Some peculiarities of metal exchange reactions in porphyrin and phthalocyanine complexes

Article abstract of DOI:10.1007/s11173-005-0004-7

Kinetics of metal exchange reaction Cd(II) → Zn(II) and Cd(II) → Cu(II) in Cd complexes with tetraphenylporphyrin in DMSO is studied. Reaction with Cu(II) nitrate occurs in both cases more vigorously as compared to that with Zn(II) nitrate. Conditions for metal exchange reactions are studied depending on the nature of metal porphyrinate, a salt (nitrates, acetates, and chlorides of Zn(II), Cu(II), and Co(II), and of organic solvent (DMSO, CH ₃CN). It is shown that Zn(II) complexes with nonplanar porphyrins do not show metal exchange Zn(II) → Cu(II) or Zn(II) → Co(II) under mild conditions in DMSO and CH₃CN.

Full text of DOI:10.1007/s11173-005-0004-7

Russian Journal of Coordination Chemistry, Vol. 31, No. 2, 2005, pp. 95–100. Translated from Koordinatsionnaya Khimiya, Vol. 31, No. 2, 2005, pp. 104–109.
Original Russian Text Copyright © 2005 by D. Berezin, Shukhto, Nikol’skaya, B. Berezin.
Some Peculiarities of Metal Exchange Reactions
in Porphyrin and Phthalocyanine Complexes
D. B. Berezin, O. V. Shukhto, M. S. Nikol’skaya, and B. D. Berezin
Ivanovo State University of Chemical Engineering, Ivanovo, Russia
Received March 24, 2004
Abstract—Kinetics of metal exchange reaction Cd(II)
Zn(II) and Cd(II)
Cu(II) in Cd complexes
with tetraphenylporphyrin in DMSO is studied. Reaction with Cu(II) nitrate occurs in both cases more vigor-
ously as compared to that with Zn(II) nitrate. Conditions for metal exchange reactions are studied depending
on the nature of metal porphyrinate, a salt (nitrates, acetates, and chlorides of Zn(II), Cu(II), and Co(II), and of
organic solvent (DMSO, CH CN). It is shown that Zn(II) complexes with nonplanar porphyrins do not show
3
metal exchange Zn(II)
Cu(II) or Zn(II)
Co(II) under mild conditions in DMSO and CH CN.
3
The metal exchange (transmetalation) reactions in complexes, namely, the association and dissociation
porphyrin complexes represent complicated coordina- mechanisms [1, 6].
tion interactions. Although some studies [1, 2] yielded
negative results, they did not consider the nature of
these interactions. In this work, an attempt is made to
determine the nature of interactions occurring during
reactions of metal exchange on the basis of new results
obtained and the literature data.
In the previous report [1], the literature data on the
kinetics and mechanisms of metal exchange reactions
of macrocyclic complexes (MC) of a nonporphyrin
type (1) and metal porphyrinates (2) with metal salts in
organic solvents were analyzed:
The association mechanism is connected with a
high-rate formation of bimetallic intermediate:
MLåC + M'X (solv)n – 2
2
(
4)
5)
2
+
–
[
M(solv) –L –M'(solv) ] ···2X ,
m
åC
p
which further decomposes slowly
[M(solv) –L –M'(solv) ] ···2X
2
+
–
m
åC
p
(
M'LåC + MX (solv)n – 2.
2
MLMC + M'X (solv)
M'LMC + MX (solv)n – 2, (1) The high-rate stage is a bimolecular reaction, while the
2
n – 2
2
low-rate stage is a monomolecular reaction.
MP + M'X (solv)
M'P + MX (solv)n – 2. (2)
2
n – 2
2
The metal exchange is the second-order reaction in
In reactions (1) and (2), MP and M'P are metal
porpyrinates, M'X (solv) is solvated complex of a
reagents. In the case of nonrigid macrocycles, which
include heterosubstituted (N and S) crown ethers, the
formation of intermediate is slightly complicated, since
due to the low stability of coordination bonds, some of
them are transferred by ligand LMC from M to M' with
2
n – 2
–
salt with ï anion, MLMC and M'LMC are macrocyclic
complexes with azacrown, thiacrown, oxaazacrown,
and oxathiacrown ethers.
