Syntheses, spectroscopic and structural characterizations and metal ion binding properties of Schiff bases derived from 4′-aminobenzo-15-crown-5

Article abstract of DOI:10.1016/j.poly.2011.10.034

A series of benzyloxybenzaldehyde derivatives (1-4) were synthesized by the reactions of 4-(bromomethyl)benzonitrile with 4-hydroxy-3-methoxybenzaldehyde (vanillin), 2-hydroxy-3-methoxybenzaldehyde (o-vanillin), 2-hydroxy-4- methoxybenzaldehyde and 2-hydr

Full text of DOI:10.1016/j.poly.2011.10.034

Polyhedron 31 (2012) 688–696
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses, spectroscopic and structural characterizations and metal ion binding
properties of Schiff bases derived from 40-aminobenzo-15-crown-5
b
c
Hüseyin Güler a, Zeliha Hayvalı a, , Hakan Dal , Tuncer Hökelek
⇑
a Department of Chemistry, Ankara University, Faculty of Science, 06100 Tandog˘an, Ankara, Turkey
b Department of Chemistry, Anadolu University, Faculty of Science, 26470 Yenibag˘lar, Eskißsehir, Turkey
c Department of Physics, Hacettepe University, 06800 Beytepe, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of benzyloxybenzaldehyde derivatives (1–4) were synthesized by the reactions of 4-(bromo-
methyl)benzonitrile with 4-hydroxy-3-methoxybenzaldehyde (vanillin), 2-hydroxy-3-methoxybenz-
aldehyde (o-vanillin), 2-hydroxy-4-methoxybenzaldehyde and 2-hydroxy-5-methoxybenzaldehyde.
Condensation reactions among the new benzyloxybenzaldehyde derivatives (1–4) with 40-amin-
obenzo-15-crown-5 yielded the new Schiff base compounds (5–8). Sodium complexes (5a–8a) and
potassium complexes (5b–8b) were prepared with NaClO4 and KI, respectively. All of these synthesized
compounds were characterized on the basis of FT-IR, 1H and 13C NMR, mass spectrometry and elemental
analyses data. The solid state structures of compounds 8 and 5a were determined by X-ray crystallogra-
phy. The extraction abilities of compounds 5–8 were also evaluated in CH2Cl2 by using several main
Received 18 August 2011
Accepted 24 October 2011
Available online 12 November 2011
Keywords:
Crown ethers
Crystal structure
Solvent extraction
NMR spectroscopy
Sodium complexes
Potassium complexes
group and transition metal picrates, such as Na+, K+, Pb2+, Cr3+, Ni2+, Cu2+ and Zn2+
.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
and 2-hydroxy-5-methoxybenzaldehyde. Condensation reactions
among the four benzyloxybenzaldehyde derivatives (1–4) with 40-
aminobenzo-15-crown-5 resulted in new crown ether imine com-
pounds (5–8) (Scheme 1), then sodium complexes (5a–8a) and
potassium complexes (5b–8b) were obtained using NaClO4 and KI,
respectively (Schemes 2 and 3). Compound structures have been
characterized by elemental analyses, FT-IR, 1H NMR, 13C NMR and
mass spectral data. The crystal structures of 8 and 5a were estab-
lished using X-ray crystallographic techniques. The complexation
The first series of crown ethers synthesized by Pedersen play
an important role in the development of supramolecular chemis-
try [1–4]. In the field of supramolecular chemistry, crown ethers
belong to the group of most popular host molecules since their
inclusion complexes provide a lot of practical applications [5,6].
Crown ethers containing hydrophobic exteriors are lipophilic
hosts, which can include cations, especially alkali and alkaline
earth metal ions, in their cavities via an ion–dipole interaction
[7–12]. This ability underlies the use of crown compounds as
selective ligands for metal cations, in particular, for extraction in
ion selective electrodes [13–15] and in photosensitive systems
[16–21]. Besides, the containing, binding and extraction abilities
of crown ethers are critically affected by several factors including
the symmetry of the crown ether, the size-fit relationship, the
type of the donor atoms, multiplicity and cooperative effects of
neighboring binding sites. Crown ether containing Schiff bases
are known to bind alkali metal cations in the crown ether cavity
in addition to the coordination of a transition metal center
through the NO donor atom [22–24].
abilities of the crown ether compounds 5–8 towards Na+, K+, Pb2+
,
Cr3+, Ni2+, Cu2+ and Zn2+ were measured by the solvent extraction
method.
2. Experimental
2.1. Reagents and techniques
4-Hydroxy-3-methoxybenzaldehyde (vanillin), 2-hydroxy-3-
methoxybenzaldehyde (o-vanillin), 2-hydroxy-4-methoxybenzal-
dehyde and 2-hydroxy-5-methoxybenzaldehyde were purchased
from Aldrich and used without further purification. Compound 3
was prepared by a modification of the reported procedure [25,26].
Solvents were dried and distilled before use according to the stan-
dard procedures. Melting points were measured on a Thomas-
Hoover apparatus using a capillary tube. 1H and 13C NMR spectra
were recorded on a Varian Mercury, High Performance Digital
FT-NMR (400 MHz) spectrometer (SiMe4 as an internal standard).
Chemical shifts for proton and carbon resonances are reported in
This study, reports the syntheses of new benzyloxybenzaldehyde
derivatives (1–4) by the reactions of 4-(bromomethyl)benzonitrile
with vanillin, o-vanillin, 2-hydroxy-4-methoxy-benzaldehyde
⇑
Corresponding author. Tel.: +90 312 2126720; fax: +90 312 2232395.
E-mail address: zhayvali@science.ankara.edu.tr (Z. Hayvalı).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.10.034

H. Güler et al. / Polyhedron 31 (2012) 688–696
689
CHO
H3CO
OH
NaOH
R
2
5
O
CHO
CHO
Br
3
1
6
DMF
( 1-4 )
+
4
NC
OCH
NC
3
+
O
H CO
3
Na
O
NH
2
O
MeOH
O
O
O
Comp.
( 1, 5 )
( 2, 6 )
4-OR 3-OCH3
2-OR 3-OCH3
R
O
2
5
O
O
N
O
O
3
1
6
( 3, 7 ) 2-OR 4-OCH3
( 4, 8 )
O
4
2-OR 5-OCH3
NC
H CO
3
( 5-8 )
Scheme 1. Structures of the aldehydes 1–4 and the crown ether Schiff bases 5–8.
