Protolytic dissociation mechanisms and comparative acid stabilities of palladium(II), zinc(II), copper(II), and nickel(II) complexes of alkylated dipyrrins

Article abstract of DOI:10.1007/s11243-014-9852-7

Complexes of Pd(II), Cu(II), Ni(II), and Zn(II) with alkylated dipyrrins (Hdpm) were synthesized and characterized by physicochemical and spectroscopic methods. Protolytic dissociation kinetics of these complexes in benzene in the presence of acetic and trichloroacetic acid was studied. A protonated dipyrrin is the reaction product of protolytic dissociation of the complexes in acid solutions. The observed and true dissociation rate constants, as well as activation reaction parameters, were calculated. Kinetic models of the processes are proposed, and the patterns of influence of the ligand nature on dissociation kinetics were determined. The Pd(II) complexes proved to be much more stable than other those of the other metals, according to the results of the kinetic studies. The lability of the complexes strongly depends on the length and position of the alkyl substituent of the ligand. The dissociation of the Ni(II) complex gives a heteroligand complex at low concentrations of acid, but the complex undergoes full protolytic dissociation at higher concentrations of acid. The dissociation of the complex of Cu(II) is an equilibrium process, involving formation of the protonated form of the ligand.

Full text of DOI:10.1007/s11243-014-9852-7

Transition Met Chem (2014) 39:699–704
DOI 10.1007/s11243-014-9852-7
Protolytic dissociation mechanisms and comparative acid
stabilities of palladium(II), zinc(II), copper(II), and nickel(II)
complexes of alkylated dipyrrins
Evgeniy V. Rumyantsev Yuriy S. Marfin
•
Received: 7 May 2014 / Accepted: 17 June 2014 / Published online: 28 June 2014
Springer International Publishing Switzerland 2014
ꢀ
Abstract Complexes of Pd(II), Cu(II), Ni(II), and Zn(II)
with alkylated dipyrrins (Hdpm) were synthesized and
characterized by physicochemical and spectroscopic meth-
ods. Protolytic dissociation kinetics of these complexes in
benzene in the presence of acetic and trichloroacetic acid was
studied. A protonated dipyrrin is the reaction product of
protolytic dissociation of the complexes in acid solutions.
The observed and true dissociation rate constants, as well as
activation reaction parameters, were calculated. Kinetic
models of the processes are proposed, and the patterns of
influence of the ligand nature on dissociation kinetics were
determined. The Pd(II) complexes proved to be much more
stable than other those of the other metals, according to the
results of the kinetic studies. The lability of the complexes
strongly depends on the length and position of the alkyl
substituent of the ligand. The dissociation of the Ni(II)
complex gives a heteroligand complex at low concentrations
of acid, but the complex undergoes full protolytic dissocia-
tion at higher concentrations of acid. The dissociation of the
complex of Cu(II) is an equilibrium process, involving for-
mation of the protonated form of the ligand.
photophysical properties. Dipyrrin-based metal complexes
have been reported recently as components for the construction
of interesting metal–organic architectures and frameworks [3,
4]. In this regard, an important practical task is to optimize the
physicochemical properties of dipyrrin complexes, to select
compounds with the photophysical and spectral characteristics
required for practical applications on the one hand, and high
chemical stability on the other hand. Data describing the sta-
bility of these substances to photochemical reactions, resistance
to solvents, acids (solvolytic, protolytic, and solvoprotolytic
dissociation), thermo-oxidative degradation, and other physical
and chemical effects are necessary for the development of their
applications [5, 6]. For example, mesomaterial and nanoma-
terial based on dipyrrin complexes synthesized by solgel
technology depend on the resistance of the complexes to acid
and alkaline media [7, 8]. Despite the relatively large amount of
available chemical data, there are currently only a few studies
[9–12] in which the stability of dipyrrin complexes toward
various acidic media has been investigated. Comparative
studies of the physical and chemical properties of metal com-
plexes with the same type of coordination environment, but
different ligand structures, are therefore of great interest.
In this paper, the spectral properties and protolytic dis-
sociation kinetics of palladium(II), zinc(II), nickel(II), and
Introduction
0
copper(II) complexes with alkyl-substituted 2,2 -dipyrrin
The coordination chemistry of linear oligopyrroles with di-
pyrrin chelating units is currently undergoing rapid develop-
ment [1, 2]. These ligands are currently being studied in a
variety of applications, including optical materials, photosen-
sitizers, antioxidants, etc., due to their interesting spectral and
ligands were studied. The structure of these complexes is
shown in Scheme 1.
