Turn-on colorimetric sensing of fluoride ions by a cationic triarylborane bearing benzothiazolium

Article abstract of DOI:10.1016/j.jorganchem.2012.04.020

A cationic triarylborane Lewis acid bearing benzothiazolium moiety, 2-(4′-dimesitylborylphenyl)-3-methylbenzo[d]thiazol-3-ium ([2] ⁺) was prepared from a neutral borane (2a) and their crystal structures were determined from X-ray diffraction studies. While 2a shows a blue-shift with a small decrease in the UV-vis absorption band upon fluoride binding to the boron atom, [2]⁺ undergoes a red-shift of the absorption band which tails over visible region, giving rise to the color change of the solution from colorless to yellow. The fluoride binding constant in THF/H₂O (9/1, v/v) is calculated to be 1.3 × 10⁴ M^-1 that is much greater than that of a neutral borane 2a (K = 4.2 × 10² M^-1). DFT calculation results suggest that the absorption process in the fluoroborate (2F) is involved with π(Mes)→π*(phenylbenzothiazolium) intramolecular charge transfer and the greater elevation of the π(Mes) donor level is responsible for the turn-on colorimetric response of [2]⁺.

Full text of DOI:10.1016/j.jorganchem.2012.04.020

Journal of Organometallic Chemistry 713 (2012) 89e95
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Turn-on colorimetric sensing of fluoride ions by a cationic triarylborane bearing
benzothiazolium
,
a,
**
Ki Cheol Song a, Kang Mun Lee a, Hyungjun Kim a, Yoon Sup Lee a, Min Hyung Lee b , Youngkyu Do
*
a Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
b Department of Chemistry and Energy Harvest-Storage Research Center, University of Ulsan, Ulsan 680-749, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
A cationic triarylborane Lewis acid bearing benzothiazolium moiety, 2-(40-dimesitylborylphenyl)-3-
methylbenzo[d]thiazol-3-ium ([2]þ) was prepared from a neutral borane (2a) and their crystal struc-
tures were determined from X-ray diffraction studies. While 2a shows a blue-shift with a small decrease
in the UVevis absorption band upon fluoride binding to the boron atom, [2]þ undergoes a red-shift of the
absorption band which tails over visible region, giving rise to the color change of the solution from
colorless to yellow. The fluoride binding constant in THF/H2O (9/1, v/v) is calculated to be 1.3 ꢀ 104 Mꢁ1
Article history:
Received 1 February 2012
Received in revised form
5 April 2012
Accepted 19 April 2012
Keywords:
that is much greater than that of a neutral borane 2a (K ¼ 4.2 ꢀ 102
M
ꢁ1). DFT calculation results suggest
(Mes)/ *(phenyl-
(Mes) donor level is
Benzothiazolium
Colorimetric sensing
Fluoride
Triarylborane
Turn-on sensor
that the absorption process in the fluoroborate (2F) is involved with
benzothiazolium) intramolecular charge transfer and the greater elevation of the
responsible for the turn-on colorimetric response of [2]þ.
p
p
p
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
involved with the introduction of electron-withdrawing groups
such as o-carborane cages [34,35] and cationic substituents
[30,36e46] into triarylboranes or with the use of metal chelation
[31,47], all of which were found to be effective in enhancing the
Lewis acidity of a boron center. In particular, the cationic triar-
ylboranes have been shown to increase largely the fluorophilicity of
the boron center enough to be operating in aqueous media due to
the favorable Coulombic attractions that assist BꢁF dative inter-
actions [38,39,41,43e45].
