Metal-organic frameworks built from achiral cyclohex-1-ene-1,2- dicarboxylate: Syntheses, structures and photoluminescence properties

Article abstract of DOI:10.1039/c2ce06415d

Using achiral ligand cyclohex-1-ene-1,2-dicarboxylic acid (H₂L), in the presence or absence of N-donor coligands, five metal(ii) complexes formulated as Zn(L)(4,4′-bpy)·H₂O (1), Cu(L)(4,4′-bpy)_0.5 (2), Cd₂(L)₂(phen) ₂(H₂O)₂·3H₂O (3), Zn ₅(L)₄(μ₃-OH)₂(H ₂O)₂ (4) and Cd(L)(H₂O) (5) have been synthesized and structurally characterized by single-crystal X-ray diffractions. Complex 1 shows a homochiral network based on novel twisted hexanuclear cyclic units, which are fused by sharing metal ions into a 3D framework. Complex 2 exhibits a 2D framework constructed by 1D chain linked by 4,4′-bpy. 4,4′-bpy exhibits twisted and planar conformations in complexes 1 and 2. In complex 3, alternating 1D left-handed and right-handed helical chains are observed. Complexes 4 and 5 display different 2D frameworks based on a Zn ₅ unit and Cd-O-Cd infinite linkage, respectively. Complex 1 shows a strong CD signal, and complexes 4-5 are achiral. The twisted 4,4-bpy plays an important role in the formation of the chiral complex 1. The five complexes show different absorption and photoluminescence properties as well as different thermal stabilities.

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PAPER
Metal–organic frameworks built from achiral cyclohex-1-ene-1,2-
dicarboxylate: syntheses, structures and photoluminescence properties{{
Yun Gong,a Tao Wu,a Jianhua Lin*a and Baoshan Wangb
Received 24th October 2011, Accepted 25th April 2012
DOI: 10.1039/c2ce06415d
Using achiral ligand cyclohex-1-ene-1,2-dicarboxylic acid (H2L), in the presence or absence of
N-donor coligands, five metal(II) complexes formulated as Zn(L)(4,49-bpy)?H2O (1),
Cu(L)(4,49-bpy)0.5 (2), Cd2(L)2(phen)2(H2O)2?3H2O (3), Zn5(L)4(m3-OH)2(H2O)2 (4) and
Cd(L)(H2O) (5) have been synthesized and structurally characterized by single-crystal X-ray
diffractions. Complex 1 shows a homochiral network based on novel twisted hexanuclear cyclic
units, which are fused by sharing metal ions into a 3D framework. Complex 2 exhibits a 2D
framework constructed by 1D chain linked by 4,49-bpy. 4,49-bpy exhibits twisted and planar
conformations in complexes 1 and 2. In complex 3, alternating 1D left-handed and right-handed
helical chains are observed. Complexes 4 and 5 display different 2D frameworks based on a Zn5 unit
and Cd–O–Cd infinite linkage, respectively. Complex 1 shows a strong CD signal, and complexes 4–
5 are achiral. The twisted 4,4-bpy plays an important role in the formation of the chiral complex 1.
The five complexes show different absorption and photoluminescence properties as well as different
thermal stabilities.
one-dimensional (1D) helical chain. If the helical units have the
Introduction
same chirality, they are linked into higher-dimensional homo-
In recent years, homochiral metal–organic frameworks (MOFs)
chiral frameworks by interactions with discriminative directions
have attracted much attention because of their intriguing
architectures1 and their potential applications in second-order
such as coordination bonding, hydrogen bonding and p–p
stacking.8–10
nonlinear optical materials, chiral separation, asymmetric
In comparison to the well-resolved helical unit in spontaneous
heterogeneous catalysts, ion exchange and so on.2 Among the
resolution, other chiral units such as twisted molecular assembly
essential approaches that exist for the preparation of chiral
are not usually reported. The chirality transfer of the twisted unit
MOFs,3 self-resolution crystallization offers great advantages as
also attracts our interest. In the present work, based on an
it does not use chiral components.4 Chiral crystallization from
an achiral source in spontaneous resolution is common,4,5 the
achiral ligand, cyclohex-1-ene-1,2-dicarboxylic acid (H2L) and
4,49-bipyridine (4,49-bpy), we got
a homochiral network,
main challenge that should be solved is to how to generate
chiral units from achiral components and induce the chiral
information of the chiral units into higher dimensional chiral
MOFs without any chiral auxiliary.5 Generally, the chiral unit
in spontaneous resolution is helicate. The term helicate was
introduced by Lehn to describe a discrete metal-based helix
comprising one or more covalent organic strands wrapped and
coordinated to a series of ions defining the helical axis.6 Helical
chirality is generated by the secondary structure of the
molecule,7 and is expressed as right- and left-handed forms.
