Synthesis of three new branched octadentate (N8) Schiff Base and competitive Lithium-7 NMR study of the stoichiometry and stability constant of Mn2+, Zn2+ and Cd2+ complexes in acetonitrile – [(BMIM)(PF6)] mixture

Article abstract of DOI:10.1016/j.molstruc.2019.126965

Three new branched hexadentate amines, have been synthesized. Condensation with picolinaldehyde in methanol leads to produce three new Schiff base, with two 2-pyridylmethyl pendant arms. ⁷Li NMR spectroscopy was used to investigation the stability and stoichiometry information of a Li⁺ complex with three symmetrical branched Schiff base (Sc.B.¹), (Sc.B.²) and (Sc.B.³) in 0–100, 25–75, 50-50 and 75–25 w/w% acetonitrile – [(BMIM)(PF₆)] ionic liquid mixture solution. A competitive ⁷Li NMR manner was also used to probe the complexation of Schiff bases with Mn²⁺, Zn²⁺ and Cd²⁺ ions in the same solvent systems. The stability constants of the resulting complexes were estimated from computer fitting of the mole ratio information to an equation that relates the observed chemical shifts to the stable constant. There is a reverse relevance of the complex stability and the amount of ionic liquid in the solvent mixture. In of the all experimented solvent mixture, the stability of the resulted 1:1 complexes were found to change in the order M-Sc.B.¹>M-Sc.B.²>M-Sc.B.³ and Cd²⁺> Mn²⁺> Zn²⁺.

Full text of DOI:10.1016/j.molstruc.2019.126965

Journal of Molecular Structure 1199 (2020) 126965
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis of three new branched octadentate (N ) Schiff Base and
8
competitive Lithium-7 NMR study of the stoichiometry and stability
2þ
2þ
2þ
constant of Mn , Zn and Cd complexes in acetonitrile e
(BMIM)(PF )] mixture
[
6
Abbas Amini Manesh , Mohammad Hasan Zebarjadian b
a, *
a
Department of Chemistry, Payame Noor Universtiy, P. O. Box, 19395-3697, Tehran, Islamic Republic of Iran
Farhangian University, Tehran, Islamic Republic of Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2019
Received in revised form
Three new branched hexadentate amines, have been synthesized. Condensation with picolinaldehyde in
7
methanol leads to produce three new Schiff base, with two 2-pyridylmethyl pendant arms. Li NMR
þ
spectroscopy was used to investigation the stability and stoichiometry information of a Li complex with
20 August 2019
1
2
3
three symmetrical branched Schiff base (Sc.B. ), (Sc.B. ) and (Sc.B. ) in 0e100, 25e75, 50-50 and 75e25
Accepted 20 August 2019
Available online 22 August 2019
7
6
w/w% acetonitrile e [(BMIM)(PF )] ionic liquid mixture solution. A competitive Li NMR manner was also
2
þ
2þ
2þ
used to probe the complexation of Schiff bases with Mn , Zn and Cd ions in the same solvent
systems. The stability constants of the resulting complexes were estimated from computer fitting of the
mole ratio information to an equation that relates the observed chemical shifts to the stable constant.
There is a reverse relevance of the complex stability and the amount of ionic liquid in the solvent
mixture. In of the all experimented solvent mixture, the stability of the resulted 1:1 complexes were
Keywords:
8
Octadentate (N ) schiff base
Competitive 7Li NMR
Stoichiometry
Stability
1
2
3
2þ
2þ
2þ
found to change in the order M-Sc.B. >M-Sc.B. >M-Sc.B. and Cd > Mn > Zn .
Complexation
Acetonitrile
© 2019 Published by Elsevier B.V.
6
[(BMIM)(PF )] mixture
1
. Introduction
bioinorganic chemistry [8], and have various applications in catal-
ysis [9e11], and as antibacterial agents [12e14] or chemosensors
[15e18]. Many Schiff base have been designed and synthesized not
only to mimic the functions of natural carriers in transporting metal
ions but also for the development of new methodologies in sepa-
ration science. Most of the work has been concerned with alkaline
and alkaline earth cations, and transport studies have been made in
parallel with the development of multidentate carriers containing
oxygen donor atoms, in particular crown ethers and cryptands
[19e23]. In contrast, much less attention has been devoted to
transport processes involving d or f metal ions.
