Metal exchange between cadmium complexes with natural porphyrins and cobalt chloride in ethanol

Article abstract of DOI:10.1134/S1070328410120080

Metal exchange reactions between Cd chlorophyll (a) or Cd pheophorbide (a) and cobalt chloride in ethanol were studied by spectrophotometry. The order of the reaction was determined. It was found that the metal exchange follows the stoichiometric mechanis

Full text of DOI:10.1134/S1070328410120080

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2010, Vol. 36, No. 12, pp. 913–917. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © S.V. Zvezdina, M.B. Berezin, B.D. Berezin, 2010, published in Koordinatsionnaya Khimiya, 2010, Vol. 36, No. 12, pp. 925–929.
Metal Exchange between Cadmium Complexes with Natural
Porphyrins and Cobalt Chloride in Ethanol
S. V. Zvezdina*, M. B. Berezin, and B. D. Berezin
Institute of Chemistry of Solutions, Russian Academy of Sciences, Ivanovo, Russia
*Eꢀmail: svvr@iscꢀras.ru
Received March 3, 2010
Abstract—Metal exchange reactions between Cd chlorophyll (a) or Cd pheophorbide (a) and cobalt chloride
in ethanol were studied by spectrophotometry. The order of the reaction was determined. It was found that
the metal exchange follows the stoichiometric mechanism.
DOI: 10.1134/S1070328410120080
All manifestations of life in the Earth are associated
with photosynthetic function of plants because of the
presence of a porphyrinꢀbased compound. Metal
exchange is an essential reaction of porphyrin comꢀ
plexes [1]. Despite much research concerned with
transmetalation of metal porphyrins, the kinetics and
mechanisms of these reactions and the role of the solꢀ
vent in them still remain unclear. Accumulation of
experimental evidence in this area is of current interest
since an exchange of metal ions between complexes is
used in isotope exchange [2], in the study of the stabilꢀ
ity order of complexes with those ligands that cannot
be characterized by stability constants [3], in the
design of metal clusters [4] and organometallic comꢀ
pounds [5], in the synthesis of coordination comꢀ
pounds [6], and in medicine as antidotes against cyaꢀ
nide derivatives [7]. To determine the distinctive feaꢀ
tures of metal exchange between cadmium complexes
with porphyrins of the chlorophyll group and Co(II)
salts in organic solvents and reveal the effects of the
solvent and the metal salt on the exchange rate, we
studied transmetalation reactions of Cd chlorophyll
(a) (CdChl(a)) and Cd pheophorbide (a) (CdPhe(a))
with cobalt chloride in ethanol. The reaction equation
has the form
CH
CH3
CH2
C2H5
N
N
N
N
Cd
H
CH3
H3C
H
O
O
COOCH3
CH2
CH2
C
OR
R = C20H39 for CdChl(a) and R = H for CdPhe(a)
EXPERIMENTAL
The starting reagents CdChl(a) or CdPhe(a) were
prepared and purified as described in [8]. Ethanol was
purified as described in [9].
Metal exchange reactions (1) for the porphyrin
complexes were studied by spectrophotometry.
Kinetic experiments were carried out as follows. A
solution of cadmium porphyrinate (CdP) and СоСl2
with a known concentration was placed in a temperaꢀ
tureꢀcontrolled cell of an SFꢀ26 spectrophotometer at
a given temperature. The optical density of the soluꢀ
tion at the absorption peak wavelength (
λ
=
max
(1)
CdP + CoCl Solv ꢀ CoP + CdCl Solv ,
(
)
(
)
660.4 nm for CdChl(a) and 655.2 nm for CdPhe(a))
was measured at regular intervals. The temperature
range was 298–318 K; the concentration of the salt
2
2
4
4
was (2.09–3.92)
for CdChl(a) and CdPhe(a), respectively.
× ×
10–2 and (1.23–2.3) 10–3 mol/l
where CdP is CdChl(a) or CdPhe(a) and Solv is ethaꢀ
nol.
913

