Synthesis and studies of hydrolysis of p-nitrophenyl picolinate catalysed by schiff base zinc (ii) complexes with aza-crown ether or morpholinopendants

Article abstract of DOI:10.3184/030823409X450525

Four Schiff base zinc (II) complexes with either benzo-10-aza-crown ether pendants or morpholino-pendants have been synthesised and employed as models for hydrolase enzymes by studying the kinetics of their hydrolysis reactions with p-nitrophenyl picolina

Full text of DOI:10.3184/030823409X450525

JOURNAL OF CHEMICAL RESEARCH 2009
JUNE, 345–350
RESEARCH PAPER 345
Synthesis and studies of hydrolysis of p-nitrophenyl picolinate catalysed
by Schiff base zinc (II) complexes with aza-crown ether or morpholino-
pendants
Xue-song Fenga, Ying Tangb, Jian-zhang Li*a, Jin Zhangb and Sheng-ying Qinc
aKey Laboratory of Green Chemistry and Technology, Department of Chemistry, Sichuan University of Science & Engineering,
Zigong, Sichuan 643000, P.R. China
bDepartment of Chemistry and Environment Science, Chongqing University of Arts and Science, Yongchuan, Chongqing 402160,
P.R. China
cDepartment of Chemistry, Sichuan University, Chengdu, Sichuan 610064, P. R. China
Four Schiff base zinc (II) complexes with either benzo-10-aza-crown ether pendants or morpholino-pendants have
been synthesised and employed as models for hydrolase enzymes by studying the kinetics of their hydrolysis
reactions with p-nitrophenyl picolinate (PNPP). A kinetic model of PNPP cleavage catalysed by these complexes is
proposed. The effects of complex structures and reaction temperature on the rate of catalytic PNPP hydrolysis have
been also examined. The rate increases with pH of the buffer solution; all four complexes exhibited high activity
in the catalytic PNPP hydrolysis. Compared with their crown-free analogues, the crowned Schiff base complexes
exhibit higher catalytic activity. The catalytic activity of a phenyl-bridged Schiff base complex is larger than that of
an ethyl-bridged Schiff base complex under the same substitute groups and metal ion.
Keywords: synthesis, benzo-10-aza-crown ether, Schiff base zinc (II) complexes, mimic hydrolase, p-nitrophenyl picolinate
Research on the hydrolysis of carboxyl and phosphoric esters
is particularly important for environment and biological
applications.1,2 Much research has been aimed to develop
high efficiency and selectivity of catalysts in order to achieve
environment-friendly and high-economy processes. Studies
on mimic hydrolytic metalloenzymes have used metal ions
cobalt(III),3,4 cobalt(II),5,6 copper(II),7,8 nickel(II),9 manganese
(III),10,11 and lanthanum(III),12,13 etc. as the active centre of
the mimic system, respectively; but use of zinc(II) complexes as
the synthetic model of metalloenzymes has been infrequent.
Studies of hydrolase mimics is an important area in enzyme
simulation research, such as studies of hydrolyses of carboxyl
esters.14-18 The artificial systems that have been used have
similar catalytic function to natural enzymes, but they are
structurally more simple and stable than enzymes, and they
can provide information on mechanistic aspects of enzyme
action. In recent years, various chemical systems have been
used as models for natural enzymes.19-21 Crown ethers are
generally considered as first generation biomimetics due to
their host–guest recognition properties. Therefore, crown ether
complexes may provide useful models for hydrolase enzymes
because the crown ether can offer a hydrophobic environment.
