Synthesis, characterization and catalytic activity toward dye decolorization by manganese (II) mononuclear complexes

Article abstract of DOI:10.1016/j.apcata.2012.12.036

Manganese(II) perchlorate reacts with L1 = N,N,N′-tris(2- methylpyridyl)-N′-hydroxyethyl-ethylenodiamine and L2 = N-(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-N′-hydroxyethyl- ethylenodiamine in ethanol to form the corresponding complexes [MnL1(H ₂O)](ClO₄)₂ and [MnL2]ClO₄. Both complexes crystallize in the monoclinic system and space group P 2 ₁/n. In [MnL1(H₂O)]²⁺ and [MnL2]⁺ the manganese(II) ion is heptacoordinated and hexacoordinated, respectively. FTIR, elemental analysis and conductance measurements were all in accordance with the molecular structures of [MnL1(H₂O)](ClO₄) ₂ and [MnL2]ClO₄. The ability of [MnL1(H ₂O)](ClO₄)₂ and [MnL2]ClO₄ to catalyze the oxidation of the dyes methyl orange and reactive orange 16 was studied in aqueous alkaline solution (carbonate/bicarbonate buffer). Both complexes were active catalysts promoting significant enhancement of dye solution decolorization in comparison to control reactions.

Full text of DOI:10.1016/j.apcata.2012.12.036

Applied Catalysis A: General 454 (2013) 11–20
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Synthesis, characterization and catalytic activity toward dye
decolorization by manganese (II) mononuclear complexes
Camila A. Wegermanna, Paola Strapassona, Stela M.M. Romanowskia,
Adailton Bortoluzzib, Ronny R. Ribeiroa, Fábio S. Nunesa,∗, Sueli M. Drechsela
a Departamento de Química, Universidade Federal do Paraná, Cx. Postal 19081, 81531-990 Curitiba, PR, Brazil
b Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Manganese(II)
ethylenodiamine
perchlorate
and
reacts
with
L1 = N,N,Nꢀ-tris(2-methylpyridyl)-Nꢀ-hydroxyethyl-
L2 = N-(2-hydroxybenzyl)-N,Nꢀ-bis(2-methylpyridyl)-Nꢀ-hydroxyethyl-
Received 5 September 2012
Received in revised form
15 December 2012
Accepted 18 December 2012
Available online xxx
ethylenodiamine in ethanol to form the corresponding complexes [MnL1(H2O)](ClO4)2 and [MnL2]ClO4.
Both complexes crystallize in the monoclinic system and space group P 21/n. In [MnL1(H2O)]2+ and
[MnL2]+ the manganese(II) ion is heptacoordinated and hexacoordinated, respectively. FTIR, elemental
analysis and conductance measurements were all in accordance with the molecular structures of
[MnL1(H2O)](ClO4)2 and [MnL2]ClO4. The ability of [MnL1(H2O)](ClO4)2 and [MnL2]ClO4 to catalyze
the oxidation of the dyes methyl orange and reactive orange 16 was studied in aqueous alkaline
solution (carbonate/bicarbonate buffer). Both complexes were active catalysts promoting significant
enhancement of dye solution decolorization in comparison to control reactions.
Keywords:
Manganese
Hydroxyethyl-ethylenodiamine
Crystal structure
EPR
© 2013 Elsevier B.V. All rights reserved.
Dye decolorization
H2O2
1. Introduction
the oxidation of substrates by H2O2 and molecular oxygen at low
temperatures [1,3].
Hydrogen peroxide and dioxygen can be used as oxidants in
industrial processes. They can be used into chemical oxidation
for remediation of pollutants as the removal of color from tex-
tile dye baths. Unfortunately, the reactivity of these oxidants is
not kinetically favorable although they are potent oxidants in the
thermodynamic point of view [1].
Among the catalytic oxidation processes, bleaching using H2O2
is of great interest [2]. These processes, in which the modification
of chromophores occurs by oxidation, are extremely important in
the waste water treatment, paper industry, textile and detergent
manufacturing.
Laundry bleaching processes are developed to destroy
unwanted stains in a cleaning effect, but also to destroy dyestuff
in solution in order to prevent migration to other clothes in the
same washing solution [3]. The percarbonate or perborate laundry
detergents, even with activator systems which release peroxy
acids after perhydrolysis, show reasonable bleach results only
above 40 ◦C. This difficulty drives to the development of soluble
complexes of transition metals, which are capable of catalyzing
Manganese complexes are widely used as oxidation catalysts
based on their rich redox chemistry. However, the use of com-
plexes of transition metals in industrial processes is limited by its
low catalytic action under the conditions used for the bleaching:
aqueous medium, high pH and high temperature [3]. Among the
complexes already described are the manganese complexes with
the ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (tmtacn) which
have shown catalytic activity in bleaching processes [4], but also in
a number of other processes like oxidation of phenols [5], epoxi-
dation [6–8], dihydroxilation of alkenes [9], activation of CH bond
[10] and oxidation of dyes (azo compounds) [11]. To this family of
catalysts it is estimated that the active species involve mono- and
binuclear complexes in high oxidation states [1,4,5,9].
In the present work the synthesis and characterization of two
mononuclear manganese complexes containing the N,O-donor L1 =
N,N,Nꢀ-tris(2-methylpyridyl)-Nꢀ-hydroxyethyl-ethylenediamine
and
L2 = N-(2-hydroxibenzyl)-N,Nꢀ-bis(2-methylpyridyl)-Nꢀ-
hydroxyethyl-ethylenediamine and the study of their catalytic
performance in bleaching of azo dyes (Methyl Orange MO or
Reactive Orange 16 RO) in alkaline and low temperature (25 ◦C)
are presented.
The ligands structures are based on N-hydroxyethyl-
ethylenodiamine group with pyridine and/or phenolic arms.
∗
Corresponding author. Tel.: +55 4133 613 180.
