Dioxygen affinities and catalytic oxidation performance of unsymmetrical bis-Schiff base transition-metal complexes with aza-crown pendant groups

Article abstract of DOI:10.3184/030823408X340654

Co^II and Mn^III complexes with aza-crown substituted, unsymmetrical bis-Schiff base ligands have been synthesised starting from monoaza-15-crown-5 or benzo-10-aza-15-crown-5. The saturated oxygen uptakes of the Co^II complex

Full text of DOI:10.3184/030823408X340654

JOURNAL OF CHEMICAL RESEARCH 2008
AUGUST, 461–464
RESEARCH PAPER 461
Dioxygen affinities and catalytic oxidation performance of unsymmetrical
bis-Schiff base transition-metal complexes with aza-crown pendant groups
Xing-yue Weia* and Sheng-ying Qinb
aCollege of Environmental and Biological Engineering, Chongqing Technology and Business University, Chongqing 400067, P.R. China
bFaculty of Chemistry, Sichuan University, Chengdu, Sichuan 610064, P.R. China
CoII and MnIII complexes with aza-crown substituted, unsymmetrical bis-Schiff base ligands have been synthesised
starting from monoaza-15-crown-5 or benzo-10-aza-15-crown-5. The saturated oxygen uptakes of the CoII complexes
[CoL1]–[CoL4]in diethyleneglycol/dimethyl ether solution were determined at different temperatures. The oxygenation
constant (Ko2) and thermodynamic parameters ('H°, 'S°) were calculated. The MnIII complexes ([MnL1Cl]–[MnL4Cl])
were employed to catalyse styrene oxidation using molecular oxygen at ambient temperature and pressure. The
modulation of O2-binding capabilities and catalytic oxidation performance by the aza-crown ether pendant groups
in [ML3] and [ML4] were investigated as compared with the parent complexes [ML1] and morpholino-substituted
analogue [ML2]. The results indicate that the dioxygen affinities and catalytic oxidation activities of [CoL3] and [CoL4]
have been much more enhanced by aza-crown pendants. Moreover, the O2-binding capabilities of [CoL3] and [CoL4]
were also improved by adding alkali metal cations (Li+, Na+ and K+) to the system, and adding Na+ shows the most
significant enhancement of dioxygen affinities. Likewise, [MnL3Cl] and [MnL4Cl] exhibit the best catalytic activities:
the conversion of styrene to benzaldehyde are up to 41.2% and 45.8% with more than 99% selectivity.
Keywords: XQV\PPHWULFDOꢀELVꢁ6FKLIIꢀEDVHVꢂꢀD]DꢁFURZQꢀHWKHUꢂꢀWUDQVLWLRQꢁPHWDOꢀFRPSOH[HVꢂꢀGLR[\JHQꢀDI¿QLWLHVꢂꢀFDWDO\WLFꢀR[LGDWLRQ
standard. Mass spectra were obtained on Finnigan MAT 4510 or
Finnigan LCQ-DECA spectrometers. Elemental analysis was performed
on a Carlo Erbo-1160 elemental analyser. Molar conductance was
obtained on a DDS–11A conductivity meter. GC analysis was carried
out on a Varian CP-3800 gas chromatograph (OV-1 column). The CoII
and MnIII contents were obtained on a IRIS-Advantage ICP emission
