Synthesis, structural and spectroscopic properties of asymmetric schiff bases derived from 2,3-diaminopyridine

Article abstract of DOI:10.1515/chempap-2016-0058

Two Schiff base derivatives, 4-(2-amino-3-pyridyliminomethyl)phenol (I) and 3-(2-amino-3-pyridyliminomethyl)nitrobenzene (II), were synthesised and characterised by spectroscopy. The structure of I was determined by single crystal X-ray diffraction studies. The asymmetric Schiff base derived from 2,3-diaminopyridine selectively recognise transition and heavy metal cations, and some anion. Ligands I and II form stable complexes with Cu²⁺, Zn²⁺, Pb²⁺, Al³⁺ whereas ligand I also binds F^- ions. The stoichiometry for the host: cation is 1: 1 and 2: 1. The addition of F^- ion in CH₃CN to ligand I causes a colour change of the solution from colourless to yellow. The binding behaviour of ligand I towards several ions was investigated using density functional theory calculations.

Full text of DOI:10.1515/chempap-2016-0058

Chemical Papers
DOI: 10.1515/chempap-2016-0058
ORIGINAL PAPER
Synthesis, structural and spectroscopic properties of asymmetric
Schiff bases derived from 2,3-diaminopyridine
a
a
b
aAgnieszka Pazik*, Beata Kamin´ska, Anna Skwierawska, Lꢀ ukasz Ponikiewski
aDepartment of Chemistry and Technology of Functional Materials, bDepartment of Inorganic Chemistry, Gdansk University of
Technology, Narutowicza Street 11/12, 80-233 Gdansk, Poland
Received 26 October 2015; Revised 21 January 2016; Accepted 4 February 2016
Two Schiff base derivatives, 4-(2-amino-3-pyridyliminomethyl)phenol (I) and 3-(2-amino-3-
pyridyliminomethyl)nitrobenzene (II ), were synthesised and characterised by spectroscopy. The
structure of I was determined by single crystal X-ray diffraction studies. The asymmetric Schiff
base derived from 2,3-diaminopyridine selectively recognise transition and heavy metal cations, and
some anion. Ligands I and II form stable complexes with Cu2+, Zn2+, Pb2+, Al3+ whereas ligand
I also binds F− ions. The stoichiometry for the host : cation is 1 : 1 and 2 : 1. The addition of
F− ion in CH3CN to ligand I causes a colour change of the solution from colourless to yellow. The
binding behaviour of ligand I towards several ions was investigated using density functional theory
calculations.
c
ꢀ 2016 Institute of Chemistry, Slovak Academy of Sciences
Keywords: asymmetric Schiff bases, X-ray crystal structure, cation complexation, fluoride, UV-
VIS spectroscopy, 1H NMR spectroscopy
Introduction
al., 2012)). Their antibacterial (Amin et al., 2012;
Dai & Mao, 2013), antifungal (Abdel-Rahman et al.,
2013; Carren´o et al., 2015), antiviral (Kumar et al.,
2010) and anti-tumour (Qiao et al., 2011) activi-
ties have been revealed. Furthermore, heteroaromatic
Schiff bases as complexones have often been applied
to the analytical determination of heavy metal ions in
the environmental samples (Afkhami et al., 2012) as
well as ionophores for ion-selective electrodes (Jeong
et al., 2005; Yuan et al., 2012).
The recognition and sensing of ions by appropri-
ately designed ion receptors is the primary objective of
a number of groups of chemists. Due to the wide vari-
ety of cations that are important for living organisms.
A number of detection methods are time-consuming
and require high-quality analytical instrumentation,
hence it is important to design and synthesise sensing
materials for transition metal ions and heavy metal
ions. Sensors that change colour upon host-guest in-
teractions are of use in simple naked-eye applications
without requiring any expensive equipment. Moreover,
Schiff bases have received a great deal of attention
in a wide variety of fields due to their simple syn-
thesis and various applications. They can be readily
synthesised by simple one-pot condensation of aldehy-
des and primary amines in an alcoholic solvent under
anhydrous conditions (Schiff, 1866). Schiff bases have
played an important role in the development of coor-
dination chemistry, as they can form stable complexes
with most of the transition metals. One of the oldest-
known classes of Schiff base ligands used in organic
synthesis is the salen derivatives (Kleij et al., 2005;
S¸ahin et al., 2010).
