Article
pubs.acs.org/IC
11
B Solid-State NMR Interaction Tensors of Linear Two-Coordinate
Boron: The Dimesitylborinium Cation
Amanda E. Alain,† Yoshiaki Shoji,‡ Takanori Fukushima,‡ and David L. Bryce*,†
†
Department of Chemistry and Biomolecular Sciences & Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa,
Ontario K1N 6N5, Canada
‡
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
S Supporting Information
*
ABSTRACT: Borinium cations (R2B+) are of particular fundamental and applied interest in part due to their pronounced Lewis acidity
which enables unique chemical transformations. Solid-state NMR
spectroscopy of magic-angle spinning and stationary powdered
samples of the dicoordinate boron cation in the recently reported
dimesitylborinium tetrakis(pentafluorophenyl)borate compound
(Shoji et al. Nature Chem. 2014, 6, 498) is applied to characterize
the 11B electric field gradient (EFG) and chemical shift (CS) tensors.
The experimental data are consistent with linear CB+C
geometry. The 11B quadrupolar coupling constant, 5.44 ± 0.08
MHz, and the span of the CS tensor, 130 ± 1 ppm, are both
particularly large relative to literature data for a variety of boron
functional groups, and represent the first such data for the linear CB+C borinium moiety. The NMR data are similar to
those for the neutral tricoordinate analogue, trimesitylborane, but contrast with those of the Cp*2B+ cation. Quantum chemical
calculations are applied to provide additional insights into the relationship between the NMR observables and the molecular and
electronic structure of the dimesitylborinium cation.
■
INTRODUCTION
The boron sites within neutral molecules, with three
substituents and a partially full octet, can still be relatively
reactive. If one substituent is removed, then the borinium ion
(two-coordinate boron cation) adopts a positive charge, and
the reactivity increases significantly. The high reactivity of the
boron cation means it can be difficult to produce a stable
sample. This instability has made the synthesis of boron cations
a challenge for chemists for over 50 years.1−3
A variety of different types of boron cations have been
studied, including three- and four-coordinate boron cations3,4
and gas-phase boron cations.5 Chemists have succeeded in
synthesizing and studying borinium ions where the directly
bonded atoms are nitrogen, oxygen, or a transition metal;
sometimes, one of the directly bonded atoms has been a
carbon, which is part of a bulky group.6−8 However, it was not
until recently that borinium ions were synthesized and studied,
wherein both of the directly bonded atoms are carbon.9 It is
this type of borinium ion that is our focus, specifically
dimesitylborinium (Mes2B+, Figure 1).
Dimesitylborinium is different from the other borinium ions
that have previously been synthesized. Unlike the other
borinium ions, dimesitylborinium is relatively stable, yet still
extremely reactive.9,10 It is stabilized by π donation rather than
lone pair donation, which allows it to keep its high reactivity,
making it a strong Lewis acid. Because of its high reactivity, it is
extremely air- and moisture-sensitive. Dimesitylborinium
© 2015 American Chemical Society
tetrakis(pentafluorophenyl)borate (1) features a perfectly linear
CB+−C angle of 180° in the solid state.
Boron-11 NMR spectroscopy has a long history, and there is
a wealth of data available for various functional groups,
particularly in solution.11−15 Solid-state nuclear magnetic
resonance (SSNMR) can provide information on the structure
of a molecule, along with detailed information about bonding
environments. Boron has two isotopes that can be observed by
NMR, 10B (I = 3, N.A. = 19.9%, Ξ = 10.744%) and 11B (I =
3/2, N.A. = 80.1%, Ξ = 32.084%). The nuclear quadrupole
moment, Q, is moderate for both nuclei (Q(10B) = 84.59 mb;
Q(11B) = 40.59 mb).16 The preferred nucleus for NMR is 11B
because of its high natural abundance, manageable quadrupole
moment, and availability of its central transition (mI = +1/2↔
−1/2). One problem with observing either boron isotope is
that the stator in many commercial probes contains boron,
which may show as a signal in the NMR spectrum. Echo pulse
sequences are used to minimize this unwanted signal.
