Synthesis, characterization, spectral property, Hirshfeld surface analysis and TD/DFT calculations of 2, 6-disubstituted benzobisoxazoles

Article abstract of DOI:10.1016/j.molstruc.2019.07.041

An effective and clean aerobic oxidative method for the synthesis of 2,6-disubstituted benzobisoxazole using the free radical catalysis was obtained. 2, 6-Di(pyridin-4-yl)-benzo[1,2-d:4,5-d']bisoxazole was synthesized and characterized by ¹H an

Full text of DOI:10.1016/j.molstruc.2019.07.041

Accepted Manuscript
Synthesis, characterization, spectral property, Hirshfeld surface analysis and TD/DFT
calculations of 2, 6-disubstituted benzobisoxazoles
Qi Hu, Yong-Hao Yue, Lan-Qin Chai, Li-Jian Tang
PII:
S0022-2860(19)30881-6
DOI:
https://doi.org/10.1016/j.molstruc.2019.07.041
MOLSTR 26794
Reference:
To appear in: Journal of Molecular Structure
Received Date: 3 May 2019
Revised Date: 8 July 2019
Accepted Date: 9 July 2019
Please cite this article as: Q. Hu, Y.-H. Yue, L.-Q. Chai, L.-J. Tang, Synthesis, characterization, spectral
property, Hirshfeld surface analysis and TD/DFT calculations of 2, 6-disubstituted benzobisoxazoles,
Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.07.041.
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ACCEPTED MANUSCRIPT
Graphical Abstract
An effective and clean aerobic oxidative method for the synthesis of
2
,6-disubstituted benzobisoxazole using the free radical catalysis was obtained. 2,
-Di(pyridin-4-yl)-benzo[1,2-d:4,5-d']bisoxazole was synthesized and
6
1
13
characterized by H and C NMR spectroscopy, elemental analysis, UV-vis and
emission spectroscopy, as well as by single-crystal X-ray crystallography. In the
crystal structure, a self-assembled two-dimensional (2-D) layer supramolecular
frameworks is formed with the help of intermolecular hydrogen bonds and π···π
stacking interactions. On the basis of experimental results, a plausible reaction
mechanism of 2,2'-disubstituted benzoxdiazoles with 4-methoxy-TEMPO as the
catalyst is proposed. To obtain more insights into the structure and bonding, DFT
calculations were performed. The obtained calculation results were compared with
the X-ray diffraction data. The electronic transitions and spectral features of 5a
and 5b were discussed by TD-DFT calculations. In addition, the noncovalent
interactions among compounds have been also analyzed using Hirshfeld surface
analysis.

ACCEPTED MANUSCRIPT
Synthesis, characterization, spectral property, Hirshfeld surface
analysis and TD/DFT calculations of 2, 6-disubstituted
benzobisoxazoles
a
b
a,
a
Qi Hu , Yong-Hao Yue , Lan-Qin Chai *, Li-Jian Tang
a
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
b
State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of Chemistry and Chemical
Engineering, Lanzhou University, Lanzhou, Gansu, 730000, China
ABSTRACT
An effective and clean aerobic oxidative method for the synthesis of 2,6-disubstituted
benzobisoxazole
using
the
free
radical
catalysis
was
obtained.
2,
1
6
-Di(pyridin-4-yl)-benzo[1,2-d:4,5-d']bisoxazole was synthesized and characterized by H
1
3
and C NMR spectroscopy, elemental analysis, UV-vis and emission spectroscopy, as well as
by single-crystal X-ray crystallography. In the crystal structure, a self-assembled
two-dimensional (2-D) layer supramolecular frameworks is formed with the help of
intermolecular hydrogen bonds and π···π stacking interactions. On the basis of experimental
results, a plausible reaction mechanism of 2,2'-disubstituted benzoxdiazoles with
4
-methoxy-TEMPO as the catalyst is proposed. To obtain more insights into the structure and
bonding, DFT calculations were performed. The obtained calculation results were compared
with the X-ray diffraction data. The electronic transitions and spectral features of 5a and 5b
were discussed by TD-DFT calculations. In addition, the noncovalent interactions among
compounds have been also analyzed using Hirshfeld surface analysis.
Keywords: 4-Methoxy-TEMPO Free Radical; Spectroscopic studies; Density functional
theory; Hirshfeld surface analysis; 2,6-disubstituted benzobisoxazole; X-ray crystal structure
*
Corresponding author.
E-mail: chailanqin@163.com (L.-Q. Chai).
1

