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 κ¹<i>P</i>,η¹<i>C</i>,η²<i>C</i>,<i>C</i>′. The (alkenylphosphonio)­phenyl complex further undergoes inversion of the coordination face of the alkene moiety at <i>p</i>-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