Trigonal-bipyramidal geometry induced by an external water ligand in a sterically hindered iron salen complex, related to the active site of protocatechuate 3,4-dioxygenase

Article abstract of DOI:10.1021/ic060650p

A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces the endogenous His₂Tyr₂ donor set with water as an external ligand. X-ray crystal structures of a series of iron model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies with the external ligand that is bound in the order Cl ? EtO < H ₂O, which is consistent with the spectrochemical series. The distortion in the model system is not due to steric repulsions but electronic interactions between the external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.

Full text of DOI:10.1021/ic060650p

Inorg. Chem. 2006, 45, 7709−7721
Trigonal-Bipyramidal Geometry Induced by an External Water Ligand in
a Sterically Hindered Iron Salen Complex, Related to the Active Site of
Protocatechuate 3,4-Dioxygenase
Takuya Kurahashi,† Kenji Oda,‡ Manabu Sugimoto,§ Takashi Ogura,‡ and Hiroshi Fujii*,†
Institute for Molecular Science & Okazaki Institute for IntegratiVe Bioscience, National Institutes
of Natural Sciences, Myodaiji, Okazaki 444-8787, Japan, and Department of Life Science,
Graduate School of Life Science, UniVersity of Hyogo, Koto 3-2-1, Kamigori, Ako, Hyogo
678-1297, Japan, and Graduate School of Science and Technology, Kumamoto UniVersity,
Kumamoto 860-8555, Japan
Received April 17, 2006
A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-
dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces
the endogenous His2Tyr2 donor set with water as an external ligand. X-ray crystal structures of a series of iron
model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted
trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural
change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies
with the external ligand that is bound in the order Cl
, EtO < H2O, which is consistent with the spectrochemical
series. The distortion in the model system is not due to steric repulsions but electronic interactions between the
external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model
complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density
functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal
geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique
distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically
induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by
the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.
Introduction
for this class of enzymes, which reveal that quite a number
of them share a basic coordination motif around the iron
center. For example, naphthalene 1,2-dioxygenase, which
catalyzes the cis-dihydroxylation of aromatic double bonds,
contains a mononuclear iron center coordinated by His208
and His213, bidentately by Asp362, and by a water molecule
in a framework comprised of a distorted octahedral bipyramid
with one unoccupied coordination site.3 Other oxygen-
activating mononuclear non-heme iron enzymes, such as
R-keto acid- and pterin-dependent enzymes, exhibit a similar
octahedral iron center that is facially coordinated by two His,
one Asp or Glu, and water molecules.2 The His2Asp/Glu
coordination motif might be required for the generation of a
Nature has evolved to produce a variety of metalloen-
zymes, which play pivotal roles in cooperation with each
other.1 Each metalloenzyme is believed to be specifically
designed to carry out a particular chemical reaction assigned
to it. Insight into active site structures and the resulting
reactions is thus of prime importance from the bioinorganic
perspective. In particular, structure-reactivity relationships
for mononuclear non-heme iron enzymes are one of the most
active areas in recent years.2 Fueling this interest is an
explosion in the number of X-ray crystal structures available
* To whom correspondence should be addressed. E-mail: hiro@ims.ac.jp.
† National Institutes of Natural Sciences.
‡ University of Hyogo.
(2) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004,
104, 939-986.
(3) Kauppi, B.; Lee, K.; Carredano, E.; Parales, R. E.; Gibson, D. T.;
Eklund, H.; Ramaswamy, S. Structure 1998, 6, 571-586.
§ Kumamoto University.
(1) For example, see: Lippard, S. J.; Berg, J. M. Principles of Bioinorganic
Chemistry; University Science Books: New York, 1994.
10.1021/ic060650p CCC: $33.50
Published on Web 08/25/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 19, 2006 7709

Kurahashi et al.
ide but also the axial Tyr447 ligand, as shown in Figure 1.2,9
It is noteworthy that the iron coordination geometry is altered
from a distorted trigonal-bipyramidal to a square-pyramidal
geometry. The formation of a bidentate complex with the
structural change is critical for the oxidation of a catechol
substrate through a characteristic substrate-activation mech-
anism not an oxygen-activation mechanism, which is usually
employed by other mononuclear non-heme iron enzymes.10
Although details of the oxidation sequence for 3,4-PCD have
been studied, structure-function relationships for 3,4-PCD
remain unclear. In particular, it is not clear why 3,4-PCD
forms a unique distorted trigonal-bipyramidal geometry for
the iron-binding site and why the active-site structure is
altered during the substrate-binding process. A drastic shift
in the Tyr447 residue suggests the possibility that the
distorted trigonal-bipyramidal structure may be enforced by
the protein structure of 3,4-PCD, and the displacement of
the Tyr447 residue induces a structural change in the active
site to a square-pyramidal geometry.
Figure 1. Resting 3,4-PCD and the 3,4-PCD/catecholate complex, created
with coordinates obtained from PDB files 2PCD and 3PCA.
key oxoiron(IV) intermediate, as suggested from recent
model studies.4,5
In contrast, protocatechuate 3,4-dioxygenase (3,4-PCD),
which is classified as an intradiol catechol dioxygenase,
contains a distinctively different mononuclear non-heme iron
center,6 although an extradiol catechol dioxygenase, 2,3-
dihydroxybiphenyl 1,2-dioxygenase, falls into the category
of enzymes with a His2Asp/Glu active site.7 As shown in
Figure 1, the resting form of 3,4-PCD exhibits a unique
distorted trigonal-bipyramidal ferric iron center with four
endogenous protein ligands (3,4-PCD from Pseudomonas
putida; Tyr408, Tyr447, His460, and His462). Among these
ligands, Tyr447 and His462 are located in axial positions
and Tyr408 and His460 serve as equatorial ligands. The other
equatorial position is occupied by a solvent-derived ligand,
which has been shown to be hydroxide.8 A series of studies
of a range of inhibitor and substrate complexes of both wild-
type and mutant 3.4-PCD9 indicate that the oxidation
mechanism is also quite different from other oxygen-
activating mononuclear non-heme iron enzymes that contain
the His2Asp/Glu active site. The oxidation sequence is
initiated by the binding of a catechol substrate, which
involves the displacement of not only the equatorial hydrox-
To date, a number of model systems have been success-
fully constructed to mimic the structure and function of 3,4-
PCD.2,11-21 Particularly important in elucidating the oxidation
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7710 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
Figure 2. Sterically hindered mesityl iron salen complexes 1-4 and less-
hindered xylyl salen complexes 5-9.
mechanism of 3,4-PCD are structurally well-defined iron-
(III) catecholate complexes as models for the enzyme/
substrate complex. The reaction of several iron(III) catecho-
late complexes with O2 affords intradiol cleavage products
in high yield, supporting the proposed substrate-activation
mechanism.11-21 However, no model complex reproducing
the distorted trigonal-bipyramidal iron site in resting 3,4-
PCD was known until our previous report in 2002.22 As a
model for resting 3,4-PCD, we designed and prepared a
sterically hindered salen ligand and successfully obtained the
aqua complex 3 (Figure 2), which reproduces the endogenous
His2Tyr2 donor set with water as an external ligand for 3,4-
PCD reasonably well. As previously reported, complex 3
discards the square-pyramidal geometry usually preferred for
iron salen complexes and instead adopts a distorted trigonal-
Figure 3. 3-D views for X-ray crystal structures of (a) 1, (b) 2, and (c) 3.
Hydrogen atoms and incorporated solvent molecules are omitted for the
sake of clarity.
bipyramidal geometry that is closely related to the active
site in 3,4-PCD, while the geometry of the iron center in
the chlorine complex 1 is rather close to a usual square-
pyramidal structure. In relation to 3,4-PCD, it is interesting
to clarify the driving force for the conversion of a preferred
square-pyramidal to a distorted trigonal-bipyramidal geom-
etry in our model system. Herein, we report on an extensive
investigation, intended to clarify the electronic and steric
effects that lead to a distorted trigonal-bipyramidal geometry
in 3, using a series of sterically hindered mesityl iron salen
complexes 1-4 and another series of less-hindered xylyl
salen complexes 5-9. A close inspection of the X-ray crystal
structures of 3,4-PCD and 3,4-PCD-inhibitor complexes in
light of the present model study suggests a new possibility
that a unique distorted trigonal-bipyramidal iron site might
not be preorganized by a 3,4-PCD protein but could be
electronically induced upon the binding of an external
hydroxide ligand to the iron(III) center.
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1042.
Results
Effect of External Ligands on Distortion around the
Iron Center. Figure 3 shows 3-D views of the X-ray crystal
structures of 1-3. As reported in the previous paper,22
3
shows a unique distorted trigonal-bipyramidal geometry in
contrast to the typical square-pyramidal geometry for the
chlorine complex 1. Further inspection of the crystal structure
of 3 indicates that the Fe-O (water) bond length in 3 (2.009
(22) Fujii, H.; Funahashi, Y. Angew. Chem., Int. Ed. 2002, 41, 3638-
3641.
