Synthesis, chemical stability, and electrocatalytic properties of zinc(II) and cobalt(II) complexes of N-methyltetraphenylporphine

Article abstract of DOI:10.1134/S0036023615100046

Distortion of flat ligand structure in metal complexes of porphyrins caused by N-methyl substitution considerably decreases the kinetic stability of these compounds in DMSO-HOAc proton-releasing medium ((AcO)Zn(N-Me)TPP (Ib) < (AcO)Co^II(N-Me)TPP (Ia) < ZnTPP (IIb) < Co^IITPP (IIa)) and their resistance to thermal destruction in crystal state {(AcO)Co^II(N-Me)TPP (Ia) < (AcO)Zn(N-Me)TPP (Ib) < H(N-Me)TPP (I) < H₂TPP (II) < Co^IITPP (IIa) < ZnTPP (IIb)}, however, exerts a positive effect on the electrocatalytic activity of the compounds in the electroreduction of molecular oxygen, which changes in the following order: II < I < IIa < Ib ? Ia. It was proved by thermogravimetry and electron spectroscopy that N-substituted metal complexes with Zn(II) and Co(II), in contrast to the corresponding ligands, undergo dealkylation to form metal porphyrins (MP) when heated above 200°C under both an inert and air oxygen atmosphere.

Full text of DOI:10.1134/S0036023615100046

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2015, Vol. 60, No. 10, pp. 1267–1274. © Pleiades Publishing, Ltd., 2015.
Original Russian Text © D.B. Berezin, Vu Thi Txao, A.A. Azorina, O.V. Shukhto, S.S. Guseinov, N.M. Berezina, 2015, published in Zhurnal Neorganicheskoi Khimii, 2015, Vol. 60,
No. 10, pp. 1385–1393.
PHYSICAL METHODS
OF INVESTIGATION
Synthesis, Chemical Stability, and Electrocatalytic Properties
of Zinc(II) and Cobalt(II) Complexes
of NꢀMethyltetraphenylporphine
D. B. Berezina, Vu Thi Txaoa, A. A. Azorinaa,
O. V. Shukhtoa, S. S. Guseinovb, and N. M. Berezinaa
a Research Institute of Macroheterocycles, Ivanovo State University
of Chemistry and Technology, Sheremetevskii pr. 7, Ivanovo, 153000 Russia
b Krestov Institute of Solution Chemistry, Russian Academy of Sciences,
Akademicheskaya ul. 1, Ivanovo, 153045 Russia
eꢀmail: berezin@isuct.ru
Received October 15, 2014
Abstract—Distortion of flat ligand structure in metal complexes of porphyrins caused by Nꢀmethyl substituꢀ
tion considerably decreases the kinetic stability of these compounds in DMSO–HOAc protonꢀreleasing
medium ((AcO)Zn(NꢀMe)TPP (Ib) < (AcO)CoII(NꢀMe)TPP
(
Ia) < ZnTPP (IIb) < CoIITPP
(
IIa)) and
Ia) < (AcO)Zn(NꢀMe)TPP
IIa) < ZnTPP (IIb)}, however, exerts a positive effect on
the electrocatalytic activity of the compounds in the electroreduction of molecular oxygen, which changes in
the following order: II IIa Ib << Ia. It was proved by thermogravimetry and electron spectroscopy that
Nꢀsubstituted metal complexes with Zn(II) and Co(II), in contrast to the corresponding ligands, undergo
their resistance to thermal destruction in crystal state {(AcO)CoII(NꢀMe)TPP
Ib) < H(NꢀMe)TPP (
) < H2TPP (II) < CoIITPP
(
(
I
(
<
I
<
<
dealkylation to form metal porphyrins (MP) when heated above 200°C under both an inert and air oxygen
atmosphere.
DOI: 10.1134/S0036023615100046
Nꢀsubstituted analogs of porphyrins (H(Nꢀ was also unclear whether the presence of atmospheric
oxygen affects the stage of thermal destruction (3).
