Crystal structure, magnetic and catalytic oxidation properties of manganese(III) tetrakis(ethoxycarbonyl)porphyrin

Article abstract of DOI:10.1039/c5ra15943a

A new meso-tetraalkyl porphyrin manganese complex, 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrinatomanganese (Mn^IIITECPCl), had been prepared and characterized by X-ray structure determination. Mn^IIITECPCl exists as a coordinated dimer in its crystal structure with a weak antiferromagnetic coupling between two Mn(iii) ions. The catalytic oxidation of styrene by Mn^IIITECPCl was carried out in acetonitrile. Mn^IIITECPCl was found to be recyclable with a high conversion efficiency when using TBHP as an oxidant and the major product was benzaldehyde. Mn^IIITECPCl was also reusable by using an PhIO oxidant, but the major products turned out to be phenyl acetaldehyde and styrene epoxide.

Full text of DOI:10.1039/c5ra15943a

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PAPER
Crystal structure, magnetic and catalytic oxidation
properties of manganese(III) tetrakis-
Cite this: RSC Adv., 2015, 5, 97391
(ethoxycarbonyl)porphyrin†
a
a
ab
a
a
Hua-Hua Wang, Hui-Qing Yuan, Mian H. R. Mahmood, Yi-Yu Jiang, Fan Cheng,
c
a
Lei Shi and Hai-Yang Liu*
A
new meso-tetraalkyl porphyrin manganese complex, 5,10,15,20-tetrakis(ethoxycarbonyl)
III
porphyrinatomanganese (Mn TECPCl), had been prepared and characterized by X-ray structure
III
determination. Mn TECPCl exists as
a coordinated dimer in its crystal structure with a weak
III
antiferromagnetic coupling between two Mn(III) ions. The catalytic oxidation of styrene by Mn TECPCl
Received 9th August 2015
Accepted 30th October 2015
III
was carried out in acetonitrile. Mn TECPCl was found to be recyclable with a high conversion efficiency
III
when using TBHP as an oxidant and the major product was benzaldehyde. Mn TECPCl was also
DOI: 10.1039/c5ra15943a
reusable by using an PhIO oxidant, but the major products turned out to be phenyl acetaldehyde and
styrene epoxide.
www.rsc.org/advances
manganese porphyrin catalyzed oxidation reaction. Man-
Introduction
25–27
28–30
III
31–33
ganese(V)-oxo,
manganese(IV)-oxo
and Mn –HOOR
From a structural viewpoint, synthetic manganese porphyrins are suggested as the active intermediates. Interestingly, trans-
V
27
resemble the active centre of heme-containing biological dioxo-Mn -porphyrin is found inert.
systems. Thus, manganese porphyrins are also widely used in
Meso-substitution has signicant effects on the reactivity of
34,35
1
2
the biomimetic studies of cytochrome P-450, catalases or other metal porphyrin, especially free radicals involved reactions.
3
enzymes. However, manganese porphyrins usually undergo Meso-tetraalkyl porphyrins are useful probes for understanding
3
6
oxidative destruction in homogeneous catalytic oxidation the structure–property relationship of porphyrin molecules.
4–6
systems. The oxidative destruction of a manganese porphyrin Although tetraalkylporphyrins may be prepared with good
37–39
can be signicantly improved with electron withdrawing yields,
there is the less reports on the application of
7–9
40,41
substituents such as b-position halogenation. The other way manganese tetraalkylporphyrin in catalytic oxidation.
to avoid oxidative destruction is by immobilizing the catalysts 5,10,15,20-tetrakis(ethoxycarbonyl)porphyrin (TECP) is a conve-
1
0
42
onto a rigid, inert support such as montmorillonite, multi- nient precursor for porphine. Recently, we have prepared
11
12
43
walled carbon nanotube, magnetite nanoparticles, meta- b-uorinated TECP. Here, we wish to report the preparation
halloysite, resin and silica gel. Assembled manganese and catalytic styrene oxidation of manganese(III) TECP
porphyrin crystalline micro-sized porous coordination poly- (Scheme 1, Mn TECPCl). The X-ray crystal structure showed
13
14
15
III
III
16
17,18
mers and metal–organic framework (MOF)
are also proved that Mn TECPCl exists as a dimer with plenty of hydrogen
robust catalysts. Although these heterogeneous catalysts are bonding interactions. There is a weak antiferromagnetic
recyclable and stable, the synthetic procedure is complicated coupling between two Mn(III) ions in the dimer. The catalytic
III
with limited type of ligands and the catalytic oxidation is time oxidation of styrene by Mn TECPCl was carried out in aceto-
consuming. On the other hand, lots of oxidants such as t-butyl nitrile solution at room temperature using TBHP and PhIO as
III
1
9
20
hydroperoxide (TBHP), iodosylbenzene (PhIO), iodobenzene oxidants. Mn TECPCl was found recyclable with high conver-
2
1
22
diacetate (PhI(OAc)
2
), m-chloroperbenzoic acid (m-CPBA),
sion efficiency when using these oxidants. Plausible mechanism
dioxygen and hydrogen peroxide are effective in the has been proposed to explain this observation.
