Synthesis, structure, and characterization of a new metal-organic framework containing meso-helices

Article abstract of DOI:10.1002/zaac.201200474

The new metal-organic framework [Cd(bqdc)(phen)]_n (1) (bqdc = 2, 2'-biquinoline-4, 4'-dicarboxylate, phen = 1, 10-phenanthroline) was hydrothermally synthesized by hetero-ring carboxylate ligand and N-containing auxiliary ligand. The structure of 1 was determined by single-crystal X-ray diffraction analysis and further characterized by elemental analyses, IR spectroscopy, X-ray powder diffraction, and TG analysis. Single-crystal X-ray structural analysis shows that complex 1 is an infinite two-dimensional framework containing aesthetic meso-helix. These two-dimensional coordination networks are interconnected through aromatic π-π stacking and C-H...O hydrogen bonding interactions to afford a 6-connected (4¹².6 ³) three-dimensional supramolecular architecture. Furthermore, the photoluminescent property of complex 1 was studied. Copyright

Full text of DOI:10.1002/zaac.201200474

ARTICLE
DOI: 10.1002/zaac.201200474
Synthesis, Structure, and Characterization of a New Metal-Organic
Framework containing Meso-Helices
Linlin Fan,[a] Haiyan Chen,[b] Dongrong Xiao,*[b] and Enbo Wang[c]
Keywords: Meso-helices; Metal-organic frameworks; Crystal structure; Photoluminescence properties; Cadmium
Abstract. The new metal-organic framework [Cd(bqdc)(phen)]n (1) that complex 1 is an infinite two-dimensional framework containing
(bqdc = 2,2Ј-biquinoline-4,4Ј-dicarboxylate, phen = 1,10-phenan-
aesthetic meso-helix. These two-dimensional coordination networks
throline) was hydrothermally synthesized by hetero-ring carboxylate are interconnected through aromatic π–π stacking and C–H···O hydro-
ligand and N-containing auxiliary ligand. The structure of 1 was deter-
mined by single-crystal X-ray diffraction analysis and further charac-
terized by elemental analyses, IR spectroscopy, X-ray powder diffrac-
tion, and TG analysis. Single-crystal X-ray structural analysis shows
gen bonding interactions to afford a 6-connected (412.63) three-dimen-
sional supramolecular architecture. Furthermore, the photoluminescent
property of complex 1 was studied.
ture and in everyday use, as known for tendrils of a variety of
plants, hair braid, and telephone wires.[40–43]
Introduction
The crystal engineering of metal-organic frameworks (MOFs)
has recently attracted considerable interest, owing to their tre-
mendous potential applications in gas storage, chemical sepa-
rations, ion exchange, microelectronics, nonlinear optics, and
heterogeneous catalysis.[1–7] Further, these coordination poly-
mers always have intriguing variety of vagarious frameworks,
new topologies and intricate entangled motifs.[1–7] In this field,
the self-assembly of helical structures from transition metal
cations and organic ligands is an interesting ongoing chal-
lenge.[8–22] Much attention is paid to them because compounds
with helical structures have broad potential applications, such
as the fundamental role of helicity in biology, the potential
applications of such structures in the fields of supramolecular
chemistry, asymmetric catalysis, and nonlinear optical mate-
rial.[1,21–38] More importantly, these helical compounds also
have aesthetical and appealing structural topologies.[39] Conse-
quently, many single-, double- and higher-stranded helical
complexes have been generated by self-assembly processes.[39]
In contrast, the occurrence of meso-helical coordination poly-
mers is still rare, although meso-helices are often found in na-
In order to obtain new network with helix structure as well
as interesting topologies, we chose the 2,2Ј-biquinoline-4,4Ј-
dicarboxylate (bqdc) and 1,10-phenanthroline (phen) as mixed
ligands. Biquinoline-4,4Ј-dicarboxylate is fascinating to us
based on the following considerations: (1) there is a single
bond between two quinoline rings leading to conformational
freedom of the two rings;[44,45] (2) there are oxygen atoms
of carboxylate groups and nitrogen atoms of quinoline rings
providing different potential coordination sites and diverse co-
ordination chemistry;[44,45] (3) with the number of protonated
and/or deprotonated carboxyl groups, it can act not only as
hydrogen-bond donors but also as acceptors;[46,47] (4) π–π
stacking interactions between the aromatic rings may facilitate
ordered, non-interpenetrating networks;[46,47] (5) the 2,2Ј-bi-
quinoline derivatives are promising for non-linear optical prop-
erties as recently are reported in the literature.[46–49]
After continuous exploration of experiments and optimiza-
tion of reaction conditions, we have successfully synthesized
a new coordination polymer of meso-helical characters,
[Cd(bqdc)(phen)]n (1). The structure was determined by sin-
gle-crystal X-ray diffraction analysis and further characterized
by IR spectroscopy and thermogravimetric analysis. Complex
1 is an infinite two-dimensional framework with aesthetic
meso-helix and these two-dimensional coordination networks
are further connected by aromatic π–π stacking and C–H···O
hydrogen bonding interactions to form a 6-connected (412.63)
three-dimensional supramolecular architecture.
