Insertion of a hydroxido bridge into a diphenoxido dinuclear copper(II) complex: Drastic change of the magnetic property from strong antiferromagnetic to ferromagnetic and enhancement in the catecholase activity

Article abstract of DOI:10.1021/ic300319s

A diphenoxido-bridged dinuclear copper(II) complex, [Cu₂L ₂(ClO₄)₂] (1), has been synthesized using a tridentate reduced Schiff base ligand, 2-[[2-(diethylamino)-ethylamino]methyl] phenol (HL). The addition of t

Full text of DOI:10.1021/ic300319s

Article
pubs.acs.org/IC
Insertion of a Hydroxido Bridge into a Diphenoxido Dinuclear
Copper(II) Complex: Drastic Change of the Magnetic Property from
Strong Antiferromagnetic to Ferromagnetic and Enhancement in the
Catecholase Activity
Apurba Biswas,† Lakshmi Kanta Das,† Michael G. B. Drew,‡ Carmen Diaz,§ and Ashutosh Ghosh*,†
†Department of Chemistry, University College of Science, University of Calcutta, 92 APC Road, Kolkata 700 009, India
‡School of Chemistry, The University of Reading, P.O. Box 224, Whiteknights, Reading RG 66AD, U.K.
§Departament de Química Inorgan
̀
ica, Universitat de Barcelona, Marti i Franques 1-11, 08028 Barcelona, Spain
S
* Supporting Information
ABSTRACT: A diphenoxido-bridged dinuclear copper(II)
complex, [Cu2L2(ClO4)2] (1), has been synthesized using a
tridentate reduced Schiff base ligand, 2-[[2-(diethylamino)-
ethylamino]methyl]phenol (HL). The addition of triethyl-
amine to the methanolic solution of this complex produced a
novel triple bridged (double phenoxido and single hydroxido)
dinuclear copper(II) complex, [Cu2L2(OH)]ClO4 (2). Both
complexes 1 and 2 were characterized by X-ray structural
analyses, variable-temperature magnetic susceptibility measurements, and spectroscopic methods. In 1, the two phenoxido
bridges are equatorial−equatorial and the species shows strong antiferromagnetic coupling with J = −615.6(6.1) cm−1. The
inclusion of the equatorial−equatorial hydroxido bridge in 2 changes the Cu···Cu distance from 3.018 Å (avg.) to 2.798 Å (avg.),
the positions of the phenoxido bridges to axial−equatorial, and the magnetic coupling to ferromagnetic with J = 50.1(1.4) cm−1.
Using 3,5-di-tert-butylcatechol as the substrate, the catecholase activity of the complexes has been studied in a methanol solution;
compound 2 shows higher catecholase activity (kcat = 233.4 h−1) than compound 1 (kcat = 93.6 h−1). Both complexes generate
identical species in solution, and they are interconvertible simply by changing the pH of their solutions. The higher catecholase
activity of 2 seems to be due to the presence of the OH group, which increases the pH of its solution.
tic.1d,f There are only a few ferromagnetically coupled
INTRODUCTION
■
phenoxido/hydroxido-bridged copper(II) complexes.1f,3−5 For
the diphenoxido-bridged complexes, the presence of large
substituents in the ligand that creates a large phenyl out-of-
plane angle or a large hinge distortion of the Cu2O2 framework
has been proposed to be responsible for ferromagnetic
coupling.3 In some dihydroxido-bridged complexes, the
presence of axially bridging ligands or a large out-of-plane
shift of hydrogen atoms created by a weak interaction between
hydrogen atoms to anionic ligands and/or solvent molecules is
taken into account in order to explain the ferromagnetic
coupling.4,5 Thus, it is a great challenge to design a
ferromagnetic copper(II) complex containing double-phenox-
ido or -hydroxido bridges.
Phenoxido/hydroxido-bridged dinuclear copper(II) complexes
have received much attention for their interesting magnetic
properties1 as well as significant catecholase activities.2 Their
magnetic properties have been extensively studied in order to
provide clear insight into the relationship between their
structural features and the strength of the magnetic exchange
interaction between the metal ions because copper(II) ions
have an S = 1/2 spin, which make them easier to deal with from
a theoretical point of view. It is now well established that the
exchange coupling of diphenoxido/hydroxido-bridged dinuclear
copper(II) complexes containing Cu2O2 cores depends on
various structural features such as the coordination geometry of
the copper ions, the Cu−O(R)−Cu angle, Cu−O bond
lengths, Cu···Cu distances, out-of-plane shift of the phenyl
rings or hydrogen atoms in the bridge, and torsion angles.1a−c
For equatorial−equatorial bridged copper(II) systems, the Cu−
O(H)−Cu angle seems to be the most important of these
dimensions. In general, antiferromagnetic character is found for
complexes with a Cu−O(H)−Cu angle larger than 97°, while
ferromagnetism appears for smaller values. The literature data
show that, in most of these complexes, this angle is greater than
this value, and consequently the coupling is antiferromagne-
Catechol oxidase is a type III active site protein containing
copper that catalyzes the oxidation of a wide range of o-
diphenols (catechols) to the corresponding o-quinones in a
process known as catecholase activity.6 The ability of dicopper
complexes to oxidize phenols and catechols is well-known.
Various factors such as metal−metal distances, electrochemical
Received: February 13, 2012
Published: September 10, 2012
© 2012 American Chemical Society
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Article
then evaporated to dryness.16 The reduced Schiff base ligand HL was
extracted from the solid mass with methanol, and this methanol
solution (ca. 20 mL) was used for the preparation of complexes.
