Synergetic oxidation of ethylbenzene to acetophenone catalyzed by manganese(II) complexes bearing pendant iodophenyl groups

Article abstract of DOI:10.1016/j.jorganchem.2017.10.034

Five tetradentate ligands, L_1–5, bearing the moiety of bis(pyridin-2-ylmethyl)amine (1) and their complexes with Mn(II) were prepared. All the compounds and metal complexes were appropriately characterized. The five manganese(II) complexes (3a–e) are of the formula, [Mn(II)L_xCl₂] (x = 1-5), as suggested by the crystal structure of complexes 3b and 3c. All the ligands except for L₃ possess two iodobenzene groups via an ether linkage (except for L₁) with various lengths. By using oxone as an oxidant, the catalytic activity of these complexes on the oxidation of ethylbenzene to acetophenone in acetonitrile/water at room temperature was studied. Our results showed that the pendant iodophenyl groups play a synergetic role with the metal center in the catalysis, and complex 3b possesses the most appropriate length of the linkage between the iodobenzene group and the metal center. EPR and FTIR data suggest that the metal center of the active species should be Mn(IV) after the oxidation of oxone under the reaction conditions. A catalytic mechanism was also proposed based on the experimental observations.

Full text of DOI:10.1016/j.jorganchem.2017.10.034

Accepted Manuscript
Synergetic oxidation of ethylbenzene to acetophenone catalyzed by manganese(II)
complexes bearing pendant iodophenyl groups
Yiwen Yang, Wei Zhong, Binmei Nie, Jiangmin Chen, Zhenhong Wei, Xiaoming Liu
PII:
S0022-328X(17)30621-6
10.1016/j.jorganchem.2017.10.034
JOM 20154
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 16 August 2017
Revised Date: 4 October 2017
Accepted Date: 24 October 2017
Please cite this article as: Y. Yang, W. Zhong, B. Nie, J. Chen, Z. Wei, X. Liu, Synergetic oxidation of
ethylbenzene to acetophenone catalyzed by manganese(II) complexes bearing pendant iodophenyl
groups, Journal of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.10.034.
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ACCEPTED MANUSCRIPT
Synergetic oxidation of ethylbenzene to acetophenone
catalyzed by manganese(II) complexes bearing pendant
iodophenyl groups
Yiwen Yang a, Wei Zhong a, Binmei Nie b, Jiangmin Chen a, Zhenhong
Wei b, Xiaoming Liu*a
a College of Biological, Chemical sciences and Engineering, Jiaxing University
No. 118, Jiahang Road, Jiaxing 314001, P. R. China.
b School of Chemistry
Nanchang University
No. 999, Xuefu Road, Nanchang 330031, P. R. China.
E-mail: xiaoming.liu@mail.zjxu.edu.cn

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Abstract: Five tetradentate ligands, L1–5, bearing the moiety of
bis(pyridin-2-ylmethyl)amine (1) and their complexes with Mn(II) were prepared. All
the compounds and metal complexes were appropriately characterized. The five
manganese(II) complexes (3a–e) are of the formula, [Mn(II)LxCl2] (x = 1-5), as
suggested by the crystal structure of complexes 3b and 3c. All the ligands except for
L3 possess two iodobenzene groups via an ether linkage (except for L1) with various
lengths. By using oxone as an oxidant, the catalytic activity of these complexes on the
oxidation of ethylbenzene to acetophenone in acetonitrile / water at room temperature
was studied. Our results showed that the pendant iodophenyl groups play a synergetic
role with the metal center in the catalysis, and complex 3b possesses the most
appropriate length of the linkage between the iodobenzene group and the metal center.
EPR and FTIR data suggest that the metal center of the active species should be
Mn(IV) after the oxidation of oxone under the reaction conditions. A catalytic
mechanism was also proposed based on the experimental observations.
Keywords: Manganese complex • iodophenyl • oxone • ethylbenzene • acetophenone

