Dicyanovinyl substituted triarylboranes: A rational approach to distinguish fluoride and cyanide ions

Article abstract of DOI:10.1039/c3dt53638f

Two new dicyanovinyl (DCV) functionalized triarylboranes (Mes ₂B-π-spacer-DCV, for 1: π-spacer = C₆H₄, for 2: π-spacer = 2,3,5,6-tetramethyl-phenyl) are reported. The molecular structures of 1 and 2 are similar except for the spacer which connects the boryl and DCV units. This small structural perturbation induces drastic changes in the optical properties of 1 and 2. Compound 2 shows weak dual fluorescence emission in nonpolar solvents and a stronger emission in polar solvents. Compound 1 is weakly fluorescent in polar environments but shows an intense single luminescence peak in less polar environments. Compound 1 exhibits a turn-off fluorescence response for both fluoride and cyanide: in contrast, 2 shows a turn on fluorescence response for both anions with different fluorescence signatures. The NMR titration studies reveal that for compound 2, fluoride binds to the boron centre and cyanide binds to the DCV unit. For compound 1, the fluoride ion binds to the boron center, whereas the CN ^- binds to both the Ar₃B and DCV units.

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ARTICLE TYPE
Dicyanovinyl Substituted Triarylboranes: A Rational Approach to
Distinguish Fluoride and Cyanide Ions
George Rajendra Kumar and Pakkirisamy Thilagar*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
ABSTRACT: Two new dicyanovinyl (DCV) functionalized triarylboranes (Mes2BꢀπꢀspacerꢀDCV, for
1: πꢀspacer = C6H4, for 2: πꢀspacer = 2, 3, 5, 6ꢀtetramethylꢀphenyl) are reported. The molecular structures
of 1 and 2 are similar except the spacer which connects the boryl and DCV units. This small structural
perturbation induces drastic changes in the optical properties of 1 and 2. Compound 2 shows weak dual
10 fluorescence emission in nonpolar solvents and a stronger emission in polar solvents. Compound 1 is
weakly fluorescent in polar environments but shows intense single luminescence peak in less polar
environments. Compound 1 exhibits turnꢀoff fluorescence response for both fluoride and cyanide: in
contrast, 2 shows “turn on” fluorescence response for both the anions with different fluorescence
signatures. NMR titration studies reveal that for compound 2, fluoride binds to boron centre and cyanide
15 binds to the DCV unit. For compound 1, fluoride ion binds to boron center, whereas the CNꢀ binds to both
Ar3B and DCV units.
For example, Aldridge and coꢀworkers utilized a mixture of redox
Introduction
active dyes and metallocene based organoboranes for colorimetric
50 discrimination of fluoride/cyanide5g. In spite of these efforts, a
single TAB based molecular sensor which could discriminate
both fluoride and cyanide has not been reported.
Triarylborane (TAB) containing functional materials continue
to attract a lot of attention owing to their intriguing optical
20 properties1. The empty pꢀπ orbital on tricoordinate boron can
facilitate π conjugation among the neighbouring organic π units.
Thus, the photoꢀphysical features of TAB containing
molecules/polymers can be conveniently fineꢀtuned by
controlling the interactions between boron centre and organic π
25 systems. This concept has been well exploited in the area of
electroluminescent materials2, energy harvesters3, nonꢀlinear
optics4, and chemosensors5.
It occurred to us that, if different receptors with different
affinities towards Fꢀ and CNꢀ ions are tethered in a single
55 molecule, the anions may preferentially bind to different receptor
centers owing to their difference in polarisabilities. In this
context, dicyanovinylꢀtriarylborane conjugate caught our
attention. The electrophillic carbon centre in dicyanovinyl (DCV)
moiety has strong affinity towards cyanide ion; hence it can
60 selectively bind to CNꢀ (carbon centred electrophile)9.
