Copper-mediated oxygenolysis of flavonols via endoperoxide and dioxetan intermediates

Article abstract of DOI:10.1002/1099-0682(200209)2002:9<2287::AID-EJIC2287>3.0.CO;2-3

[Cu(phen)₂(fla)]ClO₄ was prepared by treating [Cu(CH₃CN)₄]-ClO₄ with flavonol (flah) in the presence of 1,10-phenanthroline (phen) as a co-ligand. Its oxygenation in DMF (or CH₃CN) solution at elevated temperature gave the (O-benzoylsalicylato)copper(II) complex [Cu(phen)₂(O-bs)]ClO₄ (bs = benzoylsalicylato) and carbon monoxide via an endoperoxide intermediate. Crystallographic characterisation of [Cu(phen)₂(O-bs)]ClO₄ as the CH₂Cl₂ solvate [triclinic, space group P1, a = 10.499(3) A, b = 12.556(4) A, c = 17.094(5), A α = 72.69(2), β = 89.35(2), γ = 69.19(2)° v = 1999.7(10) A³, Z = 2, R1 = 0.0962] shows that the molecule has a distorted trigonal-bipyramidal structure (T = 0.96). The oxygenolysis was monitored by spectrophotometry, and the pseudo-first-order rate constant k′_phen was found to be (2.47 ± 0.11) × 10^-4 s^-1 at 120°C. Complexes of [Cu(L)(4′R-fla)₂] (L = phen, bpy, TMEDA; R = H, OCH₃, CH₃, Cl) were also prepared by treating the complexes Cu(4′R-fla)₂ with nitrogen-containing co-ligands. Their oxygenation resulted in the corresponding complexes Cu(L)(2HOpg)₂ (2HOpg =2-hydroxyphenylglyoxylate) derived by spontaneous hydrolysis of Cu(L)(bpg)2 (bpg = 2-benzoatophenylglyoxylate). The (phenylglyoxylato)-copper complexes were probably formed via 1,2-dioxetan intermediates, since the oxygenation of Cu(phen)(fla)₂ showed chemiluminescence with bands at 506, 546, and 578 nm in the emission spectrum due to the decomposition of a 1,2-dioxetan species. Labelling experiments with an ¹⁸O₂/¹⁶O₂ mixture (1:3) showed the incorporation of both ¹⁸O atoms of ¹⁸O₂ into the flavonolate ligand. The kinetics of the oxygenolysis of Cu(L)(fla)₂ gave rate constants according to the rate law -d[Cu(L)(4′R-fla)₂]/dt = k[Cu(L)(4′R-fla)₂] [O₂]: k/m^-1 s^-1 = (9.50 ± 0.60) × 10^-2 (L = phen), (2.40 ± 0.10) × 10^-2 (L = bpy), (2.00 ± 0.10) × 10^-2 (L = TMEDA) m^-1 s^-1 at 353.16 K. The oxygenolysis of the complexes Cu(phen)(4′R-fla)₂ fits a Hammett linear free energy relationship and an increase of the electron density on the copper ion makes the oxygenation reaction faster. Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002.

Full text of DOI:10.1002/1099-0682(200209)2002:9<2287::AID-EJIC2287>3.0.CO;2-3

FULL PAPER
Copper-Mediated Oxygenolysis of Flavonols via Endoperoxide and Dioxetan
Intermediates; Synthesis and Oxygenation of [CuII(Phen)2(Fla)]ClO4
and [CuII(L)(Fla)2] [FlaH ؍ Flavonol; L ؍ 1,10-Phenanthroline
(Phen), 2,2Ј-Bipyridine (Bpy), N,N,NЈ,NЈ-Tetramethylethylenediamine
(TMEDA)] Complexes
[a]
[a]
[b]
[b]
´
´
´
´
Eva Balogh-Hergovich, Jozsef Kaizer, Jozsef Pap, Gabor Speier,*
[c]
[c]
´
´
Gottfried Huttner, and Laszlo Zsolnai
Keywords: Enzyme models / Copper / Peroxides / Kinetics
[Cu(phen)2(fla)]ClO4 was prepared by treating [Cu(CH3CN)4]-
ylate) derived by spontaneous hydrolysis of Cu(L)(bpg)2
ClO4 with flavonol (flaH) in the presence of 1,10-phenanthro- (bpg = 2-benzoatophenylglyoxylate). The (phenylglyoxylato)-
line (phen) as a co-ligand. Its oxygenation in DMF (or
CH3CN) solution at elevated temperature gave the (O-
copper complexes were probably formed via 1,2-dioxetan in-
termediates, since the oxygenation of Cu(phen)(fla)2 showed
benzoylsalicylato)copper(II) complex [Cu(phen)2(O-bs)]ClO4 chemiluminescence with bands at 506, 546, and 578 nm in
(bs = benzoylsalicylato) and carbon monoxide via an endo- the emission spectrum due to the decomposition of a 1,2-di-
peroxide intermediate. Crystallographic characterisation of oxetan species. Labelling experiments with an 18O2/16O2
[Cu(phen)2(O-bs)]ClO4 as the CH2Cl2 solvate [triclinic, space
mixture (1:3) showed the incorporation of both 18O atoms of
group P1, a = 10.499(3) A, b = 12.556(4) A, c = 17.094(5) A, 18O2 into the flavonolate ligand. The kinetics of the oxy-
˚
˚
˚
¯
3
˚
α = 72.69(2), β = 89.35(2), γ = 69.19(2)°, V = 1999.7(10) A , Z =
2, R1 = 0.0962] shows that the molecule has a distorted tri-
gonal-bipyramidal structure (τ = 0.96). The oxygenolysis was
monitored by spectrophotometry, and the pseudo-first-order
rate constant kЈphen was found to be (2.47 0.11) × 10−4 s−1
at 120 °C. Complexes of [Cu(L)(4ЈR-fla)2] (L = phen, bpy,
genolysis of Cu(L)(fla)2 gave rate constants according to the
−1
rate law −d[Cu(L)(4ЈR-fla)2]/dt = k[Cu(L)(4ЈR-fla)2][O2]: k/
m
s−1 = (9.50 0.60) × 10−2 (L = phen), (2.40 0.10) × 10−2 (L =
bpy), (2.00 0.10) × 10−2 (L = TMEDA)
m
s−1 at 353.16 K.
