Syntheses and structures of three novel coordination compounds assembled with 1,1′-(2,2′-oxybis(ethane-2,1-diyl))bis(1H-imidazole)

Article abstract of DOI:10.1016/j.molstruc.2008.03.034

A new long flexible ligand, 1,1′-(2,2′-oxybis(ethane-2,1-diyl))bis(1H-imidazole) (L), has been prepared. The hydrothermal reactions of L with various metal ions (M(II) = Cu(II), Cd(II) and Ni(II)) yielded three new coordination compounds consisting of 0D, 1D and 2D structures. Compound 1 is a dinuclear M₂L₂ metallocycle. Compound 2 is a double-stranded chain running along b axis direction. Compound 3 affords highly undulated 2D (4, 4) layers. In addition, complex 2 exhibits strong blue photoluminescence.

Full text of DOI:10.1016/j.molstruc.2008.03.034

Journal of Molecular Structure 891 (2008) 98–102
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Syntheses and structures of three novel coordination compounds assembled
with 1,10-(2,20-oxybis(ethane-2,1-diyl))bis(1H-imidazole)
*
*
Yan Qi, Yunxia Che , Jimin Zheng
Department of Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
A new long flexible ligand, 1,10-(2,20-oxybis(ethane-2,1-diyl))bis(1H-imidazole) (L), has been prepared.
The hydrothermal reactions of L with various metal ions (M(II) = Cu(II), Cd(II) and Ni(II)) yielded three
new coordination compounds consisting of 0D, 1D and 2D structures. Compound 1 is a dinuclear M2L2
metallocycle. Compound 2 is a double-stranded chain running along b axis direction. Compound 3 affords
highly undulated 2D (4,4) layers. In addition, complex 2 exhibits strong blue photoluminescence.
Ó 2008 Published by Elsevier B.V.
Article history:
Received 21 January 2008
Received in revised form 6 March 2008
Accepted 10 March 2008
Available online 1 April 2008
Keywords:
Coordination compounds
Flexibility
N ligand
Photoluminescence
1. Introduction
of this ligand is necessary to enrich and develop this field. In this
paper, we report our efforts in the design and synthesis of the long
The self-assembly of metal–organic frameworks (MOFs) have
attracted much attention, due to their elegant structural topologies
and wide potential applications as functional materials [1]. Studies
in this field have been focused on the design and preparation, as
well as the structure–property relationships. Significant progress
have been achieved [2], however, it is still a great challenge to
rationally prepare and predict the exact structures and composi-
tion of target products in crystal engineering. The resulting MOF
structures are determined by several factors, including the coordi-
nation nature of metal ions, ligand structure, counterions, and so
on. The design of ligand is usually a useful way of manipulating
the generation of the molecular architecture.
flexible organic ligand, L, combined with different metal cations to
assemble into diverse architectures consisting of 0D, 1D and 2D
structures. Three new compounds were obtained: [Cu2(HCOO)4
(L)2]ꢀ2H2O(1), [Cd(L)2(NO3)2](2) and [Ni(BDC)(L)](3) (H2BDC = 1,4-
benzenedicarboxylic acid). All compounds are characterized by
elemental analysis, IR spectrum, thermogravimetric analyses and
X-ray crystallography. In addition, compound 2 exhibit strong blue
photoluminescence.
2. Experimental
2.1. Materials and general procedures
Up to now, a large number of beautiful MOFs of ingenious de-
sign based on flexible bis(imidazole) ligands, such as (N-im)2(CH2)n
(n = 1–4) [3,4], have been constructed. These ligands bearing alkyl
spacers are good choices of N-donor ligands, because the flexible
nature of spacers allows the ligands to bend and rotate when it
coordinates to metal centers. The structures and properties also
can be modified by changing the spacer groups, for an instance,
the length of the spacer. We designed and prepared a long ligand,
[1,10-(2,20-oxybis(ethane-2,1-diyl))bis(1H-imidazole)] (L) (Scheme
1), which is longer than 1,4-bis(imidazol-1-yl)-butane (bimb) due
to the introduction of an O atom into the methylene (–CH2–)4 skel-
eton. The increasing length may control the physical dimensions of
the crystalline architecture and, accordingly, affect the internal
chemistry of the coordination polymers. Therefore, the exploration
Solvents and starting materials for synthesis were purchased
commercially and used as received. The ligand L was prepared
according to reported procedures [5]. The IR spectrum was
recorded as KBr pellets on a Nicolet Magna-FT-IR 560 spectrometer
in the 4000–400 cmꢁ1 region. Elemental analysis for C, H and N
was performed on a Perkin-Elmer 240 analyzer. The photolumines-
cence measurements were carried out on crystalline samples at
room temperature and the spectra were collected with a Hitachi
F-2500FL spectrophotometer. The thermogravimetric analyses
were investigated on a standard TG analyzer under a nitrogen flow
at a heating rate of 5 °C/min for all measurements.
