Mono- and binuclear complexes of iron(II) and iron(III) with an N 4O ligand: Synthesis, structures and catalytic properties in alkane oxidation

Article abstract of DOI:10.1039/b516697g

Three mononuclear iron complexes and one binuclear iron complex, [Fe(tpoen)Cl]·0.5(Fe₂OCl₆) (1), [Fe(tpoen)Cl]PF₆ (2), Fe(tpoen)Cl₃ (3) and [{Fe(tpoen)}₂(-O)](ClO₄)₄ (4) (tpoen = N-(2-pyridylmethoxyethyl)-N,N-bis(2-pyridylmethyl)amine), were synthesized as functional models of non-heme iron oxygenases. Crystallographic studies revealed that the Fe(ii) center of 1 is in a pseudooctahedral environment with a pentadentate N₄O ligand and a chloride ion trans to the oxygen atom. The Fe(iii) center of 3 is ligated by three nitrogen atoms of tpoen and three chloride ions in a facial configuration. Each Fe(iii) center of 4 is coordinated with four nitrogen atoms and an oxygen atom of tpoen with the Fe-O-Fe angle of 172.0(3) A. Complexes 2, 3 and 4 catalysed the oxidation of cyclohexane with H₂O₂ in the total TNs of 24-36 with A/K ratios of 1.9-2.4. Under the same conditions they also catalysed both the oxidation of ethylbenzene to benzylic alcohol and acetobenzene with good activity (30-47 TN) and low selectivity (A/K 0.7), and the oxidation of adamantane with moderate activity (15-18 TN) and low regioselectivity (3°/2° 3.0-3.2). With mCPBA as oxidant the catalytic activities of 2, 3 and 4 increased 1.8 to 2.3-fold for the oxidation of cyclohexane and ethylbenzene and 6.3 to 7.5-fold for the oxidation of adamantane. Drastic enhancement of the regioselectivity was observed in the oxidation of adamantane (3°/2° 18.5-30.3). The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516697g

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Mono- and binuclear complexes of iron(II) and iron(III) with an N O ligand:
4
synthesis, structures and catalytic properties in alkane oxidation†
a
a
b
a
a
a,c
Fei Li, Mei Wang,* Chengbing Ma, Aiping Gao, Hongbo Chen and Licheng Sun*
Received 24th November 2005, Accepted 6th March 2006
First published as an Advance Article on the web 15th March 2006
DOI: 10.1039/b516697g
Three mononuclear iron complexes and one binuclear iron complex, [Fe(tpoen)Cl]·0.5(Fe
2
OCl ) (1),
6
[
Fe(tpoen)Cl]PF (2), Fe(tpoen)Cl (l-O)](ClO
6
3
(3) and [{Fe(tpoen)}
2
4
)
4
(4) (tpoen = N-(2-
pyridylmethoxyethyl)-N,N-bis(2-pyridylmethyl)amine), were synthesized as functional models of
non-heme iron oxygenases. Crystallographic studies revealed that the Fe(II) center of 1 is in a
pseudooctahedral environment with a pentadentate N O ligand and a chloride ion trans to the oxygen
4
atom. The Fe(III) center of 3 is ligated by three nitrogen atoms of tpoen and three chloride ions in a
facial configuration. Each Fe(III) center of 4 is coordinated with four nitrogen atoms and an oxygen
˚
atom of tpoen with the Fe–O–Fe angle of 172.0(3) A. Complexes 2, 3 and 4 catalysed the oxidation of
cyclohexane with H in the total TNs of 24–36 with A/K ratios of 1.9–2.4. Under the same
2
O
2
conditions they also catalysed both the oxidation of ethylbenzene to benzylic alcohol and acetobenzene
with good activity (30–47 TN) and low selectivity (A/K 0.7), and the oxidation of adamantane with
◦
◦
moderate activity (15–18 TN) and low regioselectivity (3 /2 3.0–3.2). With mCPBA as oxidant the
catalytic activities of 2, 3 and 4 increased 1.8 to 2.3-fold for the oxidation of cyclohexane and
ethylbenzene and 6.3 to 7.5-fold for the oxidation of adamantane. Drastic enhancement of the
◦
◦
regioselectivity was observed in the oxidation of adamantane (3 /2 18.5–30.3).
1
2
Introduction
N
5
-pentadentate ligands, such as [Fe(N4Py)(CH
3
CN)](ClO
CN)](ClO
4
)
)
2
2
,
,
13
14
[
Fe(PMA)(CH
3
CN)](ClO
4
)
2
and [Fe(PaPy
3
)(CH
3
4
The development of transition metal based efficient and selective
catalysts for environmentally benign alkane oxidation is one of
the important and challenging issues in homogeneous catalysis
and synthetic chemistry. In view of the special capability of
iron-containing oxygenases such as cytochrome P450, methane
monooxygenase (MMO), Rieske dioxygenase and iron bleomycin
(
H
Fig. 1), were found to catalyse alkane and alkene oxidations with
or TBHP as oxidant. Many l-oxo- and carboxylato-bridged
2
O
2
dinuclear iron complexes have proven to be efficient catalysts
for alkane hydroxylation combined with peroxide oxidants.
Moreover, the monoiron complexes [Fe(tpmen)Cl]PF
15,16
6
and
(
FeBLM) in the catalysis of hydrocarbon oxidation reactions,
the bio-inspired iron chemistry has received intensive attention
in recent decades. To mimic the structures and functions of
non-heme iron enzymes, many mono- and diiron complexes with
various multidentate N-based ligands were prepared and their
1–3
catalytic properties were widely investigated.
The outcome
of previous studies suggested that the metal centers of the iron
complexes can incorporate hydro- or alkyl-peroxide to form Fe(III)
peroxide species, which are capable of activating the C–H bonds
4–9
in alkanes via a high valent iron oxo intermediate. The high
performance catalysts of biomimetic non-heme mononuclear iron
complexes for alkane oxidation reported so far are those with N
tetradentate ligands, such as [Fe(TPA)(CH CN) ](ClO
[Fe(bpmen)(CH CN)
4
-
3
2
4
)
2
(TPA =
6,10
tris(2-pyridylmethyl)amine),
3
2
](ClO
4 2
)
ꢀ
ꢀ
(
bpmen = N,N -dimethyl-N,N -bis(2-pyridylmethyl)ethane-1,2-
11
diamine) and their derivatives. The monoiron complexes bearing
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Zhongshan Road 158-46, Dalian, 116012, China. E-mail: symbueno@
dlut.edu.cn; Tel: +86-(0)411-88993886
b
State Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, Fuzhou, 350002, China
c
KTH Chemistry, Organic Chemistry, Royal Institute of Technology, 10044,
Stockholm, Sweden
4
Fig. 1 Structures of the N O ligand used in the present work and the
†
Dedicated to Professor Victor Riera on the occasion of his 70th birthday.
analogous N
5
- and N
3
O-ligands used in non-heme iron model complexes.
