Catalysis and molecular magnetism of dinuclear iron(iii) complexes with N-(2-pyridylmethyl)-iminodiethanol/-ate

Article abstract of DOI:10.1039/c3dt53376j

The reaction of N-(2-pyridylmethyl)iminodiethanol (H₂pmide) and Fe(NO₃)₃·9H₂O in MeOH led to the formation of a dimeric iron(iii) complex, [(Hpmide)Fe(NO₃)] ₂(NO₃)₂·2CH₃OH (1). Its anion-exchanged form, [(pmide)Fe(N₃)]₂ (2), was prepared by the reaction of 1 and NaN₃ in MeOH, during which the Hpmide ligand of 1 was also deprotonated. These compounds were investigated by single crystal X-ray diffraction and magnetochemistry. In complex 1, one iron(iii) ion was bonded with a mono-deprotonated Hpmide ligand and a nitrate ion. The two iron(iii) ions within the dinuclear unit were connected by two ethoxy groups with an inversion center. In 2, one iron(iii) ion was coordinated with a deprotonated pmide ligand and an azide ion. The Fe(pmide)(N₃) unit was related by symmetry through an inversion center. Both 1 and 2 efficiently catalyzed the oxidation of a variety of alcohols under mild conditions. The oxidation mechanism was proposed to involve an Fe^IVO intermediate as the major reactive species and an Fe^VO intermediate as a minor oxidant. Evidence for this proposal was derived from reactivity and Hammett studies, KIE (k_H/k_D) values, and the use of MPPH (2-methyl-1-phenylprop-2-yl hydroperoxide) as a mechanistic probe. Both compounds had significant antiferromagnetic interactions between the iron(iii) ions via the oxygen atoms. 1 showed a strong antiferromagnetic interaction within the Fe(iii) dimer, while 2 had a weak antiferromagnetic coupling within the Fe(iii) dimer.

Full text of DOI:10.1039/c3dt53376j

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PAPER
Catalysis and molecular magnetism of dinuclear
iron(III) complexes with N-(2-pyridylmethyl)-
iminodiethanol/-ate†
Cite this: Dalton Trans., 2014, 43,
3999
a
b
b
c
Jong Won Shin, Jeong Mi Bae, Cheal Kim* and Kil Sik Min*
The reaction of N-(2-pyridylmethyl)iminodiethanol (H
formation of a dimeric iron(III) complex, [(Hpmide)Fe(NO
form, [(pmide)Fe(N )] (2), was prepared by the reaction of 1 and NaN
2 3 3 2
pmide) and Fe(NO ) ·9H O in MeOH led to the
)]
3 2
3
(NO )
2 3
·2CH OH (1). Its anion-exchanged
3
2
3
in MeOH, during which the
Hpmide ligand of 1 was also deprotonated. These compounds were investigated by single crystal X-ray
diffraction and magnetochemistry. In complex 1, one iron(III) ion was bonded with a mono-deprotonated
Hpmide ligand and a nitrate ion. The two iron(III) ions within the dinuclear unit were connected by two
ethoxy groups with an inversion center. In 2, one iron(III) ion was coordinated with a deprotonated pmide
3
ligand and an azide ion. The Fe(pmide)(N ) unit was related by symmetry through an inversion center.
Both 1 and 2 efficiently catalyzed the oxidation of a variety of alcohols under mild conditions. The oxi-
IV
dation mechanism was proposed to involve an Fe vO intermediate as the major reactive species and an
V
Fe vO intermediate as a minor oxidant. Evidence for this proposal was derived from reactivity and
Received 30th November 2013,
Accepted 22nd December 2013
Hammett studies, KIE (k /k ) values, and the use of MPPH (2-methyl-1-phenylprop-2-yl hydroperoxide)
H
D
as a mechanistic probe. Both compounds had significant antiferromagnetic interactions between the
iron(III) ions via the oxygen atoms. 1 showed a strong antiferromagnetic interaction within the Fe(III) dimer,
while 2 had a weak antiferromagnetic coupling within the Fe(III) dimer.
DOI: 10.1039/c3dt53376j
www.rsc.org/dalton
a dinuclear iron(III) complex with a tetradentate ligand N,N-
bis(2-pyridylmethyl)-N-(2-hydroxyethyl)amine (Hbpha) was
Introduction
Tetradentate ligands including pyridine and alcohol groups reported as a biomimetic model that displayed catechol intra-
4
are very useful for preparation of metal coordination com- diol-cleaving dioxygenase activity. Furthermore, we have
pounds. The ligands can be formed from interesting dimers, reported the structures and magnetic properties of dinuclear
trimers, and tetramers through direct deprotonation of iron(III) complexes bridged with methoxy groups, i.e., [Fe(2,3-
1
hydroxyl groups. In particular, metal complexes containing pyma)(N
3
)
2
(μ-OCH
3
)]
2
·CH
3
OH
and
2
[Fe(2,3-pyma)(NCS) -
an Fe(III) ion with tetradentate ligands could exhibit significant (μ-OCH
3
)]
2
(2,3-pyma
= (2-pyridylmethyl,3-pyridylmethyl)
5
catalytic activity, magnetic properties, and coordination chem- amine). In this context, we explored the formation of discrete
2
istry. Of these, dinuclear Fe(III) complexes bridged with alkoxo iron(III) dinuclear complexes bridged with ethoxide ions that
moieties have been studied in order to demonstrate the mecha- displayed catalytic properties and magnetic couplings. In par-
nism of action of metalloenzymes (e.g., purple acid phos- ticular, we were interested in the oxidation of alcohols using
phatases and methane monooxygenase) and investigate the iron(III) dinuclear complexes. Herein, we report the synthesis,
3
superexchange couplings between the iron centers. Recently, crystal structure, catalytic activity, and magnetic properties of
bis-ethoxy-bridged diiron(III) complexes, [(Hpmide)Fe(NO )] -
3
2
(
3 2 3 3 2 2
NO ) ·2CH OH (1) and [(pmide)Fe(N )] (2) (H pmide =
a
N-(2-pyridylmethyl)iminodiethanol).
