PH-dependent imidazolato bridge formation in dicopper complexes: Magnetic, electrochemical, and catalytic repercussions

Article abstract of DOI:10.1002/ejic.201200167

The dicopper complexes of two dinucleating ligands derived from 2-methylimidazole and 2 equiv. of an L-amino acid are presented (H ₃bphenim derived from L-phenylalanine and H₃bleuim derived from L-leucine). For each ligand L, three different protonation states for the complex, Cu₂HL²⁺, Cu₂L⁺, and Cu ₂LOH, have been characterized in solution by potentiometric, spectrophotometric, and electrochemical techniques. X-ray diffraction studies enabled the characterization, in the solid state, of the [Cu ₂(bphenim)]⁺ and [Cu₂Hbleuim]²⁺ cations. Magnetic measurements showed that while only a very slight coupling could be detected in [Cu₂Hbleuim]²⁺, [Cu ₂(bphenim)]⁺ is strongly antiferromagnetically coupled. The extent of this coupling can also be appreciated from room temperature, pH-dependent EPR spectroscopic studies, where the signal is quenched at pH values where the imidazolato-bridged species is predominant. These observations are consistent with the cyclic voltammetric studies. The catecholase activity of the complexes in the form [Cu₂L]⁺ is low, however, an important increase in activity is observed above pH = 8.5 where the concentration of the species [Cu₂LOH] starts to become significant. Copyright

Full text of DOI:10.1002/ejic.201200167

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
FULL PAPER
DOI: 10.1002/ejic.201200167
pH-Dependent Imidazolato Bridge Formation in Dicopper Complexes:
Magnetic, Electrochemical, and Catalytic Repercussions
Inés Monte-Pérez,[a] Ana M. Sosa,[b] Sylvain Bernès,[c] Núria Aliaga-Alcalde,[d]
Víctor M. Ugalde-Saldívar,[a] and Laura Gasque*[a]
Keywords: Copper / N ligands / Enzymes / Amino acids / Cyclic voltammetry / Antiferromagnetism
The dicopper complexes of two dinucleating ligands derived
from 2-methylimidazole and 2 equiv. of an -amino acid are
presented (H3bphenim derived from -phenylalanine and
H3bleuim derived from -leucine). For each ligand L, three
different protonation states for the complex, Cu2HL2+, Cu2L+,
and Cu2LOH, have been characterized in solution by
potentiometric, spectrophotometric, and electrochemical
techniques. X-ray diffraction studies enabled the characteri-
zation, in the solid state, of the [Cu2(bphenim)]+ and
[Cu2Hbleuim]2+ cations. Magnetic measurements showed
that while only a very slight coupling could be detected in
[Cu2Hbleuim]2+, [Cu2(bphenim)]+ is strongly antiferromag-
netically coupled. The extent of this coupling can also be ap-
preciated from room temperature, pH-dependent EPR spec-
troscopic studies, where the signal is quenched at pH values
where the imidazolato-bridged species is predominant.
These observations are consistent with the cyclic voltam-
metric studies. The catecholase activity of the complexes in
the form [Cu2L]+ is low, however, an important increase in
activity is observed above pH = 8.5 where the concentration
of the species [Cu2LOH] starts to become significant.
L
L
L
Introduction
SOD. Lippard and coworkers prepared a series of imid-
azolato-bridged complexes and studied their solution equi-
libriums, their SOD activity, and their magnetic behavior.
They observed that in complexes where imidazolato is a
discrete unit, the bridge is quite prone to dissociating in
solution, but that if this moiety was incorporated in the
ligand framework a much more robust bridged structure
is produced.[5] More recent reports have corroborated this
observation.[6]
The antiferromagnetic coupling observed among para-
magnetic metal ions when bridged by the imidazolato moi-
ety has encouraged numerous studies that have focused on
elucidating the factors determining the extent of this cou-
pling,[7] which may gain relevancy since there is current
interest in the potential applications of this bridging moiety
in supramolecular chemistry.[8]
In our group, we have been studying the dicopper com-
plexes of a series of imidazole-derived ligands and their cat-
echolase activity. The ligands presented in this paper
(Scheme 1) contain one imidazole unit condensed with two
amino acid equivalents (phenylalanine or leucine) by means
of a Mannich reaction. Since these ligands promote the for-
mation of imidazolato-bridged dinuclear complexes, we
have investigated the spectroscopic and electrochemical evi-
dence of the formation of the bridge as a function of pH,
as well as their magnetic behavior. The potentiometric and
spectrophotometric speciation of the system allowed the
identification of the catalytically active form of the com-
plexes in the oxidation of di-tert-butylcatechol.
Imidazole is a ubiquitous ligand in biological systems as
it occurs in proteins as part of the side chain of the amino
acid histidine found coordinated to the metal centers in
most copper proteins. However, imidazolate is compara-
tively scarce because of the very weak acid character of the
imidazole moiety (pKa = 14.4).[1] It is known that metal
coordination of the pyridinic nitrogen dramatically in-
creases the acidity of the pyrrolic site, which encourages its
coordination to a second metal ion.[2] Imidazolato-bridged
transition-metal complexes have been known since the early
1960s,[3] and attracted a lot of interest when the X-ray crys-
tal structure of the enzyme bovine erythrocyte superoxide
dismutase (SOD) showed that the active site consisted of a
Zn–Cu bimetallic pair bridged by the imidazolato group of
a deprotonated histidine residue.[4]
Many dinuclear imidazolato-bridged complexes have
been prepared with the aim of modeling the active site of
[a] Departamento de Química Inorgánica y Nuclear, Facultad de
Química, Universidad Nacional Autónoma de México,
Ciudad Universitaria, México D.F. 04510
E-mail: gasquel@unam.mx
[b] Dirección General del Colegio de Ciencias y Humanidades,
Ciudad Universitaria,
México D.F. 04510, México
[c] DEP Facultad de Ciencias Químicas, UANL,
Guerrero y Progreso S/N, Col. Treviño, 64570 Monterrey, NL,
México
[d] ICREA – Departament de Química Inorgànica, Universitat de
Barcelona,
Diagonal, 645, 08028 Barcelona, Spain
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
[CuN2O3] coordination geometry may be described as dis-
torted square pyramidal, as reflected by the τ index[13] com-
puted for this center, τ(Cu1) = 0.27, with the apical position
occupied by the water molecule O18. A MOGUL analy-
sis[14] shows that this molecule is found at the expected dis-
tance, Cu1–O18 2.161(13) Å, while the basal bond lengths
are rather short. For example, Cu1–N2 1.938(8) Å, while
the basal water molecule O19 is bonded at a distance
1.944(11) Å shorter than Cu–OH2(basal) distances in re-
lated complexes with a [CuN2O3] core.[10,15] This strong co-
ordination, and the angle restrictions imposed by the two
five-membered chelate rings formed upon coordination, en-
sure that the CuII center is displaced by 0.33 Å from the
basal plane. The bridging aromatic imidazolate ring (Im),
although having a low C2 crystallographic symmetry, is al-
most perfectly planar, and CuII ions are placed 0.06 Å
above and below the Im mean plane. Such planar CuII–(μ-
Im)–CuII fragments have been described in other polymetal-
lic complexes with various imidazole derivatives, forming
either monomeric,[16] cyclic-oligomeric,[17] or polymeric[18]
systems. The Cu···Cu separation is found in the rough range
5.6–6.2 Å in the complexes bearing a CuII–(μ-Im)–CuII
fragment. The distance observed in the cation [Cu2(bphen-
im)(H2O)4]+ is Cu···Cu 6.056(3) Å, which is within the ex-
pected range. Actually, the shortest metal–metal separation
in the crystal is 5.180(3) Å, which involves two neighboring
cations related through a 21 screw axis.