Synthesis of the Complex [Cu2L2(ClO4)2] (1). An extracted
methanolic solution of HL as prepared above was added to a solution
of Cu(ClO4)2·6H2O (1.850 g, 5 mmol) in methanol (20 mL). The
mixture was stirred for 1 h and filtered. The filtrate was kept
undisturbed at room temperature. The resultant solution upon
standing overnight at room temperature yielded green single crystals
suitable for X-ray analysis. Yield: 1.441 g, 75%. Anal. Calcd for
C26H42Cl2Cu2N4O10: C, 40.63; H, 5.51; N, 7.29. Found: C, 40.30; H,
properties of the complexes, exogenous bridging ligands, the
ligand structure, and the pH are known to influence the
catecholase activity.2 In the dinuclear complexes, a Cu−Cu
distance of 2.9−3.2 Å has been suggested to provide maximum
catecholase activity because of the requirement of a steric
match between the substrate and catalyst.7 The number and
nature of the bridging ligands between the copper centers in a
complex play an important role in the catecholase activity. It is
reported that bridging ligands such as hydroxido,8 alkoxido or
phenoxido,9 and carboxylato2c,10 enhance but halogen bridges
retard the catecholase activity.11 Regarding the number of
bridges, it has been found that double-phenoxido or -hydroxido
bridges sometimes lead to a catalytically inactive species,8b but
these may be taken as exceptional because many other similar
double-bridged compounds show considerable catalytic activi-
ty.8b,9a−c,12 Moreover, there are two possible mechanisms for
the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-
tert-butylquinone (3,5-DTBQ). The first one proceeds through
the reaction between the dicopper(II) species and substrate to
form one molecule of quinone and a dicopper(I) species, which
subsequently reacts with an oxygen molecule to generate a
peroxodicopper(II) adduct, which then oxidizes a second
molecule of the substrate to quinone. Water is produced as a
byproduct by this four-electron-reduction process.13 The
second mechanism involves the formation of a copper(II)−
copper(I) semiquinonate as an intermediate species.8a,12,14 Its
subsequent reaction with dioxygen results in the reoxidation of
the copper(I) ion and release of the quinone molecule and
hydrogen peroxide in the two-electron-reduction process.8d
However, there are disparate views as to whether the formation
of hydrogen peroxide is continued during the whole course of
the oxidation reaction8a or is stopped after a few minutes.2f The
peroxide can also participate in the catalytic cycle by reoxidizing
the reduced dicopper(I) species to dicopper(II) and itself is
converted into water.2f,13
In this paper, we report the synthesis, crystal structures,
magnetic properties, and catecholase activity of two dinuclear
copper(II) compounds, [Cu2L2(ClO4)2] (1) and
[Cu2L2(OH)]ClO4 (2), derived from a tridentate reduced
Schiff base ligand, 2-[[2-(diethylamino)ethylamino]methyl]-
phenol (HL). Compound 1 is a typical antiferromagnetically
coupled double-phenoxido-bridged dimer that shows moderate
catecholase activity. However, inclusion of an additional
hydroxido bridge into this compound results in a novel
triple-bridged dinuclear copper(II) complex, 2, in which the
magnetic coupling changes to ferromagnetic. The catecholase
activity of 2 is higher than that of 1, a fact that may be related to
the higher pH of its solution. To our knowledge, 2 is the first
triple-bridged Cu2O3 core whose catecholase activity has been
investigated.
−
5.74; N, 6.95. IR (KBr, cm−1): ν(N−H) 3217, ν(ClO4 ) 1047, 1102.
λmax, nm (εmax, dm3 mol−1 cm−1; methanol): 650 (407), 424 (2380).
Synthesis of the Complex [Cu2L2(OH)]ClO4 (2). Compound 1
(0.768 g, 1 mmol) was dissolved in methanol (10 mL), and
triethylamine (0.28 mL, 2 mmol) was added in a 1:2 molar ratio.
The mixture was stirred for 1 h and filtered. The filtrate was kept
undisturbed at room temperature. The resultant solution upon
standing overnight at room temperature yielded green single crystals
of 2 suitable for X-ray analysis. Yield: 0.480 g, 70%. Anal. Calcd for
C26H43Cu2ClN4O7: C, 45.51; H, 6.32; N, 8.16. Found: C, 45.29; H,
−
6.65; N, 7.92. IR (KBr, cm−1): ν(N−H) 3253, ν(ClO4 ) 1090. λmax
,
nm (εmax, dm3 mol−1 cm−1; methanol): 655 (373), 417 (1838).
Alternative Method for the Synthesis of Complex 2. A
methanolic solution of HL (20 mL), prepared using the same quantity
of reactant as stated above was added to a solution of Cu-
(ClO4)2·6H2O (1.850 g, 5 mmol) in methanol (20 mL). Triethyl-
amine (1.05 mL, 10 mmol) was added dropwise to this solution with
constant stirring. The mixture was stirred for 1 h and filtered. The
filtrate was kept undisturbed at room temperature. The resultant
solution upon standing overnight at room temperature yielded green
single crystals suitable for X-ray analysis. Yield: 1.235 g, 72%. Anal.
Calcd for C26H43Cu2ClN4O7: C, 45.51; H, 6.32; N, 8.16. Found: C,
−
45.17; H, 6.66; N, 7.88. IR (KBr, cm−1): ν(N−H) 3253, ν(ClO4 )
1090. λmax, nm (εmax, dm3 mol−1 cm−1; methanol): 655 (373), 417
(1838).
Catalytic Oxidation of 3,5-DTBC. In order to study the
catecholase activity of the complexes, 10−4 M solutions of 1 and 2
in methanol were treated with 100 equiv of 3,5-DTBC in methanol
under aerobic conditions at room temperature. The absorbance versus
wavelength (wavelength scan) of these solutions was recorded at
regular time intervals of 5 min in the wavelength range of 300−600
nm. To determine the dependence of the rate on the substrate
concentration and various kinetic parameters, a 10−4 M solution of
complexes was treated with 10, 30, 50, 70, and 100 equiv of substrate.
The reactions were followed spectrophotometrically by monitoring the
increase in the absorbance at 390 nm (quinone band maximum) as a
function of time (time scan).
To detect the formation of hydrogen peroxide during the catalytic
reaction, we followed a reported method.2c Reaction mixtures were
prepared as in the kinetic experiments. During the course of the
oxidation reaction, the solution was acidified with H2SO4 to pH 2 to
stop further oxidation after a certain time and an equal volume of
water was added. The formed quinone was extracted three times with
dichloromethane. To the aqueous layer were added 1 mL of a 10%
solution of KI and three drops of a 3% solution of ammonium
−
molybdate. The formation of I3 could be monitored spectrophoto-
metrically because of the development of the characteristic I3− band (λ
EXPERIMENTAL SECTION
Materials. The reagents and solvents used were of commercially
available reagent quality.
■
= 353 nm, ε = 26000 M−1 cm−1).
Physical Measurements. Elemental analyses (C, H, and N) were
performed using a Perkin-Elmer 240C elemental analyzer. IR spectra
in KBr pellets (4500−500 cm−1) were recorded using a Perkin-Elmer
RXI FT-IR spectrophotometer. Electronic spectra in methanol (1200−
350 nm) were recorded in a Hitachi U-3501 spectrophotometer.
Electrochemical studies were carried out using Sycopel model AEW2
1820F/S instrument. The measurements were performed at 300 K in
acetonitrile solutions containing 0.2 M TEAP and a 10−3 M complex
deoxygenated by bubbling with nitrogen. The working, counter, and
reference electrodes used were a platinum wire, a platinum coil, and a
saturated calomel electrode, respectively. The electrospray ionization
Caution! Perchlorate salts of metal complexes are potentially explosive,
and caution should be exercised when dealing with such derivatives.