Department of Chemistry and Green-Nano Materials Research Center, Kyungpook
National University, Daegu, 702-701, Republic of Korea
b
Department of Fine Chemistry, Seoul National University of Science and Technology,
Seoul, 139-743, Republic of Korea. E-mail: chealkim@snut.ac.kr
c
Department of Chemistry Education and Green-Nano Materials Research Center,
Experimental
General
Kyungpook National University, Daegu, 702-701, Republic of Korea.
E-mail: minks@knu.ac.kr
†
Electronic supplementary information (ESI) available. CCDC 912273 and
55381. For ESI and crystallographic data in CIF or other electronic format see
All chemicals used in the synthesis and oxidation reaction
were of reagent grade and used without further purification.
9
DOI: 10.1039/c3dt53376j
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2
N-(2-Pyridylmethyl)iminodiethanol (H pmide) was prepared Table 1 Summary of the crystallographic data for 1 and 2
6
according to the literature procedure. UV/Vis absorption
spectra were recorded with a SCINCO S-2100 spectrophoto-
meter. Infrared spectra were recorded with a Thermo Fisher
Compound
1
2
Empirical formula
C
22
H
38Fe N O
2 8 18
C
20 2 10 4
H28Fe N O
−
1
Scientific IR200 spectrophotometer (±1 cm ) using a KBr Formula weight
814.30
Monoclinic
P2 /n
8.5580(17)
18.487(4)
11.217(2)
108.06(3)
1687.2(6)
2
1.603
0.75000
96(2)
1.091
844
584.22
Monoclinic
P2 /c
10.541(2)
8.3690(17)
13.436(3)
98.13(3)
1173.4(4)
2
Crystal system
disk. Elemental analyses were carried out using a Fissons/
Space group
1
1
Carlo Erba EA1108 instrument. Magnetic susceptibilities were
a (Å)
measured in an applied field of 5000 Oe between 2 and 300 K b (Å)
on a Quantum Design MPMS superconducting quantum inter-
c (Å)
β (°)
3
ference device (SQUID) magnetometer. Diamagnetic correc-
V (Å )
−
6
−1
tions were made [387.5 (1) and 296.6 × 10 (2) emu mol
]
Z
d
calc (g cm−
3
)
1.654
using Pascal’s constants. Susceptibility data of 1 and 2 were
λ (Å)
T (K)
0.65000
100(2)
1.007
604
15 079
4085
simulated with the julX program package for magnetic
models. Product analyses for the oxidation reaction were per- μ (mm−1)
7
F(000)
Reflections collected
Independent reflections
formed on either a Hewlett-Packard 5890 II Plus gas chromato-
graph interfaced with a Hewlett-Packard Model 5989B mass
14 596
3919
spectrometer or a Donam Systems 6200 gas chromatograph Reflections with I > 2σ(I)
3524
1.093
3779
1.145
2
Goodness-of-fit on F
equipped with a FID detector using a 30 m capillary column
a
Final R indices [I > 2σ(I)]
R
1
= 0.0678
wR = 0.1885
= 0.0716
wR = 0.1935
R
1
= 0.0299
wR = 0.0882
= 0.0325
wR = 0.0895
(Hewlett-Packard, HP-1, HP-5, and Ultra 2).
2
2
a
Final R indices [all data]
R
1
R
1
Syntheses of compounds
(Hpmide)Fe(NO )] (NO
tion (5 mL) of Fe(NO ) ·9H O (192 mg, 0.476 mmol) was added
2
2
CCDC no.
912273
955381
[
3
2
3 2 3
) ·2CH OH (1). To a MeOH solu-
a
2
o
2 2
2 2 1/2
R
1
= ∑||F
o
| − |F
c
||/∑|F
o
|, wR
2
= [∑w(F
− F
c
) /∑w(F
o
) ]
.
3
3
2
dropwise a MeOH solution (5 mL) of H
2
pmide (100 mg,
0
.476 mmol); the color became dark red, and the solution was
least-squares refinement using the SHELXL-2013 computer
stirred for 30 min at room temperature. Greenish yellow crys-
tals of 1 were obtained by diffusion of diethyl ether into the
red solution for several days, collected by filtration, washed
1
1
program. The positions of all non-hydrogen atoms were
refined with anisotropic displacement factors, except C11 and
O9 in 1 which have relatively large thermal ellipsoids. 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 of hydroxyl groups in 1. The crystallographic data
and the result of refinements of 1 and 2 are summarized in
Table 1.
with diethyl ether and dried in air. Yield: 194 mg (50%). FT-IR
KBr, cm− ): 3471, 3084, 2926, 2884, 1608, 1384, 1066, 904,
1
(
7
1
63. Anal. Calcd for C20
4.25. Found: C, 30.74; H, 4.22; N, 14.24.
(pmide)Fe(N )] (2). To a MeOH solution (3 mL) of 1
34 8 2
H N Fe O18: C, 30.55; H, 4.36; N,
[
3 2
(
(
75 mg, 0.09 mmol) was added dropwise a MeOH solution
2 mL) of NaN (13 mg, 0.2 mmol); the color became dark red,
3
and the solution was stirred for 3 h at room temperature. Dark
red crystals of 2 were obtained by diffusion of diethyl ether
into the red solution for several days, collected by filtration,
washed with MeOH and diethyl ether and dried in air. Yield:
Synthesis of 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH)
1
2
MPPH was synthesized according to the literature method.
