Triarylborane-dipyrromethane conjugates bearing dual receptor sites: The synthesis and evaluation of the anion binding site preference

Article abstract of DOI:10.1039/c3dt52565a

The synthesis and optical properties of four new triarylborane- dipyrromethane (TAB-DPM) conjugates (3a-d) containing dual binding sites (hydrogen bond donor and Lewis acid) have been reported. The new compounds exhibit a selective fluorogenic response towards the F^- ion. The NMR titrations show that the anions bind to the TAB-DPM conjugates via the Lewis acidic triarylborane centre in preference to the hydrogen bond donor (dipyrromethane) units.

Full text of DOI:10.1039/c3dt52565a

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DOI: 10.1039/C3DT52565A
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ARTICLE TYPE
TriarylboraneꢀDipyrromethane Conjugates Bearing Dual Receptor
Sites: Synthesis and Evaluation of Anion Binding Site Preference
P. Chinna Ayya Swamy, Ragam N. Priyanka and Thilagar Pakkirisamy*
5
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India. Eꢀmail:
thilagar@ipc.iisc.ernet.in;Fax: 0091ꢀ80ꢀ23601552; Tel: 0091ꢀ80ꢀ22933353
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Synthesis and optical properties of four new triarylborane–dipyrromethane (TABꢀDPM) conjugates (3aꢀ
10 d) containing dual binding sites (hydrogen bond donor and Lewis acid) have been reported. The new
compounds exhibit selective fluorogenic response towards F− ion. The NMR titrations show that the
anions bind to TABꢀDPM conjugates via the Lewis acidic triarylborane centre in preference to hydrogen
bond donor (dipyrromethane) units.
Introduction
Results and Discussion
15 Fluoride ion is well known for its advantageous (e.g. dental care,
anesthetics, osteoporosis and psychiatric drugs)1ꢀ4 and injurious
Synthesis and Spectral Characterisation
The protocol employed for the synthesis of compounds 3a, 3b, 3c
(water contamination5 and chemical warfare agent6) role in both 50 and 3d is shown in Scheme 1. Reaction of substituted aryl
health care and environment. Synthesis of selective sensors for
fluoride detection has become an important area of current
20 research. In the past three decades, numerous hydrogen bond
donor based fluoride sensors such as oligopyrrole7, amide8,
bromides (2ꢀbromoꢀ4ꢀ(diethoxymethyl)benzene for 2a, 1ꢀ
bromo(4ꢀ(diethoxymethyl)benzene for 2b, 2ꢀbromoꢀ4ꢀ(1,1ꢀ
diethoxyethyl)benzene
for
2c
and
1ꢀbromoꢀ4ꢀ(1,1ꢀ
diethoxyethyl)benzene for 2d with nꢀBuLi and BMes2F followed
indolocarbazole9, guanidium cation10, imidazolium cation11 and 55 by acidification gave the key precursors 2a, 2b, 2c and 2d. The
urea/thiourea12 have been reported. Anion receptors such as
organostannanes13, organosilanes14 and organostibanes15 have
25 also received considerable attention owing to their high
selectivity towards fluoride ion. Recently pnictonium16 ions have
Lewis acid catalyzed condensation reaction of pyrrole with
corresponding boryl functionalized benzaldehyde/acetophenone
(2aꢀd) afforded the target compounds 3a, 3b, 3c and 3d.
Compounds 3a, 3b, 3c and 3d are colourless solids and soluble in
been employed for the detection of fluoride ion in aqueous 60 common organic solvents. All the compounds were characterized
medium.
by NMR (1H, 13C, 11B) and high resolution mass spectrometry
(HRMS). The CꢀH and NꢀH proton signals of pyrrole moiety for
3c and 3d are upfield shifted compared to the corresponding
resonances for 3a and 3b. The 11B NMR shifts also follow the
Owing to their inherent Lewis acidity, triarylboranes17 have
30 received much attention in recent decades. Several boron
containing DonorꢀAcceptor (DꢀA) dyads18, boron containing
polyaryls19, boryl appended metallocenes20 and conjugated21/nonꢀ 65 same trend. This may be due to the positive inductive effect of
conjugated22 boron containing polymers have been investigated
for fluoride ion binding studies. Gabbai et al elegantly
35 demonstrated the utility of ammonium23/phosphonium24 ion
decorated triarylboranes for the detection of fluoride/cyanide.
Though several Hꢀbond donor based and Lewis acid based
fluoride sensors are known in the literature, the dual Lewis
acid/hydrogenꢀbond donor receptors have not been reported25.
40 Recently we became interested in developing colorimetric sensor
for fluoride and cyanide ions.26 As a part of the ongoing program
we have synthesized triarylborane–dipyrromethane (TABꢀDPM)
conjugates which can act as dual receptor ( containing Lewis
acidic boron and Hꢀbonding NꢀH binding sites) for fluoride
45 detection and studied their optical properties. The results of
these studies are reported in this paper.
the methyl group at the mesoꢀcarbon of 3c and 3d. The structure
of 3c was confirmed by single crystal Xꢀray diffraction studies.
