Spectrophotometric determination of the formation constants of the cupric halogen complexes with 1,3-propanediamine and 1,4-butanediamine in methanol solution and their activity on the oxidative coupling of the 2,6-di-tert-butylphenol

Article abstract of DOI:10.1016/j.ica.2013.02.021

In order to understand the effect that diamine ligands induce in the cupric complexes used as catalyst in the oxidative coupling of 2,6-di-tert- butylphenol, a spectrophotometric study of the speciation of copper(II) with 1,3-propanediamine and 1,4-butanediamine both with Cl^- or Br ^- in methanol solution, was carried out at 303 K. The formation constants and the individual calculated electronic spectra of each species in methanol solution were obtained. The obtained formation constants for the copper(II)-1,3-propanediamine system are: log β₁₁₀ = 6.70 ± 0.06, log β₁₂₀ = 11.59 ± 0.09, while for the ternary-chloro or -bromo complexes are log β₁₁₁ = 10.23 ± 0.06 and log β₁₁₁ = 10.39 ± 0.02, respectively. On the other hand, for the copper(II)-1,4-butanediamine system are log β₁₁₀ = 7.44 ± 0.04, log β₂₁₀ = 11.07 ± 0.04, log β₁₂₀ = 11.86 ± 0.05, log β₂₃₀ = 22.53 ± 0.09, though for the ternary systems copper(II)-1,4-butanediamine-chloro or -bromo, the formation constants cannot be determined. A comparative study of the kinetics for the oxidation reaction of 2,6-di-tert-butylphenol, catalyzed using the different cupric complexes with 1,3-propanediamine or 1,4-butanediamine and -chloro or -bromo, has been carried out. The catalytic activity of each complex on the oxidation of 2,6-di-tert-butylphenol was monitored in methanol solution, following the corresponding quinone formation, at 418 nm (ε = 3.95 × 10⁴ L mol^-1 cm^-1 at 303 K). These kinetic studies indicate that all the copper(II)-1,4-butanediamine complexes, show a remarkable high activity, while the complexes with 1,3-propanediamine are less active although much more active than 1,2-ethylenediamine complexes.

Full text of DOI:10.1016/j.ica.2013.02.021

Inorganica Chimica Acta 400 (2013) 184–190
Contents lists available at SciVerse ScienceDirect
Inorganica Chimica Acta
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Spectrophotometric determination of the formation constants of the
cupric halogen complexes with 1,3-propanediamine and
1,4-butanediamine in methanol solution and their activity
on the oxidative coupling of the 2,6-di-tert-butylphenol
José J.N. Segoviano-Garfias a, Guillermo Mendoza-Díaz b, Rafael Moreno-Esparza a,
⇑
a Facultad de Química (UNAM), Edificio B. Av. Universidad 3000, Coyoacán, México D. F. 04510, Mexico
b División de Ciencias e Ingenierías, Departamento de Ingenierías Química, Electrónica y Biomédica, Universidad de Guanajuato, Lomas del Bosque No. 103, Col. Lomas del Campestre,
León, GTO 37150, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2012
Received in revised form 24 December 2012
Accepted 13 February 2013
Available online 27 February 2013
In order to understand the effect that diamine ligands induce in the cupric complexes used as catalyst in
the oxidative coupling of 2,6-di-tert-butylphenol, a spectrophotometric study of the speciation of
copper(II) with 1,3-propanediamine and 1,4-butanediamine both with Clꢀ or Brꢀ in methanol solution,
was carried out at 303 K. The formation constants and the individual calculated electronic spectra of each
species in methanol solution were obtained. The obtained formation constants for the copper(II)–1,3-pro-
panediamine system are: log b110 = 6.70 0.06, log b120 = 11.59 0.09, while for the ternary-chloro or -
bromo complexes are log b111 = 10.23 0.06 and log b111 = 10.39 0.02, respectively. On the other hand,
for the copper(II)–1,4-butanediamine system are log b110 = 7.44 0.04, log b210 = 11.07 0.04, log
b120 = 11.86 0.05, log b230 = 22.53 0.09, though for the ternary systems copper(II)–1,4-butanedi-
amine–chloro or –bromo, the formation constants cannot be determined.
