Methylene Bridging Effect on the Structures, Lewis Acidities and Optical Properties of Semi-planar Triarylboranes

Article abstract of DOI:10.1002/chem.202003319

Three synthetic methods towards semi-planar triarylboranes with two aryl rings connected by a methylene bridge have been developed. The fine-tuning of their stereoelectronic properties and Lewis acidities was achieved by introducing fluorine, methyl, methoxy, n-butyl and phenyl groups either at their exocyclic or bridged aryl rings. X-ray diffraction analysis and quantum-chemical calculations provided quantitative information on the structural distortion experienced by the near planar hydro-boraanthracene skeleton during the association with Lewis bases such as NH₃ and F^?. Though the methylene bridge between the ortho-positions of two aryl rings of triarylboranes decreased the Gibbs free energies of complexation with small Lewis bases by less than 5 kJ mol^?1 relative to the classical Lewis acid BAr₃, the steric shielding of the CH₂ bridge is sufficient to avoid the formation of Lewis adducts with larger Lewis bases such as triarylphosphines. A newly synthesized spirocyclic amino-borane with a long intramolecular B?N bond that could be dissociated under thermal process, UV-irradiation, or acidic conditions might be a potential candidate in Lewis pairs catalysis.

Full text of DOI:10.1002/chem.202003319

Accepted Article
Accepted Article
Title: Methylene Bridging Effect on the Structures, Lewis Acidities and
Optical Properties of Semi-Planar Triarylboranes
Authors: Thu-Hong Doan, Aurélien Chardon, Arnaud Osi, Damien
Mahaut, Nikolay Tumanov, Johan Wouters, Benoît
Champagne, and Guillaume Berionni
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To be cited as: Chem. Eur. J. 10.1002/chem.202003319
Link to VoR: https://doi.org/10.1002/chem.202003319
01/2020

10.1002/chem.202003319
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Methylene Bridging Effect on the Structures, Lewis Acidities and
Optical Properties of Semi-Planar Triarylboranes
Thu-Hong Doan, Aurélien Chardon, Arnaud Osi, Damien Mahaut, Nikolay Tumanov, Johan Wouters,
Benoît Champagne and Guillaume Berionni*
Dedicated to Professor Suzanne A. Blum
Dr. T.-H. Doan, Dr. A. Chardon, A. Osi, D. Mahaut, Dr. N. Tumanov, Prof. J. Wouters, Prof. B. Champagne and Prof. G. Berionni
Department: Department of Chemistry, Namur Institute of Structured Matter
Institution: University of Namur
Address: 61, rue de Bruxelles, 5000 Namur, Belgium
E-mail: guillaume.berionni@unamur.be
Supporting information for this article is given via a link at the end of the document.
Abstract: Three synthetic methods towards semi-planar
triarylboranes with two aryl rings connected by a methylene
bridge have been developed. The fine-tuning of their
stereoelectronic properties and Lewis acidities was achieved by
introducing fluorine, methyl, methoxy, nbutyl and phenyl groups
either at their exocyclic or bridged aryl rings. X-ray diffraction
analysis
and
quantum-chemical
calculations
provided
quantitative information on the structural distortion experienced
by the near planar hydro-boraanthracene skeleton during the
association with Lewis bases such as NH3 and F–. Though the
methylene bridge between the ortho-positions of two aryl rings of
triarylboranes decreased the Gibbs free energies of
complexation with small Lewis bases by less than 5 kJ mol–1
relative to the classical Lewis acid BAr3, the steric shielding of
the CH2 bridge is sufficient to avoid the formation of Lewis
adducts with larger Lewis bases such as triarylphosphines. A
Scheme 1. From propeller-shaped to fully planar triarylboranes.
In contrast to the well-established synthetic methods to produce
common triarylboranes Ar3B,[6] only few synthetic routes toward
semi-bridged triarylboranes are known (Scheme 2).
Previous methods to produce semi-planar triarylboranes were
mostly based on the introduction of the boron atom via a
transmetallation of an organo-stannane or -silane in the
presence of BCl3 or BBr3 (Scheme 2a).[5a-d] Another approach is
newly synthesized spirocyclic amino-borane with
a long
intramolecular B-N bond that could be dissociated under thermal
process, UV-irradiation, or acidic conditions might be a potential
candidate in Lewis pairs catalysis.
based on
a
cyclization between
a
pre-functionalized
arylboronate FG-Ar-B(OR)2 and a bis-organolithium intermediate
which tolerates only a limited number of functionals groups
(FG).[5e-g]
Introduction
We now describe three synthetic methods towards
functionalized semi-planar triarylboranes (Scheme 2c) and
report the evaluation of their Lewis acidity, stereoelectronic and
photophysical properties.