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Introduction
Alkane oxidation is one of the most fundamental and important transformations in
synthetic chemistry. Of the various alkane oxidation, the direct benzylic oxidation of
alkylarenes into the corresponding carbonyl compounds has attracted considerable
attention since the carbonyls could be key intermediates for the preparation of
pharmaceuticals and agrochemicals [1-3]. However, the direct oxidation of a saturated
sp3 C−H bond under mild conditions poses a great challenge to organic chemists due
to its notorious inactivity [4, 5]. Traditionally, benzylic oxidation often required the
use of heavy metals oxidants in stoichiometric amounts, such as cerium triflate,
potassium permanganate or potassium dichromate, which often leads to the formation
of a lot of byproducts and the residual waste from the oxidants [6]. Recently, great
progress has been made to improve the transformation [7-21]. Metal-based catalysts
involving CrMCM-41, Mn(III), Co(OAc)2, Bi(0), RuCl3, RuCl2(PPh3)3, Rh2(cap)4,
FeCl3 and Au have been reported [7-15]. Transition-metal-free oxidants, such as
N-Hydroxyphthalimide (NHPI), 2-iodoxybenzoic acid (IBX), NaClO / TBHP,
[PhIO]n / KBr and iodine-pyridine-tert-butylhydroperoxide (I2-Py-TBHP), avoided
the involvement of heavy metals and were also employed in the transformation
[16-21]. Despite the progress, employment of toxic and/or precious metal, harsh
conditions are still involved in most of these systems. Therefore, direct benzylic
oxidation of alkylarenes under mild and environmentally benign conditions with both
economic and atomic efficiently remains highly desirable.
Hypervalent iodine compounds are a category of oxidants with availability,
environmentally friendliness, mild reactivity, high stability and easy handling [22-33].
PhIO2, derived from PhI oxidation by oxone [34-37], is one of the common
hypervalent iodine oxidants. Due to the distinct feature of PhIO2 oxidant, it plays an
important role in the oxidation of hydrocarbons and has received considerate attention
in the past decades [38]. It is believed that the synergetic interaction of transition
metal complexes with PhIO2 could greatly improve its oxidizing capability due to the
catalytic role of the metal complex. Indeed, immobilizing both hypervalent iodine and

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RuCl3 onto the surface of solid silica gel particles did improve the catalytic efficiency
for ethylbenzene oxidation [39]. The improvement in catalysis was believed to be
attributed to that both PhIO2 and the metal center were brought into such a proximity
by fixing them onto the surface of silica gel that they could act concertedly. However,
the proximity created in this manner between the metal center and the hypervalent
iodine could not appropriately be controlled and was arbitrary. If both the oxidant and
the metal center are incorporated into a molecule, the proximity could be tuned well
by molecular design and hence the catalytic efficiency could be well controlled.
To this end, we attempted to design a serial of multi-dentate ligands bearing
iodobenzene motif to achieve this controlled regulation, in which the iodobenzene
group is expected to be oxidized into PhIO2 motif in situ. It is hoped that a synergetic
interaction between the hypervalent iodine and the metal center could be achieved in
oxidation. Herein, we report the synthesis and characterization of four tetradentate
ligands bearing iodobenzene moiety with various proximity (L1-2, L4-5) and their
complexes with Mn(II) (3a-b, 3d-e). For comparison, ligand L3 which bears no iodo
groups and is analogous to L2 and its manganese complex (3c) were also synthesized.
Of the five complexes, 3b and 3c were crystallographically analyzed. The catalysis of
these complexes on the oxidation of ethylbenzene was assessed by using oxone as an
oxidant. Our results demonstrated that the integration of the hyperiodine derived from
the in-situ oxidation of iodobenzene and the metal center improved significantly the
catalysis. It turned out that the proximity between the hyperiodine and the catalyst
controlled by the chain length of their link dictated the synergetic catalysis. Based on
LC-MS analysis, a catalytic mechanism was proposed and highest oxidation state
Mn(IV) is possibly involved during the catalysis as suggested by EPR and FTIR
analysis.
Results and Discussion
Synthesis

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The synthetic routes of ligands L1–5 and complexes 3a–e are shown in Schemes 1 and
2, respectively. The building blocks 2a–d of L2-5 were prepared from 2-iodophenol
(or phenol) and 1,2-dibromoethane (or 1,4-dibromohexane, or 1,6-dibromohexane) in
refluxing acetone by using potassium carbonate as a base. Compound 1 was
synthesized from ethylenediamine and picolinaldehyde in refluxing methanol and
subsequent reduction by NaBH4. Then compound
1
reacted with
1-(bromomethyl)-2-iodobenzene (or 2a–d) to afford ligand L1 (or L2–5) in refluxing
acetonitrile in the presence of potassium iodide by using potassium carbonate as a
base. Reacting ligands L1–5 with MnCl2 4H2O produced manganese (II) complexes
3a–e, respectively.
Scheme 1 Synthesis of ligands L1–5 and their Mn(II) complexes 3a–e.
Structural analysis
The spectroscopic data, elemental analyses for complexes 3a-e and the crystal
structures of complexes 3b and 3c allow the assignment of the suggested structures
(Scheme 1). The structures of complexes 3b and 3c were analyzed by X-ray single
crystal diffraction (Figs. 1 and 2). Crystallographic data and structural parameters are

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presented in Tables S1 and S2, respectively. As shown in Figs. 1 and 2, the two
complexes adopt an octahedral geometry with the donor-set “N4Cl2”. Similar to other
multidentate pyridinyl manganese complexes [40-44], the distances of the Mn–N(Py)
bond in complexes 3b and 3c are about 0.12 Å shorter than the Mn–N (methylene)
bonds.
Fig. 1 Crystal structure of complex 3b (Mn1–N1 = Mn1–N1A = 2.280(2) Å, Mn1–N2
= Mn1–N2A = 2.403(2) Å, Mn1–Cl1 = Mn1–Cl1A = 2.4404(8) Å).
Fig. 2 Crystal structure of complex 3c (Mn1–N1 = Mn1–N1A = 2.2715(13) Å,
Mn1–N2 = Mn1–N2A = 2.4069(13) Å, Mn1–Cl1 = Mn1–Cl1A = 2.4451(4) Å).

