Synthesis and Properties of Triarylhalostibonium Cations

Article abstract of DOI:10.1021/acs.inorgchem.7b00293

As part of our fundamental interest in the chemistry of main-group Lewis acids, we have decided to target stibonium cations whose Lewis acidity is enhanced by the presence of a halogen substituent directly bound to antimony. Starting from Ph₃Sb

Full text of DOI:10.1021/acs.inorgchem.7b00293

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Synthesis and Properties of Triarylhalostibonium Cations
Mengxi Yang and Franco̧ is P. Gabbaï*
Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
S
* Supporting Information
ABSTRACT: As part of our fundamental interest in the
chemistry of main-group Lewis acids, we have decided to
target stibonium cations whose Lewis acidity is enhanced by
the presence of a halogen substituent directly bound to
antimony. Starting from Ph3Sb(OTf)2 (1) and Mes3Sb(OTf)2
(2), we successfully prepared the triflate derivatives Ph3SbF-
(OTf) (3) and Mes3SbF(OTf) (4). We also synthesized the
hexachloroantimonate salt of [Mes3SbCl]+ (6), an analogue of
the known [Ph3SbCl][SbCl6] (5). The structures of these
complexes have been investigated experimentally as well as
computationally using density functional theory methods.
While direct interaction is observed between the anion and the
stibonium center in compounds 3−5, compound 6 exists as an
ionic solid with the four-coordinate [Mes3SbCl]+ cation separated from the [SbCl6]− anion. The structural difference observed
between the two hexachloroantimonate derivatives 5 and 6 is ascribed to the increased steric protection provided by the larger
mesityl substituents. To understand how these structural differences affect the properties of these antimony species, we have
compared their catalytic activity in two simple reactions, namely, the polymerization of tetrahydrofuran and the Friedel−Crafts
dimerization of 1,1-diphenylethylene. These studies show that 5 is the most active catalyst for both reactions, suggesting that the
reactivity of these species is controlled by both the coordinating nature of the counteranion and the steric accessibility of the
reactive antimony center.
sensing11 and catalysis. With respect to the latter, it has long
INTRODUCTION
■
been known that simple stibonium cations such as [Ph4Sb]+
can catalyze the addition of isocyanates to epoxides.12 It has
more recently been shown that stibonium cations can also be
used to promote hydrosilylation or allylstannation of
aldehydes13 among other reactions.14 On the basis of the
prediction that a higher Lewis acidity could be obtained
through the use of electron-withdrawing substituents, we
synthesized the stibonium salt [Sb(C6F5)4]+ (B+; Chart 1)
and found that it is sufficiently acidic to polymerize
tetrahydrofuran (THF) or abstract a fluoride anion from
[SbF6]−.15 In a continuation of these studies and inspired by
the work of Stephan and co-workers on fluorophosphonium
cations,4,5 we have now decided to investigate the synthesis and
properties of stibonium cations whose Lewis acidity is
enhanced by a halogen substituent (C+; Chart 1).
Electron-deficient group 15 compounds are attracting growing
interest in the field of Lewis acid catalysis. While it has been
shown that simple phosphonium cations can promote a range
of reactions including aldol additions,1 the cyanosilylation of
aldehydes,2 and hydroformylation reactions,3 this field of
research has attracted a renewed interest prompted by the
introduction of highly electron-deficient fluorophosphonium
cations such as [(C6F5)3PF]+ (A+; Chart 1)4 and [(SIMes)-
Chart 1
RESULTS AND DISCUSSION
■
Triarylfluorostibonium Triflates. Realizing the stabilizing
influence of the counteranion, we decided to first consider
halostibonium salts with anions of intermediate coordinative
ability. Based on the knowledge that stibonium triflates
PFPh2]2+ (SIMes = 1,3-dimesitylimidazolidin-2-ylidene).5
These electrophilic phosphorus cations6 are highly reactive
and catalyze hydrodefluorination,4 hydrosilylation,7 hydro-
deoxygenation,8 Friedel−Crafts-type dimerization,5 and dehy-
drocoupling reactions.9
In parallel to these developments, several groups including
ours have become interested in the Lewis acidic properties of
antimony(V) compounds10 and their use in the field of anion
Special Issue: Advances in Main-Group Inorganic Chemistry
Received: February 2, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b00293
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sometimes adopt molecular rather than ionic structures in the
solid state, we investigated the reaction of Ar3SbF2 (Ar = Ph,
Mes)16 with a stoichiometric amount of trimethylsilyl triflate
(TMSOTf). In situ 19F NMR measurements confirmed the
formation of fluorostibonium triflates, as indicated by the
presence of a SbF resonance at −156 ppm for Ph3SbF(OTf)
(3) and −145 ppm for Mes3SbF(OTf) (4) in CH2Cl2. Efforts
to isolate the pure salts from these mixtures were complicated
by the oily nature of the residue, which impeded purification.
