G Model
CCLET-3116; No. of Pages 3
2
X.-Y. Wei et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Cl
Cl
, NNM
N
N
OH
CCH2
HN
O
OH
NH
O
HOOCH2C
O
CH2COOH
N
CH2C
n
O
O
O
O
N
N
N
N
HO
O
OH
Cl
O
O
N
N
O
O
n
O
O
O
O
n
O
DMAP , NH2OH.HCl
O
O
n=0 L1H2; n=1 L2H2
n=0 1a; n=1 1b
n=2 1c; n=3 1d
n=2 L3H2; n=3 L4H2
Scheme 1. Synthetic route of the cobalt complexes with aza-crowned dihydroxamic acid.
The oxidation of p-xylene to p-toluic acid (PTA) catalyzed by
CoL1–CoL4 were conducted in a normal gas–liquid apparatus.
Air was bubbled into the mixture of p-xylene (40 mL) and CoL
(1 ꢁ 10ꢀ3 mol Lꢀ1) with a 2.0 L minꢀ1 flow rate at 110 8C. The
reaction mixture (0.1 mL) was sampled by pipette periodically
before the precipitates appeared (in 5 h), and diluted to 15 mL
with ethanol. The accumulative concentration of PTA was
determined by the standard acid–base titration. The selective
oxidation for PTA was determined by HPLC. The catalytic
oxidation performances of CoL1–CoL4 were shown in Table 2.
calcd. for C18H32N4O11: C 45.00, H 6.71, N 11.66; found: C 45.21, H
6.77, N 11.80.
CoL1: Yield: 70.6%, m.p. 261 8C (dec.), IR (KBr, cmꢀ1):
vmax 3112,
1732, 1595, 1420, 1122, 982; Anal. calcd. for C12H18N4O8Co: C
35.57, H 4.48, N 13.83, Co 14.54; found: C 35.39, H 4.39, N 14.03, Co
14.70. Lm (S cm2 molꢀ1): 2.56.
CoL2: Yield: m.p. 248 8C (dec.), IR (KBr, cmꢀ1):
vmax 3109, 1733,
1601, 1421, 1122, 978; Anal. calcd. for C14H22N4O9Co: C 37.43, H
4.94, N 12.47, Co 13.12; found: C 37.31, H 5.02, N 12.58, Co 13.56.
Lm (S cm2 molꢀ1): 5.71.
CoL3: Yield: m.p. 227 8C (dec.), IR (KBr, cmꢀ1):
vmax 3110, 1733,
3. Results and discussion
1605, 1422, 1120, 978; Anal. calcd. for C16H26N4O10Co: C 38.95, H
5.31, N 11.36, Co 11.95; found: C 38.77, H 5.46, N 11.19, Co 12.18.
Lm (S cm2 molꢀ1): 7.26.
The new compounds were characterized as follows.
L1H2: Yield: 87.9%, m.p. 162–163 8C, 1H NMR (CDCl3):
d 10.32 (s,
CoL4: Yield: 64.1%, m.p. 221 8C (dec.), IR (KBr, cmꢀ1):
vmax 3107,
2H, OH, D2O exchangeable), 4.50 (s, 4H, COOCH2), 3.72 (s, 4H,
NCH2CON), 3.59 (s, 4H, COCH2N), 3.11 (s, 4H, NCH2CH2N); IR (KBr,
cmꢀ1): vmax 3226, 3109, 1732, 1628, 1425, 1122, 965; MS (m/z):
348 (M+); Anal. calcd. for C12H20N4O8: C 41.38, H 5.79, N 16.06;
found: C 41.49, H 5.72, N 16.25.
1731, 1607, 1420, 1122, 979; Anal. calcd. for C18H30N4O11Co: C
40.23, H 5.63, N 10.43, Co 10.97; found: C 40.15, H 5.47, N 10.64, Co
11.21. Lm (S cm2 molꢀ1): 4.85.
The oxygenation constants in Table 1 were of significant
difference. The order of the O-binding activity is: CoL3 > CoL2 >
CoL4 > CoL1, which should be attributed to the structural diversity.
