Magnetic and catalytic properties of a new copper(II)-Schiff base 2D coordination polymer formed by connected helical chains

Article abstract of DOI:10.1016/j.ica.2011.06.024

A dicyanamide bridged 2D polynuclear complex of copper(II) having molecular formula [Cu₂(L)(μ_1,5-dca)₂]_n (1) has been synthesized using the Schiff base ligand N,N′-bis(salicylidene)- 1,3-diaminopentane, (H₂

Full text of DOI:10.1016/j.ica.2011.06.024

Inorganica Chimica Acta 376 (2011) 245–254
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Inorganica Chimica Acta
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Magnetic and catalytic properties of a new copper(II)–Schiff base 2D
coordination polymer formed by connected helical chains
Dipali Sadhukhan a, Aurkie Ray a, Ray J. Butcher b, Carlos J. Gómez García c, Bülent Dede d, Samiran Mitra a,
⇑
a Department of Chemistry, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700032, West Bengal, India
b Department of Chemistry, Howard University, 2400 Sixth Street, NW Washington, DC 20059, USA
c Instituto de Ciencia Molecular ICMol, Universidad de Valencia, Parque Científico, 46980 Paterna, Spain
d Faculty of Sciences and Arts, Chemistry Department, Süleyman Demirel University, 32260 Isparta, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A
dicyanamide bridged 2D polynuclear complex of copper(II) having molecular formula [Cu2(L)
(l
1,5-dca)2]n (1) has been synthesized using the Schiff base ligand N,N0-bis(salicylidene)-1,3-diaminopen-
Received 10 February 2011
Received in revised form 2 June 2011
Accepted 10 June 2011
tane, (H2L) and sodium dicyanamide (dca). The complex presents a 2D hexagonal structure formed by
1,5-dca singly bridged helical chains connected through double 1,5-dca bridges. The chelating character-
istics of the H2L Schiff base ligand results in the formation of copper(II) dimer with a double phenoxo
Available online 16 June 2011
bridge presenting
a very strong antiferromagnetic coupling in the copper(II) derivative (1)
Keywords:
2D coordination polymer
Helical chains
Copper(II)–Schiff base complex
Antiferromagnetic interaction
Catecholase activity
(J = ꢀ510 cmꢀ1). The dimeric asymmetric unit of 1 is very similar to the active site of the catechol oxidase
and, as expected, also presents catalytic activity for the oxidation of 3,5-di-tert-butylcatechol to 3,5-di-
tert-butylquinone in presence of O2, as demonstrated by kinetic studies of this oxidation reaction mon-
itored by absorption spectroscopy resulting in high turnover number (Kcat = 259 hꢀ1).
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
to bridge transition metal–Schiff base complexes in order to ex-
plore and modify the magnetic properties and the network topol-
The design and synthesis of transition metal coordination poly-
mers are drawing the attention of coordination chemists not only
because of their interesting structural features as observed in the
metal-organic frameworks or porous coordination polymers but
also for their many possible applications in catalysis [1], magne-
tism [2], light emission and electron transport processes [3]. One
of the possible building blocks that can be used to construct these
materials are the salen type Schiff base ligands. In this context, we
[4–6] and others [7–12] have prepared a large number of metal
complexes with tridentate (N2O donor) and tetradentate (N2O2
donor) Schiff base ligands. These studies have shown the role
played by the different transition metal ions and the Schiff base li-
gands in the formation of complexes with different nuclearities,
topologies and dimensionalities. Thus, N2O2 donor Schiff bases as
N,N0-bis(salicylidene)-1,3-diaminopropane, N,N0-bis(salicylidene)-
ogy as well as to increase the dimensionality of the resulting
coordination polymers [17–19]. The flexibility in the coordination
modes shown by the dicyanamide anion (dca) [20] has been exten-
sively used to create many different molecular architectures [21–
25] with interesting magnetic properties ranging from weak anti-
ferromagnetic to strong ferromagnetic couplings and even long
range magnetic ordering [26–28]. Among the several coordination
modes of dca, the l1,5-bridging mode is by far the most usual one
with ca. 450 known structures out of the ca. 650 known com-
pounds containing dca either as ligand or as counter ion. In fact
this coordination mode has already been observed in various com-
plexes with NNO donor Schiff base by us and other research groups
[29–33].
Besides their magnetic properties, copper(II) complexes are also
extensively studied for their bio-catalytic activity since copper
plays a very important role in many biological systems. Thus, it
is usually found in metalloenzymes involved in processes like
hydroxylation, oxygen transport, electron transfer, and catalytic
oxidation [34–36]. Among these copper enzymes, catecholoxidases
are ubiquitous plant enzymes belonging to the oxido-reductase
class, with a dinuclear copper(II) center at the active site. This cat-
alyzes the two-electron oxidation of a broad range of o-diphenols
(catechols) to the corresponding quinones, coupled with the reduc-
tion of molecular oxygen to water (Scheme 1) [37,38]. This reaction
1,3-diaminopentane
and
N,N0-bis(2-hydroxyacetophenone)-
propylenediimine have shown a remarkable tendency to form
trinuclear phenoxo bridged complexes in presence of bridging
anions or coligands [4,13–16]. Pseudohalide coligands, such as cya-
nide, azide, thiocyanate, and dicyanamide (dca), have been utilized
⇑
Corresponding author. Tel.: +91 33 2414 2779; fax: +91 33 2414 6414.
E-mail address: smitra_2002@yahoo.com (S. Mitra).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.06.024

246
D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
2.2. Syntheses
2.2.1. Synthesis of the Schiff base ligand (H2L)
OH
OH
O
O
The Schiff base ligand, OHC6H4CH@NCH2CH2CH(CH2CH3)N@
CHC6H4OH [N,N0-bis(salicylidene)-1,3-diaminopentane (H2L)] was
prepared using a published procedure (Scheme 2) [15]. Anal. Calc.
for C19H22N2O2: C, 73.55; H, 7.09; N, 9.03. Found: C, 73.58; H, 7.11;
N, 9.04%.
catalyst
O2
+
Scheme 1. Oxidation of the 3,5-di-tert-butylcatechol by the catalyst.
