Synthesis, characterization, electrochemical properties and catalytic reactivity of N-heterocyclic carbene-containing diiron complexes

Article abstract of DOI:10.1039/c4ra15150j

(μ-dmedt)[Fe(CO)₃]₂ (I, dmedt = 2,3-butanedithiol) was chosen as the parent complex. A series of new model complexes, N-heterocyclic carbene (NHC) substituted (μ-dmedt)[Fe-Fe]-NHC (II, (μ-dmedt)[Fe(CO)₂]₂[I_Me(CH₂)₂I_Me], I_Me = 1-methylimidazol-2-ylidene; III, {(μ-dmedt)[Fe₂(CO)₅]}₂[I_Me(CH₂)₂I_Me]; IV, (μ-dmedt)[Fe₂(CO)₅]IMes, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; V, (μ-dmedt)[Fe₂(CO)₅]IMe, IMe = 1,3-dimethylimidazol-2-ylidene) as mimics of the [Fe-Fe]-H₂ase active site were synthesized from I and characterized using solution IR spectroscopy, NMR spectroscopy, elemental analysis and single-crystal X-ray diffraction. The electrochemical properties of complexes I-V, with and without the addition of HOAc, were investigated by cyclic voltammetry in the coordinating solvent CH₃CN to evaluate the effects of different NHC ligands on the redox properties of the iron atoms of the series of complexes. It was concluded that all the new complexes are electrochemical catalysts for proton reduction to hydrogen. The symmetrically substituted cisoid basal/basal coordination complex II displays the most negative reduction potential owing to the stronger δ-donating ability of the NHC and the orientation of the NHC donor carbon as a result of the constraints of the bridging bidentate ligands. A new application for the [Fe-Fe]-NHC model complexes in the direct catalytic hydroxylation of benzene to phenol was also studied. Under the optimized experimental conditions (II, 0.01 mmol; benzene, 0.1 mL; CH₃CN, 2.0 mL; H₂O₂, 6.0 mmol; 60 °C, 3 h), the maximal phenol yield was 26.7%.

Full text of DOI:10.1039/c4ra15150j

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Synthesis, characterization, electrochemical
properties and catalytic reactivity of N-heterocyclic
carbene-containing diiron complexes†
Cite this: RSC Adv., 2015, 5, 29022
ab
ab
Yanhong Wang,ab Tianyong Zhang,
Bin Li,
Shuang Jiangab and Liao Shengab
*
*
(m-dmedt)[Fe(CO)3]2 (I, dmedt ¼ 2,3-butanedithiol) was chosen as the parent complex. A series of new
model complexes, N-heterocyclic carbene (NHC) substituted (m-dmedt)[Fe–Fe]–NHC (II, (m-dmedt)
[Fe(CO)2]2[IMe(CH2)2IMe], IMe ¼ 1-methylimidazol-2-ylidene; III, {(m-dmedt)[Fe2(CO)5]}2[IMe(CH2)2IMe]; IV,
(m-dmedt)[Fe2(CO)5]IMes, IMes
¼
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene; V, (m-dmedt)
[Fe2(CO)5]IMe, IMe ¼ 1,3-dimethylimidazol-2-ylidene) as mimics of the [Fe–Fe]–H2ase active site were
synthesized from I and characterized using solution IR spectroscopy, NMR spectroscopy, elemental
analysis and single-crystal X-ray diffraction. The electrochemical properties of complexes I–V, with and
without the addition of HOAc, were investigated by cyclic voltammetry in the coordinating solvent
CH3CN to evaluate the effects of different NHC ligands on the redox properties of the iron atoms of the
series of complexes. It was concluded that all the new complexes are electrochemical catalysts for
proton reduction to hydrogen. The symmetrically substituted cisoid basal/basal coordination complex II
displays the most negative reduction potential owing to the stronger d-donating ability of the NHC and
the orientation of the NHC donor carbon as a result of the constraints of the bridging bidentate ligands.
A new application for the [Fe–Fe]–NHC model complexes in the direct catalytic hydroxylation of
benzene to phenol was also studied. Under the optimized experimental conditions (II, 0.01 mmol;
benzene, 0.1 mL; CH3CN, 2.0 mL; H2O2, 6.0 mmol; 60 ꢀC, 3 h), the maximal phenol yield was 26.7%.
Received 24th November 2014
Accepted 13th March 2015
DOI: 10.1039/c4ra15150j
www.rsc.org/advances
(tdt ¼ thiodithiolate), and their derivatives have been exten-
Introduction
sively studied both in terms of structure and functional mimics
of the enzyme active site.
Hydrogenase enzymes, which are found in a variety of micro-
organisms and can efficiently catalyze the reduction of protons
to hydrogen, can be classied into [Fe]–H2ases (Hmd H2ases),
[Ni–Fe]–H2ases and [Fe–Fe]–H2ases (H-cluster). Chemists are
interested in studying the biomimetic chemistry of [Fe–Fe]–
H2ases as they have the highest and fastest catalytic capability
for the production of hydrogen among the three classes.1
Moreover, [Fe–Fe]–H2ases are the most amenable to small
molecule model studies because of their resemblance to well
known organometallic complexes of the type (m-SR)2[FeI(CO)2-
L]2.2 To date, the diiron models (m-pdt)[Fe(CO)3]2 (pdt ¼ 1,3-
propanedithiolato), (m-edt)[Fe(CO)3]2 (edt ¼ 1,2-ethanedithio-
lato), (m-adt)[Fe(CO)3]2 (adt ¼ 2-azapropane-1,3-dithiolato),
(m-odt)[Fe(CO)3]2 (odt ¼ oxadithiolate) and (m-tdt)[Fe(CO)3]2
N-Heterocyclic carbenes (NHCs) are electronic, sterically
tunable and stable with a range of transition metal complexes.