Inorganic Chemistry, Vol. 45, No. 19, 2006 7711

Kurahashi et al.
Table 1. Bond Distances (Å) and Bond Angles (deg) for 1-3a
1
2
3
Fe-X
2.234(3)
1.878(5)
1.881(6)
2.087(6)
2.102(6)
1.850(5)
1.879(4)
1.869(3)
2.080(4)
2.096(5)
2.009(3)
1.863(3)
1.876(3)
2.060(3)
2.084(3)
Fe-O1
Fe-O2
Fe-N1
Fe-N2
O1-Fe-N2
O2-Fe-N1
τ
143.3(3)
154.1(3)
0.18
132.6(2)
159.6(2)
0.45
137.7(1)
166.6(1)
0.48
a Labeling of the ligand atoms in 1-3 is shown in Figure 3.
Å) is longer than those (1.87-1.93 Å) in hydroxide model
complexes23 and the resting state of 3,4-PCD6 but much
shorter than those (2.21-2.30 Å) in the aqua complexes in
related model systems.24 The iron-bound water ligand in 3
seems to be considerably deprotonated via hydrogen-bonding
-
interaction with the ClO4 counteranion and is more like
hydroxide than water. To evaluate the effect of the external
ligand more precisely, an X-ray crystallographic analysis was
conducted for the iron complex 2 bearing ethanol as an
external ligand in the same mesityl salen framework. As can
clearly be seen, the ethanol complex 2 adopts a distorted
trigonal-bipyramidal structure similar to that of complex 3.
The structural parameters for 2 are listed in Table 1,
compared with those previously reported for 1 and 3. For a
quantitative evaluation of the extent of distortion around the
five-coordinate iron center, the geometric parameter τ is
employed (eq 1),25 and the τ values calculated for 1-3 are
included in Table 1. The τ value can be conveniently utilized
to estimate the degree of distortion from square-pyramidal
to trigonal-bipyramidal structures. In the case of an ideal
square-pyramidal geometry, the τ value is equal to zero,
while it becomes unity for a perfect trigonal-bipyramidal
geometry.
τ ) (â - R)/60
R, â: O1-Fe-N2, O2-Fe-N1, â > R (1)
The τ values for 1-3 reveal that the distortion around the
iron center is indeed correlated with the external ligand and
that the order of distortion upon binding of external ligands
is Cl , EtO < H2O. The order of Cl , EtO < H2O is
identical with that for a well-known spectrochemical series.1
Solid-State Studies of Less-Hindered Iron Salen Com-
plexes. As suggested from Figure 3, steric repulsion between
the methyl groups at the para positions of the 3- and 3′-
mesityl groups in the mesityl salen ligand might induce
distortion from a preferred square-pyramidal geometry of the
salen framework. Thus, we designed another salen frame-
Figure 4. X-ray crystal structures of (a) 5, (b) 6, (c) 8, and (d) 9. Thermal
ellipsoids represent the 50% probability surfaces. Hydrogen atoms and
incorporated solvent molecules are omitted for the sake of clarity.
work bearing xylyl groups at positions 3 and 3′ instead of
mesityl groups and prepared 5-9 in order to unambiguously
examine the effect of the external ligand. The X-ray crystal
structures of 5, 6, 8, and 9 in Figure 4 show that there is
indeed no steric repulsion between the xylyl groups at the 3
and 3′ positions. Table 2 summarizes selected bond distances
and angles.
An X-ray crystal structure of the chlorine complex 5 in
Figure 4a indicates that 5 contains a square-pyramidal iron
center closely resembling that of 1. Comparison of the τ
values shows that 5 (τ ) 0.16) is slightly less distorted than
(23) (a) Ogo, S.; Yamahara, R.; Roach, M.; Suenobu, T.; Aki, M.; Ogura,
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Chem. 2002, 41, 5513-5520. (b) MacBeth, C. E.; Gupta, R.; Mitchell-
Koch, K. R.; Young, V. G., Jr.; Lushington, G. H.; Thompson, W.
H.; Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2004, 126,
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Inorg. Chem. 1985, 24, 1647-1653. (b) Shyu, H.-L.; Wei, H.-H.; Lee,
G.-H.; Wang, Y. J. Chem. Soc., Dalton Trans. 2000, 915-918.
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7712 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
Table 2. Bond Distances (Å) and Bond Angles (deg) for 5, 6, 8, and
Table 3. Bond Distances (Å) and Bond Angles (deg) for Optimized
5
9a
Structures of the High-Spin Complexes (S ) /2)a
5
6
8
9
X ) Cl
X ) OMe
X ) OH
X ) OH2
M-X
2.2369(7)
1.896(2)
1.885(2)
2.096(2)
2.111(2)
1.840(2)
1.890(2)
1.894(2)
2.112(2)
2.094(2)
2.170(2)
1.987(2)
1.943(2)
2.101(2)
2.112(2)
2.299(4)
1.866(3)
1.847(4)
1.974(4)
1.980(4)
Fe-X
2.331
1.892
1.898
2.124
2.139
1.827
1.926
1.930
2.127
2.158
1.837
1.920
1.924
2.125
2.158
2.089
1.863
1.873
2.101
2.116
M-O1
M-O2
M-N1
M-N2
Fe-O1
Fe-O2
Fe-N1
Fe-N2
O1-M-N2
O2-M-N1
τ
153.68(8)
144.29(8)
0.16
158.38(9)
136.92(9)
0.36
165.24(7)
154.21(8)
0.18
175.8(2)
174.1(2)
0.03
O1-Fe-N2
O2-Fe-N1
τ
140
157
0.28
137
157
0.33
136
157
0.35
143
165
0.37
a Labeling of the ligand atoms in 5, 6, 8, and 9 is shown in Figure 4.
a Labeling of the ligand atoms is shown in Figure 5.
Table 4. Bond Distances (Å) and Bond Angles (deg) for Optimized
3
Structures of the Intermediate-Spin Complexes (S ) /2)a
X ) Cl
X ) OMe
X ) OH
X ) OH2
Fe-X
2.376
1.890
1.870
1.941
1.967
1.853
1.934
1.888
1.948
1.997
1.865
1.932
1.882
1.941
2.007
2.176
1.833
1.838
1.943
1.953
Fe-O1
Fe-O2
Fe-N1
Fe-N2
Figure 5. Labeling of the ligand atoms for DFT calculations.
the corresponding chlorine complex 1 (τ ) 0.18), probably
because of reduced steric repulsion. As shown in Figure 4b,
the iron center in the methanol complex 6 exhibits a
considerable structural change from a preferred square-
pyramidal to a distorted trigonal-bipyramidal geometry
(τ ) 0.36), compared to the corresponding ethanol complex
2 (τ ) 0.45). Unfortunately, the effect of water as an external
ligand in the xylyl salen framework could not be investigated
because we were unable to obtain crystals of the water
complex 7 suitable for an X-ray crystal analysis. However,
the distortion around the iron center observed for xylyl iron
salens 5 and 6 is exactly parallel to that observed for the
mesityl iron salen series. This clearly indicates that the
structural change from a square-pyramidal to a distorted
trigonal-bipyramidal geometry is not the result of steric
repulsions but is induced electronically upon the binding of
an external ligand.
A further insight was obtained from the X-ray crystal
structure of the iron(II) complex 8 bearing methanol as an
external ligand (Figure 4c). As shown in Table 2, the Fe-
OMe bond length (2.170 Å) is consistent with the binding
of methanol to the ferrous iron center, and the phenolate
O-Fe bonds in the iron(II) complex 8 are longer than those
in the iron(III) complexes 5 and 6. The iron center in 8 is
rather close to a square-pyramidal geometry (τ ) 0.18),
suggesting that the electronic effects by external ligands are
specific for the iron(III) site. External ligands do not induce
any distortion around the manganese(III) center either
because the manganese salen complex 9 bearing a weakly
bound water as an external ligand shows a square-pyramidal
manganese(III) center (τ ) 0.03), as shown in Figure 4d.
Density Functional Theory (DFT) Calculations. The
electronic nature of the distortion around the iron center was
investigated by means of DFT calculations. DFT calculations
were conducted for nonsubstituted iron salens with Cl, MeO,
OH, and H2O as an external ligand (Figure 5) to focus
particular attention on the electronic interaction between
external ligands and the iron center as well as to sim-
plify the system for calculation. Table 3 shows selected
parameters for optimized structures of the high-spin com-
O1-Fe-N2
O2-Fe-N1
τ
152
166
0.23
145
166
0.35
142
168
0.43
163
172
0.15
a Labeling of the ligand atoms is shown in Figure 5.
plexes (S ) 5/2). Selected parameters for optimized structures
of the intermediate-spin complexes (S ) /2) are listed in
3
Table 4.
