Supramolecular isomeric flat and wavy honeycomb networks: Additive agent effect on the ligand linkages

Article abstract of DOI:10.1039/c2ce26386f

Two genuine supramolecular isomers of Cd(ii) coordination polymers with an identical formula of [Cd(pptp)(ox)]_n (1a and 1b) (pptp = 2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine, H₂ox = oxalic acid) have been hydrothermally synthesized and characterized. Both compounds possess the rod-like binuclear [Cd₂(pptp)₂] subunit, which are linked by the ox ligand to afford either a flat or wavy two-dimensional (2D) 6³-hcb network, depending upon whether the additive agent is used for the reaction. The photoluminescence behaviour of the compounds was also discussed.

Full text of DOI:10.1039/c2ce26386f

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PAPER
Supramolecular isomeric flat and wavy honeycomb
networks: additive agent effect on the ligand linkages3
Cite this: CrystEngComm, 2013, 15,
349
Liming Fan,ac Xiutang Zhang,*ac Dacheng Li,a Di Sun,*b Wei Zhangc
and Jianmin Dou*a
Two genuine supramolecular isomers of Cd(II) coordination polymers with an identical formula of
[Cd(pptp)(ox)]n (1a and 1b) (pptp = 2-(3-(4-(pyridin-4-yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine, H2ox = oxalic
acid) have been hydrothermally synthesized and characterized. Both compounds possess the rod-like
binuclear [Cd2(pptp)2] subunit, which are linked by the ox ligand to afford either a flat or wavy two-
dimensional (2D) 63-hcb network, depending upon whether the additive agent is used for the reaction.
The photoluminescence behaviour of the compounds was also discussed.
Received 29th August 2012,
Accepted 27th October 2012
DOI: 10.1039/c2ce26386f
www.rsc.org/crystengcomm
For example, Du and coworkers reported one elegant case in
which two supramolecular isomers, discrete bimetallocycle
1. Introduction
In a state-of-the-art investigation on supramolecular chemistry
and crystal engineering, especially in the field of coordination
polymers, a sought-after goal is the supramolecular isomer-
ism, which was firstly coined by Zaworotko.1 Subsequently, in
an extensive review,2 they classified supramolecular isomers
into four types, namely, structural, conformational, catenane
(also known as topological) and optical isomerism. Following
this pursuit, the Chen group have achieved significant
advances and emphasized the significance of genuine supra-
molecular isomerism in their recent review.3 Supramolecular
isomers have different network structures, but the whole
crystals or the coordination networks have the same chemical
composition.4 The occurrence of supramolecular isomerism
indicates that the free energy gaps between different crystal-
line forms are small and the kinetic factors are responsible for
the crystal growth, and thus the generation of supramolecular
isomers depends on the subtle variation of assembly environ-
ments, such as solvent, reaction temperature and concentra-
tion effect.5 Although these factors have been well-recognized
in a number of examples,6 the control of the supramolecular
isomers accompanying the change of coordination modes of
one or more components by the additive agent is still scarce.7
and infinite helical chain, were formed under the induction of
different organic acids as additive agents.7a On the other hand,
many so-called supramolecular isomers are based on the
coexistence of different guest molecules, which produce
different chemical compositions; thus, it would be more
suitable to categorize these coordination polymers as pseudo-
polymorphs rather than genuine isomers.8 In contrast,
genuine supramolecular isomers have been only sporadically
reported.9
Our previous work has indicated that the 2-(3-(4-(pyridin-4-
yl)phenyl)-1H-1,2,4-triazol-5-yl)pyridine (pptp) ligand has mul-
tiple coordination sites, such as Npyridine and Ntriazole, and have
both bridging and chelating coordination modes to bind metal
centers.10 The diverse binding modes of the oxalate are also
quite favorable to the construction of various networks.11
Based on the above-mentioned points and our previous work,
we combined pptp and ox ligands in a Cd(II) coordination
polymer system, and two genuine supramolecular isomers
[Cd(pptp)(ox)]n (1a and 1b) (pptp = 2-(3-(4-(pyridin-4-yl)phenyl)-
1H-1,2,4-triazol-5-yl)pyridine, H2ox
=
oxalic acid) were
obtained. Both compounds are either a flat or wavy two-
dimensional (2D) 63-hcb networks, depending upon whether
the additive agent is used for the reaction (Scheme 1).