E-mail addresses: fsnunes@ufpr.br, fsnunes@pq.cnpq.br (F.S. Nunes).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.12.036

12
C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
The hydroxyethyl group may promote a more labile site for
H2O2 activation in the metal coordination sphere since in copper
and vanadium complexes with the same ligands the hydroxyl
groups remains uncoordinated [12,13]. Ligand L1 contains three
nitrogen donor atoms from pyridine residues and ligand L2 can
coordinate through two pyridines and one phenolic group. The
coordination ability of the ligands L1 and L2 was established thor-
ough monocrystal X ray crystallography, and we have considered
their relative potential to stabilize higher oxidation states of the
manganese ion, usually observed as intermediates in oxidation
reactions [1,4,5,9]. The performances of the catalysts herein stud-
ied were discussed based on the spectroscopic analysis (by EPR)
of the reaction intermediates, the determined values of reaction
rates and the percentage of decolorization of two azo dyes under
mild conditions of temperature.
2.2.3. Diperchlorate of aqua(N,N,Nꢀ-tris(2-methylpyridyl)-
Nꢀ-hydroxiethyl-ethylenediamine manganese(II),
[MnL1(H2O)](ClO4)2
Mn(ClO4)2·6H2O (5.42 g, 1.50 mmol), NaClO4·H2O (0.51 g,
4.17 mmol) and L1 (6.08 g, 1.61 mmol) were dissolved in 40 mL of
anhydrous ethanol and kept under reflux at 40 ◦C for 1 h. After
cooling to room temperature and slow addition of diethyl ether,
a light yellow powder precipitated. It was then washed with cold
water and dried under vacuum. Suitable crystals for X ray diffrac-
tion analysis were obtained by slow diffusion of diethyl ether to an
acetonitrile solution of the complex. The yield was 4.68 g (48.05%).
Elemental analysis for C22H29N5Cl2O10Mn: (649.34 g mol−1) calcd.
C, 40.69; H, 4.50; N, 10.79%; found C, 40.70; H, 4.50; N, 10.80%. I.R.
(cm−1): 3440s ␯(OH) H2O and alcohol; 1604s, 1573m, 1437s and
1482 m (pyridine ring, ␯C N, ␯C C); 1311m (␦O H); 1010s (alco-
hol, ␦C O); 777m and 763 m (␦C H); 479w and 410w (␯Mn N);
1100vs and 625s (␯Cl O).
2. Experimental
2.2.4. Perchlorate of N-(2-hydroxibenzyl)-N,Nꢀ-bis(2-
2.1. Materials
methylpyridyl)-Nꢀ-hydroxiethyl-ethyleno diamine manganese(II),
[MnL2]ClO4
The chemicals used in this work were purchased from
Sigma–Aldrich and used as supplied.
Caution: perchlorate salts are potentially explosive in the pres-
ence of organic substances. Avoid heating the solid samples.
Triethylamine (3.5 mL, 25 mmol) and 5.94 g (15.1 mmol) of
L2 were dissolved in anhydrous ethanol at 65 ◦C for 5 min.
Then, Mn(ClO4)2·6H2O (5.43 g, 15.0 mmol) and NaClO4·H2O (0.42 g,
3.43 mmol) were added and the solution was refluxed for 1 h. A
brown solid was separated by filtration, washed with cold water,
ethanol, ether and dried under vacuum. The yield was 4.3 g (52.6%).
Suitable crystals for X ray diffraction analysis were obtained as
described for [MnL2]ClO4. Elemental analysis for C23H27N4ClO6Mn:
(544.87 g mol−1) calcd. C, 50.51; H, 5.01; N, 10.68%; found C, 50.42;
H, 5.33; N, 10.23%. I.R. (cm−1): 3409s ␯(OH); 1603s, 1484s and
1439s; (pyridine ring ␯C N) + (pyridine and phenol ␯C C); 1265s
␯(C O); 1097vs (␯Cl O); 755s (␦C H); 617s (␯Cl O); 479w and
400w (␯Mn N).
2.2. Synthesis of ligands and complexes
2.2.1. N,N,Nꢀ-tris(2-methylpyridyl)-
Nꢀ-hydroxyethyl-ethylenodiamine, (L1)
The preparation of L1 was carried out as published [13,14].
Ten milliliters (0.10 mol) of 2-formylpyridine were mixed with
10.39 g (0.10 mol) of 2-(2-aminoethylamino)ethanol in 50 mL of
methanol and kept under reflux at 40 ◦C for 22 h. The system was
cooled in ice bath and 4.68 g (0.12 mol) of NaBH4 were added
under stirring in a period of 4 h. After concentration of the solu-
tion under vacuum, a dark yellow oil was produced. This oil was
mixed to 32.74 g (0.20 mol) of 2-picolyl chloride dissolved in 50 mL
of sodium hydroxide (4 mol L−1) at 65 ◦C, followed by the step-
wise addition of 25 mL of the sodium hydroxide solution for 1 h
until pH 10. The resulting dark red solution was then cooled
in an ice bath and extracted with chloroform (18× 50 mL). The
extracts were concentrated to produce a brown oil, which was used
as obtained to prepare the manganese complex as described in
Section 2.2.3.
2.3. Physical measurements
X-ray diffraction analyses of complexes [MnL1(H2O)](ClO4)2
and [MnL2]ClO4 were carried out with an Enraf-Nonius CAD4
diffractometer with graphite-monochromated Mo K␣ radiation
˚
(ꢀ = 0.71069 A), at room temperature. Cell parameters were deter-
mined from 25 carefully centered reflections using a standard
procedure [15]. Intensities were collected using the ω − 2Â scan
technique. All data were corrected for Lp effects [16] and for
absorption using semi-empirical psi-scan procedure [17]. The
structures were solved by direct methods and refined by full-matrix
least-squares methods using the SIR97 [18] and SHELXL97 [19]
programs, respectively. H atoms attached to C atoms were placed
at their idealized positions using standard geometric criteria. All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Complex [MnL1(H2O)](ClO4)2: H atoms of the water
molecule were found from Fourier difference map and treated with
riding model. A disordered system was found for three oxygen
atoms of one perchlorate counterion, where these atoms occupy
two alternative positions and their refined site occupancies were
0.61(2) and 0.39(2). Complex [MnL2]ClO4: the asymmetric unit
consists of two independent molecules of MnII complex and two
perchlorate ions. Due to a highly disordered solvate, squeeze cor-
rection was applied to the structure factors with PLATON software
[20]. Further relevant crystallographic data for the complexes are
summarized in Table 1 and selected bond lengths (Å) and angles (◦)
for complexes in Table 2.
