Dinuclear CuII complexes with a new phenol-based ligand bearing pyridine and thiophene substituents: Synthesis, characterization and interaction with catechol substrates

Article abstract of DOI:10.1002/ejic.200400092

The reaction of the phenol-based ligand 4-methyl-2,6-bis{[(2-methylpyridyl) (2-methylthiophenyl)amino]methyl}phenol-(Hpy2th2s), containing pyridine and thiophene substituents, with copper(II) chloride and bromide yields two new dinuclear complexes with the composition [Cu₂py2th2s)X₃], where X = Cl or Br. In both complexes, the copper(II) ions are pentacoordinate and bridged by the deprotonated phenolate anion and by one halogen anion, Both complexes exhibit geometric asymmetry, as the coordination environment around one of the two copper ions is square-pyramidal, whereas the geometry around the other can be best described as a distorted trigonal bipyramid. The complexes were characterized by means of X-ray single-crystal diffraction, ligand field and EPR spectroscopy, mass spectrometry, and electrochemically. Magnetic susceptibility measurements indicate an antiferromagnetic coupling between the two metal centers (2J ≈ -200 cm^-1). The interaction of the complexes with model substrates 3,5-di-tert-butylcatechol and tetrachlorocatechol is reported. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400092

FULL PAPER
Dinuclear CuII Complexes with a New Phenol-Based Ligand Bearing Pyridine
and Thiophene Substituents: Synthesis, Characterization and Interaction with
Catechol Substrates
Iryna A. Koval,[a] Mieke Huisman,[a] Arno F. Stassen,[a] Patrick Gamez,[a] Olivier Roubeau,[b]
Catherine Belle,*[c] Jean-Louis Pierre,[c] Eric Saint-Aman,[d] Matthias Lüken,[e]
Bernt Krebs,[e] Martin Lutz,[f] Anthony L. Spek,[f] and Jan Reedijk*[a]
Keywords: Copper / Dinuclear complexes / Catechol oxidase / Magnetochemistry / Electrochemistry
The reaction of the phenol-based ligand 4-methyl-2,6-bis{[(2-
methylpyridyl)(2-methylthiophenyl)amino]methyl}phenol-
(Hpy2th2s), containing pyridine and thiophene substituents,
with copper(II) chloride and bromide yields two new dinu-
scribed as a distorted trigonal bipyramid. The complexes
were characterized by means of X-ray single-crystal diffrac-
tion, ligand field and EPR spectroscopy, mass spectrometry,
and electrochemically. Magnetic susceptibility measure-
clear complexes with the composition [Cu2(py2th2s)X3], ments indicate an antiferromagnetic coupling between the
where X = Cl or Br. In both complexes, the copper(II) ions are
pentacoordinate and bridged by the deprotonated phenolate
anion and by one halogen anion. Both complexes exhibit
geometric asymmetry, as the coordination environment
around one of the two copper ions is square-pyramidal,
whereas the geometry around the other can be best de-
two metal centers (2J ഠ −200 cm−1). The interaction of the
complexes with model substrates 3,5-di-tert-butylcatechol
and tetrachlorocatechol is reported.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
[3-(3,4-dihydroxyphenyl)alanine] (cresolase activity) and its
subsequent oxidation to dopaquinone (catecholase ac-
The copper-containing proteins with a type-3 active site, tivity), the initial steps in melanin biosynthesis. Catechol
e.g. hemocyanin, tyrosinase and catechol oxidase, have the oxidase lacks the cresolase activity of tyrosinase and only
ability to reversibly bind and activate dioxygen at ambient catalyzes the aerial oxidation of o-diphenols to the corre-
conditions.[1] For this reason, modeling of the active site sponding o-diquinones.
of these proteins has attracted significant scientific interest
The crystal structure of catechol oxidase has been deter-
during the last few decades.[2Ϫ8] Hemocyanin functions as a mined in 1998 by Krebs and co-workers.[9] Its active site is
dioxygen-carrier protein in some mollusks and arthropods, characterized by a dinuclear copper center, in which each
whereas tyrosinase and catechol oxidase utilize dioxygen to copper ion is surrounded by nitrogen donor atoms of three
perform selective oxidation of various organic substrates. histidine residues from a protein backbone. A catalytic cycle
Tyrosinase catalyzes the hydroxylation of tyrosine to dopa for catechol oxidase activity has been proposed from a com-
bination of biochemical, spectroscopic, and structural
[a]
data[10] and by comparison with the tyrosinase-related cy-
Leiden Institute of Chemistry, Leiden University,
P. O. Box 9502, 2300 RA Leiden, The Netherlands
cle.[1] It begins with the interaction of the catechol substrate
Fax: (internat.) ϩ 31-71-527-42-61
with the met form of the enzyme, which contains two cop-
per() ions singly bridged by a hydroxo group, resulting in
the formation of a stoichiometric amount of the quinone.
Dinucleating phenol-based compartmental ligands have
been previously successfully applied to model the type-3 ac-
tive site.[6,7,11,12] Recent studies on the copper complexes of
H-BPMP ligand, published by some of us,[7] allowed to pro-
pose a new catalytic mechanism, emphasizing the role of
the µ-hydroxo bridge between the two metal centers. It in-
cludes the monodentate coordination of the substrate to
one of the metal centers with the concomitant cleavage of
the OH bridge, and the subsequent proton transfer from
E-mail: reedijk@chem.leidenuniv.nl
[b]
[c]
Centre de Recherche Paul Pascal Ϫ CNRS UPR 8641,
115 avenue du Dr. A. Schweitzer, 33600 Pessac, France
´
´
LEDSS, Chimie Biomimetique, UMR CNRS 5616, Universite
J. Fourier,
B. P. 53, 38041 Grenoble Cedex, France
E-mail: Catherine.Belle@ujf-grenoble.fr
Laboratoire d’Electrochimie Organique et de Photochimie
[d]
[e]
[f]
´
Redox, UMR CNRS 5630, Universite J. Fourier,
B. P. 53, 38041 Grenoble Cedex, France
Institut für Anorganische und Analytische Chemie der
Westfälischen Wilhelms-Universität Münster,
Wilhelm-Klemm-Strasse 8, 48149 Münster, Germany
Bijvoet Center for Biomolecular Research, Crystal and
Structural Chemistry, Utrecht University,
Padualaan 8, 3584 CH Utrecht, Netherlands
4036
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ejic.200400092
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Dinuclear CuII Complexes with a New Phenol-Based Ligand
FULL PAPER
the second OH group of the catechol substrate to the ligand Hpy2th2s with two pendant arms, containing pyri-
hydroxy group bound to the second copper center.[7] The dine and thiophene residues. The synthesis of this ligand is
release of a water molecule results in a bridging coordi- depicted in Figure 1.