−1
The oxygenolysis of the complexes Cu(phen)(4ЈR-fla)2 fits a
Hammett linear free energy relationship and an increase of
TMEDA; R = H, OCH3, CH3, Cl) were also prepared by treat- the electron density on the copper ion makes the oxygena-
ing the complexes Cu(4ЈR-fla)2 with nitrogen-containing co-
tion reaction faster.
ligands. Their oxygenation resulted in the corresponding
( Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany,
complexes Cu(L)(2HOpg)2 (2HOpg =2-hydroxyphenylglyox- 2002)
Introduction
flavonols (1) to a depside (phenolic carboxylic acid esters,
2) with concomitant evolution of carbon monoxide [Equa-
tion (1)], was purified from culture filtrate of Aspergillus
niger.[4b]
Flavonols are widely distributed in vascular plants,[1,2]
and flavonoids form active constituents of a number of
herbal and traditional medicines.[3] Flavonols are readily
degraded by microorganisms, especially by fungi such as
Aspergillus or Pullularia species.[4] Flavonol 2,4-dioxygenase
(quercetinase), which catalyses the oxidative degradation of
(1)
[a]
Research Group for Petrochemistry, Hungarian Academy of
Sciences,
´
8201 Veszprem, Hungary,
[b]
[c]
´
Department of Organic Chemistry, University of Veszprem,
´
8201 Veszprem, Hungary,
This reaction is rather exceptional as it involves the oxy-
genolytic cleavage of two carbonϪcarbon bonds. Carbon
monoxide forming enzymes are rare. To date, three prokar-
yotic dioxygenases are known that catalyse a dioxygenolytic
release of CO in a manner analogous to that of fungal quer-
Fax: (internat.) ϩ 36-88/427-492
E-mail: speier@almos.vein.hu
Anorganisch Chemisches Institut, Universität Heidelberg,
69120 Heidelberg, Germany
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295  WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002 1434Ϫ1948/02/0909Ϫ2287 $ 20.00ϩ.50/0
2287

E. Balogh-Hergovich, J. Kaizer, J. Pap, G. Speier, G. Huttner, L. Zsolnai
FULL PAPER
cetinase, these being 3-hydroxyquinaldin-4(1H)-one 2,4-di- aimed at elucidating the mechanism of the stoichiometric
oxygenase, 3-hydroxyquinolin-4(1H)-one 2,4-dioxygenase,[5] oxygenation of the two systems are also reported on.
and 1,2-dihydroxy-5-(methylthio)pent-1-en-3-one anion
1,3-dioxygenase.[6] These prokaryotic enzymes were claimed
not to contain any metal ion or organic cofactor[5b] and Results and Discussion
thus differ fundamentally from the copper-containing fla-
Synthesis and Characterisation of the Complex
[Cu(phen)2(fla)]ClO4
vonol 2,4-dioxygenase. However, it was reported recently,
that the 1,2-dihydroxy-5-(methylthio)pent-1-en-3-one anion
1,3-dioxygenase is a Ni-containing enzyme.[6c]
Complex [Cu(phen)2(fla)]ClO4 was isolated as a green
solid by the reaction of [Cu(CH3CN)4]ClO4, flavonol (1)
and 1,10-phenanthroline monohydrate at room temperature
in acetonitrile under dioxygen. Its IR spectrum shows the
principal band corresponding to the coordinated flavonol-
ate at 1581 cmϪ1. The decrease of approximately 20 cmϪ1 of
the ν(CO) band compared to that of free flavonol [ν(CO):
1602 cmϪ1] is due to chelation and formation of a stable
five-membered ring.[13] The absorption bands at 1082 and
626 cmϪ1 can be assigned to the perchlorate anion. A very
strong band at 1608 cmϪ1 [ν(CϭN)] is due to the coordin-
ated 1,10-phenanthroline. The electronic spectrum of the
complex in the charge transfer region exhibits a band of
the coordinated flavonolate ligand at 419 nm. The band at
702 nm is associated with the dϪd transition of CuII.
Room-temperature magnetic measurements yielded a value
of µB ϭ 1.92, consistent with a copper() ion.
For the oxidative cleavage reaction of the heterocycle in
flavonols, two pathways have been proposed (Scheme 1),[7]
the first via an endoperoxide 3 (route a) and the other via
a 1,2-dioxetan 4 (route b).
Synthesis and Characterisation of Cu(L)(4ЈR-fla)2
Cu(4ЈR-fla)2 (R ϭ H, CH3, OCH3, Cl) complexes reacted
with 1,10-phenanthroline at room temperature in acetonitr-
ile under argon to give the corresponding complexes Cu-
(phen)(4ЈR-fla)2 in fairly good yields. Similarly, in the pres-
ence of TMEDA or bpy co-ligands Cu(TMEDA)(fla)2 or
Cu(bpy)(fla)2 was obtained. Satisfactory elemental analyses
were obtained for all of the new complexes. Compounds
Cu(L)(fla)2 have very similar IR and electronic spectra. The
IR spectrum shows the principal bands corresponding to
the coordinated flavonolate in the 1560Ϫ1580 cmϪ1 region.
Decreases of approximately 40 cmϪ1 of the ν(CO) bands
compared to that of the 4Ј-substituted flavonols [ν(CO): ca.
1605 cmϪ1] are due to chelation and formation of a stable
five-membered ring.[13] In the case of complexes with 2,2Ј-
bipyridine and 1,10-phenanthroline, a strong band in the
1590Ϫ1600 cmϪ1 region is seen due to the coordinated ni-
trogen donor ligands ν(CϭN). The πϪπ* transitions in the
420Ϫ435 nm region arise from the coordinated flavonolate
ligands.
Scheme 1
Since flavonol 2,4-dioxygenase is a copper-containing
metalloenzyme, metal complexes of copper[8,9] and co-
balt[10] have been used in model reactions. Autoxidation re-
actions of potassium[11] and zinc flavonolates[12] have also
resulted in enzyme-like products, and some efforts have
been made to elucidate the mechanism of the reaction.