Owing to their unique chemical, physical and photophysical
properties, trivalent organoboron compounds have become a
mainstream interest in chemistry.[1] In particular, triarylboranes
represent an invaluable class of Lewis acids, as demonstrated
by their numerous applications in materials chemistry[2] and by
their ability to catalyze a wide variety of transformations.[3]
Whereas typical triarylboranes possess a planar trigonal
propeller-like structure (Scheme 1a), an enforced planarization
of their aryl rings with three covalent linkers results in completely
planar triarylboranes (Scheme 1c). These fully planar boron
Lewis acids exhibit high chemical stability, low Lewis acidity, and
are air, moisture, and chromatography compatible. They have
recently found widespread applications in anions sensing,
optoelectronic devices and boron doped -conjugated
materials.[4]
In contrast, the structures, reactivities and stereoelectronic
properties of highly functionalized semi-planar triarylboranes
with a single methylene bridge between two aryl rings have been
less explored (Scheme 1b).[5]
Scheme 2. Reported synthesis and our approach to the semi-cyclic boranes
with different substituents on hydro-boraanthracene scaffold and different aryl
rings attached to boron atom.
1
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Experimental investigations combined with quantum chemical
calculations of their associations with Lewis bases of various
sizes and natures (halides, amines, phosphines) shed light on
their steric properties and Lewis acidities, providing insights on
their potential uses in chemical catalysis (frustrated Lewis pairs)
and materials sciences (anion and Lewis base sensors).
The formation of the ammonia protected triarylboranes 3a-g via
addition of various organomagnesium on the 9-aminoethoxy-
9,10-dihydro-boraanthracene 2 was next investigated (Scheme
4). Our initial experimental conditions were based on an
example reported by Shaver on a similar Ar2B(ethanolamine)
derivative.[7b,7g]
Despite extensive reaction condition optimizations, the ammonia
protected boranes 3a-g were obtained with isolated yields in the
27-82% range. Increasing the excess of the organomagnesium
reagents ArMgX up to 8 equivalents did not improve the yields
further and the use of aryllithium reagents only produced the 3 in
traces amounts. Despite of the moderate yields, the method was
quite general and several ortho-substituents were tolerated (3b,
3d-g), even the strongly electron deficient pentafluorophenyl-
magnesium produced the pentafluorinated borane 3g in 48%
yield. As expected, di-ortho-trifluoromethyl or di-ortho-chloride
substituents were not tolerated because of the excessive steric
hindrance (3h and 3i).
Results and Discussion
In order to avoid air and moisture sensitive halogeno-borane
intermediates, we envisaged a practical access to semi-planar
arylboranes via a water tolerant precursor 2 (Scheme 3). To the
best of our knowledge only few boranes were synthesized by
this approach,[5b,7] but a methodological investigation of the
scope of this synthetic approach was not reported so far.
The spirocyclic quaternary ethanolamino complex 2 was
prepared by a one-pot borylative cyclization reaction starting
from the bis(2-bromophenyl)-methane 1a (Scheme 3). The
formation of the bis-organomagnesium reagent derived from 1a
under Barbier conditions in the presence a trivalent boron
electrophile B(OnBu)3, followed by acidic treatment and addition
of ethanolamine, produced 2 in 75% yield. With a shorter
reaction time (16 h), an acceptable yield of 60% of 2 could be
obtained (see the ESI). Despite of the purification of 2 by
crystallization, some ethanolamine traces often remained in the
final product, this can be avoided by reducing the amount of
ethanolamine, however the yield was found to be lower (see the
SI for details).
Scheme 3. Synthesis of the ethanolamine protected borinic acid 2.
The X-ray diffraction analysis on a single-crystal of 2 showed a
bent hydro-boraanthracene scaffold with an interplanar angle of
141.1° between two aryl rings (Figure 1). The value for the other
asymmetric unit can be found in the ESI. The oxygen atom is
located at the concave face of this tricyclic scaffold with the
shorter B-O bond length (1.468(3) Å) compared to that in the
non-bridged Ph2B(ethanolamine) adduct ((1.476(2) Å)[8a] and
(1.484(3) Å)[8b]). The B-N bond length of 1.657(3) Å is similar to
the bond lengths of 1.655(2) Å and 1.653(3) Å in the analogous
non cyclic derivative Ph2B(ethanolamine).[8]
Scheme 4. Scope and limitation of the synthesis of ammonia-protected semi
cyclic triarylboranes 3a-i. a) The addition of the Grignard reagent was
performed at 0 °C instead of – 94 °C.
The interplanar angles between the mean planes of the two aryl
rings of the bent hydro-boraanthracene skeleton in the
triarylborane ammonia-complexes 3a, 3d and 3f are 128.1°,
140.6° and 150.2°, respectively (Figure 2a). The smaller angle of
3a compared to that of the spiro compound 2 (141.1°) is linked
to the higher boron pyramidalization in 3a (92%)[9] than in 2
(73%). The B-N bond lengths in 3a, 3d and 3f are of 1.619(3),
1.629(5) and 1.625(2) Å, respectively (Figure 2b), slightly
smaller than that of the Ph3B-NH3 Lewis adduct (1.639(2) Å).[10]
Figure 1. Molecular structure of compound 2 (one of the two in asymmetric
unit, for the other unit, see the SI) with the selected interplanar angle (left) and
B-N and B-O bond lengths (right). Thermal ellipsoids are set at 50% probability
level with the omitted hydrogens for clarity.
