Synthesis, Characterization, and Catalytic Studies of Mn(III)-Schiff Base-Dicyanamide Complexes: Checking the Rhombicity Effect in Peroxidase Studies

Article abstract of DOI:10.1155/2017/5465890

The condensation of 3-methoxy-2-hydroxybenzaldehyde and the diamines 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane and 1,3-propanediamine yielded the dianionic tetradentate Schiff base ligands N,N′-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diphenylendi

Full text of DOI:10.1155/2017/5465890

Hindawi
Journal of Chemistry
Volume 2017, Article ID 5465890, 10 pages
https://doi.org/10.1155/2017/5465890
Research Article
Synthesis, Characterization, and Catalytic Studies of
Mn(III)-Schiff Base-Dicyanamide Complexes: Checking
the Rhombicity Effect in Peroxidase Studies
Manuel R. Bermejo,1 Rocío Carballido,2 M. Isabel Fernández-García,2
Ana M. González-Noya,1 Gustavo González-Riopedre,2 Marcelino Maneiro,2
and Laura Rodríguez-Silva2
1Department of Inorganic Chemistry, Faculty of Chemistry, Universidade de Santiago de Compostela,
15782 Santiago de Compostela, Spain
2Department of Inorganic Chemistry, Faculty of Sciences, Universidade de Santiago de Compostela, 27002 Lugo, Spain
Correspondence should be addressed to Manuel R. Bermejo; manuel.bermejo@usc.es
Received 2 February 2017; Accepted 4 July 2017; Published 16 August 2017
Academic Editor: Albert Demonceau
Copyright © 2017 Manuel R. Bermejo et al. is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
e condensation of 3-methoxy-2-hydroxybenzaldehyde and the diamines 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane
and 1,3-propanediamine yielded the dianionic tetradentate Schiff base ligands N,Nꢀ-bis(2-hydroxy-4-methoxybenzylidene)-1,2-di-
phenylendiimine (H L1), N,Nꢀ-bis(2-hydroxy-4-methoxybenzylidene)-1,2-diamino-2-methylpropane (H L2) and N,Nꢀ-bis(2-
2
2
hydroxy-4-methoxybenzylidene)-1,3-diaminopropane (H L3) respectively. e organic compounds H L1 and H L2 have been
2
2
2
1
characterized by elemental analysis, IR, H and 13C NMR spectroscopies and mass spectrometry electrospray (ES). e crystal
structure of H L2 in solid state, solved by X-ray crystallography, is highly conditioned in the solid state by two N-H∙∙∙N intramolec-
2
ular interactions. e synthesis of three new manganese(III) complexes 1–3, incorporating these ligands, H L1–H L3, and
2
2
dicyanamide (DCA), is reported. e complexes 1–3 have been physicochemically characterized by elemental analysis, IR and
paramagnetic 1H NMR spectroscopy, ESI mass spectrometry, magnetic moment at room temperature and conductivity measure-
ments. Complex 1 has been crystallographically characterized. e X-ray structure shows the self-assembly of the Mn(III)-Schiff
base-DCA complex through ꢀ-aquo bridges between neighbouring axial water molecules and also by ꢁ-ꢁ stacking interactions,
establishing a dimeric structure. e manganese complexes were also tested as peroxidase mimics for the H O -mediated reaction
2
2
with the water-soluble trap ABTS, showing complexes 1-2 relevant peroxidase activity in contrast with 3. e rhombicity around
the metal ion can explain this catalytic behaviour.
1. Introduction
and ring opening of epoxides [1–8]. Over the past years,
a variety of applications of this type of complexes has
also been reported, including a broad range of biological
activities (antibacterial, antifungal, anticancer, antioxidant,
anti-inflammatory, etc.) [9–13].
During our search for new metal catalysts contain-
ing salen-type ligands, we have observed a better cat-
alytic behaviour when the substrate molecule can be
easily coordinated by the complex and this is favoured
when the catalyst has either a vacancy in the coordina-
tion sphere or a labile ligand [14–17]. To achieve this,
Schiff base ligands stabilize different metal ions in solution
to yield metal complexes with a variety of properties and
applications. For instance, one of these ligands, the chelating
salen, is known by the ability to significantly decrease the
Mn(III)/Mn(II) redox potentials, and the resulting com-
plexes constitute suitable systems to catalyse multiple redox
reactions such as asymmetric epoxidation of unfunctional-
ized olefins, catalase reaction, water photolysis, Diels-Alder
cycloaddition, enantioselective cyclopropanation of styrenes

2
Journal of Chemistry
OMe
OMe
OMe
OMe
OMe
OMe
HO
OH
HO
OH
HO
OH
N
N
N
N
N
N
HL
HL
HL
Figure 1: Structures of the Schiff base ligands H L1–H L3.
