Potentiometric and spectrophotometric studies on the binding ability of a flexible tripodal catecholamine ligand toward iron(III)

Article abstract of DOI:10.1021/je101338m

The tripodal catecholamine ligand N¹,N³,N ⁵-tris(2-(2,3-dihydroxybenzylamino)ethyl)cyclohexane-1,3, 5-tricarboxamide (CYCOENCAT, L) has been synthesized and characterized. The ligand was investigated as a chelator for Fe(I

Full text of DOI:10.1021/je101338m

ARTICLE
pubs.acs.org/jced
Potentiometric and Spectrophotometric Studies on the Binding
Ability of a Flexible Tripodal Catecholamine Ligand toward Iron(III)
Suban K. Sahoo,*,† Minati Baral,‡ and B. K. Kanungo*,§
†Department of Applied Chemistry, S.V. National Institute of Technology (SVNIT), Surat, Gujrat, India
‡Department of Chemistry, National Institute of Technology (NIT), Kurukshetra, Haryana, India
§Department of Chemistry, Sant Longowal Institute of Engineering & Technology (SLIET), Longowal, Punjab, India
ABSTRACT: The tripodal catecholamine ligand N1,N3,N5-tris(2-(2,3-dihydroxybenzylamino)ethyl)cyclohexane-1,3,5-tricarbox-
amide (CYCOENCAT, L) has been synthesized and characterized. The ligand was investigated as a chelator for Fe(III) in an
aqueous medium of 0.1 M KCl at (25 ( 1) °C by potentiometric and spectrophotometric methods. Six protonation constants for
the ligand were determined and were used as input data to determine the stability constants of the metal complexes. The stability
constants for the FeLH3, FeLH2, FeLH, and FeL complexes are reported. The ligand showed the potential to form a tris(catecholate) type
complex with a pFe value of 24.76 at pH = 7.4.
1. INTRODUCTION
2. EXPERIMENTAL SECTION
Biomimetic approaches in research have gained immense
interest in recent years due to the interface between chemistry
and biology. Many biomolecules are identified in living systems,
which act as chelators for the assimilation and transport of metal
ions. These natural chelators have received considerable interest
due to their selective and strong binding efficiency for a specific
metal ion in the presence of other metal ions in living systems.
Therefore, to design new chelators for a metal ions, effort has
been made to mimic the molecular structure and binding sites of
natural chelators through simple synthetic molecules.1 Also, such
synthetic biomimetic molecules are implemented for metal ion
sequestration due to their similar selective and strong binding
properties toward the metal ion. One of the most studied bio-
molecules for designing biomimetic synthetic chelators for
iron(III) is enterobactin (Scheme 1), which is produced and
excreted by bacteria in iron deficient media to bind and assimilate
extracellular iron.2 It contains three catechol groups appended to
tripodal cyclic L-serine through an amide linkage, which has
been found to be the best iron-chelating agent with the highest
formation constant (log K = 52).3 The enormous iron(III)
complexing ability and effective selectivity has led to the synthesis
of new biomimetic analogs containing three catechol units in
the tripod for: (i) their potential applications as clinical iron
removal agents (iron overload is one of the most common types
of poisoning),4 (ii) the use of their iron complexes in agriculture
for the prevention or treatment of chlorosis,5 and (iii) to mimic
the essential structural features that are responsible for the different
biological functions of natural compounds.6
2.1. Materials and Measurements. The triamine CYCOEN
was synthesized by following a reported method.7 All chemicals
required for the synthesis: 1,3,5-benzenetricarboxylic acid, platinum
dioxide, ethylenediamine, 2,3-dihydroxybenzaldehyde, and sodium
borohydride were obtained from Sigma-Aldrich and were used
directly. Anhydrous FeCl3, absolute ethanol, methanol, KOH, HCl,
and KCl were obtained from Ranbaxy Chemicals Ltd., India.
Proton NMR spectra were recorded on a Bruker DPX-300
spectrometer in DMSO-d6 or D2O, and chemical shifts were
reported relative to Me4Si. IR (KBr pellets, (450 to 4000) cmꢀ1
)
spectra were recorded on a Perkin-Elmer RX I FT-IR spectrom-
eter. The electronic spectra were recorded on an Agilent-8453
diode array spectrometer. Elemental analyses were determined
for C, H, and N using the Exeter Analytical CE-440.
