Spectrophotometric determination of formation constants for the Cu-ethylenediamine-halogen (chloride and bromide) system and their catalytic effect on the oxidative coupling of 2,6-di-tert-butyl-phenol

Article abstract of DOI:10.1016/j.ica.2010.06.050

In order to explain the mechanism of the dimerization of 2,6-di-tert-butyl-phenol when catalyzed by the copper-ethylenediamine complexes, a spectrophotometric study of the speciation of copper(II) complexes in methanol of Cu(II), ethylendiamine and Cl^- or Br^- was carried out at 303 K. The formation constants obtained for the copper chloride system are: log β₁₀₁ = 2.90 ± 0.03, log β₁₀₂ = 6.39 ± 0.03 and log β₁₀₃ = 8.62 ± 0.04, for the copper bromide system are log β₁₀₁ = 3.01 ± 0.10, log β₁₀₂ = 5.50 ± 0.08, for the copper-ethylendiamine complexes are log β₁₁₀ = 6.13 ± 0.05 and log β₁₂₀ = 10.54 ± 0.08, and for the ternary copper-ethylenediamine chloride or bromide systems are log β₁₁₁ = 10.21 ± 0.03 and log β₁₁₁ = 10.07 ± 0.03, respectively. Knowing the speciation of the copper-ethylenediamine-halide systems, the kinetic studies can be correlated with the species in solution. Comparative studies of the oxidation reaction of 2,6-di-tert-butyl-phenol using different copper(II) complexes with chloride or bromide and ethylenediamine as catalyst are reported. Their catalytic activity in the oxidation of 2,6-di-tert-butyl-phenol was monitored in methanol solution, following the corresponding quinone formation, at 418 nm ( = 3.95 × 10⁴ mol^-1 L cm^-1 at 303 K). The results indicate that the most active species are [Cu(en)X]⁺, where X is bromide or chloride, Both complexes have similar activity.

Full text of DOI:10.1016/j.ica.2010.06.050

Inorganica Chimica Acta 363 (2010) 3461–3468
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Spectrophotometric determination of formation constants for the
Cu–ethylenediamine–halogen (chloride and bromide) system and their catalytic
effect on the oxidative coupling of 2,6-di-tert-butyl-phenol
José J.N. Segoviano-Garfias a, Rafael Moreno-Esparza b, Guillermo Mendoza-Díaz a,
*
a Departamento de Química, Unidad Noria Alta, División de Ciencias Naturales y Exactas, Universidad de Guanajuato, Noria Alta s/n, Guanajuato, Gto 36050, México
b Facultad de Química (UNAM), Edificio B. Av. Universidad 3000, Coyoacán, México D.F. 04510, México
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to explain the mechanism of the dimerization of 2,6-di-tert-butyl-phenol when catalyzed by the
copper–ethylenediamine complexes, a spectrophotometric study of the speciation of copper(II) com-
plexes in methanol of Cu(II), ethylendiamine and Clꢀ or Brꢀ was carried out at 303 K. The formation con-
stants obtained for the copper chloride system are: log b101 = 2.90 0.03, log b102 = 6.39 0.03 and
log b103 = 8.62 0.04, for the copper bromide system are log b101 = 3.01 0.10, log b102 = 5.50 0.08, for
the copper–ethylendiamine complexes are log b110 = 6.13 0.05 and log b120 = 10.54 0.08, and for the
ternary copper–ethylenediamine chloride or bromide systems are log b111 = 10.21 0.03 and
log b111 = 10.07 0.03, respectively. Knowing the speciation of the copper–ethylenediamine–halide sys-
tems, the kinetic studies can be correlated with the species in solution. Comparative studies of the oxi-
dation reaction of 2,6-di-tert-butyl-phenol using different copper(II) complexes with chloride or bromide
and ethylenediamine as catalyst are reported. Their catalytic activity in the oxidation of 2,6-di-tert-butyl-
phenol was monitored in methanol solution, following the corresponding quinone formation, at 418 nm
Received 5 March 2010
Received in revised form 18 June 2010
Accepted 29 June 2010
Available online 24 July 2010
Keywords:
Copper(II) complexes
Catalysis
Oxidative coupling
Speciation
Solution equilibria
Non-aqueous solvents
(e
= 3.95 ꢁ 104 molꢀ1 L cmꢀ1 at 303 K). The results indicate that the most active species are [Cu(en)X]+,
where X is bromide or chloride, Both complexes have similar activity.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
per(II) complex [5,6]; from this suggestion some research groups
studying this polymerization have considered that the oxidative
coupling is catalyzed by a biomimetic dinuclear copper species
(tyrosinase-like active site) [7]. However, many other groups, be-
lieve that a mononuclear copper species produce a phenoxyl radi-
cal followed by the coupling reaction [8].
