Synthesis, characterization and magnetic properties of an oxido-bridged tetranuclear copper(II) complex

Article abstract of DOI:10.1016/j.poly.2013.04.042

A copper(II)-Schiff-base complex, [Cu₄(O)(L)₂(CH ₃COO)₄] (1) where HL = 4-methyl-2,6-bis(((2-tri- fluoromethyl)phenyl)methyliminomethyl)phenol, has been prepared. It has been characterized by elemental analysis, different spectroscopic methods and single crystal X-ray diffraction study. Single crystal analysis of 1 reveals that four copper atoms coordinate with one oxygen atom forming a distorted tetrahedron in which the oxygen atom occupies the center and copper atoms reside in the corners. The temperature dependent magnetic property of complex 1 shows strong antiferromagnetic interactions between the copper atoms.

Full text of DOI:10.1016/j.poly.2013.04.042

Polyhedron 59 (2013) 101–106
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and magnetic properties of an oxido-bridged
tetranuclear copper(II) complex
Mahendra Sekhar Jana a, Sudipto Dey a, José L. Priego b, Reyes Jiménez-Aparicio b,
,
⇑
Tapan Kumar Mondal a, , Partha Roy a,
⇑
⇑
a Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
b Departamento de Química Inorgánica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria, E28040 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
A copper(II)–Schiff-base complex, [Cu4(O)(L)2(CH3COO)4] (1) where HL = 4-methyl-2,6-bis(((2-tri-fluo-
romethyl)phenyl)methyliminomethyl)phenol, has been prepared. It has been characterized by elemental
analysis, different spectroscopic methods and single crystal X-ray diffraction study. Single crystal analysis
of 1 reveals that four copper atoms coordinate with one oxygen atom forming a distorted tetrahedron in
which the oxygen atom occupies the center and copper atoms reside in the corners. The temperature
dependent magnetic property of complex 1 shows strong antiferromagnetic interactions between the
copper atoms.
Received 1 March 2013
Accepted 20 April 2013
Available online 3 May 2013
Keywords:
Copper
Schiff-base ligand
Tetranuclear complex
Antiferromagnetic
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
compounds have shown catalytic activity in the epoxidation of ole-
fins [7,8,12,18], oxidation of alkanes [19], sulfoxidation [20,21],
Multinuclear complexes of copper(II) are of great interest to
researchers because of their diverse structural aspects as well as
for their application in the fields of magnetism, biology, catalysis,
etc. [1–14]. Multinuclear complexes can be prepared by choosing
appropriate ligand systems with suitable donor atoms. A mononu-
clear complex can be transformed into a multinuclear complex by
the judicial choice of the bridging ligand, metal ion and reaction
conditions. Schiff-base ligands are well known in the syntheses of
multinuclear complexes. Schiff-bases are able to stabilize various
metal ions in different oxidation states. A Schiff-base ligand of a
dialdehyde, 4-methyl-2,6-diformylphenol, was reported by Robson
for the first time in 1970 [15,16]. Since then a huge number of
multinuclear complexes of Schiff-base ligands of 4-methyl-2,6-dif-
ormylphenol have been reported in a number of research articles. A
very recent review article has well documented acyclic and cyclic
compartmental ligands of this aldehyde, along with other ligands
[17]. Schiff-base ligands of such aldehydes are suitable to produce
multinuclear complexes where the phenolic oxygen atom bridges
two metal ions and ancillary ligands, e.g. halide, carboxylate etc.,
coordinate to the metal ions to further stabilize the system.
catechol oxidation [22,23] etc. On the other hand, copper(I) com-
plexes have been used as catalysts in the ‘‘click reaction’’ or Huis-
gen 1,3-dipolar cycloaddition reaction of organic azides and
alkynes. This reaction has been widely applied to generate triazole
for different purposes.
