Structures and single crystal to single crystal transformations of cadmium frameworks using a flexible tripodal ligand

Article abstract of DOI:10.1039/c7nj05125e

A highly flexible tris-imidazole ligand N(CH₂-m-C₆H₄-CH₂-imidazole)₃ (L) constructed using a tris(xylyl) backbone was synthesized. Its reactions with three cadmium(ii) salts gave various Cd(ii) coordination polymers, namely [CdL₂](ClO₄)₂ (1), [CdLCl₂] (2), and [CdL(OAc)₂] (3). X-ray studies revealed that 1 crystallized in the trigonal space group R3 with high symmetry, while 2 and 3 crystallized in lower symmetrical space groups due to Cd-anion coordination. The flexibility of ligand L was evidenced in these structures, showing different conformations of L upon coordination and crystallization. In addition, complex [CdL₂]Cl₂ (4) was obtained by the anion exchange of 1 with sodium chloride via a single crystal to single crystal transformation. Similar to 1, complex 4 also crystallized in the trigonal space group R3 with a = 13.0945(3), b = 37.4353(10), and V = 5559.2(2). Importantly, this single crystal to single crystal transformation is crucial to afford this Cd(ii) coordination polymer with unbound Cl^- anions, as the direct reaction of L and CdCl₂ gives complex 2 with Cd-bound Cl^-. This work shows the importance of flexible ligands in constructing various coordination polymer structures through the regulation of anions.

Full text of DOI:10.1039/c7nj05125e

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The role of atropisomers on the photo-reactivity and fatigue of
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Structures and Single Crystal to Single Crystal
Transformations of Cadmium Frameworks Using a Flexible
Tripodal Ligand
Zongyao Zhang,a Lijin Xu,*a and Rui Cao*ab
aDepartment of Chemistry, Renmin University of China, Beijing 100872, China.
bKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an
710119, China.
*Correspondence Email: xulj@chem.ruc.edu.cn; ruicao@ruc.edu.cn

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Abstract
A highly flexible trisꢀimidazole ligand N(CH2ꢀmꢀC6H4ꢀCH2ꢀimidazole)3 (L)
constructed using a tris(xylyl) backbone was synthesized. Its reactions with three
cadmium(II) salts gave various Cd(II) coordination polymers, namely [CdL2](ClO4)2 (1),
[CdLCl2] (2), and [CdL(OAc)2] (3). Xꢀray studies revealed that 1 crystallized in the
trigonal space group R
3 with high symmetry, while 2 and 3 crystallized in lower
symmetrical space groups due to the Cdꢀanion coordination. The flexibility of ligand L
was evidenced in these structures, showing different conformations of L upon
coordination and crystallization. In addition, complex [CdL2]Cl2 (4) was obtained by the
anion exchange of 1 with sodium chloride via a single crystal to single crystal
transformation. Similar to 1, complex 4 also crystallized in the trigonal space group R
3
with a = 13.0945(3), b = 37.4353(10), and V = 5559.2(2). Importantly, this single crystal
to single crystal transformation is crucial to afford this Cd(II) coordination polymer with
unbound Cl− anions, as the direct reaction of L and CdCl2 gave complex 2 with Cdꢀbound
Cl−. This work shows the importance of flexible ligands in constructing various
coordination polymer structures through the regulation of anions.
