Synthesis, structural characterization and extraction studies of 17-, 18-, 19- and 20-membered N2O2-donor macrocyclic Schiff bases

Article abstract of DOI:10.1007/s10847-016-0621-4

Two new macrocyclic Schiff bases m3 and m4 (m3?=?1,5-diaza-2,4:7,8:17,18-tribenzo-9,16-dioxa-cyclononadeca-1,5-dien; m4?=?1,5-diaza-2,4:7,8:18,19- tribenzo-9,17-dioxa-cycloeicosa-1,5-dien) were synthesized by reactions of the appropriate dialdehyde and di

Full text of DOI:10.1007/s10847-016-0621-4

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-016-0621-4
ORIGINAL ARTICLE
Synthesis, structural characterization and extraction
studies of 17-, 18-, 19- and 20-membered N2O2-donor
macrocyclic Schiff bases
1
Tomislav Balic Brunislav Matasovic Berislav Markovic Anamarija Ster
1
1
1
•
ˇ
•
•
•
´
´
Marija Stivojevic Dubravka Matkovic-Calogovic
´
1
2
ˇ
ˇ
•
´
´
´
Received: 9 March 2016 / Accepted: 5 May 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Two new macrocyclic Schiff bases m3 and m4
(m3 = 1,5-diaza-2,4:7,8:17,18-tribenzo-9,16-dioxa-cy-
clononadeca-1,5-dien; m4 = 1,5-diaza-2,4:7,8:18,19-
tribenzo-9,17-dioxa-cycloeicosa-1,5-dien) were syn-
thesized by reactions of the appropriate dialdehyde and
diamine. The compounds were characterized by means
of FT-IR and NMR spectroscopy, TG/DSC and ele-
mental analysis. The crystal and molecular structures
were determined by the single crystal X-ray diffraction
method. Both molecules are N2O2-donor macrocyclic
Schiff bases with 19 and 20 atoms in their inner
macrocyclic rings and of a similar molecular structure.
A different crystal packing arrangement of the mole-
cules is caused by the differences in macrocycle pla-
percentage was determined for the Hg2? ion in the case
of the m2 macrocycle.
Keywords Schiff base macrocycles Á N2O2-donor Á Crystal
and molecular structure Á Metal ion extraction
Introduction
Environmental pollution is one of the major problems of
the modern society. Halogenated organic compounds,
especially those with more than one benzene ring, and
heavy metals [1] are major pollutants of air, water and soil.
It is, therefore, of the major scientific and engineering
interests to find various, specific ways of degrading [2–4]
or removing pollutants from the environment [5, 6]. In the
studies related to the macrocyclic compounds, the potential
environmental protection importance rises from their
structure, i.e. from their cavities which may selectively
extract the pollutants from the sample [7, 8]. In this case
the pollutants in question are heavy metals whose toxicity
is well known and documented [9].
narity.
The
extraction
experiments
of
the
aforementioned compounds were performed by using
metallic picrate salts, and simultaneously for the pre-
viously synthesized 17- and 18- membered macrocyclic
analogues (m2 and m1). The metal cation extractability
was determined for m1, m2, m3 and m4 by UV–Vis
spectrophotometry in order to find correlation of
structural features and the extraction capabilities of the
Schiff bases. The highest metal cation extraction
The chemistry of macrocycles has been studied for
almost five decades due to the pronounced affinity of these
compounds towards formation of unusually stable transition
metal complexes. This stability is defined in the literature as
the macrocyclic effect [10]. There are several factors that
can affect complexation abilities by influencing the overall
stability of the macrocyclic complex such as: macrocyclic
ring size, donor atom properties, ring substituents, metal ion
properties (ionic radius, overall hardness/softness) and some
others [11, 12]. Although some of these factors, especially
the concept of the ‘‘best fit’’ (ionic metals radius vs. inner
macrocyclic cavity size), are somewhat disputed by a part of
the scientific community, it is still very interesting to
Electronic supplementary material The online version of this
article (doi:10.1007/s10847-016-0621-4) contains supplementary
material, which is available to authorized users.
´
& Berislav Markovic
bmarkovi@kemija.unios.hr
1
Department of Chemistry, Josip Juraj Strossmayer University
of Osijek, Cara Hadrijana 8/A, 31000 Osijek, Croatia
2
Division of General and Inorganic Chemistry, Department of
Chemistry, Faculty of Science, University of Zagreb,
Horvatovac 102a, 10000 Zagreb, Croatia
123

J Incl Phenom Macrocycl Chem
investigate them in the context of potential applications.
