Dinuclear Iron(III) and Nickel(II) Complexes Containing N-(2-Pyridylmethyl)-N′-(2-hydroxyethyl)ethylenediamine: Catalytic Oxidation and Magnetic Properties

Article abstract of DOI:10.1002/chem.201604498

Dinuclear Fe^III and Ni^II complexes, [(phenO)Fe(N₃)]₂(NO₃)₂ (1) and [(phenOH)Ni(N₃)₂]₂ (2), were prepared by treating Fe(NO₃)₃?9 H₂O and Ni(NO₃)₂?6 H₂O in methanol, respectively, with phenOH (=N-(2-pyridylmethyl)-N′-(2-hydroxyethyl)ethylenediamine) and NaN₃; both 1 and 2 were characterized by elemental analysis, IR spectroscopy, X-ray diffraction, and magnetic susceptibility measurements. Two ethoxo-bridged Fe^III and two azido-bridged Ni^II were observed in 1 and 2, respectively; corresponding antiferromagnetic interaction via the bridged ethoxo groups and strong ferromagnetic coupling via the bridged end-on azido ligands within the dimeric unit were observed. Complex 1 did not exhibit any catalytic activity, while 2 exhibited excellent catalytic activities for the epoxidation of aliphatic, aromatic, and terminal olefins.

Full text of DOI:10.1002/chem.201604498

A Journal of
Accepted Article
Title: Dinuclear iron(III) and nickel(II) complexes containing N-(2-
pyridylmethyl)-N'-(2-hydroxyethyl)ethylenediamine: catalytic
oxidation and magnetic properties
Authors: Kil Sik Min, Ah Rim Jeong, Jong Won Shin, Jong Hwa Jeong,
Kwon Hee Bok, Cheal Kim, Shinya Hayami, Jaeheung Cho,
and Donghyun Jeong
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To be cited as: Chem. Eur. J. 10.1002/chem.201604498
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Dinuclear iron(III) and nickel(II) complexes containing N-(2-
pyridylmethyl)-N'-(2-hydroxyethyl)ethylenediamine: catalytic
oxidation and magnetic properties
Ah Rim Jeong,[a] Jong Won Shin,[a] Jong Hwa Jeong,*[a] Kwon Hee Bok,[b] Cheal Kim,*[b] Donghyun
Jeong,[c] Jaeheung Cho,[c] Shinya Hayami,[d] Kil Sik Min*[e]
Dedicated to Professor Karl Wieghardt on the occasion of his 75th birthday
Abstract:
Dinuclear
Fe(III)
and
Ni(II)
complexes,
binding modes, such as end-on, end-to-end, and terminal.[19-21]
Versatile azide ions have been extensively investigated for
[(phenO)Fe(N3)]2(NO3)2 (1) and [(phenOH)Ni(N3)2]2 (2) were
prepared by treating Fe(NO3)3•9H2O and Ni(NO3)2•6H2O in methanol, dimers, clusters, and polymers that exhibit significant magnetic
respectively, with phenOH (= N-(2-pyridylmethyl)-N'-(2-hydroxyethyl)
ethylenediamine) and NaN3; both 1 and 2 were characterized by
elemental analysis, IR spectroscopy, X-ray diffraction, and magnetic
susceptibility measurement. Two ethoxo-bridged Fe(III) and two
properties, such as ferro- and antiferromagnetic interactions,
because they may induce interesting magnetic coupling
attributed to two bonding modes, i.e., end-on (-1,1,
ferromagnetic) and end-to-end (-1,3, antiferromagnetic).[22-31]
A
azido-bridged Ni(II) were observed in
1
and 2, respectively;
zigzag tetranuclear iron(III) complex with bridged azide and
methoxy ions and N,N-bis(2-pyridylmethyl)-2-aminoethanolate
(bpaeO−) ions,[32-33] exhibiting unique magnetic properties
attributed to both ferromagnetic and antiferromagnetic
interactions, has been reported. In addition, the bpaeO−
tetradentate ligand as a capping molecule has been utilized for
corresponding antiferromagnetic interaction via the bridged ethoxo
groups and strong ferromagnetic coupling via the bridged end-on
azido ligands within the dimeric unit were observed. Complex 1 did
not exhibit any catalytic activity, while 2 exhibited excellent catalytic
activities for the epoxidation of aliphatic, aromatic, and terminal
olefins.
the formation of
complex as
a
bis(-alkoxo)-bridged dinuclear iron(III)
a
biomimetic model for intradiol-cleaving
dioxygenase enzymes.[34] Recently, double phenoxido-bridged
dinuclear nickel(II) complexes have been reported,[35] exhibiting
catecholate activity via nickel(II) centers. Huang et al. have
reported one-dimensional homochiral nickel(II) coordination
polymers based on Ni(salen) metalloligands,[36] exhibiting
catalytic activity for alkene epoxidation; these polymers can be
used as recyclable heterogeneous catalysts.
Introduction
Acyclic tetradentate ligands have been used for preparing cis
and trans octahedral transition metal complexes;[1-6] the
coordination modes of tetradentate ligands significantly depend
on the types of functional groups, transition metal ions, and
anions.[7-13] For instance, hydroxyl-group-containing tetradentate
ligands can be deprotonated depending on metal ions and
anions, affording multinuclear metal complexes.[14-18] As anionic
ligands, azide ions have been utilized for constructing various
structures in coordination compounds, attributed to their different
As continuation of our previous study on the dinuclear iron(III)
complex
with
N-(2-pyridylmethyl)-N'-(2-hydroxyethyl)
ethylenediamine (phenOH),[37] herein, two novel transition metal
−
complexes containing Fe and Ni using phenOH and N3 ,
[(phenO)Fe(N3)]2(NO3)2 (1) and [(phenOH)Ni(N3)2]2 (2),
respectively, were prepared. In addition, their structural
characterization and magnetic properties were discussed. In
particular, complex 2 exhibited good oxidation catalytic activities
for the epoxidation of aliphatic and aromatic olefins, as well as
terminal olefins. The catalytic activities of 2 were reported in
detail.