2.2.2. N-(2-hydroxibenzyl)-N,Nꢀ-bis(2-methylpyridyl)-
Nꢀ-hydroxiethyl-ethylenodiamine, (L2)
Thirteen milliliters (0.12 mol) of salicylaldehyde were mixed
with 12.95 g (0.12 mol) of 2-(2-aminoethylamino)ethanol in 50 mL
of methanol and kept under reflux at 40 ◦C for 21 h. The sys-
tem was cooled in ice bath and 4.68 g (0.12 mol) of NaBH4 were
added under stirring in a period of 4 h. After concentration of
the solution under vacuum, an orange oil was produced and to it
was added 60 mL of ethanol and 30 mL of hydrochloric acid (37%)
until pH 2. A white solid was isolated and recrystallized from an
ethanol/chloroform (1:1, v/v) solution. This solid (19.4 g, 0.10 mol)
was mixed to a solution of 2-picolyl chloride 16.4 g (0.10 mol) dis-
solved in 25 mL of sodium hydroxide (4 mol L−1), followed by the
stepwise addition of the sodium hydroxide solution until pH 10.
The resulting dark red solution was then cooled in an ice bath,
extracted in chloroform (13× 70 mL) and dried with CaCl2. The
extracts were concentrated to produce a dark brown oil, which was
used as obtained to prepare the manganese complex as described in
Section 2.2.4.
X-band electron paramagnetic resonance (EPR) spectra were
recorded on a Bruker EMX micro X spectrometer from solid sam-
ples and from MeCN solutions at 77 K. The samples were placed
in standard 4 mm o.d. EPR quartz tubes and the low temperature

C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
13
Table 1
Crystal data and structure refinement for complexes [MnL1(H2O)](ClO4)2 and [MnL2]ClO4.
[MnL1(H2O)](ClO4)2
[MnL2]ClO4
Empirical formula
Formula weight
Temperature
C22H29Cl2MnN5O10
649.34
C23H26ClMnN4O6
544.87
293(2) K
293(2) K
˚
˚
Wavelength
0.71069 A
0.71069 A
Crystal system
Space group
Unit cell dimensions
Monoclinic
P21/n
a = 9.9949(9) A
Monoclinic
P21/n
a = 14.4554(14) A
˚
˚
˚
˚
b = 16.621(4) A
b = 13.3053(10) A
˚
˚
c = 20.589(5) A
c = 22.0656(14) A
˛ = 90◦
˛ = 90◦
ˇ = 99.367(13)◦
ˇ = 102.770(5)◦
ꢁ = 90◦
ꢁ = 90◦
3
3
˚
˚
Volume
2701.5(7) A
5170.4(14) A
Z
4
8
Density (calculated)
Absorption coefficient
F(0 0 0)
1.597 Mg/m3
1.400 Mg/m3
0.751 mm−1
1340
0.658 mm−1
2256
Crystal size
Theta range for data collection
Index ranges
0.50 mm × 0.40 mm × 0.36 mm
0.50 mm × 0.40 mm × 0.26 mm
1.83–25.07◦
1.54–25.08◦
−11 ≤ h ≤ 11,
−16 ≤ h ≤ 17,
−19 ≤ k ≤ 0,
−15 ≤ k ≤ 0,
−24 ≤ l ≤ 0
−26 ≤ l ≤ 0
Reflections collected
4919
9431
Independent reflections
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2sigma(I)]
R indices (all data)
4782 (Rint = 0.0154)
27.05◦, 100.0%
Psi-scan
0.7738 and 0.7053
Full-matrix least-squares on F2
4782/58/380
9182 (Rint = 0.0384)
25.08◦, 99.9%
Psi-scan
0.848 and 0.734
Full-matrix least-squares on F2
9182/0/631
1.027
0.966
R1 = 0.0503, wR2 = 0.1353
R1 = 0.0829, wR2 = 0.1515
0.528 and −0.595e A
R1 = 0.0602, wR2 = 0.1512
R1 = 0.1494, wR2 = 0.1693
0.494 and −0.416e A
−3
−3
˚
˚
Largest diff. peak and hole
Molar conductance of 10−3 mol L−1 solutions in methanol or
methanol/H2O mixtures at room temperature were measured with
a Digimed model DM-32 apparatus equipped with a platinum elec-
spectrum was obtained using a quartz finger Dewar. Spectra simu-
lations were carried out using EasySpin software package [21].
Infrared spectra were obtained with a FTS3500GX Bio-Rad
Excalibur series spectrophotometer in the region 4000–400 cm−1
in KBr pellets.
trode in a glass cell with a constant of 1 cm−1
.
Microanalyses were recorded with a Perkin–Elmer CHN 2400
equipment.
The spectrophotometric measurements were performed in a
Varian Cary-100 UV–Vis spectrophotometer.
Table 2
Selected bond lengths (Å) and angles (◦) for complexes [MnL1(H2O)](ClO4)2, and
[MnL2]ClO4.
2.4. Dye decolorization studies
[MnL1(H2O)](ClO4)2
[MnL2]ClO4
In a typical kinetic experiment the dye solution (Methy−l Orange
MO or Reactive Orange 16 RO) at 0.1 mol L−1 HCO3−/CO3 buffer,
pH = 9.7 was mixed with 1 mL acetone solution of the complex
([MnL1(H2O)](ClO4)2 or [MnL2]ClO4) in a 10 mL volumetric vessel
equipped with a magnetic stirrer. The mixture temperature was
kept at 25 ◦C. The reaction was started by the addition of H2O2
for each reaction condition (see Table 5). Kinetic measurements
were carried out under pseudo-first order conditions with respect
to H2O2. The kinetics of the oxidation reactions were monitored by
the decay of the bands with maximum wavelengths of 465 nm for
MO and 494 nm for RO. Values of kobs were obtained on the basis
of at least two repeated experiments and calculated from linear
plots of ln A vs. time. The rate constants were calculated using Ori-
gin version 6.1®. The dye decolorization percentage was calculated
by (Ai − At)/Ai × 100, where Ai is the initial absorbance and At the
absorbance at 1 h.
