Effect of pyrrole bromo-substitution in tetraphenylporphine on the kinetics of complexation with zinc acetate in pyridine and acetic acid

Article abstract of DOI:10.1023/A:1021626127175

Rate constants of zinc acetate complexation with 2,7,12,17-tetrabromo-5,10, 15,20-tetraphenylporphine in toluene, pyridine, or acetic acid following the reaction H₂TPP(β-Br)₄ + Zn(Ac)₂ → ZnTPP(β-Br)₄ + 2HAc are determined at four temperatures in the interval of 298-328 K, and the activation energies and entropies are calculated. The effect of bromo-substitution in pyrrole fragments of a tetraphenylporphine molecule on the complexation kinetics and mechanism is discussed.

Full text of DOI:10.1023/A:1021626127175

Russian Journal of Coordination Chemistry, Vol. 28, No. 12, 2002, pp. 834–838. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 12, 2002, pp. 890–895.
Original Russian Text Copyright © 2002 by Berezin, Karmanova, Gromova, Semeikin.
Effect of Pyrrole Bromo-Substitution in Tetraphenylporphine
on the Kinetics of Complexation with Zinc Acetate in Pyridine
and Acetic Acid
B. D. Berezin, T. V. Karmanova, T. V. Gromova, and A. S. Semeikin
Ivanovo State Academy of Chemical Technology, Ivanovo, Russia
Received February 20, 2001
Abstract—Rate constants of zinc acetate complexation with 2,7,12,17-tetrabromo-5,10,15,20-tetraphenylpor-
phine in toluene, pyridine, or acetic acid following the reaction ç2TPP(β-Br)4 + Zn(Ac)2
ZnTPP(β-Br)4 +
2HAc are determined at four temperatures in the interval of 298–328 K, and the activation energies and entro-
pies are calculated. The effect of bromo-substitution in pyrrole fragments of a tetraphenylporphine molecule on
the complexation kinetics and mechanism is discussed.
Complexation reactions of the d-metal salts with
Since the π-system of a macrocycle and the π-sys-
porphyrins in organic solvents proceed at low rates. tems of phenyl rings interact only weakly and can be
However, changes in the molecular structure of tet- regarded as independent π-chromophores [15], the
raphenylporphine (H2TPP), in particular, 2,7,12,17-tet- effect of the functional substitution –ë6ç5
rabromo-5,10,15,20-tetraphenylporphine (ç2TPP(β-
Br)4, I), essentially affect the rate and mechanism of
complexation reaction [1–11]:
−ë6ç4ï in ç2TPP also turned out to be insignificant.
Half-isolated double bonds ëβ=ëβ in the porphyrin pyr-
role fragments are relatively weak, but they are less
independent chromophores and produce a certain effect
on the macrocyclic chromophore and its reaction sites
(H2N4, MN4). Polarization of these bonds through bro-
mination of pyrrole fragments strongly affects the mac-
rocyclic chromophore due to the transfer of the induc-
tion effect of bromine atoms to this macrocyclic chro-
mophore and the polarization of the whole π-system of
a macrocycle [16, 17]. In this work, the effect of tetra-
bromo-substitution in the β-positions of tetraphe-
nylporphine on its coordination properties in reaction
(1) with zinc acetate in toluene, pyridine, or acetic acid
was studied for the first time.
H2TPP(X)m + [MA2(S)m – 2
MTPP(X)m + 2HA,
]
(1)
where åÄ2(S)m – 2 is the metal-salt solvate with coordi-
nation number m in solvent (S), A is the salt anion, and
X is the electron-donor or electron-acceptor substitu-
ent.
Br
NH
Br
N
N
EXPERIMENTAL
Br
H2TPP(β-Br)4 was synthesized as described in [16].
Porphyrin ligand was purified chromatographically on
Al2O3 (activity II) using chloroform eluent. The IR and
electronic absorption spectra of I agree with the litera-
ture data.
NH
Br
(ç2TPP(β-Br)4)
Toluene and pyridine were purified according to the
standard procedures [18, 19]; the water content was
0.02 and 0.06%, respectively. Acetic acid was dehy-
drated by boiling with a calculated amount of anhy-
dride and further distillation. The reagent grade zinc
acetate was recrystallized from ethanol and acetic acid
and dried at 373 K to a constant weight.
