Mono-Schiff base complexes with aza-crown ether or morpholino pendants as synthetic hydrolases for p-nitrophenyl picolinate cleavage

Article abstract of DOI:10.1007/s11243-012-9672-6

Mono-Schiff base complexes of Co(II) and Mn(III) with either benzo-10-aza-crown ether pendants (MnL ₂ ¹ Cl, MnL ₂ ² Cl) or morpholino pendants (MnL ₂ ³ Cl, CoL ₂ ³) ha

Full text of DOI:10.1007/s11243-012-9672-6

Transition Met Chem (2013) 38:149–155
DOI 10.1007/s11243-012-9672-6
Mono-Schiff base complexes with aza-crown ether or morpholino
pendants as synthetic hydrolases for p-nitrophenyl picolinate
cleavage
Ying Tang
Sheng-tian Huang
•
Jin Zhang
•
Jian-zhang Li
•
Received: 19 August 2012 / Accepted: 6 November 2012 / Published online: 29 November 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract Mono-Schiff base complexes of Co(II) and
Mn(III) with either benzo-10-aza-crown ether pendants
their specific attributes for hydrolysis of biological molecules.
Studies of synthetic molecules as hydrolase mimetics have
received considerable attention [1]. Recently, there has
been much effort to design and synthesize model com-
plexes for promoting the hydrolysis of carboxylate and
phosphate esters, due to important environmental and
biological applications [2, 3]. In recent years, various
chemical systems have been used as models for natural
enzymes [4–9]. These artificial hydrolases have similar
catalytic function to natural enzymes owing to their active
groups and structures which are similar to the natural
enzymes; moreover, they are structurally more simple and
stable than natural enzymes and can provide information
on the mechanistic aspects of enzyme action. Our research
group has focused on such enzyme mimics, especially
transition metal complexes with crowned Salen-type or
unsymmetrical bis-Schiff base ligands as catalysts for the
hydrolysis of esters in buffered solution [10–14]. The
crown ether-containing Schiff bases have attracted much
attention because the crown rings could endow functional
molecules with novel performance and character, owing to
the hydrophobicity of the outer ethylene groups and orderly
arrangement of inner oxygen atoms [15].
1
2
3
(
MnL Cl, MnL Cl) or morpholino pendants (MnL Cl,
2 2 2
3
CoL ) have been employed as models for hydrolase enzymes
2
by studying the kinetics of their hydrolysis reactions with
p-nitrophenyl picolinate (PNPP). A kinetic model of PNPP
cleavage catalyzed by these complexes is proposed. The
effects of complex structures and reaction temperature on the
rate of catalytic PNPP hydrolysis have also been examined.
The rate increases with pH of the buffer solution; all four
complexes exhibited high activity in the catalytic PNPP
3
hydrolysis. Compared with the crown-free analogs MnL Cl
2
3
2
1
2
and CoL , the crowned Schiff base complexes (MnL Cl,
2
MnL Cl) exhibited higher catalytic activity. The pseudo-first-
2
order rate constant for the PNPP hydrolysis catalyzed by the
1
complex MnL Cl, containing a benzo-10-aza-crown ether
2
3
pendant, is 1.84 9 10 -fold higher than that of spontaneous
-4
hydrolysis of PNPP at pH 7.60, 25 °C, [S] = 2.0 9 10
mol dm .
