JOURNAL OF CHEMICAL RESEARCH 2010
OCTOBER, 581–584
RESEARCH PAPER 581
An efficient aerobic oxidation for p-xylene to p-toluic acid by
unsymmetrical Schiff base manganese(III) complexes with pendant
benzo-10-aza-crown ether or morpholino-groups
a
a
a
a
b
Jian-Zhang Li *, Zhu-Zhu Yang , Xi-Yang He , Jun Zeng and Sheng-Ying Qin
a
Key Laboratory of Green Catalysis of Higher Education Institutes of Sichuan, Department of Chemistry, Sichuan University of Science &
Engineering, Zigong, Sichuan 643000, P. R. China
Department of Chemistry, Sichuan University, Chengdu, Sichuan 610064, P. R. China
b
Unsymmetrical Schiff base Mn(III) complexes with pendant aza-crown or morpholino groups have been synthesised
and studied as catalysts in aerobic oxidation of p-xylene to p-toluic acid (PTA). The oxidation of p-xylene to p-toluic
acid with air at 120 °C under normal atmospheric pressure occurred efficiently in the presence of aza-crown ether
substituted unsymmetrical Schiff base Mn(III) complexes. Significant selectivity (up to ~90%) and conversion levels
(
up to ~40%) were obtained. The effect of the pendant aza-crown ether group in Mn(III) Schiff base complexes on
the oxidation of p-xylene was also investigated by comparison with the pendant morpholino-group analogues. The
addition of Na cation accelerated the rate of conversion of p-xylene to p-toluic acid.
+
Keywords: benzo-10-aza-15-crown-5, unsymmetrical salicylaldimine bis-Schiff base, aerobic oxidation, Schiff base manganese
(
III) complexes, oxidation of p-xylene
With the increasing environmental and economical concerns in
recent years, much attention has been directed toward the
development of environmentally benign catalytic reactions
that use molecular oxygen as a primary oxidant, which is
morpholino-groups and report the homogeneous direct cata-
lytic oxidation of p-xylene to p-toluic acid by air in the
presence of the Schiff base Mn(III) complexes under mild con-
ditions. The structures of unsymmetrical Schiff base Mn(III)
complexes are shown in Fig. 1.
1
–3
readily available and produces water as the sole byproduct.
The oxidation of p-xylene to p-toluic acid is an industrially
important process since substituted benzoic acids are very
important materials in the chemical and pharmaceutical indus-
tries. In recent years, many studies on the oxidation of p-xylene
Experimental
Melting points were determined on a Yanaco MP-500 micro-melting
point apparatus and are uncorrected. IR spectra were recorded on a
Nicolet-1705X spectrometer. H NMR spectra were recorded on a
Bruker AC-200MHz spectrometer using Me Si as internal standard.
Mass spectra were obtained on a Finnigan LCQ spectrometer.
The metal ion content was measured using an IRIS-Advantage ICP
1
to p-toluic acid catalysed by Co(OAc) /NaBr/AcOH or
2
Co(C H O ) (cobalt stearate)/NH Br have been reported,
4
18
35
2
2
4
however, under these conditions, the selectivity (<25%) for
4
,5
p-toluic acid is low. Eastman–Kodak Tory and Amoco/
emission spectrometer. The halogen analysis was measured using a
mercury titration method,
6,7
Mid-Century processes are used for the manufacture of
p-toluic using a Co(OAc) / Mn(OAc) /NaBr catalyst in liquid
16,17
and other elementary analyses were per-
2
2
formed on a Carlo Erba 1106 elemental analyser. Molar conductances
were obtained on a DDS-11A conductivity meter in DMF solutions
phase with air as oxidant. Both of these processes operate
at 200 °C and 30 atm air to give terephthalic acid (>97%) at a
p-xylene conversion of 99%. However, because of the environ-
mental issues associated with the production of large volume
chemicals and the corrosive nature of the by-products in the
above catalyst system, many researchers have been trying to
develop highly efficient and selective catalysts for the oxida-
tion of p-xylene to p-toluic acid and to achieve environmen-
tally friendly and highly economic processes. The biomimetic
catalytic oxidation of p-xylene to p-toluic acid using Co(II)
hydroxamates complexes in liquid phase with air as oxidant
−
3
−1
(1.0 × 10 mol L ). The molar magnetic susceptibility was obtained
on a magnetic balance T3-200 at 25 °C.
