Iron(II) complexes with bio-inspired N,N,O ligands as oxidation catalysts: Olefin epoxidation and cis-dihydroxylation

Article abstract of DOI:10.1002/chem.200700573

The Rieske dioxygenases are a group of non-heme iron enzymes, which catalyze the stereospecific cis-dihydroxylation of its substrates. Herein, we report the iron(II) coordination chemistry of the ligands 3,3-bis(1- methylimidazol-2-yl)propionate (L1) and its neutral propyl ester analogue propyl 3,3-bis(1-methylimidazol-2-yl)-propionate (PrL1). The molecular structures of two iron(II) complexes with PrL1 were determined and two different coordination modes of the ligand were observed. In [Fe^II(PrL1)₂]- (BPh₄)₂ (3) the ligand is facially coordinated to the metal with an N,N,O donor set, whereas in [Fe^IIPrL1)₂- (MeOH)₂](OTf)₂ (4) a bidentate N.N binding mode is found. In 4. the solvent molecules are in a cis arrangement with respect to each other. Complex 4 is a close structural mimic of the crystallographically characterized nonheme iron(II) enzyme apocarotcnoid15-15′-oxygenase (APO). The mechanistic features of APO are thought to be similar to those of the Rieske oxygenases, the original inspiration for this work. The non-heme iron complexes [Fe^II(PrL1)₂](OTf)₂ (2) and [Fe ^II-(PrL1)₂](BPh₄)₂ (3) were tested in olefin oxidation reactions with H₂O₂ as the terminal oxidant. Whereas 2 was an active catalyst and both epoxide and cis-dihydroxylation products were observed, 3 showed negligible activity under the same conditions, illustrating the importance of the anion in the reaction.

Full text of DOI:10.1002/chem.200700573

DOI: 10.1002/chem.200700573
Iron(II) Complexes with Bio-Inspired N,N,O Ligands as Oxidation Catalysts:
Olefin Epoxidation and cis-Dihydroxylation
Pieter C. A. Bruijnincx,[a] Inge L. C. Buurmans,[a] Silvia Gosiewska,[a]
[a]
Marcel A. H. Moelands,[a] Martin Lutz,[b] Anthony L. Spek,[b] Gerardvan Koten, and
Robertus J. M. Klein Gebbink*[a]
Abstract: The Rieske dioxygenases are
a group of non-heme iron enzymes,
which catalyze the stereospecific cis-di-
hydroxylation of its substrates. Herein,
we report the iron(II) coordination
chemistry of the ligands 3,3-bis(1-
nated to the metal with an N,N,O
donor set,whereas in [Fe
II(PrL1)2-
(MeOH)2](OTf)2 (4) a bidentate N,N
be similar to those of the Rieske oxy-
genases,the original inspiration for this
work. The non-heme iron complexes
AHCTREUNG
G
ACHTREUNG
binding mode is found. In 4,the sol-
vent molecules are in a cis arrangement
with respect to each other. Complex 4
is a close structural mimic of the crys-
tallographically characterized non-
heme iron(II) enzyme apocarotenoid-
15–15’-oxygenase (APO). The mecha-
nistic features of APO are thought to
[FeII
(PrL1)2]
A
N
A
ACHTREUNG
olefin oxidation reactions with H2O2 as
the terminal oxidant. Whereas 2 was
an active catalyst and both epoxide and
cis-dihydroxylation products were ob-
served, 3 showed negligible activity
under the same conditions,illustrating
the importance of the anion in the re-
action.
methyl
A
(L1)
and its neutral propyl ester analogue
propyl 3,3-bis(1-methylimidazol-2-yl)-
propionate (PrL1). The molecular
structures of two iron(II) complexes
with PrL1 were determined and two
different coordination modes of the
Keywords: hydrogen bonds · iron ·
N,N,O ligands · olefins · oxidation
ligand were observed. In [FeII
(BPh4)2 (3) the ligand is facially coordi-
A
ACHTREUNG
Introduction
these metalloenzymes are the non-heme iron oxygenases.[2,3]
A specific subset that utilizes a mononuclear,non-heme
iron(II) center coordinated by the so-called 2-His-1-carbox-
ylate facial triad has recently emerged as a common,versa-
tile platform for these oxidative transformations.[4] The
Rieske dioxygenases belong to this class of enzymes and cat-
alyze the stereospecific cis-dihydroxylation of arenes as the
first step in the biodegradation of these compounds. Naph-
thalene 1,2-dioxygenase (NDO) was the first Rieske dioxy-
genase to be characterized crystallographically.[5] The active
site of NDO with bound substrate (Figure 1) features a
mononuclear iron center coordinated by two histidines and
a bidentate aspartate in a variation on the 2-His-1-carboxyl-
ate facial triad.[5] This bidentate binding mode is found in
some,but not all,crystallographically characterized Rieske
dioxygenases.[6] Figure 1 also shows the cis-dihydroxylation
reaction.
The selective and catalytic oxidation of organic substrates is
an important area of research in both academia and indus-
try.[1] Nature has developed several strategies for dioxygen
activation and uses metalloenzymes to selectively oxidize
and functionalize hydrocarbons. An important group of
[a] Dr. P. C. A. Bruijnincx,I. L. C. Buurmans,Dr. S. Gosiewska,
M. A. H. Moelands,Prof. Dr. G. v. Koten,
Prof. Dr. R. J. M. Klein Gebbink
Chemical Biology & Organic Chemistry,Department of Chemistry
Faculty of Science,Utrecht University
Padualaan 8,3584 CH Utrecht (The Netherlands)
Fax :(+31)30-252-3615
G
E-mail: r.j.m.kleingebbink@uu.nl
+
[b] Dr. M. Lutz,Prof. Dr. A. L. Spek
Bijvoet Center for Biomolecular Research,Crystal and Structural
Chemistry,Department of Chemistry,
Faculty of Science,Utrecht University
The unique reactivity of non-heme iron enzymes has in-
spired the development of synthetic non-heme iron com-
plexes as potential oxidation catalysts.[1,2] The family of tpa-
and bpmen-based catalysts (tpa: tris(2-pyridylmethyl)amine;
bpmen: N,N’-dimethyl-N,N’-bis(2-pyridylmethyl)ethane-1,2-
Padualaan 8,3584 CH Utrecht (The Netherlands)
E-mail: a.l.spek@uu.nl
[+] Correspondence pertaining to the crystallographic studies.
1228
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Chem. Eur. J. 2008, 14,1228 – 1237

FULL PAPER
plexes were found to be active catalysts in the epoxidation
and cis-dihydroxylation of olefinic substrates.