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2427–2434 | 2427

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[
Fe(tpmpn)Cl]PF
6
(Fig. 1) exhibit interesting properties as
Preparation of the ligand and the iron complexes
catalysts for the hydroxylation of aromatic compounds although
they are not efficient catalysts for alkane hydroxylation and alkene
N - (2 - Pyridylmethoxyethyl) - N,N - bis(2 - pyridylmethyl)amine
(tpoen). 2-Picolyl chloride hydrochloride (2.94 g, 18 mmol) was
17
+
epoxidation. The complexes [FeLCl] (L = tpmen, tphen, Fig. 1)
dissolved in H
of the solution was adjusted to 7 by the addition of saturated
aqueous solution of K CO with vigorous stirring. The mixture
2
O (6 mL) and cooled with an ice-bath. The pH value
are capable of benzylic oxidation with a peroxide oxidant and the
2
+
complex [Fe(tpaen)Br] (Fig. 1) is an efficient DNA cleaver when
18,19
2
3
activated by H
2
2
O .
was then extracted with THF (20 mL) to give solution A. Sodium
hydride (52%, 0.92 g, 20 mmol) was gradually added to the
solution of ethanolamine (0.37 g, 6 mmol) in dry THF (20 mL)
Although mono- and diiron complexes of all-N multidentate
ligands were extensively studied, to the best of our knowledge, only
two iron complexes with ether-containing multidentate ligands
20,21
under N atmosphere. The mixture was stirred for 30 min to
2
were reported active for alkane oxidation.
that the l-oxo diiron complex [{Fe(mebpa)Cl}
exhibited high activity and good selectivity for the oxidation of
cyclohexane in the presence of H . The N O-coordinate diiron
complex [{Fe(tfPy) Cl} (l-O)](ClO (Fig. 1) was also reported to
be catalytically active in alkane oxidation reactions with H as
It was described
afford solution B. Solution A was added to Solution B and
the resulting mixture was refluxed for 18 h. After filtration, the
solvent was removed under reduced pressure and the residue was
2
O](ClO (Fig. 1)
4 2
)
2
O
2
3
extracted with CH
was washed with NaOH aqueous solution, dried over Na
concentrated in vacuo. The crude product obtained was purified
by silica gel chromatography with CH Cl –MeOH–Et N as eluent
20 : 1 : 0.1, v/v) to give an orange oil (1.1 g, 55%). H NMR
400 MHz, CDCl , 293 K): d 8.52–8.49 (m, 3H, Py), 7.81–7.73 (m,
H, Py), 7.58 (d, 2H, Py), 7.41 (d, 1H, Py), 7.30–7.23 (m, 3H, Py),
.54 (s, 2H, OCH Py), 3.86 (s, 4H, NCH Py), 3.67 (t, 2H, CH O),
.80 (t, 2H, NCH ).
2
Cl
2
(3 × 5 mL). The combined organic layer
2
)
4 2
2
SO and
4
2
O
2
the oxidant. The encouraging results of these reports stimulated
us to explore the structures and the catalytic properties of iron
2
2
3
1
(
(
complexes featuring an N O potentially pentadentate ligand. The
4
3
O atom, a weak donor compared to the N atom, may, in the
multidentate ligand, create a hemi-labile site on the iron center and
thus influences the catalytic property of the iron complex. Here
we describe the preparation of three monoiron and a l-oxo diiron
3
4
2
2
2
2
2
complexes, [Fe(tpoen)Cl]·0.5(Fe
2
OCl
6
) (1), [Fe(tpoen)Cl]PF
6
(2),
[
Fe(tpoen)Cl]·0.5(Fe
the ligand tpoen (0.34 g, 1 mmol) was added to a solution of
FeCl O (0.16 g, 1.1 mmol) in ethanol (5 mL). The yellow
2
OCl ) (1). A 10 mL ethanol solution of
6
Fe(tpoen)Cl
3
(3) and [{Fe(tpoen)}
2
(l-O)](ClO
)
4 4
(4) (Fig. 1), the
molecular structures of complexes 1, 3 and 4, and the catalytic
properties of 2, 3 and 4 in the oxidation reactions of cyclohexane,
ethylbenzene and adamantane with H
chloroperbenzoic acid) as oxidants.
2
·H
2
precipitate appeared immediately. After stirring for 30 min, the
precipitate 1 (0.27 g, 82%) was collected by filtration, washed with
ethanol and diethyl ether, and dried in vacuo. Single crystals of
2
2
O and mCPBA (m-
1
·0.5CH
3
CN were obtained by slow vapor diffusion of diethyl
ether into the acetonitrile solution. Anal. Calc. (found) for
Experimental
C
20
H
22Cl
4
Fe
2
N
4
O
1.5·0.5CH
3
CN: C, 40.9 (40.9); H, 3.8 (3.6); N,
2
+
1
4
0.2 (9.9)%. MS(API-ES): m/z 195.1 [M − 0.5(Fe
2
OCl )–Cl] ,
6
General procedures
+
25.1 [M − 0.5(Fe
2
OCl
6
)] .
All reactions and operations related to organometallic complexes
were carried out under a dry, oxygen-free dinitrogen atmosphere
with standard Schlenk techniques. Solvents were distilled prior
to use with routine drying agents. Other commercially available
chemical reagents were used as received.