Reactions (1) and (2) have intricate activation mech- a change in the ligand conformation.
anisms due to a simultaneous formation of several M'–
LMC and M'P bonds, dissociation of some M–LMC and
MP bonds in the course of a metal exchange, and
destruction of one solvated acido complex with the for-
mation of another one. Obviously, reactions (1) and (2)
occur through multicenter interactions typical of com-
plexes with macrocyclic effect [3–5]. These interac-
tions are determined by a mechanism underlying reac-
tions (1) and (2). In most cases, complexes MLMC have
low stability and undergo solvolytic dissociation in
solutions of the strong donor solvents (DMSO, DMF,
Py, etc.):
When the formation of an intermediate is compli-
cated due to some reasons, for example, when the M–
LMC bond is considerably stronger than the M'–LMC
bond or when the coordination sphere of M'X (solv)
2
n – 2
solvate is stable, the metal exchange reaction can fol-
1
low the dissociation mechanism:
2
+
2–
MLMC + nsolv
[M(solv)
] + [LMC] , (6a)
n
2
–
[LMC] + M'X
(solv)n – 2 M'LMC
2
(
6b)
–
+
2X + (n – 2)solv.
2
–
Provided that the stability constants of macrocycles
of both metals and the rates of their complexation with
2
+
MLMC + nsolv
[M(solv) ] + L
.
MC
(3)
n
In this connection, two possible stoichiometric mecha-
nisms of metal exchange were established for such
1
Charges in reactions (6a) and (6b) are omitted.
1
070-3284/05/3102-0095 © 2005 Pleiades Publishing, Inc.

9
6
BEREZIN et al.
LMC are known, one can easily establish the limiting
stage of the overall reaction (6b).
Macrocyclic effect (its geometric component)
reduces the rate of reaction (7) as compared to that of
6
reaction (6b) as low as 10 times. Macrocyclic effect
The most interesting situation is observed when
none of the above mechanisms is realized although the
metal exchange still occurs. In this case, the metal
exchange reaction follows simultaneous (self-consis-
prevents solvatoprotolytic dissociation of metal por-
phyrinates even to a greater extent [3]:
+
MP + 2H (solv)
H P + MX (solv) .
(8)
2
2
2
tent) bimolecular mechanism S 2 with association–
EN
The rate of dissociation reaction (8) decreases as
compared to that of dissociation of monodentate, che-
late, and nonrigid macrocyclic complexes as low as
dissociation activation. According to Langford–Gray
classification [7], metal exchange reactions follow the
Ia, d mechanism.
15
30
10 –10 times and even lower, depending on rigidity
of porphyrin molecule. Therefore, macrocyclic effect
of metal porphyrinates can completely stop reaction (2).
This occurs actually in most of the systems under study.
Unlike ML , metal exchange is significantly com-
MC
plicated in metal porphyrin complexes and even occurs
with greater difficulties in metal phthalocyanine com-
plexes due to their high stability and a rigid macrocy-
However, labile porphyrin and phthalocyanine com-
clic effect responsible for screening of a reaction center plexes exchange their coordinated metal for another
MN from reagents [8]. As a result, interactions of metal. This case is described particularly in [1]. The
4
2
+
reagents in reaction of formation of metal porphyri- Mg ion in magnesium(II)tetra-tert-butylphthalocyan-
2
+
nates are greatly complicated:
inate Mg(4-t-Bu)Pc (I) was replaced by the Cd ion,
when the latter ion was in acetonitrile (AN) solvate of
ç P + MX (solv)
MP + 2HX + (n – 2)solv.(7) cadmium nitrate [Cd(AN) (NO ) ].
