New mononuclear copper(II) complex based on a salen derivative ligand with an unusual coordination and its catecholase activity

Article abstract of DOI:10.1016/j.inoche.2013.09.039

The new mononuclear copper(II) complex [Cu^II(H ₂LDA)(ClO₄)](ClO₄) (1) ([H₂LDA = N,N′-[bis-(2-hydroxy-3-formyl-5-methylbenzyl)(dimethyl)]-ethylenediamine]) with an unusual coordination mode of a salen derivative ligand is reported. The most interesting feature of 1 is that the ligand is doubly protonated and presents significant intermolecular π-stacking interactions, contributing to the dimer structure stabilization in the solid state and in CH₃CN and methanolic solutions. The complex was characterized by X-ray crystallography and shows catecholase-like activity in the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-dtbc), with the formation of H₂O ₂, which kinetic parameters are similar to those observed in conventional dinuclear bridged Cu^II complexes.

Full text of DOI:10.1016/j.inoche.2013.09.039

Inorganic Chemistry Communications 37 (2013) 34–38
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
New mononuclear copper(II) complex based on a salen derivative ligand
with an unusual coordination and its catecholase activity
b
b
Tiago Pacheco Camargo a, Rosely A. Peralta a, , Raphaell Moreira , Eduardo E. Castellano ,
⁎
Adailton J. Bortoluzzi a, Ademir Neves a,
⁎
a
Universidade Federal de Santa Catarina, Departamento de Química, Laboratório de Bioinorgânica e Cristalografia - LABINC, Campus Universitário, Trindade, 88040-900 Florianópolis, SC, Brazil
b
Instituto de Física de São Carlos, Universidade de São Paulo, C.P. 369, CEP 13560-970, São Carlos, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
The new mononuclear copper(II) complex [CuII(H2LDA)(ClO4)](ClO4) (1) ([H2LDA = N,N′-[bis-(2-hydroxy-3-
formyl-5-methylbenzyl)(dimethyl)]-ethylenediamine]) with an unusual coordination mode of a salen derivative
ligand is reported. The most interesting feature of 1 is that the ligand is doubly protonated and presents signifi-
cant intermolecular π-stacking interactions, contributing to the dimer structure stabilization in the solid state and
in CH3CN and methanolic solutions. The complex was characterized by X-ray crystallography and shows
catecholase-like activity in the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-dtbc), with the formation
of H2O2, which kinetic parameters are similar to those observed in conventional dinuclear bridged CuII
complexes.
Article history:
Received 4 May 2013
Accepted 22 September 2013
Available online 29 September 2013
Keywords:
Salen derivative ligand
Copper(II) complex
π-Stacking
© 2013 Elsevier B.V. All rights reserved.
Catecholase
Introduction. Active copper and iron centers dominate the field of bio-
logical oxygen chemistry and play a vital role in catalysis [1]. The oxida-
tion of organic substrates with molecular oxygen under mild conditions
is of great interest for industrial and synthetic processes, and the ability
of copper complexes to oxidize phenols and catechols has been known
for at least 40 years [2].
Experimental. The ligand N,N′-[bis-(2-hydroxy-3-formyl-5-meth-
ylbenzyl)(dimethyl)]-ethylenediamine (H2LDA) was obtained by
nucleophilic substitution of 2-hydroxy-3-chloro-methyl-5-meth-
ylbenzaldehyde and N,N'-dimethylethylenediamine in dichloro-
methane [8]. The product was recrystallized from ethanol to give
light yellow crystals, yield 78%. 1H-NMR (CDCl3): 2.26 (s, 6H, CH3);
2.29 (s, 4H, CH3); 2.69 (s, 6H, CH2 amine); 3.68 (s, 4H, CH2); 7.22 (s,
2H, CHAr); 7.26 (s, 2H, CHAr); 7.38 (s, 2H,OHphenol); 10.18 (s, 2H,
CHald). C22H28N2O4 calcd: C 68.73; H 7.34; N 7.29. Found: C 68.17; H
7.84; N 7.26. MP: 131–132 °C.