Cu(II) complexes produce variable and distorted geometries and
have a simple electronic configuration. These properties encourage
researchers to explore the magnetic properties of multinuclear me-
tal–organic complexes of Cu(II). Schiff-base complexes with a
l4-oxido–Cu4 moiety have been reported in the literature and
the magnetic properties of these complexes have been extensively
studied. The four copper atoms arrange themselves in the corners
and the oxygen atom occupies the center of a distorted tetrahe-
dron. Previously, Ray and co-workers reported the temperature
dependent magnetic properties of
in detail [2,3,24,25]. There are recent reports on the synthesis of
4-O–Cu(II) complexes and their applications in catalysis [10,11].
l4-O-bridged Cu(II) complexes
l
Study of the magnetic properties of these complexes has shown
them to be antiferromagnetic in nature.
Here we report the synthesis, characterization and magnetic
properties of [Cu4(O)(L)2(CH3COO)4] (1) where HL = 4-methyl-
2,6-bis(((2-tri-fluoromethyl)phenyl)methyliminomethyl)phenol.
Single crystal analysis of 1 has revealed the existence of four cop-
per atoms coordinating with one oxygen atom, forming a distorted
tetrahedron. Temperature dependent magnetic properties of com-
plex 1 show strong antiferromagnetic interactions amongst the
copper atoms.
Cu(II)–Schiff-base complexes have found numerous applica-
tions in catalysis, biological sciences, magnetism etc. Copper(II)
⇑
Corresponding authors. Tel.: +91 33 2457 2267; fax: +91 33 2414 6414
(T. Kumar Mondal, P. Roy), fax: +34 91 394 4352 (R. Jiménez-Aparicio).
E-mail addresses: reyesja@quim.ucm.es (R. Jiménez-Aparicio), tkmondal@
chemistry.jdvu.ac.in (T. Kumar Mondal), proy@chemistry.jdvu.ac.in (P. Roy).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.042

102
M.S. Jana et al. / Polyhedron 59 (2013) 101–106
diffraction analysis. Single crystal data collection was performed
with an automated Bruker SMART APEX CCD diffractometer using
graphite monochromatized Mo radiation (k = 0.71073 Å).
Reflection data were recorded using the scan technique. Unit cell
2. Experimental
K
a
2.1. Materials and physical methods
x
parameters were determined from least-squares refinement of set-
ting angles with h in the range 1.62 6 h 6 26.50°. Out of 34748
data collected, 2575 with I > 2r(I) were used for the structure solu-
Copper(II) acetate monohydrate and 2-trifluoromethyl-1-phen-
ylmethanamine were purchased from Sigma Aldrich and used as re-
ceived. All other reagents were of analytical reagent grade. The
solvents used for the spectroscopic studies were purified and dried
by standard procedures before use [26]. The ESI-MS was recorded
on a Qtof Micro YA263 mass spectrometer. FT-IR spectra were ob-
tained on an RX-1 Perkin Elmer spectrophotometer with the sam-
ples prepared as KBr pellets. Elemental analysis was carried out in
a 2400 Series-II CHN analyzer, Perkin Elmer, Norwalk, CT. Absorp-
tion spectra were recorded on a Lambda 25 Perkin Elmer spectro-
photometer. The luminescence property was measured using an
LS-55 Perkin Elmer fluorescence spectrophotometer at room tem-
perature (298 K) in acetonitrile solution with a 1 cm path length
quartz cell. Fluorescence lifetimes were measured using a time-re-
solved spectrofluorometer from IBH, UK. The instrument uses a
picoseconds diode laser (NanoLed-03, 370 nm) as the excitation
source and works on the principle of time-correlated single photon
counting [27]. The goodness of fit was evaluated by v2 criterion and
a visual inspection of the residuals of the fitted function to the data.
The variable-temperature magnetic susceptibilities were measured
on a polycrystalline sample with a Quantum Design MPMSXL SQUID
(Superconducting Quantum Interference Device) susceptometer.
All the experiments were carried out in air and room tempera-
ture unless reported otherwise. 4-Methyl-2,6-diformylphenol was
synthesized following a published method [28].
tion within the hkl parameters ꢀ17 6 h 6 17, ꢀ14 6 k 6 17 and
ꢀ41 6 l 6 37. The structure was solved and refined by full-matrix
least-squares technique on F2 using the SHELXS-97 program [31].
The absorption corrections were done by the multi-scan technique.
