Synthesis, characterization, and electrochemical study of two new macroacyclic Schiff bases and their copper(II) and zinc(II) complexes

Article abstract of DOI:10.1080/00958972.2010.527963

Two new potentially octadentate N₂O₆ Schiff-base ligands 2-((E)-(2-(2-(2-((E)-2-hydroxy-3-methoxybenzylideneamino)phenoxy) phenoxy)phenylimino)methyl)-6-methoxyphenol H₂L¹ and 2-((E)-(2-(2-(2-((E)-2-hydroxy-3-methoxybenzylideneamino)phenoxy) -4-tert-butylphenoxy)phenylimino)methyl)-6-methoxyphenol H₂L ² were prepared from the reaction of O-Vaniline with 1,2-bis(2'-aminophenoxy)benzene or 1,2-bis(2'-aminophenoxy)-4-t-butylbenzene, respectively. Reactions of H₂L¹ and H₂L ² with copper(II) and zinc(II) salts in methanol in the presence of N(Et)3 gave neutral [CuL1] 0.5CH₂Cl₂, [CuL²], [ZnL¹] 0.5CH₂Cl₂, and [ZnL²] complexes. The complexes were characterized by IR spectra, elemental analysis, magnetic susceptibility, ESI-MS spectra, molar conductance (m), UV-Vis spectra and, in the case of [ZnL¹] 0.5CH₂Cl₂ and [ZnL²], with ¹H- and ¹³C-NMR. The crystal structure of [ZnL1] 0.5CH2Cl2 has also been determined showing the metal ion in a highly distorted trigonal bipyramidal geometry. The electrochemical behavior of H₂L² and its Cu(II) complex, [CuL²], was studied and the formation constant of [CuL²] was evaluated using cyclic voltammetry. The logarithm value of formation constant of [CuL ²] is 21.9.

Full text of DOI:10.1080/00958972.2010.527963

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Synthesis, characterization, and
electrochemical study of two new
macroacyclic Schiff bases and their
copper(II) and zinc(II) complexes
Hassan Keypour a , Maryam Shayesteh a , Davood Nematollahi a ,
Laura Valencia b & Hadi Amiri Rudbari c
a Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174,
Iran
b Departamento de Química Inorgánica, Facultad de Química,
Universidade de Vigo, 36310 Vigo, Pontevedra, Spain
c Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica
Fisica, Universita' di Messina, 98166 Messina, Italy
Published online: 20 Nov 2010.
To cite this article: Hassan Keypour , Maryam Shayesteh , Davood Nematollahi , Laura Valencia
& Hadi Amiri Rudbari (2010) Synthesis, characterization, and electrochemical study of two new
macroacyclic Schiff bases and their copper(II) and zinc(II) complexes, Journal of Coordination
Chemistry, 63:23, 4165-4176, DOI: 10.1080/00958972.2010.527963
To link to this article: http://dx.doi.org/10.1080/00958972.2010.527963
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Journal of Coordination Chemistry
Vol. 63, No. 23, 10 December 2010, 4165–4176
Synthesis, characterization, and electrochemical study of two
new macroacyclic Schiff bases and their copper(II) and
zinc(II) complexes
HASSAN KEYPOUR*y, MARYAM SHAYESTEHy,
DAVOOD NEMATOLLAHIy, LAURA VALENCIAz and
HADI AMIRI RUDBARIx
yFaculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran
zDepartamento de Quı
mica Inorganica, Facultad de Quımica,
´ ´ ´
Universidade de Vigo, 36310 Vigo, Pontevedra, Spain
xDipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
Universita’ di Messina, 98166 Messina, Italy
(Received 6 April 2010; in final form 30 August 2010)
Two new potentially octadentate N2O6 Schiff-base ligands 2-((E)-(2-(2-(2-((E)-2-hydroxy-3-
H2L1
methoxybenzylideneamino)phenoxy)phenoxy)phenylimino)methyl)-6-methoxyphenol
and 2-((E)-(2-(2-(2-((E)-2-hydroxy-3-methoxybenzylideneamino)phenoxy)-4-tert-butylphe-
noxy)phenylimino)methyl)-6-methoxyphenol H2L2 were prepared from the reaction of
O-Vaniline with 1,2-bis(20-aminophenoxy)benzene or 1,2-bis(20-aminophenoxy)-4-t-butylben-
zene, respectively. Reactions of H2L1 and H2L2 with copper(II) and zinc(II) salts in methanol in
the presence of N(Et)3 gave neutral [CuL1] ꢀ 0.5CH2Cl2, [CuL2], [ZnL1] ꢀ 0.5CH2Cl2, and [ZnL2]
complexes. The complexes were characterized by IR spectra, elemental analysis, magnetic
susceptibility, ESI–MS spectra, molar conductance (Ãm), UV-Vis spectra and, in the case
of [ZnL1] ꢀ 0.5CH2Cl2 and [ZnL2], with 1H- and 13C-NMR. The crystal structure of
[ZnL1] ꢀ 0.5CH2Cl2 has also been determined showing the metal ion in a highly distorted
trigonal bipyramidal geometry. The electrochemical behavior of H2L2 and its Cu(II) complex,
[CuL2], was studied and the formation constant of [CuL2] was evaluated using cyclic
voltammetry. The logarithm value of formation constant of [CuL2] is 21.9.
