Kinetics of metal exchange between cadmium mesoporphyrin and zinc and cobalt salts in organic solvents

Article abstract of DOI:10.1023/B:RUCO.0000022806.85114.93

Transmetalation of Cd-mesoporphyrin with cobalt chloride in acetonitrile and with zinc chloride in dimethyl sulfoxide was performed and studied by spectrophotometric method.

Full text of DOI:10.1023/B:RUCO.0000022806.85114.93

Russian Journal of Coordination Chemistry, Vol. 30, No. 4, 2004, pp. 291–295. Translated from Koordinatsionnaya Khimiya, Vol. 30, No. 4, 2004, pp. 312–316.
Original Russian Text Copyright © 2004 by B. Berezin, Rumyantseva, M. Berezin.
Kinetics of Metal Exchange Between Cadmium Mesoporphyrin
and Zinc and Cobalt Salts in Organic Solvents
B. D. Berezin, S. V. Rumyantseva, and M. B. Berezin
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
Received February 27, 2003
Abstract—Transmetalation of Cd-mesoporphyrin with cobalt chloride in acetonitrile and with zinc chloride in
dimethyl sulfoxide was performed and studied by spectrophotometric method.
The metal exchange reactions are typical of com-
plexes with simple and chelating ligands (MLn) and in
CH3
C2H5
1
general form can be written as follows
H3C
H3C
C2H5
CH3
NH
N
N
M'Ln + M
MLn + M' (å' = Cd; M = Co, Zn).(1)
These are composite association–dissociation reac-
tions of the metal ion (or ligand) exchange that are used
in isotopic exchange [1] for determining stabilities of
complexes with the ligands, which cannot be character-
ized by the stability constants, as well as in the synthe-
sis of metal clusters [2] and organometallic compounds
[3]. These reactions are only rarely used for synthesiz-
ing complexes in liquid and gas phases [4–7].
The exchange reactions (1) usually involve salts of
transition metals that occur as different solvated com-
plexes in solutions [8]. Therefore, when discussing the
mechanism of reaction (1), it should be written as
HN
CH2
CH2
CH2
CH2
COOCH3
COOCH3
(ç2P)
C H
2
CH
3
5
H C
3
C H
2 5
M(L)n + M'(Solv)n
M'(L)n + M(Solv)n,
(2)
N
N
N
N
Cd
where Solv—solvent (ç2é, water–organic mixture,
organic solvent in liquid state or melt), M(Solv)n—sol-
vated complex of cation å2+, å3+ that does not contain
ï– anions in the coordination sphere, n is the actual
coordination number realized in the given solvent.
H C
3
CH
3
CH
2
CH
2
The reactions of metal exchange in metalloporphy-
rins (MP) are more involved. Although the reactions of
MP transmetalation have been studied sufficiently well,
the questions of kinetics and mechanisms of these reac-
tions and the role of a solvent remain still unclear [9].
The aim of this work was to establish the kinetics and
mechanisms of transmetalation of Cd-mesoporphyrin
(CdP) with ëÓCl2 in acetonitrile (AN) and with ZnCl2
in dimethyl sulfoxide (DMSO).
CH
2
CH
2
COOCH
3
COOCH
3
(CdP)
EXPERIMENTAL
CdP was prepared, purified, and isolated as
described in [12]. The high-purity grade acetonitrile
CdP is a labile ionic complex. The stabilities of the was twice distilled over ê2é5. The reagent grade dime-
Mg2+, Zn2+, and Cd2+ complexes with mesoporphyrin in thyl sulfoxide was distilled under vacuum.
tert-butanol–trichloroacetic acid mixture were previ-
The kinetic measurements were performed on a
ously studied in [10]. It was shown that CdP is less sta-
Specord M40 spectrophotometer with thermostatted
ble then MgP and ZnP, i.e., ZnP > MgP > CdP.
cell where the solution of CdP and the salt with the
Classical structures of mesoporphyrin (H2P) and
Cd-mesoporphyrin are as follows:
known concentration were placed at the given tempera-
ture. The optical density of a solution (A) was measured
in definite time intervals. The rate of metal exchange
reaction (Keff) was found from a decrease in the optical
1
The charges are omitted for simplicity.
1070-3284/04/3004-0291 © 2004 åÄIä “Nauka /Interperiodica”