R
Comp.
O
2
O
3
4-OR 3-OCH3
O
O
( 5a )
( 6a )
N
CH
Na+
1
6
O
2-OR 3-OCH3
2-OR 4-OCH3
4
( 7a )
( 8a )
NC
O
5
H3CO
2-OR 5-OCH3
ClO4
( 5a-8a )
Scheme 2. Structures of the sodium complexes 5a–8a.
ppm (d). IR spectra were obtained from a PEL-DATA spectrum 100
series spectrometer. Carbon, nitrogen and hydrogen analyses were
performed on a LECO CHNS-932 elemental analyzer. Mass spectro-
metric analyses were performed on a Waters 2695 Alliance ZQ
LC/MS spectrometer. UV–Vis spectra were recorded using a Unicam
UV2-100 series spectrometer.
2.3. Extraction studies
Sodium and potassium picrate were prepared according to the
methods suggested in the pertinent literature [31]. Transition metal
picrates were prepared by the stepwise addition of a 1 ꢀ 10ꢁ2
M
transition metal nitrate solution to a 1.17 ꢀ 10ꢁ4 M aqueous picric
acid solution, followed by vigorous shaking at room temperature
for 2 h. Then equal volumes(10 mL) of a CH2Cl2 solutionof thecrown
ether (1 ꢀ 10ꢁ3 M) and an aqueous alkali or transition metal picrate
(1.17 ꢀ 10ꢁ4 M) were introduced into an Erlenmeyer flask, which
was then stoppered and vigorously shaken with mechanical stirring
for 12 h at 25 °C in a water bath. Measurements were taken after 2 h
of shaking and subsequent 2 h of standing. After complete phase
separation, the concentration of the picrate ion in the aqueous phase
was determined spectrophotometrically (kmax = 356 nm). For each
metal:crown ether system, the absorbance measurements were re-
peated three times. Blank experiments showed that no picrate
extraction occurred in the absence of crown ether. The percentage
of extraction (E%) was determined based on the absorbance of the
picrate ion in the aqueous solutions. The extractabilities of the alkali
and the transition metal picrates into the organic phase (CH2Cl2)
were calculated by the expression [32–36]:
2.2. X-ray data collection and structure refinement
.Crystallographic data were recorded on a Bruker Kappa APEXII
CCD area-detector diffractometer using MoK
a
radiation
(k = 0.71073 Å) at T = 100(2) K. Absorption corrections by multi-scan
[27] were applied. Structures were solved by direct methods [28]
and refined by full-matrix least squares against F2 using all data
[28]. All non-H atoms were refined anisotropically. Only the H15 atom
(for compound 8) and H15, H61, H62, H71 and H72 atoms (for com-
pound 5a) were located in the difference syntheses and refined iso-
tropically. The remaining H atoms were positioned geometrically at
distances of 0.95 Å (CH), 0.98 Å (CH2) and 0.99 Å (CH3) (for compound
8) and 0.93 Å (CH), 0.97 Å (CH2) and 0.96 Å (CH3) (for compound 5a)
from the parent C atoms; a riding model was used during the refine-
ment process and the Uiso(H) values were constrained to be
1.2Ueq(carrier atom) for CH and CH2 and 1.5Ueq(carrier atom) for
CH3. Computer programs: SHELXS97 and SHELXL97 [28], ORTEP-3 [29].
Source of atomic scattering factor: International Table for X-ray
Crystallography vol. IV, 1974 [30].
E% ¼ 100 ꢀ ðA0 ꢁ AÞ=A0
where A0 is the absorbance in the absence of the crown ether and A
denotes the absorbance in the aqueous phase after extraction.

690
H. Güler et al. / Polyhedron 31 (2012) 688–696
R
O
2
5
O
O
N
CH
3
1
6
O
Comp.
( 5b )
( 6b )
4
O
NC
NC
O
4-OR 3-OCH3
2-OR 3-OCH3
OCH3
-
K+
I
( 7b ) 2-OR 4-OCH3
( 8b )
2-OR 5-OCH3
OCH3
O
O
O
O
O
CH
( 5b-8b )
Scheme 3. Structures of the potassium complexes 5b–8b.
N
O
R
Liquid–liquid extraction processes involving transition metal
picrates are usually characterized by an equilibrium that is de-
scribed by the following equation:
suitable for X-ray analysis were obtained by slow evaporation of
CH3OH at room temperature.
2.4.3. General methods for preparing NaClO4 complexes 5a–8a
Mnaqþ þ nðPicꢁÞaq þ mLorg¡½MðLÞ ðPicÞ ꢂ
m
n org
To
a solution of the corresponding ligand (5–8) (0.1 g,
0.18 mmol) in methanol (50 mL) was slowly added a solution of
NaClO4 (0.02 g, 0.18 mmol) in methanol (10 mL), with stirring for
1 h. The crude product was filtered and then left to dry in the open
air. Single crystals of compound 5a suitable for X-ray analysis were
obtained by slow evaporation of CH3OH at room temperature.
where the subscripts (aq) and (org) stand for aqueous and organic
phases (CH2Cl2), respectively. The overall extraction equilibrium
constant (Kex) can be expressed as:
½MðLÞmðPicÞnꢂorg
Kex
¼
n
m
nþ
½M
ꢂ ½Picꢂaq½Lꢂorg
aq
2.4.4. General methods for preparing KI complexes 5b–8b
To a solution of compounds 5–8 (0.2 g, 0.37 mmol) in CH3OH
(20 mL) was added a solution of KI (0.03 g, 0.18 mmol) in CH3OH
(10 mL), with stirring for 1 h. The formed yellow crystals were fil-
tered and then left to dry in the open air.
