Solid-state 11B and 13C NMR, IR, and X-ray crystallographic characterization of selected arylboronic acids and their catechol cyclic esters

Article abstract of DOI:10.1002/mrc.3815

Nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (boroxines) are investigated using ¹¹B solid-state NMR spectroscopy at three different magnetic field strengths (9.4, 11.7, and 21.1T). Through the analysis of spectra of static and magic-angle spinning samples, the ¹¹B electric field gradient and chemical shift tensors are determined. The effects of relaxation anisotropy and nutation field strength on the ¹¹B NMR line shapes are investigated. Infrared spectroscopy was also used to help identify peaks in the NMR spectra as being due to the anhydride form in some of the arylboronic acid samples. Seven new X-ray crystallographic structures are reported. Calculations of the ¹¹B NMR parameters are performed using cluster model and periodic gauge-including projector-augmented wave (GIPAW) density functional theory (DFT) approaches, and the results are compared with the experimental values. Carbon-13 solid-state NMR experiments and spectral simulations are applied to determine the chemical shifts of the ipso carbons of the samples. One bond indirect ¹³C-¹¹B spin-spin (J) coupling constants are also measured experimentally and compared with calculated values. The ¹¹B/¹⁰B isotope effect on the ¹³C chemical shift of the ipso carbons of arylboronic acids and their catechol esters, as well as residual dipolar coupling, is discussed. Overall, this combined X-ray, NMR, IR, and computational study provides valuable new insights into the relationship between NMR parameters and the structure of boronic acids and esters. Copyright

Full text of DOI:10.1002/mrc.3815

Research Article
Received: 2 December 2011
Revised: 27 February 2012
Accepted: 2 March 2012
Published online in Wiley Online Library: 12 April 2012
(wileyonlinelibrary.com) DOI 10.1002/mrc.3815
Solid-state 11B and 13C NMR, IR, and X-ray
crystallographic characterization of selected
arylboronic acids and their catechol cyclic esters
Se-Woung Oh,a,c Joseph W. E. Weiss,a# Phillip A. Kerneghan,b Ilia Korobkov,a
Kenneth E. Malyb and David L. Brycea*
Nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (boroxines) are
investigated using 11B solid-state NMR spectroscopy at three different magnetic field strengths (9.4, 11.7, and 21.1 T).
Through the analysis of spectra of static and magic-angle spinning samples, the 11B electric field gradient and chemical shift
tensors are determined. The effects of relaxation anisotropy and nutation field strength on the 11B NMR line shapes are
investigated. Infrared spectroscopy was also used to help identify peaks in the NMR spectra as being due to the anhydride
form in some of the arylboronic acid samples. Seven new X-ray crystallographic structures are reported. Calculations
of the 11B NMR parameters are performed using cluster model and periodic gauge-including projector-augmented
wave (GIPAW) density functional theory (DFT) approaches, and the results are compared with the experimental values.
Carbon-13 solid-state NMR experiments and spectral simulations are applied to determine the chemical shifts of the ipso
carbons of the samples. One bond indirect 13C-11B spin-spin (J) coupling constants are also measured experimentally and
compared with calculated values. The 11B/10B isotope effect on the 13C chemical shift of the ipso carbons of arylboronic acids
and their catechol esters, as well as residual dipolar coupling, is discussed. Overall, this combined X-ray, NMR, IR, and
computational study provides valuable new insights into the relationship between NMR parameters and the structure of
boronic acids and esters. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: nuclear magnetic resonance; solid-state NMR; boron-11; carbon-13; boronic acids; boronate esters; relaxation anisotropy;
isotope effects; quadrupolar coupling constant; chemical shift tensor
Ten boronic acids and ester derivatives were investigated in
Introduction
our previous study.[1] In the present work, we report the exper-
As highlighted previously,[1] boronic acids and boronate esters are
particularly important classes of compounds that have a wide range
of uses in organic synthesis and catalysis, as well as in biochemistry,
pharmaceutical chemistry, industry, crystal engineering, and various
other applications.[2–5] Boronic acids are used extensively in organic
chemistry as chemical building blocks[6] and intermediates, and the
most widely used example is the Suzuki–Miyaura coupling,[7] which
is a useful synthetic route to biaryl compounds. The diverse
number of applications of boronic acids and esters means that
an understanding of the structural and electronic properties of
these compounds is imperative. Boron-11 solid-state nuclear
magnetic resonance (SSNMR) spectroscopy can provide impor-
tant information about such properties. Boron has two quadru-
polar NMR-active nuclides, 10B (I = 3; n.a. = 19.9%; Ξ ꢀ 10.744 %)
and 11B (I = 3/2; n.a. = 80.1%; Ξ ꢀ 32.084%). Both isotopes have
small-to-moderate nuclear electric quadrupole moments, Q [Q
imental characterization and interpretation of 11B chemical
shift (CS) and electric field gradient (EFG) tensors in a novel
series of solid arylboronic acids and their catechol cyclic esters
(Scheme 1; Table 1), with an added focus on understanding
the origins of spurious peaks in some of the spectra as being
caused by impurities or the conditions used for spectral acquisi-
tion. The compound numbering scheme in Table 1 uses ‘A’ for
acids, ‘B’ for esters, and ‘C’ for anhydrides. Boron-11 SSNMR
experiments were performed under stationary and magic-angle
spinning (MAS) conditions at magnetic field strengths (B0) of
9.40, 11.75, and 21.1 T. At 21.1 T, the influence of the chemical
shift (CS) tensor on the observed SSNMR line shapes becomes
*
Correspondence to: David L. Bryce, Department of Chemistry, University of
Ottawa, Ottawa, ON, K1N 6N5 Canada. E-mail: dbryce@uottawa.ca
(
10B) = 8.459 fm2; Q(11B) = 4.059 fm2].[8] The 11B nucleus is more
#
Current address: Bruker Biospin, Milton, ON, L9T 1Y6 Canada
receptive to NMR spectroscopy because of its higher natural
abundance, lower value of Q, and availability of a central
transition (CT) (i.e. mI = +1/2 ↔-1/2). Some interesting recent 11B
solid-state NMR studies have highlighted advanced methods for
measuring dipolar and/or J coupling constants between pairs of
11B spins in lithium diborate[9] and bis(catecholato)diboron.[10]
a Department of Chemistry, University of Ottawa, Ottawa, ON, Canada
b Department of Chemistry, Wilfrid Laurier University, Waterloo, ON, Canada
c
On leave from the Department of Chemistry, Mokpo National University,
Muan, Chonnam, Korea
Magn. Reson. Chem. 2012, 50, 388–401
Copyright © 2012 John Wiley & Sons, Ltd.

11B and 13C NMR of solid arylboronic acids and their esters
gauge-including projector-augmented wave (GIPAW) DFT calcula-
tions of the NMR parameters of the samples studied are performed
using the atomic coordinates and cell parameters available from
refinements of the X-ray crystallographic data.
Conventions
The 11B SSNMR spectra presented here are affected by nuclear
magnetic shielding (s) and by the quadrupolar interaction
between the nuclear quadrupole moment (Q) and the EFG at
the nucleus. The relevant theory for interpreting the 11B NMR
spectra reported in this work has been summarized recently by
Weiss and Bryce.[1] We present here only the conventions used
for reporting tensor data. Magnetic shielding may be represented
by a second-rank tensor, s. Diagonalization of the symmetric
portion of s provides the orientation of its principal axis system
(PAS) relative to a reference axis system. In its PAS, the three
principal components of s are ordered as follows: s11 ≤ s22 ≤ s33.
The three principal components of the CS tensor are ordered as
follows: d11 ≥ d22 ≥ d33. For both the s and CS tensors, the
isotropic value is the average of the three principal components.