Schiff bases and their metal complexes are cata-
lysts for many reactions (e.g. oxidation (Ourari et al.,
2012), epoxidation (Grivani & Akherati, 2013), reduc-
tion (Ji et al., 2008), polymerisation (Zhang et al.,
2008), ring-opening polymerisation (Yao et al., 2012)
and Diels–Alder (Jarvo et al., 2005), Henry (Zhou
et al., 2012a) and Mizoroki–Heck reactions (Heo et
*Corresponding author, e-mail: mikaga20@wp.pl
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A. Pazik et al./Chemical Papers
Fig. 1. Chemical structure of ligands I and II.
in recent research a number of Schiff bases have been
used as highly selective fluorescent chemosensors (De-
varaj et al., 2012; Azadbakht et al., 2013; Aziz, 2013;
Jiménez-Sánchez et al., 2013) and colorimetric sensors
for cations (Gupta et al., 2013; Yang et al., 2013).
However, only a few reports describe ligands with ap-
propriate applications in aqueous media (Udhayaku-
mari et al., 2012; Zhou et al., 2012b).
It is also worth noting the important role played by
anions; because of their significant impact on human
lives, and chemical and biological processes, the easy
detection of such species by readily accessible (syn-
thetic) receptors is desired. To date, cationic receptors
are widely known, while anion receptors remain ob-
scure. To the best of our knowledge, only a few Schiff
bases serving as anion receptors have been synthesised
to date (Erdemir et al., 2013; Huang et al., 2013; Ku-
mar et al., 2013; Lin et al., 2013; Liu & Shao, 2013;
Reena et al., 2013; Sen et al., 2013).
Fig. 2. Synthesis of Schiff bases I–II.
Diaminopyridine, 4-hydroxybenzaldehyde and 3-nitro-
benzaldehyde were obtained from Sigma–Aldrich (Po-
land). The metal salts used for UV-VIS spectropho-
tometric titrations (perchlorates: Zn(ClO4)2 ·6H2O,
Cu(ClO4)2 · 6H2O, Co(ClO4)2 · 6H2O, Ni(ClO4)2 ·
6H2O, Cd(ClO4)2 · 6H2O, Pb(ClO4)2 · 3H2O) and
Al(NO3)3 ·6H2O were of analytical grade and were
purchased from Sigma–Aldrich. All anions, in the form
of tetrabutylammonium salts (from Sigma–Aldrich),
were stored in desiccators under vacuum. Acetonitrile
was dried over calcium hydride prior to use. Other
solvents employed in the synthesis and experiments
were of extra-pure grade and used as received without
further purification.
The present study was focused on the synthesis of
ligands for the selective recognition of either transi-
tion metal or heavy metal cations, and the detection
of anions. Ligands I, II with different positions of the
substituent attached to the molecule relative to the
CN double bond were investigated and their selectiv-
ity towards ions was compared (Fig. 1). To the best
of our knowledge, the ion recognition ability of I and
1H NMR spectra were recorded for the samples
dissolved in DMSO-d6 at 500 MHz on a Varian in-
strument. The chemical shifts (δ) are given in parts
per million (ppm) relative to TMS and the coupling
constants (J) are reported in hertz (Hz). Elemental
analyses were performed on an EAGER 200 appara-
tus. The FT-IR spectra for samples in dry KBr were
taken on a Genesis II FT-IR (Mattson) instrument.
Gaussian 03 software was used for all theoretical cal-
culations (Frisch, 2003). The ground-state optimisa-
tion was carried out using Gaussian 03 software with
B3LYP and 6-31++G(dp) basis set at the density
functional theory (DFT) level. Thin layer chromatog-
raphy (TLC) analyses were performed on aluminium
plates coated with silica gel 60 F254 (Merck). UV-VIS
measurements were carried out at ambient tempera-
ture with the use of a UNICAM UV 300 series spec-
trophotometer. UV-VIS titrations were carried out in
a 1 cm path length quartz cuvette maintaining a con-
stant volume for the I and II solutions (2 mL).
1
II has not been examined using UV-VIS or H NMR
spectroscopy and has not yet received attention in the
literature. Furthermore, according to recent research,
some Cu(II), Zn(II) and Ni(II) complexes with Schiff
base I exhibit antibacterial activities (Jeewoth et al.,
1999). UV-VIS spectroscopic studies show that both
compounds (I and II) are able to form complexes with
Cu2+, Zn2+, Pb2+ and Al3+ ions. In addition, com-
pound I is a potential sensing F− receptor. Ligand I
with strong hydrogen bonds and the para-substituted
hydroxyl group, upon the addition/influence of an an-
ion guest changes colour in association with changes
in the electronic properties of chromogenic units. The
present study also includes a theoretical investigation
of the nature of the binding behaviour of compounds
using the DFT method.
General experimental procedure for synthesis
of ligands I–II
Experimental
The synthesis procedure was adapted from the lit-
erature (Dubey & Ratnam, 1977; Cimerman et al.,
1997). To a heated solution of 2,3-diaminopyridine
Schiff base ligands I and II were synthesised
following the procedure presented in Fig. 2. 2,3-
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A. Pazik et al./Chemical Papers
iii
(5 mmol, 0.546 g) dissolved in dry MeOH (10 mL) a so-
lution of the appropriate aldehyde (5 mmol in 10 mL of
MeOH) was slowly added drop-wise. The mixture was
stirred overnight at 50◦C. After that time, the solvent
was removed under reduced pressure and the result-
ing crude product was recrystallised from 2-propanol.
