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)PnBr 3 and [( py CDC)PnBr 2 ][Br] (CDC = carbodicarbene; Pn = Sb or Bi; py = pyridyl). The reaction of (CDC)SbBr 3 ( 1 ) with one or two equivalents of AgNTf 2 (NTf 2 = bis(trifluoromethanesulfonyl)imide) or AgSbF 6 gives stibaalkene mono- and dications of the form [(CDC)SbBr 3- n ][A] n ( 2-4 ; n = 1,2; A = NTf 2 or SbF 6 ). The stibaalkene trication [(CDC) 2 Sb][NTf 2 ] 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 ( py CDC) was used as a ligand to access [( py CDC)PnBr 2 ][Br] ( 10 , 12 ) and trications [( py CDC)Pn][NTf 2 ] 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][NTf 2 ] 2 ( 4 ) has an acceptor number (AN) (84) that is comparable to quintessential Lewis acids such as BF 3 , 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 SbF 5 (85.1 kcal/mol), which suggest that these cations are Lewis superacids.