Syntheses, crystal structures and optical properties of six homochiral coordination networks based on phenyl acid-amino acids

Article abstract of DOI:10.1039/c1ce05422h

Six homochiral metal-organic coordination networks based on two new phenyl acid-amino acids, [ZnL¹₂(H₂O)₂] (1), [Cu₂L¹₂(H₂O)₃] (2), [Zn₂L¹₂] (3), [CuL²(H ₂O)₂]·2H₂O (4), [ZnL²] (5) and [Cd₃NaL²₃(ClO₄)]·3H ₂O·DMF (6), where L¹ = (S)-4-((1-carboxy-2- methylpropylamino)methyl)benzoic acid, and L² = (S)-4-((1-carboxy-2- hydroxyethylamino)methyl)benzoic acid, have been synthesized and characterized. Compound 1 is a supramolecular network of hydrogen bondings linking up discrete zinc coordination units, and compound 2 presents a 2D lattice motif of water molecules linking up polymeric chains, while compound 3 is a 3D network constructed of interesting left-hand triple helices. Compound 4 consists of 1D coordination polymeric chains, while compound 5 adopts an intriguing 2D diamondoid network built from two kinds of left-handed helices, and compound 6 is a 3D framework constructed from homochiral cages. In addition, the second-order nonlinear optics (NLO) properties of compounds 1-6 were investigated. Compound 1 has a powder SHG intensity of 3.4 versus a-quartz, and the powder SHG intensities of compounds 2-4 approximate to that of a-quartz, while the powder SHG intensities are 7.8 and 8.4 versus a-quartz for compounds 5 and 6, respectively. Compounds 1, 3, 5 and 6 in the solid state exhibit blue fluorescent emissions at room temperature, mainly attributed to the ligand-centered transitions.

Full text of DOI:10.1039/c1ce05422h

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PAPER
Syntheses, crystal structures and optical properties of six homochiral
coordination networks based on phenyl acid-amino acids†
a
a
a
Xiu-Li Yang, Ming-Hua Xie, Chao Zou and Chuan-De Wu
ab
*
Received 8th April 2011, Accepted 20th July 2011
DOI: 10.1039/c1ce05422h
Six homochiral metal–organic coordination networks based on two new phenyl acid-amino acids,
[ZnL1 (H2O)2] (1), [Cu2L1 (H2O)3] (2), [Zn2L1 ] (3), [CuL2(H2O)2]$2H2O (4), [ZnL2] (5) and
2
2
2
[Cd3NaL2 (ClO4)]$3H2O$DMF (6), where L1 ¼ (S)-4-((1-carboxy-2-methylpropylamino)methyl)
3
benzoic acid, and L2 ¼ (S)-4-((1-carboxy-2-hydroxyethylamino)methyl)benzoic acid, have been
synthesized and characterized. Compound 1 is a supramolecular network of hydrogen bondings linking
up discrete zinc coordination units, and compound 2 presents a 2D lattice motif of water molecules
linking up polymeric chains, while compound 3 is a 3D network constructed of interesting left-hand
triple helices. Compound 4 consists of 1D coordination polymeric chains, while compound 5 adopts an
intriguing 2D diamondoid network built from two kinds of left-handed helices, and compound 6 is
a 3D framework constructed from homochiral cages. In addition, the second-order nonlinear optics
(NLO) properties of compounds 1–6 were investigated. Compound 1 has a powder SHG intensity of
3.4 versus a-quartz, and the powder SHG intensities of compounds 2–4 approximate to that of a-quartz,
while the powder SHG intensities are 7.8 and 8.4 versus a-quartz for compounds 5 and 6, respectively.
Compounds 1, 3, 5 and 6 in the solid state exhibit blue fluorescent emissions at room temperature,
mainly attributed to the ligand-centered transitions.
ways is to react with aldehyde, such as salicylaldehyde and pyr-
Introduction
idinecarboxaldehyde, to generate Schiff base type ligands.6,7
During the course of our studies on the synthesis of homo-
chiral coordination networks for chiral related applications, we
have designed and synthesized a series of new ligands based on
4-pyridinecarboxaldehyde and amino acids.2d,8 To further
improve the bridging abilities of amino acid derivatives, we have
used 4-formylbenzoic acid instead of 4-pyridinecarboxaldehyde
to synthesize two new ligands consisting of multiple carboxylate
groups (L1 ¼ (S)-4-((1-carboxy-2-methylpropylamino)methyl)
benzoic acid and L2 ¼ (S)-4-((1-carboxy-2-hydroxyethylamino)
methyl)benzoic acid; Scheme 1). We expected that the two
carboxylate groups per ligand should provide multiple coordi-
nation modes to generate some interesting coordination
networks. Herein we report the synthesis, crystal structures and
optical properties of six homochiral coordination networks
of [ZnL1 (H2O)2] (1), [Cu2L1 (H2O)3] (2), [Zn2L1 ] (3),
Because homochirality is an essential element in life and indis-
pensable for many biological processes, the constructions of
homochiral coordination networks have attracted remarkable
attention in the past decade.1 The functionality and porosity of
homochiral metal–organic coordination networks can be
precisely controlled by modulating the synthetic strategies, which
can subsequently be applied for asymmetric catalysis2 and
enantiomeric separation.3
Due to their economical efficiency and activities in many chiral
recognition processes, the natural L-amino acids have been used
to fabricate many interesting coordination networks.4 However,
the limited amino and carboxylate groups in an amino acid have
confined the diversity of coordination frameworks. In order to
generate interesting homochiral coordination networks for
potential applications, great effort has been spent to improve the
bridging ability of natural amino acids.5 One of the convenient
2
2
2
[CuL2(H2O)2]$2H2O (4), [ZnL2] (5) and [Cd3NaL2 (ClO4)]$
3
3H2O$DMF (6).
aDepartment of Chemistry, Zhejiang University, Hangzhou, 310027, P. R.
