1.232(3) A].13 Due to coordination to the manganese ion there are
˚
Table 1 Kinetic data for the oxygenation of metal flavonolates
DH#/ DS#/
also changes in the bond lengths of the pyranone ring. The O1–C2
˚
˚
ka/M21 s21 kJ mol21 J mol21 K21
[1.374(3) A] and C10–C15 [1.393(3) A] bond lengths become
longer, and the C1–C9 bond length [1.460(3) ] is somewhat shorter,
which may be assigned to delocalization of the p-system over the
whole molecule.
2
0.08
0.50
49
40
53
—
2137
2144
2138
—
This work
This work
1
CuII(fla)2
0.0087
9
—
CuII(fla)2(py)n 0.04
Tris(ethoxo)iron(III)
reacted
with
49-methoxyflavonol
a
(49MeOflaH) in acetonitrile at room temperature to give pure,
homoleptic tris(49-methoxyflavonolato)iron(III) (2) in 85% yield. It
shows absorption in the visible region at 411 nm, and n(CO)
absorption band at 1547 cm21 in the IR spectrum assignable to the
flavonolate ligand, A shoulder at 680 and a maximum at 530 nm
characteristic of an octahedral arrangement around the ferric ion,
In DMF at 100 uC.
product permits identification of the place of the dioxygen
incorporation. As a result of oxygenations, carried out under an
atmosphere containing y60% 18,18O2, the 18O-benzoylsalicylic
acid derivative gave a molecular ion at m/z 260 (256 + 4), showing
that both 18O atoms of 18,18O2 are incorporated into the carboxylic
acid from the molecular oxygen, and the gas phases showed only
the presence of unlabeled CO [eqn. (2)]. The relative abundances of
m/z 260 to that at m/z 256 parallel the 18,18O2 enrichments used in
the experiments.
6
4
6
4
can be assigned to the A1g A T1g and A1g A T2g transition,
respectively. The crystal structure of 2, shown in Fig. 2{ together
with selected data, shows a distorted octahedral geometry around
the iron(III) center, with all coordination sites being occupied by
the bidentate 49-methoxyflavonolate ligands. The iron–oxygen
˚
bond distances are in the range of 1.955–2.109 A, somewhat longer
than those in CuII(fla)2, but somewhat shorter than those in
MnII(fla)2(py)2. Complex 2 crystallizes as two independent
molecules with each Fe cation lying on a three-fold axis.
After the characterization of the prepared complexes, their
flavonol 2,4-dioxygenase activity was examined. The manganese
and iron flavonolate complexes 1 and 2, in DMF solutions are
stable under anaerobic conditions and oxidized upon addition of
dioxygen at 95 uC. The CO content was determined in both cases
by GC-MS (80–90%). The formation of O-benzoylsalicylic acid
from flavonol requires dioxygen but no apparent dioxygen uptake
was observed since the absorption of dioxygen and the liberation
of carbon monoxide compensate each other. The GLC-MS
analysis of the residue of the hydrolyzed complexes, after treatment
with ethereal diazomethane, showed the presence of the
O-benzoylsalicylic acid methylester. Provided that a mixture of
18,18O2 and 16,16O2 is used in the oxygenation, the labelling of the
ð2Þ
Reactions of 1 and 2 with dioxygen were performed in DMF
solutions at 85–120 uC, and the concentration change of 1 and 2
was followed by electronic spectroscopy measuring the absorbance
of the reaction mixture at 432 and 411 nm. Kinetic studies on the
oxygenation of the manganese and iron flavonolate complexes
established second-order overall rate expressions –d[1 or 2]/dt = [1
or 2][O2], and both reactions were entropy driven (Table 1)
indicating that the rate-determining step is bimolecular.
As a conclusion it can be said that in the enzyme-like oxygena-
tion of the coordinated flavonolate ligand of manganese(II) and
iron(III) (1, 2), the formation of endoperoxide (3) in bimolecular
reactions can be assumed, and the unique decomposition of this
species accompanied by loss of carbon monoxide results in the
corresponding O-benzoylsalicylate complexes (4) as a good mimic
of the enzyme action. On the basis of the k values (compared to
our earlier copper-containing systems), it can be said that the
reactivity order is Fe . Mn . Cu. Work is still in progress on
model studies to disclose more details of this cleavage reaction.
We thank the Hungarian Research Fund (K-67871), Ce´lker and
Budaconsum Ltd for financial support.
Notes and references
{ Intensity data were measured on a Bruker-Nonius Kappa CCD single-
crystal diffractometer, using graphite-monochromated Mo-Ka radiation
˚
(l = 0.71073 A) and w scan technique at 293 K. The structures were solved
by direct and difmap methods (SIR92),14 and refined on F2 by using full-
matrix least-squares methods.15
Crystal data: Compound 1: C40H28MnN2O6, Mw = 687.58, triclinic,
¯
˚
space group P1, a = 8.310(6), b = 10.095(6), c = 11.167(5) A, a =
3
˚
113.361(4), b = 97.542(4), c = 106.255(4)u, V = 793.82(8) A , Z = 1, Dc =
Fig. 2 The molecular structure of one of the two independent co-
˚
1.438 g cm23, m(Mo-Ka) = 4.70 cm21, 2882 reflections measured, 223
crystallized molecules of 2 with selected bond distances (A) and angles (u):
parameters refined on F2 using 2667 unique reflections to final indices
Fe1–O1 2.109(8), Fe1–O2 1.955(7), O1–C1 1.306(12), O2–C9 1.299(13),
O3–C7 1.302(13), O3–C8 1.373(13), C1–C9 1.367(14), C1–C2 1.490(15),
C2–C7 1.454(16), C8–C9 1.404(15); O2–Fe1–O1 80.0(3), O1–C1–C9
120.1(9). Ellipsoids are shown at the 50% probability level.
R [F2 . 2s(F2)] = 0.0444, wR = 0.1354, w = 1/[s2(Fo ) + (0.0912P)2
+
2
0.3239P], P = (Fo + 2Fc2)/3.
Compound 2: C48H33FeO12, Mw = 857.59, trigonal, space group P31c,
2
3
˚
˚
a = 15.488(5), b = 15.488(5), c = 19.997(5) A, c = 120(4)u, V = 4154.2(2) A ,
5236 | Chem. Commun., 2007, 5235–5237
This journal is ß The Royal Society of Chemistry 2007