2.2. Synthesis of CYCOENCAT. The Schiff base intermediate
was prepared by dropwise addition of 2,3-dihydroxybenzaldehyde
(0.39 g, 28.24 mmol) in 5 mL of absolute ethanol to a magnetically
stirred suspended solution of CYCOEN (0.32 g, 9.33 mmol) in
10 mL of absolute ethanol. A bright yellow coloration appeared
immediately. The mixture was refluxed for one hour. A dark
yellow solution was obtained. On cooling in a freezer, a dark
yellow precipitate was obtained, which was filtered off and
washed with cold ethanol followed by ether and then dried in a
vacuum. Yield = 0.49 g (75 %); IR (KBr pellet, cmꢀ1): 3372,
3215, 2918, 2717, 1623, 1581, 1512, 1445, 1354, 1310, 1235, 1155,
1115, 1021, 962, 851, 774, 652, and 636. 1H NMR (DMSO-d6,
δ ppm): 1.58 (3H, d), 1.83 (3H, d), 2.21 (3H, t), 3.42 (6H, t),
3.69 (6H, t), 6.62 (3H, m), 6.76 (3H, q), 6.86 (3H, m), and 8.30
(3H, s).
In this communication, a novel biomimetic tripodal catecho-
lamine ligand N1,N3,N5-tris(2-(2,3-dihydroxybenzylamino)ethyl)-
cyclohexane-1,3,5-tricarboxamide (CYCOENCAT, L) was in-
vestigated as a chelator for iron(III) in an aqueous medium of
0.1 M KCl at (25 ( 1) °C by potentiometric and spectro-
photometric methods. The stability constants of the various
complexes and their possible structures in solution are explained.
To the suspension of Schiff base (0.50 g, 0.71 mmol) in 50 mL
of dry methanol, sodium borohydride (0.22 g, 5.81 mmol) was
Received: December 25, 2010
Accepted: April 15, 2011
Published: May 02, 2011
r
2011 American Chemical Society
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Scheme 1. Molecular Structures of Enterobactin and CYCOENCAT (L)
added with stirring over a period of two hours under nitrogen
atmosphere. When the solid phase disappeared and the solution
became colorless, 2 mL of concentrated HCl taken in 10 mL
methanol was added and kept overnight at ꢀ40 °C. A white residue
was removed by filtration. The solvent was evaporated under
reduced pressure. The crude product obtained was dissolved
in dry methanol and evaporated. This process was repeated
3ꢀ4 times, and then the residue was dissolved again in dry
methanol and to it decolorizing charcoal was added. After
vigorous shaking, the charcoal was filtered off. The filtrate was
collected, and the solvent was evaporated under reduced
pressure. A white product was obtained, which was dried
under vacuum. Yield = 0.39 g (65 %); IR (KBr pellet, cmꢀ1):
3374, 3220, 2922, 2718, 1634, 1576, 1504, 1446, 1356, 1304,
1240, 1160, 1126, 1066, 1020, 960, 838, 764, 696, 640, and
616. 1H NMR (D2O, δ ppm): 1.35 (3H, q), 1.83 (3H, d), 2.38
(3H, t), 3.12 (6H, t), 3.46 (6H, t), 4.15 (s, 3H), 6.70ꢀ6.85
(6H, m), and 6.87 (3H, d). Elemental analysis: Anal. Calcd.
(Found) for C36H48N6O9 3HCl 2H2O: C, 50.58 (50.62); H,
ferric complexes.10 The method adopted for the spectrophotometric
titrations are similar to the potentiometric titrations except that dilute
solutions of ligand (4.02 10ꢀ5 M) and metal ion (4.02 10ꢀ5 M)
were used. After each adjustment of pH, an aliquot was removed,
and spectra were recorded.
3
3
3. RESULTS AND DISCUSSION
3.1. Ligand Synthesis. The synthetic procedure of CYCOEN-
CAT (L) is presented in Scheme 2. The ligand was isolated as a
hydrochloride salt, and care was taken to avoid contact with the
atmosphere throughout the procedure. This compound is highly
hygroscopic and soluble in water, methanol, and ethanol but
insoluble in chloroform, ether, and acetonitrile. The ligand was
characterized on the basis of elemental analyses and various
spectral (electronic, IR, and 1H NMR) data.