Schiff base ligands have played an important role in the devel-
opment of coordination chemistry. Metal complexes of these li-
gands are ubiquitous due to their facile synthesis, wide applications
and the possibility of diverse structural modifications [1e5].
Schiff base ligands have played an important role in the devel-
opment of coordination chemistry. Metal complexes of these li-
gands are ubiquitous due to their facile synthesis, wide applications
and the possibility of diverse structural modifications [6]. Schiff
base complexes are considered to be among the most important
stereochemical models in main group and transition metal coor-
dination chemistry due to their preparative accessibility and
structural variety [7]. The chemistry of Schiff base ligands and their
metalcomplexes has expanded enormously and encompasses a vast
area of organometallic compounds and various aspects of
The complexation properties of macrocyclic amines, such as
crown ethers and cryptands, in pure and mixed solvents, have been
investigated extensively during the last three decades [24]. To date,
our understanding of the macrocyclic and macroacyclic amines
solution chemistry has been based on their behavior in molecular
solvents, such as water, methanol and acetonitrile. The part of the
solvent is very important, as the cation complexation competes
þ
*
Corresponding author.
with its solvation by solvent molecules. Stability constants of Li
E-mail addresses: a_aminima@yahoo.com (A.A. Manesh), Zebarjadian@gmail.
ion complexes with macroacyclic Schiff base were prescribe in a
com (M.H. Zebarjadian).
https://doi.org/10.1016/j.molstruc.2019.126965
0
022-2860/© 2019 Published by Elsevier B.V.

2
A.A. Manesh, M.H. Zebarjadian / Journal of Molecular Structure 1199 (2020) 126965
number of solvents. During recent decade, there has been an
increasing interest in room temperature ionic liquids, which can be
regarded as a new class of liquid media [25,26].
In this work, we report the Synthesis of three new branched
8
octadentate (N ) Schiff Base ligands with two 2-pyridylmethyl
the filtrate volume reduced to 20 mL by rotary evaporator. Excess
water was added and the product was extracted with chloroform
(3 ꢂ 25 mL); the combined chloroform solutions were separated
and dried by magnesium sulfate. The chloroform was separated by
rotary evaporator to give up yellow oil (90%). The resulting product
pendant arms. These Schiff Bases werecharacterized by several
physicochemical techniques and Li NMR was used to determine
2 3
(10 mmol) was disolved in acetonitrile (20 mL) and K CO (2.76 g,
7
20 mmol) was added. The mixture was refluxed and then a solution
of N-(3-bromopropyl) phthalimide (3.217 g, 12.0 mmol) in aceto-
nitrile (40 mL) was added. The mixture was refluxed for 24 h and
then filtered while it was warm. The filtrate was reduced to dry-
ness. The resulting product was boiled under reflux for about 20 h
in aqueous HCl (25%, ~100 mL). Then the solution was evaporated to
low volume (ca. 20 mL) under vacuum and cooled in a refrigerator
for several hours. The solid result was filtered off, and the rest
solvent on the solid product was evaporated to dryness under
vacuum. Water (~40 mL) was added to the mixture and the pH was
controlled to 12 with sodium hydroxide. The result product was
extracted with chloroform (3 ꢂ 25 mL), and then the combined
chloroform solutions were separated, and dried over magnesium
sulfate. The chloroform was removed by vacuum desiccator to leave
the product as brown oil [28,29].
the stability and stoichiometry action of complexation reaction of
1
2
3
2þ
2þ
2þ
(
Sc.B. ), (Sc.B. ) [27,28] and (Sc.B. ) with Cd , Mn and Zn ions
in binary 100e0, 75e25, 50-50 and 25e75 wt% acetonitrile - 1-
Butyl-3-methylimidazolium hexafluorophosphate ion liquid
(
[BMIM][PF
6
]) mixture solutions.
2
. Materials and methods
2.1. Materials
The butane-1,4-diamine, sodium borohydride, ethane-1,2-
diamine, propane-1,3-diamine, N-(3-bromopropyl) phthalimide,
picolinaldehyde were used of the maximum purity available from
Fluka or Merck. Lithium perchlorate, Cadmium(II), Zinc(II), and
Manganese(II) nitrate salts of all other cations were used of the
maximum purity available from Merck and used without any extra
purification except for vacuum drying over silicagel blue crystals.
