Comparative Lewis acidity in fluoroarylboranes: B(o-HC6F 4)3, B(p-HC6F4)3, and B(C6F5)3

Article abstract of DOI:10.1021/om3011195

The Lewis acidic fluoroarylborane B(o-HC₆F₄) ₃ (2) was prepared and its Lewis acid strength assessed in comparison to the known, related boranes B(C₆F₅)₃ (1) and B(p-HC₆F₄

Full text of DOI:10.1021/om3011195

Article
pubs.acs.org/Organometallics
Comparative Lewis Acidity in Fluoroarylboranes: B(o‑HC6F4)3,
B(p‑HC6F4)3, and B(C6F5)3
Matthew M. Morgan, Adam J. V. Marwitz,† Warren E. Piers,* and Masood Parvez
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
S
* Supporting Information
ABSTRACT: The Lewis acidic fluoroarylborane B(o-HC6F4)3 (2) was prepared and its Lewis acid strength assessed in
comparison to the known, related boranes B(C6F5)3 (1) and B(p-HC6F4)3 (3). Experimental methods based on spectroscopic
probes and equilibrium measurements were used to show that B(C6F5)3 is the strongest Lewis acid of the three; while the Lewis
acidities of 2 and 3 are comparable, the p-H-substituted isomer is slightly stronger in the tests employed. This contrasts with
predictions made on the basis of computed bond formation energies, as recently reported by Durfey and Gilbert.
he concept of Lewis acidity is one of the most enduring
and useful in the discipline of chemistry.1 However, unlike
bond strengths between the acids and some standard Lewis
bases. They found, not surprisingly, an additive relationship
between Lewis acidity and both the number and position of the
F atom substitution on the aryl rings. However, they did not
find that 1 was the strongest Lewis acid in the family; rather,
the borane in which one o-F on each aryl ring was replaced by
H, 2, was predicted to be the most potent Lewis acid on the
basis of computed bond formation energies with EMe3 (E 
N, P) bases. The rationale for this finding was that, because H is
smaller than F, the steric penalty for pyramidalization was less
than in the slightly more sterically demanding C6F5 rings of 1.
Since the performance of these boranes in a variety of
applications is directly related to their Lewis acid strength,20,25
we set out to prepare 2, which to our knowledge has not been
studied experimentally, to test its efficacy as a Lewis acid in
comparison to fully fluorinated 1. We also utilized the known
borane in which one p-F of each ring is replaced with H, 3,26 in
these studies as a comparison; it has been shown both
experimentally26 and computationally24 that 3 is slightly less
Lewis acidic than 1. In this case, the difference is due only to
electronic effects, since para substitution has no steric impact
on pyramidalization at boron.
T
the more specific concept of Brønsted acidity, which can be
straightforwardly quantified through pKa measurements, placing
various Lewis acids on a relative scale of Lewis acid strength is
nontrivial and indeed likely not possible in an absolute sense.
Various spectroscopic2−4 and computational5−8 methods have
been put forward that provide useful semiquantitative
information, but the many steric and electronic factors that
go into determining the strength of a given Lewis acid−Lewis
base interaction mean that Lewis acid strength is situation
dependent and cannot necessarily be placed on an absolute
scale.