[a]
[b]
A. R. Jeong, J. W. Shin, Prof. J. H. Jeong
Department of Chemistry, Kyungpook National University
Daegu 41566 (Republic of Korea)
E-mail: jeongjh@knu.ac.kr
K. H. Bok, Prof. C. Kim
Department of Fine Chemistry
Seoul National University of Science and Technology
Seoul 01811 (Republic of Korea)
E-mail: chealkim@seoultech.ac.kr
D. Jeong, Prof. J. Cho
Department of Emerging Materials Science, DGIST
Daegu 42988 (Republic of Korea)
Prof. S. Hayami
Department of Chemistry, Kumamoto University
Kumamoto 860-8555 (Japan)
Prof. K. S. Min
Results and Discussion
[c]
[d]
[e]
Synthesis and characterization
Air-stable complex 1 was obtained in moderate yield (30%) from
the reaction between one equivalent of phenOH with
Fe(NO3)3•9H2O and 2 equiv of NaN3 in MeOH under aerobic
conditions (Scheme 1). Previously we have observed that an
iron(II) ion was readily oxidized to an iron(III) ion upon
coordination of a tetradentate N3O-type ligand.[32-33] Due to the
higher Lewis acidity of the iron(III) ion, the -OH group in the
tetradentate N3O-type ligand was deprotonated and led to an
Department of Chemistry Education, Kyungpook National University
Daegu 41566 (Republic of Korea)
E-mail: minks@knu.ac.kr
Supporting information for this article is given via a link at the end of
the document.
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ethoxy-bridged iron(III) complex. Likewise, 1 was formed as an
ethoxy-bridged iron(III) dimer through the deprotonation of the -
OH group in phenOH (vide infra). As can be observed in the IR
one iron(III) ion was bonded to a deprotonated phenO− ligand
and azide ion. In other words, an Fe(phenO)(N3) unit resulted
from the bridging of two iron(III) ions through two terminal Oalkoxo
of the tetradentate phenO− ligands. The two Fe(phenO)(N3) units
were related by symmetry through an inversion center situated
at the barycenter of the complex. Each iron(III) ion adopted a
six-coordinate distorted octahedral configuration with an N4O2
donor set. Pyridine nitrogen, one of amine nitrogen atoms, and
the bridged ethoxyl oxygen atom were present in the plane of
the Fe2O2 core.[18,32,34] Another amine nitrogen atom (N2) and a
−
spectrum of 1 in KBr, peaks characteristic of NO3 and
coordinated phenO−, as well as NN absorption characteristic of
−
N3 , were observed. In addition, a strong absorption peak was
observed at 2070 cm−1, attributed to azido ligands.[38] Moreover,
peaks were observed at 3035 and 2913 cm−1, attributed to the
C−H of the phenO− ligand. Furthermore, a strong peak was also
−
observed at 1384 cm–1, corresponding to NO3 . On the other
hand, air-stable complex 2 was prepared in moderate yield (30%) nitrogen atom (N4) of the azide ion occupied two axial positions
from the reaction between one equivalent of phenOH with
Ni(NO3)2•6H2O and 3 equiv. NaN3 in MeOH under aerobic
conditions (Scheme 1). First, we tried to prepare 2 using the
same method as 1. However, 2 was formed in low yield (~13%)
and/or with monomeric pink complex. Thus we found the best
synthetic condition as changing the ratio of azide ion. That is, 2
was clearly obtained in high yield using 3 equiv of azide ion as
described above. Contrary to 1, the -OH group of the phenOH
ligand in 2 was not deprotonated. This is attributed to the lower
in the octahedron. The average Fe−NphenO, Fe−OphenO, and
Fe−Nazido bond distances were 2.172(1), 1.988(1), and 1.965(1)
Å, respectively. The average bite distance and angle of the five-
membered chelate rings were 2.080(1)
respectively. The two iron(III) ions within the dinuclear unit were
linked by two deprotonated ethoxy groups. The
Å
and 87.68(1)o,
Fe1−O2−Fe1(−x+2, −y, −z) angle was 105.06(4)o. The shortest
Fe•••Fe contact within the dimer was 3.156(1) Å. Furthermore,
the oxygen atom of uncoordinated NO3− formed hydrogen bonds
with the nitrogen atom of amine in the phenO− ligand (N2•••O2
2.9107 Å, ∠N2−H2−O2 149.7o; N3•••O4(x−1/2, −y+1/2, z+1/2)
2.8861 Å, ∠ N3−H3−O4 165.1o).[39] Because of these
interactions, 1 exhibited a 2-D supramolecular network structure
(Figure S1).
Lewis acidity of the nickel(II) ion. As can be observed in the IR
−
spectrum of
2
in KBr, peaks characteristic of N3 and
coordinated phenOH were observed. In addition, a strong
absorption peak was observed at 2049 cm−1, attributed to azido
ligands.[38] Furthermore, peaks were observed at 3081 and 2944
cm−1, attributed to the C−H band of the phenOH ligand, and a
peak was observed at 3367 cm−1, attributed to the O−H of the
phenOH ligand. The composition of 1 and 2 was also identified
by elemental analysis and single crystal X-ray diffraction.
Crystal structure of 2
̅
Compound 2 crystallized in the triclinic P space group, and the
unit cell included two dinuclear complex molecules. Figure 2
shows the atom-labeling ORTEP drawing of 2, and Table 2 lists
its selected bond lengths and angles. In the asymmetric unit of 2,
one nickel(II) ion was bonded to the phenOH ligand and two
azide ions. In other words, a Ni(phenOH)(N3)2 unit resulted from
the bridging of two nickel(II) ions through two 2-N bridging azide
ions. The two Ni(phenOH)(N3)2 units were related by symmetry
through an inversion center situated at the barycenter of the
complex molecule. Both nickel(II) ions were coordinated by three
nitrogen atoms attributed to azide ions. Each nickel(II) ion
adopted a six-coordinate distorted octahedral configuration with
an N6 donor set. The amine nitrogen atoms (N1 and N3) from
the phenOH ligand were present in the plane of the Ni2N2
core.[20] The amine nitrogen atom (N2) from the phenOH ligand
and the nitrogen (N7) of the azide ion occupied two axial
positions in the octahedron. The pyridine nitrogen atoms of the
phenOH ligands were trans to the 2-N bridging azide ion. The
average Ni−NphenOH and Ni−Nazido bond distances were 2.092(2)
and 2.106(2) Å, respectively. The two nickel(II) ions within the
dinuclear unit were connected by two 2-N bridging azide ions
with an inversion center. The Ni1−N4−Ni1(−x+1, −y+1, −z+1)
angle was 98.78(14)o. The shortest Ni•••Ni contact within the
dimer was 3.209(1) Å. Furthermore, the hydroxyl oxygen atom of
phenOH formed a hydrogen bond with the nitrogen atom of the
azide ion (O1•••N18(−x, −y+1, −z) 2.770 Å, ∠ O1−H1−N18
164.9o).[39] Because of these interactions, 2 exhibited a 1-D
supramolecular ladder-like structure (Figure S2).
