Weakly distorted 8-quinolinolato iron(III) complexes as effective catalysts for oxygenation of organic compounds by hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2015.10.017

This paper first discloses that two heteroleptic 8-quinolinolato iron(III) complexes (Qa₁Qb₂Fe^III, Qa₂Qb₁Fe^III) could be synthesized conveniently via the coordination of FeCl₂·6H₂O with 2 equivalents of 5,7-dichloro-8-hydroxyquinoline (Qb) or 5-chloro-8-hydroxyquinoline (Qa) under N₂ and then 1 equivalent of Qa or Qb under air. In comparison with the two homoleptic counterparts (Qa₃Fe^III and Qb₃Fe^III), the proposed heteroleptic Q₃Fe^III complexes possessed similar coordination features to the Qb₃Fe^III one but showed similar catalysis performances to the Qa₃Fe^III one in the oxygenation of cyclohexane to cyclohexanol and cyclohexanone by hydrogen peroxide (H₂O₂) in acetonitrile. More importantly, both heteroleptic Q₃Fe^III complexes showed a better accelerating effect on this reaction and provided a slightly higher conversion than the Qa₃Fe^III and especially Qb₃Fe^III ones. Furthermore, this predominance in catalytic activity was more strikingly apparent upon both-catalyzed oxygenations of benzene, toluene, ethylbenzene or thioanisole by H₂O₂. This should be due to a structurally distorted effect of the heteroleptic Q₃Fe^III complexes that is induced by the different in ligand environment, as supported by DFT B3LYP/6-311G (d) calculation. Based the present reaction and UV-vis spectral characterization results, a free radical mechanism for the present catalysis system was proposed.

Full text of DOI:10.1016/j.molcata.2015.10.017

Accepted Manuscript
Title: Weakly distorted 8-quinolinolato iron(III) complexes as
effective catalysts for oxygenation of organic compounds by
hydrogen peroxide
Author: Yongjun Wang Xu Wen Chunying Rong Senpei Tang
Wenfeng Wu Chao Zhang Yachun Liu Zaihui Fu
PII:
S1381-1169(15)30118-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.10.017
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5-10-2015
12-10-2015
Please cite this article as: Yongjun Wang, Xu Wen, Chunying Rong, Senpei
Tang, Wenfeng Wu, Chao Zhang, Yachun Liu, Zaihui Fu, Weakly distorted 8-
quinolinolato iron(III) complexes as effective catalysts for oxygenation of organic
compounds by hydrogen peroxide, Journal of Molecular Catalysis A: Chemical
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Weakly distorted 8-quinolinolato iron(III) complexes as effective
catalysts for oxygenation of organic compounds by hydrogen
peroxide
Yongjun Wanga,b, Xu Wena, Chunying Ronga, Senpei Tanga, Wenfeng Wua, Chao Zhanga,
Yachun Liu a *, Zaihui Fu a *
aNational & local united engineering laboratory for new petrochemical materials &
fine utilization of resources, Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education of China) and Key Laboratory of
Resource Fine-Processing and advanced materials of Hunan Province, College of
Chemistry and Chemical Engineering, Hunan Normal University, Changsha 410081,
China
bHunan Changling Petrochemical S&T Developing Co.,LTD, Yueyang 414011,
China
Corresponding Author
*E-mail: yacl315@163.com. Telephone: +86 731 88872576. Fax: +86 731 88872531.
* E-mail: fzhhnnu@126.com. Telephone: +86 731 88872576. Fax: +86 731 88872531.
1

Graphical abstract
Two heteroleptic 8-quinolinolato iron (III) (Qa1Qb2FeIII, Qa2Qb1FeIII ) complexes designed by us
were found to show a higher activity for the selective oxidation of organic compounds by
hydrogen peroxide compared to the corresponding homoleptic ones, which should be due to their
weakly distorted effect.
2

Highlights


Two heteroleptic 8-quinolinolato iron(III) complexes were synthesized.
The heteroleptic complexes showed a superiority for the catalyzed-oxygenation of
hydrocarbons.


