Design, Synthesis and Hydrogen Bonding of B3N6-[4]Triangulene

Article abstract of DOI:10.1002/anie.202109326

Replacement of the allylic C=C?C unit with a N?B?N unit at each of the three zigzag edges of [4]triangulene gives rise to B₃N₆-[4]triangulene, which is envisioned to represent a key structural unit of a new hypothetical boron carbon nitride (BC₄N). A tert-butylated B₃N₆-[4]triangulene has been successfully synthesized by three-fold nitrogen-directed borylation, and the X-ray crystallographic analysis indicates that its slightly bent triangular polycyclic framework can be viewed as a 1,3,5-triphenylbenzene connected by three 4π-electron N?B?N units. The HN?B?NH moiety provides a dual hydrogen-bond donor, which forms H-bonds with halide or carboxylate anions in solution, and form DD-AA hydrogen-bond arrays with 2,7-di(tert-butyl)-pyrene-4,5,9,10-tetraone in the co-crystal. Moreover, the blue fluorescence of B₃N₆-[4]triangulene in solution is responsive to binding p-nitrobenzoate anion through hydrogen bonds.

Full text of DOI:10.1002/anie.202109326

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Accepted Article
Title: Design, Synthesis and Hydrogen Bonding of B3N6-
[4]Triangulene
Authors: Mengna Zhao and Qian Miao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202109326
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Design, Synthesis and Hydrogen Bonding of B3N6-[4]Triangulene
Mengna Zhao and Qian Miao*
[*]
M. Zhao, Prof. Q. Miao
Department of Chemistry
The Chinese University of Hong Kong
Shatin, New Territories, Hong Kong, China
E-mail: miaoqian@cuhk.edu.hk
Supporting information for this article is given via a link at the end of the document.
Abstract: Replacement of the allylic C=C−C unit with a N–B–N unit
at each of the three zigzag edges of [4]triangulene gives rise to
B3N6-[4]triangulene, which is envisioned to represent a key structural
unit of a new hypothetical boron carbon nitride (BC4N). A tert-
butylated B3N6-[4]triangulene has been successfully synthesized by
three-fold nitrogen-directed borylation, and the X-ray crystallographic
analysis indicates that its slightly bent triangular polycyclic
framework can be viewed as a 1,3,5-triphenylbenzene connected by
three 4π-electron N−B−N units. The HN–B–NH moiety provides a
dual hydrogen-bond donor, which forms H-bonds with halide or
carboxylate anions in solution, and form DD-AA hydrogen-bond
arrays with 2,7-di(tert-butyl)-pyrene-4,5,9,10-tetraone in the co-
crystal. Moreover, the blue fluorescence of B3N6-[4]triangulene in
solution is responsive to binding p-nitrobenzoate anion through
hydrogen bonds.
[4]triangulene with an open-shell electronic structure, B3N6-
[4]triangulene is isoelectronic to the closed-shell trianion of
[4]triangulene while B6N3-[4]triangulene is isoelectronic to its
closed-shell trication.
Combining carbon with boron and nitrogen atoms in a
hexagonal lattice can offer various two-dimensional (2D)
[1, 2]
materials
because B, C, and N atoms can substitute with
each other due to their similar atomic parameters. As predicted
by theoretical studies,[3, 4] the electronic structure of hexagonal
boron carbon nitride (BxCyNz) depends on the composition and
atomic arrangement of B, C, and N in the lattice. [5] BxCyNz were
early synthesized by pyrolysis of precursors,[6, 7] and are now
typically synthesized by chemical vapor deposition (CVD),[2, 8, 9]
which commonly lead to phase segregation of boron nitride and
carbon domains.[3] These methods result in structures that lack
the atomic precision necessary to fully control and reproduce the
predicted properties. On the other hand, the concept of bottom-
[ 10 ]
up approach from BN-containing polycyclic arenes
by
surface-assisted polymerization and cyclodehydrogentation
reactions has been proved to give 2D heteroatomic networks of
atomically precise substitution patterns of B and N atoms.[11]
[1, 3, 7]
Herein, we propose a new type of BC4N
consisting of
benzene and B3N3 units as shown in Figure 1a, and envision
that B6N3-[4]triangulene and B3N6-[4]triangulene (Figure 1c)
represent the key structural units of this hypothetical BC4N.