-3
Introduction
Enzyme mimics are artificial compounds whose structures
are based on our current understanding of enzymes and
Previously, the catalytic performance of mono-Schiff
base complexes with 3-substituted aza-crown ether or
morpholino pendants in p-nitrophenyl picolinate (PNPP)
hydrolysis was reported [16, 17]. With hopes of further
investigating the reaction kinetics and mechanism of PNPP
hydrolysis catalyzed by mono-Schiff base complexes with
Y. Tang ꢀ J. Zhang
College of Materials and Chemical Engineering,
Chongqing University of Arts and Science, Yongchuan,
Chongqing 402160, People’s Republic of China
5-substituted aza-crown ether or morpholino pendants,
J. Li (&) ꢀ S. Huang
mono-Schiff base Co(II) and Mn(III) complexes with
1
2
Key Laboratory of Green Catalysis of Higher Education
Institutes of Sichuan, School of Chemistry and Pharmaceutical
Engineering, Sichuan University of Science and Engineering,
Zigong 643000, Sichuan, People’s Republic of China
e-mail: sichuanligong2006@hotmail.com
benzo-10-aza-crown ether pendants (MnL Cl, MnL Cl),
2
2
3
2
3
2
and the analogs with morpholino pendants (MnL Cl, CoL )
have been employed as models to mimic enzymatic
hydrolysis of PNPP. In this paper, we have investigated the
123

1
50
Transition Met Chem (2013) 38:149–155
O
HC
N
R
HC
O
N
Cl
O
CH2
CH2
N
O
N
O
O
O
Mn Cl
M
N
O
O
O
O
N
O
N
CH2
O
CH2
R
N
CH
O
Cl
CH
R= NO
2
, MnL1
2
Cl ; R= Cl, MnL2
2
Cl
M= MnCl, MnL3
2
Cl ; M=Co, CoL3
2
Fig. 1 Structures of mono-Schiff base complexes
effects of the position of substituents and the kind of metal
on the catalytic performance of the Schiff base complexes
with aza-crown ether or morpholino pendants for PNPP
hydrolysis. The structures of these mono-Schiff base
complexes are shown in Fig. 1.
straight lines, where (rate) is the initial rate of PNPP
0
hydrolysis and [ML] is the initial concentration of the
0
complex.
Each kinetic run was initiated by injecting an acetonitrile
solution of concentration PNPP at the desired concentration
3
into a 1-cm cuvette containing 3 cm of the complex at the
desired concentration. 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 20-fold excess of substrate over catalyst. The
Experimental
Materials and methods
-
1
-1
molar extinction coefficients (e, unit as L mol cm ) for
p-nitrophenol at 400 nm were obtained by measuring the
absorption of desired concentrations of p-nitrophenol at
various pH values and listed as follows: pH 7.60,
Kinetic and thermodynamic measurements were taken by
UV–vis methods with a GBC 916 UV–vis spectropho-
tometer (GBC Co., Australia) equipped with a thermostatic
cell holder. All reagents, unless otherwise indicated, were
of analytical grade and used without further purification.
Trihydroxymethylaminomethane (Tris) was purchased
from Aldrich. Buffer solutions were made from standard-
ized nitric acid. Water was obtained on a Water Purifica-
tion System (Nex Power 1000, Human Corporation, South
4
4
4
1.37 9 10 ; pH 7.90, 1.58 9 10 ; pH 8.20, 1.72 9 10 ; pH
4
4
8.50, 1.81 9 10 ; pH 8.80, 1.86 9 10 .
Results and discussion
-
1
Korea) to achieve a resistivity of at least 16 MX cm . The
ionic strength of the buffer solution was maintained at
Rate constants for PNPP hydrolysis
-
3
0
.1 mol dm KNO throughout the experiments. The pH
3
The pseudo-first-order rate constants obtained for catalytic
PNPP hydrolysis by these Schiff base complexes are shown in
value of the buffer solution was measured at 25 °C using a
Radiometer PHM 26 pH meter (made in China) fitted with
G202C glass and K4122 calomel electrodes. The following
compounds were prepared according to the literature:
PNPP [18], and the four mono-Schiff base Co(II) and
Table 1. The pseudo-first-order rate constant (k ) for PNPP
0
-5 -1
s
hydrolysis in the absence of catalyst is 1.95 9 10
-4
at
-3
pH = 7.60, 25 °C, [S] = 2.0 9 10 mol dm . Hence, the
rate of the PNPP hydrolysis catalyzed by the complexes
1
2
2
2
3
2
3
2
Mn(III) complexes (MnL Cl, MnL Cl, MnL Cl, CoL )
3
increases by a factor of ca. 1.84 9 10 times for MnL Cl,
1
2
[
19, 20]. PNPP stock solution for kinetics was prepared in
3
.74 9 10 times for MnL Cl, 1.40 9 10 times for MnL Cl,
2
3
3
1
2
2
acetonitrile.
