Synthesis, spectral characterization, and DFT studies on Co(II), Ni(II), and Cu(II) complexes possessing naphthoquinone signaling unit for their use as selective colorimetric fluoride ion sensor

Article abstract of DOI:10.1080/15533174.2013.799191

Co(II), Ni(II), and Cu(II) complexes of a new Schiff base have been prepared and characterized by elemental analyses, FT-IR, UV-Vis, ESR, conductivity, magnetic susceptibility measurements, and thermal analyses. The results indicated a square planar geome

Full text of DOI:10.1080/15533174.2013.799191

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Synthesis, Spectral Characterization, and DFT Studies
on Co(II), Ni(II), and Cu(II) Complexes Possessing
Naphthoquinone Signaling Unit for their Use as
Selective Colorimetric Fluoride ion Sensor
Selvaraj Madhupriya a & Kuppanagounder P. Elango a
a Department of Chemistry , Gandhigram Rural Institute (Deemed University) , Gandhigram ,
India
Accepted author version posted online: 17 Dec 2013.Published online: 26 Mar 2014.
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To cite this article: Selvaraj Madhupriya & Kuppanagounder P. Elango (2014) Synthesis, Spectral Characterization, and DFT
Studies on Co(II), Ni(II), and Cu(II) Complexes Possessing Naphthoquinone Signaling Unit for their Use as Selective Colorimetric
Fluoride ion Sensor, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:8, 1104-1114, DOI:
10.1080/15533174.2013.799191
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 44:1104–1114, 2014
Copyright ꢀ Taylor & Francis Group, LLC
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ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2013.799191
Synthesis, Spectral Characterization, and DFT Studies
on Co(II), Ni(II), and Cu(II) Complexes Possessing
Naphthoquinone Signaling Unit for their Use as Selective
Colorimetric Fluoride ion Sensor
Selvaraj Madhupriya and Kuppanagounder P. Elango
Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram, India
Co(II), Ni(II), and Cu(II) complexes of a new Schiff base have
been prepared and characterized by elemental analyses, FT-IR,
UV-Vis, ESR, conductivity, magnetic susceptibility measurements,
and thermal analyses. The results indicated a square planar geom-
etry around the metal ion in these complexes. The applicability of
the ligand and the complexes toward colorimetric sensing of fluo-
1
ride ions was also been screened by employing UV-Vis, H NMR,
and fluorescence studies. 1H NMR studies indicated that the fluo-
ride ion sensing by the receptors is due to the formation of H-bonds
Where R = aromatic/aliphatic/heterocyclic moiety.
between F−ions and the -NH moiety of the receptors.
It is presumed that such a Schiff base, when in an amine
group (receptor unit), is directly, attached to an electron defi-
cient quinone moiety (signaling unit) would act a chromogenic
sensor for simple visual detection of anions. A literature survey
revealed that, recently, substantial efforts were being made to
design and syntheses colorimetric sensors for anions in general
and for fluoride ions in particular. Since among the biologi-
cally functional anions, the beneficial role of fluoride ions is
concentration dependent.[10] Hence, selective visual detection
of fluoride ions becomes essential.
Also, recently, we established that coordination of metal ions
with similar amine receptor unit would enhance the acidity of the
amine hydrogen atom and thereby make the hydrogen bond for-
mation between fluoride ion and the H-atom, more feasible.[11]
Consequently the intramolecular charge transfer (ICT) transi-
tion between the amine group (receptor unit) and the quinone
moiety (signaling unit) would be relatively easy, which is the
pre-requisite for a good colorimetric sensor. This is because
any change in electron density on the N-atom of the amine
group would alter the electronic and thus the optical properties
of the signaling unit, quinone.[12] Though very few coordina-
tion complexes have been reported as colorimetric sensors for
anions,[13–15] this is the first attempt to utilize the remarkable
ability of Schiff bases to attach metal ions with sensors.
Keywords complex, naphthoquinone, Schiff base, sensor
INTRODUCTION
Schiff bases have been used extensively as ligand in the
field of coordination chemistry. A large number of Schiff bases
and their complexes have been investigated for their interest-
ing and important properties, such as their ability to reversibly
bind oxygen, catalytic activity in the hydrogenation of olefins,
photochromic and thermochromic properties, and complexing
ability toward some toxic metals. Further Schiff bases are im-
portant class of compounds in medicinal and pharmaceutical
field. They showed promising applications in various biological
activities and biological modeling.[1–8]
Recently, we have demonstrated a simple, intermolecular
charge transfer facilitated method for the synthesis of the fol-
lowing type of Schiff bases of a weak nucleophile 2,3-diamino-
1,4-naphthaquinone.[9]
Received 22 February 2013; accepted 17 April 2013.
The authors thank m/s Orchid Chemicals and Pharmaceuticals Ltd.,
R&D Centre, Chennai for providing thermal analysis facilities.
Address correspondence to Kuppanagounder P. Elango, Depart-
ment of Chemistry, Gandhigram Rural Institute (Deemed University),
Gandhigram 624302, India. E-mail: drkpelango@rediffmail.com
Color versions of one or more of the figures in the article can be
found online at www.tandfonline.com/lsrt.
The main objective, therefore, of the present endeavor
is to synthesize Co(II), Ni(II), and Cu(II) complexes of
2-(2-hydroxybenzylideneamino)-3-aminonapthalene-1,4-dione
(HL) ligand and characterize with conventional analytical,
thermal, electrochemical, and various spectral techniques.
Utility of the ligand and the complexes, as colorimetric sensors,
1104