4 3 2
2
2
n – 2
R
R
R1
R1
R
X
M
X
M
X
R1
R
R1
R
N
N
N
N
N
N
N
X
X
1 N
X
R1
R1
R1
R1
R
R
R
1
I. X = N, R = t-Bu, M = Mg
II. X = N, R = t-Bu, M = Cd
IV. X = CH, R = H, M = Cd
III. R = R = Ph, M = Zn
1
1
–
V. R = Ph, R = H, X = CH , X = AcO , M = Zn
VI. R = Ph, R = H, M = Zn
3
1
The solvate [Cd(DMSO) (NO ) ] turned nonreact- of four coordinated nitrogen atoms CdN . The discov-
4
3 2
4
ing, since its solvate sphere is very stable, as compared ery in reaction (9) of a spectrally detected intermediate
to acetonitrile sphere. The main question concerning [(AN) Cd–(4-t-Bu)Pc–Mg(AN)···(NO ) ], which is
m
3 2
the activation mechanism of the reaction
formed at a high rate [1] and then slowly dissociates at
the bonds Mg–Pc(4-t-Bu), indicates that in reaction (9),
Mg(4-t-Bu)Pc + [Cd(AN) (NO ) ]
4
3
2
2+
molecules of complex I are reactive whose Mg ion
(I)
(9)
deviates from the N plane as a result of vibronic move-
4
ments. This is favored by coordination of extra ligand
Cd(4-t-Bu)Pc + [Mg(AN) (NO ) ],
6
3
2
CH CN on the side opposite to the direction of attack
(II)
3
2+
2+
Cd
(4-t-Bu)PcMg. As a result of Mg deviation
is how Cd2+ is coordinated to Mg phthalocyaninate, if
all four phthalocyanine N atoms in I are involved in
coordination bonds Mg–N with sufficient covalent frac-
tion. In the proton–donor solvents, complex I is signif-
from the plane of phthalocyanine, the effective negative
charge on endocyclic N atoms increases. These charges
attract a positively charged Cd ion, which loses NO
–
2
+
3
icantly more stable than cadmium(II)tetra-tert- anions and a part of its solvation shell in the course of
butylphthalocyaninate Cd(4-t-Bu)Pc (II) [1, 9] (disso- attraction.
ciation constants in a 17.3 M HAc at 298 K are 0.998 ×
Taking these aspects of activation of reaction (9)
into account, one can conclude that no exchange in
–
3
–3 –1
1
0 and 48.72 × 10 s for I and II, respectively).
It thus follows that complex II is either less covalent reaction (2) will occur, when the starting M(4-t-Bu)Pc
2+
than I, or Cd , due to its large radius, is out of a plane (or MP) is highly covalent and vibronic movements of
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 2 2005

SOME PECULIARITIES OF METAL EXCHANGE REACTIONS
97
Table 1. Conditions of metal exchange in metal porphyrinates dissolved in DMSO with a ratio csalt/cMP ≥ 100*
Complex
Salt
Zn(β-Ph) TPP
(Ac)Zn(N-CH )TPP
CdTPP
CdTBP
Cd(4-t-Bu)Pc
8
3
Cu(NO3)2 Does not exchange
298 K, instantaneously
298–308 K
CuAc2
CuCl2
CoAc2
CoCl2
NiAc2
NiCl2
Does not exchange Does not exchange
Does not exchange Does not exchange
Does not exchange
Does not exchange
Does not exchange
298 K
Does not exchange slowly
Does not exchange Does not exchange
Does not exchange Does not exchange
Does not exchange Does not exchange
Does not exchange slowly
Does not exchange Does not exchange
Does not exchange slowly
Zn(NO3)2
ZnAc2
298–318 K
313 K
Does not exchange
Does not exchange
298 K
298 K
*
Lines where not temperature is indicated refer to an interval 198–343 K.
M are limited, when it is weakly solvated with a sol- ters of reaction (2) were calculated by classic kinetic
vent, and when the entering metal cation (M') is in the equations.
composition of a stable complex solvate. However, one
should note that as the solvating ability of a solvent
increases, the stability of a solvation shell also
increases as a rule, which hampers metal exchange.
Therefore, one can suggest that the most efficient sol-
vents in reaction (2) will be those with moderate elec-
tron–donor function.
RESULTS AND DISCUSSION
The possibility of exchange reactions (2) was stud-
ied in [16] for systems MP–metal salt–solvent with par-
ticipation of metal porphyrinates II–VI in DMSO with
nitrates, acetates, and chlorides of Cu(II), Co(II),
Ni(II), and Zn(II), as well as in acetonitrile. The spec-
tral data obtained are presented in Tables 1, 2.