Compound I was purified by gradient column chro-
matography on silica gel, using a CHCl3–MeOH sol-
vent mixture.
Table 1. Summary of crystal data, data collection and refine-
ment for 4-(2-amino-3-pyridyliminomethyl)phenol
Parameter
Value
Empirical formula
Formula mass/Da
T/K
C24H22N6O2
426.48
298.15
Crystal system
Space group
Monoclinic
P 21/n
Unit cell dimensions
˚
a/A
X-ray structure analysis of ligand I
9.8550(9)
13.9442(7)
16.2245(10)
104.975(7)
˚
b/A
4-(2-Amino-3-pyridyliminomethyl)phenol (I) sin-
gle crystal was grown from a CHCl3–MeOH mixture
by a slow solvent evaporation. For the X-ray studies, a
transparent crystal of needle shape was selected. The
experimental data were collected on a KUMA KM4
diffractometer with graphite monochromated MoKα
radiation using a Sapphire-2 CCD detector. The ap-
paratus was equipped with an open flow thermostat
(Oxford Cryosystems, UK) which made it possible for
the experiments to be carried out at 298 K. Data
reduction, space group determination, solution and
refinement were effected using the CrysAlisPro soft-
ware package (Version 1.171, 2008, Oxford Diffraction,
Abingdon, UK). The structure was resolved by the di-
rect method and refined by full-matrix least-squares
on F2 (all data) using the shelxl program package
(Sheldrick, 2008). The basic crystal data, descriptions
of the diffraction experiment and details of the struc-
ture refinement are given in Table 1.
˚
c/A
β/◦
3
˚
V /A
2153.9(3)
Z
4
ρ/(g cm−3
μ/(mm−1
)
1.325
0.088
)
F (0 0 0)
896
Crystal size/mm3
Index ranges
0.23 × 0.22 × 0.20
–12 ≤ h ≤ 12
–15 ≤ k ≤ 17
–7 ≤ l ≤ 20
7954
Reflections collected
Unique reflections
Parameters
θ, range for data collections/◦
S (goodness-of-fit)
R1 [I > 2σ(I)]
4224 (Rint = 0.0260)
321
2.58–28.49
1.053
R1 = 0.0516;
wR2 = 0.1228
R1 = 0.0814;
wR2 = 0.1456
0.244 and –0.160
R indices (all data)
All non-hydrogen atoms were refined anisotropi-
cally. The N—H, O—H hydrogen atoms and H6A,
H18A were located in a difference Fourier map and re-
Largest differe−nc3es in peak
˚
and hole/(e A
)
˚
˚
fined freely; N2—H2B 0.92(3) A, N2—H2C 0.84(3) A,
˚
˚
N5—H5B 0.93(3) A, N5—H5C 0.88(3) A, O1—H1A
˚
˚
˚
1.00(3) A, O2—H2A 0.95(3) A, C6—H6A 1.03(2) A,
˚
C18—H18A 1.02(2) A. Other H atoms were positioned
an intramolecular hydrogen bond.
geometrically and refined using a riding model C—
H = 0.93 A, Uiso(H) = 1.2Ueq(C).
The synthesised compounds were characterised on
the basis of spectroscopic methods including FT-IR,
UV-VIS, 1H NMR and elemental analysis (Tables 2
and 3). For ligand I, the X-ray structure was also de-
termined. The presence of free amino groups in the
molecules was confirmed by the appearance of a singlet
at δ around 5.83 and 6.13 in the 1H NMR (DMSO-d6)
spectroscopy. The addition of D2O caused the loss of
signals corresponding to the 2-amino protons in pyri-
dine.
˚
Results and discussion
Synthesis and X-ray characterisation
The Schiff bases I and II were synthesised follow-
ing the procedure detailed in the literature by heat-
ing 2,3-diaminopyridine and appropriate aldehyde in
dry methanol (Fig. 2). Investigations into this group
of compounds began as early as 2005. This problem
was studied by Kleij et al. (2005), who synthesised the
compounds under milder conditions and with higher
yields of products (70 %). The reaction was carried
out in methanol at ambient temperature for 24 h. The
reaction can be adopted as a general method for syn-
thesised compounds with an O—H group in the ortho
positions relative to an imino group. The yield of the
reaction appears to depend on the existence of a strong
The singlet at δ of 8.52 (I) and 8.85 (II ) is as-
—
cribed to the CH N proton (Waldeck, 1991; Schilf et
—
al. 2004; Yıldız et al., 2009). The OH proton in ligand
I appears at δ of 10.1. The chemical shifts, observed
for I between the δ of 6.54–7.82 and for II 6.59–7.80
are ascribed to the aromatic protons. Comparing the
1H NMR data of aromatic protons of compounds I
and II, the different location of substituent attached
to the molecule can clearly be seen.