China
bState Key Laboratory of Structural Chemistry, Chinese Academy of
Sciences, Fuzhou, 350002, P. R. China. E-mail: cdwu@zju.edu.cn
† Electronic
supplementary
information
(ESI)
available:
Thermogravimetry results (Fig. S1–S6), RXPD patterns (Fig. S7–S12),
IR spectra (Fig. S13–S18). CCDC reference numbers 821192–821197.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c1ce05422h
Scheme 1
6422 | CrystEngComm, 2011, 13, 6422–6430
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solvent of CH3CN (3 mL) and H2O (3 mL) was heated at 95 ꢀC
for one day to form colorless rod-like crystals of 1, which were
filtered, washed with H2O, CH3CN and Et2O, and dried at room
temperature. Yield: 72% based on L1. Anal. Calcd. for 1 (%): H,
6.03; C, 51.88; N, 4.65. Found: H, 6.15; C, 52.06; N, 4.69. IR
(KBr pellet, v/cmꢁ1): 3399s, 3056m, 2964m, 1604s, 1560s, 1511w,
1466w, 1382s, 1334w, 1247w, 1218m, 1180w, 1114w, 1044w,
1020w, 996w, 969w, 905w, 860m, 820w, 779s, 713s, 664m, 543m,
502m, 454w.
Experimental
Materials and methods
All of the chemicals were obtained from commercial sources and
were used without further purification, except the chiral ligands
L1 and L2 were synthesized according to a literature method.9
Elementary analyses were performed on a ThermoFinnigan
Flash EA 1112 Element Analyzer. IR spectra were recorded from
KBr pellets on a FTS-40 spectrophotometer. Thermogravimetric
analyses (TGA) were carried out under N2 atmosphere on
a NETZSCH STA 409 PC/PG instrument at a heating rate of
10 ꢀC minꢁ1. Powder X-ray diffraction data (PXRD) were
Synthesis of [Cu2L1 (H2O)3] (2)
2
A mixture of Cu(ClO4)2$6H2O (18.6 mg, 0.05 mmol), L1
(12.6 mg, 0.05 mmol) and NaOH (1.0 mg, 0.025 mmol) in
a mixed solvent of CH3CN (3 mL) and H2O (3 mL) was heated at
95 ꢀC for three days to form blue plate crystals of 2, which were
filtered, washed with H2O, CH3CN and Et2O, and dried at room
temperature. Yield: 58% based on L1. Anal. Calcd. for 2 (%): H,
5.34; C, 45.95; N, 4.12. Found: H, 5.39 C, 46.14; N, 4.09. IR
(KBr pellet, v/cmꢁ1): 3405s, 3133s, 2964s, 1685s, 1636s, 1529s,
1506w, 1449w, 1399s, 1358w, 1338w, 1321m, 1275w, 1239w,
1182m, 1158w, 1120w, 1073m, 1018w, 993m, 927w, 906w, 843w,
794m, 770m, 712m, 657m, 590m, 543w, 498w.
recorded on
a RIGAKU D/MAX 2550/PC for Cu-Ka
1
ꢀ
(l ¼ 1.5406 A). H NMR spectra were recorded on a 500 MHz
spectrometer in (CD3)2SO solution and the chemical shifts were
reported relative to internal standard TMS (0 ppm). The
following abbreviations are used to describe peak multiplicity
where appropriate: d ¼ doublet, t ¼ triplet, m ¼ multiplet. The
second-order nonlinear optical intensities were estimated by
measuring powder samples of 90–120 mm in diameter relative to
a-quartz. A pulsed Q-switched Nd: YAG laser at a wavelength of
1064 nm was used to generate second-order harmonic generation
(SHG) signals. The backscattered SHG light of 532 nm was
collected and detected with a photomultiplier through a mono-
chromator. Luminescent spectra for the solid samples were
recorded with a Hitachi F4500 fluorescence spectrometer. The
photomultiplier tube voltage was 700 V, and the scan speed was
240 nm minꢁ1. The slit widths were both 2.5 nm for the excitation
and emission spectra.
Synthesis of [Zn2L1 ] (3)
2
A mixture of Zn(ClO4)2$6H2O (18.6 mg, 0.05 mmol), L1
(12.6 mg, 0.05 mmol) and NaOH (2.0 mg, 0.05 mmol) in H2O
(5 mL) was sealed in a Teflon-lined stainless autoclave and
ꢀ
heated at 150 C for 36 h, and then gradually cooled to room
temperature at a rate of 3.3 ꢀC hꢁ1 to form colorless needle
crystals of 3, which were filtered, washed with H2O, EtOH and
Et2O, and dried at room temperature. Yield: 37% based on L1.
Anal. Calcd. for 3 (%): H, 4.81; C, 49.62; N, 4.45. Found: H, 4.73;
C, 49.47; N, 4.32. IR (KBr pellet, v/cmꢁ1): 3431s, 3305s, 2967s,
1676s, 1612s, 1593s, 1550s, 1508w, 1466w, 1431s, 1407s, 1358m,
1328m, 1305s, 1272w, 1223m, 1182m, 1165w, 1117w, 1067m,
1042s, 1017w, 950s, 929w, 914s, 864w, 848m, 808w, 773s, 711s,
650m, 569m, 529m, 457m.