1
Scheme 1. General formula for the ligands and H NMR spectro-
scopic identification.
Results and Discussion
Synthesis of the Ligands and Complexes
We have previously described the dicopper complexes of
a series of related, but significantly different ligands ob-
tained from the same type of Mannich reaction. When the
amine employed is a primary amine[9] or a nonchiral amino
acid (glycine[10] or β-alanine[11]) the product is a 10-mem-
bered cyclic diazecine, incorporating two imidazole and two
amine moieties. However, when an l-amino acid[12] is used
as the amine source, a ligand comprising only one imidazole
moiety, similar to the ones described here, is obtained. The
ligands used in the present work are H3bphenim (derived
from 2-methylimidazole and l-phenylalanine) and H3bleuim
(derived from 2-methylimidazole and l-leucine).
In principle, at least two forms of each one of the two
dinuclear complexes can be isolated depending on the pH
of the medium. When 1 equiv. of the ligand is combined
with an aqueous solution containing 2 equiv. of Cu(NO3)2
or Cu(ClO4)2, without the addition of a base, the pH drops
to approximately 1.5 and a complex with a cation of the
acidic form [Cu2HL]2+ is formed. If the pH is increased to
above 6, the complex cation is of the deprotonated form
[Cu2L]+. Crystals suitable for X-ray determination of the
structure could be obtained for the acidic form – [Cu2-
HL]2+ – only for the leucine derivative and for the basic
form – [Cu2L]+ – only for the phenylalanine derivative.
Figure 1. ORTEP-like view[20] of the cation [Cu2(bphenim)-
(H2O)4]+ in compound 1, with ellipsoid displacements at the 20%
probability level. H atoms have been omitted for clarity. Atoms
belonging to the asymmetric unit are labeled.
Perchlorate and lattice water molecules are distributed
within the crystal between the cations. As mentioned in the
Exp. Section, they present very high thermal motion pa-
rameters and are prone to disorder, which indicates that
they interact poorly with the cations. Hence, it may be an-
ticipated that the dominant magnetic interaction for 1
Crystal Structures
[Cu2(bphenim)(H2O)4]ClO4·2.86(H2O)
The complex cation [Cu2(bphenim)(H2O)4]+ in com- should be sought in the planar CuII–(μ-Im)–CuII moiety of
pound 1 is build up of the H3bphenim molecule, after de- the cation, rather than in the inter-cation contacts.
protonation of both ammonium and imidazole functionali-
Interestingly, the same cation could be crystallized with
ties, allowing the coordination of two CuII ions. The main nitrate counterions in place of perchlorate, affording the
ligand lies on a twofold axis, resulting in an asymmetric compound [Cu2(bphenim)(H2O)4]NO3·3.6(H2O). This was
unit limited to a half-cation. The metal center coordinates performed with the hope of getting better single crystals
three heteroatoms in the ligand, and the coordination than those of 1. Unfortunately, the opposite result was ob-
sphere is completed by two water molecules (Figure 1). The tained. The crystals belong to the same cubic space group,
2
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
Imidazolato Bridge Formation in Dicopper Complexes
P4132, but only the position of the cation was determined.
The most important difference between 2 and 1 is that
The nitrate anions, which have a C3 axis in common with a the organic ligand in 2 features an imidazole, ImH, ring
perchlorate (tetrahedral point group), are probably located rather than an imidazolate, Im. As a consequence, ImH co-
–
on the same sites as ClO4 in compound 1, but disordered ordinates a single CuII ion in 2. The independent coordina-
by rotation. Moreover, lattice water is systematically disor- tion site Cu2 is bonded to the amine group N8, the nitrate
dered, lowering the scattering power of these crystals. The O atom O31, one water molecule O29, and two carboxylate
above quoted formula, including 3.6 H2O per complex, was O atoms (O11 and O12) belonging to two symmetry-related
obtained from a SQUEEZE refinement,[19] and should be Hbleuim ligands. The bridging μ-carboxylate-κ2O:OЈ func-
seen as a rough estimation of the actual water content.
tionality of Hbleuim gives rise to zigzag chains of Cu2-
based monomers, running along the 21 screw axis parallel
to [100], while Cu1-based groups are arms placed along the
Cu2-polymer backbone (Figure 2, inset). The five-coordi-
nate Cu2 center is strongly distorted from an ideal square-
pyramidal geometry, with τ(Cu2) = 0.38. The apical posi-
tion is occupied by the nitrate O atom, with a long bond
length Cu2–O31 2.333(5) Å, and the metal is placed 0.21 Å
out of the basal mean plane. However, in contrast to the
Cu1 metal center, nothing is found trans to the apical posi-
tion, ruling out the interference of a Jahn–Teller effect in
this case. It should be noted that few occurrences of bime-
tallic copper-bridged complexes with such different coordi-
nation geometries are found in the CSD.[21] Such asym-
metry seems to be generally a consequence of π–π interac-
tions[22] (τ = 0.07 and 0.53) or restrictions from a polychel-
ated connectivity[23] (τ = 0.07 and 0.25).
With regard to the metal–metal distances along the poly-
mer, the Cu2 centers are separated by 4.5986(9) Å, and Cu1
ions in the arms are separated by 7.3647(16) Å (Cu2 and
Cu1 stacks in the inset of Figure 2). All other Cu···Cu sepa-
rations within each chain are more than 8 Å long. Finally,
interchain contacts are intermediate: the shortest Cu···Cu
separation involving two neighboring chains in the crystal
is Cu1···Cu2i 5.4230(8) Å (symmetry code i: 1/2 – x, –y, 1/2
+ z).
[Cu2(Hbleuim)(H2O)3(NO3)]NO3·H2O
Complex 2 is a polymeric compound, build up of the
H3bleuim molecule, after deprotonation of only the ammo-
nium groups. The asymmetric unit contains one [Cu2-
(Hbleuim)(H2O)3(NO3)]+ cation, one uncoordinated nitrate,
and one lattice water molecule, all placed in general posi-
tions and ordered in the crystal. Metal center Cu1 (see Fig-
ure 2) has a [CuN2O3] coordination sphere close to that de-
scribed in 1. Distortion from the square-pyramidal geome-
try is, however, less pronounced, and indeed very close to
an ideal geometry, with τ(Cu1) = 0.02.[13b] The apical posi-
tion is occupied by the water molecule O27, with a long
bond length, Cu1–O27 2.330(6) Å, and basal bond lengths
are short, in the range 1.937(3)–1.997(4) Å. The metal is
displaced by 0.15 Å from the basal plane. The nitrate coun-
terion is located in the position trans to the apical water
molecule O27, with
a large nonbonding separation,
Cu1···O37 2.721(8) Å. The Cu1 can thus alternatively be
seen as an octahedral CuII ion with a huge Jahn–Teller dis-
tortion.
Both X-ray structures show that H3bphenim and
H3bleuim are comparable ligands, and that coordination
behavior is strongly related to two factors: the pH at which
the coordination is realized and the ability of the carboxyl-
ate functionality to bridge metal centers. In the case of 1,
bphenim behaves as a bis-tridentate ligand, and coordinates
two CuII ions in a symmetric arrangement. Once six coordi-
nation sites are used, there is no chance of forming a poly-
meric structure. In contrast, Hbleuim is a less symmetric
ligand because the imidazole ring is protonated and thus
provides a single coordination site (assuming that no tauto-
meric equilibrium is reached in solution). In that case, one
carboxylate O atom is available to bridge two metal centers,
and a 1D polymeric structure is stabilized.