B6N3-[4]triangulene and B3N6-[4]triangulene not only contain
structural information of the BC4N but also can in principle be
used as monomers in a bottom-up approach to synthesize
porous BC4N (Figure 1b) through on-surface dehydrogenation
reactions. B3N6-[4]triangulene and B6N3-[4]triangulene have
[12, 13, 14, 15, 16]
[17, 18]
three N–B–N
and B–N–B
units, respectively,
replacing the allylic C=C−C units in [4]triangulene, which
represents a triangular graphene fragment containing three
unpaired electrons delocalized around the zigzag edge. Different
[19, 20,
from the isoelectronic replacement of C=C with B−N units,
21, 22, 23, 24]
Figure 1. (a) Lattice structure of BC4N with the two structural units highlighted
in light grey and light green; (b) lattice structure of porous BC4N with the two
structural units highlighted; (c) structures of B6N3-[4]triangulene, B3N6-
[4]triangulene and t-butylated B3N6-[4]triangulene (1).
embedding N–B–N or B–N–B units in a polycyclic
aromatic hydrocarbon changes the number of π-electrons while
leaving the skeleton geometry almost unchanged. Unlike
1
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Herein, we report the first derivative of B3N6-[4]triangulene,
1 (Figure 1c), which has tert-butyl groups to increase the
solubility. B3N6-[4]triangulene is also a new higher analog of a
family of [3]-heterotriangulenes,[25, 26] which present versatile
singlets at 6.74, 6.31, 1.42 ppm, corresponding to the aromatic
CH protons, heterocyclic NH protons and tert-butyl CH protons
respectively, in agreement with the D3h symmetry of 1.
chemistry and are utilized as building blocks for supramolecular
[ 27 , 28 ]
assemblies
and as functional materials for various
applications, such as organic light emitting diodes,[29] perovskite
solar cells,[30] and liquid crystals.[31] Another interesting aspect of
B3N6-[4]triangulene is that the NH–B–NH moiety provides two
proximate and polarized N–H fragments, capable of forming two
hydrogen bonds similar to N,N’-disubstituted ureas.[32] Although
[33, 34, 35]
NBN-embedded polycyclic arenes
can offer interesting
hydrogen bonds, their hydrogen bonding has been rarely
studied.[36] Moreover, as found from density functional theory
(DFT) calculations at the B3LYP/6-31G(d, p) level, B3N6-
[4]triangulene is of D3h symmetry (Figure S5 in the Supporting
Information), which suggests hydrogen bonds between B3N6-
[4]triangulene and proper hydrogen bond acceptors may
potentially enable hydrogen-bonded 2D networks. Detailed
below are the synthesis, structural analysis and properties of 1,
and a study on its hydrogen bonding in solution and solid states.
Figure 2. Crystal structure of 1 • (CH3CN)2: (a) top view and side view of the
polycyclic framework of 1 with bond lengths; (b) herringbone packing of 1; (c)
edge-to-face interaction between molecules of 1 and hydrogen bonds between
1 and acetonitrile. (C, N and B atoms are shown as ellipsoids at the 50%
probability level; the tert-butyl groups in panel a and c and H atoms in panel a
are removed for clarity.)
Single crystals of 1 • (CH3CN)2 were grown by slow
diffusion of acetonitrile vapor into its solution in chloroform. X-ray
[39]
crystallographic analysis
revealed an orthorhombic unit cell
with an R factor of 7.51%. In this crystal, 1 has a slightly bent
triangular polycyclic framework as shown in Figure 2a with
dihedral angles of 5.6 ~ 5.8° between one peripheral benzenoid
ring (ring C in Figure 2a) and the central benzenoid ring (ring A
in Figure 2a). As a result, the B3N6-[4]triangulene backbone in 1
has approximate C3v symmetry in the crystal rather than D3h
symmetry as predicted by DFT calculations for a planar
polycyclic framework. The central benzenoid ring has six C–C
bonds of essentially identical bond lengths (1.403–1.408 Å),
which clearly indicate its aromatic character. The peripheral
benzenoid rings (ring C) have four C–C bonds (r1–r4) of 1.388 ~
1.395 Å long and two slightly longer C–C bonds (r5 and r6) as
shown in Figure 2a. In contrast, the C–C bonds (r7) between the
central benzenoid ring and the peripheral benzenoid rings are
significantly longer (1.447 Å). The B−N bonds in 1 have bond
Scheme 1. Synthesis of tert-butylated B3N6-[4]triangulene (1). Reagents and
conditions: (a) Pd(PPh3)4, K2CO3, tetrabutylammonium bromide, toluene, H2O,
reflux, 72h; (b) SnCl2, EtOH/HCl, 100 °C, 3h; (c) BCl3, o-dichlorobenzene,
triethylamine, 180 °C, 48h.