SYNTHESIS, SPECTRAL CHARACTERIZATION, AND DFT STUDIES
1105
SCH. 1. Synthesis of HL.
SCH. 2. Synthesis of M(II) complexes.
for the visual detection of fluoride ions has also been screened.
Attempts have also been made to explain the colorimetric
sensing of the recognition event using Density Functional
Theory computations.
Synthesis of Metal (Co(II), Ni(II) and Cu(II)) Complexes
To a stirred solution of HL in DCM, an ethanolic solution
of metal chloride was slowly added. The reaction mixture was
stirred at RT for 1 h and the reaction mass was monitored by
TLC. Once the reaction has been completed, a solid compound
which separated out was then filtered, dried, and recrystallized
from acetonitrile (Scheme 2).
EXPERIMENTAL
Chemicals
All the chemicals used in the present study were of high purity
analytical grade (Merck, India) and were used as received. The
solutions of anions were prepared from their analytical grade
tetrabutylammonium salts.
RESULTS AND DISCUSSION
Facile condensation of salicyladehyde and 2, 3-
diaminonaphthaquinone in
a 1:1 molar ratio afforded
the Schiff base ligand 2-(2-hydroxybenzylideneamino)-3-
aminonaphthalene-1, 4-dione (general abbreviation HL, where
H atom is dissociable phenolic hydrogen), which has been em-
ployed as a chelating ligand in the present work.
Instrumentation
The UV-Vis spectra were recorded on a double beam spec-
trophotometer (JASCO 460 Plus). Fluorescence spectra were
1
recorded on a JASCO FP-6200 spectrofluorometer. H NMR
spectra were recorded on a Bruker NMR (300 MHz) with
DMSO-d6 as solvent. EPR spectra were recorded at Madurai
Kamaraj University, Madurai on JEOL FA3000, X-Band Mi-
crowave spectrometer using Mn marker as the standard. Ele-
mental analysis for CHN was performed at the Sophisticated
Analytical Instrument Facility, Cochin University of Science
and Technology, Kochi (Elementar Vario EL III). The geometri-
cal optimization of the receptors was performed at the SCF level
with the Gaussian program. The geometries were optimized in
the RHF framework with 3-21G basis sets.
Synthesis of 2-(2-hydroxybenzylideneamino)-3-
aminonaphthalene-1,4-dione (HL)
The ligand was synthesized and characterized by re-
The reaction of Co(II), Ni(II) and Cu(II) chlorides with the
ported method[9] (Scheme 1). To a stirred solution of 2,3- ligand, HL, resulted in the formation of [M(L)Cl].xH2O. All the
diaminonapthaquinone[16] (2 g, 0.016 mol) in 10 mL of complexes are stable and could be stored without any apprecia-
ethanol, HMB (1.897 g, 0.0116 mol) was added and the re- ble change. The complexes are colored and soluble in acetoni-
action mixture was stirred at RT for 15 min. Afterwards 2- trile (ACN), DMF, and DMSO. The physical characteristic and
hydroxybenzaldehyde (2.595 g, 0.0212 mol) was slowly added analytical data of the complexes are giv−en3 in Table 1. The mo-
into the reaction mixture. The reaction mixture was stirred at RT lar conductivity values measured for 10 M solutions in ACN
for 1 h and the reaction mass was monitored by Thin layer chro- are in the range of 13–20 Ohm−1 cm2 mol−1, indicating that
matography. Once the reaction has been completed 20 mL of the complexes are non-electrolytic in nature.[17] The observed
n-hexane was poured into the crude reaction mixture and stirred magnetic moment values indicated that the Ni(II) complex is
for 5 min. Finally the reaction mixture was filtered through the diamagnetic, as expected correspond to square planar geometry
filter paper under vacuum and washed with 20 mL of n-hexane around the metal ion.[17] The analytical data (CHN) are in good
to get the pure product as a dark brown solid.
agreement with the proposed stoichiometry of the complexes.