Figure 2. ORTEP-like view[34] of cation [Cu2(Hbleuim)(H2O)3-
(NO3)]+ in compound 2, with ellipsoid displacements for non-hy-
drogen atoms at the 40% probability level. C-bonded H atoms have
been omitted, and the asymmetric unit is labeled. Nonlabeled
atoms are connected through the 21 axis parallel to [100], to form
the 1D polymer. Part of this chain is represented in the inset. Both
coordinated and uncoordinated nitrate ions are displayed, while
lattice water molecules are omitted.
Magnetic Studies
Solid-state, variable-temperature (2.0–300.0 K) dc mag-
netic susceptibility data using a 0.05 T field from 2 to 30 K
and a 1.0 T field from 30 to 300 K were collected from a
polycrystalline sample of 1. The resulting data are plotted
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
as χMT vs. T and χM vs. T (inset) in Figure 3. The χMT Table 1. Coupling constants and selected geometrical parameters
for imidazolato-bridged complexes.
values found for 1 match well with that of a dinuclear com-
J [cm–1]
α1 [°]
θ1 [°]
Ref.
plex with a strong antiferromagnetic interaction. The mag-
netic data were measured using two fields, 0.05 T (from 2 K
to 30 K) and 1.0 T (from 30 K to 300 K). The χMT value
Compound
[7b,25]
Cu3(iz–)2(iz)8(ClO4)4
Cu2(TMDT)(iz)(ClO4)2
–117
–52
162.9
161.9
158.9
157.5
176.2
159.4
176.5
176.9
176.6
70.0
91.8
68.8
5.8
4.7
86.5
0.7
at 300.0 K is 0.65 cm3 mol–1 K smaller than that expected Cu(macro)2+
–41
–38
–163
–48
–112
–140
–158
[5a]
[26]
Cu2(Gly–GlyO)2(iz)–
for two independent CuII centers (0.75 cm3 mol–1 K when g
= 2.0), implying that a strong coupling is already evident
at high temperatures. The magnetic susceptibility decreases
abruptly upon cooling to ca. 0.02 cmmol–1 K at 40 K and
then reaches a plateau that smoothly drops close to zero at
2 K. The best-fit parameters using the Hamiltonian in the
form H = –JS1S2, are: J = –158Ϯ2 cm–1 and g =
Cu2(bpim)3+
[7c,28]
[7d]
Cu2(phen)4(im)3+
Cu(LЈ)Cu(tfacac)2
[Cu(LЈ)Cu(tebima)]PF6
Cu2(bphenim)+
[29]
[29]
0.7
26.5
this work
Even though 2 exhibits a complicated structural compo-
sition (a chain of Cu2+ ions where each metal center is con-
nected, at the same time, to a terminal Cu2+ system), the
magnetic data shows a straightforward behavior character-
istic of a very weakly coupled Cu2+ species. Taking only the
two crystallographically different Cu2+ centers into ac-
count, the χMT value at 300 K was found to be
1.03 cm3 mol–1 K higher than that expected for two uncou-
pled S = 1/2 spins assuming g = 2.00 (0.75 cm3 mol–1 K).
The χMT value varies slightly from the highest temperatures
until approximately 50 K (0.98 cm3 mol–1 K), then drops
faster to 0.33 cm3 mol–1 K at 2 K. Because of the complexity
of the structure, fitting of the data was not feasible. Still,
the disposition of the magnetic orbitals in the structure
corroborates the almost nonexistent coupling between the
Cu centers. This way, in the chain, the Cu centers are con-
nected to each other through the O atoms from the carb-
oxylate group of the ligand in a syn-anti conformation. In
general, weak interactions are typical in carboxylate-
bridged CuII arrays where the carboxylate groups adopt
syn-anti conformations. Here, the dx2–y2 orbital of one Cu
atom related with the dz2 orbitals of the neighbors (equato-
rial–axial interactions) making the coupling almost inexis-
tent.[30] Furthermore, each Cu2+ in the chain coordinates to
a terminal mononuclear Cu2+ system through the ligand.
This time, the orbitals relating through L are both dx2–y2 in
nature, however, the ligand lies in such a way that a relevant
angle forms (113.45°) between the two planes containing
the magnetic orbitals and as a result the magnetic com-
munication is very poor. Overall, this unique 1D structure
depicts a very weak magnetic behavior, which in nature
seems to appear antiferromagnetic.
2.04Ϯ0.01 (TIP
=
120ϫ10–6 cm3 mol–1 and
R
=
7.6ϫ10–4). The J value corroborates the shape of the
graph, indicating antiferromagnetic coupling and the χM vs.
T data exhibits a maximum at approximately 140 K, char-
acteristic of strong AF coupled systems.[24] This J value is
similar to the most negative J value reported in the litera-
ture for imidazolato-bridged dicopper complexes, which is
J = –163 cm–1.[7c]
Figure 3. Fitting of the χMT vs. T and χM vs. T (inset) of 1 between
2.0 and 300.0 K. The experimental data are shown as black circles
and the red line corresponds to the theoretical values.
Several attempts have been made[7a,7b,7d] to correlate the
magnitude of the coupling constant with diverse structural
factors such as the Cu–N bond length and two angles: α,
the Cu–N–N angle and θ, the dihedral angle between the
imidazolate plane and the copper coordination plane, con-
sidering a square or square-pyramidal coordination geome-
try. A stronger coupling is expected when α approaches 180°
since this maximizes a σ interaction, while θ is expected to
be relevant only in the hypothesis of a π Cu–imidazolate
interaction; in which case a value of 90° would be optimal.
Table 1 shows these structural parameters for several
Solution Speciation Studies
Potentiometry, UV/Vis, and EPR Spectrophotometry
Since in our group there is interest in the effect of the
imidazolato-bridged dicopper complexes described in the solvent on catecholase activity, we pursued the pH-depend-
literature, with their corresponding J value. The large nega- ent speciation of our catalysts in the two solvent mixtures
tive value found for 1 is in agreement with previous analy- in which the catalytic studies were performed, MeOH/H2O
ses,[7b,7d] which have proposed that the α angles are the most and MeCN/H2O (1:1 v/v).
important of these parameters; the closer these angles are
to 180°, the stronger the coupling. This correlation suggests achieved through potentiometric titrations and treatment of
that σ bonding is the main path for the coupling.
the data with the program Hyperquad.[31] Four pKa values
The determination of the equilibrium constants was
4
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
Imidazolato Bridge Formation in Dicopper Complexes
were obtained for both ligands in MeOH/H2O correspond- in both of the solvent mixtures explored. The bridged spe-
ing to the two aliphatic amine groups, the imidazole, and cies predominates over the nonbridged one above pH =
two equivalent carboxylate groups. For H3bphenim these logKeq(1), as can be appreciated from the species predomi-
values are 9.64, 6.89, 3.95, and 2.64, while for H3bleuim the nation diagrams in Figure 4. In the four cases, this bridged
values are 10.0, 7.71, 4.47, and 3.5, respectively. For MeCN/ species is predominant through a wide pH range, roughly
H2O, only three pKa values could be refined for the two from pH 5–9, the point from where the monohydrolized
ligands. The values obtained for H3bphenim are 9.37, 6.73, form [Cu2LOH] becomes the most abundant. The corre-
and 3.56, and for H3bleuim they are 9.97, 7.53, and 3.74.
sponding pKa value for the deprotonation of a coordinated
The overall formation constants for the dicopper com- water molecule can be estimated from logβ201 – logβ2–11
plexes of these ligands in their different protonation states (Table 2).
in the two mentioned media are shown on Table 2. From
these values it is possible to estimate the value of the con-
stants of two chemically relevant equilibriums. The first one
corresponds to the formation of the bridged species with
the deprotonation of the pyrrolic nitrogen:
The formation of the imidazolato bridge can account for
the significant changes observed in the UV/Vis spectra of
both dinuclear complexes upon base addition. Very similar
behavior was observed for both complexes, both in MeOH/
H2O and in MeCN/H2O. Figure 5 shows the results ob-
tained for Cu2bleuim in MeCN/H2O. Consistent with what
was found through the potentiometric studies, the mixing
of one ligand equivalent with two Cu2+ equivalents causes
a marked decrease in pH because of the coordination of
both the carboxylate and the aliphatic amine groups.