Almost all of the aforementioned factors can be intentionally
tuned in order to achieve highly selective macrocyclic sys-
tems [11, 13]. The unusual stability and rational design of
macrocyclic systems can be utilized for construction of very
stable and selective macrocyclic ligands and therefore these
compounds have promising applications as chemical sensors
[14] and selective separating agents (e.g. for liquid–liquid
extractions) [15, 16].
in order to compare structural features of compounds with
metal ion selectivity and the extraction capabilities.
Experimental
Chemicals and apparatus
All commercially available chemicals were of reagent
grade. IR spectra were recorded on Shimadzu FTIR 8400S
spectrophotometer using a DRS 8000 attachment, in the
4000-400 cm-1 region. Approximately 3 mg of samples
were mixed with 100 mg of KBr (IR grade), placed in the
sample cup and FTIR data were collected using the diffuse
reflectance technique. Thermogravimetric analyses were
performed using a simultaneous TGA-DSC analyzer
(Mettler-Toledo TGA/DSC 1). The samples (*20–30 mg)
were placed in aluminum pans (100 lL) and heated in a
nitrogen atmosphere (200 mL min-1) up to 550 °C at a
rate of 10 °C min-1. The data collection and analysis was
performed using the program package STARe Software
10.0 [30]. C, H, N analyses were performed on a Perk-
Historically, there are two major groups of macrocycles
that have been extensively investigated for the last couple
of decades: all-oxygen (crown ethers) [17] and all-nitrogen
macrocycles introduced by several scientific groups [18,
19]. The oxa-aza macrocycles (N, O-donor) have been
introduced as a part of the crown ether studies (aza cryp-
tands) [20], after which they have become an integral part
of many studies [21, 22]. Until now, the most convenient
preparation route for the synthesis of oxa-aza macrocycles
was found to be a cyclocondenzation reaction of aldehyde
and amine that results in a formation of macrocyclic Schiff
bases [23, 24]. This preparation route usually does not
require employment of high-dilution technique or the
templating agents, and therefore these methods were
extensively used for syntheses of polydentate oxa-aza
macrocycles [22, 25]. The relatively simple preparation of
an oxa-aza macrocyclic Schiff base along with possibility
of the sophisticated design and multipurposeness, have
endorsed these compounds as attractive materials for
design of a novel chemical sensors, pharmaceuticals and
liquid crystals [26].
1
inElmer 2400 Series II CHNS/O system. The H NMR
spectra were recorded on an NMR (300 MHz) Bruker
instrument, using deuterated chloroform as solvent at the
ˇ
´
NMR Laboratory of the Ruder Boskovic Institute, Zagreb.
¯
The UV–Vis analyses were conducted using Shimadzu
Scientific UV–Vis spectrophotometer UV-1800 at 356 nm
which is an absorption wavelength of picrates used in
experiments.
The complexation ability of an oxa-aza Schiff base
macrocycle, and indirectly its application, is dependent upon
the previously mentioned factors. The additional factor that
can influence binding properties of the oxa-aza macrocycles
is conformational analysis (orientation of the potential donor
atoms, torsion angles associated with these atoms, distances
between donor atoms etc. [27]). Considering orientation of
the donor atoms, previous investigations [28, 29] have
shown that the most stable conformation of the oxa-aza
Schiff base macrocycles is endodentante for oxygen and
exodentante for nitrogen atoms. In addition, planarity of the
macrocyclic system (puckering amplitude) and conse-
quently, flexibility of a molecule can also influence the
metal–ligand interactions.
X-ray crystallography
The single-crystal X-ray diffraction data were collected at
room temperature (293 K) for the macrocyclic compounds
m3 and m4 on an Oxford Diffraction Xcalibur 3 CCD
diffractometer with graphite-monochromated Mo-Ka radi-
˚
ation (k = 0.71073 A) using x-scans. Suitable crystals of
the macrocyclic ligands were glued to a thin glass needle.
The data reduction was performed using the CrysAlis
software package [31]. The structures were solved using
SIR2004 program [32]. Refinement and analysis of the
structures was done using the programs integrated within
the WinGX system [33]. The refinement procedure was
performed by the full-matrix least-squares method based
on F2 against all reflections using SHELXL-97 [34]. All
non-hydrogen atoms were refined anisotropically and
hydrogen atoms in the structures were placed in calculated
positions and refined using the riding model. Geometrical
calculations were done using PLATON [35, 36] and the
structure drawings with ORTEP and MERCURY [37]
programs. The crystallographic data are summarized in
Table 1.