Previously, the catalytic performances of bis-Schiff base
manganese(III) and cobalt(II) complexes with aza-crown
ethers or morpholino-pendants in p-nitrophenyl picolinate
(PNPP) hydrolysis have been reported.22-26 However, zinc
(II) Schiff base complexes containing crown ethers, which
can offer the hydrophobicity of outer ethylene groups
and hydrophilicity of inner oxygen atoms in an orderly
arrangement, have been seldom reported as synthetic models
of metalloenzymes. In this paper, the synthesis of four Schiff
base zinc (II) complexes with either benzo-10-aza-crown ether
pendants (ZnL1, ZnL2) or morpholino-pendants (ZnL3, ZnL4)
have been synthesised and employed as models for hydrolase
enzymes by studying the kinetics of their hydrolysis reactions
with p-nitrophenyl picolinate (PNPP). Compared with the
crown-free analogues ZnL3 and ZnL4, the effect on PNPP
catalytic hydrolysis of ligand structure and the crown ether
ring in the complexes has been also been examined. The
proposed structures of the Schiff base–zinc (II) complexes are
shown in Fig. 1.
Cl
N=CH
O
Cl
CH=N
O
Zn
CH2
N
CH2
N
O
O
O
O
O
O
O
O
ZnL1
Zn
N=CH
O
Cl
Cl
CH=N
O
CH2
N
CH2
N
O
O
O
O
O
O
O
O
ZnL2
N=CH
Cl
Cl
CH=N
O
Zn
O
CH2
N
H2C
N
O
O
ZnL3
Zn
N=CH
O
Cl
Cl
CH=N
O
CH2
N
H2C
N
O
O
ZnL4
* Correspondent. E-mail: sichuanligong2006@hotmail.com
Fig.1 Structure of the Schiff base zinc complexes.

346 JOURNAL OF CHEMICAL RESEARCH 2009
H 5.51, N 5.66, Cl 7.19, Zn 6.28. ZnC50H54N4O10Cl2 requires C
59.64, H 5.37, N 5.57, Cl 7.06, Zn 6.46%); Lm(S.cm2 mol-1): 8.21.
ZnL2: Straw yellow, 72% yield, m.p. >300°C.IR (KBr, cm-1)
Experimental
Methods and materials
Melting points were determined on a Yanaco MP-500 micro-melting
point apparatus and were uncorrected. Infrared spectra were recorded
on a Nicolet-1705X spectrometer (KBr, film). 1H NMR spectra were
determined on a Bruker AC-200 MHz spectrometer using Me4Si as
an internal standard. Mass spectra were obtained on a Finnigan MAT
4510 s and Finnigan LCQ-DECA spectrometers. The zinc (II) content
was measured by an IRIS-Advantage ICP emission spectrometer. The
halogenanalysiswasmeasuredusingthemercurytitrationmethod.27,28
Other elementary analyses were performed on a Carlo Erba 1106
elemental analyser. Molar conductance was obtained on a DDS-
11A conductivitimeter. Kinetic studies were carried out by UV-vis
methods with a GBC 916 UV-vis spectrophotometer equipped with a
thermostatic cell holder.
n
max: 1618, 1225, 1126; ESI-MS m/z: 959(M+ + 1). (Found C 57.53,
H 5.82, N 5.99, Cl 7.24, Zn 6.59. ZnC46H54N4O10Cl2 requires C
57.62, H 5.64, N 5.85, Cl 7.41, Zn 6.78%). Lm(S.cm2 mol-1): 9.51.
ZnL3: Straw yellow, 63% yield, m.p. 235–237°C. IR (KBr, cm-1)
n
max: 1614, 1226, 1130; ESI-MS m/z: 647(M+ + 1). (Found C 55.91,
H 4.54, N 8.49, Cl 10.81, Zn 10.19. ZnC30H30N4O4Cl2 requires C 55.72,
H 4.64, N 8.67, Cl 10.99, Zn 10.06%). Lm(S.cm2 mol-1): 8.52.
ZnL4: Straw yellow, 68% yield, m.p. 211–213°C. IR (KBr, cm-1) nmax
:
1616, 1235, 1125. ESI-MS m/z: 599(M+ + 1). (Found C 52.33, H 5.21,
N, 9.18, Cl 11.71, Zn 10.69. ZnC26H30N4O4Cl2 Calcd C 52.17, H 5.02, N
9.36, Cl 11.87, Zn 10.87%). Lm (S.cm2 mol-1) 9.26.