Usually, helical units contain discrete oligonuclear helicate and
Zn(L)(4,49-bpy)?H2O (1). Complex 1 is constructed by chiral
twisted hexanuclear cyclic units, which are further fused by
sharing metal ions into three-dimensional (3D) framework.
Since the generation of chiral unit and the transfer of chirality
in spontaneous resolution of MOFs are usually governed by a
simultaneous and synergistic cooperation of many factors, the
outcomes are often beyond prediction. Thus, it is necessary to
gather more relevant data to understand the principal bridging
between the structure of building units and the resultant
molecular packing. Herein, changing the species of N-donors
or metal ions, in the presence or absence of coligands, we got
another four achiral metal(II)-L complexes: Cu(L)(4,49-bpy)0.5
(2), Cd2(L)2(phen)2(H2O)2?3H2O (phen = phenanthroline) (3),
Zn5(L)4(m3-OH)2(H2O)2 (4) and Cd(L)(H2O) (5). They have been
structurally characterized by single-crystal X-ray diffraction. The
solid-state circular dichroism (CD) spectrum of complex 1 and
the absorption and photoluminescence properties as well as the
thermal stability of complexes 1–5 have been investigated.
aDepartment of Chemistry, College of Chemistry and Chemical
Engineering, Chongqing University, Chongqing, 400030, P. R. China.
E-mail: jhlin@cqu.edu.cn; jhlin@pku.edu.cn; Tel: +86-023-65102316
bTechnical Institute of Physics and Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P. R. China
{ Electronic supplementary information (ESI) available. CCDC refer-
ence numbers 800776 and 848601–848604. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2ce06415d
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crystals is ca. 37% based on Zn. Elemental anal. Found: C, 35.90;
H, 3.53%. Calc. for Zn5C32H38O20: C, 35.94; H, 3.55%. IR
(cm21): 3584(w), 3436(m), 2933(m), 2859(w), 2834(w), 1605(w),
1574(s), 1550(s), 1417(s), 1386(s), 1335(m), 1268(w), 1179(w),
1072(w), 968(w), 887(w), 835(w), 807(m), 782(w), 735(w), 642(w),
608(w), 517(w).
Experimental section
General considerations
All chemicals purchased were reagent grade and used without
further purification. C, H, N elemental analyses were performed
on an Elementar Vario MICRO E III analyzer. IR spectra were
recorded as KBr pellets on a PerkinElmer spectrometer. The
powder XRD (PXRD) data were collected on a RIGAKU
DMAX2500PC diffractometer using Cu-Ka radiation. CD
spectrum was measured on a bio-logic MOS 450 AF/CD circular
dichroism spectrometer. UV-Vis spectra were measured on a
HITACHI U-4100 UV-vis spectrophotometer. Solid-state
photoluminescence spectra of all the compounds were measured
at room temperature with an Edinburgh FLS920 fluorescence
spectrometer. The instrument is equipped with an Edinburgh
Xe900 xenon arc lamp as the exciting light source. All
measurements were performed at room temperature. TGA was
performed on a NETZSCH STA 449C thermogravimetric
Synthesis of Cd(L)(H2O) (5):
The synthesis of complex 5 is similar to that of complex 1 except
that Cd(NO3)2?4H2O was utilized instead of Zn(NO3)2?6H2O in
the absence of 4,49-bpy. The yield of the colorless cubic crystals
is ca. 48% based on Cd. Elemental anal. Found: C, 32.15; H,
3.36%. Calc. for CdC8H10O5: C, 32.18; H, 3.38%. IR (cm21):
3379(m), 2936(m), 2864(w), 1653(w), 1543(s), 1425(m), 1406(m),
1134(w), 1071(w), 1038(w), 970(w), 890(w), 816(m), 744(m),
625(w), 550(w), 484(w).