In 1959, Allan S. Hay discovered the oxidative polymerization of
2,6-di-methyl-phenol, (1) catalyzed by the CuCl/pyridine system
under oxygen, leading to the poly(2,6-dimethyl-1,4-phenylene
oxide), (2) as a principal product with a small amount of the C–C
coupled product, 3,30,5,50-tetra-tert-methyl-4,40-dibenzoquinone
(3) (Scheme 1) [1].
The copper-catalyzed oxidative coupling of (1) will produce (2),
which is a valuable industrial thermoplastic. This polymer is still
one of the most important commercially produced nowadays, with
a yearly increasing turnover of around one billion dollars at the end
of the last century. However, after almost 50 years of intensive re-
search, the mechanism of this reaction is not yet elucidated [7,8].
We are interested to contribute to the understanding of this
mechanism and to achieve this; we have studied the copper–ethy-
lenediamine–halide system in methanol, in order to obtain the for-
mation constants and the distribution diagram species of these
systems. These diagrams will help to understand which catalytic
species are present in solution under the conditions employed in
the coupling oxidation reaction. It is important to mention that
the formation constants for copper–ethylenediamine have been re-
ported in water [9], whereas for copper–halides reports exist in
several solvents [10–14]; but as far as we know, there is not a re-
port of the determination of the formation constants of the ternary
system copper–ethylenediamine–halides in methanol.
A wide variety of diamines have been tested as ligands for the
copper catalyst in this polymerization reaction, some of them
are: N,N,N0,N0-tetramethylethylenediamine, N,N0-di-tert-amylethy-
lenediamine, N,N0-di-isopropyl-ethylendiamine and N,N0-di-tert-
butyl-ethylenediamine. [2]. This reaction can be carried out in
several solvents such as methanol, benzene, 1,2-di-chloro-benzene
and others [3,4]. Later, it was found that copper(II)-di-tert-butyl-
ethylenediamine dibromide is presumably the best catalyst for this
reaction. This compound was synthesized and characterized in
solid phase [1,2].
Several studies assume that copper complex with N,N0-di-tert-
butyl-ethylenediamine, is able to form a
l- :g
g2 2-peroxodicop-
* Corresponding author. Tel.: +52 473 734 6077.
E-mail address: mendozag@quijote.ugto.mx (G. Mendoza-Díaz).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.06.050

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J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 363 (2010) 3461–3468
Scheme 1.
Several diamines has been tested as ligands in the copper com-
experiment. The final copper concentration was set constant at
0.0075 and 0.0039 M, respectively. The spectral region observed
was from 450 to 850 nm for both experiments.
plexes used as catalysis in the oxidative coupling of 2,6-di-methyl-
phenol [2], as far as we know, there is no reported the use of
[Cu(en)]2+ in the oxidative coupling of 2,6-di-methyl-phenol or
2,6-di-tert-butyl-phenol. However, copper-bis-ethylenediamine,
has been tested in the oxidation of 2,4-di-tert-butyl-phenol to bi-
phenyl-diols [15,16].
2.4. Copper–ethylenediamine–chloride and copper–ethylenediamine–
bromide equilibrium studies
Finally, for the copper–ethylenediammine–chloride system,
two stock solutions of ethylenediamine (0.2093 and 0.1941 M),
Cu(NO3)2 3H2O (0.1867 and 0.01941 M) and sodium chloride
2. Experimental
.
(0.0224 and 0.0291) were prepared. For the copper–ethylenedia-
mine–bromide system, the stock solutions of ethylenediamine
(0.0747 and 0.0373 M), Cu(NO3)2 3H2O (0.0747 and 0.0373 M)
Methanol HPLC grade (Tecsiquim) was used as purchased as
solvent for the preparation of the different solutions, Cu(NO3)2 3-
.
.