Concomitantly, the triarylborane moiety at the other end can
accommodate Lewis bases thereby perturbing the interaction
between neighboring π systems. We also envisioned that if the
steric crowding around boron could be tuned for fluoride, then
65 one can conveniently manipulate TABꢀDCV conjugate for the
selective discrimination of both Fꢀ and CNꢀ. To evaluate our
hypothesis, we designed and synthesised two new dicyanovinyl
(DCV) functionalized triarylboranes (Mes2Bꢀπꢀ spacerꢀDCV, for
1: πꢀ spacer = C6H4, for 2: πꢀ spacer= 2, 3, 5, 6ꢀtetramethylꢀ
70 phenyl). Our anticipation is realised with compound 2 which can
selectively discriminate both Fꢀ and CNꢀ and the results are
reported in this manuscript.
Triarylboranes been increasingly exploited for selective detection
of fluoride and cyanide5 ions owing to their higher selectivity
30 towards these anions, Detection and quantification of Fꢀ and CNꢀ
is an active area of research. Several TAB based optical
(colorimetric and fluorescence) sensors for the selective detection
of fluoride and cyanide have been independently developed by
Wang, Yamaguchi and others5. Gabbai and coꢀworkers elegantly
35 utilized the heterobimetallic Lewis acids for the detection of Fꢀ
and CNꢀ ions6. Recently, as part of our onꢀgoing program on
boron based luminescent materials; we have developed several
dyads and triads for the selective detection of fluoride and
cyanide7. The optical properties of these dyads are highly
40 dependent on their molecular conformations. Dyads with
different degrees of steric crowding showed different optical
response for fluoride ions (“turn on” fluorescence response or
Result and discussion
colorimetric response)7 a, c
.
Several TAB based sensors failed to distinguish Fꢀ and CNꢀ due
45 to their strong interference. To overcome this problem
considerable research efforts have been made to design new
receptors which could discriminate both fluoride and cyanide8.
Synthesis & characterization
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DOI: 10.1039/C3DT53638F
CN
Fwt
402.33
458.43
O
H
T (K)
296(2)
273(2)
NC
R
CH2Cl2/C2H5OH
Piperidine/1h
R
R
R
R
0.71073
Monoclinic
P 2 1/C
0.71073
Monoclinic
P 2 1/n
λ(Å)
R
CH2(CN)2
Crystal lattice
Space group
R
R
B(Mes)2
B(Mes)2
a = 18.030(2)
b = 8.1868(10)
c = 15.998(2)
a = 10.2325(11)
b = 17.2007(17)
c = 16.2381(18)
1a: R = H
1: R = H
2: R = CH3
Unit cell dimensions (a,
b, c) (Å),
2a: R = CH3
β = 100.873(6)°
2319.0(9)
β = 104.175(3)°
2771.0(5)
V (Å3)
Scheme 1: Synthetic scheme for preparation of 1 and 2.
4, 1.152
0.066
4, 1.099
0.063
Z, ρ
ꢀ (cmꢀ1)
The procedure employed for the synthesis of 1 and 2 is shown in
scheme 1. Boryl aldehydes (1a and 2a) are synthesised based on
recent reports by Thilagar et al7. DCV functionalised TAB are
synthesised by adopting known literature procedures9.
Compounds 1 and 2 are characterized by NMR spectroscopy,
ESIꢀmass spectrometry. The molecular structures of 1 and 2 are
confirmed by single crystal Xꢀray diffraction studies.
F(000)
856
984
5
size (mm)
θ range (°)
TMax & Tmin
Unique refln
0.15×0.13×0.13
1.15ꢀ31.17
0.991, 0.990
5510
0.14×0.12×0.12
3.12ꢀ27.6
0.992, 0.991
4212
10 The 1H and 13C NMR of 1 and 2 are consistent with their
molecular composition. The vinylic CꢀH signal for compound 2
(8.29 ppm) is more downꢀfield shifted compared to 1 (7.78 ppm).
