PH-switchable porphyrin receptor for binding halide ions

Article abstract of DOI:10.1134/S1070363211060272

Main properties of 5,10,15,20-tetra(4-chlorophenyl)porphyrin in the acetonitrile-perchloric acid system were studied by the method of spectrophotometric titration at standard temperature. The protonation of nitrogen atoms of the tetrapyrrole macrocycle wa

Full text of DOI:10.1134/S1070363211060272

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 6, pp. 1231–1238. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © Yu.B. Ivanova, O.M. Kulikova, N.Zh. Mamardashvili, 2011, published in Zhurnal Obshchei Khimii, 2011, Vol. 81, No. 6, pp. 1038–
1045.
pH-Switchable Porphyrin Receptor for Binding Halide Ions
Yu. B. Ivanova, O. M. Kulikova, and N. Zh. Mamardashvili
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: ybi@isc-ras.ru
Received May 25, 2010
Abstract—Main properties of 5,10,15,20-tetra(4-chlorophenyl)porphyrin in the acetonitrile–perchloric acid
system were studied by the method of spectrophotometric titration at standard temperature. The protonation of
nitrogen atoms of the tetrapyrrole macrocycle was shown to occur in two steps, through the formation of mono-
and diprotonated forms. The corresponding ionization constants and concentration intervals were determined.
The diprotonated form of porphyrin was shown to bind effectively iodide, bromide and chloride ions, the
stability constants of the complexes of 1:1 and 1:2 composition were determined.
DOI: 10.1134/S1070363211060272
The building up of effective receptors for detection
of chemical and biological compounds is a topical
problem of the modern science [1]. The protonated
forms of tetrapyrrole macrocycles, being excellent
chromophores, bind selectively anions by their
conformationally labile complexing cavity [2–5]. Pro-
nounced electron donor or electron acceptor properties
of the created receptors are defined largely by the
acidity function of the medium. The use of relatively
simple methods of spectral analysis to determine
anions in solution makes these studies extremely
popular. Due to the increasing interest in the
development of selective tetrapyrrole receptors for
anions, in this study we carried out a direct titration of
the 5,10,15,20-tetra(4-chlorophenyl)porphyrin (H2P, I)
diprotonated (H4P2+) forms with solutions of (C4H9)4NI,
(C4H9)4NBr, and (C2H5)4NCl. We found that the H4P2+
effectively binds halide anions in acetonitrile at 298 K.
The stability constants of the formed complexes of 1:1
and 1:2 composition and the concentration ranges of
their existence were determined.
perchloric acid is fully dissociated and the protonation
occurs with a solvated proton [7].
Cl
NH
N
N
Cl
Cl
HN
Cl
5,10,15,20-Tetra(4-chlorophenyl)porphyrin (I).
kb1
H2P + H+
H3P+,
(1)
kb2
The porphyrin (H2P) molecules in acid acetonitrile
media [2,6] are protonated at the tertiary nitrogen
atoms with the formation of mono- (H3P+) and
dications (H4P2+) [Eqs. (1), (2)]. We studied by the
spectrophotometric titration method the basic
properties of compound I. The titration was carried out
in acetonitrile medium with perchloric acid of the
concentration of 0.01 M. The perchloric acid solution
was prepared in acetonitrile. In these circumstances
H3P+ + H+
H4P2+,
(2)
(3)
H2P–CH3СN–HClO4.
Upon the titration of the tetrapyrrole macrocycle
with perchloric acid (Fig. 1) the electron absorption
spectra (EAS) indicate successive formation of the
1231

1232
IVANOVA et al.
A
2
0
300
400
500
600
700
λ, nm
Fig. 1. Spectrophotometric titration of I (Cporph. = 3.5×10–5 M) in the system of acetonitrile–perchloric acid in the region of HClO4
concentrations from 0 to 1×10–4 M.
H3P+ and H4P2+ forms. At the increase in the con-
centration of perchloric acid the peaks of the five-band
absorption spectrum of the molecular form of I suffer a
red shift with the gradual transformation of the
spectrum first into a four-band (H3P+), and then in two-
band (H4P2+) spectrum (Fig. 1). The parameters of
EAS of compound I in the system (3) for various
forms of the ligand are given in Table 1.
The corresponding titration curve (Fig. 2) includes
two steps, to each one corresponds its family of spec-
tral curves in the EAS. Determining the coordinates of
the inflection point in the titration curve as described
in [6], we separated the protonation processes in the
system (3) into the steps, and calculated the correspond-
ing constants for the processes (1), (2) according to
formula (4).
