Synthesis and catalytic properties of a series of cobalt porphyrins as cytochrome P450 model: The effect of substituents on the catalytic activity

Article abstract of DOI:10.1007/s10847-012-0205-x

A series of cobalt porphyrins derived from hemin was prepared as cytochrome P450 models. Effects of substituents at the cobalt deuteroporphyrin-propionate side chains are investigated in oxidation of toluene with air to benzaldehyde and benzyl alcohol without the use of solvent and sacrificial co-reductant. The catalytic activity of cobalt porphyrins depends on the type of substituents. When the electron-withdrawing groups like -Cl, -Br, were introduced into the double propionate side chains, they can increase the catalyst stability and selectivity to benzaldehyde. In comparison with these electron-withdrawing groups, the electron-donor groups, such as -CH₃, -S-S- and -NH ₂ groups, can improve their catalytic activities. Moreover, the electron-donor group containing an unpaired electron (such as -S-S-, -NH ₂) is benefit for improving its catalytic efficiency and promoting the electron delivery. It can be concluded that the double propionate side chains in the deuteroporphyrin complex may participate in oxidation process and effect electron transfer from the high-valent metalloporphyrin species to the substrate.

Full text of DOI:10.1007/s10847-012-0205-x

J Incl Phenom Macrocycl Chem (2013) 76:345–352
DOI 10.1007/s10847-012-0205-x
ORIGINAL ARTICLE
Synthesis and catalytic properties of a series of cobalt porphyrins
as cytochrome P450 model: the effect of substituents
on the catalytic activity
Bingcheng Hu
Zuliang Liu
•
Chengguo Sun
•
Quanzhi Deng
•
Received: 3 May 2012 / Accepted: 14 June 2012 / Published online: 5 July 2012
Ó The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract A series of cobalt porphyrins derived from
hemin was prepared as cytochrome P450 models. Effects
of substituents at the cobalt deuteroporphyrin-propionate
side chains are investigated in oxidation of toluene with air
to benzaldehyde and benzyl alcohol without the use of
solvent and sacrificial co-reductant. The catalytic activity
of cobalt porphyrins depends on the type of substituents.
When the electron-withdrawing groups like –Cl, –Br, were
introduced into the double propionate side chains, they can
increase the catalyst stability and selectivity to benzalde-
hyde. In comparison with these electron-withdrawing
effective strategy to increase the stability of metallopor-
phyrins against oxidative degradation and develop new
porphyrin catalysts with enhanced catalytic activity for
hydroxylation of hydrocarbon [1–4]. The full understand-
ing of the relationship between the catalytic activity and
the structure of metalloporphyrins turns out to be an
important issue in designing catalysts for industrial appli-
cations. As a result, a lot of efforts have been invested to
synthesize multi-substituted porphyrins, trying to elucidate
the role of the substituents and the changes of catalytic
performances of catalysts [5, 6]. Indeed, the several major
studies have proved that halogenated, nitrified metallo-
porphyrins are more active as catalysts and more resistant
to oxidative degradation than their unsubstituted analogues
[7, 8]. Although the wide studies have focused on the effect
of substituents of meso-tetraphenylporphyrins on their
catalytic activity, the relationship between the structure of
porphyrin and the effect of peripheral substituents, espe-
cially at the propionate side chains, is still unclear, because
synthetic meso-tetraphenylporphyrin analogues deviate
dramatically from natural porphyrins [9].
groups, the electron-donor groups, such as –CH , –S–S–
3
and –NH groups, can improve their catalytic activities.
2
Moreover, the electron-donor group containing an unpaired
electron (such as –S–S–, –NH ) is benefit for improving its
2
catalytic efficiency and promoting the electron delivery. It
can be concluded that the double propionate side chains in
the deuteroporphyrin complex may participate in oxidation
process and effect electron transfer from the high-valent
metalloporphyrin species to the substrate.
Keywords Deuteroporphyrin Á Oxidation of toluene Á
Cobalt porphyrin Á Hemim Á Catalyst
In the previous work, the oxidation of cyclohexane with
air catalyzed by hemin derivatives has been investigated
and the propionate side chains are important factors influ-
encing the catalytic efficiency of the metallodeuteroporph-
yrin dimethyl ester [10, 11]. For a systematic study, in this
paper we have prepared a series of cobalt deuteropor-
phyrin derivatives (Co-DPs) by modification of the hemin-
propionate side chains (Scheme 1) and investigated the
effects of peripheral substituents at the deuteroporphyrin-
propionate side chains in the oxidation of toluene under the
conditions of 150 °C and 0.7 MPa using air as oxygen
donor. To our knowledge, this is the first time to investigate
the effect of substituents of hemin derivatives on catalyzing
the oxidation of toluene.