The Maryland convention is used for describing the s and CS
tensor parameters.[15] The span (Ω) is defined as[15]
Scheme 1. Samples studied in the present work (Table 1).
extremely important as its line shape contribution (in Hz) is
proportional to B0.[1,11]
Carbon-13 chemical shifts of the ipso carbon, which is attached
to boron of arylboronic acids and their catechol cyclic esters have
not been reported with high precision.[12,13] This is due to residual
dipolar and J coupling to the quadrupolar nuclei (10/11B) attached
to the carbon.[14] The 13C NMR peak therefore exhibits fine structure
because of the directly bonded quadrupolar nucleus. The fine
structure may or may not be clearly resolved from other peaks
in the 13C MAS NMR spectrum, depending on the compound. If
the fine structure overlaps with other peaks, it is very difficult to de-
termine the chemical shift of the ipso carbon. In the present work,
we report accurate chemical shifts of the ipso carbons of arylboro-
nic acids and their catechol cyclic esters, and one bond spin-spin
coupling constants between 13C and 11B in the same compounds.
Single-crystal X-ray crystal structures of seven arylboronic acids
are also presented to assist in our understanding of the NMR
data. Gauge-including atomic orbital (GIAO) cluster model and
Ω ¼ s33 ꢁ s11 ꢀ d11 ꢁ d33
The skew (k) is defined as[15]
(1)
3ðsiso ꢁ s22Þ 3ðd22 ꢁ diso
Þ
:
k ¼
¼
(2)
Ω
Ω
The EFG tensor may be diagonalized to provide its principal com-
ponents and the orientation of its PAS. The principal components
of the EFG tensor are defined as follows: |V33| ≥ |V22| ≥ |V11|. The
nuclear quadrupolar coupling constant (CQ) is given by
Table 1. Samples studied in the present work (Scheme 1)
Arylboronic acids
w
x
y
z
A1
A2
A3
A4
A5
A6
A7
A8
A9
Phenylboronic acid
H
H
H
H
H
4-Methoxyphenylboronic acid
3,4-Dimethoxyphenylboronic acid
4-Methylphenylboronic acid
2-Methylphenylboronic acid
4-Fluorophenylboronic acid
H
OCH3
OCH3
CH3
H
H
H
H
H
H
H
CF3
H
z
H
OCH3
H
H
H
H
H
H
H
x
CH3
H
F
4-Chlorophenylboronic acid
2-Chloro-5-(trifluoromethyl)phenylboronic acid
4-(Methylthio)phenylboronic acid
H
Cl
Cl
H
H
SCH3
y
Arylboronic acid catechol cyclic esters
w
B1
B2
B3
B4
B5
B6
B7
Phenylboronic acid catechol cyclic ester
H
H
H
H
H
H
H
H
H
H
4-Methoxyphenylboronic acid catechol cyclic ester
3,4-Dimethoxyphenylboronic acid catechol cyclic ester
4-Methylphenylboronic acid catechol cyclic ester
2-Methylphenylboronic acid catechol cyclic ester
4-Fluorophenylboronic acid catechol cyclic ester
4-Chlorophenylboronic acid catechol cyclic ester
H
H
OCH3
OCH3
CH3
H
H
OCH3
H
H
H
H
H
CH3
H
F
H
Cl
2,4,6-triarylboroxines (anhydrides)
y
C1
C2
2,4,6-Triphenylboroxine
2,4,6-Tris(4-fluorophenyl)boroxine
H
F
Magn. Reson. Chem. 2012, 50, 388–401
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S.-W. Oh et al.
(11B) = 128.38 MHz, nL(13C) = 100.62 MHz) at room temperature.
Spectra were acquired using TopSpin 2.1 software. All experiments
were performed using a Bruker 4.0mm HXY triple-resonance MAS
probe. For the 11B NMR experiments, experimental referencing,
calibration, and setup were done using solid powdered sodium
borohydride (NaBH4), which has a chemical shift of ꢁ42.06 ppm
relative to the primary standard, liquid F3BꢃO(C2H5)2 (where d(11B)=
0.00 ppm).[21] For both MAS and stationary samples, the Hahn echo
(p/2 - t1 - p - t2 -ACQ) pulse sequence[22,23] was used with high
power proton decoupling. Echo delays ranged from approximately
40 to 80 ms for experiments on all spectrometers and probes. The
MAS frequency was 10kHz. The typical 11B p/2 pulse length
for solid NaBH4 was 24.0 ms. For all samples in this study, the ‘solid
p/2’ pulse was used (e.g., 24.0ms/(I + 1/2) = 12.0 ms, where I = 3/2 for
11B). Recycle delays of 10–90 s were employed. The probe exhibited
a small but manageable background 11B signal as a result of boron
nitride in the stator. The Hahn echo pulse sequence was generally
found to be effective at suppressing this background signal. For
13C NMR experiments, the referencing, calibration, and setup
procedures were identical to those performed on the 4.7 T
NMR spectrometer. CP and DD pulse sequences were used to
measure the chemical shifts of the ipso carbons of the samples
under investigation.
eV33Q
h
CQ
¼
;
(3)
where e is the fundamental charge, and h is the Planck constant.
The asymmetry parameter of the EFG tensor is defined as
V11 ꢁ V22
ꢀQ
¼
:
(4)
V33
Experimental and Computational Details
Sample preparation
As shown in Scheme 1 and Table 1, nine arylboronic acids, which
have different aryl substituents were purchased from Aldrich. All
arylboronic acids except A6 were recrystallized from hot water in
the air, filtered and dried at room temperature, and then stored
in a refrigerator at 5 ꢂC. A6 converted to its trimeric anhydride
(C2) when it was recrystallized from hot water, isopropanol, or
tetrahydrofuran in the air. It is well known that most arylboronic
acids may be converted easily to their trimeric anhydride forms
when they are heated (Scheme 2).[2,16–18] Seven arylboronic acid
cyclic catechol esters were synthesized by combining the
corresponding boronic acids with catechol in toluene and
heating at reflux with a Dean Stark trap. After 3 h, the reaction
mixture was cooled to room temperature and concentrated
to dryness. The resulting boronate esters were used without
further purification or purified by recrystallization from hexanes
as needed. These esters were stored in a dry place at room
temperature and were maintained at room temperature during
the NMR experiments.
11.75 T NMR experiments
Boron-11 NMR experiments were conducted using a Bruker Avance
500 NMR spectrometer (B0 = 11.75 T, nL(11B)= 160.46 MHz) at room
temperature. Spectra were acquired using TopSpin 1.3 software. A
Bruker 4 mm HXY MAS probe was used. The referencing, calibra-
tion, and setup procedures were identical to those performed on
the 9.4 T NMR spectrometer. For both MAS and stationary samples,
the Hahn echo pulse sequence was used. The MAS frequency was
10 kHz. Typical p/2 pulse lengths and recycle delays were the same
as those at 9.4 T.
Instrumentation and measurements
4.7 T NMR experiments
Carbon-13 NMR experiments were conducted using a Bruker
Avance III 200 NMR spectrometer (B0 = 4.7 T, nL(13C) = 50.31 MHz)
at room temperature. Spectra were acquired using Bruker
TopSpin 2.1 software. A Bruker 7.0 mm HXY triple-resonance MAS
probe was used for all experiments. Experimental referencing,
calibration, and setup were done using glycine. The carbon-13
chemical shifts were referenced with respect to tetramethylsilane
(TMS) by setting the carboxylic carbon peak of external solid
glycine to 176.40 ppm. Cross polarization (CP), total suppression
of spinning sidebands (TOSS),[19] and dipolar dephasing (DD) pulse
sequences[20] were used to obtain the chemical shifts of the ipso
carbons of the samples under investigation. A dephasing delay of
40 ms and a MAS frequency of 4.5kHz were used.
