Kinetics of the formation and dissociation of complexes of cobalt(II) with meso-Phenylocta(methyl)porphyrin derivatives in organic solvents

Article abstract of DOI:10.1134/S0036024410090128

The influence of the deformation of the tetrapyrrole aromatic macrocycle and the electronic effects of substituents on the kinetic parameters in the formation and dissociation reactions of cobalt complexes of porphyrins are discussed on the basis of a study of the kinetics of the formation and dissociation of cobalt complexes with meso-phenyl octa(methyl)porphyrin derivatives: 5,10,15-triphehyl octa(methyl) porphyrin; 5,15-bis-(3′, 5′-di-tert-butyl phenyl) octa(methyl)porphyrin; 5,10,15-tris-(3′, 5′-di-tert-butyl phenyl) octa(methyl)porphyrin; and 5,10,15,20-tetrakis- (3′,5′-di-tert-butyl phenyl) octa(methyl)porphyrin in acetic acid and pyridine.

Full text of DOI:10.1134/S0036024410090128

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2010, Vol. 84, No. 9, pp. 1516–1519. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © E.M. Kuvshinova, A.S. Semeikin, S.A. Syrbu, O.A. Golubchikov, 2010, published in Zhurnal Fizicheskoi Khimii, 2010, Vol. 84, No. 9, pp. 1666–1669.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Kinetics of the Formation and Dissociation of Complexes
of Cobalt(II) with mesoꢀPhenylocta(methyl)porphyrin Derivatives
in Organic Solvents
E. M. Kuvshinova, A. S. Semeikin, S. A. Syrbu, and O. A. Golubchikov
Ivanovo State University of Chemical Technology, Ivanovo, 153460 Russia
eꢀmail: gornoukhina@isuct.ru
Received July 6, 2009
Abstract—The influence of the deformation of the tetrapyrrole aromatic macrocycle and the electronic
effects of substituents on the kinetic parameters in the formation and dissociation reactions of cobalt comꢀ
plexes of porphyrins are discussed on the basis of a study of the kinetics of the formation and dissociation of
cobalt complexes with mesoꢀphenyl octa(methyl)porphyrin derivatives: 5,10,15ꢀtriphehyl octa(methyl) porꢀ
phyrin; 5,15ꢀbisꢀ(3',5'ꢀdiꢀtertꢀbutyl phenyl) octa(methyl)porphyrin; 5,10,15ꢀtrisꢀ(3',5'ꢀdiꢀtertꢀbutyl phenyl)
octa(methyl)porphyrin; and 5,10,15,20ꢀtetrakisꢀ(3',5'ꢀdiꢀtertꢀbutyl phenyl) octa(methyl)porphyrin in acetic
acid and pyridine.
DOI: 10.1134/S0036024410090128
INTRODUCTION
the formation and dissociation of cobalt (II) comꢀ
plexes with mesoꢀphenyl octa(methyl)porphyrin
derivatives in acetic acid and pyridine were studied in
a continuation of our investigations of the structural
It was shown in [1–5] that introducing a large
number of peripheral substituents into the mesoꢀ and
ꢀpositions of the tetrapyrrole porphyrin macrocycle
leads to a substantial change in the latters physicoꢀ
chemical and coordination properties. the kinetics of properties.
β
features of porphyrins I–IV on their coordination
R
Me
Me
Me
I
: R = R1 = R2 = Ph, R3 = H;
Me
R3
Me
R1
NH
N
II: R = R2 = 3,5ꢀdiꢀtertꢀbutylꢀPh, R1 = R3 = H;
III: R = R1 = R2 = 3,5ꢀdiꢀtertꢀbutylꢀPh, R3 = H;
IV: R = R1 = R2 = R3 = 3,5ꢀdiꢀtertꢀbutylꢀPh.
N
HN
Me
Me
R2
Me
EXPERIMENTAL
Porphyrins Н2Р (I IV) were synthesized by the
methods described in [6] and purified by repeated colꢀ
umn chromatography on silica gel and aluminum
oxide. Their identity was verified by thin layer chromaꢀ
tography on silufole. the electronic absorption spectra
Purity was controlled by the electronic absorption
spectra.
Cobalt (II) acetate (chemically pure grade) was
purified by recrystallization from aqueous acetic acid
and dehydrated. To accomplish this, cobalt acetate
was boiled for three hours with acetic anhydride and
filtered. Salt crystals were filtered; the filter cake was
–
of the porphyrins were identical to those presented in washed with hexane and dried in a vacuum desiccator
[6]. the cobalt complexes were obtained by boiling containing potassium hydroxide.
porphyrins with a 50ꢀfold molar excess of cobalt aceꢀ
tate in DMF and purified by chromatography over
aluminum oxide of level III Brockmann activity.
Pyridine (pure grade) was held over potassium
hydroxide and distilled twice on a dephlegmator [7].
Acetic acid was dehydrated by fractional freezing and
Chloroform was used as the developer and the eluent. distilled on the dephlegmator. the water content in the
1516