2

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Introduction
Selfꢀassembled coordination polymers, particularly metalꢀorganic frameworks,1ꢀ3
have gained considerable attention in recent years for not only their intriguing structural
diversities but also their applications in various fields, such as magnetism,4,5 optics,6,7
catalysis,8ꢀ11 hostꢀguest recognition,12ꢀ15 drug delivery,16 gas adsorption and separation.17ꢀ
19
In general, ligands with rigid backbones and optimized binding angles are favored for
constructing selfꢀassembled coordination polymers because it is relatively straightforward
to predict and construct desired structures. In contrast to rigid ligands, flexible ligands
have much less been used in assembling coordination polymers. The ligand flexibility is
caused by the presence of sp3 hybridized carbon atoms, which connect rigid components
with rotationally free single bonds. Such carbon atoms can be considered as flexible
joints. Ligands with one flexible joint in total or one flexible joint in each coordinating
group have been explored for coordination polymer synthesis, showing conformational
variations of ligands and structural diversities of the resulted coordination polymers.20ꢀ28
However, there are very few examples of ligands bearing more than two flexible joints
for assembling coordination polymers in literature.29
Recently, Cao and Lippard investigated the coordination chemistry of a series of
multidentate tripodal ligands and their uses in assembling highꢀnuclearity metal
clusters.30ꢀ34 By functionalizing either the ortho or the meta positions of a tris(xylyl)
backbone with different ligand appendages, including triazacyclononane and
dipicolylamine, Cao and Lippard reported the applications of the metal complexes of
these tripodal ligands in biologically relevant ꢁ3ꢀoxoanion binding,30,32 phosphate ester
hydrolysis,30,33 carbon dioxide fixation,32 and anion immobilization and trapping.31 For
3

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the tripodal ligands functionalized at the meta positions, there are two kinds of flexible
joints (Scheme 1). One is the three methylene groups in the tris(xylyl) backbone
connecting the central N atom and the three benzyl arms, and the other one is the three
methylene groups in each of the three arms connecting the binding site and the benzyl
group. As a consequence, these multidentate tripodal ligands have six flexible joints in
total, and thus they will have in theory four different conformations, including upward-
upward-upward (UUU), upward-upward-downward (UUD), upward-downward-
downward (UDD), and downward-downward-downward (DDD). These flexible joints
and conformations bring possibilities of distinct structures. Although the designed
synthesis of coordination polymers using ligands with more than two flexible joints is
challenging, we consider that the various binding affinities and the diverse conformations
of ligands using the tris(xylyl) backbone is appealing to make different coordination
polymers through the control of synthetic conditions.
Scheme 1. Schematic view of ligand L and its four different conformations.
4

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For this purpose, we herein report a new tripodal ligand N(CH2ꢀmꢀC6H4ꢀCH2ꢀ
imidazole)3 (L, Scheme 1) and its coordination chemistry with Cd(II) salts. The choose of
Cd(II) is benefited by its d10 electronic structure, which has no crystal field stabilization
energy.35 Therefore, the binding between Cd(II) and L can be easily reorganized and
rearranged during the coordination and crystallization. Direct reactions of L with three
Cd(II) salts gave various Cd(II) coordination polymers, namely [CdL2](ClO4)2 (1),
[CdLCl2] (2), and [CdL(OAc)2] (3). Complex 1 has a twoꢀdimensional honeycombꢀlike
structure with high symmetry. Complex 2 has a lowꢀsymmetrical 2D structure due to the
Cd−Cl− binding, and 3 presents 1D double chains with Cd−OAc− bonds. In these
structures, ligand L adopted three kinds of conformations, depending on coordination
modes and counter anions.
−
Importantly, because the ClO4 counter anions of 1 are not coordinated and are
located in the cavity of the frameworks, they are potentially exchangeable with other
anions. Anions play significant roles in regulating a variety of physiological processes,
and they are potential toxins in natural environment.36ꢀ38 Complexes and materials that
can trap or immobilize anionic pollutants have therefore attracted extensive interests for
chemists and material scientists.19,39ꢀ45 Coordination polymers have shown anion
recognition and anion exchange properties.46ꢀ48 In this work, we also studied the anion
exchange property of 1. Anion exchange of 1 with sodium chloride was monitored by
infrared spectroscopy and powder Xꢀray diffraction methods. Significantly, this exchange
proceeds via a single crystal to single crystal (SCꢀSC) transformation, leading to
[CdL2]Cl2 (4) with the maintenance of the highꢀsymmetrical structure of 1. SCꢀSC
transformation is an attractive property of coordination complexes, usually induced by
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the change of metal ions, anions or other guest species.49ꢀ53 Anionꢀexchangeꢀinduced SCꢀ
SC transformations are rare and are usually observed for coordination polymers
constructed by rigid ligands.54ꢀ56 As a result, the SCꢀSC transformation from 1 to 4
represents a very rare example using coordination polymers with highly flexible ligands.