2
2
a careful design of the environment around the metal ion is
necessary.
balance, calibrated using mercury tetrakis(isothiocyana-
to)cobaltate(II), Hg[Co(NCS) ]. Measurements of conduc-
4
tivity were made with 10−3 M solutions of complexes in
dimethylformamide using a Crison microCM 2200 conduc-
tivimeter.
In this article, we report the synthesis and characteri-
zation of three new complexes containing both salen-type
ligands and ancillary dicyanamide ligands (see Figure 1).
e two imine nitrogen and two phenol oxygen atoms of
the tetradentate Schiff bases H L1–H L3 constitute an inner
2.3. Synthesis of H2L1. Schiff base ligand H L1 was prepa-
2
2
2
ONNO site. e presence of the outer methoxy groups facil-
itates the extension of the supramolecular network through
hydrogen-bond interactions [18, 19]. Our scheme consists of
inducing tetragonal elongation around the manganese ion
red according to the literature by condensation of 3-me-
thoxy-2-hydroxybenzaldehyde (1.12 g, 10.35 mmol) with 1,2-
phenylenediamine (0.54 mL, 5.18 mmol) in methanol [28].
e mixture of diamine and salicylaldehyde was refluxed in
a round bottom flask fitted with a Dean-Stark apparatus.
Afer heating for 3 h, the solution was concentrated to yield
an orange solid. e product was collected by filtration,
washed with diethyl ether, and dried in air. Yield was almost
quantitative. M.p. 172.5∘C. Anal. Calc. for C H N O : C,
with the combination of the H Lꢁ and dicyanamide ligands.
2
ese complexes may behave like enzyme mimics, and
their peroxidase activity is reported in this work. Peroxidases
catalyse oxidation of a broad range of substrates by hydrogen
hydroxide, so they may be successfully used for different
industrial processes since peroxidases are capable of degrad-
ing aromatic rings in lignin, dyes, and polycyclic aromatic
hydrocarbons [15, 16, 20, 21]. In this sense, dyes are regarded
as one of the most important contaminants to be removed
from the industrial effluents, and peroxidases arise as a
way to treat these pollutants. Here we check the peroxidase
activity of Mn(III)-Schiff base complexes in function of the
rhombicity (or tetragonal elongation) around the manganese
ion.
22 20
2
4
70.2; H, 5.3; N, 7.4. Found: C, 70.1; H, 5.3, N, 7.5%. MS ES
(m/z): 377 (L + H+)+; IR (KBr, cm−1): ](O-H) 3461, ](C=N)
1611, ](C-O) 1253. 1H NMR (DMSO-d , ppm): ꢂ 12.95 (s, 2H),
6
8.88 (s, 2H), 7.42 (m, 4H), 7.36 (d, 2H), 7.13 (d, 2H), 6.88 (t,
2H), 3.79 (s, 6H). 13C NMR (DMSO-d , ppm): ꢂ 164.8 (C=N),
6
151.4 (C-OCH ), 142.8 (C-OH), 116-128.5 (Car), 40.2 (CH ).
3
3
2.4. Synthesis and Crystallization of H2L2. e tetradentate
Schiff base H L2 was prepared following the procedure
2
already described for H L1, using 3-methoxy-2-hydroxyben-
2
zaldehyde (2.00 g, 13.14 mmol) and 1,2-diamine-2-methyl-
propane (0.68 mL, 6.57 mmol) in methanol. e yellow prod-
uct was collected by filtration, washed with diethyl ether, and
2. Materials and Methods
2.1. Materials. All solvents, 3-methoxy-2-hydroxybenzalde-
hyde, 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane,
1,3-propanediamine, manganese(II) acetate, sodium dicyana-
mide, hydrogen peroxide, and 2,2ꢀ-azinobis-(3-ethylbenzo-
thiazoline)-6-sulfonic acid, are commercially available and
were used without further purification.
dried in air. Yellow crystals of H L2, suitable for single crystal
2
X-ray diffraction studies, were obtained by slow evaporation
of the methanol solution of the ligands. Yield was almost
quantitative, 94%. M.p. 122.9∘C. Anal. Calc. for C H N O :
20 24
2
4
C, 67.4; H, 6.8; N, 7.9. Found: C, 67.3; H, 6.7, N, 7.8%. MS ES
(m/z): 357 (L + H+)+, 379 (L + Na+)+; IR (KBr, cm−1): ](O-H)
3441, ](C=N) 1630, ](C-O) 1256. 1H NMR (DMSO-d , ppm):
6
ꢂ 8.53, 8.50 (s, 2H), 6.77–7.01 (m, 6H), 3.83 (m, 6H), 3.76 (t,
2.2. Physical Measurements. Elemental analyses of C, H, and
N were performed on a Carlo Erba EA-1108. IR spectra were
recorded as KBr discs on a Bio-Rad FTS135 spectrophoto-
meter in the range 4000–400 cm−1. 1H NMR and 13C NMR
spectra were recorded on a Bruker AC-300 spectrometer at
4H), 1.37 (t, 6H), 13C NMR (DMSO-d , ppm): ꢂ 163.3 (C=N),
6
148.9 (C-OCH ), 152.9 (C-OH), 25.7 (CH ).