2.2. Preparation
Synthesis of [Cu2(HCOO)4(L)2]ꢀ2H2O(1). An aqueous solution
* Corresponding authors. Tel./fax: +86 22 23508056.
E-mail address: jmzheng@nankai.edu.cn (J. Zheng).
(15 mL) of formic acid (0.10 g, 0.2 mmol) was added slowly with
0022-2860/$ - see front matter Ó 2008 Published by Elsevier B.V.
doi:10.1016/j.molstruc.2008.03.034

Y. Qi et al. / Journal of Molecular Structure 891 (2008) 98–102
99
Table 1
O
Crystallographic data and structure refinement details for 1–3
Compound
Formula
Fw
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
a (°)
b (°)
1
2
3
N
N
C24H36Cu2N8O12
755.69
Monoclinic
C2/c
15.799(3)
15.952(3)
14.885(3)
90
115.36(3)
90
3389.9(11)
4
1.481
293(2)
1560
0.1298
0.992
C20H28CdN10O8
648.92
Triclinic
C72H72N16Ni4O20
1716.30
Monoclinic
Cc
6.9627(14)
19.317(4)
13.806(3)
90
95.51(3)
90
1848.3(7)
1
1.542
293(2)
888
0.0333
1.045
0.0253, 0.0643
ꢀ
P1
N
N
8.7924(18)
8.8320(18)
10.444(2)
84.20(3)
68.23(3)
61.91(3)
661.8(2)
1
1.628
293(2)
330
0.0347
Scheme 1. 1,10-(2,20-oxybis(ethane-2,1-diyl))bis(1H-imidazole) (L).
c (°)
constant stirring to a solution of CuCO3ꢀCu(OH)2ꢀH2O (0.12 g,
0.5 mmol) in water (15 mL). After the precipitation was dissolved
completely, L ligand (0.21 g, 1.0 mmol) also in water was added
in drops. The resulting mixture was stirring for 5 h at room tem-
perature. About one month later blue block-shaped crystals were
obtained. Yield: 48% (based on Cu). Elemental analysis (%): Calcd.
for (1): C 38.14, H 4.80, N 14.83. Found: C 38.08, H 4.85, N 14.86.
IR (KBr): m (cmꢁ1) = 3430(s), 3130(s), 2918(m), 1609(s), 1523(m),
1405(m), 865(m), 657(m).
V (Å3)
Z
D (mg cmꢁ3
T (K)
)
F(000)
Rint
GOF
1.012
0.0312, 0.0807
R1, wR2 [I > 2r(I)]
0.0872, 0.2416
Synthesis of [Cd(L)2(NO3)2](2). An aqueous solution (15 mL) of
Cd(NO3)2ꢀ4H2O (0.31 g, 1.0 mmol) was added slowly with constant
stirring to a solution of L (0.21 g, 0.1 mmol) in water (20 mL). The
reactionmixturewasthenheatedtorefluxfor3 h. Theresultingmix-
ture was left to stand at room temperature for three weeks. Colorless
crystalline product was obtained. Yield: 51% (based on Cd). Elemen-
tal analysis (%): Calcd. for (2): C 37.02, H 4.35, N 21.59. Found: C
37.09, H 4.40, N 21.51. IR (KBr): m (cmꢁ1) = 3130(s), 3037(m),
2906(m), 1528(m), 1380(s), 1085(m), 840(m), 825(s), 656(m).
