Oxidative double dehalogenation of tetrachlorocatechol by a bio-inspired CuII complex: Formation of chloranilic acid

Article abstract of DOI:10.1002/chem.200701878

Copper(II) complexes of the potentially tripodal N,N,O ligand 3,3-bis(1-methylimidazol-2-yl)propionate (L1) and its conjugate acid HL1 have been synthesised and structurally and spectroscopically characterised. The reaction of equimolar amounts of ligand and Cu^II resulted in the complexes [Cu(L1)]_n(X)_n (X = OTf^-, PF ₆^-; n = 1,2), for which a new bridging coordination mode of L1 is inferred. Although these complexes showed moderate catecholase activity in the oxidation of 3,5-di-tert-butylcatechol, surprising reactivity with the pseudo-substrate tetrachlorocatechol was observed. A chloranilato-bridged dinuclear Cu^II complex was isolated from the reaction of [Cu(L1)]_n(PF₆)_n with tetrachlorocatechol. This stoichiometric oxidative double dehalogenation of tetrachlorocatechol to chloranilic acid by a biomimetic copper(II) complex is unprecedented. The crystal structure of the product, [Cu₂(Ca)Cl₂(HL1) ₂], shows a bridging bis-bidentate chloranilato (ca) ligand and ligand L1 coordinated as its conjugate acid (HL1) in a tridentate fashion. Magnetic susceptibility studies revealed weak antiferromagnetic coupling (J = -35 cm^-1) between the two copper centres in the dinuclear complex. Dissolution of the green complex [Cu₂(Ca)Cl₂(HL1) ₂] resulted in the formation of new pink-purple mononuclear compound [Cu(Ca)(HL1)(H₂O)], the crystal structure of which was determined. It showed a terminal bidentate chloranilato ligand and N,N-bidentate coordination of ligand HL1, which illustrates the flexible coordination chemistry of ligand L1.

Full text of DOI:10.1002/chem.200701878

FULL PAPER
DOI: 10.1002/chem.200701878
Oxidative Double Dehalogenation of Tetrachlorocatechol by a Bio-Inspired
CuII Complex: Formation of Chloranilic Acid
Pieter C. A. Bruijnincx,[a] Marta Viciano-Chumillas,[b] Martin Lutz,[c] Anthony L. Spek,[c]
Jan Reedijk,[b] Gerard van Koten,[a] and Robertus J. M. Klein Gebbink*[a]
Abstract: Copper(II) complexes of the
potentially tripodal N,N,O ligand 3,3-
bis(1-methylimidazol-2-yl)propionate
(L1) and its conjugate acid HL1 have
been synthesised and structurally and
spectroscopically characterised. The re-
action of equimolar amounts of ligand
chloranilato-bridged dinuclear CuII
complex was isolated from the reaction
conjugate acid (HL1) in a tridentate
fashion. Magnetic susceptibility studies
revealed weak antiferromagnetic cou-
pling (J=ꢀ35 cmꢀ1) between the two
copper centres in the dinuclear com-
plex. Dissolution of the green complex
of [Cu(L1)]n(PF6)n with tetrachloroca-
G
techol. This stoichiometric oxidative
double dehalogenation of tetrachloro-
catechol to chloranilic acid by a bio-
mimetic copper(II) complex is unpre-
cedented. The crystal structure of the
[Cu2(ca)Cl2
mation of new pink-purple mononu-
clear compound [Cu(ca)(HL1)(H2O)],
A
and CuII resulted in the complexes
ꢀ
[Cu(L1)]n(X)n (X=OTfꢀ, PF6
;
n=
product, [Cu2(ca)Cl2
A
A
ACHTREUNG
1,2), for which a new bridging coordi-
nation mode of L1 is inferred. Al-
though these complexes showed mod-
erate catecholase activity in the oxida-
tion of 3,5-di-tert-butylcatechol, surpris-
ing reactivity with the pseudo-substrate
tetrachlorocatechol was observed. A
bridging bis-bidentate chloranilato (ca)
ligand and ligand L1 coordinated as its
the crystal structure of which was de-
termined. It showed a terminal biden-
tate chloranilato ligand and N,N-biden-
tate coordination of ligand HL1, which
illustrates the flexible coordination
chemistry of ligand L1.
Keywords: bioinorganic chemistry ·
copper · dehalogenation · hydrogen
bonds · N,O ligands
Introduction
centres.[2] The type-3 active sites of copper proteins, for in-
stance, consist of two copper(II) ions in close proximity,
both of which are coordinated by three histidine residues.