The complex 1 was prepared according to the following procedure:
To a 20 mL acetonitrile solution containing 0.192 g (0.5 mmol) of the li-
gand, it was added, 15 mL of a acetonitrile solution of copper(II) per-
chlorate (0.187 g, 0.5 mmol) was slowly added. After 20 min, a few
drops of an aqueous solution of perchloric acid (2 M) were added and
the solution was allowed to stand. After 3 days, green crystals were ob-
tained with 67% yield. CuC22H28Cl2N2O12 calcd: C 40.85; H 4.36; N 4.33.
Found: C 40.70; H 4.31; N 4.29.
Elemental analysis (CHN) was performed on a Carlo Erba E-1110 an-
alyzer. Electronic absorption spectra in the 200–1200 nm range were
recorded on a Perkin-Elmer Lambda 19 spectrophotometer. Electrochem-
ical measurements were obtained using a Bas Epsilon potentiostat/
galvanostat. Square-wave voltammograms were obtained for the com-
plex in acetonitrile solution containing 0.1 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte under an argon atmo-
sphere. The electrochemical cell employed was of a standard three-
electrode configuration: glassy carbon electrode (working), platinum
wire (counter), Ag/AgCl (reference). The Fc+/Fc couple (E1/2 = 400 mV
vs NHE) was used as the internal standard [9]. The X-band (9.76 GHz)
Since the first reports of complexes using salen (salen =
di(salicylidene)ethylenediamine) as ligand [3,4], the area of metal–
salen catalysis has expanded greatly. A significant number of
metal–salen like complexes have been reported in the literature
(more than 4000 entries were found on the ISI Web of knowledge
under the topic salen) and many of them have been used as catalysts
in different oxidation reactions [5,6]. The easy synthesis and modifi-
cation of the salen ligand skeleton contribute to making it a good op-
tion for the design of new catalysts. Salen-type ligands generally
coordinate to the metal with N2O2 atom donors. Herein, we report
the synthesis and characterization of a new salen derivative ligand
containing a carbonyl group attached to the phenol group, the X-
ray structure of its CuII complex (Fig. 1) and the catecholase activity
of the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-dtbc).
Interestingly, in the present structure of 1, the CuII center is coordi-
nated by the oxygen atoms of the ligand H2LDA while the amine ni-
trogens are protonated. It is important to emphasize here that only
one other example of this sort of ligand and its CuII complex has
been reported in the literature [7].
⁎
Corresponding authors. Tel.: +55 48 37216849; fax: +55 48 37216850.
E-mail addresses: rosely.peralta@ufsc.br (R.A. Peralta), ademir.neves@ufsc.br (A. Neves).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.09.039

T.P. Camargo et al. / Inorganic Chemistry Communications 37 (2013) 34–38
35
Fig. 1. ORTEP view of the dimer structure (left, symmetry code: −x + 0.5, −y + 0.5, −z + 1) and cation complex 1 (right). Ellipsoids are shown at 40% probability level and hydrogen
atoms were omitted for clarity. Selected bond distances (Å): Cu1–O4 1.900(3); Cu1–O2 1.901(3); Cu1–O3 1.939(3); Cu1–O1 1.955(3); Cu1–O11 2.471(4); Cu1 O2′ 2.822(3); Cu1–Cu1′
3.4744(9).
measurement was performed on a BRUKER EleXsys E580 spectrometer at
5 K using continuous flow liquid-helium cryostats (Oxford Intruments
ESR 900).
500 μL) of a 3,5-dtbc solution ([3,5-dtbc]final = 0.9–9 mM). All experi-
ments were carried out in an air saturated methanolic/water (32:1) solu-
tion. To take into account the spontaneous oxidation of the substrate,
correction was carried out using a reference cell under identical condi-
tions but without the addition of the catalyst. The initial rate was obtained
from the slope of the absorbance versus time plot over the first 20 min of
the reaction. The Michaelis–Menten model was applied and the kinetic
parameters were obtained from nonlinear least squares fit.
