On the mechanism of the metal exchange in natural cadmium porphyrins

Article abstract of DOI:10.1134/S1070328407070032

The kinetics of the exchange of the Cd²⁺ ions in the complexes with the natural protogroup porphyrins (protoporphyrin, mesoporphyrin, deuteroporphyrin, and hematoporphyrin) for the Zn²⁺ ions is studied. The stoichiometric ratios of components in the reaction of cadmium exchange for zinc in acetonitrile and DMSO are established. The results obtained are compared with the previously published data for the reaction withthe Co²⁺ ions. The activation mechanism of the metal exchange reaction is considered, depending on the effect of the nature of substituents in porphyrin, d-metal ions, and a solvent.

Full text of DOI:10.1134/S1070328407070032

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2007, Vol. 33, No. 7, pp. 488–492. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © M.B. Berezin, S.V. Zvezdina, B.D. Berezin, 2007, published in Koordinatsionnaya Khimiya, 2007, Vol. 33, No. 7, pp. 499–503.
On the Mechanism of the Metal Exchange
in Natural Cadmium Porphyrins
M. B. Berezin, S. V. Zvezdina, and B. D. Berezin
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, 153045 Russia
Received July 26, 2006
Abstract—The kinetics of the exchange of the Cd2+ ions in the complexes with the natural protogroup porphy-
rins (protoporphyrin, mesoporphyrin, deuteroporphyrin, and hematoporphyrin) for the Zn2+ ions is studied. The
stoichiometric ratios of components in the reaction of cadmium exchange for zinc in acetonitrile and DMSO
are established. The results obtained are compared with the previously published data for the reaction with the
Co2+ ions. The activation mechanism of the metal exchange reaction is considered, depending on the effect of
the nature of substituents in porphyrin, d-metal ions, and a solvent.
DOI: 10.1134/S1070328407070032
Previously [1, 2], we studied the metal exchange [1, 2], use was made of aprotic solvents and therefore
kinetics, using the reactions of cadmium mesoporphy-
rin and protoporphyrin with Co(II) and Zn(II) chlorides
in acetonitrile and DMSO, and the changes in the met-
alloporphyrin spectra during this process. The metal
exchange reaction can be written as follows:
protons did not participate in exchange reaction (1). It
has been shown also that reaction (1) of cadmium
exchange for zinc and cobalt is a two-stage bimolecular
reaction, which is described by the kinetic equation of
the second-order reaction:
CdP + MCl2(Solν)n – 2
CdCl2(Solν)n – 2 + MP, (1)
–dÒCdP/dt = kν[CdP][MCl2].
(4)
where CdP and MP are metalloporphyrins,
MCl2(Solν)n – 2 are the solvation complexes of metal
chlorides [3].
The bimolecular (true) rate constant (kn) depends
not only on the nature of a solvent and metal salt, but
also on the metal salt concentration.
It is known [4] that in centimolar solutions of acetic
acid in ethanol, CdChl (Chl is the chlorophyll-ligand)
dissociates readily with Cd2+ elimination. The rate of
the Cd complex dissociation increases rapidly with the
increasing ëç3ëééç concentration. This is
explained by an increase in the concentration of sol-
R
CH
3
H C
3
R
vated proton ë2ç5éH+2 and by the growth in the chem-
N
N
N
N
ical activity of the ëç3ëééç molecules, which lose
their solvation shell as the molar fraction of ë2ç5éç is
decreased in a binary solution. We assume that the dis-
sociation of cadmium porphyrins in ethanol–acetic acid
mixture occurs in two routs:
Cd
H C
3
CH
3
H COOH C
CH COOCH
3 3
3
3
CdP + 2C2H5OH+2
and
CdP + 2CH3COOH
[Cd(C2H5OH)2]2+ + H2P, (2)
CdDP: R = H — cadmium deuteroporphyrin;
CdGP: R = CH(OCH )CH3 — cadmium hematoporphyrin;
3
CdMP: R = C2H5 — cadmium mesoporphyrin;
CdPP: R = CH=CH2 — cadmium protoporphyrin.
Cd(CH3COO)2 + H2P. (3)
Cadmium porphyrins are known not to undergo sol-
volytic dissociation [1]. The ëç3ëééH+2 concentra-
tion is negligible and, hence, the CdP dissociation can-
not follow the other route. The above factors of CdP
dissociation in solutions should be taken into consider-
ation, when the other sources of solvated protons can
In this work, the rates of reaction (1) of cadmium
exchange in its complexes with proto-, meso-, hemato-,
and deuteroporphyrins for zinc are measured in aceto-
nitrile and DMSO and the kinetic parameters—kν, E,
appear in reaction (1). In experiments performed in and ∆S# of this reaction are calculated.
488