These type-3 sites in copper proteins are represented by
haemocyanin, tyrosinase, and catechol oxidase, all of which
have now been characterised crystallographically.[3–5] Hae-
mocyanin reversibly binds dioxygen for transport, whereas
the other proteins are enzymes that are involved in the oxi-
dation of phenols to catechols (tyrosinase) and subsequent
two-electron oxidation of the catechols to quinones (both ty-
rosinase and catechol oxidase). To understand the mecha-
nisms by which these closely related enzymes catalyse their
respective transformations, many structural and functional
modelling studies have been reported. Most of these deal
with model compounds of catechol oxidase,[6–11] and they
have recently been reviewed.[2]
Copper-containing metalloenzymes play a major role in the
activation of dioxygen in nature.[1] Currently, up to seven
different types of copper sites in proteins can be discerned
on the basis of structure and spectroscopy of their active
site, and they feature mono-, di-, tri- or tetranuclear copper
[a] Dr. P. C. A. Bruijnincx, Prof. Dr. G. van Koten,
Prof. Dr. R. J. M. Klein Gebbink
Chemical Biology & Organic Chemistry
Department of Chemistry, Faculty of Science, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
Fax :(+31)30-252-3615
G
E-mail: r.j.m.kleingebbink@uu.nl
[b] M. Viciano-Chumillas, Prof. Dr. J. Reedijk
Leiden Institute of Chemistry, Leiden University
P. O. Box 9502, 2300 RA Leiden (The Netherlands)
Some of us earlier reported copper[12,13] and iron[14,15] com-
plexes of the family of monoanionic, substituted 3,3-bis(1-al-
kylimidazol-2-yl)propionate ligands as biomimetic oxidation
catalysts. Homogeneous and immobilised copper(II) com-
plexes of parent ligand L1 (Figure 1) have, for instance,
been tested in the oxidation of benzyl alcohol.[13] Whereas a
[c] Dr. M. Lutz, Prof. Dr. A. L. Spek
Bijvoet Centre for Biomolecular Research, Crystal and Structural
Chemistry
Department of Chemistry, Faculty of Science, Utrecht University
Padualaan 8, 3584 CH Utrecht (The Netherlands)
E-mail: a.l.spek@chem.uu.nl
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5567

1259, 1224, 1148 and 1028 cmꢀ1 are indicative of the pres-
ence of uncoordinated triflate anions.[20–22] Further insight
into the structure of [1](OTf) was obtained from its ESI-MS
G
spectrum recorded in acetonitrile. Major ions were observed
at m/z 295.99 and 740.94, corresponding to the dimeric cat-
ions [Cu2(L1)2]2+ (calcd 296.03) and [Cu2(L1)2(OTf)]+
U
(calcd 741.02), respectively. In addition, a signal observed at
m/z 518.63 can be attributed to trinuclear copper species
[Cu3(L1)3
(OTf)]2+ (calcd 518.53). The molar conductivity of
C
Figure 1. Ligand L1 and 2:1 ligand/metal complex [Cu(L1)2].[12]
G
trile lies in the reported range for 2:1 electrolytes.[23] In addi-
tion, a d–d transition was observed at 636 nm in the elec-
1:1 ligand to copper complex was found inside a zeolite
framework,[13] we mainly observed 2:1 species [Cu(L1)2] in
solution and single-crystal X-ray diffraction studies
(Figure 1).[12]
tronic spectrum of [1]
A
To obtain further insight into the structure of the 1:1 Cu-
AHCTREUNG
We have now expanded these studies on the coordination
chemistry of L1 with copper(II) ions. A study of the cate-
cholase activity of these complexes resulted in the unexpect-
ed and unprecedented formation of chloranilic acid from
tetrachlorocatechol. This observation has environmental
relevance. Tetrachlorocatechol is part of a bigger group of
pollutants, such as chlorinated phenols, guaiacols (2-meth-
A
ular weight fraction of waste generated by, for instance, the
paper bleaching industry.[16,17] These highly chlorinated or-
ganic compounds include some of the most toxic and persis-
tent organic pollutants. A copper-mediated oxidative deha-
logenation reaction that provides a new approach to the
chemical degradation of these pollutants is described herein.
ACHTREUNG
ence of both dinuclear and higher nuclearity species. We
therefore suggest the formation of a polynuclear structure
through the initial formation of dimers followed by equilib-
rium polymerisation (Figure 2). Herein we will refer to the
1:1 complex of Cu
(OTf).
N
G
AHCTREUNG
Results
Synthesis of copper(II) complexes: The coordination
chemistry of L1 towards copper(II) ions was found to be
stoichiometry dependent, as was previously observed for
other transition metal cations, such as zinc(II).[18] Earlier, we
reported complex [Cu(L1)2] (Figure 1), which can be ob-
tained by the addition of two equivalents of L1 to a cop-
ACHTREUNG
AHCTREUNG
isolation of a light blue complex. According to elemental
analysis, a complex of stoichiometric amounts of copper(II),
ligand and anion is formed, which we label as [1](OTf) (see
G
below).
Previous studies showed that the positions of the symmet-
ric and asymmetric carboxylato stretching vibrations in the
IR spectrum are indicative of the binding mode of the
ligand.[12,18] The IR spectrum of the precipitated product
from the 1:1 reaction of Cu(OTf)2 and L1, however, showed
U
a distinct absorption pattern that had not previously been
observed. The asymmetric and symmetric stretching vibra-
tions were found at 1555 and 1433 cmꢀ1, respectively. As a
result, n˜asꢀn˜s is only 122 cmꢀ1. This is substantially less than
(n˜asꢀn˜s)ionic (188 cmꢀ1) and points to a bridging carboxylato
group.[19] Furthermore, the four sharp bands observed at
Figure 2. Proposed structure of the cationic part of [1](X) (X=OTf or
PF6): a polynuclear material formed from dinuclear units.
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Chem. Eur. J. 2008, 14, 5567 – 5576

Oxidative Double Dehalogenation of Tetrachlorocatechol
FULL PAPER
For solubility reasons, the triflate anion in [1]
exchanged for a PF6 anion by addition of an excess of po-
A
Tetrachlorocatechol binding studies: The mode of binding
ꢀ
of catechols to [1]
ACHTREUNG
tassium hexafluorophosphate to an aqueous solution of [1]-
techol (H2tcc) to a solution of [1]
ACHTREUNG
A
G
chlorocatechol has a higher oxidation potential than
U
G
H2dtbc[26,27] due to its electron-withdrawing substituents and
is commonly used as a pseudo-substrate resistant to oxida-
and hence, the same structural formulation is inferred. Un-
fortunately, numerous attempts to obtain single crystals of
tion. Upon titration of a solution of [1](PF6) with H2tcc, a
A
[1]
A
new band appears at 430 nm and concomitantly the intensity
of the d–d transition decreases (Figure 4).
Catecholase activity: The (partial) dinuclear character of
[1](X) (X=OTf, PF6) in solution prompted us to study the
catecholase activity of these complexes in the oxidation of
benchmark substrate 3,5-di-tert-butylcatechol (H2dtbc) to
3,5-di-tert-butylbenzoquinone (dtbq) in acetonitrile. The
product dtbq has a strong absorption band at about 395 nm,
and therefore the reaction can be easily monitored by UV/
Vis spectroscopy. Because [1]
slowly in acetonitrile, the studies described below were per-
formed with [1](PF6). To estimate the catecholase-type ac-
tivity of [1](PF6), a solution of the catalyst in acetonitrile
A
ACHTREUNG
ACHTREUNG
was treated with 25 equiv of H2dtbc. Formation of dtbq was
followed over time by UV/Vis spectroscopy (Figure 3).
Figure 4. UV/Vis spectral changes upon addition of tetrachlorocatechol
(0–1 equiv in 0.125 equiv increments) to a solution of [1](PF6) in acetoni-
A
trile. Arrows indicate a decrease or increase in absorption upon addition.
The inset shows the spectral changes in the d–d transition region.
The spectral changes correspond to the observed colour
change of the solution from blue to yellow upon addition of
H2tcc. The two isosbestic points at 581 and 785 nm indicate
the presence of only two absorbing species in solution.
