Influence of the structure of the organic moiety of copper(II) porphyrins on their reactivity toward acids

Article abstract of DOI:10.1007/s11176-005-0250-x

The acid-induced dissociation of copper(II) porphyrin complexes with a high degree of meso- and β-substitution in the ligand was studied. The copper(II) complexes with octaethylporphyrins containing 0, 1, 2, 3, and 4 meso-phenyl substituents were ranked with respect to the stability. Octaβ-alkyl and unsymmetrical meso-phenyl substitution affects not only the quantitative characteristics of dissociation of the complexes but also the kinetic relationships of the process. 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11176-005-0250-x

Russian Journal of General Chemistry, Vol. 75, No. 3, 2005, pp. 461 467. Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 3, 2005,
pp. 497 503.
Original Russian Text Copyright
2005 by Kosareva, Klyueva, Lomova, Suslova.
Influence of the Structure of the Organic Moiety
of Copper(II) Porphyrins on Their Reactivity toward Acids
O. V. Kosareva, M. E. Klyueva, T. N. Lomova, and E. E. Suslova
Institute of Chemistry of Solutions, Russian Academy of Sciences, Ivanovo, Russia
Received June 6, 2003
Abstract The acid-induced dissociation of copper(II) porphyrin complexes with a high degree of meso-
and -substitution in the ligand was studied. The copper(II) complexes with octaethylporphyrins containing 0,
1, 2, 3, and 4 meso-phenyl substituents were ranked with respect to the stability. Octa- -alkyl and unsym-
metrical meso-phenyl substitution affects not only the quantitative characteristics of dissociation of the
complexes but also the kinetic relationships of the process.
A versatile procedure for preparing metal porphy-
rins [1] is the reaction of free porphyrin bases with
salts of double-charged metal cations. This procedure
allows preparation of complexes with the majority of
natural and synthetic porphyrins under similar condi-
tions. On the contrary, the conditions for the reverse
reaction, dissociation of the complexes, vary within
very wide limits: in some cases, the acidity of water
or alcohol is sufficient to effect the dissociation, and
in other cases sulfuric acid monohydrate is required.
With variation of the metal cation, the thermodynamic
and kinetic stabilities of metal porphyrins vary in
parallel [2]. The series of kinetic stability were ob-
tained for complexes of a synthetic porphyrin, tetra-
phenylporphine [2], and of natural pheophytin [2].
The kinetic stability of complexes of p and d metals
with pheophytin in AcOH H2SO4 varies within the
range spanning seven orders of magnitude depending
on the electronic structure and oxidation state of the
metal.
was studied previously [6 8]. Thus, we obtained the
stability data for a full series of Cu(II) complexes with
octaethylporphines differing in the degree of meso-
substitution with phenyl groups.
R2
R1
R2
R1
R5
R1
R1
R1
N
N
N
R3
Cu
R4
N
R1
R1
I VIII
I, R1 = CH3, R2 = C6H5, R3 = R4 = R5 = H; II, R1 = C2H5,
R2 = R3 = C6H5, R4 = R5 = H; III, R1 = C2H5, R2 = R3 =
R4 = C6H5, R5 = H; IV, R1 = CH3, R2 = R3 = R4 = R5 =
C6H5; V, R1 = C2H5, R2 = C6H5, R3 = R4 = R5 = H; VI,
R1 = C2H5, R2 = R4 = C6H5, R3 = R5 = H; VII, R1 = C2H5,
R2 = R3 = R4 = R5 = C6H5; VIII, R1 = C2H5, R2 = R3 =
The stability of porphine complexes depends to
a still greater extent on substituents in the meso- and
-positions. For example, octa- -ethylporphine forms
a stable complex with Pb(II) [4], whereas attempted
synthesis of the related meso-tetraphenylporphine
complex results in formation of a mixture of two com-
plexes: that of Pb(II) with the reduced form of tetra-
phenylporphine (meso-tetraphenylchlorin) and that of
Pb(IV) with tetraphenylporphine [5]. The effect of
porphine substitution on the stability of the complexes
was not studied systematically.