On the other hand, development of a new design principle that
may lead to the efficient detection of fluoride at a low level of
concentration has been also equally regarded important from an
analytical point of view. In most cases, triarylboranes are conju-
gated with aromatic fluorophore (or chromophore) through the
empty pp orbital of boron atom. Fluoride binding disrupts the
extended conjugation because of population of the boron pp orbital,
thus giving rise to the quenching of the absorption or fluorescence
intensity of the triarylboranes. When triarylborane displays an
initial absorption in the visible region, which makes the solution
colored, disappearance of the color can be accompanied by the
fluoride binding [28]. Although such turn-off responses have been
widely observed in the usual triarylborane-based sensors, turn-on
sensors have also attracted great attention due to their better
suitability for practical uses [48e54]. For example, Wang et al.
reported a turn-on fluorescence sensor that utilizes intramolecular
electron transfer from triarylamine donor to triarylborane acceptor
[32] and also investigated turn-on phosphorescence responses
The recognition and quantification of fluoride anions in chem-
ical and physiological systems are of great interest because they
play important roles in dental health and in treating osteoporosis
but are toxic when exposed to overdoses [1e3]. A number of
fluoride sensors based on optical and electrochemical methods
have been developed for the detection of fluoride [4e11]. Among
them, triarylborane compounds have been particularly intriguing
because of their high fluorophilicity [5,6,8,9,12e29]. Moreover,
optical changes in color [13,28,30], absorption, or fluorescence
[19,27,31e33] before and after a binding event of fluoride to the
boron center of triarylboranes have provided a simple and easy
detection of fluoride. While most of triarylborane sensors for
fluoride developed to date have successfully operated in organic
media, such sensors showed a limited binding response in the
presence of water that interferes with the fluoride binding. Since
fluoride anion is present in abundance in aqueous media, detection
of fluoride in aqueous or water-compatible media is of great
importance in real applications. To address this issue, many efforts
have been devoted to enhance the Lewis acidity of triarylborane to
overcome hydration of fluoride. Some of the plausible strategies are
* Corresponding author. Tel.: þ82 52 259 2335; fax: þ82 52 259 2348.
** Corresponding author. Tel.: þ82 42 350 2829; fax: þ82 42 350 2810.
E-mail addresses: lmh74@ulsan.ac.kr (M.H. Lee), ykdo@kaist.ac.kr (Y. Do).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.04.020

90
K.C. Song et al. / Journal of Organometallic Chemistry 713 (2012) 89e95
using Ru(II)- [55] and Pt(II)-borane [31,56,57] conjugates. Huang
et al. demonstrated that turn-on switching from phosphorescence
to fluorescence can be achievable using Ir(III)-borane conjugate
[58]. Yoon et al. reported a ratiometric fluorescence change as well
as enhanced fluorophilicity in the bidentate receptor bearing ortho-
boronic acid and imidazolium moiety [59]. Gabbaï et al. have
shown that blocking of a PET (photo-induced electron transfer)
process from organic fluorophores to borane by fluoride binding
can induce a fluorescence turn-on response of phosphonium
boranes which can be operating in aqueous solution [37]. We also
reported very recently that metal-to-ligand charge transfer (MLCT)
phosphorescence can be effectively turned on by preventing a PET
process from MLCT state to the auxiliary borane ligand upon fluo-
ride binding [60]. In addition to the turn-on response of emission
mentioned above, Gabbaï et al. reported a colorimetric turn-on
sensor based on cationic pyridinium boranes, which can extract
fluorides biphasically in water/CHCl3 mixture [30]. They have
shown that the turn-on color change is attributed to the intra-
molecular charge transfer (CT) from borate to pyridinium moiety,
which has been newly induced in the visible region after fluoride
binding.
Since the colorimetric turn-on response may provide the most
simple naked eye detection of fluoride, the cationic boranes that
can exhibit a colorimetric turn-on response, as well as can operate
in aqueous solution at a low level of fluorides will be promising in
real applications. For this purpose, we have been interested in the
heterocyclic benzothiazole which has been widely used in dyes and
can be easily converted into a cationic form [61e63]. In particular,
benzothiazole and benzothiazolium derivatives have been effec-
tively utilized as a colorimetric or fluorescent signaling unit in
metal cation sensing [64e69] and very recently in fluoride sensing
[70,71]. Thus, if the cationic borane with benzothiazolium moiety
induces a low-energy intramolecular CT (ICT) transition after
fluoride binding, one may expect turn-on response toward fluoride
as similarly demonstrated in the pyridinium boranes.