This predominance was due to a structural distortion effect of both.
A free radical mechanism for the catalytic system was proposed.
3

Abstract
This paper first discloses that two heteroleptic 8-quinolinolato iron(III) complexes (Qa1Qb2FeIII,
Qa2Qb1FeIII) could be synthesized conveniently via the coordination of FeCl2·6H2O with 2
equivalents of 5,7-dichloro-8-hydroxyquinoline (Qb) or 5-chloro-8-hydroxyquinoline (Qa) under
N2 and then 1 equivalent of Qa or Qb under air. In comparison with the two homoleptic
counterparts (Qa3FeIII and Qb3FeIII), the proposed heteroleptic Q3FeIII complexes possessed similar
coordination features to the Qb3FeIII one but showed similar catalysis performances to the Qa3FeIII
one in the oxygenation of cyclohexane to cyclohexanol and cyclohexanone by hydrogen peroxide
(H2O2) in acetonitrile. More importantly, both heteroleptic Q3FeIII complexes showed a better
accelerating effect on this reaction and provided a slightly higher conversion than the Qa3FeIII and
especially Qb3FeIII ones. Furthermore, this predominance in catalytic activity was more strikingly
apparent upon both-catalyzed oxygenations of benzene, toluene, ethylbenzene or thioanisole by
H2O2. This should be due to a structurally distorted effect of the heteroleptic Q3FeIII complexes the
is induced by the different in ligand environment, as supported by DFT B3LYP/6-311G (d)
calculation. Based the present reaction and UV-Vis spectral characterization results, a free radical
mechanism for the present catalysis system was proposed.
Keywords: Heteroleptic 8-quinolinolato iron (III) complexes; Distorted effect; Catalytic
oxygenation; Hydrogen peroxide; Organic compounds
4

1. Introduction
The selective oxygenation of inactivated hydrocarbons by hydrogen peroxide (H2O2) and
especially dioxygen (O2)/ or air constitutes an extremely important field of contemporary
chemistry [1-5] due to the large demand for their oxidized products. And yet the well-known
inertness of such hydrocarbons makes their transformation be a challenge from a basic science
viewpoint. For example, the current industrial process for producing cyclohexanone and
cyclohexanol (i.e. KA oil), important raw materials for the production of caprolactam and adipic
acid, mainly relies on the cobalt-catalyzed oxidation of cyclohexane with air under harsh
conditions (temperatures of 443~503 K and pressures of 10~15 bar), which should suppress
cyclohexane conversion (lower than 10%) to obtain a relatively high selectivity for KA oil
(ca.70%) [4, 6-9].
In recent decades, the oxygenation of hydrocarbons through the action of metal complexes
seems to be a very promising field, particularly when the complexes act as catalysts. This is
mainly because the oxygenation of hydrocarbons by O2 and especially H2O2 with metal complexes
as catalysts occurs at relatively low temperatures and can be selective [9-24]. Among the transition
metal complexes chiefly used as catalysts, metallo-porphyrin complexes are currently of
considerable interest to promote these oxygenation reactions [10-14, 25-29]. The relatively high
cost of this type of complexes, however, still hampers their commercialized applications.
Recently, our groups have reported that homoleptic 8-quinolinolato manganese(III) complexes
(Q3MnIII) combined with H2O2, constitute a valuable new family of heterogeneous systems for the
epoxidation of olefins [30], the selective oxygenation of alcohols [31], thioanisole [32] and
ethylbenzene [33]. This should be due to the distorted octahedral structure of these homoleptic
Q3MnIII complexes [30-32]. But more cost-effective homoleptic 8-quinolinolato iron(III)
5

complexes (Q3FeIII) show a poor activity for the epoxidation of olefins by H2O2 due to their
symmetrically octahedral geometry [30]. More recently, our groups reported that visible light can
obviously accelerate the homoleptic Q3FeIII complexes-catalyzed oxygenation of cyclohexane by
H2O2, which may originate from that the excited Q3FeIII complexes react with H2O2 more easily
than the ground-state counterparts [34]. It is interesting to explore the preparation of heteroleptic
Q3FeIII complexes with a distorted octahedral geometry and their potential advantages in catalysis
oxygenations. To achieve this goal, for the first time the two heteroleptic Q3FeIII complexes were
designed and synthesized via the centered iron ions successively coordinated 8-hydroxyquinoline
ligands with different substituents and both catalytic performances were checked and compared
with those of the homoleptic counterparts using the selective oxygenation of benzene, toluene,
ethylbenzene, thioanisole and especially cyclohexane by H2O2.
2. Experiment Section
2.1. Materials and apparatus
Materials and reagents used in this study were cyclohexane, benzene, toluene, ethylbenzene
thioanisole, n-hexanol，acetonitrile (MeCN), anhydrous tetrahydrofuran (THF), ethanol, 30%
aqueous hydrogen peroxide (H2O2), iron (III) trichloride (FeCl3·6H2O), iron (II) dichloride
(FeCl2· 4H2O), 5-chloro-8-hydroxyquinoline (Qa), 5, 7-dichloro-8-hydroxyquinoline (Qb), all of
which were of analytical grade. Distilled water was used throughout this experiment.
Transmission FT-IR and UV-Vis diffuse reflectance spectra of samples were recorded from
400 to 4000 cm-1 on a Nicolet Nexus 510 P FT-IR spectroscopy as KBr disc and from 220 to 1000
nm on a DUV-370D spectrophotometer with BaSO4 as a standard (Shimadzu, Japan), respectively.
6