NHCs can activate some bonds (C–H, C–C and C–N) at the N
atom functionalities in the presence of transition metals.3
Moreover, iron atoms combined with NHCs are more electron-
rich and more easily accept protons for catalytic H2 production
due to the strong d-electron-donating with negligible
aSchool of Chemical Engineering and Technology, Tianjin Key Laboratory of Applied
Catalysis Science and Technology, Tianjin University, Tianjin 300072, China.
E-mail: tyzhang@tju.edu.cn; libin@tju.edu.cn
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, China
† Electronic supplementary information (ESI) available. CCDC 1035522 (II) and
1035526 (IV), electrochemistry results and GC gures. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra15150j
Fig. 1 The NHCs widely used in diiron model complexes.
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p-electron-accepting ability and great potential of NHCs.3b,4 The
The IR spectra of the ve complexes displayed three or four
powerful ligands used for the design of diiron systems are listed absorption bands in the range 2073–1886 cmꢁ1 (Fig. 2) for their
in Fig. 1.5
terminal carbonyls in CH2Cl2. The bands of II–V were consid-
Herein, we chose (m-dmedt)[Fe(CO)3]2 (dmedt ¼ 2,3-butane- erably shied towards lower energy compared to those of their
dithiol) as the parent complex due to its greater durability upon parent complex I, which is obviously due to their CO ligands
electrochemical cycling.6 Donovan6 synthesized complex I by being substituted by stronger electron-donating NHC ligands.
the reaction of iron dodecacarbonyl with 2,3-butanedithiol. Previous studies state that iron atoms become more electron-
However, we used iron pentacarbonyl reacted with 2,3-butane- rich and more easily accept protons for catalytic H2 produc-
dithiol to yield the same complex I.7 Typical NHC ligands of tion aer the hexacarbonyl parent complex is substituted by
IMe, IMes and IMe(CH2)2IMe which are different to each other in stronger d-donating ligands.5b,15 As a result, stronger back-
structure were chosen to synthesise NHC-containing [Fe–Fe]– bonding from the metal to the CO ligands is formed, then the
H2ase models. The main aims of this study were: (i) to prepare nC]O absorbance bands shi towards lower frequencies. So, we
the rst NHC-containing [(m-dmedt)Fe2(CO)6] models, both estimate that the nC]O bands can be considered as a useful
monosubstituted and disubstituted model complexes were indicator for evaluating the variation in the electron density of
included; (ii) to examine the inuences of different NHC ligands the Fe atoms. However, the electron donating capacity of the
upon the structures and properties of the NHC-containing bidentate dimer IMe(CH2)2IMe which connects to the two iron
complexes.
atoms of the [Fe–Fe] active center is obviously much higher than
Iron complexes containing NHC ligands were rst used as that of the monodentate dimers, IMes or IMe, mainly due to not
homogeneous catalysts in 2000 and have since become highly only the presence of the more electronegative nitrogen atom in
attractive.8 Various organic transformations catalyzed by Fe– the latter two ligands, but also the orientation of the NHC donor
NHC have been achieved, including hydrosilylation reactions,9 carbon as a result of the constraints of the bridging bidentate
aryl Grignard–alkyl halide cross-coupling reactions,10 Kumada- ligands.
type alkyl–alkyl cross-coupling reactions11 and ring-opening
2 equiv. of the imidazolium salt 1 was reacted with 7.2 equiv.
polymerization12 etc. However, only [Fe–Fe]–NHC has been t-BuOK in THF at room temperature followed by treatment of
used for the reduction of protons to hydrogen until now.13 In the resulting mixture with 1 equiv. of the parent complex I to
order to study the catalytic reactivity of [Fe–Fe]–NHC complexes, ensure the complex I completely transformed. The chelated
the direct hydroxylation of benzene to phenol via Scheme 1 by a bidentate NHC substituted model complex II and di-NHC
series of NHC substituted diiron complexes in mild conditions ligand substituted III were afforded in low yields (Scheme 2)
was investigated.
which were isolated aer purication by aluminium oxide
chromatography. However, no complex with a similar congu-
ration to II was produced in the similar studies by Morvan4c and
Song.16 This demonstrates that the bridged dithiolate cofactors
in (m-dmedt)[Fe(CO)3]2, (m-pdt)[Fe(CO)3]2 (pdt ¼ 1,3-propane-
dithiolate) and [(m-SCH2)2(Nbu-t)][Fe(CO)3]2 play a key role in
the production of these two types of model complexes. The IR
spectrum of II is very close to those reported for the bis-cyanide
complex [(m-pdt)Fe2(CN)2(CO)4]2ꢁ (1965, 1924, 1884 cmꢁ1),2b,17
Results and discussion
Synthesis and characterisation of the diiron complexes
The imidazolium salts [IMe(CH2)2IMe]$2HBr (1), IMes$HCl (2)
and IMe$HI (3), which are easily deliquescent white solids, were
prepared with high yields as previously reported.4a,14 The
ethidene-bridged imidazolium salt [IMe(CH2)2IMe]$2HBr was
characterized by H NMR and shows a singlet at 8.84 ppm for
the protons of the two NCHN units in its two bridged imidazole
rings.