EXPERIMENTAL
As follows from Table 1, in the medium of a highly
Synthesis and purification of porphyrins were con-
coordinated DMSO, Cu(NO ) instantaneously enters
ducted as follows: metal porphyrinate II according to
3 2
the exchange reaction with CdTPP (VI) even at 298 K,
while it reacts with a more stable CdTBP (IV) at a mea-
[
10], zinc(II)dodecaphenylporphyrin [Zn(β-Ph) íêP]
8
(
III) by the procedure [11], cadmium(II)tetrabenzopor-
surable rate; more stable acidosolvates CuAc and
phyrin [CdTBP] (IV), cadmium(II)tetraphenylporphy-
rin [CdTPP] (VI), as described in [12], and N-meth-
yltetraphenylporphyrin zinc(II)acetate [(Ac)Zn(N-
2
CoCl practically do not participate in the exchange. A
2
CH )TPP] (V) as reported in [13].
3
Table 2. Conditions of metal exchange in metal porphyrins
The solvents were purified in accordance with rec-
ommendations [14]. DMSO was kept above calcined
CaO and distilled in vacuum (≈1 mmHg). Acetonitrile
was boiled with ê é and refluxed at 81.5°ë. Reagent
dissolved in acetonitrile with a ration c /c ≥ 100
salt MP
Complex
CdTBP (Ac)Zn(N-CH )TPP
Salt
2
5
CdTPP
Slowly*
3
grade acetic acid was freezed out several times, boiled
with the calculated amount of acetic anhydride, frac- Cu(NO3)2 298 K
tionated at 117.5°ë.
CuAc2
298 K Does not exchange* 298 K, at a high
rate
Reagent grade metal nitrates, chlorides, and acetates
were also used in experiments. The salts were addition- CuCl2
298 K Does not exchange*
298 K Does not exchange*
298 K
298 K
ally recrystallized before kinetic measurements [15].
CoCl2
The kinetic measurements were performed using
spectrophtometric method [3]. An aliquot of a solution
of the investigated complex in DMSO or acetonitrile
was placed in temperature-controlled cell of a Hitachi
U2000 spectrophotometer. Then, an equal amount of
thermostatted solution of a metal salt in the correspond-
ing solvent with the established concentration was
added, and the changes in the electronic absorption
spectra of a system were recorded. The kinetic parame-
Zn(NO3)2 298 K
298 K, instanta-
neously
ZnAc2
298 K
298 K
298 K
MgAc2
Does
not exchange*
ZnCl2
298 K, at a high
rate
* In a temperature interval of 298–343 K.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 2 2005

9
8
BEREZIN et al.
Table 3. Kinetic parameters of metal exchange in metal porphyrinates IV and VI in DMSO
System
csalt × 103,
3
–1
kv, s–1 l mol–1
≠
λexp, nm
T, K
keff × 10 , s
Ea, kJ/mol
60 ± 5
∆S , J/(mol K)
(
cMP, mol/l)
mol/l
Zn(NO ) –CdTPP 614
1.12
298
308
0.18 ± 0.01
0.44 ± 0.01
0.85 ± 0.02
0.37 ± 0.01
0.84 ± 0.01
1.60 ± 0.08
0.98 ± 0.02
1.92 ± 0.06
3.50 ± 0.22
0.17 ± 0.01
0.40 ± 0.01
0.76 ± 0.02
0.16 ± 0.01
0.38 ± 0.04
0.72 ± 0.04
0.18 ± 0.01
0.34 ± 0.01
0.63 ± 0.04
–124 ± 16
–125 ± 13
–142 ± 3
3
2
–
5
(
2.76 × 10 )
3
18
2
5
.24
.60
298
58 ± 4
50 ± 1
3
3
08
18
298
3
3
08
18
Cu(NO ) –CdTPP 614
Instantaneously at T = 298 K
3
2
Zn(NO ) –CdTBP 630
Does not exchange in a temperature interval 298–343 K
3
2
Cu(NO ) –CdTBP 630
0.61
293
298
0.26 ± 0.05
1.14 ± 0.01
2.77 ± 0.01
0.80 ± 0.14
1.96 ± 0.01
3.79 ± 0.03
1.14 ± 0.13
2.28 ± 0.01
4.19 ± 0.03
0.42 ± 0.21
1.86 ± 0.01
4.54 ± 0.02
0.52 ± 0.18
1.28 ± 0.01
2.48 ± 0.02
0.41 ± 0.11
0.83 ± 0.01
1.53 ± 0.01
135*
3
2
–
5
(
1.43 × 10 )
3
08
1
2
.53
.75
293
87*
2
3
98
08
293
71*
2
3
98
08
*
Estimated values.