Kinetics studies
All reagents, unless otherwise indicated, were of analytical grade
and used without further purification. Tris[tris(hydroxymethyl)
aminomethane] was purchased from Aldrich. Buffers were made
from standardised nitric acid. The water used for kinetics was obtained
by doubly distilling deionised water. The ionic strength of the buffers
was maintained at 0.1 mol dm-3 KNO3. The pH of the buffers was
measured at 25°C using a Radiometer PHM 26 pH meter fitted
with G202C glass and K4122 calomel electrodes. The following
compounds were prepared according to the literature: PNPP (p-nitro-
phenylpicolinate),29 N-(2-hydroxy-3-formyl-5-chlorobenzyl)benzo-10-
aza-15- crown-5 and N-(2-hydroxy-3-formyl-5-chlorobenzyl)morpholi
ne.30 PNPP stock solution was prepared in acetonitrile.
The pseudo-first-order rate constants (kobs) were obtained based
on the initial rate method, i.e. according to the equations: (rate)0 =
-(dC/dt)0 = (dA/dt)0/e and (rate)0 = kobs[ML]0. (rate)0 was calculated
initially, then the Fig. of (rate)0 versus [ML]0 was plotted; the pseudo-
first-order rate constants were obtained from the slope of the straight
line in the Fig., where (rate)0 is the initial rate of PNPP hydrolysis
and [ML]0 is the initial concentration of the complex.
Each kinetic run was initiated by injecting an acetonitrile solution
of PNPP at the desired concentration into a 1 cm cuvette containing
3 cm3 of the desired concentration of the complex. The pseudo-
first-order rate constants for PNPP hydrolysis were determined
by monitoring the release of p-nitrophenol at 400 nm under the
conditions of more than 10-fold excess of substrate over catalyst
concentration, and each kinetic run was reproducible to within 3%
error. The molar extinction coefficients (e) of p-nitrophenol were
examined by measuring absorption values of the desired concentration
of p-nitrophenol at the various pH of the solutions. The ionic strength
of all reaction systems was maintained at 0.1 mol dm-3 KNO3.
Synthesis of Schiff base ligands H2L1–H2L4
Schiff base ligand H2L: A solution of 1, 2-phenylenediamine (1.08 g,
10 mmol) and N-(2-hydroxy-3-formyl-5-chlorobenzyl)benzo-10-
aza-15-crown-5 (8.72 g, 20 mmol) in anhydrous EtOH (40 cm3) was
stirred for 4 h under an N2 atmosphere at 80°C, and then the mixture
was cooled. The resulting yellow precipitate was filtered and washed
with EtOH. After recrystallisation from EtOH, yellow crystals
(8.20 g, yield 87%) were obtained. m.p. 94–96°C. 1H NMR(200 MHz,
CDCl3) d: 9.90(s, 2H, OH, D2O exchangeable), 8.33(s, 2H, N=CH),
7.76–6.80(m, 16H, ArH), 4.18–3.68(m, 28H, OCH2, NCH2Ar), 2.84
(t, J = 6.1 Hz, 8H, NCH2); IR (KBr, cm-1)nmax: 3412, 2949, 2868,
1631, 1596, 1502, 1256, 1128, 1050, 930; ESI-MS m/z: 944 (M+ + 1);
(Found C 61.49, H 6.14, N 5.78, Cl 7.71. C50H56 N4O10Cl2 requires C
61.63, H 5.94, N 5.94, Cl 7.53%).