Single Crystal Xꢀray Diffraction Studies
The molecular structure of 3c is shown in Figure 1. The
70 asymmetric unit cell of 3c contains two crystallographically
distinct molecules. The geometrical parameters of both the
molecules are almost the same. The boron centre in BAr3 unit
adopts trigonal planar geometry with the sum of CꢀBꢀC angles
being 360° which is inꢀline with the reported values for related
75 triarylboranes27. The dihedral angle between the BꢀC(ipso)3 plane
and spacer ꢀC6H4 is 72.5°. In the solid state the intermolecular Nꢀ
Hꢀꢀꢀπ (centre of pyrrole unit)
interaction between the
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neighboring molecules generates an interesting 3D 20 nm. The fluorescence quantum yields of 3aꢀd are in the range,
supramolecular structure. The Me groups on ꢀBMes2 unit
effectively protrude into the hydrophobic voids present in the 3D
structure. (The relevant data and figure are given in ESI).
0.30ꢀ0.41. The fluorescence emission maxima of 3b (498 nm) is
∼20 nm red shifted compared to that of 3a (469 nm), 3c (464 nm)
and 3d (470 nm). This is remarkable considering their structural
similarities. Fluorescence studies were carried out in solvents
25 with different polarities (Figure S15ꢀ Figure S18). The absorption
and emission profile of 3b is sensitive to solvent polarity while
the other compounds do not show any changes in their optical
properties.
5
30
Figure 2: Comparison of absorption (left) and emission (right) spectra of
dichloromethane solutions of 3aꢀd (1×10ꢀ5M), λex = 315 nm.
Scheme 1: Synthesis of 3a – 3d
Table 2: Photophysical Properties of triarylborane–
dipyrromethane (TAB-DPM) conjugates (3a-d)
Compound
λabs(nm) (ε/Mꢀ1
cmꢀ1)
λem (nm)
QF
3a
3b
310 (3.1 × 104)
260 (3.9 × 104)
318 (3.5× 104)
258 (3.0× 104)
469
498
0.38
0.30
3c
312 (3.7× 104)
260 (4.1× 104)
318 (4.5× 104)
262 (4.4× 104)
464
470
0.41
0.39
3d
35 aQuinine sulfate was used as the reference dye (Φ = 0.58 in 0.1 M H2SO4)
for the measurement of ΦF
The Lippert–Mataga plot4b was derived for all the four
compounds using the relationship between solvent polarity
parameter ꢁf and Stoke shift ꢁν (eqꢀ1). The excited state dipole
40 moments are higher than the ground state dipole moments. The
larger value for the change in dipole moment (ꢁꢁ) for 3b
clearly indicates that CT characteristics are stronger for 3b
compared to 3a, 3c and 3d. TDꢀDFT optimization results showed
significant reorganisation of the pyrrole units with respect to
45 BMes2 unit. Hence, the large Stokes shift observed for 3aꢀd are
due to significant geometric rearrangements in their excited
states. The distinct behavior of 3b (larger Stokes shift compared
to 3a, 3c and 3d) is possibly due to the combination of both CT
and excited state structural reorganizations.
10 Figure 1: Molecular structure of 3c
Table 1: Bond angles and bond lengths of 3c obtained from single crystal
Xꢀray diffraction studies
Bond length (Å)
Bond angle (º)
C1ꢀB1ꢀC26
B1ꢀC1
1.560(4)
1.574(5)
1.574(4)
1.538(4)
1.518(4)
1.510(3)
1.538(4)
115.3(2)
122.3(2)
122.2(2)
108.1(2)
108.0(2)
111.7(2)
109.5(3)
B1ꢀC26
B1ꢀC17
C3ꢀC7
C15ꢀC7
C8ꢀC7
C1ꢀB1ꢀC17
C17ꢀB1ꢀC26
C15ꢀC7ꢀC3
C8ꢀC7ꢀC16
C8ꢀC7ꢀC3
C16ꢀC7
C16ꢀC7ꢀC15
50
3
(νa ꢀ νf) = [2ꢁꢁ2 /hcao ] ∆f + A ꢀꢀꢀꢀꢀ eq 1
Optical Properties
ꢁꢁ is the electric dipole moment change upon an electronic
transition (ꢁꢁ) ꢁg ꢀ ꢁe, where ꢁg and ꢁe are the dipole moments
in the ground and excited states, respectively and, h, c, aO, and A
55 are the Planck’s constant, the speed of light, the Onsager radius
of 3aꢀd (since we don’t have crystal structure for 3a, 3b and 3d, a
value is assumed as ~8.16 Å based on crystal structure of 3c), and
UVꢀvisible and fluorescence spectra of compounds 3aꢀd are
15 shown in Figure 2. The absorption spectra show two major bands
corresponding to the dominant πꢀpπ (B) transition (~318 nm) and
π→π* transition (~260 nm) respectively. Upon excitation at ∼315
nm, dichloromethane solutions of 3aꢀd show single broad
emission band with maxima at 470 nm with stokes shift of ∼152
2
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a constant, respectively.
∆f = [Ds – 1/2Ds + 1] ꢀ [n2 – 1/2n2 + 1]
where Ds and n are the dielectric constant and refractive index of
the solvent, respectively.