ON THE MECHANISM OF THE METAL EXCHANGE
489
EXPERIMENTAL
1
A
1.0
2
The porphyrin-ligands and their complexes were
prepared by a known procedure [5]. Pure grade aceto-
nitrile was distilled twice over ê2é5; chemically pure
grade DMSO was distilled under reduced pressure. The
kinetic measurements were performed on Specord M40
and SF-26 spectrophotometers in the temperature inter-
val 298–318 K at ZnCl2 concentration (2.09–3.92) ×
10–2 mol/l. The reaction rate was determined from the
change in the optical density of CdGP and CdDP solu-
tions in the electronic absorption spectra at the operat-
ing wavelengths λ = 554 and 550 nm, respectively. The
procedure used for calculation of the CdP current con-
centration and of the kinetic parameters was described
in [2]. The changes in the electronic absorption spectra
of a CdDP solution in DMSO during metal exchange
reaction are shown in Figs. 1, 2; the kinetic data are
given in Tables 1, 2.
2
1
0.5
470
650
λ, nm
Fig. 1. The changes in the electronic absorption spectra of
CdDP solution in reaction with ZnCl in DMSO at c0
=
2
CdDP
RESULTS AND DISCUSSION
0
–5
–2
4.6 × 10 , cZnCl = 7.83 × 10 mol/l: (1) at the initial
2
The metal exchange reaction (1) in the coordina-
tion center of metalloporphyrins is the most complex
reaction of all the known reaction types [6]. This reac-
tion involves all types of interactions, except for redox
(transfer of lone electrons) and acid-base (transfer of
protons) interactions. According to classification [6],
reaction (1) is the association–dissociation complex
moment, (2) in 120 min.
ical event consists of simultaneously occurring disso-
ciation of one complex and the formation of the other
complex. The above classification is based on the fact
formation reaction. In this case, the elementary chem- that the principal objects of the chemical event are the
Table 1. The kinetic parameters of metal exchange reaction (1) between Cd(II) complexes with porphyrins of protoporphyrin
group and chlorides of Co(II) [1, 2] and Zn(II) in acetonitrile
k2v98 of reaction (1),
Activation energy Activation entropy
Cd porphyrinate
CdMP
csalt × 104, mol/l
T, K
E, kJ/mol
∆S#, J/(mol K)*
l mol–1 s–1
CoCl2
1.73
298
308
318
0.84
1.28
1.78
30
–210
CoCl2
3.24
298
308
318
0.35
0.53
0.71
27
31
31
31
29
–214
–228
–222
–227
–224
CdPP
CoCl2
0.245
298
308
318
13.0
20.1
28.3
CoCl2
0.460
298
308
318
13.6
21.3
30.1
ZnCl2
6.13
298
308
318
0.048
0.071
0.104
ZnCl2
11.50
298
308
318
0.047
0.070
0.099
* The activation energy and entropy were determined to within 8 kJ/mol and 16 J/(mol K), respectively.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 7 2007

490
BEREZIN et al.
Table 2. The kinetic parameters of metal exchange reaction (1) between Cd(II) complexes with porphyrins of protoporphyrin
group and Zn(II) chloride in DMSO
k2v98 of reaction (1),
Activation energy Activation entropy
Cd porphyrinate
CdMP
csalt × 104, mol/l
T, K
E, kJ/mol
∆S#, J/(mol K)*
l mol–1 s–1
ZnCl2
1.17
298
308
318
0.45
1.00
1.86
55
–135
ZnCl2
2.19
298
308
318
0.48
0.91
1.97
56
59
52
29
30
26
28
66
66
67
70
–130
–142
–137
–173
–173
–173
–173
–95
CdPP
CdGP
ZnCl2
209
298
308
318
0.71
1.36
3.15
ZnCl2
392
298
308
318
0.74
1.40
2.77
ZnCl2
209
298
308
318
0.015
0.022
0.035
ZnCl2
261
298
308
318
0.014
0.022
0.035
ZnCl2
339
298
308
318
0.015
0.027
0.035
ZnCl2
392
298
308
318
0.015
0.023
0.036
CdDP
ZnCl2
209
298
308
318
0.015
0.035
0.081
ZnCl2
261
298
308
318
0.016
0.035
0.084
–93
ZnCl2
339
298
308
318
0.017
0.038
0.091
–90
ZnCl2
392
298
308
318
0.016
0.038
0.092
–90
* The activation energy and entropy were determined to within 8 kJ/mol and 16 J/(mol K), respectively.
metal cations, in particular, their salts. If the principal tions, unlike the addition and destruction reactions. It is
object of reaction (1) is taken to be the porphyrin
ligand and the ligands in the inner coordination sphere
of the d-metal salt, then reaction (1) will be the ligand
exchange reaction.
most reasonable to use the above considerations in
order to establish the mechanism of reaction (1). The
first conclusions on the mechanism were made in [1, 2,
8, 9] and the reactions of metal exchange between Mg
and Cd porphyrin complexes and bivalent metal chlo-
In accordance with the classical concepts [7], both
approaches described above lead to substitution reac- rides were shown to be two-stage bimolecular reac-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 7 2007