Results and discussion
Synthesis
Compared with the IR spectra of the ligand, the IR spectra of
complexes were almost at the same frequency, except for the C=N
stretching vibration shifted 16–19 cm-1 to lower frequency and
its intensity was greater than that of free ligand. This indicated the
formation of a N–O…Zn coordination bond; the absence of an OH
stretching vibration in the complexes indicated deprotonation of
OH of the ligand after complex formation. The C–O–C stretching
vibrations in the crown ether ring for the complexes were almost at
the same frequency as those of the free ligand. The observed molar
conductance of all complexes in DMF solution (1.0 ¥ 10-3 mol L-1)
at 25°C showed that they were non-electrolytes.31 The ESI-MS Mass
spectra and elemental analysis of the complexes indicated that Schiff
base H2L formed 1:1 (ligand/metal) complexes (ML). The facts
above showed that Schiff base ligands can coordinate with zinc ion
as in Fig. 1.
Ligand H2L2 was prepared as described for H2L1 except starting
with ethylenediamine instead of 1, 2-phenylenediamine to give a
yellow solid, in yield 85%, m.p. 85–86°C. 1H NMR(200 MHz, CDCl3)
d: 9.89 (s, 2H, OH, D2O exchangeable), 8.27 (s, 2H, N=CH), 7.57–
6.87(m, 12H, ArH), 4.16–3.76 (m, 32H, OCH2, NCH2Ar, C=NCH2),
2.83 (t, J = 5.4 Hz, 8H, NCH2); IR (KBr, cm-1)nmax: 3448, 2949, 2864,
1635, 1600, 1500, 1256, 1128, 1050, 928; ESI-MS m/z: 896 (M+ + 1);
(Found C 61.45, H 6.15, N 6.41, Cl 7.78. C46H56N4O10Cl2 requires C
61.68, H 6.26, N 6.26, Cl 7.93%).
Ligand H2L3 was prepared as described for H2L1 except starting
with N-(2-hydroxy-3-formyl- 5-chlorobenzyl)morpholine instead
of N-(2-hydroxy-3-formyl-5-chlorobenzyl)benzo-10-aza-15-crown-
5, to give a yellow solid, in yield 83%, m.p. 215–217°C. 1H NMR
(200 MHz, CDCl3) d: 10.23 (s, 2H, OH, D2O exchangeable), 8.64(s,
2H, N=CH), 7.46–7.20 (m, 8H, ArH), 3.79–3.67 (m, 12H, OCH2,
Pseudo-first-order rate constants of catalytic PNPP hydrolysis at 25°C
The pseudo-first-order-rate constants obtained for catalytic PNPP
hydrolysis are shown in Table 1. The pseudo-first-order-rate constant
(k0) for PNPP hydrolysis in the absence of catalyst is 7.8 ¥ 10-6 s-1 at
pH = 7.00, 25°C, [S] = 2.0 ¥ 10-4 mol dm-3. The rate for the PNPP
hydrolysis catalysed by the complexes (see Table 1) increases by a
factor of ca 1.05 ¥ 103 times for ZnL1, 9.48 ¥ 102 times for ZnL2, 7.56
¥ 102 times for ZnL3, and 6.79 ¥ 102 times for ZnL4 under the same
experimental conditions. The catalytic activity followed the order
NCH2Ar), 2.69 (t, J = 5.6 Hz, 8H, NCH2); IR (KBr, cm-1)nmax
:
3448, 2959, 2842, 1632, 1599, 1504, 1229, 1118, 1036, 928; ESI-
MS m/z: 584 (M+ + 1); (Found C 61.55, H 5.65, N 9.41, Cl 12.18.
C30H32N4O4Cl2 requires C 61.75, H 5.49, N 9.61, Cl 12.19%).
Ligand H2L4 was prepared as described for H2L3 except starting
with ethylenediamine instead of 1, 2-phenylenediamine to give a
ZnL1> ZnL2> ZnL3> ZnL4
.
1
yellow solid, in yield 85%, m.p. 204–206°C. H NMR(200 MHz,
Kinetic model of the catalytic PNPP hydrolysis
The catalytic hydrolysis of PNPPin aqueous solution of the complexes
can be expressed as Scheme 1 involving equations (1) and (2):
CDCl3) d: 10.15 (s, 2H, OH, D2O exchangeable), 8.56 (s, 2H,
N=CH), 7.45–7.23 (m, 4H, ArH), 3.86–3.76 (m, 16H, OCH2,
NCH2Ar, C=NCH2), 2.76 (t, J = 5.5 Hz, 8H, NCH2); IR (KBr, cm-1)
n
max: 3442, 2959, 2842, 1632, 1597, 1501, 1230, 1129, 1036, 930;
H2O
M
H2O
M
ESI-MS m/z: 536(M+ + 1); (Found C 58.45, H 6.15, N 10.31, Cl 13.18.
