Synthesis and metal ion complexation of acyclic Schiff base podands with lipophilic amide and ester end groups

Article abstract of DOI:10.1007/s10847-010-9878-1

A series of acyclic Schiff base podands 14-19 with lipophilic amide and ester end groups were synthesized in good yield and in a simple way. Their transition metal ions complexation was studied using conductometric method in acetonitrile (AN) at 25 °C. Schiff base podands 14-16 showed a continuous decrease in the molar conductances in their complexation with Hg²⁺, Pb²⁺, Cu²⁺, Zn²⁺ and Cd²⁺ which begins to level off at a mole ratio of 1:1 crown-to-metal indicating the formation of a stable 1:1 complexes. The order of the stability constants of the metal ions studied with the Schiff base podands 14, 15 and 16 is: Hg ²⁺ > Pb²⁺ > Cu²⁺ > Zn²⁺ > Cd²⁺ > Ag⁺. Metal ion complexation by acyclic diamide or diester podands involves presumably the oxygen atoms of the carbonyl groups in addition to the nitrogen atoms of the imino groups.

Full text of DOI:10.1007/s10847-010-9878-1

J Incl Phenom Macrocycl Chem (2011) 70:135–142
DOI 10.1007/s10847-010-9878-1
ORIGINAL ARTICLE
Synthesis and metal ion complexation of acyclic Schiff base
podands with lipophilic amide and ester end groups
•
Shehadeh Mizyed Muhammad Ashram
Received: 30 May 2010 / Accepted: 22 September 2010 / Published online: 10 October 2010
Ó Springer Science+Business Media B.V. 2010
Abstract A series of acyclic Schiff base podands 14–19
with lipophilic amide and ester end groups were synthesized
in good yield and in a simple way. Their transition metal ions
complexation was studied using conductometric method in
acetonitrile (AN) at 25 °C. Schiff base podands 14–16
showed a continuous decrease in the molar conductances in
their complexation with Hg2?, Pb2?, Cu2?, Zn2? and Cd2?
which begins to level off at a mole ratio of 1:1 crown-to-
metal indicating the formation of a stable 1:1 complexes.
The order of the stability constants of the metal ions studied
their selectivity toward a variety of metal ions, a large
number of podands with different end groups and chain
lengths were synthesized. Vogtle and Werber reported a
variety of podand ligands such as 2 and 3 (Fig. 1) which
having quinoline end groups [2]. Some of podands contain
ester, amide or pyrone linkages or residues such as 4–9
(Fig. 2) [3–9].
On the other hand, Schiff bases such as 10–13 (Fig. 3)
are considered as a very important class of organic com-
ponent which have a wide applications in many biological
aspects [10–13]. These wide applications have generated a
great deal of interest in metal complexes. Schiff base-
transition metal complexes are one of the most adaptable
and thoroughly studied systems. These complexes have
also applications in clinical and analytical fields [13, 14].
The synthesis of Schiff-base acyclic ligands and their solid
complexes with transition metal ions were characterized
using NMR, MS, X-ray and IR spectroscopy [15–18].
In this report, we have designed and synthesized a new
series of podands 14–19 (Fig. 4) by introducing an ester or
amide residues as proper ligating groups into the Schiff
bases. It is known from calixarene chemistry that the ester
and amide groups are one of the most powerful groups
introduced into the lower rim of calixarenes in order to
enhance their complexing abilities, selectivity and to
enhance their solubility in regular organic solvents [19–22].
The complexation properties of podands 14–19 with
some of transition metal ions were studied using conduc-
tometric technique in AN at 25 °C.
with the Schiff base podands 14, 15 and 16 is: Hg2?
[
Pb2? [ Cu2? [ Zn2? [ Cd2? [ Ag?. Metal ion com-
plexation by acyclic diamide or diester podands involves
presumably the oxygen atoms of the carbonyl groups in
addition to the nitrogen atoms of the imino groups.