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Optimization of reaction conditions of ethylbenzene oxidation
The catalytic activity of the complexes was assessed using the alkane oxidation
reaction. To optimize the reaction conditions, complex 3b and the ethylbenzene was
employed as the catalyst and substrate, respectively. The reaction was found to give
the best result in the mixture of CH3CN and H2O to react for 16 h. In this reaction
medium, both the dosage of the catalyst and an oxidant were examined. The reaction
was analyzed by a gas chromatography. It turned out that 0.1 equivalent of the
catalyst and 6 equivalent of the oxidant gave the best result (Table 1, entry 2). The
best ratio of CH3CN over H2O for the reaction is 1:1 ratio (Table 1). Screening
Oxidants showed that oxone is the most appropriate oxidant compared to other ones,
H2O2, TBHP (tert-butyl hydroperoxide), CAN [Ce(NH4)2(NO3)6], NaIO4 and I2O5
(entries 2 and 7–11, Table 1).
Table 1 Optimization of reaction conditions using complex 3b as the catalyst.a
Entry
1
Cat. / Ox. (eq.)
Solvents
b Yield (%)
Conversion (%)
23
3b / Oxone = 0.05 / 6
MeCN-H2O
(1:1)
12
2
3
4
5
6
7
3b / Oxone = 0.1 / 6
3b / Oxone = 0.15 / 6
3b / Oxone = 0.1 / 3
3b/Oxone = 0.1 / 6
3b / Oxone = 0.1 / 6
3b / H2O2 = 0.1 / 6
MeCN-H2O
(1:1)
89
68
34
24
10
0
91
75
40
28
40
0
MeCN-H2O
(1:1)
MeCN-H2O
(1:1)
MeCN-H2O
(4:1)
MeCN-H2O
(1:2)
MeCN-H2O
(1:1)

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8
3b / TBHP = 0.1 / 6
3b / CAN = 0.1 / 6
3b / NaIO4 = 0.1 / 6
3b /I2O5 = 0.1 / 6
3b (0.1)
MeCN-H2O
(1:1)
41
43
8
9
MeCN-H2O
(1:1)
3
10
11
12
MeCN-H2O
(1:1)
14
0
20
0
MeCN-H2O
(1:1)
MeCN-H2O
(1:1)
0
0
a Reaction conditions: ethylbenzene (0.5 mmol), solvents (4 mL), 25 oC, 16 h in open
air.
b Yield by GC analysis.
Synergetic catalysis on the oxidation of ethylbenzene
To confirm whether synergetic interaction exists for the manganese(II) complexes
bearing iodobenzene motif, four ligands (L1–2, L4–5) pendant with two iodobenzene
groups derived from a tetradentate ligand (1) were designed and synthesized. In the
four ligands, between the iodo group and secondary amine N atom is separated by the
distance of 4, 6, 8, 10 bonds. Their complexes (3a, 3b, 3d, 3e) with Mn(II) were
analogously prepared. Under the same reaction conditions described in previous
section, their catalytic efficiency towards the oxidation of ethylbenzene was examined.
The results shown in Table 2 indicate clearly that the length of the linkage between
the iodo group and the coordinating sphere matters so much that the conversion of the
reaction catalyzed by 3a decreased by nearly 50% compared to 3b (Table 2, entries 1
and 2). In complexes 3d and 3e, the linkage was further lengthened, the conversion of
the reaction declined slightly (Table 2, entries 5 and 6).
The variation in conversion rate with the length of the linkage suggests
straightforward that both metal center and the hyperiodo derived from the oxidation of