Their liquid UV-Vis spectra in MeCN were recorded from 200 to 800 nm on UV-2450
spectrophotometer (Shimadzu, Japan). X-ray powder diffraction (XRD) analysis of the samples
was carried out on a X’PERT3 powder diffractometer apparatus using Cu Kα radiation (λ=0.15406
nm) and a graphite monochromator, operated at a voltage of 40 kV and a current of 40 mA.
Diffraction patterns were recorded in the angular range of 5-50˚ with scanning rate 0.05˚/s.
2.2. Preparation of 8-quinolinolato iron (III) complexes
The preparation procedure for two homoleptic 8-quinolinolato iron(III) complexes (Qa3FeIII and
Qb3FeIII ) could be found in our recent publication [34].The preparation procedure for two
heteroleptic Q3FeIII complexes is described as follows: the ligand Qb or Qa (4 mmol) was
dissolved in 20 ml of anhydrous THF to obtain a solution, designated as “A”, and FeCl2· 4H2O (2
mmol) was dissolved in 20 ml of THF to form the solution “B”, to which an excess amount of iron
powder was added to prevent oxidation of ferrous ions to ferric ions. The solution “A” was added
dropwise to the solution "B" under N2 and then the reaction mixture was stirred at room
temperature until the ligand Qb or Qa was consumed completely. After the iron powder was
removed with a magnetic separation method, another lignad Qa or Qb (2 mmol) in anhydrous THF
(10 mL) was added dropwise to the above reaction mixture under air, and then the reaction
mixture was stirred at room temperature until the ligand Qa or Qb was consumed completely.
Finally, the resulting precipitate was filtrated and washed with ethanol for three times. After
air-drying, a black solid (denoted as Qa1Qb2FeIII or Qa2Qb1FeIII) was obtained in 90-95% yield.
The Fe contents of four 8-quinoliolato FeIII complexes were determined by conventional EDTA
titration and the results are listed in Table 1. Herein, the found values for Qa3FeIII and Qb3FeIII
7

were in good agreement with those calculated from the corresponding formulae of the hexadentate
structures. However, the found value for Qa1Qb2FeIII or Qa2Qb1FeIII was obviously higher than it’s
calculated one, this is likely because a small amount of iron powder or tetradentate Qa2FeII
/Qb2FeII complexes still remains in the heteroleptic Q3FeIII complex with a hexadentate structure.
2.3. General procedure for oxygenation of organic compounds
30 % Aqueous H2O2 solution (0.5-4 mmol) was added to a stirred mixture of acetonitrile
(MeCN, 3mL), substrate (1 mmol) and catalyst Q3FeIII (0.01 mmol) and then the reaction mixture
o
was stirred magnetically at 12-35 C. After the desired reaction time had elapsed, the addition of
MnO2 decomposed the residual H2O2 and the solid residue was filtrated from the reaction solution,
the resulting filtrate was subjected to an analysis of the oxygenated products. The content of
various oxygenated products was measured on an Agilent 6890N gas chromatography with a
DB-17 polysiloxane capillary column (30 m × 0.32 mm × 0.50 µm) and flame ionization detector
(FID) using n-hexanol as an internal standard. Both the injector and detector temperature were
250℃, and the column temperature was 80-120℃. Each experiment was conducted in triplicate,
the data deviation between three parallel experiments was lower than ca. 5%.
3. Results and discussion
3.1. Characterization of Q3FeIII complexes
The structure of Q3FeIII complexes was investigated by means of the density functional theory
method (DFT B3LYP) with 6-311G(d) basis set and all the calculations were conducted on the
Ganssinan-03 version C02 package with tight self-consistent field (SCF) convergence and
ultrafine integration grids. Table 1 lists the Fe-O bond distances and the highest occupied and
8

