Thermal stability of Pd(1,4-bis(2-hydroxyethyl)piperazine)Cl2 and its role in the catalysis of base hydrolysis of α-amino acid esters

Article abstract of DOI:10.1080/00958972.2015.1061659

Pd(BHEP)Cl₂ was synthesized and characterized (BHEP = 1,4-bis(2-hydroxyethyl)piperazine). The complex decomposes in two steps, leaving a residue of palladium metal. Amino acid ester (L) reacts with [Pd(BHEP)(H₂O)₂]²⁺ (BHEP = 1,4-bis(2-hydroxyethyl)piperazine), giving mixed-ligand complexes, [Pd(BHEP)L]²⁺. The kinetics of hydrolysis of [Pd(BHEP)L]²⁺ have been studied by pH-stat technique, and rate constants were obtained. Rate acceleration observed for glycine methyl ester is high. The effect with methionine methyl ester is much less marked, as the mixed-ligand complexes with these ligands do not involve alkoxycarbonyl donors. Possible mechanisms for these reactions are considered.

Full text of DOI:10.1080/00958972.2015.1061659

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Thermal stability of Pd(1,4-bis(2-
hydroxyethyl)piperazine)Cl and its role in the
2
catalysis of base hydrolysis of α-amino acid esters
a
a
b
b
Mohamed R. Shehata , Mohamed M. Shoukry , Shokry A. Shokry & Mahmoud A. Mabrouk
a
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
b
Science and Technology Center of Excellence (STCE) Ministry of Military Production,
Cairo, Egypt
Accepted author version posted online: 15 Jun 2015.
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stability of Pd(1,4-bis(2-hydroxyethyl)piperazine)Cl and its role in the catalysis of base hydrolysis of α-amino acid esters,
2
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2015.1061659
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Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2015.1061659
Thermal stability of Pd(1,4-bis(2-hydroxyethyl)piperazine)Cl and its role in
2
the catalysis of base hydrolysis of α-amino acid esters
MOHAMED R. SHEHATA†, MOHAMED M. SHOUKRY*†, SHOKRY A. SHOKRY‡ and
MAHMOUD A. MABROUK‡
†
‡
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
Science and Technology Center of Excellence (STCE) Ministry of Military Production, Cairo, Egypt
th
Dedicated to Prof. Rudi van Eldik on the occasion of his 70 birthday.
Pd(BHEP)Cl
2
was synthesized and characterized (BHEP = 1,4-bis(2-hydroxyethyl)piperazine).
The complex decomposes in two steps, leaving a residue of palladium metal. Amino acid ester
2
+
(
L) reacts with [Pd(BHEP)(H O) ] (BHEP = 1,4-bis(2-hydroxyethyl)piperazine), giving mixed
2 2
2
+
2+
ligand complexes, [Pd(BHEP)L] . The kinetics of hydrolysis of [Pd(BHEP)L] have been
studied by pH-stat technique and rate constants were obtained. Rate acceleration observed for
glycine methyl ester is high. The effect with methionine methyl ester is much less marked, as the
mixed-ligand complexes with these ligands do not involve alkoxycarbonyl donors. Possible
mechanisms for these reactions are considered.
Keywords: 1,4-Bis(2-hydroxyethyl)piperazine; Thermal stability; Amino acid ester hydrolysis;
Pd(II); pH-stat Technique
1
. Introduction
Piperazines or cyclizines are generally considered as ethylenediamine derivatives or cyclic
ethylenediamines (cyclizines); Piperazines are a broad class of compounds with many important
pharmacological properties. This dinitrogen moiety has been a component of a plethora of drugs.
Piperazines have chemical similarity with piperidine, a constituent of piperine in the black piper
plant (Piper nigrum). Piperazine was introduced into medicine as a solvent for uric acid [1].
Metal ions in metalloenzymes such as carboxypeptidase A [2], carbonic anhydrase [3] and
*Corresponding author. Email: shoukrymm@hotmail.com
1