The high-spin complexes are calculated to be more stable
than the corresponding intermediate-spin complexes by 0.24
(Cl), 0.42 (MeO), 0.39 (OH), and 0.01 eV (H2O), which is
in agreement with the experimental observation that all of
the iron salen complexes investigated herein are in a high-
spin electronic state (see the next section). As shown in Table
3, DFT calculations indicate that the coordination of water
to the high-spin iron(III) center causes the largest structural
change from a square-pyramidal to a distorted trigonal-
bipyramidal structure (τ ) 0.37). Along with a spectro-
chemical series of external ligands, the distortion is calculated
to become smaller (τ ) 0.35 for OH, τ ) 0.33 for MeO,
and τ ) 0.28 for Cl). The order of distortion, as estimated
from DFT calculations, is thus exactly the same as the
experimental observation. Computationally reproduced dis-
tortion in the absence of any steric interaction unambiguously
indicates that the distortion around the iron(III) center is
induced electronically by the external ligand. It is interesting
to note that water coordination to the hypothetical intermedi-
ate-spin iron(III) center does not cause such distortion (Table
4), suggesting that the d5 high-spin electron configuration
might be responsible for the unique structural feature of the
water complex 3 as a model for 3,4-PCD.
Solution Studies. To further study the structures in
solution, we characterized 1-7 in dichloromethane using
various spectroscopic methods as shown in Table 5. Salen
complexes, 1-7, showed quasi-reversible redox peaks at
room temperature. The redox potential for FeII/FeIII is quite
sensitive to the external ligand. The chlorine complexes 1
and 5 show the same redox potential at -0.48 V vs SCE,
Inorganic Chemistry, Vol. 45, No. 19, 2006 7713

Kurahashi et al.
Table 5. Electrochemical and Spectral Data for 1-7
1a
2a
3b
4b
5a
6a
7a
redox potential E1/2 vs SCE/Vc
UV-vis
-0.48
-1.04
0.00
-0.97
-0.48
-1.09
0.01
335 (16.7)
490 (4.2)
550 (3.9)
-67.4
460 (7.2)
516 (7.0)
370 (12.1)
331 (15.3)
433 (4.5)
535 (4.6)
363 (11.9)
435 (6.0)
485 (5.0)
365 (11.8)
λmax/nm
(ꢀ/103 M-1 cm-1
)
1H NMR 5H shifts/ppmd
6H shifts/ppm
-69.5, -65.6
56.8
-47.7
44.0
53.7 (76.7e)
56.1 (80.2e)
85.1 (121.4)
88.9 (125.7)
8.1 6.7
41.6 (63.4e)
58.3 (80.2)
(90.2)
89.9 (127.4)
(136.0)
9.2
42.2 (60.2)
40.5
4H shifts/ppm
72.9 (103.2)
(108.6)
9.5
73.7 (99.5)
(105.5)
9.4
89.7
91.6
6.8
76.4
79.9
EPR
g
9.3
4.3
9.4
4.2
3.5 5.2
4.3
4.3
4.3
5.3
(E/D)
1.8 2.0
(0.25)
(0.21)
(0.25)
2.0
(0.23)
(0.24)
(0.10) (0.03)
1310
1340
(0.03)
resonance Raman
ν
1316
1340
1304
1338
1327
C-O/cm-1, at 297 K
a UV-vis and EPR spectra were measured in CH2Cl2 at 297 and 1.5 K, respectively. Resonance Raman and 1H NMR spectra were measured in CD2Cl2.
b UV-vis and EPR spectra were measured in THF/H2O (7/3) at 297 and 1.5 K, respectively. Resonance Raman and 1H NMR spectra were measured in
CD2Cl2. c Reduction potentials are determined by cyclic voltammetry at 297 K in CH2Cl2 containing 0.10 M tetrabutylammonium perchlorate supporting
electrolyte. Conditions: a saturated calomel reference electrode; a glassy carbon working electrode; a platinum-wire counter electrode; scan rate 100 mV
s-1 d 1H NMR spectra were measured in CD2Cl2 at 297 K or at 203 K (in parentheses). e The signal is too broad to accurately determine the position.
.
which is close to those of other ferric salen complexes.26
2
and 6, with RO as an external ligand, show an increased
redox potential at -1.04 and -1.09 V, respectively. The
water complexes 3 and 7 show a redox potential of around
0.00 V. Although the corresponding xylyl iron salen complex
7 was not characterized by X-ray crystal analysis, the
similarity of the redox potential of 7 suggests that the water
ligand is coordinated as in 3. The OH complex 4 shows an
increased redox potential at -0.97 V, consistent with
coordination of an anionic OH ligand.
The salen complexes, 1-7, show ligand-to-metal charge-
transfer (LMCT) bands in a visible region, and the peak
positions are also dependent on the external ligand. A red
solution of the chlorine complex exhibits two absorption
maxima in the visible region at 460 and 516 nm for 1 and at
435 and 485 nm for 5. In the case of the RO complexes 2
and 6, the absorption in the visible region is diminished
considerably, generating an orange solution (λmax 370 nm
for 2 and λmax 365 nm for 6). Both 3 and 7, with water as an
external ligand, give a purple solution in dichloromethane.
UV-vis spectra of 3 and 7, in the visible region, show two
broad absorption maxima extending to 800 nm (λmax 433
and 535 nm for 3 and λmax 490 and 550 nm for 7).
In 1H NMR spectra, salen complexes, 1-7, show alterna-
tive paramagnetic shifts for phenolic protons, as in the case
of other monomeric iron(III) salen complexes.12c,27 According
to the previous report, the most downfield-shifted signal is
assigned to 4H (4H′), and the other is assigned to 6H (6H′)
in the phenolate group (see Figure 2 for the numbering of
the phenolate protons). In the case of the xylyl iron salens,
the peak assigned to 5H is observed upfield. The chlorine
complex 1 shows separate 1H NMR signals for both 4H, 4H′
and 6H, 6H′, indicating that 1 has two distinct phenolate
rings in solution. Likewise, the xylyl iron salen complex 5
exhibits separate signals for 4H, 4H′ and 5H, 5H′ but a single
Figure 6. 1H NMR spectra of 3 in wet CD2Cl2 at 297 K (a) and in dry
CD2Cl2 at 297 K (b) and 203 K (c).
signal for 6H, 6H′. On the other hand, the RO complexes 2
and 6, the H2O complexes 3 and 7, and the OH complex 4
show single signals for protons in two phenolate rings at
297 K. However, at 203 K, signals for 4H and 4H′ for those
complexes are also observed separately.
1
Figure 6a shows H NMR spectra of 3 in wet CD2Cl2 at
297 K. Downfield-shifted signals are observed at 58.3 and
89.9 ppm for 3, indicating a monomeric iron(III) form in
solution. We previously failed to obtain a 1H NMR spectrum
at low temperature below 223 K because of extreme line
broadening.22 However, the use of anhydrous aluminum-
treated CD2Cl2 suppresses line broadening and enables a
NMR measurement to be made for the mesityl iron salen
series including 3 at 203 K. As shown in Figure 6c, signals
for 6H and 4H, which appear at 59.3 and 90.7 ppm as one
set of single signals at 297 K, are observed separately at
80.2 and 90.2 ppm (6H) and 127.4 and 136.0 ppm (4H) at
203 K, consistent with 3 having two distinct phenolate rings
in solution. In anhydrous CD2Cl2, an additional pair of signals
of low intensity is observed at 37.5 and 76.4 ppm at 297 K
(26) Lubben, M.; Meetsma, A.; van Bolhuis, F.; Feringa, B. L.; Hage, R.
Inorg. Chim. Acta 1994, 215, 123-129.
(27) La Mar, G. N.; Eaton, G. R.; Holm, R. H.; Walker, F. A. J. Am. Chem.
Soc. 1973, 95, 63-75.
7714 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
and at 55.8 and 114.0 ppm at 203 K. These signals are readily
assigned to H2O-depleted 3. In a similar manner, the EtO
complex 2 and the OH complex 4 exhibit separated signals
especially for 4H, 4H′. On the other hand, the single signals
for the xylyl salen complexes were not separated even at
low temperature, although the X-ray crystal structures show
an asymmetric environment for the two phenolate groups in
the salen ligands. This is because of the more rapid
pseudorotation of the xylyl salen ligand than the mesityl salen
ligand.28
Electron paramagnetic resonance (EPR) spectra are also
dependent on the external ligand for both the mesityl and
xylyl iron salen series. As we recently reported,5,22 an EPR
spectrum of mesityl iron salen 1 shows two sets of signals
from high-spin iron(III) centers, a major component with g
) 8.1, 3.5, and 1.8 (E/D ≈ 0.10) and a minor component
with g ) 6.7, 5.2, and 2.0 (E/D ≈ 0.03). On the other hand,
the less-hindered xylyl iron salen 5 apparently shows only
one set of EPR signals at g ) 6.8, 5.3, and 2.0 (E/D ≈ 0.03).
Only one set of EPR signals for 5 might indicate a single
stable structure in solution for the less-hindered xylyl iron
salen complex 5. The RO complexes show more rhombic
EPR signals at g ) 9.5 and 4.3 (E/D ≈ 0.25) for 2 and g )
9.3 and 4.3 (E/D ≈ 0.23) for 6. The H2O and OH complexes
show similar EPR signals at g ) 9.2 and 4.3 (E/D ≈ 0.21)
for 3, g ) 9.4 and 4.2 (E/D ≈ 0.24) for 7, and g ) 9.4 and
4.3 (E/D ≈ 0.25) for 4.