3
and 1.32 9 10 times for CoL , and the catalytic activity
3
2
1
2
2
2
3
2
3
2
follows the order MnL Cl [ MnL Cl [ MnL Cl [ CoL
Kinetics studies
under the same experimental conditions.
In contrast, the values ofk for PNPP hydrolysis catalyzed
obs
The pseudo-first-order rate constants (kobs) were obtained
based on the initial rate method, that is, according to the
equations:
1
2
3
by thefree Schiff bases HL , HL , andHL inbufferedsolution
-4
-4
-4 -1
are 5.93 9 10 , 5.26 9 10 , and 4.97 9 10 s ,
respectively, and the kobs values for the hydrolysis cata-
lyzed by the simple metal salts in buffered solution are
ðrateÞ ¼ ꢁðdC=dtÞ ¼ ðdA=dtÞ =e
0
0
0
-
4
-1
-4 -1
8
.16 9 10
s
for CoCl and 8.13 9 10
2
s
for MnCl2
ðrateÞ ¼ kobs½MLꢂ ;
Plots of (rate) versus [ML] were used to obtain the
0
0
under the same conditions. Hence, these Schiff base com-
plexes are much better catalysts than the free Schiff base
ligands or simple metal salts.
0
0
pseudo-first-order rate constants from the slope of the
1
23

Transition Met Chem (2013) 38:149–155
Kinetic model of PNPP hydrolysis
151
Since the rate of PNPP spontaneous hydrolysis is much
lower than that of catalytic hydrolysis, the products of
spontaneous hydrolysis of PNPP can be neglected in the
kinetic calculations. Hence, Scheme 1 leads to the rate
equation:
The catalytic hydrolysis of PNPP by these complexes can be
expressed as in Scheme 1 involving Eqs. (1) and (2), where
S is the substrate PNPP, ML is the hydrated complex, MLS
represents the intermediate adduct of ML and PNPP,
P represents the product p-nitrophenol, K is the association
constant between the PNPP and ML, and k is the first-order
rate constant for the product formation, which is pH-
Rate ¼ k½MLSꢂ
The association constants K can be expressed in terms of
ð3Þ
concentrations:
dependent. The constant k is the pseudo-first-order rate
0
K ¼ ½MLS]=½MLꢂ½Sꢂ
According to the material balance, we have:
ð4Þ
ð5Þ
constant for PNPP hydrolysis in the absence of catalyst.
