Flavonolate complexes of MII (M = Mn, Fe, Co, Ni, Cu, and Zn). structural and functional models for the ES (Enzyme-substrate) complex of quercetin 2,3-dioxygenase

Article abstract of DOI:10.1021/ic400972k

A series of flavonolate complexes [M^IIL(fla)] (M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5), and Zn (6), LH: 2-{[bis(pyridin-2-ylmethyl)amino] methyl}benzoic acid, fla: flavonolate) have been synthesized as structural and functional models for the ES (enzyme-substrate) complexes of the active site of various M^II-containing quercetin 2,3-dioxygenase (2,3-QD) and their structures, spectroscopic features, and redox properties, as well as the reactivity toward molecular oxygen, have been investigated. The metal centers of [Fe^IIL(fla)]·H₂O (2), [Co^IIL(fla)] ·CH₃OH (3), and [Ni^IIL(fla)] (4) exhibit a distorted octahedral geometry with two oxygen atoms of fla, one oxygen atom of the benzoate group of ligand L, and three nitrogen atoms of ligand L, in which oxygen atom of the carbonyl group of fla and one of the pyridine nitrogen atoms occupy the axial positions. The complexes [M^IIL(fla)] exhibit relatively high reactivity in the oxidative ring-opening of the bound flavonolate at lower temperature, presumably due to the existing carboxylate group in the supporting ligand. Thus, our complexes act as good functional ES models of various metal(II)-containing 2,3-QD. In addition, complexes [Fe ^IIL(fla)]·H₂O (2), [Co^IIL(fla)] ·CH₃OH (3), and [Ni^IIL(fla)] (4) are the first structurally characterized Fe^II-, Co^II-, and Ni ^II-flavonolate complexes, as an active site ES model of Fe ^II-, Co^II-, and Ni^II-containing 2,3-QD, respectively. The model complexes exhibit notably different reactivity in the order of Fe (2) > Cu (5) > Co (3) > Ni (4) > Zn (6) > Mn (1). The differences in the reactivity among them may be attributed to the redox potential of the coordinated flavonolate of the complexes, which are remarkably influenced by the Lewis acidity of the metal ion and its coordination environment. Our study is the first example of the metal ion effects on the enzyme-like dioxygenation reactivity, providing important insights into the metal ion effects on the enzymatic reactivity of various metal(II)-containing 2,3-QD.

Full text of DOI:10.1021/ic400972k

Article
pubs.acs.org/IC
Flavonolate Complexes of MII (M = Mn, Fe, Co, Ni, Cu, and Zn).
Structural and Functional Models for the ES (Enzyme−Substrate)
Complex of Quercetin 2,3-Dioxygenase
Ying-Ji Sun,*,† Qian-Qian Huang,† Tetsuro Tano,‡ and Shinobu Itoh‡
†School of Chemistry, Dalian University of Technology, 2 Linggong Road, Dalian 116024, China
‡Department of Material and Life Science, Division of Advanced Science and Biotechnology, Graduate School of Engineering, Osaka
University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: A series of flavonolate complexes [MIIL(fla)]
(M = Mn (1), Fe (2), Co (3), Ni (4), Cu (5), and Zn (6),
LH: 2-{[bis(pyridin-2-ylmethyl)amino]methyl}benzoic acid,
fla: flavonolate) have been synthesized as structural and
functional models for the ES (enzyme−substrate) complexes
of the active site of various MII-containing quercetin 2,3-
dioxygenase (2,3-QD) and their structures, spectroscopic
features, and redox properties, as well as the reactivity toward
molecular oxygen, have been investigated. The metal centers of
[FeIIL(fla)]·H2O (2), [CoIIL(fla)]·CH3OH (3), and [NiIIL(fla)] (4) exhibit a distorted octahedral geometry with two oxygen
atoms of fla, one oxygen atom of the benzoate group of ligand L, and three nitrogen atoms of ligand L, in which oxygen atom of
the carbonyl group of fla and one of the pyridine nitrogen atoms occupy the axial positions. The complexes [MIIL(fla)] exhibit
relatively high reactivity in the oxidative ring-opening of the bound flavonolate at lower temperature, presumably due to the
existing carboxylate group in the supporting ligand. Thus, our complexes act as good functional ES models of various metal(II)-
containing 2,3-QD. In addition, complexes [FeIIL(fla)]·H2O (2), [CoIIL(fla)]·CH3OH (3), and [NiIIL(fla)] (4) are the first
structurally characterized FeII-, CoII-, and NiII-flavonolate complexes, as an active site ES model of FeII-, CoII-, and NiII-containing
2,3-QD, respectively. The model complexes exhibit notably different reactivity in the order of Fe (2) > Cu (5) > Co (3) > Ni (4)
> Zn (6) > Mn (1). The differences in the reactivity among them may be attributed to the redox potential of the coordinated
flavonolate of the complexes, which are remarkably influenced by the Lewis acidity of the metal ion and its coordination
environment. Our study is the first example of the metal ion effects on the enzyme-like dioxygenation reactivity, providing
important insights into the metal ion effects on the enzymatic reactivity of various metal(II)-containing 2,3-QD.
tetrahedral geometry without the direct coordinative interaction
between copper(II) ion and the carboxylate group of Glu73
(major conformation 70%). In its principal conformation,
Glu73 acts as a hydrogen bond acceptor for the metal-bound
water molecule.2 The substrate quercetin is bound to the
copper(II) ion through the deprotonated 3-hydroxy group of
flavonolate, with displacement of the water molecule, forming
an ES (enzyme−substrate) complex with a distorted square
pyramidal geometry under anaerobic conditions.3 The carbox-
ylate group of Glu73 probably acts as an active site base for the
deprotonation of the substrate, stabilizes the bound substrate
via a hydrogen bonding interaction, modulates the redox
potential of the metal ion in the ES adduct, thus lowering the
energy barriers.2,4 The site directed mutagenesis study has
indicated that the enzyme is inactive without Glu73.2
INTRODUCTION
■
Quercetin 2,3-dioxygenase (2,3-QD) activates molecular oxy-
gen to catalyze the oxygenative ring-opening reaction of the O-
heterocycle of quercetin (3′,4′,5,7-tetrahydroxyflavonol, QUE)
to the corresponding depside (phenolic carboxylic acid esters)
and carbon monoxide (Scheme 1).1
The mononuclear copper(II) active site of resting 2,3-QD
from Aspergillus japonicus (homo bicupin glycoprotein) exhibits
two distinct geometries: a distorted trigonal-bipyramidal
geometry consisting of three histidine imidazoles (His66,
His68, and His112), one water molecule and a carboxylate
group of Glu73 (minor conformation 30%), and a distorted
Scheme 1
The active site of FeII-containing 2,3-QD from Bacillus
subtilis (hetero bicupin glycoprotein) has been shown to exhibit
Received: April 19, 2013
Published: September 17, 2013
© 2013 American Chemical Society
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similar structures. The one has a distorted trigonal-bipyramidal
geometry consisting of three histidine imidazoles (His62,
His64, and His103), a water molecule and one Glu69 at a 2.10
Å distance in the N-terminal cupin motif, and the other has a
square pyramidal geometry consisting of three histidine
imidazoles (His234, His236, His275), a water molecule, and
a weakly coordinating Glu241 at a 2.44 Å distance in the C-
terminal cupin motif.5a Previous metal-replacement experi-
ments have shown that the catalytic activity of the MnII- and
CoII-containing enzymes were 35- and 24-fold more active than
the FeII-containing native enzyme, respectively. Thus, MnII is a
preferable metal cofactor for 2,3-QD from B. subtilis.6a The EPR
studies indicate that the Mn(II)-containing 2,3-QD from
reactivity was observed, and they required higher temperature
(100 °C) to induce oxidative degradation of the substrate.
With respect to other transition-metal 2,3-QD model
complexes, only a few examples have been reported such as
[MnII(fla)2(py)2],11 [FeIII(4′-MeOfla)3],11 [FeIII(fla)(salen)],9b
[CoIII(fla)(salen)],12 [(6-Ph2TPA)MII(fla)]X (M = Mn, Fe, Co,
−
Ni, Cu, Zn; X: OTf− or ClO4 ; 6-Ph2TPA: N,N-bis((6-phenyl-
2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine),13a,b and
[ZnII(fla)(idpa)]ClO4,14 among which [(6-Ph2TPA)MII(fla)]X
is the only example containing a series of divalent first row MII
ions. However, no reactivity study has been done on these
model complexes.13a
While our manuscript was under review and revision
(submitted April 19, 2013), a series of the ES models of 2,3-
QD supported by a ligand having a carboxylate group
[M(fla)(L1)] (M = Mn, Fe, Co, Ni, Cu; L1: N-propanoate-
N,N-bis(2-pyridylmethyl)amine) have been reported by Ka-
izer’s group (received on May 23, 2013).13c The structure of
the model ligand is very similar to that of ours, both of them are
a N,N-bis(2-pyridylmethyl)amine N3O donor set ligands while
another side arm is different. Kaizer’s ligand has a N-propanoate
side arm but ours has a N-benzonate side arm. The
dioxygenation reactivity of the Kaizer’s complexes (at 80 °C)
are lower than ours (at 70 °C).