SSNMR spectra of 11B are largely governed by the nuclear
magnetic shielding (σ) and the quadrupolar interaction
between the electric field gradient (EFG) and Q. From these
two interactions, two second-rank tensors can be measured.
Diagonalization of the symmetric part of a tensor gives the
orientation of its principal axis system (PAS) in the molecular
Received: September 19, 2015
Published: December 1, 2015
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Inorganic Chemistry
and electronic structure and the observed NMR parameters.
Since this type of borinium ion is fairly unexplored, yet fairly
easily produced in the laboratory, work in this area will lay the
groundwork for further understanding of the electronic
structure and reactivity of related borinium cations. In this
work, the complete 11B EFG and CS tensors are measured
experimentally and interpreted. These tensors are also
calculated using cluster models in Gaussian 09,18 using periodic
boundary conditions and the gauge-including projectoraugmented wave (GIPAW) formalism in CASTEP,19 and
interpreted using natural localized molecular orbital (NLMO)
analyses with Amsterdam Density Functional (ADF) software.20
■
Sample Preparation. Dimesitylborinium tetrakis(pentafluorophenyl)borate (1) was prepared as described in ref 9.
All subsequent handling of the sample was done in a dry argon-filled
glovebox. The sample was ground into a fine powder and packed in a 4
mm outer diameter ZrO2 rotor for SSNMR experiments.
Solid-state 11B NMR. Data were acquired at the University of
Ottawa using a Bruker AVANCE III spectrometer at a magnetic field
strength of 9.4 T (ν0(11B) = 128.38 MHz) and at the National
Ultrahigh-field NMR Facility for Solids in Ottawa using a Bruker
AVANCE II spectrometer at a magnetic field strength of 21.1 T
(ν0(11B) = 288.80 MHz). Chemical shift referencing and set up were
done with powdered sodium borohydride, which has a chemical shift
of −42.06 ppm relative to boron’s primary chemical shift standard,
F3B·O(C2H5)2. All data are acquired using Bruker’s TopSpin software.
At 9.4 T, 11B data were acquired using a 4 mm HXY triple
resonance magic-angle-spinning (MAS) probe with a Hahn echo pulse
sequence and high-powered proton decoupling (π/2 − τ1 − π −
acquire). The echo suppresses the background signal, though it does
not remove it entirely. A recycle delay of 5 s was used with a 30 μs
delay between the two pulses. 15 808 scans were collected.
At 21.1 T, 11B spectra of stationary and MAS samples were acquired
using a 4 mm HBN triple resonance MAS probe with a Hahn echo
pulse sequence and proton decoupling (π/2 − τ1− π − acquire).
Again, the echo reasonably suppresses the background signal, though it
can still be seen. The spinning speeds used for subsequent MAS
experiments were 7, 8, 9, and 10 kHz. A CT-selective π/2 pulse length
of 2 μs was used. Recycle delays of 5 to 10 s were used. 512 scans were
recorded for the MAS spectra and 2048 scans were recorded for the
static spectrum.
Spectral fitting was carried out using both WSolids121 and the
Solids Line Shape Analysis Guide (SOLA) built into TopSpin 3.0.
WSolids1 was used for the spectra of stationary samples, while SOLA
was used for the MAS spectra.
Quantum Chemical Computations. Calculations of the EFG
and magnetic shielding tensor parameters were performed using
Gaussian 0918 and CASTEP (v. 4.4).19,22 Initial models for Gaussian
09 calculations were generated using standard bond lengths and angles
in Gaussview 3.0. Unless otherwise indicated, models were optimized
in Gaussian 09 using the same functional that is used for the NMR
calculation. The B3LYP or RHF methods were used for all Gaussian
09 calculations, normally with the 6-311++G(d,p) basis set unless
otherwise indicated. Models for GIPAW calculations were generated in
Materials Studio (v. 4.4.0.0) using the crystal structure of Shoji and coworkers.9 “On-the-fly” pseudopotentials were used with a 1 × 1 × 3
Monkhorst−Pack grid using two k-points. The basis set cutoff energy
was set to 500 eV. The generalized gradient approximation (GGA)
with the PBE functional was used for all CASTEP calculations. Further
GIPAW calculations with higher energy cut-offs or more k-points were
impractical with our resources due to the size of the unit cell. Relevant
EFG and magnetic shielding tensor information was parsed from the
output files using EFGShield (v. 4.2).23
Natural localized molecular orbital (NLMO) calculations were
accomplished using the ADF molecular modeling suite (v. 2012.01).
