Synthesis, crystal structure and reactivity studies of iron complexes with pybox ligands

Article abstract of DOI:10.1016/j.ica.2014.08.050

Iron(II) complexes, [Fe(2,6-bis(4,4-dimethyl-1,3-oxazolin-2-yl)pyridine)Cl₂] ((Fe(Me₂-pybox)Cl₂), 3) and [Fe(2,6-bis(4,4-diphenyl-1,3-oxazolin-2-yl)pyridine)Cl₂] ((Fe(Ph₂-pybox)Cl₂), 4), ha

Full text of DOI:10.1016/j.ica.2014.08.050

Inorganica Chimica Acta 423 (2014) 320–325
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structure and reactivity studies of iron complexes with
pybox ligands
Tao Chen a,b, , Limin Yang b,c, Dirong Gong b,d, Kuo-Wei Huang b,
⇑
⇑
a MOE Key Laboratory of Advanced Textile Materials & Manufacturing Technology, National Base for International Cooperation in Science & Technology of Textiles and
Daily Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, PR China
b King Abdullah University of Science and Technology (KAUST), Division of Physical Sciences and Engineering and KAUST Catalysis Center, Thuwal 23955-6900, Saudi Arabia
c College of Materials, Chemistry & Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, PR China
d Department of Polymer Science and Engineering, Faculty of Materials Science and Chemical Engineering, Key Laboratory of Specialty Polymers, Grubbs Institute, Ningbo
University, Ningbo 315211, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 June 2014
Received in revised form 8 August 2014
Accepted 13 August 2014
Available online 6 September 2014
Iron(II) complexes, [Fe(2,6-bis(4,4-dimethyl-1,3-oxazolin-2-yl)pyridine)Cl2] ((Fe(Me2–pybox)Cl2), 3) and
[Fe(2,6-bis(4,4-diphenyl-1,3-oxazolin-2-yl)pyridine)Cl2] ((Fe(Ph2–pybox)Cl2), 4), have been synthesized
and characterized by X-ray crystallographic analysis. Upon treatment of complex 3 with silver triflate
and 4 with acetonitrile, [Fe(Me2–pybox)(CH3CN)OTf2] (5) and [Fe(Ph2–pybox)(CH3CN)2Cl][FeCl3] (6) were
obtained, respectively. The bulkier phenyl substitutes were found not only to cause the elongation of the
N–Fe bonds but also influence the reactivity of the Fe center.
Keywords:
Synthesis
Ó 2014 Elsevier B.V. All rights reserved.
Crystal structure
Reactivity
Iron complex
Pybox ligand
1. Introduction
The development of iron-based catalysts has recently become
an active research area due to their low cost and low toxicity, as
Since Nishiyama’s first report on the tridentate nitrogen ligand
pybox in 1989 [1], the use of chiral pybox ligands derived from
aminoalcohols in transition metal catalysis has received consider-
able attention due to the convenient preparation, ready availability
of the chiral precursors, and their excellent performance [2–9]. The
pybox usually serves as a tridentate ligand [10,11], although a few
examples as bidentate ligands (involving pyridine and one of the
two oxazoline nitrogen atoms) [12–15], and as monodentate
ligands have been reported [16,17]. These observations make the
synthesis and structural characterization of pybox transition metal
complexes highly interesting and useful as they may allow one to
understand the factors that influence the coordination nature of
ligands, and therefore the reactivity of the corresponding com-
plexes [18–23].
well as benign environmental impact [24–28]. A number of Fe/
pybox catalyst systems were developed for various organic trans-
formations such as the asymmetric Nazarov cyclization of divinyl
ketones [29], the asymmetric aziridine formation reaction [30],
Mukaiyama–Aldol reaction [31,32], the enantioselective conjugate
addition [33,34], olefin polymerization [35,36], etc. Despite the
wide applications of these catalysts, structural studies of well-
defined iron–pybox complexes are scarce [30,36,37]. Herein, we
report the synthesis, crystal structure and reactivity studies of
two iron(II)–pybox complexes with different steric hindrance.
2. Experimental
2.1. Materials and methods
Commercially available reagents were used without further
purification. 2-Amino-2,2-diphenylethanol was prepared by reduc-
tion of 2,2-diphenylglycine according to the literature procedure
[38]. Column chromatographic purifications were performed using
Merck silica gel 60. 1H, and 13C{1H} NMR spectra were recorded
using Bruker AVIII 400 or AVIII 500 spectrometer. Chemical shifts
in 1H NMR and 13C{1H} NMR were reported in parts per million
⇑
Corresponding authors. Address: MOE Key Laboratory of Advanced Textile
Materials & Manufacturing Technology, National Base for International Cooperation
in Science
& Technology of Textiles and Daily Chemistry, Zhejiang Sci-Tech
University, Hangzhou 310018, PR China. Tel.: +86 571 86843779; fax: +86 571
86843666 (T. Chen).