From our earlier results obtained both with redox and non-
redox metal-containing systems, the conclusion could be
drawn that the oxygenolysis of the flavonolate ion in aprotic
solvents takes place via an endoperoxide inter-
mediate.[9eϪ9h,11,12]
Flavonol 2,4-dioxygenase has been claimed to contain
nonblue type 2 CuII ions at its active site.[4b] Recent crystal
structure determination of quercetin 2,3-dioxygenase from
Aspergillus japonicus reveal that it forms homodimers. The
mononuclear type 2 copper centre displays two distinct geo-
metries: a distorted tetrahedral coordination, formed by
Oxygenation of [Cu(phen)2(fla)]ClO4
Treatment of [Cu(phen)2(fla)]ClO4 with dioxygen in
three histidine residues and one water molecule, and a dis- DMF or CH3CN solution at elevated temperature led to
torted trigonal-bipyramidal environment, which addition- the corresponding (O-benzoylsalicylato)copper complex
ally comprises a glutamate.[4i] In this paper we report details [Cu(phen)2(O-bs)]ClO4 (10) and carbon monoxide contam-
for the synthesis and characterisation of [Cu(phen)2(fla)]- inated with a small amount of CO2 [Equation (2)]. The
ClO4 (6) and [Cu(L)(4ЈR-fla)2] [L ϭ phen (7), bpy (8), formation of 10 from 6 required dioxygen, which was meas-
TMEDA (9); R ϭ H (a), OCH3 (7b), CH3 (7c), Cl (7d)] ured by volumetry (no apparent dioxygen uptake was ob-
complexes, which are models for the enzymeϪsubstrate served, because the absorption of dioxygen and the libera-
(ES) complex. Spectrophotometric and kinetic studies tion of CO compensate each other).
2288
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295

Copper-Mediated Oxygenolysis of Flavonols
FULL PAPER
X-ray Crystal Structure of [Cu(phen)2(O-bs)]ClO4
The molecular structure and atom numbering scheme for
[Cu(phen)2(O-bs)]ClO4 is shown in Figure 2, and selected
bond lengths and angles are listed in Table 1. The copper
ion has a slightly distorted trigonal-bipyramidal geometry
with τ ϭ 0.96.[9g] Two nitrogen atoms of the 1,10-phen-
anthroline ligands [Cu(1)ϪN(1) 2.176(5), Cu(1)ϪN(3)
(2)
˚
2.074(5) A] and one oxygen atom of the O-benzoylsalicylate
The oxygenation of 6 was monitored by UV/Vis spectro-
scopy. The time course of the variation of [Cu(phen)2(fla)]-
ClO4 (6) in DMF solution at 120 °C is shown in Figure 1.
Plots of log[Cu(phen)2(fla)]ClO4 vs. time show a linear de-
pendence under pseudo-first-order conditions[14] (constant
dioxygen pressure), in agreement with a rate equation of
first-order dependence with respect to the complex 6. The
pseudo-first-order rate constant kЈ was found to be (2.47 Ϯ
0.11) ϫ 10Ϫ4 sϪ1 at 120 °C. This value is very close to that
found for the oxygenation of [Cu(idpaH)(fla)]ClO4 [id-
paH ϭ 3,3Ј-iminobis(N,N-dimethylpropylamine)] in DMF
˚
ligand [Cu(1)ϪO(1) 1.993 A] occupy basal positions. The
two other nitrogen atoms of the phenanthroline ligands
[Cu(1)ϪN(2) 1.992(5) and Cu(1)ϪN(4) 1.987(5)] are in ap-
ical positions. The CuϪN distances in the basal plane are
longer than those in apical positions.
[kЈ120 °C ϭ (1.45 Ϯ 0.06) ϫ 10Ϫ4 Ϫ1)].[9g]
s
Figure 1. Time course for the oxygenation of [Cu(phen)2(fla)]ClO4
in DMF (o) and plot of log[{Cu(phen)2(fla)]ClO4}] (•) vs. reaction
time
for
the
oxygenation
of
[Cu(phen)2(fla)]ClO4
Figure 2. Molecular structure of [Cu(phen)2(O-bs)]ClO4 1.5CH2Cl2
([{Cu(phen)2(fla)]ClO4}]0 ϭ 1.35 ϫ 10Ϫ3 , [O2] ϭ 6.1 ϫ 10Ϫ3 ,
120 °C)
˚
Table 1. Selected bond lengths [A] and angles [°] for [Cu-
(phen)2(O-bs)]ClO4·1.5CH2Cl2
Characterisation of [Cu(phen)2(O-bs)]ClO4
Cu(1)ϪN(4)
Cu(1)ϪN(2)
Cu(1)ϪO(1)
1.987(5) Cu(1)ϪN(3)
1.992(5 Cu(1)ϪN(1)
1.993(4)
2.074(5)
2.176(5)
The complex [Cu(phen)2(O-bs)]ClO4 can also be pre-
pared by the reaction of [Cu(CH3CN)4]ClO4, O-benzoylsal-
icylic acid and 1,10-phenanthroline monohydrate at room
N(4)ϪCu(1)ϪN(2) 176.4(2)
N(4)ϪCu(1)ϪO(1)
N(4)ϪCu(1)ϪN(3)
90.7(2)
81.4(2)
temperature in acetonitrile under dioxygen. Its IR spectrum N(2)ϪCu(1)ϪO(1)
92.5(2)
96.8(2)
97.5(2)
92.9(2)
N(2)ϪCu(1)ϪN(3)
O(1)ϪCu(1)ϪN(3) 148.0(2)
N(2)ϪCu(1)ϪN(1) 80.6(2)
N(3)ϪCu(1)ϪN(1) 118.8(2)
shows bands corresponding to the coordinated O-benzoyl-
N(4)ϪCu(1)ϪN(1)
salicylate at 1741 [ν(CO), indicating that the carbonyl group
O(1)ϪCu(1)ϪN(1)
of the benzoyl group is not coordinated], 1572 and 1388
[ν(CO2)] cmϪ1. The difference between the asymmetric and
symmetric stretching frequencies of the carboxylato group
Oxygenation of the Complexes Cu(L)(4ЈR-fla)2
[∆ν˜ ϭ νas(CO2) Ϫ νs(CO2)] is 184 cmϪ1, rendering these to
a monodentate carboxylate bonding mode. Absorption
Treatment of the complexes Cu(L)(4ЈR-fla)2 (L ϭ bpy,
bands at 1068 and 635 cmϪ1 are associated with the per- phen, TMEDA; R ϭ H, CH3, OCH3, Cl) with dioxygen in
chlorate anion. In the UV/Vis spectrum the dϪd transition DMF or CH3CN solution at elevated temperature led to
may be assigned to the 694 nm absorption and the ligand- the corresponding (2-hydroxyphenylglyoxylato)copper com-
to-metal charge transfer at 271 and 294 nm. The magnetic plexes Cu(L)(2HOpg)2 and 4-substituted benzoic acids
moment of µB ϭ 2.06 is in agreement with a d9 CuII ion [Equation (3)]. According to gas-volumetric and GC meas-
without magnetic interactions.
urements, the stoichiometry of the oxygenation reactions
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295
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E. Balogh-Hergovich, J. Kaizer, J. Pap, G. Speier, G. Huttner, L. Zsolnai
FULL PAPER
the real emitter is Cu(phen)(bpg)2 (11), which could not be
isolated due to its extreme sensitivity toward hydrolysis.
