Cationic Triarylchlorostibonium Lewis Acids

Organopnictogen cations show promise as powerful, tunable main-group Lewis acid catalysts. The synthesis, solid-state structures, and reactivity of a series of weakly coordinated triarylchlorostibonium salts [Ar3SbCl][B(C6F5)4] (Ar = Ph, 3-FC6H4, 4-FC6H4, 3,5-F2C6H3, 2,4,6-F3C6H2) are reported. The cation in each adopts a tetrahedral coordination environment of antimony, with near complete separation from the anion. Structural, computational, and reactivity studies reveal that the Lewis acidity of [Ar3SbCl]+ generally increases with increased fluorination of the Ar substituents, with a secondary quenching effect from para fluorination. [Ar3SbCl]+ is reduced to Ar3Sb in the presence of Et3SiH, and the mechanism of this reaction has been modeled computationally. Preliminary studies demonstrate that they are useful catalysts for the dimerization of 1,1-diphenylethylene and the Friedel–Crafts alkylation of benzene.


■ INTRODUCTION
Organopnictogen derivatives are gaining increasing attention due to their catalytic potential, including in previous transition metal-dominated redox catalysis and as a new generation of tunable Lewis acid catalysts. 1−5 Electrophilic group 15 cations in particular are versatile Lewis acids, with potential applications in catalysis and anion sensing. Fluorophosphonium salts such as [(C 6 F 5 ) 3 PF][B(C 6 F 5 ) 4 ] (A, Figure 1) and its derivatives have been shown to catalyze a diverse range of organic transformations, including C−X activation and C−C bond forming reactions. 6−12 These compounds benefit from possessing a well-defined site of Lewis acidity, namely, the lowlying σ* P−F lowest unoccupied molecular orbital (LUMO), trans to the fluoride substituent, the accessibility of which relies on the use of a bulky, non-coordinating anion. Related Pn(V) cations of the heavier pnictogens (Pn = As, Sb, and Bi) are attractive targets as the increased electropositivity of these elements compared to P offers the possibility of considerably increased Lewis acidity, as well as potentially divergent reactivity compared with P congeners. A small number of organostibonium cations have been investigated for their catalytic reactivity, including [(C 6 F 5 ) 4 Sb][B(C 6 F 5 ) 4 ], which promotes the hydrodefluorination of compounds containing strong C sp3 −F bonds in the presence of Et 3 SiH. 13 Dicationic bis-stibonium ions (B for example, Figure 1) and phosphinesupported stibonium derivatives have also been shown to activate aldehydes and catalyze the transfer hydrogenation of quinolines. 14−17 Very few halide-substituted stibonium cations have been reported. 18−21 Of these, [Mes 3 SbCl][SbCl 6 ] (C, Figure 1) is the only example not supported by cation−anion interactions in the solid state; the structure of [Ph 3 SbCl]-[SbCl 6 ] contains Sb···ClSbCl 5 contacts (3.231(6) Å). 18,19 Both of these chlorostibonium salts slowly promote the polymerization of tetrahydrofuran (THF); however, while [Ph 3 SbCl]-[SbCl 6 ] efficiently catalyzes the dimerization of 1,1-diphenylethylene (DPE) (Scheme 5b), C is inactive in this reaction, attributed to the lower electron deficiency of the Sb center. 18 We reasoned that weakly coordinated cations of the form [R 3 SbX] + (X = halide) would have the threefold advantages of a highly Lewis acidic cationic Sb(V) center, a well-defined Lewis acidic site trans to the halide substituent, and the potential to modulate reactivity by varying the organic substituents, making them ideal targets in the design of tunable Lewis acid catalysts.
Here, we report the synthesis of a series of triarylchlorostibonium salts featuring near-tetrahedral [Ar F 3 SbCl] + cations with different degrees of fluorination at the aryl substituents and an investigation of their Lewis acidity and reactivity using both computational and experimental methods.