Keywords:
Copper(II) complexes
Catalysis
Oxidative coupling
Speciation
A comparative study of the kinetics for the oxidation reaction of 2,6-di-tert-butylphenol, catalyzed
using the different cupric complexes with 1,3-propanediamine or 1,4-butanediamine and –chloro or –
bromo, has been carried out. The catalytic activity of each complex on the oxidation of 2,6-di-tert-butyl-
phenol was monitored in methanol solution, following the corresponding quinone formation, at 418 nm
Solution equilibrium
Non-aqueous solvents
(
e
= 3.95 ꢁ 104 L molꢀ1 cmꢀ1 at 303 K). These kinetic studies indicate that all the copper(II)–1,4-butane-
diamine complexes, show a remarkable high activity, while the complexes with 1,3-propanediamine are
less active although much more active than 1,2-ethylenediamine complexes.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
the CuCl/pyridine system. Later, a wide variety of copper com-
plexes, with diamines as ligands: diethylamine, di-n-butylamine,
After the discovery of the oxidative coupling of 2,6-di-methyl-
phenol by Hay [1], it was found that this reaction generate two
products, the poly(2,6-dimethyl-1,4-phenylene oxide) as the main
product and the 3,30,5,50-tetra-methyl-4,40-dibenzoquinone as a
byproduct. The poly(2,6-dimethyl-1,4-phenylene oxide), has a very
important industrial application, when blended with polystyrene
the noryl resin is obtained [2]. This reaction can be carried out in
several solvents such as methanol, benzene, 1,2-di-chloro-benzene
and many others [1,3]. Several paths have been proposed to the
oxidative coupling of 2,6-di-methylphenol, although the mecha-
nism which drives this reaction is not well understood [2,4].
The copper complexes used as catalysts for the oxidative
polymerization of 2,6-di-methylphenol were diverse; starting with
N,N,N0,N0-tetramethylethylenediamine, N,N0-di-isopropyl-ethylen-
diamine, N,N0-di-tert-amylethylenediamine, and N,N0-di-tert-buty-
lethylenediamine; were tested, being this last ligand, the one
used for the synthesis of the patented copper catalyst [2,5].
The solution system containing the copper complex with this
amine and bromide, is presumably the best catalytic system for
this reaction [2,5]. Several studies postulate that the copper
complex can be activated if binds a dioxygen species and create
a peroxodicopper complex [6,7]; from this idea, some research
groups have considered that the oxidative coupling is catalyzed
by a biomimetic binuclear copper species (tyrosinase- or laccase-
like active site) [4,6–8]. However, other groups, consider that a
mononuclear copper complex is capable to form the active species
for this reaction [4,9].
The ligand design of the copper catalyst for this reaction, has
been extensively discussed, nevertheless, after 50 years of
⇑
Corresponding author. Tel.: +52 55 5622 3793; fax: +52 55 5616 2010.
E-mail addresses: moresp@unam.mx, rmorenoe@me.com (R. Moreno-Esparza).
0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2013.02.021

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
185
intensive research, the reasons why the N,N0-di-tert-butylethylen-
2. Experimental
ediamine is so active, remains unknown, and the catalytic copper
active species for this reaction is not yet elucidated [4,9]. More
studies are required in order to understand why N,N0-di-tert-buty-
lethylenediamine presents higher activity.
2.1. Materials
Methanol spectrophotometric and HPLC grade (Tecsiquim,
Mexico) were used as purchased as solvent for the formation con-
stant determination and the kinetic studies, respectively.
Cu(NO3)2ꢂ3H2O, sodium chloride and sodium bromide (J.T. Baker);
1,3-propanediamine, 1,4-butanediamine and 2,6-di-tert-butylphe-
nol (Sigma–Aldrich), were analytical grade, and used without fur-
ther purification. Due to problems with the solubility presented
with several counterions, no ionic strength salt was employed.
Our interest is to understand how the copper complex works in
the oxidative coupling of the 2,6-di-tert-butylphenol system. By
using this system as a model of the polymerization of 2,6-di-meth-
ylphenol, we expect, it will allow us to understand the catalytic
properties of the copper complex, and eventually, obtain a better
knowledge of the role of the copper complex in the oxidative cou-
pling formation of the polymer. Hopefully, this will let us find sev-
eral ways to improve the activity. Finding better ligands with a
lower cost represent a significant task with an immediate indus-
trial application.
2.2. Spectrophotometric equilibrium studies
A review of equilibrium constants for the copper(II) complexes
with 1,3-propanediamine and 1,4-butanediamine under several
conditions in water is presented in Table 1. Apparently, the stabil-
ity of the copper–1,3-propanediamine complexes increase as the
ionic strength growths. Of course temperature plays an important
role as well and as expected an increase in temperature decreases
the stability of the complex. On the other hand, the copper–1,3-
propanediamine protonated complex, has a high stability value,
this can be linked to the rich electron environment of the complex.
At the opposite, the hydroxide complex has a very low stability va-
lue. Also, this last behavior is observed for the copper–1,4-butane-
diamine complexes.
In this work, we report the formation constants in methanol of
the copper complexes with 1,3-propanediamine and 1,4-butanedi-
amine, and their respective halogen ternary complexes, in order to
determinate the predominant species of these systems and
correlate these with their kinetic behavior in the coupling reaction.