The initial addition of base does not cause any change
in the position of the absorption maximum and a decrease
in absorbance is attributed merely to the dilution of the
complex while the free H+ is neutralized. Starting at pH =
3.9, the maximum begins to shift to smaller wavenumber
values, with a considerable increase in the intensity of the
absorption, similar to what has been described by other au-
thors.[5d,32]
[Cu2L+][H+]
(1)
(1)
Keq.
=
logKeq = logβ211 – logβ210
[Cu2LH2+
]
While the second one corresponds to the hydrolysis of one
of the metal ions in the complex:
An additional confirmation of the imidazolato-bridge
formation came from EPR spectroscopy. Room tempera-
ture MeOH/H2O solution and aqueous solution spectra are
shown in Figure 6. The initial spectrum may be associated
with the overlap of two sets of peaks corresponding to two
different copper(II) complexes in tetragonal environments,
the first in which the imidazole nitrogen is coordinated to
the metal together with the neighboring amino acid ligand
moiety, and the second in which only the other amino acid
ligand donor atoms are coordinated. As the pH rises upon
[Cu2L(OH)][H+]
(2)
Khyd
=
;
[Cu2L(H2O)+]
(2)
–logKhyd = logβ210 – log21–1 = pKa(hyd)
From the values obtained for Keq(1), it is evident that the the addition of base, the formation of the imidazolato
formation of the bridged [Cu2L]+ species is strongly favored bridge quenches the EPR signal because of the strong anti-
over the nonbridged [Cu2LH]2+ one, for both of the ligands ferromagnetic coupling. It is interesting to mention that this
Table 2. Equilibrium constants in MeOH/H2O and MeCN/H2O.
[CuxHyLz]
[Cu2+]x[H+]y[L]z
[Cu2L(H2O)6–i(OH)i][H+]
(i)
β =
Ka
=
[Cu2L(H2O)6–i+1(OH)i–1
]
H3bphenim
MeOH/H2O
H3bleuim
MeOH/H2O
Related equilibrium
MeCN/H2O
MeCN/H2O
logβ221
24.026Ϯ0.057
logβ211
20.503Ϯ0.051
logβ201
=
logβ221 = 21.689Ϯ0.035
logβ211 = 17.623Ϯ0.016
logβ201 = 12.286Ϯ0.020
2Cu2+ + L– + 2 H+ Ǟ Cu2H2L
=
logβ211
18.063Ϯ0.041
logβ201
=
logβ211 = 17.472Ϯ0.063 2Cu2+ + L– + H+ Ǟ Cu2HL
=
=
logβ201 = 12.581Ϯ0.053 2Cu2+ + L– Ǟ Cu2L
15.937Ϯ0.055
13.800Ϯ0.036
logKeq = 4.56
logKeq 5.33
(Cu2LH)2+ Ǟ Cu2L+ + H+
(1)
(1)
(1)
(1)
logKeq = 4.26
logKeq = 4.89
logβ2–11
=
logβ2–11
=
logβ2–11 = 2.855Ϯ0.027
logβ2–11 = 2.822Ϯ0.061 2Cu + L + OH Ǟ Cu2L(OH)+
pKa(hyd) = 9.759
Cu2L(H2O) Ǟ Cu2L(OH) + H+
6.860Ϯ0.056
pKa(hyd) = 9.077
4.557Ϯ0.037
pKa(hyd) = 9.243
pKa(hyd) = 9.431
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
Figure 4. Top: species distribution diagrams as a function of pH for 2Cu2+:H3bleuim in MeCN/H2O (left) and in MeOH/H2O (right).
Bottom: species distribution diagrams as a function of pH for 2Cu2+:H3bphenim in MeCN/H2O (left) and in MeOH/H2O (right).
Figure 5. UV/Vis spectra of a solution of 2Cu2+:1H3bleuim in
MeCN/H2O, upon base additions.
quenching is more pronounced when the spectra are re-
corded in an aqueous solution, and also at room tempera-
ture.
Figure 6. Room-temperature EPR spectra of 2Cu2+:1H3bphenim at
variable pH. Top: in MeOH/H2O, bottom: in H2O.
Cyclic Voltammetry
The presence of two reduction signals in dinuclear cop-
per complexes can be attributed to two different chemical between two metal ions in identical chemical environ-
environments for each copper ion, or to a strong coupling ments.[5e,33]
6
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
Imidazolato Bridge Formation in Dicopper Complexes
Figure 7 shows the cyclic voltammograms of the MeCN/ cleating ligands derived from imidazole and amines.[9b,9c,34]
H2O solutions of 2Cu2+:1H3bphenim, at two different pH These complexes have shown important catecholase ac-
values. On top at pH = 3.8, when from the corresponding tivity, in spite of the steric impossibility for the two Cu ions
speciation diagram only scarce formation of the bridged to act concertedly on the substrate. They all exhibit a slight
species is expected, it can be assumed that the dominant antiferromagnetic coupling in the solid state and their fro-
copper species is that labeled A in equilibrium (1). The two zen-solution EPR spectra display seven hyperfine lines in
observed reduction signals appear at Epc(1) = +0.05 V and the parallel region, associated with the coupling of an elec-
Epc(2) = –0.51 V, ΔE = 0.56 V. The first one can be assigned tron with two nuclei with S = 3/2, regardless of a separation
to the reversible reduction of the less coordinatively satu- between the two copper ions of more than 7 Å. Now, since
rated Cu2+ ion, i.e. the one coordinated to only two of the catecholase is a type III copper enzyme – EPR silent – with
three potentially available donor atoms the ligand has for two strongly coupled CuII centers and the complexes de-
each metal ion. The second signal can then be assigned to scribed in this paper have similar magnetic behavior, we de-
a more strongly coordinated Cu2+ ion bound to three donor cided to test their catecholase activity as well.
atoms from the ligand.
The catalytic oxidation of 3,5-di-tert-butylcatechol
(H2DTBC) has been widely studied as a model reaction for
the catecholase activity of tyrosinase and catechol oxidase.
This model substrate is oxidized to the corresponding
quinone (Scheme 2), while the CuII catalyst is reduced to
CuI and reoxidized by atmospheric oxygen. The kinetic
studies on the oxidation of H2DTBC were carried out by
the method of initial rates, monitoring the increase in the
characteristic quinone (DTBQ) absorption band at 400 nm
as a function of time.
Scheme 2. Catechol oxidation by oxygen.
Since the dicopper complexes of the ligands described in
this work undergo drastic changes as a function of pH be-
cause of the formation of the imidazolato bridge, we
studied the variation in catecholase activity of the com-
pounds Cu2bphenim and Cu2bleuim as a function of pH in
two solvent mixtures, MeCN/H2O and MeOH/H2O. This is
shown in Figure 8 where the effect of the buffered medium
in the absence of catalyst can be appreciated.