2
+
similar behavior is shown by chlorides and acetates of Zn occurs in bimolecular mode and requires a consid-
Co(II), Ni(II), and Zn(II).
erable activation of reagents to form a strongly polar-
≠
ized intermediate (–∆S ).
We determined the kinetic parameters for reactions
Metal porphyrin IV is significantly more stable than
of Zn(NO ) with VI and Cu(NO ) with IV (Table 3).
3
2
3 2
of
VI [17], but in DMSO, Cu(NO ) is more reactive than
The
exchange
reaction
CdTPP
with
3 2
Zn(NO ) . Therefore, the exchange reaction of
[
Zn(DMSO) (NO ) ] occurs at 298 K with a true rate
3 2
4
3 2
–
1
–1
Zn(NO ) with IV does not take place, whereas
constant k = 0.17 ± 0.01 s l mol , the average activa-
3 2
v
≠
Cu(NO ) reacts with VI at a rate one order of magni-
tion energy E = 56 kJ/mol, and activation entropy ∆S =
3 2
2
+
tude higher than that of the reaction between Zn(NO )
3
2
–
130 kJ/(mol K). Obviously, the exchange of Cd for
and VI. The exchange reaction is bimolecular. How-
ever, k in the system Cu(NO ) –CdTBP noticeably
v
3 2
decreases as the salt concentration increases (Table 3).
This can be associated with the existence of one more
course of the exchange reaction following the mono-
molecular (associative) mechanism.
A
1
5
73
6
14
5
60
Figure 1 shows a change in the electronic absorption
spectra of solutions in the Zn[(DMSO) (NO ) ]–
4
3 2
6
00
CdTPP system in DMSO. One can see that a more sta-
ble complex ZnTPP is formed and the absorption bands
exhibit hypsochromic shift (spectral criterion of stabil-
ity of metal porphyrinates [3]). The effective stability
constant of Cd exchange in CdTPP for Zn in DMSO
vs. zinc salt concentration is presented in Fig. 2 in a
temperature range of 298–318 K. In these concentra-
tion and temperature intervals, the exchange reaction
has the second order.
2
+
5
00
600
700 λ, nm
Fig. 1. Changes in electronic absorption spectrum of metal
exchange reaction in Zn[(DMSO) (NO ) ]–CdTPP system
in DMSO.
4
3 2
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 31 No. 2 2005

SOME PECULIARITIES OF METAL EXCHANGE REACTIONS
99
Table 4. Kinetic stability of Zn complexes with nonplanar porphyrins at T = 298 K
3
–1
Complex
keff dis × 10 , s
Ionizing medium
References
18]
[
Zn(β-Ph) TPP
51.39 ± 3.13
26.65 ± 2.03
0.26 ± 0.02
5.25 M CH COOH in DMSO
8
3
(
Ac)Zn(N-CH )TPP
CH COOH
[18]
[3]
3
3
ZnTPP
CH COOH
3
In accordance with the above considerations on the Zn2+ to Cd ) brings about its off-plane arrangement
activation mechanism, the exchange reactions occur in and thus, more efficient solvation of the ion by the coor-
acetonitrile more readily than in DMSO (Table 2). dinating solvent. All these facts together with the acti-
CdTBP enters the exchange reaction even with Cu(II) vation of off-plane vibrations of the N–M bonds facili-
and Zn(II) acetates and Co(II) and Zn(II) chlorides. The tates reaction (2).
2+
exchange reaction with CdTPP proceeds even more
vigorously.
ACKNOWLEDGMENTS
The metal exchange in
a
weakly stable
The authors are grateful to P. Stuzhin for providing
equipment time and to the Fund of Assistance to
Domestic Science for financial support.
(
Ac)Zn(N-CH )TPP (as compared to ZnTPP) (Table 4)
3
does not occur even in CH CN, which forms solvates
3
less stable than DMSO forms (Table 2) [19]. It is not
excepted that this exchange occurs under more rigid
conditions (at T > 373 K) [2]. Data in Table 4 indicate
that the kinetic stability of metal porphyrinate under
given conditions is not a single factor determining its
capability toward metal exchange. For instance, com-
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3
2
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–
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–
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2.6
3.0
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4
3 2
tration at T = (1) 298, (2) 308, and (3) 318 K.
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