ISSN 1070-3632, Russian Journal of General Chemistry, 2006, Vol. 76, No. 11, pp. 1848 1850.
Pleiades Publishing, Inc., 2006.
Original Russian Text
pp. 1932 1934.
M.E. Klyueva, Yu.N. Lebedev, A.A. Nikitin, A.S. Semeikin, 2006, published in Zhurnal Obshchei Khimii, 2006, Vol. 76, No. 11,
LETTERS
TO THE EDITOR
A First Example of Oxidation of Manganese(III) Porphyrin
in Sulfuric Acid Solutions
b
a
a
a
M. E. Klyueva , Yu. N. Lebedev , A. A. Nikitin , and A. S. Semeikin
a
Ivanovo State University of Chemical Technology,
pr. F. Engel’sa 7, Ivanovo, 153000 Russia
b
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
e-mail: mek@isc-ras.ru
Received September 2, 2005
DOI: 10.1134/S1070363206110351
We studied spectrophotometrically the state and
reactions of the Mn(III) complex with 2,7,12,17-tetra-
methyl-3,8,13,18-tetra(n-butyl)porphine [(Cl)MnP] (I)
in acid solutions. The spectrum of I in acetic acid
143 K exhibit a characteristic signal of manganese
with a hyperfine structure (g 2.007, A 9.8 mT). This
fact indicates that the oxidation state of the central
atom changed upon dissolution in acids, because for
Mn(III) porphyrins, as well as for other systems with
the high-spin d4 electronic configuration, recording
of the ESR spectra is impossible [2].
(
AcOH) is similar to that in toluene, which indicates
that the Mn(III) complex is stable in AcOH. However,
its spectrum in concentrated H SO [ max, nm (log ):
2
4
4
27 (5.03), 571 (3.93), 620 (3.82), 768 (3.79)] differs
The product with
in the mixed solvent AcOH H SO and in concen-
427 429 nm (II) is stable
max
essentially from the known spectra of porphyrins and
their Mn(III) complexes in acids. In the mixed solvent
AcOH H SO at 233 253 K, the electronic absorp-
tion spectrum of I changes with time: The characteris-
tic bands of manganese(III) porphyrin decrease and
2
4
trated H SO in the cold for a long time (more than
2
4
2
4
3
months) and under heating. By reprecipitation from
H SO onto ice, we isolated the Mn(III) porphyrin.
2
4
When complex I is dissolved in deaerated sulfuric
acid, the spectral pattern is different, and product II
is not formed.
disappear, and a very strong band appears with
max
429 nm. The spectrum evolution in characterized by
well-defined isobestic points. After the process com-
pletion, the spectrum becomes identical to that in con-
centrated aqueous sulfuric acid. Such a pattern is
observed with a Mn(III) porphyrin for the first time.
The apparent rate constants of the process, calculated
with a first-order equation, nonlinearly grow with an
increase in the initial concentration of sulfuric acid in
acetic acid. The negative logarithm of the rate con-
stant at 25 C linearly correlates with the acidity func-
Analysis of our results and published data [3]
shows that compound II is an oxo complex (O)Mn P
formed by one-electron oxidation of the central man-
ganese atom with oxygen in the presence of solvated
protons.
IV
The increase in the manganese oxidation state in
porphyrin complexes in the presence of oxidants is
a well-known phenomenon [3], but in strong acids the
formation of a Mn(IV) complex is observed for the
first time. All the previously studied Mn(III) com-
plexes with porphyrins containing various substituents
in and meso positions of the macroring dissociate
in concentrated sulfuric acid and its acetic acid solu-
tions with the cleavage of metal nitrogen bonds and
release of the metal-free porphyrin in the diprotonated
form [4].
tion of the mixed solvent H [1]. The slope of the
0
straight line in these coordinates is close to 2, i.e., the
experimental rate equation involves the total solution
acidity squared:
2
dC /d = kC h ,
I
I
0
where k is the rate constant; C , concentration of
complex I; and h , acidity of the medium.
I
0
The ESR spectra of sulfuric acid solutions of I at
Thus, in this study we found the first Mn(III) por-
1848
Klyueva