292
log(c0CdP/cCdP
BEREZIN et al.
log(c0CdP/cCdP
)
)
0.20
0.16
0.12
0.08
0.04
0.6
0.5
0.4
0.3
0.2
0.1
2
3
1
2
3
1
0
200
400
600
800
1000
τ, s
Fig. 1. The plots of log(c0CdP /c ) vs. time of CoCl reac-
tion with CdP in AN at T = (1) 298, (2) 308, and (3) 318 K
CdP
2
0
1000
2000
3000
τ, s
0
0
–5
−4
( cCdP = 4.63 × 10 mol/l, cCoCl = 6.48 × 10 mol/l).
2
Fig. 2. The plots of log(c0CdP /c ) vs. time of ZnCl reac-
CdP
2
tion with CdP in DMSO at T = (1) 298, (2) 308, and (3)
density of a solution at λ = 541.7 nm corresponding to
the maximum of absorption band due to CdP. The mea-
surements were carried out in the temperature interval
298–318 K with the salt concentration 1.73 × 10–4–
3.24 × 10–4 mol/l (ëÓCl2) and 1.17 × 10–4–2.19 ×
10−4 mol/l (ZnCl2). The current concentration of CdP
was determined from the equation
0
0
–5
–4
318 K ( cCdP = 4.63 × 10 mol/l, cZnCl = 4.37 × 10 mol/l).
2
for transmetalation of CdP with ëÓël2 in AN and with
ZnCl2 in DMSO at 298–318 K are listed in Tables 1 and 2.
c = c0(A∞ – Aτ)/(A∞ – A0),
(3)
RESULTS AND DISCUSSION
Cd-porphyrins are mainly the ionic, kinetically
unstable porphyrin complexes. In organic solvents,
they rapidly dissociate under the action of both strong
and weak acids [13–15]. Thus, unlike Mg-chlorophyll
(MgCh), which dissociates in ethanol upon addition of
2–10 M ëç3ëééç, its Cd-analog (CdCh) dissociates
in ethanol with Cd2+elimination by the reaction
where A0, Aτ, and A∞ are the optical densities at the
beginning, at time τ, and after completion of the reac-
tion, respectively; c0 and c are the starting and current
concentrations of CdP.
The rate of metal exchange reaction for CdP is
described by the first-order reaction in concentration,
which is confirmed by the linear dependence of
log(c0/c) vs. reaction time τ (Figs. 1 and 2). Therefore,
the effective rate constants keff are calculated as follows
CdCh + 2ëç3ëééç
Cd(ëç3ëéé)2 + ç2Ch (7)
in 0.02 M ëç3ëééç with the rate constant of the
298
eff
pseudofirst order k
= 1.8 × 10–4 s–1, in 0.2 M
dcCdP/dτ = keffcCdP
.
(4)
298
ëç3ëééç with k = 2.3 × 10–2 s–1.
eff
The activation energy (Ea) is calculated from the
Arrhenius equation
–logkeff
3.1
3.0
T1T2
k2
----
k1
----------------
T2 – T1
Ea = 19.1
log
.
(5)
2.9
2.8
1
The change in the activation entropy (∆S≠) is calcu-
lated from the following equation
2.7
2.6
2.5
2
3
Ea
298
∆S≠ = 8.314lnk298 + -------- – 253.22.
(6)
2.4
2.3
The reaction order in salt (n) is determined as the
slope of the plot logkeff vs. logcsalt and is equal to 1
(Figs. 3 and 4). The changes in the electronic absorp-
tion spectra in the course of transmetalation of CdP
with ëÓël2 in AN and of CdP with ZnCl2 in DMSO are
shown in Figs. 5 and 6, respectively. The values of keff
3.4
3.5
3.6
3.7
3.8
–logcCoCl
2
Fig. 3. The plots of logk vs. logc0
at T = (1) 298, (2)
eff
CoCl2
308, and (3) 318 K.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004

