Synthesis, spectral characterization, and structural investigation of mononuclear salen-type Cu(II) and Zn(II) complexes of a potentially octadentate N2O6 Schiff base ligand derived from binaphthol

Article abstract of DOI:10.1007/s11243-013-9728-2

A new potentially octadentate N₂O₆ Schiff base ligand, H₂L derived from the condensation of 2,2′-(1,1′- binaphthyl-2,2′-diylbis(oxy))dianiline and o-vanillin, along with its copper(II) and zinc(II) complexes, is synthesized and has been characterized by elemental analyses, IR, UV-vis, ¹H and ¹³C NMR spectra, as well as conductivity measurements. H₂L forms mononuclear complexes of 1:1 (metal:ligand) stoichiometry with Cu(II) and Zn(II), and conductivity data confirm the non-electrolyte nature of these complexes. The [ZnL] and [CuL] complexes display very different solid-state structures, as determined by X-ray crystallography. While the [ZnL] complex has a distorted octahedral geometry about the metal, the [CuL] complex displays a distorted square planar geometry about the copper, with long Cu-O(ether) distances of 2.667 A.

Full text of DOI:10.1007/s11243-013-9728-2

Transition Met Chem (2013) 38:611–616
DOI 10.1007/s11243-013-9728-2
Synthesis, spectral characterization, and structural
investigation of mononuclear salen-type Cu(II)
and Zn(II) complexes of a potentially octadentate
N2O6 Schiff base ligand derived from binaphthol
•
Hassan Keypour Maryam Shayesteh
•
•
Reza Golbedaghi Allan G. Blackman
•
Scott A. Cameron
Received: 19 February 2013 / Accepted: 6 May 2013 / Published online: 7 June 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract A new potentially octadentate N2O6 Schiff base
ligand, H2L derived from the condensation of 2,20-(1,10-
binaphthyl-2,20-diylbis(oxy))dianiline and o-vanillin, along
with its copper(II) and zinc(II) complexes, is synthesized
and has been characterized by elemental analyses, IR, UV–
vis, 1H and 13C NMR spectra, as well as conductivity
measurements. H2L forms mononuclear complexes of 1:1
(metal:ligand) stoichiometry with Cu(II) and Zn(II), and
conductivity data confirm the non-electrolyte nature of
these complexes. The [ZnL] and [CuL] complexes display
very different solid-state structures, as determined by X-ray
crystallography. While the [ZnL] complex has a distorted
octahedral geometry about the metal, the [CuL] complex
displays a distorted square planar geometry about the
metals in a variety of oxidation states [1–5]. Among such
ligands, tetradentate salen, derived from the condensation of
two equivalents of salicylaldehyde with one equivalent of
ethylenediamine [17], has been of particular importance in
coordination chemistry, especially in homogeneous transi-
tion metal-mediated catalysis [6–16]. Transition metal
complexes of salen have been known since at least 1931
[17–20], but extensive investigations have been carried out
in the years since chiral variants were first applied to asym-
metric catalysis in the early 1990s [21, 22]. Modified salen
complexes have served as catalysts in a variety of reactions
such as asymmetric epoxidation [7], copolymerization of
carbon dioxide with epoxides [10, 11], and hydrolytic kinetic
resolution of racemic epoxides [23, 24], depending on the
metal employed. In the present work, we report the synthesis
and characterization of a new salen-derived ligand, H2L,
containing 8 potential donor atoms. The X-ray structures of
the Cu(II) and Zn(II) complexes of this ligand are reported;
somewhat surprisingly, the two complexes display very
different geometries about the metal centers.
˚
copper, with long Cu–O(ether) distances of 2.667 A.
Introduction
Schiff base ligands play an important role in inorganic
chemistry as they form stable complexes with most transition
Experimental
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9728-2) contains supplementary
material, which is available to authorized users.
Materials and measurements
H. Keypour (&) ꢀ M. Shayesteh
2,20-(1,10-Binaphthyl-2,20-diylbis(oxy))dianiline was syn-
thesized according to the literature procedure [25–29].
Solvents, 1,10-binaphthyl-2,20-diol, o-vanillin, 1-fluoro-2-
nitrobenzene, and metal salts were purchased from Merck
and used without further purification.