This may be due to electronic isolation of DCV unit from the rest
of molecule in 2 by sterically crowded duryl spacer. The aryl CꢀH
15 protons of mesitylene moieties in 2 give rise to a doublet (6.74
ppm) in CDCl3, indicating the nonꢀequivalence of the two CꢀH
protons of mesityl unit. From the single crystal structure of 2, it is
found that the duryl spacer and –B(Mes)2 are twisted by an angle
of 59.3 °; possibly this twisted arrangement is retained in solution
20 rendering the two CꢀH protons of the mesitylene group nonꢀ
equivalent. The NMR spectrum of 2 shows three different signals
for the ꢀCH3 (two for mesityl ortho CH3 and one for duryl orthoꢀ
methyl) moieties ortho to boron centre, which also support our
above inference. However, the orthoꢀmethyl groups in compound
25 1 shows only one resonance confirming the chemical equivalence
of both mesityl units.
R1 = 0.0750
R1 = 0.0615
R1[a], wR2[b] (I > 2sI)
WR2 = 0.2419
WR2 = 0.2067
Largest diff. peak and
hole ( e.Aꢀ3)
0.700, ꢀ0.688
1.026
0.224, ꢀ0.168
1.358
GooF
[a]R1 = Σ││Fo│ − │Fc││/ Σ│Fo│. [b]wR2 = [Σ{w(Fo2 –
Fc2)2}/Σ{w(Fo2)2}]1/2
The molecular structures of 1 and 2 are shown in Figure 1. The
35 boron centre in both 1 and 2 adopts trigonal planar geometry with
sum of (∑C−B−C) angles being ~360°. The dihedral angle between
BC2 (mesityl carbon connected to boron) and ꢀC6H4ꢀ spacer and
the angle between DCV unit and C6H4 spacer is significantly
higher for 2 compared to 1. The bulky duryl spacer causes
40 significant twisting of the –BMes2 and DCV units in 2 and hence
the electronic interaction between boryl unit and DCV moiety in
2 is less than that in 1.
Molecular Structures
Table 2: Selected bond lengths in compounds 1 and 2
Bond lengths (Å)
B1ꢀC8
C4ꢀC5
C1ꢀC3
C2ꢀC3
C3ꢀC4
C6ꢀC7
C9ꢀC10
1
2
1.578(3)
1.454(2)
1.442(3)
1.438(3)
1.352(3)
1.387(3)
1.384((3)
1.589(2)
1.479(3)
1.423(4)
1.441(3)
1.336(3)
1.393(2)
1.402(3)
45
Optical Properties
Figure 1: ORTEP plots for 1 and 2 (30% probability ellipsoids,
30 hydrogens omitted for clarity). C = Black, B = Red, N = Blue.
Table 1: Crystallographic data for 1 and 2
1
2
Empirical formula
C28 H27 B N2
C32 H35 B N2
2
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Figure 2: UVꢀVis absorption (left) and fluorescence spectra
(right) of 1 and 2 in CHCl3 (1×10ꢀ5 M). Inset shows enlarged (20
times) PL spectrum of 2.
Table 3: Optical properties of 1 and 2 in CHCl3 (1×10-5 M)
and phenyl spacer) thereby facilitating better π conjugation which
can also cause red shift in emission. In contrast, upon increasing
45 the solvent polarity, compound
2 does not show any
bathochromic shift since it cannot undergo structural
modification upon excitation due to rigid molecular
conformation. From single crystal structure, it is observed that
compound 2 assumes a twisted geometry because of the presence
50 of the sterically encumbered duryl spacer. This twisted geometry
can facilitate intramolecular charge transfer (ICT) transition in
polar medium. Hence the higher energy emission band may be
local emission (LE) and less intense broad low energy emission
may be due to ICT process in the emitting state.
