Lead ion induced chemodosimeter approach of a tripodal hydroxyl-quinoline based phospho-ester through P-O bond cleavage

Article abstract of DOI:10.1039/c6dt00941g

Lead ion induced P-O bond breaking with instant colour change was observed in a tripodal hydroxyl-quinoline based phosphor-ester (HQP). A new penta-coordinated lead chelate complex [Pb₄HQ₆(ClO₄)₂] was found. The hydrolysis reaction followed by P-O bond cleavage with 'Pb-O' and 'Pb-N' bond formation proved the chemodosimeter approach.

Full text of DOI:10.1039/c6dt00941g

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Lead ion induced chemodosimeter approach of a
tripodal hydroxyl-quinoline based phospho-ester
through P–O bond cleavage†
Cite this: DOI: 10.1039/c6dt00941g
Dibyendu Sain,‡ Chanda Kumari, Ashish Kumar, Hari Pada Nayek and Swapan Dey*
Received 9th March 2016,
Accepted 25th April 2016
Lead ion induced P–O bond breaking with instant colour change was observed in a tripodal hydroxyl-
quinoline based phosphor-ester (HQP). A new penta-coordinated lead chelate complex [Pb4HQ6(ClO4)2]
was found. The hydrolysis reaction followed by P–O bond cleavage with ‘Pb–O’ and ‘Pb–N’ bond for-
mation proved the chemodosimeter approach.
DOI: 10.1039/c6dt00941g
www.rsc.org/dalton
has recently become an active research field.11 In particular,
lead(II) present in drinking water, has severe negative effects
Introduction
Phospho-ester containing compounds have significant roles in on physical and mental development of children.12 It inter-
a variety of biological processes as major constituents of feres with a variety of body processes and is toxic to many
nucleic acids and nucleotides.1 These compounds have elec- organs like the heart, bones, intestines, kidneys and reproduc-
tron accepting tendencies to the ‘P’ centre, which promotes tive system.13 Therefore, the selective and rapid detection of
nucleophiles to attack,2 leading to phospho-ester (P–O) bond Pb2+ in water or physiological samples is highly demanding
cleavage.3 There are lots of previous reports on P–O bond clea- and a challenging research area. A number of chemodosi-
vage reactions.4 A variety of research studies have also been meters have been previously reported for different metal ions14
focused on phosphate esters regarding their hydrolysis and but still very few chemodosimeters are reported for the Pb2+
alcoholysis reactions.5 Selective nucleophilic attacks in phos- ion,15 although, various chemosensors of Pb2+ ions based on
phate esters and P–O bond cleavage reactions under acidic6 peptide, protein and calixarene molecules have been reported
and alkaline7 conditions have been reported. 8-quinolyl phos- previously.16 For example, Ghosh et al. have recently described
phate and phosphate monoester are well known and the hydro- a triazole motif linked rhodamine-based chemosensor, which
lysis of the phosphate-ester by trivalent lanthanide ions has recognized both Hg2+ and Pb2+ ions.17 Amino acid-based fluo-
been employed to regulate biological activity.8 The interaction rescent chemosensors have also been designed and syn-
between metal ions and nucleoside phosphates in chemistry thesized for the fluorescence turn-on detection of Pb2+ ions in
and biochemistry is a well established phenomenon and the aqueous media.18
metal ion promoted reactions of phosphate derivatives have
been thoroughly reviewed.9 But still, there is no report of lead bond cleavage reaction of
ion induced P–O bond cleavage reaction in a tripodal hydroxyl- phospho-ester followed by the formation of a metal–chelate
quinoline based phospho-ester. complex, Pb4(HQ)6(ClO4)2. Hydroxyquinoline phosphate (HQP)
Herein, we describe, for the first time, a Pb2+ induced P–O
8-hydroxyquinoline based
a
In this regard, chemodosimeters are employed to identify and naphthol phosphate (NP) were synthesized by the reaction
analyte selectively through irreversible chemical reaction with of 8-hydroxyquinoline and 1-naphthol, accordingly, with POCl3
a signal which is observable to the naked eye.10 The develop- in the presence of dry Et3N (Scheme 1).
ment of chemodosimeters for monitoring toxic metal cations
Results and discussion
Department of Applied Chemistry, Indian School of Mines, Dhanbad, Jharkhand
826004, India. E-mail: dey.s.ac@ismdhanbad.ac.in, hpnayek@yahoo.com;
X-ray crystallography analysis
Fax: +91 326 2296563; Tel: +91 326 2235607
†Electronic supplementary information (ESI) available: Experimental pro-
Solid state structures of the HQP, NP and Pb4(HQ)6(ClO4)2
compounds were determined by X-ray crystallographic analy-
cedures, spectra, extensive tables, graphs and calculations. CCDC 1413010,
1413011 and 1413012 for HQP, Pb4HQ6(ClO4)2 and NP. For ESI and crystallo-
sis. HQP crystallized in a cubic system with the P213 space
graphic data in CIF or other electronic format see DOI: 10.1039/c6dt00941g
group, whereas, NP was a monoclinic crystal system with the
‡Present address: College of Pharmacy, Sunchon National University, Suncheon,
P21/c space group. The unit cells of both crystals contained
South Korea.
