Effect of ligand nonplanarity and solvent nature on the kinetic stability of zinc porphyrin complexes

Article abstract of DOI:10.1134/S1070363208050277

Planarity disturbance in metal porphyrin macrorings produces destabilization of complexes in dimethyl sulfoxide-acetic acid and benzene-acetic acid systems, except for cases where the coordination center is additionally stabilized by endocyclic substituents and/or extra ligands. The first dissociation stage of complexes with N-substituted analogs of porphyrins involves substitution of the acido ligand in the strongly shielded coordination sphere by an electron-donor solvent molecule. The revealed dependences of dissociation rate constants on acid concentration in DMSO, untypical of most metal porphyrins, are explained by a change in the type of the attacking species with changing solvent composition. The dissociation rate constants of complexes in an electron donor solvent can be lower by several orders of magnitude than in a weakly solvating medium.

Full text of DOI:10.1134/S1070363208050277

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 5, pp. 997–1004. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © D.B. Berezin, O.V. Shukhto, P.A. Shatunov, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 5, pp. 854–861.
Effect of Ligand Nonplanarity and Solvent Nature on the Kinetic
Stability of Zinc Porphyrin Complexes
D. B. Berezin, O. V. Shukhto, and P. A. Shatunov
Ivanovo State University of Chemical Technology,
pr. F. Engel’sa 7, Ivanovo, 153000 Russia
e-mail: berezin@isuct.ru
Received October 15, 2007
Abstarct— Planarity disturbance in metal porphyrin macrorings produces destabilization of complexes in
dimethyl sulfoxide–acetic acid and benzene–acetic acid systems, except for cases where the coordination center
is additionally stabilized by endocyclic substituents and/or extra ligands. The first dissociation stage of
complexes with N-substituted analogs of porphyrins involves substitution of the acido ligand in the strongly
shielded coordination sphere by an electron-donor solvent molecule. The revealed dependences of dissociation
rate constants on acid concentration in DMSO, untypical of most metal porphyrins, are explained by a change
in the type of the attacking species with changing solvent composition. The dissociation rate constants of
complexes in an electron donor solvent can be lower by several orders of magnitude than in a weakly solvating
medium.
DOI: 10.1134/S1070363208050277
The main research activities in porphyrin (Н2Р)
chemistry initially focused on preferentially planar
molecules, such as bioporphyrins (hemes, chloro-
philles, etc.) isolated from natural objects [1, 2]. Over
the recent years, in view of the growing interest in
biological processes involving conformationally
flexible in vivo porphyrins and their metal complexes
[3], molecules greatly differing from “classical” [4] in
geometry and π-electron characteristics have received
much attention.
characterized by X-ray diffraction deviations of β
(saddle сonformation) and meso (waved) atoms of the
mean plane of the porphyrin nucleus (ΔСβ and ΔСmeso
,
1
respectively; Table 1) [3]. As shown by Н NMR
spectroscopy, in going from solid phase to solution
nonplanar porphyrins scarcely change their structure
[3, 5, 7, 9]. Information on the molecular geometry of
nonplanar porphyrins can also be obtained from
quantum-chemical data [10].
The mechanisms of functioning sterically distorted
metal porphyrins in biological systems are impossible
to understand not knowing their state and stability in
solutions [11]. Therefore, in the present work we set
ourselves the goal to explore the kinetic stability of
zinc complexes with nonplanar porphyrins of various
structure: dodecasubstituted tetraphenyltetrabenzopor-
phine ZnTPhTBP (III), octabromotetraphenylporphine
Zn(β-Br)8TPhP (IV), dodecaphenylporphine Zn(β-Ph)8·
TPhP (V), and octaethyltetraphenylporphine Zn(β-Et)8·
TPhP (VI), as well as with N-methyloctaethylpor-phine
(AcO)Zn·(NMe)(β-Et)8P (VII) and N-methyltetra-
phenylporphine (AcO)Zn(NMe)TPhP (VIII) in media
on the basis of acetic acid.
There are several structural groups of synthetic
porphyrins with a defined nonplanar structure [3, 5].
These are largely сompounds with bulky substituents
at the nitrogen atom in the coordination center,
specifically N-substituted porphyrins [6], or com-
pounds in which most or all β- and meso-hydrogen
atoms in the Н2Р molecule are substituted by various
hydrocarbon residues or functional groups [5, 7].
Strong distortion takes place in complexes of
porphyrins with metal cations whose size is smaller or,
vice versa, larger than the diameter of the coordination
cavity of the ligand (4.02 0.02 Å) [7].
Nonplanar porphyrins and their complexes in
crystal and in solutions can adopt, depending on
macroring structure, several typical conformations
some of which, such as structures I–VIII, can be
The ligands in the studied porphyrin complexes
prefer the saddle conformation both in solid phase and
in solutions (Table 1, Fig. 1), and, therewith, in
997