The distribution ratios for each cation, D, defined as the ratios
between the extracted amounts of the alkali and the transition me-
tal picrate (in organic phase) and the remaining metal picrate in
the aqueous phase, were computed using the expression:
½MðLÞmðPicÞnꢂorg
D ¼
nþ
3. Results and discussion
½M
ꢂ
aq
In conclusion, the equation which presents the relation be-
3.1. Syntheses
tween Kex and D is as follows:
The synthetic procedure for the preparation of the benzyloxy-
benzaldehydes 1–4 consisted of two steps. Firstly, vanillin,
o-vanillin, 2-hydroxy-4-methoxybenzaldehyde and 2-hydroxy-5-
methoxybenzaldehyde were reacted with NaOH in DMF to give
the corresponding sodium alkoxide. Secondly, the obtained sodium
salts were converted into the corresponding new benzyloxybenzal-
dehydes 1–4 by reacting with 4-(bromomethyl)benzonitrile. The
compounds 1–4 were successfully synthesized with yields in the
range 86–90% (Scheme 1). The substituted benzyloxybenzaldehy-
des 1–4 were allowed to react with 40-aminobenzo-15-crown-5
to afford corresponding Schiff bases 5–8. Sodium and potassium
complex formation was carried out by combining the respective
crown compound with an equimolar amount of the corresponding
metal salt. The sodium complexes (5a–8a) were prepared from the
substituted benzo-15-crown-5 with NaClO4 in methanol. The spec-
troscopic data indicated that the stoichiometry of the complex
formed between Na+:benzo-15-crown-5 is 1:1 (M:L). According
to the results of the elemental analysis data, the M:L ratio is 1:2
for the potassium complexes 5b–8b. Since the potassium ions with
benzo-15-crown-5 tend to form sandwich type complexes, it can
be suggested that the complexes with the ligands 5–8 are also of
the sandwich type, with the potassium cation coordinating the
ether oxygen atoms of, apparently, two neighboring ligand mole-
cules. Analytical physical data of the aldehydes (1–4), crown ether
Schiff bases (5–8), sodium complexes (5a–8a) and potassium com-
plexes (5b–8b) are presented in Table 1. All of the compounds
n
m
D ¼ Kex½Picꢂaq½Lꢂorg
2.4. Syntheses of the benzyloxybenzaldehydes 1–4, crown ether Schiff
bases 5–8, sodium complexes 5a–8a and potassium complexes 5b–8b
2.4.1. General methods for preparing the benzyloxybenzaldehydes 1–4
The corresponding vanillin or methoxysalicylaldehyde deriva-
tives (0.38 g, 2.55 mmol) were dissolved in DMF (20 mL). NaOH
(0.10 g, 2.55 mmol) was added in small amounts and the resulting
reaction mixture was refluxed for 1 h. Subsequently, 4-(bromo-
methyl)benzo-nitrile (0.5 g, 2.55 mmol) in DMF (20 mL) was added
dropwise to the reaction mixture. The resulting solution was re-
fluxed for 35 h, and the complete consumption of the starting
material was observed by TLC (silica, eluent: CHCl3). Water was
then added until the solution became cloudy. The white precipitate
was isolated and recrystallized from ethanol.
2.4.2. General methods for preparing the crown ether Schiff bases 5–8
To a solution of the corresponding aldehyde (1–4) (0.18 g,
0.7 mmol) in methanol (50 mL) was slowly added with continuous
stirring 40-aminobenzo-15-crown-5 (0.2 g, 0.7 mmol). The result-
ing solution was stirred under reflux for 3 h, and then the reaction
mixture was allowed to stand for 2 h at room temperature. The
crude product was recrystallized from CH3OH. Single crystals of 8

H. Güler et al. / Polyhedron 31 (2012) 688–696
691
were fully characterized by melting points, elemental analyses, FT-
IR, 1H and 13C NMR and mass spectrometry.
metrical stretching vibrations of
m
(CN) appear as intensive, strong
single bands at 2224, 2225, 2232 and 2225 cmꢁ1, respectively. Fur-
thermore, characteristic aldehyde
bonyl groups were observed at 1680, 1698, 1666 and 1682 cmꢁ1
respectively. In the imine compounds 5–8, relatively strong bands
attributable to (C@N), were detected at 1599, 1598, 1607 and
1619 cmꢁ1, respectively. The aliphatic
(C–H) vibrations exerted
by –OCH2 and –OCH3 groups for compounds 1–8 were observed
in the range 3078–2831 cmꢁ1, and the
as(C–O–C)aliph. and s(C–
m(C@O) vibrations for the car-
,
3.2. Mass spectra
m
The molecular ion peaks and fragments of the ligands and com-
plexes support the proposed structures. In the mass spectra of all
the aldehydes 1–4, the respective molecular ion peaks [M+H]+
were detected at m/z 268.7 (100%). The major fragments for com-
pounds 2–4 were obtained at m/z 286.0 (48.4%, 51.0% and 47.3%),
corresponding to [M+H2O]+ and [M+2H2O]+, respectively. The
molecular ion peak [M+H]+ for the free Schiff base 5 was detected
at m/z 533.2 (100%). The major fragments for 5 were obtained at m/
z 567.2 (58.6%), 301.1 (52.3%) and 284.1 (72.5%), corresponding to
[M+2H2O]+, [C16H13N2O2+2H2O] and [C14H18NO5+3H]+, respec-
tively. In the mass spectra of the sodium complexes 5a and 7a,
the dominant peak at m/z 555.8 (100%) corresponds to the ligand
plus sodium [M+Na]+. The major fragments for compound 5a were
detected at m/z 557.8 (25.9%), 533.8 (5.6%) and 306.7 (32.7%), cor-
responding to [M+Na+2H]+, [M+H]+ and [C14H21NO5+Na]+, respec-
tively. For the potassium complex 5b, the ligand plus potassium
peak was not observed, but important fragments were detected
at 533.6 (6.3%) and 284.3 (13.3%), corresponding to [M+H]+ (ligand
molecular ion peak) and [C14H18NO5+3H]+, respectively. The
molecular ion peaks and fragments of the ligands and complexes
support the proposed structures.
m
m
m
O–C)aliph. vibrations appear between 1193 and 1030 cmꢁ1. The
FT-IR spectra for compounds 5–8 exhibit a broad absorption in
the 3000–3500 cmꢁ1 region, indicating the presence of the water
molecules in the compounds. The anion ClO4ꢁ in the sodium com-
plexes (5a–8a) gives rise to IR absorption bands at 1081, 1095,
1084 and 1085 cmꢁ1 and 624, 626, 623 and 620 cmꢁ1, respectively.
These bands are attributable to the asymmetric
m
as(Cl–O) stretch-
as(O–Cl–O)
ing modes (1095–1081 cmꢁ1) and the asymmetric
m
bending modes (626–620 cmꢁ1), suggesting that both bonded
and uncoordinated free anions are present. The vibration modes
of the free ligand undergo substantial shifts and splittings in the
sodium (5a–8a) and potassium complexes (5b–8b), and these were
most distinct for the
between 1167 and 1030 cmꢁ1. Frequency shifts were found to be in
the range 20–70 cmꢁ1
m(C–O–C)aliph. absorptions of the ether bonds
.