Table 1 Pseudo-first-order rate constants (kobs) of catalytic PNPP
hydrolysis in buffer solutions
½MLꢂ ¼ [ML] þ [MLS]
Combination of Eqs. (4) and (5) leads to:
T
4
Complex 10 [PNPP]/
2
-1
10 kobs/s
-
mol dm )
3
(
K½Sꢂ½MLꢂ
T
pH
7
pH
7.90
pH
8.20
pH
8.50
pH
8.80
½
MLSꢂ ¼
ð6Þ
.60
1 þ K½Sꢂ
1
2
MnL
MnL
MnL
Cl 2.00
3.58
4.68
5.86
6.97
7.96
3.40
4.32
5.51
6.55
7.46
2.73
3.43
4.36
5.15
5.97
2.58
3.31
3.96
4.96
5.61
8.74 23.42 38.67 52.29
11.81 28.24 51.96 68.53
14.19 35.11 60.58 82.21
16.45 41.95 68.11 90.32
19.09 49.82 76.92 99.78
7.99 21.87 36.96 48.98
11.12 27.17 45.96 62.59
13.24 32.35 54.58 75.21
15.84 37.13 63.45 87.38
16.41 46.54 73.92 89.18
7.14 20.14 36.96 44.94
9.42 23.84 44.34 57.22
10.89 28.71 51.58 65.21
13.12 34.79 58.45 73.38
15.14 38.41 69.92 82.18
6.76 18.44 36.26 43.99
9.17 22.24 43.14 54.72
9.97 26.56 49.58 62.91
12.72 32.21 56.05 71.18
14.11 34.19 66.98 76.18
Combination of Eqs. (6) and (3) followed by rearrangement
gives:
2
3
4
4
.67
.33
.00
.67
kK½Sꢂ½MLꢂ
T
rate ¼
¼ kobs½MLꢂT
ð7Þ
ð8Þ
1
þ K½Sꢂ
2
2
1
1
1
Cl 2.00
¼ k
þ
kobs
Kk½Sꢂ
2
3
4
4
.67
.33
.00
.67
In the above equations, [ML] and [ML] are the free and
T
total concentration of the hydrated complex, respectively;
[S] is the concentration of free substrate and can be
3
2
Cl 2.00
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; and kobs is the pseudo-first-order rate
constant.
2
3
4
4
.67
.33
.00
.67
3
2
CoL
2.00
Based on the experimental data (see Table 1) and
Eq. (8), the relationships between 1/kobs versus 1/[S] were
plotted in Fig. 2, showing a good linear relationship
2
3
4
4
.67
.33
.00
.67
2
between the variables, with r [ 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
calculated values of k are listed in Table 2.
-
3
Conditions: (25 ± 0.1) °C, I = 0.1 mol dm KNO
3
, [complex] =
-
5
.0 9 10 mol dm
-3
1
Scheme 1 The catalytic
hydrolysis process of PNPP in
buffered solution of the
complexes
H O
H O
2
2
Ligand
Metal
NO2
K
k
+
S
PNPP
( S )
P
(1)
O
C
O
N
PNPP
(
ML )
(MLS)
p-Nitrophenol picolinate (PNPP) ( S )
O N
ko
O-
2
(
2)
P
-
p-Nitrophenol(PNPO ) ( P )
123

1
52
Transition Met Chem (2013) 38:149–155
pH 7.60
pH 7.90
pH 8.20
pH 8.50
pH 8.80
pH 7.60
pH 7.90
pH 8.20
pH 8.50
pH 8.80
30
2
2
2
1
1
1
7
4
1
8
5
2
9
6
3
0
2
7
24
2
1
1
1
1
8
5
2
9
6
3
0
2
000
3000
4000
1
5000
2000
3000
4000
5000
-1
-1
2
1
/[S](L.mol ) for MnL
2
Cl
1/[S](L.mol ) for MnL 2Cl
pH 7.60
pH 7.60
pH 7.90
pH 8.20
pH 8.50
pH 8.80
pH 7.90
pH 8.20
pH 8.50
pH 8.80
35
3
5
3
0
3
2
2
1
1
0
5
0
5
0
5
0
25
2
1
1
0
5
0
5
0
2
000
3000
4000
3
5000
2000
3000
4000
5000
-
1
-1
3
1
/[S](L.mol ) for MnL
2
Cl
1/[S](L.mol ) for CoL
-
5
-3
Fig. 2 Effect of substrate concentration on first-order rate constants for
1
[complex] = 1.0 9 10 mol dm , filled diamond pH 7.60, filled
square pH 7.90, filled triangle pH 8.20, filled circle pH 8.50, asterisk
pH 8.80
2
3
catalytic PNPP hydrolysis by the complexes MnL Cl, MnL
2
2 2
Cl, MnL Cl,
3
-3
-3
, [Tris] = 0.1 mol dm ,
and CoL
2
at 25 °C, I = 0.1 mol.dm KNO
3
-
Table 2 k (s ) of the catalytic hydrolysis of PNPP by mono-Schiff
1
base complexes in buffer solutions at various pH
pH
pH 7.60 pH 7.90 pH 8.20 pH 8.50 pH 8.80
H2O
OH
H O
2
Ka
k1
1
2
k(MnL
k(MnL
k(MnL
Cl) 1.155
Cl) 0.842
Cl) 0.525
1.582
1.313
0.793
0.685
2.299
1.784
1.176
1.028
2.973
2.527
1.686
1.474
3.378
2.901
2.073
1.754
P +
2
2
3
2
PNPP
( MLS- ) PNPP
(MLS)
( ML )
3
2
k (CoL
)
0.439
Scheme 2 The process of proton transfer at the rate-determining step
pH profile and stoichiometry
from 7.60 to 8.80. This implies that the reaction process
contains a proton transfer at the rate-determining step in
Scheme 1, which can be shown as in Scheme 2.
