Kinetic stability of complexes of some d-metals with 3,3'- bis(dipyrrolylmethene) in the binary proton-donor solvent acetic acid-benzene

Article abstract of DOI:10.1134/S1070363212070195

The kinetics of dissociation of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) binuclear homoleptic double-stranded helicates with bis(2,4,7,8,9- pentametyldipyrrolylmethen-3-yl)methane (H₂L) of the [M ₂L₂] composition

Full text of DOI:10.1134/S1070363212070195

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 7, pp. 1293–1297. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © L.A. Antina, G.B. Guseva, A.I. V’yugin, E.V. Antina, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 7, pp. 1195–
1200.
Kinetic Stability of Complexes of Some d-Metals
with 3,3'-Bis(dipyrrolylmethene)
in the Binary Proton-Donor Solvent Acetic Acid–Benzene
L. A. Antina, G. B. Guseva, A. I. V’yugin, and E. V. Antina
Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: eva@isc-ras.ru
Received May 5, 2011
Abstract—The kinetics of dissociation of Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) binuclear
homoleptic double-stranded helicates with bis(2,4,7,8,9-pentametyldipyrrolylmethen-3-yl)methane (H2L) of the
[M2L2] composition in the binary proton-donor solvent acetic acid–benzene was studied. Protolytic dissociation
of the helicates [M2L2] proceeds in accordance with the third order kinetic equation. Effect of the complexing
metal nature is manifested in increased kinetic inertness in the following series of the helicates: [Cd2L2] <
[Hg2L2] < [Cu2L2] < [Ni2L2] < [Zn2L2] < [Co2L2]. By the lability in similar environmental conditions the
helicates [M2L2] are comparable to the metal complexes of distorted porphyrins and are much more inert than
dipyrrolylmethenate mononuclear complexes.
DOI: 10.1134/S1070363212070195
In the past two decades extensive data was accu-
mulated on the synthetic chemistry of the chromo-
phoric linear oligopyrroles [1–3], among which the bis
(dipyrrolylmethenes) and their metallocomplexes are
of particular interest. The limitless possibilities of
varying molecular structure of bis(dipyrrolylmethenes)
by varying functional substituents and spacers between
pyrrole rings place this class of compounds among a
group of the most promising ones for the creation of
new materials with practically useful properties. Along
with considerable success in the synthesis of bis
(dipyrrolylmethenes) and their metal complexes, there
is a global lag in the research of their most important
physical and chemical properties. There are no
available information on the lability of complexes of
bis(dipyrrolylmethenes) in the proton-donor environ-
ment, which is necessary for solving a variety of
problems at their preparation and practical application.
In this connection, we studied for the first time the
kinetics of dissociation of the Co(II), Ni(II), Cu(II),
Zn(II), Cd(II), and Hg(II) helicates with bis- (2,4,7,8,9-
pentametildipyrrolylmethen-3-yl)methane of the [M2L2]
composition in binary proton-donor solvent
CH3COOH–C6H6 (298.15–318.15 K) with a large excess
of the acid with respect to the complex.
The 3,3'-bis(dipyrrolylmethene) helicates of the
[M2L2] composition, like metalloporphyrins [4], are
intramolecular complexes formed with the fragments
connected through covalent and donor-acceptor bonds,
Me
Me
Me
Me
Me
Me
N
N
N
N
CH2
Me
Me
Me
Me
Me
Me
Me
Me
M
M
CH2
N
N
N
N
Me
Me
Me
Me
Me
Me
M = Co(II), Ni(II),Cu(II), Zn(II), Hg(II), Cd(II).
1293

