Chemical composition of donor–acceptor complexes of hydroxyoxo(5,10,15,20-tetraphenylporphinato)molybdenum(V) with 3,5-dimethylpyrazole and equilibrium constants for their formation

Article abstract of DOI:10.1134/S0036024417100272

The results from a quantitative study of reactions between hydroxyoxo(5,10,15,20-tetraphenylporphinato)molybdenum(V) (O=Mo(OH)TPP) and 3,5-dimethylpyrazole, a biologically active base, in toluene are presented. The chemical structure and key parameters of intermediates and reaction products are determined by spectral means. The equilibrium constant (K = 51.3 L/mol) is calculated and a full kinetic description of simple reactions that occur in this system during complex transformation is obtained. The prospect of using a mixed porphyrin-containing complex as a receptor for 3,5-dimethylpyrazole, a building block for alkaloids and pharmaceutical preparations, is substantiated.

Full text of DOI:10.1134/S0036024417100272

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2017, Vol. 91, No. 11, pp. 2085–2091. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © E.V. Motorina, T.N. Lomova, 2017, published in Zhurnal Fizicheskoi Khimii, 2017, Vol. 91, No. 11, pp. 1837–1844.
CHEMICAL KINETICS
AND CATALYSIS
Chemical Composition of Donor–Acceptor Complexes
of Hydroxyoxo(5,10,15,20-tetraphenylporphinato)molybdenum(V)
with 3,5-Dimethylpyrazole and Equilibrium Constants
for Their Formation
E. V. Motorina and T. N. Lomova*
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
*е-mail: tnl@isc-ras.ru
Received November 8, 2016
Abstract—The results from a quantitative study of reactions between hydroxyoxo(5,10,15,20-tetraphenyl-
porphinato)molybdenum(V) (O=Mo(OH)TPP) and 3,5-dimethylpyrazole, a biologically active base, in tol-
uene are presented. The chemical structure and key parameters of intermediates and reaction products are
determined by spectral means. The equilibrium constant (K = 51.3 L/mol) is calculated and a full kinetic
description of simple reactions that occur in this system during complex transformation is obtained. The
prospect of using a mixed porphyrin-containing complex as a receptor for 3,5-dimethylpyrazole, a building
block for alkaloids and pharmaceutical preparations, is substantiated.
Keywords: molybdenum(V) porphyrin, 3,5-dimethylpyrazole, physicochemistry of complexation
DOI: 10.1134/S0036024417100272
INTRODUCTION
imidazole, and piperidine) and hydrogen sulfide in
biomedicine and the food industry. They also showed
that the coordination of the pyridyl residue of sub-
stituted fullerene C60 by the central atom in
For a number of reasons, the physicochemistry of
molybdenum compounds has developed considerably
since the first attempts to study them in 1778 (made by
Carl Wilhelm Scheele). In the field of advanced tech- O=Mo(OH)TPP allows us to obtain supramolecular
nology, the interest in these compounds is driven by
the use of molybdenum-containing alloying additives
(intended to improve the hardness and viscosity of
steels [1]) and industrial catalysts (in the processes of
cracking, hydrotreatment, and oil reforming). Homo-
and heterometallic methoxide complexes of molybde-
num in different valence states and mixed-valence
Mo(III)–Mo(IV) and Mo(III)–Mo(V) clusters acti-
vate and reduce molecular nitrogen under mild condi-
tions [2]. The cited work presented the first systematic
study of successive stages for obtaining protonic nitro-
gen-fixing systems. The biological role of the trace ele-
ment Mo also attracts the attention of scientists. So far,
calibration curves have been obtained for the determina-
tion of molybdenum via diffuse reflection spectroscopy
and colorimetry using heterocyclic azo compounds [3].
Using hydroxyoxo(5,10,15,20-tetraphenylporphi-
nato)molybdenum(V) (O=Mo(OH)TPP) as an exam-
ple, the authors of this work showed in [4–7] that
molybdenum compounds can be used to identify bio-
systems in which long-lived states with separated
charges can be generated for optoelectronics [8, 9].
