Azido bridge mediated catecholase activity, electrochemistry and magnetic behavior of a dinuclear copper(II) complex of a phenol based "end-off" compartmental ligand

Article abstract of DOI:10.1016/j.ica.2015.07.038

Abstract A dinuclear Cu(II) species [Cu₂L₂(H₂O)₂(N₃)](NO₃)₂ (L = 2,6-bis(N-ethylpyrrolidine-iminomethyl)-4-methyl-phenolato) where two Cu centers are bridged by phenoxido and μ_1,1-azido bridges with Cu-Cu separation of ～3 ? have been synthesized with the view to explore the role of azido bridge on catecholase activity and electrochemical property and the roles of both the bridging groups on magnetic coupling of two copper centers. The complex exhibits excellent catecholase activity in acetonitrile as well as in DMSO medium not only by oxidizing 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a catechol which is very thorny to oxidize, under aerobic conditions and becomes the first example of its own kind. CV study reveals three quasi-reversible reductive couples which are tentatively assigned as Cu₂^II to Cu^IICu^I and Cu^ICu^I reduction followed by reduction of Cu^ICu^I complex to Cu⁰Cu⁰ species. Variable temperature magnetic study suggests the presence of an antiferromagnetic spin-exchange interaction between Cu(II) ions in the dimer via double bridge where the antiferromagnetic contribution of phenoxido bridge predominates over the ferromagnetic interaction of azido bridge.

Full text of DOI:10.1016/j.ica.2015.07.038

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Inorganica Chimica Acta 436 (2015) 139–145
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Azido bridge mediated catecholase activity, electrochemistry and
magnetic behavior of a dinuclear copper(II) complex of a phenol based
‘‘end-off” compartmental ligand
Prateeti Chakraborty a, Ishani Majumder a, Hulya Kara b, Shyamal Kumar Chattopadhyay c,
Ennio Zangrando d, Debasis Das a,
⇑
a Department of Chemistry, University of Calcutta, 92 A. P. C. Road, Kolkata 700 009, India
b Department of Physics, Faculty of Science and Art, Balikesir University, 10145 Balikesir, Turkey
c Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah, India
d Department of Chemical and Pharmaceutical Sciences, University of Trieste, Via L. Giorgieri 1, 34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 June 2015
Received in revised form 29 July 2015
Accepted 30 July 2015
Available online 4 August 2015
A
dinuclear Cu(II) species [Cu2L2(H2O)2(N3)](NO3)2 (L = 2,6-bis(N-ethylpyrrolidine-iminomethyl)-4-
methyl-phenolato) where two Cu centers are bridged by phenoxido and 1,1-azido bridges with Cu–Cu
l
separation of ꢀ3 Å have been synthesized with the view to explore the role of azido bridge on catecholase
activity and electrochemical property and the roles of both the bridging groups on magnetic coupling of
two copper centers. The complex exhibits excellent catecholase activity in acetonitrile as well as in DMSO
medium not only by oxidizing 3,5-di-tert-butylcatechol (3,5-DTBC) but also tetrachlorocatechol (TCC), a
catechol which is very thorny to oxidize, under aerobic conditions and becomes the first example of its
own kind. CV study reveals three quasi-reversible reductive couples which are tentatively assigned as Cu2II
to CuIICuI and CuICuI reduction followed by reduction of CuICuI complex to Cu0Cu0 species. Variable tem-
perature magnetic study suggests the presence of an antiferromagnetic spin–exchange interaction
between Cu(II) ions in the dimer via double bridge where the antiferromagnetic contribution of phenox-
ido bridge predominates over the ferromagnetic interaction of azido bridge.
Keywords:
Schiff-base
Cu(II) complex
Catecholase activity
Magnetic property
Cyclic voltammetry
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
may exert the similar effect as of hydroxo bridge. We are successful
in our search when we worked with an ‘‘end-off” compartmental
Dinuclear copper complexes with Cu–Cu separation of ꢀ3 Å are
interesting as corroborative model of the active site of catechol oxi-
dase to elucidate the functional mechanism of the native enzyme
and to develop bio-inspired catalytic systems [1–9]. When that
separation is associated with bridging azido ligand the species
becomes of particularly interesting for gaining deeper insight into
magneto-structural correlation, a study very much essential for
developing new functional molecule based materials [10–14].