23
24
Experimental
a
Department of Chemistry, South China University of Technology, Guangzhou 510641,
China. E-mail: chhyliu@scut.edu.cn; Fax: +86 020 22236805; Tel: +86 020 22236805 Materials and methods
b
Department of Chemistry, University of Education, Lahore 54770, Pakistan
All reagents were purchased from commercial sources and used
c
Department of Chemistry, Guangdong University of Education, Guangzhou 510303,
China
without further purication, unless otherwise mentioned.
Styrene was passed through silica gel column prior to use.
Electronic spectra were recorded on Hitachi 3900H UV-vis
†
CCDC 1059156. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/c5ra15943a
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and 5,10,15,20-tetraphenylporphyrin manganese(III) chloride
III
(
Mn TPPCl) (Scheme 1) were prepared using literature proce-
44,45
dures.
The synthesis of 5,10,15,20-tetrakis(ethoxycarbonyl)
porphyrin ligand (TECP) was performed by our recently re-
43
ported procedure.
III
Mn TECPCl was obtained by reuxing TECP with
Mn(OAc) $4H O in DMF solution for 2 h. Aer that it was
diluted with CH Cl and washed with saturated aqueous solu-
2
2
2
2
tion of NaCl and HCl several times. The organic layer was
collected and dried over anhydrous Na SO . The ltrate was
2
4
concentrated and the crude product was puried on silica gel
100–200 mesh) using CH Cl /CH OH (95 : 5 v/v) as eluent.
(
III
III
2
2
3
Scheme 1 Molecule structure of Mn TECPCl and Mn TPPCl.
Black solid was obtained aer recrystallization from CH
hexane (1 : 4 v/v). Yield: 85%. UV-vis (CH Cl ) lmax (log 3): 364
4.80), 474 (4.90), 570 (4.07); HR-MS (ESI) ([M ꢀ Cl] ): calcd for
28MnN 651.1282, found 651.1285, with an isotope
2 2
Cl /
2
2
+
(
Spectrophotometer in 1 cm optical path length quartz cell at
room temperature. Fourier translation infrared spectra (FT-IR)
were recorded on a Perkin-Elmer spectrophotometer by using
KBr pellets. HR-MS spectra were recorded on Bruker maxis
C
32
H
4 8
O
distribution pattern the same as the calculated one.
III
impact mass spectrometer with an ESI source. Single-crystal X- X-ray diffraction studies of Mn TECPCl
ray diffraction data were recorded on a Rigaku R-AXIS SPIDER
A suitable crystal was selected and measurement on an Xcali-
IP diffractometer with Cu Ka radiation. All cyclic voltammo-
grams (CV) were performed in N,N-dimethylformamide (DMF)
solution containing 0.1 M tetrabutylammonium perchlorate
TBAP) and Mn TECPCl (3 mM) using CHI-660E electro-
chemical analyzer under nitrogen atmosphere at ambient
temperature. The scan rate was 100 mV s . A three-electrode
system consisting of a glassy carbon working electrode, a plat-
inum wire counter electrode and saturated Ag/AgNO electrode
as the reference electrode were employed. The Ag/AgNO elec-
trode contained 1 M TBAP in DMF. Half-wave potentials (E1/2
bur, Sapphire3, Gemini ultra diffractometer using Cu K radi-
a
ation. The crystal was kept at 150(5) K during data collection.
46
Using Olex2, the structure was solved with the Olex2.solve
III
(
structure solution program using charge ipping and rened
47
with the XL renement package using least squares minimi-
ꢀ
1
zation. All non-hydrogen atoms of the complex were rened
anisotropically. The hydrogen atoms were generated by the
riding mode. Data collection and structural renement
parameters are given in Table 1. CCDC 1059156 contained the
3
3
)
III
supplementary crystallographic data for Mn TECPCl.†
for reversible or quasi-reversible redox processes were calcu-
lated as E1/2 ¼ (Epa + Epc)/2, where Epa and Epc represent the
anodic and cathode peak potentials, respectively. The E1/2 value
for the ferrocene couple under these conditions was 0.47 V.
