Complexation studies of Mn2+, Zn2+ and Cd 2+ ions with a series of tetradentate (N4) Schiff base ligands containing pyridine moiety in acetonitrile and nitromethane solutions by a competitive NMR technique using 7Li nucleus as a probe

Article abstract of DOI:10.1007/s13738-013-0248-y

Lithium-7 NMR spectroscopy was used to investigate the stoichiometry and stability of a Li⁺ complex with N¹,N²- bis(pyridin-2-ylmethylene)ethane-1,2-diamine (L¹), N ¹,N³-bis(pyridin-2-ylmethylene)propane-1,3-diamine (L ²) and N¹,N⁴-bis(pyridin-2-ylmethylene) butane-1,4-diamine (L³) in acetonitrile (AN) and nitromethane (NM) solutions. A competitive ⁷Li NMR method was also employed to probe the complexation of Mn²⁺, Cd²⁺ and Zn²⁺ ions with L¹, L² and L³ in the same solvents. The formation constants of the resulting complexes were evaluated from computer fitting of the mole ratio data to an equation that relates the observed chemical shifts to the formation constant. In both solvents, the stability of the resulting 1:1 complexes were found to vary in the order Zn²⁺> Cd²⁺>Mn²⁺>Li⁺. In addition, the stability of M-L complexes of M²⁺ with the Schiff base ligands found to vary in the order M²⁺-L¹ > M²⁺-L² > M²⁺-L³.

Full text of DOI:10.1007/s13738-013-0248-y

J IRAN CHEM SOC
DOI 10.1007/s13738-013-0248-y
ORIGINAL PAPER
Complexation studies of Mn2+, Zn2+ and Cd2+ ions with a series
of tetradentate (N4) Schiff base ligands containing pyridine moiety
in acetonitrile and nitromethane solutions by a competitive NMR
7
technique using Li nucleus as a probe
•
Hassan Keypour Mohammad Hassan Zebarjadian
•
•
Majid Rezaeivala Mojtaba Shamsipur
•
Seyyed Javad Sabounchei
Received: 11 January 2013 / Accepted: 9 March 2013
Ó Iranian Chemical Society 2013
Abstract Lithium-7 NMR spectroscopy was used to
investigate the stoichiometry and stability of a Li? com-
plex with N1,N2-bis(pyridin-2-ylmethylene)ethane-1,2-
diamine (L1), N1,N3-bis(pyridin-2-ylmethylene)propane-
1,3-diamine (L2) and N1,N4-bis(pyridin-2-ylmethylene)
butane-1,4-diamine (L3) in acetonitrile (AN) and nitro-
Introduction
Schiff base ligands have been extensively studied in
coordination chemistry mainly due to their facile syntheses,
easily tunable steric, electronic properties and good solu-
bility in common solvents [1]. Transition metal complexes
with oxygen and nitrogen donor Schiff-bases are of par-
ticular interest [2] because of their ability to possess unu-
sual configurations and their sensitivity to molecular
environments [3]. Schiff base ligands have proven to be
very effective in constructing supramolecular architectures
such as coordination polymers, double helixes, and triple
helicates [4, 5]. The stability constants provide the infor-
mation required to calculate the concentrations of the
complexes in solution. There are many areas of application
in many fields such as industrial chemistry [6], environ-
mental studies [7], medicinal [8] and analytical chemistry
[9]. Therefore, complexation reactions of metal ions with
different ligands have been widely studied [10–12]. Several
methods such as potentiometric titration [13], conductom-
etry [14], and spectrophotometry [15] have been reported
for the determination of stability constants [16]. Our
interest in the physicochemical properties of the Schiff
bases encouraged us to study the formation and stability of
a number of transition metal Schiff base complexes. In
recent years, we have introduced some spectrophotometric
methods for the evaluation of the formation constants of
some transition metal cations with new synthetic Schiff
base ligands in various solvents [16, 17]. In this paper, we
used 7Li NMR as a very sensitive probe to study the
complexation of Mn2?, Zn2? and Cd2? ions with N1,N2-
bis(pyridin-2-ylmethylene)ethane-1,2-diamine(L1), N1,N3-
7
methane (NM) solutions. A competitive Li NMR method
was also employed to probe the complexation of Mn2?