Kr
Knr
(1×108 sꢀ1)
ΦF
(%)
λabs (nm)
λem (nm)
τ (ns)
(1×106 sꢀ1)
4.29 (λem
509 nm)
,
1
2
334
340
509
3.9
9.1
2.24
9.21
1.08 (λem
,
391, 493
0.35
4.07
391 nm)
5
Compounds 1 and 2 show similar absorption features in the
region 270ꢀ 400 nm (maxima at ∼336 nm). The peaks in this
region can be attributed to both π to pπ* (boron) and π to π*
transitions of DCV units. The absorption features of 1 are slightly
55
Table 4: Quantum yield of 1 and 2 in solvents of different polarity
ΦF (%)a
10 broader than that of 2 (Figure 3). Their emission profiles differ
significantly. Compound 1 shows a single emission band with a
peak maximum at 509 nm (stokes shift, ꢁν = 10294 cmꢀ1), while
2 shows dual fluorescence bands at 391 (ꢁν = 3836 cmꢀ1) and 493
nm (ꢁν = 9131 cmꢀ1) respectively. The fluorescence quantum
15 yields indicate that compound 1 is a better fluorophore than 2.
The twisted arrangement of DCV and boryl unit in 2 may be
responsible for its weak fluorescence signature. This is in line
with the recent report by Xie et al11. They demonstrated that DCV
unit substituted with less crowded substituents exhibits better
20 luminescance than the sterically encumbered DCV derivatives.
The steady state time resolved fluorescence studies clearly
indicate that the non radiative process is dominant for photo
excited 2 compared to 1 (Knr for 1: 2.24×108 sꢀ1, for 2: 9.2 ×108 sꢀ
1). Hence, the fluorescence quantum yield for 2 is significantly
25 less compared to 1.
Hexane
6.8
Toluene
6.4
CH2Cl2
3.0
CHCl3
3.9
DMSO
0.1
1
2
0.18
0.27
0.44
0.35
1.0
aQuantum yields are calculated using anthracene as standard.
DFT Computational Studies:
To gain further insight into the electronic nature of compounds
60 1 and 2, DFT computational studies were performed12, 13 using
B3LYP functional and 6ꢀ31G(d) as basis set.
Figure 3: Fluorescence spectra of 1 (left, λex = 334 nm) and 2
(right, λex = 340 nm) in solvents (1×10ꢀ5 M) of different polarity.
Figure 4: Frontierꢀmolecularꢀorbitals for 1 and 2 (isovalue =
65 0.04). Hydrogen atoms are omitted for clarity.
To obtain further insights into electronic nature of 1 and 2, optical
30 studies were carried out in solvents with different dielectric
constants. There were no changes in the absorption profiles of 1
and 2 (supporting information). However, the fluorescence
emission bands show significant changes. For compound 1, the
fluorescence emission band undergoes considerable red shift with
35 significant decrease in quantum yield (table 4) upon increasing
solvent polarity (ꢁν = 7850 cmꢀ1 and 12580 cmꢀ1 in hexane and
DMSO respectively). This clearly indicates the involvement of
intramolecular charge transfer process (ICT) in the excited state
of 1. Thus compound 1 shows lower quantum yield in polar
40 solvents due to the stabilization of ICT state in polar
environments, which is in line with literature reports. Moreover,
upon excitation, 1 may be able to attain a planar geometry (DCV
The frontier molecular orbitals of 1 and 2 are shown in figure 4.
