Structure of Phenyl Derivatives of Octaethylporphyrin and Dissociation Kinetics of Their Mn3+, Co2+, and Cu2+ Complexes in Acetic Acid

Article abstract of DOI:10.1023/A:1025673427341

Octaethyl-, 5-phenyloctaethyl-, 5,15-diphenyloctaethyl-, 5,10,15,20-tetraphenyloctaethyl, and dodecaphenylporphyrins were prepared, and their geometries were optimized by the method of molecular mechanics. The deformation of the porphyrin macroring grows

Full text of DOI:10.1023/A:1025673427341

Russian Journal of General Chemistry, Vol. 73, No. 4, 2003, pp. 652 654. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 4, 2003,
pp. 691 693.
Original Russian Text Copyright
2003 by Kuvshinova, Kuz’min, Pukhovskaya, Semeikin, Golubchikov.
Structure of Phenyl Derivatives of Octaethylporphyrin
and Dissociation Kinetics of Their Mn3+, Co2+, and Cu2+
Complexes in Acetic Acid
E. M. Kuvshinova, D. L. Kuz’min, S. G. Pukhovskaya,
A. S. Semeikin, and O. A. Golubchikov
Ivanovo State University of Chemical Engineering, Ivanovo, Russia
Received May 5, 2001
Abstract Octaethyl-, 5-phenyloctaethyl-, 5,15-diphenyloctaethyl-, 5,10,15,20-tetraphenyloctaethyl, and
dodecaphenylporphyrins were prepared, and their geometries were optimized by the method of molecular
mechanics. The deformation of the porphyrin macroring grows as the number of phenyl groups is increased.
The dissociation kinetics of the manganese, cobalt, and copper complexes of the porphyrins in solutions of
sulfuric acid in acetic acid were studied. The kinetic parameters of the dissociation of metal porphyrins were
analyzed in relation to the porphyrin structure and nature of the metal.
Kinetic studies of formation and dissociation of
copper complexes with octaethylporphyrin I and its
5-phenyl (II), 5,15-diphenyl (III), and 5,10,15,20-tet-
raphenyl (IV) derivatives, and also with dodecaphen-
ylporphyrin V, showed that the reaction kinetics very
strongly depends on the extent of deformation of the
porphyrin macroring.
matic tetrapyrrole macroring grows as the number of
bulky peripheral substituents is increased.
The dissociation kinetics of the copper (CuP),
cobalt (CoP), and manganese [(AcO)MnP] complexes
of I V were studied in solutions of H2SO4 in acetic
(a)
0.2
R1
0
R1
R1
0.2
(b)
R1
R3
R1
R1
R5
0.2
0
0.2
0.4
N
HN
NH
R1
N
(c)
R1
0.4
0.2
R1
R4
0
0.2
0.4
I V
R1 = Et, R2 = R3 = R4 = R5 = H (I); R1 = Et, R2 = Ph,
R3 = R4 = R5 = H (II); R1 = Et, R2 = R4 = Ph, R3 = R5 =
H (III); R1 = Et, R2 = R3 = R4 = R5 = Ph (IV); R1 = R2 =
R3 = R4 = R5 = Ph (V).
2
3
(d)
12
13
1.2
0.8
0.4
1
N21 4
N23
11
10
14
5
15
The molecular-mechanics calculations (MM+ force
field, see figure) suggest that macrocycle I is virtually
planar. In II, the -carbon atoms of the first, third, and
fourth pyrrole rings in the porphyrin core deviate by
0.1, 0.3, and 0.25 , respectively. In III, the -carbon
atoms of the first and third pyrrole rings deviate up-
wards, and those of the second and fourth pyrrole
rings, downwards from the least-squares plane by up
to 0.55 . In IV, this deviation reaches 1.4 . Thus,
the calculations show that the deformation of the aro-
20
19
0
0.4
0.8
1.2
6
9
16
N22
N24
7
8
17
18
Deviation of the carbon and nitrogen atoms of the
porphyrin macroring from the mean plane drawn through
four nitrogen atoms: (a) I, (b) II, (c) III, and (d) IV. The
atom numbering is shown for IV as example.
1070-3632/03/7304-0652$25.00 2003 MAIK Nauka/Interperiodica