,
Cd2? and Zn2? ions with L1, L2 and L3 in the same sol-
vents. The formation constants of the resulting complexes
were evaluated from computer fitting of the mole ratio data
to an equation that relates the observed chemical shifts to
the formation constant. In both solvents, the stability of the
resulting 1:1 complexes were found to vary in the order
Zn2?[Cd2?[Mn2?[Li?. In addition, the stability of M–L
complexes of M2? with the Schiff base ligands found to
vary in the order M2?–L1 [ M2?–L2 [ M2?–L3.
7
Keywords Competitive Li NMR ꢀ Schiff base ligand ꢀ
Complexation ꢀ Nitromethane ꢀ Acetonitrile ꢀ
Complexation study
H. Keypour (&) ꢀ M. H. Zebarjadian ꢀ S. J. Sabounchei
Faculty of Chemistry, Bu-Ali Sina University,
65174 Hamedan, Iran
e-mail: haskey1@yahoo.com
M. Rezaeivala
Department of Chemical Engineering, Hamedan University
of Technology, 65155 Hamedan, Iran
bis(pyridin-2-ylmethylene)propane-1,3-diamine(L2)
N1,N4-bis(pyridin-2-ylmethylene)butane-1,4-diamine(L3) in
and
M. Shamsipur
Department of Chemistry, Razi University, Kermanshah, Iran
acetonitrile (AN) and nitromethane (NM) by a competitive
123

J IRAN CHEM SOC
technique. It should be noted that transition metal ions used
have unsuitable NMR properties such as low receptivity,
high quadrupole moment, insensitive chemical shifts, etc.
Our interest in the physicochemical properties of the
Schiff bases encouraged us to study the formation and
stability of a number of transition metal Schiff base com-
plexes. In recent years, we have introduced some spectro-
photometric methods for the evaluation of the formation
constants of some transition metal cations with new syn-
thetic Schiff base ligands in various solvents [16, 17]. In
experiments, a 4.0 M aqueous LiCl solution was used as
7
external standard. The Li NMR spectra of the resulting
solutions were recorded at 25.0 ( 0.1) °C.
Synthesis of Schiff base ligands
The tetradentate Schiff base ligands were prepared
according to literature method [19]. A solution of ethane-
1,2-diamine, propane-1,3-diamine and butane-1,4-diamine
(10 mmol) in dry EtOH (30 mL) was added dropwise to a
warm solution of pyridine-2-carbaldehyde (2.14 g,
20 mmol) in dry EtOH (30 mL) over a period of 2 h. The
mixture was refluxed under stirring for 12 h and then
allowed to cool to room temperature (Scheme 1).
7
this paper, we used Li NMR as a very sensitive probe to
study the complexation of Mn2?, Zn2?, Cd2? and Mn2?
ions with N1,N2-bis(pyridin-2-ylmethylene)ethane-1,2-dia-
mine (L1), N1,N3-bis(pyridin-2-ylmethylene)propane-1,3-
diamine (L2) and N1,N4-bis(pyridin-2-ylmethylene)butane-
1,4-diamine (L3) in acetonitrile (AN) and nitromethane
(NM) by a competitive technique. It should be noted that
transition metal ions used have unsuitable NMR properties
such as low receptivity, high quadrupole moment, insen-
sitive chemical shifts, etc.
Sample preparation
Stock solutions of the Schiff base ligands were prepared by
weighting an appropriate amount of the ligands at con-
centrations of 0.10 M in AN and NM. Appropriate quan-
tities of metal salts at concentrations of 0.01 M dissolved in
the mentioned solvents were added up to the [Schiff base]/
[Metal] mole ratio of 4, using a microsyringe. After mixing
Experimental
7
of the solutions under ultrasonic for 5 min, their Li NMR
Materials
spectra were recorded at 25.0( 0.1) °C.
Ethane-1,2-diamine, propane-1,3-diamine, butane-1,4-dia-
mine and pyridine-2-carbaldehyde were used from Fluka.