R4 = R5 = H.
Complexes I and II do not decompose in glacial
acetic acid at 298 K and on heating; complex III dis-
sociates very slowly, with the conversion half-time of
30 days. On adding small amounts of 100% H2SO4 to
acetic acid, porphyrins I III (CuP) dissociate at meas-
urable rates in accordance with Eq. (1):
CuP + 4H+
H4P+2 + Cu2+.
(1)
In this study we examined the reactivity toward
acids of Cu(II) complexes with highly substituted (in
the meso- and -positions) porphine ligands (I IV).
The kinetics of dissociation of complexes V VIII
The occurrence of reaction (1) causes changes in
the electronic absorption spectra of the complexes in
the course of dissociation. An example for complex II
is shown in Fig. 1. The presence of well-defined iso-
1070-3632/05/7503-0461 2005 Pleiades Publishing, Inc.

462
KOSAREVA et al.
bestic points (
386, 425, and 471 nm) is indica-
tive of the coexmisatxence of two colored species in the
complex acid system throughout the process. The
electronic absorption spectra in the UV visible range
allow the initial and final compounds to be identified
as the starting metal porphyrin and the porphyrin
double-protonated at the nitrogen atoms, respectively.
The linear plots ln(c0/c ) = f( ) (Fig. 2; c0 and c are
the concentrations of the complexes in the initial time
and in time , respectively) show that reaction (1) is
first-order with respect to the metal porphyrin. The
corresponding apparent rate constants kapp are given
438
408
A
in Table 1. For each of complexes I III, as for the
previously studied complexes IV VIII, the dissocia-
tion rate grows with an increase in the H2SO4 concen-
tration. However, the dependence of the reaction rate
on the initial concentration of H2SO4 in AcOH for
copper porphyrins I VIII is different, suggesting
different reaction mechanisms.
For complexes II and III, kapp is a nonlinear func-
tion of the H2SO4 concentration (Fig. 3), fairly ade-
quately described by a power function (2). In Eq. (2),
the parameters const1 and const2 have the sense of the
true rate constant k and the reaction order n with
respect to the acid:
kapp = const1(c 01)const = k(c 01)n,
(2)
2
, nm
Fig. 1. Variation of the electronic absorption spectrum
of complex II in the course of dissociation in AcOH
0.004 M H SO (293 K).
where c01 is the initial H2SO4 concentration.
2
4
When plotted in the log log coordinates, Eq. (2)
gives a linear correlation (R2 0.996 0.998) with a
slope close to 2. The true rate constants calculated for
n = 2 and the corresponding activation parameters are
given in Table 2, together with the true second-order
(with respect to c01) rate constants for complexes V
and VI, calculated from the kapp values given in [6, 7].
ln (c0/c )
1
1.0
2
0.8
0.6
6
5
4
3
The experimentally found kinetic equation (3) for
complexes II and III is as follows:
7
dc2/d = kc2(c 01)2,
(3)
0.4
0.2
0
8
where c2 is the concentration of copper porphyrin.
In the case of I, the slope of the correlation bet-
ween logkapp and logc01 corresponds to the reaction
order with respect to c01 of 1.5 (Fig. 5a). The fractional
reaction order with respect to the acid concentration
indicates that the kinetic equation for the dissociation
of I is more complex. A linear correlation observed in
the coordinates kapp/c01 c01 (Fig. 5b) allows us to write
the following equations for the rate constant and rate
of dissociation of complex I:
4
8
12
16
20
102, s
0
Fig. 2. Kinetic curves [ln(c /c ) vs. time ] of dissocia-
tion of complexes (1, 2) III, (3 5, 7) II, and (6, 8) I. c ,
M: (1) 0.00033, (2) 0.00016, (3, 5) 0.003, (4) 0.004,
(6) 0.100, (7) 0.002, (8) 0.025. T, K: (1, 2, 5, 7) 298,
(3) 308, (4) 288, (6) 303, and (8) 313. (c ) Initial H SO
0
1
0
1
2
4
2
concentration; approximation quality R > 0.98.
