the first example of a bis
binds and then ortho-hydroxylates phenolates, thus mimick-
ing the reactivity of tyrosinase. We have trapped and
A
N
Nevertheless, addition of 10 equivalents of the sodium
salt of p-chlorophenol at ꢀ908C caused immediate bleach-
ing of the spectral features associated with 2. Acidic work-
up and HPLC analysis revealed that 4-chlorocatechol was
formed in 67% yield with respect to the initial dicopper
complex. The identity of the product was further confirmed
spectros
ing from the low-temperature reaction of sodium p-chloro-
phenolate (p-Cl-C6H4ONa) with a bis(m-oxo)dicopper(III)
copically characterized a metastable species result-
1
species preceding phenolate hydroxylation. Finally, kinetic
analysis of the hydroxylation reaction has been performed
to obtain activation parameters that can be compared with
those found for tyrosinase.[12]
by isolation with preparative HPLC and H NMR analysis.
ꢀ
ꢀ
Neither quinone nor C C or C O coupling products were
obtained. Similar reactions with p-carbomethoxyphenolate
and p-cyanophenolate also show the formation of the corre-
sponding catechol as the sole oxidation product. Therefore,
We recently found that the reaction of [CuI2(m-
XYLMeAN)]
A
2 constitutes a rare example of a bis
N
N
species capable of performing the phenolate hydroxylation
to form a catechol, thus mimicking the activity exhibited by
tyrosinase.
Insight into the reaction mechanism was obtained by trap-
ping at very low temperature (ꢀ908C) a metastable reaction
intermediate formed after reaction of 2 with sodium p-
chloro
reaction in acetone showed that the initial features corre-
sponding to the bis(m-oxo) species (2) immediately disap-
A
AHCTREUNG
pear after phenolate addition (Figure 1, top). Concomitantly,
new spectral features appear at 390 and 563 nm correspond-
ing to a new species 3Cl. The latter is thermally very sensitive
and rapidly decomposes (t1/2 ꢁ 20 s) at ꢀ908C. Neverthe-
less, resonance Raman experiments (Figure 1, bottom) of
frozen solutions of 3Cl with laser excitation at 407 nm show a
resonance enhanced feature at 597 cmꢀ1 that experiences a
ꢀ26 cmꢀ1 shift when 18O2 is used in the generation of 2. This
feature is not enhanced when 568 nm laser excitation is used
in the experiment. Moreover, no isotope-sensitive features
that could be assigned to a (m-h2:h2-peroxo)dicopper(II) spe-
cies were observed in the 700–770 cmꢀ1 region.[6a] On the
other hand, laser excitation at 568 nm shows intense peaks
at 1264, 1409, and 1642 cmꢀ1, characteristic of phenolate vi-
bration modes.[15] These vibrational features are not affected
by the use of 18O2, and they are not enhanced with laser ex-
citation at 407 nm. The Raman data thus provide direct evi-
dence for phenolate binding to the Cu2O2 core in 3Cl.
Scheme 2. The reactions described in this work.
THF resulted in the fast formation of a deep yellow species
2 that is characterized by two intense UV/Vis features at
lmax =308 nm
(20000mꢀ1 cmꢀ1)
and
lmax =413 nm
The accumulated data can be interpreted with two differ-
(28000mꢀ1 cmꢀ1).[13] Resonance Raman experiments of
frozen acetone solutions using laser excitation at 413 nm re-
vealed a characteristic Cu2O2 breathing vibration peak at
600 cmꢀ1 that showed a downshift of 23 cmꢀ1 when 18O2 was
ent scenarios. The first is that 3Cl is actually a mixture of re-
sidual bis
phenolate species. Alternatively, 3Cl may be formulated as
[CuIII2(m-O)2(p-Cl-C6H4O)(m-XYLMeAN)]+, where bis
(m-oxo)
A
N
A
ACHTREUNG
used. These are common spectral features for a CuIII
A
and phenolate vibrations are uncoupled. We favor the latter
hypothesis on the basis of the following observations. Kinet-
ic analysis (vide infra) indicates that reaction of p-Cl-
C6H4ONa is fast even for stopped-flow methodology and no
residual 2 should be present under the experimental condi-
tions used to prepare the resonance Raman sample. Also,
the UV/Vis spectrum of 3Cl does not change upon varying
the concentration of phenolate. Furthermore, the features
core[11,14] that led us to formulate 2 as [CuIII
ACHTREUNG
[13]
XYLMeAN)]2+
.
Use of several solvents (THF, diethyl ether,
ꢀ
ꢀ
CH2Cl2, or acetone) and counterions (ClO4 , CF3SO3 ,
ꢀ
BArFꢀ, and SbF6 ) did not change the nature of 2, and no
experimental evidence for the isomeric (m-h2:h2-peroxo)di-
copper(II) species was observed. This was further substanti-
ated by DFT calculations at the B3LYP level, which indicat-
ed that the peroxo form was 35.5 kJmolꢀ1 higher in
energy.[13] Extraction and analysis of the ligand after thermal
decay of 2 did not show any evidence for ligand modifica-
tion, indicating that intramolecular aromatic hydroxylation
does not take place in this meta-xylyl-based system.
associated with the bis(m-oxo) core (390 nm) and with the
A
phenolate (563 nm), decay with the same kinetic behavior
(as monitored by UV/Vis spectroscopy). In addition, we
have observed negligible perturbations in the energy of the
Cu2O2 breathing mode in the resonance Raman spectra of
3536
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 3535 – 3538