21.1 T NMR experiments
Boron-11 NMR experiments were conducted using a Bruker Avance
II 900 NMR spectrometer (B0 = 21.1 T, nL(11B)= 288.80MHz) at room
temperature. Spectra were acquired using TopSpin 1.3 software at
the National Ultrahigh-Field NMR Facility for Solids in Ottawa
(www.nmr900.ca). A Bruker 4 mm HXY MAS probe was used. The
referencing, calibration, and setup procedures were identical to
those performed at 9.4T. For both MAS and stationary samples,
the solid echo (p/2 - t1 – p/2 - t2 -ACQ) pulse sequence was used.
The MAS frequency was 10 kHz. All solid p/2 pulse lengths were
5.0 ms, and pulse delays were 10s.
FT-IR ATR experiments
9.4 T NMR experiments
All IR spectra were recorded using a Varian 640 Fourier transform
infrared (FT-IR) spectrometer with PIKE MIRacle™ ATR (attenuated
total reflectance) accessories having a single reflection ZnSe
ATR crystal fixed at an incident angle of 60ꢂ with a resolution
of 4 cm-1 in the spectral region of 4000–600 cm-1. The ATR
accessories were fitted with a high pressure clamp, providing
contact between the sample and the ATR crystal. Varian Resolu-
tions Pro (ver. 5.1.0.829) has been installed on the spectrometer.
FT-IR ATR data for each sample were collected with 64 scans at
room temperature. Background signal was obtained for every
sample with 64 scans. Atmospheric contributions from carbon
Boron-11 and carbon-13 NMR experiments were conducted
using a Bruker Avance III 400 NMR spectrometer (B0 = 9.4 T, nL
Scheme 2. Interconversion of phenylboronic acid and its anhydride.
wileyonlinelibrary.com/journal/mrc
Copyright © 2012 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2012, 50, 388–401

11B and 13C NMR of solid arylboronic acids and their esters
dioxide and water vapor were not accounted for beyond a
simple background subtraction.
Spectral simulation and computation
Spectral processing and simulation
Data were processed using TopSpin 1.3. FIDs were left-shifted to
the echo maxima when necessary, apodized using a Gaussian
function of 50 Hz for MAS samples and 100 Hz for stationary
samples, and then Fourier transformed. Stack plots were produced
with DMFit.[26] All spectral simulations were performed using the
WSolids1 program,[27] which incorporates the space-tiling algo-
rithm of Alderman et al.[28]
Single crystal X-ray diffraction
Crystal structures were collected at the University of Ottawa in
the X-ray Diffraction Laboratory. The crystals were mounted on
thin glass fibres using paraffin oil. Data for A5, A9, and B7 were
collected at 296.15 K. Crystals of the other four compounds were
cooled to 200.15 K prior to data collection. Data were collected
on a Bruker AXS SMART single crystal diffractometer equipped
with a sealed Mo tube source (wavelength = 0.71073 Å) and an
APEX II CCD detector. Raw data collection and processing were
performed with the APEX II software package from Bruker
AXS.[24] All hydrogen atoms were treated as idealized contribu-
tions. All scattering factors are contained in several versions of
the SHELXTL program library, with the latest version used being
6.12.33.[25] Crystallographic data and selected data collection
parameters are reported in Table 2.
Compounds A5, A8, B7, and C2 consistently formed very small
crystals, and the results presented here are the best from a series
of collection attempts for each compound. In addition to small
sample size, crystals of compound C2 demonstrated non-
merohedral twinning. Separate two-component orientation
matrices for non-merohedral twinning were found by analyzing
positions of reflections in reciprocal space using the RLATT
software package and were confirmed by CELL-NOW software
data analysis.[25] The structure was solved by direct methods,
completed with difference Fourier synthesis, and refined with
full-matrix least-squares procedures based on F2. Twinning was
accounted for by refining the final structural model against split
reflections and intensity data with the twinning parameter
refined to 11.5% for the smaller component.
Quantum chemical calculations
Single crystal X-ray data for all compounds studied in this work
except B2, B3, and B5 were used for NMR calculations. The
geometries of those three compounds are not known experimen-
tally and, instead, were generated using standard bond lengths
in Gaussview 3.0 and, then, fully optimized using a hybrid DFT
functional (B3LYP)[29] and the 6-311 + G* basis set for all atoms
using Gaussian 09.[30] All compounds for which experimental
structures are known were subjected to geometry optimization
of the hydrogen positions only (B3LYP/6-311 + G*). The optimized
structures were subjected to further magnetic shielding, EFG, and
spin-spin coupling constant calculations. Conformational effects
on the boron NMR parameters were investigated by varying the
C-C-B-O dihedral angle (fCCBO). EFG and magnetic shielding
tensors were also calculated using the GIPAW DFT approach,
after optimization of hydrogen atom positions, as implemented
in the CASTEP code, using Materials Studio (ver. 3.2.0.0) and the
PBE functional.[31,32] All calculations used a plane wave basis set
with an ‘ultrafine’ cutoff energy of 610 eV, and the ‘fine’ setting
was used to establish the number of k-points used to sample
the Brillouin zone (Monkhorst–Pack mesh).[33]
Table 2. Crystallographic data for some boronic acids and esters under studya
A3
A5
A8
A9
C2b
B6
B7
Formula
C8H11BO4
181.98
Monoclinic
P21/c
C7H9BO2
135.95
Triclinic
P-1
C7H5BClF3O2
224.37
Monoclinic
P21/c
C7H9BO2S
168.01
Monoclinic
P21/c
C18H12B3F3O3
365.71
Monoclinic
P21/m
C12H8BFO2
213.99
Monoclinic
P21/n
C12H8BClO2
230.44
Orthorhombic
Pnma
FW
Crystal system
Space group
a/Å
b/Å
c/Å
a/ꢂ
b/ꢂ
g/ꢂ
5.5550(18)
9.532(3)
17.406(5)
90
4.975(5)
6.180(6)
12.206(11)
84.67(3)
85.00(3)
88.61(3)
372.2(6)
2
4.930(6)
5.704(7)
32.72(4)
90
11.5921(3)
5.15100
14.5958(4)
90
3.8571(4)
20.8400(17)
10.4372(10)
90
12.362(8)
4.735(3)
17.487(11)
90
19.6321(15)
11.0200(9)
4.9630(3)
90
95.269(4)
90
90.007(15)
90
112.345(2)
90
97.423(5)
90
98.611(7)
90
90
90
V/Å3
Z
917.8(5)
4
920.2(19)
4
806.09(3)
4
832.20(14)
2
1012.0(11)
4
1073.72(14)
4
D
calc/g cm-3
1.317
1.213
1.620
1.384
1.459
1.405
1.426
T/K
200(2)
26.43
296(2)
28.38
200(2)
21.96
296(2)
28.30
200(2)
200(2)
24.99
296(2)
θ(max)/deg
24.72
24.72
F(000)
384
144
448
352
372
440
472
GOF on F2
R1[I > 2s(I)]
wR2
1.047
1.037
1.063
1.020
1.071
1.012
1.015
0.0354
0.1046
0.0734
0.1754
0.0379
0.0872
0.0341
0.0893
0.0714
0.1821
0.0558
0.1198
0.0626
0.1310
aCrystallographic data for samples A1, A2, and B4 obtained using XRD in this work were consistent with those in the literature. These data are
excluded from this table.
bTrimeric anhydride form made from the recrystallization of A6 from hot water.
