Triazole-appended BODIPY-piperazine conjugates and their efficacy toward mercury sensing

Article abstract of DOI:10.1039/c4nj01625d

An expeditious synthesis of new click reaction-based BODIPY-piperazine conjugates separated by alkyl spacers (4-7) has been described. The compounds under investigation have been thoroughly characterized by various physicochemical techniques viz., element

Full text of DOI:10.1039/c4nj01625d

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Triazole appended BODIPY-piperazine conjugates and their efficacy toward
mercury sensing
Roop Shikha Singh, Rakesh Kumar Gupta, Rajendra Prasad Paitandi, Arvind Misra, and Daya
Shankar Pandey*
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi - 221 005
(U.P.) India
___________________________________________________________________
Abstract
An expeditious synthesis of new click based BODIPY-piperazine conjugates separated by alkyl
spacers (4−7) has been described. The compounds under investigation have been thoroughly
characterized by various physicochemical techniques viz., elemental analyses, IR, HRMS, NMR
(1H, 13C, 11B and 19F), electronic absorption, emission and theoretical studies. Comparative
sensing ability of 4−7 toward a range of metal ions has been investigated by various methods.
Amongst these compounds only 7 exhibited appreciable selectivity towards Hg2+, while other
remain silent in presence of various metal ions. Binding constant and Job’s plot analysis
indicated 1:1 stoichiometry between 7 and Hg2+. HRMS data and theoretical studies undoubtedly
indicated formation of 7·Hg2+ complex and interaction of Hg2+ with the probe through nitrogen
atom from the piperazine and triazole units.
----------------------------------------------------------------------------------------------------------------
* To whom correspondence should be addressed. E mail: dspbhu@bhu.ac.in. Phone + 91 542
6702480; FAX + 91 542 2368174

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Introduction
Contemporary chemical research has seen a large surge toward development of novel systems
for detection of heavy and transition metal ions (HTMs) because of their association with the
environmental and health issues.1 Among these, Hg2+ in its various forms is known as a
detrimental neurological toxin which is continuously being introduced in the environment by
various natural processes, industrial releases, and anthropogenic activities.2 Its bio-accumulation
in the living tissues of human and animal bodies via food chain causes mercury poisoning,
serious neural disorder, and other diseases.1,2 Numerous reports dealing with detection of Hg2+
are based on fluorescence quenching, because Hg2+ induces spin-orbit coupling3 and is spectro-
scopically and magnetically silent (filled d10 orbitals). Therefore, common analytical techniques
like nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) are not
adequately proficient for detection of Hg2+ in the environment and biological systems. In this
context, efforts have been made toward development of various optochemical sensors employing
diverse chemical transducers and optical principles.4 Moreover, development of naked-eye
chemosensors for detection of HTMs have drawn special attention, since “naked-eye” detection
can offer desired qualitative and quantitative information. In this regard, chemosensors based on
rhodamine5, fluorescein5, naphthalimide5, porphyrin5 and 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (BODIPY) etc. have drawn attention of many research groups. Among these,
BODIPYs are preferred in the development of fluorescent sensors due to their robustness against
light and chemicals, high extinction coefficients, excellent quantum yield, and good solubility.
Moreover, spectroscopic and photophysical properties of BODIPYs can be fine tuned by
modifying dipyrrin core.6
Additionally, Cu(I)-catalyzed click reactions between terminal alkynes and azides have been
extremely popular in creating molecular diversity. These simple transformations lead to highly
regioselective 1,4-di-substituted triazoles in excellent yield and are beneficial in construction of
the ligands with cation-chelating ability.7 Moreover, substituted piperazine-based ligands have
been demanding since these adopt various conformations and can control geometry of the metal
complexes.8,16 With an objective of developing selective and sensitive fluorescent sensors
containing a fluorophore and metal recognition unit i.e, fluoroionophore a series of triazole
appended BODIPY-piperazine conjugates 4, 5, 6 and 7 have been successfully synthesized.
Possibility of the metal recognition has been explored by varying fluoroionohpores from un-