For both the compounds, HOMO’s are mainly localised on the π
orbitals of mesityl units but the LUMO’s differ slightly. In 1,
70 LUMO is distributed over DCV unit, C6H4 spacer and boron’s
empty p orbital equally. On the other hand, in 2, LUMO is
localised on DCV unit with minor contribution from spacer and
tricoordinated boron center. Apparently the twisted conformation
of 2 plays a significant role on the distribution of molecular
75 orbital coefficients. To understand the electronic transitions, TDꢀ
DFT calculations were performed (see SI for data). In case of 1,
the theoretically predicted excitation wavelength (328 nm, f =
0.7562) nearly matches with the experimental value of 334 nm
and has a great contribution from transition between HOMOꢀ4 to
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LUMO. In contrast, 2 shows two excitation wavelengths (365 40 the boron center. As stated videꢀsupra, the broad absorption band
nm, f = 0.222 and 339 nm, f = 0.1073). The higher energy peak
has more contribution from transition between HOMOꢀ5 to
LUMO and the lower energy transition has a significant
contribution from HOMOꢀ1 to LUMO transition. HOMOꢀ1 is
at 334 nm is the combination of both π→pπ* (boryl centered) and
DCV centred π→π* transitions. The fluoride binding at Ar3B
center quenches the boryl centered absorption leaving the DCV
centred π→π* transition unaffected. The slight bathochromic
5
mainly localised on πꢀorbital of one of the mesityl ring 45 shift observed for the new peak can be rationalized in the
(supporting information). Hence the additional low energy
transition can be a charge transfer (CT) transition from the
mesityl unit to the DCV moiety and is facilitated by twisted
following way. Upon fluoride binding, Ar3B unit is transformed
from trigonal planar to tedrahedral geometry. This geometry
change may alter the dihedral angel between the borate unit and
DCV unit. Thus, the electronic communication between DCV and
10 conformation of 2. Since CT transition is sensitive towards
environment, it may be attenuated in solution medium. Hence 50 the phenyl spacer could be facilitated.
UVꢀVis absorption spectrum of 2 shows single absorption band
Under similar titration conditions, the optical features of 2
with a maximum at 340 nm (spanning between 275 to 380 nm).
differed significantly from those of 1. Upon fluoride binding, the
absorption peak at 340 nm is blue shifted to 330 nm with a slight
increase in the absorbance. Upon excitation at 340 nm in the
55 presence of fluoride, the intensity of the lower energy
fluorescence (493 nm) band increased significantly, while the
intensity of higher energy band is unchanged under titration
conditions (Figure 6). These results clearly indicate that
compound 2 could be a potential fluorescence “turn on” sensor
60 for fluoride (For 2 and 2+Fꢀ; ΦF = 0.35 and 1.6%). There is no
further change in fluorescence intensity beyond the addition of ∼
1 eq of fluoride (Figure 6) indicating the formation of 1:1 fluoride
complex (2⋅F) under titration conditions. Increase of low energy
emission (CT transition) band intensity is presumably due to the
65 facilitation of CT transition in the 2·F adduct by the bulky duryl
spacer. UVꢀVis absorption titration also supports our inference.
Upon incremental addition of fluoride a low energy absorption
band at ~400 nm is evolved (figure 6). The 19F NMR titration of 2
against TBAF in CDCl3 shows a broad peak at ꢀ152.5 ppm
70 (supporting information) and the peak intensity saturates upon
addition of 1 equivalent of TBAF. The binding constant value
15 Anion Binding Studies
Figure 5: UVꢀVis absorption (left) and fluorescence (right)
titrations of 1 (1×10ꢀ5 M, λex = 334 nm) in CHCl3 against TBAF
(2ꢂL = 0.1 eq).
ꢀ
(KF = 1.79 × 105 Mꢀ1) for 2 is significantly less than the value
obtained for 1 (KFꢀ = 7.07 × 105 Mꢀ1). These results show that the
aryl spacers have significant influence on fluoride binding
75 affinity of 1 and 2.
20
.
Figure 6: UVꢀVis absorption (left) and fluorescence (right,)
titrations of 2 (1×10ꢀ5 M, λex = 340 nm) in CHCl3 against TBAF
(2ꢂL = 0.1 eq).
Upon addition of TBAF (Tetrabutylammonium fluoride) in
25 CHCl3) to compound 1, the absorption band at 334 nm decreased
in intensity with a simultaneous evolution of a new peak at 370
nm (Figure 5). Similarly, in photoluminescence spectrum (PL),
fluoride binding quenches the band at 509 nm (for 1 and 1+Fꢀ ; ΦF
= 3.9 and 0.9% respectively). The quenching of boryl centred
30 absorption and emission bands is due to the disruption in the
Figure 7: UVꢀVis absorption (left) and fluorescence (right, λex
=
334 nm) titrations of 1 (1×10ꢀ5 M) in CHCl3 against TBACN
(2ꢂL = 0.1 eq).
electronic communication between boron
p
orbital and
neighbouring π units upon fluoride binding at the boron centre.