Synthesis of the Reduced Schiff Base Ligand 2-[[2-
(Diethylamino)ethylamino]methyl]phenol (HL). The Schiff base
ligand was synthesized by refluxing a solution of salicylaldehyde (0.52
mL, 5 mmol) and N,N-diethylethylenediamine (0.70 mL, 5 mmol) in
methanol (30 mL) for 1 h.15 The solution was cooled to 0 °C, and
solid sodium borohydride (210 mg, 6 mmol) was added slowly to this
methanolic solution with stirring. After completion of the addition, the
resulting solution was acidified with concentrated HCl (5 mL) and
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mass spectra (ESI-MS) were recorded on a Micromass Q-TOF mass
spectrometer. The magnetic measurements were carried out in the
Scheme 1. Formation of the Complexes
̀
“Unitat de mesures magnetiques dels SCT (Universitat de Barcelona)”
on polycrystalline samples (20 mg) with a Quantum Design SQUID
MPMSXL magnetometer in applied fields of 10000 and 500 G in the
temperature ranges of 2−300 and 2−30 K, respectively. The
diamagnetic corrections were evaluated from Pascal’s constants.
Crystal Data Collection and Refinement. Crystal data for the
two crystals are given in Table 1. A total of 16201 and 11514
Table 1. Crystal Data and Structure Refinement of
Complexes 1 and 2
1
2
formula
M
C26H42Cl2Cu2N4O10
768.64
orthorhombic
P21ca
C26H43Cu2ClN4O7
686.17
cryst syst
space group
a/Å
triclinic
P1
̅
39.388(2)
8.7010(5)
18.2535(8)
90
12.912(4)
13.044(4)
19.528(6)
81.778(4)
84.012(4)
89.282(4)
3237.4(17)
4
b/Å
c/Å
α/deg
β/deg
90
that when a methanolic solution of compound 1 is treated with
triethylamine in a 1:2 molar ratio, a hydroxido group
coordinates to the equatorial positions of both copper(II)
atoms and consequently acts as an additional bridge to result in
a very rare three-single-atom (two phenoxido and one
hydroxido)-bridged copper(II) dimer. Compound 2 can be
produced directly by reacting HL and copper(II) perchlorate in
the presence of triethylamine (2:2:3 molar ratio) in a methanol
solution. Complex 1 was also obtained by adding HClO4 to a
methanolic solution of complex 2 in a 1:1 molar ratio. In this
context, it should be noted that unreduced N,N,O-donor Schiff
bases upon reaction with copper(II) perchlorate in the
presence of triethylamine, i.e., by a method that is similar to
the synthesis of 2, produces μ3-OH-bridged trinuclear copper-
(II) complexes.21
IR and Electronic Spectra. A moderately strong, sharp
peak due to a N−H stretching vibration, at 3217 and 3253
cm−1 for complexes 1 and 2, respectively, shows that the imine
group of the Schiff base is reduced. This reduction is also very
clearly indicated by the absence of the strong band due to imine
vibration, which appears in the region 1620−1650 cm−1 for the
complexes of the unreduced Schiff bases.22 In complex 2, there
is a single band at 3575 cm−1, indicating the presence of a
hydroxido group. The IR spectra show bands due to the ν3
mode of perchlorate stretching at 1047 and 1102 cm−1 for
complex 1 and a single band at 1090 cm−1 for complex 2,
corroborating the presence of coordinated and ionic per-
chlorate, respectively.
The electronic spectra of these two compounds were
recorded in a methanolic solution as well as in the solid state
(Figure S1 in the Supporting Information). In methanol
solutions, the electronic spectra show single absorption bands
at 650 and 655 nm due to a d−d transition for compounds 1
and 2, respectively. In the higher-energy region, the ligand-to-
metal charge-transfer band was located for compound 1 at 424
nm, which is at a higher wavelength than that of compound 2 at
417 nm. Upon the addition of an increasing amount of
triethylamine to the solutions of the complexes, these bands
gradually shifted to lower wavelength regions for both
complexes (Figures S2 and S3 in the Supporting Information).
In the solid state, the electronic spectra show d−d transition
bands at 600 and 650 nm and ligand-to-metal charge-transfer
γ/deg
V/Å3
90
6255.8(6)
8
Z
Dc/g cm−3
μ/mm−1
F(000)
R(int)
1.632
1.408
1.591
1.442
3184
1432
0.110
0.048
total reflns
unique reflns
I > 2σ(I)
R1, wR2
temp/K
33822
23371
16201
11514
9527
7193
0.0683, 0.1120
150
0.0682, 0.2228
293
independent reflection data were collected with Mo Kα radiation at
150 and 293 K, respectively, using the Oxford Diffraction X-Calibur
CCD system. The crystals were positioned at 50 mm from the CCD. A
total of 321 frames were measured with a counting time of 10 s. Data
analyses were carried out with the CrysAlis program.17 The structures
were solved using direct methods with the SHELXS97 program.18 The
non-hydrogen atoms were refined with anisotropic thermal parame-
ters. The hydrogen atoms bonded to carbon were included in
geometric positions and given thermal parameters equivalent to 1.2
times (or 1.5 times for methyls) those of the atom to which they were
attached. Absorption corrections were carried out using the ABSPACK
program.19 In 2, one perchlorate was refined with two sets of positions
for the four oxygen atoms, and the other was disordered over two
distinct sites; each was refined with 50% occupancy. The structures
were refined on F2 to R1 = 0.0683 and 0.0682 and wR2 = 0.1120 and
0.2228 for 9527 and 7193 data, respectively, with I > 2σ(I).
RESULTS AND DISCUSSION
■
Synthesis of the Complexes. The condensation of N,N-
diethylethylenediamine in a 1:1 molar ratio with salicylaldehyde
afforded the Schiff base 2-[[2-(diethylamino)ethylimino]-
methyl]phenol, which upon reduction with sodium borohy-
dride readily produced the reduced Schiff base HL (Scheme 1).
HL upon reaction with copper(II) perchlorate in a 1:1 molar
ratio yielded the diphenoxido-bridged compound 1. It is to be
noted that the formation of such diphenoxido-bridged
copper(II) dimers is a very common feature for the complexes
of copper(II) with NNO-donor Schiff bases or reduced Schiff
bases.1h−j,20 However, a unique feature of the present work is
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bands at 426 and 417 nm for compounds 1 and 2, respectively.
The electronic spectral data for both of the complexes were
found to be almost identical in solutions, although the solid-
state spectra show some differences. Therefore, the different
structural identities of 1 and 2 that have been found in the solid
state are probably lost in the solution.
Crystal Structures of Complexes 1 and 2. The structure
of 1 contains two molecules in the asymmetric unit, called A
and B, with equivalent structures of the formula
[Cu2L2(ClO4)2]. The structure of A is shown in Figure 1,
together with the atomic numbering scheme. Dimensions in the
metal coordination spheres of A and B are given in Table 2.