Results
Figure 1. Active site of naphthalene 1,2-dioxygenase (NDO), a Rieske di-
oxygenase,with bound substrate (1O7G.pdb). The catalyzed cis-dihy-
droxylation reaction is also shown.
Synthesis andcharacterization of the iron( II) complexes
1–4: We have previously reported the synthesis and
copper(II) coordination chemistry of the new bis(1-alkylimi-
dazol-2-yl)propionate ligand family.[22,23] In analogy to the
[CuL2] complexes,we synthesized the 2:1 iron(II) complex
with ligand L1 (Figure 2). The addition of 0.5 equiv
diamine) developed by Que et al.,for instance,catalyze the
epoxidation and/or cis-dihydroxylation of olefins with H2O2
as oxidant.[7–9] These catalysts therefore serve as excellent
functional models of the non-heme iron oxygenases. Other
mononuclear iron systems capable of olefin epoxidation
have also been reported.[10–14] All ligands employed in these
studies provide the metal center with an all-N donor set,
which does not accurately reflect the Nim,Nim,Ocarb ligand en-
vironment found at the active site of the enzymes. For this
reason,attention has been devoted recently to the develop-
ment of iron complexes with mixed donor ligands.[15–19] Bur-
zlaff et al.,for example,have reported on the iron coordina-
tion chemistry of the bispyrazolylacetate N,N,O ligand
system,[15,16] and Que et al. recently communicated a very ef-
fective olefin cis-dihydroxylation catalyst based on the bis(2-
pyridyl)methylbenzamide ligand.[18] A different approach to
the isolation of mononuclear iron(II) complexes with a
N,N,O carboxylato donor set has also been reported recently; it
involves the use of sterically hindered bidentate N and mon-
odentate O donor ligands.[20] Very recently,a mononuclear
iron(II) complex has been reported with an N,N,O ligand,
which accurately captures the bidentate coordination mode
of the carboxylate,as found in some of the Rieske dioxyge-
nases. This complex showed olefin cis-dihydroxylation,
albeit with poor reactivity amounting to less than one turn-
over.[21]
As part of our efforts to build suitable models that mimic
the active site of non-heme iron(II) enzymes,which exhibit
the so-called 2-His-1-carboxylate facial triad,[2,4] we have
been studying the coordination chemistry of the substituted
bis(1-alkylimidazol-2-yl)propionate ligand family[22,23] and
have synthesized mononuclear iron complexes that accurate-
ly mimic the coordination environment of the 2-His-1-car-
boxylate facial triad.[24] Here,we describe the synthesis and
structural characterization of bio-inspired iron(II) com-
plexes with the ligand 3,3-bis(1-methylimidazol-2-yl)propio-
nate (L1) and its neutral ester analogue propyl 3,3-bis(1-
methylimidazol-2-yl)propionate (PrL1). The latter com-
Figure 2. Iron(II) complexes [FeII(L1)2] (1) and [FeII
A
U
FeII
(OTf)2·2MeCN to a solution containing the tetrabutyl-
N
ammonium salt of L1 (3,3-bis(1-methylimidazol-2-yl)propio-
nate) in methanol resulted in the formation of neutral
[FeII(L1)2] (1). The white product was analyzed by ESI-MS,
IR spectroscopy,and elemental analysis. The ESI-MS and
elemental analysis data point to the formation of a mononu-
clear,neutral species with a 2:1 ligand/metal ratio. The
major peaks at m/z 523.00 and 261.98 in the ESI-MS spec-
trum correspond to the [Fe(L1)2+H]+ and [Fe(L1)2+2H]2+
ions,respectively. The IR absorption spectrum obtained for
the iron(II) complex is similar to that of the copper(II) com-
plex[23] and,most importantly,the asymmetric stretching vi-
bration of the carboxylato group is found at the same fre-
quency (n˜as =1580 cmÀ1). This indicates that the carboxylato
group is bound to the metal in the same fashion. The sym-
metric stretch of the carboxylato group is assigned to the ab-
sorption at 1392 cmÀ1,which results in a
D
U
188 cmÀ1. It has been shown recently that this D value is de-
termined by the coordination mode symmetry and thus pro-
vides a useful structural probe.[25] The D
G
identical to D(n˜as-n˜s)ionic obtained for the corresponding free
G
carboxylate K[L1][23] and thus is indicative of a monodentate
binding mode of the carboxylate.[25] Based on these data,it
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1229

R. J. M. Klein Gebbink et al.
is proposed that [FeII(L1)2] (1) is isostructural with the cor-
responding copper(II) complex [CuII(L1)2][23] (Figure 2).
Two L1 ligands facially cap the iron(II) metal center in 1
through both imidazole N and the carboxylato O donor
atoms,very similarly to the coordination observed for the 2-
His-1-carboxylate facial triad.[2,4] The solution magnetic
moment of 1 in D2O as determined by Evansꢁ NMR
method[26,27] amounts to 5.2mB,which is consistent with a
high-spin configuration (S=2) at ambient temperature. Un-
fortunately,attempts at obtaining single crystals of complex
1 for X-ray analysis failed.
again reflects the coordination of the carbonyl group of the
ester functionality to the iron(II) metal center. [Fe
A
AHCTREUNG
From the data given above it can be concluded that 2 and 3
have isostructural cations in the solid state.
Crystal structure of [Fe
N
N
tals of 3 suitable for X-ray diffraction were obtained by slow
evaporation of a dichloromethane solution of 3. The molec-
ular structure of the cation of 3 is depicted in Figure 3; se-
Preliminary catalytic results in alkane and alkene oxida-
tions with H2O2 showed that 1 is not an active catalyst for
these kinds of transformations (vide infra). Therefore,we
also synthesized iron(II) complexes with the neutral ligand
PrL1 (propyl 3,3-bis(1-methylimidazol-2-yl)propionate), the
propyl ester precursor of L1. The mononuclear complex
[FeII
(2 equiv) with FeII
A
ACHTREUNG
ACHTREUNG
off-white solid that could be recrystallized from acetonitrile/
diethyl ether. In the IR absorption spectrum of 2 the
n˜
of 35 cmÀ1 to lower wavenumbers compared to the free
ligand (n˜
(C=O): 1724 cmÀ1). A similar shift has been ob-
(C=O) absorption frequency appears at 1689 cmÀ1,a shift
ACHTREUNG
served previously upon coordination of an ester carbonyl
functionality to an iron(II) metal center.[17] Furthermore,the
four sharp single vibrations at 1259 (n˜asSO3),1216 ( n˜sCF3),
1153 (n˜asCF3),and 1030 ( n˜sSO3) cmÀ1 are indicative of the
presence of noncoordinated triflate anions.[17,28,29] This im-
plies that each PrL1 is coordinated as a tridentate N,N,O
ligand (Figure 2). This notion is further corroborated by the
X-ray crystal structure determination of the complex [FeII-
Figure 3. Molecular structure of the [FeII(PrL1)2]2+ cation of 3 in the
A
crystal. All hydrogen atoms,the tetraphenylborate anions,and disordered
solvent molecules have been omitted for clarity. Displacement ellipsoids
are drawn at the 50% probability level. Symmetry operation a: 1Àx,
1Ày, 1Àz.
