A new nitrogen heterocyclic carbene containing diiron complex as bio-inspired catalyst for proton reduction and benzene hydroxylation

Article abstract of DOI:10.1016/j.jorganchem.2016.10.020

A new diiron complex (μ-dmedt)[Fe₂(CO)₅IPr] (2, dmedt = 2,3-butanedithiol, IPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene) was synthesized as analogue of the active site of [Fe-Fe]-H₂ases. The sterically bulky nitrogen heterocyclic carbene ligand of IPr coordinated to the iron center was introduced via CO/L substitution reaction in mild conditions. The reactivity of electrocatalytic reduction of proton and catalytic hydroxylation of benzene were explored for the developed diiron complex. Complex 2 undergoes two irreversible reduction events at ca. -2.17 V and ?2.50 V and acts as an efficient electrocatalyst for proton reduction to hydrogen with both HOAc and H₂O as proton source. The catalytic hydroxylation of benzene by complex 2 achieved phenol yield of 32.4% with almost 100% selectivity. The enhanced catalytic activity compared to the all carbonyl precursor and related derivatives was attributed to the good electron donating ability of introduced IPr ligand.

Full text of DOI:10.1016/j.jorganchem.2016.10.020

Accepted Manuscript
A new nitrogen heterocyclic carbene containing diiron complex as bio-inspired
catalyst for proton reduction and benzene hydroxylation
Yanhong Wang, Yiwen Yang, Tianyong Zhang, Xia Zhang, Shuang Jiang, Guanghui
Zhang, Bin Li
PII:
S0022-328X(16)30468-5
10.1016/j.jorganchem.2016.10.020
JOM 19664
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 7 June 2016
Revised Date: 12 October 2016
Accepted Date: 15 October 2016
Please cite this article as: Y. Wang, Y. Yang, T. Zhang, X. Zhang, S. Jiang, G. Zhang, B. Li, A
new nitrogen heterocyclic carbene containing diiron complex as bio-inspired catalyst for proton
reduction and benzene hydroxylation, Journal of Organometallic Chemistry (2016), doi: 10.1016/
j.jorganchem.2016.10.020.
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ACCEPTED MANUSCRIPT
Table of Contents
A nitrogen heterocyclic carbene containing diiron complex, (µ-dmedt)[Fe2(CO)5IPr], is
synthesized and used as a bio-inspired catalyst for both electro-chemical proton reduction
and highly selective hydroxylation of benzene.

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A New Nitrogen Heterocyclic Carbene Containing Diiron Complex as
Bio-inspired Catalyst for Proton Reduction and Benzene Hydroxylation
Yanhong Wanga,b, Yiwen Yanga, Tianyong Zhang*a,c, Xia Zhanga, Shuang Jianga, Guanghui Zhanga, Bin
Li*a,c
a School of Chemical Engineering and Technology, Tianjin Key Laboratory of Applied Catalysis Science
and Technology, Tianjin University, Tianjin 300072, China.
b Sinopec Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, China.
c Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China.
*Corresponding Author
E-mail: libin@tju.edu.cn; tyzhang@tju.edu.cn
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Abstract
A
new diiron complex (ꢀ-dmedt)[Fe2(CO)5IPr] (2, dmedt
=
2,3-butanedithiol, IPr
=
1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene) was synthesized as anologue of the active site of
[Fe-Fe]-H2ases. The sterically bulky nitrogen heterocyclic carbene ligand of IPr coordinated to the iron
center was introduced via CO/L substitution reaction in mild conditions. The reactivity of electrocatalytic
reduction of proton and catalytic hydroxylation of benzene were explored for the developed diiron complex.
Complex 2 undergoes two irreversible reduction events at ca. -2.17 V and -2.50 V and acts as an efficient
electrocatalyst for proton reduction to hydrogen with both HOAc and H2O as proton source. The catalytic
hydroxylation of benzene by complex 2 achieved phenol yield of 32.4% with almost 100% selectivity. The
enhanced catalytic activity compared to the all carbonyl precursor and related derivatives was attributed to
the good electron donating ability of introduced IPr ligand.
Keywords:
[Fe-Fe]-H2ases, Nitrogen heterocyclic carbene, Electrochemical property, Proton reduction, Catalytic
hydroxylation
2

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1. Introduction
Hydrogenases are widespread in nature, efficiently and reversibly catalyze the reduction of protons
to hydrogen, and the reverse oxidation of hydrogen with release of protons and electrons. With active
sites comprised of iron and nickel, they hold out the promise of the development of base metal catalysis
for these processes of significance to fuel cells [1, 2-5]. As early as in 1990s, the crystal of
[Fe-Fe]-H2ases is extracted from pasterurianum (CpI) and Desulfovibrio desulfuricans (DdH) [6][7].
The structure of [Fe-Fe]-H2ase and the pathways for dihydrogen and H+ transfer are given in Fig. 1.
[Fe-Fe]-H2ase consists of a diiron subcluster [2Fe2S] and a cubane configuration [4Fe4S] cluster [12].
These two clusters are connected by a thiolate of a cysteine residue [2, 8]. Each Fe atom of [2Fe2S]
subunit features two diatom ligands, CO and CN, both in terminal coordination model [8, 9]. [4Fe4S]
cluster forms the electronic channel from protein to active site. The two S atoms of [2Fe2S] are
connected by 2-azapropane-1,3-dithiolato (adt). The bridge group NH, intramolecular base acts as a
proton relay and form secondary amine to capture protons which heterolytic cleavage from H2 [9-11].
The variety of the structure of [Fe-Fe]-H2ase active site enabled the development of electronic and steric
effect controllable models complexes, which may served as powerful bio-inspired redox catalyst.
Fig. 1 The structure of the active sites of [Fe-Fe]-H2ases and the pathways for dihydrogen and H+
transfer.
Besides the attractive functions, the air sensitivity of [Fe-Fe]-hydrogenase still remains a challenge
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for its catalytic applications. It was demonstrated the oxygenation of [FeFe]-hydrogenase active site
analogues would occur at the dithiolato-sulfur site or the Fe-Fe bond,[12-14] forming the S-oxygenated
products in which the oxygen atom is relatively stable or the Fe-based oxygenated products
(FeII-ꢀ-O-FeII oxidative addition products) which are thermodynamically favored but not detected in the
oxygenation. Since the highly active iron-based oxygenated ꢀ-oxo diiron derivatives are accepted as the
crucial intermediates to transfer oxygen in the biomimetic hydrocarbon hydroxylation.[15-18] Our
previous studies demonstrate the Fe-Fe bond based oxygenated ꢀ-O complex was capable of
transferring oxygen atom to the aromatic substrates based on the DFT predicted results and the catalytic
activity towards hydroxylation[19]. We found that the different catalytic activity of such complexes was
derived from their different Fe-Fe bond electron density. The Fe-Fe bond in the complex with the better
electron-donating ligand could be oxidized more easily by H2O2 to form the oxygen-transfer
intermediate while the less electron-rich complexes tended to be less reactive with the oxidants.
Aiming at enhancing the reactivity of catalytic hydroxylation of benzene, we try to introduce
N-heterocyclic carbene ligand as a better electron donor. A new diiron analogue of
(ꢀ-dmedt)[Fe2(CO)5IPr] (2, dmedt = 2,3-butanedithiol, IPr = 1,3-bis(2,6-diisopropylphenyl)
imidazol-2-ylidene) is synthesized and characterized in this paper. To date, IPr as a ligand is the first
time to be used in (ꢀ-SR)2[FeI(CO)3]2. As a molecular electrocatalyst, the electrochemical properties on
proton reduction with both HOAc (acetic acid) and H2O as proton source are also investigated. And its
stability is monitored by solution IR. Complex 2 as a homogeneous catalyst is used for the
hydroxylation of benzene. The synthetic route of complex 2 is shown in Scheme 1.
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Scheme 1 Synthetic route of (ꢀ-dmedt)[Fe2(CO)5(IPr)] (2).
2. Experimental Section
2.1 General Comments
Some reactions and operations were carried out using standard Schlenk and vacuum-line techniques
under an atmosphere of nitrogen. The Fe(CO)5 was obtained as a gift from Jiangsu Tianyi Ultra-fine
Metal Powder Co., Ltd (China). The other solvents and reagents were purchased from Guangfu
Chemicals and Sigma-Aldrich. Solvents were dried and distilled prior to use according to the standard
methods. The purified solvents were stored with molecular sieves under N2 for no more than 1 week
before use. THF, hexane, CH2Cl2, toluene and acetonitrile were stirred over molecular sieves under N2
for 24 h prior to further purification. THF and hexane were distilled from sodium/benzophenone.
Acetonitrile were distilled from NaH. All the solvents were deoxygenated before every use.
1
The NMR spectra were measured on a Bruker AVANCE III 400 MHz NMR spectrometer. H NMR
and 13C NMR shifts are referenced to residual solvent resonances according to literature values.
Solution IR spectra were recorded on a Shimadzu FTIR-8400 spectrometer using 0.1 mm KBr sealed
cells. Elemental analysis was carried out on a Heraeus CHN-Rapid, fully automatic elemental analyzer
with TCD detection, type: TMT CHN, BESTELL–NR 2215001.
2.2 Preparation of (µ-dmedt)Fe2(CO)6 (1)
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The hexacarbonyl parent 1 was synthesized as our previous reports and its characterization data were
also given in our previous reports [20].
2.3 Preparation of (µ-dmedt)Fe2(CO)5(IPr) (2)
Solid 1 (80 mg, 0.20 mmol) and salt IPr·HCl (213 mg, 0.50 mmol) were combined, dried in vacuo for
30 min and then dissolved in THF (4 mL). After stirring for another 30 min, the previously dried
t-BuOK (112 mg, 1.00 mmol) in 3 mL of THF was added, and the resulting mixture was stirred for an
additional 30 min. The reaction was monitored by IR spectroscopy to confirm that the reaction had
reached completion. The solvent was then removed, and the red residue was dissolved in CH2Cl2 and
loaded onto a silica gel column. Elution with petroleum ether removed excess starting material 1. The
product was then eluted with CH2Cl2. Solvent was evaporated from the dark red eluant to yield product
1
2 (113.5 mg, 70.6%). IR (CH2Cl2) νCO=2030, 1971, 1909 cm-1. H NMR(400 MHz, Acetone-d6):
7.59-7.51 (m, NCH=CHN); 7.43-7.38 (m, ArH); 2.83 (s, p-ArCH); 1.79 (m, SCH(CH3)CH(CH3)S);
1.46-0.68 (m, CH3). 13C NMR (400 MHz, Acetone-d6): 215.5 and 211.8 (CO); 188.8 (Ccarbene);
146.5-17.2 (C6H2); 124.0 (NCH=CHN); 54.8 and 47.7 (SCH(CH3)CH(CH3)CHS); 25.3-21.8 (C(CH3)2),
19.9-19.5 (C(CH3)2), 18.4 (SCHCH3). Elemental analysis: Calc. for C36H44Fe2N2O5S2: C, 56.85; H, 5.83;
N, 3.68. Found: C, 56.75; H, 5.84; N, 3.67.
2.4 X-Ray Crystallography
The single-crystal X-ray diffraction data were collected with a Rigaku MM-007 diffractometer
equipped with a Saturn 724CCD. Data were collected at 113 K using a confocal monochromator with
Mo-Kα radiation (λ=0.71075 Å). Data collection, reduction and absorption correction were performed
with the CRYSTALCLEAR program [21]. Structure was solved by direct methods using the
SHELXS-97 program [22] and refined by full-matrix least-squares techniques (SHELXL-97) [23]on F2.
Hydrogen atoms were located by geometrical calculation. Details of crystal data, data collections and
structure refinements are summarized in Table S1 and Fig. S1.