■ RESULTS AND DISCUSSION
In order to isolate cations of the form [Ar 3 SbCl] + with an accessible acidic site, minimizing the formation of cation− anion interactions, salts of the form [Ar 3 SbCl][B(C 6 F 5 ) 4 ] were targeted due to the weakly coordinating and chemically inert nature of the borate anion. 22 The triarylstibine dichlorides Ar 3 SbCl 2 (2-Ar) were synthesized by oxidative chlorination of the arylstibines 1-Ar (Scheme 1). Adding a solution of 2-a to a suspension of a slight excess of [(Et 3 Si)(C 7 H 8 )][B(C 6 F 5 ) 4 ] in toluene, followed by recrystallization, yielded analytically pure [Ph 3 SbCl][B(C 6 F 5 )] (3-a) (Scheme 2). The synthesis of four other triarylchlorostibonium salts 3-b to 3-e was carried out in an equivalent manner. Similar treatment of 2-f and 2-g resulted in multiple products, predominantly unreacted triarylstibine dichlorides. It appears that in these cases, the presence of highly electron withdrawing fluorinated substituents makes chloride abstraction by the silyl cation unfavorable.
The 1 H, 13 C{ 1 H}, and 19 F NMR spectra of 3-a to 3-e showed signals in the expected regions for the cations, with only small changes in the chemical shift compared to the respective dichlorides 2-Ar (see Electronic Supplementary Information, ESI). In all cases, the ipso C could not be identified in the 13 C{ 1 H} NMR spectrum due to broadening from coupling with quadrupolar Sb nuclei. The presence of the anion was confirmed by the characteristic 19 F resonance at −133.7 ppm, though anion resonances in the 13 C{ 1 H} spectra were weak and broad, presumably due to extensive coupling with 19 F and 11 B, and only the sharpest para-F resonances have been assigned as these are characteristic. The solid-state structures of 3-Ar were determined by X-ray crystallography (Figures 2 and S6−S9). Key structural parameters for the series are summarized in Table 1. In all cases, the cation adopts a distorted tetrahedral geometry, with mean Cl−Sb−C angles ranging between 105.1°and 108.1°, in marked contrast to 98.3°in the near-trigonal bipyramidal [Ph 3 SbCl][SbCl 6 ]. The borate counterions are very weakly coordinating, with between two and four long Sb···F−B contacts that vary significantly in length and angle of approach within the 3-Ar series, including between the two independent cations within the structure of 3d ( Figure S14), 23 suggesting that the dominant factor dictating these contacts is packing effects rather than any directional interaction. The minimum Sb···F−B distance in 3-a is 3.80 Å, considerably longer than the Sb···Cl−Sb distance of 3.20 Å in the [SbCl 6 ] salt of this anion despite the smaller radius of F. However, some Sb···F−B contacts remain within the sum of the van der Waals radii of Sb and F (3.93 Å), meaning that while the cations in 3-Ar cannot be described as fully uncoordinated, they can be considered to be very weakly coordinated, with a vacant acidic site trans to Sb−Cl, which is not significantly blocked by anion interactions. Across the series, the length of the covalent Sb−Cl bond decreases slightly  Ellipsoids are shown at 50% probability, and hydrogen atoms have been omitted for clarity. One phenyl group in 3-a is disordered over two positions, and only one is shown here.
with increased fluorination of the aryl substituents as would be expected for an increasingly electron deficient Sb center (with the exception of 3-b, in which the bond length is surprisingly slightly longer than that in 3-a), and are generally around 0.2 Å shorter than those in the parent stibine dihalides 2-Ar. 24 During early attempts to synthesize 3-a in which 2-a was used in slight excess, small amounts of a dimeric monocation of the form [(Ph 3 SbCl) 2 (μ-Cl)][B(C 6 F 5 ) 4 ] (4-a) were also isolated and characterized crystallographically ( Figure 2). By using an appropriate reaction stoichiometry, 4-a was directly targeted and isolated in 62% yield (Scheme 2), though elemental analysis suggests some 3-a may be present as a minor product, potentially exchanging in solution with 4-a on the spectroscopic timescale, resulting in only one set of NMR resonances.