3
3
2.5. Synthesis of H2L3. e synthesis and characterization of
H L3 have been already previously reported and they will not
296 K using DMSO-d as deuterated solvent and tetramethyl-
2
6
be discussed in this paper [22].
silane as an internal reference. Positive electrospray-ioniza-
tion (ESI) mass spectra of the ligands and complexes were
recorded on a LC-MSD 1100 Hewlett-Packard instrument
2.6. Complex Preparation. Mn2L12(H2O) (DCA) (1): to a
(positive-ion mode, 98 : 2 CH OH/HCOOH as the mobile
methanolic solution of H L1 (0.53 mmol, 20.20 g),20.53 mmol
3
2
phase, 30–100 V). Magnetic susceptibilities were measured
at room temperature using a Sherwood-Scientific MK1
(0.13 g) of Mn(CH COO) was added and the stirred solution
3
2
changed its initial yellow colour to brown. Afer 30 minutes of

Journal of Chemistry
3
Table 1: Crystal data and structure refinement for H L2 and com-
stirring with gentle heating, 0.53 mmol (0.05 g) of NaN(CN)
2
2
in 10 mL of methanol was added. e solution was then
concentrated by slow evaporation. e complex was obtained
as brown crystals, which were isolated by filtration, washed
with diethyl ether, and dried in air. Yield 60%. Anal. Calcd. for
C H Mn N O (1026.8): C, 56.1; H, 3,9; N, 13.6. Found:
plex 1.
H L2
Complex 1
2
Chemical formula
Formula weight
Crystal system
Space group
C H N O
C H MnN O
20 24
2
4
24 20
5
5
356.41
513.39
48 40
2
10 10
C, 55.5; H, 3.9; N, 13.4%. MS ES (m/z): 429 [MnL]+; 496
Triclinic
ꢋ1
Triclinic
ꢋ1
[MnL(DCA)]+. IR (KBr, cm−1): ](O-H) 3433 (m), ](C=N)
1600 (vs), ](C-O) 1258 (s), ]sym(C≡N) 2151 (vs), ]asym(C≡N)
Temperature (K)
100
100(2)
1
˚
2254 (m). H NMR (DMSO-d , ppm): ꢂ ꢃ14.9 (H4), ꢃ20.0
ꢌ (A)
8.3632(6)
10.5626(7)
11.4846(7)
108.689(2)
95.176(2)
103.965(2)
917.00(11)
2
8.3157(4)
11.9711(6)
12.7633(10)
114.321(3)
102.633(3)
97.626(2)
1093.74(11)
2
6
(H5). ꢀ = 4.8 BM. Conductivity (in DMF) ΛM = 45 ꢀS.
[MnL2(H2O)(DCA)](H2O) (2) was obtained by the same
˚
ꢍ (A)
˚
ꢎ (A)
procedure as 1, using 0.46 mmol (0.16 g) of H L2, 0.46 mmol
2
ꢄ (∘)
ꢏ (∘)
ꢐ (∘)
(0.11 g) of Mn(CH COO) , and 0.46 mmol (0.04 g) of
3
2
2
NaN(CN) in 10 mL of methanol which were added. Yield
2
58%. Anal. Calcd. for C H MnN O (511.41): C, 51.6; H,
3
22 26
5
6
˚
Volume (A )
ꢑ
5.1; N, 13.7. Found: C, 52.1; H, 5.2; N, 13.5%. MS ES (m/z)
408.5 [MnL]+, 884.0 [Mn L (DCA) ]+; IR (KBr, cm−1):
2
2
Radiation type
Mo Kꢄ
Mo Kꢄ
](O-H) 3425, ](C=N) 1610, ](C-O) 1260, ]sym(C≡N) 2148,
ꢀ (mm−1
)
0.09
0.653
]
asym(C≡N) 2266; 1H NMR (DMSO-d , ppm): ꢂ ꢃ15.2 (H4),
6
Crystal size (mm)
0.2 × 0.11 × 0.03 0.12 × 0.11 × 0.06
ꢃ16.8/ꢃ25.0 (H5). ꢀ = 5.0 BM. Conductivity (in MeOH) Λ
=
M
ꢒ
min, ꢒ
0.942, 0.997
380
0.9619, 0.9858
528
max
46 ꢀS.