[
Fe(tpoen)Cl]PF
6
(2). The salt FeCl
2
·H
2
O (0.117 g, 0.81 mmol)
was dissolved in methanol (5 mL) and added to a 5 mL methanol
solution of tpoen (0.277 g, 0.83 mmol). The orange solution was
stirred under N
added. The mixture was stirred for 3 h and a yellow precipitate
appeared during this time. The precipitate was collected by
filtration and extracted with acetonitrile. A yellow powder 2
2
for 30 min and NH
4
PF (0.116 g, 0.71 mmol) was
6
1
H NMR Spectra were collected on a Varian INOVA 400NMR
apparatus. Mass spectra of iron complexes were recorded on an
HP1100 MSD instrument. Elemental analyses were performed
on a ThermoQuest-Flash EA 1112 elemental analyzer. Gas
chromatography (GC) analyses were performed on an HP6890GC
instrument equipped with a flame ionization detector and an
HP-5 column (30 m × 0.32 mm) and GC-MS analyses were
carried out on an HP6890GC/5973MS apparatus. Cyclic voltam-
mograms (CVs) were recorded using a BAS-100W electrochemical
(
0.37 g, 81%) was obtained by removing solvent in vacuo. Anal.
Calc. (found) for C20 FeN OP: C, 42.1 (42.2); H, 3.9 (3.9);
N, 9.8 (9.8)%. MS(API-ES): m/z 195.4 [M–PF
H
22ClF
6
4
2
+
6
–Cl] , 425.5 [M–
+
PF ] .
6
Fe(tpoen)Cl
3
(3). The salt FeCl
3
·6H
2
O (0.270 g, 1 mmol) in
absolute ethanol (10 mL) was added to a solution of tpoen (0.311 g,
−
1
potentiostat at a scan rate of 100 mV s under argon. Studies
were carried out in CH CN solution using 0.05 M n-Bu NPF
Fluka, electrochemical grade) as the electrolyte with 10 M of
0
3
.93 mmol) in absolute ethanol (10 mL). A yellow precipitate
(0.42 g, 90%) appeared immediately, which was isolated by
3
4
3
6
−
(
filtration, and washed with ethanol and diethyl ether. The crystals
of 3 suitable for X-ray diffraction analysis were collected by slow
vapor diffusion of diethyl ether into the acetonitrile solution of
complex 3. Anal. Calc. (found) for C20
H, 4.5 (4.5); N, 11.3 (11.15)%. MS (API-ES): m/z 130.1 [M–3Cl] ,
iron complexes in a three-electrode cell. The working electrode
was a glass carbon disc (diameter 3 mm) successively polished
with 3 and 1 lm diamond pastes and sonicated in ion-free water
for 10 min. The reference electrode was a non-aqueous Ag/Ag
electrode (0.01 M AgNO
was a platinum wire.
H
22Cl
3
FeN
4
O: C, 48.4 (48.6);
+
3+
3
in CH
3
CN) and the auxiliary electrode
+
460.0 [M–Cl] .
Caution: Metal perchlorates are potentially explosive and
should be handled with care!
[{Fe(tpoen)}
2
(l-O)](ClO
4
)
4
(4). The salt Fe(ClO
4
)
2
·xH
2
O
(0.195 g, 0.75 mmol) in absolute ethanol (20 mL) was slowly
2
428 | Dalton Trans., 2006, 2427–2434
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dropped into an ethanol solution (5 mL) of tpoen (0.218 g,
.65 mmol). A yellow precipitate formed immediately. After filtra-
tion, the residue was dissolved in acetonitrile and the diffusion of
(1 mmol) was added in 3 min with rapid stirring under N . After
2
0
1.5 h at room temperature, an internal standard (1,2-dichloro-
benzene) was added to the reaction mixture. The conversions of
substrates were measured by GC and the structures of the products
were verified by GC-MS. In the case of the oxidation reaction of
diethyl ether into the solution afforded dark red crystals 4 (0.15 g,
3
4
8
1
6%). Anal. Calc. (found) for C40
1.0 (40.8); H, 3.9 (3.9); N, 10.4 (10.4)%. H NMR (DMSO-d
.49 (br s, 3H, Py), 7.76 (br s, 3H, Py), 7.56 (d, 2H, Py), 7.40 (d,
H, Py), 7.27 (br s, 3H, Py), 4.52 (s, 2H, OCH Py), 3.89 (s, 4H,
Py), 3.67 (br s, 2H, CH O), 2.83 (br s, 2H, NCH ).
H
44Cl
4
Fe
2
N
8
O
19·1.25CH
3
CN: C,
1
6
): d
adamantane, CH
3
CN–CH
2
2
Cl (3 and 2 mL, respectively) was used
as the solvent due to the poor solubility of adamantane in CH CN.
3
2
NCH
2
2
2
Results and discussion
X-Ray crystallography
Synthesis
Crystallographic data were measured on a Siemens SMART Sys-
tem CCD diffractometer using graphite monochromated MoKa
The new ligand tpoen, which contains three pyridine nitrogens,
one tert-amine nitrogen and one ether oxygen, was conveniently
prepared using cheap reagents ethanolamine and 2-picolyl chlo-
˚
radiation with a wavelength (k) of 0.71073 A. Data processing was
22
2−
accomplished with the SAINT processing program. Intensity
data were corrected for absorption by the SADABS program.
The structures were solved by direct methods and refined on F
ride. Complex 1 with [Fe
2
OCl
6
]
as a counteranion was readily
in ethanol. To avoid
the effect of the diiron counteranion in complex 1 on the oxidation
23
prepared by the reaction of tpoen and FeCl
2
2
2
−
against full-matrix least-squares methods using the SHELXTL
7 program. All non-hydrogen atoms were refined anisotropically.
experiment, complex 2 was prepared by replacement of [Fe
2
OCl ]
6
−
9
in complex 1 with PF
6
. The mononuclear Fe(III) complex 3 was
The hydrogen atoms were placed geometrically and held in the
riding mode. Machine parameters, crystal data and data collection
parameters are summarized in Table 1. Crystallographic data of 1,
prepared in a similar manner to that for complex 1 by the reaction
of tpoen and FeCl . The yellow powder directly precipitated
from the reaction of Fe(ClO O and tpoen in ethanol is
paramagnetic, evidenced by broad and wide-ranged H NMR
signals. Recrystallization of the paramagnetic yellow powder only
afforded the l-oxo diiron complex 4, which is diamagnetic. The
structure of the paramagnetic species formed initially was not
determined because of its instability in solution. It is assumed
3
4
)
2
·xH
2
1
3
and 4 have been deposited with the Cambridge Crystallographic
Data Center. CCDC reference numbers 247052–247054. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b516697g.
to be a monoiron complex [Fe(tpoen)(sol)](ClO
the similar spontaneous conversion of [Fe(SBPy
4
)
2
in the light of
Catalytic oxidation of alkanes
3
)(DMF)](ClO
)
4 3
24
In a typical run, an acetonitrile solution (4 mL) of cyclohexane
(Fig. 1) to a diiron complex [{Fe(SBPy
3
)}
2
(l-O)](ClO ) . All four
4
4
(
5 mmol) or ethylbenzene and the catalyst (5 lmol) was precooled
complexes are air stable in the solid state. They are quite soluble
in acetonitrile and sparingly soluble in methanol.
