A flexible tri-carboxylic acid derived zinc(II) 3D helical metal-organic-framework and a cadmium(II) interwoven 2D layered framework solid

Article abstract of DOI:10.1002/zaac.201300677

Starting from a flexible tri-carboxylic acid ligand H₃L {H ₃L = 2,4,6-tris[(4'-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene}, a 3D metal-organic framework [Zn(HL)(H₂O)]_n (1) and a 2D polymeric layered solid {[(

Full text of DOI:10.1002/zaac.201300677

ARTICLE
DOI: 10.1002/zaac.201300677
A Flexible Tri-carboxylic Acid Derived Zinc(II) 3D Helical Metal-Organic-
Framework and a Cadmium(II) Interwoven 2D Layered Framework Solid
Pratap Vishnoi[a] and Ramaswamy Murugavel*[a]
Dedicated to Professor C. N. R. Rao on the Occasion of His 80th Birthday
Keywords: Flexible tri-carboxylate ligand; Helical structures; Metal-organic frameworks; Tetrahedral arrangement; Twofold
interpenetration; Layered compounds
Abstract. Starting from a flexible tri-carboxylic acid ligand H3L {H3L
2,4,6-tris[(4Ј-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene},
tion. The X-ray structural analyses reveal that the complex 1 is an
example of a doubly interpenetrated 3D helical MOF with mutually
=
a
3D metal-organic framework [Zn(HL)(H2O)]n (1) and a 2D polymeric entwined right- and left-handed helices to form an overall racemic
layered solid {[(CH3)2NH2)]2[Cd2L2(DMA)2]}n (2) were hydrother-
mally synthesized. The complexes were characterized by elemental
analyses, FT-IR spectroscopy, TGA, and single-crystal X-ray diffrac-
framework. Complex 2 is made up of fourfold interwoven 2D nets in
the solid state.
Introduction
In recent years, metal-organic framework (MOF) materials
have attracted widespread attention owing to their interesting
properties and potential applications as gas storage and separa-
tion materials,[1,2] luminescent sensors,[3–5] magnetic materi-
als,[6] and catalysts.[7] MOFs with helical structure are of par-
ticular interest because of their relevance to bio-molecules
such as DNA and proteins, which living organisms utilize for
coding genetic information.[8,9]
It is well studied in MOF chemistry that several factors such
as metal coordination arrangement, solvent, reaction condi-
tions, and pH etc. contribute to the topology of the desired
framework.[10,11] Therefore, a judicious selection of building
units can lead to helical MOFs with specific structures and
functions. Towards synthesizing helical MOFs, one of the most
contributing factors is the denticity and flexibility of the ligand
system(s). The flexible multidentate acid ligands have been
known to adopt various conformations and can curve when
coordinate with metal ions. In this regard, aromatic multiden-
tate ligands are good connectors and have opportunity to form
helical structures.[13–16] Over past few years, the chemistry of
helical structures have been well developed employing flexible
C2 symmetric ligands.[17–19]
Scheme 1. Coordination modes of H3L in complexes 1 and 2.
and structural characterization of a 3D MOF of ZnII with 1D
helical channels, [Zn(HL)(H2O)]n (1) and a 2D layered MOF
of CdII, {[(CH3)2NH2)]2[Cd2L2(DMA)2]}n (2), were achieved.
Owing to the flexibility of the ligand, helical features were
introduced into 1, where each carboxylate anion of the ligand
exhibits monodentate mode of coordination (Scheme 1). The
chelating mode of the carboxylates in 2 produces a 2D struc-
ture. While this work has been in progress, other framework
structures of H3L with ZnII and CdII ions have been reported;
these structures have however been obtained using different
reaction conditions and co-ligands, exhibiting different molec-
ular topologies.[20,21]
To the best of our knowledge, helical MOFs with C3 sym-
metric ligands are rare. In the presented study the synthesis
* Prof. Dr. R. Murugavel
Fax: + 91-022-2576-7152
E-Mail: rmv@chem.iitb.ac.in
Results and Discussion
[a] Department of Chemistry
Indian Institute of Technology Bombay
Mumbai – 400 076, India
Synthesis
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300677 or from the au-
thor.