Keywords Acyclic ligands ꢀ Schiff base ꢀ Podands ꢀ
Transition metals ꢀ Stability constant ꢀ Conductivity
Introduction
Podands are a family of noncyclic multidentate ligands such
as acyclic ligand 1 (Fig. 1) [1]. They generally form com-
plexes with smaller stability constants than those of corre-
sponding macrocyclic complexes and thus are considered as
poor ligands. To enhance their complexing abilities and
S. Mizyed
Department of Chemistry, Yarmouk University,
Irbid, Jordan
Experimental
M. Ashram (&)
Department of Chemistry, Mutah University,
Melting points were obtained on electrothermal melting
point apparatus. 1H and 13C NMR spectra were recorded on
Mutah, Al-Karak, Jordan
e-mail: ashram_1961@yahoo.com
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J Incl Phenom Macrocycl Chem (2011) 70:135–142
O
O
O
N
N
N
N
OH
HO
1
O
O
OH HO
O
O
O
O
O
N
12
10
O
N
O
N
N
N
N
N
N
N
2
3
O
O
OH
HO
Fig. 1 Structures of podands 1–3
13
11
Fig. 3 Structures of podands 10–13
N
O
N
O
O
O
O
N
measurements were recorded on Shimadzu UV-2400PC
Series. All reagents were of analytical grade and used
without further purification. The solvents were used
without further purification. The following salts were
purchased from the companies indicated: AgNO3
(Degussa), Cu(ClO4)2ꢀ6H2O (Aldrich), Pb(ClO4)2ꢀ3H2O
(Aldrich), Zn(ClO4)2ꢀ6H2O (Aldrich), Cd(ClO4)2 (Aldrich),
Hg(ClO4)2ꢀxH2O(Aldrich). For the conductivity measure-
ments, acetonitrile was used after fractional distillation.
The conductivity of the CH3CN was less than 1 9
10-7 S cm-1. The description of the conductometer and
the details of the conductance measurements have been
given previously [23]. Compounds 22, 26, 27 were pre-
pared according to the literature procedures [24, 25].
Uv–visible spectrophotometric method was used to con-
struct the Jobs plot. The detailed procedure and the NMR
complexation measurements were treated according to the
published methods [26–28].
O
N
O
O
O
N
O
O
O
5
4
OR
RO
R
O
R
O
O
O
O
O
O
O
O
Ph
Ph
Ph
O
Ph
R = OH or OCH3
6
R = CH3 or C2H5
7
O
O
O
N
O
O
N
General method for synthesis of Schiff base podands
14–19
X
X
Schiff base podands were prepared according to the known
method from the condensation of ethylenediamine or 1,2-
diaminobenzene with the corresponding aldehyde in a
molar ratio of 1:2, respectively, using ethanol as a solvent
at reflux temperature [13]. The reaction mixture was stirred
for 3 h. The solvent was evaporated under reduced pressure
using rotary evaporator. The product was dried in a des-
iccator over phosphorous pentoxide. The Schiff bases were
8 X = O
9 X = S
Fig. 2 Structures of podands 4–9
200 and 50 MHz NMR spectrometers, respectively. Unless
otherwise noted, samples were dissolved in CDCl3.
Chemical shifts are reported in ppm relative to TMS used
as internal standard. Bruker Avance 400 MHz spectrometer
was used for the complexation study. Spectrophotometric
1
obtained in good yields. The H- and 13C-NMR, infrared
analysis and elemental analysis measured for the obtained
products are consistent very well with the corresponding
Schiff base formula.
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J Incl Phenom Macrocycl Chem (2011) 70:135–142
137
Fig. 4 Structures of
synthesized podands 14–19
X
X
X
X
N
N
N
N
O
O
O
O
O
O
O
O
Y
Y
Y
Y
14 X = H; Y = OC2H5
15 X = H; Y = N(C2H5)2
16 X = Br; Y = N(C2H5)2
17 X = H; Y = OC2H5
18 X = H; Y = N(C2H5)2
19 X = Br; Y = N(C2H5)2
Podand 14
(CDCl3): 14.1, 61.0, 62.2, 65.0, 113–132,157.8, 170.1.
Elemental analysis, found (calculd): H 6.35 (6.41), C 66.93
(65.44), N 6.36 (6.48).