Mn N1
Mn N4
Mn N22
Mn N12
Mn N32
Mn O42
Mn O1W
N1 Mn O42
N4 Mn O42
N1 Mn O1W
N4 Mn O1W
N12 Mn N32
N1 Mn N4
N1 Mn N12
N12 Mn O42
2.346(3)
2.379(3)
2.412(3)
2.273(3)
2.280(3)
2.420(3)
2.194(3)
128.50(12)
67.76(11)
145.17(11)
138.69(11)
173.58(12)
75.75(12)
72.97(11)
80.09(11)
Mn N1
Mn N4
2.272(4)
2.312(4)
2.251(4)
2.334(4)
2.309(4)
2.270(4)
2.283(4)
2.251(4)
2.055(3)
2.176(3)
2.106(3)
2.149(3)
161.01(15)
158.07(13)
149.48(14)
Mn N51
Mn N54
Mn N22
Mn N32
Mn N72
Mn N82
Mn O10
Mn O40
Mn O60
Mn O90
N4 Mn 010
N54 Mn O60
N1 Mn O40
O1W Mn O42
76.07(11)
N51 Mn O90
168.98(15)
174.61(14)
80.24(14)
143.65(14)
N22 Mn N32
N72 Mn N82
N1 Mn N4
N54 Mn N51
O40 Mn O10
O60 Mn O90
O10 Mn N22
O60 Mn N82
N1 Mn N22
N51 Mn N72
78.64(14)
3. Results and discussion
116.62(12)
118.57(12)
98.60(14)
87.96(14)
74.90(14)
3.1. Synthesis and characterizations of the catalysts
The ligands L1 and L2 (Scheme 1) were prepared by
three step procedure. In the first step, 2-formylpirydine (to
a
75.04(13)

14
C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
OH
N12 Mn1 O1W: 90.79(11)◦, N12 Mn1 O42: 80.09(11)◦ and
N12 Mn1 N1: 72.97(11)◦.
OH
N
N
N
The bond distances between the manganese ion and
the nitrogen atoms from the ethylenediamine fragment are
N
N
OH
N
N
N
˚
˚
Mn1 N1: 2.346(3) A and Mn1 N4: 2.379(3) A, close to the values
L1
described by Anxolabehere-Mallart for cis-[(L4)MnIICl2] (L4 = N,Nꢀ-
N
L2
ꢀ
˚
dimethyl-N,N -bis-(2-pyridilmethyl)ethane-1,2-diamine): 2.366 A
and
[(L5)MnIICl(OH2)](ClO4) (L5 = N,Nꢀ-dimethyl-N,Nꢀ-bis(2-
pyridilmethyl)propane-1,3-diamine): 2.326 A [23]. Both L4 and L5
are tetradentate ligands. The values reported by Anxolabehere-
Scheme 1. Ligands L1 and L2.
˚
˚
prepare L1) or salicylaldehyde (to prepare L2) react with 2-(2-
aminoethylamino)ethanol in methanol at 40 ◦C, in a typical Schiff
base reaction to produce the corresponding imines. This is followed
by reduction at low temperature with sodium borohydride to give
the amines. Finally, the insertion of 2-methylpyridine (L1 and L2)
groups completes the preparation. Scheme 2 summarizes the syn-
thetic route to prepare L1 and L2.
Manganese(II) complexes of L1 and L2 were prepared by the
mixture with equimolar amounts of Mn(ClO4)2·6H2O in ethyl alco-
hol to yield 48% of [MnL1(H2O)](ClO4)2 and 53% of [MnL2]ClO4.
Preparation of [MnL2]ClO4 required triethylamine to deprotonate
the phenol, as evidenced by the absence of the out of-plane angu-
lar deformation mode at 1366 cm−1 upon coordination to the
manganese. The positive charges of the complexes were coun-
terbalanced by the perchlorate anion as indicated by the intense
stretching mode at 1100 cm−1. The compounds are fairly stable.
Complexes with ligands L1 and L2 afforded suitable single crystals
for structure determinations.
Mallart et al. for the Mn N(pyridine) distances (2.278 and 2.311 A)
are also similar to the values found for [MnL1(H2O)](ClO4)2,
˚
˚
Mn1 N12 2.273(3) A and Mn1 N32 2.280(3) A. Note that
these values for Mn N(pyridine) axial distances are signifi-
cantly shorter than the equatorial Mn N22(pyridine) distance
˚
observed: 2.412(3) A, most likely caused by steric effects. A
similar argument was reported by Chen et al. on the crystal struc-
ture of the heptacoordinated complex [Mn(mim)3htren]2+
,
where (mim)3htren is the tridentate ligand tris{2-[2-(1-
methyl)imidazole]methylaminoethyl} amine. In this case, the
˚
˚
Mn N(ethylenediamine) bond distances are 2.369 A and 2.51 A
[24].
˚
The shortest bond distance observed for Mn O1W: 2.194(3) A,
is probably
a result of the small size of the ligand water.
Naskar et al. observed comparable values: 2.1361(15) and
˚
2.1516(15) A for [Mn(dapS2)(H2O)2]·DMF (dapS2 = pentadentate
bis(phenylhydrazone) ligand) [25]. The alcohol is trans to the pyri-
dine in equatorial position and shows a bond distance Mn O42:
˚
In order to help elucidate the composition of the com-
plexes, molar conductance measurements were carried out from
acetonitrile solutions at 298 K. Complexes [MnL1(H2O)](ClO4)2
and [MnL2]ClO4 showed conductivities values of 301 and
143 S cm2 mol−1, respectively at 298 K, as expected for a 1:2 and
1:1 electrolytes [22].
2.410(3) A, that is as big as the Mn N22(pyridine) distance:
˚
2.412(3) A. This result is consistent with the coordination of the
alcohol in the protonated form.
The molecular structure of [MnL2]ClO4 is represented in Fig. 1
(bottom). It crystallizes in the same crystal system and space group
as does [MnL1(H2O)](ClO4)2, but in this case the manganese(II) is
hexacoordinated in a distorted octahedral environment. The metal
ion is coordinated to four nitrogen and two oxygen atoms from L2.