All data were corrected for Lorentz and polarization effects, and the
non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were generated using SHELXL-97 [31] and their positions cal-
culated based on the riding mode with thermal parameters equal
to 1.2 times that of the associated C atoms, and they participated
in the calculation of the final R-indices.
2.5. Magnetic measurements
The variable-temperature magnetic susceptibilities were mea-
sured on a polycrystalline sample with a Quantum Design MPMSXL
SQUID (Superconducting Quantum Interference Device) suscep-
tometer over a temperature range of 2–300 K at constant field of
1 T. Each raw data set was corrected for the diamagnetic contribu-
tion of both the sample holder and the complex to the susceptibil-
ity. Molar diamagnetic corrections were calculated on the basis of
Pascal constants. The fitting of the experimental data was carried
out using the MATLAB V.5.1.0.421 program.
2.2. Synthesis of 4-methyl-2,6-bis(((2-tri-
fluoromethyl)phenyl)methyliminomethyl)phenol(HL)
3. Results and discussion
3.1. Synthesis and FT-IR spectra
The ligand HL was synthesized following a literature method
with a slight modification [29,30]. 2-Trifluoromethyl-1-phenylme-
thanamine (0.857 g, 8 mmol) in 10 mL of acetonitrile was added to
a solution of 4-methyl-2,6-diformylphenol (0.656 g, 4 mmol) in
15 mL of acetonitrile. The reaction mixture was refluxed for 4 h.
The solution was filtered, concentrated on a rotary evaporator to
dryness and kept at 4 °C overnight. The yellow colored solid Schiff
base ligand was recrystallized from acetonitrile. (Yield = 1.7 g, 89%)
Anal. Calc. For C25H20N2OF6: C, 62.76; H, 4.21; N, 5.86. Found: C,
62.73; H, 4.10; N, 5.92%. 1H NMR dH (300 MHz, CDCl3, Me4Si):
2.31 (3H, s, Ar–CH3), 5.01 (4H, s, CH), 7.26–7.72 (10H, m, Ar–CH),
8.70 (2H, s, HC@N). HRMS: m/z (ESI) 479.09 ([HL+H]+ requires
479.1513).
The synthetic procedure for complex 1 is shown in Scheme 1.
The ligand HL was synthesized by the Schiff-base condensation be-
tween 4-methyl-2,6-diformylphenol and 2-trifluoromethyl-1-
phenylmethanamine in a 1:2 ratio in acetonitrile. Complex 1 was
prepared by the reaction between one equivalent of HL and two
equivalents of the copper(II) salt in acetonitrile. No external base
was used to deprotonate the phenolic proton of the ligand.
Table 1
Crystal data for complex 1.
Complex 1
Formula
Formula weight
C58H50Cu4F12N4O11
1461.22
2.3. Synthesis of [Cu4(O)(L)2(CH3COO)4] (1)
T (K)
293(2)
Copper(II) acetate monohydrate (0.6 mmol, 0.120 g) was added
to an acetonitrile solution (10 mL) of 4-methyl-2,6-bis(((2-tri-fluo-
romethyl)phenyl)methyliminomethyl)phenol (0.3 mmol, 0.144 g)
under stirring conditions. The mixture was allowed to stir for 1 h
while the solution became green in color. After that it was refluxed
for 1 h. The mixture was then cooled to room temperature and fil-
tered to remove any precipitate or undissolved material, if any. The
filtrate was kept at room temperature. Single crystals suitable for
X-ray diffraction were obtained within a few days. (Yield = 0.26 g,
60%) Anal. Calc. For C58H50Cu4F12N4O11: C, 47.68; H, 3.45; N, 3.83.
Found: C, 47.63; H, 3.40; N, 3.88%.