A
mixture of 30% hydrogen peroxide (57.5 mL, 0.66 mol) and
sulfuric acid (1.5 mL) was added to a flat-bottomed boiling
flask by stirring for 10 min under an ice bath system. 2-Methyl-
−
1
7
1
8 mg (67%). FT-IR (KBr, cm ): 3058, 2955, 2859, 2036, 1601,
476, 1359, 1085, 904, 788, 606. Anal. Calcd for
1-phenyl-2-propanol (17.2 μL, 0.11 mol) was added slowly to
C
20
H
28Fe
2
N
10
O
4
: C, 41.12; H, 4.83; N, 23.98. Found: C,
the mixture by stirring over 10 min, and then stirred at 45 °C
for 12 h. After distilled water (150 mL) was added to the reac-
tion mixture, the resulting solution was extracted with pentane
4
0.72.74; H, 5.05; N, 24.12.
X-ray crystallographic data collection and refinement
(
75 mL) to obtain an oil layer. The solvent was removed by stir-
Single crystals of 1 and 2 were coated with paratone-N oil and
the diffraction data measured at 96(2) and 100(2) K, respect-
ively, with synchrotron radiation (λ = 0.75000 and 0.65000 Å for
1
ring with magnesium sulfate to afford an oil product. H NMR
CDCl , 400 MHz): δ 1.211 (s, 6H), 2.888 (s, 2H), 7.220 (m, 3H),
(
7
3
.269 (m, 2H), 7.370 ppm (s, 1H).
1
and 2) 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 was used for data collection (detector distance is
Catalytic alcohol oxidations by 1 and 2 with tert-butyl
hydroperoxide (t-BuOOH)
8
3 mm, omega scan; Δω = 1°, exposure time is 1 s per frame) To a mixture of substrate (0.3 mmol), 1 or 2 (1.2 × 10− mmol),
3
6
9
and HKL3000sm (Ver. 703r) was used for cell refinement, and solvent (CH
3
CN, 1 mL) was added t-BuOOH (0.03 mmol).
reduction and absorption correction. The crystal structures The mixture was stirred at 50 °C (1 day for 1 and 3 days for 2).
1
0
were solved by direct methods and refined by full-matrix Reaction products were monitored by GC/Mass analysis of
4000 | Dalton Trans., 2014, 43, 3999–4008
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2
0 μL aliquots withdrawn periodically from the reaction
mixture. All reactions were run at least three times and the
average product yields are presented.
Competitive alcohol oxidations by 1 and 2 with t-BuOOH
To a mixture of benzyl alcohol (0.15 mmol), sec-phenethyl
−
3
alcohol (0.15 mmol), 1 or 2 (1.2 × 10 mmol), and solvent
CH CN, 1 mL) was added t-BuOOH (0.03 mmol). The mixture
(
3
was stirred for 1 day at 50 °C. Reaction products were moni-
tored by GC/Mass analysis of 20 μL aliquots withdrawn periodi-
cally from the reaction mixture. The reaction was run at least
three times and the average product ratio is presented.
Kinetic isotope effect for the benzyl alcohol oxidation by 1 and
Scheme 1 Synthesis of iron(III) dinuclear complexes 1 and 2.
2
with t-BuOOH
In order to improve the accuracy for measuring the amount of
the deuterated benzyl alcohol product, a 1 : 5 mixture of benzyl
Results and discussion
alcohol and deuterated benzyl alcohol was used. The reaction Synthesis and characterization
conditions are as follows: to a mixture of benzyl alcohol
The reaction of 1 equiv. H pmide and 1 equiv. Fe(NO ) ·9H O
in methanol under aerobic conditions afforded the iron(III)
dinuclear complex 1 in moderate yield (Scheme 1). As
2
3 3
2
(
(
0.05 mmol), deuterated benzyl alcohol (0.25 mmol), 1 or 2
1.2 × 10− mmol), and solvent (CH CN, 1 mL) was added
3
3
t-BuOOH (0.03 mmol). The mixture was stirred at 50 °C (1 day
for 1 and 3 days for 2), respectively. Reaction conversion was
monitored by GC/Mass analysis of 20 μL aliquots withdrawn
periodically from the reaction mixture. All reactions were run
at least in triplicate and the average KIE value is presented.
expected,
a bis-ethoxy-bridged diiron(III) compound was
obtained. That is, both iron(III) ions in 1 were connected by the
oxygen atoms of the two ethoxy groups supplied by the
Hpmide ligand. The composition of 1 was determined by
elemental analysis, infrared spectroscopy, and X-ray diffrac-
tion. The IR spectrum (KBr pellet) of 1 showed νNO of the
−1
nitrate ions at 1384 cm
and νOH of the Hpmide at
The compound displayed CH peaks corres-
ponding to the pyridine and ethoxy groups at 3084, 2926, and
Competitive reactions of benzyl alcohol and para-substituted
benzyl alcohols for the Hammett plot
471 cm−
.
1 14
3
To a mixture of benzyl alcohol (0.02 mmol), para(X)-substi-
−1
2
884 cm . Compound 2 was obtained by the reaction of 1 and
tuted benzyl alcohol (0.02 mmol) (X = –OCH
3
, –CH
3
, –Cl and
NaN3 in MeOH (Scheme 1). Interestingly, the reaction was
accompanied by anion exchange of the nitrate to azide ions, as
well as deprotonation of the Hpmide ligand of 1. The compo-
sition of 2 was also identified by elemental analysis, infrared
spectroscopy, and X-ray diffraction. The IR spectrum (KBr
pellet) of 2 exhibited the νNN of the azide ions at 2036 cm
however, no ν
observed. Complex 2 exhibited CH peaks corresponding to
the pyridine and ethoxy groups at 3058, 2955, and 2859 cm
−
3
–
CF ), 1 or 2 (1.2 × 10 mmol), and solvent (CH CN, 1 mL)
3
3
was added t-BuOOH (0.03 mmol). The mixture was stirred for
day at 50 °C. The amounts of benzyl alcohols before and
after the reactions were monitored by GC/Mass analysis of
0 μL aliquots withdrawn periodically from the reaction
1
2
−1
;
mixture. The relative reactivities were determined using the fol-
−1
for the Hpmide near 3471 cm
was
OH
lowing equation: k /k = log(X /X )/log(Y /Y ) where X and X are
14
x
y
f
i
f
i
i
f
each initial and final concentration of para-substituted sty-
renes and Y and Y are each initial and final concentration of
−1
.
i
f
1
3
styrene.