2.2.2. Synthesis of the complex [Cu2(L)(l1,5-dca)2]n (1)
is of great importance in medical diagnosis for the determination
of the hormonally active catecholamines such as adrenaline,
noradrenaline and dopamine [39]. Quinones thus produced are
very much reactive to autopolymerize resulting in melanin, a
brown pigment, responsible to protect tissues against pathogens
and insects.
In order to synthesize 1 Cu(NO3)2ꢁ3H2O (0.362 g, 1.5 mmol) was
dissolved in 20 mL of warm methanol. About 10 mL of methanolic
solution (yellow) of the Schiff base (H2L) (0.354 g, 1 mmol), was
added to it drop wise with stirring. An aqueous solution of sodium
dicyanamide, Na(dca) (0.133 g, 1.5 mmol) was added drop wise
into the stirring solution. After dca addition was completed the
mixture was allowed to stir for 5 min with gentle warming to avoid
precipitation of the complex. The dark green solution was filtered
and kept at room temperature for crystallization by slow evapora-
tion for overnight to yield brown prismatic crystals suitable for X-
ray crystallography. Crystals were isolated by filtration and air-
dried. Yield: 0.2715 g (75%). Anal. Calc. for C23H20Cu2N8O2: C,
48.63; H, 3.52; N, 19.73. Found: C, 48.70; H, 3.59; N, 19.82%.
In the isolated catecholoxidases, the copper was found to be
EPR-silent and was assigned to an antiferromagnetically spin-cou-
pled copper(II)–copper(II) pair [40]. XAS investigations on the na-
tive met forms of catecholoxidases from Lycopus europaeus and
Ipomoea batatas have revealed that the active site consists of a
dinuclear copper(II) core [41], 2.9 Å apart and are bridged by a
hydroxide ion. Each metal is further coordinated by three histidine
residues. Although the active site composition of different forms of
catechol oxidases are established from X-ray crystallography, its
catalytic mechanism is not fully understood yet. Krebs and his
group proposed a monodentate asymmetric coordination of the
substrate to the dicopper core of the enzyme where as a bidentate
bridging substrate coordination was predicted by Solomon and his
group [34–36]. After that Mukherjee and Mukherjee reported that
copper(II) complexes are reduced to copper(I) during catalysis and
electron transfer from catechol to the copper(II) complex occurs
via the formation of a copper–catechol intermediate [38]. For the
present work the coordination environment of the copper(II) com-
2.3. Physical measurements
The Fourier transform infrared spectra were recorded in the
range 4000–400 cmꢀ1 on a Perkin-Elmer RX I FT-IR spectropho-
tometer with solid KBr pellets. The electronic spectra were
recorded at 300 K on a Perkin-Elmer Lambda 40 (UV–Vis) spec-
trometer by diffuse reflectance technique using paraffin oil matrix
in a 1 cm quartz cuvette in the range 200–800 nm. Electrochemical
measurements were performed at 20 °C on a VersaStat-Potentio-
Stat II cyclic voltammeter using HPLC grade DMF as solvent where
tetrabutylammonium perchlorate was used as supporting electro-
lyte. Platinum and saturated calomel electrodes (SCE) were the
working and the reference electrodes in the process, respectively.
C, H and N microanalyses were carried out with a Perkin-Elmer
2400 II elemental analyzer. The magnetic susceptibility measure-
ments were carried out in the temperature range 2–300 K with
an applied magnetic field of 0.1 T on a polycrystalline sample of
1 (mass = 43.76 mg) with a Quantum Design MPMS-XL-5 SQUID
susceptometer. The isothermal magnetization was performed on
the same sample at 2 K with magnetic fields up to 5 T. The suscep-
tibility data were corrected for the sample holder previously mea-
sured using the same conditions and for the diamagnetic
contributions of the salt as deduced by using Pascal’s constant ta-
plex [Cu2(L)(l1,5-dca)2]n (1) resembles that of the catecholoxidase
as the asymmetric unit contains strongly antiferromagnetically
coupled dicopper metal core coordinated by N/O donor ligand.
Moreover the penta coordinated copper(II) centers are able to pro-
vide vacant coordination sites for the binding of 3,5-di-tert-butyl-
catechol (3,5-DTBC) in a bridging mode. Hence this structural
similarity prompted us to investigate whether 1 can act as a model
system mimicking the activity of catecholoxidase.
In the context of these two potential applications, here we re-
port the synthesis, X-ray crystal structure and physicochemical
properties of a new polymeric copper(II) complex formed with a
N2O2 donor Schiff base ligand and dicyanamide (dca) as coligand,
formulated as [Cu2(L)(l1,5-dca)2]n (1). Thus, the copper(II) deriva-
dia = ꢀ312.8 emu molꢀ1). The spectrophotometric measure-
tive (1) represents a 2D coordination polymer formed by chiral
helical chains connected through double dca bridges to form a hex-
agonal layer with very strong antiferromagnetic interactions and
catecholoxidase-like activity.
bles (
v
ments for the catalytic studies were carried out with a Perkin-
Elmer Lambda 20 UV–Vis spectrophotometer. Mass spectra of 1
was analyzed in a Qtof Micro YA263 mass spectrometer.
