Kinetics of the formation of copper β-octaphenylporphyrin complexes in pyridine and acetic acid

Article abstract of DOI:10.1134/S0036023610090275

The formation kinetics of copper β-octaphenylporphyrin complexes in pyridine and acetic acid is reported and is compared with that of copper β-octamethylporphyrin and dodecaphenylporphyrin complexes. The introduction of electron-donating or electron-withdrawing substituents in the β-positions of the porphyrin macrocycle change the rate of the complexation reaction by at most one order of magnitude. On passing from the planar porphyrin macrocycle to the heavily distorted one, the rate of the reaction in pyridine (electron donor solvent) and acetic acid (electron acceptor solvent) increases and decreases, respectively, by several orders of magnitude.

Full text of DOI:10.1134/S0036023610090275

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 9, pp. 1494–1498. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.G. Pukhovskaya, V.A. Efimovich, A.S. Semeikin, O.A. Golubchikov, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 9, pp. 1580–1584.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Kinetics of the Formation of Copper βꢀOctaphenylporphyrin
Complexes in Pyridine and Acetic Acid
S. G. Pukhovskaya, V. A. Efimovich, A. S. Semeikin, and O. A. Golubchikov
Ivanovo State University of Chemistry and Technology, pr. Engelsa 7, Ivanovo, 153460 Russia
Received February 25, 2009
Abstract—The formation kinetics of copper
reported and is compared with that of copper
plexes. The introduction of electronꢀdonating or electronꢀwithdrawing substituents in the
porphyrin macrocycle change the rate of the complexation reaction by at most one order of magnitude. On
passing from the planar porphyrin macrocycle to the heavily distorted one, the rate of the reaction in pyridine
electron donor solvent) and acetic acid (electron acceptor solvent) increases and decreases, respectively, by
β
ꢀoctaphenylporphyrin complexes in pyridine and acetic acid is
ꢀoctamethylporphyrin and dodecaphenylporphyrin comꢀ
ꢀpositions of the
β
β
(
several orders of magnitude.
DOI: 10.1134/S0036023610090275
The most important coordination property of the and on the electronic structure and geometry of the
porphyrins is their capacity to form a complex with porphyrin macrocycle [1–7].
any metal of the periodic table. The most favorable
Continuing the systematic investigation of the
substances for porphyrin coordination are salts of douꢀ effect of the system of substituents in the porphyrin
macrocycle on its coordination properties, we studied
bly charged transition metal ions [1].
the kinetics of complexation between ꢀoctapheꢀ
β
Studies of complexation between porphyrins and
nylporphyrin ( ) and copper(II) acetate in pyridine
I
3d
ꢀmetal salts in organic solvents demonstrated that and acetic acid and compared the results with earlier
the rate of this reaction depends markedly on the comꢀ data for
βꢀoctamethylporphyrin (II) and dodecapheꢀ
position and structure of the metal solvato complex nylporphyrin (III) [3, 4].
Me
Me
Me
Me
N
HN
N
HN
N
HN
NH
N
NH
N
NH
N
Me
Me
Me
Me
(
I)
(II)
(III)
EXPERIMENTAL
,3,7,8,12,13,17,18 Octaphenylporphyrin (I) Ethꢀ
ylmagnesium bromide (2.13 g, 16 mmol) dissolved in
diethyl ether (10 ml) was added to 2ꢀdimethylamiꢀ
nomethylꢀ3,4ꢀdiphenylpyrrole (4.15 g, 15.4 mmol) 0.41 mg (0.043 mmol), or 51.8%. Porphyrin
acid was stirred in a nitrogen atmosphere for 24 h.
Thereafter, the solution was cooled and was neutralꢀ
2
ꢀ
ized with sodium carbonate. The resulting crystals of
were collected on a filter. The yield of the product was
was
I
I
dissolved in xylene (200 ml), and the mixture was purified chromatographically on alumina, activity II
stirred for 20 h in a nitrogen atmosphere. The resulting
precipitate was filtered and was then purified by
extraction with a benzoquinone solution in benzene in
a Soxhlet apparatus and by recrystallization from dimꢀ
ethylformamide [8]. A solution of the magnesium
and III. The eluent was chloroform. The purity of the
product was checked by thin layer chromatography on
aluminum plates coated with a 0.5ꢀmmꢀthick F254 silꢀ
ica gel layer (Merck). The mobile phase was CHCl –
3
complex of
I
(0.80 mg, 0.083 mmol) in glacial acetic
C
6
H
6
(1 : 1). Electronic absorption spectrum,
λ
max
1494