Colorless bulk crystals of [Zn(HL)(H2O)]n (1) were ob-
tained by hydrothermal treatment of H3L with Zn(NO3)2·6H2O
Z. Anorg. Allg. Chem. 2014, 640, (6), 1075–1080
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P. Vishnoi, R. Murugavel
ARTICLE
Scheme 2. Synthetic routes for 1 and 2.
in DMF/H2O solvent mixture (Scheme 2). However the hydro- Table 1. Selected bond lengths /Å for 1 and 2.
thermal synthesis involving Cd(NO3)2·4H2O performed in
DMF/H2O solvent mixture yielded only an amorphous powder.
1
Zn(1)–O(5)
1.941(3)
1.948(4)
Zn(1)–O(8)
Zn(1)–O(10)
1.967(4)
2.005(4)
The replacement of DMF with DMA resulted in X-ray quality Zn(1)–O(6)
2
colorless crystals of {[(CH3)2NH2)]2[Cd2L2(DMA)2]}n (2).
Cd(1)–O(10)
Molecular compositions of the complexes were determined by
Cd(1)–O(8)
2.281(4)
2.306(4)
2.350(4)
2.362(4)
2.408(4)
2.414(4)
2.488(4)
Cd(2)–O(16)
Cd(2)–O(14)
Cd(2)–O(19)
Cd(2)–O(20)
Cd(2)–O(18)
Cd(2)–O(17)
Cd(2)–O(15)
2.216(4)
2.266(4)
2.313(4)
2.345(4)
2.437(4)
2.553(4)
2.577(4)
elemental analyses, FT-IR spectroscopy, and single-crystal X-
ray diffraction studies.
Cd(1)–O(6)
Cd(1)–O(5)
Cd(1)–O(4)
Cd(1)–O(7)
Cd(1)–O(9)
FT-IR and Thermogravimetric Analysis
The formation of metal carboxylate bonds in 1 and 2 were
verified with the aid of FT-IR spectra (Figure S1, Supporting
Information). Two intense peaks appearing at around 1600 and
around 1390 cm–1 correspond to the antisymmetric and sym-
metric C=O (carboxylate) stretching vibrations.
Thermogravimetric analyses were performed at heating rate
of 10 °C·h–1 from room temperature to 1000 °C under con-
tinuous flow of nitrogen gas. Both complexes show similar
thermal decomposition behavior and exhibit two weight losses
(Figure S2, Supporting Information). A rather continuous
weight loss observed up to ca. 250 °C is attributable to the loss
of solvent molecules (coordinated/uncoordinated) present in
the complexes. After removal of the solvent molecules, gradual
decomposition of organic ligands occurs in the temperature
range of 330–500 °C.
Crystal Structure Analyses
Crystal Structure of [Zn(HL)(H2O)]n (1)
A detailed X-ray structure analysis suggests that the com-
plex crystallizes as a 3D helical network in the monoclinic C2/
c space group. The fundamental structural unit consists of one
tetra-coordinated ZnII ion, one doubly deprotonated HL2– li-
gand, and one terminally coordinated aqua ligand (Figure S3,
Supporting Information). Three coordination sites of ZnII ion
are occupied by carboxylic oxygen atoms and the fourth one
is occupied by an aqua ligand, thus providing a tetrahedral
arrangement around the central ZnII ions. In other words, each
Figure 1. A view of a 3D network in complex 1 with hexagonal chan-
nels (helices) viewed down crystallographic b axis.
HL2– ligand binds with three different ZnII ions through mono- carboxylate groups is protonated at O9 as the C–O bond (C33–
dentate coordination modes, which can be described as (η1-η1- O9) is longer than the C33–O8 bond length. Because of the
η1)-μ3 mode of coordination.
flexibility, the three different donor sites of the ligand are not
The Zn–O(carboxylate) distances fall in the range of 1.947– in the same plane (oriented towards crystallographic b axis)
1.968 Å (Table 1), whereas the Zn–O(water) bonds were found and hence generate a 3D network with helical channels along
to be slightly longer with bond lengths of 2.009 Å. One of the the same direction (Figure 1). TOPOS[22] analysis indicates
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Z. Anorg. Allg. Chem. 2014, 1075–1080

Tri-carboxylic Acid Derived ZnII and CdII Metal-Organic-Frameworks
that the framework has uninodal 6-connected plane nets with
point symbol of (33.59.62.7) and sda topology.[23]
An interesting structural feature of the complex is that the
two 3D helical nets with opposite handedness are mutually
entwined and extended along the crystallographic b axis with
same helix pitch of 8.394(4) Å. Thus, the overall structure of
the complex is a pair of enantiomers, leading to the racemiza-
tion of the whole framework (Figure 2 and Figure 3). The sol-
vent accessible volume of the complex 1 in each unit cell is
5207.0 Å3, which is 50.8% of the total volume of the unit
cell (10242.0 Å3), and occupied by severely disordered solvent
molecules (DMF and water).