Me)TPP (I)) are strongly nonplanar molecules where
macrocycle distortion is achieved by the introduction
of bulky intracyclic substituent (Rꢀ) at the N atom,
while nonplanarity extent is controlled by the size of R
and aromaticity of initial unsubstituted porphyrin (for
(AcO)M(NꢀMe)P + HOAc
(1)
H(NꢀMe)P + M(OAc)2,
(X)M(NꢀR)P + RNH2
MP + [R2NH2]+X–, (2)
example, H2ТРP (II)) [1–8]. The distortion leads to
Δ
electron density redistribution and macrocycle polarꢀ
ization [6], changes the hybrid state of substituted
nitrogen atom, and sharply changes complexes reacꢀ
tivity [1, 2, 5, 7, 8]. A change in molecular structure
considerably affects the resistance of Nꢀsubstituted
ligands H(NꢀR)P and their complexes (X)M(NꢀR)P
toward different physical factors and chemical
reagents. Thus, the complexes with Nꢀsubstituted
ligands are known to be unstable, the most stable comꢀ
plexes undergo dissociation when exposed to protons
of acids (1) even at pH > 3 [1, 9]. In the medium of
organic bases such as aliphatic amines, (X)M(NꢀR)P
undergo nucleopnilic substitution reactions (2) and
eliminate Nꢀsubstituent [1], whereas their ligands do
not undertake dealkylation. The authors [7] supposed
similar dealkylation process takes place also in the iniꢀ
tial stage of thermoꢀoxidative destruction (3) of
Nꢀsubstituted metal complexes. However, no products
(X)M(NꢀR)P
MP + RX.
(3)
The interest in Nꢀsubstituted porphyrins is caused
by not only their high biological activity [1] but also
the ability of Nꢀsubstituted porphyrins to stabilize
metal ions in the lowest oxidation states [8–10], which
can be used, for example, in metalꢀcomplex catalysis
and electrocatalysis of chemical processes [11]. To
assess the applicability of Nꢀsubstituted compounds
for practical applications, one needs information on
their stability under different conditions, which is not
available in the literature [7, 8].
Therefore, with the aim to analyze the effect of
Nꢀmethylation and metal nature in (X)M(NꢀR)P on
the physicochemical properties of macroheterocycles,
we synthesized zinc(II) and cobalt(II) complexes of
Nꢀmethyltetraphenylporphyrin (compounds Ia and
Ib) and studied their stability toward dissociation in
solutions containing protonꢀreleasing component and
of reaction (3) were identified by spectral methods, it in solid state under polythermal conditions (thermal
1267

1268
BEREZIN et al.
destruction and thermoꢀoxidation) by chemical kinetꢀ pounds in the electroreduction of molecular oxygen
ics and thermogravimetry, respectively; moreover, we using cyclic voltammetry (CVA).
obtained data on the catalytic activity of the comꢀ
Ph
Ph
Ph
N
N
N
N
N
N R
N Me
M
N
Ph
Ph
Ph
M
Ph
Ph
Ph
N
N
N
HN
OAc
Ph
. R = Me; II. R = H
Ph
Ph
I
Ia. M = Co; Ib. M = Zn
IIa. M = Co; IIb. M = Zn
EXPERIMENTAL
Ecат + E
an ) were determined from voltammetric
2
H(NꢀMe)TPP (
of unsubstituted tetraphenylporphyrin (Н2ТРР
with dimethyl sulfate in refluxing
I
) was obtained by direct alkylation
II
ꢀxylene in the presꢀ
curves recorded in the range from –1.5 to 0.5 V as averꢀ
age from the series of 5–6 parallel experiments. Relative
error in Ered/ox determination was not higher 3%.
,
)
о
ence of K2CO3 [12]. Its complexes with cobalt(II) and
zinc(II) (compounds Ia and Ib) were prepared by
reacting the ligand with tenfold molar excess of the
Organic solvents (СHCl3 (reagent grade), DMF
(analytical grade), HOAc (pure grade), DMSO (anaꢀ
lytical grade)), and water used in experiment were
purified according to recommendations [15].