Since the only difference among the cobalt complexes was the size
of the crown ether ring (or the number of the ethyleneoxy linkers),
it could be deduced that the macrocyclic effect of crown ring,
which possesses a special configuration, would favor oxygen
molecule to approach the active center of the Co(II) complexes and
form the Co–O2 bond through its hydrophobic outer ethylene
groups and orderly arranged inner aza oxa atoms. However, CoL4,
which possesses the largest crown ring, did not show the highest
O2-binding activity. The reason may that the large crown ring of
CoL4 would be distorted and create steric hindrance around the
active center, which is disadvantageous for the binding of O2
L2H2: Yield: 90.2%, m.p. 147–148 8C, 1H NMR (CDCl3):
d 10.30 (s,
2H, OH, D2O exchangeable), 4.39 (m, 4H, COOCH2), 3.95 (s, 4H,
NCH2CON), 3.66–3.78 (s, 8H, COCH2N, COOCH2CH2O), 3.25 (s, 4H,
NCH2CH2N); IR (KBr, cmꢀ1): vmax 3226, 3105, 1730, 1628, 1427,
1122, 962; MS (m/z): 392 (M+); Anal. calcd. for C14H24N4O9: C
42.86, H 6.17, N 14.28; found: C 42.55, H 6.11, N 14.36.
L3H2: Yield: 86.3%, m.p. 129–130 8C, 1H NMR (CDCl3):
d 10.30
(s, 2H, OH, D2O exchangeable), 4.37 (t, 4H, COOCH2), 3.92 (s, 4H,
NCH2CON), 3.64–3.77 (s, 8H, COCH2N, COOCH2CH2O), 3.60 (s, 4H,
OCH2CH2O), 3.25 (s, 4H, NCH2CH2N); IR (KBr, cmꢀ1): vmax 3222,
3106, 1732, 1630, 1425, 1120, 961; MS (m/z): 436 (M+); Anal.
calcd. for C16H28N4O10: C 44.03, H 6.47, N 12.84; found: C 44.15, H
6.37, N 12.97.
[11]. Moreover, the
O2–Co(II) bond. The Co(II) complexes with smaller
D
H8 seems to contribute to the formation of
H8 show larger
L4H2: Yield: 85.1%, m.p. 113–114 8C, 1H NMR (CDCl3):
d 10.28
D
(s, 2H, OH, D2O exchangeable), 4.35 (t, 4H, COOCH2), 3.91 (s, 4H,
NCH2CON), 3.65–3.77 (s, 8H, COCH2N, COOCH2CH2O), 3.61 (s, 8H,
OCH2CH2O), 3.25 (s, 4H, NCH2CH2N); IR (KBr, cmꢀ1): vmax 3225,
3105, 1730, 1630, 1422, 1122, 960; MS (m/z): 480 (M+); Anal.
oxygenation constants. The relevance might be used to judge the
O2-binding activity of CoL.
As shown in Table 2, The oxidation of p-xylene catalyzed by
CoL1–CoL4 experienced a certain induction period (0.3–0.4 h),
which indicated there may be a connection between the dioxygen
affinity and induction period and the coordination of molecular
oxygen to the central cobalt ions is necessary to the initiation of
the catalytic oxidation reaction. Meanwhile, same as the order
of the O2-binding activity of CoL1–CoL4, CoL3 showed the highest
catalytic oxidation activity and selectivity followed by CoL2, and
then CoL4 and CoL1. This result indicated that on one hand, a proper
size of crown ether ring could offer a favorable microenvironment
or even some kind of substrate specificity to significantly enhance
the catalytic oxidation performance of the cobalt complexes.
On the other hand, the steric hindrance of the crown ether ring
would shield the active center, which is not advantageous to the
formation of the active oxygen affinity species and the inclusion
to the substrate. It also seemed to indicate that the crown ring
configuration played an important role in the modulation of the
catalytic oxidation, which would be researched in our future
structural studies of the cobalt complexes.
Table 1
Oxygenation constants and thermodynamic parameters (
D
H8 and
DS8).
Complexes
CoL1
B
T (8C)
LnKO2
D
H8 (kJ molꢀ1
)
D
S8 (J Kꢀ1 molꢀ1
)
(mmꢀ1
)
Py
ꢀ5
10
20
ꢀ4.59
ꢀ5.45
ꢀ5.98
ꢀ36.56 ꢂ 0.03
ꢀ171.48 ꢂ 0.04
ꢀ182.27 ꢂ 0.05
ꢀ182.51 ꢂ 0.05
ꢀ191.37 ꢂ 0.03
CoL2
CoL3
CoL4
Py
Py
Py
ꢀ5
10
20
ꢀ3.72
ꢀ4.68
ꢀ5.27
ꢀ40.56 ꢂ 0.04
ꢀ41.78 ꢂ 0.04
ꢀ43.72 ꢂ 0.05
ꢀ5
10
20
ꢀ3.20
ꢀ4.18
ꢀ4.78
ꢀ5
10
20
ꢀ3.65
ꢀ4.69
ꢀ5.34
Please cite this article in press as: X.-Y. Wei, et al., Synthesis, O2-binding ability and catalytic oxidation performance of cobalt(II)