2. Experimental
2.4. X-ray crystallography
2.1. Materials
The X-ray diffraction study for 1 was carried out at 110(2) K on
a brown prismatic single crystal (0.42 ꢂ 0.18 ꢂ 0.15 mm3). The
intensity data of 1 were collected with ‘Xcalibur, Ruby, Gemini’ dif-
All solvents were of reagent grade and used without further
purification. 1,3-Diaminopentane and salicylaldehyde were pur-
chased from Aldrich Chemical Company. Sodium dicyanamide
was bought from Fluka. Copper nitrate trihydrate was purchased
from E. Merck, India and was used as received. The chemicals
3,5-DTBC (3,5-di-tert-butylcatechol), MES (2-(N-morpholino)
ethanesulfonicacid), Hepes (2-[4-(2-hydroxyethyl)piperazin-1-
yl]ethanesulfonicacid) and Tris (2-amino-2-hydroxymethyl-pro-
pane-1,3-diol) used in catalytic activity studies were purchased
from Acros Organics.
fractometer device using
x scan technique with Cu Ka radiation
(k = 1.54184 Å). The structure was solved by direct methods
using the SHELXS-97 program and refined by full-matrix least-
squares methods in SHELXL-97 program [42]. All non-hydrogen
atoms were refined anisotropically by full-matrix least-squares
based on F2. The H atoms were generated geometrically and were
included in the refinement in the riding model approximation. The
lattice constants were refined by least-square refinement using
10 463 total reflections (4.44 < h < 74.02°), 4618 unique reflection

D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
247
CH3
CH3
Refluxed for 2 h
N
N
+
OHC
H2N
NH2
OH
1 mmol
2 mmol
OH
HO
H2L
-2H+
CH3
N
N
O-
-O
N2O2 donor set with phenoxo bridging character
Scheme 2. Synthesis of the Schiff base H2L and its coordination mode.
(Rint = 0.0244). Structure solution and refinement based on 4618
observed reflections with I > 2 (I) and 337 parameters gave final
R = 0.0609, wR = 0.1557 with goodness-of-fit 1.101. Selected crys-
tallographic data, experimental conditions and relevant features
of the structural refinements for the complex are summarized in
Table 1.
were evaluated from Lineweaver–Burk double reciprocal plots. To
reduce the statistical error, every measurement was carried out
at least thrice.
r
3. Results and discussion
3.1. Fourier transform infrared spectroscopy
2.5. Methodology for catecholase-like activity of 1
The IR spectrum of 1 was analyzed and compared with that of
the free ligand H2L in the region 4000–400 cmꢀ1. A strong sharp
absorption band around 1631 cmꢀ1 in the spectrum of the Schiff
base ligand can be assigned to the C@N stretching. Upon complex-
ation with the metal, this band is shifted to 1618 cmꢀ1 in 1 as a re-
sult of the coordination of the imine nitrogen to the metal center
[43–45]. The ligand showed a well defined band at 3537 cmꢀ1
due to O–H stretching which disappeared in the complex, indicat-
ing the deprotonation of the Schiff base ligand upon complexation.
The strong phenolic C–O absorption band at 1214 cmꢀ1 observed
in the spectrum of the protonated free ligand, H2L, is shifted to
lower frequencies (1156 cmꢀ1 in 1), supporting the deprotonation
and the coordination of the phenolic oxygen atoms to the metal
centers, as confirmed by the structure of the complex [45].
Complex 1 showed sharp absorption bands in the region 2160–
2380 cmꢀ1 which indicate the presence of the dicyanamide ligand.
The dicyanamide anion in NaN(CN)2 showed three sharp and
strong characteristic bands in the frequency region 2290–
The catalytic oxidation of the model substrate 3,5-di-tert-butyl-
catechol (3,5-DTBC) by 1 was evaluated spectrophotometrically in
dioxygen-saturated DMF along with blank experiments in pres-
ence of dioxygen. A pH-dependence study was carried out to deter-
mine the pH value at which catecholase activity is maximum. For
this pH dependence studies, catalyst aqueous solutions were pre-
pared with MES (pH 5.5, 6.0 and 6.5), Hepes (pH 7.0, 7.5, 8.0 and
8.5) and Tris (pH 9.0) and standardized NaOH or HNO3 were mixed
with equal volumes of 3,5-DTBC solution in DMF. In all cases, a ra-
pid increase in the reaction rate is observed above pH ꢃ 7.5. To re-
duce the influence of the deprotonation of 3,5-DTBC at high pH, the
catalytic activity experiments were performed at pH 8.0 and to re-
move the effect of pH on the spontaneous reaction the same solu-
tion was used without adding the complex as an internal reference.
The catalytic oxidation rate of 3,5-DTBC (3,5-di-tert-butylcate-
chol) by 1 was monitored spectrophotometrically recording the in-
crease in absorbance at 395 nm, corresponding to the formation of
the quinone product 3,5-DTBQ (3,5-di-tert-butyl-o-benzoquinone).
The initial reaction rates were determined from the slope of the
trace at 395 nm during the first 30 min of the reactions, when
the absorption at 395 nm increases linearly. To determine the
dependence of the rates on the substrate concentration and various
kinetic parameters, 5.75 ꢂ 10ꢀ5 M solution of 1 was treated with
10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 equivalent of 3,5-DTBC
in DMF under aerobic condition. A kinetic treatment on the basis
of the Michaelis–Menten approach was applied and the results
2170 cmꢀ1 which are attributed to mas
+ ms (C„N) combination
modes (2286 cmꢀ1), mas (C„N) (2232 cmꢀ1
) and ms (C„N)
(2179 cmꢀ1) [20]. Upon complexation these bands shift towards
higher frequencies along with further splitting. In 1 the doubly
splitted mas
+ ms (C„N) bands were located at 2371 (m, with shoul-
der on the high frequency side) and 2306 (s) cmꢀ1. The mas (C„N)
band appeared at 2240 (s) cmꢀ1 and the bifurcated
ms (C„N) bands
were detected at 2172 (s) and 2169 (s) cmꢀ1 (s, strong; m, med-
ium). The splitting and displacement of the bands towards higher

248
D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
Table 1
Table 2
Crystal data for [Cu2(L)(
l1,5-dca)2]n (1).