[MnL3(H2O)(DCA)](H2O) (3) was obtained by the same
ꢆ(000)
procedure as 1, using 0.58 mmol (0.20 g) of H L3, 0.58 mmol
Number of measured reflections
Number independent reflections
Reflections observed [ꢓ > ꢉꢊ(ꢓ)ꢔ
11933
3607
2055
0.054
0.617
0.049
0.128
247
0.31, ꢃ0.28
20087
4293
3209
0.0374
0.0752
0.0630
0.0816
332
2
(0.14 g) of Mn(CH COO) , and 0.58 mmol (0.05 g) of
3
2
NaN(CN) in 10 mL of methanol which were added. Yield
2
65%. Anal. Calcd. for C H MnN O (497.37): C, 50.7; H,
ꢇ
21 24
5
6
int
−1
4.8; N, 14.1. Found: C, 49.8; H, 4.6; N, 13.8%. MS ES (m/z)
˚
(sin ꢕꢖꢅ)
(A
)
max
395.3 [MnL]+; IR (KBr, cm−1): ](O-H) 3412, ](C=N) 1612,
2
2
ꢇ[ꢆ > ꢉꢊ(ꢆ )ꢔ
1
](C-O) 1256, ]sym(C≡N) 2144, ]asym(C≡N) 2252; H NMR
2
ꢈꢇ(ꢆ )
(DMSO-d , ppm): ꢂ ꢃ16.4 (H4), ꢃ18.6 (H5). ꢀ = 4.7 BM.
6
Number of parameters
Conductivity (in DMF) ΛM = 42 ꢀS.
−3
˚
Δ
max, Δmin (e A
)
0.338, ꢃ0.423
2.7. Peroxidase Probes. Peroxidase activity for complexes 1–3
was tested according to the literature procedure [15, 16]
by study of the oxidation of 2,2ꢀ-azinobis-(3-ethylbenzo-
thiazoline)-6-sulfonic acid (ABTS) with H O at ca. pH 6.8
2
of reflections for refinement. ꢇ-factors based on ꢆ are
statistically about twice as large as those based on ꢆ, and ꢇ-
factors based on all data will be even larger.
2
2
in the presence of the complexes.
3. Results and Discussion
2.8. Crystallographic Studies. Single crystals of H L2 and
2
Schiff bases H L1-H L2 have been obtained in almost quan-
complex 1, suitable for X-ray diffraction studies, were
obtained by slow evaporation of the methanolic solution at
room temperature.
2
2
titative yield and characterized by elemental analysis, IR,
1H, and 13C NMR spectroscopies, and mass spectrometry
electrospray (ES). e synthesis and characterization of H L3
Table 1 collects detailed crystal data collection and refine-
2
ment for H L2 and complex 1. Measurements were made on a
2
have been already reported [22].
Bruker-Smart CCD-1000 diffractometer employing graphite-
Complexes 1–3 were prepared in high yield as detailed
in the Materials and Methods. Elemental analysis establishes
the general formula [MnLꢁ(H O)(DCA)ꢔ(H O) , with ꢗ =
˚
monochromated Mo-Kꢄ radiation (ꢅ = 0.71073 A) at 100 K.
e structures were solved by direct methods [29] and
2
2
ꢂ
2
finally refined by full-matrix least-squares base on ꢆ . An
1–3 and ꢘ = 0-1. Analytical and spectroscopic data are in
accordance with this empirical formula. All the complexes
seem to be stable in air and thermally stable, melting
above 300∘C without decomposition. ey are insoluble or
sparingly soluble in water but partially soluble in common
organic solvents such as methanol and totally soluble in polar
coordinating solvents such as pyridine, DMF, and DMSO.
ESI (electrospray-ionization) mass spectra registered in
methanol show, for complexes 1–3, a peak corresponding
empirical absorption correction was applied using SADABS
[30]. All nonhydrogen atoms were included in the model at
geometrically calculated positions.
2
Refinement of ꢆ against all reflections: the weighted ꢇ-
factor ꢈꢇ is based on ꢆꢉ, and conventional ꢇ-factors ꢇ are
2
based on ꢆ, with ꢆ set to zero for negative ꢆ . e threshold
2
2
expression of ꢆ > 2ꢊ(ꢆ ) is used only for calculating ꢇ-
factors(gt) and so forth and is not relevant to the choice

4
Journal of Chemistry
∘
2
×105
Table 2: Selected geometric parameters (A, ) for H L .
˚
2
408.5102
C12-N2
1.272(3)
1.351(3)
6
C14-O3
C12-N2-C9
C7-N1
4
2
0
122.9(2)
1.279(3)
1.354(3)
106.65(19)
C1-O1
N2-C9-C8
884.0985
316.3185
300 400
500
600
700
800
m/z
Table 3: Hydrogen bonds for H L2.