In our previous work [29] we have described the
molecular and crystal structures of 17- and 18- membered
macrocyclic Schiff bases prepared by a cyclocondenzation
reaction of dialdehydes and m-phenylenediamine (m2 and
m1). The present work describes synthesis and structural
characterization of novel 19- and 20-membered analogues
prepared by similar methods. The solvent extraction
properties of the newly synthesized (m3 and m4) and
previously reported compounds (m1 and m2) are presented
123

J Incl Phenom Macrocycl Chem
Table 1 Crystallographic data and structure refinement details for all compounds
Compound
m1
m2
m3
m4
CCD number
Formula
1039600
1039601
C24H22N2O2
370.44
1445992
C26H26N2O2
398.49
1445995
C27H28N2O2
412.51
C
25H24N2O2
Formula weight
Crystal system
Space group
T, (K)
384.46
Monoclinic
P 21/n
Monoclinic
P 21/c
Monoclinic
P 21/c
Orthorhombic
P nam
294(2)
15.493(1)
7.889(7)
16.783(1)
91.136(9)
2050.9(3)
4
294(2)
293(2)
293(2)
˚
a (A)
15.671(1)
7.4720(7)
33.315(2)
92.029(6)
3898.5(3)
8
24.322(1)
10.953(5)
16.463(8)
99.072(4)
4330.9(3)
8
8.0121(1)
15.5284(1)
18.920(2)
˚
b (A)
˚
c (A)
b/8
˚
V/A
3
2353.9(5)
4
Z
Dcalc (g cm-3
)
1.245
1.262
1.222
1.164
l (mm-1
)
0.079
0.081
0.077
0.073
F(000)
816
1568
1696
880
Total data
13174
26701
28886
11033
2776
Number of unique data
Number of parameters
Ra1, [Fo C 4r(Fo)]
wRb2
4009
7634
8491
262
536
596
145
0.0654
0.1192
1.063
0.0757
0.0714
0.0641
0.1120
1.019
0.1170
0.0958
Goodness of fit on F2, S
1.072
0.971
-3
)
˚
Max. and min. electron density (e A
0.114; -0.134
0.161; -0.138
0.126; -0.132
0.122; -0.119
a
R = R||Fo|-|Fc||/R|Fo|
wR = [ (Fo2-F2c)2/ w(Fo2)2]1/2
P
P
b
Extraction experiments
Syntheses
For the extraction studies, aqueous solutions of various
picrates were used. They were extracted by dichlor-
omethane solutions of macrocyclic compounds. Metallic
picrates solutions were prepared by mixing the aqueous
solution of picric acid with aqueous solutions of various
ionic salts of the respective metals. Both solutions, i.e.
picric acid and metallic ion solutions were prepared by
dissolving solid salts and acid in water in order to achieve
the same final concentration for every solution. Solutions
of Ni2?, Ca2?, Al3?, Mg2?, Cd2?, Zn2?, Ag?, Na?, K?,
Co2?, Hg2?, Ba2? and Cu2? were prepared for these
analyses. The final concentration of picrates prior to the
extraction study, which was used in the UV–Vis spec-
The syntheses of dialdehydes 1, 2 were previously reported
[38, 39]. Details regarding preparation and characterization
of macrocyclic compounds m1 and m2 were recently
reported by our group [29]. The preparation route for all
compounds is represented on Scheme 1.
Preparation of the dialdehydes 3 and 4
2-[6-(2-formylphenoxy)hexoxy]benzaldehyde (3) 10.5 mL
of salicylaldehyde (0.1 mol) and 13.9 g of K2CO3,
(0.1 mol) were mixed in 50 mL of dimethylformamide and
the mixture was heated to boiling temperature
(150–160 °C). 7.7 mL (0.05 mol) of 1,6-dibromohexane
were dissolved in 20 mL of dimethylformamide and slowly
added to the mixture of salicylaldehyde and K2CO3. The
mixture was refluxed at boiling temperature for 4 h and
stirred at room temperature for additional 4 h. After the
reaction was completed, approximately 500 mL of water
were added and the resulting suspension was left in the
refrigerator. After a few days the precipitate was filtered
and washed with deionized water. Yield: 10.92 g (70 %),
troscopy analyses, was approx. 8 9 10-5 mol dm-3
.