2
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Removal of the coordinated ammonia from
3 was then
performed in ethereal HCl or HBF4 solutions, [7b] but we noticed
partial degradation of the desired boranes by 11B NMR
measurements. In contrast, the methylation of NH3 moeity with
–
the Meerwein salt Me3O+BF4 in CH2Cl2 proceeded smoothly
(Scheme 6), and filtration of the methyl-ammonium chloride salt
provided the analytically pure triarylboranes 4b-d as white
crystalline solids in 66-71% isolated yields after solvent removal.
Scheme 6. Deprotection of ammonia-borane complexes 3 and formation of
the free triarylboranes 4b-d. (a) Due to the highly sensitive nature of 4b, some
impurities have been observed in the 1H and 13C NMR spectra (see the ESI for
details).
Figure 2. Molecular structures of compound 3a, 3d and 3f with: a) the
selected dihedral angles, b) the B-N bond lengths. Thermal ellipsoids are set
at 50% probability level with the omitted hydrogens and minor components for
clarity; c) the intermolecular N-H…F interactions of 3f.
The exocyclic aryl rings in 3a and 3d are located in the convex
face of the hydro-boraanthracene skeleton in a skewed fashion
for maximizing van der Waals attractions with the peri-
hydrogens of the hydro-boraanthracene motif. However, the aryl
ring in 3f is in a nearly perpendicular orientation (Figure 2a).
Interestingly, the short intermolecular N-H…F interactions of
2.336(8) Å in 3f (Figure 2c), which cannot be found in 3d,
reduce notably the exocyclic aryl ring twisting. In details, the C-
C-B-N torsion angles of the ortho-fluorinated boranes 3d and 3f
are 51.7° and 4.7°, respectively, which are significantly smaller
than that of the non-fluorinated analogue 3a (90.3°).
The same strategy was attempted to introduce the bulky mesityl
group in the ammonia protected triarylborane system. However,
no desired compound with the coordination between B atom and
NH3 was formed. In this case, the unprotected sterically
hindered triarylborane 4a was directly obtained, though in low
yield together with the dimerization side product 5 (Scheme
5),[11] showing the limitations of this method with strongly
hindered aryl Grignard reagents. The dimer 5 is presumably
formed by oxidative homocoupling of two organomagnesium
species or via a single-electron transfert between 4a and
MesMgBr.[12]
Although four semi-planar triarylboranes 4a-d have been
prepared, the long synthetic route and the limitations of the
method for synthesizing the ammonia-protected triarylboranes
with bulky aryl rings attached to the boron atom, are the
disadvantages of the methodology.
In order to overcome these drawbacks, a straightforward
method for introducing a mesityl group attached to boron atom
has been developed (Scheme 7) based on the method of
Kawashima (Scheme 2b). The synthesis pathway starts with the
Br/Li exchange reaction of bis-(2-bromophenyl)-methane 1a with
n-BuLi, followed by the addition of mesityl(dimethoxy)borate
MesB(OMe)2 that was prepared from 2-bromomesitylene and
trimethylborate, and was used directly without further purification
(see the SI).
The desired compound 4a was obtained in a yield of 65% and
the side product 6a with a n-butyl chain has been formed and its
structure was confirmed by X-ray diffraction analysis (see the SI).
Although the side product 6a was formed, it was possible to
perform the purification of the target compound 4a by flash
chromatography and also to isolate 6a with a yield of 8%
The same method was applied to synthesize the new spirocyclic
amino hydro-boraanthracene 8 (Scheme 7). This compound was
purified by crystallization from CH2Cl2/n-pentane at -20 °C due to
its decomposition on SiO2 and Al2O3.
Scheme 5. Synthesis of the B-substituted mesityl semi-planarized borane 4a
and formation of the dimerization side-product 5.
3
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Figure 3. Structure of compound 8 with the selected dihedral angle (left) and
B-N bond length (right). Thermal ellipsoids are set at 50% probability level with
the omitted hydrogens for clarity.
A third method was then developed to obtain mesityl-hydro-
boraanthracenes by performing the successive reaction of two
Grignard reagents on a boron electrophile in a one-pot fashion
(Scheme 9). The bis-organomagnesium reagent produced by
the reaction between 1a and Mg0 was reacted with
trimethylborate, followed by the addition of MesMgBr to give 4a
with the yield of 20%.
Scheme 7. Synthesis of the mesityl (4a) and the amino spirocyclic
boraanthracene (8) derivatives by using organolithium reagent.
The formation of 6a should results from the deprotonation of 4a
by an unreacted amount of n-BuLi followed by a nucleophilic
substitution at the n-butylbromide released in the reaction during
the Br/Li exchange on 1a.
This was confirmed by the
independent synthesis of the semi-planar derivative 6a with a
yield of 96% (traces amount of 6b have been detected) by
deprotonation of 4a and addition of n-butylbromide. This result
shows that the functionalization at C9 position of mesityl-
hydroantracenylboranes can open an access to new semi-planar
triarylboranes 6a functionalized at the bridge (Scheme 8).