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INFLUENCE OF THE STRUCTURE OF THE ORGANIC MOIETY
463
Table 1. Apparent rate constants of dissociation of Cu(II)
complexes I IV with meso-phenyloctaalkylporphyrins in
AcOH H2SO4
we can suggest the following scheme of successive
transformations (6) (9) describing overall reaction (1):
k1
slowly
CuP + H2SO4
[CuH2P]2+ + HSO42
,
(6)
0
1
Comp. no. c
,
M
T, K
kapp 103, s
H2SO4
k2
[CuH2P]2+ + 2H2SO4
H4P2+ + 2HSO + Cu2+,
(7)
(8)
(9)
4
fast
I
0.025
0.050
0.075
0.100
323
313
303
298
323
313
303
298
323
313
303
298
323
313
303
298
308
298
288
308
298
288
308
298
288
308
298
288
308
298
293
298
293
288
298
293
288
288
298
0.11 0.01
K1
H2SO4 + AcOH
HSO4 + AcOH
AcOH+2 + HSO4 ,
0.070 0.006
0.046 0.004
0.037 0.001
0.26 0.02
0.19 0.01
0.13 0.01
0.108 0.002
0.65 0.05
0.42 0.02
0.25 0.02
0.193 0.015
0.97 0.09
0.60 0.04
0.35 0.02
0.25 0.02
0.51 0.04
0.32 0.04
0.20 0.03
1.07 0.04
0.68 0.03
0.42 0.04
1.99 0.06
1.26 0.07
0.84 0.04
2.7 0.2
1.8 0.1
1.20 0.05
2.1 0.2
1.5 0.1
1.1 0.1
5.1 0.4
3.4 0.2
2.1 0.1
8.4 0.4
5.9 0.4
3.8 0.3
6.6 0.6
Unstable
K2
AcOH+2 + SO42 .
improbable
The kinetic equation for the slow step has the form
dc2/d = kc2c1, (10)
where c1 is the equilibrium concentration of H2SO4.
1
kapp 103, s
(a)
1
2.5
2
2.0
II
0.002
0.003
0.004
0.005
1.5
3
1.0
0.5
0
1
2
3
4
5
c01 103, M
1
kapp 103, s
III
0.00015
0.00025
0.00033
(b)
4
9
8
5
6
7
6
5
4
3
2
1
0.00041
0
IV
kapp = k1c1 + k2(c 01)2,
dc2/d = [k1c 01 + k2(c 10)2]c2.
(4)
(5)
0
0
1
2
3
4
c01 103, M
It follows from the linear dependences kapp/c01 c01
(Fig. 5b) that k2 exceeds k1 by more than an order of
magnitude (Table 3).
0
Fig. 3. Plots of k
vs. c for complexes (a) II and (b)
app
1
III. T, K: (1) 308, (2, 4) 298, (5) 293, and (3, 6) 288
2
Taking into account the spectrophotometric data,
(R > 0.995).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 3 2005

464
KOSAREVA et al.
log kapp
(a)
(a)
1
2
3
2
4.3
4.1
3.8
3.6
3
3.9
3.7
1
3.4
3.5
3.3
3.1
3.2
2.8
2.6
2.4
2.9
2.7
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6
log c01
2.2
2.3
2.4
2.5
2.6
2.7
log c01
(b)
kapp/c0I
(b)
6
0.010
5
4
0.008
0.006
0.004
0.002
0
2.8
2.6
6
5
4
2.4
2.2
2.0
1.8
0.02
0.04
0.06
0.08
0.10 c01
3.3
3.4
3.5
3.6
3.7
3.8
log c01
0
0
Fig. 5. Plots of (a) logk
vs. logc and (b) k /c vs.
app
1
app
1
0
0
Fig. 4. Plots of logk
and (b) III. T, K: (1) 308, (2, 4) 298, (5) 293, and
(3, 6) 288 (R > 0.995).
vs. logc for complexes (a) II
c
for complex I. T, K: (1, 6) 303, (2, 5) 313, and
app
1
1
2
(3, 4) 323 [R (1, 2) 0.99, (3) 0.98, (4) 0.93, (5) 0.95,
and (6) 0.94].