5
Table 3: Excited state and ground state dipole moments of 3aꢀd
Compounds
(ꢂg) (D)a
1.2
1.8
1.7
1.7
(ꢂe) (D)
~7.1
~8.9
~7.5
~7.9
ꢂeꢀ ꢂg (D)
~5.8
~7.1
~5.8
~6.1
30
3a
3b
3c
3d
Figure 5: Absorption (left) and emission (right) spectra of 3b (10 ꢃM in
CH2Cl2, λex = 315 nm) in the presence of various anions (1×10ꢀ3M)
aground state dipole moment obtained from DFT
Table 4: Stern–Volmer quenching constant of 3aꢀd
Compounds
Quenching constant(KSV
)
3a
3b
3c
3d
1.8×105
1.3×105
1.6×105
2.3×105
35 High selectivity is mandatory for a superior sensor. Achieving
high selectivity for anions of interest over other potentially
competing species is an challenging task in the area of anion
probe development. To evaluate the selectivity of the new hybrid
sensors 3a, 3b, 3c and 3d, spectrophotometric titrations were
40 carried out in the presence of various anions (Figure 4 and ESI).
As depicted in Figure 3, only the addition of Fꢀ resulted in
prominent decrease in fluorescence intensity at ∼480 nm, whereas
10
Figure 3. LippertꢀMataga plots for 3aꢀd.
ꢀ
addition of an excess of anions such as Clꢀ and H2PO4 caused
Anion Binding Studies
very little change in the emission spectra. Other competing anions
45 such as Brꢀ, Iꢀ, NO3 , HSO4 , ClO4 and CNꢀ caused almost no
changes in the emission intensity. This is quite remarkable
considering the possible hydrogen bonding interaction between
NꢀH of DPM unit of the host and the guest anions. This shows
that the compounds 3aꢀd are highly selective towards fluoride
50 ions. The quenching constants of 3aꢀd were calculated from
Stern–Volmer plot (ESI and Table 4). The results indicate that 3d
shows the highest Stern–Volmer quenching constant (KSV) and
3b the least. Thus 3d is the superior fluoride sensor among these
four compounds.
ꢀ
ꢀ
ꢀ
All the compounds showed similar changes in absorption profile
in the presence of fluoride ions (Figure 3 and supporting
15 information). Upon addition of fluoride ion, the intensity of
characteristic boryl absorption band at 315 nm decreased;
concomitantly the intensity of the band at ~230 nm (π → π*
absorption of mesitylene unit) increased. This indicates that the
coordination of the Fꢀ ion to the boron centre of triarylborane
20 moiety disrupts the π →pπ conjugation between the boron centre
and the attached π conjugated system. These results are in line
with those reported for TABs17. Fluorescence emission studies
revealed that upon addition of TBAF, the broad emission band of
3a, 3b, 3c and 3d show gradual quenching as observed for
25 triarylborane based fluoride sensors.
55
NMR Titrations
To get further insight into the anion binding event, the 19F and
1H NMR titrations were carried out in CDCl3. In 19F NMR
titrations (5×10ꢀ2 M), upon addition of TBAF to 3aꢀd, a new peak
60 at ꢀ172 ppm corresponding to FꢀBAr3 was observed28. Addition of
more than 1.25 Eq of fluoride ion gave rise to an additional peak
at −81 ppm corresponding to FꢄꢄꢄHꢀNꢀR unit. Based on these
results one can tentatively conclude that the initially fluoride
prefers to bind at the triarylboron centre rather than the hydrogen
65 bonding donor sites (dipyrromethane) in 3aꢀd. However, the
intensity of the peak corresponding to R3BꢀF is still growing
(with onset of peak corresponding to NꢀH—F) upon addition of
more than 1eq of fluoride source. Addition of 1 eq of TBAF also
resulted in a downꢀfield shift of the NꢀH resonance of the
70 receptors. The 1H and 19F NMR data show that there is a
competition between the two binding sites (Ar3B and pyrrole Nꢀ
H), with exchange of the Fꢀ anion between them. Thus, NMR data
do not exclude the possibility of binding of fluoride to NꢀH units
before saturating the Ar3B moiety. The 1H and 19F NMR data also
Figure 4: Absorption (left) and emission (right) spectra of 3b (10 ꢃM in
CH2Cl2, λex = 315 nm) in the presence of TBAF(1×10ꢀ3 M)
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indicate that titration of the receptors 3aꢀd is complete when 3 eq
of Fꢀ were added. This result is consistent with one BꢀF and two
NꢀH...F interactions (Chart 1 ‘A’).