ON THE MECHANISM OF THE METAL EXCHANGE
491
tions. At the first bimolecular stage, the intermediate
binuclear complex (intermediate) is formed:
A
1.0
1
2
CdP + MX2(Solν)n – 2
(Solν)mCdP ·MX2(Solν)n – 4. (5)
2
The intermediate can form immediately after the
solutions are poured together and can be easily identi-
fied by the spectral method. However, this stage can be
also retarded.
1
At the second slow monomolecular stage, the inter-
mediate undergoes dissociation:
0.5
(Solv)mCdP ⋅ MX2(Solν)n – 4
[X2Cd2+···P2–···M2+(Solν)m]#
(6)
CdX2(Solν)m + MP(Solν)n – 4
.
This stage is accompanied by the change in the
absorption band intensities in the electronic absorption
spectra with time (Fig. 1, 2).
470
650
λ , nm
The situation may happen, when the energetic and
kinetic conditions for entering of a new metal (M') into
the complex and removal of an old metal (M) from the
complex are the same. In the Cd complexes under
study, the above situation was not observed. However,
the concentrations of components (metalloporphyrin–
metal salt–solvent) can be chosen in such a way, that
the above conditions will be realized. In this case, the
exchange reaction will be bimolecular, but will proceed
in one stage though a single transition state (6). We did
not observe such cases in our experiments so far.
Fig. 2. The change in the electronic absorption spectra of
CdGP in reaction with ZnCl in acetonitrile at c0
=
2
CdDP
0
–5
–2
4.6 × 10 , cZnCl = 7.83 × 10 mol/l: (1) at the initial
2
moment, (2) in 100 min.
conditions (k = 4 × 10–9 s–1 l mol–1 at 25°ë). It thus fol-
lows that the ZnChl complex strength determined by
the stability constant is 106 times as high as that of the
CdChl complex [11].
In a series of reactions (5) and (6) with porphyrins
of protoporphyrin group (blood group), of particular
interest are the reactions with the alkyl substituents in
2,4-positions of porphyrin. The functional substitution
The thermodynamic and kinetic parameters of the
CdChl dissociation were estimated in [11]. The energy
of cleavage of the Cd–N bonds is compensated due to
the energy of Cd2+ solvation and protonation of two N
atoms of the P2– anion in the transition state. The Cd–
Chl bond has the ionic nature.
in a series hydrogen atom
ethyl
vinyl
1-methoxyethyl affects both the rates of the reactions of
metalloporophyrin formation
form
k
v
H2P + MX2(Solν)n – 2
2HX + MP + (n – 2)Solν, (7)
Cd (+2–4δ)
which in the most cases are chemically irreversible
reactions, and on the rates of solvatoprotolytic dissoci-
ation reactions
(–δ)N
N (–δ)
–
2X
MP + 2H+(Solν)
which are also irreversible.
H2P + MX2(Solν)n – 2
,
(8)
(–δ)N
N(–δ)
The effect of substituents, both of σ-electron-donors
(ë2ç5) and π-electron-donors (ëç=ëç2) should man-
ifest in the strength of the porphyrin N–M bonds and
hence, in the rate of formation (7) and dissociation (8)
of Cd and Zn porophyrins. The rates of CdP and ZnP
formation are almost identical [10]; the rates of their
dissociation reactions differ greatly. The cadmium
complex with a natural chlorophyll-ligand (CdChl) dis-
The conclusions made in [11] apply to the Cd and
Zn complexes with the protogroup porphyrins and,
hence, to the exchange reaction (1). As follows from
[10, 11], the exchange of Cd2+ for Zn2+ is energetically
advantageous. A considerable negative charge on the
CdP N atoms involved in coordination favors the attack
of Zn(X)2(Solν)n – 2 solvation complex and the forma-
tion of the intermediate by reaction (5) at a high rate,
sociates at a rate of 2 × 10–4 and 4 × 10–3 s–1 l mol–1 in provided that the energy of the Zn–N bond will com-
solutions of a 0.04 M and a 0.10 M acetic acid in etha- pensate the energy of elimination of two DMSO mole-
nol, respectively (at 25°ë); according to the extrapola- cules from its solvation salt. Such conditions are real-
tion data, ZnChl does not almost dissociate under these ized due to the occurrence of zinc salts in two coordina-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 7 2007