5
Bacillus subtilis has a high-spin (S = /2) mononuclear Mn(II)
center in a octahedral geometry in the resting state.6a
On the other hand, recent study of 2,3-QD from Streptomyces
sp. FLA (monocupin) indicated that the highest and next
highest level of reactivity was observed when the enzyme was
overexpressed in E. coli in the LB medium containing NiII and
CoII salts, but less reactivity was observed with MnII, FeII, CuII,
or ZnII salts.6b The EPR studies indicate that the Co(II)-
containing 2,3-QD from Streptomyces sp. FLA has a high-spin
3
(S = /2) mononuclear Co(II) center in a trigonal bipyramidal
or tetrahedral geometry in the resting state.6b These results
clearly indicate that 2,3-QD enzymes have surprising variability
in metal cofactor selectivity and structural flexibility for the
metal-cofactor, and the catalytic activity of the enzymes are
largely influenced by the metal ions.5,6 The metal ion may
control the orientation of the bound substrates, modulate the
reduction potential of the bound substrate,4 and provide
electrostatic stabilization of the intermediates.6b However, little
is known about the precise roles of metal ion and the
carboxylate group as well as the catalytic mechanism of the
individual enzymatic systems from the different origins. Similar
phenomenon are also found in other enzyme systems such as
ARD (Acireductone dioxygenase) (Fe, Ni, Co, Mn, and Mg)7a
and extra-diol catechol dioxygenase (Fe, Co, and Mn).7b The
metal diversity among the various enzymes for catalysis and the
variability in metal cofactor selectivity in the same enzyme are a
very interesting and poorly understood feature and will be a
topic of further study.
In order to get insights into the effects of metal ions as well
as the carboxylate group on the chemical functions of 2,3-QD,
we herein synthesized a series of MII-flavonolate complexes
supported by the carboxylate ligand L− [MIIL(fla)] (LH: 2-
{[bis(pyridin-2-ylmethyl)amino]methyl}benzoic acid (Figure
1), fla: flavonolate, M = Mn (1), Fe (2), Co (3), Ni (4), Cu
Figure 1. Structure of ligand LH.
(5), and Zn (6)) as structural and functional models of ES-
complexes of various metal(II)-containing 2,3-QD. The
structure, physicochemical properties, redox properties, and
reactivity toward molecular oxygen have been examined in
detail.
There have been reported several biomimetic studies of 2,3-
QD,8−14 most of which, however, dealt with copper(II)
complexes with polyamine supporting ligands.8,9a,10 In those
studies, little attention has been paid to the role of active site
carboxylate (Glu). To the best of our knowledge, there are only
two examples demonstrating that addition of excess free
carboxylate to N-chelating ES-model complexes
[CuII(fla)(idpa)]ClO4 (idpa: 3,3′-iminobis(N,N-dimethylpro-
pylamine)9a and [FeIII(fla)(salen)] (salenH2: 1,6-bis-(2-hydrox-
yphenyl)-2,5-diaza-hexa-1,5-diene)9b accelerates the dioxygena-
tion of the bound substrate flavonolate. The additional
coordination by an exogenous carboxylate coligand may
displace the carbonyl oxygen atom of the bidentate chelating
flavonolate to monodentate ligand, enhancing the reactivity
toward molecular oxygen. Two types of the model ligands
having a carboxylate group have also been reported, [CuIILCl]
(L: 1-benzyl-4-acetato-1,4,7-triazacyclononane),10a
[CuII(BPEA)Obs] (BPEA: N,N-bis(2-pyridylmethyl)amino-2-
ethanoate, Obs: O-benzoylsalicylic acid),10b as a resting enzyme
and an enzyme−product (EP) complex model, respectively.
However, no effect of the ligand carboxylate group on the
EXPERIMENTAL SECTION
■
General and Physical Methods. All chemicals used in this study
except the ligand and the complexes were commercial products of the
highest available purity and were further purified by the standard
method,15 if necessary. FT-IR spectra were recorded with a Nicolet
6700 spectrophotometer. UV−vis spectra were measured using an
Agilent Technologies HP8453 diode array spectrophotometer. ESI-MS
(electrospray ionization mass spectra) measurements were performed
on a PE SCIEX API 150EX or an Agilent Technologies HP1100LC-
MSD, and the sample was prepared by adding a little amount of HOAc
or NH3·H2O, if necessary. 1H NMR spectra were recorded on a JEOL
FT-NMR Lambda 300WB and 400WB or Bruker 400WB. EPR spectra
were obtained on a Bruker MEX-Plus or Bruker A200 spectrometer
fitted with a liquid helium cooled probe. The spectra of the complexes
(2 or 4 mM in 0.5 mL DMF) were recorded at −150 °C for CuII-
complex and −173 °C for MnII-, FeII-, and CoII-complexes, about 20
mW microwave powers and about 9.40 GHz. The sample was put into
a nitric acid washed quartz EPR tube and was frozen in liquid nitrogen.
The data were simulated with WINEPR software. Cyclic voltammetry
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1
data was collected using a CHI620b system. All CV data were obtained
under N2 in DMF with a complex concentration of 2 mM and KClO4
(0.5 M) as the supporting electrolyte. The scan rate was 50 mV s−1.
The experimental setup consisted of a glassy carbon working electrode,
an SCE reference electrode, and a platinum wire auxiliary electrode. All
potentials are reported versus SCE. The organic reaction products
were analyzed by HPLC-MS and 1H NMR. HPLC-MS measurements
were performed on a Thermo Fisher Scientific LTQ Orbitrap XL
HPLC-MS. The production of the gas product CO during
dioxygenation reaction was detected by online COx analyzer (SICK-
MAIHAK-S710, Germany).
By H NMR. The [FeIIL(fla)] (1.0 × 10−3 M in 200 mL DMF)
solution was reacted with O2 at 100 °C for 1 day. The reaction mixture
was evaporated, esterificated by CH3OH in the presence of H2SO4,
and then extracted with CH2Cl2. The products were separated by
using silica gel column chromatography and analyzed by 1H NMR (in
CDCl3).
CO Analysis. A homemade reactor was linked with a pure N2 and an
O2 gas tank and an online COx analyzer. Dry N2 gas (100 mL min−1)
was continuously supplied by bubbling to a deaerated DMF solution
(30 mL) of [FeIIL(fla)] (1.0 × 10−3 M) in the reactor for 30 min to
degas the CO2, CO, and O2 in solution, reactor, and all of the system
lines. The solution was kept at 70 °C for several minutes until the CO
and CO2 concentrations were lower than 1 and 9 ppm, respectively.
Then dry O2 and N2 gas (30:70) were continuously supplied by
bubbling, the analyses were started, and the online CO and CO2 gas
concentrations were recorded for 6.5 h. The reaction was monitored at
70 °C during the initial 50 min. After 50 min, the reaction became very
slow, so we increased the reaction temperature to 100 °C. After 342
min, the CO concentration was lower than 1 ppm, we stopped the
detection of CO. The total CO concentration was integrated for the
whole reaction time.
X-ray Crystallography. The single crystals were mounted on a
glass capillary. Data of X-ray diffraction were collected by a Rigaku
RAXIS-RAPID imaging plate two-dimensional area detector using
graphite-monochromated Mo Kα radiation (λ = 0.71075 Å) to 2θ
max
of 55.0°. The crystallographic calculations were performed using the
Crystal Structure software package of the Rigaku Corporation and
Molecular Structure Corporation for 238 and 339 and SHELXTL 97 for
4.40 All non-hydrogen atoms and hydrogen atoms were refined
anisotropically and isotropically, respectively. Since the disordered
H2O solvent molecules in 4 could not be unambiguously modeled, the
Platon Squeeze option was utilized to remove all of them.41 Squeeze
indicates 2 solvent regions in the cell corresponding to about 86
electrons/cell or approximately 2 H2O molecules per formula.
Kinetic Measurements. [MIIL(fla)] Dependence. The reactions of
the complexes [MIIL(fla)] (M = Mn, Fe, Co, Ni, Cu, and Zn) with O2
were carried out in a 10 mm path length UV−vis cell that was held in a
Unisoku thermostatted cell holder USP-203 (a desired temperature
can be fixed within 0.5 °C). After the deaerated solution (3 mL) of
[MIIL(fla)] (0.8−2.0 × 10−4 M) in the cell was kept at a desired
temperature (70 °C) for several minutes, dry dioxygen gas was
continuously supplied by gentle bubbling from a thin needle. The time
courses of the reactions were followed by monitoring the absorption
change at a λmax due to the π → π* transition of the coordinated
flavonolate. The initial concentrations of [MIIL(fla)] were determined
by using the ε value at the λmax of each complex (see Table 3).
O2 Dependence. After the deaerated solvent DMF (58 mL) in a
thermostatted reaction vessel was kept at a desired temperature (30−
80 0.5 °C) and a constant dioxygen pressure through a manometer
for several minutes, the deaerated solution of [MIIL(fla)] (3 mM in 2
mL DMF) was added. The reaction mixture was then taken by syringe
periodically (ca. every 2 min), and the time courses of the reactions
were followed by monitoring the absorption change at a λmax due to
the π → π* transition of the coordinated flavonolate. The O2
concentrations were calculated from literature data16 taking into
account the partial pressure of DMF17 and assuming the validity of
Dalton’s law.
Synthesis of the Model Complexes. Ligand LH was prepared
according to the reported procedures.18
[MnIIL(fla)]·H2O (1). A dry CH2Cl2 (1.0 mL) solution of flavonol
(72.2 mg, 0.3 mmol) was added dropwise to a dry MeOH/acetonitrile
(3:1) solution (4.0 mL) of Mn(OAc)2·4H2O (74.7 mg, 0.3 mmol) at
room temperature under Ar. After stirring for 1 day, the solvent was
removed by evaporation, and the resulting mixture was dissolved in
CH2Cl2 (2.0 mL). A dry CH2Cl2 solution (1.0 mL) containing ligand
LH (100.0 mg, 0.3 mmol) was added dropwise to the above solution,
then the mixture was stirred for another 1 day under Ar. The
[MnIIL(fla)] was isolated as yellow-green powder by filtration. Light
yellow thin crystals were obtained by slow diffusion of hexane into the
dichloromethane solution of the complex at room temperature (96.0
mg, 51%). ESI-MS: m/z (pos.) = 625.3 ([MnIIL(fla)]H+), 647.3
([MnIIL(fla)]Na+), 663.2 ([MnIIL(fla)]K+) (main peak). Anal. calc for
C35H29MnN3O6 (642.56): C, 65.42; H, 4.55; N, 6.54. Found: C,
65.23; H, 4.63; N, 6.89. FT-IR (solid sample: KBr, cm−1): 3488 (m),
1609 (s), 1562 (s), 1480 (w), 1444 (w), 1389 (s), 1105 (m), 764 (m).