Extraction of the cations from the water layer to the
organic layer by macrocyclic compounds was done by
shaking the samples in the vials on the mechanical shaker
for three hours at room temperature (*22 °C). After
shaking, the samples were left to stabilise until the next day
when the spectrophotometric measurements were carried
out at 356 nm.
123

J Incl Phenom Macrocycl Chem
Scheme 1 Synthetic pathway for the synthesis of compounds 1–4 and m1–m4
mp 77 °C (DSC, from EtOH). IR mmax (cm-1): 2949(m),
2858(m), 1681(s), 1597(s), 1247(s), 1037(m), 760(s).
singlet, d doublet, t triplet and m multiplet. Chemical shifts
were given in respect to TMS (1H NMR, 0 ppm) signals.
Anal. Calc. for C25H24N2O2: C, 78.36; H, 6.58; N, 7.03;
Found: C, 78.25, H, 6.66; N, 6.92 %.
2-[7-(2-formylphenoxy)heptoxy]benzaldehyde (4) 1.05 mL
of salicylaldehyde (0.01 mol) and 1.4 g of K2CO3,
(0.01 mol) were mixed in 20 mL of dimethylformamide
and the mixture was heated to boiling temperature
(170–180 °C). 0.854 mL (0.005 mol) of 1,7-dibromhep-
tane were dissolved in 5 mL of dimethylformamide and
dropwise added to the mixture of salicylaldehyde and
K2CO3. The mixture was refluxed for 4 h and stirred at
room temperature for additional 15 h. After the reaction
was completed, approximately 250 mL of deionized water
were added and the resulting suspension was left in the
refrigerator. After a few days the precipitate was filtered
and washed with deionized water. Yield: 0.9 g (55 %), mp:
55 °C (DSC, from EtOH). IR mmax (cm-1): 2937(m),
2856(m), 1683(s), 1601(s), 1244(s), 1058(m), 756(s).
Preparation of m4 0.475 g of dialdehyde 4 (1.4 mmol)
were dissolved in 40 mL of absolute ethanol and 0.274 mL
(2 mmol) of triethylamine were added to this solution. The
solution was heated to reflux temperature and 0.243 g
(2.24 mmol) of m-phenylendiamine dissolved in 25 mL of
absolute ethanol were added dropwise. The resulting
solution was heated at reflux temperature for 3 h. After the
reaction was completed the suspension was left at room
temperature for 24 h. After cooling, a yellow needle-like
crystalline product was formed. This suspension was left at
room temperature for 2 weeks, during which time, larger
crystals gradually formed. Single crystals suitable for
X-ray diffraction experiments were obtained directly from
the reaction mixture. Yield: 0.46 g (80 %), mp 198 °C
(DSC). IR mmax (cm-1): 2931(m), 2854(m), 1622(m),
1599(s), 1578(s), 1456(s), 1261(s) 1238(s), 1043(m),
Preparation of macrocycles m3 and m4
1
Preparation of m3 0.658 g of dialdehyde 3 (2 mmol)
were dissolved in 50 mL of absolute ethanol and 0.280 mL
(2 mmol) of triethylamine were added to the solution. The
solution was heated to the reflux temperature (78 °C) and
0.270 g (2.5 mmol) of m-phenylendiamine dissolved in
25 mL of absolute ethanol were gradually added. The
resulting solution was heated at reflux temperature for 3 h.
After the reaction was completed the suspension was left in
a refrigerator for 2 weeks. During this time, larger crystals
gradually formed. Single crystals suitable for X-ray
diffraction experiments were obtained directly from the
reaction mixture. Yield: 0.6 g (75 %), mp 194 °C (DSC).
IR mmax (cm-1): 2937(m), 2894(m), 1618(m), 1597(s),
1572(s), 1454(s), 1263(s) 1240(s), 1043(m), 798(m),
785(m), 760(s). H NMR (CDCl3), d ppm: 8.98 (s, 2H,
H7), 8.16 (dd, 2H, H22), 1.42 (m, 4H, 18), 1.63 (m, 4H,
H19), 4.79 (m, 4H, H20), 4.04 (t, 4H, H21), 6.78 (s, 1H,
H4), 6.90 (d, 2H, H25), 7.02 (t, 2H, H27), 7.23 (m, 2H,
H6), 7.38 (m, 1H, H1), 7.43 (m, 2H, H26). A numbering
scheme was taken from Fig. S3b; the following abbrevia-
tions were used: s singlet, d doublet, t triplet and m mul-
tiplet. Chemical shifts were given in respect to TMS (1H
NMR, 0 ppm) signals. Anal. Calc. for C27H28N2O2: C,
78.61; H, 6.84; N, 6.79; found: C, 78.41, H, 6.95; N,
6.71 %.