Description of crystal structures
Structure of 1. Compound 1 crystallized in the monoclinic
Analysis of the O–O bond cleavage products from cyclohexanol
oxidation by MPPH in the presence of 1 and 2
P2 /n space group, and the unit cell included two dinuclear
1
complex molecules. The ORTEP drawing of 1 is shown in
Fig. 1, and selected bond lengths and angles are listed in
To a mixture of cyclohexanol (0.3 mmol), 1 or 2 (1.2 × 10−
3
mmol), and solvent (distilled CH CN, 1 mL) was added MPPH Table 2. In the asymmetric unit of 1, one iron(III) ion was
3
(
0.03 mmol). The mixture was stirred at 50 °C for 1 day (cata- bonded with a mono-deprotonated Hpmide ligand and a
lyst 1) and 3 days (catalyst 2), respectively. Reaction products nitrate ion. That is, an Fe(Hpmide)(NO ) unit resulted from
were determined by GC/Mass analysis of 20 μL aliquots with- the bridging of the two iron(III) ions through the terminal
drawn periodically from the reaction mixture. All reactions alkoxo of the tetradentate Hpmide ligand. The two
were run at least in triplicate and the average product yields Fe(Hpmide)(NO ) units were related by symmetry through an
3
O
3
are presented.
inversion center situated at the barycenter of the complex.
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Fig. 1 ORTEP (40% probable thermal ellipsoid) view of 1. H atoms,
−
MeOH molecules, and non-coordinated NO
3
anions are omitted for
Fig. 2 ORTEP (50% probable thermal ellipsoid) view of 2. H atoms are
omitted for clarity (’ = 2 − x, 1 − y, −z).
clarity, except for the hydrogen atom of O1 (’ = 2 − x, −y, −z).
Table 2 Selected bond distances (Å) and angles (°) for 1
iron(III) ions within the dinuclear unit were connected by
two deprotonated ethoxy groups with an inversion center. The
Fe1–O2–Fe1(2 − x, −y, −z) angle was 108.05(10)°. The shortest
Fe⋯Fe contact within the dimer was 3.212(1) Å and the
shortest Fe⋯Fe distance between the dimers was 8.472(2) Å.
Furthermore, the hydroxyl oxygen atom of the Hpmide formed
a strong hydrogen bond with the uncoordinated oxygen atom
Fe1–O1
Fe1–O2
Fe1–O2
2.001(2)
1.999(2)
1.970(2)
3.212(1)
Fe1–O3
Fe1–N1
Fe1–N2
2.061(2)
2.080(3)
2.229(3)
i
i
Fe1–Fe1
i
i
O2–Fe1–O2
O1–Fe1–O2
71.95(10)
103.40(10)
102.17(10)
81.18(9)
151.92(9)
91.64(10)
104.76(10)
88.47(10)
O1–Fe1–N1
O3–Fe1–N1
O2 –Fe1–N2
151.77(11)
90.68(11)
150.39(9)
78.72(9)
78.61(10)
128.43(9)
77.97(11)
i
O1–Fe1–O2
i
O2 –Fe1–O3
O2–Fe1–O3
O1–Fe1–O3
O2 –Fe1–N1
O2–Fe1–N1
O2–Fe1–N2
O1–Fe1–N2
O3–Fe1–N2
N1–Fe1–N2
of the free nitrate ion (O1⋯O6 2.569(4) Å, ∠O1–H11–O6
16
171.46°).
i
Structure of 2. Compound 2 crystallized in the monoclinic
i
Fe1 –O2–Fe1
108.05(10) P2
/c space group, and the unit cell included two dinuclear
1
complex molecules. The ORTEP drawing of 2 is shown in
Fig. 2, and the selected bond lengths and angles are listed in
Table 3. In the asymmetric unit of 2, one iron(III) ion was co-
ordinated with a deprotonated pmide ligand and an azide ion,
Symmetry transformations used to generate equivalent atoms: (i) 2 −
x, −y, −z.
Interestingly, both iron(III) ions were coordinated by two i.e., Fe(pmide)(N ). In addition, the two Fe(pmide)(N ) units
3 3
different oxygen atoms which were supplied by the same were connected through two Oalkoxo of the pmide ligands
Hpmide ligand, i.e., one was protonated and the other deproto- belonging to each unit in which the dimeric iron(III) complex
nated. Each iron(III) ion adopted a six-coordinate distorted was formed. As shown in Fig. 2, each iron(III) ion adopted a
octahedral configuration with an N
2 4
O
2 4
donor set. The amine six-coordinate distorted octahedral configuration with an N O
nitrogen and the bridging ethoxy oxygen atom were situated in donor set. The amine nitrogen and the bridged ethoxy oxygen
4
,15
4,15
the plane of the Fe
2
O
2
core.
The pyridine nitrogen atom atom were situated in the plane of the Fe
2
O
2
core.
The pyri-
and protonated ethoxy oxygen atom occupied the two axial dine nitrogen atom and the non-bridged ethoxy oxygen atom
positions in the octahedron. The oxygen atoms of the co- occupied two axial positions in the octahedron. The nitrogen
ordinated nitrate ions were trans to the deprotonated ethoxy atoms of the coordinated azide ions were trans to the
oxygen from the other Hpmide ligand. The Fe–NHpmide and ethoxy oxygen from the other pmide ligand. Thus, the struc-
Fe–OHpmide bond distances were in the ranges of 2.080(3)– ture of 2 was isomorphous to that of 1. Remarkably, the co-
2
.229(3) and 1.970(2)–2.001(2) Å, respectively. The Fe–Onitrato ordinated pmide ligands in 2 were deprotonated fully, in
1
5
bond distance was 2.061(2) Å. The bite distance and angle of contrast to
1
with mono-deprotonated Hpmides.