X-ray crystallography
analyzer in flowing N2 with a heating rate of 10 uC min21
.
Single-crystal X-ray data for complexes 1–5 were collected on an
Synthesis of Zn(L)(4,49-bpy)?H2O (1):
Oxford Xcalibur Eos diffractometer using graphite monochro-
˚
mated Mo-Ka (l = 0.71073 A) radiation at room temperature.
A mixture of Zn(NO3)2?6H2O (0.1 mmol, 0.030 g), H2L
(0.1 mmol, 0.017 g), 4,49-bpy (0.1 mmol, 0.016 g), H2O (1 mL)
and DMF (3 mL) was heated at 120 uC in Teflon-lined autoclaves
for 3 days, followed by slow cooling to room temperature. The
resulting yellow block crystals were filtered off (yield: ca. 35%
based on Zn). Elemental anal. Found: C, 53.01; H, 4.46; N, 6.86%.
Calc. for C18H18ZnN2O5: C, 53.03; H, 4.45; N, 6.87%. IR (cm21):
3419(s), 3096(w), 2935(m), 1709(w), 1612(s), 1582(s), 1488(w),
1419(s), 1329(w), 1221(m), 1071(m), 1047(w), 1015(w), 889(w),
826(m), 786(w), 746(w), 642(m), 574(w), 522(w).
Empirical absorption correction was applied. The structures
were solved by direct methods and refined by the full-matrix
least-squares methods on F2 using the SHELXTL-97 software.11
All non-hydrogen atoms were refined anisotropically. All of the
hydrogen atoms were placed in the calculated positions. The
solvent molecules in complex 1 were highly disordered and were
impossible to refine using conventional discrete-atom models,
thus the contribution of solvent electron densities were removed
by the SQUEEZE routine in PLATON.12 The final chemical
formula was estimated from the SQUEEZE results combined
with the TGA results. The crystal data and structure refinements
for complexes 1–5 are summarized in Table 1. Selected bond
lengths and angles for complexes 1–5 are listed in Table S1 in the
ESI.{ Further details are provided in the ESI.{ The CCDC
reference numbers are the following: 848603 for complex 1,
848604 for complex 2, 848601 for complex 3, 848602 for complex
4 and 800776 for complex 5.
Synthesis of Cu(L)(4,49-bpy)0.5 (2):
The synthesis of complex 2 was carried out as described above
for complex 1 except that Cu(NO3)2?3H2O was utilized instead
of Zn(NO3)2?6H2O. The yield of the blue cubic crystals is ca.
26% based on Cu. Elemental anal. Found: C, 54.38; H, 3.86; N,
4.55%. Calc. for C13H12CuNO4: C, 54.40; H, 3.90; N, 4.52%. IR
(cm21): 3420(s), 2931(w), 1647(w), 1625(s), 1615(s), 1524(w),
1491(m), 1435(s), 1417(m), 1361(m), 1317(m), 1265(w), 1222(w),
1168(w), 1078(m), 1036(w), 891(w), 816(m), 784(w), 747(m),
652(w), 582(w), 537(w).
Results and discussion
Crystal structure of Zn(L)(4,49-bpy)?H2O (1)
Complex 1 crystallizes in the trigonal space group R32 with a
Flack parameter of 0.07(5). Its asymmetric unit contains one
independent Zn(II) with 50% occupancy, half L and half
4,49-bpy. The crystallographically independent Zn(1) lies on a
twofold axis and it exhibits a distorted octahedral coordination
geometry defined by four oxygen atoms from two chelating
Synthesis of Cd2(L)2(phen)2(H2O)2?3H2O (3):
The synthesis of complex 3 was carried out as described above
for complex 1 except that Cd(NO3)2?4H2O and phen were
utilized instead of Zn(NO3)2?6H2O and 4,49-bpy, respectively.
The yield of the colorless block crystals is ca. 35% based on Cd.
Elemental anal. Found: C, 47.45; H, 4.19; N, 5.52%. Calc. for
C40H42Cd2N4O13: C, 47.49; H, 4.18; N, 5.54%. IR (cm21):
3447(m), 2928(w), 1999(w), 1623(w), 1560(s), 1540(w), 1517(w),
1427(m), 1405(m), 1332(w), 1144(w), 1103(w), 859(w), 729(m),
576(w), 514(w), 495(w).
