Copper monooxygenase models. Aromatic hydroxylation by a dinuclear copper(I) complex containing methionine sulfur ligands

Article abstract of DOI:10.1039/a705225a

A dinuclear copper(I) complex 1 containing a bis(imine) ligand derived from the condensation between benzene-1,3-dicarbaldehyde and two molecules of L-methionine has been prepared. When this compound reacts with dioxygen a partial aromatic hydroxylation of the ligand occurs, giving a dinuclear μ-phenoxoμ-hydro-dicopper(II) complex 2, together with simple copper oxidation products. Definitive evidence of the monooxygenase activity of the present sulfur-containing model system results from the crystallographic characterisation of the dinuclear copper(II) complex 3 of the hydroxylated dicarbaldehyde, [Cu₂{C₆H₃(CHO)₂O}-(ClO ₄)₂], which forms upon hydrolysis of the imine groups of 2. In this complex two deprotonated 1,3-diformylphenoxide ligands bind two copper(II) ions, with di-μ-phenoxo bridges. Each copper is essentially square pyramidal, with a basal O₄ donor set, including two phenoxide and two carbonyl oxygen atoms from two 2-hydroxybenzene-1,3-dicarbaldehyde ligands. Two perchlorate oxygen atoms are bound in axial positions on opposite sides of the Cu₂O₆ plane. A minor fraction (15-20%) of 2 contains 5-oxygenated methionine residues. However, oxygenation at sulfur is a secondary process, resulting from the reaction of H₂O₂, formed according to the simple copper(I) oxidation pathway, and the dinuclear copper(II) complex 2.

Full text of DOI:10.1039/a705225a

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Copper monooxygenase models. Aromatic hydroxylation by a
dinuclear copper(I) complex containing methionine sulfur ligands
Gloria Alzuet,a Luigi Casella,*,a Maria Laura Villa,a Oliviero Carugoa and Michele Gullottib
a Dipartimento di Chimica Generale, Università di Pavia, Via Taramelli 12, 27100 Pavia, Italy
b Dipartimento di Chimica Inorganica, Metallorganica e Analitica, Università di Milano,
Centro CNR, Milano, Italy
A dinuclear copper() complex 1 containing a bis(imine) ligand derived from the condensation between
benzene-1,3-dicarbaldehyde and two molecules of -methionine has been prepared. When this compound
reacts with dioxygen a partial aromatic hydroxylation of the ligand occurs, giving a dinuclear µ-phenoxo-
µ-hydroxo-dicopper() complex 2, together with simple copper oxidation products. Definitive evidence of the
monooxygenase activity of the present sulfur-containing model system results from the crystallographic
characterisation of the dinuclear copper() complex 3 of the hydroxylated dicarbaldehyde, [Cu2{C6H3(CHO)2O}-
(ClO4)2], which forms upon hydrolysis of the imine groups of 2. In this complex two deprotonated 1,3-diformyl-
phenoxide ligands bind two copper() ions, with di-µ-phenoxo bridges. Each copper is essentially square
pyramidal, with a basal O4 donor set, including two phenoxide and two carbonyl oxygen atoms from two
2-hydroxybenzene-1,3-dicarbaldehyde ligands. Two perchlorate oxygen atoms are bound in axial positions on
opposite sides of the Cu2O6 plane. A minor fraction (15–20%) of 2 contains S-oxygenated methionine residues.
However, oxygenation at sulfur is a secondary process, resulting from the reaction of H2O2, formed according to
the simple copper() oxidation pathway, and the dinuclear copper() complex 2.
Dinuclear copper complexes with ligands of biological rele-
vance and containing metal centres in close proximity have been
extensively studied1–3 since this structural unit is present in the
active sites of several copper proteins involved in dioxygen
transport and activation, e.g. hemocyanin and tyrosinase.4 The
recent characterisation of several dinuclear peroxocopper()
complexes represents a topical achievement that has provided
valuable information about the binding mode of dioxygen to
the dicopper sites of the proteins. The redox chemistry associ-
ated with such characterised or putative peroxodicopper()
complexes has produced a number of systems in which acti-
vation of dioxygen results in ligand oxygenation reactions.