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substituted to N-substituted piperazine and triazole units employing single (4−6) to double click
(7) reactions. The fluorophore–spacer–chelator construct working on the principle of photo-
induced electron transfer (PET) based fluorescence quenching enables 7 to serve as a selective
fluoro- and chromogenic probe for Hg2+. Through this contribution we present a small library of
BODIPY-piperazine conjugates 4, 5, 6 and 7. Also, we describe herein their comparative sensing
ability toward various metal ions. The studies advocated that 7 exhibits significant chromo- and
fluorogenic response toward Hg2+.
Results and discussion
As shown in scheme 1, azido BODIPY 2 has been synthesized following literature procedures.9
Piperazine derivatives reacted with propargyl bromide in presence of K2CO3 to afford terminal
alkynes 3a−d in good yield. Finally, single click reaction between the azido BODIPY 2 and 3a−c
in presence of CuSO4 and sodium ascorbate afforded 4, 5, and 6 while a double click between 3d
and 2 gave 7 in reasonably good yield.9 The synthetic pathway adopted for 4−7 is summarized in
Scheme1.
Scheme 1 Synthesis of 2−7

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The ensuing products have been purified by flash silica gel column chromatography to afford
bright orange coloured compounds in good yield. All these have been thoroughly characterized
11
19
1
by IR, 1H, 13C, B, F NMR and HRMS spectral studies. Triazolyl protons (Hh) in H NMR
spectra of the respective compounds resonated at ~ 7.7 ppm (7.78, 4; 7.78, 5; 7.75, 6; 7.70, 7)
while those associated with piperazine (Hjk, Hj′k′) appeared between 2.6−3.6 ppm (2.89, 4; 2.90,
δ
δ
5; 2.67 and 3.57, 6; 2.60, 7). Notably, piperazine protons for 7 resonated as a broad singlet while
those for 4−6 emerged as splitted signals. It may be attributed to relatively high symmetry of 7
due to presence of the triazolyl rings at either end of piperazine, while 4−6 involve piperazine
nitrogen with diverse substituents. It is noteworthy to mention that 6 showed two different
signals for piperazine protons, signals due to Hkk′ protons appeared at
Hjj′ resonated at 2.67 ppm. Three different types of methylene protons affirmed occurrence of
the click reaction wherein Hf (2H) resoanted at ~ 4.8 ppm (4.82, 4; 4.83, 5; 4.81, 6; 4.79, 7)
δ 3.57 ppm and those for
δ
δ
while Hg protons (2H) appeared at ~
methylene protons (Hi) resonated at ~
δ
δ
3.8 (3.82, 4; 3.82, 5; 3.76, 6; 3.71, 7) ppm. Similarly,
4.4 (4.47, 4; 4.48, 5; 4.47, 6; 4.45, 7) and pyrrole ring
protons (6H) at ~
δ 6.5, 6.9 and 7.9 ppm. In the NMR spectrum of 6 signal due to pyridyl ring
11
19
protons appeared at
δ
6.68 and 8.17 ppm. Moreover, B and F NMR spectra of 4−7 showed
typical triplet (11B) and quartet (19F) due to B-F coupling (Figs. S1-S9, ESI†).10
Simulated and observed isotopic mass distribution patterns for 4−7 [molecular ion peak [M +
H]+ at [492.2495 (Obs) 492.2492 (calcd), 4; 506.2657 (Obs) 506.2649 (calcd), 5; 555.2601 (Obs)
555.2602 (calcd), 6; 869.3757 (Obs) 869.3757 (calcd), 7] (Figs. S10−S13, ESI†) and fragmented
peaks are in excellent agreement with respective formulations.
Photophysical properties of 4−7 have been investigated by electronic absorption and
emission spectral studies (CH3CN:H2O; 3:2, v/v). The electronic absorption spectra of 4−7
displayed common spectral features with high (386, 4; 385, 5; 386, 6; 350 nm, 7) and low (494,
4; 495, 5; 496, 6; 493 nm, 7) energy bands along with a shoulder at ~473 nm (Fig. 1). The
position and intensity of bands at ~495 nm signify π→π* transitions, recurrently present in
analogous systems.11 Likewise, 4−7 exhibited typical emission bands due to BODIPY π−π*
transitions at 516, 515, 513 and 521 nm upon excitation at 494, (SS, 22 nm), 495 (SS, 20 nm),
496 (SS, 17 nm) and 493 nm (SS, 28 nm), respectively (Fig. 1; Table S1, ESI†).11

