Syntheses, crystal structures, and characterization of four homochiral coordination polymers based on two chiral reduced Schiff base ligands

Article abstract of DOI:10.1007/s11243-013-9706-8

Four homochiral coordination polymers incorporating two chiral reduced Schiff base ligands, namely, [Cu(L¹)(H₂O)]·H ₂O (1), [Zn₂(L²)₂] (2), [Co(L ²)(H₂O)] (3), and [Ni(L²)(H₂O)] (4) (H₂L¹ = N-(4-carboxyl)benzyl-l-alanine, H ₂L² = N-(4-carboxyl)benzyl-l-leucine) have been obtained by hydrothermal methods and characterized by physico-chemical and spectroscopic methods. X-ray crystallographic analysis reveals that complex 1 exhibits a chain structure with 1D channels. Complexes 2-4 all are 3D network structures with 1D channels in which the isobutyl group of the ligand points toward to the channel. Complex 2 displays strong photoluminescent emission in the purple region.

Full text of DOI:10.1007/s11243-013-9706-8

Transition Met Chem (2013) 38:413–418
DOI 10.1007/s11243-013-9706-8
Syntheses, crystal structures, and characterization of four
homochiral coordination polymers based on two chiral reduced
Schiff base ligands
•
Shao-Ming Ying Xiao-Hui Huang
Received: 7 January 2013 / Accepted: 11 February 2013 / Published online: 2 March 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Four homochiral coordination polymers incor-
porating two chiral reduced Schiff base ligands, namely,
[Cu(L1)(H2O)]ꢀH2O (1), [Zn2(L2)2] (2), [Co(L2)(H2O)] (3),
and [Ni(L2)(H2O)] (4) (H2L1 = N-(4-carboxyl)benzyl-L-
alanine, H2L2 = N-(4-carboxyl)benzyl-L-leucine) have
been obtained by hydrothermal methods and characterized
by physico-chemical and spectroscopic methods. X-ray
crystallographic analysis reveals that complex 1 exhibits a
chain structure with 1D channels. Complexes 2–4 all are
3D network structures with 1D channels in which the
isobutyl group of the ligand points toward to the channel.
Complex 2 displays strong photoluminescent emission in
the purple region.
nonlinear optical (NLO) materials [10–17]. Compared with
centrosymmetric complexes, NCS complexes are hard to
obtained because the inorganic–organic hybrid systems
normally tend to arrange in opposite directions to give cen-
trosymmetric structures. Several approaches have been
developed to resolve this problem: These include the intro-
duction of Jahn–Teller-distorted cations (suchas Ti4?, Nb5?
,
W6?) or lone-pair cations (such as Pb2?), the uses of chiral or
asymmetric ligands and the introduction of polar guest
molecules into the system [18]. To induce NCS structures,
the use of the chiral ligands is a convenient and efficient
method [19]. For this purpose, we have studied reduced
Schiff base ligands incorporating a chiral carbon atom. Such
reduced Schiff base ligands can be obtained by reaching
amino acids which have a chiral carbon atom with 4-carb-
oxybenzaldehyde to provide the corresponding Schiff bases,
then reducing the C=N bond. To data, to the best of our
knowledge, examples of NCS complexes based on reduced
Schiff base ligands of this type are still rare [20, 21]. In order
to extend these studies, we have synthesized two chiral
reduced Schiff base ligands by the reactions of 4-carboxy-
benzaldehyde with L-alanine or L-leucine, and four NCS
complexes have been obtained. Herein, we report their
syntheses, characterization, and crystal structures.
Introduction
In recent years, functional coordination complexes have
received much attention because of their potential applica-
tions in fields such as catalysis, ion exchange, intercalation
chemistry, photochemistry, selective separation, physical,
and materials chemistry [1–9]. Such complexes can be
divided into two categories according to the space group,
namely, centrosymmetric complexes or noncentrosymmet-
ric (NCS) complexes. NCS complexes are of great interest
because of their potential applications in areas such as
pyroelectricity, ferroelectricity, and especially second-order
Experimental
Materials and instruments
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9706-8) contains supplementary
material, which is available to authorized users.
All the chemicals were obtained from commercial sources
and used without further purification. H2L1 and H2L2 were
synthesized according to the procedures previously
described [21–23]. C, H, and N elemental analyses were
performed on a German Elementary Vario EL III
S.-M. Ying (&) ꢀ X.-H. Huang
Department of Chemistry, Ningde Normal University,
Ningde 352100, People’s Republic of China
e-mail: yingshaoming@126.com
123