N
S
N
S
(i)
(iii)
Br
B
(ii)
1
2a
CF3SO3
N
S
N
S
(iv)
B
B
F
[2]OTf
2F
Scheme 1. (i) n-BuLi, THF, ꢁ78 ꢂC; (ii) Mes2BF, THF, 25 ꢂC; (iii) Methyl triflate, ether,
25 ꢂC; (iv) TBAF, CHCl3, 25 ꢂC.
moiety (Scheme 1). While the formation of [2]OTf has been charac-
terized by multinuclear NMR spectroscopy, elemental analysis, and
mass spectrometry, X-ray diffraction studies unequivocally revealed
the presence of N-methylated benzothiazolium moiety as well as
P
a trigonal planar boron center (
¼ 359.9ꢂ) (Fig. 1, right), the
(CꢁBꢁC)
latter of which is also consistent with the 11B signal observed at
d
75.3 ppm. In contrast to the almost planar arrangement between
the phenyl and benzothiazole ring fragments in 2a, the two rings in
[2]þ shows a large distortion (:dihedral ¼ 51.8(2)ꢂ) probably due tothe
steric hindrance between phenylene CꢁH and N-methyl group of
the benzothiazolium moiety. To gain insight into the fluoride binding
properties, the cationic [2]þ was further converted into its fluoride
adduct (2F) from the reaction with one equiv of tetra-n-butylammo-
nium fluoride (TBAF) in CHCl3. The zwitterionic 2F could be easily
obtained as a yellow solid by simple filtration of a reaction mixture.
While the 1H and 13C NMR spectroscopy and elemental analysis
showed the formation of fluoride adduct, the 11B NMR signal at
In this report, we prepared a triarylborane containing benzo-
thiazolium moiety and investigated its fluoride binding properties
in THF/H2O (9/1, v/v) solution. The details of synthesis, character-
ization, and colorimetric turn-on sensing properties for fluorides at
a low level of concentration (w4 ppm) are described.
d
9.1 ppm confirmed the presence of a four-coordinated boron center.
It is also notable that 2F forms a yellow solution different from the
colorless [2]þ.
2. Results and discussion
2.2. Fluoride sensing properties
2.1. Synthesis and characterization
To investigate fluoride sensing properties of [2]þ, UVevis titra-
tion experiments were carried out. The cationic borane [2]þ
features a low-energy absorption band at 325 nm (log ε ¼ 4.33) in
THF, as similarly observed in the usual Mes2B-based triarylboranes
(Fig. 2a) [5,8,25,28]. Upon addition of incremental amounts of
fluoride, this band was gradually quenched while forming a new
absorption band at the lower energy region (ca. 355 nm) which tails
over visible region (450 nm). The absorption band after the addi-
tion of one equiv of fluoride was also found to be exactly matched
with that of 2F, confirming the binding of fluoride to the boron
atom of [2]þ.
The reaction between the lithium salts derived from 2-(4-
bromophenyl)benzo[d]thiazole (1) [72] and dimesityboron fluo-
ride (Mes2BF) in THF afforded the triarylborane containing benzo-
thiazole moiety, 2-(4-bromophenyl)benzo[d]thiazole (2a) as a pale
yellow solid in moderate yield (66%, Scheme 1). The identity of 2a
has been fully characterized by multinuclear NMR spectroscopy,
elemental analysis, mass spectrometry, and X-ray diffraction
methods. While the 1H and 13C NMR spectra show the expected
resonances corresponding to the benzothiazole and Mes2B moie-
ties, the 11B NMR signal detected at
presence of a three-coordinated boron center. Furthermore, X-ray
diffraction studies revealed the molecular structure of 2a (Fig. 1 and
d
73.8 ppm confirmed the
To elucidate the absorption change of [2]þ after fluoride binding,
the structures of [2]þ and 2F were optimized using density functional
theory (DFT) methods. To include the solvent effects of THF, the
Polarizable Continuum Model (PCM) [73e75] was used. As shown in
Fig. 3, the highest occupied molecular orbitals (HOMOs) of both
compounds are mainly located on the mesityl groups while the
lowest unoccupied molecular orbitals (LUMOs) are localized on
the phenylbenzothiazolium moiety. Further inspection of LUMOs
indicates that the empty pp(B) orbital of [2]þ largely contributes to
LUMOdelocalization, butnomoreconjugationin2F due to occupancy
Table 1). The boron atom of the triarylborane moiety adopts
P
a trigonal planar geometry (
¼ 359.9ꢂ), as consistent with
(CꢁBꢁC)
the 11B resonance. The phenyl and benzothiazole ring fragments
forms a dihedral angle of 3.5(1)ꢂ, indicating a planar arrangement
between the two rings.