The unit cell contains two independent molecules of [MnL2]ClO4
and then the average of bond distances and angles are discussed.
It is interesting to note that the alcohol group is protonated, but
3.2. Structural descriptions
The complex [MnL1(H2O)](ClO4)2 crystallizes in the monoclinic
system and space group P21/n. A projection of the molecular struc-
ture with an atom-labeling scheme is shown in Fig. 1 (top). The
structure is of a mononuclear complex with the metal ion bound to
five nitrogen atoms and one oxygen atom from L1. In addition, Mn2+
is bound to a water molecule and occupies the center of a distorted
pentagonal bipyramid, with N12 and N32 at the axial positions as
confirmed by the angle N12 Mn1 N32 at 173.58(12)◦. The angles
in the equatorial plane are in the range 67–84◦ as observed for
N4 Mn1 O42: 67.76(11)◦ and for O1W Mn1 N22: 83.70(11)◦,
for example. Other main angles are N12 Mn1 N22: 99.23(12)◦,
˚
with a Mn O bond distance of 2.163 A, much shorter than the value
˚
observed for [MnL1(H2O)](ClO4)2 complex: 2.420(3) A, an outcome
clearly determined by the heptacoordination of L1 as discussed
above.
For the [MnL2]ClO4 complex, the bond distance between the
manganese ion and the nitrogen atom from the ethylenedi-
˚
amine fragment is Mn N(en): 2.292 A. It is, therefore, shorter
˚
than the mean value observed for [MnL1(H2O)](ClO4)2: 2.363 A.
Apparently this is consistent with the heptacoordination of
2
OH
Cl
OH
N
N
MeOH
N
N
N
H2
N
H
N
N
O
OH
+
65 oC,NaOH
NaBH4
N
N
N
H
N
H
H
L1
OH
OH
OH
O
2
OH
Cl
OH
N
N
MeOH
NaBH4
N
H2
N
H
N
N
+
OH
65 oC,NaOH
N
N
H
H
H
N
L2
Scheme 2. Reaction steps for the preparation of the ligands L1 and L2.

C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
15
Fig. 1. View of the structure of the cations of [MnL1(H2O)](ClO4)2 (top) and of [MnL2]ClO4 (bottom) with the atom labeling. Displacement ellipsoids were drawn at the 50%
probability level.
Mn2+ species with N6O coordination sphere (D = 0.127 cm−1) [26].
For hexacoordinate Mn2+ complexes with N/O ligand atoms, D is
found in the range (0.001–0.175) cm−1 [26]. The determination of
the magnetic species for the MeCN solution of [MnL1(H2O)](ClO4)2
was not straightforward. The expected spin Hamiltonian for a
mononuclear MnII high-spin species could not generate a satis-
factory simulated spectra, nor a MnIV (S = 3/2), and a good fitting
was obtained for a binuclear MnIIMnII magnetic model simulation
(Fig. 2(B)). For this purpose, a model system comprising two spins
S = 5/2 coupled by an isotropic Heisenberg exchange interaction
was employed. The exchange coupling generates five magnetically
relevant spin states, S = 1, 2, . . ., 5 (Table 4). Each of these spin states
is described by an effective spin Hamiltonian (Eq. (1)), comprised
of ZFS interaction and isotropic Zeeman interaction:
L1 causing a more distorted geometry and weaker bonds. On
the other hand, Mn-pyridine(axial) bond distances are prac-
tically the same for both complexes with means values of
˚
˚
2.277 and 2.278 A. Manganese–oxygen (Mn OC6H4: 2.081 A and
˚
Mn O(H) CH2 : 2.163 A) bond distances are significantly shorter
than the Mn N(en) and Mn N(py) in accordance with the coordi-
nation of phenolato and alcoholato groups.
The manganese experiences a strong distortion from the octahe-
dral geometry as indicated by the angles (en)N Mn N(en): 79.44◦,
H4C6O Mn OCH2: 117.60◦ and (en)N Mn OC6H4: 159.54◦ in the
equatorial plane and pyN Mn Npy: 171.61◦ related to the axial
ligands. Angles containing donor atoms from equatorial and axial
positions, such as, N1 Mn N22: 74.90(14)◦ are also consistent
with an irregular geometry environment around the metal ion.
H(S) = S · D(S) · S + ꢂBg(S)BmS
(1)
3.3. EPR characterization of the complexes
This Hamiltonian generates five spectra, each described by
the three parameters D(S), E(S) and g(S). The total spectrum
X-band EPR investigation of [MnL1(H2O)](ClO4)2 and
[MnL2]ClO4 from both powder and MeCN solutions were car-
ried out and the EPR spectra are shown in Figs. 2 and 3 together
with the respective spectral simulations.
Simulation of the EPR powder spectrum of [MnL1(H2O)](ClO4)2,
in accordance to a Mn2+ high-spin species, S = 5/2, demonstrates a
large axial Zero Field Splitting (ZFS) interaction (Table 3) along with
an expected non-negligible rhombicity characterized by the inter-
mediate value of the ratio between the rhombic and the axial ZFS
parameters, E/D (Fig. 2(A)). The obtained value of D = 0.137 cm−1
is very close to the one observed for a similar heptacoordinated
Table 3
Spin Hamiltonian parameters for the X-band EPR spectral simulations of
[MnL1(H2O)](ClO4)2 and [MnL2]ClO4, (MnII high-spin species) from powder and
MeCN solutions at 77 K.
Sample
giso
D [cm−1
]
E/D
[MnL1(H2O)](ClO4)2 powder
[MnL2]ClO4 powder
[MnL2]ClO4 solution
1.997
2.022
2.038
0.137
0.046
0.053
0.093
0.204
0.135

16
C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
B
A
0
200
400
600
800
1000
0
200
400
600
800
1000
Magnetic Field [mT]
Magnetic Field [mT]
Fig. 2. 77 K X-band EPR spectra of [MnL1(H2O)](ClO4)2, powder (A) and MeCN solution (B). Thin line represents the simulation ((A) Table 3; (B) Table 4).