These results led us to choose a pH of 8.5, in both sol-
Figure 7. Voltammograms of
a
MeCN/H2O solution of vent mixtures, to determine the kinetic parameters (Table 3)
2Cu2+:1H3bphenim at pH = 3.8 (top) and pH = 7.5 (bottom).
since at this value the largest increment in activity was ob-
served compared to the activity of the medium without the
catalyst. The initial rates method was used for the analysis
of the kinetic profile obtained for the aerobic oxidation of
DTBC, which follows the Michaelis–Menten model. As we
previously reported for similar catalytic systems[9c] the cata-
lytic efficiency is significantly better in MeOH/H2O than in
MeCN/H2O: This was previously explained by electrochem-
ical studies that showed that these catalysts are easier to
regenerate, being oxidized from CuI to CuII, in MeOH/H2O
than in MeCN/H2O, where CuI is stabilized.
At pH = 7.5 (bottom), the two nonreversible peaks shift
to lower potential values and Epc(1) = –0.50 V and Epc(2)
= –0.64 V, and the separation becomes smaller, ΔE = 0.14 V.
At this pH the predominant species is B in equilibrium (1),
the two copper ions are in identical chemical environments,
bridged by the imidazolato group. The intense electronic
communication between the two copper ions, evidenced by
magnetic susceptibility and EPR spectroscopic measure-
ments, causes the presence of these two reduction signals.
At the optimal pH for the catalytic reaction we can see
from the species distribution diagrams (Figure 4) that in
both solvent mixtures and with both ligands, the dominant
Catecholase Activity
In our group we have been interested in the catecholase species is type “B” in equilibriums (1) and (2). However,
activity of a series of dinuclear copper complexes with dinu- this species is predominant over a wide pH range below 8.5,
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
7

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
Figure 8. The pH effect on the catecholase activity of Cu2bphenim in MeOH/H2O (top, left) and MeCN/H2O (top, right). pH effect on
the catecholase activity of Cu2bleuim in MeOH/H2O (bottom, left) and MeCN/H2O (bottom, right). (Concentrations in MeOH/H2O
were 10 times lower due to solubility).
Table 3. Kinetic parameters for Cu2bphenim and Cu2bleuim at pH
= 8.5 in MeOH/H2O and MeCN/H2O.
Conclusions
The dinucleating ligands presented here promote the for-
Cu2bphenim
Cu2bleuim
mation of imidazolato-bridged dicopper complexes, which
are stable over a wide range of pH values. A strong antifer-
romagnetic coupling between the copper ions in the bridged
species is observed, both in the solid state and in solution.
In the solid state, magnetic susceptibility studies gave a
value of J = –158 cm–1 for this coupling. The extent of the
coupling in solution was assessed by the quenching of the
EPR spectroscopic signal that accompanies the formation
of the bridged species. The cyclic voltammograms of these
solutions are also consistent with a strong electronic com-
munication between the two metal centers. Important cate-
cholase activity was found for these complexes at pH = 8.5,
which led us to propose a hydrolyzed species of the type
[Cu2LOH] as being catalytically active.
MeOH/H2O MeCN/H2O MeOH/H2O MeCN/H2O
R2
KM
Vmax
0.989
0.987
0.994
0.997
0.00098
0.00207
2.98ϫ10–6
2ϫ10–5
0.149
0.00120
1.86ϫ10–6
1ϫ10–5
0.186
0.0032
3.76ϫ10–6
2ϫ10–5
0.188
2.92ϫ10–6
[Cu2L] [m] 1ϫ10–5
kcat
kcat/KM
0.292
299
72
155
59
at which the catalytic efficiency is not as high. Besides, it is
possible to appreciate from the diagrams that at pH = 8.5,
the species of type “C” is present at a lower, but definitively
relevant, proportion.
It has been mentioned before that the OH– group coordi-
nated to the copper ions at the active site of catecholase[35]
has a basic role in the mechanism of catechol oxidation. In
many model systems described in the literature[36] this has
proven to be the case. The model substrate, H2DTBC is not
fully deprotonated[37] below pH = 10. We therefore propose
that the catalytically active form of the complex is “C”,
where the imidazolato bridge is formed and one of the coor-
dinated water molecules has been deprotonated.
Experimental Section
4,5-Bis(phenylalanyl-N-methyl)-2-methylimidazole (H3bphenim) and
4,5-Bis(leucyl-N-methyl)-2-methylimidazole (H3bleuim): The ligands
H3bphenim and H3bleuim were prepared according to a procedure
described previously.[38] 2-Methylimidazole (2.05 g, 0.025 mol) was
dissolved in distilled water (150 mL) together with the correspond-
ing amino acid (0.05 mol, phenylalanine for H3bphenim and leu-
cine for H3bleuim). A concentrated KOH solution was added drop-
wise until these reactants were completely dissolved. Formaldehyde
(6 mL, 0.075 mol, 37% aqueous solution) was added dropwise to
this mixture while stirring. Finally, the mixture pH was adjusted to
12 with a concentrated KOH solution and the solution was stirred
The slightly higher catalytic efficiency of Cu2bpheim
compared to Cu2bleuim must be related to the aromatic
moiety in the former interaction, which can participate in a
π–π interaction with the substrate and in which the ali-
phatic residue of the latter cannot engage.
8
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
Imidazolato Bridge Formation in Dicopper Complexes
at 55 °C for 24 h. After this time, the solution was neutralized with
acetic acid. The solid obtained was collected and washed a couple
of times with water and ethanol.
Table 4. Crystal data for the complexes.
1
2
Empirical formula
Mr [gmol–1]
Color, habit
Crystal size [mm]
Space group
a [Å]
C24H38.72ClCu2N4O14.86 C18H38Cu2N6O14
783.57
H3bphenim: C24H28N4O4·2H2O: calcd. C 61.00, H 6.83, N 11.86;
689.62
1
found C 61.24, H 7.00, N 12.06. H NMR (D2O) (see Scheme 1):
green, irregular
0.6ϫ0.4ϫ0.2
P4132
21.549(2)
–
blue, prism
0.42ϫ0.20ϫ0.18
P212121
7.3647(16)
19.050(2)
21.0189(19)
2948.9(8)
4, 1
1.553
1.514
4–52.5
5643
δ = 2.37 [s, 3 H, –CH3; (a)], 3.10 [o, 4 H, –CH2-φ; (d)], 4.23 [s, 4
H, –CH2–Im; (b)], 4.27 [t, 2 H, alpha C=O, (c)], 7.1 [m, 10 H,
phenyl; (e)]; yield 28%.
b [Å]
H3bleuim: C18H32N4O4·2H2O: calcd. C 53.47, H 7.92, N 13.86;
c [Å]
–
1
found C 53.91, H 8.07, N 14.12. H NMR (D2O): δ = 1 [q, 12 H,
V [Å3]
10006.6(18)
12, 1/2
1.560
1.427
4–45
CH3; (h)], 1.8 [m, 4 H, –CH2–; (f)], 1.95 [m, 2 H, –CHϽ; (g)], 2.7
(s, 3 H, CH3–Im), 4.4 [t, 2 H, alpha C, (c)], 4.7 [c, 4 H, –CH2–Im;
(b)]; yield 19%.
13C NMR spectroscopic data are available in the Supporting Infor-
mation.
Z, ZЈ
ρcalcd. [gcm–3]
μ [mm–1]
2θ range [o]
Reflections collected
Independent reflections
9608
2192 (0.058)
5057 (0.022)
(Rint
)
[Cu2(bphenim)(H2O)2]NO3 and [Cu2(bphenim)(H2O)2]ClO4: Crys-
tals of the copper complex Cu2bphenim with nitrate or perchlorate
can be obtained as described previously,[38] from a 50% mixture of
an aqueous solution buffered to pH 7.5 with HEPES and acetoni-
trile (MeCN + H2OpH 7.5), mixing H3bphenim·2H2O (0.47 g,
1 mmol) with Cu(NO3)2·2.5H2O (0.464 g, 2 mmol), or Cu(ClO4)2·
6H2O (0.741 g, 2 mmol). The dark blue solutions obtained were
allowed to stand for several days until crystallization occurred.