Lithium perchlorate (Merck) was purified and dried as
reported elsewhere [18]. Except for vacuum drying,
Reagent grade salts (all from Fluka) were applied with no
further purification. Acetonitrile and nitromethane were
commercially available from the Merk and used as
received. A Jeol FX90Q FT-NMR spectrometer imple-
Results and discussion
The 7Li chemical shifts were monitored as a function of the
three Schiff base ligands (L1–L3) to lithium ion mole ratios
in acetonitrile and nitromethane solutions, in the absence
and presence of equimolar concentration of different M2?
ions used. In all cases, only one resonance line for the Li?
ion was observed irrespective of the [Schiff base]/[Li?]
mole ratio, indicating a fast exchange between the solvated
7
menting Li resonance at 33.74 MHz was used to measure
all nuclear magnetic resonance with a field strength 2.12 T
equipped with a temperature controller. Typical acquisition
parameters were 4,000 Hz sweep width, 1 s relaxation
delay, 1 s acquisition time, 15 ls pulse width (45° pulse) in
90.0 MHz and 32 k = 32,768 words. In all 7Li NMR
7
and complexed cation sites. Typical Li NMR spectra at
varying [L1]/[Li?] mole ratios in acetonitrile in the absence
and presence of Zn2? ion are shown in Figs. 1 and 2,
respectively.
(CH2)n
O
N
N
N
(CH2)n
n=2,3,4
2
+
H2N
NH2
N
N
n=2, L1
n=3, L2
n=4, L3
Scheme 1 Synthesis of the tetradentate Schiff base ligands
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J IRAN CHEM SOC
Fig. 1 Lithium-7 NMR spectra
of 0.01 M LiClO4 in acetonitrile
at various [Li?]/[L1] mole ratios
at 25.0 °C
indicating the higher stability of all three Schiff base-Li?
complexes in nitromethane.
7
The variations of Li chemical shift with the [Schiff
base]/[Li?] mole ratio were used to calculate the formation
constants of Li? complexes with L1, L2 and L3. The for-
mation constant of 1:1 complexes are calculated from the
variation of metal chemical shift with the [Schiff base]/
[Li?] mole ratio [20]. The observed chemical shift of the
7Li? (dobs) is a mass average of the characteristic chemical
shifts of Lithium ion at each site (i.e., Li? in the bulk
solution and Li? in the complex). Assuming that a fast
exchange occurs between these two sites with respect to the
NMR time scale [21].
dobs ¼ PLidLi þ P
d
ð1Þ
Fig. 2 Lithium-7 chemical shifts as a function of the L1, L2 and L3 to
lithium ion in acetonitrile and nitromethane solutions
ðLiꢁLÞ ðLiꢁLÞ
where dLi and d(Li–L) are the characteristic chemical shifts
for Li? in the bulk solution and in the complex, respec-
tively, and PLi and P(Li–L) are the respective mole fractions
of these species.
All resulting chemical shift-mole ratio plots are illus-
trated in Figs. 3, 4, 5 and 6. As is seen from Fig. 3, in the
absence of M2? ions, addition of Schiff bases L1, L2 and L3
to lithium ion solutions causes an almost linear paramag-
netic shift, which begins to level off at mole ratios greater
than unity. The slope of the corresponding mole ratio plots
changes sharply at a point where the ligand-to-cation mole
ratio is equal to one, emphasizing the formation of a 1:1
complex. It is interesting to note that, in nitromethane
solution, the inflection points of the mole ratio plots are
clearly sharper than those obtained in acetonitrile solution,
Knowing that PLi ? P(Li–L) = 1 and PLi = [Li?]/CLi
then:
dobs ¼ PLidLi þ ð1 ꢁ PLiÞ d
ð2Þ
ð3Þ
ðLiꢁLÞ
dobs ¼ PLiðdLi ꢁ dðLiꢁLÞÞ þ d
½Liþꢂ
ðLiꢁLÞ
dobs
¼
ðdLi ꢁ dðLiꢁLÞÞ þ d
ð4Þ
ð5Þ
ðLiꢁLÞ
CLi
CLi ¼ ½Liꢂ þ ½Li ꢁ Lꢂ
123

J IRAN CHEM SOC
È
dobs ¼ ½ðKfCLi ꢁKfCL ꢁ1ÞþðKf2CL2: þKf2CL2i ꢁ2Kf2CL:CLi
1=2
þ2KfCL þ2KfCLi þ1Þ ꢂðdLi ꢁdLiꢁLÞ=2KfCLig
þdLiꢁL
ð8Þ
where Kf is the formation constant for the 1:1 complex, CL:
and CLi are the total concentrations of the Schiff base and
lithium ion, respectively, dLi and dLiꢁL are the respective
chemical shifts of the free and complexed lithium ion. The
non-linear least-squares curve fitting program KINFIT [22]
was used to evaluate Kf and dLiꢁL values for the 1:1 the
calculations.