PNPP
C26H32N4O4Cl2 requires C 58.32, H 5.98, N 10.47, Cl 13.27%).
K
k
(1)
(2)
+
P
PNPP
( S )
General methods for preparation of Schiff base–zinc(II) complexes
A solution of Schiff base ligand (1.0 mmol) and ZnCl2 (1.1 mmol)
in EtOH (15 cm3) was stirred for 2 h under a N2 atmosphere at
70°C, then the mixture was cooled and filtered, washed with ethanol
to give the complexes, and the pure product was obtained after
recrystallisation from ethanol.
Ligand
(MLS)
Ligand
( ML )
k0
S
P
ZnL1: Straw yellow, 81% yield, m.p. >300°C.IR (KBr, cm-1)nmax
:
1615, 1226, 1124; ESI-MS m/z: 1007(M+ + 1); (Found C 59.45,
Scheme 1 The process of PNPP hydrolysis.

JOURNAL OF CHEMICAL RESEARCH 2009 347
Table 1 The pseudo–first-order rate constants (kobs) of catalytic PNPP hydrolysis in buffer solutions
104 [PNPP]/
mol dm-3
Complex
102kobs(s-1)
pH 7.00
7.30
7.60
7.90
8.20
ZnL1
2.00
2.67
3.33
4.00
4.67
2.00
2.67
3.33
4.00
4.67
2.00
2.67
3.33
4.00
4.67
2.00
2.67
3.33
4.00
4.67
0.82
1.07
1.34
1.59
1.89
0.74
0.98
1.22
1.45
1.63
0.59
0.75
0.92
1.12
1.33
0.53
0.67
0.83
0.98
1.18
1.63
2.01
2.67
3.26
3.58
1.52
1.98
2.46
2.99
3.32
1.33
1.81
2.27
2.52
2.84
1.16
1.59
1.91
2.18
2.47
3.49
4.42
5.72
6.87
7.67
3.29
4.32
5.59
6.37
6.89
2.73
3.63
4.46
5.12
5.83
2.53
3.35
3.94
4.65
5.42
8.55
11.58
13.72
16.92
18.99
8.06
22.94
28.14
35.54
43.11
49.28
22.42
25.64
33.21
39.32
46.76
20.47
22.61
29.89
34.49
38.31
19.94
21.34
26.36
32.11
34.03
ZnL2
ZnL3
ZnL4
10.51
12.59
14.68
17.62
7.33
9.98
10.65
13.82
14.99
7.19
9.41
10.13
12.98
14.19
Condition: 25±0.1°C, I = 0.1 mol dm-3 KNO3, [complex] = 1.0 ¥ 10-5 mol dm-3
Where S is the substrate PNPP, ML is the hydrated complex, MLS
represents the intermediate made from ML and S, P represents the
product p-nitrophenol, K is the association constant between the
substrate(S) and the hydrated complex (ML), and k is first-order-rate
constant for the product formation, and is pH-dependent. k0 is the
pseudo-first-order-rate constant in absence of catalyst.
The rate of spontaneous hydrolysis is much lower than that of
catalytic hydrolysis in buffered aqueous solution, so the products
of PNPP spontaneous hydrolysis can be neglected in the kinetics
calculations. From the previous report, 24 and Scheme 1, we have:
MLS gives the products. The rate for PNPP catalytic hydrolysis
depends on the stabilisation of the intermediate MLS–. According to
the principles of chemical equilibration, it is more favourable for the
intermediate MLS to form the intermediate MLS– and generate the
products in alkaline solution. Hence, the first-order-rate constants (k)
increase with the increase of pH value in the reaction system.