1-3 show two resonance Raman bands, both of which
can be assigned to C-O stretching bands, in contrast to a
single C-O stretching band at 1337 cm-1 reported for the
iron salen complex.12f As shown in Figure 7a, 3 exhibits two
resonance Raman bands at 1304 and 1338 cm-1 in CD2Cl2
at the excitation wavelength of 457.9 nm. These two bands
are also observed at 1306 and 1339 cm-1 in the solid state,
suggesting that the solid-state structure of 3 is also maintained
in solution. As reported for 3,4-PCD,29 these two bands
follow different excitation profiles as shown in Figure 7b.
The 1338-cm-1 band exhibits a maximum enhancement at
around 500 nm, while the 1304-cm-1 band approaches a
maximum at a wavelength beyond 600 nm. The phenolate
with higher energy vibrations gives rise to the higher energy
LMCT band. Both 1 and 2, in addition to 3, show two
resonance Raman bands, further supporting the view that
1-3 bear two distinct phenolate rings in solution. The
resonance Raman spectrum of 4 shows a single broadened
νC-O band for some as yet unknown reason.
Figure 7. (a) Resonance Raman spectrum of 3 in CD2Cl2. The excitation
wavelength is 457.9 nm. The band from the solvent is denoted with s. (b)
Excitation profiles for the 1304- and 1338-cm-1 bands of 3 in CD2Cl2.
Peak intensities are normalized relative to the 1047-cm-1 band from CD2-
Cl2.
Figure 8. τ values calculated for 1-3 in comparison with 3,4-PCD.
other structural characteristics with the active site of the
resting state of 3,4-PCD. As reported for 3,4-PCD,6 3 has a
shorter axial Fe-O2 bond than the equatorial Fe-O1 bond,
although the bond length difference of 0.013 Å is smaller
than the reported value for 3,4-PCD (0.09 Å). The O2-Fe-
N1 angle of 166.6° in 3 is nearly identical with that in 3,4-
PCD (167°). The Fe-O(water) bond length in 3 (2.009 Å)
is slightly longer than that in 3,4-PCD (1.90 Å). As pointed
out in the previous section, however, the iron-bound water
ligand in 3 is more like OH than H2O, considering the
previous reports for the Fe-OH (1.87-1.93 Å)23 and Fe-
OH2 (2.21-2.30 Å)24 bond lengths in model and enzyme
systems. The bond lengths between iron and ligand atoms
in 3,4-PCD are longer than those in 3. Thus, the electron
density at the iron(III) center in 3,4-PCD is smaller than that
Discussion
Trigonal-Bipyramidal Distortion of Ferric Iron Salen
Complexes. The iron salen complex 3 is the first inorganic
model complex in which the unique distorted trigonal-
bipyramidal ferric iron center in 3,4-PCD is reproduced, as
is clearly seen in the X-ray crystal structure and from the τ
values in Figure 8. The iron salen complex 3 shares several
(28) Berry, R. S. J. Chem. Phys. 1960, 32, 933-938.
(29) Siu, D. C.-T.; Orville, A. M.; Lipscomb, J. D.; Ohlendorf, D. H.; Que,
L., Jr. Biochemistry 1992, 31, 10443-10448.
Inorganic Chemistry, Vol. 45, No. 19, 2006 7715

Kurahashi et al.
affected at all, even by the water ligand. DFT calculations
also indicate that the hypothetical intermediate-spin iron-
(III) center is not altered to a distorted trigonal-bipyramidal
geometry upon the binding of the water ligand (Figure 9).
All of these results suggest that a d5 high-spin configuration
is essential for inducing the structural change and that the
electron-electron interactions between unpaired electrons in
the d orbitals and electrons in the ligand induce this structural
change.
The binding of a weak donor ligand at the axial position
does not significantly affect the energy levels of the d orbitals
of an iron salen complex. However, the binding of a strong
donor ligand at the axial position significantly destabilizes
some of the d orbitals and increases the total energy of the
salen complex. To compensate for the increased total energy,
the iron salen complex will tend to stabilize other d-orbital
energies via a structural change around the iron. To ef-
fectively lower the total energy, the d orbital with the highest
Figure 9. τ values calculated for optimized structures by DFT calculations.
in 3. The difference in Lewis acidity of the iron(III) center
results in the deprotonation of the iron-bound water ligand
to form hydroxide in 3,4-PCD but results in partial depro-
tonation in 3.
It is generally believed that a salen iron complex consis-
tently maintains a square-pyramidal geometry,30 and the
distorted trigonal-bipyramidal geometry for 3 is unusual and
unexpected. It is thus worth discussing in detail the driving
force for the unprecedented distortion of the iron(III) salen
complex from a preferred square-pyramidal to a unique
trigonal-bipyramidal geometry, in the context of structure-
reactivity relationships of 3,4-PCD. Figure 8 summarizes the
τ values for 1-3 in comparison with that for 3,4-PCD. A
clear correlation can be seen between distortion and the
external ligand, and distortion is increased in the order Cl
, EtO < H2O. As was already mentioned, this order is
parallel to the spectrochemical series. In other words, as the
ligand field of the external ligand of the iron salen complex
is stronger, the geometry around the iron center is more
distorted to trigonal-bipyramidal. Furthermore, DFT calcula-
tions for nonsubstituted high-spin iron salens accurately
reproduce the structural distortion of the salen complexes,
as shown in Figure 9. A correlation between distortion and
the position of the external ligand in the spectrochemical
series clearly shows that the distortion is mainly due to
electronic interactions between the external ligand and the
iron center. In fact, the τ value (0.44) for the resting state of
3,4-PCD is very close to those (0.45 and 0.48) for 2 and 3
(Figure 8). On the other hand, the distortion does not
originate from steric repulsion between the p-methyl groups
of the 3,3′-mesityl groups. This is because the xylyl iron
salen complexes 5 and 6 also show a similar trend of
distortion.
2
2
energy, dx -y , is stabilized by the distortion of the salen
ligand from a preferred square-pyramidal to a trigonal-
bipyramidal geometry. Because the ligand-field splits of the
d orbitals for iron(II) and manganese(III) salen complexes
are smaller than that for the iron(III) salen complex, the effect
of the external ligand is not so drastic for the iron(II) and
manganese(III) salen complexes. As a result, these complexes
show the square-pyramidal geometries, even when a metha-
nol or water ligand is present. For the manganese(III) salen
2
2
complex, the empty dx -y orbital no longer contributes to
the stabilization.
An insight into the solution structures for 1-4 concerning
1
distortion around the iron(III) center is obtained from H
NMR and EPR data. The 1H NMR spectra of 1-4 at 297 K
are somewhat contradictory, and in the spectrum of the less
distorted 1, the 6H and 4H signals are separated, although
the more distorted 2 and 3 as well as 4 show single signals.
This is because the pseudorotation of the salen ligand is
relatively slow in the Cl complex 1 compared to the EtO
complex 2, the H2O complex 3, and the OH complex 4 at
297 K on the NMR time scale.28 However, at 203 K, single
signals for 4H are clearly separated for 2-4 in addition to
1. Most importantly, the separation between two signals for
4H is on the order of 3 (8.6 ppm) > 4 (6.0 ppm) > 2 (5.4
ppm) >1 (4.3 ppm), suggesting that the structural change
from a square-pyramidal to a distorted trigonal-bipyramidal
geometry in the solid state also holds for a solution. On the
other hand, the fast pseudorotation of the salen ligand
suggests that the energy gap between the square-pyramidal
and distorted trigonal-bipyramidal structures is very small.
EPR spectra of the frozen CH2Cl2 solutions of 1-4 are also
consistent with this because 2 (E/D ≈ 0.25), 3 (E/D ≈ 0.21),
and 4 (E/D ≈ 0.25) are much more rhombic than 1 (E/D ≈
0.03 and 0.10). Two νC-O bands in the resonance Raman
spectra indeed indicate two distinct phenolate groups in the
iron salen complex but do not appear to reflect the difference
in distortion among 1-4.
The electronic configuration of the metal center partici-
pates in the creation of a distorted trigonal-bipyramidal
structure. The coordination of methanol to the iron(II) center
retains a square-pyramidal geometry and the manganese(III)
salen complex with the d4 electronic configuration is not
(30) (a) Gerloch, M.; Mabbs, F. E. J. Chem. Soc. A 1967, 1598-1608. (b)
Gerloch, M.; Mabbs, F. E. J. Chem. Soc. A 1967, 1900-1908. (c)
Gerloch, M.; McKenzie, E. D.; Towl, A. D. C. J. Chem. Soc. A 1969,
2850-2858. (d) Coggon, P.; McPhail, A. T.; Mabbs, F. E.; McLachlan,
V. N. J. Chem. Soc. A 1971, 1014-1019. (e) Elerman, Y.; Paulus, H.
Acta Crystallogr., Sect. C 1996, 52, 1971-1973. (f) Elerman, Y.;
Kabak, M.; U¨ lku¨, D. Acta Crystallogr., Sect. C 1997, 53, 712-714.