Structural changes in tetraphenylporphine can be
produced by substitution in phenyl groups or in the β-
positions of pyrrole rings. Numerous data are available
today on the effect of substitution in the phenyl groups
of ç2TPP on the kinetics of complexation with the d-
metal salts [3, 4], on the kinetic stability of tetraphe-
nylporphine complexes [12–14], solubility and solva-
The complexation reaction was studied using a Hita-
tion in organic solvents [9], on the electronic absorption chi U-2000 spectrophotometer equipped with a thermo-
spectra [7–10], and on other properties.
statted cell and connected to a computer. To carry out
1070-3284/02/2812-0834$27.00 © 2002 åÄIä “Nauka /Interperiodica”

EFFECT OF PYRROLE BROMO-SUBSTITUTION IN TETRAPHENYLPORPHINE
835
kinetic measurements, the reagents were mixed in a
temperature-controlled cell and the reaction course was
determined from the changes in the electronic absorp-
tion spectra in the range of 300–800 nm by performing
20–30 measurements corresponding to different extents
of the process (Figs. 1, 2).
The concentration of metalloporphyrin in the course
of experiments was determined from the formula
A
1.0
As – A∞
cMP = c0
,
(2)
------------------
H P A0 – A∞
2
0.8
0.6
0.4
where c0H P and cMP are the initial and current concen-
2
trations of porphyrin and metalloporphyrin in a solu-
tion and A0, As, and A∞ are the optical densities of a
solution at the initial moment, at time τ, and at the end
of a reaction, respectively.
The rate of reaction (1) was found from the changes
in the optical density of the solutions at the maxima of
absorption bands of the obtained zincporphyrin (ZnP)
at λ = 522, 536, and 567 nm for toluene, pyridine, and
acetic acid, respectively.
0.2
0
Kinetic studies have shown that reaction (1) is
described by the first-order reaction in porphyrin
500 550
600 650 700
750 800
(Fig. 3) (with a linear dependence of log( c0H P/ cH P) =
λ, nm
2
2
f(τ)). The reaction was carried out with a significant salt
Fig. 1. Changes in the elecronic absorption spectrum of
ç TPP(β-Br) in pyridine with addition of Zn(Ac) ; c
excess (cZn(Ac) = 10–2–10–4 mol/l) as compared to por-
2
=
2
4
2
H2P
phyrin (cH P ≈ 10–5 mol/l); therefore, the effective rate
–4
–3
2
1.68 × 10 mol/l, cZn(Ac) = 2.74 × 10 mol/l, T = 298 K.
2
constants of complexation were calculated from the
first-order equation:
RESULTS AND DISCUSSION
keff = (1/τ)log(c0H P/cH P),
(3)
2
2
Pyridine acts as the most strong inhibitor of reaciton
(1) for porphyrins of the chlorophyll group, protopor-
phyrins, alkylporphyrins, tetraphenylporphine, and tet-
rabenzoporphine [7–10]. The above porphyrin ligands
can be considered to be porphyrins with a weak (low-
polarity) NH bond. The first-stage dissociation con-
stants of these NH-acids in DMSO change from 25.30
to 18.53 [9, 10]. On the contrary, analogs of porphyrins
with an active NH group and highly polarized (proto-
nated) N–H bond, such as tetraazaporphyrins (H2TAP),
phthalocyanines (H2PC), and their derivatives, give
complexes with the d-metal salts in pyridine at higher
rates as compared to the majority of other organic sol-
vents. Obviously, this occurs due to the internal ioniza-
tion of the NH group up to its complete dissociation [7,
8, 21, 22].
where τ is the duration of a reaction. The reaction order
in a salt was found graphically as the slope of logkeff
=
(1/τ)logcZn(Ac) (Fig. 4). Experimental data show that it
2
is close to unity. The virtual rate constant was deter-
mined according to the following formula (with
account for the zinc salt concentration)
kv = keffcZn(Ac)
.
(4)
2
The activation energy of the reaction (Ea) was found
from the Arrhenius equation
T1T2
k2
----
k1
----------------
T2 – T1
Ea = 2.3R
log
,
(5)
where k1 and k2 are the rate constants obtained with Eq.
(3) at temperatures T1 and T2.
The introduction of a large number of electron
acceptors like Cl, Br, NO2, SO2OH, and C6H4SO2OH
into H2TAP still further activates the NH group and
accelerates reaction (1) by hundreds and even thou-
sands of times. In porphine, tetraphenylporphine, and
similar ligands, NH bonds (that are fully inert to the
The entropy of activation (∆S#) can be calculated
using the equation [20]
∆S# = 19.12logk + Ea/(298 – 253.22).
(6)
The results of kinetic studies of reaction (1) are action of bases), after the replacement of the H atom in
given in Tables 1–3. pyrrole nuclei by the electron acceptors, become capa-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

836
BEREZIN et al.