Pale brown semi-solid (79%); FT-IR (KBr, cm-1) 1751
(C=O), 1604 (C=N); 1H-NMR (CDCl3): d 1.28 (t, 6H,
J = 8 Hz, CH2CH3), 4.31 (q, 4H, J = 8 Hz, CH2CH3),
4.89 (s, 4H, OCH2CO), 6.96–7.88 (m, 12H, Ar), 8.81
(s, 2H, CH=N); 13C-NMR (CDCl3): 14.3, 61.5, 65.8, 116.9,
117.9, 119.8, 132.7, 133.1, 160.8, 164.2, 167.9. Elemental
analysis, found (calculd): H 5.86 (5.78), C 68.47 (68.84), N
5.81 (5.73).
Podand 18
Pale yellow semi-solid (93%); FT-IR (KBr, cm-1) 1652
1
(C=O), 1601 (C=N); H-NMR (CDCl3): d 1.13 (m, 12H,
CH2CH3), 3.39 (m, 8H, CH2CH3), 3.98 (s, 4H,
NCH2CH2N), 4.69 (s, 4H, OCH2CO), 6.90 (d, J = 3 Hz,
2H), 6.99 (d, 2H), 7.34 (d, 2H), 7.95 (d, 2H), 8.78 (s, 2H,
CH=N); 13C-NMR (CDCl3): 12.9, 14.4, 40.4, 41.5, 62.2,
67.9, 112.4, 121.7, 125.1, 127.6, 131.9, 157.5, 158.4,
166.8. Elemental analysis, found (calculd): H 7.59 (7.74),
C 67.48 (67.99), N 11.14 (11.33).
Podand 15
Pale yellow semi-solid (76%); FT-IR (KBr, cm-1) 1682
1
(C=O), 1601 (C=N); H-NMR (CDCl3): d 1.11 (m, 12H,
CH2CH3), 3.35 (m, 8H, CH2CH3), 4.61 (s, 4H, OCH2CO),
6.89–7.86 (m, 12H, Ar), 8.77 (s, 2H, CH=N); 13C-NMR
(CDCl3): 12.7, 13.9, 39.9, 41.2, 61.9, 67.2, 111.9, 120.2,
127.4, 131.7, 141.1, 158.2, 167.3. Elemental analysis,
found (calculd): H 7.14 (7.06), C 70.59 (70.83), N 10.11
(10.32).
Podand 19
Pale brown semi-solid (85%); FT-IR (KBr, cm-1) 1656
1
(C=O), 1589 (C=N); H-NMR (CDCl3): d 1.15 (m, 12H,
CH2CH3), 3.32 (m, 8H, CH2CH3), 3.96 (s, 4H,
NCH2CH2N), 4.69 (s, 4H, OCH2CO), 6.77 (d, 2H), 7.40 (d,
2H), 8.06 (s, 2H), 8.67 (s, 2H, CH=N); 13C-NMR (CDCl3):
12.9, 14.5, 40.4, 41.5, 61.8, 67.9, 114.4, 126.8, 130.2,
134.3, 156.6, 157.3, 166.4. Elemental analysis, found
(calculd): H 5.34 (5.56), C 51.41 (51.55), N 8.68 (8.59), Br
24.23 (24.49).
Podand 16
Brown semi-solid (81%); FT-IR (KBr, cm-1) 1657 (C=O),
1593 (C=N); 1H-NMR (CDCl3): d 1.13 (m, 12H, CH2CH3),
3.36 (m, 8H, CH2CH3), 4.64 (s, 4H, OCH2CO), 6.93-7.81
(m, 12H, Ar), 8.79 (s, 2H, CH=N); 13C-NMR (CDCl3):
12.9, 13.8, 40.1, 42.1, 62.2, 67.8, 112.3, 120.6, 127.5,
132.0, 142.0, 159.7, 167.8. Elemental analysis, found
(calculd): H 5.24 (5.18), C 54.69 (54.87), N 7.88 (8.0), Br
22.63 (22.81).
Results and discussion
Synthesis
Podand 17
The first synthetic approach investigated toward 21 or 17 is
outlined in Scheme 1. Diamine 20 was envisioned as being
suitable precursor to condense with ethyl bromoacetate or
chloroacetamide. However, all attempts at synthesizing 17
or 21 by this route were failed to recover only the starting
material 20. Thus an alternative route (Schemes 2, 3) was
applied to prepare Schiff base podands 14, 17, 21 and 25.