KINETICS OF METAL EXCHANGE
293
–logkeff
4.1
3.9
3.7
3.5
3.3
3.1
2.9
1
A
1.2
1
2
3
2
2
2.7
3.6
3.7
3.8
3.9
4.0
–logcZnCl
0.6
2
Fig. 4. The plots of logk vs. logc0
at T = (1) 298, (2)
eff
ZnCl2
308, and (3) 318 K.
The rate of dissociation (7) rapidly increases with
concentration of ëç3ëééç in ethanol. It should be
noted that in the polar donor–acceptor solvents (ç2é,
ROH, dimethylformamide, DMSO, etc), Cd-porphy-
rins, which are labile (kinetically and thermodynami-
cally unstable) complexes, usually do not exhibit the
solvatolytic dissociation
450
650
λ, nm
Fig. 5. Changes in the electronic absorption spectra in the
course of metal exchange reaction between of CdP with
ëÓCl in AN (1) in the initial time, (2) in 90 min ( c0
CdP + n(Solv)
[Cd(Solv)n]2+ + P2–,
(8)
=
2
CdP
0
–5
–4
since the anions of porphyrins cannot occur in solu-
tions [10].
4.63 × 10 mol/l, cCoCl = 6.48 × 10 mol/l).
2
For the metalloporphyrin dissociation to occur, an
acid is required for realization of solvatoprotolytic dis-
sociation (ç3é+ or ç+(Solv)):
0.02 M ëç3ëééç is equal to 2.3 × 10–4; in 0.1 M
ëç3ëééç, it is equal to 5 × 10–3. Reaction (7) is sol-
vatoprotolytic reaction, which follows from the depen-
dence of Kst on concentration of ëç3ëééç.
åP + 2ç3é+
å2+(ç2é)n + ç2P.
(9)
It was established in [16] that the stability constant
(Kst, the equilibrium constant of the formation reaction)
found from the kinetic data in ethanol for CdCh in complexes in acetonitrile and DMSO are lacking.
The data on dissociation and formation of the Cd2+
Table 1. Kinetic parameters of Cd2+ exchange for Cd2+ in CdP complex in AN ( c0CdP = 4.63 × 10–5 mol/l, c0CoCl = 6.48 ×
2
10–4 mol/l)
cCoCl × 104, mol/l
T, K
keff × 103, s–1
kv, l/(mol s)
Ea, kJ/mol
30 ± 1
∆S≠, J/(mol K)
–210 ± 2
2
1.73
2.16
2.81
3.24
298
308
318
298
308
318
298
308
318
298
308
318
1.5 ± 0.24
2.3 ± 0.01
3.2 ± 0.03
1.3 ± 0.17
1.9 ± 0.03
2.7 ± 0.08
1.1 ± 0.17
1.6 ± 0.06
2.3 ± 0.03
1.0 ± 0.24
1.5 ± 0.09
2.0 ± 0.06
0.84
1.28
1.78
0.62
0.90
1.28
0.43
0.63
0.90
0.35
0.53
0.71
29 ± 2
29 ± 2
27 ± 2
–211 ± 1
–213 ± 1
–214 ± 2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004

294
BEREZIN et al.
[ZnCl2(Solv)n – 2], [ZnCl(Solv)n – 1]+, and [Zn(Solv)n]2+.
This follows from the data in [8] and references therein.
Both DMSO and AN solvents almost do not solvate
anions (including Cl–) but readily solvate cations.
A
1.0
1
2
Data are available in the literature that the reactions
of metal exchange in macrocyclic [4] and porphyrin [9,
17] complexes are likely to follow the stoichiometric
mechanism, namely, the associative and dissociative
stoichiometric mechanism.
2
The results obtained on the kinetics of the cadmium
ion exchange for cobalt and zinc in mesoporphyrin
(Tables 1 and 2) allows one to conclude that in both
cases, reaction (10) follows the associative mechanism.
Obviously, an intermediate is formed at the first stage
0.5
1
CdP + [M(Solv)nCl2]
[Cd–P–M(Solv)n – 2]2+···2Cl– + 2Solv,
(11)
rapidly
where M = Co2+ or Zn2+; Solv = AN or DMSO, respec-
tively.
470
650
λ, nm
Fig. 6. Changes in the electronic absorption spectra in the
course of metal exchange reaction of CdP with ZnCl in
DMSO (1) in the initial time, (2) in 120 min ( c0CdP = 4.63 ×
At the second stage, this intermediate slowly decays
2
[Cd–P–M(Solv)n – 2]2+···2Cl– + 2Solv
(12)
slowly
0
–5
–4
MP + [CdCl2(Solv)n].
10 mol/l, cZnCl = 4.37 × 10 mol/l).
2
The experimental data revealed that the general
transmetalation reaction is bimolecular reaction, which
obeys the kinetic second-order equation
The ëÓCl2 and ZnCl2 salts are the attacking reagents
in the metal exchange reaction
–dcCdP/dτ = kv[CdP][MCl2].
(13)
CdP + åël2(Solv)n – 2
åP + CdCl2(Solv)n – 2 (10)
In equation (13), kv is a true constant or the second-
order rate constant, CdP is cadmium mesoporphyrin,
åCl2 is the form of the salt that is the main form in AN,
in organic solvents (in ç2é at high concentrations of
these metal chlorides) and occur as complex species
[CoCl2(Solv)n – 2], [CoCl(Solv)n – 1]+, [CoCl3(Solv)n – 3]–, and possibly, in DMSO. Data in Tables 1 and 2 show
Table 2. Kinetic parameters of Cd2+ exchange for Zn2+ in CdP complex in DMSO (c0CdP = 4.63 × 10–5 mol/l, c0ZnCl = 4.37 ×
2
10–4 mol/l)
cZnCl × 104, mol/l
T, K
k
eff × 104, s–1
kv, l/(mol s)
Ea, kJ/mol
56 ± 2
∆S≠, J/(mol K)
–130 ± 3
2
2.19
1.89
1.46
1.17
298
308
318
298
308
318
298
308
318
298
308
318
1.9 ± 0.04
3.6 ± 0.05
7.8 ± 0.05
1.6 ± 0.05
3.1 ± 0.03
6.2 ± 0.01
1.1 ± 0.07
2.2 ± 0.05
5.8 ± 0.05
1.0 ± 0.08
2.3 ± 0.07
4.1 ± 0.05
0.48
0.91
1.97
0.46
0.90
1.80
0.41
0.81
2.10
0.45
1.00
1.86
53 ± 5
66 ± 8
55 ± 3
–131 ± 2
–134 ± 1
–135 ± 2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004

