Influence of fluorination and boronic group synergy on the acidity and structural behavior of o-phenylenediboronic acids

Article abstract of DOI:10.1021/om401146p

The solid-state and solution structural properties and acidity of a series of fluorinated 1,2-phenylenediboronic acids were investigated. Solution NMR studies indicate that these compounds equilibrate with their dehydrated forms, in the simplest case presumably possessing the cyclic benzoxadiborole structure. Ab initio calculations showed that the stability of such cyclic semianhydrides is improved by fluorination of the aromatic ring and complexation of one of the boron centers with water. This was demonstrated by the crystal structure determination of tetrafluoro-1,2-phenylenediboronic acid. The coordinated water molecule participates in very strong intermolecular hydrogen bonding with the OH group bonded to the four-coordinate boron center (d_O...O = 2.423(2) A, E_int = -87 kJ mol^-1). This indicates that in fact this compound is an oxonium, i.e., Bronsted acid, which is exceptional for boronic acids. Under different crystallization conditions, tetrafluoro-1,2-phenylenediboronic acid dimerizes by aggregation of boronic groups, which leads to the formation of an uncommon eight-membered B ₄O₄ ring. Such a coordination dimer exists as the boat conformer, featuring π-π interactions of fluorinated aromatic rings. The enhanced acidity of 1,2-phenylenediboronic acids can be rationalized in terms of a synergic effect of two adjacent boronic groups and is manifested by relatively low pK_a values ranging from 6.0 (1,2-phenylenediboronic acid) to only 3.0 for the perfluorinated derivative.

Full text of DOI:10.1021/om401146p

Article
pubs.acs.org/Organometallics
Influence of Fluorination and Boronic Group Synergy on the Acidity
and Structural Behavior of o‑Phenylenediboronic Acids
Krzysztof Durka,*,† Sergiusz Lulinski,*,† Janusz Serwatowski,† and Krzysztof Wozniak‡
́
́
†Department of Chemistry, University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland
‡Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
S
* Supporting Information
ABSTRACT: The solid-state and solution structural proper-
ties and acidity of a series of fluorinated 1,2-phenyl-
enediboronic acids were investigated. Solution NMR studies
indicate that these compounds equilibrate with their
dehydrated forms, in the simplest case presumably possessing
the cyclic benzoxadiborole structure. Ab initio calculations
showed that the stability of such cyclic semianhydrides is
improved by fluorination of the aromatic ring and complex-
ation of one of the boron centers with water. This was
demonstrated by the crystal structure determination of
tetrafluoro-1,2-phenylenediboronic acid. The coordinated
water molecule participates in very strong intermolecular hydrogen bonding with the OH group bonded to the four-coordinate
boron center (dO···O = 2.423(2) Å, Eint = −87 kJ mol−1). This indicates that in fact this compound is an oxonium, i.e., Brønsted
acid, which is exceptional for boronic acids. Under different crystallization conditions, tetrafluoro-1,2-phenylenediboronic acid
dimerizes by aggregation of boronic groups, which leads to the formation of an uncommon eight-membered B4O4 ring. Such a
coordination dimer exists as the boat conformer, featuring π−π interactions of fluorinated aromatic rings. The enhanced acidity
of 1,2-phenylenediboronic acids can be rationalized in terms of a synergic effect of two adjacent boronic groups and is manifested
by relatively low pKa values ranging from 6.0 (1,2-phenylenediboronic acid) to only 3.0 for the perfluorinated derivative.
counterparts, boron−oxygen compounds exist typically as
monomers. A rare example is tetramethyldiboroxane
(Me2B)2O, which dimerizes in the solid state by forming a
four-membered B2O2 ring.16 A useful method for tuning the
Lewis acidity of arylboronic acids is fluorination of the aromatic
ring. Fluorine atoms are structurally comparable to hydrogen
atoms (similar atomic radii), and they do not participate in
strong intermolecular interactions, although, weak contacts
with fluorine atoms are very desirable in supramolecular
chemistry.17 This is often beneficial from the point of view of
crystal engineering. Recently, we have shown that the boron
atom in fluorinated 1,4-phenylenediboronic acids becomes a
stronger Lewis acid, attracting more electronegative partners
such as oxygen and fluorine atoms as well as π-electron density
from aromatic rings, which affects molecular organization in the
crystal.18 To extend the chemistry in this area, we turned our
attention to fluorinated 1,2-phenylenediboronic acids (Chart
1). In this paper we show that the specific structural behavior of
these compounds in the solid state and solution is dictated by
the superposition of two major factors: the vicinity of two
boronic groups and the strong effect of fluorine substituents.
This observation was also supported by theoretical calculations
and additionally discussed in terms of varying the acidity of the
INTRODUCTION
■
Interest in arylboronic acids is stimulated by their increasing
application in organic synthesis, medicine, materials science,
and analytical chemistry.1 Fluorinated boranes, due to their
Lewis acidic properties, were used in the field of catalysis for
organic transformations,2 polymerization processes,3 and
chemical sensing.4 In the case of 1,2-bifunctional diboranes
the bidentate Lewis acid chelation effect from two boron-
containing groups was crucial for catalytic (polymerization of
olefins,5 activation of group 4 metallocene dialkyls,6 or
7
reduction of CO2 with H2 ) and F−/CN− anion sensing
properties.8 It was also shown that such compounds can chelate
a variety of anions to form new classes of weakly coordinating
anions.5e,9 The related 9,10-dihydro-9,10-diboraanthracenes
have found application in the fields of catalysis (bidentate
Lewis acid for the activation of 1,2-diazines in the inverse-
electron-demand Diels−Alder reaction,10 chelate ditopic Lewis
bases,11 polymerization of olefins12), crystal engineering,13 and
materials chemistry (construction of semiconductors14 and
organic light emitting devices15).
Organoboron Lewis acidity should also be reflected by the
solid-state properties of the compounds presented here.
However, compounds bearing boron−oxygen bonds such as
arylboronic acids are not strong Lewis acids, due to the so-
called back-bonding effect, leading to the partial electronic
saturation of the boron atom. Thus, unlike their aluminum
Received: November 25, 2013
© XXXX American Chemical Society
A
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and 3-B (Figure 1a), whose geometrical parameters differ only
marginally. They are linked via dimeric B(OH)2···2(HO)B
Chart 1. Studied 1,2-Phenylenediboronic Acids
studied compounds, which was evaluated using experimental
pKa measurements.