Scheme 1. Ligand used and dinuclear complexes prepared.
Crystal structure of 1
Compound 1 crystallized in the monoclinic P21/n space group,
and the unit cell included two dinuclear complex molecules.
Figure 1 shows the ORTEP drawing of 1, and Table 1 lists its
selected bond lengths and angles. In the asymmetric unit of 1,
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Table 1. Selected bond distances (Å ) and angles (°) for 1.
Fe1−N1
Fe1−N2
Fe1−N3
Fe1•••Fe1i
2.1533(11)
2.1772(10)
2.1832(10)
3.1558(7)
Fe1−N4
Fe1−O1
Fe1−O1i
1.9648(12)
1.9999(9)
1.9762(9)
N1−Fe1−N2
N1−Fe1−N3
N1−Fe1−N4
N1−Fe1−O1
N1−Fe1−O1i
N2−Fe1−N3
N2−Fe1−N4
Fe1−O1−Fe1i
75.47(4)
113.49(4)
90.55(5)
162.55(4)
89.84(4)
78.28(4)
157.80(5)
105.07(4)
N2−Fe1−O1
N2−Fe1−O1i
N3−Fe1−N4
N3−Fe1−O1
N3−Fe1−O1i
N4−Fe1−O1
N4−Fe1−O1i
O1−Fe1−O1i
96.63(4)
93.40(4)
92.10(5)
79.25(4)
151.75(4)
101.20(5)
103.88(5)
74.93(4)
Figure 2. ORTEP view of 2. The atoms are represented by 20% probable
thermal ellipsoids. The above dimeric structure is one of two independent
dimeric complexes. Hydrogen atoms are omitted for clarity except for the
hydrogen atom of the hydroxyl groups. Symmetric code: (´ = −x+1, −y+1,
−z+1).
Symmetry transformations used to generate equivalent atoms: (i) −x+2, −y,
−z.
Magnetic properties
The magnetic susceptibilities, (T), of 1 and 2 were measured
between 2 and 300 K (external field of 5000 Oe). Figure 3 plots
eff versus T. The effective magnetic moment eff [= (8MT)1/2] of
1 at 300 K was 6.19 B/Fe2; it continuously decreased with
decreasing temperature until 2 K, indicative of significant
antiferromagnetic interaction. The eff value at room temperature
was less than the spin-only value of 8.37 µB/Fe2, as expected for
independent two S = 5/2, Fe(III) spins. Notably, as temperature
decreased from 300 to 2 K, µeff(T) continuously decreased,
implying antiferromagnetic interaction in 1. The µeff(T) data were
simulated with the julX program package for exchange-coupled
systems.[40] The simulation was based on the spin Hamiltonian
operator for a dinuclear system consisting of two high-spin
iron(III) ions with spins Sa = Sb = 5/2 (H = −2J(Sa•Sb)).[41] With the
best fit had, J (= Jab) = −17.3 cm−1 and g (= ga = gb) = 2.0. The
Figure 1. ORTEP view of 1. The atoms are represented by 50% probable
thermal ellipsoids. Hydrogen atoms and noncoordinated NO3 anions are
omitted for clarity.
–
coupling constant
J through bridging oxygen atoms was
expected to be antiferromagnetic, which is consistent with the
value reported for [(phenO)Fe(SO4)]2•2CH3OH (phenOH = N-(2-
Table 2. Selected bond distances (Å ) and angles (°) for 2.
pyridylmethyl)-N'-(2-hydroxyethyl)ethylenediamine,
cm−1).[37] Complex 1 exhibited two exchange pathways, i.e., two
μ2-ethoxo groups. μ2-Ethoxo bridges can transmit
antiferromagnetic interaction depending on Fe-O(ethoxo)-Fe
bridging angles. In 1, the Fe1−O1−Fe1(−x+2, −y, −z.) angle was
105.07(4)°, indicating antiferromagnetic interaction through the
O1 atom. The bridging angles were greater than 93.5° (magic
angle).[42] As a result, the iron(III) dinuclear complex exhibits
antiferromagnetic coupling (J = −17.3 cm−1); this value is smaller
J = −17.9
Ni1[a]−N1
Ni1−N2
Ni1−N3
Ni1−N4
2.063(3)
2.088(3)
2.124(3)
2.106(3)
Ni1−N4i
Ni1−N7
Ni1•••Ni1i
2.121(3)
2.079(4)
3.2091(12)
N1−Ni1−N2
N1−Ni1−N3
N1−Ni1−N4
N1−Ni1−N4i
N1−Ni1−N7
N2−Ni1−N3
N2−Ni1−N4
N2−Ni1−N4i
80.57(14)
93.91(13)
167.44(13)
92.10(12)
99.52(15)
84.38(14)
88.77(14)
90.04(14)
N2−Ni1−N7
N3−Ni1−N4
N3−Ni1−N4i
N3−Ni1−N7
N4−Ni1−N4i
N4−Ni1−N7
N4i−Ni1−N7
Ni1−N4−Ni1i
179.17(14)
91.61(13)
171.02(13)
96.43(14)
81.22(14)
91.04(14)
89.14(14)
98.78(14)
than those of [Fe2(dbe)2(bz)2](ClO4)2 (dbe
=
2-[bis(2-
benzimidazolylmethyl)amino]ethanolato; bz = benzoate, J =
20.5 cm1), [PH(t-Bu)3]2[Fe2(μ-OEt)2Cl6] (24.6 cm1), and
[(Hpmide)Fe(NO3)]2(NO3)2•2CH3OH
(H2pmide
=
N-(2-
This
pyridylmethyl)iminodiethanol,
observation indicates that
23.5
cm1).[18,43-44]
1
exhibits comparatively weak
antiferromagnetic coupling and less efficient exchange
interaction.