414
Transition Met Chem (2013) 38:413–418
instrument. FTIR spectra were recorded on a Perkin-Elmer
Spectrum 2000 spectrometer using KBr pellets in the range
of 4,000–400 cm-1. Thermogravimetric analyses were
carried out on a NETZSCH STA 449C unit at a heating rate
of 10 °C/min under a nitrogen atmosphere from room
temperature to 900 °C. Powder X-ray diffraction (PXRD)
data were collected on a Bruker D8 Advance instrument
1,175(vs), 1,103(m), 1,080(m), 1,038(m), 1,018(m), 930(s),
893(s), 864(m), 775(m), 710(m).
Synthesis of [Ni(L2)(H2O)] (4)
The synthesis of complex 4 was similar to that for 2, except
that Ni(NO3)2ꢀ6H2O (0.058 g, 0.2 mmol) was used instead
of Zn(NO3)2ꢀ6H2O. Green rod crystals of complex 3 were
recovered in ca. 30 % yield based on H2L2. Elemental
analyses for 4, C14H19NNiO5 (Mr = 340.01): C, 49.4; H,
5.5; N, 4.1 %; Calcd.: C, 49.4; H, 5.6; N, 4.1 %. IR data
(KBr, cm-1): 3,326(m), 2,951(m), 1,605(vs), 1,562(s),
1,403(s), 1,369(s), 1,208(m), 1,175(m), 1,139(m), 1,102(m),
1,038(m), 1,019(m), 933(m), 902(m), 863(m), 847(m),
769(m), 712(m).
˚
using Cu–Ka radiation (k = 1.54056 A) at room temper-
ature. Photoluminescence spectra were collected on an
Edinburgh FLS920 spectrometer equipped with a contin-
uous Xe900 Xenon lamp at room temperature.
Synthesis of [Cu(L1)(H2O)]ꢀH2O (1)
To a mixture of Cu(NO3)2ꢀ3H2O (48 mg, ca. 0.2 mmol),
H2L1 (43 mg, ca. 0.2 mmol), EtOH (6 mL), and deionized
water (6 mL) were slowly added 1 M NaOH solution drop
by drop with stirring until the suspension changed to a
transparent solution. The mixture was then filtered, and the
filtrate was kept at room temperature. After about 2 weeks,
blue block crystals of complex 1 were obtained in ca.
50 % yield based on H2L1. Elemental analyses for 1,
C11H15CuNO6 (320.78): C, 41.3; H, 4.3; N, 4.4 %; Calcd.:
C, 41.2; H, 4.7; N, 4.4 %. IR (KBr pellet, cm-1): 3,217(s),
1,614(s), 1,556(s), 1,452(s), 1,371(s), 1,269(m), 1,184(m),
1,134(m), 1,082(s), 1,009(m), 847(m), 812(m), 771(s), 727(m).
X-ray analysis
Data collections for complexes 1–4 were performed on a
Bruker Smart Apex CCD diffractometer equipped with a
˚
graphite monochromated Mo–Ka radiation (k = 0.71073 A).
Intensity data for the four complexes were collected using x-
scans at 173 K. The data sets were corrected for Lorentz and
polarization factors as well as for absorption using the SAD-
ABS program. All structures were solved by direct methods
and refined by full-matrix least-squares fitting on F2 by
SHELX-97 [24]. All nonhydrogen atoms were refined with
anisotropic thermal parameters, whereas all hydrogen atoms
were generated geometrically and refined isotropically. Atoms
C26, C27, and C28 which comprise the isobutyl group in
complex 2 were twofold disordered, and therefore refined in
two positions. Crystallographic data and structural refinements
are summarized in Table 1. Selected bond lengths are listed in
Table 2.
Synthesis of [Zn2(L2)2] (2)
A mixture of Zn(NO3)2ꢀ6H2O (0.059 g, 0.2 mmol), H2L2
(0.027 g, 0.1 mmol), DMF (1 mL), EtOH (3 mL) and
deionized water (3 mL) was sealed into a steel bomb
equipped with a Teflon liner (15 mL), and then heated at
90 °C for 3 days. White needle crystals of complex 2 were
recovered in ca. 35 % yield based on H2L2. Elemental
analyses for 2, C28H34N2O8Zn2 (Mr = 657.31): C, 51.2; H,
5.0; N, 4.2 %; Calcd.: C, 51.1; H, 5.2; N, 4.3 %. IR data
(KBr, cm-1): 3,267(s), 2,958(s), 1,672(vs), 1,616(s),
1,593(s), 1,552(s), 1,412(vs), 1,327(s), 1,252(s), 1,205(s),
1,182(m), 1,045(m), 1,094(m), 970(m), 933(m), 850(s),
771(m), 715(m).
Results and discussion
As shown in Fig. 1, complex 1 contains one Cu(II) atom,
an anionic L1 ligand, a coordinated water ligand and a
lattice water molecule in its asymmetric unit. The Cu(II)
atoms are four-coordinated by two oxygen atoms and a
nitrogen atom from two L ligands, and a water ligand in a
distorted tetrahedral geometry. The Cu–O distances range
Synthesis of [Co(L2)(H2O)] (3)
˚
from 1.917(3) to 1.940(3) A, while the Cu–N distance is
˚
The synthesis of complex 3 was similar to that for 2, except
that Co(NO3)2ꢀ6H2O (0.058 g, 0.2 mmol) was used instead of
Zn(NO3)2ꢀ6H2O. Pink rod crystals of complex 3 were
recovered in ca. 30 % yield based on H2L2. Elemental anal-
yses for 3, C14H19CoNO5 (Mr = 340.23): C, 49.4; H, 5.5; N,
4.1 %; Calcd.: C, 49.4; H, 5.6; N, 4.1 %. IR data (KBr, cm-1):
3,327(s), 2,953(s), 1,609(vs), 1,564(s), 1,366(vs), 1,209(s),
2.013(3) A, which are comparable to the values reported
for other Cu(II) complexes [25]. The L1 chelates a Cu(II)
atom by means of a nitrogen atom and an oxygen atom of
the carboxyl group and also bridges with an other Cu(II)
atom. By means of these bridging ligands, a 1D left-hand
helical chain is formed (Fig. 2). These chains are further
linked by hydrogen bonds between the water molecules
123