Color
Crystal system
Space group
a (Å)
green
tetragonal
I41/a
13.6052(3)
13.6052(3)
32.8012(13)
6071.6(3)
4
0.24 ꢁ 0.24 ꢁ 0.23
0.708–0.711
2952
1.599
0.71073
25.00–1.62
34748, 2575
multi-scan
0.0375
b (Å)
c (Å)
V (Å3)
Z
Crystal dimensions (mm)
Minimum and maximum transmission factors
F(000)
Dcalc (g cmꢀ3
k(Mo K
h (°)
Reflection collected/unique
Absorption correction
Rint
)
a
) (Å)
2.4. X-ray data collection and structure determination
Final R indices [I > 2
r
(I)]
R1 = 0.0826, wR2 = 0.1978
1.050
The crystal data of complex 1 are summarized in Table 1. The
structure of the complex was determined by single crystal X-ray
Goodness-of-fit (GOF) on F2

M.S. Jana et al. / Polyhedron 59 (2013) 101–106
103
NH2
CH3
CH3
CF3
1:2, reflux 4 h
CH3CN
+
OH
N
N
OH
O
O
CF3
F3C
Cu(OAc)2
CH3
O
N
N
Cu
Cu
Cu
O
O
CF3
CF3
O
O
O
O
F3C
F3C
= acetate
O
O
O
O
O
Cu
N
N
O
CH3
Complex 1
Scheme 1. Synthesis of complex 1.
The FT-IR spectra of the ligand and complex 1 were recorded
with the samples prepared as KBr pellets. The FT-IR spectrum of
the ligand shows strong mC–H bands in the region 2800–
3000 cmꢀ1. The FT-IR spectrum of 1 shows the retention of these
bands in the said region [32,33]. The ligand and complex exhibit
very strong bands at 1637 and 1623 cmꢀ1 respectively, confirming
the presence of the C@N bond in the ligand and the retention of
this bond in the complex. Complex 1 shows a band at around
600 cmꢀ1, indicating that the ligand is coordinated to the metal
atoms [34]. This band is absent in the spectrum of the ligand, as
anticipated.
complexes with different transition metal ions using different
bridging ligands under various reaction conditions. Different tran-
sition metal ions produce complexes with this type of ligand with
varied nuclearity. Hexanuclear zinc [30], dinuclear copper [13], tet-
ranuclear copper [11–13,24,25], tetranuclear cobalt [35,36],
polymeric copper [18] etc. have been reported. Among these, tetra-
nuclear copper(II) complexes have a similar structural metal–
donor connectivity to complex 1.
Complex 1 crystallizes in the tetragonal space group with aceto-
nitrile. A perspective view of 1 with a partial atom numbering
scheme is shown in Fig. 1. Selected bond lengths and bond angles
are given in Table 2. It is found to be a discrete tetranuclear com-
plex of Cu(II). It has the crystallographic site symmetry 4-. Only
one copper atom is present in the asymmetric unit. Four copper
3.2. Description of the crystal structure of complex 1
An N2O donor Schiff-base ligand (HL), 4-methyl-2,6-bis(((2-tri-
fluoromethyl)phenyl)methyliminomethyl)phenol, was used to
obtain complex 1. HL has not been used to synthesize any other
transition metal complexes, but similar types of Schiff-base ligands
with an N2O donor system have been used to get multinuclear
atoms arrange themselves around the l4-oxido oxygen atom
(O1) in a distorted tetrahedral geometry. The metal–oxygen–metal
bond angles range from 103.23(5)° to 112.68(2)°. The copper atom
is in a pentacoordinated environment. It is bonded to the
l4-oxido
oxygen atom O1, one oxygen atom O2 and one nitrogen atom N1

104
M.S. Jana et al. / Polyhedron 59 (2013) 101–106
complex, the band at 347 nm is shifted to a lower energy, viz.
410 nm, suggesting that the nitrogen atom of the C@N group is
coordinated to the metal ion. This band may be attributed to a li-
gand to metal charge transfer transition. In addition to these two
bands, another broad band appears in the region 657–755 nm. This
band may be attributed to the d–d transition usually observed in
copper(II) complexes.
The emission spectra of the ligand and complex 1 were re-
corded in methanol at room temperature. The emission spectrum
of complex 1 is shown in Fig. 3. The free ligand shows an emission
band at 548 nm when excited at 347 nm, whereas complex 1
exhibits an emission band at 474 nm when excited at 410 nm.