lowest unoccupied molecular orbital (HOMO and LUMO) energies for the Q3FeIII complexes. It is
seen from Table 1 that the three Fe–O bonds of Qa3FeIII or Qb3FeIII complex were exactly equal to
each other in length, indicating that the two homoleptic Q3FeIII complexes possess a symmetrically
octahedral geometry. On the contrary, the three Fe–O bonds of Qa1Qb2FeIII or Qa2Qb1FeIII
complex were not equal to each other in length and there was one Fe–O bond to be the longest
among them, indicating that both the heteroleptic Q3FeIII complexes have a distorted octahedral
geometry. In comparison with the differences among the Mn-O bonds in the Q3MnIII complexes
previously reported by us [31, 32], both heteroleptic Q3FeIII complexes exhibited much smaller
differences among their Fe-O bonds (see  Fe-O value listed in Table 1), illustrating that such two
complexes belong to a weakly distorted octahedral geometry. The HOMO or LUMO energy of
these Q3FeIII complexes followed an ascending order: Qa3FeIII> Qa2Qb1FeIII>Qa1Qb2FeIII>Qb3FeIII,
implying that the reactivity of Q3FeIII complexes with H2O2 should be the same sequence with
their HOMO or LUMO energy.
Fig. 1 shows the FT-IR spectra of four Q3FeIII complexes and the corresponding ligands. It is
seen from Fig. 1 that the FT-IR spectra of ligands (Qa and Qb) were similar to each other,
consisting of one O-H stretching at 3192 cm-1, three C=C stretching bands near 1400-1510 cm-1,
one C=N stretching band at 1589 cm-1 and one C-O stretching band near 1045-1094 cm-1 [35-37].
Such characteristic IR bands in the corresponding Q3FeIII complexes all took place different
degree of shift due to the coordinating effect. Moreover, disappearance of the fundamental
stretching frequencies characteristic of OH group in the complexes suggests the occurrence of
deprotonation process as the ligand reacts with iron ion [38].
9

Fig. 2 provides the UV-Vis diffuse reflectance spectra (DRS) of these samples in the 220-800
nm regions. The two ligands displayed a higher energy band near 328 nm, which should originate
from the * transitions of ligands [39]. The Q3FeIII complexes exhibited two lower energy
bands in the 450-640 nm region in addition to the * band assigned to the ligands. In that, one
band in the region of 450-490 nm should be assigned to the ligand to metal charge-transfer
transition (LMCT), as observed in an UV-Vis spectrum of the salen Fe(III) complex [40]. Another
band in the region of 570-640 nm is responsible for the intense colors of the complex [41].
Notably, the two low energy bands for the Qb3FeIII was obviously stronger and shifted toward the
longer wavelength region compared to both bands for the Qa3FeIII, illustrating that the Qb3FeIII
generates the charge-transfer transition of ligand to FeIII ion more easily than the Qa3FeIII. The
complexes Qa1Qb2FeIII and Qa2Qb1FeIII displayed very similar UV-Vis spectral features to the
Qb3FeIII, implying that the ligand Qb plays a dominant role in the coordination features of both
heteroleptic Q3FeIII complexes.
Crystal phase of these samples was measured by X-ray diffraction (XRD) techniques. As
shown in Fig. S1, the homoleptic Qa3FeIII complex exhibited some strong and sharp XRD peaks in
the angular range of 8-30˚ in its XRD pattern, indicating that this complex has a good crystallinity.
On the contrary, the XRD pattern of another homoleptic Qb3FeIII complex only showed several
very weak and broad XRD peaks in 5-30˚, illustrating that this complex has a poor crystalline
quality. Notably, the Qa2Qb1FeIII and especially Qa1Qb2FeIII complexes exhibited very similar
XRD peaks to the Qb3FeIII, indicating that the crystalline behavior of both heteroleptic Q3FeIII
complexes is mainly dominated by the ligand Qb, which is consistent with the results from
UV-Vis characterization. We have tried our best to improve the crystalline quality of heteroleptic
10