Treatment of 2a with excess MeOTf in Et2O resulted in the
formationof cationictirarylborane([2]þ)containingbenzothiazolium

K.C. Song et al. / Journal of Organometallic Chemistry 713 (2012) 89e95
91
Fig. 1. Crystal structures of 2a (left) and [2]þ in [2] [OTf] (right) (40% ellipsoid). The H atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg). 2a: BꢁC(1) 1.562(5),
BꢁC(14) 1.588(5), BꢁC(20) 1.576(5), C(1)ꢁBꢁC(14) 119.4(3), C(1)ꢁBꢁC(20) 117.3(3), C(14)ꢁBꢁC(20) 123.2(3), C(7)ꢁNꢁC(9) 105.2(3), C(7)ꢁSꢁC(8) 89.12(19). [2]þ: BꢁC(1) 1.585(7),
BꢁC(14) 11.573(7), BꢁC(20) 1.583(7), NꢁC(32) 1.477(5), C(1)ꢁBꢁC(14) 119.3(4), C(1)ꢁBꢁC(20) 115.5(4), C(14)ꢁBꢁC(20) 125.1(4), C(7)ꢁNꢁC(9) 113.4(4), C(7)ꢁSꢁC(8) 91.3(2), C(7)ꢁ
NꢁC(32) 125.3(4), C(9)ꢁNꢁC(32) 121.0(4).
of pp(B) orbital by fluorine atom. These results suggest that the strong
absorption of [2]þ at 325 nm is largely involved with charge transfer
pyridinium boranes [30]. Consequently, the intense absorption in the
visible region after fluoride binding gave rise to the vivid color change
of the solution from colorless to yellow, which can be easily observed
with naked eye (Fig. 4). This suggests that [2]þ could be utilized as
a colorimetric turn-on sensor for fluoride.
In contrast, the absorption feature of neutral 2a is quite different
from that observed for [2]þ. While the neutral 2a exhibits a major
absorption band at 342 nm, it undergoes an overall blue-shift after
fluoride binding (labs ¼ 329 nm) but with a small decrease in the
absorption intensity, indicating that the fluoride binding to the
(CT) transition from
*(phenylbenzothiazolium) system. However, after fluoride binding
to the boron atom of [2]þ, the major absorption process appears to
change to (Mes) / *(phenylbenzothiazolium) ICT transition,
resulting in a red-shift of the absorption band probably due to greater
elevation of (Mes) level than * as a resultofincreased charge on the
p(Mes) to pp(B) orbital which is delocalized with
p
p
p
p
p
dimesitylfluoroborate group (Fig. 3). Huang et al. reported that the
charge on the boron atom in 2-(4-dimesitylborylphenyl)quinoline
compound is significantly increased after fluoride binding and
thereby the red-shifted CT transition from fluoroborate moiety to
quinoline can be induced [76]. A similar red-shift in the absorption
spectra after fluoride binding has also been observed in the arylbor-
onic acid or arylboronate ester compounds [50,77]. The electron-
withdrawing boronic acid or ester groups were changed into
electron-donating groups upon fluoride binding, giving rise to the
red-shifted ICT transition. In fact, the absorption features involving
ICT transition in [2]þ and 2F are very similar to those found for
Table 1
Crystallographic data and parameters for 2a and [2]OTf.