˚
carboxylate groups from two L [Zn–O 1.919(1)–2.385(1) A] and
˚
two nitrogen atoms from two 4,49-bpy [Zn–N 2.065(5) A]
(Fig. 1a and Table S1{). In complex 1, the two carboxylate
groups in the ortho-position of L are deprotonated and L acts as
a 60u bent linker connecting two Zn(II) centers in a bis-chelating
…
˚
mode into a Zn2 unit with a Zn Zn separation of 5.79 A
(Fig. 1a and Scheme 1a). 4,49-bpy acts as a m2-bridge linking two
Zn2 units with a dihedral angle of 45.9u between the two pyridine
rings. Three Zn2 units are connected by three twisted 4, 49-bpy
Synthesis of Zn5(L)4(m3-OH)2(H2O)2 (4):
The preparation of complex 4 is similar to that of complex 1
except in the absence of 4,49-bpy. The yield of the colorless block
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Table 1 Crystal data and structure refinements for complexes 1–5
Complex
1
2
3
4
5
Empirical formula
M
Crystal system
Space group
C
18H18ZnN2O5
C13H12CuNO4
309.78
Monoclinic
P21/c
C40H42Cd2N4O13
1011.50
Monoclinic
P21/n
Zn5C32H38O20
1069.44
Triclinic
CdC8H10O5
298.54
Orthorhombic
Pbca
407.70
Trigonal
R32
¯
P1
˚
a/A
18.2300(4)
18.2300(4)
17.1118(5)
90
90
120
4924.9(2)
9
1.237
1.141
1843
1606
1800
1.082
R1 = 0.0794,
wR2 = 0.2133
0.07(5)
10.0635(5)
6.1226(2)
19.9763(9)
90
103.340(5)
90
1197.62(9)
4
1.718
1.832
2102
1733
632
0.993
R1 = 0.0308,
wR2 = 0.0799
12.1516(5)
10.0138(3)
17.5544(5)
90
107.844(4)
90
2033.33(1)
2
1.636
1.115
3217
2738
1000
1.064
R1 = 0.0242,
wR2 = 0.0662
7.1646(3)
9.1391(5)
14.6408(7)
74.030(4)
79.564(4)
83.527(4)
904.40(8)
1
1.956
3.355
3182
2629
9.2264(3)
7.7926(3)
25.4538(9)
90
90
90
1830.07(1)
8
2.153
2.378
1616
1473
1152
1.275
R1 = 0.0347,
wR2 = 0.0884
˚
b/A
˚
c/A
a (u)
b (u)
c (u)
3
˚
V/A
Z
Dcalc./g cm23
m/mm21
No. of unique reflcns
Reflcns used [I . 2s(I)]
F(000)
536
1.070
R1 = 0.0507,
wR2 = 0.1413
Goodness of fit on F2
Final R indicesa [I . 2s(I)]
Flack
a
R1 = g||Fo|2|Fc||/g|Fo|; wR2 = g[w(Fo22Fc2)2]/g[w(Fo2)2]1/2
˚
into a chiral hexanuclear cyclic unit, and the chiral cyclic unit lies
about a threefold axis (Fig. 1a).
[Cu–O 1.9434(2)–2.3701(2) A] (Fig. 2 and Table S1{). The L in
complex 2 shows a tridentate coordination mode with one
chelating and one bridging monodentate carboxylate groups
(Scheme 1b). Two Cu(II) ions are bridged by the monodentate
carboxylate group of L to give a paddle-wheel Cu2 unit with a
The chirality of the hexanuclear unit is the result of the folding
and twisting of the molecular assembly, and it can be well
understood in view of chemical topology. If a molecular
assembly is schematically represented on paper as a nonplanar
graph with crossings, and the nonplanar graph cannot be
deformed to form its mirror image without cutting, the assembly
is topologically chiral.13 For example, as shown in Scheme 2a
and 2b, the two graphs represent two enantiomers of a trefoil
knot.13 Chiral folded molecular assemblies are found everywhere
in daily life, such as twisted decorations, as shown in Scheme 2c.
However, it is rarely reported in spontaneous resolution of
MOFs. All the hexanuclear units in the single crystal of complex
1 adopt the same chirality, and they are fused by sharing the
inorganic corners into a 3D network (Fig. 1b).