FT-IR (solution sample: in ethanol, cm−1): 3331 (m), 1600 (w), 1583
(s), 1560 (s), 1481 (w), 1444 (w), 1385 (s), 1125 (w), 759 (m).
[FeIIL(fla)]·H2O (2). This compound was synthesized by a similar
manner as for the synthesis of [MnIIL(fla)] except using Fe(OAc)2
(54.9 mg, 0.3 mmol) instead of Mn(OAc)2·4H2O. The [FeIIL(fla)]
was isolated as dark red powder by filtration. Dark red single crystals
suitable for X-ray crystallographic analysis were obtained from the
filtrate by slow evaporation of the solvent (95.7 mg, 51%). ESI-MS: m/
z
(pos.) =
626.2 ([FeIIL(fla)]H+) (main peak), 648.2
([FeIIL(fla)]Na+). Anal. calc for C35H29FeN3O6 (643.48): C, 65.33;
H, 4.54; N, 6.53. Found: C, 65.50; H, 4.67; N, 6.32. FT-IR (solid
sample: KBr, cm−1): 3374 (m), 1609 (m), 1589 (m), 1544 (s), 1504
(w), 1488 (s), 1440 (w), 1416 (s), 1356 (s), 1321 (w), 1213 (m), 753
(m). FT-IR (solution sample: in ethanol, cm−1): 3331 (m), 1613 (m),
1588 (m), 1545 (s), 1504 (w), 1489 (s), 1440 (w), 1417 (s), 1355 (s),
1321 (w), 1215 (s), 754 (m).
Determination of the Activation Parameters. The activation
parameters for the degradation of [MIIL(fla)] were obtained from an
Eyring plot (Table 4, Supporting Information Figures S4 and S12)
with [MIIL(fla)] (0.1 × 10−4 M) in the temperature range of 55−95
°C. The experiments procedures were similar to kinetic study
described above.
Reaction Product Analysis. By HPLC-MS. After the deaerated
DMF solution (2 mL) of [MIIL(fla)] (1.0 × 10−3 M) in the cell was
kept at 70 °C for several minutes, dry dioxygen gas was continuously
supplied by gentle bubbling from a thin needle for 8 h. After the
reaction, the mixture was concentrated by evaporation, and the
remaining residue was dissolved in 1.9 mL MeOH. o-Methylbenzoic
acid (10 mM in 100 μL MeOH solution, total 0.5 mM in 2.0 mL
solution) was added to the above solution as an internal standard, and
the reaction products were analyzed with a Thermo Fisher Scientific
LTQ Orbitrap XL HPLC-MS with an online UV−vis detector (λ: 210
nm). A Hypersil GOLD C18 column (Thermo Fisher Scientific 150
mm × 2.1 mm, 5 μm) was used for the HPLC analysis at room
temperature with a mobile phase (MeOH and 5 mM HOAc−
NH4OAc buffer (pH = 3.00)) with some gradient at a constant flow
rate of 0.8 mL min−1. The yields of the reaction products were
calculated by using the standard calibration curve.
[CoIIL(fla)]·CH3OH (3). This compound was synthesized by a similar
manner as for the synthesis of [MnIIL(fla)] except using the
corresponding Co(OAc)2·4H2O (75.5 mg, 0.3 mmol) instead of
Mn(OAc)2·4H2O. The complex was isolated as reddish-yellow
powder. Wine red single crystals suitable for X-ray crystallographic
analysis were obtained by slow diffusion of hexane into the
dichloromethane solution of the complex at room temperature (88.3
mg, 47%). ESI-MS: m/z (pos.) = 629.0 ([CoIIL(fla)]H+), 651.2
([CoIIL(fla)]Na+), 667.2 ([CoIIL(fla)]K+) (main peak). Anal. calc for
C36H31CoN3O6 (660.57): C, 65.46; H, 4.73; N, 6.36. Found: C, 65.63;
H, 4.53; N, 6.67. FT-IR (solid sample: KBr, cm−1): 3449 (m), 1610
(m), 1592 (s), 1561 (s), 1481 (w), 1439 (w), 1412 (s), 1377 (s), 1217
(m), 760 (m). FT-IR (solution sample: in ethanol, cm−1): 3331 (m),
1605 (m), 1588 (s), 1553 (s), 1481 (m), 1435 (w), 1414 (w), 1378
(s), 1219 (w), 768 (w).
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Figure 2. ORTEP representation of (a) [FeIIL(fla)]·H2O (2), (b) [CoIIL(fla)]·CH3OH (3), and (c) [NiIIL(fla)] (4). Ellipsoids are shown at the
50% probability level. Hydrogen atoms and solvents molecules are omitted for clarity.
[NiIIL(fla)]·2H2O (4). This compound was synthesized by a similar
flaH in the reaction. All the complexes are relatively stable
manner as for the synthesis of [MnIIL(fla)] except using Ni(OAc)2·
under air in the solid state but readily react with O2 in solution
4H2O (74.7 mg, 0.3 mmol) instead of Mn(OAc)2·4H2O. The complex
(see below). Single crystals of complexes [FeIIL(fla)]·H2O (2),
was isolated as yellow-green powder. Yellow-green single crystals
[CoIIL(fla)]·CH3OH (3), and [NiIIL(fla)] (4) suitable for X-
suitable for X-ray crystallographic analysis were obtained by slow
ray crystallographic analysis were successfully obtained as
diffusion of ether into the dichloromethane solution of the complex at
shown in Figure 2a, b, and c, respectively. A summary of the
crystallographic data is shown in Table 1 together with the
selected bond lengths and angles listed in Table 2. The FeII-
and CoII-complexes crystallize in the orthorhombic system and
Pbca space group, while the NiII-complex crystallizes in the
monoclinic system and P2(1)/c space group. These ternary
complexes exhibit a very similar structure, where the metal
center of each complex has a distorted octahedral geometry
involving two oxygen atoms of fla− (O(3): 3-hydroxylate and
O(4): 4-carbonyl), one oxygen atom of ligand L− (O(1):
benzoate) and three nitrogen atoms of ligand L−, where O(4)
and N(2) (one of the pyridine nitrogen atoms) occupy the axial
positions.
room temperature (109.2 mg, 58%). ESI-MS: m/z (pos.) = 628.2
([NiIIL(fla)]H+), 650.3 ([NiIIL(fla)]Na+) (main peak), 666.2 ([NiIIL-
(fla)]K+). Anal. calc for C35H31N3NiO7 (664.33): C, 63.28; H, 4.70; N,
6.33. Found: C, 63.22; H, 4.77; N, 6.26. FT-IR (solid sample: KBr,
cm−1): 3438 (m), 1605 (s), 1591 (s), 1553 (s), 1479 (m), 1417 (m),
1368 (s), 1224 (m), 761 (m). FT-IR (solution sample: in ethanol,
cm−1): 3331 (m), 1605 (w), 1587 (s), 1548 (s), 1484 (m), 1417 (m),
1397 (s), 1221 (m), 754 (w).
[CuIIL(fla)]·3CH3OH (5). This compound was synthesized by a
similar manner as for the synthesis of [MnIIL(fla)] except using
Cu(OAc)2·H2O (60.5 mg, 0.3 mmol) instead of Mn(OAc)2·4H2O.
The complex was isolated as dark green powder. Pure sample crystals
were obtained by slow diffusion of hexane into the dichloromethane
solution of the complex at room temperature (59.5 mg, 31%). ESI-MS:
m/z (pos.) = 633.3 ([CuIIL(fla)]H+). Anal. calc for C38H39CuN3O8
(729.28): C, 62.58; H, 5.39; N, 5.76. Found: C, 62.74; H, 5.65; N,
5.40. FT-IR (solid sample: KBr, cm−1): 3447 (m), 1609 (s), 1554 (s),
1487 (m), 1446 (m), 1384 (s), 1213 (m), 759 (m). FT-IR (solution
sample: in ethanol, cm−1): 3331 (m), 1612 (w), 1589 (m), 1538 (s),
1490 (s), 1441 (w), 1420 (s), 1357 (s), 1316 (w), 1215 (m), 750 (w).
[ZnIIL(fla)]·3CH3OH (6). This compound was synthesized by a
similar manner as for the synthesis of [MnIIL(fla)] except using
Zn(OAc)2·2H2O (66.6 mg, 0.3 mmol) instead of Mn(OAc)2·4H2O.
The complex was isolated as bright yellow powder (165.5 mg, 75%).