alkaline phosphatase [4] play key roles in many biochemical processes [5]. In these metal-
containing metalloenzymes, metal ions at the active sites are considered to serve as a primary
catalytic centre for bringing substrate and nucleophile together through formation of a
coordination complex, to activate the substrate carbonyl group facilitating attack of the
nucleophile as hydroxide. In order to probe the mechanism by which the metalloenzyme may
operate and consequently provide a theoretical base for designing effective artificial
metalloenzymes, previous reports [6-8] have developed biomimetic models for metalloenzyme,
which catalyze the hydrolysis of carboxylic acid esters in biomimetic models.
Work in our laboratory [9-14] has focused on catalysis of the hydrolysis of various amino
2
+
acid esters by metal complexes. [Pd(en)L] undergoes hydrolysis by water and hydroxide ion
15]. It is therefore of considerable interest to extend this work to study the thermal stability of
[
Pd(II) complexes with 1,4-bis(2-hydroxyethyl)piperazine and its role in the catalysis of amino
acid esters. The results of this investigation are expected to support the biological activity of the
piperazine derivative.
2
2
. Experimental
.1. Materials and reagent
2 4
All reagents were of Analar grade. K PdCl and 1,4-bis(2-hydroxyethyl)piperazine are provided
by Aldrich. The glycine- and methionine methyl esters were purchased from Fluka. Carbonate-
free NaOH was prepared and standardized against potassium hydrogen phthalate solution. All
solutions were prepared in deionized H O.
2
2
.2. Preparation of the complex
Pd(BHEP)Cl was prepared by dissolving K PdCl (0.92 g, 2.82 mmol) in 10 ml water with
2
2
4
2
-
4
stirring. The clear solution of [PdCl ] was filtered and 1,4-bis(2-hydroxyethyl)piperazine
(
0.491 g, 2.82 mmol) dissolved in 10 ml H O was added dropwise to the stirred solution. The pH
2
was adjusted to 2-3 by addition of HCl and/or NaOH. A yellowish-brown precipitate of
Pd(BHEP)Cl was formed and stirred for a further 30 minute at 50 °C. After filtering off the
precipitate, it was thoroughly washed with H O, ethanol and diethyl ether. An orange crystalline
precipitate was obtained after recrystallization from 2:1 methanol/water mixture. (Found: C,
7.3; H, 5.3; N, 7.8 %. Calc. for C PdCl (F.Wt = 351.55): C, 27.33; H, 5.17; N,
2
2
2
8
H
18
N
2
O
2
2
2

-
1
-1
7
4
.97 %). IR: ν
(st) at 3429 cm , ν (st) at 3018, 2983, 2949 and 2891 cm , ν
at
O-H
C-H
Pd–N
-1
-1
72 cm , ν
at 333 and 324 cm .
Pd–Cl
2
Aqueous solutions of the diaqua form of Pd(BHEP)Cl were prepared in situ by addition
of slightly less than two mole equivalents of AgNO to a solution of a known amount of the
3
dichloro complex and stirred overnight. The white precipitate of AgCl that formed was filtered
off using a 0.1 µm pore membrane filter. Great care was taken to ensure that the resulting
+
solution was free of Ag ion and that the dichloro complex had been converted completely into
the diaqua species.
2
.3. Instrumentation and measurements
IR spectra were recorded on a 8001-PC FT-IR Shimadzu spectrophotometer using KBr pellets.
Thermogravimetric measurements (TGA, DTA, 20–1000 °C) were recorded on a DTG-50
Shimadzu thermogravimetric analyzer at a heating rate of 10 °C/min and nitrogen flow rate of
2
0 ml/min.
The kinetics of hydrolysis was monitored using a Metrohm 751 Titrino operated with the
SET mode (titration to a preset end point). The titroprocessor and electrode were calibrated with
standard buffer solutions according to NIST [16]. Hydrolysis kinetics of glycine- and methionine
2
+
methyl esters in the presence of [Pd(BHEP)(H
2
O)
2
] are investigated by pH-stat technique
3
2+
[
(
17, 18]. After equilibrating a solution mixture (40 cm ) containing [Pd(BHEP)(H O) ]
2
2
-3
5.0×10 M) (the concentration of Pd complex was taken twice that of ester to ensure that the
-3
ester is completely coordinated), ester (2.5×10 M) and NaNO (0.1 M) at the required
3
temperature under nitrogen flow and the pH was brought to the desired value by addition of
0
.05 M NaOH solution. The hydrolysis was then followed by addition of 0.05 M NaOH solution
to maintain the given pH constant. The data fitting was performed with the OLIS KINFIT set of
programs [19] as described previously [20]. The precision of the kinetic data was estimated from
the plot as obtained from the OLIS program output. The accepted residual values are less than
-2
1
0 . Values of the hydroxide ion concentration were estimated from the pH using pK = 13.997
w
and an activity coefficient of 0.772 was determined from the Davies equation [21]. At the
variable temperature studies, the following values of pK and γ were employed [22] at 15 °C
w
(pK
w
= 14.35, γ = 0.776), at 20 °C (pK
w
= 14.16, γ = 0.774), at 25 °C (pK = 14.00, γ = 0.772),
w
at 30°C (pK = 13.83, γ = 0.770) and at 35 °C (pK = 13.68, γ = 0.768).
w
w
3