Table 1. Parameters of electron absorption spectra of some forms of compound I (Cporph ~2×10–5 M) and their complexes
with halide ions in acetonitrile in the presence of HClO4 (0 to 10–3 M)
Form
λ1, nm (log ε)
413 (5.26)
414 (5.24)
439 (5.24)
442 (4.20)
444 (4.11)
445 (5.14)
451 (5.09)
443 (5.18)
448 (5.16)
λ2, nm (log ε)
λ3, nm (log ε)
λ4, nm (log ε)
λ5, nm (log ε)
644 (4.21)
668 (4.20)
660 (4.41)
671 (3.57)
680 (3.55)
660 (4.39)
671 (4.37)
666 (4.38)
668 (4.36)
Н2Р
512 (4.41)
546 (4.31)
587 (4.27)
Н3P+
446 (5.03)
512 (4.31)
–
–
–
–
–
–
–
–
Н4P2+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
H4PI+
H4PI2
H4PBr+
H4PBr2
H4PCl+
H4PCl2
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pH-SWITCHABLE PORPHYRIN RECEPTOR FOR BINDING HALIDE IONS
(a)
1233
A
A
2
(b)
A
400
λ, nm
400
500
λ, nm
0
6
C(HClO4)×105, M
Fig. 2. Spectrophotometric titration of I in acetonitrile by perchloric acid at the analytical wavelength λ = 445 nm; Cporph = 3.5×
10–5 M, the region of HClO4 concentrations from 0 to 1×10–4 M: (a) the step of the formation of H3P+, (b) 2nd step, the formation
of H4P2+.
log kb = log (Ind) – nlog cac,
(4)
The results of spectrophotometric titration showed
that system (3) passes three areas of concentration of
perchloric acid, where I exists as H2P (≤3×10–7 M),
H3P+ (4.1×10–5 to 4.4×10–5 M) and H4P2+ (≥7.5×
10–5 M). The data obtained allowed us the performance
of a direct titration of the dication I solutions with
(C4H9)4NI, (C4H9)4NBr and (C2H5)4NCl in the systems
(6)–(8) at 298 K for the investigation of a possibility to
use the protonated porphyrins as receptors for halide
ions X–.
where kb is the basicity constant for the respective
first or second step, Ind is the indicator ratio H3P+/H2P
for the first step and H4P2+/H3P+ for the second step;
cac is the analytical value of the perchloric acid con-
centration in the solution, M, n = 1 (the number of added
protons formed at the dissociation of perchloric acid).
At each point the values of the reagent analytical
concentrations was corrected for dilution of the
working solution according to Eq. (5).
Н2Р–HClO4–(С4Н9)4NI–CH3CN,
Н2Р–HClO4–(С4Н9)4NBr–CH3CN,
Н2Р–HClO4–(С2Н5)4NCl–CH3CN.
(6)
(7)
(8)
Ат = А0т/(1 +ΣΔV/V0),
(5)
where V0 is the initial volume of the solution, ΣΔV is
the total value of the added titrant, and AT is the
current value of the optical density at the analytical
wavelength with accounting for the added titrant, A0T is
the current value of the optical density at the analytical
wavelength.
The results of titration of H4P2+ with the iodide ion
[system (6)] are shown as the ESP pattern (Fig. 3) and
the titration curve (Fig. 4). Upon increase in the iodide
ion [I–] concentration, in the EAS of the H4P2+ solution
a sequential formation occurs of two series of spectral
curves (each series has its isobestic point), which
probably belong to the equilibria (9), (10).
The values of the protonation constants of the first
and second stages are 1.6×105 and 2.1×104 M–1,
respectively. The error in the determination of the
constants was calculated as the arithmetic mean value
of the constants obtained in three parallel experiments,
and amounted to 5–7%.
k1
H4P2+ + I–
(H4P+)I,
(9)
k2
(H4P+)I + I–
H4PI2.
(10)
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IVANOVA et al.
A
1
500
600
700
λ, nm
Fig. 3. Changes in the electron absorption spectra of the system H4P2+ (C = 5.5×10–5 M) – (C4H9)4NI (C = 0–1.2×10–3 M) – HClO4
(C = 9.9×10–5 M) – CH3CN.
(b)
A
A
1.5
1.2
1.0
1.4
1.3
1.2
1.1
1.0
0.9
0.8
0.6
(a)
0.4
A
600
700
λ, nm
1
500
600
700
λ, nm
0
5
10
C(Bu4NI)×104, M
Fig. 4. Titration curve of the I dication in the system (6) at the analytical wavelength λ = 662 nm: (a) step 1, formation of H4PI+;
(b) step 2, formation of H4PI2.