Magn. Reson. Chem. 2012, 50, 388–401
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S.-W. Oh et al.
have an ortho substituent, give the most twisted structures. This
is attributable to the steric effect of the ortho substituent. A5 has
the shortest B-C bond length of all compounds investigated in this
study. This may be due to the electron-donating methyl group
located at the ortho position. The bond length between B and C
atoms of C1 is marginally shorter than that of A1. The same trend
was found for A4 and its anhydride structure.[34] This may indicate
a strengthening of this bond by the electron-withdrawing nature of
the ring in the trimeric anhydride form.[35]
Results and Discussion
X-ray crystallography
Among the arylboronic acids and catechol cyclic esters studied in
this work, seven new crystallographic structures were obtained by
single-crystal XRD. They are listed in Table 2. A1 and A3 each have
two molecules in the asymmetric unit, that is, two nonequivalent
boron sites, whereas the remaining samples have one molecule
in their asymmetric unit. C2 is the trimeric anhydride form of A6,
obtained when the latter was recrystallized from hot water.
FT-IR
Carbon-boron bond lengths, intermolecular hydrogen bond
lengths, and the dihedral angle formed by the C, C, B, and O
atoms (fCCBO) are summarized in Table 3. As shown, the
carbon–boron bond lengths are in the range of 1.526 to 1.575 Å
and do not vary greatly within each compound class. All arylboro-
nic acids studied have geometries which indicate the ability
to form intermolecular hydrogen bonds [r(O•••H) < 2 Å], as
confirmed by IR spectroscopy (vide infra). When the idealized
hydrogen atom positions reported in the XRD structures are
optimized computationally, the intermolecular hydrogen bonding
distances in solid arylboronic acids decreased by 0.080–0.201 Å.
The ꢁ B(OH)2 plane and the aromatic ring are twisted relative to
each other for most arylboronic acids, while they are almost
coplanar for the phenylboronic acid catechol cyclic esters.
The CCBO dihedral angle in the arylboronic acids varied from
3.4ꢂ for one form of A4 up to 40.0ꢂ for A5. On the other hand,
CCBO dihedral angles of arylboronic acid catechol cyclic esters
were below 3.5ꢂ and those in trimeric phenylboronic acid
anhydrides were 8.6ꢂ for C1 and 4.9ꢂ for C2. A5 and A8, which
FT-IR ATR spectra of representative samples are shown in Fig. 1.
The purpose of acquiring IR spectra was to provide an indepen-
dent perspective on the formation of anhydrides, impurities, or
decomposition products, so as to aid in the interpretation of
the solid-state NMR spectra. Assignment of all vibrational bands
is not our intention in this work. The C ꢁ H stretching mode of
the benzene ring around 3000 cm-1 from the FT-IR ATR spectra
was very weak compared with normal FT-IR spectra[16,38] of A1
and A4 obtained using a KBr disk. As shown in Fig. 1, pure A1,
A3, B5, and C1 did not give any evidence that they contained
any water or boric acid impurities. Boric acid was found to be a
common impurity in boronic acids in our previous study.[1] Other
pure arylboronic acid samples have given the same result [see
Supplementary Material (SM)]. Many arylboronic acids usually
have their corresponding anhydride forms as an impurity in the
solid state. These anhydride forms may either already be present
as impurities or self-assemble from the acids as a result of heating
(Scheme 2). To characterize the boronic acid compounds in the
Table 3. Summary of B-C and B-O bond lengths, intermolecular O•••H hydrogen bond lengths, and dihedral angles composed of C, C, B, and O
atoms for the samples studied in this work
Sample
Site
r(B-C)/Å
r(B-O)/Å
r(O•••H)/Å
f(CCBO)/ꢂ
Referencea
T,34,35
XRD data
Optimization of H position
A1
a
1.566
1.560
1.560
1.569
1.554
1.575
1.526
1.552
1.558
1.563
1.536
1.533
1.532
1.533
1.542
1.535
1.534
1.536
1.544
1.542
1.538
1.539
1.539
1.364, 1.370
1.367, 1.368
1.362, 1.374
1.355, 1.371
1.354, 1.377
1.352, 1.362
1.379, 1.396
1.357, 1.364
1.347, 1.364
1.354, 1.366
1.394, 1.394
1.396, 1.397
1.397, 1.398
1.383, 1.387
1.396, 1.399
1.394, 1.394
1.387, 1.387
1.386, 1.390
1.379, 1.384
1.386, 1.389
1.379, 1.379
1.373, 1.386
1.373, 1.386
1.891, 1.896
1.753, 1.761
5.34, 6.63
20.42, 22.02
19.64, 21.28
21.59, 22.47
3.37, 4.63
20.28, 22.95
36.44, 40.04
26.19,26.28
36.01, 38.49
17.62, 19.55
0.37, 0.37
0, 0
b
1.891, 1.896
1.753, 1.761
A2
A3
A4
—
—
A
1.969, 1.969
1.806, 1.806
T, 36
T
2.003, 2.003
1.826, 1.826
1.921, 1.927
1.766, 1.777
37
B
1.921, 1.927
1.766, 1.777
A5
A7
A8
A9
B1
B2
B3
B4
B5
B6
B7
C1
—
—
—
—
—
—
—
—
—
—
—
1
1.944, 2.193
1.776, 1.776
T
1.860, 1.860
1.754, 1.754
T
1.948, 1.948
1.778, 1.778
T
1.905, 1.905
1.830, 1.830
T
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
T, 38
G
0, 0
G
2.52, 3.45
0, 0
T, 39
G
0.42, 2.25
0.69, 0.69
2.10, 3.61
2.87, 3.38
7.07, 8.56
0.77, 0.77
3.64, 4.92
3.64, 4.92
T
T
40
2
3
C2
1
T
2
3
aT means this work, and G means results obtained from geometry optimization (B3LYP/6-311 + G*).
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Magn. Reson. Chem. 2012, 50, 388–401

11B and 13C NMR of solid arylboronic acids and their esters
Figure 2. Short oxygen–hydrogen contacts, some of which could result
in weak intramolecular hydrogen bonding in A2 (left) and A5 (right).
Figure 1. IR spectra of representative boronic acids, a boronic ester, and
an anhydride, along with spectra for boric acid (b) and water (w).
obtained in this work suggests that intramolecular hydrogen bond-
ing may occur between O and the H atoms of the methoxy group
of A2; on the other hand, it may occur between O and H atoms of
different OH groups for A5.
When A1 was heated in an electric oven at 80 ꢂC overnight, its
IR spectrum changed exactly to that of C1 (Fig. 1). When C1 was
recrystallized from hot water, its IR spectrum changed exactly to
that of A1. That is, phenylboronic acid and its trimeric anhydride
can convert reversibly as shown in Scheme 2.
solid state, it is necessary to purify them. It is well known that the
acid form and the anhydride form of arylboronic acids show
different characteristic IR patterns.[2,12,13,35–37] Pure arylboronic
acids have
a
strong H-bonded O ꢁ H stretching mode
(3300–3200 cm-1) and a very strong band attributed to the B–O
stretching mode (1380–1310 cm-1).[2] Upon anhydride forma-
tion, the O ꢁ H stretching mode disappears, and a new strong
absorption appears at 680–705 cm-1.[2,16,36] Additionally, the BO2
out-of-plane deformation mode[38] at ~630 cm-1 in arylboronic
acids disappears in anhydrides. This information is particularly
diagnostic of the presence of boronic anhydrides in the IR spectra.
Before performing 11B NMR experiments on pure solid arylboronic
acids, it may be useful and convenient to confirm that an O ꢁ H
stretching mode around 3300 cm-1 and a BO2 deformation mode
around 630 cm-1 exist and that ring deformation modes of trimeric
anhydrides around 700 cm-1 do not exist. Unlike the other arylboro-
nic acids, only A1 shows a vibrational mode around 700cm-1,
which can also be seen in the IR spectrum of C1, a trimeric
anhydride form of A1. This may be due to the out of plane phenyl
ring deformation of monosubstituted benzenes[38] rather than
because of the ring deformation mode of trimeric anhydride. In
addition to three vibrational bands, a Bꢁ O ꢁ H deformation
mode[38] around 1000 cm-1 and a B ꢁ C stretching mode around[38]
1100 cm-1 can be observed for the pure arylboronic acids. However,
the latter is also observed for C1. So the latter modes may not be
useful to diagnose the anhydride form of arylboronic acids.