The mono- and bi-NHC substituted complexes were synthe-
sized by the facile reaction of the all-carbonyl complex I
(Scheme 2) with the corresponding imidazolium salt and a
strong base (t-BuOK) in THF at room temperature under suffi-
cient stirring and monitored by solution IR spectroscopy. All the
new model complexes could be handled in air and stored in
atmospheric conditions for a couple of weeks, as both the solids
and solutions of these complexes are relatively stable to air and
water and are not hygroscopic.
1
but shied to lower frequencies than those of the disubstituted
IMe complex [(m-pdt)Fe2(CO)4(IMe)2] (1967, 1926, 1888 cmꢁ1
)
5a
and the bis-phosphine complex [(m-pdt)Fe2(CO)4(PMe3)2] (1979,
1940, 1898 cmꢁ1).18 These IR data demonstrate that the biden-
tate IMe(CH2)2IMe ligand exhibits a better electron-donating
ability relative to that of CN, IMe and PMe3. In addition, the
IR spectrum of III displays almost the same pattern in the
carbonyl region at very similar frequencies to the mono-
substituted complexes IV and V, which suggests that III is also a
monosubstituted complex.4c
Although we tried our best, single crystals of complexes III
and V suitable for X-ray detection were not achieved. The
structures of complexes II and IV were conrmed by X-ray
analyses of single crystals obtained from hexane–dichloro-
methane solutions at low temperature, and the selected bond
lengths and angles are given in Tables 1 and 2. The Fe2S2
skeleton of the two complexes shares the well-known buttery
conformation in which each iron center adopts a distorted
square–pyramidal coordination geometry. The two Fe atoms of
complex II (Fig. 3) are bridged by a bi-NHC ligand with a
Scheme 1 Direct hydroxylation of benzene to phenol by [Fe–Fe]–
NHC complexes.
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Scheme 2 Synthetic route of complexes II–V.
Table 2 Selected bond angles (Deg) for complexes II and IV
II
IV
S(1)–Fe(1)–S(2)
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(10)–N(2)
N(1)–C(8)–N(2)
N(3)–C(13)–N(4)
N(2)–C(9)–C(10)
N(3)–C(10)–C(9)
79.36(3)
54.73(2)
54.46(2)
79.62(3)
54.90(19)
54.74(2)
70.54(2)
70.65(2)
—
103.10(2)
103.70(2)
115.80(2)
113.90(3)
78.88(7)
54.97(5)
54.77(5)
78.89(7)
54.98(5)
54.77(5)
70.27(6)
70.25(6)
101.90(5)
—
—
—
—
Fig. 2 The v(CO) region of the IR spectra of complexes I–V (observed
in CH2Cl2 solution).
Table 1 Selected bond lengths (A˚) for complexes II and IV
II
IV
Fe(1)–Fe(2)
Fe(1)–S(1)
Fe(1)–S(2)
Fe(2)–S(1)
Fe(2)–S(2)
Fe(1)–C(1)
Fe(1)–C(8)
Fe(2)–C(5)
Fe(2)–C(10)
Fe(2)–C(13)
2.5952(6)
2.2474(8)
2.2503(8)
2.2471(8)
2.2381(8)
1.7490(3)
1.9910(3)
—
2.5870(12)
2.2450(18)
2.2510(19)
2.2504(17)
2.2452(19)
1.8000(8)
—
1.7540(7)
2.0000(6)
—
—
1.9690(3)
Fig. 3 X-ray crystal structure of complex II (ellipsoids at 30% proba-
bility level). The hydrogen atoms are omitted for clarity.
symmetrically chelated cisoid basal/basal coordination pattern
and adopt a distorted square-pyramidal geometry. The Fe–Fe
˚
˚
{Fe(CO)2NHC} subunits (II, 1.7490(3) A; IV, 1.7540(7) A) are
signicantly shorter than those of the carbonyl of the parent
˚
bond distance in II (2.5952(6) A) is shorter than that of the same
˚
NHC substituted model complex A (Fig. 5) (2.6253(4) A) made by
7
˚
complex I, which are in the range 1.7887(19)–1.8090(2) A. This
indicates an enhanced Fe–CO p-back-donation from the Fe
Morvan,4c but is longer than that of complex IV (Fig. 4)
˚
(2.5870(12) A). The Fe–C bond distances of the carbonyl of the
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Cyclic voltammograms
The electrochemical properties of complexes I–V, with and
without the addition of HOAc (acetic acid), were investigated by
cyclic voltammetry (CV) in the coordinating solvent CH3CN in
the presence of Bu4NPF6 as the supporting electrolyte (Fig. 6).
The redox potentials (referenced to Fc/Fc+ ¼ 0.00 V) of I–V are
given in Table 3 and were assigned according to previous
reports.4c,21,22
The hexacarbonyl parent I has quasi-reversible and irre-
versible reduction events at ca. ꢁ1.73 V and ꢁ2.23 V, respec-
tively. It is obvious that the electrocatalysis occurs at the second
reduction potential which corresponds to an Fe0Fe0 species in
the presence of HOAc. The FeII–Hꢁ species that results from the
oxidative addition of a proton to Fe0Fe0 is set up to accept
another proton, generating an (h2-H2)FeII–Fe0 complex. This
process is accounted for by an EECC (E ¼ Electrochemical, C ¼
Chemical) mechanism.6,23
However, a typical similar characteristic can be observed
from the CVs that all members of the series display a two-
electron reduction event at ca. ꢁ2.57, ꢁ2.14, ꢁ2.18 and ꢁ2.17
V, respectively. These events are shied in a cathodic direction
by ca. 860, 430, 470 and 460 mV for II–V, respectively, compared
to the rst reduction event of I. These shis are consistent with
the increase in electron density at the diiron centers as CO
groups were replaced by the better electron donor of NHC
ligands. Moreover, II appears at the most negative potentials
among the known diiron complexes because the two basal CO
groups of I have been substituted by the better electron donor
ligand IMe(CH2)2IMe. The reduction potentials of III, IV and V
under the CV conditions are very similar to each other, which is
consistent with the similarity in electron density readily
observed in the CO region of the IR spectra (Fig. 2). This
suggests that the introduction of the bidentate NHC ligand
bridged with two Fe atoms exerts a stronger inuence on the
redox potentials of the diiron complexes than the introduction
of a monodentate carbene.