The authors of [10–12] considered reactions
between metalloporphyrins (MPs) and 3,5-dimeth-
ylpyrazole (3,5-DMP), which is widely used in the
pharmaceutical industry and, as an intermediate, in
the production of azine dyes. Mechanisms of intramo-
lecular and intermolecular ligand exchange have been
identified in the reaction between a base and ruthe-
nium complex, Ru(CO)TPP at different temperatures
[10]. It was established in [11] that the equilibrium
constant for the reversible donor–acceptor complex-
ation of zinc-5,15-di(o-nitrophenyl)-octaalkylpor-
phyrin with 3,5-dimethylpyrazole is (5.89 0.8) × 103
L/mol [11]. To optimize the search for effective recep-
tors of biologically active N-bases, it is especially
important to provide a quantitative description of the
equilibria, rates, and mechanisms of reactions
logically active nitrogen-containing bases (pyridine, between N-bases and metalloporphyrins.
2085

2086
MOTORINA, LOMOVA
The equilibrium constants of reversible stages were
calculated from formula (1), which was derived using
the law of mass action and the Beer–Lambert–Bou-
guer law for a mixture of two colored compounds, i.e.,
initial complex (I) and the product of its reaction with
base II:
O
CH3
Mo
N
N
HN
(I)
(II)
CH3
N
(Aeq − Ao)/(A∞ − Ao)
K =
N
N
1 − (Aeq − Ao)/(A∞ − Ao)
OH
(1)
1
×
.
n
Ap − Ao
A∞ − Ao
⎛
⎞
CL − CM0 P
⎜
⎝
⎟
⎠
The equilibrium and kinetics of elementary reac-
tions that occur during the complex transformation of
O=Mo(OH)TPP (I) in the presence of 3,5-dimeth- Here,
0
and СL are the initial concentrations of I
СMP
ylpyrazole (II) in toluene are studied in this work. The
composition and spectral properties of the reaction
products are also studied. The prospects for using the
porphyrin complex as a receptor for 3,5-dimethylpyr-
azole are analyzed.
and II, respectively, in toluene; Ао, Аeq, and А∞ are the
optical densities measured at the working wavelength
at τ = 0 for the solutions of I, the equilibrium mixture
at a given concentration of II, and the reaction prod-
uct, respectively; and n is the number of bonded mol-
ecules of base II.
The effective rate constants for the reaction at dif-
ferent concentrations of II were calculated using the
equation for a formally first-order reaction,
EXPERIMENTAL
Complex I was synthesized according to the proce-
dure from [13]: 0.1 g (0.16 mmol) of H2TPP and
0.076 g (0.53 mmol) of MoO3 reacted for 4 h in 0.8 g
of phenol at 454 K, which is the boiling point of the
mixture. The complex was isolated in solid form via
the vacuum distillation of phenol and then dissolved in
CHCl3 and chromatographed twice on Al2O3 columns
using CHCl3. The solid complex was obtained with a
yield of 60% via crystallization from a solution of it in
CHCl3. The electronic absorption spectrum (UV–
Vis) in chloroform (λmax, nm (log ε)) was 620.0 (2.94),
584.0 (2.92), and 456.0 (3.78). The IR spectrum in
KBr (ν, cm−1) was vibrations of benzene rings, 701,
752 (γC–H); 1070, 1177 (δC–H); and 1484, 1596 (νC=C);
vibrations of pyrrole rings, 800 (γC–H); 1018 (C3–C4,
Ao − A∞
Aτ − A∞
1
τ
(2)
keff = ln
,
where Ао, Аτ, А∞ are the optical densities of the reac-
tion mixture at the working wavelength at the initial
time, time τ, and after terminating the reaction,
respectively.
