Carbonic anhydrase inspired phosphatase model complexes derived from the tripodal ligand tris(hydroxy-2-benzimidazolyl)amine: The hydrolysis of p-nitrophenyl acetate

Article abstract of DOI:10.1080/15533174.2010.522651

The water soluble ligand tris(hydroxyl-2-benzimidazolyl)amine NTBOH was used for the preparation of a series of aqua transition metal complexes and their hydroxo derivatives of the types [NTBOH-M(OH₂)]²+ 1-3 and [NTBOH-M(OH)]+ 4-6 (M

Full text of DOI:10.1080/15533174.2010.522651

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Carbonic Anhydrase Inspired Phosphatase Model
Complexes Derived from the Tripodal Ligand
Tris(hydroxy-2-benzimidazolyl)amine: The Hydrolysis
of p-nitrophenyl Acetate
a b
a
a
a
Mohamed M. Ibrahim
, Hatem. A. Eissa , Hosny Y. El-Baradie & Gamal E. Kamel
a
Chemistry Department, Faculty of Education , Kafr El-Sheikh University , Kafr El-
Sheikh, Egypt
b
Chemistry Department, Faculty of Science , Taif University , Taif, Saudi Arabia
Published online: 03 Dec 2010.
To cite this article: Mohamed M. Ibrahim , Hatem. A. Eissa , Hosny Y. El-Baradie & Gamal E. Kamel (2010) Carbonic
Anhydrase Inspired Phosphatase Model Complexes Derived from the Tripodal Ligand Tris(hydroxy-2-benzimidazolyl)amine:
The Hydrolysis of p-nitrophenyl Acetate, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry,
40:10, 869-878
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:869–878, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.522651
Carbonic Anhydrase Inspired Phosphatase Model
Complexes Derived from the Tripodal Ligand
Tris(hydroxy-2-benzimidazolyl)amine: The Hydrolysis
of p-nitrophenyl Acetate
Mohamed M. Ibrahim,1,2 Hatem. A. Eissa, Hosny Y. El-Baradie, and
1
1
1
Gamal E. Kamel
1
Chemistry Department, Faculty of Education, Kafr El-Sheikh University, Kafr El-Sheikh, Egypt
Chemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia
2
of hydrophilic interactions of the substrates with polar protein
The water soluble ligand tris(hydroxyl-2-benzimidazolyl)amine side chains and water molecules in the active site of the enzyme.
NTBOH was used for the preparation of a series of aqua transi- Therefore, a further development of the ligand NTB to provide
tion metal complexes and their hydroxo derivatives of the types
a hydrophobic cavity within a hydrophilic system bearing H-
bonding groups on the benzimidazole rings and being soluble
in aqueous solution has been recently done. First attempts were
2+
+
2+
2+
[
NTBOH-M(OH
2
)] 1-3 and [NTBOH-M(OH)] 4-6 (M = Zn
,
2+
2+
Cu , and Co ]. Solid state studies of these model complexes
were investigated using FT-IR spectroscopy, elemental analysis,
and thermal analysis. Solution studies have also been carried out the synthesis of the water soluble ligand NTBsulf(Scheme 1) was
to investigate the coordination behavior of the ligand towards the recently made.[
10,11]
The insertion of the hydrophilic anionic
metal ions as well as the evidence for the protonation/deprotonation
−
sulfonate groups (SO ) on the neutral ligand NTB allowed the
3
of the coordinated water molecules. The effect of these model com-
plexes as potential catalysts, especially the nature of metal ions on
the hydrolysis of p-nitrophenyl acetate (NA), has also been inves-
tigated. The results indicate that copper(II) complex 3 is the most afforded the desired N4-coordination array around zinc, which
active hydrolytic catalyst among complexes 1–3, presumably a re- was more appropriate biomimetic zinc compound in aqueous
flection of the effective electron-withdrawing as well as the greatest
isolation of zinc complexes very stable towards hydrolysis and
soluble in water polar solvents like wet methanol. This ligand
solution. This model complex showed the largest rate constant
electrophilicity of cobalt(II) ion.
among them carbonic anhydrase model compounds reported so
far[ towards the hydration of CO2.
11]
Keywords carboxylate ester hydrolysis, thermal analysis, transition
Due to the anionic charge effect of NTBsulf, an-
metal complexes, water soluble ligand
other new water soluble ligand, namely tris(hydroxy-2-
benzimidazolyl)amine NTBOH, has been obtained by the intro-
duction of the phenolic OH group on each benzimidazole ring
INTRODUCTION
of NTB.[ This new ligand with the hydrophilic OH groups
11]
The use of tripod-like ligand, tris(2-benzimidazolyl-
afforded new water zinc model complexes, and structurally and
functionally mimicked the active site of carbonic anhydrase,
[1]
methyl)amine, NTB , (Scheme 1) with hydrophobic benzene
rings can provide protected ‘pockets’ for metal ions. It was
successfully utilized for the preparation and isolation of differ-
ent structural zinc(II) enzyme model complexes
also suitable for the synthesis of biomimetic coordination com-
which has (i) pseudotetrahedral structure around zinc, (ii) hy-
−
drohibic environment around zinc-bound H2O / OH , (iii) the
[
2−6]
and was
solubility in both water and organic solvents, and (iv) neutral
ligand to prevent the charge effect on the enzymatic reaction.