The intense study on synthetic analogs of active site of catechol
oxidase in last decade reveals that successful models must possess
some characteristic structural features and one of the most impor-
tant features is the presence of hydroxo bridge as is noticed in the
crystal structure of the active site of the native enzyme in the met
state [6–9]. We were in search whether any other bridging entity
ligand selecting azide as the bridging entity. The selection of an
‘‘end-off” compartmental ligand is to full fill the basic requirement
of keeping two copper atoms in the optimum distance of ꢀ3 Å and
that of azide is just to exploit its versatile coordination modes. It is
obvious that azide either in l1,3 or l1,1 mode may act as bridging
ligand [10,11]. The azido bridge is generally antiferromagnetic for
the
l1,3-N3 (end-to-end, EE) mode, though, in the recent past, some
exceptions have been reported [15–18]. For the
l
1,1-N3 (end-on,
EO) bridging mode, ferromagnetic ordering is established when
the Cu–N–Cu angle is small, which has been attributed to a spin-
polarization effect [19–22]. We are reporting herein synthesis,
structural characterization of [Cu2L2(H2O)2(l1,1-N3)](NO3)2
(L = 2,6-bis(N-ethylpyrrolidine-iminomethyl)-4-methyl-phenolato),
catecholase activity in DMSO and acetonitrile medium using
3,5-di-tert-butylcatechol (3,5-DTBC) and tetrachlorocatechol
(TCC) as substrate, electrochemistry and variable temperate
magnetic study and magneto-structural correlations.
⇑
Corresponding author.
E-mail address: dasdebasis2001@yahoo.com (D. Das).
http://dx.doi.org/10.1016/j.ica.2015.07.038
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

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P. Chakraborty et al. / Inorganica Chimica Acta 436 (2015) 139–145
Table 1
2. Experimental section
2.1. Physical methods and materials
Crystallographic data and details of refinements for complex 1.
Empirical formula C21H35Cu2N9O9 F(000)
1416
24.71
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
684.66
monoclinic
P21/n
8.8950(11)
17.1010(13)
19.0790(19)
97.475(10)
2877.5(5)
4
hmax (°)
Reflections collected 8822
Elemental analyses (carbon, hydrogen and nitrogen) were per-
formed using a Perkin–Elmer 240C elemental analyzer and copper
content was estimated gravimetrically. Infrared spectra were
recorded on KBr disks (400–4000 cmꢁ1) with a Perkin–Elmer RXI
FTIR spectrophotometer. Electronic spectra (200–800 nm) were
measured at room temperature on a Shimadzu UV-3101PC by
using dry methanol/DMSO as medium. Cyclic voltammetric and
DPV measurements were performed by using a CH1106A poten-
tiostat with glassy carbon (GC) as working electrode, Pt-wire as
counter electrode and Ag, AgCl/sat KCl as reference electrode.
Magnetic susceptibility measurements over the temperature range
5–300 K was performed at a magnetic field of 0.0750 T using a
MPMS SQUID-VSM dc magnetometer. Correction for the sample
holder, as well as the diamagnetic correction, which was estimated
from the Pascal constants, [23] was applied.
All solutions were purged with dinitrogen prior to measure-
ments. All other chemicals used in this study were obtained from
commercial sources and used as received. Solvents were dried
according to standard procedure and distilled prior to use. 4-
Methyl-2,6-diformylphenol was prepared according to the litera-
ture method [24]. N-(2-Aminoethyl)pyrrolidine was purchased
from Sigma–Aldrich Chemical Company and used as received.
Unique reflections
Rint
Observed I > 2
4687
0.044
2733
383
r(I)
Parameters
b (°)
Goodness of fit (F2)
0.917
V (Å3)
R1 (I > 2
r(I))
0.0498
0.1298
ꢁ0.545, 0.372
Z
wR2
Dcalc (mg mꢁ3
)
1.542
Dq
(e Å3)
3. Results and discussion
3.1. Synthesis and characterization
The complex was synthesized adopting template synthesis
technique in which a methanolic solution of copper(II) nitrate tri-
hydrate was treated with the Schiff-base formed in situ by the reac-
tion between 4-methyl-2,6-diformylphenol and N-(2-aminoethyl)
pyrrolidine. Complex 1 was obtained after the addition of a
water–methanolic solution of sodium azide (four times with
respect to the copper salt). The IR spectrum of the complex shows
a band due to C@N stretch at 1646 cmꢁ1 and skeletal vibration at
1553 cmꢁ1. Broad band centered at 1383 cmꢁ1 indicates the pres-
ence of weakly coordinated nitrate ion in the complex. A strong
band observed at 2079 cmꢁ1 confirms the presence of bridging
azide ion in the complex (see Table 2).