The systems where an aromatic hydroxylation of the ligand
occurs5–7 can be considered as functional models of tyrosinase,
while those performing an aliphatic hydroxylation in the
ligand8 mimic the activity of dopamine β-hydroxylase9 or
peptidylglycine α-amidating monooxygenase.10
going aromatic hydroxylation also undergoes subsequent sulfur
oxidation at the thioether group, according to a secondary
pathway involving hydrogen peroxide.
Results and Discussion
Complex 1 belongs to a family of relatively simple dicopper()
model compounds of general structure 4 where the ligands are
derived from the condensation between benzene-1,3-dicarb-
aldehyde and substituted amines.7 These ligands provide two
co-ordinating atoms to each copper() centre, but related
macrocyclic complexes reported by Martell and co-workers7c
contain triamine residues and hence three-co-ordinated metal
centres. All these complexes react with dioxygen performing, at
least to some extent, an aromatic hydroxylation reaction which
leads to the corresponding µ-phenoxo-µ-hydroxo-dicopper()
complexes, as shown by structure 2 in Scheme 1. The ligand
oxygenation is accompanied by simple copper() oxidation, and
both the ratio between oxygenation and oxidation products and
the rate of oxygenation depend on the nature of the X donor
group and the reaction solvent.7e,11 The idea to introduce a
methionine sulfur donor in the copper() complex was stimu-
lated by the possibility that such a ligand could be present in the
co-ordination sphere of the copper monooxygenase dopamine
β-hydroxylase.12 This feature has been subsequently confirmed
by more detailed studies.9c–f Other recent studies disclose strong
structural similarities between dopamine β-hydroxylase and
peptidylglycine α-amidating monooxygenase, and hence the
possibility that such a methionine–copper bond can be present
also in the latter enzyme.10c In our model studies we also
performed several attempts to extend the investigation to
analogues of 1 containing -cysteine or S-methyl--cysteine
residues in place of -methionine, but all these were unsuccess-
ful, probably because the resulting copper() compounds would
contain less sterically favourable five-membered chelate rings.
For the reaction of complex 1 with dioxygen, at least in dilute
methanol solution, the ratio between the ligand oxygenation
product (2) and copper oxidation product is about 1:1.7e This
In a communication7e we briefly reported the ligand
hydroxylation reaction undergone by a series of dicopper()
complexes with the bis(imines) derived from benzene-1,3-
dicarbaldehyde and substituted amines carrying various types
of donor groups in the alkyl chain in the presence of dioxygen.
Among these complexes the most interesting was certainly that
derived from -methionine (1, see Scheme 1) because of the
presence of thioether sulfur donors. The relevance of this
feature is related to the identification of a methionine ligand in
the co-ordination sphere of the dioxygen-binding, CuB site of
dopamine β-hydroxylase9e and peptidylglycine α-amidating
enzyme.10c Since the monooxygenase activity promoted by the
Cu᎐S centres of 1 is unprecedented in copper biomimetic chem-
istry a more complete characterisation of this system, for
instance with respect to the possible oxidation of the sulfur
donor, is a necessary step to assess the catalytic significance of
the methionine–copper bonding. Here we report the character-
isation of the oxygenation product of the model monooxy-
genase reaction, 2, the X-ray structural characterisation of the
dinuclear copper() complex 3 resulting from hydrolytic cleav-
age of 2, and show that a small fraction of the complex under-
J. Chem. Soc., Dalton Trans., 1997, Pages 4789–4794
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2+
O
N
O
N
Cu
S
Cu
HO2C
CO2H
O
O
O
O
H
H
N
S
N
CuI
CuI
Me
Me
S
S
λ
δ
Me
Me
5
6
1
2+
CO2H
Cu
O
N
O
N
Cu
HO2C
CO2H
S
S
N
O
N
Me
Me
H
H
Cu
Cu
HO2C
S
O
H
S
δ
λ
Me
Me
7
8
2
Scheme 2
2+
weak CD activity above 400 nm. The other prominent CD band
of negative sign at 290 nm is due to a benzenoid π → π*
transition of the ligand.