2
From the equation for the constant of equilibri-
um (8), we obtain
shifted to the right, i.e., c01 is approximately equal
to the equilibrium concentration of AcOH+2. Thus,
Eq. (12) is identical to the experimental kinetic equa-
tion (3) for complexes II and III, and the constant k
in Eq. (3) is equal to k1/K1. From the known value of
K1 (pK1 4.3 [9]), we can calculate the rate constants
c1 = (1/K1)(c3)2,
(11)
where c3 is the equilibrium concentration of AcOH+2.
for the limiting step (6): k2198 3.83 10 and 3.71 for
3
Taking into account (11), Eq. (10) can be trans-
formed into
complexes II and III, respectively.
dc2/d = k1(1/K1)c2(c3)2.
(12)
The spectra in Fig. 1, the lack of the spectrum of
the intermediate complex [CuH2P]2+ with the biden-
tate coordination of the macrocycle in the system of
the electronic absorption spectra of the reaction mix-
4
At low concentrations of H2SO4 in AcOH (10
10 M), equilibrium (8) is virtually completely
3
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INFLUENCE OF THE STRUCTURE OF THE ORGANIC MOIETY
465
Table 2. Kinetic parameters of the dissociation of complexes II VI in the mixed solvent AcOH H2SO4
1
1
1
Comp. no.
c 01, M
T, K
k, l mol 1 s
Ea, kJ mol
33 1
S , J mol 1 K
II
0.002 0.005
308
298
288
298
293
288
313
303
298
293
333
323
313
303
298
120 3
77 3
49 2
105 3
14 6
III
V
0.00015 0.00041
0.05 0.15
(74 3) 103
(52 3) 103
(36 3) 103
0.15 0.02
0.08 0.02
0.055 0.02
0.04 0.01
2.0 0.2
52 2
51 4
105 13
VI
0.01 0.05
1.1 0.1
0.64 0.05
0.38 0.03
0.28 0.01
33 1
150 3
ture CuP acid, and the low stability of such SAT
complexes known from the literature [2] confirm that
step (6) with transition state IX is the limiting step:
k2 = k2/K1, and c01 c3. The latter fact indicates that,
in the expression c01 = c3 + c1, the equilibrium concen-
tration of H2SO4 can still be neglected.
x
The above examples of the dissociation of metal
porphyrins show that changes in the detailed mechan-
ism of the process are associated with changes in the
acid base properties of the mixed solvent AcOH
H2SO4. In a wide range of solvent compositions, more
significant changes in the kinetics of dissociation of
metal porphyrins should be expected.
N
N
Cu
N
H
N
+
y
+
O
H
O
S
The reactivity (k298) of copper complexes with
meso-phenyl-substituted octaethylporphine increases
with an increase in the number of phenyl groups.
Indeed, as found previously [6, 7], complex VIII dis-
sociates only on reaching the H2SO4 concentration
in AcOH of 0.10 M. Complex VIII dissociates at a
measurable rate even in 100% AcOH without H2SO4
O
O
IX
The above-considered scheme of reactions (6) (9)
is apparently valid for complex I also. However, in
going to a medium with a higher concentration of
H2SO4, in which complex I dissociates (Table 1), it
becomes necessary to take into account the concenta-
tion of molecular H2SO4 species in equilibrium (8).
In this case, the starting complex I will decompose
along two pathways:
298
4
1
additions (kapp 4.7 10 s ). Thus, the complexes
can be ranked in the following order of decreasing
kinetic stability (increasing k298): VIII > V > VI >
II > III > VII.
AcOH+2
[CuH2P]2+ + AcO ,
Table 3. Constants in Eq. (5) and activation parameters of
the dissociation of complex I
k1
(13)
CuP
H2SO4
[CuH2P]2+ + SO24 .
1
1
T, K
k1, l mol 1 s
k2, l2 mol 2 s
k2
The reaction rate will be determined in accordance
with (14):
323
313
303
0.0022
0.0017
0.0014
18 3
0.078
0.046
0.023
49 5
122 17
dc2/d = (k 1c3 + k 2c1)c2.