The phenolic groups in NTBOH could increase the solubility
and recognize the water molecules through hydrogen bond in
solution.
[
7,8]
pounds having superoxide dismutase
and carbonic anhy-
drase activities.[
1,9−11]
Since the catalytic activity of the “biological zinc” cannot
only be understood as a result of hydrophobic effects but also
The ultimate aim of this work was to show how the lig-
and NTBOH may assume, upon complexation with metal ion, a
basket-like structure, bind substrates in water, and eventually,
cleavage them under physiological conditions. Therefore our
target was (i) the synthesis of a series of water soluble aqua
Received 16 December 2009; accepted 2 August 2010.
Address correspondence to Mohamed M. Ibrahim, Chemistry De-
partment, Faculty of Education, Kafr El-Sheikh University, Kafr El-
Sheikh 33516, Egypt. E-mail: ibrahim652001@yahoo.com
2+
2+
2+
metal (Zn , Cu , and Co ) complexes and their hydroxide
869

870
M. M. IBRAHIM ET AL.
◦
jected to dynamic TGA scans at a heating rate of 10 C / min
◦
in the temperature range of ambient to 800 C under a dynamic
atmosphere of dry nitrogen gas (flow rate = 10 ml/min). The
kinetic parameters of the dehydration and decomposition steps
were determined from the TGA thermograms using the Coats-
Redfern[ equations in the following form:
12]
ꢀ
ꢁ
ꢀ
ꢁ
1
−n
1
− (1 − α)
AR
2RT
Ea
log
= log qEa 1 −
−
2
T (1 − n)
Ea
2.3 RT
Where α is the fraction of the sample decomposed at time t, T
is the derivative peak temperature (K), n is the order of reaction,
A is the frequency factor, R the molar gas constant, Ea is the
activation energy. A plot of log[-log(1-α)T ] versus 1/T gives
the slope for evaluation of the activation energy. The thermo-
SCH. 1. The ligand NTBOH and its metal complexes 1–3.
derivatives (Scheme 2) as structural and functional models of
the active site of hydrolase enzymes, (ii) analysis and character-
ization of the obtained compounds in the solid state using FT-IR
spectroscopy, elemental analysis, and thermal analysis, (iii) so-
lution studies of the metal-ligand interactions for determining
the stoichiometry and the pKa value of the coordinated water
2
dynamic parameters, ꢀH, ꢀS, and ꢀG were computed using
the relationships[ : ꢀH = E − RT, ꢀS = R[ln(Ah/kT) ], and
13]
−1
ꢀ
G = ꢀH − TꢀS where k is Boltzmann’s constant and h is
the Planck’s constant.
1
molecule using complexometric titration by using H NMR and
UV-visible spectroscopies, and (iv) application of the obtained
metal complexes as structural and functional catalyst towards
the hydrolytic reactions using UV-visible spectroscopy.
UV-visible Measurements
The stoichiometry of ligand-metal complexes were calcu-
lated by applying the “mole ratio” method. The stoichiomet-
ric ratios between ligand and metal ions for NTBOH with
Zn(ClO4)2·6H2O, Cu(NO3)2·3,5H2O and Co(NO3)2·6H2O with
EXPERIMENTAL
−4
Materials and General Methods
(1×10 ) mole/L in aqueous methanolic solution (50%, v/v).
The pka of the coordinated water molecules of the obtained
aqua metal complexes 1–3 has been determined by recording
the absorbance of the generated hydroxo complexes at different
pH values. The measurements have been carried out in aqueous
ethanol (50%, v/v) and the pH values were adjusted by using
dilute solutions of NaOH and HNO3.
The
NTBOH, and its aqua and hydroxo zinc complexes
NTBOHZn(OH2)]·(ClO4)2 · 6H2O 1 and [NTB(OH)](ClO4)·1.5
ligand
tris(hydroxyl-2-benzimidazolyl)amine,
[
H2O 4, respectively, have been previously synthesized and
described.[
6]
The IR absorption spectra (Figure 1) were
recorded using FT-IR Bruker Vector 22 Spectrometer, Germany
−1
1
in the range of 400–4000 Cm . The H NMR measurements
were recorded by using Mercury-300 BB ‘NMR300’ and Syntheses
Gemini-200 ‘NMR’. All UV-visible measurements were
recorded by UV-240 Shimadzue, Japan and quartez cuvette.
[
NTBOH Cu(OH2)](NO3)23H2OCH3OH 2
TBAOH (0.103 g, 0.20 mmol) in methanol (10 ml) was added
to a stoichiometric amount of Cu(NO3)2·3.5 H2O (0.05 g, 0.2
Thermal Analysis
mmol) in methanol (10 ml). The mixed solution was stirred
Thermogravimetric analyses (Figures 2 and 3) were per- for around 2 h. A dark brown precipitate was formed formed,
formed using Shimadzu Stand-Alone Thermal Analyzer Instru- filtered off, washed with diethyl ether, and dried in vacuo. Yield
◦
ments (TGA-50H) Japan. About 5 mg of pure sample was sub- 114 mg (78 %), m. p. 265 C. Anal. calcd. for C25H33O14N9Cu
∗
SCH. 2. The hydrolysis reaction of p-nitrophenylacetate by using the hydroxo metal complexes [NTBOH
−
M(OH)] 4–6.

