Zinc-1,4-benzenedicarboxylate-bipyridine frameworks - Linker functionalization impacts network topology during solvothermal synthesis

Article abstract of DOI:10.1039/c1jm14791a

Substitution of 1,4-benzenedicarboxylate (bdc) with additional alkoxy chains is the key to construct a family of metal-organic frameworks (MOFs) of the type [Zn₂(fu-bdc)₂(bipy)]_n (fu-bdc = functionalized bdc; bipy = 4,4′-bipyridine) exhibiting a honeycomb-like topology instead of the default pillared square-grid topology. Both the substitution pattern of the phenyl ring of the fu-bdc linker and the chain length of the alkoxy substituents have a major impact on the structure of the derived frameworks. Substitution at positions 2 and 3 leads to the trivial pillared square-grid framework, and substitution at positions 2 and 5 or 2 and 6 yields MOFs with the honeycomb-like topology. Also, simple methoxy substituents lead to the construction of a pillared square-grid topology, whereas longer substituents like ethoxy, n-propoxy, and n-butoxy generate honeycomb-like framework structures. These honeycomb MOFs feature one-dimensional channels, which are tuneable in diameter and functionality by the choice of substituent attached to the bdc-type linker. Pure component sorption isotherms indicate that the honeycomb-like frameworks selectively adsorb CO₂ over N ₂ and CH₄. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1jm14791a

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PAPER
Zinc-1,4-benzenedicarboxylate-bipyridine frameworks – linker
functionalization impacts network topology during solvothermal synthesis†
a
Sebastian Henke, Andreas Schneemann, Shobhna Kapoor, Roland Winter and Roland A. Fischer
a
b
b
a
*
Received 26th September 2011, Accepted 18th October 2011
DOI: 10.1039/c1jm14791a
Substitution of 1,4-benzenedicarboxylate (bdc) with additional alkoxy chains is the key to construct
a family of metal–organic frameworks (MOFs) of the type [Zn2(fu-bdc)2(bipy)]n (fu-bdc ¼
functionalized bdc; bipy ¼ 4,40-bipyridine) exhibiting a honeycomb-like topology instead of the default
pillared square-grid topology. Both the substitution pattern of the phenyl ring of the fu-bdc linker and
the chain length of the alkoxy substituents have a major impact on the structure of the derived
frameworks. Substitution at positions 2 and 3 leads to the trivial pillared square-grid framework, and
substitution at positions 2 and 5 or 2 and 6 yields MOFs with the honeycomb-like topology. Also,
simple methoxy substituents lead to the construction of a pillared square-grid topology, whereas longer
substituents like ethoxy, n-propoxy, and n-butoxy generate honeycomb-like framework structures.
These honeycomb MOFs feature one-dimensional channels, which are tuneable in diameter and
functionality by the choice of substituent attached to the bdc-type linker. Pure component sorption
isotherms indicate that the honeycomb-like frameworks selectively adsorb CO2 over N2 and CH4.
MOF topologies, by covalent functionalization of the linker
molecules, because this allows the intrinsic properties of the
linker to be influenced.9 A prerequisite is that the functionali-
zation should not interfere with the formation of an open
framework structure. Therefore, particular functionalization of
a linker can modify the conformational flexibility, for example by
affecting rotational barriers, which can be the key for the
construction of formerly inaccessible network structures.
Introduction
The rapidly growing family of metal–organic frameworks
(MOFs) or porous coordination polymers (PCPs) represents
highly porous hybrid materials constructed from inorganic
building units and organic linkers.1 A modular building principle
allows the isoreticular synthesis of network topologies with large
specific surface areas and high pore volumes. In contrast to
strictly inorganic porous materials, like zeolites, additional
functional groups or substituents can be implemented in the
pores via extensive modification of the organic building blocks.2
This allows a targeted tailoring of specific properties, focussing
on a great variety of possible applications, for example in gas
storage,3 gas separation,4 catalysis,5 optics,6 or chemical sensing.7
Due to the great diversity in the accessible inorganic and
organic building units an enormous number of different MOFs
has been prepared so far.1,2 It has been shown that the diversity
of MOF structures can not only be controlled by the choice of
inorganic and organic building bricks, but also by the utilized
solvent, the reaction temperature, or molecular templates.8
However, an important field is the targeted construction of novel
A well-studied family of MOFs are the pillared layer-based
networks of composition [M2L2P]n with M ¼ Zn, Cu, Ni, Co;
L ¼ linear dicarboxylate linker (e.g., 1,4-benzenedicarboxylate,
1,4-naphthalenedicarboxylate); P ¼ neutral pillar (e.g., 1,4-dia-
zabicyclo[2.2.2]octane, 4,40-bipyridine).10,11 All these structures
are constructed from dinuclear metal paddlewheel units, which
are linked by L to construct two-dimensional sheets of compo-
sition [M2L2]n, which are most often of the square-grid type.