III
Table 1 Crystal data and structure refinement for Mn TECPCl
III
Analysis of oxidation products was recorded by gas chromato- Compound
Mn TECPCl
graph (Echrom A90) equipped with an Agilent HP-5 capillary
Empirical formula
Formula weight
Temperature/K
32 28 4 8
C H N O MnCl
686.97
150(5)
column (30.0 m ꢁ 320 mm ID: 0.25 mm lm thickness) coupled
with FID detector. The carrier gas was nitrogen and the chro-
matographic conditions were as follows: the oven temperature Crystal system
was retained at 60 C for 4 min, and then increased at a rate of Space group
Monoclinic
P2 /c
14.9631(3)
9.6880(3)
21.4177(5)
90.00
100.638(2)
90.00
3051.41(13)
4
ꢂ
1
ꢀ
ꢂ
ꢂ
a/A
1
2
0 C min (from 60 to 250 C); the injector temperature was set
ꢀ
b/A
ꢂ
ꢂ
30 C while the detector temperature was kept 250 C. The
ꢀ
c/A
injection volume was 1.0 mL and the products were conrmed
ꢂ
ꢂ
a/
by the retention time using standard samples under the same b/
GC conditions. The yields of products were reported with g/ꢂ
3
ꢀ
Volume/A
respect to the amount of oxidant used. The solid-state magnetic
susceptibility was measured under helium on a Quantum
Design PPMS-9 magnetometer from 2 to 300 K at a eld of 1 T.
Z
r
ꢀ3
calc g cm
m/mm
1.495
4.832
ꢀ1
The powder sample was installed in the paralm and the F(000)
1416.0
ꢀ
magnetic moment was obtained aer background correction. Radiation
Cu Ka (l ¼ 1.54184 A)
Reections collected
Independent reections
Data/restraints/parameters
10 067
The experimental susceptibilities were corrected for the
diamagnetism of the constituent atoms (Pascal's table).
4591 [Rint ¼ 0.0372, Rsigma ¼ 0.0453]
4591/0/527
1.049
2
Goodness-of-t on F
Synthesis of catalysts
Final R indexes [I $ 2s(I)]
Final R indexes [all data]
Largest diff. peak/hole/e A
R
R
1
¼ 0.0400, wR
2
2
¼ 0.1002
¼ 0.1044
1
¼ 0.0447, wR
The metalloporphyrin catalysts 5,10,15,20-tetrakis(ethoxy-
carbonyl)porphyrin manganese(III) chloride (Mn TECPCl)
ꢀ3
ꢀ
0.50/ꢀ0.42
III
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ꢀ
General procedure for catalytic oxidation
parallel with a distance of 3.2845 A, indicating the existence of
p–p stacking, in addition to coordination interactions. The
short intermacrocycle distances are the consequence of two
supplementary structural features. The rst one is weak coor-
dination bond between manganese ion and oxygen atom from
another macrocycle. The second one is the nearly coplanar
orientation of the meso-5,10,15,20-tetra-ethoxycarbonyl groups
towards the macrocycle, which facilitates the intermolecular
approach, despite their spatial proximity. This distance is
comparable to the same meso-ethoxycarbonyl groups corrole
A mixture of organic substrate (1.0 mmol), oxidant (0.1 mmol)
and catalyst (1.0 mmol) in 2 mL acetonitrile was stirred in a 10
mL glass ask at room temperature. Aer an appropriate reac-
tion time, chlorobenzene (5.0 mL) was added to this reaction
mixture as internal standard. The conversion efficiency and
selectivity were monitored by GC analysis.
Typical procedure for the reuse of catalyst in the catalytic
oxidation of styrene
49
free base.
III
A mixture of catalyst (1.0 mmol), styrene (1 mmol) and TBHP (0.1
Selected bond distances and angles for Mn TECPCl are
ꢀ
mmol) in CH
3
CN (2 mL) was sealed in a 10 mL glass ask, listed in Table 2. The Mn–N bond length range from 2.0159(2) A
ꢀ
ꢀ
evaporated under vacuum aer stirring at room temperature for to 2.0275(2) A with an average value of 2.0224(9) A, and the Mn–
ꢀ
2
.5 h. The resulting solid was washed with hexane for several Cl bond length is 2.4235(8) A, both of the bond length were
times, and detected by GC analysis. The recovered catalyst solid comparable to 5,10,15,20-tetrakis(4-hydroxyphenyl) porphyrin
III
III
was used in the successive run.
manganese chloride (Mn TOHPPCl) or Mn TPPCl found in
48,50
ꢀ
the literature.
The Mn–O bond length is 2.6290(2) A, and is
ꢀ
weaker than covalent bond (2.16 A).
The stability of catalyst
Hydrogen bonding plays an important role in constructing
multi-dimensional structure as well as in catalytic oxidation.