Keywords: Schiff base; Copper(II) complexes; Zinc(II) complexes; O-Vaniline; X-ray crystal
structure; Cyclic voltammetry
1. Introduction
Schiff bases have been extensively studied in coordination chemistry due to their facile
syntheses, easily tunable steric and electronic properties, and good solubility in common
solvents [1]. Azomethine compounds have wide applications in biology, for example,
proteins [2], visual pigment [3], enzymic aldolization [4], decarboxylation reactions [5],
in pyridoxal phosphate degradation [6], and in the field of color photography [7].
Transition metal complexes with oxygen and nitrogen donor Schiff bases are of
*Corresponding author. Email: haskey1@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2010 Taylor & Francis
DOI: 10.1080/00958972.2010.527963

4166
H. Keypour et al.
particular interest [8–13] because of their ability to possess unusual configurations, be
structurally labile, and their sensitivity to molecular environments [14]. Schiff-base
complexes containing Ni, Co, Zn, and Cu have been studied in great detail for
structures, structure–redox relationships, enzymatic reactions, mesogenic characteris-
tics, and catalytic properties [15–17]. Zinc has a significant role in bioinorganic
chemistry, involving a large number of enzymatic and catalytic functions [18, 19]. In this
article, synthesis of potentially octadentate Schiff bases H2L1 and H2L2 and their Cu(II)
and Zn(II) complexes are described. Analytical, structural, spectral, magnetic, molar
conductivity measurements, and electrochemical studies were used to investigate the
structures of these ligands and their complexes.
2. Experimental
2.1. Starting materials
1,2-Bis(20-aminophenoxy)benzene and 1,2-bis(20-aminophenoxy)-4-t-butylbenzene were
synthesized according to the literature method [20–24]. o-Vaniline, solvents, metal salts,
and other reagents were purchased from Merck and used without purification.
2.2. Physical measurements
Infrared (IR) spectra were collected using KBr pellets on a BIO-RAD FTS-40A
spectrophotometer (400–4000 cmꢁ1). A Perkin-Elmer, Lambda 45 (UV-Vis) spectro-
photometer was used to record electronic spectra. Electron impact (20 eV) mass spectra
for ligands were recorded on a Shimadzu, QP1100EX spectrometer. ESI–MS spectra
were recorded using a Kratos-MS-50T spectrometer connected to a DS90 data system
using 3-nitrobenzyl alcohol as the matrix. Magnetic susceptibility measurements were
performed at 25ꢂC using a Johnson Matthey Alfa MSBMKI Gouy balance. CHN
analyses were carried out using a Perkin-Elmer, CHNS/O elemental analyzer model
2400. Conductance measurements were performed using a Hanna HI 8820 conductivity
meter. 1H- and 13C-NMR spectra were taken in CDCl3 and DMSO on a Bruker Avance
300 and 500 MHz and a Jeol 90 MHz spectrometer using Si(CH3)4 as an internal
standard. Cyclic voltammetry was performed using an Autolab model PGSTAT 20
potentiostat/galvanostat. The working electrode used in the voltammetry experiments
was a Pt disc (1.8 mm diameter) and platinum wire was used as the counter electrode.
Working electrode potentials were measured versus Ag wire (all electrodes from AZAR
Electrodes). A stream of pure nitrogen was used to remove the dissolved oxygen in the
solution before voltage scanning.