Figure 3. Time course for the oxygenation of the complexes Cu(L)-
(fla)2 (7Ϫ9) (L ϭ phen (᭜), bpy (•), TMEDA (o); [Cu(L)(fla)2]0 ϭ
0.4 mmol in 50 mL of DMF, [O2] ϭ 6.91 ϫ 10Ϫ3 , 80 °C)
corresponds to Equation (3). Figure 3 shows typical dioxy-
gen uptakes vs. time curves for the complexes [Cu(L)(fla)2]
(L ϭ bpy, phen, TMEDA). No other oxidation products
could be detected.
Figure 4. Electron spectrum of the emitted light during the oxy-
genation of Cu(phen)(fla)2 in DMF
(3)
Figure 5. Fluorescence spectrum of Cu(phen)(2HOpg)2 in DMF
Characterisation of the Complexes Cu(L)(2HOpg)2 (L ؍
phen, bpy, TMEDA)
Satisfactory elemental analyses were obtained for the
three new (2-hydroxyphenylglyoxylato)copper complexes,
namely Cu(phen)(2HOpg)2 (14), Cu(bpy)(2HOpg)2 (15),
and Cu(TMEDA)(2HOpg)2 (16). The IR spectrum shows
the presence of the coordinated α-oxocarboxylato ligand
The GC and GC-MS analyses of the reaction mixture
obtained by the oxygenation of Cu(phen)(fla)2, after treat-
ment with dilute HCl solution (10%) and ethereal diazo-
methane, show the presence of methyl benzoate {136 (23)
[Mϩ], 105 (100), 77 (87)}, coumaronedione {148 (3) [Mϩ],
120 (100), 92 (59), 64 (22), 63 (14)}, and methyl 2-hydroxy-
phenylglyoxylate {180 (7) [Mϩ], 121 (100), 93 (11), 65 (11)}.
In the case of complex Cu(phen)(fla)2, during the oxygena-
tion reaction chemiluminescence was observed, which is
crucial evidence for the decomposition of a 1,2-dioxetan[15]
intermediate to carbonyl compounds. Figure 4 shows the
electron spectrum of the emitted light (λ ϭ 506, 546,
578 nm). Compound Cu(phen)(2HOpg)2 does not seem to
be involved in the chemiluminescence process since its
fluorescence spectrum in DMF excited by 430 nm shows
bands at 480 and 535 nm with a shoulder at 557 nm (Fig-
ure 5), which does not match the spectrum of the emitted
{ca. 1616 [ν(CO)], ca. 1575, ca. 1375 [ν(CO2)] cmϪ1
,
respectively}.
Kinetic Measurements
The kinetics of the oxygenation of Cu(phen)(fla)2 were
monitored by electron spectroscopy at 432 nm. Experi-
mental conditions are summarised in Table S1 (Supporting
Information). To determine the rate dependence on the two
reactants, oxygenation runs were performed at various ini-
tial Cu(phen)(fla)2 concentrations and different dioxygen
pressures. A simple rate law for the reaction between Cu-
(phen)(fla)2 and O2 is as shown in Equation (4).
Ϫd[Cu(phen)(fla)2]/dt ϭ d[Cu(phen)(2HOpg)2]/dt ϭ
light of the chemiluminescence process. It seems likely that kphen [Cu(phen)(fla)2]m[O2]n
(4)
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Eur. J. Inorg. Chem. 2002, 2287Ϫ2295

Copper-Mediated Oxygenolysis of Flavonols
FULL PAPER
Under pseudo-first-order conditions (constant dioxygen
The activation parameters for the oxygenation reactions
concentration) for the oxygenation of 7 a typical time were determined from the temperature dependence of the
course and first-order plot are shown in Figure 6; the reac- kinetic constant kphen in the temperature range of
tion remained first-order in 7 during the whole time in 348.16Ϫ363.16 K. From the straight line in the Eyring plot
which the experiment was monitored (81% conversion, [Figure S2 (Supporting Information)], with a correlation
0.6 h). The first-order dependence on the reaction rate on coefficient of 99.7% the activation parameters ∆H‡ ϭ 79 Ϯ
7 could also be confirmed by plotting the initial reaction 16 kJ molϪ1 and ∆S‡ϭ Ϫ40 Ϯ 44 J molϪ1 KϪ1 were calcu-
rate Ϫd[Cu(phen)(fla)2]/dt vs. initial substrate concentra- lated at 353.16 K.
tion. The straight line obtained [as shown in Figure S1
Reaction rates on the oxygenation of the 4Ј-substituted
(Supporting Information)] validates the first-order depend- complexes Cu(phen)(4ЈR-fla)2 under identical conditions
ence in Cu(phen)(fla)2 with the pseudo-first-order rate con- were also determined (with various electron-withdrawing or
stant kЈphen ϭ 6.86 ϫ 10Ϫ4 sϪ1 and a correlation coefficient releasing substituents R) in order to find out electronic ef-
of 99.5%.
fects on the reaction rate. The Hammett plot obtained is
shown in Figure 8. Electron-releasing substituents en-
hanced the reaction rate, and the reaction constant ρphen
was found to be Ϫ1.03 (R ϭ 98.3%).
Figure 6. Time course for the oxygenation of Cu(phen)(fla)2 in
DMF (᭜) and plot of log[Cu(phen)(fla)2] (ᮀ) vs. reaction time for
the oxygenation of Cu(phen)(fla)2
Experiments made at different dioxygen pressures show
that pO2 appreciably influences the rate of the reaction. Kin-
etic measurements of the reaction rate indicate a first-order
dependence with respect to the dioxygen concentration.