2.2. Synthesis of the complex
2.2.1. [Cu2(L)(H2O)2(N3)](NO3)2 (1)
3.2. Description of crystal structure
A methanolic solution (5 mL) of N-(2-aminoethyl)pyrrolidine
(0.256 g, 2 mmol) was added dropwise to a heated methanolic
solution (10 mL) of 4-methyl-2,6-diformylphenol (0.164 g,
1 mmol), and the resulting mixture was refluxed for half an hour.
Then, a methanolic solution (15 mL) of Cu(NO3)2ꢂ3H2O (0.604 g,
2.5 mmol) was added to it and the reflux was further continued
for 2 h. After cooling down the reaction mixture at room tempera-
ture a water–methanolic (1:1) solution (5 mL) of sodium azide
(0.325 g, 5 mmol) was added dropwise at stirring condition. The
resulting mixture was allowed to stir for 3 h and filtered. After fil-
tration, the clear deep-green solution was kept in a CaCl2 desicca-
tors in dark. Square-shaped deep-green crystals, suitable for X-ray
analysis, were obtained from the filtrate after a few days (yield
70%). Anal. Calc. for C21H35Cu2N9O9: C, 36.84; H, 5.15; N, 18.41;
The structural analysis of compound 1 shows that it comprises
of a dicationic species [Cu2L(H2O)2(N3)]2+ counterbalanced by two
nitrate anions. The ORTEP drawing of the complex is shown in
Fig. 1, while a selection of coordination geometry parameters is
reported in Table 3. The metals are chelated by the ligand L
through the phenoxido oxygen, the imino and amino pyrrolidine
nitrogen donors, and bridged by a l1,1 azide, completing their
square pyramidal coordination sphere through aqua ligands. The
Cu–O(phenoxo) bond lengths are comparable being of 1.982(3)
and 1.960(3) Å for Cu1 and Cu2, respectively. The length of the
Cu–N(imino) bond distances (1.924(4) and 1.916(5) Å) are similar
to those involving the bridging azide (1.983(4) and 1.998(5) Å).
On the other hand the Cu–N(pyrrolidine) ones are slightly longer,
of 2.037(4) and 2.028(4) Å, for the different hybridization of N
atom donors. On the other hand the water molecules at apical
Found: C, 36.94; H, 4.89; N, 18.47%; IR:
(skeletal vibration) 1553 cmꢁ1 (H2O) 3434 cmꢁ1
2079 cmꢁ1
m
(C@N) 1653 cmꢁ1
(Nꢁ3 1
)
; m
;
m
;
m
.
Table 2
2.3. X-ray data collection and crystal structure determination
Coordination bond lengths (Å) and angles (°) for complex 1.
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–O(1)
Cu(1)–N(5)
Cu(1)–O(1w)
Cu(1)–O(8)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–N(5)
N(1)–Cu(1)–O(1w)
N(2)–Cu(1)–O(1)
N(2)–Cu(1)–N(5)
N(2)–Cu(1)–O(1w)
O(1)–Cu(1)–N(5)
O(1)–Cu(1)–O(1w)
N(5)–Cu(1)–O(1w)
Cu(1)–O(1)–Cu(2)
1.924(4)
2.037(4)
1.982(3)
1.983(4)
2.330(4)
2.753(5)
86.0(2)
91.4(2)
167.0(2)
95.8(2)
176.5(2)
103.4(2)
94.1(2)
78.8(2)
88.6(2)
Cu(2)–N(3)
Cu(2)–N(4)
Cu(2)–O(1)
Cu(2)–N(5)
Cu(2)–O(2w)
Cu(2)–O(4)
N(3)–Cu(2)–N(4)
N(3)–Cu(2)–O(1)
N(3)–Cu(2)–N(5)
N(3)–Cu(2)–O(2w)
N(4)–Cu(2)–O(1)
N(4)–Cu(2)–N(5)
N(4)–Cu(2)–O(2w)
O(1)–Cu(2)–N(5)
O(1)–Cu(2)–O(2w)
N(5)–Cu(2)–O(2w)
Cu(1)–N(5)–Cu(2)
1.916(5)
2.028(4)
1.960(3)
1.998(5)
2.337(5)
2.979(6)
86.0(2)
91.6(2)
169.2(2)
96.4(2)
168.5(2)
102.3(2)
96.3(2)
78.9(2)
95.1(2)
The single crystal of complex 1 was mounted on a glass fiber
and coated with epoxy resin. Intensities data were collected at
room temperature on a Nonius DIP-1030H system with Mo K
a
radiation (k = 0.71073 Å). Maximum diffraction angle theta was
24.7° (completeness of 96%) due to poor diffracting crystals. Cell
refinement, indexing and scaling of the data sets were carried
out using Denzo and Scalepack [25]. The structure was solved by
direct methods and subsequent Fourier analyses [26] and refined
by the full-matrix least-squares method based on F2 with all
observed reflections using SHELX-97 [26] software. The non-hydro-
gen atoms were refined anisotropically. All calculations were car-
ried out using WINGX System version 1.80.05 [27].