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the iodobenzene act concertedly on the catalytic oxidation of ethylbenzene. To reveal
how important such concerting interaction is in the catalysis, ligand with no
iodobenzene group (L3), an analogue of ligand L2, and its Mn (II) complex (3c) were
synthesized. Complex 3c gave 17% conversion rate (Table 2, entry 4). By adding two
equivalents of iodobenzene to constitute equivalent system analogous to complex 3b,
the reaction conversion rate was doubled (Table 2, entry 3), which is still lower than
half of that obtained by complex 3b. These results suggest that most contribution of
the conversion is attributed to the concerting intermolecular interaction between the
metal complex and PhIO2. In other word, the intramolecular concerted interaction in
the catalysis dominates the catalysis. Despite that using MnCl2 alone produced better
result than complex 3c, incorporating iodobenzene into the complexes enhances
significantly their catalytic efficiency (Table 2, entries 3, 4 and 10). It is not surprising
that either ligand L2 or iodobenzene alone produced no detectable products (Table 2,
entries 7 and 8). No product was detected without catalyst (Table 2, entry 9). Under
the acidic condition resulted from the addition of excess oxone, it is possible that the
manganese (II) catalysts collapse in the oxidation reaction to form MnO2 which could
catalyse the oxidation, too. Since we cannot rule out the possibility of forming MnO2,
confirming the catalytic activity of MnO2 becomes important. As shown by the results
(Table 2, entries 12-14), MnO2 does show some activity but both yield and conversion
rate are much less than those observed for catalyst 3b (Table 2, entry 2). By
considering that forming MnO2 during the catalysis is not significant, its contribution
to the catalysis can be neglected.
Table 2 The reaction efficiency under various catalytic conditions.a
Entry
Catalyst (eq.)
3a (0.1)
b Yield (%) Conversion (%)
1
2
3
4
46
89
24
9
58
91
39
17
3b (0.1)
3c (0.1) + PhI (0.2)
3c (0.1)

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5
6
3d (0.1)
3e (0.1)
82
75
0
88
84
0
7
L2 (0.1)
8
PhI (0.1)
0
0
9
none
0
0
10
11
12c
13
14
MnCl2 (0.1)
17
12
0
30
41
25
66
34
MnCl2 ( 0.1 ) + PhI ( 0.2 )
MnO2 ( 0.1 )
MnO2 ( 0.1 )
MnO2 ( 0.1 ) + PhI ( 0.2 )
46
16
a Ethylbenzene (0.5 mmol) in MeCN-H2O (1:1), oxone ( 6.0 eq.), 25 oC, 16 h in open
air.
b Yield by GC analysis.
c Without oxone.
To understand how the pendant iodobenzene groups play a role in the synergetic
catalysis, we first examined the oxidation products of ligand L2, complexes 3b and 3c
using high-resolution LG-MS. The mass spectra are shown in Figs. S1-3. By common
knowledge in oxidation chemistry, one major oxidation product for the oxidations is
tentatively proposed in Scheme S1 and in Table S3, further fragments are also
proposed. For comparison, we used ChemDraw software to predict the peak
distribution of those proposed fragments. As shown in Table S3, for all the proposed
fragments, there are generally good agreements between the predicted signals and the
observed ones.
The fragments found in the MS spectra indicate that the iodo is oxidized to high
valent iodine (ArIO2). Under the oxidative condition, the metal center is likely
oxidized to Mn(IV) to form Mn(IV)=O bond as suggested by the EPR spectrum of the
oxidation product (Fig. S4) [45] and its IR absorption band at 921 cm–1 (Fig. S5)
[46].These observed fragments suggest also that under the oxidation conditions two

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bound chlorides were cleaved despite the report that such a chloride could involve in
reaction with the iodobenzene [47]. Oxidizing the analogue (3c) produced metal-free
fragments (Scheme S1 and Table S3). This suggests strongly that the high valent
iodine stabilizes the oxidized product via direct interaction between the metal center
and hyperiodide. This may explain how the high valent iodine involves in the
synergetic catalysis between the iodine and the metal center and why the linkage
length matters in the catalysis. This is the ground for us to proposed a mechanism for
the catalysis (vide infra). It is particularly noteworthy that examining the LC-MS
spectrum of the catalytic reaction reveals a fragment which may bear the
half-oxidized intermediate (3b-P, Scheme S1 and Table S3). Very likely, the substrate
is bound at the iodo center rather than the metal center.
Proposed mechanism for the synthesis of acetophenone
The oxidation of ethylbenzene into ketone involves oxo transfer insertion and
hydrogen abstraction. The hypervalent iodine must involve in either process. Since
iodine is more electronegative than manganese, the oxo group of the hypervalent
iodine ought to be less nucleophilic than that of Mn(IV)=O. Bearing with this
argument in mind, it is feasible to speculate that the metal oxo undertake the proton
abstraction from the substrate while the hypervalent iodine helps complete the second
electron transfer from the substrate to form the product, ketone. With above
mechanistic outline, a mechanism for the synergetic oxidation of ethylbenzene is
proposed (Scheme 2).