The infrared spectrum of the unreduced form of CYCOEN-
CAT showed a weak band at 1623 cmꢀ1 for υCdN that merged
with the broad υCdO band. Upon reduction, the infrared spectrum
of L showed only the υCdO band at 1634 cmꢀ1. Usually, the
normal position for the stretching vibration of non-hydrogen
bonded or free hydroxyl groups absorbs strongly in the range of
3
3
6.47 (6.49); N, 9.89 (9.84).
2.3. Titration Procedure. The apparatus used, experimental
details, calibration of the electrode, and titration procedures were
as described before.8 A standard hydrochloric acid solution
was titrated with a standard KOH solution, and the calculated
hydrogen ion concentrations (pKw = 13.77) were used to convert
the pH-meter reading to hydrogen ion concentration. All solu-
tions were prepared prior to the experiments in double-distilled
deoxygenated water. KOH solution of 0.1 M was prepared and
standardized against potassium hydrogen phthalate. HCl solu-
tion (0.1 M) was prepared and standardized against standard
KOH. The ionic strength was maintained at 0.1 M by adding the
appropriate amount of 1 M KCl. Stock solutions of 0.01 M ligand
and 0.01 M iron(III) were also prepared in deoxygenated water.
(3700 to 3584) cm .
ꢀ1 11 In the present case, the ligand showed a
broad νO—H stretching vibration at 3374 cmꢀ1 indicating the
presence of intramolecular hydrogen bonding. The deformation
δO—H peak was observed at 1356 cmꢀ1. The band observed at
1240 cmꢀ1 was attributed to νC—O. Aromatic carbonꢀcarbon
stretching vibrations were observed at (1574 and 1504) cmꢀ1
,
whereas two bands at (838 and 764) cmꢀ1 were assigned to the
ring substituted bending vibrations.
The experimental 1H NMR spectrum of the unreduced form
of CYCOENCAT displayed a resonance signal at 8.30 ppm due
to imine hydrogens and three distinct peaks at (1.58, 1.83, and
2.21) ppm for three different types of protons present in the
cyclohexane ring. Triplets at (3.42 and 3.69) ppm were char-
acterized for the methylene groups attached to the imine and
amide functional groups, respectively. Other peaks at (6.62,
6.76, and 6.86) ppm were characterized for the aromatic
protons. Upon reduction, the characteristic imine band at 8.30
ppm disappeared, and the 1H NMR spectrum of CYCOENCAT
(Figure 1) showed the presence of benzylic signals at 4.15 ppm.
The final concentrations of ligand (1 10ꢀ3 M) and metal
3
(1 10ꢀ3 M or 5 10ꢀ4 M) were maintained for the different
3
3
potentiometric titrations. The following titrations with metal-to-
ligand molar ratios of CFe/CL = 0:1, 1:1, 1:2 were carried out, and
the potentiometric data were refined using the computer program
Hyperquad 20009 to get the protonation constants of the ligand
and the formation constants of the metal complexes. Literature
data (log β) on the iron(III) hydroxo complexes were included in
the refinement process to obtain the formation constants of the
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Scheme 2. Synthesis of CYCOENCAT. Reagents and Conditions: (a) i. EtOH, Dry HCl; ii. PtO2/H2, Room Temp., 24 h;
(b) Ethylenediamine (en); (c) 2,3-Dihydroxybenzaldehyde, EtOH; (d) NaBH4, Conc. HCl, MeOH
Figure 1. 1H NMR spectrum of CYCOENCAT (D2O, 300 MHz).
The spectrum showed three distinct peaks for the cyclohexane
ring protons. A quartet and doublet were obtained at 1.35 ppm and
1.83 ppm for the axial and equatorial protons of methylene
groups, respectively, whereas a triplet at 2.38 ppm was observed
for the three axial protons attached with the appended groups in
the cyclohexane ring, which confirmed a cis,cis-equatorial config-
uration for the ligand. Other peaks at (3.12 and 3.46) ppm were
characterized for the methylene groups attached to the amine
and amide functional groups, respectively. Peaks due to aromatic
protons showed multiplets between (6.70 to 6.85) ppm and a
doublet at 6.87 ppm.