1106
S. MADHUPRIYA AND K. P. ELANGO
TABLE 1
Analytical and physical data of the ligand (HL) and its complexes
Elemental analysis, found (calcd.)%
Molar conductance
Ohm−1cm2mol−1
Compound
C
H
N
Color μeff.(B. M.)
HL
69.54 (69.86)
52.91 (52.94)
44.59 (44.63)
45.89 (45.95)
4.10 (4.14)
2.86 (2.87)
4.12 (4.19)
3.81 (3.86)
9.37 (9.58)
7.23 (7.26)
6.09 (6.12)
6.27 (6.30)
dark brown
Violet
Violet
Dark Green
—
—
13
16
20
[Co(L)Cl]
[Ni(L)Cl]
[Cu(L)Cl]
1.53
Dia
1.72
FT-IR Spectra
UV-Vis Spectra
The FT-IR spectrum of the ligand was compared with those
The electronic spectra of the ligand (HL) and its complexes
of the metal complexes in order to confirm the binding mode with Co(II), Ni(II), and Cu(II) ions were recorded in ACN. The
of the ligand to the metal ion in the complexes (Table 2). The electronic spectral data along with assignment of bands are col-
free ligand showed a strong band at 1571 cm−1, which is the lected in Table 3. The electronic spectrum of the free ligand
characteristic frequency of the azomethine ν(C N) group.[18] exhibited four bands. The absorption bands appeared at 207
In all the metal complex, the ν(C N) band is shifted to lower and 276 nm correspond to π→ π∗ transitions of the substituted
frequency 1517–1552 cm−1, indicating coordination of the lig- benzene ring. These bands suffered shifts on coordination. The
and through the azomethine nitrogen atom. A band observed at band at 347 nm is assigned to n→π∗ transition involving molec-
3328 cm−1 in the free ligand has been assigned to ν(N-H) of ular orbitals of the azomethine (C N) group. Electronic spectra
NH2 group. On complexation, this band is shifted to lower fre- of Schiff base ligands, in general, exhibit band in the region
quency (3166–3295 cm−1) indicating coordination of the ligand of 370–410 nm for the CH N chromophore.[21] In the present
with the metal ion through the NH2 group.[19] The ν(OH) fre- study the observed blue shift of the band (347 nm) may be due
quency observed as broad band at 3426 cm−1 in the spectrum of to the existence of ICT transition between azomethine N-atom
the ligand, is absent in the Co(II), Ni(II), and Cu(II) complexes and quinone ring. The n→π∗ band undergo a blue shift in the
indicating coordination of phenolic oxygen atom to the metal metal complexes and appear at 292 ([Co(L)Cl]); 303, 312 (as a
ions via deprotonation.[18] The two ν(C O) bands observed doublet, [Ni(L)Cl]); and 306 nm (broad, [Cu(L)Cl]). This blue
at 1674 and 1602 cm−1 in the spectrum of the ligand remain shift is due to the polarization within the CH N chromophore
unchanged in the complexes suggesting the non-involvement group caused by the metal-ligand interaction which clearly indi-
of quinone oxygen atoms in the complex formation. The two cates the coordination of azomethine N-atom to metal ion.[22,23]
peaks observed for the two carbonyl groups of the quinone may Another important characteristic in the electronic spectrum of
be due to the unsymmetrical substitution at 2 and 3 positions of free HL, is the appearance of a broad absorption band at 488 nm.
the naphthaquinone. Moreover the spectra of all the complexes This band is due to ICT transition involving N-atoms, which are
showed new bands in 530–505 cm−1 region due to ν(M-O) and directly attached to the quinone system.[24] This band is blue
in the 446–405 cm−1 region due to ν(M-N).[20] These results shifted and appeared at 334 nm (Co(II)), 350 nm (shoulder,
indicated that the Schiff base ligand is coordinated to these Ni(II)), and 399 nm (shoulder, Cu(II)). This observation sug-
metal ions through the phenolic oxygen, amine and azomethine gested that both azomethine and amino N-atoms are coordinated
nitrogen atoms.
to the metal ions, thereby shifts the ICT transition to a relatively
TABLE 2
FT-IR spectral data of the ligand (HL) and its complexes (cm−1
)
Complex
νNH2
νOH
C N
C O
M-O
M-N
HL
3328
3166
3268
3295
3426
1571
1517
1552 1680,1594
1524
1671, 1602
1675,1597
515
—
512
—
[Co(L)Cl]
[Ni(L)Cl]
[Cu(L)Cl]
465
481
475
—
—
1679,1596
525