E-mail addresses: tao.chen@zstu.edu.cn (T. Chen), hkw@kaust.edu.sa
(K.-W. Huang).
http://dx.doi.org/10.1016/j.ica.2014.08.050
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

T. Chen et al. / Inorganica Chimica Acta 423 (2014) 320–325
321
(ppm). The residual solvent peak was used as an internal reference:
1H NMR (CHCl3 in chloroform-d d 7.26) and 13C{1H} NMR (chloro-
form-d d 77.0). Multiplicity was indicated as follows: s (singlet), d
(doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of dou-
blet), br s (broad singlet). Coupling constants were reported in
Hertz (Hz). The NMR data of iron complexes are not available
due to the paramagnetic properties of the Fe(II) complexes. Mass
spectra were recorded on an Accela-LCQ Fleet mass chromato-
graph, using Electro Spray Ionization (ESI) mode. HRMS analysis
was carried out using a LTQ-Orbitrap-MS (LTQ Orbitrap Velos,
Thermo Scientific) with ESI in positive ionization mode. Elemental
analysis was performed using a Flash 2000 – Thermo Scientific
CHNO Analyzer. MW of PE was determined by a PL-GPC 220 type
high-temperature chromatograph equipped with three Plgel
2 and L2, that have Rf similar values. The white solid was treated
with solution of NaOH (0.2 g) in water (6 mL), methanol
a
(12 mL) and dichloromethane (12 mL) at room temperature for
24 h. The mixture was extracted with dichloromethane
(10 mL ꢀ 3). The combined organic extracts were washed with
brine and dried over MgSO4. The crude product was purified by col-
umn chromatography on silica gel (hexanes:ethyl acetate = 4:1 as
eluent) to afford a white solid in 41% overall yield. 1H NMR
(500 MHz, CDCl3): 8.47 (d, 2H, J = 7.8 Hz), 7.92 (t, 1H, J = 7.8 Hz),
7.41–7.39 (m, 8H), 7.35–7.31 (m, 8H), 7.28–7.24 (m, 4H), 5.08 (s,
4H). 13C{1H} NMR (100 MHz, CDCl3): 161.71, 146.95, 145.71,
137.29, 128.53, 127.30, 126.78, 126.70, 80.53, 79.85. HRMS (ESI):
calcd for C35H28N3O2 m/z 522.21815 (M+H+). Found 522.21822.
10
lm Mixed-B LS type columns.
2.2.4. Iron complex 3
Under nitrogen atmosphere, to a 100 mL Schlenk flash charged
with the L1 (273 mg, 1.0 mmol) and iron(II) chloride tetrahydrate
(199 mg, 1.0 mmol), tetrahydrofuran (20 mL) was added. The mix-
ture was stirred at room temperature for 4 h. The solvent was
removed in cacuo, and the purple powder was washed with cold
ether (15 mL ꢀ 3) to provide the iron(II) pincer complex in nearly
quantitative yield (390 mg). The crystal suitable for a single-crystal
X-ray diffraction was obtained by diffusion of diethyl ether into a
dichloromethane solution of 3. MS (ESI, MeOH): 364.04 (100%,
(MꢁCl)+). Elemental analysis (%) for C15H19Cl2FeN3O2: Calc. C,
45.03; H, 4.79; N, 10.50. Found: C, 44.87; H, 4.92; N, 10.22.
2.2. Synthesis of ligands and iron complexes
2.2.1. Ligand L1
To
a solution of 2-amino-2-methyl-1-propanol (0.979 g,
11 mmol) and triethylamine (3.03 g, 30 mmol) in chloroform
(25 mL), 2,6-pyridinedicarbonyl dichloride (1.02 g, 5 mmol) solu-
tion in chloroform (10 mL) was slowly added at 0 °C with continu-
ous stirring. The mixture was stirred for 16 h at room temperature.
To the reaction mixture, thionyl chloride (3.7 mL, 50 mmol) was
added, and the mixture was heated at reflux for 5 h. The solvent
and excess thionyl chloride were removed in vacuo to give the
crude dichloride compound as a colorless oil. The oil was treated
with a solution of NaOH (0.8 g) in water (20 mL) and methanol
(40 mL) at room temperature for 3 days. The mixture was
extracted with dichloromethane (30 mL ꢀ 3). The combined
organic extracts were washed with brine and dried over MgSO4.
The crude product was purified by column chromatography on sil-
ica gel (hexanes:ethyl acetate = 1:2 as eluent) to afford a white
solid (0.69 g, 49%). 1H NMR (500 MHz, CDCl3): 8.19 (d, 2H,
J = 7.8 Hz), 7.84 (t, 1H, J = 7.8 Hz), 4.21 (s, 4H), 1.39 (s, 12H).