CARBONIC ANHYDRASE PHOSPHATASE MODEL COMPLEXES
871
FIG. 2. TGA-DTG curves of (a) Zinc(II) complex 1, (b) Copper(II) complex
FIG. 1. IR spectra of the ligand NTBOH and its zinc(II), copper(II), and
cobalt(II)-bound hydroxo complexes 1–3.
◦
2
, and (c) Cobalt (II) complex 3 in the temperature ranges 25–800 C.
(
746.54): C, 41.17; H, 4.52; N, 16.87. Found: C, 40.33; H, 5.25; mmol) in methanol (10 ml). The mixed solution was stirred
−1
N, 16.94. FT-IR (KBr): ν(cm ): 1156 (m, C-N, R), 1353 (s, for around 3 h. A pinkish white precipitate was formed formed,
NO3-), 1484 (m), 1450 (m, C=C), 1545 (m), 1599 (m), 1634 filtered off, washed with diethyl ether, and dried in vacuo. Yield
m , C=N), and 3217cm− (b, OH).
1
◦
(
123 mg (89%), m. p. > 300 C (decomp.). Anal. calcd. for
C25H29O12N9Co (705.933): C, 42.49; H, 4.10; N, 17.84. Found:
C, 40.21; H, 3.59; N, 17.34. FT-IR (KBr): ν(cm ): 1157 (m,
C-N, R), 1354 (m), 1381 (s, NO ), 1481 (m), 1449 (m, C=C),
−1
[
NTBOHCo(OH2)](NO3)2·H2O·CH3OH 3
−
TBAOH (103 mg, 0.20 mmol) in methanol (10 ml) was added
3
−1
1
545 (w), 1603 (m), 1634 (s, C=N), and 3379 cm (b, OH).
to a stoichiometric amount of Co(NO3)2·6H2O (60 mg, 0.2

872
M. M. IBRAHIM ET AL.
[
NTBOHCo(OH)](NO3)·6H2O 6
The cobalt(II) hydroxo complex 6 was precipitated by stirring
ethanolic solutions of the aqua cobalt complex 3 (0.0255 mmol)
and sodium methoxide (0.0255 mmol) for 24 hours. Filtered,
washed with diethyl ether, dried in vacuo and weighted as violet
◦
powder. Yield 12 mg (63%). m. p. > 300 C (decomp.). Anal.
calcd. for C24H34O13N8Co (700.93): Calc.: C, 41.08; H, 4.85;
N, 15.97. Found: C, 41.21; H, 4.33; N, 15.74. IR (KBr): ν =
−
1
159.4 (m, C N, R), 1356.4 (s, NO ), 1446.9 (m), 1475.6 (m,
3
C=C), 1543.6 (w), 1631.2 (m, C=N), 2924.8 (m, C H), 3219.1
(b, OH), 3379.1 (b, NH).
[NTBOHZn(OPO(OC6H4-p-NO2)2)](ClO4) 3H2O 7
Complex was prepared by the reaction of
7
[
NTBOHZn(OH2)]·(ClO4)2·6H2O 1 (169 mg, 0.2 mmol)
in methanol (10 ml) with a stoichiometric amount of bis(p-
nitrophenyl)phosphate (82 mg , 0.039 mmol) in methanol (10
ml) at pH = 9. After several days, colorless crystals were
◦
formed, filtered, and dried in vaccuo. Yield 61%, m.p. 136 C.
Anal. calcd. for C36H35O17N9ZnPCl (996.53): Calc.: C, 43.39;
1
H, 3.54; N, 12.65 %. Found: C, 43.11; H, 3.60; N, 12.58 %. H
NMR, δH (DMSO-d6): 8.09 (d, 4H, C6H4-NO2), 7.47 (d, 3H
C6H3), 7.42 (s, 3H C6H3), 7.34 (d, 4H, C6H4 NO2), 6.93 (d,
3
H C6H3), and 4.53 ppm (s, 6H, -CH2-). IR (KBr): ν = 855
−
(
(
1
m, C-N for C-NO2), 1089 (s, ClO ), 1219 (m, P-O-C), 1253
4
m, P=O), 1348 (s, NO2), 1453 (m), 1488 (s), 1517 (s, C=C),
593 (m), 1616 (m), 1637 (m, C=N), 2926 (m, C-H), 3413
(b, OH).
