Syntheses and properties of two chiral multidentate carboxylate copper coordination compounds

Article abstract of DOI:10.1080/00958972.2012.735363

A chiral ligand, (R)-2-(4-(carboxymethoxy)phenoxy)propanoic acid, is employed as building block to construct two chiral coordination compounds, [Cu(L)(bipy)(H₂O)] (1) and [Cu₂(L)₂(phen) ₄(H₂O)₁₃] (2), by the solvent-thermal method and the volatilization method, respectively. The compounds are characterized by single-crystal X-ray diffraction (XRD), IR spectra, XRD, TGA, and circular dichroism spectra. Compound 1 is comprised of right-handed helical chains and 2 shows a separated structure, both of which extend to 3-D supramolecular framework by hydrogen bonds and π...π interactions. CD indicates both compounds retain original stereo character with strong fluorescence at room temperature.

Full text of DOI:10.1080/00958972.2012.735363

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Syntheses and properties of two chiral
multidentate carboxylate copper
coordination compounds
Ying-Hui Yu a , Chang-Yue Ren a , Guang-Feng Hou a , Hantao Ye a
& Jin-Sheng Gao a
a Engineering Research Center of Pesticide of Heilongjiang
University, Heilongjiang University , Harbin 150080 , China
Accepted author version posted online: 28 Sep 2012.Published
online: 15 Oct 2012.
To cite this article: Ying-Hui Yu , Chang-Yue Ren , Guang-Feng Hou , Hantao Ye & Jin-
Sheng Gao (2012) Syntheses and properties of two chiral multidentate carboxylate copper
coordination compounds, Journal of Coordination Chemistry, 65:23, 4137-4146, DOI:
10.1080/00958972.2012.735363
To link to this article: http://dx.doi.org/10.1080/00958972.2012.735363
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Journal of Coordination Chemistry
Vol. 65, No. 23, 10 December 2012, 4137–4146
Syntheses and properties of two chiral multidentate
carboxylate copper coordination compounds
YING-HUI YU, CHANG-YUE REN, GUANG-FENG HOU,
HANTAO YE and JIN-SHENG GAO*
Engineering Research Center of Pesticide of Heilongjiang University,
Heilongjiang University, Harbin 150080, China
(Received 3 May 2012; in final form 27 August 2012)
A chiral ligand, (R)-2-(4-(carboxymethoxy)phenoxy)propanoic acid, is employed as building
block to construct two chiral coordination compounds, [Cu(L)(bipy)(H2O)] (1) and
[Cu2(L)2(phen)4(H2O)13] (2), by the solvent-thermal method and the volatilization method,
respectively. The compounds are characterized by single-crystal X-ray diffraction (XRD), IR
spectra, XRD, TGA, and circular dichroism spectra. Compound 1 is comprised of right-handed
helical chains and 2 shows a separated structure, both of which extend to 3-D supramolecular
framework by hydrogen bonds and ꢀ ꢀ ꢀ ꢀ ꢀ interactions. CD indicates both compounds retain
original stereo character with strong fluorescence at room temperature.
Keywords: X-ray; Coordination; Chiral; Helical
1. Introduction
Inorganic–organic hybrid compounds with broad applications have received significant
attention [1]. The design and synthesis of chiral metal–organic coordination compounds
have especially evoked interest owning to unique properties [2]. Building logical non-
centrosymmetric or chiral coordination compounds is still a challenge for materials
and synthetic chemists. Chiral coordination compounds can be obtained by two routes.
The first is spontaneous resolution upon crystallization without any chiral sources,
which is reported to be used in construction of some chiral coordination compounds [3].
However, spontaneous resolution is still a relatively scarce phenomenon and cannot be
predicted a priori because the mechanisms of the processes are not fully understood.
Moreover, spontaneous resolution will result in a racemic mixture which cannot be
easily separated. The second is stereoselective synthesis using enantiopure chiral
species. Compared to the first way, this route is more feasible and effective. Recently,
ligands containing single or multiple asymmetric centers are used to synthesize chiral
coordination compounds, most of which have helical chains or channel structures with
good predictability of space configuration [4].
*Corresponding author. Email: gaojins@hlju.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.735363