2.3. X-ray crystallography
Single crystals of [ZnL1] ꢀ 0.5CH2Cl2 complex were obtained from dichloromethane
by n-hexane diffusion. The single crystal X-ray diffraction analysis was performed
on
a
Bruker SMART APEX2 CCD area detector at 150(1) K using

Macroacyclic Schiff bases
4167
Table 1. Crystal data and structure refinement for [ZnL1] ꢀ 0.5CH2Cl2.
Empirical formula
Formula weight
Temperature (K)
C34.50H27ClN2O6Zn
666.40
150(1)
0.71073
Monoclinic
C 2/c
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions (A, )
ꢂ
˚
a
b
c
ꢀ
ꢃ
ꢄ
24.6500(11)
12.1260(4)
22.4350(11)
90
118.7811(17)
90
5877.5(4), 8
1.506
3
˚
Volume (A ), Z
Calculated density (g cmꢁ3
)
Absorption coefficient (mmꢁ1
F(000)
)
0.978
2744
Crystal size (mm3)
ꢅ range for data collection (ꢂ)
Index ranges
0.22 ꢃ 0.20 ꢃ 0.06
3.01–27.55
ꢁ32 ꢄ h ꢄ 31, ꢁ15 ꢄ k ꢄ 15, ꢁ24 ꢄ l ꢄ 29
17807
Reflections collected
Independent reflections
Completeness to ꢅ ¼ 25.24 (%)
Absorption correction
Max. and min. transmission
Refinement method
6629 [R(int) ¼ 0.0652]
99.1
Multi-scan
0.965 and 0.879
Full-matrix least-squares on F2
6629/0/407
Data/restraints/parameters
Goodness-of-fit on F2
1.022
Final R indices [I 4 2ꢆ(I)]
R indices (all data)
Largest difference peak and hole (e A
R1 ¼ 0.0581, wR2 ¼ 0.1201
R1 ¼ 0.1311, wR2 ¼ 0.1472
0.764 and ꢁ0.467
ꢁ3
˚
)
˚
graphite-monochromated Mo-Kꢀ X-ray radiation (ꢁ ¼ 0.71073 A). Details of the X-ray
experiments and crystal data are summarized in table 1.
2.4. Synthesis
2.4.1. H2L1. 1,2-Bis(20-aminophenoxy)benzene (0.292 g, 1 mmol) in methanol (20 mL)
was added dropwise with stirring to a solution of o-Vaniline (0.304 g, 2 mmol) in
methanol (30 mL). The mixture was stirred and heated to reflux for 4 h. A red
precipitate was obtained that was filtered off, washed with cold methanol, and dried
in vacuo. Red crystals of H2L1 were obtained by liquid diffusion of n-hexane into a
solution of the ligand in chloroform. Yield: 0.5 g (89%); m.p. 151ꢂC. Anal. Calcd for
C34H28N2O6: C, 72.8; H, 5.0; N, 5.0. Found: C, 72.4%; H, 5.2%; N, 5.4%. IR (cmꢁ1
KBr): 1615 (s, ꢂC¼N). UV-Vis [ꢁ (nm), " ((mol L
345(29560). Mass spectral parent ion; m/z 560.
,
)
ꢁ1 ꢁ1 cmꢁ1)]: 271(123700), 287(sh),
2.4.2. H2L2. In a manner similar to that for H2L1, a methanol solution (20 mL) of
1,2-bis(20-aminophenoxy)-4-t-butylbenzene (0.348 g, 1 mmol) was added dropwise with
stirring to a solution of o-Vaniline (0.304 g, 2 mmol) in methanol (30 mL). The mixture

4168
H. Keypour et al.
was stirred and heated to reflux for 4 h. A red solid product that formed was washed with
cold methanol and dried in vacuo. Red crystals of H2L2 were obtained by liquid diffusion
of n-hexane into a solution of the ligand in chloroform. Yield: 0.4 g (65%); m.p. 132ꢂC.
Anal. Calcd for C38H36N2O6: C, 74.0; H, 5.9; N, 4.5. Found: C, 73.7%; H, 5.9%;
N, 4.9%. IR (cmꢁ1, KBr): 1616 (s, ꢂC¼N). UV-Vis [ꢁ (nm), " ((mol L
)
ꢁ1 ꢁ1 cmꢁ1)]:
234(50390), 281(26420), 338(22310). Mass spectral parent ion; m/z 616.