Obtained experimental results and calculation data
are represented in Tables 1–3 and Figs. 1–4.
corresponding metal acetate on slight heating (50°С)
in DMF solution for 30 min [1, 7]. Next, the reaction
mixture was cooled, diluted with water, and the comꢀ
plex was extracted with chloroform. The reaction was
monitored by spectrophotometry (electronic absorpꢀ
tion spectrum), the purity and individuality of the
compounds were monitored by thin layer chromatogꢀ
raphy (silica, CHCl3). The yield of complexes
RESULTS AND DISCUSSION
NꢀSubstituted porphyrin analogs produce several
types of complexes, simple and bridging including.
Simple chelate Nꢀsubstituted metal derivatives
{(X)M(NꢀR)P}, like metal porphyrins (MP), form
only from 21Nꢀmonosubstituted ligands, for example,
(AcO)CoII(NꢀMe)TPP
Me)TPP (IIb) was 83 and 61%, respectively.
(
IIa)
and (AcO)Zn(Nꢀ
Thermogravimetric study was performed on a DSC
204 F1 differential scanning calorimeter equipped
with a NETZSCH TG 209 F1 Iris thermobalances
(Germany). A weighed ~5ꢀmg crystalline sample was
placed into a platinum crucible and heated under a
static argon atmosphere at a rate of 10 K/min in the
range 298–1223 K. The studied samples were dried for
several hours in vacuum (<1 mmHg) at ambient temꢀ
perature prior to experiment.
compound
I [1]. The feature of H(NꢀR)P ligands is
their ability to produce complexes with metals in oxiꢀ
dation states that are unstable for MP itself (Co(II),
Mn(II), Fe(II)) [1, 8]. The Nꢀsubstituted ligand in
fact is a tridentate and forms formally one covalent
and two strong coordination bonds with metal in the
complexes. The second metal covalence is compenꢀ
sated due to acido ligand X. The bond of metal with
the substituted nitrogen atom, which seems to be in р3
valence state [1, 2, 5, 8], is the weakest coordination
The kinetics of dissociation reaction (1) for comꢀ
plex Ia was registered spectrophotometrically in
HOAc–DMSO medium according to the previously
described procedure [13].
bond of aminne type (l(M–N) = 2.26–2.80 Å [1]).
In the context of monoanionic structure of the Nꢀ
substituted ligand, one should expect that its comꢀ
plexes will be reactive in the processes of substitution
The active masses of catalyst based on compounds I,
Ia, and Ib as well as electrochemical and electrocataꢀ
lytic studies were performed by the previously
described procedure [14] using cyclic voltammetry
(CV) in 0.1 M KOH (reagent grade) aqueous solution
upon its saturation with argon or oxygen. The values of
redox potentials of working graphite electrode coated
of charged extra ligand Х– ОАс– in our case). Howꢀ
(
ever, as we showed previously [5, 8, 18], solvent nature
has no considerable effect on the position and intenꢀ
sity of bands in electronic absorption spectra and fluoꢀ
rescence emission for complexes, in particular, in
coordinating and nonꢀcoordinating solvents. The
with a layer of catalytically active paste (
=
Ered ox
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SYNTHESIS, CHEMICAL STABILITY, AND ELECTROCATALYTIC PROPERTIES
1269
438
418
448
A
b
a
2.5
c
2.0
1.5
1.0
0.5
0
611
c
547
657
560
400
500
600
λ
, nm
Fig. 1. Electronic absorption spectra of zinc complexes with porphyrins I and II in CHCl solution: ZnTPP (a) obtained by heatꢀ
3
ing of (AcO)Zn(NꢀMe)TPP (c) up to 300°C and direct chemical synthesis (b).