nation of the catecholate, which undergoes an oxidation to
The starting compound for the ligand synthesis, (2-picol-
quinone. In order to further demonstrate the importance yl)(2-thiophenyl)amine, was prepared by reaction of the
of a µ-hydroxo bridge on the catalytic cycle, we report the commercially available 2-formylthiophene and (pyridin-2-
synthesis of two dinuclear copper() chloride and bromide ylmethyl)amine, followed by the reduction of the in situ
complexes with the new phenol-based ligand 4-methyl-2,6- generated imine by NaBH4. Another starting compound
bis[[(2-methylpyridyl)(2-
2,6-bis(chloromethyl)-4-methylphenol was prepared as de-
methylthiophenyl)amino]methyl}phenol (abbreviated as scribed previously.[16] The reaction of 2,6-bis(chloromethyl)-
Hpy2th2s), their structural, spectroscopic, magnetic and 4-methylphenol with a stoichiometric amount of (2-picol-
electrochemical properties, and their interaction with the yl)(2-thiophenyl)amine in the presence of excess NEt3 re-
model substrates 3,5-di-tert-butylcatechol (3,5-DTBC) and sulted in the formation of the ligand, which was isolated as
tetrachlorocatechol (TCC). In these complexes, the two me- a transparent light yellow oil. The reaction of Hpy2th2s
tal ions are doubly bridged by the oxygen atom of the phe- with copper() chloride and bromide led to the formation
nolate group and a halogen anion. It can be expected that of two new dinuclear copper complexes, which were isolated
the substitution of the µ-hydroxo group by a halogen anion as dark brown and dark purple crystals, respectively.
should hamper the cleavage of the bridge and the sub-
sequent proton transfer, thereby preventing the coordi-
nation of the catecholate in the bridging bidentate fashion,
Characterization of the Complexes
and thus hampering the catalytic cycle.
Crystal Structure Descriptions
Results and Discussion
[Cu2(py2th2s)Cl3]·CH3OH (1)
Rectangular reddish-brown crystals of complex 1 were
obtained by diethyl ether diffusion into a methanol solution
containing stoichiometric amounts of copper() chloride
and the ligand. A Platon[17] projection of the crystal struc-
ture of the complex is shown in Figure 2. Selected bond
lengths and angles are presented in Table 1. The complex
crystallizes as methanol solvate in space group P21/n with
four formula units present per unit cell. The dinuclear core
is composed of two copper() ions, bridged on the one side
by the endogenous cresolato bridge and on the other side
by the chloride anion. The Cu···Cu separation in the com-
Synthesis
An interesting structural feature encountered in the ac-
tive site of catechol oxidase from sweet potatoes (Ipomoea
batatas)[9] and in active sites of some hemocyanins[13,14] is
an unusual thioether bond between a carbon atom of one
of the histidine ligands and the sulfur atom of a nearby
cysteine residue from the protein backbone. There is also
sequence homology evidence for this type of bridge in tyro-
sinase from Neurospora crassa.[15] Such a moiety was also
encountered in the mononuclear copper enzyme galactose
oxidase, in which it was proposed to stabilize the tyrosine
radical generated during catalysis. However, the absence of
this bridge in hemocyanins of arthropods and in human
tyrosinase does not support the hypothesis about its direct
involvement in the electron transfer process.[9]
˚
plex is 3.185(1) A. Both copper() ions are pentacoordinate
with an identical N2OCl2 donor sets. However, the complex
possesses geometrical asymmetry, as the coordination ge-
ometry around Cu1 is almost a regular square pyramid (τ ϭ
0.10),[18] whereas the coordination environment around the
Cu2 ion can be best described as a heavily distorted trigonal
bipyramid (τ ϭ 0.63).[18] The basal plane of the square
pyramid around the Cu1 ion is made up of the nitrogen
atom N1 of the tertiary amine group, the N2 atom of the
pyridine ring, the oxygen atom O1 of the cresolate moiety,
and the bridging chloride atom Cl1, whereas another chlo-
ride anion Cl2 occupies the apical position. In the case of
the Cu2 ion, the nitrogen atom N3 of the tertiary amine
group, the bridging chloride anion Cl1, and the chloride
anion Cl3 form the equatorial plane of the trigonal bipyra-
mid. The axial positions are occupied by the nitrogen atom
N4 of the pyridine ring and the oxygen atom O1 of the
cresolate group. The thiophene rings of the ligand remain
Although a very large number of synthetic models of the
type-3 active site appeared in the literature in the past few
decades, relatively little attention has been paid to this thio-
ether bond. In an attempt to mimic this quite unusual struc-
tural feature we prepared a new dinucleating phenol-based
˚
noncoordinated; the Cu···S separations are Ͼ 5 A. One of
the thiophene rings shows disorder; the sulfur atom is re-
fined on two independent positions, with occupancy factors
Figure 1. Reaction scheme of the synthesis of the ligand Hpy2th2s of 78% (S1) and 22% (S1a). One noncoordinated disordered
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C. Belle, J. Reedijk et al.
FULL PAPER
Figure 2. Platon[17] projection of the crystal structure [Cu2(py2th2s)Cl3]·CH3OH; hydrogen atoms and noncoordinated solvent molecule
are omitted for clarity
formula units. As in the case of the chloride complex 1, two
copper() ions are doubly bridged by the oxygen atom of
the cresolate moiety and the halogen anion. The Cu···Cu
Table 1. Selected bond lengths and bond angles of the complex
[Cu2(py2th2s)Cl3]·CH3OH (1)
˚
Bond lengths [A]
˚
distances are 3.2710(10) and 3.2394(10) A, respectively. The
copper ions are pentacoordinate, with N2OBr2 donor sets.
One of the two independent formula units (Cu11, Cu12)
possesses geometrical asymmetry. The geometry around the
Cu11 ion is a slightly distorted square pyramid (τ ϭ
0.17),[18] whereas the geometry around the Cu12 ion is a
very distorted trigonal bipyramid (τ ϭ 0.58).[18] The nitro-
gen atom N11 of the tertiary amine group, the nitrogen
atom N12 of the pyridine ring, the bridging oxygen atom
O11, and the bridging bromide anion Br11 form the basal
plane of the square pyramid around the Cu11 ion, whereas
the monocoordinated bromide anion Br12 occupies the ax-
ial position. The equatorial plane of the trigonal bipyramid
around the Cu12 ion is formed by the nitrogen atom N13
of the tertiary amine group and the bromide atoms Br11
and Br13. The axial positions of the bipyramid are occu-
pied by the bridging oxygen atom of the cresolate group
and the nitrogen atom N14 of the pyridine ring. One of the
noncoordinated thiophene rings is rotationally disordered.