3.4. 1H and 13C NMR spectra
The 1H NMR spectral data are summarized in Table 3. The 1H
NMR spectra of the aldehydes (1–4), crown ether imine com-
pounds (5–8), sodium complexes (5a–8a) and potassium com-
plexes (5b–8b) indicated singlets between 3.52 and 4.04 ppm,
and between 5.19 and 5.33 ppm, corresponding to –OCH3 and –
OCH2 protons, respectively. The singlets at 9.91, 10.32, 10.40 and
10.60 ppm belong to aldehyde protons (–CHO) for 1–4. The aro-
matic protons of the aldehydes, crown ether imine compounds, so-
dium complexes and potassium complexes appear as multiplets in
the ranges 6.49–7.89 (for 1–4), 6.48–8.13 (for 5–8), 6.40–7.97 (for
5a–8a) and 6.37–7.85 ppm (for 5b–8b), respectively. For the Schiff
bases 5–8, the characteristic azomethine protons (HC@N) were ob-
served as singlets at 8.39, 8.38, 8.88 and 8.89 ppm, respectively.
The 1H NMR spectra indicated that the substituted –OCH3 group
was effective for the imine proton shift because the imine proton
was seen at 8.38 ppm for 6, while this peak was detected
0.50 ppm upfield (d = 8.88 and 8.89 ppm) for compounds 7 and 8,
respectively. The imine protons for the sodium complexes were
approximately the same as the ligands imine protons, but for the
potassium complexes the chemical shifts for the imine protons
were detected between 0.03 and 0.29 ppm upfield. The crown
ether protons (–OCH2–CH2O–) appeared as multiplets between
3.78 and 4.23 ppm for compounds 5–8. In the 1H NMR spectra of
the sodium complexes (5a–8a), small downfield or upfield chemi-
cal shifts were detected for the crown ether protons (–OCH2–
CH2O–). These small shifts may be attributed to the differences
in the longer distances between Na+ and the five oxygen atoms
of the benzo-15-crown-5 [3,37]. The 1H NMR spectra of the potas-
sium complexes (5b–8b) were approximately the same as that of
the free crown ether Schiff bases, except that the imine protons,
the aromatic protons, the crown ether protons and methyl protons
were slightly shifted downfield.
3.3. FT-IR spectra
Selected IR data for the benzyloxybenzaldehydes (1–4), crown
ether Schiff bases (5–8), sodium complexes (5a–8a) and potassium
complexes (5b–8b) are listed in Table 2. For aldehydes 1–4, asym-
Table 1
Physical characterizations and analytical data for compounds 1–8, sodium complexes
5a–8a and potassium complexes 5b–8b.
Compounds Color
m.p.
(°C)
Yield
(%)
Found (Calcd.)
C
H
N
1
White
White
White
White
Gray
141
102
118
110
109
86
88
90
87
64
68
61
68
53
56
54
55
51
49
53
54
72.19
4.92
5.40
(5.24)
5.36
(5.24)
5.12
(5.24)
5.28
(5.24)
5.41
(5.26)
5.21
(5.26)
5.38
(5.26)
5.43
(5.26)
4.27
(4.28)
4.34
(4.28)
4.36
(4.28)
4.46
(4.28)
4.27
(4.55)
4.35
(4.55)
4.42
(4.55)
4.31
(71.90)
72.21
(4.90)
4.96
2
(71.90)
72.13
(4.90)
4.76
3
(71.90)
72.29
(4.90)
4.82
4
(71.90)
67.52
(4.90)
6.28
5
(67.66)
67. 74
(67.66)
67.54
(6.06)
6.32
(6.06)
6.13
6
Yellow 105
Gray 78
7
(67.66)
67.55
(6.06)
6.24
8
Yellow 102
Yellow 74
(67.66)
55.08
(6.06)
4.91
5a
6a
7a
8a
5b
6b
7b
8b
(55.01)
54.92
(4.92)
5.12
White
76
(55.01)
55.30
(4.92)
4.86
Yellow 98
(55.01)
55.40
(4.92)
4.79
White
161
(55.01)
58.30
(4.92)
5.57
The 13C NMR data of compounds 1–8, 8a, 6b and 8b were re-
corded in CDCl3 solution (Table 4). In all of the ligands (1–8), the
characteristic aliphatic –OCH3 carbons were detected at 56.11,
56.33, 55.73, 55.85, 56.10, 56.10, 55.57 and 55.90 ppm, respec-
tively. The aldehyde carbons (–CHO) of compounds 1–4 were ob-
served at 190.79, 189.96, 187.87 and 189.00 ppm, respectively.
The signals of the aromatic carbons were detected in the expected
Yellow 138
(58.53)
58.41
(5.24)
5.59
White
99
(58.53)
58.36
(5.24)
5.33
Yellow 129
Yellow 107
(58.53)
58.21
(5.24)
5.51
(58.53)
(5.24)
(4.55)

692
H. Güler et al. / Polyhedron 31 (2012) 688–696
Table 2
IR spectra of compounds 1–8; (w: weak; m: medium; s: strong).
ꢁ
Compounds
m(C–H)aliph.
m
(C–O–C)arom.
m(C–O–C)aliph.