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Y.-H. Yu et al.
Coordination polymers based on chiral carboxylic acid ligands showed advantageous
properties as magnetic materials, porous materials, nonlinear optical materials,
and in asymmetric synthesis [5]. Carboxylates are efficient for generation of helical
coordination polymers [6]. Inspired by the aforementioned considerations, we select
chiral carboxylates as building blocks [7]. We synthesized a chiral ligand with phenoxy
propanoic acid structure to construct new coordination compounds and introduce
auxiliary ligands to extend delocalization of ꢀ electrons and enhance molecular rigidity.
Auxiliary ligands can enhance performance considering 2,2-bipy and phen exhibit
fluorescence enhancement.
In this article, we report two chiral coordination compounds, [Cu(L)(bipy)(H2O)] (1)
and [Cu2(L)2(phen)4(H2O)13] (2). Compound 1 is comprised of right-handed helical
chains and 2 shows a separated structure, both of which extend to 3-D supramolecular
framework by hydrogen bonds and ꢀ ꢀ ꢀ ꢀ ꢀ interactions. The optical activities and the
luminescence properties are investigated.
2. Experimental
2.1. Materials and methods
All reagents and solvents were purchased from commercial sources and used without
purification. IR spectra were recorded from 4000 to 400 cmꢁ1 using KBr pellets on a
Spectrum One FT-IR spectrometer. 1H NMR spectra were performed on a Bruker AV
300 MHz spectrometer. X-ray powder diffraction (XRPD) data were collected on a
Bruker D8 AVANCE diffractometer. Elemental analyses were performed on a CARLO
ERBA 1106 analyzer. Circular dichroism (CD) spectra were recorded on a JASCO
J-810. Thermogravimetric measurements were carried out on a Perkin-Elmer TGA-7
analyzer. Luminescence spectra for solid samples were measured at room temperature
on a Perkin-Elmer LS 55 luminance meter.
2.2. Preparation of the ligand
H2L (scheme 1) was synthesized by a two-step procedure. In the first step, methyl
chloroacetate (18 g, 163 mmol) was dropped into a mixture of methyl R-(þ)-2-(4-
hydroxyphenoxy)propionate (21.6 g, 110 mmol), anhydrous K2CO3 (64 g, 463 mmol),
CH3
CH3
O
CH3
Cl
O
H
H
HO
O C
H3COOH2C O
O C
COOCH3
COOCH3
CH3
HCl
HOOCH2C O
O
C
H
COOH
L
H2
Scheme 1. Synthesis route of the ligand.