As shown in Fig. 1, any O ꢁ H stretching mode around 3300 cm-1,
which appears for pure arylboronic acids did not appear in the IR
spectra of pure solid arylboronic acid catechol cyclic esters studied
in this work. Most vibration modes are observed as in the litera-
ture,[39] that is, C ꢁ O stretching modes were observed around
1237 and 1030 cm-1, whereas B ꢁ O stretching modes were
observed around 1329 and 1070 cm-1. As in the case of the
arylboronic acids, the C ꢁ C stretching mode was observed
around 1600 cm-1 for arylboronic acid catechol cyclic esters.
11B NMR
Shown in Figs. 3–6 are 11B NMR spectra acquired under MAS and
static conditions at three different fields of 9.4, 11.75, and 21.1 T
for some arylboronic acids and their catechol cyclic esters.
Several more spectra for the other compounds are given in the
SM. Presented in Table 4 are the 11B EFG and CS tensor values
obtained via analytical simulations of the 11B NMR spectra of
arylboronic acids (A1 through A9) and their catechol cyclic esters
(B1 through B7). By simultaneously fitting data at three different
fields, the CQ, ꢀQ, and diso values were determined from the MAS
NMR spectra. The fitting is done using WSolids1 software in a
heuristic fashion by cycling through the spectra acquired at all
three fields and adjusting a single set of parameters manually
until self-consistent results are obtained for all three spectral data
sets. Spectra of stationary samples were then analyzed to deter-
mine the Ω, k, and Euler angles relating the CS tensor PAS to
Alver[13] has observed four O ꢁ H stretching modes from 3480 to
3262 cm-1 in the FT-IR spectrum of 2,4-dimethoxyphenylboronic
acid. He suggested that the two sharper peaks at higher wavenum-
bers are caused by the formation of intramolecular hydrogen
bonds, and the two broader peaks at lower wavenumber are
caused by the formation of intermolecular hydrogen bonds.
Among the various arylboronic acids studied in this work, three
samples (A2, A3, and A5) have an additional sharp peak in their
IR spectra. This may be caused by the formation of weak intramo-
lecular hydrogen bonds, which can be expected in arylboronic
acids substituted with a methoxy group or ortho-substituted by a
methyl group. As shown in Fig. 2, the X-ray crystal structure
the EFG PAS.[40] As shown in Table 4, a small range in CQ, diso
,
and Ω is observed. Incidentally, the clear resolution of the two
crystallographically non-equivalent boron sites in A1 and A4
was not possible (Supplementary Material, Fig. 4S); however,
slightly improved simulations were obtained using a two-site
Magn. Reson. Chem. 2012, 50, 388–401
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S.-W. Oh et al.
Figure 3. Boron-11 MAS (left) and static (right) NMR spectra of A2 acquired at different three fields (21.1T, 11.75 T, and 9.4T from the top). Experimental
spectra are in black, and simulated spectra are in red. The MAS rate was 10 kHz, and asterisks (*) indicate sidebands.
Figure 4. Boron-11 MAS (left) and static (right) NMR spectra of A8 acquired at different three fields (21.1T, 11.75 T, and 9.4T from the top). Experimental
spectra are in black, and simulated spectra are in red. The MAS rate was 10 kHz, and asterisks (*) indicate sidebands.
model to fit the data, and so, two sets of parameters are listed in
Table 4 for each of these compounds. Depending on the sample,
the CQ values vary from 2.64 MHz for compound B6 to 3.05 MHz
for compound A5; the diso values vary over a very small range,
from 28.6 ppm for A2 to 31.5 ppm for B3, and the Ω values vary
from 17.7 ppm for site a of A1 to 28.3 ppm for A5. Although the
11B NMR parameters vary over a small range, it is possible to
divide the systems studied into two groups, that is, arylboronic
acids and arylboronic acid catechol cyclic esters, using their
NMR parameters, which is consistent with the trends reported
previously for boronic acids and esters bearing different substi-
tuents.[1] The CQ, ꢀQ, and k values for the arylboronic acids fall in
the ranges 2.95–3.05 MHz, 0.30–0.45, and 0.35–1.0, respectively,
whereas those for the arylboronic acid catechol esters fall in the
ranges 2.64 –2.71 MHz, 0.74–0.08, and ꢁ0.85 to ꢁ0.12, respec-
tively. As seen previously[1] for diso and Ω, there is overlap in
the ranges for each group of compound. The Ω values vary over
a larger range (10.7 ppm) for arylboronic acids than for
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11B and 13C NMR of solid arylboronic acids and their esters
Figure 5. Boron-11 MAS (left) and static (right) NMR spectra of B1 acquired at different three fields (11.75 T and 9.4 T from the top). Experimental
spectra are in black, and simulated spectra are in red.
Figure 6. Boron-11 MAS (top) and static (bottom) NMR spectra of B3, B4, and B5 acquired at 9.4 T. The MAS rate was 10 kHz, and asterisks (*) indicate
sidebands. Experimental spectra are in black, and simulated spectra are in red.
arylboronic acid catechol cyclic esters (5 ppm). The Euler angles
suggest that the largest components of the CS and EFG tensors
are coincident or nearly coincident.
also revealed that a particularly long and weak pulse was optimal
for achieving ideal lineshapes (i.e. up to 48 ms for the CT-selective
90-degree pulse) and that short pulses (e.g. 1.5 ms for the 90-degree
pulse) contributed to the unwanted extra spectral intensity at the
high-frequency edge of the MAS line shape.
In the 11B MAS NMR spectra, an anomalous peak on the high-
frequency edge of the powder pattern was sometimes observed,
as shown for B6 and B7 (see SM). We were interested to deter-
mine whether this was caused by an impurity or the experimental
conditions used to acquire the NMR spectra. Several verifications
of sample purity were performed using IR spectroscopy and X-ray
crystallography (vide supra) to ensure that the unexplained peaks
were not due to boric acid or another decomposition product.
We then decided to investigate the dependence of the second-
order quadrupolar line shape under MAS conditions as a function
of relaxation delay and of the excitation pulse power and length.
Some spectra demonstrating the results are presented in the
Supplementary Material. Some of the findings are summarized in
Fig. 7 for a typical case, compound B6, where it is seen that the
high-frequency and low-frequency discontinuities of the powder
pattern correspond to nuclei in crystallites which relax marginally
more quickly (T1 = 19 ꢄ 2s and 16 ꢄ 3 s for the high-frequency and
low-frequency edges, respectively) than do the nuclei in crystallites
corresponding to the central two maxima (T1 = 22 ꢄ 1 s). Investiga-
tion of the effects of pulse length on the line shape (see SM, Fig. 12S)
For the anhydrides (C1 and C2), 11B MAS and static NMR
spectra obtained at two magnetic fields are shown in Fig. 8 and
in the Supplementary Material. Although the MAS NMR spectra at
9.4 T closely resemble those expected for an isolated 11B nucleus,
spectral analysis was not pursued further because of the existence
of homonuclear 11B-11B dipolar couplings. More advanced meth-
ods may be amenable to analyzing the MAS NMR spectra;[10,41]
however, the analysis of the spectra of the stationary samples is
complicated additionally by anisotropic chemical shift contribu-
tions from the multiple crystallographically non-equivalent sites.
13C NMR
Most of the samples studied in this work showed different 13C
CP/MAS spectra before and after recrystallization. Phenylboronic
acid (A1) is one such example. Shown in the Supplementary
Material (Fig. 14S) are 13C CP/MAS spectra for the commercial
sample and for the purified sample. The two spectra are completely
Magn. Reson. Chem. 2012, 50, 388–401
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S.-W. Oh et al.