However, Kagome type sheets can be synthesized for some Zn
derivatives.12 The pillar P coordinates to the axial positions of the
paddlewheel building units to interconnect the two-dimensional
sheets and to generate a three-dimensional network. Note-
worthy, if longer pillars are used, for example 4,40-bipyridine
(bipy), 1,4-bis(4-pyridyl)benzene or N,N0-di(4-pyridyl)-1,4,5,8-
naphthalenetetracarboxydiimide, the frameworks are commonly
interpenetrated by a second independent network.11
aLehrstuhl fu€r Anorganische Chemie II, Organometallics & Materials
Chemistry, Ruhr-Universita€t Bochum, Universita€tsstraße 150, 44780
Bochum, Germany. E-mail: roland.fischer@rub.de; Fax: +49 234-32-
Recently, we have communicated that specific functionaliza-
tion of the linker 1,4-benzenedicarboxylate (bdc) can trigger
the formation of an unexpected honeycomb-like network of
analogous [M2L2P]n composition.13 The framework [Zn2(2,5-
BME-bdc)2(bipy)]n (1; with 2,5-BME-bdc ¼ 2,5-bis(2-methoxy-
ethoxy)-1,4-benzenedicarboxylate) crystallizes in the trigonal
14174; Tel: +49 234-32-24174
b
Lehrstuhl fu€r Physikalische Chemie I, Technische Universita€t Dortmund,
442277 Dortmund, Germany
† Electronic supplementary information (ESI) available: Single crystal
X-ray diffraction data, PXRDs, TGAs, NMR and IR spectra. CCDC
reference numbers 834509 and 834510. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1jm14791a
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J. Mater. Chem., 2012, 22, 909–918 | 909

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ꢀ
space group R3c and features one dimensional porous channels,
reactions of 2,5-BME-bdc with the shorter co-ligand 1,4-dia-
zabicyclo[2.2.2]octane yield the expected pillared square-grid
type network.14
which are populated by the flexible 2-methoxyethoxy substitu-
ents (Fig. 1). These flexible groups act as molecular gates for
incoming guest molecules. Therefore, CO2 is very selectively
adsorbed in 1 compared to N2 and CH4. We have shown that the
major prerequisite for the formation of this honeycomb-like
framework structure instead of the default pillared layer-based
structures is substitution of the bdc linker in positions 2 and 5.13
Thus a rotation of the carboxylate groups from the plane of the
phenyl ring is facilitated in 2,5-BME-bdc. In the solid-state
structure of 1 one half of the 2,5-BME-bdc linkers have
a conformation, whereby the carboxylate groups are twisted
approx. 90ꢀ (Fig. 1, top). Obviously, that kind of conformation
is very unfavourable for conventional bdc, and also for the
mono-substituted linker ME-bdc (2-methoxyethoxy-1,4-benze-
In this contribution we discuss the impact of the substitution
pattern as well as the substituent chain length on the network
topology of these [Zn2L2(bipy)]n coordination polymers more
comprehensively. Therefore, the two substitution isomers of 2,5-
BME-bdc, namely 2,3-bis(2-methoxyethoxy)-1,4-benzenedi-
carboxylate (2,3-BME-bdc) and 2,6-bis(2-methoxyethoxy)-1,4-
benzenedicarboxylate (2,6-BME-bdc), were utilized as linker L in
MOF synthesis together with zinc nitrate and bipy. In order to
gain information about the impact of the substituent chain length
we used four different alkoxy-functionalized bdc derivatives as
linker L, in particular 2,5-dimethoxy-1,4-benzenedicarboxylate
(DM-bdc), 2,5-diethoxy-1,4-benzenedicarboxylate (DE-bdc),
2,5-dipropoxy-1,4-benzenedicarboxylate (DP-bdc), and 2,5-
dibutoxy-1,4-benzenedicarboxylate (DB-bdc). The derived
MOFs of the general composition [Zn2L2(bipy)]n were charac-
terised via single crystal X-ray diffraction and/or powder X-ray
diffraction and spectroscopic techniques. Furthermore, the
sorption properties of [Zn2(DE-bdc)2(bipy)]n and [Zn2(DB-
bdc)2(bipy)]n were compared with the properties of the proto-
typic compound 1.
nedicarboxylate).