◦
to 0 C. Oxidant H
2
O
2
(1 mmol, 5 M in CH CN) or mCPBA
3
Table 1 Crystallographic data and processing parameters for 1·0.5CH
3
3
CN, 3 and 4·2CH CN
Complex
1·0.5CH
3
CN
Fe
3
4·2CH
3
CN
Fe
Formula
C
21
H
23.50Cl
4
2
N
4.50
O
1.50
C
20
H
22
C
l3FeN
4
O
C
44
H
50Cl
4
2 10 19
N O
M
w
616.44
Monoclinic
P2/c
14.651(3)
8.674(3)
20.993(4)
90.00
90.27(2)
90.00
496.62
Monoclinic
P2 /n
12.004(4)
13.4247(10)
13.155(6)
90.00
92.890(3)
90.00
2117.24(16)
4
1276.44
Triclinic
P-1
Crystal system
Space group
a/A˚
1
12.6658(5)
12.9257(5)
19.7839(8)
72.835(1)
76.497(1)
65.272(1)
2788.19(19)
2
b/A˚
c/A˚
a/
◦
◦
◦
b/
c /
V/ A˚ 3
2667.8(12)
4
1.535
Z
−
3
q
calcd/m
1.558
1.520
T/K
l/mm
293(2)
1.512
298(2)
1.111
273(2)
0.792
−
1
F (000)
min/max
1252
1.94/25.03
3384
1020
2.17/28.28
5192
1312
1.95/25.21
9680
◦
h
/
Reflns measured
Reflns observed [I > 2r (I)]
2442
300
1.002
0.0406
0.0981
0.0295
4794
262
1.007
0.0497
0.1202
0.0245
7700
712
1.128
0.0867
0.1959
0.0240
Parameters refined
2
GOF (on F )
1
a
R
[I > 2r(I)]
b
b
wR [I > 2r(I)]
2
R
int
a
2
2
2
2 2 1/2
R
1
= (R||F
o
| − |F
c
||)/(R|F
o
|). wR
2
= [Rw(F
o
− F
c
o
) /Rw(F ) ] .
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2427–2434 | 2429

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The API-ES positive mode mass spectrometry and the elemental
analyses of complexes 1–4 are in good agreement with the pro-
posed compositions. Both complexes 2 and 3 display paramagnetic
by an oxygen atom, the N
ion in an approximate C
mentioned all p-coordinate N
Fe(II) complexes all possess C
4
O ligand coordinates to the Fe(II)
symmetry (Fig. 2), while the above-
s
5
-ligands in the previously reported
1
5,7,19
H NMR signals in the 0–140 for 2 and 0–130 ppm range for 3,
1
symmetry.
The coordination
-ligands
containing a carboxamide or an imine nitrogen
2
5,26
indicative of the high spin Fe(II) (2) and Fe(III) (3) complexes.
geometry of tpoen in complex 1 resembles that of the N
PaPy and SBPy
5
1
The H NMR of complex 4 in DMSO-d
6
exhibits sharp signals
3
3
in the diamagnetic field, as found for the antiferromagnetically
coupled l-oxo diiron(III,III) complexes.
in the arm of the ligand, in which three pyridine nitrogen atoms
and one tertiary amine nitrogen atom ligate to an Fe(III) ion
with p bonds and the carboxamide with a r bond or an imine
27,28
−
X-Ray crystal structures
with a p bond is trans to the readily exchangeable ligand Cl
at the sixth site. As reported for other N
5
-pentadentate Fe(II)
The molecular structures of 1, 3 and 4 were determined by single
crystal X-ray analyses. Each crystal cell of complex 1 consists
complexes, the Fe–Npy bond distances of 1 (2.180(3), 2.194(4)
˚
and 2.156(4) A) are shorter than the Fe–Namine distance (2.226(4)
+
2−
of two cations of [Fe(tpoen)Cl] , an [Fe
and a solvent molecule CH CN. The structure of the cation of
is shown in Fig. 2 and the selected bond lengths and angles
2
OCl
6
]
counteranion
˚
˚
A). Both the Fe–Npy(av) distance (2.177(1) A) and the Fe–Namine
bond length are comparable to those found in the N -coordinate
3
5
1
+
high spin Fe(II) analogues [FeLCl] (L = tpmen, tphen, tpaen
are summarized in Table 2. The Fe(II) center of 1 is in a six-
coordinate pseudooctahedral environment. Four nitrogen atoms
5,7,19
and tpmpn),
but considerably longer than that noted for N -
5
2+
coordinate low spin Fe(II) complexes, [Fe(N4Py)(MeCN)] (Fe–
and an oxygen atom of the ligand constitute an N
around the metal ion and the left coordination position is occupied
by a chloride ion. Although the structure of the N O ligand tpoen is
quite similar with the N -ligands tpmen, tphen, tpaen and tpmpn,
the coordination configuration of tpoen around the Fe(II) ion
apparently differs from that of the analogous N -ligands. With
4
O donor set
˚
28
2+
N
N
N
py(av) 1.971(8), Fe–Namine 1.961(3) A), [Fe(SBPy
˚
3
)(MeCN)] (Fe–
+
py(av) 1.970(9), Fe–Namine 1.996(3) A) and [Fe(SBPy
3
)(CN)] (Fe–
24
4
py(av) 1.967(4), Fe–Namine 1.9922(13) A). The chloride ion is trans
˚
◦
5
to the oxygen atom with an O(1)–Fe–Cl(1) angle of 172.69(10)
and the amine N atom lies cis to the chloride ion with a N(2)–
◦
+
5
Fe–Cl(1) angle of 111.97(11) in the cation [Fe(tpoen)Cl] . While
replacement of the nitrogen atom in the arm of the N
5
-ligand
+
in the N
5
-coordinate analogues [FeLCl] (L = tpmen, tphen,
tpaen and tpmpn) the chloride ion is trans to the amine N atom
5,7,19
attached to two pyridylmethyl groups in the ligand.