Thus, the first orientation has an occupancy factor of 85%,
while the 180° rotated orientation has an occupancy factor
of 15%.
Cu1ϪO1
Cu1ϪN2
Cu1ϪN1
Cu1ϪCl1
Cu1ϪCl2
Cu1ϪCu2
1.9209(19)
1.975(2)
2.103(2)
2.3922(8)
2.3931(10)
3.185(1)
Cu2ϪO1
Cu2ϪN4
Cu2ϪN3
Cu2ϪCl1
Cu2ϪCl3
1.911(2)
1.972(2)
2.083(2)
2.4717(9)
2.3636(10)
Bond angles [°]
O1ϪCu1ϪN2
O1Cu1ϪN1
159.33(9)
O1ϪCu2ϪN4 168.80(9)
90.47(8)
81.53(9)
82.70(6)
95.90(7)
153.25(6)
100.79(7)
99.74(7)
106.18(6)
100.50(3)
O1ϪCu2ϪN3
N4ϪCu2ϪN3
O1ϪCu2ϪCl1
N4ϪCu2ϪCl1
N3ϪCu2ϪCl1 131.18(6)
O1ϪCu2ϪCl3
N4ϪCu2ϪCl3
90.59(8)
82.01(9)
80.76(6)
97.76(7)
N2ϪCu1ϪN1
O1ϪCu1ϪCl1
N2ϪCu1ϪCl1
N1ϪCu1ϪCl1
O1ϪCu1ϪCl2
N2ϪCu1ϪCl2
N1ϪCu1ϪCl2
Cl1ϪCu1ϪCl2
94.77(7)
96.43(7)
N3ϪCu2ϪCl3 130.53(6)
Cl1ϪCu2ϪCl3 98.17(4)
methanol molecule is present per formula unit, which is hy-
drogen-bonded to the chloride anion Cl3.
In the second independent formula unit, both copper()
ions (Cu21 and Cu22) have significantly distorted square-
[Cu2(py2th2s)Br3] (2)
Very dark purple needles of complex 2 were obtained by pyramidal surroundings (τ ϭ 0.26 for the Cu21 ion and
slow concentration of an acetonitrile solution containing 0.45 for the Cu22 ion)[18]. The basal plane around the Cu21
stoichiometric amounts of copper() bromide and the li- ion is made up of, as in the case of the Cu11 ion, the two
gand. A Platon[17] projection of the crystal structure is nitrogen atoms N21 and N22, the oxygen atom O21 of the
shown in Figure 3. Selected bond lengths and angles are cresolate group, and the bridging halogen atom Br21,
presented in Table 2. The complex crystallizes in space whereas the bromide anion Br22 occupies the apical posi-
¯
group P1. The asymmetric unit consists of two independent tion. For the Cu22 ion, the nitrogen atom N23 of the ter-
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 4036Ϫ4045

Dinuclear CuII Complexes with a New Phenol-Based Ligand
FULL PAPER
Figure 3. Platon[17] projection of the two independent formula units of [Cu2(py2th2s)Br3]; hydrogen atoms are omitted for clarity; three
of the thiophene rings are rotationally disordered
tiary amine group, the nitrogen atom N24, the bridging pyramidal copper() complexes.[19] In acetonitrile solution,
oxygen atom O21, and the bridging bromide anion Br21 the charge-transfer band is observed at 472 nm for complex
form the basal plane of the square pyramid, and its apical 1 (ε ϭ 1851 Ϫ1·cmϪ1) and at 500 nm for complex 2 (ε ϭ
position is occupied by the Br23 atom. Thus, two square 1812 Ϫ1·cmϪ1). The d-d band is located at 771 nm (ε ϭ
pyramids share one side through the atoms O21 and Br21, 464 Ϫ1·cmϪ1) for complex 1 and 791 nm (ε ϭ 534
with their apical positions trans-located to each other. Both Ϫ1·cmϪ1) for complex 2. A small shift of the d-d bands
thiophene rings of the ligand remain noncoordinated upon dissolution of the complexes in acetonitrile may sug-
˚
(Cu···S separations are Ͼ 5 A) and exhibit rotational dis- gest the coordination of the solvent to the metal centers.
order. Thus, the thiophene ring at S21 was refined on two However, the partial dissociation of the complexes due to
orientations with occupancies of 0.72 and 0.28, respectively, the exchange of the halide anions with acetonitrile mol-
and the thiophene ring at S22 with occupancies of 0.55 and ecules can also not be excluded.
0.45, respectively.
EPR and Magnetic Susceptibility Studies
Physical Methods
Both complexes 1 and 2 are EPR-silent in the solid state
and in a frozen acetonitrile solution at 100 K, suggesting an
antiferromagnetic coupling between the copper ions. Such a
Ligand Field Spectroscopy
The ligand field spectra of both complexes, recorded in behavior is confirmed by the temperature dependence of the
the solid state (diffuse reflectance), exhibit almost identical molar magnetic susceptibility χM depicted in Figure 4,
features. Both spectra are characterized by the presence of which is typical for complexes of antiferromagnetically cou-
the charge-transfer band from the phenolate moiety to the pled copper() dimers. Indeed the χM vs. T curves present
copper ions[11] around 500 nm (483 nm for complex 1 and a broad maximum centered around 160 and 190 K for 1
521 nm for complex 2) and the d-d transition band of the and 2, respectively, while the values of χM at high tempera-
copper() ions. The latter band, observed at 774 nm for the tures (2.10 ϫ 10Ϫ3 and 1.85 ϫ 10Ϫ3 cm3·molϪ1 at 300 K,
chloride complex and at 806 nm for the bromide complex, respectively) are slightly lower than expected for two un-
is asymmetric in both cases, with a shoulder observed at coupled copper() ions (2.5 ϫ 10Ϫ3 cm3·molϪ1 for g ϭ 2).
higher wavelengths. As shown previously, such spectro- Below 40 K, a Curie tail ascribed to paramagnetic impurity
scopic behavior (high-energy absorption band in the visible is observed, which is usual in such copper() complexes.
region with a low-energy shoulder) is typical for square- These experimental data were reproduced correctly using
Eur. J. Inorg. Chem. 2004, 4036Ϫ4045
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C. Belle, J. Reedijk et al.