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2.5 Cyclic Voltammograms
Cyclic voltammograms were obtained in a three-electrode cell under N2 using a CHI 660B
electrochemical workstation. The working electrode was a glassy carbon disk (diameter 3 mm) polished
with 3 ꢀm and 1 ꢀm polishing powder and sonicated in ion-free water for 20 min prior to use. The
reference electrode was a non-aqueous Ag/Ag+ (in a CH3CN solution of 0.01 M AgNO3/0.1 M
n-Bu4NPF6) electrode and the counter electrode was platinum wire. A solution of 0.1 M n-Bu4NPF6 in
CH3CN was used as supporting electrolyte, which was degassed by bubbling with dry CO for 10 min
before measurement. Ferrocene was used as an internal standard under the same measuring conditions
and all potentials were referenced to the Cp2Fe+/0 couple at 0 V. During the electrocatalytic experiments
≧
under N2, increments of HOAc (chromatographic grade, 99.8%, water 0.15% by Karl Fischer method)
were added by microsyringe.
2.6 Catalytic Hydroxylation
Hydroxylation of benzene with H2O2 was carried out in a 25 mL round bottom flask equipped with a
reflux condenser and a magnetic stirrer. In a typical reaction, 2 (7.6 mg, 0.01 mmol) was dissolved in
CH3CN (2.0 mL). After the mixture was heated to the desired temperature, benzene (0.1 mL, 1.1 mmol)
was added to the mixture. A certain amount of H2O2 was added by dropwise to start the reaction and the
mixture was stirred for several hours. All the experiments were conducted at ambient pressure.
Measurements of GC (9890B) equipped with a FID detector and a capillary column (OV-1701; 30 m ×
0.25 mm × 0.25 ꢀm) were performed to analyze the product mixture. Internal standard method was used
to quantitative analysis and chlorobenzene was chosen as the internal standard substance. TPD analysis
was carried out, from 80 °C to 200 °C by a ramp of 10 °C, then from 200 °C to 280 °C by a ramp of
30 °C. This reaction system appeared to have a high selectivity since phenol was the only product
detected by GC.
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3. Results and Discussion
3.1 Synthesis and Characteristics of (µ-dmedt)[Fe2(CO)5(IPr)] (2)
All-carbonyl parent complex (ꢀ-dmedt)[Fe2(CO)6] (1) reacted with imidazolium salt IPr·HCl and
strong base t-BuOK in THF at room temperature under sufficient stir, and the goal bio-inspired complex
(ꢀ-dmedt)[Fe2(CO)5(IPr)] (2) was obtained as an air-stable red solid powder (Scheme 2). Complex 2
was characterized by solution IR spectra, NMR spectra, elemental analysis and single-crystal X-ray
diffraction.
The C=O absorption bands of complex 2 shift towards lower frequency by 43-91 cm-1 compared to
those of the parent complex 1 due to the ligand substitution reaction and the stronger δ-donating ability
of IPr. The IR spectrum of complex 2 display three singlet peaks and a shoulder peak (Fig. 2) for their
terminal carbonyls in CH2Cl2. The IR spectrum of complex 2 is very close resemblance with the
previous reported (ꢀ-dmedt)[Fe2(CO)5(IMes)] (IV, Fig. 3) (2036, 2028, 1971, 1909 cm-1) [20]. νC=O
Bands can be identified as an indicator for evaluating the δ-donating ability of ligands and the electron
density of the Fe atoms [24]. Hence, we deduced that IPr ligand is almost the same as IMes ligand in
this diiron complex, due to the influence of both δ-donating ability and steric hinderance.
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Fig. 2 The v(CO) region of IR spectra of complexes 1 and 2 (observed in CH2Cl2 solution).
Fig. 3 The structures of complexes II, IV and V. [20].
Crystal of complex 2 used for X-ray analysis of single crystals was obtained from ethanol-acetone
solution in low temperature. Combined with the preparation process of complex 2, it is hardly soluble in
nonpolar solvents, but soluble and stable in polar solvent. Thus, we can conclude that complex 2 is a
polar compound. The structure of complex 2 is as expected and given in Fig. 4. The crystallographic
data and refinement parameters are listed in Table S1. And the unit cell of complex 2 is given in Fig. S1.