To gain further insight into the possible fluoride adducts, variable
1
temperature H and 19F NMR titrations were carried out in the
1
range 20 to ꢀ50°C in CDCl3. The H NMR spectrum of a 1:1
20 mixture of receptor and TBAF shows a broad NꢀH signal at 8
ppm, which becomes sharp upon decreasing the temperature from
20 to ꢀ50 °C. For the 1:3 mixtures of receptor (1 eq) and TBAF (3
eq), this signal (at 8 ppm) is shifted downꢀfield (3a: 10.3, 3b is
11.0, 3c is 10.3 and 3d is 10.9 ppm) at 20° C. This signal is
25 further shifted downꢀfiled and progressively broadened upon
decreasing the temperature from 20 to ꢀ40 °C. At ꢀ50° C this peak
could not be observed. A new triplet at 16 ppm with a coupling
constant of 123 Hz is appeares in the temperature range ꢀ10 to ꢀ50
° C. The chemical shift and the coupling constant of this new
30 peak are close to those observed for F2Hꢀ species reported
elsewhere34. One can envisage the formation of HF2 by two
different routes. (1) reaction of adventious water in the medium
with the fluoride source (ii) deprotonation of CꢀH of
triarylmethine units in receptors 3aꢀb . The observation of the
35 signal corresponding to triarylmethine units in the 1H NMR
spectra of 3aꢀb under titration conditions, excludes the possibility
of CꢀH deprotonation (Figure S72 and S73). Hence the only
plausible interpretation of the new triplet at 16 ppm is the
formation HF2− ion due to adventious water in the medium35. This
40 inference is also consistent with results of variable temperature
19F NMR measurements carried out for 1:3 mixtures of receptor
and TBAF. The proton coupled 19F NMR shows two 19F signals
at ꢀ80.5 (doublet, JNꢀHꢀF = 54 Hz for 3a, 53 Hz for 3b, 53 Hz for
3c and 53 Hz for 3d) and broad signal at ꢀ172 ppm respectively.
45 In proton decoupled experiment, compounds 3aꢀd give rise to a
singlet at ꢀ80.5 ppm, but there is no change in the peak at ꢀ172
ppm at 20 °C. Thus, the signal at ꢀ172 is assigned to Ar3BꢀF unit
and the peak at ꢀ80 ppm is attributed to NꢀHꢀꢀꢀF unit (Figure S37
to S44). Upon decreasing the temperature from 20 °C to ꢀ50 °C,
50 in addition to ꢀ80 and ꢀ172 ppm peaks, a new signal at ꢀ156 ppm
gradually gained in intensity and the peak at ꢀ172 disappeared
completely. In proton decoupled 19F NMR titrations, the peak at
ꢀ156 ppm shows two singlets at ꢀ154 and ꢀ155 ppm respectively,
whereas in proton coupled 19F NMR spectrum, the peak at ꢀ154
55 become doublet (JHꢀF = 123 Hz) and the signal at ꢀ155 ppm
remains singlet in the same region. The signal at ꢀ155 is
attributable to Ar3BꢀF and the doublet at ꢀ154 ppm is attributed to
F2H−. As reported elsewhere the signal corresponding to F2H− ion
could be observed only at low temperatures36.
5
Figure 6: 19F NMR titration spectra of 3b solution in CDCl3 (5 × 10ꢀ2M )
against increasing TBAF (2.1 M in CDCl3) (0 to 5 Eq). a represents
resonance of Fꢀ BAr3, b represents resonance of NꢀHꢄꢄꢄF and c represents
resonance of the anion source TBAF.
10
Figure 7: 1H NMR of 3b (5×10ꢀ2 M in CDCl3) with various anions (2.1 M
in CDCl3)
60
Figure 9: DFT B3LYP/6ꢀ31G(d) optimised structures of 3aꢀd (from left
to right respectively; Colour codes, C = Black, H = Yellow, N = Blue, B
= Magenta)
65 DFT Computational Studies:
15 Figure 8: 1H NMR titration spectra of 3b solution in CDCl3 (5 × 10ꢀ2M )
against increasing TBAF (2.1 M in CDCl3) (0 to 10 Eq) – NꢀH region
To understand the electronic structure of 3aꢀd, DFT
computational studies were performed. The hybrid B3LYP
4
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functional30 has been used in all computations as incorporated in
bromophenyl)ethanone,
were purchased from Avra
Gaussian 09 package,31 mixing the HartreeꢀFockꢀtype exchange
with Becke’s exchange functional32 as proposed by LeeꢀYangꢀ
Parr for the correlation contribution33 We have considered 6ꢀ
chemicals(India) and pyrrole was purchased from SRL (India).
All reactions were carried out under an atmosphere of purified
nitrogen using Schlenck techniques. THF and pyrrole were
5
31G(d) basis set for all the atoms. Visualizations of the optimized 50 distilled over sodium. Chlorinated solvents were distilled over
structures and the MOs were performed using Gaussview5.0. The
harmonic force constants were computed at the optimized
geometries to characterize the stationary points as minima. TDꢀ
DFT vertical excitation calculations were performed for 3aꢀd
CaH2 and subsequently stored over 4Å molecular sieves. The
NMR spectra were recorded on a Bruker Avance 400 MHz NMR
1
spectrometer. All solution H and 13C spectra were referenced
internally to the solvent signal. 11B and 19F NMR spectra were
55 externally referenced to BF3.Et2O (δ=0) in C6D6.UVꢀVisible
absorption data were acquired on Lambda 750ꢀPerkin Elmer UVꢀ
10 based on their ground state optimized structures.
visible spectrophotometer. Solutions were prepared in
a
microbalance (± 0.1 mg) and volumetric glasswares and then
charged in quartz cuvettes with sealing screw caps.