The formations of Schiff base units in the FT-IR
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Table 2. Data characterising prepared compound
wi(calc.)/%
wi(found)/%
Yield
%
M.p.
◦C
Compound
Formula
Mr
C
H
N
I
C12H11N3O
C12H10N4O2
213.09
242.08
67.59
67.52
59.50
59.55
5.28
5.31
4.16
4.19
19.71
19.68
23.13
23.15
41
45
217–220
198–200
II
Table 3. Spectral data of compounds
Compound
Spectral data
I
IR, ν˜/cm−1: 452, 496, 520, 598, 629, 644, 697, 773, 794, 836, 919, 953, 983, 1032, 1100, 1136, 1161, 1161, 1178, 1178,
1251, 1252, 1272, 1305, 1410, 1463, 1505, 1574, 1582, 1618, 2340, 2678, 3358, 3463
1H NMR (500 MHz, DMSO-d6), δ: 5.83 (s, 2H, NH2), 6.54 (dd, J1 = 4.88 Hz, J2 = 7.33 Hz, 1H), 6.86 (d, J = 8.3
Hz, 2H), 7.31 (d, J = 7.33 Hz, 1H), 7.79 (d, J = 4.89 Hz, 1H), 7.82 (d, J = 8.79 Hz, 2H), 8.52 (s, 1H), 10.1 (s, 1H,
OH)
1H NMR (500 MHz, DMSO + D2O), δ: 6.59 (dd, J1 = 4.88 Hz, J2 = 7.32 Hz, 1H), 6.85 (d, J = 8.3 Hz, 2H), 7.30
(d, J = 7.33 Hz, 1H), 7.72 (d, J = 4.88 Hz, 1H), 7.80 (d, J = 8.3 Hz, 2H), 8.46 (s, 1H)
IR, ν˜/cm−1: 428, 524, 564, 668, 693, 731, 768, 793, 850, 923, 968, 1099, 1180, 1255, 1311, 1347, 1433, 1467, 1518,
1605, 3127, 3241, 3471
II
1H NMR (500 MHz, DMSO-d6), δ: 6.13 (s, 2H, NH2), 6.58 (dd, J1 = 4.88 Hz, J2 = 7.33 Hz, 1H), 7.45 (d, J = 7.33
Hz, 1H), 7.78 (t, J = 8.30 Hz, 1H), 7.88 (d, J = 4.88 Hz, 1H), 8.33 (d, J = 7.81 Hz, 1H), 8.47 (d, J = 7.82 Hz,
1H), 8.79 (s, 1H), 8.85 (s, 1H)
1H NMR (500 MHz, DMSO + D2O), δ: 6.61 (dd, J1 = 5.37 Hz, J2 = 7.32 Hz, 1H), 7.35 (d, J = 6.35 Hz, 1H), 7.72
(t, J = 8.30 Hz, 1H), 7.80 (d, J = 3.42 Hz, 1H), 8.26 (d, J = 7.81 Hz, 1H), 8.34 (d, J = 7.81 Hz, 1H), 8.69 (s, 1H),
8.73 (s, 1H)
Fig. 3. ORTEP plot of I, showing atom-labelling scheme. Dis-
placement ellipsoids are drawn at 50 % probability
level.
spectrum can be observed in the absence of the bands
between 1715–1645 cm−1 assignable to the character-
—
—
istic C O stretching vibration’s. The CH N groups
—
—
stretching vibration appears between 1618–1605 cm−1
for both compounds. The FT-IR spectra of the lig-
ands I and II show strong-intensity absorption bands
at 3463–3471 cm−1 assigned to the N—H stretch-
ing mode. The spectrum of compound I also showed
bands at about 2678 cm−1 and 2340 cm−1 which are
assigned to the stretching hydrogen motions in the
intramolecular O—H····N hydrogen bonding between
the OH proton and the nitrogen atoms.
Fig. 4. X-ray crystal packing for ligand I.
A drawing of the X-ray structure and crystal pack-
ing of I is shown in Figs. 3–5. X-ray analysis con-
firmed that the compound only existed in the phenol-
imine form. The asymmetric unit cell contains two
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v
Fig. 5. Formation of left-handed and right-handed helix of ligand I.
unique molecules of compound I presented in Fig. 3.
They are joined by the hydrogen bond formed be-
tween the hydroxyl group and the nitrogen of the pyri-
dine ring of the second molecule of the compound.