ESI-MS: m/z (pos.) = 634.1 ([ZnIIL(fla)]H+). Anal. calc for
C38H39ZnN3O8 (731.12): C, 62.43; H, 5.38; N, 5.75. Found: C,
62.65; H, 5.56; N, 5.52. FT-IR (solid sample: KBr, cm−1): 3427 (m),
1610 (w), 1590 (s), 1563 (s), 1484 (m), 1441 (s), 1410 (s), 1383 (s),
1217 (m), 759 (s). FT-IR (solution sample: in ethanol, cm−1): 3331
(m), 1610 (w), 1591 (s), 1558 (s), 1485 (m), 1442 (m), 1409 (m),
The bond lengths of M−O(1) (benzoate of L−) are
2.0256(18) Å for 2, 2.0165(14) Å for 3, and 2.076(2) Å for
4, respectively, which are slightly shorter than those of the
resting FeII- and CuII-containing 2,3-QD (2.10 Å)2,5a and much
shorter than that of the ES adduct of the CuII-containing 2,3-
QD (2.28 Å),3 but longer than that of [FeIII(fla)(L1)]ClO4
(1.909(5) Å).13c The bond lengths of Fe−O(3) and Fe−O(4)
(oxygen atoms of fla−) in 2 are 2.0297(16) and 2.1893(17) Å,
respectively, which are longer than those of the reported FeIII-
flavonolate complexes [FeIII(fla)(L1)]ClO4 (1.912(5) and
2.100(5) Å),13c [FeIII(4′-MeOfla)3] (1.955(7) and 2.109(8)
Å),11a [FeIII(fla)(salen)] (1.955(4) and 2.139(4) Å),9b
[FeIII(fla)2Cl(MeOH)] (1.958(12) and 2.127(63) Å),19 and
[(6-Ph2TPAFeIII(fla))2(μ-O)](ClO4)2 (1.9683(64) and
2.1257(15) Å).13a The average Fe−N bond length is 2.23 Å,
which is also slightly longer than those of [FeIII(fla)(L1)]ClO4
(2.15 Å)13c and [(6-Ph2TPAFeIII(fla))2(μ-O)](ClO4)2 (2.21
Å).13a These results indicate that the oxidation state of Fe ion is
+2 in 2, which is in good agreement with the calculated
valence20 of the Fe ion (2.02) and the EPR results (silent)
described below. The difference [Δd(Fe−O)] between Fe−
O(3) and Fe−O(4) is 0.16 Å, which is similar to those of
[FeIII(fla)(L1)]ClO4 (0.19 Å),13c [FeIII(4′-MeOfla)3] (0.15
Å),11a [FeIII(fla)(salen)] (0.18 Å),9b [FeIII(fla)2Cl(MeOH)]
(0.17 Å),19 and [(6-Ph2TPAFeIII(fla))2(μ-O)](ClO4)2 (0.17
Å).13a
1
1381 (s), 1217 (w), 757 (s). H NMR (CDCl3, 400 MHz): δ (ppm)
8.82 (d, J = 2.4 Hz, 2 H), 8.61 (t, J = 2.0 Hz, 2 H), 8.05 (d, J = 6.0 Hz,
1 H), 7.91 (d, J = 6.0 Hz, 1 H), 7.66 (d, J = 8.8 Hz, 2 H), 7.20−7.60
(m, 11 H), 7.06 (t, J = 4.8 Hz, 1 H), 6.97 (d, J = 8.0 Hz, 1 H), 3.92 (s,
4 H), 3.48 (s, 2 H).
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. All the
model complexes [MIIL(fla)] (M = Mn (1), Fe (2), Co (3),
Ni (4), Cu (5), and Zn(6)) were synthesized by treating the
neutral ligand LH, flavonol (flaH), and MII(OAc)2 under Ar
without adding any base. In these cases, acetate ions (OAc−) of
MII(OAc)2 may act as a proton acceptor both from LH and
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Table 1. Summary of X-ray Data Collection and Refinement
Table 2. Selected Bond Lengths (Å) and Bond Angles (deg)
for the Complexes
[CoIIL(fla)]·
[FeIIL(fla)]·H2O
(2)
CH3OH
[NiIIL(fla)]
(4)
[FeIIL(fla)]·
H2O (2)
[CoIIL(fla)]·
CH3OH (3)
[NiIIL(fla)]
(4)
(3)
empirical formula C35H29FeN3O6
C36H31CoN3O6
660.57
orthorhombic
Pbca
C35H27N3NiO5
628.31
M(1)−O(1)
M(1)−O(3)
M(1)−O(4)
M(1)−N(1)
M(1)−N(2)
M(1)−N(3)
O(3)−C(21)
O(4)−C(27)
C(21)−C(22)
2.0256(18)
2.0297(16)
2.1893(17)
2.297(2)
2.191(2)
2.189(3)
1.316(3)
1.261(3)
1.372(3)
2.0165(14)
1.9993(14)
2.1797(14)
2.2173(17)
2.1360(15)
2.1371(17)
1.313(2)
2.076(2)
2.0145(16)
2.0604(16)
2.114(2)
2.053(2)
2.095(2)
1.311(3)
1.259(3)
1.381(3)
Mr
643.48
orthorhombic
Pbca
crystal system
space group
a/Å
monoclinic
P2(1)/c
11.3376(5)
32.5833(12)
8.8492(4)
90
9.560(4)
20.519(7)
30.232(9)
90
9.690(3)
21.045(7)
30.112(13)
90
b/Å
c/Å
α/deg
β/deg
γ/deg
V/Å3
1.259(2)
90
90
101.996(3)
90
1.376(2)
90
90
5931(4)
8
6140(4)
8
3197.7(2)
4
O(1)−M(1)−O(3)
O(1)−M(1)−O(4)
O(3)−M(1)−O(4)
O(1)−M(1)−N(1)
O(1)−M(1)−N(2)
O(1)−M(1)−N(3)
O(3)−M(1)−N(1)
O(3)−M(1)−N(2)
O(3)−M(1)−N(3)
O(4)−M(1)−N(1)
O(4)−M(1)−N(2)
O(4)−M(1)−N(3)
N(1)−M(1)−N(2)
N(1)−M(1)−N(3)
N(2)−M(1)−N(3)
108.78(7)
87.74(7)
78.02(7)
86.64(7)
89.53(7)
155.61(7)
160.81(7)
90.59(7)
90.59(7)
114.93(7)
166.72(7)
87.23(7)
77.85(7)
73.96(8)
100.40(8)
103.08(5)
85.22(5)
79.30(5)
88.07(6)
88.48(6)
157.83(6)
166.52(6)
93.22(6)
95.30(6)
109.55(5)
168.84(6)
86.14(6)
79.37(6)
75.68(6)
102.91(6)
94.62(7)
93.11(8)
81.90(6)
92.29(8)
84.59(8)
171.12(8)
173.05(8)
97.98(8)
93.91(8)
98.41(7)
177.69(8)
85.65(8)
82.00(8)
79.21(8)
96.67(8)
Z
d
calcd/g cm−3
1.437
1.427
1.305
T/K
153
163
296(2)
crystal habit
color
prism
prism
block
dark red
wine red
yellow-green
crystal size
0.20 × 0.20 ×
0.40 × 0.20 ×
0.10
0.30 × 0.15 ×
0.10
(mm3)
0.050
μ/mm−1
0.561
54.96
99.9
0.606
54.96
99.9
0.652
52.00
100
2θmax/deg
completeness to
θ (%)
reflections
53872
6803
55197
7027
23266
6274
collected
independent
reflections
Rint
0.0590
433
0.0500
445
0.0414
397
variable
parameters
lengths (corresponding to C(4)O(4-carbonyl) of the
enzymatic substrate, Scheme 1) are 1.261(3) Å for 2,
1.259(2) Å for 3, and 1.259(3) Å for 4, respectively, which,
on the other hand, are slightly elongated relative to free
flavonol (1.232(3) Å).21 The C(21)C(22) bond lengths
(corresponding to C(2)C(3) of the enzymatic substrate,
Scheme 1) are 1.372(3) Å for 2, 1.376(2) Å for 3, and 1.381(3)
Å for 4, respectively, which are slightly elongated relative to free
flavonol (1.363(4) Å),21 indicating that the CC bond is
elongated by the metal coordination, which may promote the
C(21)C(22) bond cleavage during reaction with O2
described below (Scheme 3). These results indicate that the
C(22) atom (corresponding to C(2) of the enzymatic
substrate, Scheme 1) increases sp3 character by pyramidaliza-
tion as found in the ES adduct of CuII-containing 2,3-QD from
A. japonicas.3 It has been proposed that such a structural feature
stabilizes substrate radical produced from the reaction with O2.3
There are only four structurally characterized iron(III)−
flavonolate complexes [FeIII(fla)(L1)]ClO4,13c [FeIII(4′-MeO-
fla)3],11a [FeIII(fla)(salen)],9b and [FeIII(fla)2Cl(MeOH)],19 but
their oxidation state or/and structures are very different from
a
b
R1 /wR2
0.0417/0.0835
1.048
0.0286/0.0368
1.015
0.0435/0.1078
1.033
goodness-of-fit
on F2
Δρmax/min/e Å−3
1.070/−0.606
0.86/−0.34
0.259/−0.285
a
b
R1 = ∑(∥Fo| − |Fc∥)/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo )2]1/2
,
2
2
where w = 1/[σ2(Fo ) + (aP)2 + bP].
The Co−O(3) and Co−O(4) distances in 3 are 1.9993(14)
and 2.1797(14) Å, respectively, which are close to that of
[CoII(6-Ph2TPA)(fla)]ClO4 (1.956(2) and 2.172(2) Å, re-
spectively).13a The difference [Δd(Co−O)] between Co−O(3)
and Co−O(4) is 0.18 Å, which is also similar to that of 2 and
[CoII(6-Ph2TPA)(fla)]ClO4 (0.22 Å).13a The average Co−N
distance is 2.16 Å, which is close to those of 2 and [CoII(6-
Ph2TPA)(fla)]ClO4 (2.22 Å).13a These results indicate that the
oxidation state of Co ion in 3 is also +2, which is consistent
with the calculated valence20 of the Co ion (1.86) and the EPR
results described below.
The Ni−O(3) and Ni−O(4) bond lengths in 4 are
2.0145(16) and 2.0604(16) Å, respectively, which are close
to those of [NiII(6-Ph2TPA)(fla)]ClO4 (1.996(2) and 2.031(2)
Å, respectively).13a The difference [Δd(Ni−O)] between Ni−
O(3) and Ni−O(4) is 0.046 Å, which is shorter than those of 2
and 3 and similar to that of [NiII(6-Ph2TPA)(fla)]ClO4 (0.035
Å).13a The average Ni−N bond length is 2.09 Å, which is close
to those of 2, 3, and [NiII(6-Ph2TPA)(fla)]ClO4 (2.16 Å).13a
The C(21)−O(3) bond lengths (corresponding to C(3)
O(3-hydroxylate) of the enzymatic substrate, Scheme 1) are
1.316(3) Å for 2, 1.313(2) Å for 3, and 1.311(3) Å for 4,
respectively, which are slightly contracted relative to free
flavonol (flaH) (1.357(3) Å).21 The C(27)O(4) bond
that of the ES adduct of the FeII-containing 2,3-QD.