492
BEREZIN et al.
tion forms (tetrahedral and octahedral) and due to the the ëç(éëç3)ëç3 group in hematoporphyrin is
equilibrium
equivalent in the electron-donor properties to the H
atom, i.e., is electroneutral, whereas the ethyl group of
mesopoephyrin and the vinyl group of protoporphyrin
show the electron-donor properties only (+I- and +C-
electron effects), which favor the metal exchange in
DMSO. The results of Cd exchange for Co (Table 1)
were discussed in [1, 2].
Zn(X)2(Solν)4
Zn(X)2(Solν)2 + 2Solν.
(9)
The rate of the intermediate formation is expected to
depend only slightly on the nature of the protogroup
porphyrin, since the transition state has low energy and
is mainly determined by rearrangement of the solvation
salt structure. System (5) should further overcome a
higher energy barrier. At this stage, the CdP dissocia-
tion, which depends on the nature of the functional
groups [10], occurs simultaneously with the formation
of ZnP, which also depends on the nature of porphyrin
[12].
REFERENCES
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naya khimiya solvatokompleksov solei perekhodnykh
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Complexes of Transition Metal Salts), Moscow: Nauka,
1992.
Based on the data available for the reactions of for-
mation and dissociation of Cd and Zn porphyrins and
the properties of solvation complexes of these metals
with DMSO [3], one can tentatively predict subsequent
transformations of the intermadiate into complexes.
The removal of Cd as solvation salt or pure solvate
Cd(DMSO)26+ is energetically disadvantageous (the
replacement of Cd–N by Cd–O requires some energy)
and is kinetically hampered, since the DMSO attack of
the Cd−N bond is required under sterically unfavorable
conditions. The above removal is favorable at the high-
est vibronic level at the moment when Cd(II) is at max-
imum distance from the porphyrin plane [13]. Solvate
of Zn(II) reacting with cadmium porphyrin should pre-
liminarily dissociate at the Zn–X bonds (in this work,
X = Cl–), which is rather difficult in DMSO, since the
latter readily solvates Zn2+, but poorly solvates anions,
particularly Cl– and another halide ions. Therefore, the
removal of both Cl– and some DMSO from the inner
sphere of Zn2+ is difficult. However, the energy spent on
the above kinetically hindered processes is compen-
sated by the energy of formation of stable ZnP com-
plex.
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kompleksnykh soedinenii (Dissociation of Complex
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p. 95].
The data in Table 1, 2 show that the rate constants of
formation of Zn porphyrins from CdP are low and
under standard conditions, they are equal to 0.46
(CdMP) and 0.73 (CdPP) l mol–1 s–1, while for hemato-
and deuteroporphyrins, the rate constants are
0.015 l mol–1 s–1 on the average. As follows from
10. Berezin B.D., Coordination Compounds of Porphyrins
and Phtalocyanines, New York: Wiley, 1981, p. 286.
11. Berezin, B.D., Klopova, L.V., and Drobysheva, A.N.,
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12. Berezin, B.D. and Khelevina, O.G., Porfiriny: Struktura.
Svoistva. Sintez (Porphyrins: Structure, Properties, and
Synthesis), Moscow: Nauka, 1985.
Table 2, the value of k2v98 does not depend on the salt
concentration, which means that it is exactly the form
of a salt, whose concentration remains unchanged
under the experimental conditions, that enters the reac-
tion. It is important that in the exchange reraciton (1),
13. Bersuker, I.V., Effekt Yana–Tellera i vibronnye vzaumo-
deistviya v sovremennoi khimii (The Jahn–Teller Effect
and Vibronic Interactions in Modern Chemistry), Mos-
cow: Nauka, 1987.
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