Introduction
During the past decades, introduction of peripheral sub-
stituents on the porphyrin macrocycle has been used as an
B. Hu (&) Á C. Sun Á Q. Deng Á Z. Liu
School of Chemical Engineering, Nanjing University of Science
and Technology, Nanjing 210094, Jiangsu, China
e-mail: hubingcheng@yahoo.com
123

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J Incl Phenom Macrocycl Chem (2013) 76:345–352
NH
N
N
+
LiAlH , THF
4
NH
N
N
PBr or SOCl2
NH
N
N
CH OH, H ,1 h
3
NH
N
N
3
rt, 2 h
HN
HN
CH Cl ,
reflux, 4 h
Ultrasound
HN
2
2
HN
O
O
R
R
O
O
OH HO
OH HO
OCH3 H CO
3
R= Cl; DP(-Cl)2
(1)CH CH OH,CHCl
DP
DPDME
R= Br
3 2 3
DHPP
DP(-Br)2
H NCSNH , reflux, 8
2
2
o
Reflux; SOCl2;
(2) Na CO , 80 C, 30
2 3
3
h
2
CH Cl2
min
O
COCl
(
1) R= 1,6-Hexylenediamime
NH
o
N
HN
N
N
HN
N
100 C, 30 min
N
N
S
S
Co
CH2 6
NH
NH
N
N
(
2) CoCl2 H O, DMF,
2
NH
COCl
reflux, 4 h
DP acyl chloride
O
Co-DP(-NHC H12NH-)
CoCl2 H O,
2
DMF, reflux, 4 h
o
1
00 C; Acohol or
DP(-SS-)
6
Amine
3
0 min
R
N
HN
N
CoCl2 H O
N
N
N
2
Co-DP(-NHBA)2
S
R= CONHCH2
R= CONH2
Co
NH
N
DMF, reflux, 4 h
Co-DP(-NH2)2
S
R
DP(-NHBA)2
R= CONHCH2
Co-DP(-SS-)
Scheme 1 Schematic structure of the Cobalt deuteroporphyrin derivatives in this study
Experimental
extent of degradation of catalyst in reaction conditions. IR
spectra were obtained by a Perkin–Elmer 681 instrument
with KBr optics. GC–MS analyses were performed on a
Thermo Trace DSQ mass spectrometer. The electrochemical
determination was tested in a Chen Hua CHI 660 Electro-
chemical Workstation. Redox potentials of the compounds
General methods
All reagents and solvent used were of analytical grate.
Deuteroporphyrin (DP) was synthesized and purified
according to our recent report [12]. Tetrahydrofuran (THF)
was dried by distillation from sodium and benzophenone.
-
3
(10 mol/L) in dried DMF containing 0.1 mol/L tetrabu-
tylammonium perchlorate (TBAP) as supporting electrolyte
were determined at room temperature by cyclic voltammetry
using platinum–carbon electrode, Pt and SCE as working,
counter and reference electrodes, respectively.
1
H NMR was recorded on a Bruker 500 MHz spectrome-
ters with tetramethylsilane (TMS) as an internal standard.
LC–MS/MS (ESI) mass spectra were recorded on a Finn-
igan TSQ Quantum ultra AM mass spectrometer in con-
Ò
junction with a Finnigan Surveyor LC system comprising
Preparation of deuteroporphyrin dimethyl ester
DP(–OCH3)2
of an HPLC with a built-in degasser, autosampler and PDA
detector (Thermo Electron Corporation, San Jose, CA,
USA). Column effluent was introduced into the electro-
spray chamber with a 0.1 mm internal diameter fused silica
capillary. Electrospray ionization spray voltage was
To a stirred mixture of DP(–OCH ) (0.20 g, 0.37 mmol)
3
2
in dry THF (30 mL) was added a solution of LiAlH4
(0.050 g, 1.32 mmol) in dry THF (10 mL). The mixture
was stirred at rt for 2 h after which ethyl acetate (3 mL)
was added. The reaction mixture was further stirred for
another 10 min and adjusted the PH of solution to 6.0 by
diluted hydrochloric acid. The organic layer was extracted
with CH Cl , dried over sodium sulfate. Column chroma-
3
,800 V in positive mode. Nitrogen was used as sheath gas
at 30 psi and as the auxiliary gas at 10 psi, and argon as the
collision gas at 1.2 mTorr, with the capillary temperature
2
70 °C. Analytes were introduced into the electrospray
source via an Thermo DB-C18 2.1 mm 9 150 mm column
at a flow rate of 20 lL/min (methanol/water = 80/20).