Scheme 8. Synthesis of functionalised semi-planar triarylboranes 6a and 6b.
The structure of 8 with the intramolecular B-N coordination was
confirmed by X-ray diffraction analysis (Figure 3). The
interplanar angle between two aryl rings of hydro-
boraanthracene moiety is 146.3°. The B-N bond length of
1.712(2) Å is consistent with a B-N coordination and is
comparable to that of the similar systems containing the
borafluorene scaffold instead of the boraanthracene skeleton
(1.712(2) Å[13] and 1.673(1) Å[14]). In comparison to the
ethanolamine protected borinic acid 2 shown in Figure 1 with B-
N bond length of 1.657(3) Å, the B-N bond in compound 8 is
longer. In addition, the bond length between B atom and the
adjacent C atom of the five-membered ring is much longer than
the B-O bond in the compound 2 (1.624(2) Å compared to
1.468(3) Å). These data reveal that the five-membered ring in
the spirocyclic amino hydro-boraanthracene 8 is less stable than
the five-membered ethanolamine ring in compound 2 and might
be opened under thermal process, UV-irradiation, or acidic
conditions.[15] By fine-tuning the electronic and structural
properties of the boraanthracene scaffold and the Lewis base,
this benchmark structure should find some applications in
frustrated Lewis pair chemistry or bifunctional metal free
catalysis.
Scheme 9. One-pot method to reach mesityl-hydro-boraanthracenes 4a-h by
successive addition of two Grignard reagents.
Based on this method, the introduction of different substituents
like methyl or fluoride in one side of the hydro-boraanthracene
skeleton has been applied successfully to give 4e and 4f in the
yields of 37% and 15%, respectively. The insertion of two
methoxy groups was also possible, however, the pure
4
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compound 4g was not obtained due to the same Rf with the
formed bis-(2-bromo-4-methoxyphenyl)methane side product.
In addition, this method also worked well with the starting
material 1e containing a phenyl group at the benzylic position to
produce 4h in the yield of 25%, and analogue of 6a.
bearing three and five fluorides on the aryl rings show the most
twisted angles of 67.1° and 66.8°, respectively.
The X-ray diffraction crystallographic structures of compounds
4a, 4c-f and 4h are shown in Figure 4. The structure of 4h was
also previously reported in the literature.[16] Considering 4a, 4e,
4f and 4h having the mesityl ring attached to the B atom, the
similar bond lengths in the range of 1.543 – 1.546 Å between B
atom and the adjacent C atoms of the hydro-boraanthracene
skeletons as well as the similar distances in the range of 1.574 –
1.577 Å between B atom to the mesityl moiety show the minor
effects of substituents on B-C bonds length. The distances from
B atom to the mesityl ring of 4a, 4e, 4f and 4h are similar to the
same distance of 1.575(3) Å in 9-phenyl-hydro-boraanthracene
4b.[17] The longer distances in the cases of 4c and 4d compared
to 4a (1.585(2) and 1.597(3) Å vs 1.577(2) Å, respectively)
exhibit the effect of F atom on the structures. The more the
number of fluorine atoms, the longer B-C distances have been
obtained.
Figure 5. Molecular structures of compounds 4a, 4c-f and 4h with the
selected dihedral angles. Thermal ellipsoids are set at 50% probability level
with omitted hydrogens and minor component of disorder in 4f omitted for
clarity.
This difference in terms of rotation angles in the case of 4c and
4d are potentially due to H…F intermolecular interactions
between two nearby molecules with distances of 2.449(0) Å and
2.524(0) Å, respectively (Figures 6a and 6c). Interestingly, face-
to-face stacking between the parallel hydro-boraanthracene
units exists in the structure of 4d in the solid state with and
interplanar distance of 3.637(0) Å (Figure 6b).
Figure 4. Structures of compounds 4a, 4c-f and 4h with the selected B-C
bond lengths. Thermal ellipsoids are set at 50% probability level with omitted
hydrogens and minor component of disorder in 4f omitted for clarity. Selected
bond lengths L1 and L2 in Å for 4a: 1.546(2) and 1.544(2); 4c: 1.537(3) and
1.536(3); 4d: 1.525(3) and 1.533(3); 4e: 1.543(3) and 1.550(3); 4f: 1.543(0)
and 1.543(0); and 4h: 1.544(2) and 1.545(2), respectively.
Figure 6. a) The intermolecular H…F interactions in the structure of 4c; b) the
interplanar distance between two parallel hydro-boraanthracene units of 4d; c)
the intermolecular H…F interactions and the face-to-face stacking between the
hydro-hydro-boraanthracene units in the structure of 4d. Thermal ellipsoids
are set at 50% probability.
The hydro-boraanthracene skeleton in 4c-f and 4h are almost
planar as shown by the angles between two aryl rings of 173.9°,
175.9°, 178.8°, 176.6° and 175.9°, respectively (Figure 5).