The
the five-membered chelate rings were in the ranges of 2.685(4)– Fe–Npmide and Fe–Opmide bond distances were in the ranges of
.713(4) Å and 77.97(11)–78.72(9)°, respectively. The two 2.1891(12)–2.2427(10) and 1.8629(10)–2.0723(8) Å, respectively.
2
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Table 3 Selected bond distances (Å) and angles (°) for 2
i
Fe1–O1
Fe1–O2
Fe1–N1
Fe1–Fe1
1.8629(10)
2.0723(8)
2.1891(12)
3.1987(8)
Fe1–O2
Fe1–N2
Fe1–N3
1.9395(9)
2.2427(10)
2.0628(10)
i
i
O1–Fe1–O2
108.21(4)
93.32(4)
99.12(4)
100.63(4)
74.30(4)
165.84(4)
154.62(4)
97.10(4)
N3–Fe1–N1
O2–Fe1–N1
O1–Fe1–N2
84.33(4)
84.09(3)
81.23(4)
151.99(3)
106.73(4)
78.08(4)
75.34(4)
105.70(4)
O1–Fe1–N3
O2 –Fe1–N3
O1–Fe1–O2
O2 –Fe1–O2
N3–Fe1–O2
O1–Fe1–N1
i
i
O2 –Fe1–N2
i
N3–Fe1–N2
O2–Fe1–N2
N1–Fe1–N2
i
i
O2 –Fe1–N1
Fe1 –O2–Fe1
Symmetry transformations used to generate equivalent atoms: (i)
x + 2, −y + 1, −z.
−
The Fe–Nazido bond distance was 2.0628(10) Å. The bite dis-
tance and angle of the five-membered chelate rings were in the
ranges of 2.688(1)–2.721(1) Å and 75.34(4)–81.23(4)°, respect-
ively. The two iron(III) ions within the dinuclear unit were
linked by two deprotonated ethoxy groups. The angle of Fe1–
O2–Fe1(2 − x, 1 − y, −z) was 105.70(4)°. The shortest Fe⋯Fe
contact within the dimer was 3.199(1) Å and the shortest
Fe⋯Fe distance between the dimers was 6.739(1) Å.
Fig. 3 Temperature dependence of μeff for 1 (×) and 2 (○) at 5000 Oe.
The solid lines are the best-fit curves, as discussed in the text.
This value was less than the spin-only value of 8.37 µ /Fe
B 2
III
Magnetic properties. Variable-temperature (2–300 K) mag- expected for two independent Fe spins (g = 2, S = 5/2), indi-
netic susceptibility, χ, measurements on solid samples of 1 cative of significant antiferromagnetic coupling. eff(T)
µ
and 2 were performed on a SQUID magnetometer (external decreased with decreasing temperature, consistent with the
field 5000 Oe). For 1, at room temperature, the effective presence of an antiferromagnetic interaction between the
1
/2
magnetic moment, µeff [= (8χ
M
T) ], was 5.70 µ
/Fe
two independent Fe spins (g = 2, S = 5/2), indicative of sig- based on the Hamiltonian H = −2JS
B
/Fe
2
. This value iron(III) ions (Fig. 3). The µeff(T) data were fitted to an analytical
was less than the spin-only value of 8.37 µ
B
2
expected for expression for χ(T) for a coupled S = 5/2 dinuclear spin model
III
7
1
·S
2 1 2
(S = S = 5/2). The
−
1
nificant antiferromagnetic coupling. As shown in Fig. 3, µeff(T) best fit had J = −11.6 cm , g = 2.0, and spin impurities, ρ =
decreased with decreasing temperature, consistent with the 0.013 (TIP = 400 × 10− emu mol ). The experimentally deter-
6
−1
−
1
presence of an antiferromagnetic interaction between the iron(III) mined J value (−11.6 cm ) for 2 was almost half that of 1
ions. The µeff(T) data were fitted to an analytical expression (−23.5 cm− ). This means that the magnetic interaction within
for χ(T) for a coupled S = 5/2 dinuclear spin model (eqn (1)) the dimeric unit for 1 was much stronger in antiferromagnetic
1
7
based on the Hamiltonian H = −2JS ·S (S = S = 5/2). The terms than that for 2. The difference in coupling constants
1
2
1
2
−
1
best fit to eqn (1) had J = −23.5 cm , g = 2.0, and spin impuri- was attributed to the different environments around the iron(III)
ties, ρ = 0.024 (TIP = 400 × 10 emu mol−1).
−
6
ions (i.e., nitrate and azide ions, protonated and deprotonated
ꢀ
6 expð2J=TÞ þ 30 expð6J=TÞ þ 84 expð12J=TÞ þ 180 expð20J=TÞ þ 330 expð30J=TÞꢁ
ꢀ
ꢁ
Nβ2g2
Nβ2g2
3kT
χM ¼
ð1 ꢀ ρÞ þ 2ρ
SðS þ 1Þ ð1Þ
3
kT
1 þ 3 expð2J=TÞ þ 5 expð6J=TÞ þ 7 expð12J=TÞ þ 9 expð20J=TÞ þ 11 expð30J=TÞ
It should be noted that the J value of 1 indicates that the ethoxy groups), as well as the Fe–O–Fe′ angle (108.05° for 1
7
a
ethoxo groups mediate the antiferromagnetic couplings and 105.70° for 2).
between the two iron(III) ions separated by ca. 3.212 Å. The Catalytic properties. Currently, there is strong interest in
coupling constant of was comparable to −20.5 and the biomimetic oxidation chemistry of non-heme iron com-
(bz) ][(ClO ] and [PH(t-Bu) [Fe plexes, as they have been identified or implicated in a number
2-[bis(2-benzimidazolylmethyl)amino]- of non-heme iron enzymes which activate dioxygen. As part
1
−
24.6 cm− for [Fe
1
2
(dbe)
2
2
4
)
2
3
]
2
2
-
1
8
(
μ-OEt) Cl ] (dbe
=
2
6
1
5,17
19
ethanolato; bz = benzoate), respectively.