1
.
2.4.3. [CuL ] 0.5CH2Cl2. A methanol solution (15 mL) of Cu(ClO4)2 ꢀ 6H2O (0.370 g,
1 mmol) and a moderate excess of NEt3 were added to a warm solution of H2L1
(0.560 g, 1 mmol) in methanol (50 mL). The mixture was stirred and heated to reflux for
4 h. The resultant green solid was collected by filtration and washed with diethyl ether
and cold methanol. Green crystals of [CuL1] ꢀ 0.5CH2Cl2 were obtained by liquid
diffusion of n-hexane into a solution of the complex in dichloromethane. Yield: 0.5 g
(75%); m.p. 260ꢂC. Anal. Calcd for C34H26CuN2O6 ꢀ 0.5 CH2Cl2: C, 62.3; H, 4.1; N,
4.2. Found: C, 62.3%; H, 4.2%; N, 4.5%. IR (cmꢁ1, KBr): 1611 (s, ꢂC¼N), 427–470
(M–N), 540–582 (M–O). UV-Vis [ꢁ (nm), " ((mol L )
ꢁ1 ꢁ1 cmꢁ1)]: 301(39030),
415(13840), 690(60). ÃM ¼ 9 ꢀꢁ1 cm2 molꢁ1. ESI–MS (m/z): 622 ([CuL1 þ H]þ, 100%).
2.4.4. [CuL2]. Similarly, H2L2 (0.617 g, 1 mmol) and Cu(ClO4)2 ꢀ 6H2O yielded
green crystals. Yield: 0.4 g (59%); m.p. 285ꢂC. Anal. Calcd for C38H34CuN2O6: C,
67.3; H, 5.0; N, 4.1. Found: C, 67.3%; H, 5.2%; N, 4.4%. IR (cmꢁ1, KBr): 1610 (s,
ꢂC¼N), 445–476 (M–N), 523–553 (M–O). UV-Vis [ꢁ (nm), " ((mol L
)
ꢁ1 ꢁ1 cmꢁ1)]:
298(33110), 425(8220), 651(109). ÃM ¼ 11.5 ꢀꢁ1 cm2 molꢁ1. ESI–MS (m/z): 678
([CuL2 þ H]þ, 100%).
1
2.4.5. [ZnL ] 0.5CH2Cl2. In the same way, a methanol solution (15 mL) of
.
Zn(NO3)2 ꢀ 6H2O (0.297 g, 1 mmol) and a moderate excess of NEt3 were added to a
warm solution of H2L1 (0.560 g, 1 mmol) in methanol (50 mL). The resultant yellow
solid was collected by filtration and washed with cold methanol. Yellow crystals of
[ZnL1] ꢀ 0.5CH2Cl2 were obtained by liquid diffusion of n-hexane into a solution of
the complex in dichloromethane. Yield: 0.4 g (60%); m.p. 266ꢂC. Anal. Calcd for
C34H26ZnN2O6 ꢀ 0.5CH2Cl2: C, 62.2; H, 4.1; N, 4.2. Found: C, 62.2%; H, 4.3%;
N, 4.4%. IR (cmꢁ1, KBr): 1612 (s, ꢂC¼N), 440–473 (M–N), 532–560 (M–O). UV-Vis
[ꢁ (nm), " ((mol L
)
ꢁ1 ꢁ1 cmꢁ1)]: 317(30560), 435(15220). ÃM ¼ 12.4 ꢀꢁ1 cm2 molꢁ1
.
ESI–MS (m/z): 561 ([H2L1 þ H]þ, 100%).
2.4.6. [ZnL2]. Similarly, H2L2 (0.617 g, 1 mmol) and Zn(NO3)2 ꢀ 6H2O (0.297 g,
1 mmol) yielded yellow crystals. Yield: 0.5 g (73%); m.p. 242ꢂC. Anal. Calcd for
C38H34ZnN2O6: C, 67.1; H, 5.0; N, 4.1. Found: C, 67.3%; H, 5.2%; N, 4.2%. IR (cmꢁ1
,
KBr): 1610 (s, ꢂC¼N), 437–466 (M–N), 529–548 (M–O). UV-Vis [ꢁ (nm), "
((mol L
)
ꢁ1 ꢁ1 cmꢁ1)]: 315(19710), 435(11750). LM ¼ 14.2 ꢀꢁ1 cm2 molꢁ1
. ESI–MS
(m/z): 617 ([H2L2 þ H]þ, 100%).

