Dimesitylborylphenyl substituted 1,3,4-oxadiazole: Synthesis, characterization and sensitivity to F-

Article abstract of DOI:10.1016/j.cclet.2010.10.053

A new 1,3,4-oxadiazole compound with a dimesitylboryl group was synthesized and characterized by ¹H NMR, MS, element analysis and X-ray single-crystal diffraction. It exhibits good blue emission at 425 nm and high sensitivity to fluoride anion.

Full text of DOI:10.1016/j.cclet.2010.10.053

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Chinese Chemical Letters 22 (2011) 599–602
www.elsevier.com/locate/cclet
Dimesitylborylphenyl substituted 1,3,4-oxadiazole: Synthesis,
characterization and sensitivity to Fꢀ
*
Xiang Hong Li , Bo Xu, Kang Le Lv, Ai Qing Zhang
College of Chemistry and Materials, South-Central University for Nationalities, Wuhan 430074, China
Received 31 August 2010
Available online 2 March 2011
Abstract
A new 1,3,4-oxadiazole compound with a dimesitylboryl group was synthesized and characterized by 1H NMR, MS, element
analysis and X-ray single-crystal diffraction. It exhibits good blue emission at 425 nm and high sensitivity to fluoride anion.
# 2010 Xiang Hong Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: 1,3,4-Oxadiazole; Dimesitylboryl; Fluoride anion; Chemo-sensor
Owing to the vacant p-orbital of heteroatom boron, dimesitylboryl compounds containing three-coordinated
organoboron groups exhibit interesting fluorescence emission. Design and synthesis of new dimesitylboryl
compounds have been of interest to chemists for recent years because of their potential applications in OLEDs [1–3],
fluoride ion chemo-sensors [4–6] and optical materials [7]. To explore diverse properties of dimesitylboryl
compounds, thiophene [1,3b], fluorene [3b], naphthylphenylamine [2,3a], SiPh4 group [6b] and heavy metal
complexes [5,6] were introduced respectively. However, there were no reports about oxadiazole compounds
containing dimesitylboryl groups.
Herein, we designed phenyl as p-bridges and synthesized a new 1,3,4-oxadiazole compound with dimesitylboryl
group: 2-(4-tert-butylphenyl)-5-(4-(dimesitylboryl)phenyl)-1,3,4-oxadiazole (abbreviated as TBDO, Scheme 1). 4-
Bromophenylterazole was synthesized according to Ref. [8]. When 4-bromophenylterazole and 4-tert-Butylbenzoyl
chloride were refluxed in pyridine under argon, 2-(4-bromophenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(abbreviated as BTBO) was obtained with a yield of 90% [9]. Then, n-BuLi was added slowly into a suspension
of the BTBO in anhydrous THF at ꢀ78 8C. Dimesitylboron fluoride (FB(Mes)2) in THF was added by injection
successively. The mixture was stirred overnight without a cooling bath to give TBDO as white solids (yield: 54%) [10].
From X-ray single-crystal diffraction, the colorless crystal belongs to monoclinic. As shown in Fig. 1, for
compound TBDO, the central boron and its three bonded carbon atoms are also perfectly coplanar and form a trigonal
BC3 plane, with the sum of the three C–B–C angles (359.998) being very close to 3608. This indicates a sp2 hybrid
format for the atomic orbitals of the central boron. Around the central boron, three benzene ring planes are arranged
in a propeller-like fashion, which can form a good protection to the central boron. The bond lengths of B1–C7 and
* Corresponding author.
E-mail address: lixhchem@yahoo.com.cn (X.H. Li).
1001-8417/$ – see front matter # 2010 Xiang Hong Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.053