3
. Results and discussion
The analytical data indicated that the complex is of 1:1 stoichiometry and of formula
Pd(BHEB)Cl , which is given in scheme 1. IR spectra of BHEP and Pd(BHEP)Cl are compared
2
2
-1
(
figure 1). The stretching vibration of OH group of BHEP shows a broad band at 3151 cm . This
may be explained on the premise that the OH groups are involved in hydrogen bonding. This
-1
band is sharp in the complex and occurs at 3429 cm , which indicates that the OH groups are not
involved in coordination. The stretching vibration band corresponding to νPd–N was assigned at
-
1
4
72 cm . The stretching vibration band corresponding to ν
was found to be a doublet at 333
Pd–Cl
-1
and 324 cm , which confirm cis configuration of the two Pd-Cl bonds [23, 24].
Cl
Cl
Pd
N
N
HO
OH
Scheme 1. Structure of [Pd(BHEP)Cl2].
3
.1. Thermal measurements
Thermogravimetric studies (TG) for the complex were carried out from room temperature up to
000 °C with a heating rate of 10 degree/min. TG data are displayed in figure 2. The determined
1
temperature ranges and percent mass losses of the solid complex on heating are given table 1.
Analysis of the data revealed the following findings:
The TG curve of Pd(BHEP)Cl exhibits a first estimated mass loss of 49.1% (calcd.
2
4
8 18 2 2
9.56%) at 150–360 °C, which may be attributed to the liberation of C H N O as gases. The
second estimated mass loss of 20.3% (calcd. 20.17 %), at 360–560 °C, may be attributed to the
liberation of 2Cl atoms as gases. The residue corresponds to palladium metal 30.6% (calcd.
3
0.26%).
3
.3. The catalysis of base hydrolysis of α-amino acid esters
4

2
+
α-Amino acid esters react with [Pd(BHEP)(H O) ] according to equilibrium (1). The
2
2
II
equilibrium constant is expected to be >> 1. This is due to the high affinity of Pd to N-ligands
2
+
[
10]. The resulting mixed-ligand complexes [Pd(BHEP)L] [L = NH
2
CH(R)CO
2
R`] undergo
hydrolysis by water and hydroxide ion according to equations (2) and (3).
2
+
[
[
[
Pd(BHEP)(H O) ] + L
2 2
[
Pd(BHEP)L]2+ + 2H O
(1)
(2)
2
H O
2
[Pd(BHEP)L`] + R`OH + H+
+
2
+
Pd(BHEP)L] + H O
2
kOH
+
Pd(BHEP)L]2+ + OH-
[Pd(BHEP)L`] + R`OH
(3)
-
where L = NH CH(R)CO R` and L` = NH CH(R)CO .
2
2
2
2
The kinetic data, viz. the volume of base added to keep the pH constant versus time, could be
fitted by a single exponential function as shown in figure 3. Various other kinetic models were
tested without satisfactory fits of the data. A plot of kobs versus hydroxide ion concentration is
linear as shown in figures 4 and 5 (for data see table 2) and follows the rate expression given in
equation (4):
-
kobs = k + k [OH ] (4)
o
OH
The k term is suggested to arise from spontaneous hydrolysis with water. The linear dependence
o
−
−
of the rate on the OH concentration is consistent with direct attack of OH on the ester group
figures 4 and 5).
For the coordinated glycine methyl ester, the catalytic ratio is 1.45×10 . A catalytic ratio
(
5
of this magnitude is consistent with the formation of the mixed ligand complex of structure I,
II
where there is a direct interaction between Pd and the carbonyl group of the ester.
5