In this study, we suggest that the intermediary species of the
oxidative coupling reaction is affected by the chain carbon size in
the diamine. This is the second part of a study initiated to explore
the nature of the copper complexes and their catalytic oxidation
capabilities in the oxidative coupling reaction. In our first study,
we explain how the copper(II)–1,2-ethylenediamine catalytic sys-
tem was affected by the formation of the halide ternary complexes
[10].
An HP-8453 UV–Vis spectroscopy system, equipped with a HP-
89090A Peltier temperature-controlled cell holder was employed
in the spectral studies, with a standard quartz cell with 1 cm opti-
cal length and 3 mL volume at 303 K, each run was made by trip-
licate. The determination of the formation constants was
performed refining the spectrophotometric data with the program
HYPERQUAD [11]. The observed spectral region was from 450 to
950 nm for all the experiments.
2.3. Copper(II)–1,3-propanediamine equilibrium studies
Experiments were performed using two different stock solu-
tions of 1,3-propanediamine (0.0478 and 0.0956 M). Cu(NO3)2ꢂ3H2O
was used to prepare copper stock solutions (0.0318 M and
0.0636 M). The 1,3-propanediamine concentrations were varied
from 0.0005 to 0.0096 M and 0.0001 to 0.0191 M, respectively, in
each experiment. The final copper concentration was set constant
at 0.0032 and 0.0064 M, respectively. A total of 39 spectra were
used for the refinement in HYPERQUAD
.
2.4. Copper(II)–1,3-propanediamine-chloride and copper(II)–1,3-
propanediamine–bromide equilibrium studies
In this case, two stock solutions of 1,3-propanediamine (0.0478
and 0.0956 M), Cu(NO3)2ꢂ3H2O (0.0318 and 0.0636 M) and sodium
chloride or sodium bromide (0.0154 and 0.0308 M) were prepared.
For both experiments, the final copper concentration was set con-
stant at 0.0032 and 0.0064 M and the 1,3-propanediamine at
0.0034 and 0.0067 M, respectively. The halide concentrations were
varied from 0.0002 to 0.0031 M, and 0.0003 to 0.0062 M, respec-
tively. The number of obtained spectra for the system copper(II)–
1,3-propanediamine–chloride and –bromide were 30 and 39,
Table 1
Comparison of the stability constants of the copper(II) complexes with 1,3-propane-
diamine and 1,4-butanediamine under several conditions.
Complex
Log b
Ionic strength/ Solvent
salt
Reference
[Cu(pn)]2+
11.65,
0 °C
0.5 (KNO3)
Water/dioxane
(1:1)
[27]
respectively, and all were used for the refinement in HYPERQUAD
.
10.78,
25 °C
21.77,
0 °C
[Cu(pn)2]2+
2.5. Copper(II)–1,4-butanediamine equilibrium studies
20.06,
25 °C
Experiments were performed using three different stock solu-
tions of 1,4-butanediamine (0.0397, 0.0793 and 0.1500 M).
Cu(NO3)2ꢂ3H2O was used to prepare copper stock solutions
(0.0378, 0.0757 and 0.0753 M). The 1,4-butanediamine concentra-
tions were varied from 0.0004 to 0.0079, 0.0008 to 0.0159 and
0.0015 to 0.0301 M, respectively, in each experiment. The final
copper concentration was set constant at 0.0038, 0.0076 and
0.0075 M, respectively. Although several attempts were made to
obtain the formation constants of the copper(II)–1,4-butanedi-
amine–halogen complexes, however due to the very small absor-
bance changes observed in the spectrum when an halide is
added, it was not possible to obtain a reliable value of these forma-
tion constants with this method and therefore is not possible to as-
sure if the halide is coordinated to the copper center. The total
[Cu(pn)]2+
[Cu(pn)2]2+
[Cu(pn)]2+
[Cu(pn)2]2+
10.58
19.66
0.5 (KNO3)
Water
Water
Water
[26]
[25]
[14]
9.85
17.18
0.15 (NaClO4)
0.1 (NaClO4)
[CuH(pn)]3+
[Cu(pn)]2+
[Cu(pn)2]2+
15.78
9.68
16.79
[Cu(pn)2(OH)]1+ 2.04
[Cu(pn)3]2+
21.66
[CuH(bn)]3+
[Cu(bn)]2+
[Cu(bn)2]2+
15.83
8.62
13.40
0.1 (NaClO4)
Water
[13,15]
[Cu(bn)2(OH)]1+ 0.065

186
J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
number of spectra obtained and employed for the HYPERQUAD refine-
ment was 45.