Hydrolases are found to be very sensitive to the acidity of
the physiological microenvironment. The spatial confor-
mation of the enzyme may be transformed by the changes
in the pH, and the stability of the intermediate (MLS)
should also be changed. From the results obtained in the
present study, it is clear that the first-order rate constant
K is the acid dissociation constant of H O coordinated
a
2
to the metal center, and k is the first-order rate constant,
1
which is pH-independent. In this process, the intermediate
MLS was first ionized and then internal electron transfer
within the intermediate MLS formed the products. The rate
for PNPP catalytic hydrolysis depends on the stabilization
(
k) is pH-dependent. It is most likely that k is related to the
acid dissociation constant K of H O coordinated to the
a
2
-
metal center. The value of k increased with increasing pH
of the intermediate MLS . According to the principles of
1
23

Transition Met Chem (2013) 38:149–155
153
chemical equilibration, it is more favorable for the inter-
-
mediate MLS to form the intermediate MLS and generate
Effects of the complex structures
the products in alkaline solution. Hence, the first-order rate
constants (k) increase with the increase in pH value in the
reaction system.
Factors affecting the activity of Schiff base complexes as
enzyme mimics in ester hydrolysis include steric hindrance,
and electronic effects of the substituents on the Schiff base
ligand provide a hydrophobic microenvironment for PNPP
close to the catalytic site. The hydrolase mimics reported in
this paper exhibit the similar structural effects to those of
the natural hydrolase in PNPP catalytic hydrolysis. Tables 1
and 2 show that the catalytic activities of the crowned Schiff
complexes are higher than those of crown-free analogs
This leads to the following analysis:
þ
ꢁ
K ¼ ½H ꢂ½MLS ꢂ=½MLS]
ð9Þ
a
According to the material balance, we have:
ꢁ
½
MLSꢂ ¼ ½MLSꢂ þ ½MLS ꢂ
Combination of Eqs. (9) and (10) leads to:
ð10Þ
t
2
3
(MnL
Cl [ MnL Cl). This can be attributed to the crown
2
2
ether substituent in the Schiff base complexes. Its hydro-
phobic environment, provided by its outer ethylene groups,
means that the hydrophobic PNPP molecule interacts more
favorably with these complexes; on the other hand, as
hydrogen bonds may be formed between the oxygen atoms
of the crown ether and H O in the intermediate MLS, H O
Ka½MLSꢂ
ꢁ
½
MLS ꢂ ¼
ð11Þ
þ
½
H ꢂ þ Ka
The rate equation in Scheme 2 can then be expressed as:
k½MLSꢂ ¼ k1½MLSꢁꢂ
ð12Þ
2
2
coordinated to the metal can be activated synergistically by
both the metal and crown; hence, the formation of the
Combination of Eqs. (12) and (13) and rearrangement leads
to:
-
intermediate MLS may be faster in the crown Schiff
1
1
1
þ
complexes. This result is, however, opposite to that pub-
lished previously [21]. We think that the differences in
geometric factors and steric hindrance between benzoaza-
¼ k
þ
½H ꢂ
ð13Þ
k
k1Ka
1
-
where [MLS ] is the dissociated concentration of the
15-crown-5 and morpholine may be definitive factors when
intermediate (MLS), and [MLS] is the undissociated con-
t
these substituents occupy position 3, whereas the difference
in hydrophobicities between these two substituents may be
the definitive factor when they occupy position 5. The steric
hindrance of the 5- substituents for the approach of PNPP is
smaller than that of the 3-substituted analogs. However, the
hydrophobic microenvironment provided by benzoaza-15-
crown-5 when at position 5 may be the dominant factor for
PNPP, rather than steric hindrance. It can also be seen that
the catalytic activity of the Mn(III) complex is higher than
that of the corresponding Co(II) complex with the same
centration of the intermediate (MLS). On the basis of
Eq. (13), the k and K values can be obtained from the
1
a
?