The optimization of K and keff values and the deter-
mination of their mean square deviations were done
according to the method of ordinary least squares
(OLS) in Microsoft Excel. The relative error in deter-
mining the K and keff values did not exceed 20 and
10%, respectively. An example of calculating K con-
stants for the II concentration range of 1.6 × 10−3–
1.13 × 10−1 mol/L in one of the corresponding parallel
experiments is given in Table 1.
The UV–Vis spectra of the solutions and the IR
spectra in a solid chaotic layer on silicon wafers were
recorded on a UV–Vis Agilent 8453 spectrophotome-
ter and a VERTEX 80v spectrometer, respectively. The
IR spectra for chaotic layers on silicon wafers were
obtained by evaporating liquid from solutions of I in
toluene or in a mixture of toluene and 2.98 × 10−2
mol/L II after all chemical reactions in the mixture
were complete.
ν
C–N, δC–H); 1336 (νC–N); 1441 (νC=N); 441 (Мо–N);
659 (Мо–O); 928 (Мо=O).
The reaction between complex I and base II was
studied spectrophotometrically using molar ratios and
excess concentrations methods. Solutions of I and II in
freshly distilled toluene (Tb = 110.6°C) were prepared
immediately before use to avoid the formation of perox-
ides in the solvent. The original method first presented
in [14] was used for spectrophotometric titration and
determination of the reaction rate in one series of exper-
iments. The optical density of a series of solutions with
a constant СI concentration (2.3 × 10−5 mol/L) and dif-
ferent concentrations of base II that ranged from 1.6 ×
10−3 to 1.13 × 10−1 mol/L was measured at the operat-
RESULTS AND DISCUSSION
3,5-Dimethylpyrazole is mildly basic (pKb = 9.74
ing wavelength of 462 nm at the initial time (τ = 0) and [15]), due to the nitrogen atom at position 2 of the
over time. The solutions were held at a constant tem- molecule (formula II). This makes its interaction with
perature of 298 K (with an accuracy of 0.1 K) in the coordinatively unsaturated Mo atom in complex I
closed quartz cells inside a special compartment of the highly probable. The UV–Vis spectra of I in toluene at
spectrophotometer.
different concentrations of base II (Fig. 1) do indeed
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Table 1. Sample calculations of equilibrium constants for
the reaction between О=Мо(ОН)ТРР and 3,5-dimeth-
ylpyrazole at 298 K
indicate a reversible interaction. As the concentration
of the base was gradually increased, a reversible drop
in the optical density was observed in the region of the
Soret band (λmax = 462 nm) along with the emergence
of a new maximum at 430 nm. Note that the substitu-
tion of axial anionic ligands in complex I was accom-
panied by the emergence of a new band (or shoulder)
to the right of the Soret band [13]. As the EASes trans-
form with the rising level of the base, an isobaric point
is observed at λ = 478 nm. The equilibrium, character-
ized by the K constant of 51.3 9.6 L/mol (Table 1), is
established in the system immediately upon adding the
solutions. Despite the emergence of a new maximum
in the final spectrum (Fig. 1), the Soret band retains
almost the same position and has fairly high intensity,
as if initial compound I is not completely titrated. This
indicates that the system cannot be described by the
simple transition of one colored compound (complex I)
to another (the colored product). It is well known [16,
17] that the UV–Vis spectra of porphyrin complexes of
molybdenum in different degrees of oxidation are very
specific. In particular, the spectra in which the Soret
band lies in the region of 445–460, near 430, and near
420 nm, belong to the Н2TPP complexes with
С3,5-DMP × 102,
Аeq
K1, L/mol
mol/L
0
0.91962
0.88731
0.86304
0.82371
0.77378
0.74027
0.70649
0.65446
0.65285
0.64151
0.6225
0.16
0.34
0.68
0.82
1.03
1.40
1.76
1.87
1.95
2.03
2.30
2.76
2.98
35.63
32.96
47.06
50.56
49.00
58.23
55.48
58.03
64.54
62.96
54.23
57.27
41.10
0.60971
0.60552
0.58924
K
δK = 51.3 9.6, L/mol.