Considering that only two colored species plus Cu2+ were found,
the formation constants calculation was achieved using this
model:
2.6. Catalytic oxidation studies
2þ
Cu2þ þ pn ꢀ ½CuðpnÞꢃ
log b110
ð1Þ
All kinetic runs were performed at 303 K, in methanol solution,
using pseudo-first-order conditions for the copper complex
(1:255). The formation of 3,30,5,50-tetra-tert-butyl-4,40-dibenzoqui-
2þ
Cu2þ þ 2pn ꢀ ½CuðpnÞ2ꢃ
log b120
ð2Þ
The logarithmic values of the formation constants and the sum-
mary of the experimental parameters are reported in Table 2,
according to the format suggested by the IUPAC Tuck [29].
The reported values for the stability constants in aqueous sys-
tems [14] are usually higher than those reported in pure methanol,
in addition, the formation of the tris-1, 3-propanediamine-cupric
complex is not observed in methanol under the conditions
employed in this study, probably due to the fact that methanol is
none, at 418 nm (
e
= 3.95 ꢁ 104 L molꢀ1 cmꢀ1) was followed
spectrophotometrically. Stock solutions of Cu(NO3)2ꢂ3H2O, 1,3-
propanediamine or 1,4-butanediamine, sodium chloride or sodium
bromide were prepared separately at the same concentration
(0.0225 M); 2,6-di-tert-butylphenol was prepared at 0.0229 M. Ali-
quots of each reactive were used in order to obtain a final concen-
tration of 0.00003 M for the corresponding catalyst and 0.0077 M
for the substrate. At least three runs were averaged for each
experiment. In order to compare these results with those published
earlier [10,12], we have been employed the same concentrations.
Table 2
Summary of experimental parameters for the system: copper(II)–1,3-propanediamine
and copper(II)–1,3-propanediamine–halogen complexes in methanol.
3. Results and discussion
Solution composition
[TL] range from 0.0005 to
0.0096 mol Lꢀ1 and from
0.0001 to 0.0191 mol Lꢀ1
[TM] constant at
In this study, it is essential to avoid the use of ionic strength in
all the equilibrium determinaꢀtions, forꢀif the ionic strength is ad-
justed with either PF6ꢀ, ClO4 or NO3 salts; early precipitation
of the copper complexes occurs. This means of course, that the for-
mation constants obtained here, should not be considered as ther-
modynamic stability constants. In order to compare the data
obtained here with our last study [10] we have kept essentially
the same conditions in these experiments.
0.0032 mol Lꢀ1 and
0.0064 mol Lꢀ1
Ionic strength,
electrolyte
pH range
Not
used
Not
used
Experimental method
Spectrophotometric
titration.
The ligands bn (1,4-butanediamine) and pn (1,3-propanedi-
amine) have been studied coordinated to copper(II), in water under
several conditions and the stability constants reported [13–15].
The synthesis and some spectroscopic and crystallographic studies
of several copper(II)–1,3-propanediamine complexes have been re-
ported as well [16–24]. There are no reports about pn or bn as part
of the copper(II) catalyst in the oxidative coupling of 2,6-di-tert-
butylphenol.
Spectrophotometer
calibrated with holmium
oxide glass filters [36]
30 °C
Cu complexation: 39
solution spectra
Temperature
Total number of data points
Method of calculation
HYPERQUAD [11]
Log b
Species
Equilibrium
r
[Cu(pn)]2+
Cu2+ + pn ꢀ [Cu(pn)]2+
Log
0.0399
b110 = 6.70 0.06
Log
b120 = 11.59 0.09
[Cu(pn)2]2+
Cu2+ + 2 pn ꢀ [Cu(pn)2]2+
3.1. Formation constants of the copper(II)–1,3-propanediamine and
copper(II)–1,3-propanediamine–halogen complexes
Solution composition
[TX] range from 0.0002 to
0.0031 mol Lꢀ1 and from
0.0003 to 0.0062 mol Lꢀ1
[TL] constant at
Copper(II)–1,3-propanediamine complexes have been studied
in aqueous systems, at several ionic strengths. The existence of
the complexes [Cu(pn)]2+, [Cu(pn)2]2+ or [Cu(pn)3]2+ with their
respective formation constants have been reported at ionic
strength of 0.1 (NaClO4) [14]; 0.15 (NaClO4) [25]; 0.5 (KNO3)
[26]; and in a water/dioxane (1:1) system with ionic strength of
0.5 (KNO3) [27]. However, in non-aqueous systems, these com-
plexes have been less studied, and there are no reports concerning
the formation constants of these complexes in methanol. On the
other hand, several studies points out that in some alcoholic solu-
tions, when the ligand to metal molar ratio is higher than 2, the
[Cu2(pn)5]4+ is generated [16,25,24].
0.0032 mol Lꢀ1 and
0.0067 mol Lꢀ1
[TM] constant at
0.0032 mol Lꢀ1 and
0.0064 mol Lꢀ1
Ionic strength,
electrolyte
pH range
Not
used
Not
used
Experimental method
Spectrophotometric
titration.