aCollege of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng,
252059, China. E-mail: jmdou@lcu.edu.cn
bSchool of Chemistry and Chemical Engineering, Shandong University, Jinan,
2. Experimental
Shandong, 250100, China. E-mail: dsun@sdu.edu.cn
cAdvanced Material Institute of Research, Department of Chemistry and Chemical
Engineering, Qilu Normal University, Jinan, 250013, China.
E-mail: xiutangzhang@yahoo.com.cn
2.1. Materials and methods
All chemicals were purchased from Jinan Henghua Sci. & Tec.
Co. Ltd. without further purification. C, N, and H analyses
were performed on an EA1110 CHNS-0 CE 65 elemental
analyzer. IR (KBr pellet) spectra were recorded on a Nicolet
Magna750FT-IR spectrometer. Powder-XRD measurements
Electronic supplementary information (ESI) available: The preparation of
3
compounds 1a and 1b, Tables S1 and S2, IR, TGA and powder XRD patterns.
X-ray crystallographic files in CIF format for the 1a and 1b. CCDC 898678 and
898679. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2ce26386f
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2.2.3. SYNTHESIS OF 1B. The procedure was similar to the
preparation of 1a, except that 4,49-bipyridine (35 mg, 0.20
mmol) was used as an additive agent. Pale yellow crystals of 1b
were obtained in 59% yield. Elemental analysis (%): calcd for
C
20H13N5O4Cd (499.75): C 48.02, H 2.60, N 14.01; found: C
47.84, H 2.79, N 13.74. IR (KBr pellet, cm21): 1599 (vs), 1461
(m), 1400 (m), 1300 (w), 1154 (m), 970 (w), 809 (m), 755 (m).
3. Results and discussion
Scheme 1 Synthetic procedures of two supramolecular isomers.
3.1. X-Ray crystallography
Single crystals of the complexes 1a and 1b with appropriate
dimensions were chosen under an optical microscope and
mounted on a glass fiber for data collection. Single-crystal
X-ray diffraction was performed using a Bruker Apex II CCD
diffractometer equipped with a fine-focus sealed-tube X-ray
source (Mo Ka radiation, graphite monochromated). In both
cases, the highest possible space group was chosen. Both
structures were solved by direct methods using SHELXS-9712
and refined on F2 by full-matrix least-squares procedures with
SHELXL-97.13 Atoms were located from iterative examination
of difference F-maps following least squares refinements of
the earlier models. Hydrogen atoms were placed in calculated
positions and included as riding atoms with isotropic
displacement parameters 1.2–1.5 times Ueq of the attached C
atoms. Both structures were examined using the Addsym
subroutine of PLATON14 to assure that no additional
symmetry could be applied to the models. Pertinent crystal-
lographic data collection and refinement parameters are
collated in Table 1. Selected bond lengths and angles for 1a
and 1b are collated in Table 2.
were recorded on a D/Max-2500 X-ray diffractometer using Cu
Ka radiation. The fluorescent spectra were measured on an
F-4500 FL Spectrophotometer. Thermogravimetric analyses
(TGA) were performed on a Netzsch STA 449C thermal analyzer
from room temperature to 800 uC under nitrogen atmosphere
at a heating rate of 10 uC min21
.