92.5(2)
101.8(2)
89.5(2)
100.5(2)
The CCDC number of the complex is 1033058. Pertinent crystal-
lographic data and refinement details are summarized in Table 1.

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141
3.3. Catecholase activity
Catechol oxidase, a type-3 copper protein can binds oxygen
reversibly at room temperature and so it can be utilized to oxidize
phenols to respective o-benzoquinones. As a model of the enzyme
we have taken one dinuclear azido bridged complex of Cu(II) and
study it’s efficiency towards oxidation of 3,5-DTBC to 3,5-DTBQ
and TCC to TCQ, respectively. Interestingly, it displays significant
catalytic activity towards the oxidation of 3,5-di-tert-butylcatechol
(3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ) and tetra-
chlorocatechol (TCC) to tetrachloroquinone (TCQ) in DMSO as well
as in acetonitrile medium. Before proceeding into detailed kinetic
study we have checked the ability of our complex to mimic the
active site of catechol oxidase by treating 1 ꢃ 10ꢁ4 mol dmꢁ3 solu-
tions of complex 1 with 1 ꢃ 10ꢁ2 mol dmꢁ3 (100 equivalents) of
3,5-DTBC and TCC under aerobic condition. The course of the reac-
tion was followed by UV–Vis spectroscopy (Fig. 3). The time depen-
dent UV–Vis spectral scan was performed in pure acetonitrile and
DMSO medium. The complex shows a smooth conversion of 3,5-
DTBC to 3,5-DTBQ in both solvent. It is also able to convert TCC
to TCQ which is hard to oxidize under ordinary condition. Fig. 3
(a) and (b) show the spectral change for our complex upon addition
of 100-fold 3,5-DTBC (1 ꢃ 10ꢁ2 M) observed at interval of 5 min in
DMSO medium. The kinetics of the 3,5-DTBC and TCC oxidation
was determined by monitoring the increase of the concentration
of the product 3,5-DTBQ and TCQ. The experimental conditions
were the same as we reported earlier [1]. The complex showed sat-
uration kinetics and a treatment based on the Michaelis–Menten
model seemed to be appropriate. The binding constant (KM), max-
imum velocity (Vmax), and rate constant for dissociation of sub-
strates (i.e., turnover number, kcat) were calculated by using the
Lineweaver–Burk graph of 1/V versus 1/[S] (Figs. S2–S5 and S8–
S11), using the equation 1/V = {KM/Vmax}{1/[S]} + 1/Vmax, and the
kinetic parameters are presented in Tables S1–S2 in SI file.
Fig. 1. ORTEP drawing of complex cation of 1 (30% ellipsoid probability).
position of the square pyramidal coordination geometry are at
sensibly longer distance (Cu1–OH2 = 2.330(4) and Cu2–
OH2 = 2.337(5) Å). The bridging Cu(1)–O(1)–Cu(2) and Cu(1)–N
(5)–Cu(2) bond angles are of 101.83(16)° and 100.5(2)°, respec-
tively, leading to a metal–metal separation of 3.0604(9) Å. The
intermetallic distance is ca. 0.1 Å longer than the values of
2.918(2) and 2.9450(4) Å measured in the correspondent com-
plexes [Cu2(L0)(OH)(H2O)(NO3)2] where the phenol is a 4-chloro-
or 4-tert-butyl derivative, respectively, and metals are bridged
by a hydroxyl OH group rather than the azide [2]. It is worth of
note the crystal packing showing nitrate anions H-bond con-
nected by the coordinated water molecules so that to form a
polymeric chain elongated along axis a (Fig. 2, range of Ow. . .O
distances 2.736(8)–2.848(8) Å). The supramolecular motif gener-
ated from the intermolecular H-bonds can be designated as a
R24ð12Þ synthon [28]. The position of nitrate anions is such that
an oxygen is located at 2.753(5) and 2.979(6) Å from copper
Cu1 and Cu2, respectively. These long distances indicate a possi-
ble weak interaction of nitrate with copper ions, also confirmed
by the slight displacement of metals by ca. 0.10 and 0.15 Å from
the mean basal plane towards these species.