Cyclic voltammetry data for the red-ox process of 1 at platinum electrode.
Formula
Formula weight
T (K)
C23H20Cu2N8O2
567.55
110(2)
1.54184
monoclinic
P2(1)/c
Scan rate
Epc
Epa
D
Ep
Eꢄ1=2
V
(mV sꢀ1
)
V (vs. SCE)
V (vs. SCE)
V
Step 1
10
20
50
100
k (Å)
ꢀ0.468
ꢀ0.569
ꢀ0.651
ꢀ0.728
ꢀ0.026
0.442
0.560
0.642
0.719
ꢀ0.247
ꢀ0.289
ꢀ0.329
ꢀ0.368
Crystal system
Space group
Unit cell dimensions
a (Å)
ꢀ0.00875
ꢀ0.00875
ꢀ0.009
9.7603(3)
b (Å)
c (Å)
12.0653(3)
19.9097(6)
90
Step 2
10
20
ꢀ1.123
ꢀ1.164
ꢀ1.141
ꢀ1.029
ꢀ1.052
0.071
0.129
–
ꢀ1.087
a
(°)
ꢀ1.035
ꢀ1.099
b (°)
91.720(3)
90
2343.53(12)
4
1.609
50
100
–
–
–
–
c
(°)
–
V (Å3)
D
Ep = Epa ꢀ Epc, Eꢄ1=2 ¼ ðEpa þ EpcÞ=2;
D
Eꢄ1=2 ¼ Eꢄ1=2 ðstep 1Þ ꢀ E1ꢄ=2 ðstep 2Þ.
Z
2
Kcom ¼ ½CuIICuIꢅ =½CuIICuIIꢅ½CuICuIꢅ ¼ exp½nFð
D
Eꢄ1=2Þ=RTꢅ ¼ 2:84 ꢂ 1014 (for scan rate
Dcalc (mg/m3)
10 mV sꢀ1) and 8.66 ꢂ 1013 (for scan rate 20 mV sꢀ1).
Absorption coefficient (mmꢀ1
F(0 0 0)
)
2.562
1152
Crystal size (mm3)
h (°)
0.42 ꢂ 0.18 ꢂ 0.15
4.44–74.02
ꢀ12 6 h 6 12, ꢀ11 6 k 6 14,
ꢀ24 6 l 6 20
10 463
4618 [Rint = 0.0244]
97.2%
semi-empirical
1.00000 and 0.71030
reductions via two one electron charge transfer steps with a sepa-
ration of ca. 0.35–0.80 V.
Index ranges
Reflections collected
þe
Step 1 : CuIICuII ꢀ CuIICuI
Independent reflections
Completeness to h = 74.02°
Absorption correction
Maximum and minimum
transmission
ꢀe
þe
Step 2 : CuIICuI ꢀ CuICuI
ꢀe
Refinement method
full-matrix least-squares on F2
4618/9/337
1.101
R1 = 0.0609, wR2 = 0.1557
R1 = 0.0677, wR2 = 0.1592
0.827 and ꢀ0.658
It is also seen that the second step is electrochemically reversible at
slow scan rates but not at high scan rates whereas the first step has
a greater probability of being electrochemically reversible than the
second step [52]. It is examined for phenoxo bridged binuclear mac-
rocyclic complexes [55] that the first couple (CuIICuII) shows near
Nernstian behavior while the second couple (CuIICuI) undergoes
adsorption at the electrode. For the present complex (1) containing
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r(I)]
Largest difference peak and hole
(e Åꢀ3
)
a bis(l
1,1-phenoxo)bridged dinuclear CuII core, during the cathodic
scan two well separated reduction peaks (A and B in Fig. 1) corre-
sponding to the steps 1 and 2 were observed at all scan rates (Table
2). During the reverse scan two quasi reversible oxidation peaks
were observed at 10 and 20 mV sꢀ1 scan rates (B0 and A0 in Fig. 1)
which correspond to the reverse processes of steps 2 and 1, respec-
frequencies clearly show the bidentate bridging mode of dca ligand
[46] in 1 as is evidenced by the X-ray crystal structure. The ligand
coordination to the metal center is substantiated by a band appear-
ing at 450 cmꢀ1 in 1 which is mainly attributed to mCu–N in the
complex.
3.2. UV–Vis spectroscopy
UV–Vis spectrum of the complex was recorded at 300 K with
the diffuse reflectance technique in paraffin oil matrix. Three dis-
tinct CT bands appeared in the spectrum of 1 at 209, 263 and
345 nm. The high energy band at 209 nm in 1 can be considered
as an aromatic
p–
p⁄ intraligand CT transition [47]. The band at
263 nm can be attributed to the n–p⁄ transition within the ligand
[48]. The L ? M charge transfer transition band appeared around
345 nm [47]. For a square pyramidal copper(II) environment the
one electron orbital sequence is dxz ¼ dyz < dxy < dz2 < dx2
and
ꢀy2
generally a broad visible band appears in the range 550–660 nm.
In contrast for a TBP complex the d–d band usually appears at
k > 800 nm [49–51]. A broad d–d transition band of much weaker
intensity was found in the spectrum of 1 at 670 nm supporting that
the copper(II) centers have square-pyramidal geometry.
3.3. Cyclic voltammetry
The electrochemical behavior of 1 was studied in DMF medium
at platinum electrode with tetrabutylammonium perchlorate as
supporting electrolyte at 10, 20, 50 and 100 mV sꢀ1 scan rates.
The electrochemical data are summarized in Table 2 and the cyclic
voltammograms at 10 and 50 mV sꢀ1 scan rates are shown in Fig. 1.
