Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)OCP][OCP]

The synthesis and thermal redox chemistry of the first antimony (Sb)- and bismuth (Bi)-phosphaketene adducts are described. When diphenylpnictogen chloride [PhPnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)], tetraphenyldipnictogen (PhPn-PnPh) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the -heterocyclic carbene adduct (NHC)-PnPhCl is combined with [Na[OCP]·(dioxane)], Sb- and Bi-phosphaketene complexes are isolated. Thus, NHC serves as an essential mediator for the reaction. Immediately after the formation of an intermediary pnictogen-phosphaketene NHC adduct [NHC-PnPh(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC-C(O)P-PnPh [Pn = Sb () or Bi ()]. In the solid state, and are dimeric with short intermolecular Pn-Pn interactions. When compounds and are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn is thermally reduced to Pn to form tetraphenyldipnictines (PhPn-PnPh) and an unusual -carbene-supported OCP salt, [(NHC)OCP][OCP] (). The formation of compound and PhPn-PnPh from or is unique in comparison to the known thermal reactivity for group 14 carbene-phosphaketene complexes, further highlighting the diverse reactivity of [OCP] with main-group elements. All new compounds have been fully characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy (H, C, and P), infrared spectroscopy, and elemental analysis (, , and ). The electronic structure of and the mechanism of formation were investigated using density functional theory (DFT).