III
A mixture of Mn TECPCl and oxidant in 2 mL acetonitrile was
50
stirred in a 10 mL glass ask at room temperature. Aer an
appropriate reaction time, 100 mL of the mixture was diluted to 3
mL acetonitrile solution in a quartz cell with 1 cm optical path
length and UV-vis spectrum was obtained.
ꢀ
ꢂ
Table
2
Selected bond distances (A) and bond angles ( ) for
III
Mn TECPCl
ꢀ
Results and discussion
X-ray structure of Mn TECPCl
Bond distances (A)
Mn1–Cl1
Mn1–N2
2.4235(7)
2.0275(19)
2.0160(2)
Mn1–N1
Mn1–N3
Mn1–O5A
2.0270(2)
2.0200(19)
2.6290(2)
III
III
Mn1–N4
Single crystal of Mn TECPCl, suitable for X-ray diffraction
analysis, was obtained by slow evaporation of dichloromethane/
hexane solution. Single crystal of Mn TECPCl was monoclinic
ꢂ
Bond angles ( )
N3–Mn1–Cl1
III
94.65(6)
89.33(8)
88.91(8)
89.67(8)
170.48(8)
127.23(16)
127.91(16)
125.65(17)
127.32(15)
N3–Mn1–N1
N1–Mn1–Cl1
N4–Mn1–Cl1
N4–Mn1–N1
N2–Mn1–Cl1
C11–N3–Mn1
C4–N1–Mn1
C9–N4–Mn1
C16–N2–Mn1
169.66(8)
95.69(6)
90.72(6)
90.38(8)
98.79(6)
126.80(15)
126.14(17)
126.80(16)
126.55(16)
in the space group P2
1
/c, with nonplanar porphyrin core, like N3–Mn1–N2
48
Mn(III) tetraphenylporphyrin. The molecular structure of N1–Mn1–N2
III
N4–Mn1–N3
N4–Mn1–N2
C14–N3–Mn1
C1–N1–Mn1
Mn TECPCl is depicted in Fig. 1a. The 24-membered macro-
cyclic core of the porphyrin is coplanar, and the displacement of
each atom in the equatorial mean plane is within 0.15 A. The
distance between the Mn ion to the mean plane is 0.1701 A. C6–N4–Mn1
The six-coordinate manganese atom located at the centre of this C19–N2–Mn1
ꢀ
3
+
ꢀ
porphyrin macrocycle and is bonded to four nitrogen, one
chlorine and one oxygen atom to construct a distorted octahe-
dral. Two manganese porphyrin molecules combined in
a dimer (Fig. 1b) and two manganese porphyrin core planes are
III
Fig. 1 ORTEP plots of Mn TECPCl. (a) Monomer. (b) Dimer (showing
III
5
0% probability thermal displacement ellipsoids, hydrogen atoms are Fig. 2 Unit cell of Mn TECPCl show hydrogen bonding interaction.
omitted for clarity).
Hydrogen atoms not involved in these contacts are omitted for clarity.
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III
Hˆ ¼ ꢀ2J Sˆ Mn Sˆ Mn + D Sˆ zMn2
Fig. 2 shows the packing of Mn TECPCl constructed from
hydrogen bonding. Apparently, there are two types of hydrogen
bonding i.e. intramolecular and intermolecular bonding. The
former exists between oxygen of carbonyl and hydrogen of
pyrrole, which was also observed in the meso-ethoxycarbonyl
(1)
2
ꢀ
ꢁ
3
1
8
28
ꢀ 3
3
y
3y
4y
8 þ 2e
þ
ꢀ
e
þ 12e
0
:3749
1 6
y
3
7
2
6
7
5
cm
¼
geff p ꢁ
4
3
y
4y
T
3
2 þ 2e þ e
f
49
groups corroles. While the latter exists between the oxygen of
the ester group from one manganese porphyrin and hydrogen
of pyrrole or ethyl group from another manganese porphyrin.