Stick drawing and space-filling models of complex 2 in three perspectives are shown in Fig. S2.
Obviously, the [2Fe2S] skeleton of the complex shares the well-known butterfly conformation in which
each iron center adopts distorted square-pyramidal coordination geometry. The selected bond lengths
and angles are given in Table 1. Similar to our previously prepared complexes II, IV and V (Fig. 3) [20],
IPr ligand also lies in the basal position. The two anti-configuration methyls in ꢀ-dmedt hinder ligand to
attack the CO in apical positions, thus, ligand prioritizes attacking the base position own to its relatively
small steric hinderance. This conclusion is consistent with the Singleton’s [25]. The experimental results
made by Capon [26] also prove this conclusion very well.
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Fig. 4 X-ray crystal structure of complex 2 (ellipsoids at 20% probability level). The hydrogen atoms are omitted for
clarity.
Table 1 Selected bond length and angle for complex 2
Bond length (Å)
Bond angle (Deg.)
Fe(1)- Fe(2)
2.5647(12)
S(1)-Fe(1)-S(2)
79.15(6)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe(1)-C(2)
Fe(1)-C(1)
Fe(1)-C(6)
Fe(2)-C(4)
N(1)-C(9)
2.2360(16)
2.2491(16)
2.2478(17)
2.2526(17)
1.7640(7)
1.7660(6)
1.9910(5)
1.7770(7)
1.4450(7)
S(1)-Fe(1)-Fe (2)
S(2)-Fe(1)-Fe (2)
S(2)-Fe(2)-S(1)
S(2)-Fe(2)-Fe (1)
S(1)-Fe(2)-Fe (1)
Fe(1)-S(1)-Fe (2)
Fe(1)-S(2)-Fe (2)
N(1)-C(6)-N(2)
S(1)-C(34)-C(33)
55.33(5)
55.33(5)
78.83(6)
55.21(5)
54.90(4)
69.78(5)
69.46(5)
101.70(4)
107.50(5)
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3.2 Cyclic Voltammograms
Electrochemical properties of complex 2, also with HOAc or H2O were investigated by cyclic
voltammetry (CV) in coordinating solvent CH3CN in the presence of Bu4NPF6 as supporting electrolyte
under CO atmosphere at room temperature. The redox potentials are referenced to Fc/Fc+ = 0.00 V. The
CVs of HOAc in CH3CN at 0.2 V/s as a control is shown in Fig. S3. Measuring in CO saturated solution
can not only eliminate oxygen but also inhibit carbonyl dissociation from [2Fe2S] active site [27, 28].
CVs of complex 2 at various scan rates are given in Fig. 5(a). CV of complex 2 at selected scan rate (0.2
V/s) in subsequent experiments is shown separately in Fig. 5(c). With the increment of the scan rate, the
peak currents of both reduction and oxidation peak current are increased, which is close to the
concentration gradient of complex 2 around the glassy carbon electrode, indicating that the intensity of
the catalytic reaction is increased (Fig. 5(a)). Complex 2 undergoes two irreversible reduction events at
ca. -2.17 V and -2.50 V and no return oxidation peak is observed on the return scan under CV
≧
conditions. The second reduction event becomes to merger at high scan rates ( 0.3 V/s) (Fig. 5(c)) [29].
The two reduction events are probably associated with FeIFeI → FeIFe0 couple and FeIFe0→ Fe0Fe0
couple respectively. A possible quasi-reversible two-electron oxidation FeIFeI → FeIIFeII couple at ca.
A
C
0.17 V is observed in the CV of complex 2 (ꢁE = EP - EP = 70 mV > 59/2 mV = 29.5 mV, Fig. 5(a))
according to the previous report [29]. Whilst the small coupling reduction peak observed at ca. 0.10 V is
associated with the oxidation production [5]. The oxidation peak current at ca. 0.17 V shows linear
dependence on the square root of the scan rate (Fig. 5(b)). This linear plot indicates the reaction is
quasi-reversible and under diffusion control [30-32]. It is worth pointing out that the two catalytic
potentials of complex 2 are all less than the two corresponding potentials of complex 1 (-1.73 V and
-2.23 V) [20] as a result of the existence of a strong δ-donating IPr ligand.