O
O
O
O
O
O
Cl
O
It is interesting that the sign of the optical activity of the 2-
hydroxyphenylimino CD band at 375 nm depends on the
chirality of the amino acid chelate ring.17 For imine complexes
containing -amino acid residues, negative CD activity within
this band correlates with a preferred conformation chirality of
sign λ.17 In the case of 2 this indicates that the -methionine
residue is either bound as a substituted glycine, with an anionic
carboxylate group in the co-ordination plane and an axial
thioether chain (5), or as a substituted γ-aminopropyl thioether,
with an unbound, protonated carboxyl group in equatorial dis-
position (8) (Scheme 2). The latter agreement is to be preferred
because, as indicated by the analytical data and the IR spec-
trum, the carboxyl groups in 2 are protonated.
As stated above, solutions of complex 2 are unstable to
hydrolysis of the imine bonds by traces of water. This process
can be followed easily in solution by UV spectroscopy, where
a progressive shift of the 362 nm band to higher energy, 344
nm, typical of the hydroxydialdehyde, can be observed. Upon
standing, methanolic solutions of 2 deposit blue crystals of the
dinuclear copper() complex of the anion of 2-hydroxy-
benzene-1,3-dicarbaldehyde, 3. The IR spectrum of 3 exhibits
strong bands corresponding to vibrations of the carbonyl (1640
cmϪ1) and the phenolate groups (1560 cmϪ1) and the character-
istic splitting of the ≈1100 cmϪ1 band of co-ordinated perchlor-
ate ions. The structure of this compound definitively confirms
the monooxygenase reaction undergone by 1 (Fig. 1). The
geometry around each copper is essentially undistorted from a
tetragonal pyramid with a basal O4 donor set which includes
two phenolate oxygen atoms from two 2-hydroxybenzene-1,3-
dicarbaldehyde ligands. Two perchlorate oxygen atoms occupy
the axial positions on opposite sides of the Cu2O6 plane. The
distances and bond angles in the co-ordination spheres (Table
1) are normal for copper().16e,18
O
Cl
O
O
Cu
Cu
O
O
O
O
3
2+
N
X
N
CuI
CuI
X
4
Scheme 1
ratio decreases when, as it has been done here, the reaction is
performed on a larger scale, and operating under heterogeneous
conditions. The two products can be separated by fractional
precipitation or, better, chromatography on Sephadex LH-20,
but both suffer the inconvenience of being somewhat unstable
to hydrolysis of the imine groups in solution by the presence of
even trace amounts of water. This reaction has been often
observed for imine complexes (see e.g. ref. 13) and in the present
case it may be favoured by the weak binding properties of the
thioether sulfur donors towards CuII. The characterisation of 2
as the ligand oxygenation product is based on the moderately
intense UV band near 360 nm, typical for the 2-hydroxyphenyl-
imino chromophore,7a and the medium-intensity IR band near
1560 cmϪ1, due to the ν(C᎐O) vibration of the phenolate group,
which assumes a partial double-bond character by conjugation
with the imine group.14 The CD spectrum of 2 in the UV region
shows several resolved features. It is dominated by a negative
band at 375 nm, which corresponds to the optical absorption at
362 nm, and is essentially due to the low-energy conjugated
imine π → π* transition. The apparent red shift in the CD
peak with respect to the optical absorption is due to partial
cancellation with a weaker CD band of opposite sign near 330
nm, which is probably contributed both by the thioether
S(σ)→CuII ligand-to-metal charge transfer (LMCT)15 and the
higher-energy component of the phenolate to CuII LMCT, or
metal-to-ligand charge transfer (MLCT),16 the lower-energy
component of this charge transfer being responsible for the
The presence of a sulfur donor in the ligand of complex 1
raises the problem of the possible oxidation at this site, in paral-
lel or alternatively to the aromatic hydroxylation reaction.