Experimental
Apparatus
E. V. Rumyantsev ꢀ Y. S. Marfin (&)
Department of Inorganic Chemistry, Ivanovo State University of
Chemistry and Technology, Ivanovo 153000, Russia
e-mail: marfin@isuct.ru
UV–Vis electronic absorption spectra (EAS) were recorded
in the range of 350–800 nm on an SF-104 spectrophotometer
123

7
00
Transition Met Chem (2014) 39:699–704
CH3
CH3
T1T2
kT2
kT1
Ea ¼ 19:1
lg
ð2Þ
ð3Þ
T ꢁ T
2
1
R2
R1
H C
CH3
6
T
DS ¼ 19:1 lg k þ E =T ꢁ 19:1 lg T ꢁ 205
a
N
N
N
N
CH3
R1
R2
Solvents
M
3
Organic solvents and acetic acid (Chimmed, Russia) were
all of analytical grade purified by standard techniques [13].
The residual water content (\0.02 %) was determined by
the Karl Fischer method [14].
H C
3
CH3
CH3
Scheme 1 The structural formula of the investigated [ML
2
R = C
2
] com-
1
plexes: 1: R = CH
1
M = Pd(II); 2: R = H,
Synthesis of Zn(II), Ni(II), and Cu(II) complexes
3
,
2
H
5
,
2
R = CH
1
, M = Zn(II); 3: R = CH
2
, R = H, M = Zn(II); 4:
3
3
1
R = CH
2
1
2
0
0
0
3 2 5 3 2 5
, R = C H , M = Zn(II); 5: R = CH , R = C H ,
Complexes of zinc(II) with 3,3 ,4,4 ,5-pentamethyl-2, 2 -
0
1
M = Ni(II); 6: R = CH
2
, R = C
3
H
2 5
, M = Cu(II)
0
0
dipyrrin (2), 3,3 ,4,5,5 -pentamethyl-2,2 -dipyrrin (3), and
3,3 ,4,5,5 -pentamethyl-4 -ethyl-2,2 -dipyrrin (4), nickel(II)
0 0 0 0
0
0
0
0
3
,3 ,4,5,5 -pentamethyl 4 -ethyl-2,2 -dipyrrin (5), and cop-
per(II) 3,3 ,4,4 ,5,5 -hexamethyl-2,2 -dipyrrin (6), free
0 0 0 0
(
Aquilon, Russia) controlled with a PC using the software
0
0
0
package UVWin 5.1.0. The accuracy of the measurements
was ± 0.03 on the scale of optical density; wavelength
accuracy was ± 0.05 nm. The fluorescence spectra were
obtained with a Cary Eclipse fluorescence spectrometer
3,3 ,4,4 ,5-pentamethyl-2,2 -dipyrrin ligand (HL), and its
hydrofluoride salt ([H L]F) were synthesized, purified, and
2
characterized in accordance with the procedures described in
the literature [1, 15]. Beforecharacterization, the compounds
were crushed and dried to constant weight under vacuum.
(
Varian-Agilent, US–Australia) controlled with a PC using
the software package Cary Eclipse Scan Application 1.1.
Spectra were measured in the wavelength range
Synthesis of Pd(II) complexes
4
95–650 nm, and the excitation wavelength was 490 nm.
The slit widths for excitation and emission ranged from 2.5 to
nm. Investigations were carried out in quartz cuvettes with
0
0
The complex of palladium(II) with 3,3 ,4,5,5 -pentamethyl-
0
0
5
4 -ethyl-2,2 -dipyrrin (1) was synthesized by the following
procedure. The hydrobromide salt of the free ligand
a thickness of the absorbing layer of 2 and 10 mm. All
experiments were performed in a temperature-controlled cell
with Peltier PTC-2 module at fixed temperatures of 298, 308,
-
(0.03 g, 2 9 10 mol) was deprotonated using triethyl-
4
-
amine (0.04 g, 4 9 10 mol) in methanol (20 ml); then,
4
-
PdCl (0.04 g, 4 9 10 mol) was added, and the reaction
4
3
18, and 328 K. The IR spectra of samples in KBr tablets
2
were recorded with an FTIR spectrophotometer Avatar 360
FT–IR ESP (Thermo Nicolet, USA) in the frequency range
mixture was heated for *30 min under reflux. Completion
of the reaction was checked for by disappearance of the
absorption band of the ligand and by cessation of the
increase in the complex absorption band intensity in the
EAS of the reaction mixture. The reaction mixture was
then evaporated, and the main product was isolated and
purified by column chromatography on silica gel using
benzene as eluent. The complex was isolated by evapora-
tion of benzene, washed with water, and dried in a vacuum
oven at 40 ꢁC. The yield was 0.041 g (70 %).