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Fig. 5 (a) CVs of complex 2 (2 mM) in CH3CN solution (0.1 M n-Bu4NPF6) under CO at various scan rates. (b) Plot
of peak current (Ip) vs square root of the scan rate. (c) CV of complex 2 (0.2 mM) in CH3CN solution (0.1 M
n-Bu4NPF6) under CO at 0.2 V/s. All potentials are scaled to Fc/Fc+=0.00 V.
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In the presence of HOAc, both of the two reduction peak currents are increased, which indicated the
electrocatalysis of proton reduction occurs at the two reduction potentials may correspond to FeIFe0 (Epc
≈ -2.14 V, process I) and Fe0Fe0 species (Epc ≈ -2.14 V, process II) (Fig. 6(a)). This suggests that both
FeIFe0 and Fe0Fe0 species can combine with protons and be active toward electrocatalytic H2 production
from HOAc. Moreover, the second reduction peak current increases faster than the first. It indicates that
the reactivity of Fe0Fe0 specie is stronger than that of FeIFe0 specie. Such double electrocatalytic
processes for proton reduction to hydrogen by [Fe-Fe]-H2ases model complexes were consistent to the
previous report [33, 34]. The current heights of the first single cathodic events in the CVs of complex 2
almost increase linearly with the concentration of HOAc (Fig. 6(b)). The steeper slope displayed by the
line of current-concentration indicates greater sensitivity to acid concentration, which indicate the
electrocatalytic reactions occurred [28, 35]. Additionally, CVs of complex 2 with H2O as proton source
are investigated in coordinating solvent CH3CN in the presence of Bu4NPF6 as supporting electrolyte
under CO atmosphere (Fig. 6(c)). Similarly, both of the two reduction peak currents increase as the
increased concentration of H2O. However, the magnitude of the peak current increase is less obvious
than that when HOAc was used as proton source. The possible electrocatalysis mechanism with H2O as
proton source is similar to that with HOAc as proton source. It is worth noting: When HOAc was added
to the electrocatalytic system, electrode potentials of the two reduction peaks shift to positive by 30 mV
and 50 mV, with the increased concentration of HOAc, the electrode potentials return to original
positions gradually. However, when H2O was added, electrode potentials of the two reduction peaks
remain unchanged. It is concluded that the addition of HOAc can affect electrode potential of electric
catalytic peak, this effect will disappear gradually with the increased concentration of HOAc. IR spectra
of complex 2 before and after electrocatalytic reaction are shown in Fig. 6(d). There is no significant IR
change was observed. Thus, complex 2 can implement the entire catalytic cycle of electrocatalytic
proton reduction and back to the original state. Although complex 2 is synthesized under anhydrous
anaerobic condition, it is relatively stable to air, water and HOAc during the catalytic process.
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Fig. 6 (a) CVs of complex 2 (2 mM) with HOAc (0, 4, 6, 8, 12, 16 and 20 mM) in CH3CN solution (0.1 M n-Bu4NPF6)
under CO at 0.2 V/s. (b) Dependence of current heights (Ip) of first reduction peak for complex 2 (2 mM) on HOAc
concentration (4, 6, 8, 12, 16 and 20 mM) in CH3CN solution. (c) CVs of complex 2 (2 mM) with H2O (0, 10, 20 and
40 ꢀL) in CH3CN solution (10 mL, 0.1 M n-Bu4NPF6) under CO at 0.2 V/s. (d) The v(CO) region of IR spectra of
complex 2 (observed in CH3CN solution): (a) black, before CV test; (b) red, after addition of HOAc; (c) blue, after
addition of H2O. All potentials are scaled to Fc/Fc+=0.00 V.