Figure 1. (a) A simple representation of the dimesitylborinium cation.
(b) Model of dimesitylborinium tetrakis(pentafluorophenyl)borate
generated from the X-ray crystal structure coordinates.9 Carbon atoms
are shown in light gray, boron in taupe, and fluorine in green.
Hydrogen atoms are omitted for clarity.
frame. In its PAS, the principal components of the shielding
tensor are ordered σ11 ≤ σ22 ≤ σ33. Similarly, the chemical shift
(CS) tensor components are ordered δ11 ≥ δ22 ≥ δ33. The
average of the three components provides the familiar isotropic
value:
δiso = (δ11 + δ22 + δ33)/3
(1)
The span (Ω) of the magnetic shielding or chemical shift
tensor is given by the following:17
Ω = σ33 − σ11 ≈ δ11 − δ33
and the skew (κ) is given by the following:
(2)
17
3(σiso − σ22)
3(δ22 − δiso)
=
(3)
Ω
Ω
Analogously, the EFG tensor can be diagonalized to give the
orientation of its PAS and its principal components. The
principal components are ordered |V33| ≥ |V22| ≥ |V11|. These
principal components may instead be expressed as a
quadrupolar coupling constant and asymmetry parameter.
The quadrupolar coupling constant (CQ) is defined by the
following:
κ=
eV33Q
(4)
h
where e is the fundamental charge, and h is Planck’s constant.
The asymmetry parameter (η) is given by the following:
CQ =
η=
V11 − V22
V33
EXPERIMENTAL SECTION
(5)
The motivation of this work is to provide further insight into
the nature of the CB+C functional group in dimesitylborinium ions, and into the connection between the local molecular
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The model comprised one dimesitylborinium cation with the atomic
coordinates from the crystal structure of Shoji and co-workers.9 The
GGA revPBE method was used, with the TZP basis set on all atoms.
■
RESULTS AND DISCUSSION
The 11B solid-state MAS NMR spectra of 1 acquired at 21.1 T
are shown in Figure 2, with spinning speeds of 7, 8, 9, and 10
Figure 2. Solid-state boron-11 MAS NMR spectra of powdered
dimesitylborinium tetrakis(pentafluorophenyl)borate at 21.1 T at MAS
rates of 7, 8, 9, and 10 kHz ((a), (c), (e), (g)). The best fit simulations
are shown in (b), (d), (f), and (h) using the parameters in Table 1.
The simulations account for the cation, the anion, and a trace impurity
of sodium borohydride. A baseline distortion, resulting in some small
discrepancies in the experimental and simulated intensities, is seen in
the region between ∼50 and −50 ppm.
Figure 3. Solid-state boron-11 stationary NMR spectra of a powdered
sample of dimesitylborinium tetrakis(pentafluorophenyl)borate at (a)
9.4 T and (d) 21.1 T. The best fit simulations are shown in (b) and
(e), using the parameters in Table 1. The asterisks denote signal from
residual 11B probe background. The simulations shown in (b) and (e)
account for the cation, the anion, and a trace impurity of sodium
borohydride. The simulations of only the cation site shown in (c) and
(f) show only the effect of the quadrupolar interaction; the span of the
chemical shift tensor is set to zero.
kHz. The 11B solid-state NMR spectra of the central transition
of a stationary sample of 1 at magnetic field strengths of 9.4 and
21.1 T are shown in Figure 3. The spectra are somewhat
complicated because of the interplay between quadrupolar and
CS tensors, and because the spinning speed is not sufficient to
remove all of the spinning sidebands. In addition, the baseline
shows some unwanted residual broad signal due to the boron
background in the probe (marked by an asterisk in Figure 3).
