Effect of the composition of the H2SO4-AcOH binary solvent on the dissociation kinetics of metal porphyrins

Article abstract of DOI:10.1023/B:RUGC.0000007662.38908.e2

The kinetics and mechanism of dissociation of copper(II) complexes with octaethylporphyrin and of manganese(III) with tetraphenylporphyrin were studied in relation to the composition of an H₂SO₄-AcOH binary solvent. Concentration ran

Full text of DOI:10.1023/B:RUGC.0000007662.38908.e2

Russian Journal of General Chemistry, Vol. 73, No. 8, 2003, pp. 1303 1308. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 8, 2003,
pp. 1377 1382.
Original Russian Text Copyright
2003 by Klueva, Suslova, Lomova.
Effect of the Composition of the H2SO4 AcOH Binary Solvent
on the Dissociation Kinetics of Metal Porphyrins
M. E. Klueva, E. E. Suslova, and T. N. Lomova
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received October 1, 2001
Abstract The kinetics and mechanism of dissociation of copper(II) complexes with octaethylporphyrin
and of manganese(III) with tetraphenylporphyrin were studied in relation to the composition of an H2SO4
AcOH binary solvent. Concentration ranges of H2SO4 in the composition of the binary solvent, where the
kinetics of dissociation of metal porphyrins obeys common kinetic regularities, were found using data for
porphyrin complexes of transition metals and indium. The reaction order with respect to proton and the nature
of the active form of this latter change in the range of solvent compositions, where the H0 c0H SO dependence
2
4
is nonlinear.
In aprotic solvents, porphyrin complexes with p-
and d-metal cations behave as nonelectrolytes. In
solutions containing a proton-donor component, they
dissociate into a solvated metal cation (or a solvated
salt) and a free or protonated porphyrin [1, 2]. The
different location of strong absorption bands in the
electronic absorption spectra of porphyrins and their
metal complexes makes possible the kinetics of dis-
sociation of metal porphyrins to be studied by spec-
trophotometry [3]. As a rule, dissociation of com-
plexes is impossible to study kinetically in pure
proton-donor solvents. When using mixed solvents,
one can vary their composition and component con-
centrations to obtain a solvent with necessary acidity
and solution power.
in glacial acetic acid (
564 and 529 nm) and in
max
chloroform almost coincide; the spectrum in AcOH
does not change on heating. The spectra of CuOEP in
the AcOH 0.1 0.25 M H2SO4 system vary with time,
passing into the spectrum of H4OEP2+ with isosbestic
points preserved (Fig. 1), which corresponds to reac-
tion (1).
CuOEP + 4H+solv
H4OEP2+ + Cu
.
(1)
2+
solv
Reaction (1) is first-order with respect to
metal porphyrin concentration (Fig. 2). As seen from
the apparent parameters of CuOEP dissociation at
298 353 K (Table 1, Fig. 3), the reaction rate is
proportional to the concentration of H2SO4. This
points to a first order of the reaction with respect to
sulfuric acid concentration. However, sulfuric acid
behaves as a weak electrolyte in AcOH. With regard
to pKa 4.3 [10], we can ascertain the equation
The H2SO4 AcOH system has most commonly
been used for studying dissociation of stable com-
plexes [1, 4 7]. However, as a rule, kinetic measure-
ments were carried out within narrow ranges of
H2SO4 concentrations in AcOH. In this work we have
studied the kinetics of dissociation of copper(II)
complexes of octaethylporphirin (CuOEP) and of
manganese(III) with tetraphenylporphyrin
[(Cl)MnTPP], which differ greatly in their stability.
The study was carried out within various ranges of
H2SO4 concentrations in the H2SO4 AcOH binary
solvent. We have also summarized dissociation
kinetics data for a series of metal porphyrins over a
wide range of concentrations of the binary solvent.
0
2
0
+
cH SO = cAcOH /Ka (cH SO is the initial concentration
2
4
2
2
2
4
+
2
of H2SO4 in AcOH, cAcOH is the equilibrium con-
centration of protonated acetic acid, and Ka is the
acidic dissociation constant of sulfuric acid in acetic
acid) at low concentrations of H2SO4 in AcOH. Then
we can write the overall CuOEP dissociation kinetic
equation in form (2).
dcCuOEP
d
= k(1/Ka)cCuOEP cA2cOH+
(2)
.
2
Here
is time and k is the true dissociation rate
The electronic absorption spectra of the synthesized
metal porphyrins are consistent with published data
[8, 9].
constant.
A linear dependence of apparent dissociation rate
constant (kapp) on initial concentration of sulfuric acid
in acetic acid was found previously for the complexes
The location of bands in the spectrum of CuOEP
1070-3632/03/7308-1303$25.00 2003 MAIK Nauka/Interperiodica