Kinetic Studies for the Hydrolysis Reactions of the
Carboxy Ester p-nitrophenyl Acetate (NA)
The hydrolysis rates of both p-nitrophenyl acetate (NA) in
aqueous methanol (40% v/v)) by using the obtained model com-
plexes 1–3 were measured by recording the increase in 402 nm
absorption of the released yellow p-nitrophenolate as a hy-
drolyzed product.[
4
11,14−17]
The mixed solvent (CH3OH / H2O,
0%) was used to prevent precipitation of the metal complex
as well as increasing the solubility of the substrates. The pH
FIG. 3. TGA-DTG curves of (a) Zinc(II) complex 4, (b) Copper(II) complex value of each sample was adjusted by using a concentrated
◦
5
, and (c) Cobalt (II) complex 6 in the temperature ranges 25–800 C.
solutions of NaOH (1M) or HNO (1M), and not by using
3
buffers. In the course of hydrolysis, the pH value of each sam-
ple was checked and was found that the difference compared
to the adjusted pH value at the beginning of hydrolysis was
not larger than 0.3. The measurements of the hydrolysis reac-
[
NTBOHCu(OH)](NO3)·4H2O 5
The copper hydroxo complex 5 was precipitated by stirring
◦
methanolic solutions of the aqua copper(II) complex 2 (0.0479
mmol) in ethanol (15 ml) and sodium methoxide (0.0479 mmol)
for 24 hours. Filtered, washed with ethanol and dried in vacuo.
tions were carried out at 25± 0.2 C. Release of only one p-
nitrophenolate per the substarte NA molecules was considered.
The catalytic reaction was initiated by rapid injection of 0.3 ml
◦
−
4
Yield 1.9 mg (59 %). m. p. > 300 C (decomp.). Anal. calcd. for
acetonitrile solution of both NA (3 × 10 mol/L) into 2.7 ml
−6
C24H31O11N8Cu (670.54): Calc.: C, 42.95; H, 4.62; N, 16.70.
Found: C, 41.21; H, 4.57; N, 16.12. IR (KBr): ν = 1161.2 (m,
C-N R), 1381.1(s, NO3), 1443.7 (m), 1479 (m, C=C), 1544.3
of the metal complex (72 × 10 mol / L) in quartez covette
3
ml. The UV absorption increase was recorded immediately,
the concentration of p-nitrophenolate was measured at inter-
vals of times. Reaction rates are corrected by blank experiments
(m), 1631.3 (m, C=N), 2926.3 (w, C-H), 3380.2 (b, OH).

CARBONIC ANHYDRASE PHOSPHATASE MODEL COMPLEXES
873
that were made up similarly but without the presence of metal
complexes.
RESULTS AND DISCUSSION
Characterization of the Model Complexes 1–7
Complexation of transition metal cations is favored by the
presence of soft donor atoms such as nitrogen or sulfur, but
also ligands containing the harder oxygen atoms can bind these
cations. We examined the complexation of both Zn(II), Cu(II)
and Co(II) ions with the ligand NTBOH and their complex struc-
tures as well as the determination of the pka values for the
coordinated water molecules. The ligand NTBOH has three ben-
zimidazole nitrogen atoms and on tertiary amine nitrogen atom,
allowing them to act as tetradentate donors. The aqua metal(II)
complexes [NTBOH-M(OH2)]2 1–3 (M = Zn, Cu, and Co) were
obtained by the reaction of the ligand NTBOH with equimolar
amounts of Zn(ClO4)2, Cu(NO3)2, and (Co(NO3)2, respectively,
in absolute methanol. Analogous to zinc (II) complex 1 (Figure
+
[11]
1
), the IR spectra (Figure 1) of both copper(II) complex 2 and
cobalt(II) complex 3 showed very broad OH absorption bands
−
1
around 3217 and 3279 cm ,respectively. They also showed
ν(NO) absorption bands of the nitrate anion at 1353 and 1354
1
FIG. 4. H NMR spectra of the ligand NTBOH and its zinc(II)-bound aqua
◦
−1
and zinc(II)-bound hydroxo complexes in DMSO-d6 at 25 C.
cm , respectively. These complexes were also characterized
using elemental analysis, in which the observed values were
in good fits with the calculated ones. Attempts to propose the Solution Studies
structure of the isolated aqua copper(II) complex come from The binding ability of NTBOH with these metal ions was spec-
1
the investigation using thermal analysis studies in its solid state troscopically examined in aqueous methanol using H NMR and
as well as from the solution studies using UV-visible spec- UV-visible spectroscopies.
troscopy. Attempts to prepare the metal-bound hydroxo com-
In order to get a better insight into the complex species
+
2+
1
plexes of the formula [NTBOHMOH] 4–6 in methanol solu- formed in NTBOH / Zn system, H NMR study was car-
tion using Me4NOH·5H2O condition similar to those utilized ried out for the free ligand, its aqua zinc(II) complex, and
1
for the preparation of the monomeric hydroxide complexes of its hydroxo zic(II) complex 4. The H NMR spectrum of
[18]
similar ligand system failed to give clean crystalline product. zinc(II) complex 1 (Figure 4) showed that the proton sig-
However, using sodium methoxide, MeONa as a base, treatment nals of the methylene and benzimidazolyl protons were shifted
of the ligand NTBOH with equimolar amounts of the perchlo- down filed compared to those found for the free ligand, in-
rate or nitrate salts of metal ions (Zn2 , Cu , and Co ) and dicating the formation of zinc(II) complex 1. The observed
+
2+
2+
1
MeONa under an aerobic conditions, then followed by workup
H NMR spectrum for the zinc(II) complex 4 showed that
and crystallization from MeOH/CH2Cl2 yielded the monomeric the chemical shifts of the methylene and benzimidazolyl pro-
hydroxide complexes as crystalline solid with good yields. The tons were shifted upfield. This shift is not found in the lig-
chemical characterization of these hydroxide derivatives was and itself, indicating the formation of zinc(II)-bound hydroxo
1
determined by H NMR and FT-IR spectroscopices as well as complex 4.
elemental and thermal analyses.