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pH-SWITCHABLE PORPHYRIN RECEPTOR FOR BINDING HALIDE IONS
1235
A possibility of the existence of the porphyrin
associates of this kind was established [8] by X-ray
analysis.
pX
7.0
6.5
In the electron absorption spectrum the maximal
red shift (~20 nm) of the absorption band with the
maximum at the analytical wavelength λ = 662 nm is
observed (Fig. 3)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1
It has been published that alkilammonum halide
salts dissociate in the acid–acetonitrile solutions and as
by-product of associates form the associates of the type
HnXm [4, 9–11], where X is halide. Effect of the HnXm
associates in the process of binding the halide anions
by tetrapyrrole macrocycle halide cations to form AH
in the range pH = 3–18 was studied potentiometrically
using a glass electrode, reference silver chloride
electrode, and the ion-selective electrodes for halide
ions (Cl–, Br–, I–) in a standard spectropotentiometric
cell [4, 10, 11].
2
3
The straight-line relationship pI = f(pH) (Fig. 5)
shows that at the concentration of perchloric acid
7.5×10–5 M (experimental conditions) among the
byproducts in the system (6) there are no associates of
the HnXm type with iodine. Taking this into account we
calculated the formation constants of H4PI+ and H4PI2
for the equilibria (9), (10) by Eq. (11).
3
6
9
12
15
18 pH
Fig. 5. Dependences of pX = f(pH) in acetonitrile at 298 K,
X = (1) Cl, (2) I, and (3) Br[16].
log k = log (Ind) – nlog cх,
(11)
where k is the association constant of the first or
second steps (7), (8), respectively; Ind is Indicator ratio
(H4P+X)/H4P2+ for the first stage, and (H4PH2)/(H4P+X)
for the second; cx is the analytical concentration of the
halide salt in solution in M, n = 1 (the number of the
added halide ions). The error in the determination of
the constants has been calculated as the arithmetic
mean value of the constants obtained in three parallel
experiments, it amounted to 5–7%. The values of the
constants are listed in Table 2.
The results of titration of H4P2+ with the bromide
ions [system (7)] are seen from the EAS and the
titration curve (Fig. 6). Upon increase in the con-
centration of bromide [Br–], in the EAS of the H4P2+
solutions occurs a sequential formation of two series of
spectral curves. Each series has its own isosbestic
point [the processes (12) and (13)].
During the entire titration a gradual red shift (11 nm)
of the absorption bands was observed. The intensity of
the bands decreased, and the extinction coefficients
(Table 1) gradually diminished. After determining the
coordinates of the inflection points on the titration
curve the association processes in the system (7) were
separated into steps and by formula (11) the
corresponding constants for the processes (12), (13)
were calculated. At the concentration of perchloric
acid 7.5×10–5 M (experimental conditions) among the
byproducts of the system (7) the hydrobromic as-
Table 2. Association constants of the complexes (H4P+)X–
and H4RX2 in CH3CN
[X–]
[I–]
k1
k2
2.45×104
9.55×104
1.38×106
1.29×103
6.92×103
1.99×105
k1
H4P2+ + Br–
(H4P+)Br,
H4PBr2.
(12)
(13)
[Br–]
[Cl–]
(H4P+)Br + Br–
k2
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IVANOVA et al.
A
A
0.9
A
0.8
0.7
(a)
400
450
(b)
λ, nm
0.6
400
450
λ, nm
0
3
C(Bu4NBr)×104, M
Fig. 6. Changes in the electron absorption spectra of the system (7): H4P2+ (5.6×10–6 M)–(C4H9)4NBr (0–3.5×10–4 M)–HClO4
(1.1×10–5 M) – CH3CN and the titration curve of the compound I dication in the system (7) at the analytical wavelength λ = 440 nm:
(a) first step, formation of H4PBr+, (b) second step, formation of H4PBr2.
sociates were not detected (Fig. 5). The numerical
values of the constants of formation of (H4P+)Br and
H4PBr2 are listed in Table 2.
The results of titration of H4P2+ with chloride ions
[system (8)] are seen from the data of the EAS and the
titration curve (Fig. 7). Upon increase in the con-
centration of chloride ions [Cl–], in the EAS of the
H4P2+ solutions also the consecutive formation of two
series of spectral curves was observed, each series had
its isobestic point [processes (14) and (15)].
were calculated according to Eq. (9) taking into
account the formation of associates of the form HnClm.
At the perchloric acid concentration 7.5×10–5 M (ex-
perimental conditions) among the byproducts of the
system (8) were found the associates like HCl, H2Cl+,
HCl2–, HCl32– [processes (16)–(19)] with the following
values of the association constants: K(H2Cl+) (pK =
4.76), KHCl (pK = 8.39), K(HCl2–) (pK = 2.65), and
K(HCl32–) (pK = 3.61).