KINETICS OF THE FORMATION AND DISSOCIATION OF COMPLEXES
1517
Table 1. Effective rate constants for the complexation of porphyrins
is the analytical wavelength)
I
–IV with cobalt acetate in pyridine and acetic acid
(
λ
Pyridine
cCo(OAc) = 4.7
10–3 mol/l
Acetic acid
×
×
cCo(OAc) = 2.7
10–3 mol/l
Porphyrin
Т, К
2
2
λ
, nm
kef
104, s–1
×
λ
, nm
kef
×
104, s–1
I
318
328
338
526
1.40 0.13
2.8 0.3
5.4 0.2
654
550
660
1.31 0.05
2.93 0.17
6.7 0.5
II
2.0 0.1
5.6 0.3
15.4 0.7
0.53 0.03
1.56 0.07
4.51 0.18
III
IV
318
328
338
318
328
338
522
406
0.245 0.015
0.82 0.02
2.43 0.14
0.55 0.03
1.42 0.09
3.5 0.2
Note: Electronic absorption spectrum of the reaction mixture did not change over 5 days at 338 K for II in pyridine and for IV in acetic acid.
Table 2. Kinetic parameters of the formation reaction of cobalt complexes of porphyrins
I
–IV
Acetic acid
Porphyrin
Pyridine
k3218
×
104, l/(mol s)
k3218
×
104, l/(mol s)
290 30
Еа, kJ/mol
А
, s–1
Еа, kJ/mol
А
, s–1
I
480 20
740 40
74
91
98
1
2
2
4.77
6.68
2.36
60
1
2.1
×
108
II
III
IV
185
9
52
4
6
102
82
2
2
3.15
3.29
110
Note: The reaction rate is very slow for IV in acetic acid and for II in pyridine.
solvents was controlled by the Fisher titration method tion reactions of cobalt complexes of porphyrins
[8] and was no higher than 0.03%. Trifluoroacetic acid are presented in Tables 1–3.
(TFA) (pure grade) was mixed with concentrated sulꢀ
I–IV
furic acid at a ratio of 10 : 1. Dehydrated TFA was disꢀ
tilled from the mixture on the dephlegmator at atmoꢀ
spheric pressure; the fraction with boiling point 72–
73°С was collected and then redistilled.
RESULTS AND DISCUSSION
The complexation of porphyrins
I–IV with
Со(АсО)2
:
The rate of the formation and dissociation of cobalt
porphyrin complexes (
I
–
IV) was spectrophotometriꢀ
Н2Р + Со(АсО)2
obeys the kinetic equation
СоР + 2АсОН
(1)
cally measured using a Hitachi Uꢀ2000 spectrophoꢀ
tometer with thermostatted cells in the temperature
range of 308—338 K. The temperature did not exceed
0.1 K.
d
[H2P]/dt =–
k
[H2P][Co(AcО)2]n,
(2)
where [Н2Р] is porphyrin, СоР is its cobalt complex,
and is the rate constant of reaction (1).
Clear isobestic points were observed in the spectra
of all of the reaction systems. The optical density of the
solution was measured 15–20 times at fixed time
intervals on a single wavelength for each experiment.
The rms error related to determining the effective rate
constants (keff) was 4–8% at a confidence level of 95%.
k
In every case, the formation of cobalt porphyrin
complexes IV is of the first kinetic order with
I–
respect to porphyrin. The experiments were conꢀ
ducted under the conditions of a large (~200ꢀfold)
The kinetic parameters in the formation and dissociaꢀ excess of salt relative to porphyrin. This allowed us to
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KUVSHINOVA et al.
Table 3. Kinetic parameters of the dissociation reaction of cobalt complexes of porphyrins
tem ( is the analytical wavelength; TFA = 0.1 mol/l)
I
–
IV in an acetic acid–TFA sysꢀ
, s–1
2.47
λ
c
318
1.5
СоР
λ
, nm
Т, К
kef
×
103, s–1
Е
а, kJ/mol
А
k
×
103, l/(mol s)
I
554
505
658
308
318
328
308
318
328
308
318
328
1.58 0.08
1.93 0.12
2.33 0.14
0.13 0.07
0.21 0.01
0.346 0.017
0.63 0.04
0.92 0.06
1.33 0.08
5.8 0.3
0.63 0.03
2.77 0.18
16
40
30
2
2
1
II
III
2344
234
Note: IV dissociates when dissolved in water.
calculate the effective constants (keff) using the equaꢀ
tion
The kinetics of dissociation of porphyrins
complexes,
СоР + 4Н+
(where Н4Р2+ is porphyrin dication) was investigated
I
–
IV
Со2+ + Н4Р2+
(5)
keff = (1/τ)ln [А0
–
А∞]/[
А
–
А∞],
(3)
where А0, А, and А∞ are the optical densities of the
in acetic acid solutions of TFA with an approximately
1000ꢀfold molar excess of TFA relative to CoP. Under
these conditions, the reaction is of the first kinetic
order with respect to CoP, as is demonstrated by the
ꢀ
solution at the initial point in time, at time
the end of the reaction, respectively.
τ
, and at
The dependence of the CoP I and CoP II rate of
formation on the concentration of salt was studied to
determine the kinetic order of the reaction with
respect to cobalt acetate in pyridine and acetic acid.
These curves were linear lines in logarithmic coordiꢀ
linear dependence in coordinates
–
log(cCoP/cCoP)
τ
ꢀ
where сСоР and
are the current and initial concenꢀ
cCoP
trations of cobalt complexes of porphyrins
I–IV, and
τ
nates. The slopes of the lines tangent to CoP I in pyriꢀ
is time. The effective rate constants (keff) of dissociaꢀ
tion calculated from Eq. (3) remain satisfactorily staꢀ