FULL PAPER
Table 2. Selected bond lengths and bond angles of the complex
[Cu2(py2th2s)Br3] (2)
˚
Bond lengths [A]
Cu11ϪO11
Cu11ϪN12
Cu11ϪN11
Cu11ϪBr11
Cu11ϪBr12
Cu21ϪO21
Cu21ϪN22
Cu21ϪN21
Cu21ϪBr21
Cu21ϪBr22
Cu11ϪCu12
1.925(3) Cu12ϪO11
1.977(4) Cu12ϪN14
2.099(4) Cu12ϪN13
2.5620(9) Cu12ϪBr11
2.5314(8) Cu12ϪBr13
1.931(3) Cu22ϪO21
1.968(4) Cu22ϪN24
2.112(4) Cu22ϪN23
2.5535(10) Cu22ϪBr21
2.5166(9) Cu22ϪBr23
3.2710(10) Cu21ϪCu22
1.931(4)
1.986(5)
2.096(4)
2.5226(8)
2.4870(9)
1.918(4)
1.981(5)
2.113(4)
2.5666(9)
2.4953(9)
3.2394(10)
Bond angles [°]
Figure 4. χM vs. T curve for complexes 1 (squares) and 2 (circles);
see text for further details
O11ϪCu11ϪN12 159.37(17) O11ϪCu12ϪN14 165.93(17)
O11ϪCu11ϪN11
N11ϪCu11ϪN12
91.14(16) O11ϪCu12ϪN13
81.55(17) N13ϪCu12ϪN14
91.70(17)
81.61(19)
Br11ϪCu11ϪO11 81.00(11) Br11ϪCu12ϪO11 81.95(10)
Br11ϪCu11ϪN12 95.53(13) Br11ϪCu12ϪN14 93.07(13)
Br11ϪCu11ϪN11 149.47(13) Br11ϪCu12ϪN13 130.96(12)
Br12ϪCu11ϪO11 97.77(11) Br13ϪCu12ϪO11 96.52(11)
Br12ϪCu11ϪN12 102.84(12) Br13ϪCu12ϪN14 97.54(13)
Br12ϪCu11ϪN11 108.72(13) Br13ϪCu12ϪN13 119.60(12)
CuϪO bonds and the large CuϪOϪCu angles of the phe-
nolate bridge in 1 and 2, relative to the long CuϪX bonds,
point at a dominant coupling through the phenolate bridge.
The coordination sphere around one of the two copper ions
is intermediate between a square pyramid and a trigonal
bipyramid, while the other has a square-pyramidal environ-
ment. In the former case, the unpaired electron of copper()
Br11ϪCu11ϪBr12 101.58(3)
Br11ϪCu12ϪBr13 109.44(3)
O21ϪCu21ϪN22 162.14(18) O21ϪCu22ϪN24 164.78(18)
O21ϪCu21ϪN21
N21ϪCu21ϪN22
91.15(16) O21ϪCu22ϪN23
81.54(17) N23ϪCu22ϪN24
90.99(17)
81.71(19)
2
2
2
occupies either the dz or the dx Ϫy orbital, which would
both point along the CuϪO(phenolate) bond. Short CuϪO
bonds and large CuϪOϪCu angles in square-pyramidal
phenolate-bridged Cu dimers result in a strong overlap of
Br21ϪCu21ϪO21 83.16(12) Br21ϪCu22ϪO21 83.04(11)
Br21ϪCu21ϪN22 93.90(14) Br21ϪCu22ϪN24 93.39(15)
Br21ϪCu21ϪN21 146.37(13) Br21ϪCu22ϪN23 137.81(12)
Br22ϪCu21ϪO21 99.57(12) Br23ϪCu22ϪO21 95.20(11)
Br22ϪCu21ϪN22 98.26(13) Br23ϪCu22ϪN24 99.98(14)
Br22ϪCu21ϪN21 110.29(13) Br23ϪCu22ϪN23 122.65(12)
2
2
dx Ϫy orbitals and thus produce strong to very strong anti-
ferromagnetic couplings.[20] On the other hand, if one of
the copper ions in such dimers has a trigonal-bipyramidal
environment, weaker overlap is expected, and even weak
ferromagnetic coupling has been observed.[21,22] Therefore,
the smaller singlet-triplet energy gap in 1 can be related to
a smaller CuϪOϪCu angle in 1 (ca. 112°) relative to 2 (on
average over the two dimeric units about 115.3°), but also
to a more distorted trigonal-bipyramidal coordination
sphere in 2 (τ ϭ 0.58, Cu12) than in 1 (τ ϭ 0.63, Cu2).
Most likely, the difference in the singlet-triplet energy gap
results from both structural differences.
Br21ϪCu21ϪBr22 103.34(3)
Cu11ϪO11ϪCu12 116.04(18) Cu21ϪO21ϪCu22 114.63(19)
Cu11ϪBr11ϪCu12 80.08(3) Cu21ϪBr21ϪCu22 78.50(3)
Br21ϪCu22ϪBr23 99.51(3)
the modified BleaneyϪBowers equation [Equation (1)],
where χM is the magnetic susceptibility per dimer. In this
equation, 2J corresponds to the singlet-triplet energy gap,
ρ to the fraction of paramagnetic impurity, and TIP to a
term to account for temperature-independent paramagnet-
ism, while NA, β, kB, and g have their usual meaning. The
paramagnetic impurity was assumed to be a mononuclear
copper() species and g was fixed to 2. The best-fit param-
eters were then obtained as 2J ϭ Ϫ177(2) cmϪ1, ρ ϭ
1.8(1)% and TIP ϭ 2.9(1) ϫ 10Ϫ4 cm3·molϪ1 for complex
1, and 2J ϭ Ϫ219(1) cmϪ1, ρ ϭ 0.6(1)% and TIP ϭ 2.0(1)
ϫ 10Ϫ4 cm3·molϪ1 for complex 2.
Electrochemistry
The electrochemical behavior of both complexes was in-
vestigated by cyclic voltammetry (CV) and rotating disk
electrode voltammetry (RDE) in acetonitrile solution, with
tetra-n-butylammonium perchlorate (TBAP) as supporting
electrolyte (0.1 ). The potentials were referred to an Ag/
10 m AgNO3 ϩ CH3CN ϩ0.1  TBAP reference elec-
trode.
