Borazatruxenes

Article abstract of DOI:10.1039/c9sc02489a

We report the synthesis and characterization of a series of arene-borazine hybrids called borazatruxenes. These molecules are BN-isosteres of truxene whereby the central benzene core has been replaced by a borazine ring. The straightforward three step synthesis, stability and their chiroptical and electronic properties recommend them as new scaffolds for BN-carbon hybrid materials. Computational studies at DFT level, closely matching the experimental data, provided insights in the electronic structure of these molecules.

Full text of DOI:10.1039/c9sc02489a

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Borazatruxenes†
Simone Limberti,‡ Liam Emmett,‡ Anamaria Trandafir, Gabriele Kociok-Kohn
Cite this: DOI: 10.1039/c9sc02489a
¨
*
and G. Dan Pantos
¸
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
We report the synthesis and characterization of a series of arene-borazine hybrids called borazatruxenes.
These molecules are BN-isosteres of truxene whereby the central benzene core has been replaced by
a borazine ring. The straightforward three step synthesis, stability and their chiroptical and electronic
properties recommend them as new scaffolds for BN–carbon hybrid materials. Computational studies at
DFT level, closely matching the experimental data, provided insights in the electronic structure of these
molecules.
Received 22nd May 2019
Accepted 27th August 2019
DOI: 10.1039/c9sc02489a
rsc.li/chemical-science
mitigated by steric bulk,44,45 introduction of electron rich
substituents or structural connement of the borazine ring.45–52
Introduction
The interest in polycyclic aromatic hydrocarbons (PAHs) has
grown over the past decade due to their stability, tuneable
properties and versatility.1–3 This has made them ideal candi-
dates for use as semiconductors in the next generation of
organic electronic components.4,5 Moreover, the electronic
properties of PAHs are intimately related to their structure;
therefore small modications in their scaffold can lead to vastly
different optical and electronic properties.6–14
Truxene is a C3-symmetric PAH that has been intensively
studied for the past 15–20 years.15 In particular, it has been used
as a precursor to polyarenes,16,17 liquid crystals18,19 and hemi-
fullerenes,20,21 all of which have potential to be used in organic
electronics.22 Many truxene derivatives have been syn-
thesised23–25 and utilized for their light emitting and light har-
vesting properties26–28 while other analogues have found
applications in dendrimers,29–31 gels,32,33 photoresists,34,35 uo-
rescent probes,36,37 and two photon absorbers.38,39
Results and discussion
Herein we introduce a new class of borazine-PAHs that are
moisture stable§ and easy to synthesise. Borazatruxenes are
truxene analogues in which the central benzene core has been
replaced by borazine. The B atom is directly linked to the phe-
nylene rings which are connected via a methylene bridge to
the N atom of the borazine core. This particular arrangement is
The B–N bond is quasi-isosteric and isoelectronic with the
C]C bond,40,41 except that it is more polarized42 due to the
difference in electronegativity between the B and N atoms.
Therefore, replacing a C]C with a B–N in a PAH will lead to
quasi-identical geometries but drastically different electronic
structures.43 This concept has been at the core of the develop-
ment of BN doped graphenes and BN-PAHs. Borazine contain-
ing PAHs are prone to hydrolytic decomposition which delayed
the development of this eld.43 In most cases, this reactivity is
Scheme 1 Synthesis of borazatruxenes 1–3 from substituted 2-for-
mylphenylboronic acids; the isolated yields for each derivative are
given after their descriptor. (a) (i) H2O, 45 ꢀC, 15 min, (ii) NH2OMe$HCl
(1.3 equiv.), 10% NaOH (w/w%) pH 7, (iii) reflux, 15 min; (b) LiAlH4 (3
equiv.) THF, ꢁ78 ꢀC, then room temperature, followed by heating to
reflux, 3 h; (c) PhMe, mW irradiation, 2 h, 180 ꢀC. The numbering
scheme adopted for the borazatruxenes is also shown for compounds
1–3.
Department of Chemistry, University of Bath, Bath, BA2 7AY, UK. E-mail: g.d.pantos@
bath.ac.uk
† Electronic supplementary information (ESI) available: Detailed synthetic
protocols, modelling and analytical data. CCDC 1843607, 1843608 and 1865474.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9sc02489a
‡ These authors have made equal contribution to this work.
