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

Abstract

The synthesis and thermal redox chemistry of the first antimony (Sb)- and bismuth (Bi)-phosphaketene adducts are described. When diphenylpnictogen chloride [Ph2PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)x], tetraphenyldipnictogen (Ph2Pn-PnPh2) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N-heterocyclic carbene adduct (NHC)-PnPh2Cl 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-PnPh2(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC-C(O)P-PnPh2 [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 PnIII is thermally reduced to PnII to form tetraphenyldipnictines (Ph2Pn-PnPh2) and an unusual bis-carbene-supported OCP salt, [(NHC)2OCP][OCP] (5). The formation of compound 5 and Ph2Pn-PnPh2 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 (1H, 13C, and 31P), 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).