1304
KLUEVA et al.
The second order with respect to solvated proton,
ln (c0/c )
moderate activation energies, and negative S values
make it possible to define the stage of protonation of
two nitrogen atoms of the coordinated porphyrin as a
limiting stage, which results in destruction of the
complex in the second, fast stage [scheme (3)].
1.0
2
0.8
0.6
0.4
0.2
0
5
3
slowly
CuOEP + 2AcOH+2
(AcOH)2CuH2OEP
fast
1
2+
(3)
Cu
+ H2OEP.
solv
4
It is seen from Fig. 1 that H2OEP is not accumu-
lated in the reaction mixture, i.e. the formation of the
H4OEP2+ dication, too, occurs very fast.
In the case of (Cl)MnTPP, a hypsochromic shift of
the absorption bands in the electronic spectrum takes
place on passing from chloroform to proton-donor
2
10
20
30
40
50
10 , s
0
Fig. 1. Plot of ln (c /c ) vs.
(1, 4) (Cl)MnTPP and (2, 3, 5) CuOEP in the H SO
AcOH binary solvent [c
(4) 7.18, and (1) 7.85]. Temperature, K: (1) 308, (2) 343,
(3) 333, (4) 298, and (5) 323.
for dissociation of
2
4
solvents:
of the first absorption band is 616, 606,
max
0
, M: (5) 0.15, (2, 3) 0.2,
H2SO4
and 602 in chloroform, glacial AcOH, and AcOH
H2SO4 (c0H SO 5 M), respectively. In AcOH H2SO4
2
4
(c0H SO 0 5 M), the spectrum is time invariant.
2
4
A
1.0
At the H2SO4 concentrations in AcOH higher than
5.0 M, the electronic spectrum of the complex
gradually transforms into the spectrum of the doubly
protonated tetraphenylporphyrin H4TPP2+ upon
thermostating at 298 K (Fig. 4). The kinetic charac-
teristics of (Cl)MnTPP dissociation [Eq. (4)] in the
AcOH H2SO4 binary solvent (c0H SO 3 8 M) at
4
288 318 K are given in Table 2 and2Fig. 5. At H2SO4
concentrations of 8.5 10 M, the complex dissociates
instantaneously to give the doubly protonated ligand
[scheme (4)].
0
450
500
550
600
650 , nm
Fig. 2. Changes in the electronic absorption spectrum of
2+
CuOEP upon thermostating in the H SO AcOH binary
(Cl)MnTPP + 4Hs+olv
H4TPP2+ + ClMn . (4)
solv
2
4
0
solvent (c
0.15 M, 323 K).
H2SO4
The absence of ionization or substitution of the Cl
ligand replacement in the coordination sphere of
manganese tetraphenylporphyrin was experimentally
confirmed by the invariance of the absorption spec-
trum of (Cl)MnTPP in CHCl3 after isolation of the
(Ac)FeChl [Chl is the dianion of pheophytin (a)] and
(Cl)PtDP and (Cl)PtMP (DP and MP are the dianions
of proto- and mesoporphyrin) at H2SO4 concentrations
0.2 1.0 and 0.4 2 M, respectively [11, 12].
Table 1. Apparent kinetic parameters of CuOEP dissociation in the H2SO4 AcOH binary solvent
1
kapp 104, s
298
c 0H SO
,
4
kapp 105,
Ea,
kJ/mol
S ,
2
a
1
1
s
J mol 1 K
M
313 K
323 K
333 K
343 K
353 K
0.10
0.15
0.20
0.25
1.8 0.1
2.1 0.1
5.0 0.6
7.6 0.7
3.2 0.3
0.85 0.05
1.6 0.2
3.1 0.1
5.6 0.3
58 1
59 12
52 1
44 3
64 4
60 41
83 3
0.74 0.03 1.3 0.1
1.60 0.09 2.9 0.2
1.3 0.1
2.2 0.1
4.0 0.5
110 9
a
Found by extrapolation of the k
1/T plot.
app
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