Consequently,
[Zn2(bdc)2(bipy)]n
and
[Zn2(ME-bdc)2(bipy)]n crystallize in the default interpenetrated
pillared square-grid framework.10b–d,13
An additional and important requirement for the construction
of the novel honeycomb-like topology is that the metrics of the
linker L and the co-ligand P must be similar. Note that analogous
Experimental section
General techniques, materials, and methods
All chemicals were purchased from commercial suppliers (Sigma-
Aldrich, Fluka, Alfa Aesar, and others) and used without further
purification. Elemental analyses were performed on a vario EL
instrument from Elementar Hanau and AAS analyses were per-
formed on an AAS 6 vario from Analytik Jena in the Microan-
alytical Laboratory of the Department of Analytical Chemistry
€
at the Ruhr-Universitat Bochum. Liquid phase NMR spectra
were recorded on a Bruker Avance DPX-250 spectrometer (1H,
250.1 MHz; 13C, 62.9 MHz) at 293 K in [D6]DMSO for the linkers
and in DCl/D2O/[D6]DMSO for the digested MOF samples. 13
C
NMR spectra were measured using an attached proton test
(ATP) pulse program. Chemical shifts are given relative to TMS
and were referenced to the solvent signals as internal standards.
Solid-state 13C MAS NMR spectra were recorded on a Bruker
DSX-400 MHz spectrometer in ZrO2 rotors of 2.5 mm diameter,
applying pulse programs written by H.-J. Hauswald of the
€
Department of Analytical Chemistry of the Ruhr-Universitat
Bochum. Infrared (IR) spectra were recorded on a Bruker Alpha-
P FT-IR instrument in the ATR geometry equipped with a dia-
mond ATR unit (~n ¼ 4000–375 cmꢁ1) inside a glovebox (Ar
atmosphere). Advanced IR measurements were performed at
25 ꢀC with a Nicolet 5700 FT-IR spectrometer equipped with
a liquid nitrogen cooled MCT (HgCdTe) detector using a sample
cell with CaF2 windows that were separated by 50 mm mylar
spacers. Thin pellets of the dried MOF samples were incorpo-
rated between the two windows separated by the spacer. The
spectrometer was purged continuously with dry air to remove
water vapour. Typically, FT-IR spectra of 512 scans were taken
with a resolution of 2 cmꢁ1 and corresponding processing like
background correction and base lining was performed using
GRAMS software (Thermo Electron). The time between the
Fig. 1 Structure of as synthesized [Zn2(2,5-BME-bdc)2(bipy)]n (1as).13
Top: view of the infinite rod-like building unit consisting of Zn2+ centres,
which are syn–anti bridged by carboxylates. The 2-methoxyethoxy
groups have been omitted for clarity because they are highly disordered.
Therefore, they could only be located partially in the structure. Bottom:
ꢁ
view of the trigonal unit cell in the [001] direction (a ¼ b ¼ 53.188(3) A,
ꢀ
ꢀ
ꢁ
ꢀ
c ¼ 17.813(2) A; a ¼ b ¼ 90 , g ¼ 120 ; space group R3c). Carbon,
nitrogen, oxygen and zinc atoms are shown in dark grey, blue, red, and
yellow, respectively. The polyhedra around the zinc centres are shown in
yellow. Note that not all flexible 2-methoxyethoxy chains and no guest
molecules could be localized in the electron density map due to severe
disorder. Hydrogen atoms are omitted for the sake of clarity.
910 | J. Mater. Chem., 2012, 22, 909–918
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assembly of cell and first spectrum collection was approximately
as short as 2 min, which helped in minimizing the exposure of the
MOFs to water vapour. For the conformational analysis of
the CH2 wagging modes of the alkoxy chains in the MOFs, the
spectral region from 1400–1300 cmꢁ1 was used. After baseline
correction, the spectra were fitted using vibrational bands cor-
responding to kinks + gtg, eg, dg and CH3 conformers using
Gaussian and Lorentzian contributions applying GRAMS
8.0 software. During the fit analysis, the band positions were
restricted to ꢂ2 cmꢁ1 of their initial values given for each MOF.