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174,
Iran
e-mail: haskey1@yahoo.com
R. Golbedaghi
Chemistry Department, Payame Noor University,
Tehran 19395-4697, Iran
Infrared spectra were collected using KBr pellets on a
BIO-RAD FTS-40A spectrophotometer (4,000–400 cm-1).
A Perkin-Elmer Lambda 45 (UV–Vis) spectrophotometer
was used to record the electronic spectra. CHN analyses
A. G. Blackman ꢀ S. A. Cameron
Department of Chemistry, University of Otago, P.O. Box 56,
Dunedin, New Zealand
123

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Transition Met Chem (2013) 38:611–616
C50H36Cl6CuN2O6
Table 1 Crystal data and
structure refinement for
[ZnL]ꢀCH3CN and
Empirical formula
C50H37N3O6Zn
Formula weight
Temperature
841.20
1037.05
89(2) K
0.71073
Monoclinic
C2/c
[CuL]ꢀ2CHCl3 complexes
89(2) K
0.71069
Monoclinic
P2(1)/c
˚
Wavelength (A)
Crystal system
Space group
Unit cell dimensions
˚
a (A)
14.434(1)
15.527(1)
18.784(1)
90
17.6692(13)
14.6492(12)
18.1301(14)
90
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
111.151(3)
90
103.929(4)
90
3
˚
Volume (A )
3926(1)
4554.8(6)
4
Z
4
Calculated density
Absorption coefficient
F(000)
1.423 Mg/m3
0.684 mm-1
1744
1.512 Mg/m3
0.885 mm-1
2116
Crystal size
0.47 9 0.27 9 0.27 mm3
0.43 9 0.33 9 0.18 mm3
Theta range for data collection
Limiting indices
2.02–26.44°
-18 B hB18, -19 B kB19,
-23 B lB23
2.31 to 28.26°
-21 B hB 23, -19 B k B 19,
-24 B l B 24
Reflections collected/unique
Completeness to theta = 25.00
Absorption correction
69469/8050 [R(int) = 0.0656]
46244/5559 [R(int) = 0.0602]
99.9 %
98.4 %
Semi-empirical from equivalents Semi-empirical from equivalents
Max. and min. transmission
Refinement method
1.000000 and 0.884555
Full-matrix least-squares on F2
8050/0/544
1.000000 and 0.876297
Full-matrix least-squares on F2
5559/117/365
Data/restraints/parameters
Goodness-of-fit on F2
1.088
1.164
Final R indices [I [ 2sigma(I)] R1 = 0.0426, wR2 = 0.1096
R1 = 0.0586, wR2 = 0.1476
R1 = 0.0791, wR2 = 0.1599
-3
R indices (all data)
R1 = 0.0561, wR2 = 0.1236
0.513 and -0.524 e.A-3
˚
Largest diff. peak and hole
1.159 and -0.923 e.A
were carried out using a Perkin-Elmer Model 2400 CHNS/O
elemental analyzer. Conductance measurements were per-
formed using a Hanna HI 8820 conductivity meter. 1H and
13C NMR spectra were obtained in CDCl3 on a Bruker
Avance 300 MHz and Jeol 90 MHz spectrometer using
Si(CH3)4 as an internal standard.
using SIR-97 [32] within the WinGX [33] package, and
weighted full-matrix refinement on F2 was carried out using
SHELXL-97 [34]. Hydrogen atoms were included in calcu-
lated positions and refined as riding with individual (or group,
if appropriate) isotropic displacement parameters. Disorder
was apparent in the naphthalene rings of [CuL]ꢀ2CHCl3, with
the rings displaying slight conformational mobility about
C15; this was successfully modeled as a 50:50 distribution of
the two conformations. Details of the X-ray experiments and
crystal data are summarized in Table 1.