Similar observations were reported for triarylboron systems by
others research groups.
35 The formation of 1•F complex is confirmed by the appearance of
a characteristic Ar3B•F peak at ꢀ173.2 ppm in 19F NMR titrations
in CDCl3. In the presence of fluoride, no changes were observed
1
for vinylic proton of DCV unit (7.78 ppm) in H NMR titrations.
From NMR studies one can conclude that fluoride binds only to
80
4
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Figure 8: UVꢀVis absorption (left) and fluorescence (right, λex
=
340 nm) titrations of 2 (1×10ꢀ5 M) in CHCl3 against TBACN
(2ꢂL = 0.1 eq).
Figure 7 and 8 represent changes in the optical features of 1 and 2
5
in
presence
of
various
equivalents
of
TBACN
(Tetrabutylammonium cyanide). For compound 1, the binding of
CNꢀ quenches the absorption band (334 nm) with two isosbestic
point at 270 (for 1 eq of CN) and 250 nm (for 2 eq of CN). The
observed decrease in the intensity of the absorption band clearly
10 suggests that, cyanide ion binds to the boron center as well as
DCV unit, thereby disturbing π conjugation in 1. Upon excitation
at 334 nm in the presence of 1 eq of CNꢀ, the intensity of
emission band at 509 nm decreases and is red shifted to 590 nm.
Addition of another eq of CNꢀ quenches the new band at 590 nm
15 (For 1, 1+CNꢀ and 1+2CNꢀ; ΦF = 3.9, 0.31 and 0.28%
respectively). These results clearly indicate that cyanide ion binds
to 1 in more than one mode.
1
Figure 9: H NMR titration spectrum of 2 (2.5×10ꢀ2 M) against
TBACN (0.625 M, 2ꢂL = 0.1 eq) in CDCl3.
60 The upfield shifted proton signal appeared as doublet (J = 8 Hz)
which may be due to existence of two isomeric species (cyanide
bonded carbon centre becomes chiral). Binding constant of 2
To get further insight into the binding process, 1H NMR titrations
were carried out. In general, upon cyanide binding the vinyl CꢀH
20 proton resonance of DCV unit shifts to downfield region (~ 6 to 8
ppm to ~ 4 to 5 ppm)9. Similar changes were observed for
compound 1 in the presence of TBACN in CDCl3. The intensity
of peaks corresponding to DCV unit and mesityl ring CꢀH
protons decreased; concomitantly six new peaks appeared at 4.29,
25 4.36, 6.79, 6.60, 6.56 and 7.46 ppm respectively. This result
suggests that CNꢀ binds concomitantly to both Ar3B unit as well
as DCV unit producing multiple species (supporting information).
For the 1:2 mixture of 1 and TBACN in CDCl3, the peaks
corresponding to mesityl ring CꢀH and DCV unit disappeared and
30 a single peak at 6.54, ppm and 4.23 ppm are observed. No
changes were observed by the addition of further equivalents of
CNꢀ to 1.
ꢀ
ꢀ
(KCN = 1.70 × 105 Mꢀ1) is higher than 1 (KCN = 0.96 × 105 Mꢀ1)
indicates DCV unit in 2 has more affinity towards cyanide ion.
CN
CN
65
CN
Ha
NC
H b
NC
TBACN
B
B
Scheme 2: Binding mode of 2 with CNꢀ.
Addition of TBACN to compound 2, decreased the intensity of
absorption band slightly with a 10 nm blue shift. The minimal
35 changes in the absorption spectrum clearly indicate that there is
very much less electronic communication between boron and
attached organic π systems. In contrast to absorption spectrum of
1 in the presence of fluoride, observation of a blue shift of
absorption band of 2 in the presence of cyanide indicates that,
40 cyanide ion binds to DCV unit and not to the boron center.