EFFECT OF THE COMPOSITION OF THE H2SO4 ACOH BINARY SOLVENT
1305
kapp 104,
complex from the reaction mixture at instant
(
1/2
1/2
1
1
is the half-life period of the metal porphyrin).
Moreover, the kinetic studies of dissociation of
Mn(III)TPP with a series of acido ligands (Cl , Br ,
AcO , and HSO4) in the AcOH H2SO4 binary solvent
(c0H SO 3 5 M) [13, 14] showed that quantitative
s
7
6
5
2
4
parameters of dissociation of different extra com-
plexes are different. This means that the manga-
nese(III) complexes dissociate by metal nitrogen
bonds without initial ionization of metal acido ligand
bonds. The absence of an appreciable concentration of
[MnTPP]+ ions in the solution is confirmed by the
high dissociation activation energy and entropy of
(Cl)MnTPP [Eq. (4)] compared to H2TPP complexes
with doubly charged metal ions (Table 2) [1, 3].
2
4
3
3
Using the above results, we can examine the dis-
sociation rate constants of (Cl)MnTPP as a function
2
of solvent composition over a wide range of H2SO4
298
app
concentrations. The value of log k
for dissociation
1
of (Cl)MnTPP in 4 8 M H2SO4 are linearly related to
the initial H2SO4 concentration in AcOH and with the
acidity function H0 [15] (Fig. 5). The correlations
seem to be associated with the fact that H0 is pro-
portional to c0H SO in this very range of compositions
0
0.15
0.20
0.25 c0H SO , M
2 4
Fig. 3. Plot of apparent CuOEP dissociation rate con-
stants (k ) vs. initial concentration of H SO in AcOH.
2
4
of the binary solvent [15]. The slope of the straight
app
2
4
298
app
line in the log k
H0 coordinates is close to two
Temperature, K: (1) 343, (2) 333, and (3) 323.
(1.74), i.e. the kinetic equations includes the total
acidity h0, rather than the concentration of any active
form of proton in the solution [Eq. (5)]. The total
acidity of the medium is determined by the presence
of the species AcOH+2, H3SO4+, H2SO4, and AcOH and
of ion pairs [15]. In the range of compositions of the
H2SO4 AcOH binary solvent under consideration, the
activity of the more acidic species AcOH+2 and H3SO4+
in reaction (4) is suppressed because of the presence
A
0.5
0
450
500
550
600
650
700
750
, nm
Fig. 4. Changes in the electronic absorption spectrum of (Cl)MnTPP in the course of thermostating in the H SO AcOH
2
4
0
binary solvent (c
6.61 M, 298 K).
H2SO4
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