A
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
400
500
600
700
800
λ, nm
Fig. 2. Changes in the electronic absorption spectrum of ç TPP(β-Br) in acetic acid with addition of Zn(Ac) ; c
= 9.02 ×
2
4
2
H2P
–5
–3
10 mol/l, cZn(Ac) = 5.43 × 10 mol/l, T = 298 K.
2
ble of acid-base interaction with pyridine, DMSO,
DMF, and other strong basic solvents, as was observed
in the case of tetraazaporphine [23]:
log(cH0 P/cH P
)
2
2
0.5
0.4
0.3
0.2
0.1
0
H2TAP(X)4 + 2Py
TAP(X)2–···(H+···Py)2. (7)
4
An example of this is the β-tetrabromo substitution
in ç2TPP. It produces strong activation of a molecule in
the complexation reaction:
H2TPP(β-Br)4 + [ZnAc2(Py)m – 2
]
(8)
ZnTPP(β-Br)4 + 2HAc + (m – 2)Py.
Reaction (8) is peculiar in that under standard con-
ditions (298 K and with strongly diluted porphyrin and
metal salt solutions), its rate in pyridine is 103 times
higher than that with unsubstituted ç2TPP: k2v98 = 0.45 ±
0
500
1000 1500 2000
τ, s
0.05 and 8.7 × 10–3 l/(mol s) [24] for H2TPP(β-Br)4 and
H2TPP, respectively.
Fig. 3. Time dependence of log(c0H P/cH P) for complex-
2
2
ation of ç TPP(β-Br) with Zn(Ac) in pyridine at c0
=
2
4
2
H2P
Reaction (1) in pyridine occurs with a low activation
energy due to the active participation of the solvent
–5
5.03 × 10 mol/l, T = 298 K.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

EFFECT OF PYRROLE BROMO-SUBSTITUTION IN TETRAPHENYLPORPHINE
837
Table 1. Kinetic parameters of ç2TPP(β-Br)4 complexation with Zn(Ac)2 in the toluene–pyridine mixture (cPy = 4.41 mol/l)
298 K 308 K 318 K 328 K
keff,
10–3 s–1
keff
,
keff
,
keff
,
kv,
l/(mol s)
kv,
l/(mol s)
kv,
l/(mol s)
kv,
l/(mol s)
10–3 s–1
10–3 s–1
10–3 s–1
2.67 1.1 ± 0.07 0.42 ± 0.02 1.6 ± 0.08 0.62 ± 0.03 2.2 ± 0.10 0.82 ± 0.04 2.7 ± 0.12 1.01 ± 0.05 23 ± 1 –217 ± 13
4.46 2.0 ± 0.1 0.44 ± 0.02 2.8 ± 0.15 0.63 ± 0.03 3.6 ± 0.13 0.82 ± 0.04 4.6 ± 0.24 1.04 ± 0.05 22 ± 1 –217 ± 15
8.92 4.2 ± 0.2 0.47 ± 0.02 5.8 ± 0.3 0.64 ± 0.04 7.3 ± 0.32 0.82 ± 0.04 9.4 ± 0.46 1.05 ± 0.05 22 ± 1 –217 ± 17
11.6
5.5 ± 0.25 0.47 ± 0.03 7.5 ± 0.04 6.5 ± 0.03 9.6 ± 0.45 0.83 ± 0.05 1.2 ± 0.50 1.06 ± 0.06 21 ± 1 –217 ± 11
Table 2. Kinetic parameters of ç2TPP(β-Br)4 complexation with Zn(Ac)2 in pyridine
298 K
308 K
318 K
328 K
keff,
10–3 s–1
keff,
10–3 s–1
keff,
10–3 s–1
keff,
10–3 s–1
kv,
l/(mol s)
kv,
l/(mol s))
kv,
l/(mol s)
kv,
l/(mol s)
0.56 0.45 ± 0.02 0.80 ± 0.03 0.73 ± 0.03 1.30 ± 0.06 0.99 ± 0.04 1.75 ± 0.07 1.4 ± 0.07 2.48 ± 0.12 30.6 ± 0.3 –214.8 ± 10
0.86
1.02
2.74
5.05
0.7 ± 0.04 0.81 ± 0.04 1.1 ± 0.05 1.38 ± 0.04 1.5 ± 0.05 1.74 ± 0.08 2.2 ± 0.11 2.54 ± 0.18 31.2 ± 1.5 –214.6 ± 8
0.9 ± 0.05 0.88 ± 0.04 1.4 ± 0.07 1.37 ± 0.06 1.9 ± 0.11 1.86 ± 0.08 2.8 ± 0.11 2.74 ± 0.11 30.5 ± 1 –214.8 ± 9
2.4 ± 0.10 0.87 ± 0.04 3.7 ± 0.15 1.35 ± 0.07 4.95 ± 0.28 1.8 ± 0.09 7.9 ± 0.48 2.88 ± 0.12 32.1 ± 0.3 –214.4 ± 8
4.3 ± 0.22 0.87 ± 0.03 6.8 ± 0.30 0.35 ± 0.06 9.0 ± 0.45 1.78 ± 0.03 13.0 ± 0.78 2.57 ± 0.14 31.9 ± 0.5 –214.5 ± 11
Table 3. Kinetic parameters of ç2TPP(β-Br)4 complexation with Zn(Ac)2 in acetic acid