The effective magnetic moment for 2 at 300 K was 4.75 µB/Ni2,
which is greater than the spin-only value of 4.00 µB/Ni2, as
[a] The Ni1 ion is one of two independent dimeric nickel(II) complexes.
Symmetry transformations used to generate equivalent atoms: (i) −x+1,
−y+1, −z+1.
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expected for independent two high-spin nickel(II) ions (g = 2, S =
1); this value is attributed to significant ferromagnetic interaction
and/or high g value; with decreasing temperature to 28 K, the
μB/Ni2 gradually increased to 5.37 μB/Ni2. The µeff(T) data were fit
to an analytical expression for a coupled S = 1 dinuclear spin
model based on the Hamiltonian H = −2J(Sa•Sb) (Sa = Sb = 1).[40-
41] With the best fit had, J (= Jab) = +33.0 cm−1, g = 2.19, and spin
impurities,  = 0.005. The coupling constant Jab through the
bridging nitrogen atoms was expected to be ferromagnetic,
which is consistent with the value reported for [Ni2(-N3)2(232-
N4)2](ClO4)2 (Ni•••Ni 3.423 Å, Ni-N 2.167 Å, Ni-N-Ni, 103.8o, J =
+33.8 cm−1);[45] this value is greater than those of
[{Ni(terpy)(N3)2}2]•H2O (Ni-N-Ni: 101.6o and J = +22.8 cm−1) and
[Ni2(terpy)2(N3)3(H2O)]ClO4•H2O (Ni-N-Ni: 102.5o and J = +13.6
cm−1) (terpy = 2,2´:6´,2´´-terpyridine).[46] In 2, the bridging angle
of Ni1-N4-Ni1(−x+1, −y+1, −z+1) was 98.78(14)°. Interestingly,
the magnetic properties of 2 indicated ferromagnetic coupling
between nickel(II) within the dinuclear unit. This result can be
expected as ferromagnetic coupling is attributed to the end-on
bridging mode of azide ion, attributed to spin polarization.[41]
Complex 2 exhibited more efficient exchange pathways as
compared with those of other nickel(II) dinuclear complexes.[45-46]
for 1 min at room temperature. Control experiments suggested
that the direct oxidation of the substrate by MCPBA is negligible
and that 2 is sufficiently stable during catalytic reactions
according to a UV-vis spectroscopic study. Without catalyst 2,
yield of epoxide was less than 10%. NiCl2 or Ni(NO3)2 did not
catalyze the epoxidation reaction under the same conditions.
Table 3 summarizes the results obtained from the epoxidation.
Cyclic olefins, such as cyclopentene, cycloheptene, and
cyclooctene, were oxidized to the corresponding epoxides in
good yields (68.1%-92.5%, entries 1-3, respectively), with 100%
conversion. Cyclohexene afforded cyclohexene oxide (65.3%)
as the major product, with low amounts of 2-cyclohexen-1-one
(4.2%) and 1-cyclohexen-1-ol (2.9%, entry 4). This result
indicates that free-radical oxidation also occurs with
epoxidation.[50-51] Terminal olefins, such as 1-hexene and 1-
octene, which are well known to be substrates that are difficult to
oxidize, were oxidized to their corresponding epoxides in good
yields (50.5% and 48.3%, entries 5 and 6, respectively).[52-54]
cis-2-Hexene and cis-2-octene were also oxidized to cis-2-
hexene oxide (26.7%) and cis-2-octene oxide (23.1%), in
addition to large amounts of trans-2-hexene oxide (56.5%) and
trans-2-octene oxide (61.9%; entries 7 and 9), respectively.
Hence, 2 exhibits a comparatively strong ferromagnetic coupling. trans-2-Hexene and trans-2-octene were oxidized to trans-2-
hexene oxide (66.5%) and trans-2-octene oxide (61.9%), in
addition to low amounts of cis-2-hexene oxide (15.8%) and cis-
2-octene oxide (13.3%; entries 8 and 10), respectively. Notably,
7
1
2
to the best of our knowledge, high amounts of cis-epoxides from
trans-olefins are extremely rare in metal-catalyzed olefin
epoxidation, and their formation is indicative that the lifetime of
the Ni-oxo-olefin-radical intermediate is sufficiently long for
rotation of the trans-olefin radical to cis-olefin one.[55]
6
5
4
3
2
1
0

Styrene was oxidized to styrene epoxide (82.2%), with some
amount of benzaldehyde (12.1%) and phenylacetaldehyde
(5.0%; entry 11). cis-Stilbene was converted to cis-stilbene oxide
(6.8%; entry 12) and trans-stilbene oxide (64.6%), in addition to
some benzaldehyde (16.1%) and 2-phenylacetophenone (4.9%).
trans-Stilbene was converted to trans-stilbene oxide (64.5%) as
the major product with minor amounts of cis-stilbene oxide
(9.4%), benzaldehyde (17.8%), and 2-phenylacetophenone
(4.3%; entry 13). For olefins containing aromatic rings, the
substrates produced aldehyde and ketone as by-products in
moderate yields and exhibited low stereochemical retention. On
the other hand, the catalytic hydroxylation of alkanes such as
ethylbenzene and cyclohexane in the presence of 2 was
conducted, but trace amounts (~5%) of alcohols were observed.
Based on product analysis, Scheme 2 shows the most plausible
mechanism for the epoxidation of olefins with 2. First, peracid
reacts with nickel(II) complex 2 to afford a mononuclear Ni-
acylperoxo species (NiII-OOC(O)R (3)). After the epoxidation
reaction, ESI-mass spectrometry analysis showed the formation
of a mononuclear LNiII(MCBA) species. The intermediate 3 then
undergoes either heterolysis or homolysis to afford a NiIV=O (4)
or NiIII=O (5) species, respectively; the reactive NiIV=O
intermediate 4 is possibly responsible for the epoxidation of
olefins, whereas NiIII=O complex 5 is responsible for radical-type
oxidation, affording allylic oxidation products, with the loss of
stereospecificity.