3.2. Ligand Protonation Constants. Potentiometric and
spectrophotometric titrations of the ligand CYCOENCAT were
carried out in an aqueous medium to determine the protonation
constants. In the potentiometric method, the ligand was first
acidified with 10-fold excess of a measured amount of standard
HCl and then titrated against standard KOH at an ionic strength
of μ = 0.1 M KCl and (25 ( 1) °C. Equilibrium points (pH =
ꢀlog10[Hþ]) were collected by adding base into the acidified
solution of ligand. The titration curve of the ligand is shown in
Figure 2. The analysis of the potentiometric titration curve using
the program Hyperquad 2000 gave the best fit for six protonation
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of data in the least-squares fitting program pHAb13 resulted in six
protonation constants (Table 1), which accords well with the
potentiometric results. The predicted electronic spectra of the
species whose protonation constants have been determined are
shown in Figure 4b. According to these results, the six proton-
ation constants calculated for CYCOENCAT can now be assigned
to the three amine groups and three most acidic hydroxyl groups
of the catechol units.
3.3. MetalꢀLigand Complex Formation. Potentiometric
titrations of the ligand CYCOENCAT (L) were carried out in
the presence of Fe(III) at 1:1 and 1:2 metal-to-ligand molar
ratios to study the formation of metal complexes. The titrations
were carried out in an aqueous medium at an ionic strength of
μ = 0.1 M KCl and (25 ( 1) °C. The potentiometric curve for the
1:1 metalꢀligand molar ratio is shown in Figure 2. The solid symbols
represent equilibrium points collected when no solid phase was
present in solution, while dotted lines represent points collected
when turbidity or precipitation appeared in the solution. The
deviation in the metalꢀligand titration curves from the free
ligand curve implies the formation of metal complexes. As the pH
increases, precipitation/solid phase starts to appear from pH =
∼8.0. Considering these preliminary observations, the experi-
mental data represented in symbols were only tested in the
minimization program with many sets of possible models. The
best-fit models were obtained, when the formation of the species
FeLH3, FeLH2, FeLH, and FeL were considered. No additional
species were detected from the data obtained between the 1:1
and the 1:2 metalꢀligand molar ratios. The overall formation
constants (log β) of the species were calculated using the
Hyperquad 2000 program and are summarized in Table 2. The
equilibrium reactions for the overall formation of the complexes
are given below by the eqs 2 to 5 (charges are omitted for simplicity):
Figure 2. Potentiometric titration curves of CYCOENCAT (L) in the
absence and presence of Fe(III) at a 1:1 ligandꢀmetal molar ratio.
constants (Table 1) for the ligand as proposed by the eq 1 (charges
are omitted for simplicity):
½LHnꢁ
LHn ꢀ 1 þ H h LHn
Kn ¼
ð1Þ
½LHn ꢀ 1ꢁ½Hꢁ
The ligand CYCOENCAT contained nine deprotonation sites:
six catechols and three sec-amines. However, due to the very low
acidity of the catechol (pKa1 = 9.13 and pKa2 = 13.00)12 subunits,
only one proton could be deprotonated. Therefore, the neutral
CYCOENCAT was considered as triprotic acid and addressed as
LH3, whereas the fully protonated form as LH6þ3. The species
distribution diagram as a function of pH (Figure 3) indicates that
in acidic solution CYCOENCAT initially exists 100 % in the fully
protonated form as LH63þ below pH < 4.0. As the pH increases,
3
½FeLH3ꢁ½Hꢁ
Fe þ LH6 h FeLH3 þ 3H
FeLH3 h FeLH2 þ H
FeLH2 h FeLH þ H
FeLH h FeL þ H
β113
¼
ð2Þ
ð3Þ
ð4Þ
ð5Þ
½Feꢁ½LH6ꢁ
½FeLH2ꢁ½Hꢁ
½FeLH3ꢁ
2þ
β112
¼
deprotonation starts with the formation of LH5 which exists
between pH 5ꢀ8 with a maximum concentration (70 %) at pH ∼7.