STRUCTURE OF PHENYL DERIVATIVES OF OCTAETHYLPORPHYRIN
653
Kinetic parameters of dissociation of copper, cobalt, and manganese complexes of porphyrins I V in acetic acid
containing 2 wt % H2SO4
1
kapp 103, s
1
a
Porphyrin E, kJ mol
, nm
288 K
298 K
CuP
308 K
318 K
328 K
I
35.7 1.3
27.7 0.1
22.1 0.7
18.2 0.7
559
562
563
562
574
0.12 0.02
0.178 0.016
0.81 0.03
4.5 0.2
0.18 0.01
0.25 0.04
1.07 0.03
5.7 0.3
0.281 0.018
II
III
IV
V
0.12 0.02
0.61 0.03
3.6 0.2
b
b
b
CoP
I
56 2
38 2
34.5 0.4
29.4 0.3
556
533
540
569
0.99 0.02
1.65 0.16
2.60 0.2
5.80 0.2
2.00 0.03
2.76 0.05
4.11 0.17
8.61 0.19
4.15 0.15
4.57 0.12
II
III
IV
V
1.60 0.1
3.87 0.13
(AcO)MnPc
I
139.1 1
125 1
116 1
80 0.7
549
559
564
578
0.302 0.024
0.45 0.02
0.591 0.027
19.20 0.11
1.71 0.11
2.07 0.16
2.4 0.17
8.30 0.3
8.9 0.21
9.4 0.14
II
III
IV
V
2.22 0.03
6.85 0.13
a
b
c
Analytical wavelength.
The reaction is instantaneous in CH CO H.
H SO content 10 wt %.
2 4
3
2
acid. The H2SO4 content was 2% for CuP and CoP
and 10% for (AcO)MnP. All the metal complexes MP
dissociate in these proton-donor media to form the
double-protonated species H4P2+:
Comparison of the results of MM+ structural calcu-
lations for I V with the kinetic parameters of dissoci-
ation of the corresponding Mn3+, Co2+, and Cu2+
complexes in the binary solvent CH3CO2H H2SO4
shows that, as the extent of deformation of the por-
phyrin macroring grows, the dissociation rate constant
increases and the activation energy decreases. The
complexes of the most deformed dodecaphenylporphy-
rin V [3] instantaneously dissociate even in straight
CH3CO2H.
MP + 4H+
M2+ + H4P2+.
(1)
The electronic absorption spectra of the reacting
systems exhibit clear isobestic points: 566, 578, 567,
and 597 nm for CuP; 508, 513, 516, and 518 nm for
CoP; 555, 572, 454, and 667 nm for (AcO)MnP (data
for I IV, respectively). The kinetic experiments were
performed at a 100-fold excess of H2SO4 relative to
MP, which allowed calculations of the apparent rate
constants (kapp) by Eq. (2):
It is known that deformation of the porphyrin
macroring decreases its aromaticity and increases the
-electron density on the central nitrogen atoms [4 6],
with a sharp increase in the basicity of the porphyrins
[7]. When metal complexes of I V are formed, the
deformation becomes somewhat weaker, but the pat-
tern as a whole remains the same [8]. Hence, as the
distortion of the planar structure of the metal porphy-
rin increases, the M N bonds become more accessible
for the attack of solvated proton, and the complex as
a whole becomes less stable.
kapp = (1/ )ln [(A0
A )/(A
A )],
(2)
where A0, A , and A are the optical densities of
solutions at the initial moment, at time , and after re-
action completion, respectively. The activation energy
E was calculated by Eq. (3):
As for the influence of the central cation, the dis-
sociation rate of the metal complexes increases in the
order Mn(I V) < Cu(I V) < Co(I V). As expected,
the complexes of triple-charged manganese appeared
to be the most stable in proton-donor media, which is
E = 8.3T1T2/(T2
T1)ln (k2/k1).
(3)
The kinetic parameters of dissociation of CuP,
CoP, and (AcO)MnP are listed in the table.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 4 2003

654
KUVSHINOVA et al.
due to the high strength of the Mn N covalent bonds.
The higher stability of the copper complexes, as com-
pared to cobalt complexes, is due to the tendency of
the Cu2+ d9 cation to form square planar complexes
[9] fitting the geometry of the porphyrin ligands. The
Co2+ d7 cation tends to form octahedral complexes
[9]. Apparently, when the Co N bonds are attacked
by a solvated proton, the released solvent molecules
(CH3CO2H) solvate the metal cation, facilitating dis-
sociation.
328 K. The temperature fluctuations in the tempera-
ture-controlled chamber did not exceed 0.1 K.
The structural calculations were performed by the
method of molecular mechanics (MM+). The devia-
tions of the carbon atoms of the porphyrin macroring
from the least-squares plane drawn through four N
atoms are given in the figure.
REFERENCES
1. Dudkina, N.S., Shatunov, P.A., Kuvshinova, E.M.,
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p. 2042.
EXPERIMENTAL
Chemically pure grade acetic acid was dehydrated
by refluxing with the calculated amount of acetic
anhydride for 20 h and distilled with a Vigreux col-
umn. The water content in the solvent, determined by
Fischer titration, was 0.03%. Sulfuric acid monohy-
drate was prepared by saturation of 95% H2SO4 with
oleum; the water content was monitored potentiomet-
rically.
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Semeikin, A.S., and Golubchikov, O.A., Zh. Obshch.
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skii, Yu.P., Izv. Vyssh. Uchebn. Zaved., Ser. Khim.
Khim. Tekhnol., 1998, vol. 41, no. 5, p. 113.
Octaethylporphyrin I, 5-phenyloctaethylporphyrin
II, 5,10-diphenyloctaethylporphyrin III, 5,10,15,20-
tetraphenyloctaethylporphyrin IV, and dodecaphenyl-
porphyrin V were prepared according to [1].
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J. Am. Chem. Soc., 1988, vol. 110, no. 22, p. 7566.
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plexes of porphyrins I V were prepared by refluxing
porphyrins with excess Cu(OAc)2, Co(OAc)2, and
Mn(OAc)3 in DMF. The metal porphyrins were puri-
fied by chromatography on Al2O3 (Brockmann grade
III, eluent chloroform). The purity of the complexes
was checked by electronic absorption spectroscopy
and TLC.
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Chem. Soc., 1990, vol. 112, no. 24, p. 8851.
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khimii i biokhimii makrotsiklicheskikh soedinenii (Conf.
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p. 2378.
The kinetics of solvoprotolytic dissociation of the
metal porphyrins in acetic acid containing H2SO4 was
studied spectrophotometrically (SF-46 and Specord
M-400 devices, quartz cells with ground-glass stop-
pers). In all the systems, clear isobestic points were
observed. The experiments were performed at 288
9. Golubchikov, O.A. and Berezin, B.D., Zh. Fiz. Khim.,
1986, vol. 60, no. 9, p. 2113.
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