998
BEREZIN et al.
R'
OAc
R
R'
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Zn
N
N
N
N
N
N
N
N
N
N
N
N
R
Zn
R
R'
R'
Zn
R'
R'
R'
Me
R
R
R'
I, III
II, IV−VI
VII, VIII
R = H (I); R = H, R' = Ph (II); R = Ph (III); R = Br, R' = Ph (IV); R = R' = Ph (V); R = Et, R' = Ph (VI); R = Et, R' = H
(VII); R = H, R' = Ph (VIII).
Table 1. Mean deviations of β- and meso-carbon atoms in Н2Р and metal porphyrins on the initial macroring plane by X-ray
diffraction data [3]
Ligand
Zinc complex II
Compound
Conformation
ΔСβ, Å
ΔСmeso, Å
ΔСβ, Å
ΔСmeso, Å
–
–
Planar
–
0.23
0.77
0.92
0.96
0.12
–
II
Saddle
–
III
IV
V
Saddle–waved
Saddle
1.26
1.28
1.17
~0.1а
~0.5а
0.32
0.07
0.25
0.01
Saddle
0.03
–
1.09
–
–
–
VI
VII
VIII
Planar
–
–
–
Saddle
a Quantum-chemical data [8].
compound IV there is an essential fraction of the
waved conformation [3]. Substitution at the endocyclic
NH group, along with tetrameso- or octa-β-substitution
(compounds VII and VIII) makes the porphyrin ligand
markedly nonplanar with the N-substituted pyrrole ring
slightly displaced from the macroring plane, but the
distorted saddle сonformation is generally preserved
[3, 5, 8]. The nonplanar saddle conformation of the
ligand is also stabilized by complex formation with
zinc ion [3].
The effect of nonplanarity of a porphyrin complex
on its ability for dissociation is controlled by various
factors [11]. Disturbed planarity of the aromatic
macroring attenuates the macrocyclic effect [5, 12],
thus isolating the pyrrole rings and increasing the
positive charge on the endocyclic nitrogen atoms [7]
and facilitating their interaction with solvated proton.
Electron-donor substituents in the β-, meso-, or N-
positions accelerates dissociation of the complex due
to transfer of additional electron density on macroring
nitrogens, but the macroring nonplanarity not
infrequently creates steric hindrances to interaction of
endocyclic porphyrin nitrogens with solvated proton.
A combination of these two factors, electronic and
steric [11], defines the rate of solvoprotolytic dissocia-
tion reactions of sterically distorted porphyrins.
0
+
+
+
−
+
−
−
−
−
−
−
−
+
0
NH
N
NH
N
−
+
0
0
0
0
N
0
HN
−
+
N
HN
−
+
−
+
+
+
+
0
Fig. 1. (Left) Waved and (right) saddle nonplanar con-
formations of porphyrin ligands (+ and – related to atoms
located above, below, and in the macroring plane,
respectively).
The coordination bonds in zinc porphyrin
complexes are largely formed due to the covalent
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