1
1
O=MoV(OH), O=MoIV, and O=MoV(Cl), respec-
tively. The hypsochromic position of the band in the
latter complex is due to the lower covalency of the
Mo–Cl bond, compared to that of Mo–O (the oxygen
in the OH group of the first complex).
molecule of base II participates in the reaction for
each molecule of I (Fig. 2b, tanα = 0.99).
One step on the titration curve—the I : II stoichio-
metric ratio of 1 : 1—allows us to interpret the reaction
between I and II in toluene as the formation of a
donor–acceptor octacoordinate complex of molybde-
num(V), O=Mo(OH)(3,5-DMP)TPP. The above
Figure 2 shows the titration curve and the curve for
Aeq − A
A∞ − Aeq
dependence
0 –logCL, indicating that one
log
462
A
1
0.9
430
0.5
0.1
420
460
500
λ, nm
−5
Fig. 1. UV–Vis spectra of O=Mo(OH)TPP in toluene at different levels of 3,5-dimethylpyrazole: С
= 2.3 × 10 mol/L;
O=Mo(OH)TPP
−3
−1
С
, mol/L. (1) 0; the other lines correspond to the concentration range of С
= 1.6 × 10 –1.13 × 10 .
3,5-DMP
3,5-DMP
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MOTORINA, LOMOVA
Aeq − A0
A∞ − Aeq
log
A
0.9
−0.2
(a)
(b)
0.7
0.5
−0.8
−1.4
tanα
2.55
5.05
7.55 CL × 102, mol/L
−2.75
−1.75 logCL
Fig. 2. Curves for the spectrophotometric titration of O=Mo(OH)TPP with 3,5-dimethylpyrazole in toluene: (a) С
=
3,5-DMP
−3
−1
2
1.6 × 10 –1.13 × 10 mol/L and (b) the respective log((A − A )/(A − A ))−log C (R = 0.986) dependence.
eq
0
∞
eq
L
A
1.0
419
A
1.0
462
1
2
0.6
0.2
462
λ, nm
λ, nm
400
500
600
–3
Fig. 3. UV–Vis spectra of O=Mo(OH)TPP in toluene at 1.83 × 10 mol/L 3,5-DMP: (1) τ = 0, (2) τ = 24 h. The insert shows
the time dependence of the band.
spectral pattern of the transformation (Fig. 1), how- Reaction rate constant k is equal to 6.24 ×
ever, indicates a more complex process whose nature 10‒5 s‒1 mol−1 L−1. Table 2 and Fig. 4 show that the
becomes evident when studying the reactive system reaction rate slows with an increase in the concentra-
over time.
tion of II.
A slow, irreversible process that is accompanied by
To establish the nature of the conversion products,
a shift of the Soret band in UV–Vis spectra to the
short-wave region (419 nm) is observed in all of the
reaction mixtures over time (Fig. 3).
we performed a comparative study of the IR spectra
for the initial O=Mo(OH)TPP (Fig. 5, line 1) and the
product of the reversible reaction (Fig. 5, line 2) (the
equilibrium mixture at the point of equivalence, where
initial complex I is completely titrated) after the irre-
versible reaction was complete. We observed new sig-
nals with the coordinated base II vibration frequencies
of 737, 779, 1307, 1422, 1466, 1663, 2789, 3108, 3130,
and 3199 cm−1 in the IR spectrum of the product. The
abovementioned frequencies correspond to the fre-
quencies of (719, 754), 800, 1338, (1410, 1441), 1485,
1650, 2784, 3108, 3130, and 3198 cm−1 in the IR spec-
trum of base II, which was recorded in our study under
comparable conditions. Signals with frequencies of
659 and 928 cm−1, which correspond to the vibrations
of the Мо–О and Мо=О bonds in the initial complex
I (Fig. 5, line 1), are retained in the spectrum of the
Because of the considerable difference between the
times needed to establish equilibrium and complete
the subsequent irreversible reaction, we were able to
anatomize the chemical reaction and thereby study the
above equilibrium without allowing for the subsequent
slow conversion of the equilibrium mixtures, and to
study the kinetics of the latter process. Table 2 shows
the values of the effective rate constants kobs that
were found for different initial concentrations of II.