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1
. Introduction
Plane heterocycles define an important class of chemical entities in life science research.
Benzoxazole are benzoheterocyclic compounds containing nitrogen and oxygen atoms in a
molecular structure. Benzoxazole derivatives possess rigid backbone and high conjugated
system. Such as this kind of five-membered heterocyclic ring compounds always show good
biological and therapeutic activities, for example antiproliferative [1], anti-inflammatory, and
the efficacy of kill the plant virus [2] and coordination properties [3]. Besides they also used
widely in the field of medicine [4,5], pesticide, coordination catalysis and in synthetic
pharmaceutical [6] and agrochemical compounds [7]. Benzoxazole structure also has
significant influence on the fluorescence of conjugate system [8,9], is considered to be the
potential materials of electron transfer in polymer light-emitting diode and hole conduction
block [10].
Compared with benzoxazoles, 2, 6-disubstituted benzobisoxazoles have larger
conjugated systems. Herein, an effective and more eco-friendly new aerobic approach for the
synthesis of 2, 6-disubstituted benzobisoxazole using a one-pot reaction of an organic
aminoxyl radical as the catalyst is reported (Scheme
1
)
.
Especially, 2,
1
13
6
-di(pyridin-4-yl)-benzo[1, 2-d:4, 5-d']bisoxazole (5a) was characterized by H and C NMR
spectroscopy, UV-visible and photoluminescence spectroscopy, as well as by single-crystal
X-ray crystallography. On the basis of experimental results, a plausible reaction mechanism
for 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-methoxy-TEMPO) catalyzed aerobic
oxidative synthesis of 2, 2'-disubstituted benzoxdiazoles is proposed. Moreover, DFT
calculations have been also carried out to corroborate the experimental results. DFT coupled
with TD-DFT calculations ascertain the nature of orbitals involved in transition processes and
to correlate the structural parameters. In addition, by means of Hirshfeld surface analysis are
useful for the understanding of the noncovalent forces and for rationalizing their influence in
the crystal packing.
2
2
. Experimental Section
.1 Materials and physical measurements
Pyrocatechol and pyridine-4-carbaldehyde were purchased from Alfa Aesar. All
1
chemicals were of analytical reagent grade and were used without further purification. H and
1
3
C NMR spectra were measured on Mercury Plus (400 MHz) spectrometers. Chemical shifts
2

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1
of H NMR were expressed in parts per million relative to the singlet (d = 7.26) for CDCl .
3
1
3
Chemical shifts of C NMR were expressed in parts per million relative to the central line of
the triplet (d = 77.0 ppm) for CDCl and TMS as internal standard. HRMS spectra were
3
obtained with a Bruker APEX II FT-MS instrument using CD Cl as solvent. UV/Vis spectra
2
2
were recorded on a Shimadzu UV-2550 spectrometer. The fluorescence spectra were taken on
a 970 CRT spectrofluorometer (Spectro, Shanghai/China) with 370 nm excitation.
Single-crystal X-ray structures were determined using an Agilent SuperNova Dual area
detector diffractometer. Elemental analysis was performed on an Elementar Vario EL
Elemental Analysis instrument.
2
.2 Synthesis of 2, 6-disubstituted benzobisoxazoles (5a-5d).
Scheme 1 The detailed synthetic route of the target molecules 5a-5d.
4, 5-dimethoxycyclohexa-3, 5-diene-1, 2-dione (1). Lead(IV) oxide (19.12 g, 80 mmol)
and sodium hydroxide (2.0 g, 50 mmol) were dissolved following the atmosphere was
replaced with argon, and stirred rapidly in anhydrous methanol (100 mL) at room temperature.
The solution of catechol (2.2 g, 20.0 mmol) added dropwise slowly into the reaction system
and stirred for 15 min. After the reaction was stopped, the residual solid PbO was removed
2
by suction filtration under reduced pressure, then the pH of the filtrate was adjusted to 5.0
with 50 % H SO , and the solution was filtered again with suction to remove the precipitated
2
4
Pb in the solution. The filtrate was extracted for three times with an appropriate amount of
dichloromethane, the extracts were combined, and 90 % of the solvent was distilled off under
reduced pressure, then acidified with glacial acetic acid under an argon atmosphere, and
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cooled to 5 ꢀ at the same time a large amount of crystals were formed. Yellow crystalline
1
solids (1.61 g) were obtained by vacuum drying at 50 ꢀ. Yield 48.0 %. H NMR (CDCl ,
3
4
00 MHz): δ = 5.77 (s, 2H), 3.90 (s, 6H).
, 5-dimethoxycyclohexa-2, 5-diene-1,4-dione (2). 1 (336.0 mg, 2.0 mmol) were
2
stirred rapidly in anhydrous methanol (100 mL), then concentrated sulfuric acid (5 mL) was
added dropwise to the solution slowly, and the resulting solution was further stirred for 4 h at
room temperature. The insoluble solids were precipitated sufficiently resting 0.5 h, then
filtered under normal pressure. The crude products were washed with a large amount of
secondary distilled water and absolute ethanol. Yellow powdery solids (322.5 mg) were
1
obtained by vacuum drying at 50 ꢀ. Yield 96.0 %. H NMR (CDCl , 400 MHz): δ = 5.87 (s,
3
2
H), 3.85 (s, 6H).
, 5-diaminocyclohexa-2, 5-diene-1,4-dione (3). 2 (308.0 mg, 2.0 mmol) was placed in
2
a three-necked flask (250 mL), and an argon atmosphere was used to replace completely the
air with the purpose of ensuring the air was eliminated as much as possible. Strong ammonia
solution (1.0 mL) and anhydrous ethanol (100 mL) in the system was maintained refluxing for
3
h meanwhile producing a large amount of ammonia gas. The color of the mixing solution
turned light purple gradually from yellow. The precipitates were washed with distilled water
and absolute ethanol after filtering and sheet purple crystals (148.7 mg) were obtained by
1
vacuum drying at 50 ꢀ. Yield 58.8 %. H NMR (d-DMSO, 400 MHz): δ = 6.12 (s, 2H), 3.29
(s, 4H).
2, 5-diaminobenzene-1, 4-diol (4). 3 (276.0 mg, 2.0 mmol) and reducing agent sodium
thiosulfate (1044.6 mg, 6.0 mmol) were added to distilled water (20 mL) in a two-necked
flask (100 mL) under an argon atmosphere. The solution changed from pale purple to light
yellow after the mixture was stirred for 1 h at 40 ℃. The resulting solution was filtered under
atmospheric pressure, and the precipitates were washed with distilled water and anhydrous
ethanol several times. Powdery yellow solids (272.2 mg) were obtained by vacuum drying at
1
5
0 ꢀ. Yield 97.2 %. H NMR (d-DMSO, 400 MHz): δ = 7.96 (s, 2H), 6.06 (s, 2H), 3.75 (s,
1
3
4
H); C NMR (d-DMSO, 400 MHz): δ = 137.98, 126.58, 105.06.
, 6-disubstituted benzobisoxazoles (5). To o-xylene solution (25 mL) of 4 (280.0 mg,
.0 mmol), pyridine-4-carbaldehyde (a) (0.5 mL, 4.5 mmol) was added dropwise and refluxed
2
2
for 5 h at 120 ꢀ under argon atmosphere, then cooled at room temperature.
4