Yellow solid (83%); mp 76–78 °C; FT-IR (KBr, cm-1
)
1741 (C=O), 1634 (C=N); 1H-NMR (DMSO-d6): d 1.20 (t,
6H, J = 7.8 Hz, CH2CH3), 3.90 (s, NCH2CH2N) 4.21 (q,
4H, J = 7.8 Hz, CH2CH3), 4.92 (s, 4H, OCH2CO),
7.0–7.91 (m, 8H, Ar), 8.70 (s, 2H, CH=N); 13C-NMR
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J Incl Phenom Macrocycl Chem (2011) 70:135–142
O
O
N
N
Br
Cl
OC2H5
X
N
N
O
N
N
O
NH2
K2CO3, CH3CN
X
OH HO
O
O
O
O
K2CO3, CH3CN
O
O
C2H5O
H2N
OC2H5
NH2
17
21
20
Scheme 1 Synthesis of podands 17 and 21
Scheme 2 Synthesis of
podands 14 and 17
H
O
O
O
Br
H
OC2H5
O
H2N
NH2
N
N
OH
C2H5OH, reflux
K2CO3, acetone
O
O
O
O
O
OC2H5
C2H5O
OC2H5
22
14
NH2
H2N
N
N
O
O
O
O
C2H5O
OC2H5
17
H
O
O
Cl
X
NH2
X
X
X
O
NH2
K2CO3, KI, acetone
N
N
O
H2N
H
C2H5OH, reflux
O
OH
O
O
O
O
NH2
H2N
NH2
X = H
X = Br
23 X = H
24 X = Br
21 X = H
25 X = Br
Scheme 3 Synthesis of podands 21 and 25
In this route, salicylaldehyde or 5-bromosalicylaldehyde
were condensed with ethyl bromoacetate or chloroacet-
amide to give the monomers 22, 23 and 24 in high yield.
Treatment of two equivalents of 22 with 1,2-diaminoben-
zene or 1,2-diaminoethane successfully gave the Schiff
base podands 14 and 17 respectively, in high yield. In
similar fashion treatment of two equivalents of either 23 or
24 with 1,2-diaminoethane gave both 21 and 25, respec-
tively. Podands 14 and 17 are soluble in most regular
organic solvents while podands 21 or 25 are only sparingly
soluble in DMSO. This poor solubility of 21 and 25 in
regular organic solvents such as methanol or acetonitrile
causes a limitation in their metal complexation studies.
Therefore, in order to enhance their solubility, alkyl groups
(ethyl groups) were introduced into the nitrogen atoms of
amido groups. This done by reacting of N,N-diethylchlo-
roacetamide with either salicylaldehyde or 5-bromosali-
cylaldehyde to give 26 and 27, respectively as shown in
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J Incl Phenom Macrocycl Chem (2011) 70:135–142
139
H
O
O
O
Cl
X
X
X
X
O
N
N
O
N(C2H5)2
K2CO3, KI, acetone
H2N
NH2
H
C2H5OH, reflux
O
O
OH
O
O
N(C2H5)2
(C2H5)2N
N(C2H5)2
X = H
X = Br
26 X = H
27 X = Br
15 X = H
16 X = Br
NH2
H2N
X
X
N
N
O
O
O
O
(C2H5)2N
N(C2H5)2
18 X = H
19 X = Br
Scheme 4 Synthesis of podands 15, 16, 18 and 19
365
350
335
320
305
290
275
260
245
230
215
375
360
345
330
315
300
285
270
255
240
225
210
195
180
165
Ag+
Cd2+
Hg2+
Pb2+
Zn2+
Cu2+
Ag+
Cd2+
Hg2+
Cu2+
Zn2+
Pb2+
200
0
0.5
1
1.5
[L]T/[M]T
2
2.5
3
0
0.5
1
1.5
2
2.5
3
[L]T/[M]T
Fig. 6 Molar conductance–mole ratio plots for 15 and metal cations
in CH3CN at 25 °C
Fig. 5 Molar conductance–mole ratio plots for 14 and metal cations
in CH3CN at 25 °C
nature of the interactions taking place. We employed such
measurements to establish the stoichiometry of the com-
plexes formed between several individual metal cations
and compounds 14–19 and also to determine the stability
constants of these complexes. To determine the stability
constant in this study, a simplex program, which is spe-
cially designed for 1:1 complexes, is applied [29–33].