The UV–Vis spectra were measured by a Shimadzu
UV-240 spectrophotometer. The absorption changes versus
time at the kmax of Co-DPs have been used to determine the
2
2
tography (silica gel/dichloromethane:methanol = 10:1)
followed by usual workups gave the product. Yield: 0.13 g
(0.27 mmol, 73 %). m.p: [250 °C; 1HNMR (500 MHz,
DMSO-d ) d/ppm : = -4.01 (s, 2H, –NH); 2.38 (m, 4H);
6
1
23

J Incl Phenom Macrocycl Chem (2013) 76:345–352
347
1
3
4
1
.48 (dd, 4H, J = 7.5 Hz); 3.61, 3.66, 3.73, 3.77 (4s, 12H);
DP(-Cl) , m.p: [250 °C; H NMR (500 MHz, CDCl )
2
3
.15 (t, J = 7.5 Hz, 4H); 4. 81 (t, 2H); 9.33, 9.35(2s, 2H);
d/ppm = -3.86 (s, 2H), 2.75 (t, J = 7.5 Hz, 4 H),
3.62–3.84 (m, 16 H), 4.26 (t, J = 7.5 Hz, 4 H), 9.10 (s, 2
-
1
0.29, 10.30, 10.32, 10.33(4s, 4H); IR (KBr, cm ): 3450
?
H), 10.03, 10.07, 10.14, 10.16 (4s, 4H). ESI -MS (m/z):
(
m, OH), 3320 (m, NH), 2932 (m, CH), 1639 (w, C=C),
043 (m), 1061 (m), 1001 (m), 845 (m), 742 (m), 534 (s);
?
519.07 [M ? H] , Anal. Calcd for C H Cl N : C, 69.36;
1
3
0
32
2 4
?
ESI -MS (m/z): 483.04 [M ? H] . Anal. Calcd for
?
H, 6.21; N, 10.78. Found: C, 69.11; H, 6.69; N, 10.97.
C H N O : C, 74.66; H, 7.10; N, 11.61; Found: C, 74.92;
30 34 4 2
H, 7.60; N, 11.53.
Synthesis of 2,7,12,18-tetram ethyl-13,17-
di(3-disulfidepropyl) porphyrin DP(–SS–)
Preparation of 2,7,12,18-tetram ethyl-13,17-di(3-
hydroxypropyl) porphyrin DHPP
To a solution of thiourea (0.60 g) in ethanol (50 mL) were
added a solution of DP(-Br) (0.50 g) in CHCl (25 mL).
2
3
The mixture was stirred magnetically under reflux condi-
tion for 8 h. the process of reaction was monitored by TLC
until the DP(-Br)2 was consumed. Then, the saturated
Na CO solution was added to the mixture until the pH was
To a stirred mixture of DP(–OCH ) (0.20 g, 0.37 mmol) in
3
2
dry THF (30 mL) was added a solution of LiAlH (0.050 g,
4
1
.32 mmol) in dry THF (10 mL). The mixture was stirred at rt
2
3
for 2 h after which ethyl acetate (3 mL) was added. The
reaction mixture was further stirred for another 10 min and
adjustedthePHofsolution to6.0bydilutedhydrochloric acid.
The organic layer was extracted with CH Cl , dried over
9
.0. After stirring at 80 °C for 30 min, the mixture was
diluted with CH Cl (60 mL) and then washed with H O
2
2
2
(100 mL). The organic layers were decanted, combined,
2
2
dried, over NaSO , filtered and concentrated to yield the
4
sodium sulfate. Column chromatography (silica gel/dichlo-
romethane:methanol = 10:1) followed by usual workups
gave the product. Yield: 0.13 g (0.27 mmol, 73 %). m.p:
crude product, which was further purified by silica gel
chromatography using CH Cl as an eluent to provide the
2
2
1
desired product in 79 % (0.33 g) yield. m.p: [250 °C; H
[
250 °C; 1HNMR (500 MHz, DMSO-d ) d/ppm : = -4.01
6
NMR (500 MHz, CDCl ) d/ppm = -2.03 (s, 2 H), 2.76 (s,
3
(
s, 2H, –NH); 2.38 (m, 4H); 3.48 (dd, 4H, J = 7.5 Hz); 3.61,
4
H), 3.07 (t, J = 7.5 Hz, 4 H), 3.62, 3.65 (2s, 6 H),
3
.66, 3.73, 3.77 (4s, 12H); 4.15 (t, J = 7.5 Hz, 4H); 4. 81
3.73–3.75 (2s, 6 H), 4.32–4.35 (m, 4 H), 9.24, 9.26 (2s, 2
(
t, 2H); 9.33, 9.35(2s, 2H); 10.29, 10.30, 10.32, 10.33(4s, 4H);
-1
H), 10.53 (s, 1 H), 10.58–10.59 (2s, 2 H), 11.61 (s, 1 H). IR
IR (KBr, cm ): 3450 (m, OH), 3320 (m, NH), 2932 (m, CH),
639 (w, C=C), 1043 (m), 1061 (m), 1001 (m), 845 (m), 742
-
KBr, cm ): 3693 (m), 2925 (m), 1739 (s), 1554 (m), 1224
1
(
1
?