Faced with these difficulties, we considered an alternative
approach based on ligand redistribution starting from Ar3SbF2
and the corresponding Ar3Sb(OTf)2. This approach was
inspired by the elegant work of Burford and co-workers, who
recently described Ph3Sb(OTf)2 (1).10d,e For the purpose of
this study, we synthesized the mesityl analogue, namely,
Mes3Sb(OTf)2 (2). This new compound, which could be
conveniently obtained by the reaction of Mes3SbBr2 with
AgOTf in CH2Cl2, has been spectroscopically characterized.
Gratifyingly, we found that 1 and 2 react with their
By contrast, 1H and 13C NMR of 4 show the expected
resonances, with no sign of impurities or decomposition,
suggesting that 4 is kinetically stabilized by the bulky mesityl
ligands. Despite the asymmetry introduced by the presence of
two different axial ligands in 4, the o-methyl groups from the
mesityl substituents are not differentiated, giving rise to a single
1H NMR resonance at 2.50 ppm in CD2Cl2. The equivalence of
these resonances is rationalized by invoking the rapid rotation
of the mesityl substituents about the Sb−Cipso bonds. In
agreement with this view, we observed a single aromatic CH
resonance at 7.10 ppm.
Colorless single crystals of compounds 2−4 could be
obtained by layering a CH2Cl2 solution of the stibonium salt
with hexanes at −20 °C. The structure of 2 resembles that
reported for 1 (Figure 1 and Table 1). The antimony center
adopts a trigonal-bipyramidal geometry, with the triflate anions
occupying the apical sites. It is interesting to note that the Sb−
O distances in 2 [2.178(2) Å] are almost equal to those in 1
[2.1708(14) Å]10d despite the larger steric demand of the
mesityl substituents. The structures of 3 and 4 (Figure 1) are
again best described as trigonal-pyramidal, with the fluoride and
triflate anions spanning the apical sites. However, the Sb−O
distances of 2.2493(15) Å in 3 and 2.325(9) Å in 4 are longer
than those in 1 and 2, thus indicating that the triflate anions are
more loosely coordinated to the antimony atom. This
lengthening is distinctly more acute in the case of 4, a factor
that we assign to the bulk of the mesityl substituents. The
structures of 1−4 have been optimized using density functional
theory (DFT) methods, as implemented in Gaussian 09.17
These optimizations were carried out using the M06 func-
tional18 and mixed basis set [6-31g(d) for carbon, hydrogen,
and oxygen, 6-31g(d′) for fluorine, 6-311g* for sulfur and
chlorine, and cc-pVTZ-PP19 with Stuttgart relativistic small-
core effective core potential (ECP)20 for antimony] to produce
structures that closely match those determined by X-ray
diffraction. The optimized structures were also subjected to
natural bond orbital (NBO) analysis to extract natural
population analysis (NPA) charges (Table 2). These charges,
combined with the Sb−F and Sb−O Wiberg bond indices
(WBIs), are in good agreement with the structural results and
show that the antimony centers of 3 and 4 have greater cationic
character while forming a weaker bond with the triflate anion.