compound
2a
[2]OTf
Formula
C31H30BNS
459.43
Triclinic
Pꢁ1
C33H33BF3NO3S2
623.53
Monoclinic
P21/c
8.354(3)
48.295(16)
7.924(3)
90
96.001(16)
90
3179.3(19)
4
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
7.697(3)
8.388(3)
20.3961(7)
93.339(13)
90.103(14)
98.683(13)
1299.4(8)
2
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
V (Å3)
Z
rcalc(g cmꢁ3
)
1.174
0.144
1.303
0.219
m
(mmꢁ1
)
F(000)
T (K)
488
296
1304
296
Scan mode
Multi
Multi
hkl range
ꢁ8 / þ8, e9 / þ9, ꢁ11 / þ11, e63 / þ65,
ꢁ23 / þ23
16339
ꢁ10 / þ8
37397
Measd reflns
Unique reflns [Rint
Reflns used for refinement 4111
]
4111 [0.0473]
8342 [0.2251]
8432
Refined parameters
313
395
0.0815
0.2260
1.005
R1a (I > 2
s
(I))
0.0642
0.2180
1.053
0.423, ꢁ0.451
wR2b all data
Fig. 2. Spectral change in the UVevis absorption of a solution of (a) [2]þ (2.5 ꢀ 10ꢁ5 M)
and (b) 2a (1.5 ꢀ 10ꢁ5 M) in THF upon addition of TBAF. The inset shows the absor-
bance at 325 nm for [2]þ and at 342 nm for 2a as a function of [Fꢁ]. The line corre-
sponds to the binding isotherm calculated with K ¼ 1.0 ꢀ 109 Mꢁ1 for [2]þ and
3.2 ꢀ 107 Mꢁ1 for 2a.
GOF on F2
r
fin (max/min) (e Åꢁ3
)
0.530, ꢁ0.406
P
P
a
R1 ¼ jjFoj ꢁ jFcjj/ jFoj.
P
P
b
wR2 ¼ {[ w(Fo2 ꢁ Fc2)2]/[ w(Fo2)2]}1/2
.

92
K.C. Song et al. / Journal of Organometallic Chemistry 713 (2012) 89e95
fluoride binding to the boron center of [2]þ in THF/H2O (9/1, v/v) and
the binding constant was calculated to be 1.3 ꢀ 104 Mꢁ1. This K value
is shown to be much greater than that of 2a (K ¼ 4.2 ꢀ 102 Mꢁ1) and
those of other neutral boranes such as Mes3B (K ¼ 1.0 Mꢁ1) [23,25]
and Mes2B(p-F-Ph) (K ¼ 3.3 ꢀ 101 Mꢁ1) [35] under the same condi-
tions indicating a dramatic increase in the fluorophilicity of [2]þ. This
result thus clearly suggests that the introduction of benzothiazolium
moiety into triarylborane notably enhances the fluorophilicity of the
boron atom probably due to Coulombic and inductive effects of
benzothiazolium group [8,39,41,43,44,46].
From the high fluorophilicity of [2]þ in THF/H2O (9/1, v/v), we
further investigate the colorimetric sensing ability of [2]þ in the
presence of small amounts of fluorides. As shown in the TOC figure,
the colorless solution of [2]þ (1.0 ꢀ 10ꢁ4 M) turned faint yellowish
upon addition of 200 mM (3.8 ppm) of fluorides which is similar to
the fluoride contamination level (4 ppm) in drinking water set by
Environmental Protection Agency (EPA) [78]. This observation may
suggest that a THF solution of [2]þ can be used for turn-on colori-
metric sensing of fluorides dissolved in pure water.
Finally, the change in the UVevis absorption of cationic [2]þ
was examined in the presence of various anions (Fig. 6). While the
addition of one equiv of Fꢁ gave rise to the large quenching of the
absorption band with a red-shift, the presence of one equiv of
other anions such as Clꢁ, Brꢁ, Iꢁ, OAcꢁ, NOꢁ3 , HSO4ꢁ, ClOꢁ4 did not
significantly affect the band. This result indicates high selectivity of
[2]þ toward fluoride, as usually observed in the Mes2B-based tri-
arylborane receptors [5,33,44,76,79].