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also displayed that all four lead atoms were penta co-ordinated
and two of them were connected with two bridging moieties
and one chelating HQ moiety (PbO3N2 unit), whereas the other
two Pb atoms were attached to one bridging moiety and three
chelating HQ moieties (PbO4N unit). Two perchlorate counter
anions (ClO4−) were connected with the Pb4(HQ)6 unit through
several short contacts (see ESI, Table S2†).
Visual colour change experiment
The complexation study of HQP and NP was carried out with a
series of perchlorate salts of Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, Bi3+
and Ag+ in CH3OH–H2O (1 : 1). HQP immediately changed its
colour from colourless to yellowish green upon addition of 2.0
equivalents of Pb2+ (Fig. 2). HQP did not show any interesting
response upon addition of a large excess of other cations, even
up to 10.0 equivalents.
Scheme 1 Synthesis of HQP and NP. Reagents and conditions: dry
Et3N, dry benzene, reflux, 10 h, (i) 8-hydroxyquinoline, 70% yield;
(ii) 1-naphthol, 75% yield.
four molecules (Z = 4). It was found that three quinoline and
naphthalene moieties of HQP and NP respectively, were equi-
distance and far apart from each other. The PvO moiety was
located at the apex of the molecule and the central P atom was
connected with the three O atoms of the quinoline and
naphthalene moieties of HQP and NP respectively (Fig. 1a and
b). A new Pb4(HQ)6(ClO4)2 chelate complex crystal was found,
when Pb(ClO4)2 was introduced with HQP in a CH3OH–H2O
Complexation study using UV-vis experiments
The complexation studies using UV-Vis spectroscopy were
recorded for HQP (2.08 × 10−4 M) with all mentioned cations
in CH3OH–H2O (1 : 1). HQP itself showed absorption maxima
at λmax = 281 nm. With the gradual addition of Pb2+, the absor-
bance at 281 nm decreased regularly along with the increase of
a new absorption peak at λmax = 371 nm (Fig. 3a). This change
was associated with the appearance of one isosbestic point at
330 nm indicating the complex formation reaction. No positive
response of HQP was found for other cations in the UV-vis
absorption spectra (Fig. 4a). For the calculation of association
constant (K) using the nonlinear method,19 the intensity of the
UV-vis absorption (I) of HQP at 281 nm was plotted against the
concentration of Pb2+ following the non-linear curve fitting
ˉ
solvent with pH 2 to 3. It was a triclinic crystal with the P1
space group. The structure showed that three P–O bonds of
HQP were broken and four lead(II) ions were co-ordinated by
six hydroxyquinoline (HQ) moieties through the Pb–O and Pb–N
bonds (Fig. 1c). The three-dimensional network structure
Fig. 2 Visual colour change of HQP (c = 2.085 × 10−4 M) with different
cations in CH3OH–H2O (1 : 1).
Fig. 1 Single crystal X-ray ORTEP diagrams of the (a) HQP, (b) NP and
(c) Pb4(HQ)6(ClO4)2 compounds (black for C, grey for H, blue for N, red
for O, orange for P, green for Cl and pink for Pb).
Fig. 3 (a) UV-Vis absorption spectra and (b) fluorescence emission
spectra of HQP with Pb2+ (λexc = 281 nm), in a CH3OH–H2O (1 : 1)
solvent.
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almost the same in basic conditions, which suggested that
HQP had very good sensitivity in the neutral to basic region
and the least sensitivity in acidic conditions. This was prob-
ably due to the phosphoester bond cleavage resulting in lower
conjugation. But the present scenario changed totally when
Pb2+ was introduced with HQP, where both the UV-vis and
fluorescence intensities were very high in acidic conditions
(pH 1 to 6) because acidic conditions were favorable for phos-
Fig. 4 Competitive (a) UV-Vis and (b) fluorescence spectral experi- phoester hydrolysis of HQP for the formation of the Pb4(HQ)6
ments of HQP with Pb2+ and other ions.
complex. The intensity changes of the complex were negligible
from pH 7 to 12 as the basic conditions had no effect on
complex formation. The pH titration curves also suggested
that the UV-vis absorbance intensity of the complex was lower
equation: Y = (I0 + AKX)/(1 + KX), where X = [Pb2+], Y = intensity
of UV Vis absorption, I0 = initial intensity of HQP. The very
high association constant value (K = 1.23 × 105 M−1) of HQP
with Pb2+ (see ESI†) suggested a strong interaction between
them.
than that of HQP but the fluorescence intensity of the complex
was higher than that of HQP.