Triarylchlorostibonium Hexachloroantimonates. Fol-
lowing the observation that the triflate anions remain
Scheme 1. Synthesis of the Ditriflate and Fluorotriflate
Derivatives
corresponding difluorides to afford 3 and 4 (Scheme 1). The
19F NMR spectra of both 3 and 4 display two sets of
resonances, which correspond to the triflate CF3 and SbF
moieties, respectively. These two signals, which appear at −78
and −156 ppm for 3 and −78 and −145 ppm for 4, show the
expected 3:1 intensity ratio. It is interesting to note that the
chemical shifts of the antimony-bound fluorine atoms are
distinctly more upfield than those measured for Ph3SbF2 (−148
ppm) and Mes3SbF2 (−100 ppm). The origin of the larger
change observed in the chemical shift of the antimony-bound
fluorine atom upon conversion of Mes3SbF2 into 4 has not
been elucidated. Compounds 3 and 4 have also been
1
investigated by H and 13C NMR spectroscopy. Clean spectra
of compound 3 could not be obtained even when starting from
thoroughly dried CD2Cl2 and recrystallized materials. We
assign these difficulties to the reactive nature of this compound.
Figure 1. Structures of 2 (left), 3 (middle), and 4 (right) in the crystal. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are
omitted for clarity. The asymmetric unit of 4 contains two independent molecules, one of which resides about the C3 axis. The molecule of 4 shown
in the figure does not reside in a crystallographic special position.
B
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Table 1. Selected Structural Parameters for Compounds 1−4
∠O1−Sb−F1/O4
(deg)
avg. ∠C−Sb−C
avg. ∠O1−Sb−C
avg. ∠F1/O4−Sb−C
compd
110d
Sb1−O1 (Å)
2.1716(14)
Sb1−F1/O4 (Å)
avg. Sb−C (Å)
(deg)
(deg)
(deg)
2.1708(14)
2.084
173.21(5)
120.0
89.8
89.8
2
2.173(2)
2.178(2)
2.123
174.91(9)
120.0
89.3
90.7
3
2.2493(15)
2.0003(15)
2.085
177.75(6)
119.9
87.9
92.1
a
4
2.325(9) [2.344(15)]
1.948(7) [1.958(12)]
2.096 [2.142]
176.5(3) [175.2(7)]
119.9 [120.0]
88.2 [88.9]
91.8 [91.8]
a
The metrical parameters given in square brackets correspond to the molecule that resides about the C3 axis.
Table 2. Calculated NBO Partial Charges (NPA) and WBIs
of Compounds 1−4 in the Gas Phase
WBI
compd
NPA on Sb
Sb−O
Sb−F
1
2
3
4
2.2926
2.3226
2.5806
2.3635
0.2976
0.2778
0.2646
0.2297
0.4601
0.4232
coordinated to the antimony center of 3 and 4, we decided to
investigate the use of a more weakly coordinating counteranion.
A review of the literature shows that monohalostibonium
cations such as [Ph3SbCl]+ have been previously isolated as
hexachloroantimonate ([SbCl6]−) salts. Given the weakly
coordinating nature of this anion, we decided to revisit some
aspects of this chemistry. While it has been shown previously
that Ph3SbCl2 reacts with SbCl5 in CCl4 to afford [Ph3SbCl]-
[SbCl6] (5; Scheme 2),21 we found that this synthesis could be
Figure 2. Structure of 6 in the crystal. Ellipsoids are drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths (Å) and angles (deg) for 6: Sb1−Cl1 = 2.322(2), Sb1−
C1 = 2.112(9), Sb1−C10 = 2.092(10), Sb1−C19 = 2.105(9), average
∠C−Sb−C = 116.3, and average ∠Cl−Sb−C = 101.2.