0
50
100
150
200
250
300
Temperature, T, K
Figure 3. Temperature dependence of the effective magnetic moment, eff(T),
of 1 (O) and 2 (x). Solid lines represent best fits by using parameters given in
the text.
Catalytic properties
Olefin epoxidation catalyzed by nickel complex 2
The catalytic epoxidation of various olefins was performed with
MCPBA in the presence of 2 under mild conditions.[47-49] First,
catalyst 2 (0.001 mmol) and substrate (0.035 mmol) were
dissolved in a mixture of CH3CN/CH2Cl2 (1:1 v/v; 1 mL). Second,
MCPBA (0.1 mmol) was added to the mixture, which was stirred
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Scheme 2. Plausible mechanism for the olefin epoxidation reaction with the catalyst 2.
Table 3. Olefin epoxidations by MCPBA with nickel complex
2 in
trans-stilbene oxide
benzaldehyde
64.6±2.6
16.1±0.6
4.9±0.1
CH2Cl2/CH3CN (1:1) at room temperature.[a]
(phenOH)2Ni2(N3)4
2-
Entry Substrate
Product
Conversion(%)[b]
Yield(%)[b]
75.5±9.4
92.5±3.7
68.1±4.4
65.3±4.4
phenylacetophenone
trans-
stilbene
13
trans-stilbene oxide
cis-stilbene oxide
benzaldehyde
100
64.5±7.0
9.4±0.2
1
2
3
4
cyclopentene epoxide
cycloheptene epoxide
100
100
100
100
17.8±1.3
4.3±0.6
cyclooctene
cyclohexene
epoxide
epoxide
2-
phenylacetophenone
[a] Reaction conditions: olefins (0.035 mmol), catalysts (0.001 mmol), MCPBA
(0.10 mmol), and solvent (1 mL, CH2Cl2/CH3CN = 1:1). [b] Based on substrate.
2-cyclohexene-1-
one
4.2±0.4
2-cyclohexene-1-ol
epoxide
2.9±0.3
Terminal olefins have been considered as difficult-to-oxidize
olefins, attributed to their electron-deficient nature,[52-54,56-60] while
their corresponding products 1,2-epoxides are known as
versatile starting materials for the synthesis of more complicated
compounds. Hence, the epoxidation of terminal olefins using
catalyst 2 is investigated because 1-hexene and 1-octene are
reasonably oxidized to their corresponding epoxides. Table 4
summarizes the results. All of the terminal olefins were oxidized
to the corresponding epoxides in moderate yields (43.1%-
76.5%), with conversion ranging from 87.4% to 100%, indicating
that our catalytic system can be useful for the epoxidation of
various terminal olefins. Importantly, this is the first example that
Ni complex catalyzed efficiently the terminal olefin epoxidation.
5
6
7
1-hexene
1-octene
100
96.9±0.1
100
50.5±1.9
epoxide
48.3±0.3
26.7±1.3
cis-2-hexene cis-oxide
trans-oxide
56.5±5.0
66.5±4.8
trans-2-
8
trans-oxide
hexene
100
100
100
100
cis-oxide
15.8±0.3
23.1±1.7
9
cis-2-octene
cis-oxide
trans-oxide
50.1±0.5
61.9±1.1
trans-2-
octene
Table 4. Terminal olefin epoxidations by MCPBA with nickel complex 2 in
10
11
trans-oxide
CH2Cl2/CH3CN (1:1) at room temperature.[a]
cis-oxide
13.3±0.2
82.2±3.2
Entry Substrate
Product
Conversion(%)[b] Yield(%)[b]
styrene
epoxide
benzaldehyde
phenylacetaldehyde
cis-stilbene oxide
12.1±0.5
1
2
1-hexene
1-octene
1,2-epoxyhexane
1,2-epoxyoctane
100.0±0.0
96.9±0.1
60.5±1.9
58.3±0.3
5.0±0.4
6.8±0.1
12
cis-stilbene
97.8±0.1
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Product distribution of the O−O bond cleavage of PPAA
with catalyst 2
3
4
5
6
7
8
1-nonene
1,2-epoxynonane
1,2-epoxydecane
1,2-epoxyundecane
1,2-epoxydodecane
1,2-epoxytridecane
93.1±0.3
91.3±1.2
94.2±1.5
93.2±3.2
92.1±1.2
59.8±1.3
54.0±1.0
54.6±1.8
52.0±1.3
58.1±0.7
43.1±2.7
1-decene
For gaining insight into the partition of the heterolytic and
homolytic O-O bond cleavage of Ni-OOC(O)R species 3,
phenylperacetic acid (PPAA) was utilized as a mechanistic
probe. The O-O bond cleavage mode could be easily
determined by the quantitative analysis of the degradation
products of a nickel-acylperoxo intermediate (Ni-OOC(O)R) 3
formed during the reaction between nickel complex 2 and PPAA
(Scheme 3). The O-O bond of the Ni-acylperoxo species was
1-undecene
1-dodecene
1-tridecene
1-tetradecene
1,2-epoxytetradecane 93.5±0.1
1,2-
9
1-pentadecene
91.9±1.3
55.0±0.6
epoxypentadecane
heterolytically cleaved, affording Ni(IV)=O species
4 and
1,2-
phenylacetic acid (PAA, 6) (pathway a), while the O-O bond was
homolytically cleaved, affording an acyloxyl radical; this radical
afforded benzaldehyde (7), benzyl alcohol (8), and toluene (9)
via the rapid β-scission of the acyloxyl radical (pathway c).[62-65]
First, we carried out a control experiment using catalyst 2 with
PPAA in the absence of substrate (Table 5, entry 1). The
heterolytic cleavage product 6 (46.4%), as well as the homolytic
cleavage products 7 (30.8%) and 8 (6.0%), was formed. These
results indicate that the NiII-OOC(O)R intermediate 3 underwent
the partition of heterolysis (55.8%) and homolysis (44.2%),
affording high-valent complexes. Importantly, the presence of
high-valent NiIII=O and NiIII-oxyl species has been proposed in
only few studies.[47-49,66-69] Notably, the high-valent complexes
(NiIII=O and NiIV=O) are observed using the mechanistic probe
PPAA. On the other hand, the dangling -OH groups in 2 might
act as a general acid catalysis to facilitate O-O bond heterolysis
to produce the intermediate 4.