[FeII(fla)(L1)]13c and [(6-Ph2TPA)FeII(fla)]ClO4
are the
13a
only two examples of iron(II)−flavonolate complexes mimick-
ing the reduced state of FeII-containing 2,3-QD. However, they
were not structurally characterized, and the dioxygenation
reactivity of [FeII(fla)(L1)] was lower and required higher
temperature. [(6-Ph2TPA)FeII(fla)]ClO4 reacted with O2 to
give a (μ-oxo)diiron(III) complex [(6-Ph2TPAFeIII(fla))2(μ-
O)](ClO4)2, and no enzyme-like dioxygenation reactivity
(dioxygenation of the bound flavonolate) study has been
done on this complex. There is only one structurally
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Table 3. Summary of Spectroscopic and CV Data for the Complexes
[MnIIL(fla)] (1)
[FeIIL(fla)] (2)
[CoIIL(fla)] (3)
[NiIIL(fla)] (4)
[CuIIL(fla)] (5)
[ZnIIL(fla)] (6)
FT-IR
1561
ν(CO)/cm−1
νas(CO2)/cm−1
νs(CO2)/cm−1
Δν/cm−1
1562
1561
1553
1554
1563
1609
1589
1592
1591
1609
1610
1389
1356
1377
1368
1384
1383
220
233
215
223
225
227
UV−vis
275 (14347)
422 (10695)
275 (13516)
398 (9384)
486 (2250)
570 (371)
278 (16231)
417 (13281)
510 (548)
607 (247)
278 (15650)
440 (15184)
567 (47)
775 (4.4)
972 (24)
275 (23253)
423 (10205)
658 (212)
278 (14356)
417 (9909)
λ/nm
(ε/M−1 cm−1
)
ESI-MS m/z
663.2
626.2
667.2
650.3
633.3
634.1
([MnIIL(fla)]K+)
([FeIIL(fla)] H+)
([CoIIL(fla)]K+)
([NiIIL(fla)]Na+)
([CuIIL(fla)]H+)
([ZnIIL(fla)]H+)
EPR
g: 2.01
A: 95 G
g: 5.62
5.94
silent
g: 5.60
g: 2.25
g//: 2.25
g⊥: 2.06
A//: 159 G
1.99
μeff/B.M.
4.13
3.12
CV
E1/2 (V)
Epa (V)
Epc (V)
E1/2 (V)
−0.094
+0.169
+0.091
+0.130
−0.062
+0.396
+0.216
+0.306
−0.427
+0.243
+0.469
+0.401
+0.438
characterized example of the cobalt(II)−flavonolate complex
[CoII(6-Ph2TPA)(fla)]ClO4,13a but it only exhibits ligand
exchange reactivity. No enzyme-like dioxygenation reactivity
study is reported on this complex. There are two structurally
characterized examples of the nickel(II)−flavonolate complexes
[NiII(Py)2(fla)2]22 and [NiII(6-Ph2TPA)(fla)]ClO4,13a but no
enzyme-like dioxygenation reactivity study on these complexes
is reported. Thus, our complexes [FeIIL(fla)]·H2O (2),
[CoIIL(fla)]·CH3OH (3), and [NiIIL(fla)] (4) are the first
examples of structurally characterized MII-flavonolate com-
plexes, which exhibit an enzyme-like dioxygenation reactivity as
demonstrated below.
Spectroscopic and Redox Properties of the Com-
plexes. Infrared Spectroscopy. The FT-IR spectra of the solid
samples (Table 3, Supporting Information Figure S1-C for 2)
show a CO stretching vibration ν(CO) of the coordinated
carbonyl group of fla− around 1560 cm−1, which is lower by
40−60 cm−1 as compared with that of free flavonol (1602
cm−1). A similar phenomenon found in other flavonolate
complexes so far has been reported.8−14 The asymmetric
νas(COO−) and symmetric νs(COO−) stretching frequencies of
the carboxylate group of L− in the complexes appear at ∼1600
and ∼1380 cm−1, respectively, which are also lower as
compared with those of free ligand LH (1696 and 1594
cm−1, respectively). The difference between them (Δν =
νas(COO−) − νs(COO−)) is about 220 cm−1, suggesting a
monodentate carboxylate binding mode,23,24 consistent with
the X-ray structures shown in Figure 2.
monodentate carboxylate binding mode in solution.23,24
These results clearly indicate that the complexes keep their
mononuclear structures in solution, which is in agreement with
other spectroscopic results such as ESI-MS, solution magnetic
moment, solution EPR, and NMR described bellow.
UV−vis Spectroscopy. Complexes [MIIL(fla)] exhibit an
intense absorption band at 398−440 nm (Mn: 422 nm (ε = 1.1
× 104 M−1 cm−1), Fe: 398 nm (ε = 9.4 × 103 M−1 cm−1)
(Figure 4a red line), Co: 417 nm (ε = 1.3 × 104 M−1 cm−1), Ni:
440 nm (ε = 1.5 × 104 M−1 cm−1), Cu: 423 nm (ε = 1.0 × 104
M−1 cm−1), Zn: 417 nm (ε = 9.9 × 103 M−1 cm−1) Supporting
Information Figure S2 and Table 3), which can be assigned to
the π → π* transition of the B ring (Scheme 1) of the
coordinated flavonolate ligand.25 The λmax of our model
complexes [MIIL(fla)] are blue-shifted by 3−17 nm as
compared to those of [MII(6-Ph2TPA)(fla)]ClO4 (Mn: 431
nm, Fe: 415 nm, Co: 422 nm, Ni: 443 nm, Cu: 428 nm, Zn:
420 nm),13a [MII(fla)(L1)] (Mn: 430 nm, Fe: 402 nm, Co: 423
nm, Ni: 450 nm, Cu: 432 nm),13c and other synthetic model
complexes such as [FeIII(fla)(salen)] (407 nm),9b [FeIII(4′-
MeOfla)3] (411 nm),11a and [MnII(fla)2(py)2] (432 nm).11a
The shift of the λmax of [FeIIL(fla)] (2) is especially larger (4−
17 nm) than those of [FeII(6-Ph2TPA)(fla)]ClO4,13a [FeIII(¯a)-
(salen)],9b and [FeIII(4′-MeOfla)3],11a due to the existing
negative charge of the caboxylate group of our supporting
ligand and lower charge (+2 relative to +3) and low spin state
of Fe(II) ion (see below). The bands are blue-shifted relative to
those of free flavonolate (458 nm for Me4Nfla13a and 465 nm
for Kfla26) about 20−50 nm and in the order of Ni < Cu < Mn
< Co ∼ Zn < Fe, which is consistent with the order observed
with [MII(fla)(L1)]13c and [(6-Ph2TPA)MII(fla)]ClO4,13a in-
dicating that the degree of the blue-shift is markedly affected by
the nature of the M(II) ion. The electron distribution of the
coordinated substrate flavonolate may be altered by the nature
of the M(II) ion via the “electronic bridge” conferred by
C(21)C(22) double bond (corresponding to C(2)C(3) of
the enzymatic substrate, Scheme 1) as found in the enzyme
system.6b This tendency is similar to those seen in the
enzymatic2−6 and other synthetic model systems.8−14 The Fe-,
In order to get insight into the solution structure of the
complexes, FT-IR spectra of the complexes were also recorded
in ethanol (Supporting Information Table S1, Figure S1-B for
2). The solution spectrum of each complex is similar to that of
the corresponding solid sample. The complexes also show a
CO stretching vibration ν(CO) of the coordinated carbonyl
group of fla− at ∼1550 cm−1 and the asymmetric νas(COO−)
and symmetric νs(COO−) stretching frequencies of the
carboxylate group of L− at ∼1590 and ∼1380 cm−1,
respectively. The difference between them (Δν = νas(COO−)
− νs(COO−)) is about 210 cm−1, also suggesting a
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Co-, Ni-, and Cu-complexes also exhibit one or two weak d−d
absorption bands in the visible region (Table 3).
suggesting that [CuIIL(fla)] also has a similar mononuclear
square pyramidal geometry with a monodentate coordinated
substrate flavonolate. The observed g value (g// > g⊥ > ge) are
typical for a mononuclear copper(II) (3d9) ion in axial
symmetry geometry with the unpaired electron lying in the
Our binary complex [FeIIL(OAc)] (without fla−) was very
sensitive to air in solution, being easily auto-oxidized to form a
μ-oxo dimer [LFeIII(μ-O)(μ-OAc)FeIIIL](OAc) (characterized
by ESI-MS (m/z (pos.): 851.2 ([LFeIII(μ-O)(μ-OAc)FeIIIL]+))
and single crystal X-ray diffraction, unpublished results). Our
ternary complex [FeIIL(fla)] (398 nm) was also very sensitive
to air in solution, which could be oxidized to [FeIIIL(fla)]+ (m/
z (pos.): 625.2). In this case, however, no oxo or flavonolate
bridged dimer was observed in the ESI-MS spectrum
(Supporting Information Figure S3). When a [FeIIL(fla)]
solution was exposed to air or O2, the band was shifted to 402
nm, which is similar to the Fe(III) analogue [FeIIIL(fla)](OAc)
(2A) (406 nm) described bellow (Supporting Information
Table S4, Figure S12a). The spectrum of [FeIIL(fla)] (2) is
very similar to those of [FeII(fla)(L1)] (402 nm)13c and [FeII(6-
dx −y orbital at the ground state.29
2
2
The EPR spectrum of [MnIIL(fla)] (1) displayed a strong 6-
fold hyperfine splitting centered at g = 2.01 with a hyperfine
coupling constant of 95 G (Supporting Information Figure S4d-
A), corresponding to the +1/2 → −1/2 transition of the 5/2 spin
of 55Mn. This clearly indicates that complex 1 is a typical
octahedrally coordinated monomuclear MnII complex with
nitrogen and oxygen donors.30 The spectral data are also very
similar to those of the native MnII-containing enzyme (g = 2.0,
A = 93 G; g = 9.0, A = 92 G) and its ES adduct from B.
subtilis,6a suggesting that [MnIIL(fla)] (1) also has a
mononuclear distorted octahedral geometry.