Compound 4a is the most bent hydro-boraanthracene structure
with the corresponding angle of 160.7°.
The mesityl moieties in 4a, 4e and 4h are almost
perpendicular to the hydro-boraanthracene scaffold with the
corresponding angles of 87.8°, 88.2° and 84.9°, respectively.
The presence of fluoride leads to the rotation of aryl rings
attached to the B atom. In details, 4f containing one fluoride
substituent has the rotated angle of 83.5°, while 4c and 4d
The UV/Vis absorption and photoluminescence (PL) spectra of
4a, 4c, 4e, 4f and 4h in CH2Cl2 are presented in Figure 7. Their
spectroscopic data are listed in Table 2.
The bridged triarylborane 4c containing three F atoms displays
the most pronounced red-shifted absorption and emission
spectra with Abs, max = 324 and 289 nm, and PL, max = 424 nm.
5
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The slightly bathochromic shifted spectra of 4c compared to that
of 4a (with Abs, max = 315 and 282 nm and PL, max = 405 nm)
show the insignificant effect of different groups on exo-aryl ring
on photophysical behaviors. Similar minor effect was obtained
when changing different substituents (Me and F) in the cases of
4e and 4f, or adding phenyl group at the benzylic position in 4h.
The appearance of absorbance bands at longer wavelength for
these five compounds compared to that of Ph3B (Abs, max = 287
nm in methylcyclohexane/isopentane)[18] underlines the role of
methylene bridge in the extent of -delocalization by imposing
the planarization of the system and enhancing the conjugation of
the aryl rings through the boron atom.
cm-1 of the Stokes shift compared to 4a, whereas in the case of
4f, fluoride as the weak electron withdrawing group at the same
position makes a minor difference of 40 cm-1. The highest value
of 7440 cm-1 associated to 4h proves the role of phenyl group at
the position 10 of the 9-hydro-boraanthracene skeleton in
increasing intramolecular charge transfer in the system.
Surprisingly, these Stokes shifts are comparable to the value of
6680 cm-1 of the fully planarized triarylboranes measured in THF
and reported by Yamaguchi, and nearly double compared to the
value of 3380 cm-1 of Mes3B with a typical propeller-like
structure.[19]
We next determined the Lewis acidities of selected semi-
planar triarylboranes 4 with respect to various Lewis bases by
calculating their fluoride anion affinities (FIAs) and NH3 affinities,
which are well-established Lewis acidity scales for comparing
extensive sets of Lewis acids, due to the small size of these
Lewis bases which result in negligible influence of sterics on the
acidity measurements.[20] The F– and NH3 affinities computed at
the M06-2X/6-311G(d) level of theory (Scheme 10) showed that
the introduction of a methylene bridge has a negligible influence
on the Lewis acidity of the boron atom since the F– or NH3
affinities of 4b are only 4 kJ mol–1 smaller than those of
triphenylborane 9a. Furthermore, the evaluation of the global
and local electrophilicity indexes also indicated a slight decrease
of acidity upon addition of a methylene bridge.[21] Increasing the
number of fluorine atoms on the exocyclic aryl ring expectedly
increased the Lewis acidity of the boron atom, as illustrated by
an increase of FIA amplitude of almost 40 kJ mol–1 for 4d (Ar =
C6F5) versus 4b (Ar = Ph).
UV, 4a
PL, 4a, ex = 315 (nm)
50000
UV, 4c
1,0
PL, 4c, ex = 325 (nm)
UV, 4e
40000
PL, 4e, ex = 330 (nm)
UV, 4f
PL, 4f, ex = 330 (nm)
30000
UV, 4h
PL, 4h, ex = 320 (nm)
0,5
20000
10000
0,0
0
250
300
350
400
450
500
550
600
650
Wavelength (nm)
In addition, the G0 of association of the mesityl-borane 4a
with NH3 is nearly equal to 0 kJ mol–1, which is in full agreement
with the experimental observation that the Lewis adduct
between 4a and ammonia was not formed (Scheme 10). In
agreement with the experimentally reported formation of the
Ph3P⦁BPh3 Lewis adduct,[22] the Gibbs free energy of
association of the large Lewis base PPh3 with BPh3 9a was
found to be negative (G0 = -7 kJ mol–1). The association of
PPh3 with the semi-planar borane 4b is however
thermodynamically unfavorable (G0 = +20 kJ mol–1, Scheme
10). Since 4b is as Lewis acidic as BPh3 9a, it highlights that the
presence of the methylene bridge in 4b strongly shields the
boron atom and prevents the system from conformational
changes that would reduce the steric repulsions, thus resulting
in a frustrated Lewis pair.[23]
Figure 7. UV/Vis (solid line) and photoluminescence (PL) (dashed line)
spectra of the synthesized semi-planar triarylboranes 4a, 4c, 4e, 4f and 4h in
CH2Cl2.
Table 2: Spectroscopic data of the semi-planar triarylboranes 4a, 4c, 4e, 4f
and 4h in CH2Cl2.
Abs max
(nm)
e
PL max
(nm)
Stokes shift
(cm-1)
Comp.