-
of 400–4.000 cm at room temperature. H NMR spectra of
1
1
the compounds in CDCl were obtained with a Bruker NMR
3
spectrometer (Germany) (500 MHz).
Kinetic measurements
The observed rate constants (k ), energy (E ), and entropy
obs
a
=
(
(
DS ) of activation were calculated according to Eqs. (1)–
3):
abs
EAS k , nm (log e): in C H 396 (3.78), 507 (4.25); in
max
6
6
CHCl 395 (3.97), 506 (4.42); in ethanol 391 (3.87), 503
3
ꢀ
ꢁ
-
1
1
A0 ꢁ A1
As ꢁ A1
where A , A , and A are the absorbance of the solution at
(4.25). IR spectrum (KBr tablets) m, cm : 3,420 w, 3,059
w, 3,026 w, 2,922 s, 2,851 w, 1,600 w, 1,491 w, 1,449 w,
kobs ¼ s ln
ð1Þ
1
1
,065 s. NMR H (CDCl , d), 6.73 singlet (2H, ls-CH),
3
0
?
s
2
.67 singlet (12H, CH ), 2.58 singlet (12H, CH ), 2.52
3
3
the absorption maximum at the start, end, and intermediate
time points, respectively,
quartet (4H, –CH –CH3), 2.24 singlet (6H, CH ), 1.04
2
3
triplet (6H, –CH –CH ). [C H N Pd] Elemental analysis:
3
2
32 42 4
1
23

Transition Met Chem (2014) 39:699–704
701
1
1
.6
.2
0.15
492 506
1
1
0.1
A
A
0.8
2
2
0.05
0
.4
0
3
0
3
50
400
450
500
550
600
3
50
400
450
500
550
λ, nm
λ, nm
Fig. 2 Changes in EAS of benzene solution of 1 under the addition of
trichloroacetic acid (6 9 10 mol/l): 1 the initial time moment, 2 the
final time moment
Fig. 1 EAS of complexes in benzene: 1 complexes of Zn(II) (the
spectra for the Pd(II) complex are similar), 2 complex of Ni(II), 3
complex of Cu(II)
-4
2
1
0
calculated, %: C 64.0, H 7.2, N 10.0; found, %: C 64.0, H
7
.2, N 10.0.
Results and discussion
Pd(II) complexes
The interaction of PdCl with Hdpm in methanol leads to
2
0
1
2
3
4
the formation of [Pd(dpm) ] with high yield. The reaction
2
7
C2 ·10 mol/l
acid
is complete within *30 min when excess PdCl is used.
2
The IR spectra of crystalline samples of the compounds do
not show any features arising from NH groups, indicating
deprotonation and formation of M–N bonds. The same
Fig. 3 The dependence of kobs values for the dissociation of the
complex 1 on trichloroacetic acid concentration
1
situation was observed in the H NMR spectra; the proton
and, accordingly, the range and quantitative characteristics
of its kinetic stability. Changes in EAS of the reaction
mixtures during the protolytic dissociation are shown in
Fig. 2. The occurrence and growth of an absorption max-
imum at 492 nm indicate destruction of the complex under
the acidic conditions, with the formation of protonated
ligand. The linear decrease of the absorption band of the
dipyrrin complex with respect to the logarithm of time
indicates first-order dependence on the metal complex
concentration, while the dependence of the kobs on the
square of the acid concentration (Fig. 3) shows that the
reaction rate is the second order with respect to CCl3-
COOH concentration:
signals typical for the free ligands and belonging to the NH
groups are absent in the spectra of the complex.
In the UV–Vis spectrum, a maximum at *505 nm,
corresponding to intraligand p–p* transitions, and a max-
imum in the near-UV at *393 nm (charge transfer) are
observed in the spectra of complex 1 (Fig. 1). Previously, it
was determined [16] that the value of the bathochromic
shift of the dipyrrin absorption maximum after the coor-
dination with a metal ion correlates with the stability of the
resulting complexes and indicates the prevalence of ionic
and/or covalent bonding contributions in their stabilization.
