Regioselective hydroxylation of the xylyl linker in a diiron(III) complex having a carboxylate-rich ligand with H2O2

Article abstract of DOI:10.1039/b304171a

Reaction of a diiron(III) complex having a xylta^4- ligand (N,N,N′,N′-m-xylylenediamine tetraacetate) with H₂O₂ resulted in regioselective hydroxylation of the m-xylyl linker. The reaction mimics the self-hydroxylation of a

Full text of DOI:10.1039/b304171a

Regioselective hydroxylation of the xylyl linker in a diiron(III) complex
having a carboxylate-rich ligand with H O †
2
2
a
a
a
a
a
Hideki Furutachi, Mizue Murayama, Amane Shiohara, Satoshi Yamazaki, Shuhei Fujinami,
a
a
b
c
d
Akira Uehara, Masatatsu Suzuki,* Seiji Ogo, Yoshihito Watanabe and Yonezo Maeda
Department of Chemistry, Faculty of Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192,
Japan. E-mail: suzuki@cacheibm.s.kanazawa-u.ac.jp
a
b
c
d
Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka
565-0871, Japan
Department of Chemistry, Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602,
Japan
Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higasi-ku, Fukuoka 812-0053,
Japan
Received (in Cambridge, UK) 15th April 2003, Accepted 10th June 2003
First published as an Advance Article on the web 25th June 2003
Reaction of a diiron(III) complex having a xylta4 ligand
2
electronic spectral change showed that the reaction quickly
(
N,N,NA,NA-m-xylylenediamine tetraacetate) with H
2
O
2
re-
proceeds within several seconds at 0 °C and no detectable
7
sulted in regioselective hydroxylation of the m-xylyl linker.
The reaction mimics the self-hydroxylation of a phenyl-
alanine side chain found for ribonucleotide reductase (R2-
W48F/D84E).
intermediate was observed at present stage (Fig. S2).† The
ESI-TOF/MS spectrum of the solution diluted by acetonitrile
(1 : 1) showed two signals at m/z (%): 608.9652 (100) and
506.9423 (29) attributable to the hydroxylated species having a
2
(
m-phenoxo)diiron(III) core {[Fe
accurate mass : m/z = 608.9743) and a {[Fe
(accurate mass : m/z = 506.9426), respectively, together with a
2
(xylta-O)(CH
3
CO
2
)
2
]} (1,
2
Hydroxylation of alkanes and arenes catalyzed by iron com-
plexes is of current interest for understanding the dioxygen
activation mechanisms of non-heme diiron proteins such as
methane monooxygenase, ribonucleotide reductase, and fatty
acid desaturase.1 Recently, self-hydroxylation of a phenyl-
alanine side chain, which is located in close proximity to the
2
(xylta-O)(O)]}
2
2 3 2
signal of {[Fe (xylta)(O)(CH CO )]} (I = 27%) (Fig. S1).†
Addition of a methanol solution of (n-Bu) NBr into the red
aqueous solution afforded red crystals (n-Bu) N[Fe (xylta-
O)(CH CO ]·3H O (1·Bu N) (yield = 62%) suitable for X-
4
,2
4
2
3
2
)
2
2
4
diiron center, has been reported for ribonucleotide reductase
ray crystallography (Fig. 1).‡ The crystal structure of 1 clearly
shows that the xylyl linker of xylta is hydroxylated and the
3
42
(R2-W48F/D84E). Many model compounds have been devel-
oped and their reactivity toward various substrates has been
investigated.2 Even though excellent systems for hydroxyla-
tion of arenes have been developed for the copper complexes,2
only a limited number of examples for efficient hydroxylation
of arene groups by iron complexes have been known. The
hydroxylation of a phenyl group in the ligand was reported for
some diiron complexes having a carboxylate-rich coordination
resulting phenolate oxygen bridges the two iron atoms. Each
,4
iron atom has a distorted octahedral geometry with the N
donor set and is triply bridged by the phenolate oxygen and two
acetate groups providing a bis(m-acetato)(m-phenoxo)diiron(III
core as found for a closely related complex Me N[Fe (5Me-
1 5
O
,5
)
6
4
2
8
HXTA)(CH
Isotope labeling experiments using H
showed that the phenolate oxygen of 1 comes from the
3 2 2
CO ) ].
