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

We herein report the synthesis and characterization of a series of PNP-coordinated rhodium vinylidene complexes with a carbazole-based PNP pincer ligand framework. The availability of the reactive 14-valence electron [(PNP)RhI] complex 1 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 1 to form an isolable corresponding alkynyl(hydrido)rhodium(III) intermediate (2a–5a), from which intramolecular conversion to the vinylidene complexes 2b–5b occurred. The equilibrium of 2a–5a 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 2b toward electrophiles and nucleophiles was investigated, uncovering a remarkable inertness of the vinylidene-rhodium unit in 2b.