cation of 6 is closer to a tetrahedral geometry than the
[Ph3SbCl]+ cation of 5, with τ4 values22 of 0.84 for 5 and 0.89
for 6, respectively. We attribute the absence of such short
contacts between the [SbCl6]− anion and the stibonium center
of 6 to the protection offered by the o-methyl groups of the
mesityl substituents, as illustrated in Figure 3. The antimony
Scheme 2. Synthesis of the Triarylchlorostibonium
Hexachloroantimonates
easily carried out in CH2Cl2, leading to a moderate yield (55%)
of this salt. This approach also proved to be well adapted to the
synthesis of [Mes3SbCl][SbCl6] (6), which was obtained in
75% yield by the reaction of Mes3SbCl2 with SbCl5 in CH2Cl2.
1
In the H NMR spectrum, the methyl groups of 6 give rise to
two resolved resonances at 2.45 ppm (6H) and 2.42 ppm (3H),
respectively. Similar to compound 4, we propose that the
detection of only two methyl resonances originates from the
rapid rotation of the mesityl substituents about the Sb−Cipso
bonds. In agreement with this view, we observe a single
aromatic CH resonance at 7.22 ppm (2H). We also note that
this resonance is more downfield than that measured for the
ditriflate derivative 1 (7.11 ppm) and the fluorotriflate
derivative 4 (7.09 ppm).
In the crystal, we observe that 6 exists as a salt with no short
contact between the [SbCl6]− anion and the stibonium cation
(Figure 2). This is in contrast to the previously reported
structure of 5 in which the [SbCl6]− anion interacts with the
stibonium center via a long Sb−Cl bond of 3.231(6) Å.21
Congruent with the different degrees of cation−anion
interactions observed in 5 and 6, we find that the [Mes3SbCl]+
Figure 3. Space-filling model of cations [Ph3SbCl]+ (left) and
[Mes3SbCl]+ (right).
atom is 0.30 Å above the plane defined by the three ipso-carbon
atoms in the structure of 5 and 0.41 Å in that of 6. The
distorted tetrahedral geometry of 6 is further characterized by
average ∠Cl−Sb−C and ∠C−Sb−C angles of 101.2° and
116.3°, respectively. The Sb−Cl bond distance in [Mes3SbCl]+
[2.322(2) Å] is as short as that in [Ph3SbCl]+ [2.325(7) Å],21
pointing to a very electrophilic antimony center. This view is
corroborated by the fact that this bond distance is shorter than
C
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that measured in (C6F5)4SbCl (2.45 Å),15 Ph3SbCl2 (av. 2.49
after 2 h, as established by 1H NMR spectroscopy. By contrast,
5 and 6 were significantly more active, leading to conversions of
17% [turnover frequency (TOF) = 1.6 min−1] for 5 and 15%
(TOF = 1.3 min−1) for 6 in 2 h. Letting the reaction run for
longer periods gave viscous polymer solutions, thereby
complicating NMR analysis. These results point to the
inhibitory role played by the triflate anion, which may
coordinate too strongly to the Lewis acidic antimony center.
Such limitations do not seem to affect 5 and 6, in accordance
with the more weakly coordinating nature of the [SbCl6]−
counteranion. The higher activity displayed by 5 most likely
originates from steric effects, with the smaller phenyl
substituents affording a more exposed and thus more reactive
antimony center. This argument is supported by the observed
catalytic activity of these compounds in the Friedel−Crafts
dimerization of 1,1-diphenlyethylene (Chart 2). Compound 5
was the only one to show any activity, leading to a 99% yield of
1-methyl-1,3,3-triphenyl-2,3-dihydro-1H-indene after 20 min,
with a catalyst loading of only 5%. Compounds 1, 2, 4, and 6
showed no activity even after 1 day. The fact that 6 was inactive
also ruled out the possibility of the hexachloroantimonate anion
being responsible for the catalytic properties of 5. While a
similar reactivity has been reported for electrophilic phospho-
niums and fluorosulfoxonium cations,5,25 this is the first use of a
stibonium catalyst for this reaction.