KINETICS OF THE FORMATION
1495
0
H P H P
A
.5
log(
c
/c
)
2
2
2
2
1
1
0
1
1.0
0.8
0.6
.0
.5
.0
.5
0
2
0.4
3
0.2
0
50
100
150
200
250
, min
τ
450 500 550 600 650 700 750
λ
, nm
0
Fig. 2. log(c /c
)
versus
τ
for the formation of copꢀ
H P H P
Fig. 1. Evolution of the electronic absorption spectrum in
porphyrin coordination to copper acetate in acetic acid at
2
2
I
per–porphyrin
and (3) 328 K.
I
complexes in pyridine at ( ) 348, ( ) 338,
1
2
298 K.
and current porphyrin concentrations; Fig. 2). Kinetic
experiments were carried out under conditions of a
hundredfold excess of the salt over the porphyrin. This
allowed the apparent rate constant (kapp) of reaction
(
logε ): 635 (3.47), 583 (3.95), 550 (4.04), 518 (4.23),
23 (5.13) (chloroform). These data are in excellent
4
agreement with the literature [8, 9].
Copper acetate (analytical grade) was purified by
recrystallization from aqueous acetic acid and by
dehydration at 380–390 K [10].
Acetic acid was dehydrated by fractional freezing
followed by fractional distillation. The residual water
content of acetic acid was 0.02% or below, as deterꢀ
mined by Fischer titration. Pyridine (reagent grade)
was purified using a standard technique. The residual
(
1) to be calculated via the equation
kapp = (1/ )ln[(A0 A∞)/(
where А0 and А∞ are, respectively, the initial absorꢀ
τ
–
A
–
A∞)],
(3)
, А,
bance of the solution and those at the point in time
t
and after the reaction (Table 1).
The order of the reaction with respect to the salt (n)
water content of pyridine was not higher than 0.03% is 0.5 in acetic acid [7] and 0.4 in pyridine [13]. The
[
11, 12].
The rate of complexation of
fractional order of the reaction in the former case is
was measured on due to the fact that copper acetate in acetic acid exists
I
Specord Mꢀ400 and Hitachi Uꢀ2000 spectrophotomꢀ as an equilibrium system of dinuclear and mononuꢀ
eters between 288 and 348 K using temperatureꢀconꢀ clear solvato complexes, with the equilibrium shifted
6
trolled cells with groundꢀglass joints. The temperature
variations did not exceed 0.1 K. Wellꢀdefined isosꢀ
bestic points were observed in the spectra of all reactꢀ
ing systems (Fig. 1).
toward the dinuclear complexes (Kdim > 10 L/mol)
Table 1. Apparent rate constants for porphyrin I coordination
to copper acetate in acetic acid and pyridine
RESULTS AND DISCUSSION
3
–1
kapp
×
10 , s in acetic acid
298 K 308 K
21 ± 2
10 , s in pyridine
338 K 348 K
103, λa*,
cCu(OAc)2
×
The complex between porphyrin
tate forms via the reaction
I and copper aceꢀ
mol/L
nm
288 K
H P + Cu(OAc)
2
→
CuP + 2AcOH
(1)
2
0.0625
651 3.4 ± 0.5 7.8 ± 0.5
and obeys the rate law
5
–1
n
kapp
×
d[H P]/d
2
t
= –
k
[H P][Cu(OAc) ] ,
2
(2)
2
where H2P is the porphyrin, CuP is its complex, and
is the rate constant of reaction (1). The complexation
reaction of ꢀoctaphenylporphyrin is firstꢀorder with
k
3
28 K
β
1.48
513 6.6 ± 0.4 12.3 ± 0.6 26.4 ± 0.2
respect to the ligand. This is indicated by the linearity
0
0
H2P
of log(c /c
)
as a function of
τ
(
c
are the initial * Analytical wavelength.
H P
H P
2
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