13a
Ph2TPA)(fla)]ClO4 (also have a shoulder peak at near 500
The EPR spectrum of [CoIIL(fla)] (3) showed a very week
signal (Supporting Information Figure S4e-A) at g ∼ 5.60 and
2.25, which is similar to that of the native high-spin CoII-
containing enzyme and its ES adduct from Streptomyces sp. FLA
(g = 6.0, A = 95 G; g = 3.8 and 2.3)6b and B. subtilis (g = 6.5).6a
However, the signal is too week at the liquid nitrogen
temperature to be analyzed in detail.
nm), but different from that of the μ-oxo dimer [LFeIII(μ-
O)(μ-OAc)FeIIIL](OAc) (317 and 484 nm). These results
indicate that [FeIIL(fla)] keeps a mononuclear structure in
solution even after auto-oxidation, which is consistent with the
solution FT-IR, ESI-MS, solution EPR, and solution magnetic
moment results (see below).
ESI-MS Spectroscopy. The solution structures of the
complexes were also examined by ESI-MS. The complexes
exhibit a peak cluster as a main peak at m/z (pos.) = 663.2
([MnIIL(fla)]K+) for 1, 626.2 ([FeIIL(fla)]H+) for 2, 667.2
([CoIIL(fla)]K+) for 3, 650.3 ([NiIIL(fla)]Na+) for 4, 633.3
([CuIIL(fla)]H+) for 5, and 634.1 ([ZnIIL(fla)]H+) for 6,
respectively (Table 3) but did not give any carboxylate or
flavonolate bridged dimer peak. Since the iron(II) complex
[FeIIL(fla)] (2) was very sensitive to air in solution, we
obtained the overlapped peaks of [FeIIIL(fla)]+ (m/z (pos.) =
625.2) and [FeIIL(fla)]H+ (m/z (pos.) = 626.2) when the
sample was prepared in a glove box and measured under air
(Supporting Information Figure S3a), but we did not observe
any oxo or flavonolate bridged dimer structure. Such the auto-
oxidation products were not detected in the case of other
complexes, indicating that the complexes are relatively stable
under air in solution and remaining their oxidation state except
the Fe(II)-complex, being consistent with EPR and CV results
described below. The m/z values and the isotope distribution
patterns of all the peaks are in good agreement with the
calculated values. These results indicate that the complexes
remain their mononuclear structures in solution, which is
consistent with the solution FT-IR, solution EPR, and solution
magnetic moment results.
Solution Magnetic Moment. The solution magnetic
susceptibility of the paramagnetic complexes were examined
by NMR using Frei−Bernstein’s method,31 with TMS as a
reference and CDCl3 as a solvent. The solution magnetic
moments of the paramagnetic complexes were calculated and
listed in Table 3. The magnetic moments of the paramagnetic
MnII-, CoII-, NiII-, CuII-complexes are 5.94, 4.13, 3.12, and 1.99
μB, respectively, which are consistent with or a little larger (due
to the contribution of the orbital moment) than the
corresponding calculated values using the spin-only equation
of the high-spin MnII (5.92 μB), CoII (3.87 μB), NiII (2.83 μB),
and CuII (1.73 μB) centers, respectively, but in the range of the
experimental data MnII (4.3−5.2 μB), CoII (4.3−5.2 μB), NiII
(2.8−3.5 μB), CuII (1.7−2.2 μB), respectively. Since the
diamagnetic (low-spin) [FeIIL(fla)] was very air-sensitive in
solution, we observed some paramagnetically shifted reference
resonance due to the auto-oxidation of Fe(II) to Fe(III) during
measurement. In order to get insight into the nature of the
Fe(III)-complex that initially formed upon reaction with O2, we
reexamined [FeIIL(fla)] solution after exposure to O2 for 10
min. The magnetic moment is 5.79 μB, which is similar to the
Fe(III) analogue [FeIIIL(fla)](OAc) (2A) (5.76 μB, Supporting
Information Table S4) and is in line with the calculated value
using the spin-only equation of the high-spin FeIII center (5.92
μB), indicating that [FeIIL(fla)] is oxidized to high-spin
[FeIIIL(fla)]+ by O2 and keeps a mononuclear structure in
solution even after oxidation. These results also indicate that
our model complexes keep their oxidation state (except Fe) and
a mononuclear structure in solution.
Solution EPR Spectroscopy. The X-band EPR spectra of the
Mn, Fe, Co, and Cu-complexes in DMF were examined under
N2 at 100 K (Supporting Information Figure S4). Complex
[FeIIL(fla)] (2) was EPR silent under N2 (Supporting
Information Figure S4a-A), indicating that the Fe(II)-complex
is in the low-spin oxidation state, which is similar to the spin
state of FeII in the native enzyme6a and other FeII−quercetin
complexes.27
1
1H NMR Spectroscopy. H NMR spectra of the Fe(II)-,
Co(II)-, Ni(II)-, and Zn(II)-complexes were examined in
CDCl3 for 2 and 6, and in d7-DMF for 3 and 4 at ambient
temperature (Supporting Information Figure S5). Both spectra
of the paramagnetic Co(II)- and Ni(II)-complexes contain
several paramagnetically shifted weak and broad resonances
over a range of −7−17 ppm, which are similar to those of the
corresponding [MII(6-Ph2TPA)(fla)]ClO4 (M = Co and
Ni),13a respectively. Since [FeIIL(fla)] was very air sensitive in
solution, we observed several paramagnetically shifted weak and
The EPR spectrum of [CuIIL(fla)] (5) displayed a typical
axial symmetry signal with the EPR parameters of g// = 2.25, g⊥
= 2.06, A// = 159 G (Supporting Information Figure S4c-A, the
parameters were obtained by the simulation using a WIN EPR
software), which are similar to those of the enzyme-flavonol
adduct (g// = 2.31 and A// = 142 G, square pyramidal geometry
with a monodentate coordinated substrate) from A. japonicus,28
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Figure 3. Cyclic voltammograms of [FeIIL(fla)] in DMF at room temperature: (a) under N2, when (b) appearance and (c) disappearance of the
second couple under slightly aerobic condition.
Scheme 2. Reaction of the Model Complexes with O2 and Their Reaction Products
broad resonances over a range of −7−17 ppm due to the auto-
oxidation of Fe(II) to Fe(III) during the measurement, which is
also similar to that of [FeII(6-Ph2TPA)(fla)]ClO4.13a
oxygenation reaction could be tentatively assigned to one-
electron oxidation from fla− to fla• on the O2-generated
[FeIIIL(fla)]+. The redox couple of fla− to fla• were also
observed in the CoII-complexes at E1/2 = +0.306 V (ΔEp = 180
mV, ipc/ipa = 0.94) (Supporting Information Figure S6b). In
other complexes [MIIL(fla)] (M = Mn, Ni, Cu, and Zn), we
only observed an irreversible oxidation wave but did not
observe any reduction wave of the fla• to fla− even under O2.
The oxidation wave of the fla− to fla• appeared at +0.469 V for
[MnIIL(fla)] (1), +0.169 V for [FeIIL(fla)] (2), +0.396 V for
[CoIIL(fla)] (3), +0.401 V for [NiIIL(fla)] (4), +0.243 V for
[CuIIL(fla)] (5), and +0.438 V for [ZnIIL(fla)] (6),
respectively. The oxidation potential Epa of the coordinated
flavonolate of the complexes are much negative than those of
reported [MII(6-Ph2TPA)(fla)]OTf (M = Mn, Fe, Ni, Cu, and
Zn) (+0.765 ∼ +1.06 V in CH2Cl2)13a (Mn: +0.765 V, Ni:
+0.906 V, Cu: +1.06 V, and Zn: +0.934 V, respectively),
[FeII(fla)(H2O)4]Cl (+0.84 V), and [CuII(fla)(H2O)2]Cl
(+0.87 V) (in MeOH), respectively,32 due to the existing
negative charge of carboxylate in the supporting ligand L−.
Namely, the bound flavonolate are more easily oxidized in our
model system.
The Epa of the coordinated flavonolate of the complexes are
in the range of +0.169−+0.469 V (over a range of 300 mV) and
in the order of Fe (2) < Cu (5) < Co (3) < Ni (4) < Zn (6) <
Mn (1), which is just reverse with the order of the Lewis acidity
order of the metal ion (ZnII < CuII > NiII > CoII > FeII > MnII)
provided by the Irving−Williams series33 except FeII and CoII.
The Lewis acidity of the CoII and FeII are weaker than NiII but
the FeII (low-spin) and CoII (high-spin) become to the stronger
FeIII (high-spin) and CoIII (low-spin), respectively, during
reaction. [FeIIL(fla)] and [MnIIL(fla)] have the lowest and
highest potential, respectively, indicating that the flavonolate is
a better and worse reductant toward O2 in [FeIIL(fla)] and
[MnIIL(fla)], respectively, than in other MII-complexes [MIIL-
(fla)]. These results indicate that the Epa of the bound substrate
flavonolate are remarkably influenced by the Lewis acidity of
Cyclic Voltammetry. The redox properties of [MIIL(fla)]
were examined by cyclic voltammetry at room temperature
under N2. The results are summarized in Table 3. All potentials
are reported versus SCE. There is a quasi-reversible redox
couple in the Fe(II)-complex [FeIIL(fla)] (2) at E1/2 = −0.094
V (ΔEp = 69 mV, ipc/ipa = 1.00) (Figure 3a), which can be
assigned to the one-electron oxidation from FeII to FeIII. The
E1/2 value of the [FeIII/IIL(fla)]+/0 couple is more negative than
that of [(6-Ph2TPA)FeIII/II(fla)]2+/+ (+0.35 V)13a due to the
existing negative charge (carboxylate) in the present ligand
system L−. The Co(II)-complex [CoIIL(fla)] (3) also has a
quasi-reversible redox couple at E1/2 = −0.062 V (ΔEp = 201
mV, ipc/ipa = 0.94) under O2 (Supporting Information Figure
S6a), which can be assigned to the one-electron oxidation from
CoII to CoIII. In [CuIIL(fla)] (5), a quasi-reversible redox
couple appeared at E1/2 = −0.427 V (ΔEp = 120 mV, ipc/ipa
=
2.32), which can be assigned to one-electron reduction of CuII
to CuI (Figure S6c). The E1/2 value of [CuII/IL(fla)]0/− couple
is similar to that of [CuII(6-Ph2TPA)(fla)]OTf (−0.500 V, ΔEp
= 377 mV).13a The ipc/ipa value of [CuII/IL(fla)]0/− is large,
which is similar to that of [CuII(6-Ph2TPA)(fla)]OTf (ipc/ipa
∼
3).13a Such the metal redox couple were not observed with the
MnII-, NiII-, and ZnII-complexes within the range of −1.5−+1.5
V.