Experimental Section
General methods and materials. Manipulations of airꢀ and moistureꢀsensitive
materials were performed under an atmosphere of nitrogen gas using standard Schlenk
line techniques. All materials were commercially available and were used as received
1
13
unless otherwise noted. H and C NMR spectroscopic measurements were made on a
13
Brüker spectrometer operating at 400 MHz (100 MHz for C spectra). Infrared spectra
(2% sample in KBr pellet) were recorded using a Brüker spectrophotometer running the
OPUS software. ESIꢀMS measurements were performed on a Brüker fourier transform
ion cyclotron resonance mass spectrometer APEX IV. Powder Xꢀray diffraction (PXRD)
patterns were recorded at room temperature on Shimadzu XRDꢀ7000 diffractometer (Cu
Kα, 40 kV and 40 mA). Elemental analysis was performed on Elementar Vario ELꢀIII
analyzer. Thermogravimetric data were collected on TA Q50 analyzer in N2 at heating
rate of 10 °C min−1. Fluorescence spectra were measured on Hitachi Fꢀ4600 fluorescence
spectrophotometer at room temperature. The tripodal backbone N(CH2ꢀmꢀC6H4ꢀCH2Br)3
was synthesized according to our previous work.30
Synthesis of 2-cyanoethyl imidazole. A mixture of imidazole (2.46 g, 36.1
mmol) and acrylonitrile (5 mL, 75.5 mmol) was taken up in toluene (50 mL). The
mixture was kept reflux overnight, and the solvent was then removed in vacuo. The
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resulting oily residue was extracted with dichloromethane and dried over Na2SO4. The
solution was filtered, and removal of solvent under reduced pressure gave clear yellow oil
1
as the pure product (3.28 g, yield 75%). HꢀNMR (CDCl3, 400 MHz): δ = 7.52 (s, 1H),
6.95ꢀ7.09 (m, 2H), 4.22 (t, 2H), 2.78 (t, 2H).
Synthesis of N(CH2-m-C6H4-CH2-imidazole)3. To a 50 mL dry chloroform
solution under N2 was added N(CH2ꢀmꢀC6H4ꢀCH2Br)3 (0.566 g, 1.0 mmol). Then, 2ꢀ
cyanoethyl imidazole (0.363 g, 3.3 mmol) dissolved in 100 mL of chloroform was added
dropwise to the solution under continuous vigorous stirring. The mixture was stirred
under N2 at room temperature overnight, and the solvent was then removed in vacuo. The
resulting oily product was crude product of quaternary ammonium salt, which was used
directly without purification. The crude product was dissolved in 20 mL of water, and 10
mL of 1 M NaOH solution was added dropwise. After stirring for 2 h, the light yellow
oily product formed as a separate layer. The mixture was extracted with chloroform. The
organic layers were combined and dried over Na2SO4, and filtered. The solvent was then
removed on a rotary evaporator, giving a yellow sticky liquid. The final product was
purified from alumina column (1:10 methanolꢀdichloromethane) to give a light yellow oil
1
(0.45 g, yield 85%). HꢀNMR (CDCl3, 400 MHz): δ = 7.62 (s, 3H), 7.29ꢀ7.34 (m, 3H),
13
7.21ꢀ7.25 (m, 3H), 7.03ꢀ7.13 (m, 9H), 6.90 (s, 3H), 5.14 (s, 6H), 3.45 (s, 6H). C NMR
(CDCl3, 100 MHz): δ = 140.23, 137.44, 136.30, 129.58, 128.97, 128.52, 127.18, 126.10,
119.41, 57,67, 50.71. Anal. calcd. for C33H33N7: C, 75.11; H, 6.30; N, 18.58. Found: C,
75.08; H, 6.34; N, 18.52. ESIꢀMS for [M+H]+: calcd. 528.2875; found, 528.2869.
Synthesis of complex 1. To a yellow solution of ligand L (52.7 mg, 0.10 mmol)
in 10 mL of methanol, was added Cd(ClO4)2·6H2O (20.5 mg, 0.05 mmol) dissolved in 5
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mL of water. Then, 10 mL of acetonitrile was added. The mixture was heated at 60 °C for
3 h under stirring, from which a light yellow solution with white precipitate was obtained.
Then the solvent was evaporated and the residue was dissolved in 10 mL of acetonitrile.
The resulted solution was filtered. Removal of the solvent gave the product as a white
solid. Slow diffusion of ether at low temperature into the yellow filtrate afforded
colorless hexagonal crystals of 1. (30 mg, yield 44%) Anal. calcd. for C66H66CdCl2N14O8:
C, 58.00; H, 4.87; N, 14.35. Found: C, 57.81; H, 4.94; N, 14.10. FTIR (KBr pellet, 2%):
1606(w), 1515(s), 1438(s), 1398(w), 1361(w), 1288(w), 1157(m), 1067(vs), 968(w),
935(w), 846(m), 734(s), 696(w), 597(m).