For 2, the effective of our efforts to model the non-heme iron enzymes, we exam-
magnetic moment µeff at room temperature was 6.91 µ
B
/Fe
2
.
ined the reactivity of 1 and 2 in the epoxidation and alcohol
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Table 4 Oxidation reaction of various alcohols catalyzed by iron cata- reveals the high catalytic activities for the oxidation of a variety
a
13,22
3
lysts 1 and 2 with t-BuOOH in CH CN
of alcohols by catalysts 1 and 2 under mild conditions.
With catalyst 1, cyclohexanol, cis-2-methylcyclohexanol, and
endo-norborneol were almost quantitatively oxidized to the
corresponding ketones (92–98%, entries 1, 2, and 4), whereas
sterically hindered alcohols such as trans-2-methylcyclo-
hexanol and exo-norborneol were less efficiently oxidized to
b
Yield (%)
1
Entry
1
Substrate
Product
2
98 ± 5
84 ± 5
1
3
ketones (compare entries 2 and 3, and entries 4 and 5). The
linear aliphatic secondary alcohol 2-hexanol was converted to
the corresponding 2-hexanone in moderate yield (62%, entry 6).
Primary and secondary benzylic alcohols also showed high
reactivity for the oxidative dehydrogenation (entries 7 and 8).
It was notable that benzyl alcohol was smoothly oxidized to
afford benzaldehyde, with only a very small amount of the
further oxidation product of the aldehyde, benzoic acid. In
competition experiments of benzylic alcohols, the primary
benzyl alcohol was about twice as reactive as sec-phenethyl
alcohol, perhaps because of steric hindrance. The catalytic
2
3
4
92 ± 7
69 ± 1
95 ± 3
81 ± 1
35 ± 4
96 ± 3
1
3
system was also applicable in the oxidation of aliphatic
5
65 ± 1
27 ± 0
23
allylic alcohols (entry 9).
The secondary allylic alcohol
3-buten-2-ol afforded the corresponding enone (90%) with a
small amount of epoxide (9%). Catalyst 2 also yielded
similar results, albeit with longer reaction times (3 days). Two
exceptions were the substrates trans-2-methylcyclohexanol and
exo-norborneol, which afforded yields of the corresponding
products reduced by ca. 50%. Presumably, the fast catalytic
reaction with 1 might be due to an effect of the protonated
oxygen of the ethanol moiety in 1. The protons (pKa1 = 3.03
and pKa2 = 7.03, Fig. S3 in the ESI†) might promote the
fast O–O bond cleavage of the initially formed intermediate
6
7
62 ± 1
90 ± 5
55 ± 4
88 ± 0
5
± 1
3 ± 1
(
t-BuOO–Fe(III)).
As kinetic isotope effects (KIE) have been used extensively
kH/kD
4.4 ± 0.2
100
5.0 ± 0.0
88 ± 2
8
9
as a mechanistic probe to understand the nature of the oxidiz-
ing species in alkane hydroxylation reactions catalyzed by
1
3,24
90 ± 6
87 ± 5
8 ± 0
both iron-containing enzymes and model complexes,
we
carried out a competitive intermolecular alcohol oxidation
with benzyl alcohol and deuterated benzyl alcohol. The KIEs
for benzaldehyde formation by 1 and 2 with t-BuOOH as
the oxidant were determined to be 4.4 and 5.0, respectively
9
± 1
(
entry 7, Table 4), which indicated that C–H bond cleavage by
a
Reaction condition: catalyst; 1 mg, alcohol; 0.3 mmol, t-BuOOH;
the oxidizing species was the rate-determining step for
alcohol oxidation. These values were higher than those of a
corresponding hydrogen abstraction reaction by the tert-butoxy
radical (3.6) and by the combination of an iron(III)
porphyrin complex ([F20TPP]FeCl) (F20TPP = tetrakis(penta-
0
3
.03 mmol, dodecane; 0.01 mmol, solvent; CH CN (1 mL) at 50 °C.
b
Yields are based on the oxidant.
oxidation of a wide range of olefin and alcohol substrates with fluorophenyl)porphyrin) and m-chloroperoxybenzoic acid
environmentally friendly oxidants such as H O and tert-butyl (3.3). However, the results were lower than those of oxoiron(IV)
2
2
2
0
IV
•+
hydroperoxide (t-BuOOH). The combination of catalyst 1 or 2 porphyrin π-cation radicals (21 for [(TDCPP)Fe vO] at
IV
•+
with t-BuOOH was effective for alcohol oxidation, and CH
3
CN −100 °C and 20 for [(TMP)Fe vO] at −60 °C) and mono-
was the best solvent for the reactions. To a mixture of substrate nuclear non-heme oxoiron(IV) complexes (48 for [(N4Py)-
0.3 mmol) and catalyst (1 or 2; 1.2 × 10 mmol) in CH CN Fe vO] at 0 °C and 58 for [(tpa)Fe vO] at −40 °C). These
3
−
3
IV
IV
(
was added t-BuOOH (0.03 mmol), and the mixture was stirred results led us to propose that the major reactive species
at 50 °C (1 day for 1 and 3 days for 2). We confirmed that the responsible for alcohol oxidation in our catalytic systems
direct alcohol oxidation by t-BuOOH was negligible (see may be a high-valent iron oxo species that is produced by
2
1
Fig. S1†), and catalysts 1 and 2 were sufficiently robust the O–O bond cleavage in the iron(III)-peroxide intermediate
III
during the catalytic reactions, as shown in Fig. S2.† Table 4 (t-BuOO–Fe ).