RESULTS AND DISCUSSION
■
Synthesis. 1,2-Phenylenediboronic acid (1)19 and its
perfluorinated analogue 520 were synthesized according to
published procedures. 4,5-Difluoro-1,2-phenylenediboronic
acid (3) was synthesized by protecting 2-bromo-4,5-difluor-
ophenylboronic acid with N-butyldiethanolamine (BDEA) to
give 2-(2′-bromo-4′,5′-difluorophenyl)-6-butyl[1,3,6,2]-
dioxazaborocan (2a). This kind of protection of the B(OH)2
group was used previously for the generation of various
aromatic21 and heteroaromatic boron−lithium reagents.15d,22
Thus, 2a was subjected to Br/Li exchange in the mixed solvent
Et2O/THF, resulting in the intermediate 2a-Li. The reaction
was performed at low temperature (−90 °C) to avoid the risk
of degradation or isomerization of 2a-Li due to the basicity-
gradient driven migration of lithium to one of the positions
ortho to fluorine atoms. Finally, 2a-Li was treated with
B(OMe)3. The synthesis of the 3,6-difluoro derivative 4 started
from 1,4-difluoro-2-iodobenzene. It was subjected to lithiation/
silylation, and the intermediate arylsilane was ipso-iododesily-
lated with ICl to give 1,4-difluoro-2,3-diiodobenzene (2c). The
key step was accomplished by a double iodine/lithium
exchange reaction using an excess of n-BuLi at low temperature
(−110 °C), as it was suspected that at higher temperatures the
dilithio intermediate may readily decompose via an aryne
mechanism or undergo coupling with the byproduct n-BuI.
Subsequent boronation with B(OMe)3 and quench with
ethereal HCl afforded diboronate 2d. Free acids 3 and 4
were obtained by the aqueous hydrolysis of the corresponding
1,2-phenylenediboronate precursors. It should noted that the
obtained 1,2-phenylenediboronic acids are readily soluble in
water, which should be taken into account during their
isolation. All reactions are shown in Scheme 1.
Figure 1. Molecular structures of (a) 3, (b) 5a, and (c) 5b together
with atom-labeling schemes. Thermal ellipsoids were generated at the
50% level of probability. Hydrogen bonds are shown as red dashed
lines. Selected bond distances (Å) and angles (deg) for 3: C(1)−B(1)
= 1.579(2), C(2)−B(2) = 1.582(2), C(7)−B(3) = 1.583(2), C(8)−
B(4) = 1.588(2), B(1)−O(1) = 1.371(2), B(1)−O(2) = 1.364(2),
B(2)−O(3) = 1.370(2), B(2)−O(4) = 1.366(2), B(3)−O(5) =
1.364(2), B(3)−O(6) = 1.373(2), B(4)−O(7) = 1.373(2), B(4)−
O(8) = 1.353(2); C(1)−C(2)−B(1) = 124.9(1), C(2)−C(1)−B(2) =
125.5(1), C(8)−C(7)−B(3) = 127.0(1), C(7)−C(8)−B(4) =
127.9(1), C(6)−C(1)−B(1)−O(1) = 27.3(1), C(3)−C(2)−B(2)−
O(4) = −45.1(1), C(12)−C(7)−B(3)−O(5) = 23.5(1), C(9)−
C(8)−B(4)−O(8) = −10.8(1). Selected bond distances (Å) and
angles (deg) for 5a: C(1)−B(1) = 1.572(2), C(2)−B(2) = 1.628(1),
B(1)−O(1) = 1.353(1), B(1)−O(2) = 1.372(1), B(2)−O(2) =
1.491(1), B(2)−O(3) = 1.492(1), B(2)−O(4) = 1.452(1); C(6)−
C(1)−B(1) = 136.7(1), C(3)−C(2)−B(2) = 133.1(1), C(1)−B(1)−
O(2) = 109.8(1), C(2)−B(2)−O(2) = 103.3(1), B(1)−O(2)−B(2) =
112.1(1), C(1)−B(1)−O(1) = 125.9(1), C(2)−B(2)−O(3) =
109.2(1), C(2)−B(2)−O(4) = 118.4(1). Selected bond distances
(Å) and angles (deg) for 5b: C(1)−B(1) = 1.58(2), C(2)−B(2) =
1.62(1), C(7)−B(3) = 1.61(1), C(8)−B(4) = 1.60(1), B(1)−O(1) =
1.45(1), B(1)−O(2) = 1.53(1), B(1)−O(4) = 1.50(1), B(2)−O(3) =
1.44(1), B(2)−O(5) = 1.47(1), B(2)−O(2) = 1.54(1), B(2)−O(4) =
1.366(2), B(3)−O(4) = 1.53(1), B(3)−O(6) = 1.44(1), B(3)−O(7)
= 1.57(1), B(4)−O(7) = 1.52(1), B(4)−O(8) = 1.41(1), B(4)−O(5)
= 1.52(1); B(1)−O(2)−B(2) = 116.1(7), B(2)−O(5)−B(4) =
130.5(7), B(4)−O(7)−B(3) = 116.1(7), B(3)−O(4)−B(1) =
127.9(7).
Crystal Structure Description. The asymmetric part of
the unit cell of 3 comprises the two independent molecules 3-A
Scheme 1. Synthesis of Fluorinated 1,2-Phenylenediboronic Acids
B
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hydrogen-bonded interactions, forming [010] chains (Figure
2a) typical of diboronic acids.23 The geometrical parameters of
the symmetrical one, as the O(3)−H(3B) bond is significantly
lengthened (dO−H = 1.01(1) Å) while the H(3B)···O(4)
contact is very short (dH···O = 1.42(1) Å). The position of the
H(3B) proton was visible on the difference density map and
was refined without any constraints (Figure S2a in the
Supporting Information). The calculated energy profile
(MP228/6-31g(d,p)29 level of theory) for linear proton motion
across the O(3)−H(3B)···O(4) hydrogen bond gives one
broad minimum (Figure S2b in the Supporting Information).
Presumably, the H(3B) proton can easily migrate from one
oxygen to another, which is characteristic for very strong H
bonds.30 The estimated interaction energy of this H bond is
very high at −87 kJ mol−1. This also indicates that 5a can
effectively act as a Brønsted acid, which should readily lose the
proton in solution to give the corresponding anionic species.
