C–H Activations with a T‑Shaped 14 Electron PNP-Rhodium(I) Complex: Step-by-Step Formation and Reactivity of (Vinylidene)rhodium Complexes

The availability of the reactive 14-valence electron [(PNP)Rh^I] complex <b>1</b> allowed a step-by-step monitoring of the metal-centered conversion of the terminal alkyne to the coordinated vinylidene. In a first step, the R–C≡CH (R = H, Ph, C₆H₄F, cyclohex-1-enyl) substrate oxidatively added to <b>1</b> to form an isolable corresponding alkynyl(hydrido)rhodium(III) intermediate (<b>2a</b>–<b>5a</b>), from which intramolecular conversion to the vinylidene complexes <b>2b</b>–<b>5b</b> occurred.

The equilibrium of <b>2a</b>–<b>5a</b> with the preceding η²-π-coordinated alkyne complexes was not observed, the latter being destabilized by >5 kcal/mol as indicated by DFT modeling. This enabled easy monitoring of the vinylidene generation via a 1,3-hydrogen shift mechanism, as evidenced by kinetic measurements in combination with DFT calculations. Some aspects of the reactivity of the parent Rh=C=CH₂ vinylidene complex <b>2b</b> toward electrophiles and nucleophiles was investigated, uncovering a remarkable inertness of the vinylidene-rhodium unit in <b>2b</b>.