18O
(in H
16O)
2
2
2
environment such as edtp in their reactions with either H
2
O
2
or
6a
18
2
O
2
.
It was also found that the reaction of a mononuclear
2 3 2 2
hydrogen peroxide ({[Fe (xylta- O)(CH CO ) ]} (m/z (%):
t
iron(II) complex of 6Ph-tpa (N
4
-donor set) with BuOOH
610.9687 (100)) (Fig. S3).† The yield and regioselectivity for
6b
1
hydroxylates a phenyl pendant of the supporting ligand. Since
the diiron centers of the above enzymes share carboxylate-rich
coordination environment, it is of particular interest to in-
vestigate the oxidation reactions of diiron complexes having
carboxylate-rich ligands. Herein, we report a regioselective
hydroxylation of the xylyl linker were assessed by the H NMR
spectrum after the reaction mixture was treated by sodium
1
dithionite and potassium cyanide. The H NMR spectrum of the
reaction mixture clearly showed only two sets of signals arising
from xylta4 and the oxidized ligand (xylta-O ), and there was
2
52
hydroxylation of a xylyl linker by the reaction of a diiron(III
)
(±2)% of xylta4 was converted into xylta-O (H
2
52
complex bearing carboxylate-rich coordination environment
(
N,N,NA,NA-m-xylylenediamine tetraacetate: xylta4
2
)
with
2 2
H O
, where two iron(III) ions are linked by a xylyl group. The
reaction mimics the self-hydroxylation of a phenylalanine side
chain found for R2-W48F/D84E.3
To an aqueous suspension containing 1 equiv. of H
equiv. of Et N, and 2 equiv. of FeCl was added 6 equiv. of
sodium acetate to generate a dark yellowish green solution
Scheme 1). The negative ion ESI-TOF/MS spectrum of the
4
xylta, 4
3
3
(
solution diluted by acetonitrile (1 : 1) in the range of m/z = 20
to 1000 showed a signal at m/z (%): 550.9614 (100) with a
groups (pyxyl) afforded a very different tetranuclear iron(III)
characteristic distribution of isotopomers attributable to a
2
diiron(III) species {[Fe
2
(xylta)(O)(CH
3
CO
2
)]} (accurate mass
:
m/z = 550.9688) (Fig. S1).† All attempts to crystallize this
species were in vain so far. Addition of 5 equiv. of H to the
solution at 0 °C resulted in a rapid color change to red. The
2 2
O
†
Electronic supplementary information (ESI) available: experimental
1
details, ESI-mass, UV–vis, and H NMR spectra. See http://www.rsc.org/
suppdata/cc/b3/b304171a/
Scheme 1 Reaction scheme for iron(III) complexes with H
2 2
O .
1
900
CHEM. COMMUN., 2003, 1900–1901
This journal is © The Royal Society of Chemistry 2003

curred at the 2-position of the xylyl linker in xylta4 appears to
be mostly due to the proximity effect of the metal center
depending upon their core structures in solution.
2
In summary, we have succeeded for the first time in the
regioselective hydroxylation of the xylyl linker of the carbox-
ylate-rich ligand (xylta4 ) by a Fe /CH
2
3+
CO /H O /H O
2
3 2 2 2
2
system. The reaction mimics the self-hydroxylation of a
phenylalanine side chain found for R2-W48F/D84E. Un-
fortunately, no intermediate was observed for the present
system due to the rapid hydroxylation of xylta4 , although a
peroxo intermediate has been detected for R2-W48F/D84E
system. The present ligands xylta4 and pyxyl afford two types
of the intramolecular diiron and intermolecular tetrairon
complexes, depending upon the side arms of the xylyl linker,
whose core structures seem to be responsible for hydroxylation
of the xylyl linker in this system.
2
2
Financial support of this research by the Ministry of
Education, Science, and Culture Grant-in-Aid for Scientific
Research to M. S. and Y. W. is gratefully acknowledged.