The Fe–
˚
O distance of 2.285(3) A and Fe–Cl bond length of 2.3354(14)
5,19–21
˚
A are close to those noted for related iron complexes.
angles of N–Fe–Cl in the cis position are larger than 96 (96–
12 ), being considerably greater than the corresponding angles
-coordinate analogues.
cation [Fe(tpoen)Cl] suggests that the coordination of the N
All
◦
◦
1
5,14,19
of its N
5
The crystal structure of the
O-
+
4
pentadentate ligand to an Fe(II) ion could provide sufficient room
for lodging not only an end-on but also a side-on peroxide
coordination unit.
The molecular structure of complex 3 in Fig. 3 shows the poten-
tially pentadentate ligand tpoen is tridentate here, with an amino
nitrogen and two pyridyl nitrogen atoms of the ligand in a facial
configuration. The remaining facial sites are occupied by three
chloride ligands. The hexacoordinated Fe(III) center possesses a
pseudooctahedral geometry, which is similar to that reported for
+
+ 7
[
Fe(tpmpnH )Cl
3
] . The selected bond lengths and angles of 3 are
summarized in Table 3. The Fe–N distances (2.207(3), 2.191(3) and
+
Fig. 2 Molecular structure of the cation [Fe(tpoen)Cl] in complex 1
with thermal ellipsoids at 30% probability. Hydrogen atoms are omitted
for clarity.
◦
Table 2 Selected bond lengths ( A˚ ) and angles ( ) for 1·0.5CH
3
CN
Fe(1)–N(1)
Fe(1)–N(2)
2.180(3)
2.226(4)
Fe(1)–N(4)
Fe(1)–O(1)
2.156(4)
2.285(3)
Fe(1)–N(3)
2.194(4)
Fe(1)–Cl(1)
2.3354(14)
83.96(13)
75.32(14)
100.69(12)
96.33(11)
97.06(10)
111.97(11)
172.69(10)
N(4)–Fe(1)–N(1)
N(4)–Fe(1)–N(3)
N(1)–Fe(1)–N(3)
N(4)–Fe(1)–N(2)
N(1)–Fe(1)–N(2)
N(3)–Fe(1)–N(2)
N(4)–Fe(1)–O(1)
N(1)–Fe(1)–O(1)
101.35(14)
100.47(15)
151.73(16)
147.34(16)
75.27(15)
76.65(15)
72.03(15)
85.89(13)
N(3)–Fe(1)–O(1)
N(2)–Fe(1)–O(1)
N(4)–Fe(1)–Cl(1)
N(1)–Fe(1)–Cl(1)
N(3)–Fe(1)–Cl(1)
N(2)–Fe(1)–Cl(1)
O(1)–Fe(1)–Cl(1)
Fig. 3 Molecular structure of Fe(tpoen)Cl
3
(3) with thermal ellipsoids at
3
0% probability. Hydrogen atoms are omitted for clarity.
2
430 | Dalton Trans., 2006, 2427–2434
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◦
Table 3 Selected bond lengths ( A˚ ) and angles ( ) for complex 3
Fe–N(1)
Fe–N(2)
Fe–N(3)
2.207(3)
2.191(3)
2.282(3)
78.20(10)
92.12(7)
91.99(7)
77.59(9)
75.71(10)
165.25(7)
165.14(7)
90.16(7)
Fe–Cl(1)
Fe–Cl(2)
Fe–Cl(3)
Cl(2)–Fe–Cl(3)
N(3)–Fe–Cl(3)
N(2)–Fe–Cl(1)
N(1)–Fe–Cl(1)
Cl(2)–Fe–Cl(1)
N(3)–Fe–Cl(1)
Cl(3)–Fe–Cl(1)
2.3154(9)
2.2762(8)
2.2946(9)
97.54(3)
90.70(7)
89.06(7)
164.22(7)
97.81(3)
92.62(7)
N(2)–Fe–N(1)
N(2)–Fe–Cl(2)
N(1)–Fe–Cl(2)
N(2)–Fe–N(3)
N(1)–Fe–N(3)
Cl(2)–Fe–N(3)
N(2)–Fe–Cl(3)
N(1)–Fe–Cl(3)
100.71(4)
+
+
˚
2
.282(3) A) of 3 close to those in [Fe(tpmpnH )Cl
3
] are indicative
of the high spin Fe(III) ion of 3. The Fe–Cl distances of 2.3154(9),
˚
2
.2762(8) and 2.2946(9) A are comparable to the lengths of the
+
+
Fe–Cl bonds in [Fe(tpmpnH )Cl
3
] and in the Fe(III) complexes
In the coordination sphere, the
7
,29,30
with TPA or other N
3
ligands.
angles between the chloride ion and the trans-positioned nitrogen
◦
atom are around 165 , and those between the chloride ion and the
◦
cis-positioned nitrogen atom are close to 90 . The angles between
◦
the chloride ions are larger than 97 .
Fig. 4 Molecular structure of [{Fe(tpoen)} (l-O)](ClO ) (4) with ther-
2
4 4
The structure of the l-oxo diiron complex 4 is shown in Fig. 4
and the selected bond lengths and angles are given in Table 4.
All four nitrogen atoms and an oxygen atom in the ligand are
coordinated to one of the Fe(III) centers together with a l-
oxo ligand, to form a distorted octahedral geometry on each
Fe(III) ion. The two parts of the dimer are unsymmetric in the
spatial orientation. The oxygen in the ligand is trans to the l-
oxo group to construct a slightly zigzag O(2)–Fe(1)–O(1)–Fe(2)–
O(3) linkage with the bond angles of 171.76(18) for O(2)–Fe(1)–
mal ellipsoids at 30% probability. Hydrogen atoms are omitted for clarity.
2
0,24,28
˚
oxo diiron(III,III) complexes (1.77–1.82 A).
It is noteworthy
˚
that the Fe–Npy(av) bond length (2.122(7) A) and the Fe–Namine(av)
˚
(2.183 A) of 4 are very close to the corresponding values found
in the antiferromagnetically coupled complex [{Fe(N4Py)} (l-
2
24,28
˚
) (Fe–Npy(av) 2.125 and Fe–Namine(av) 2.244 A).