[Cu2(C24H25N4O4)(H2O)4]NO3·4H2O: calcd. C 37.60, H 5.34, N
9.13; found C 37.98, H 4.97, N 9.07. [Cu2(C24H25N4O4)(H2O)4]-
ClO4·4H2O: calcd. C 35.84, H 5.1, N 6.97; found C 36.1, H 4.92,
N 7.08.
Transmission factors
(min., max.)
Final R indices
[IՆ2σ(I)] R1, wR2
0.220–0.298
0.094, 0.215
0.244–0.280
0.044, 0.099
0.062, 0.107
Final R indices (all data) 0.119, 0.226
R1, wR2
Goodness-of-fit on F2
Data/restraints/
parameters
1.84
2192/24/210
1.06
5057/102/379
Largest difference peak/ 0.603, –0.628
0.442, –0.375
hole [eÅ–3]
Residuals Rint, R1, and wR2 are standard indices computed by
SHELXL
[Cu2(Hbleuim)(H2O)3(NO3)]NO3·H2O: H3bleuim·2H2O (0.404 g,
1 mmol) was dissolved in water (20 mL), which contained Cu(NO3)2·
2.5H2O (0.464 g, 2 mmol), and was left to stand. The pH of the
solution was approximately 1.5. Deep blue crystals were found after
2 w. [Cu2(C18H30N4O4)(NO3)(H2O)]NO3·H2O: calcd. C 31.35, H
5.52, N 12.19; found C 31.54, H 5.63, N 11.93.
ual considering that dϾ0.9 Å. However, a sensible geometry is ob-
tained for the cation, which is the residue of interest.
Complex 2 was much easier to refine, since all residues are ordered,
including one lattice water molecule (O38). Nitrate ions, however,
present high displacement parameters, which were refined through
similarity restraints and forced to approximate an isotropic behav-
ior. C- and N-bonded H atoms were treated as in 1, and water H
atoms were found in a difference map, and refined with free coordi-
nates and restrained geometry. In both crystals, the absolute config-
uration was determined by refining a Flack parameter, x = –0.03(6)
for 1 and x = 0.00(2) for 2, which accounts for S chiral centers in
the ligands (C6 in 1 and C9/C19 in 2).
Crystal Structures: X-ray diffraction data for complexes [Cu2-
(bphenim)(H2O)4]ClO4·2.86(H2O) (1) and [Cu2(Hbleuim)(H2O)3-
(NO3)]NO3·H2O (2) were collected at room temperature with a
Bruker P4 diffractometer[39] using Mo-Kα radiation (Table 4). Raw
data were corrected for absorption effects[40] and the structures re-
fined with the SHELX programs.[41]
Complex 1 only gave very poorly diffracting single crystals and
resolution for data was limited to d = 0.93 Å (2θmax = 45°), since
beyond this angle no diffraction was observed at all. The position
of the cation was easily determined by direct methods, while other
residues were difficult to locate as a consequence of their very large
and anisotropic displacement parameters. The perchlorate anion is
equally disordered over two sites in the asymmetric unit, both with
a threefold symmetry. The geometry of the anions was regularized
through suitable restraints on Cl–O bond lengths and O···O separa-
tions. Similarity restraints on ADP were applied on O atoms, and
atom O31 was refined isotropically with a displacement parameter
fixed to 0.4 Å2. At this point, the largest peak in a difference map
was assigned to a water lattice molecule (O41) with full occupancy.
All C- and N-bonded H atoms were placed in idealized positions
(riding refinement). Positions for water H atoms were first refined
freely, although with restricted geometries, and eventually forced to
ride on the O atoms in the last least-squares cycles. This model (R1
= 10% for observed reflections) still contained solvent accessible
regions and was further refined after applying the SQUEEZE pro-
cedure,[19] which recovered 103 electrons per cell, assigned to disor-
dered water molecules (0.858 H2O for each cation). The final re-
finement converges to R1 = 9.38% [IՆ2σ(I)], a disappointing resid-
Full details about these models may be obtained from the archived
CIF file, and diffraction data are available on request. CCDC-
866651 (for 1) and -866652 (for 2) contain the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Magnetic Measurements: Magnetic measurements were carried out
in the “Unitat de Mesures Magnètiques (Universitat de Barce-
lona)” on polycrystalline samples with a Quantum Design SQUID
MPMS-XL magnetometer working in the 2.0–300 K range. The
magnetic fields were 0.05 T (from 2.0 to 30 K) and 1.0 T (from 30
to 300 K) for compounds 1 and 2. The diamagnetic corrections
were evaluated from Pascal’s constants. The fit was performed by
minimizing the function R (agreement factor defined as):[24]
Σ[(χMT)exp – (χMT)calcd]2 / Σ[(χMT)exp 2
]
Potentiometric Studies: For all solution studies spectrophotometric
grade solvents were mixed with bidistilled water.
Potentiometric titrations were performed in two different media:
MeOH/H2O (1:1 v:v) and MeCN/H2O (1:1 v:v) using a Methrom
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
9

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
702 Titrino set including a digital pH meter coupled to a PC, fitted
with a Methrom Ag|AgCl combined electrode that was calibrated
differently in each media.
benzoquinone (3,5-DTBQ) over time (εMeOH/H2O = 1640 m–1 cm–1
and εMeOH/H2O = 1630 m–1 cm–1) was monitored spectrophotomet-
rically with an Agilent 8453 diode-array spectrophotometer with a
magnetically stirred and thermostatted quartz cell with a 1 cm path
length at (25Ϯ0.1) °C. The catalytic activity of the complexes was
determined by the method of initial rates by obtaining the slope of
the A vs. t plot over the first 15 to 120 s of the reaction.
In the solvent mixtures a single point calibration was made using
a succinate buffer in MeOH/H2O (pH = 4.94)[42] and a biphthalate
buffer in MeCN/H2O (pH = 5.37)[43] followed by a HClO4 titration
in the solvent mixture where values of pH and E were collected for
each titration point in order to obtain the parameter of cell effi-
ciency Ef by means of the equation:[44]
Three sets of experiments were performed in each medium. In the
first one, aimed to observe the effect of the catalyst concentration
on the initial rate of the reaction, the concentration of catalyst was
varied from 1ϫ10–5 to 6ϫ10–5 m at a constant pH and the concen-
pHcal – pHexp
pHcorr = pHexp + [
]Ef
tration of 3,5-DTBC was 6ϫ10–3 m in MeOH/H2O and 2ϫ10–3
m
pHcal
where pHcorr is the corrected pH, pHexp the experimental pH, pHcal
the pH value used for the calibration of the electrode and Ef the cell
efficiency by which all data should be corrected. Ef was obtained by
setting the pH vs. E slope to the Nernst slope (–59.16).
in MeCN/H2O. This series of experiments was performed at pH =
7.0, 7.5, 8.0, 8.5, 9.0, and 9.5 fixed with HEPES (pH 7.0, 7.5, 8.0)
and TRIS (pH 8.5, 9.0, 9.5) buffers (0.1 m).