These marked changes are consistent with previously ob-
served UV/Vis spectral changes upon titration of dinuclear
copper(II) complexes with tetrachlorocatechol[2,28,29] and in-
dicate binding of the catechol to the complex.
To obtain further insight into the binding mode of tetra-
chlorocatechol, we tried to obtain single crystals of the
adduct. However, the crystallisation solutions gradually
changed colour from yellow to brown to green over several
weeks. A crystal structure determination on the green crys-
tals that were obtained revealed that tetrachlorocatechol
was no longer intact. Instead, a chloranilato (ca) moiety was
found to bridge two copper(II) centres in [Cu2(ca)Cl2-
Figure 3. Oxidation of 3,5-di-tert-butylcatechol by [1](PF6), monitored by
N
UV/Vis spectroscopy. Spectra were recorded at regular time intervals
(5 min) over a period of 120 min. The inset shows the increase in absorp-
tion at 395 nm with time.
The results show that [1](PF6) does catalyse the oxidation
G
of H2dtbc, but only with a very modest activity (2.6 turn-
overs with respect to copper after 120 min). The activity was
tested at different substrate concentrations. Michealis–
Menten-type substrate saturation behaviour, which is com-
monly observed at higher substrate concentrations in related
model studies,[2,3,24] was not observed and the reaction rate
was found to be independent of substrate concentration
over the range studied (5–50 equiv). This phenomenon has
been observed before,[7,11,25] and particularly strong binding
of the substrate to the catalyst was proposed as a possible
explanation.[11]
C
(HL1)2]·8MeCN·2H2O:
2
G
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5569

R. J. M. Klein Gebbink et al.
ꢀ
tures result in essentially equal Cu O distances and a sym-
metric binding mode of the dianion. Some end-capped di-
meric [Cu2(ca)] structures have been reported before, and
the observed bond lengths of the dicopper(II) chloranilato
moiety in the structure of 2·8MeCN·2H2O compare well
with the reported values.[30–34] These end-capped dinuclear
complexes were all synthesised by self-assembly of their
components, that is, they were obtained from mixtures of
chloranilic acid, a copper(II) source, and a bi- or tridentate
nitrogen donor ligand.
The neutral HL1 ligand is coordinated to copper through
both 1-methylimidazole N donor atoms and the carbonyl O
ꢀ
atom of the carboxyl group. This is supported by C O bond
ꢀ
ꢀ
lengths of 1.210(2) and 1.326(2) Š for C11 O1 and C11
O2, respectively, and involvement of O2 in a hydrogen-
bonding interaction of the OH group of the acid with a co-
Figure 5. Molecular structure of [CuII2(ca)Cl2
(HL1)2]·8MeCN·2H2O in
A
ꢀ
crystallised water molecule (Figure 6). The Cu Ocarbonyl dis-
tance in 2·8MeCN·2H2O (2.7911(12) Š) is much longer
the crystal. All CH hydrogen atoms and co-crystallised acetonitrile and
water solvent molecules have been omitted for clarity. Displacement el-
lipsoids are drawn at the 50% probability level. Symmetry operation a:
1ꢀx, 1ꢀy, 1ꢀz. The side view of the dimer in the inset shows the planari-
ty of the bridging chloranilato dianion.
ꢀ
than the Cu Ocarboxylato distance observed in the related com-
plex [CuII(L1)2]·2H2O (2.400 Š),[12] in which the monoanion-
ic L1 ligand is coordinated through its carboxylato O anion.
The two co-crystallised water molecules are involved in
identical hydrogen-bonding patterns that connect the dimer-
version centre and is bound as a bis-bidentate bridging
ligand. Each copper(II) atom is facially capped by a proton-
ated, neutral HL1 ligand. A chlorido anion completes the
N2O2O’Cl donor set to result in a severely distorted octahe-
dral coordination geometry around each copper(II) centre.
The basal plane is defined by the two nitrogen donor atoms
and the chloranilato oxygen donor atoms. The copper atom
lies 0.2713(2) Š above this plane towards the chlorido
ligand. The complex is centrosymmetric with the crystallo-
graphic inversion centre coinciding with the centre of the
chloranilato ring. The intramolecular Cu···Cu separation is
7.7628(5) Š. The chloranilato moiety is planar and its struc-
tural features are consistent with delocalisation of the nega-
tive charges to the lower and upper regions of the ring. This
ic units into
(Figure 6, Table 2).
a one-dimensional ladder-like structure
The carboxylic acid proton H2O is involved in a hydrogen
bond with oxygen atom O5 of a lattice water molecule. The
water molecule is in turn involved in a hydrogen bond with
a chlorido ligand of a second dinuclear unit. Finally, the
water molecule forms an additional hydrogen bond with one
of the co-crystallised acetonitrile molecules.
Chloranilic acid formation: The observed conversion of tet-
rachlorocatechol to chloranilic acid is very unusual. Because
tetrachlorocatechol is stable in acetonitrile under ambient
conditions, complex [1](PF6) apparently mediates the stoi-
A
ꢀ
ꢀ
is reflected in the C12 C13 and C12a C14 (1.393(2) and
chiometric oxidative dehalogenation of tetrachlorocatechol
to chloranilic acid (Scheme 1). Both the protons and chlo-
ride anions that are released during the reaction can be
found in the final product. The monoanionic ligands L1 are
ꢀ
ꢀ
1.398(2) Š) and the C13 O3 and C14 O4 bond lengths
ꢀ
(1.2627(19) and 1.259(2) Š). The C13 C14 bond length of
1.530(2) Š is indicative of a single bond. All of these fea-
Table 1. Selected bond lengths [Š] and angles [8] for 2·8MeCN·2H2O and 3. Symmetry operation a: 1ꢀx, 1ꢀy, 1ꢀz.