1
1'
A separation solution of all the brown oil amines, N , N -
1
(ethane-1,2-diyl) bis [N -(pyridin-2-ylmethyl) propane-1,3-
1
1
1’
1
diamine] (A ), N , N - (propane-1,3-diyl) bis [N -(pyridin-2-
2
1
1'
Spectroscopic
grade
1-Butyl-3-methylimidazolium
hexa-
ylmethyl) propane-1,3-diamine] (A ) and N , N -(butane-1,4-diyl)
1
3
fluorophosphate [(BMIM)(PF
6
)] ionic liquid] and acetonitrile (all
bis [N -(pyridin-2-ylmethyl) propane-1,3-diamine] (A ), (1 mmol)
was added to a warm solution of picolinaldehyde (0.214 g,
2.00 mmol) in dry MeOH (~30 mL) over a period of 2 h. The mixture
was refluxed under stirring for 10 h and then let to cool to room
temperature. A low volume of diethyl ether was slowly added into
the solution. The product Schiff bases were filtered off, washed with
cold diethyl ether and dried under vacuum (Scheme 1).
from Merck), were used to do up prepare the solvent mixture by
weight.
3
3'
1
1'
2
.3.2. Synthesis of Schiff base (N Z, N Z)-N , N -(ethane-1,2-diyl)
1
3
bis[N -(pyridin-2-ylmethyl)-N -(pyridin-2-ylmethylene) propane-
1
1
,3-diamine] (Sc.B. )
Yield: (71%). Anal. Calc. for C32
.16; N, 20.96 Found: 71.64; H, 7.32; N, 20.71. IR (Nujol mull, cm ):
38 8
H N (MW: 534.32): C, 71.88; H,
2.2. Instrumentation
ꢀ
1
7
1
1
647 (
ppm, 90 MHZ):
d,d'), 7.683 (m, 2H, He,e'), 7.276 (m, 2H, Hf,f'), 8.476 (d, 2H, Hg,g'),
3.641 (t, 4H, Hh,h'), 1.876 (p, 4H, Hi,i'), 2.601 (t, 4H, Hj,j'), 8.339 (s, 2H,
k,k'), 7.980 (d, 2H, Hm,m'), 7.767 (m, 2H, Hn,n'), 7.581 (m, 2H, Ho,o'),
n
]
C N
Schiff base), 1585 (
C N 6
n ] pyridine). H NMR (DMSO‑d ,
CHN analyses were carried out using a Carlo-Erba Ea, CHN
d
2.683 (s, 4H, Ha,a'), 3.753 (s, 4H, Hb,b'), 7.138 (d, 2H,
elemental analyzer. Infrared spectra were measured using KBr
pellets on a Shimadzu FT-IR 8000 series spectrophotometer
H
ꢀ
1
(
4000e400 cm ). FAB mass spectra were recorded using a Kratos-
H
8
MS-50 T spectrometer connected to a DS90 data system using 3-
nitrobenzyl alcohol as the matrix. 1H and 13C NMR spectra were
13
.631 (d, 2H, Hp,p'). C NMR (DMSO‑d
6
, ppm, 300 MHz): d 54.66
(
(
(
C ,a'), 58.44 (Cb,b'), 155.66 (Cc,c'), 128.68 (Cd,d'), 145.47 (Ce,e'), 124.51
a
recorded on a Bruker 500 and JEOL FX-90 Q using DMSO‑d
6
and
Cf,f'), 148.10 (Cg,g'), 52.51 (Ch,h'), 25.07 (Ci,i'), 58.28 (Cj,j'), 162.26
k,k'), 155.77 (Cl,l'), 125.39 (Cm,m'), 141.14 (Cn,n'), 136.93 (Co,o'), 149.33
CDCl as solvent. All NMR evaluations were made on a Jeol FX-FT
3
C
NMR 90Q spectrometer with a temperature controller. The tem-
ꢁ
(Cp,p').
perature of the probe was fixed to an accuracy of ±0.1 C using the
temperature controller in the spectrometer. At this spectrometer,
lithium-7 resonates at 33.742 MHz.