Synthesis of complex 2. To a yellow solution of ligand L (52.7 mg, 0.10 mmol)
in 10 mL of methanol, was added CdCl2·2.5H2O (22.8mg, 0.10 mmol) dissolved in 5 mL
of water. The mixture was heated at 60 °C for 3 h under stirring, during which white
precipitate formed. The solvent was removed on a rotary evaporator, and 5 mL of a 1:1
DMFꢀacetonitrile solution was added to redissolve the precipitate. The solution was then
filtered over celite. Single crystals suitable for Xꢀray diffraction studies were obtained by
slow evaporation of the solution under low temperature (28 mg, yield 39%). Anal. calcd.
for C66H66Cd2Cl4N14: C, 55.75; H, 4.68; N, 13.79. Found: C, 55.60; H, 4.82; N, 13.65.
FTIR (KBr pellet, 2%): 1606(w), 1515(s), 1438(s), 1398(w), 1361(w), 1288(w), 1157(m),
968(w), 935(w), 846(m), 734(s), 696(w), 597(m).
Synthesis of complex 3. To a yellow solution of ligand L (52.7 mg, 0.10 mmol)
in 10 mL of methanol, was added Cd(OAc)2·2H2O (23.4 mg, 0.10 mmol) dissolved in 5
mL of water. The mixture was heated at 60 °C for 3 h under stirring. The resulted
solution was filtered, and the solvent was removed on a rotary evaporator. The oily
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product was then redissolved in acetonitrile. Single crystals suitable for Xꢀray diffraction
studies were obtained by slow diffusion of ether into the solution of the crude product.
(28 mg, yield 37%) Anal. calcd. for C74H78Cd2N14O8: C, 58.61; H, 5.18; N, 12.93. Found:
C, 58.38; H, 5.03; N, 12.70. FTIR (KBr pellet, 2%): 1630(w), 1564(s), 1477(m),
1411(m), 1384(w), 1230(s), 1157(m), 1062(s), 948(s), 871(s), 763(s), 719(m) 669(s),
644(s), 616(m), 540(w).
Synthesis of complex 4. A freshly prepared crystalline sample of complex 1 (10
mg) was placed into an aqueous solution of NaCl (2.0 M), and this mixture was kept for 7
days without stirring. The crystalline product was found to keep its size and trigonal
shape. The product was characterized by single crystal diffraction, and the bulk sample
was washed with water and dried in vacuo for further analysis (9 mg, yield 83%). Anal.
calcd. for C66H66CdCl2N14: C, 64.00; H, 5.37; N, 15.83. Found: C, 63.81; H, 5.50; N,
15.66. FTIR (KBr pellet, 2%): 1606(w), 1515(s), 1438(s), 1398(w), 1361(w), 1288(w),
1157(m), 968(w), 935(w), 846(m), 734(s), 696(w), 597(m).
Caution: Perchlorate salts are highly explosive and must be handled with care.
X-ray crystallography. Single crystals of ligand LꢀClO4 and complexes 1ꢀ4
(CCDC 1811703ꢀ1811707) suitable for Xꢀray analysis were each coated with ParatoneꢀN
oil, suspended in a small fiber loop, and placed in a cold gaseous nitrogen stream on a
Bruker D8 VENTURE Xꢀray diffractometer performing φꢀ and ωꢀscans at 150(2) K.
Diffraction intensities were measured using graphite monochromated Mo Kα radiation (λ
= 0.71073 Å). Data collection, indexing, initial cell refinements, frame integration, and
final cell refinements were accomplished using the program APEX2.57 Absorption
corrections were applied using the SADABS.58 Scattering factors and anomalous
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dispersion corrections were taken from the International Tables for X-ray
Crystallography. All structures were solved by direct methods using SHELXS59 and
refined against F2 on all data by fullꢀmatrix least squares with SHELXL60,61 following
established refinement strategies.62
SQUEEZE program was used in the refinement of complexes 2, 3 and 4.63 All
nonꢀhydrogen atoms were refined anisotropically. All hydrogen atoms binding to carbon
were included into the model at geometrically calculated positions and refined using a
riding model. The isotropic displacement parameters of all hydrogen atoms were fixed to
1.2 times the U value of the atoms they are linked to (1.5 times for methyl groups).