O)](ClO
4
4
Electronic spectra of complexes 1–4
◦
O(1), 172.34(18) for O(3)–Fe(2)–O(1) and 172.0(3) for Fe(1)–
The UV-Vis spectra of complexes 1–4 were recorded in acetonitrile.
Complex 1 exhibits two bands at 260 (17000) and 317 nm (sh, 10
O(1)–Fe(2). The structure of 4 is reminiscent of that reported
for [{Fe(SBPy
3
◦
)}
2
24
(l-O)](ClO
4
)
4
, in which the bond angle of Fe–
−
1
−1
5
[Fe
00 mol cm ), which are assigned to the characterization of the
O–Fe is 175.4 . The average distance of the Fe–Oether bonds
2
−
31
˚
2
OCl
6
]
counteranion, and the absorption bands from the
in 4 (2.163(9) A) are shorter than the Fe–O bond of complex
+
˚
cation [Fe(tpoen)Cl] are enveloped by the two intense absorption
bands of the anion [Fe
258 (10300) and 382 nm (1280 mol cm ) in close analogy to the
1
(2.283(4) A), reflecting the weaker trans effect of the l-oxo
2
−
OCl ] . Complex 2 displays two bands at
6
group relative to the chloride ion. The Fe–Ol-oxo bond lengths
2
−
1
−1
˚
of 1.786(4) and 1.790(4) A fall in with those found for other l-
7
UV-Vis spectrum of [Fe(tpmpn)Cl](PF
6
). The first band is due
◦
to a pyridyl p–p* transition and the second band is assigned to
the metal-to-ligand charge-transfer (MLCT) band, which shows a
Table 4 Selected bond lengths ( A˚ ) and bond angles ( ) for 4·2CH
3
CN
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–N(4)
Fe(1)–O(1)
Fe(1)–O(2)
2.092(5)
2.192(5)
2.128(5)
2.142(5)
1.786(4)
2.156(4)
150.3(2)
102.9(2)
99.4(2)
Fe(2)–N(5)
Fe(2)–N(6)
Fe(2)–N(7)
Fe(2)–N(8)
Fe(2)–O(1)
Fe(2)–O(3)
N(5)–Fe(2)–N(6)
N(5)–Fe(2)–N(7)
N(5)–Fe(2)–N(8)
N(5)–Fe(2)–O(1)
N(5)–Fe(2)–O(3)
N(6)–Fe(2)–N(7)
N(6)–Fe(2)–N(8)
N(6)–Fe(2)–O(1)
N(6)–Fe(2)–O(3)
N(7)–Fe(2)–N(8)
N(7)–Fe(2)–O(1)
N(7)–Fe(2)–O(3)
N(8)–Fe(2)–O(1)
N(8)–Fe(2)–O(3)
O(3)–Fe(2)–O(1)
2.091(5)
2.173(5)
2.140(6)
2.140(6)
1.790(4)
2.171(4)
slight blue shift compared to that observed for [Fe(tpmen)Cl](PF )
6
4,5
(
k
max = 399 nm). The intensity of the MLCT band gives a
further clue to the high spin state of the Fe(II) ion in complex
2. The electronic spectrum of complex 3 shows three bands in
4
the range of 200–500 nm. Like complex 2, the first band of 3
N(1)–Fe(1)–N(2)
N(1)–Fe(1)–N(3)
N(1)–Fe(1)–N(4)
N(1)–Fe(1)–O(1)
N(1)–Fe(1)–O(2)
N(2)–Fe(1)–N(3)
N(2)–Fe(1)–N(4)
N(2)–Fe(1)–O(1)
N(2)–Fe(1)–O(2)
N(3)–Fe(1)–N(4)
N(3)–Fe(1)–O(1)
N(3)–Fe(1)–O(2)
N(4)–Fe(1)–O(1)
N(4)–Fe(1)–O(2)
O(2)–Fe(1)–O(1)
Fe(1)–O(1)–Fe(2)
150.9(2)
−1
−1
at 255 nm (19100 mol cm ) is due to the p–p* transition of
99.1(2)
102.8(2)
101.76(19)
73.36(19)
77.6(2)
the ligand. The two other broad bands at 290 (sh, 8400) and
−
1
−1
99.71(19)
73.03(19)
76.3(2)
384 nm (5000 mol cm ) are assigned to the chloro-to-Fe(III)
7
charge transfer. These absorption bands are quite similar to those
found for the other N -tridentate high spin Fe(III) complexes.
7,29,30
3
76.9(2)
76.2(2)
The UV-Vis spectrum of 4 is dominated by two bands at 254
109.9(2)
77.30(19)
153.2(2)
95.40(19)
90.13(19)
95.28(19)
82.32(19)
171.76(18)
172.0(3)
107.29(19)
77.56(19)
153.8(2)
93.4(2)
81.78(19)
96.11(19)
90.79(18)
172.34(18)
−
1
−1
(39500) and 354 nm (10900 mol cm ). A red shift of ca. 40 nm is
observed for the second band in the spectrum of 4 compared
to the corresponding bands in the UV-Vis spectra of the N
coordinate l-oxo diiron complexes [{Fe(SBPy )} (l-O)](ClO
and [{Fe(N4Py)} (l-O)](ClO
The positions and intensities of the two absorptions of 2 in
the UV-Vis spectrum in CH CN are identical to those measured
5
-
3
2
4
)
4
24,28
2
4
4
) .
3
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2427–2434 | 2431

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in MeOH. The UV-Vis spectra of 4 in CH CN and MeOH gave
3
similar results. The UV-Vis spectra suggest that the ether oxygen
of the tpoen ligand in complexes 2 and 4 is not displaced by the
molecule of acetonitrile in solution.
Redox properties of complexes 2 and 4
The redox properties of complexes 2 and 4 were studied by cyclic
voltammetry. Complex 2 exhibits a reversible one-electron wave
in CH
3
CN with an E1/2 value of +0.37 V vs Ag/AgNO for the
3
II
III
redox potential of Fe /Fe couple. Compared with the E1/2 value
of +0.57 and +0.64 V vs SCE for the monoiron complexes of the
closely related N
4,7,17
5
ligands, tpmen and tpmpn,
complex 2 shows
Fig. 5 Cyclic voltammogram of [{Fe(tpoen)}
2
(l-O)](ClO
4
1
)
.