3
3'
1
1'
1
2.3.3. Synthesis of (N Z, N Z)-N , N -(propane-1,3-diyl) bis [N -
3
(
pyridin-2-ylmethyl)-N -(pyridin-2-ylmethylene) propane-1,3-
2
2
2
.3. Synthesis
diamine] (Sc.B. )
40 8
Yield: (65%). Anal. Calc. for C33H N (MW: 548.34): C, 72.23; H,
7.35; N, 20.42%. Found: C, 71.83; H, 7.24; N, 20.29%. IR (Nujol mull,
.3.1. General synthesis of the branched octadentate Schiff bases
1
2
3
ꢀ1
1
(
Sc.B. ), (Sc.B. ) and (Sc.B. )
cm ) 1650 (
n
]
C N
Schiff base), 1587 ( pyridine). H NMR
n
]
C N
The general method for the synthesis of the pentadentate Schiff
(DMSO‑d , ppm, 90 MHz):
6
d
1.658 (m, 2H, H ), 2.693 (t, 4H, Hb,b'),
a
bases are as follows. A solution of butane-1,4-diamine, propane-
,3-diamine and ethane-1,2-diamine (10 mmol) in dry Methanol
MeOH) (40 mL) was added slowly to a warm solution of picoli-
naldehyde (2.14 g, 20.0 mmol) in dry MeOH (40 mL) over a period of
h. The mixture was refluxed under stirring for 10 h and then
allowed to cool to room temperature. The sodium borohydride
3.78 g, 100 mmol) was then added slowly and the reaction mixture
heated at reflux for a further 2 h. The solution was filtered and then
3.569 (s, 4H, Hc,c'), 7.127 (d, 2H, He,e'), 7.683 (m, 2H, Hf,f'), 7.273 (m,
2H, Hg,g'), 8.470 (d, 2H, Hh,h'), 2.403 (t, 4H, Hi,i'), 1.590 (m, 4H, Hj,j'),
2.403 (t, 4H, Hk,k'), 8.333 (s, 2H, Hl,l'), 7.969 (m, 2H, Hn,n'), 7.765 (m,
1
(
13
6
2H, Ho,o'), 7.575 (m, 2H, Hp,p'). C NMR (DMSO‑d , ppm, 300 MHz):
2
d
25.63 (C ), 53.96 (Cb,b'), 60.38 (Cc,c'), 158.66 (Cd,d'), 125.26 (Ce,e'),
a
141.59 (Cf,f'), 124.39 (Cg,g'), 147.68 (Ch,h'), 50.29 (Ci,i'), 20.77 (Cj,j'),
60.38 (Ck,k'), 163.41 (Cl,l'), 155.17 (Cm,m'), 126.76 (Cn,n'), 147.11 (Co,o'),
140.54 (Cp,p'), 147.68 (Cq,q').
(

A.A. Manesh, M.H. Zebarjadian / Journal of Molecular Structure 1199 (2020) 126965
3
Scheme 1. Synthesis of the branched octadentate Schiff bases showing the lettering scheme for NMR assignments.
3
3'
1
1'
1
2
(
.3.4. Synthesis of (N Z, N Z)-N , N -(butane-1,4-diyl) bis [N -
respectively) were added to the [Sc.B.]/[Metal salt] mole ratio of 4,
using a micro syringe, then the mixture was under ultrasonic for
10 min. Throughout the whole temperature range used, it was
found that 10 min thermosetting prior to information accumulation
was enough for each sample to give temperature equilibrium
3
pyridin-2-ylmethyl)-N -(pyridin-2-ylmethylene) propane-1,3-
3
diamine] (Sc.B. )
42 8
Yield: (62%). Anal. Calc. for C34H N (MW: 562.35): C, 72.57; H,
.52; N, 19.91%. Found: C, 72.34; H, 7.38; N, 19.52%. IR (Nujol mull,
7
ꢀ
1
1
7
cm ) 1650 (
DMSO‑d , ppm, 90 MHz):
.605 (s, 4H, Hc,c'), 6.931 (d, 2H, He,e'), 7.567 (m, 2H, Hf,f'), 7.076 (m,
n
C
]
N
Schiff base), 1587 (
n
]
C N
pyridine). H NMR
(Scheme 2). The Li-NMR spectra of the solutions were recorded at
25.0 (±0.1) ºC.