This is not common for boronic acids, which are generally weak
Lewis acids accepting OH− anions to a small extent by the
heterolytic cleavage of water molecules.31 A notable exception
is the cyclic hemiester of 2′-hydroxybiphenyl-2-boronic acid,
where the proton abstraction is favored, as it retains the
aromaticity of the boraheterocycle.32 Weaker H-bonding
interactions link molecules of 5a to form hydrogen-bonded
dimers resembling the structural motif typical of benzoxabor-
oles.33 Within this motif the endocyclic oxygen atoms (O2) act
as acceptors, whereas external OH groups bonded to the three-
coordinate boron center are donors of H bonding (O1−H1···
O2). Under different crystallization conditions (slow evapo-
ration of 1/2 acetone/toluene solution), the process of
aggregation of boronic groups in 5 goes further, involving
two molecules and leading to the formation of the unique eight-
membered B4O4 ring (Figure 1c). Thus, the aggregation in 5b
is not based on H bonding but, primarily, on Lewis acid−base
interactions between boron and oxygen atoms. It should be
noted that the only other reported example of a structure
comprising the related B4O4 ring is a tetramer resulting from
the self-assembly of the initially formed 4-fluoro-1,3-dimethoxy-
1H-1λ3-benzo[d][1,2,3]iodoxoborole.34
In the molecular structure of 5b, all four boron atoms are
four-coordinate, linked with four bridging hydroxyl groups. The
B−O bond lengths within the eight-membered boron−oxygen
skeleton show a wide range of lengths (dB−O = 1.47−1.54 Å)
and are slightly shorter between monomers in comparison to
those within the same monomeric unit. For comparison, the
B−O bond distances in previously reported compounds
possessing the B−(μ−OH)−B moiety also fall in the range
1.47−1.54 Å.35 As expected, bond distances between boron
atoms and the corresponding terminal OH groups are shorter,
ranging from 1.41 to 1.46 Å. The molecule 5b features a boat
conformation, which means that the aromatic rings are located
above each another. The interplanar angle between aromatic
ring planes is equal to 14.4°, and the distance between
centroids of the two rings is 3.42(1) Å, which indicates a weak
π−π stacking interaction. It seems that the strength of this
intramolecular interaction depends on the number of fluorine
substituents. According to the computations, the hypothetical
chair conformer (see Computational Studies) is less stable than
the boat form by about 60 kJ mol−1. However, when the
number of fluorine substituents decreases, these differences are
less pronounced and, for the nonfluorinated derivative 1, both
hypothetical forms, i.e., chair and boat, have comparable
stabilities.
Figure 2. Hydrogen-bonded motifs in (a) 3 and (b) 5a. Hydrogen
bonds are shown as red dashed lines.
hydrogen bonds are given in Table S2 in Supporting
Information. The chains are linked by lateral H bonds derived
from molecules 3-A, resulting in a double-chain motif. The
further propagation of these motifs into 2D layers is hampered,
as molecules 3-B participate in H-bonding interactions with
acetone molecules, which resembles the situation observed for
1,2-ethynediylbis(4,1-phenylene)diboronic acid solvated with
THF molecules.24
Tetrafluoro-1,2-phenylenediboronic acid crystallizes in two
completely different forms. The first one is the semianhydride
5a with one water molecule coordinated to one of the boron
atoms (Figure 1b). It crystallized from the aqueous solution.
One of the boron atoms in 5a is three-coordinate (B1) and the
other is four-coordinate (B2). The geometry of the three-
coordinate boron atom is flat with a C1−B1−O2 endocyclic
angle close to 110° and C1−B1−O1 and O1−B1−O1 angles
both close to 125°. The geometry of the four-coordinate boron
atom is tetrahedral, and all B−O bonds are of a comparable
length. It should be noted that the formally dative B(2)−O(3)
bond length in 5a is much shorter in comparison to the values
found for the analogous distance in the crystal structures
comprising molecules of the related species (C6F5)3BOH2
25
́
(1.55−1.63 Å). Hence, it seems that the molecular structure
of 5a can be properly interpreted as the oxonium acida cyclic
tautomer of the classical open form of 5, which would possess
two B(OH)2 groups, as observed for acids 119 and 3. The
tautomerization would involve the formation of the B−O bond
due to the strong donor−acceptor interaction of the Lewis
acidic B atom with the O atom of the neighboring B(OH)2
followed by proton transfer to one of the O atoms bonded to
the tetracoordinate B atom. It should be noted that the related
heterocyclic systems featuring the B−(μ-OR)−B moiety (R =
H, Me) were isolated as products of hydrolysis or alcoholysis of
perfluorinated diborane (o-C6F4[9-BC12F8]2).5d Another nota-
ble example constitutes 1H,3H-naphth[1,8-cd][1,2,6]-
oxadiborin,26 which is an anhydride of 1,8-naphthalenedibor-
onic acid featuring a six-membered heterocyclic ring with a B−
O−B moiety. The O(3)−H(3B)···O(4) H bond with the very
short intermolecular contact of dO···O = 2.423(2) Å occurs
between the coordinated water molecule (proton donor) and
the OH group bonded to the four-coordinate boron center of
the adjacent molecule. This distance is close to the lower limit
observed for strong anion-supported H bonds in adducts
formed by the strong Brønsted acid (C6F5)3BOH2 and its
conjugated base (C6F5)3BOH−, as the corresponding O···O
contacts vary in the range 2.40−2.52 Å, depending on the given
crystal structure.27 The short H bridge in 5a is in fact close to
Acidity of o-Phenylenediboronic Acids. To quantita-
tively describe the cooperative effect of two adjacent boronic
C
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groups and the additional influence of fluorination on the
acidity, we have measured the pKa values of all studied 1,2-
phenylenediboronic acids. We have also compared them with
those obtained for phenylboronic (PhBA), 1,4-phenylenedibor-
onic (1,4-dBA), and thiophene-2,3-diboronic acids (2,3-
ThdBA) (Table 1). The measurements were done by a
compound 4, pKa = 4.1, i.e., it is stronger acid than the parent
compound 1 and also the isomeric derivative 3, which reflects
the potent acidifying effect of two fluoro substituents at the
position ortho to two boronic groups. For compound 5, pKa =
3.0, which makes it the most acidic boronic acid known to date,
with an acidity comparable to that of a related aromatic
dicarboxylic acid: namely, phthalic acid (pKa1 = 2.9).37 It should
be noted that the differential titration curve for 3 shows a
distinct maximum close to the expected half-neutralization
point. This means that the pKa obtained for this compound is
rather a rough approximation and may not correspond to the
behavior of 3 in solution. We suppose that this may be due to
the formation of a fairly stable 1:1 adduct between the anionic
and neutral forms of 3. The details concerning the pKa value
estimations are given in the Supporting Information.