2.2. Syntheses
2.2.1. SYNTHESIS OF 1A. A mixture of Cadmium(II) sulfate
octahydrate (51.2 mg, 0.067 mmol), pptp (60 mg, 0.20 mmol),
oxalic acid (36 mg, 0.40 mmol), sodium hydroxide (16 mg, 0.40
mmol) and 14 mL of H2O were sealed in a 25 mL Teflon-lined
stainless steel autoclave and heated at 170 uC for 3 days, and
then slowly cooled to room temperature. Colorless crystals of 1
were obtained in 67% yield. Elemental analysis (%): calcd for
C20H13N5O4Cd (499.75): C 48.02, H 2.60, N 14.01; found: C
47.68, H 2.71, N 13.92. IR (KBr pellet, cm21): 1615 (vs), 1453
(m), 1300 (s), 1161 (m), 977 (w), 824 (m), 778 (m).
Table 1 Crystal data for 1a and 1b
Empirical formula
C20H13CdN5O4
C20H13CdN5O4
Formula weight
Temperature/K
Crystal system
Space group
a/Å
b/Å
499.75
296(2)
499.75
296(2)
Monoclinic
P21/c
8.6361(11)
10.8402(15)
19.795(3)
90.00
95.624(3)
90.00
1844.3(4)
4
Monoclinic
P21/c
9.955(6)
19.393(11)
10.077(6)
90.00
108.590(11)
90.00
1843.9(18)
4
c/Å
a (u)
b (u)
c (u)
Volume/Å3
Z
rcalc/mg mm23
1.800
1.225
1.800
1.225
m/mm21
F(000)
992.0
0.46 6 0.18 6 0.15
4.14 to 50
210 ¡ h ¡ 9, 212 ¡ k ¡ 12, 219 ¡ l ¡ 23
9116
3246 [Rint = 0.0323]
3246/0/271
0.989
R1 = 0.0251, wR2 = 0.0549
R1 = 0.0309, wR2 = 0.0560
0.32/20.56
992.0
0.36 6 0.25 6 0.21
4.2 to 50
211 ¡ h ¡ 10, 223 ¡ k ¡ 22, 211 ¡ l ¡ 10
9213
3221 [Rint = 0.0506]
3221/0/271
0.915
R1 = 0.0358, wR2 = 0.0704
R1 = 0.0464, wR2 = 0.0740
0.74/20.97
Crystal size/mm3
2h range for data collection (u)
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I . = 2s (I)]
Final R indexes [all data]
Largest diff. peak/hole/e Å23
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Table 2 Selected bond lengths (Å) and angles (u) for 1a and 1b
Complex 1aa
Cd1–O2i
Cd1–O3
2.325(2)
2.3546(18)
2.358(2)
2.3718(19)
71.78(6)
143.77(7)
135.15(7)
69.99(6)
141.71(6)
80.61(7)
96.37(8)
98.53(7)
101.06(8)
83.69(8)
94.71(8)
Cd1–N5ii
Cd1–N1
Cd1–O1
2.382(2)
2.422(2)
2.4423(19)
Cd1–N2
Cd1–O4i
O2i–Cd1–O3
O2i–Cd1–N2
O3–Cd1–N2
O2i–Cd1–O4i
O3–Cd1–O4i
N2–Cd1–O4i
O2i–Cd1–N5ii
O3–Cd1–N5ii
N2–Cd1–N5ii
O4i–Cd1–N5ii
O2i–Cd1–N1
O3–Cd1–N1
N2–Cd1–N1
O4i–Cd1–N1
N5ii–Cd1–N1
O2i–Cd1–O1
O3–Cd1–O1
N2–Cd1–O1
O4i–Cd1–O1
N5ii–Cd1–O1
N1–Cd1–O1
84.14(7)
70.07(8)
100.95(7)
168.89(8)
137.67(7)
68.15(6)
77.63(7)
147.29(6)
76.92(8)
94.27(7)
a
Fig. 1 (a) ORTEP representation of 1a showing the local coordination environ-
ment around the Cd(II) center with 50% thermal ellipsoid probability. (Symmetry
codes: (i) 2x + 2, y 2 1/2, 2z + 1/2; (ii) 2x + 1, 2y + 1, 2z.) (b) and (c) Ball-and-
Stick representation of the flat 2D 63-hcb network along different directions. (e)
Perspective view of the packing structure of 2D flat networks.