The values of the absorption maxima of the present com-
pound indicates the predominantly covalent character of
the Pd–N bonds.
2
kobs ¼ k ꢀ ½CCl3COOHꢂ
ð4Þ
Preliminary experiments showed that the complex 1 is
inert to protolytic dissociation in C H –CH COOH mix-
Thus, the protolytic dissociation of the Pd(II) complex
proceeds with the formation of the protonated form of the
ligand. The overall kinetic equation is then third order:
6
6
3
tures. Palladium(II) complexes are more resistant to prot-
olytic dissociation than analogous complexes with
d½PdðdpmÞ ꢂ
3
d metals [11]. Benzene and trichloroacetic acid mixtures
2
2
ꢁ
¼ k ꢀ [Pd(dpm) ] ꢀ ½CCl3COOH]
ð5Þ
2
ds
were used to study the protolytic dissociation reactions of 1
123

7
02
Transition Met Chem (2014) 39:699–704
Table 1 Kinetic and activation parameters of the protolytic dissoci-
ation reaction for the investigated compounds
a rate constant k–1 under the conditions of the experiment is
kinetically insignificant, Eq. (9) becomes the following:
=
=
T, K
k
E
mol)
a
(kJ/
DS [J/
(mol K)]
DH
(kJ/mol)
d½PdðdpmÞ ꢂ
2
þ
ꢁ 2
ꢁ
¼ k1 ꢀ ½Pd(dpm) ꢂ ꢀ ½H . . .A ꢂ
ð10Þ
2
ds
0
0
0
0
0
1
3
4
5
: [Pd(dpm) ] dpm = 3,3 ,4,5,5 -3,3 ,4,5,5 -pentamethyl-4 -ethyl-
,2 -dipyrrin in mixture C
2
Thus, the limiting step (6) makes the main contribution
0
2
6
6
H –CCl
3
COOH
to the overall rate of reaction, and the resulting activation
parameters are related to the formation of the correspond-
ing transition state.
2
3
3
98
4,700 ± 200
7,320 ± 360
18,788 ± 393
53 ± 2
-6.0 ± 0.3
45 ± 2
08
18
0
0
0
: [Zn(dpm)
mixture C
2
] dpm = 3,3 ,4,5,5 -pentamethyl-2,2 -dipyrrin in
6
H
6
–CH COOH
3
Zn(II) complexes
298
308
318
0.0001
0.0002
0.0005
61 ± 2
-120 ± 10
59 ± 2
The EAS of zinc complexes 2–4 (Fig. 1) are characterized
by intense absorption bands at *500 nm, and low-inten-
sity bands in the near-UV region at *370 nm. Changes in
the EAS of benzene solutions of the zinc complexes are
similar to those for the Pd(II) complex. Decrease in the
intensity of the long wavelength absorption maximum
0
0
0
0
: [Zn(dpm)
in mixture C
2
] dpm = 3,3 ,4,5,5 -pentamethyl-4 -ethyl-2,2 -dipyrrin
6
6
H –CH
3
COOH
298
308
318
0.0003
0.0005
0.0007
52 ± 3
-141 ± 12
50 ± 3
(
kabs = 497–506 nm) and the appearance of a protonated
ligand H L absorption band at 485–488 nm are observed
0
0
0
: [Ni(dpm)
CH COOH
2
] dpm = 3,3 ,4,4 ,5-pentamethyl-2, 2 -dipyrrin in
?
2
3
in the EAS. Thus, the protolytic dissociations of [ZnL2]
2
3
3
98
0.29 ± 0.02
0.42 ± 0.03
0.83 ± 0.05
44 ± 3
-115 ± 13
41 ± 3
proceed with the formation of the protonated forms of di-
?
pyrrin ligands, namely H L .
18
28
2
Under the condition of minimum amount of acid addi-
tion to complex 2, its instantaneous dissociation takes
place. The data obtained allowed us to determine the
equilibrium constant for this protolytic dissociation
including ligand protonation, according to the following
scheme:
Values of kobs and k at different temperatures and the
corresponding activation parameters are given in Table 1.
?