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-Methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-methoxy-TEMPO) (47.0 mg, 5.0 mmol)
4
as a free radical catalyst was added carefully and stirred for 15 h at 120 � under oxygen
atmosphere. The resulting solution was evaporated under vacuum to remove the solvent, and
the excess o-xylene was extracted by chloroform (3 × 20 mL). The crude product was washed
with distilled water (3 × 20 mL) and dried over anhydrous sodium sulfate. The residue was
purified by chromatography using petroleum ether-ethyl acetate (3:2) to give white crystalline.
1
13
The identidy and purity of the product was confirmed by H and C NMR spectroscopic
analysis.
5b, 5c and 5d were prepared using similar procedure as for 5a except for using
thiophene-2-carbaldehyde (b), furan-2-carbaldehyde (c) or picolinaldehyde (d) instead of
pyridine-4-carbaldehyde.
1
2
, 6-di(pyridin-4-yl)-benzo[1, 2-d:4, 5-d']bisoxazole (5a).Yield 81 % (508.7 mg). H
13
NMR (400 MHz, CDCl , 25°C, TMS): δ = 8.87 (d, 4H), 8.14 (d, 4H), 8.04 (s, 2H) ppm. C
3
NMR (400 MHz, CDCl ): δ = 163.32, 149.88, 142.41, 136.25, 133.26, 121.53, 116.19 ppm.
3
MS (EI): m/z = 314. Analysis for C H N O (332.32): calcd. C, 65.06; H, 3.64; N, 16.86.
1
8
12
4
3
Found, C, 65.13; H, 3.76; N, 16.94.
, 6-di(thiophene-2-yl)-benzo[1, 2-d:4, 5-d']bisoxazole (5b). Yield 87.2 % (535.9 mg).
H NMR (400 MHz, CDCl , 25°C, TMS): δ = 7.97 (q, J = 4.8 Hz, J = 3.2 Hz, 2H), 7.86 (s,
2
1
3
1
2
13
2
H), 7.61 (q, J = 6.4 Hz, J = 1.6 Hz, 2H), 7.23 (q, J = 10.18 Hz, J = 3.6 Hz, 2H) ppm. C
1 2 1 2
NMR (400 MHz, CDCl ): δ = 160.22, 148.32, 140.28, 136.25, 130.61, 130.17, 129.47, 128.40,
3
1
00.66 ppm. MS (EI): m/z = 324.
, 6-di(furan-2-yl)-benzo[1, 2-d:4, 5-d']bisoxazole (5c). Yield 67.1 % (391.9 mg). H
NMR (400 MHz, CDCl , 25°C, TMS): δ = 7.89 (s, 2H), 7.71 (t, J = 0.8 Hz, 2H), 7.33 (t, J=
1
2
3
1
3
1
.6 Hz, 2H), 6.66 (t, J=1.8 Hz, 2H) ppm. C NMR (400 MHz, CDCl ): δ = 156.46, 148.08,
3
1
46.07, 142.35, 140.06, 114.76, 112.44, 101.11 ppm. MS (EI): m/z = 292.
1
2
, 6-di(pyridin-2-yl)benzo[1, 2-d:4, 5-d']bisoxazole (5d). Yield 81.0 % (508.7 mg). H
NMR (400 MHz, CDCl , 25 ꢀ, TMS): δ = 8.55 (t, J = 1.2 Hz, 2H), 7.93 (m, 2H), 7.38 (t, J=
3
1
3
1
1
2
.8 Hz, 2H), 7.19 (m, 2H) ppm. C NMR (400 MHz, CDCl ): δ = 165.24, 155.80, 151.46,
3
37.93, 133.61, 126.35, 121.48, 118.26 ppm. MS (EI): m/z = 314.
.3 Computational method.
In order to understand the electronic structure of compounds 5a and 5b further, Density
5