Å),23 and Ph4SbCl (2.69 Å).24
On the basis of the knowledge that the Lewis acidity of group
15 compounds originates from low-lying σ* orbitals centered
on the pnictogen atom,15 we examined the molecular orbitals of
both [Ph3SbCl]+ and [Mes3SbCl]+. The geometries of the
cations were optimized with the M06 functional and mixed
basis sets [6-31+g(d′) for carbon, hydrogen, and chlorine and
cc-pVTZ-PP19 with Stuttgart relativistic small-core ECP20 for
antimony]. The lowest unoccupied molecular orbital (LUMO)
of both [Ph3SbCl]+ and [Mes3SbCl]+ is centered on the
antimony atom and is dominated by σ*(Sb−Cl) character.
Smaller components of the LUMO reside on the ortho and
para positions of the arene substituents, suggesting a small
degree of σ*−π conjugation (Figure 4). The LUMO energy of
CONCLUSION
■
Our results demonstrate that halostibonium cations of the
general formula [Ar3SbX]+ can be readily synthesized using
either anion-exchange or anion-abstraction reactions. The high
electrophilic character of the antimony center in these species is
reflected by the tendency of these stibonium cations to interact
with the counteranion as in the case of 3 and 4, which exist as
molecular rather than ionic solids. The formation of
halostibonium salts necessitates the use of the more weakly
coordinating hexachloroantimonate anion as well as the steric
protection of the antimony center, as in the case of 6.
Computational studies show that the [Ph3SbCl]+ present in 5 is
the most electron-deficient cation investigated in this study.
This view is supported by the highest reactivity that 5 displays
in the polymerization of THF and the Friedel−Crafts
dimerization of 1,1-diphenlyethylene. These results show that
the reactivity of such species is controlled by the coordinating
nature of the counteranions, the steric accessibility of the
reactive antimony center, and possibly the greater electron-
withdrawing ability of the phenyl substituents present in 5.
Figure 4. LUMO of [Ph3SbCl]+ (left) and [Mes3SbCl]+ (right).
Isovalue = 0.02.
[Ph3SbCl]+ is 42 kJ/mol lower that of [Mes3SbCl]+, consistent
with the greater electron-withdrawing ability of Ph versus Mes
as well as with the experimental observation that [Ph3SbCl]+
strongly interacts with the [SbCl6]− anion in the solid state.
Hence, electronic effects may also be partly responsible for the
absence of a direct interaction between the mesitylstibonium
[Mes3SbCl]+ cation and the [SbCl6]− anion in the solid state of
6.
Reactivity Studies. We have investigated the reactivity of
all compounds described above, with the exception of 3, whose
1H and 13C NMR spectra showed that it could not be reliably
handled in solution (vide supra). We first decided to assess the
relative reactivity of these derivatives by investigating their
ability to polymerize THF (Chart 2). With a catalyst loading of
0.1 mol % in neat THF, we observed that 1, 2, and 4 were
almost inactive, with only 0.1% of the monomer polymerized
EXPERIMENTAL SECTION
■
General Considerations. Caution! Antimony compounds are
potentially toxic and should be handled accordingly. Air-sensitive
experiments were carried out using standard glovebox or Schlenk
techniques in the absence of oxygen and moisture. All glassware was
dried in an oven and cooled under vacuum before use.