Plots of kЈphen vs. [O2] for the above experiments give a
straight line with kphen ϭ 0.10 Ϫ1 sϪ1 and a correlation
coefficient of 99.1% (Figure 7). On the basis of the results
above, it can be concluded that the reaction follows the rate
law of Equation (4) with m ϭ n ϭ 1, from which a mean
value of the kinetic constant kphen of (9.50 Ϯ 0.60) ϫ 10Ϫ2
Ϫ1 sϪ1 at 353.16 K can be obtained. In the case of the
oxygenation of Cu(bpy)(fla)2 and Cu(TMEDA)(fla)2 the
mean values of the kinetic constants at 353.16 K were found
Figure 8. Hammett plot for the oxygenation of Cu(phen)(4ЈR-
fla)2 complexes
18O2 Labelling Experiments
Oxygenation of Cu(phen)2(fla) was also carried out with
a mixture of 16O2 and 18O2 (3:1) in acetonitrile. After stir-
ring the reactants at 25 °C for 48 h, the unchanged starting
material was filtered off (ca. 50%), and the green solution
was concentrated to dryness, giving a mixture of Cu-
(phen)(16O-bpg)2 and Cu(phen)(18O-bpg)2 {IR (Nujol): 1685
[ν(C18O)], 1716 [ν(C16O)] cmϪ1}, Cu(phen)(18O-2HOpg)2
and Cu(phen)(16O-2HOpg)2, respectively. The GLC-MS
analysis of the residue, after treatment with ethereal diazo-
methane, showed the presence of 16O- and 18O-containing
methyl benzoate: m/z (%) ϭ 136 (23) [Mϩ], 105 (100), 77
(87); 138 (8) [Mϩ ϩ 2], 181 (9), 107 (26), 105 (100), 77 (87);
methyl 2-methoxyphenylglyoxylate: m/z (%) ϭ 194 (32)
[Mϩ], 179 (22), 135 (100), 120 (29), 105 (15), 92 (15); 196
(12) [Mϩ ϩ 2], 181 (9), 135 (100), 120 (29), 105 (15), 92
(28); methyl 2-hydroxyphenylglyoxylate: m/z (%) ϭ 180 (7)
[Mϩ], 121 (100), 93 (17), 65 (23); 182 (2.2) [Mϩ ϩ 2], 121
(100), 93 (17), 65 (23); and coumaronedione: m/z (%) ϭ 148
(4.3) [Mϩ], 120 (100), 92 (69), 64 (27), 63 (30); 138 (8) [Mϩ
ϩ 2], 107 (26), 105 (100), 77 (87); in the appropriate ratio.
On the bases of the above results it can be said that both
18O atoms of 18O2 are incorporated into the primary oxy-
genated product 2-benzoatophenylglyoxylic acid from mo-
to be kbpy ϭ (2.40 Ϯ 0.10) ϫ 10Ϫ2 Ϫ1 sϪ1 and kTMEDA
(2.00 Ϯ 0.10) ϫ 10Ϫ2 Ϫ1 sϪ1
ϭ
.
Figure 7. Plot of pseudo-first-order rate constant kЈ vs. O2 pressure lecular oxygen (Scheme 2).
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295
2291

E. Balogh-Hergovich, J. Kaizer, J. Pap, G. Speier, G. Huttner, L. Zsolnai
FULL PAPER
Cu(phen)(fla)2 revealed that both 18O atoms of the 18O2
molecule are incorporated into the substrate. A reaction
mechanism that nicely fits the chemical, photochemical,
spectroscopic, labelling, kinetic, and thermodynamic data is
shown in Scheme 3.
Scheme 2
All experimental data reported in the previous sections
can be used for the interpretation of the mechanism of cop-
per-assisted oxygenolysis of coordinated flavonolate to O-
benzoylsalicylate or 2-benzoatophenylglyoxylate. Flavonol-
ate as a chelating ligand forms stable complexes with copper
ions. In most cases they are stable toward dioxygen in the
solid form or even in solution at ambient conditions. At
elevated temperatures the oxygenolysis proceeds reasonably
Scheme 3
The second-order overall rate expression for the oxygena-
rapidly. Recently, we have found that copper() flavonolate tion of the flavonolato complexes 7 supports the proposal
complexes with simple N-containing tridentate ligands re- that in a fast pre-equilibrium the copper() flavonolate
acted with dioxygen to the corresponding enzyme-like (O- complex 7 undergoes intramolecular electron transfer from
benzoylsalicylato)copper complexes and CO. On the basis the ligand flaϪ to CuII, resulting in the copper() flavonoxy
of kinetic measurements of the reaction of the complexes radical complex 17. The equilibrium is largely shifted to the
above with dioxygen, a mechanism was proposed to pro- left (K1 is rather small). In 17 there are two redox-active
ceed via an endoperoxide intermediate according to route centres, the radical ligand and the copper() ion. The birad-
a in Scheme 1.[9g,9h] Treatment of the (flavonolato)copper ical dioxygen may react at both sites; in an oxidative addi-
complex [Cu(phen)2(fla)]ClO4 with dioxygen in DMF or tion to the copper ion [leading to a (superoxo)copper()
CH3CN solution at elevated temperature leads also to an complex 18], or in a radical-radical reaction with the fla-
(O-benzoylsalicylato)copper
complex
[Cu(phen)2(O- vonoxy ligand. We believe that the former is the rate-deter-
bs)]ClO4 and carbon monoxide, indicating that the mechan- mining step. The Hammett relationship shows that higher
ism of this reaction is the same as that proposed for copper- electron density at the copper ion enhances the rate of the
containing model systems investigated earlier by us.
reaction. This is consistent with an electrophilic attack of
Recently, we have published that the oxygenolysis of Cu- dioxygen at the copper() centre.