(
6
d
1.426 (t, 4H, Ha,a'), 2.592 (t, 4H, Hb,b'),
3
2
2
2
d
H, Hg,g'), 8.273 (d, 2H, Hh,h'), 2.590 (t, 4H, Hi,i'), 1.504 (m, 4H, Hj,j'),
.420 (t, 4H, Hk,k'), 8.135 (s, 2H, Hl,l'), 7.772 (m, 2H, Hn,n'), 7.688 (m,
3. Results and discussion
1
3
H, Ho,o'), 7.483 (m, 2H, Hp,p'). C NMR (DMSO‑d
24.94 (Ca,a'), 54.18 (Cb,b'), 60.48 (Cc,c'), 160.50 (Cd,d'), 122.30 (Ce,e'),
36.48 (Cf,f'), 121.68 (Cg,g'), 148.69 (Ch,h'), 51.90 (Ci,i'), 28.17 (Cj,j'),
0.89 (Ck,k'), 154.42 (Cl,l'), 161.86 (Cm,m'), 121.14 (Cn,n'), 124.60 (Co,o'),
22.74 (Cp,p'), 149.30 (Cq,q').
6
, ppm, 300 MHz):
The Lithium-7 chemical shifts were studied as a function of the
1
2
3
mole ratio of complexant, Schiff Base (Sc.B. ), (Sc.B. ) and (Sc.B. ) in
1
3
1
binary 100e0, 75e25, 50-50 and 25e75 %wt acetonitrile e Ion
6
liquid ([BMIM][PF ]) mixture solution, in the absence and presence
2
þ
of equimolar concentration of different M ions used. In all cases,
7
show of only a single Li resonance locates that the exchange rate of
þ
2.4. Methods
Li ion between the bulk solution and the complexed sites is fast on
the NMR time scale (Fig. 1).
An aqueous 4.0 M Lithium chloride solution was used as the
All the resulting chemical shift - mole ratio plots are illustrated
in Figs. 1 and 2. As can be seen, in the absence of other cations
(Fig. 1-A), addition of Schiff base to the lithium ion solution causes
external reference and the related lithium-7 chemical shifts are
referenced to this solution. The downfield shift from the reference
is designated as being positive. The concentration of all cations
used was 0.02 M. Stock solutions of the Schiff bases were prepared
by weighting adaption amounts at concentrations of 0.2 M in Ion
liquid - Acetonitrile mixtures. Appropriate quantities of metal ions
at concentrations of 0.02 M (dissolved in the same solvents,
7
an almost linear paramagnetic shift in the observed Li chemical
shift,
shifts of Li ion in the bulk solvent,
complexed with Schiff base, Li-Sc. B, which are quite different from
each other due to differences in the electron densities surrounding
dobs, as a population - averaged combination of the chemical
þ
þ
d
Li, and that of the Li ion
d

4
A.A. Manesh, M.H. Zebarjadian / Journal of Molecular Structure 1199 (2020) 126965
Scheme 2. Synthesis of M2 -Schiff base octadentate complexes.
þ
Fig. 1. 7Li NMR spectra of 0.02 M LiClO
equilimolar concentration of Cd ion at 25.0 ± 0.1 C.
in binary 50e50% acetonitrile e (BMIM)(PF
) ionic liquid mixtures at various [Li ]/[Sc.B. ] mole ratios in (A): the absence and (B) presence of
þ
1
4
6
2þ
ꢁ
the probe nucleus in the different two sites, as
d
obs ¼ PLi
d
Li þ PLi
d
Li-
mole ratio in acetonitrile-ion liquid mixture solutions are shown in
Fig. (2). As is obvious from Figs. (1-A, 1-B and 2), unlike the cases
Sc.B. (where, PLi þ PLi-Sc.B. ¼1 in the form of titration of metal ion
þ
with the Schiff base). However, as is obvious from Fig. (1-A), the
d
obs
involving in the stability of 1:1 Li -Sc.B. complex in whichever the
þ
value begins to level off at mole ratios bigger than unity, because
change in chemical shift with the Sc.B$/Li mole ratio is quite linear
þ
after formation of a 1:1 complex almost all Li ions present in
at mole ratio <1, here the mole ratio plots in the presence of other
þ
2þ
þ
(
[
LieSc.B.) complex (PLi ¼ 0 and PLi-Sc.B. ¼1 so that
d
obs
¼
d
Li-Sc.B.
