Kinetics of Complexation of meso-Diphenyl-Substituted Alkylporphyrins with Zinc Acetate in Acetonitrile

Article abstract of DOI:10.1023/A:1023300404659

Kinetics of interaction of diphenyl-substituted porphyrins and their derivatives with substituents in the phenyl fragments with zinc acetate in acetonitrile solution is studied. The effect of the solvent on the rate of coordination is shown. Kinetic param

Full text of DOI:10.1023/A:1023300404659

Russian Journal of Coordination Chemistry, Vol. 29, No. 4, 2003, pp. 231–233. Translated from Koordinatsionnaya Khimiya, Vol. 29, No. 4, 2003, pp. 247–249.
Original Russian Text Copyright © 2003 by Malkova, Storozheva, Andrianov, Semeikin.
Kinetics of Complexation of meso-Diphenyl-Substituted
Alkylporphyrins with Zinc Acetate in Acetonitrile
O. V. Malkova, Yu. Yu. Storozheva, V. G. Andrianov, and A. S. Semeikin
Ivanovo State University of Chemical Technology, Ivanovo, Russia
Received June 14, 2001
Abstract—Kinetics of interaction of diphenyl-substituted porphyrins and their derivatives with substituents in
the phenyl fragments with zinc acetate in acetonitrile solution is studied. The effect of the solvent on the rate of
coordination is shown. Kinetic parameters of complexation are calculated.
The kinetics of porphyrin complexation with metal 0.6 mmol) was stirred with a solution of SnCl2 · 2H2O
salts in nonaqueous solutions has been studied in [1–3]. (2.5 g, 11 mmol) in 20 ml of concentrated hydrochloric
The use of the metal complexes in various branches of acid and 20 ml of methanol for 10 h; the resulting dark
science and technique (for example, as catalysts and green solution was poured into a solution of KOH (20 g)
semiconductors) is of interest [1].
in 50 ml H2O. After cooling, the solution was filtered
off, the resulting solid was washed with water, dried
and chromatographed on Al2O3 (Brockman III type of
activity, eluent CHCl3). The dark red zone was concen-
trated, and aminoporphyrin was precipitated with
petroleum ether. The yield was 410 mg (86%); Rf (silu-
fol)—0.3, 0.48 (chloroform); electronic absorption
spectrum [λmax, nm (logε)]: 625 (3.50); 574 (3.92); 541
(3.85); 509 (4.26); 410 (5.33) (chloroform).
Porphyrin molecules (H2P) show their biological
and catalytic activities only in combined state in metal
complexes. Formation of metalloporphyrins is accom-
panied by the removal of two protons from two NH
groups, almost complete destruction of the coordina-
tion sphere of the initial metal salt, and by the formation
of a fundamentally new coordination sphere with
extremely strong metal–porphyrin interaction [1].
The mixture of atropoisomers was separated by
double chromatography on Al2O3 (Brockman III type
of activity, eluent benzene).
Properties of metalloporphyrins depend on the
nature of the substituents bonded to the porphyrin ring
and of the metal, which coordinates the donor atoms of
the macromolecule. The mechanism of transfer of the
5α,15β-Bis(2'-aminophenyl)-3,7,13,17-tetrame-
electronic effects of the substituents to the reaction cen- thyl-2,8,12,18-tetra-n-buthylporphine (II). Rf (silu-
ter of the molecule is still unclear; further investigation fol)—0.83 (benzene); electronic absorption spectrum
of coordination properties of porphyrins is necessary.
The purpose of this work was to study Zn(Ac)2 com-
plexation with phenyl-substituted porphyrins in the
[λmax, nm (logε)]: 626 (2.58); 575 (3.94); 542 (3.77);
508 (4.16); 410 (5.23) (chloroform).
5α,15α-Bis(2'-aminophenyl)-3,7,13,17-tetrame-
medium of a weakly coordinated solvent (in the 288– thyl-2,8,12,18-tetra-n-buthylporphine (III). Rf (silu-
318 K range).
fol)—0.4 (benzene); electronic absorption spectrum
[λmax, nm (logε)]: 625 (3.50); 574 (3.92); 541 (3.85);
509 (4.26); 410 (5.33) (chloroform).
5,15-diphenyltetramethyltetrabutylporphine (IV)
was prepared as reported elsewhere [4].
EXPERIMENTAL
Synthesis of substituted alkylporphyrins was per-
formed according to the following procedures.
Bu
Bu
5,15-Bis(2'-nitrophenyl)-3,7,13,17-tetramethyl-
2,8,12,18-tetra-n-buthylporphine (a mixture of atro-
poisomers) (I) was prepared by condensation of 3,3'-
dibuthyl-4,4'-dimethyldipyrrolylmethane with Ó-
nitrobenzaldehyde according to the procedure reported
in [4]. The yield was 60%; Rf(silufol)—0.22, 0.29 (1 : 1
benzene–hexane); electronic absorption spectrum
[λmax, nm (logε)]: 632 (3.62); 579 (3.91); 546 (3.90);
512 (4.23); 411 (5.24) (chloroform).
1
R1
R2
R3
R
Me
Me
Me
Me
NH
N
N
HN
R
Bu
Bu
5,15-Bis(2'-aminophenyl)-3,7,13,17-tetramethyl-
2,8,12,18-tetra-n-buthylporphine (a mixture of atro-
poisomers) (II and III). A portion of complex I (0.5 g,
I: R = R2 = NO2,
R1 = R3 = H or R = R3 = NO2,
1070-3284/03/2904-0231$25.00 © 2003 åÄIä “Nauka /Interperiodica”