(1)
When scanning towards the negative region of potentials,
the CV curve for both complexes 1 and 2 is characterized
by three successive electrochemical signals (Figures 5 and
These medium antiferromagnetic couplings should be 6). The first signal for 1, at Epc ϭ Ϫ0.52 V, is irreversible,
correlated to a significant overlap between the two magnetic and re-oxidation of the reduced species is seen on the re-
orbitals of the two copper() ions in complexes 1 and 2, verse scan at ϩ0.23 V (Figure 5, curve a), while that for 2
through both the halide and phenolate bridges. The short appears to be weakly reversible at E1/2 ϭ Ϫ0.34 V (∆Ep ϭ
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Dinuclear CuII Complexes with a New Phenol-Based Ligand
FULL PAPER
0.12 V, Figure 6, curve a). Coulometric titrations give n ϭ
The anodic behavior of complexes 1 and 2 differs. The
1 for the exchanged electron per complex, allowing to attri- anodic part of the CV curve for 1 (Figure 5, b) is charac-
bute this electrochemical process to the complexed CuI2I,II
/
terized by two electrochemical signals, whereas three signals
Cu2II,I redox couple. The UV/Vis spectra of the one-electron are observed on the CV curve for 2 (Figure 6, b). For 1, the
reduced complexes exhibit one very broad band around first signal at E1/2 ϭ 0.64 V (∆Ep ϭ 0.26 V) corresponds to
680 nm. The lack of reversibility of the first electrochemical a one-electron quasi-reversible oxidation of 1. The electron
reduction of 1 and 2 suggests a change in the coordination transfer is likely to be centered on the phenolate bridge,
sphere around the metal center upon reduction, likely a par- as previously shown for other phenolate-bridged dinuclear
tial dissociation of the exogenous halide ligand and a copper() complexes.[11,23] This process is followed by the
change in the geometry of the coordination sphere. Ad- oxidation of the chloride anions, which is seen as a shoulder
ditional electrochemical measurements performed at Ϫ40 at the negative foot of the over-oxidation wave of the com-
°C showed that the course of these coupled chemical reac- plex (free chloride anions are reversibly oxidized at 0.72 V
tions is not prevented at low temperature. The second elec- under our experimental conditions). The CV curve recorded
trochemical signal, weakly reversible at E1/2 ϭ Ϫ0.78 V for in a solution of 2 shows an additional fully irreversible one-
1 and at E1/2 ϭ Ϫ0.72 V for 2, also corresponds to a one- electron process at Epa ϭ 0.47 V. This is assumed to be
electron process, leading to the complexed Cu2II,I/Cu2I,I redox due to the one-electron oxidation of bromide anions; free
couple. Finally, the third electrochemical signal at Epc
ϭ
bromide anions are irreversibly oxidized at Epa ϭ 0.39 V
Ϫ1.45 V for 1 and Ϫ1.04 V for 2 corresponds to a two- under our experimental conditions. The one-electron phe-
electron process, suggesting the reduction of CuI ions to nolate-based oxidation of complex 2, similarly to 1, is ob-
the Cu0 state and the deposition of metallic copper on the served as a quasi-reversible pair of peaks at E1/2 ϭ 0.76 V
electrode surface. Accordingly, an additional sharp anodic (∆Ep ϭ 0.18 V), and is followed at higher potentials by the
peak is observed on the reverse scan, which is caused by over-oxidation of the complex.
the redissolution of metallic copper.
These results have been confirmed by rotating disk elec-
trode (RDE) voltammetric experiments (Figures 5 and 6).
The RDE curves display one anodic wave at E1/2 ϭ 0.84 V
for 1 and two successive well-behaved anodic waves at
E1/2 ϭ 0.42 V and 0.82 V (Figure 6, curve c) for 2. For both
complexes, three successive cathodic waves are seen. For 1
and 2, the first one at E1/2 ϭ Ϫ0.42 V and Ϫ0.40 V, respec-
tively, is followed by a second ill-behaved wave at E1/2
ϭ
Ϫ0.78 V and Ϫ0.80 V, respectively. The third cathodic wave
corresponding to the deposition of metallic copper onto the
electrode surface is observed at E1/2 ϭ Ϫ1.22 V and Ϫ1.20
V, respectively.
Figure 5. Electrochemical curves recorded in a 0.67 m solution of
1 in CH3CN ϩ TBAP 0.1  on a Pt disc [diameter ϭ 5 (a, b) or 3
(c) mm]; curves a, b: CV curves, ν ϭ 0.1 V·sϪ1; curve c: RDE curve,
N ϭ 600 rpm; V vs. Ag/AgNO3 m ϩ CH3CN ϩ TBAP 0.1 ;
scale s ϭ 20 µA (curve a), 120 µA (curve b) or 10 µA (curve c)
Conductivity Measurements
The conductivity measurements of the complexes 1 and
2 were performed on 0.5 m solutions of the complexes in
acetonitrile. The calculated values for equivalent conduc-
tivities of both compounds are roughly the same and are
equal to 52 cm2·molϪ1·ΩϪ1 for complex
1 and 55
cm2·molϪ1·ΩϪ1 for complex 2. Previously, the conductivity
range for complexes corresponding to the 1:1-type electro-
lytes was suggested to lie between 120 and 160
cm2·molϪ1·ΩϪ1 if acetonitrile was utilized as a solvent.[24]
Although values as low as 90 cm2·molϪ1·ΩϪ1 were also re-
ported in some cases, the observed conductivities of both
complexes 1 and 2 are still too low for them to be con-
sidered as 1:1 electrolytes. As shown previously,[25] such
small conductivity values in acetonitrile are often found for
nonelectrolytes, due to coordinating and solvating proper-
ties of this solvent. Some authors[25] argue that they may
indicate a partial exchange of counter anions with the sol-
vent molecules. Small changes in the positions of UV/Vis/
NIR d-d bands of the complexes upon their dissolution in
acetonitrile, observed in our case, appear to sustain this as-
sumption.
Figure 6. Electrochemical curves recorded in a 0.67 m solution of
2 in CH3CN ϩ TBAP 0.1  on a Pt disc [diameter ϭ 5 (a, b) or 3
(c) mm]; curves a, b: CV curves, ν ϭ 0.1 V·sϪ1; curve c: RDE curve,
N ϭ 600 rpm; V vs. Ag/AgNO3 m ϩ CH3CN ϩ TBAP 0.1 ;
scale s ϭ 20 µA (curve a), 40 µA (curve b, c)
Eur. J. Inorg. Chem. 2004, 4036Ϫ4045
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C. Belle, J. Reedijk et al.
FULL PAPER
Interaction of the Complexes with the Model Substrates
Catecholase Activity Measurements
To evaluate the ability of the complexes to behave as
functional models of catechol oxidase, they were tested as
catalysts in the oxidation of 3,5-di-tert-butylcatechol (3,5-
DTBC), a widely used model substrate, to the respective
quinone. Both complexes exhibit only negligible catecholase
activity (TON Ͻ 1 after 30 min), making a detailed kinetic
analysis dispensable. These results are as expected, con-
firming that the substitution of the µ-hydroxo bridge by a
halogen anion precludes catecholase activity. However,
these results do not provide information on whether or not
the binding of the substrate to the metal centers takes place
at all. Reim et al. have previously shown that, in the case
of essentially inactive complexes, no interaction between the
dimetal core and the substrate occurred.[6] To evaluate
whether or not the first step of the catalytic cycle, e.g. the
binding of catechol to the metal centers, takes place, we
studied the interaction of the complexes with tetrachloroca-
Figure 7. UV/Vis titration of complex 1 (0.5 m solution in aceto-
nitrile) with TCC (from 0 to 4 equiv.); the inset shows the enlarge-
ment of the spectroscopic curves in the range 530Ϫ780 nm
techol (TCC). The latter catechol has a very high oxidation ing catecholate were previously described,[8,26] no magnetic
potential due to its electron-withdrawing substituents and properties of these compounds have been reported. How-
is not oxidized by copper complexes.
ever, we believe that the cleavage of the halide bridge would
lead to a decrease in strength of the magnetic coupling,
similarly to the behavior seen in previously reported µ-
hydroxo complexes with phenol-based ligands[7] after the
cleavage of the hydroxo bridge. Thus, the reaction of TCC
with the chloride complex probably results in the substi-
tution of at least one of the apical chloride anions.