Complexes other than 1:1 stoichiometry will not fit with
this program.
Scheme 4. Indeed, condensation of two equivalents of 26
or 27 with either 1,2-diaminobenzene or 1,2-diaminoethane
gave 15 and 16; 18 and 19, respectively which are soluble
in most organic solvents.
Complexation studies
The molar conductance K (S cm2 mol-1) of the per-
chlorate salts of Cu2?, Cd2?, Zn2?, Pb2?, and Hg2? and the
nitrate of Ag? in acetonitrile was monitored as a function
of [L]T/[M]T at 25 °C are given in Figs. 5, 6, 7, 8, 9 and 10.
In principle, measurements of the variation of electrical
conductance with the concentration of metal salts and
receptors can determine the strength, stoichiometry and
stability constants of complex formation and to assess the
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375
360
345
330
315
300
285
270
255
240
225
210
195
180
165
150
380
365
350
335
320
305
290
275
260
245
230
215
200
185
Ag+
Cd2+
Cu2+
Pb2+
Hg2+
Zn2+
Ag+
Pb2+
Hg2+
Cu2+
Zn2+
Cd2+
0
0.5
1
1.5
2
2.5
3
[L]T/[M]T
Fig. 7 Molar conductance–mole ratio plots for 16 and metalcations
in CH3CN at 25 °C
0
0.5
1
1.5
2
2.5
3
[L]T/[M]T
Fig. 10 Molar conductance–mole ratio plots for 19 and metal cations
in CH3CN at 25 °C
390
375
360
345
330
315
300
285
270
255
240
225
210
195
180
165
150
0.2
0.15
0.1
Ag+
Cu2+
Zn2+
Hg2+
Pb2+
Cd2+
0.05
0
0
0.2
0.4
0.6
0.8
1
X 14
-0.05
0
0.5
1
1.5
[L]T/[M]T
2
2.5
3
Fig. 11 Job’s plot for ligand 14 with Hg2? metal cation in CH3CN at
25 °C
Fig. 8 Molar conductance–mole ratio plots for 17 and metal cations
in CH3CN at 25 °C
Mole ratio of
+ Hg+2
14
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0
370
355
340
325
310
295
280
265
0
1
2
3
4
5
6
7
[Hg+2]/[
]
14
250
Ag+
Fig. 12 Plot of chemical shift changes Dd versus [Hg2?]/[14] metal
cation for CH3 protons of the ethyl group of 14 in CD3CN at 25 °C
235
Cu2+
220
205
Hg2+
Pb2+
190
175
160
145
Cd2+
Zn2+
[L]T and [MT are the total concentrations of the ligands
14–19 and of the metal cations, respectively.
0
0.5
1
1.5
[L]T/[M]T
2
2.5
3
It is obvious from Figs. 5, 6 and 7 that the addition of
podands 14, 15, or 16 (where the bridge is benzene ring)
to Hg2?, Pb2?, Cu2?, Zn2?, or Cd2? metal solutions causes
a continuous decrease in the molar conductance which
Fig. 9 Molar conductance–mole ratio plots for 18 and metal cations
in CH3CN at 25 °C
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141
Table 1 log Kassoc for complexation of Ag?, Cu2?, Zn2?, Hg2?, Pb2? and Cd2? with podands 14, 15 and 16 in acetonitrile at 25 °C
Compound
Ag?
Cu2?
Zn2?
Pb2?
Hg2?
Cd2?
14
15
16
17
18
19
8
a
3.95 0.10
4.14 0.11
4.24 0.11
b (1:2)
2.95 0.07
4.24 0.11
4.26 0.12
4.38 0.13
b (2:1)
4.95 0.13
2.81 0.07
a
3.29 0.08
5.05 0.14
1.83 0.05
0.95 0.02
1.67
3.71 0.10
4.43 0.11
2.79 0.07
1.29
b
2.48
b
a
4.10
b
b (2:1)
5.81
6.33
NA
NA
3.29
b
5.67
NA
NA
4.52
b
NA
NA
3.72 0.10
3.16 0.07
NA
NA
9
3.22 0.08
3.16 0.07
a
No change in molar conductance was observed
The data cannot be fitted in equation
b
Figure 8 shows that addition of podands 17 to Hg2?, Cu2?
or Zn2? metal solutions causes a continuous increase in the
molar conductance which does not level off at any ratio.