4
Fig. 1 ORTEP view (40% probability) of a complex anion of 1·Bu N.
Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and
angle (°): Fe1…Fe2 3.458(1), Fe1–O1 2.017(1), Fe1–O2 1.982(2), Fe1–O4
Notes and references
1
2
2
.958(2), Fe1–O10 2.045(2), Fe1–O12 1.975(1), Fe1–N1 2.168(2), Fe2–O1
.029(1), Fe2–O6 1.981(1), Fe2–O8 1.955(2), Fe2–O11 1.970(2), Fe2–O13
.035(2), Fe2–N2 2.174(2); Fe1–O1–Fe2 117.43(6).
‡ Crystal data for 1·Bu
group P2 /a with Z = 4, a = 15.986(5), b = 17.475(3), c = 15.193(3) Å,
b = 91.69(2)°,V = 4243(1) Å , rcalcd = 1.418 g cm , R = 0.040, R
.061 for 6979 data with I > 3s(I). Crystal data for 2 at 2120 °C;
13Cl Fe 27, orthorhombic, space group Pnma with Z = 4, a =
4.961(2), b = 28.608(3), c = 12.617(4) Å, V = 9009(2) Å , rcalcd = 1.448
4
63 3 2
N at 2120 °C; C36H N Fe O16, monoclinic, space
1
3
23
w
=
0
C
2
75
H
83
N
4
4
O
4 2 2 3 2 4
complex, [Fe (O) (pyxyl) (CH CO )
]4+ (2), whose crystal
3
structure was determined by X-ray crystallography (Fig. 2).‡
Complex 2 has a dimer of dimer structure where two (m-
oxo)bis(m-acetato)diiron(III) cores are linked by a xylyl linker as
23
g cm , R = 0.064, R
w
= 0.106 for 6628 data with I > 3s(I). CCDC
2
00159 and 200160. See http://www.rsc.org/suppdata/cc/b3/b304171a/ for
crystallographic data in .cif format.
9
found for closely related complexes. The reaction of 2 with
H
2
O
2
in CH
3
CN/H
2
O (1 : 1) resulted in no hydroxylation of the
1 (a) M. Merkx, D. A. Kopp, M. H. Sazinsky, J. L. Blazyk, J. Müller and
S. J. Lippard, Angew. Chem., Int. Ed., 2001, 40, 2782; (b) B. J. Wallar and
J. D. Lipscomb, Chem. Rev., 1996, 96, 2625; (c) E. I. Solomon, T. C.
Brunold, M. I. Davis, J. N. Kemsley, S.-K. Lee, N. Lehnert, F. Neese, A.
J. Skulan, Y.-S. Yang and J. Zhou, Chem. Rev., 2000, 100, 235.
xylyl linker of the pyxyl ligand and only original ligand was
recovered, which was confirmed by the ligand recovery
experiment. The results show a sharp contrast with those of the
xylta4 system. The reactivity of xylyl linkers in the xylta and
2
42
2
Bioinorganic Catalysis, 2nd edn,. eds. J. Reedijk and E. Bouwman,
Marcel Dekker, New York, 1999; Biomimetic Oxidations Catalyzed by
Transition Metal Complexes, ed. B. Meunier, Imperial College Press,
London, 1999; Metal-oxo and Metal-Peroxo Species in Catalytic
Oxidations, ed. B. Meunier, Springer, Berlin, 2000.
pyxyl ligand seems to be regulated by their structures in
solution; ESI-TOF/MS spectroscopy of 2 in CH CN revealed
3
that the tetranuclear unit remains intact (Fig. S5).† The crystal
structure of 2 indicates that the closest carbon atom of the xylyl
linker is apart from the diiron core by ~ 5 Å, which is too far for
3 J. Baldwin, W. C. Voegtli, N. Khidekel, P. Moënne-Loccoz, C. Krebs, A.
S. Pereira, B. A. Ley, B. H. Huynh, T. M. Loehr, P. J. Riggs-Gelasco, A.
C. Rosenzweig and J. M. Bollinger Jr., J. Am. Chem. Soc., 2001, 123,
the hydroxylation. In contrast, the hydroxylated carbon atom of
the xylyl linker in xylta42 is expected to be close to the diiron
7
017.
core as postulated for the hydroxylation of [Cu
2
(XYL–H)]2
+
4
M. Fontecave, S. Ménage and C. Duboc-Toia, Coord. Chem. Rev., 1998,
with dioxygen.5a Thus the regioselective hydroxylation oc-
1
2
78–180, 1555; M. Costas, K. Chen and L. Que Jr., Coord. Chem. Rev.,
000, 200–202, 517; K. Chen, M. Costas and L. Que Jr., J. Chem. Soc.,
Dalton Trans., 2002, 672.