1496
PUKHOVSKAYA et al.
Table 2. Kinetic parameters of βꢀoctaphenylporphyrin (I), βꢀoctamethylporphyrin (II), and dodecaphenylporphyrin (III) coꢀ
ordination to copper acetate in acetic acid and pyridine
2
.4
98
298
1.5
105,
≠
≠
S ,
k1
×
Δ
E
,
Δ
S
,
k
,
Δ
E
,
Δ
L0.4 mol–0.4 s–1
kJ/mol
J/(mol K)
L0.5 mol–0.5 s–1
kJ/mol
J/(mol K)
Porphyrin
Reference
Pyridine
Acetic acid
I
6.3
0.10
919
±
±
±
0.6
0.01
20
69
153
21
±
±
±
3
1
1
–101
148
±
±
±
10
2
0.987
0.368
±
±
0.005
0.001
67
±
±
5
–28
±
±
15
3
This work
[3, 4]
II
III
28.3
1.1 –166
–202
3
No complexation
[3, 4]
[
14]. Dinuclear copper(II) carboxylates are kinetically
The coordination properties of porphyrins with
inert in complexation with porphyrins [15–17], so the respect to metal salts depend directly on the nature of
complex formation reaction involves only the monoꢀ the substituents bonded to the porphyrin core. As was
nuclear solvated species, whose concentration is proꢀ demonstrated in earlier works [3–7], the introduction
portional to the square root of the total salt concentraꢀ of a large number of bulky substituents not only
tion. In pyridine, the dimerization equilibrium takes changes the effective charge of the reaction center of
place in parallel with the electrolytic salt dissociation the molecule through an electronꢀdonating or elecꢀ
equilibrium. The copper monoacetate solvato comꢀ tronꢀwithdrawing effect, but also causes a marked disꢀ
tortion of the planar structure of the aromatic
polyamine. However, the PM3 calculation of the
plexes are at least two orders of magnitude more active
in complexation with porphyrins than the diacetate
complexes [18]. This is why the kinetic order of the
reaction with respect to the salt in pyridine takes a still
smaller value of 0.4. The porphyrin concentrations in
kinetic experiments are so low that they cannot exert
any effect on the equilibrium speciation of the salt.
Accordingly, the kinetic order of the reaction with
geometry of
ꢀoctaphenylporphyrin (with an energy
β
gradient of 0.02 kJ/(mol Å) as the count stopping criꢀ
terion) demonstrated that the macrocycle of the porꢀ
phyrin has a nearꢀplanar structure in spite of the presꢀ
ence of eight bulky substituents (Fig. 3).
The phenyl fragments in
I
make an angle of 75°
≈
respect to the salt is independent of the nature of the with the macrocycle plane. Therefore, they can proꢀ
porphyrin.
In acetic acid, transitionꢀmetal salts form comparꢀ
duce an effect on the reaction center N4 only through
the system of
which exert a
σ
bonds. The eight phenyl substituents,
I effect, reduce the negative charge on
–
atively labile solvato complexes, which decompose
readily when interacting with a porphyrin to yield a
metalloporphyrin. Pyridine, a strong electron donor,
forms a stable coordination sphere around the metal
cation. The partial decomposition of this coordination
sphere in complexation with a porphyrin takes a conꢀ
siderable amount of energy, thus slowing down the
the central nitrogen atoms. The methyl groups in the
ꢀpyrrole positions of the porphyrin macrocycle (in
octamethylporphyrin II) show electronꢀdonating
properties ( effect), reducing the electron density on
the atoms of the reaction center. However, it is obvious
that these factors have no considerable effect on the
complexation rate (Table 2). The rate of reaction (1)
varies within one order of magnitude. The introducꢀ
β
+I
process. In acetic acid, the copper
ꢀoctaphenylporꢀ
β
phyrin complex forms several orders of magnitude tion of four phenyl substituents in the meso positions
more rapidly than it does in pyridine.
of the porphyrin macrocycle causes substantial geoꢀ
metric changes: the molecule of dodecaphenylporꢀ
phyrin III has a soꢀcalled saddleꢀdistorted configuraꢀ
tion, as is shown in Fig. 3. The distortion of the macꢀ
rocycle has a marked effect on the basic properties of
the porphyrin as a ligand [19] and increases and
decreases the complexation rate by several orders of
magnitude in basic and acidic solvents, respectively.
The ( + 1)thꢀorder rate constants calculated via
n
Eq. (4) are listed in Table 2, where they are compared
with earlier data for
βꢀoctamethylporphyrin and
dodecaphenylporphyrin.
n
Cu
kn + 1
=
kapp
/
c
OAc 2
.
(4)
(
)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

