The Careful Combination of Bismuth Compounds and CAAC: Formal Halonium and Nitrenium Transfer via Radical Pathways

Cyclic (alkyl)(amino)carbenes (CAACs) are key compounds en route to a plethora of unprecedented low-valent and radical complexes of transition metals and main group elements. For p-block elements of the sixth period, however, low-valent and radical complexes of CAACs are extremely rare, and even simple adducts with CAACs are difficult to access. Here we report the full characterization of the first adduct between a CAAC and a bismuth trihalide, [BiCl₃(^Me2CAAC)] (1), which has previously only been spectroscopically characterized. 1 and its in situ generated bromido analog form intensely colored solutions in solvent molecule (THF) and undergo formal halonium ion (i.e., X⁺) transfer from the Bi atom to the carbene carbon atom to give the complex cations [CAAC─X]⁺ (X = Cl, Br). Starting from bismuth amide complexes, formal nitrenium ion (i.e., [NR₂]⁺) transfer reactions are observed to give [CAAC─NR₂]⁺. EPR spectroscopic reaction monitoring indicates the viability of radical processes and suggests the involvement of unprecedented α-aminyl radicals and mononuclear Bi⁰ species. Our investigations reveal easily accessible radical pathways via peerless molecular complexes, which proceed in the absence of external reducing agents, as a key factor on the way to exploit the full potential of carbene ligands in the chemistry of sixth row p-block elements.