Table 4. Experimental 11B EFG and CS tensor parameters of the boronic acids and esters in this worka
Sample
CQ/MHz
ꢀQ
diso
(
11B)/ppm
Ω/ppm
k
a/ꢂ
b/ꢂ
g/ꢂ
A1(a)b
A1(b)b
A2
3.00 ꢄ 0.10
3.03 ꢄ 0.10
3.02 ꢄ 0.05
2.97 ꢄ 0.05
3.00 ꢄ 0.10
3.03 ꢄ 0.10
3.05 ꢄ 0.05
3.03 ꢄ 0.05
2.97 ꢄ 0.05
2.95 ꢄ 0.05
2.71 ꢄ 0.05
2.68 ꢄ 0.17
2.68 ꢄ 0.05
2.68 ꢄ 0.05
2.64 ꢄ 0.05
2.64 ꢄ 0.05
2.69 ꢄ 0.05
0.45 ꢄ 0.10
0.40 ꢄ 0.10
0.43 ꢄ 0.05
0.40 ꢄ 0.05
0.42 ꢄ 0.10
0.40 ꢄ 0.10
0.43 ꢄ 0.05
0.40 ꢄ 0.05
0.30 ꢄ 0.05
0.37 ꢄ 0.05
0.78 ꢄ 0.05
0.80 ꢄ 0.10
0.78 ꢄ 0.05
0.78 ꢄ 0.05
0.80 ꢄ 0.05
0.74 ꢄ 0.05
0.76 ꢄ 0.05
30.5 ꢄ 1.0
31.0 ꢄ 1.0
28.6 ꢄ 0.6
28.7 ꢄ 0.6
30.2 ꢄ 1.0
30.7 ꢄ 1.0
30.4 ꢄ 0.5
29.7 ꢄ 0.5
28.9 ꢄ 0.5
28.8 ꢄ 0.5
30.8 ꢄ 0.5
30.8 ꢄ 0.1
31.5 ꢄ 0.5
30.4 ꢄ 0.5
30.1 ꢄ 0.5
30.0 ꢄ 0.7
30.3 ꢄ 0.5
17.7 ꢄ 1.3
18.7 ꢄ 1.3
23.0 ꢄ 1.2
27.0 ꢄ 1.2
17.7 ꢄ 1.4
18.7 ꢄ 1.4
28.3 ꢄ 1.0
26.0 ꢄ 1.0
24.0 ꢄ 1.0
25.7 ꢄ 1.0
26.0 ꢄ 1.0
23 ꢄ 1
0.80 ꢄ 0.20
1.00 ꢄ 0.20
0.73 ꢄ 0.13
0.35 ꢄ 0.12
0.80 ꢄ 0.20
1.00 ꢄ 0.20
0.38 ꢄ 0.13
0.52 ꢄ 0.10
0.57 ꢄ 0.10
0.40 ꢄ 0.10
ꢁ0.25 ꢄ 0.10
ꢁ0.45 ꢄ 0.05
ꢁ0.85 ꢄ 0.10
ꢁ0.15 ꢄ 0.10
ꢁ0.15 ꢄ 0.10
ꢁ0.22 ꢄ 0.10
ꢁ0.12 ꢄ 0.10
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
90
90
60
40
0
0
A3
10
5
A4(a)b
A4(b)b
A5
5
0
8
40
40
50
0
A7
13
8
A8
A9
14
0
B1
0
B2c
B3
5
0
25.5 ꢄ 1.0
28.0 ꢄ 1.0
28.0 ꢄ 1.0
27.5 ꢄ 1.0
27.7 ꢄ 1.0
4
0
B4
0
0
B5
0
0
B6
0
0
B7
0
12
aThe errors in the angles a and g are approximately ꢄ 50 degrees, whereas the error in b is ꢄ 10 degrees.
b(a) or (b) indicates one of two molecules in the asymmetric unit. Site (a) indicates a flat molecule in which the dihedral angle (fCCBO) is below 10
degrees, whereas site (b) indicates a twisted molecule in which a dihedral angle is larger than 10 degrees.
cData for B2 are from Weiss, J. W. E., M.Sc. Thesis, University of Ottawa, 2011.
Figure 7. Boron-11 T1 relaxation anisotropy for B6. All data were
taken from spectra obtained at 9.4 T. Data are fit to curves of the form
Figure 8. Boron-11 MAS and static NMR spectra of C1 acquired at two
I = I0ꢁfexp(ꢁt/T1). Values of T1 obtained are 19 ꢄ 2, 22 ꢄ 1, 22 ꢄ 1, and
fields (21.1 T and 9.4 T).
16 ꢄ 3 s for the positions of the MAS powder pattern labelled A, B, C,
and D, respectively.
and their catechol cyclic esters. Of particular interest presently
different, and the as-received commercial sample was found to
be mostly the anhydride form of phenylboronic acid. This
phenomenon was also detected in the FT-IR spectra (vide supra).
Presented in the Supplementary Material (Fig. 15S) are repre-
sentative 13C CP/MAS NMR spectra of pure arylboronic acids
are the chemical shifts of the ipso carbons attached to boron. It
can be difficult to determine these values because there is often
spectral overlap with other aromatic carbons; the resonance is
split into four peaks in principle due to coupling with 11B or, alter-
natively, is split because of coupling to the other quadrupolar
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Magn. Reson. Chem. 2012, 50, 388–401

11B and 13C NMR of solid arylboronic acids and their esters
boron isotope (10B). It is well known that spin-1/2 nuclei which
are spin-coupled to quadrupolar nuclei are subjected to residual
dipolar coupling effects under MAS conditions.[14] That is, the
dipolar coupling between the spin-1/2 nucleus and the quadru-
pole is not completely averaged as a result of MAS. This leads
to broadening and/or splittings of the isotropic resonance of
the spin-1/2 nucleus. These splittings are influenced by the values
of the dipolar coupling constant between the two nuclei, the
J coupling between them, the EFG tensor at the quadrupole, and
the relative orientation of the EFG and dipolar tensors. Therefore,
under favourable conditions, novel information on some or all of
these NMR parameters may be determined through spectral line
shape simulations. The NMR spectra here were fit using the dipolar
coupling constants derived from the X-ray crystallographic atomic
coordinates for boron and carbon (Table 3), the 11B quadrupolar
coupling data determined experimentally (Table 4), and an approx-
imately perpendicular orientation of the largest component of the
11B EFG tensor with respect to the 13C-11B dipolar vector. A
maximum of a few percent adjustment downwards in the value
of the static dipolar coupling constant was permitted in the simula-
tions to account for vibrational averaging and a small anisotropy in
the indirect spin-spin coupling tensor (ΔJ). In all cases, the spectral
simulations showed that the experimental data were consistent
with a positive value of CQ(11B) and a positive value of RDD
(13C,11B), which is consistent with our quantum chemical data
(vide infra) and our previous work.[1]
isotopes to the ipso carbon is shown in the Supplementary Material
for A2. In the simulation process, the isotope shift[42] between 13
C
bonded to the two different boron isotopes, 11B and 10B, was
also considered. The one-bond 13C isotope shift,[1]Δ13C(11/10B), is
defined as the chemical shift of 13C nuclei which are directly
bonded to 11B less the chemical shift of 13C nuclei which are
directly bonded to 10B.[43,44] On the basis of the theory of the
isotope shift and from tabulated values for spin pairs of comparable
mass, we anticipated a small negative isotope shift of less than
0.1 ppm.[44] Simulations of all data show that a negative value of
magnitude less than about 0.2ppm for the isotope shift reproduces
the experimental spectra. Spectral simulations are shown in detail
for A2 in the Supplementary Material (Fig. 18S). The appropriate
natural abundance ratio (80.1% for 11B and 19.9% for 10B) was
applied to all simulations. When this simulation was performed,
one bond indirect nuclear spin-spin coupling constants (1J)
between 11/10B and 13C were also determined. The 1J(13C,11B)
values are summarized with the 13C isotropic chemical shifts of
the ipso carbons in Table 5. Isotope effects on the reduced coupling
constants were assumed to be negligible. A weak correlation
(R2 = 0.79) was found between the value of 1J(13C,11B) and the
boron–carbon bond length (Fig. 10b); however, closer inspection
of the data reveals that this apparent trend is mainly because the
data for the boronic acids cluster together (long bond lengths
and smaller coupling constants) and those for the boronate esters
form a separate cluster (shorter bond lengths and larger coupling
constants). Within each of the two compound classes, the experi-
mental J coupling data are not precise enough to establish a clear
correlation with bond length.