Fig. 4 X-ray crystal structure of complex IV (ellipsoids at 30% prob-
ability level). The hydrogen atoms are omitted for clarity.
Fig. 5 Structure of complex A.
4b,19
center to the Ccarbene
.
The Fe–Ccarbene bond distances are
A quasi-reversible two-electron reduction FeIFeI / Fe0Fe0
couple at ca. ꢁ2.14 and ꢁ2.17 V was observed in the CVs of
complexes III and V, respectively.5b,21 Interestingly, for III and V,
in the presence of HOAc, new reduction peaks were observed at
the more negative peaks ca. ꢁ2.25 and ꢁ2.43 V when detected
under N2, but were not observed when the CV measurement of V
was carried out under CO (Fig. S6†). So we suppose that the
additional reduction events under N2 can be attributed to the
FeIFeI/Fe0Fe0 couple of a coordinated solvent substituted
species ((m-dmedt)[Fe–Fe](NCCH3)) which is likely to result from
a radical chain reaction initiated by the production of the
radical anion process,1b,3a,23c,23d not because the two-electron
reduction process divided into two one-electron reduction
processes. Moreover, the process of CO dissociation from the
labile Fe0Fe0 radical anion was inhibited during the measure-
ment in a CO saturated solution.5a The current heights of the
rst reduction peaks of III and V at ca. ꢁ2.07 and ꢁ2.17 V
showed a slight increase with added increments of HOAc, while
the second new reduction peaks showed a signicant electro-
catalytic response, which is typical of an electrocatalytic
typical of bonds having s character.4b,19 The Fe–Ccarbene bond
˚
˚
lengths of II (1.991(3) A, 1.969(3) A) are shorter than that of
˚
complex IV (2.000(6) A) owing to the relatively large space steric
effect of the IMes ligand substituent.5a
Notably, in the crystal structure of complex IV (Fig. 4), the
plane of the IMes ligand twists to a large extent to reduce steric
interactions with the surrounding CO ligands. The IMes
group in complex IV lies in a basal position in accordance with
(m-dmpdt)[Fe(CO)3][Fe(CO)2IMes] (nCO ¼ 2030, 1971, 1947,
1913 cmꢁ1, in THF) and (m-depdt)[Fe(CO)3][Fe(CO)2IMes] (nCO
¼
2025, 1971, 1946 and 1911 cmꢁ1 in THF),20 but in contrast with
the apical positions of the same ligand in (m-pdt)[Fe(CO)3]
[Fe(CO)2IMes] (nCO ¼ 2035, 2027, 1969, 1947, 1916 cmꢁ1, in
THF)21 and (m-odt)[Fe(CO)3][Fe(CO)2IMes] (nCO ¼ 2039, 1969,
1947, 1924 cmꢁ1, in KBr).16 It is probable that there is a strong
steric repulsion between the IMes and dithiolate cofactor when
the steric bulk is added to the bridgehead carbons of m-SRS. But
this different dithiolate cofactor does not drastically change the
thiolate donor ability according to the IR data.
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Fig. 6 Cyclic voltammograms of the complexes with and without HOAc (2, 4, 6, 8 and 10 equiv.) in CH3CN solution (0.1 M n-Bu4NPF6) under N2
at room temperature: (a) I, 2 mM, scan rate, 200 mV sꢁ1; (b) II, 2 mM, scan rate, 100 mV sꢁ1; (c) III, 1 mM (poor solubility), scan rate, 100 mV sꢁ1; (d)
IV, 2 mM, scan rate, 200 mV sꢁ1; and (e) V, 2 mM, scan rate, 200 mV sꢁ1. All potentials are scaled to Fc/Fc+ ¼ 0.00 V.
process. This suggests that both the Fe0Fe0 and [(m-dmedt)[Fe– electron more readily than the Fe–Fe manifold.16,22b The CV of IV
Fe](NCCH3)] species can combine with protons and be active shows that the two electron process of electrocatalytic H2
toward electrocatalytic H2 production, but the reactivity of the production (one electron at the Fe–Fe center and one electron
latter is stronger than that of the former.
on the IMes ligand) with a quasi-reversible reduction at ca.
The conjunction of the Fe and IMes ligand valence orbital ꢁ2.18 V can be assigned to the FeIFeI(IMes)0 / Fe0FeI(IMes)ꢁ1
permits the uptake of two electrons at the same potential. reduction. In the presence of HOAc, the Fe0 of the [Fe0–
What’s more, in addition to a one-electron Fe–Fe reduction, FeI(IMes)ꢁ1 2ꢁ
species is protonated to afford the Fe–H species
]
DFT shows that the aryl-substituted NHC can accept a second of the monoanion [FeII(H)–FeI(IMes)ꢁ1]ꢁ. A second protonation
Table 3 Redox potentials of complexes I–V
Epc/V
Epc/V
Complex
vs. Fc/Fc+, E: FeIFeI / Fe0Fe0
vs. Fc/Fc+, FeIFeI / FeIIFeI, FeIIFeI / FeIIFeII
I
ꢁ1.73a, ꢁ2.23b
ꢁ2.57
0.93, —
II
III
IV
V
ꢁ0.11, 0.86
0.38, 0.75
0.14, 0.95
0.28, 0.71
ꢁ2.14
ꢁ2.18c
ꢁ2.17
a
E1: FeIFeI / Fe0FeI. b E2: Fe0FeI / Fe0Fe0. c FeIFeI(IMes)0 / Fe0FeI(IMes)ꢁ1
.