In the second one, the pH was varied from 6.0 to 9.5 and its influ-
ence on the initial rate of the reaction was evaluated with and with-
Each solvent mixture’s autoionization constant was determined by
performing the titration of standardized HClO4 solutions with
standardized NaOH and refining the corresponding pKi with Hy-
perquad. pKi for MeOH/H2O was 14.28 and for MeCN/H2O was
14.74.
out a catalyst. The concentration of 3,5-DTBC used was 2ϫ10–3
m
in MeOH/H2O and 2ϫ10–2 m in MeCN/H2O, when needed, the
concentration of the catalyst used was 3ϫ10–5 m in MeOH/H2O
and 3ϫ10–4 m in MeCN/H2O. The pH was fixed with buffers
HEPES and TRIS as stated above and values under pH = 7.0 were
fixed with MES buffer, all at a 0.1 m concentration.
In all the solution studies, standardized copper perchlorate and
perchloric acid solutions were used. In each medium two systems
of each ligand were titrated (three times each): L + 4H+ (ligand
fully protonated) and L + 2CuII + 4H+ (ligand protonated in the
presence of copper). The ionic strength was maintained at 0.1 m
with NaClO4. Titrations were performed the usual way: 25 mL of
the studied solution was contained in a 75 mL jacketed glass cell
thermostated at 25.00(Ϯ0.05) °C by a circulating constant-tem-
perature water bath and purged with a purified nitrogen stream.
Additions of a standardized ca. 0.1 m carbonate free NaOH water
solution were performed and the pH was determined after each
addition.
In the last set of experiments the concentration of substrate was
varied from 1ϫ10–4 m to 2.5ϫ10–3 m in MeOH/H2O and from
6ϫ10–4 m in MeCN/H2O at a constant pH = 8.5 fixed with HEPES
(0.15 m) and a 1ϫ10–5 m in MeOH/H2O and 2ϫ10–5 m in MeCN/
H2O concentration of catalyst in order to observe the influence of
the substrate concentration on the initial rate of the reaction. This
last set of experiments was fitted to a Michaelis–Menten model
and the parameters Vmax, kcat, and KM were determined.
Supporting Information: χMT vs. T and χM vs. T curves for 2. 13C
NMR spectroscopic data for ligands.
UV/Vis Spectrophotometric Studies: Titrations of the L + 2CuII
+
4H+ system with a carbonate free NaOH water solution in H2O,
MeOH/H2O, and MeCN/H2O were followed simultaneously
potentiometrically and spectrophotometrically. After each NaOH
addition, absorption spectra were recorded with an Agilent 8453
spectrophotometer and pH values were measured with a Metrohm
Ag/AgCl combined electrode that had been previously calibrated.
Acknowledgments
N. A.-A. thanks the Ministerio de Educación y Ciencia (CTQ2009-
06959/BQU) for financial support. Also, the work performed at the
Universitat de Barcelona by Dr. Aliaga-Alcalde was supported by
ICREA (Institució Catalana de Recerca i Estudis Avançats). L. G.
and I. M.-P. thank UNAM-DGAPA for economic support via pro-
ject PAPIIT 210109. L. G. wishes to thank Prof. Peter Gans for
assistance on the use of HYPERQUAD.
EPR Studies: X-band EPR spectra of systems Cu/L, 2:1 in H2O
and MeOH/H2O with different pH values ranging from 2.5 to 9
(adjusted with NaOH and/or HNO3) were recorded with a Bruker
ER200-SRC spectrometer with a cylindrical cavity at room tem-
perature.
[1] G. Yagil, Tetrahedron 1967, 23, 2855.
Electrochemical Studies: Cyclic voltammetric studies were per-
formed on the Cu/L, 2:1 systems with an AUTOLAB PGSTAT 100
potentiostat/galvanostat in MeCN/H2O at pH values of 3.8 and
7.5. The experiments were carried out under a nitrogen atmosphere
in a conventional three-electrode cell composed of: a reference elec-
trode Ag/AgCl(s)/0.1 m KCl in the corresponding solvent mixture;
a platinum wire as auxiliary electrode, and a glassy carbon disk as
working electrode. Cyclic voltammograms were performed over a
potential range of –1.0 to 1.2 V vs. Ag/AgCl at a sweep rate of
100 mVs–1 and were initiated from an open circuit potential (Ei=0).
The 10–3 m complex solutions were prepared in situ. The buffer also
played the role of the supporting electrolyte.
[2] C. R. Johnson, W. W. Henderson, R. E. Sheperd, Inorg. Chem.
1984, 23, 2754.
[3] a) R. B. Martin, J. T. Edsall, J. Am. Chem. Soc. 1960, 82, 1107;
b) J. E. Bauman Jr., J. C. Wang, Inorg. Chem. 1964, 3, 368.
[4] J. S. Richardson, K. A. Thomes, B. H. Rubin, D. C. Richard-
son, Proc. Natl. Acad. Sci. USA 1975, 72, 1349.
[5] a) C.-L. O’Young, J. C. Dewan, H. R. Lilienthal, S. J. Lippard,
J. Am. Chem. Soc. 1978, 100, 7291; b) P. K. Coughlin, J. C.
Dewan, S. J. Lippard, E.-i. Watanabe, J. M. Lehn, J. Am. Chem.
Soc. 1979, 101, 265; c) P. K. Coughlin, S. J. Lippard, A. E.
Martin, J. E. Bulkowski, J. Am. Chem. Soc. 1980, 102, 7616; d)
G. Kolks, C. R. Firhart, P. K. Coughlin, S. J. Lippard, Inorg.
Chem. 1981, 20, 2933; e) P. K. Coughlin, S. J. Lippard, Inorg.
Chem. 1984, 23, 1446.
[6] a) Q. Yuan, K. Cai, Z.-P. Qi, Z.-S. Bai, Z. Su, W.-Y. Sun, J.
Inorg. Biochem. 2009, 103, 1156; b) Z.-P. Qi, K. Cai, Q. Yuan,
T. Okamura, Z.-S. Bai, W.-Y. Sun, N. Ueyama, Inorg. Chem.
Commun. 2010, 13, 847.
Catecholase Activity: The catecholase activity of the complexes was
evaluated by the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC)
in MeOH/H2O and MeCN/H2O. The increase of the absorption
band at 400 nm from the formation of the 3,5-di-tert-butyl-o-
10
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
Imidazolato Bridge Formation in Dicopper Complexes
[7]
[26] M. G. B. Drew, M. McCann, S. M. Nelson, J. Chem. Soc., Dal-
ton Trans. 1981, 1868.
[27] K. Matsumoto, S. Ooi, Y. Nakao, W. Mori, A. Nakahara, J.
Chem. Soc., Dalton Trans. 1981, 2045.
a) A. Bencini, C. Benelli, D. Gatteschi, C. Zanchini, Inorg.
Chem. 1986, 25, 398; b) C. Benelli, R. K. Bunting, D. Gattes-
chi, C. Zanchini, Inorg. Chem. 1984, 23, 3074; c) G. Kolks, S. J.
Lippard, J. V. Waszczak, H. R. Lilienthal, J. Am. Chem. Soc.
1982, 104, 717; d) S. M. Morehouse, Inorg. Chim. Acta 1996,
243, 327.
a) R. Watanabe, N. Koyama, T. Nogami, T. Ishida, T. Kogane,
Chem. Lett. 2009, 38, 30; b) N. Koyama, R. Watanabe, T. Ish-
ida, T. Nogami, T. Kogane, Polyhedron 2009, 28, 2001.
a) G. Mendoza-Díaz, W. L. Driessen, J. Reedijk, Acta Crys-
tallogr., Sect. C 1996, 52, 960; b) L. Gasque, V. M. Ugalde-
Saldívar, I. Membrillo, J. Olguín, E. Mijangos, S. Bernes, I.
González, J. Inorg. Biochem. 2008, 108, 1227; c) L. González-
Sebastián, V. M. Ugalde-Saldívar, E. Mijangos, M. R. Men-
doza-Quijano, L. Ortiz-Frade, L. Gasque, J. Inorg. Biochem.