In Fig. 15S (see SM), the 13C CPMAS spectra of A2 and A3 show
well-resolved peaks for the ipso carbons, whereas there is only
partial resolution in the spectrum for A9, and there is no resolution
in the spectrum of A1. To obtain the 13C isotropic chemical shift of
the ipso carbons, fitting was straightforward in cases where the
resonance was clearly resolved from other peaks. In the case of
A1 or A9, for example, CP/DD spectra were used instead of normal
CP spectra to remove peaks from protonated carbons in the
aromatic region. TOSS/DD spectra were used in some cases to deal
with overlapping spinning sidebands (Supplementary Material).
Spectra obtained at both 4.7 and 9.4 T were used for the simula-
tions. Fits of the 13C CP/MAS NMR spectra of A2 and A3 are
presented in Fig. 9, and an example of the deconvolution of the
contributions caused by coupling to each of the two boron
Quantum chemical calculations
Calculations of NMR parameters were carried out using two
different approaches. The first is a cluster model approach, where
one molecule of the compound of interest was used in the calcu-
lation. A second series of calculations using this approach was
carried out using a dimer rather than a monomer in the case of
some of the boronic acids, which are known to be hydrogen
bonded according to their crystal structures. The second
approach is the GIPAW DFT method, where the atomic coordinates
Figure 9. Fits of 13C CP/MAS NMR peaks of the ipso carbon attached to a boron atom for A2 (left) and A3 (right). Experimental spectra are in black, and
simulated spectra are in red.
Magn. Reson. Chem. 2012, 50, 388–401
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S.-W. Oh et al.
Table 5. Experimental and calculated 13C chemical shifts of the ipso
carbons and one-bond 13C-11B spin-spin coupling constants for the
boronic acids and catechol cyclic esters studied in this work
Sample
diso
(
13C)/ppm
1J(13C,11B)/Hzb
Experiment Calculationa Experiment Calculation
A1 Site a
129.4 ꢄ 0.5
130.8 ꢄ 0.5
123.8 ꢄ 0.2
124.8 ꢄ 0.2
123.0 ꢄ 0.5
128.0 ꢄ 0.5
132.0 ꢄ 0.5
128.0 ꢄ 0.5
130 ꢄ 0.5
135.1
135.9
128.6
128.5
127.8
131.9
134.7
133.1
136.0
129.3
131.4
122.6
125.5
130.8
125.8
127.2
86 ꢄ 10
86 ꢄ 10
85 ꢄ 5
86.4
85.9
87.2
86.7
78.9
84.0
103.9
85.9
89.8
85.8
100.1
101.0
97.9
98.7
99.8
98.2
Site b
A2
A3
85 ꢄ 5
A4 Site a
Site b
A5
79 ꢄ 10
84 ꢄ 20
104 ꢄ 10
86 ꢄ 10
90 ꢄ 10
86 ꢄ 10
100 ꢄ 10
100 ꢄ 10
98 ꢄ 10
100 ꢄ 10
100 ꢄ 10
98 ꢄ 10
A7
A8
A9
127.5 ꢄ 0.5
126.4 ꢄ 0.5
116.5 ꢄ 0.5
123.0 ꢄ 0.5
125.3 ꢄ 0.5
122.1 ꢄ 0.5
123.9 ꢄ 0.5
B1
B3
B4
B5
B6
B7
aMonomer model, B3LYP/6-311 + G*. Shielding constants were placed
on the chemical shift scale using the absolute shielding constant of
184.1 ppm for TMS (A. K. Jameson, C. J. Jameson, Chem. Phys. Lett.
1987, 134, 461).
bOne bond spin-spin coupling constants [(1J(13C,11B)] were calculated
using the heavy atom coordinates from XRD crystallographic data
with the optimization of hydrogen positions.
from the crystal structure as well as the unit cell parameters are
treated using periodic boundary conditions so as to model an
infinite crystalline lattice.
Figure 10. (a) Relation between experimental and calculated one
bond spin-spin coupling constants for arylboronic acids and their cate-
chol cyclic esters. 1J(13C,11B) calc = 0.9701(1J(13C,11B) expt) + 2.9426 Hz;
R² = 0.9897. (b) Correlation between experimental values of 1J(13C,11B)
and the experimental boron–carbon bond lengths. 1J(13C,11B) =
ꢁ453.87r + 795.04 Hz; R² = 0.7862.
Calculated 13C chemical shifts and 1J(13C,11B) values are
summarized in Table 5. The 13C chemical shifts are systematically
overestimated by the B3LYP calculations by about 2–5 ppm when
using a monomer as the model. The values of 1J(13C,11B) are very
well reproduced by the calculations (Fig. 10a), with a R² value of
0.9897. Calculated 11B NMR parameters are listed in Table 6.
Comparison with the experimental data reveals several points
of interest. First, in the cases where there are two crystallograph-
ically distinct molecules in the unit cell (compounds A1, A4), the
calculations are consistent with the experimental finding that the
values of CQ(11B), ꢀQ, and diso are too close to distinguish the two
boron sites. Second, as shown in Fig. 11, the experimental trends
in CQ(11B) are well reproduced using a monomer (cluster, B3LYP),
dimer (cluster, B3LYP), or infinite lattice (GIPAW DFT), with essen-
tially negligible improvements noted upon increasing the model
size from a monomer to a dimer. The GIPAW DFT CQ values are
systematically larger than the values obtained using a cluster
model, and the former are also further from the experimental
values. In the case of ꢀQ, both the cluster model and the periodic
models reproduce the experimental data fairly well, with the
GIPAW DFT method providing slightly better agreement with
experiment both in terms of absolute values and in terms of
the correlation coefficient (see SM). Likely as a result of the small
overall range in the values of diso and Ω, both types of calculations
do not provide good correlations with the experimental data.
Experimental chemical shifts are overestimated by both types of
calculations when the experimental absolute shielding scale of
Jackowski et al. is used (s = 110.9ppm for liquid BF3OEt2).[45]
Calculated CS tensor spans are in approximate agreement with
the experimental data, but the trends are not reproduced.
Overall, given that the use of a dimer model did not signifi-
cantly alter the results obtained when using a monomer model,
it would seem that the notably different values obtained from
the GIPAW DFT approach are largely because of the difference
in the density functional and basis set/energy cutoff, rather
than because of the inclusion or exclusion of the surrounding
molecules from the model used for the calculation. This is
perhaps not surprising given that the boron atoms in the mole-
cules studied presently are covalently bound within the centre
of each molecule, and there are no ions in the lattice.
Boron-11 NMR parameters were calculated for A1, A4, and A5
using a monomer model as the CCBO dihedral angle was varied.