Here the activity of the complex is higher in acetonitrile med-
ium than in DMSO medium. It can be explained by considering
the higher coordinating ability of the solvent which retarded the
possibility of catechol–substrate adduct formation.
In one of our earlier study [1] we reported the catecholase activ-
ity of a similar hydroxo-bridged complex in acetonitrile and
Table 3
Geometrical parameters (Å/°) for H bonds.
Donor–H
A
D–H
H. . .A
D. . .A
D–H. . .A
Symmetry code of A
O(1w)–H(1a)
O(1w)–H(1b)
O(2w)–H(2a)
O(2w)–H(2b)
O(4)
O(7)
O(8)
O(6)
0.83(6)
0.85(6)
0.85(4)
0.84(7)
1.98(6)
2.05(6)
2.12(7)
2.32(8)
2.789(8)
2.848(8)
2.817(9)
2.736(8)
164(8)
156(8)
140(7)
111(8)
–
1 + x, y, z
–
ꢁ1 + x, y, z
Fig. 2. Polymeric chain elongated along axis a built by H bond formed between aqua ligands and nitrate anions (H atoms not shown for sake of clarity).

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5
4
3
2
1
0
3,5-DTBQ
Complex
300
400
500
600
Wavelength(nm)
(a)
(b)
Fig. 3. Changes observed in UV–Vis spectra of complex (1 ꢃ 10ꢁ4 M) (a) in acetonitrile and (b) in DMSO medium upon addition of 100-fold 3,5-DTBC (1 ꢃ 10ꢁ2 M).
Fig. 4. CV spectrum of complex 1 (a) in negative potential, (b) positive potential, at the GC electrode at 100 mV sꢁ1 scan rate.
methanol medium where the complex forms an adduct with 3,5-
DTBC in acetonitrile medium and was unable to oxidize TCC to
TCQ. But our newly synthesized azido-bridged complex is being
able to oxidize 3,5-DTBC to 3,5-DTBQ as well as TCC to TCQ, respec-
tively in both acetonitrile and DMSO medium. To the best of our
knowledge this is the first azido bridged complex which is able
to oxidize TCC to TCQ both in acetonitrile and in DMSO medium.
dioxygen or the intramolecular electron transfer. Since in our case
the d–d band remains intact with slight blue shift (Fig. S12) during
the catalytic reaction the intramolecular electron transfer is most
probably the rate determining step. The role of dioxygen is very
crucial for regeneration of the catalyst with its concomitant reduc-
tion of either kind: (i) two electron reduction to H2O2 or (ii) four
electron reduction to water. Our spectral study after treating with
iodide fails to detect I3ꢁ (Fig. S13) and thereby clearly eliminate the
possibility of dioxygen reduction to H2O2 in our case.
3.3.1. Mechanistic interpretation on catecholase activity exhibited by
complex 1
The reports so far available on mechanism of catecholase activ-
ity of the synthetic analogs of catechol oxidase reveal that the very
first step of the mechanism is the substrate–copper center(s) inter-
action. Coordination of catechol may be monodentate asymmetric
type, bridging bidentate or combination of both. In our present
case the spectral study is not distinct enough to assign the binding
mode of catechol to the copper center as we noticed in our earlier
studies. However, a pre-equilibrium of the free complex and the
substrate prior to the intramolecular electron transfer is a key step
of the mechanism which follows reduction of Cu(II) to Cu(I) with
concomitant oxidation of catechol to quinone. Now the rate deter-
mining step may be either the re-oxidation of copper(I) by
3.4. Cyclic voltammetric study
The electron transfer behavior of the complex was investigated
by cyclic voltammetry at a GC electrode in MeCN solution. The bin-
uclear Cu(II) complex shows three quasi-reversible reductive cou-
ples (Fig. 4) at 0.06 V (92 mV), ꢁ0.26 V (76 mV) and ꢁ0.55 V
(115 mV), the last couple showing cathodic peak current value
which is almost twice that of the first couple. We tentatively assign
the first two couples to successive one electron reductions of the
CuI2I to CuIICuI and CuICuI species, whereas the couple at ꢁ0.55 V
is assigned to reduction of CuICuI complex to Cu0Cu0 species. At
more negative potentials, at ꢁ1.12 V and ꢁ1.78 V, ligand based