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Scheme 2 Possible mechanism for the concerted oxidation of ethylbenzene (sol =
solvent).
The applicability of the complexes on the activation of sp3 C–H bonds
To examine the applicability of the Mn(II) complexes on the activation of sp3 C–H
bonds, we checked the catalysis of complex 3b on a number of substrates which were
arbitrarily chosen. As shown in Table 3, complex 3b exhibited generally better
catalytic performance on benzylic C–H bonds compared to cyclohexane. This is
apparently due to that the benzylic C–H bonds are more reactive than aliphatic C–H
bonds of cyclohexane. For those compounds of benzylic type C–H bonds, the
reactivity is much lower compared to ethylbenzene, which may be attributed to both
electronic and steric reasons.

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Table 3 Oxidation of various hydrocarbons under the catalysis by complex 3b.a
Entry Substrate
Product
b Yield (%)
1
12
4a
2
3
4
5
6
54
65
61
35
57
4b
4c
4d
4e
4f
a Reaction conditions: substrate (1 eq.), complex 3b (0.1 eq.), oxone (6 eq.), 4 mL
MeCN-H2O (1:1, v/v) , 25 oC, 16 h.
b GC yield.
Conclusions
In summary, we reported four tetradentate ligands L1 , L4 and Mn(II)
–2 –5
complexes 3a, 3b, 3d and 3e. Related compounds and complexes were fully
characterized. In all the ligands, iodobenzene group was introduced to examine
the synergetic catalysis on the ethylbenzene oxidation by oxone between the
metal center and the hypervalent iodine. It was found that cooperation between
the two functionalities enhanced greatly the reaction. The most striking
evidence was that the linkage length between the tetradentate ligand moiety and
the iodobenzene affects severely the reactivity. It turned out that ligand L2

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presence the most appropriate linkage. Further evidence came for the poor
performance of the analogue (3c) of complex 3b, in which a phenyl ring
replaced the iodobenzene in L2. EPR and FTIR data (Figs. S4 and S5) suggests
Mn(IV) as the active oxidation state.
Experimental Section
Materials and Methods
All commercially available reagents and solvents were obtained from the commercial
providers unless otherwise stated and used without further purification. Preparation
and handling of air-sensitive compounds were performed under an argon atmosphere
using standard Schlenk techniques or in an MBraunLabmaster 120 inert atmosphere
(<1 ppm O2, <1 ppm H2O) glovebox filled with nitrogen. Deoxygenation of solvents
was effected by either repetitive freezing/pumping/thawing cycles or bubbling with
argon for 45-60 min. Elemental analyses were performed on Quantitative
1
Technologies, Inc. (QTI, Whitehouse, NJ) or Desert Analytics (Tucson, AZ). H
NMR and 13C NMR spectra were recorded using a Varian 400 MHz spectrometer.
Chemical shifts were reported relative to internal tetramethylsilane (0.00 ppm) for 1H
and CDCl3 (77.0 ppm) for 13C. IR spectra were obtained on an Agilent 640 FT-IR
spectrophotometer. High-resolution mass spectra were obtained on a Waters LCT
Premier XE LC-MS spectrometer (ESI). GC analysis was performed on GC-2010
equipped with a flame ionization detector using a 0.2 mm × 30 m capillary column.
Flash column chromatography was performed on 300-400 mesh silica gel.
Crystallographic data of complexes 3b and 3c (CCDC 1522129 for 3b and 1522130
for 3c) were collected on a Bruker SMART CCD diffractometer with
graphite-monochromated Mo–Kα radiation (λ = 0.71073 Å). The crystal structures
were solved using direct methods in SHELXS and refined by full-matrix least-squares
routines, based on F2, using the SHELXL package [48].
Synthesis of ligands and complexes