X-ray crystallography
Vapor diffusion of diethyl ether into solutions of [CuL] and
[ZnL] in chloroform and acetonitrile, respectively, afforded
green and yellow prismatic crystals suitable for study by
X-ray crystallography. Single-crystal X-ray diffraction anal-
yses were performed on a Bruker Kappa APEX-II system at
89(2) K using graphite monochromated Mo–Ka X-ray radi-
ation with exposures over 0.5°. Data were corrected for Lor-
entz and polarization effects using SAINT [30] and for
absorption using SADABS [31]. All structures were solved
Synthesis of H2L
A solution of 2,20-(1,10-binaphthyl-2,20-diylbis(oxy))diani-
line (0.468 g, 1 mmol) in methanol (20 ml) was added
dropwise with stirring to a solution of o-vanillin (0.304 g,
2 mmol) in methanol (30 ml). The mixture was stirred and
heated to reflux for 12 h. The resulting yellow precipitate was
123

Transition Met Chem (2013) 38:611–616
613
filtered off, washed with cold methanol, and dried in vacuo.
Yield: 0.5 g (68 %). M.p. 108 °C. Anal. Calc. for
C48H36N2O6: C, 78.2; H, 4.9; N, 3.8. Found: C, 78.9; H, 4.7;
N, 4.1 %. IR (cm-1, KBr): 1614 (s, mC=N). 1H NMR
(90 MHz, CDCl3): d 3.8 (s, 6H, H23), 6.4–8.0 (m, 26H, aro-
matics), 8.1 (s, 2H, CH=N), 13.2 (s, 2H, OH). UV–Vis
[k (nm), e (M-1 cm-1)]: 275 (48723).
Synthesis of [CuL]ꢀ2CHCl3
A methanol solution (30 ml) of Cu(NO3)2ꢀ6H2O (0.296 g,
1 mmol) was added to a warm solution of [H2L] (0.737 g,
1 mmol) in methanol (50 ml). The mixture was stirred and
heated to reflux for 12 h. The resultant green solid was col-
lected by filtration, washed with cold methanol, and dried in
vacuum. Yield: 0.7 g (68 %). M.p. 299 °C. Anal. Calc. for
C50H36Cl6CuN2O6 (MW: 1037.09): C, 57.9; H, 3.5; N, 2.7.
Found: C, 57.5; H, 3.7; N, 3.1 %. IR (cm-1, KBr): 1606
(s, mC=N). UV–Vis [k (nm), e (M-1 cm-1)]: 294 (39445),
398 (14850), 658 (62). Km = 1.3 cm2 X-1 mol-1
.
Fig. 1 Structure of H2L ligand, along with NMR numbering
Synthesis of [ZnL]ꢀCH3CN
complexes are soluble in CHCl3, EtOH, MeOH, and THF
and are non-electrolytes, as determined by molar conduc-
tivity measurements.
The preparation of the yellow microcrystalline complex
followed the same procedure described for [CuL]ꢀ2CHCl3
by using a solution of Zn(NO3)2ꢀ6H2O (0.297 g, 1 mmol)
in 30 mL of methanol. Yield: 0.6 g (71.3 %). M.p. 315 °C.
Anal. Calc. for C50H37ZnN3O6 (MW:841.26): C, 71.4; H,
Spectroscopic and conductivity data
4.4; N, 5.0. Found: C, 71.5; H, 4.3; N, 4.8 %. IR (cm-1
,
The IR spectra of both complexes show a sharp band in the
range 1,606–1,608 cm-1, attributed to m(C=N), which is
shifted to lower frequency compared to the free ligand
(1,614 cm-1). This is indicative of coordination of the
imine nitrogen with the metal. Furthermore, the absence of
C=O and N–H stretching vibrations in the spectra of the
complexes confirms the Schiff base condensation. Depro-
tonation of the phenolic functions is confirmed by the lack
of O–H stretching bands in the region 3,400–3,300 cm-1
for both complexes [35, 36]. A band at 1,179 cm-1 for
[H2L] is ascribed to the phenolic C–O stretching vibrations;
this band is shifted to lower frequencies (1,168–1,170) in
the complexes due to O-metal coordination.
1H and 13C NMR data for the free Schiff base, H2L, and the
diamagnetic complex [ZnL] in CDCl3 are given in Table S1,
while the NMR numbering of the atoms is shown in Fig. 1.