(L mol-1 cm-1)
315[a]
9180
32570
13050
9240
405
282
4a
(at ex = 315
7060
7280
6280
(nm))
245[a]
324[a]
289
424
46850
18500
9300
4c
4e
(at ex = 325
(nm))
246
321
402
279
245[a]
39870
14530
10940
12150
39500
14120
11870
(at ex = 330
(nm))
320
414
297[a]
276
244[a]
313[a]
4f
(at ex = 330
7100
7440
(nm))
408
(at ex = 320
4h
282
46450
(nm))
[a]: shoulder.
Interestingly, large Stokes shifts were observed in all cases,
ranging from 6280 to 7440 cm-1. The presence of the weak
electron donating methyl group at position 7 of the 9-hydro-
boraanthracene skeleton in 4e leads to a strong decrease of 780
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preclude Lewis acid-base adduct formation with triarylphoshines.
These Lewis acid/base pairs and the newly synthesized
spirocyclic amino-borane might be potential candidate in
frustrated Lewis pair catalysis due to their weak B-P and B-N
bonds, which should dissociate under thermal process, UV-Vis
irradiation, or acidic conditions.
Experimental Section
Experimental procedures for the synthesis of the final compounds and
intermediates as well as their characterizations, NMR spectra, UV/Vis
and photoluminescence spectra, DFT calculations and X-ray
crystallographic data are described in detail in the Supporting Information.
Acknowledgements
We gratefully acknowledge the University of Namur, the Namur
Institute of Structured Matter (NISM) and research college
(NARC) and the FNRS (2018-2020 grant MIS F.4513.18 for GB
and FRIA PhD grants for DM and AO) for generous funding. The
DFT calculations were performed on the computing facilities of
the “Consortium des ꢀquipements de Calcul Intensif”, in
particular at the High-Performance Computing technological
platform (University of Namur), for which we acknowledge the
financial support of the FNRS-FRFC of the Walloon Region. We
thank the PC2 technological platform at the University of Namur
for access to X-ray diffraction and NMR instruments.
Scheme 10. M06-2X/6-311G(d) gas phase fluoride ion affinities (FIA, H0 in
kJ mol–1), affinities with ammonia and triphenylphosphine (Gibbs free energies,
G0 in kJ mol–1), global (, in eV) and local (boron, B, in eV) electrophilicity
indexes of the borane derivatives 9a-d and 4a-d. As a comparison, the
corresponding affinities (in kJ mol-1) of B(C6F5)3 for F- (H0), NH3 (G0) and
PPh3 (G0) are -466, -117 and -62 respectively while its global and local
electrophilicity indexes are respectively 2.79 eV and -1.23 eV.[24] In
parenthesis are indicated the reorganization energies necessary for the
borane to reach its final geometry in the Lewis adduct.
Keywords: semi-planar triarylboranes • boron Lewis acidity •
Lewis adduct • structure property relationship
[1]
[2]
a) W. E. Piers, Adv. Organomet. Chem. 2005, 52, 1; b) “Synthesis
and Application of Organoboron Compounds”, E. Fernández, A.
Conclusion
Whiting, Eds. Springer, New York, 2015
.
a) L. Ji, S. Griesbeck, T. B. Marder, Chem. Sci., 2017, 8, 846; b) A.
Escande, M. J. Ingleson, Chem. Commun., 2015, 51, 6257; c) A.
Lorbach, A. Hübner, M. Wagner, Dalton Trans., 2012, 41, 6048; d)
C. R. Wade, A. E. J. Broomsgrove, S. Aldridge, F. Gabbai, Chem.
Rev., 2010, 110, 3958; e) F. Jäkle, Chem. Rev. 2010, 110, 3985.
For recent reviews on the synthesis and applications of
triarylboranes see: a) J. L. Carden, A. Dasgupta, R. L. Melen,
Chem. Soc. Rev. 2020, 49, 1706; b) J. R. Lawson, R. L. Melen,
Inorg. Chem. 2017, 56, 8627; c) J. Hermeke, J. Mohr, M. Oestreich,
Chem. Soc. Rev. 2015, 44, 2202; d) M. Hirai, N. Tanaka, M. Sakai,
S. Yamaguchi, Chem. Rev. 2019, 119, 8291.
Three synthetic methods towards semi-planar triarylboranes
have been developed. Four semi-planar triarylboranes with
phenyl, mesityl and fluoroaryl moieties attached to boron atom,
six functionalized mesityl-hydro-boraanthracenes and one
spirocyclic amino-borane derivative have been synthesized
successfully by these methods.
The fine-tuning of the triarylboranes stereoelectronic properties
and Lewis acidities was achieved by introducing fluorine, methyl,
nbutyl, phenyl and methoxy groups either on their exocyclic aryl
ring, bridged aryl rings, or methylene bridge. First, the
photophysical properties of these semi-planar triarylboranes
have been systematically evaluated. The presence of different
substituents displays a minor effect on the UV-Vis and PL
spectra, whereas the introduction of a methylene bridge at ortho-
position induces a large Stokes shift in the similar range of the
planar triarylboranes described by Yamaguchi.