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Synthesis of N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (1) [49]: To a
solution of ethylenediamine (1.50 g, 25 mmol) in CH3OH (60 mL) was added
picolinaldehyde (5.35 g, 50 mmol). The clear yellow solution was stirred and refluxed
at 80 oC for 12 h. At ice-temperature, NaBH4 (1.43 g, 37.5 mmol) was added in small
o
portions, and then refluxed at 80 C for 17 h. When the reaction was completed as
monitored by TLC, the solvent was evaporated to produce a residue to which water
(50 mL) was added. The solution was extracted with CH2Cl2 (3 × 20 mL). The
organic phases were combined and dried with anhydrous Na2SO4. Evaporation of the
solvents gave light-brown solid. Purification by silica flash chromatography (ethyl
acetate / methanol / triethylamine = 5:1:0.1, v / v / v) gave the desired product as
1
pale-yellow oil 1 (4.48 g, 74%). H NMR (400 MHz, CDCl3, ppm): δ = 8.48 (d, J =
4.4 Hz, 2H), 7.54 (t, J = 7.6 Hz, 2H), 7.25 (d, J = 8.0 Hz, 2H) 7.11-7.06 (m, 2H), 3.86
(s, 4H), 2.77 (s, 4H), 2.54 (s, 2H).
Synthesis of 2a, 2c and 2d: To a solution of 2-iodophenol (4.4 g, 20 mmol) and 1,
2-dibromoethane (or 1, 4-dibromoethane, or 1,6-dibromohexane) (120 mmol) in
acetone (50 mL) was added potassium carbonate (5.52 g, 40 mmol). The mixture was
o
refluxed at 80 C for 24 h. After being cooled to room temperature, any insoluable
solids were filtered off and the solvent was evaporated from the filtrate to give a
residue which was purified by column chromatography (ethyl acetate / petroleum
ether = 1:50, v / v) to afford the desired product 2a ( or 2c, or 2d).
1
1-(2-Bromoethoxy)-2-iodobenzene (2a): pale-yellow oil ( 4.44 g, 68%). H NMR
(400 MHz, CDCl3, ppm): δ = 7.77 (d, J = 7.6 Hz, 1H), 7.28 (t, J = 7.8 Hz, 1H), 6.79
(d, J = 8.4 Hz, 1H), 6.74 (t, J = 7.6Hz, 1H), 4.29 (t, J = 6.4 Hz, 2H), 3.66 (t, J = 6.4
13
Hz, 2H). C NMR (100 MHz, CDCl3, ppm): δ = 156.7, 139.7, 129.6, 123.4, 112.9,
87.0, 69.2, 29.7. IR (neat, cm–1): 3065, 2926, 2854, 1582, 1475, 1439, 1246, 1050,
1018, 650, 579.
1
1-(4-bromobutoxy)-2-iodobenzene (2c) [50]: yellow oil ( 6.10 g, 85%). H NMR
(400 MHz, CDCl3, ppm): δ = 7.76 (d, J = 8.0 Hz, 1H), 7.27 (t, J = 7.2 Hz, 1H), 6.77
(d, J = 8.4 Hz, 1H), 6.70 (t, J = 7.6Hz, 1H), 4.02 (t, J = 5.8 Hz, 2H), 3.53 (t, J = 6.6
Hz, 2H), 2.20-2.08 (m, 2H), 2.03-1.92 (m, 2H).

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1
1-(6-Bromohexyloxy)-2-iodobenzene (2d) [50]: yellow oil (5.97 g, 78%). H NMR
(400 MHz, CDCl3, ppm): δ = 7.75 (d, J = 8.0 Hz, 1H), 7.26 (t, J = 7.8 Hz, 1H), 6.78
(d, J = 8.0 Hz, 1H), 6.68 (t, J = 7.6Hz, 1H), 4.00 (t, J = 6.0 Hz, 2H), 3.42 (t, J = 6.8
Hz, 2H),1.95-1.75 (m, 4H), 1.65-1.45 (m, 4H).
Synthesis of (2-bromoethoxy) benzene (2b) [51]:To a solution of phenol (1.88 g, 20
mmol) and 1,2-dibromoethane (22.56 g, 120 mmol) in acetone (50 mL) was added
potassium carbonate (5.52 g, 40 mmol ) under stirring. The mixture was refluxed at
o
80 C for 24 h. After being cooled to room temperature, any insoluable solids were
filtered off and the solvent was evaporated from the filtrate to give a residue which
was purified by column chromatography (ethyl acetate / petroleum ether = 1:50, v /
v ) to afford the desired product as pale-yellow oil (2b) (1.11 g, 44%). 1H NMR (400
MHz, CDCl3, ppm): δ = 7.29 (t, J = 8.0 Hz, 2H), 6.98 (t, J = 7.4 Hz, 1H), 6.91 (d, J =
8.8 Hz, 2H), 4.28 (t, J = 6.2 Hz, 2H), 3.63 (t, J = 6.4 Hz, 2H).
Synthesis of N1,N2-bis(2-iodobenzyl)-N1,N2-bis((pyridin-2-yl)methyl) ethane-1,2-
diamine (L1): To a solution of compound 1 (0.73 g, 3 mmol) and
1-(bromomethyl)-2-iodobenzene (1.78 g, 6 mmol) in acetonitrile (50 mL) were added
potassium carbonate (1.66 g, 12 mmol ) and potassium iodide (0.01g, 0.06 mmol )
under stirring. The mixture was refluxed at 80 oC for 15 h. After being cooled to room
temperature, any insoluable solids were filtered off and the solvent was evaporated
from the filtrate to give a residue which was purified by column chromatography
(petroleum ether / ethyl acetate / triethylamine = 20:5:4, v / v / v ) to afford the
desired product as yellow oil (L1) (0.55 g, 27%). 1H NMR (400 MHz, CDCl3, ppm): δ
=8.44 (d, J = 4.0 Hz, 2H), 7.75 (d, J = 8.0 Hz, 2H), 7.51 (t, J = 7.6 Hz, 2H ), 7.44 (d, J
= 7.6 Hz, 2H), 7.39 (d, J = 7.6 Hz, 2H), 7.19 (t, J = 7.6 Hz, 2H ), 7.06 (t, J = 5.8 Hz,
2H ), 6.87 (t, J = 7.4 Hz, 2H ), 3.73 (s, 4H), 3.62 (s, 4H), 2.72 (s, 4H). 13C NMR (100
MHz, CDCl3, ppm): δ = 159.7 (2C), 148.8 (2C), 141.3 (2C), 139.3 (2C), 136.4 (2C),
130.0 (2C), 128.6 (2C), 128.0 (2C), 122.7 (2C), 121.9 (2C), 100.1 (2C), 63.6 (2C),
60.6 (2C), 52.2 (2C). IR (neat, cm–1): 3061, 2927, 2823, 1591, 1570, 1474, 1436,
1363, 1147, 1046, 617 .