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and internal electron transfer from the reduced IMes ligand to phenol was obtained. This means that 1 molecule of catalyst
the iron center result in the release of dihydrogen and regen- produced 25 molecules of product, which provides evidence for
eration of the FeIFeI starting material. On the basis of previous a catalytic process. However, adding more catalyst did not
reports, the electrochemical process of IV is also inferred as an change the yield of phenol obviously, only increased the rate. It
EECC mechanism.4c,16,21
is possible that the reaction is indiscriminate and gives several
In the presence of HOAc, the current heights of the single oxidized products of which phenol is just one (possibly the
cathodic events in the CVs of the ve complexes almost increase other oxidized products have very high retention times and are
linearly with the concentration of HOAc (Fig. S8†). The steeper not detected by GC). Fig. S9† illustrates that under the opti-
slope displayed by the line of current–concentration indicates mized experimental conditions (IV, 0.01 mmol; benzene,
greater sensitivity to acid concentration. That is to say electro- 0.1 mL; CH3CN, 2.0 mL; H2O2, 6.0 mmol; 60 ꢀC; 3 h), the
catalytic reactions occurred.23c,23d
maximal phenol yield was 25.9%.
Complex I and its NHC ligand substituted derivatives were
used as homogeneous catalysts to catalyze the hydroxylation of
benzene with H2O2, and phenol was detected as the main
product in all the reactions (Table 5). Although there were small
differences in the yields of phenol, they are consistent with the
rst oxidation events presumed to be FeIFeI/FeIIFeI for I–V
which are at ca. 0.93 V, ꢁ0.11 V, 0.38 V, 0.14 V and 0.28 V
respectively as shown in the CVs (Fig. 6). From the infrared
spectra (Fig. 2) we can observe the nCO of complexes III, IV and V
are little different due to the similar electron densities of Fe–Fe,
which should be consistent with their almost similar activity for
the hydroxylation of benzene. It is notable that, for complex II,
the bridging ligand [IMe(CH2)2IMe] reduced the exibility of the
diiron complex and hindered the rotation of the subunits which
against H2O2 attack Fe–Fe bond to form the oxygen-transfer
intermediate.24 On the other hand, although the increasing
electron donor effect due to the disubstituted ligands should
enhance its catalytic ability, the effect of the steric hindrance of
the bridging ligands overcomes the electron donor effect.
Therefore, the yield of phenol catalyzed by complex II increased
little.
To our surprise, for the hydroxylation of benzene to phenol
with m-CPBA by FeSO4 no phenol was detected by gas chro-
matography (GC) under the same experimental conditions
(Table 5 and Fig. S12†). Therefore, we inferred that the mech-
anism of hydroxyl radical oxidation for the hydroxylation of
benzene to phenol may be ruled out here. DFT calculations
carried out by Darensbourg24 state that the iron-based oxygen-
ated product FeII-(m-O)-FeII is more stable than the S-oxygenated
product; this indicates that the FeII-(m-O)-FeII isomer is ther-
modynamically favored in the diiron model complexes. Hence,
we infer that the FeII-(m-O)-FeII active intermediate may be more
Catalytic hydroxylation
Our previous study indicated that the Fe–Fe bond in a complex
with an electron-donating ligand could be oxidized more easily
by H2O2 to form an oxygen-transfer intermediate.7 Therefore,
the better electron-donating NHC ligand substituted diiron
model complexes were used as catalysts for the hydroxylation of
benzene.
Hydroxylation of benzene with H2O2 (30%, w/w, in H2O) was
carried out in a 25 mL round bottom ask equipped with a
reux condenser and a magnetic stirrer. All the experiments
were conducted at ambient pressure. Blank controls (without
catalyst or H2O2) were also conducted, and no phenol was
detected.
At rst, different oxidants (O2, H2O2, PhIO, and m-CPBA)
were used as hydroxylation reagents (Table 4). The phenol yield
with H2O2 was more than twice that with m-CPBA under the
same conditions. However, O2 and PhIO were not able to oxidize
benzene to phenol under the given experimental conditions.
Thus, H2O2 was chosen as the oxidant in the following
hydroxylation. Some parameters which would affect the phenol
yield such as reaction time, temperature, amount of catalyst and
H2O2 were also investigated (Fig. S9†). The temperature had a
strong effect on the phenol yield. The reaction rate increased
with temperature increments up to 60 ꢀC, leading to an
increased phenol yield. However, the phenol yield decreased
with further increase of the temperature, which is probably
partly attributable to side reactions (Fig. S11†). Due to the
complexity of the oxidation reactions, it is hard to identify all
the by-products. Especially as some of the by-products could
probably not be detected under the special GC analytical
conditions. In the presence of 0.01 mmol of catalyst and 1 mmol
of benzene – i.e. 100 equivalents of benzene – a yield of 25%
Table 5 Effects of different catalysts and oxidants on the hydroxyl-
ation of benzene to phenola
Table 4 Effects of different oxidants on the hydroxylation of benzene
to phenola
Entry
Catalyst
Catalyst (mmol)
Oxidant
Yield (%)
5
6
7
8
9
10
I
I
II
III
0.05
0.01
0.01
0.01
0.01
0.01
H2O2
H2O2
H2O2
H2O2
H2O2
m-CPBA
7.57
0
26.7
23.4
24.3
0
Entry
Catalyst
Oxidant
Yield (%)
1
2
3
4
IV
IV
IV
IV
H2O2
m-CPBA
PhIO
O2
25.9
12.1
0
V
0
FeSO4
a
a
Catalyst, 0.01 mmol; benzene, 0.1 mL; CH3CN, 2.0 mL; oxidant, 6.0
mmol; 60 ꢀC; 3 h.