Both 1H and 13C NMR spectra of the H2L and [ZnL] show
only a single imine resonance (1H: 8.1 ppm (H2L), 8.0 ppm
(ZnL); 13C:166.1 ppm (H2L), 168.7 ppm (ZnL)), demon-
strating the equivalence of the two imine environments. The
1H NMR spectrumofthe complex [ZnL] isverysimilartothat
of the free ligand, with only slight shifts to lower fields seen
for the ligand protons in coordination with zinc(II). Most
notably, the signal for the imine proton in the zinc complex is
shifted downfield with respect to the corresponding signal in
KBr): 1608 (s, mC=N). 1H NMR (300 MHz, d6-DMSO): d
3.7 (s, 6H, H23), 6.5–7.8 (m, 26H, aromatics), 8.0 (s, 2H,
CH=N). UV–Vis [k (nm), e (M-1 cm-1)]: 300(30213),
403(13221). Km = 2.5 cm2 X-1 mol-1
.
Results and discussion
A new Schiff base H2L (Fig. 1) has been prepared from the
reaction of 2,20-(1,10-binaphthyl-2,20-diylbis(oxy))dianiline
and o-vanillin, in refluxing methanol. Elemental analysis and
NMR spectrometry (see below) confirmed its formula and
purity. Complexes of this ligand with Cu(II) and Zn(II) were
synthesized by direct reactions of Cu(NO3)2ꢀ6H2O and
Zn(NO3)2ꢀ6H2O with the ligand in an equimolar ratio in
methanol solvent or template reactions starting from the
respective metal salts, o-vanillin, and 2,20-(1,10-binaphthyl-
2,20-diylbis(oxy))dianiline in 1:1:1 mol ratio. Characteriza-
tion of the complexes was achieved by IR spectroscopy,
elemental analysis, molar conductance, UV–Vis spectra,
NMR spectrometry (Zn complex), and X-ray crystal dif-
fraction, all of which gave results consistent with the pro-
posed formulations. The compositions of the products were
not affected by changing the mole ratio of the reactants. The
123

614
Transition Met Chem (2013) 38:611–616
Table 2 Electronic spectroscopy data (nm) for ligand H2L and
related complexes
Compound
max(nm) (e)ak
Intraligand (LL)
CT
d–d
[H2L]
[CuL]
[ZnL]
275 (48723)
294 (39445)
300 (30213)
398 (14850)
403 (13221)
658 (62)
a
Mol-1 cm-1
the free ligand, indicating that the metal-nitrogen bond is
retained in solution.
A total of 22 peaks are observed in the aromatic region of
the 13C NMR spectrum (112.8–152.9 ppm) of the [ZnL]
complex, consistent with the presence of twofold symmetry
in this complex. In the 13C NMR spectrum of the free Schiff
base, the region corresponding to aromatic rings carbons
1
(115.4–153.4 ppm) has 11 peaks instead of 22. The H
spectrum of [ZnL] shows the loss of the phenolic OH signal
observed at 13.2 ppm for free H2L, indicating deprotonation
Fig. 2 Molecular structure of [ZnL] showing 30 % probability
thermal ellipsoids using ORTEP
of the phenolic OH in coordination with Zn2?
.
The electronic spectra of the ligand and its metal com-
plexes were recorded in CHCl3. The bands below 300 nm
are attributable to intraligand p ? p* and n ? p* transi-
tions. In the electronic spectra of the complexes, the in-
traligand transitions are slightly shifted as a result of
coordination. The electronic spectrum of the Cu(II) com-
plex shows an absorption at 658 nm attributed to the
2
2Eg ? T2g transition, characteristic of square planar
geometry [37, 38]. The Zn(II) complex is diamagnetic
without any d–d transitions. The spectra of both complexes
show an intense band around 400 nm, due to a charge
transfer transition [39–42]. The electronic spectral details
of the complexes are given in Table 2.
The molar conductivity (KM) values of the complexes
were 1.3–2.5 X-1 mol-1 cm2, measured at 25 °C using
10-3 M solutions in CHCl3 solvent. These low values
indicate that both complexes are non-electrolytes [43, 44].
Hence, the Schiff base ligand is most probably coordinated
with the copper(II) and zinc(II) centers as a doubly charged
anion. This is consistent with deprotonation and coordi-
nation of the two phenolic OH groups [45].