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responsible for the borazatruxenes' hydrolytic stability. Bor- 95%), without requiring a further deprotection step (Scheme 2
azatruxenes 1–3 were synthesised according to the synthetic and ESI†). Using more robust protecting groups, such as pina-
pathway illustrated in Scheme 1.
col or neopentyl glycol, results in the reduction of the CN group
Commercially available meta- and para- substituted 2-for- to the corresponding 1ꢀ amine while the protected boronic acid
mylbenzeneboronic acids were reacted with methoxyamine remains intact under the reduction conditions utilised. Tri-
hydrochloride under reuxing conditions for 15 minutes in merisation of amine-boranes 18–21, using microwave assisted
water (pH 7) to produce compounds 8–10 in high yields (70– synthesis in dry toluene at 180 ꢀC for 2 hours, provided bor-
90%). LiAlH4 in THF was added dropwise to a solution of phe- azatruxenes 4–7 in 50–70% yields. Two additional borazatrux-
nylboronic acid derivatives 8–10 in THF at ꢁ78 ꢀC, the mixture enes, 8 and 9 (Scheme 3), have been synthesised in order to
was allowed to warm to room temperature followed by heating expand the scope of the reaction to larger PAHs and chiral
to reux for 3 hours. In all cases, the amine-borane53 product derivatives (vide infra), respectively.
11–13, a BN analogue of indane, was isolated in excellent yields
Borazatruxene 1 is soluble in organic solvents of medium
(85–90%). Trimerisation54 of 11–13 under microwave-assisted polarity. This is in stark contrast to the parent truxene, which
conditions afforded the desired borazatruxenes 1–3 in 56–65% has very poor solubility in most common solvents.15 The halo-
yields as white solids aer a simple ltration/washing protocol. genated borazatruxene derivatives have signicantly lower
Due to the limited number of commercially available 2-for- solubility compared to 1, which is likely due to their increased
mylphenylboronic acids and their relatively lengthy syntheses, molecular weight. The 1H-NMR of 1 (CDCl3, 300 MHz) indicates
a second pathway was devised in order to obtain functionalised that the aromatic protons closest to the borazine rings are most
borazatruxenes (Scheme 2). 2-Cyanobenzene-boronic esters 14– deshielded (8.03–8.05 ppm) while the external ones experience
17 were synthesised from meta- and para- substituted benzo- a lower inuence of the BN anisotropy with chemical shis in
nitriles. The benzonitriles were reacted with lithium tetrame- the range of 7.42–7.59 ppm. The 11B NMR spectrum (CDCl3, 96
thylpiperidine (LTMP) and B(OiPr)3 at ꢁ78 ꢀC, followed by slow MHz) displays a peak resonating at 37.2 ppm which is in good
warming to room temperature (20 ꢀC) overnight to afford the agreement with a typical 11B chemical shi of a substituted
respective ortho- substituted 2-cyanophenylboronic acid. The borazine. The UV-vis spectrum of 1 shows a very intense and
reaction was quenched using AcOH (2.2 equiv.) followed by an broad absorbance centered at 250 nm followed by three distinct
in situ protection of the boronic acid with 1,3-propanediol. The peaks of lower intensity at 279.5, 272.0 and 265.0 nm. These
use of a protecting group is key for isolation of 14–17, and the peaks are blue shied compared to corresponding truxene
choice of 1,3-propanediol allows the reduction to the corre- ones, thus highlighting that introducing BN bonds into the all-
sponding amine-borane products 18–21 in very good yields (65– carbon system increases the HOMO–LUMO gap (Fig. 3). The
borazatruxenes have higher quantum yields than the
Scheme 3 Synthesis of naphthylborazatruxene 8 from 1-naphthoni-
trile and the chiral trimethylborazatruxene 9, the reported yields are
over three steps. (a) (i) LTMP (in situ: n-BuLi 2.5 M in n-hexane (1.5
Scheme 2 Synthesis of borazatruxenes 4–7 from substituted ben- equiv.) and TMP (1.5 equiv.)), THF, ꢁ10 ꢀC, (ii) B(OiPr)3 (2.0 equiv.),
zonitriles; the isolated yields for each derivative are given after their ꢁ78 ꢀC, benzonitrile (1.0 equiv.), then room temperature, o.n, (iii)
descriptor. (a) (i) LTMP (in situ: n-BuLi 2.5 M in n-hexane (1.5 equiv.) and AcOH (2.2 equiv.), 1,3-propanediol (6.0 equiv.); (b) LiAlH4 (3.0 equiv.)