The recognition and
nitrophenyl)phosphate (BNPP ) in complex
The spectra of NTBOH with Co(II) ions in aqueous methanol
bis(p- (33%, v/v) show d-d absorptions at 539 nm, which suggests that
coordination
of
−
7
was cobalt ions have the characteristics of a penta- coordinate com-
possible by the reaction of the aqua complex plex with distortion.[ As can be seen from Figure 5, an increase
21]
[
NTBOHZn(OH2)]·(ClO4)2·6H2O 1[11] with an equiv- in absorbance is observed upon addition of increasing quantities
alent amount of the deprotonated form of bis(p- of cobalt(II) ions to the ligand solution, whereas the absorption
2+
nitrophenyl)phosphate in water. It was isolated analytically intensity changes as a function of the [Co ]/[NTBOH] mole
pure with a good yield. Its chemical characterization showed ratio. These changes could be attributed to the complexation be-
−
−1
absorption of ν(ClO ) at 1089 cm and absorptions of 1253 tween the ligand and copper(II) ions. From the inflection point
4
−1
−1
2+
cm (s, P=O) and 1219 cm (s, P O C), and are typical in the absorbance/mole ratio plots at [Co ]/[ NTBOH] values
characterization bands of coordinated phosphate to metal between 0.8 and 1.0, it can be inferred that 1:1 complex species
complexes.[
15−20]
are formed.

874
M. M. IBRAHIM ET AL.
−
4
−4
FIG. 5. UV study for NTBOH (1 × 10 M) with different ratio of Cu(NO3)2
FIG. 7. Time course for the hydrolysis of p-nirophenyl aceate (3 × 10
−
4
−6
3
2
.5H2O (1 × 10 M) in aqueous MeOH (33%, v/v) at I = 0.1 M NaNO3 and
M) catalyzed by copper(II) complex 2 (72 × 10 M) at pH = 8.5 in aqueous
◦
◦
5 C.
methanol (33%, v/v) atI = 0.1 M NaNO and 25 C.
3
+
+ [19]
The absorbance indicates a sigmoidal
The generation of the hydroxo complexes [NTBOH-M(OH)]
[NTBOH-Cu(OH)] .
4
-6 (M = Zn, Cu, and Co) were also investigated in solu- change in the pH range 8 to 9, which corresponds to a change
2+ +
tion by determining the pka values of the coordinated water from [NTBOH-Cu(H2O)] to [NTBOH-Cu(OH)] . From the fit-
molecule of their aqua complexes 2 and 3. The absorbance of ting curve, the obtained pka value for the coordinated water
these complexes has been measured at different pH values in molecule of complex 3 is ca 8.86 ± 0.1.
aqueous methanol (33%, v/v). The pH values were adjusted
by using dilute solutions of HNO3 and NaOH. The obtained
UV-Vis spectra in solution for cobalt(II) complex are shown
Thermogravimetric Analysis
The TGA curve of complex 1 (Figure 2a) shows four de-
composition steps within the temperature range 40–900 C. The
in Figure 6 as an representative example. The copper(II) com-
◦
plex [NTBOH-Cu(H2O)]2 3 is characterized by a visible band
+
first step of the decomposition is within the temperature range
ꢀ
at 639 nm, which can be assigned to the A →E” transition
◦
4
0–180 C, corresponding to the loss of six water molecules of
[22−24]
for a five-coordinate D3h Cu(II) complex.
A new band
crystalization with a mass loss of 12.689% (calcd. 12.773%).
The second step within the temperature range 210–325 C is
equivalent to the loss of coordinated water molecule with a
mass loss of 2.144% (calcd. 2.28%). The third and the fourth
gradually appeared at 349 nm on increasing the pH. This indi-
cates that the geometry distorts from trigonal bipyramidal to a
square pyramidal/octahedral structure due to the formation of
◦
−
4
FIG. 6. Absorbance-pH graph for the determination pka value for (a) [TBAOH
FIG. 8. Hydrolysis rate constant, kobs of p-nitrophenyl acetate (3 × 10
2+
−4
2+
−6
Cu(OH2)] (1 × 10 M), and (b) [TBAOH Co(OH2)] in aqueous MeOH
M) catalyzed by using complexes 1-3 (72 × 10 M) at different pH values in
◦
◦
(
33%, v/v) at I = 0.1 M NaNO3 and 25 C.
aqueous methanol (33%, v/v) at I = 0.1 M NaNO3 and 25 C.