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KLUEVA et al.
Table 2. Apparent kinetic parameters of (Cl)MnTPP dissociation in the AcOH H2SO4 binary solvent
1
kapp 104, s
c 0H SO
,
4
Ea,
kJ/mol
S ,
2
1
J mol 1 K
298 Ka
333 K
343 K
353 K
M
3.00
4.00
5.00
0.0050 0.0003
0.0047 0.0003 0.4030 0.0005
0.031 0.006
0.153 0.008
0.36 0.01
1.23 0.02
5.67 0.08
0.79 0.01
3.47 0.03
80 2
105.4 0.8
100 4
105 4
22 2
30 10
1.910 0.002
288 K
298 K
308 K
318 K
6.00
6.61
7.13
7.85
1.3 0.1
3.3 0.2
5.8 0.4
9.1 0.9
3.5 0.2
0.24 0.02
0.50 0.04´
1.8 0.2´
0.87 0.05
1.4 0.1
3.6 0.2
97 5
100 7
67 4
3 16
11 24
93 13
a
b
Found by extrapolation of the k
1/T plot.
k
at 290 K.
app
app
of higher concentrations of molecular H2SO4 which is
less active in this reaction.
The participation of molecular H2SO4 along with
solvated protons in reaction (4) increases E and
S
(Tables 1 and 2) compared with the reaction in media
with a low H2SO4 concentration [reaction (1)]. When
H2SO4 molecules participate in the limiting stage
(dissociation of two metal nitrogen bonds), the energy
consumed for activation of the reacting system is not
compensated for by metal cation solvation on the way
to transition state. Such a compensation takes place
during attack of a metal porphyrin by solvated protons
[Eq. (3)] whose electron-donor solvent molecule has
a higher affinity to the metal cation than H2SO4.
dc(Cl)MnTPP/d = kc(Cl)MnTPPh20.
(5)
Here c(Cl)MnTPP is the concentration of manganese
porphyrin and h0 is the acidity of the medium.
Comparison of Eqs. (2) and (5) shows that the
stoichiometric
mechanisms
of
CuOEP
and
(Cl)MnTPP dissociation are common.
H0
At H2SO4 concentration in AcOH of 3 and 4 M,
the (Cl)MnTPP dissociation rate at 298 K is the same,
3.5 4.0 4.5 5.0 5.5 6.0 6.5
while the H0 values are different. As c0H SO increases
4
from 3 to 4 M, the activation energy 2increases by
6.5
25.4 kJ/mol, and the activation entropy, by
2
1
1
83 J mol 1 K . The sharp change in E and S points
6.0
5.5
5.0
to decreased role of solvation of the transition state of
reaction (4), which, in its turn, provides evidence for
the conclusion that the solvated protons AcOH+2 are
involved in the reaction at low H2SO4 concentrations.
298
4.5
4.0
3.5
3.0
A fair linear correlation between log k
and H0
app
was found for (AcO)CrTPP, (Cl)2ZrTPP, and
(Cl)2HfTPP and for the indium complex of tetra-
phenylchlorine (Cl)InTPC (Table 3). Their dissocia-
tion rates were determined previously [2, 5, 6, 16].
The complexes dissociate at a measurable rate in
AcOH H2SO4 mixtures at c0H SO 1.5 4.0 M. The
4
5
6
7
8
9
2
4
c0H SO , M
order of these dissociations with respect to the total
acidity h00 is close to unity (Table 3).
2
4
Fig. 5. Semilogarithmic plots of apparent (Cl)MnTPP
Another group of complexes characterized by a
similar linear correlation and by a second dissociation
order with respect to h0 comprises, along with the
(Cl)MnTPP complex discussed above, (Cl)RhTPP
298
dissociation rate constants (k
) vs. (1) acidity function
app
of the solvent and (2) initial concentration of H SO in
2
4
AcOH (298 K).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