…
˚
Cu Cu separation of 3.43 A, which is further connected by L
into a 1D chain (Fig. 2a). Different chains are linked by 4,49-bpy
to form a two-dimensional (2D) layer (Fig. 2b). In complex 2,
4,49-bpy is a planar molecule and it lies about an inversion
center, which is different from that in complex 1. The Cu2 unit
also lies about an inversion center, giving rise to the centrosym-
metric structure of complex 2.
As described above, though the Zn complex 1 and the Cu
complex 2 are constructed from the same achiral ligands, they
show different architectures. It is expected that the twisted
conformation of 4,49-bpy in complex 1 plays an important role
for the formation of the chiral complex. The two examples
indicate that different metal(II) ions in the structure can induce
different conformation of ligand.
As shown in Fig. 1a and 1b, each Zn(II) ion is connected to
four neighboring Zn(II) through two L and two 4,49-bpy ligands,
thus Zn(II) can be considered as
a 4-connnected node.
Topological analysis14 of complex 1 reveals that it is a uninodal
4-connected network with {64.82}-nbo topology (Fig. 1c).
As we know, achiral H2L and coplanar 4,49-bpy both possess
symmetrical structures. Utilizing symmetrical ligands to form
chiral MOFs, the prerequisite is to remove the inversion center
during molecular packing. Based on the consideration, we extend
our work to investigate the generation of inversion center in
molecular packing.
Crystal structure of Cd2(L)2(phen)2(H2O)2?3H2O (3)
In order to further explore the effect of the N-donor on the
formation of the chiral framework, phen, as a planar molecule,
was utilized instead of 4,49-bpy in the synthesis. Complex 3 was
obtained and it crystallizes in the monoclinic space group P21/n
with one Cd(II), one L, one phen, one coordinated water and one
and a half uncoordinated water molecules in the asymmetric
unit. Cd(1) exhibits a distorted octahedral coordination geome-
try, being coordinated by three oxygen atoms from one chelating
and one monodentate carboxylate groups of two L, one oxygen
Crystal structure of Cu(L)(4,49-bpy)0.5 (2)
When Cu(II) was introduced in place of Zn(II), complex 2 was
obtained. Complex 2 crystallizes in the monoclinic space group
P21/c, its asymmetric unit contains one independent Cu(II) atom,
one L and half 4,49-bpy. Cu(1) exhibits a distorted tetragonal
pyramidal geometry supplied by one nitrogen atom from
˚
atom from terminal aqua ligand [Cd–O 2.263(2)–2.409(2) A] and
˚
two N atoms from phen [Cd–N 2.355(2)–2.357(2) A] (Fig. 3 and
Table S1{). In complex 3, the L possesses one monodentate and
one chelating carboxylate groups (Scheme 1c) to link two Cd(II)
ions. Phen acts as a terminal ligand, occupying two coordination
sites of the Cd(II) centers. Different Cd(II) ions are connected by
˚
4,49-bpy [Cu–N 1.961(2) A] and four oxygen atoms from two
monodentate and one chelating carboxylate groups of three L
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Scheme 1 The coordination modes of L in complexes 1 (a), 2 (b), 3(c),
4(d, e) and 5(f).
Scheme 2 Projections of enantiomers of trefoil knots (a, b), chiral
decoration (c).
Fig. 1 Chiral hexanuclear cyclic unit constructed by Zn(II), L and
twisted 4,49-bpy (H atoms omitted for clarity) (a); 3D homochiral
network (b); Schematic illustrating the {64.82}-nbo topology of the
uninodal 4-connected 3D architecture in complex 1 (c).
L ligands to give a 1D helical chain (Fig. 3). The helical structure
is attributed to the angular geometry of the two carboxylate
˚
groups of L, and the period of the helical chain is 10.014 A with
…
the Cd(II) ions functioning as hinges. Strong p p stacking
interactions are observed between two neighbouring chains. For
example, plane 1 is constructed by C15–C19, N2, plane 2 is
constructed by C12i–C15i, C19i, C20i (atoms with the additional
label i refers to the symmetry operation: 1 2 x, 2y, 2z), the
Fig. 2 1D chain consisting of Cu(II) and L in complex 2 (H atoms and C
atoms of 4,49-bpy omitted for clarity) (a); 2D framework built from 1D
chain linked by 4,49-bpy (H atoms omitted for clarity) (b).