4004 | Dalton Trans., 2014, 43, 3999–4008
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Based on these results and KIE values, we propose the most
plausible mechanism for alcohol oxidation by catalysts 1 and 2
as shown in Scheme 2. t-BuOOH reacts with an iron(III)
complex to form an initial iron(III)-peroxide species (t-BuOO–
III
Fe (3)). The O–O bond of the coordinated anion of t-BuOOH
can then cleave heterolytically to form a high-valent iron(V)–
oxo complex (4) and t-BuOH (pathway (a)), or homolytically to
•
form a high-valent iron(IV)–oxo complex (5) and t-BuO radical
(
pathway (b)).
To determine whether the iron(III)-peroxide species t-BuOO–
III
Fe (3) decomposes by heterolysis or homolysis, we used
MPPH (2-methyl-1-phenyl-2-propyl hydroperoxide) as
a
mechanistic probe, given the well-established use of MPPH in
distinguishing the heterolytic versus homolytic cleavage of the
2
7
alkyl peroxide O–O bond. If the O–O bond of the t-BuOO–
III
III
Fe is cleaved heterolytically, then t-BuOO–Fe would yield
-methyl-1-phenyl-2-propanol (MPPOH (6)) (Scheme 3). In con-
2
III
trast, the homolytic cleavage of the O–O bond in t-BuOO–Fe
would generate an alkoxy radical (MPPO ), which would
undergo an extremely fast β-scission (2.2 × 10
•
8
−1
s ) to
produce benzyl alcohol (7), benzaldehyde (8), and toluene (9).
Therefore, the mechanism of the O–O bond cleavage of
alkyl hydroperoxides can be determined by analyzing the
products derived from the decomposition of MPPH by iron
complexes.
The catalytic oxidation of cyclohexanol by MPPH was
3
carried out in the presence of 1 and 2 in CH CN, and the
product distribution results are shown in Table 5. In the cyclo-
hexanol oxidation catalyzed by 1 and MPPH, the homolytic
cleavage products benzaldehyde (69.2%) and benzyl alcohol
(
16.7%) were the predominant degradants of the oxidant, with
a small amount of the heterolytic cleavage product, MPPOH
5.3% based on MPPH) (entry 1). These results demonstrate
(
Fig. 4 Hammett plots for relative reactivities depending on para-
III
that the MPPOO–Fe species 3 generated from the reaction of
and MPPH underwent partitioning between 94% homolysis
p
substituent constants σ of benzyl alcohol; (a) for catalyst 1 and (b) for
1
catalyst 2.
and 6% heterolysis. Before we were confident of the 94 : 6
ratio, we carried out another control experiment to determine
whether MPPH was completely consumed by catalyst 1 during
the reaction period (1 day). Therefore, we added triphenylphos-
To gain further information about the nature of the reactive phine (PPh ) into the reaction solution to trap any possibly
3
intermediates responsible for these alcohol oxidation systems, remaining MPPH after 1 day; it is well known that alkyl hydro-
we conducted the competitive oxidation of benzyl alcohol and peroxides react quickly with PPh3 to give the corresponding
para-substituted benzyl alcohols (see the Experimental section alcohols. Nearly identical product distributions were obtained
for details). As shown in Fig. 4, a significant electronic effect with and without PPh (entries 1 and 2, Table 5), suggesting
3
on the reaction rate was observed, showing that more electron- that MPPH decomposed completely during the alcohol oxi-
rich alcohols reacted faster than electron-deficient alcohols. dation reactions. Therefore, these results indicate that homo-
III
Hammett plots afforded negative ρ values of −0.52 for 1 and lytic cleavage (94%) of the t-BuOO–Fe intermediate (3) is the
−0.40 for 2, suggesting that the reactive intermediates respon- major pathway, with a small portion (6%) of heterolytic cleav-
sible for alcohol oxidation were electrophilic. These ρ values age. These results led us to conclude that most of the alcohol
IV
were higher than those determined from the alcohol oxidation was oxidized to ketone by the Fe vO intermediate (4) and the
reactions by mononuclear non-heme oxoiron(IV) complexes, tert-butoxy radical, and that a small amount of the alcohol was
IV
IV
25
V
[
(tpa)Fe vO] (−0.06) and [(N4Py)Fe vO] (−0.07),
and oxidized to the ketone by the Fe vO intermediate. Nearly
similar to those investigated in cytochrome P450 enzymes identical results were also obtained for 2 (entries 3 and 4,
2
6
(
around −0.4) and heme model compounds such as oxoiron(IV) Table 5), indicating that both catalysts 1 and 2 catalyzed
IV
•+
porphyrin π-cation radicals, [(TDCPP)Fe vO] (−0.39) and alcohol oxidation by the same reaction mechanism. Moreover,
(TMP)Fe vO] (−0.43).
IV
•+
25
[
the nearly identical product distribution from MPPH by
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Scheme 2 Plausible mechanism on the formation of reactive species responsible for alcohol oxidation by iron complexes with t-BuOOH.
Scheme 3 Possible O–O bond cleavage mechanism of MPPH by iron complexes.