13C{1H} NMR (125 MHz, CDCl3): 160.83, 146.94, 137.20, 125.68,
79.71, 67.99, 28.41.
2.2.5. Iron complex 4
The synthetic procedure is same to that of iron complex 3. Iron
complex 4 was obtained as a light purple powder. The single crys-
tal suitable for X-ray diffraction studies was obtained by diffusion
of diethyl ether into a tetrahydrofuran solution of 4. MS (ESI,
MeOH): 612.13 (100%, (MꢁCl)+). Elemental analysis (%) for C35H27-
Cl2FeN3O2ꢂ1Et2O: Calc. C, 64.83; H, 5.16; N, 5.82. Found: C, 64.55;
H, 4.88; N, 6.11.
2.2.6. Iron complex 5
Under nitrogen atmosphere, to a 25 mL Schlenk flash charged
with iron complex
3 (20 mg, 0.05 mmol) and silver triflate
2.2.2. Compound 1
(30 mg, 0.12 mmol), acetonitrile (2 mL) was added. The mixture
was stirred at dark for 1 h and then stirred under light for another
20 min to decompose surplus silver triflate. Filtration was carried
out, and diethyl ether was added to precipitate 5 as an orange
solid. The single crystal of 5 suitable for X-ray diffraction studies
was grown by vapor diffusion of n-hexane into an acetonitrile solu-
tion of 5. MS (ESI, MeOH): 477.98 (100%, (MꢁOTfꢁCH3CN)+). Ele-
mental analysis (%) for C19H22F6FeN4O8S2: Calc. C, 34.14; H, 3.32;
N, 8.38. Found: C, 33.87; H, 3.50; N, 8.64.
To a solution of 2-amino-2,2-diphenylethanol (2.0 g, 9.4 mmol)
and triethylamine (2.8 g, 28.2 mmol) in chloroform (30 mL), 2,6-
pyridinedicarbonyl dichloride (0.96 g, 4.7 mmol) solution in chlo-
roform (10 mL) was slowly added at 0 °C with continuous stirring.
The mixture was stirred for 16 h at room temperature. Water
(50 mL) was added and the reaction mixture was extracted with
dichloromethane (30 mL ꢀ 3). The combined organic extracts were
washed with brine and dried over MgSO4. The crude product was
purified by column chromatography on silica gel (hexanes:ethyl
acetate = 1:1 as eluent) to afford a white solid (1.88 g, 72%). 1H
NMR (500 MHz, d6-DMSO): 9.08 (s, 2H), 8.20–8.17 (m, 3H), 7.42–
7.40 (m, 8H), 7.31–7.29 (m, 8H), 7.27–7.22 (m, 4H), 5.36 (t, 2H,
J = 5.4 Hz), 4.40 (d, 4H, J = 5.4 Hz). 13C{1H} NMR (125 MHz, CDCl3):
162.23, 149.41, 142.85, 139.91, 128.05, 127.27, 126.80, 124.57,
66.17, 65.99. HRMS (ESI): calcd for C35H32N3O4 m/z 558.23928
(M+H+). Found 558.23934.
2.2.7. Iron complex 6
Iron complex 6 was obtained by vapor diffusion of diethyl ether
into acetonitrile solution of 4 under nitrogen atmosphere. MS (ESI,
MeOH): 694.12 (100%, (MꢁFeCl3)+). Elemental analysis (%) for C39-
H33Cl4Fe2N5O2: Calc. C, 54.64; H, 3.88; N, 8.17. Found: C, 54.37; H,
4.02; N, 8.43.
2.3. General ethylene polymerization procedure
2.2.3. Ligand L2
To a solution of compound 1 (220 mg, 0.4 mmol) in chloroform
(5 mL), thionyl chloride (240 lL) was added. The mixture was stir-
Ethylene polymerizations were performed in a 100 mL glass
reactor equipped with a septum adapter and a magnetic stir bar.
red at reflux for 2 h. Water (10 mL) was added to quench the reac-
tion. The mixture was extracted with dichloromethane
(10 mL ꢀ 3). The combined organic extracts were washed with
brine and dried over MgSO4. The crude product was purified by col-
umn chromatography on silica gel (hexanes:ethyl acetate = 8:1 as
eluent) to afford a white solid, which is a mixture of compounds
Iron complex (5 lmol) was added to the reactor and kept under
vacuum for 30 min and then purged with N2. The reactor was then
charged with toluene (50 mL) under N2 and then pressurized with
ethylene (18 bar) at the 50 °C with stirring. The polymerization
was started by the addition of MAO (Al/Fe = 1000). After 1 h, the
reaction was quenched by the addition of acidified methanol fol-

322
T. Chen et al. / Inorganica Chimica Acta 423 (2014) 320–325
lowed by precipitation in methanol. The polymer was then washed
3 times with methanol and dried in a vacuum oven to constant
weight.
solution of NaOH in a mixture of solvent (H2O/MeOH/CH2Cl2) at
room temperature for 24 h to afford the pure L2 as a white solid
in 42% overall yield.