Details of the data quality and a summary of the residual values for the refinements are
listed in Table S1.
Anion exchange experiments. In a typical experiment, 100 mg freshly prepared
crystalline sample of complex 1 was placed into an aqueous solution of NaCl, NaBPh4,
Na2ReO4 or NaPF6 (2.0 M), and this mixture was kept one day without stirring. The
exchange process was monitored by infrared spectroscopy: a small portion of the solid
was taken out, washed with water (× 3), dried in vacuo, and then analyzed by FTIR. After
the anion exchange reaction was confirmed to be complete, powder Xꢀray diffraction
method was used to analyze the purity and crystallinity of the product.
Results and Discussion
Synthesis and characterization. Imidazole is often used in ligands with rigid
backbones for the synthesis of coordination polymers largely due to its simple ligation
modes.64 Thus, we decided to introduce three imidazole binding groups to the tris(xylyl)
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backbone. However, direct reaction between imidazole and N(CH2ꢀmꢀC6H4ꢀCH2Br)3 did
not give the desired product but gave a mixture of oligomeric products because of the
presence of two N sites on each imidazole. To prevent this phenomenon, one of the N
atoms of imidazole was protected in advance by reacting with acrylonitrile. The protected
2ꢀcyanoethyl imidazole was then reacted with N(CH2ꢀmꢀC6H4ꢀCH2Br)3, and the product
in terms of quaternary ammonium salt was precipitated as a white solid from chloroform.
Deprotection with 2.0 M NaOH and subsequent purification by chromatography using
neutral alumina gave the desired ligand L, whose identity and purity were confirmed by
13
1H and C NMR, ESIꢀMS (Figure S1ꢀS3) and elemental analysis. Efforts to get crystals
of ligand L suitable for single crystal Xꢀray diffraction analysis were not successful
because of its structural flexibility. As a result, we obtained the Xꢀray quality single
crystal samples of protonated L by addition of HClO4.
Complexes 1ꢀ3 were synthesized by direct reaction of ligand L and the
corresponding Cd(II) salts. The rather low yields of 1ꢀ3 (ca. 40%) were likely caused by
the inherent flexibility of ligand L to produce undesired coordination polymers. Complex
4 was obtained by anion exchange of 1 with NaCl solution. The successful exchange of
−
ClO4 anions in 1 by Cl− was confirmed by FTIR. More importantly, the resulted product
maintained the crystallinity, which is suitable for single crystal Xꢀray diffraction analysis.
This feature is significant as such anionꢀexchangeꢀinduced SCꢀSC transformations are
rare and are usually observed for coordination polymers constructed by rigid ligands.
X-ray structures of L and 1-3. Slow evaporation of the methanolꢀacetonitrile
solution of L in the presence of HClO4 afforded colorless single crystals. This perchlorate
salt of ligand L crystallized in the triclinic space group P1 (see Table S1 for details). In
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−
each asymmetric unit, there are two ligand molecules and three ClO4 anions, leading to a
formula of L·1.5HClO4. The thermal ellipsoid plot of the Xꢀray structure of L is depicted
in Figure 1, showing a UUU conformation.
Figure 1. Thermal ellipsoid plot (50% probability) of the Xꢀray structure of L. All
hydrogen atoms are omitted for clarity.
Crystallographic studies revealed that 1 crystallized in the trigonal space group R
3
with Z = 3. Each ligand binds three Cd(II) atoms, and the extending of these Cd−N
bonds gives a 2D cationic hexagonal honeycombꢀlike framework (Figure 2). The
asymmetric unit contains one third of [Cd3L] (Figure S4). In the Xꢀray structure of 1,
each Cd(II) atom is sixꢀcoordinated with six N atoms from the ligand imidazole moiety,
creating a regular octahedral geometry. The symmetryꢀrequired C3 axis passes though the
Cd(II) atom along c axis, and all the Cd−N bond distances are 2.342(5) Å. In consistent
with the coordination environment of Cd(II) and highꢀsymmetrical crystal packing,
ligand L takes the highly symmetrical UUU conformation. The perchlorate anions are
uncoordinated and are located in the cavity of the framework structure.
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Figure 2. Ballꢀandꢀstick representation of the Xꢀray structure of 1, showing the 2D
honeycombꢀlike framework. All hydrogen atoms are omitted for clarity. Color code: Cd,
yellow; N, blue; C, grey.