These values indicate a reaction of the first order with
respect to the initial substance. The order of the reac-
tion with respect to base II was found to be −1 (Fig. 4).
The experimental kinetic equation had the form
−dCO=Мо(OH)TPP/dτ = kCO=Мо(OH)TPP(C(3,5-DMP))−1. (3)
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CHEMICAL COMPOSITION OF DONOR–ACCEPTOR COMPLEXES
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Table 2. Effective rate constants (kobs) for the reaction
between O=Мо(OH)TPP and 3,5-DMP in toluene at 298 K.
The range of С3,5-DMP concentrations is 0.0176–
0.0298 mol/L
final reaction product (Fig. 5, line 2). However, they
are split into two components (620, 662, and 930,
1011 cm−1, respectively). New signals with frequencies
of 620 and 1011 cm−1 indicate the emergence of new
forms of the molybdenum complex with weak Мо–O
bonds (a sharp drop in frequency) and stronger Mo=O
bonds (a sharp rise in frequency). However, the most
important difference between the IR spectrum of the
reaction product and that of complex I is the notable
broadening and smoothing of most signals.
The quantitative study of the equilibrium and kinet-
ics of the reaction between metalloporphyrin I and base
II in combination with the accompanying spectral
results allows us to interpret this transformation. The
substitution of ligands in complex I is complicated by
the partial reduction of the central atom. This reduction
in turn is accompanied by the release of an oxidized
form of the base (the nature of this form is unknown).
С3,5-DMP × 102, mol/L
(kobs δkobs) × 103, s−1
1.76
1.87
1.95
2.03
2.30
2.76
2.98
3.4 0.20
3.2 0.20
2.8 0.10
2.6 0.08
2.5 0.20
2.2 0.20
1.9 0.10
The reaction is immeasurably slow in the concentration range of
0.0016–0.014 mol/L.
acceptor complex O=Мо(OH)(3,5-DMP)TPP is,
according to reaction scheme (4)–(6) in equilibrium
not only with the initial complex O=МоV(OH)TPP
but also with its own reduced form, O=MoIVTPP,
which is formed in rapidly established equilibrium (5).
The experimentally observed isosbestic point at 478
nm is apparently associated with slight differences in
the parameters of the Soret band in the spectra of
O=Mo(OH)TPP and O=Мо(OH)(3,5-DMP)TPP.
The МоV → МоIV transition cases for porphyrin com-
O=МоV (OH)TPP + 3,5-DMP
(4)
K1
V
⎯⎯→
O=Мо (OH)(3,5-DMP)TPP,
←⎯⎯
O=МоV OH 3,5-DMP TPP
(
)(
)
(5)
(6)
K2
ox
IV
⎯⎯→
←⎯⎯
O=Mo TPP + 3,5-DMP + H2O,
(
)
O=МоV (OH)(3,5-DMP)TPP
V
+
−
k3
⎯⎯→ [O=Mo (3,5-DMP)TPP] OH .
slow
MoV forms complex compounds in which its coor- plexes are known in the literature. For instance, the
reduction of O=MoV(OEt)TPP to О=МоIVTPP was
described in [19]. Porphyrin complexes of molybde-
dination numbers are 5, 6, 7, and 8, with relative dis-
tributions of 13, 32, 0, and 7, respectively, according to
X-ray diffraction data [18]. The proposed transition of
heptacoordinate complex
I
to octacoordinate
O=Мо(OH)(3,5-DMP)TPP (4) does not contradict
the general trend. The mentioned above donor–
1
2
logkobs
−2.55
−2.65
−1.7
−1.6
logCL
3000
2000
1000 ν, сm−1
Fig. 5. IR spectra recorded in a solid chaotic layer on sili-
con wafers for (1) O=Mo(OH)TPP and (2)
О=Мо(ОН)(3,5-DMP)ТРР.
