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

Robert J. Gilliard,Guocang Wang,Jacob E. Walley,Erik Kertész,Diane A. Dickie,Levi S. Warring,Zoltán Benkő

doi:10.1021/acs.inorgchem.0c03683

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).

Highlights

Due to their unique electron distribution, heteroketenes show versatile and fascinating chemistry
The most important of these are summarized as follows. (i) Dimer formation: the P C bond of phosphaketenes is prone to cycloaddition, which results in 4-membered rings; this process can be minimized with the incorporation of bulky substituents, or with heteroatoms.[5] (ii) Formation of constitutional isomers: due to its ambident reactivity, the [OCP]− anion may bind through the P or the O center. Oxophilic species, such as s-3c,6 or f-block elements,[7] favor the oxyphosphaalkyne isomer, while soft Lewis acidic elements, for example, the heavy group 14 elements (Ge, Sn, Pb),4b,8 and Ga4c,9 favor the phosphaketene isomer. Both O- and P-bound isomers are known for B4a,10 and Si,4b illustrating the ambident nature of the [OCP]− anion. (iii) Redox chemistry: the OCP anion is prone to oxidation by many metals due to its reductive nature and the electrophilic character of the metals.3a,11 While point (i) can be circumvented using sterically demanding substituents to stabilize the phosphaketene, points (ii) and (iii) are more challenging to avoid because the inherent electrophilic properties of the main-group elements differ widely across the periodic table
Compound 5 represents the first example of an ionic compound where the cation and anion each possess an OCP unit. These results further demonstrate the utility of the 2phosphaethynolate ion as a reductant and contrast with the chemistry observed for the group 14 (Sn and Ge) analogues, which undergo decarbonylation to yield phosphinidenyl species

Summary

■ INTRODUCTION

Due to their unique electron distribution, heteroketenes show versatile and fascinating chemistry. The formation of the tetraphenyldibismuthine may indicate a radical mechanism, in which the first step would be the homolytic dissociation at the P−Bi bond of adduct 4, or alternatively, the free Ph2BiPCO Both reactions are highly endothermic (ΔE = 53.0 and 51.7 kcal/mol, respectively); they are unlikely to happen even at higher temperature. This reaction is rather endothermic and proceeds via an activation barrier of 27.9 kcal/mol (Figure 9), resulting in a weakly bound complex of NHC and Ph2BiPCO at the energy of 27.2 kcal/mol Even though this reaction is likely shifted toward the side of the starting adduct, the formation of small amounts of free carbene is expected, especially if the entropy factor is taken into account (dissociation Gibbs free energy: 18.9 kcal/mol). Ph2BiPCO Ph2BiBiPh2 formed in step 1 delivers the dibismuthine as well as the [OCP]− anion for compound 5

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES