Direct synthesis of phenol by novel [FeFe]-hydrogenase model complexes as catalysts of benzene hydroxylation with H2O2

Article abstract of DOI:10.1039/c6ra27831k

Three new [FeFe]-hydrogenase model complexes, μ-(SCH(CH₂CH₃)CH₂S)-Fe₂(CO)₆ (complex 1), μ-(SCH(CH₂CH₃)CH₂S)-Fe₂(CO)₅PCy₃ (complex 2) a

Full text of DOI:10.1039/c6ra27831k

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Direct synthesis of phenol by novel [FeFe]-
hydrogenase model complexes as catalysts of
Cite this: RSC Adv., 2017, 7, 2934
benzene hydroxylation with H O †
2
2
Xia Zhang, Tianyong Zhang,* Bin Li,*abc Guanghui Zhang, Li Hai, Xiaoyuan Ma
abc
abc
a
a
a
a
and Wubin Wu
Three new [FeFe]-hydrogenase model complexes, m-(SCH(CH
2 3
CH )CH
2 2 6
S)–Fe (CO) (complex 1), m-
(
SCH(CH CH )CH S)–Fe (CO) PCy (complex 2) and m-(SCH(CH
2
3
2
2
5
3
2 3
CH )CH
2
S)–Fe
2
(CO)
5
PPh
1
3
13
(complex 3)
31
were prepared. The structures of complexes 1–3 were characterized by FT-IR, UV-vis, H, C, P NMR
spectra and single-crystal analyses. The electron density of these model complexes was studied by IR
spectra, UV spectra and electrochemical analysis and evaluated against their respective catalytic
performances. The CV (cyclic voltammetry) study of complex 2 showed a less positive oxidation event at
0
.6 V and a more negative reduction event at ꢀ1.94 V, which is in accordance with the enlargement of
electron density at diiron centers when CO were substituted by better electron donor ligands. Of all
these three complexes, complex 2 exhibited the best catalytic activity, with a yield of phenol of up to
Received 6th December 2016
Accepted 22nd December 2016
24.6% and selectivity up to 92%, which is consistent with its higher electron density of the Fe–Fe bond.
This study revealed the correlations between the electron density of the catalytic site of catalysts and
their performance in catalytic hydroxylation of benzene. Based on these experimental results, a catalytic
DOI: 10.1039/c6ra27831k
2+
2+
www.rsc.org/advances
oxidation mechanism via an Fe –m-O–Fe intermediate as oxygen transfer reagent has been proposed.
been found to be lower than produced through the traditional
method. The hydroxylation process can not avoid the excessive
Introduction
Being an important industrial feedstock used in production of oxidation and subsequent formation of by-products derived
bres, resins, antioxidants, plastics, agrochemicals and medi- from phenol thereby leading to an overall decline in desirable

1
9
cines, phenol is in high demand. The current method for yield. Therefore, the seeking for novel catalysts with high
industrial scale production of phenol is mostly based on the selectivity and efficiency for the direct catalytic hydroxylation of
cumene process, which produces a large amount of acetone as benzene to phenols under mild circumstances has been an
10,11
the by-product. Since the market demand for phenol is much ongoing process and a challenge.
higher than for acetone, a method of direct oxidation of
Among diversied catalysts, iron-based catalysts attracted
benzene to phenol is expected to bring better economic bene- great research interest for many years due to the convenience,
2
–4

ts. The direct catalytic hydroxylation of benzene to phenol low cost, environmental amity and low toxicity of iron. As re-
12–14
under mild conditions has attracted great interest and has been ported in the previous studies,
iron-based catalysts usually
extensively investigated due to the reduced expense of the base have high selectivity and efficiency in a series of chemically
materials. Some new catalysts have already been tried in this challenging oxidative processes. Especially, iron-based biomi-
5–8
reaction, like iron complexes, TS-1 and Py
4
PMo11V. However, metic catalysts with low barriers to suitable redox states for
the yield of phenol by the direct hydroxylation of benzene has selective oxidations, have been studied. These catalysts mimic
the active site of natural oxygenase enzymes and serve as high-
11,15–17
performance catalysts for selective hydrocarbon oxidation.
FeFe]-hydrogenases are also iron-based bio-catalysts which
a
Tianjin Key Laboratory of Applied Catalysis Science and Technology, School of
[
Chemical Engineering and Technology, Tianjin University, Tianjin 300354, China.
E-mail: tyzhang@tju.edu.cn; libin@tju.edu.cn
are much more efficient in the evolution and oxidation of H
2
17–24
b
under mild conditions.
However, oxygenation is found to
Collaborative Innovation Center of Chemical Science and Engineering, Tianjin
3
00354, China
occur at the dithiolato-sulfur or at the Fe–Fe bond site of the
c
Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin 300354, [FeFe]-hydrogenase active site model complexes.25,26 This
China
oxygenation may lead to S-oxygenated products which having
†
Electronic supplementary information (ESI) available: CCDC 1470792, 1502264
relatively stable O atom. The other route is the formation of Fe-
and 1502265 contains the supplementary crystallographic data of complexes 1–3
for this paper. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c6ra27831k
2
+
2+
based oxidative addition products of Fe –m-O–Fe . Although
this species are demonstrated thermodynamically favourable,
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they are not isolated to date. According to the reaction kinetics reactivity of the catalysts could be improved by enhancing the
research on [FeFe]-hydrogenase active site model complexes, electron density of active sites in the catalysts.
+
+
the diiron Fe –Fe subsite is thermodynamically favored to be
2
+
2+
25,27
oxidized to the active Fe –m-O–Fe species.
The iron-based
Results and discussion
2
+
2+
highly active Fe –m-O–Fe species as the crucial intermediates
Synthesis and spectroscopic characterization of complexes 1–
3
during oxygenation could be used for transfer of oxygen in the
28–31
biomimetic hydrocarbon hydroxylation.
This is due to the
fact that the [FeFe]-hydrogenases model complexes have an The reaction of Fe(CO)5 with 1,2-butanedithiol in reuxing
active site that remains unconstrained towards a suitable redox toluene, yielded the anticipated m-(SCH(CH CH )CH S)–
2
3
2
state. This also implies high activity towards the oxygenation
reactions. Our group's previous research showed the Fe–Fe
bond based oxygenated m-O complex has the ability of trans-
ferring oxygen atom to the aromatic substrates. Supporting DFT
Fe
CH
PPh
PCy
2
(CO)
S)–Fe
, triphenyl phosphine) were obtained by treating 1 with
and PPh respectively, along with Me NO in THF at
6
(1). Mono-substituted complexes m-(SCH(CH
2
3
CH )
2
2
(CO)
5
L (2: L ¼ PCy
3
, tricyclohexyl phosphine; 3: L ¼
3
3
3
3
calculations and the experimental catalytic activity towards ambient temperature with good yields through sufficient stir-
32
1
hydroxylation tend to conrm the mechanism. Although the ring. The H NMR spectra of complexes 1–3 consist of the ex-
selectivity for phenol is high, the yield is not ideal yet. Chen's pected signals for the 1,2-butanedithiol bridges and the
1
3
group showed the benzene conversions by the catalytic system phosphine ligands. In the C NMR spectrum, complexes 1–3
can be improved by adding PPh analogues rather than using
3
show peaks at d ¼ 208.82, 210.86 and 210.82 ppm, respectively.
33
the catalysts alone. Thus, it is essential to prepare some new
FeFe]-hydrogenase model compound with a PPh3 analogue
These peaks can be safely assigned to the Fe–CO carbon atom.
3
1
[
The P NMR spectrum of 2 and 3 was display a singlet at d ¼
0.89 and 63.95 ppm, respectively. The observations from the
elemental analysis in all products agree well with the assumed
ligand aimed at enhancing the yield of catalytic hydroxylation of
benzene.
7
Herein,
SCH(CH CH
with excess 1,2-butanedithiol. For studying the impact hexane solution show distinct peaks between 2100 and 1900
a
new dinuclear [2Fe
2
S] mimic complex m- compositions.
(
2
3
)CH S)–Fe (CO) (1) was obtained by reaction of
2
2
6
The IR spectra for the compounds 1–3 obtained in the
Fe(CO)
5
ꢀ
1
of the ligand at the active site of [FeFe]-hydrogenase model
cm , characteristic of stretching vibrational modes of terminal
34
compound, the complexes namely m-(SCH(CH CH )CH S)–
2
3
2
metal carbonyls. As a result of the electron-donating phos-
35
Fe (CO) L (2: L ¼ PCy , tricyclohexyl phosphine; 3: L ¼ PPh ,
2
5
3
3
phine ligands, the IR spectra of complexes 2–3 are shied by
ꢀ1
ꢀ1
triphenyl phosphine) have been prepared by controllable CO
substitution of 1 with PCy and PPh , as shown in Scheme 1. compared to the all-carbonyl compound 1 (Fig. 1). The appli-
The structural elucidation of these model compounds featured cation of n(CO) bands is regarded to be a great tool for identi-
n
CO ¼ 30 cm and nCO ¼ 27 cm towards lower frequency
3
3
1
13
31
the use of UV-vis, FT-IR, H, C, P NMR spectra and single- fying the alteration of electron density on Fe atoms in the model
36,37
crystal analyses. For investigated the correlations between the compounds,
the CO stretches of the monomeric complexes
–3 are lower in wavenumber in comparison to the all-carbonyl
catalytic reactivity and the electronic features of the catalysts,
their electrochemical properties, FT-IR, UV-vis and their cata-
lytic performance have been discussed. The results indicate that
complex 2 has the greatest electron density and also the best
catalytic reactivity. This nding suggests that the catalytic
2
compound 1 which is indicative of the strong donating
Scheme 1 Synthetic route of complexes 1–3.
Fig. 1 IR spectra of complexes 1–3 in hexane solution (CO region).
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37
3 3
character of PCy and PPh group. The red-shi value order for
the n(CO) bands of 2–3 is in accordance with electron-donation
capabilities of diverse phosphine ligands, PCy ligand reveals its
3
strong electron-donation character compared to the PPh3
ligand. These showing a distinct ranking of 2 > 3 > 1 in electron
density on active sites.
To further clarify the electronic effect of the terminal ligands
(L) on the activity of the complexes, UV-vis spectra of the
compounds 1–3 were recorded in dichloromethane. As shown
in the Fig. 2, prominent absorption peaks of complexes 1–3 are
observed at 326 nm, 360 nm and 354 nm, respectively. These
peaks in the visible region can be attributed to p–p* transi-
38
tion. The electronic transitions of compound 2 (360 nm) and 3
(
(
354 nm) were found to be red-shied compared to compound 1
326 nm). These indicated more electron-donation ligand will
improve the electron densities at the diiron core. The electronic
transition signal compound 2 is about 6 nm red-shied
compared to compound 3, which indicated the PCy3 ligand
has the strong electron-donation than the PPh ligand. These
3
are in accordance with the result derived from the IR spectra
analysis and showed a distinct ranking of 2 > 3 > 1.
Molecular structures of complexes 1–3
X-ray quality single crystals of complexes 1–3 were obtained by
slow evaporation of the respective hexane–dichloromethane
solutions at low temperature under normal atmospheric pres-
sure. The structures of the compounds 1–3 are illustrated in
Fig. 3. As the coordination locations for phosphine ligands on
the diiron compounds are not identiable only via the spec-
troscopic data, the X-ray diffraction study was conducted for
1–3. The [Fe
2 2
S ] skeleton of compounds 1–3 have the famous
buttery structure where every iron center takes on the twisted
39
square-pyramidal coordination shape. Some of the important
bond lengths and angles are displayed in Table S1,† while
crystallographic data are shown in Table S2.† The Fe–Fe
˚
˚
distances (2.5090 (4) A in 2, and 2.5126(11) A in 3) were found to
Fig. 3 Crystal structures of the complexes (ellipsoids are drawn at the
50% probability level).
be slightly longer compared to the Fe–Fe distances in complex 1
˚
(
2.5054 A in 1). The PCy
3
or PPh
3
substituent group has a minor
or PPh ligand was located
impact on the Fe–Fe distances. PCy
3
3
in the apical site of Fe atom and was placed trans to the Fe–Fe
˚
bond. The Fe–P bond distances of complex 2(2.2734(5) A) are
˚
longer than that of complex 3 (2.2447(15) A), showing the larger
steric effects on the coordination geometry following the order
PCy
implied that the replacement of one CO by tertiary phosphine
ligands on the m-(SCH(CH CH )CH S)–Fe (CO) offers solely an
3 3
> PPh . X-ray crystallographic analysis for complexes 2 and
3
2
3
2
2
6
apical isomer as illustrated in Fig. 3. The displacement of CO by
the tertiary phosphine ligands (PCy or PPh ), which are better
3
3
electron-donating, increases the electron density on the iron
centers of a diiron dithiolate model compound, thereby further
strengthening the strong back-bonding from the Fe atoms to
Fig. 2 UV-visible spectra of 1 (black), 2 (red) and 3 (blue) recorded in
CH Cl
2
2
.
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the CO ligands and simultaneous weakening of CO triple Table 1 Redox potentials of the complexes
11,40,41
bonds.
+
+
Complex
Epc (V) vs. Fc/Fc
Epa (V) vs. Fc/Fc
Electrochemistry
1
2
3
ꢀ1.78
ꢀ1.94
ꢀ1.84
0.74
0.60
0.75
The electrochemical properties of complexes 1–3 were investi-
gated via cyclic voltammogram (Fig. 4). CV data are shown in
Table 1. The cyclic voltammogram of complex 1 displays an
irreversible oxidation process at Epa ¼ 0.74 V and a quasi-
reversible process at Epc ¼ ꢀ1.78 V. The rst oxidation
I
I
II
I
process is ascribed to the process from Fe Fe to Fe Fe , and the
initial process is able to be allocated to the Fe Fe /Fe Fe
reduction. The CV of the PCy -substituted complex 2 and
I
I
I
0
11
3
PPh -substituted complex 3 in CH CN solution display one
3
3
I
I
I
0
irreversible reduction process (Fe Fe /Fe Fe ) at Epc ¼ ꢀ1.94 V
and ꢀ1.84 V, and a quasi-reversible oxidation process at Epa
¼
0
.6 V and 0.75 V, respectively. Compared with the rst reduction
3
and oxidation event of complex 1, the PCy -substituted complex
2
3
and PPh -substituted complex 3, reveal cathodic shis in the
reduction potential, all in accordance with the enlargement of
electron density in diiron centers when CO were substituted by
11,12
better electron donating ligands.
Hydroxylation reactions of aromatic compounds
Although many iron-based catalysts have been studied, but the
[
FeFe]-hydrogenase model complexes have rarely been used for
11
the direct production of phenol from benzene. In the present
work, the [FeFe]-hydrogenase model compounds 1–3 were used
as catalysts in the single-step hydroxylation from benzene into
2 2 2
phenol with oxidant. We choose O , H O , PhIO, m-CPBA as
oxidants in the single-step hydroxylation. Among the oxidants,
H O has a higher phenol yield than m-CPBA and O , PhIO were
2
2
2
not have the ability to catalyze the single-step hydroxylation
from benzene into phenol with these complexes under the given
experimental conditions, as summarized in Table S3.† Thus,
2 2
we used H O as the oxidant in the single-step hydroxylation
from benzene into phenol with [FeFe]-hydrogenase model
compounds. The catalytic performance of complex 1 was
assessed by following the procedures well established in liter-
42–45
ature
ters which can potentially impact the catalytic properties, like
the amount of H , amount of catalyst, the reaction time, as
in acetonitrile using H O as oxidant. Some parame-
2 2
2 2
O
well as the temperature, were also examined.
The phenol yield and selectivity is signicantly affected by
the amount of H
amount of H
phenol was 10.5% when the amount of H O was 5.0 mmol. But
2
O
2
(Fig. 5). The yield increased with increase in
2 2
O from 0–5.0 mmol. The maximum yield of
2
2
with the further raise in the amount of H O , the yield and
2
2
selectivity of phenol has an obvious decreased, because too
much H may be resulted in excessive oxidation of phenol.
2 2
O
Therefore, the best amount of H
is 5.0 mmol.
2 2
O applicable for this reaction
The phenol yield as well as selectivity is displayed with the
variation temperature in Fig. 6. The result showed none of
ꢁ
phenol is detected below 40 C, since induction for a longer
period of time at a low-temperature blocks the creation of
phenol. The yield of phenol increased with temperatures growing
from 40 to 60 C. But with further rise in temperature, a clear
Fig. 4 Cyclic voltammograms of the diiron complexes 1–3 (1.0 mM) in
ꢀ1
ꢁ
3 4 6
CH CN solution (0.1 M n-Bu NPF ; scan rate 200 mV s ).
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Fig. 7 Effects of the amount of complex 1 on the phenol yield in the
2 2
Fig. 5 Effects of the amount of H O on the phenol yield in the
hydroxylation of benzene (H
2
O
2
, 5.0 mmol; benzene, 1.12 mmol;
hydroxylation of benzene (1, 30 mmol; benzene, 1.12 mmol; CH
3
CN,
ꢁ
ꢁ
3
CH CN, 2.0 mL; 60 C; reaction time, 1 h). All the experimental data are
2.0 mL; 60 C; reaction time, 1 h). All the experimental data are the
the average value of three sets of duplicate experiments.
average value of three sets of duplicate experiments.
The yield went to a maximum (12.6%) aer 3 h. Declining
selectivity due to phenol overoxidation was also discovered with
31
extension in the reaction time. Under the optimized circum-
ꢁ
stances (1, 30 mmol; 60 C; benzene, 1.12 mmol; CH CN, 2.0 mL;
3
H
2
O
2
, 5.0 mmol; reaction time, 3 h), the selectivity and yield of
one-step conversion of benzene to phenol with catalyst complex
is 12.6% and 85%, respectively.
1
12
According to previous results, the catalytic hydroxylation
activity of such diiron complexes is based on their special
structural and electronic properties, which was noticed rstly
in some precious fundamental studies of the oxygen sensitivity
of the [FeFe]-hydrogenase active site models, which was carried
25
out by Darensbourg' group. They found the iron based
oxidized species was thermodynamically favored in the
diiron model complexes, which was further proven by the
Fig. 6 Effects of the reaction temperature on the phenol yield in the experimental and theoretical studies. Just based on this idea, we
hydroxylation of benzene (1, 30 mmol; benzene, 1.12 mmol; CH
.0 mL; H , 5.0 mmol; reaction time, 1 h). All the experimental data
are the average value of three sets of duplicate experiments.
3
CN,
designed the catalytic system of these diiron complexes as an
2
2 2
O
decrease in yield and selectivity of phenol could be noticed, the
probable reason being the excessive oxidation of phenol high
temperatures, leading to by-products. Summing up from the
ꢁ
tests the best appropriate reaction temperature was 60 C.
The effect of amount of catalyst in the one-step hydroxylation
of benzene into phenol was tested and the results shown in the
Fig. 7. The phenol yield increased from 7.2% to 10.5% when the
catalyst amount was changed from 10 mmol to 30 mmol. And the
selectivity of phenol was no signicant change when the catalyst
amount was changed from 10 mmol to 30 mmol. Moreover, with
the increased of catalyst amount to 40 mmol caused an obvious
decrease in yield and selectivity of phenol which may be
attributed to the oxidation on all possible sites of benzene or
excessive oxidation of phenol caused by the excessive catalyst.
So, the best amount of catalyst is 30 mmol.
Fig. 8 Effects of the reaction time of complex 1 on the phenol yield in
the hydroxylation of benzene (H
O
2 2
, 5.0 mmol; benzene, 1.12 mmol;
The effect of reaction time on the yield and selectivity of
ꢁ
3
CH CN, 2.0 mL; 60 C; 1, 30 mmol). All the experimental data are the
phenol was also studied and the ndings are shown in Fig. 8. average value of three sets of duplicate experiments.
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oxygen transfer species. Actually, the Fe–(m-O)–Fe species as the
iron–oxygen reactive species in the catalytic oxidation was also
46
studied by Can Li' group. They identied the Fe–(m-O)–Fe sites
in Fe/ZSM-35 by in situ resonance Raman spectroscopy. Fe–
(m-O)–Fe species worked as an oxygen transfer species.
Under the optimized reaction conditions, performances of
complexes 1–3 as catalysts were investigated and the results are
summarized in Table 2, their active properties decline in the
ranking of 2 > 3 > 1 under identical conditions. The complexes
1
–3 showed better phenol yield and comparable selectivity to
11
that we reported recently, namely [(m-dmedt)-{Fe(CO) } ]
3
2
Fig.
9
Proposed reaction mechanism for the hydroxylation of
[
[
3
dmedt ¼ SCH(CH )CH(CH )S] (yield 7.5%; selectivity 92.5%),
3
3
benzene to phenol.
(m-dmest)-{Fe(CO)
3
}
2
] [dmest ¼ SCH(CH
3 3
)CH(CH )S(O)] (yield
.7%; selectivity 95.6%). These observations can probably be
attributed to the different activities of 1, 2 and 3 correlations
with the diverse Fe–Fe electron densities. The Fe–Fe bond of the
complex 2 or complex 3 with the electron-donating PCy
3
ligand
or PPh ligand could be oxidized more easily by H O to form
3
2 2
the iron–oxygen reactive species, whereas the less electron-
donating complexes were found to be less reactive with this
oxidant. These proved the electron density of the Fe–Fe bond of
43,45
the complexes would affect the catalytic performances.
According to the inference of the experimental results and
25,27,46
based on previously published theoretical results,
a prob-
able mechanism has been proposed for hydroxylation of
benzene to phenol. The [FeFe]-hydrogenase model complex has
the ability to catalyze the hydroxylation of different substrates to
phenol, which revealed that hydroxylation reaction involves an
electrophilic addition process and the proposed reaction
mechanism for the hydroxylation of benzene to phenol is shown
in Fig. 9. In the process of hydroxylation, the nucleophilic diiron
+
+
center of [Fe Fe ] complex tended to react with the oxidant of
due to the high electron density of the diiron center.
