Ligand exchange in the complexone-porphyrin macrocycle system in reactions of copper(II) ethylenediaminetetraacetate with porphyrins and phthalocyanines

Article abstract of DOI:10.1134/S1070328406070116

Complexation between 5,10,15,20-tetraphenylporphine H₂TPP and tetra(tert-butyl)phthalocyanine H₂(t-Bu)₄Pc with copper(II) ethylenediaminetetraacetate in DMSO was studied by spectrophotometry. The kinetic parameters of the

Full text of DOI:10.1134/S1070328406070116

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2006, Vol. 32, No. 7, pp. 529–533. © Pleiades Publishing, Inc., 2006.
Original Russian Text © G.M.Mamardashvili, B.D. Berezin, 2006, published in Koordinatsionnaya Khimiya, 2006, Vol. 32, No. 7, pp. 552–556.
Ligand Exchange in the Complexone–Porphyrin Macrocycle
System in Reactions of Copper(II) Ethylenediaminetetraacetate
with Porphyrins and Phthalocyanines
G. M. Mamardashvili and B. D. Berezin
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received June 23, 2005
Abstract—Complexation between 5,10,15,20-tetraphenylporphine H2TPP and tetra(tert-butyl)phthalocyanine
H2(t-Bu)4Pc with copper(II) ethylenediaminetetraacetate in DMSO was studied by spectrophotometry. The
kinetic parameters of the reaction were calculated and the mechanism of ligand exchange in the complexone–
porphyrin macrocycle system was proposed. The reactivities of H2TPP and H2(t-Bu)4Pc in reactions with cop-
per ethylenediaminetetraacetate and some other copper chelate complexes were compared.
DOI: 10.1134/S1070328406070116
Complexonates and complexes with macrocyclic
ligands that form several adjoining chelate rings are
known to be highly stable due to the polychelating and
macrocyclic effects [1–2].
Complexes with macrocyclic ligands are generally
more stable than complexonates (for metal porphyrins,
1035–40 [6]).
Since the Kst values of complexonates and metal
porphyrins differ by more than 10 orders of magnitude
and complexonates, in contrast to complexes with mac-
rocycles, can undergo sequential opening of the metal
chelate rings [7–8], ligand exchange in a complexone–
macrocyclic ligand system is theoretically possible. In
real systems, this occurs very infrequently because of
great steric hindrances due to the large sizes of interact-
ing molecules (complexone and porphyrin). Our stud-
ies showed that out of six d metal ethylenediaminetet-
raacetates, only copper complexonate is involved in
ligand exchange with porphyrin in DMSO. Cobalt(II),
cadmium(II), mercury(II), zinc(II), and nickel(II) eth-
ylenediaminetetraacetates did not react with porphyrins
even if they are soluble in DMSO.
For instance, stable complexes are formed by such
complexones as ethylenediaminetetraacetic acid
ç4EDTA and its disodium salt Na2H2EDTA (trilon B)
[3]. In complexation between ç4EDTA and metal ions,
four carboxy groups become ionized and the metal ion
forms a distorted octahedron surrounding composed of
two N atoms and four ionized O atoms
2–
O
C
CH
N
2
CH
2
O
C O
H C
2
O
O
Cu
O
H C
2
We studied complexation between copper(II) ethyl-
enediaminetetraacetate and macrocyclic ligands with
two porphyrin macrocycles as examples, namely,
5,10,15,20-tetraphenylporphine ç2TPP (“porphyrin pro-
per”) and tetra(tert-butyl)phthalocyanine ç2(t-Bu)4Pc
(unsubstituted phthalocyanine is virtually insoluble in
organic solvents). Although porphyrins and phthalocy-
anines are related compounds, they behave differently
in complexation reactions.
N
C O
CH
2
C
CH
2
O
2–
2–
Cu(EDTA)
Cu(ùÑíÄ)
The high stabilities of 3d metal ethylenediaminetet-
raacetates (e.g., the stability constant Kst of
Cu(EDTA)2– in water is 7.24 × 1018 [4]) by no means
imply that the M–N and M–O bonds are strong. The
high stabilities of these complexes in solutions are
mostly due to high shielding of the metal ion by adja-
cent atomic groups [5].
EXPERIMENTAL
The porphyrins ç2TPP and ç2(t-Bu)4Pc
529