Scheme 1 shows the synthesis of tert-butylated B3N6-
[4]triangulene (1) from 1,3,5-phenyltriboronic acid trispinacol
[38]
ester (2) [37] and 2-bromo-5-(tert-butyl)-1,3-dinitrobenzene (3)
.
First, the Suzuki-Miyaura cross-coupling between 2 and 3 gave
the corresponding triarylbenzene 4 in a yield of 51%. Then,
reduction of the nitro groups in 4 with tin (II) chloride under an
acid condition afforded 5 with six amino groups. Finally, three-
fold nitrogen-directed borylation of 5 was achieved by heating it
with boron trichloride and excessive triethylamine at 180 °C,
resulting in 1 as a white solid in a yield of 18% after purification
by column chromatography on silica gel and recrystallization
from CH2Cl2 and ethyl acetate. In comparison to the yield for
each borylation (56% as calculated from the overall yield of
lengths of 1.422 ~ 1.428 Å, which are shorter than the B−N bond
[40]
in 1,2-dihydro-1,2-azaborine (1.443 Å).
The B−C and C−N
bond lengths are both longer than the corresponding bond
lengths in 1,2-dihydro-1,2-azaborine. The above bond lengths
indicate that B3N6-[4]triangulene backbone in 1 can be viewed
as a 1,3,5-triphenylbenzene connected by three N−B−N units,
which involve equivalent resonance contributors containing B–N
double bonds (Figure S1 in the Supporting Information), or, in
other words, a three-center-four-electron π bond. In the crystal,
1 adopts a herringbone packing mode (Figure 2b) involving
18%), the reported yield for borylation with formation of a NH–B–
[35]
NH moiety under the same condition varied from 32%
to
90%.[34] The 1H NMR spectrum of 1 in CDCl3 exhibited three
2
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edge-to-face interaction (Figure 2c) between adjacent molecules
with H···B (3.12 Å) and H···C (2.73 Å) contacts (shown with red
dashed lines) shorter than the corresponding sum of van der
Waals radii. In the crystal, each molecule of 1 interacts with two
molecules of acetonitrile with hydrogen bonds (shown with
magenta dashed lines in Figure 2c), which involve a H-to-N
distance of 2.24 Å, a N-to-N distance of 3.27 Å and a N–H···N
angle of 161.6°.
binding with the different anions. In a control experiment,
addition of ethyl p-nitrobenzoate (12 equivalents) into the
solution of 1 in CDCl3 did not change the chemical shift of the
NH in 1 as shown in Figure S9. This indicates that p-
nitrobenzoate anion is hydrogen-bonded to 1 through the
carboxylate anion rather than the nitro group. The above results
provide qualitative evidences to support the formation of
hydrogen bonds between the NH–B–NH moiety in 1 and
different ions and suggest the relative binding affinity to these
anions. However, we were not able to determine the
corresponding binding constant on the basis of NMR titration
due to the complicity of stoichiometry.
The aromaticity of B3N6-[4]triangulene was also studied by
calculating nucleus-independent chemical shift (NICS) with the
GIAO method at the B3LYP/6-31+G* level of DFT.[41, 42] It is
found that the central benzenoid ring (ring A) in 1 has a negative
NICS(1) value (−10.3 ppm) typical for an aromatic ring, while the
peripheral benzenoid rings (ring C) have slightly lower
aromaticity with a NICS(1) value of −8.2 ppm. In contrast, the
1,2-azaborine ring (ring B) is essentially non-aromatic with a
NICS(1) value of −1.1 ppm. In comparison to B3N6-[4]triangulene,
9,10-dihydro-9-aza-10-boraphenanthrene was reported to have
a higher aromaticity in its 1,2-azaborine ring with a NICS(1)
value of −4.6 ppm. [43] Such difference can be attributed to the
[4]triangulene framework. The calculated NICS values are in
agreement with the bond lengths found in the crystal structure.