Probable mechanism for complex formation
The chemodosimeter approach of HQP with Pb2+ was pro-
posed from the above results as the colourless solution of HQP
was turned into a visually detectable greenish-yellow colour
due to the formation of a ligand–metal charge transfer
complex21 through phosphoester (P–O) bond cleavage and the
formation of new Pb–O and Pb–N bonds. This P–O bond
breaking process was initiated by the hydrolysis reaction of the
phosphoester compound (HQP) by the nucleophilic attack of
solvent water (Scheme 2). The role of ring N-atoms in the qui-
noline moiety was very crucial in the bond cleavage reaction as
it provided an extra force for the breaking of P–O bonds in
HQP through complexation with Pb2+. The interaction of the
N and O atoms of the quinoline moiety with Pb2+ facilitated
the charge transfer (CT) process from the heterocyclic quino-
line ring to the lead ion. It indeed pronounced the enhance-
ment of electrophilicity to the central P atom to make it easier
for the nucleophilic attack of water. This observation was
proven when we compared the whole experiment using
naphthol phosphate (NP) as a dummy molecule. No such
metal ion induced phosphor-ester bond cleavage had taken
place with no characteristic colour change due to the absence
of a ring N atom in the naphthalene moiety. This observation
was also helpful for the selective detection of Pb2+ by HQP in
the presence of other heavy metal ions.
Complexation study using fluorescence experiments
The complexation study of HQP with different cations was also
carried out by fluorescence spectroscopic analysis in CH3OH–
H2O (1 : 1). When a solution of HQP (2.085 × 10−4 M) was
excited at λmax = 281 nm, an emission spectrum with peak
maxima at 407 nm was obtained. Sharp fluorescence enhance-
ment with a slight peak shift from 407 nm to 420 nm was
found for HQP in the presence of Pb2+ (Fig. 3b) only. The fluo-
rescence response of HQP with other cations was not signifi-
cant enough (Fig. 4b). The limit of detection (LOD)
calculation20 using fluorescence experiments pointed out that
HQP could sense Pb2+ up to a very low concentration (1.47 ×
10−7 M) (see ESI†).
The competitive experiment using the UV-vis and fluo-
rescence methods was also performed and confirmed no such
interference by other metal ions in the colorimetric and fluo-
rescence response of HQP to Pb2+ (Fig. 4).
pH titration experiment
To investigate the practical applicability, the effect of pH on
the UV-vis and fluorescence intensities of HQP in the presence
and absence of Pb2+ were studied at different pH conditions
(pH 1.0 to 12.0). Fig. 5 shows that the intensity of free HQP
increases from acidic to neutral conditions and remained
Quantum chemical DFT calculation
For the support of experimental work, quantum chemical
(DFT) calculations were performed using the Gaussian 0922
Fig. 5 pH titration curve for (a) the UV-vis absorbance intensity (at
281 nm for free HQP and 371 nm for HQP + Pb2+) and (b) the fluor-
escence intensity at 407 nm (λex = 281 nm) of HQP in the presence and
absence of Pb2+ at different pH conditions (pH 1.0–12.0).
Scheme 2 Complex formation reaction of HQP with Pb2+
.
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program package with the aid of the Gauss-View 5.023 visual-
ization program. The structures of the compounds were opti-
mized using Density Functional Theory (DFT) and B3LYP
procedures24 in the restricted forms, following 6-31G and
LanL2MB basis sets. DFT calculations and molecular model-
ling studies determined the energy optimized structures and
electronic properties of the phospho-esters. The energy mini-
mised structures of HQP and NP showed that the three quino-
line and naphthalene moieties were located apart from each
other, similar to the single crystal X-ray structures of HQP and
NP (Fig. 6). The two-dimensional stable network structure of
Pb4(HQ)6 displayed that four lead(II) ions were co-ordinated by
six hydroxyquinoline(HQ) moieties through Pb–O and Pb–N
bonds, where all four lead atoms were penta co-ordinated
(Fig. 6).
Molecular Electrostatic Potential (MEP) map diagrams of
the compounds highlighted the most negative regions in a
deep red colour and positive regions in a blue colour (Fig. 6).