2·8MeCN·2H2O
Cu1ꢀCl1
Cu1ꢀN1
Cu1ꢀN4
Cu1ꢀO1
Cu1ꢀO3
Cu1ꢀO4
2.5171(5)
Cl1-Cu1-O1
O3-Cu1-N1
O4-Cu1-N4
O1-Cu1-N1
O1-Cu1-N4
O1-Cu1-O3
O1-Cu1-O4
174.62(3)
161.09(6)
165.93(6)
77.88(5)
77.19(5)
84.66(5)
89.79(4)
Cl1-Cu1-O3
Cl1-Cu1-O4
Cl1-Cu1-N1
Cl1-Cu1-N4
O4-Cu1-N1
N1-Cu1-N4
N4-Cu1-O3
O3-Cu1-O4
96.99(4)
95.53(4)
101.06(4)
97.59(4)
91.85(5)
90.60(6)
92.57(5)
80.79(5)
C11ꢀO1
C11ꢀO2
C13ꢀO3
C14ꢀO4
1.210(2)
1.326(2)
1.2627(19)
1.259(2)
C13ꢀC12
C12ꢀC14a
C13ꢀC14
1.393(2)
1.398(2)
1.530(2)
1.9607(14)
1.9541(14)
2.7911(12)
2.0234(12)
2.0027(12)
3
Cu1ꢀO1
Cu1ꢀN11
Cu1ꢀN41
Cu1ꢀO12
Cu1ꢀO22
2.302(2)
1.971(2)
1.950(2)
1.9485(19)
1.9700(19)
O1-Cu1-N11
O1-Cu1-N41
O1-Cu1-O22
O1-Cu1-O12
O12-Cu1-N41
91.35(9)
102.75(8)
101.11(8)
88.56(8)
O22-Cu1-N11
O12-Cu1-N11
N11-Cu1-N41
N41-Cu1-O22
O22-Cu1-O12
166.99(9)
C12ꢀO12
1.272(3)
1.267(3)
1.227(3)
1.243(3)
1.518(4)
C22ꢀC32
C32ꢀC42
C42ꢀC52
C52ꢀC62
C62ꢀC12
1.390(4)
1.407(4)
1.550(4)
1.418(4)
1.375(4)
93.49(8)
90.85(9)
90.03(8)
83.21(8)
C22ꢀO22
C52ꢀO42
C42ꢀO32
C12ꢀC22
167.80(9)
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Chem. Eur. J. 2008, 14, 5567 – 5576

Oxidative Double Dehalogenation of Tetrachlorocatechol
FULL PAPER
formed under an argon atmosphere or under water-free con-
ditions, that is, both water and dioxygen are required for for-
mation of 2.
Independent synthesis of [CuII (ca)Cl2
(HL1)2] (2): Complex
A
2
2 can also be directly synthesised from its components. Ad-
dition of chloranilic acid to a solution containing equimolar
amounts of CuCl2·H2O and [Bu4N][L1] in methanol resulted
in the precipitation and isolation of 2 as a dark green
powder.
The IR spectrum of 2 is identical to that obtained for the
product isolated from the reaction of [1](PF6) and tetra-
A
chlorocatechol (see above). The carboxylato group of L1 is
protonated upon formation of the complex, as indicated by
the asymmetric stretching band at 1731 cmꢀ1. Bands at 1525
and 1383 cmꢀ1 can be attributed to the chloranilato group
and agree well with the approximate D2h symmetry of this
group.[30] The electronic absorption spectrum of a powder
sample of 2, which has a green colour, shows two absorp-
tions at 403 and 629 nm, which can be attributed to a
charge-transfer band and a d–d transition, respectively.[30]
The complex is insoluble in acetonitrile. Interestingly, dis-
solution of green complex 2 in methanol yielded a blue-
purple solution, whereas in DMF a pinkish purple solution
was obtained. Two bands are observed in the UV/Vis ab-
sorption spectrum of a solution of 2 in DMF: one at 518 nm
(e=725mꢀ1 cmꢀ1) and a less intense band at 679 nm (e=
150mꢀ1 cmꢀ1). The transition at 518 nm can be assigned to a
chloranilato-to-copper charge-transfer transition of a chlor-
Figure 6. Hydrogen-bonding network of 2·8MeCN·2H2O resulting in a
ladder-like structure. All CH hydrogen atoms and co-crystallised solvent
molecules that do not participate in hydrogen bonding have been omitted
for clarity.
Table 2. Selected hydrogen-bond lengths [Š] and angles [8] for
2·8MeCN·2H2O and 3. Symmetry operations for 3: i) ꢀx, ꢀy, ꢀz; ii) 1ꢀx,
1ꢀy, 1ꢀz; iii) 1+x, 1+y, 1+z.
ꢀ
ꢀ
D H
ꢀ
Donor H···Acceptor
H···A
D···A
D H···A
2·8MeCN·2H2O
ꢀ
O2 H2O···O5
0.73(3)
0.93(3)
0.87(3)
1.91(3)
2.04(3)
2.28(3)
2.627(2)
2.932(3)
3.1111(18)
170(3)
161(3)
159(3)
anilato moiety with ortho-benzoquinone character.[30] Cre-
ꢀ
O5 H5A···N8
ꢀ
O5 H5B···Cl1
dence for this assignment and further insight into the struc-
ture of the pink-purple chromophore was obtained from an
X-ray crystal structure determination on single crystals of
3
i
ꢀ
O1 H1O···O42
0.71(5)
0.85(5)
0.96(5)
2.05(5)
2.07(5)
1.65(5)
2.742(3)
2.919(3)
2.594(3)
166(5)
173(5)
169(5)
ii
ꢀ
O1 H2O···O21
iii
ꢀ
[Cu(ca)
N
N
O11 H11O ···O32
tonitrile/methanol (see below). Apparently, a new equilibri-
um of both mono- and dinuclear species is established upon
dissolution of 2 that ultimately results in crystallisation of
mononuclear complex 3. Ions from both dinuclear and
A
N
of 2. The higher extinction coefficient associated with the
mononuclear pink-purple chromophore, however, causes the
colouration of the solution.
Scheme 1. Formation of chloranilic acid in the reaction of [1](PF6) with
tetrachlorocatechol.