Table 1. pKa Values of Studied Boronic Acids
PhBA36 1,4-dBA 2,3-ThdBA
1
3
4
5
pKa
9.0 7.9 7.0
6.0
5.3
4.1
3.0
potentiometric titration with 0.1 M aqueous NaOH in H2O/
MeOH (1/1) or by conductometry in the case of 4 and 5. For
1 and 3, pKa values were estimated to be 6.0 and 5.3,
respectively, which means that these compounds are signifi-
cantly stronger acids than phenylboronic acid and its many
functionalized derivatives, where typical pKa values are about
9.36 The pKa value for 1,4-phenylenediboronic acid (1,4-dBA)
is 7.9; i.e., it is a much weaker acid than the isomeric compound
1. The stronger acidity of 1 can be attributed to the increased
stabilization of its anion resulting from the proximity of two
boronic groups. However, the rationale for this stabilization
remains debatable, as it depends on the structural formulation
of the anionic form of 1. Proposed structures can include the
symmetrical arrangement with the bridging OH− binded
simultaneously to two boron atoms (Chart 2a), the unsym-
Solution NMR Studies. In our recent study on the
structure of 1,2-phenylenediboronic acid (1), we have
concluded that this compound undergoes partial dehydration
when dissolved in deuterated acetone, THF, or DMSO.38 The
respective 1H NMR spectra indicate the formation of additional
species. For instance, in acetone-d6 signals of 1 (a sample was
obtained by dissolving the crystalline dihydrate) appear at 7.88
and 7.36 ppm. The less abundant component (ca. 50% in
comparison to 1) features resonances of aromatic protons at
7.69 and 7.44 ppm and a strongly deshielded broadened singlet
at 8.73 ppm, which can be assigned to the OH group. The
addition of D2O to solutions of 1 resulted in the simplification
of the spectra, showing only resonances of 1 irrespective of the
kind of deuterated solvent used. The 11B NMR spectrum of 1
in acetone-d6 comprises the signal at 34 ppm partially
overlapping with the major peak at 30 ppm. The former peak
can be assigned to a dehydrated form of 1, as it disappears after
addition of D2O. On the basis of these observations, we
proposed that in solution 1 coexists in equilibrium with its
cyclic semianhydride form, possessing the structure of
benzoxadiborole and featuring an increased 11B NMR chemical
Chart 2. Proposed Structures of 1,2-Phenylenediboronate
Anions
1
shift with respect to that of 1 (Table 2). In addition, the H
−
NMR spectrum of crystalline anhydrous 1 in acetone-d6
showed that the proportions of 1 and its dehydrated form are
comparable. However, additional signals are observed at 8.12
and 7.53 ppm. They become much stronger when a sample of 1
was subjected to dehydration in vacuo at 100 °C. This indicates
that additional dehydration processes occur already in solutions
of anhydrous acid 1.
metrical form, where neutral B(OH)2 and the anionic B(OH)3
are linked by an intramolecular H-bond (Chart 2b), and the
cyclic semianhydride complexed with OH− (Chart 2c). The
stability of all these forms was confirmed by theoretical
calculations (see the Supporting Information). The vicinity of
two boronic groups apparently accounts for the acidity of
thiophene-2,3-diboronic acid (pKa = 7.0). However, in this case
the anion stabilization is less effective in comparison to that for
1. This can be simply understood by considering that two
Lewis acidic boron atoms are more separated in thiophene-2,3-
diboronic acid than in 1 (by ca. 0.15 Å according to X-ray
structure measurements).19,22 This is due to a wider angle
between the two B−C vectors in the former compound. For
The results obtained for 1 have prompted us to study the
1
solution behavior of its fluorinated analogues 3−5. The H
NMR spectrum of 3 in acetone-d6 exhibits a broad resonance at
7.72 ppm as a major signal. In the 19F NMR spectrum, the
major broadened signal at −139.0 ppm is accompanied by a set
of four smaller broad resonances in the range −140 to −135
Table 2. Summary of 11B NMR Data for Studied Systems
11B chem shift (ppm)
b
c
d
compd
acetone-d6
acetone-d6 + D2O
29.9
D2O
D2O + K2CO3
D2O + NaOH
D2O + NaOH
a
e
1
34.0, 29.7 (20:80)
30.1
32.9
27.5
23.4
18.0, 7.6 (50:50)
17.8, 6.5 (85:15)
16.6, 6.8 (90:10)
13.8, 6.9 (60:40)
7.8, 1.5 (85:15)
10.1, 5,6 (85:15)
9.7, 5.6 (85:15)
9.3, 5.6 (85:15)
9.0, 1.8 (80:20)
8.7, 4,6 (60:40)
8.7, 4.6 (70:30)
9.2, 5.4 (5:95)
3
4
5
28.9
28.6
28.6
28.5
23.7, 16.3, 2.3 (20:20:60)
23.5, 16.4, 2.2 (30:30:40)
a
b
c
d
See ref 19. 0.05 M solutions of 1 and 3−5 in 0.15 M K2CO3/D2O. 0.05 M solutions in 0.10 M NaOH/D2O. 0.05 M solutions in 0.30 M NaOH/
e
D2O. Approximate integral ratios are given in parentheses; however, they may not represent the relative proportions of species due to strongly
varying peak half-widths.
D
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ppm. The 13C NMR spectrum also features broadened signals.
However, at lower temperatures (below 0 °C) a well-resolved
triplet at 7.72 ppm and other minor resonances in the range
7.8−7.4 ppm appear in the 1H NMR spectrum of 3. This is also
reflected by significant changes in the VT 19F NMR spectra
recorded in the range +25 to −30 °C. On the basis of the
results obtained previously for 1,19 the distinctive labile
behavior of 3 can be attributed to the equilibrium with its
dehydrated forms. This is supported by the fact that the 1H and
19F NMR spectra of a solution of 3 in wet acetone-d6 (i.e.,
containing a drop of D2O) exhibit triplets instead of the
aforementioned broad resonances. The dehydration could be
simply interpreted as the formation of a cyclic semianhydride,
but an analysis of the spectra (e.g., the number of peaks in the
19F NMR spectra) indicates that the situation is more
complicated, presumably due to other processes involving
aggregation of dehydrated forms of 3. However, the observed
processes are not reflected by the 11B NMR spectrum, which
shows only one signal in the range typical of a three-coordinate
boron atom in arylboronic acids39 (Table 2).