1294
ANTINA et al.
Table 1. Kinetic parameters of dissociation processes of the complexes [Co2L2], [Zn2L2], [Ni2L2], and [Cu2L2] in binary
solvent acetic acid–benzenea
k
obs×103, s–1
308.15 K
Е,
–∆S#,
c0AcOH, M
kJ mol–1
J mol–1 K–1
298.15 K
303.15 K
313.15 K
318.15 K
[Cо2L2]
4.523
5.915
7.307
8.699
10.439
0.13
0.23
0.37
0.52
0.74
0.21
0.35
0.54
0.78
1.14
0.32
0.53
0.82
1.20
1.73
0.47
0.76
1.17
1.72
2.49
0.65
1.05
1.65
2.37
3.47
61
60
60
60
62
122
124
126
124
118
[Zn2L2]
[Ni2L2]
2.610
3.914
5.219
6.524
7.829
0.11
0.26
0.45
0.69
1.04
0.21
0.49
0.79
1.32
1.91
0.29
0.66
1.14
1.77
2.66
0.42
0.87
1.57
2.54
3.84
0.54
1.22
2.20
3.48
5.17
75
72
71
75
73
75
79
80
61
64
3.131
3.914
5.219
6.524
7.829
0.2
0.31
0.5
0.85
1.37
1.96
0.5
0.75
1.17
2.12
3.31
4.63
0.99
1.62
2.82
4.38
6.27
64
64
65
64
64
107
109
106
109
109
0.33
0.57
0.90
1.28
0.76
1.36
2.18
3.09
[Cu2L2]
0.190
0.253
0.291
0.316
0.342
0.380
2.10
3.81
4.87
5.75
6.80
8.44
2.50
4.19
5.80
6.83
8.10
9.82
2.83
4.89
6.43
7.72
9.20
11.33
3.14
5.70
3.58
6.24
22
18
23
23
23
22
231
237
221
219
217
220
7.47
8.20
8.78
10.34
12.53
9.66
11.44
14.26
a The errors in determining the values are: kobs 3–5%, E and ΔS≠ 10% or less.
They do not exhibit the ability to solvolytic dissocia-
tion into ions in neutral media. In the [M2L2] mole-
cules the tetrapyrrole chains of each of the two
helicands braid two coordinated metal atoms. Each
atom is coordinated to four nitrogen atoms of two
dipyrrolylmethene domains of both ligands. Due to the
presence of four dipyrrolylmethene domains in the
[M2L2] molecules the compounds possess pronounced
chromophore properties.
helicate concentration (c0, M) is described by a kinetic
equation of first order, dc/dt = kobscM2L2, which is
confirmed by the linearity of the dependences in the
coordinates ln (c0/cτ)–τ, ρ = 0.998–0.999 (Fig. 2), and
permanency of the rate constants during the whole
reaction. Table 1 lists the observed rate constants
obtained by the treatment of linear dependences
ln(c0/cτ)–τ and the values of the activation energy and
entropy.
The study showed that the kinetically controlled
dissociation of the complexes [Co2L2], [Ni2L2],
[Cu2L2], and [Zn2L2] in benzene solution proceeds in
the presence of a large excess of acetic acid, from 0.19
to 10.4 M, depending on the nature of complexing
agent (Table 1), and at lower concentrations of acid the
reaction either does not occur, or is equilibrial. The
process is accompanied by the transformation of the
EAS of the [M2L2] into the spectrum of the protonated
ligand H2L·2AcOH with retention of several isosbestic
points (Fig. 1).
It was difficult to estimate the total kinetic pattern
of dissociation for the helicates [Cd2L2] and [Hg2L2]
on the following reasons. At 298.15 K the complexes
[Hg2L2] and [Cd2L2] are stable in mixtures containing
AcOH less than 5×10–3 and 5×10–4 M respectively.
When the content of the acid in the mixed solvent was
higher than 1×10–2 and 5×10–3 M, respectively, the
dissociation of the complexes [Hg2L2] and [Cd2L2]
proceeded immediately with the formation of the
protonated ligand (H2L·2AcOH). At the intermediate
concentration of the acid the dissociation of the
helicates [Hg2L2] and [Cd2L2] proceeds with the simul-
taneous decomposition of the formed protonated ligand
The dependence of the dissociation rate of [M2L2]
[M = Co(II), Ni(II), Cu(II), Zn(II)] on the initial
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 7 2012