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Fig. 1 Absorption (a) and emission (b) spectra of 4−7 (10 ꢀM) in CH3CN/H2O (3 : 2, v/v) .
The compounds under study give coloured solution and are expected to show distinctive
colour change upon interaction with various metal ions which may form the basis for naked eye
detection. To affirm this, interaction of 4−7 (10 ꢀM) with various metal ions viz., Na+, K+, Ag+,
Mg2+, Ca2+, Ni2+, Zn2+,Cd2+, Cu2+, Pb2+, Hg2+, Al3+, Fe3+, and Cr3+ [1 mM; CH3CN/H2O; (3:2,
v/v).] has been investigated. Interestingly, among these, only 7 displayed instantaneous
photophysical response toward Hg2+ with visible colour change from pale yellow to orange red
(Fig. 2). This colorimetric response toward Hg2+ persisted for longer durations and we conclude
that 7 can act as a potential chromogenic sensor selectively for Hg2+.
Fig. 2 Top: chromogenic response of 7 (c = 100 ꢀM) toward tested metal ions (10 equiv.)
Bottom: fluorogenic response of 7 (c = 100 ꢀM) for tested metal ions (10 equiv.) under UV light
(λex. 365 nm).
The electronic absorption spectra of 4−6 do not show any appreciable changes in the position
of bands (Fig. S17, ESI†), however, 7 showed considerable shift upon addition of Hg2+. The
absorption spectra of 7 upon successive additions of Hg2+ (1−10 equiv) displayed decrease and
increase in intensity of the bands at 493, and 472 nm, respectively (Fig. 3) along with appearance
of a new band at ~548 nm. Presumably strong interaction between triazole-piperazine receptor

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and Hg2+ enhances π delocalization, in turn, reduces energy of the π–π* transitions.12 This causes
a new noticeable absorption band at ~548 nm and changes the colour of solution from pale
yellow to orange red. Here we believe that distinctive color changes may arise due to formation
of 7·Hg2+ complex which has further been supported by HRMS studies (Figs. S14−S15, ESI†).
Fig. 3 (a) Absorption spectra of 7 (10 ꢀM) in presence of 10 equiv of various metal ions in
CH3CN/H2O (3:2, v/v) (b) the changes in absorption spectrum of 7 (10 ꢀM) in CH3CN/H2O (3:2,
v/v) upon addition of Hg2+ (1−10 equiv).
In an analogous manner, addition of Hg2+ to a solution of 4−7 did not show any alteration in
the position and intensity of emission maxima except for 7, which displayed significant decrease
in intensity of the band at 521 nm (Fig. 4). The results suggested that 7 may serve as a selective
fluorescent sensor for Hg2+. Further, gradual addition of the Hg2+ (1.0−10.0 equiv.) to a solution
of 7 led to a decrease in relative fluorescence intensity by ~ 10 and quantum yield by ~9 fold (7,
Φ
fl, 28%; 7ꢀHg2+,
Φfl, 3%). The fluorescence quenching thus observed may be attributed to
binding of the Hg2+ with electron donating piperazine and triazolyl nitrogen and formation of a
new species 7·Hg2+.13
Moreover, interference studies performed by addition of tested metal ions to a solution of
7·Hg2+ did not show any significant change in its emission pattern (Fig. S19a, ESI†).
Stoichiometry between the probe 7 and Hg2+ have been deduced from Job’s plot analysis by
plotting χ·ꢁI vs molar fraction of 7 keeping total concentration constant. Maximum emission
intensity has been obtained at 0.5 molar fractions. The fluorescence changes substantiated 1:1
binding between 7 and Hg2+ (Fig. S18a, ESI†). The 1:1 binding constant for 7·Hg2+ has been