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Functional Theory (DFT) calculation of the target 2,6-disubstituted benzobisoxazoles has
been carried out using Gaussian 09 program package [11], such as optimized geometries and
corresponding molecule orbitals (MOs) energy for 5a and 5b performed with the aid of the
Gauss View 5.0 visualisation software at the B3LYP level [12,13]. The 6-31G(d) basis set for
C, H, O, N and S atoms were employed. Vertical electronic excitations based on the
DFT-B3LYP optimised geometry were calculated using the Time-Dependent Density
Functional Theory (TD-DFT) formalism [14,15] in CH Cl using conductor-like polarisable
2
2
continuum model (CPCM) [16]. Molecular Hirshfeld surface calculations were performed by
using the CrystalExplorer 3.1 program, and all the Hirshfeld surfaces were generated using a
high surface resolution[17].
2
.4 X-ray Crystallography
Suitable colorless needle-like crystals of 5a were obtained by vapour diffusion of ethyl
acetate and methanol solution for two weeks. X-ray diffraction measurement for 5a was
collected on Agilent SuperNova diffractometer equipped with graphite monochromated
Mo-Kα radiation (λ = 0.71073 Å). Data reduction and cell parameter refinement were
performed using OLEX2 [18]. The structures were solved using Fourier difference techniques
2
and refined by full-matrix least-squares methods on F data [19,20]. All hydrogen atoms were
added in calculated positions except for the hydrogen atoms of the water molecule. Relevant
crystallographic data together with refinement details are listed in Table 1.
Table 1 Crystallographic data and structure refinement parameters of 5a.
Empirical formula
C H N O
18 12 4 3
Formula weight
332.32
Temperature (K)
273.4(6)
0.71073
Wavelength (Å)
Crystal system
monoclinic
C2/c
Space group
Unit cell dimensions (Å, º)
a
b
c
β
12.7875(17)
3.7629(6)
30.418(4)
94.125(13)
1459.9(4), 4
1.512
3
Volume (Å ), Z
3
Calculated density(Mg /m )
–
1
Absorption coefficient (mm )
0.107
F(000)
688
3
Crystal size (mm )
0.19 × 0.05 × 0.04
6

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θ Range for data collection (°)
4.0240–24.9920
Index ranges
–15 ≤ h ≤ 9; –4 ≤ k ≤ 2; –35 ≤ l ≤ 37
Reflections collected
Independent reflections
Completeness to θ = 26.32 (%)
Data/restraints/parameters
2524
1440 [R(int) = 0.0336]
99.54
1440/0/118
2
Goodness-of-fit on F
1.055
Final R indices [I >2σ(I)]
R indices (all data)
R = 0.0448, wR = 0.1163
1
2
R = 0.0574, wR = 0.1231
1
2
-3
Largest difference peak and hole(e Å )
0.191 and – 0.302
2
2
2
2
2
w = 1/[s (F )+ (0.0697P) +1.6634P], Where P = (F + 2F )/3.
o
o
c
3
. Results and discussion
As shown in Scheme 1, compounds 5a-5d were synthesized with 4-methoxy-TEMPO as
a free radical catalyst using a one-pot reaction and 5b only reported previously [21].
-Methoxy-TEMPO radical is widely used for the selective oxidation of primary or secondary
4
alcohols [22,23]. 4-Methoxy-2, 2, 6, 6-tetramethyl-1-piperidinyloxy (4-methoxy-TEMPOH)
can be slowly oxidized to 4-methoxy-TEMPO radical in air at room temperature, so a strategy
for the aerobic catalytic oxidative synthesis of benzoxazole using 4-methoxy-TEMPO as the
catalyst was developed [4].
3
.1 Benzodioxazole synthesis mechanism
As shown in Scheme 2, the synthesis of mono-benzoxazole catalyzed aerobicly by
-methoxy-TEMPOH oxygen free radical was proposed [24]. The catalytic reaction begins
4
with the reaction of 4-methoxy-TEMPO with 2-aminophenol [25,26]. In the first step,
2
4
-aminophenol reacts with of the aromatic aldehyde through Schiff base reaction. The
-methoxy-TEMPO catalyst attack the H atom on the phenolic hydroxyl group to generate
active phenoxy radicals and 4-methoxy-TEMPOH [27,28]. At the same time, the generated
-methoxy-TEMPOH can be reoxidized by air O to 4-methoxy-TEMPO and reused [29]. In
4
2
the third step, the phenoxy free radical attack molecules within the imine double bond, a
five-membered ring addition reaction takes place and generates reactive amine radical. In the
fourth step, 4-methoxy-TEMPO catalyst offensive activity amino radical on the tertiary
carbon atoms in the H atom, regenerate the imine double bond, and yield the final product of
the target mono-benzoxazole molecule.
7