Relevance to the Structure and Reaction of 3,4-PCD.
The present model study reveals a clear correlation between
distortion and the position of the external ligand in the
7716 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
Figure 10. Relevance of the iron salen complex 3 to the active site in
resting 3,4-PCD.
spectrochemical series and thus unambiguously indicates that
a structural change from a square-pyramidal to a distorted
trigonal-bipyramidal geometry is electronically induced upon
the binding of the external ligand to the iron salen complex.
The same electronic effect would be possibly operative in
the active site in 3,4-PCD, which bears a N2O2 coordination
sphere similar to that of the salen complex (Figure 10). The
external ligand of the resting state of 3,4-PCD is hydroxide.
Although we have not shown the crystal structure of the
hydroxide complex 4, the correlation found in this study
suggests that the hydroxide ligand in 4 distorts the iron center
to trigonal-bipyramidal geometry as the ethoxide ligand in
2 and the partially deprotonated water ligand in 3. In fact,
as expected from the position of the OH ligand in the
spectrochemical series, the τ value for the resting state of
3,4-PCD is closely similar to those for 2 and 3, as shown in
Figure 11. Role of the water ligand on the oxidation process of a catechol
substrate by 3,4-PCD.
indicated that the Lewis acidity of the iron center is primarily
important for catalysis,12i,j while the recent mutagenesis study
for 3,4-PCD indicated that efficient catalysis stands on a
delicate balance of the Lewis acidity, which must be high
enough to promote the charge localization on a catechol
substrate necessary for electrophilic O2 attack yet low enough
to allow the necessary ligand rearrangements for rapid
turnover.9f For the oxidation mechanism of 3,4-PCD, the
present results suggest that the electronic effect of the water
ligand in the active site of 3,4-PCD controls not only the
distorted trigonal-bipyramidal structure of the resting state
but also the oxidation process of a catechol substrate. Because
of the high Lewis acidity of the iron center of 3,4-PCD, the
iron-bound water in 3,4-PCD is deprotonated to a hydroxide.
The electronic effect of the iron-bound hydroxide contributes
to the trigonal-bipyramidal distortion, as discussed above.
The distorted trigonal-bipyramidal geometry may serve to
open the binding site for the catechol substrate. The iron-
bound hydroxide further acts as a base to accept a proton
from the OH group in a catechol substrate, which also assists
in the monodentate binding of the substrate. The protonation
of the iron-bound hydroxide would weaken the electronic
effect, resulting in the conversion of the iron center to an
octahedral geometry with substrate binding. This is supported
by the X-ray crystal structures of 3,4-PCD phenolic inhibitors
and 3,4-PCD under acidic conditions, in which water is still
bound to the iron center to form an octahedral geometry.9e
With the binding of the substrate, the Lewis acidity of the
iron center is further decreased by electron donation from
the substrate and iron-bound water, which fosters the
dissociation of Try447 on binding of the substrate in a
bidentate fashion. The formation of the bidentate complex
drastically decreases the Lewis acidity of the iron center via
an intramolecular electron transfer from the substrate. As a
result, the iron-bound water is dissociated. The dissociation
of the water ligand is quite important not only to create a
site for the subsequent oxygen reaction with the substrate
but also to increase the Lewis acidity of the iron center to
make it reactive with oxygen as suggested by previous model
studies.12i,j In fact, the X-ray crystal structural studies show
that the water ligand is dissociated with bidentate binding
1
Figure 8. The present H NMR study also suggests that 4
has a distorted trigonal-bipyramidal geometry similar to those
of 2 and 3 and the distortion is increased in the order of 2 <
4 < 3. This order is identical with the order expected from
the DFT calculations, as well as the spectrochemical series.
Close examinations of the X-ray crystal structures of 3,4-
PCD further support our proposal for the unique distorted
trigonal-bipyramidal geometry. Because there are no steric
and strong hydrogen-bonding interactions to the iron-bound
hydroxide from protein residues,6 the hydroxide ligand should
be located at the most stable position in energy. Furthermore,
as shown by the X-ray crystal structures of 3,4-PCD with
various inhibitors, 3,4-PCD is sufficiently flexible to shift
the protein ligands to form an octahedral geometry.9 Al-
though the role of the 3,4-PCD protein on the active-site
geometry could not be completely ruled out, all of these
findings suggest a new possibility that the unique distorted
trigonal-bipyramidal geometry in the 3,4-PCD active site
could be electronically induced by the iron(III) center
carrying a N2O2 coordination sphere with OH as an external
ligand. The electronic effect of the hydroxide ligand changes
the energies of the iron d orbitals, leading to the ligand shifts
to a distorted trigonal-bipyramidal geometry, as observed in
this model study.
Figure 11 shows the latest view for the oxidation process
of a catechol substrate by 3,4-PCD, which is proposed from
a series of X-ray crystal structures of 3,4-PCD9 as well as a
number of model studies.2,11-21 The previous model studies
Inorganic Chemistry, Vol. 45, No. 19, 2006 7717

Kurahashi et al.
Scheme 1
of a catechol substrate, while it is not dissociated with
bidentate binding of an electron-poor inhibitor such as
4-nitrocatechol, 2-hydroxyisonicotinic acid, or 6-hydroxyni-
cotinic acid.9b,e The water ligand also plays an important role
in the product release process. When the catechol is oxidized
to a dicarboxylate compound on the iron center, a water
molecule in an active site may facilitate the dissociation of
one of the carboxylate moieties from the iron center with
protonation. Subsequent binding of the formed hydroxide
to the iron center would electronically induce a trigonal-
bipyramidal distortion. This accelerates the rebinding of
Try447 with dissociation of the other carboxylate moiety,
resulting in an efficient release of the product.
Conclusion
Salen iron complexes, which reproduce the unique dis-
torted trigonal-bipyramidal ferric active site in 3,4-PCD, were
closely examined with various physicochemical methods.
Steric repulsion between introduced mesityl groups in 3 is
not the dominant factor in determining geometry, but the
binding of an external ligand to the iron(III) center is the
key to the unique trigonal-bipyramidal distortion that occurs.
An external ligand with a higher position in the spectro-
chemical series (Cl , RO < H2O) induces a larger degree
of distortion. As a result of the combination of DFT
calculations, we propose that an electronic interaction
between the external ligand and the iron center induces the
structural change from a preferred square-pyramidal to a
distorted trigonal-bipyramidal geometry in the iron(III) salen
complex. The present model study provides a new viewpoint
for electronic interactions between the external ligand and
the central iron(III), which results in a unique distorted
trigonal-bipyramidal geometry in 3,4-PCD. The new concept
established herein for the construction of a 3,4-PCD model
will contribute to revealing the relatively unexplored substrate-
binding and product-releasing step by 3,4-PCD.
through activated alumina under an argon atmosphere just prior to
use. Sodium hydride (60% dispersion in paraffin liquid) was
purchased from Nacalai Tesque and was utilized as received.
Tetrakis(triphenylphosphine)palladium(0) was purchased from Na-
calai Tesque. 2-Mesitylmagnesium bromide (1.0 M in diethyl ether)
and 2,6-dimethylphenylmagnesium bromide (1.0 M in THF) were
purchased from Aldrich. n-Butyllithium (1.6 M in hexane) and tert-
butyllithium (1.5 M in pentane) were purchased from Kanto
Chemical Co., Inc. Alkyllithium concentrations were determined
just prior to use according to a previously reported method.31 Silver
tetrafluoroborate (99.99+%) was purchased from Aldrich. Other
reagents were purchased from Kanto Chemical Co., Inc., or Wako
Pure Chemical Industries, Ltd., and were used as received. Caution!
The perchlorate salts used in this study are potentially explosiVe
and should be handled in small amounts with great care.
Experimental Section
Synthesis of Bis(3,5-dimesitylsalicylidene)-1,2-dimesityleth-
ylenediamine (18). 18 was synthesized according to Scheme 1.