(14)
1
Ea, kJ mol
1
After transformations similar to those described
S , J mol 1 K
249 8
above, taking into account Eq. (5), we obtain k1 = k1,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 75 No. 3 2005

466
KOSAREVA et al.
Table 4. Electronic absorption spectra in CHCl3 and Rf
values for complexes I IV
The steric effect of phenyl substituents also follows
from the comparison of the kinetic stabilities of com-
plex II and its symmetrically substituted isomer VI.
The rates of dissociation of II and VI are equal at
different concentrations of H2SO4 in AcOH: in the
case of VI, the H2SO4 concentration ensuring the
same dissociation rate is higher by a factor of 15.
Comp. no.
max, nm (log )
Rf
I
II
III
IV
395 (4.97), 530 (4.33), 567 (4.44) 0.444
414 (5.48), 544 (4.15), 579 (4.08) 0.665
424 (4.94), 556 (4.12), 591 (3.93) 0.725
Octamethylporphine, its meso-phenyl derivatives,
and their copper complexes are very poorly soluble in
organic solvents; some of them are virtually insoluble.
Therefore, experiments in solution were possible only
with monophenyl- and tetraphenyl-substituted octa-
methylporphine (complexes I and IV). The stability
of these compounds in solutions appeared to be simi-
lar to the stability of the related octaethylporphine
derivatives. Tetraphenyl-substituted complexes IV
and VII decompose at room temperature in 100%
AcOH containing no H2SO4 additions. Monophenyl-
substituted complexes I and V dissociate at equal
rates in 0.05 0.1 M solutions of H2SO4 in AcOH.
430 (4.65), 568 (3.90)
0.595
a
Alufol, eluent hexane benzene, 1 : 1.
This order shows that a decrease in the kinetic
stability is due to the electron-withdrawing properties
of phenyl substituents. The phenyl substitution de-
creases the electron density on the bonds and atoms of
the macroring and the strength of the N Cu donor
acceptor bonds. This conclusion is confirmed by the
relationship between the activation parameters of the
complexes. For the reactions with isoentropic activa-
tion (complexes II and V), the activation energy in-
creases with an increase in the stability of the com-
plex. For VI, in accordance with the stability order,
we could expect an intermediate value of Ea between
In dissociation of the compounds of the octaethyl-
porphine series (II, III, V, VI) in the mixed solvent,
the limiting step is first-order with respect to H2SO4
[Eq. (10)]. Contrastingly, in dissociation of the com-
pounds of the octamethylporphine series (with I as
example), the kinetic equation for the limiting step
[Eq. (14)] includes, along with c1, also the [AcOH+2]1.
The increased order with respect to the acid reagent
in the latter case is apparently due to the increase in
the basicity of the macroring and coordinated nitrogen
atoms in going from ethyl to methyl substituents.
1
1
the values for II (33 kJ mol ) and V (51 kJ mol ).
However, actually Ea for VI is very low because of
sharply decreased S (increased role of solvation of
the transition state). Complex III surpasses in stability
only VII. However, its dissociation is characterized
by high Ea. This complex dissociates in a medium
4
with minor additions of H2SO4 ( 10 M), in which
the concentration of charged species is low. Apparent-
ly, this accounts for the decreased role of solvation of
the transition state (positive S ) and increased Ea.
EXPERIMENTAL
Metal porphyrins I IV were prepared by the reac-
tion of CuAc2 H2O (analytically pure grade) with
the corresponding porphyrins in refluxing DMF for
20 30 min and were isolated by extraction with chlo-
roform and repeated washing of the chloroform solu-
tion with water. The compounds were purified by
column chromatography on Al2O3 (Brockmann grade
II) with CHCl3 as eluent. The purity of the complexes
was judged from the absence of changes in the elec-
tronic absorption spectra after successive purification
steps (Table 4). The required porphyrins were synthe-
sized by A.S. Semeikin as described in [12, 13].