ꢂ
ꢃ
2
x
6x
12x
20x
2
1
e
þ 10e þ 28e þ 60e
þ q
2x 6x 12x 20x
þ 3e þ 5e þ 7e þ 9e
Magnetic properties
þ ð1 ꢀ p ꢀ qÞ ꢁ 2:917 þ TIP
g
III
Magnetic data for Mn TECPCl are reported in Fig. 3 in the form
of cm and meff versus T. The value of meff varies from 4.66 m at
00 K to 3.35 mB at 2 K. The magnetic moment clearly shows
B
(2)
3
D ðcmꢀ1Þ
J ðcmꢀ1Þ
where,
y ¼ 1:44
,
x ¼ 1:44
and
a platform (ꢃ4.64 m ) at high temperatures (300–38 K). With
B
T
T
pffiffiffiffiffiffiffiffiffi
decreasing the temperature, the meff decreases slowly and rea- meff ¼ 2:828 cmT.The term p (or q) is the fraction of Mn(III) [or
ches its terminal value of 4.48 m
B
at about 12 K, and aer this Mn(III)–Mn(III) dimer], geff is the effective g value, TIP is the
the meff decrease rapidly and reaches its lowest value of ꢃ3.35 m
B
temperature independent paramagnetism. J is the coupling
at 2 K. The abrupt meff rise in the range of temperature 2 < T < 12 exchange parameter and D is the parameter which describes the
K is characteristic of a compound with signicant zero-eld effects of the axial eld. The best ts, as represented in Fig. 3,
51
ꢀ1
splitting (ZFS). The effective magnetic moment of 4.66 m at gave the values geff ¼ 1.75, p ¼ 0.70, D ¼ 7.34 cm , q ¼ 0.08,
B
ꢀ
1
ꢀ4
3
ꢀ1
room temperature is lower than the spin-only moment of 4.9 mB 2J ¼ ꢀ0.84 cm and TIP ¼ ꢀ2.7 ꢁ 10 cm mol , with an
for an S ¼ 2 system, consistent with other high spin Mn(III) agreement factor of 0.61%, showing the high reliability of data.
52
complexes in which g < 2. The magnetic susceptibilities of The R value is dened as
III
Mn TECPCl in the whole temperature range obeys Curie–
2
2
R ¼ S(cobsdT ꢀ ccaldT) /S(c
obsd
T) ꢁ 100%
(3)
Weiss law with small negative Weiss constant q ¼ ꢀ0.91 K and
ꢀ
1
Curie constant C ¼ 2.71 emu K mol , indicating the occurrence
ꢀ
1
53
III
The small negative J value (|J| < 20 cm
)
suggests a weak
of weak antiferromagnetic interactions in Mn TECPCl.
III
antiferromagnetic coupling between the Mn(III) and Mn(III)
The molecular structure of Mn TECPCl shows that the
connected by the coordination bridge. Within the dimer, Mn/Mn
metal (Mn) centers are linked by weak O5A–Mn coordination
bonds to form a dimer (Fig. 1), and such interactions are able to
transmit magnetic interactions. On the basis of the binuclear
III
ꢀ
separation of 6.56 A in Mn TECPCl precludes direct metal–
metal bonding, the superexchange through O5A–Mn coordina-
tion bonding in the Mn(III)/Mn(III) dimer must be responsible
for this antiferromagnetic interaction. The shortest interdimer
III
III
Mn –Mn (S ¼ 2 and 2) model, the magnetic susceptibilities
III
for Mn TECPCl can be tted by eqn (2) derived from the
III
ꢀ
52,53
Mn/Mn distance of 10.53 A in Mn TECPCl indicates that
isotropic exchange spin Hamiltonian (eqn (1)).
interdimer interactions are negligible. This is similar to the
previous observation for Mn(2-NC
3 5
H NCTPP)Br complex, in
which the antiferromagnetic coupling constant J between
ꢀ1
Mn(III)–Mn(III) ions is ꢀ0.02 cm with an intradimer Mn/Mn
ꢀ
52
separation of 8.177 A linked by C–H/Br hydrogen bonding.
Electrochemistry
III
The electrochemical characteristics of Mn TECPCl and
III
Mn TPPCl were investigated by cyclic voltammetry in DMF
solutions. Fig. 4 represents the cyclic voltammograms of
III
III
Mn TECPCl and Mn TPPCl in DMF containing 0.1 M TBAP
and ferrocene as internal standard. Manganese porphyrins
show a single one-electron reversible or quasi-reversible
oxidation and a series of irreversible reductions within the
III
scan window (ꢀ2.0 to 2.0 V). Mn TECPCl exhibits four redox
couples located at E1/2 ¼ ꢀ1.831, ꢀ1.531, ꢀ0.96 and 0.021 V.
The voltammogram is in close resemblance to that of
Mn TPPCl, which exhibits the same redox couples located at
III
Fig. 3 Temperature dependence of magnetic susceptibility in the
III
forms of meff and c
00 K. The red solid lines represent the best-fitting results. Inset:
Temperature dependence of the magnetic susceptibility in the form of
m
versus T for Mn TECPCl at 1 T between 2 and
E1/2 ¼ ꢀ1.948, ꢀ1.836, ꢀ1.310, ꢀ0.19 V respectively. The elec-
3
trochemical properties of manganese porphyrins are well
5
4–56
ꢀ1
III
documented in the literature.
Here, the assignments of
c
m
versus T for Mn TECPCl between 2 and 300 K. The solid line was
III
obtained from the best fit by the Curie–Weiss expression.
