Lewis Superacidic Heavy Pnictaalkene Cations: Comparative Assessment of Carbodicarbene-Stibenium and Carbodicarbene-Bismuthenium Ions.

We report a comprehensive assessment of Lewis acidity for a series of carbone-stibenium and -bismuthenium ions using the Gutmann-Beckett (GB) method. These new antimony and bismuth cations have been synthesized by halide abstractions from (CDC)PnBr3 and [(pyCDC)PnBr2][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr3 (1) with one or two equivalents of AgNTf2 (NTf2 = bis(trifluoromethanesulfonyl)imide) or AgSbF6 gives stibaalkene mono- and dications of the form [(CDC)SbBr3-n][A]n (2-4; n = 1,2; A = NTf2 or SbF6). The stibaalkene trication [(CDC)2Sb][NTf2]3 (5) was also isolated and collectively these molecules fill the gap among the series of cationic pnictaalkenes. The Sb cations are compared to the related CDC-bismaalkene complexes 6-9. With the goal of preparing highly Lewis acidic compounds, a tridentate bis(pyridine)carbodicarbene (pyCDC) was used as a ligand to access [(pyCDC)PnBr2][Br] (10, 12) and trications [(pyCDC)Pn][NTf2]3 (Pn = Sb (11), Bi (13)), forgoing the need for a second CDC as used in the synthesis of 5. The bonding situation in these complexes is elucidated through electron density and energy decomposition analyses in combination with natural orbital for chemical valence theory. In each complex, there exists a CDC-Pn double bonding interaction, consisting of a strong σ-bond and a weaker π-bond, whereby the π-bond gradually strengthens with the increase in cationic charge in the complex. Notably, [(CDC)SbBr][NTf2]2 (4) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF3, and tricationic pnictaalkene complexes 11 and 13 exhibit strong Lewis acidity with ANs of 109 (Pn = Sb) and 84 (Pn = Bi), respectively, which are among the highest values reported for any antimony or bismuth cation. Moreover, the calculated fluoride ion affinities (FIAs) for 11 and 13 are 99.8 and 94.3 kcal/mol, respectively, which are larger than that of SbF5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.

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 [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).

Carbodicarbene Bismaalkene Cations: Unravelling the Complexities of Carbene versus Carbone in Heavy Pnictogen Chemistry.

We report a combined experimental and theoretical study on the first examples of carbodicarbene (CDC)-stabilized bismuth complexes, which feature low-coordinate cationic bismuth centers with C=Bi multiple-bond character. Monocations [(CDC)Bi(Ph)Cl][SbF6 ] (8) and [(CDC)BiBr2 (THF)2 ][SbF6 ] (11), dications [(CDC)Bi(Ph)][SbF6 ]2 (9) and [(CDC)BiBr(THF)3 ][NTf2 ]2 (12), and trication [(CDC)2 Bi][NTf2 ]3 (13) have been synthesized via sequential halide abstractions from (CDC)Bi(Ph)Cl2 (7) and (CDC)BiBr3 (10). Notably, the dications and trication exhibit C ⇉ Bi double dative bonds and thus represent unprecedented bismaalkene cations. The synthesis of these species highlights a unique non-reductive route to C-Bi π-bonding character. The CDC-[Bi] complexes (7-13) were compared with related NHC-[Bi] complexes (1, 3-6) and show substantially different structural properties. Indeed, the CDC ligand has a remarkable influence on the overall stability of the resulting low-coordinate Bi complexes, suggesting that CDC is a superior ligand to NHC in heavy pnictogen chemistry.