Fluoroaryl boranes9,10 are an important class of Lewis acids
with a wide variety of applications,11−17 largely as a function of
their strong Lewis acidity and hydrolytic stability.18,19 These
properties stem not only from the fact that the three-coordinate
boron center is electron deficient but also from the strongly
electron withdrawing nature of the fluoroaryl groups. Thus, it is
not surprising that for boranes B(C6H5−nFn)3, the measured or
calculated Lewis acid strength tends to correlate with the
number of F substituents present in the molecule.20 Use of
larger fluoroaryl groups can also boost acidity, but only to a
point; as these groups become sterically larger, the energy
required to pyramidalize the boron center upon interaction
with a Lewis base overwhelms the electron-withdrawing effects
of high fluorine content.21
In the B(C6H5−nFn)3 series, the parent borane B(C6F5)3
(1)22,23 was thus considered to be the strongest Lewis acid in
the family. However, recently Durfey and Gilbert reported a
computational study24 in which they assessed the Lewis acid
strength of all possible members of this series by calculating the
Received: November 21, 2012
© XXXX American Chemical Society
A
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solvent. These bases were chosen on the basis of their differing
steric properties and donor strength. The adducts 2·NCCH3
and 2·PEt3 were both characterized by X-ray crystallography,
and ORTEP depictions of each compound are shown in Figure
1, while selected metrical parameters are given in Table 1. The
RESULTS AND DISCUSSION
■
The new fluoroarylborane 2 was synthesized via a route
analogous to that reported for the p-H-substituted borane 3.26
Thus, the necessary 2,3,4,5-tetrafluorophenyl Grignard reagent
was generated in situ from the corresponding bromide and
27
iPrMgCl in diethyl ether and quenched with /3 equiv of
1
Table 1. Selected Metrical Parameters for Lewis Base
BF3·OEt2 (Scheme 1); careful control of stoichiometry is
Adducts of B(C6F5)3, B(o-HC6F4)3, and B(p-HC6F4)3 with
a
Acetonitrile and Triethylphosphine
Scheme 1
L
param
(deg)
B(C6F5)3
B(o-HC6F4)3 B(p-HC6F4)3
NCCH3
∑
342.9
339.37
337.8
C−B−C
B−N (Å)
N−C (Å)
1.616(3)
1.124(3)
337.9
1.604(4)
1.130(4)
331.74
1.589(3)
1.132(3)
334.5
PEt3
∑
(deg)
C−B−C
B−P (Å)
2.081(4)
2.057(2)
2.078(2)
a
For an explanation of the values in italics, see text.
structures of the adducts for both of these Lewis bases with
B(C6F5)3 (1) have been reported previously (NCCH3;31
PEt332), but only that of 3·PEt3 has been disclosed.26 Therefore,
we also prepared and crystallized the acetonitrile adduct of 3
(the ORTEP drawing is also shown in Figure 1) in order to
have all six structures for comparative analysis.
For the acetonitrile adducts of 1−3, three particular metrical
parameters might be expected to correlate with relative Lewis
acid strength toward this base: the B−N bond length, the C−N
bond length, which is expected to get shorter upon complex-
ation with at Lewis acid,31 and the extent of pyramidalization of
the boron center, as measured by the sum of the C−B−C
angles. In the case of the PEt3 adducts, only the B−P distance
and the sum of the C−B−C angles are relevant. For the
acetonitrile adducts, the values in italics in Table 1 indicate that
the boron center is most pyramidalized and the B−N distance
is shortest in the adduct with the p-H-substituted borane 3,
suggesting acetonitrile is most tightly bound to this LA.
Conversely, it is the fully fluorinated borane that shortens the
C−N triple bond the most in this series; in free NCCH3, the
C−N bond is 1.141(2) Å.33 Thus, no clear trend emerges from
these data, and indeed, the spread in the bond distance data is
rather narrow and differences may be due to packing factors. In
the PEt3 adducts, the extent of pyramidalization at boron is
greater than in the corresponding acetonitrile adducts,
reflecting the greater steric bulk and donor strength of this
base; the B−P bond distances are typical for such
compounds.32,34 Here, the data for the adduct 2·PEt3 imply
that the strongest interaction is with this Lewis acid, although
important to avoid excessive production of tetraarylborate salts.