EFFECT OF LIGAND NONPLANARITY AND SOLVENT NATURE
999
contribution, but the ionic contribution still takes place
[2]. The complexes in study dissociate according to
Eqs. (1) and (2).
The concentration dependences of the apparent
dissociation rate constants of the complexes in the
DMSO–AcOH medium (Fig. 2а, Table 2) proved to be
untypical of classical porphyrins studied previously
[2]. For example, the apparent dissociation rate
constants of complexes V, VI, and VIII vary
unproportionally in the concentration of acetic acid
with the DMSO–АсОН system (Fig. 2а). As a result,
there is no sense to compare the stabilities of the
complexes in glacial acetic acid for constructing their
stability series in dissociation reactions (1) and (2).
Therefore, as an arbitrary criterion for stability of the
complexes we chose the concentration of АсОН in
DMSO, at which the dissociation rate is 1×10–4 s–1.
Under this condition, complexes I–VI can be arranged
in the following series in terms of their decreasing
stability: ZnTBP (I) > ZnTPhP (II) > (AcO)Zn(NMe)·
(β-Et)8P (VII) > ZnTPhTBP (III) > Zn(β-Br)8TPhP
(IV) > (AcO)Zn(NMe)TPhP (VIII) > Zn(β-Et)8TPhP
(VI) > Zn(β-Ph)8TPhP (V).
ZnP + 2 AcОН → H2P + Zn(OAc)2,
(1)
(OAc)Zn(NMe)P + AcОН → H(NMe)P + Zn(OAc)2. (2)
The dissociation kinetics data for сompounds III–
VIII (Table 2) give evidence showing that the
complexes of zinc with strongly distorted porphyrin
ligands dissocate much faster than their planar analogs
[11–15], both in the coordinating (DMSO–АсОН) and
in the weakly coordinating (C6H6–АсОН) solvents.
Disturbed planarity of the macroring in complexed
Н2Р unfavors mutual adjustment of the metal and
ligand reaction centers, produces additional
polarization of N–M bonds, and adversely affects
stability of the complexes. The kinetic stability of the
complexes in proton-donor media decreases as their
nonplanarity is enhanced over series of compounds
[13–16] with increasing number of peripheral
substituents and in nonplanar complexes whose ligands
belong to various structural types (compounds III–VI
and VII–VIII) (Table 2).
The dependences of the apparent dissociation rate
complexes of the zinc complexes on the concentration
of acetic acid have two different curve patterns. The
Table 2. Kinetic parameters of dissociation reactions (1) and (2) of complexes III–VIII in the DMSO–АсОН and C6H6–
АсОН systems, 298 K (cМР 4 × 10–5 M)
kapp × 103, s–1
Еа, kJ mol–1
ΔS≠, J mol–1 K–1
Solvent system
cАсОН, M
Compound V
DMSO–АсОН
С6Н6–АсОН
2.60
3.50
4.40
0.21 0.01
0.32 0.01
8.88 0.50
85 6
71 4
36 2
33 2
–2 0.1
–92 6
5.25
51.39 3.53
2.51 0.11
7.45 0.63
10.08 0.87
78 3
14 3
30 5
37 5
37 7
64 4
1.31·10–2
2.62·10–2
4.36·10–2
6.09·10–2
–255 22
–193 18
–169 15
–163 24
17.05 0.17
Compound VI
DMSO–АсОН
С6Н6–АсОН
5.25
6.50
0.31 0.01а
0.17 0.01а
1.51 0.07а
0.40 0.02а
1.50 0.04а
1.09 0.05
2.73 0.02
4.35 0.32
11.66 0.69
34 1
39 1
25 1
38 1
24 1
51 10
26 4
21 6
13 3
–125 4
–107 2
–136 7
–104 4
–142 8
–139 35
–215 15
–228 30
–248 11
8.80
10.9
12.8
2.62·10–2
4.36·10–2
6.09·10–2
8.69·10–2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