Electrospray ionization mass spectrometry (ESI-MS) of 1 dissolved
in an ultrapure acetonitrile solution (500 nM) was performed using an
amaZon X Ion Trap MS instrument (Bruker Daltonics) with an ion
spray source using electrospray ionization in positive-ion mode. The
ion source condition was an ion spray voltage of 4500 V. Nitrogen was
used as the nebulizing gas (20 psi) and curtain gas (10 psi). The sam-
ples were directly infused into the mass spectrometer at a flow rate
of180 μL/h. The scan range was m/z 200–3000. The simulated Spectrum
was calculated using the Mmass software [10,11].
Potentiometric studies of 1 were carried out in an acetonitrile/water
mixture (1:1, v/v) with a Corning-350 research pH meter fitted with
blue-glass and Ag/AgCl reference electrodes, calibrated to read −log
[H+] directly, designated as the pH. Equilibrium measurements were
performed in a thermostated cell, purged with argon, containing
50.00 mL of the acetonitrile/water (1:1) solution and 0.03 mmol of
Results and discussion. The synthesized complex 1 afforded suitable
crystals for the structure determination by X-ray analysis (See Supple-
mentary Material - Table S1 and Table S2). The structure of 1 (Fig. 1,
right) reveals that the CuII center is “4 + 2” coordinated, exhibiting a
highly distorted tetragonal geometry in which the basal plane is occupied
by two oxygen atoms from the phenolates and two oxygen atoms from
the carbonyl group of the H2LDA ligand. One perchlorate group and one
phenolate oxygen from another [CuII(H2LDA)(ClO4)](ClO4) molecule, in
axial positions, complete the “4 + 2” pseudo-coordination geometry.
The Cu–O distances in the basal plane are between 1.900 and 1.955 Å
and the Cu–Operchlorate in the apical position is of 2.471(4) Å. The trans
O–Cu–O angles in the basal plane are of 176.13(12) and 178.41(13)º,
showing a very small distortion in the geometry (τ = 0.01) [13]. The
bond lengths around the CuII ion in the basal plane (O4-donor site - aver-
age 1.923 Å) of complex 1 are similar to the corresponding distances (av-
erage 1.918 Å) observed around the Cu2 center in the dinuclear
[Cu2L(dmf)2]2+ cation containing a similar coordination environment,
in which 1,3-propanediamine forms the backbone of the ligand L [7].
For the ligand described in this work, the addition of perchloric acid to
the reaction mixture as well as steric crowding around the tertiary
amines probably prevents a second CuII from being accommodated.
In fact, the packing analysis in the unit cell shows that two molecules
of the [CuII(H2LDA)(ClO4)](ClO4) complex are very close with respect to
each other and thus, a dimeric structure (face to face) is formed consid-
ering the weak inter-dimer electrostatic Cu–O(phenolate) interactions in
this centrosymmetric arrangement (Fig. 1 left). In the dimer, the Cu1–
O2′ and Cu1…Cu1′ distances are of 2.822(3) Å and 3.4744(9) Å, respec-
tively, which are significantly shorter than those distances found in the
bis(salicylaldehydato)CuII complex (3.13 and 4.05 Å, respectively),
most probably due to the electrostatic interactions between the Cu–O
phenolates and the protonated amines of neighboring molecules in 1
[14,15]. Indeed this Cu…Cu distance is comparable to those distances
found in many dinuclear CuII oxygen bridged complexes [16–19].
Thus, in the solid state the complex is a dimer, where each CuII ion is
surrounded by six oxygen atoms. In addition, intermolecular neighbor-
ing phenolate rings are coplanar to each other, with a dihedral angle
the complex. The temperature was maintained at 25.00
0.05 °C,
and the experimental solutions were adjusted to an ionic strength of
0.100 M through the addition of KCl. Computations of the triplicate re-
sults were carried out with the BEST7 program, and species diagrams
were obtained with the SPE and SPEPLOT programs [12].