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Fig. 3 Alternating left-handed and right-handed 1D helical chains in
complex 3 (H atoms omitted for clarity).
centroid–centroid and perpendicular distances between the two
˚
planes are 3.850 and 3.554 A, respectively. Due to the strong
…
p
p stacking interaction between phen molecules, alternating
left-handed and right-handed helical chains are in an orderly
arrangement, resulting in the whole structure of complex 3 being
nonchiral (Fig. 3).
Fig. 4 1D chain constructed by Zn(II) and pentadentate L together with
m3-OH in complex 4 (a); 1D chain constructed by Zn(II) and tridentate L
(b); 2D layer built from Zn(II) ions and pentadentate (denoted in pink)
and tridentate L (denoted in blue) (c). (Atoms with additional labels A or
B refer to the symmetry operations: A, 2 2 x, 1 2 y, 1 2 z; B, x, 21 + y,
z. H atoms and disordered C atoms omitted for clarity).
Crystal structure of Zn5(L)4(m3-OH)2(H2O)2 (4)
Complex 4 was obtained in the absence of N-donor. It
¯
crystallizes in the triclinic space group P1 with two and a half
independent Zn(II), two L, one m3-OH and one coordinated
water molecule in the asymmetric unit. Zn(1) sits on the
inversion center and is six-coordinated by four oxygen atoms
of four carboxylate groups from four L and two m3-OH to give a
distorted octahedral coordination geometry. Zn(2) is five-
coordinated by three oxygen atoms of three carboxylate groups
from three L, one m3-OH and one terminal oxygen atom from
one aqua ligand to furnish a distorted triangular-bipyramidal
coordination geometry. Zn(3) exhibits a tetrahedral coordination
environment supplied by three oxygen atoms of three carbox-
ylate groups from three L and one m3-OH [Zn–O 1.924(3)–
bidentate mode (Scheme 1d). 1D chain is constructed by Zn(II)
ions and the pentadentate L together with m3-OH, in which the
…
Zn5 unit is observed with Zn(1) Zn(2), Zn(1) Zn(3) and
…
…
˚
Zn(3) Zn(2A) separations of 3.063, 3.313 and 3.431 A,
respectively (atoms with the additional label A refers to the
symmetry operation: A, 2 2 x, 1 2 y, 1 2 z) (Fig. 4a). Another L
shows a tridentate fashion with two carboxylate groups bonded
to two and one Zn(II) ions bidentately and monodentately,
respectively (Scheme 1e) to give a 1D chain (Fig. 4b). Holding
two kinds of L together by Zn(II) ions results in a 2D framework
(Fig. 4c). The structure is similar to the Co(II)-L complex
reported previously.15 The inversion center is occupied by the
metal(II) center, which leads to the centrosymmetric structures of
the complex.
˚
2.162(3) A] (Fig. 4a and b and Table S1{). In complex 4, the two
crystallographically independent L ligands exhibit different
coordination modes. The C atoms of the cyclohexene ring from
one L are disordered over two locations (C12A and C12B, C13A
and C13B) and each site is half-occupied. One C atom from
another cyclohexene ring is disordered over two locations (C4A
and C4B). The occupancy factors of C4A and C4B are 0.8 and
0.2, respectively. One L shows a pentadentate fashion with one
carboxylate group bridging two Zn(II) center bidentately and
another carboxylate group linking three Zn(II) ions in a bridging
Crystal structure of Cd(L)(H2O) (5)
Similarly, complex 5 was also obtained in the absence of an
N-donor except that Cd(II) was utilized instead of Zn(II) in the
synthesis of complex 4. Complex 5 was obtained and it
crystallizes in the orthorhombic space group Pbca, its asym-
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metric unit contains one unique Cd(II), one L and one
coordinated water molecule. The crystallographically indepen-
dent Cd(1) exhibits slightly distorted pentagonal bipyramid
geometry, being coordinated by six oxygen atoms of five
carboxylate groups from four L and one oxygen atom from
inversion centers in the structures. It is expected that 1,2-
dicarboxylate ligand in the absence of an N-donor possesses the
preference for the formation of achiral framework with metal–
oxygen–metal linkage.16
˚
aqua ligand [Cd–O 2.281(3)–2.469(3) A] (Fig. 5a and Table S1{).