Macroacyclic Schiff bases
4169
3. Results and discussion
Two new potentially octadentate N2O6 Schiff-base ligands, H2L1 and H2L2, have been
prepared from the reaction of o-Vaniline with 1,2-bis(20-aminophenoxy)benzene or
1,2-bis(20-aminophenoxy)-4-t-butylbenzene, respectively. The analytical and spectral
data are consistent with the proposed formulation. The neutral Cu(II) and Zn(II)
complexes of these ligands were also synthesized. All complexes were characterized by
IR spectra, elemental analyses, magnetic susceptibility measurements, mass spectra,
molar conductances (Ãm), UV-Vis spectra and, in the case of [ZnL1] ꢀ 0.5CH2Cl2, with
X-ray diffraction.
3.1. IR spectra
IR spectra of H2L1 and H2L2 (400–4000 cmꢁ1) show a strong absorption band at
1615 cmꢁ1 and 1616 cmꢁ1, respectively, assigned to C¼N stretch, indicating the
formation of the Schiff-base linkages. The absence of C¼O and N–H stretches in
spectra of the ligands, related to aldehyde and diamine, respectively, indicate Schiff-
base condensation. Reactions of copper(II) or zinc(II) salts with H2L1 and H2L2 yield
[CuL1] ꢀ 0.5CH2Cl2, [CuL2], [ZnL1] ꢀ 0.5CH2Cl2, and [ZnL2]. Deprotonation of all
phenolic functions is confirmed by lack of O–H stretching bands at 3300–3400 cmꢁ1
for all complexes [25, 26]. The strong ꢂ(C¼N) bands are shifted to lower frequencies
compared with free imine bonds, indicating coordination of the ligands to copper(II)
and zinc(II) via azomethine nitrogen. Bands at 1188 cmꢁ1 for H2L1 and H2L2 ascribed
to phenolic C–O stretches are shifted to lower frequencies due to O-metal coordination.
Conclusive evidence of the bonding is also shown by the observation that new bands in
IR spectra of the complexes appear at 525–590 and 410–485 cmꢁ1 assigned to ꢂ(M–O)
and ꢂ(M–N) [27–30].
3.2. Magnetic measurements
The magnetic moment values (at room temperature) of [CuL1] ꢀ 0.5CH2Cl2 and [CuL2]
are 1.63 and 1.68 BM, respectively, in agreement with the magnetic moment for a single
unpaired electron [31]. Due to a weak intermolecular antiferromagnetic exchange
interaction between the copper centers in the crystal structure, these values are slightly
less than the spin-only values [20, 32]. Zn(II) complexes are diamagnetic as expected.
3.3. NMR spectra
1H- and 13C-NMR spectroscopies have proved useful in establishing the nature and
1
structure of many Schiff bases as well as their complexes in solutions. The H- and
13C-NMR spectra of the Schiff bases were recorded in chloroform (CDCl3) using
tetramethylsilane (TMS) as an internal standard; 1H-NMR spectra of
[ZnL1] ꢀ 0.5CH2Cl2 and [ZnL2] and 13C-NMR spectrum of [ZnL1] ꢀ 0.5CH2Cl2 were
recorded in DMSO-d6. 1H- and 13C-NMR spectra of H2L1 and H2L2 and their
diamagnetic Zn(II) complexes are listed in table S1. The NMR numbering of the atoms
is shown in figure 1. 1H-NMR spectrum of H2L1 shows only single –CH¼N– resonance

4170
H. Keypour et al.
(a)
(b)
13
17„
17
14„
17
OCH3 H3CO 15'
14
15
16
OCH3 H3CO
13„
12„
15
16
13
12
16'
12
10
OH
HO
11„
11
OH
HO
O
11
„
10
10
N
N
N
N
9„
9
8„
7„
8
9
4
4„
O
O
3„
8
4
O
3
3
7
7
5
5„
2
2„
6
5
6„
2
6
1
18
1„
1
19
Figure 1. Structure of (a) H2L1 and (b) H2L2, along with NMR numbering.
at ca 8.35 ppm, demonstrating equivalence of the two imine environments. The
13C-NMR spectrum shows that the imine carbon (162.8 ppm) are chemically equivalent.