Although co-ordination to a metal ion is expected to make the
thioether group less easily oxidisable, by reducing the electron
density at the sulfur atom, the necessity to establish the integrity
of this residue in a system, like 1, capable of performing a
carbon oxygenation reaction is certainly important in the con-
text of dopamine β-hydroxylase chemistry. The presence of
methionine sulfoxide was examined by isolation of the amino
acids upon decomposition of the various fractions resulting
from chromatography or fractional precipitation of the prod-
ucts of oxygenation of several samples of 1. Somewhat surpris-
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J. Chem. Soc., Dalton Trans., 1997, Pages 4789–4794

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and the copper()–triazacyclononane complexes of Tolman and
co-workers.8h,i We performed several low-temperature oxygen-
ation experiments (down to Ϫ60 ЊC) with complex 1 to try to
characterise an analogous dioxygen complex, but under these
conditions we either observed no reaction or an extremely slow
reaction to give 2, according to the temperature and solvent
used. This indicates that either the oxygenation equilibrium lies
almost totally to the side of the dicopper() complex even at low
temperature, or the reaction of 1 with dioxygen is extremely
slow, while the subsequent reaction to 2 is faster. The latter
possibility was recently proposed19 on the basis of kinetic
studies for the reaction between a dicopper() complex with a
macrocyclic tetraimine reported by Menif et al.7c and dioxygen.
We can add that the difficulty to build up dioxygen adducts for
bis(imine) complexes of type 4 is not surprising in view of the
sluggish reactivity these systems exhibit towards carbon mon-
oxide.7a Regarding the different routes that the putative dioxy-
gen intermediate(s) can take, our proposal is summarised in
Scheme 3. The regiospecific aromatic hydroxylation yielding 2
can only be accounted for by an intramolecularly bound dioxy-
gen complex 10, formally a peroxodicopper() species. This
species may be preceded by an initially formed 1:1 Cu:O2
complex 9, formally a mixed-valence copper()–copper()
superoxo species. The latter species can alternatively form an
intermolecular Cu2O2 complex (not shown in Scheme 3) which
is responsible for the path leading to copper() oxidation, but is
unable to perform any oxygenase chemistry on the ligand. Oxy-
genation at sulfur is a secondary process involving the hydrogen
peroxide formed upon copper() oxidation and specifically acti-
vated by the dinuclear complex 2, where it can bind intra-
molecularly as a bridging hydroperoxo ligand in 12. Similar
µ-phenoxo, µ-1,1-hydroperoxo, or acylperoxo dicopper() com-
plexes reported by Karlin et al.20 have been shown to be able
quantitatively to transfer an oxygen atom to exogenous phos-
phines or sulfides through analogous reactions.
Table 1 Selected bond distances (Å) and angles (Њ) for [Cu2{C6H3-
(CHO)2O}2(ClO4)2]
Cu᎐O(1)
Cu᎐O(1I)
Cu᎐O(2)
1.951(1)
1.958(3)
1.923(4)
Cu᎐O(3I)
Cu᎐O(12)
1.928(4)
2.504(5)
O(1)᎐Cu᎐O(1I)
O(1)᎐Cu᎐O(2)
78.3(1)
92.8(2)
O(1I)᎐Cu᎐O(3I)
92.0(2)
O(1I)᎐Cu᎐O(12) 94.8(1)
O(1)᎐Cu᎐O(3I) 169.4(2)
O(2)᎐Cu᎐O(3I)
O(2)᎐Cu᎐O(12)
96.6(1)
88.4(2)
O(1)᎐Cu᎐O(12)
90.9(1)
O(1I)᎐Cu᎐O(2) 170.5(2)
O(3I)᎐Cu᎐O(12) 94.1(2)
Symmetry relation: I Ϫx, Ϫy, Ϫz.