spectrometer. N-(3-formyl-4-hydroxybenzyl)morpholine (2b) and
N-(3-formyl-4-hydroxybenzyl)benzo- 10-aza-15-crown-5 (2d) were
prepared according to our report.14 2-hydroxy-5-chloromethyl-
benzaldehyde,19 monoaza-15-crown-5,20 compounds [CoL1] and
[MnL1Cl] were synthesised according to the literature method.21
Synthesis of N-(3-formyl-4-hydroxybenzyl aza-15-crown-5 (2c):
To a stirred mixture of aza-15-crown-5 (0.01 mol) and K2CO3
(0.011 mol) in acetonitrile (40 ml), a solution of 2-hydroxy-5-chloro-
methylbenzaldehyde (0.01 mol) in acetonitrile (10 ml) was added
GURSZLVHꢃꢀ 7KHꢀ PL[WXUHꢀ ZDVꢀ UHÀX[HGꢀ IRUꢀ ꢄꢅꢀ Kꢂꢀ FRROHGꢀ DQGꢀ ¿OWHUHGꢃꢀ
The solvent was evaporated and the residual mass was chromato-
graphed on a silica gel column (eluent: ethyl acetate) to give the pure
product as an oil, 86% yield. IR (neat) vmax: 3256, 1658, 1256, 1126
cm-1; 1H NMR (CDCl3ꢆꢇꢀ įꢄꢅꢃꢈꢉꢊVꢂꢀ ꢄ+ꢂꢀ 2+ꢂꢀ '2O exchangeable),
ꢋꢃꢋꢈꢊVꢂꢀ ꢄ+ꢂꢀ &+ 1ꢆꢂꢀ ꢌꢃꢍꢎ±ꢈꢃꢉꢄꢀ ꢊPꢂꢀ ꢏ+ꢂꢀ $U+ꢆꢂꢀ ꢍꢃꢄꢎ±ꢏꢃꢈꢉꢊPꢂꢀ ꢄꢉ+ꢂꢀ
OCH2, NCH2Ar), 2.86 (t, 4H, Jꢀ ꢀꢐꢃꢐꢀ+]ꢂꢀ1&+2); MS (m/z): 353 (M+).
Anal. Calcd for C18H27NO6: C 61.19, H 7.65, N 3.97. Found: C 61.30,
H 7.51, N 4.04%.
The transition-metal complexes of Schiff bases represent one
of the most successful classes of synthetic oxygen carrier
due to their structural similarity to those found in biological
systems.1,2 They have been extensively studied as models of
dioxygen-carrying metalloenzymes3 and oxygenases,4 such
as hemoglobin and cytochrome P–450,5 which play important
roles in the catalytic oxidation of organic substrates.6 However,
many of these complexes easily dimerise and lose activity
after O2 absorption.7 Avdeef and coworkers8 demonstrated
that formation of stable dioxygen adducts would be favoured
LIꢀ WKHVHꢀ FRPSOH[HVꢀ ZHUHꢀ PRGL¿HGꢀ E\ꢀ SURSHUꢀ VXEVWLWXHQWVꢀ
to obtain additional stereo hindrance and hydrophobicity.
Thus, various substituents have been employed to improve
O2-binding capabilities and catalytic oxidation activities
of the complexes.9-11 Among them, crown ether-containing
Schiff bases have attracted much attention because they can
bind both alkali and transition metal guest cations through
the crown ether cavity and the N2O2 donor atoms.12-15
In addition, co-complexation of alkali cations close to the
central transition-metal ion is known to be important in
LQÀXHQFLQJꢀ LWVꢀ GLR[\JHQꢁELQGLQJꢀ SURSHUWLHVꢃ16 Our recent
works have indicated that symmetrical axa-crowned bis-
Schiff bases transition-metal complexes are good receptors
for alkali cations17 and showed much improved O2-binding
activity and catalytic oxidation activity due to the special
FRQ¿JXUDWLRQꢀDQGꢀIXQFWLRQꢀRIꢀWKHꢀFURZQꢀHWKHUꢀULQJꢃꢀ0RUHRYHUꢂꢀ
adding alkali metal cations (Li+, Na+ and K+) will also favour
their O2-binding activity.