Formation of protonated ligand (H dpm ) during the dis-
2
sociation reaction and the third-order kinetics of this
equation allow us to consider the process of protolytic
dissociation of the complex as a set of three steps involving
sequential protonation of the ligand nitrogen donor atoms:
þ
ꢁ
½
where [ZnL ] is complex 2. The equilibrium constant for
ZnL2ꢂ þ 4AcOH ¼ Zn(OAc) þ 2½H2L . . .OAc ꢂ
2
2
-6
2
2
this equation was 5 9 10 l /mol , which is consistent
with the known data on the thermodynamic stability of the
complexes [17]. The reaction shows the first-order depen-
dence on the complex concentration and second-order
n
o
k1
þ
ꢁ
2þ
ꢁ
½
PdðdpmÞ ꢂ þ 2H . . .A ꢀ ½Pd(dpm) ꢀ 2Hꢂ . . .2A
2
2
kꢁ1
ð6Þ
T
dependence on AcOH concentration. The k values
ꢂ
ꢃ
k2
2þ
ꢁ
½
PdðdpmÞ ꢀ 2H] . . .2A !2Hdpm þ PdA2
ð7Þ
obtained at various temperatures and the corresponding
activation parameters are shown in Table 1.
2
k3
þ
ꢁ
þ
ꢁ
Hdpm þ H . . .A !½H2dpm . . .A ꢂ
ð8Þ
Thus, protolytic dissociation of the palladium(II) and
zinc(II) complexes occurs in the same manner, with the
formation of the protonated form of the ligands. The total
kinetic equation is the third order:
?
-
where H …A are protonating forms of the acid, reactions
6) and (8) have low rates, and reaction (7) has a high rate.
(
The kinetic equation for the reaction of protolytic disso-
d½ZnL ꢂ
ciation of [Pd (dpm) ] under quasi-steady-state conditions
2
2
2
ꢁ
¼ kobs ꢀ [ZnL ꢂ ꢀ ½AcOHꢂ
2
can be written in the following form:
ꢀ
ds
ꢁ
The data obtained are in agreement with the literature
d½PdðdpmÞ ꢂ
k1k2
2
þ
ꢁ 2
ꢁ
¼
ꢀ ½Pd(dpm) ꢂ ꢀ ½H . . .A ꢂ
2
[7] and indicate the participation of two acid molecules in
the rate-limiting step of the reaction. The resulting rate
constants and activation parameters could be used for the
determination of the influence of the various alkyl sub-
stituents of the ligands on the kinetic stability of complexes
ds
kꢁ1 þ k2
ð9Þ
This corresponds to the experimentally found equation
5). Assuming that the reversible stage of reaction (6) with
(
1
23

Transition Met Chem (2014) 39:699–704
703
ꢂ
ꢃ
6
¼ k2
2
–4. The absence of a substituent in the a-position of the
NiðdpmÞ ðAcOHÞ !½NiðdpmÞðAcOÞꢂ þ Hdpm
2
pyrrole ring leads to a significant increase in complex
lability. The inductive (?I) effect of the alkyl group
located in the a-position of the pyrrole ring has a strong
effect on the rate of Zn–N bond dissociation. This is in line
with the proposed reaction mechanism, since the stability
k3
Hdpm þ AcOH!½H dpmꢂðAcOÞ
2
Such that the first step is rate-determining. The kinetic
scheme for the equilibrium of the heteroligand complex
2
?
-
dissociation to give Ni(OAc) includes the following steps:
2
of the intermediate {[ZnL ꢀ2H] …2OAc } is determined
2
ꢃ
k
primarily by two intermolecular hydrogen bonds, whose
strength depends on the electron density on the nitrogen
atoms of the ligand. Strengthening of the ?I-effect of the
a-substituents will increase the effectiveness of such
interactions. Increasing the length of the alkyl groups leads
to a damping of the ?I-effect, decreasing the stability of
the intermediate complex. This is confirmed by the results
available in the literature for the dibutyl-substituted di-
pyrrin complex [9]. Thus, the ability of the coordinated
nitrogen atoms of the ligand to react with acid has the
dominant influence on the kinetic stability of dipyrrin
zinc(II) complexes. The higher stability of the palladium
complexes compared with zinc can be explained by the
energetically favorable square-planar coordination unit in
complex 1, reducing the possibility of coordination of the
acid anions to the palladium(II) center.