Fig. 1 View of the complex [Cu2{C6H3(CHO)2O}2(ClO4)2] 3, showing
the structure of the copper() centres and the numbering scheme of the
ligands atoms
Experimental
Materials and methods
ingly, we consistently found methionine sulfoxide, in an amount
of 15–20% with respect to methionine, only in the fraction of 2
undergoing carbon hydroxylation, while only methionine was
present in the fractions containing simple copper oxidation
product. Since a simultaneous dioxygenation reaction at such
separated carbon and sulfur centres of 1 is unlikely to occur, we
investigated the possibility that oxidation at sulfur could
involve reactivity of 2 in a step subsequent to the monooxygen-
ase reaction. As 2 is stable to dioxygen, such reactivity may
depend on the presence of hydrogen peroxide. This can be
formed by two-electron reduction of dioxygen by the fraction
of 1 undergoing simple oxidation at copper(). In Scheme 3 the
two oxidative pathways undergone by 1, leading to ligand
hydroxylation and simple copper oxidation, are separated. The
latter pathway may be complex, and involve both partial reduc-
tion of dioxygen to hydrogen peroxide (stoichiometry 2 CuI :1
O2) and complete reduction of dioxygen to water (4 CuI :1 O2).
In order to check the possible involvement of H2O2 in the
sulfur oxygenation, the reaction between complex 2 and H2O2
was studied. When 2 was treated with hydrogen peroxide, a
rapid oxidation of the methionine ligand to methionine sulf-
oxide occurred, through the probable intermediacy of the
hydroperoxide complex 12 (Scheme 4). The increase in the con-
tent of methionine sulfoxide in the amino acid mixture, after
decomposition of the product complex, corresponds to the
amount of hydrogen peroxide used in the reaction. Blank
experiments using copper() salt and methionine showed that
no sulfoxide is formed by hydrogen peroxide through non-
specific reactions under the same (mild) conditions.
Tetrakis(acetonitrile)copper() perchlorate was prepared
according to a literature method.21 Methanol was dried with
molecular sieves and degassed by repeated vacuum/purge cycles
or bubbling argon directly through the solvent. Elemental
analyses were performed by the microanalytical laboratory of
the University of Milano. Infrared spectra of solid compounds
were obtained as Nujol mulls with a Mattson Galaxy model
5020 FT-IR instrument, optical spectra on a HP-8452 diode-
1
array spectrophotometer, H NMR spectra at 200 MHz on a
Bruker spectrometer and circular dichroism spectra on a
JASCO 710 spectrometer.
Preparation of complex 1ؒ0.5MeCN
The synthesis of this copper() complex was carried out in
Schlenk glassware under an atmosphere of purified argon. -
Methionine (373 mg, 2.5 mmol) was added to a solution of
benzene-1,3-dicarbaldehyde (166 mg, 1.25 mmol) in methanol–
water (2:1 v/v, 30 cm3) and heated to reflux temperature. The
mixture was stirred for 0.5 h and then, after cooling, a slight
excess of solid Cu(MeCN)4ClO4 (845 mg, 2.58 mmol) was
quickly added. The resulting light yellow solution was stirred
for 1 h at room temperature and subsequently concentrated to a
small volume under reduced pressure, until a yellow precipitate
appeared. Then degassed methanol was added and the solid
present was filtered off, washed with a small amount of
degassed methanol, and dried under vacuum (yield 65%)
(Found: C, 30.41; H, 3.68; Cu, 17.6; N, 4.50. C18H24Cl2Cu2-
N2O12S2ؒ0.5CH3CN requires C, 30.71; H, 3.46; Cu, 17.1; N,
4.71%). ν max/cmϪ1 (Nujol) 1700 (CO H), 1634 (C᎐N) and 1097
A final comment is deserved by the preliminary step in the
monooxygenase reaction, i.e. dioxygen binding to the copper()
complex, since so far this step has been thoroughly character-
ised only for a binuclear copper() complex by Karlin et al.6c
᎐
2
(ClO4).