Due to chemical balance principle, we have:
Ka = [H+] [MLS -]/[MLS]
(4)
According to the material balance, we have:
1
1
1
=
+
(3)
[MLS] = [MLS]t + [MLS-]
(5)
kobs
k
Kk[S]
In the above equations, [ML] and [ML]T are the free and the total
concentration of the hydrated complex, respectively; [S] is the
concentration of free substrate and can be substituted by the initial
concentration of the substrate based on the initial rate method; [MLS]
is the concentration of the intermediate formed by the substrate and
the hydrated complex in the buffered solution; kobs is the pseudo-first-
order-rate constant.
Combination of equations (4) and (5) leads to:
K [MLS]
−
a
[MLS ] =
(6)
+
[H ]+ K
a
The rate equation in Scheme 2 can be expressed as:
Based on the experimental data (see Table 1) and equation (3),
the relationships between 1/kobsd versus 1/[S] were plotted in Fig. 2,
showing a good linear relationship between the variables, with r2>
0.98. The plots have a positive intercept that evaluates the first-
order-rate constant k with relative standard deviation of less than 1%.
The k of calculation has been listed in Table 2.
k[MLS] = k1 [MLS-]
(7)
(8)
Combination of equations (6) and (7) leads to:
K k1
a
k =
[H+ ]+ K a
pH-rate profile and stoichiometry of the kinetic constants
From the results obtained, it was shown that the first-order-rate
constant (k) was pH-dependent. In other words, k is related to the
acid dissociation constant Ka of H2O coordinated to the metal ion.
The k increases along with the increase of the pH value from 7.00 to
8.20. This implies that the reaction process contains proton transfer
in the rate-determining step in Scheme 1, which can be shown as
Scheme 2.
Ka is the acidic dissociation constant of H2O coordinated to the
metal ion; k1 is the first-order-rate constant, which is pH-independent.
In this process, the intermediate MLS is first ionised or undergoes
proton transfer, and then electronic transfer inside the intermediate
Rearrangement of equation (8) gives:
+
1
1
1
+ [H
=
]
(9)
k
k1 k1Ka
[MLS–] is the dissociated concentration of the intermediate (MLS),
[MLS]t is the undissociated concentration of the intermediate (MLS).
On the basis of equation (9), the k1 and Ka values can be afforded
from the slope and intercept of the plot of 1/k versus [H+] (Fig. 3)
The results give k1 and pKa values of 4.66 s-1 and 7.98 for ZnL1,
2.51 s-1 and 8.00 for ZnL2, and 1.83 s-1 and 8.09 for ZnL3, 1.41 s-1 and
8.15 for ZnL4, respectively.
Table 2 k (s-1) of the catalytic hydrolysis of PNPP by Schiff
base zinc complexes in buffer solution
Effects of the complex structures on catalytic PNPP hydrolysis
The mimic hydrolase used in this paper exhibits similar structural
features to those of the natural hydrolase in PNPP catalytic hydrolysis.
Tables 1 and 2 show that: (1) the catalytic activity of the crown Schiff
complexes is higher than that of the crown-free analogous. On the
one hand, this can be attributed the crown ether ring in the Schiff base
pH
7.00
7.30
7.60
7.90
8.20
k(ZnL1)
k(ZnL2)
k(ZnL3)
k (ZnL4)
0.454
0.231
0.135
0.093
0.751
0.409
0.257
0.180
1.200
0.651
0.430
0.313
2.587
1.095
0.659
0.483
3.463
2.026
1.150
0.742

348 JOURNAL OF CHEMICAL RESEARCH 2009
pH 7.00
pH 7.30
pH 7.60
pH 7.90
pH 8.20
pH 7.00
150
150
120
90
60
30
0
pH 7.30
pH 7.60
pH 7.90
pH 8.20
120
90
60
30
0
2000
3000
1/[S](L.mol-1) for ZnL2
4000
5000
2000
3000
4000
5000
1/[S](L.mol-1) for ZnL1
pH 7.00
pH 7.30
pH 7.60
pH 7.90
pH 8.20
pH 7.00
210
210
pH 7.30
pH 7.60
pH 7.90
pH 8.20
180
150
120
90
180
150
120
90
60
60
30
30
0
0
2000
3000
1/[S](L.mol-1) for ZnL3
4000
5000
2000
3000
1/[S](L.mol-1) for ZnL4
4000
5000
Fig. 2 Effect of substrate concentration on first-order rate constants for catalytic PNPP hydrolysis by the complexes ZnL1, ZnL2,
ZnL3, ZnL4 in buffer solutions at 25ꢀ°C in the buffer solution pH7.00 ꢀpH7.30 ꢀpH7.60 ꢀpH7.90 *pH8.20.