SYNTHESIS, SPECTRAL CHARACTERIZATION, AND DFT STUDIES
1107
TABLE 3
voltammogram produced two reversible waves at –0.5954 and
–0.8095 V. The first wave is attributed to the addition of an elec-
tron to the quinone (Q) to produce a semiquinone radical ion
(Q−) and the second one to subsequent addition of an electron to
the semiquinone radical ion, producing a dianion hydroquinone
(Q2−).[24] The E1/2 value for the second wave is higher than that
for the first, as expected. The results in Table 4 indicate that
the coordination of metal ions to the quinone ligand signifi-
cantly alerted the E1/2 values of both the waves. This suggested
that the electronic charge on the quinone moiety, in these com-
plexes, is different for different metal ions. The E1/2 values for
the M(III)/M(II) redox couple were found to be 0.348, 0.60,
and 0.350 V in Co(II), Ni(II), and Cu(II) complexes, respec-
tively.[30–32]
Electronic spectra of ligand (HL) and its complexes
Compound (F.wt.)
HL
Band (nm)
Assignment
log ε
207
276
347
488
208
238
292
334
536
π→π∗
π→π∗
n→π∗
ICT
3.548
3.380
0.557
0.219
3.357
3.418
3.487
2.573
2.544
1.891
3.278
3.456
[Co(L)Cl]
π→π∗
π→π∗
n→π∗
ICT
d→d
688sh
267
d→d
[Ni(L)Cl]
[Cu(L)Cl]
π→π∗
n→π∗
3.458
ICT
Thermal Study
303
All the three complexes were subjected to thermogravimet-
ric analysis with a view to find out the presence of water of
hydration. These complexes showed decomposition in the tem-
perature range: 44.63–111.51 for Co(II); 36.78–49.95 for Ni(II);
34.89–56.01◦C for Cu(II) complex indicating presence of water
of hydration.
The foregoing analytical, spectral, electrochemical, and ther-
mal studies indicated that the Schiff base ligand is coordinated
to the chosen divalent metal ions through the imine and amine
N-atoms and phenolic –OH group, after deprotonation, to form
square planar complexes.
n→π∗
350sh
584
2.8
d→d
2.592
3.569
2.918
2.077
306b
399sh
586w
π→π∗
n→π∗
d→d
sh = shoulder, b = broad, w = weak.
higher region. In these complexes the d-d bands were observed at
536, 688 nm ([Co(L)Cl]); 584 nm ([Ni(L)Cl]); and 586 nm (very
weak [Cu(L)Cl]). In the case Cu(II) complex the d-d band is ob-
servable only at higher concentrations. The electronic spectral
data coupled with magnetic moment values indicated a square
planar geometry for these M(II) complexes.[17,25–28]
EPR Spectra
The EPR spectrum of the powdered samples of the Cu(II)
complex was recorded at room temperature and is shown in
Figure 1. The EPR spectrum of the complex was found to be
isotropic consisting of one ‘g’ value with no appearance of hy-
perfine structure characteristic of regular geometry. The broad
universal spectral signal could be attributed to super exchange
interaction between metal centers in the solid state. The spec-
trum exhibited a single ‘g’ value (i.e., giso centered at 2.145).[29]
Where x = 1 in Co(II), 4 in Ni(II), and 3 in Cu(II) complex.
Electrochemical Study
Fluoride Ion Sensing
The redox potentials of the HL and the complexes were mea-
The fluoride ion sensing behavior of the ligand HL and its
sured by differential pulse voltammetry at room temperature Co(II), Ni(II), and Cu(II) complexes has been investigated in
using a glassy carbon working electrode in acetonitrile solvent acetonitrile using various spectral techniques (UV-Vis, fluores-
1
and 0.1 M tetrabutylammonium perchlorate as the supporting cence, and H NMR) and DFT calculations. The colorimetric
electrolyte. The voltammograms were recorded in the poten- sensitivity of HL and its complexes toward various anions such
tial range from +1.2 to –1.2 V versus Ag/AgCl. The half-wave as F−, Cl−, Br−, I−, AcO−, NO3−, H2PO4−, and CN− was mon-
potential values E1/2 were evaluated from the voltammograms itored visually and the results are indicated in Figure 2. As
obtained at a scan rate of 50 mVs−1, as E1/2 = (Epa + EPc)/2, seen from the figure, there is an observable color change in
where Epa and Epc correspond to anodic and cathodic peak po- the solutions of HL and Co(II) complex after the addition of
tentials, respectively. The E1/2 values computed for the ligand fluoride ions. However, their color remained unchanged after
and the complexes are collected in Table 4. For the HL, the the addition of other chosen anions. This observation indicated