Results and discussion
Syntheses
1
752(s). H NMR (CDCl3), d ppm: 9.19 (s, 2H, H7), 8.23
(dd, 2H, H21), 1.68 (m, 4H, H17/18), 1.87 (m, 4H, H16/
19), 4.07 (t, 4H, H15/20), 6.92 (d, 2H, H24), 7.06 (t, 2H,
H26), 7.17 (s, 1H, H4), 7.32 (m, 2H, H6), 7.41 (m, 1H,
H1), 7.46 (m, 2H, H25). The numbering scheme was taken
from Fig. S3a; the following abbreviations were used: s
Preparation route for synthesis of the Schiff base macro-
cycles m1–m4 is shown in Scheme 1. Reaction of a, x-
dibromo alkanes with salicylaldehyde resulted in a
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J Incl Phenom Macrocycl Chem
formation of dialdehydes 1–4 which were used for the
[1 ? 1] cyclocondensation reaction with m-phenylenedi-
amine. By employment of these reactions, a series of the
oxa-aza Schiff base macrocycles with different ring size
(17–20 membered) was obtained.
melting point of the compound (Fig. S7). The TG and DSC
results are summarized in Table 2.
Crystal structures
Single crystals of both ligands were obtained directly from
the reaction mixture. Selected bond lengths, valence angles
and torsion angles for the synthesized macrocycles are
listed in Table S1, and ORTEP drawings and crystal
packing of m3 and m4 are represented in Figs. 1 and 2,
respectively (molecular structure of the m3 conformer 2 is
represented in Fig. S8).
Spectroscopy
The IR spectra of the m3 and m4 are similar. The stretching
vibrations of the imine group appear at 1618 and
1622 cm-1 for m3 and m4, respectively. The aliphatic C–H
stretching vibrations are in the region 2937–2854 cm-1
.
The strongest vibrations in both spectra are at 1263 (m3)
and 1261 cm-1 (m4) and are assigned to stretching vibra-
tion of the C–O–C group. C–H bending vibration bands of
the meta and ortho substituted benzene rings appear in the
Both molecules are tetradentante N2O2 macrocyclic
Schiff bases with 19 (m3) and 20 (m4) atoms in the inner
macrocyclic ring. The relative inner macrocyclic hole size
˚
in m3 is in the interval of 2.449 to 3.037 A and 2.817 to
region 798– 760 cm-1
.
3.144 A for m4 (Table 3). The asymmetric unit of m3 is
˚
1
The H NMR spectra show singlet signals at 9.19 and
composed of two symmetrically independent conformers
(conformer 1 and 2). The two conformers are different
regarding the orientation of the aliphatic chain and dihedral
angles between the benzene rings (Table 4). In conformer
2, the aliphatic chain is disordered over two positions with
an occupancy factor of 0.58(2) for the major site. The
nitrogen atoms are exo-oriented and oxygen atoms endo-
oriented, in both molecules, as previously observed in other
N2O2 Schiff base macrocycles [28, 29]. The deviations
from planarity in the investigated macrocycles can be seen
8.98 ppm corresponding to the CH=N group of m3 and m4,
1
respectively. Other H NMR chemical shifts are in accor-
dance with the proposed molecular structures of the com-
pounds and are represented in Figs. S3 and S4.
Thermal analysis
The TG curve of m3 indicates one–step thermal decom-
position of the compound in the temperature interval
347–544 °C that is accompanied by a 75 % mass loss
(Fig. S5). The exothermic peak at 393 °C is related to the
thermal decomposition event. Prior to the decomposition
event, two distinct endothermic peaks are observed on the
DSC curve (first at 170 °C and second at 194 °C). In order
to further investigate these two thermal events, the scan
was repeated under modified conditions (5 °C min-1, up to
250 °C, blank curve subtracted). The corresponding DSC
curve displays three distinct thermal events at 167 °C
(DH = 13.07 kJ mol-1), 170 °C (DH = 24.48 kJ mol-1),
and 193 °C (DH = 16.29 kJ mol-1) (Fig. S6). This type of
DSC curve indicates possibility of polymorphism of the
compound m3. The three corresponding events can be
attributed to the melting of Form I at 167 °C, crystalliza-
tion of Form II (170 °C) and melting of Form II (193 °C).