In order to further confirm this hypothesis, the conduc-
tivity of complex 1 in acetonitrile solution before and after
addition of TCC was studied. The equivalent conductivity
of TCC itself is negligible.
The values of the equivalent conductance observed after
addition of 1Ϫ4 equiv. of TCC to 0.5 m solution of com-
plex 1 are listed in Table 3. As can be seen, a substantial
increase is observed after 1 equiv. of TCC has been added.
Further addition of TCC leads to a slight increase in con-
ductivity, until a final value of 118 cm2·molϪ1·ΩϪ1 is re-
ached. This value fits perfectly in a range suggested for the
1:1 electrolytes in acetonitrile.[24] The interaction of 1 with
TCC leads to the release of one chloride anion in solution;
complete release is reached in the presence of an excess of
TCC as shown by the slight increase in the conductivity
above 1 mol-equiv. added. The reversible nature of this pro-
cess is further confirmed by UV/Vis titration experiments
TCC Binding Studies
The titration of 5 ϫ 10Ϫ4  solutions of the complexes
in acetonitrile with TCC was monitored by means of UV/
Vis and EPR spectroscopy. The changes observed in the
UV/Vis spectra of complex 1 upon addition of TCC (up to
4 equiv.) are depicted in Figure 7. Quite significant changes
in the spectrum of the original complex indicate the interac-
tion between the substrate and the metal centers. The pres-
ence of two isosbestic points at 570 nm and 724 nm shows
the presence of only two absorbing species in solution. The
CuII d-d transition band shifts gradually from 789 nm to
718 nm, whereas its absorbance decreases to about 50% of
its initial value. Also, the extinction coefficient of the
LMCT band decreases from 1851 to about 1600 Ϫ1·cmϪ1
(Figure 7).
Whereas the chloride complex obviously reacts with
TCC, no changes were observed in the UV/Vis spectrum
upon addition of TCC to the bromide complex. Taking into
account the great structural similarity between the two
complexes, it seems reasonable to us to suggest that the dif-
ference in their behavior is caused by the different halogen
anions coordinated to the copper ions. The CuϪCl bond
has a more ionic character than the CuϪBr bond, which
facilitates the substitution of (at least one of) the chloride
anions by catechol, whereas no exchange of Br anions by
TCC occurs.
Table 3. Calculated values of equivalent conductivity upon treating
0.5 m solution of 1 in acetonitrile with TCC
Titration of complex 1 with TCC, monitored by X-band
EPR spectroscopy, revealed that no evolution of the EPR
signal is observed upon addition of TCC. This indicates
that the two copper ions are strongly antiferromagnetically
coupled, suggesting that they probably remain doubly
bridged by the phenolate group and the chloride anion. Al-
though a few structures of copper() complexes with bridg-
Number of equiv. of TCC Equivalent conductivity [cm2·molϪ1·ΩϪ1
]
0
1
2
3
4
52
100
110
117
118
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 4036Ϫ4045

Dinuclear CuII Complexes with a New Phenol-Based Ligand
FULL PAPER
that showed that the maximal perturbation in the UV/Vis water molecule. We have shown that the reaction of catechol
spectra was reached in the presence of at least 3 equiv. of with the complexes 1 and 2 does not result in a cleavage of
TCC (Figures 7 and 8).
the bridge, although binding of the substrate to the chloride
complex undoubtedly takes place. These results thus em-
phasize the influence of the bridging group between the
copper centers on the catecholase activity of the complexes
and underline the importance of the exogenous hydroxo
bridge for the catalytic mechanism. This hydroxo ligand ap-
pears to be the key factor to achieve the complete depro-
tonation of catechol, leading to a bridging catecholate prior
to the electron transfer. Upon substitution of the hydroxo
bridge with the halogen anion, no proton transfer can oc-
cur, precluding the binding of catecholate in a bidentate
bridging fashion, and thus the subsequent catalytic cycle.
Conclusion
Figure 8. Decrease in the LMCT band (474 nm) upon titration of
the solution of 1 (0.5 m) in acetonitrile with TCC
The synthesis of a new phenol-based ligand Hpy2th2s
bearing thiophene- and pyridine-containing pendant arms
was successfully accomplished. Two dinuclear copper()
complexes with this ligand were isolated and characterized
by means of single-crystal X-ray analysis, UV/Vis and EPR
spectroscopy, magnetochemistry and electrochemistry. Both
complexes were found to be essentially inactive in the cata-
lytic oxidation of 3,5-di-tert-butylcatechol to the respective
quinone. The presence of chloride and bromide anions as
bridging ligands between the two metal centers precludes
the deprotonation of catechol and its binding in a bidentate
bridging fashion to the copper ions, and thus from further
oxidation. Studies to establish a better structure-activity re-
lationship for dinuclear copper() complexes as structural
models of catechol oxidase are currently in progress.
Not surprisingly, additions of TCC to the bromide com-
plex do not lead to an increase in conductivity, in complete
agreement with the hypothesis that substitution of BrϪ
anions by TCC is not possible.
General Discussion
Previously, many attempts to establish a correlation be-
tween the structural parameters of the complexes and their
catalytic activity in the oxidation of catechols were de-
scribed in the literature. Three most important criteria in-
fluencing the catalytic properties of the complexes can now
be distinguished: (i) the distance between the metal cen-
ters,[11,27,28] (ii) the nature of the bridging group between
the metal ions,[29,30] (iii) the electronic properties of the
complexes.[23] Some other factors, e.g. the imposed strain in
Experimental Section
the complex due to the rigidity of either the ligand and/or General Remarks: All starting materials were commercially avail-
the bridging group, may also play a role.[6] Results pub-
able and used as purchased. 2,6-Bis(chloromethyl)-4-methylphenol
was prepared as described previously.[16] The NMR spectra were
recorded with a JEOL FX-200 (200 MHz) FT NMR spectrometer.