CD spectrum of complex 1
The C atoms of the cyclohexene ring from L are disordered over
two locations (C4A and C4B, C5A and C5B). The occupancy
factors of C4A and C5A are 0.8, and the occupancy factors of
C4B and C5B are 0.2. In complex 5, the completely deprotonated
L bridges four Cd(II) centers in a pentadentate coordination
mode: one carboxylate group adopts a chelating bridging
coordination mode connecting two Cd(II) ions, whereas another
carboxylate group links one of the above two Cd(II) ions and
another two Cd(II) centers bidentately (Scheme 1f and Fig. 5a).
Different Cd(II) ions are bridged by the carboxylate groups of L
into a 2D Cd–O–Cd layer which consists of vertex- and edge-
As discussed above, complex 1 belongs to the chiral hexagonal
space group R32 with a Flack parameter of 0.07(5), suggesting
the homochirality of the 3D framework. The solid-state CD
spectrum of complex 1 was recorded on one single crystal with a
KCl pellet between 200 and 400 nm (Fig. 6). The CD spectrum
exhibits an obvious Cotton effect at 246 nm. The CD signal
suggests that the single crystal of 1 is a same-handed conforma-
tion, which further demonstrates the validity of structural
analysis. No CD signal is observed when eight single crystals
were randomly picked, indicating the bulk sample of complex 1
contain a 50 : 50 (racemic) mixture of two enantiomorphs in the
spontaneous crystallization.
…
sharing {CdO7} pentagonal bipyramids with Cd Cd separation
˚
of 4.237 and 3.678 A, respectively (Fig. 5b). A pair of edge-
sharing pentagonal bipyramids lie about an inversion center,
leading to the centrosymmetric structure of the complex. As
discussed above, both complex 4 and 5 were obtained in the
absence of an N-donor. Though the framework of complex 5 is
different from that of complex 4, both of them are achiral with
UV-vis absorption spectra of complexes 1–5
The XRD powder patterns of complexes 1–5 are shown in Fig.
S1 in the ESI.{ All the peaks of the five compounds can be
indexed to their respective simulated XRD powder patterns,
indicating each of the five compounds is in pure phase. The UV-
vis absorption spectra of complexes 1–5 together with the free
organic ligands in the solid state at room temperature are shown
in Fig. S2.{ As shown in Fig. S2,{ H2L exhibits strong
absorption with a maximum at ca. 250 nm in the range of
240–400 nm, which probably corresponds to the p electron
17a
transitions of the CLC and CLO bonds.
The absorption of
4,49-bpy is strong in a similar range with one peak at 290 nm.
And the absorption band of phen extends from 200 to 400 nm
with two peaks at 254 and 335 nm. They may be ascribed to the
n–p* or p–p* transition.17 As for complex 1, it displays weak
absorption in the range of 240–400 nm, in which four maxima at
246, 320, 334 and 357 nm are present. The CD spectrum of
complex 1 matches well with its absorption spectrum, it exhibits
strong CD signal in a similar range.
Fig. 5 Ball-stick (a) and polyhedral diagram (b) of 2D layer constructed
by Cd(II) and L in complex 5 (H atoms omitted for clarity).
Fig. 6 The solid-state CD spectrum of complex 1.
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As for complexes 2 and 3, they exhibit absorption with
maxima at 344 and 304 nm, respectively, in the same range,
which are different from those of free ligands, inferring that the
absorption of the complexes may originate from the intraligand
transition (ILCT), ligand-to-ligand change transfer transition
(LLCT) or metal-to-ligand charge-transfer transition (MLCT).17
Similarly, complexes 4 and 5 show strong absorption in the range
of 200–300 nm and weak absorption in the range from 300 to 400
nm, which are different to that of free H2L.
loss of water molecules (Fig. S4e{). The desolvated sample
remained stable up to y320 uC without any weight loss. The
decomposition of the host framework occurred in the rang of
320–660 uC (calc.: 56.4%; observed: 56.2%) (Fig. S4e{).
Conclusion
In conclusion, based on achiral ligand H2L, we got five metal–L
complexes in the presence or absence of N-donor coligands. L
exhibits different coordination modes in the five complexes.