KINETICS OF THE FORMATION
1497
(I)
(II)
(III)
Fig. 3. Structure pf porphyrins I–III calculated by the PM3 method: different projections.
This is unambiguous evidence that the distortion of
the planar porphyrin molecule is of great significance
in the complexation kinetics.
REFERENCES
1
. B. D. Berezin, Coordination Compounds of Porphyrins
and Phthalocyanine (Nauka, Moscow, 1978; Wiley,
New York, 1981).
ACKNOWLEDGMENTS
2
. E. M. Kuvshinova, S. G. Pukhovskaya, L. Zh. Guseva,
et al., Zh. Fiz. Khim. 72 (6), 1040 (1998) [Russ. J. Phys.
Chem. 72 (6), 926 (1998)].
This study was supported by the Russian Foundaꢀ
tion for Basic Research, grant no. 07ꢀ03ꢀ00635ꢀa.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010

1498
PUKHOVSKAYA et al.
3
4
5
6
7
. N. S. Dudkina, P. A. Shatunov, E. M. Kuvshinova, 12. I. Gyenes, Titration in NonꢀAqueous Media (Akademiai
et al., Zh. Obshch. Khim. 68 (12), 2042 (1998).
Kiado, Budapest, 1967; Mir, Moscow, 1971).
. O. A. Golubchikov, S. G. Pukhovskaya, and E. M. Kuvshiꢀ
nova, Usp. Khim. 74 (3), 263 (2005).
. E. M. Kuvshinova, D. A. Kuz’min, N. S. Dudkina,
et al., Zh. Obshch. Khim. 72 (1), 142 (2002).
. E. M. Kuvshinova, D. A. Kuz’min, N. S. Dudkina,
et al., Zh. Obshch. Khim. 73 (4), 691 (2003).
. E. M. Kuvshinova, S. G. Pukhovskaya, A. S. Semeikin,
1
3. E. M. Kuvshinova, N. S. Dudkina, and O. A. Golubꢀ
chikov, Zh. Obshch. Khim. 68 (12), 2042 (1999).
14. H. Grasdalen and I. Svare, Acta Chem. Scand. 25 (3),
1089 (1971).
15. O. A. Golubchikov, O. I. Koifman, and B. D. Berezin,
Zh. Fiz. Khim. 50 (6), 1469 (1976).
and O. A. Golubchikov, Zh. Fiz. Khim. 79 (6), 1010
1
1
1
6. O. A. Golubchikov, T. A. Ageeva, E. L. Ponomareva,
(
2005) [Russ. J. Phys. Chem. 79 (6), 871 (2005)].
and B. D. Berezin, Koord. Khim. 13 (1), 37 (1987).
8. M. Friedman, J. Org. Chem. 30, 859 (1965).
7. O. A. Golubchikov, T. A. Ageeva, E. L. Ponomareva,
9
. P. A. Shatunov, Candidate’s Dissertation in Chemistry
and B. D. Berezin, Koord. Khim. 13 (7), 886 (1987).
(
IGKhTU, Ivanovo, 2002).
0. Yu. V. Karyakin and I. I. Angelov, Pure Chemicals
Khimiya, Moscow, 1974) [in Russian].
1. A. Weisberger, E. Proskauer, J. Riddick, and E. Toops,
8. O. A. Golubchikov and B. D. Berezin, The Coordinaꢀ
tion Chemistry of Solvato Complexes of TransitionꢀMetal
Salts (Nauka, Moscow, 1992) [in Russian].
1
1
(
Techniques of Organic Chemistry, Vol. 7: Organic Solꢀ 19. S. G. Korovina, V. B. Sheinin, O. A. Golubchikov, and
vents: Physical Properties and Methods of Purification
Interscience, New York, 1955; Inostrannaya Literꢀ
atura, Moscow, 1958).
B. D. Berezin, Proceedings of III AllꢀUnion Conference
on the Chemistry and Biochemistry of Macroheterocyclic
Compounds (Ivanovo, 1988), p. 170 [in Russian].
(
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 9 2010