12
9
5.5
5
6
4.5
4
3
3
3.1 3.2
3.3
3.4
3.5
0
1/T(10-3K-1)
0
20
40
60
80
100
109[H+](mol.L-1)
Fig. 4 The plot of -lnkobs versus 1/T at pH = 7.00 [S]=2.0 ¥ 10-4
mol dm-3, [complex] = 1.0 ¥ 10-5 mol dm-3. ( ZnL1 ꢀZnL2
ZnL3 * ZnL4).
ꢀ
Fig. 3 pH-rate profile for the catalytic hydrolysis of PNPP by
the Schiff base zinc complexes in the buffer solution at 25°C
(
ZnL1 ꢀZnL2 ꢀZnL3 ꢀZnL4).
Effect of temperature and apparent activation energy on catalytic
PNPP hydrolysis
complexes which offers a hydrophobic environment owing to its outer
ethylene groups, so the hydrophobic PNPP molecule interacts more
favourably with these complexes; on the other hand, as hydrogen
bonds may be formed between the oxygen atoms of the crown ether
ring and H2O in the intermediate MLS, H2O coordinated to the metal
can be activated synergistically by both the metal ion and crown, the
rate of formation of the intermediate MLS– may be faster in crown
Schiff base complexes than that in crown-free Schiff base complexes;
(2) it can be seen that the catalytic activity of phenyl-bridged Schiff
base complex is larger than that of ethyl-bridged Schiff base complex
under the same substitute groups and metal ion. In other words, the
catalytic activity follows the order ZnL1 > ZnL2, and ZnL3 > ZnL4,
respectively. This perhaps can be attributed to rigidity, since phenyl-
bridged Schiff bases possess more rigidity than that of ethyl-bridged
Schiff bases due to the different bridge chain.
In order to investigate the effect of temperature on catalytic hydrolysis,
the pseudo-first-order-rate constants (kobs) of PNPP hydrolysis were
obtained at five different temperatures from 15 to 55°C at pH = 7.00,
[S] = 2.0 ¥ 10-4mol dm-3. According to the Arrhenius equation,
straight lines for –ln kobs versus 1/T were obtained (see Fig. 4).
The apparent activation energy (Ea) values were determined from the
slopes of the straight lines. The Ea values are 16.05 kJ mol-1 for ZnL1,
17.76 kJ mol-1 for ZnL2, 19.53 kJ mol-1 for ZnL3, and 20.92 kJ mol-1
for ZnL.4 Since the formation rate of the intermediate, MLS and
MLS–, should increase with rising reaction temperature, the rate of
PNPP catalytic hydrolysis is enhanced.