The diethyl ether adduct of 2 was isolated in 59% yield from
this reaction upon workup. This crude product was carried on
without further purification and treated with an excess of
Me2Si(H)Cl, which removes the coordinated ether via borane-
catalyzed silation of the C−O ether bonds.28−30 Removal of all
volatiles gave an off-white solid that sublimed under high
vacuum at 120 °C to afford the free borane in 53% yield as a
white microcrystalline solid. The 19F NMR spectrum shows the
expected four resonances in a 1:1:1:1 ratio, while the 11B NMR
spectrum in CD2Cl2 is comprised of a broad resonance at 62.3
ppm, comparable to the value found for 3 (58.5 ppm).26
We were unable to obtain X-ray-quality crystals of 2 in its
unligated state, but adducts of 2 with the Lewis bases
acetonitrile and triethylphosphine were straightforwardly
prepared by reacting the two components in a nondonor
Figure 1. Thermal ellipsoid diagrams (35% probability level) of 2·PEt3 (left), 2·NCCH3 (middle), and 3·NCCH3 (right). Ligand hydrogen atoms
are omitted for clarity. Color scheme: B, orange; C, gray; N, blue; F, green; P, pink; aryl H, small green spheres.
B
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again the data for all three adducts are similar enough that firm
conclusions regarding Lewis acid strength are not possible.
Given the inconclusive nature of the structural data, we
assessed the Lewis acidity of each compound in solution using
the Childs2 and Gutmann−Beckett3,4 methods. Both of these
methods measure the perturbation in an NMR chemical shift of
a probe Lewis base upon coordination to a given Lewis acid. In
the Child method, the base is crotonaldehyde and the
resonance probed is that of the γ proton; in the Gutmann−
Beckett method, the base is Et3PO, and the shift in the
position of the 31P NMR resonance is monitored. The results of
both of these tests are summarized in Table 2. At the outset, it
and 42 ppm, respectively, shifted more toward that expected for
a neutral, four-coordinate borane adduct.
A final evaluation of the relative Lewis acidities of these
boranes was performed by allowing pairs of Lewis acids to
compete for 1 equiv of acetonitrile. The lability of this base
allows for rapid establishment of the equilibria shown in
Scheme 3, and measurement of the equilibrium constants was
accomplished by integration of the 19F NMR spectra obtained.
Although complex, the spectra of all four species in each
equilibrium are known and thus easily distinguished in the
spectra of the mixtures (see the Supporting Information). The
samples were prepared by weighing 1 equiv each of an isolated
acetonitrile adduct and a free borane and dissolving in a
measured amount of C6D6; each equilibrium was approached
from both the left-hand and right-hand sides, providing
consistent results as summarized in Scheme 3, in which the
favored side of the equilibrium is on the left.
Table 2. Lewis Acidities of Boranes 1−3 in CD2Cl2 As
Measured by the Childs and Gutmann−Beckett Methods
In each case, all four species are present in easily measurable
quantities, underscoring the small differences in overall Lewis
acidity between these closely related Lewis acids toward
acetonitrile. However, the results in Scheme 3 clearly show
that the o-H-substituted variant 2 is again the weakest of the
series, at least when in competition for a rodlike Lewis base of
minimal steric demand. Most interesting is the equation in
Scheme 3c, in which the p-H-substituted borane 3 competes
more effectively for the Lewis base than Lewis acid 2, which is
predicted to be the stronger Lewis acid.24
a
Data taken from ref 21.
In conclusion, we have prepared the (fluoroaryl)borane B(o-
HC6F4)3 (2) and fully characterized its adducts with acetonitrile
and triethylphosphine. By a variety of experimental spectro-
scopic and thermodynamic measurements we find that,
contrary to predictions on the basis of computed bond
formation energies,24 compound 2 is not more Lewis acidic
than the parent borane B(C6F5)3 (1). Indeed, it is a slightly
weaker Lewis acid than the related compound B(p-HC6F4)3
(3), as measured by the methods described above. Thus, it
seems that electronic effects dominate the Lewis acid properties
of these boranes and that the cumulative steric effects of
substituting three o-fluorines with slightly smaller hydrogen
atoms are minimal, at least as far as the Lewis bases utilized
herein are concerned. It is possible that, for larger Lewis bases,
the steric advantage found in 2 may become important.