In order to investigate the catalysts stability of complexes
–3, further investigations were carried out through solution IR
2 2
H O
1
spectrum before and aer the hydroxylation reaction in CH CN.
3
As shown in Fig. 10, the CO-stretching band positions of
complexes 1–3 didn't change, only the intensities of IR spec-
trum of complexes 1–3 has slightly changes before and aer the
hydroxylation reaction in CH CN. This indicates the preserva-
3
tion of molecular structure of complexes 1–3 in the hydroxyl-
ation reaction in CH CN. Further research on the catalysts
3
stability of complexes 1–3 is concluded by the TG analysis.
The TG analysis patterns of complexes 1–3 are shown in
Fig. 11, these showed great heat resistance of complexes 1–3
Table 2 Effects of various catalysts for the direct hydroxylation of
a
2 2
benzene to phenol with H O
b
Catalysts
Yield (%)
Selectivity (%)
Fig. 10 The IR spectra of complexes 1–3 before benzene hydroxyl-
ation (A) and after benzene hydroxylation (B) (recorded in CH
solution).
3
CN
1
2
3
12.6
24.6
18.2
85
92
87
a
ꢁ
Catalysts, 30 mmol; 60 C; benzene, 1.12 mmol; CH
3
CN, 2.0 mL;
ꢁ
b
below 100 C. Among them complexes 2 and 3 have better heat
resistance than complex 1. A rapid weight loss for complexes 2
and 3 originated from the decomposition ranging from 200 to
oxidants, 5.0 mmol; reaction time, 3 h. Selectivity: yield of phenol/
benzene conversion.
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Guangfu Chemical. PPh
3
, PCy
3
CDCl
3
, CD
2
Cl
2
, benzene, and
AgNO , 1,2-butanedithiol, and n-Bu NPF were bought from
3
4
6
Sigma-Aldrich.
General methods
1
13
31
The H NMR, C NMR and P NMR spectrum are acquired
with Bruker AVANCE III 400 MHz NMR spectrometer. Solution
IR spectrum was recorded by a Shimadzu FTIR-8400 spec-
trometer with 0.1 mm KBr sealed battery. Element analysis was
conducted in a Heraeus CHN-Rapid, a fully automatic factor
analyzer along with TCD inspection, type: TMT CHN, BESTELL-
NR 2215001. By using a Rigaku MM-007 diffractometer along
with a Saturn 724CCD, the single-crystal X-ray diffraction data
were gathered at 293 K or 113 K with a confocal monochromator
˚
in Mo-Ka radiation (l ¼ 0.71073 A). Data gathering, decrease as
well as absorption rectication was conducted in the CRYS-
Fig. 11 The TG analysis of complexes 1–3.
47
TALCLEAR program. Structural issues were addressed by
47
direct approaches in SHELXL-97 program and also rened by
2
SHELXL-97 on F . The UV/visible spectra of the compounds
ꢁ
3
00 C. From the results it can be concluded that the structure
determined using the UV-vis diffuse reectance spectra (UV-
of the catalyst in hydroxylation reaction has not changed.
3600) at room temperature. TG was performed using a TA
SDT-Q600 apparatus and conducted in dynamic temperature at
ꢁ
ꢀ1
ꢁ
Conclusions
10 C min from room temperature to a temperature of 400 C
in nitrogen circumstance. The conductivities of the complexes
1
In this work, we successfully prepared three dinuclear [2Fe–2S]
bio-mimetic complexes m-(SCH(CH CH )CH S)–Fe (CO) (1),
–3 were on a Delsa™ Nano C Particle Analyzer (Beckman
Coulter Commercial Enterprise Co., Ltd, USA).
Electrochemical examinations were performed with
a CHI760B electrochemical workstation. The working electrode
diameter 3 mm) was polished in an a-alumina suspension,
washed with CH CN, and then sonicated with ion-free water for
0 min before usage. The counter electrode referred to a Pt wire.