It is seen in the literature [52–54] that binuclear CuII complexes
with endogenous diphenoxo bridges undergo electrochemical
Fig. 1. Cyclic voltammogram of 1 in DMF at a platinum electrode at 20 °C, black: at
10 mV sꢀ1 scan rate, red: at 50 mV sꢀ1 scan rate. (For interpretation of the
references to color in this figure legend, the reader is referred to the web version
of this article.)

D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
249
tively. At higher scan rates (50 and 100 mV sꢀ1) no oxidation peak
was observed for the reverse process of step 2. Ep increases at
D
higher scan rates (Table 2), showing the quasi reversible nature of
the electron transfer processes. All these behaviors are in accor-
dance with the redox properties of similar kind of doubly phenoxo
bridged dinuclear CuII complexes reported previously [52,55].
The stability of the mixed valent species (CuIICuI species) in the
equilibrium mixture is rationalized by the comproportionation
constant (Kcom), which is 2.84 ꢂ 1014 for 10 mV sꢀ1 and is
8.66 ꢂ 1013 for 20 mV sꢀ1 scan rates (Table 2). The Kcom values
are in good agreement with those reported for doubly phenoxo
bridged dinuclear copper complexes [56,57] and signify consider-
able interaction between the two phenoxo bridged copper centers
in 1 [58].
3.4. ESI mass spectrum of 1
Positive ion electrospray mass spectrum of 1 in dilute acetoni-
trile solution give two intense signals at m/z 372 and m/z 765
(Fig. 2). The former signal corresponds to the species [LCu]+ and
the latter can be attributed to a combined species [{Cu2(L)(l1,5
-
dca)2}Cu(dca)2]+. The structural assignments for such ions are also
supported by their characteristic isotopic distributions particularly
affected by the presence of copper (63Cu:65Cu of 1:0.44). It is
Fig. 3. ORTEP view of the asymmetric unit of 1. Displacement ellipsoids drawn at
the 40% probability level. Suffix A symmetry code: ꢀx, y + 1/2, ꢀz + 1/2; suffix B
symmetry code: ꢀx, ꢀy + 1, ꢀz.
possible that a portion of the molecular ion [Cu2(L)(l
1,5-dca)2]+
undergoes fragmentation in two components [LCu and Cu(dca)2].
The Cu (dca)2 component being associated with the molecular
ion [Cu2(L)(l
1,5-dca)2]+ appear as a signal at m/z 765. It is not very
and bond angles around the metal centers are listed in Tables 3
and 4, respectively. The asymmetric unit of 1 contains two diposi-
tive Cu ions, a Schiff base ligand connecting both metal ions
through a double phenoxo bridge and two independent dca anions
that connect each copper(II) dimer with its three closest neighbors.
certain that whether this fragmentation occurs due to dissociation
mechanism in acetonitrile solution or during the ionization process
within the mass spectrometer. If the molecular fragmentation of 1
occurs in the gas phase during the ionization process [59] then it is
possible that the solid state structure of 1 retains in solution also.
On the other hand it is also reportrd that ESI-MS is a very gentle
technique to transfer preformed ions from solution to the gas
phase [60]. Hence there is another possibility that the fragmenta-
tion occurs during solvation in acetonitrile and a mono metallic
All dca ligands are bridging in the l1,5 mode and generate a 2D
structure formed by interconnected dimers with two types of in-
ter-dimer connections: a single dca bridge (centered at N5) con-
necting each dimer with two neighboring copper(II) dimers along
the y-direction and a double one (centered at N2) connecting each
dimer with its closest copper(II) dimer along the z-direction
(Fig. 4).
species [LCu] and
a trimetallic species [{Cu2(L)(l1,5-dca)2}Cu
(dca)2] exist in equilibrium. Then it is evident that at least the
phenoxo bridged dinuclear core of the asymmetric unit retains in
solution.
The dimeric asymmetric unit can be formulated as [Cu2(L)(l1,5
-
dca)2] and it comprises a dimetallic core consisted of two five coor-
dinate metal centers linked by the phenoxo oxygen atoms of the
Schiff base. The metal centers (Cu1 and Cu2) enclosed by the Schiff
base are closest to square pyramidal geometry as defined by the
3.5. Crystal structure of [Cu2(L)(l1,5-dca)2]n (1)
Addison parameter,
= |b ꢀ |/60° where b and
central atom; = 0 and 1 for perfect square pyramidal and trigonal
s
= 0.13 and 0.14 for Cu1 and Cu2, respectively,
The ORTEP diagram of the asymmetric unit of the complex
[Cu2(L)( 1,5-dca)2]n (1) is shown in Fig. 3 and the bond distances
[s
a
a
are the two largest angles around the
l
s
bipyramidal geometries, respectively] [61]. In 1 the Cu1 is chelated
by the deprotonated Schiff base ligand (L2ꢀ) through its two phen-
oxo oxygen atoms (O1 and O21) and two imino nitrogen atoms (N9
and N13) (Fig. 5). The axial position of this Cu1 ion is occupied by
Table 3
Selected bond distances (Å) for 1.
Cu1–O21
Cu1–N9
Cu1–O1
Cu1–N13
Cu1–N3#1
Cu1–Cu2
Cu2–N1
Cu2–N4
Cu2–N6#2
Cu2–O1
1.948(3)
1.953(5)
1.965(3)
1.986(6)
2.347(4)
2.9737(9)
1.958(4)
1.980(4)
2.154(4)
1.994(3)
1.998(3)
Cu2–O21
Fig. 2. Positive ion ESI mass spectrum of 1 in acetonitrile solution.

250
D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
Table 4
Selected bond angles (°) for 1.