Spectrophotometer
calibrated with holmium
oxide glass filters [36]
30 °C
Cu-ligand to halide
complexation: 30 solution
spectra for chloride ion and
39 solution spectra for
bromide ion
The electronic spectra of the methanol solutions of copper(II)–
1,3-propanediamine are available as Supplementary material in
Fig. S1. For this system, a maximum at 791 nm starts to appear
at low ligand concentration, and as the 1,3-propanediamine con-
centration increases, a hyperchromic effect was observed with a
hypsochromic change up to 559 nm.
The determination of the formation constants bjkl, correspond-
ing to the successive equilibrium between Cu2+ and 1,3-propanedi-
amine, was made by processing all the obtained spectra of the two
experiments at high and low concentrations simultaneously and
takes into account a large array of data. The observed absorbance
values at different wavelengths were recorded at 303 K.
Temperature
Total number of data points
Method of calculation
HYPERQUAD [11]
[Cu(pn)Cl]+
[Cu(pn)Br]+
Cu2+ + pn + Clꢀ ꢀ [Cu(pn)Cl]1+
Log
0.0274
0.2618
b111 = 10.23 0.06
Cu2+ + pn + Brꢀ ꢀ [Cu(pn)Br]1+ Log
b111 = 10.39 0.02

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
187
better nucleophile than water [30,31], the formation of the supe-
rior complexes is less favored.
When an equimolar solution of cupric nitrate and 1,3-propane-
diamine in methanol is made, the [Cu(pn)]2+ complex has about
80% formation relative to copper, as can be seen below. In order
to study the ternary system of copper(II)–1,3-propanediamine–ha-
lide, we employed the equimolar Cu:pn concentrations, where the
[Cu(pn)]2+ complex has the highest formation percentage, varying
the halide concentration.
The observed electronic spectra of the copper(II)–1,3-propane-
diamine–chloride and –bromide solutions are also available as
Supplementary material in Figs. S2 and S3, respectively. For the
copper(II)–1,3-propanediamine–chloride system, a maximum at
643 nm appears at low chloride ion concentration, as the chloride
ion concentration increases, a hyperchromic/bathochromic effect
is observed in the region near to 665 nm. For the copper(II)–1,3-
propanediamine–bromide system, a maximum at 665 nm appears
at low bromide ion concentration, when bromide ion concentra-
tion increases, the hyperchromic and bathochromic effect also ap-
pears at 665 nm.
The determination of the formation constants bjkl, correspond-
ing to the successive equilibriums between copper, 1,3-propanedi-
amine and each halide were made using the same methodology as
described above.
Fig. 1. Formation curves of the copper(II)–1,3-propanediamine complexes in
methanol. [Cu]2+ = 0.0064 M and 1,3-propanediamine range from 0.0010 to
0.0191 M.
The observed absorbance values at different wavelengths were
recorded at 303 K, considering that one additional colored species
was found plus Cu2+, [Cu(pn)]2+ and [Cu(pn)2]2+ the determination
of the formation constants was achieved using the next model:
Cu2þ þ pn þ Clꢀ ꢀ ½CuðpnÞClꢃþ log b111
ð1Þ
Cu2þ þ pn þ Brꢀ ꢀ ½CuðpnÞBrꢃþ log b111
ð2Þ
The logarithmic values of the formation constants are presented
in Table 2. The calculated electronic spectra for the [Cu(pn)]2+ dis-
play an absorption maximum at 658 nm with
cmꢀ1, while the complex [Cu(pn)Cl]+ exhibit a maximum at
672 nm with
= 91.79 L molꢀ1 cmꢀ1, and the complex [Cu(pn)Br]+
at 677 nm with
= 207.56 L molꢀ1 cmꢀ1. The [Cu(pn)Br]+ has a
e
= 67.29 L molꢀ1
e
e
greater extinction coefficient than the complex [Cu(pn)Cl]+, proba-
bly due to the fact that bromide ion is a softer base than chloride
ion, this is consistent with the expected higher charge transfer from
the bromide ion coordinated to the copper. Finally, [Cu(pn)2]2+
reveal an absorption maximum at 569 nm with e
= 123.05 L molꢀ1
cmꢀ1. The calculated electronic spectra of the copper(II)–1,3-pro-
panediamine complexes are presented in Fig. S4 and the –chloro
and –bromo complexes, in Fig. S5, respectively.
Fig. 2. Formation curves of the copper(II)–1,3-propanediamine–chloro complexes
in methanol, [Cu]2+ = 0.0064 M, [1,3-propanediamine] = 0.0067 M and sodium
chloride range from 0.0003 to 0.0061 M.
3.2. Distribution curves of the copper(II)–1,3-propanediamine and
copper(II)–1,3-propanediamine–halogen complexes
chloro and copper–1–3-propanediamine–bromo complex, respec-
tively. In both systems, when the halide concentration is increased
more than one molar equivalent per copper, both systems become
insoluble, suggesting that when the second halide gets into the
coordination sphere a neutral species is formed. This neutral com-
plex precipitates immediately, the same behavior was observed in
the copper(II)–1,2-ethylenediamine–halide system reported before
[10].