The crystal of L2 suitable for a single-crystal X-ray diffraction
was obtained by slowly evaporation of solvent of its dichlorometh-
ane solution. The molecular structure of L2 shows a twisted confor-
mation with the N atoms of the two oxazoline rings rotating away
from the central pyridine (Fig. 1). The oxazoline rings are nearly
planar with a dihedral angle of 50.89° between them. The dihedral
angles between the two phenyl rings are 76.73° for PlaneC1/PlaneC7
2.4. X-ray diffraction studies
Single-crystal X-ray diffraction data were collected with a Bru-
ker SMART diffractometer equipped with a CCD area detector (Mo
Ka, k = 0.71073 Å). The crystals were mounted on a glass fiber and
coated with paratone-N oil. Data collection was carried out at
100(2) K or 223(2) K to minimize solvent loss, possible structural
disorder and thermal motion effects. Structure was solved using
SHELXS-86 (Patterson) and refined using SHELXL-97 (full-matrix
least-squares on F2). All non-hydrogen atoms were anisotropically
and 67.05° for PlaneC30/PlaneC24
.
3.2. Iron complexes synthesis and crystallographic studies
Iron complexes 3 and 4 were prepared by stirring iron (II) chlo-
ride tetrahydrate and the corresponding pybox ligand at a ratio of
1:1 in THF at room temperature (Scheme 2). During the prepara-
tion of the Me2–pybox complex 3, the resultant solution turned
dark blue–violet instantly. Upon removal of the solvent in vacuo,
iron complex 3 was achieved in almost quantitative yield. In the
case of the Ph2–pybox complex 4, a light violet solution was
observed and 4 was obtained as a light purple solid in a slightly
lower yield of 65%, presumably due to the larger steric hindrance
of the L2 ligand making its coordination to the iron center less
favorable.
refined. Hydrogen atoms on carbon atoms were placed in idealized
0
positions and refined as riding atoms, with CꢁH 0.93 ÅA and Uiso
(H) = 1.2 Ueq (C). The program SHELXTL was used to prepare molec-
ular graphics images. Data collection and structures refinement
parameters are presented in Table 1.
3. Results and discussion
3.1. Ligand synthesis
L1 (Me2–pybox) and L2 (Ph2–pybox) were prepared according
to the literature procedure with some modification (Scheme 1)
[39], which allows the use of pyridine-2,6-dicarbonyl dichloride
and the corresponding amino alcohol as the starting materials. Pyr-
idine-2,6-dicarbonyl dichloride was synthesized by mixing pyri-
dine-2,6-dicarboxylic acid in SOCl2 at reflux. Condensation with
2-amino-2-methyl-1-propanol in the presence of Et3N was then
carried out, followed by cyclization using SOCl2. All the crude inter-
mediates were used without purification, and final product L1 was
obtained in 49% yield after recrystallization in hexane–EtOAc as a
white solid. Synthesis of L2 started with the condensation of pyri-
dine-2,6-dicarbonyl dichloride and 2-amino-2,2-diphenylethanol
using Et3N as base to afford diamide 1 as a white solid in 72% yield.
2-Amino-2,2-diphenylethanol was readily derived from 2,2-
diphenylglycine by the LiAlH4 reduction [38]. Heating diamide 1
and SOCl2 in CHCl3 at reflux for 2 h afforded a mixture of 2 and
L2, which have similar Rf values. The mixture was treated with a
1H NMR analysis results of the iron complexes 3 and 4 were
similar to those of analogous compound in the literature [30]. Only
highly shifted resonances appearing as broad singlet peaks were
observed, indicating that they are paramagnetic complexes. The
structures of complexes 3 and 4 were elucidated by X-ray crystal-
lography. The crystals suitable for single-crystal X-ray diffraction
were obtained by diffusion of diethyl ether into their solutions
(CH2Cl2 in case of 3, THF in the case of 4). Both of these complexes
exhibit distorted trigonal bipyramidal geometry (Fig. 2 and Fig. 3).
As expected, in complex 3 the L1 ligand is coordinated in a tri-
dentate fashion to the iron center via three N atoms, with bond dis-
tances of N–Fe being 2.240, 2.145 and 2.212 Å. The two Fe–Cl bond
distances are both 2.3488 Å. Similar to the (R,R)-Et–pybox Zinc
complex [40], the atoms of N(2), Cl(1), Cl(2), Fe(1) are approxi-
mately coplanar with N(1) and N(3) in the axial orientation to com-
plete the trigonal bipyramidal coordination. The pyridine and two
oxazoline rings were found to be nearly planar and the dihedral
Table 1
Crystallographic data and structure refinement parameters for complexes L2, 3–6.