J. Chem. Soc., Dalton Trans., 1997, Pages 4789–4794
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2+
HO2C
CO2H
N
S
N
CuI
CuI
S
Me
Me
1
O2
2+
2+
HO2C
CO2H
HO2C
CO2H
N
S
N
N
N
CuI
Cu
Cu
Cu
O2
S
S
O2
S
Me
Me
Me
Me
9
10
2+
2+
Me
S
Me
S
HO2C
CO2H
N
O
N
N
O
N
Cu
Cu
CuII
CuII
S
O
H
S
O
O
O
Me
Me
11
2
Scheme 3
Sephadex LH-20 column (1 × 20 cm) using methanol as eluent.
The fractions collected were controlled by optical spectroscopy,
by measuring the A260 :A362 ratio. The oxygenated product was
precipitated by addition of diethyl ether to the fraction that
showed the minimum value of A260 :A362 (about 5:1). The prod-
uct was filtered off and dried under vacuum (Found: C, 28.44;
H, 3.31; N, 3.54. C18H24Cl2Cu2N2O14S2 requires C, 28.66; H,
3.21; N, 3.71%). ν max/cmϪ1 (Nujol) 3285 (OH), 1728 (CO2H),
2+
HO2C
CO2H
N
S
O
Cu
N
Cu
S
O
Me
Me
H
2
1640 (C᎐N), 1562, 1237 (Ph᎐O), 1098 and 624 (ClO ). UV/VIS:
᎐
4
λmax/nm (ε/dm3 molϪ1 cmϪ1) (MeOH) 260 (30 000), 362 (6000)
and 670 (230). CD: λmax/nm (∆ε/dm3 molϪ1 cmϪ1) (MeOH) 270
(ϩ1.3), 290 (Ϫ4.0), 330 (ϩ0.3), 375 (Ϫ4.8), 420 (ϩ0.2) and 650
(Ϫ0.9).
H2O2
2+
HO2C
CO2H
Isolation of the amino acids from the oxygenation/oxidation
products
N
O
N
Cu
Cu
Complex 2, isolated as described before, or other fractions sep-
arated by chromatography or fractional precipitation from the
mixture obtained upon oxygenation of 1 (about 10 mg) were
treated with dilute HCl and the dialdehyde was extracted six
times with dichloromethane. The aqueous phase was concen-
trated and chromatographed on Chelex 100 (sodium form,
1 × 8.0 cm), using water as eluent, in order to remove the
copper() ions. The recovered solution was evaporated to dry-
ness to give a white solid residue. The presence of methionine
sulfoxide can be detected by TLC, but the amino acid com-
S
S
O
Me
Me
OH
12
Scheme 4
Isolation of complex 2
A stirred suspension of complex 1 (70 mg) in degassed meth-
anol (100 cm3) was exposed to dioxygen (1 atm, ca. 101 325 Pa)
and left to stir overnight at room temperature. The resulting
green solution was filtered, to discard the small amounts of blue
solid present [identified as copper() methioninate], and con-
centrated to a small volume. Then it was chromatographed on a
1
position was established more conveniently by H NMR spec-
troscopy in D2O, e.g. from the relative intensity of the signals of
the S-methyl groups (δ 2.13 for the methyl group of methionine,
2.76 for that of methionine sulfoxide).
4792
J. Chem. Soc., Dalton Trans., 1997, Pages 4789–4794

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by systematic extinctions. Lorentz-polarisation corrections
were applied. The structure was solved by direct methods.23 All
the atoms were refined by full-matrix least squares (the non-
hydrogen ones anisotropically, the hydrogen ones isotropically).
Secondary extinctions24 were applied. Atomic scattering factors
were taken from ref. 25. Empirical absorption corrections were
applied according of ref. 26. Pertinent experimental details are
given in Table 2.