Ultimately, the differences in Lewis acid strength among these
three boranes, while measurable, is quite small and the relative
acidities might change when other bases are employed.
should be noted that the differences in LA strength for these
three boranes are quite small; however, both methods show
that the fully fluorinated borane 1 is the strongest Lewis acid in
the series. Perhaps more surprising is the finding that the o-H
substituted borane 2 is the weakest of the three, in contrast to
the findings of Durfey and Gilbert.24 Again, although the
differences are small, the trend is reproducible over several
measurements.
This relative ordering in LA strength was confirmed by
measuring the equilibrium constants for coordination of one
equivalent of ethyl benzoate (Scheme 2).35 This has been done
Scheme 2
EXPERIMENTAL SECTION
■
All manipulations were performed under a purified argon atmosphere
using vacuum line techniques or in an argon-atmosphere glovebox
unless otherwise specified. Toluene and hexane were dried and
purified using the Grubbs/Dow purification system38 and stored in
evacuated glass vessels over sodium/benzophenone ketal. All other
solvents were dried over the appropriate drying agents (CaH2, Na/
benzophenone) and vacuum-distilled prior to use. 1-Bromo-2,3,4,5-
tetrafluorobenzene, BF3·Et2O, and triethylphosphine oxide were used
as received from Aldrich. Crotonaldehyde, acetonitrile, and triethyl-
phosphine were distilled prior to use. All NMR spectra were recorded
from solution in dry, oxygen-free C6D6 or CD2Cl2 on a Bruker UGI-
400 MHz or Bruker RDQ-400 MHz spectrometer operating at 400
MHz (1H), 376 MHz (19F), 100 MHz (13C), or 128 MHz (11B).
Details on the X-ray analyses can be found in the deposited .cif files in
the Supporting Information or via the Cambridge Crystallographic
Data Centre (CCDC 911794−911796).
previously for the parent borane 136 and was accomplished
37
1
using established H NMR spectroscopic methods. The data
shows that the equilibrium constant is largest for 130,36 followed
closely by that measured for the p-H-substituted borane 3.
Interestingly, the Keq value measured for 2 was only [0.3(1)] ×
102, 3−6 times smaller than those found for 1 and 3. Consistent
with this low equilibrium constant, the 11B NMR spectrum of a
1:1 mixture of ethyl benzoate and 2 showed a signal at 60 ppm,
not far perturbed from the shift of the free borane at 62.3 ppm;
by comparison, the 11B chemical shifts for the averaged signals
in the 1·OC(OEt)Ph and 3·OC(OEt)Ph equilibria are 19.235
C
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Scheme 3
Sythesis of B(o-HC6F4)3 (2). The procedure below is modified
from a literature report for B(p-HC6F4)3.26 iPrMgCl (2.0 M in Et2O,
1.05 mL, 2.1 mmol) was added to a stirred solution of 1-bromo-
2,3,4,5-tetrafluorobenzene (500 mg, 2.2 mmol) in diethyl ether (20
mL) at room temperature. After 6 h of stirring, an aliquot of the
reaction mixture was quenched with H2O and the full conversion to
Grignard reagent confirmed by 19F NMR spectroscopy. Trifluorobor-
ane etherate was then added quickly (94 mg, 0.66 mmol), which
caused heating of the reaction vessel momentarily. This mixture was
stirred overnight; the mixture was then concentrated in vacuo to
NMR (CD2Cl2): −132.51 (m), −141.14 (m), −157.73 (m), −158.21
(m). 11B NMR (CD2Cl2): −6.5 (br s).