slope and intercept of the plot 1/k versus [H ] (Fig. 3).
-
This gives k and pK values of 3.64 s and 7.95 for
1
1
a
1
2
-1
MnL Cl, 3.30 s and 8.07 for MnL Cl, 2.32 s and 8.14
2
2
-1
3
-1
for MnL Cl, and 2.07 s and 8.18 for CoL , respectively.
1
2
2
3
3
ligands (MnL Cl [ CoL ). This can be explained by the
2
2
2
2
1
1
0
0
0
.4
.0
.6
.2
.8
.4
.0
higher charge density of Mn(III) compared to Co(II),
resulting from the smaller radius and higher positive charge
of Mn(III). The smaller the metal radius, the stronger the
ability of activating H O and binding PNPP, increasing rate
2
of PNPP hydrolysis; on the other hand, the higher the
positive charge of central metal, the faster the proton
transfer of coordinated H O to give coordinated hydroxide
2
-
OH . In this way, coordinated water becomes a good
nucleophile in the form of hydroxide around neutral pH for
the attack of PNPP, as observed in metallohydrolases [22].
The obtained k and k values listed in Tables 1 and 2 show
obs
1
2
2
2
that MnL Cl possesses a higher activity than MnL Cl. This
0
4
8
12
16
20
24
28
could be due to the positive charge density of the metal
being enhanced by an electron-withdrawing effect of the
substituent on the amino aromatic ring. Since the electron-
withdrawing effect of the substituents follows the order
9
+
-1
1
0 [H ](mol.L )
Fig. 3 pH rate profile for the catalytic hydrolysis of PNPP by the
1
complexes in the buffer solution at 25 °C (filled diamond MnL
2
Cl,
2
2
3
2
3
2
1
filled square MnL
Cl, filled triangle MnL
Cl, filled circle CoL
)
NO [ Cl, the positive charge on Mn(III) in MnL Cl will
2
2
123

1
54
Transition Met Chem (2013) 38:149–155
2
2
1
2
be higher than that in MnL Cl; hence, MnL Cl readily
Effect of temperature
accelerates the deprotonation of the water ligand, which can
1
be confirmed by the smaller pK value (7.95) of MnL Cl
2
Temperature is another important factor that influences
enzyme efficiency. However, an enzyme may become
denatured and lose all functions at higher temperatures. In
order to investigate the effects of temperature on the present
system, the pseudo-first-order rate constants (kobs) of PNPP
hydrolysis were obtained at five different temperatures from
a
2
relative to that (8.07) of MnL Cl. Compared with catalytic
2
activity of mono-Schiff base complexes bearing an elec-
tron-donating methyl group, as reported in the literature
[
23], the catalytic activities of these mono-Schiff base
complexes bearing electron-withdrawing substituent groups
NO or Cl) are larger under comparable conditions.
-
4
-3
(
25 to 65 °C at pH = 7.60, [S] = 3.33 9 10 mol dm
According to the Arrhenius equation, straight lines were
.