It could be expected that the electronic effect of the
meso-phenyl substitution in the above series will re-
sult in increased contribution of the Cu N -dative
bonding to the stability of the complexes. However,
actually this is not the case. Apparently, the geometric
factor in substitution should also be taken into account
along with the electronic effects of phenyl groups.
According to single crystal X-ray diffraction studies
[10, 11] and optimization of the molecular geometries
by the ZINDO 1 method [6] taking into account the
-dative interaction of the metal cation with the por-
phyrin ligand, the coordination center of octaethyl-
porphine remains planar on introduction of one phenyl
substituent. Upon further meso-substitution, the defor-
mation of the macroring grows, and the structure of
the complex of tetraphenyl-substituted octaethylpor-
phine becomes crimped. With an increase in the defor-
mation of the aromatic macroring, the conditions for
-dative bonding Cu N get worse, which results in
destabilization of the complexes.
Solutions of H2SO4 in AcOH were prepared by
adding an accurately weighed portion of sulfuric acid
monohydrate into the required amount of AcOH.
Acetic acid (chemically pure grade) was dehydrated
by stepwise freezing thawing. Sulfuric acid mono-
hydrate was prepared from 30% oleum (chemically
pure grade) and concentrated H2SO4 (chemically pure
grade) with conductometric monitoring.
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INFLUENCE OF THE STRUCTURE OF THE ORGANIC MOIETY
467
The rates of reactions of I III with acids were
determined spectrophotometrically. The electronic
absorption spectra of the solutions were recorded on
Hitachi U-2000 and SF-26 spectrophotometers with a
temperature-controlled cell. The accuracy of the tem-
perature control was 0.1 K. The variations of the
optical density of solutions of I III in a proton-donor
solvent in the course of thermostating were monitored
at a working wavelength in the vicinity of the absorp-
tion maximum of the released porphyrin ligand.
3. Lomova, T.N., Volkova, N.I., and Berezin, B.D., Zh.
Neorg. Khim., 1987, vol. 32, no. 4, p. 969.
4. Ferguson, J.A., Meyer, T.J., and Whitten, D.G., Inorg.
Chem., 1972, vol. 11, no. 11, p. 2767.
5. Molodkina, O.V., Lomova, T.N., and Mozhzhukhi-
na, E.G., Izv. Ross. Akad. Nauk, Ser. Khim., 1998,
no. 10, p. 2052.
6. Suslova, E.E., Cand. Sci. (Chem.) Dissertation,
Ivanovo, 2001.
7. Klyueva, M.E., Suslova, E.E., and Lomova, T.N., Zh.
Obshch. Khim., 2003, vol. 73, no. 8, p. 1377.
The apparent (kapp) and true (k) rate constants and
the reaction order with respect to proton-donor species
(n) were determined with the Microsoft Excel program
by optimization of the dependences ln(c0/c ) and
kapp c01, respectively. The activation energies were
found with the Arrhenius equation. The activation
entropies were calculated with the basic equation of
the transition state theory, transformed to the follow-
ing form [14]:
8. Suslova, E.E., Lomova, T.N., and Klyueva, M.E.,
Abstracts of Papers, XXI nauchnaya sessiya Rossii-
skogo seminara po khimii porfirinov i ikh analogov
(XXI Scientific Session of the Russian Workshop on
the Chemistry of Porphyrins and Their Analogs),
Ivanovo, 2000, p. 30.
9. Fialkov, Yu.A., Rastvoritel’ kak sredstvo upravleniya
khimicheskim protsessom (Solvent as a Means for
Controlling a Chemical Process), Moscow: Znanie,
1988.
S
= 19.11log k T + Ea/T
19.1log T
205. (15)
ACKNOWLEDGMENTS
10. Sparks, L.D., Medforth, C.J., Park, M.-S., Cham-
berlain, J.R., Ondrias, M.R., Senge, M.O.,
Smith, K.M., and Shelnutt, J.A., J. Am. Chem. Soc.,
1993, vol. 115, no. 2, p. 581.
The study was made within the framework of the
Integrated Program of the Russian Academy of Sci-
ences Purposeful Synthesis of Substances with the
Preset Properties and Development of Functional
Materials Based on Them and was supported by the
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