H2O, sodium chloride and sodium bromide (J.T. Baker), ethylenedi-
amine (Sigma–Aldrich), were analytical grade, and used without
further purification. 2,6-di-tert-butylphenol (Sigma–Aldrich) was
recrystallized twice in methanol HPLC grade. No ionic strength
was used.
and sodium bromide (0.0373 and 0.0187 M) were prepared.
For the copper–ethylenediammine–chloride system, the final
concentrations of copper and ethylenediamine were maintained
equal and constant in each experiment (0.0075 and 0.0019 M).
The chloride concentrations were varied from 0.0045 to
0.0089 M, and 0.0003 to 0.0058 M, respectively. The observed
spectral region was from 450 to 850 nm for both experiments.
For the copper–ethylenediamine–bromide system, the final
concentrations of copper and ethylenediamine were maintained
equal and constant in each experiment (0.0075 and 0.0037 M).
The bromide concentrations were varied from 0.0004 to
0.0075 M and 0.0002 to 0.0037 M, respectively. The observed spec-
tral region was from 550 to 1000 nm for both experiments.
2.1. Spectrophotometric equilibrium studies
An Agilent 8453 UV–Visible Spectroscopy System, equipped
with a temperature-controlled cell holder was utilized in the spec-
tral studies, in a standard quartz cell with 1 cm optical length and
3 mL volume at 303 K. The determination of the formation con-
stants was performed refining the spectrophotometric data with
the program HYPERQUAD [17].
2.5. Catalytic oxidation studies
2.2. Copper chloride and copper bromide equilibrium studies
All kinetic runs were performed using pseudo-first-order
conditions respect to the copper complex, at 303 K, in methanol
solution. The formation of 3,30,5,50-tetra-tert-methyl-4,40-dibenzo-
Experiments were carried out using two different copper con-
centrations. Stock solutions of NaCl and NaBr were prepared each
with two different concentrations (0.0690 and 0.1370 M). The hal-
ogen concentration was varied from 0 to 0.0137 M and 0.0014 to
0.0274 M for two experiments, respectively. Two stock solutions
quinone (5), at 418 nm (
e
= 39.5 ꢁ 103 molꢀ1 L cmꢀ1) was followed
.
spectrophotometrically. Stock solutions of Cu(NO3)2 3H2O, ethy-
lenediamine, sodium chloride, and sodium bromide were prepared
separately at the same concentration (0.0225 M); 2,6-di-tert-butyl-
phenol was prepared at 0.0229 M. Aliquots of each reactive were
used in order to obtain a final concentration of 0.00003 M for the
corresponding catalyst and 0.0077 for the substrate (Scheme 2).
.
of Cu(NO3)2 3H2O were used (0.0342 M and 0.0684 M, respec-
tively). The final copper concentration was set constant at
0.00342 and 0.00684 M, for each experiment. The spectral region
observed was from 600 to 1100 nm for copper chloride solutions
and 700 to 1000 nm for copper bromide solutions.
2.3. Copper ethylenediamine equilibrium studies
3. Results and discussion
Experiments were also performed using two different stock
solutions of ethylenediamine (0.0418 and 0.0598 M). Cu(NO3)2 3-
3.1. Formation constants of the copper(II) halide complexes
.
H2O was used to prepare copper stock solutions (0.1870 M and
0.0400 M). The ethylenediamine concentration was varied from
0.00084 to 0.0167 M and 0.0006 to 0.0119 M, respectively, in each
In all determinations made it was necessary to avoid the use of
ionic strength, because when it was attempted an early precipita-
tion of copper was observed making impossible the present study.

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 363 (2010) 3461–3468
3463
Scheme 2.
Therefore, it was decided that a comparative study under similar
conditions could help in clarify the objective proposed in this
study.
The observed absorbance values at different wavelengths were
recorded at 303 K and the evaluation of the formation constants
was achieved using the next model:
The observed visible spectra of copper(II) chloride solutions at
different halide concentrations, are presented in Fig. 1, the ob-
served visible spectra for copper(II) bromide are similar and they
are available as Supplementary material.