The white solid of 1 dissolves in common organic solvents
forming colorless solution with strong blue fluorescence upon
irradiation with UV light. As shown in Figure S3 in the Supporting
Information, the UV/Vis absorption spectrum of 1 in CH2Cl2
exhibits two strong, vibronic-structured bands with the
absorption maxima at 253 nm and 327 nm, respectively. The
solution of 1 in CH2Cl2 exhibits blue fluorescence in the range of
370 to 500 nm with a quantum yield of 0.38, which was
measured using anthracene as a reference. The electrochemical
behavior of 1 was studied by cyclic voltammetry (CV). In the test
window, 1 exhibits three irreversible oxidation waves but does
not show any reduction waves as shown in Figure S4. On the
basis of the first oxidation potential at 0.24
V versus
ferrocenium/ferrocene, the highest occupied molecular orbital
(HOMO) energy level of 1 is estimated as −5.34 eV.[44] DFT
calculations at the B3LYP level with 6-311++G(d, p) basis set
reveal that 1 has degenerate frontier orbitals including lowest
unoccupied molecular orbitals (LUMO) and LUMO+1 with the
same energy level of −1.51 eV and HOMO and HOMO−1 with
the same energy level of −5.10 eV, which is roughly in
agreement with experimental value.
Having two proximate and polarized N–H units, the NH–B–
NH moiety in 1 is presumably capable of binding anions with
formation of two hydrogen bonds. In order to test this
assumption, a tetrabutylammonium salt of halide or carboxylate
(p-nitrobenzoate and acetate) was added into a solution of 1 (3.4
mM) in CDCl3, which was monitored with 1H-NMR spectroscopy.
It was found that addition of the fluoride (1.2 equivalents) led to
white precipitation and the 1H-NMR spectrum showed only a
trace amount of 1 remaining in solution with its signal of NH
shifted from 6.34 ppm to 7.10 ppm (Figure S8 in the Supporting
Information). In contrast, addition of the other halides and
carboxylates (12 equivalents) to the solution of 1 all gave clear
solutions and resulted in downfield shift of the signal of NH in 1
as shown in Figure 3a. The observed shifts (ΔδNH) in a
descending order are 1.74 ppm for p-nitrobenzoate, 1.49 ppm
for acetate, 1.36 ppm for chloride, 1.24 ppm for bromide and
0.87 ppm for iodide. In contrast, the resonance of aromatic
protons (CH) exhibited almost negligible downfield shifts upon
Figure 3. (a) Partial 1H-NMR spectra of 1 (3.4 mM) in CDCl3 without and with
various anions (12 equiv., as the tetrabutylammonium salt) at 23 ±1 °C; (b)
emission spectra of 1 (0.34 mM) in CH2Cl2 without and with 1.0 mM of ethyl p-
nitrobenzoate, tetrabutylammonium acetate or tetrabutylammonium p-
nitrobenzoate (excited at 352 nm).
It was found that formation of hydrogen bonds between 1
and p-nitrobenzoate anion not only changed the chemical shift of
NH in the 1H NMR spectrum, but also led to significant decrease
in the blue fluorescence of 1 (0.34 mM in CH2Cl2) as shown in
Figure 3b. In the control experiments, addition of the same
amount of tetrabutylammonium acetate (3 equiv.) or ethyl p-
nitrobenzoate (3 equiv.) led to much smaller or almost negligible
decrease in the fluorescence of 1 (Figure 3b), respectively.
These results indicate that the emission of 1 is responsive to
binding p-nitrobenzoate anion through hydrogen bonds because
the fluorescence of 1 is quenched by photo-induced electron
transfer from 1 (an electron donor) to p-nitrobenzoate anion (an
3
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electron acceptor). In contrast, acetate is not a good electron
acceptor although it also binds 1 with hydrogen bonds. The ester
group or nitro group in ethyl p-nitrobenzoate cannot bind 1 with a
essentially flat. The bond lengths in the polycyclic framework of
1 in this co-crystal are similar to those found in the crystal of 1 •
(CH3CN)2 (Figure S2a in the Supporting Information) although
the larger R factor (12.37%) may not allow analysis of the
structure of 1 in the co-crystal with high accuracy. All the π-
planes of 1 and 6 are essentially perpendicular to the c axis
forming a layered structure (Figure S2b). Within each layer,
each molecule of 1 forms dual hydrogen bonds with two
molecules of 6 and hydrogen bonds with two molecules of
acetonitrile as shown in Figure 4b. The dual hydrogen bond
between 1 and 6 involves H-to-N distances of 2.22 Å and 2.38 Å,
N-to-O distance of 2.99 Å and 3.15 Å, and N-H···O angles of
148.2° and 149.4°. The two hydrogen bonds between 1 and
acetonitrile are the same involving a H-to-N distance of 2.22 Å, a
N-to-N distance of 3.06 Å and a N-H···N angle of 168.2°. These
lengths and angles indicate that the hydrogen bonds between 1
and 6 or acetonitrile are of moderate strength.[45] Unlike the
proposed hydrogen-bonded 2D network, each molecule of 6 in
the co-crystal has only two oxygen atoms forming hydrogen
bonds with 1. To achieve the proposed hydrogen-bonded 2D
network, it is necessary to tune the size of TDHBA and the
substituting groups on B3N6-[4]triangulene and TDHBA. The π-
planes of 1 and 6 in adjacent layers stack in a face-to-face
manner but are not strictly parallel to each other. The average π-
π distance between 1 and 6 is 3.45 Å as measured from the sp2
carbon atoms of 6 to the plane of B3N6-[4]triangulene in 1. This
reflects π-donor/acceptor interactions between 1 and 6.[46] The
slightly different polycyclic frameworks of 1 in the two crystals
are presumably due to crystal packing, and in particular, a flat π-
backbone of 1 is favored by π-donor/acceptor interactions.