+
2
+2
OCH3
COOMe
H
C
O
N
N
NH2
S
N
Pd
CH2
Pd
CH-CH3
N
C
H
NH2
2
H C
3
I
II
The catalytic ratio for methionine methyl ester complexes is 13.2 (table 3). The relatively low
catalytic ratio suggests that the carbonyl group is not bound to Pd(II). The methionine methyl
ester complex is expected to have structure II, where the amino and the sulfur are donors. Earlier
studies [25-27] have shown that the formation of such complexes with a pendant ester group
leads only to relatively small catalytic effects.
Comparative values of kOH at 25 °C for the base hydrolysis of the glycine methyl ester
2
+
4
3
-1 -1
incorporated in [Pd(en)L] is 4.88×10 dm mol s [18], where en = ethylenediamine. The k
OH
2
+
5
2+
value for [Pd(BHEP)L] (1.85×10 ) is higher than that of [Pd(en)L] . The enhanced hydrolysis
is probably due to stabilization of the mixed-ligand complex through involvement of the OH
groups of BHEP in hydrogen bonding with the coordinated amino acid ester.
The activation parameters for the hydrolysis of coordinated glycine methyl ester were
determined using Eyring plots of ln(k/T) versus 1/T, figure 6 (for data see table 4). The
#
−1
#
activation parameters for kOH were calculated to be ΔH = 28.0(±0.2) kJ mol and ΔS =
-1
-1
-
49.5((±0.3) JK mol . The activation parameters for k may be inaccurate since the rate
o
constants were determined by an extrapolation procedure. For base hydrolysis of the free glycine
#
−1
#
methyl ester, the activation parameters were reported [18, 28] to be ΔH = 39.7 kJ mol and ΔS
-1
-1
=
-117 JK mol . The enhanced rate for base hydrolysis of the ester coordinated to the complex
#
#
is therefore due to a decrease in ΔH and an increase in ΔS .
4
. Conclusion
Thermogravimetric studies (TG) for Pd(BHEP)Cl confirmed the structure and the absence of
2
water of crystallization. The kinetic parameters (Supplementary Material) of thermal degradation
process calculated using Coats–Redfern and Horowitz–Metzger methods show high values of the
energy of activation of the complex reveal the high stability of such chelates and all the
6