General method for synthesis of unsymmetrical bis-Schiff base
ligands HL2–HL4
A mixture of compound 1 (10 mmol and 2b–d (10 mmol) were
GLVVROYHGꢀLQꢀPHWKDQROꢀꢊꢐꢅꢀPOꢆꢀDQGꢀUHÀX[HGꢀIRUꢀꢈꢀKꢀXQGHUꢀ12, then
FRROHGꢀ DQGꢀ ¿OWHUHGꢀ WRꢀ JLYHꢀ WKHꢀ FUXGHꢀ SURGXFWꢀ DVꢀ DQꢀ RUDQJHꢀ VROLGꢃꢀ
The crude product was dissolved in the minimum of ethyl acetate and
then chromatographed on a silica gel column (eluent:ethyl acetate)
to give the pure product as a yellow solid.
However, to the best of our knowledge, no reports about
O2-binding activity and biomimetic catalytic oxidation
performance of crown ether-containing unsymmetrical
Schiff base complexes have appeared. As an extension of
our former studies,18 we herein report a facile way to prepare
unsymmetrical bis-Schiff base complexes bearing aza-crown
HWKHUVꢀDQGꢀGLVFXVꢀWKHꢀLQÀXHQFHꢀRIꢀWKHꢀD]DꢁFURZQꢀSHQGDQWꢀRQꢀ
the O2-binding activity and catalytic oxidation performance.
HL2: Yellow crystals; Yield 82%. m.p.178–180°C. 1H NMR
(CDCl3ꢆꢀįꢄꢍꢃꢄꢎꢀꢊVꢂꢀꢎ+ꢂꢀ2+ꢂꢀ'22ꢀH[FKDQJHꢆꢂꢀꢉꢃꢎꢎꢀꢊVꢂꢀꢄ+ꢂꢀ&+ 1ꢆꢂꢀ
7.15–6.86 (m, 15H, ArH), 3.71–3.65 (m, 6H, OCH2, NCH2Ar), 2.92–
2.76 (t, 4H, NCH2); IR(KBr) vmax: 3445, 1608, 1280, 1156 cm-1;
MS m/z: 526(M+). Anal. Calcd for C31H28N3O3Cl: C 70.72, H 5.32,
N 7.98. Found C 70.91, H 5.17, N 7.75%.
HL3: Yellow crystals; Yield 61%. m.p. 126–128°C. 1H NMR
(CDCl3ꢆꢀ įꢇꢀ ꢄꢍꢃꢄꢄꢊVꢂꢀ ꢎ+ꢂꢀ 2+ꢂꢀ '22ꢀ H[FKDQJHꢆꢂꢀ ꢉꢃꢎꢐꢊVꢂꢀ ꢄ+ꢂꢀ 1 &+ꢆꢂꢀ
7.22–6.85(m, 15H, ArH), 4.10–3.33(m, 18H, OCH2, NCH2Ar),
2.81(t, J = 5.6 Hz, 4H, NCH2); IR(KBr) vmax: 3441, 1608, 1255,
1133; ESIMS m/z: 658 (M+). Anal. Calcd. For C37H40N3O6Cl: C
67.48, H 6.08, N 6.38. Found C 67.65, H 6.11, N 6.26%.
Experimental
Melting points were determined on a Yanaco-500 micro-melting
point apparatus and are uncorrected. IR spectra were recorded on a
Nicolet-1705X IR spectrometer. 1H NMR spectra were recorded on a
Bruker AC-200 MHz spectrometer using tetramethylsilane as internal
HL4:Yellow crystals;Yield 66%. m.p.144–146°C. 1H NMR(CDCl3)
įꢇꢀꢄꢍꢃꢎꢉꢊVꢂꢀꢎ+ꢂꢀ2+ꢂꢀ'22ꢀH[FKDQJHꢆꢂꢀꢉꢃꢏꢐꢊVꢂꢀꢄ+ꢂꢀ1 &+ꢆꢂꢀꢌꢃꢐꢎ±ꢈꢃꢈꢈꢊPꢂꢀ
19H, ArH), 4.12–3.31(m, 14H, OCH2, NCH2Ar), 2.81(t, J = 5.6 Hz,
4H, NCH2); IR(KBr) vmax: 3443, 1608, 1256, 1131; ESIMS m/z: 706
(M+). Anal. Calcd. For C41H40N3O6Cl: C 69.69, H 5.67, N 5.98. Found
C 69.82, H 5.46, N 5.74%.
