Spectrophotometric study of acid-base and complexing properties of 5,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin in acetonitrile

Article abstract of DOI:10.1134/S1070363214060255

Acid-base and complexing properties of 5,10,15-trinitro-2,3,7,8,12,13,17, 18-octaethylporphyrin have been studied by means of spectrophotometry in the acetonitrile/1,8-diazabicyclo[5.4.0]undec-7-ene, acetonitrile/perchloric acid, and acetonitrile/zinc ace

Full text of DOI:10.1134/S1070363214060255

ISSN 1070-3632, Russian Journal of General Chemistry, 2014, Vol. 84, No. 6, pp. 1207–1211. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © Yu.B. Ivanova, Dao Tkhe Nam, N. V. Chizhova, N.Zh. Mamardashvili, 2014, published in Zhurnal Obshchei Khimii, 2014, Vol. 84,
No. 6, pp. 1030–1034.
Spectrophotometric Study of Acid-Base
and Complexing Properties
of 5,10,15-Trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin
in Acetonitrile
a
b
a
a
Yu. B. Ivanova , Dao Tkhe Nam , N. V. Chizhova , and N. Zh. Mamardashvili
a
Krestov Institute of Solutions Chemistry, Russian Academy of Sciences,
ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: jjiv@yandex.ru
b
Ivanovo State University of Chemistry and Technology, Ivanovo, Russia
Received September 16, 2013
Abstract—Acid-base and complexing properties of 5,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin
have been studied by means of spectrophotometry in the acetonitrile/1,8-diazabicyclo[5.4.0]undec-7-ene,
acetonitrile/perchloric acid, and acetonitrile/zinc acetate media. The ionization constants and concentration
ranges of ionized forms existence have been determined, spectral parameters of the studied porphyrin and its
zinc complex have been elucidated, and kinetic parameters of the zinc complex formation have been analyzed.
Keywords: porphyrin, metal complex, kinetics, deprotonation
DOI: 10.1134/S1070363214060255
Tetrapyrrole compounds are important components
of various biological systems providing for numerous
vital functions of living organisms [1–3]. Depending
on the external conditions such compounds can act as
acids or bases, or undergo redox transformations [3, 4].
Available reference data show that in living organisms
tetrapyrrole molecules act in the charged (anionic or
cationic) form [5, 6]. Such species cannot be easily
isolated from the reaction medium. Stable ionic forms
of tetrapyrrole molecules can be prepared in vitro via
electron detachment from neutral molecule or
introduction of excessive electron into the porphyrin
macrocycle. Study of the behavior of the so prepared
cations and anions allows elucidation of compounds
reactivity, intermediates structure, and mechanism of
various biochemical, photochemical, complex forma-
tion, and other reactions.
Taking into account the recently reported structural
data on porphyrins acting in enzyme systems, model
studies of the effect of porphyrin macrocycle
deformation on spectral, acid-base, and coordination
properties of the molecule are of topical importance. In
this work we used spectrophotometric titration [7, 8] to
investigate acid-base and complexing properties of
5
(
,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin
I) in the acetonitrile/1,8-diazabicyclo [5.4.0]undec-7-
ene (DBU) (A), acetonitrile/HClO4 (B), and
acetonitrile/Zn(OAc) (C) media (Scheme 1).
2
Deprotonation and protonation of transannular
nitrogen atoms of compound I are in general two-stage
processes, Eqs. (1)–(4).
k
a1
–
+
H
2
Р
⇄
HР + H ,
(1)
(2)
(3)
(4)
k
a2
–
2–
+
HР
⇄
Р + H ,
The introduction of various substituents (electron
donors, electron acceptors, hydrophilic or lipophilic
groups) at the porphyrin ring is an efficient means to
alter spectral, coordination, catalytic, and other
properties of the tetrapyrrole molecules.
k
b1
+
+
H
2
Р + H
⇄
H
3
Р ,
k
b2
+
+
2+
H
3
Р + H
⇄
H
4
Р .
1
207