m(CHO)
m
(C@N)
m
(C@C)
m
(CN)
m(ClO4 )
1
2
3
4
5
6
7
8
5a
6a
7a
8a
5b
6b
7b
8b
2971–2831(w)
2973–2843(w)
3078–2770(w)
3075–2862(w)
2945–2868(w)
2945–2869(m)
2902–2869(w)
2968–2871(w)
2912–2880(w)
2922–2877(w)
2936–2881(w)
2935–2869(w)
2906–2871(w)
2927–2871(w)
2916–2869(w)
2926–2870(w)
1178–1030(s)
1193–1066(s)
1177–1030(s)
1165–1045(s)
1123–1029(s)
1118–1030(s)
1128–1035(s)
1124–1046(s)
1172–1032(s)
1173–1043(s)
1158–1040(s)
1154–1046(s)
1176–1030(s)
1188–1045(s)
1167–1038(s)
1168–1045(s)
1680(s)
1698(s)
1666(s)
1682(s)
1589–1519(s)
1584(m)
1607(s)
1615(m)
1512(s)
1511(s)
1511(s)
1506(m)
1512(s)
1510(m)
1512(m)
1496(s)
1510(s)
1508(s)
1510(s)
1506(s)
2224(m)
2225(m)
2232(m)
2225(m)
2230(m)
2229(m)
2227(w)
2227(w)
2230(m)
2228(w)
2230(w)
2224(w)
2228(w)
2229(w)
2230(w)
2228(w)
1231(s)
1232(s)
1258(s)
1214(s)
1234(m)
1220(m)
1223(m)
1220(s)
1225(s)
1220(s)
1226(s)
1220(s)
1599(m)
1598(m)
1607(m)
1619(w)
1596(w)
1602(w)
1609(m)
1622(m)
1589(m)
1621(m)
1607(m)
1615(m)
1081s; 624(m)
1095s; 626(m)
1084s; 623(m)
1084s; 620(m)
Table 3
1H NMR spectral data. Chemical shifts (d) are reported in ppm, (in CDCl3) (s: singlet, m: multiplet).
Compoundss
–OCH3
–OCH2
–CHO
HC@N
–OCH2CH2O–
Ar–H
1
2
3
4
5
6
7
8
5a
6a
7a
8a
5b
6b
7b
8b
3.96 (s,3H)
3.92 (s,3H)
3.87 (s,3H)
3.86 (s,3H)
4.02 (s,3H)
4.04 (s,3H)
3.98 (s,3H)
4.00 (s,3H)
4.03 (s,3H)
3.52 (s,3H)
3.84 (s,3H)
3.71 (s,3H)
3.97 (s,6H)
4.01 (s,6H)
3.95 (s,6H)
3.89 (s,6H)
5.31 (s,2H)
5.22 (s,2H)
5.25 (s,2H)
5.23 (s,2H)
5.28 (s,2H)
5.30 (s,2H)
5.23 (s,2H)
5.22 (s,2H)
5.31 (s,2H)
5.19 (s,2H)
5.31 (s,2H)
5.23 (s,2H)
5.33 (s,4H)
5.21 (s,4H)
5.28 (s,4H)
5.23 (s,4H)
9.91 (s,1H)
10.32 (s,1H)
10.40 (s,1H)
10.60 (s,1H)
7.73–6.95 (m,7H)
7.69–7.18 (m,7H)
7.89–6.49 (m,7H)
7.78–6.96 (m,7H)
7.82–6.80 (m,10H)
7.70–6.78 (m,10H)
8.13–6.48 (m,10H)
7.71–6.76 (m,10H)
7.73–6.92 (m,10H)
7.72–6.63 (m,10H)
7.97–6.54 (m,10H)
7.78–6.40 (m,10H)
7.83–6.37 (m,20H)
7.78–6.56 (m,20H)
7.85–6.45 (m,20H)
7.76–6.74 (m,20H)
8.39 (s,1H)
8.38 (s,1H)
8.88 (s,1H)
8.89 (s,1H)
8.42 (s,1H)
8.59 (s,1H)
8.86 (s,1H)
8.86 (s,1H)
8.63 (s,2H)
8.67 (s,2H)
8.94 (s,2H)
8.92 (s,2H)
4.22–3.81 (m,8H)
4.23–3.80 (m,8H)
4.19–3.78 (m,8H)
4.21–3.81 (m,8H)
4.29–3.74 (m,8H)
4.22–3.74 (m,8H)
4.27–3.29 (m,8H)
4.23–3.53 (m,8H)
3.94–3.75 (m,16H)
3.98–3.69 (m,16H)
4.06–3.72 (m,16H)
4.02–3.68 (m,16H)
Table 4
13C NMR spectral data. Chemical shifts (d) are reported in ppm (in CDCl3).
Compounds –OCH3
–OCH2
69.82
–CHO
HC@N
–CN
–OCH2CH2O–
Ar–CH
1
2
3
4
5
56.11
56.33
55.73
55.85
56.10
190.79
118.55
118.87
119.49
118.50
152.88, 150.15, 141.47, 132.58, 130.89,
127.46, 126.35, 112.47, 112.12, 109.64
153.00, 150.67, 142.24, 132.62, 130.17,
128.59, 124.97, 119.82, 118.29, 112.29
166.13, 161.91, 141.32, 132.72, 131.33,
127.45, 119.37, 112.21, 106.27, 99.43
155.02, 154.49, 141.67, 132.59, 127.52,
125.60, 123.38, 114.76, 112.17, 110.93
150.23, 150.06, 149.63, 147.65, 145.94,
142.11, 132.49 130.71, 127.48, 123.74, 114.66,
113.06, 112.77, 111.87, 109.54, 107.88
150.23, 150.06, 149.63, 147.65, 145.94,
142.11, 132.49 130.71, 127.48, 123.74, 114.66,
113.06, 112.77, 111.87, 109.54, 107.88
159.17, 153.97, 149.59, 147.42, 146.88,
141.88, 132.57, 129.25, 127.50, 118.70,
114.77, 112.52, 112.01, 108.26, 106.30, 99.59
152.53, 149.61, 147.86, 142.28, 132.58,
132.52, 127.56, 126.03, 123.38, 119.66,
114.75, 114.59, 112.78, 11.95, 110.57, 108.31
151.85, 147.36, 145.36, 141.81, 131.71,
127.16, 124.80, 122.29, 117.90, 116.16,
114.47, 113.80, 112.67, 110.25, 107.26, 100.58
154.99, 152.61, 148.13, 142.64, 132.40,
132.24, 130.02, 128.66, 128.33, 124.82,
124.75, 119.06, 118.13, 115.01, 11.72, 107.42
154.61, 154.39, 152.63, 146.08, 142.37,
132.58, 127.72, 125.56, 123.38, 119.80,
114.78, 113.81, 112.12, 111.79, 110.89, 107.27
75.13
68.30
70.15
71.14
189.96
187.87
189.00
158.05 118.66 71.11–68.94
158.04 118.66 71.11–68.95
163.42 118.56 71.09–68.92
154.51 118.58 71.09–68.98
153.86 118.74 77.82–66.01
168.62 118.70 77.29–67.01
166.61 118.61 77.29–67.43
6
56.10
55.57
55.90
55.02
55.98
55.90
71.15
71.12
71.12
69.49
74.75
70.50
7
8
8a
6b
8b

H. Güler et al. / Polyhedron 31 (2012) 688–696
693
region, and were equal to the number in the proposed structures of
compounds 1–8. As expected, the imine carbons (HC@N) of com-
pounds 5–8 were observed at 158.05, 158.04, 163.42 and
154.51 ppm, respectively. The signals of the eight crown ether –
OCH2 carbons were observed between at 68.92–71.11 ppm, as
expected. In the 13C NMR spectra of complexes 8a, 6b and 8b up-
field chemical shifts were detected for the crown ether carbons
(–OCH2–CH2O–). In addition, the spectra indicated the characteris-
tic signals for the –OCH3, –OCH2, –HC@N, –CN and aromatic ring
carbons.