* Prof. Dr. D. Xiao
Fax: +86-23-68254000
E-Mail: xiaodr98@yahoo.com.cn
[a] College of Materials Science and Engineering
Qiqihar University
Qiqihar, 161006, P. R. China
[b] College of Chemistry and Chemical Engineering
Southwest University
Chongqing, 400715, P. R. China
[c] Key Laboratory of Polyoxometalate Science of Ministry
of Education
Department of Chemistry
Northeast Normal University
Experimental Section
Changchun, 130024, P. R. China
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201200474 or from the au-
thor.
Materials and Methods: All chemicals were commercially purchased
and used without further purification. Elemental analyses (C, H and
N) were performed with a Perkin-Elmer 2400 CHN Elemental Ana-
558
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Z. Anorg. Allg. Chem. 2013, 639, (3-4), 558–562

New Metal-Organic Framework containing Meso-Helices
lyzer. The IR spectrum was recorded in the range 400–4000 cm–1 with Supporting Information (see footnote on the first page of this article):
an Alpha Centaurt FT/IR Spectrophotometer using KBr pellets. TG Photoluminescent spectra of Na2bqdc, XRPD patterns, IR spectrum,
analysis was performed with a Perkin-Elmer TGA7 instrument in flow-
ing nitrogen with a heating rate of 20 °C·min–1. Fluorescence measure-
ments were performed with an F-4500 FL fluorescence spectropho-
tometer.
and additional structural figures.
Results and Discussion
Structure of [Cd(bqdc)(phen)]n (1)
Synthesis of [Cd(bqdc)(phen)]n (1): A mixture of Cd(OAc)2·2H2O
(0.25 mmol), Na2bqdc (0.125 mmol), phen (0.25 mmol), NH4F
(0.25 mmol), and distilled water (10 mL) was stirred about 15 min in
air. Afterwards, the mixture was transferred and sealed in a 17 mL
Teflon-lined autoclave, which was heated at 150 °C for 96 h. After
slow cooling to room temperature, light yellow block crystals of com-
plex 1 were filtered off, washed with distilled water, and dried at ambi-
ent temperature (yield: 54% based on Na2bqdc). Elemental analyses
for C32H18CdN4O4: calcd. C 60.53; H 2.86; N 8.82%; found: C 60.27;
H 2.98; N 8.61%. IR (KBr): ν˜ = 3422 (w), 3073 (w), 2360 (w), 2340
(w), 1971 (w), 1858 (w), 1565 (s), 1546 (s), 1513 (m), 1448 (w), 1429
(s), 1383 (s), 1348 (m), 1279 (w), 1208 (w), 1154 (w), 1142 (w), 1100
(w), 1075 (w), 1049 (w), 1020 (w), 962 (w), 929 (w), 886 (w), 862
(w), 844 (m), 812 (w), 788 (w), 760 (m), 725 (m), 670 (m), 655 (w),
637 (w), 542 (w), 524 (w), 474 (w), 421 (w) cm–1.