A
Magnetic and EPR properties of 2: The EPR spectrum of a
powder sample of independently synthesised 2 recorded at
room temperature and at 77 K showed an anisotropic signal
with gk =2.26 and g? =2.08 (weighted average g value of
2.14). The spectrum does not show the hyperfine splitting
normally observed for mononuclear copper(II), and no indi-
cations of triplet spectra, expected for isolated dinuclear
copper(II), were seen. Apparently, relatively short intermo-
lecular Cu···Cu distances are responsible for exchange nar-
rowing, resulting in a pseudo-axial spectrum without hyper-
fine splitting.
coordinated as their conjugate acid HL1 and the chloride
anions are each coordinated to one of the copper(II) ions of
the dimer. The reaction can also be performed on a larger
scale. Upon addition of tetrachlorocatechol to a blue solu-
tion of [1](PF6) in acetonitrile/water (100/1 v/v), an immedi-
A
ate colour change to brown-yellow was observed and after
stirring the reaction for a week under ambient conditions a
green solution was obtained. The product was finally collect-
ed as a green crystalline solid in a yield of 41%. It was ana-
lysed by elemental analysis and the spectroscopic data were
compared with those of independently synthesised 2 (see
below). No product is obtained when the reaction is per-
To find out more about the presence and exchange cou-
pling of this dinuclear species, variable-temperature magnet-
ic susceptibility data were collected for a powdered sample
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R. J. M. Klein Gebbink et al.
of 2 at 0.1 Tin the temperature range of 2 to 300 K. The
plot of cMT and cM versus temperature is shown in Figure 7.
The cMT product of 2 at 300 K is 0.76 cm3 Kmolꢀ1, which
is within the range expected for the spin-only value for two
for chloranilato-bridged dinuclear copper(II) com-
plexes.[31,32,36]
Crystal structure of [CuII(ca)
A
E
tals of 3 suitable for X-ray diffraction were obtained from a
blue-purple solution of 2 in acetonitrile/methanol upon
standing. The molecular structure of 3 is depicted in
Figure 8, and selected bond lengths and angles are presented
in Table 1.
*
*
Figure 7. Plots of cM versus T ( ) and cMT versus T ( ) for 2 in the range
of 2–300 K in 0.1 Tfield. The inset shows the plot of cꢀM1 versus T with
the corresponding fitting.
Figure 8. Molecular structure of [CuII(ca)
N
N
All CH hydrogen atoms have been omitted for clarity. Displacement el-
lipsoids are drawn at the 50% probability level.
non-interacting S=1= copper(II) centres with g=2
2
(0.75 cm3 Kmolꢀ1). The value extrapolated to high T agrees
with a g value of 2.14 (from EPR) and was used to fit the
data for dinuclear copper(II). The cMT value decreases upon
cooling, and the decrease is more pronounced from 65 K to
0.02 cm3 Kmolꢀ1 at 5 K; this behaviour is characteristic for
antiferromagnetic coupling of the two copper(II) ions. The
temperature dependence of the magnetic susceptibility of 2
was fitted to the Curie–Weiss expression with a C value of
0.83 cm3 Kmolꢀ1 and a q value of ꢀ30 K. The negative q
value is again indicative of antiferromagnetic coupling be-
tween the metal centres.
The crystal structure consists of a mononuclear, five-coor-
dinate copper(II) complex in which the dianionic chloranila-
to ligand acts as a bidentate terminal ligand. The protonat-
ed, neutral HL1 ligand is coordinated in an N,N-bidentate
fashion to the copper centre through its 1-methylimidazole
N donor atoms. The acid group is pointing away from the
metal centre, that is, it is uncoordinated, and is involved in
hydrogen-bonding interactions (see below). Completion of
the coordination sphere around the copper ion by a water
molecule results in an N2O2O’ donor set. The coordination
geometry around the metal centre is slightly distorted
square-pyramidal with a t value[37] of 0.01. The basal plane
is made up of the two chloranilato oxygen atoms and the
two nitrogen donor atoms of the HL1 ligand. The water
molecule occupies the apical position. The copper atom is
displaced 0.2024(3) Š from the best least-squares plane
through the equatorial atoms towards the water molecule.
The slight distortion from ideal symmetry is illustrated by
the increased O1-Cu1-N41 and O1-Cu1-O22 angles
(102.75(8) and 101.11(8)8, respectively). This is the result of
the involvement of the water molecule in two hydrogen-
bonding interactions, which move the oxygen atom slightly
from the centre of the pyramid. The structural features of 3
The experimental cMT data were fitted to the equation for
dinuclear copper compounds derived from the Hamiltonian
h=J
ACHTREUNG
ci ¼
1ꢀð2zJ0=Ng2b2Þci
in which ci is given by Equation (2):
ꢀ
ꢁ
2NAb2g2
kBT
NAg2b2
2kBT
ci ¼ ð1ꢀ1Þ
½3þexpðꢀJ=kBTÞꢁꢀ1þ1
þTIP
in which zJ’ covers the intermolecular interactions (between
different dinuclear species), and ci is the magnetic suscepti-
bility considering the paramagnetic impurities (1) and the
temperature-independent paramagnetism (TIP) of the CuII
ions. The fit shown in the graph was based on g=2.14 (fixed
from the EPR experimental data), J=ꢀ35 cmꢀ1, zJ’=
ꢀ0.1 cmꢀ1 and 1=0.02 (from a fit of the experimental data)
with a reliability factor (R) of 4.6”10ꢀ4. A TIP of 60”
10ꢀ6 cm3 molꢀ1 (for each copper(II) ion) was taken into ac-
count. The negative J value arises from the antiferromagnet-
ic interactions between the two metal centres and is typical
compare well to those reported for [CuII(ox)
(H2O)],[38] which features the related 3,3-bis(imidazol-2-yl)-
propionate (Hbip) ligand and has an oxalato (ox) group as
A
AHCTREUNG
ꢀ
the other bidentate, dianionic ligand. Similar Cu N and
ꢀ
Cu OH2 distances are found in both complexes.
As a result of the different binding modes of the chlorani-
lato anion, that is, terminal bidentate in 3 versus bis-biden-
ꢀ
ꢀ
tate in 2, the observed sequences of C C and C O bond
ꢀ
lengths are different. The C O bond lengths in 3 are
5572
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5567 – 5576

Oxidative Double Dehalogenation of Tetrachlorocatechol
FULL PAPER
zoquinone. Chlorido-bridged dinuclear CuII complexes were
found as one of the products in these reactions.[42,43] These
conversions are the consequence of the well-established in-
stability of tetrachlorobenzoquinone with respect to both re-
duction and dehalogenation.[17,44,45] The principal products of
this conversion are tetrachlorocatechol and chloranilic acid.