Synthesis of 2·PEt3. In the glovebox, borane 2 (30 mg, 0.06
mmol) was taken up in hexanes (3 mL). Et3P (∼5 equiv) was then
added, and the solution was stirred for 10 min. The solvent was then
removed in vacuo and the white solid redissolved in hexanes. The
hexanes were allowed to slowly evaporate in the glovebox until clear
crystals (34 mg, 100%) suitable for an X-ray structure determination
formed. 1H NMR (CD2Cl2): δ 6.252 (br mult, 1 × o-H borane), 1.829
(t, 2 × CH2), 1.101 (q, 3 × CH3). 19F NMR (CD2Cl2): δ −124.78 (3
3
× o-F, t JFF = 23 Hz), −140.34 (nonresolved multiplet), −157.50 (t,
1
approximately /4 of its initial volume. Addition of hexanes (20 mL)
3JFF = 23 Hz) −158.37 (nonresolved multiplet). 31P NMR (CD2Cl2):
resulted in the formation of a sticky white precipitate. The slightly
yellow supernatant was transferred to another flask, and the white
precipitate was washed three more times by dissolution in benzene (30
mL) and precipitation of the residue with hexanes (20 mL). The
combined filtrates were evaporated to dryness in vacuo to leave a dark
yellow oil. This oil was dissolved in benzene and transferred into a
sublimator, from which the benzene was sublimed away at 0 °C to give
a brown powder. The brown powder was then sublimed in vacuo at
120 °C to give a fluffy white product (180 mg, 0.39 mmol, crude,
59%). The crude product was then slurried in hexanes and treated with
Me2SiHCl (4 mL), and this mixture was stirred for 3 h. All volatiles
were removed in vacuo to leave a white solid, which was sublimed two
additional times in vacuo at 120 °C to give the final product (160 mg,
0.35 mmol, 53%) free of any ether. 1H NMR (CD2Cl2): δ 6.97 (broad
1
δ 1.89 (d of m). 11B NMR (CD2Cl2): −10.4 (d, JBP = 70 Hz).
Synthesis of 3·NCCH3. In the glovebox, borane 3 (20 mg) was
taken up in acetonitrile (2 mL) and the mixture stirred for 10 min. The
solvent was then removed in vacuo, and the white solid was
redissolved in dry dichloromethane. Through slow evaporation of
the DCM into toluene crystals suitable for an X-ray structural
determination (22 mg, 100%) were recovered from the bottom of the
1
vial. H NMR: δ 7.01 (br m, 3 × p-H), 2.66 (br s, 3 × NCCH3). 19F
NMR: δ −135.19 (m, 6 × o-F), −141.63 (m, 6 × m-F). 11B NMR:
−10.1 (br s).
Childs Lewis Acidity Tests in CD2Cl2. In a sealable NMR tube,
crotonaldehyde (1.6 uL) was added to CD2Cl2. The 1H NMR
spectrum was then recorded at both 253 and 298 K. One equivalent of
B(o-HC6F4)3 or B(C6F5) was then added, and again the spectra were
recorded at 253 and 298 K. An excess of borane was then added to
ensure that all crotonaldehyde was coordinated, and the spectra were
multiplet). 19F NMR (CD2Cl2): −125.5, −138.7, −146.8, −155.1. 11
B
=
1
NMR (CD2Cl2): 62.3. 13C{1H} NMR (CD2Cl2): δ 151.66 (dd JCF
1
again recorded. H NMR (253 K): H3CCHCHCHO reference δ
252 Hz, 2JCF = 10 Hz, 3 × o-CF), 147.87 (dd 1JCF = 251 Hz, 2JCF = 10
6.89 (m, 1H); H3CCHCHCHO·B(C6F5)3 adduct δ 7.89 (m, 1H),
reference shift Δδ = 1.00; H3CCHCHCHO·B(o-HC6F4)3 adduct δ
7.85, reference shift Δδ = 0.96; Lewis acidity relative to B(C6F5)3
1
Hz, 3 × m-CF), 144.58 (dm JCF = 262 Hz, 3 × m-CF), 141.37 (dm
1JCF = 278 Hz, 3 × p-CF), 123.70 (nonresolved, broad), 118.79 (d,
2JCF = 16 Hz, o-CH).