Complex 2 crystallized in the monoclinic space group P21/n with Z = 2. The
asymmetric unit contains one molecule of [Cd2L2Cl4] (Figure S5). The two ligands have
different conformations. One takes the conformation of UUD, and the other takes UDD.
This result is consistent with the much lower symmetry observed for the structure of 2 as
compared with 1. For the two Cd(II) atoms, Cd1 is coordinated by two imidazole N
atoms and two Cl− atoms. The two N atoms are oriented from one upward imidazole
belonging to the UDD backbone and one downward imidazole belonging to the UDD
backbone. The Cd−N bond distances are 2.206(18) and 2.204(16) Å, and Cd−Cl bond
distances are 2.418(10) and 2.408(14) Å. As a consequence, Cd1 takes a slightly distorted
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tetrahedral geometry. Cd2 is fiveꢀcoordinated with three imidazole N atoms and two Cl−
atoms, giving a distorted trigonal bipyramidal geometry. The three N atoms are oriented
from one downward imidazole belonging to the UUD backbone, one downward
imidazole belonging to the UDD backbone, and another one upward imidazole from the
UUD backbone with Cd−N bond distances of 2.245(14), 2.232(16) and 2.197(20) Å. The
Cd−Cl bond distances are 2.577(10) and 2.614(10) Å. There are several uncoordinated
water molecules located in the Xꢀray structure. The extending of the Cd−N bond
interactions gives a 2D framework (Figure 3). The subunit of the building block can be
viewed as [Cd8L4] squares. Because of the presence of Cd−Cl bonds, one imidazole
group from a UUD ligand in each asymmetric unit is uncoordinated.
Complex 3 crystallized in the triclinic space group P1 with Z = 2. The asymmetric
unit contains one molecule of [Cd2L2(OAc)4] (Figure S6). In each asymmetric unit, the
two ligands have different conformations. One takes the conformation of UUD, and the
other takes UDD. For the two Cd atoms, Cd1 is coordinated by one upward imidazole N
atom from a UUD ligand and two downward imidazole N atom from one UDD ligand
with Cd−N bond distances of 2.300(8), 2.304(9) and 2.348(2) Å. The Cd1 atom is further
coordinated with two acetate groups through four O atoms with Cd−O bond distances of
2.327(8), 2.389(9), 2.567(7) and 2.698(6) Å. Cd2 is sixꢀcoordinated by one downward
imidazole N atom from a UUD ligand, one upward imidazole N atom from a UDD ligand,
and four O atoms of two acetate groups. The Cd−N bond distances are 2.205(1) and
2.248(9) Å, and Cd−O bond distances are 2.239(10), 2.277(12), 2.497(8) and 2.549(10)
Å. In the crystal structure, the double chain structure is formed by [Cd8L4] squares
(Figure 4). Two Cd1 are paired by UDD ligands, creating a Cd···Cd separation of 9.281
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Å. Two such Cd1ꢀCd1 pairs act as the joints of the framework, while two Cd2 act as the
vertexes of the framework.
Figure 3. Xꢀray structure of 2. (a) Ballꢀandꢀstick representation of the [Cd8L4] square. (b)
The packing diagram. (c) Ballꢀandꢀstick representation of the overall structure of 2,
showing the 2D framework. All hydrogen atoms are omitted for clarity. Color code: Cd,
yellow; N, blue; C, grey; Cl, green.
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Figure 4. Ballꢀandꢀstick representation of the Xꢀray structure of 3, showing the 1D
double chain structure. All hydrogen atoms are omitted for clarity. Color code: Cd,
yellow; N, blue; C, grey; O, red.
As shown in these structures, the conformation of ligand L takes an important role
in constructing these frameworks. In these structures, ligand L takes three different
−
conformations as presented in Scheme 1. When nonꢀcoordinating ClO4 was introduced,
ligand L takes UUU conformation and forms a honeycombꢀlike 2D framework. The
situation became complicated when Cl− and OAc− anions were used in the synthesis. In
both structures of 2 and 3, ligand L have UUD and UDD conformations with a ratio of
1:1, although their framework structures are different. Complex 2 has a 2D framework
structure, while complex 3 has a 1D double chain structure. The reaction between
Cd(ClO4)2 and L leads to UUU conformations in favor of more metalꢀligand coordination
interactions, while the reaction of Cd(OAc)2 or CdCl2 with L leads to a combination of
UUD and UDD conformations in favor of more metalꢀanion coordination interactions.