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estimated by plotting 1/I−I0 vs. 1/Hg2+ by B-H (Benesi-Hildebrand method) method. The Ka
value came out to be 5.2 × 104 M−1 for 7·Hg2+ (Fig. 4 inset). The detection limit for Hg2+ has
been determined by plotting fluorescence intensity vs concentration of the Hg2+ which emerged
to be 20 ꢀM, and permits 7 to detect micromolar levels of Hg2+ (Fig. S19b, ESI†).
Fig. 4 (a) Fluorescence spectra of 7 (10 ꢀM) in presence of 10 equiv of various metal ions in
CH3CN/H2O (3:2, v/v) (b) the changes in the fluorescence spectrum of 7 (10 ꢀM) in
CH3CN/H2O (3:2, v/v) with varying amounts of Hg2+ (1−10 equiv) (λex = 493 nm). Inset
showing B-H plot for 7·Hg2+.
Aqueous solution of metal nitrates can alter the pH and consequently, photophysical
properties of the probe. To ensure that the observed photophysical changes arise exclusively due
to presence of Hg2+ and not because of change in pH of the probe, pH titration studies using 10
ꢀM solution of 7 in CH3CN/H2O (3:2, v/v) have been performed. The pH of probe 7 was varied
from 2−12 by adding 10−1 M solution of HCl and NaOH with concommitant monitoring of the
absorbance and emission intensity. It displayed insignificant change in these properties within
the aforesaid pH range (Fig. S20−21, ESI†) suggesting insensitivity of the probe toward pH
variations.
Reversibility in recognition offers an ideal platform for development of metal sensors as it
improves their applicability. Considering the strong binding ability of EDTA towards Hg2+ the
reversibility of probe 7 has been examined by addition of EDTA. Addition of a large excess of
EDTA (50 equiv) to a solution of 7ꢀHg2+ (7 + 10 equiv Hg2+) in CH3CN/H2O (3:2, v/v) resulted
in restoration of absorption band at 493 and 472 nm along with disappearance of the band at 548

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nm. Simultaneously ~ 88 % restoration in emission intensity of band at 521 nm was observed.
(Fig. S22, ESI†). These results established the probe 7 as a reversible sensor for Hg2+ and also
eliminated the possibility of it being a chemodosimeter.
Being a heavy metal ion, Hg2+ can alter position of the proton signals close to the binding
1
sites.14a To explicitly explore the complexation of 7 with Hg2+, H NMR titration studies have
1
been performed. In a typical experiment H NMR spectra of 7 has been acquired in CD3CN by
gradually increasing amount of Hg2+ (Fig. 5). Proton corresponding to the triazolyl ring
resonated at
δ
7.83 ppm, which upon addition of 0.3 equiv of Hg2+ shifted downfield to appear at
7.90 ppm (ꢁ , 0.07) as a merged signal with pyrrolic proton. At the same time, methylene
4.74; Hg, 3.66; Hi, 4.46 ppm) too downfield shifted (Hf, 4.76; Hg, 3.90; Hi,
δ
δ
protons (Hf,
δ
δ
δ
δ
δ
δ
4.47 ppm). With an increase in the concentration of Hg2+ (0.6 equiv) triazolyl proton signal
started differentiating from pyrrolic proton along with an additional downfield shift in the
position of methylene protons. A total addition of 0.9 equiv of Hg2+ resulted in an appreciable
downfield shift for triazoly proton to appear at
Hf, 4.87 ppm and Hg , 4.49 ppm.
δ 8.28 ppm (ꢁδ, 0.46) and methylene proton at
δ
δ
Fig. 5 1H NMR spectrum of 7 in CD3CN with increasing amounts of Hg2+ showing formation of
7·Hg2+.