Symmetry codes: (i) 2x + 2, y 2 1/2, 2z + 1/2; (ii) 2x + 1, 2y + 1,
2z.
Complex 1bb
Cd1–O1
2.249(3)
2.285(3)
Cd1–N5ii
Cd1–N2
2.342(3)
2.383(3)
Cd1–O2i
Cd1–O4i
2.292(3)
Cd1–N1
2.385(3)
O1–Cd1–O2i
O1–Cd1–O4i
O2i–Cd1–O4i
O1–Cd1–N5ii
O2i–Cd1–N5ii
O4i–Cd1–N5ii
O1–Cd1–N2
O2i–Cd1–N2
164.49(9)
93.28(10)
71.71(10)
85.07(11)
91.14(11)
90.93(11)
99.87(10)
95.43(10)
O4i–Cd1–N2
N5ii–Cd1–N2
O1–Cd1–N1
O2i–Cd1–N1
O4i–Cd1–N1
N5ii–Cd1–N1
N2–Cd1–N1
165.99(10)
94.96(11)
91.38(11)
96.09(11)
104.50(11)
164.36(11)
70.60(11)
geometry. Both Cd–N (2.358(2)–2.422(2) Å) and Cd–O
(2.325(2)–2.4423(19) Å) bond lengths are well-matched to
those observed in similar complexes.16 The pair of pptp
ligands with a m3-bridging + chelating mode bind two Cd(II)
centers to form a rod-like binuclear [Cd2(pptp)2] subunit,
which is further extended by m1,2,3,4-ox ligand to form the
resulting 2D network. To better understand the structure of 1a,
the topological analysis approach is employed. If all nodes in
one net have identical connectivity, then, according to Wells, it
is a platonic uniform net and can be represented by the symbol
(n, p), where n is the size of the shortest circuit and p is the
connectivity of the nodes.17 In the sheet of 1a, all Cd(II) ions
are 3-connecting and the shortest circuit is a six-membered
ring. So this 2D sheet can be simplified to a 63-hcb net with the
window size of ca. 15.1 6 17.1 Å (Fig. 1b and 1c). The adjacent
nets further interdigitated with each other to form the
resulting 3D supramolecular framework (Fig. 1d) through
b
Symmetry codes: (i) x, 2y + 1/2, z + 1/2; (ii) 2x + 2, 2y, 2z + 2.
3.2. Synthesis and general characterization
Colorless crystals of 1a and pale yellow 1b were synthesized by
solvothermal reaction of CdSO4?6H2O, pptp and H2ox in a
similar process, but without and with 4,49-bipyridine as an
additive agent, respectively (see ESI ). Phase purity of 1a and
1b is supported by the powder X-ray diffraction patterns (Fig.
3
S1, see ESI ). For 1a and 1b, most of the peak positions of
3
simulated and experimental patterns are in good agreement
with each other. The dissimilarities in intensity may be due to
the preferred orientation of the crystalline powder samples.
…
classic N–H O hydrogen bonds (2.724(3) Å). We previously
obtained a 2D A 2D interpenetrated 63-hcb network with the
window dimensions of 21.3 6 25.1.18 However, no interpene-
tration was observed in 1a, which may be caused by the
relatively smaller window dimensions in 1a precluding the
insertion of rods of an adjacent window.
The solid FT-IR spectra (Fig. S2, see ESI ) of 1a and 1b show
3
characteristic absorption bands for carboxyl groups.