Table 2 Crystallographic details for [Cu2{C6H3(CHO)2O}2(ClO4)2]
Formula
Mr
C8H5ClCuO7
312.12
System
Space group
Monoclinic
P21/c
a/Å
b/Å
8.206(1)
6.890(2)
c/Å
18.707(3)
97.52(1)
1048.7(4)
CCDC reference number 186/751.
β/Њ
U/Å3
Z
4
Acknowledgements
Dc/g cmϪ3
Radiation (λ/Å)
1.977
Cu-Kα (1.541 84 Å),
The authors thank the European Community (contracts
ERBCHRXCT920014 and ERBCHBICT930312 under the
Human Capital and Mobility Programme) and Ministero dell’
Università e della Ricerca Sientifica e Tecnologica for financial
support.
grapite monochromated
µ/cmϪ1
T/K
55.7
293(2)
0.1 × 0.1 × 0.4
ω–2θ
Crystal size/mm
Scan type
Scan speed/Њ minϪ1
Scan width/Њ
Reflections measured
3.3
0.7 ϩ 0.14 tan θ
hkl, hk Ϫ l; 0 < h <10, 0 < k < 8,
Ϫ22 <l < 22
2085
1407
175
0.045
0.041
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Observed reflections [I > 1.5σ(I)]
Refined parameters
R = Σ Fo| Ϫ |Fc /Σ|Fo|
RЈ = Σw(|Fo| Ϫ |Fc|)2/Σw(Fo)2
Weighting scheme
w = 1.0
0.21, Ϫ0.14
0.08
Final Fourier map features/e ÅϪ3
(shift/e.s.d.)max
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Reactions with hydrogen peroxide
Complex 2 (68 mg), obtained upon oxygenation of 1, was
diluted in MeOH–water (1:1, 20 cm3) and then an equimolar
amount of aqueous H2O2 was added. The solution was stirred
for 10 min and subsequently evaporated to dryness. The prod-
uct was treated with dilute HCl (50 cm3) and the dialdehyde
was extracted six times with dichloromethane (100 cm3). The
aqueous phase was then chromatographed on Chelex 100 (1 × 10
cm) using water as eluent, in order to remove the copper()
ions. The recovered solution was evaporated to dryness to give
a white solid residue, which was analysed by 1H NMR
spectroscopy as described above.
A blank experiment was carried out as follows. A solution of
-methionine (1 mmol) in water (30 cm3) was treated at room
temperature with an equimolar amount of Cu(ClO4)2ؒ6H2O
and stirred for 1 h. Then aqueous H2O2 (0.5 mmol) was added
with stirring. After 10 min of reaction the mixture was concen-
trated to about 20 cm3 under vacuum, acidified with HCl, and
chromatographed on Chelex 100 (1 × 10 cm) using water as elu-
ent. The recovered solution was then evaporated to dryness to
give a white solid residue, which was identified as methionine.
CAUTION: Although the compounds reported in this paper
seem to be stable to shock and heat, care should be used in
handling them because of the potentially explosive nature of
perchlorate salts.
X-ray crystallography
Green prismatic crystals of compound 3 were obtained by slow
evaporation of a concentrated oxygenated solution of 1 in
methanol [spectral data for the crystals: UV λmax/nm (MeOH)
344 nm; ν max/cmϪ1 3160 (CH), 1640 (C᎐O), 1560 (Ph᎐O) and
᎐
1080, 1115, 1140 and 623 (ClO4)]. Unit-cell parameters and
intensity data were obtained on a Enraf-Nonius CAD-4 dif-
fractometer. Calculations were performed with the SDP22a and
MOLEN22b software on a MicroVax-3100 computer.
The cell dimensions were determined by least-squares fitting
of 25 centred reflections monitored in the range 30 < θ < 40Њ.
No damage of the crystal was observed during the data collec-
tion (maximum decay = 1.0%). The space group was obtained
J. Chem. Soc., Dalton Trans., 1997, Pages 4789–4794
4793

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Received 21st July 1997; Paper 7/05225A
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