Similar dinuclear fluorobismuthonium cations were recently reported, along with mononuclear and trinuclear examples, though in their case, aggregation appears to be entirely controlled by steric factors rather than the precursor ratio. 25 A fluoride-bridged tetraarylstibonium dimer with a related structure was also reported recently. 26 The structural parameters of the two trigonal bipyramidal Sb centers in 4-a are similar, with terminal Sb−Cl bond lengths (mean 2.380 Å) shorter than those in the neutral 2-a (2.481 Å) but longer than those in the monomeric 3-a cation (2.282 Å), indicating an intermediate positive charge at each Sb. A small amount of the tetraarylstibonium salt [(4-FC 6 H 4 ) 4 Sb][B(C 6 F 5 ) 4 ] was also isolated from a solution of 3-c after standing for several days and was crystallographically characterized ( Figure S11).
We proceeded to investigate the Lewis acidity of the 3-Ar series both computationally and experimentally. Fluoride ion affinity (FIA) is one measure that can be used to compare the Lewis acidities of a series of compounds. 27 FIAs were calculated for the cationic fragments of the salts 3-Ar (Ar = a−g) in dichloromethane (DCM) and are included in Tables 1  and S2. While the figures obtained suggest significant Lewis acidity in all cases, it is unsuitable to quantitively compare them with FIAs reported for other Lewis acids, which are calculated using different levels of theory. However, within the 3-Ar series, some general trends can be identified. Increasing fluorination of the aryl substituents leads to an increased FIA, with fluoride in the meta position having a stronger effect than fluoride in the para position, in line with their respective Hammett parameters; 28 hence, 3c and 3e have lower predicted FIAs than 3-b or 3-d. As expected, FIAs for the unobtainable 3-f and 3-g are the highest in the series, though they remain lower than that of [Et 3 Si(tol)] + . However, it is notable that the LUMO energies of these two cations are the only ones significantly more negative than [Et 3 Si(tol)] + , perhaps providing an explanation for the failure to synthesize them from [Et 3 Si(tol)][B(C 6 F 5 ) 4 ] (Table S2).
A close examination of the Kohn−Sham orbitals of [Ar 3 SbCl] + (Figure 3 and ESI) demonstrates that in all cases, the LUMO has a predominantly σ* Sb−Cl character, similar to those previously computed for cations of this type 18 and reminiscent of the σ* P−F orbital that has been shown to act as the Lewis acidic site in related fluorophosphonium species. 6 The large component trans to the Sb−Cl bond gives a likely angle of approach for Lewis base interactions.
The Gutmann−Beckett method is a semi-quantitative measure of effective Lewis acidity based on the change in the 31 P NMR shift of Et 3 PO in the presence of a Lewis acid. 29 This method assumes simple adduct formation; however, the reactivity of 3-Ar in the presence of Et 3 PO appears more complicated, with multiple peaks observed in the 31 P NMR spectrum in some cases. Crystals obtained from a mixture of 3e and Et 3 PO in CD 2 Cl 2 were determined to be highly disordered 2-e co-crystallized with a molecule of Et 3 PO ( Figure S12). This reactivity is reminiscent of A (Figure 1), which forms the respective phosphine difluoride (C 6 F 5 ) 3 PF 2 on mixing with dimethylformamide. 6 We hypothesize that the source of Cl is another molecule of 3-e and that a Schlenk-like equilibrium may exist (Scheme 3).
While it was not possible to isolate the [B(C 6 Figure S13). While X-ray structural data are of poor quality, the previously reported trigonal bipyramidal dication 31 can be unambiguously identified, giving further weight to this interpretation.