Compared to the IR spectra of 1a and 1b, the spectra of 1a
and 1b demonstrate slight but significant differences, due to
somewhat different coordination modes of oxalate ligands
[bis(bidentate)-chelating vs. bidentate + monodentate brid-
ging, respectively].15
3.4. Structure description of 1b
The supramolecular isomeric 2D network of 1b was obtained
under similar reaction conditions but with 4,49-bipyridine as
an additive agent. Single-crystal X-ray analysis revealed that it
crystallized in the same space group to that of 1a and the
asymmetric unit of 1b consists of completely identical
components to those in 1a. In contrast to 1a, the coordination
geometry of Cd(II) in 1b is a six-coordinated octahedron,
completed by three O atoms and three N atoms (Fig. 2a). The
similar rod-like binuclear [Cd2(pptp)2] subunits also exist in
1b, which were connected by m1,2,3-ox ligands to form the
resulting 2D 63-hcb net (Fig. 2c). The different arrangement of
3.3. Structure description of 1a
The crystal structure of 1a was determined by single-crystal
X-ray diffraction analysis (see ESI ). Complex 1a crystallizes in
3
the monoclinic space group of P21/c with an asymmetric unit
that contains one Cd(II) ion, one pptp ligand, and one ox
dianion. As shown in Fig. 1a, the Cd1 is seven-coordinated by
four O atoms from two ox ligands and three N atoms from two
pptp ligands, displaying a distorted pentagon-bipyramidal
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with the help of ConQuest version 1.3,20 only 45 hits among
2440 ox-based coordination compounds show the m1,2,3-mode.
To the best of our knowledge, most of the supramolecular
isomers reported so far usually show networks with completely
different geometries or topologies.21 One representative
example is isomeric binary silver(I)–imidazolates with
a
stoichiometry of 1 : 1 metal : ligand molar ratio based on 2D
63-hcb and 44-sql networks.22 Recently, silver(I)–2-methylimi-
dazolates including isomeric zigzag chains, helical chain, and
S-shaped chain were also unmasked by the same group.7b The
Zaworotko group also showed two supramolecular isomers
based on the identical paddlewheel secondary building units
´
(SBUs) but with Kagome and NbO topologies, induced by
different solvents.23 However, only
a
few examples of
supramolecular isomers based on networks with the same
topology have been reported.24 The Chen group reported a pair
of supramolecular isomers showing 3D frameworks with the
same eight-connected bcg topology but different magnetic
behaviour due to the slightly different linear trinuclear
Co3(RCOO)8 SBU.24a Hence, 1a and 1b presented here are an
unprecedented example showing supramolecular isomerism
based on networks with the same 63-hcb topology.
Fig. 2 (a) ORTEP representation of 1b showing the local coordination environ-
ment around the Cd(II) center with 50% thermal ellipsoid probability. (Symmetry
codes: (i) x, 2y + 1/2, z + 1/2; (ii) 2x + 2, 2y, 2z + 2.) (b) and (c) Ball-and-Stick
representation of the wavy 2D 63-hcb network along different directions. (e)
Perspective view of the packing structure of 2D wavy networks.
3.5. Structural comparison of 1a and 1b
1a and 1b are genuine supramolecular isomers having
identical chemical compositions for not only the coordination
network but also the whole crystal. Although, topologically,
both 1a and 1b are 2D 63-hcb nets, there are three different
characters for 1a and 1b: (i) coordination geometry of the
central Cd(II) ion, pentagon-bipyramid vs. octahedron; (ii) ox
linkage fashion, m1,2,3,4 vs. m1,2,3; (iii) flat vs. wavy network
(Table 3). As described above, both the conformation and
coordination mode of the pptp are very similar in 1a and 1b,
resulting in the same rod-like binuclear [Cd2(pptp)2] subunit,
within slight differences due to the rotation of C–C single
bonds, hence, such negligible geometric differences are not
the two ox ligands around one Cd(II) center in 1a and 1b made
the resultant 63-hcb nets different in their geometries. The 2D
net in 1b is more corrugated than that in 1a, reflecting a large
dihedral angle of 86.99u between adjacent hexagons (Fig. 2b).
The 2D nets in 1b pack into a 3D supramolecular framework
with a ‘peak-and-valley’ mode (Fig. 2d). Another noteworthy
point is the unusual m1,2,3 coordination mode of the ox ligand.
As we know, the more frequently encountered coordination
modes of ox are m1,2,3,4, m1,2, m1,1,2 and m1,1,2,3, however, the ox
ligand acting as a m1,2,3-tridentate bridging ligand is scarce.19
As indicated by a CSD (Cambridge Structure Database) survey
Table 3 Comparison of coordination geometry of Cd(II) ion and coordination modes of ox ligand in 1a and 1ba
1a
1b
Cd(II)
ox
a
purple: Cd; red: O; blue: N.