1
dual hydrogen bond, in agreement with the H NMR experiment
results (Figure S9), and thus collisions between 1 and ethyl p-
nitrobenzoate in a dilute solution (0.34 mM) are not able to
quench the fluorescence.
In summary, the above study has explored B3N6-
[4]triangulene, which is regarded as a key structural unit of a
new hypothetical boron carbon nitride (BC4N) and an
isoelectronic motif for the trianion of [4]triangulene. The tert-
butylated derivative of B3N6-[4]triangulene (1) has been
successfully synthesized by threefold nitrogen-directed
borylation, and its slightly bent triangular polycyclic framework in
the single crystal of 1 • (CH3CN)2 can be viewed as a 1,3,5-
triphenylbenzene connected by three 4π-electron N−B−N units.
With each N–B–N moiety providing a dual hydrogen-bond donor,
one molecule of 1 can form up to six hydrogen bonds. As found
from 1H NMR spectrum, 1 forms H-bonds with halide or
carboxylate anions in solution; and the X-ray crystallography
indicates 1 forms DD-AA hydrogen bond arrays with 2,7-di(tert-
Figure 4. (a) Proposed hydrogen-bonded 2D network of NBN-[4]triangulene
and a twofold dual-hydrogen-bond acceptor (TDHBA) and the structure of 6;
butyl)-pyrene-4,5,9,10-tetraone
in
the
co-crystal.
The
(b) the hydrogen bonds of 1 with 6 and acetonitrile in the crystal of 1 • (6)2
•
fluorescence response of 1 to binding p-nitrobenzoate anion
suggests that NH–B–NH embedded polycyclic arenes can form
(CH3CN)2 • (THF)2 (C, N, B and O atoms are shown as ellipsoids at the 50%
probability level, one tert-butyl group involves two sets of disordered atoms).
a
new structural basis for designing hydrogen-bonded
fluorescent sensors.
With a threefold symmetry, B3N6-[4]triangulene can in
principle form a 2D network with a twofold dual-hydrogen-bond
acceptor (TDHBA) through DD-AA hydrogen bonds as shown in
Figure 4a. To test this hypothesis, we attempted to grow co-
Acknowledgements
crystals of
1
with different TDHBAs including sodium
We thank Ms. Hoi Shan Chan and Dr. Chun Fai Ng (the Chinese
University of Hong Kong) for the single crystal crystallography,
and Ms. Qi Gong for help on NMR spectroscopy. This work was
supported by the Research Matching Grant from the University
Grants Committee.
terephthalate, p-dinitrobenzene and 2,7-di(tert-butyl)-pyrene-
4,5,9,10-tetraone (6 in Figure 4a). Black plate co-crystals of 1
and 6 were obtained by slow diffusion of acetonitrile into a mixed
solution of 1 and 6 in THF, while other TDHBAs failed in forming
co-crystals with 1. X-ray crystallographic analysis reveals the co-
crystal has a composition of 1 • (6)2 • (CH3CN)2 • (THF)2 and a
Keywords: BN heterocycles • arenes • polycycles • hydrogen
[39]
tetragonal unit cell,
where the polycyclic framework of 1 is
bond
4
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10.1002/anie.202109326
Angewandte Chemie International Edition
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Entry for the Table of Contents
B3N6-[4]triangulene, which is isoelectronic to the trianion of [4]triangulene, represents a key structural unit of a new hypothetical boron
carbon nitride. The tert-butylated derivative of B3N6-[4]triangulene is synthesized by threefold nitrogen-directed borylation. Its
fluorescence in solution is responsive to binding p-nitrobenzoate anion through H-bonds, and it forms DD-AA H-bond arrays with
di(tert-butyl)-pyrenetetraone in the co-crystal.
Institute and/or researcher Twitter usernames: The Miao Group
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