Complex 1, Zn(L)(4,49-bpy)?H2O, is a homochiral network
based on twisted hexanuclear cyclic units, which are fused by
sharing metal ions into a 3D architecture. The homochirality of
the compound is evidenced by the CD spectrum. The other four
complexes, formulated as Cu(L)(4,49-bpy)0.5 (2), Cd2(L)2(phen)2
(H2O)2?3H2O (3), Zn5(L)4(m3-OH)2(H2O)2 (4) and Cd(L)(H2O)
(5), are achiral compounds. Complex 2 exhibits a 2D framework
constructed by 1D chain linked by 4,49-bpy, in which a paddle-
wheel Cu2 unit with inversion center is observed. 4,49-bpy in
complex 2 is a planar molecule, which is different from the
Photoluminescence properties of complexes 1–5
The emission spectra of the free organic ligands 4,49-bpy and
phen (slit width = 5 nm) are depicted in Fig. S3a.{ Strong
photoluminescence emission bands at 466 and 442 nm are
observed for free 4,49-bpy (lex = 390 nm) and phen (lex
=
370 nm), respectively (Fig. S3a{). As for free H2L and complexes
1, 3–5, the emission bands are very weak under similar
conditions (slit width = 5 nm). When the slit width is adjusted
to be 10 nm, emission bands at 440, 486 and 450 nm are observed
for free H2L (lex = 350 nm), complex 4 (lex = 310 nm) and
complex 5 (lex = 330 nm), respectively (Fig. S3b{). Complexes 1
and 3 shows similar emission bands at 445 and 493 nm when
excitation at 390 nm (Fig. S3b{). Complex 2 is non-emissive
under similar conditions, indicating the emissions of organic
ligands have been quenched completely in the complex, which is
due to the emission quencher of Cu(II) in the structure.18
The emission peaks of complexes 1, 3–5 are red- or blue-
shifted compared with those of the free organic ligands, inferring
their emission bands may be assigned to the LLCT, admixing
with MLCT and ligand-to-metal charge-transfer transition
(LMCT) as previously reported.19 The results are in agreement
with the absorption spectra of the complexes.
twisted conformation in complex 1. In complex 3, due to the
…
p
p stacking of phen, 1D alternating left-handed and right-
handed helical chains are in an orderly arrangement, making the
whole structure nonchiral. Complexes 4 and 5 were obtained in
the absence of an N-donor and they display different 2D
frameworks based on a Zn5 unit and Cd–O–Cd infinite linkage,
respectively. In complex 4, inversion centers are occupied by
Zn(II) ion. In complex 5, it exists within a pair of edge-sharing
{CdO7} pentagonal bipyramids. It is expected that the achiral
framework with metal–oxygen–metal linkage is favored when the
1,2-carboxylate ligand L was utilized in the absence of N-donor.
The twisted 4,4-bpy plays an important role in the formation of
the chiral MOF.
Thermal stability of complexes 1–5
Acknowledgements
In order to examine the thermal stability of the five complexes,
thermogravimetric analyses (TGA) were carried out. The
samples were heated up to 900 uC in N2. It is found that the
cavity of complex 1 is occupied by five water molecules per unit,
as estimated by SQUEEZE and TGA.12 The TGA curve of
complex 1 shows a one step weight loss of 4.5% between 30 and
100 uC, corresponding to the loss of lattice water molecules (calc.
4.4 wt%). Decomposition of the organic components began at
140 uC (Fig. S4a{).
Financial support from the National Key Basic Research Project
of China (Grant 2010-CB833103), the Fundamental Research
Funds for the Central Universities (No. CDJZR10 22 00 09),
Natural Science Foundation Project of Chongqing (No. CSTC
2011jjA50001) and the sharing fund of Chongqing university’s
large-scale equipment (No. 2 011 063 001) are gratefully
acknowledged.
As for complex 2, no weight loss is observed in the
temperature range of 30–180 uC (Fig. S4b{), which is in good
agreement with the crystal structure of complex 2, in which no
solvent is included. Complex 2 shows one step of weight loss in
the temperature range of 180–400 uC with a loss of 79.6 wt%
(calc. 79.4 wt%), corresponding to the decomposition of the
organic components.
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