60 Spectrophotometric titrations were performed on solutions of 3a,
3b, 3c and 3d in CH2Cl2. Tetrabutylammonium fluoride (TBAF)
(1M) in THF, diluted in CH2Cl2 was added as the Fꢀ ion source
and the absorption/fluorescence emission spectra of the samples
were recorded. The excitation wavelength was λex = 315 nm, the
65 excitation and the emission slit widths were set at 3.0 nm, and the
emission spectra were recorded. Absorption and emission spectra
were recorded sequentially until no further changes were
observed. Singleꢀcrystal Xꢀray diffraction studies were carried
out with a Bruker SMART APEX diffractometer equipped with
70 3ꢀaxis goniometer. The data was integrated using SAINT, and an
empirical absorption correction was applied with SADABS. The
structure was solved by direct methods and refined by full matrix
leastꢀsquares on F2 using SHELXTL software29. All the nonꢀ
hydrogen atoms were refined with anisotropic displacement
75 parameters, while the hydrogen atoms were refined isotropically
on the positions calculated using a riding model. CCDC number
for 3c: 951177.
Figure 10: DFT B3LYP/6ꢀ31G(d) obtained FMOs (Frontier molecular
orbitals) of 3aꢀd (from left to right respectively, isovalue = 0.02)
The ground state optimized structures for compound 3aꢀd are
15 shown in figure 9. Evidently, the opposite orientations of the
neighboring pyrrole moieties are consistent with the Xꢀray crystal
structure of 3c. The FMOs of the compounds are localised on
individual chromophoric units indicating virtually no electronic
communication between the two individual moieties. However,
20 the HOMOs 3aꢀd are concentrated on the pyrrolic moieties and
the LUMOs are localised on the triarylborane moieties which
apparently indicates the possibility of a dipyrromethane to BMes2
(i.e. HOMO→LUMO) charge transfer process (~380ꢀ390 nm).
However, TDꢀDFT (†ESI) vertical excitation calculations show
25 that these transitions have low oscillator strengths with respect to
BMes2 centered πꢀpπ* transitions at ~350 nm and thus appears
only as a very weak feature in their UVꢀVis absorption spectra.
The HOMOꢀLUMO band gaps (Figure 10) for 3aꢀd are close to
each other which would explain their identical UVꢀVis absorption
30 profile.
Synthesis of 1a
3ꢀBromobenzaldehyde (9.5 g, 51.41 mmol), triethylorthoformate
80 (113.24 mmol) and catalytic amount of conc. HCl were dissolved
in ethanol and the resultant solution was refluxed for 4 h. After
all the 3ꢀBromobenzaldehyde was consumed, reaction mixture
was brought to room temperature and extracted with mixture of
cold water/ethyl acetate. The combined organic layer was washed
85 with brine and dried over anhydrous Na2SO4. The volatiles
removed under reduced pressure afforded 1 as a colourless liquid.
1
Yield: 14.2 g, 99%. H NMR (399.99 MHz, CDCl3) δ (ppm):
Conclusions
7.65 (s, 1H (oꢀPh)), 7.35 (m, 2H (o, mꢀPh)), 7.28 (d, J = 12 Hz
1H (pꢀPh)) 5.48 (s, 1H (Methine CꢀH)), 3.76ꢀ3.63(m, 4H (ꢀCH2)),
90 1.28 (t, J= 6.8 Hz, 7.2 Hz, 6H (ꢀCH3)).
We have successfully synthesized and characterized four
triarylboron decorated dipyrromethanes (3a, 3b, 3c and 3d).
Their anion sensing abilities were studied using UV/visible,
35 fluorescence and NMR (1H and 19F) spectroscopy. DFT
computational studies show that the individual chromophores in
3aꢀd are electronically isolated. The new conjugates exhibit high
selective response towards fluoride ions compared to other
anions. The NMR measurements show that there is a competition
40 between the two binding environments (Ar3B and pyrrole NꢀH),
with exchange of the Fꢀ anion between them.
Synthesis of 1b
Compound 1b was prepared following a procedure similar to that
used for 1a. The quantities involved and characterization data are
as follows. 4ꢀBromobenzaldehyde (9.5 g, 51.41 mmol),
95 triethylorthoformate (113.24 mmol), catalytic amount of con.
HCl. Yield: 14.0 g, 98%. 1H NMR (399.99 MHz, CDCl3) δ
(ppm): 7.49 (d, J = 6.8 Hz, 2H (oꢀPh)), 7.35 (d, J = 8 Hz, 2H (mꢀ
Ph)), 5.46(s, 1H (Methine CꢀH)), 3.63ꢀ3.49(m, 4H (ꢀCH2)), 1.24
(t, J= 6.8 Hz, 7.2 Hz, 6H (ꢀCH3)).