Mn TECPCl redox couples are made accordingly. The redox
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Table 3 Catalytic oxidation of styrene by manganese porphyrins
Product yield %
a
Entry
Oxidant
Catalyst
BA
PA
SO
Total
III
1
2
3
4
5
6
None
m-CPBA
Mn TECPCl
0
0
0
0
III
Mn TECPCl
0.72
1.32
4.67
3.20
71.47
37.01
42.40
14.00
2.62
37.56
29.68
3.25
1.63
1.57
17.43
6.48
33.31
41.37
14.73
11.07
3.68
32.15
10.42
75.54
74.25
89.45
49.71
47.65
III
Mn TPPCl
III
PhIO
Mn TECPCl
III
Mn TPPCl
III
TBHP
Mn TECPCl
b
III
7
8
Mn TECPCl
III
Mn TPPCl
a
Reaction conditions: catalyst (0.001 mmol), oxidation (0.1 mmol),
styrene (1 mmol) in 2 mL acetonitrile stirring at room temperature.
Yield and selectivity of product were detected by GC, chlorobenzene
b
as internal standard. Under argon atmosphere.
III
III
Fig. 4 Cyclic voltammograms of Mn TECPCl and Mn TPPCl in DMF
ꢀ1
containing 0.1 M TBAP, scan rate ¼ 0.1 V s (ferrocene as internal
standard).
7
manganese porphyrins. PhIO or m-CPBA is a single-oxygen
donor and favors the formation of the high-valent (oxo)
IV
29
manganese species Mn ]O, which is responsible for oxygen
insertion into the carbon–carbon double bond of styrene. When
using TBHP as oxidant, the main product was benzaldehyde
couple at E1/2 ¼ 0.021 V may be assigned to Mn(III)/Mn(II). The
other more negative potential redox couples at E1/2 ¼ ꢀ0.96,
ꢀ
1.53 and ꢀ1.83 V are assigned to porphyrin ring-centered
(
Table 3, entry 6), which may be attributed to free radical
redox processes related to formation of Mn(II) p-anion
radical, p-dianion and p-trianions radicals species. The Mn(III)/
63–65
involved oxidation.
The yield of benzaldehyde and total
oxidation yield was signicantly lowered under argon (Table 3,
III
Mn(II) redox couple of Mn TPPCl appeared at more negative
III
entry 7), further conrming Mn TECPCl/TBHP system inclines
III
E1/2 (ꢀ0.19 V). This suggests Mn TECPCl may have a higher
45
to free radical involved oxidation. For all tested oxidants, the
III
stability and catalytic oxidation activity than Mn TPPCl. In
III
catalytic activity of Mn TECPCl was observed much higher
addition, an additional oxidation process was observed at E1/2
III
than control Mn TPPCl, this may be rationalized by the
III
¼
1.64 V for Mn TPPCl, which is assigned to the formation of
electron-withdrawing effect of the four meso-ethoxycarbonyl
57,58
the porphyrin radical cation.
This redox peak was not
III
groups in Mn TECPCl, which leads to a higher catalytic
III
observed for Mn TECPCl within the same electrochemical
activity. This may also be conrmed from the anticathode shi
window, possibly because of the electron-decient virtue of
3
+
2+
III
of its Mn /Mn couple as compared to Mn TPPCl (Fig. 4).
It is known the periphery electron-withdrawing group will
not only enhance the catalytic activity of metalloporphyrin, but
III
Mn TECPCl.
Catalytic oxidation activity
59
also its anti-oxidation stability. To check the anti-oxidation
It is well documented that high-valent metalloporphyrins stability of Mn TECPCl, direct reaction of catalysts with PhIO
III
having electron-withdrawing groups at the four meso positions and TBHP were carried out and the reaction were monitored by
5
9
III
are efficient oxidation catalysts. Styrene is a commonly used UV-vis spectroscopy. As shown in Fig. 5A, Mn TECP displays
substrate.
23,60,61
Previous literatures and our group studies classical electronic spectra of manganese porphyrin spectrum.
showed that acetonitrile is an ideal solvent for manganese cat- Its UV-vis characteristics may well be depicted using Gouter-
19,45,62
66
III
alysed oxidation of organic substrate.
Here, the catalytic man's four-orbital model. Electronic spectra of Mn TPPCl
oxidation of styrene was carried out in acetonitrile with
different oxidants at room temperature. The main oxidation
products are benzaldehyde (BA), phenyl acetaldehyde (PA) and
styrene epoxide (SO).
The nature and the relative yields of the products formed by
catalytic oxidation of styrene using porphyrins vary consider-
ably depending on the catalyst and oxidant. In this work, three
oxidants TBHP, m-CPBA and PhIO were used, and the experi-
mental conditions were kept the same in all catalytic reactions.