530
MAMARDASHVILI, BEREZIN
t-Bu
t-Bu
N
NH
N
NH
N
N
HN
N
N
N
HN
N
t-Bu
t-Bu
H2íPP
H2(t-Bu)4Pc
were prepared and purified as described in [9, 10].
RESULTS AND DISCUSSION
Dimethyl sulfoxide was purified according to a stan-
dard procedure. The copper salt Cuç2(EDTA) was syn-
thesized from copper(II) acetate and Na2H2EDTA. The
parameters of the electronic absorption spectra of the
Cu(EDTA)2– ion in water and DMSO are given in
Table 1.
The reaction of ç2TPP with Cu(EDTA)2– in DMSO
can be generally written as follows:
H2TPP + Cu(EDTA)2– + 2H+
(1)
CuíPP + H4EDTA.
Kinetic studies of the complexation between por-
phyrins and Na2Cu(EDTA) by spectrophotometry were
performed as described in [11–15]. The pseudofirst
order of the reaction in porphyrin was confirmed by the
linear dependence ln(c0/cτ) = f(τ). The effective rate
constants keff were calculated from the changes in the
optical density of the solution at the peak wavelengths of
metal porphyrin over certain intervals by the equation
Since, according to the calculation, the concentration of
free Cu2+ ions (at cCu(EDíÄ)2– = 10–4 mol/l) does not
exceed 10–11 mol/l, only complexonate Cu(EDTA)2–
rather than its dissociation products can act as the kinet-
ically active species in reaction (1).
As noted above, the Cu(EDTA)2– ion is highly stable
because of shielding of the metal ion by adjacent
groups of the complexone, which prevents water,
ç3é+, etc. from having access to the central atom and
from causing dissociation of donor–acceptor chemical
bonds.
The electronic state of the Cu2+ ion in Cu(EDTA)2–
only slightly depends on the solvent nature, in contrast
to many other salts (in particular, acetates), which can
also be attributed to this shielding. For instance, when
passing from water to DMSO, the absorption band cor-
responding to the forbidden d−d* transition character-
istic of most transition metals with the incomplete d
level experiences a hypsochromic shift by 47 nm in the
electronic absorption spectrum of Cu(éAc)2 (the band
intensity increases six times), while the absorption
D0 – D∞
-- -------------------
,
c0
cτ
1
--
τ
1
τ
keff
=
ln---- = ln
Dτ – D∞
where τ is the current time of the reaction and D0, Dτ,
and D∞ are the optical densities at the initial, current,
and final points, respectively. The error in keff determi-
nation was 3%. In all cases, the reaction order in the salt
(calculated from the slope of the graph logkeff
=
f( logcsalt ) was close to unity. Activation energies Ea
were calculated by the Arrhenius equation from the
temperature dependences of keff obtained in the range
from 338 to 363 K for ç2TPP and from 288 to 313 K
for ç2(t-Bu)4Pc.
Spectrophotometric measurements were carried out band in the spectrum of Cu(EDTA)2– is shifted only by
on a Specord M40 spectrophotometer. 3.5 nm (its intensity increases 1.4 times). The electronic
Table 1. Parameters (λ, nm (ε)) of the electronic absorption spectra of Cu(OAc)2 and Cu(EDTA)2– in water and DMSO
Solvent
H2O
H2O : DMSO = 1 : 1
DMSO
Complex
Cu(OAc)2
Cu(EDTA)2–
764.5 (30)
734.1 (47)
717.3 (180)
734.1 (162)
739.6 (112)
734.1 (161)
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 7 2006

LIGAND EXCHANGE IN THE COMPLEXONE–PORPHYRIN
531
absorption spectra of Cu(EDTA)2– in pure DMSO and same time, two protons of the coordination center of the
water–DMSO (1 : 1) are virtually identical (Table 1).
porphyrin molecule affected by the metal ion field go
out of the macrocycle plane so that they are on the one
side of it. The N–H bonds become more polarized, also
because of solvation by DMSO.
Reaction (1), which has the first order in the salt and
the first order in porphyrin, is bimolecular and, like all
other complexation reactions of porphyrins, obviously
follows self-consistent bimolecular mechanism with
associative–dissociative activation of both reagents [6].
Apparently, the stereochemical configuration of cop-
per(II) ethylenediaminetetraacetate is more “favorable”
for an interaction with the porphyrin macrocycle than
the configurations of other metal complexonates. When
Cu(EDTA)2– and ç2TPP approach each other, the force
field of porphyrin deforms the complexonate by
At a certain distance between ç2TPP and
Cu(EDTA)2– (peak of the potential barrier), two adja-
cent bonds (M–N or M–O) undergo cleavage resulting
in opening of the chelate rings. Two liberated coordina-
tion sites are occupied by the tertiary N atoms of por-
phyrin. Then the reaction system converts into a stable
intramolecular chelate complex CuTPP. Reaction (1)
extending two nearest bonds (M–O or M–N). At the can be schematically represented as follows:
O
C
CH
N
2
CH
2
O
C O
NH
N
N
H C
2
O
O
+
Cu
O
HN
H C
2
N
C O
CH
2
C
O
CH
2
+ Solv
#
#
O
O
C
CH
CH
N
C
O
2
2
CH
CH
2
2
–
O
O
O
C O
N
C
H C
H C
H
Solv
Solv
2
2
N
O
O
N
Solv H N
Solv
N
or
Cu
O
Cu
H
N
–
N
H C
2
H
H C
2
O
O
N
N
N
N
C O
C
O
CH
CH
2
2
C
C
H C
2
CH
2
O
O
Transition state
Transition state
N
N
N
CH2 N(CH2COOH)2
CH2 N(CH2COOH)2
+
Cu
N
CuíPP; Solv stands for the solvent molecule
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 7 2006