Fig. 3 Molecular structure of [CuL] showing 30 % probability
thermal ellipsoids using ORTEP
The structure of [CuL]ꢀ2CHCl3 comprises a Cu(II) center
coordinatedwiththetwoimineNatomsandthetwophenolate
OatomsoftheSchiffbaseligand. TheN2O2 donoratomsliein
whatisbestdescribedasadistortedsquareplanararrangement
(despitethes4 valueof0.42, supposedlyindicativeofaseesaw
geometry[46])withthetwoNandtwoOdonorsbothadopting
a mutually pseudo-trans arrangement (O–Cu–O and N–Cu–N
bond angles of 143.22(12)° and 158.23(14)°, respectively).
The other angles about the metal are in the range 92.42(9)–
X-ray crystal structures
Suitable crystals of [ZnL]ꢀCH3CN and [CuL]ꢀ2CHCl3 were
obtained from acetonitrile and chloroform solutions of the
respective complexes by slow diffusion of diethyl ether.
ORTEP views of the complexes are shown in Figs. 2 and 3,
respectively. Crystallographic data and structure refine-
ment parameters are given in Table 1, and selected bond
distances and angles are given in Table 3.
˚
94.41(10)°. The Cu(1)–O(1) distance (1.9070(19) A) is con-
sistent with deprotonation and formal coordination as
123

Transition Met Chem (2013) 38:611–616
615
˚
positioned mutually cis to each other. Coordination of the
Table 3 Selected bond lengths (A) and bond angles (°) for com-
plexes [CuL]ꢀ2CHCl3 and [ZnL]ꢀCH3CN
ether O atoms with the zinc is confirmed by bond distances of
3
2.4062(17) A and 2.4181(17) A. The Zn–N (imine)
˚
˚
Complex [ZnL]
Complex [CuL]
˚
(2.037(2)–2.0642(19) A) and Zn–N (phenolate) (1.9556(17)–
˚
1.9620(18) A) bond lengths are comparable to corresponding
Bond
Distance
Bond
Distance
N(1)–Zn(1)
N(2)–Zn(1)
O(2)–Zn(1)
O(3)–Zn(1)
O(4)–Zn(1)
O(5)–Zn(1)
2.0642(19)
2.037(2)
Cu(1)–O(1)#1
Cu(1)–O(1)
1.9070(19)
1.9070(19)
1.976(3)
distances reported in the literature (see footnote 1).
1.9556(17)
2.4062(17)
2.4181(17)
1.9620(18)
Cu(1)–N(1)
Conclusion
Cu(1)–N(1)#1
1.976(3)
In the present work, we have synthesized and characterized a
new potentially octadentate N2O6 Schiff base ligand and its
complexes with zinc(II) and copper(II). Physico-chemical
measurements confirm the 1:1 metal to ligand stoichiometry
of the complexes. The molecular structures of both com-
plexes have been revealed by single-crystal X-ray analysis.
Both of these complexes are neutral and found to have quite
different geometries. Thus, the zinc complex is in a distorted
octahedral geometry, while the copper complex has distorted
square planar geometry.
Bond
Angle
Bond
Angle
O(2)–Zn(1)–O(5) 105.76(7) O(1)#1–Cu(1)–O(1)
O(2)–Zn(1)–N(2) 102.60(7) O(1)#1–Cu(1)–N(1)
143.22(12)
92.42(9)
O(5)–Zn(1)–N(2)
O(2)–Zn(1)–N(1)
O(5)–Zn(1)–N(1)
94.46(7) O(1)–Cu(1)–N(1)
93.38(8) O(1)#1–Cu(1)–N(1)#1
96.69(8) O(1)–Cu(1)–N(1)#1
94.41(10)
94.41(10)
92.43(9)
N(2)–Zn(1)–N(1) 157.24(8) N(1)–Cu(1)–N(1)#1
O(2)–Zn(1)–O(3) 157.01(7)
158.23(14)
O(5)–Zn(1)–O(3)
N(2)–Zn(1)–O(3)
N(1)–Zn(1)–O(3)
O(2)–Zn(1)–O(4)
94.52(7)
86.17(7)
73.24(7)
86.52(7)
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 financial
support.
O(5)–Zn(1)–O(4) 164.89(6)
N(2)–Zn(1)–O(4)
N(1)–Zn(1)–O(4)
O(3)–Zn(1)–O(4)
73.91(7)
91.23(7)
75.43(6)
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