10
11
12
1-hexadecene
1-octadecene
vinylcyclohexane
89.3±0.2
76.5±3.9
75.9±9.5
62.7±0.2
epoxyhexadecane
1,2-epoxyoctadecane 89.3±1.3
vinylcyclohexane
87.4±0.6
oxide
[a] Reaction conditions: olefins (0.035 mmol), catalysts (0.001 mmol), MCPBA
(0.10 mmol), and solvent (1 mL, CH2Cl2/CH3CN =1:1). [b] Based on substrate.
Competition experiments of styrene and para-substituted
styrenes for the Hammett plot
For determining the nature of the reactive species responsible
for epoxidation, we examined the effect of the electronic effects
of the substituents on the reaction rate using styrene and four
para-substituted styrenes. The Hammett plot analysis showed a
 value of -1.00, indicating that the active oxidant is electrophilic
For examining the concentration dependence on the ratio of
heterolysis to homolysis, the concentration of the easy-to-oxidize
substrate, cyclohexene, was gradually increased from 0 to 160
(Figure 4); this value is slightly less than that ( = −1.35)
mM. If the NiII-OOC(O)R species
3 contributed to the
reported for [Ni(TpCF Me)(κ2-mCPBA)] (TpCF
= hydrotris(3-
Me
3
3
trifluoromethyl-5-methylpyrazolyl)borate).[61]
epoxidation reaction (pathway b), the ratio of heterolysis to
homolysis would vary according to the concentration of
substrate used. As direct olefin epoxidation by NiII-OOC(O)R
intermediate 3 generates the heterolytic cleavage product 6, the
ratio of heterolysis to homolysis would increase. The percentage
of heterolysis slightly increased from 55.8% to 64.8% as shown
in Table 5 (entries 1-5). These results suggest that the NiII-
OOC(O)R species is, to some extent, involved in olefin
epoxidation, attributed to the increase of cyclohexene
concentration.
1.00
-OCH3
0.60
-CH3
0.20
The substrate was changed from cyclohexene to 1-octene,
which is known as a difficult-to-oxidize olefin. In this case, the
partition of heterolysis and homolysis slightly varied, attributed to
the increase of 1-octene concentration (entries 1-5; Table 6).
These results demonstrate that the NiII-OOC(O)R species is
slightly involved in the epoxidation of 1-octene, even with high
concentration of 1-octene. Taken all together, the nickel catalyst
promoted O-O bond cleavage to heterolysis (60%) and
homolysis (40%), affording NiIV=O and NiIII=O species,
respectively. Moreover, the intermediate NiII-OOC(O)R species
might slightly contribute, to some extent, to catalytic epoxidation
in the presence of the easy-to-oxidize substrate, such as
cyclohexene, while it might be inactive to the difficult-to-oxidize
substrate, 1-octene.
-Cl
-H
-0.20
-0.60
-1.00
-CN
0.8
-0.8
-0.4
0
0.4
Figure 4. Hammett plot for selective reactivity of styrene to para-substituted
styrenes by 2 with MCPBA ( = -1.00).
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Table 5. Yield of products derived from PPAA mediated by nickel catalyst 2 in the absence and the presence of cyclohexene in a mixture of
CH3CN/CH2Cl2 (1/1) at room temperature.[a]
Heterolysis[b]
Homolysis[b]
Oxidation products[c]
ol
cyclohexene
[mM]
Hetero/Homo
[6/(7+8+9)]
Entry
6
7
8
9
oxide
-
one
-
1
2
3
4
5
0
46.4±0.6
49.5±1.4
49.7±0.7
50.1±1.0
51.9±0.4
30.8±0.5
6.0±0.4
5.1±0.5
4.8±0.3
4.3±0.1
5.3±0.2
-
1.26(55.8/44.2)
1.46 (59.3/40.7)
1.66 (62.4/37.6)
1.73 (63.4/36.7)
1.84 (64.8/35.2)
-
20
40
80
160
28.9±1.2
25.2±0.7
24.6±1.0
22.9±0.6
-
-
-
-
21.3±2.7[d]
15.7±3.8[d]
17.7±3.0[b]
21.3±3.1[b]
3.7±0.1[d]
2.0±0.3[d]
2.1±0.1[b]
2.9±0.4[b]
3.0±0.2[d]
2.1±0.6[d]
1.9±0.1[b]
2.8±0.4[b]
[a] Reaction conditions: substrate (0-0.16 mmol), catalysts (0.001 mmol), PPAA (0.04 mmol), and solvent (1 mL, CH3CN/CH2Cl2=1:1). [b] Based on PPAA;
6-9 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene, respectively. [c] oxide, -ol, and -one indicate cyclohexene oxide, 2-cyclohexen-
1-ol, and 2-cyclohexen-1-one, respectively. [d] Based on substrate.
Table 6. Yield of products derived from PPAA mediated by nickel catalyst 2 in the absence and the presence of 1-octene in a
mixture of CH3CN/CH2Cl2 (1/1) at room temperature.[a]
Heterolysis[b]
Homolysis[b]
Oxidation products
1-octene oxide
1-octene
[mM]
Hetero/Homo
[6/(7+8+9)]
Entry
6
7
8
9
1
2
3
4
5
0
20
46.4±0.6
47.0±6.2
48.7±0.9
48.0±0.1
51.9±4.3
30.8±0.5
6.0±0.4
5.7±0.1
5.5±0.2
5.1±0.3
5.0±0.6
-
1.26(55.8/44.2)
1.39 (58.2/41.8)
1.48 (59.7/40.3)
1.51 (60.2/39.8)
1.53 (60.5/39.5)
-
28.1±1.3
27.5±1.1
26.7±1.6
24.3±0.1
-
-
-
-
9.4±0.1[c]
7.1±0.3[b]
9.2±0.5[b]
9.7±0.2[b]
40
80
160
[a] Reaction conditions: substrate (0-0.16 mmol), catalysts (0.001 mmol), PPAA (0.04 mmol), and solvent (1 mL,
CH3CN/CH2Cl2=1:1). [b] Based on PPAA; 6-9 indicate phenylacetic acid, benzaldehyde, benzyl alcohol, and toluene,
respectively. [c] Based on substrate.