1,3-Dibromo-2-(methoxymethoxy)benzene (11). A solution of
10 (25.53 g, 0.101 mol) in dry ether (50 mL) was added slowly to
a suspension of NaH (4.1 g, 0.103 mol) in dry ether (200 mL) and
dry DMF (50 mL) under an argon atmosphere. Chloromethyl methyl
ether (9.3 mL, 0.122 mol) in ether (10 mL) was then carefully added
with vigorous stirring. After 1 h, the solution was quenched with
ethanol (10 mL) and then with water (100 mL). The aqueous layer
was extracted with ether (100 mL) three times. The combined ether
layer was washed with saturated aqueous NaHCO3 (100 mL),
distilled water (100 mL), and finally saturated aqueous NaCl (100
mL). The resulting solution was dried over MgSO4 and then
evaporated. The residue was purified by vacuum distillation (5
mmHg, 120 °C) to give 11 (20.2 g, 0.068 mol) in 67% yield as an
Instrumentation. UV-vis spectra were recorded on an Agilent
8453 (Agilent Technologies). 500-MHz 1H NMR spectra were
measured on a LA-500 spectrometer (JEOL). Resonance Raman
spectra were measured with a quartz spinning cell, maintained at
room temperature, using a MC-100DG 100-cm single polychro-
mator (Ritsu Ohyo Kogaku) equipped with an intensified photodiode
array (PAR, 1421HQ). An Ar ion laser (NEC, GLG3200) was
utilized as an excitation source. The laser power at the sample was
about 7 mW. Raman shifts were calibrated with indene. EPR spectra
were recorded in a quartz cell (d ) 5 mm) at 4 K on an E500
continuous-wave X-band spectrometer (Bruker) with an ESR910
helium-flow cryostat (Oxford Instruments). Elemental analyses were
conducted on a CHN corder MT-6 (Yanaco). Cyclic voltammo-
grams were measured with an ALS612A electrochemical analyzer
(ALS).
1
oil. H NMR (500 MHz, CDCl3): δ 3.49 (s, 3H), 5.20 (s, 2H),
Materials. Ether, tetrahydrofuran (THF), N,N-dimethylforma-
mide (DMF), dimethoxyethane, methanol, and ethanol were pur-
chased from Kanto Chemical Co., Inc., or Wako Pure Chemical
Industries, Ltd., as anhydrous solvents and were stored over 3A or
4A molecular sieves. CD2Cl2 was purchased from Acros and was
stored in the presence of 4A molecular sieves. CD2Cl2 was passed
7.02 (d, J ) 8.9 Hz, 1H), 7.33 (dd, J ) 2.5 and 8.9 Hz, 1H), 7.66
(d, J ) 2.5 Hz, 1H). GC-MS (EI, ether): calcd for C8H8Br2O2
m/z 295.9; found m/z 296.1.
(31) Kofron, W. G.; Baclawski, L. M. J. Org. Chem. 1976, 41, 1879-
1880.
7718 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
1,3-Dimesityl-2-(methoxymethoxy)benzene (12). Palladium-
catalyzed Grignard cross-coupling was employed to incorporate
mesityl groups.32 To a solution of 11 (16.19 g, 54.7 mmol) in dry
THF (100 mL) was added tetrakis(triphenylphosphine)palladium
(3.27 g, 2.8 mmol) under an argon atmosphere. A 1.0 M solution
of 2-mesitylmagnesium bromide in ether (120 mL, 120 mmol) was
then slowly added with stirring. After the ether was distilled off
upon heating, the solution was refluxed for 12 h. The resulting
solution was quenched with careful addition of a 1 M NaHCO3
aqueous solution (100 mL). The aqueous layer was extracted with
ether (100 mL) three times. The combined organic layer was washed
with saturated aqueous NaCl, dried over MgSO4, and then
evaporated. The residue was passed through a silica column
(hexane:ethyl acetate ) 20:1). Vacuum distillation (1 mmHg, 200
°C) gave pure 12 (10.72 g, 28.6 mmol) in 52% yield as a viscous
atmosphere. Particular attention was paid to preventing the solution
temperature from exceeding 10 °C. The resulting reddish solution
was stirred at 0 °C for 30 min. At room temperature, trimethylsilyl
chloride (50 g, 460 mmol) in dry THF (400 mL) was then slowly
added. After the solution was stirred for 30 min at room temperature,
it was evaporated. The residue was purified by vacuum distillation
(1 mmHg, 100 °C) to give 16 (64.8 g, 295 mmol) in 74% yield. 1H
NMR (500 MHz, CDCl3): δ 0.24 (s, 9H), 2.26 (s, 3H), 2.38 (s,
6H), 6.83 (s, 1H), 6.84 (s, 1H), 9.32 (s, 1H). GC-MS (EI, ether):
calcd for C13H21NSi m/z 219.1; found m/z 219.3.
1,2-Dimesitylethylenediamine (17). The coupling of 16 was
carried out in the same manner as the reported procedure.35 To an
orange solution of NbCl4(THF)2 (52.0 g, 137 mmol) dissolved in
dry dimethoxyethane (1.8 L) was added 16 (30.0 g, 137 mmol) in
dimethoxyethane (200 mL) under an argon atmosphere. The
resulting solution was stirred for 6 h at room temperature, becoming
yellow in color. Dimethoxyethane was removed in vacuo, and the
residue was treated with 10% aqueous KOH (500 mL) for 20 min.
The aqueous layer was extracted with ether (500 mL) three times.
The combined organic layer was washed with saturated aqueous
NaCl, dried over Na2SO4, and then evaporated. The residue was
dissolved in methanol, and the addition of water with stirring yielded
1
oil. H NMR (500 MHz, CDCl3): δ 2.03 (s, 6H), 2.04 (s, 6H),
2.30 (s, 3H), 2.31 (s, 3H), 3.35 (s, 3H), 5.09 (s, 2H), 6.80 (d, J )
2.1 Hz, 1H), 6.91 (s, 2H), 6.92 (s, 2H), 7.05 (dd, J ) 2.1 and 8.6
Hz, 1H), 7.24 (d, J ) 8.6 Hz, 1H). GC-MS (EI, ether): calcd for
C26H30O2 m/z 374.2; found m/z 374.5.
3,5-Dimesityl-2-(methoxymethoxy)benzaldehyde (13). Ortho-
directing lithiation by a methoxymethoxy group is utilized for
regioselective incorporation of a formyl group.33 A 1.59 M solution
of n-BuLi (17 mL, 27 mmol) was added dropwise with stirring to
a solution of 12 (8.43 g, 22.5 mmol) in dry THF (47 mL) at -80
°C under an argon atmosphere. The solution was gradually warmed
to -20 °C for 1 h to give a reddish solution. Dry DMF (3.5 mL,
45.2 mmol) was then added at -20 °C, resulting in an immediate
color change from reddish to pale orange. After 1 h, the resulting
solution was quenched with ethanol (0.5 mL) and then distilled
water (120 mL). The aqueous layer was extracted with ether (100
mL) three times. The combined organic layer was washed with
saturated aqueous NaCl, dried over MgSO4, and then evaporated.
The residue was purified with a silica column chromatography
(hexane:ethyl acetate ) 20:1) to give 13 (6.18 g, 15.4 mmol) in
1
17 (26.3 g, 88.7 mmol) in 65% yield as a white solid. H NMR
(500 MHz, CDCl3): δ 1.62 (s, 6H), 2.16 (s, 6H), 2.65 (s, 6H),
4.70 (s, 2H), 6.52 (s, 2H), 6.78 (s, 2H).
18. To a solution of 17 (1.18 g, 4.0 mmol) in dry methanol (150
mL) was added 14 (2.87 g, 8.0 mmol). The mixture was heated to
reflux for 1 h. After cooling, the resulting precipitate was filtered,
washed with a small amount of methanol, and dried in vacuo. 18
was obtained in 77% yield as a yellow solid (3.00 g, 3.1 mmol).
1H NMR (500 MHz, CDCl3): δ 1.82 (s, 6H), 1.86 (s, 6H), 1.96 (s,
6H), 1.97 (s, 6H), 2.08 (s, 6H), 2.18 (s, 6H), 2.28 (s, 6H), 2.29 (s,
6H), 2.65 (s, 6H), 5.66 (s, 2H), 6.61 (s, 2H), 6.80 (s, 2H), 6.83 (s,
2H), 6.86 (s, 2H), 6.87 (s, 2H), 6.90 (s, 2H), 6.92 (s, 4H), 8.40 (s,
2H), 13.06 (s, 2H). 13C NMR (125.65 MHz, CDCl3): δ 20.23,
20.31, 20.69, 20.78, 20.84, 20.98, 21.01, 21.13, 22.61, 72.03,
118.92, 127.93, 127.99, 128.46, 129.19, 131.04, 131.15, 131.57,
134.22, 134.33, 134.71, 136.34, 136.41, 136.45, 136.63, 136.76,
137.09, 138.02, 156.57, 165.51. MS (FAB): calcd for C70H77N2O2
m/z 977.6; found m/z 977.7.
1
68% yield as a white solid. H NMR (500 MHz, CDCl3): δ 2.01
(s, 6H), 2.08 (s, 6H), 2.29 (s, 3H), 2.30 (s, 3H), 3.19 (s, 3H), 4.68
(s, 2H), 6.91 (s, 2H), 6.93 (s, 2H), 7.13 (d, J ) 2 Hz, 1H), 7.64 (d,
J ) 2 Hz, 1H), 10.49 (s, 1H). GC-MS (EI, ether): calcd for
C27H30O3 m/z 402.2; found m/z 402.4.