232
MALKOVA et al.
Selected kinetic parameters of formation of metalloporphyrins in acetonitrile
kv , s–1 mol–1 l*
Ea,
kJ/mol
∆S≠,
J/(mol K)
Porphyrin
288
298
308
318
I
–
4.72
5.44
10.34
7.61
6.73
20.2
5.79
20.66
9.92
9.59
57 ± 6
26 ± 3
28 ± 3
34 ± 3
34 ± 3
–49 ± 5
II
III
IV
V
3.37
3.19
8.03
2.30
–156 ± 16
–146 ± 15
–118 ± 12
–129 ± 13
4.85
12.89
3.41
* Accuracy of k determination is not less than 10%.
v
R1 = R2 = H (mixture of atropoisomers);
II: R = R2 = NH2, R1 = R3 = H;
III: R = R3 = NH2, R1 = R2 = H;
IV: R = R1 = R2 = R3 = H.
Introduction of two phenyl fragments in the meso-
position of the porphyrin molecule results in accelera-
tion of reaction (1) as compared to unsubstituted alkyl-
porphyrin
3,7,13,17-tetramethyl-2,8,12,18-tetrabu-
tylporphine ç2P(ëç3)4(ë4ç9)4 (V) without phenyl
rings in the meso-position of the ring [2]. This may
occur due to the electron-accepting properties of the
phenyl fragments, connected with the conjugated π-
system of the porphyrin; they appear as weakening of
the N–H bond in the pyrrol rings, which decreases the
electronic density on the annular nitrogen atoms.
Weakening of the N–H bond, as a rule, increases the
rate of porphyrin coordination with metal salts. A
decrease in the electronic density on the annular nitro-
gen atoms may also decelerate complexation. If the
reaction rate increases upon introduction of the elec-
tron-accepting substituent, the main factor of the pro-
cess is the weakening of the N–H bonds of the annular
nitrogen atoms. In the poorly coordinated solvent AN,
the electron-accepting substituents decelerate the reac-
tion of coordination [3]. Introduction of amino groups
in the phenyl fragments of the porphyrin molecule also
decreases the rate of complexation. Probably, protona-
tion of amino groups proceeds in the solution.
Acetonitrile (AN) was purified and dried as reported
elsewhere [5]. Reagent grade zinc acetate Zn(Ac2) was
recrystallized from glacial acetic acid as described in [6].
Kinetic measurements were carried out on a Hita-
chi-U-2000 spectrophotometer with a temperature-
controlled cell section by standard technique [7]. The
virtual reaction rate constants (kv) were calculated from
the equation of the pseudofirst order.
RESULTS AND DISCUSSION
It was found in the systematic study of complex-
ation [1] that the reaction:
MXm · (n – m)Solv + H2P pSolv
(1)
MP qSolv + 2HX 2Solv
+ (n – m + p – q – 2r)Solv
The protonated amino group is the electron accep-
tor; this also decreases electronic density on the ternary
nitrogen atoms. Experimental data show that different
arrangement of the substituents (α, β) and (α, α) has no
significant effect on the state of the π-electronic struc-
ture of the macrocycle.
Low values of entropy change (∆S≠) suggest
strengthening of solvation of the transition state of the
system as compared to its initial state. The resulting
values of Ea and ∆S≠ allow us to propose that solvation
ability of the solvent has significant effect on the com-
proceeds via a consistent mechanism with associative–
dissociative activation of the NH bond, and its kinetic
parameters drastically depend on the structure of the
porphyrin molecule and on the metal and solvent
nature.
Depending on the donor–acceptor properties of sub-
stituents in the porphyrin macrocycle, the rate of com-
plexation may change by hundreds of times; however,
it is difficult to foresee the changes in reactivity of the
porphyrin ligand versus the substituent nature.
Kinetics of reaction (1) depends on the solvating plexation rate. The experimental data show that com-
ability of the solvent. Composition and stability of a plex I in the transition state is less solvated [∆S≠ =
solvate shell of both porphyrin and transition state in −49 J/(mol K)] as compared to complexes II and III,
the reaction change with the change of the substituent where solvation of the transition state is –156 or
nature.
The experimental data (table) allow to find the effect
−146 J/(mol K), respectively.
For all systems under study, energy of formation of
of substitution in the meso-position and in the phenyl the transition state in complexation is compensated by
rings of the substituent on the kinetic parameters of solvation of the system with the solvent molecules. Lin-
reaction (1).
ear character of this dependence (figure) for all porphy-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 29 No. 4 2003