TMP (1.5 equiv.)), THF, ꢁ10 ꢀC, (ii) B(OiPr)3 (2.0 equiv.), ꢁ78 ꢀC, ben- THF, ꢁ78 ꢀC, then room temperature, followed by reflux, 48 h; (c)
zonitrile (1.0 equiv.) then room temperature, 16 h, (iii) AcOH (2.2 PhMe, mW irradiation, 2 h, 180 ꢀC; (d) NH2OMe$HCl (1.2 equiv.), MeOH,
equiv.), 1,3-propanediol (6.0 equiv.); (b) LiAlH4 (5.0 equiv.) THF, ꢁ78 ꢀC, 36 ꢀC, 16 h; (e) LiAlH4 (5.0 equiv.) THF, ꢁ78 ꢀC, then room temperature,
then room temperature, followed by mW irradiation, 1.5 h, 90 ꢀC; (c) followed by mW irradiation, 1.5 h, 90 ꢀC; (f) PhMe, mW irradiation, 2 h,
PhMe, mW irradiation, 2 h, 180 ꢀC.
180 ꢀC.
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corresponding all-carbon derivatives. Borazatruxenes 5 and 8
aggregate at concentrations higher than 0.05 mM, while
compounds 3, 5 and 6 are non-emissive. The variable temper-
ature UV-vis and emission spectra, as well as the excitation
spectra for the molecules that exhibited emission, are collated
in the ESI, Fig. S10–S18.† The extinction coefficients, quantum
yields and the excitation and emission wavelengths are collated
in Table 1.
There are four possible optical isomers of borazatruxene 9
despite having three stereogenic centres. This is because the
molecule has C3 symmetry which makes the (R,R,S) isomer
identical with the (R,S,R) and (S,R,R) isomers. Therefore, the only
possible isomers are the enantiomeric pairs syn: (R,R,R) and
(S,S,S) with all three methyl groups located on the same side of
the borazatruxenes plane, and anti: (R,R,S) and (S,S,R), where one
methyl is on the opposite side of the plane with respect to the
other two (Scheme 3). The syn and anti enantiomeric pairs can be
readily separated because the syn isomer is more soluble in an
8 : 2 n-hexane : CH2Cl2 mixture and thus it can be isolated
through recrystallisation. Furthermore, all four stereoisomers
can be separated using a Chiralpak OD chiral column (details in
the ESI†). The selectivity factor for the syn enantiomers is 1.92
(98 : 2 n-hexane : propan-2-ol, 0.5 ml minꢁ1) while the anti
enantiomers are separated with a selectivity factor of 1.17 (99 : 1
n-hexane : propan-2-ol, 0.5 ml minꢁ1). The Circular Dichroism
(CD) spectra of the two enantiomeric pairs syn (R,R,R and S,S,S)
and anti (R,R,S and R,S,S) of chiral derivative 9 display strong Fig. 1 Top: UV spectrum of (R,R,R)-9 ( ); bottom: the CD spectra of
the two enantiomers of the syn isomer of 9, along with the computed
CD spectrum (sTDDFT) of the (R,R,R)-9.
Cotton effects on both the arene and borazine absorbance. The
identication of all four isomers was possible by comparing the
experimental (Fig. 1) with the computed (vide infra) CD spectra.
The molecular structure of BN-indane 11 (Fig. 2) was obtained
from the X-ray analysis of single crystals grown by slow evapora-
tion from an ethyl acetate solution. This compound crystallised in
the centrosymmetric monoclinic space group P21/n with four
molecule units per unit cell packed in staggered array motif.
˚
N/Ph(C2–C7) distance of 3.239 A and N/Ph(C3, C4, C5)
˚
distance of 3.339 A.
Structural proof of borazatruxene 1 came from an X-ray
diffraction analysis of single crystals grown by slow evapora-
tion from a CH2Cl2 solution. Fig. 3A shows a top view of the X-
ray structure of 1, which crystallises in a P21/c space group. This
compound forms a staggered L-shaped conguration in the
unit cell (Fig. 3C) where weak CH/p bonding is seen between
˚
The distance between boron and nitrogen atom is 1.637 A
consistent with an N / B Lewis-type interaction. There are two
weak NH/p intermolecular interactions between the amine
group with the aromatic rings of two neighbouring molecules:
˚
C6 and C19 with the close contact distance being 3.801 A. The p
surfaces are also aligned parallel to one another with the closest
˚
distance between B2 and C16 being 3.561 A.