Triphenylantimony and potassium fluoride were purchased from
EMD Millipore, 2-bromomestyliene, n-butyllithium (2.2 M in
hexanes), and 1,1-diphenlyethylene were purchased from Alfa Aesar,
and antimony pentachloride was purchased from Acros Organics. All
commercially available chemicals were used without further
Chart 2. Catalytic Reactions Investigated
26
purification. SbMes3, PhICl2,27 Ph3SbCl2,28 Ph3SbBr2,29 Ph3Sb-
(OTf)2 (1),10d and Mes3SbBr226 were prepared according to reported
procedures. The solvents were dried by passing through an alumina
column (pentane and CH2Cl2), distillation under N2 over Na/K
(Et2O, n-hexane, and THF), or distillation under N2 over CaH2
(CDCl3, CD2Cl2, CD3CN, and CH3CN). All other solvents were ACS
D
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reagent grade and were used as received. NMR spectra were recorded
overnight. The supernatant was discarded by cannula filtration,
affording a yellow precipitate, which was dried in vacuo. This
precipitate was identified as 5 (558 mg, 55% yield). The product was
stored in the −40 °C freezer in the glovebox. Single crystals were
grown at −40 °C in 3−5 days after a CH2Cl2 solution of 5 was layered
with pentanes. The unit cell matches the reported data. 1H NMR (500
MHz, CD2Cl2): δ 7.97−7.94 (m, 2H), 7.93−7.89 (m, 1H), 7.88−7.83
(m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 135.80 (s, p-Ph),
134.96 (s, o-Ph), 132.37 (s, m-Ph), 127.31 (quaternary). Elem anal
Calcd: C, 29.90; H, 2.09. Found: C, 29.52; H, 2.40.
1
on a Varian Unity Inova 400 FT NMR (399.52 MHz for H, 375.92
MHz for 19F, 161.74 MHz for 31P, and 100.46 MHz for 13C) or a
1
Varian Unity Inova 500 FT NMR (499.42 MHz for H, 469.86 MHz
for 19F, 202.18 MHz for 31P, and 125.60 MHz for 13C) at ambient
temperature. Chemical shifts (δ) are given in ppm and are referenced
against residual solvent signals (1H and 13C) or external standards
[BF3·Et2O for 19F (−153 ppm) and 85% H3PO4 for 31P (0 ppm)].
Elemental analyses were performed at Atlantic Microlab (Norcross,
GA).
Synthesis of [Mes3SbCl][SbCl6] (6). A solution of SbCl5 (179 mg,
0.59 mmol) in CH2Cl2 (5 mL) was slowly added to a solution of
Mes3SbCl2 (304 mg, 0.59 mmol) in CH2Cl2 (5 mL) in a 50 mL
Schleck tube. The reaction mixture was allowed to stir for 10 min, at
which point a large amount (∼40 mL) of hexanes was added while the
reaction was still stirring. The resulting suspension was left to stand at
−40 °C overnight. The supernatant was discarded by cannula
filtration, affording a white precipitate, which was dried in vacuo.
This precipitate was identified as 6 (380 mg, 75% yield). Single crystals
were grown at −40 °C in 3−5 days after a CH2Cl2 solution of 6 was
layered with hexanes. 1H NMR (500 MHz, CD2Cl2): δ 7.22 (s, 2H, m-
Mes), 2.45 (s, 6H, o-CH3), 2.42 (s, 3H, p-CH3). 13C{1H} NMR (126
MHz, CD2Cl2): δ 146.30 (s, p-Mes), 142.84 (s, o-Mes), 132.41 (s, m-
Mes), 131.27 (s, quaternary), 24.60 (s, o-CH3), 21.05 (s, p-CH3). Elem
anal. Calcd: C, 38.19; H, 3.92. Found: C, 37.90; H, 3.80.
Synthesis of Mes3Sb(OTf)2 (2). A suspension of AgOTf (720 mg,
2.80 mmol) in CH2Cl2 (5 mL) was added to a solution of Mes3SbBr2
(883 mg, 1.38 mmol) in CH2Cl2 (5 mL) in a 50 mL Schleck tube.