KINETICS OF METAL EXCHANGE
295
2. Gubin, S.P., Khimiya klasterov (Chemistry of Clusters),
that the exchange reaction (10) with ëÓCl2 in AN can
be described by the dependence of “true” rate constant
k2v98 on the salt solvate concentration that is well known
in the coordination chemistry of porphyrins [11]. This
dependence is more pronounced for ëÓël2 in AN, in
which several forms of reagents occur and the quantita-
tive ratio of these forms depends on the ëÓël2 concen-
tration.
Moscow: Nauka, 1987, pp. 98, 129, 212.
3. Reutov, O.A., Beletskaya, I.P., and Sokolov, V.I., Mekha-
nizmy reaktsii metalloorganicheskikh soedinenii (Mech-
anisms of Reactions of Organometallic Compounds),
Moscow: Khimiya, 1972.
4. Yatsimirskii, K.B. and Lampeka, Ya.D., Fizikokhimiya
kompleksov metallov s makrotsiklicheskimi ligandami
(Physical Chemistry of Metal Complexes with Macrocy-
clic Ligands), Kiev: Naukova Dumka, 1985, p. 217.
It should be noted that all Co2+ salts exhibit the con-
centration dependence in all the studied complexation
reactions with porphyrins [11]. This dependence was
also significantly pronounced in the Zn2+ salts [11].
However, in the case of Cd2+ exchange for Zn2+ in CdP,
5. Langford, C.H. and Gray, H.B., Ligand Substitution Pro-
cesses, New York: Benjamin, 1965.
6. Kukushkin, V.Yu. and Kukushkin, Yu.N., Teoriya i prak-
tika sinteza koordinatsionnykh soedinenii (Theory and
Practice of the Synthesis of Coordination Compounds),
Leningrad: Nauka, 1990.
7. Pryamoi sintez koordinatsionnykh soedinenii (Direct
Synthesis of Coordination Compounds), Skopenko, V.V.,
Ed., Kiev: Venturi, 1997.
8. 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.
k2v98 is a true constant. No concentration dependence
was shown by ZnCl2. Therefore, [Zn(DMSO)n]2+ is the
prevailing form of zinc salt in DMSO solution.
The electronic absorption spectra obtained in the
course of Co- and Zn-mesoporphyrin formation
(Figs. 5 and 6) coincide with those for individual com-
pounds. It was found that the replacement of weakly
solvating and poorly coordinating AN by DMSO in the
transmetalation reaction of CdP with ëÓël2 resulted in
instantaneous formation of an intermediate and very
slow formation of CoP due to the fact that the salt sol-
vates formed by Co2+ with DMSO are significantly
more stable than the acetonitrile solvated complexes.
An increase in concentration of ëÓCl2 or ZnCl2 by one
order of magnitude leads to an instantaneous metal
exchange of Cd2+ in CdP. This confirms that the reac-
tion of metal exchange (10) follows very intricate acti-
vation mechanism. Its explanation requires additional
experimental data on the reactions of this type. That is
why the thermodynamic parameters (Ea and ∆S≠) in
Tables 1 and 2 are not discussed in this work.
9. Berezin, B.D., Shukhto, O.V., and Berezin, D.B., Zh.
Neorg. Khim., 2002, vol. 47, no. 8, p. 1305.
10. Berezin, B.D., Drobysheva, A.N., and Karmanova, L.P.,
Zh. Fiz. Khim., 1977, vol. 51, no. 6, p. 1344.
11. Berezin, B.D., Koordinatsionnye soedineniya porfirinov
i ftalotsianina (Coordination Compounds of Porphyrins
and Phthalocyanine), Moscow: Nauka, 1978.
12. Berezin, M.B., Semeikin, A.S., Koifman, O.I., and Kre-
stov, G.A., Izv. Vyssh. Uchebn. Zaved., Khim. Khim.
Tekhnol., 1987, vol. 30, no. 1, p. 48.
13. Berezin, B.D. and Drobysheva, A.N., Zh. Fiz. Khim.,
1970, vol. 44, no. 11, p. 2804.
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Zh. Fiz. Khim., 1976, vol. 51, no. 5, p. 1194.
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Neorg. Khim., 1971, vol. 46, no. 8, p. 2053.
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 30 No. 4 2004