EFFECT OF THE COMPOSITION OF THE H2SO4 ACOH BINARY SOLVENT
1307
[17], (AcO)MnTPP [14], (AcO)MnTPP(X)4 (X = n-Br,
n-F, n-OMe) [18] and CuTPP [4]. As a rule, these
complexes dissociate at higher concentrations of
H2SO4 compared to the complexes whose dissociation
order with respect to h0 is close to unity (Table 3). An
exception is CuTPP whose dissociation order with
respect to h0 (2.33) relates to low-acidity media
(c0H SO 0.5 2.0 M). At the same time, the linear
Table 3. Concentration range of H2SO4 in the AcOH
H2SO4 binary solvent for dissociation of metal porphyrins
and numerical values of coefficients in the equation
298
log k
= nH0
log k298
app
c 0H SO
,
4
Coefficients
2
Complex
M
n
log k298
2
4
298
kapp f(c0H SO ) correlation typical of other complexes
2
4
(AcO)CrTPP
(Cl)2ZrTPP
(Cl)2HfTPP
(Cl)InTPC
(Cl)RhTPP
(AcO)MnTPP
1.43 2.86
2.0 4.0
2.0 4.0
2.0 3.0
3.0 5.0
3.0 5.0
0.89
1.10
1.29
0.69
1.66
1.76
1.94
1.97
2.11
2.33
7.7
8.7
10.3
6.2
11.1
13.8
13.7
14.9
16.5
11.5
in such media [11, 12], is not observed with CuTPP.
The increased dissociation order in the latter case is
probably explained by the fact that in CuTPP there is
free access of reagents from both sides of the
macroring plane. All the other complexes under con-
sideration have an axial acido ligand in the coordina-
tion sphere, which creates steric hindrance for attack-
ing M N bonds.
(AcO)MnTPP(OMe-p)4 4.3 5.4
(AcO)MnTPP(F-p)4
(AcO)MnTPP(Br-p)4
CuTPP
4.3 6.3
5.4 7.0
0.5 2.0
The above regularities allow us to recognize the
ranges of H2SO4 concentrations in the AcOH H2SO4
binary solvent, where different metal porphyrins have
common rate equations including the concentration of
solvated protons or the total acidity in the degrees of
1 or 2. These ranges are: 0.15 2 M, proton donor
species AcOH+2, reaction order in AcOH+2 concentra-
tion 2; 2 4 M, all the proton species, order in h0 1;
and 4 8 M, all proton species, order in h0 ~2.
synthesized by the procedures [19, 20]. The com-
plexes were obtained by reacting Cu(AcOH)2 H2O
(analytical grade) with H2OEP and MnCl2 4H2O
(analytical grade) with H2TPP in boiling DMF for
10 15 min. CuOEP was isolated by precipitation on
dilution of the reaction mixture with water.
(Cl)MnTPP was extracted with chloroform from the
reaction mixture diluted with equal volume of H2O
and repeatedly washed with water to remove excess
salt and the solvent. Spectrally pure complexes were
obtained by double chromatography on a column of
Al2O3 (eluent chloroform). Electronic absorption
spectrum (CHCl3), max, nm (log ): CuOEP: 563
(4.54), 528 (4.26), 400 (5.88); and (Cl)MnTPP: 616
(3.03), 582 (4.00), 478 (4.98).
As seen, as the H2SO4 concentration increases, the
AcOH+2 cation which is the most active in the solvo-
protolytic dissociation and is the only reagent at low
c0H SO concentrations is replaced by other species,
2
4
H2SO4 molecules inclusive. In media with sufficiently
high H2SO4 concentrations, two parallel dissociation
reactions seem to occur, involving AcOH+2 and H2SO4.
Then the experimental order in total acidity h0
depends on the relative rates of these reactions. When
the rates of the two reactions become commensurate,
the order in h0 becomes close to two. The first order
in proton donor in each of these reactions is accounted
for by screening of the coordination center of the
metal porphyrins in question (except for CuTPP) by
the acido ligand on one side of the macroring plane.
Acetic acid solutions of H2SO4 of specified con-
centration were prepared by the gravimetric procedure
from 100% AcOH and sulfuric acid monohydrate.
100% AcOH was obtained from glacial acetic acid
(chemically pure grade) by stepwise defrosting,
sulfuric acid monohydrate was obtained from 60%
oleum and H2SO4 (chemically pure grade) by the
conductometric procedure.
In conclusion we note that partial kinetic equations
for the limiting stage of dissociation of metal por-
phyrins at various compositions of the AcOH H2SO4
solvent can be obtained only when the equilibrium
compositions of species over a wide range of solvent
compositions will become available, as is the case
with concentrated aqueous H2SO4.
The dissociation rates were determined spectro-
photometrically [3]. The electronic absorption spectra
were recorded on Hitachi-2000 and SF-26 spectro-
photometers. The apparent rate constants (kapp) were
refined by the least-squares method. The k
298
app
values
for CuOEP were determined by extrapolation of the
log k 1/T plot to the standard temperature. The
activation energies Ea were determined by least-
squares fit of the log k 1/T plot. The activaton en-
EXPERIMENTAL
Octaethylporphyrin
H2OEP
(provided
by
A.S. Semeikin) and tetraphenylporphyrin H2TPP were
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 73 No. 8 2003

1308
KLUEVA et al.
tropies S were calculated by the basic equation of
the transition state theory, transformed to form (6).
9. Boucher, L.J., Coord. Chem. Rev., 1972, vol. 7, no. 1,
p. 289.
10. Fialkov, Yu.A., Rastvoritel’ kak sredstvo upravleniya
khimicheskim processom (Solvent as a Means for
Controlling Chemical Reaction), Moscow: Znanie,
1988.
S
= 19.1log k T + Ea/T
19.1log T
205. (6)
Here kT is the true dissociation rate constant at
temperature T.
11. Berezin, B.D. and Drobysheva, A.N., Zh. Fiz. Chim.,
1968, vol. 42, no. 11, p. 2821.
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