The catecholase-like activity of the complex was determined by mea-
suring the oxidation of the substrate 3,5-di-tert-butylcatechol (3,5-dtbc)
in a UV–Vis Varian Cary 50 BIO fitted with a thermostated water-
jacketed cell holder. The reactions were accompanied by formation of
3,5-di-tert-butylquinone (3,5-dtbq) at 400 nm(ε = 1900 M−1 cm−1
)
and 25.0 °C; less than 5% of conversion of substrate to product were
monitored and the data were treated by the initial rate method.
Initially, pH-dependent studies were carried out to determine the pH
value at which catecholase-like activities reached a maximum. The influ-
ence of pH on the reaction rate in the oxidation of 3,5-dtbc catalyzed by 1
was determined over the pH range of 5.0–9.0 at 25 °C. The following re-
agents were placed in a 1-cm-path quartz cell: 100 μL of an aqueous solu-
tion ([B]final = 100 mM) of buffer [MES (pH 5.0–6.5) and TRIS (pH 7.0–
9.0)], 30 μL of a methanolic complex solution ([1]final = 58 μM), and
1500 μL of air-saturated methanol. The reaction was initiated with the ad-
dition of 50 μL of a methanolic substrate solution ([3,5-dtbc]final
=
5.00 mM) and monitored for 20 min. The kinetic experiments under con-
ditions of excess substrate were performed as follows: a total of 100 μL of
aqueous buffer TRIS at pH 8.5 ([B]final = 100 mM), 30 μL of a methanolic
complex solution ([1]final = 58 μM), and oxygen-saturated methanol (to
complete 1530 μL) were added to a 1 cm path-length cell at 25 °C. The
reaction was initiated with the addition of known volumes (from 50 to

36
T.P. Camargo et al. / Inorganic Chemistry Communications 37 (2013) 34–38
Fig. 3. Positive ion ESI mass spectrum for 1 in CH3CN.
Fig. 2. Distribution of species for complex 1 according to Scheme 1.
Potentiometric titration studies of 1 in a water/acetonitrile (1:1) so-
lution showed the neutralization of 3 mol of KOH/mol of complex in the
pH range 3–12. Three pKa values were found and speciation plots are
shown in the Fig. 2.
The first two constants pKa1 = 4.27 and pKa2 = 5.59 can be attrib-
uted to the deprotonation of the uncoordinated amine nitrogen atoms,
which are protonated even in the solid state structure. The pKa3 at
11.39 can be tentatively attributed to the deprotonation of a water mol-
ecule coordinated to the CuII center in the Jahn–Teller position as shown
in Scheme 1.
between the planes and a mean distance between them of 8.62(18)°
and 3.520 Å, respectively, suggesting the occurrence of intermolecular
π-stacking, which also contribute to stabilize the dimer. Unlike other
salen-derivative–metal complexes, the two nitrogen atoms, from the
amines, are not coordinated to the metal center and are protonated.
The cyclic voltammograms of the complex in CH3CN solution show
an irreversible oxidation Epc = −0.55 V vs. NHE which can be attribut-
ed to the CuII/I couple (Fig. S 1). This value is in good agreement with
other mononuclear CuII complexes with two phenolates coordinated
to the metal center [20,21].
In order to establish unequivocally the relevant species in solution,
ESI-MS studies were carried out in CH3CN. Fig. 3 shows the spectrum
in CH3CN, whileFig. S 4 (See Supplementary Material) shows the ob-
served and expected isotopic distribution for the assigned species
under these experimental conditions. As can be observed in Fig. 3,
three main groups of peaks are observed at mass to charge (m/z) ratios
of 466.2, 446.1, and 385.2. The peak at m/z = 465.2 can be attributed to
[(CuIH2LDA)2(H2O)]2+, (reduction of CuII to CuI under the conditions of
the electrospray ionization), which corresponds to a dimer form of the
complex, in agreement with the crystal structure and the four protonat-
ed nitrogen atoms from the amines observed in the equilibrium studies
(Scheme 1 and Fig. S 5). The peak at m/z = 446.1 can be attributed to
the [(CuIIHLDA)]+ species while the peak at 385.2 correspond to the
free ligand. Therefore, it can be concluded that the dimer form of
The electronic spectrum of 1 in CH3OH solution (Fig. S 2) shows
an intense band centered at 380 nm (ε = 3200 M−1 cm−1) attrib-
uted to a ligand-to-metal charge transfer (LMCT) transition between
the two phenolates and the CuII ion and a broad band at ~640 nm
(ε = 65 M−1 cm−1), which is typical of CuII d–d transitions.