914
ZVEZDINA et al.
c0
Table 1. Rates of the exchange of Cd2+ for Co2+ in the complex CdPhe(a) in ethanol (
= 2.3 × 10–5 mol/l)
CdPhe a
( )
keff
103, s–1
×
103, mol/l
≠
S , J/(mol K)
kv,
l/(mol s)
Еа, kJ/mol
Т, К
×
cCoCl
Δ
2
2.30
298
308
318
298
308
318
298
308
318
298
308
318
1.4 0.09
4.3 0.22
14 0.94
1.2 0.04
3.6 0.28
12 0.26
0.99 0.61
2.7 0.10
9.4 0.24
0.73 0.13
2.2 0.16
7.4 0.19
0.61
1.87
6.09
0.610
1.81
6.03
0.64
1.76
6.14
0.59
1.79
6.02
91
91
5
7
–4 18
1.99
1.53
1.23
–4 23
89 12
–14 41
91
7
–6 25
The current concentrations of CdChl(a) and
CdPhe(a) were calculated by the equation
Ea
298
ΔS = 8.314lnkv298
≠
+
− 253.22.
(5)
(2)
are the initial optical density of
c = c A − A
A − A ,
(
) (
)
Experimental data are given in Tables 1 and 2. The
metal exchange reaction of CdPhe(a) with cobalt
chloride in ethanol (Fig. 1) gave the more stable comꢀ
plex CoPhe(a) (its absorption bands experience hypꢀ
sochromic shifts). This conclusion follows from the
satisfaction of the condition for the spectroscopic criꢀ
terion of the stability of metal porphyrins [11].
0
τ
∞
0
∞
where А0
,
А
, and
А
τ
∞
the solution, its density at the instant
τ
, and the final
density, respectively; с0 and are the initial and current
с
concentrations of the Cd(II) complexes, respectively.
The effective rate constants keff of reaction (1) were
calculated by the equation
(3)
−dcCdP dτ = keffcCdP
.
RESULTS AND DISCUSSION
The energy of activation Еа was calculated by the
The concepts of chemical bonding in metal porꢀ
phyrins, phthalocyanines, and chlorophyll [11] and
the concepts of the properties of solvate complexes of
Arrhenius equation:
T1T2
T2 −T1 k1
k2
(4)
Ea = 19.1
lg ,
d
metals [12], which have been developed in the study
of the formation and dissociation of metal porphyrins
[8, 11–14], help to explain unsuccessful results in the
transmetalation of stable porphyrin complexes. That is
S were why later investigations into cation exchange have
where k1 and k2 are the true rate constants
kv keff сsalt
The changes in the entropy of activation
=
/
.
≠
Δ
calculated by an equation from the absolute reaction been focused on complexes with a considerable ionic
rate theory [10]: character of the M–N bonds [15–18]. Such are
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
No. 12
2010

METAL EXCHANGE BETWEEN CADMIUM COMPLEXES
915
c0
Table 2. Rates of the exchange of Cd2+ for Co2+ in the complex CdChl(a) in ethanol (
= 2.3 × 10–5 mol/l)
CdChl a
( )
keff
103, s–1
×
102, mol/l
≠
S , J/(mol K)
kv,
l/(mol s)
Еа, kJ/mol
Т, К
×
cCoCl
Δ
2
3.92
298
308
318
298
308
318
298
308
318
298
308
318
3.8 0.19
5.0 0.63
6.6 0.46
3.4 0.20
4.5 0.41
5.9 0.17
2.6 0.16
3.4 0.37
4.5 0.27
2.04 0.12
2.7 0.22
3.5 0.14
0.097
0.128
0.168
0.100
0.133
0.174
0.099
0.130
0.172
0.096
0.129
0.167
18
18
18
18
4
4
4
4
–201 16
–201 14
–201 19
–201 17
3.39
2.61
2.09
Mg(II), Cd(II), Hg(II), Pb(II), and Zn(II) comꢀ where kv is the true constant or the secondꢀorder rate
plexes.
constant, CdP is CdPhe(a) and CdChl(a), and МCl2 is
the dominant form of the salt in ethanol.
We found that the rates of the metal exchange reacꢀ
tions of CdPhe(a) and CdChl(a) with СоCl2 in ethanol
are described by a firstꢀorder equation with respect to
the concentration of the complex, which is evident
0
from a linear dependence of
tion time (Figs. 2, 3).
The order of the reaction with respect to the salt
concentration is unity (Figs. 4, 5). The order was
determined from the slope of the linear plot of
Our experimental data (Tables 1, 2) suggest that the
metal exchange between CdP and СоСl2 in ethanol
follows an associative mechanism (Eqs. (7), (8)). In
the first, bimolecular step, a binuclear intermediate is
produced:
on the reacꢀ
cCdP
)
lg c
(
CdP
CdP + MX Solv
ꢀ
(
)
2
n−2
lg keff
(7)
1
ꢀ Solv CdP ⋅ MX Solv
.
n−4
(
)
(
)
0
2
m
vs.
(for the reaction of CdPhe(a) with СоСl2 in
lgc
CoCl2
ethanol) and from the slope of the linear plot of
lg keff
2
The formation of this intermediate can immediꢀ
ately result from the mixing of the reagents (in this
case, it can easily be detected by spectroscopic methꢀ
0
vs.
(for the reaction of CdChl(a) with СоСl2 in
lgcCoCl
ethanol). 2
The metal exchange is a bimolecular reaction and ods) or this step can be retarded.
obeys the secondꢀorder kinetic equation
The second, monomolecular step involves slow disꢀ
sociation of the intermediate:
(6)
−dcCdP dτ = k CdP MCl .
][
[
]
v
2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
No. 12
2010