Synthesis of [Ni(BDC)(L)](3). The mixture of NiSO4ꢀ6H2O (0.26 g,
1.0 mmol), 1,4-H2BDC (0.17 g, 1.0 mmol) and L (0.21 g, 1.0 mmol)
was dissolved in 8 mL of distilled water. The pH value was then ad-
justed to 5.5 with 1 M NaOH solution and the resulting mixture was
transferred and sealed in a 25 mL Teflon-lined stainless steel vessel.
This was then heated to 160 °C for 2 days. After the reactor was
slowly cooled to room temperature, green block-shaped crystals
were filtered off and dried in air. Yield: 42% (based on Ni). Elemental
analysis (%): Calcd. for (3): C 50.39, H 4.23, N 13.06. Found: C 50.48, H
4.19, N 13.13. IR (KBr): m (cmꢁ1) = 3126(s), 3040(s), 2915(m),
1585(s), 1548(m), 1469(m), 1405(m), 1085(m), 840(m), 656(m).
Table 2
Selected bond lengths (Å) and angles (°) for complexes 1–3a
1
Cu(1)–O(3)
Cu(1)–N(4)#1
Cu(1)–O(1)#1
O(3)–Cu(1)–N(1)
N(1)–Cu(1)–N(4)#1
N(1)–Cu(1)–O(1)
O(3)–Cu(1)–O(1)#1
1.974(5)
1.982(6)
2.322(5)
90.2(2)
167.5(2)
90.8(2)
Cu(1)3–N(1)
Cu(1)–O(1)
1.978(6)
1.983(5)
O(3)–Cu(1)–N(4)#1
O(3)–Cu(1)–O(1)
N(4)#1–Cu(1)–O(1)
O(1)–Cu(1)–O(1)#1
89.7(2)
171.1(2)
91.3(2)
77.9(2)
93.2(2)
2
Cd(1)–N(4)
Cd(1)–O(1)
2.286(2)
Cd(1)–N(2)#3
2.3173(19)
2.405(3)
N(4)–Cd(1)–N(4)#2
N(4)–Cd(1)–N(2)#4
N(2)#4–Cd(1)–O(1)
N(4)#2–Cd(1)–O(1)
O(1)–Cd(1)–O(1)#2
180.000(1)
88.85(8)
86.60(10)
85.37(9)
N(4)–Cd(1)–N(2)#3
N(2)#3–Cd(1)–N(2)#4
N(4)–Cd(1)–O(1)
91.15(8)
180.000(1)
94.63(9)
N(2)#3–Cd(1)–O(1)
93.40(10)
180.00(11)
3
N(1)–Ni(1)
Ni(1)–O(1)
2.0348(14)
2.1661(11)
88.23(8)
Ni(1)–O(2)
2.0815(11)
N(1)#5–Ni(1)–N(1)
N(1)–Ni(1)–O(2)#5
N(1)#5–Ni(1)–O(1)#5
O(2)#5–Ni(1)–O(1)#5
O(1)#5–Ni(1)–O(1)
N(1)–Ni(1)–O(2)
95.93(5)
161.08(6)
93.05(5)
104.04(5)
97.63(5)
158.03(5)
62.16(4)
O(2)#5–Ni(1)–O(2)
N(1)–Ni(1)–O(1)#5
O(2)–Ni(1)–O(1)#5
2.3. Crystallographic data collection and refinement
93.84(6)
X-ray Crystallographic Measurements for 1–3. Accurate unit
cell parameters were determined by a least-squares fit of 2h values,
and intensity data were measured on a rigaku r-axis rapid IP area
detector with MoKa radiation (k = 0.71073 Å) at room temperature.
The intensities were corrected for Lorentz and polarization effects
as well as for empirical absorption based on multi-scan technique.