Next, X-ray crystallographic analysis, NMR investigations and
DFT calculations showed that the covalent methylene bridge
connecting two aryl rings of these boranes by their ortho-
positions has a strong effect on their structural flexibility, and on
the steric shielding around the boron atom, though their Lewis
acidity is nearly unaffected as shown by the respective energies
of associations with the small NH3 and F– Lewis bases in
comparison with those of BPh3. The steric shielding of the boron
atom by the methylene bridge in ortho-position is shown to
[3]
[4]
[5]
a) V. M. Hertz, N. Ando, M. Hirai, M. Bolte, H.-W. Lerner, S.
Yamaguchi, M. Wagner, Organometallics, 2017, 36, 2512; b) A.
Shuto, T. Kushida, T. Fukushima, H. Kaji, S. Yamaguchi, Org. Lett.
2013, 15, 6234; c) J. F. Araneda, B. Neue, W. E. Piers, Angew.
Chem. Int. Ed. 2012, 51, 9977.
a) P. Jutzi. J. Organomet. Chem., 1969, 1; b) R. Van Veen, J. F.
Bickelhaupt, J. Organomet. Chem., 1970, 24, 589; c) R. A. Lee, R.
J. Lachicotte, G. C. Bazan, J. Am. Chem. Soc. 1998, 120, 6037; d)
T. K. Wood, W. E. Piers, B. A. Keay, M. Parvez, Angew. Chem. Int.
Ed. 2009, 48, 4009; e) T. Agou, J. Kobayashi, T. Kawashima, Org.
Lett., 2006, 8, 2241; f) M. Kranz, F. Hampel, T. Clark, J. Chem.
Soc., Chem. Commun., 1992, 1247; g) T. Agou, J. Kobayashi, T.
Kawashima, Chem. Commun., 2007, 3204; h) N. Ando, T. Kushida,
S. Yamagushi, Chem. Commun., 2018, 54, 5213.
[6]
a) K. Smith, Chem. Soc. Rev. 1974, 3, 443; b) A. G. Massey, A. J.
Park, J. Organomet. Chem. 1964, 2, 245; c) S. Yamaguchi, S.
Akiyama, K. Tamao, J. Am. Chem. Soc. 2000, 122, 6335; d) M. V.
7
This article is protected by copyright. All rights reserved.

10.1002/chem.202003319
Chemistry - A European Journal
FULL PAPER
Metz, D. J. Schwartz, C. L. Stern, P. N. Nickias, T. J. Marks, Angew.
Chem. Int. Ed. 2000, 39, 1312.
b) W. J. Layton, K. Niedenzu, P. M. Niedenzu, S. Trofimenko, Inorg.
Chem. 1985, 24, 1454.
[7] a) R. Van Veen, F. Bickelhaupt, J. Organometal. Chem. 1971, 30,
C51; b) M. Wang, F. Nudelman, R. R. Matthes, M. P. Shaver, J. Am.
Chem. Soc. 2017, 139, 14232; c) R. Van Veen, F. Bickelhaupt, J.
Organometal. Chem. 1972, 43, 241; d) B. A. Haag, C. Samann, A.
Jana, P. Knochel. Angew. Chem. Int. Ed. 2011, 50, 7290; e) M. A.
Oberli, S. L. Buchwald, Org. Lett. 2012, 14, 4606; f) A. Music, A. N.
[23] As a proof as concept, the association of boranes 4c, 4d with
moderate to highly bulky phosphines have been studied
experimentally. For a recent review about water tolerant FLPs, see:
V. Fasano, M. J. Ingelson, Synthesis. 2018, 50, 1783. For selected
reviews on frustrated Lewis pair see: a) D. W. Stephan, G. Erker,
Angew. Chem. Int. Ed., 2015, 54, 6400; b) D. W. Stephan, Science
2016, 354, 1248.
Baumann, P. Spie
ß
, N. Hilgert, M. Kꢁllen, D. Didier, Org. Lett. 2019
,
21, 2189; g) C. Gerleve, A. Studer, Angew. Chem. Int. Ed. 2020
,
[24] A. Chardon, A. Osi, D. Mahaut, T-H. Doan, N. Tumanov, J.
Wouters, L. Fusaro, B. Champagne, G. Berionni, Angew. Chem. Int.
Ed, 2020, 59, 12402.
DOI: 10.1002/anie.202002595; h) F. Rauch, P. Endres, A. Friedrich,
D. Sieh, M. Hähnel, I. Krummenacher, H. Braunschweig, M. Finze,
L.
Ji,
T.
B.
Marder.
Chem.
Eur.
J
2020
,
DOI:
10.1002/chem.202001985.
[8]
[9]
a) S. J. Rettig, J. Trotter, Can. J. Chem. 1973, 51, 1288; b) S. J.