1108

SYNTHESIS, SPECTRAL CHARACTERIZATION, AND DFT STUDIES
1109
FIG. 3. UV-Vis spectra of a 10−5M solution of ligand (HL) with anions in
acetonitrile.
FIG. 2. Color changes of (a) ligand (HL) and (b) Co(II) complex with anions.
cated that, in HL, all the three H-atoms (azomethine, amine and
phenolic –OH) may be involved in the sensing process. The
the selectivity HL and its Co(II) complex toward fluoride ions. bathochromic shift observed in the azomethine n→π∗ and the
Since, Ni(II) and Cu(II) complexes of the Schiff base don’t pro- ICT peaks (see Table 3) may be due to the fact that H. . .F− units
duce any such color change with fluoride ions, they were omitted are better electron donors than the corresponding free functional
from further sensing studies.
groups.[12] In order to get a better insight into the mechanism
The interaction of HL and its Co(II) complex with these of the sensing process, under investigation, 1H NMR study has
anions was studied using electronic spectral changes and the also been carried out in DMSO-d6 (Figure 7). The results indi-
results are shown in Figures 3 and 4, respectively. As evidenced cated that, after the addition of fluoride ions to HL, the char-
from these figures, addition of anions, except fluoride, did not acteristic proton signals were absent. This is due to the fact
produce any significant change in the electronic spectra of HL that the HL senses fluoride ions through the formation of H-
and its Co(II) complex. However, addition of fluoride ions dras- bonds.[12] The Co(II) complex also exhibited similar electronic
tically changed the electronic spectra of these two compounds spectral behavior with incremental addition of fluoride ions
UV-Vis titration experiments were also performed with the ad- (Figure 6).
dition of incremental amounts of fluoride ions to the solutions
The stoichiometry and association constants for the receptor-
of HL and its Co(II) complex. The results obtained are shown F− ion complexes were determined by using fluorescence spec-
in Figures 5 and 6, respectively. It is evident from the Figure 5 tral studies. In both HL and its Co(II) complex cases, curve
that the absorbance of the peaks at 276 and 347 nm decreased with a maximum at 0.5 mole fraction (Figure 8) in the Job’s
gradually with concurrent increase in absorbance of two new plot,[33] indicated the formation of 1:1 (receptor:F−) complex.
peaks at 417 and around 600 nm. A clear isobestic point was The fluorescence responses of the interaction of HL and its
observed at 385 nm. Such as electronic spectral change indi- Co(II) complex with fluoride ions are shown in Figures 9 and
TABLE 4
Electrochemical properties of ligand (HL) and Co(II), Ni(II), and Cu(II) complexes
I Wave
Epc
II Wave
Epc
M(III)/M(II)
‘E1/2
Compound
Epa
E1/2
Epa
E1/2
HL
–0.625
–0.716
–0.532
–0.537
–0.565
–0.656
–0.428
–0.433
–0.595
–0.686
–0.48
–0.832
–1.109
–0.757
–0.716
–0.787
–1.1
–0.712
–0.632
–0.809
–1.104
–0.734
–0.674
—
[Co(L)Cl]
[Ni(L)Cl]
[Cu(L)Cl]
0.348
0.060
0.350
–0.485