A
B
0
200
400
600
800
1000
0
200
400
600
800
1000
Magnetic Field [mT]
Magnetic Field [mT]
Fig. 3. 77 K X-band EPR spectra of [MnL2]ClO4 powder (A) and MeCN solution (B). Thin line represents the simulation (Table 3).
corresponds to the sum of these individual spectra, considering the
Boltzmann coefficients at thermal equilibrium (Eq. (2)):
powder spectrum, in opposition to an expected elimination of the
ZFS strains upon solution, the E/D ratio was found to be smaller in
the solution, which confirms a diminishing of the complex rhom-
bicity in solution.
exp[−JS(S + 1)/kT]
ꢀ
n(S, J, T) =
(2)
(2Si + 1) exp[−JSi(Si + 1)/kT]
From the fitting of the experimental data, one can extract the
value of the isotropic Heisenberg exchange coupling, J. The deter-
mined value of J is around −2.14 cm−1, which characterizes an
antiferromagnetic coupling, with the spin state S = 5 being the
highest energy state. The axial ZFS parameter of the S = 2 spin com-
ponent, D (S = 2), is known to be linearly correlated to the MnIIMnII
distance, r [27], and the value obtained from the simulation points
3.4. Dyes decolorization and EPR study of the reaction media
The oxidation of dyes methyl orange (MO) and reactive orange
16 (RO) (Scheme 3) were performed in the presence of H2O2 and
catalytic amounts of the manganese complexes at pH 9.7 in a car-
bonate/bicarbonate buffer.
Typical spectral changes observed during reaction with MO and
RO as substrates are presented in Fig. 4 and the decrease of the
absorbance with time reflects de oxidative degradation process of
the substrate. For MO the decreasing of the absorbance at 465 nm
was accompanied by an absorbance increasing at 330 nm indicat-
ing the oxidation of the amino group Fig. 4(A). For RO, absorbance
decreasing was observed over all the wavelength range, typical of
a complete decomposition of the chromophore group Fig. 4(B). The
reaction rates were determined using the absorbance decreasing
at 494 nm. Good linear plots of ln A vs. time were obtained under
the pseudo-first order conditions, with the H2O2 concentrations
in the range 1.5 × 10−3 to 3.0 × 10−2 mol L−1 (molar proportion
1:20:1000–1:20:20,000 catalyst:substrate:oxidant).
The observed constants (kobs) using the linearized equation
were determined for each reaction condition and the obtained
results are presented in Table 5 and Fig. 5. The rates of uncat-
alyzed reactions are practically ten times smaller than those
observed for catalyzed reactions. The best result was observed
for the [MnL1(H2O](ClO4)2 complex for both dyes at the con-
dition 1:20:10,000 catalyst:substrate:oxidant and the following
˚
to r = 3.5 A, which is in agreement to binuclear bis(␮2-alkoxide) or
bis(␮2-phenoxide) structures indicating a possible dimerization by
the alcohol group [28–30].
The recorded EPR spectrum of [MnL2]ClO4 from frozen solution
is very similar to the one recorded from solid sample, which sug-
gests the complex maintain its structure after dissolution (Fig. 3).
This behavior is not observed for the [MnL1(H2O)](ClO4)2 complex
as discussed above. Although the D value determined from MeCN
frozen solution spectrum was found to be higher than that for the
Table 4
Spin Hamiltonian parameters for a MnIIMnII dimer species X-band EPR spectral
simulations of [MnL1(H2O)](ClO4)2 from MeCN solution at 77 K.
Spin component
g
D [cm−1
]
E/D
Linewidth [mT]
S = 1
S = 2
S = 3
S = 4
S = 5
2.055
1.860
2.031
1.970
2.298
0.055
0.073
0.091
0.086
0.118
>0.001
>0.001
0.016
0.178
0.088
53.7
76.9
74.4
76.4
79.5

C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
17
Scheme 3. Structures of dyes methyl orange (MO), reactive orange 16 (RO) and the amine oxide of MO.
observed rate constants: kobs = 4.3 × 10−2 min−1 (for MO) and
kobs = 8.6 × 10−2 min−1 (for RO) It is important to emphasize
that the Mn(ClO4)2·6H2O salt also showed similar behavior with
kobs = 3.3 × 10−2 min−1 and 5.2 × 10−2 min−1 for MO an RO, respec-
tively.
The results in Table 5 show that the reaction is first order with
respect to H2O2. Hydrogen peroxide-based bleaching processes
have been frequently associated with radicals, especially •OH. The
well described model of a radical mechanism of hydrogen bleaching
involves the transfer of an electron from a donor to H2O2 inducing a
radical chain reaction. Metal cations or metal complexes are known
candidates for the role of electron donors forming an oxidized form
of the metal and radicals (Reaction (R1)) [31,32].
was observed. This outcome suggests that the formation of the
MnIV O species occurs through the reaction with percarbonate
(HCO4−) (Eqs. (3) and (4)), a behavior also reported by Ember, van
Eldik et al. for the oxidation of several azo dyes with H2O2 [34].
HCO3− + H2O2 → HCO4− + H2O
[MnIIL1]2+ + HCO4− → [L1MnIV O]2+ + HCO3
(3)
(4)
−
Even for the well known Fenton reaction, a process described
over a century ago, the nature of the reactive oxidizing species able
to degrade the pollutants is still under discussion and it has been
a subject of controversy in the literature. Two reaction pathways
have been considered: a radical pathway, which considers an •OH
radical production and a non-radical pathway considering a ferryl
ion production [35].
[MnIIL1]2+ + H2O2 → [MnIIIL1]3+ + HO− + •OH
(R1)
In order to investigate the possible formation of higher-
oxidized manganese active species during the bleaching reactions,
the reaction medium containing [MnL1(H2O)](ClO4)2 was moni-
tored using EPR spectroscopy at 77 K. Fig. 5(A) shows an X-band
EPR spectrum of a sample collected from an undergoing oxida-
tion reaction containing [MnL1(H2O)](ClO4)2 (10 min reaction).