The X-band EPR spectrum recorded in a frozen methanolic solution
of 1 shows four lines, typical of CuII ions, as a result of the hyperfine in-
teraction between the unpaired electron (S = ½) and the CuII nucleus
(I = 3/2) (Fig. S 3). The parameters A‖ = 240 × 10−4 G, g‖ = 2.28
and g? = 2.07 were obtained from the simulated spectrum and since
g‖ N g?, a square pyramidal geometry can be proposed for complex 1,
which is consistent with the geometry found from the X ray analysis
and also with the structure proposed in solution from the potentiomet-
ric titration, ESI-MS (vide infra) and the electronic spectroscopic results.
Scheme 1. Proposed equilibria for 1 in water/acetonitrile (1:1) solution.

T.P. Camargo et al. / Inorganic Chemistry Communications 37 (2013) 34–38
37
Fig. 4. (A) Dependence of the initial rate on the pH for the oxidation of the 3,5-dtbc promoted by 1. (B) Dependence of the initial rate on the 3,5-dtbc concentration for the oxidation pro-
moted by 1.
the complex containing one CuII-coordinated water molecule (in-
stead of a pseudo-coordinated ClO−4 ) is maintained in CH3CN or
CH3OH solutions.
copper(I) species back to the active copper(II) species occurs with a
1:1 (O2:3,5-dtbc) stoichiometry and concomitant formation of hydro-
gen peroxide.
As mentioned above, several metal–salen complexes have been test-
ed as catalysts for oxidation processes [5,22], although, a few number of
such complexes have been employed in the oxidation of catechols. Con-
sidering that the dimer form of the complex presented herein is
maintained in CH3OH solutions, we investigated the catalytic activity
of the complex 1 in the oxidation of 3,5-dtbc. The reactions were carried
out in CH3OH solutions, using the initial rate method by monitoring the
increase in the characteristic quinone (3,5-dtbq) absorption band at
400 nm (ε = 1900 M−1 cm−1), which is sufficiently stable under
these experimental conditions. Before the kinetic experiments the sol-
vent was saturated with O2.
Thus, it can be concluded that the significant catecholase activity of
the complex described herein can be explained by the fact that the
two CuII centers within the dimer are close enough together to facilitate
the binding of two phenolic oxygen atoms of catechol and mediate the
redox reaction.
Acknowledgments. The authors are grateful for grants awarded to sup-
port this research from CNPq, FAPESC, CAPES-PROCAD, and INCT-Catálise
(Brazil).
The dependence of the oxidation reaction catalyzed by complex on
the pH was investigated within the range of 5.0–9.0 in order to deter-
mine the pH value at which the catecholase-like activity is at a maxi-
mum and a sigmoidal shaped profile was obtained, as seen in Fig. 4A.
The data were fitted using a Boltzmann model, and a sigmoidal fit of
Appendix A. Supplementary materials. The details of the X-ray analysis
(Table S1 and Table S2), Cyclic voltammetry (Fig. S 1), Electronic spec-
troscopy (Fig. S 2), EPR (Fig. S 3) and Mass spectrometry (Fig. S 4 and
Fig. S 5) are given in Supplementary material. CCDC 936899 contains
the supplementary crystallographic data for this paper. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk. Supplementa-
ry materials related to this article can be found online at http://dx.doi.
org/10.1016/j.inoche.2013.09.039.
the curve revealed a kinetic pKa value of 7.53
0.1 which is in relative
good agreement with the deprotonation of the catechol in presence of
CuII [23].
The complex exhibited saturation kinetics behavior for initial rates
(V0) vs 3,5-dtbc concentrations (Fig. 4B), consistent with the binding
of the substrate to the metal complex. The good fitting of the data to
the Michaelis–Menten model was observed, with the kinetic parameters
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