Chiral multidentate carboxylate copper
4139
sodium iodide (16 g, 107 mmol), and disodium hydrogen phosphate (5 g, 35.4 mmol) in
anhydrous acetone (350 mL). After refluxing for 10 h, the mixture was filtered to
remove unreacted K2CO3. Acetone was then distilled off and product was obtained [8].
In the second step, hydrochloric acid (2 g, 56 mmol) was added to a solution of methyl
R-(þ)-2-(4-hydroxy phenoxy)propionate (12.8 g, 50 mmol) in ethanol and water (1 : 1).
The mixture was stirred for 5–6 h at 60ꢂC. The ethanol was then distilled off and
PEG-400 (1 g) was added. After heating for 7–8 h the mixture was cooled to room
temperature to get light yellow powder. The powder was recrystallized with water to get
white crystals (94% ee). Elemental Anal. Calcd for C11H12O6 (%): C, 55.00; H, 5.04.
Found (%): C, 55.02; H, 5.01. IR data (KBr, cmꢁ1): 3047 (m), 2584 (w), 1717 (s), 1507
1
(s), 1428 (m), 1208 (s), 1138 (s), 1078 (m), 821 (s) (figure S1). H NMR (DMSO-d6,
400 MHz): ꢁ 6.88 (m, 4H), 4.71 (q, J ¼ 6.8 Hz, 1H), 4.58 (s, 2H), 1.47 (d, J ¼ 6.8 Hz, 3H)
(figure S2).
2.3. Syntheses of 1 and 2
2.3.1. Synthesis of [Cu(L)(bipy)(H2O)] (1). A mixture of Cu(AcO)2 ꢀ H2O (0.399 g,
0.2 mmol), H2L (0.048 g, 0.2 mmol), and 2,2-bipy (0.0396 g, 0.2 mmol) was dissolved in
2 mL of distilled water and 4 mL of DMF. The resulting mixture was then transferred
and sealed in a 20 mL Teflon-lined stainless steel vessel and heated at 90ꢂC for 2 days.
After the reactor was slowly cooled to room temperature, purple blue block crystals
were filtered off, washed with distilled water, and dried in air. Yield: 78%. IR data
(KBr, cmꢁ1): 3423 (m), 3060 (w), 2898 (w), 1676 (m), 1630 (s), 1506 (s), 1419 (m), 1316
(m), 1216(s), 950 (w), 822 (m), and 776 (m). Elemental Anal. Calcd for C21H20CuN2O7
(%): C, 53.00; H, 4.24; N, 5.89. Found (%): C, 53.06; H, 4.21; N, 5.90.
2.3.2. Synthesis of Cu2(L)2(phen)4(H2O)13 (2). Compound 2 was synthesized by the
reaction of H2L (0.048, 0.2 mmol) and Cu(AcO)2 ꢀ H2O (0.399 g, 0.2 mmol) in CH3OH
(15 mL) which was stirred for 0.5 h. The precipitate collected by filtration was
redissolved in a solution of 1,10-phenanthroline (0.0396 g, 0.2 mmol) in 20 mL of water
and stirred for 1 h. After filtration, green stick crystals suitable for X-ray diffraction
(XRD) were obtained by slow diffusion for several days [9]. Yield: 53%. IR data (KBr,
cmꢁ1): 3368 (w), 3050 (w), 1598 (s), 1505 (m), 1427 (m), 1391 (m), 1207 (m), 1091 (w),
848 (m), 723 (s), 569 (w). Elemental Anal. Calcd for C70H78Cu2N8O24 (%): C, 54.61; H,
4.91; N, 7.28. Found (%): C, 54.58; H, 4.89; N, 7.30.
3. Results and discussion
XRD measurements for 1 and 2 were carried out on a Rigaku R-AXIS RAPID imaging
˚
plate diffractometer with graphite-monochromated Mo-Ka (ꢂ ¼ 0.71073 A) radiation at
293 K. Empirical absorption corrections based on equivalent reflections were applied.
The structures of 1 and 2 were solved by direct methods using SHELXS 97 [10].
All non-hydrogen atoms were refined anisotropically by full-matrix least-squares on F2.
Hydrogen atoms bound to C were placed in calculated positions and treated as riding.