Compound 2 exhibits a “turn on” fluorescence response towards
cyanide ion (For 2 and 2+CNꢀ; ΦF = 0.35 and 3.87 %
respectively). Upon excitation at 340 nm, the intensity of dual
emission bands increased in presence of cyanide (Figure 8) (the
45 peak positions remains same). The 1H NMR spectrum of the 1: 1
mixture of compound 2 and TBACNꢀ in CDCl3 shows that, vinyl
proton resonance (8.29 ppm) was shifted to 4.89 ppm (Figure 9).
Similarly, the resonances of ring CꢀH protons of mesityl units are
also down fieldꢀshifted (From 6.74 to 6.3 ppm). Addition of
50 TBACN beyond 1 eq. caused no changes in the 1H NMR
spectrum of 2. The 11B NMR spectrum of 2+CN- does not show
any peak corresponding to Ar3B·CNꢀ species. Based on these
results one can conclude that the boron centre in 2 does not
interact with cyanide ion. The changes observed in the optical
55 features of 2+CNꢀ are solely due to the binding of cyanide to the
DCV unit.
70 Figure 10: Fluorescence spectrum of 1 (1×10ꢀ5 M, left) and 2
(1×10ꢀ5 M, right) in presence of various anions of 10 equivalents
in CHCl3.
When compounds 1 and 2 were titrated against different anions;
no significant changes in their optical features were observed.
75 This confirms that compound 1 and 2 are highly selective for Fꢀ
and CNꢀ.
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Figure 11: Photographs of 1 and 2 in presence of fluoride and 60 column chromatography (Hexane/ethylacetateꢀ99:1).Yield: 0.15 g
cyanide under UVꢀlight.
in 66.0 %. 1H NMR (400 MHz, CDCl3, δ ppm) 8.29 ( s, 1H), 6.74
(d, J = 4 Hz, 4H), 2.26 (s, 6H), 2.12 (s, 6H), 2.01 (s, 6H), 1.97 (s,
6H), 1.92 (s, 6H). 13C NMR (100 MHz, CDCl3, δ ppm) 166.57,
144.25, 144.24, 141.51, 140.27, 136.87, 131.68, 129.51, 113.21,
Conclusions
In summary, two new dicyanovinyl substituted triarylboranes 65 111.65, 91.61, 23.75, 23.19, 21.69, 20.34, 17.73. ESI mass calcl:
C32H35BN2 (M +Na+) 481.2786 found 481.2716. Anal. Calcd for
C32H35BN2: C, 83.84; H, 7.70; N, 6.11. Found: C, 83.73; H, 7.70;
N, 6.39.
5
are reported. It is found that molecular conformations have a
significant influence on optical properties. Compounds 1 and 2
act as selective sensors for fluoride and cyanide. Compound 2
exhibits “turn on” response towards both fluoride and cyanide
with different fluorescence features. The presence of two
Acknowledgement
10 different receptors in the rigid molecular framework of 2 enabled
us to distinguish the interfering anions such as fluoride and
cyanide. Anion binding mechanisms were established by NMR
experiments. For compound 1, Fꢀ binds to Ar3B unit and CNꢀ bind
to both Ar3B and DCV units and gives similar response. For
15 compound 2, fluoride selectively binds to boron centre and
cyanide binds to only vinylic carbon centre thereby giving
different fluorescence responses.
70 PT thanks; the Department of Science and Technology (DST)
New Delhi, and IIScꢀSTC Bangalore for the financial support.
GRK thanks UGC New Delhi for SRF.
Notes and references
Inorganic and Physical Chemistry Department, Indian Institute of
75 Science, Bangalore-560012, India. Fax: (+91) 8023601552; Tel: +91-80-
2293-3353; E-mail: thilagar@ipc.iisc.ernet.in
† Electronic Supplementary Information (ESI) available: NMR titrations.