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Scheme 3. Possible degradation pathways of PPAA by nickel complex 2.
H218O experiments and spectroscopic studies for observing
the high-valent Ni=O species
Conclusions
In this study, two dinuclear iron(III) and nickel(II) metal
complexes 1 and 2 containing phenO/phenOH and azido ligands
were prepared and characterized. Complex 1 was a dinuclear
iron(III) complex bridged with ethoxo groups and exhibited
antiferromagnetic interaction between the iron(III) ions within the
dimeric unit, complex 2 was an end-on azido-bridged dinuclear
nickel(II) complex and exhibited strong ferromagnetic interaction
via the azide ions between the nickel(II) ions. Complex 1 formed
a 2-D extending structure via hydrogen-bonding interactions,
while 2 exhibited hydrogen bonding interactions leading to a 1-D
ladder-like structure. Complex 2 shows excellent oxidation
catalytic activities for various aliphatic and aromatic olefins as
well as terminal olefins. Particularly, cycloheptene was oxidized
to the corresponding epoxide in very high yield (92.5%). Based
on product analysis and H218O experiments, NiIV=O and NiIII=O
species are proposed as key intermediates in olefin epoxidation
using these catalytic systems. Moreover, further study for the
clear characterization and reactivity of the red species (possibly
NiIII=O or NiIV=O species) observed at -40 oC, which reacted with
cyclohexene, is ongoing.
Isotope labeling experiments were carried out in order to
understand the source of oxygen atoms found in oxidation
products.[62] Epoxidation of cyclohexene (10 equiv) by nickel
complex 2 (1 mM) and MCPBA (20 equiv) was conducted in
presence of excess H218O (1111 equiv) in a mixture of distilled
CH3CN and anhydrous CH2Cl2 (v/v, 1:1). The cyclohexene oxide
product contained a small amount of (~5%) of 18O derived from
H218O. The results of the labeled-water experiments
demonstrate that although high-valent Ni=O species are
involved in the reactions of the catalysts with MCPBA, oxygen-
atom transfer of Ni=O species to substrate might be much faster
than oxygen exchange between Ni=O species and H218O.
To trap the possible Ni=O species at low temperature, UV-vis
spectroscopic study was carried out at -40 °C. The reaction of 2
with 6 equiv of MCPBA in a solvent mixture of CH2Cl2 and
CH3CN led to the fast formation of a red species characterized
by a broad absorption band at ~500 nm (Figure S3); this species
decayed within 30 min at -40 °C (natural decay). Upon the
addition of 200 equiv of cyclohexene to the red species at -
40 °C, its characteristic UV-vis absorption bands disappeared
within 5 min, which was much faster than the natural decay
(Figure S4). After the reaction was completed, the red species
was converted to its Ni(II)-O2CPhCl complex, which was
confirmed by ESI-MS. The EPR spectrum of a frozen solution of
the red intermediate showed a rhombic signal with g values of
2 1
Experimental Section
2.195, 2.139, and 2.027, which is a typical for Ni(III) (dz )
electron configuration (Figure S5). Further study for the clear
characterization and reactivity of the red species is ongoing.
General procedures
All reagent-grade chemicals for synthesis were used without further
purification.
N-(2-Pyridylmethyl)-N'-(2-hydroxyethyl)ethylenediamine
(phenOH) was prepared according to a previously reported study.[16]
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Olefins, epoxides, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, benzaldehyde,
acetonitrile, dichloromethane, dodecane, and MCPBA (65%) were
purchased from Aldrich Chemical Co. and were used without further
purification. PPAA was synthesized according to previously reported
methods.[70-71] UV/Vis absorption spectra were recorded on a SCINCO S-
2100 spectrophotometer and a Perkin Elmer model Lambda 25 UV/Vis
spectrometer. Infrared spectra were recorded on a Thermo Fisher
Scientific IR200 spectrophotometer (±1 cm−1) using a KBr disk. Elemental
analyses were performed using a Fisons/Carlo Erba EA1108 instrument
First, MCPBA (0.03 mmol) was added to a mixture of styrene (0.02
mmol) and para(X)-substituted styrene (0.02 mmol, X = -OCH3, -CH3, -Cl,
and -CN), 2 (0.001 mmol), and solvent (CH3CN/CH2Cl2 1:1 v/v; 1 mL).
Second, the mixture was stirred for 10 min at room temperature. Next,
the amount of styrene before and after reaction was measured by GC.
The relative reactivities were determined by the following equation: kx/ky
= log(Xf/Xi)/log(Yf/Yi), where Xi and Xf denote the initial and final
concentrations of para-substituted styrenes, and Yi and Yf denote the
initial and final concentrations of styrene.[72]
1
in air. H-NMR spectra were measured on a Bruker AVANCE digital 400
spectrometer. Magnetic susceptibilities were measured at an applied field
of 5000 Oe between 2 and 300 K on a Quantum Design MPMS
superconducting quantum interference device (SQUID) magnetometer.
Diamagnetic corrections were made [354.9 (1) and 353.6  10−6 (2)
emu/mol] by using Pascal’s constants. The analysis of the products
obtained from the epoxidation of olefins, as well as from the PPAA
experiment, was conducted on a Perkin Elmer gas chromatograph
equipped with an FID detector using a 30 m capillary column (Hewlett-
Packard, DB-5 or HP-FFAP).
Analysis of the O-O bond cleavage products from the oxidation of
substrates by PPAA in the presence of 2
First, PPAA (0.04 mmol) was added to a mixture of substrate (0−0.16
mmol), 2 (0.001 mmol), and solvent (anhydrous CH3CN/CH2Cl2 1:1 v/v; 1
mL). Second, the mixture was stirred for 10 min at room temperature.