1110
S. MADHUPRIYA AND K. P. ELANGO
FIG. 4. UV-Vis spectra of a 10−5M solution of Co(II) complex with anions in
acetonitrile.
FIG. 6. UV-Vis titration curves of Co(II) complex in acetonitrile solution with
fluoride ions.
10, respectively. It is evident from the figures that, in both the
cases, with the addition of incremental amount of fluoride ions
to the receptors, the emission intensity increased gradually.
From the fluorescence enhancement data the binding con-
stants of the receptor-F−complexes can be determined using the
Benesi-Hildebrand equation[34]
:
(Fα − Fo)/(Fx − Fo) = 1/K[F−]
Where Fo, Fx, and Fα are the fluorescence intensities of the
receptor in the absence of fluoride ions, at a given concentration
FIG. 7. 1H NMR spectrum of HL in (a) the absence and (b) presence of fluoride
ions.
FIG. 5. UV-Vis titration curves of HL in acetonitrile solution with fluoride
ions.
FIG. 8. Job’s plots for ligand (HL) and Co(II) complex.

SYNTHESIS, SPECTRAL CHARACTERIZATION, AND DFT STUDIES
1111
FIG. 11. Benesi-Hildebrand plots of the Ligand (HL) and Co(II) complex.
FIG. 9. Fluorescence titration curves of a 10−5M solution of ligand (HL) with
fluoride ions in acetonitrile λex = 489 nm.
Theoretical Study
The foregoing results and discussion indicated that the lig-
and HL and its Co(II) complex selectively sense fluoride ions
through H-bond formation. In order to get a better insight into
the mechanism of sensing event, the structural and electronic
properties of HL and its [Co(L)Cl] were also investigated using
density functional theory calculations as implemented in Gaus-
sian 03 package.[35] The geometry optimizations were carried
out using B3LYP/3-21G basis sets. The optimized geometries
of the ligand and its complexes are shown in Figure 12. The rel-
evant frontier molecular orbitals of the HL and [Co(L)Cl] and
those with fluoride ions are shown in Figure 13.
and concentration for complete interaction, respectively. In the
present study, in both the cases, plots of (Fα –Fo)/(Fx–Fo) versus
1/[F−] are linear (Figure 11; r >0.98). The binding constants
computed were found to be 2.2 × 104 and 5.7 × 104 M−1 for the
ligand and Co(II) complex, respectively. The observed relatively
higher binding constant in the case of the Co(II) complex may
be due to the fact that complexation process would makes the
amine and imine hydrogen atoms more acidic and consequently
as a better H-donor toward fluoride ions.
In the case of HL, the HOMO to LUMO excitation is re-
sponsible for intramolecular charge transfer (ICT) transition
observed at 488 nm in the electronic spectrum (Figure 5). The
energy gap (ꢁE = EHOMO – ELUMO) between these two MOs is
0.09279 eV. After the formation of H-bond with fluoride ions,
in HL-F− complex, the HOMO-LUMO energy gap is decreased
(0.09075 eV). Hence, the ICT transition occurs at a relatively
FIG. 10. Fluorescence titration curves of a 10−5M solution of Co(II) complex
FIG. 12. The optimized geometries of the ligand and its complexes.
in with fluoride ions in acetonitrile, λex = 538 nm.