M
M
M
2þ þ Lꢀ ¼ MLþ log b101
2þ þ 2Lꢀ ¼ ML2 log b
ð1Þ
ð2Þ
ð3Þ
102
2þ þ 3Lꢀ ¼ MLꢀ log b103
3
For the copper–chloride system a maximum appear at 814 nm
for the lowest chloride concentration, as the concentration is
raised, a shift up to 907 nm is observed for the highest concentra-
tion, while in the case of the bromide solutions this shift goes from
823 to 908 nm. This red shift is also observed for the copper chlo-
ride system in water [14], trimethyl phosphate, N,N-dimethylform-
amide [12], propylene carbonate, dimethyl sulfoxide [11] and
methanol [13] solutions. The same behavior is observed also for
the copper bromide in methanol [13]. Copper (II) bromide com-
plexes have been less studied than copper(II) chlorides complexes.
Although the experimental spectra of both chloride and bro-
mide complexes resemble each other, it is to be noted that the
intensity of absorption in the visible region is higher for the bro-
mide copper complexes than chloride copper complexes for equiv-
alent concentrations of copper and halides in each experiment.
The determination of the formation constants bjkl, correspond-
ing to the successive equilibriums between Cu2+ and either halide,
takes into account a large array of data, made with all spectra of
both experiments (low an high concentrations) simultaneously.
The obtained formation constants are reported in Table 1. In
general, it is observed that the calculated formation constants of
copper (II) halides in methanol are much higher than those in
water, although lower than those obtained in trimethyl phosphate
and N,N-dimethylformamide solutions, this is explained first con-
sidering the difference on the dielectric constant of each solvent,
this is, water with its high dielectric constant makes the ionic spe-
cies more stable [18]. On the other hand, the observed formation
constants are higher than those reported with ionic strength of
1.0 M [13], such effect can be explained in terms of the competition
for the copper cation among the anions present in this solution
(perchlorate vs. halides).
In addition, data in Table 1, show that in the same medium,
bromides are in general more stable than chlorides, this is prob-
ably due to the fact that bromide is better nucleophile than chlo-
ride. It is also worth to mention, that with the metal and ligand
concentration range studied, only mononuclear species are
formed.
Fig. 1. Absorption spectra of copper(II) chloride in methanolic solution (high concentration experiment): for spectrum (1) [Cu(II)] = 0.004 M and ligand concentration = 0 M.
For the rest: [Cu(II)] = 0.0068 M. Ligand concentration (M): (2) 0.0014; (3) 0.0027; (4) 0.0041; (5) 0.0055; (6) 0.0065; (7) 0.0082; (8) 0.0096; (9) 0.0109; (10) 0.0123; (11)
0.0137; (12) 0.0150; (13) 0.0164; (14) 0.0178; (15) 0.0190; (16) 0.0205; (17) 0.0219; (18) 0.0233; (19) 0.0246; (20) 0.0230.

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Table 1
Results of the spectrophotometric data refinement of the copper chloride and copper bromide solutions.
Chloro
complexes in
methanol
Chloro complexes in
methanol using ionic
strength [13]
Chloro
complexes in
water [14]
Chloro complexes
in TMP solutions
[10]
Chloro complexes in
N,N-dimethylformamide complexes in
[10]
Bromo
Bromo complexes in
methanol using ionic
strength [13]
methanol
log b101 2.90 0.03
log b102 6.39 0.03
log b103 8.62 0.04
2.34
3.54
3.58
0.6
0.67
0.29
5.8
9.4
12.7
4.4
9.0
12.0
3.01 0.10
5.50 0.08
2.08
2.81
r
0.0428
0.0119
Fig. 2. Formation curves of the (a) chloro and (b) bromo copper complexes. [Cu]2+ was maintained constant at 0.00685 M and the concentration of sodium halide was varied
from 0.00137 to 0.0274 M.
3.2. Distribution curves of the halide copper complexes
maximum at 780 nm appears at dilute ligand concentration. As
the ethylenediamine concentration increases, an increment in
absorbance was observed with a blue shift to 550 nm.
Determination of the formation constants of the complexes of
copper with halides and copper–ethylendiamine is an important
issue for the determination of the speciation in methanol of the
copper–ethylenediamine–halide systems, this formation constants
were included in the refinement of the models of the copper–eth-
ylenediamine–halide systems, (vide infra).