* Correspondent. E-mail:boshiweer@yahoo.com

462 JOURNAL OF CHEMICAL RESEARCH 2008
General method for synthesis of Co(II)/Mn(III) unsymmetrical bis-
Catalytic oxidation of styrene
Schiff base complexes ML
To a solution of styrene(1.0 mmol), the imidazole as co-catalyst
(0.1 mmol), reducing agent Zn (1.0 mmol) and CH3CN:CH2Cl2
(15 ml, 9.5:0.5, v/v) was added [MnLCl] as catalyst (0.01 mmol).
Then the mixture was vigorously stirred at 25°C under O2 for 0.5 h,
and the proton donor acetic acid (0.069 ml, 1.1 mmol) was injected.
The mixture was stirred for 1.5 h at 25°C. The oxidation products
ZHUHꢀLGHQWL¿HGꢀE\ꢀ*&ꢀDQDO\VHVꢀDQGꢀFRQ¿UPHGꢀE\ꢀ06ꢃ
A
solution of HL2–HL4 (1.0 mol) and Co(OAc)2·4H2O or
MnCl2·6H2O(1.0 mol) in methanol (15 ml) was stirred for 2 h under
N2ꢀDWꢀUHÀX[ꢂꢀWKHQꢀWKHꢀPL[WXUHꢀZDVꢀFRROHGꢀDQGꢀ¿OWHUHGꢂꢀZDVKHGꢀZLWKꢀ
methanol and then with diethyl ether to give the metal complexes.
The pure product was obtained after recrystallisation from ethanol.
[CoL2]: Red solid, 71% yield, IR (KBr) vmax: 1616, 1281, 1156 cm-1;
MS m/z: 583(M+). Anal. Calcd for C31H26N3O3ClCo: C 63.81, H
4.46, N 7.20, Co 10.12. Found C 63.59, H 4.25, N 7.38, Co 10.04%.
ȁm(S·cm2·mol-1): 4.33.
Result and discussion
All of the unsymmetrical bis-Schiff base ligands HL2–HL4 have
been conveniently prepared by condensation of the semi-ligand 1
and morpholine or aza-crown ether substituted salicylaldehyde. The
procedure is illustrated in Scheme 1. The elemental analyses and
ESIMS of the complexes indicate that they are 1:1 (metal/ligand)
complexes. Moreover, the molar conductance of all complexes in
DMF solution (10-3 mol·dm-3) are respectively in the 4.33–6.82 range
(cobalt complexes) and 79.68–93.44 range (manganese complexes)
S·cm2·mol-1at 25°C. The facts suggest the cobaltII complexes are non-
electrolytes and manganeseIII complexes are 1:1 electrolytes.24 The IR
spectra show most ligand absorptions are still in the same frequency
H[FHSWꢀ WKHꢀ & 1ꢀ VWUHWFKꢀ DEVRUSWLRQꢀ VKLIWVꢀ VOLJKWO\ꢀ ꢊꢉ±ꢄꢎꢀ FP-1)
to higher frequency, which is different from that of the symmetrical
analogues we previously reported.25 The OH stretches (ca 3442 cm-1)
disappear after complex formation. The proposed structure is shown
in Fig. 1.
[CoL3]: Red solid, 79% yield, IR(KBr) vmax: 1620, 1255, 1132;
ESIMS m/z: 715(M+). Anal. Calcd. for C37H38N3O6CoCl: C 62.09,
H 5.31, N 5.87, Co 8.25. Found C 61.93, H 5.22, N 5.90, Co 8.39%.
ȁm(S·cm2·mol-1): 6.82.