Table 2. Bond Distances (Å) and Angles (deg) in the Metal
Coordination Spheres for Complex 1
A
B
Cu1−O31
Cu1−O11
Cu1−N22
Cu1−N19
Cu1−O51
Cu1−Cu2
Cu2−O11
Cu2−O31
Cu2−N39
Cu2−N42
Cu2−O61
1.956(5)
1.968(5)
2.031(6)
2.007(6)
2.345(5)
3.016(1)
1.945(5)
1.972(5)
1.991(6)
2.053(5)
2.356(6)
1.978(5)
1.946(5)
2.036(6)
2.003(6)
2.350(5)
3.020(1)
1.939(5)
1.974(4)
2.001(6)
2.040(5)
2.316(5)
O31−Cu1−O11
O31−Cu1−N19
O31−Cu1−N22
O31−Cu1−O51
O11−Cu1−N19
O11−Cu1−N22
O11−Cu1−O51
N19−Cu1−N22
N19−Cu1−O51
N22−Cu1−O51
O11−Cu2−O31
O11−Cu2−N39
O11−Cu2−N42
O11−Cu2−O61
O31−Cu2−N39
O31−Cu2−N42
O31−Cu2−O61
N39−Cu2−N42
N39−Cu2−O61
N42−Cu2−O61
Cu1−O11−Cu2
Cu1−O31−Cu2
76.7(2)
161.9(2)
97.8(2)
99.0(2)
92.1(2)
156.5(3)
88.7(2)
86.9(2)
94.8(2)
114.9(2)
76.9(2)
165.9(2)
99.8(2)
95.0(2)
92.3(2)
158.7(2)
98.4(2)
87.0(2)
95.6(2)
102.9(2)
100.9(2)
100.4(2)
76.5(2)
161.4(2)
98.5(2)
99.0(2)
91.7(2)
157.5(2)
88.3(2)
86.9(2)
94.8(2)
114.2(2)
76.7(2)
164.3(2)
99.4(2)
94.7(2)
91.7(2)
159.3(2)
94.1(2)
87.6(2)
96.8(2)
106.6(2)
102.1(2)
99.7(2)
Figure 1. Structure of molecule A in 1 with ellipsoids at 30%
probability. Hydrogen bonds are shown as dotted lines. The structure
of molecule B is equivalent.
The bonding of the perchlorate is facilitated by the formation
of a hydrogen bond between an oxygen atom and the
secondary nitrogen in the ligand. Thus, dimensions of N···O,
N−H···O, and H···O for N19−H19 to O52 are 2.982(8) Å,
153°, 2.14 Å in A and 2.973(8) Å, 153°, and 2.13 Å in B, and
those of N39−H39···O62 are 2.924(9) Å, 147°, and 2.12 Å in A
and 2.957(9) Å, 152°, and 2.12 Å in B.
In each dimer, the two metal atoms are bonded to three
donor atoms of one ligand L and the oxygen atom of a second
ligand L, which therefore bridges both metals. These four
atoms form an equatorial plane, and in addition, there is a
perchlorate oxygen atom in an axial position, thus completing a
square-pyramidal structure. The Addison parameter (τ)23 of the
pentacoordinated copper(II) is 0.09 (average), indicating that
the geometry around the metal is slightly distorted square
pyramidal. Bond lengths from the metal to donor atoms of the
ligand are in the ranges 1.991(6)−2.007(6) Å to secondary
nitrogen and 2.031(6)−2.053(5) Å to tertiary nitrogen. There
is little difference between the bond lengths to the two bridging
oxygen atoms with all Cu−O distances in the range 1.945(5)−
1.978(5) Å. The axial bond lengths to the perchlorate oxygen
atom are in the range 2.316(5)−2.356(6) Å. The four donor
atoms in the equatorial plane show root-mean-square (rms)
deviations of 0.064, 0.092, 0.051, and 0.071 Å, with the copper
atoms 0.343(3), 0.279(3), 0.334(3), and 0.288(3) Å from the
relevant planes for Cu1A, Cu2A, Cu1B, and Cu2B, respectively,
in the direction of the axial atom. In the dimers, the two
equatorial planes intersect at 46.2(1)° for A and 45.6(1)° for B,
thus allowing the two copper atoms to be in close proximity at
3.016(1) Å in A and 3.020(1) Å in B.
The structure of 2 contains two [Cu2L2(OH)]+ cations
together with two perchlorates, one of which is disordered over
two distinct sites. The two cations, called A and B, have
equivalent geometries, described in Table 3, and the structure
of cation A is shown in Figure 2.
Each dimer contains two equivalent copper atoms both with
pentcoordinate square-pyramidal environments. The metals are
bonded to three donor atoms of ligand L together with a
hydroxido, which bridges the two metal atoms that constitute
the equatorial planes. Like 1, the tridentate ligand L coordinates
to the metal ion in a meridional configuration. The equatorial
planes show a slight tetrahedral distortion with rms deviations
of 0.104, 0.120, 0.108, and 0.130 Å, respectively, for Cu1A,
Cu2A, Cu1B, and Cu2B. The metal atoms are 0.030(5),
0.041(5), 0.043(5), and 0.038(5) Å from the respective
equatorial planes. In the dimers, the two square planes intersect
at 56.8(3)° for A and 55.8(3)° for B, thus allowing the two
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2.062(10)−2.083(9) Å. In addition, there is a weak interaction
in an approximate axial position to the oxygen atom of the
second ligand, thus giving Cu1−O31 of 2.448(7) and 2.447(8)
Å and Cu2−O11 of 2.445(8) and 2.453(8) Å in A and B,
respectively. The Addison parameter (τ) of the pentacoordi-
nated copper(II) is 0.06 (average), indicating that the geometry
around the metal is slightly distorted square pyramidal. The
bond lengths in the equatorial plane are slightly smaller than
those in complex 1, but axial bond lengths are greater.
Table 3. Bond Distances (Å) and Angles (deg) in the Metal
Coordination Spheres for Complex 2
A
B
Cu1−O1
1.931(7)
1.949(8)
1.977(9)
2.083(9)
2.448(7)
1.937(7)
1.925(8)
1.993(10)
2.071(9)
2.447(8)
Cu1−O11
Cu1−N19
Cu1−N22
Cu1−O31
The secondary nitrogen forms hydrogen bonds to oxygen
atoms of the disordered perchlorate centered on Cl6, from
cation A to oxygen atoms in one orientation and from cation B
to oxygen atoms in the other (Figure 3). Dimensions for N···O,
O1−Cu1−O11
O1−Cu1−N19
O1−Cu1−N22
O11−Cu1−N19
O11−Cu1−N22
N19−Cu1−N22
84.4(3)
174.9(4)
97.3(4)
93.3(4)
172.3(4)
85.6(4)
85.3(3)
175.6(4)
96.3(4)
92.7(4)
171.2(4)
86.2(4)
Cu2−O1
1.937(7)
1.945(8)
1.981(10)
2.062(10)
2.445(8)
2.804(2)
1.934(7)
1.944(7)
1.990(9)
2.066(10)
2.453(8)
2.792(2)
Cu2−O31
Cu2−N39
Cu2−N42
Cu2−O11
Cu2−Cu1
O1−Cu2−O31
O1−Cu2−N39
O1−Cu2−N42
O31−Cu2−N39
O31−Cu2−N42
N39−Cu2−N42
Cu1−O11−Cu2
Cu1−O31−Cu2
Cu1−O1−Cu2
85.2(3)
175.1(4)
96.6(4)
93.4(4)
170.6(4)
85.5(4)
78.4(3)
78.4(3)
93.8(3)
85.8(3)
174.4(4)
96.0(4)
92.9(4)
170.2(4)
86.2(4)
78.3(3)
78.1(3)
92.4(3)
Figure 3. Hydrogen bonding in the polymeric structure of compound
2 (all hydrogen atoms except H1, H19, and H39 are omitted for
clarity).