Fig. 3. Molecular orbital diagrams of HOMO and LUMO of [2]þ and 2F with their
relative energies from DFT calculation (Isovalue ¼ 0.04).
boron atom does not affect much the absorption feature of 2a
(Fig. 2b). There is no color change in the solution containing 2a
before and after fluoride addition due to the change of absorption
wavelength in the UV region only (Fig. 4). This finding may suggest
that the absorption in 2a mainly arises from
phenylbenzothiazole group into the * of which the empty pp(B)
orbital is delocalized to form LUMO. Upon fluoride complexation,
however, the extended conjugation of * orbital of the phenyl-
p
ꢁp* transition in the
p
p
benzothiazole group with the pp(B) is disrupted, resulting in the
blue-shift of the absorption band.
Remarkably, the fluoride binding constant (K) for [2]þ, estimated
from the 1:1 binding isotherm, exceeded the measurable range
(K > 107 Mꢁ1) in THF (Fig. 2a). Comparison of the binding constant
with that of the neutral compound 2a (K ¼ 3.2 ꢀ 107 Mꢁ1 in THF)
which shows a comparable binding constant to that of Mes2BPh
(K ¼ 5.0ꢀ106 Mꢁ1 inTHF)[18,35]havinga similarstericenvironment
around the boron center reveals an increase in the fluoride binding
affinity of [2]þ. To provide more quantitative measure of the fluoride
ion affinity of [2]þ, UVevis titrations were carried out in a more
competing medium, i.e., THF/H2O (9/1, v/v). As shown in Fig. 5, the
intensity of the absorption band gradually decreases as a result of
Fig. 5. Spectral change in the UVevis absorption of a solution of (a) [2]þ and (b) 2a in
THF/H2O (9:1 v/v) (2.0 ꢀ 10ꢁ5 M) upon addition of TBAF. The inset shows the absor-
bance at 311 nm for [2]þ and at 342 nm for 2a as a function of [Fꢁ]. The line corre-
sponds to the binding isotherm calculated with K ¼ 1.3 ꢀ 104 Mꢁ1 for [2]þ and
4.2 ꢀ 102 Mꢁ1 for 2a.
Fig. 4. Color change of the solution of 2a (left) and [2]þ (right) (5.0 ꢀ 10ꢁ4 M) before
and after the addition of one equiv of fluoride in THF.

K.C. Song et al. / Journal of Organometallic Chemistry 713 (2012) 89e95
93
for 1H, 100.62 MHz for 13C, 128.38 MHz for 11B) at ambient
temperature. Chemical shifts are given in ppm, and are referenced
against external Me4Si (1H, 13C) and BF3$Et2O (11B). Elemental
analyses were performed on an EA1110 (Fisons Instruments) by
the Environmental Analysis Laboratory at KAIST. HR EI-MS and HR
FAB-MS measurement (JEOL JMS700) was carried out at Korea
Basic Science Institute (Daegu).
4.1.1. Synthesis of 2-(40-Dimesitylborylphenyl)benzo[d]thiazole (2a)
A hexane solution of n-BuLi (2.5 M, 0.74 mL, 1.84 mmol) was
added at ꢁ78 ꢂC to a solution of 1 (0.510 g, 1.75 mmol) in THF
(20 mL), and the mixture was stirred for 1 h at that temperature. A
solution of Mes2BF (0.524 g, 1.75 mmol) in THF (5 mL) was added to
the reaction mixture at ꢁ78 ꢂC. After stirring for 1 h, the reaction
mixture was allowed slowly to warm to room temperature, and
stirred overnight. After removal of all volatiles, the residue was
extracted with CH2Cl2 and recrystallized from CH2Cl2/MeOH to
afford a pale yellow crystal of 2a (0.53 g, 66%). 1H NMR (CDCl3):
Fig. 6. Spectral change in the UVevis absorption of a solution of [2]þ (2.5 ꢀ 10ꢁ5 M in
d
8.07 (d, J ¼ 7.6,1H), 8.04 (d, J ¼ 8.0, 2H), 7.90 (d, J ¼ 7.2,1H), 7.62 (d,
J ¼ 8.0, 2H), 7.48 (t, J ¼ 7.2, 1H), 7.38 (t, J ¼ 7.0, 1H), 6.82 (s, 4H), 2.30
(s, 6H), 2.00 (s, 12H). 13C NMR (CDCl3):
167.88, 154.21, 148.77,
141.51, 140.88, 139.03, 136.661, 136.17, 135.19, 128.28, 127.01, 126.41,
125.39, 123.41, 121.64, 23.46, 21.24. 11B NMR (CDCl3):
73.8. HR EI-
THF) after addition of one equiv of various anions.