Synthesis of L1
4-Formylbenzoic acid (1.50 g, 10 mmol) and NaOH (0.40 g, 10
mmol) weredissolved ina mixed solvent of EtOH(50 mL) and water
(30 mL), which was subsequently added to the solution of L-Valine
(1.17 g, 10 mmol) and NaOH (0.40 g, 10 mmol) in water (20 mL).
After the resulting yellow solution was stirred for 3 h at room
temperature, sodium borohydride (0.76 g, 20 mmol) was added to
the mixture under stirring. After the reaction mixture was stirred for
2 h at room temperature, the pH value of the solution was adjusted
to 5.0 by using 10% HCl aqueous solution. The resulting white solid
was filtered, washed with ethanol and diethyl ether, and recrystal-
lized in water. Yield: 53%. 1HNMR[500MHz, (CD3)2SO]d¼ 0.89–
0.91 (t, 6H, J ¼ 6.7 Hz), 1.86–1.92 (m, 1H), 2.82–2.83 (d, 1H, J ¼ 5.6
Hz), 3.63–3.66 (d, 1H, J ¼ 14.3 Hz), 3.90–3.93 (d, 1H, J ¼ 14.3 Hz),
7.46–7.47 (d, 2H, J ¼ 8.1 Hz), 7.89–7.90 (d, 2H, J ¼ 8.1 Hz).
Synthesis of [CuL2(H2O)2]$2H2O (4)
A mixture of Cu2(OH)2CO3 (22.0 mg, 0.1 mmol) and L2
(23.9 mg, 0.1 mmol) in 10 mL distilled water was stirred at room
temperature for 1 h, which was subsequently filtered. Blue needle
crystals of 4 formed after two days, which were filtered, washed
with H2O, EtOH and Et2O, and dried at room temperature.
Yield: 83% based on L2. Anal. Calcd. for 4 (%): H, 5.14; C; 35.44;
N, 3.76. Found: H, 5.02; C, 35.68; N, 3.68. IR (KBr pellet,
v/cmꢁ1): 3372s, 3133s, 2940s, 1648s, 1608s, 1550s, 1510w, 1455w,
1382s, 1315w, 1303w, 1212m, 1176m, 1137m, 1090w, 1048m,
1016m, 992w, 953w, 873w, 843w, 808w, 771s, 733w, 711w, 660m,
614w, 583w, 538s, 480w, 442m.
Synthesis of L2
The synthesis procedure of L2 is similar tothat of L1, except L-Serine
1
(1.05 g, 10 mmol) was used instead of L-Valine. Yield: 64%. H
NMR [500 MHz, (CD3)2SO] d ¼ 3.20–3.22 (m, 1H), 3.64–3.72
(m, 2H), 3.95–3.98 (d, 1H, J ¼ 13.8 Hz), 4.05–4.07 (d, 1H, J ¼ 13.8
Hz), 7.53–7.54 (d, 2H, J ¼ 8.1 Hz), 7.91–7.93 (d, 2H, J ¼ 8.1 Hz).
Synthesis of]ZnL2] (5)
A mixture of ZnO (8.2 mg, 0.1 mmol) and L2 (23.9 mg, 0.1 mmol)
in 5 mL distilled water was stirred at room temperature for 2 h,
which was subsequently filtered. 5 mL DMF was added into the
filtrate, which was sealed in a capped vial and heated at 95 ꢀC for
Synthesis of [ZnL1 (H2O)2] (1)
2
A mixture of Zn(ClO4)2$6H2O (18.6 mg, 0.05 mmol), L1
(25.1 mg, 0.1 mmol) and NaOH (2.0 mg, 0.05 mmol) in a mixed
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two days. Colorless sheet crystals of 5 were filtered, washed with
H2O, EtOH and Et2O, and dried at room temperature. Yield:
75% based on L2. Anal. Calcd. for 5 (%): H, 3.66; C, 43.66; N,
4.63. Found: H, 3.75; C, 43.76; N, 4.50. IR (KBr pellet, v/cmꢁ1):
3248s, 3221s, 2963m, 2880w, 1698w, 1610s, 1557s, 1513w, 1468w,
1416s, 1303m, 1235w, 1203m, 1180m, 1130m, 1095s, 1065m,
1021s, 961w, 926s, 882m, 866s, 817m, 777s, 714s, 645m, 597s,
560m, 468m, 436m, 413m.
vibrations, while the bands at 3305–3057 cmꢁ1 are assigned to the
N–H stretching vibrations.13 The weak bands in the range of 2967–
2869 cmꢁ1 correspond to the C–H stretches. The distinct bands
between 1698 and 1400 cmꢁ1 are due to the asymmetric and
symmetric COOꢁ stretches, respectively.6b,14 The peaks in the 1593–
1431 cmꢁ1 region are ascribed to the stretching vibrations of the
benzene rings. The weak peaks at 960–906 cmꢁ1 should be due tothe
O–H out-plane bending vibrations. The bands in the 1275–
1007 cmꢁ1 region correspond to the C–C, C–N and C–O stretching
vibrations, while the peaks in the ranges of 1358–1303 and 900–
650 cmꢁ1 are attributed to the C–H bending vibrations of alkane and
out-plane bending vibrations of benzene rings, respectively.