(m), 887 (m), 848 (m), 788 (m); ESI -MS (m/z): 512.97
+
m), 534 (s); ESI -MS (m/z): 483.04 [M ? H] . Anal. Calcd
?
(
?
M ? H] , Anal. Calcd for C H N S : C, 70.27; H, 6.29;
[
30 32 4 2
forC H N O : C, 74.66;H, 7.10; N, 11.61;Found:C, 74.92;
30 34 4 2
N,10.93. Found: C, 70.37; H, 6.21; N, 11.23.
H, 7.60; N, 11.53.
General procedure for the preparation of DP(–NHBA)2,
DP(–NHC H NH–) and DP(–NH )
Synthesis of 2,7,12,18-tetram ethyl-13,17-di(bromic
or chloric-propyl) porphyrin DP(-Br) , DP(-Cl)
6
12
2 2
2
2
To a solution of deuteroporphyrin (0.20 g, 0.39 mmol) in
dry CH Cl (30 mL) was added a solution (0.2 mL) of
A solution of DHPP (0.20 g, 0.41 mmol) in 40 mL of
dichloromethane was treated at 39 °C with 0.4 mL of PBr3
or SOCl in 5 mL of CH Cl under the condition of stirring
2
2
SOCl , which dissolved in CH Cl (5 mL). Then, the
2
2
2
2
2
2
reaction mixture was stirred under reflux for 3 h. After the
reaction was complete, CH Cl was evaporated, the excess
for 4 h. After completion, the reaction mixture was washed
with water, extracted with CH Cl . The solution was con-
2
2
2
2
amine (benzylamine, 1,6-hexylenediamime or NH ÁH O)
3
2
centrated and purified by silica gel chromatography
CH Cl :EtOAc = 10:1) to give 0.21 g (0.35 mmol, 84 %)
dissolved in N,N-dimethylformamide 10 mL was added and
heated at 100 °C for 30 min. At the end of the reaction, the
mixture solution was poured into distilled water. The resi-
due was filtered, dried in vacuum and purified by silica gel
column chromatography with CH Cl /ethyl acetate (5:2)
(
2
2
of DP(–Br) and 0.17 g (0.33 mmol, 81 %) of DP(-Cl) as
2
2
black solids, respectively.
1
DP(–Br) , m.p: [250 °C; H NMR (500 MHz, CDCl )
2
3
2
2
d/ppm = -3.87 (s, 2H), 2.85–2.88 (t, J = 6.5 Hz, 4 H),
for DP(–NHBA)2 (0.21 g, 0.31 mmol, 78 %) and with
CH Cl /CH OH (10:1) for DP(–NHC H NH–) (0.11 g,
3
.71–3.78 (m, 16 H), 4.29 (t, J = 6.5 Hz, 4 H), 9.12, 9.13
2
2
3
6 12
?
2s, 2 H), 10.08, 10.12, 10.19, 10.24 (4s, 4H). ESI -MS (m/
(
0.19 mmol, 48 %) and DP(–NH2)2 (0.13 g, 0.26 mmol,
1
?
z): 606.85 [M ? H] , Anal. Calcd for C H Br N : C,
3
0
32
2
4
66 %). DP(–NHBA) : m.p:[300 °C. H NMR (500 MHz,
2
5
5
9.22; H, 5.30; Br, 26.27; N, 9.21. Found: C, 59.71; H,
.44; N, 9.70.
DMSO-d ): d/ppm = -3.96 (s, 2 H, NH), 3.60, 3.64, 3.75,
6
3.78 (4s, 12 H), 4.11–4.13 (d, 4 H,), 4.40 (s, 1 H), 6.61,
123

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48
J Incl Phenom Macrocycl Chem (2013) 76:345–352
?
?
6
1
2
.74 (2s, 2 H), 8.38 (s, 2 H), 9.36–9.37 (s, 2 H), 10.30,
-
ESI -MS (m/z): 568.93 [M] . UV–Vis (CH Cl ), k (nm):
2
2
max
1
0.35, 10.41 (3s, 4 H); IR (KBr, cm ): 3294 (m, N–H),
917 (w, C–H), 1633 (s, C=O), 1548 (m), 1453 (m), 1374
391, 521, 547.