Either podands 17 or 18 form with Pb2? 2:1 (L:M) stable
complexes, while podand 19 forms 1:1 complex.
begins to level off at a mole ratio of 1:1 crown-to-metal
indicating the formation of a stable 1:1 complexes. This
decreasing indicates a lower mobility of the complex than
free solvated metal ions. To confirm the 1:1 complexation
ratio obtained by conductivity measurements, the com-
plexation between ligand 14 and Hg2? was studied, as an
example, using Job’s plot (Fig. 11). This figure shows that
a maxima of 0.5 is obtained indicating 1:1 ratio. Additional
proof for the 1:1 ratio of 14-Hg2? complex comes also
from the mole ratio plot using NMR titration method.
Figure 12 shows that the change in d levels off near the
ratio of 1 for CH3 protons of the ethyl group. Figures 5 and
6 show that no detectable changes in molar conductance
occured upon addition of either a solution of 14 or 15 to
Ag? solution. This indicates that either no complexation
occurred or the mobility of the podand-Ag? complex is
equal to the mobility of free solvated Ag?. An anomalous
behavior was observed in the complexation of Hg2?, Cu2?
and Zn2? ions with either 18 or 19 (where the bridge is
ethylene group). As is obvious from Figs. 9 and 10, addi-
tion of either podands 18 or 19 to Hg2?, Cu2? or Zn2? ions
causes the molar conductivity to initially decrease until the
mole ratio reaches 1:1 and then to increase. Such behavior
may be described according to the following equilibria:
As shown in Table 1, the order of the stability constants
of metal cations with 14, 15, or 16 is: Hg2?
[
Pb2? [ Cu2? [ Zn2? [ Cd2? [ Ag?. It is noteworthy
that there is no big differences in the stability constants
whether the podand ended with alkoxy groups (podand
14) or ended with diethyl amino group (podands 15, 16).
This presumably indicates that the metal ion coordinates
the oxygen atoms of the carbonyl groups in addition to
the nitrogen atoms of the imino groups. This finding was
observed also in the complexation studies of transition
metal cation with podands 8 and 9 (Fig. 2). It was con-
cluded that the oxygen or sulfur, but not the nitrogen
atoms of the end groups participate in metal ion coordi-
nation [3].
For comparison purposes, the stability constants of some
acyclic ligands 8 or 9 with Cu2?, Ag? and Pb2? were
incorporated (Table 1). It can be noticed that the stability
constants of these acyclic ligands with Cu2? and Pb2? are
lower than acyclic Schiff base ligands 14–16.
À
Á
M2þ; ClOꢁ4 þ podand ꢀ M2þ podand; ClO4ꢁ
ð1Þ
M2þ podand ; ClOꢁ4 þ podand ꢀ M2þðpodandÞ2 þ ClO4ꢁ
Conclusions
ð2Þ
Three new acyclic Schiff base ligands 14–19 have been
synthesized in a simple way and in a very good yield. These
ligands showed good complexation ability for Cu2?, Zn2?
It seems that addition of the podand to metal ion solution
results in the formation of a relatively stable 1:1 complex
(I) which is present as an ion-pair. Addition of the second
podand to ion-pair complex, causes formation of a stable
1:2 complex with a sandwich structure (II) which decreases
the interaction of chlorate anion with metal cations and
results in dissociation of ion-pair. Similar behavior has
been observed by others in their complexation studies of
variety of crown ethers with several metal cations [34, 35].
,
Hg2?, Pb2? and Cd2? but they showed low selectivity
toward metal cations studied. The order of the stability
constants of metal cations with the three Schiff base ligands
14–16 is: Hg2? [ Pb2? [ Cu2? [ Zn2? [ Cd2? [ Ag?.
Schiff base ligands 18 and 19 showed an anomalous
behavior in the complexation with Hg2?, Cu2? and Zn2?
ions.
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Acknowledgement We thank Deanship of the Academic Research
at Mutah University and Scientific Research for financial support.
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