And the reference electrode referred to a non-aqueous Ag/Ag
0.01 M AgNO and 0.1 M n-Bu NPF in CH CN) reference
electrode. A liquid of 0.1 M n-Bu NPF with CH CN was applied
as auxiliary electrolyte that was degassed with dry N for 10 min
2
3
2
2
6
m-(SCH(CH CH )CH S)–Fe (CO) PCy (2) and m-(SCH(CH CH )
2
3
2
2
5
3
2
3
CH
2
S)–Fe
2
5 3
(CO) PPh (3). The molecular structures of 1, 2 and 3
1
13
31
were well characterized by IR, H, C, P NMR, UV-vis spectra
and single crystal X-ray analysis, respectively. These complexes
were tested as catalysts in the study of the one-step oxidation of
(
3
2
benzene by H
2
O
2
to phenol. The study rstly revealed the
+
correlations between the electron densities of the catalytically
active sites of the complexes and the catalytic performance. On
comparing the catalytic performance of complexes, the complex
(
3
4
6
3
4
6
3
2
2
was found to be leading to highest phenol yield (24.6%) and
prior to examination. Ferrocene was applied as an internal
phenol selectivity (92%), which has the highest electron densi-
ties of the Fe–Fe bond. This study demonstrates the catalytic
abilities of [FeFe]-hydrogenase model complexes. This is a new
research direction for the design and development of new
criterion in the identical examining situation and each poten-
+
tial was referred to the Cp Fe /0 couple at 0 V.
2
2 2
Hydroxylation of benzene with H O was conducted in
a 25 mL ask that is equipped by a condenser as well as
a magnetic agitator. In a characteristic reaction, the composite 1
[FeFe]-hydrogenase model complexes applied on the one-step
oxidation of benzene by H O to phenol which has the excel-
2
2
(4 mg, 0.01 mmol) was mixed in CH CN (2.0 mL). Aer heating
3
lent phenol selectivity.
to the required temperature, benzene (0.1 mL, 1.12 mmol) was
2 2
added into the blend. A given mass of H O was added at last for
beginning the reaction, and in the meanwhile the blend was
agitated for a couple of hours. The research was conducted in
atmospheric pressure. Examinations of GC (9890B) equipped by
Experimental section
Materials
Reactions and measurements related to [FeFe]-hydrogenase a FID sensor as well as a capillary pillar (OV-1701; 30 m ꢂ 0.25
model compounds were performed in an inert (N
) atmo- mm ꢂ 0.25 mm) were conducted to investigate the product
ꢁ ꢁ
sphere with Schlenk line technology. THF, hexane, CH composition. TPD analysis was performed, from 70 C to 180 C
2
2
Cl
2
,
ꢁ
ꢁ
ꢁ
toluene, and acetonitrile were purchased from Jiangtian by a variation of 10 C, and then from 180 C to 280 C by
Chemical and dried and distilled by standard methods. THF, a variation of 30 C.
ꢁ
hexane, CH Cl , toluene, and acetonitrile were deoxygenated
Synthesis of m-(SCH(CH CH )CH S)–Fe (CO) (1). The
2 3 2 2 6
2
2
before every use. The Fe(CO) was acquired from Jiangsu Tianyi complex 1 was prepared according to the published procedures
5
11,12
Ultra-ne Metal Powder Co., Ltd. H
2
O
2
(30%) and Me
3
NO (tri- with a modied procedure.
1,2-Butanedithiol (1.39 g, 11.4
methylamine oxide) were directly applied aer purchasing from mmol) was added to the solution of 1,2-butanedithiol (1.39 g,
2940 | RSC Adv., 2017, 7, 2934–2942
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1
1.4 mmol) in 30 mL toluene. The mixture was stirred at reux NMR (CD
2
Cl
2
): dCO ¼ 210.82, ddalkyl ¼ 13.76, 29.57, 40.51, 54.44,
3
1
for 72 h under an atmosphere of nitrogen. The solvent was
evaporated under 110 C. Then the residue was puried by 63.95(s).
d
Ar
¼ 128.65, 130.29, 133.16, 135.5. P NMR (400 MHz, CDCl ):
3
ꢁ
a silica gel column and eluted with hexane. Single crystals of
product 1 was obtained by slow diffusion of hexane into CH
2
Cl
2
Acknowledgements
solution at ꢀ20 ꢁC. Yield: 1.1 g (24%). C10
H O S Fe (anal.
8 6 2 2
calcd): C 30.03, H 2.02, O 24.00; found: C 29.98, H 2.01, O 23.89. We are grateful to the National Natural Science Foundation of
ꢀ1
IR (hexane, cm ): v(CO) 2076(s), 2035(m), 2006(s), 1992(m), China (21276187), Tianjin Science and Technology Innovation
1
3
1
4
3
983(m). C NMR (CD
2
Cl
2
): dCO ¼ 208.82, ddalkyl ¼ 13.69, 30.18, Platform Program (14TXGCCX00017), and the Tianjin Munic-
1
1.86, 56.44. H NMR (400 MHz, CD Cl ): d ¼ 1.10, 1.08, 1.06 (d, ipal Natural Science Foundation (16JCYBJC20800) for nancial
2
2
H, CH ), d ¼ 1.63, 1.61, 1.59, 1.58, 1.56, 1.54, 1.52, 1.51 (m, 2H, support of this work.