N−H···O, and H···O are as follows: for N19A−H19A to O66a
(a = x, y − 1, z), 3.17(3) Å, 160°, and 2.27; for N39A−H39A to
O68, 3.08(3) Å, 166°, and 2.19 Å; for N19B−H19B to O62b (b
= x + 1, y, z), 3.11(3) Å, 172°, and 2.21 Å; for N39B−H39B to
O64, 3.19(3) Å, 166°, and 2.30 Å. The two hydroxides also
form hydrogen bonds but to the perchlorate disordered over
two sites: O1A−H1A to O82c (c = 1 − x, −y, −z), 2.86(3) Å,
158°, and 1.97 Å; O1B−H1B to O73, 2.93(3) Å, 146°, and 2.11
Å (Table 4).
Magnetic Properties. Temperature-dependent molar
susceptibility measurements of powdered samples 1 and 2
were carried out in an applied field of 10000 G for 1 and 7000
G for 2 and also 500 G for 1 and 400 G for 2 in the
temperature ranges of 2−300 and 2−30 K, respectively. The
χmT vs T plots of complexes 1 and 2 are shown in Figures 4
and 5, respectively; in both cases, χmT is the magnetic
susceptibility per Cu2 unit. At room temperature, the χmT
values of 1 and 2 are 0.15 and 0.85 cm3 mol−1 K, values that are
respectively considerably less and slightly higher than the 0.75
Figure 2. Structure of cation A in 2 with ellipsoids at 30% probability.
The structure of cation B is equivalent. The weak axial bonds are
shown as open bonds.
copper atoms to be in closer proximity at 2.804(2) Å in A and
2.792(2) Å in B than is found in 1. Dimensions around all four
copper atoms in the two cations are equivalent with bond
lengths to bridging hydroxido O1 in the range 1.915(8)−
1.934(7) Å, ligand oxygen 1.937(7)−1.948(8) Å, secondary
nitrogen 1.977(9)−1.993(10) Å, and tertiary nitrogen
1
cm3 mol−1 K calculated for two uncoupled S = /2 spins (g =
2.00). When the samples are cooled, the χmT values decrease
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Inorganic Chemistry
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Information). On the contrary, the isothermal magnetization
measurements at 2 K for compound 2 confirm the presence of
ferromagnetic coupling with a S = 1 ground spin state (Figure
S4 in the Supporting Information). Its magnetization value
tends to 2.0 NμB at 5 T, and it is not yet saturated.
Table 4. Hydrogen-Bonding Distances (Å) and Angles (deg)
for Complexes 1 and 2
a
∠D−
D−H A···H
H−A
complex
D−H···A
(Å)
(Å)
D···A (Å)
(deg)
Assuming the isotropic Hamiltonian H = −JS1S2, the
experimental data were fitted to the equation given in the
literature for dinuclear copper compounds.24
1
N19A−H19A···O52A
N19B−H19B···O52B
N39A−H39A···O62A
N39B−H39B···O62B
N19A−H19A···O66a
N19B−H19B···O62b
N39A−H39A···O64
N39B−H39B···O68
O1A−H1A···O82c
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.91
0.93
0.93
2.14
2.13
2.12
2.12
2.27
2.21
2.3
2.982(8)
2.973(8)
2.924(9)
2.957(9)
3.17(3)
3.11(3)
3.19(3)
3.26(3)
2.86(3)
2.93(3)
153
153
147
152
169
172
166
136
158
146
−1
⎡
⎤
2
2zJ′
χ = 2Ng2β2 kT −
⎢
⎥
⎦
3 + exp(−J/kT)
⎣
[3 + exp(−J/kT)]−1
2.54
1.97
2.11
O1B−H1B···O73
The parameters N, g, β, k, and T in the equation have their
usual meanings, while J = singlet−triplet splitting and zJ′ is the
interaction between neighboring dinuclear identities. The best-
fit parameters for reproducing satisfactorily the experimental
data, as shown in Figures 4 and 5, are J = −615.6(6.1) cm−1 and
g = 2.07 with R = 8.69 × 10−4 for 1 (the parameter zJ′ was
considered to be equal to zero) and J = 50.1(1.4) cm−1, g =
2.06, and zJ′ = −0.15 cm−1 with R = 3.30 × 10−5 for 2 (R =
∑i(χTi,calc − χTi,exp)2/∑i(χTi,exp)2).
a
D = donor atom; A = acceptor atom. Symmetry operations: a, x, y −
1, z; b, x + 1, y, z; c, 1 − x, −y, −z.
Magnetostructural Correlations. The magnetic proper-
ties of dinuclear copper complexes containing a Cu2O2 core
depend on its structural properties. Several parameters, such as
the coordination geometry of the copper ions, the Cu−O−Cu
angle, the Cu−O bond lengths, the Cu···Cu distances, and the
torsion angle, have been postulated in the literature as
influencing the J values of the spin coupling in diphenoxido-
bridged dinuclear copper(II) complexes.1a,b It has been shown
that the overlap between the metal and phenoxido bridging
oxygen is a factor that controls spin coupling and is correlated
not only with the bridging angle but also with the M−O bond
distances. A literature survey of phenoxido-bridged dinuclear
copper(II) complexes reveals that when the Cu−O bond
distance is less than 1.98 Å, strong antiferromagnetic coupling
is observed and the strength of this coupling is linearly
dependent on the Cu−O bond distances.1b The conformation
of the phenyl rings is also important: in the syn conformation,
the out-of-plane shift of the phenyl ring can influence the
magnetic coupling significantly.1a Another important structural
factor in pentacoordinated copper(II) complexes is the Addison
parameter (τ) because an increase in τ can be correlated with a
decrease in the antiferromagnetic coupling.22 The strong
antiferromagnetic coupling in 1 is anticipated considering the
low value of τ = 0.09 (average), the large Cu−O−Cu angle of
100.75° (average), the short Cu−O distance of 1.960 Å
(average), and finally the syn conformation of the phenyl
groups with an out-of-plane shift of the phenyl ring of 26.05°
(average).