The speciation diagrams of the copper(II)–1,3-propanediamine,
copper(II)–1,3-propanediamine–chloride and the copper(II)–1,3-
propanediamine–bromide systems are shown in Fig. 1, Fig. 2 and
Fig. 3, respectively. A solution with equimolar concentration of
copper(II) and 1,3-propanediamine (at 0.0032 M or 0.0064 M, for
low and high concentration experiments, respectively), roughly
yields 80% of the copper(II)–1,3-propanediamine complex and less
than 20% of the mixture of free copper and the bis complex. Yet
having two molar equivalents of 1,3-propanediamine per copper
generates virtually 100% of the bis complex.
3.3. Formation constants of the copper(II)–1,4-butanediamine
complexes
According to Fig. 2 and Fig. 3, solutions with equimolar concen-
trations of copper(II), 1,3-propanediamine and sodium chloride or
copper(II), 1,3-propanediamine and sodium bromide (at 0.0032 M
or 0.0064 M, for low and high concentrations, respectively), gener-
ates around 71% and 78% of the copper–1–3-propanediamine–
The copper(II)–1,4-butanediamine system has been scarcely
studied, and there are just a few examples reported of cop-
per(II)–1,4-butanediamine complexes in aqueous systems, where
the existence of the complexes [Cu(bn)]2+ and [Cu(bn)2]2+ with

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J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
Table 3
Summary of experimental parameters for the system copper(II)–1,4-butanediamine
complexes in methanol.
Solution composition
[TL] range from 0.0004 to
0.0079, 0.0008 to 0.0159 and
0.0015 to 0.0301 mol Lꢀ1
[TM] constant at 0.0038, 0.0076
and 0.0075 mol Lꢀ1
Ionic strength,
electrolyte
pH range
Not
used
Not
used
Experimental method
Spectrophotometric titration.
Spectrophotometer calibrated
with holmium oxide glass
filters [36]
Temperature
30 °C
Total number of data points
Cu complexation: 45 solution
spectra
Method of calculation
HYPERQUAD [11]
Species
Equilibrium
Log b
r
[Cu(bn)]2+
[Cu(bn)2]2+
Cu2+ + bn ꢀ [Cu(bn)]2+
Cu2+ + 2bn ꢀ [Cu(bn)2]2+
Log b110 = 7.44 0.04 0.0423
Log
b120 = 11.86 0.05
Log
b210 = 11.07 0.04
Fig. 3. Formation curves of the copper(II)–1,3-propanediamine–bromo complexes
in methanol. [Cu]2+ = 0.0064 M, [1,3-propanediamine] = 0.0067 M and sodium
bromide range from 0.0003 to 0.0061 M.
[Cu2 (bn)]4+
[Cu2(bn)3]4+
2Cu2+ + bn ꢀ [Cu2(bn)]4+
2Cu2+ + 3bn ꢀ [Cu2(bn)3]2+ Log
b230 = 22.53 0.09
their respective formation constants of log b110 = 8.62 and log
b120 = 13.40, had been established [13–15]. Even so, in non-aque-
ous solvents, the copper–1,4-butanediamine system has been even
less studied, and to our knowledge, there are no reports concerning
the formation constants of copper(II)–1,4-butanediamine com-
plexes in methanol.
The electronic spectra of copper(II)–1,4-butanediamine solu-
tions are available as Supplementary material in Fig. S6. For this
system, a maximum at 776 nm appears at low ligand concentra-
tion. As the 1,4-butanediamine concentration increases, also a
hyperchromic effect was observed with a hypsochromic change
to 589 nm. The determination of the stepwise formation constants
bjkl, corresponding to the successive equilibriums between Cu2+
and 1,4-butanediamine, takes into account a large array of data
and was made using all spectra at low an high concentrations
simultaneously, the observed absorbance values at different wave-
lengths were recorded at 303 K. Four additional colored species
plus Cu2+ were identified and the evaluation of the formation con-
stants was achieved using the next model:
of copper atoms per molecule. The calculated spectra are shown
in Fig. S7.