possessing one of the structures proposed in Chart 2. The
second peak can be attributed to the dianionic form 1,2-
Ph[B(OH)3 ]2, which is in agreement with the 11B NMR
−
−
chemical shift of the PhB(OH)3 anion in D2O (2.9 ppm). It
should be noted that the proportions of mono- vs dianionic
species depend on the base concentration but even in the
presence of a large excess of base the dianion is not formed
quantitatively. When K2CO3 was used as the base, the 11B
NMR spectra of 3 and 4 showed a broad resonance at ca. 17
ppm and a less intense and more narrow peak at ca. 7 ppm. In
the case of 5 one observes two peaks possessing comparable
intensities at 13 and 7 ppm. These results may point to the
formation of complexes of acids 3−5 with carbonate anion. It
should be noted that, despite the presence of two anionic forms
1
in solution, the H NMR spectra of 3 and 4 show only one
signal with a well-resolved multiplet structure. On the other
hand, the broad 19F NMR resonance of 3 indicates that some
dynamic process still persists. In the case of 3 the triplet at 7.22
ppm observed in D2O is shifted to 7.06 ppm in K2CO3/D2O
and 7.02 ppm in NaOH/D2O. For 4, the triplet at 6.84 ppm in
D2O is shifted to 6.65 ppm in K2CO3/D2O and 6.46 ppm in
1
The H NMR analysis of 4 indicates that this acid is also
1
NaOH/D2O. The different values of H NMR chemical shifts
labile in acetone-d6 but gives a well-resolved multiplet at 6.84
ppm in D2O. Thus, it seems that 4 also undergoes reversible
dehydration, in acetone. As found for 3, the 11B NMR spectrum
in acetone-d6 is not a good probe of the observed equilibrium
and shows only a single resonance at 28.6 ppm. For 5, the
situation was more complicated, as the NMR spectra strongly
differ depending on the solvent used. Thus, the 19F NMR of a
sample of 5 in acetone-d6 shows two complex sets of signals at
−130 to −138 ppm and −153 to −163 ppm. However, in D2O
one observes only two relatively narrow resonances (albeit still
lacking the multiplet structure resulting from expected F−F
couplings) at −136.8 and −156.8 ppm. Accordingly the 11B
NMR spectrum in acetone-d6 shows three signals at 23.7, 16.3,
and 2.3 ppm. We suppose that this may be due to the
occurrence of species bearing a tetracoordinate boron atom
such as the cyclic tautomer 5a or the dimeric species 5b, which
should clearly give rise to an upfield shift (with respect to the
classical form of boronic acid). In contrast, a solution of 5 in
D2O shows one signal at 23 ppm. A significant upfield shift with
respect to the values typical of arylboronic acids (ca. 30 ppm)
may suggest coordination of water to the boron atoms, as
observed in the crystal structure of 5a. However, two separate
resonances expected for three- and four-coordinate boron
atoms in 5a are averaged for the solution spectrum. Thus, it is
clear that 5 is fluxional in D2O. The situation is complicated by
the low pKa of 5 consistent with a significant ionization in D2O,
which results in the formation of its anionic form, presumably
by proton abstraction from the coordinated water molecule
(see structure c in Chart 2). Interestingly, upon addition of 1
drop of concentrated aqueous HCl two broad resonances at 20
and 13 ppm of approximately equal intensities appear, which
may suggest that some aggregation of 5 occurs under these
conditions.
point again to a different nature of boronate anions generated
upon addition of hydroxide vs carbonate anion.
Computational Studies. In order to get more information
about the relative stability of different conformers of diboronic
acids and their dimers and anhydrides, we performed ab initio
calculations at the MP228 level of theory with a 6-31G(d,p)29
basis set using Gaussian09.40 To get more details about the role
of the fluorination on the formation of cyclic semianhydrides
and coordination dimers, all appropriate species derived from 1
and 3−5 were studied. The studied transfomations are depicted
in Figure 3, and the results of appropriate calculations are given
Figure 3. Schematic representation of equilibria between various forms
of o-phenylenediboronic acids.
in Table 3. The theoretical calculation on single molecules of 1
and 3−5 revealed that o-diboronic acid displays two stable
conformations. The basic form found in the crystal structures of
1 and 3 is stabilized by an intramolecular H bond. In the
second form, the boronic groups are significantly twisted along
the B−C bonds and a B···O dative interaction between them
occurs. The O−H···O conformer for 1 and 3 is more stable by
ΔEc ≈ 15 kJ mol−1, but electron-withdrawing fluoro
substituents in the positions ortho to boronic groups increase
the boron Lewis acidity, which results in the relative
stabilization of the B···O conformer. Thus, for derivatives 4
and 5 both forms have similar stability. One can expect that the
occurrence of a B···O conformation should facilitate the
formation of an oxadiborole heterocycle featuring the B−O−
Further work was performed to study the behavior of acids
3−5 under aqueous alkaline conditions. Approximately 0.05 M
solutions in 0.1 M NaOH/D2O were studied by 11B NMR
spectroscopy. In all cases we observed the major broad
resonance at ca. 9−10 ppm accompanied by a smaller but
more narrow peak at ca. 5−6 ppm (Table 2). When the
concentration of base is higher (0.3 M), the former resonance
becomes less abundant and almost disappears for 5. We
suppose that it can be assigned to a monoanionic form
E
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Article
behavior of studied compounds. It is based on their reversible
dehydration, presumably resulting in the formation of cyclic
semianhydrides and other species. Quantum chemical calcu-
lations show that the dehydration process is rather disfavored,
but the formed semianhydride is stabilized by a coordinated
water molecule. This was confirmed by the X-ray structure of
5a, which can be described as a semianhydride with a water
molecule coordinated to one of the boron atoms. The very
strong hydrogen bond between the coordinated water molecule
and the OH group bonded to the four-coordinate boron center
(dO···O = 2.423(2) Å, Eint = 87 kJ mol−1) together with the low
pKa value and high proton mobility across the O3−H3···O4
bond indicate that 5 can act effectively as a Brønsted oxonium
acid which readily loses a proton to give the corresponding
anionic species. The enhanced Lewis acidic properties of boron
atoms in 5 are also responsible for the formation of the unique
dimeric self-aggregate featuring a rare eight-membered B4O4
ring stabilized by the intramolecular π−π interaction of two aryl
rings. Unlike the case for 5, the molecular structure of 3 is
typical of boronic acids and similar to that observed in 1, as it
features two B(OH)2 groups linked by an intramolecular O−
H···O bond. This indicates that two fluorine atoms remote
from both boronic groups are not sufficient to affect any
significant structural changes, as the Lewis acidity of the boron
atoms in 3 is only slightly higher than that in 1. An extension to
this work currently in progress in our laboratory is the
utilization of o-phenylenediboronic acids as effective Lewis acid
catalysts in selected organic transformations.
Table 3. Results of Quantum-Chemical Calculations at the
MP2/6-31G(d,p) Level of Theory
a
b
b
ΔEc
ΔEan
ΔEan_H2O
ΔEHB
ΔED
ΔEBC
1
3
4
5
16.6
15.4
−1.2
−2.3
50.1
56.8
43.9
37.0
14.2
10.9
−49.3
−48.4
−51.0
−53.0
43.4
44.5
4.6
1.7
−9.7
−15.5
−13.9
−33.4
48.0
65.9
a
All energy values are given in kJ mol−1. Definitions: ΔEc = energy
difference between H-bonded and B···O-bonded conformers of
monomeric diboronic acids; ΔEan = dehydration energy; ΔEan_H2O
energy of the coordination of water molecule to semianhydride; ΔEHB
=
= energy of the formation of the hydrogen-bonded dimer; ΔED
=
energy of the formation of the coordination dimer; ΔEBC = energy
difference between boat and chair conformations of the coordination
b
dimer. Energy of the formation of a dimer from two monomeric
molecules.
B linkage. Nevertheless, the dehydration process is energetically
disfavored, as evidenced by positive ΔEan values (Table 3).