O21–Cu1–N9
O21–Cu1–O1
N9–Cu1–O1
167.77(19)
75.45(13)
92.76(19)
91.0(2)
99.5(2)
160.1(2)
89.20(15)
94.32(17)
90.79(15)
103.82(18)
94.82(16)
158.07(15)
89.56(15)
92.45(15)
149.84(15)
73.71(14)
100.10(16)
102.60(17)
99.89(14)
104.85(15)
O21–Cu1–N13
N9–Cu1–N13
O1–Cu1–N13
O21–Cu1–N3#1
N9–Cu1–N3#1
O1–Cu1–N3#1
N13–Cu1–N3#1
N1–Cu2–N4
N1–Cu2–O1
N4–Cu2–O1
N1–Cu2–O21
N4–Cu2–O21
O1–Cu2–O21
N1–Cu2–N6#2
N4–Cu2–N6#2
O1–Cu2–N6#2
O21–Cu2–N6#2
Symmetry transformations used to generate equiva-
lent atoms in 1: #1 ꢀx, ꢀy + 1, ꢀz; #2 ꢀx, y + 1/2,
ꢀz + 1/2; #3 ꢀx, y ꢀ 1/2, ꢀz + 1/2.
Fig. 4. View of the 2D structure of 1 showing the interconnected dimers. Only the
dca units and the coordination sphere of the copper(II) ions are shown for clarity.
the N3 atom of dca bridge connecting the Cu1 ion with the Cu2 ion
of a neighboring dimer. In the structure of 1 Cu1 is 0.17 Å off the
square plane defined by O1, N9, N13 and O21. The square plane
around Cu2 is formed by two bridging phenoxo oxygens (O1 and
O21) from the deprotonated Schiff base ligand (L2ꢀ) and two nitro-
gen ends (N1 and N4) of two l1,5-bridging dca anion and another
dicyanamide nitrogen end (N6) occupies the apical position. The
Cu2-axial dca N bond is elongated due to Jahn–Teller distortion
[2.154(4) Å]. Cu2 is 0.406 Å off the mean square plane passing
through O1, N4, N1 and O21 displaced towards N6.
The four intradimer Cu–O bond distances are in the range
1.948–1.998 Å in 1 and the two Cu–O–Cu bond angles are very
similar (97.36° and 97.78°) in 1 with a dihedral angle in the central
Cu2O2 unit of ca. 29.6°. The Cuꢁ ꢁ ꢁCu separation is 2.9737(9) Å in 1.
Here the bridging character of dca is worth mentioning. The N3
Fig. 5. Coordination sphere of the two Cu ions in the copper(II) dimer of 1 showing
the main bond distances (in Å) and angles (in °).
atom of a
l1,5-bridging dca which occupies the axial position of
Cu1 ion connects that Cu1 ion with the Cu2 ion of a neighboring
dimer (A). The N4-containing dca bridge connects the Cu2 with an-
other Cu2 atom of the neighboring dimer B (Fig. 6). Finally, the N6
containing dca bridge connects Cu2 with a Cu2 atom of a third
ple, the
v
mT product linearly decreases down to ca. 150 K to reach
mT
a value of ca. 0.015 emu K molꢀ1 (Fig. 8). Below 150 K the
v
neighboring [Cu2(L)(l1,5-dca)2] dimer (C). Thus each dimer is con-
product remains almost constant down to ca. 50 K and below this
temperature it shows an additional decrease to reach a value of ca.
0.005 emu K molꢀ1 which remains almost constant down to 2 K.
This behavior indicates that 1 presents a strong antiferromagnetic
coupling, which is already operative at room temperature, respon-
nected to its three neighboring dimers (A, B and C) through a dou-
ble dca bridge (with dimer A) and through two single dca bridges
(with dimers B and C) in form of six membered metallacyclic rings
(counting each dimetallic core as one node). These six membered
rings extend in two dimensions giving rise to an undulating sheet
in the 1 0 0 plane shown in Fig. 4. Between the undulating sheets
the benzene rings in the Schiff base interdigitate but in 1 the cen-
sible for the linear decrease observed in the
vmT product from
room temperature down to ca. 150 K. The residual value observed
in the temperature range 150–50 K can be attributed to the pres-
ence of a small amount of ‘‘monomeric’’ impurities, as is customary
in many polymeric copper(II) complexes. This ‘‘monomeric’’ impu-
rity is further coupled through a weaker antiferromagnetic interac-
tion, responsible for the additional decrease observed below 50 K.
This behavior is clearly confirmed by the thermal variation of vm
that shows a broad rounded maximum at ca. 300 K, corresponding
to a strong antiferromagnetic coupling and a second maximum at
ca. 40 K that corresponds to a weaker antiferromagnetic coupling
between the monomeric impurities (inset in Fig. 8). At low temper-
atures vm shows a divergence corresponding to the presence of a
small amount of paramagnetic isolated impurities.
troid to centroid distance exceeds 3.9 Å which is too long for p–p
interactions. The connectivity of the single dca bridges from Cu2
to symmetry equivalent centers can be viewed as a helical chain
running parallel to the y axis. As expected from the achiral space
group, there is an equal number of right and left handed helical
chain (Fig. 7).
3.6. Magnetic properties of 1
The thermal variation of the molar magnetic susceptibility per
copper(II) dimer times the temperature (vmT) for 1 shows at room
temperature a value of ca. 0.16 emu K molꢀ1 (Fig. 8). This value is
much lower than the expected one (ca. 0.75 emu K molꢀ1) for
two isolated copper(II) S = 1/2 ions. When cooling down the sam-
According to the crystal structure, 1 consists of copper(II) di-
mers where the copper(II) ions are connected through double
phenoxo bridges connecting two equatorial positions of the square

D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
251
0.20
0.0010
0.0008
0.0006
0.15
0.0004
0.0002
0.10
0.0000
0
50
100 150 200 250 300
T (K)
0.05
0.00
0
50
10 0
150
T (K)
200
250
300
Fig. 8. Thermal variation of the
vmT product per copper(II) dimer for 1. Inset shows
the thermal variation of m. Solid line is the best fit to the S = 1 dimer model.