According to the Burger–Ramberger heat of fusion rule
[40] the relation between these two forms is monotropic.
Unfortunately, we were not able to separate these forms by
crystallization from different solvents or solvent systems,
therefore no final conclusion regarding differences in
molecular or crystal structure can be done. The TG curve
of m4 is similar to m3, it reveals a continuous mass loss
between 315 and 514 °C accompanied by an exothermic
peak on the DSC curve at 410 °C. This thermal event can
be attributed to thermal decomposition of m4. The
endothermic peak on the DSC curve at 198 °C is the
˚
from analyses of puckering amplitudes for m3 (1.452(4) A)
˚
and m4 (1.709(2) A) [41], and by comparison of dihedral
angles between the benzene rings (Table 4). The analysis
indicates that there is a more pronounced deviation from
planarity in m4 then in m3. By comparison of these com-
pounds to the previously synthesized analogues (m1 and
m2), it can be seen that the relative inner macrocyclic hole
size increases with addition of a number of methylene
groups in the aliphatic chain, as expected. The hole size of
the macrocyclic ring in compound m4 is large enough to
3
˚
create voids in the crystal structure (*2353.9 A per unit
cell—7.1 % [36]). A similar effect was previously
3
observed for m1 (2050.9 A per unit cell—1.6 %). Pres-
˚
ence of porosity in the crystal structures of m4 and m1 is a
consequence of inefficient packing of molecules caused by
deviation from planarity of the macrocycles. As described
in our previous publication [29], molecules of planar
macrocyclic systems can pack in a manner that enables
formation of strong pÁÁÁp and C–HÁÁÁp interactions that are
not possible in the case of non-planar macrocyclic systems.
It is interesting to note that macrocycles with an even
number of atoms in the inner macrocyclic ring (m1 and
m4) have more pronounced deviations from planarity.
Considering the molecular features of the synthesized
compounds it can be expected that these molecules can
form coordination compounds with large transition metal
123

J Incl Phenom Macrocycl Chem
Table 2 TG and DSC data of
compounds
Compound
TG results
DSC results
Trange (°C)
Mass loss (%)
Tpeak (°C)
Thermal event
m1
336–500
68 (Decomposition)
272
407
405
170
194
393
198
410
Melting point
Decomposition
Decomposition
Melting point Form I
Melting point Form II
Decomposition
Melting point
m2
m3
340–500
347–544
55 (Decomposition)
75 (Decomposition)
m4
315–514
81 (Decomposition)
Decomposition
cations (i.e. mercury(II), silver(II)) and lanthanides due to
the large relative macrocyclic hole size. Considering donor
set atoms (N, O) there is a possibility of formation of
transition metal and alkali metal complexes. However, the
orientation of donor atoms could favour formation of exo–
coordinated compounds, and due to rigidity of the prepared
macrocycles it is questionable whether these compounds
can change conformation in presence of a metal cation.
Considering crystal packing, the macrocycle molecules
are linked by weak C–HÁÁÁp, C–HÁÁÁN and C–HÁÁÁO inter-
actions in m3 and by weak C–HÁÁÁp interactions in m4. In
m3 the molecules of conformer 1 are connected along the
crystallographic axis c by weak C–HÁÁÁN interactions
forming a staircase like motif. Molecules of conformer 2
are connected by C5A-H5AÁÁÁp (centroid of C20A-C26A)
interactions along axis c. The two independent conformers
are linked by a C-HÁÁÁO and C14A-H14CÁÁÁp (centroid of
C8–C13) interactions along the b-axis (Fig. 1b; Table S2).
In m4 no classical intermolecular interactions were
observed. A possible manner of molecular linkage is
through C12-H12BÁÁÁp (centroid of C6–C11) interactions
along the a-axis. Solvent accessible voids are formed due
to weak intermolecular interactions and a bowl-shape of
the molecule (Fig. 2b; Table S2).
metals can be especially advantageous for separation pur-
poses, although for removing pollutants such specificity is
not always necessary.