1112
S. MADHUPRIYA AND K. P. ELANGO
TABLE 5
signaling unit (quinone moiety), any slight change that occurs in
its electronic property can easily be exhibited in the form of color
change etc. Parallel to this observation, in the case of [Co(L)Cl],
addition of fluoride ions to the N-H moiety through H-bonding
decreased the ꢁE (0.09075 eV) when compared to that of free
complex (ꢁE = 0.10701 eV) and consequently makes the vi-
sual recognition of fluoride ions possible. It is interesting to note
that, as explained in the electronic spectral study, the magnitude
of ꢁE in the free Co(II) complex is higher than that in free HL
indicating relatively easier ICT transition in free HL. And the
same trend is observed in the position of the ICT band in the
electronic spectra of the two compounds.
Mulliken charges on selected H- atoms in HL and its
complexes
Compound
O-H
CH N
NH2
HL
0.352
0.363
—
—
—
0.218
0.264
0.283
0.280
0.278
0.269
0.327
0.309
0.386
0.354
0.383
0.366
0.358
0.415
0.403
0.415
0.400
0.395
HL with F−
[Co(L)Cl]
[Co(L)Cl] with F−
[Ni(L)Cl]
[Cu(L)Cl]
—
As the selective colorimetric recognition of the analyte by
these receptors is through H-bonding, the Mulliken charges on
the H-atoms of the possible functional groups that took part in
such bond formation were also computed and are collected in
Table 5. The results indicated that the positive charge on these
H-atoms increased as a result of complexation (i.e., H-atoms
become more acidic). This is in line with our persuption as spelt
in the introduction section of this article. In the case of HL-
fluoride complex the charge on the three H-atoms (phenolic,
imine and amine) are relatively higher than that in free HL,
indicating the possible involvement of these three H-atoms in
H-bond formation with F− ions.
higher wavelength, imparting a visible color change after the
addition of fluoride ions to HL. This may be due to the fact
that N-H. . .F− unit is a better electron donor than free N H
group.[.12, 36]
.
Also, as the N-H group is directly attached to the
The same has been proved by the result of 1H NMR study. In
the case of the Co(II) complex one of the amine hydrogen may be
involved in such bond formation. In the case of receptor-fluoride
complexes, as the H-atom becomes more positive through H-
bond formation with fluoride ions, the N-H. . .F− unit would
acts as a better electron donor than free N-H group and thus
making the sensing event easier. In the case of Ni(II) and Cu(II)
complexes, though coordination of metal ion with the Schiff
base made the H-atoms more acidic, as we presumed, they could
not colorimetrically sense the fluoride ions. This may be due to
the fact that the new absorption band, due to the N-H. . .F−
unit, in the electronic spectra, in these cases, may overlap with
other bands, resulting in no observable change in color after
the addition of fluoride ions. It is observed that the results of
the theoretical study corroborate well with the experimental
findings.
CONCLUSION
The new Co(II), Ni(II), and Cu(II) complexes of a novel
naphthoquinone based Schiff base ligand have been prepared
and characterized using analytical, thermal, electrochemical and
various spectral techniques. Based on the results of those inves-
tigations a square planar geometry has been proposed for these
complexes. The Schiff base and its Co(II) complex showed high
selectivity for visual fluoride ion detection over other anions
such as Cl−, Br−, I−, AcO−, NO3−, H2PO4−, and CN− in ace-
tonitrile. The ligand senses fluoride ions through H-bond for-
mation and such H-bond formation was found to increase as
a result of coordination of the metal ion with the ligand. The
FIG. 13. Frontier MOs of HL and [Co(L)Cl].

SYNTHESIS, SPECTRAL CHARACTERIZATION, AND DFT STUDIES
1113
results obtained from the DFT calculations agree well with the
experimental observations.
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FUNDING
The authors thank the Council of Scientific and Industrial
Research, New Delhi, for financial assistance to carry out the
research work. Dr. Madhupriya is thankful to the University
Grant Commission, New Delhi, for the award of UGC-BSR
Fellowship in Sciences for Meritorious Students.
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