1
208
IVANOVA et al.
Scheme 1.
NO2
NO2
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
NO2
Et
Et
Et
Et
NH
N
N
NH
N
N
NH
N
N
NO2
NO2
NO2
Et
HN
HN
HN
Et
Et
Et
Et
NO2
Et
Et
Et
Et
A
B
C
–
Hereinafter, Н Р stands for porphyrin I; HР and
The combined constant of the first and the second
stages of acid ionization was calculated according to
Eq. (5).
2
2
–
+
Р
are deprotonated forms of compound I; H Р and
3
2
+
H Р are protonated forms of compound I.
4
We determined the concentration ranges of ionized
forms existence of compound I and the corresponding
ionization constants. With increasing concentration of
the titrant (DBU) in system A from 0 to 1.22 ×
log k
A
= log Ind + 2log cDBU
.
(5)
In Eq. (5), kA stands for the combined acid
ionization constant to be calculated; Ind is the indicator
ratio, [P ]/[H P]; c
of DBU in the solution, mol/L.
2–
is the analytical concentration
–4
2
DBU
1
0 mol/L the electron absorption spectrum of neutral
form of compound I [H P, λmax, nm (log ε): 382 (4.54),
2
With increasing concentration of the titrant
HClO ) in system B from 0 to 1.5 × 10 mol/L the
electron absorption spectrum of neutral form of
5
10 (3.74), 541 (3.63), 584 (3.53)] was smoothly
–
4
2
–
(
transformed into that of the dianion [P , λmax, nm
logε): 376 (4.31), 502 (4.07)] (Fig. 1a). The
4
(
compound I was smoothly transformed into that of the
spectrophotometric titration curve (λ 382 nm) was
step-like, and the presence of two isosbestic points
indicated the two-stage deprotonation, Eqs. (1) and (2).
2
+
dication [Н P , λmax, nm (log ε): 447 (4.64), 593
4
(
3.78), 648(3.69)] (Fig. 1b). The spectrophotometric
(
а)
(b)
А
А
А
А
5
с
DBU × 10 , mol/L
5
с(HClO
4
) × 10 , mol/L
λ, nm
λ, nm
Fig. 1. Evolution of electron absorption spectra and the corresponding titration curves in the course of titration of the porphyrin I
–5
–5
4 I
with DBU (a) and with HClO (b) in acetonitrile media at 298 K. cDBU = 0 to 8.22 × 10 mol/L and c = 4.02 × 10 mol/L (a);
–5
–
4
4 I
c(HClO ) = 0 to 1.5 × 10 mol/L and c = 5.30 × 10 mol/L (b).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 6 2014

SPECTROPHOTOMETRIC STUDY OF ACID-BASE AND COMPLEXING PROPERTIES
1209
titration curve (λ 382 nm) was step-like, and the
presence of two isosbestic points showed the two-
staged protonation, Eqs. (3) and (4). The combined
constant of the first and the second stages of base
ionization was calculated according to Eq. (6).
all cases clear isosbestic points were observed in the
spectral series. Parameters of electron absorption
spectra of the studied zinc complex coincided with the
reference data [10, 11].
The formation of zinc complex of porphyrin I can
be expressed by Eq. (7).
log k
B
= log Ind – 2log c(HClO
4
).
(6)
In Eq. (6), kB stands for the combined base
ionization constant to be calculated; Ind is the indicator
Н P + [ZnХ (Solv)n–2] → ZnР + 2НХ + (n – 2)Solv, (7)
2
2
where X, acidic ligand; Solv, solvent molecule; n, zinc
ion coordination number.
2
+
ratio, [H P ]/[H P]; c(HClO ) is the analytical
4
2
4
concentration of HClO in the solution, mol/L.
4
The reaction kinetic parameters were calculated
following the well known method, expressed by
Eqs. (8)–(11). First, the effective reaction rate constant
keff was determined following Eq. (8):
0
The calculated values of the combined acidity and
basicity constants of compound I at 298 K are col-
lected in the table below along with the corresponding
data on the previously studied 5,10,15,20-tetranitro-
keff = (1/t)ln [с (H
2
P)/c(H
2
P)]
)],
2
1
,3,7,8,12,13,17,18-octaethylporphyrin II and 2,3,7,8,-
2,13,17,18-octaethylporphyrin III [9] (the constants
=
(1/t)ln [(A – A )/(A – A
0
∞
∞
(8)
determination error was ±3–5%).
with t, time passed from the reaction start; A , A , and
0
t
A∞ absorption of the reaction mixture: initial, at time t,
and final, correspondingly. The true reaction rate on
the (n + 1) order kn+1 was calculated using Eq. (9).
n
log k
I
II
III
–
log k
А
–10.90
10.23
–10.44
9.56
kn+1 = keff/сsalt
,
(9)
log k
B
11.85
Comparison of the determined acidity and basicity
constants of compounds I–III showed that in
acetonitrile medium the acid-base properties of the
studied porphyrins were dependent on the number of
nitro groups in the molecule, due to combination of
steric and electronic effects of the substituents [9]. In
particular, increasing the nitro groups number in the
porphyrin led to the growth of the combined proto-
nation constant in the III > I > II series and to
decrease of the combined deprotonation constant in the
same series (spectrophotometric titration of compound
III with the organic base did not detect the presence of
any anionic species).
where c , zinc acetate concentration.
salt
The activation energy was calculated from the
reaction rate constants determined at different
temperatures using integrated form of the Arrhenius
equation (10); the activation entropy was calculated
using Eq. (11) from the rate constant at 298 K.
А
The complexing properties of structurally different
β-substituted tetraphenylporphyrins in pyridine or
acetic acid medium are known to be dependent on the
number of bromine substituents: with more bromine
substituents the complex formation constant increases
[13]. This unusual effect of the substituents on the
properties on the tetraphenylporphyrin has been
explained by change of the macrocycle electronic
structure due to its spatial distortion and polarization of
the N–H bonds.
λ, nm
In order to elucidate the nitro substituents effect on
complexing properties of octaethylporphyrinspor-
phyrins, we investigated kinetics of formation of zinc
complex with I in system C at 298–318 K (Fig. 2). In
Fig. 2. Kinetic series of electron absorption spectra in the
course of coordination of the porphyrin I with zinc acetate
in acetonitrile medium at 298 K.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 84 No. 6 2014