3.4. Distribution curves of the copper(II)–1,4-butanediamine
complexes
The distribution diagram of copper(II)–1,4-butanediamine com-
plexes is exhibited in Fig. 4. A solution with a molar equivalent of
copper and a half molar equivalent of 1,4-butanediamine (at
0.0038 and 0.0019 M or 0.0075 and 0.0038 M or 0.0076 and
0.0038 M, respectively, for low, medium and high concentration)
yields about 80% of the [Cu2(bn)]4+ complex and 10% of a mixture
of free copper and [Cu(bn)]2+. An equimolar solution of copper(II)
and 1,4-butanediamine (at 0.0038, 0.0075 or 0.0076 M, for low,
medium and high concentrations, respectively), roughly yields
45% of the copper(II)–1,4-butanediamine complex and about 25%
of [Cu2(bn)]4+ and [Cu2(bn)3]4+each, the remaining 5% of copper
is a mixture of Cu2+ and [Cu(bn)2]2+. Having two molar equivalents
of 1,4-butanediamine per copper (at 0.0038 and 0.0076 M, 0.0075
and 0.01507 M or 0.0076 and 0.0152 M, respectively, for low, med-
ium and high concentration) generates almost 70% of the [Cu2
(bn)3]4+ complex and about 15% of a mixture of [Cu(bn)]2+ and
[Cu(bn)2]2+ each. Yet having three molar equivalents of 1,4-butane-
diamine per copper generates virtually 100% of the bis complex.
The formation of binuclear copper(II)–1,4-butanediamine com-
plexes in methanol has been not reported yet. In the studies of this
system in water [13–15], the binuclear complex was not detected.
This can be explained considering that a solvent with a low dielec-
tric constant, such as methanol, stabilizes the ionic pairs required
for the binuclear complex, while in water, with a high dielectric
constant, the necessary ionic pairs are not formed and this may
prevent that the second copper coordinate to the 1,4-butanedi-
amine ligand. Further studies should be completed in order to cor-
roborate this hypothesis and explain the type of solvent
environment that requires the binuclear complexes to be gener-
ated. From the ligands employed in this work and the previous
study, it is clear that increasing the diamine chain up to increase
the affinity of the ligand for the copper. As the ligand chain in-
creases, the formation constants increase, for the monocomplexes
2þ
Cu2þ þ bn ꢀ ½CuðbnÞꢃ
Cu2þ þ 2bn ꢀ ½CuðbnÞ2ꢃ
2Cu2þ þ bn ꢀ ½Cu2ðbnÞꢃ
log b110
ð1Þ
ð2Þ
ð3Þ
ð4Þ
2þ
4þ
log b120
log b210
4þ
2Cu2þ þ 3bn ꢀ ½Cu2ðbnÞ ꢃ
log b230
3
The logarithmic values of the formation constants and the sum-
mary of the experimental parameters are reported in Table 3. The
reported values for the stability constants are higher in aqueous
systems [13–15] than in pure methanol. This can be explained in
terms of the nucleophilic character of the methanol [28,30,31].
Low values for the formation constants in methanol denote that
the ligand has to compete with stronger bonded solvent molecules.
The calculated electronic spectra for the [Cu(bn)]2+ has an
absorption maximum at 655 nm with
e ,
= 51.84 L molꢀ1 cmꢀ1
while the complex [Cu2(bn)]4+ has a maximum at 722 nm with
e
e
= 91.09 L molꢀ1 cmꢀ1, and the complex [Cu(bn)2]+ at 584 nm with
= 86.31 L molꢀ1 cmꢀ1. Finally, the [Cu2(bn)3]4+ has the absorption
maximum at 595 nm with
e . The [Cu2
= 150.45 L molꢀ1 cmꢀ1
(bn)3]4+ has a greater extinction coefficient because the amount

J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
189
and copper(II)–halogen complexes; a solution with an equimolar
mixture of copper(II), 1,3-propanediamine and sodium bromide
(at 0.00003 M), with 78% of [Cu(pn)Br]+, 15% of [Cu(pn)]2+ and less
than 10% of [Cu(pn)2]2+ and copper(II)–halogen complexes.
We also test an equimolar mixture of copper(II) and 1,4-butane-
diamine (at 0.00003 M), with 45% of the complex [Cu(bn)]2+, about
26% of [Cu2(bn)]4+ and [Cu2(bn)3]4+ each, and about 2% of Cu2+ and
[Cu(bn)2]2+; a mixture of 1 M equivalent of Cu(II) and two molar
equivalent of 1,4-butanediamine (at 0.00003 and 0.00006 M,
respectively), with 67% of [Cu2(bn)3]4+, 16.5% of [Cu(bn)]2+ and
[Cu(bn)2]2+ each; a mixture of 1 M equivalent of 1,4-butanedi-
amine and 2 M equivalent of Cu(II) (at 0.00003 and 0.00006 M,
respectively), with 78% of [Cu2(bn)]4+, 11% of free copper and 11%
of [Cu(bn)]2+; and finally a mixture of 3 M equivalent of 1,4-
butanediamine and 1 M equivalent of Cu(II) (at 0.00009 and
0.00003 M, respectively), with 94% of [Cu(bn)2]2+, and 6% of
[Cu2(bn)3]4+
.