Single crystal X-ray diffraction analysis reveals that 1 crys-
tallizes in the triclinic system with P1 space group. The asym-
¯
metric unit of 1 contains one cadmium atom, two half bqdc
ligands, and one phen ligand (Figure 1), Cd1 is seven-coordi-
nated by five oxygen atoms from three carboxylate groups of two
different bqdc ligands [Cd(1)–O(1) 2.317(2) Å, Cd(1)–O(2)
2.442(3) Å, Cd(1)–O(3) 2.585(3) Å, Cd(1)–O(4) 2.339(2) Å,
Cd(1)–O(3)#1 2.333(2) Å], and two nitrogen atoms from one
phen ligand [Cd(1)–N(3) 2.358(3) Å, Cd(1)–N(4) 2.354(3) Å]
in a distorted pentagonal-bipyramidal coordination environ-
ment. The carboxylic groups of one bqdc ligand coordinate to
cadmium ions in chelating mode (Scheme 1a); those of the
other bqdc ligand coordinate in μ2-η1:η2 bridging mode to
connect with four cadmium ions (Scheme 1b). Notably, the
bqdc acting as bis(tridentate chelating-bridging) ligand is dif-
ferent from most usual coordination modes of bqdc ligands,
which were previously reported, such as bis-bidentate chelat-
ing, bidentate and monodentate chelating, bis-monodentate
chelating, and tetradentate ligand with bidentated-bidentate
mode etc.[44–47,53–57]
In the crystal structure of 1, each dinuclear cadmium build-
ing unit is formed by two cadmium atoms linked through four
carboxylic groups from two different bqdc ligands, then afford-
ing a two dimensional coordination network with regular
quadrilateral that lies parallel to [1,1,–1] plane (Figure 2).
These two-dimensional coordination networks are inter-
connected by means of aromatic π–π stacking interactions of
phen ligands and C–H···O hydrogen bonds [C(28)···O(4)
3.371(6) Å] to afford a three-dimensional supramolecular
architecture, as shown in Figure 3 and Figure S1 (Supporting
Information). From a topological perspective, if each dinuclear
cadmium building unit is taken as a single node, the 3D struc-
ture of 1 can be regarded as a 6-connected (412.63) 3D net
(Figure 4). Four bqdc ligands connect with dinuclear cadmium
building units as four linkers, another two linkers are hydrogen
bonding interactions between the hydrogen atoms of phen and
the oxygen atoms of carboxylate in dinuclear cadmium build-
ing unit [C(28)···O(4) = 3.371(6) Å] (the perspective and sim-
plified views are shown in Figure S2, Supporting Information).
The most fascinating and peculiar structural feature of 1 is
that there are single-stranded meso-helices in the two-dimen-
sional coordination network. The meso-helical chains are run-
ning along the [–1 1 0] direction with a pitch of 12.436 Å and
formed by the same bqdc ligands and cacmium atoms (Fig-
ure 5). It is encouraging that the meso-helical motifs are rare
and especially the meso-helices are formed by bqdc ligands in
coordination compounds.
X-ray Crystallography: A well shape single crystal of complex 1 was
glued on a glass fiber. Diffraction intensity data were collected with a
Bruker Smart Apex CCD diffractometer with Mo-Kα monochromated
radiation (λ = 0.71073 Å) at 293 K. Absorption correction was applied
using the multi-scan technique. The structures were solved by the di-
rect method and refined by the full-matrix least-squares method on F2
using the SHELXL-97 software.[52] All of the non-hydrogen atoms
were refined anisotropically. The hydrogen atoms were generated geo-
metrically. A summary of crystal data and structure refinement for
complex 1 is provided in Table 1. The selected bond lengths and angles
for complex 1 are listed in Table 2.
Table 1. Crystal data and structure refinement for compound 1.
Formula
C32H18CdN4O4
Fw
T /K
634.90
293(2)
λ /Å
0.71073
triclinic
P1
Crystal system
Space group
a /Å
b /Å
c /Å
¯
10.901(2)
11.999(2)
12.111(2)
64.14(3)
81.38(3)
65.58(3)
1297.1(4)
2
α /°
β /°
γ /°
V /Å3
Z
Dc /mg·cm–3
μ /mm–1)
R1 [I Ͼ 2σ(I)]
wR2 [I Ͼ 2σ(I)]
R1 (all data)
wR2 (all data)
1.626
0.889
0.0385
0.0777
0.0511
0.0839
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting the
depository number CCDC-906925 (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Thermal Stability Analysis
To study the thermal stability of complex 1, TG analysis
(TGA) was performed on two samples in a nitrogen atmo-
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L. Fan, H. Chen, D. Xiao, E. Wang
ARTICLE
Table 2. Selected bond lengths /Å and angles /° for compound 1.