In contrast to tetrachlorobenzoquinone, tetrachlorocatechol
is commonly used as a chemically inert catechol derivative
because of its stability with respect to oxidation. Indeed, the
formation of stable complexes with this electronically deac-
tivated (pseudo)-substrate has been used to advantage in
modelling studies.[11,42,46] The downside of this chemical sta-
bility is limited biodegradation
ꢀ
ꢀ
1.272(3) (C12 O12) and 1.267(3) Š (C22 O22) for the coor-
ꢀ
dinated (anionic) oxygen atoms and 1.227(3) (C52 O42)
ꢀ
and 1.243(3) Š (C42 O32) for the uncoordinated (carbonyl)
oxygen atoms. The C C single-bond lengths observed for
C12 C22 and C42 C52 together with the intermediate C C
distances for the other C C bond lengths in the chloranilato
ring indicate charge delocalisation over the lower and upper
regions of the ring. These structural features are similar to
those observed in the few other reported copper complexes
with a terminal bidentate chloranilato ligand.[30,33,39–41]
Three different hydrogen-bonding interactions are ob-
served in the crystal structure of 3 (Table 2, Figure 9). The
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
of tetrachlorocatechol.
A plausible mechanism for
the transformation of tetra-
chlorocatechol to chloranilic
acid involves its initial oxida-
tion to tetrachlorobenzoqui-
none by [1](PF6) (Scheme 2).
G
Subsequently, two consecutive
nucleophilic attacks of water
result in dehalogenation and
formation of chloranilic acid
with release of two equivalents
of HCl. In the presence of di-
oxygen the CuI ions formed in
the first step are reoxidised to
CuII, and dimeric complex 2 can
Figure 9. Hydrogen-bonding network of 3 resulting in infinite strands; all CH hydrogen atoms have been omit-
ted for clarity. Inset: p–p stacking in the crystal structure of 3. Symmetry operations: i) ꢀx, ꢀy, ꢀz; ii) 1ꢀx,
1ꢀy, 1ꢀz; iii) 1+x, 1+y, 1+z.
carboxyl oxygen atom of HL1 acts as a hydrogen-bond
donor to uncoordinated chloranilato oxygen atom O32 of a
second monomeric unit, and in this way an infinite, ladder-
like linear chain is formed along the [1,1,1] body diagonal.
The infinite linear chain is connected by two hydrogen
bonds to a second infinite chain. The water molecule acts as
a donor for two single hydrogen bonds to both the carbonyl
oxygen atom O21 of the acid group and the other uncoordi-
nated chloranilato oxygen atom O42. The two linear chains
run antiparallel to each other, and the strand is further held
together by a p–p stacking interaction between the chlorani-
lato rings with an interplanar distance of 3.49 Š.
Scheme 2. Proposed mechanism for the degradation of tetrachlorocate-
chol (H2tcc) to chloranilic acid (H2ca).
be assembled. Importantly, the chloranilic acid product has
low acute toxicity and is unlikely to pose a major environ-
mental threat.[17] Further development of the stoichiometric
reaction reported herein might provide a new method for
the chemical oxidation and dechlorination of halogenated
aromatic compounds in waste-treatment and remediation
processes.[47–49]
Interestingly, a biological precedent is known for the ob-
served formation of chloranilic acid from tetrachlorocate-
chol. The lignin-degrading white-rot fungus Coriolus versi-
colour converts tetrachloroguaiacol to tetrachlorocatechol
and finally degrades the latter to, amongst other products,
chloranilic acid.[16] The enzyme that is thought to be in-
volved in this transformation is laccase. Laccase has a so-
called type-4 copper active site,[2] which comprises a trinu-
clear copper cluster that is composed of a type-2 mononu-
Discussion
The copper(II) coordination chemistry of ligand L1 and the
reactivity with tetrachlorocatechol is summarised in
Scheme 3.
Isolation of a chloranilato-bridged dinuclear copper(II)
complex from a reaction mixture containing [1](PF6) and
G
tetrachlorocatechol was unexpected and is very unusual. In
fact, the observed oxidative dehalogenation is, to the best of
our knowledge, the first report of this type of chemical deg-
radation of tetrachlorocatechol. Dehalogenation has been
reported in reactions of CuI complexes with tetrachloroben-
Chem. Eur. J. 2008, 14, 5567 – 5576
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5573

R. J. M. Klein Gebbink et al.
Experimental Section
Infrared spectra were recorded with a Perkin–Elmer Spectrum One
FTIR instrument. Elemental microanalyses were carried out by the Mi-
croanalytisches Laboratorium Dornis & Kolbe, Mulheim a.d. Ruhr (Ger-
many). ESI-MS spectra were recorded on a Micromass LC-TOF mass
spectrometer at the Biomolecular Mass Spectrometry group, Utrecht
University. Solution UV/Vis spectra were recorded on a Varian Cary 50,
and diffuse reflectance UV/Vis spectra on a Perkin–Elmer Lambda 900.
Magnetic susceptibility measurements (2–300 K) were carried out with a
Quantum Design MPMS-5 5TSQUID magnetometer at 0.1 T. Data were
corrected for the magnetisation of the sample holder and for diamagnetic
contributions, which were estimated from Pascal constants. X-band
powder EPR spectra were obtained on polycrystalline samples with a
JEOL RE2X EPR spectrophotometer with 2,2-diphenyl-1-picrylhydrazyl
(DPPH, g=2.0036) as reference. The potassium and tetrabutylammoni-
um salts of 3,3-bis(1-methylimidazol-2-yl)propionate (L1) were prepared
according to a published procedure.[12] Tetrachlorocatechol was recrystal-
lised from toluene before use. All other chemicals were obtained com-
mercially and used as received. All concentrations of the solutions and
the equivalents added in the reactivity and binding studies were calculat-
ed and are expressed with respect to the quantity of copper ions in solu-
tion.
[Cu(L1)]n
A
G
N
in dry methanol (5 mL) was added to a solution of K[L1] (289 mg,
1.06 mmol) in warm, dry methanol (10 mL), and the resulting deep blue
solution was stirred at 508C for 30 min, during which a light blue precipi-
tate formed. The suspension was allowed to cool to room temperature
and was stirred overnight, after which the blue precipitate was separated
by centrifugation. The precipitate was washed with dry methanol (2”
20 mL) and the product was obtained as a blue powder (391 mg, 82%).