Synthesis of [Cd3NaL2 (ClO4)]$3H2O$DMF (6)
3
Heating a mixture of Cd(ClO4)2$6H2O (21.0 mg, 0.05 mmol), L2
(12.0 mg, 0.05 mmol) and NaOH (2.0 mg, 0.05 mmol) in a mixed
solvent of DMF (3 mL) and H2O (3 mL) at 95 ꢀC yielded
colorless triangular crystals of 6 after three days, which were
filtered, washed with H2O, EtOH and Et2O, and dried at room
temperature. Yield: 41% based on L2. Anal. Calcd. for 6 (%): H,
3.57; C, 33.30; N, 4.31. Found: H, 3.62; C, 32.98; N, 4.20. IR
(KBr pellet, v/cmꢁ1): 3431s, 3240s, 2919m, 1676w, 1598s, 1547s,
1511w, 1491w, 1405s, 1319w, 1304w, 1283w, 1236w, 1185m,
1110s, 1090s, 1007m, 948m, 900m, 861m, 805m, 773s, 709m,
662w, 617m, 597m, 556w, 523w.
The reaction of L1 and Zn(ClO4)2$6H2O in a mixed solvent of
water and CH3CN afforded colorless crystals of [ZnL1 (H2O)2]
2
(1). Single crystal X-ray diffraction analysis has revealed that
compound 1 crystallizes in the monoclinic C2 space group with
one half zinc(II) atom, one L1 ligand and one water ligand in the
asymmetric unit (Fig. 1). Each Zn atom coordinates to two
carboxyl groups of two monodentate L1 ligands (Zn–O ¼ 1.964
ꢀ
ꢀ
(3) A) and two water molecules (Zn–O ¼ 1.975(4) A) in distorted
tetrahedral geometry (Scheme 3a). The discrete coordination
units are further self-linked into a 1D supramolecular ribbon-
shaped network by strong hydrogen bonding interactions
between the aqua ligand and uncoordinated carboxyl oxygen
X-Ray crystal data collections and structure determinations
ꢀ
atom (O–H/O ¼ 2.702(5) A; Fig. 2a). Subsequently, the 1D
The determinations of the unit cells and data collections for the
crystals of 1–6 were performed using the CrysAlisPro program.
The data were collected using graphite–monochromatic Mo-Ka
tapes are joined together by hydrogen bondings between
the aqua ligand and uncoordinated carboxyl oxygen atom
ꢀ
(O–H/O ¼ 2.610(5) A), and between the amino group and
ꢀ
radiation (l ¼ 0.71073 A) at 293 K for compounds 1, 2, 4–6, and
ꢀ
uncoordinated carboxyl oxygen atom (N–H/O ¼ 2.839(5) A) to
ꢀ
using graphite–monochromatic Cu radiation (l ¼ 1.54178 A) at
form a 3D supramolecular network (Fig. 2b). TGA of 1 showed
ꢀ
293 K for compound 3. The data sets were corrected by empirical
absorption correction using spherical harmonics, implemented in
SCALE3 ABSPACK scaling algorithm.10 All structures were
solved by direct methods, and refined by full-matrix least-square
methods with the SHELX-97 program package.11 All non-
hydrogen atoms including solvent molecules were located
successfully from Fourier maps and were refined anisotropically
for compounds 1–5. The solvent molecules in compound 6 are
highly disordered, and attempts to locate and refine the solvent
peaks were not successful. SQUEEZE subroutine of the PLA-
TON software suit was used to remove the scattering from the
highly disordered guest molecules.12 The resulting new file was
used to further refine the structure. The H atoms on C atoms
were generated geometrically. The H atoms of the water mole-
cules and amino groups were clearly visible in difference maps
and were handled in the subsequent refinement with fixed
isotropic displacement parameters. The data collection parame-
ters, crystallographic data and final agreement factors are
collected in Tables 1 and 2, respectively. CCDC- 821192 (1),
821193 (2), 821194 (3), 821195 (4), 821196 (5) and 821197 (6).
that the weight loss of 6.1% occurring between 35 and 230 C
corresponds to the loss of the aqua ligands (expected 6.0%).
There was almost no weight loss until 258 ꢀC. Above 258 ꢀC,
compound 1 began to lose L1 ligand to decompose.
The reaction of L1 and Cu(ClO4)2$6H2O in a mixed solvent of
water and CH3CN afforded blue crystals of [Cu2L1 (H2O)3] (2).
2
Single crystal X-ray diffraction analysis revealed that compound
2 crystallizes in the monoclinic C2 space group with one copper
(II) atom, one L1 ligand, and one and a half water ligand in the
asymmetric unit (Fig. 3). The CuII atom coordinates to one
ꢀ
amino group (Cu–N
¼
2.024(3) A), two monodentate
carboxylate groups of two L1 ligands (Cu–O ¼ 1.935(3)–
ꢀ
1.961(3) A) and three water molecules (Cu–O ¼ 1.942(3)–2.473
ꢀ
(1) A) in a distorted square-pyramidal coordination geometry.
Each L1 ligand acts as a tridentate ligand, utilizing the neigh-
boring carboxyl and amino groups to chelate a copper atom, and
further to bridge the neighboring copper atom via the phenyl
carboxyl group to propagate into a 1D polymeric chain running
along two orientations (Scheme 3b, Fig. 4a). Subsequently, two
directional polymeric chains are coupled by water molecules to
form a 2D cross-grid lamellar framework (Fig. 4b and 4c).