v
Fig. 6. Perspective view of 1 showing a complete Cu dimer and the four Cu atoms
that are connected to it. Color code: N = blue, C = brown, O = pink, Cu = green. Only
the major component of the disordered ethyl group is shown and H atoms are
omitted for clarity. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
ent an antiferromagnetic coupling, this extra contribution is very
difficult to simulate since it corresponds to a very small mono-
meric fraction and the coupling constant is also much smaller than
the intradimer one. Although it is possible to fit the magnetic prop-
erties in the whole temperature range (with quite similar parame-
ters, except the paramagnetic impurity that decreases from 2.1% to
ca. 0.7%), the weak antiferromagnetic coupling appearing as a max-
imum at ca. 40 K is not well reproduced (dashed line in Fig. 8). The
low g value found can be attributed to the very low signal of the
sample at high temperatures, as a consequence of the very high
antiferromagnetic coupling. This low signal precludes an accurate
determination of the true contribution of the sample since the dia-
magnetic contributions are very similar to the paramagnetic one at
high temperatures.
The isothermal magnetization at 2 K shows a very small satura-
tion value at 5 T (ca. 0.014 lB, coming from the small amount of
paramagnetic impurity), confirming the strong antiferromagnetic
coupling in this compound.
As already described, 1 can be considered as a copper(II) dimer
with a double equatorial–equatorial phenoxo bridge (Fig. 5). This
kind of bridge is well known to provide strong antiferromagnetic
coupling since the magnetic orbitals (x2–y2) present a good overlap
with the orbitals of the oxygen atoms. In fact, magneto-structural
correlations for copper(II) complexes with double phenoxo bridges
indicate that the magnetic exchange linearly depends on the Cu–
O–Cu bond angle and on the Cu–Cu distance. The larger the Cu–
O–Cu bond angle, the stronger the antiferromagnetic coupling,
with a crossing point at ca. 95.7° [64–66]. In 1 if we apply these
magneto-structural correlations to the two Cu–O–Cu bridging an-
gles (ca. 97.8° and 97.4°) we expect a coupling constant of ca.
ꢀ450 cmꢀ1, in agreement with the experimental value found in
this compound (ꢀ510 cmꢀ1). Regarding the Cu–Cu distance, the
application of the magneto structural correlations to the Cuꢁ ꢁ ꢁCu
distance in 1 (2.9736(8) Å) leads to an expected coupling of ca.
ꢀ400 cmꢀ1, also in quite good agreement with the experimental
value.
Fig. 7. View of the two helical chains with different chirality in 1. Only the Cu1 and
the single dca bridges are shown for clarity.
pyramidal copper(II) ions. These copper(II) dimers are further con-
nected through dicyanamide (dca) bridges to three neighbor di-
mers to construct a hexagonal-2D undulating sheet-like polymer
(Fig. 4). This structure leads to a quite complex 2D exchange path-
way with at least three different exchange constants since one of
the three interdimer connections is made through two equivalent
equatorial–axial dca bridges (centered at N2) whereas the other
two interdimer connections are made through equivalent single
equatorial–axial dca bridges (centered at N5). Fortunately, to a first
approximation, we can consider that the intradimer double phen-
oxo bridges connecting two equatorial positions give rise to a
much stronger (antiferromagnetic) coupling since they present a
strong overlap with the magnetic (x2–y2) orbitals on both cop-
per(II) ions. Assuming that this coupling is by far the most impor-
tant one, we can neglect the dca bridges and fit the magnetic
properties to the simple Bleaney–Bowers model for an S = 1/2 di-
mer plus a monomeric S = 1/2 contribution [62,63]. This model sat-
isfactorily reproduces the magnetic properties of 1 above 50 K with
3.7. Catecholase-like activity of 1
Among the different catechols used in catecholoxidase model
studies, 3,5-di-tert-butylcatechol (3,5-DTBC) is the most widely
used substrate due to its low redox potential for the quinone
–catechol couple, which facilitates its oxidation to the correspond-
ing quinone 3,5-di-tert-butylquinone (3,5-DTBQ), and to the pres-
ence of bulky substituents, that slow down further oxidation
g = 1.854(9), J = ꢀ734(6) K = ꢀ510(4) cmꢀ1 and
a paramagnetic
impurity of 2.1% (solid line in Fig. 8). Note that although at low
temperatures the small amount of paramagnetic impurities pres-

252
D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
reactions such as ring opening. The detection of the oxidation of
3,5-DTBC to the corresponding 3,5-DTBQ can be easily followed
by the development of the absorption band at about 400 nm
[67,68].
Catalytic studies were performed at 25 °C in N,N-dimethylform-
amide solution owing to the good solubility of the complex as well
as substrate. Blank experiments showed that in absence of catalyst
the transformation of 3,5-DTBC to 3,5-DTBQ does not take place.
Evaluating the catalytic property of 1, we note that it exhibits con-
siderable catecholase activity in presence of dioxygen. Therefore,
initially, a pH-dependence study was carried out to determine
the pH value at which catecholase activity is at a maximum.
Although in all cases, catecholase activity was observed, 1 exhibits
almost no catalytic activity below pH 7.5 with a fast increase of the
reaction rate above this pH value (Fig. 9). Hence, the catalytic activ-
ity experiments were performed at pH 8.0. The pH dependence of
the catechol oxidation reaction is due to the fact that at alkaline pH
the water molecules present in the medium (present in solvent or
in buffer or generated by the reduction of O2 during the catalytic
cycle) form hydroxide ions which help in deprotonation of the
3,5-DTBC substrate. Thus the catecholate anions coordinate to
the Cu(II)–Cu(II) moiety of complex 1 either by coordinating at
the sixth coordination sites of the two Cu(II) ions or by replacing
the dca ligands [69,70]. In order to minimize the effect of pH on
the spontaneous reaction, the same solution was used without
adding the complex as an internal reference.