A4 and A5 are arylboronic acids, which have only methyl groups
as substituents on the phenyl ring. The results are shown in
Fig. 12 and in the SM. The observed dependence and its relation-
ship with the electronic structure at boron has been discussed by
Weiss and Bryce.[1] The inclusion of a methyl group as a para
substituent (A1 vs A4) did not alter the manner in which the
energy and the 11B NMR parameters vary as a function of the CCBO
dihedral angle; however, a difference in the behaviour of the
computed parameters when the methyl group was substituted at
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Magn. Reson. Chem. 2012, 50, 388–401

11B and 13C NMR of solid arylboronic acids and their esters
Table 6. Calculated 11B EFG and magnetic shielding tensor parameters for arylboronic acids, esters, and anhydrides
Sample
Number of molecules
included in the cluster
model for GIAO calculation
CQ/MHz
ꢀQ
siso
(
11B)/ppm
Ω/ppm
k
GIAOb
GIPAW
A1a
1 site a
1 site b
2 site a
2 site b
3.130
3.165
3.132
3.155
3.513
3.563
3.132
3.155
3.525
3.099
3.114
3.469
3.119
3.150
3.135
3.142
3.582
3.956
3.182
3.195
3.593
3.170
3.161
3.585
3.113
3.116
3.559
3.129
3.137
3.517
2.768
3.120
2.786
3.123
2.753
3.083
2.760
3.109
3.301
3.314
3.305
3.616
3.677
3.392
3.283
3.277
3.277
3.673
3.678
3.678
0.452
0.467
0.508
0.529
0.399
0.431
0.459
0.590
0.415
0.652
0.511
0.370
0.296
0.434
0.515
0.494
0.289
0.356
0.759
0.636
0.542
0.440
0.502
0.402
0.311
0.372
0.283
0.439
0.477
0.396
0.880
0.855
0.859
0.806
0.853
0.824
0.840
0.812
0.678
0.691
0.659
0.638
0.617
0.577
0.645
0.643
0.643
0.586
0.583
0.583
74.64
73.74
74.48
73.41
65.16
64.87
73.98
74.13
66.61
76.00
74.55
66.46
74.54
74.62
74.54
74.21
64.88
60.32
77.73
70.85
62.83
74.19
74.02
65.21
74.56
74.19
66.18
74.61
74.98
67.11
71.09
64.71
71.22
65.12
70.89
65.39
71.50
64.92
73.06
73.27
73.45
66.10
66.38
66.82
74.11
74.22
74.22
66.85
66.58
66.58
24.95
28.19
27.41
31.53
22.17
30.34
27.29
26.54
30.90
28.10
31.46
30.73
28.36
27.45
26.86
30.54
32.15
29.54
37.83
39.81
38.81
29.30
32.25
31.32
33.36
32.41
29.55
26.43
25.81
29.90
24.77
34.24
25.25
34.08
23.86
31.59
24.09
33.39
25.92
26.14
24.68
32.89
31.89
29.71
24.43
25.26
25.26
34.19
30.70
30.70
0.79
0.89
0.02
0.11
Site a
Site b
0.87
0.54
A2
1
3
0.88
0.60
0.73
A3
1
2
0.58
0.13
0.55
A4a
1 site a
1 site b
2 site a
2 site b
0.45
0.91
0.12
0.11
Site a
Site b
0.68
0.25
A5
A7
A8
A9
1
2
0.36
0.00
0.31
1
2
0.91
0.18
0.65
1
2
0.58
0.41
0.85
1
3
0.90
0.55
0.75
B1
B4
B6
B7
C1
1
1
1
1
ꢁ0.71
ꢁ0.04
ꢁ0.83
ꢁ0.06
ꢁ0.77
ꢁ0.04
ꢁ0.73
0.00
1 site a1
1 site a2
1 site a3
ꢁ0.47
ꢁ0.42
ꢁ0.40
ꢁ0.05
0.02
Site a1
Site a2
Site a3
0.22
C2
1 site a1
1 site a2c
1 site a3c
ꢁ0.39
ꢁ0.39
ꢁ0.39
0.10
Site a1
Site a2c
Site a3c
0.08
0.08
aTwo molecules exist in the asymmetric unit for these samples.
bNumerical value of 2 or 3 means that intermolecular H-bonding was involved in the model used for the GIAO calculation. That is, the calculation
was performed for a dimer or trimer.
cSites a2 and a3 are crystallographically equivalent.
Magn. Reson. Chem. 2012, 50, 388–401
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/mrc

S.-W. Oh et al.
A5 versus A1 and A4. This is in agreement with the experimental
data in Tables 3 and 4. Additional calculations of various 11B and
13C NMR parameters for boronic acids as a function of geometry
are presented in the Supplementary Material.
Conclusions
In conclusion, this study has revealed new insights into the
relationship between the NMR parameters of boronic acids and
boronate esters, and their molecular structures. X-ray crystal
structures of seven compounds have been newly determined.
Along with X-ray crystallography, IR spectroscopy of powdered
samples was very useful in identifying anhydride impurities in
the samples and to understand the conversion between acid
and anhydride forms of various samples. Proper consideration
of T1 relaxation anisotropy and excitation pulse power was found
to be critical for the acquisition of undistorted 11B solid-state
NMR spectra. Boron-11 chemical shift and electric field gradient
tensors were characterized for several samples, and, in agreement
with our previous work, it was noted that the acids and esters
may be easily differentiated on the basis of the value of the 11B
quadrupolar coupling constant. Small boron chemical shift
anisotropies were measureable because of the use of spectrome-
ter equipped with a 21.1 T magnet. The magnitude of these small
anisotropies was well reproduced with quantum chemical
calculations; however, the experimental trends were not. The
importance of the CCBO dihedral angle and the local hydrogen
bonding environment was noted. Careful analysis of the 13C CP/
MAS NMR spectra of the same samples allowed for the accurate
measurement of ipso carbon chemical shifts as well as boron-
carbon J coupling constants. Both of these values were well-
reproduced with quantum chemical calculations, suggesting the
potential utility of the NMR parameters for providing insight into
structure in boronic acids and esters.
Figure 11. Correlations between experimental and calculated 11B quadru-
polar coupling constants for boronic catechol acids and esters. Results are
presented using Gaussian 09 software [cluster model; monomer (red circles)
and dimer (open black circles)] as well as CASTEP (GIPAW-DFT) using
periodic boundary conditions (blue triangles). The equations of best fit are
as follows: CQ(calc) = 1.13CQ(expt)
–
0.26 MHz, adjusted R2 = 0.986
(monomer); CQ(calc) = 1.27CQ(expt) – 0.27 MHz, adjusted R2 = 0.977 (GIPAW
DFT).
Acknowledgements
D. L. B. and K. E. M. thank the Natural Sciences and Engineering
Research Council (NSERC) of Canada for funding. S.-W. O. thanks
Mokpo National University, Korea, for an award under the profes-
sors’ training program (Kiseonghoe 2010). We thank Dr Victor
Terskikh, Dr Eric Ye, Dr Glenn Facey, and Dr Andy Lo for technical
assistance, and Prof. R. Tom Baker for use of the IR spectrometer.
Access to the 900MHz NMR spectrometer was provided by the
National Ultrahigh-Field NMR Facility for Solids (Ottawa, Canada);
a national research facility funded by the Canada Foundation
for Innovation, the Ontario Innovation Trust, Recherche Québec,
the National Research Council Canada, and Bruker BioSpin and
managed by the University of Ottawa (www.nmr900.ca). NSERC is
acknowledged for a Major Resources Support grant.
Supporting Information
Additional 11B and 13C solid-state NMR spectra, figure showing
the effects of pulse delay and pulse length on 11B NMR spectra,
additional computational results, correlations between experi-
mental and calculated data, cif data.
Figure 12. Calculated (B3LYP/6-311 + G*) 11B magnetic shielding tensor
parameters versus dihedral angle (CCBO) for A5.
the ortho position (A5) was noted. The computed energy explains
why the experimental value of fCCBO of A5 is large (~ 40ꢂ) relative
to the values for A1 and A4, as shown in Table 3. Inspection of the
plots of computed energy and 11B NMR parameters as a function of
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