DTG, %/min
TG, %
100
b
a
a
b
0
90
80
70
60
50
–0.5
–1.0
–1.5
a
) DTG peak: 745.6
С
° b
–2.0
–2.5
–3.0
–3.5
a
) DTG peak: 244.4
°С
40
a
) DTG peak: 516.7°С
a
b
) DTG peak: 282.7°С
b
) DTG peak: 524.7°С
100
200
300
400
500
T, °C
600
700
800
900
Fig. 2. Derivatogram of complexes: a, (AcO)Co(NꢀMe)TPP (Ia); b, (AcO)Zn(NꢀMe)TPP (IIa) under argon atmosphere (TG,
solid line; DTG, dashed line).
electronic absorption spectra of complexes Ia and Ib siderable (by 50
°
C
under inert (argon) atmosphere
under air oxygen atmosphere [7])
decrease of onset degradation temperature ( o) for
retain threeꢀband structure in the region 550–700 nm,
in contrast to unsubstituted MPs, which show two
bands (500–600 nm) and characteristic split Soret
band at 430–450 nm (Fig. 1).
and by 150°C
T
porphyrin (Table 1). Under thermal degradation conꢀ
ditions in the absence of О2, this is mainly caused by
the localization of
π
bonds in the macrocycle due to
As would be expected, the methylation of the cenꢀ
tral nitrogen atom in the macrocycle II leads to a conꢀ decrease in its aromaticity (which is small as evidenced
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 10 2015

1270
BEREZIN et al.
(а)
(b)
A
2
eff, s–1
4
k
672
3
0.0008
0.0006
0.0004
0.0002
0
2
1
1
10
СHOAc, mol/L
0
400
500
600
700
λ
, nm
II
Fig. 3. (a) Alteration of electronic absorption spectrum of (AcO)Co (NꢀMe)TPP complex in the course of dissociation in
DMSO–HOAc solution (Δτ = 4 min, = 308 K, = 17.5 mol/L); (b) dependences of efficient rate constant for complex
Ia in reaction (1) on acetic acid concentration.
T
С
НОАс
T, K:
1, 298;
2, 308;
3, 318;
4, 328.
I, mA
OH–
HO2
O2
–
–1.5
–1.0
–0.5
0
III
2
Co3+/Co2+
II
I
1
2
1
0.5
1.0
1.5
0.4 0.2
0
–0.2 –0.4 –0.6 –0.8 –1.0 –1.2 –1.4
, V
E
II
Fig. 4. Voltammetric curves for working electrode coated with (AcO)Co (NꢀMe)TPP (Ia), electrolyte is 0.1 M KOH (1, in argon
–1
medium; 2, O ). Scan rate is 0.02 V s . I–III are electroreduction stages.
2
by electronic absorption spectrum and 1H NMR data absence of oxygen is always higher: for ligands by 65
[8]) due to the outꢀofꢀplane location of the Nꢀmethyl for Н2ТРР (II)) and 160 for its Nꢀmethylated anaꢀ
group and therefore the nonplanarity of the molecule log ( ), for complexes by 160 for ZnTPP (IIb),
as a whole. In its turn, the molecule nonplanarity and 135 for CoTPP (IIa), and 37 for (AcO)Zn(Nꢀ
unstable hybrid state of the substituted nitrogen atom Me)TPP (Ib). Thermal degradation conditions (Ar,
lead to the growth of electron donation of macrocycle O2) seem to have low effect on the thermal conditions
which considerably decreases o of thermoꢀoxidation. of Nꢀdemethylation stage for complex Ib (Table 1)
°C
°C
I
°C
°C
°C
I,
T
The thermal stability of the studied compounds in the because it proceeds via not oxidative mechanism.
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SYNTHESIS, CHEMICAL STABILITY, AND ELECTROCATALYTIC PROPERTIES
1271
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 10 2015

1272
BEREZIN et al.