The 2D grids are stacked together via strong hydrogen bonding
interactions to form a 3D supramolecular network. The hydrogen
bonds are between the aqua ligand and carboxyl oxygen atom (O–
Results and discussion
Compounds 1–6 were synthesized under the optimized conditions as
shown in Scheme 2. The formulae of these compounds were estab-
lished by single-crystal X-ray diffraction studies, TGA and
elementary analyses. The bulk purities of the as-synthesized samples
were confirmed by PXRD. All of the typical vibration bands of
compounds 1–6 are clearly visible in the IR spectra. The broad
bands in the 3448–3372 cmꢁ1 region are due to the O–H stretching
ꢀ
H/O ¼ 2.673(3)–2.699(4) A) and between the amino group and
ꢀ
carboxyl oxygen atom (N–H/O ¼ 2.841(5) A). TGA of 2 showed
that there is no weight loss up to 138 ꢀC. Above 138 ꢀC,
compound 2 began to lose coordinated aqua and L1 ligands to
decompose.
6424 | CrystEngComm, 2011, 13, 6422–6430
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Table 1 Crystal data and structure refinements for 1, 2 and 3a
Compound
1
2
3
Formula
Formula weight
Crystal system, space group
C26H36N2O10Zn
601.94
C26H36N2O11Cu2
679.65
C26H30N2O8Zn2
629.26
Monoclinic, C2
21.960(5)
5.8873(13)
10.731(3)
102.46(2)
1354.7(5)
2
Monoclinic, C2
21.574(3)
5.9897(3)
14.1692(18)
127.501(19)
1452.6(3)
2
Orthorhombic, P212121
6.6534(1)
18.9600(3)
22.5591(3)
90
2845.80(7)
4
1.469
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
b (ꢀ)
Volume/A
3
ꢀ
Z
Calculated density (g cmꢁ3
)
1.476
632
1.554
704
F(000)
1296
2.490
Absorption coefficient (mmꢁ1
Reflections collected
Data/parameters
)
0.966
2916 [R(int) ¼ 0.0455]
1.525
3288 [R(int) ¼ 0.0264]
10549 [R(int) ¼ 0.0323]
1866/177
0.924
2078/186
1.007
4568/343
1.084
Goodness-of-fit on F2
R1 (wR2) [I > 2s(I)]
R1 (wR2) (all data)
0.0410 (0.0666)
0.0691 (0.0710)
0.045(19)
0.0309 (0.0651)
0.0383 (0.0667)
0.006(17)
0.0363 (0.0791)
0.0444 (0.0833)
ꢁ0.19(4)
Absolute structure parameter
P
P
P
P
a
R1 ¼ (|Fo| ꢁ |Fc|)/ |Fo|, wR2 ¼ [ w(Fo ꢁ Fc )2/ w(Fo2)2]0.5
.
2
2
The reaction of L1 and Zn(ClO4)2$6H2O in water afforded
careful inspection of the framework structure, we have found
that the 3D framework comprises of left-hand triple helices with
colorless crystals of [Zn2L1 ] (3). Single crystal X-ray diffraction
2
ꢀ
a pitch of 19.97 A (Fig. 6c and 6d). The homochiral triple
analysis has revealed that compound 3 crystallizes in the ortho-
rhombic P212121 space group with two zinc(II) atoms and two L1
ligands in the asymmetric unit (Fig. 5). In compound 3, all L1
ligands comprise the same coordination mode to bridge and
chelate four ZnII atoms (Scheme 3c). Each Zn atom coordinates
stranded helices are held together by the Zn knots with the
ꢀ
average Zn/Zn distance of 6.66 A. TGA of 3 showed that there
is no weight loss up to 330 ꢀC. Above 330 ꢀC, compound 3 began
to lose coordinated L1 ligand to decompose.
ꢀ
to one amino group (Zn–N ¼ 2.188(3)–2.154(3) A) and four
The reaction of L2 and Cu2(OH)2CO3 in water yielded blue
crystals of [CuL2(H2O)2]$2H2O (4). Single crystal X-ray
diffraction analysis has revealed that compound 4 crystallizes in
the monoclinic C2 space group with one copper(II) atom, one L2
ligand, two aqua ligands and two lattice water molecules in the
asymmetric unit (Fig. 7). The CuII center adopts a distorted
octahedral coordination environment by coordinating to two
carboxylate groups of four L1 ligands (Zn–O ¼ 1.984(3)–
ꢀ
2.098(3) A) in a distorted square-pyramidal geometry. Subse-
quently, the Zn atoms are bridged by one carboxyl oxygen of the
amino acid to form a 1D polymeric chain, which is further
coupled by the phenyl carboxylate groups of the neighboring L1
ligands to propagate into a 3D framework structure (Fig. 6a, 6b
and 6e). The 3D framework contains two kinds of cavities
that are occupied by the isopropyl groups in an interesting .