2010, 104, 1112.
G. Mendoza-Díaz, W. L. Driessen, J. Reedijk, S. Gorter, L.
Gasque, K. R. Thompson, Inorg. Chim. Acta 2002, 339, 51.
L. Gasque, unpublished results.
L. Gasque, E. Mijangos, S. Bernes, Acta Crystallogr., Sect. E
2005, 61, o284.
a) A. W. Addison, T. N. Rao, J. Reedijk, J. v. Rijn, G. C. Ver-
schoor, J. Chem. Soc., Dalton Trans. 1984, 1349; b) E. L. Muet-
terties, I. J. Guggenberger, J. Am. Chem. Soc. 1974, 96, 1748.
I. J. Bruno, J. C. Cole, M. Kessler, J. Luo, W. D. S. Motherwell,
L. H. Purkis, B. R. Smith, R. Taylor, R. I. Cooper, S. E. Harris,
A. G. Orpen, J. Chem. Inf. Comput. Sci. 2004, 44, 2133.
B.-L. Lee, M. D. Kärkäs, E. V. Johnston, A. K. Inge, L.-H.
Tran, Y. Xu, Ö. Hansson, X. Zou, B. Åkermark, Eur. J. Inorg.
Chem. 2010, 5462.
[28] J. C. Dewan, S. J. Lippard, Inorg. Chem. 1980, 19, 2079.
[29] N. Matsumoto, T. Akui, H. Murakami, J. Kanesaka, A.
¯
Ohyoshi, H. Okawa, J. Chem. Soc., Dalton Trans. 1988, 1021.
[8]
[9]
[30] a) H. Arora, F. Lloret, R. Mukherjee, Inorg. Chem. 2009, 48,
1158; b) J. Cano, P. Alemany, S. Alvarez, M. Verdaguer, E.
Ruiz, Chem. Eur. J. 1998, 4, 476.
[31] P. Gans, A. Sabatini, A. Vacca, Talanta 1996, 43, 1739.
[32] a) B. Verdejo, S. Blasco, E. García-España, F. Lloret, P. Ga-
viña, C. Soriano, S. Tatay, H. R. Jiménez, A. Domenech, J.
Latorre, Dalton Trans. 2007, 4726; b) R. N. Patel, N. Singh,
K. K. Shukla, V. L. N. Gundla, Spectrochim. Acta Part A 2005,
61, 1893.
[33] a) H. Ohtsu, Y. Shimazaki, A. Odani, O. Yamauchi, W. Mori,
S. Itoh, S. Fukuzumi, J. Am. Chem. Soc. 2000, 122, 5733; b)
G. Tabbi, W. L. Driessen, R. Reedijk, R. P. Bonomo, N. Veld-
man, A. L. Spek, Inorg. Chem. 1997, 36, 1168.
[34] M. R. Mendoza-Quijano, G. Ferrer-Sueta, M. Flores-Álamo,
N. Aliaga-Alcalde, V. Gómez-Vidales, V. M. Ugalde-Saldívar,
L. Gasque, Dalton Trans. 2012, DOI: 10.1039/C2DT12155G.
[35] C. Gerderman, C. Eicken, B. Krebs, Acc. Chem. Res. 2002, 35,
183.
[36] a) J. Kaizer, T. Csay, G. Speier, M. Giorgi, J. Mol. Catal. A
2010, 329, 71; b) A. Granata, E. Monzani, L. Casella, J. Biol.
Inorg. Chem. 2004, 9, 903; c) C. Fernandes, A. Neves, A. J.
Bortoluzzi, A. S. Mangrich, E. Rentschler, B. Szpoganicz, E.
Schwingel, Inorg. Chim. Acta 2001, 320, 12; d) C. Belle, K.
Selmeczi, S. Torelli, J.-L. Pierre, C. R. Chim. 2007, 10, 271.
[37] S. V. Jovanovic, K. Konya, J. C. Scaiano, Can. J. Chem. 1995,
73, 1803.
[10]
[11]
[12]
[13]
[14]
[15]
[16] S. Li, D. Li, D. Yang, Y. Li, J. Huang, K. Yu, W. Tang, Chem.
Commun. 2003, 880.
[17] T. Higa, M. Moriya, Y. Shimazaki, T. Yajima, F. Tani, S. Kara-
sawa, M. Nakano, Y. Naruta, O. Yamauchi, Inorg. Chim. Acta
2007, 360, 3304.
[38] A. M. Sosa, V. M. Ugalde-Saldívar, L. Gasque, I. González, J.
Electroanal. Chem. 2005, 579, 103.
[39] XSCAnS (rel. 2.21), User Manual, Siemens Analytical X-ray
Instruments Inc., Madison, WI, USA, 1996.
[40] A. C. T. North, D. C. Phillips, F. S. Mathews, Acta Crystallogr.,
Sect. A 1968, 24, 351.
[41] G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112.
[42] T. Mussini, A. K. Covington, P. Longhi, S. Rondinini, Pure
Appl. Chem. 1985, 57, 865.
[18] J.-L. Lin, Y.-Q. Zheng, Z. Kristallogr. New Cryst. Struct. 2004,
219, 431.
[19] a) A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7; b) P.
van der Sluis, A. L. Spek, Acta Crystallogr., Sect. A 1990, 46,
194.
[20] C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington, P.
McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J.
van de Streek, P. A. Wood, J. Appl. Crystallogr. 2008, 41, 466.
[21] F. H. Allen, Acta Crystallogr., Sect. B 2002, 58, 380.
[22] P. Kopel, Z. Trávnícˇek, J. Marek, M. Korabik, J. Mrozinski,
Polyhedron 2003, 22, 411.
[23] M. Lubben, R. Hage, A. Meetsma, K. Bÿma, B. L. Feringa,
Inorg. Chem. 1995, 34, 2217.
[24] O. Kahn, Molecular Magnetism, Wiley-VCH, New York, 1993.
[25] G. Ivarson, B. K. S. Lundberg, N. Ingri, Acta Chem. Scand.
1972, 26, 3005.
[43] S. Rondinini, P. R. Mussini, T. Mussini, Pure Appl. Chem.
1987, 11, 1549.
[44] J. M. Islas-Martínez, D. Rodríguez-Barrientos, A. Galano, E.
Ángeles, L. A. Torres, F. Olvers, M. T. Ramírez-Silva, A. Rojas-
Hernández, J. Phys. Chem. B 2009, 113, 11765.
[45] I. Puigdomenech, MEDUSA (Making Equilibrium Diagrams
Using Sophisticated Algorithms), http://www.kemi.kth.se/
medusa/
Received: February 20, 2012
Published Online: ᭿
Eur. J. Inorg. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
11

Job/Unit: I20167
/KAP1
Date: 29-05-12 10:08:19
Pages: 12
L. Gasque et al.
FULL PAPER
Dicopper Complexes
The dicopper complexes of two dinucleat-
ing ligands undergo a strong antiferromag-
netic coupling upon formation of an imid-
azolato bridge. UV/Vis and EPR spec-
troscopy as well as electrochemical studies
confirmed the potentiometric solution spe-
ciation.
I. Monte-Pérez, A. M. Sosa, S. Bernès,
N. Aliaga-Alcalde, V. M. Ugalde-Saldívar,
L. Gasque* ...................................... 1–12
pH-Dependent Imidazolato Bridge Forma-
tion in Dicopper Complexes: Magnetic,
Electrochemical, and Catalytic Reper-
cussions
Keywords: Copper / N ligands / Enzymes /
Amino acids / Cyclic voltammetry / Anti-
ferromagnetism
12
www.eurjic.org
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 0000, 0–0