916
ZVEZDINA et al.
log(c0
/cCdChl(a))
CdChl(a)
A
1
1.0
3
CdPhe(a)
2
3
2
CoPhe(a)
2
1
0
1
200
400
600
800
1000
τ, s
0
Fig. 3. Plots of
vs. the reaction time
)
lg c
(
cCdChl a
CdChl a
( )
( )
for the reaction of СоCl with CdChl(a) in ethanol
λ
, nm
400
700
2
–2
(
= 3.92 ×10 mol/l) at Т= (1) 298, (2) 308, and (3)
c
CoCl2
Fig. 1. Electronic absorption spectra of the reaction mixꢀ
ture in the metal exchange reaction of CdPhe(a) with
318 K.
c0
5
CoCl in ethanol (
= 2.3
×
10⎯ mol/l,
=
CoCl2
c
CdPhe a
( )
2
When the strong coordinating dipolar aprotic solꢀ
vent (DMSO) is replaced by protic ethanol, the rates
of the metal exchange reactions of CdPhe(a) and
CdChl(a) with СоСl2 increase sharply. We found that
CdPhe(a) and CdChl(a) are virtually inert to metal
exchange with ZnСl2 in ethanol. Comparison of the
transmetalation rates for the reactions of CdPhe(a)
and CdChl(a) with СоСl2 in ethanol showed that the
former is more reactive under these conditions. This is
due to the effect of the substituent in position 4 of the
macrocyclic molecule: the OH group in Н2Phe(а) is
–2
2.3
×
10 mol/l) for
τ
= (
1
) 0 and (
2
) 120 min;
λ
= 655.2
and 644 nm correspond to
respectively.
λ
CdPhe(а),
λ
and CoPhe(a),
I
Solv CdP ⋅ MX Solv
(
ꢀ
n−4
)
(
)
2
m
#
2+
2−
2+
⎡
⎤
...
(8)
ꢀ X2Cd
P ...M Solv
→
(
)
m
⎣
⎦
→ CdX Solv + MP Solv
.
(
)
(
)
2
m
n−4
This step is characterized by the changing band
intensities in the electronic absorption spectra the stronger donor of
σ
ꢀelectrons than the group
(Fig. 1).
ОС20Н39 in Н2Chl(а), which makes the complex
log(c0
/
cCdPhe(a)
)
–
logkeff1
CdPhe(a)
3
2
1
1.6
1.2
0.8
0.4
0
3
3
2
2
1
1
2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95
log
200
400
600
800
1000
, s
–
c
CoCl
τ
2
0
Fig. 2. Plots of
vs. the reaction time
cCdPhe a
)
lg c
(
CdPhe a
( )
( )
0
for the reaction of СоCl with CdPhe(a) in ethanol
2
Fig. 4. Plots of
vs.
in the metal exchange
lg kэфф
lg cCoCl
1
2
–2
(
= 2.3
×
10 mol/l) at
Т
= (
1) 298, (
2) 308, and (
3)
reaction of CdPhe(a) with СоСl in ethanol at
Т
= (1) 298,
c
CoCl2
2
318 K.
(
2) 308, and (
3) 318 K.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
No. 12
2010

METAL EXCHANGE BETWEEN CADMIUM COMPLEXES
917
(Mechanisms of Organometallic Reactions), Moscow:
Khimiya, 1972.
–
lgkeff
2
3
2
1
2.7
2.6
2.5
2.4
2.3
2.2
2.1
6. Kukushkin, Yu.N. and Kukushkin, V.Yu., Teoriya i
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Veshchestva (Pure Chemicals), Moscow: Khimiya,
1974.
1.40
1.45
1.50
1.55
1.60
1.65
log
1.70
c
CoCl
2
10. Entelis, S.G. and Tiger, R.P., Kinetika reaktsii v zhidkoi
faze (Reaction Kinetics in the Liquid Phase), Moscow:
Khimiya, 1973.
–
0
Fig. 5. Plots of
reaction of CdChl(a) with СоСl in ethanol at Т = (1) 298,
(2) 308, and (3) 318 K.
vs.
in the metal exchange
lg kэфф
lg cCoCl
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ftalotsianina (Coordination Compounds of Porphyrins
and Phthalocyanine), Moscow: Nauka, 1978.
2
2
2
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naya khimiya solvatokompleksov solei perekhodnykh
metallov (Coordination Chemistry of the Solvate Comꢀ
plexes of Transition Metal Salts), Moscow: Nauka,
1992.
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tending toward metal exchange with СоСl2 in ethanol.
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tura. Svoistva. Sintez (Porphyrins. Structure. Properꢀ
ties. Synthesis), Moscow: Nauka, 1985, p. 49.
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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2010