)
M
ions used show a small change in chemical shift of Li ion at
þ þ
29]. Lithium complexes in Fig. (1-B) show that the slope of the
Sc.B$/Li mole ratios at Li ion between 0 and 1, followed by a sharp
corresponding mole ratio plots change significantly at the point
where the Schiff base to cation mole ratio is equal to one, empha-
sizing the formation of a relatively stable 1:1 complex in solution.
linear increase in
>2. Such more or less similar
d
obs, whichever begins to level off at mole ratios
þ
d
obs Sc.B$/Li mole ratio behavior
2
þ
2þ
2þ
observed for all three Cd , Zn and Mn ions in the presence of
þ
þ
2þ
The stability constants of 1:1 (LieSc.B.) complexes were studied
Li ion clearly indicate that all 1:1 M -Sc.B. complexes formed in
7
þ
þ
from the difference of the Li chemical shift with Sc.B$/Li mole
ratio. It has been shown formerly in Refs. [28e33].
both acetonitrile-ionic liquid mixtures are more stable than the Li -
Sc.B. complex. Here the resulting mole ratio plots show that, due to
higher stability of M -Sc.B. complexes over Li -Sc.B, the majority
of Schiff base molecules interact with M ion at Sc.B$/Li -mole
ratios <1 so that most of the Li ions present remain free in solution
and, therefore, the Li chemical shift changes only slightly. How-
ever, at mole ratios >1, where the number of free M ions in so-
lution has greatly diminished, the Li ions present begin to form
2
þ þ
It is well known that, most transition metals nuclei cannot be
accurately used in an NMR study of their macroacyclic complexes.
This is mainly because of their high quadruple moments and low
2þ
þ
þ
7
7
receptivities [34]. Therefore, in this work, we decided to employ Li
2
þ
NMR as a much sensitive probe [35e39] to investigate the complex
2
þ
2þ
2þ
1
2
þ
stability of Cd , Zn and Mn ions with (Sc.B. ), (Sc.B. ) and
2
(
Sc.B. ) in the mixture solutions used. In this case, known concen-
trations of each of the Li and M ions (i.e., 0.02 M) present in
solution are simultaneously titrated with each of Schiff bases
their 1:1 complexes with the added Schiff base ligands and, thus,
þ
2þ
the
dobs mole ratio behavior at mole ratios >1 shows a trend more or
þ
less similar to that observed in the case of titration of Li ion alone
with the Schiff base (see Fig. 1), and levels off at mole ratios >2
indicating the quantitative stability of 1:1 complexes of the Schiff
1
2
3
(
Sc.B. ), (Sc.B. ) and (Sc.B. ), while controlling the changes in the
7
chemical shift of Li nucleus in solution. The resulting plots of
variation of 7Li chemical shift in the presence of equimolar con-
bases with both Li and all three M ions studied.
þ
2þ
2
þ
þ
þ
centrations of M ions and Li (0.02 M) as a function of Sc.B$/Li
Another point which worth to notice in Figs. 1 and 2 is that, in all

A.A. Manesh, M.H. Zebarjadian / Journal of Molecular Structure 1199 (2020) 126965
5
Fig. 2. 7Li NMR chemical shifts as a function of the Sc.B. , Sc.B. and Sc.B.3 to metal ions mole ratios in (A) 100% AN, (B) 75% AN e 25% [(BMIM)(PF
1
2
)], (C) 50% AN e 50%
6
ꢁ
[(BMIM)(PF
6
)], (D) 75% AN e 25% [(BMIM)(PF
6
)] mixture solutions at 25.0 ± 0.1 C.
cases, the addition of nitrate salts of each of all ions to a solution
containing equimolar of LiClO causes some changes of about 0.2 to
about 1 ppm in Li NMR chemical shift of free Li ions at Sc.B$/Li
in this work, we used the same salt concentration of 0.02 M with
the same counter anions, in order to have a constant influence of
counter anion and salt concentration on all apparent stability
constants of the complexes of interest.