The model that best fits the spectrum corresponds to a dom-
inant MnIV species, as demonstrated by spectral simulation
However, many manganese(II) and iron(II) complexes react
with H2O2 in a two electron transfer process also producing MIV
O
species (Reaction (R2)) which can transfer oxygen to the substrate
[33].
[MnIIL1]2+ + H2O2 → [L1MnIV O]2+ + H2O
(R2)
Decolorization reactions were also carried out in
Na2HPO4/NaH2PO4 buffer (0.1 mol L−1 pH = 7.0), but no oxidation
Table 5
Observed rate constants for the oxidation of MO and RO catalyzed by [MnL1(H2O)](ClO4)2 or [MnL2]ClO4 at different H2O2 concentrations.
MO
kobs, min−1
Proportion 1 cat: 20 subst: X H2O2
[H2O2] mol L−1
[MnL1(H2O)](ClO4)2
[MnL2]ClO4
Control reactionsa
1000
5000
10,000
15,000
20,000
1.5 × 10−3
7.5 × 10−3
1.5 × 10−2
2.25 × 10−2
3.0 × 10−2
3.3 × 10−3
1.2 × 10−2
4.3 × 10−2
3.9 × 10−2
3.6 × 10−2
7.2 × 10−3
1.4 × 10−2
1.8 × 10−2
2.2 × 10−2
2.4 × 10−2
1.9 × 10−3
2.1 × 10−3
5.0 × 10−3
7.4 × 10−3
1.3 × 10−2
RO
kobs, min−1
Proportion 1 cat: 20 subst: X H2O2
[H2O2] mol L−1
[MnL1(H2O)](ClO4)2
[MnL2]ClO4
Control reactionsa
1000
5000
10,000
15,000
20,000
1.5 × 10−3
7.5 × 10−3
1.5 × 10−2
2.25 × 10−2
3.0 × 10−2
4.1 × 10−2
4.6 × 10−2
8.6 × 10−2
7.5 × 10−2
2.8 × 10−2
2.0 × 10−2
3.5 × 10−2
4.3 × 10−2
3.9 × 10−2
3.4 × 10−2
3.7 × 10−3
4.0 × 10−3
5.5 × 10−3
9.1 × 10−3
5.6 × 10−3
awithout catalyst.

18
C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
0.8
0.6
1.2
1.0
0.8
MnL1
MnL2
(A)
0.4
0.2
0.0
0.6
0.4
300
400
500
600
Wavelength (nm)
0.2
0.0
0
10
20
30
40
50
60
t (min)
MnL1
MnL2
0.3
0.2
0.1
1.2
1.0
0.8
(B)
Fig. 6. Effect of the H2O2 concentration on the percentage decolorization in 1 h of
MO (top) and RO (bottom). Reactions conditions: 0.1 mol L−1 HCO3−/CO3− buffer, pH
9.7; 3 × 10−5 mol L−1 MO or RO, 1.5 × 10−6 mol L−1 [MnL1(H2O)](ClO4)2, [MnL2]ClO4
and Mn(ClO4)2·6H2O, 25 ◦C.
0.6
0.4
0.2
0.0
0.0
300
400
500
600
Wavelength (nm)
spectrum, typical of the mixed valence MnIIIMnIV complex and the
radical signal centered at g = 2.
The presence of MnIV species indicates a two electron process
between the MnII complex and the oxidant (Reaction (R2)). The
binuclear MnIIIMnIV may be formed by the reaction of MnIV = O and
MnIII ions, the last one produced as given in Reaction (R1) above.
Alternatively, the binuclear MnIIIMnIV could be formed from the
oxidation of the dimmer MnIIMnII, however, the epr spectrum only
detected this species in acetonitrile solutions. It was not observed
in the carbonate/bicarbonate buffer aqueous solutions used for the
catalytic oxidation of the dyes.
0
10
20
30
40
50
60
t (min)
Fig. 4. Spectral changes (using [MnL1(H2O)](ClO4)2) and absorbance decay with
time for degradation of methyl orange (A) and reactive orange 16 (B). Reac-
tions conditions: H2O2 1.5 × 10−2 mol L−1; HCO3−/CO3 0.1 mol L−1 buffer, pH 9.7;
−
MO or RO 3 × 10−5 mol L−1; [MnL1(H2O)](ClO4)2 or [MnL2]ClO4 1.5 × 10−6 mol L−1
;
cat:dye:H2O2 = 1:20:10,000; 25 ◦C.
The percentage of decolorization was determined for a 1 h
reaction as seen in Fig. 6. In all cases, a large enhancement in
the decolorization percentage was observed for the catalyzed
reactions compared to the control reactions. For the MO reac-
tions under the conditions 1:1000 and 1:5000 (cat:oxidant), the
(giso = 2.110; D [cm−1] = 0.095; E/D = 0.067), but the hyperfine
modulations around g = 2 are indeed due to a MnIIIMnIV dimer
species (Fig. 5(B)) [23,36,37]. The same behavior was also observed
for the [MnL2](ClO4 as seen in Fig. S3), which shows the 16 line epr
A
B
0
200
400
600
800
1000
250
300
350
400
Magnetic Field [mT]
Magnetic Field [mT]
Fig. 5. (A) 77 K X-band EPR spectrum of catalysis solution of [MnL1(H2O)](ClO4)2 (10 min). Thin line represents the simulation according to a MnIV species. (B) Hyperfine
modulations extracted from (A). These hyperfine modulations are attributed to a MnIIIMnIV dimer species.

C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
19
since there is always a large excess of H2O2, the percentage of
oxidation is much less sensitive to the nature of the catalyst. This
was observed for conditions in Fig. 6 (top) (conditions 1:10,000
through to 1:20,000), where it is clear that the difference between
the efficiencies of both complexes decreases as the concentration
of H2O2 increases (See the yellow and the red bars). This trend was
also observed for most of the conditions in Fig. 6 (bottom).