CCDC no’s 975319 and 975320 See DOI: 10.1039/b000000x/
Experimental Section
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85
20
nꢀButyllithium (1.6 M in hexanes) and Malanonitrile were
purchased from Acros (india) and Spectrochem (India)
respectively. All reactions were carried under an atmosphere of
purified Nitrogen using standard schlenck techniques. THF was
distilled over sodium prior to use. HPLC grade solvents were
25 used for absorption and emission spectroscopic measurements.
All the UVꢀVis absorption and fluorescence titrations are carried
out at room temperature in freshly distilled solvents including
NMR titrations. Quantum yields are calculated using anthracene
as standard. Single crystals suitable for Xꢀray diffraction10 studies
30 are obtained by slow evaporation of CH2Cl2 solutions of 1 and 2
90
at 25°C. The (400 MHz) H NMR, (376 MHz) 19F NMR, (100
1
MHz) 13C NMR and (160 MHz) 11B NMR were recorded on a
Bruker Advance 400 MHz NMR spectrometer. Electronic
absorption spectra were recorded on a Perkin Elmer LAMBDA
35 750 UV/visible spectrophotometer. Fluorescence emission studies
were done using Horiba JOBIN YVON Fluoromaxꢀ4
spectrometer. Singleꢀcrystal Xꢀray diffraction studies were
carried out with a Bruker SMART APEX diffractometer
equipped with 3ꢀaxis goniometer.
95
100
105
110
115
120
125
40
Synthesis of 1: 4ꢀdimesitylborylbenzaldehyde (0.2 g, 0.564
mmol) and malanonitrile (0.55 mL, 0.53 mmol) are taken in a
mixture of DCM/ethanol (1:99) mixture. Catalytic amount of
piperidine was added and the reaction mixture stirred for one
hour. Evaporation of all the volatiles gave crude product which is
2
45 further purified by silica gel column chromatography
(Hexane/ethylacetateꢀ99:1). Yield: 0.17 g in 77.2 %. H NMR
1
(400 MHz, CDCl3, δ ppm) 7.85 (d, J = 8 Hz, 2H), 7.78 (s, 1H),
7.63 (d, J = 8 Hz, 2H), 6.83 (s, 4H), 2.31 (s, 6H), 1.97 (s, 12H).
13C NMR (100 MHz, CDCl3, δ ppm) 160.1, 141.2, 140.1, 136.7,
50 133.4, 130.4, 128.9, 114.1, 112.9, 84.0, 23.9, 21.7. ESI mass
calcl; C28H27BN2 (M +Na+) 425.2160 found 425.2032. Anal.
Calcd for C28H27BN2: C, 83.59; H, 6.76; N, 6.96. Found: C,
83.54; H, 7.05; N, 7.35.
Synthesis of 2: 4ꢀdimesitylboryl 2, 3, 5, 6ꢀ
55 tetramethylbenzaldehyde (0.2 g, 0.487 mmol) and malanonitrile
(0.55 mL, 0.53mmol) are taken in a mixture of DCM/ethanol
(1:99) mixture. Catalytic amount of piperidine was added and the
reaction mixture stirred for one hour. Evaporation of all the
volatiles gave crude product which is further purified by silica gel
Funct.
Mater.,
2013,
DOI:
10.1002/adfm.201302871.
6
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DOI: 10.1039/C3DT53638F
Dicyanovinyl Substituted Triarylboranes: A Rational Approach to Distinguish Fluoride
and Cyanide Ions
George Rajendra Kumar and Pakkirisamy Thilagar*
Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore-
560012, India. Phone: +91-80-2293-3353, Fax: (+91) 8023601552, E-mail:
Thilagar@ipc.iisc.ernet.in.
Two new dicyanovinyl (DCV) functionalized triarylboranes (Mes2BꢀπꢀDCV, for 1: π = C6H4, for 2, π
= 2, 3, 5, 6ꢀtetramethylꢀphenyl) are prepared and structurally characterized. A small structural
perturbation induced drastic changes in the optical properties of 1 and 2. The dual receptor
2
selectively discriminate Fꢀ and CNꢀ ions.