For compounds 5–8, the calculated extraction equilibrium con-
stants log(Kex) and distribution ratio (D) values are presented in
Tables 6 and 7, respectively. The extraction constants for the Cr3+
ion are higher than for the other metal ions for all the crown ether
compounds (5–8), and the extraction constants were calculated as
16.36, 16.22, 16.30 and 16.08, respectively. These results indicate
that the hard metal ion is very effective with regards to extraction
efficiency. Consequently, the high extraction constant values may
result from the stabilities of the formed complexes.
3.6. Description of the structures of compounds 8 and 5a
3.5. Extraction studies using UV–Vis spectrophotometry
Experimental data for compounds 8 and 5a are listed in Table 8,
and selected bond lengths and angles, and hydrogen-bond geome-
tries are given in Tables 9 and 10, respectively. The X-ray structural
determinations of compounds 8 and 5a confirm the assignments of
their structures from spectroscopic data. The molecular structures,
along with the atom-numbering schemes, are depicted in Figs. 2
and 3, respectively. Compound 8 (Fig. 2) consists of a benzo-15-
crown-5 ring and side-arm, which contains an imine group. The
macrocyclic ring contains five etheral-O atoms and the ligand cav-
ity plays an important role in metal–ion selectivity. In the 15-
membered crown ring, atoms O1, O2, O3 and O4 are almost planar,
with a maximum deviation of ꢁ0.015(2) Å for atom O3, and the
atom O5 is oriented in an axial position. Atom O5 is ꢁ1.110 (2) Å
away from the best least-squares plane. The relative macrocyclic
inner-hole size, estimated as being twice the mean distance of
the donor atoms from their centroid, is approximately 1.59 Å, using
the modified covalent radius (0.76 Å) of the O3sp atoms, as demon-
strated in the literature [41–43]. The polyhedral volume of the 15-
membered crown ring is calculated as 20.57(5) Å3. The C@N imine
bond length [N1–C15 1.277(3) Å] and C@N–C bond angle [C1–N1–
C15 117.3(2)°] have smaller values than the corresponding ones
[1.287(2) Å and 121.7(1)°] in 4-{[(1E)(2-hydroxyphenyl)methyli-
dene]amino}-1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazole-3-
one [44]. The UCN torsion angle (C1–N1–C15–C16) is 176.9(2)°,
which shows that the configuration about the N1–C15 bond is anti
(1E). The rings A (C1–C6), B (C16–C21) and C (C24–C29) are, of
course planar, and they are oriented at dihedral angles of
A/B = 55.45(8)°, A/C = 47.59(8)° and B/C = 23.62(9)°. There is an
intramolecular C–Hꢃ ꢃ ꢃO hydrogen bond (Table 8) in the 15-mem-
bered crown ring. In the crystal structure, intermolecular
C–Hꢃ ꢃ ꢃO hydrogen bonds link the molecules into dimers. There also
Extraction is a simple and useful method for evaluating the
complexation abilities of the crown ethers 5–8 with alkali and
transition metal cations. The results, expressed as percentage of
cation extracted (E%), are presented in Table 5 and are shown
graphically in Fig. 1. The main group metal cations (Na+, K+ and
Pb2+) and transition metal cations (Cu2+, Ni2+, Zn2+ and Cr3+) com-
plexation abilities with the crown ethers (5–8) were assessed by
the solvent extraction of aqueous alkali and transition metal pi-
crates with CH2Cl2 solution. The data in Table 5 indicates that com-
pounds 5–8 show the highest extractabilities towards Na+, K+ and
Cr3+ when compared to the other cations. On the other hand, the
levels of extraction for Ni2+, Zn2+, Pb2+ and Cu2+ were low for all
of the compounds (5–8). The extraction trends observed for the
crown compounds 5–8 consisted of approximately similar extrac-
tion profiles. As can be seen in Table 5, compounds 5–7 have higher
selectivities for K+ (39.3%, 54.5% and 48.4%, respectively), but com-
pound 8 had higher selectivity for Na+ (39.8%).
It can be concluded that the extractability results of the ligands
are different. According to the results of the present study, the
match between the alkali and transition metal cations and ben-
zo-15-crown-5 cavity dimensions do not seem to be an important
factor in cation selectivity. The complexation abilities of the ligand
with various alkali, main group and transition metal ions may de-
pend on the flexibility of the macrocyclic ring substituents which
cause changes of the conformation and the flexibility [9]. On the
other hand, the hard and soft acids and bases principle seems to
be an important factor in selectivity for some compounds [38]. Be-
cause all the Schiff bases 5–8 contain hard oxygen donor atoms,
they showed a preference for Cr3+ (especially for compound 5).
Consequently, for the observed extraction selectivity, no simple
explanation is apparent from these results, and hence other factors
involving the hosts and guests must be considered. Different
substituted groups and side arms of the crown ether rings lead
to different arrangements of the donor atoms in the crown ethers,
and also the metal cations have different geometrical requirements
and different extraction ranges [39,40].
exists a weak C–Hꢃ ꢃ ꢃ
p interaction (Table 10).