1
6¼
½
NiðdpmÞðOAcÞꢂ þ AcOH ꢀ fNiðdpmÞðOAcÞðAcOHÞg
kꢃ
ꢁ
1
ꢃ
k
2
6
fNiðdpmÞðOAcÞðAcOHÞg !Ni(OAc) þ Hdpm
2
k3
Hdpm þ AcOH!½H2dpmꢂðOAcÞ
Here again, the first step is slow relative to the following
two steps. The observed first-order dependence on acid
concentration indicates that the formation of the heteroli-
gand complex limits the overall process of dissociation,
which allows us to neglect the dissociation rate of this
complex. Assuming a steady state then leads to the fol-
lowing kinetic equation:
ꢀ
ꢁ
d½NiðdpmÞ ꢂ
k k
1 2
2
ꢁ
¼
ꢀ [Ni(dpm) ꢂ ꢀ ½AcOHꢂ
2
ds
k
þ k2
This equation corresponds to the experimentally found
ꢁ
1
equation such that
Ni(II) complexes
k1k2
kobs
¼
EAS of benzene solutions of complex 5 are characterized by
three absorption peaks: a high intensity peak at 530 nm,
another at 464 nm, and a third at 300 nm, assigned to charge
transfer (Fig. 1). Protolytic dissociation of the nickel(II)
complex proceeds with a measurable velocity in the range of
kꢁ1 þ k2
From the temperature dependence of the rate constants,
the activation parameters were determined (See Table 1).
The activation energy for this complex is higher than that
of the nickel complex of dibutyl-substituted dipyrrin [9].
The kinetic stability of the complexes increases due to the
stronger induction effect of the methyl and ethyl groups,
resulting in higher electron-donating effect of the pyrrole
nitrogen.
-
4
acid concentration from 2.4 9 10 to 0.2 mol/l. At low
-
3
acid concentration ([AcOH] \2 9 10 mol/l), only
absorption peak corresponding to heteroligand complex is
fixed in the EAS [15]. At higher acid concentrations
-
3
(
[AcOH][2 9 10 mol/l), the heteroligand complex is no
longer observed in the reaction mixture, and [Ni(dpm)2]
undergoes full dissociation. In this case, the reaction shows
first-order dependencies on both metal complex and acid,
corresponding to the following kinetic equation:
Cu(II) complex
EAS of benzene solutions of complex 6 are similar to those
of complex 5. The spectrum is characterized by three
absorption maxima at 527, 460, and 300 nm. Significant
decrease in the intensity of the absorption bands of the
d½NiðdpmÞ ꢂ
2
ꢁ
¼ kobs ꢀ [Ni(dpm) ꢂ ꢀ ½AcOHꢂ
2
ds
Each of the two [Ni(dpm) ] protolytic dissociation
2
complex, together with the appearance of bands due to the
?
dpm ligand at 488 nm, is
equilibria can be represented by a kinetic model compris-
ing successive protonation of coordinated ligand nitrogen
atoms. The kinetic scheme for the equilibrium with the
formation of the heteroligand complex [Ni(dpm)(AcO)]
includes the following steps:
protonated form of the H
2
observed in the EAS (Fig. 4) when small amounts of acetic
acid ([AcOH] = 0.04 mol/l) are added to a solution of
[Cu(dpm) ] (the estimated reaction time is B1 s). Addition
2
of the strong base triethylamine to the reaction mixture
leads to the reverse process, i.e., the formation of
[Cu(dpm) ]. Thus, the dissociation of [Cu(dpm) ] in these
k1
ꢂ
ꢃ
6¼
½
NiðdpmÞ ꢂ þ AcOH ꢀ NiðdpmÞ ðAcOHÞ
2
2
2
2
kꢁ1
123

7
04
Transition Met Chem (2014) 39:699–704
0
.4
.3
Considering the different metals, complexes of zinc(II)
and nickel(II) are relatively stable. Protolytic dissociation
leads to the formation of heteroligand complexes for
nickel(II). Cobalt(II) complexes are relatively stable; their
stabilities are similar to those of zinc(II) complexes. Pal-
ladium(II) complexes are much more stable, trifluoroacetic
acid being required for their dissociation.