[CoL4]: Red solid, 85% yield, IR(KBr) vmax: 1618, 1252, 1131;
ESIMS m/z: 763(M+). Anal. Calcd. for C41H38N3O6CoCl: C 64.48,
H 4.98, N 5.50, Co 7.73. Found C 64.35, H 5.25, N 5.32, Co 7.47%.
ȁm(S·cm2·mol-1): 5.26.
[MnL2Cl]: Brown solid, 81% yield, m.p.>300°C. IR (KBr)
v
max: 1617, 1278, 1155 cm-1; ESIMS m/z: 614(M+). Anal. Calcd for
C31H26N3O3Cl2Mn: C 60.49, H 4.23, N 6.83, Mn 8.96. Found C
ꢈꢅꢃꢎꢎꢂꢀ+ꢀꢍꢃꢏꢄꢂꢀ1ꢀꢈꢃꢐꢉꢂꢀ0Qꢀꢉꢃꢌꢏꢑꢃꢀȁm(S·cm2·mol-1): 93.44.
[MnL3Cl]: Brown solid, 87% yield, m.p.>300°C. IR(KBr)
v
max: 1620, 1254, 1130; ESIMS m/z: 746(M+). Anal. Calcd. for
C37H38N3O6MnCl2: C 59.52, H 5.09, N 5.63, Mn 7.37. Found C
ꢈꢅꢃꢅꢎꢂꢀ+ꢀꢐꢃꢄꢄꢂꢀ1ꢀꢐꢃꢌꢅꢂꢀ0Qꢀꢌꢃꢎꢄꢑꢃꢀȁm(S·cm2·mol-1): 82.04.
Considering that the pyridine (Py) solutions of [MnL1Cl]–
[MnL4Cl] are readily oxidised in air to yield [(Py)L2Mn(III)-O-
Mn(III)L2(Py)] at ambient temperature, the oxygenation constants
(Ko2) of [MnL1Cl]–[MnL4Cl] cannot be accurately determined.26
:HꢀKDYHꢀRQO\ꢀVWXGLHGꢀWKHꢀGLR[\JHQꢀDI¿QLWLHVꢀRIꢀ[CoL1]–[CoL4]. The
oxygenation constants (Ko2) and thermodynamic parameters 'H0 and
'S0 of [CoL1]–[CoL4] are given in Table 1. It shows, as compared
with [CoL1] and [CoL2]ꢂꢀ WKHꢀ GLR[\JHQꢀ DI¿QLW\ꢀ RIꢀ D]DꢁFRZQHGꢀ
complexes [CoL3] and [CoL4] are obviously improved. This result
is due to the aza-crown pendants increasing the steric hindrance and
hydrophobicity of the complexes. Furthermore, the macrocycle effect
of aza-crown ring, rather than the morpholino-substituents, which
SRVVHVVHVꢀDꢀVSHFLDOꢀFRQ¿JXUDWLRQꢂꢀSUREDEO\ꢀDOORZVꢀWKHꢀ22 molecule
to approach the coordinated CoII of the complexes, and then form the
Co–O2 bond through the hydrophobicity of the outer ethylene groups
and the orderly arrangement of the inner aza-oxa atoms.18
[MnL4Cl]: Brown solid, 82% yield, m.p.>300°C. IR(KBr)
v
max: 1618, 1255, 1128; ESIMS m/z: 794(M+); Anal. Calcd. for
C41H38N3O6Cl2Mn: C 61.89, H 4.78, N 5.28, Mn 6.93. Found C
ꢈꢎꢃꢅꢌꢂꢀ+ꢀꢍꢃꢐꢏꢂꢀ1ꢀꢐꢃꢅꢏꢂꢀ0Qꢀꢌꢃꢄꢄꢑꢃꢀȁm(S·cm2·mol-1): 79.68
Oxygen uptake measurements
The oxygenation constants and thermodynamic parameters of
[CoL1]–[CoL4] were determined by the known equipment and
method:22,23 diglyme (O2 saturated) as solvent, 1.0 mol·dm-3 of
pyridine as axial ligand (B), 5 × 10-3 mol·dm-3 of complexes, 9.7 × 104
Pa of oxygen partial pressure. The equilibrium constant (Ko2) was
calculated as follows.