Compound
L2
3
4
5
6
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C
35H27N3O2
C15H19Cl2FeN3O2
400.08
Monoclinic
P2(1)/C
10.0639(11)
15.7554(19)
11.2901(13)
90.00
C39H37Cl2FeN3O3
722.47
Monoclinic
P2(1)/C
9.0563(4)
28.3096(14)
13.8556(7)
90.00
C19H22F6FeN4O8S2
668.38
Monoclinic
Cc
15.3382(18)
9.8899(11)
17.930(2)
90.00
C39H33Cl4Fe2N5O2
857.20
Monoclinic
P2(1)/C
16.4907(7)
8.7045(4)
27.1368(11)
90.00
521.60
Monoclinic
P2(1)/C
10.8032(17)
12.1835(19)
20.626(3)
90.00
b (Å)
c (Å)
a
(°)
b (°)
102.317(4)
90.00
2652.3(7)
4
98.515(2)
90.00
1770.4(4)
4
93.2330(10)
90.00
3546.6(3)
4
98.143(3)
90.00
2692.4(5)
4
95.8110(10)
90.00
3875.3(3)
4
c
(°)
V (Å3)
Z
T (K)
100(2)
1.306
0.082
block, colorless
0.60 ꢀ 0.34 ꢀ 0.30
1.93–27.50
6076
100(2)
1.501
1.164
block, dark-red
0.38 ꢀ 0.10 ꢀ 0.04
2.05–27.50
4095
223(2)
1.353
0.617
block, colorless
0.58 ꢀ 0.36 ꢀ 0.20
1.44–27.50
8142
223(2)
1.649
0.807
block, orange
0.18 ꢀ 0.10 ꢀ 0.08
2.29–25.00
3894
223(2)
1.469
1.066
rod, orange
0.28 ꢀ 0.06 ꢀ 0.04
1.24–25.00
6835
Density (calc.) (Mg mꢁ3
)
Absorption coefficient (mmꢁ1
Crystal habit, colour
Crystal size (mm)
h Range (°)
)
Independent reflections
Rint
0.0303
0.0502, 0.1275
0.0496
0.0484, 0.1025
0.0518
0.0578, 0.1378
0.0486
0.0599, 0.1413
0.0694
0.0617, 0.1476
Final R, Rwa
a
R =
R
||Fo| ꢁ |Fc||/
R
|Fo| for ‘‘observed’’ reflections having F2 > 2d (F2). Rw = [
R
w(F2o ꢁ F2c)2/
R
w(F2o)2]1/2 for all data.

T. Chen et al. / Inorganica Chimica Acta 423 (2014) 320–325
323
Scheme 1. Synthesis of pybox ligands L1 and L2.
+
FeCl3
O
O
N
CH3CN
O
O
N
N
N
Fe
Ph
Ph
Ph
Ph
N
N
N
Fe
Cl Cl
N
Ph
Ph
Cl
C
Ph
Ph
C
H3C
CH3
4
6
Scheme 4. Synthesis of iron complex 6.
Fig. 1. Ellipsoid plot of L2. Thermal ellipsoids are drawn at the 30% probability
level. All hydrogen atoms are omitted for clarity.
.
FeCl2 4H2O
THF, RT
O
O
O
O
N
N
N
N
N
N
Fe
Cl Cl
R
R
R
R
R
R
R
R
3
4
(R = Me) and (R = Ph)
Scheme 2. Synthesis of iron complexes 3 and 4.
Fig. 2. Ellipsoid plot of 3. Thermal ellipsoids are drawn at the 30% probability level.
All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:
Fe1–N1 2.240(2), Fe1–N2 2.145(2), Fe1–N3 2.212(2), Fe1–Cl1 2.3488(8), Fe1–Cl2
2.3488(8); N2–Fe1–N3 73.51(9), N2–Fe1–N1 73.08(8), N3–Fe1–N1 146.06(8), N2–
Fe1–Cl2 139.63(7), N3–Fe1–Cl2 99.49(6), N1–Fe1–Cl2 101.96(6), N2–Fe1–Cl1
98.83(6), N3–Fe1–Cl1 98.04(6), N1–Fe1–Cl1 92.78(6), Cl2–Fe1–Cl1 121.54(3).