A
N
A
ACHTREUNG
5.1mB,consistent with a high-spin iron( II) metal center.
To exclude any possible coordination of the anion to the
metal in solution and/or participation of the anion in hydro-
gen bonding,such as that observed in 4 (vide infra),we ex-
changed the triflates in 2 for
lected bond lengths and angles of 3 are presented in Table 1.
The crystal structure of 3 consists of discrete mononuclear
molecules. The iron(II) metal ion is situated on a crystallo-
graphic inversion center and two neutral PrL1 ligands are
the noncoordinating tetraphe-
nylborate anions. [Fe(PrL1)2]-
(BPh4)2 (3) was synthesized by
Table 1. Selected bond lengths [Š] and angles [8] for [Fe
(4)
G
U
N
G
C
AHCTREUNG
ACHTREUNG
Bond length
addition of a methanolic solu-
tion of NaBPh4 to a solution of
2 in methanol followed by ad-
dition of water,which resulted
in immediate precipitation of
the crude product. The prod-
uct was recrystallized from an
acetonitrile/diethyl ether mix-
ture and characterized by IR
spectroscopy,ESI-MS,elemen-
tal analysis,and X-ray crystal
structure determination. The
sharp carbonyl stretch vibra-
tion observed at 1677 cmÀ1
3
À
Fe1 N1
Fe1 N4
Fe1 O1
C9 O1
2.122(2)
2.100(2)
2.228(2)
1.216(4)
1.332(4)
N1-Fe1-N
N4-Fe1-O1
N1-Fe1-O1
86.35(9)
87.94(8)
86.60(9)
N1-Fe1-N4a
N1-Fe1-O1a
N4-Fe1-O1a
93.66(9)
93.41(9)
92.06(8)
À
À
À
À
C9 O2
4
À
Fe1 N11
Fe1 N12
Fe1 N31
Fe1 N32
Fe1 O1
Fe1 O2
2.161(2)
2.154(2)
2.135(2)
2.170(2)
2.1941(19)
2.209(2)
N32-Fe1-O2
N11-Fe1-N12
N31-Fe1-O1
171.23(8)
175.78(8)
171.65(8)
N31-Fe1-N32
N32-Fe1-O1
O1-Fe1-O2
O2-Fe1-N31
N12-Fe1-N32
N32-Fe1-N11
N11-Fe1-O2
O2-Fe1-N12
101.75(8)
84.80(8)
86.62(8)
86.96(8)
84.63(8)
91.48(8)
90.33(8)
93.27(8)
À
À
À
À
N11-Fe1-N31
N31-Fe1-N12
N12-Fe1-O1
O1-Fe1-N11
84.58(8)
97.79(8)
87.89(8)
90.13(8)
À
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Catalytic Olefin Epoxidation and cis-Dihydroxylation
FULL PAPER
arranged centrosymmetrically around the metal. The ligands
cap the metal center facially through all three donor groups,
that is,two 1-methylimidazole nitrogen atoms and the car-
bonyl oxygen of the ester functionality,resulting in a six-co-
ordinate iron(II) metal center with an N4O2 donor set and
nearly ideal octahedral geometry. The deviation from ideal
octahedral geometry is reflected in the (diminished) angles,
which are mainly dictated by the inherent geometrical re-
À
strictions imposed by the tripodal ligand. The Fe N distan-
À
À
ces (Fe1 N1 2.122(2) Š,Fe1 N4 2.100(2) Š) are character-
istic of a high-spin iron(II) metal center and are comparable
to those found in high-spin iron(II) complexes with polyimi-
dazole ligands.[30,31] The structure is similar to the iron(II)
complex [FeII
A
N
thylbenzamide) reported recently by Que et al.,[18] featuring
an N4O2 donor set of four pyridines and two amide carbonyl
À
oxygens. Although the average Fe N bonds in 3 are shorter
than those in [FeII
(Ph-dpah)2]
N
À
the Fe1 O1 bond (2.228(2) Š) in 3 is considerably longer
than the amide carbonyl oxygen to iron bond (2.043 Š).
This difference in the coordination strength of an amide car-
bonyl and an ester carbonyl to an iron(II) metal center has
been observed before.[32] The coordination of an ester group
to an iron(II) metal center is quite rare and only a few struc-
turally characterized examples have been reported.[17,32–34]
Crystal structure of [Fe
A
G
N
tallization of [Fe(PrL1)2]
A
ACHTREUNG
ethyl ether resulted in the formation of complex 4,whose
structure was determined by X-ray diffraction. In [Fe-
A
G
A
Figure 4. a) Molecular structure of the [FeII
N
N
ties of both ligands are not coordinated to the FeII metal
À
4 in the crystal. C H hydrogen atoms and triflate anions have been omit-
ted for clarity. Only the major disorder component (62.1% occupancy) of
the disordered propoxy group is depicted. Displacement ellipsoids are
drawn at the 50% probability level. b) The hydrogen bonding pattern in
center. IR analysis of the crystals showed that the carbonyl
stretch vibration n˜(C=O) is split into two signals at 1732 and
A
1724 cmÀ1 of almost equal intensity,close to the value of the
free ligand. The two signals are probably the result of the
slightly different orientations of the ligands around the
metal ion,as seen in the crystal structure of 4. The solution
[FeII
(PrL1)2
(MeOH)2]
N
magnetic moment of [Fe
A
G
N
[D6]acetone is 5.2mB,consistent with a high-spin iron( II)
complex. Colorless crystals of 4 suitable for X-ray diffrac-
tion were obtained by slow diffusion of diethyl ether into a
solution of 2 in methanol. The molecular structure of 4 is
given in Figure 4; selected bond lengths and angles are pre-
sented in Table 1.