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The shift in Hi proton signals could not be monitored at higher concentration of Hg2+ (0.6 and
0.9 equiv) as these merged with the broad signal of piperazine and solvent protons. Downfield
shift for the triazolyl and methylene protons may arise due to complexation of the Hg2+ through
triazolyl nitrogen as it is expected to withdraw electron density from the triazole-piperazine core
14b
and to some extent from the vicinal methylene protons
Moreover, Hg2+-probe interaction is
achieved by the orientation changes14c in the probe, which may also contribute to the shift in
proton probe signals.
−
Characteristic vibrations due to NO3 at 1384 cm-1 in the IR spectrum of 7·Hg2+ indicated its
presence in the ensuing species (Fig. S16, ESI†). Formation of 7·Hg2+ has further been supported
by HRMS studies. In its HRMS, 7·Hg2+ dispalyed a peak associated with [M + H]+ at 1231.3483
−
−
(7 + Hg2+ + 2NO3 + 2H2O) and 1174.3593 (7 + Hg2+ + NO3 + CH3CN) and ardently
evidenced formation of the aforesaid species (Figs. S14−S15, ESI†).
Major Hg2+ probes are known to exhibit ‘turn off’ response15 due to Hg2+ induced spin–orbit
coupling, in turn, CHEQ (chelation-enhanced quenching).3a Our probe exhibited remarkable
fluorescence quenching which may be attributed to the complexation of 7 with Hg2+ through
piperazine and triazole nitrogens,15,13c consequently, reducing electron density on the fluorophore
(BODIPY), leading to oxidative photoinduced electron transfer ( PET) (Fig. S24, ESI†).
To account for rather inert behavior of 4−6 toward various metal ions and to have better
understanding of binding mode and optical response of 7 toward Hg2+ energy optimization and
frequency calculations have been performed on 4−7 and 7·Hg2+. Optimized structures clearly
exhibited that piperazine unit adopted chair form in 4−6 in contrary to the twisted boat form in 7
(Fig. 6). This twisted boat form of piperazine unit in 7 clearly accounted for binding of this
molecule to the metal ion. Two triazole units in 7 created by double click reaction forces the
piperazine nitrogen to come closer, thereby facilitate interaction of the metal ion with donor
atoms. After Hg2+ coordination, two previously diagonally placed triazole units in 7 become
facial and facilitate complexation. Hg2+ binds to 7 through two nitrogens one each from
piperazine (3.88, 3.96 Å) and triazole units (4.02, 3.87 Å).16
In many BODIPY-based probes, incorporation of a chelator at meso-position of the BODIPY
fluorophore decouples two subunits owing to almost perpendicular arrangement of fluorophore
and chelator.6c In our case too, optimized structure revealed perpendicular arrangement of the

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BODIPY unit and piperazine-triazole chelator. It also supported oxidative-PET induced
quenching upon interaction with Hg2+ ion.
Fig. 6 (a) DFT-optimized structure of 7 (b) 7·Hg2+ complexes.
Conclusion
In summary, through this work we have described piperazine conjugates embracing BODIPY as
fluorophore and piperazine along with triazole unit as chelater, separated by alkyl spacers. The
compounds 4, 5, 6 and 7 have been synthesized by click reactions between azido BODIPY and
alkynes imparting substituted and unsubstituted piperazine. Amongst these 7 displays selective
response toward Hg2+ by oxidative PET, which has been supported by various studies,
particularly optical (UV-vis, Fluorescence), HRMS and 1H NMR titration.
Experimental Section
Reagents. . The solvents were dried and distilled prior to their use following standard literature
procedures.17 Pyrrole, trifluoroacetic acid, 4-hydroxybenzaldehyde, propargyl bromide,
piperazine, 1-methylpiperazine, 1-ethylpiperazine, 1-(2-pyridyl)piperazine, 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ), triethylamine and boron trifluoride diethyl etherate were
procured from Sigma Aldrich India and used as received without further purifications. Metal
nitrates viz., NaNO3, KNO3, Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, FeCl3, Co(NO3)2·6H2O,
Ni(NO3)2·6H2O, Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, AgNO3, Pb(NO3)2,
Hg(NO3)2, Cr(NO3)3·9H2O were obtained from s.d. fine chem India Pvt. Ltd and used without
further purifications. The azido BODIPY 2 was synthesized following literature procedure.18