All spectra were iteratively simulated to find a unique set of
EFG and CS tensor parameters which fit all data simultaneously. These parameters are reported in Table 1. In addition
to the dominant signal from the dimesitylborinium cation,
signals from the relatively symmetric tetrakis(pentafluorophenyl)borate anion and a trace amount of sodium
borohydride at −42 ppm are also taken into account in the
simulations. BX4− anions typically give rise to sharp peaks in
11
B SSNMR spectra due to their relatively high symmetry and/
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Table 1. Experimental 11B Quadrupolar and Chemical Shift Tensor Parameters for Dimesitylborinium
Tetrakis(pentafluorophenyl)borate and Selected Other Boron Compoundsa
|CQ( B)|/MHz
ηQ
δiso/ppm
Ω/ppm
κ
β/dege
11
dimesitylboriniumb
trimesitylboranec
[Cp*2B]+d
Cp2*BMed
5.44(0.08)
0.02(0.02)
91.2(0.2)
130(1)
0.90(0.10)
0 (5)
4.75(0.01)
0.0
77.4(0.5)
121(1)
1.0
0
1.14
0.10(4)
−41.3(1)
73.0(3)
0.98(2)
0
4.52(2)
0.11(1)
81.9(1)
146.0(3)
0.75(4)
0
a
See the SI for the same chemical shift tensor information expressed according to the Haeberlen convention. bData are for the cation of
dimesitylborinium tetrakis(pentafluorophenyl)borate. Parameters used to fit the spectral features due to the anion are as follows: CQ < 0.70 MHz; ηQ
< 0.04; δiso = −17.2 ppm; Ω < 70 ppm. cFrom Bryce et al.29 dFrom Schurko et al.28 eα and γ are omitted because, for dimesitylborinium, they were
determined to not affect the line shape due to the axial symmetry of the tensors.
Table 2. Calculated 11B Quadrupolar and Magnetic Shielding Tensor Parameters for Compound 1
11
CQ( B)/MHz
ηQ
σiso/ppm
Ω/ppm
κ
β/deg
RHF/6-311+Ga
RHF/6-311G++(d,p)a
B3LYP/6-311+Ga
B3LYP/6-311G++(d,p)a
GIPAWb
−6.11
0.02
31.2
154
0.98
0
−6.50
0.03
27.0
155
0.97
0
−6.06
0.01
31.8
149
1.00
0
−5.91
0.00
12.4
127
1.00
0
−6.55
0.02
0.1
129
0.94
0
a
Calculations carried out using Gaussian 09 on a single dimesitylborinium cation. See Experimental section for details. bCalculations carried out on
the whole crystal structure with periodic boundary conditions using CASTEP. See Experimental section for details.
or due to rapid reorientation.24 Dipolar couplings to 19F,
particularly in the case of the stationary samples, result in
somewhat broadened spectral discontinuities. The isotropic
chemical shift, δiso(11B), of the cation is 91.2 ppm, showing a
small difference relative to the value measured in odichlorobenzene solution, 93.45 ppm.9 A shift of similar
magnitude is noted for the anion (−17.2 ppm in the solid vs
−16.49 ppm in solution).9 The value of 91.2 ppm for the cation
is high when compared to the data available for many other
small molecules,11 although higher values can be found. In
particular, boron-containing cluster compounds often exhibit
boron chemical shifts in the range from 100 to over 200
ppm.25,26
The CQ(11B) measured for the dimesitylborinium cation,
5.44 ± 0.08 MHz, approaches the largest such measured value,
to our knowledge, 5.58 ± 0.02 MHz for the 1b site in boron
carbide.27 The largest values that have been reported for
molecular solids are typically less than 5 MHz. There is at least
one reported 11B span that is slightly larger in bis(pentamethylcyclopentadienyl)methylborane, with a span of
146.0 ppm.28 It is worth noting that the dimesitylborinium
cation and trimesitylborane29 have similar NMR parameters, as
shown in Table 1. The values for the quadrupolar coupling
constant, isotropic chemical shift, and span are all slightly larger
in the dimesitylborinium cation. While the skew of the boron
CS tensor in 1 (0.90 ± 0.10) is axially symmetric within
experimental error, a small deviation from perfect axial
symmetry in this tensor would be consistent with the lack of
perfect axial symmetry in the crystal structure as one considers
additional atoms and ions beyond the CB+C axis.