1000
BEREZIN et al.
Table 2. (Contd.)
Solvent system
cАсОН, M
kapp × 103, s–1
Compound IV
0.11 0.01
Еа, kJ mol–1
ΔS≠, J mol–1 K–1
DMSO–АсОН
С6Н6–АсОН
10.9
12.8
14.1
1.59
2.28
2.91
3.49
4.36
53 4
27 1
27 2
19 4
16 1
15 4
20 6
21 7
–66 10
–128 9
1.55 0.12
3.73 0.22
–122 12
–243 14
–248 1
1.58 0.05
3.16 0.09
6.54 0.24
–244 16
–228 20
–224 25
6.79 0.03
6.80 0.05
Compound III
–
–
DMSO–АсОН [15]
С6Н6–АсОН
13.35
15.04
5.57
0.24 0.06
1.99 0.06
0.44 0.07
1.07 0.25
1.30 0.02
2.80 0.03
49 3
42 13
56 4
50 2
40 7
45 3
–145 9
–178 44
–123 15
–140 7
9.06
11.88
15.02
17.5
–168 22
–148 15
АсОНice
5.12 0.62
Compound VII [16]
DMSO–АсОН
17.5
16.2
12.8
63 4
0.97 0.06
112.3 9.6
144 10
284 20
0.11 0.01
159.1 11.3
Compound VII [16]
DMSO–АсОН
17.5
16.2
14.1
10.9
9.43
26.65 0.39
34.18 0.07
47.36 0.48
30.88 0.64
9.19 0.97
44.5 2.3
47.3 3.2
66.3 5.1
74.1 4.3
38.4 7.0
–58 4
–48 3
16 1
39 2
–163 24
a Calculated by the Arrhenius equation.
dissociation rates of complexes ZnTPhTBP (III),
Zn(β-Br)8TPhP (IV), Zn(β-Ph)8TPhP (V), and (AcO)·
Zn(NMe)(β-Et)8P (VII) either linearly increase with
the concentration of acetic acid or this dependence is
fitted by a curve that looks like a branch of a parabola.
The dependences of the apparent dissociation rate
constants of complexes VIII [16] and VI look like
curves with their maxima at 11–14 M AcОН (Fig. 2а).
Such kapp = f(сАсОН) dependences are uncharacteristic
of porphyrins and probably explained by existence, at
the given concentration of acetic acid in DMSO, of
several species active in the dissociation reaction,
specifically DMSO·Н+, АсОН2+, АсОН, etc., whose
equilibrium concentrations are unknown. According to
reagents, and the dependence has a diffuse maximum
at the AcOH concentration of 9–15 M. The intricate
concentration dependences in reactions (1) and (2) can
be associated with a change in the attacking species
with varied composition of the solvent; a mixed
dissociation mechanism is also possible. Reactions (1)
and (2) may take different pathways, including those
induced by undissociated acetic acid [18].
The stability series of the porphyrin complexes in
reactions (1) in the benzene–acetic acid medium is as
follows (Table 2, Fig. 2b): ZnTВP (I) > ZnTPhP (II) >
ZnTPhTBP (III) > Zn(β-Br)8TPhP (IV) > Zn(β-Et)8·
TPhP (VI) > Zn(β-Ph)8TPhP (V).
This series is consistent with the concept that the
stability of zinc porphyrin complexes decreases (Table 1)
with enhancing degree of their planarity disturbance. A
[17], the exponent of autoprotolysis constant рKаpр
=
–log Kарp smoother varies with the composition of the
DMSO–АсOH system at small fractions of each of the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

EFFECT OF LIGAND NONPLANARITY AND SOLVENT NATURE
(a) (b)
1001
kapp × 103,
s–1
kapp × 103,
V
50
40
30
20
10
s–1
16
12
8
VI
IV
II
4
III
VI
IV
VI
0
0.05 0.10 5.1 10.1 15.1
0
2
4
6
8
10 12 14 16
сАсОН, M
сАсОН, M
Fig. 2. Plots of the rate constants of reactions (1) and (2) vs. concentration of acetic acid in (a) DMSO–АсОН and (b) C6H6-АсОН
for compounds III–VIII. сМР 4 × 10–5 M, 298 K.
planar zinc tetrabenzoporphine (I) undergoes no
dissociation even in glacial acetic acid [2]. A fairly
conformation (ΔСβ = 0.96 Å) compared with complex
VI (Table 1).
planar zinc tetraphenylporphine (II) (ill-defined saddle
The order of reaction (1) in complexes III–VIII is
always 1, as evidenced by the straight-line nature of
the log(с0/с) = f(τ) plots (Fig. 3, plot 3).
298
conformation) dissociates in glacial acetic acid, k
=
app
(0.26 0.02)×10–3 s–1 [2], whereas a markedly dis-
torted zinc tetraphenyltetrabenzoporphine (III) (ΔСβ =
–dCMP/dτ = kapp·CMP, kapp = kv·СnH+.
(3)
0.77 Å) under the same conditions is 20 times more
solv
298
active in reaction (1) [k = = (5.12 0.62) ×10–3 s–1,
app
The reactions orders in proton–donor species,
determined by Eq. (3) as the slopes of the log kapp
Table 2, Fig. 2]. A still more facile dissociation is
characteristic of zinc octabromotetraphenylprophine
(IV) whose reaction (1) in the benzene–acetic acid
medium can be studied by classical kinetic methods
only in the range cAcOH 0.5–5.0 М. The fact that
complex IV is more stable than complexes V and VI
(Table 2, Fig. 2) can be explained by a considerable
contribution of polarization factor (–I effect of
bromine) which operates here along with the
macroring distortion factor whose contribution is
slightly smaller (ΔСβ = 0.92 Å, the. A less planar zinc
octaethyltetraphenylporphine (VI) is even less stable
and dissociates at cAcOH 0.02–0.10 M. Probably, the
eight alkyl groups increase, due to their +I effect, the
effective charge of porphyrin nitrogens, thus
additionally driving reaction (1). Zinc dodecaphenyl-
porphine (V) was found be the most unstable in the
C6H6–АсОН medium among the complexes studied:
Its dissociative ability is enhanced by a still stronger
destabilizing effect of the β-phenyl substitution (Fig. 2).
Stability decrease is observed in spite of the fact that
=
f(log cАсОН) straight-line plots, are 1.99 0.03 for III,
1.54 0.20 for IV, and 1.42 0.03 for V. With zinc
log (с0/с)
4
3.0
2.5
3
2
2.0
1.5
1.0
0.5
0.0
1
0
200
400
600
800
τ, s
Fig. 3. Plots of log (с0/с) vs. time of reaction (1) in C6H6–
АсОН for zinc octaethyltetraphenylporphine (VI). 298 K,
сАсОН, M: (1) 2.62 × 10–2; (2) 4.36 × 10–2; (3) 6.09 × 10–2;
and (4) 8.69 × 10–2.
compound
V
has
a
slightly flattened saddle
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