Species LH4þ, LH3, and LH2ꢀ are formed successively with an
increase in pH and show a maximum concentration of 55 %,
70 %, and 55 % at pH∼ 8, ∼9, and ∼10, respectively. At pH ∼11,
the species LH2- shows the maximum concentration of 50 %, and
thereafter the deprotonated ligand L3ꢀ starts to predominate.
Spectrophotometrically, the electronic spectra [(225 to 340) nm]
of the ligand were recorded in the pH range 3.2ꢀ10.2 (Figure 4a).
No appreciable change in the electronic spectra was observed
below the experimental pH of 3.2, and the spectra overlapped
each other. With an increase in pH, the spectra showed a batho-
chromic shift in the absorption band for the πfπ* transition
from (281 to 288) nm with a concomitant increase in absor-
bance. The shift continued until pH 8.12, and within this range,
two isosbestic points at (266 and 286) nm were observed. The
formation of isosbestic points indicate the state of equilibrium
between the deprotonated and the protonated ligand and can be
attributed to the formation of catecholate ions. Since the catechol
group is the only significant chromophore in CYCOENCAT, the
red shift in the electronic spectra is attributable to deprotonation
of the hydroxy group. As the pH rises further, a hyperchromic
shift was prominent, and no more isosbestic points were observed.
This observation can be characterized for the deprotonation
process from ꢀNH2þ groups. The inclusion of the whole range
½FeLHꢁ½Hꢁ
β111
¼
½FeLH2ꢁ
½FeLꢁ½Hꢁ
½FeLHꢁ
β110
¼
The interaction of Fe(III) with CYCOENCAT was also studied
by a spectrophotometric method to explain the different possible
coordination modes of the species formed in solution. The
spectrometric titration was carried out with a 1:1 metal-to-ligand
ratio with the ligand and metal ion concentrations equal to
4.02 10ꢀ5 M and varying the pH between 2.5 and 8.0. The
3
experimental electronic spectra of L in the presence of Fe(III) is
shown in Figure 5. Major spectral changes were observed after
pH ∼3.2 with the appearance of a charge-transfer band at 575 nm
due to the formation of ferric complexes. A study of species
distribution curves (Figure 6) for the LꢀFe(III) system indicates
that below pH 3, the free ligand, and the species FeLH3 are
present in appreciable amounts. Hence, the peak at 575 nm is
due to the formation of FeLH3. On addition of the Fe(III) ion,
3þ
three protons from LH6 are released upon complexation to
give the species FeLH3. These three deprotonation/coordination
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Table 1. Protonation Constants (log K) of CYCOENCAT (L), TAMCHCAT,8 and TRENCAT21 Obtained at (25 ( 1) °C
and μ = 0.1 M (A = Potentiometry and B = Spectrophotometry)
Figure 3. pH-dependent species distribution curves for CYCOENCAT
(L) at [L]total = 5 10ꢀ4 M.
3
processes should have resulted from the three most acidic ortho-
hydroxy groups of the catechol to form a capped-type geometry.
Thus, the peak at 575 nm can now be assigned to be a charge-transfer
transition from the ligand to metal (catecholfFeIII). Similar
observations have been made for other tripodal catecholate
ligands,14 in which on coordination of the three acidic hydroxyl
groups with Fe(III) in acetonitrile, a peak at 528 nm results due
to ligand-to-metal charge transfer (LMCT).
At higher pH, the other species FeLH2, FeLH, and FeL are
formed in steps with the release of three protons from FeLH3
(Figure 6). The release of these protons from FeLH3 can either
take place from the NH2þ groups for which the protonation constant
values have been evaluated, or from the remaining ꢀOH groups
of catechols, for which the protonation constant values are unknown.