Proposed mechanism for hydrolysis of PNPP catalysed by base
complexes with azacrown ether pendant
The proposed mechanism of the catalytic hydrolysis of PNPP by
ZnL1, which is used as an example of PNPP catalytic hydrolysis
by Schiff base complexes, is outlined in Scheme 2 on the basis of
previous report32,33 and pKa values of complexes above. According

JOURNAL OF CHEMICAL RESEARCH 2009 349
Cl
Cl
Cl
Cl
CH=N
O
CH=N
N=CH
N=CH
HO
NO2
Zn
Zn
O
H
O
N
O
N
CH2
CH2
H2C
N
O
H2C
N
O
O
k1
N
O
N
O
O
O
O
O
O
O
O
C
O
C
O
O
O
O
O
O
O
OH
H2O
N
NO2
MLS-
O
C
quick
OH
PNPP
S
Cl
Cl
Cl
Cl
CH=N
CH=N
O
N=CH
O
N=CH
O
Zn
Zn
Ka
H+
O
CH2
CH2
H2C
N
O
H2C
N
O
O
O
N
O
N
O
H
H
H
O
O
O
O
O
O
O
O
O
O
O
O
ML
O
S
PNPP
C
OH
N
quick
H2O
K
Cl
Cl
Cl
Cl
CH=N
O
N=CH
CH=N
O
N=CH
HO
NO2
Zn
Zn
O
N
O
CH2
CH2
H2C
N
H2C
N
O
O
O
N
H
k
N
O
N
O
H
O
O
O
O
O
C
O
O
O
O
C
O
O
O
O
O
O
OH
NO2
MLS
Scheme 2 Proposed mechanism for hydrolysis of PNPP catalysed by Schiff base zinc II) complex with benzo-10-aza-crown ether pendant.
to the complex structure, H2O could be coordinated to the zinc (II)
ion, which would result in the formation of the hydrated complex
in aqueous solution. This hydrated complex may be the real active
species for the PNPP catalytic hydrolysis.34 Therefore, we assume
that the mechanism of PNPP catalytic hydrolysis is similar to the
hydrolytic reaction catalysed by hydrolytic metalloenzymes.
by ZnL1 containing a benzo-10-aza-crown ether ligand is 1.05 ¥ 103
times than that of spontaneous hydrolysis of PNPP at pH = 7.00, [S]
= 2.0 ¥ 10-4 mol dm-3. The studies also indicate that the Schiff base
complexes are stable in the range of temperature investigated and the
apparent activation energy (Ea) values follow the order ZnL1 <ZnL2
< ZnL3< ZnL4.
Scheme 2 shows that: (1) the H2O coordinated to the zinc(II) ion
is activated synergistically by the zinc(II) ion and crown ring, and
then the intramolecular hydroxide is generated; (2) the N atom of the
pyridine ring in a PNPP molecule is coordinated to the zinc(II) ion
in the complex, forming the intermediate MLS; (3) the coordinated
hydroxide OH- in the intramolecular zinc(II) attacks the positive C
atom of the ester carbonyl group in the PNPP molecule to promote
the departure of the forming p-nitrophenol with a first-order-rate
constant (k), which is the rate-determining step of the total reaction;
and (4) H2O is bonded to zinc(II) ion quickly again and the picolinic
acid molecule coordinated to the zinc(II) ion is released.
The authors gratefully acknowledge financial support from
China National Natural Science Foundation (No: 20072025),
Key Project of Sichuan Province Education Office
(No: 2005D007) and Key Scientific and Technological Project
Issued by Ministry of Education of China (No. 208118).
Received 29 November 2008; accepted 29 March 2009
Paper 08/0311 doi: 10.3184/030823409X450525
Published online: 30 June 2009
Conclusion
In this report, we have investigated the catalytic hydrolysis of a
carboxylic ester by Schiff base zinc(II) complexes with either benzo-
10-aza-crown ether pendants (ZnL1, ZnL2), or morpholino-pendants
(ZnL3, ZnL4). The rate of catalytic PNPP hydrolysis increases with
increasing of pH of the buffer solution compared with the crown-
free analogues ZnL3 and ZnL4. The crown Schiff base complexes
(ZnL1, ZnL2) exhibit higher catalytic activity; the catalytic activity
of phenyl-bridged Schiff base complex is larger than that of ethyl-
bridged Schiff base complex under the same substitute groups and
metal ion. The pseudo-first-order-rate for PNPP hydrolysis catalysed
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