(
m), 1295 (w), 1266 (w), 1195 (m), 1227 (s), 1109 (m),
?
028 (w), 967 (m), 833 (w), 720 (s), 694 (m); ESI -MS
Oxidation of toluene catalyzed by Cobalt porphyrins
with air
1
?
(
m/z): 689.21 [M ? H] . Anal. Calcd for C H O N : C,
44 44 2 6
7
1
6.72; H, 6.44; N, 12.20. Found:C, 76.33; H, 6.67; N,
2.68.
Oxidation of toluene was carried out in a stainless steel
autoclave of 1 L volume equipped with a mechanical
stirrer, an internal thermocouple and cooling coils for
regulating the reaction temperature. In a typical experi-
ment, organic phase consisting of 300 mL toluene and
1
DP(–NHC H NH–): m.p:[300 °C. H NMR (500MHz,
6
12
DMSO-d ): d/ppm = -4.04 (s, 2 H, NH), 1.72–1.88
6
(
m, 8 H), 2.15–2.28 (m, 4 H), 3.62, 3.66 (2s, 6 H), 3.73–3.77
-
5
(
2s, 6 H), 4.35 (s, 2 H,), 9.33, 9.35 (s, 2 H), 10.30 (s, 1 H),
3.0 9 10 mol/L of metalloporphyrin were mixed with
50 mL nitrobenzene as an internal standard substance. The
experiment was performed at 150 °C under the air pressure
of 0.7 MPa. The reaction mixture was sampled by an ‘‘on-
line’’ means every 30 min until the yield decreased mark-
edly. The samples and the final products were analyzed by
GC–MS.
-
0.32(s, 1 H), 10.55 (2s, 2 H). IR (KBr, cm ): 3306 (m, NH),
1
1
2
925 (s), 1633 (s, C=O), 1544 (m), 1439 (m), 1375 (w), 1228
?
w), 1107 (m), 1035 (w), 840 (w). ESI -MS (m/z): 591.17
(
?
M ? H] . Anal. Calcd for C H N O : C, 73.19; H, 7.17;
[
3
6 42 6 2
N, 14.23. Found: C, 72.49; H, 7.33; N, 14.72.
1
DP(-NH ) : m.p:[250 °C. H NMR (500 MHz, DMSO-
2
2
d6): d/ppm = -4.06 (s, 2 H, NH), 3.03 (t, J = 7.5 Hz, 4 H),
3.63, 3.66, 3.73, 3.76 (4s, 12 H, 2-, 7-, 12-, 18-CH ), 4.33 (t,
3
J = 7.5 Hz, 4 H), 6.89, 6.93 (2s, 4 H), 7.50 (s, 2 H), 9.33 (s, 2
Results and discussion
-1
H), 10.28, 10.31, 10.35, 10.40 (4s, 4 H). IR (KBr, cm ): 3309
m, NH), 3171 (m, NH), 2917 (w), 1651 (s, C=O), 1406 (m),
282 (m), 1229 (w), 1135 (w), 1107 (m), 971 (m), 842 (w), 803
(
Stability of catalyst
1
?
?
(s), 728 (m), 666 (w). ESI -MS (m/z):509.10 [M ? H] . Anal.
Generally, porphyrin macrocycles are subject to oxidation
degradation in strong oxidizing medium [2, 13, 14]. The
stability of cobalt metalloporphyrins has been studied in the
presence of m-CPBA in dichloromethane solvent and UV–
Vis spectroscopy has been used to determine the extent of
degradation of catalyst. In this reaction, we have used cat-
alyst and m-CPBA in 1:200 molar ratios. The changes in the
UV–Vis spectra of a solution of Co-DP(–OCH3)2 in
dichloromethane at various time intervals after the addition
of m-CPBA have been shown in Fig. 1. The results indicate
that approximately 78.5 % of Co-DP(–OCH ) has been
Calcd for C H O N : C, 70.84; H, 6.34; N, 16.52. Found:C,
30 32 2 6
71.35; H, 6.61; N, 15.97.