2
Figure 2: ESI mass spectrum for complex 2.
˚
˚
˚
˚
˚
D-H⋅ ⋅ ⋅ A (A) D-H (A) H⋅ ⋅ ⋅ A (A) D⋅ ⋅ ⋅ A (A) D-H⋅ ⋅ ⋅ A (A)
O1-H1⋅ ⋅ ⋅ N1
0.96
0.93
1.73
1.72
2.55
2.5839
2.5891
3.4451
146
154
151
to the fragment [MnL]+, indicating the coordination of the
Schiff base ligand to the metal centre. Other minor signals,
assigned to [MnL1(DCA)]+ and [Mn L2 (DCA)2]+ units,
O3-H3 ⋅ ⋅ ⋅ N2
C8-H8B⋅ ⋅ ⋅ O1 0.99
2
2
also confirm the coordination of the dicyanamide ion to the
manganese ion and point to the formation of dimeric species
(Figure 2).
twisted about the central aliphatic chain which acts as a
spacer (Figure 5). e C12-N2 and C7-N1 distances of 1.272(3)
e values for the magnetic moment at room temperature
are very close to the spin-only value of 4.89 BM, as expected
for a high-spin magnetically diluted d4 manganese(III) ion.
All the complexes show similar IR spectral features,
exhibiting a strong band between 1600 and 1612 cm−1
attributable to ](CNimine) band, which is shifed 3–7 cm−1
lower than in the free Schiff base ligand. ere is also a
positive shif (3–7 cm−1) of the ](COphenol) mode with respect
to the free ligand (Figure 3). ese data suggest the coordi-
nation of the Schiff base in its dianionic form through the
inner phenol oxygen and the imine nitrogen atoms [14, 15,
22]. Moreover, two new bands at about 2150 and 2260 cm−1
are assigned, respectively, to the symmetric and asymmetric
dicyanamide modes [16, 31].
˚
and 1.279(3) A, respectively, are practically identical and
consistent with a C=N double bonding (Table 2), indicating
both imine groups fully localized in the molecule [34]. e
two oxygen O1 and O3 atoms are forming phenolic groups,
˚
and they present C-O distances of 1.354(3) A and 1.351(3),
respectively, corresponding to the expected single bonds. e
C12-N2-C9 angle of 122.9(2)∘ confirms the sp2 character for
N2 atom [32].
e disposition adopted by the free ligand in the solid
state is conditioned by two strong intramolecular hydrogen-
bond interactions which form two six-membered rings.
ese interactions arise from the contact between the
hydroxy and the imine groups (see Table 3). As expected, the
O1-H1O∙∙∙N1 hydrogen bonding establishes an almost planar
N1-C7-C6-C1-O1-H1 ring, the same for the O3-H3O∙ ∙ ∙N2
hydrogen bonding which establishes other almost planar N2-
C12-C13-C14-O3-H3 rings.
1
Paramagnetic H NMR spectra of 1–3 were registered
using DMSO-d as solvent. e data interpretation, based on
6
previous findings for manganese(III) complexes with related
Schiff base ligands [24], indicates [32, 33] the formation of
Despite the mentioned intramolecular hydrogen inter-
actions and a partial conjugation of the molecule, this one
is not planar, probably due to the packing based on the
intermolecular hydrogen bonds and the ꢁ-stacking inter-
actions between the salicyl residues of the neighbouring
molecules (see Figure 6). e aromatic rings are arranged
almost perpendicular to each other. Clearly, rotation about
the C8-C9 bond to give a near-eclipsed or gauche con-
formation would achieve quasi-square-planar coordination
environments using the inner ONNO site. According to this
crystal structure study, this ligand should act as tetradentate
donor in the equatorial plane about an octahedral metal
centre. However, we cannot predict the role of the outer
phenoxy oxygen atoms of each aldehyde residue during
complexation as they could coordinate, for instance, with the
metal centre by an axial position, but also their role could be
limited to intermolecular hydrogen-bond interactions.
1
the manganese(III) complexes. e paramagnetic H NMR
spectra of the complexes contain, for all cases, two upfield
proton resonances, outside the diamagnetic region (ꢂ =
0–14 ppm), due to the isotropic shifing of the ligand protons
for high-spin manganese(III) complexes in an octahedral
field. e signals must arise from the protons of the aromatic
phenoxy rings. In the case of the symmetric complexes 1 and
3, the ꢃ20.0 (for 1)/ꢃ18.6 (for 3) signals arise from the protons
in the orthopositions to the methoxy substituent, while the
resonances from ꢃ14.9 (for 1)/ꢃ16.4 (for 3) ppm arise from
protons in the metapositions to the methoxy substituent. For
complex 2 the signal from the protons in the orthopositions
to the methoxy substituent appears split in two resonances at
ꢃ16.8 and ꢃ25.0 ppm (Figure 4).