4
88
0
516
460
A
0.2
2
1
0
.1
Acknowledgments The authors thank S. Alyoshin, the graduate
student, for his help in routine spectral and kinetic measurements. The
work was supported by the grant of the President of the Russian
Federation for young scientists and graduate students engaged in
advanced research and development in priority directions of mod-
ernization of the Russian economy (2013–2015) (Grant No. SP-
0
360
410
460
510
560
λ, nm
Fig. 4 Changes in the EAS of benzene solution of 6 under the
addition of acetic acid (0.04 mol/l; 1 the initial time moment, 2 the
final time moment)
1
742.2013.1).
References
conditions is a reversible equilibrium. This allowed us to
determine the equilibrium constant of the reaction by
spectrophotometric titration:
1
2
. Wood TE, Thompson A (2007) Chem Rev 107:1831
. Loudet A, Burgess K (2007) Chem Rev 107:4891
3. Beziau A, Baudron SA, Fluck A, Hosseini MW (2013) Inorg
Chem 52:14439–14448
½
CuðdpmÞ ꢂ þ 4AcOH ¼ 2½H2dpm](OAc) þ Cu(OAc)
The value of the equilibrium constant K was determined
2
2
4. Beziau A, Baudron SA, Rogez G, Hosseini MW (2013) Cryst Eng
Commun 15:5980–5985
5
. Mula S, Ray AK, Banerjee M, Chaudhuri T, Dasgupta K,
Chattopadhyay S (2008) J Org Chem 73:2146
-
7
2
2
as (7.5 ± 0.4) 9 10 l /mol , which is consistent with the
data [17] for the thermodynamic stability of dipyrrin cop-
per(II) complexes. During the dissociation of the fully
methylated dipyrrin copper(II) complex studied in this
paper, even when adding trace amounts of acid, formation
of a mixed coordination sphere was not obtained. This is in
contrast to the butyl-substituted analogue, for which het-
eroligand complex formation is observed at low acid con-
centrations [9].
6. Makarova SP, Rumyantsev EV, Antina EV (2008) Russ J Gen
Chem 78:1770
7
. Kuznetsova R, Samsonova L, Kopylova T, Mayer G, Sikorskaya O,
Aksenova Y, Zulina N, Antina E, Yutanova S, Antina L, Pavich T,
Solovyov K, Arabei S (2010) CAOL 2010 international conference
on advanced optoelectronics and lasers, 10–14 September, Sevas-
topol, Ukraine, p 228
8
9
. Kuznetsova RT, Kopylova TN, Mayer GV, Sikorskaya OO,
Ermolina EG, Guseva GB, Antina LA (2011) Opt Spectrosc
110:385
. Guseva GB, Antina EV, Berezin MB (2003) Russ J Coord Chem
2
9:690
0. Banuelos J, Lopez Arbeloa F, Arbeloa T, Salleres S, Vilas JL,
Amat-Guerri F, Liras M, L o´ pez Arbeloa I (2008) J Fluoresc
1
Conclusions
1
8:899
The results of this study reveal the kinetic stabilities of
various dipyrrin complexes in an acidic environment. The
influence of ligand alkyl substituents on the kinetic stability
of the complexes can be explained in terms of their
inductive effects on the electron density on the coordinated
ligand nitrogen atoms. Increasing the length of the alkyl
groups leads to damping of the ?I-effect and, accordingly,
decreasing stability of the intermediate complexes. These
results are in good agreement with those obtained earlier
data [9]. Thus, the ability of the coordinated nitrogen atoms
of the ligand to interact with acid has a dominant influence
on the kinetic stability of the dipyrrin complexes.
11. Rumyantsev EV, Aleshin SN, Marfin YS (2013) Russ J Phys
Chem A 87:323
1
2. Rumyantsev EV, Aleshin SN, Desoki A, Marfin YS, Antina EV
2013) Russ J Inorg Chem 58:596
(
13. Weissberger A, Proskauer ES, Riddick JA, Toops EE Jr (1955)
Organic solvents. Physical properties and methods of purification.
Interscience Publishers, Inc., New York
1
4. Bruttel P, Schlink R (2003) Water determination by Karl Fischer
titration. In: Metrohm Monograph 8.026.5003
15. Berezin MB, Semeikin AS, Antina EV, Pashanova NA, Lebedeva
NS, Bukushina GB (1999) Russ J Gen Chem 69:1949
1
6. Rumyantsev EV, Desoky A, Antina EV (2010) Russ J Inorg
Chem 55:932
1
7. Rumyantsev EV, Desoki A, Antina EV (2009) Russ J Inorg Chem
54:1735
1
23