The determination of the formation constants bjkl, correspond-
ing to the successive equilibria between Cu2+ and ethylenediamine,
was made using in the refinement, all spectra of both experiments
(at two concentrations) simultaneously. The observed absorbance
values at different wavelengths were recorded at 303 K and the
evaluation of the formation constants was achieved using the next
model:
The formation curves of chloro and bromo copper complexes
are shown in Fig. 2a and b, respectively. Whereas chloro complexes
display a distribution between the mono-, di- and tri-chloro com-
plexes in the concentration range, the bromo complexes are char-
acterized by the presence of only the mono- and di-bromo
complexes.
The calculated electronic spectra for each halide copper com-
plex are available as Supplementary material. Monochloro and
monobromo complexes, both have a band ca. 860 nm, however,
they differ in intensity, being the extinction coefficient of 91.5 for
the chloro system and 87.1 L molꢀ1 cmꢀ1 for the bromo complexes.
The dichloro copper complex has an absorption maximum at
890 nm with
per complex has the maximum at 915 nm with stronger intensity,
= 122 L molꢀ1 cmꢀ1. The trichloro complex has an absorption
maximum at 910 nm with
= 76.3 L molꢀ1 cmꢀ1. It is important
e
= 51.7 L molꢀ1 cmꢀ1. In contrast, the dibromo cop-
e
Cu2þ þ en ¼ CuðenÞ2þ log b110
Cu2þ þ 2en ¼ CuðenÞ22þ log b120
ð1Þ
ð2Þ
e
to mention that tribromo species was not detected under these
experimental conditions.
The values of the logarithm of the formation constants are re-
ported in Table 2. Copper–ethylenediamine stability constants
have been reported in aqueous systems, as well in mixtures water
methanol systems. In aqueous systems, copper–ethylenediamine
have higher stability values than in pure methanol, this is probably
because the methanol is more nucleophilic than water.
When an equimolar solution of cupric nitrate and ethylenedia-
mine in methanol is made, the [Cu(en)]2+ complex has about a 60%
of formation relative to copper, as can be seen in Fig. 5. In order to
study the ternary system of copper–ethylenediamine–halide, we
used this equimolar concentration because at this concentrations,
the [Cu(en)]2+ complex has the highest percentage of formation,
therefore, only the halide concentration was varied.
3.3. Formation constants of the copper(II) ethylenediamine and
copper(II) ethylenediamine halide complexes
Copper–ethylenediamine complexes have been extensively
studied in aqueous systems, the existence of the complexes
,
[Cu(en)]2+ [Cu(en)2]2+ and [Cu(en)3]2+ with their respective
formation constants of log b110 = 10.75, log b120 = 20.03 and
log b130 = 20.93, have been well established [9]. However, in non-
aqueous systems, the copper–ethylenediamine has been less
studied, and as far as we know, there are no reports about the for-
mation constants of copper–ethylenediamine complexes in pure
methanol.
The observed electronic spectra of the copper–ethylenedia-
mine and copper–ethylenediamine chloride solutions are shown
in Figs. 3 and 4, respectively. The observed electronic spectra of
The electronic spectra of copper–ethylenediamine solutions are
shown in Fig. 3. For the copper–ethylenediamine system,
a

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 363 (2010) 3461–3468
3465
Fig. 3. Absorption spectra of copper(II) ethylenediamine system in methanolic solution (high concentration experiment): for all the spectra [Cu(II)] = 0.0075 M and
ethylenediamine concentration (M): (1) 0.0008; (2) 0.0017; (3) 0.0025; (4) 0.0033; (5) 0.0042; (6) 0.0050; (7) 0.0059; (8) 0.0067;(9) 0.0075; (10) 0.0084; (11) 0.0092; (12)
0.0100; (13) 0.0109; (14) 0.0117; (15) 0.0125; (16) 0.0137; (17) 0.0142; (18) 0.0151; (19) 0.0159; (20) 0.0167.
In a similar way, when bromide concentration increases, the
absorbance increase and a red shift occur up to 678 nm.
The determination of the formation constants bjkl, correspond-
ing to the successive equilibria between copper, ethylenediamine
and either halide were made using the same methodology as men-
tioned above.
The observed absorbance values at different wavelengths were
recorded at 303 K and the evaluation of the formation constants
was achieved using the next model:
Table 2
Results of the spectrophotometric data refinement of the copper–ethylenediamine in
solution.