CARBONIC ANHYDRASE PHOSPHATASE MODEL COMPLEXES
875
SCH. 3. Proposed mechanism for the hydrolysis of p-nitrophenylacetate by using the hydroxo-bound zinc(II), copper(II), and cobalt(II) complexes 1–3.
◦
steps of the decomposition within the temperature range 330– 900 C corresponded to (CuO + 2C) with a mass loss of 14.41%
◦
7
34 C, corresponding to the loss of the perchlorate anions and (calcd 14.49%). The TGA curve of complex 3 (Figure 2c) shows
the organic part of the complex with a mass loss of 77.46% four decomposition steps within the temperature range 30–90 C
◦
(
7
calcd 77.36%). The final residue within the temperature range (Scheme 3). The first step of the decomposition within the tem-
34–900 C corresponded to (Zn) metal with a mass loss of perature range 30–100 C corresponded to the loss of one water
◦
◦
7
.703% (calcd 7.732%). The TGA curve of complex 2 (Fig- molecule of lattice with a mass loss of 2.711% (calcd 2.67%).
◦
ure 2b) shows four decomposition steps within the temperature The second step within the temperature range 260–278 C cor-
range 25–900 C. The first step of the decomposition within the responded to the loss of one coordinated water molecule with
temperature range 25–100 C corresponded to the loss of three a mass loss of 2.513% (calcd 2.67%). The third and the fourth
◦
◦
water molecules of lattice with a mass loss of 7.368% (calcd steps of the decomposition within the temperature range 282–
◦
7
2
.55%). The second step within the temperature range 120– 515 C corresponded to the loss of nitrate anions and the organic
◦
20 C corresponded to the loss of coordinated water molecule part of complex with a mass loss of 76.57% ( calcd 76.42%).
with a mass loss of 2.32% (calcd 2.5%). The third and the fourth The final residue within the temperature range 515–900 C cor-
◦
steps of the decomposition within the temperature range 222– responded to (CoO + 4C) with a mass loss of 18.21% (calcd
◦
4
97 C corresponded to the loss of the nitrate anions and the 18.19%).
organic part of the complex with a mass loss of 75.89% (calcd
The TGA curve of complex 4 (Figure 3a) shows five de-
◦
75.433%). The final residue within the temperature range 497– composition steps within the temperature range 40–900 C. The

876
M. M. IBRAHIM ET AL.
TABLE 1
Kinetic and thermodynamic parameters of the aqua metal(II) complexes 1–3
Complex
Step
st
T(k)
A
r
ꢀE#
ꢀH#
ꢀS#
ꢀG#
1
2
1
338
563
638
798
317
398
513
612
323
533
583
658
1.6×10
0.977
0.912
0.979
0.994
0.825
0.992
0.982
0.985
0.958
0.965
0.996
0.998
91.26
20.89
77.81
161.93
101.56
44.84
219.76
298.94
123.65
23.74
88.34
17.97
74.89
159.01
98.64
21.92
216.85
296.02
120.73
20.82
125.27
336.41
−0.021
−0.146
−0.082
−0.120
0.036
95.43
100.16
127.20
254.77
87.01
2
nd
3rd
th
st
nd
3rd
th
st
2nd
rd
th
719416
7
1
183×10
6
4
22×10
1.49×10
2018.2
1
0
1
2
−0.192
78.33
8
2
3
1.87×10
0.130
150.07
329.06
95.27
97.03
169.57
202.82
1
0
4
5.06×10
2.42×10
−0.054
0.078
−0.143
−0.076
0.203
1
0
1
4
95×10
7
3
119×10
128.19
339.33
9
4
1.5×10
first step of the decomposition within the temperature range 40– with a mass loss of 10.909 % (calcd. 10.737 %). The second
◦ ◦
0 C corresponds to the loss of one and half water molecules step within the temperature range 205–225 C corresponds to the
7
of lattice with a mass loss of 3.91% (calcd. 4.11%). The second loss of hydroxo group with a mass loss of 2.8% (calcd. 2.53%).
◦
step within the temperature range 180–280 C corresponds to The third and fourth steps of the decomposition within the tem-
◦
the loss of hydroxo group with a mass loss of 2.989 % (calcd. perature range 250–550 C corresponds to the loss of organic
2
.59 %). The third, fourth and fifth decomposition steps of the part of complex with a mass loss of 73.76% (calcd 74.86%).
◦
◦
decomposition within the temperature range 300–820 C cor- The final residue within the temperature range 550–900 C cor-
responded to the loss of organic part of complex with a mass responded to CuO with a mass loss of 12.528% (calcd. 11.86
loss of 75.22% (calcd. 75.39%). The final residue within the %). The TGA curve of 6 (Figure 3c) shows five decomposition
◦
◦
temperature range 820–900 C corresponds to a mixture of zinc steps within the temperature range 50–900 C. The first step of
◦
oxide and carbon, ZnO + 3C with a mass loss of 18.15% (calcd. the decomposition within the temperature range 50–89 C cor-
17.89%). The TGA curve of 5 (Figure 3b) shows four decom- responds to the loss of four water molecule of lattice with a
◦
position steps within the temperature range 31–900 C. The first mass loss of 15.928% (calcd. 15.41%). The second step within
step of the decomposition within the temperature range 31– the temperature range 200–242 C corresponds to the loss of
◦
◦
200 C corresponds to the loss of four water molecules of lattice hydroxo group with a mass loss of 2.473% (calcd. 2.43%).