In Eq. (8) Values of CLi, CL: and dLi are known. dLiꢁL:
and Kf are the two unknowns. These parameters are ini-
tially estimated and used to calculate dcal for different
values of [Schiff base]/[Li?] mole ratio with the aid of Eq.
(8). The calculated chemical shifts are iteratively fitted to
the observed chemical shifts and the values of dLiꢁL and Kf
are varied to obtain the best fit. A sample computer fit of
7
the Li chemical shift-mole ratio data is shown in Fig. 7a
Fig. 3 Computerfit oflithium-7chemical shiftvs. L1:Li? moleratiofor
Li?–L2 (a) and Li?–L2–Cd2? (b) systems in nitromethane solution.
(x) Experimental point; (o) calculated point; (=) experimental and
calculated points are the same within the resolution of the plot
and all the calculated log Kf values are summarized in
Table 1. As seen from Fig. 7a, there is a satisfactory
agreement between the calculated and observed chemical
shifts, supporting the formation of a complex with 1:1
stoichiometry between Li? and L2 in solution.
CSB ¼ ½Lꢂ þ ½Li ꢁ Lꢂ
ð6Þ
ð7Þ
It is well known that most transition metal nuclei cannot
be accurately used in NMR studies of their macroacyclic
complexes, because of their low receptivities and high
Liþ þ L ¼ Li ꢁ L KLi ¼ ½Li ꢁ Lꢂ=½Liþꢂ½Lꢂ
By substitution of Eqs. (5), (6) and (7) in Eq. (4), one
obtains the following equation:
7
quadrupole moments [23]. Thus, in this work, Li NMR
Fig. 4 Lithium-7 NMR spectra
of 0.01 M LiClO4 in acetonitrile
at various [Li?]/[L1] mole ratios
at 25.0 °C in the presence of
equilimolar concentration of
Zn2? ion
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J IRAN CHEM SOC
Fig. 7 Lithium-7 chemical shifts as function of L3:Li? mole ratio in
pure acetonitrile and nitromethane solutions in the presence of
different M2? ions
Fig. 5 Lithium-7 chemical shifts as function of L1:Li? mole ratio in
pure acetonitrile and nitromethane solutions in the presence of
different M2? ions
ratios more than 2, because of the quantitative formation of
1:1 complexes of Schiff bases with both Li? and M2? ions.
It is interesting to note that, in cases of Mn2?, Zn2? and
Cd2? ions in both solvent systems, the mole ratio plots also
show a distinct inflection point at a [Schiff base]/[Li?]
mole ratio of about 1 (Figs. 4, 5, 6), most probably due to
significant differences in the formation constants of the
Li?-Schiff base and M2?-Schiff base complexes in solution
[27, 28].
The competitive complex formation equilibria, where
only the 1:1 complexes between the Schiff bases and both
Li? and M2? ions are shown in Eqs. (7) and (9).