The ligand field spectra of the solids (300Ϫ2000 cmϪ1, diffuse re-
flectance) were taken with a PerkinϪElmer 330 spectrophotometer
equipped with a data station. The ligand field spectra in solution
were recorded with a Varian Cary 50 Scan UV/Vis spectrophoto-
meter. Electrospray mass spectra (ESI-MS) in acetonitrile solutions
were recorded with a Thermo Finnigan AQA apparatus. X-band
electron paramagnetic resonance (EPR) measurements were per-
lished by Reim et al.[6] indicate that complexes with strained
structures show catalytic activity, whereas complexes pre-
sent in relaxed and energetically favored conformations are
essentially inactive towards catechol oxidation. In the chlo-
ride and bromide complexes 1 and 2, the metalϪmetal dis-
tance and the redox potentials of the metal ions are similar
to the values previously determined for catalytically active
complexes;[6,11,29] thus, the absence of catalytic activity is
not likely to be due to these two factors. Also, the coordi- formed at 77 K in the solid state with a Jeol RE2x electron spin
resonance spectrometer, using DPPH (g ϭ 2.0036) as a standard,
and at 100 K as acetonitrile frozen solutions with a Bruker ESP
300E spectrometer operating at 9.4 GHz (X-band). The conduc-
tivity measurements were performed with a Philips PW9526 digital
conductivity meter and PW 9552/60 measuring cell with 0.5 m
solutions of the complexes in acetonitrile. The electrochemical be-
havior of the complexes was investigated in a 0.1  solution of
tetra-n-butylammonium perchlorate (TBAP) in acetonitrile using
an EGG 273 potentiostat coupled with a Kipp&Zonen x-y re-
corder. The experiments were performed at room temperature or
nation spheres around the copper ions in both complexes
exhibit quite a large distortion from a regular square-pyr-
amidal geometry, resulting in relatively strained structures.
Thus, the main reason for the absence of catalytic activity
lies in the nature of the bridging groups between the two
copper ions in the complexes. As discussed above, the cru-
cial steps of the mechanism that was proposed earlier[7] is
the cleavage of the OH bridge between the two metal ions
and the subsequent proton transfer from the catechol sub-
strate to the hydroxy anion, leading to the release of one at Ϫ40 °C in a three-compartment cell. Potentials are referred to
Eur. J. Inorg. Chem. 2004, 4036Ϫ4045
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C. Belle, J. Reedijk et al.
FULL PAPER
an Ag/10 m AgNO3 ϩ CH3CN ϩ0.1  TBAP reference electrode.
The working electrode was a platinum disk with a 5 mm diameter
for the cyclic voltammetry (CV, 0.1 V/s) experiments or 3 mm di-
ameter for the rotating disk electrode (RDE, 600 rpm) voltammetry
experiments. The working electrode was polished with 1 µm dia-
mond paste prior to each recording. Bulk magnetizations of po-
lycrystalline samples were performed on the crystals of complexes
1 (11.11 mg) and 2 (16.88 mg) in the temperature range 5Ϫ400 K
with a Quantum Design MPMS-5S SQUID magnetometer in a 1
kG applied field. The data were corrected for the experimentally
determined contribution of the sample holder. Corrections for the
diamagnetic responses of the complexes, as estimated from Pascal’s
constants, were applied.[31]
7.53 (d, 2 H, 5Јth), 7.66 (t, 2 H, 4Јpy), 8.56 (d, 2 H, 6Јpy), 10.40 (1
H, OH) ppm. ESI-MS: m/z ϭ 541 [M ϩ Hϩ].
[Cu2(py2th2s)(µ-Cl)Cl2]·CH3OH (1): Hpy2th2s (0.15 g, 0.28 mmol)
and copper chloride (0.10 g, 0.54 mmol) were dissolved in meth-
anol (10 mL). Addition of diethyl ether (20 mL) resulted in the
precipitation of the complex as a dark brown powder. Yield:
102 mg (46%). Single crystals of the complex were obtained by slow
diffusion of diethyl ether into a 0.01  solution of the complex.
[Cu2(symth2py2)Cl3]·CH3OH.
C32H36Cl3Cu2N4O2S2
(806.2):
calcd. C 47.73, H 4.38, N 6.96, S 7.96; found C 45.73, H 4.14, N
7.25, S 7.91. ESI-MS: m/z ϭ 737 [Cu2(py2th2s)Cl2]ϩ.
[Cu2(py2th2s)(µ-Br)Br2] (2): Hpy2th2s (0.15 g, 0.28 mmol) and cop-
per bromide (0.12 g, 0.54 mmol) were dissolved in of methanol (10
mL). After addition of diethyl ether to the resulting solution, the
complex precipitated as a dark purple crystalline powder. Yield:
155 mg (64%). Crystals suitable for X-ray diffraction were obtained
by slow diffusion of diethyl ether into a 0.01  solution of the
complex in acetonitrile. C31H31Br3Cu2N4OS2 (906.5): calcd. C
40.95, H 3.76, N 5.97, S 6.83; found C 39.76, H 3.42, N 6.07, S
6.50. ESI-MS: m/z ϭ 827 [Cu2(py2th2s)Br2]ϩ.
Catecholase Activity Study: The catecholase activity of the com-
plexes was evaluated by reaction with 3,5-di-tert-butylcatechol at
25 °C. The absorption at 400 nm, characteristic of the formed quin-
one, was measured as a function of time. The experiments were
performed under 1 atm of dioxygen; 3 mL of 2.5·10Ϫ4  solution
of complex in acetonitrile was placed in 1-cm path-length cell, and
75 µL of a 1  solution of the substrate in the same solvent was
added. After thorough shaking, the changes in UV/Vis spectra were
recorded over 30 min. The titration of the complexes with tetra-
chlorocatechol (TCC) was carried out by adding 3 µL aliquots of
0.1  solution of TCC (corresponding to 0.198 equiv. of TCC/1
equiv. of the complex) to 3 mL of 5 ϫ 10Ϫ4  solution of complex
in acetonitrile.
X-ray Structures
Compound 1: A single crystal of [Cu2(py2th2s)(µ-Cl)Cl2]·CH3OH
(1) was mounted at 150 K on a ruker AXS SMART 6000 dif-
˚
fractometer, equipped with Cu-Kα radiation (λ ϭ 1.54184 A).