However, in the case of 4 and 5, the coordination of H2O to
one of the boron atoms in oxadiborole is energetically favorable
(negative ΔEan_H2O values for these two compounds). It is
notable that a significant stabilization through Lewis base
coordination was also observed for the boroxine complexes,
where N-donor ligands, such as pyridine, facilitate the
trimerization of arylboronic acids under mild conditions.41
However, as already mentioned, the formation of the structure
of oxonium acid occurs rather through cyclic tautomerization of
the classical open form of diboronic acid than by the simple
coordination of water molecules to one of the boron center.
We have also performed ab initio calculations on
coordination dimers possessing either a boat conformation
similar to that of 5b or the hypothetical chair structure and
referenced the results to the energy of independent molecules
and H-bonded dimers. It appears that the coordination dimers
(in both conformations) are much less stable than classical H-
bonded forms and, for molecules 1 and 3, are even less stable
than two noninteracting independent molecules of 1,2-
diboronic acid. As observed for semianhydrides, fluorine
substituents in the ortho positions significantly increase the
stabilities of coordination dimers, as for 5 the energy difference
between the dimer 5b and the classical H-bonded form does
not exceed 20 kJ mol−1. We also suppose that the stability of
the coordination dimer is strongly enhanced by the crystal field.
It is notable that crystals composed of the monomeric species
5a were obtained by crystallization from water, while the
structure 5b came from acetone/toluene (1/2). However, the
crystallization mechanism and the role of the solvent are still
unclear. The chair conformation is unstable for the 4 and 5
derivatives (positive ΔEBC values), which may suggest the
contribution of π−π stacking interactions of aromatic rings to
the stabilization of the boat conformer.
EXPERIMENTAL SECTION
■
General Comments. All reactions involving air- and moisture-
sensitive reagents were carried out under an argon atmosphere.
Organic solvents were stored over sodium wire before use. Key
reagents, including 2-bromo-4,5-difluorophenylboronic acid, 1,4-
difluoro-2-iodobenzene, 1,2-dibromotetrafluorobenzene, n-BuLi (10
M solution in hexanes), trimethyl borate, N-butyldiethanolamine,
diisopropylamine, chlorotrimethylsilane, and iodine chloride, were
received from Aldrich and used without additional purification.
Phenylboronic acid and 1,4-phenylenediboronic acid were also
received from Aldrich. Thiophene-2,3-diboronic acid,22 1,2-phenyl-
enediboronic acid,19 and tetrafluoro-1,2-phenylenediboronic acid20
were obtained according to the literature procedures.
The NMR chemical shifts are given relative to TMS using known
chemical shifts of residual proton (1H) or carbon (13C) solvent
resonances. In the 13C NMR spectra the resonances of boron-bound
carbon atoms were not observed in most cases due to their broadening
caused by partially relaxed B−C couplings. 19F NMR chemical shifts
are given relative to Et2O·BF3 and CFCl3, respectively.
Synthesis. 2-(2′-Bromo-4′,5′-difluorophenyl)-6-butyl[1,3,6,2]-
dioxazaborocan (2a). A mixture of 2-bromo-4,5-difluorophenylbor-
onic acid (11.9 g, 0.05 mol), N-butyldiethanolamine (8.5 g, 0.052
mol), anhydrous MgSO4 (5 g), and acetone (50 mL) was stirred for 1
h at 35 °C. The mixture was filtered and concentrated under reduced
pressure. To the remaining solid residue was added hexane (25 mL)
followed by filtration of the resultant slurry. The crystalline product
was washed with hexane (2 × 20 mL) and dried in vacuo at 50 °C to
1
CONCLUSIONS
give the title compound. Yield: 17.0 g (94%). Mp: 115−117 °C. H
■
NMR (CDCl3, 400.1 MHz): δ 7.62 (dd, J = 11.0, 10.0 Hz, 1 H, Ph),
7.32 (dd, J = 9.5, 7.0 Hz, 1 H, Ph), 4.19−4.10 (m, 4 H, CH2O), 3.32−
3.24 (m, 2 H, CH2N), 3.04−3.11 (m, 2 H, CH2N), 2.63−2.58 (m, 2H,
NCH2CH2CH2CH3), 1.62−1.52 (m, 2 H, NCH2CH2CH2CH3),
1.27−1.16 (m, 2 H, NCH2CH2CH2CH3), 0.87 (t, J = 7.0 Hz, 3 H,
NCH2CH2CH2CH3) ppm. 13C NMR (CDCl3, 100.6 MHz): δ 149.7
(dd, J = 250.0, 10.5 Hz), 148.5 (dd, J = 247.0 Hz, 11.0 Hz), 129.3 (dd,
J = 19.0 Hz, 4.0 Hz), 121.2, 115.1 (dd, J = 19.0, 4.0 Hz), 63.1, 59.8,
57.4, 26.8, 20.0, 13.7 ppm. 11B NMR (CDCl3, 64.16 MHz): δ 12.1
The relatively high acidity of o-phenylenediboronic acids is due
to the proximity of two boronic acid groups. Hence, they can be
regarded as bidentate Lewis acids and their acidity increases
continuously in the order 1 < 3 < 4 < 5, reaching the upper
limit for boronic acids (for 5 pKa = 3.0). Regarding solution
NMR studies, it should be stressed that the spectra of 1 and 3−
5 vary significantly depending on the water content in a sample.
This is consistent with the nature of the observed labile
F
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Article
ppm. Anal. Calcd for C14H18BBrF2NO2 (362.02): C, 46.45; H, 5.29;
N, 3.87. Found: C, 46.31; H, 5.39; N, 3.80.
hydrolyzed by the addition of water (0.3 g) to the solution in Et2O (18
mL). The resulting mixture was evaporated to dryness, and the
remaining solid was washed with Et2O (2 × 3 mL) and dried in vacuo
to give the title compound. Yield: 0.85 g (28%). Mp: 132−135 °C dec.
4,5-Difluoro-1,2-phenylenediboronic Acid (3). A solution of 2
(9.05 g, 0.025 mol) in 1/2 THF/Et2O (50 mL) was added to a
previously prepared solution of n-BuLi (10 M solution in hexane, 2.60
mL, 0.026 mol) in Et2O (80 mL) at −90 °C. The lithiated
intermediate precipitated gradually to give a brown slurry. It was
stirred for ca. 30 min at −90 °C followed by the dropwise addition of
B(OMe)3 (3.2 mL, 0.026 mol). The mixture was stirred for ca. 30 min
at −75 °C and hydrolyzed with 10 wt % aqueous HCl (30 mL). The
water phase was separated, followed by extraction with Et2O (2 × 15
mL). The extracts were added to the organic phase, which was
concentrated under reduced pressure. The solid residue was filtered
and washed with toluene (5 mL) and hexane (5 mL). Drying in vacuo
afforded the title compound as a white powder, which was
recrystallized from toluene. Yield: 3.67 g (73%). Mp: 123−125 °C.