298 K 308 K 318 K 328 K
keff,
10–3 s–1
keff,
10–3 s–1
keff,
10–3 s–1
keff,
10–3 s–1
kv,
l/(mol s)
kv,
l/(mol s)
kv,
l/(mol s)
kv,
l/(mol s)
1.81 0.6 ± 0.03 3.3 ± 0.15 1.0 ± 0.05 5.5 ± 0.27 1.5 ± 0.10 8.3 ± 0.4 2.4 ± 0.1 13.3 ± 0.60 36.0 ± 1.5 –213.5 ± 8
3.62 1.3 ± 0.08 3.6 ± 0.15 2.1 ± 0.10 5.8 ± 0.25 3.0 ± 0.15 8.3 ± 0.4 4.9 ± 0.25 13.5 ± 0.65 35.9 ± 0.6 –213.5 ± 10
5.43 1.9 ± 0.10 3.5 ± 0.15 3.1 ± 0.15 5.7 ± 0.20 4.7 ± 0.23 8.6 ± 0.4 7.2 ± 0.3 13.3 ± 0.62 35.8 ± 0.9 –213.5 ± 10
molecules in the porphyrin solvation and the removal of shifts the equilibrium between fully localized structures
protons of the NH groups in the coordination site in the
transition state. This is confirmed by the low values of
the activation entropy. Reaction (1) in pyridine is per-
fectly described by the second-order equation, i.e., first
order with respect to the salt concentration, which fol-
lows from Fig. 3 and from kv remaining constant with
the zinc salt concentration changing by 4.3 times
(Table 1), and first order with respect to the H2P con-
centration. Zinc acetate does not change its coordina-
tion sphere in this concentration interval and is likely to
have the composition [Zn(ÄÒ)2(Py)2] [25].
(the proton from the NH bond is localized) and their
delocalized electron isomers (the proton from the NH
logkeff
–1.0
–1.5
–2.0
–2.5
Reaction (1) for porphyrins with a low-activity NH
bond, such as tetraphenylporphine, proceeds at the
maximum rate in glacial acetic acid and at the mini-
mum rate in pyridine [7, 8]. For the NH-active porphy-
rins (ç2TAP, ç2PC, and their derivatives), the situation
is reversed. This interesting fact in the chemistry of por-
phyrins has been explained as resulting from a depen-
dence of the electronic structure of a macrocycle and its
reaction site N4H2 on the nature of the medium, which
–3.0
–2.6 –2.2 –1.8 –1.4
logcZn(Ac)
2
Fig. 4. Plot of logkeff = f( logcZn(Ac) ) for complexation of
2
ç TPP(β-Br) with Zn(Ac) in pyridine (n = 1.2).
2
4
2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002

838
BEREZIN et al.
11. Berezin, B.D., Zh. Neorg. Khim., 1992, vol. 37, no. 6,
bond is delocalized due to a change in the type of chem-
ical bond or due to complete intramolecular NH ioniza-
tion). These questions are considered in [7–10, 22–24].
p. 1260.
12. Berezin, B.D., Shormanova, L.P., Tsvetkov, G.A., and
Koifman, O.I., Zh. Fiz. Khim., 1979, vol. 53, no. 11,
p. 2716.
As is seen from Tables 1–3, tetrabromo-substitution
in the β-positions of ç2TPP changes the electronic
structure of a macrocycle such that the polarization of
the N–H bond is not suppressed in either pyridine or
glacial acetic acid. These data are of great interest for
the development of structural chemistry of aromatic
macrocycles with the N–H, C–H, S–H and other bonds
capable of giving complexes with the d-metal salts.
13. Berezin, B.D., Shormanova, L.P., Tsvetkov, G.A., and
Koifman, O.I., Voprosy sol’vatatsii i kompleksoobrazo-
vaniya (Problems of Solvation and Complexing), Kre-
stov, G.A., Ed., Ivanovo, 1979, p. 62.