In the region corresponding to aromatic ring carbons (114.672–151.761 ppm), 15 peaks
are observed as expected. The 1H-NMR spectrum of H2L2 shows two singlet –CH¼N–
resonances at ca 8.30 and 8.35 ppm, supporting the non-equivalence of the two imine
environments. 13C-NMR spectrum shows that in the region corresponding to signals of
aromatic rings (114.479–151.827 ppm) 22 peaks instead of 30 expected peaks are
observed, due to similarity of some carbons and low resolution of the instrument. The
spectra of the complexes are examined in comparison with those of the parent Schiff
base. Phenolic-OH signals of H2L1 and H2L2 at 13.692 and 13.794 ppm disappear in
spectra of their Zn(II) complexes, indicating that OH protons are removed completely
upon coordination. Also, an upEeld shift of 0.1 ppm for imine proton in
[ZnL1] ꢀ 0.5CH2Cl2 and 0.2 and 0.3 ppm for two imine protons of [ZnL2] was observed.
All the spectral changes supported the formation of the complex between Zn(II) and
these ligands. Representative spectra of [ZnL1] ꢀ 0.5CH2Cl2 and [ZnL2] are presented
in figure S1.
3.4. Electronic absorption spectroscopy
Electronic spectra of the ligands and complexes were recorded in CHCl3. Bands below
345 nm are attributable to intramolecular ꢇ ! ꢇ* and n ! ꢇ* transitions. Electronic
spectra of the Cu(II) complexes show an absorption at 651–690 nm attributed to the
2
2Eg ! Tg transition, characteristic for square-planar geometry [33, 34]. The band
assigned to d–d transitions provides a rough estimate of the ligand Eeld strength, since
one electronic transition in the band envelope is dx2ꢁy2 ꢁ dxy and the energy associated
with this transition is 10Dq-C [28, 33, 34]. Spectra of the Zn(II) and Cu(II) complexes
show an intense band at ca 415–435 nm, which can be assigned to a charge transfer
transition [35–38].

Macroacyclic Schiff bases
4171
Figure 2. The observed and simulated isotopic patterns of ESI–MS spectra of (a) [CuL1] and (b) [CuL2].
3.5. Molar conductivity
The molar conductivities (Ãm) of the Cu(II) and Zn(II) complexes in CH2Cl2 at
10ꢁ3 mol Lꢁ1 were 9–14.2 ꢀꢁ1 cm2 molꢁ1, indicating non-electrolytes [39, 40]. The molar
conductance values of these complexes indicate that H2L1 and H2L2 are coordinated
to copper(II) and zinc(II) as doubly negatively charged anions. Therefore, two phenolic
OH have been deprotonated and bonded to the metals [41].
3.6. ESI–MS spectra
ESI–MS spectra of ([CuL1] ꢀ 0.5CH2Cl2, [CuL2], [ZnL1] ꢀ 0.5CH2Cl2, and [ZnL2])
confirm formation of the complexes. In the ESI–MS spectra of the Cu complexes,
the peaks at m/z 622.12 and 678.18 correspond to [CuL1 þ H]þ and [CuL2 þ H]þ species,
respectively. The spectra of latter complexes are shown in figure 2. The observed
isotopic patterns of the ESI–MS spectra of these complexes are completely consistent
with simulated patterns. The ESI–MS spectra of the Zn complexes showed the parent

4172
H. Keypour et al.
ion peak at m/z 561.20 and 617.26, corresponding to [H2L1 þ H]þ and [H2L2 þ H]þ
species, respectively, which compares well with the calculated formula weight of metal
free ligands. These results seem to be quite common, since ESI–MS is very rich in
energy and may push the equilibrium to the uncomplexed side [42].
3.7. X-ray structure
Slow diffusion of n-hexane into dichloromethane solution of [ZnL1] ꢀ 0.5CH2Cl2 resulted
in single crystals suitable for X-ray structure determination. An ORTEP of the Zn(II)
complex is shown in figure 3. The complex is Eve coordinate with four normal bonds and
one weak interaction, resulting in a ZnN2O3 core with highly distorted trigonal
bipyramidal geometry. The base of the trigonal bipyramid is occupied with two nitrogens
of 1,2-bis(20-aminophenoxy)benzene and one oxygen of phenolate. Two oxygens, one of
phenolate and one of the mentioned amine occupy the apical sites. The four normal
bonds are from phenolate oxygen and imine nitrogen of ligand toward Zn(II).