The red coloured negative regions were located mainly around
the O and N atoms of HQP. The appearance of these negative
regions around the O and N atoms of HQP encouraged Pb2+ to
participate in P–O bond breaking reactions. This negative
region around the naphthalene ring was absent for NP, which
made NP inert in the reactions with metal ions.
Fig. 7 HOMO and LUMO diagrams of HQP and NP with their orbital
energies.
Conclusions
Theoretically calculated highest occupied molecular orbi-
tals (HOMOs) and the lowest unoccupied molecular orbitals
(LUMOs) of HQP and Pb4(HQ)6 were represented in Fig. 7. It
was suggested that the HOMOs were located around the quino-
line ring including O and N atoms. On the other hand, the
LUMOs were present mainly around the electropositive central
P atom. HOMO–LUMO structures of Pb4(HQ)6 suggested that
the electronic transition took place from electronegative O and
N atoms of the quinoline moiety to the electropositive Pb2+
ions. The extra electron donating capability of the quinoline N
atoms facilitated P–O bond breaking through coordination
with Pb2+.
In conclusion, two phospho-ester compounds (HQP and NP)
based on 8-hydroxyquinoline and 1-naphthol moieties were
individually designed and synthesized. The single crystal X-ray
crystallographic analysis showed the actual solid state stable
structures of the HQP, NP and Pb4(HQ)6(ClO4)2 complexes.
The three-dimensional network of the Pb4(HQ)6(ClO4)2
complex proved the hydrolysis of HQP followed by phosphoe-
ster (P–O) bond cleavage under the influence of Pb2+ ions. The
formation of new ‘Pb–O’ and ‘Pb–N’ bonds associated with
colour change from colourless to yellowish green upon
addition of Pb2+ ions was observed. The bathochromic shift of
HQP (λmax 281 to 371 nm with Pb2+) and isobestic point for-
mation found in the electronic spectra strongly supported the
complex formation selectively with Pb2+. All of the above
results concluded the chemodosimeter approach of HQP selec-
tively towards Pb2+ ions. The complexation with a dummy
phospho-ester compound (NP) did not show any response
towards complexation with metal cations due to the absence of
N. The structural aspects of these compounds were investi-
gated by quantum chemical calculations with MEP diagrams
for the optimization of stable structures and HOMO–LUMO
interactions.
Experimental
All reagents (AR grade) for synthesis were obtained commer-
cially and used without further purification. Solvents were
dried following standard procedures. UV-grade CH3OH was
used for the UV-vis and fluorescence titrations. 1H NMR
spectra were recorded on a Bruker AV400 instrument at
400 MHz with TMS as an internal standard. ESI-MS measure-
Fig. 6 Optimized structure of HQP, NP and Pb4(HQ)6 and MEP map
diagrams of HQP and NP by DFT/B3LYP/6-31G and LanL2MB method.
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ments were carried out using a microTOF-Q II 10330 mass structures reported in this paper have been deposited with the
spectrometer. IR spectra were measured using a Spectrum Cambridge Crystallographic Data Centre with supplementary
2000 Perkin-Elmer Spectrometer. UV–Vis spectra and Fluo- publication no. CCDC 1413010, 1413011 and 1413012. The
rescence spectra were recorded using a UV-1800 Shimadzu crystallographic data are summarized in Table S1 (ESI†).
Spectrophotometer (1.0 cm quartz cell) and Perkin-Elmer LS
55 Fluorescence spectrometer, respectively. Melting points
were determined using Remco hot-coil stage melting point Acknowledgements
apparatus and are uncorrected.
DS, CK and AK are sincerely thankful to the Indian School of
Mines for the Junior Research Fellowship. SD acknowledges
Erusmus Mundus Action 2 AREAS+ for providing Postdoctoral
General procedure for the synthesis of hydroxyquinoline
phosphate (HQP) and naphthol phosphate (NP)
8-Hydroxyquinoline (1.42 g, 9.78 mmol) was dissolved in 10 ml
fellowship.
dry benzene, followed by the addition of dry Et3N (1 ml). After
cooling the system in an ice bath, POCl3 (0.3 ml, 3.26 mmol)
was added dropwise for 10 minutes. Then the ice bath was
removed and the solution was refluxed for 10 h. After com-
Notes and references
pletion of the reaction, monitored by TLC, the benzene solvent
was removed under vacuum. Then CHCl3 was added to the
reaction mixture and it was filtered. The filtrate was dried and
the crude solid was purified using the column chromatography
technique using chloroform (CHCl3) as eluent to get pure
HQP. For the synthesis of NP, 1-naphthol (1.41 g, 9.78 mmol)
was used instead of 8-hydroxyquinoline and the reaction was
carried out in the same way.
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