Transition Met Chem (2013) 38:413–418
415
Table 1 Summary of crystal data and structural refinements for complexes 1–4
Complex
1
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
C11H15CuNO6
320.78
C28H34N2O8Zn2
657.31
C14H19CoNO5
340.23
C14H19NNiO5
340.01
Tetragonal
P43
Orthorhombic
P21212
Orthorhombic
P212121
Orthorhombic
P212121
˚
a(A)
7.5156(6)
7.5156(6)
22.679(3)
1281.0(2)
4
21.1766(18)
22.4239(19)
6.5093(6)
3091.0(5)
4
5.9014(3)
11.8762(6)
22.0534(11)
1545.64(13)
4
5.795(3)
˚
b(A)
11.807(6)
21.974(10)
1503.4(13)
4
˚
c(A)
3
˚
V(A )
Z
Dc (g cm-3
)
1.663
1.412
1.462
1.502
l(Mo–Ka) (mm-1
)
1.727
1.600
1.130
1.310
F(000)
660
1,360
708
712
Measured-reflections
Unique-reflections [Rint
GOF on F2
6,614
11,927
6,719
6,806
]
2,475(0.0601)
1.032
5,723(0.0715)
1.010
2,437(0.0445)
1.181
2,434(0.0849)
0.0856
hkl range
-6 B h B 9
-9 B k B 9
-27 B l B 26
0.0414/0.1091
0.0416/0.1096
-0.005(16)
-25 B h B 24
-27 B k B 27
-8 B l B 2
0.0552/0.1227
0.0767/0.1381
0.02(3)
-7 B h B 7
-14 B k B 14
-25 B l B 18
0.0432/0.1194
0.0675/0.1626
-0.04(4)
-7 B h B 6
-14 B k B 14
-25 B l B 24
0.0485/0.0758
0.0738/0.0849
0.00(2)
R1, wR2 [I [ 2r(I)]
R1, wR2 (all data)
Flack parameter
ꢀ
ꢁ
1=2
h
i
h
i
2
2
P
P
P
P
2
2
2
R1 ¼ kFojꢁjFck= jFoj; wR2 ¼
w ðFoÞ ꢁ ðFcÞ
=
w ðFoÞ
˚
Table 2 Selected bond lengths (A) for 1–4
Complex 1
Cu(1)–O(2W)
Cu(1)–N(1)#1
Complex 2
1.917(3)
2.013(3)
Cu(1)–O(3)#1
1.925(3)
Cu(1)–O(2)
1.940(3)
Zn(1)–O(1)#2
Zn(1)–O(7)
1.960(5)
2.092(4)
1.987(4)
2.243(5)
Zn(1)–O(3)
1.980(4)
2.240(4)
2.007(4)
Zn(1)–O(5)#3
Zn(2)–O(8)
2.003(4)
1.941(4)
2.040(5)
Zn(1)–N(2)#3
Zn(2)–O(3)#4
Zn(2)–O(5)#3
Zn(2)–N(1)#4
Complex 3
Zn(2)–O(2)#5
Co(1)–O(2)
2.069(5)
2.088(5)
Co(1)–O(3)#6
Co(1)–O(4)#8
2.084(5)
2.164(4)
Co(1)–O(4)#7
Co(1)–N(1)#8
2.094(4)
2.213(6)
Co(1)–O(1 W)
Complex 4
Ni(1)–O(1)#9
Ni(1)–O(1W)
2.029(4)
2.057(3)
Ni(1)–O(4)
2.043(4)
2.115(3)
Ni(1)–O(2)#10
Ni(1)–N(1)#11
2.049(3)
2.159(4)
Ni(1)–O(2)#11
Symmetry transformations used to generate equivalent atoms: #1 y - 1, -x ? 1, z ? 1/4, #2 x - 1/2, -y ? 3/2, -z ? 4, #3 -x ? 1/2, y - 1/
2, -z ? 3, #4 x, y, z - 1, #5 x - 1/2, -y ? 3/2, -z ? 3, #6 -x ? 1, y - 1/2, -z ? 1/2, #7 -x ? 3/2, -y ? 2, z - 1/2, #8 -x ? 2, y - 1/2,
-z ? 1/2, #9 -x ? 2, y - 1/2, -z ? 3/2, #10 -x ? 3/2, -y ? 1, z ? 1/2, #11 -x ? 1, y - 1/2, -z ? 3/2
and the oxygen atoms of the ligands into a supramolecular
structure (Fig. 3).
nitrogen atom from four L ligands in a distorted trigonal
bipyramid geometry. The Zn–O distances range from
˚
1.960(5) to 2.092(4) A, while the Zn–N distances are
˚
2.240(4) and 2.243(5) A, which are comparable to values
Complex 2 contains two Zn(II) atoms and two anionic
L2 ligands in its asymmetric unit (Fig. 4). The Zn(II) atoms
all are five-coordinated by four oxygen atoms and a
reported for other Zn(II) complexes [26]. The coordination
123