TABLE 2
Kinetic and thermodynamic parameters of the metal-bound hydroxo complexes 4–6
Complex
Step
T(k)
A
r
ꢀE#
ꢀH#
ꢀS#
ꢀG#
1
st
318
503
621
742
889
353
513
568
648
347
477
586
677
828
13095.2
26635.4
98715.7
0.982
0.922
0.991
0.957
0.978
0.899
0.968
0.995
0.977
0.977
0.925
0.996
0.958
0.991
8.28
13.5
14.14
5.36
−0.174
−0.172
−0.163
−0.004
−0.062
0.014
60.69
97.48
112.44
225.15
415.02
99.02
2nd
10.58
11.22
222.19
359.11
103.99
7.17
295.96
103.11
98.6
4
3rd
9
4
5
th
th
2.6×10
225.11
362.03
106.91
10.09
298.88
106.02
101.52
5.25
1
0
2.6×10
1
1
1
st
nd
3rd
th
st
nd
3rd
1.09×10
2
15063.04
−0.177
97.97
1
0
5
7.07×10
0.235
161.94
201.60
98.25
4
4
40×10
−0.152
0.001
1
1
1
2.2×10
2
1900.7
2.33
317.51
359
−0.194
94.86
1
0
6,
5.3×10
2.2×10
4.388
320.43
361.91
38.23
0.029
200.51
205.53
241.48
9
4
5
th
th
0.226
−0.249
35.31

CARBONIC ANHYDRASE PHOSPHATASE MODEL COMPLEXES
877
TABLE 3
−
1
−4
The observed pseudo-first-order rate constants, kobs/s for the hydrolysis of NA (3 × 10 M) catalyzed by using complexes
6
◦
1
–3 (72 × 10− M) at different pH values in aqueous MeOH (33 %, v/v) at I = 0.1 M NaNO3 and 25 C
4
Complex 2 Kobs×10−4
Complex 3 Kobs×10−4
Blank Kobs×10−4
PH
7
Complex 1 Kobs×10−
2.184
2.576
3.878
6.148
19.336
35.127
2.570
2.833
4.167
9.08
55.122
91.289
4.119
9.526
13.345
31.378
112
—
7.5
8
8.5
9
9.5
1.561
3.012
5.939
8.804
15
177.8
−
6
The third, fourth and fifth steps of the decomposition within complex 2 (72 × 10 M) as a function of time time at pH = 8.5
◦
◦
the temperature range 265–600 C correspond to the loss of or- in aqueous methanol (33%, v/v) atI = 0.1 M NaNO and 25 C.
3
ganic part of complex with a mass loss of 73.595% (calcd. The pseudo-first-order rate constants,k , were obtained from
obs
7
9
8
3.673%). The final residue within the temperature range 600– the plot of ln(A /A − A ) versus time. The pH-dependence
∞
∞
t
◦
00 C corresponds to Co with a mass loss of 8.034% (calcd. of the observed pseudo-first-order rate constants, k studied
obs
.4%).
The kinetic parameters (ꢀE ) and thermodynamic param- The cobalt(II) complex 3 was found to be the most effective one
over the pH range 7.0–9.5 are shown in Table 3 and Figure 8.
#
#
#
#
eters ꢀS , ꢀH , and ꢀG also have been calculated for ev- towards the hydrolysis of NA. This phenomenon may be due to
ery step for the decomposition of the aqua complexes 1–3 and the higher electrophilic character comparing to the other metal
their hydroxide derivatives 4–6. The results are shown in Ta- ions.
bles 1 and 2. The low value of E for each water molecule
The pH-rate constant kobs profiles for the hydrolysis reac-
in the first dehydration process indicates that the first six wa- tions displayed sigmoidal curves with inflection points around
ter molecules are not coordinated to the zinc metal. Whereas, the pK values of the coordinated water molecules in com-
a
the relatively high values for the second decomposition step, plexes 1–3. Such pH-rate profile was observed in a number
[
26]
as well as the third and fourth decomposition steps, indicate of phosphate ester hydrolysis promoted by using copper(II),
that the coordinated water molecule and the organic part of the cobalt(II),[
27−29]
11,15−18,30]
and are in-
and zinc(II) complexes,[
ligand are strongly coordinated to the zinc(II) ion. The nega- dicative of the involvement of metal-hydroxo species in the
#
tive values of ꢀS in the second and the other following steps catalytic process. The pk values obtained from these inflec-
a
indicate that the activated complex have more ordered struc- tion points are in good agreement with the pk values deter-
a
ture than the reactants and the reactants are slower than the mined from the previously mentioned solution studies of these
metal.[
25]
complexes by using UV-visible spectroscopy. Therefore, the
metal(II)-bound hydroxo species of these complexes are be-
lieved to be active catalyst for hydrolyzing the carboxy ester
NA.