The crystal structure of 4 consists of discrete mononuclear
molecules. In the complex cation of 4,two ligands PrL1
bind to the metal center in an N,N bidentate fashion
through the N donor atoms of the 1-methylimidazole
groups. The propoxy group of one of the noncoordinated
ester groups is disordered over two positions with occupan-
cies of 62.1% and 37.9% for the major and minor compo-
nent,respectively. The orientation of the ligands resembles
that found in [FeII(L)2(OH)]BF4 (L: bis(1-methylimidazol-2-
yl)-3-methylthiopropanol),in which the thioether tail is sim-
ilarly pointing away from the metal center.[30] The fifth and
sixth coordination sites are occupied by two cis-positioned
methanol molecules,completing the distorted octahedral
À
N4O2 coordination sphere around the iron(II) ion. The Fe
N bond lengths are rather different,ranging from 2.135(2) Š
À
À
for Fe1 N31 to 2.170(2) Š for Fe1 N32. The imidazole
groups at the longest and shortest distance are both located
À
trans to a methanol molecule. The average Fe N distance is
2.16 Š,characteristic of a high-spin iron(II) species. The
À
Fe N distances are greater than those observed in [Fe-
À
A
U
nol molecules are similar and on average amount to 2.20 Š,
À
a value that compares well to the Fe O distance found for
the methanol molecule located trans to a pyridine donor
À
group
(Fe O=2.195 Š)
in
[FeII
N
U
A
drogen bond,acting as a hydrogen bond donor with a tri-
flate oxygen as acceptor. The hydrogen bonding pattern is
shown in the Figure 4 inset and relevant angles and distan-
ces are given in Table 2.
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R. J. M. Klein Gebbink et al.
Table 2. Selected hydrogen bond lengths [Š] and angles [8] for [FeII-
(PrL1)2(MeOH)2]
(OTf)2 (4).[a]
A
N
ACHTREUNG
[b]
À
À
D H
À
D H···A
H···A
D···A
D H···A
À
O1 H10··· O13a
0.79(3)
0.75(3)
1.93(3)
2.07(3)
2.705(3)
2.812(3)
166(3)
172(3)
À
O2 H20···O14
[a] Symmetry operation a: xÀ1, y, z). [b] D: donor; A: acceptor.
Solution structures of 2–4 (ESI-MS, solution IR, and
19F NMR spectra): The crystal structures of 3 and 4 illustrate
the possibility of different binding modes of the ligand PrL1
to an FeII ion. To determine the structure of the complexes
in solution,the ESI-MS,solution IR,and
19F NMR spectra
of complexes 2–4 were recorded. The position of the carbon-
yl stretch vibration was found to be indicative of the coordi-
nation mode of the ligand. Vibrations at distinct wavenum-
bers were observed for the coordinated and noncoordinated
ester carbonyl group vibrations in the solid state (Table 3).
The energies of these vibrations corroborate the structural
information provided by the crystal structures. The solution
IR spectra were found to be highly solvent-dependent. Un-
fortunately,due to the limited solubility of [Fe
A
ACHTREUNG
obtained only for an acetonitrile solution of this compound.
Interestingly,the IR spectra of 2 and 3 in acetonitrile are
identical in the carbonyl absorption region (Figure 5c). Two
absorptions at 1737 and 1702 cmÀ1 were observed,corre-
sponding to noncoordinated and coordinated ester carbonyl
Figure 5. a) Electrospray ionization mass spectrum for 2 in methanol;
b) measured and calculated isotope distribution patterns for the {[FeII-
A
N
broad and several shoulders
were observed. This indicates
that not all triflate anions are
noncoordinated,but some of
them are either directly bound
to the metal center or involved
in hydrogen bonds with coor-
dinated solvent molecules. The
Table 3. Solid-state and solution IR vibrations of the carbonyl groups in complexes 2–4.[a]
(C=O)
n
A
2
3
4
[b]
A
]
[Fe
G
G
[Fe
A
(BPh4)2
[Fe
G
G
G
1689
1737,1702
1740,1704 (sh)
1727 (sh), 1694
1677
1737,1702
i.s. [c]
[d]
MeCN
MeOH
CH2Cl2
n.d.
1740,1707 (sh)
1732, 1697
i.s.
[a] The signal with the strongest absorption is reported in bold. [b] Free ligand PrL1: 1724 and 1735 cmÀ1 for
solid-state and MeCN solution,resp. [c] i.s.: insufficiently soluble. [d] n.d.: not determined.
19F NMR spectrum of
2 in
methanol points to the latter
option. Again one single,sharp
groups,respectively. Furthermore,four sharp vibrations at
signal at d=À80.2 is observed,which excludes direct bind-
ing to the metal since coordination to a paramagnetic center
would cause a significant shift. Finally,spectra recorded in
the noncoordinating solvent dichloromethane showed the
carbonyl vibration around 1695 cmÀ1 for both 2 and 4,indi-
cating coordination of the ester carbonyl functionalities. A
small shoulder on this signal is observed at 1727 cmÀ1,indi-
cating carbonyl group exchange to a small extent. This is
corroborated by the 19F NMR spectrum of 2 in dichlorome-
thane,which shows a single feature at d=À65.7. As the
temperature is decreased the signal slightly broadens and
gradually shifts to d=À75.5 upon cooling to À808C. This
points to a rapid equilibrium between coordinated and non-
coordinated triflates,in which the latter predominate. When
the temperature is decreased,the signal shifts to the value
expected for free triflates and little or no fluxional exchange
of the carbonyl group for triflate occurs. This coordinative
À1
1272,1227,1156,and 1035 cm
were found for the triflate
anions of 2. Indeed,the 19F NMR spectrum of 2 in acetoni-
trile shows one single,sharp signal at d=À78.6,correspond-
ing to free triflate.[9] The data suggest that dissociation of
the ester groups and subsequent replacement with acetoni-
trile solvent molecules happens to the same extent for both
2 and 3. This is important,since vacant sites on the metal
center are a necessary requirement for metal-based catalysis
(vide infra). Identical spectra were also obtained for metha-
nolic solutions of [Fe
A
G
N
A
ACHTREUNG
1740 cmÀ1. This suggests that the ligand rearrangement pro-
cess in methanol solution is quite facile and the structure of
the cation of 2 and 4 in solution might resemble more the
structure of the cation observed in the crystal structure of 4.
The vibrations associated with the triflate anions are rather
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Chem. Eur. J. 2008, 14,1228 – 1237

Catalytic Olefin Epoxidation and cis-Dihydroxylation
FULL PAPER
saturation of 2 in dichloromethane causes the complex to be
almost inactive in the oxidation reactions (vide infra).