1002
BEREZIN et al.
octaethyltetraphenylporphine VI, the reaction order in
proton–donor species decreased from 1.93 at 298 K to
1.12 at 318 K. The noninteger dissociation reaction
orders in active reagent, as well as their tendency to
change with increasing temperature point to a mixed me-
chanism of dissociation in the C6H6–АсОН medium.
solvent. As seen from the titration curves for complex
VIII and its corresponding ligand (Fig. 4), protonation
of the ligand initiates already at сАсОН about 5 M, but
the equilibrium of dissociation reaction (4) shifts to
ligand formation only at сАсОН > 8 M.
The resulting data suggest that complex VIII
dissociates along Eq. (2) [scheme (5)]. According to
this mechanism, dissociation in the DMSO–C6H6
medium begins with exchange of an extra ligand (in
our case, acetate ion) for DMSO in the inner
coordination sphere of the complex.
N-Methyl substitution not only disturbs planarity
of the porphyrin macroring, but also produces a strong
shielding of the reaction center [6]; as a result, the
reaction center in complexes VII and VIII can only be
attacked along the z axis, which makes them
kinetically more stable.
The Zn–N bonds in the metal-protonated complex
are therewith strongly polarized and weakened, which
accelerates reaction (2). As seen from scheme (5), the
dissociation reaction involves only one proton-donor
molecule. Dissociation by two concurrent pathways,
under the action of solvated proton and undissociated
АсОН [18], results in a peaked kapp = f(сАсОН) plot for
compound VIII in the DMSO–АсОН system. If the
solvent unfavors (like C6H6–АсОН) extra ligand
substitution [scheme (5)], compound VIII dissociates
equilibrially along pathway (4).
The dissociation of compound VIII in the C6H6–
АсОН medium was found to be reversible, i.e. Eq. (2)
for this complex transforms into Eq. (4).
(ОАс)Zn(NMe)P + AcOH ↔ H(NMe)P + Zn(OAc)2. (4)
In acetic acid, complex VIII immediately forms
the ligand Н(NMe)TPhP with doubly protonated endo-
cyclic nitrogen atoms. To obtain evidence for this
conclusion, we performed a spectrophotometric titra-
tion of Н(NMe)TPhP in the C6H6–АсОН binary
AcO
Ph
DMSO
+H+solv
Ph
Z+n
Zn
DMSO
N
N
N
N
N
N
N
N
Ph
Ph
Ph
Ph
−AcO−
Me
Me
Ph
Ph
Z+n
Ph
Ph
DMSO
fast
N
N
N
N
N
N
N
Ph
Ph
Ph
(5)
Ph
−Zn(DMSO)+
HN
slow
H
CH3
CH3
Ph
Ph
Comparing the state of complexes III–VIII in the two
binary systems (Table 2, Fig. 5) we can conclude that
the stability series are generally analogous, and
dissociation in C6H6–AcОН is much faster than in
DMSO–АсОН. These findings suggest that in the
weakly solvating solvent an acidoprotolytic
dissociation mechanism [18] is operative along with
solvoprotolytic [2]. The more nonplanar is the
complex, the more different acid concentrations are
required for it to dissociate at equal rates in the two
systems studied (Table 1, Fig. 5).
EXPERIMENTAL
Compounds III–VIII were prepared, purified, and
identified as described in [6, 19–21].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