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Scheme 2 A proposed mechanism for 4-methoxy-TEMPO-catalyzed aerobic oxidative synthesis of
2-substituted benzoxazoles.
Scheme 3 A proposed mechanism for 4-methoxy-TEMPO-catalyzed aerobic oxidative synthesis of 2,
′-disubstituted benzoxdiazoles.
2
The synthesis mechanism of benzobisoxazole derivatives containing heteroaromatic ring
substituents catalyzed by 4-methoxy-TEMPO was further studied as shown in Scheme 3.
Firstly, the 2, 5-diaminohydroquinone reacts with twice the equivalent of aromatic aldehyde
to generate a Schiff-base containing two symmetrical imine double bonds [30].
4
-Methoxy-TEMPO catalyst then attacks the H atom on the two phenolic hydroxyl groups on
the benzene ring to form two active phenoxy radicals and 4-methoxy-TEMPOH [31]. At the
same time, the generated 4-methoxy-TEMPOH can be re-oxidized by O in the air to
2
8

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-methoxy-TEMPO for reuse; and then two phenoxy oxygen free base respectively attack the
4
molecule within its adjacent imine double bond, five-membered cycloaddition reaction, while
generating two active amine free radicals. Finally, 4-methoxy-TEMPO catalyst attacks the H
atom on the tertiary carbon atom in the molecule and regenerates the imine double bond to
finally obtain the target benzoxobioxazole derivative [32].
In synthetic experiments, we found that when 2-aldehyde-based imidazole and 2,
5
-diaminohydroquinone were reacted under the same experimental conditions, the target
molecule could not be obtained. One of the reasons is that the solubility of 2-allyl imidazole
in the solvent o-xylene is very small. Therefore, the reaction between the 2-aldehyde
imidazole and 2, 5-diamino-p-phenol did not occur. The second reason is that due to the
presence of active H atoms on the amine groups in the 2-aldehyde-based imidazole molecules,
they are readily linked to 4-methoxy-TEMPO thereby reducing the free radical catalyst and 2,
5
-diaminohydroquinone phenolic hydroxyl reaction efficiency, and ultimately lead to the
failure of free-radical catalyst. On the other hand, this further confirms the rationality of the
proposed mechanism of using the free radical catalysis.
3
.2 Crystal structure determination
X-ray crystallographic analysis reveals that 5a crystallizes in the form of a hydrate and in
the monoclinic system with space group C2/c. The molecular structure and intermolecular
hydrogen bonds of 5a are depicted in Fig. 1. Selected bond lengths and angles from X-ray
diffraction as well as DFT optimization geometric parameters of 5a are compared in Table 2.
Fig. 1 Crystal structure and intermolecular hydrogen bond of 5a with 30 % thermal ellipsoids probability.
The dihedral angles defined by the central benzo[1, 2-d:4, 5-d']bisoxazole ring plane
both left and right side planes have the same value, which is only 6.32°. The distance between
the center of both sides pyridine ring is about 0.198 Å, showing that 5a has a good coplanar
property owing to the absence of steric hindrance of alkyl chains [33]. There are two pyridine
rings and two oxazole rings in the crystal structure of 5a. The dihedral angles between the
9