1
1H NMR (acetone-d6, 499.9 MHz): δ 7.05 (broad, 2H, Ph) ppm. H
NMR (acetone-d6 + D2O, 400.1 MHz): δ 7.73 (t, J = 6.0 Hz, 2H, Ph)
1
ppm. H NMR (D2O, 200.0 MHz): δ 6.87 (t, J = 5.5 Hz, 2H, Ph)
ppm. 1H NMR (D2O + K2CO3, 200.0 MHz): δ 6.65 (t, J = 5.5 Hz, 2H,
Ph) ppm. 1H NMR (D2O + NaOH, 200.0 MHz): δ 6.48 (t, J = 5.0 Hz,
2H, Ph) ppm. 13C NMR (acetone-d6, 100.6 MHz): δ 161.8 (d, J =
213.0 Hz), 117.7 ppm. 13C NMR (acetone-d6 + D2O, 100.6 MHz): δ
161.9 (d, J = 222.0 Hz), 117.7 ppm. 19F NMR (acetone-d6, 470.4
MHz): δ −114.1 ppm (broad). 19F NMR (acetone-d6 + D2O, 376.5
MHz): δ −112.8 ppm (t, J = 6.0). 11B NMR (acetone-d6, 64.16 MHz):
δ 28.6 ppm. 11B NMR (acetone-d6 + D2O, 64.16 MHz): δ 28.5 ppm.
11B NMR (D2O, 64.16 MHz): δ 31.3 ppm. 11B NMR (D2O + K2CO3,
64.16 MHz): δ 16.6, 6.8 ppm. 11B NMR (D2O + NaOH, 64.16 MHz):
δ 9.7, 5.6 ppm. Anal. Calcd for C6H6B2F2O4 (201.73): C, 35.72; H,
3.00. Found: C, 35.42; H, 3.21.
1
1H NMR (acetone-d6, 499.9 MHz): δ 7.67 (broad, 2H, Ph) ppm. H
NMR (acetone-d6 + D2O, 400.1 MHz) δ 7.71 (t, J = 10.5 Hz, 2H, Ph)
1
ppm. H NMR (D2O, 400.1 MHz): δ 7.22 (t, J = 10.0 Hz, 2H, Ph)
1
Tetrafluoro-1,2-phenylenediboronic Acid (5). 13C NMR (acetone-
d6, 100.6 MHz): δ 149.5 (d, J = 236.0 Hz), 140.8 (d, J = 251.5 Hz)
ppm. 13C NMR (D2O + K2CO3, 100.6 MHz): δ 151.7 (d, J = 241.0
Hz), 144.6 (d, J = 240.0 Hz) ppm. 19F NMR (acetone-d6, 376.5 MHz):
δ −130 − −138 (broad), −153 − −157 (broad),−157 − −163
(broad) ppm. 19F NMR (acetone-d6 + D2O, 376.5 MHz): δ −132.8
(2F), −156.8 (2F) ppm. 19F NMR (D2O, 376.5 MHz): δ −138.7
(broad, 2F), −160.2 (broad, 2F) ppm. 19F NMR (1 M HCl in D2O,
471.0 MHz): δ −136.8 (2F), −156.8 (2F) ppm. 11B NMR (acetone-d6,
64.16 MHz): δ 23.7, 16.3, 2.3 ppm. 11B NMR (acetone-d6 + D2O,
64.16 MHz): δ 23.5, 16.4, 2.2 ppm. 11B NMR (D2O, 64.16 MHz): δ
23,4 ppm. 11B NMR (D2O + K2CO3, 64.16 MHz): δ 13.8, 6.9 ppm.
11B NMR (D2O + NaOH, 64.16 MHz): δ 9.3, 5.6 ppm. 11B NMR (1
M HCl in D2O, 64.16 MHz): δ 19.3, 13.4 ppm. Anal. Calcd for
C6H4B2F4O4 (237.71): C, 30.32; H, 1.70. Found: C, 30.45; H, 1.72.
Crystallization and Structural Measurement Details. Single
crystals of 3 were obtained upon crystallization from acetone solution.
The crystallization of 5 from water solution results in the formation of
the semianhydride 5a. In turn, by slow evaporation of a 1/2 acetone/
toluene solution the dimeric form 5b was obtained. All attempts to
grow crystals of 4 were unsuccessful. Despite numerous solvent and
temperature conditions tested, we always obtained amorphous
powders or glasses. The crystal structure of 5a contains three H2O
molecules per one molecule of 5a incorporated into the lattice during
crystallization. In the case of 5b there are two H2O molecules and one
acetone molecule per one molecule of 5b. A detailed discussion of
supramolecular networks is given in the Supporting Information.
Single-crystal X-ray measurements were performed on a Kuma
KM4CCD κ-axis diffractometer with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) and equipped with an Oxford Cryosystems
nitrogen gas-flow apparatus. The crystals were positioned at 45 mm
from the KM4CCD camera. The data were corrected for Lorentz and
polarization effects. Data reduction and analysis were carried out with
the Oxford Diffraction Ltd. suit of programs.42 All structures were
solved by direct methods using SHELXS-97 and refined using
SHELXL-97.43 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms of C−H bonds were placed in idealized positions.
The position of the H(3B) proton in 5a was refined without any
constraints. The structure of 5b is of moderate quality, due to the fact
that this compound crystallizes as tiny, small needles. The problem
with crystallization is ascribed to the labile behavior of this compound
in solution. During the refinement several atoms were restrained so
that their displacement parameters approximate isotropic behavior
(ISOR instruction with SHELXL). Selected crystal data and geometry
of hydrogen bonds for all crystals are summarized in Tables S1 and S2
of the Supporting Information.
ppm. H NMR (D2O + K2CO3, 400.1 MHz): δ 7.07 (dd, J = 12.0, 8
Hz, 2H, Ph) ppm. 1H NMR (D2O + NaOH, 200.0 MHz): δ 7.02 (t, J
= 8.5 Hz, 2H, Ph) ppm. 13C NMR (acetone-d6, 100.6 MHz): δ 151.7
(d, J = 240.0 Hz), 138.9 (broad), 123.8 (broad) ppm. 13C NMR
(acetone-d6 + D2O, 100.6 MHz): δ 151.7 (dd, J = 249.0, 13.6 Hz),
138.4 (broad), 124.3 (t, J = 249.0, 5.5 Hz) ppm. 13C NMR (D2O +
K2CO3, 100.6 MHz): δ 168.4, 154.0 (d, J = 240 Hz), 119.6 ppm. 19F
NMR (acetone-d6, 470.4 MHz): δ −135.5 (broad), −136.5 (broad),
−138.0 (broad), −139.0 (broad), −140.0 (broad) ppm. 19F NMR
(acetone-d6 + D2O, 376.5 MHz): δ −138.7 (t, J = 10.5 Hz) ppm. 19F
NMR (D2O + K2CO3, 376.5 MHz): δ −143.0 (broad) ppm. 11B NMR
(acetone-d6, 64.16 MHz): δ 28.9 ppm. 11B NMR (acetone-d6 + D2O,
64.16 MHz): δ 28.6 ppm. 11B NMR (D2O, 64.16 MHz): δ 32.9 ppm.
11B NMR (D2O + K2CO3, 64.16 MHz): δ 17.8, 6.5 ppm. 11B NMR
(D2O + NaOH, 64.16 MHz): δ 10.1, 5.6 ppm. Anal. Calcd for
C6H6B2F2O4 (201.73): C, 35.72; H, 3.00. Found: C, 35.55; H, 3.10.
