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)_<i>x</i>], tetraphenyldipnictogen (Ph₂Pn-PnPh₂) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the <i>N</i>-heterocyclic carbene adduct (NHC)-PnPh₂Cl is combined with [Na[OCP]·(dioxane)_<i>x</i>], 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 (<b>3</b>) or Bi (<b>4</b>)]. In the solid state, <b>3</b> and <b>4</b> are dimeric with short intermolecular Pn-Pn interactions. When compounds <b>3</b> and <b>4</b> are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn^III is thermally reduced to Pn^II to form tetraphenyldipnictines (Ph₂Pn-PnPh₂) and an unusual <i>bis</i>-carbene-supported OCP salt, [(NHC)₂OCP][OCP] (<b>5</b>). The formation of compound <b>5</b> and Ph₂Pn-PnPh₂ from <b>3</b> or <b>4</b> 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 (<b>1</b>, <b>2</b>, and <b>5</b>). The electronic structure of <b>5</b> and the mechanism of formation were investigated using density functional theory (DFT).