3
CH ), d ¼ 1.89, 1.88, 1.86, 1.85 (m, H, CH), d ¼ 2.80, 2.78, 2.77,
2
2
.75, 2.67, 2.65, 2.64, 2.62, 2.60 (m, 2H, CH
Synthesis of m-(SCH(CH CH )CH S)–Fe
complex m-(SCH(CH CH )CH S)–Fe (CO) PCy
according to the published procedures with a modied proce-
2
).
(CO)
(2) was prepared
Notes and references
2
3
2
2
5 3
PCy (2). The
2
3
2
2
5
3
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11,12
dure.
The mixing of complex 1 178.8 mg (0.44 mmol) and
ꢁ
Me NO 34.8 mg (0.46 mmol) in 5 mL THF was carried out at 0 C
3
for 30 min under N , the color turned into black from red. Then
3 Y. K. Zhong, G. Y. Li, L. F. Zhu, Y. Yan, G. Wu and C. W. Hu,
J. Mol. Catal. A: Chem., 2007, 272, 169–173.
2
PCy (30.8 mg, 0.11 mmol) was added in the THF solution. Aer
3
the mixture was stirred for 1 h at room temperature under N
2
4 J. K. Joseph, S. Singhal, S. L. Jain, R. Sivakumaran, B. Kumar
and B. Sain, Catal. Today, 2009, 141, 211–214.
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and then the mixture solution was evaporated to dryness in
vacuum. The residue was puried by a silica gel column and
eluted with hexane. The second red band eluted with ethyl
acetate gave a red product of complex 2 (129.8 mg, 45%). Single
crystals of product 2 was obtained by slow diffusion of hexane
into CH Cl solution at room temperature. Anal. calcd (%) for
6 T. Kusakari, T. Sasaki and Y. Iwasawa, Chem. Commun., 2004,
8, 992–993.
2
2
C H Fe O PS : C 49.71, H 6.33, O 12.26; found(%): C 49.62, H
7 J. Yang, G. Sun, Y. Gao, H. Zhao, P. Tang, J. Tan, A. H. Lu and
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2
7
41
2
5
2
ꢀ
1
6
1
1
C
.29, O 12.23. IR (hexane, cm ): v(CO) 2046(s), 2039(m),
1
980(s), 1961(m), 1932(m), 1924(m). H NMR (CD
2
Cl
2
): d ¼ 1.09,
.33, 1.46, 1.62, 1.77, 1.90, 1.99, 2.13, 2.28, 2.50, 2.64 (m, 41H,
1
3
6
H11, SCH(CH
2
CH
3
)CH
2
S). C NMR (CD
2
Cl
2
): dCO ¼ 210.86,
ddalkyl ¼ 14.09, 29.43, 42.67, 55.57, dCy ¼ 26.49, 29.13, 30.63,
3
1
38.24. P NMR (400 MHz, CDCl ): 70.89(s).
3
Synthesis of m-(SCH(CH CH )CH S)–Fe (CO) PPh (3). The
2
3
2
2
5
3
complex m-(SCH(CH CH )CH S)–Fe (CO) PPh (3) was prepared
2
3
2
2
5
3
11,12
according to the published with a modied procedure.
11 X. Wang, T. Y. Zhang, Q. S. Yang, S. Jiang and B. Li, Appl.
Organomet. Chem., 2014, 28, 666–672.
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3
NO (37.7 mg, 0.49
ꢁ
mmol) in 5 mL THF was stirred at 0 C for 0.5 h under N
2
, the 12 Y. H. Wang, T. Y. Zhang, B. Li, S. Jiang and L. Sheng, RSC
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1123–1130.
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3
2
diffraction quality crystals of product 3 were obtained by
slowly evaporating the hexanes/dichloromethane mixture solu-
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27
H
23Fe
2
O
5
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ꢀ
1
3
1
7
1
988(m), 1968(s), 1946(m). H NMR (400 MHz, (CD Cl )): d ¼
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.41, 2.39, 2.38, 2.29, 2.25, 2.24, 2.20, 2.18 (m, 2H, SCH
2
2
d ¼ 0.81, 0.79, 0.78, (d, 3H, trans isomer, CH
3
cis isomer, CH
3
2
3
1
2
3
2
3
2
3
1
3
2
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