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middle benzene ring and each side oxazole rings are equal 0.22°. The distance between the
middle benzene ring and each side pyridine ring are equal 6.04 Å. The two torsion angles
C4–C3–C6–O1 and C2–C3–C6–N1 are practically the same (the mean angle is 180°). Thus it
can be seen that the molecule is center symmetrical structure on the benzene ring centroid as
the center of symmetry.
Table 2 Selected bond distances (Å) and angles (º) from X-ray diffraction and DFT optimization of 5a.
Bond lengths
O1–C6
Experimental Theoretical Bond lengths
Experimental Theoretical
1.351(4)
1.306(4)
1.336(5)
1.383(5)
1.385(5)
1.379(5)
1.371(4)
1.3750
1.3016
1.3422
1.3935
1.4032
1.3914
1.3844
O1–C7
N1–C8
N2–C5
C2–C3
C3–C6
C7–C8
C8–C9
1.389(4)
1.392(3)
1.342(5)
1.392(4)
1.452(4)
1.402(4)
1.386(4)
1.3736
1.3893
1.3443
1.4023
1.4606
1.4168
1.4009
N1–C6
N2–C1
C1–C2
C3–C4
C4–C5
C7–C17
Bond angles
C6–O1–C7
C1–N2–C5
C1–C2–C3
C2–C3–C6
C3–C4–C5
O1–C6–N1
N1–C6–C3
O1–C7–C17
N1–C8–C7
C8–C7–C9
Experimental Theoretical Bond angles
Experimental Theoretical
104.4(2)
116.7(3)
118.9(3)
121.8(3)
118.8(3)
115.2(3)
124.9(3)
128.4(2)
107.8(2)
122.9(2)
104.6674
117.8061
118.5496
121.5872
118.6088
114.9381
127.5594
127.5685
108.4299
121.5050
C6–N1–C8
N2–C1–C2
C2–C3–C4
C4–C3–C6
N2–C5–C4
O1–C6–C3
O1–C7–C8
C8–C7–C17
N1–C8–C9
C8–C9–C7
105.3(3)
123.6(3)
118.0(3)
120.2(3)
124.0(3)
119.9(3)
107.4(2)
124.2(3)
129.3(2)
113.0(2)
104.9958
123.3547
118.3592
120.0536
123.3216
117.5025
106.9689
125.4626
130.0651
113.0282
Torsion angles Experimental Theoretical Torsion angles Experimental Theoretical
O1–C7–C8–N1 –0.2(3)
C8–N1–C6–C3 –178.6(3)
C1–N2–C5–C4 0.5(5)
C1–C2–C3–C6 –179.7(3)
C2–C3–C6–O1 6.4(5)
C3–C4–C5–N2 –0.1(5)
C4–C3–C6–N1 3.7(5)
C6–O1–C7–C8 –0.1(3)
C6–N1–C8–C7 0.4(3)
C6–C3–C4–C5 180.0(3)
C7–O1–C6–C3 178.5(3)
0.0042
180.0012
0.0007
–180.0
0.0002
0.0007
0.0019
–0.0038
–0.0027
180.0
O1–C7–C8–C9 –179.9(3)
N2–C1–C2–C3 –0.5(5)
C1–C2–C3–C4 0.9(5)
C2–C3–C4–C5 –0.7(5)
C2–C3–C6–N1 –175.7(3)
C4–C3–C6–O1 –174.3(3)
C5–N2–C1–C2 –0.2(5)
C8–N1–C6–O1 –0.5(4)
C6–N1–C8–C9 –179.9(3)
C7–O1–C6–N1 0.4(4)
180.0035
0.0004
0.0011
–0.0016
–180.001
–179.9
–0.0013
0.0002
–180.002
0.0024
–180.0
Table 3 Hydrogen-bonding distances (Å) and angles (º) of 5a.
D−H···A
d(D–H)
d(H···A)
d(D···A)
∠ D–H···A
Symmetry code
x, y, z
O2−H2A···N2
1.00(5)
2.09(6)
2.970(4)
146(5)
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0

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There is only one intermolecular O2–H2A···N2 hydrogen bond, which between the
pyridine ring and non-coordinated water molecule (Fig. 1, Table 3). Molecules of 5a are
connected by extensive π···π stacking interactions, which play significant roles in the crystal
packing modes, face-to-face π···π stacking interactions between neighboring benzene rings
and oxazole rings with distances between the centroids of 3.763 Å and 3.764 Å [34] (Fig. 2,
Table 4). Overall, with the help of intermolecular hydrogen bonds and π···π stacking
interactions, adjacent molecules also can be linked together into an infinite two-dimensional
(2-D) layer supramolecular structure along the bc-plane (Fig. 3).
Fig. 2. Face-to-face π···π stacking of 5a.
Table 4 π-π stacking interactions (Å, º) in the crystalline state of 5a.
RingI
Cg1
Cg1
Cg2
Cg3
Cg3
RingJ
Cg1
Cg3
Cg2
Cg1
Cg3
CgI···CgJ
3.7631(18)
3.7638(18)
3.763(2)
CgI−Perp
Cg−JPerp
Slippage
1.630
−
3.3916(12) −3.3917(12)
3.3880(12) 3.3943(12)
3.4589(13) −3.4588(13)
3.3943(12) 3.3882(12)
3.3915(12) 3.3915(12)
1.482
−
3.7640(18)
3.7629(18)
1.630
Cg1, Cg2 and Cg3 are the centroids for O1–C6–N1–C8–C7, N2–C1–C2–C3–C4–C5 and
C7–C8–C9–C7A–C8A–C9A, respectively. CgI···CgJ is the distance between ring centroids. CgI-Perp is the
perpendicular distance of CgI on ring J. CgJ-Perp is the perpendicular distance of CgJ on ring I. Slippage is
the distance between CgI and perpendicular projection of CgJ on ring I.
11

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Fig. 3 View of an infinite 2-D layer supramolecular structure of 5a (hydrogen atoms are omitted for clarity
except those forming hydrogen bonds).
3
.3 UV–Vis absorption spectra and TDDFT calculations
UV–Vis absorption spectra of 5a and 5b were determined in 5.0 ×10 mol L CH Cl
–
5
–1
2
2
solution at room temperature. As shown in Fig. 4 and Fig.S1, there were very similar triple
absorption peak appeared between 320 and 400 nm of two compounds. Compared with 5b,
–1
–1
three peaks were observed at 328, 341 and 357 nm (ε = 1,4860, 1,9998, 1,5720 mol L cm ),
respectively in the spectrum of 5a, while three peaks centered at 344, 360 and 381 nm (ε =
–1
–1
1
,9040, 2,3962, 1,8864 mol L cm ), respectively in 5b. The maximum UV absorption peaks
of two compounds produce a blue shift, mainly because the pyridine ring substituent is an
electron-withdrawing group, which can reduce the charge density on the benzoxazole skeleton
and reduce the degree of conjugation. In contrast, the thiophene ring substituent is an
electron-donating group, which leads to an increase in the charge density on the benzoxazole
skeleton and an increase in the degree of conjugation, so that the maximum UV-absorbing
peak will undergo a significant red shift [21, 35]. The absorption edge (λedge) of 5a and 5b are
opt
at 370 and 385 nm, corresponding to an optical band gap (Eg ) of 3.351 and 3.221 eV [36].
Time dependent DFT calculations have been done on the optimised geometry to further
understand the electronic transitions in UV–Vis spectra of compounds in CH Cl and
2
2
compared with experimentally observed absorption bands. The UV–Vis spectral analysis of
a and 5b are calculated by TDDFT/B3LYP method and the calculated vertical electronic
5
excitation energy (eV) and oscillator strength (f) are summarised in Table 5. The experimental
absorption bands of 5a and 5b have been explained with help of TD-DFT calculations. The
peak observed at 357 nm is identified to intra-ligand charge transfer (ILCT) character
corresponds to (70 %) HOMO → LUMO transitions respectively in 5a. The peaks observed at
1
2

