DISSOCIATION KINETICS OF COPPER AND COBALT COMPLEXES
135
uents destabilize metal porphyrins in proton-donor
media. In II IV, the phenyl rings are perpendicular to
in a 10 : 1 ratio, with subsequent distillation from the
mixture with a Vigreux column. The fraction boiling
the porphyrin macrocycle plane; therefore, they can at 72 73 C was collected and distilled again.
affect the MN4 core only via the system of bonds.
Octaethylporphyrin, 5-phenyloctaethylporphyrin,
Sequential introduction of phenyl substituents exhib-
5,15-diphenyloctaethylporphyrin,
5,10,15,20-tetra-
iting a I effect decreases the effective charge on the
nitrogen atoms that are subject to attack of solvated
proton. Therefore, the dissociation rate of metal por-
phyrins decrease on introducing phenyl groups into
meso positions of octaethylporphyrin. However, actu-
ally the trend is opposite.
phenyloctaethylporphyrin, and dodecaphenylporphyrin
were prepared by procedures described elsewhere [6].
The copper and cobalt complexes were prepared by
the reactions of the porphyrins with the corresponding
metal acetates, taken in excess, in boiling DMF and
were purified chromatographically (Al2O3, activity
grade III, eluent chloroform). The product purity was
checked by TLC and spectrophotometrically.
According to a single crystal X-ray diffraction
study [12], octaethylporphyrin has an essentially
planar structure. In octamethyldiphenylporphyrin,
deviations of the carbon atoms of the porphyrin core
from the mean plane of the macroring are relatively
small [13]. Tetraphenyloctaethylporphyrin, dodeca-
phenylporphyrin, and their metal derivatives are es-
sentially nonplanar [14, 15]. Deformation of the por-
phyrin ring decreases the aromaticity of the ligand and
breaks the conjugation between the -electron systems
of the pyrrole fragments. As a result, the electron
density on the nitrogen atoms increases, which, as
already noted, facilitates solvoprotolytic dissociation
of metal porphyrins.
The kinetics of metal porphyrin dissociation were
studied spectrophotometrically with Specord M-40
and SF-46 spectrophotometers in temperature-con-
trolled ( 0.1 K) cells with ground-glass stoppers at
288 338 K. The dissociation rate constants, averaged
over two or three parallel runs, are listed in Table 1.
REFERENCES
1. Berezin, B.D., Koordinatsionnye soedineniya porfiri-
nov i ftalotsianina (Coordination Compounds of
Porphyrins and Phthalocyanines), Moscow: Nauka,
1978.
As the deformation of the tetrapyrrole macrocycle
increases, steric hindrance to interaction of solvated
proton with the nitrogen atoms of the reaction center
of metal porphyrin becomes less significant. This
essentially structural effect should be especially sig-
nificant in the second, limiting stage of interaction of
HMP+ with Hs+olv. Increasing deformation of the
macrocycle makes more favorable the conditions for
solvation of the reacting system in the transition state.
This is apparently the major factor responsible for a
drastic decrease in the activation entropy in the order
I > II > III > IV.
2. Lavallee, D.K., Chemistry and Biochemistry of N-
Substituted Porphyrins, New York: VCH, 1987.
3. Simonis, U., Walker, F.A., and Lee, P.L., J. Am.
Chem. Soc., 1987, vol. 109, no. 9, pp. 2659 2668.
4. Medforth, C.J., Berber, M.D., and Smith, K.M.,
Tetrahedron Lett., 1990, vol. 31, no. 26, pp. 3719
3722.
5. Kuvshinova, E.M., Golubchikov, O.A., and Bere-
zin, B.D., Zh. Obshch. Khim., 1991, vol. 61, no. 8,
pp. 1799 1804.
6. Dudkina, N.S., Shatunov, P.A., Kuvshinova, E.M.,
Pukhovskaya, S.G., Semeikin, A.S., and Golubchi-
kov, O.A., Zh. Obshch. Khim., 1998, vol. 68, no. 12,
pp. 2042 2047.
EXPERIMENTAL
Analytically pure grade copper acetate and chemi-
cally pure grade cobalt acetate were recrystallized
from aqueous acetic acid and dehydrated: Cu(OAc)2,
by heating to constant weight at 370 390 K, and
Co(OAc)2, by refluxing for 3 h with acetic anhydride,
with subsequent filtration, washing with hexane on
the filter, and drying in a vacuum desiccator.
7. Pukhovskaya, S.G., Golubchikov, O.A., and Bere-
zin, B.D., Kinet. Katal., 1992, vol. 33, no. 1, pp. 21
26.
8. Berezin, B.D., Uspekhi khimii porfirinov (Advances
in Porphyrin Chemistry), Golubchikov, O.A., Ed., St.
Petersburg: Nauchno-Issled. Inst. Khimii, Sankt-
Peterb. Gos. Univ., 1997, vol. 1, pp. 94 128.
Chemically pure grade acetic acid was dehydrated
by refluxing with the calculated amount of acetic
anhydride for 20 h and distilled with a Vigreux column
The water content was 0.02 wt %, as determined by
Fischer titration. Pure grade trifluoroacetic acid was
dehydrated by mixing with concentrated sulfuric acid
9. Golubchikov, O.A., Mamardashvili, N.Zh., Semei-
kin, A.S., and Zdanovich, S.A., Koord. Khim., 1994,
vol. 20, no. 10, pp. 747 751.
10. Golubchikov, O.A., Klopova, L.V., and Mamardashvi-
li, N.Zh., Abstracts of Papers, I Mezhdunarodnaya
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 1 2002