Figure 4. Plot of χmT vs T in the range 2−300 K for 1.
Figure 5. Plot of χmT vs T in the range 2−300 K for 2.
nearly to zero at 120 K in compound 1, indicating strong
antiferromagnetic coupling between copper(II) ions. However,
in the case of compound 2, when the temperature is lowered,
the χmT values increase gradually to reach a maximum value of
1.048 cm3 mol−1 K at 8 K, a behavior that is typical for a
ferromagnetically coupled dinuclear copper(II) compound.
Below this temperature, the χmT values decrease to a value of
0.955 cm3 mol−1 K at 2 K, a drop that may originate from
intermolecular antiferromagnetic interactions.
The strong antiferromagnetic coupling in 1 leads to a
diamagnetic (S = 0) ground state, as confirmed by the
isothermal magnetization measurements at 2 K, which show a
value of 0.013 NμB at 5 T (Figure S4 in the Supporting
In the literature, numerous diphenoxido-bridged copper(II)
complexes have been reported, but only 11 contained an
additional bridge between the two copper(II) atoms to make
the complexes triple-bridged.20,25 In nine of them, the
additional bridge is formed through three atoms of polyatomic
coordinated ligands like acetate, perchlorate, nitrate, etc.,25 but
only in two complexes are there single-atom bridges via the
oxygen atom of a methanol or water molecule.20 However, in
all such triple-bridged complexes, the phenoxido-bridged
Cu2O2 core remained equatorial−equatorial and the addition-
ally coordinated ligand occupied the axial position. As a
2
2
consequence, in all such cases, the magnetic orbitals dx −y of
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a
Table 5. Magnetostructural Data for Antiferromagnetic Dicopper(II) Complexes with a Single Hydroxido Bridge
compound
Cu···Cu (Å)
Cu−O−Cu (deg)
J (cm−1
)
geometry
reference
[Cu2(bpy)4(OH)](ClO4)3
3.645
3.437
3.370
3.740
3.757
3.642
3.90
141.60
131.11
125.74
150.60
155.97
143.7
174.0
137.9
141.7
128.1
145.7
166.1
93.10
−322
−334
−296
−510
−691
−500
−865
−360
−240
−374
−303
−880
50
Tbpeq
Spl
27b
27a
Na[Cu2(L1)2(OH)]·2H2O
K[Cu2(L1)2(OH)]·0.2H2O
Spl
27a
[Cu2(L2)(OH)](ClO4)3·1.5H2O
[Cu2([22]py4pz)OH](ClO4)3·H2O
[Cu2(L3)(OH)(ClO4)](ClO4)2·CHCl3
[Cu2(L4)(OH)](CF3SO3)3
Tbpax
Tbpax
Spy
27c
1g
27d
27e
27g
27h
27i
Tbpax
Spy
[Cu2(L5)(dpm)(OH)](ClO4)3·2H2O
[Cu2(L6)(OH)](ClO4)2·H2O
[Cu2(dien)2(ClO4)3(OH)]
3.663
3.57
Spy
3.435
3.642
3.89
Spl
[Cu2(terpy)2(H2O)(ClO4)3(OH)]
[Cu2(L7)(OH)](CF3SO3)(BPh4)2
[Cu2L2(OH)]ClO4
Spy
27j
Tbpeq
27k
2.798
Spy
this work
a
Spl = square planar, Spy = square pyramidal with OH− in the equatorial position, Tbpax = trigonal bipyramidal with OH− in the axial position, and
Tbpeq = distorted trigonal bipyramidal with OH− in the equatorial position. Abbrevations: H2L1 = 2,6-bis[N-(phenyl)carbamoyl]pyridine; L2 =
tetraimine Schiff base of tris(2-aminoethyl)amine and 2,5-diformylfuran; L3 = 1,4,7,13,16,19-hexaaza-10,22-dioxatetracosane; L4 = octaamine from
−
BH4 reduction of the Schiff base of tris(2-aminoethyl)amine and 2,5-diformylfuran; L5 = 1,1,2,2-tetrakis(2-pyridyl)ethylene; dpm = bis(2-
pyridyl)methane; L6 = Schiff base of 2,6-diacetylpyridine and 3,6-dioxooctane-1,8-diamine; L7 = partially hydrolyzed Schiff base of 2,6-
diacetylpyridine and tris(2-aminoethyl)amine; HL = 2-[[2-(diethylamino)ethylamino]methyl]phenol; dien = diethylenetriamine; terpy = 2,2′;6′,2″-
terpyridine; tpmc = 1,4,8,11-tetrakis(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane.
the two copper(II) atoms interact via the diphenoxido bridge,
whereas the axially coordinated bridged atom does not
contribute significantly toward magnetic coupling. Therefore,
magnetic interactions through Cu−Ow or Cu−OMeOH of the
two dinuclear triply oxido-bridged copper(II) complexes20 can
be considered to be negligible because the distances are too
long and the corresponding oxygen atoms are coordinated at
the axial positions. The magnetic interaction in both
compounds is antiferromagnetic, as is expected for bis(μ-
phenoxido)dicopper complexes. In addition, another dinuclear
copper(II) complex,1d containing two short hydroxido bridges
and an additional H2O bridge with a long axial distance, is
reported. The magnetic interaction in this compound is very
weak and antiferromagnetic.
this result was proposed by Hoffmann and co-workers.26 In
such complexes, a transition from antiferromagnetism to
ferromagnetism is observed for Cu−O(H)−Cu angles smaller
than 97° because of an “accidental orthogonality” of the orbitals
bearing the unpaired electrons.5d According to Ruiz et al., there
are two possible mechanisms accountable for the exchange
interaction in the hydroxo-bridged dinuclear copper(II)
complexes: direct interaction between the copper atoms and
superexchange interaction through the hydroxido bridge.1c In
compound 2, two additional axial−equatorial phenoxido
bridges, although not very effective in magnetic coupling,
seem to cause a decrease in the Cu···Cu distance, which, in
turn, reduces the Cu−O(H)−Cu angle below 97°, and the
coupling becomes ferromagnetic. The literature data show that,
although there are few ferromagnetically coupled double
hydroxido-bridged copper(II) dimers (Table S1 in the
Supporting Information),4,5 in all of the single hydroxido-
bridged dicopper(II) complexes that have been reported so far,
the Cu−O(H)−Cu angles are above the limit, causing
moderate to strong antiferromagnetic coupling (Table 5)
between the metal ions.1g,27 Thus, to the best of our
knowledge, compound 2 is the first example of a copper(II)
dimer in which ferromagnetic coupling takes place through the
single hydroxido bridge. In summary, we can conclude that the
magnetic results obtained for complex 1 are in accordance with
the literature data, and the ferromagnetic behavior of complex 2
is probably due to the small Cu−O−Cu angle of 93.10°.