Compound 5a (Fig. 3) consists of a benzo-15-crown-5 ring and a
side-arm, whichcontains an imine group. The cavity of the host mac-
rocyclic ring containing five etheral-O atoms is occupied by the
guest Na+ ion. The two water molecules are also coordinated to the
Na+ ion. Compound 5a also contains a perchlorate anion. In the 15-
membered crown ring, atoms O1, O2, O3 and O5 are almost planar,
with a maximum deviation of 0.006(4) Å for atom O1, and atoms O4
and O7 are oriented in axial positions. Atoms Na1, O4, O6 and O7 are
ꢁ0.790(2), 1.395(4), ꢁ1.804(4) and ꢁ3.032(6) Å away from the best
least-squares plane, respectively. The relative macrocyclic inner-
hole size, estimated as being twice the mean distance of the donor
atoms from their centroid, is approximately 1.49 Å, using the mod-
ified covalent radius (0.76 Å) of the O3sp atoms as demonstrated in
the literature [41–43]. The polyhedral volume of the 15-membered
crown ring is calculated as 20.90(15) Å3. The C@N imine bond length
[N1–C15 1.282(7) Å] and C@N–C bond angle [C1–N1–C15 118.8(5)°]
have smaller values than the corresponding ones [1.287(2) Å and
121.7(1)°] in 4-{[(1E)(2-hydroxyphenyl)methylidene]amino}-1,5-
dimethyl-2-phenyl-1,2-dihydro-3H-pyrazole-3-one [44]. The UCN
torsion angle (C1–N1–C15–C16) is 179.6(5)°, which shows that the
configuration about the N1–C15 bond is anti (1E). The rings A (C1–
C6), B (C16–C21) and C (C24–C29) are, of course planar, and they
Table 5
Alkali and transition metal picrate extractions for compounds 5–8 into the dichlo-
romethane phase.a
Metal ion
Extractability (%)b
5
6
7
8
Na+
K+
19.5 0.8
39.3 0.7
1.2 0.1
3.4 0.9
2.7 0.7
6.3 0.1
30.6 0.6
29.3 0.7
54.5 0.1
2.4 0.3
1.3 0.1
8.3 0.3
5.1 0.9
24.8 0.8
40.5 0.5
48.4 0.6
1.6 0.2
4.8 0.1
13.4 0.5
7.7 0.2
28.0 0.1
39.8 0.6
35.3 0.3
5.2 0.8
2.6 0.2
7.8 0.7
5.7 0.9
18.6 0.6
Ni2+
Zn2+
Pb2+
Cu2+
Cr3+
a
Temperature 25 1 °C; aqueous phase (10 mL); [picꢁ] = 1.17 ꢀ 10ꢁ4 M, organic
phase (10 mL CH2Cl2); [L] = 1 ꢀ 10ꢁ3 M. [M+] picrate; 1.17 ꢀ 10ꢁ4 M.
b
Average for three samples.

694
H. Güler et al. / Polyhedron 31 (2012) 688–696
Fig. 1. Percentage extractions (E%) of different metal picrates [M+] picrate; 1.17 ꢀ 10ꢁ4 M into CH2Cl2 at 25 1 °C.
Table 6
Table 8
The extraction constants (log Kex) for compounds 5–8.
Crystallographic data, details of data collection and structure refinement parameters
for compounds 8 and 5a.
a
Compounds
Extraction equilibrium constant (log Kex)
8
5a
3+
Na+
K+
Cr
Ni2+
Cu2+
Zn2+
Pb2+
Empirical formula
Color/shape
Formula weight
T (K)
C
30H32N2O7
C30H36ClN2NaO13
yellow/block
691.05
5
6
7
8
8.05
8.16
8.31
8.30
12.63
12.74
12.69
12.63
16.36
16.22
16.30
16.08
11.81
11.83
11.82
11.87
11.88
11.86
11.89
11.87
11.84
11.81
11.86
11.84
11.93
11.91
11.98
11.90
yellow/block
532.58
100(2)
Radiation used, graphite monochr.
MoKa (k = 0.71073 Å)
a
Crystal system
Space group
a (Å)
b (Å)
c (Å)
orthorhombic
Pbca
monoclinic
P21/n
Extraction equilibrium constants were calculated taking into consideration the
benzo-15-crown-5 cavity and metal ion diameter (complex compositions (L:M) are
1:1 for Na+, Cr3+, Ni2+, Cu2+, Zn2+ and Pb2+, only the K+ complex composition is 2:1)
[39,40].
20.5815(5)
7.9405(2)
32.8557(9)
90
5369.5(2)
8
8.9798(5)
19.8067(8)
18.7290(8)
101.323(5)
3266.3(3)
4
b (°)
V (Å3)
Z
Table 7
Absorption coefficient (mmꢁ1
)
0.094
1.318
0.199
1.405
The distribution constants (D) for compounds 5–8.
Dcalc (Mg mꢁ3
)
Compounds
Distribution constant (D)
Maximum crystal dimension (mm) 0.22 ꢀ 0.22 ꢀ 0.35 0.14 ꢀ 0.23 ꢀ 0.26
3+
h (Maximum) (°)
Reflections measured
Range of h, k, l
28.45
6762
28.22
7954
ꢁ11 < h < 11
ꢁ26 < k < 21
ꢁ21 < l < 24
2595
Na+
K+
Cr
Ni2+
Cu2+
Zn2+
Pb2+
5
6
7
8
0.24
0.41
0.68
0.66
0.65
1.20
0.94
0.55
0.44
0.33
0.39
0.23
0.01
0.02
0.02
0.05
0.07
0.05
0.08
0.06
0.04
0.01
0.05
0.04
0.03
0.09
0.15
0.08
ꢁ27 < h < 25
ꢁ10 < k < 10
ꢁ36 < l < 44
2684
Lorentz-polarization
Direct methods
No of reflection with I > 2
Corrections applied
Structure solution
Treatment of H atoms
No. of parameters var.
Goodness-of-fit (GOF)
R = ||Fo| ꢁ |Fc||/|Fo|
Rw
r(I)
Geom. Calc. and Diff. Map.