4
(4), 1.0 mM
a slightly higher Fe(II)/Fe(III) redox potential (+0.65 V vs SCE)
−
in 0.05 M n-Bu NPF –CH CN at a scan rate of 100 mV s
4
6
3
32
if the measured potential is corrected to the values of SCE. It
can be attributed to the higher acidity of the Fe(II) center of 2,
resulting from the replacement of one of the nitrogen atoms in the
adamantine, with H
at room temperature under a N
2
O
2
and mCPBA as oxidants in acetonitrile
atmosphere. The results are
2
N
5
ligand with an oxygen atom. Cyclic voltammogram of complex
summarized in Table 5. Complexes 2, 3 and 4 catalysed the
oxidation of cyclohexane to cyclohexanol (A) and cyclohexanone
4
is shown in Fig. 5, which displays two reduction peaks typical
33
of a l-oxo diiron core. The quasi-reversible reduction peak at
0.11 V with an E1/2 value of +0.06 V vs Ag/AgNO is ascribed to
(K). As illustrated in Table 5, when H
2
O was employed as an
2
+
3
oxidant, the catalysts performed with the best reactivity in a molar
ratio of 1 : 200 : 1000 for catalyst : oxidant : substrate. Complex
the one-electron reduction process of Fe(III)Fe(III) to Fe(III)Fe(II).
The irreversible reduction peak at −0.83 V is assigned to the one-
electron reduction process of Fe(III)Fe(II) to Fe(II)Fe(II).
2
bearing an N O-pentadentate ligand catalysed cyclohexane
4
to 17.2 TN (turnover number) of cyclohexanol and 7.2 TN
of cyclohexanone (A/K = 2.4) with 12.2% conversion based
Catalytic oxidation of alkanes
on oxidant (entry 3). Complex 3, with an N and 3Cl fac-
3
The catalytic activities of complexes 2, 3 and 4 were explored
for the oxidation of alkanes, e.g., cylcohexane, ethylbenzene and
coordination configuration, exhibited a catalytic activity of 25
TN for cyclohexanol and 11.1 TN for cyclohexanone (A/K = 2.3)
a
Table 5 Oxidation of alkanes catalysed by complexes 2, 3 and 4
b
c
Entry
Catalyst
Oxidant
Product (TN)
Conversion (%)
Remarks
Cyclohexane
Alcohol
6.6
8.3
17.2
37.9
8.9
11.5
25.0
54.1
15.6
40.3
2-ol
15.7
37.9
12.7
33.6
16.4
38.1
1-ol
7.9
Ketone
3.7
3.8
7.2
11.8
5.8
A/K
1.8
2.2
2.4
3.2
1.5
1.6
2.3
3.0
1.9
2.9
d
1
2
3
4
5
6
7
8
9
2
H
H
H
2
2
2
O
O
O
2
2
2
10.3
6.1
12.2
24.9
14.7
9.3
18.1
36.2
11.9
27.1
e
2
2
2
3
3
3
3
4
4
mCPBA
d
H
H
H
2
2
2
O
O
O
2
2
2
e
7.1
11.1
18.3
8.2
mCPBA
H O
2 2
mCPBA
f
1
0
13.8
2-one
22.7
46.0
17.2
33.4
26.2
38.1
2-ol + 2-one
7.4
14.8
8.9
15.5
8.2
Ethylbenzene
1
1
1
1
1
1
1
2
3
4
5
6
2
2
3
3
4
4
H
2
O
2
19.2
41.9
14.9
33.5
21.3
38.1
0.7
0.8
0.7
1.0
0.7
1.0
mCPBA
H O
2 2
mCPBA
2 2
H O
mCPBA
g
◦
◦h
Adamantane
3 /2
1
1
1
2
2
2
7
8
9
0
1
2
2
2
3
3
4
4
H
2
O
2
7.7
53.1
8.9
57.8
8.3
3.2
18.5
3.0
19.4
3.1
mCPBA
H O
2 2
mCPBA
2 2
H O
91.3
8.9
100.1
8.4
mCPBA
95.0
9.4
52.2
30.3
a
◦
Reaction conditions: catalyst 5 lmol; catalyst : oxidant : substrate in a molar ratio of 1 : 200 : 1000; reaction time 90 min; reaction temperature 20 C;
b
c
d
solvent CH
3
CN 4 mL. TN = mol of product/mol of catalyst. Conversions are based on oxidant. The molar ratio of catalyst : oxidant : substrate
e
f
is 1 : 100 : 100. The molar ratio of catalyst : oxidant : substrate is 1 : 200 : 100 and the corresponding conversions are based on substrate. For all
g
h
◦
◦
reactions with complex 4 as catalyst, 2.5 lmol of catalyst was used. The reactions were carried out in 3 mL CH
1
3
CN and 2 mL CH
2
Cl
2
.
3 /2
=
-adamantanol/(2-adamantanol + 2-adamantanone) multiplied by 3.
2
432 | Dalton Trans., 2006, 2427–2434
This journal is © The Royal Society of Chemistry 2006

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◦
◦
with 18.1% conversion (entry 7), which is higher than complex 2.
In contrast, the analogous Fe(III) complex with an N and 3Cl
mer-coordination configuration did not show catalytic activity
(3 /2 = 3.0–3.2, entries 17, 19 and 21) compared to the iron
11
3
complexes of tpa and tpa-derived ligands. The total TNs and
the regioselectivity of 2, 3 and 4 are similar to those displayed
by [Fe(N4Py)(CH
are only half of that performed by the iron complex of the N4Py
ligand. Appealingly, complexes 2, 3 and 4 showed much higher
efficiency in adamantane oxidation with mCPBA as the oxidant
34
12
in the oxidation of cyclohexane with H
Although high turnover numbers were obtained in the oxidation
reactions catalysed by 2 and 3, the conversions based on H
were lower than those observed for non-heme mononuclear iron
complexes bearing well-known N - and N -ligands, e.g., TPA,
The conversions with 2 and 3 under
similar oxidation conditions are higher than those found for other
2
O
2
and t-BuOOH.
3
CN)](ClO
4
)
2
,
but the H
2
2
O -based conversions
2
O
2
4
5
relative to H
2
2
O . The total TNs reached 106.2 for 2, 115.6 for
2,11,12
bpmen and N4Py.