The weak interaction was from one oxygen of the 1,2-bis(20-aminophenoxy)benzene.
Figure 3. Molecular structure of [ZnL1] ꢀ 0.5CH2Cl2 showing 50% probability thermal ellipsoids using
ORTEP.

Macroacyclic Schiff bases
4173
Selected bond lengths and angles are given in table 2. It has been noted from O– and N–
bound Zn(II) complexes in the literature that the common coordination number is four,
and when this number increases, the additional coordinations are generally weak with
˚
Zn(II) ꢀ ꢀ ꢀ O distances ranging from 2.34 to 2.91 A and the corresponding geometries are
highly distorted [39–43]. Such observations include the Zn(II) ꢀ ꢀ ꢀ Ophosphato distance of
˚
2.71 A observed in
a
Zn(II)–ATP complex [43] (distorted octahedral),
˚
Zn(II) ꢀ ꢀ ꢀ Ocarboxylato distance of 2.91 and 2.79 A observed in two Zn(II)–histidino
complexes [44, 45] (distorted octahedral), Zn(II) ꢀ ꢀ ꢀ Operchlorato distance of 2.528 and
˚
2.818 A observed with the two crystallographically independent molecules present in the
lattice of a Zn(II)–triazacyclododecane complex [46] (distorted trigonal bipyramid),
˚
Zn(II) ꢀ ꢀ ꢀ Ocrown ether distances of 2.34, 2.56 and 2.60 A observed for a Zn–crown ether
complex [47] (highly distorted trigonal bipyramid). In this case, Zn(II) exhibits a
distorted trigonal bipyramidal geometry with a trans angle (Ophenoxo–Zn–Ophenolic) of
168.87(10)ꢂ (156.8(3)ꢂ and 146.1(6)ꢂ [47] and 150.7(1)ꢂ [48]). The average angle observed
in the basal plane of 118.05ꢂ (65.1ꢂ and 68.1ꢂ [47] and 112.0(1)ꢂ [48]) and the average
angle between apical and basal atoms of 90.69ꢂ are all within the reported range.
However, if only the four normal bonds are considered, the Zn2þ complex revealed a
highly distorted tetrahedral geometry, wherein five of the angles (95.31ꢂ, 94.30ꢂ, 96.09ꢂ,
122.59ꢂ and 137.26ꢂ) are quite off from the expected ones.
Sixty-three structures with [ZnN2O3] core have been described. In 61 cases, the Zn(II)
coordination is a trigonal bipyramid (ꢈ ¼ 0.51–1.0) and only in two cases does N occupy
the axial position [49, 50].
3.8. Electrochemical studies
Electrochemical behavior of H2L2 and its Cu(II) complex, [CuL2], were studied by
cyclic voltammetry. Figure S2 shows the cyclic voltammogram obtained for 1 mmol Lꢁ1
H2L2 in acetonitrile, containing 0.2 mol Lꢁ1 tetra-n-butylammonium perchlorate
(Bu4NClO4) as supporting electrolyte. At this condition, cyclic voltammogram of
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( )
for [ZnL1] ꢀ 0.5CH2Cl2.
Zn(1)–O(5)
Zn(1)–O(2)
Zn(1)–N(1)
Zn(1)–N(2)
Zn(1)–O(1)
1.938(3)
1.967(2)
2.013(3)
2.045(3)
2.421(2)
O(5)–Zn(1)–O(2)
O(5)–Zn(1)–N(1)
O(2)–Zn(1)–N(1)
O(5)–Zn(1)–N(2)
O(2)–Zn(1)–N(2)
N(1)–Zn(1)–N(2)
O(5)–Zn(1)–O(1)
O(2)–Zn(1)–O(1)
N(1)–Zn(1)–O(1)
N(2)–Zn(1)–O(1)
103.47(11)
122.59(10)
95.31(11)
94.30(11)
96.09(11)
137.26(12)
85.18(9)
168.87(10)
73.95(10)
90.15(10)

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H. Keypour et al.