The kinetic studies on the oxidation of 3,5-DTBC were carried
out by the method of initial rates by monitoring the increase in
the characteristic quinone (3,5-DTBQ) absorption band at 395 nm
as a function of time (Fig. 10). A linear relationship for the initial
rates and complex concentration is observed, meaning a first-order
dependence on the catalyst concentration for the system. At low
concentrations of 3,5-DTBC a first-order dependence of the sub-
strate concentration could be observed. Also saturation kinetics
was found at high substrate concentrations. In order to determine
the kinetic parameters, the Michaelis–Menten approach which was
originally developed for enzyme kinetics, was applied. The results
were evaluated from Lineweaver–Burk double reciprocal plots
from which the parameters such as maximum velocity
(VMAX = 1.8 ꢂ 10ꢀ7 M sꢀ1), Michaelis binding constant (KM = 7.8 ꢂ
10ꢀ4 M) and the turnover number (Kcat = 259 hꢀ1) which is actually
the dissociation constant for the complex–substrate intermediate
were obtained for 1. The turnover number for catechol oxidase is
8254.8 ꢂ 103 hꢀ1. Though 1 showed much less turnover number
compared to catechol oxidase the value is quite comparable to
the dinuclear copper(II) complexes reported in the literature
[38,70,71].
Fig. 9. Dependence of the reaction rates on pH for the oxidation of 3,5-DTBC
catalyzed by 1. The reactions were performed in DMF/aqueous buffer saturated
with O2 [MES (pH 5.5, 6.0 and 6.5), Hepes (pH 7.0, 7.5, 8.0 and 8.5) and Tris (pH
9.0)], [complex] = 5.75 ꢂ 10ꢀ5 M and [3,5-DTBC] = 2.87 ꢂ 10ꢀ3 M at 25 °C.
Some of the crucial factors dictating the catecholase activity can
now be highlighted based on many investigations correlating var-
ious structural parameters of the dicopper(II) complexes with their
activity; viz., the distance between the copper(II) centers [72,73],
the nature of the bridging group between the copper ions
[38,69,74], electronic properties of the complexes [34–41,75,76]
and the geometric change of the dicopper core [77]. Furthermore,
the factors such as the number of donor sites, nature of the donors,
and the rigidity of the ligand or the bridging moiety imposing
strain in the complex have also been found to play a vital role [67].
Dinuclear copper complexes are generally more reactive to-
wards the oxidation of catechols than are the corresponding mono-
nuclear-species [78] and these complexes catalyze oxidation if the
Cuꢁ ꢁ ꢁCu distance is <5 Å [79]. The synthesized complex 1 showed
high catalytic activity regarding the oxidation of 3,5-di-tert-butyl-
catechol to 3,5-di-tert-butylquinone as evident from the observed
hyperchromism of the absorption band at 395 nm recorded every
30 min time interval during the course of the reaction (Fig. 10).
As observed in the structure of 1, the metallic site of the dinuclear
asymmetric unit is easily accessible and the short phenoxo bridged
intra dimer Cu1–Cu2 distance (2.9736(8) Å) will allow a bridging
catechol coordination compatible with the distance between the
two o-diphenol oxygen atoms (a Cu–Cu distance of 3.25 Å was
measured for the unique o-catecholate-bridged dicopper complex
described in the literature) [80]. It is emphasized that this distance
is 2.9 Å in the enzyme, which is also a
l-hydroxo dicopper com-
plex. Similar conclusions had been reached by Krebs [67].
Fig. 10. Increase of the quinone band at 395 nm after addition of 3,5-DTBC (2.87 ꢂ 10ꢀ3 M) to a solution of 1 (5.75 ꢂ 10ꢀ5 M) in DMF at 25 °C. The spectra were recorded
every 30 min. Inset shows the variation of absorbance with time (min).

D. Sadhukhan et al. / Inorganica Chimica Acta 376 (2011) 245–254
253
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In this paper, we have described the synthesis of a new dicyan-
amide bridged 2D polymeric complex of copper with a N2O2 donor
Schiff base ligand and its applications in magnetic and catalytic
fields. The antiferromagnetically coupled dicopper metal core with
Cu–Cu distance 2.974 Å resembles the active site of the plant en-
zyme catechol oxidase and show good catecholase activity. Thus
a relevant structure–function correlation has been established
among the structure of 1 and its magnetic and catalytic properties.
Though most of the activities of the complex are due to the metal
and the Schiff base ligand fragment the dicyanamide linker also
plays some crucial role to form a 2D undulating sheet like metal or-
ganic framework. Furthermore, the presence of vacant coordina-
tion sites on the square planar chelated Cu1 atom and specially
on the non-chelated Cu2 ion offers up to 1 + 3 bridging points (if
we assume penta-coordination for the copper(II) ions). The addi-
tion of a highly potentially bridging ligand as dca has led, as ex-
pected, to the formation of up to four dca bridges connecting the
vacant positions of the copper ions. This coordination mode gener-
ates a 2D coordination polymer with helical –Cu–dca–Cu– chains
that are further connected through double dca bridges to form a
2D layer with a hexagonal topology.
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Acknowledgements
D. Sadhukhan is thankful to University Grants Commission, New
Delhi, Government of India for financial assistance. R.J.B. wishes to
acknowledge the NSF-MRI program (Grant CHE-0619278) for funds
to purchase the Oxford Diffraction Gemini diffractometer. We also
acknowledge the European Union (MAGMANet network of excel-
lence), the Spanish Ministerio de Educación y Ciencia (CSD 2007-
00010 Consolider-Ingenio in Molecular Nanoscience) and the Gen-
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Appendix A. Supplementary material
CCDC 754866 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge from The Cambridge
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