C32H35Cl3Cu2N4O2S2, M ϭ 802.17, rectangular reddish-brown
˚
˚
plate, 0.23 ϫ 0.21 ϫ 0.05 mm, a ϭ 7.984(2) A, b ϭ 34.589(7) A,
(2-Picolyl)(2-thiophenyl)amine: A solution of (pyridin-2-ylmethyl)-
amine (2.00 g, 18.5 mmol) was added dropwise, while stirring, to a
solution of 2-formylthiophene (2.08 g, 18.5 mmol) in MeOH. The
resulting solution was stirred at room temperature overnight. After-
wards, NaBH4 (2.1 g, 56 mmol, 3 equiv. per 1 CHϭN) was added
slowly, and the resulting solution was heated at 50 °C for 3 h. After
concentration, the obtained oil was redissolved in a mixture of di-
chloromethane and water. The organic and aqueous layers were
separated, and the water layer was washed twice with a small
amount of dichloromethane. After combining and drying of the
dichloromethane layers with Na2SO4 and concentration, the pure
product was obtained as a light yellow oil. The product is light-
sensitive and should preferably be stored in darkness at low tem-
peratures. Yield: 3.59 g (95%). 1H NMR (CDCl3): δ ϭ 2.28 (s, 1
H, NH), 3.95 (s, 2 H, NHCH2th), 4.03 (s, 2 H, NHCH2py), 6.93
(d, 1 H, 3Јth), 6.96 (s, 1 H, 4Јth), 7.20 (d, 1 H, 5Јth), 7.15 (t, 1
H, 5Јpy), 7.30 (d, 1 H, 3Јpy), 7.62 (t, 1 H, 4Јpy), 8.55 (d, 1 H,
6Јpy) ppm.
3
˚
˚
c ϭ 12.554(3) A, β ϭ 94.31(3)°, Z ϭ 4, V ϭ 3457(2) A , ρcalcd.
ϭ
1.541 g·cmϪ3, µ ϭ 5.068 mmϪ1, absorption correction with SAD-
ABS,[32] monoclinic, space group P21/n (no. 14), 19864 reflections
collected, 6317 independent reflections (Rint ϭ 0.0386). The struc-
ture was solved by direct methods and refined using the SHELX
program package.[33,34] All hydrogen atoms were placed on ideal-
ized positions riding on their carrier atoms with isotropic displace-
ment parameters. The final cycle of full-matrix least-squares refine-
ment, including 438 parameters, converted into R1 ϭ 0.0352 (R1 ϭ
0.0430 all data) and wR2 ϭ 0.1009 (wR2 ϭ 0.1038 all data) with a
maximum (minimum) residual electron density of 0.535 (Ϫ0.466)
Ϫ3
˚
e·A
.
Compound 2: C31H31Br3Cu2N4OS2 ϩ solvent, M ϭ 906.53, red
¯
needle, 0.60 ϫ 0.06 ϫ 0.03 mm, triclinic, P1 (no. 2), a ϭ 8.4207(2)
˚
˚
˚
A, b ϭ 17.9812(4) A, c ϭ 24.2238(6) A, α ϭ 71.2709(9), β ϭ
3
˚
81.2708(7), γ ϭ 80.6146(11)°, V ϭ 3407.66(14) A , Z ϭ 4, ρcalcd. ϭ
1.767 g·cmϪ3, 43713 measured reflections, 12061 unique reflections
(Rint ϭ 0.0734), 8080 observed reflections [I Ͼ 2σ(I)], 775 refined
parameters, no restraints; R (obsd. refl.): R1 ϭ 0.0461, wR2 ϭ
0.1121; R (all data): R1 ϭ 0.0776, wR2 ϭ 0.1286; S ϭ 1.085; re-
4-Methyl-2,6-bis{[(2-methylpyridyl)(2-methylthiophenyl)amino]-
methyl}phenol (Hpy2th2s): A solution of (2-picolyl)(2-thiophenyl)-
amine (0.7 g, 3.4 mmol) and Et3N (0.7 g, 7 mmol) in THF was ad-
ded dropwise, while stirring, to a solution of 2,6-bis(chloromethyl)-
4-methylphenol (0.35 g, 1.7 mmol) in THF. After refluxing for 2
h, the solution was cooled to room temperature. Filtration of the
triethylammonium salt and subsequent evaporation of the solvent
resulted in an oil, which was dissolved in acidified water and
washed with dichloromethane. The water layer was made alkaline
by adding a concentrated aqueous solution of NH3, and the re-
sulting suspension was extracted three times with dichloromethane.
The combined organic layers were dried with Na2SO4 and the sol-
sidual electron density between Ϫ1.08 and 1.17 e·AϪ3. Intensities
˚
were measured with a Nonius KappaCCD diffractometer with rot-
˚
ating anode (Mo-Kα, λ ϭ 0.71073 A) at a temperature of 150 K.
An absorption correction based on multiple measured reflections
was applied (µ ϭ 4.920 mmϪ1, 0.59Ϫ0.86 transmission). The struc-
ture was solved with direct methods with the program SHELXS-
97,[35] and refined with the program SHELXL-97[34] against F2 of
all reflections up to a resolution of (sin /λ)max. ϭ 0.60 AϪ1. Non-
˚
hydrogen atoms were refined freely with anisotropic displacement
parameters. Hydrogen atoms were refined as rigid groups. Three of
vents evaporated under reduced pressure. The resulting oil was the thiophene rings were rotationally disordered and refined with
found to be the pure product. Yield: 0.77 g (84%). 1H NMR
(CDCl3): δ ϭ 2.28 (s, 3 H, CH3), 3.82 (s, 4 H, NCH2th), 3.85 (s, 4
occupancies of 0.85:0.15, 0.72:0.28, and 0.55:0.45, respectively. The
crystal structure contains large voids (150.9 A /unit cell) filled with
3
˚
H, phCH2N), 3.89 (s, 4 H, NCH2py), 6.95 (m, 4 H, 3Јth ϩ 4Јth), disordered solvent molecules. Their contribution to the structure
7.04 (s, 2 H, 3Јph ϩ 5Јph), 7.15 (t, 2 H, 5Јth), 7.24 (d, 2 H, 3Јpy), factors was secured by back-Fourier transformation (program
4044
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 4036Ϫ4045

Dinuclear CuII Complexes with a New Phenol-Based Ligand
FULL PAPER
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CCDC-230014 (1) and -230015 (2) contain the supplementary crys-
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charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
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Acknowledgments
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Support from the NRSC Catalysis (a Research School Combi-
nation of HRSMC and NIOK) is kindly acknowledged. Also sup-
port and sponsorship from COST Action D21/003/2001 is kindly
acknowledged. This project was financially supported by the Dutch
Economy, Ecology, Technology (EET) program, a joint program of
the Ministry of Economic Affairs, the Ministry of Education, Cul-
ture and Science, and the Ministry of Housing, Spatial Planning
and the Environment. This work was supported in part (M. L., A.
L. S.) by The Netherlands Foundation for Chemical Sciences (CW)
with financial aid from the Netherlands Organization for Scientific
Research (NWO).
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Early View Article
Published Online August 17, 2004
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