Ethylenediamine complexes in
methanol
Ethylenediamine complexes in
water [9]
log b110 6.13 0.05
log b120 10.54 0.08
log b130
10.75
20.03
20.93
r
0.1169
Cu2þ þ en þ Clꢀ ¼ ½CuðenÞClꢂ1þ log b111
Cu2þ þ en þ Brꢀ ¼ ½CuðenÞBrꢂ1þ log b111
ð1Þ
ð2Þ
copper–ethylenediamine–bromide solutions are similar and they
are available as Supplementary material. For the copper–ethy-
lenediamine–chloride system, a maximum at 657 nm appears at
dilute chloride concentration, and as chloride concentration in-
creases, increment in absorbance and a red shift is observed up
to 672 nm. For the copper–ethylenediamine–bromide system, a
maximum at 663 nm appears at dilute bromide concentration.
The values of the logarithms of formation constants are reported in
Table 3.
The calculated electronic spectra for the [Cu(en)]2+ (see Supple-
mentary material) has an absorption maximum at 680 nm and
e
= 53.64 L molꢀ1 cmꢀ1, meanwhile the complex [Cu(en)Cl]1+ have
his maximum at 677 nm and
e
= 66.01 L molꢀ1 cmꢀ1, and the
Fig. 4. Absorption spectra of copper(II) ethylenediamine chloride system in methanol solution (high concentration experiment): for all the spectra [Cu(II)] = 0.0075 M,
[ethylenediamine] = 0.0075 and chloride concentration (M): (1) 0.0005; (2) 0.0009; (3) 0.0013; (4) 0.0018; (5) 0.0022; (6) 0.0027; (7) 0.0031; (8) 0.0036; (9) 0.0040; (10)
0.0045; (11) 0.0049; (12) 0.0054; (13) 0.0058.

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Fig. 5. Formation curves of the (a) copper–ethylenediamine, (b) copper–ethylenediamine–chloride and (c) copper–ethylenediamine–bromide complexes. (a)
[Cu]2+ = 0.0075 M, ethylenediamine range from 0.00084 to 0.0167 M. (b) [Cu]2+ = 0.007465 M, [ethylenediamine] = 0.007465 M, chloride concentration ranges from
0.00045 to 0.00896 M and (c) [Cu]2+ = 0.007465 M, [ethylenediamine] = 0.007465 M of bromide concentration ranges from 0.00037 to 0.007465 M.
complex [Cu(en)Br]1+ has the maximum at 682 nm and
e
= 80.89
halide is reached, the formation of the [Cu(en)X] complex is ob-
served in more that 80%. This information will be very helpful in
the determination of which are the catalytic species in the oxida-
tive coupling of the 2,6-di-tert-butyl-phenol.
L molꢀ1 cmꢀ1. The [Cu(en)Br]1+ has a stronger intensity than the
complex [Cu(en)Cl]1+ which is consistent with a higher charge
transfer from the bromide to the copper. Copper bis ethylenedia-
mine has the absorption maximum at 551 nm and a
e = 70.13
L molꢀ1 cmꢀ1
.
3.5. Catalytic oxidation studies
3.4. Distribution curves of the copper(II)–ethylenediamine and
copper(II)–ethylenediamine–halides in methanol
The catalytic activity of the copper(II) nitrate, copper(II) halides,
copper(II) ethylenediamine and copper(II) ethylenediamine halide
systems in the oxidative coupling of 2,6-di-tert-butyl-phenol (4) to
3,30,5,50-tetra-tert-butyl-4,40-dibenzoquinone (5), was measured at
418 nm (Fig. 6).
To find out which are the active catalytic species in the coupling
reaction, we use solutions with equimolar concentrations of metal
and ligand, in order to obtain the complexes as follow: a solution
with an equimolar mixture of copper and ethylenediamine will
generate [Cu(en)]2+ with 60% abundance, [Cu(en)2]2+ and free cop-
per each with 20% abundance; a solution with an equimolar mix-
ture of copper, ethylenediamine and sodium chloride, will
The distribution species curves of the copper–ethylenediamine,
copper–ethylenediamine–chloride and the copper–ethylenedia-
mine–bromide are shown in Fig. 5. In both systems, it was possible
to detect only the 111 complex because in attempts to increase the
halide concentration always we observe the precipitate formation.