3.3 Hydroxylation of Benzene to Phenol
Diiron complexes are well known for hydrocarbon oxidation and our previous studies indicate that
[Fe-Fe]-H2ases model complexes are capable of transferring oxygen, therefore, complex 2 is used to
catalyze hydroxylation of benzene to phenol [19, 20]. From the data in Table 2 we can conclude that the
more electron-rich complex tends to be more reactive with the oxidant. Thus we speculate that the yield
of phenol catalyzed by complex 2 is higher than that by complex 1, but equally with that by complex
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IV.
Table 2 Yield of phenol catalyzed by different catalysts
Complex
Absorbance bands νC=O of (cm-1)
2073, 2032, 2000, 1990
2082, 2043, 2020, 1997, 1988
2049, 1988, 1967, 1945
2036, 2028, 1971, 1909
2030, 1971, 1909
Yield of phenol (%)
7.5 a
3.7 a
1
1-(S=O)
1-PPh3
IV
9.6 a
25.9 b
32.4 c
2
a ref 19; b ref 20; c this work.
Some of the H2O2 decomposed when added all at once to the reaction system in our previously
reported experiments [19, 20]. However, now, H2O2 is added dropwise to ensure that effective H2O2
presents in reaction system. In this operating mode, Some parameters which affected the catalytic
activity of the catalyst such as the amount of H2O2, the temperature and the reaction time were also
investigated. As illustrated in Fig. 7, the phenol yield reaches the maximum in 5 h. The decreased yield
is obtained resulting from the further oxidation of phenol under longer reaction time. The phenol yield
is plotted as a function of temperature (Fig. 8). No product is detected under 40 °C because a longer
induction period under low temperature prevents the formation of phenol. The decreased selectivity
with the increase of temperature is attributed to the accelerated further oxidation of phenol. Phenol yield
is greatly influenced by the amount of H2O2 (Fig. 9) and increases initially with added increments of
H2O2 then decreases as a result of further oxidation of phenol to the by-products of dihydroxyhenzenes
or benzoquinone with excess H2O2. Under the optimized experimental conditions (benzene / catalyst 2
(mol), 10/1; CH3CN, 2.0 mL; H2O2, 8 eq. of benzene; 60 °C; 5 h), the maximal yield of phenol is 32.4%
and the selectivity is almost 100% detected by GC.
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Figure 7 Effects of reaction time on hydroxylation of benzene to phenol
Figure 8 Effects of reaction temperature on hydroxylation of benzene to phenol
Figure 9 Effects of H2O2 amount on hydroxylation of benzene to phenol
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4. Conclusions
In summary, a new bio-inspired [Fe-Fe]-H2ases model complex 2, (ꢀ-dmedt)[Fe2(CO)5IPr], that
relatively stable to air, water and weak acid, is synthesized by CO/L substitution reaction under mild
condition. The X-ray crystallographic reveals that monodentate IPr ligand in complex 2 is located in a
basal position. Complex 2 was able to serve as an efficient electrocatalyst for H2 production, and
implement the entire catalytic cycle of electrocatalytic proton reduction and back to the original state
with both HOAc and H2O as proton source. In addition, complex 2 acts as an effective homogeneous
catalyst for the hydroxylation of benzene to phenol with the maximum phenol yield of 32.4%.
Supporting Information
Procedures, electrochemistry, hydrogenation results and X-ray crystallographic data (CIF) obtained
from the structure determination and a full list of metric parameters for complex 2, CCDC 1064131.
Crystallographic
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Acknowledgements
We are grateful to the National Natural Science Foundation of China (21103121 and 21276187), the
Research Fund for the Doctoral Program of Higher Education of China (20110032120011) and the
Tianjin Municipal Natural Science Foundation (16JCYBJC20800) for financial support of this work.
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Highlight
1. A new N-heterocyclic carbene substituted diiron complexes was synthesized and
characterized.
2. This complex could electro-catalyzed the reduction of proton to hydrogen from
both HOAc and H2O.
3. The catalytic hydroxylation of benzene by this complex achieved a phenol yield of
32.4% and a good selectivity.