EFFECT OF LIGAND NONPLANARITY AND SOLVENT NATURE
1003
(a)
(b)
А
0.6
0.5
0.4
0.3
0.2
0.1
0.0
А
1.0
0.6
0.2
0
5
10
15
сАсОН, M
0
2
4
6
8
10 12 14 16
сАсОН, M
Fig. 4. Plots of the optical density of solutions of (a) com-plex VIII, λ 665 nm, and (b) ligand Н(NMe)TPhP, λ 669 nm, in C6H6–
AcOH vs. concentration of АсОН (298 K).
(a)
(b)
kapp × 103,
s–1
kapp × 103,
s–1
1
5
16
12
8
2
3
4
3
2
1
1
2
4
5
6
4
0
0.04 0.08
5
7
9 11 13 15
5
7
9
11
13
15
сАсОН, M
17
сАсОН, M
Fig. 5. Plots of the apparent rate constants vs. concentration of acetic acid for dissociation reactions (1) of (a) zinc
octaethyltetraphenylporphine (VI) in C6H6–AcОН at (1) 318, (2) 308 K, and (3) 298 K and in DMSO–АсОН at (4) 328, (5) 318,
and (6) 308 K and of (b) zinc tetraphenylbenzoporphine (III) (298 K) in (1) C6H6–AcОН and (2) DMSO–AcОН.
Acetic acid of pure grade was twice frozen out,
refluxed over acetic anhydride, and distilled at 117.5–
118°С. Dimethyl sulfoxide of pure grade was dried
over calcined СаО and distilled in a vacuum. Benzene
of analytical grade of refluxed over P2O5 and distilled [22].
gravimetric procedure. A 4 × 10–5 M benzene solution
of a complex (1 ml) was placed into a tube, and the
solvent was evaporated. The acid solutions were
thermostated, added to the dry residue, let it to dissolve
completely, after which the optical density of the
solution was monitored in time using Hitachi U-2000,
Hitachi U-3000, or SF-46 spectrophotometers. The
trend in the concentration of the complex during
dissociation was followed by the optical density of the
long-wave absorption band.
Spectrophotometric study of dissociation of
complexes III-VIII. Series of DMSO–АсОН and
C6H6–AcОН solutions with a preset concentration of
acetic acid were preliminarily prepared by the
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008

1004
BEREZIN et al.
Smith, K.M., and Guilard, R., New York: Academic,
ACKNOWLEDGMENTS
2000, vol. 1, p. 3.
The authors are grateful to Dr. Chem. Sci. P.A.
Stuzhin (Ivanovo State University of Chemical
Technology) for providing the instrument time
(Hitachi U-2000).
10. Ghosh, A., Ibid., vol. 7, p. 1.
11. Berezin, B.D. and Lomova, T.N., Reaktsii dissotsiatsii
kompleksnykh soedinenii (Dissociation Reactions of
Complex Compounds), Moscow: Nauka, 2006.
12. Lomova, T.N. and Berezin, D.B., Problemy khimii
The work was financially supported by the Russian
Foundation for Basic Research (project no. 06-03-
96327) and the Foundation for Support of Domestic
Science.
rastvorov. Biologicheski aktivnye veschestva
v
rastvorah: struktura, termodinamika, reaktsionnaya
sposobnost’ (Problems of Solution Chemistry.
Biologically Active Substances in Solutions: Structure,
Thermodynamics, Reactivity), Kutepov, A. M., Ed.,
Moscow: Nauka, 2001, p. 326.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 5 2008