All structures were solved by direct methods and refined by full-
matrix least-squares fitting on F2 by SHELX-97. All non-hydrogen
atoms were refined with anisotropic thermal parameters. Crystal-
lographic data for the three compounds are summarized in Table
1 and selected bond lengths and angles are listed in Table 2.
a
Symmetry transformations used to generate equivalent atoms: #1 ꢁx,ꢁy,ꢁz for
1; #2 ꢁx,ꢁy + 2,ꢁz + 2; #3 ꢁx,ꢁy + 1,ꢁz + 2; #4 x,y + 1,z for 2; #5 ꢁx + 1,y,ꢁz + 1/2
for 3.
are 1.978(6) and 1.982(6) Å. The L ligands exhibit cis configuration
with a pair of imidazole groups inclined by 16.19°. Two L ligands
connect two Cu(II) atoms to achieve a 24-numbered M2L2 metalla-
cyclic ring. The formate anions adopt two coordination modes. One
occupies terminal positions as unidentate ligand, the other bridges
two Cu ions through one oxygen atom only, which results in
Cuꢀ ꢀ ꢀCu separation of 3.354 Å. The distances from 2.716 to
3.013 Å between the O atoms of the free water molecules and
the O atoms of formate anions indicate the formation of O–Hꢀ ꢀ ꢀO
hydrogen bonds, although the hydrogen atoms of water molecules
could not be found [4c]. These hydrogen bonds connected the adja-
cent Cu2L2 metallacycles to form a 1D supramolecular chain run-
ning along caxis direction (Fig. 1B).
3. Results and discussion
3.1. Descriptions of the structures
3.1.1. [Cu2(HCOO)4(L)2]ꢀ2H2O(1)
X-ray crystallographic analysis provides direct evidence of M2L2
metallocyclic structure of compound 1, as shown in Fig. 1A. Each
Cu(II) atom exhibits a trigonal–bipyramidal (CuO3N2) coordination
environment with three O atoms from three formate anions and
two N atoms from two L ligands, respectively. The Cu–O bond dis-
tances vary from 1.983(5) to 2.322(5) Å, while Cu–N bond lengths
3.1.2. [Cd(L)2(NO3)2](2)
ꢀ
Polymer 2 crystallizes in triclinic space group of P1. As shown in
Fig. 2A, each Cd(II) atom is coordinated by four N atoms from four L

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Y. Qi et al. / Journal of Molecular Structure 891 (2008) 98–102
Fig. 1. (A) Copper coordination environment with major atom numbering scheme in 1 showing 24-numbered metallocyclic Cu2L2 unit (the free water molecule is omitted for
clarity). (B) 1D supramolecular chain formed by O–Hꢀ ꢀ ꢀO hydrogen bonds indicated by blue dashed lines.
Fig. 2. (A) Cadmium coordination environment with major atom numbering scheme in 2showing 24-numbered metallocyclic Cd2L2 unit in the 1D chain. (B) View along b axis
direction of 1D double-stranded chain.
ꢁ
ligands in the equatorial positions and two O atoms from two NO3
anions at the axial positions to give octahedral environment. The
Cd–N bond lengths are 2.286(2) and 2.3173(19) Å and Cd–O bond
length is 2.405(3) Å, which fall in the normal range found in other
Cd complexes [6]. Comparing to those in 1, the L ligands exhibit cis
configuration with a pair of imidazole groups inclined by 81.77°
and link two adjacent Cd atoms to form a 24-numbered M2L2
metallocyclic ring with Cdꢀ ꢀ ꢀCd distance being 8.832 Å. In contrast
to 1, the metallacycles of 2 are repeated to generate a 1D double-
stranded chain running along the b axis direction (Fig. 2B).
ence on the formation of resulting structure.The chemical nature
of the long spacer tends to favour cis conformation due to the ori-
entation of C–O–C bond, which facilitates the formation of metal-
locyclic structures, as found in complexes 1 and 2. On the other
hand, a twist of the two imidazole rings relative to the N–
(CH2)2–O–(CH2)2–N chain can lead to helical chains, as found in
complex 3. Such helical chains may be formed by the ring-opening
polymerization of M2L2 metallocycle [8].