The 11B EFG and σ tensors from ab initio and DFT
calculations carried out on an isolated dimesitylborinium cation
are shown in Table 2. The method showing the best agreement
with the experimental quadrupolar coupling constant and CS
tensor span is B3LYP/6-311++G(d,p). The asymmetry
parameter, skew, and angle β between the unique components
of the EFG and CS tensors are all within experimental error.
The span is just outside of the experimental error range and the
quadrupolar coupling constant is experimentally lower.
To assess the impact of including the tetrakis(pentafluorophenyl)borate anion in the computational model,
the 11B EFG and σ tensor results for the dimesitylborinium
boron site using the full unit cell of the crystal structure of 1,
and periodic boundary conditions, are shown in Table 2. These
GIPAW calculations suggest that while the quadrupolar
coupling constant may be somewhat influenced by the nature
and the location of the counteranion, the boron chemical shift
tensor reports much more locally on the nature of the borinium
functional group. Note, for example, the excellent correspondence between the calculated CS tensor spans for the B3LYP/6311++G(d,p) calculation (127 ppm), the GIPAW DFT
calculation (129 ppm), and the experimental value (130(1)
ppm).
A natural localized molecular orbital (NLMO) calculation
shows that the two boron−carbon bonding orbitals make the
largest contributions to the electric field gradient tensor and
thus to the quadrupolar coupling constant (Figure 4). The
largest component of the EFG tensor, V33, is aligned along the
CB+C axis. Similarly, the largest component of the
magnetic shielding tensor, σ33, (and smallest component of
the chemical shift tensor, δ33) is oriented along the CB+C
axis (Figure 4). The two other components, σ11 and σ22, are
affected by paramagnetic contributions which may be
rationalized29,30 with the aid of the molecular orbital diagram
presented in ref 9. The HOMO−2 and HOMO−3 orbitals
feature strong contributions from π-type bonds between the
mesityl groups and boron. Virtual 90° rotation of these orbitals
about each of two orthogonal axes which lie perpendicular to
the C−B+−C axis results in favorable overlap with one of the
two orthogonal vacant 2p orbitals of boron (LUMO and
LUMO+1). This virtual mixing of orbitals related by the
application of an angular momentum operator causes para11892
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similarities and differences in the 11B EFG and CS tensors
across the three compounds. First, the isotropic shift increases
from −41.3 ppm for Cp*2B+ to 77.4 ppm for trimesitylborane
to 91.2 ppm for 1. These average values can be properly
understood by considering the values of the three principal
components of the CS tensors, which are depicted graphically
in Figure 5, as well as the CS tensor orientations in the
molecular frame of reference. All three compounds show axially
symmetric CS tensors (κ ≈ 1). Inspection of the tensor
components shows a strong similarity between 1 and
trimesitylborane, and rather large differences between 1 and
Cp*2B+, for each principal component. Conversely, the
orientation of the CS tensor for 1 is analogous to that seen
for Cp*2B+, i.e., δ33 lies along the CB+C direction, whereas
in trimesitylborane this most shielded component lies
perpendicular to the three boron−carbon bonds. In this
sense, the CS tensor for 1 is unique and diagnostic of the
linear CB+C borinium functional group, but both the
tensor magnitude and orientation must be considered. Such
information and differentiation is not necessarily as readily
available from an inspection only of the isotropic chemical shift.
For example, δiso for neutral tricoordinate Cp*2BMe,28 at 81.9
ppm, is not far from the 91.2 ppm observed for 1 (Table 1).
Consideration of CS tensor components and isotropic chemical
shifts known for other boron functional groups, such as boronic
acids and boronate esters, also show a clear differentiation with
respect to the borinium ion (Figure 5).
The boron CS tensor spans shown in Table 1 include, to our
knowledge, the largest such values measured in solids, i.e., 121
ppm for trimesitylborane, 130 ppm for 1, and 146 ppm for
Cp*2BMe. It is perhaps somewhat surprising that the linear
geometry at boron in 1 is not accompanied by a larger Ω(11B).