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Alkynes 3a−3d were synthesized by conventional propargylation reactions using propargyl
bromide and K2CO3.
Synthesis of 4: Azide functionalized BODIPY 2 (0.353 g, 1 mmol) and alkyne 3a (0.138g, 1
mmol) were dissolved in THF/H2O (1:1) in a round bottom flask and CuSO4·5H2O (0.012 g,
0.05 mmol per propargyl group) and sodium L-ascorbate (0.02 g, 0.1 mmol per propargyl group)
were added with stirring. The reaction mixture was allowed to stir at room temperature for 24h.
After completion of the reaction (ensured by TLC) DCM was added to the reaction mixture and
washed with saturated aqueous NH4Cl (2 × 10 mL), water (10 mL) and brine (10 mL). The
organic layer was separated and dried over Na2SO4 and concentrated to drynes to afford the
crude product. It was purified by flash chromatography and desired product isolated as bright
orange solid. Yield: 0.428 g, 88% Analytical data: Anal. Calc. for C25H28BF2N7O: C, 61.11; H,
5.74; N, 19.95. Found: C, 61.05; H, 5.53; N, 19.54%. 1H NMR (CDCl3, δ ppm): 2.60 (3H, s,
methyl-H), 2.89 (8H, m, piperazine-H), 3.82 (2H, s, methylene), 4.47 (2H, s, methylene-H), 4.82
(2H, s, methylene-H), 6.47 (2H, s, pyrrolic-H), 6.93 (2H, s, pyrrolic-H), 7.02 (2H, d, J = 8.4 Hz,
aromatic-H), 7.50 (2H, m, aromatic-H), 7.79 (1H, s, triazolyl-H), 7.91 (2H, s, pyrrolic-H). 13C
NMR (CDCl3, δ ppm): 49.6, 52.4, 52.9, 66.6, 114.5, 118.5, 124.1, 127.2, 131.3, 132.4, 134.8,
11
143.7, 146.8, 160.2. B NMR (CDCl3, δ ppm): 0.25 (t, J (B,F) = 28.8 Hz). 19F NMR (CDCl3, δ
ppm): -145. IR (KBr pellet, cm-1): 2926, 1603, 1575, 1544, 1413, 1387, 1261, 1117. UV-Vis.
(λmax nm, ε M-1 cm-1); 386 (8 × 103), 474 (1.4 × 104), 494 (3.2 × 104).
Synthesis of 5: It was prepared following the above procedure for 4 using 3b (0.152 g, 1 mmol)
in place of 3a. Yield: 0.462 g, 91% Analytical data: Anal. Calc. for C26H30BF2N7O: C, 61.79; H,
5.98; N, 19.40. Found: C, 61.51; H, 5.64; N, 19.11%1H NMR (CDCl3, δ ppm): 1.31 (3H, t,
methyl-H), 2.90 (10H, br, piperazine-H and methylene proton), 3.82 (2H, s, methylene-H), 4.48
(2H, t, methylene-H), 4.83 (2H, d, methylene-H), 6.55 (2H, s, pyrrolic-H), 6.93 (2H, s, pyrrolic-
H), 7.03 (2H, d, J = 8.7 Hz, aromatic-H), 7.54 (2H, d, J = 8.4 Hz, aromatic-H), 7.78 (1H, s,
13
triazolyl-H), 7.92 (2H, s, pyrrolic-H); C NMR (CDCl3, δ ppm): 49.7, 51.3, 52.0, 52.2, 66.5,
114.6, 118.5, 127.2, 131.3, 132.5, 134.8, 143.7, 146.8, 160.1. 11B NMR (CDCl3, δ ppm): 0.25 (t,
19
J (B,F) = 28.8 Hz). F NMR (CDCl3, δ ppm): -145. IR (KBr pellet, cm-1): 2925, 1603, 1575,
1544, 1412, 1387, 1260, 1117, 1078. UV-Vis. (λmax nm, ε M-1 cm-1); 385 (8.9 × 103), 473 (1.5 ×
104), 495 (3.4 × 104).