dine and CoP II in acetic acid were equal to 1 within
the determined error. The kinetic order with respect to
Со(АсО)2 in pyridine and acetic acid in the porphyrin
I–IV complex formation reaction is therefore of the
first order. The second order rate constants were calꢀ
culated according to Eq. (4):
ble (Table 3). The 1 +
n order rate constants were calꢀ
culated from the equation
n
.
(6)
k1 + n = keff/cТFA
The dependency of keff on cТFA for CoP II shown in
Figure 2 demonstrates that the kinetic order of reacꢀ
tion (5) with respect to a solvated proton is equal to 0.5
within the determined error. Apparently the fractional
order of the reaction is due to the TFA behaving as a
weak electrolyte in acetic acid solutions.
.
(4)
k = keff
c
Co(AcO)2
−
logkeff
The activation energies for the formation and disꢀ
3.5
sociation of CoP
I–IV were calculated using the
Arrhenius equation:
Е/RT
Ае–
k
=
.
(7)
The preꢀexponential factor was calculated from the
equation
α
3.4
3.3
ln
Judging from the data presented in Tables 1–3, the
reactivity of porphyrin IV and their cobalt comꢀ
A
= lnk1 +
+
Е/
RT
.
(8)
n
I–
plexes is determined by two main factors: the degree of
the deformation of the porphyrin macrocycle due to
its periphery being overloaded with bulky substituents
and the electronic influence of the latter.
0.75
0.85
0.95
−
logcТFA
The logkeff dependence on the logcTFA of the dissociaꢀ
tion of CoP II in acetic acid at 328 K.
The tertꢀbutyl groups in porphyrins II
I effect that leads to an increase in the basicity of the
–IV exert a
+
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KINETICS OF THE FORMATION AND DISSOCIATION OF COMPLEXES
1519
porphyrins and a strengthening of their protonated tion energy of complexation reactions in pyridine and
NH bonds in the transition state of complexation acetic acid, confirming once again the validity of the
reactions. In metalloporphyrins, the fractional negaꢀ proposed interpretation of our results.
tive charge rises on the nitrogen atoms being attacked
by solvated protons in the dissociation reactions of the
complexes.
In the dissociation of cobalt complexes, the most
distorted porphyrin (the one containing the four
tertꢀbutyl groups of IV) reacts incomparably faster
than any of the others, as might be expected (Table 3).
The least distorted CoP (II) exhibits the least activity.
The distortion from the planarity of the aromatic
ring of
tion to the number of mesoꢀaryl substituents [1, 2]. In
our case, II III IV. On the one hand, deformaꢀ
βꢀocta(methyl)porphyrin increases in proporꢀ
We might expect CoP III to be less stable than CoP
but this is not true. The solvation factors are different
in complexes that contain and do not contain tert
butyl groups, and it is possible that these factors influꢀ
I,
<
I
≈
<
tion is accompanied by increased accessibility of the
porphyrin H2N4 coordination centre to the attack of
ꢀ
solvated metal complexes. On the other hand, it leads ence the reaction kinetics.
to isolation of the pyrrole nucleus, an increase in the
density of electrons on the nitrogen atom, and conseꢀ
quently raises the basicity of the porphyrins.
REFERENCES
In the case of pyridine, the reactivity series for the
formation of cobalt complexes increases I > IV > III > II;
i.e., the least active porphyrin is the one least distorted:
1. N. S. Dudkina, P. A. Shatunov, E. M. Kuvshinova,
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porphyrin II. The most active porphyrin, however, is
not the one most deformed (IV); it is the I porphyrin.
The electronic effects of the tertꢀbutyl groups
strengthening the NH bonds were obviously dominant
in this case.
Porphyrin IV has extremely low reactivity induced
by complex formation in acetic acid. In this case, the
+I effect of the tertꢀbutyl groups and the effect of
deformation are additive. As a result, the basicity of
the tertiary nitrogen atoms increases sharply, and they
form strong hydrogen bonds with the AcOH moleꢀ
cules blocking the coordination centre of the porphyꢀ
rin. As might be expected, the least deformed porphyꢀ
rin (II) is the most active substance in the reaction
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1972).
with Со(АсО)2. The reactivity ratios of porphyrins
I
and III is determined by the +I effect of the substituꢀ
ents.
It should be noted that the growth rate constants
were accompanied by a regular decrease in the activaꢀ
8. I. Denesh, Titration in Nonaqueous Media (Mir, Mosꢀ
cow, 1971), p. 130 [in Russian].
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2010