Catalyst; benzene, 0.1 mL; CH3CN, 2.0 mL; oxidant 6.0 mmol;
60 ꢀC; 3 h.
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likely to be capable of transferring an oxygen atom to the literature values. Solution IR spectra were recorded on a Shi-
aromatic substrate in the hydroxylation process. Although the madzu FTIR-8400 spectrometer using 0.1 mm KBr sealed cells.
isolation of the FeII-(m-O)-FeII species based on our diiron Elemental analysis was carried out on a Heraeus CHN-Rapid,
complexes has not been achieved to date, the Fe-(m-O)-Fe sites fully automatic elemental analyzer with TCD detection, type:
in Fe/ZSM-35 as the oxygen transfer species in the catalytic TMT CHN, BESTELL-NR 2215001.
oxidation has been identied by C. Li’s group using in situ
resonance Raman spectroscopy.25
A mechanism for the
Preparation of IMe(CH2)2IMe$2HBr (1)
hydroxylation of benzene was proposed based on the theoretical
and experimental results in our previous work,7 which suggests
that the reaction involves an electrophilic addition process
between the FeII-(m-O)-FeII species and the benzene substrate.
Combined with the electrophilic addition process, a further
hydrogen-atom shi occurs, and then the oxygen atom is
transferred to the substrate.
A solution of 1-methylimidazole (2.49 g, 30 mmol) and 1,2-
dibromoethane (2.82 g, 15 mmol) in CH3CN (20 mL) was
reuxed for 72 h under a nitrogen atmosphere. The precipitate
was ltered aer cooling through a double-ended needle, and
washed with THF (3 ꢂ 10 mL). Following drying in vacuo,
product 1 was obtained as a white, hygroscopic powder (4.96 g,
93.9%). The bromide salt must be stored under a nitrogen
1
atmosphere in a dryer due to its easy deliquescence. H NMR
Conclusions
(400 MHz, D2O): d ¼ 3.962 (2CH3–), 4.825 (–CH2CH2–), 7.501
(2CH3NCH]CHN), 7.562 (2CH3NCH]CHN), 8.840 (2NCHN).
Four new monodentate and bidentate NHC-containing iron
complexes (II–V) were synthesized by facile carbonyl replace-
ment on the parent complex I in THF at room temperature. The
Preparation of IMes$HCl (2)
X-ray crystallography results revealed that the monodentate Glyoxylaldehyde (40%, 6.27 g, 43.21 mmol) and 50 mL anhy-
NHC ligand in IV is coordinated to one of the two Fe atoms and drous alcohol were added dropwise over 1 h to an anhydrous
located in a basal position. Whereas, the chelated bidentate alcohol (50 mL) solution of 2,4,6-trimethylaniline (4.63 g, 33.57
ligand IMe(CH2)2IMe in II is connected to the two Fe atoms with a mmol). The mixture was stirred at room temperature for 12 h
symmetrically substituted cisoid basal/basal coordination and then a yellow solid was separated out; the insoluble mate-
pattern. It is worth pointing out that all of the four complexes rial was ltered and then washed with cold alcohol. Aer drying
were found to be catalysts for proton reduction to hydrogen in a vacuum, 3.01 g of the yellow solid of N,N0-(ethane-1,2-
under electrochemical conditions and the results conrmed diylidene) bis(2,4,6-trimeth-ylaniline) (imine) in 61.5% yield
that the more d-donating of NHC, the easier the reduction. The was achieved.
reduction potential of the novel model complex II at ca. ꢁ2.57 V
In a ask, the above imine (1.50 g, 5.13 mmol) was dissolved
is more negative than the other diiron complexes we have in THF (12.5 mL), followed by the dropwise addition of chlor-
known. The electrocatalytic mechanism for the proton reduc- omethyl ethyl ether (0.612 g, 6.15 mmol). Then, the mixture was
ꢀ
tion to H2 by [Fe–Fe]–NHC will be investigated in our further stirred under N2 at 40 C for 18 h. The precipitate was ltered
studies. In addition, the abilities of these complexes to act as aer cooling, and washed with ethyl ether (2 ꢂ 20 mL). The
homogeneous catalysts for the hydroxylation of benzene to residue was dissolved in as little anhydrous alcohol as possible,
phenol (with a maximum phenol yield of 26.7%) were correlated and then ethyl ether (25 mL) was added to the extract. The above
with the initial potentials of their oxidation events.
operation was repeated until a white solid was obtained. The
white solid was dried under vacuum to afford salt 2 (0.95 g,
54.1%).
Experimental section
General comments
Preparation of IMe$HI (3)
Some reactions and operations were carried out using standard
Schlenk and vacuum-line techniques under a nitrogen atmo-
sphere. All solvents and reagents were purchased from Guangfu
Chemical and Sigma-Aldrich, respectively. Fe(CO)5 was
obtained as a gi from Jiangsu Tianyi Ultra-ne Metal Powder
Co., Ltd (China). Solvents were dried and distilled prior to use
according to the standard methods. The puried 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
purication. THF and hexane were distilled from sodium/
benzophenone. Acetonitrile was distilled from NaH. All the
solvents were deoxygenated before every use.