O
O
O
N
AgOTf
O
O
N
N
S
N
Fe
O
O
O
O
N
N
CH3CN
N
Fe
Cl Cl
C
O
S
H3C
CF3
F3C
3
5
to those of complex 3 (2.3370 and 2.510 Å in 4 vs. 2.240 and
2.212 Å in 3). It is noteworthy that the two N–Fe bonds distances
are considerably different, 2.510 of N(3)–Fe(1) is significantly
longer than the other one. This may be caused by the steric repul-
sions between the phenyl groups. The dihedral angles between the
two oxazoline rings, pyridine ring and two oxazoline rings are
16.07°, 11.40° and 12.00° respectively, indicating that the planar
structure was greatly distorted because of the phenyl substituents.
Scheme 3. Synthesis of iron complex 5.
angle between two oxazoline rings is 6.20°, while the dihedral
angles between pyridine ring and two oxazoline rings are 4.55°
and 6.51°, respectively. The L2 ligand in complex 4 is also coordi-
nated to the iron center in a tridentate mode, although the N–Fe
bonds between the oxazoline rings and Fe are elongated compared

324
T. Chen et al. / Inorganica Chimica Acta 423 (2014) 320–325
Fig. 3. Ellipsoid plot of 4. Thermal ellipsoids are drawn at the 30% probability level.
Solvent and all hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Fe1–N1 2.3370(19), Fe1–N2 2.120(2), Fe1–Cl1 2.2560(7), Fe1–Cl2
2.2573(7); N2–Fe1–Cl1 110.81(6), N2–Fe1–Cl2 124.82(6), Cl1–Fe1–Cl2 124.11(3),
N2–Fe1–N1 74.79(7), Cl1–Fe1–N1 100.51(5), Cl2–Fe1–N1 98.53(5).
Fig. 5. Ellipsoid plot of 6. Thermal ellipsoids are drawn at the 30% probability level.
[FeCl3]ꢁ and all hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Fe1–N1 2.288(3), Fe1–N2 2.183(3), Fe1–N3 2.406(3), Fe1–N4
2.143(4), Fe1–N5 2.249(4), Fe1–Cl1 2.2885(13); N2–Fe1–N1 72.80(13), N2–Fe1–N3
71.66(12), N2–Fe1–N4 85.80(13), N2–Fe1–N5 84.10(13), N3–Fe1–N1 144.35(12),
N4–Fe1–N5 169.90(14), N1–Fe1–Cl1 105.06(10), N2–Fe1–Cl1 177.02(10), N3–Fe1–
Cl1 110.56(9), N4–Fe1–Cl1 96.15(11), N5–Fe1–Cl1 93.95(11).
3.3. Reactivity studies
Iron complex 3 is stable both as a solid and in solution under
inert atmosphere. To test its reactivity, it was treated with silver
triflate in acetonitrile in the absence of light. An instant color
change from dark blue-violet to orange was observed. After expo-
sure to light for 20 min, the unreacted silver reagent and AgCl were
removed by filtration. Complex 5 was obtained as a light orange
solid (Scheme 3). The single crystal of 5 suitable for X-ray diffrac-
tion studies was grown by vapor diffusion of diethyl ether into the
acetonitrile solution of 5 (Fig. 4). The X-ray structure shows that
the two chloride ligands were replaced by two triflate ligands
and one acetonitrile molecule was coordinated to the iron center
forming a distorted octahedral iron(II) complex. Two O and the S
atoms of one of the OTf were found to be disordered at two posi-
tions with occupancy ratio of approx. 72:28. The bond lengths of
Fe(1)–N(1) 2.214 Å, Fe(1)–N(2) 2.143 Å, Fe(1)–N(3) 2.208 Å and
the trans angle of N(3)–Fe(1)–N(1) 146.4° are similar to those in
complex 3.
In contrast to 3, iron complex 4 decomposed in the similar reac-
tion conditions, which may be explained by the weak coordination
between more steric Ph2–pybox ligand with the iron center. The Fe–
Cl bond of iron complex 4 could be cleaved in mild conditions
(Scheme 4). In the absence of silver triflate, diffusion of ether into
the solution of 4 in acetonitrile afforded complex 6 (Fig. 5). It was
speculated that one of the Fe–Cl bonds on complex 4 was cleaved,
and the released chloride reacted with FeCl2 to form the FeClꢁ3 coun-
ter anion in solution. Two molecules of acetonitrile were then coor-
dinated to the iron center to afford a hexa-coordinated complex
with an octahedral geometry. It appears that the geometrical
change from trigonal bipyramid to octahedron leads to a smaller
steric repulsion: the Fe(1)–N(1) and Fe(1)–N(3) bonds distances
of 2.288 and 2.406 Å respectively, are now considerably shorter
than those in complex 4; and the dihedral angle between the two
oxazoline rings, pyridine ring and two oxazoline become 10.09°,
3.65° and 7.25° respectively, also smaller than those in complex 4.