(fla)2 in DMF resulted in the Cu(O-bs)2 complex and
The reaction constant of the oxygenation reaction is al-
CO.[9e] We found that this reaction proceeded in a bimo- most the same (ρphen ϭ Ϫ1.03, R ϭ 98.2%) as that found
lecular step via an endoperoxide intermediate (Scheme 1, for the oxygenation of copper() chloride in pyridine (ρ ϭ
route a), and the kinetic constant kobs was found to be 9.90 Ϫ1.24).[16] The crucial evidence for a bimolecular reaction
Ϯ 0.80 ϫ 10Ϫ3 molϪ1 dm3 sϪ1 at 90 °C.[9e] This value is step is the negative ∆S‡ value (Ϫ40.3 J molϪ1 KϪ1). The
much smaller than that obtained for the oxygenolysis for (superoxo)copper() complex 18 is possibly then trans-
Cu(phen)(fla)2 (21.3 Ϯ 1.8 ϫ 10Ϫ2 molϪ1 dm3 sϪ1 at 90 °C). formed in a fast consecutive coupling reaction to 19, which
This difference in the reaction rates indicates that there are reacts by an intramolecular nucleophilic addition on C3 to
two different pathways for the above reactions. The forma- give the 1,2-dioxetan 20. The main difference between the
tion of (oxocarboxylato)copper() complexes 14Ϫ16 in the two types of mechanistic routes can be explained by this
oxygenation of the complexes Cu(L)(fla)2 (L ϭ phen, bpy, step. The nucleophilic attack of the bound peroxide could
TMEDA) also suggest that the oxidative cleavage reaction also take place on the 4-CϭO group as well giving rise to
of the coordinated flavonolate ligand can only proceed via an endoperoxide intermediate, but in that case the higher
a 1,2-dioxetan intermediate, which, upon the usual decom- electron density on the 4-CϭO carbon atom compared to
position and hydrolysis of the ester linkage, leads to the that on the 3-CϭO carbon atom renders this pathway un-
corresponding (oxocarboxylato)copper complexes 14Ϫ16. likely. The 1,2-dioxetan 20 is then believed to decompose in
The 18O2 labelling experiments on the oxygenation of a fast step to the (oxocarboxylato)copper complex 21. Ow-
2292
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295

Copper-Mediated Oxygenolysis of Flavonols
FULL PAPER
added and stirred under argon for 1 h, and then under dioxygen
(0.1 MPa) for 2 h. A green solid formed, which was filtered, washed
with acetonitrile, and dried under vacuum (0.66 g, 87%). The prod-
uct was recrystallised from acetonitrile. M.p. 195 °C. IR (KBr): ν˜ ϭ
1608 (vs, CϪN), 1581 (vs, CϪO), 1511, 1493, 1416, 1207, 1141,
1082 (vs, ClO4), 845, 756, 719, 626 cmϪ1 (vs, ClO4). UV/Vis
(DMF): λmax (ε/L molϪ1 cmϪ1) ϭ 229 (67608), 269 (69183), 419
(8318), 702 nm (51). µB ϭ 1.92. C39H25ClCuN4O7 (760.7): calcd. C
61.58, H 3.31, N 7.36; found C 61.04, H 3.16, N 7.18.
ing to a ligand scrambling reaction, the starting copper fla-
vonolate complex 7 and the product (2-hydroxyphenyl-
glyoxylato)copper complex 14 are formed by hydrolysis of
the ester linkage in complex 11. In order to prove the
formation of 1,2-dioxetan during the oxygenolysis the
chemiluminescence of the reaction was tested. In the emis-
sion spectrum bands at 506, 546, and 578 nm were found,
which were assigned to the decomposition of the 1,2-dioxe-
tan species.[15] Light emission is a result of the transition of
the exited complex 21 to its ground state. Assuming steady-
state conditions for species 17 the rate according to Equa-
tion (5) can be deduced, which after some simplification
(k2[O2] ϽϽ k1 ϩ kϪ1 and [Cu] means the total copper con-
centration) is in good agreement with an overall second-
order dependence according the experimental data obtained
in the kinetic measurements.
Preparation of Cu(phen)(fla)2 (7a): To a solution of Cu(fla)2
(0.538 g, 1 mmol) in acetonitrile (35 mL) was added 1,10-phen·H2O
(0.198 g, 1 mmol) and the reaction mixture stirred under argon for
4 h. A green precipitate formed, which was filtered off, washed with
ether, and dried under vacuum (0.65 g, 90%). M.p. 236 °C. IR
(KBr): ν˜ ϭ 3059 vw, 3044 vw, 1595 vs, 1564 vs, 1508 s, 1485 m,
1465 m, 1408 vs, 1350 m, 1310 s, 1221 vs, 1187 m, 1144 m, 1109 m,
1077 m, 1033 m, 995 w, 913 m, 859 s, 774 s, 747 s, 735 m, 721 w,
698 s, 571 m, 533 m, 499 m cmϪ1. UV/Vis (DMF): λmax (ε/
L molϪ1 cmϪ1) ϭ 265 (64125), 432 nm (63658). µB ϭ 1.98.
C42H26CuN2O6 (718.2): calcd. C 70.24, H 3.64, N 3.90; found C
69.88, H 3.48, N 3.98.
(5)
Cu(phen)(4ЈOCH3-fla)2 (7b): Yield 0.66 g (85%). M.p. 200 °C. IR
(KBr): ν˜ ϭ 3052 vw, 2924 vw, 2823 w, 1591 vs, 1568 vs, 1542 s,
1491 s, 1416 vs, 1355 m, 1305 s, 1249 vs, 1216 vs, 1181 s, 1116 s,
1046 s, 1003 m, 923 m, 852 s, 761 s, 742 s, 691 m, 642 m, 578 m,
529 m cmϪ1. UV/Vis (DMF): λmax (ε/L molϪ1 cmϪ1) ϭ 271
(70018), 434 (67844) nm. µB ϭ 2.01. C44H30CuN2O8 (778.3): calcd.
C 67.90, H 3.38, N 3.82; found C 67.29, H 3.48, N 3.98.
Conclusion
From the results obtained the conclusion may be drawn
that the oxidative ring-cleavage reaction of the flavonolate
ligand coordinated to the copper ion may take place by two
different reaction routes, namely through an endoperoxide
or a 1,2-dioxetan intermediate. We believe that the coor-
dination mode and the ligand environment around the
metal centre, and the Lewis acidity of the copper ion influ-
ence the mechanism of this unusual reaction. Both com-
plexes Cu(phen)(fla)2 and [Cu(phen)2(fla)]ClO4 are
sterically very crowded so that the discrimination of the in-
tramolecular nucleophilic peroxide group between the 4-Cϭ
O and 3-CϭO carbon atoms seems to be governed by the
different electron densities on the carbon atoms, which is
influenced by changing the ligand environment. Further
work is in progress to better understand the reasons and
factors affecting the selectivity between this two types of
CϪC cleavage reactions.