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Synthesis of 6: It was prepared following the above procedure for 4 using 3c (0.201 g, 1 mmol)
in place of 3a. Yield: 0.440 g, 78% Analytical data: Anal. Calc. for C29H29BF2N8O: C, 62.83; H,
1
5.27; N, 20.21. Found: C, 62.59; H, 5.14; N, 20.08%. H NMR (CDCl3, δ ppm): 2.67 (4H, m,
piperazine-H), 3.57 (4H, m, piperazine-H), 3.76 (2H, s, methylene-H), 4.48 (2H, t, methylene-
H), 4.82 (2H, d, J = 4.5 Hz, methylene-H), 6.60 (3H, m, 2 pyrrolic-H and 1 pyridyl-H), 6.97
(4H, dd, 2 pyrrolic-H and 2 aromatic-H), 7.49 (3H, m, 2 aromatic-H and 1 pyridyl-H), 7.75 (1H,
s, triazolyl-H), 7.92 (2H, s, pyrrolic-H), 8.17 (2H, t, pyridyl-H). 11B NMR (CDCl3, δ ppm): 0.25
(t, J (B,F) = 28.8 Hz). 19F NMR (CDCl3, δ ppm): -145. IR (KBr pellet, cm-1): 2925, 1668, 1594,
1544, 1480, 1436, 1387, 1247, 1117. UV-Vis. (λmax nm, ε M-1 cm-1); 386 (9.4 × 103), 472 (1.5 ×
104), 496 (3.5 × 104).
Synthesis of 7: This compound was prepared following the above procedure for 4 using 3d
(0.162 g, 1 mmol) in place of 3a. The alkyne and azide were taken in 1:2 ratio. Yield: 0.701 g,
80%. Analytical data: Anal. Calc. for C44H42B2F4N12O2: C, 60.85; H, 4.87; N, 19.35. Found: C,
1
60.69; H, 4.72; N, 19.14% H NMR (CDCl3, δ ppm): 2.60 (8H, m, piperazine-H), 3.71 (2H, d, J
= 7.2 Hz, methylene-H), 4.45 (2H, s, methylene-H), 4.79 (2H, s, methylene-H), 6.55 (2H, s,
pyrrolic-H), 6.92 (2H, s, pyrrolic-H), 6.99 (2H, d, J = 8.4 Hz, aromatic-H), 7.51 (2H, d, J = 8.4
Hz, aromatic-H), 7.69 (1H, s, triazolyl-H), 7.92 (2H, s, pyrrolic-H). 13C NMR (CDCl3, δ ppm):
49.6, 52.4, 52.9, 66.5, 109.4, 114.5, 118.5, 127.2, 131.3, 132.4, 134.7, 143.7, 146.8, 160.1.11B
19
NMR (CDCl3, δ ppm): 0.25 (t, J (B,F) = 28.8 Hz); F NMR (CDCl3, δ ppm): -145. IR (KBr
pellet, cm-1): 2933, 1603, 1875, 1545, 1413, 1388, 1261, 1118. UV-Vis. (λmax nm, ε M-1 cm-1);
350 (2.1 × 104), 470 (1.7 × 104), 493 (3.2 × 104).
Theoretical studies. Quantum chemical calculations were performmed at the B3LYP Density
Functional Theory (DFT) level using B3LYP/6-31G** for 4−7 and B3LYP/LANL2DZ for
7·Hg2+ complex.19 All the geometry optimizations and frequency calculations (to verify a
genuine minimum energy structure) were performed using Gaussian 09 suits of program.20
Acknowledgments
Financial support from the Department of Science and Technology (DST), New Delhi through
the Scheme SR/S1/IC-25/2011 is gratefully acknowledged. RSS acknowledges the University
Grants Commission, New Delhi, India for a Senior Research Fellowship (19-12/2010(i)EU-IV).
We also thank the Head Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, India for extending laboratory facilities.

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