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Synthesis of L2–5: To a solution of compound 1 (0.73 g, 3 mmol) and compound 2a
(2b, 2c, 2d) (9 mmol) in acetonitrile (50 mL) were added potassium carbonate (1.66 g,
12 mmol) and potassium iodide (0.015 g, 0.09 mmol ) under stirring. The mixture was
o
refluxed at 80 C for 15 h. After being cooled to room temperature, any insoluable
solids were filtered off and the solvent was evaporated from the filtrate to give a
residue which was purified by column chromatography (ethyl acetate / methanol /
triethylamine = 50:5:4, v / v / v ) to afford the desired product L2 (L3, L4, L5).
N1,N2-bis(2-(2-iodophenoxy)ethyl)-N1,N2-bis(pyridin-2-ylmethyl)
ethane-1,2-
1
diamine (L2): yellow oil (1.04 g, 47%). H NMR (400 MHz, CDCl3, ppm): δ = 8.45
(d, J = 4.8 Hz, 2H), 7.70 (d, J = 7.6 Hz, 2H), 7.55-7.45 (m, 4H), 7.18 (t, J = 7.6 Hz,
2H ), 7.06 (t, J = 5.4 Hz, 2H), 6.69-6.60 (m, 4H), 4.01 (t, J = 5.6 Hz, 4H), 3.90 (s, 4H),
13
3.01 (t, J = 5.6 Hz, 4H), 2.87 (s, 4H). C NMR (100 MHz, CDCl3, ppm): δ = 160.1
(2C), 157.4 (2C), 148.8 (2C), 139.4 (2C), 136.3 (2C), 129.3 (2C), 123.0 (2C), 122.4
(2C), 121.8 (2C), 111.9 (2C), , 86.5 (2C), 67.8 (2C), 61.5 (2C), 53.6 (2C), 53.5 (2C).
IR (neat, cm–1): 3062, 2932, 2825, 1583, 1470, 1438, 1365, 1275, 1122, 1050, 617.
N1,N2-bis(2-phenoxyethyl)-N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine (L3):
1
brown oil (0.64 g, 44%). H NMR (400 MHz, CDCl3, ppm): δ = 8.45 (d, J = 4.8 Hz,
2H), 7.54 (t, J = 7.6 Hz, 2H), 7.43 (d, J = 7.6 Hz, 2H), 7.19 (t, J = 8.0 Hz, 4H), 7.09
(d, J = 6.0 Hz, 2H), 6.87 (t, J = 7.4 Hz, 2H), 6.76 (d, J = 8.0 Hz, 4H), 4.01 (t, J = 5.6
13
Hz, 4H), 3.92 (s, 4H), 3.01 (t, J = 5.0 Hz, 4H), 2.91 (s, 4H). C NMR (100 MHz,
CDCl3, ppm): δ = 158.5 (2C), 148.9 (2C), 136.5 (2C), 129.35 (4C), 123.1 (2C), 122.1
(2C), 120.7 (2C), 114.4 (4C), 65.9 (2C), 53.5 (2C), 52.9 (2C), 45.8 (4C). IR (neat,
cm–1): 3159, 2985, 2927, 1599, 1497, 1474, 1390, 1244, 1173, 1038.
N1,N2-bis(4-(2-iodophenoxy)butyl)-N1,N2-bis(pyridin-2-ylmethyl)
1
ethane-1,2-diamine (L4): yellow oil (0.26 g, 11%). H NMR (400 MHz, CDCl3,
ppm): δ =8.45 (d, J = 3.2 Hz, 2H), 7.71 (d, J = 7.6 Hz, 2H), 7.55 (t, J = 6.8 Hz, 2H ),
7.42 (d, J = 7.6 Hz, 2H), 7.22 (t, J = 6.6 Hz, 2H) , 7.08 (t, J = 5.2 Hz, 2H) , 6.72-6.63
(m, 4H), 3.90 (t, J = 6.0 Hz, 4H), 3.77 (s, 4H), 2.73 (s, 4H), 2.57 (s, 4H), 1.80-1.65 (m,
8H). 13C NMR (100 MHz, CDCl3, ppm): δ = 157.4 (2C), 157.0 (2C), 148.8 (2C),