Fig. 5 presents the kinetics runs of each of the mixtures of this
work and those obtained in our previous work. From these exper-
iments, it is clear that increasing the length of the carbon chain of
the amine from 2 to 4 carbons increase the rate of the coupling
reaction. Addition of the halide also increases the rate of the reac-
tion. On the other hand, as the amount of the amine is increased
(this is, as the concentration of the bis complex increase) decrease
the activity. This means that the bis complex is less active that the
corresponding mono complex. This effect is possibly related to the
steric hindrance in these complexes. Then the presence of the sec-
ond ligand, may prevent the substrate molecule approach close to
the copper catalytic site, weakening the interaction with the phe-
nol molecule, decreasing the activity. This effect is not quite clear
in the case of the 1,4-butanediamine probably due to the fact that
the mono complex is less abundant than in the other two amines.
Then, it is clear that the 1,2-ethylenediamine complexes have a
very slow catalytic rate, while the 1,3-propanediamine is four
times faster; the 1,4-butanediamine is 11 times faster. The
tendency observed on the activity growth may be related to an in-
crease in the conformational flexibility of the complex, this also
can be observed in those complexes containing halogen. Appar-
ently, conformational flexibility has been evaluated only in the
case of the bis-1,3-propanediamine cupric complexes [17–19].
However, conformational flexibility is a parameter dependant
on the size of the ligand and is correlated with a steric control in
several catalysts [33–35]. If this is the case then, probably the
Fig. 4. Formation curves of the copper(II)–1,4-butanediamine complexes in meth-
anol. Cu2+ = 0.0076 M and 1,4-butanediamine range from 0.0015 to 0.0301 M.
[Cu(en)]2+ (6.13), [Cu(pn)]2+ (6.7) and [Cu(bn)]2+ (7.44). For the bis-
complexes, same behavior can be observed for [Cu(en)2]2+ (10.54),
[Cu(pn)2]2+ (11.59) and [Cu(bn)2]2+ (11.86).
The observed tendency in the formation constants for the three
copper(II)–diamine complexes, is not what should be expected in
terms of the chelate effect for a ring size. However a possible
explanation can be suggested considering first that methanol is
better nucleophile than water, and on the other hand, bearing in
mind that increasing the ligand chain from two to three and to four
carbons increases the conformational flexibility of the ligands
[17–19].
Then taking into account both phenomena together, this is, that
the rate of solvent exchange (kexch) of Cu in water is at least two or-
ders of magnitude faster than that of methanol (water kexch
ꢄ
1010 sꢀ1; methanol kexch = 107 sꢀ1 [28,31]), and at the same time
conformational flexibility increases from 1,2-ethanodiamine to
1,3-propanediamine to 1,4-butanediamine, is possible to suggest
that both effects contribute to compensate the changes in the
geometry and have in consequence increasing the stability of the
copper(II)–1,4-butanediamine complex. The first having more sta-
ble conformations for the larger ring, and the second allowing the
best conformational arrangement to take place.
1,4-butanediamine ligand may have
a greater potential for
3.5. Catalytic oxidation studies
The catalytic activity of the copper(II)–1,3-propanediamine,
copper(II)–1,3-propanediamine-halogen
and
copper(II)–1,4-
butanediamine systems in the oxidative coupling of 2,6-di-tert-
butylphenol to 3,30,5,50-tetra-tert-butyl-4,40-dibenzoquinone was
measured at 418 nm (a typical run is available as Supplementary
material in Fig. S8). All kinetic runs were stopped at approximately
2.3 of absorbance, values higher than this, the Lambert–Beer law
fail, and the molar absorbance of the 3,30,5,50-tetra-tert-butyl-
4,40-dibenzoquinone is no longer linear. At this absorbance value
the converted quinone has
a
concentration of about 1.75
ꢁ 10ꢀ7
M and the oxygen solubility in methanol is roughly
10.3 ꢁ 10ꢀ3 M [32], showing that the oxygen is not depleted.
Several solutions were tested: a equimolar mixture of copper(II)
and 1,3-propanediamine (at 0.00003 M) generate [Cu(pn)]2+ with
80% abundance, [Cu(pn)2]2+ and free Cu2+, each with 10% abun-
dance; a solution with an equimolar mixture of copper(II), 1,3-pro-
panediamine and sodium chloride (at 0.00003 M), with 71% of
[Cu(pn)Cl]+, 17% of [Cu(pn)]2+ and less than 10% of [Cu(pn)2]2+
Fig. 5. Kinetic curves of the catalyzed oxidation of 2,6-di-tert-butylphe-
nol (0.00763 M) at 303 K, in methanol solution, monitored at 418 nm
[complex] = 0.00003 M.

190
J.J.N. Segoviano-Garfias et al. / Inorganica Chimica Acta 400 (2013) 184–190
processes where significant changes in coordination number and
geometry are desired.
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copper(II)–1,4-butanediamine complex.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2013.02.021.