ACCEPTED MANUSCRIPT
41 and 328 nm by experimentally are also attributable to ILCT character amounts to (69 %)
3
HOMO–1 → LUMO and (70 %) HOMO → LUMO+1 transitions in 5a. The simulated broad
peak at 388.12 nm whereas experimental peak at 380 nm in 5b corresponds to (70 %) HOMO
→
LUMO transitions [37]. Conclusively, theoretical simulation shows good agreement with
the experimental phenomenon.
−
5
−1
Fig. 4 UV–Vis absorption spectra in CH Cl (5 × 10 mol L ): 5a (a). Theoretical simulation of UV–vis
2
2
absorption spectra: 5a (b) and 5b (c), and the oscillator strength of main excited states shown in the red
color.
Table 5 TD-DFT calculated electronic excitations of 5a and 5b in CH Cl .
2
2
Excitation (eV) λexcitation (nm) Osc.strength (f)
Key transitions
Character
λexpt
5
a
3.3951 eV
3.8761 eV
4.0753 eV
365.18
319.87
304.24
1.5199
0.0373
0.0090
(70%) HOMO → LUMO
L (π) → L (π*)
357
341
328
1
1
(69%) HOMO
–
1 1
1 → LUMO L (π) → L (π*)
(70%) HOMO → LUMO+1 L (π) → L (π*)
1
1
5
b
3.1945
4.0672
4.6072
388.12
304.84
269.11
1.5685
0.0211
0.0604
(70%) HOMO → LUMO
L (π) → L (π*)
380
361
344
1
1
(68%) HOMO
(64%) HOMO
–
–
2 → LUMO L (π) → L (π*)
1 1
4 → LUMO L (π) → L (π*)
1
1
3
.4 Fluorescence spectra
–5
–1
Fluorescent behaviors of compounds were recorded in 5.0 ×10 mol L CH Cl solution
2
2
at ambient temperature (Fig. S2). Both of 5a and 5b showed broad emission bands indicating
charge transfer nature of the transitions. The fluorescence spectrum of 5a shows a maximum
peak at 413 nm, a shoulder peak at 436 nm and another peak at 392 nm upon excitation at 360
nm. Similarly, 5b shows a maximum peak at 408 nm, a shoulder peak at 428 nm and another
peak at 387 nm. The Stokes shift between the maximum wavelength of the absorbance and
fluorescence spectrum is 72 nm of 5a while 47 nm of 5b [21, 34]. Due to the benzoxazole
ring participates in the conjugate molecule, it enhances the degree of conjugation and
promotes the enhancement of the fluorescence intensity [38].
1
3

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3
.5 Theoretical investigations
To gain deeper insight into the frontier molecular orbital energies of the compounds, the
structures were optimized by the Gaussian 09 package using density functional theory (DFT)
method with the basis set of B3LYP/6-31G(d) based on the single-crystal X-ray diffraction
structure data of 5a and 5b. The selected geometrical parameters are presented in Table 2 and
the data are well replicated with the experimental data having minor deviations in bond
lengths and angles. The optimized geometry of two planar symmetric compounds are shown
in Fig. 5. As shown in Fig. 6, the lowest unoccupied molecular orbitals (LUMO) and the
highest occupied molecular orbitals (HOMO) of the compounds are not significantly
influenced by the substituents, and these molecular orbitals are clearly not different between
5
a and 5b [39,40]. The energies of some selected molecular orbits for 5a and 5b are shown in
Table 6, from LUMO–1 to HOMO+1 principally. According to Table 6, the frontier molecular
orbital energies of the HOMO–1, HOMO, LUMO, and LUMO+1 for 5a are –7.138, –6.515,
–
2.693, and –2.011 eV, respectively. The mentioned energy HOMO–LUMO gap is 3.822 eV,
which implies high kinetic stability and low chemical reactivity [41]. The HOMO energy
level of 5b is –5.922 eV while the LUMO energy level of 5b is –2.342 eV. The
HOMO–LUMO gap of 5b is 3.580 eV. In other words, it is energetically disadvantageous to
add electron to the high-lying LUMO (–2.693, –2.342 eV), or to extract electron from the
low-lying HOMO (–6.515, –5.922 eV) [42–44]. The occupied molecular orbital energies are
all negative, demonstrating the chemical stability of the structure [45]. A theoretical
investigation utilizing DFT allows rationalization of experimental findings [46].
Fig. 5 Optimized molecular structure of 5a (left) and 5b (right).
1
4