After stirring in the dark for 1 h, the solution was filtered with a
cannula and transferred to another Schlenk tube. Next, a large amount
of hexanes (∼40 mL) was added to the solution, and the resulting
suspension was left to stand at −40 °C overnight. The supernatant was
discarded by cannula filtration, affording a white precipitate, which was
dried in vacuo. This precipitate was identified as 2 (800 mg, 75%
yield). Single crystals were grown at −40 °C in 3−5 days after a
1
CH2Cl2 solution of 2 was layered with hexanes. H NMR (399 MHz,
CD2Cl2): δ 7.11 (s, 2H, m-Mes), 2.38 (s, 6H, o-CH3), 2.36 (s, 3H, p-
CH3). 19F NMR (376 MHz, CD2Cl2): δ −79.50 (s). 13C{1H} NMR
(100 MHz, CD2Cl2): δ 145.43 (s), 143.41 (s, o-Mes), 131.90 (s),
130.83 (s, m-Mes), 119.84 (q, JC−F = 318.3 Hz, OTf), 24.07 (s, o-
CH3), 21.36 (s, p-CH3). Elem anal. Calcd for 2·CH2Cl2: C, 41.78; H,
4.09. Found: C, 42.09; H, 4.67.
Catalytic Polymerization of THF. In the glovebox, the antimony
catalyst (0.01 mmol) was dissolved in THF (1 mL) in a vial. Aliquots
from the reaction mixture were transferred into NMR tubes, mixed
Synthesis of Ph3SbF(OTf) (3). A solution of 1 (187 mg, 0.28
mmol) in CH2Cl2 (5 mL) was added to a solution of Ph3SbF2 (141
mg, 0.36 mmol) in CH2Cl2 (5 mL) in a 50 mL Schleck tube. The
reaction mixture was allowed to stir for 10 min, at which point a large
amount (∼40 mL) of hexanes was added while the reaction was still
stirring. The resulting suspension was left to stand at −40 °C
overnight. The supernatant was discarded by cannula filtration,
affording a white precipitate, which was dried in vacuo. This
precipitate was identified as impure 3 (250 mg). Single crystals were
grown at −40 °C in 4 days after a CH2Cl2 solution of 3 was layered
1
with CDCl3, and analyzed by H NMR. The progress of the reaction
was derived from the integrated signals of the monomer and polymer.
Catalytic Friedel−Crafts Dimerization of 1,1-Diphenylethy-
lene. In the glovebox, a NMR tube was charged with a CH2Cl2
solution (0.6 mL) of the antimony catalyst (0.025 mmol). Next, 1,1-
diphenlyethylene (64 μL, 0.49 mmol) was added to the NMR tube via
1
a microsyringe. The reaction was monitored by in situ H NMR.
Crystallographic Measurements. All crystallographic measure-
ments were performed at 110(2) K using a Bruker SMART APEX II
diffractometer with a CCD area detector (graphite-monochromated
Mo Kα radiation, λ = 0.71073 Å) at 110 K. In each case, a specimen of
suitable size and quality was selected, coated with paratone oil, and
mounted onto a nylon loop. Crystals of 3 and 4 were mounted on a
nylon loop over dry ice to avert decomposition. The semiempirical
method SADABS was applied for absorption correction. The structures
were solved by direct methods and refined by a full-matrix least-
squares technique against F2 with anisotropic temperature parameters
for all non-hydrogen atoms. All hydrogen atoms were geometrically
placed and refined using the riding model approximation. Data
reduction and further calculations were performed using the Bruker
SAINT and SHELXTL-NT program packages.30
Theoretical Calculations. All structures were optimized starting
from the crystal structure geometries, using the program and level of
theory specified in the Results and Discussion section. Cartesian
coordinates of the optimized structures are provided in the Supporting
Information. For compounds 2 and 4, weakly negative frequencies
associated with methyl group rotation were observed. Because these
rotations affect peripheral groups, efforts to carry out additional
optimization cycles were not considered. None of the other structures
displayed imaginary frequencies, indicating that a local minimum on its
potential energy hypersurface had been reached. The optimized
structures were also subjected to NBO31 analysis. The molecular
orbitals were visualized and plotted in the Jimp2 program.32
1
with hexanes. H NMR (500 MHz, CD2Cl2): δ 8.06−8.02 (m, 2H),
7.74−7.65 (m, 3H). 19F NMR (470 MHz, CD2Cl2): δ −78.57 (s, 3F,
OTf), −156.27 (s, 1F, SbF). 13C{1H} NMR (126 MHz, CD2Cl2): δ
134.16 (d, o-Ph, J = 2.9 Hz), 133.42 (s, p-Ph), 130.35 (s, m-Ph),
119.04 (q, JC−F = 318.3 Hz, OTf). Elem anal. Calcd: C, 43.79; H, 2.90.