4140
Y.-H. Yu et al.
Table 1. Crystallographic data for 1 and 2.
1
2
Chemical formula
Formula weight
Crystal system
Space group
C21 H20CuN2O7
475.93
Orthorhombic
P21
C70 H78Cu2N8O24
1558.48
Triclinic
P1
ꢂ
˚
Unit cell dimensions (A, )
a
b
c
ꢃ
ꢄ
ꢅ
6.9652(14)
15.046(3)
19.065(4)
90
90
90
1998.1(7), 4
1.582
980
10.306
12.007
16.177
71.060
72.800 (19)
87.217 (18)
1806.3(16), 1
1.433
812
12,004
3
˚
Volume (A ), Z
Calculated density (g cmꢁ3
F(000)
)
Reflection collection
Unique reflections
Number of parameters
Goodness-of-fit on F2
R indices [I 4 2ꢆ(I)]
19,449
4557 [R(int) ¼ 0.0235]
9449 [R(int) ¼ 0.0636]
281
1.028
939
0.996
R1 ¼ 0.0216
wR2 ¼ 0.0540
R1 ¼ 0.0234
wR2 ¼ 0.0545
R1 ¼ 0.0715
wR2 ¼ 0.1831
R1 ¼ 0.1236
wR2 ¼ 0.2206
R indices (all data)
The crystal parameters, data collection, and refinement results are summarized in
table 1, selected bond lengths and angles are listed in table 2, and hydrogen-bond
parameters are provided in table S1. CCDC reference numbers for 1 and 2 are 872,686
and 872,687, respectively.
3.1. Descriptions of structures
3.1.1. [Cu(L)(bipy)(H2O)] (1). X-ray analysis reveals that 1 crystallized in the
chiral orthorhombic space group P21. The asymmetric unit contains one crystallo-
graphically unique Cu(II), two L, a 2,2-bipy, and a water molecule. The Cu(II)
possesses a coordination number of five, coordinating with two oxygen atoms from two
different L and two nitrogen atoms from one 2,2-bipy, as shown in figure 1, in a
distorted rectangular pyramid. All L exhibit monodentate coordination; uncoordinated
˚
carboxylic oxygen atoms in L have weak interactions with Cu(II) at 2.87 A. Compound
1 is chiral consistent with L, angle O1–C2–O3 is 112.0ꢂ, and angle C2–C1–O3 is 120.3ꢂ,
which forms a right-handed helical chain along the b-axis, as shown in figure 2(a).
˚
The pitch of the helix is 19.065 A. The helical character in 1 is probably due to
the space angles produced by the chiral center of L. The prominent feature of 1
˚
˚
is the helical channels along the c-axis with a width of 10.868 A ꢃ 5.227 A, with
potential as asymmetric catalysts. An analogous achiral ligand, namely 2,20-(1,4-
phenylenebis(oxy))diacetic acid, reacting with 2,2-bipy and Cu(OAc)2 could produce
a coordination compound with a zigzag chain [11]. O–H ꢀ ꢀ ꢀ O interactions bridge
adjacent helical chiral chains into 2-D network which is further connected to 3-D
supramolecular framework by ꢀ–ꢀ interactions of 2,2-bipy.