The hydroxyl group acts as the donor of the hydro-
gen bond to nitrogen atom of pyridine with O····N
The hydrogen bonds form a one-dimensional single
chain along the b axis. The amino group of the same
pyridine ring acts as a donor of the next hydrogen
bond to the oxygen of the hydroxyl group of the next
molecule of the compound with N····O distances of
˚
3.058(19) A (N5····O2) and N5—H5C····O2 angles
˚
˚
distances of 2.763(2) A (O1····N4) and 2.723(2) A
(O2····N1) and O—H····N angles of 172(3)◦ (O1—
H1A····N4), and 171(3)◦ (O2—H2A····N1) (Table 4).
of 140(2)◦. The presence of the hydrogen bond results
in the formation of a left-handed helix built by three
single chains. The compound so obtained crystallises
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A. Pazik et al./Chemical Papers
in the monoclinic system and belongs to the P21/n
space group. Hence, the unit cell contains both chains
– left-handed and right-handed helix (Fig. 5).
In the case of a mono Schiff base derived from
2,3-diaminopyridine and salicylaldehyde, the strong
intramolecular H-bonding is formed between O—
Fig. 6. Naked-eye colour change of ligand I (c = 3 × 10−5
mol L−1) after addition of 10 eq. of various anions (c =
10−4 mol L−1) in CH3CN solvent.
—
H····N C (Cimerman et al., 1992). The bond length
—
˚
O····N corresponding to a double bond is 2.649(4) A
which for compound I is not very probable. The X-ray
powder diffractograms show similarities in the pyri-
dine and benzene rings.
colourless to pale yellow in the presence of F− ions
(10 eq.) (Fig. 6). Ligand II did not exhibit any de-
tectable colour change upon the addition of various
ions. The presence of the —NO2 substituent exhibit-
ing an electron-withdrawing effect on the hydrogen
properties did not provide colorimetric and spectral
sensing ion recognition.
In the preliminary experiments, the anion bind-
ing properties of ligands in the UV-VIS spectrum
in CH3CN were investigated. The most significant
changes for ligand I in the absorption spectra were
observed in the presence of tetra−1-butylammonium
fluoride. UV-VIS titrations were carried out to esti-
mate the stability constant of the respective complex.
The spectral changes of compound I under various
concentrations of F− ions are shown in Fig. 7a. The
Spectroscopic studies
Nowadays, the design of a potential sensor for an-
ions and cations such as transition metal ions, and
heavy metal ions is very important from the medical
perspective. High selectivity is a matter of necessity
in an excellent sensor. The ion recognition properties
of ligands I–II were investigated visually by colour
change, 1H NMR and UV-VIS spectroscopic methods.
The visual colour change in CH3CN was investi-
gated upon the addition of various ions. No colour
and spectral changes were observed in the presence
of alkali metal and alkaline earth metal cations. In
the naked eye experiments, ligand I (c = 3 × 10−5
mol L−1) showed a noticeable colour change from
Table 4. Hydrogen bond geometry for I
D—H
H····A
D····A
D—H····A
D—H····A
◦
˚
A
O1—H1A····N4
O2—H2A····N1a
N5—H5B····N3b
N5—H5C····O2c
1.00(3)
0.95(3)
0.93(3)
0.88(3)
1.77(3)
1.78(3)
2.66(3)
2.33(3)
2.763(2)
2.723(2)
3.483(3)
3.058(3)
172(3)
171(3)
148.4(19)
140(2)
Symmetry codes: a) –x + 1/2, y + 3/2, –z + 1/2; b) –x + 1, –y, –z + 1; c) –x + 1/2, y – 1/2, –z + 1/2.
Fig. 7. Changes in absorption spectra upon titration of I (c = 3 × 10−5 mol L−1) with TBAF (c = 0–2.10 × 10−4 mol L−1
(lines 1–30) in CH3CN (a). Titration plot to determine stability constant of complex between I and F− at 400 nm (b).
)
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vii
addition of F− ions caused a gradual decrease in the
intensity of the peak at 285 nm and a batochromic
shift from 362 nm to 400 nm (Δλ = 38 nm), followed
by a change in colour of the solution from colourless
to yellow. In the spectrum, there is only one isosbestic
point at 257 nm. On the basis of the titration exper-
iments, two types of equilibrium of strong complexes
in CH3CN were found (Fig. 7b). The stoichiometries
of the complexes were 2 : 1 and 1 : 4 (I : F−). It
was not possible to calculate the binding constants of
the fluoride complexes. Moreover, ligand I showed no
UV-VIS absorption spectral changes and no colour in
the presence of other anions in the sample of F− ions
(Fig. 8) as inspected by the naked eye. This is very im-
portant because many sensors for F− ions suffer from
interference from AcO− and H2PO4− ions.
Fig. 8. UV-VIS spectra of sensor I (c = 3 × 10−5 mol L−1
)
and changes to F− ions (100 eq.) in the absence and
presence of 100 eq. of various anions in CH3CN.