ABAB. stacking mode as viewed down the a axis (Fig. 6b). By
ꢀ
monodentate carboxylate groups (Cu–O ¼ 1.962(3)–1.979(3) A),
ꢀ
2.561(3) A), two water
one hydroxy group (Cu–O
¼
ꢀ
molecules (Cu–O ¼ 1.983(3)–2.349(3) A) and one amino group
Table 2 Crystal data and structure refinements for 4, 5 and 6a
Compound
4
5
6
Formula
Formula weight
Crystal system, space group
C11H19CuNO9
372.81
C11H11NO5Zn
302.58
C36H46Cd3ClN4NaO23
1298.41
Monoclinic, C2
21.5285(10)
5.9677(3)
11.6873(6)
98.289(5)
1485.85(13)
4
Monoclinic, P21
8.3240(8)
6.4377(4)
11.3504(8)
101.988(9)
594.97(8)
2
1.689
308
2.076
2326 [R(int) ¼ 0.0318]
1774/163
1.084
0.0475 (0.0969)
0.0724 (0.1018)
0.04(3)
Cubic, P213
17.0146(1)
17.0146(1)
17.0146(1)
90
4925.67(5)
4
1.751
ꢀ
a/A
ꢀ
b/A
ꢀ
c/A
b (ꢀ)
Volume/A
3
ꢀ
Z
Calculated density (g cmꢁ3
)
1.667
772
F(000)
2584
1.429
Absorption coefficient (mmꢁ1
Reflections collected
Data/parameters
)
1.516
3531 [R(int) ¼ 0.0293]
7138 [R(int) ¼ 0.0494]
2478/204
0.876
3222/181
1.010
Goodness-of-fit on F2
R1 (wR2) [I > 2s(I)]
R1 (wR2) (all data)
0.0370 (0.0512)
0.0562 (0.0538)
0.036(18)
0.0365 (0.0823)
0.0551 (0.0847)
0.03(4)
Absolute structure parameter
P
P
P
P
a
R1 ¼ (|Fo| ꢁ |Fc|)/ |Fo|, wR2 ¼ [ w(Fo ꢁ Fc )2/ w(Fo2)2]0.5
.
2
2
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Fig. 2 a) View of the 1D supramolecular ribbon of the coordination
units interlinked by hydrogen bonding contacts; b) view of the 3D
supramolecular network slight off the b axis (Zn, cyan; O, red; N, blue; C,
gray; hydrogen bonds are shown in dotted lines).
Scheme 2
Fig. 3 ORTEP representation of the symmetry expanded local structure
for 2. Displacement ellipsoids are drawn at the 35% probability level
[symmetry code: i) x + 1/2, y ꢁ 1/2, z].
Fig. 1 ORTEP representation of the symmetry expanded local structure
for 1. Displacement ellipsoids are drawn at the 35% probability level
[symmetry code: i) ꢁx, y, 2 ꢁ z].
Fig. 4 a) View of the 1D polymeric chain in 2 (Cu, green; O, yellow; N,
blue; C, gray; H, light gray); b) A view of the lamellar network of 2
formed by water molecules (red ball) linking up linear chains down the
c axis; c) schematic representation of the 2D framework of 2.
Scheme 3
between the carboxyl oxygen atom and aqua ligand (O–H/O ¼
ꢀ
2.682(3) A) (Fig. 8c). Most interestingly, two lattice water
2
ꢀ
molecules, four aqua ligands and two uncoordinated phenyl
(Cu–N ¼ 2.006(3) A). Each L ligand chelates one copper atom
via one monodentate carboxylate group of amino acid, one
hydroxy group and one amino group (Scheme 3d). Subsequently,
the phenyl carboxylate group monodentately bridges the neigh-
boring copper atom to extend into 1D polymeric chains running
along two orientations (Fig. 8a and 8b).
carboxyl
oxygen
atoms
between
the
neighboring
layers are hydrogen bonding linked to form an octa-atomic unit
ꢀ
(O–H/O ¼ 2.558(4)–2.893(4) A; Fig. 8d). The octa-atomic
synthons further join the lamellae into a 3D supramolecular
network (Fig. 8e). Additionally, the hydrogen bondings between
the hydroxy group and lattice water molecule (O–H/O ¼ 2.615
Hydrogen bonding interactions play a very important role in
the formation and stabilization of the 3D supramolecular
network of compound 4. The neighboring chains are joined
together to form a lamellar network by strong hydrogen bonding
ꢀ
(10) A), between the lattice water molecule and carboxyl oxygen
ꢀ
atom (O–H/O ¼ 2.972(3) A), and between the amino group and
ꢀ
uncoordinated carboxyl oxygen atom (N–H/O ¼ 2.979(5) A)
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Fig. 7 ORTEP representation of the symmetry expanded local structure
for 4. Displacement ellipsoids are drawn at the 35% probability level
[symmetry code: i) x + 1/2, y ꢁ 1/2, z].
Fig. 5 ORTEP representation of the symmetry expanded local structure
for 3. Displacement ellipsoids are drawn at the 35% probability level
[symmetry codes: i) 2 ꢁ x, y ꢁ 1/2, 1/2 ꢁ z; ii) x ꢁ 1, y, z; iii) 1 ꢁ x, y ꢁ 1/2,
1/2 ꢁ z; iv) 1/2 ꢁ x, 1 ꢁ y, z ꢁ 1/2].
Fig. 8 a) View of the 1D linear polymeric chain in 4 (Cu, cyan; O, red; N,
blue; C, gray; H, light gray); b) schematic representation of the
arrangement of the polymeric chains in 4; c) view of the 2D supramo-
lecular network in 4; d) view of the hydrogen bonding interlinked octa-
atomic unit; e) view of the 3D supramolecular network of 4 (hydrogen
bonds are shown in pink dotted lines).