14. Berezin, B.D., Tsvetkov, G.A., and Shormanova, L.P.,
Zh. Neorg. Khim., 1980, vol. 25, no. 10, p. 2645.
15. Berezin, D.B., Cand. Sci. (Chem.) Dissertation,
Ivanovo: Ivanovo State Academy of Chemical Technol-
ogy, 1997.
ACKNOWLEDGMENTS
The authors are grateful to P.A. Stuzhin for his help
in recording electronic absorption spectra on a Hitachi-
2000 spectrophotometer.
16. Bhyrappa, P. and Krishnan, V., Inorg. Chem., 1991,
vol. 30, no. 2, p. 239.
17. Callot, H.J., Bull. Soc. Chim. Fr., 1974, vol. 8, p. 1492.
18. Techniques of Organic Chemistry, vol. 2: Organic Sol-
vents. Physical Properties and Methods of Purification,
Weissberger, A. and Proskauer, E.S., Eds., New York:
Interscience, 1955. Translated under the title Orga-
nicheskie rastvoriteli. Fizicheskie svoistva i metody
ochistki, Moscow: Inostrannaya Literatura, 1958.
REFERENCES
1. Berezin, B.D. and Koifman, O.I., Zh. Fiz. Khim., 1972,
vol. 46, no. 1, p. 42.
2. Berezin, B.D., Koifman, O.I., and Golubchikov, O.A.,
Voprosy kinetiki i kataliza (Problems of Kinetics and
Catalysis), Krestov, G.A., Ed., Ivanovo, 1974, no. 2,
p. 94.
3. Koifman, O.I., Berezin, B.D., Shormanova, L.P., and
Kuvshinova, L.M., Koord. Khim., 1978, vol. 4, no. 10,
p. 1494.
19. Ogranikum; Organisch-Chemisches Grundpraktikum,
Berlin: VEB Deutscher Verlag der Wissenschaften,
1976, vol. 2. Translated under the title Organikum: Prak-
tikum po organicheskoi khimii, Moscow: Mir, 1979,
vol. 2.
4. Berezin, B.D., Golubchikov, O.A., Shormanova, L.P.,
and Kuvshinova, L.M., Izv. Vyssh. Uchebn. Zaved.,
Khim. Khim. Tekhnol., 1978, vol. 21, no. 2, p. 208.
5. Karavaeva, E.B., Potapova, T.I., and Berezin, B.D., Izv.
Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol., 1978,
vol. 21, no. 8, p. 1099.
6. Koroleva, T.A., Koifman, O.I., and Berezin, B.D.,
Voprosy kinetiki i kataliza (Problems of Kinetics and
Catalysis), Krestov, G.A., Ed., Ivanovo, 1978, p. 54.
7. Berezin, B.D., Koordinatsionnye soedineniya porfirinov
i ftalotsianina (Coordination Compounds of Porphyrines
and Phthalocyanine), Moscow: Nauka, 1978.
20. Knorre, D.G., Kurs khimicheskoi kinetiki (Course of
Chemical Kinetics), Moscow: Vysshaya Shkola, 1984.
21. Berezin, B.D., Zh. Fiz. Khim., 1965, vol. 39, no. 2,
p. 321.
22. Stuzhin, P.A., Khelevina, O.G., and Berezin, B.D., Phth-
alocyanines: Properties and Applications, Leznoff, C.C.
and Lever, A.B.P., Eds., New York: VCH, 1996, vol. 4,
p. 19.
23. Khelevina, O.G., Petrov, O.A., and Berezin, B.D.,
Koord. Khim., 1996, vol. 22, no. 12, p. 898.
24. Kharitonov, S.V., Cand. Sci. (Chem.) Dissertation,
Ivanovo; Ivanovo State Inst. of Chemical Technology,
1982.
8. Berezin, B.D., Coordination Compounds of Porphyrin
and Phtalocyanines, New York: Wiley, 1981.
9. Berezin, B.D. and Enikolopyan, N.S., Metalloporfiriny
25. Berezin, B.D. and Golubchikov, O.A., Koordinatsion-
naya khimiya sol’vatokompleksov solei perekhodnykh
metallov (Coordination Chemistry of Solvation Com-
plexes Formed by Transition-Metal Salts), Moscow:
Nauka, 1992.
(Metalloporphyrins), Moscow: Nauka, 1988.
10. Porfiriny: struktura, svoistva, sintez (Porphyrins: Struc-
ture, Properties, and Synthesis), Enikolopyan, N.S., Ed.,
Moscow: Nauka, 1985.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 12 2002