General procedure for the metallation of porphyrin
Cobalt porphyrins were synthesized by the reaction of free
base porphyrin (0.10 g) with CoCl Á6H O (0.30 g) in DMF
2
2
(
30 mL) to afford high yields of more than 95 %. The
reaction condition was under reflux for 4 h. After the com-
pletion of the reaction, the reaction was quenched with water
3
2
degraded 8 min after the addition of m-CPBA. Under the
(
(
300 mL) and the mixture was extracted with CH Cl2
2
same conditions, 13.3 % of Co-DP(–NHBA) has been
2
100 mL). The organic layer was dried with magnesium
degraded (Fig. 1). The extent of degradation of other cobalt
porphyrins, which was measured through the change in
absorbance at the Soret band kmax of metalloporphyrins, has
been shown in Table 1. The results indicated the stability of
Co-DPs in the order Co-DP(–NHBA) [ Co-DP(–Br) [
sulfate and the solvents were evaporated. The crude com-
pound was purified by chromatography on silica gel using
CH Cl and methanol (10:1–10:3) as the mobile phase. All
2
2
products were analyzed and characterized by LC–MS to
?
indicate a high state of purity. (a) Co-DP(–OCH ) , ESI -
2
2
3
2
Co-DP(–NHC H NH–) [ Co-DP(–SS–) [ Co-DP(–Cl)
6
12
2
?
MS (m/z): 595.08 [M] . UV–Vis (CH Cl ), k
5
(nm):390,
[ Co-DP(–NH ) [ Co-DP(–OCH ) . It is observed that
2
2
max
2 2
3 2
?
12, 546; (b) Co-DP(-Br) , ESI -MS (m/z): 664.89 [M] .
?
2
with the exception of Co-DP(–NH–C H NH–) and Co-
6 12
UV–Vis (CH Cl ), k
(nm):395, 499, 547; (c) Co-DP
max
DP(–SS–), there is an approximate linear relationship
between the stability of the cobalt porphyrins and the
electronic effect of substituents at the propionate side
chains. The Co-DP(–NH ) and Co-DP(–OCH ) with the
2
2
?
?
(
–Cl) , ESI -MS (m/z): 575.15 [M] . UV–Vis (CH Cl ),
2 2 2
?
kmax (nm): 390, 517, 548; (d) Co-DP(–NHBA) , ESI -MS
2
?
(
m/z): 745.14 [M] . UV–Vis (CH Cl ), k (nm): 415, 527,
2
2
max
2 2
3 2
?
58; (e) Co-DP(–NHC H NH–), ESI -MS (m/z): 647.11
5
electron-donor groups are less stable than Co-DP(–Br) and
2
6
12
?
[
M] . UV–Vis (CH Cl ), k
(nm): 412, 526, 556; (f)
max
Co-DP(–Cl)2 bearing with electron-withdrawing groups.
Interestingly, although the group electronegativity of
–NHC H NH– and –SS– was lower than that of –NH and
2
2
?
Co-DP(–NH ) , ESI ? -MS (m/z): 565.03 [M] . UV–Vis
2
2
(
DMF), kmax (nm): 414, 525, 558; (g) Co-DP(-SS-)2,
6
12
2
1
23

J Incl Phenom Macrocycl Chem (2013) 76:345–352
349
2
1
1
0
0
.0
.6
.2
.8
.4
selected as catalysts in the oxidation of toluene with air as
oxygen donor under the conditions of 150 °C and 0.7 MPa.
Table 2 summarizes the data obtained from our catalytic
experiments. It may be seen that the selected Co-DPs
appeared suitable catalysts for the oxidation of toluene
under the chosen conditions. For the cobalt porphyrins with
different substituents at the hemin-propionate side chains,
the main products are benzaldehyde and benzyl alcohol. As
shown in Table 2, the reaction yield varies with the change
of the substituents, and the molar ratios of benzaldehyde to
benzyl alcohol changes obviously. Based on the total yield
of benzaldehyde and benzyl alcohol, it is concluded that
the order of catalytic activity of Co-DPs for the oxidation
of toluene is as follows: Co-DP(–SS–) [ Co-DP(–NH ) [
390
415
1
1
0
0
0
.6
.2
.8
.4
.0
Co-DP(-OCH )
Co-DP(-NHBA)
2
3
2
3
20 400 480 560 640 720 320
400
480
560
640
Wavelengh (nm)
2
2
Fig. 1 UV–Vis spectrum of a solution of Co-DP(–OCH
3
)
2
and Co-
DP(–NHBA) in CH Cl and their changes upon addition of m-CPBA
Co-DP(–OCH ) [ Co-DP(–NHBA) [ Co-DP(–NHC
3
2
2
6
2
2
2
H NH–) [ Co-DP(–Cl) [ Co-DP(–Br) .
1
2
2
2
with 2 min time intervals up to 8 min
To our surprise, the order of catalytic activity disfavors
the conclusion that observed the effect of electron-with-
drawing substituents at meso-postion of tetraphenylpor-
phyrin, which suggests that the double propionate side
chains play a special role in the oxidation of toluene.