Fig. 6 a) View of the 1D polymeric chain of the carboxyl oxygen atom
bridging Zn atoms (Zn, pink); b) view of the .ABAB. sequence
composed 3D network of 3 as viewed down the a axis (Zn, cyan; O, red;
N, blue; C, gray); c) and d) The left-hand triple helix motifs in 3 as viewed
along the b axis; e) perspective view of the 3D framework of 3 along the
a axis.
to three carboxyl groups of three L2 moieties (Zn–O ¼ 1.932(5)–
also play very important roles for the stabilization of the 3D
2
ꢀ
ꢀ
2.025(5) A) and one amino group (Zn–N ¼ 2.040(5) A). Each L
ligand tridentately bridges and chelates three ZnII atoms to
propagate into a lamellar network, which consists of two kinds of
left-handed helices (Scheme 3e, Fig. 10a and 10b). The first kind
is formed by using the monodentate phenyl carboxyl group and
amino group to link up zinc atoms running along the b axis with
supramolecular network. TGA of 4 showed that a weight loss of
ꢀ
19.2% occurring between 30 and 186 C corresponds to the loss
of the aqua ligands and lattice water molecules (expected 19.3%).
There is almost no weight loss up to 196 ꢀC. Above 196 ꢀC,
compound 4 began to lose L2 ligand to decompose.
The reaction of L2 and ZnO in a mixed solvent of water and
DMF yielded colorless crystals of [ZnL2] (5). Single crystal X-ray
diffraction analysis has revealed that compound 5 crystallizes in
the monoclinic P21 space group with one zinc(II) atom and one L2
ligand in the asymmetric unit (Fig. 9). The ZnII center adopts
a distorted tetrahedral coordination environment to coordinate
ꢀ
a period of 6.4377 A. Another kind is built up from the
carboxylate group of the amino acid bridging zinc atoms along
the b axis. Consequently, two kinds of helical chains are joined
together via sharing the zinc connecting nodes to propagate into
a 2D diamondoid network.
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Each CdII atom octa-coordinates to one amino group
ꢀ
ꢀ
(Cd–N ¼ 2.321(5) A), one hydroxy group (Cd–O ¼ 2.548(5) A),
ꢀ
one oxygen atom of perchlorate (Cd–O ¼ 2.315(4) A) and five
carboxyl oxygen atoms of three carboxylate groups
I
ꢀ
(Cd–O ¼ 2.247(5)–2.505(4) A). Each Na atom hexa-coordinates
to six carboxyl oxygen atoms of six carboxylate groups
2
ꢀ
(Na–O ¼ 2.318(6)–2.356(5) A). Consequently, each L ligand
performs as a pentadentate ligand to bridge and chelate three Cd
and two Na atoms to form a composite unit (Scheme 3f).
Subsequently, three of the composite units are joined together by
one perchlorate to coordinate three Cd atoms of three composite
units to form a cage subunit (Fig. 12a). Moreover, three Cd
atoms of three neighboring cages are linked together by three
carboxyl oxygen atoms of three amino acids, which let the cage
subunits to propagate into a 3D porous network structure
(Fig. 12b–12d). Each octahedrally coordinated Na atom is three-
face-shared with three Cd polyhedra of the trinuclear unit. TGA
of 6 ꢀshowed that a weight loss of 9.7% occurred between 27 and
303 C, corresponding to the loss of three water and one DMF
molecules (expected 9.8%). Above 303 ꢀC, compound 6 began to
lose coordinated L2 ligand to decompose.
Fig. 9 ORTEP representation of the symmetry expanded local structure
for 5. Displacement ellipsoids are drawn at the 35% probability level
[symmetry codes: i) 4 ꢁ x, y ꢁ 1/2, 1 ꢁ z; ii) 4 ꢁ x, y + 1/2, ꢁz].
Preliminary Kurtz powder SHG measurements were
performed on compounds 1–6 to evaluate their potentials as
second-order NLO materials.15 Detailed experiments indicate the
crystalline samples of 1–6 are SHG-active. Compound 1 has
a powder SHG intensity of 3.4 versus a-quartz, and the powder
SHG intensity of compounds 2–4 approximate to that of
a-quartz, while the powder SHG intensities are of 7.8 and 8.4
versus a-quartz for compounds 5 and 6, respectively (Fig. 13). It
has been known that the second-order NLO response is origi-
nated from the molecular quadratic hyperpolarizability of the
donor–acceptor system and their asymmetric organization in
a framework structure.16 As these compounds are constructed
from the similar donor–acceptor systems, the distinct SHG
responses are mainly affected by the distinct array of the chiral
donor–acceptor systems. By careful inspection of the crystal
structures of these compounds, we found that the metal sites
Fig. 10 a) View of the diamondoid lamellar framework of 5 constructed
from two kinds of left-handed helices down the a axis (Zn, yellow); b)
schematic representation of the arrangement of the left-handed helices; c)
schematic representation of the paralleled diamondoid lamellar networks
of 5; d) view of the 3D supramolecular network connected by hydrogen
bondings along the b axis (hydrogen bonds are shown in pink dotted
lines).
The 2D layered lattices are further stacked into a 3D supra-
molecular network through strong hydrogen bonding
interactions (Fig. 10c and 10d). The hydrogen bonds are
between the hydroxy group and carboxyl oxygen atom
ꢀ
(O–H/O ¼ 2.678(8) A), and between the amino group and
ꢀ
hydroxy group (N–H/O ¼ 2.793(9) A). TGA of 5 showed that
there is no weight loss up to 340 ꢀC. Above 340 ꢀC, compound 5
began to lose coordinated L2 ligand to decompose.