Rettig, J. Trotter, Can. J. Chem. 1976, 54, 3130.
a) H. Höpfl, J. Organomet. Chem. 1999, 581, 129. Slightly different
values resulted, when the tetrahedral character was calculated
according to Oki’s equation: b) S. Toyota, M. Ōki, Bull. Chem. Soc.
Jpn. 1992, 65, 1832.
[10] M. Dabrowski, K. Durka, S. Lulinski, J. Serwatowski, J. Warkocka,
Acta. Cryst., E. 2011, 67, 3098.
[11] The structure of
5
was confirmed by NMR spectroscopy and X-ray
similar dimerization reaction under
diffraction analysis, for
a
irradiative conditions see: S. C. Lapin, B.-E. Brauer, G. B.Schuster,
J. Am. Chem. Soc. 1984, 106, 2092.
[12] A. Krasovskiy, A. Tishkov, V. del Amo, H. Mayr, P. Knochel, Angew.
Chem. Int. Ed. 2006, 45, 5010.
[13] T. Matsumoto, K. Tanaka, K. Tanaka, Y. Chujo, Dalton Trans. 2015
,
44, 8697.
[14] T. Matsumoto, H. Takamine, K. Tanaka, Y. Chujo, Mater. Chem.
Front. 2017,1, 2368.
[15] For selected examples see: a) K. Matsuo, S. Saito, S. Yamaguchi,
J. Am. Chem. Soc., 2014, 136, 12580; b) T. Matsumoto, H.
Takamine, K. Tanaka, Y. Chujo, Mater. Chem. Front., 2017, 1,
2368; c) N. Kano, J. Yoshino, T. Kawashima, Org. Lett., 2005, 7,
3909; d) K. Ansorg, H. Braunschweig, B. Engels, D. Gamon, M.
Hügel, T. Kupfer, K. Radacki, Angew. Chem. Int. Ed., 2011, 50,
2833.
[16] P. Finocchiaro, A. Recca, F. A. Bottino, F. Bickelhaupt, R. Van
Veen, H. Schenk, J. D. Schagen, J. Am. Chem. Soc. 1980, 102,
5594.
[17] R. A. Lee, R. J. Lachicotte, G. C. Bazan, J. Am. Chem. Soc. 1998
,
120, 6037.
[18] D. S. Miller, J. E. Leffler, J. Phys. Chem. 1970, 74, 2571.
[19] Z. Zhou, A. Wakamiya, T. Kushida, S. Yamaguchi, J. Am. Chem.
Soc. 2012, 134, 4529.
[20] a) C. Laurence, J. F. Gal, "Lewis Basicity and Affinity Scales: Data
and Measurement" Wiley, 2009; b) I. B. Sivaev; V. I. Bregadze,
Coord. Chem. Rev. 2014, 270, 75; c) L. A. Mück, A. Y. Timoshkin
and G. Frenking, Inorg. Chem. 2012, 51, 640; d) L. O. Meller, D.
Himmel, J. Stauffer, G. Steinfeld, J. Slattery, G. Santiso-Quinones, V.
Brecht; I. Krossing, Angew. Chem. Int. Ed. 2008, 47, 7659; e) T. E.
Mallouk, G. L. Rosenthal, G. Mueller, R. Brusasco, N. Bartlett,
Inorg. Chem. 1984, 23, 3167; f) M. F. Smith, S. J. Cassidy, I. A.
Adams, M. Vasiliu, L. Gerlach, D. A. Dixon, P. A. Rupar,
Organometallics. 2016, 35, 3182; g) K. O. Christe, D. A. Dixon, D.
McLemore, W. W. Wilson, J. A. Sheehy, J. A. Boatz, J. Fluorine
Chem. 2000, 101, 151.
[21] a) A. R. Jupp, T. C. Johnstone, D. W. Stephan, Dalton Trans. 2018
,
47, 7029; b) A. R. Jupp, T. C. Johnstone, D. W. Stephan, Inorg.
Chem. 2018, 57, 14764; c) L. V. Szentpály, S. Liu, R. G. Parr, J.
Am. Chem. Soc, 1999, 121, 1922; d) W. J. Mortier, W. Yang, J. Am.
Chem. Soc, 1986, 108, 5708; e) P. Perez, A. Toro-Labbé, A.
Aizman, R. Contreras, J. Org. Chem. 2002, 67, 4747; f) U. Sarkar,
D. R. Roy, P. K. Chattaraj, Chem. Rev. 2006, 106, 2065.
[22] The BPh3 Lewis acid is known to react with PPh3 to form a stable
Lewis adduct, see: a) A. Vogler, Inorg. Chem. Comm. 2016, 72, 83;
8
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Entry for the Table of Contents
A series of semi-planar triarylboranes were obtained by three procedures. The effects of the planarization induced by the methylene
bridge on their steric shielding, Lewis acidities and optical properties have been investigated by experiments and quantum chemical
calculations. These methods can be extended to the preparation of functionalized spirocyclic amine-boranes and unsymmetrical
triarylboranes, which might be used in frustrated Lewis pair chemistry or as pH and photo-responsive boron Lewis acids.
Institute and/or researcher Twitter usernames: @Dr_Berionni
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