To investigate the anion-binding properties of lig-
and I in the presence of the anions studied and the role
of the para-substituted hydroxyl group in the Schiff
1
base, the H NMR titration experiment in deuterated
deprotonation. In the case of the imine group in the
—
DMSO made it possible to distinguish the free sensor
I and its complex with F− ions. The spectra differed
significantly, as shown in Fig. 9 and Table 5. Upon
the addition of 2 eq. of TBAF, the signal at δ of 10.1
disappeared which may be due to the formation of
an intramolecular hydrogen bond between the oxy-
gen atom of the hydroxyl group and the F− ions after
absence of TBAF, the CH N proton appeared as a
—
singlet at δ of 8.52 (Fig. 9a), whereas in the presence
of 2 eq. of F− ions (Fig. 9b), the singlet was shifted
up-field to δ of 8.07. There were significant changes
in the ligand I spectrum in the aromatic part upon
complexation were. All the aromatic hydrogen atoms
of the pyridine units were shifted up-field. Analysis
Fig. 9. 1H NMR spectra of sensor I with addition of TBAF in DMSO-d6 solution. I only (a); I + F− (2.0 eq.) (b).
Table 5. Chemical shift changes in 1H NMR spectra (500 MHz) of sensor I in the presence of 2 eq. of TBAF in DMSO-d6
1
2
3,3ꢀ
4
5
6,6ꢀ
7
8
Compound
δ (Split)
I
5.83 (s)
5.55 (s)
–0.28
6.54 (dd)
6.47 (dd)
–0.07
6.86 (d)
5.98 (d)
–0.88
7.31 (d)
7.07 (d)
–0.24
7.79 (d)
7.59 (d)
–0.20
7.82 (d)
7.33 (s)
–0.49
8.52 (s)
8.07 (s)
–0.45
10.1
–
–
I + F−
Δδ
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Fig. 10. Absorption spectra recorded in CH3CN solution containing ligand I (c = 3 × 10−5 mol L−1) and Al(NO3)3 (c = 0–5.24 ×
10−5 mol L−1) (lines 1–23) (a). Dependence of absorbance at 352 nm for ligand I with Al3+ ions (b).
Fig. 11. UV-VIS spectral changes of ligand I (c = 3 × 10−5 mol L−1) upon addition of Pb(ClO4)2 (c = 0–4.74 × 10−5 mol L−1
(lines 1–19) (a). Dependence of absorbance at 350 nm for ligand I with Pb2+ ions (b).
)
of the chemical shift changes for the aromatic pro-
ton signals confirmed the phenolate form, which can
be seen in the up-field shift of 3,3’ (Δδ = –0.88) and
6,6’ (Δδ = –0.49) protons. Simultaneously, two pro-
tons 6,6’ (doublet) at δ of 7.82 were coupled to each
other and formed a single peak at δ of 7.33. In the
presence of 3 eq. of TBAF a new triplet at δ of 16.1
with J = 124 Hz appeared, which was due to the de-
protonation of the chromogenic sensor and may be at-
tributed to the HF−2 dimer. Changes in the chemical
shift values of the aromatic protons, the amino group
and the disappearance of the —OH singlet upon 2
eq. of TBAF indicated a strong H-bond interaction
between the sensor-recognition site and the F− ions
resulting in the appearance of the yellow colour.
In addition, ligands I–II were treated with transi-
tion metal ions and heavy metal ions to evaluate the
optical response in CH3CN. The most notable changes
Pb2+, Zn2+ and Cu2+ ions (Figs. 10–16). The addi-
tion of metal ions such as Ni2+, Co2+ and Fe2+ to
the solution of ligands in CH3CN did not exhibit any
notable spectral change; the absorption spectra of the
ligands only increased the intensity of the main band.
Figs. 10–12 show that the additions of Al3+, Pb2+
,
Cu2+ ions to I induced similar spectral changes. Lig-
and I had absorption peaks at about 285 nm and
362 nm in the absence of cations. With the increase in
cations’ concentration, the band intensity at 285 nm
decreased and the intensity of the absorbance at about
350 nm increased. The formation of two clear isos-
bestic points at 298 nm and 388 nm (for Al3+ and
Pb2+ ions) and one at 374 nm for Cu2+ indicates the
formation of a stable complex with a certain stoichio-
metric ratio between the ligands and cations examined
(Table 6). Although ligand I exhibited similar spectral
changes for selected cations at the same wavelength,
the intensity of the spectral response was different
in the absorption were found in the presence of Al3+
,
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A. Pazik et al./Chemical Papers
ix
Fig. 12. Changes in UV-VIS absorption spectrum of ligand I (c = 3 × 10−5 mol L−1) with Cu(ClO4)2 (c = 0–4.87 × 10−5
mol L−1) (lines 1–20) in CH3CN solution (a). Titration plot to determine stability constant of complex between Iand
Cu2+ (1 : 1) at 285 nm (b).