3.1. Crystallographic Characterization of H2L2 and Complex 1
3.1.1. Crystal Structure of H2L2. e crystal structure reveals
3.1.2. Crystal Structure of 1. Single crystals of complex 1, suit-
able for X-ray diffraction studies, were obtained as described
in the Materials and Methods. e crystal structure is shown
that H L2 exists as discrete molecules, and it is composed
2
of two identical 3-methoxy-2-hydrobenzylimine moieties

Journal of Chemistry
5
](O-H)
887
](O-H)
3425
]
ＭＳＧ(C-N)
]
；ＭＳＧ(C≡N)
2266
](C-O)
](C=N)
1260
]；ＭＳＧ(C-N)
](C-O)
](C=N)
1300
]
ＭＳＧ(C≡N)
1610
2148
4000
3500
3000
2500
2000
1500
1000
500
4000 3500 3000 2500 2000 1500 1000
500
Wavenumber (＝Ｇ−1
)
Wavenumber (＝Ｇ−1
)
(a)
(b)
Figure 3: Comparison between the spectra for the free ligand H L2 (a) and complex 2 (b).
2
C19
O4
C5
C6
C4
C3
C15
O3
C14
C16
C17
C7
C2
O2
C1
C13
C12
N1
N2
C20
C8
C18
−15.0
−20.0
−25.0
C9
O1
1(ppm)
Figure 4: Paramagnetic H NMR spectrum for complex 2.
C10
C11
∘
˚
Table 4: Selected bond lengths (A) and angles ( ) for complex 1.
Figure 5: ORTEP representation (50% probability) of H L2.
2
Mn(1)-O(830)
Mn(1)-N(1)
1.870(15)
1.9816(19)
2.257(2)
1.8705(15)
1.9940(19)
2.2859(18)
91.03(7)
Mn(1)-N(20)
Mn1-O(130)
Mn1-N(8)
Mn1-O(30)
O(830)-Mn(1)-O(130)
N(8)-Mn(1)-O(9)
O(830)-Mn(1)-N(1)
N(1)-Mn(1)-O(9)
O(130)-Mn(1)-N(1)
O(830)-Mn(1)-O(9)
86.02(6)
176.10(7)
84.56(6)
92.72(7)
90.13(6)
in Figure 7 and main bond distances and angles are shown in
Table 4.
e monomer is composed by an approximately octa-
Figure 6: View along the ꢍ-axis for H L2.
hedral [MnL1(H O)(DCA)] unit. e planar tetradentate
2
2

6
Journal of Chemistry
C4
C3
C5
C2
O30
C6
C7
N8
Mn1
O130
O140
C141
C81
C82
C12
C17
C16
C87
C13
C86
O830 C83
C15
C85
C84
N20
C840
C21
C841
C23
N22
N24
Figure 7: ORTEP view of the environment around the manganese ion in complex 1.
Table 5: Hydrogen bonds for complex 1.
˚
˚
˚
˚
˚
D-H⋅ ⋅ ⋅A (A)
D-H (A)
0.83(2)
0.83(2)
0.76(3)
H⋅ ⋅ ⋅ A (A)
D⋅ ⋅ ⋅ A (A)
D-H⋅ ⋅ ⋅ A (A)
O(30)-H(30A)⋅ ⋅ ⋅ O(130)∗
2.43(2)
2.12(2)
2.40(2)
2.23(3)
2.51
3.061(2)
2.908(2)
3.034(3)
2.922(3)
3.449(4)
134(2)
158(2)
141(2)
152(2)
160
O(30)-H(30A)⋅ ⋅ ⋅ O(141)∗
O(30)-H(30B)⋅ ⋅ ⋅ O(830)∗
O(30)-H(30B)⋅ ⋅ ⋅ O(840)∗
0.76(3)
0.98
C(841)-H(84A)⋅ ⋅ ⋅ N(24)∗∗
∗ = −ꢂ, −ꢃ, and 2 − ꢄ; ∗∗ = 1 − ꢂ, 1 − ꢃ, and 2 − ꢄ.