Table 2. Kinetic parameters for reaction (1) of Nꢀsubstituted compounds Ia and Ib dissociation in HOAc–DMSO solution,
= 298 K
T
Activation entropy
HOAc concentraꢀ Efficient rate conꢀ
Activation energy,
≠
Compound
(AcO)CoII(NꢀMe)TPP (Ia
change ΔS ,
tion, mol/L
stant keff
×
103, s–1
E
, kJ/mol
J mol–1 K–1
–186 35
)
10.48
12.58
14.68
16.77
17.50
9.43
0.05*
45 10
0.06*
50
48
49
1
3
8
–167
1
0.12*
–176 10
–171 29
–160 46
–163 24
0.06*
0.04 0.0004
9.19 0.97
30.88 3.11
47.36 3.72
34.18 0.32
26.65 2.03
53 14
(AcO)ZnII(NꢀMe)TPP (Ib) [17]
38.4 7.0
72.8 14.3
66.0 15.1
47.8 11.2
27.9 6.3
10.90
14.10
16.20
17.50
–38
9
–57 10
–120 35
–189 30
*Calculated by Arrhenius equation.
Table 3. Potentials of redox transformations (in V) for electrodes coated with Nꢀsubstituted analog of macrocycle [H(Nꢀ
Me)TPP], its metal complexes [(X)M(NꢀMe)P], and their unsubstituted derivatives (scan rate
= 0.02 V s–1)
V
M3+
M2+
Process I
EaIn ErIed ox
Process II
Process III
E
1/2(O2)
Compound
II
II
red ox
III
III
EcIat
E
E
II
cat
III
cat
Ecat Ean Ered/ox
E
E
E
E
red ox
an
an
H
2TPP (II) [11, 14]
–0.664 –0.575 –0.620 –1.081 –0.968 –1.025 –0.275
–1.210 –1.000 –1.110 –0.230
0.130 0.260 0.200
CoTPP (IIa
[11, 14]
)
–0.580* –0.480* –0.530*
–0.573 –0.454 –0.514 –0.772 –0.619 –0.695 –1.072
–0.552 –0.440 –0.496 –0.754 –0.638 –0.696 –1.089
H(NꢀMe)TPP(
(AcO)Zn(Nꢀ
I)
–
–
–
–
–0.242
–0.189
Me)TPP (Ib
)
(AcO)CoII(Nꢀ
0.172 0.309 0.241 –0.551 –0.476 –0.514 –0.742 –0.613 –0.678 –1.103 –0.917 –1.010 –0.110
Me)TPP (Ia
)
2+
1+
–
*Transition
M
M
or L
L .
The number of peaks on DTG curve, i.e., the numꢀ gen and argon atmosphere proceeds through lowꢀtemꢀ
ber of destruction stages, is dependent on the nature of
ligand and metal. Thus, two stages of thermal destrucꢀ
tion under an argon atmosphere are observed for
perature (220–295°С under inert atmosphere) stage of
elimination of the Nꢀsubstituent (R) and covalently
bound extra ligand (X) (3), in our case СН3ꢀ and
CH3COOꢀ, with evaporation of methyl acetate moleꢀ
cule (Fig. 2, Table 1). Thus, for example, this is eviꢀ
denced by weight loss value under inert atmosphere,
which for complexes (AcO)Zn(NꢀMe)TPP (Ib) and
(AcO)Co(NꢀMe)TPP (Ia) agrees well with calculated
H(NꢀMe)TPP (To = 421 and 479
complexes irrespective of ligand nature. Onset
destruction temperatures are 475 and 715 for
CoTPP (IIa) and 481 and 689 for (AcO)Co(Nꢀ
Me)TPP (Ia) (Table 1, Fig. 2).
°C) and for cobalt
°C
°C
value (
Δ
mt and me equal to 9.85, 9.93% and 9.52,
Δ
As distinct from Nꢀsubstituted ligand, the destrucꢀ
tion of the corresponding complexes under both oxyꢀ 10.50%, respectively, Table 1). On account of dealkyꢀ
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SYNTHESIS, CHEMICAL STABILITY, AND ELECTROCATALYTIC PROPERTIES
1273
lation stage, the thermal stability of the Nꢀsubstituted drop of stability for the complexes with Nꢀsubstituted
complexes in contrast to unsubstituted compounds ligand ( ) toward dissociation in ionizing medium as
considerably decreases (by 110 in argon and 230– compared with similar planar complexes (IIa IIb) [3]
250 in air).
I
°С
,
°
С
corresponds to the notion on the negative effect of
nonplanar structure of macrocycle and factors accomꢀ
panying distortion on the stability of its complexes [17,
18, 21].