Methylimidazole (10.2 g, 0.125 mol) and iodomethane (17.8 g,
0.125 mol) were dissolved in 100 mL toluene and stirred at
reux for 12 h under an nitrogen atmosphere. Aer cooling to
room temperature toluene was ltered off through a double-
ended needle then a white solid was formed. The residue was
dissolved in as little anhydrous alcohol as possible, and then
ethyl ether (50 mL) was added to the extract. The above opera-
tion was repeated until a white solid was obtained. The white
solid was dried in vacuo to yield 27.2 g (97.1%) of 3. The same as
IMe(CH2)2IMe$2HBr, IMe$HI must be stored under a nitrogen
atmosphere in a dryer.
Preparation of (m-dmedt)Fe2(CO)6 (I)
The NMR spectra were measured on a Bruker AVANCE III 400
MHz NMR spectrometer. 1H NMR and 13C NMR shis were Fe(CO)5 (13.4 g, 68.6 mmol) and 2,3-butanedithiol (2.77 g,
referenced to residual solvent resonances according to 22.7 mmol) were added to 55 mL toluene and stirred at reux for
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29 h under a nitrogen atmosphere. Then, the solvent was monitored by IR spectroscopy to conrm that the reaction had
distilled under 140 ꢀC, and the residue was dissolved in a little reached completion. The solvent was then removed, and the red
toluene and loaded onto a silica gel column. The product was residue was dissolved in CH2Cl2 and loaded onto a silica gel
then eluted with hexane. The solvent was evaporated from the column. Elution with petroleum ether removed the excess
dark red eluant to yield I as a red powder (3.439 g, 37.9%). IR starting material I. The product was then eluted with CH2Cl2.
(CH2Cl2) nCO ¼ 2073, 2032, 2000, 1990 cmꢁ1. 1H NMR (400 MHz, The solvent was evaporated from the dark red eluant to yield
CDCl3) d ¼ 2.24(s, CH]CH), 1.36(m, CH3C–CCH3). 13C NMR product IV (140 mg, 89%). IR (CH2Cl2) nCO ¼ 2036, 2028, 1971,
(400 MHz, CDCl3) d ¼ 208.63 (CO); 55.21 and 49.37 (SCH(CH3) 1909 cmꢁ1. 1H NMR (400 MHz, acetone-d6) d ¼ 7.46 and 7.36 (s,
CH(CH3)CHS); 20.61 and 19.14 (SCH(CH3)CH(CH3)CHS). NCH]CHN); 7.08 and 7.06 (d, ArH); 2.35 (s, p-ArCH3); 2.21–2.16
Elemental analysis: calc. for C10H8O6Fe2S2: C, 30.03; H, 2.02. (m, o-ArCH3); 1.79 (m, SCH(CH3)CH(CH3)S); 1.04–0.92 (m,
Found: C, 30.11; H, 2.01.
SCH(CH3)CH(CH3)S). 13C NMR (400 MHz, acetone-d6) d ¼
216.02, 212.43 and 211.88 (CO); 187.48 (Ccarbene); 138.99, 137.93,
136.06 (C6H2); 128.94 and 125.65 (NCH]CHN); 53.58 and 47.65
(SCH(CH3)CH(CH3)CHS); 20.23, 19.89, 19.43, 18.42, 18.14, 18.03
and 17.83 (CH3). Elemental analysis: calc. for C30H32O5N2Fe2S2:
C, 53.27; H, 4.78; N, 4.14. Found: C, 53.13; H, 4.79; N, 4.15.
Preparation of (m-dmedt)Fe2(CO)4(IMe(CH2)2IMe) (II) and
[(m-dmedt)Fe2(CO)5]2 (IMe(CH2)2IMe) (III)
IMe(CH2)2IMe$2HBr (160 mg, 0.46 mmol) was dried in vacuo
under N2 for 30 min in a 50 mL Schenk ask, then t-BuOK (184
mg, 1.64 mmol) and THF (7 mL) were added to the mixture aer
dried in vacuo under N2 for 20 min. Aer 3 h of vigorous stirring
at room temperature, a THF (7 mL) solution of I (77 mg, 0.23
mmol) was transferred to the reaction mixture over 35 min by a
double-ended needle at room temperature. The resulting
mixture was stirred at room temperature for 1.25 h, and
monitored by IR spectroscopy to conrm that the reaction had
reached completion. The solution was evaporated to dryness in
vacuo. The resulting red solid was dissolved in a minimum
amount of CH2Cl2 and applied to an aluminium oxide column.
The rst red band was eluted with a CH2Cl2–hexane (1/2)
mixture. Slow evaporation of the solvent gave III as an orange
powder (26 mg, 29.0%). A second red band eluted with CH2Cl2–
hexane (1/1) gave complex II as a red powder (11 mg, 12.4%)
aer slow evaporation of the solvents.
Preparation of (m-dmedt)Fe2(CO)5(IMe) (V)
Complex V was prepared by a procedure similar to that of IV
from IMe$HI (585 mg, 2.61 mmol), I (1.00 g, 2.50 mmol) and t-
BuOK (569 mg, 5.07 mmol). The product was obtained as an
orange solid: yield 1.01 g, 86.1%. IR (CH2Cl2) nCO ¼ 2038, 2032,
1971, 1911 cmꢁ1
.
