Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP]

The synthesis and thermal redox chemistry of the first antimony (Sb)- and bismuth (Bi)-phosphaketene adducts are described. When diphenylpnictogen chloride [Ph 2 PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane) x ], tetraphenyldipnictogen (Ph 2 Pn-PnPh 2 ) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N -heterocyclic carbene adduct (NHC)-PnPh 2 Cl is combined with [Na[OCP]·(dioxane) x ], 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 2 (PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC-C(O)P-PnPh 2 [Pn = Sb ( 3 ) or Bi ( 4 )]. In the solid state, 3 and 4 are dimeric with short intermolecular Pn-Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn III is thermally reduced to Pn II to form tetraphenyldipnictines (Ph 2 Pn-PnPh 2 ) and an unusual bis -carbene-supported OCP salt, [(NHC) 2 OCP][OCP] ( 5 ). The formation of compound 5 and Ph 2 Pn-PnPh 2 from 3 or 4 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 ( 1 H, 13 C, and 31 P), infrared spectroscopy, and elemental analysis ( 1 , 2 , and 5 ). The electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).