[CoLBO2]
[CoLB] Po2
Ko2
CoLBO2
=
CoLB CoLB + O2
CoL + B
.
Table 1 also shows that adding alkali metal ions (as LiNO3,
NaNO3 and KNO3) to the oxygenation system (the concentration of
the alkali metal ions is equal to the CoL concentration) improves the
O2-binding capability of [CoL3] and [CoL4]. In contrast, in terms
of the uncrowned analogs [CoL1] and [CoL2], there was almost no
LQÀXHQFHꢃꢀ7KLVꢀUHVXOWꢀUHYHDOVꢀWKDWꢀDONDOLꢀPHWDOꢀLRQVꢀLPSURYHꢀWKHꢀ22-
Complexes are expressed in terms of molarities and the dioxygen
concentration are expressed as partial pressure (Po2) in torr.
Thermodynamic parameters 'H°and 'S° for the oxygenation
reactions were calculated from Ko2 and K’o2 over a range of
temperatures.
R
OHC
HO
methanol
=
N
HC
=
C
N
N
=
NH
2
C
N
+
R
HO
Cl
OH
Cl
OH
O
O
O
O
O
O
3
1
N
CH2
CH2
N
N
CH2
CH2
HL : R=
HL : R= H
2a: R= H
2c: R=
O
O
O
O
1
O
O
O
O
2
4
O
HL : R= -CH -
-CH -
HL : R=
N
O
N
2b: R=
2d: R=
2
2
O
O
Scheme 1
O
O
O
ML3: R=
ML4: R=
ML1: R= H
ML2: R=
N
N
CH2
CH2
O
O
C
N
=
N= HC
O
O
O
M
O
-CH -
N
2
Cl
O
R
O
M = Co (II), Mn (III)Cl
ML
Fig. 1 Structure of the unsymmetrical Schiff base complexes ML.

JOURNAL OF CHEMICAL RESEARCH 2008 463
Table 1 Oxygenation constants and thermodynamic parameters ('H° and 'S°) of CoL
CoL
B
T/°C
Ions added
lnKo2/mm–1
'H°/kJ mol–1
'S°/J K–1 mol–1
[CoL1]
Py
–5
0
15
25
–5
0
15
25
25
25
25
–5
0
15
25
25
25
25
–5
0
–
–4.36
–4.64
–5.45
–5.94
–3.12
–3.44
–4.33
–4.83
–4.82
–4.83
–4.82
–2.83
–3.19
–3.85
–4.75
–4.67
–3.68
–4.39
–2.32
–2.65
–3.64
–4.31
–4.22
–3.25
–3.99
–36.25
–171.51
–
–
–
[CoL2]
[CoL3]
[CoL4]
Py
Py
Py
–
–41.12
–42.51
–44.34
–179.37
–182.11
–172.72
–
–
–
Li+
Na+
K+
–
–
–
–
Li+
Na+
K+
–
–
15
25
25
25
25
–
–
Li+
Na+
K+
Table 2 Catalytic oxidation of styrene catalysed by [MnLCl]
Catalyst
Convention/wt.%
TOF/h-1
Selectivity/wt.%
A
B
[MnL1Cl]
11.6
13.3
41.2
47.1
45.8
53.5
5.80
6.65
20.6
23.6
22.9
26.8
6.7
8.4
99.1
99.2
99.3
99.2
93.3
91.6
0.9
[MnL2Cl]
[MnL3Cl]
[MnL3Cl] + Na+
[MnL4Cl]
0.8
0.7
0.8
[MnL4Cl] + Na+
A = benzaldehyde, B = epoxy product TOF = Turnover frequency (mole styrene converted per mol catalyst per hour).