Hydrolysis Reactions of p-nitrophenyl Acetate (NA) by
Using the Model Complexes 1–3
On the basis of the above mentioned kinetic studies of the
The hydrolysis reactions of p-nitrophenyl acetate (NA) have carboxylate ester NA in aqueous methanol and the isolation
been examined using the model complexes 1–3. The hydrolysis of the aqua model complexes 1–3 and their hydroxide deriva-
reactions were investigated in aqueous methanol (40%, v/v). tives 4–6, we proposed a mechanism for the hydrolysis of NA.
−
Upon hydrolysis, NA decompose into p-nitrophenolate, (NP ) At the first step: the coordinated water molecule in 1–3 can
and acetate anions, respectively (Scheme 3). The catalytic reac- deprotonat at pka = 8.5 −9.0 to produce the metal(II)-bound
tions were followed by an increase in absorption maximum at hydroxo species.[
11,18]
Second: the metal center delivers the
4
02 nm for generation of p-nitrophenolate. The pH of each sam- coordinated hydroxide that can nucleophically attack the NA
ple was adjusted by using 1 M NaOH / HNO3, and not by using molecule, while the metal ion simultaneously withdraw elec-
buffers in order to avoid any catalytic or inhibition effect. In the tron density away from the carbon atom by interacting with the
course of hydrolysis, the pH of each sample was checked and carbonyl oxygen, forming pentaccordinated intermediate. Fi-
was found that the difference compared to the adjusted pH value nally, the p-nitrophenolate is released and the metal(II)-bound
at the beginning of the hydrolysis was not larger than 0.3. Figure acetate intermediate is replaced by water to form again the start-
7
shows a absorption intensities of the releasedp-nitrophenolate ing catalyst aqua metal(II) complexes, which are ready to start
for the hydrolysis of p-nitrophenyl acetate by using copper(II) another catalytic cycle.

8
78
M. M. IBRAHIM ET AL.
8. Xiang, D.F., Duan, C.Y., Tan, X.S., Hang, Q.W., and Tang, W.X. J. Chem.
CONCLUSION
A series of aqua metal(II) complexes 1–3 and their hydrox-
ide derivatives 4–6, containing different transition metal ions
Soc., Dalton Trans., 1998, 1201–1204.
9. Ichikawa, K., Nakata, K., and Ibrahim, M.M. Chem. Lett., 2000, 796–797.
10. Ibrahim, M.M., Nakata, K., and Ichikawa, K. Yoeki Kagaku Shinpojiumu
2
+ 2+ 2+
(
Co , Cu , and Zn ) have been synthesized and fully char-
Koen Yoshishu, 1999, 149–150.
acterized as structural models of the active sites of hydrolytic en- 11. Echizen, T., Ibrahim, M.M., Nakata, K., Izumi, M., Ichikawa, K., and Shiro,
zymes. Solid state studies of these aqua/hydroxo complexes 1–6
were investigated using FT-IR spectroscopy, elemental analysis,
and thermogravimetric analysis. Solution studies of the metal-
ligand interactions for determining the stoichiometry and the
M. J. Inorg. Biochem., 2004, 98, 1347–1360.
1
1
2. Coats, A.W., and Redfern, J.P. Nature 1964, 201, 68–69.
3. Mahfouz, R.M., Monshi, M.A., Alshehri, S.A., El-Salam, N.A., and Zaid,
A.M.A. Synth. React. Inorg. Met. Org. Chem., 2001, 31, 1873–1886.
4. Ibrahim, M.M., Ichikawa, K., and Shiro, M. Inorg. Chem. Commun., 2003,
6, 1030–1034.
1
1
pKa value of the coordinated water molecule using H NMR and
UV-visible spectroscopies. The reactivity of these aqua model 15. Ibrahim, M.M., Shimomura, N., Ichikawa, K., and Shiro, M., Inorg Chim
Acta., 2001, 313, 125–136.
complexes as potential bioinorganic catalysts, especially the
nature of metal ions on the hydrolysis of both p-nitrophenyl
acetates (NA), has been investigated. The results showed that
1
6. Ibrahim, M.M., Ichikawa, K., and Shiro, M. Inorg. Chim. Acta., 2003, 353,
87–196.
7. Ibrahim, M.M. Inorg. Chem. Commun., 2006, 9, 1215–1218.
1
1
cobalt(II) complex 3 is the most effective hydrolytic catalyst 18. Ibrahim, M.M., and Mersal, G.A.M. J. Inorg. Organomet. Polym., 2009,
among complexes 1–3. This phenomenon may be due to the
fact that cobalt(II) ion is likely to be the most effective electron-
withdrawing ion and has the greatest electrophilicity. Therefore,
it can activate the nucleophile OH most effectively. Zinc(II) is
the opposite and has the lowest activity.
19, 549–557.
1
2
9. Hathway, B.J. J. Chem. Soc., Dalton Trans., 1972, 1196–1202.
0. Menzer, S.M., Phillips, J.R., Slawin, A.M.Z., Williams, D.J., and Woollins,
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UK, 1998.
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2