Kinetic Studies for Catecholase Activity. In most studies
of the catecholase activity of model complexes, 3,5-DTBC has
been chosen as the substrate. The oxidation product 3,5-di-tert-
butylquinone (3,5-DTBQ) is very much stable and exhibits a
maximum absorption at 390 nm in pure methanol. Prior to a
detailed kinetic study, it is necessary to check the ability of the
dinuclear complexes to oxidize 3,5-DTBC. For this purpose,
10−4 M solutions of complexes 1 and 2 were treated with 100
equiv of 3,5-DTBC under aerobic conditions in a methanol
solution. Immediately after the addition of substrate 3,5-DTBC
to the solutions of the catalysts, the absorption peak at 390 nm
(ε = 1900 M−1 cm−1), which is indicative of oxidation from 3,5-
Our present complex 2 is unique in that the additional
hydroxido group forms an equatorial−equatorial bridge
between the two square-pyramidal copper(II) atoms, whereas
the diphenoxido bridges are axial−equatorial with the long axial
bonds at 2.446 Å (average). In square-pyramidal copper(II),
the equatorial bond lengths are shorter than the axial bond
lengths, indicating that the spin-unpaired electron is located in
2
2
2
the dx −y orbital and that the dz orbital contains spin-paired
electrons. Therefore, to a first approximation, in compound 2,
the coupling between two copper(II) atoms through the
equatorial−axial phenoxido bridges can be neglected. Con-
sequently, from a magnetic point of view, the compound can be
considered as a single hydoxido-bridged dimer with a Cu−
O(H)−Cu angle of 93.10° (average) and Cu−O(H) distances
of 1.928 Å (average).
A limited number of dihydroxido-bridged copper(II)
complexes have been reported1d,f,4,5 but most of them exhibited
relatively strong antiferromagnetic coupling.1d,f Magnetostruc-
tural correlations in dinuclear copper(II) complexes, bridged
equatorially by pairs of hydroxido groups, show that the major
1
factor controlling spin coupling between the S = /2 metal
centers is the Cu−O(H)−Cu angle. In particular, Hatfield and
Hodgson5d have shown that the experimentally determined
singlet−triplet energy gap (J) varies linearly with the Cu−
O(H)−Cu angle, and subsequently a theoretical justification of
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DTBC corresponding to quinone (3,5-DTBQ), increased in
size, thus showing considerable catecholase activity, as shown in
Figures 6 (for complex 2) and S5 in the Supporting
Figure 7. Plot of the initial rates versus substrate concentrations for
the oxidation reaction catalyzed by complex 2. The inset shows the
Lineweaver−Burk plot.
catecholase activity owing to the requirement of a steric
match between the substrate and catalyst.2a Wei et al. reported
that, within a series of oxygen-atom-bridged dicopper(II)
complexes, the highest catalytic activity was observed with
metal−metal distances of 2.9−3.0 Å, which were close to that
observed (2.9 Å) for the metal form of catechol oxidase, and
the compound that contained a Cu···Cu distance of 2.80 Å gave
the lowest catecholase activity.9b When the present two
complexes are compared, the higher catecholase activity of
complex 2, which has a very short Cu···Cu distance (average
2.798 Å) compared to that (average 3.018 Å) of 1, is at first
sight surprising. However, if we consider the presence of the
OH bridge, which has been found to enhance the catecholase
activity of some compounds,2g,29 the results can be explained.
In fact, the UV−vis spectra suggest that in solution both
compounds produce identical species. However, the pH
measurements reveal a higher value for the solution of 2,
which is expected because of the presence of a OH group.
Again, regardless of whether 1 or 2 is used as the precatalyst,
the catecholase activity increases with increasing pH of the
solutions until pH 8.20 (Figure S9 in the Supporting
Information) and thereafter self-oxidation of 3,5-DTBQ
becomes very fast. This pH dependence may be due to the
fact that in alkaline media the catechol undergoes deprotona-
tion, and consequently its coordination to the metal centers is
facilitated. Therefore, the higher catecholase activity of 2 may
be assumed to be simply due to the higher pH of its solution.
To establish the pathway for the catalytic reaction, we
checked whether dioxygen was reduced to water (as in the
enzyme) or to H2O2, as is found in various model systems. The
presence of hydrogen peroxide was clearly detected by the
oxidation of I− (see the Experimental Section) using a 1 × 10−2
M concentration of catechol with a catalyst concentration of 1
× 10−4 M for both compounds (Figure S10 in the Supporting
Information). During the course of the catalytic reaction, the
amount of H2O2 was also estimated. It was found that for both
compounds dihydrogen peroxide is formed at the initial stage of
the reactions and its concentration remains almost constant,
although the oxidation of 3,5-DTBC continues (Figure 8).
After 1 h, the amount of H2O2 is only about 30% and 20% of
3,5-DTBQ formed for compounds 1 and 2, respectively. A
Figure 6. Increase in the absorbance after the addition of 100 equiv of
3,5-DTBC to a methanolic solution containing complex 2 (10−4 M).
The spectra were recorded every 5 min.
Information (for complex 1), when compared to the inactive
Cu(ClO4)2·6H2O (Figure S6 in the Supporting Information).
Plots of absorbance versus wavelength for complexes 1 and 2
with the same time interval are shown in Figure S7 in the
Supporting Information.
The kinetic study of the oxidation of 3,5-DTBC to 3,5-
DTBQ by the complexes was carried out by monitoring the
increase in the absorbance at 390 nm by the initial rate method.
The rate constant for a particular complex−substrate mixture
was determined from the log[Aα/(Aα − At)] versus time plot.
To determine the dependence of the rates on the substrate
concentration and various kinetic parameters, solutions of
complexes 1 and 2 were studied using increasing concen-
trations of 3,5-DTBC (from 10 to 100 equiv) under aerobic
conditions at a complex concentration of 10−4 M. A first-order
dependence was observed at low concentrations of the
substrate, whereas saturation kinetics were found at higher
concentrations of the substrate for all of the complexes shown
in Figures 7 (for complex 2) and S8 in the Supporting
Information (for complex 1). This dependence on the substrate
concentration indicates a catalyst−substrate binding to be an
initial step in the catalytic mechanism. A treatment on the basis
of the Michaelis−Menten approach, originally developed for
enzyme kinetics, was therefore applied and linearized by means
of a Lineweaver−Burk plot to calculate various kinetic
parameters such as the Michaelis−Menten constant (KM) and
the maximum initial rate (Vmax).28 The kcat values can be
calculated by dividing the Vmax values by the concentration of
the corresponding complexes.