Fig. 4. log k –log C dependence for the reaction
pbs
L
between O=Mo(OH)TPP and 3,5-DMP in toluene at
2
298 K; R = 0.935.
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MOTORINA, LOMOVA
Table 3. Stepwise reactions of O=Mo(OH)TPP with organic N-bases in toluene and their respective stepwise equilibrium
constants (Kn) and rate constants (k and k–)
Base (pKb)
[27, 28]
k, L/(s mol),
k–, 1/s
Kn, L/mol
(9.1 1.2) × 103
39.3 5.2
Chemical equations for the stepwise reactions
Py (8.82)
O=Mo(OH)TPP + Py ↔ [O=Mo(Py)TPP]+OH–
k = 5.25
k– = 5.75 × 10–4
[O=Mo(Py)TPP]+ OH– + Py + H2O
k = 1.83 × 10–2
↔ [Mo(OH)(Py)2TPP]2+2OH–
k– = 4.65 × 10–4
[Mo(OH)(Py)2TPP]2+2OH– + Py ↔ [Mo(Py)3TPP]3+3OH–
1.0 0.1
k = 1.19 × 10–3
k– = 1.20 × 10–3
Im (7.03)
Pip (2.88)
O=Mo(OH)TPP + Im ↔ O=Mo(OH)(Im)TPP
O=Mo(OH)(Im)TPP + Im ↔ [O=Mo(Im)2TPP]+OH–
O=Mo(OH)TPP + Pip ↔ [O=Mo(Pip)TPP]+OH–
[O=Mo(Pip)TPP]+OH– + Pip ↔ [O=Mo(Pip)2TPP]+OH–,
(1.85 0.14) × 103
(4.80 0.20) × 102
(1.91 0.44) × 103
1.76 0.39
k1 = (1.63
0.29) × 10−4
+
NH
Pip
Pip
Pip
k1
+
+
OH−
OH−
Mo
Mo
O
O
Pyz (13.35) O=W(OH)TPP + Pyz ↔ O=W(OH)(Pyz)TPP
3.99 × 102
а
pK for 3,5-DMP is 9.74.
b
num(IV) are characterized by normal UV–Vis spectra
[20] in which the Soret bands lie near 420 nm [21–23].
−dCO=Мо(OH)(3,5-DMP)TPP /dτ
= −dCO=Мо(OH)TPP /dτ = k(CO0=Мо OH TPP
(11)
(
)
At the subsequent slow and irreversible stage, the
same donor–acceptor complex O=Мо(OH)(3,5-
DMP)TPP at its equilibrium concentration partici-
pates in reaction (6), which is the displacement of the
anionic ligand OH− into the second coordination
sphere. The following kinetic equation corresponds to
the rate of reaction (6):
− KCO=Мо OH TPPC30,5-DMP(C 3,5-DMP ox)−1).
(
)
(
)
Theoretical kinetic equation (11) accounts for the
experimentally observed first order with respect to
compound I and the negative order with respect to
base II (see experimental kinetic equation (3)). The
product of the reaction that occurs in a system com-
posed of complex I and base II is a mixture of
[O=MoV(3,5-DMP)TPP]+OH− and O=MoIVTPP.
The H2O molecule released in reaction (5) likely belongs
to the coordination sphere of O=MoIVTPP and accounts
for the emergence of a low-frequency signal (620 cm−1)
in the IR spectrum of the product. In our opinion,
the new high-frequency signal of the Mo=O that
−dCO=Мо(OH)(3,5-DMP)TPP/dτ = kCO=Мо(OH)(3,5-DMP)TPP. (7)
At rapidly established equilibria (4), (5):
0
CO=Мо(OH)(3,5-DMP)TPP
=
– CO=МоTPP. (8)
CO=Мо OH TPP
(
)
CO=МоTPP is expressed by adding the left- and right-
hand sides of equilibria (5) and (6):
occurs
at
1011
cm−1
corresponds
to
O=МоV OH TPP + 3,5-DMP
(
)
[O=MoV(3,5-DMP)TPP]+OH− complex with an
increased effective positive charge on the central atom.