Chiral multidentate carboxylate copper
4141
ꢂ
˚
Table 2. Selected bond lengths (A) and angles ( ) for 1 and 2.
Compound 1
Cu(1)–O(6)#1
Cu(1)–N(1)
Cu(1)–O(7)
O(6)#1–Cu(1)–N(1)
O(6)#1–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
O(2)–Cu(1)–O(7)
N(2)–Cu(1)–O(7)
1.957(1)
2.003(1)
2.264(1)
169.23(6)
92.26(6)
80.09(6)
90.62(5)
98.04(6)
Cu(1)–O(2)
Cu(1)–N(2)
1.966(1)
2.035(1)
91.63(5)
94.82(5)
170.41(5)
91.49(5)
97.05(6)
O(6)#1–Cu(1)–O(2)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(2)
O(6)#1–Cu(1)–O(7)
N(1)–Cu(1)–O(7)
Compound 2
Cu(1)–N(1)
Cu(1)–O(1)
Cu(1)–N(3)
Cu(2)–O(7)
Cu(2)–N(7)
N(1)–Cu(1)–N(4)
N(4)–Cu(1)–O(1)
N(4)–Cu(1)–N(2)
N(1)–Cu(1)–N(3)
O(1)–Cu(1)–N(3)
N(5)–Cu(2)–O(7)
O(7)–Cu(2)–N(8)
O(7)–Cu(2)–N(7)
N(5)–Cu(2)–N(6)
N(8)–Cu(2)–N(6)
1.993(14)
2.049(10)
2.143(14)
1.981(11)
2.026(11)
176.2(6)
95.6(4)
Cu(1)–N(4)
Cu(1)–N(2)
Cu(2)–N(5)
Cu(2)–N(8)
Cu(2)–N(6)
N(1)–Cu(1)–O(1)
N(1)–Cu(1)–N(2)
O(1)–Cu(1)–N(2)
N(4)–Cu(1)–N(3)
N(2)–Cu(1)–N(3)
N(5)–Cu(2)–N(8)
N(5)–Cu(2)–N(7)
N(8)–Cu(2)–N(7)
O(7)–Cu(2)–N(6)
N(7)–Cu(2)–N(6)
2.025(10)
2.108(14)
1.980(14)
1.999(10)
2.213(13)
88.0(5)
81.5(5)
151.2(4)
79.3(5)
112.9(4)
175.3(6)
95.2(5)
82.1(5)
95.7(5)
113.0(5)
94.7(5)
101.7(5)
95.5(5)
94.8(5)
89.4(5)
150.7(5)
81.6(5)
95.9(5)
Symmetry transformations used to generate equivalent atoms for 1: #1 ꢁ x þ 1/2, ꢁy þ 1, z þ 1/2.
Figure 1. Molecular structure of 1 displaying the crystallographic-numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level.
3.1.2. Cu2(L)2(phen)4(H2O)13 (2). X-ray analysis reveals that 2 crystallized in the
chiral triclinic space group P1. The asymmetrical unit consists of two Cu(II), two L, and
four phen. Both Cu(II) possess a coordination number of five with four nitrogen atoms
from two different phen and one oxygen atom from L, as shown in figure 3. Cu(II)
exhibits a distorted rectangular pyramid. Both L exhibit monodentate coordination and
the configuration of chiral carbon is consistent with that in L; ꢀ–ꢀ interaction plays an
important role in 2. Two independent chiral molecules connect with each other to form
an asymmetric unit of 2, many of which are further packed together into a chain by ꢀ–ꢀ
interactions. These ꢀ–ꢀ interactions occur in different positions of the ligand.
Interaction between complex molecules are on the middle phenyl rings of phen with
˚
a bond length of 3.558 A; interaction in chain construction are on the middle phenyl
˚
rings in right and on the side phenyl rings in left, with a bond length of 3.574 A. Two
˚
other kinds of ꢀ–ꢀ interactions with distance of 3.582 A and 3.751 A bridge adjacent
˚
1-D chains into a 2-D network with numerous complicated H-bonds between water

4142
Y.-H. Yu et al.
Figure 2. (a) The 2-D supramolecular network connected by H-bonds between adjacent helical chiral
chains; (b) the right-handed chiral helical along the c-axis; and (c) the right-handed helical channel.
Figure 3. Molecular structure of 2 displaying the crystallographic-numbering scheme. Displacement
ellipsoids are drawn at the 50% probability level. All hydrogen atoms and water molecules are omitted
for clarity.
molecules and carboxylates bridging the 2-D network into a 3-D supramolecular
framework. Figure 4 exhibits four different ꢀ–ꢀ interactions.
3.2. XRPD and thermal analyses
XRPD was used to determine the purity of 1 and 2. As shown in ‘‘Supplementary
material’’, the peaks of 1 closely match those in simulated patterns generated from