IR (solid): n˜ =3139.2, 2963.8, 1555.5, 1516.2, 1433.6, 1412.4, 1320.0,
1258.4, 1223.8, 1148.0, 1028.3, 958.9, 768.9, 752.9 cmꢀ1; UV/Vis (acetoni-
trile): lmax (e)=636 nm (100mꢀ1 cmꢀ1); Conductivity (1 mM in acetoni-
Scheme 3. Synthesis and reactivity of copper complexes 1–3.
clear and type-3 dinuclear copper site. In this light, the reac-
trile): lM =243 cm2 molꢀ1
296.03}, 445.98 {[Cu(L1)
(OTf)]2+, calcd 518.53}, 740.94 {[Cu2(L1)2
{[Cu3(L1)3
(OTf)2]+, calcd 1186.00}; elemental analysis calcd (%) for
W
ꢀ1; ESI-MS: m/z: 295.99 {[Cu2(L1)2]2+, calcd
(OTf)+H]+, calcd 445.99}, 518.63 {[Cu3(L1)3-
(OTf)]+, calcd 741.02}, 1186.30
tivity of [1]
ed as a functional model of the laccase involved in the de-
chlorination of tetrachlorocatechol.
A
AHCTREUNG
A
ACHTREUNG
AHCTREUNG
ACHTREUNG
C12H13CuF3N4O5S (445.86): C 32.33, H 2.94, N 12.57; found: C 32.46, H
3.10, N 12.38.
[Cu(L1)]n
water (10 mL) was added to
A
ACHTREUNG
Conclusion
a
ACHTREUNG
0.92 mmol) in water (35 mL), and immediately a light blue precipitate
formed. The suspension was stirred for 10 min and the light blue precipi-
tate was separated by centrifugation. The precipitate was washed with
water (3”20 mL) and the product was obtained as a light blue powder
(365 mg, 90%). IR (solid): n˜ =3157.1, 2962.0, 1555.1, 1527.2, 1515.8,
1433.1, 1411.0, 1320.2, 1289.1, 1213.8, 1171.4, 1148.9, 1092.4, 976.1, 958.4,
837.1, 818.7, 767.6, 742.8 cmꢀ1; UV/Vis (acetonitrile): lmax (e)=639 nm
The partial dinuclear character in solution of [1](X) (X=
OTf, PF6), which was obtained from the reaction of stoichio-
metric amounts of K[L1] and Cu(OTf)2 followed by anion
G
exchange for the PF6 salt, prompted us to study its catecho-
lase activity. Although the catecholase activity is modest, a
different and surprising conversion of tetrachlorocatechol to
chloranilic acid was discovered. Structure determination on
single crystals obtained from a solution containing tetra-
(100mꢀ1 cmꢀ1);
conductivity
ESI-MS: m/z: 296.00 {[Cu2(L1)2]2+
(PF6)]+, calcd 737.03}, 1052.06 {[Cu3(L1)3
(1 mM
in
acetonitrile):
calcd 296.03},
(PF6)+F]+,
lM =
231 cm2 molꢀ1 Wꢀ1
737.03 {[Cu2(L1)2
;
,
A
ACHTREUNG
calcd 1052.06}; elemental analysis calcd (%) for C11H13CuF6N4O2P
(441.76): C 29.91, H 2.97, N 12.68; found: C 29.82, H 3.10, N 12.54.
chlorocatechol and dinuclear [1](PF6) revealed formation of
H
a dinuclear copper compound containing a bridging bis-bi-
dentate chloranilato ligand. This ligand results from stoi-
chiometric oxidative double dehalogenation of tetrachloro-
catechol by a homogeneous copper complex to yield chlor-
A
N
mediate colour change to yellow-brown was observed. The solution was
stirred for 8 d at room temperature, during which time the colour gradu-
ally changed from yellow-brown to green. The solution was filtered and
concentrated to 3 mL. Green crystals of composition [Cu2(ca)Cl2-
A
an interesting new opportunity for the degradation of persis-
tent organic pollutants, such as polychlorinated aromatic hy-
drocarbons. Furthermore, the supporting ligand HL1 shows
flexible coordination chemistry, which ranges from N2O
(HL1)2]·6H2O grew upon standing for several weeks (35 mg, 41%). IR
(solid): n˜ =3419.7, 3135.3, 1732.0, 1522.2, 1504.6, 1383.4, 1286.0, 1148.3,
756.2 cmꢀ1; elemental analysis calcd (%) for C28H28Cu2Cl4N8O8·6H2O
(981.57): C 34.26, H 4.11, N 11.42; found: C 34.65, H 4.26, N 11.65.
binding as
N2O(H) binding as a neutral ligand in [Cu2(ca)Cl2
(2), to bidentate N2 binding in [Cu(ca)(HL1)(H2O)] (3).
a
monoanionic ligand in [1]
ACHTREUNG
Direct synthesis of [Cu2(ca)Cl2(HL1)2] (2): CuCl2·H2O (157 mg,
0.92 mmol) was added to a colourless solution of [Bu4N][L1] (438 mg,
0.92 mmol) in methanol (15 mL), and the blue solution was stirred for
AHCTREUNG
A
ACHTREUNG
5574
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 5567 – 5576

Oxidative Double Dehalogenation of Tetrachlorocatechol
FULL PAPER
5 min. A red solution of chloranilic acid (98 mg, 0.46 mmol) in methanol
(5 mL) was added to this solution, and an immediate colour change to
dark blue-green was observed. The solution was stirred overnight at
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
room temperature, during which time
a green precipitate gradually
formed. The crude product was separated by centrifugation and washed
with methanol (3”5 mL) and diethyl ether (3”20 mL). The product was
obtained as a dark green powder (354 mg, 88%). IR (solid): n˜ =3144.3,
3102.4, 2961.3, 1730.7, 1525.9, 1505.3, 1383.8, 1290.5, 1178.1, 1151.2,
1136.1, 861.3, 784.4 cmꢀ1; UV/Vis (DMF): lmax (e)=518 (725), 679 nm
(150mꢀ1 cmꢀ1). UV/Vis (diffuse reflectance): lmax =403, 629 nm; ESI-MS:
Acknowledgements
The work described here was financially supported by the National Re-
search School Combination-Catalysis (P.C.A.B.) and the Council for
Chemical Sciences of the Netherlands Organization for Scientific Re-
search (CW-NWO) (M.L., A.L.S.). Dr. Stefania Tanase-Grecea is kindly
acknowledged for her help with the magnetic measurements. Support
from the FP6 Network of Excellence “Magmanet” (contract number
515767) to J.R. is also kindly acknowledged.
m/z: 400.03 {[Cu2(ca)
A
,
calcd 400.00}, 504.00 {[Cu(ca)-
ACHTREUNG
emental analysis calcd (%) for C28H28Cu2Cl4N8O8 (873.47): C 38.50, H
3.23, N 12.83; found: C 38.64, H 3.28, N 12.72.
