubstituted vinylidene at a transition metal center. The reaction mixture at 70 °C for 69 h unexpectedly afforded a coupling product of diphenylacetylene and the dppm ligand, an (alkenylphosphonio)­phenyl complex [CpRu­{Ph2PCH2P­(C6H4)­Ph­(η2-C­(Ph)CHPh)}]+ with the extremely rare coordination mode of κ1P,η1C,η2C,C′. The (alkenylphosphonio)­phenyl complex further undergoes inversion of the coordination face of the alkene moiety at p-xylene reflux temperature. Both isomers of (alkenylphosphonio)­phenyl complexes as well as the intermediary η2-internal alkyne and η1-disubstituted vinylidene complexes were fully characterized spectroscopically and crystallographically. By considering the structure of the (alkenylphosphonio)­phenyl complexes obtained, the coupling reaction has been proposed to involve the attack of a phosphorus atom on the coordinated diphenylacetylene, which is in equilibrium with an η1-diphenylvinylidene ligand, and the C–H bond activation of a phenyl group on the cationic phosphorus atom leading to the products. Theoretical calculations support the proposed mechanism.