KINETIC STABILITY OF COMPLEXES OF SOME d-METALS
ln(c0/cτ)
1295
τ, s
Fig. 2. The ln(c0/cτ)–τ dependences in the reaction of
[Cu2L2] dissociation in proton donor binary solvent AcOH–
C6H6, c0AcOH (M) and (T, K): (1) 0.190 (298.15), (2) 0.253
(303.15), (3) 0.291 (308.15), (4) 0.316 (313.15), (5) 0.380
(318.15).
λ, nm
Fig. 1. Variation of the electron absorption spectrum of
[Ni2L2] in a mixed solvent AcOH–C6H6 in the process of
dissociation of the complex at 298.15 K, c0AcOH = 3.131 M,
c0([Ni2L2]) = 3.6×10–6 M; τ = 0 (1) and ∞ (2) (min), the
remaining curves correspond to intermediate time points.
autoprotolysis in benzene [12] and the value of
benzene dielectric constant (2.284) [13] suggest that in
solutions of AcOH in benzene the main agent serving
as a carrier of proton is the molecular form of acetic
acid [ 14, 8].
to afford monopyrroles, as evidenced by discoloration
of solutions and increased absorption in the EAS
below 350 nm. According to the literature [5–8] the
dissociation of helicate [Hg2L2] in benzene can also be
accompanied with processes of mercuration. Never-
theless, the results obtained indicate the greatest
lability of the complexes [Hg2L2] and [Cd2L2] in
comparison with other studied helicates, due probably
to the large ionic radii of Cd2+ and Hg2+ compared to
Co2+, Ni2+, and Zn2+, and in the case of the most labile
complex [Cd2L2] due to a significant contribution of
the ionic component in the coordination interaction.
Similar pattern of influence of the nature of
complexing agent has been observed previously for the
chlorophyll complexes [HgCHL] and [CdCHL] in
binary solvent AcOH–EtOH [9].
To determine the reaction order with respect to the
acid we analyzed, according to [4], the dependence of
the dissociation rate constant on the initial
concentration of acetic acid in benzene (Fig. 3). This
dependence for the complexes [Co2L2], [Zn2L2],
[Ni2L2] and [Cu2L2] is not linear, and is well described
by
a
second order power function: keff
=
const1(c0AcOH
)
const2, where the constants const1 and
const2 have the physical meaning of the true rate
constant (kV ) and reaction order (n) with respect to the
T
acid, respectively, which for all systems at all
temperatures was equal to two (ρ = 0.998–0.999).
Table 2 lists the true rate constant, reaction order with
respect to the acid, the activation energy (Ev) and
entropy (ΔS≠) of the dissociation reaction of Zn(II),
Co(II), Ni(II), and Cu(II) helicates in the proton-donor
solvent acetic acid–benzene.
Considering the general order of dissociation of the
complexes [Co2L2], [Ni2L2], [Cu2L2], and [Zn2L2] in
the medium of the mixed solvent CH3COOH–C6H6 we
took into consideration the data of [4, 11–14] which
showed that in solutions of acetic acid in benzene an
equilibrium existed between the four forms: monomer,
cyclic dimer, and two linear cis- and trans-dimers. In a
dilute solution monomers and dimers dominate. The
results of calculations performed using the data of [11]
on the dependence of the activity coefficients on the
concentration of the acid in C6H6 at 298.15–318.15 K
showed that in the studied concentration range the
activity coefficient of acetic acid in benzene was
constant and within the error was equal to one. Low
values of constants (10–10 to 10–18) of the acetic acid
Given the first reaction order with respect to the
complex and the second order on acetic acid, the rate
of dissociation process of the helicates in the range of
working concentrations of acetic acid can be expressed
by the total third-order equation: dc/dt = kobsc(M2L2)·
c2AcOH. Given the overall third order reaction and the
facts that in the reaction product formed H2L·2AcOH
and acetic acid reacted in the molecular form, the
process of the [M2L2] acid dissociation could be
regarded as a twofold protonation of the coordinated
ligand at the donor nitrogen atoms in accordance with
the scheme:
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 7 2012