Experimental Section
100 Synthesis of 1c
Compound 1c was prepared following a procedure similar to that
used for 1a. The quantities involved and characterization data are
as follows. 3 – Bromoacetophenone (5.0 g, 25 mmol), CH(OEt)3
(12.60 mL, 85 mmol), catalytic amount of HCl. Yield: 6.8 g,
Methods and Materials
nꢀbutyllithium (1.6 M in hexane), 3ꢀbromobenzaldehyde, 4ꢀ
45 bromobenzaldehyde,
1ꢀ(3ꢀbromophenyl)ethanone,
1ꢀ(4ꢀ
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5

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1
99%. H NMR (399.99 MHz, CDCl3), δ (ppm): 7.69 (s, 1H (oꢀ 60 δ (ppm): 71.7. HRMS (TOFꢀMS ES) m/z calcd for C26H29BONa
Ph)), 7.44 (m, 2H (o, mꢀPh)), 7.22 (d, J = 7.6 Hz, 1H (pꢀPh)),
3.51ꢀ3.30 (m, 4H (ꢀCH2)), 1.52 (s, 3H (ꢀCH3)), 1.22 (t, J = 2.8
Hz, 7.2 Hz, 6H (ꢀCH3)).
(M + Na)+ 391.2209, found 391.2206.
Synthesis of 2d
Compound 2d was prepared following a procedure similar to that
used for 2a. The quantities involved and characterization data are
5
Synthesis of 1d
Compound 1d was prepared following a procedure similar to that 65 as follows. 1d (3.5 g, 12.81 mmol), nꢀbutyllithium (8.8 mL (1.6
used for 1a. The quantities involved and characterization data are
as follows. 4 – Bromoacetophenone (5.0 g, 25 mmol), CH(OEt)3
(12.60 mL, 85 mmol), catalytic amount of HCl. Yield: 6.9 g, 99%
M solution in hexane), 14.09 mmol), bismesitylfluoroborane (4.1
g, 15.37 mmol), dry THF (30 mL + 15 mL), HCl (20 mL, 2N).
Recrystallization of crude product in ethyl acetate gave pure 8 as
1
10 ; 1H NMR (399.99 MHz, CDCl3), δ (ppm): 7.46 (d, J = 8 Hz, 2H
a colourless solid. Yield: 3.8 g, 80%. H NMR (399.99 MHz,
(oꢀPh)), 7.40 (d, J = 8 Hz, 2H ( mꢀPh)), 3.50ꢀ3.42 (m, 2H (ꢀCH2)), 70 CDCl3), δ (ppm): 7.90 (d, J = 8 Hz, 2H (oꢀPh)), 7.59 (d, J = 8 Hz,
3.37ꢀ3.29 (m, 2H (ꢀCH2)), 1.52 (s, 3H (ꢀCH3)), 1.23ꢀ1.18 (t, J =
4.8 Hz, J = 4.8 Hz, 6H (ꢀCH3)).
Synthesis of 2a
15 A solution of 1a (2.5 g, 9.64 mmol) in dry THF was degassed by
purging N2 for 30 min followed by cooling to ꢀ78oC (Acetone/liqꢀ
N2). nꢀbutyllithium (6.6 mL (1.6 M solution in hexane), 10.61
mmol) was added over 30 min. After 1 h, a solution of
bismesitylfluoroborane (2.9 g, 10.08 mmol) in 15 mL of dry THF
20 was added over 10 min. The reaction mixture was allowed to
2H (mꢀPh)), 6.83 (d, J = 6.4 Hz, 4H (Mes CꢀH)), 2.62 (s, 3H (ꢀ
CH3)), 2.31ꢀ1.97 (s, 18H (MesꢀCH3)). 13C NMR (100.00 MHz,
CDCl3), δ (ppm): 199.1, 141.3, 139.7, 139.4, 136.3, 128.8, 128.6,
128.1, 27.3, 23.9, 22.8, 21.8. 11B NMR (160 MHz, CDCl3), δ
75 (ppm): 74.1. HRMS (TOFꢀMS ES) m/z calcd for C26H29BONa
(M + Na)+ 391.2209, found 391.2205.
Synthesis of compound 3a
Pyrrole (9.0 mL, 102.92 mmol) and 2a (1.0 g, 2.82 mmol) were
stirred at room temperature under nitrogen atmosphere for 30 min
warm to room temperature and stirring was continued for 12 h. 80 then one drop BF3ꢄEt2O was added. The resultant mixture was
After 12 h, 30 mL of 1N HCl was added and stirring was
continued for another 4 h and extracted with ether. The combined
organic layers were washed with brine solution and dried over
25 anhydrous Na2SO4. Evaporation of the solvents under reduced
stirred for another 6 h at room temperature and then quenched
with 2N NaOH solution. The crude product was extracted with
ethyl acetate and dried over anhydrous Na2SO4. Column
chromatography on a neutral alumina (ethyl acetate and pet ether
pressure yielded crude product. Recrystallization of crude product 85 in the ratio 0.2:9.8) afforded pure 3a as a grey colour solid. Yield
in ethyl acetate gave pure 2a as a colourless solid. Yield: 2.4 g,
64% ; H NMR (399.99 MHz, CDCl3), δ (ppm): 9.89 (s, 1H (ꢀ
0.8 g, 64%. 1H NMR (399.99 MHz, CDCl3), δ (ppm): 7.92(br, Nꢀ
H), 7.45 (s, 1H (oꢀPh)), 7.41 (d, J = 4.4 Hz, 1H (mꢀPh)), 7.29 (m,
1H (o, pꢀPh)), 6.79 (s, 4H (Mes CꢀH)), 6.67 (d, J = 1.6 Hz, 2H (βꢀ
H pyrrolic)), 6.13 (d, J = 3.2 Hz, 2H (αꢀ H pyrrolic)), 5.82 (s, 2H
1
CHO)), 8.01 (m, 2H (oꢀPh)), 7.77 (d, J = 6 Hz, 2H (mꢀPh)), 7.53
30 (d, J = 7.2 Hz, 1H (mꢀPh)) 6.83 (s, 4H (Mes CꢀH)), 2.31, 1.98 (s,
18H (MesꢀCH3)). 13C NMR (100.00 MHz, CDCl3), δ (ppm): 90 (αꢀH pyrrolic)), 5.38 (s, 1H (meso CꢀH)), 2.30 (s, 6H (mesꢀCH3)),
193.4, 142.5, 141.2, 139.7, 138.6, 136.6, 132.2, 129.3, 128.9,
23.9, 21.7. 11B NMR (160 MHz, CDCl3), δ (ppm): 75.7.