ESI-MS measurements indicate the presence of mononu-
clear species in solution for all complexes (Figure 5a,b), with
dation of styrene (TON 0.6 and 1.1 for 2 and 3,respectively)
A considerable amount of the allylic oxidation product 2-cy-
clohexen-1-ol (7.9 turnovers),but no 2-cyclohexen-1-one,
was observed with cyclohexene as a substrate. The addition
of more equivalents of peroxide resulted in a gradual drop
in efficiency of the catalytic conversion. Control experi-
ments using the epoxides as potential substrates under stan-
dard reaction conditions did not result in cis-diol product
formation,which showed that the diol product did not result
from the epoxide under the experimental conditions. The
stereoselectivity of the reactions was studied by the oxida-
tion of the cis- and trans-2-heptene isomers. The epoxidation
of cis-2-heptene and trans-2-heptene occurs with high ste-
reoselectivity to the corresponding cis- and trans-epoxides,
that is,with retention of configuration of 93% and 84%,re-
spectively. In addition,the cis-dihydroxylation of both sub-
strates shows a high retention of configuration. The high ste-
reoselectivity characterizes this oxidation as a true cis-dihy-
droxylation. We also tested 2 in the oxidation of 1-octene in
dichloromethane. The use of this solvent rendered the com-
plex almost completely inactive,with TON =0.1 for epoxide
and no cis-diol formation.
the {[Fe
N
N
ions as the major species. Interestingly,the 1:1 ligand/iron
complex and free ligand are also observed in the ESI-MS
spectrum (m/z=481.021). This feature is not observed in di-
chloromethane solution (data not shown),which indicates
that a ligand dissociation–association equilibrium exists only
in coordinating solvents.
Oxidation catalysis: Complexes 1, 2,and 3 were tested in
the oxidation of several different alkenes in acetonitrile so-
lution with H2O2 as the oxidant. The catalytic reactions
were performed at ambient conditions by slow,dropwise ad-
dition of 10 equiv H2O2 over 20 min,in order to minimize
peroxide disproportionation. The oxidations of styrene and
cyclohexene were run under an N2 atmosphere to suppress
autoxidation of the substrate. [FeII(L1)2] (1) was found to be
incapable of catalyzing the oxidation of olefins with H2O2 as
oxidant. The coordination of two monoanionic ligands in 1
probably results in relatively slow ligand exchange and
hence coordinative saturation at the metal center. This in
turn hampers the interaction of the metal with either perox-
ide or substrate and makes 1 ineffective in catalysis.
Interestingly,[Fe
N
N
lyst under the reaction conditions tested. Low turnover
numbers to epoxide are obtained in the oxidation of cyclo-
octene (0.8),styrene (1.2),1-octene (0.2),and cyclohexene
(0.6). No formation of diol is observed in these reactions.
The difference in activity is remarkable,given the similar
structure of the cations of 2 and 3 in solution (Figure 5).
The influence of the anion on the accessible reaction path-
ways is therefore substantial and might be related to the
possibility of hydrogen bonding interactions with the reac-
tive iron-based oxidant and the chemical stability of the
anion under the reaction conditions. Instead,significant
amounts of biphenyl were observed in the GC traces of re-
action mixtures with 3, pointing to the degradation of the
BPh4 anion. To determine to what extent this observed dif-
ference in reactivity can be attributed to the reductive prop-
Table 4 summarizes the results obtained in the olefin oxi-
dation experiments with 2 and 3. [Fe
N
N
lyzes the epoxidation and cis-1,2-dihydroxylation of various
olefins with the conversion of H2O2 ranging from 39 to
51%. The oxidation of cyclooctene with 10 equiv peroxide
afforded a clean reaction with 1,2-epoxycyclooctane and cis-
1,2-cyclooctanediol as the only products, with 39% efficien-
cy and an epoxide/diol ratio of 2.5:1. A slight preference for
epoxidation is also observed for the substrates styrene and
cyclohexene. For 1-octene,the formation of the cis-diol
product was favored over the epoxide by a factor of almost
two. A small amount of benzaldehyde was found in the oxi-
[a]
Table 4. Oxidation of alkenes catalyzed by [Fe
A
(OTf)2 (2) and [Fe
G
N
Substrate
H2O2 [equiv]
Epoxide[b]
Diol[b]
Conversion [%][c]
Epoxide/diol ratio
2
2.8
3.8
2.3
4.9
1.6
2.4
3.3
3.0
3
5
0.5
–
0.1
–
0.1
0.2
–
6
–
–
–
–
0.1
0.1
–
–
–
2
1.1
1.5
1.7
3.4
2.7
4.5
7.0
2.1
3
0
0
0
0
–
–
–
–
–
–
–
5
7.0
–
8.0
–
7.6
10.3
–
6.2
–
6
–
–
–
–
0.7
0.6
–
–
–
2
3
8
8
12
11
–
–
–
–
–
5
75
–
81
–
77
53
–
69
–
6
–
–
–
–
8
4
–
–
–
–
–
2
5
1:14
–
1:80
–
1:76
1:52
–
1:9
–
6
–
–
–
–
1:7
1:6
–
–
–
cyclooctene
styrene[d,e]
1-octene[f]
10
20
10
20
10
20
40
10
20
20
20
0.8
1.5
1.2
2.2
–
–
–
–
–
39
27
40
41
43
35
26
51
45
28
63
2.5 : 1
2.5:1
1.4 : 1
1.4:1
1:1.7
1:1.9
1 : 2.1
1.4 : 1
1.3:1
1.4:1
1:1
cyclohexene[d,g]
0.7
–
–
5.0
3.9
trans-2-heptene
cis-2-heptene
3.2[93][h]
6.2[84][h]
–
–
–
–
2.3[91][h]
6.3[92][h]
–
–
–
–
–
–
–
–
–
–
–
–
–
[a] Selected data from published alkene oxidation experiments on the other two reported iron complexes with a mixed N,N,O-donor set: that is,[Fe
A
dpah)2] (OTf)] (6) (ref. [21] have been included for comparison. [b] Yields expressed as turnover numbers (TON=mol product/
A
ACHTREUNG
mol catalyst). [c] Percentage conversion of H2O2 into epoxide and cis-diol. [d] Reaction performed under N2 atmosphere with deoxygenated solutions.
[e] Some benzaldehyde formation was observed,with TON =0.6 and 1.1 for 2 and 3,resp. [f] Oxidation of 1-octene catalyzed by 2 in dichloromethane
(20 equiv H2O2) resulted in 0.1 equiv epoxide and no cis-diol product. [g] The allylic oxidation product 2-cyclohexen-1-ol was observed,with TON =7.9.