6[()TD$FIG] 00
X.H. Li et al. / Chinese Chemical Letters 22 (2011) 599–602
N
N
a
b
NH
N
Br
Br
CN
c
O
O
Br
B
N N
N N
BTBO
TBDO
Scheme 1. The synthetic route of the compound TBDO. Conditions and reagents: (a) NaN3, NH4Cl/DMF, 100 8C, 20 h; (b) 4-tert-butylbenzoyl
chloride, pyridine, reflux 2 h; (c) n-BuLi, FB(Mes)2, THF, ꢀ78 8C ! r.t., 24 h.
[()TD$FIG]
Fig. 1. ORTEP diagram of TBDO (CCDC 788677) with the thermal ellipsoids drawn at the 30% probability level and the H atoms removed for
clarity.
˚
˚
B1–C18 are 1.567(5) and 1.576(5) A, respectively. However, that of B1–C19 is reduced to 1.559(5) A, indicating
some additional p-bonding between trivalent boron and 1,3,4-oxadiazole.
In THF solution, TBDO shows strong absorption centered at 331 nm due to p–p* transitions. When excited at
330 nm, it exhibits good blue emission at 425 nm with quantum efficiency 0.37 (1 mol/L quinine sulfate as standard).
TBDO stands for a fluoride sensor, which shows unique blue-shift absorptions and fluorescence quenching properties
after the addition of fluoride anions (Figs. 2 and 3). As shown in Fig. 2, the addition of tetrabutylammonium fluoride
(TBAF) to the TBDO solutions leads to a significant blue-shifted absorption from 330 nm to 310 nm. And the strong
emission bands of TBDO centered at 425 nm gradually decrease (Fig. 3). It is worth to mention that the fluorescence
was quenched immediately after the addition of isometric fluoride anions (Fig. 3, inset). The fluorescence response
time of TBDO to isometric Fꢀ is only a few seconds. Generally, the binding constant can be easily calculated by fitting
the titration curve of both absorption and emission spectra [11]. Herein, the binding constant (K) of TBDO with Fꢀ was
determined as 3.1 ꢁ 105 molꢀ1. All these facts indicate that TBDO can be used as a good fluoride anion sensor with
high sensitivity.
The selectivity of the interaction was also investigated using a series of anions in addition to AcOꢀ, Clꢀ, Brꢀ, Iꢀ,
HSO4ꢀ, HCO3ꢀ and H2PO4ꢀ. However, very weak variations of absorption and emission spectra were observed upon
ꢀ
addition of Clꢀ, Brꢀ, Iꢀ, AcOꢀ, HSO4ꢀ, HCO3 and H2PO4ꢀ. Therefore, TBDO exhibited a high selectivity in
sensing Fꢀ. Furthermore, the competition experiment was also carried out by adding Fꢀ to the solutions of TBDO in
the presence of other anions. As shown in Fig. 4, whether in the absence or presence of the other anions, obvious
emission intensity changes were observed for TBDO only upon the addition of Fꢀ. The results indicate that the sensing
of Fꢀ by TBDO is hardly affected by these commonly coexistent anions. It is attributed to the strong affinity between

[()TD$FIG]
X.H. Li et al. / Chinese Chemical Letters 22 (2011) 599–602
601
1.0
330 nm
310 nm
0.8
0.6
0.4
0.2
0.0
250
300
350
400
Wavelength (nm)
Fig. 2. The absorption changes of TBDO (c = 2 ꢁ 10ꢀ5 mol/L) upon addition of Fꢀ in THF.
[()TD$FIG]
1.0
0
0.8
F-
800
0.6
0.4
0.2
600
400
200
0
2 eq.
0
5
10 15 20 25 30 35 40
t (s)
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. The emission changes of TBDO (c = 1 ꢁ 10ꢀ5 mol/L) upon addition of Fꢀ in THF (lex = 330 nm). Inset: The fluorescence response time of
the TBDO to isometric Fꢀ.
[()TD$FIG]
1.2
0.8
0.4
0.0
F- AcO-
Br- Cl-
I- HSO4- HCO3- H2PO4-
Free
Fig. 4. Luminescent responses of TBDO (c = 1 ꢁ 10ꢀ5 mol/L) to various anions in THF. Bars represent the luminescent intensity I/I0 at 425 nm.
Black bars represent the addition of anions (5 equiv.) to the solutions of TBDO. White bars represent the addition of Fꢀ (1 equiv.) to the above
solutions.

602
X.H. Li et al. / Chinese Chemical Letters 22 (2011) 599–602
trivalent B atom and Fꢀ, which are typical ‘‘acid’’ and ‘‘base’’, respectively. On the other hand, the special steric
structure of dimesityl group may make some unique contribution [4b]. This contribution is also supported by
previously reported crystal structure data [12].
Acknowledgments
We are grateful to the Hubei Provincial Natural Science Foundation (No. BZY09008) and the Natural Science
Foundation of South-Central University for Nationalities (No. YZZ06022).
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