Single crystals of the syn isomer of 9 were obtained from a 9 : 1
hexane : 2-propyl alcohol mixture. The X-ray diffraction revealed
Table 1 Collated UV-vis and fluorescence spectroscopic data of
borazatruxenes 1–9a
ꢀ
that the (R,R,R)-9 and (S,S,S)-9 co-crystallised in the R3 space
group. In this structure the two enantiomers adopt a 60ꢀ rotated
3
lem
Borazatruxene
lex (nm)
(L molꢁ1 cmꢁ1
)
(nm)
Fb (A)
1
2
3
4
5
6
7
8
9
279.5
285.5
287.5
290
290
289
278
298.5
272
10 650
10 280
12 950
6740
2880
4410
6563
7930
3177
294
301
380
305
294
298
301
304
293
0.065
0.104
–
c
0.099
c
–
c
–
0.039
c
–
0.038d
a
b
c
HPLC grade CHCl3. Determined using anthracene standard. Not
d
determined due to aggregation.
propanol 9 : 1.
The solvent was n-hexane : 2-
Fig. 2 (a) Molecular structure and (b) unit cell of BN-indane 11.
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sets as implemented in the Gaussian 16 soware. Also, TD-
DFT60,61 (sTDDFT62) methods were employed to calculate the UV-
vis and CD spectra with the above mentioned functionals
(Fig. S8 and S21†), the best prediction for the CD spectra being
obtained by M06-2X, while the closest agreement for the UV-vis
spectra being given by the M11-L functional (Table S11†).
Similarly the AICD properties (Fig. S22†) of borazatruxenes can
be predicted at the M06-2X/6-311G level. A short study of the
required geometry optimization time versus the agreement to
experimental X-ray data (Table S10, Fig. S20†) yields M06-2X
and M11 functionals along with 6-31G and 6-31G(d,p) basis
sets as excellent choices for the modelling of borazatruxenes
geometries while using reasonable computation resources.
Conclusions
In summary, we have developed the synthesis of a new class of
BN-PAHs that incorporate a borazine unit in place of the central
benzene core of a truxene. These derivatives have higher solu-
bilities than the parent all-carbon derivatives and can be syn-
thesised in three steps from commercially available starting
materials. These derivatives are air and moisture stable which is
in contrast to the majority of non-sterically hindered borazines.
We have synthesised and separated the rst chiral derivatives of
these molecules which also represents a premiere in the larger
truxene family. The borazatruxenes molecules are likely to
become important materials in molecular electronic devices
due to their unique structure which combines areas of electron-
conductance with electron-insulating domains. Further inves-
tigations into the synthesis of new borazatruxenes analogues
are currently in progress in our group.
Fig. 3 (a) Top view of the molecular structure of 1;44 (b) the molecular
structure of 1, as determined by X-ray crystallography, overlaid (RMSD
¼ 0.0603) with the computed structure for 1 in red, translated to (x + 1/
2, y, z + 1/2) with respect to the X-ray coordinates; (c) packing
structure highlighting the CH/p and aromatic p/p interactions.
face-on stacked arrangement with an intermolecular B–N
˚
distance of 3.749 A and the Me groups pointing outwards (Fig. 4).
The molecules pack in an off-set columnar arrangement with
˚
a distance of 1.981 A between two sequential planes of distinct
columns.
The X-ray determined structure of 1 allowed us to validate
a series of molecular modelling parameters used to predict the
geometry of borazatruxenes 1–9. Fig. 3b shows the translated (x
+ 1/2, y, z + 1/2) overlay of the experimentally determined (X-ray
diffraction) co-ordinates and the computed ones. Geometry
optimisations were performed using M06-2X,55 M11,56 M11-L57
or B3LYP58,59 exchange-correlation functionals and various basis
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was funded by the EPSRC (DTA SL, LME, AT) and the
University of Bath. X-ray diffraction and NMR facilities were
provided through the Chemical Characterisation and Analysis
Facility (CCAF) at the University of Bath, while the mass spec-
trometry data were acquired at the EPSRC National Mass
Spectrometry Facility at Swansea University.
Notes and references
§ The borazatruxenes are stable in solid state for at least one year if kept under N2
in close capped vials; they are stable in solution for at least two weeks with
minimal decomposition.
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