The further intramolecular rearrangement of comꢀ
pounds Ia and Ib including conversion of unstable p3
valence state of 21N atom in thermally demethylated
compounds leads to formation of virtually flat unsubꢀ
stituted complexes CoTPP and ZnTPP, which is conꢀ
The dissociation of (AcO)Co(NꢀMe)TPP) (Ia) in
DMSO–HOAc medium is described by unusual for
solvoprotolytic dissociation reaction nonlinear depenꢀ
firmed by the proximity (6°С for cobalt complexes) of
dence of efficient rate constant for reaction (1) (k
eff, s–1)
To values for the second stage of (X)M(NꢀMe)TPP
degradation to the onset degradation temperatures of
the corresponding MTPP complexes (Fig. 2, Table 1),
the similar type of thermograms in the considered
temperature range, and complete coincidence of elecꢀ
tronic absorption spectrum for the product of RX
elimination obtained by the heating of the initial Nꢀsubꢀ
on acetic acid concentration (СНОАс) (Table 2) with
maximum at about 14.5 mol/L acetic acid (Fig. 3b).
Similar dependence indicating the similarity of dissoꢀ
ciation reaction mechanisms for these MPs was
observed previously in similar solvent system for
(AcO)Zn(NꢀMe)TPP (Ib) [17] (Table 2). The comꢀ
plication of concentration dependence in reaction (1)
may be caused by the change of attacking species upon
alteration of solvent composition by either participaꢀ
tion of several species simultaneously, possibly includꢀ
stituted compound up to 300
with the spectrum of the corresponding unsubstituted
°С under inert atmosphere
max
CHCl3
metal porphyrin (
= 418, 547 nm) (Fig. 1).
λ
ing DMSO · Н+, НОАс H+, HOAc, etc.
·
Under thermoꢀoxidation conditions, the degradaꢀ
tion of the macrocyle to form gaseous nitrogen and
carbon oxides and water [19] follows immediately
after dealkylation stage and is characterized by exoꢀ
thermal peaks on DTG curve, whose number depends
on metal nature [7], whereas these processes are sepaꢀ
rated under inert atmosphere (To = 267 and 468 for
The comparison of stability characteristics for Nꢀ
substituted complexes in solution and in solid state
indicates the lack of linear correlation between them.
Thus, complex (AcO)Co(NꢀMe)TPP (Ia) is more
kinetically stable in dissociation reaction as compared
with its zinc analog, however, its destruction under
polytermal conditions begins at lower temperature
than for Ib due to Nꢀdemethylation stage.
(AcO)Zn(NꢀMe)TPP and To = 223 and 468°C for
(AcO)Co(NꢀMe)TPP) (Fig. 2, Table 1). It is seen that
the scheme of degradation of Nꢀsubstituted metal porꢀ
phyrins Ia and Ib under argon atmosphere shows a
certain feature, process mechanism is unknown.
Thus, the removal of the Nꢀsubstituent from the
molecules of Nꢀsubstituted porphyrins occurs not
only in solutions on exposure to organic bases or
strong heating [1, 8] but also in solid state on heating
irrespective of experiment medium (Ar, O2).
Another important aspect that determines the staꢀ
bility of coordination compounds in solutions is their
ability to remain in undissociated form. To analyze the
effect of nonplanar structure of metal porphyrin upon
introduction of Nꢀmethyl substituent on the chemical
stability of its complexes toward protonꢀreleasing
reagent, we studied the kinetics of dissociation reacꢀ
tion (1) for zinc(II) and cobalt(II) compounds with
On the basis of analysis of kinetic stability of
obtained complexes toward chemicals and thermal
impact, we draw conclusions on the fundamental posꢀ
sibility to use H(N–Me)TPP ligand (I) and its Co(II)
and Zn(II) complexes in the electrocatalysis of
cathodic electroreduction of molecular oxygen ((4a)–
(4c)), which leads to processes that proceed in the
components of lowꢀtemperature chemical sources of
electric energy [11, 22].