1
210
IVANOVA et al.
0
i
ln (c /c )
log keff
t, min
2
log c[Zn(ОAc) ]
0 i
Fig. 3. Kinetics of ln (с /с ) change in the course of
formation of zinc complex with the porphyrin I in
acetonitrile medium: determination of the reaction order
with respect to porphyrin. (1) 298 K; (2) 308 K; (3) 318 K;
Fig. 4. Determination of the reaction rate order, with
respect to zinc acetate, of formation of zinc complex with
the porphyrin I, at 298 K in acetonitrile medium (slope
0.999, the correlation coefficient 0.9998).
–
5
–3
I 2
с = 2.32 × 10 mol/L; с[Zn(ОAc) ] = 2.35 × 10 mol/L.
E
a
= R[T
1
T
2
/(T
2
– T
1
)]ln (k
2
/k
1
),
(10)
(11)
site, and therefore accelerate the reaction (7). The
collected experimental data show that with more of
nitro groups in the porphyrin macrocycle, the reaction
≠
S = R[ln (k298/298) + E
a
/(298R) – ln (k /h)],
B
where k , Boltzmann constant; R, gas constant; h,
Planck’s constant.
B
(
7) was indeed accelerated. The energy of the zinc
complex formation was approximately equal in the
cases of all the studied porphyrins; however, the
activation entropy of the complex formation decreased
in the III < I < II series. Likely, the nitro-containing
derivatives of octaethylporphyrin are kind of
dissipative structures, leading to alternation of the
system equilibrium due to structural rearrangement of
porphyrin molecule. The spatially distorted meso-
substituted octaalkylporphyrins reveal stronger acid
properties as compared with the unsubstituted analog.
The deformation of the macrocycle resulting from
introduction of the nitro groups decreases likely the
molecule aromaticity and isolates π-electron systems
of the pyrrole and pyrrolenine fragments, thus
enhancing acidity of compounds I and II and
accelerating the complex formation reaction.
At each temperature, the measurements were run at
least in triplicate. The reaction rate order with respect
to porphyrin was determined by analysis of the
0
log [с (H
2
P)/c(H
2
P)] plots versus time (Fig. 3). The
reaction rate order with respect to zinc acetate was
determined from the slope of the log keff versus
log с[Zn(OAc) ] plot (Fig. 4). Kinetic parameters of
2
the formation of zinc complexes with porphyrins I–III
are given in the table below [9]. It could be anticipated
that introduction of strong electron-acceptor nitro
groups should weaken the N–H bond at the reactive
Kinetic parameters of coordination reaction (7) of
porphyrins I–III with zinc acetate in acetonitrile medium
EXPERIMENTAL
2
98
≠
Comp. [Zn(OAc)
2
],
k
,
∆Е,
∆S ,
–
1
–1
–1
no.
mol/L
L mol s
kJ/mol
J mol K
Electron absorption spectra were recorded using
Shimadzu UV1800, Hitachi U2000 Cary 100 Varian
spectrophotometers.
–
–
–
3
3
3
I
2.35 × 10
2.35 × 10
2.56 × 10
73±2
81±3
64±2
50±4
54±5
47±4
–3±1
12±1
II
5
,10,15-Trinitro-2,3,7,8,12,13,17,18-octaethylpor-
phyrin was prepared as described elsewhere [10, 11].
III
–15±1
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SPECTROPHOTOMETRIC STUDY OF ACID-BASE AND COMPLEXING PROPERTIES
1211
Details of spectrophotometric experiments and the data
processing are to be found in [7, 8].
(Chemistry of Coordinated Compounds), Moscow:
Integral-Press, 2008.
6
. Shul’ga, A.M., Sinyakov, G.N., and Gurinovich, G.P.,
Biofiz., 1978, vol. 23, no. 1, p. 5.
Zinc acetate (analytically pure grade) was purified
by recrystallization from aqueous acetic acid and
dehydrated at 380–390 K [12].
7
. Ivanova, Yu.B., Churakhina, Yu.I., and Mamardashvi-
li, N.Zh., Russ. J. Gen. Chem., 2008, vol. 78, no. 4,
p. 673. DOI: 10.1134/S1070363208040269.
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