Generally, several factors are considered in the discussion
of the correlation between the structure of metalloporphy-
rins and their catalytic activity in oxidation processes:
catalyst stability, electron effects of substituent, selectivity
of oxidation products and reduction potential.
Table 1 Degradation of Co-DPs in the presence of m-CPBA at room
temperature
Co-porphyrin
Soret band
(
Degradation
(%)
Time
(min)
nm)
Co-DP(–OCH
3
)
2
390
a
78.5
56.8
43.7
40.0
28.6
23.1
13.3
8
8
8
8
8
8
8
Co-DP(–NH
Co-DP(–Cl)
2
)
2
414
390
391
412
395
415
2
Co-DP(–SS–)
Co-DP(–NHC
Comparison of the results obtained for their stability
6
H
12NH–)
(Table 1) with those of their activity quoted in Table 2
Co-DP(–Br)
2
indicates that the order of stability is nearly opposite to the
order of catalytic activity with the exception of Co-DP(–
SS–). In the case of Co-DP(–Br) and Co-DP(–Cl) with
Co-DP(–NHBA)
2
2 2
The molar ratio of catalyst: oxidation is 1:200 in CH Cl
2
2
a
The Co-DP(-NH
2
)
2
is dissolved in DMF
electron-withdrawing groups at the propionate side chains,
the yield of benzaldehyde and benzyl alcohol is lower than
other catalysts with electron-donor substituents, such as
Co-DP(–OCH ) , Co-DP(–NH ) . It is interesting to note
–
OCH , Co-DP(–NHC H NH–) and Co-DP(–SS–) are
3 6 12
more stable than Co-DP(–NH2)2 and Co-DP(–OCH3)2.
Apparently, the groups at the propionate side chains of
Co-DP(–NHC H NH–) and Co-DP(–SS–) form the closed
3 2
2 2
that the effect of substituents was inconsistent with the
substituent effects of meso-tetraphenylporphyrins. How-
ever, the electron-withdrawing substituents at the propio-
nate side chains obviously can improve their stability in the
process of oxidative degradation process.
6
12
ring that contributes to metalloporphyrins stability toward
oxidative degradation.
Investigation on the catalytic activity of cobalt
porphyrins
We also found that with electron-donor groups at the
propionate side chains among the four cobalt porphyrins,
Co-DP(–NHC H NH-), Co-DP(–SS–), Co-DP(–NH2)2
6
12
Synthetic metalloporphyrins with various groups on the
phenyl ring have been employed to elucidate the effect of
substituent properties on their catalytic activity in oxidation
of hydrocarbon compounds [4, 15, 16]. These investiga-
tions have shed light on how substituent groups induce
changes in the catalytic activity of metalloporphyrins. It is
well known that the catalytic activity of the synthetic
metalloporphyrin complexes depend on the degree of
electron-withdrawing substituents at the phenyl ring. On
the basis of these considerations, a series of Co-DPs were
and Co-DP(–OCH ), the best activity for toluene oxide was
3
obtained with Co-DP(–SS–). Co-DP(–NH ) was also more
2
2
efficient than Co-DP(–OCH ), in spite of the high elec-
3
tronegativity of –NH group in the former compared with
2
the –OCH of the latter. Namely, our results suggest that
3
the groups at the propionate side chains play an important
role in the investigated reaction and they participate in the
reaction and partially contribute to tuning of the electron
density of the activated hemin species [17]. The –SS– and
–NH groups have the unpaired electrons that are prone to
2
123

3
50
J Incl Phenom Macrocycl Chem (2013) 76:345–352
Table 2 Oxidation of toluene with air catalyzed by various Co-DPs and Half-wave potential data for the metal-centered one-electron reduction
of Co-DPs
Cobalt porphyrins
Benzaldehyde
yield (%)
Benzyl alcohol
yield (%)
Reaction
a
time (h)
E
pa1 (mV)
E
pc1 (mV)
Half-wave reduction
b
potential E1/2 (mV)
Co-DP(–SS–)
11.6
10.2
10.1
9.4
4.3
2.9
1.6
1.1
0.5
0.1
–
5.0
4.0
5.0
6.0
5.0
6.0
6.0
-877.8
-830.8
-913.2
-884.6
-872.6
-880.0
-915.2
-695.4
-591.8
-694.2
-655.2
-664.0
-677.6
-650.0
-786.6
-711.3
-803.7
-768.4
-768.3
-778.8
-782.6
Co-DP(–NH
Co-DP(–OCH
Co-DP(–NHBA)
Co-DP(–NH–C
Co-DP(–Cl)
Co-DP(–Br)
2 2
)
3 2
)
2
6
H
12NH–)
8.8
2
1.9
2
1.3
Reaction conditions: Toluene 300 mL, Co-DPs, Pressure 0.7 MPa, temperature 150 °C
a
Time is defined as the reaction time until the yield of benzaldehyde and benzyl alcohol reaches the maximum
-3
Conditions: DMF, Co-DPs = 10 mol/L, TBAB = 0.1 mol/L, scan rate 100 mV/s
b
bind the cobalt in porphyrin core by forming the intermo-
with Co-DP(–Cl) , the catalytic activity increases with the
2
lecular interaction in the reaction. This coordination mode
would be responsible for the high values of yield for
benzaldehyde observed in the Co-DP(–SS–) and Co-DP
increase of the redox potential of cobalt porphyrin. It is
important to point out that this single linear relation just
exists in the same character of elements or functional
groups, for example, –Cl and –Br in the same column of
periodic table of elements. For the whole cobalt porphyrins
bearing the various substituents, the catalytic efficiency
does not correlate with the Co(III)/Co(II) reduction
potential. Thus, we conclude that the substitution of pro-
pionate side chains act as double roles both improving the
stable of structure and affecting the electron transfer.