In the absence of oxidant, no any products could be detected
(Table 3, entry 1). When using m-CPBA or PhIO as oxidant, the
main products are phenyl acetaldehyde (PA) and styrene
epoxide (SO) (Table 3, entry 2 to 5), similar to the previously
III
III
Fig. 5 UV-vis spectra changes of Mn TECPCl (A) and Mn TPPCl (B)
upon the addition of PhIO in CH CN solution (reaction time 2.5 h.
3
reported catalytic oxidation of styrene by b-brominated [PhIO]/[catalyst] ¼ 100 : 1).
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III
III
Fig. 6 UV-vis spectra changes of Mn TECPCl (A) and Mn TPPCl (B)
upon the addition of TBHP in CH CN solution (Reaction time 12.5 h.
3
[
TBHP]/[catalyst] ¼ 500 : 1.).
6
7
(
Fig. 5B) has been assigned in earlier publication. Soret peak
0
VI (372 nm) is originated from a , a / e * and a 2u, b2u / eg
1
u
2u
g
0
transitions. Soret peak V (398 nm) is LMCT band from a
/
2u
a
a1g transition. The sharp absorption peak V (474 nm) is the
III
0
Fig. 7 The total oxidation products yield changes of Mn TECPCl
typical LMCT band assigned to a 2u, b2u / e
less intense Q bands IV (580 nm) is from a1u, a2u / e
g
transition. The
* tran-
sition, and III (617 nm) is LMCT band from a1u, a2u / e
catalyst with the different oxidants and cycles (reaction condition:
g
catalyst/oxidant/styrene ¼ 1 : 100 : 1000 in 2 mL CH
3
CN, catalyst:
g
1
mM, products were analyzed by GC after 2.5 h).
III
transition. Mn TECPCl exhibits similar absorption peaks at
58 nm (VI), 468 nm (V) and 568 nm (IV) respectively. Due to the
3
substituent effect, V and III peaks could not be resolved clearly
a
III
in the UV-vis spectra of Mn TECPCl. Which is quite similar to
the case of chloro(aquo)etioporphyrinI manganese(III)
6
7
III
complex. As compared to Mn TPPCl, the electronic spectrum
III
of Mn TECPCl is obviously blue-shied. This may be ratio-
nalized by the electron-decient property of porphyriniod
68
macrocycle. When PhIO oxidant was added to the catalyst
III
solution, Mn TPPCl was completely destructed aer 2.5 hours
as indicated by UV-vis spectrum (Fig. 5B). While only about 12%
III
Mn TECPCl decomposed at the same condition (Fig. 5A).
Interestingly, when using TBHP as oxidant, nearly no decom-
III
position of Mn TECPCl could be observed even aer 12.5 hours
with [TBHP]/[catalyst] ¼ 500 : 1 (Fig. 6A). In this situation,
III
although the stability of Mn TPPCl was also observed greatly
III
enhanced, the oxidative destruction Mn TPPCl was still very
obvious as indicated by the intensity loss of the absorption
peaks (Fig. 6B).
These observations prompted us to explore the recovery or
reuse of Mn TECPCl under the catalytic reaction conditions
III
III
Fig. 8 FT-IR spectra (KBr pellets) of recovered and original Mn TECPCl
catalyst.
using TBHP and PhIO oxidatns (Fig. 7). Aer ve cycles, the
III
total oxidation products yield by Mn TECPCl was kept above
7
0% for both oxidants. This indicates meso-ethoxycarbonyl
1
9
benzaldehyde as main product. Three types of reactive species
have been postulated for manganese porphyrin catalyzed
groups were good alternative to stabilize manganese
porphyrin in resisting oxidation. The IR spectra of recovered
Mn TECPCl is the same to original catalyst (Fig. 8), suggesting
Mn TECPCl is very stable in the catalytic reactions.
a
28
III
oxidation reactions: porphyrin manganese(IV)-oxo, porphyrin
manganese(V)-oxo and porphyrin Mn –HOOR.
manganese(V)-oxo is a transient species, it return back to
manganese(III) via manganese(IV)-oxo in a few minutes.
