Oxidative Dimerization of o-Aminophenol
283
Over the past few years, the oxidation of o-aminophenol
(AP) to 2-amino-3H-phenoxazin-3-one (APX) over differ-
ent types of transition metal salts and complexes has been
intensively studied with the aim to mimic the catalytic
activity of phenoxazinone synthase (a naturally occurring
copper-containing oxidase that catalyses the oxidative
coupling of AP compounds to phenoxazinone chromoph-
ores in soil bacteria) [23–25]. APX is one of the derivatives
of phenoxazinone, which has been found as a chromo-
phoric part of actinomycins. Actinomycins, especially
actinomycin D, have been found to inhibit DNA direct
RNA synthesis and are clinically used in the treatment of
some kinds of cancers and tumours [26, 27]. The catalytic
oxidation of AP to APX has mostly been studied in
homogeneous systems [23–25]. However, several studies
have been reported on the heterogeneous oxidation of AP
to APX. For the examples, oxidation of AP to APX has
been successfully carried out over bis(2-[a-hydroxy-
ethyl]benzimidazolato)copper(II) anchored on a polymer
and over K2[Cu(C2O4)2] complex anchored on SiO2 in the
presence of dioxygen at mild temperatures, respectively
[28, 29].
6.05 (2H, d, J = 2.4 Hz, H-6), 6.22 (2H, dd, J = 8.8 Hz,
J = 2.4 Hz, H-4), 7.08 (2H, d, J = 8.8 Hz, H-3), 8.25 (2H,
s, H-7), 13.42 (2H, broad, s, –OH). 13C NMR (acetone-d6),
dC (ppm): 12.06 (4C, –CH3), 44.04 (4C, –CH2–), 59.51
(2C, C-8), 97.84 (2C, C-6), 102.91 (2C, C-4), 108.55 (2C,
C-1), 132.81 (2C, C-3), 151.05 (2C, C-5), 163.56 (2C,
C-2), 165.26 (2C, C-7). UV–Vis DR, kmax (nm): 190–280
(p–p*), 283 (n–p*), 385 (p–p*).
2.2 Synthesis of CAS Complex
The prepared N,N0-bis[4-(N,N-diethylamino)salicylidene]
ethylenediamine (4 mmol) was stirred with copper(II) ace-
tate monohydrate(4 mmol) in acetonitrile for 2 h at room
temperature. After drying, the product was recovered and
recrystallised from a mixture of hexane and acetone (9:1) to
afford copper(II) N,N0-bis[4-(N,N-diethylamino)salicylidene]
ethylenediamine (1.79 g, 91.33 %) as a brownish black
solid. IR, t (cm-1): 3447 (OH), 3087 and 2968 (CH, sp2),
2922 (CH, sp3), 1591 (C=N), 1591 and 1541 (C=C aromatic),
1354 (C–N), 1248 (C–O). UV–Vis DR, kmax (nm): 190–300
(p-p*), 312 (n-p*), 416 (p-p* and metal–ligand charge
transfer), 557 (d–d transition of copper(II) ion).
Mesoporous silica MCM-48 was used as a support in
this study because it possesses high specific surface areas
and chemical and thermal stability as well as providing a
modifiable silanol surface for immobilising a homogeneous
complex [10, 30]. Previous studies have shown that mes-
oporous silica is a good solid support which does not
destroy or hinder the active site of the homogeneous cat-
alyst. Moreover, it can enhance the performance of cata-
lysts, such as increasing the selectivity and reusability of
catalysts [31]. In this study, the heterogenisation of the
CAS complex was realised by first modifying the MCM-48
surface by the functionalisation of the amino and sulfonic
acid group, respectively.
2.3 Heterogenisation of CAS on MCM-48
Mesoporous silica MCM-48 (Si-MCM-48) was prepared
according to the procedure published [32].
2.3.1 Preparation of Organo-functionalisation MCM-48,
NH2-MCM-48 and SO3H-MCM-48
For the preparation of amino-functionalised MCM-48 (NH2-
MCM-48), 3-(aminopropyl)trimethoxysilane (9 mmol) in
dry toluene was added into dried toluene containing dehy-
drated Si-MCM-48 (3.00 g). The resulting mixtures were
refluxed for 24 h at 110 °C. The suspension was then
recovered and soxhlet extracted with acetonitrile for 3 h.
Sulfonic acid-functionalised MCM-48 (SO3H-MCM-48)
was prepared by oxidising mercapto-functionalised
MCM-48 over tert-butyl hydrogen peroxide, TBHP (70 %
in water). Dry toluene containing 3-(mercaptopropyl)tri-
methoxysilane (9 mmol) and Si-MCM-48 (3.00 g) was
refluxed for 24 h at 110 °C. The solid was recovered and
washed with acetonitrile using the soxhlet extraction
technique for 3 h. The resulting solid (3.00 g) was then
stirred in acetonitrile containing tert-butyl hydrogen per-
oxide (11 mL) for 2 days at room temperature. The pre-
cipitate was recovered and washed with acetonitrile.
2 Experimental
2.1 Synthesis of N,N0-Bis[4-(N,N-
diethylamino)salicylidene]ethylenediamine Ligand
Ethylenediamine (15 mmol) and 4-(N,N-diethylamino)sal-
icylaldehyde (30 mmol) were refluxed in ethanol for 2 h.
The reaction product was then recrystallised in methanol
to afford N,N0-bis[4-(N,N-diethylamino)salicylidene]ethy-
lenediamine (4.97 g, 80.64 %) as yellow solid with melting
point 136.2–138.6 °C. IR, t (cm-1): 2400–3200 (OH with
presence of intramolecular hydrogen bonding), 2967, 2839
and 3081 (C–H, sp2), 2912 (-CH, sp3), 1611 (C=N), 1560
1
and 1521 (C=C aromatic), 1342 (C–N), 1241 (C–O). H
NMR (acetone-d6), dH (ppm): 1.15 (12H, t, J = 7.2 Hz,
–CH3), 3.40 (8H, q, J = 7.2 Hz, –CH2–), 3.79 (4H, s, H-8),
123