The data obtained from the Lineweaver−Burk plot model are
used for a comparison of the catalytic activity toward the
oxidation of 3,5-DTBC, as shown in Table 6. It is to be noted
that the catecholase activity of compound 2 is higher than that
of compound 1. The results are, at first sight, inconsistent with
earlier findings. In the dinuclear complexes, the Cu−Cu
distances of 2.9−3.2 Å are supposed to give maximum
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Table 6. Kinetic Parameters for Complexes 1 and 2
complex
Vmax (M min−1
)
KM (M)
kcat (h−1
)
Kass (M−1
)
Kcat/KM (M−1 h−1
)
1
2
(1.56 0.03) × 10−4
(3.89 0.17) × 10−4
(3.32 0.06) × 10−3
(4.60 0.20) × 10−3
93.6
301.2
217.4
28.2 × 103
50.7 × 103
233.4
properties of the copper(II) complexes with their catecholase
activity. The redox potential should be such as to permit the
reoxidation of the reduced copper centers by molecular oxygen
to maintain the catalytic cycle. The cyclic voltammograms in a
acetonitrile solution of both complexes display irreversible
cathodic responses, and the electrochemical data are
summarized in Table S2 in the Supporting Information. The
reduction peaks are tentatively assigned to the one-electron
processes CuIICuII → CuIICuI and CuIICuI → CuICuI. During
the anodic potential scan, both complexes show an oxidative
response at ca. −0.11 V with a very narrow width and high peak
current (Figure S21 in the Supporting Information). The
response at −0.11 V is typical of the anodic stripping of copper
and has also been observed in similar complexes.2c Thus, the
electrochemical behavior of complexes 1 and 2 is very similar,
indicating the presence of similar species in solution under
these conditions.
Figure 8. Course of 3,5-DTBQ and H2O2 formation during the
catalytic reaction at substrate-to-catalyst ratios of 100:1: (a and b)
H2O2 concentrations and (c and d) 3,5-DTBQ concentrations for
compounds 1 and 2, respectively.
CONCLUSIONS
■
The tridentate reduced Schiff base ligand HL {2-[[2-
(diethylamino)ethylamino]methyl]phenol} upon reaction
with copper(II) perchlorate afforded a phenoxido-bridged
dicopper complex, 1. The addition of triethylamine in a
methanol solution of this complex transformed it into a novel
double phenoxido- and single hydroxido-bridged dinuclear
copper(II) complex, 2. The additional hydroxido bridge shifted
the two equatorial−equatorial phenoxido bridges to axial−
equatorial positions and itself occupied the equatorial−
equatorial position. In 2, this triple-bridging arrangement
brings the two copper atoms in closer proximity and the
magnetic coupling drastically changes from strong antiferro-
magnetic to ferromagnetic. This behavior can be explained by
considering equatorial−equatorial bridging angles of average
100.75° (phenoxido) in 1 and average 93.10° (hydroxido) in 2,
which are the effective pathways for magnetic coupling. To the
best of our knowledge, complex 2 is the first ferromagnetic
dinuclear copper(II) complex in which magnetic coupling takes
place through a single hydroxido bridge. The higher catecholase
activity of complex 2 compared to 1 is rather unexpected
considering the Cu−Cu distances in the solid-state structures.
However, both of the species generate essentially identical
species in solution except that the pH of the solution of 2 is
higher because the OH group produces 1 equiv of base in its
solution. This higher pH facilitates deprotonation of catechol
and, consequently, its coordination to the metal centers. Hence,
the catecholase activity of complex 2 is higher.
similar result was obtained by Reedijk et al.,2f and they
proposed two different mechanisms: initially reduction of
dioxygen to dihydrogen peroxide and then to water are
probably operative during the catalytic oxidation.
To obtain a mechanistic inference of the catecholase activity
and to investigate possible complex−substrate intermediates,
we recorded ESI-MS spectra of both complexes and a 1:100
mixture of the complexes and 3,5-DTBC within 10 min of
mixing in a methanol solvent (Figures S11−S20 in the
Supporting Information). The spectra of complexes 1 and 2
show a base peak at m/z 283.9 (100%), which can be assigned
to the mononuclear species [CuL]+. Other peaks at m/z 604.9
and 668.8 indicate the presence of the dinuclear species
[Cu2L2Cl]+ and [Cu2L2(ClO4)]+, respectively. There is also
another peak at m/z = 642.9 in complex 2 indicating the
presence of the dinuclear species [Cu2L2Cl(OH)Na]+. After the
addition of triethylamine to 2, the spectrum shows a base peak
at m/z 102.1 (100%) due to the presence of the cationic species
[Et3NH]+. A new peak generated at m/z 662.81 indicates the
presence of the dinuclear species [Cu2L2Cl(OH)(H2O)Na]+.
After the addition of 3,5-DTBC to the solutions of the
complexes, some new peaks are generated. In both cases, the
spectra exhibit a base peak at m/z 243 (100%) corresponding
to the quinone sodium aggregate [3,5-DTBQ-Na]+. The peaks
at m/z 506.2 and 789.0 corroborate the formation of species
[CuL(3,5-DTBC)H]+ and [Cu2L2(3,5-DTBC)]+, respectively.
It is interesting to note that the mass spectra of 1 and 2 are
essentially the same after the addition of 3,5-DTBC. Hence,
one may infer that the differences in the catecholase activities of
1 and 2 might be caused simply by the presence of 1 equiv of
base in methanol solutions of 2 (originating from the hydroxido
bridge), while the copper species are essentially identical.
Electrochemistry. The electrochemical properties of both
complexes have been investigated to correlate the redox
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data in CIF format for the structures reported
and additional figures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ghosh_59@yahoo.com.
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dx.doi.org/10.1021/ic300319s | Inorg. Chem. 2012, 51, 10111−10121

Inorganic Chemistry
Article
Polyhedron 2010, 29, 2769. (g) Safei, E.; Rasouli, M.; Weyhermuller,
T.; Bill, E. Inorg. Chim. Acta 2011, 375, 158.
(6) Rompel, A.; Fischer, H.; Meiwes, D.; Buldt-Karentzopoulos, K.;
Dillinger, R.; Tuczek, F.; Witzel, H.; Krebs, B. J. Biol. Inorg. Chem.
1999, 4, 56.
Notes
̈
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(7) Oishi, N.; Nishida, Y.; Ida, K.; Kida, S. Bull. Chem. Soc. Jpn. 1980,
53, 2847.
We thank CSIR, Government of India [Senior Research
Fellowship to A.B., Sanction No. 09/028(0717)/2008-EMR-I,
and Junior Research Fellowship to L.K.D., Sanction No. 09/
028(0805)/2010-EMR-I], and DST FIST for financial support
for the X-ray diffraction facility. We also thank the British
Engineering and Physical Sciences Research Council and the
University of Reading for funds for the X-Calibur system and
the Spanish Government (Grant CTQ2009-07264) for
financial support.
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