Figure 3. Schematic representation of the twofold interpenetration
in 1.
decomposes to provide charge-balancing cations, which
eventually stabilize the anionic framework.
The coordination environment of CdII ions is completed by
six oxygen atoms from three different carboxylate groups of
three distinct L3– ligands and one oxygen atom of the coordi-
nated DMA molecule. Thus, each CdII ion is connected to
three L3– ligands in η2 mode of coordination, and each L3– is
connected to three CdII ions, resulting in 2D sheets or nets. The
structures of the 2D sheets are similar to previously reported
[Cd2(HBTB)2·(DMF)(MeOH)] (BTB = benzene-1,3,5-triben-
zoate).[24] The 2D sheets are interconnected by hydrogen
bonds formed between carboxylate groups and [(CH3)2NH2]+
cations present in the lattice cavities (Table 2). Due to the large
aperture of the hexagonal cavities in these sheets, a fourfold
interpenetration results as shown in Figure 4 (top), where the
individual sheets are held together by strong N–H···O bonding
formed between carboxylate groups and [(CH3)2NH2]+ cations.
A view down the b axis reveals that several such interwoven
2D nets are stacked one over other through N–H···O interac-
tions between them, to result in a final layered structure (Fig-
ure 4 bottom and Figure 5).
Table 2. Hydrogen bond parameters (distances in Å and angles in °)
of 2.
Figure 2. Two-fold interpenetrated 3D helical networks of 1 where
nets with right and left handedness are shown.
D–H···A
d(D–H) d(H···A)
d(D···A)
Ͻ(DHA)
N(3)–H(3D)···O(17)#1 0.90
2.09
2.45
2.63
1.89
1.94
2.830(8)
3.117(10) 131.5
3.107(8)
2.771(6)
2.827(7)
138.6
N(3)–H(3D)···O(15)#1 0.90
N(3)–H(3C)···O(4)#2 0.90
N(4)–H(4D)···O(6)#3 0.90
N(4)–H(4C)···O(5)#4 0.90
114.4
166.2
166.5
Crystal Structure of {[(CH3)2NH2)]2[Cd2L2(DMA)2]}n (2)
Single crystal structure analysis reveals that complex 2 is an
Symmetry transformations used to generate equivalent atoms: #1 –x
+ 1, –y+1, –z+1 #2 x, y, z #3 x, y, z+1 #4 –x + 2, –y+1, –z+1.
anionic 2D layer framework that crystallizes in the triclinic
¯
centrosymmetric space group P1. The asymmetric unit consists
of two different molecular units (A and B; Figure S4, Support-
The two-dimensional sheets feature uninodal 6-connected
ing Information) with slight differences in their bonding pa- plane nets with point symbol of (36.46.53) as revealed by TO-
rameters. Each unit consists of one seven-coordinate CdII ion, POS[22] analysis. The solvent accessible volume of the com-
one L3– ligand, one coordinated DMA molecule, and one plex 2 is 1096.6 Å3, which is 24% of the total volume
[(CH3)2NH2]+ cation, which stays outside the metal coordina- (4569.0 Å3) and occupied by solvent molecules (DMA and
tion sphere (Figure S4). It must be pointed out that the amide water). The Cd–O bond lengths range from 2.224(4)–
solvent (DMA) not only serves as reaction medium but also 2.554(4) Å (Table 1).
Z. Anorg. Allg. Chem. 2014, 1075–1080
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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P. Vishnoi, R. Murugavel
ARTICLE
Figure 4. Crystal structure of 2, layered structure (top) and a section
of one of the layers of 2, where four 2D nets are interwoven.
Conclusions
Figure 5. Schematic representation of the fourfold interwoven 2D nets
in 2.
We have reported two new MOFs of ZnII and CdII, which
utilize a bent and flexible tri-carboxylic acid ligand. The com-
plexes were characterized by elemental analysis, FT-IR spec-
troscopy, thermogravimetry, and single crystal XRD. The stud-
ies reveal that the diversity in metal coordination number and
ligand flexibility play a significant role in producing helical
architectures. The X-ray structure analyses reveal that ZnII ex-
hibits tetrahedral arrangement and culminates in a 3D helical
structure, whereas the seven coordinate CdII ions afford a 2D
layered framework made of interwoven nets. The large solvent
accessible pores suggest the possibility of de-solvation and
subsequent use of these materials for gas adsorption studies.