[h] Retention of configuration (in brackets)=100”(AÀB)/
A
Chem. Eur. J. 2008, 14,1228 – 1237
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
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1233

R. J. M. Klein Gebbink et al.
erties of the tetraphenylborate anion,a blank experiment
was done with sodium tetraphenylborate and hydrogen per-
oxide under the experimental conditions of the catalytic
runs. Indeed,biphenyl formation was detected but signifi-
cantly less (ꢀ25%) than observed in the catalytic runs with
3 present.
these conditions. The availability of two labile sites located
cis to each other is important,since this is a necessary re-
quirement for cis-dihydroxylation.[7,8] The combination of
(stepwise) ligand dissociation and/or rearrangement leads to
many possible isomers in solution (Figure 6).
The combination of (solution) IR data and ESI-MS meas-
urements suggests that all isomers are present in solution to
a certain extent. The fact that 2 catalyzes the cis-dihydroxy-
lation of alkenes implies that either species G or F contrib-
ute significantly to the catalytic reactions. The lack of cata-
lytic activity in dichloromethane further emphasizes the ne-
cessity for complete or partial ligand dissociation.
Discussion
The reaction of 2 equiv of ligand L1 or PrL1 with an FeII
metal source resulted in the formation of the 2:1 ligand/
metal complexes 1, 2,and 3. In [FeII(L1)2] (1),the ligand
caps the metal center facially with an N,N,O donor set, simi-
larly to the coordination observed at the active site of the
mononuclear non-heme iron(II) enzymes featuring the 2-
His-1-carboxylate facial triad.[2,4] In this respect, 1 is related
to,yet distinct from the iron( II)–bispyrazolylacetate com-
plexes developed by Burzlaff et al.[15,16] Both ligand systems
offer a monoanionic N,N,O donor set, but differ in the
actual N donor groups. The X-ray crystal structure of [Fe-
Only a few mononuclear iron complexes capable of epoxi-
dation and cis-dihydroxylation have been reported to
date.[8,9,13,36] The best-studied examples of catalysts that elicit
this type of reactivity are the prototypical tpa- and bpmen-
based catalysts,[7,8] which both feature tetradentate N4 li-
gands. Related to these complexes are the N3py catalysts re-
ported by Feringa et al.[13] Only very recently the first exam-
ples of iron complexes with N,N,O ligands capable of olefin
cis-dihydroxylation were reported.[18,21] [Fe
N
N
A
G
(2) adds a new example to this rather exclusive set of iron
metal center facially through all donor atoms. However,the
structure of 4 illustrates that the tridentate,facial capping
mode of PrL1 is not the only possible coordination mode of
the ligand. In fact,solution IR measurements indicate the
presence of both coordinated and noncoordinated carbonyl
groups. It is interesting that the weakly bound carbonyl
donor groups are located trans to each other in the solid-
state structure of 3,whereas the two solvent molecules in 4
occupy cis positions relative to each other. This requires a
rearrangement of the ligands with respect to each other.
Dissolution of both 2 and 4 in either methanol or acetoni-
trile results in the formation of identical complexes in solu-
tion,according to solution IR and ESI-MS measurements;
this indicates that such a rearrangement is quite facile under
catalysts. Given the overall structural resemblance between
[18]
[Fe
A
(OTf)2 and [Fe
G
N
ference in selectivity for epoxide or cis-diol formation is re-
markable. Whereas [Fe(Ph-dpah)2](OTf)2 yields a predomi-
A
ACHTREUNG
nantly cis-diol product (it is the most efficient cis-dihydroxy-
lation catalyst reported to date), 2 seems to favor epoxida-
tion slightly.
The epoxide/diol product ratio has been correlated to the
spin state of iron(III)–hydroperoxo intermediates,with high-
G
spin complexes leading to increased selectivity for the cis-
diol product.[7] Complex 2 features a high-spin metal center
and is expected to yield a putative high-spin hydroperoxo
intermediate upon reaction with H2O2,given the weak field
exerted by the tridentate ligand. Yet 2 is more selective for
Figure 6. Possible solution structures of 2 and 3. X denotes solvent or OTf,depending on the compound and conditions. The boxed species correspond to
the crystal structures of 3 and 4.
1234
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
Chem. Eur. J. 2008, 14,1228 – 1237

Catalytic Olefin Epoxidation and cis-Dihydroxylation
FULL PAPER
epoxide formation. The formation of a purple species was
observed in the reaction of both 2 and 3 with tert-butyl hy-
droperoxide at low temperature,which indicates the forma-
tion of a FeIII-OOtBu species[37–39] similar to the hydroper-
oxo intermediate invoked above. Interestingly,the purple
species slowly converts into a second,green,intermediate.
Further investigations of these intermediates and,for exam-
ple,their spin states are currently under way.
The influence of the anion on the observed catalytic activ-
ity of 2 and 3 is remarkable. The exchange of triflate for tet-
raphenylborate anions results in a complete loss of cis-dihy-
droxylation activity and a greatly diminished epoxidation ef-
ficiency. Solution IR studies and ESI-MS measurements,
however,suggest that the structures of the cations of 2 and
3 in acetonitrile are similar and the influence of the anions
should therefore be attributed either to second-sphere inter-
actions or to BPh4 anion degradation under the catalytic
conditions (compare biphenyl formation). Precedents for
the role of second-sphere noncovalent interactions,for ex-
ample,hydrogen bonds,are found both in Nature and in
biomimetic model systems.[40,41] The interesting catalytic
properties of 2 might therefore be correlated with the ability
of triflate anions to accept hydrogen bonds and in this way
stabilize reactive intermediates of the catalytic cycle.
apocarotenoid-15–15’-oxygenase (Figure 7). The biologically
relevant 1-methylimidazole donor groups of PrL1 mimic the
fourfold histidine coordination accurately. The average
À
Fe N bond lengths (2.16 Š) agree well with the reported
values. The second key structural feature is the availability
of two cis-positioned vacant sites,which can be occupied by
solvent molecules. The two coordinated methanol solvent
molecules in 4 mimic this structural feature of the active
site. The ester tails furthermore provide an entry into pep-
tide chemistry to mimic the protein backbone and thus
allow the inclusion of the second coordination sphere in
modeling studies.
The oxidative cleavage of carotenoids was recently estab-
lished as a dioxygenative process[48] and a side-on binding of
dioxygen to the metal center has been suggested.[42] These
mechanistic features are reminiscent of the Rieske dioxyge-
nases,[5] which served as the inspiration for the biomimetic
cis-dihydroxylation catalysts reported here. Further studies
into the reactivity of the complexes presented here might
shed light on possible mechanistic similarities between these
two classes of non-heme iron enzymes.