Cu(phen)(4ЈCH3-fla)2 (7c): Yield 0.62 g (83%). M.p. 197 °C. IR
(KBr): ν˜ ϭ 3030 vw, 2895 vw, 2841 wv, 1590 vs, 1576 vs, 1495 vs,
1476 s, 1412 vs, 1353 m, 1312 s, 1217 vs, 1189 s, 1147 m, 1112 s,
1050 w, 1029 w, 1001 m, 916 s, 861 s, 834 s, 766 s, 738 s, 689 m,
663 w, 638 m, 514
s
cmϪ1
.
UV/Vis (DMF): λmax (ε/
L molϪ1 cmϪ1) ϭ 271 (80041), 429 (63062) nm. µB ϭ 1.86.
C44H30CuN2O6 (746.3): calcd. C 70.81, H 4.05, N 3.75; found C
69.84, H 3.89, N 3.88.
Cu(phen)(4ЈCl-fla)2 (7d): Yield 0.59 g (75%). M.p. 238 °C. IR
(KBr): ν˜ ϭ 3048 vw, 1595 vs, 1568 vs, 1510 m, 1483 vs, 1410 vs,
1355 m, 1316 s, 1219 vs, 1149 m, 1109 s, 1093 m, 1022 m, 1003 m,
916 s, 849 s, 758 s, 736 s, 691 w, 661 w, 616 m, 551 w, 505 m. UV/
Vis (DMF): λmax (ε/L molϪ1 cmϪ1) ϭ 271 (73578), 424 (60173) nm.
µB ϭ 1.81. C42H28Cl2CuN2O6 (788.1): calcd. C 64.01, H 3.67, N
3.56; found C 63.68, H 3.89, N 3.66.
Cu(bpy)(fla)2 (8): Yield 0.22 g (40%). M.p 280 °C. IR (KBr): ν˜ ϭ
3023 vw, 1588 vs, 1578 vs, 1513 m, 1484 m, 1465 s, 1438 m, 1405
vs, 1348 m, 1305 vs, 1239 vs, 1142 w, 1117 w, 1078 m, 1057 vw,
1032 m, 997 w, 913 m, 857 vw, 765 vs, 742 w, 719 w, 703 s, 671 m,
Experimental Section
General Remarks: Solvents used for the reactions were purified us-
ing literature methods[17] and stored under argon. Flavonol,[18] 4Ј-
methoxyflavonol,[18] 4Ј-methylflavonol,[19] 4Ј-chloroflavonol,[19] O-
591 vw, 532 m, 507
s . UV/Vis (DMF): λmax (ε/
cmϪ1
L molϪ1 cmϪ1) ϭ 273 (67640), 431 (58829) nm. µB ϭ 1.89.
C40H26CuN2O6 (694.2): calcd. C 69.21, H 3.77, N 4.04; found C
68.79, H 3.92, N 4.11.
benzoylsalicylic acid,[20] Cu(OCH3)2,[21] [Cu(CH3CN)4]ClO4,[22]
[9b]
and Cu(fla)2
were prepared using literature methods. The li-
gands 1,10-phenanthroline, 2,2Ј-bipyridine, and N,N,NЈ,NЈ-tetra-
methylethylenediamine (Aldrich) were used as provided.
Cu(TMEDA)(fla)2 (9): Yield 0.43 g (65%). M.p. 160 °C (dec.). IR
(KBr): ν˜ ϭ 3055 vw, 2999 vw, 2873 vw, 2827 vw, 1573 vs, 1524 m,
1473 s, 1440 m, 1427 vs, 1352 m, 1318 vs, 1298 w, 1256 w, 1226 vs,
1188 w, 1150 w, 1124 w, 1086 m, 1061 vw, 1037 w, 973 w, 937 vw,
918 m, 863 vw, 822 m, 783 s, 751 vs, 749 vw, 724 w, 705 s, 692
vw, 674 m, 612 vw, 537 m, 501 s cmϪ1. UV/Vis (DMF): λmax(ε/
CAUTION: Perchlorate salts of metal complexes with organic li-
gands are potentially explosive. Only small amounts of material
should be prepared, and these should be handled with great care.
Preparation of [Cu(phen)2(fla)]ClO4 (6): [Cu(CH3CN)4]ClO4 L molϪ1 cmϪ1) ϭ 272 (51368), 430 (59979), 674 (380) nm. µB
(0.328 g, 1 mmol) was dissolved in acetonitrile (60 mL), and fla-
ϭ
1.91. C36H34CuN2O6 (654.2): calcd. C 66.09, H 5.24, N 4.28; found
vonol (0.238 g, 1 mmol) and 1,10-phen·H2O (0.396 g, 2 mmol) were C 65.79, H 4.95, N 4.21.
Eur. J. Inorg. Chem. 2002, 2287Ϫ2295
2293

E. Balogh-Hergovich, J. Kaizer, J. Pap, G. Speier, G. Huttner, L. Zsolnai
FULL PAPER
Preparation of [Cu(phen)2(O-bs)]ClO4 (10): [Cu(CH3CN)4]ClO4
(0.328 g, 1 mmol) was dissolved in acetonitrile (30 mL), then O-
benzoylsalicylic acid (0.242 g, 1 mmol) and 1,10-phen·H2O
direct methods and refined by full-matrix least-squares techniques
against F2 (SHELXL-93,[23] SHELXS-86[24]), hydrogen atoms loc-
ated geometrically, R1 ϭ 0.0962, ωR2 ϭ 0.2614 (all data) for 540
(0.360 g, 2 mmol) were added and the reaction mixture was stirred parameters, min/max residual electron density ϭ Ϫ0.642/1.448 e
under argon for 1 h and then under dioxygen (0.1 MPa) for 4 h.
A blue crystalline solid formed, which was filtered, washed with
acetonitrile, and dried under vacuum (0.69 g, 90%). Recrystallis-
Ϫ3. CCDC-179746 contains the supplementary crystallographic
˚
A
data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge
ation of the product from dichloromethane gave blue crystals. The Crystallographic Data Centre, 12, Union Road, Cambridge
dichloromethane solution produced crystals suitable for X-ray ana-
lysis upon standing at room temperature for a few days. M.p. 262
°C. IR (KBr): ν˜ ϭ 741 (vs, CϪO), 1600 (vs, CϪN), 1572 (vs, CO2),
1420, 1388 (vs, CO2), 1253, 1200, 1143, 1094, 1068 (vs, ClO4), 841,
800, 731, 719, 635 (vs, ClO4) cmϪ1. UV/Vis (DMF): λmax (ε/
L molϪ1 cmϪ1) ϭ 271 (52481), 294 (17783), 331 (2042), 694 (115)
nm. µB ϭ 2.06. C38H25ClCuN4O8 (764.6): calcd. C 59.70, H 3.29,
N 7.33; found C 58.62, H 2.98, N 7.21. The complex was also
prepared by stirring (0.76 g, 1 mmol) Cu(fla)(phen)2]ClO4 in ace-
tonitrile (30 mL) at room temperature under dioxygen (0.1 MPa)
for 25 h (0.61 g, 80%).