5
(a) K. D. Karlin, Y. Gultneh, J. C. Hayes, R. W. Cruse, J. W. McKown,
J. P. Hutchinson and J. Zubieta, J. Am. Chem. Soc., 1984, 106, 2121; E.
Pidcock, H. V. Obias, C. X. Zhang, K. D. Karlin and E. I. Solomon, J. Am.
Chem. Soc., 1998, 120, 7841; (b) Bioinorganic Chemistry of Copper, eds.
K. D. Karlin and Z. Tyeklár, Chapman & Hall, New York, 1993.
(a) S. Ménage, J.-B. Galey, G. Hussler, M. Seité and M. Fontecave,
Angew. Chem., Int. Ed. Engl., 1996, 35, 2353; S. Ménage, J.-B. Galey, J.
Dumats, G. Hussler, M. Seité, I. G. Luneau, G. Chottard and M.
Fontecave, J. Am. Chem. Soc., 1998, 120, 13370; (b) S. J. Lange, H.
Miyake and L. Que Jr., J. Am. Chem. Soc., 1999, 121, 6330; M. P. Jensen,
S. J. Lange, M. P. Mehn, E. L. Que and L. Que Jr., J. Am. Chem. Soc.,
6
7
2
003, 125, 2113.
The electronic spectral change for the reaction of 80 equiv. of H
a yellowish green aqueous mixture ( ~ 0.83 mM/[Fe ]) containing
xylta, Et N, FeCl , and CH CO Na (1 : 4 : 2 : 6) was very similar to
that for the reaction of 1 with 80 equiv. H in the presence of 4 equiv.
of CH CO Na at 0 °C (Fig. S2),† suggesting that both changes are due to
2 2
O with
2
H
4
3
3
3
2
2 2
O
3
2
the same reaction. The spectral change of 1 seems to be the formation of
a peroxo species having a phenolate bridge similar to that generated from
a closely related diiron(III) complex.8 Therefore the hydroxylation of
xylta4 seems to proceed very rapidly and the observed spectral change
Fig. 2 ORTEP view (40% probability) of a complex cation of 2. Hydrogen
atoms are omitted for clarity. Selected bond distances (Å) and angle (°):
Fe1…Fe2 3.117(1), Fe1…Fe1* 9.336(1), Fe1…Fe2* 10.259(1),
Fe2…Fe2* 9.614(1), Fe1–O1 1.791(3), Fe1–O2 2.073(3), Fe1–O4
2
2 2
may be due to the reaction of 1 with H O .
2
1
2
.038(3), Fe1–N1 2.246(3), Fe1–N2 2.173(3), Fe1–N3 2.177(4), Fe2–O1
.800(3), Fe2–O3 2.022(3), Fe2–O5 2.030(3), Fe2–N4 2.211(3), Fe2–N5
.200(3), Fe2–N6 2.141(3); Fe1–O1–Fe2 120.4(1). The atoms asterisked
8 B. P. Murch, F. C. Bradley and L. Que Jr., J. Am. Chem. Soc., 1986, 108,
5027.
9 H. Toftlund, K. S. Murry, P. R. Zwack, L. F. Taylor and O. Anderson, J.
Chem. Soc., Chem. Commun., 1986, 191; J. L. Sessler, J. D. Hugdahl, V.
Lynch and B. Davis, Inorg. Chem., 1991, 30, 334.
are those of the counter part of the dimer and generated by the symmetry
transformations: x, 1/2 2 y, z.
CHEM. COMMUN., 2003, 1900–1901
1901