We are investigating these possibilities along with efforts to
isolate new MOFs with other transition metals employing H3L
as a prototype flexible ligand.
Elemental analyses were performed with a Thermo Finnigan (FLASH
EA 1112) microanalyzer. FT-IR spectra were recorded with a Perkin-
Elmer Spectrum One Infrared Spectrometer as KBr diluted disks.
Thermogravimertic analyses were performed with a Perkin-Elmer
Pyris Diamond TGA instrument. Single crystal X-ray diffraction mea-
surements were performed using Mo-Kα radiation (λ = 0.71075 Å)
with a Rigaku AFC10K Satrun 724HG based diffractometer. PXRD
data were collected with a Rigaku SmartLab X-ray diffractometer
equipped with Cu-Kα radiation (λ = 1.54056 Å). Nitrogen adsorption /
desorption experiments were performed isothermally at 77 K on
Quantachrome autosorb-1.
Synthesis of H3L: A mixture of methyl-p-hydroxybenzoate (7.6 g,
50.0 mmol), NaOH (4.0 g, 100 mmol) and DMSO (100 mL) was
stirred at 75 °C for 1 h. To this hot mixture, 1,3,5-tris(bromomethyl)-
2,4,6-trimethylbenzene (5.98 g, 15.0 mmol) was added and the re-
sulting mixture was heated for another 24 h at 75 °C. The mixture was
cooled to room temperature and poured into 200 mL of distilled water.
Experimental Section
Methods, Materials, and Instruments: All the starting materials and
the products were found to be stable towards moisture and air, and To this, 50 mL aqueous solution of NaOH (20.0 g, 250 mmol) was
hence no specific precaution was taken while handling them. Starting
materials such as Zn(NO3)2·6H2O, Cd(NO3)2·4H2O, N,N-dimethyl-
acetamide (DMA) and N,N-dimethylformamide (DMF) were procured
from Merck, India.
added and heated for another 12 h at 75 °C. The mixture was again
cooled to room temperature and acidified with diluted HCl (final pH
ca. 2). A white precipitate formed immediately upon acidification was
filtered under suction and washed repeatedly with water and with di-
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Z. Anorg. Allg. Chem. 2014, 1075–1080

Tri-carboxylic Acid Derived ZnII and CdII Metal-Organic-Frameworks
ethyl ether and dried under vacuum to obtain analytically pure acid
Table 3. Crystallographic data and structure refinement summary for
H3L as white powder. Yield; 5.65 g, 73%. Mp 270–275 °C. C33H30O9: complexes 1 and 2.
1
C 69.46; H 5.30%; found: C, 69.34; H, 5.37%. H NMR (400 MHz,
1
2
3
[D6]DMSO, 295 K): δ = 12.69 (s, 3 H, COOH), 7.93 (d, JHH
=
3
Formula
Formula wt
C33H30ZnO10
651.94
150(2)
0.71075
monoclinic
C2/c
36.991(14)
8.394(3)
36.940(15)
90
116.750(5)
90
10242(7)
8
0.846
0.514
C39H44CdN2O10
813.16
150(2)
0.71075
triclinic
8.72 Hz, 6 H, ArH), 7.17 (d, JHH = 7.76 Hz, 6 H, ArH), 5.21 (s, 6
H, BzCH2), 2.36 (s, 9 H, ArCH3) ppm. 13C{1H} NMR (100 MHz,
[D6]DMSO, 295 K): δ = 167.0, 162.4, 139.2, 131.4, 131.2, 123.3,
Temperature /K
Wavelength /Å
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
β /°
+
114.5, 65.1, 15.6 ppm. LR-MS (ESI); m/z calcd. for C33H31O9
571.19, found 571.14 [M + H]+. FT-IR (KBr): ν˜ = 3400–2600, 1682,
¯
P1
1603, 1510, 1428, 1244, 1168 cm–1.