According to Fig. 5, having an equimolar solution of copper and
ethylenediamine, generates 60% of the copper–ethylenediamine
complex, however, in the case of the ternary systems, once that
an equimolar concentration of copper, ethylenediamine and the
generate 90% of [Cu(en)Cl]1+ and 10% of a mixture of [CuCl]1+
,
[CuCl2], Cu2+, [Cu(en)]2+ and [Cu(en)2]2+; a solution with an equi-
molar mixture of copper, ethylenediamine and sodium bromide
Table 3
Results of the spectrophotometric data refinement of the copper–ethylenediamine–
halides solutions.
will generate 88% of [Cu(en)Br]1+ with 12% of a mixture of [CuBr]1+
,
[CuBr2], Cu2+, [Cu(en)]2+ and [Cu(en)2]2+. Finally we also study the
activity of a copper nitrate solution; and mixtures of: 1 mol of Cu2+
and 4 of sodium chloride, (which generates 81% of [CuCl3]ꢀ, 15% of
[CuCl2] and less of 4% of a mixture of free Cu2+, and [CuCl]+ forma-
tion); and another of 1 mol of copper nitrate and 4 of sodium
r
Copper–ethylenediamine–halide complexes
[Cu(en)Cl]+
log b111
log b111
10.21 0.03
10.07 0.03
0.0247
0.0099
[Cu(en)Br]1+

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 363 (2010) 3461–3468
3467
Fig. 6. Oxidation of 2,6-di-tert-butylphenol, [2,6-di-tert-butylphenol] = 0.00763 M and [Cu(en)] = 0.00003 M at 303 K, monitored at 418 nm.
Fig. 7. Kinetic curves of the catalyzed oxidation of 2,6-di-tert-butylphenol (0.00763 M) at 303 K, in methanol solution, monitored at 418 nm. [Complex] = 0.00003 M.
bromide (which generate about 80% of [CuBr2]ꢀ, less of 4% of free
Cu2+ and 17% of [CuBr]+ formation).
this can be explained if it is considered that the substrate mole-
cules exchange is restricted because being a chelate ligand, ethy-
lenediamine should have a very slow exchange rate.
The kinetic profile of the reactions catalyzed by the different
copper(II) systems are illustrated in Fig. 7. The cupric ion has cat-
alytic activity even without any other ligand present. The activity
of the copper–ethylenediamine complex was higher than the cup-
ric ion, and finally the activity of the copper–ethylenediamine–ha-
lide complex was higher than the [Cu(en)]2+ complex. It is
important to mention that the kinetic behavior of [Cu(en)Cl] and
[Cu(en)Br] are essentially the same.
This is a remarkable result, because it is to be expected that the
redox potential of the complex play an important role in the cou-
pling reaction, and therefore a coordinated chloride or bromide
should shift such potential, being easier the reduction of the bro-
mide complex due that this halide is softer. The result obtained
here, indicates that the redox potential is not so relevant, and prob-
ably the geometry of the intermediate formed is more relevant.
Also, it must be considered that halides coordinate in a similar
way, and probably both induce strong back bonding effect, which
perhaps is responsible of the enhanced catalytic activity. However,
the presence of ethylenediamine is relevant due the fact that cop-
per–halide complexes are less active than the free cupric ion. Final-
ly, the bis-ethylendiamine copper(II) complex shows no activity,
In Fig. 7, a lag phase is observed for all reactions; this phase is
shorter for the copper–ethylendiamine–halide complexes. How-
ever, due to the fact that the lag phase has an elusive nature it is
difficult to suggest a mechanism, especially because this study
does not have enough data. Although when we compare our sys-
tem with similar systems where the substrate used is 2,6-di-
methyl-phenol, and copper coordinated to several diamines as cat-
alyst, a lag phase is observed in all cases. This lag phase decrease
when hydroxide ion is added at equimolar concentration to the
copper complex, and it has been suggested that this lag phase is re-
lated to the formation of the phenolate ion [19,20]. Nevertheless,
further studies are required in order to establish a reliable
mechanism.
4. Conclusions
The use of spectroscopic methods for the determination of the
formation constants, represent a powerful tool for studying metal
complex speciation in non-aqueous solution.

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J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 363 (2010) 3461–3468
Because the solubility of the complexes, the use of ionic
References
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Appendix A. Supplementary
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.06.050.