Fig. 13. 1H NMR spectra of free I and its complex with Al(NO3)3 dissolved in DMSO-d6 (I : Al3+).
upon the addition of an equivalent amount of Al3+
,
ing to the protons of the hydroxyl and imine groups
were shifted slightly downfield which suggests the for-
mation of a complex in protonated form. In the case
of the solution containing I and Al3+ ions in a mo-
lar ratio of 1 : 4, the addition of Al(NO3)3 caused
a gradual broadening of the complex signals. The
N—H protons of I δ of around 5.83 displayed a down-
field shift to δ of 6.44. The formation of new small
Pb2+ and Cu2+. The stoichiometries of the complexes
were 1 : 1 (Pb2+, Zn2+, Cu2+) and 1 : 2 for (Al3+
ions).
The mechanism of the cation-binding properties of
I with Al3+ ions was investigated by the 1H NMR
1
titration experiment in DMSO-d6 (Fig. 13). H NMR
spectral analysis showed that the signals correspond-
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x
A. Pazik et al./Chemical Papers
Table 6. Overall stability constants of Cu2+, Pb2+, Al3+ and Zn2+ complexes with ligands I–II
Compound
Cu2+,a
Pb2+,a
Al3+
Zn2+,a
I
(3.90
(5.70
0.37) × 103
(7.80
(5.30
0.12) × 103
(8.40
(3.50
0.15) × 105
1 : 2
(1.20
(1.80
0.46) × 104
Complex stoichiometry
II
Complex stoichiometry
1 : 1
1 : 1
1 : 1
0.05) × 103
1 : 1
0.20) × 103
1 : 1
0.41) × 105
0.42) × 104
1 : 1
1 : 2
a) Determined using Benesi–Hildebrand relation.
Fig. 14. UV-VIS titration of compound II (c = 3 × 10−5 mol L−1) with Al(NO3)3 (c = 0–4.87 × 10−5 mol L−1) (lines 1–20) (a).
Dependence of absorbance at 351 nm for ligand II with Al3+ ions (b).
Fig. 15. Absorption spectra recorded for CH3CN solution containing ligand II (c = 3 × 10−5 mol L−1) and Pb(ClO4)2 (c =
0–5.24 × 10−5 mol L−1) (lines 1–23) (a). Dependence of absorbance at 350 nm for ligand II with Pb2+ ions (b).
signals occurred when a 2 : 1 (I : Al(NO3)3) ratio was
achieved. A gradual addition of salt caused the largest
changes especially in the pyridine region and a slightly
throughout the complexation process.
Figs. 14–16 show the impact of various cations
on the UV spectrum of ligand II in a CH3CN solu-
tion. Compound II had certain absorption peaks at
242 nm and 381 nm. Upon the successive addition of
cations (Al3+, Pb2+, Cu2+) to ligand II, the intensity
of absorbance at 242 nm decreased, while the inten-
sity of the peak at 381 nm was shifted to a shorter
wavelength. In the case of Al3+ ions, a new maxi-
mum absorption was found at 350 nm with one clear
—
smaller contribution of CH N protons. These spec-
—
tral changes suggest that the free exchange of Al3+
ions promotes the formation of two complexes dur-
ing the titration experiments in solution, hence. It
was difficult to assess the structure of the resulting
complexes. The spectral changes observed could im-
ply that ionisation of the amino groups was achieved
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A. Pazik et al./Chemical Papers
xi
Fig. 16. Changes in UV-VIS absorption spectrum of ligand II (c = 3 × 10−5 mol L−1) with Cu(ClO4)2 (c = 0–4.87 × 10−5
mol L−1) (lines 1–20) in CH3CN solution (a). Titration plot to determine stability constant of complex between II and
Cu2+ (1 : 1) at 324 nm (b).
isosbestic point at 376 nm (Fig. 14). A molar ratio
plot revealed that, under the titration experiments,
a complex of 1 : 2 (Al3+ : II) was established. Fur-
thermore, the absorption spectra of II with Pb2+ and
Cu2+ with various concentrations of ions exhibited a
hypsochromic shift; the absorption band at 381 nm
was shifted to 350 nm and 324 nm, respectively. As
the concentration of cations increased, one isosbestic
point at 376 nm for Pb2+ and 364 nm Cu2+ com-
plex was formed. The binding constants for the 1 : 1
complexes were obtained from the Benesi–Hildebrand
equation (Table 6).
The binding ability (b) of ligands I and II with
the examined cations is in the order of b(Al3+) >
b(Zn2+) > b(Pb2+) > b(Cu2+); b(Al3+) > b(Zn2+) >
b(Cu2+) ≥ b(Pb2+), respectively. Compounds I and
II exhibit the highest binding ability with Al3+ ions.
