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,
et al., Zh. Obshch. Khim. 68, 2042 (1998) [Russ. J.
Gen. Chem. 68, 1955 (1998)].
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
2. E. M. Kuvshinova, N. S. Dudkina, S. G. Pukhovskaya,
et al., Zh. Obshch. Khim. 70, 1010 (2000) [Russ. J.
Gen. Chem. 70, 945 (2000)].
3. E. M. Kuvshinova, D. L. Kuz’min, S. G. Pukhovskaya,
et al., Zh. Obshch. Khim. 73, 691 (2003) [Russ. J. Gen.
Chem. 73, 652 (2003)].
4. E. M. Kuvshinova, S. G. Pukhovskaya, A. S. Semeikin,
et al., Zh. Fiz. Khim. 79, 1010 (2005) [Russ. J. Phys.
Chem. A 79, 871 (2005)].
5. M. W. Renner, K. M. Barkigia, and J. Fajer, Inorg.
Chim. Acta 263, 181 (1997).
6. A. S. Semeikin, T. V. Lyubimova, and S. A. Syrbu,
Khim. Geterotsikl. Soedin., No. 10, 1464 (2004).
7. A. J. Gordon and R. A. Ford, The Chemist’s Companion:
A Handbook of Practical Data, Techniques and Referꢀ
ences (Mir, Moscow, 1976, p. 541; Wiley, New York,
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].
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
Vol. 84
No. 9
2010