For example, it is known from gas-phase microwave
spectroscopic experiments on linear molecules such as FBO,
Figure 4. Orientations of the unique components of the boron electric
field gradient (V33) and magnetic shielding (σ33) tensors for the
dimesitylborinium cation (they are collinear along the direction of the
CBC bonds). Both tensors are axially symmetric, and the
remaining principal components (V11, V22 and σ11, σ22) lie in the plane
perpendicular to the CB+C bond. The natural localized molecular
orbital shown is one of two symmetry-equivalent BC bonding
orbitals which determine the magnitude and direction of the
quadrupolar coupling tensor.
magnetic deshielding along each of the two orthogonal axes
which lie perpendicular to the CB+C axis, resulting in the
large values observed experimentally for δ11 and δ22, and thus
the large CS tensor span. This relationship between electronic
structure and the CS tensor components could be particularly
valuable in characterizing novel borinium cations in the future.
It is interesting to further compare the EFG and CS tensor
data obtained for 1 to the analogous neutral tricoordinate
compound, trimesitylborane, as well as to [Cp*2B+][AlCl4−].
These two compounds are selected for comparison with 1 in
part because they all feature boron atoms directly bonded to
carbon ligands. The latter compound features a cationic boron
interacting in a η1 fashion with one of the Cp* rings and
interacting in an η5 fashion with the second Cp* ring. Despite
the quite different nature of the bonding to boron, the nearlinear geometry at boron makes for an interesting comparison
to the data obtained for 1. As seen in Table 1, there are
Figure 5. (a) Representation of the magnitude of the 11B quadrupolar coupling constant for a range of boron functional groups. The red parts of the
bars represent the typical range of values which have been observed for a given functional group. (b) Representation of the principal components of
the boron chemical shift tensor (δ11 in green, δ22 in red, and δ33 in blue) for various boron functional groups including boronic acids, boronate esters,
trimesitylborane, [Cp*2B+][AlCl4−], Cp*2BMe, and 1. Error bars are not shown but typically range from 1 to 10 ppm. Data are from refs 29−36.
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Figure 6. Calculated (B3LYP/6-311++G(d,p)) dependence of the principal components of the 11B EFG and magnetic shielding tensors on the C
B+C angle (θ) in the diphenylborinium cation. The CBCC dihedral angle was fixed at 90°. Fitted trend lines are given as follows: V33 = (−1.53 ×
10−5)(θ2) + (5.50 × 10−3)(θ) + 1.43 × 10−1, R2 = 1.00; V22 = (−3.43 × 10−6)(θ2) + (1.22 × 10−3)(θ) − 4.28 × 10−1, R2 = 1.00; V11 = (1.87 ×
10−5)(θ2) − (6.72 × 10−3)(θ) + 2.85 × 10−1, R2 = 1.00; σ33 = (−5.22 × 10−3)(θ2) + (1.82 × 100)(θ) − 6.18 × 101, R2 = 0.991; σ22 = (−6.07 ×
10−3)(θ2) + (2.21 × 100)(θ) − 2.28 × 102, R2 = 0.999; σ11 = (−1.01 × 10−2)(θ2) + (3.67 × 100)(θ) − 3.62 × 102, R2 = 0.999.
and the corresponding quadrupolar parameters provide a
unique diagnostic of the borinium functional group.
The results of calculations shown in Figure 6 demonstrate
how sensitive the boron EFG and magnetic shielding tensors
are to the CB+C angle in a model diphenylborinium
cation (see SI). Over the range from 180◦ to 160◦, the
calculated quadrupolar coupling constant increases slightly
from 6.04 to 6.10 MHz and the asymmetry parameter increases
from zero to only 0.013. The isotropic shielding constant
decreases by about 3 ppm over the same range, while the span
increases by about 4 ppm. These results suggest that the NMR
parameters should be moderately sensitive to deviations from
perfect linearity at boron. A more pronounced change in the
NMR parameters, however, is obtained by varying the nature of
the aryl or alkyl groups bonded to boron (see SI). A series of
calculations for R2B+ cations where R = H, methyl, phenyl, and
mesityl shows that quadrupolar coupling constants of up to
8.71 MHz are predicted for R = H; similarly, the span when R
is an alkyl group (e.g., Ω = 339 ppm when R = Me) is
ClBO, FBS, and BF, that boron CS tensor spans can be up to
200 to 220 ppm.31,32
The boron quadrupolar coupling data are also informative.