14.275(5)
18.902(5)
19.667(7)
67.586(16)
85.46(2)
68.972(16)
4569(3)
4
Synthesis of [Zn(HL)(H2O)]n (1): A mixture of H3L (0.057 g
0.10 mmol), Zn(NO3)2·6H2O (0.060 g 0.20 mmol), DMF (4.0 g), and
distilled H2O (1.0 g) was placed in a 20 mL glass vial and stirred for
half an hour. The mixture was transferred to a 15 mL Teflon vial. The
Teflon vial was placed inside a stainless steel autoclave and heated at
115 °C for 5 d. Microcrystalline solid was obtained upon cooling the
reaction mixture to room temperature at the ambient conditions. The
mixture was filtered and the filterate was kept at room temperature to
obtained needle like colorless crystals of 1 after a month. Yield; 0.04 g
(61% based on H3L used). Mp; Ͼ 250 °C. C33H30ZnO10: C 60.8; H
4.6%; found: C 57.2; H 4.6%.[25] FT-IR (KBr): ν˜ = 3433, 2925, 1604,
1385, 1223 cm–1.
γ /°
Volume /Å3
Z
Density (calcd) /g·cm–3
Absorption coeff /mm–1
F(000)
1.182
0.527
1680
2704
Crystal size /mm3
θ range /°
0.28ϫ0.23ϫ0.08 0.20ϫ0.11ϫ0.02
3.05 to 25.00
34298
8908 (0.0601)
98.9
7/397
2.12 to 25.00
34619
15926 (0.0500)
99.0
Reflection collected
Data (Rint)
Completeness to θ /%
Restraints/parameters
R1 [I Ͼ 2σ(I)] /all data
wR2 [I Ͼ 2σ(I)] /all data
8/938
Synthesis of {[(CH3)2NH2)]2[Cd2L2(DMA)2]}n (2): A mixture of H3L
(0.057 g, 0.10 mmol), Cd(NO3)2·4H2O (0.062 g, 0.2 mmol), DMA
(5.0 g), and distilled H2O (1.0 g) was placed in a 20 mL glass vial,
stirred for half an hour, and transferred to a 15 mL Teflon vial. The
Teflon vial was placed inside a stainless steel autoclave and heated at
115 °C for 5 d. Colorless crystals of 2 were obtained after cooling the
mixture to room temperature over 2 d. Yield; 0.06 g (74% based on
H3L used). Mp; Ͼ 250 °C. C39H44CdN2O10: C 57.6; H 5.4; N 3.4%;
found: C 56.0; H 5.6; N 2.7%.[25] FT-IR (KBr): ν˜ = 3389, 3073, 2957,
2921, 1604, 1542, 1385, 1239 cm–1.
0.0920 / 0.1072
0.2587 / 0.2725
0.0656 / 0.0879
0.1691 / 0.1838
0.494, –0.845
Largest peak and hole /e·Å–3 1.063, –0.874
Supporting Information (see footnote on the first page of this article):
FT-IR spectra, TGA profiles, metal coordination geometries, surface
area measurement results, PXRD profiles, and the table containing
selected bond distances and angles.
Acknowledgements
X-ray Crystallography: The single-crystal X-ray data were collected
with a Rigaku Saturn 724 CCD diffractometer with a Mo-Kα radiation
source (λ = 0.71075 Å) at 150 K under continuous flow of nitrogen.
The data integration and indexing were performed using Rigaku Crys-
talclear software and a multi-scan method was employed to correct for
absorption. The structures were solved by direct methods using SIR-
92[26] and refined by full-matrix least-squares fitting on F2 using
SHELXL-97.[27] All the non-hydrogen atoms were refined anisotropi-
cally. The C–H protons were placed in their geometrical idealized posi-
tions and refined isotopically as riding atoms. The hydrogen atoms
attached to oxygen were located in the difference Fourier maps and
refined as rigid atoms in their idealized positions. Fourier maps also
indicate severely disordered solvent molecules in the cavities. To ac-
count for the disordered solvent molecules, the final refinement was
performed with modification of the structure factors using the
SQUEEZE option of PLATON. Squeeze removed the solvent mol-
ecules from the voids which account for the diffrence in the elemental
analyses. The solvent accessible volumes were calculated using CALC
SOLV utility of PLATON. The data refinement details are given in
Table 3
We sincerely thank SERB, DST, New Delhi for financial support for
this work. We also thank the Department of Atomic Energy, Mumbai
for a DAE-SRC Outstanding Investigator Award to RM, which made
the purchase of a single crystal diffractometer possible. P. V. thanks
CSIR, New Delhi for research fellowship.
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Received: December 23, 2013
Published Online: March 13, 2014
1080
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Z. Anorg. Allg. Chem. 2014, 1075–1080