decomposition steps are nonspontaneous processes. Also, the negative values of ΔS* for the
degradation process indicates more ordered activated complex than the reactants or the reaction
is slow.
2
+
The hydrolysis of glycine methyl esters is catalyzed by [Pd(BHEP)(H O) ] with
2
2
5
catalysis ratio 1.45×10 . The catalytic effect is due to a direct interaction between Pd(II) and the
alkoxycarbonyl group of the ester species. The activation parameters for the hydrolysis of
coordinated glycine methyl ester were determined and compared with those of free glycine
methyl ester. The enhanced rate for base hydrolysis of the ester coordinated to the complex is
#
#
due to a decrease in ΔH and an increase in ΔS . The hydrolysis of methionine methyl ester is not
significantly catalyzed. The relatively small catalysis ratio value suggests that the
alkoxycarbonyl group is not bonded to the metal ion.
References
[1]
[2]
[3]
[4]
V.K. Jain, B. Jain, U.K. Sharma, D. Saha, Int. J. Curr. Pharm. Res., 3, 66 (2011).
W.N. Lipscomb. Acc. Chem. Res., 3, 81 (1970).
M.J. Pesando. Biochemistry, 14, 681 (1975).
T.W. Reid, I.B. Wilson, in P.D. Boyer (Ed.), The Enzymes, 3rd Edn., Academic Press,
New York, vol. 4 (1971).
[5]
D.S. Auld, in M.I. Page and A. Williams (Eds.), Enzyme Mechanisms, The Royal Society
of Chemistry, London (1987).
[
[
[
[
[
6]
7]
8]
9]
R. Jairam, M.L. Lau, J. Adorante, P.G. Potvin. J. Inorg. Biochem., 84, 113 (2001).
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J. Xia, S. Li, Y. Shi, K. Yu, W. Tong. J. Chem. Soc., Dalton Trans., 2109 (2001).
M.R. Shehata, M.M. Shoukry, F.M.H. Nasr, R. van Eldik. Dalton Trans., 779 (2008).
10] M.R. Shehata, M.M. Shoukry, F.H. Abdel-Shakour, R. van Eldik. Eur. J. Inorg. Chem.,
912 (2009).
3
[
[
[
[
[
11] M.M.A. Mohamed, A.A. Shoukry, M.M. Shoukry. Int. J. Chem. Kinet., 38, 737 (2006).
12] M.M.A. Mohamed, M.M. Shoukry. Polyhedron, 21, 167 (2002).
13] A.A. El-Sherif, M.M. Shoukry. Inorg. Chim. Acta, 360, 473 (2007).
14] M.M. Shoukry, E.M. Khairy, A. Saeed. Transition Met. Chem., 12, 315 (1987).
15] R.W. Hay, P. Banerjee. J. Chem. Soc., Dalton Trans., 362 (1981).
7

nd
[
16] R.G. Bates, Determination of pH-Theory and Practice, 2 Edn., Wiley Interscience, New
York (1975).
[
17] R.W. Hay, A.K. Basak. J. Chem. Soc., Dalton Trans., 1819 (1982).
18] R.W. Hay, P.J. Morris, in "Metal Ions in Biological Systems", H. Sigel (Ed.), Marcel
Dekker, New York, 5, 173 (1976).
[
[19] OLIS KIFET, Olis Inc., Borgart GA, 19 (1993).
[20] A.A. Shoukry, T. Rau, M.M. Shoukry, R. van Eldik. J. Chem. Soc., Dalton Trans., 3105
(
1998).
21] C.W. Davis. J. Chem. Soc., 2093 (1938).
22] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, 2 Edn., Butterworths, London
1959).
[
[
nd
(
[
23] S.N. Singh, M. Katyal, R.K. Agarwal, “Molecular Structure: A Spectroscopic
Approach”, Book Publisher: New Delhi: Discovery Publishing House, p 62 (1990).
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[
[
[
[
[
25] R.W. Hay, P. Banerjee. J. Chem. Soc., Dalton Trans., 362 (1981).
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28] R.J. Angelici, J.W. Allison. Inorg. Chem., 10, 2238 (1971).
8

Table 1. Scheme for TGA mass loss of Pd(BHEP)Cl in the temperature range
2
~33 to 600 °C with heating rate of 10 degree/min.
Assignment loss
TGA °C
DrTGA °C
%Wt. Loss
Found (Calcd.)
L(C H N O )
150-360
360-560
> 600
255
457
49.1
20.3
30.6
(49.56)
(20.17)
(30.26)
8
18
2
2
2Cl
Pd (residue)
9

2
+
Table 2. Kinetics of hydrolysis of [Pd(BHEP)(ester)] at 25 °C and 0.1 M ionic strength.
-
a
-1
Ester
pH
4.8
[OH ]
kobs (s )
Glycine methyl ester
7.94E-10
1.26E-09
2.00E-09
3.16E-09
5.01E-09
6.64E-04
8.19E-04
9.28E-04
1.17E-03
1.47E-03
5
5
5
5
.0
.2
.4
.6
Methionine methyl ester
8.8
7.94E-06
1.26E-05
2.00E-05
3.16E-05
5.01E-05
1.66E-04
2.25E-04
2.89E-04
4.06E-04
5.97E-04
9
9
9
9
.0
.2
.4
.6
10