Again, considering the first three compounds in Table 1, it is
seen that all have axially symmetry EFG tensors at boron (ηQ ≈
0). We again expect to be able to differentiate between the
functional groups in these three compounds by inspecting the
magnitude and orientation of the tensor components. The
CQ(11B) values indeed differ clearly between the compounds,
with Cp*2B+ at 1.14 MHz, trimesitylborane at 4.75 MHz, and 1
at 5.44 MHz. The orientations of the EFG tensors are
analogous to those described above for the CS tensors: in the
case of Cp*2B+ and 1, the largest and unique component (V33)
lies along the BC bond, while for trimesitylborane V33 lies
perpendicular to the plane containing the boron−carbon
bonds. These orientations are determined by the molecular
symmetry and the traceless nature of the EFG tensor.
Nevertheless, as was seen with the CS tensors, the EFG tensor
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Inorganic Chemistry
a consortium of Canadian Universities, supported by the
National Research Council Canada and Bruker BioSpin, and
managed by the University of Ottawa (http://nmr900.ca).
calculated to be substantially larger than when it is an aryl
group, presumably due to reduced π-donation in the former.
■
■
CONCLUSIONS
The present work has provided precise experimental 11B
quadrupolar and chemical shift tensor magnitudes and
orientations for a linear two-coordinate dimesitylborinium
cation by fitting six spectral data sets recorded at two different
applied magnetic field strengths. The 11B quadrupolar coupling
constant of 5.44 MHz is one of the largest known for any boron
compound. NLMO analysis shows that the large electric field
gradient at boron arises largely due to the two boron−carbon
bonds. The boron chemical shift tensor span of 130 ppm is also
large when compared to literature data for all boron functional
groups. The NMR data observed are similar to those for the
neutral tricoordinate analogue, trimesitylborane, but contrast
strongly with those of the η1,η5-Cp*2B+ cation. Quantum
chemical calculations have provided insight into the relationship between the observed NMR parameters and the local
molecular and electronic structure of the borinium cation.
Mixing of the low-lying vacant 2p orbitals centered on boron
with occupied orbitals of BC bonding character contributes
to the large CS tensor span. Further calculations show the
sensitivity of the NMR parameters to the nature of the aryl or
alkyl groups bonded to boron in the borinium ion, as well as to
the degree of linearity at boron. As the chemistry of boron
continues to expand in terms of fundamental understanding
and diverse applications, this study demonstrates the value of
the characterization of the complete boron EFG and CS tensors
via solid-state NMR spectroscopy. Such data are particularly
valuable, both for fundamental understanding and in a
predictive sense, when comparing a series of related boron
compounds, when contrasting different functional groups, and
when examining compounds for which crystallographic data are
unavailable.
■
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02161.
Results of additional calculations of NMR parameters
(PDF)
■
REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Phone: 1-613-562-5800 ext. 2018. Fax: 1-613-562-5170. Email: [email protected] (D.L.B.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
A.E.A. and D.L.B. thank the Natural Sciences and Engineering
Research Council (NSERC) for a scholarship and funding,
respectively. Y.S. and T.F. acknowledge KAKENHI 26708004
and 26102001 for funding. We acknowledge Dr. Glenn Facey,
Dr. Victor Terskikh, and Mr. Scott Southern for technical
assistance, and the High Performance Computing Virtual
Laboratory (HPCVL) on which the NLMO calculations were
performed. Access to the 21.1 T NMR spectrometer was
provided by the National Ultrahigh-Field NMR Facility for
Solids (Ottawa, Canada), a national research facility funded by
11895
DOI: 10.1021/acs.inorgchem.5b02161
Inorg. Chem. 2015, 54, 11889−11896
Article
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