We have also studied the fluorescence decay behavior of HL and
complex 1. The observed fluorescence decay fits well with a
bi-exponential nature for both the ligand and the complex (see
Supplementary information). We have used the mean fluorescence
lifetime (
tudes of the decay process) to study the excited state stability of
the complex. The fluorescence lifetime ( ) of the free ligand is
sf = a1s1 + a2s2, where a1 and a2 are the relative ampli-
s
2.99 ns and that of the metal bound ligand has been found to be
3.62 ns. The lifetime data of the compounds were recorded at
298 K in acetonitrile solution when excited at 370 nm.
1.5
0.4
Fig. 1. A perspective view of complex 1. Hydrogen atoms are omitted for clarity.
0.3
Table 2
0.2
Selected bond lengths (Å) and bond angles (°) of complex 1.
1.0
Cu1–O1
Cu1–O2
Cu1–O3
1.9243(7)
1.970(4)
2.259(6)
Cu1–O4
Cu1–N1
Cu1–Cu1
1.944(5)
1.988(6)
3.0164(14)
0.1
0.0
O1–Cu1–O4
O4–Cu1–O2
O4–Cu1–N1
O1–Cu1–O3
O2–Cu1–O3
97.50(18)
175.4(2)
92.6(3)
98.92(15)
89.2(2)
O1–Cu1–O2
O1–Cu1–N1
O2–Cu1–N1
O4–Cu1–O3
N1–Cu1–O3
78.44(14)
166.65(19)
91.1(2)
93.5(3)
89.1(2)
500
600
700
800
λ
/ nm
0.5
0.0
300
400
500
600
700
800
900
from the Schiff-base ligand (HL), and two oxygen atoms O3 and O4
from two different 2-bridging acetato ligands. The copper atom
Cu1 is in a distorted square pyramidal atmosphere, as revealed
by the trigonal index, . The value of has been found to be
λ
/ nm
l
Fig. 2. UV–Vis spectrum of complex 1 in methanol at room temperature.
s
s
0.146. The value of the trigonal index is defined as the difference
between the two largest donor–metal–donor angles divided by
60, a value which is 0 for an ideal square pyramid and 1 for an ideal
trigonal bipyramid geometry [37]. The O1, O2, N1 and O4 atoms
form the basal plane of the distorted square pyramid, whereas
O3 from the bridging acetate moiety occupies the apical position.
The Cu1–O3 bond distance (2.259(6) Å) is quite long compared to
other metal–donor bond distances. The metal–metal bond dis-
tances vary from 3.016 to 3.203 Å. All the bond distances are in
good agreement with the relevant published values [10,11].
700
350
3.3. UV–Vis and fluorescence spectra
The UV–Vis spectra of the ligand HL and complex 1 were re-
corded in methanol at room temperature. The ligand shows a peak
at 263 nm, indicating a p?
p⁄ transition. In the spectrum of the li-
0
400
gand, the band at 347 nm may be assigned to an n?p⁄ transition.
The UV–Vis spectrum of complex 1 is shown in Fig. 2. Complex 1
shows spectral bands at 265 and 411 nm. The band at 265 nm
500
600
700
λ / nm
may be assigned to a
the ligand in the metal complex. During the formation of the
p?
p⁄ transition, indicating the presence of
Fig. 3. Emission spectrum of complex 1 recorded in methanol at room temperature
(kex = 410 nm).

M.S. Jana et al. / Polyhedron 59 (2013) 101–106
105
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.008
0.006
0.004
0.002
0
50
100
150
200
250
300
T / K
Fig. 4. The variations of molar magnetic susceptibility (vM) and magnetic moment (leff) with temperature for complex 1.