It is important to remember that both substrates herein stud-
ied, (MO = 4-(4-sulfophenylazo)-N,N-dimethylaniline and RO =
6-(acetylamino)-4-hydroxy-3-[[4-[[2-(sulfooxy)ethyl]sulfonyl]
phenyl]azo]-2-naphthalenesulfonic acid disodium salt) are azo
dyes. For the MO reactions the intensity of the 465 nm peak
decreases only partially with time, suggesting that the azo group
retains its integrity. The visible absorption spectra provide good
evidence that the oxidative attack occurs at the amine group rather
than at the azo group, since the final spectrum is identical to that
known for the amine oxide derivative of dimethylaminoazoben-
zene as seen in Scheme 3. Thus the loss of color simply results
from a shift of the spectrum into the UV, rather than degradation
of the molecule into smaller fragments. This behavior is well
documented in the literature which reports the inertness of the
azo link in methyl orange [43].
Fig. 7. Comparison of turn over frequency numbers of the catalysts
[MnL1(H2O)](ClO4)2, [MnL2]ClO4, and Mn(ClO4)2·6H2O during the oxidation
of MO and RO for the condition 1:20:10,000 (cat:substrate:oxidant), 25 ◦C.
complex [MnL2]ClO4 (green bar in Fig. 6(A)) showed better results
when compared to [MnL1(H2O)](ClO4)2. It is also worth men-
tion that [MnL2]ClO4 decolorization increases continuously in all
range, from lower to higher oxidant concentrations. However, from
the condition 1:10,000 up to 1:20,000 the bleaching efficiency is
reversed and [MnL1(H2O)](ClO4)2 showed better percentages of
oxidations (Fig. 6(A)) and better TOF (Fig. 7). We have also tested
the decolorization of the Mn(ClO4)2·6H2O complex at the condition
of 1:20:10,000 (Fig. 6) and the performance is comparable with
both [MnL1(H2O)](ClO4)2 and [MnL2]ClO4 complexes, with per-
centages of 80% and 93% for MO and RO, respectively. Therefore,
Mn(II) is indeed the key species to support the radical mechanism.
The TOF values, though, are slightly higher for [MnL1(H2O)](ClO4)2
and [MnL2]ClO4 when compared to Mn(ClO4)2·6H2O for the oxida-
tion of RO (Fig. 7). In contrast, the complex [MnL2]ClO4 showed a
smaller TOF than Mn(ClO4)26H2O for the oxidation of MO, perhaps
a consequence of the quelate effect of L2 blocking the Mn(II) sites.
In order to check the role of free radical processes, experiments
using H2O2 concentration of 1.5 × 10−2 mol L−1 were performed
for both complexes and dyes in the presence of tert-butanol
(5.3 × 10−2 mol L−1), which is a strong radical scavenger [38]. The
absorbance values diminished as in the control reactions and the
reaction rates dropped down. (Figs. S1 and S2). Although high
valence MnIV species were detected, this significant decrease of the
reaction rate as a result of the addition of a radical scavenger induce
us to assume that the active species responsible of the substrate
oxidations are hydroxyl radicals formed directly by the reaction
between H2O2 and the MnII complex as in Reaction (R1) above, or
even from more oxidized forms, MnIII and MnIV [39,40].
Experiments to test the reuse of both catalysts were car-
ried out. After a complete decolorization, a second amount of
dye and oxidant were added to the used catalyst. In a typ-
ical reaction, the percentage of decolorization was over 90%
(Fig. S4 and Table S1).
4. Conclusions
Two new complexes of the hydroxiethyl-ethylenodiamine
series have been prepared and characterized. [MnL1(H2O)](ClO4)2
has a pentagonal bipyramidal structure and in [MnL2]ClO4 the
metal ion experiences a distorted octahedral environment. Bond
lengths and angles are result of the distortions, the quelate effect
of L1 and L2 and the mode of coordination of the donor atoms –
protonated/deprotonated.
Both complexes were active catalysts toward the oxidative
decolorization of organic dyes such as methyl orange and reac-
tive orange 16, promoting significant enhancement of solution
decolorization in comparison to control reactions. The catalytic
activity of both complexes depends on the concentration of
H2O2. In conditions up to the ratio 1:20:5000 (cat:dye:oxidant),
[MnL2]ClO4 showed better results than [MnL1(H2O)](ClO4)2,
probably due the ability of the phenol ring in L2 to facil-
itate the production of higher oxidation state-intermediates,
which in turn can produce hydroxyl radicals in acetonitrile
solutions.
The effect of the concentration of H2O2 on the velocity and
the percentage of decolorization was studied as seen in Table 5
and Fig. 6. Overall, for [MnL1(H2O)](ClO4)2 and [MnL2]ClO4 a con-
tinuous increase of velocity was observed up to the 1:20:10,000
condition, followed by lower reaction rates in higher H2O2
concentrations. The phenol ring was expected to improve the
ability of the complex [MnL2]ClO4 toward the stabilization of
high oxidations states of the manganese ions as intermediates
species. Indeed, it may explain the results seen for the oxida-
tions of methyl orange at low concentrations of H2O2 (Fig. 6(A),
conditions cat:oxidant 1:1000 and 1:5000). Referring again to
Eq. (1): [MnIIL] + H2O2 → [MnIIIL]+ + HO− + •OH, ligands that can
stabilize higher oxidations states favors the products of this reac-
tion, which in turn contain hydroxyls radicals as a byproduct.
At higher H2O2 concentrations, the system gets more compli-
cated because of secondary reactions. The reaction rate tend
Supplementary material
CCDC-895064 and CCDC-895065 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or
from the Cambridge Crystallographic Data Centre (CCDC), 12 Union
Road, Cambridge CB2 1EZ, UK; fax: +44(0)1223 336033; email:
deposit@ccdc.cam.ac.uk].
Acknowledgments
to decrease (Table 5) due to reaction of •OH radicals with
excess of H2O2 producing HO2 radicals (k = 2.7 × 107 M−1 s−1
)
•
The Brazilian Research Council CNPq and Fundac¸ ão Araucária
supported this work. S.M.D, F.S.N., C.A.W. thank CNPq, for research
fellowships.
which are less reactive. Also, the •OH radicals dimerize form-
ing H2O2 (k = 5.5 × 109 M−1 s−1) [41,42]. The major result is that

20
C.A. Wegermann et al. / Applied Catalysis A: General 454 (2013) 11–20
Appendix A. Supplementary data
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