Cd(1)–O(1)
Cd(1)–O(3)#1
Cd(1)–O(4)
Cd(1)–N(4)
2.317(2)
2.333(2)
2.339(2)
2.354(3)
81.12(9)
124.49(9)
117.01(9)
154.21(9)
83.63(9)
81.06(9)
94.52(10)
108.37(10)
122.66(10)
70.78(11)
54.73(9)
Cd(1)–N(3)
Cd(1)–O(2)
Cd(1)–O(3)
2.358(3)
2.442(3)
2.585(3)
O(1)–Cd(1)–O(3)#1
O(1)–Cd(1)–O(4)
O(3)#1–Cd(1)–O(4)
O(1)–Cd(1)–N(4)
O(3)#1–Cd(1)–N(4)
O(4)–Cd(1)–N(4)
O(1)–Cd(1)–N(3)
O(3)#1–Cd(1)–N(3)
O(4)–Cd(1)–N(3)
N(4)–Cd(1)–N(3)
O(1)–Cd(1)–O(2)
N(3)–Cd(1)–O(2)
O(1)–Cd(1)–O(3)
O(3)#1–Cd(1)–O(3)
O(4)–Cd(1)–O(3)
N(4)–Cd(1)–O(3)
N(3)–Cd(1)–O(3)
O(2)–Cd(1)–O(3)
O(3)#1–Cd(1)–O(2)
O(4)–Cd(1)–O(2)
N(4)–Cd(1)–O(2)
87.83(11)
84.46(9)
79.53(9)
52.80(8)
113.05(10)
171.82(9)
84.96(9)
134.41(8)
84.57(9)
141.46(9)
Symmetry transformations used to generate equivalent atoms: for 1: #1 –x,–y,–z; #2 –x–1,–y+1,–z–1; #3 –x, –y, –z + 1.
Figure 1. ORTEP diagram showing the coordination environment for
the cadmium atom in complex 1.
Figure 2. Perspective views of the 2D framework in complex 1.
Scheme 1. Two coordination modes of bqdc in complex 1.
Figure 3. Perspective views of the aromatic π–π stacking interactions
of phen ligands for the 3D supramolecular network in 1.
sphere with a heating rate of 20 K·min–1 in the temperature
range of 20–1000 °C. The TG curve of 1 is shown in Figure 6.
It gives a total weight loss of 79.82% in the range of 360–
980 °C, which agrees with the calculated value of 79.78%.
The first weight loss is 55.67% in the temperature range of
Photoluminescence Property
The luminescent property of the as-synthesized 1, was
360–490 °C, assigned to the decomposition of bqdc ligands studied in solid state at room temperature (Figure 7) and the
(calcd. 50.13%). The decomposition is going on until 980 °C XPRD pattern was measured to confirm the purity of 1 (Figure
and the total loss is 24.15% at 490–980 °C, assigned to the S3b). At room temperature, 1 exhibits intense blue emission
decomposition of phen ligands (calcd. 29.65%). Last the re- band with an emission maximum at 451 nm upon excitation at
maining weight shows 20.18% that indicates the final residue 370 nm. The luminescent property of Na2bqdc was also
was CdO (calcd. 20.22%).
studied in solid state at room temperature (Figure S3a). The
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New Metal-Organic Framework containing Meso-Helices
Figure 4. Schematic illustration of the 6-connected 3D net of (412.63)
topology for 1.
Figure 6. TG curve of complex 1.
Figure 7. Solid-state photoluminescent spectra of complex 1 at room
temperature. Ex = excitation, Em = emission.
coordination polymers, but also further confirms the aesthetic
diversity of coordinative network chemistry.
Acknowledgements
Figure 5. Perspective views of the meso-helical chain in complex 1.
This work was financially supported by the National Natural Science
spectrum of Na2bqdc is similar to complex 1, which exhibits Foundation of China (20801044), the China Postdoctoral Science
Foundation (20080430205, 200902282), the Science and Technology
Foundation of Southwest University (SWUB2007035), the Fundamen-
tal Research Funds for the Central Universities (XDJK2009B014), and
the Program for Chongqing Excellent Talents in University.
intense blue emission band with an emission maximum at
449.6 nm. Because the luminescent property of complex 1 was
not affected by coordination of d10 metal, the emission of com-
plex 1 may be assigned to intraligand charge transfer (ILCT)
transition inside bpdc.
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Received: October 26, 2012
Published Online: December 20, 2012
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