3,5-Dimesityl-2-hydroxybenzaldehyde (14). To a solution of
13 (5.45 g, 13.5 mmol) in ethanol (30 mL) was added slowly
concentrated HCl (3 mL). The resulting solution was refluxed for
30 min. Then saturated aqueous NaCl (100 mL) was added. The
aqueous layer was extracted with ether (100 mL) three times. The
combined organic layer was washed with saturated aqueous NaCl,
dried over MgSO4, and then evaporated. The residue was purified
by silica column chromatography (hexane:ethyl acetate ) 20:1) to
Synthesis of Bis(3-xylylsalicylidene)-1,2-dimesitylethylenedi-
amine (24). 24 was synthesized according to Scheme 2.
Iodo-2-(methoxymethoxy)benzene (20). 20 was synthesized in
exactly the same manner as that of 11. 19 (9.69 g, 44 mmol) was
converted to 20 (8.50 g, 32 mmol) in 73% yield as an oil. 1H NMR
(500 MHz, CDCl3): δ 3.50 (s, 3H), 5.22 (s, 2H), 6.74 (dd, J ) 7.8
and 8.3 Hz, 1H), 7.05 (d, J ) 8.3 Hz, 1H), 7.26 (dd, J ) 7.6 and
7.8 Hz, 1H), 7.76 (d, J ) 7.6 Hz, 1H). GC-MS (EI, ether): calcd
for C8H9IO2 m/z 264.0; found m/z 264.1.
1
give 14 (4.82 g, 13.4 mmol) quantitatively as a viscous oil. H
NMR (500 MHz, CDCl3): δ 2.06 (s, 12H), 2.31 (s, 6H), 6.94 (s,
2H), 6.95 (s, 2H), 7.14 (d, J ) 2 Hz, 1H), 7.35 (d, J ) 2 Hz, 1H),
9.94 (s, 1H), 11.10 (s, 1H). GC-MS (EI, ether): calcd for C25H26O2
m/z 358.2; found m/z 358.4.
N-(Trimethylsilyl)-2,4,6-trimethylbenzylideneimine (16). This
preparation was conducted according to a previously reported
procedure.34 A 1.0 M solution of lithium bis(trimethylsilyl)amide
(400 mL, 400 mmol) was added slowly at 0 °C to a solution of 15
(59.3 g, 400 mmol) in dry THF (400 mL) under an argon
1-(2,6-Dimethylphenyl)-2-(methoxymethoxy)benzene (21). 21
was synthesized in a manner similar to that of 12. To a solution of
20 (3.11 g, 11.8 mmol) in dry THF (10 mL) was added tetrakis-
(triphenylphosphine)palladium (0.66 g, 0.57 mmol) under an argon
atmosphere. Then a 1.0 M solution of 2,6-dimethylphenylmagne-
sium bromide in THF (16 mL, 16 mmol) was slowly added with
stirring. The solution was refluxed for 6 h. The resulting solution
was quenched by careful addition of 1 M aqueous Na2CO3 (50 mL).
The aqueous layer was extracted with ether (50 mL) three times.
The combined organic layer was washed with saturated aqueous
NaCl, dried over Na2SO4, and then evaporated. Vacuum distillation
(32) Tsuji, J. Palladium Reagents and Catalysts: InnoVations in Organic
Synthesis; John Wiley & Sons: Chichester, U.K., 1997.
(33) Winkle, M. R.; Ronald, R. C. J. Org. Chem. 1982, 47, 2101-2108.
(34) Colvin, E. W.; McGarry, D. G. J. Chem. Soc., Chem. Commun. 1985,
539-540.
(35) Roskamp, E. J.; Pedersen, S. F. J. Am. Chem. Soc. 1987, 109, 3152-
3154.
Inorganic Chemistry, Vol. 45, No. 19, 2006 7719

Kurahashi et al.
Scheme 2
127.05, 127.29, 128.38, 129.23, 131.06, 131.19, 133.39, 133.41,
136.69, 136.87, 136.92, 136.99, 137.34, 157.60, 165.57. MS (FAB)
calcd for C50H53N2O2 m/z 714.0; found m/z 713.5.
Synthesis of Metal Complexes. Syntheses of 1-4 are reported
elsewhere.5,22
5. To a solution of anhydrous FeCl3 (163 mg, 1.0 mmol) in dry
ethanol (10 mL) was added 24 (713 mg, 1.0 mmol). The solution
was refluxed for 30 min. The resulting dark-red solution was then
allowed to stand at room temperature. 5 was obtained in 51% yield
as black crystals (410 mg, 0.51 mmol). A crystal suitable for X-ray
crystallographic analysis was obtained by recrystallization from
acetonitrile. Anal. Calcd for C50H50N2O2ClFe: C, 74.86; H, 6.28;
N, 3.49. Found: C, 74.54; H, 6.42; N, 3.38. IR (KBr): 3449, 3014,
2953, 2920, 2856, 1605, 1555, 1460, 1415, 1384, 1329, 1303, 1270,
1224, 1120, 1090, 1061, 1034, 985, 855, 758, 691, 649, 578 cm-1
.
6. To a solution of 5 (401 mg, 0.50 mmol) in methanol (30 mL)
was added 0.1 M aqueous NaOH (5 mL). After the resulting red
precipitate was stirred for 30 min at room temperature, it was
isolated by filtration and recrystallized from pyridine/methanol to
give 6 (205 mg, 0.25 mmol) in 50% yield. A crystal suitable for
X-ray crystallographic analysis was obtained by recrystallization
from pyridine/methanol. Anal. Calcd for C52H57N2O4Fe: C, 75.26;
H, 6.92; N, 3.38. Found: C, 75.06; H, 7.18; N, 3.40. IR (KBr):
3436, 3014, 2952, 2919, 2855, 1608, 1590, 1554, 1459, 1416, 1387,
1331, 1309, 1272, 1226, 1120, 1091, 1060, 1033, 995, 853, 762,
(<1 mmHg, 120 °C) gave pure 21 (2.4 g, 9.9 mmol) in 84% yield
as a viscous oil. 1H NMR (500 MHz, CDCl3): δ 2.04 (s, 6H), 3.29
(s, 3H), 5.04 (s, 2H), 7.03 (dd, J ) 7.6 and 2.2 Hz, 1H), 7.06 (dd,
J ) 7.1 and 1.0 Hz, 1H), 7.09 (d, J ) 7.9 Hz, 2H), 7.15 (dd, J )
8.6 and 6.6 Hz, 1H), 7.21 (dd, J ) 7.3 and 0.8 Hz, 1H), 7.30 (ddd,
J ) 8.6, 7.3, and 2.2 Hz, 1H). GC-MS (EI, ether): calcd for
C16H18O2 m/z 242.1; found m/z 242.2.
687, 649, 577 cm-1
.
3-(2,6-Dimethylphenyl)-2-hydroxybenzaldehyde (23). Regi-
oselective formylation was carried out in a manner similar to that
for 13. A 1.5 M solution of t-BuLi (3.5 mL, 5.3 mmol) was added
dropwise with stirring to a solution of 21 (600 mg, 2.5 mmol) in
dry ether (25 mL) at 0 °C under an argon atmosphere. Dry DMF
(510 mg, 7.0 mmol) was then added at 0 °C. After the resulting
solution was stirred for 30 min at room temperature, it was quenched
with ethanol (25 mL), and 1 M aqueous Na2CO3 (50 mL) was then
added. The aqueous layer was extracted with ether (50 mL) three
times. The combined organic layer was washed with saturated
aqueous NaCl, dried over Na2SO4, and then evaporated. The residue
was passed through a silica column chromatography using toluene
as the eluent to give 22. To a solution of 22 in ethanol (20 mL)
was added slowly concentrated HCl (1 mL). The resulting solution
was refluxed for 5 min. Then saturated aqueous NaCl (50 mL) was
added. The aqueous layer was extracted with ether (50 mL) three
times. The combined organic layer was washed with saturated
aqueous NaCl, dried over Na2SO4, and then evaporated. The residue
was purified by silica column chromatography (toluene) to give
7. To a solution of 5 (50 mg, 0.062 mmol) in THF (5 mL) was
added a solution of silver perchlorate (15.4 mg, 0.068 mmol) in
THF (1 mL). After the mixture was stirred for 30 min at room
temperature, it was filtered, and the THF solvent was evaporated
in vacuo. The residue, dissolved in CH2Cl2, was passed through a
Teflon filter (Cosmonice Filter (S), pore size 0.45 µm) to give 7
(49.7 mg, 0.056 mmol). An analytically pure sample was obtained
by recrystallization from CH2Cl2/hexane. Anal. Calcd for C50H54N2O8-
ClFe: C, 66.56; H, 6.03; N, 3.10. Found: C, 66.41; H, 6.02; N,
3.50. IR (KBr): 3428, 3015, 2955, 2922, 2855, 1608, 1556, 1460,
1422, 1386, 1329, 1304, 1270, 1224, 1121, 1109, 1090, 1062, 985,
855, 759, 691, 651, 579 cm-1
.