Initial attempts to synthesize 3-Ar employed [(Et 3 Si) 2 (μ-H)][B(C 6 F 5 ) 4 ] prepared in situ from trityl borate and neat Et 3 SiH. 32 These reactions were not high yielding, and nuclear magnetic resonance (NMR) spectroscopy suggested that some reduction to the parent stibine (1-Ar) had occurred. This was overcome by the use of purified [(Et 3   We suspected that the generation of 1-Ar in the first instance could be attributed to the reduction of the stibonium product by residual Et 3 SiH. Direct reaction of excess Et 3 SiH with isolated 3-a and 4-a resulted in stoichiometric conversion to the parent stibine 1-a. We undertook density functional theory (DFT) calculations to elucidate the mechanism of this reduction. The most energetically accessible pathway involves nucleophilic attack on the Sb center by a hydride, yielding a trans-Ar 3 SbClH intermediate (IM1) that isomerizes to cis-Ar 3 SbClH (IM2) followed by reductive elimination of HCl ( Figure 4). Other routes involving elimination of benzene or chlorobenzene or initial chloride abstraction from IM2 were modeled and found to be kinetically inaccessible ( Figures S66  and S67). A number of fluorine-containing impurities in addition to Et 3 SiF were identified by NMR, possibly the result of decomposition of the borate anion by a transient uncoordinated Et 3 Si + cation. This reactivity is not unprecedented; [(C 6 F 5 ) 4 Sb][B(C 6 F 5 ) 4 ] also reduces to the parent stibine in the presence of Et 3 SiH, 13 and the reduction of [(C 6 F 5 ) 3 PF] + (A, Figure 1) occurs via a similar mechanism. 36 This reactivity precludes the use of these salts as direct catalysts for reactions involving silanes, such as the hydrodefluorination of halocarbons. One potentially powerful feature of group 15 cations is the ability to activate C−X bonds. The stibonium salts 3-a and 3-e and the dimer 4-a rapidly react with trityl chloride or trityl fluoride (Ph 3 CCl/Ph 3 CF) in CD 2 Cl 2 to form Ar 3 SbCl 2 / Ar 3 SbClF, respectively, demonstrating the halophilicity of these cations. The stability of the resulting trityl cation assists the reaction thermodynamically.
To probe the relative chlorophilicity of the triarylstibonium salts, competition reactions were performed in which 3-Ar were mixed with 2-Ar′ (Ar ≠ Ar′). Chloride exchange between these two species generates 2-Ar and 3-Ar′ (Scheme 4), the equilibrium favoring the formation of the dichloride with the more electron withdrawing aryl substituents and the stibonium cation with the less electron withdrawing aryl substituents. To assess the catalytic potential of 3-Ar for C−X bond activation, we chose the Friedel−Crafts alkylation of benzene as a test reaction (Scheme 5a). t BuBr was selected as an alkylating agent as we reasoned that the Sb−Br bond in the putative Ar 3 SbClBr intermediate would be weak enough to eliminate HBr on reacting with the Wheland intermediate, and the bulky t Bu would impede any carbocation rearrangements. A series of 3-Ar with increasing fluorination of the aryl substituents were tested and are moderate catalysts in this reaction (Table 2); interestingly, 3-c gives the highest yield despite crystallographic and computational data suggesting that it is of intermediate Lewis acidity. This may be due to requiring a balance between the rate of initial C−Br activation by the 3-Ar cation and that of Sb−Br cleavage from the resulting 2-Ar intermediate; however, more detailed mechanistic studies would be required to confirm this. No decomposition of the catalyst was observed in any of these reactions.

Organometallics pubs.acs.org/Organometallics Article
The stibonium cations 3-Ar also catalyze the dimerization of DPE, a common test reaction for Lewis acid catalysis (Scheme 5b). The reaction was complete after 2 h at room temperature in CD 2 Cl 2 with 5% catalyst loading. We also tested 4-a, which only gave traces of the product under the same conditions, suggesting that the dimeric structure is largely retained in solution, leaving no vacant catalytic site.