After conducting our experiments, as can be seen in
Table 5, the majority of tested metal ions could not be
extracted by our compounds and their extractability is less
than 10 %. All four macrocycles may extract around 10 %
of Al3? ions from their aqueous solutions which is some-
what odd since in almost all other cases (the exception
being Ba2? which also has similar extractability albeit
lesser) differences are much more emphasised. This can be
observed on the example of Cd2? 10 % of which is
extracted by m1, while m3 extracts almost nothing. Simi-
larly is for the Mg2? ion, only that the least extraction
occurs with m4. However, the most interesting results were
obtained for Zn2?and especially Hg2? ions. Mercury is
exceptionally well extracted by m2—more than 50 % of it
was extracted from the original solution. Extractions of
mercury by all four compounds are very high ranging from
30 % (m1) to the already mentioned 50 %. The only other
example in our study in which more than 30 % of a metal
ion has been extracted is that of Zn2? which was extracted
by m1 for as high as 31.3 %. The only other extraction
percentage worth mentioning is that for Ag? and Mg2?
which are both extracted for around 14 % by m1. Although
extractions under different conditions resulted in different
percentages, similar results were already observed for this
type of macrocycles [42]. Ligand m1 is a very interesting
example for comparison of structural features and extrac-
tion capabilities since it shows the highest percentage of
extraction for almost all metal cations (except mercury). A
possible explanation for this could be the molecular
structure of m1. As previously mentioned, m1 shows the
highest deviation from planarity (Tables 3, 4), therefore it
is possible that this ligand switches its conformation in the
presence of a metal cation or that the position of its donor
atoms is simply favourable in comparison to other ligands.
On the other hand, m1 extracts only 30 % of Hg2? ions.
Again, if we compare molecular structure to extraction
percentage of Hg2? ions, it can be noticed that the best
Metal extraction studies
Based on the very nature of macrocyclic compounds that
originates from the fact that they possess cavities in their
structure, these compounds have already been investigated
as potential extraction agents for metals in aqueous solu-
tions that simulate real environmental conditions. In the-
ory, if some of these compounds can extract metals from
their aqueous solutions then such a compound may be used
for various purposes where there is a need for separation of
one or at least a small number of cations out of a complex
mixture. Environmental samples are such an example. That
is especially true if such an extracting compound is not by
itself dangerous for the environment and/or can be easily
removed from it. To have a specific agent for one or a few
123

J Incl Phenom Macrocycl Chem
Fig. 2 a ORTEP plot of m4 with displacement ellipsoids of non-
hydrogen atoms drawn at the 50 % probability level. A, B and
C represent benzene rings. Symmetry transformation: (i) x, y,
-z ? 3/2). b Representation of crystal packing and voids (yellow)
in m4. The view is along the crystallographic axis a. C–HÁÁÁp
interactions denoted by green dashed lines. Cg2 is C6-C11 benzene
ring. (Color figure online)
Fig. 1 a ORTEP plot of m3 with displacement ellipsoids of non-
hydrogen atoms drawn at the 50 % probability level. A, B and
C represent benzene rings. b Representation of crystal packing in m3.
Blue coloured molecules represent conformer 1, and yellow con-
former 2. Green dashed lines represent C–HÁÁÁp interactions in
conformer 2, blue dashed lines C–HÁÁÁN interactions in conformer 1,
and violet dashed lines C–HÁÁÁO interactions between conformers.
Hydrogen atoms are omitted for clarity. (Color figure online)
tion percentage of the investigated macrocyclic systems
depends not only on the known complexation factors but
also on subtle molecular features of ligands, such as
planarity.
extractability is achieved for the most planar macrocycle
(m2). It can be seen that the extractability of Hg2? ions
decreases as planarity of macrocycle increases (in order
m2 [ m3 [ m4 [ m1). As can be easily seen, neither size
of the cation (Table 5) nor size of the cavity (Table 3) in
our compounds can be logically related to the percentage of
extraction, so the ‘‘best fit’’ concept is of no practical use
for these macrocycles (although in general, larger cations
are more extracted (Hg2? ion for example)). The afore-
mentioned discussion leads to the conclusion that extrac-
All of the investigated molecules have C=N groups,
therefore they are considered to be rigid molecules. In
addition, the presence of three benzene rings in each
molecule also contributes to the rigidity of these systems.