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Transition Met Chem (2013) 38:413–418
Fig. 1 The molecular structure of complex 1 with 30 % probability
displacement ellipsoids. Hydrogen atoms were omitted for clarity.
Symmetry codes for the generated atoms. A y - 1, 1 - x, 0.25 ? z,
B 1 - y, 1 ? x, z - 0.25
modes of the two L2 ligands are of the same. Each bridges
and chelates four Zn(II) atoms. Furthermore, the Zn(II)
atoms are bridged by one carboxyl oxygen of the L2 ligand
to form a 1D chain, which is further coupled by the phenyl
carboxylate groups of the neighboring L2 ligands to prop-
agate into a 3D framework structure (Fig. 5). The 3D
framework contains two kinds of cavities that are occupied
by the isobutyl groups.
Fig. 3 View the structure of complex 1 down the c axis. The
hydrogen bonds are represented by dashed lines
the thermal curves are given in the ESI Figs. S5–S8. The
TGA curve of complex 1 exhibit two stages of weight loss.
The first stage is assigned to the release of the lattice water
molecules (from 40 to 75 °C, expt.: 5.4 %, calcd.: 5.6 %);
the second stage arises from release of the water ligands
followed by decomposition of the organic ligands (from
190 to 440 °C). The TGA curve of complex 2 shows that it
is stable until 390 °C; the decomposition temperature of
this complex spans the range of 390–470 °C. According to
the TGA curve of complex 3, the water molecules are lost
in the range of 115–150 °C, expt.: 5.0 %, calcd.: 5.3 %;
the decomposition temperature of this complex is in the
range of 360–490 °C. Finally, the TGA curve of 4 shows
that the water molecules are lost in the range of
140–180 °C, expt.: 5.2 %, calcd.: 5.3 %, and the decom-
position temperature of this complex is in the range of
360–450 °C.
Complexes 3 and 4 are isostructural and also exhibit a
3D structure. The structure of 3 will be discussed in detail
as an example. As shown in Fig. 6, complex 3 contains one
Co(II) atom, an anionic L2 ligand, and a coordinated water
ligand in its asymmetric unit. The Co(II) atom is six-
coordinated by four oxygen atoms and a nitrogen atom
from four L2 ligands and an oxygen atom from the water
ligand. The Co–O distances ranges from 2.069(5) to
˚
2.164(4) A, and the Co–N distance is 2.213(6), which are
again comparable to values reported for other Co(II)
complexes [25]. Each L2 ligand chelates a Co(II) atom and
also bridges with three other Co(II) atoms. The Co(II)
atoms are bridged by the L2 ligands to a form 3D frame-
work structure with a 1D chain (Fig. 7). The 3D framework
contains two kinds of cavities that are occupied by the
isobutyl groups.
The solid-state photoluminescence spectrum of complex
2 at room temperature is depicted in Fig. 8. Complex 2
exhibits emission peaks at 414 nm upon excitation at
302 nm. The emission bands of complex 2 most likely
originate from intraligand luminescent emissions [27].
The simulated and experimental PXRD patterns of
complexes 1–4 are in good agreement (see ESI, Figs. S1–
S4), indicating the phase purity of the products. The
thermal behavior of complexes 1–4 was also studied, and
Fig. 2 The structure of the chain in complex 1
123