Figure 4. (a) Experimental electronic spectral data of CYCOENCAT (L)
These two probabilities lead to two distinct geometries: a nitrogenꢀ
oxygenencapsulated or oxygenꢀoxygen encapsulated. In the first
case, if the metal ion enters into the nitrogenꢀoxygen cavity with
the release of one/two protons from NH2þ upon coordination, it
may encounter the repulsive force from the two or one protonated
NH2þ groups, whereas in the second case, the ligand coordinates
through its more basic catecholic oxygens as an anionic donor to
as a function of pH (3.2ꢀ10.2) during a spectrophotometric titration: [L] =
0.041 mM; [KCl] = 0.1 M, and T = (25 ( 1) °C. (b) Electronic spectra of
the seven species as a function of molar absorptivity and wavelength
predicted from pHAb using experimental data for CYCOENCAT.
form a tris(catechol) type complex. The latter is more preferable
for these hard metal ions, which can be assigned from the shifting
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Table 2. Overall (log β) Formation Constants of the Metal
Complexes Formed by Ligand CYCOENCAT (L) at (25 ( 1)
°C and μ = 0.1 M KCl
equilibrium
log β
[FeLH3]/[Fe][L][H]3
[FeLH2]/[Fe][L][H]2
[FeLH]/[Fe][L][H]
[FeL]/[Fe][L]
49.82 ( 0.03
46.55 ( 0.04
42.21 ( 0.07
34.61 ( 0.02
Figure 7. Plot of pFe (ꢀlog [Fe3þ]) vs pH. pFe was calculated for [L] =
5 10ꢀ4 M, [Fe3þ] = 5 10ꢀ5 M using the deprotonation constants of
3
3
ligand L, and the complexation constants β11n
.
ligands derived from TREN possessing six oxygen functions
show that in their cationic or neutral complexes, nitrogen atoms
(imine or amide functions) are not involved in coordination.16,17
On the basis of above facts, it is suggested that the ferric ion in the
tricapped geometry of FeLH3 is not efficiently protected by the
wrapped pod and remains accessible for further complexation to
give the encapsulated tris(catecholate) complex.
Figure 5. Experimental electronic spectra at 1:1 metal-to-ligand ratio
for Fe(III)-CYCOENCAT system.
The ligand is a weak acid, and proton competition occurs depend-
ing on their pKa and the pH; therefore, the pFe (= ꢀlog[Fe3þ])
value was calculated under given conditions of pH and Mnþ and L
concentration to quantify the complexation efficiency of CYCO-
ENCAT. The calculated pFe value of CYCOENCAT at pH = 7.4
by taking [L]total = 5 10ꢀ4 M and [Fe3þ total = 5 10ꢀ5 M is pFe =
]
3
3
24.76, which infers its ability to compete with transferrin (pFe =
20.3).18 It is well-known that most of the iron circulating in blood
plasma is bound to transferrin. Because of the labile nature of the iron
transferrin complex, iron overload is cured by sequestering agents like
desferrioxamine B (DFO), deferiprone, and deferasirox.19 Concern-
ing the high iron(III) chelating ability of CYCOENCAT, aqueous
solubility over a wide range of pH and no hydroxo species with
Fe(III) make the ligand useful for iron(III) sequestration. More-
over, Santos and co-workers20 suggested that sequestering ligands
with free amines as in DFO may improve their usefulness as a
drug, where the amine groups are used as a point of attachment to
a polymeric solid matrix, which enhance the properties of
removing ‘‘hard” ions from water solutions. Such free amines
are present in CYCOENCAT and also in their complexes
depicted in solution. Further, the pH dependence of the selec-
tivity was examined owing to the fact that, in some particular
biological compartments or circumstances, pH values exceeding
7 can be observed. The plot of pFe as a function of the pH
(Figure 7) reflects that the selectivity of the ligand for Fe(III) is
maintained over a pH range 4 to 8.
Figure 6. Speciation diagram calculated for the Fe(III)-CYCOENCAT
system at an equimolar ferric ion-ligand ratio ([Fe3þ] = [L] = 10ꢀ3 M).
of the peak at 575 nm (pH ∼3.2) toward ∼500 nm characterized
for the formation of the tris(catechol) type complex. Iron(III)-
catechol usually gives the LMCT at ∼490 nm,15 which also
supports the present proposition. This type of coordination (O,O) is
expected for iron(III), because the hard metal ion Fe(III) prefers
coordination of hard negatively charged oxygen as compared to
the nitrogen donors. X-ray crystal structure studies of tripodal
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: suban_sahoo@rediffmail.com and b.kanungo@vsnl.com.
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