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

Abstract

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.

Bismuth Cations: Fluoride Ion Abstraction, Isocyanide Coordination, and Impact of Steric Bulk on Lewis Acidity

“…Electronic effects seem to play a minor role in these particular cases, since the acceptor numbers of species based on the complex fragment [BiPh 2 ] + have been reported to be similar to those of I. [17,37] The most prominent difference between I and 4 Dipp is found for the Lewis acidity towards OPEt 3 (entries 1, 2), where I gives results that are in line with previous findings, [6,17,18,37] but 4 Dipp shows a very pronounced Lewis acidity that even surpasses that of a tri-cationic bismuth species [19] and bismuth cations, in which the Lewis acidity has been increased by tuning of the ligand bite angle. [18] While acceptor numbers in experiments with two equivalents of a donor are expectedly smaller than in cases of a 1 : 1 stoichiometry, these results indicate that the simultaneous and considerable activation of two Lewis bases at the bismuth centers of 4 Dipp and I is possible.…”

“…This parallels previous findings in bismuth(V) and mixed bismuth(III/V) chemistry. [23,24] In a similar reaction, Me 2 BiCl (II) was reacted with Ag[PF 6 ] in toluene, which gave a complex mixture of products according to 1 H and 19 F NMR spectroscopy, none of which could unambiguously be identified. While bismuth cations with neutral donor ligands and [PF 6 ] À counteranions such as [BiPh 2 (NC 5 H 5 ) 2 ] + are accessible, [25] our results indicate that [PF 6 ] À is not a suitable anion for the preparation of donor-free organobismuth cations.…”

“…[32] In case of 4 Dipp , a chemical shift of 238.1 ppm (C 6 D 6 ) or 237.6 (CD 2 Cl 2 ) is observed, corresponding to an even lower field. In the 19 F NMR spectrum, a very broad signal at 120.8 ppm (C 6 D 6 ) or 122.9 (CD 2 Cl 2 ) is observed, hinting at the formation of a contact-ion pair in solution (Figures S3, S16, and S19).…”

Planar bismuth triamides: a tunable platform for main group Lewis acidity and polymerization catalysis

“…35 Analogous bismuth cations and polycations have been reported to be strong Lewis acids with applications in catalysis. [36][37][38][39][40][41][42][95][96][97][98] In a mesomeric tuning approach, one of the adjacent p-donor groups was removed to boost the metal-centred electrophilicity by precluding one of the three N(lp) / Bi(6p) interactions. 43 A third, inductive approach, has been computationally predicted for tuning the electrophilicity in these complexes over a very wide range, 34,44 but this approach remains experimentally unrealized, despite being the most convenient due to its modular nature.…”

Kinetically Stabilized Diarylpnictogenium Ions

“…[1] Unlike those, pnictogenium ions are also strong electrophiles, which can be attributed to the positive charge that attracts any nucleophile present. Consequently, the vast majority of phosphenium ions, [R 2 P] + , arsenium ions, [R 2 As] + , [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] stibenium ions, [R 2 Sb] + [52][53][54][55][56][57][58][59] and bismuthenium ions, [R 2 Bi] + , [60][61][62][63][64][65][66][67][68][69][70][71][72] reported in the literature are either affected by ion pairing or stabilized by electron-rich ligands or substituents, which inter-or intramolecularly fill the vacant p-orbital. These electronically stabilized pnictogenium ions often have a dramatically reduced Lewis acidity and cannot be regarded as genuine 6 VE species as they fulfill or even exceed the octet rule.…”