Peak C0
CuIIL
Cu0
+
2e
Cu0
+
+
L
L
(1)
(2)
(3)
–1.08 Vvs. Ag wire
Peak A1
CuII
–
2e
0.04 V vs. Ag wire
Peak C1
CuII
+
2e
Cu0
–0.43 Vvs. Ag wire
Scheme 1. Electrochemical reduction of [CuL2].
H2L2 is the same as a voltammogram of background (figure S2, inset) showing no clear
oxidation or reduction peaks.
The electrochemical behavior of [CuL2] was studied by cyclic voltammetry. Figure S3
shows the first cyclic voltammogram obtained for a 1 mmol Lꢁ1 solution. The
voltammogram exhibits one cathodic peak C0 (ꢁ1.08 V vs. Ag wire) which corresponds
to reduction of Cu(II) in [CuL2] to Cu(0) (scheme 1, equation (1)). On the reverse scan,
one anodic peak (A1) was observed at 0.04 V versus Ag wire which corresponds to
oxidation of Cu(0) to Cu(II) (scheme 1, equation (2)).
In the second cycle, a new cathodic peak (C1) appears with an Ep value of ꢁ0.43 V
versus Ag wire and the current increases with increasing number of scans. This new peak
is related to reduction of CuII to Cu0 (scheme 1, equation (3)). This peak was confirmed
with the addition of Cu(ClO4)2 to the previous solution. Under this condition, the
current of peak C1 increases.
The following equation is used to determine the formation constant of [CuL2] [51]:
DEpc ¼ ðRT=2F Þ ln K,
where K is the formation constant for:
CuII þ L
CuL:
ꢁꢁꢁꢁꢁ*
)ꢁꢁꢁꢁꢁꢁ
DEpc is [(Epc)M ꢁ (Epc)C], where (Epc)M is the cathodic peak potential of CuII
(Epc1 ¼ ꢁ0.43 V) and (Epc)C is the cathodic peak potential of [CuL2] (Epc0 ¼ ꢁ1.08 V);
R, T, and F have their usual meanings. The calculated logarithmic value of formation
constant, log K, is 21.9 (at 25ꢂC).
4. Conclusion
The condensation of o-Vaniline with 1,2-bis(20-aminophenoxy)benzene or 1,2-bis(20-
aminophenoxy)-4-t-butylbenzene afforded H2L1 and H2L2. Reactions of the Schiff
bases with copper(II) and zinc(II) in 1 : 1 molar ratio (ligand : metal) were studied. On
comparison to similar structures and on the basis of spectroscopic data, the Schiff-base
ligands in Cu(II) complexes are dianionic tetradentate through two azomethine
nitrogens and the two oxygens of phenolic groups; square-planar structures were
assigned to these complexes. The crystal structure of [ZnL1] ꢀ 0.5CH2Cl2 shows that
Zn(II) is coordinated by two azomethine nitrogens and two phenolic oxygens of the

Macroacyclic Schiff bases
4175
octadentate ligand to form a distorted trigonal bipyramidal geometry. In this complex,
one oxygen of the initial amine exhibits a weak interaction with Zn(II) giving a ZnN2O3
core. All spectroscopic data show that ZnL2 has similar structure with ZnL1. From the
study of the Schiff bases derived from salicylaldehyde (salens) and also its derivatives as
polydentate ligands, we conclude that, in the mononuclear complexes related to these
ligands, the metal ions prefer the N2O2, N2O3, and/or N2O4 sites depending on the
flexibility of the ligands and the nature of metal. In more rigid ligands, due to a stronger
coordination of azomethine nitrogens and phenolate oxygens, N2O2 is preferred and
other donor atoms are usually too far from the metal to form strong coordination
bonds. In more flexible ligands, both N2O3 and N2O4 sites can be observed.
Supplementary material
CCDC 744569 contains the supplementary crystallographic data for [ZnL1]. 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, Cambridge
CB21EZ, UK; Fax: (þ44) 1223-336-033; or Email: deposit@ccdc.cam.ac.uk.
Acknowledgments
We are grateful to the Faculty of chemistry of Bu-Ali Sina University, National
Foundation of elites (BMN), and Ministry of Science, Research & Technology of Iran
for the financial support. The authors thank Mr. Reza Farhadi for the help in the CV
measurements.
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