Liþ þ L ¼ Li ꢁ L KLi ¼ ½Li ꢁ Lꢂ=½Liþꢂ½Lꢂ
M
ð7Þ
ð9Þ
2þ þ L , M ꢁ L2þ KM ¼ ½M ꢁ L2þꢂ=½M2þꢂ½Lꢂ
The free Schiff base concentration, [L], is then obtained
Fig. 6 Lithium-7 chemical shifts as function of L2:Li? mole ratio in
pure acetonitrile and nitromethane solutions in the presence of
different M2? ions
from Eq. (10):
3
2
KLiKM½Lꢂ ꢁ fKLiKMðC
1
ꢁ CLi ꢁ CMÞ ꢁ KLi ꢁ KMg½Lꢂ
ꢁfKLiðCL ꢁ CLiÞ þLK: MðCL ꢁ CMÞ ꢁ 1g½Lꢂ ꢁ CL ¼ 0
was used as a very sensitive probe [20, 24–26] to investi-
gate the complex formation of Mn2?, Cd2? and Zn2? ions
with L1, L2 and L3 in acetonitrile and nitromethane solu-
tions. Variation of 7Li chemical shift, in the presence of an
equimolar concentrations of different M2? ions, as a
function of [Schiff base]/[Li?] for three Schiff bases in
acetonitrile and nitromethane was obtained. Typical spec-
tra are shown in Fig. 2, and the resulting chemical shift-
mole ratio plots are included in Figs. 4, 5 and 6.
ð10Þ
where CL., CLi and CM are the analytical concentrations of
L, Li? ion and M2? ion, respectively. In this case, the
observed 7Li NMR chemical shift is obtained from Eq. (1)
and then by substitution from Eq. (7) and the mass balance
equation CLi = [Li?]?[Li–L], Eq. (1) can be written as:
dobs ¼ fdLi þ dLiꢁL½LꢂKLig=f1 þ KLi½Lꢂg
ð11Þ
In spite of the cases involving the formation of 1:1 Li-
Schiff base complexes (Fig. 3), as is obvious from Figs. 4,
5 and 6, in which the change in chemical shift with the
[Schiff base]/[Li?] mole ratio is approximately linear at
mole ratios less than 1, the mole ratio plots in the presence
of M2? ions illustrate a curved relationship between 0 and
2. However, in all cases, a plateau is reached for mole
The KM values were evaluated by obtaining the free
Schiff base ligand concentration [L] from Eq. (10) and
fitting the chemical shift-mole ratio data to Eq. (11) using
the KINFIT program. A sample computer fit of the mole
ratio data is shown in Fig. 7b and the resulting log Kf
values are included in for all M2?–L complexes in Table 1.
Our assumption that the 1:1 stoichiometry of both Li?–L
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J IRAN CHEM SOC
Table 1 Formation constants for metal ion complexes with L1, L2 and L3 in acetonitrile and Nitromethane solutions at 25 °C
Cationa
Log Kf
Acetonitrile
L1
Nitromethane
L1
L2
L3
L2
3.51 0.06
L3
Li? (0.76)
Mn2? (0.83)
Cd2? (0.95)
Zn2? (0.74)
a
3.27 0.03
3.91 0.08
4.33 0.09
4.75 0.05
3.02 0.03
3.82 0.04
3.95 0.08
4.26 0.11
2.93 0.05
3.58 0.08
3.82 0.05
4.09 0.06
3.41 0.05
4.36 0.06
4.61 0.07
5.24 0.18
3.39 0.07
4.02 0.06
4.06 0.03
4.49 0.10
4.07 0.09
4.461 0.04
4.78 0.17
´
˚
Values in parentheses are the ionic sizes of cations in A
and M2?–L complexes seems reasonable in the light of a
fair agreement between the observed and calculated
chemical Shifts (Fig. 7b).
The data given in Table 1 revealed that the stability of
M2?–L complexes vary in the order Zn2?[ Cd2?[
Mn2?[ Li?. As is obvious, transition metal complexes
form stronger complexes with all Schiff bases than the Li?
ion used. This is due to the soft character of the transition
metal ions, which result in stronger interactions with the
donating nitrogens of the Schiff base as soft bases [34].
Lithium ion with the lowest charge density and a hard
character forms the least stable complex among the four
cations studied.