binding capability only when they are coordinated with the crown
O
CHO
O2 , 25oC
MnLCl
rings of the complexes. Table 1 also indicates that the O2-binding
capabilities of [CoL3] and [CoL4] are visibly enhanced by adding
NaNO3. However, the enhancement by adding equal LiNO3 and
KNO3 is very limited. Since aza-crown substituted Schiff base Co(II)
complexes can complex with alkali cations,27 the above results may
be due to the fact that Na+ ꢊGꢀ ꢀꢄꢃꢋꢅcꢆꢀPDWFKHVꢀPXFKꢀEHWWHUꢀZLWKꢀWKHꢀ
FDYLW\ꢀVL]HꢀRIꢀꢄꢐꢁFURZQꢁꢐꢀꢊGꢀ ꢀꢄꢃꢌ±ꢎꢃꢎcꢆꢀWKDQꢀGRHVꢀ/L+ ꢊGꢀ ꢀꢄꢃꢏꢈcꢆꢀ
or K+ ꢊGꢀ ꢀ ꢎꢃꢈꢈcꢆꢃꢀ 7KXVꢀ WKHꢀ HI¿FLHQWꢀ FRPSOH[DWLRQꢀ RIꢀ 1D+ with
aza-crown ethers would favour this cation to approach and control
effectively the microenviroment around the coordinated CoII and
facilitate the formation and stabilisation of Co–O2.28
+
B
A
Scheme 2
steric hindrance in the MnIII complexes, plays a critical role in
the formation of phenyl aldehyde. Further studies of the catalytic
oxidation mechanism are being carried out in our lab.
Kochi’s report demonstrated that MnIII-Schiff complexes are high-
DFWLYHꢀFDWDO\VWVꢀIRUꢀROH¿QꢀR[LGDWLRQꢃ4 Herein, we describe [MnL1Cl]–
[MnL4Cl] catalysing styrene oxidation (Scheme 2). As illustrated in
Table 2, the aza-crowned [ML3] and [ML4] show higher catalytic
oxidation activities than the uncrowned analogues [ML1] and [ML2].
The result may be due to the macrocycle effect of the azacrown rings.
The special conformation of azacrown rings in [ML3] and [ML4]
can offer large steric hindrance and a hydrophobic environment and
favour the formation and protection of the active oxidation species
ꢊ0Qꢀ ꢀ2ꢆꢃꢀ7DEOHꢀꢎꢀDOVRꢀLOOXVWUDWHVꢀWKDWꢀDGGLQJꢀ1D+ (as NaNO3 and in
the same concentration as [ML3] and [ML4]) to the complexes [ML3]
and [ML4] enhances the catalytic oxidation activities. This result also
shows that the Na+ complex, with the azacrown ring, may advantage
WKHꢀDFWLYHꢀR[LGDWLRQꢀVSHFLHVꢀꢊ0Qꢀ ꢀ2ꢆꢃ
Received 26 May 2008; accepted 9 July 2008
Paper 08/5293 doi:10.3184/030823408X340654
Published online: 27 August 2008
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An interesting fact is that aza-crowned [ML3] and [ML4] show
different catalytic selectivity from that of uncrown analogues [ML1]
and [ML2]. The major product of styrene oxidation catalysed by
[ML3] and [ML4]ꢀFRPHVꢀIURPꢀR[LGDWLYHꢀ& &ꢀFOHDYDJHꢀRIꢀWKHꢀROH¿QLFꢀ
moiety to form phenyl aldehyde with more than 99% selectivity.
In contrast, the major product of styrene oxidation catalysed by [ML1]
and [ML2] is epoxide with about 90% selectivity. This fact indicates
WKDWꢀ WKHꢀ D]DFURZQꢀ ULQJꢂꢀ ZKLFKꢀ SRVVHVVHVꢀ VSHFLDOꢀ FRQ¿JXUDWLRQꢀ DQG
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