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Anion exchange and X-ray structure of 4. Because complex 1 has a cationic
framework structure with counter anions located in the cavity of the framework, we next
studied its anion exchange property. A freshly prepared crystalline sample of 1 was first
immersed in the aqueous solution of NaCl. FTIR is useful and decisive in monitoring the
−
anion exchange process because of the characteristic absorbance of ClO4 anions. The
−
exchange of ClO4 by Cl− was confirmed by FTIR, showing the disappearance of the
−
characteristic peak of ClO4 at 1090 cm−1 (Figure 5).31 To our surprise, crystals after
anion exchange kept their trigonal shape and crystallinity. Single crystal Xꢀray diffraction
analysis showed that the exchanged sample 4 crystallized in the same trigonal space
group R 3 with similar cell parameters as compared with 1 (Figure S7). More importantly,
it kept the same 2D cationic framework with Cl− anions taking the sites once occupied by
−
ClO4 anions (Figure 6). Powder Xꢀray diffraction analysis using the bulk sample shows
the phase purity of complex 4 (Figure S8), and the powder pattern of complex 1 is also
recorded for comparison (Figure S9). To further characterize this process, luminescent
and thermogravimetric analyses of complexes 1 and 4 were measured in the solid state,
showing their similar behaviors (Figures S10ꢀS12).
In the crystal structure of 4, each Cd(II) atom is sixꢀcoordinated with six N atom
from the imidazole, creating a regular octahedral geometry. The symmetry required C3
axis passes though the Cd(II) atom along c axis, and all the Cd−N bond distances are
2.365(3) Å. The ligands take the UUU conformation. In the crystal lattice of 4, coꢀ
crystalized disordered solvent water molecules were also found, which is attributed to the
anion exchange process in the aqueous solution, and was removed using SQUEEZE.
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Structural comparison of 1 and 4 showed their almost identical Cd coordination
environment and framework structure, indicating a SCꢀSC transformation.
Figure 5. FTIR spectra of complex 1 before (a) and after (b) anion exchange with Cl−.
Characteristic peaks from perchlorate anions are highlighted.
Figure 6. Schematic illustration of the SCꢀSC transformation of the anion exchange
process with 1.
It is important to synthesize complex 4 through the anion exchange of 1 with
NaCl, as the direct reaction of ligand L and chloride salt of Cd(II) always gave complex 2
−
with Cd−Cl− bonds. In addition to Cl−, we also tried anion exchange of ClO4 in 1 with
−
−
−
other anions, including PF6 , BPh4 , and ReO4 . However, exchanges with these anions
−
−
−
were not successful. We rationalized that the size of PF6 , BPh4 , and ReO4 is larger
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−
than ClO4 . Because the framework structure of 1 has limited cavity entry size, it may
hinder the entry of these large anions.
Conclusions
In summary, we reported the synthesis and structure of a novel highly flexible
ligand N(CH2ꢀmꢀC6H4ꢀCH2ꢀimidazole)3 (L) and its four Cd(II) coordination polymers.
Ligand L bears six flexible joints, and this flexibility of ligand L is crucial for structural
diversity of its Cd(II) coordination polymers. Complex 1 crystallized in the trigonal space
group R 3 with high symmetry, while 2 and 3 crystallized in lower symmetrical space
groups due to the Cdꢀanion coordination. Anion exchange experiments using complex 1
led to a SCꢀSC transformation product of 4. The structure of 4 keeps the same cationic
backbone. This is a rare example of SCꢀSC transformation in coordination polymers
supported by flexible ligands.
Author Information. The authors declare no competing financial interests.
Acknowledgments. We are grateful for support from the “Thousand Talents Program” of
China, the National Natural Science Foundation of China (No. 21101170, 21573139 and
21773146), the Fundamental Research Funds for the Central Universities, and the
Research Funds of Renmin University of China.
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Supporting Information. Supporting Information is available free of charge on the
website, including Table S1, Scheme S1, Figure S1ꢀS14, Xꢀray data as CIF files for
ligand L and complexes 1ꢀ4.
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Table of Contents
Various cadmium coordination polymers were constructed using flexible trisꢀimidazole
ligand and showed single crystal to single crystal transformation in anion exchange.
26