Interestingly, when a DMF solution of [FeIIL(fla)] (2) was
contacted slowly with O2, a quasi-reversible redox couple at E1/2
= +0.130 V (ΔEp = 78 mV, ipc/ipa = 1.05) appeared (Figure 3b)
then disappeared (Figure 3c) slowly, which was not observed
under anaerobic conditions. In fact, a new absorption band
arose at 670 nm (Supporting Information Figure S7−blue line)
(same with reported free fla• at 676 nm26a) in Fe-complexes
during the CV measurement under O2, which could be assigned
to the flavonoxy radical fla• generated in a one-electron
oxidation of fla−. This redox couple observed in the slow
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Figure 4. Dioxygenation of the [MIIL(fla)] complexes at 70 °C under O2. (a) Spectral change observed upon introduction of O2 gas into a DMF
solution of [FeIIL(fla)] (1.0 × 10−4 M). (inset) Time course of the absorption changes of [FeIIL(fla)] at 398 nm. (b) Plot of −d[MIIL(fla)]/dt vs
[MIIL(fla)]0 (× 10−4 M at 70 °C). (c) Plot of −d[MIIL(fla)]/dt vs [O2]0 (× 10−3 M). (d) Plot of k vs Epa of the flavonolate.
the MII ion, and the stronger Lewis acidity of the metal ion, the
lower oxidation potential Epa of the coordinated flavonolate of
the complexes.
transition of the coordinated flavonolate at the corresponding
λmax (Figure 4a for [FeIIL(fla)], Supporting Information Figure
S2 and Table 3). The kinetic data and activation parameters of
the complexes are listed in Table 4 and Supporting Information
Degradation of Complexes (Enzyme-like Dioxygenation
Reactivity). The ES model complexes [MIIL(fla)] reacted with
O2 in DMF at 70 °C to give o-benzoylsalicylic acid (HObs)
(m/z (pos.): 243 (M + H)+) (1.9−21%) (Supporting
Information Figure S8b) and CO (over 68% for 2) (Supporting
Information Figure S9) as the primary products (Supporting
Information Table S2). The HObs was then hydrolyzed with a
small amount of water in the solvent and amidated by the
solvent DMF to give salicylic acid (m/z (neg.): 137 (M − H)−)
(79−97%) (Supporting Information Figure S8c and Figure
S10a), benzoic acid (m/z (neg.): 121 (M − H)−) (34−95%)
(Supporting Information Figure S8d and Figure S10b), 2-
hydroxy-N,N-dimethylbenzamide (m/z (pos.): 183 (M +
NH4)+) (0−7.0%), and N,N-dimethylbenzamide (m/z (pos.):
150 (M + H)+) (0.1−56%) as the final products (Scheme 2)
(characterized by HPLC-MS and HPLC results) (Table S2 and
Figures S8 and S10). The conversion of the model complexes
are over 95%. Two main reaction products salicylic acid and
benzoic acid were also identified by 1H NMR after esterification
and separation (Figure S10). The spectra of the products were
identical to those of the authentic samples (AIST: Integrated
Spectral Database System of Organic Compounds. (Data were
obtained from the National Institute of Advanced Industrial
Science and Technology, Japan)). Since the yields of other
reaction products were low, we could not isolate and
characterize them further. The dioxygenation reaction products
of our model complexes are very similar to the reaction
products of the native enzyme, indicating that our model
complexes [MIIL(fla)] have enzyme-like dioxygenation reac-
tivity.
Table 4. Kinetic Data (at 70 °C) and Activation Parameters
of the Complexes
102k
ΔH‡
ΔS‡
Ea
(M−1 s−1
)
(kJ mol−1
)
(J mol−1 K−1
)
(kJ mol−1
)
[MnL(fla)]
(1)
2.05 0.13
179 0.08
34.0 0.04
7.11 0.16
113 0.03
3.37 0.10
69.94
65.92
84.48
63.60
55.25
80.72
−75.82
−51.81
−8.290
−82.19
−83.67
−39.90
72.88
68.60
87.27
66.51
58.03
83.65
[FeL(fla)]
(2)
[CoL(fla)]
(3)
[NiL(fla)]
(4)
[CuL(fla)]
(5)
[ZnL(fla)]
(6)
Table S3. The plots of the initial reaction rate versus the initial
concentration of both [MIIL(fla)] (Figure 4b) and dioxygen
(Figure 4c) are linear, respectively. So the rate law can be
described as −d[MIIL(fla)]/dt = k[MIIL(fla)][O2]. The second-
order reaction rate constants k were thus determined as 2.05−
179 × 10−2 M−1 s−1 at 70 °C (ΔH‡ = 55−84 kJ mol−1, ΔS‡ =
−40 to −84 J mol−1 K−1) (Table 4 and Supporting Information
Table S3 and Figure S11). Reactivity of the reported 2,3-QD
model complexes were much lower and required higher
temperature (80−145 °C) (Table 5)8−14,34 as compared with
that of our complexes. The higher reactivity of the present
model system could be attributed to the carboxylate group
attached to the supporting ligand, which may enhance the
reactivity via lowering the redox potential of the coordinated
The dioxygenation of the complexes were followed by
monitoring the disappearance of the band due to the π → π*
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structural and functional ES model system, providing important
insights into the metal ion effects on the enzymatic reactivity of
metal-substituted 2,3-QD.
Table 5. Summary of the Kinetic Data for the Metal−
Flavonolate Model Complexes
complexes
T (°C)
102k (M−1 s−1
2.05
)
ref
Catalytic Mechanism. Importantly, when the complexes
[MIIL(fla)] were treated with O2 in the presence of excess nitro
blue tetrazolium (NBT2+), a new absorption band arose at 530
nm (Figure 5−blue line), indicating that [MIIL(fla)] reacted
[MnL(fla)] (1)
[MnIIL1(fla)]
70
80
this work
13c
0.82
50.0
179
[Mn(fla)2Py2]
100
70
11
[FeIIL(fla)] (2)
[FeIIL1(fla)]
this work
13c
with O2 to produce free O2 (superoxide radical) and NBT2+
•−
80
58.6
80.0
2.07
34.0
2.09
36
was reduced to monoformazan (MF+) by the generated O2
.
•−
[FeIII(fla)3]
100
100
70
11
[FeIII(fla)(salen)]
[CoIIL(fla)] (3)
[CoIIL1(fla)]
9b
this work
13c
80
[CoIII(fla)(salen)]
25
v = 5.50 × 10−6 M s−1
12
[NiL(fla)] (4)
70
7.11
0.43
113
this work
13c
[NiIIL1(fla)]
80
[CuL(fla)] (5)
[CuIIL1(fla)]
[CuII(Bz-tac)(fla)]ClO4
[CuII(fla)(ipr-tac)]ClO4
[CuII(fla)(idpaH)]ClO4
[CuII(fla)2]
70
this work
13c
80
1.34
0.331
0.151
0.613
1.57
2.40
2.02
9.50
18.3
3.37
0.311
120
120
100
100
80
34a
34a
34b
34c
[CuII(fla)2(tmeda)]
[CuII(fla)2(bpy)]
[CuII(fla)2(phen)]
[CuII(fla)(phen)2]ClO4
[ZnL(fla)] (6)
34d
Figure 5. Spectral change of [FeIIL(fla)] (0.5 mM in DMF) under N2
(black) and in the presence of NBT under N2 (red) and O2 (blue) at
rt.
80
34d
80
34d
120
70
34d
this work
14
The ESI-MS and ESR results of the complexes obtained after
exposure to air or O2 and after the reaction with O2 were
summarized in Table 6. When the solution of [FeIIL(fla)] (2)
and [CoIIL(fla)] (3) were exposed to air and O2, we initially
observed a peak cluster due to [FeIIIL(fla)]+ at m/z (pos.) =
625.2 (Supporting Information Figure S3b) and [CoIIIL(fla)]+
at m/z (pos.) = 628.2 (Supporting Information Figure S13b),
respectively, and a typical mononuclear FeIII signal with g =
8.02, 4.24, and 2.20 (Supporting Information Figure S4a-B and
S4b), indicating that the Fe(II) and Co(II) were oxidized by O2
to FeIII and CoIII at the first stage of the reaction. After the
[FeIIL(fla)] solution was exposed to O2 for 5 min, about 87%
FeII was converted to FeIII (Figure S4b) based on the peak area
relative to the external standard [FeIIILOMe(sal2−)] (LOMe: p-
OMe substituted at the benzoic side arm of the ligand L, sal:
salicylate, unpublished results), which also has a similar
mononuclear distorted octahedral structure (characterized by
single crystal X-ray diffraction, EA, ESI-MS, et al.). After the
reaction with O2, we observed a peak cluster due to
[FeIIIL(sal2−)]H+ (sal: salicylate, one of the oxidation products)
at m/z (pos.) = 525.3 for [FeIIL(fla)] (2) and [CoIIIL(ben)]+
(ben: benzonate, one of the oxidation products) at m/z (pos.)
= 512.1 for [CoIIL(fla)] (3), respectively, and a typical
mononuclear Fe(III) signal at g = 7.66, 4.27, and 2.06
(Supporting Information Figure S4a-C), and the weak CoII
EPR signal disappeared (Supporting Information Figure S4e-
B).
