Conceptual Extension of the Degradation–Transformation of N‑Heterocyclic Carbenes: Unusual Rearrangements on Osmium

The range of processes of degradation–transformation of NHC ligands in the coordination sphere of a transition metal has been enlarged. The NHC-acyl ligand of the complex Os­{κ2-C,C-[C­(O)­CH2ImMe]}­Cl­(PiPr3)2 (1) undergoes a complex rearrangement promoted by internal alkynes to give Os­{κ2-C,N-[CH2ImMe]}­Cl­(CO)­(PiPr3)2 (2). Mechanistic studies have revealed that the degradation involves a catalytic alkyne-mediated deinsertion of CO from the acyl moiety to afford Os­{κ2-C,C-[CH2ImMe]}­Cl­(CO)­(PiPr3)2 (3), followed by a thermally activated stoichiometric 1,2-methylene shift from N to C. The catalytic activity of the alkynes depends upon their substituents, decreasing in the sequence diphenylacetylene > 1-phenyl-1-propyne > 3-hexyne > 2-butyne. Phenylacetylene tautomerizes in the metal coordination sphere to afford the stable vinylidene Os­{κ2-C,C-[C­(O)­CH2ImMe]}­Cl­(CCHPh)­(PiPr3)2 (4), which experiences the coupling of the acyl moiety and the vinylidene ligand under a carbon monoxide atmosphere. The addition of HBF4·OEt2 to the resulting complex Os­{κ2-C,C-[C­(CHPh)­C­(O)­CH2ImMe]}­Cl­(CO)2(PiPr3) (5) leads to [Os­{κ2-O,C-[OC­(CHCHPh)­CH2ImMe]}­Cl­(CO)2(PiPr3)]­BF4 (6) containing an NHC-(α,β-unsaturated ketone) ligand.