Table 5 Yield of products derived from 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH) mediated by catalysts 1 and 2 in the presence of
a
cyclohexanol
b
Homolysis
b
Heterolysis
Triphenyl phosphine
Entry Catalyst (mM)
Benzaldehyde
MPPOH (6) (7)
Benzyl alcohol Toluene
Hetero (6)/Homo (7
+8 +9)
b
(8)
(9)
Cyclohexanone
1
2
3
4
1
1
2
2
0
100
0
5.3 ± 0.1
6.3 ± 0.4
5.1 ± 0.8
6.2 ± 0.3
69.2 ± 0.7
65.6 ± 0.7
74.0 ± 6.7
72.2 ± 4.5
16.7 ± 0.0
15.9 ± 1.8
12.1 ± 2.2
12.2 ± 1.1
0
0
0
0
5.8/94.2
7.2/92.8
5.6/94.4
6.8/93.2
62.9 ± 0.4
64.1 ± 2.6
58.4 ± 4.8
58.4 ± 5.1
c
c
100
a
Reaction conditions: catalyst; 1 mg, cyclohexanol; 0.3 mmol, MPPH; 0.03 mmol, dodecane; 0.01 mmol, solvent; CH
day for 1 and 3 days for 2 at 50 °C. MPPOH indicates 2-methyl-1-phenyl-2-propanol. Based on MPPH. Triphenyl phosphine was added into
3
CN (1 mL), reaction time;
b
c
1
the reaction solution 1 day (for 1) and 3 days (for 2), respectively, after the oxidation reaction ended.
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catalysts 1 and 2 supported our prior assumption that the
proton on the oxygen of the ethanol moiety in 1 does not
affect the partition between homolytic and heterolytic O–O
bond cleavage; rather, it may accelerate only the O–O bond
cleavage rate, in contrast to the known action of protons in
driving the heterolytic cleavage of the O–O bonds of alkyl
2369; (c) G. H. Spikes, E. Bill, T. Weyhermuller and
K. Wieghardt, Chem. Commun., 2007, 4339.
3 (a) S. Ménage and L. Que Jr., Inorg. Chem., 1990, 29, 4293;
(b) E. Ruiz, P. Alemany, S. Alvarez and J. Cano, J. Am. Chem.
Soc., 1997, 119, 1297.
4 F. Li, M. Wang, P. Li, T. Zhang and L. Sun, Inorg. Chem.,
2007, 46, 9364.
2
8
hydroperoxides.
5
J. W. Shin, J. H. Han, S. R. Rowthu, B. G. Kim and
K. S. Min, Bull. Korean Chem. Soc., 2010, 31, 3617.
C.-C. Wu, S. Datta, W. Wernsdorfer, G.-H. Lee, S. Hill and
E.-C. Yang, Dalton Trans., 2010, 39, 10160.
6
Conclusions
We prepared two new bis-ethoxy-bridged iron(III) dinuclear
complexes 1 and 2 which showed significant antiferromagnetic
couplings through the bridging ethoxy groups between the
iron(III) ions. Complex 1 showed much stronger antiferro-
magnetic interactions than 2 due to geometric effects. 2 was
obtained from 1 through an anion exchange process using
azide ions. Surprisingly, the coordinated Hpmide ligand of 1
was deprotonated during the formation of 2. The dimers 1 and
7 (a) O. Kahn, Molecular Magnetism, VCH, New York, 1993,
pp. 103–134; (b) E. Bill, 2008, http://ewww.mpi-muelheim.
mpg.de/bac/logins/bill/julX_en.php
8 A. J. Arvai and C. Nielsen, ADSC Quantum-210 ADX Program,
Area Detector System Corporation, Poway, CA, USA, 1983.
9 Z. Otwinowski and W. Minor, in Methods in Enzymology, ed.
C. W. Carter Jr. and R. M. Sweet, Academic Press,
New York, 1997, vol. 276, part A, p. 307.
2
efficiently catalyzed the oxidation of a variety of alcohols 10 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
under mild conditions. Reactivity and Hammett studies, KIE
logr., 1990, 46, 467.
(
k
H
/k
suggested that an Fe vO intermediate might be the major
reactive species responsible for the alcohol oxidation, with an 12 G. T. Rowe, E. V. Rybak-Akimova and J. P. Caradonna,
D
) values, and the use of MPPH as a mechanistic probe 11 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
IV
logr., 2008, 64, 112.
V
Fe vO intermediate as a minor oxidant. This reaction pro-
Chem.–Eur. J., 2008, 14, 8303.
vides a new, environmentally friendly, and economical route 13 J. H. Han, S.-K. Yoo, J. S. Seo, S. J. Hong, S. K. Kim and
for the conversion of alcohols to carbonyl groups.
C. Kim, Dalton Trans., 2005, 402.
1
1
4 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, John Wiley and Sons, Inc., New
Jersey, 6th edn, 2009, Part B, pp. 79–94.
Acknowledgements
5 J. D. Walker and R. Poli, Inorg. Chem., 1990, 29, 756.
This research was supported by the Basic Science Research 16 (a) C. B. Aakeröy, Acta Crystallogr., Sect. B: Struct. Sci., 1997,
Program through the National Research Foundation of Korea
NRF) funded by the Ministry of Education, Science and Tech-
nology (no. 2010-0003672, 2012001725, and 2013007314). X-ray
53, 569; (b) E. A. Archer, A. E. Sochia and M. J. Krische,
Chem.–Eur. J., 2001, 7, 2059; (c) N. V. Belkova, E. S. Shubina
and L. M. Epstein, Acc. Chem. Res., 2005, 38, 624.
(
crystallography at the PLS-II 2D-SMC beamline was supported 17 B. Chiari, O. Piovesana, T. Tarantelli and P. F. Zanazzi,
in part by MEST and POSTECH.
Inorg. Chem., 1982, 21, 1396.
1
8 (a) M. Costas, M. P. Mehn, M. P. Jensen and L. Que Jr.,
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E. V. Rybak-Akimova, Chem. Rev., 2005, 105, 2175.
9 S. H. Lee, J. H. Han, H. Kwak, S. J. Lee, E. Y. Lee, H. J. Kim,
J. H. Lee, C. Bae, S. N. Lee, Y. Kim and C. Kim, Chem.–Eur.
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