Conclusion
Finally,the molecular structure of [Fe
N
N
A
As part of our recent efforts in the structural and functional
modeling of non-heme iron enzymes featuring the 2-His-1-
carboxylate facial triad,we studied the iron(II) coordination
chemistry of the potentially tridentate, tripodal N,N,O li-
gands L1 and PrL1. The molecular structures of 3 and 4 re-
vealed two different binding modes of the new ligand PrL1,
that is, tridentate N,N,O and bidentate N,N coordination, re-
spectively. Solution studies,however,revealed the facile in-
terconversion between the two different geometries. Com-
active site of apocarotenoid-15–15’-oxygenase (ACO),a reti-
nal-forming carotenoid oxygenase.[42] Carotenoids are pre-
cursors for retinal and its derivatives,which are crucial for
vision and for the immune system.[43] ACO catalyzes the oxi-
dative cleavage of the 15–15’ double bond of its substrate.
The crystal structure of the enzyme–substrate complex re-
veals a non-heme iron(II) active site,in which the metal
center is coordinated by four histidine residues (Figure 7).[42]
plex [FeII
(BPh4)2 (3),was found to be an active catalyst in olefin oxi-
A
G
N
AHCTREUNG
dation reactions,which illustrates the importance of the
anion. Complex 2 constitutes a new example of the rather
exclusive class of non-heme iron catalysts,which are capable
of catalyzing both the epoxidation and cis-dihydroxylation
of substrates.
Experimental Section
Figure 7. First coordination sphere of the mononuclear iron(II) enzyme
apocarotenoid-15–15’-oxygenase (ACO,2BIW.pdb) and that of complex
Air-sensitive organic reactions were carried out under an atmosphere of
[Fe
A
R
U
dry,oxygen-free N using standard Schlenk techniques. THF and diethyl
2
ether were dried over sodium benzophenone ketyl and distilled under N2
before use. Methanol was dried over magnesium methoxide and distilled
under N2 before use. The air-sensitive iron complexes 1–4 were synthe-
sized and handled under an N2 atmosphere using standard Schlenk tech-
niques. Solvents were thoroughly deoxygenated with N2 before use. 1H
and 13C{1H} NMR spectra were recorded on a Varian AS400 or Varian
Inova 300 spectrometer,operating at 25 8C. Infrared spectra were record-
ed with a Perkin-Elmer Spectrum One FT-IR instrument. Solution IR
measurements were recorded with a Mettler Toledo ReactIRꢂ 1000 spec-
trometer with a SiCompꢂ probe,which was fitted in a reaction vessel
under an N2 atmosphere. GC analyses were performed on a Perkin–
Elmer Autosystem XL GC (30 m,PE-17 capillary column) and a Perkin–
This structural motif is relatively rare and has been re-
ported in only four other enzymes.[44–47] The coordination ge-
ometry around the iron in ACO is octahedral with an aver-
À
age Fe Nim distance of 2.17 Š and two cis-positioned coordi-
nation sites available for the binding of dioxygen. A water
molecule occupies one of these sites,whereas the other is
vacant in the reported crystal structure. [FeII
ACHTREUNG
A
ACHTREUNG
Chem. Eur. J. 2008, 14,1228 – 1237
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
www.chemeurj.org
1235

R. J. M. Klein Gebbink et al.
Elmer Clarus 500 GC (30 m,Econo-Cap EC-5),both with FID detector.
Elemental microanalyses were carried out by the Microanalytisches Lab-
oratorium Dornis & Kolbe,Mulheim a.d. Ruhr,Germany. ESI-MS spec-
tra were recorded on a Micromass LC-TOF mass spectrometer at the
Biomolecular Mass Spectrometry group,Utrecht University. UV/Vis
spectra were recorded on a Varian Cary 50. Solution magnetic moments
were determined by the Evans NMR method in D2O/1,4-dioxane
(solid): n˜ =3371.8,2969.5,1732.0,1723.7,1503.6,1421.6,1360.4,1281.1,
1247.0,1222.7,1194.7,1150.8,1031.6,1011.84,977.3,731.6 cm
À1; ESI-MS:
m/z=303.96 {[MÀ2OTf]2+,calc. 304.13},757.05 {[
MÀOTf]+,calc.
757.18}; solution magnetic moment (Evansꢁ method): meff =5.2 mB; ele-
mental analysis calcd (%) for C32H48F6FeN8O12S2 (970.74): C 39.59,H
4.98,N 11.54; found C 39.40,H 5.02,N 11.59.
Catalysis protocol: Substrate (1000 equiv,3 mmol) and acetonitrile (to
bring the total volume to 2.5 mL) were added to a solution of catalyst
(3 mmol) in acetonitrile (2 mL). Subsequently,0.5 mL of oxidant solution
(10 equiv,60 m m solution in acetonitrile diluted from 35% aqueous
H2O2) was added dropwise in 20 min. The reaction mixture was stirred at
room temperature and after 1 h (from the start of oxidant addition) inter-
nal standard (10 mL: cyclooctene/1,2-dibromobenzene; all other sub-
strates: bromobenzene) was added and the first sample was taken. An
aliquot of the reaction mixture was filtered over a short silica plug,after
which the short column was flushed twice with diethyl ether. The sample
was concentrated in a stream of N2 and analyzed by GC. The products
were identified and quantified by comparison with authentic compounds.
ACHTREUNG
ACHTREUNG
ACHTREUNG
The epoxides of cis- and trans-2-heptenes were synthesized by stereose-
lective epoxidation of the olefins with mCPBA. Hydrolysis of these epox-
ides by HClO4 in H2O/THF yielded the corresponding diols. All other
chemicals were obtained commercially and used as received.
[Fe(L1)2] (1): A solution of Fe
ACHTREUNG
methanol (10 mL) was added to
a
0.63 mmol) in methanol (20 mL); a white precipitate formed gradually.
The reaction mixture was stirred for 1 h at 508C,after which diethyl
ether was added to precipitate the product. The crude product was sepa-
rated by centrifugation and washed twice with diethyl ether (2”40 mL).
The product was obtained as a white powder (161 mg,98%). IR (solid):
n˜ =3120.2,2946.8,2906.3,2815.2,1580.2,1506.7,1426.0,1392.4,1310.1,
X-ray crystal structure determinations: X-ray intensities were measured
on a Nonius Kappa CCD diffractometer with rotating anode (graphite
monochromator, l=0.71073 Š) at 150 K. The structures were solved by
automated Patterson methods[50] (3) or direct methods[51] (4) and refined
with SHELXL-97[52] against F2 of all reflections. Geometry calculations
and checks for higher symmetry were performed with the PLATON pro-
gram.[53]
1286.9,1230.4,1163.0,1140.4,1045.2,953.3,771.9,753.3 cm
À1; ESI-MS:
m/z=261.97 {[M+2H]2+,calc. 262.08},523.00 {[ M+H]+,calc. 523.15}; so-
lution magnetic moment (Evansꢁ method): meff =5.2mB; elemental analysis
calcd (%) for C22H26FeN8O4 (522.34): C 50.59,H 5.02,N 21.45; found C
50.28,H 4.92,N 21.30.