1,4-Difluoro-2,3-diiodobenzene (2c). A solution of 1,4-difluoro-2-
iodobenzene (24.0 g, 0.10 mol) in THF (20 mL) was added to a
stirred solution of LDA freshly prepared from n-BuLi (10 M, 10 mL,
0.10 mol) in THF (100 mL) at −75 °C. The resulting white slurry was
stirred for 15 min, followed by the dropwise addition of Me3SiCl (12.5
mL, 0.10 mol). Then the reaction mixture was warmed slowly to ca.
−30 °C and quenched with 2 M aqueous H2SO4 (2 M solution in
Et2O, 18 mL, 0.036 mol). The mixture was concentrated in vacuo. The
residue was subjected to fractional distillation to give crude 1,4-
difluoro-2-iodo-3-(trimethylsilyl)benzene (2b; 19.3 g, 62%) as a
colorless oil. Bp: 90−95 °C (1 Torr). It was added dropwise to a
solution of ICl (11.5 g, 0.07 mol) in CHCl3 (100 mL). The resulting
dark solution was refluxed for 1 h, and then it was quenched with 10
wt % aqueous NaHSO3 (50 mL). The organic layer was separated,
washed with water, and concentrated in vacuo. The residue was
subjected to fractional distillation to give crude 2c as a pale yellow oil.
Bp: 110−115 °C (1 Torr). The product solidified when it was cooled
to room temperature. It was purified by crystallization from hexane
1
(50 mL). Yield: 16.0 g (44%). Mp: 67−69 °C. H NMR (CDCl3,
1
400.1 MHz): δ 7.11 (t, J = 11.2 Hz, 2H, Ph) ppm. H NMR (D2O,
400.1 MHz): δ 6.84 (td, J = 6.5, 2.0 Hz, 2H, Ph) ppm. 13C NMR
(CDCl3, 100.6 MHz): δ 158.1 (dd, J = 244.0, 4.0 Hz), 115.7 (m), 97.5
(m) ppm. Anal. Calcd for C6H2F2I2 (365.89): C, 19.70; H, 0.55.
Found: C, 19.5; H, 0.52.
3,6-Difluoro-1,2-phenylenediboronic Acid (4). A solution of 2c
(5.5 g, 0.015 mol) in Et2O (20 mL) was added to a stirred solution of
n-BuLi (10 M, 3.5 mL, 0.035 mol) in THF/Et2O (50 mL, 4/1) at
−110 °C. The resulting white slurry was stirred for 15 min followed by
the dropwise addition of B(OMe)3 (5.5 mL, 0.050 mol). Then the
reaction mixture was warmed slowly to ca. −100 °C, which resulted in
the formation of a white gelatinous mixture. It was warmed to −10 °C
and quenched with anhydrous HCl (2 M solution in Et2O, 18 mL,
0.036 mol). The mixture was concentrated in vacuo. The residue was
subjected to fractional distillation to give crude 1,4-difluoro-2,3-
bis(dimethoxyboryl)benzene (2d; 1.25 g) as a pale yellow oil. It was
Computational Methods. All geometry optimizations and
frequency calculations were carried out with the GAUSSIAN 09
suite of programs,40 and the MP2 method was applied28 using 6-
31G(d,p)29 basis sets. The minima were confirmed by vibrational
frequency calculations within the harmonic approximation (no
G
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Article
imaginary frequencies). The computations were performed with
Tomasi’s polarized continuum model,44 using the polarizable
conductor calculation model (SCRF(CPCM, solvent = H2O)). In
optimization processes no symmetry constraints were applied. The
hydrogen-bonded dimer interaction energies as well as the energy of
the O(3)−H(3B)···O(4) bond in 5a were calculated using supra-
molecular methods including basis set superposition error (BSSE), and
the B97D45 functional with a 6-31G(d,p) basis set was applied.
However, due to significant contribution of π stacking and other types
of weak interactions to the total interaction energy between molecules
of 5a, the O(3)−H(3B)···O(4) hydrogen bond energy could not be
directly estimated. Therefore, the coordinated water molecule together
with a second molecule of 5a was rotated by 180° along the B(2)−
O(3) bond, preserving the geometry of the hydrogen bond (Figure S3,
Supporting Information). Under these circumstances the interaction
energy decreased from −104 to −87 kJ mol−1.
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ASSOCIATED CONTENT
■
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* Supporting Information
Text, tables, figures, and CIF files giving details of X-ray
crystallographic, computational, and solution multinuclear
NMR studies and acidity constant measurements. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for K.D.: kdurka@gmail.com.
*E-mail for S.L.: serek@ch.pw.edu.pl.
(13) Durka, K.; Lulin
ski, S.; Jarzembska, K. N.; Serwatowski, J.;
Wozniak, K. Acta Crystallogr. 2014, B70, 157−171.
́
Notes
(14) (a) Januszewski, E.; Lorbach, A.; Grewal, R.; Bolte, M.; Bats, J.
W.; Lerner, H.-W.; Wagner, M. Chem. Eur. J. 2011, 17, 12696−12705.
(b) Chai, J.; Wang, C.; Jia, L.; Pang, Y.; Graham, M.; Cheng, S. Z. D.
Synth. Met. 2009, 159, 1443−1449.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(15) (a) Lorbach, A.; Bolte, M.; Li, H.; Lerner, H.-W.; Holthausen,
This work was supported by the “Iuventus Plus” program of the
Polish Ministry of Science and Higher Education (0111/IP3/
2011/71). The authors thank the Interdisciplinary Center for
Mathematical and Computational Modelling in Warsaw (G33-
14). We gratefully acknowledge the Aldrich Chemical Co.,
Milwaukee, WI, USA, for a long-term collaboration. The
authors thank Dr. Sian T. Howard for reading and correcting
the manuscript.
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