Q3FeIII complexes, but did not succeed. Therefore, the crystallographic structure information for
these complexes can not be given at present.
3.2. Catalytic oxygenation
Table 2 lists the data for the Q3FeIII complexes-catalyzed oxygenation of cyclohexane by
o
aqueous hydrogen peroxide (H2O2) in acetonitrile (MeCN) at 12 C. As predicted above, the
Qa3FeIII complex presented a good activity for cyclohexane oxygenation, achieving ca. 13.0 %
cyclohexane conversion at 8 h with cyclohexanol and cyclohexanone as mainly oxygenated
products (Entry 1). But the Qb3FeIII complex showed a poor activity for this oxygenation under the
same conditions (Entry 2). Interestingly, the Qa1Qb2FeIII and Qa2Qb1FeIII complexes possessed a
better activity for this oxygenation compared to the Qa3FeIII and especially Qb3FeIII complexes,
respectively providing ca. 19.8 and 20.3 % conversion under the same conditions (Entries 3 and 4).
If a mechanical mixture consisting of Qa3FeIII and Qb3FeIII with 1:2 molar ratio (its composition
was similar to that of Qa1Qb2FeIII) was used to catalyze this reaction, the conversion is ca. 8.6%
under the same conditions (entry 5). These findings support that the catalytic activity of
homoleptic Q3FeIII complex is directly proportional to its HOMO or LUMO energy and the
outstanding catalytic activity of heteroleptic complex is likely due to its distorted octahedral
geometry that is induced by the difference in the coordination capacity of ligands Qa and Qb to
the centered FeIII ions, as supported by the foregoing DFT calculation results.
In the following experiments, the effect of temperature on the oxygenation of cyclohexane by
H2O2 was checked using Qa1Qb2FeIII as a catalyst and the results obtained are listed in table 2. As
expected, the accelerated effect of temperature on this reaction was evident. Cyclohexane
11

conversion gradually increased from 7.8 to 22.0 % with increasing temperature from 0 to 25 °C
(Entries 6, 3 and 7). when the temperature was further increased to 35oC, the catalytic reaction
was accelerated drastically and the time to achieve ca. 23.7 % conversion was shortened to 2 h
(see entry 8).
The effect of reaction time on the oxygenation of cyclohexane by H2O2 was examined using the
Qa3FeIII and Qa1Qb2FeIII as catalysts, respectively. As shown in Fig. 3, when reaction time was
The effect of reaction time on cyclohexane oxidation was examined using the Qa3FeIII and
Qa1Qb2FeIII as catalysts, respectively. As shown in Fig. 3, when reaction time was between 2 and
12 h, the conversion of cyclohexane over Qa3FeIII increased slowly with the time, and then, it
ascended rapidly in 12-24 h and finally achieved ca. 23.2 % at 24 h. After that, the conversion
hardly increased with the time (Curve 1) likely due to the oxidative degradation of Qa3FeIII, which
can be confirmed from the facts that the dissolved and decolorized phenomena of Qa3FeIII were
observed clearly during the period of 24-33 h. The selectivity for cyclohexanol continuously and
slowly descended with the time (Curve 3), with a concomitant increase in cyclohexanone
selectivity (Curve 6). The effect of reaction time on the Qa1Qb2FeIII-catalyzed this reaction was
very similar to the Qa3FeIII catalytic system described above, but its conversion went up faster
than that of Qa3FeIII in 2-12 h and achieved ca.22.2 % at 12 h (Curves 2 vs 1 in Fig.3). When the
time was further prolonged to 33 h, the conversion was ca. 25.6 %. These findings further indicate
that the heteroleptic Qa1Qb2FeIII complex has a faster catalysis rate and achieves a slightly higher
conversion for this reaction than the homoleptic Qa3FeIII one.
12