[ZnII(fla)(idpa)] ClO4
100
flavonolate by electron-donation. Notably, the reactivity of
[FeIIL(fla)] and [CuIIL(fla)] were quite high (k = 1.79 and 1.13
M−1 s−1 at 70 °C, respectively) over the other divalent metal
complexes. Maybe it is because the Fe(II)-complex is easily
auto-oxidized to form more Lewis acidic and reactive Fe(III)-
complex during reaction with O2, and in [CuIIL(fla)] the
flavonolate ligand is coordinated via more reactive mono-
dentate coordination mode (based on EPR results) and the
Lewis acidity of the Cu(II) is stronger than others.
Although the structures of the ES-model complexes [MIIL-
(fla)] are similar, the reactivity of the [MIIL(fla)] complexes
toward dioxygen were significantly different as in the order of
Fe (2) > Cu (5) > Co (3) > Ni (4) > Zn (6) > Mn (1) (Figure
4), which is just reverse with the order of oxidation potential
Epa of the coordinated flavonolate of the complexes, and the
plot of k vs Epa is linear (R = 0.98) (Figure 4d). This order is
also approximately consistent with the Lewis acidity order of
the metal ion (ZnII < CuII > NiII > CoII > FeII > MnII) and the
Irving−Williams series except FeII and CoII. This tendency is
similar to those seen in the enzyme system6a and other
enzymatic and nonenzymatic systems such as RNA catalysis35a
and metal ion catalysis for the decomposition of tetramethyl-
1,2-dioxetane.35b The observed differences in the reactivity
among them may be attributed to the Lewis acidity of the metal
ion and its coordination environment. Maybe the metal(II) ion
could be to adjust the orientation of the bound substrate
flavonolate and the coordination environment of the MII ion, to
control the redox potential of the flavonolate, and to stabilize
the flavonoxyl radical electrostaticly, which is similar to the
enzymatic system.6b These results indicate that the stronger
Lewis acidity of the metal ion, the lower oxidation potential Epa
of the coordinated flavonolate and the higher dioxygenation
reactivity. Our study is the first example of the metal ion effects
on the enzyme-like dioxygenation reactivity of a series of
In order to get insight into the nature of the Fe(III)- and
Co(III)-complexes that initially formed upon reaction with O2,
the Fe(III) and Co(III) analogues [MIIIL(fla)](OAc) (M = Fe
(2A), Co (3A)) were synthesized independently and
characterized (Supporting Information page S4: synthesis of
the MIII-complexes [MIIIL(fla)](OAc) (M = Fe (2A), Co
(3A)); Supporting Information Table S4 and Figure S12 and
S13). The λmax of the π → π* transition of the coordinated
flavonolate of the complexes are 406 nm for 2A (Supporting
Information Figure S12a) and 425 nm for 3A (Supporting
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Table 6. ESI-MS and ESR Results of the Complexes when Exposed to Air or O2 and After the Reaction with O2
ESI-MS (m/z (pos.))
after reaction
EPR (g, A)
exposed to air/O2
exposed to O2
after reaction
[MnIIL(fla)] (1)
[FeIIL(fla)] (2)
[CoIIL(fla)] (3)
[CuIIL(fla)] (5)
536.3 ([MnIIL(dmb)]+)
g = 2.06, A = 90.5 G, g = 6.12, 16.4
g = 7.66, 4.27, 2.06
625.2 ([FeIIIL(fla)]+)
628.2 ([CoIIIL(fla)]+)
525.3 ([FeIIIL(sal2−)]H+)
512.1 ([CoIIIL(ben)]+)
g = 8.02, 4.24, 2.20
no signal
565.4 ([CuIIL(sal−)] H+·CH3OH)
g// = 2.26, g⊥ = 2.07, A// = 195 G
Scheme 3. Proposed Dioxygenation Reaction Mechanism of the Model Complexes [MIIL(fla)]
Information Figure S13a), respectively. Both 2A and 3A are 8
nm red-shifted than those of the corresponding M(II)
analogues [MIIL(fla)] (398 nm for 2 and 417 nm for 3),
respectively. The wavelength of [FeIIIL(fla)](OAc) (2A) is
similar to the initially formed [FeIIIL(fla)]+. The magnetic
moments of [MIIIL(fla)](OAc) are 5.76 μB for 2A and 0.45 μB
for 3A, respectively, which are in good agreement with the
calculated values using the spin-only equation of the high-spin
FeIII (5.92 μB) and low-spin CoIII (0.00 μB) center, respectively.
These results are consistent with the solution ESR results. The
magnetic moment of [FeIIIL(fla)](OAc) is similar to that of the
initially formed Fe(III)-complex (5.79 μB) upon reaction with
O2. These results indicate that [FeIIL(fla)] and [CoIIL(fla)] are
oxidized to high-spin [FeIIIL(fla)]+ and low-spin [CoIIIL(fla)]+,
respectively, at the first stage during the reaction with O2, and
keep a similar mononuclear structure in solution. The reactivity
of [MIIIL(fla)](OAc) toward dioxygen were also examined. The
plots of the initial reaction rate versus the initial concentration
of both [MIIIL(fla)](OAc) and dioxygen are also linear,
respectively (Figure S12 for 2A and Figure S13 for 3A). So
the rate law also can be described as −d[MIIIL(fla)(OAc)]/dt =
k[MIIIL(fla)(OAc)][O2]. The second-order reaction rate
constants k of [MIIIL(fla)](OAc) are 133 × 10−2 M−1 s−1
(ΔH‡ = 109 kJ mol−1, ΔS‡ = −52 J mol−1 K−1) for 2A at 50 °C
and 71.2 × 10−2 M−1 s−1 (ΔH‡ = 111 kJ mol−1, ΔS‡ = −75 J
mol−1 K−1) for 3A at 70 °C (Supporting Information Table S3
and Figures S12 and S13), which are about 2−3 times larger
than those of the corresponding M(II) analogues [MIIL(fla)]
(39.8 × 10−2 M−1 s−1 for 2 and 34.0 × 10−2 M−1 s−1 for 3,
respectively) at the same temperature. The enhanced reactivity
of [MIIIL(fla)](OAc) could be attributed to the enhanced
Lewis acidity of the M(III) ion and their coordination
environment.
After the reaction of [MnIIL(fla)] (1) and [CuIIL(fla)] (5)
with O2, we observed a peak cluster due to [MnIIL(dmb)]+
(dmb: N,N-dimethyl-benzamide, one of the amidated oxidation
product) at m/z (pos.) = 536.3 and [CuIIL(sal−)]H+·CH3OH
at m/z (pos.) = 565.4, respectively, and a typical sextet
mononuclear MnII signal with the EPR parameters of g = 2.06
(A = 90.5 G), g = 6.12, and 16.4 (Supporting Information
Figure S4d-B) and g// = 2.26, g⊥ = 2.07, A// = 195 G (Figure
S4c-B), respectively.
In the MnII-, NiII-, and ZnII-complexes, we did not observe
any metal ion redox couple during CV measurements even
under O2. When the complexes were exposed to O2 or reacted
with O2, we did not observe any oxidation state change on ESI-
MS and EPR. These results indicate that the MnII-, NiII-, and
ZnII-complexes are stable toward O2 in solution and keep their
oxidation state during reaction.
On the basis of the spectroscopic and kinetic results, we
suggest the dioxygenation mechanism of [MIIL(fla)] as follows
(Scheme 3).
FeII- and CoII-complexes [MIIL(fla)] (M = Fe and Co)
(Scheme 3−route a): [MIIL(fla)] may react with one O2
•−
molecule to produce [MIIIL(fla)]+ (A) and O2 quickly.
Then [MIIIL(fla)]+ reacts with another O2 molecule to generate
flavonoxy radical [MIIIL(fla•)] (B) and superoxide radical
slowly by direct one electron transfer from the coordinated
substrate fla− to O2, which are similar to the enzymatic
system.6a Then B reacts with O2 quickly and releases CO.
•−
MnII-, NiII-, and ZnII-complexes [MIIL(fla)] (M = Mn, Ni,
and Zn) (Scheme 3−route b): The reaction mechanism of
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these complexes are similar to FeII- and CoII-complexes except
the first step, and there is direct one electron transfer from the
activated flavonol (fla−) to dioxygen to generate flavonoxy
radical [MIIL(fla•)] (B) and superoxide radical slowly.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
For the copper(II)-complex [CuIIL(fla)] (Scheme 3−route
c), there is a fast equilibrium between [CuIIL(fla)] and
[CuIL(fla•)] (C), which is more favorable than dioxygen
activation. Then [CuIL(fla•)] react with one O2 molecule to
form [CuIIL(fla•)] (B) and superoxide radical slowly by direct
■
We thanks a lot Prof. Sugimoto Hideki (Osaka Univ., Japan)
for his kind help in crystal structure analysis. We are gratefully
acknowledged the financial support of the National Natural
Science Foundation of China (No. 20641002, 20771020,
20811140328) and the postdoctoral fellowship of JSPS
(Japan Society for the Promotion of Science).
•−
one electron transfer from CuI to O2. Then B reacts with O2
quickly and releases CO.
The reaction mechanism of [CuIIL(fla)] is similar to the
CuII-containing 2,3-QD,2−4 and the reaction mechanism of
[MIIL(fla)] (M = Fe, Mn, Co, and Ni) are similar to the
Fe(II)-, Mn(II)-, Co(II)-, and Ni(II)-containing 2,3-QD,
respectively.5,6 It is very similar to the intradiol and extradiol
catechol dioxygenase, respectively.37
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CONCLUSIONS
■
In conclusion, we succeeded in developing a series of structural
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ASSOCIATED CONTENT
■
S
* Supporting Information
1
The FT-IR, UV−vis, ESI-MS, EPR, H NMR, CV, LC-MS
spectra, Eyring plot, reaction products analysis data, kinetic
data, and crystallographic information (CIF) CCDC 840052
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: yingjis@dlut.edu.cn.
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