532
MAMARDASHVILI, BEREZIN
Table 2. Kinetic parameters of the complexation reactions of H2TPP and H2(t-Bu)4Pc with copper(II) ethylenediaminetet-
raacetate in DMSO
Porphyrin
(cpor, mol/l)
keff × 104, s–1
keff × 104, s–1
keff × 104, s–1
Ea, kJ/mol
(∆S, J/(mol K))
cCu(EDTA)2– , mol/l
(353 K)
(333 K)
(298 K)
H2TPP
0.00016
0.00032
0.00064
0.00016
0.00032
0.00064
0.48
0.89
1.56
14.4
22.8
35.9
0.33
0.70
1.24
13.2
20.6
32.3
0.24
0.44
0.78
10.6
16.7
26.2
11 (–233)
(8 × 10–6)
H2(t-Bu)4Pc
(9 × 10–6)
5 (–222)
Table 3. Kinetic parameters of the complexation reactions of H2TPP and H2(t-Bu)4Pc with copper(II) salts in DMSO [11–14]
keff × 104, s–1, keff × 104, s–1,
Ea,
kJ/mol
keff × 104, s–1,
Ea,
kJ/mol
(298 K)
(363 K)
(298 K)
Cu(II) complex (Ò ≈ 3–3.2 × 104 mol/l)
H2TPP
H2(t-Bu)4Pc
Cu(Dz)2 (dithizonate)
No reaction
0.9
1.2 (358 K)
3.87
Cu(Ox)2 (8-hydroxyquinolate)
Cu(Nft)2 (α-nitroso-β-naphtholate)
Cu(Gly)2 (glycinate)
0.00064
0.0024
0.00005
0.0069
0.018
110
100
130
85
3.3
0.524
2.63
8.80
3.15
3.68
2.10
[Cu(GlyGly)Cl · H2O (glycylglycinate)
[Cu(GlyGlyGly)Cl (glycylglycylglycinate)
CuCl2
32
26
8.3
6.42
80
0.079
9.43
66
Cu(OAc)2
1.10
24.7
43
36.0
Apparently, the reaction of Cu(EDTA)2– with phtha- reactive sites in porphyrin and Cu(EDTA)2–. This is evi-
locyanine occurs analogously. However, since the N–H dent from the high negative entropies of activation for
bonds in dissolved phthalocyanine can be localized [6], ç2TPP and ç2(t-Bu)4Pc (–233 and –222 J/(mol K),
the protons are removed more easily from the transition respectively (Table 2)).
state, which increases the reaction rate of copper com-
plexonate with ç2(t-Bu)4Pc as compared to ç2TPP.
Another distinctive feature of reaction (1) is a rela-
tively small difference between the complexation rates
In contrast to complexation between porphyrins and
of ç2TPP and ç2(t-Bu)4Pc (only 35 times). The reac-
acetates (or various salts with bidentate ligands such as
tivities of porphyrins and phthalocyanines in complex-
acetyl acetonates, hydroxyquinolates, amino acids,
ation with bidentate ligands usually differ by several
etc.), the rate of reaction (1) is virtually temperature-
orders of magnitude (Table 3, [14]). Phthalocyanines
independent, particularly for ç2(t-Bu)4Pc (Table 2). As
are more reactive in basic solvents such as DMSO or
the result, Cu(EDTA)2–, which at high temperatures is
pyridine, while porphyrins are more reactive in protic
superior in reactivity only to amino acid complexes, is
solvents. Apparently, the small difference between the
more reactive toward porphyrins and phthalocyanines
complexation rates of ç2TPP and ç2(t-Bu)4Pc is also
at room temperature than all Cu(II) chelate complexes
due to the major contribution from the entropy factor,
we studied (Table 3) and even than monodentate aden-
which depends only slightly on the porphyrin nature
inates and adenosinates [15].
(i.e., on the state of the N–H bonds of the reactive site).
The low enthalpies of activation of reaction (1) sug-
gest a low energy barrier to be surmounted for conver-
Thus, ligand exchange in a complexone–macrocy-
sion of the starting reagents into final products. At the cle system is mainly precluded by great steric hin-
same time, the probability of formation of such transi- drances arising when such large molecules as complex-
tion states is very low because of steric mismatch of the onates and porphyrin macrocycles approach each other.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 7 2006

LIGAND EXCHANGE IN THE COMPLEXONE–PORPHYRIN
533
5. Berezin, B.D. and Berezin, D.B., Kurs sovremennoi
organicheskoi khimii (Course of Modern Organic Chem-
istry), Moscow: Vysshaya Shkola, 2003.
Such an interaction takes place only in separate
cases, e.g., in complexation between porphyrins and
copper(II) ethylenediaminetetraacetate, whose stere-
ochemical configuration seems to be most favorable for
the formation of the transition state [H2TPP ·
Cu(EDTA)2–] among other metal ethylenediaminetet-
raacetates.
The complexation reactions of porphyrins and phtha-
locyanines with Cu(EDTA)2– in DMSO are character-
ized by very low enthalpies, high negative entropies,
and small differences between the complexation rates
of porphyrins and phthalocyanines.
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