The effects of the amount of catalyst, oxidant and substrate on cyclohexane oxygenation were
checked at 12oC using Qa1Qb2FeIII as a catalyst and the results are presented in Fig. S2, S3 and S4,
respectively. As shown in Fig. S2, S3 and S4, the increasing catalyst or oxidant amount or the
decreasing substrate amount was conducive to improve cyclohexane conversion. For example,
when the amount of catalyst was raised from 0.025 to 0.01 mmol, the conversion increased 3.5-
fold. When the amount of cyclohexane was reduced 8-fold (from 4.0 to 0.5 mmol), the conversion
increased by 23 %. Also, the distribution of oxygenated products was influenced by the variables
above-described. The selectivity for cyclohexanol continuously decreased with increasing the
amount of catalyst, oxidant or substrate, along with an increase in cyclohexanone selectivity. The
optimized amounts of catalyst, substrate and oxidant were 0.01, 1.0 and 4.0 mmol, respectively.
Finally, the Q3FeIII complexes-catalyzed oxygenation of other substrates, such as benzene,
o
toluene, ethylbenzene and thioanisole by H2O2, was examined in MeCN at 18 C. As shown in
Table 3. the catalyst Qa3FeIII was still active for the selective oxidation of benzene, toluene and
especially ethylbenzene,, respectively providing phenol for benzene, benzaldehyde for toluene and
acetophenone for ethylbenzene as their main oxygenated products (Entries 1, 5 and 9). The
reactivity of three substrates over Qa3FeIII complex followed an increasing sequence of
ethylbenzene>benzene>toluene. And yet the Qb3FeIII did not show any activity for these
oxygenations (Entries 2, 6 and 10). It is noteworthy that the catalytic activity of both heteroleptic
Q3FeIII complexes for these oxygenations was obviously superior to that of the Qa3FeIII complex at
a predetermined reaction time 8h (Entries 3 and 4 vs 1, 7 and 8 vs 5, 11 and 12 vs 9). For example,
the conversions of benzene, toluene and ethylbenzene over the Qa1Qb2FeIII complex were ca. 10.1,
9.7 and 24.3 %, being 2-, 3- and 1.0-fold higher than those over the Qa3FeIII complex, respectively.
13

Entries 13-16 illustrate that thioanisole oxygenation easily proceeded upon catalysis with these
Q3FeIII complexes, providing methyl phenyl sulfoxide as a main oxidation product. Thioanisole
conversion over both heteroleptic Q3FeIII complexes was ca. 100 %, being higher than those over
the Qa3FeIII (95.5 %) and especially Qb3FeIII (23.5 %). This further supports that the distorted
effect of heteroleptic Q3FeIII complex can help to improve its activity for the oxygenation of these
substrates by H2O2.
3.3. Catalytic mechanism
In order to explore the possible reaction pathway, a stepwise overlay of UV-Vis spectra for
Qa1Qb2FeIII catalysis system in MeCN at 20oC are illustrated in Fig. 4. As described above in
Fig.2, the complex Qa1Qb2FeIII displayed two weak and broad coordination bands in 450-600 nm
in the presence of cyclohexane (curve A in Fig. 4). When H2O2 was added to the A solution, such
two bands decreased immediately (see curve B0), which is likely due to the rapid reaction of
Qa1Qb2FeIII with H2O2. With prolonging the interaction time of H2O2 with the complex, both the
bands further decayed and were nearly disappeared after 22 min (curve B1-B4). Meanwhile the
absorbance near 350-380 nm grew gradually (see inset in Fig. 4), which is indicative of the
formation of phenoxyl radical [40]. Notably, the spectral changes above-described could be
noticed in the UV-Vis spectra of Qa3FeIII catalysis system previously reported by us [34], but both
coordinated bands of Qa1Qb2FeIII showed a much faster decay with increasing the time than those
of Qa3FeIII (Fig. 5 A vs B). This further confirms that the heteroleptic Qa1Qb2FeIII reacts with H2O2
more easily than the homoleptic Qa3FeIII.
Based on the above findings, as well as those in previous studies [42-45], we propose that
heteroleptic Qa1Qb2FeIII complex (1 in Scheme 1) easily reacts with H2O2 to form a pentadente
14