HO2−
H2O2,
4OH–.
O2 + 2e + H2O
O2 + 2e + 2H+
O2 + 4e + 4H2O
+ OH–,
(4a)
(4b)
(4c)
Under argon atmosphere, the cyclic
I
,
E
curves of
Nꢀmethylꢀsubstituted ligand H(NꢀMe)TPP (
cobalt(II) and zinc(II) complexes show three general
stages of electroreduction process ( II, and III, Table 3)
related to the addition of electrons to the electron
I), its
Nꢀsubstituted porphyrin I in DMSO–HOAc medium.
The obtained results show (Table 2) that, under comꢀ
parable conditions at acetic acid concentration in
DMSO of 9.43–17.5 mol/L, the cobalt complex with
H(NꢀMe)TPP (Ia) undergoes dissociation to porphyꢀ
rin dication Н3(NꢀМе)ТРР2+ (Fig. 3a) by a factor of
I,
π
system of the macrocycle. The first stage of the elecꢀ
troreduction of H(NꢀMe)TPP and its derivatives under
observed conditions proceeds by 0.1 V more “easily”
400–700 slower as compared with zinc complex (Ib
)
than in the case of unsubstituted macrocycle II
For (AcO)CoII(NꢀMe)ТPP complex, we detected
Co2+ Co3+ transition with Ered/ox = 0.241 V, which
.
[17]. The decrease of stability for complex Ib in comꢀ
parison with Ia agrees well with the growth of the valꢀ
ues of nonplanar shift of the corresponding metal
atoms from 0.56 to 0.65 Å and their average radii from is shifted toward higher potentials by ~40 mV as comꢀ
0.077 to 0.080 Å [1, 20], which leads to the further disꢀ pared with the closest analogs, for example CoTPP
tortion of the flat structure of the compounds. The [14, 22], which agrees well with the stabilization of
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1274
BEREZIN et al.
2. S.ꢀI. Aizawa, Y. Tsuda, Y. Ito, et al., Inorg. Chem. 32
,
metals in lower oxidation states by this ligand. We
failed to ascribe valueЕred/ox= –0.530 V to Co+
1119 (1993).
Co2+ or L
L–1 transition.
3. T. E. Clement, L. T. Nguyen, R. G. Khoury, et al., Hetꢀ
erocycles 45, 651 (1997).
4. A. M. Stolzenberg, S. W. Simerly, B. D. Steffey, and
H. G. Scott, J. Am. Chem. Soc. 119, 11843 (1997).
5. D. B. Berezin, N. V. Zharnikova, V. G. Andrianov,
et al., Russ. J. Coord. Chem. 28, 325 (2002).
6. I. V. Sazanovich, A. van Hoek, A. Yu. Panarin, et al.,
To analyze the electrocatalytic activity of
(AcO)M(NꢀMe)TPP in the process of electroreducꢀ
tion of molecular oxygen, we took
I,E curves (Fig. 4,
curve ) corresponding to the ultimate saturation of
2
electrolyte with О2. The growth of electrocatalytic
activity leads to a considerable depolarization effect
that appears as a shift of molecular oxygen electroreꢀ
J. Porph. Phthaloc. 9, 59 (2005).
7. D. B. Berezin, E. N. Mis’ko, E. V. Antina, and
duction wave E1/2(O2) to the positive values region.
M. B. Berezin, Russ. J. Gen. Chem. 76, 482 (2006).
The high electrolytic effect of H(NꢀMe)TPP
1/2(O2) = –0.242 V) and (AcO)CoII(NꢀMe)ТPP
0.111 V) as compared with the closest analogs
8. D. B. Berezin, NꢀSubstituted Porphyrinoids: Structure,
Spectroscopy, and Reactivity (LAMBERT Acad. Pubꢀ
lishing, Saarbrücken, 2012).
9. P. Hambright, The Porphyrin Handbook, Ed. by
K. M. Smith, K. M. Kadish, and R. Guilard, (Acad.
Press, San Diego, 2000).