CB2 1EZ, UK [Fax: (internat.)
deposit@ccdc.cam.ac.uk].
ϩ 44-1223/336-033; E-mail:
Physical Measurements: Electronic spectra were recorded with a
Shimadzu UV-160 (Carl Zeiss) spectrometer, infrared spectra with
a Specord IR-75 (Carl Zeiss) spectrometer. Magnetic susceptibilit-
ies were determined at room temperature with a Bruker B-E 10B8
magnetic balance. GC analyses were performed with an HP 5830A
gas chromatograph equipped with a flame ionization detector and
a CP SIL8CB column. GC-MS measurements were recorded with
an HP 5890 II, 5971 GC/MSD at 75 eV.
Kinetic Measurements: Reactions of copper flavonolate complexes
with O2 were performed in DMF solutions. In a typical experiment
CuII(phen)(fla)2 was dissolved under argon in a thermostatically
controlled reaction vessel with an inlet for taking samples with a
syringe, and connected to a mercury manometer to maintain a con-
stant pressure. The solution was then heated to the appropriate
temperature, a sample was taken by syringe, and the initial concen-
tration of CuII(phen)(fla)2 was determined by UV/Vis spectroscopy
measuring the absorbance of the reaction mixture at 432 nm (63
658) {λmax of a typical band of CuII(phen)(fla)2}. The argon was
then replaced by dioxygen, and the consumption of CuII(phen)-
(fla)2 was analysed periodically (ca. every 10 min). Experimental
conditions are summarised in Table 1. The temperature was deter-
mined with an accuracy of Ϯ0.5 °C and the pressure of dioxygen
with an accuracy of Ϯ0.5%. The O2 concentration was calculated
from literature data,[25] taking into account the partial pressure of
DMF[26] and assuming the validity of Dalton’s law.
Oxygenation of Cu(L)(fla)2 (7aϪd, 8Ϫ9): In a typical experiment,
[Cu(phen)(fla)2] (7a) (0.57 g, 0.8 mmol) in DMF (50 cm3) was
treated with dioxygen (0.1 MPa) until dioxygen uptake ceased at
80 °C. The dioxygen uptake was measured by a gas-volumetric
method. The GC-MS analyses of the reaction mixture, after treat-
ment with dilute HCl solution (10%) and ethereal diazomethane
showed the presence of methyl benzoate: m/z (%) ϭ 136 (23) [Mϩ],
105 (100), 77 (87); coumaronedione: m/z (%) ϭ 148 (3) [Mϩ], 120
(100), 92 (59), 64 (22), 63 (14); and methyl 2-hydroxyphenylglyoxyl-
ate: m/z (%) ϭ 180 (7) [Mϩ], 121 (100), 93 (11), 65 (11). Addition
of excess Et2O resulted in the deposition of [Cu(phen)(2HOpg)2]
(14) as a green microcrystalline solid: Yield 0.20 g (43%). M.p. 204
°C (dec.). IR (KBr): ν˜ ϭ 3037 w, 1616 vs, 1575 vs, 1516 s, 1462 m,
1423 s, 1375 m, 1346 m, 1326 m, 1265 w, 1213 vs, 1158 s, 1110 m,
1039 vw, 908 w, 858 vs, 785 s, 752 m, 733 s, 688 m, 629 w, 575 m.
UV/Vis (DMF): λmax (ε/L molϪ1 cmϪ1) ϭ 273 (42213), 398 (5262)
nm. µB ϭ 1.91. C28H18CuN2O8 (574.0): calcd. C 58.59, H 3.16, N
4.88; found C 57.89, H 3.33, N 5.12.
Supporting Information: Kinetic data and diagrams for the oxy-
genations are available (see also footnote on the first page of this
article).
Cu(bpy)(2HOpg)2 (15): Yield 0.18 g (41%). M.p. 176 °C. IR (KBr):
ν˜ ϭ 3057 w, 1618 vs, 1577 vs, 1550 s, 1516 s, 1496 s, 1448 m, 1428
s, 1387 m, 1352 w, 1326 m, 1258 w, 1217 vs, 1156 s, 1041 w, 770 vs,
668 m, 629 w, 573 m. UV/Vis (DMF): λmax (ε/L molϪ1 cmϪ1) ϭ
268 (29960), 305 (46121) nm. µB ϭ 1.94. C26H18CuN2O8 (550.0):
calcd. C 56.78, H 3.30, N 5.09, found C 56.49, H 3.16, N 5.12.
Acknowledgments
Financial support of the Hungarian National Research Fund
(OTKA T-30400) and Ministry of Education (FKFP-0446/1999) is
gratefully acknowledged.
Cu(TMEDA)(2HOpg)2 (16): Yield 0.12 g (30%). M.p 195 °C (dec.).
IR (KBr): ν˜ ϭ 3126 vw, 3084 vw, 3068 vw, 3045 vw, 2871 vw, 2827
vw, 1616 vs, 1571 vs, 1519 s, 1478 w, 1465 w, 1445 s, 1421 s, 1407 m,
1362 m, 1320 m, 1258 m, 1207 s, 1147 s, 1084 vw, 1062 w, 1035 m,
990 w, 913 w, 861 m, 845 w, 769 s, 758 s, 745 s, 735 m, 720 m, 671 m,
558 m. UV/Vis (DMF): λmax (ε/L molϪ1 cmϪ1) ϭ 266 (14207), 302
(19828), 349 (7650) nm. µB ϭ 1.91. C22H26CuN2O8 (510.0): calcd.
C 51.81, H 5.14, N 5.49; found C 52.23, H 4.95, N 5.57.
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Eur. J. Inorg. Chem. 2002, 2287Ϫ2295

Copper-Mediated Oxygenolysis of Flavonols
FULL PAPER
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