3.4. Magnetic properties of complex 1
the exchange interactions thought these ligands could be ne-
glected. In addition, the Cu–Cu distances between the copper
atoms joined by the asymmetric carboxylate ligands is larger
(3.203 Å) than the copper atoms bonded by the oxo ligands
(3.016 Å), which also advises the ruling out the interaction be-
tween these copper atoms in order to avoid over-parametrization
effects. Thus, the six possible interactions can be reduced to only
two, similar to some of the above-mentioned models [39,40]:
Variable-temperature magnetic susceptibility measurements
were performed on a powdered crystalline sample of complex 1
at 1 T in the range 2.0–300 K. The variations of the molar magnetic
susceptibility (vM) and magnetic moment (leff) with temperature
for 1 are illustrated in Fig. 4. The magnetic moment at room tem-
perature is 2.86 lB smaller than the spin only value for four S = ½
spins by a tetramer unit (3.46
lB). The magnetic susceptibility
One exchange interaction (J1) mediated by the
bridging the Cu(II) atoms of the tetramer unit and the other one (J2)
through the 2-O oxygen belonging to the ligand L (Scheme 2b).
l
4-O2ꢀ oxygen atom
slowly decreases from room temperature until about 100 K and
then increase mainly from 25 K. This shape suggests the existence
of a maximum above room temperature. However, the magnetic
moment shows an intense decrease with decreasing temperature.
The magnetic exchange interactions between the metallic
l
Thus, the used spin Hamiltonian is:
H ¼ ꢀ2½J1ðS1 ꢂ S3 þ S1 ꢂ S4 þ S2 ꢂ S3 þ S2 ꢂ S4Þ þ J2ðS1 ꢂ S2 þ S3 ꢂ S4Þꢃ
atoms in
a cluster can be treated with the isotropic spin
Hamiltonian.
The magnetic susceptibility expression derived from this Hamilto-
nian is shown in the following equation
X
H ¼ ꢀ2 JijSi ꢂ Sj
ꢀ
ꢁ
g2Nb2
kT
10e2x þ 2eꢀ2x þ 4eꢀ2y
i<j
vCu ¼ ð1 ꢀ vp
Þ
þ
vp
5e2x þ 3eꢀ2x þ eꢀ4x þ 6eꢀ2y þ eꢀ4y
4
In a tetranuclear cluster of the type [M4(l4-oxido)] six magnetic
g2Nb2
3kT
interaction pathways are possible (Scheme 2a), but with all these
parameters magnetic analysis of this system is not simple. In order
to avoid over-parametrization, a simplification of this system is
ꢁ
SðS þ 1Þ þ TIP
with x = J2/kT; y = J1/kT
desirable. So, the study of the magnetic properties of [Cu4(l4-O)]
complexes is usually carried out using several approximations
[24,25,38–42]. The most used simplifications are the following:
(a) To consider only four types of magnetic interaction pathways,
as has been used in the fit of the magnetic data of the complex
[Cu4(L)(
l
4-O)]ꢂH2O (L = tetranucleating macrocyclic ligand, L6ꢀ
)
[38]. (b) To consider two exchange parameters, J1 and J2 as
used [39,40] in the fit of the magnetic data of the complexes
[L2Cu4(
l
4-O)(
l2-carboxylato)4] (carboxylato = acetato, benzoato)
and [Cu4(l4-O){(py)C(CN)NO}4(O2CMe2)]. (c) To consider the tetra-
nuclear unit as being formed by two dimers with S = ½ spins and
only one exchange parameter, J. This approach has been used in
the fit of the magnetic data of the complexes [Cu4(
1,3-O2CPh)4]ꢂ2MeOH (Hcip = 2,6-bis(cyclohexyliminomethylene)
-4-methylphenol) and [Cu4( 4-O)( -bip)2(
(Hbip = 2,6-bis(benzyliminomethyl)-4-methylphenol) [24,25].
In complex 1 each bridging acetate ligand displays a short
[1.933(6) Å] and a large [2.249(6) Å] Cu–O distance and therefore
l4-O)(l-cip)2
(l
l
l
l
-O2CPh)4]ꢂ0.5CH2Cl2
(a)
(b)
Scheme 2. (a) Possible magnetic interactions pathways in [Cu4(
Magnetic interactions considered in complex 1.
l
4-O)]. (b)

106
M.S. Jana et al. / Polyhedron 59 (2013) 101–106
In this equation two terms corresponding to the presence of a
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r
2 = 6.4 ꢁ 10ꢀ4. The experi-
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CCDC 793601 contains the supplementary crystallographic data
for article. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.04.042.