found: C, 44.06; H, 2.97. Note: Despite repeated recrystallizations
1
under multiple conditions, the H and 13C NMR spectra showed the
presence of other species, leading to the conclusion that this
compound could not be isolated or handled confidently in solution.
Synthesis of Mes3SbF(OTf) (4). A solution of 2 (235 mg, 0.30
mmol) in CH2Cl2 (5 mL) was added to a solution of Mes3SbF2 (154
mg, 0.30 mmol) in CH2Cl2 (5 mL) in a 50 mL Schleck tube. The
reaction mixture was allowed to stir for 10 min, at which point a large
amount (∼40 mL) of hexanes was added while the reaction was still
stirring. The resulting suspension was left to stand at −40 °C
overnight. The supernatant was discarded by cannula filtration,
affording a white precipitate, which was dried in vacuo. This
precipitate was identified as 4 (285 mg, 74% yield). Single crystals
were grown at −40 °C in 3−5 days after a CH2Cl2 solution of 4 was
layered with hexanes. 1H NMR (500 MHz, CD2Cl2): δ 7.10 (s, 2H, m-
Mes), 2.50 (s, 6H, o-CH3), 2.35 (3H, p-CH3). 19F NMR (470 MHz,
CD2Cl2): δ −78.52 (s, 3F, OTf), −144.66 (s, br, 1F, SbF). 13C{1H}
NMR (126 MHz, CD2Cl2): δ 143.69 (s), 143.56 (s), 137.70 (s),
131.38 (s, m-Mes), 119.93 (q, JC−F = 319.6 Hz, OTf), 23.60 (s, o-
CH3), 21.30 (s, p-CH3). Elem anal. Calcd: C, 51.95; H, 5.14. Found:
C, 51.67; H, 5.09.
ASSOCIATED CONTENT
* Supporting Information
■
Synthesis of [Ph3SbCl][SbCl6] (5). This procedure is a variant of
that available in the literature.21 A solution of SbCl5 (396 mg, 1.33
mmol) was slowly added to a solution of Ph3SbCl2 (563 mg, 1.33
mmol) in CH2Cl2 (5 mL) in a 50 mL Schleck tube. The resulting
yellow solution was allowed to stir for 10 min, at which point a large
amount (∼40 mL) of hexanes was added while the reaction was still
stirring. The resulting suspension was left to stand at −40 °C
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00293.
Additional experimental data (synthesis of Ph3SbF2,
Mes3SbCl2, and Mes3SbF2), NMR spectra for com-
E
DOI: 10.1021/acs.inorgchem.7b00293
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pounds 2, 3, and 6, and Cartesian coordinates of the
Forum Article
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optimized structures (PDF)
X-ray crystallographic data of compounds 2−4, 6, and
Mes3SbF2 in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: francois@tamu.edu.
■
ORCID
Franco̧ is P. Gabbaï: 0000-0003-4788-2998
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgment is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research (Grant 56871-ND3). We also acknowledge
support from the Welch Foundation (Grant A-1423) and Texas
A&M University (Arthur E. Martell Chair of Chemistry and the
Laboratory for Molecular Simulation). We thank Dr. Guillaume
́
Belanger-Chabot for bringing the paper listed in reference 21 to
the attention of the authors and for suggesting the use of SbCl5
as a chloride abstracting agent. We thank Dr. Nattamai
Bhuvanesh, Dr. J. Stuart Jones, and Dr. Guillaume Belanger-
́
Chabot for their constructive input on the chemistry described
in this paper.
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