The results given in Table 1 clearly demonstrated that, in
100e0%, 75-25%, 50-50% and 25e75% acetonitrile-ion liquid
mixture solutions and in the cases of all three metal ions studied,
the resulting complexes with the Schiff bases change in the order of
4
7
þ
þ
mole ratio ¼ 0, as it as compared to the case of lithium salt alone.
This is most possibly due to increased ionic power of solution, as a
þ
result, in the measure of ion coupling of Li ion with the counter
anions extant. Chemical schift in Lithium ion in the presence of
2
þ
2þ
2þ
Mn ion respect to Cd and Zn is due to paramagnetic behavior
conduct of Mn2
þ
.
M-Sc.B. > M-Sc.B. > M-Sc.B. . This is due to the fact that the Schiff
1
2
3
1
2
3
The competitive complexation equilibria about the case of 1:1
bases (Sc.B. ), (Sc.B. ) and (Sc.B. ) differ in the size of their in chelate
rings in order that they can shape five, six and seven membered
rings, separately (see Scheme 3). As it is well documented in the
literature [42,43], the complexes forming a seven-membered
chelate ring are expected to be less stable than those forming a
five or six membered chelate rings.
þ
complexation between Schiff bases and Li proposed ions were
f
calculated using equation in Refs. [40,41]. The resulting log K values
are included in Table 1. The obvious apparent stability constants of
the complexes formed between the two Schiff bases and all metal
ions used in ion liquid - acetonitrile mixture solutions are expected
to significantly affected by both the concentrations of the salts used
and the nature of counter anions, due to variations in extent of ion
pairing between the metal ions and their counter anions. Therefore,
1
2
3
In fact, (Sc.B. ), (Sc.B. ) and (Sc.B. ) are a series of octadentate
8
) Schiff bases including two pyridine moiety groups at end po-
(N
sition. It has been shown that, the complexation ability of the Schiff
bases to metal ions can be considerably increased by attaching a
more rigid carbon chain in between the donor nitrogen groups
Table 1
1
2
3
[
44,45]. In the cases of Schiff bases (Sc.B. ), (Sc.B. ) and (Sc.B. ), the
2
þ
Stability constants of different M -Schiff base complexes in various acetonitrile-
ionic liquid mixtures at 25 C.
stability of the resulting metal ion complexes arises from the three
types nucleation centers: (1) the amine nitrogen's of carbon chain,
ꢁ
Complexions
f
Log K in Solvent Composition (w/w% ionic liquid in
(
2) the imine nitrogen's on Schiff bases and (3) the pyridine moiety
affects remainders, which control the strength of complexation by
-electron interactions, steric influences and by supplying three
acetonitrile)
0
25
50
75
p
1
2þ
2þ
2þ
2þ
2þ
2þ
2þ
2þ
(
(
(
(
(
(
(
(
(
MneSc.B. )
4.09 ± 0.04
4.40 ± 0.05
4.99 ± 0.08
3.94 ± 0.02
4.15 ± 0.04
4.47 ± 0.03
3.83 ± 0.08
3.91 ± 0.04
4.11 ± 0.05
3.72 ± 0.03
3.99 ± 0.04
4.62 ± 0.06
3.60 ± 0.04
3.72 ± 0.05
4.12 ± 0.04
3.33 ± 0.04
3.55 ± 0.02
3.70 ± 0.07
3.24 ± 0.04
3.62 ± 0.02
4.17 ± 0.07
3.22 ± 0.03
3.45 ± 0.06
3.76 ± 0.03
2.91 ± 0.05
3.24 ± 0.03
3.42 ± 0.08
2.81 ± 0.09
3.42 ± 0.05
3.82 ± 0.04
2.80 ± 0.04
3.24 ± 0.04
3.39 ± 0.05
2.54 ± 0.03
3.08 ± 0.06
3.21 ± 0.09
terminal nitrogen atoms [45]. Subsequently, the metal ion is sur-
rounded by the flexible carbonic 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 [46e49]. There-
1
ZneSc.B. )
1
CdeSc.B. )
2
MneSc.B. )
2
ZneSc.B. )
2
þ
2
fore, as is obvious from Table 1, in all cases the stability of M
(Sc.B. ) is more stable than M -(Sc.B. ) in the solvents.