Transition Met Chem (2013) 38:413–418
417
Fig. 6 The molecular structure of complex 3 with 30 % probability
displacement ellipsoids. Hydrogen atoms were omitted for clarity.
Symmetry codes for the generated atoms. A 2 - x, -0.5 ? y, 0.5 -
z, B 1.5 - x, 2 - y, z - 0.5, C 1 - x, y - 0.5, 0.5 - z, D 1 - x,
y ? 0.5, 0.5 - z, E 1.5 - x, 2 - y, z ? 0.5, F 2 - x, 0.5 ? y,
0.5 - z
Fig. 4 The molecular structure of complex 2 with 30 % probability
displacement ellipsoids. Hydrogen atoms were omitted for clarity.
Symmetry codes for the generated atoms. A 1.5 - x, 0.5 ? y, -z-1,
B 0.5 ? x, 0.5 - y, -z-2, C 0.5 ? x, 0.5 - y, -1-z, D x, y, z ?
1, E x - 0.5, 0.5 - y, -z-2, F x - 0.5, 0.5 - y, -z-1, G x, y,
z - 1, H 1.5 - x, y - 0.5, -z-1
Fig. 7 View of the structure of complex 3 down the a axis. Hydrogen
atoms were omitted for clarity
Fig. 5 View of the structure of complex 2 down the c axis. Hydrogen
atoms were omitted for clarity
emission
excitation
Conclusion
250
300
350
400
450
500
550
600
Wavelength(nm)
In summary, we have synthesized four homochiral coor-
dination polymers based on the chiral N-(4-carboxyl)
benzyl-L-alanine or the N-(4-carboxyl)benzyl-L-leucine
ligands. This example illustrates that NCS complexes can
be relatively easily obtained by the use of chiral ligands. In
our future work, we will concentrate our efforts on
Fig. 8 The solid-state photoluminescent spectra of complex 2
construction of the NCS complexes for potential applica-
tions in areas such as pyroelectricity, ferroelectricity, and
especially second-order nonlinear optics.
123

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Transition Met Chem (2013) 38:413–418
8. Xu WT, Chen L, Jiang FL, Hong MC (2012) Chin J Struct Chem
31:321
Supplementary material
9. Herm ZR, Swisher JA, Smit B, Krishna R, Long JR (2011) J Am
Chem Soc 133:5664
10. Evans OR, Lin W (2002) Acc Chem Res 35:511
11. Jayanty S, Gangopadhyay P, Radhakrishnan TP (2002) J Mater
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12. Prakash MJ, Radhakrishnan TP (2006) Inorg Chem 45:9758
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(2009) J Am Chem Soc 131:6894
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17. Cao GJ, Fang WH, Zheng ST, Yang GY (2010) Inorg Chem
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CCDC 902678 to 902681 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: (internet.) C44
1223 336 033; E-mail: deposit@ccdc.cam.ac.uk]. Elec-
tronic supplementary information (ESI) available: PXRD
and TGA curves of the four complexes.
Acknowledgments This work was supported by the Research
Program of Ningde Normal University (2011H103, 2011Y001 and
2011J001).
18. Li JT, Cao DK, Akutagawa T, Zheng LM (2010) Dalton Trans
39:8606
19. Du ZY, Sun YH, Xu X, Xu GH, Xie YR (2010) Eur J Inorg Chem
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