Some variations in flexibility of these compounds are
achieved by changing the number of methylene groups in
the aliphatic chain. Therefore, it is very unlikely for these
molecules to change the conformation in presence of a
metal cation and adapt the structure to the metal cation, as
some other macrocycles are capable of doing [43, 44]. In
123

J Incl Phenom Macrocycl Chem
Table 3 Factors affecting complexation for compounds m1–m4
Complexation factor/compound
m1
m2
m3 Conformer1/conformer2
m4
Number of atoms in inner macrocyclic ring
Donor atom set
18
17
19
20
N2O2
N2O2
N2O2
N2O2
˚
Relative inner macrocyclic hole size/A
2.43–2.61
N-exo
O-endo
2.35–2.44
N-exo
O-endo
2.45–3.04/2.45–3.07
N-exo
2.82–3.14
N-exo
O-endo
1.709(2)
Orientation of donor atoms
O-endo
˚
1.843(3) A
˚
1.299(4) A
Puckering amplitude
1.452(5)/1.350(9)
Table 4 Dihedral angles
between benzene rings in m1,
m2, m3 and m4 compounds
Compound
A/B
A/C
B/C
m1
m2 (conformer 1)
53.16(1)°
16.67(1)°
17.38(1)°
59.41(1)°
26.28(1)°
24.3(4)°
30.4(4)°
14.40(1)
3.41(1)
56.09(1)°
21.34(1)°
14.3(4)°
26.73(6)°
11.41(1)
20.49(1)
47.23(1)
m2 conformer 2 (major part of disordered C ring)
Conformer 2 (minor part of disordered C ring)
m3 (conformer 1)
m3 (conformer 2)
m4
6.13(1)
17.64(1)
47.23(1)
37.39(1)
Table 5 The extractability of various metal cations for compounds
m1, m2, m3 and m4
Conclusion
Extractability (%)#
m1 m2
0.2 2.8
˚
Metal ion (ionic radius in A)
Two new N2O2-donor macrocyclic Schiff bases m3 and m4
(m3 = 1,5-diaza-2,4:7,8:17,18-tribenzo-9,16-dioxa-cy-
clononadeca-1,5-dien; m4 = 1,5-diaza-2,4:7,8:18,19- tri-
benzo-9,17-dioxa-cycloeicosa-1,5-dien) were prepared and
characterized by spectroscopic, thermal and elemental
analysis methods. The molecular and crystal structure of
the prepared macrocycles was determined using single
crystal X-ray diffraction in order to correlate molecular
features of the compounds with their extraction capabili-
ties. The molecular structure of the compounds is rather
similar, with some differences in the planarity of the
macrocycles and the crystal packing arrangement. The
extraction experiments were performed using aqueous
solution of the metal picrates and organic solutions of the
ligands (newly synthesized compounds, m3 and m4, and
the previously synthesized 17- and 18–membered macro-
cyclic analogues, m1 and m2). Results of the extraction
experiments indicate that the extractability of metal cations
by the investigated macrocycles is quite similar. The best
extractability was achieved for the Hg2? ion (50.3 %) in
the case of the m2 macrocycle. By correlating the molec-
ular structure with extractability it was determined that best
results are accomplished by planar macrocyclic systems
(m2 and m3) in the case of the Hg2? ion, while other metal
ions are better extracted by non-planar macrocycles
(especially in the case of the Zn2? ion and the m1
macrocycle). From the above stated, it can be concluded
that extractability is not only dependent on known
m3
0.2
m4
5.9
Ni2? (0.72)
Ca2? (0.99)
Al3? (0.50)
Mg2? (0.65)
Cd2? (0.97)
Zn2? (0.74)
Ag? (1.26)
Na? (0.95)
K? (1.33)
Co2? (0.74)
Hg2? (1.10)
Ba2? (1.35)
Cu2? (0.72)
#
9.1
9.7
3.7
9.8
4.1
5.6
4.7
11.7
4.6
5.1
4.8
50.3
8.8
2.1
4.9
11.4
7.0
0.4
5.5
8.2
6.1
1.7
5.6
47.4
7.1
8.8
4.6
9.1
3.5
4.5
0.2
5.9
1.4
4.6
6.9
43.2
6.2
5.9
13.8
10.1
31.3
14.2
9.2
7.0
9.8
30.2
5.5
0.7
The extractability was calculated using equation:
(%) = [(A0-A)/A0] 9 100. A0 and A is absorbance in absence and
presence of the ligand, respectively
E
order to achieve higher extractability and selectivity of
metal cations from the solution it is necessary to increase
flexibility of the studied macrocycles, and this can be
done by reduction of the imino group. Although, imine
reduction is a simple organic reaction, until the present
day we were not able to conduct these reactions with
satisfactory results.
123

J Incl Phenom Macrocycl Chem
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