1H NMR (400 MHz, CD3CN) d ¼ 7.14 (s,
NCH]CHN); 3.94 (s, NCH3); 2.03 (s, SCH(CH3)CH(CH3)S), 1.23
(m, broad, SCH(CH3)CH(CH3)S), 0.88 (s, broad, SCH(CH3)
CH(CH3)S). 13C NMR (400 MHz, CD3CN) d ¼ 217.40 and 212.08
(CO); 163.52 (Ccarbene); 124.07 and 120.01 (NCH]CHN); 55.11
(NCH3); 39.16 (SCH(CH3)CH(CH3)CHS); 20.07 (CH3). Elemental
analysis: calc. for C14H16O5N2Fe2S2: C, 35.92; H, 3.45; N, 5.98.
Found: C, 36.02; H, 3.46; N, 5.96.
Complex II. IR (CH2Cl2) nCO ¼ 1961, 1924, 1886 cmꢁ1
.
1H
X-ray crystallography
NMR (400 MHz, acetone-d6) d ¼ 7.11–6.98 (m, 2 NCH]CHN),
5.85–5.43 (m, NCH2CH2N), 4.29 (m, NCH2CH2N), 3.95 (d, 2
NCH3), 2.29 (m, SCH(CH3)CH(CH3)S), 1.94 (m, SCH(CH3)
CH(CH3)S), 1.47 (d, SCH(CH3)CH(CH3)S), 1.26 (d, SCH(CH3)
CH(CH3)S). 13C NMR (400 MHz, acetone-d6) d ¼ 222.32, 221.61
and 216.47 (CO); 188.95 (Ccarbene); 123.76, 123.40, 122.98 and
122.06 (NCH]CHN); 54.90 and 54.57 (NCH2CH2N); 47.10 and
46,49 (NCH3); 39.09 and 38.85 (SCH(CH3)CH(CH3)CHS); 20.70
and 20.35 (SCH(CH3)CH(CH3)CHS). Elemental analysis: calc.
for C18H22O4N2Fe2S2: C, 40.47; H, 4.15; N, 10.49. Found: C,
40.34; H, 4.14; N, 10.52.
The single-crystal X-ray diffraction data were collected with a
Rigaku MM-007 diffractometer equipped with a Saturn 724CCD.
Data were collected at 293 K or 113 K using a confocal mono-
˚
chromator with Mo-Ka radiation (l ¼ 0.71073 A). Data collec-
tion, reduction and absorption correction were performed
using the CRYSTALCLEAR program.26 The structures were
solved by direct methods using the SHELXS-97 program27 and
rened by the full-matrix least-squares technique (SHELXL-97)28
on F2. Hydrogen atoms were located by geometrical calculation.
Details of crystal data, data collection and structure renements
are summarized in Table S1 and Fig. S1–S3.†
Complex III. IR (CH2Cl2) nCO ¼ 2040, 2034, 1973, 1905 cmꢁ1
.
1H NMR (400 MHz, CD2Cl2) d ¼ 7.11 (m, 2 NCH]CHN), 6.24 (m,
Cyclic voltammograms
NCH2CH2N), 4.05 (d, 2 NCH3), 2.06 (m, SCH(CH3)CH(CH3)S),
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 and 1 mm diamond pastes 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
0.92 (d, SCH(CH3)CH(CH3)S). Elemental analysis: calc. for C28
-
H30O10N4Fe4S4: C, 36.00; H, 3.24; N, 6.00. Found: C, 36.12; H,
3.23; N, 6.01.
Preparation of (m-dmedt)Fe2(CO)5(IMes) (IV)
Solid I (93 mg, 0.23 mmol) and salt 2 (198 mg, 0.58 mmol) were n-Bu4NPF6) electrode and the counter electrode was platinum
combined, dried in vacuo for 30 min and then dissolved in THF wire. A solution of 0.1 M n-Bu4NPF6 in CH3CN was used as the
(4.5 mL). Aer stirring for another 30 min, previously dried t- supporting electrolyte, which was degassed by bubbling with
BuOK (130 mg, 1.16 mmol) in 3.5 mL of THF was added, and the dry N2 for 10 min before the measurements. Ferrocene was used
resulting mixture was stirred for an additional 30 min, and as an internal standard under the same measuring conditions
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and all potentials were referenced to the Cp2Fe+/0 couple at 0 V.
During the electrocatalytic experiments under N2, increments of
glacial HOAc (chromatographic grade, S99.8%, water 0.15%,
by the Karl Fischer method) were added by microsyringe.
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carried out in a 25 mL round bottom ask equipped with a
reux condenser and a magnetic stirrer. In a typical reaction, IV
(7 mg, 0.01 mmol) was dissolved in CH3CN (2.0 mL). Aer the
mixture was heated to the desired temperature, benzene (0.1
mL, 1.1 mmol) was added to the mixture. Lastly, a certain
amount of H2O2 was added to start the reaction and the mixture
was stirred for several hours. All the experiments were con-
ducted at ambient pressure. Measurements of GC (9890B)
equipped with a FID detector and a capillary column (OV-1701;
30 m ꢂ 0.25 mm ꢂ 0.25 mm) were performed to analyze the
product mixture. The internal standard method was used for
quantitative analysis and chlorobenzene was chosen as the
internal standard substance. TPD analysis was carried out, from
80 ꢀC to 200 ꢀC at a ramp of 10 ꢀC, then from 200 ꢀC to 280 ꢀC at
7 X. Wang, T. Y. Zhang, Q. S. Yang, S. Jiang and B. Li, Eur. J.
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ꢀ
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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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 (13JCQNJC05800) for nancial support of this work.
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