Qa1Qb2FeIII-OOH with a pendant hydroxyl group (2) through opening its longest Fe-O bond. Then,
the 2 may undergo a homolytic cleavage of its O-OH bond to generate a Qa1Qb2FeIIIO (3) and OH
radicals in solution, which leads to spurious reactions that can degrade the catalyst [46], as
supported by the above results of catalytic reactions and UV-vis spectral characterizations. Finally,
organic substrates can be oxidized to the corresponding oxygenated products by the 3 or OH
radicals, along with the regeneration of catalyst 1. Probably, the catalytic rate of Q3FeIII complex
depends on its reactivity with H2O2. A fast catalytic rate of the homoleptic Qa3FeIII should
originate from its higher HOMO or LUMO orbital energy. While the heteroleptic Q3FeIII with a
distorted octahedral structure, should easily open the longest Fe-O bond to form the intermediate 2
in the participation of H2O2, so that it's catalysis rate is much faster than that of the homoleptic
counterpart with a symmetrically octahedral structure, as supported by the present reaction and
UV-vis spectral characterization results.
4. Conclusion
In summary, for the first time we have designed and synthesized two heteroleptic
8-quinolinolato iron (III) (Q3FeIII) complexes with a weakly distorted octahedral geometry. The
proposed two Q3FeIII complexes show faster catalysis rates and provide higher conversions for the
oxygenation of some organic compounds by H2O2 compared to the homoleptic counterparts.
Having found this kind of the heteroleptic Q3FeIII complexes with a superior catalysis oxidation
performance, we are interested in exploiting the synthesis of other heteroleptic metal complexes
and their potential applications for the selective oxygenation of organic compounds by H2O2 and
more especially dioxygen or air.
15

Acknowledgement
We acknowledge the financial support for this work by the National Natural Science Fund of
China (20873040, 21546010), the Specialized Research Fund for the Doctoral Program of Higher
Education (20124306110005), the Natural Science Fund of Hunan Province (10JJ2007, 14JJ2148),
the Innovation Platform Open Fund of Hunan College (11K044, 14K059), Program for Science
and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province,
the 100 Talents Program of Hunan Province and the Hunan Provincial Innovation Foundation for
Postgraduate of China (CX2012B208)
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19

Qa2Qb1FeIII
Qa1Qb2FeIII
250
200
150
100
50
QbFeIII
QaFeIII
Qb
0
Qa
-50
3500 3000
1000
500
-1
Wavenumber/cm
Fig. 1. FT-IR spectra of four Q3FeIII complexes and their ligands
1.8
2
1.6
1.4
1.2
1.0
0.8
0.6
1
3
1: Qa2Qb1FeIII
2: Qb3FeIII
4
5
6
3: Qa Qb2FeIII
4: Qb1
5: Qa
6: Qa3FeIII
200
300
400
500
600
700
800
Wavelength / nm
Fig. 2. UV-Vis diffuse reflectance spectra of four Q3FeIII complexes and their ligands
20

60
50
40
30
20
10
0
6
5
4
1: Conv. for Qa3FeIII
3
2: Conv. for Qa1Qb2FeIII
3: Cyclohexanol for Qa3FeIII
4: Cyclohexanol for Qa1Qb2FeIII
5: Cyclohexanone for Qa1Qb2FeIII
6: Cyclohexanone for Qa3FeIII
2
1
A: The reaction was carried out at 12oC
0
5
10
15
20
25
30
35
Time / h
Fig. 3. Reaction time-dependence of conversion and selectivity for the Q3FeIII-catalyzed
cyclohexane oxygenation by H2O2 at 12oC in MeCN
2.0
1.5
1.6
1.2
0.8
0.4
0.0
1.2
0.9
0.6
320
340
360
380
400
A
B0
B1
B2
B3
B4
300
400
500
600
700
800
Wavelength / nm
Fig. 4. UV-Vis spectra of (A) a saturated solution of Qa1Qb2FeIII in MeCN (10 ml ) and
cyclohexane (27µmol); B0: Adding 13µmol 30 % H2O2 to the solution A; B1: 1min later; B2: 2
min later; B3:12 min later; B4:. 22 min later. Inset is the amplified traces in 350-400nm.
21

B, Qa3FeIII
A: Qa1Qb2FeIII
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
1, band at 465 nm
2, band at 596 nm
1, band at 465 nm
2, band at 596 nm
1
1
2
2
-20
0
20
40
60
80 100 120 140 160
0
20
40
60
80
100
120
Time / min
Time / min
Fig. 5. Decay curves for both the bands at 465 and 596 nm of Qa1Qb2FeIII (A) and Qa3FIII (B)
complexes with interaction time of H2O2
Scheme 1. Proposed reaction pathway for the Qa1Qb2FeIII-catalyzed oxygenation
22