X-ray crystal structure determination of 3: [C28H40FeN8O4](C24H20B)2,
G
formula weight=1246.95,* colorless block, 0.15”0.15”0.10 mm3,mono-
clinic, P21/c (no. 14), a=14.9857(2), b=11.0234(2), c=24.7254(4) Š, b=
119.7779(5)8,
V=3545.14(10) Š3,
Z=2,
Dx =1.168 gcmÀ3,*
m=
[Fe
A
(OTf)2 (2):
A
solution of Fe
ACHTREUNG
0.27 mmÀ1.* 40742 Reflections were measured up to a resolution of (sinq/
l)max =0.52 ŠÀ1. The reflections were corrected for absorption on the
basis of multiple measured reflections (0.86–0.97 correction range). 4189
reflections were unique (Rint =0.0845). Non-hydrogen atoms were refined
with anisotropic displacement parameters. All hydrogen atoms were in-
troduced in calculated positions and refined with a riding model. The
crystal structure contains large voids,filled with disordered solvent mole-
cules (300.1 Š3/unit cell). Their contribution to the structure factors was
secured by back Fourier transformation with the SQUEEZE routine of
the PLATON package[53] (56 electrons/unit cell). 415 parameters were re-
fined with no restraints. R1/wR2 [I>2s(I)]: 0.0441/0.1095, R1/wR2 [all
0.91 mmol) in methanol (10 mL) was added to
a
(506 mg,1.83 mmol) in methanol (15 mL) and the reaction mixture was
stirred for 30 min. The solvent was evaporated in vacuo and the remain-
ing off-white solid was recrystallized from an acetonitrile/diethyl ether
mixture at À308C overnight. The product was obtained as a slightly
greenish crystalline solid (430 mg,52%). IR (solid): n˜ =3126.5,2973.5,
1689.1,1506.7,1404.4,1258.9,1215.6,1152.4,1029.9,948.7,782.1,
755.3 cmÀ1
;
ESI-MS: m/z=277.12 {[PrL1+H]+,calc 277.17},304.00
{[MÀ2OTf]2+,calc. 304.13},481.02 {[M ÀPrL1ÀOTf]+,calc. 481.05},
757.20 {[MÀOTf]+,calc. 757.18}; solution magnetic moment (Evansꢁ
method):
meff =5.1 mB;
elemental
analysis
calcd
(%)
for
refl.]: 0.0652/0.1197. S=1.047. Residual electron density between À0.27
C30H40F6FeN8O10S2 (906.65): C 39.34,H 4.45,N 12.36; found C 39.75,H
4.64,N 12.48.
À3
and 0.49 eŠ
.
X-ray crystal structure determination of 4: [C30H48FeN8O6](CF3SO3)2,for-
C
[Fe
(PrL1)2]
(BPh4)2 (3):
A
solution of Fe
ACHTREUNG
¯
0.56 mmol) in methanol (10 mL) was added to
(312 mg,1.13 mmol) in methanol (10 mL) and the reaction mixture was
stirred for 30 min. Subsequently,a solution of NaBPh (1.1 g,3.2 mmol)
a
b=101.274(5), g=91.638(4)8, V=2130.2(4) Š3, Z=2, Dx =1.513 gcmÀ3
,
4
m=0.55 mmÀ1. 37349 reflections were measured up to a resolution of
(sinq/l)max =0.61 ŠÀ1,and corrected for absorption on the basis of multi-
ple measured reflections (correction range 0.80–0.97); 7945 reflections
were unique (Rint =0.0436). Non-hydrogen atoms were refined with ani-
sotropic displacement parameters. All hydrogen atoms were located in
in methanol (10 mL) was added in a single portion to the reaction mix-
ture and immediately a white precipitate formed. The addition of water
(50 mL) to the suspension caused further precipitation of the product.
The suspension was stirred for 20 min,after which the solid was filtered
off and washed three times with H2O (3”20 mL) to yield an off-white
powder. The crude product was recrystallized from acetonitrile/diethyl
ether at À308C to give the product as a slightly greenish crystalline solid
(401 mg,57% yield). Single crystals of 3 suitable for X-ray diffraction
were obtained by slow evaporation of a dichloromethane solution. IR
(solid): n˜ =3139.2,3122.5,3052.5,2972.1,2860.8,1677.3,1579.5,1507.7,
1479.4,1426.5,1388.2,1287.1,1268.0,1214.6,1178.4,1118.6,1064.6,
À
the difference Fourier map. The O H hydrogen atom was refined freely
with isotropic displacement parameters; all other hydrogen atoms were
refined with a riding model. The propoxy group was refined with a disor-
der model. 604 parameters were refined with 55 restraints. R1/wR2
A
À3
sidual electron density between À0.39 and 0.31 eŠ
1029.2,946.3,850.0,732.7,704.0 cm
À1; ESI-MS: m/z=303.99 {[MÀ2-
CCDC 622570 (3) and 622571 (4) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
(BPh4)]2+,calc. 304.13},927.33 {[ MÀBPh4]+,calc. 927.42}; solution mag-
netic moment (Evansꢁ method): meff =5.0 mB; elemental analysis calcd (%)
for C76H80B2FeN8O4 (1246.97): C 73.20,H 6.47,N 8.99; found C 73.28,H
6.40,N 8.86.
[Fe
0.11 mmol) from a methanol/diethyl ether mixture at À308C resulted in
the formation of [Fe(PrL1)2(MeOH)2](OTf)2 (4) as a white,microcrystal-
line solid after several days (54 mg,50%). Slow vapor diffusion of diethyl
ether into a solution of [Fe(PrL1)2](OTf)2 (2) in methanol at room tem-
perature yielded colorless crystals suitable for X-ray diffraction. IR
A
G
G
2
(100 mg,
[*] Derived parameters do not contain the contribution of the disordered
solvent.
1236
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Chem. Eur. J. 2008, 14,1228 – 1237

Catalytic Olefin Epoxidation and cis-Dihydroxylation
FULL PAPER
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Received: April 13,2007
Revised: September 18,2007
Published online: November 19,2007
Chem. Eur. J. 2008, 14,1228 – 1237
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www.chemeurj.org
1237