Synthesis of 2b
35 Compound 2b was prepared following a procedure similar to that
used for 2a. The quantities involved and characterization data are
as follows. Compound 1b (2.5 g, 9.64 mmol), nꢀbutyllithium (6.6
1.97 (s, 12H (MesꢀCH3)). 13C NMR (100.0 MHz, CDCl3), δ
(ppm): 142.0, 141.2, 139.1, 137.0, 135.6, 133.1, 132.2, 129.8,
128.6, 117.6, 108.8, 107.6, 44.5, 23.9, 21.7. 11B NMR (160 MHz,
CDCl3), δ (ppm): 69.6. HRMS (TOFꢀMS ES) m/z calcd for
95 C33H34BN2 (M+) 469.2815, found 469.2825.
Synthesis of compound 3b
mL (1.6
M
solution in hexane), 10.61 mmol),
Compound 3b was prepared following a procedure similar to that
used for 3a. The quantities involved and characterization data are
as follows. Pyrrole (1.5 mL, 21.72 mmol), 2b (0.2 g, 0.54 mmol),
bismesitylfluoroborane (2.9 g, 10.08 mmol), 30 mL of 1N HCl.
40 Recrystallization of crude product in ethyl acetate gave pure 2b
as a colourless solid. Yield: 2.0 g, 59%. H NMR (399.99 MHz, 100 BF3⋅Et2O (one drop). The pure compound was obtained as a dirty
1
1
CDCl3), δ (ppm): 10.07 (s, 1H (ꢀCHO)), 7.85 (d, J = 8 Hz, 2H (oꢀ
Ph)), 7.66 (d, J = 8 Hz, 2H (mꢀPh)), 6.83 (s, 4H (Mes CꢀH)), 2.32,
1.98 (s, 18H (MesꢀCH3)). 13C NMR (100.00 MHz, CDCl3), δ
45 (ppm): 193.3, 141.3, 139.9, 138.4, 136.4, 129.5, 128.8, 23.9,
21.7. 11B NMR (160 MHz, CDCl3), δ (ppm): 76.7.
white colour solid. Yield 0.1 g, 55 %. H NMR (399.99 MHz,
CDCl3), δ (ppm): 7.82(br, NꢀH), 7.45 (d, J = 8 Hz, 2H ( mꢀPh)),
7.14 (d, J = 8 Hz, 2H (oꢀPh)), 6.8 (s, 4H (Mes CꢀH)), 6.56 (d, J =
1.6 Hz, 2H (βꢀH pyrrolic)), 6.11 (d, J = 3.2 Hz, 2H (αꢀ H
105 pyrrolic)), 5.8 (s, 2H (αꢀH pyrrolic)), 5.38 (s, 1H (meso CꢀH)),
2.25 (s, 6H (mesꢀCH3)), 2.01 (s, 12H (MesꢀCH3)). 13C NMR
(100.00 MHz, CDCl3), δ (ppm): 146.7, 145.0, 142.3, 141.3,
139.2, 137.4, 132.8, 128.8, 128.6, 118.0, 108.9, 108.0, 44.6, 24.0,
21.8, 21.6. 11B NMR (160 MHz, CDCl3), δ (ppm): 68.1. HRMS
110 (TOFꢀMS ES) m/z calcd for C33H34BN2 (M+) 469.2815, found
469.2816.
Synthesis of 2c
Compound 2c was prepared following a procedure similar to that
used for 2a. The quantities involved and characterization data are
50 as follows. Compound 1c (3.5 g, 12.81 mmol), n ꢀ butyllithium
(8.8 mL (1.6
M
solution in hexane), 14.09 mmol),
bismesitylfluoroborane (4.1 g, 15.37 mmol), 20 mL of 2N HCl.