Schiff base L1 is tightly bound to the manganese ion through
the inner N O compartment by the Nimine and Ophenol
2
2
˚
atoms (Mn-Nimine bond lengths of 1.9816–1.9940 A and Mn-
Ophenol of 1.87 A which are typical of such complexes [14–16]),
˚
occupying the equatorial positions. e geometry around the
manganese ion can be described as distorted and octahedral
with the axial positions of the octahedron occupied by a water
molecule and a dicyanamide molecule. e axial distances
˚
˚
of 2.2859 A (for Mn-O(30)) and 2.257 A (for Mn-N(20)) are
considerably longer than the equatorial Mn-O bond lengths
quoted earlier, resulting in a rhombicity factor, expressed
as the ratio between the Mn-O axial and Mn-O equatorial
distances in the crystal structure, of 1.22. e deviation from
an ideal octahedral geometry is also revealed by the range
of angles observed around the metal centre (from 82.93∘ to
95.13∘), as well as by the interaxial angle O(30)-Mn-N(20) of
173.92∘.
Figure 8: Stick diagram for the dimeric unit for complex 1 (man-
ganese ion in magenta, oxygen in red, nitrogen in blue, and carbon
in grey).
e octahedral [MnL1(H O)(DCA)] entities are linked
2
in pairs by ꢀ-aquo bridges between neighbouring axial water
molecules and phenoxy and methoxy oxygen atoms and also
by ꢁ-ꢁ stacking interactions to give [MnL1(DCA)(H O)]
3.2. Peroxidase Studies. e peroxidase-like activity of 1–3
was followed by the oxidation of the diammonium salt of 2,2ꢀ-
azinobis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) at
pH 6.8. During this reaction, ABTS is oxidized by hydrogen
peroxide to the radical cation ABTS∙+ [33] in the presence
of the appropriate peroxidase mimic. While ABTS is colour-
less, its radical cation ABTS∙+ is green, showing charac-
teristic absorption bands that can be easily followed by
2
2
dimeric structures (Figure 8, Table 5). As a result of these
supramolecular interactions, the Mn⋅ ⋅ ⋅ Mn distances of
˚
about 4.96 A are short for monomeric compounds. ese
types of systems have been reported in literature [15, 16] as
ꢀ-acuo dimers.

Journal of Chemistry
7
Table 6: Peroxidase activity for complexes 1–22, axial and equatorial crystal distances around the manganese ion for each complex with
solved crystallographic structure, resulting tetragonal elongation, and corresponding reference.
Crystal distances
Complex
Tetragonal elongationa
Reference
Peroxidase activityb
Mn-Oax
2.28
un.
Mn-Oeq
1.870
un.
c
1 [MnL1(D)(DCA)ꢔ2
1.22
un.
un.
1.225
1.243
1.234
1.24
33
53
2
3
is work
is work
is work
[16]
2 [MnL2(D)(DCA)](D)
3 [MnL3(D)(DCA)]
4 [MnL4(D) (DCA) ]
un.
un.
1.9 0.4
2.303
2.326
2.318
2.314
2.373
2.263
2.271
2.36
2.246
2.258
2.282
2.308
2.338
2.339
2.25
1.885
1.872
1.878
1.865
1.862
1.880
1.870
1.851
1.871
1.879
1.893
1.869
1.876
1.865
1.872
1.889
1.863
1.88
37
34
31
2
3
2
2
3
1
1
3
1
1
1
2
2
2
1
2
2
5 [MnL5(D) ](NO )(D)
[22]
[15]
[23]
[15]
[22]
[24]
[23]
[15]
[15]
[15]
[14]
[14]
[14]
[15]
[24]
[25]
2
3
6 [MnL5(D)Cl](D)
2.5
7 [MnL5(D) ](ClO )
19
2
4
8 [MnL6(D) ](NO )(D)
1.27
50
32
33
30
29
29
33
28
28
39
18
2
3
9 [MnL7(D) ](NO )(D)
1.203
1.214
1.27
2
3
10 [MnL8(D) ](NO )(D)
2
3
11 [MnL1(D) ](ClO )
2
4
12A [MnL2(D) ](NO )(D)
1.200
1.202
1.205
1.235
1.246
1.254
1.202
1.187
1.183
1.181
1.163
1.159
1.196
2
3
12B [MnL2(D)(D2)](NO )
3
13 [MnL2(D)(OAc)](D)
14A [MnL2(D)(O CEt)]
2
14B [MnL2(D)(O CEt)]
2
15 [MnL2(D)(O CPeꢁ)]
2
16 [MnL9(D) ](NO )(D)
2
2
3
2
17 [MnL10(D) ](NO )(D)
2.243
2.203
2.22
2.0 0.4
3.2 0.4
2.4 0.4
2.1 0.4
1.1 0.3
1.8 0.3
2
3
2
18 [MnL11(D)(D2)](ClO )
4
19 [MnL12(D) ](ClO )(D)
[26]
[27]
[22]
[24]
4
20 [MnL13(D) ](ClO )(D)
2.21
1.90
2
4
21 [MnL14(D) ](NO )(D)
2.206
2.258
1.904
1.888
2
3
2
22 [MnL15(D) ](NO )(D)
2
3
2
aTetragonal elongation expressed as the ratio between the manganese-axial oxygen distances and the manganese-equatorial oxygen distances. bPeroxidase
activity expressed as percentage of conversion of ABTS to ABTS measured 10 min afer mixing the solutions; cD = H O or other solvent molecule (such as
∙+
2
CH OH); un.: unavailable (structure not solved).