1296
ANTINA et al.
Table 2. The true rate constants and activation parameters
slow
[М2L2] + 2AcOH
[M2(АсОH·L)2] + 6AcOH
of the dissociation reaction of the complexes [Co2L2],
[Zn2L2], [Ni2L2], and [Cu2L2] in a binary solvent acetic acid–
benzene
fast
2(H2L·2AcOН) + 2M(AcO)2.
kTv (const1),
Ev,
–∆S#,
Т, K
n(const2)
ρ
l2 mol–2 s–1
kJ mol–1 J mol–1 K–1
Relatively low activation energy and the negative
values of entropy ΔS≠ allow us defining as the limiting
the successive stages of attack on two nitrogen atoms
of one of the coordination units [M2L2] by the acetic
acid molecules, which leads to the subsequent stages
of the fast decomposition of the complex. The reaction
of protonation of the formed ligand bis(dipyrrol-
ylmethene) to afford H2L·2AcOH is also very fast, as
follows from the fact that the molecular form of the
ligand is not detected by spectrophotometry. In view of
these data and the fact that in an environment of non-
polar benzene the molecular form of acetic acid serves
as protonating agent [10–13], the dissociation of the
helicates Co(II), Zn(II), Ni(II), and Cu(II) in the
proton-donor solvent benzene–acetic acid can be
regarded as protolytic reaction.
[Cо2L2]
298.15
303.15
308.15
313.15
318.15
6.00×10–6
1.02×10–5
1.44×10–5
2.18×10–5
3.01×10–5
2.06
2.03
2.04
2.01
2.02
0.9996 69.1±7
0.9992
128±15
109±12
121±13
221±25
0.9992
0.9985
0.9990
[Zn2L2]
298.15
303.15
308.15
313.15
318.15
1.57×10–5
3.03×10–5
4.23×10–5
5.69×10–5
7.43×10–5
2.03
2.01
2.00
2.03
2.05
0.9995 75.5±8
0.9985
0.9996
0.9978
0.9998
[Ni2L2]
298.15
303.15
308.15
313.15
318.15
2.05×10–5
3.20×10–5
5.01×10–5
2.01
2.00
2.00
0.9996 66.7±6
0.9995
0.9997
7.68×10–5
1.03×10–4
2.00
0.9997
2.00
0.9995
[Cu2L2]
Analysis of the data on the dissociation rate
constant (Tables 1, 2) indicates that in the binary
proton-donor solvent acetic acid–benzene the
investigated helicates [M2L2] can be arranged in the
following series by the kinetic stability: [Cd2L2] <
[Hg2L2] < [Cu2L2] < [Ni2L2] < [Zn2L2] < [Co2L2]. The
activation energies Ev are comparable for the
complexes [Ni2L2], [Co2L2], and [Zn2L2] and about
three times lower for the helicate [Cu2L2]. The values
of activation entropy ΔS≠ for the reactions of protolytic
dissociation of all helicates are negative, and in the
case of [Cu2L2], the value –ΔS≠ is almost two times
higher than for the other three complexes, which
298.15
303.15
308.15
313.15
318.15
5.85×10–3
6.96×10–3
7.97×10–3
9.00×10–3
9.78×10–3
2.01
2.02
2.02
2.02
2.00
0.9991 23.1±2
0.9988
0.9996
0.9996
0.9997
indicate an increase in solvation of the reacting system
in the transition state.
The true reaction rate constants of dissociation and
the lability of helicates [M2L2] in the AcOH–C6H6
medium are close to those for the corresponding
metallocomplexes with distorted (nonplanar) porphyrin
ligands [14] and significantly lower than those of
mononuclear analogs, the dipyrrolylmethene com-
plexes, under the similar environmental conditions [8].
So, while for the cobalt(II) dipyrrolylmethenate of
[CoL2] composition the true constant at 298.15 K in
the acetic acid–C6H6 mixture is 5.7×10–4 l2 mol–1 s–1,
then for the binuclear helicate [Co2L2] the value of kTv
decreases by two orders of magnitude (Table 2). Even
greater differences are observed in the lability of the
nickel(II) complexes: for dipyrrolylmethenate [NiL2],
the value of kTv reaches 2279 l2 mol–1 s–1, while for
bis(dipyrrolylmethenate) [Ni2L2] the kTv is only 5.2×
10–5 l2 mol–1 s–1, that is, decreases by eight orders of
magnitude. These results indicate a much greater inertia
toward the protolytic dissociation of the binuclear
double-stranded bis(dipyrrolylmethene) helicates com-
c0AcOH, M
Fig. 3. Dependences of kobs in the reaction of the [Zn2L2]
dissociation on the initial concentration of acetic acid in
C6H6, at different temperatures, T, K: (1) 298.15, (2)
303.15, (3) 308.15, (4) 313.15, (5) 318.15.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 7 2012

KINETIC STABILITY OF COMPLEXES OF SOME d-METALS
pared with mononuclear dipyrrolylmethenes, and
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ACKNOWLEDGMENTS
This work was supported by the Federal Program
“Research and scientific-pedagogical cadres of
innovative Russia” for 2009–2013, the state contract
no. 02.740.11.0253.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 7 2012