8. All manipulations were carried out in a glovebox. To a solution
of iron(II) acetate (34.8 mg, 0.20 mmol) in deoxygenated methanol
(60 mL) was added 24 (143 mg, 0.20 mmol). The solution was
heated and was then allowed to stand at room temperature. The
resulting red crystalline product was filtered, washed with a small
amount of methanol, and dried under an argon atmosphere. 8 was
obtained in 55% yield (97 mg, 0.11 mmol). A crystal suitable for
X-ray crystallographic analysis was obtained by recrystallization
from methanol. Anal. Calcd for C53H62N2O5Fe: C, 73.77; H, 7.24;
N, 3.25. Found: C, 73.92; H, 7.03; N, 3.21.
1
23 (550 mg, 2.4 mmol) quantitatively as a viscous oil. H NMR
(500 MHz, CDCl3): δ 2.03 (s, 6H), 7.10 (dd, J ) 7.3 and 7.6 Hz,
1H), 7.12 (d, J ) 7.6 Hz, 2H), 7.19 (dd, J ) 7.6 and 7.6 Hz, 1H),
7.34 (d, J ) 7.3 Hz, 1H), 7.59 (d, J ) 7.6 Hz, 1H), 9.96 (s, 1H),
11.13 (s, 1H). GC-MS (EI, ether): calcd for C15H14O2 m/z 226.1;
found m/z 226.3.
24. To a solution of 17 (593 mg, 2.0 mmol) in dry methanol (15
mL) was added 23 (910 mg, 4.0 mmol). The mixture was heated
to reflux for 1 h. After cooling, the resulting precipitate was filtered,
washed with a small amount of methanol, and dried in vacuo. 24
was obtained in 55% yield as a yellow solid (780 mg, 1.09 mmol).
1H NMR (500 MHz, CDCl3): δ 1.69 (s, 6H), 1.84 (s, 6H), 1.95 (s,
6H), 2.17 (s, 6H), 2.65 (s, 6H), 5.61 (s, 2H), 6.59 (s, 2H), 6.81 (s,
2H), 6.83 (dd, J ) 7.5 and 7.5 Hz, 2H), 6.96 (dd, J ) 1.7 and 7.5
Hz, 2H), 7.04 (d, J ) 7.5 Hz, 2H), 7.05 (d, J ) 7.5 Hz, 2H), 7.12
(dd, J ) 7.6 and 7.6 Hz, 2H), 7.15 (dd, J ) 1.7 and 7.6 Hz, 2H),
7.25 (s, 2H), 8.41 (s, 2H). 13C NMR (125.65 MHz, CDCl3): δ
20.24, 20.33, 20.68, 20.77, 22.56, 72.16, 118.50, 118.90, 127.03,
9. 9 was synthesized from the corresponding chloromanganese
complex. To a solution of the chloromanganese complex (21.5 mg,
0.027 mmol) in THF (0.5 mL) was added a solution of silver
tetrafluoroborate (6.27 mg, 0.032 mmol) in THF (0.5 mL). After
the mixture was stirred for 30 min at room temperature, it was
filtered, and the THF solvent was evaporated in vacuo. The residue,
dissolved in CH2Cl2, was passed through a Teflon filter (Cosmonice
Filter (S), pore size 0.45 µm) to give 9 quantitatively (21.0 mg,
0.025 mmol). A crystal suitable for X-ray crystallographic analysis
was obtained by recrystallization from CH2Cl2/hexane. Anal. Calcd
for C50H54BF4N2O4Mn: C, 67.57; H, 6.12; N, 3.15. Found: C,
67.26; H, 6.18; N, 3.40. IR (KBr): 3436, 3016, 2965, 2919, 2855,
1607, 1591, 1553, 1461, 1418, 1385, 1330, 1305, 1277, 1223, 1123,
1084, 1064, 1037, 995, 985, 855, 756, 696, 656, 581 cm-1
.
7720 Inorganic Chemistry, Vol. 45, No. 19, 2006

Sterically Hindered Iron Salen Complex
Table 6. Crystallographic Data for 5, 6, 8, and 9
5
6
8
9
formula
Mr
C50H50ClFeN2O2
802.26
C52H56FeN2O4
828.87
C53H59FeN2O5
859.91
C50H52N2O3BF4Mn
870.71
T/K
173
173
173
173
cryst size/mm
cryst syst
space group
cryst color
a/Å
0.25 × 0.20 × 0.20
monoclinic
P21/n (No. 14)
black
11.6196(5)
12.7901(5)
28.174(1)
0.40 × 0.20 × 0.15
monoclinic
P21/n (No. 14)
black
15.643(4)
11.169(3)
25.931(7)
0.30 × 0.20 × 0.20
monoclinic
P21/n (No. 14)
black
17.7864(9)
12.9730(5)
20.655(1)
0.30 × 0.20 × 0.20
monoclinic
Cc (No. 9)
black
29.053(3)
8.4182(7)
22.860(3)
b/Å
c/Å
R/deg
â/deg
93.438(3)
92.542(3)
103.958(2)
126.620(3)
γ/deg
V/Å3
4179.5(3)
4
1.275
1.86
0.060
0.156
4526(2)
4
1.216
1.82
0.078
0.178
4625.2(4)
4
1.235
2.07
0.066
0.175
4487.4(9)
4
1.289
2.01
0.061
0.160
Z value
D
calc/g cm-3
GOF
R1a
wRb
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
X-ray Crystallography. Measurements were made on a Rigaku
RAXIS-IV Imaging Plate diffractometer or Rigaku/MSC Mercury
CCD diffractometer equipped with graphite-monochromated Mo
KR radiation (λ ) 0.710 70 Å). Data were collected at 173 K under
a cold nitrogen stream. The images were processed with the
CrystalClear (Rigaku) program.36 Structures were solved by direct
methods with the TeXsan crystallographic software.37 Anisotropic
refinement was applied to all non-hydrogen atoms. Hydrogen atoms,
except for those of water molecules, were placed at the calculated
positions and refined with isotropic parameters. Corrections for
Lorentz-polarization effects and absorption were performed.
Crystallographic data for each complex are summarized in Table
6. Crystallographic data for 2 is not sufficient, mainly because of
the solvent molecules incorporated in the crystal, but is supposed
to be valid for discussion on the geometry around the iron center.
DFT Calculations. Electronic structure calculations were carried
out using the DFT method in which the B3LYP hybrid func-
tional38,39 was employed. The Gaussian98 program40 was used for
the implementation. The ground wave function was described with
the LANL2DZ basis set for Fe and Cl where their core electrons
were replaced with the effective core potentials.41 For the other
atoms, the (9s5p)/[3s2p] basis sets by Dunning and Hay42 were
used. In the present calculations, spin polarization was taken into
account. The spin contaminations in the spin-unrestricted ap-
proximation were negligibly small. For example, the expectation
values of the Sˆ2 operator for the FeIII/Cl complex were 3.7944 and
3
5
8.7650 for the lowest-spin quartet (S ) /2) and sextet (S ) /2)
states, where their deviations from the ideal values were within
the tolerance of 1.2%. The number of unpaired electrons in the
quartet and sextet states (Table S2 in the supporting Information)
indicates that they dominantly localize on the Fe atom and some
of them delocalize over the salen ligand. The convergence criteria
for geometry optimizations were due to the default values in
Gaussian98, where the maximum and root-mean-square forces were
less than 4.5 × 10-4 and 3.0 × 10-4 au, respectively. The maximum
and root-mean-square displacements were 1.8 × 10-3 and 1.2 ×
10-3 au, respectively.
Acknowledgment. We thank Prof. T. Yamada for helpful
comments on the synthesis of 17, Dr. Y. Funahashi for
assistance in the early stages of the project, Dr. M. Tomura
for helpful comments on X-ray crystallographic analysis, and
S. Makita for elemental analysis. This work was supported
by grants from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and from the Japan Science
and Technology Agency, CREST.
(36) (a) CrystalClear Software Package; Rigaku and Molecular Structure
Corp.: The Woodlands, TX, 1999. (b) Pflugrath, J. W. Acta Crys-
tallogr. 1999, D55, 1718-1725.
(37) TeXsan, version 1.11; Rigaku and Molecular Structure Corp.: The
Woodlands, TX, 2000.
(38) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(39) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. 1988, B37, 785-789.
(40) Pople, J. A.; et al. Gaussian98, revision A.11.3; Gaussian, Inc.:
Pittsburgh, PA, 2002.
(41) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270-283. (b)
Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284-298. (c) Hay,
P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(42) Dunning, T. H., Jr.; Hay, P. J. In Modern Theoretical Chemistry;
Schaefer, H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1.
Supporting Information Available: ORTEP view and crystal-
lographic data of 2, X-ray crystallographic files in CIF format for
1
2, 5, 6, 8, and 9, cyclic voltammogram, UV-vis, H NMR, EPR,
and resonance Raman spectra for 1-7, the resonance Raman
spectrum of 3 in the solid state, spin population on the Fe atom in
the spin-unrestricted DFT (B3LYP) calculations, and complete ref
40. This material is available free of charge via the Internet at
http://pubs.acs.org.
IC060650P
Inorganic Chemistry, Vol. 45, No. 19, 2006 7721