3
3.0
UV-spectroscopy. e extent of the reaction can be measured
quantitatively at ꢅ = 650 nm since ꢙ = 12000 M−1 cm−1 has
2.5
been determined [35].
In our experiments, an aqueous solution of ABTS (50 ꢀL;
2.0
0.009 M; 4.5 × 10−7 mol) and a methanolic solution of the
corresponding complexes 1–3 (10 ꢀL; 10−3 M; 10−8 mol) were
added to water. e intensity of the UV absorption bands
of ABTS started to increase immediately afer the addition
of an aqueous solution of H O (50 ꢀL; 10 M; 5 × 10−4 mol).
1.5
1.0
0.5
0.0
2
2
e extent of the reaction can be measured quantitatively at
ꢅ = 650 nm since ꢙ = 12000 M−1 cm−1 has been determined.
e peroxidase-like activity results are collected in Table 6,
where peroxidase activity is expressed as the percentage of
the starting ABTS oxidized to its radical cation measured
10 min afer mixing the solutions. e rate of formation
of ABTS∙+ of about 33–53% indicates that 1 and 2 behave
as efficient peroxidase mimics, while 3 does not present
significant catalytic activity. Figure 9 shows the UV spectrum
of the ABTS∙+ radical cation obtained afer ABTS reaction
with H O in presence of complex 2, with the characteristic
400
500
600
700
800
Wavelength (nm)
Figure 9: UV-Vis absorption spectrum of ABTS + H O + 2,
recorded 10 min afer mixing the solution.
2
2
easily coordinated by the manganese complex, and this
is favoured if the peroxidase mimic has either a vacancy
in the coordination sphere or a labile ligand [14, 15].
For the complexes used in this study, the high efficiency
2
2
absorption bands at 415, 650, 735, and 815 nm.
Our previous findings in this field indicate a better
catalytic behaviour when the substrate molecule is

8
Journal of Chemistry
4. Conclusions
8
50
40
30
20
10
0
e synthesis and structural characterization of the Schiff
base compounds N,Nꢀ-bis(2-hydroxy-4-methoxybenzyli-
15
14B
4
5
13
10
1
dene)-1,2-diphenylendiimine (H L1) and N,Nꢀ-bis(2-hydro-
2
9
6
xy-4-methoxybenzylidene)-1,2-diamino-2-methylpropane
11
14A
(H L2) have been reported. ese two organic compounds
12B
12A
2
behave as suitable ligands for coordination with metal
ions, highly versatile due to different structural features:
being tetradentate, ability to extend the hydrogen network
using the outer methoxy groups, short and bulky spacer,
etc. ree new manganese(III)-L-dicyanamide complexes
have been obtained and characterized. e short two-
carbon chain between imine groups in H L1-H L2
7
16
18
22
17
20
19
21
2
2
1.16
1.18
1.20
1.22
1.24
1.26
1.28
provokes tetragonally elongated octahedral geometries
in the resulting complexes, which may increase their
potential as catalysts for different applications. e
correlation of peroxidase activity and the rhombicity
caused by the short length of the Schiff base should be
taken into account for designing new potential catalytic
systems.
Tetragonal elongation
Figure 10: Peroxidase activity, expressed as percentage of ABTS
conversion into ABTS∙+, in function of the tetragonal elongation of
the catalyst, expressed as the ratio between the Mn-O axial and Mn-
O equatorial distances in crystal structures 1 and 4–22.
as peroxidase mimics found for complexes 1 and 2 can
be explained in this sense. Complexes 1 and 2, derived
from H L1 and H L2, respectively, have a short two-
Conflicts of Interest
2
2
e authors declare that there are no conflicts of interest
regarding the publication of this paper.
carbon chain between imine groups in the Schiff ligand.
e short chain constricts the chelate ring once
nitrogen atom coordinates with the metal and led to
a tetragonally elongated octahedral geometry for the
manganese complex. An axial water molecule in this
class of distorted geometries constitutes a quite labile
ligand, which would generate a vacant position in the
coordination sphere to accommodate the substrate molecule.
Besides, the two methyl groups located in the methylene
Acknowledgments
e authors are grateful for the financial support given by the
Xunta de Galicia (GRC2014/025).
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Research International
Catalysts
Chemistry
Spectroscopy
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Hindawi Publishing Corporation
http://www.hindawi.com
Volume 2014
Volume 2014
Volume 2014
Volume 2014
Volume 2014