Catecholase activity and tetrachlorocatechol titrations: A solution con-
taining 5, 10, 25 or 50 equiv of 3,5-di-tert-butylcatechol in acetonitrile
(0.1 mL) was added to a solution of [1]
A
G
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(2 mL, 0.75 mm) in a quartz cuvette. Upon addition of the substrate the
blue solution immediately turned yellow. Quinone formation was moni-
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were performed under ambient conditions.
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The titrations of [1]
A
N
formed by incremental additions of 10 mL aliquots of a 37.5 mm solution
of tetrachlorocatechol in acetonitrile (0.125 equiv) to a solution of the
copper(II) complex in acetonitrile (1.5 mm, 2 mL). UV/Vis spectra were
recorded after each addition.
X-ray crystal structure determination of 2·8MeCN·2H2O and 3: Reflec-
tions were measured on a Nonius Kappa CCD diffractometer with rotat-
ing anode (graphite monochromator, l=0.71073 Š) up to a resolution of
sin
(q/l)max =0.65 Šꢀ1. Intensities were integrated with EvalCCD[50] using
A
an accurate description of the experimental setup for the prediction of
the reflection contours. The structures were refined with SHELXL-97[51]
against F2 of all reflections. Non-hydrogen atoms were refined with aniso-
tropic displacement parameters. All hydrogen atoms were located in the
difference Fourier map. The OH hydrogen atoms were refined freely
with isotropic displacement parameters; all other hydrogen atoms were
refined with a riding model. Geometry calculations and checking for
higher symmetry were performed with the PLATON program.[52]
X-ray
C28H28Cl4Cu2N8O8·8MeCN·2H2O;
0.21”0.18 mm; monoclinic; P21/c (no. 14); a=10.1857(4), b=11.0587(3),
c=27.1239(14) Š; b=109.560(2)8; V=2878.9(2) Š3; Z=2; 1calcd
crystal
structure
of
2·8MeCN·2H2O:
[12] P. C. A. Bruijnincx, M. Lutz, A. L. Spek, E. L. van Faassen, B. M.
Weckhuysen, G. van Koten, R. J. M. Klein Gebbink, Eur. J. Inorg.
Chem. 2005, 779–787.
M
W =1237.93; green block; 0.24”
=
1.428 gcmꢀ3; m=0.99 mmꢀ1. 55363 reflections were measured at 150 K,
corrected for absorption, and scaled on the basis of multiple measured
reflections with the program SADABS[53] (correction range 0.79–0.84).
6601 reflections were unique (Rint =0.0418). The structure was solved
with the program DIRDIF-99[54] by using automated Patterson methods.
361 parameters were refined with no restraints. R1/wR2 [I>2s(I)]:
[13] K. Kervinen, P. C. A. Bruijnincx, A. M. Beale, J. G. Mesu, G.
van Koten, R. J. M. Klein Gebbink, B. M. Weckhuysen, J. Am.
Chem. Soc. 2006, 128, 3208–3217.
[14] P. C. A. Bruijnincx, I. L. C. Buurmans, S. Gosiewska, M. Lutz, A. L.
Spek, G. van Koten, R. J. M. Klein Gebbink, Chem. Eur. J. 2008, 14,
1228–1237.
[15] P. C. A. Bruijnincx, M. Lutz, A. L. Spek, W. R. Hagen, B. M. Weck-
huysen, G. van Koten, R. J. M. Klein Gebbink, J. Am. Chem. Soc.
2007, 129, 2275–2286.
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1181–1196.
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[20] S. P. Gejji, K. Hermansson, J. Lindgren, J. Phys. Chem. 1993, 97,
3712–3715.
[21] S. Gosiewska, J. L. M. Cornelissen, M. Lutz, A. L. Spek, G. van Ko-
ten, R. J. M. Klein Gebbink, Inorg. Chem. 2006, 45, 4214–4227.
[22] D. H. Johnston, D. F. Shriver, Inorg. Chem. 1993, 32, 1045–1047.
[23] W. J. Geary, Coord. Chem. Rev. 1971, 7, 81–122.
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Dillinger, F. Tuczek, H. Witzel, B. Krebs, J. Biol. Inorg. Chem. 1999,
4, 56–63.
0.0299/0.0745. R1/wR2 (all reflections): 0.0398/0.0789. S=1.040. Residual
ꢀ3
electron density ꢀ0.33/0.51 eŠ
.
X-ray crystal structure determination of 3: C17H16Cl2CuN4O7; MW
=
¯
522.78; dark purple block; 0.30”0.30”0.15 mm; triclinic; P1 (no. 2); a=
7.6753(3), b=11.5330(4), c=12.0878(4) Š; a=111.707(2), b=101.398(2),
g=90.896(1)8; V=969.88(6) Š3; Z=2; 1calcd =1.790 gcmꢀ3
;
m=
1.45 mmꢀ1. 13998 reflections were measured at 110 K. The crystal ap-
peared to be non-merohedrally twinned with a twofold rotation about
the crystallographic b axis as twin operation. This twin operation was
taken into account during integration of the intensities and HKLF5 re-
finement.[55] An absorption correction was not applied due to the twin-
ning. 4424 reflections were unique (Rint =0.0676). The structure was
solved for non-overlapping reflections with the program DIRDIF-99[54]
by using automated Patterson methods. 295 parameters were refined with
no restraints. R1/wR2 [I>2s(I)]: 0.0390/0.0975. R1/wR2 (all reflections):
0.0434/0.1002. S=1.177. The twin fraction refined to 0.371(3). Residual
ꢀ3
electron density ꢀ0.72/0.62 eŠ
.
CCDC-634539 (2·8MeCN·2H2O) and 634540 (3) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained free
Chem. Eur. J. 2008, 14, 5567 – 5576
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Received: November 28, 2007
Published online: April 30, 2008
5576
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