3 and 104.4 for 4 and the regioselectivity was also drastically
◦
◦
improved to the 3 /2 ratios of 18.5 for 2, 19.4 for 3 and 30.3
for 4 (entries 18, 20 and 22). The high catalytic activities and the
promising regioselectivity of complexes 2, 3 and 4 in the oxidation
of adamantane with mCPBA are comparable to the previously
reported most efficient non-heme iron catalysts, e.g. iron complexes
3
5
36
reported monoiron complexes bearing N
ligands.
3
-,
N
4
-,
N
5
- and N
3
O-
2,14,20
Under our standard reaction conditions (2 : H
2
O
2
:
2
cyclohexane = 1 : 200 : 1000), complex 2 displayed an A/K of
.4, which is superior to those for all previously reported monoiron
complexes featuring N
and PMA.
0
5
-pentadentate ligands, such as N4Py, PaPy
In those cases, the A/K values are in the range of
.9–1.4. In the molar ratios of 1 : 100 : 100 and 1 : 200 : 100 for
3
of bpmen, tpa and tpa-derived ligands and the diiron complex
2,12–14
11,38
[Fe
2
((S)-dpan)(O)(OBz)]ClO
4
.
The low A/K ratios close to 1
or mCPBA and
the poor regioselectivity with 3 /2 ratios close to 3 in adamantane
oxidation with H convey a hint of the presence of long lifetime
in the oxidation of ethylbenzene with either H
2
O
2
◦
◦
catalyst : oxidant : substrate (entries 1, 2 and 5, 6), lower activities
and selectivities were detected. These results indicate with a large
excess of substrate, 2 and 3 perform higher activities and the over-
oxidation of cyclohexanol to cyclohexanone can be depressed to
a certain extent.
It has been reported recently that the chemo- and regiose-
lectivity of the alkane oxidation can be greatly enhanced for
some non-heme iron complexes when mCPBA was used as the
2
O
2
alkyl radicals, which react rapidly with O
2
to initiate a radical chain
2
autoxidation. In contrast, significantly enhanced regioselectivity
in the oxidation of adamantane with the catalyst/mCPBA system
indicates the involvement of a metal-centered oxidant that acts
as an active intermediate in the oxidation of the C–H bonds of
adamantane.
3
7,38
oxidant.
: 200 : 1000 with mCPBA as the oxidant, the oxidation of
cyclohexane catalysed by complex 2 can be completed within
0 min with a total TN of 49.7 and a conversion of 24.9%
based on mCPBA (entry 4), which are twice as high as those
observed for H . The A/K value was improved to 3.2. A
With a catalyst : oxidant : substrate molar ratio of
1
Conclusions
2
In summary, mono- and dinuclear complexes 1–4 were synthesized
2
O
2
using an N
and 2 each possess an Fe(II) center with an N
ligand. The Fe(III) center of 3 is coordinated by three N atoms of
the N O ligand, in which the O and NPy atoms of the pendant arm
are non-coordinate. Complex 4 has an Fe(III)Fe(III) l-oxo core
featuring an N O-pentadentate ligand in each moiety. Complexes
, 3 and 4 displayed moderate activities and low to moderate
selectivities in the oxidation of cyclohexane, ethylbenzene and
adamantane with H . Although mononuclear iron complexes
with the N -meridional tridentate ligands are poor catalysts for the
oxidation of cyclohexane,
tridentate ligand, exhibited good catalytic activity. Replacement of
the oxidant H with mCPBA led to significant enhancement of
4
O multidentate ligand (tpoen). Monoiron complexes
total TN of 72.4 and a conversion of 36.2% with an A/K ratio
of 3.0 (entry 8) were obtained for oxidation of cyclohexane by
the 3/mCPBA system. With mCPBA as a surrogate of H
1
4
O-pentadentate
2
2
O ,
4
complexes 2 and 3 did not show great enhancement of the selectiv-
ity as reported for iron complexes containing N4Py and dpan
ꢀ
ꢀ
4
(
2
dpan = 3,3 -bis[N,N-bis(2-pyridylmethyl)aminomethyl]-1,1 -bi-
2
37,38
-naphthoxy) ligands.
The l-oxo dinuclear iron complex 4
displayed an activity similar to the mononuclear Fe(II) complex 2
and 2.9 for
2
O
2
and the chemical selectivity of 4 (A/K = 1.9 for H
2
O
2
3
mCPBA, entries 9 and 10) is somewhat lower than that shown by
complex 2.
34,35
complex 3, which has an N -facial
3
The catalytic activities of complexes 2, 3 and 4 were further
explored for the oxidation of ethylbenzene and adamantane
2
O
2
the catalytic activity of 2, 3 and 4. The A/K values of cyclohexane
oxidation were apparently improved and the regioselectivity of
adamantane oxidation was drastically raised. According to the
catalytic results, a radical-based oxidant was proposed to play
(
Table 5, entries 11–22). Catalytic oxidation of ethylbenzene
with H by 2, 3 and 4 gave more acetobenzene than benzylic
alcohol (A/K = 0.7) in moderate H -based conversions (entries
1, 13 and 15) as compared to the non-heme iron complexes
2
O
2
2
O
2
1
the dominant role in the oxidation of alkanes by catalyst/H
2
O
2
18
of N and O multidentate ligands for the benzylic oxidation.
Replacement of H with mCPBA led to the promotion of the
systems, while the oxidation of adamantane by catalyst/mCPBA
systems is mainly due to metal-based oxidation.
2
O
2
conversions to 41.9% for 2, 33.5% for 3 and 38.1% for 4 with
nearly equivalent benzylic alcohol and acetobenzene (entries 12, 14
and 16). Compared with other non-heme model complexes, which
normally afforded benzylic alcohol as the major product with an
Acknowledgements
1
8,37,38
A/K ratio of 1.1–7.9,
selectivities towards the alcohol product.
Oxidation of adamantane with H
afforded 1-adamantanol, 2-adamantanol and 2-adamantanone
complexes 2, 3 and 4 manifested low
We are grateful to the National Natural Science Foundation of
China (Grant nos. 20471013 and 20128005), the Swedish Energy
Agency and the Swedish Research Council for financial support
of this work.
2
2
O catalysed by 2, 3 and
4
with moderate efficiencies (TN = 15–18) and low regioselectivity
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 2427–2434 | 2433

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