-
CdeSc.B. )
3
1
2þ
2
MneSc.B. )
3
ZneSc.B. )
According to the information given in Table 1, it is immediately
obvious that for a given Schiff bases the stability of the resulting
3
2þ
CdeSc.B. )

6
A.A. Manesh, M.H. Zebarjadian / Journal of Molecular Structure 1199 (2020) 126965
Scheme 3. Purposed structures of M-Sc.B. octadentate complexes.
complex is dependent on the solvent mixture composition. In all
cases the stability increases significantly with decreasing amount of
ionic liquid in the solvent mixture. It has been well documented
that the solvating ability of the solvent, plays a role in different
8
reaction between two asymmetrical branched octadentate (N )
2
þ
2þ
2þ
Schiff base with the Cd , Zn and Mn metal ions having pyri-
1
2
3
dine moiety, (Sc.B. ), (Sc.B. ) and (Sc.B. ) in binary acetonitrile -
Ionic liquid ([BMIM][PF ]) mixtures were inspected by NMR spec-
6
7
complexation reactions [50e54]. Ionic liquid ([BMIM][PF
6
]) is a
troscopy. In all studied instances, the distinction of Li chemical
shift with the [Sc.B.]/[M] mole ratio showed the stability of 1:1
complexes in total solvents. While the synthesized ligands are
susceptible to moisture and unstable, the resulting complexes are
stable in solution and in a pure state for a long time. It was found
that, stability of the ending in complexes is powerfully dependent
on the quantity of Ionic liquid in the solution. In all cases, the sta-
bility decrease significantly with increasing the amount of Ionic
liquid in the solvent mixture. In the total solutions, the stability of
solvent of comparatively high solvating ability which can compete
with the metal ion for Schiff base. Thus, it is not unexpected that the
addition of acetonitrile as a low density solvent to ionic liquid will
increase the stability of the complex considerably.
The data given in Table 1 obviously indicate that, in a given
acetonitrileeion liquid mixture, the stability of the resulting com-
2
þ
2þ
2þ
plex changes in the order Mn <Zn <Cd . The complex stability
results from the superposition of several factors including: the
extent of interaction of donor groups of the nitrogen amine with
the cation, the consonance between the size of metal ion and the
satirical size of the Schiff base cavity formed, the extent of Schiff
base conformational changes as a consequence of complex stability,
soft-hard acid-base characters of the metal ion and the donating
groups of the Schiff base nitrogen's, desolvation of the Schiff base
and cation and solvation of the resulting complex, the two later
factors being strongly dependent on the nature of solvents used.
The size and cationic charge (the consequent charge density) be
able play a fundamental role in the complexation sequential pro-
cess since the viewpoints of ionic solvation and binding energy
with the nitrogen's giving atoms. By increasing charge density of a
cation, both its binding energy and salvation energy to the Schiff
base are expectedly increased, so that the increased binding energy
of the cation to the amine acts against its increased desolvation
energy. from the other point of view, since the change in configu-
ration of the chain Schiff base 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 deformations of Schiff base and
reduce the entropy of complexation.
2
þ
2þ
all complexes increases in the order Cd -Sc.B. > Zn -Sc.B. >
2
þ
1
Mn -Sc.B. The resulting complexes with the Schiff bases (Sc.B. ),
(Sc.B. ) and (Sc.B. ) vary in the order of M-Sc.B. > M-Sc.B. > M-
Sc.B. . This is because the Schiff bases are different in the size of the
2
3
1
2
3
chelate rings so that they can form five, six and seven membered
rings, respectively. As it is well substantiated in, the complexes
shaping a five-membered chelate ring are anticipated to be stable
more than those creating a six or seven membered chelate ring.
Acknowledgements
We are grateful to the Department of Chemistry of Payame Noor
University, Faculty of Chemistry of Farhangian and Bu Ali Sina
University, and Ministry of Science Research and Technology for the
financial support.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2019.126965.
2
þ
2þ
Meanwhile, the sequence of stability of the Zn
and Mn
2
þ
2þ
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