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sufficient in determining the formation of supramolecular
isomers. In contrast, different coordination modes of the ox
ligand can cause rather drastic differences in the coordination
geometry of the metal centers as well as the bonding direction
of it, which should be the key factor in the formation of a flat
or wavy 2D net. So, the formation of supramolecular isomers
in this system is dominated by the ox ligand linkages, which is
influenced by whether the 4,49-bipyridine is used as an
additive agent in the reaction. In fact, the isomerism arising
from the 4,49-bipyridine is fortuitously observed since the
additive is not incorporated in the final crystalline product of
1b. This fact intensively indicates that 4,49-bipyridine mole-
cules may ingeniously influence the micro-environment of the
reaction system, generating different superstructures from
those crystallized in the absence of 4,49-bipyridine mole-
cules.22 On consideration of the potential interconversion
between the 1a and 1b, we conceived of an idea that may
realize the interconversion between 1a and 1b. 1a was
obtained in the absence of 4,49-bipyridine, so we mixed it
with 4,49-bipyridine. This mixture was treated under the same
condition as the synthesis of 1b. The resulting sample (white
precipitation) was washed with water and methanol several
times. We have carried out a series of trials, but these efforts
were proved to have failed through the comparison of XRPD
Fig. 4 The photoluminescence of 1a and 1b in solid state.
ligand is nearly emission silent in the range 350–550 nm under
the excitation at 300 nm at room temperature. The emission
spectra for 1a and 1b exhibit emission peaks at 412 and 440
nm for 1a and 1b, respectively. Both emissions are neither
metal-to-ligand charge transfer (MLCT) nor ligand-to-metal
transfer (LMCT) in nature since the Cd(II) ion is difficult to
oxidize or reduce due to its d10 configuration.25 Thus, they may
be assigned to intraligand (p* A n or p* A p) emission. The
enhancement of luminescence in d10 complexes may be
attributed to ligand chelation to the metal center, which
effectively increases the rigidity of the ligand and reduces the
loss of energy by radiationless decay.26 The difference of the
emission behavior for 1a and 1b probably derives from the
differences in the rigidity of solid-state crystal packing.
patterns (Fig. S3, see ESI ), which demonstrated that inter-
3
conversion from 1a to 1b was unsuccessful and the resulting
white precipitation was neither 1a nor 1b.
3.6. TGA curves of 1a and 1b
The thermogravimetric (TG) measurements were performed in
N2 atmosphere on polycrystalline samples of 1a and 1b and
the TG curves are shown in Fig. 3. The TGA curves of 1a and 1b
suggested that they were stable up to y350 and y250 uC,
respectively. Above these temperatures, the samples suffered
an abrupt weight loss, indicating the collapse of the networks.
3.7. Photoluminescence properties of 1a and 1b
The emission spectra of 1a and 1b were examined in the solid
state at room temperature, shown in Fig. 4. The free pptp
4. Conclusions
In summary, a pair of genuine supramolecular isomers, a flat
and a wavy 2D 63-hcb net, have been synthesized and
characterized for the first time. Our analysis based on the
structural data suggests that the additive agent exerts a
profound influence on the coordination environment of
Cd(II) and linking modes of the ox ligand, resulting in the
isomerization of the 63-hcb net from a flat one to a wavy one.
Moreover, the photoluminescence behavior of the compounds
was also discussed.
Acknowledgements
This work was supported by the NSFC (grant no. 21101097 and
21201110), Natural Science Foundation of Shandong Province
(ZR2010BQ023), the Independent Innovation Foundation of
Shandong University (2011GN030), the Special Fund for
Postdoctoral Innovation Program of Shandong Province
(201101007) and the China Postdoctoral Science Foundation
(2012M511010).
Fig. 3 The TGA curves of 1a and 1b.
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