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139.3 (2C), 136.4 (2C), 129.4 (2C), 123.0 (2C), 122.3 (2C), 121.9 (2C), 112.0 (2C),
86.7 (2C), 68.8 (2C), 60.6 (2C), 54.4 (2C), 52.2 (2C), 26.9 (2C), 23.7 (2C).
N1,N2-bis(6-(2-iodophenoxy)hexyl)-N1,N2-bis((pyridin-2-ylmethyl)
ethane-1,2-
1
diamine (L5): yellow oil (1.19 g, 47%). H NMR (400 MHz, CDCl3, ppm): δ = 8.43
(d, J = 4.8 Hz, 2H), 7.69 (d, J = 8.0 Hz, 2H), 7.53 (t, J = 7.6 Hz, 2H ), 7.39 (d, J = 8.0
Hz, 2H), 7.19 (t, J = 7.0 Hz, 2H) , 7.04 (t, J = 6.2 Hz, 2H) , 6.70 (d, J = 8.4 Hz, 2H),
6.61 (t, J = 7.6 Hz, 2H) , 3.89 (t, J = 6.2 Hz, 4H), 3.68 (s, 4H), 2.60 (s, 4H), 2.43 (t, J
13
= 7.4 Hz, 4H), 1.77-1.67 (m, 4H), 1.50-1.35 (m, 8H), 1.33-1.21 (m, 4H). C NMR
(100 MHz, CDCl3, ppm): δ = 160.6 (2C), 157.5 (2C), 148.8 (2C), 139.3 (2C), 136.2
(2C), 129.3 (2C), 122.7 (2C), 122.3 (2C), 121.7 (2C), 112.1 (2C), 86.8 (2C), 69.0
(2C), 61.0 (2C), 54.9 (2C), 52.5 (2C), 29.0 (2C), 27.1 (2C), 27.0 (2C), 26.0 (2C). IR
(neat, cm–1): 3064, 3013, 2931, 2829, 1592, 1571, 1471, 1439, 1366, 1277, 1247,
1122, 1050, 621, 540.
Synthesis of metal complexes 3a–e: To a solution of compound L1 (or L2, L3, L4,
L5) (0.2 mmol) in methanol (2 mL) was added slowly the solution of manganese
chloride tetrahydrate (0.04 g, 0.2 mmol) in methanol (1 mL) under stirring. After
being stirred at room temperature for 1 h, a precipitate was collected by filtration,
washed with diethyl ether and dried under vacuum overnight and got the desired
complex 3a (or 3b, 3c, 3d, 3e).
3a (0.14 g, 90%). Calcd. for C28H28Cl2I2MnN4 (FW = 800.20): N 7.00%; C 42.03%;
H 3.53%; Found: N 6.32 %; C 41.74%; H 3.78%.
3b (0.13 g, 74%). Calcd. for C30H32Cl2I2MnN4O2 (FW = 860.25): N 6.51%; C
41.89%; H 3.75%; Found: N 6.39%; C 41.68%; H 3.80%.
3c (0.051 g, 42%). Calcd. for C30H34Cl2MnN4O2 (FW = 607.14): N 9.21%; C 59.22%;
H 5.63%; Found: N 8.7%; C 58.97%; H 5.57%.
3d (0.094 g, 51%). Calcd. for C34H40Cl2I2MnN4O2 (FW = 916.36): N 6.11%; C
44.56%; H 4.40%; Found: N 5.95%; C 44.42%; H 4.29%.
3e (0.12 g, 63%). Calcd. for C38H48Cl2I2MnN4O2 (FW = 972.47): N 5.76%; C
46.93%; H 4.98%; Found: N 5.64%; C 46.61%; H 4.86%.

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Acknowledgements
The authors would like to thank the National Natural Science Foundation of
China (Grant no. 21571083, 21301071) and the Government of Zhejiang
Province (Qianjiang Professorship for XL) for supporting this work.
Appendix A. Supplementary data
Crystallographic data of the complexes, related spectra for oxidations, possible
oxidation products, synthesis and characterization of compounds 4 and LC-MS
signal analysis for oxidations. Supplementary data associated with this article
can be found in the online version.
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Highlights
‹ Oxidation of ethylbenzene to acetophenone catalyzed by Mn(II) complexes in
good yield under mild conditions.
‹ The pendant iodophenyl groups in Mn(II) complexes play a synergetic role with
the metal center in the catalysis.
‹ The linkage length between the tetradentate ligand moiety and the iodobenzene
affects severely the catalytic reactivity.