(9)
K
ox
IV
⎯→
←⎯
O = Mo TPP + 3,5-DMP + H2O,
(
)
Note that the interaction between complex I and
organic N-bases is not universal. The reactions of
О=Мо(ОН)ТРР with bases pyrazine, imidazole, pyr-
idine, and piperidine were studied earlier, and a com-
parative analysis of them was provided in [24]. The
most important conclusion drawn from this analysis
was the existence of individual features of the stoichio-
metric reaction mechanism that depend on the nature
CO=МоTPP = KCO=Мо(OH)TPPC3,5-DMP(C(3,5-DMP)ox)–1. (10)
Since a necessary condition for titrating the solution of
complex I with base II is an excess of the latter, the
equilibrium concentration of C3,5-DMP is always the
same in each thermodynamic experiment (at each
0
titration point):
. Using Eqs. (8) and (10) and
C3,5-DMP
considering equilibria (4) and (5), we move from rate of the base (Table 3). According to the table, constants
equation (7) to of the same type of equilibria for the formation of
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3. G. A. Kochelaeva, V. M. Ivanov, and A. R. Guseinova,
donor–acceptor complexes О=Мо(ОН)ТРР with
N-bases change in a particular manner: for coordina-
tion of the base molecule without the displacement of
the OH− ligand, Im > Pyz; for coordination with the
displacement of OH− to the second coordination
sphere, Py > Pip. It is apparent that the constant for
the formation of a donor–acceptor complex grows for
the same type of base molecules (σπ ligands) along
with their basicity. However, this rule is not followed
in the case of Py and Pip: Pip, which is a relatively
strong base, has a lower formation constant because
only the properties of the σ-ligand are manifested.
The 3,5-dimethylpyrazole (σπ ligands) studied in this
work is characterized by the pKb value of 9.74 [15], so
it too does not fit into the first of the mentioned above
series: Im > Pyz > 3,5-DMP. Although the coordina-
tion of the base molecule occurs without displacement of
the OH− ligand, the coordination is coupled with the
subsequent reduction of the metal cation in the donor–
acceptor complex that is formed. In addition, the possi-
ble steric hindrances that two methyl groups can cause
during coordination should also be considered.
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CONCLUSIONS
tailCB2707394_EN.htm#2. Accessed October 7, 2016.
The complexes that compound I forms with bases
have moderately high stability constants. In the case of
3,5-DMP, these constants also differ considerably
from unity. Using the procedure described in [25], we
determined the relative optical response (Аopt = (А0 –
А∞)/А0) observed the region of the Soret band at
298 K for the 3,5-DMP concentration range of 1.6 ×
10−3–1.13 × 10−1 mol/L. Its value was 0.46, which
agrees with the data provided in the literature [25, 26]
for optical sensors based on other complexes of por-
phyrins and phthalocyanine. For instance, Aopt = 0.1
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for Pip [6] in the concentration range of 1.35 × 10−4–
(1992).
1.62 × 10−2 mol/L and Aopt = 0.44 for Im [4] in the
22. T. Diebold, B. Chevrier, and R. Weiss, Inorg. Chem.
18, 1193 (1979).
concentration range of 1.2 × 10−4–7.18 × 10−4 mol/L.
The value of the relative optical response is thus high-
est when using O=Mo(OH)TPP and 3,5-DMP.
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ACKNOWLEDGMENTS
25. J. Spadavecchia, G. Ciccerella, and R. Rella, Sens.
This work was supported by the Russian Founda-
tion for Basic Research, project no. 15-03-00646-a.
Actuators B 106, 212 (2005).
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S. Reza Mousawi, Iran. Talanta 63, 371 (2004).
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Translated by Yu. A. Modestova
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 91 No. 11 2017