The complexation ability of the Schiff base ligands to
metal ions can be considerably increased by attaching rigid
aromatic donor end groups to the amines backbone [29–
32]. In fact, the stability of its complexes arises from the
two types nucleation centers, namely, the carbon chain
nitrogen and the pyridine aromatic residues, which can
control the strength of complexation by p-electron inter-
actions, steric influences and by supplying two nitrogen
atoms [30, 31, 33]. Subsequently, the metal ion is sur-
rounded by the flexible carbon chain, which can easily
adapt to different cation sizes, and is better shielded from
the solvent and the counter ions by two terminal pyridine
groups [31–33].
Furthermore, Table 1 shows that in both acetonitrile and
nitromethane solutions, the stability of all three M2? ions
studied, the resulting complexes with the Schiff base
ligands L1, L2 and L3 vary in the order of M2?–L1 [ M2?
–
L2 [ M2?–L3. This is due to the fact that the ligands L1, L2
and L3 differ in the size of their central chelate rings so that
they can form five-, six- and seven- membered rings,
respectively (Fig. 8). It should be noted that the selectivity
order thus obtained is in according to the well known rule
which states that the seven-membered chelate rings are less
stable than six-membered ones and both are less stable than
five-membered chelate rings [35, 36].
Thus, the complex stability results from the superposi-
tion of several factors including the extent of interaction of
donor groups of the Schiff base ligands and the cations, the
consonance between the size of metal ion and the sterical
size of the ligand cavity formed, the extent of ligand
conformational changes as a consequence of complex
formation, desolvation of the Schiff base ligands and cation
and solvation of the resulting complex, the two later factors
being strongly dependent on the nature of solvents used.
The cationic charge and size (and the consequent charge
density) can also play a fundamental role in the complex-
ation process from the view points of ionic solvation and
binding energy with the Schiff base’s donating atoms. By
increasing charge density of cations, both its salvation
energy and binding energy to the ligand are expectedly
increased, so that the increased binding energy of the cat-
ion to the ligand acts against its increased desolvation
energy. On the other hand, since the change in topology of
the open chain ligand from a flexible free state in solution
to a helical rigid conformation in the complexed form leads
to a large loss of entropy, the decrease in cation size and
increase in charge density may induce the steric deforma-
tions of Schiff base and reduce the entropy of complexation
[30]. Consequently, it is expected that by increasing the
cation size, the stability of complex will be increased.
As can be seen from the data given in Table 1, solvent
plays a fundamental role in complex formation process. This
is due to the fact that the solvent molecules are directly
involved in three possible equilibria in the process complex
formation, including desolvation of the metal ion, desolva-
tion of the ligand and solvation of the resulting charged 1:1,
so that the nature of solvent is expected to affect the overall
energy balances involved in the metal ion complexation in
solution [33, 34, 37–46]. In all cases studied, the resulting
complexes are more stable in NM than in AN. It should be
noted that, while NM and AN have about the same dielectric
constants (i.e., e ¼ 35:9 for NM and e ¼ 36:1 for AN), their
solvating abilities, as expressed by the Gutmann donor
number [47], are quite different (i.e., DN = 2.7 for NM and
DN = 14.1 for AN). Thus, AN, as a solvent of higher sol-
vating ability, can compete more strongly with the Schiff
bases for the metal ions than does NM with poor solvating
ability; this will, consequently, results in the formation of
weaker complexes in the acetonitrile solvent.
123

J IRAN CHEM SOC
N
N
N
N
N
N
N
N
N
N
N
N
M
M
M
3
1
2
-
M L
-
-
M L
M L
M=Li+,Mn2+,Zn2+,Cd2+
Fig. 8 Purposed structures of M–L tetradentate Schiff base complexes
Acknowledgments The financial support Faculty of Research
Council of Bu-Ali Sina University gratefully acknowledged.
24. E. Karkhaneei, J. Zolgharnein, A. Afkhami, M. Shamsipur, J.
Coord. Chem. 46, 1 (1998)
25. M. Shamsipur, E. Karkhaneei, A. Afkhami, Polyhedron 17, 3809
(1998)
26. M. Shamsipur, E. Karkhaneei, A. Afkhami, J. Coord. Chem. 44,
23 (1998)
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