To check the possible active species involved, direction
26
III
31–33
III
Porphyrin
21,25,69
Mechanistic considerations
Manganese porphyrins possess high catalytic activity in alkene reaction the manganese porphyrin with TBHP and PhIO were
21
7
22
epoxidation using PhI(OAc)
2
,
PhIO and m-CPBA oxidants. monitored by UV-vis spectra (Fig. 9). In the presence of TBHP,
Epoxides are generally considered from the oxygen atom there was no obvious change in the spectrum, except a small
transfer (OAT) reaction of high-valent (oxo)manganese new shoulder peak appeared at Soret band and obvious
27
porphyrin and alkene. It has been observed manganese decrease in the absorption intensity. Similar spectral changes
III
porphyrin catalyzed oxidation of styrene by TBHP oxidant gave were observed for Mn TPPCl too. This spectra change may be
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III
benzaldehyde in styrene/Mn TECPCl/TBHP system and there
III
is no spectroscopic sign that the reaction of Mn TECPCl and
IV
TBHP will generate porphyrin Mn ]O intermediate. We
suggest here the catalytic oxidation of styrene by TBHP is a free-
70
radical involved process. p-Benzoquinone (p-BQ) is a well-
known reactive oxygen radical species (RORS) trapping
71
III
agent. When p-BQ was added to the styrene/Mn TECPCl/
TBHP system, the catalytic oxidation reaction was totally sup-
pressed (Fig. 10), indicating RORS were indispensable active
intermediate. When using PhIO as oxidant, the addition of p-BQ
will also lower the total product yield remarkably. This suggests
RORS is also involved in the oxidation of styrene by PhIO.
Interestingly, p-BQ cannot suppress the styrene oxidation
III
III
Fig. 9 UV-vis spectra changes of Mn TECPCl (left) and Mn TPPCl
right) upon the addition of TBHP and PhIO in acetonitrile [Mn
porphyrin]/[Oxd] ¼ 1/10.
(
III
completely when using Mn TPPCl catalyst and TBHP oxidant.
III
When using PhIO as oxidant, the catalytic activity of Mn TPPCl
III
was not inhibited as much as that of Mn TECPCl. This shows
the catalytic pathway not only depends on the oxidant, but the
virtue of the catalyst also. Detailed mechanism still needs
further investigation.
The plausible mechanism of styrene oxidation catalyzed by
manganese porphyrin is depicted in Scheme 2. The reaction
III
III
III
between TBHP and Mn TECPCl or Mn TPPCl gives Mn -
HOOR intermediate, which undergo heterolysis to form RORS
which oxidizes styrene to give BA, PA and SO. When using PhIO
IV
as oxidant, porphyrin Mn ]O formed and subsequently
transfers its oxygen to the carbon–carbon double bond of
IV
styrene to form SO and PA. An alternate pathway is that Mn ]O
reacts with styrene to give radical intermediate which will
initiate oxygen involved free radical chain propagation
28,29
reaction.
Fig. 10 Change in the yield of products after adding p-BQ in different
catalysts/oxidants systems (+ ¼ present; ꢀ ¼ absent).
Conclusions
In summary, we have reported the synthesis, structure charac-
terization, magnetic properties of Mn TECPCl, and its catalytic
III
ascribed to the formation of Mn –HOOR. In the presence of
III
III
PhIO, the UV-vis spectra of Mn TECPCl exhibited a new Soret
3
activities for styrene in CH CN in homogenous system. In its
band at 387 nm and the LMCT (ligand to metal charge transfer)
band at around 475 nm disappeared, indicating the formation
III
crystal, Mn TECPCl is linked through O–Mn coordination
bonds to form dimer, and displays weak antiferromagnetic
intradimer coupling between Mn(III) ions. As compared to tetra-
IV
31
of high-valent porphyrin Mn ]O intermediate. Similar
III
phenomena were observed for Mn TPPCl. From these obser-
vations, it can be concluded that Mn TPPCl and Mn TECPCl
react with PhIO yield high-valent Mn ]O intermediate, while
TBHP coordinates to the porphyrin manganese metal centre to
form axial ligated complexes. As the main product is
III
III
aryl manganese porphyrin Mn TPPCl, Mn TECPCl exhibits
III
III
III
better catalytic activity and anti-oxidation capacity. Mn TECPCl
IV
can be recycled with high catalytic activity even aer ve cycles
in the oxidation of styrene by TBHP and the plausible reactive
intermediate is reactive oxygen radical species generated from
III
Scheme 2 Plausible mechanism for oxidation of styrene by Mn TECPCl.
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III
III
the heterolysis of porphyrin Mn –HOOR. Mn TECPCl was also 22 Z. Solati, M. Hashemi and L. Ebrahimi, Catal. Lett., 2011,
reusable in the oxidation of styrene by using PhIO oxidant.
141, 163–167.
2
2
3 C. G. Sun, B. C. Hu and Z. L. Liu, Chem. Eng. J., 2013, 232, 96–
103.
Acknowledgements
4 N. Amiri, P. le Maux, H. Srour, H. Nasri and G. Simonneaux,
This work was supported by National Natural Science Founda-
tion of China (21171057, 21371059).
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25 J. T. Groves, J. Lee and S. S. Marla, J. Am. Chem. Soc., 1997,
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1
2
6 W. Nam, I. Kim, M. H. Lim, H. J. Choi, J. S. Lee and
H. G. Jang, Chem.–Eur. J., 2002, 8, 2067–2071.
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