d
3. Conclusion
d
MS: m/z Calcd for C31H30BNS, 459.2192; Found, 459.2194. Anal.
Calcd for C31H30BNS: C, 81.04; H, 6.58; N, 3.05. Found: C, 81.01; H,
6.30; N, 3.06.
We have synthesized a novel cationic triarylborane bearing
benzothiazolium moiety. In contrast to a neutral borane, the
cationic borane underwent a red-shift of the UVevis absorption
band upon addition of fluorides. Theoretical studies suggested that
4.1.2. Synthesis of 2-(40-Dimesitylborylphenyl)-3-methylbenzo[d]
thiazol-3-ium triflate ([2]OTf)
greater elevation of the
p(Mes) donor level of fluoroborate in
p
(Mes) / *(phenylbenzothiazolium) ICT transition is respon-
p
To a solution of 2a (0.20 g, 0.43 mmol) in Et2O (10 mL) was
added excess MeOTf (0.20 mL, 1.7 mmol). The mixture was stirred
overnight at room temperature. The white solid precipitated was
filtered, washed with Et2O (3 ꢀ 10 mL) and pentane (3 ꢀ 10 mL).
Recrystallization from CH2Cl2/Et2O afforded [2]OTf as a pale yellow
sible for the red-shift in the absorption. The cationic borane was
shown to bind fluoride ions in THF/H2O (9/1, v/v) with the high
binding constant of 1.3 ꢀ 104 Mꢁ1 that is much greater than that of
neutral borane (K ¼ 4.2 ꢀ 102
M
ꢁ1). In particular, the fluoride
binding led to the color change of the solution from colorless to
yellow, which could be easily observed with naked eye even in the
presence of very low level of fluoride ions (w4 ppm) in THF/H2O
(9/1, v/v). The fluoride binding was also selective over other
anions. These results suggest that the introduction of benzothia-
zolium moiety into triarylborane can give rise to the large
enhancement of fluorophilicity of the boron atom probably due to
Coulombic and inductive effects of benzothiazolium group, as well
as the turn-on colorimetric response, all of which are suitable for
the use of the given cationic borane as a colorimetric turn-on
sensor for fluoride.
crystal (0.16 g, 60%). 1H NMR (CDCl3):
d
8.15 (d, J ¼ 8.4, 2H),
7.86e7.71 (m, 6H), 6.84 (s, 4H), 4.37 (s, 3H), 2.30 (s, 6H), 1.99 (s,
12H). 13C NMR (CDCl3):
174.01, 152.45, 142.49, 140.99, 139.85,
d
1368.55, 130.52, 129.70, 129.55, 129.28, 128.59, 126.89, 123.63,
122.11, 118.93, 117.84, 117.84, 38.42, 23.54, 21.24. 11B NMR (CDCl3):
d
75.3. HR FAB-MS: m/z Calcd for C32H33BNSþ, 474.2421; Found,
474.2425. Anal. Calcd for C32H33BF3NO3S2: C, 63.56; H, 5.33; N, 2.25.
Found: C, 63.26; H, 5.56; N, 2.19.