1
96.0%. H NMR (298 K): H3CCHCHCHO reference δ 6.87 (m,
Synthesis of 2·NCCH3. In the glovebox, borane 2 (20 mg) was
taken up in acetonitrile (2 mL) and the mixture stirred for 10 min. The
solvent was then removed in vacuo, and the white solid was
redissolved into dry dichloromethane. Crystals of 2·NCCH3 were
obtained via the slow evaporation of a CH2Cl2 solution of 2·NCCH3
1H); H3CCHCHCHO·B(C6F5)3 adduct δ 7.84 (m, 1H), reference
shift Δδ = 0.97; H3CCHCHCHO·B(o-HC6F4)3, adduct δ 7.83;
reference shift Δδ = 0.96; Lewis acidity relative to B(C6F5)3 98.9%.
Gutmann−Beckett Lewis Acidity Test. In a sealable NMR tube,
triethylphosphine oxide (1 mg) was dissolved in CD2Cl2. To this was
added 3 equiv of borane. The 31P NMR spectra of initial and
(22 mg, 100%). 1H NMR (CD2Cl2): 6.33 (m, o-H), 2.66 (s, CH3). 19
F
D
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coordinated triethylphosphine oxide was recorded at 25 °C for
B(C6F5)3, B(o-HC6F4)3, and B(p-HC6F4)3. 31P{1H} NMR: Et3PO
reference δ 51.2; Et3PO·B(C6F5)3 reference adduct δ 77.8, reference
shift Δδ = 26.6; Et3PO·B(o-HC6F4)3 δ 76.7, reference shift Δδ =
25.5; Lewis acidity strength relative to B(C6F5)3 97.3%; Et3PO·B(p-
HC6F4)3 δ 77.4, Δδ = 26.2; Lewis acidity strength relative to B(C6F5)3
98.5%.
Present Address
†1253 University of Oregon, Eugene, OR 97403.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Acetonitrile Competition Reactions General Procedure. A
1:1 mixture of borane−acetonitrile adduct and free borane was loaded
into a sealable J. Young NMR tube and dissolved in CD2Cl2 (0.7 mL).
The 19F NMR spectra of each reaction mixture were then recorded.
For complete fluorine spectra and assignment of peaks, see the
Supporting Information (Figures S20−S22).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
W.E.P. acknowledges the NSERC of Canada for a Discovery
Grant and the Canada Council of the Arts for a Killam
Research Fellowship (2012−2014).
Synthesis and Equilibrium Study of 2−Ethyl Benzoate. To a
solution of 2 (85 mg, 0.17 mmol) in toluene was added ethyl benzoate
(24.32 uL, 0.17 mmol), and the mixture was stirred for 10 min. The
solvent was then removed, the white product was washed three times
with hexanes (10 mL), and the final product was isolated as a fluffy
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1
1
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1
1
δ 0.816, free ethyl benzoate H δ 1.004, adduct H δ 0.874.
X-ray Crystallography. Colorless prismatic crystals of 2·PEt3,
2·NCCH3, and 3·NCCH3 were used for data collection. The crystals
were coated with Paratone 8277 oil (Exxon) and mounted on glass
fibers. All measurements were made on a Nonius Kappa CCD
diffractometer with graphite-monochromated Mo Kα radiation. The
data were collected using ω and φ scans and corrected for Lorentz and
polarization effects and for absorption using the multiscan method.
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Fourier techniques. The non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were included at geometrically idealized
positions and were not refined. An o-F atom in 2·NCCH3 was
disordered over sites F1 and F5 in a 0.586(5):0.414(5) ratio with H2
and H6 occupying 0.414(5) and 0.586(5) site occupancy factors,
respectively. In the final cycles of full-matrix least-squares refinement
using SHELXL97 the weighting schemes were based on counting
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* Supporting Information
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