(
E
⎯
(
H2TPP (–0.275 V) and CoIITPP (–0.230 V) [14] is
caused by the high electronꢀreleasing ability of the Nꢀ
substituted macrocycle.
The efficient number of electrons calculated by the
Randles–Sevcik equation is close to four, typical value
for oxygen electroreduction in the presence of cobalt
complex Ia, this indicates that the reaction proceeds
via fourꢀelectron mechanism (4c) [11, 14, 22].
Thus, the electrocatalytic activity of the studied
tetrapyrrole compounds in the reaction of electroreꢀ
duction of molecular oxygen increases in the following
10. D. B. Berezin and E. N. Mis’ko, Russ. J. Inorg. Chem.
48, 1354 (2003).
11. J. Masa, K. Ozoemena, W. Schuhmann, and
J. H. Zagal, J. Porph. Phthaloc. 16, 761 (2012).
·
12. L. LatosꢀGrams
z
y
nski, Inorg. Chem. 24, 1681 (1985).
13. D. B. Berezin, O. V. Shukhto, Vu Thi Thao. et al., Russ.
J. Inorg. Chem. 59, 1522 (2014).
14. M. I. Bazanov, N. M. Berezina, D. R. Karimov, and
order: H2TPP
(AcO)Zn(NꢀMe)TPP
<
H(NꢀMe)TPP
< CoTPP <
D. B. Berezin, Russ. J. Electrochem. 48, 905 (2012).
<
(AcO)CoII(NꢀMe)ТРP
)
15. K. Burger, Solvation, Ionic and Complex Formation
Reactions in Nonaqueous Solvents (Akademiai Kiado,
Budapest, 1983; Mir, Moscow, 1984).
16. E. V. Antina, V. P. Barannikov, M. B. Berezin, and
A. I. V’yugin, in Problems of Solution Chemistry and
LiquidꢀPhase Materials Processing (Institute of Soluꢀ
tion Chemistry, Ivanovo, 2001), p. 217 [in Russian].
(Table 3); Nꢀsubstituted cobalt(II) complex (comꢀ
pound Ia) shows the best ratio of electrocatalytic
activity and thermodynamic stability in solution and
solid state. One can suppose that the ability of complex
(AcO)CoII(NꢀMe)ТPP) to undergo electrochemiꢀ
cally reversible reductive migration of the Nꢀsubstituꢀ
ent to metal to form unstable in this case Co3+ state
increases the electrocatalytic activity [1].
17. D. B. Berezin, O. V. Shukhto, and P. A. Shatunov, Russ.
J. Gen. Chem. 78, 997 (2008).
18. D. B. Berezin, Macrocyclic Effect and Structural Chemꢀ
istry of Porphyrins (Krasand, Moscow, 2010) [in Rusꢀ
sian].
19. W. W. Wendlandt, Thermal Methods of Analysis (Wiley,
NewꢀYork, 1974; Mir, Moscow, 1978).
20. Yu. Yu. Lur’e, Handbook of Analytical Chemistry
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ
tion for Basic Research (project nos. 13ꢀ03ꢀ00557 a
(synthesis and kinetic studies of compounds) and 14ꢀ
03ꢀ31232 mol_a (electrochemical studies)) and the
Ministry of Education and Science of the Russian
Federation (thermogravimetric measurements).
(Khimiya, Moscow, 1979) [in Russian].
21. O. A. Golubchikov, S. G. Pukhovskaya, and E. M. Kuvshiꢀ
nova, Advances in Porphyrin Chemistry, Ed. by O. A. Goꢀ
lubchikov (St. Petersburg State Univ., St. Petersburg,
2004), Vol. 4, pp. 45–75 [in Russian].
REFERENCES
22. M. I. Bazanov, Izv. Vyssh. Uchebn. Zaved., Khim.
1. D. K. Lavallee, The Chemistry and Biochemistry of
NꢀSubstituted Porphyrins (VCH Publishers, New York,
1987).
Khim. Tekhnol. 48 (7), 31 (2004).
Translated by I. Kudryavtsev
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 60 No. 10 2015