Н+ + НСl → H2Cl+, K(H2Cl+),
Н+ + Сl– → HCl, KHCl
(16)
(17)
(18)
(19)
,
k1
H4P2+ + Cl–
(H4P+)Cl,
(14)
НСl + Cl– → HCl2–, K(HCl2–),
НСl2– + Cl– → HCl32–, K(HCl32–).
k2
(H4P+)Cl + Cl–
H4PCl2.
(15)
In the EAS throughout the titration a gradual red
shift of the absorption bands (8–9 nm) was observed.
The intensity of the bands decreased and, con-
sequently, the extinction coefficient gradually di-
minished (Table 1). It should be noted that the red shift
was the lowest compared with the values of a similar
shift in the spectra of iodide and bromide associates
(Table 1).
From the Eqs. (16)–(19) and the material balance
Eq. (20), we can construct Eq. (21), which connects all
the current chlorine concentration.
С0 =Cl + НСl + H2Cl+ +2 HCl2– + 3HCl32–,
(20)
C0 = {1+ [а]KHCl + [а]2KHClK(H2Cl+)}[Cl–]
+ 2 [а]KHClK(HCl2–)[Cl–]2
+ 3 [а]KHClK(HCl2–)K(HCl32–) [Cl–]3
(21)
,
For system (8) the formation constants (Table 2) of
where C0 is the analytical concentration of Et4NCl at
H4PCl+ (k1) and H4PCl2 (k2) [equilibria (14) and (15)]
each point, [Cl–] is the current concentration of
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pH-SWITCHABLE PORPHYRIN RECEPTOR FOR BINDING HALIDE IONS
1237
A
(a)
A
1
λ, nm
1
(b)
A
1
400
0
λ, nm
5
10 C(C2H5)4NCl×104, M
Fig. 7. Changes in the electron absorption spectra at the analytical wavelength λ = 440nm for the system (8): H4P2+ (6×10–6 M)–
(C2H5)4NCl (0–8×10–4 M)–HClO4 (1.1×10–5 M)–CH3CN and the titration curve of the compound I dication in the system (8) on the
analytical wavelength λ = 440 nm: (a) the first step, formation of H4PCl+, (b) second step, formation of H4PCl2.
chlorine, [a] is the active concentration of hydrogen
ions measured by a glass electrode. Substituting in this
equation the values of analytical concentration of
Et4NCl at each point and the value of active
concentration of hydrogen ion, we obtain a system of
cubic equations like (22).
complicated compared with the bromide and chloride
associates. One can assume that the iodide anion due to
its high volume and low ionic potential cannot
approach the coordination center of the dication to a
distance sufficient for the formation of hydrogen bonds
with hydrogen atoms of the NH groups, whereas for
the anions of bromine and chlorine with lower values
of ionic radii these processes are more probable. In all
three systems the spectral responses of the system to
the change in its state can easily be recorded and
indentified. The sensitivity of the proposed
macroheterocycle (the band shift and change in
intensity in the visible region of the absorption spectra)
covers a wide concentration range of the measured
analyte concentrations (from 10–6 to 10–3 M), which
allows to use the porphyrin chromophores as the basic
compounds for the creation of pH-switchable
molecular sensors capable of forming an analytical
signal due to chemical interaction with the receptor
molecules of the substrate.
C0 = p[Cl–] + [Cl–]2[Cl–]3.
(22)
This system can be simply enough solved to obtain
the true values of the [Cl–] concentration in the
solution.
Table 1 contains the parameters of the EAS in a
series of I––Br––Cl– for the transitions of (H4P+)X to
H4PH2 and H4P2+ to H4PH2. In the first case, the band
shift values were 7, 5 and 3 nm, in the latter case 20,
11 and 8 nm, respectively. The data in Table 2 show
that the association constant of diprotonated forms of
compound I are maximal in the case of chloride
associates and is minimal for iodide associates. The
formation of the iodide associates is much more
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IVANOVA et al.
EXPERIMENTAL
“Research and scientific-pedagogical personnel
of innovation Russia in 2009-2013,” GK no.
02.740.11.0106 and GK no. P1105.
Acetonitrile from Aldrich was not additionally
purified. The water content was determined by Fischer
method, and it did not exceed 0.04%. Solutions of
(C4H9)4NI and perchloric acid were prepared in
acetonitrile. The choice of tetraalkylammonium salts
was determined by the value of the association
constants in acetonitrile [12–15]. All salts were
purified in a Soxhlet apparatus with acetonitrile and
dried in a vacuum (1.33 Pa) at 393K to constant
weight. The method of spectro-photometric titration in
acetonitrile was described in detail in [2, 6].
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ACKNOWLEDGMENTS
This work was supported by Russian Foundation
for Basic Research (grants nos. 09-03-00040-a, 09-03-
97500-r_center_a) and the Federal Target Program
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 6 2011