■ CONCLUSIONS
In conclusion, we have reported the synthesis of a family of distorted tetrahedral chlorostibonium ions with electron withdrawing aryl substituents. Both experimental and computational investigations of their reactivity demonstrate that they are halophilic Lewis acids with well-defined LUMO sites that are accessible due to very weakly coordinating borate anions. Acidity was seen to increase with increasing fluorination of the aryl substituents, with a slight quenching effect from para fluorides. The chlorostibonium ions are reduced in the presence of Et 3 SiH, this reduction being proceeded through Sb−H containing intermediates. We have conducted the first investigation into the catalytic potential of these types of cations for C−C bond-forming Friedel−Crafts alkylation reactions, demonstrating that the catalyst efficiency is highly dependent on the aryl substituent. ■ EXPERIMENTAL METHODS General Considerations. Caution: All antimony-containing compounds should be treated as toxic. All manipulations were performed under an atmosphere of dry N 2 using standard Schlenk or glovebox (Mbraun Unilab 2000) techniques unless otherwise stated. All glassware was dried in an oven at 150°C and cooled under vacuum before use. THF, DCM, toluene, and n-hexane were dried using an Mbraun MB SPS5. All deuterated solvents were dried and stored over 4 Å molecular sieves. SbCl 3 was sublimed in vacuo at 40− 65°C before use. Triethylsilane was distilled over CaH 2 , degassed by freeze/pump/thaw, and stored over 4 Å molecular sieves. All other reagents were used as received unless otherwise stated. [Et 3 Si-(C 7 H 8 )][B(C 6 F 5 ) 4 ] was synthesized according to literature methods 32,37,38 and recrystallized in toluene at −10°C or by the addition of Et 3 SiH to a solution of [Ph 3 C][B(C 6 F 5 ) 4 ] in toluene. 35 Synthetic details for the preparation of 1-Ar and 2-Ar are given in the Supplementary Information.

3-a [Ph 3 SbCl][B(C 6 F 5 ) 4 ]. To a stirring solution of [(Et 3 Si)C 7 H 8 ]-
[B(C 6 F 5 ) 4 ] (0.416 g, 0.47 mmol) in toluene (30 mL) was added a solution of Ph 3 SbCl 2 (0.165 g, 0.39 mmol) in toluene (15 mL) at room temperature. An off-white oily suspension formed instantly. This was stirred at room temperature for 90 min. The oil was allowed to settle, and the solution was decanted. The oil was dried in vacuo to an oily solid, which was dissolved in CH 2 Cl 2 and layered with hexane to give colorless crystals suitable for X-ray diffraction (XRD), which were isolated by filtration (0.065 g, 0.06 mmol, 15%). 1 (30 mL) was added a solution of (3-FC 6 H 4 ) 3 SbCl 2 (0.186 g, 0.39 mmol) in toluene (15 mL) at room temperature. A red oil formed instantly, and the reaction mixture was stirred at room temperature for 90 min. The oil was allowed to settle, and the solution was decanted. The oil was dried under vacuum and triturated with hexane to give a white waxy solid, which was dissolved in CH 2 Cl 2 (5 mL) and layered with hexane (30 mL) to give colorless crystals suitable for XRD, which were isolated by filtration (0.120 g, 0.1 mmol, 25%). 1 (30 mL) was added a solution of (4-FC 6 H 4 ) 3 SbCl 2 (0.162 g, 0.34 mmol) in toluene (15 mL) at room temperature. A red oil formed instantly, and the reaction mixture was stirred at room temperature for 90 min. The oil was allowed to settle, and the solution was decanted. The oil was dried under vacuum to give a white foam, which was dissolved in CH 2 Cl 2 (10 mL), giving a dark solution. The solution was filtered through celite and layered with hexane to give clear colorless crystals suitable for XRD, which were isolated by filtration (0.056 g, 0.05 mmol, 15%). 1  15 mol) in toluene was added dropwise to a solution of (2,4,6-F 3 C 6 H 2 ) 3 SbCl 2 (0.0879 g, 0.15 mmol) in toluene (2 mL), which resulted in the formation of a red-orange oil, which was stirred for 1 h. The solvent was removed in vacuo to afford a red oil, which was washed with hexane. The oil was dissolved in CH 2 Cl 2 (∼10 mL) and layered with hexane (∼15 mL) to afford green clear crystals suitable for XRD, which were isolated by filtration (0.078 g, 0.06 mmol, 40%). 1

4-a [(Ph 3 SbCl) 2 (μ-Cl)][B(C 6 F 5 ) 4 ].