As iron complexes have displayed some moderate to good activ-
ities for ethylene polymerization [41–45], preliminary investiga-
tions to apply iron complex 3 and 4 as ethylene polymerization
catalysts were carried out. They were both found to be effective
towards ethylene polymerization in the presence of MAO (Al/
Fe = 1000) at 50 °C under 18 bar. Iron complex 4 shows a higher
catalytic activity (1.01 kg/mol (Fe)ꢂh) than 3 (0.51 kg/mol (Fe)ꢂh)
and the resultant polymer obtained from 4 possesses a higher
molecular weight (8.9 ꢀ 104) than that from 3 (5.9 ꢀ 104), indicat-
ing bulky phenyl groups are beneficial to the catalytic activity and
favoring the higher molecular weight. These results are compara-
ble to those with the reported isopropyl analogue [35]. However,
the catalytic activities are still considered low, presumably due
to the alkylation of the pybox ligand in the presence of co-catalyst
MAO [46].
Fig. 4. Ellipsoid plot of 5. Thermal ellipsoids are drawn at the 30% probability level.
All hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:
Fe1–N1 2.214(6), Fe1–N2 2.143(5), Fe1–N3 2.208(6), Fe1–N4 2.168(7), Fe1–O3
2.091(4), Fe1–O6 2.181(5); O3–Fe1–N2 170.5(2), O3–Fe1–N4 87.9(2), N2–Fe1–N4
97.5(2), O3–Fe1–O6 84.3(2), N2–Fe1–O6 90.4(2), N4–Fe1–O6 172.1(2), O3–Fe1–N3
99.4(2), N2–Fe1–N3 73.1(2), N4–Fe1–N3 88.4(2), O6–Fe1–N3 94.5(2), O3–Fe1–N1
114.1(2), N2–Fe1–N1 73.6(2), N4–Fe1–N1 91.7(2), O6–Fe1–N1 90.0(2), N3–Fe1–N1
146.4(2).
4. Conclusions
We have successfully synthesized the iron (II) complexes
[Fe(Me2–pybox)Cl2] (3) and [Fe(Ph2–pybox)Cl2] (4) by the reaction

T. Chen et al. / Inorganica Chimica Acta 423 (2014) 320–325
325
[16] D. Cuervo, J. Díez, M.P. Gamasa, S. García-Granda, J. Gimeno, Inorg. Chem. 41
(2002) 4999.
[17] M.A. Panera, J. Diez, I. Merino, E. Rubio, M.P. Gamasa, Inorg. Chem. 48 (2009)
of iron (II) chloride tetrahydrate with the corresponding pybox
ligands. X-ray crystallographic analysis and reactivity studies of
these pybox–Fe complexes were performed. The results indicate
that the coordination abilities of pybox ligands to the iron center
as well as the reactivity of the pybox–Fe complexes are dependent
on the structure of the pybox ligand employed.
´
11147.
[18] H. Nishiyama, T. Naitoh, Y. Motoyama, K. Aoki, Eur. J. 5 (1999) 3509.
[19] Y. Motoyama, O. Kurihara, K. Murata, K. Aoki, H. Nishiyama, Organometallics
19 (2000) 1025.
[20] D. Cuervo, J. Díez, M.P. Gamasa, J. Gimeno, Organometallics 24 (2005) 2224.
[21] P. Paredes, J. Díez, M.P. Gamasa, Organometallics 27 (2008) 2597.
[22] K. Matsumoto, K. Suzuki, T. Tsukuda, T. Tsubomura, Inorg. Chem. 49 (2010)
4717.
Acknowledgments
[23] D. Cuervo, E. Menéndez-Pedregal, J. Díez, M.P. Gamasa, J. Organomet. Chem.
696 (2011) 1861.
[24] R. Langer, Y. Diskin-Posner, G. Leitus, L.J.W. Shimon, Y. Ben-David, D. Milstein,
Angew. Chem. Int. Ed. 50 (2011) 9948.
We are grateful for the financial support from King Abdullah
University of Science and Technology. We thank Ms Geok Kheng
Tan (NUS) for her kindly help in X-ray crystallographic analysis.
Financial supports from the Natural Science Foundation of China
(21302170, 21304050), the Science Foundation of Zhejiang Sci-
Tech University (1201839-Y), the Program for Zhejiang Leading
Team of Science and Technology Innovation (2010R50038-20)
were also greatly appreciated.
[25] M. Mayer, A. Welther, ChemCatChem 3 (2011) 1567.
[26] S. Blanchard, E. Derat, M. Desage-El Murr, L. Fensterbank, M. Malacria, V.
Mouriès-Mansuy, Eur. J. Inorg. Chem. 10 (2012) 376.
[27] S.M. Paradine, M.C. White, J. Am. Chem. Soc. 134 (2012) 2036.
[28] K. Umehara, S. Kuwata, T. Ikariya, Inorg. Chim. Acta 413 (2014) 136.