KINETICS OF COMPLEXATION OF meso-DIPHENYL-SUBSTITUTED ALKYLPORPHYRINS
233
∆S≠, J/(mol K)
detachment of the protons at the pyrrol nitrogen atoms.
In addition, the solvating properties of acetonitrile have
significant effect on coordination.
I
–60
–80
REFERENCES
1. Berezin, B.D., Koordinatsionnye soedineniya porfirinov
i ftalotsianina (Coordination Compounds of Porphyrins
and Phthalocyanine), Moscow: Nauka, 1978.
–100
–120
IV
III
2. Malkova, O.V., Bazlova, O.V., Andrianov, V.G., and
–140
Berezin, B.D., Koord. Khim., 1998, vol. 24, no. 6, p. 473.
3. Bazlova, I.Yu., Cand. Sci. (Chem.) Dissertation,
Ivanovo, Inst. of Chemistry of Nonaqueous Solutions,
Russ. Acad. Sci., 1998.
II
–160
25
30
35
40 45
50
55
60
Ea, kJ/mol
4. Semeikin, A.S., Lyubimova, T.V., and Golubchikov, O.A.,
Zh. Prikl. Khim. (St. Petersburg), 1993, vol. 66, no. 3,
p. 710.
Kinetic compensation effect of complexation of porphyrins
with Zn(Ac) in acetonitrile.
5. Hofmanova, A. and Angelis, K., Chem. Listy, 1978,
2
vol. 72, no. 3, p. 306.
6. Karyakin, Yu.V. and Angelov, I.I., Chistye khimicheskie
veshchestva (Pure Chemical Substances), Moscow:
Khimiya, 1974.
rins investigated point to similar mechanisms of com-
plexation.
Therefore, in the poorly coordinated (acetonitrile)
solvent, the limiting stage of complexation is the
7. Malkova, O.V., Cand. Sci. (Chem.) Dissertation,
Ivanovo, Inst. for Chemical Technology, 1988.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 29 No. 4 2003