3
-1 -1
2+
Table 3. Hydrolysis data (dm mol s ) of [Pd(BHEP)(ester)] at 25 °C and 0.1 M ionic strength.
(
ester)a
(ester)
Ester
k
OH
k
H O
k
OH
kOH/kOH
2
Glycine methyl ester
1.86E+05 1.00E-05
10.1 1.71E-06
1.28
0.767
1.45E+05
13.2
Methionine methyl ester
a
Data from ref. (9).
11

3
-1 -1
2+
Table 4. Hydrolysis data (dm mol s ) of [Pd(BHEP)(GlyOMe)]
at different temperatures in aqueous solution at pH=5.4.
T °C
k
OH
k
H O
2
15
20
25
30
35
1.24E+05
1.51E+05
1.86E+05
2.41E+05
2.76E+05
9.664E-06
9.832E-06
1.000E-05
1.017E-05
1.034E-05
12

100
%
T
r
80
a
n
s
60
m
i
t
40
i
o
n
20
0
4000
3400
2800
2200
Wavenumbers (cm )
1600
1000
400
-1
2
Figure 1a. Infrared spectrum of [Pd(BHEP)Cl ] (-) and ligand BHEP (. . .).
1
8
6
4
2
0
00
%
T
r
a
n
s
m
i
t
i
o
n
0
0
0
3
24
0
3
33
4
00
350
300
Wavenumbers
250
200
(cm-1)
2
Figure 1b. Far-infrared spectrum of [Pd(BHEP)Cl ].
13

1
1
20
00
0.0500
-0.0500
-0.1500
-0.2500
8
6
4
2
0
0
0
0
0
4
57
2
55
0
100
200
300
400
500
600
700
800
o
Temperature ( C)
2
Figure 2. TGA (solid) and DrTGA (dotted) curves of Pd(BHEP)Cl .
14

0
0
.006
.003
0
-
-
0.003
0.006
1
1
1
1
1
0
.80
.60
.40
.20
.00
.80
b
a
s
e
a
d
d
e
d
0
1000
2000
3000
4000
Seconds
Figure 3. Typical plot of volume of base added vs. time in seconds fitted with a single
2
+
exponential function for the hydrolysis of [Pd(BHEP)(GlyOMe)] at 25 °C and 0.1 M ionic
strength.
15

2.0E-03
1.5E-03
1.0E-03
y = 1.86E+05x + 5.55E-04
5.0E-04
0.0E+00
0.0E+00
2.0E-09
4.0E-09
6.0E-09
-
[
OH ]
Figure 4. Kinetic data for the hydrolysis of Pd(BHEP)-glycine methyl ester at 25 °C and 0.1 M
ionic strength.
16

8.0E-04
6.0E-04
4.0E-04
y = 1.01E+01x + 9.01E-05
2.0E-04
0.0E+00
0.0E+00
2.0E-05
4.0E-05
6.0E-05
-
[
OH ]
Figure 5. Kinetic data for the hydrolysis of Pd(BHEP)-methionine methyl ester at 25 °C and
.1 M ionic strength.
0
17

7
6.75
y = -3.37E+03x + 1.78E+01
6.5
6
.25
6
0.0032
0.0033
0.0034
0.0035
1/T
Figure 6. Effect of temperature on the hydrolysis of the coordinated glycine methyl ester.
18

Graphical Abstract
THERMAL STABILITY OF Pd(1,4-BIS(2-HYDROXYETHYL)PIPERAZINE)Cl AND
2
ITS ROLE IN THE CATALYSIS OF BASE HYDROLYSIS OF α-AMINO ACID ESTERS
MOHAMED R. SHEHATA†, MOHAMED M. SHOUKRY*†, SHOKRY A. SHOKRY‡ and M.A. MABROUK‡
†
Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt
Science and Technology Center of Excellence (STCE) Ministry of Military Production, Cairo, Egypt
‡
1
1
20
00
0.0500
-0.0500
-0.1500
-0.2500
8
6
4
2
0
0
0
0
0
4
57
2
55
0
100
200
300
400
500
600
700
800
o
Temperature ( C)
The thermal stability of Pd(BHEP)Cl complex was investigated. The role of the complex in the
2
catlalysis of amino acid esters was studied.
19