[29] M. Kawatsura, K. Kajita, S. Hayase, T. Itoh, Synlett (2010) 1243.
[30] M. Redlich, M.M. Hossain, Tetrahedron Lett. 45 (2004) 8987.
[31] J. Jankowska, J. Paradowska, J. Mlynarski, Tetrahedron Lett. 47 (2006) 5281.
[32] J. Jankowska, J. Paradowska, B. Rakiel, J. Mlynarski, J. Org. Chem. 72 (2007)
2228.
[33] M. Kawatsura, Y. Komatsu, M. Yamamoto, S. Hayase, T. Itoh, Tetrahedron Lett.
48 (2007) 6480.
[34] M. Kawatsura, Y. Komatsu, M. Yamamoto, S. Hayase, T. Itoh, Tetrahedron 64
(2008) 3488.
[35] K. Nomura, W. Sidokmai, Y. Imanishi, Bull. Chem. Soc. Jpn. 73 (2000) 599.
[36] J.D. Nobbs, A.K. Tomov, R. Cariou, V.C. Gibson, A.J.P. White, G.J.P. Britovsek,
Dalton Trans. 41 (2012) 5949.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.08.050.
References
[37] A.M. Tondreau, J.M. Darmon, B.M. Wile, S.K. Floyd, E. Lobkovsky, P.J. Chirik,
Organometallics 28 (2009) 3928.
[38] B.T. Hahn, R. Fröhlich, K. Harms, F. Glorius, Angew. Chem. Int. Ed. 47 (2008)
9985.
[39] H. Nishiyama, M. Kondo, T. Nakamura, K. Itoh, Organometallics 10 (1991) 500.
[40] J. Li, J. Zhao, Struct. Chem. 17 (2006) 617.
[41] G.J.P. Britovsek, V.C. Gibson, O.D. Hoarau, S.K. Spitzmesser, A.J.P. White, D.J.
Williams, Inorg. Chem. 42 (2003) 3454.
[42] V.C. Gibson, C. Redshaw, G.A. Solan, A.J.P. White, D.J. Williams,
Organometallics 26 (2007) 5119.
[43] V.K. Appukuttan, Y. Liu, B.C. Son, C.-S. Ha, H. Suh, I. Kim, Organometallics 30
(2011) 2285.
[1] H. Nishiyama, H. Sakaguchi, T. Nakamura, M. Horihata, M. Kondo, K. Itoh,
Organometallics 8 (1989) 846.
[2] D.A. Evans, Z.K. Sweeney, T. Rovis, J.S. Tedrow, J. Am. Chem. Soc. 123 (2001)
12095.
[3] D.W. Dodd, H.E. Toews, F.d.S. Carneiro, M.C. Jennings, N.D. Jones, Inorg. Chim.
Acta 359 (2006) 2850.
[4] G. Desimoni, G. Faita, M. Toscanini, M. Boiocchi, Eur. J. 14 (2008) 3630.
[5] P.K. Singh, V.K. Singh, Org. Lett. 12 (2009) 80.
[6] H. Kim, C. Lee, Org. Lett. 13 (2011) 2050.
[7] K. Junge, K. Möller, B. Wendt, S. Das, D. Gördes, K. Thurow, M. Beller, Chem.
Asian J. 7 (2012) 314.
[8] J. Wang, M. Frings, C. Bolm, Angew. Chem. Int. Ed. 52 (2013) 8661.
[9] J. Wang, M. Frings, C. Bolm, Eur. J. 20 (2014) 966.
[10] K.B. Jensen, R.G. Hazell, K.A. Jørgensen, J. Org. Chem. 64 (1999) 2353.
[11] A.B. Kazi, G.D. Jones, D.A. Vicic, Organometallics 24 (2005) 6051.
[12] P.J. Heard, C. Jones, J. Chem. Soc. Dalton Trans. (1997) 1083.
[13] P.J. Heard, D.A. Tocher, J. Chem. Soc. Dalton Trans. (1998) 2169.
[14] J. Díez, M.P. Gamasa, M. Panera, Inorg. Chem. 45 (2006) 10043.
[15] M. Panera, J. Díez, I. Merino, E. Rubio, M.P. Gamasa, Eur. J. Inorg. Chem. (2011)
393.
[44] Q. Xing, T. Zhao, Y. Qiao, L. Wang, C. Redshaw, W.-H. Sun, RSC Adv. 3 (2013)
26184.
[45] J. Ma, C. Feng, S. Wang, K.-Q. Zhao, W.-H. Sun, C. Redshaw, G.A. Solan, Inorg.
Chem. Front. 1 (2014) 14.
[46] A.V. Chuchuryukin, R. Huang, M. Lutz, J.C. Chadwick, A.L. Spek, G. van Koten,
Organometallics 30 (2011) 2819.