Chiral multidentate carboxylate copper
4143
Figure 4. The 2-D supramolecular network of 2 connected by four kinds of ꢀ ꢀ ꢀ ꢀ ꢀ interactions; water
molecules are omitted for clarity.
single-crystal diffraction data, indicating high phase purity of the product. The peaks
of 2 displayed in measured pattern seem not to be accord with those in the simulated
patterns completely. The intensity of some peaks changed and a few peaks disappeared.
The phenomenon is probably due to loss of crystallized water molecules in 2 during the
process of heating in XRPD analysis.
To study the stability of these compounds, TGA of 1 and 2 are performed
(Supplementary material). Compound 1 lost a coordination water molecule from 163ꢂC
(Calcd: 3.78%, Found: 6.47%), while 2 immediately collapsed at the beginning of
heating, owing to loss of crystallized water (Calcd: 15.01%, Found: 13.11%). The loss
of L in 1 appears from 229ꢂC to 311ꢂC (Calcd: 46.59%, Found: 49.61%). The curve of 2
has a rapid weight loss at 243ꢂC, which indicates loss of L (Calcd: 33.46%, Found:
32.38%). The third weight loss is from auxiliary ligand (Calcd: 32.82%, Found: 32.66%
for 1; Calcd: 46.26%, Found: 43.25% for 2). Compound 1 decomposed completely at
500ꢂC and the residue is presumed to be CuO (Calcd: 16.81%, Found: 16.19%).
Compound 2 is more stable and continued to decompose up to 565ꢂC (Calcd: 10.27%,
Found: 11.26%).
3.3. CD spectra and photoluminescent properties
CD spectrum is a powerful technique to obtain conformational information of chiral
molecules. As shown in figure 5, the CD spectra of 1, 2, and the ligand measured
in DMF show strong signals corresponding to the absorptions of the UV spectra. The
UV spectra are shown in inset. The maximum peaks of coordination compounds have

4144
Y.-H. Yu et al.
Figure 5. CD spectra of ligand, 1, and 2 in DMF (inset: UV absorption spectra in DMF).
Figure 6. Solid-state fluorescence emission spectra of ligand, 1, and 2 at room temperature.
a slightly blue shift relative to ligand from 265 nm to 273 nm, which arises from the
presence of a large conjugated ꢀ-system of the molecule. The negative maximum peaks
in accord with the X-ray results indicate that 1 and 2 maintained the right-handed
configuration during coordination.
We also investigated the luminescence properties of 1 and 2 for potential applications
as light-emitting diodes. The solid-state fluorescence emission spectra at room
temperature are shown in figure 6. Compared with ligand, in which a fluorescence
emission is observed at ꢂmax ¼ 465 nm upon excitation at 350 nm, 1 exhibits a strong
blue emission band at 468 nm and a green band at 513 nm upon excitation at 350 nm.
Compound 2 shows a blue emission band at 469 nm and a broad strong green band
at 510 nm with excitation at 335 nm. We speculate that the intensity of blue bands

Chiral multidentate carboxylate copper
4145
are probably due to strong luminescence of ligand, the auxiliary ligands can obviously
affect the luminescence and produce co-fluorescence effect which manifest as the green
bands. These observations indicate that 1 and 2 may be candidates for photoactive
materials in asymmetric reactions.
4. Conclusion
Chiral coordination polymers can be achieved from achiral or racemic ligands by
spontaneous resolution upon crystallization, which is difficult to predict [12]. Therefore,
we used enantiopure chiral carboxylic acid as the source of chirality to construct
coordination polymers.
In particular, we prepared two chiral metal–organic frameworks from a chiral ligand,
Cu(OAc)2, and two auxiliary ligands (phen and 2,2-bipy). Compound 1 is comprised
of right-handed chains assembled by the solvent-thermal method and 2 shows
separated structure containing four different kinds of ꢀ-bonds. Compounds 1 and 2
are extended to 3-D supramolecular sheets by hydrogen bonds and ꢀ ꢀ ꢀ ꢀ ꢀ interactions.
Both compounds exhibit significant CD and can be excited directly and emit strong
fluorescence at room temperature. Our study provides insight into the synthesis of
chiral coordination polymers from chiral ligands.
Acknowledgments
We acknowledge the support of the Natural Science Foundation of Heilongjiang
Province (B201108).
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