2
obtained from plots of –ln kobs versus 1/T (Fig. 4). The
apparent activation energies (E ) were determined from the
a
slopes of the straight lines, giving E values of
a
3
2
2
2
.2
.9
.6
.3
2
-
1
1
-1
2
1
6.01 kJ mol for MnL Cl, 16.34 kJ mol for MnL Cl,
2 2
-
1
3
-1
7.50 kJ mol for MnL Cl, and 18.23 kJ mol for CoL .
3
1
2
2
This further indicates that all four complexes are stable in
the temperature range investigated. Since the formation rate
-
of the intermediates, MLS and MLS , should increase with
increasing temperature, the rate of PNPP catalytic hydro-
lysis is enhanced.
Proposed mechanism for catalytic hydrolysis
2
.9
3
3.1
3.2
-1
3.3
3.4
The proposed mechanism of the catalytic hydrolysis of
1
PNPP by MnL Cl, which is used as an example, is outlined
2
-3
1/T(10 K )
-
1
in Scheme 3 on the basis of previous reports [24–26] and
Fig. 4 The plot of -ln kobs versus T at pH = 7.60, [S] = 3.33 9
-
4
-3
mol dm , [complex] = 1.0 9 10 mol dm . (filled diamond
2
Cl, filled square MnL Cl, filled triangle MnL
-5
-3
the pK values of the complexes noted above. Coordination
a
1
MnL
0
1
2
3
2
3
2
Cl, asterisk
of H O to the Mn(III) atom would result in a hydrated
2
2
CoL )
complex in aqueous solution, which may be the active
HC
N
NO
2
HC
N
NO
O
2
H
2
C
O
C
P
2
H C
OH
O
N
O
O
N
O
C
N O
O
Mn
N
HO
NO2
O
N O
Mn
N
O
O
O
O
O
O
O
H
O
O
O
N
O
N
O
O
(
II-2) k1
O N
CH
CH2
O
O N
CH
CH2
2
2
MLS-
d
e
te
H
2
O
r
m
in
in
quick
III )
H+
g
O
C
r
a
te
Ka
s
(
te
p
OH
HO
NO2
k
( II-1)
N
(
II )
HC
N
NO
2
S
HC
N
NO2
PNPP
2
H C
H C
2
O
H
O
H
H
O
N
O
C
N O
O
Mn
N O
O
O
( I)
K
O
Mn
N
O
O
O
O
O
O
H
O
O
N
O
O
O
N
O
O
O N
N
CH
CH2
O
2
CH
CH2
2
O N
ML
MLS
Scheme 3 Proposed mechanism for the hydrolysis of PNPP catalyzed by mono-Schiff base complexes with benzo-10-aza-crown ether pendant
1
23

Transition Met Chem (2013) 38:149–155
155
species for the PNPP catalytic hydrolysis [27]. Therefore,
we assume that the mechanism of PNPP catalytic hydrolysis
is similar to that of analogous hydrolytic metalloenzyme.
Acknowledgments The authors gratefully acknowledge financial
supportfromChinaNational NaturalScience Foundation(No.20072025),
Key Scientific and Technological Project Issued by Ministry of Education
of China (No. 208118), and the Opening Project of Key Laboratory of
Green Catalysis of Sichuan Institutes of High Education (No. LZJ01,
LZJ1101).
The coordinated H O is then synergistically activated
2
by the metal and crown ring, forming an intramolecular
hydrated complex (ML). Coordination of PNPP then gives
the intermediate MLS (step I). Next, the coordinated
hydroxide attacks the carbonyl group of PNPP with a first-
order rate constant (k) (step II), which is the overall rate-
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2
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-
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of PNPP by mono-Schiff base Mn(III) and Co(II) com-
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pendants. The rate of the catalytic PNPP hydrolysis was
increased with increasing pH of the buffer solution. The
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3
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2
1
3
complex MnL Cl is 1.84 9 10 times than that of spon-
2
2
taneous hydrolysis of PNPP. These mono-Schiff base
complexes are stable in the range of temperature of
1
2
2
5–65 °C in this work.
123