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Fig. 6 Surface plots of some selected HOMO-LUMO structures molecular orbitals of 5a (left) and 5b
(right).
Table 6 Absorption spectra and HOMO-LUMO structures orbital energies of 5a and 5b.
opt
a
Compound λedge (nm) Eg (eV)
EHOMO–1 (eV) EHOMO (eV) ELUMO (eV)
ELUMO+1 (eV) ꢀEgap (eV)
5
5
a
370
385
3.351
3.221
–7.138
–6.929
–6.515
–5.922
–2.693
–2.342
–2.011
–1.698
3.822
3.580
b
a
opt
Eg estimated from the UV–vis absorption spectrum.
3
.6 Hirshfeld surfaces analysis (HSA)
Intermolecular forces between molecules packed in lattice were computed using the CIF
under HSA condition to figure out the surface spots (Figs. 7 and 8). Fig. 7 indicates surfaces
which have been mapped over dnorm, shape index and curvedness. To visualize the molecular
moiety, around which they were calculated, the surfaces are shown as transparent. The
detected of H-bonds types and number such as H···H, C···H/H···C, O···H/H···O,
N···H/H···N and S···H/H···S surfaces of 5a and 5b were recorded as main H–bond type [47].
Fig. 8 shows the two-dimensional fingerprint plots generated correspond to the H···H, C···H,
1
5

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O···H, N···H and S···H interactions appearing as distinct spikes of 5a and 5b. Additionally,
the H···H contacts are discovered to be the largest contribute with 34.9 % and 23.7 % of 5a
and 5b, respectively. Comparatively, the proportion of O···H/H···O, N···H/H···N, C···C and
C···H/H···C interactions comprising 15.4 %, 15.0 %, 12.3 % and 11.4 % of the entire HSA
surface for each molecule of 5a, respectively. Simultaneously, 5b also has the extents of
S···H/H···S, C···H/H···C, N···H/H···N and O···H/H···O interactions cover 15.8 %, 13.4 %,
1
2.3 % and 10.2 %, respectively. Due to the existence of these hydrogen bonding contacts, the
structures of both compounds can be stabilized [48].
Fig. 7. Hirshfeld surfaces mapped with dnorm and shape index of 5a and 5b.
1
6

ACCEPTED MANUSCRIPT
Fig. 8. 2D fingerprint plots of 5a and 5b showing the percentages of contacts contributed on the total HSA
surface.
4
. Conclusion
In summary, we have successfully synthesized and characterized 2, 6-disubstituted
benzobisoxazole by using an effective and clean aerobic oxidative approach with
4
-methoxy-TEMPO as the catalyst. Especially, 2, 6-di(pyridin-4-yl)-benzo[1, 2-d:4,
-d']bisoxazole (5a) was confirmed by single-crystal X-ray diffraction analyses. On the basis
5
1
7

ACCEPTED MANUSCRIPT
of experimental results, a plausible reaction mechanism of 4-methoxy-TEMPO catalyzed
aerobic oxidation to 2,2'-disubstituted benzoxdiazoles was proposed. DFT and TD-DFT
calculations have also been performed to determine the origin of the spectral behavior of
compound 5a and 5b. Experimental XRD-crystal packing and Hirshfeld surface computed
analysis agreed on several hydrogen-bonds formation in compounds lattice. We will focus on
the synthesis of other useful heterocycles based on this material, as well as device
construction and evaluation of the electronic, photoluminescence and thermostability
properties of oligomers and polymers containing the benzobisoxazole moiety.
Appendix A. Supplementary data
CCDC 1442771 contains the supplementary crystallographic data for compound a. These
data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif., or from
the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge, B2 1EZ, UK; fax
+
44 1223 336033; or E-mail: deposit@ccdc.cam.ac.uk.
Acknowledgments
This work was supported by the Fundamental Research Funds of Gansu Province
Universities [grant number 214152]; the Foundation of A Hundred Youth Talents Training
Program of Lanzhou Jiaotong University [grant number 152022]; and the Graduate Student
Education Innovation Project of Lanzhou Jiaotong University [grant number 16001201],
which are gratefully acknowledged.
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HIGHLIGHTS
•
2, 6-Disubstituted benzobisoxazole using an effective and clean aerobic
oxidative approach were synthesized and structural characterized by
spectroscopic methods and single-crystal X-ray diffraction.
•
•
•
A plausible reaction mechanism for 4-methoxy-TEMPO-catalyzed aerobic
oxidative synthesis of 2, 2'-disubstituted benzoxdiazoles is proposed.
The structures of compounds are interesting and Hirshfeld surface analyses of
5a and 5b were also measured and discussed.
Time dependent DFT calculations on the optimized structure of compounds 5a
and 5b allowed for the assignment of specific electronic transitions in the
UV–Vis spectra.