3.2. Photoluminescent properties
3.1.3. [Ni(BDC)(L)](3)
Single-crystal X-ray structural analysis of 3 shows that each
Ni(II) atom exhibits a distorted octahedral environment, composed
Photoluminescent measurements of the compound 2 were car-
ried out in the solid state at room temperature. The solid-state
photoluminescent spectra of 2 is depicted in Fig. 4. It can be seen
that the intense photoluminescence emission at 448 nm
(kex = 350 nm) is exhibited. The emission of 2 may be due to r-
donation from the coordination environment of the Cd(II) centers
and thus be assigned as ligand-to-metal charge-transfer (LMCT)
[9]. The emission in the blue region indicates that compound 2 ap-
pears to be good candidate for promising blue-light emitting
material.
of four carboxylic
O atoms from two 1,4-BDC anions (Ni–
O = 2.0815(11) and 2.1661(11) Å) and two N atoms from two L
ligands (Ni–N = 2.0348(14) Å), as shown in Fig. 3A. The L ligand
adopts a cis configuration with two imidazole rings inclined by
54.63° and gives a Niꢀ ꢀ ꢀNi separation of 6.963 Å. Different from
the structures mentioned earlier, only one L ligand connects two
adjacent Ni atoms. Ni atoms are arranged in left- and right-handed
helices running along a axis direction (Fig. 3B) instead of 24-mem-
bered M2L2 metallocycles. Each 1,4-BDC anion coordinates to two
Ni atoms in the l2-g3-chelation mode [7]. Thus, the 1,4-BDC anions
bridge two adjacent left- and right-handed helical chains to afford
highly undulated 2D (4,4) layers that contain 42-membered rings
with metal ions at each corner and a molecule L and a 1,4-BDC an-
ion at each edge, respectively (Fig. 3C). The lengths of the opposite
edges are equal, with the distances being 10.672 Å and 6.963 Å,
respectively, and the diagonal distances are 15.814 Å and
8.639 Å. The (4,4) networks of 3 stack in a parallel fashion. It is sur-
prisingly, no interpenetration occurs.
3.3. Thermogravimetric analyses
To study the stability of these polymers, thermogravimetric
analyses (TGA) of complexes 1–3 were performed (Fig. 5). The
TGA curve of complex 1 indicates two main weight losses. The first
one from 34 to 152 °C corresponds to the loss of two lattice water
molecule and one L ligand. The observed weight loss of 33.28% is in
agreement with the calculated one (32.35%). The second weight
loss can be detected from 212 to 529 °C that is attributed to the
complete decomposition of the other L ligand and formate anions.
The remaining residue is presumed to be CuO (calcd: 21.17%;
found: 21.58%). In comparison with 1, compound 2 is more stable
Through comparison of the structures of 1–3, it is apparent that
the steric geometry of the flexible L ligand has a significant influ-

Y. Qi et al. / Journal of Molecular Structure 891 (2008) 98–102
101
Fig. 3. (A) Nickle coordination environment with major atom numbering scheme in 3. (B) The 1D left- and right-helical structure constructed by nickel centers and L ligands.
(C) Front (left) and side (right) view of the highly undulated 2D (4,4) network.
Fig. 5. Thermogravimetric analyses (TGA) curve of compounds 1–3.
Fig. 4. Solid-state emission spectra of compound 2 at room temperature.
4. Conclusion
up to 260 °C, where the decomposition of the framework starts. A
rapid weight loss of 66.12% from 247 to 385 °C corresponds to the
burning of the L ligand (calcd: 66.57%) and the resulting residue re-
mains as CdO (calcd: 19.79%; found: 18.49%). The framework of
compound 3 begins to collapse from 393 to 440 °C and the remain-
ing residue is presumed to be NiO (calcd: 17.41%; found: 14.89%).
Therefore, compound 3 is the most stable one among three
compounds.
The use of the new flexible bis(imidazole) ligand (L) and diva-
lent transitional metal ions [Cu(II), Cd(II) and Ni(II)] affords three
interesting architectures, demonstrating again the aesthetic diver-
sity of coordination chemistry. The results of this study not only
illustrate that the nature of the neutral ligands play an important
role in the construction of coordination polymers but also indicates

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Y. Qi et al. / Journal of Molecular Structure 891 (2008) 98–102
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Crystallographic data for the structures in this paper have been
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plementary publication no. CCDC-674675 (1), 674676 (2) and
674677 (3). These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Table S1 (hydrogen-bond geometry for com-
pound 1), Fig. S1 (Fluorescence emission spectrum of L ligand at
room temperature) and Fig. S2 (the measured and simulated XRPD
patterns for compound 2) in the Supplementary material. Supple-
mentary data associated with this article can be found, in the on-
line version, at doi:10.1016/j.molstruc.2008.03.034.
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