The reaction of L2 and Cd(ClO4)2$6H2O in a mixed solvent
of water and DMF afforded colorless crystals of
[Cd3NaL2 (ClO4)]$3H2O$DMF (6). Single crystal X-ray
3
Fig. 11 ORTEP representation of the symmetry expanded local struc-
ture for 6. Displacement ellipsoids are drawn at the 35% probability level
[Symmetry codes: i) ꢁz ꢁ 1/2, ꢁx ꢁ 1, y + 1/2; ii) ꢁz, x ꢁ 3/2, ꢁy ꢁ 3/2;
iii) 1/2 ꢁ x, ꢁy ꢁ 2, z ꢁ 1/2; iv) y + 1, z ꢁ 1, x ꢁ 1; v) z, x ꢁ 1, y + 1; vi)
y + 1, z ꢁ 1, x; vii) ꢁy ꢁ 1, z ꢁ 1/2, ꢁx ꢁ 1/2].
diffraction analysis has revealed that compound 6 crystallizes in
the cubic P213 space group with three cadmium(II) atoms, one
natrium(I) atom, one perchlorate and three L2 ligands in the
asymmetric unit besides the solvent molecules (Fig. 11).
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Fig. 14 Solid luminescent spectra for compounds 1,
(lex ¼ 324 nm).
3
and L1
therefore studied the emission properties of 1, 3, 5 and 6 in the
solid-state. To make a comparison, the photoluminescent spectra
of L1 and L2 were also measured. Compounds 1 and 3 exhibit
blue fluorescent emissions around the band of 395 nm excited at
324 nm (Fig. 14). Compound 6 displays a fluorescent emission
band at 400 (lex ¼ 326 nm), while the fluorescent emission bands
of compound 5 are at 365 and 412 nm (lex ¼ 326 nm) (Fig. 15).
Compared with the photoluminescent spectra of the free ligands
L1 and L2, compounds 1, 3, 5 and 6 have similar red shifts. Since
Zn2+ or Cd2+ ions are difficult to be oxidized or reduced due to
their d10 configuration,19 the emissions of compounds 1, 3, 5 and
6 are neither metal-to-ligand charge transfer (MLCT) nor ligand-
to-metal charge transfer (LMCT) in nature, which should be
ascribed to the intra-ligand p / p* or p/ n* transitions arising
from the phenyl acid-amino acid ligands. Different emission
intensities of 1, 3, 5 and 6 are probably due to the variations of
the metal ions and the ligand environments, which can increase
or decrease the loss of energy by radiationless decay, because the
photoluminescence behavior is closely associated with the coor-
dination ligands.20
Fig. 12 a) View of the cage unit in 6; b) view of the linkage between the
cage units joined by three Cd and one Na atoms; c) and d) the 3D
framework structure of 6 built up from the cage units.
Fig. 13 Comparison of the SHG intensities of a-quartz (set as unit) and
compounds 1–6 (l ¼ 532 nm).
comprise of distinct coordination environments. In compounds 2
and 3, each metal site coordinates to one amino group and four
oxygen atoms in an approximate C4v symmetry. However, the
polarity was cancelled out by the neighboring metal sites,
because the two metal atoms have pseudo inverse center
symmetry. In compound 4, the six O/N atoms around the octa-
hedrally coordinated Cu atom have a pseudo inverse center. The
Zn atoms in 1 and 5 comprise of the tetrahedral coordinated
environments, which thus have at most C2v symmetry. In
compound 6, the Cd atom is coordinated to eight donor atoms in
Cs symmetry. The noncentrosymmetric coordination environ-
ments of the metal centers can subsequently improve the polarity
of the solids. Additionally, the relatively high powder SHG
responses of compounds 5 and 6 should be attributed to the
entire helical arrangement of the chiral ligands.17
It has been shown that the coordination polymers constructed
from d10 metal centers, such as zinc(II) and cadmium(II) atoms,
often exhibit an interesting luminescent property.18 We have
Fig. 15 Solid luminescent spectra for compounds 5,
(lex ¼ 326 nm).
6
and L2
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CrystEngComm, 2003, 5, 34; (d) J. A. Gould, J. T. A. Jones,
J. Bacsa, Y. Z. Khimyak and M. J. Rosseinsky, Chem. Commun.,
2010, 46, 2793.
Conclusions
In summary, we have synthesized six homochiral MOFs based
on two new phenyl acid-amino acids. Compared with the
homochiral coordination networks based on the ligands derived
from 4-pyridinecarboxaldehyde and amino acids, we have found
that the new homochiral coordination networks are inclined to
form higher dimensional frameworks because of the increased
bridging ability of the phenyl carboxylate group versus the
pyridine group. In addition, besides the L1 and L2 ligands and
their arrangements in the framework structures, the distinct
SHG-activities of compounds 1–6 are very sensitive to the
coordination symmetries of metal nodes. Compounds 1, 3, 5 and
6 present blue fluorescent emissions at room temperature in the
solid state, mainly attributed to the ligand-centered transition
processes. This contribution provided valuable information on
the following designed synthesis of functional homochiral coor-
dination networks based on amino acid derivatives. Following
this lead, some interesting homochiral coordination networks
can be obtained by using other kinds of phenyl acid-amino acid
derivatives, which is underway in our laboratory.
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We are grateful for financial support from the NNSF of China
(Grant Nos. 21073158 & J0830413), Zhejiang Provincial Natural
Science Foundation of China (Grant No. Z4100038), the
Specialized Research Fund for the Doctoral Program of Higher
Education of China (Grant No. 20090101110017), and the
Fundamental Research Funds for the Central Universities
(Grant No. 2010QNA3013).
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