Recrystallization of crude product in ethyl acetate gave pure 2c as
a colourless solid. Yield: 4.0 g, 85%. H NMR (399.99 MHz,
55 CDCl3), δ (ppm): 8.07 (m, 2H (o, pꢀPh)), 7.71 (d, J = 6.4 Hz, 1H
Synthesis of compound 3c
Compound 3c was prepared following a procedure similar to that
used for 3a. The quantities involved and characterization data are
1
(oꢀPh)), 7.47 (d, J = 8 Hz, 1H (mꢀPh)), 6.83 (s, 4H (Mes CꢀH)), 115 as follows. Pyrrole (1.5 mL, 21.72 mmol), 2c (0.2 g, .54.03
2.54 (s, 3H (ꢀCH3)), 2.31ꢀ1.98 (s, 18H (MesꢀCH3)). 13C NMR
(100.00 MHz, CDCl3), δ (ppm): 199.2, 141.3, 139.6, 137.4,
136.3, 131.9, 128.9, 27.3, 24, 21.8. 11B NMR (160 MHz, CDCl3),
mmol) BF3ꢄEt2O (one drop). Yield: 50 mg, 19%. 1H NMR
(399.99 MHz, CDCl3), δ (ppm): 7.82 (s, br, 2H (NꢀH)), 7.48 (s,
1H (oꢀPh)), 7.41 (d, J = 4.4 Hz, 1H (mꢀPh)), 7.30 (m, 2H (o, pꢀ
6
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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DOI: 10.1039/C3DT52565A
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Ph)), 7.13 (d, J = 4 Hz, 1H (pꢀPh)), 6.84 (s, 4H (Mes CꢀH)), 6.67
55
(d, J = 2.8 Hz, 2H (βꢀH pyrrolic)), 6.18 (d, J = 2.8 Hz, 2H (αꢀ H
pyrrolic)), 5.92 (d, J = 1.6 Hz, 2H (βꢀH pyrrolic)), 2.35(s, 6H
(mesꢀCH3)), 1.97 (s, 12H (MesꢀCH3)), 1.34 (s, 3H (mesoꢀCH3)).
13C NMR (100.00 MHz, CDCl3), δ (ppm): 151.5, 142.2, 139.0,
137.4, 136.8, 128.6, 127.2, 117.4, 108.7, 106.8, 23.9, 21.7. 11B
NMR (160.4 MHz, CDCl3), δ (ppm): 62.0. HRMS (TOFꢀMS ES)
m/z calcd for C34H37BN2Na (M + Na)+ 507.2947, found 507.2942
and 418.2677 (M+_C4H4N).
5
60
65
70
75
80
85
10 Synthesis of compound 3d
Compound 3d was prepared following a procedure similar to that
used for 3a. The quantities involved and characterization data are
as follows. Pyrrole (1.5 mL, 21.72 mmol), 2d (0.2 g, 0.54 mmol),
BF3ꢄEt2O (one drop). Yield: 45 mg, 17%. 1H NMR (399.99 MHz,
15 CDCl3), δ (ppm): 7.83 (s, br, 1H (NꢀH)), 7.45 (d, J = 8 Hz, 2H (oꢀ
Ph)), 7.15 (d, J = 8 Hz, 2H (mꢀPh)), 6.79 (s, 4H (MesꢀCH)), 6.57
(t, J = 2.4 Hz, 2H (αꢀH pyrrolic)), 6.11 (d, J = 2.4 Hz, 2H (βꢀH
pyrrolic)), 5.81 (s, 2H (βꢀH pyrrolic)), 2.27 (s, 6H (MesꢀCH3)),
1.99 (s, 12H (MesꢀCH3)), 1.32 (s, 3H (MesoꢀCH3)). 13C NMR
20 (100.00 MHz, CDCl3), δ (ppm): 171.9, 146.7, 145.0, 142.3,
141.3, 139.2, 137.3, 132.8, 128.8, 118.0, 108.9, 44.6, 42.0, 24.0,
21.8, 14.8. 11B NMR (160 MHz, CDCl3), δ (ppm): 64.2. HRMS
(TOFꢀMS ES) m/z calcd for C34H37BN2Na (M + Na)+ 507.2942,
found 507.2950 and 418.2677 (M+ꢀC4H4N).
25 ACKNOWLEDGMENTS
PT thanks the Department of Science and Technology (DST),
New Delhi, CSIR, New Delhi, and the ISROꢀIISc Space
Technology Cell for the financial support. Mr CAS thanks IISc
for SRF and authors are grateful to Mr. Sanjoy Mukherjee for
30 help in the theoretical calculations.
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Notes and references
aDepartment of Inorganic and Physical Chemistry, Indian Institute of
Science, Bangalore, India. Fax: 0091ꢀ80ꢀ23601552; Tel: 0091ꢀ80ꢀ
22933353 Eꢀmail: thilagar@ipc.iisc.ernet.in
35 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
40 spectral data, and crystallographic data.
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DOI: 10.1039/C3DT52565A
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DOI: 10.1039/C3DT52565A
Triarylborane-Dipyrromethane Conjugates Bearing Dual
Receptor Sites: Synthesis and Evaluation of Anion Binding Site
Preference
Ragam N. Priyanka, P. Chinna Ayya Swamy and Thilagar Pakkirisamy*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012,
India. E-mail: thilagar@ipc.iisc.ernet.in;Fax: 0091-80-23601552; Tel: 0091-80-22933353
Synthesis and optical properties of four new triarylborane–dipyrromethane (TAB-DPM)
conjugates (3a-d) containing dual binding sites (hydrogen bond donor and Lewis acid) have
been reported. The titration results show that the anions bind to TAB-DPM conjugates via the
Lewis acidic triarylborane centre in preference to hydrogen bond donor (dipyrromethane)
units.