A solution of Ph 3 SbCl 2 (0.254 g, 0.60 mmol) in toluene (15 mL) was added to a suspension of [Et 3 Si][B(C 6 F 5 ) 4 ] in toluene (10 mL), freshly prepared from [Ph 3 C][B(C 6 F 5 ) 4 ] (0.231 g, 0.25 mmol), to yield a reddish oily suspension, which was stirred at room temperature for 90 min. Then, the solution was decanted and stood at room temperature, yielding white crystals, which were washed with n-hexane (3 × 5 mL) and dried in vacuo to yield a white crystalline solid (0.022 g, 0.015 mmol). 1 4 ], identified by XRD, were obtained by layering a DCM solution with n-hexane. The red oil that dried in vacuo was then washed with n-hexane (3 × 5 mL) and dried in vacuo to give a light red powder, which was spectroscopically identical to 4-a. Combined yield: 0.222 g, 0.156 mmol, 62%. A reliable elemental analysis could not be obtained probably due to Organometallics pubs.acs.org/Organometallics Article impurities including 3-a formed as a minor product in the synthesis, which would be difficult to distinguish spectroscopically. Gutmann−Beckett Method. The stibonium salt (0.05 mmol) and Et 3 PO (0.0012 g, 0.001 mmol) were mixed in CD 2 Cl 2 and loaded into a J-Young NMR tube. 1  Friedel−Crafts Alkylation of Benzene. An NMR tube was loaded with t BuBr (14 mg, 0.10 mmol), benzene (16.0 mg, 0.20 mmol), and the catalyst (0.005 mmol) in CDCl 3 (0.7 mL). The mixture was allowed to stand for 30 min, and then a 1 H NMR spectrum was obtained. Conversion was determined by relative integration of the t BuBr (δ H = 1.81 ppm) and t BuPh (δ H = 1.34 ppm) peaks.

■ COMPUTATIONAL METHODS
All calculations were performed using Gaussian 09 Revision E0.01. 39 All geometries were optimized in the presence of a self-consistent reaction field, specifically the solvation model based on density (SMD, DCM) without imposing symmetry constraints at the M062X/def2-SVP level of theory in conjunction with the associated effective core potential on Sb. London dispersion effects were included via Grimme's D3 atom pairwise correction. 40−42 Counteranions were omitted from all calculations. Subsequent analytical vibrational frequency calculations on optimized geometries were utilized to confirm the nature of stationary points (zero and exactly one imaginary mode for minima and transition states, respectively). Moreover, within the ideal gas/rigid rotor/harmonic approximation, these calculations also provided thermal and entropic corrections to the Gibbs free energy at 1 atm and 298.15 K. Electronic energies were obtained from single-point calculations at the M062X-D3/def2-QZVPP (in conjunction with the associated effective core potential on Sb) level of theory including a polarizable continuum model (SMD) to account for solvent effects (parameters corresponding to those of DCM). 43 The Kohn−Sham orbitals were visualized using Chemcraft. 44 Single-point energies for FIAs were calculated in the gas phase using M062X-D3/def2-QZVPP 40,45 and subsequently corrected for the solvent effect via the self-consistent reaction field (SMD with parameters corresponding to DCM). FIAs were then calculated using an isodesmic reaction, which was referenced against the defluorination reaction COF 3 − → COF 2 + F − . The enthalpy change for this anchor reaction was considered to be 208.8 kJ mol −1 from the experiment. 27 ■ X-RAY CRYSTALLOGRAPHY All crystallographic measurements were performed at 150 K using an Oxford Diffraction single-crystal diffractometer with a Sapphire 3 CCD plate (graphite-monochromated Mo Kα radiation, λ = 0.71073 Å or Cu Kα radiation λ = 1.54184 Å). In each case, a specimen of suitable size and quality was selected, coated with the Fomblin Y oil, and mounted onto a nylon loop. Unit cell finding, data collection, data reduction, and space group determination were performed using CrysAlis Pro. The analytical numeric absorption correction using a multifaceted crystal model was implemented. 46 The empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was applied for absorption correction. 47 Using Olex2, 48 the structure was solved with the ShelXT structure solution program using intrinsic phasing and refined with the ShelXL refinement package using least-squares minimization. 49,50 All hydrogen atoms were geometrically placed and refined using the riding model approximation.
■ ASSOCIATED CONTENT * sı Supporting Information