Table 1 The iron contents and Fe-O bond distances of Q3FeIII complexes
Fe contenta / wt% Fe-O bond distancesb (Å)
HOMOb
(kcal.mol-1) (kcal.mol-1)
LUMOb
Sample
found
theoretical
Fe-O1
1.942
Fe-O2
1.942
Fe-O3
1.942
-49.45
-133.46
Qa3FeIII
9.46
9.49
d
Fe-O bondc
=0
⊿
84.01
-56.26
82.68
-54.67
82.42
-52.62
1.944
Fe-O bondc
1.944
1.944
=0
-138.94
Qb3FeIII
8.01
8.06
8.42
d
⊿
1.947
1.951
1.933
-137.09
Qa1Qb2FeIII
10.01
d
Fe-O bondc
0.011
1.940
0.016
⊿
1.936
-135.36
1.954
Qa2Qb1FeIII
10.55
8.95
d
Fe-O bondc
⊿
82.74
aThe iron content of Q3FeIII complexes was measured by EDTA complexometric titration, their theoretical iron
content was calculated based on Q3FeIII complexes; b Their Fe-O bond distances and HOMO LUMO energy were
calculated by the B3LYP/6-311+G (d) basis set; cFe-O bond value for Q3FeIII = the longest Fe-O bond –(additive
value of other two Fe-O bond )/2; d⊿ value = LUMO energy – HOMO energy.
Table 2 Various Q3FeIII complexes-catalyzed oxygenation of cyclohexane by H2O2 in MeCN a
Entry
Catalyst
Temp.
/ oC
Time
/ h
Yield of oxygenated products ( % )
Conv.
/ %
Cyclohexanol
cyclohexanone
1
2
Qa3FeIII
Qb3FeIII
12
12
8
8
13.0
0.5
6.5
0.3
6.5
0.2
Qa1Qb2FeIII
12
19.8
10.2
9.6
8
3
4
5
Qa2Qb1FeIII
Mixtureb
12
12
8
8
20.3
8.6
10.1
4.3
10.2
4.3
6
7
8
Qa1Qb2FeIII
Qa1Qb2FeIII
Qa1Qb2FeIII
0
8
8
2
7.8
3.8
4.0
25
35
22.0
22.7
10.5
9.9
11.5
12.8
a Reaction conditions: substrate (1 mmol), catalyst (0.02 mmol), 30 % H2O2 (4 mmol), MeCN (3 ml ). Conversion
b
and yield for products were determined by GC; Using a mechanical mixture of Qa3FeIIIand Qb3FeIII with 1: 2
molar ratio
23

Table 3 Q3FeIII complexes-catalyzed oxidation of other organic compounds by H2O2 in MeCN a
Entry
Catalyst
Substrate
Conv./ %
Main oxygenated products (yield / % )
1b
Qa3FeIII
Qb3FeIII
Qa1Qb2FeIII
Qa2Qb1FeIII
Qa3FeIII
3.3
0
Phenol (3.2)
----
2
Benzene
3b
4b
5
10.1
9.8
Phenol (9.6)
Phenol (9.5)
2.3
0
Benzaldehyde (2.3)
----
6
Qb3FeIII
Toluene
7c
8c
9d
Qa1Qb2FeIII
Qa2Qb1FeIII
Qa3FeIII
9.7
10.8
12.0
Benzaldehyde (6.5)
Benzaldehyde (6.7)
Acetophenone (7.3)
10
Qb3FeIII
0
----
Ethylbenzene
11d
12d
Qa1Qb2FeIII
Qa2Qb1FeIII
24.3
26.4
Acetophenone (17.2)
Acetophenone (16.5)
13e
14
Qa3FeIII
Qb3FeIII
95.5
23.5
100
100
Methylphenyl sulfoxide (91.7)
Methylphenyl sulfoxide (23.5)
Methylphenyl sulfoxide (92.1)
Methylphenyl sulfoxide (93.6)
Thioanisole
15e
16e
Qa1Qb2FeIII
Qa2Qb1FeIII
a Reaction conditions: substrate (1 mmol), catalyst (0.02 mmol), 30 % H2O2 (4 mmol), MeCN (3 ml ), the reaction
temperature, 18oC, the reaction time, 8h. The oxygenation products were identified by GC-MS and the conversion
b
c
and product yield were determined by GC; A trace amount of benzoquinone was detected; A small amount of
hydroxylated products (o- and m-cresols) was detected; dA small amount of benzaldehyde, phenylacetaldehyde and
phenethyl alcohol was detected; eA small amount of methylphenyl sulfone was detected.
24