The similar patterns of spectroscopic changes in the
absorption spectra may suggest that the substituent
in the phenyl ring does not participate in the com-
plexation process. Moreover, it was presumed that
cations are incorporated between the imino and amino
groups. However, the different patterns of spectro-
scopic changes in the absorption spectra between the
F− ions and the previously reported studies may con-
firm the hydroxyl group to be essential in the com-
plexation properties towards anions.
Table 7. Calculated parameters for complex of I
Compound I
Cu2+ : I
Pb2+ : I
Zn2+ : I
N1····X
N2····X
N3····X
C6····X
O1····X
5.565
3.447
3.512
4.228
6.406
5.833
4.045
3.781
4.343
6.430
5.247
3.143
3.121
3.886
6.369
spectroscopy have not yet received any attention in
the literature.
Molecular simulation
Computational simulations of the Schiff base lig-
and and the metal complexes were carried out us-
ing Gaussian 03 software with the B3LYP and 6-
31++G(dp) basis set at the density functional the-
ory (DFT) level. The optimised calculated geometry
in Fig. 17 reveals molecule I not to be in the same
plane. The binding affinity of this ligand was deter-
mined toward several cations and F− ions. The in-
troduction of cations (Cu2+, Pb2+, Zn2+) into ligand
I shows that, in all cases, cations were located in the
cavity formed between the imino bonds and the amino
groups. The absence of changes in the hydroxyl group
suggests that it is not engaged in the complex, thereby
confirming the earlier hypothesis. Table 7 presents the
important parameters of the experimental data be-
tween I and the metal cations studied. The calculated
distances between the imino bonds and the cations
are the shortest for Zn2+ ions. The bond lengths in
the case of different complexes increase with the big-
ger and different geometrical shape of the guest. The
optimised global minima of the Schiff base ligand I
The metal Schiff base complexes derived from sal-
icylaldehyde and diaminopyridine have been widely
studied. The Schiff base ligands behave like O, N donor
bidentate for copper, nickel, iron, cobalt, zinc and
ruthenium ions. Asymmetrical Schiff bases with Cu,
Ni and Fe(III) complexes have also been prepared. In
the event that the free amino group is still present, co-
—
—
ordination also occurs through the amino and CH
N
groups. However, the coordination chemistry and be-
haviour of these compounds as sensors using UV-VIS
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xii
A. Pazik et al./Chemical Papers
Fig. 17. DFT optimised structure of compound I.
Fig. 19. Optimised geometry of Al3+ : I complex.
method. UV-VIS measurements showed that ligand
I selectively recognised F− ions. A naked-eye colour
change was not observed upon the addition of other
anions. Furthermore, the synthesised ligands were
used as sensing materials for transition metal cations
and heavy metal cations. The addition of Al3+, Zn2+
,
Pb2+ and Cu2+ ions caused a gain and hypsochromic
shift effect of the maximum absorption band of the
compounds presented. The values of the binding con-
stant are highest for Al3+ complexes. The presence of
the amino and hydroxyl groups strongly influences the
complexing ability of molecules.
Acknowledgements. The financial support for this work re-
ceived from Gdansk University of Technology, grant no. BW
020331/006 DS 020223/003 is gratefully acknowledged.The au-
thors wish to thank Professor J. Biernat for the invaluable as-
sistance in the implementation of research and preparation of
the manuscript.
Fig. 18. Optimised structure of Cu2+ : I complex.
and Cu2+, Al3+ complexes are presented in Figs. 18–
19. In addition, the introduction of F− ions into lig-
and I led to a reorganisation of the structure to a
more rigid system. Clearly, the O—H moieties in the
structure I are involved in the complexation of F−
ions by relatively strong hydrogen interactions. The
distance between the intramolecular hydrogen bonds
Supplementary data
Complete crystallographic data for the structure
reported in this paper were deposited with the Cam-
bridge Crystallographic Data Centre as Supplemen-
tary Publication No CCDC 988906. Copies of the
data can be obtained free of charge from the CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (tel.:
+44-1223-336-408; fax: +44-1223-336-033, e-mail: de-
posit@ccdc.cam.ac.uk).
˚
(O—H····F) is 2.109 A. Fluorine ion is the smallest
ion and can fit between two molecules and form a 2 :
1 stable complex.
Conclusions
The spectroscopic properties of Schiff bases’ be-
haviour depend on the nature and position of the sub-
stituent attached to the molecule. Induction of the
electron-withdrawing group such as NO2 into the sen-
sor facilities the creation of a possible sensitive and
complexing molecule. In order to increase the selec-
tivity of the ligand, the free hydroxyl group was in-
troduced the para position on the aromatic ring. Ac-
cordingly, a new mono Schiff base derived from 2,3-
diaminopyridine I–II was synthesised. The spectro-
scopic properties were investigated by UV-VIS and
1H NMR spectroscopic methods and by the DFT
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