358
443
are oriented at dihedral angles of A/B = 46.3(2)°, A/C = 12.7(2)° and
B/C = 33.7(2)°. In the crystal structure, intermolecular O–Hꢃ ꢃ ꢃN and
C–Hꢃ ꢃ ꢃO hydrogen bonds link the molecules into a three-dimen-
sional network, and the perchlorate molecules fill in the cavities be-
0.958
0.0532
0.1000
0.298
ꢁ0.312
0.975
0.0898
0.1934
1.100
ꢁ0.446
(
(
D
D
/
/
q
q
) maximum (e Aꢁ3
) minimum (e Aꢁ3
)
)
tween the molecules. The
pꢃ ꢃ ꢃp contacts between the benzene rings
Cg1ꢃ ꢃ ꢃCg3i and Cg2ꢃ ꢃ ꢃCg2ii [symmetry codes: (i) ꢁx, ꢁy, ꢁz, (ii) 1 ꢁ x,
ꢁy, ꢁz, where Cg1, Cg2 and Cg3 are centroids of the rings A(C1–C6), B
(C16-C21) and C (C24-C29), respectively] may further stabilize the
structure, with centroid–centroid distances of 3.769(3) and
The calculated relative macrocyclic inner hole sizes are 1.59 Å
(for compound 8) and 1.49 Å (for compound 5a). To make a com-
parison, the corresponding macrocyclic inner-hole size for unsub-
stituted 15-crown-5 is estimated to be 1.70–2.20 Å [45,46], while
3.876(3) Å, respectively. There also exists a weak C–Hꢃ ꢃ ꢃ
p interac-
tion (Table 10).

H. Güler et al. / Polyhedron 31 (2012) 688–696
695
Table 9
Selected bond lengths (Å) and angles (°) for compounds 8 and 5a.
8
5a
O1–C3
O1–C7
O2–C8
O2–C9
O3–C10
O3–C11
O4–C12
O4–C13
O5–C4
1.361(3)
1.439(3)
1.422(3)
1.440(3)
1.420(3)
1.421(3)
1.429(3)
1.423(3)
1.372(3)
1.435(3)
1.419(3)
1.277(3)
1.368(6)
1.437(6)
1.425(6)
1.429(6)
1.431(6)
1.424(7)
1.432(6)
1.424(6)
1.401(6)
1.438(6)
1.423(6)
1.282(7)
2.455(4)
2.550(4)
2.447(4)
2.473(4)
2.659(4)
2.388(5)
2.423(6)
O5–C14
N1–C1
N1–C15
Na1–O1
Na1–O2
Na1–O3
Na1–O4
Na1–O5
Na1–O6
Na1–O7
Fig. 2. An ORTEP-3 [29] drawing of compound 8 with the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level.
C3–O1–C7
C8–O2–C9
C10–O3–C11
C12–O4–C13
C4–O5–C14
C1–N1–C15
O1–C3–C4
O5–C4–C3
118.83(19)
111.62(19)
111.0(2)
114.00(19)
117.70(19)
117.3(2)
118.2(4)
112.0(4)
113.5(4)
111.8(4)
111.3(4)
118.8(5)
115.2(5)
118.4(5)
114.7(2)
116.0(2)
Table 10
Hydrogen-bond geometries (Å, °) for compounds 8 and 5a.
D–Hꢃ ꢃ ꢃA
D–H
Hꢃ ꢃ ꢃA
Dꢃ ꢃ ꢃA
D–Hꢃ ꢃ ꢃA
8
C6–H6ꢃ ꢃ ꢃO7i
0.95
0.99
0.99
0.95
0.95
2.39
2.52
2.42
2.44
2.81
3.315(3)
3.185(3)
3.350(3)
3.154(3)
3.724(3)
164
124
156
131
162
C8–H8aꢃ ꢃ ꢃO3
C9–H9aꢃ ꢃ ꢃO4ii
C26–H26ꢃ ꢃ ꢃO1i
C5–H5ꢃ ꢃ ꢃCg3iii
5a
O6–H61ꢃ ꢃ ꢃN2iv
O6–H62ꢃ ꢃ ꢃN1v
C6–H6ꢃ ꢃ ꢃO6vi
1.00(4)
0.94(6)
0.93
0.97
0.97
1.98(5)
2.10(6)
2.57
2.35
2.64
2.958(7)
2.989(7)
3.345(7)
3.222(7)
3.506(6)
168(5)
156(5)
141
150
149
C7–H7Aꢃ ꢃ ꢃO8vii
C14–H14bꢃ ꢃ ꢃCg3vi
[Cg3 is the centroid of the ring C (C24–C29)].
Fig. 3. An ORTEP-3 [29] drawing of compound 5a with the atom-numbering
scheme. Displacement ellipsoids are drawn at the 50% probability level.
Symmetry codes: (i) 3/2 ꢁ x, ½ + y, z, (ii) x, y ꢁ 1, z, (iii) ½ ꢁ x, ½ + y, z, (iv) ꢁx, ꢁy,
ꢁz, (v) ½ + x, ½ ꢁ y, ½ + z, (vi) 1 ꢁ x, ꢁy, ꢁz, (vii) x ꢁ ½, ½ ꢁ y, z ꢁ ½.
tures of the synthesized compounds were identified by FT-IR, 1H and
13C NMR spectra, MS and elemental analyses and their results
showed that all reactions were completed successfully. The solid
state structures of compounds 8 and 5a were determined by sin-
gle-crystal X-ray crystallography.
hole sizes of 1.68, 1.57, 1.56 and 1.44 Å have been found for four
benzo-15-crown-5 Schiff bases corresponding to compounds 8
and 5a [10,47–49]. Hence, the inner hole sizes of compounds 8
and 5a are in accordance with the reported values [42,50].
Acknowledgements
4. Conclusion
The authors gratefully acknowledge the financial assistance of
the Scientific and Technical Research Council of Turkey (TUBITAK),
Grant No: TBAG 210T122. We also thank Anadolu University and
Medicinal Plants and Medicine Research Center of Anadolu Univer-
sity Eskisßehir, for allowing us to use the X-ray facility.
In conclusion, the substituted benzyloxybenzaldehyde deriva-
tives (1–4) were prepared by reacting the appropriate hydroxy
benzaldehydes with 4-(bromomethyl)benzonitrile in DMF in the
presence of NaOH. These compounds reacted with 40-aminobenzo-
15-crown-5 to afford the corresponding crown ether imine com-
pounds(5–8). Mononuclearsodium complexes (5a–8a) of the crown
ether imine compounds were prepared with NaClO4, while potas-
sium complexes (5b–8b) were prepared with KI. Spectroscopic re-
sults showed that intermolecular 1:2 (potassium ion:crown ether)
sandwich complexes are formed. The ligand complexing properties
were studied by solvent extraction of selected alkali and transition
metals (Na+, K+, Cr3+, Ni2+, Cu2+, Zn2+ and Pb2+). The molecular struc-
Appendix A. Supplementary data
CCDC 827235 and 827236 contain the supplementary crystallo-
graphic data for compounds8 and 5a. These data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,

696
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