Generally, in the catalytic cycle of P450, Compound I is
(
–NH ) systems. Such a conclusion proposed in this study
2 2
is reminiscent of the mechanism proposed by Idemori et al.
18] in the Mn(III) aminophenylporphyrins-catalyzed oxi-
[
dation of cyclohexane with iodosylbenzene and iodoben-
zene diacetate.
As shown in Table 2, it has been found that in case of
Co-DPs, introduction of electron-donor substituents at the
propionate side chains increases the selectivity of benzyl
alcohol and the order of ratio of benzaldehyde to benzyl
alcohol is opposite to the order of catalytic activity. The
low ratio of benzaldehyde to benzyl alcohol was observed
in the oxidation of toluene catalyzed by Co-DP(–SS–) and
Co-DP(–NH ) . This phenomenon may be attributed to the
IV
believed to be a Fe oxyferryl species with two unpaired
electrons in an iron-oxo moiety and a third unpaired elec-
tron in a porphyrin. The theoretical and experimental
studies indicated that the propionate side chains are crucial
in determining the nature of the third unpaired electron [19,
20]. Additionally, these results proposed a pathway
2
2
fact that the –SS–or –NH groups with unpaired electrons
2
can affect the porphyrin–cation radical complexes on the
mechanism of electron transfer. It should be noted that the
yield of benzyl alcohol is negligible when the –Br group is
introduced at the propionate side chains.
Co-DP (-N H-C H12N H-)
6
Co-DP (-B r)2
Co-DP (-C l)2
Furthermore, according to the reported literature, the
catalytic activity of metalloporphyrins has the linear rela-
tionship with their reduction potential [4, 15]. Figure 2
shows the cyclic voltammograms for these Co-DPs. All of
them have exhibited an irreversible wave corresponding to
the Co(III)/Co(II) reduction. Table 2 shows the half-wave
potentials (E1/2) for the one-electron reduction of Co-DPs.
Obviously, the half-wave potentials (E1/2) for the first
reduction of Co-DP(–OCH ), Co-DP(–NH ) and Co-DP
Co-DP (-S S-)
Co-DP (-N H2)2
Co-DP (-O CH3)
2
Co-DP (-NH BA)2
3
2 2
(
–Br)2 corresponding to the Co(III)/Co(II) couple were
located, respectively, at E1/2 = -803.7, -711.3 and
728.6 mV versus SCE in DMF. There is no linear rela-
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1.0
-1.1
-1.2
E/V vs. SCE
-
-
3
Fig. 2 Cyclic voltammograms of Co-DPs (10 mol/L) in dried
DMF containing TBAP (0.1 mol/L) at room temperature (scan rate
100 mV/s)
tionship between the catalytic efficiency and the Co(III)/
Co(II) potential. However, if we compare the Co-DP(–Br)2
1
23

J Incl Phenom Macrocycl Chem (2013) 76:345–352
351
e-
Acknowledgments The authors are grateful to Jiangsu Natural
Science Foundation (No. BK2009386), NUST research founding
(No. 2010GJPY043) and Jiangsu Graduate Innovation Project
(No. CXZZ11_0263.) for financial support.
δ
O-
H3C
e-
H C
2-
O
O
3
N
N
1
M
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
N
N
O
CH3
L
CH3
O-
Scheme 2 The electron transfer from the porphyrin system into the
metal center and from the propionate lone pairs into the porphyrin
system
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