4.1.3. Synthesis of Dimesityl(4-(3-methylbenzo[d]thiazol-3-ium-2-
yl)phenyl)fluoroborate (2F)
The solution of [2]OTf (15 mg, 0.024 mmol) in CHCl3 (1 mL) was
treated with TBAF (6.91 mg, 0.026 mmol). After stirring for 1 h at
room temperature, the yellow precipitate was filtered and washed
with CHCl3 (3 ꢀ 2 mL). Drying under vacuum afforded a yellow
4. Experimental section
4.1. General considerations
solid 2F (9.95 mg, 84%). 1H NMR (DMSO-d6):
d
8.44 (d, J ¼ 8.0, 1H),
All operations were performed under an inert nitrogen atmo-
sphere using standard Schlenk and glove box techniques. Anhy-
drous grade solvents (Aldrich) were dried by passing them
through an activated alumina column and stored over activated
molecular sieves (5 Å). Spectrophotometric-grade tetrahydrofuran
(THF) was used as received from Aldrich. Commercial reagents
were used without any further purification after purchasing from
Aldrich (4-Bromobenzaldehyde, 2-aminothiophenol, dimesi-
tylboron fluoride (Mes2BF), n-BuLi (2.5 M solution in n-hexane),
methyl triflate (MeOTf), tetra-n-butylammonium fluoride trihy-
drate (TBAF)). 2-(4-Bromophenyl)benzo[d]thiazole (1) [72] was
analogously synthesized according to the reported procedures.
CDCl3 from Cambridge Isotope Laboratories was used after drying
over activated molecular sieves (5 Å). NMR spectra of compounds
were recorded on a Bruker Avance 400 spectrometer (400.13 MHz
8.30 (d, J ¼ 8.4, 1H), 8.02 (br s, 1H), 7.90 (t, J ¼ 7.4, 1H), 7.81 (t, J ¼ 7.6,
1H), 7.64 (br s, 2H), 7.55 (br s, 1H), 6.42 (s, 4H), 4.25 (s, 3H), 2.08 (s,
6H), 1.85 (s, 12H). 13C NMR (DMSO-d6):
d
175.87, 153.45, 142.70,
140.48, 139.93, 139.29, 134.35, 130.80,129.41, 128.86,128.06,127.44,
124.17, 119.81, 117.34, 38.24, 24.92, 20.50. 11B NMR (DMSO-d6):
9.1.
d
Anal. Calcd for C32H33BFNS: C, 77.88; H, 6.74; N, 2.84. Found: C,
77.41; H, 6.60; N, 2.60.
4.2. UVevis absorption titrations
UVevis absorption spectra were recorded on a Jasco V-530
spectrophotometer. Fluoride titration experiments were performed
in THF or in THF/H2O (9/1, v/v) with a 1-cm quartz cuvette at
ambient temperature. The detailed conditions are given in the
Figure captions.

94
K.C. Song et al. / Journal of Organometallic Chemistry 713 (2012) 89e95
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a
Bruker Apex II-CCD area detector diffractometer, with
¼ 0.71073 Å). The
scan frames with
l
u
0.3ꢂ/frame and an exposure time of 10 s/frame. Cell parameters
were determined and refined by SMART program. Data reduction
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was applied using the SADABS program. The structure was solved
by direct methods and all nonhydrogen atoms were subjected
to anisotropic refinement by full-matrix least-squares on F2 by
using the SHELXTL/PC package. Hydrogen atoms were placed at
their geometrically calculated positions and refined riding on the
corresponding carbon atoms with isotropic thermal parameters.
The detailed crystallographic data and selected bond lengths and
angles are given in Table 1.
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This work was supported by the Basic Science Research Program
(No. 2010-0016243 and 2011-0001213 for Y.S. Lee, No. 2010-
0008264 for Y. Do, and No. 2010-0007796 for M.H. Lee) and the
Priority Research Centers program (No. 2009-0093818 for M.H. Lee)
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology.
Appendix A. Supplementary material
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CCDC 862766 (2a) and 862767 ([2]OTf) contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk) via www.ccdc.cam.ac.
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