Each reaction was monitored by GC/mass analysis by periodically
withdrawing 20 μL aliquots from the reaction mixture. Internal standard
dodecane was used for quantifying the yields of products and conversion
of substrates. All reactions were performed at least three times, and the
average conversion and product yield were obtained. Conversion and
product yield were based on the substrate or PPAA.
Synthesis of [(phenO)Fe(N3)]2(NO3)2 (1)
First, a methanol solution (2 mL) of phenOH (40 mg, 0.20 mmol) and a
methanol solution (2 mL) of NaN3 (26 mg, 0.40 mmol) were added to a
methanol solution (2 mL) of Fe(NO3)3•9H2O (80 mg, 0.20 mmol). Second,
the mixture was stirred for 10 min, and the color turned dark red. After
filtration, red crystals of 1 were obtained by the diffusion of diethyl ether
into the red solution for several days; these crystals were washed with
diethyl ether and dried in air. Yield: 42 mg (30%). FT-IR (KBr, cm−1):
3213, 3151, 3035, 2913, 2877, 2070, 1610, 1384, 1358, 1335. Anal
Calcd for C20H33N14Fe2O6.5: C, 33.49; H, 4.64; N, 27.34. Found: C, 33.22;
H, 4.68; N, 27.41.
X-ray crystallographic data collection and refinement
Single crystals of 1 and 2 were coated with paratone-N oil, and diffraction
data were measured at 100(2) K with synchrotron radiation ( = 0.63000
Å) on an ADSC Quantum-210 detector at 2D SMC with a silicon (111)
double crystal monochromator (DCM) at the Pohang Accelerator
Laboratory, Korea. The ADSC Q210 ADX program[73] was utilized for
collecting data (detector distance is 63 mm, omega scan; ∆ω = 1º,
exposure time is 1 s per frame) and HKL3000sm (Ver. 703r)[74] was used
for cell refinement, reduction, and absorption correction. Crystal
structures were solved by direct methods[75] for 1 and 2 and refined by
full-matrix least-squares refinement using the SHELXL-2013 computer
program.[76] The positions of all non-hydrogen atoms were refined with
anisotropic displacement factors. All hydrogen atoms were placed using
a riding model, and their positions were constrained relative to their
parent atoms using the appropriate HFIX command in SHELXL-2013,
except the hydrogen atoms of hydroxyl groups in 2. Table 7 summarizes
the crystallographic data and the result obtained from refinement.
Synthesis of [(phenOH)Ni(N3)2]2 (2)
First, a methanol solution (2 mL) of phenOH (78 mg, 0.40 mmol) and a
methanol solution (2 mL) of NaN3 (78 mg, 1.20 mmol) were added to a
methanol solution (2 mL) of Ni(NO3)2•6H2O (116 mg, 0.40 mmol). Second,
the mixture was stirred for 10 min at room temperature, and the color
turned blue. Finally, blue crystals of 2 were obtained by the diffusion of
diethyl ether into the blue solution for several days; these crystals were
collected by filtration and washed with water and diethyl ether and dried
in air. Yield: 82 mg (30%). FT-IR (KBr, cm−1): 3367, 3258, 3081, 2944,
2884, 2041, 1606. Anal Calcd for C20H36N18Ni2O3: C, 34.61; H, 5.23; N,
36.33. Found: C, 34.84; H, 4.97; N, 36.24.
Table 7. Summary of the crystallographic data for 1 and 2.
1
2
Empirical formula
C20H32Fe2N14O8
708.29
Monoclinic
P21/n
C20H34N18Ni2O2
676.07
Epoxidation of olefin by nickel complex 2 with MCPBA
Formula weight
Crystal system
Space group
a (Å )
Triclinic
First, MCPBA (0.1 mmol) was added to a mixture of substrate (0.035
mmol), nickel catalyst 2 (0.001 mmol), and solvent (CH3CN/CH2Cl2 1:1
v/v; 1 mL). Second, the mixture was stirred for 10 min at room
temperature, although the reaction was completed within 1 min. Each
reaction was monitored by GC/mass analysis by periodically withdrawing
20 μL aliquots from the reaction mixture. Internal standard dodecane was
used for quantifying the yields of products and conversion of substrates.
All reactions were performed in triplicate, and the average conversion
and product yield were obtained. Conversion and product yield were
based on the substrate.
̅
P
8.8810(18)
12.394(3)
13.090(3)
90
9.915(2)
11.502(2)
12.626(3)
82.55(3)
88.65(3)
88.34(3)
1426.9(5)
2
b (Å )
c (Å )
 (°)
93.26(3)
90
 (°)
 (°)
V (Å 3)
1438.5(5)
2
Z
Competitive reactions of styrene and para-substituted styrenes for
the construction of a Hammett plot
Dcalc (g cm−3
T (K)
)
1.635
1.574
100(2)
100(2)
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10.1002/chem.201604498
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0.63000
0.63000
0.989
704
 (Å )
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65, 2221-2233.
 (mm−1
F(000)
)
0.773
732
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4008.
Reflections collected
Independent reflections
Reflections with I>2σ(I)
Goodness-of-fit on F2
R1[a] (4 data)
22689
6533
15823
8216
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0.0368
0.1021
1477550
1.234
0.0850
0.2513
1477551
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Entry for the Table of Contents
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Dinuclear iron(III) complex 1 is
ethoxo-bridged, while dinuclear
nickel(II) complex 2 is azido-
bridged. 1 shows an
antiferromagnetic interaction
through the bridged ethoxo
groups, while 2 displays a strong
ferromagnetic coupling through
the bridged end-on azido
ligands. Interestingly, 2 showed
excellent catalytic activities for
olefin epoxidations and terminal
olefin epoxidations as a
Ah Rim Jeong, Jong Won
Shin, Jong Hwa Jeong*, Kwon
Hee Bok, Cheal Kim*,
Donghyun Jeong, Jaeheung
Cho, Shinya Hayami, Kil Sik
Min*
Page No. – Page No.
Dinuclear iron(III) and nickel(II)
complexes containing N-(2-
pyridylmethyl)-N'-(2-
homogeneous catalyst.
hydroxyethyl)ethylenediamine:
catalytic oxidation and
magnetic properties
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