Salicylaldehyde, 1,2-phenylenediamine and p-xylene were pur-
chased in China. Standard samples of p-toluic acid, p-tolualdehyde,
terephthalic acid, 5-chlorosalicylaldehyde, 5-bromosalicylaldehyde
and 5-methylsalicylaldehyde were obtained from Aldrich Co. N-
(
2-hydroxy-3-formyl-5-chlorobenzyl)morpholine and N-(2-hydroxy-
3
-formyl-5-bromo-benzyl)morpholine were prepared according to the
13
2
literature; unsymmetrical Schiff base ligand H L was prepared
2
18
1
4
according to the literature,
H
2
L , H
L and their Mn(III) complexes
2
1
4
19
(MnL Cl and MnL Cl) were prepared according to the literature.
p-Xylene was purified prior to use. Other reagents were of analytical
grade and were used without further purification.
8,9
has been reported. To the best of our knowledge, previous
studies on the Schiff base complexes with pendant aza-crown
group as catalyst in aerobic oxidation of p-xylene to p-toluic
Synthesis of unsymmetrical Schiff base ligands
1
0,11
acid are rare.
ties conferred by the hydrophobicity of the outer ethylene
Crown ether rings endow interesting proper-
(
For formulae and structure of ligands see Fig. 1)
3
1
Ligand H L was prepared as described for H L except starting with
2
2
1
2,13
groups and orderly arrangement of inner oxygen atoms.
N-(2-hydroxy-3-formyl-5-methylbenzyl)benzo-10-aza-15-crown-5
instead of N-(2-hydroxy-3-formyl-5- chlorobenzyl)benzo-10-aza-15-
crown-5 to give a yellow solid, yield 74%, m.p. 84–86 °C. H NMR
δ (p.p.m.): 12.53 (s, 1H, OH, D O exchange), 9.98 (s, 1H, OH, D O
exchange), 8.62 (s, 1H, N=CH), 8.40 (s, 1H, N=CH), 7.67–6.64 (m,
Crown ether-containing Schiff bases are known to bind cations
in the crown ether cavity in addition to coordination of a transi-
tion metal center through the N O donor atoms. Co-complex-
1
2
2
2
2
ation of a hard cation close to the transition metal centre is
believed to play an important role in improving its oxygen
1
4
2
4H, ArH), 4.12–3.71 (m, 14H, OCH , NCH Ar), 2.82 (t, J=5.6Hz,
2 2
−1
H, NCH ), 2.32 (s, 3H, CH ); IR (KBr, cm ) vmax: 3430, 3228, 2968,
2
3
14,15
binding properties.
As part of a research program aimed at
930, 2858, 1617, 1602, 1500, 1250, 1129, 1052, 930; ESI-MS m/z:
+
studying the effects of the aza-crown ether group pendant in
Mn(III) Schiff base complexes and the addition of alkali metal
cations into the reactive system on the catalytic oxidation per-
formance of p-xylene, we have synthesised novel unsymmetri-
cal Schiff base Mn(III) complexes with pendant aza-crown or
675 (M +1). Found: C, 70.77; H, 6.52; N, 6.54. C H N O . Calcd:
36 39 3 6
C, 70.94; H, 6.40; N, 6.70%.
5
4
Ligand H L was prepared as described for H L except starting
2
2
with N-(2-hydroxy-3-formyl-5-bromobenzyl)morpholine instead of
N-(2-hydroxy-3-formyl-5-chlorobenzyl) morpholine to give a yellow
solid, yield 73%, m.p. 77–79 °C. H NMR δ (p.p.m.): 13.06 (s, 1H,
1
OH, D O exchange), 10.15 (s, 1H, OH, D O exchange), 8.60 (s, 1H,
N=CH), 8.46 (s, 1H, N=CH), 7.56–6.88 (m, 10H, ArH), 3.72–3.49 (m,
2
2
*
Correspondent. E-mail: sichuanligong2006@hotmail.com