The integrated areas under the subbands were normalized with
respect to the area of the standard reference band at 1320 cmꢁ1
(C–O stretch). Details of the method and analysis can be found
elsewhere.15 Single crystal X-ray diffraction was performed on an
Oxford Excalibur 2 diffractometer in a nitrogen cold stream
dimethyl-2,3-dihydroxy-1,4-benzenedicarboxylate
(S4),
dimethyl-2,5-dihydroxy-1,4-benzenedicarboxylate (S5), and
dimethyl-2,6-dihydroxy-1,4-benzenedicarboxylate (S6), which
1
were dried in vacuum and characterized by H and 13C NMR
spectroscopy. The particular linkers were synthesized via Wil-
liamson etherification of the compounds S4 to S6. In a typical
reaction S4, S5, or S6 (4.40 mmol) and K2CO3 (20.0 mmol) were
suspended in DMF (35 mL). RX (9.68 mmol, see Table 1 for the
used halides) was added drop wise and the mixture was heated
under stirring to 85 ꢀC for 3 h. Afterwards the solvent was
evaporated under reduced pressure at 60 ꢀC and the residue was
refluxed in H2O (40 mL) with NaOH (10.0 mmol) for 4 h. After
cooling to room temperature the solution was acidified with
aqueous HCl (ꢃ15%) and the precipitate was collected by filtra-
tion, washed with water (20 mL) and dried in vacuo at 80 ꢀC for 16
h (see Table 1 for the obtained yields). 2,3-Bis(2-methoxyethoxy)-
ꢁ
(approx. 107 K) using Mo Ka radiation (l ¼ 0.71073 A). The
1
crystal structures were solved by direct methods using SHELXS-
97 and refined against F2 on all data by full-matrix least squares
with SHELXL-97 (SHELX-97 program package, Sheldrick,
1,4-benzenedicarboxylic acid, H2(2,3-BME-bdc): H NMR (250
MHz, DMSO): d ¼ 7.38 (s, 2H, Ar-H), 4.16–4.09 (m, 4H, OCH2),
3.67–3.61 (m, 4H, OCH2), 3.29 (s, 6H, OCH3) ppm. 13C NMR (63
MHz, DMSO): d ¼ 166.64, 151.34, 130.57, 124.36, 72.87, 70.96,
57.99 ppm. 2,6-Bis(2-methoxyethoxy)-1,4-benzenedicarboxylic
acid, H2(2,6-BME-bdc): 1H NMR (250 MHz, DMSO): d ¼ 7.22
(s, 2H, Ar-H), 4.20–4.11 (m, 4H, OCH2), 3.68–3.60 (m, 4H,
OCH2), 3.30 (s, 6H, OCH3) ppm. 13C NMR (63 MHz, DMSO):
d ¼ 166.62, 165.90, 155.32, 132.73, 118.97, 106.16, 70.20, 68.30,
58.33. 2,5-Dimethoxy-1,4-benzenedicarboxylic acid, H2(DM-
bdc): 1H NMR (250 MHz, DMSO): d ¼ 7.30 (s, 2H, Ar-H), 3.78
(s, 6H, OCH3) ppm. 13C NMR (63 MHz, DMSO): d ¼ 166.68,
151.07, 124.95, 114.41, 56.39 ppm. 2,5-Diethoxy-1,4-benzenedi-
carboxylic acid, H2(DE-bdc): 1H NMR (250 MHz, DMSO): d ¼
7.26 (s, 2H, Ar-H), 4.04 (q, J ¼ 6.9 Hz, 4H, OCH2), 1.29 (t, J ¼
6.9 Hz, 6H, CH3) ppm. 13C NMR (63 MHz, DMSO): d ¼ 166.77,
150.31, 125.61, 115.73, 64.99, 14.64 ppm. 2,5-Dipropoxy-1,4-
benzenedicarboxylic acid, H2(DP-bdc): 1H NMR (250 MHz,
DMSO): d ¼ 7.26 (s, 2H, Ar-H), 3.94 (t, J ¼ 6.3 Hz, 4H, OCH2),
Universitat Gottingen, 1997).16 The data of 2as were treated with
the ‘‘squeeze’’ routine in Platon17 to account for the disordered 2-
methoxyethoxy chains and solvent molecules. Powder X-ray
diffraction (PXRD) patterns were recorded on a Bruker
D8 Advance AXS diffractometer with Cu Ka radiation (l ¼
€
€
ꢁ
€
1.54178 A) and a Gobel mirror in q ꢁ 2q geometry with a posi-
tion-sensitive detector in a 2q range from 5–50ꢀ at a scan speed of
1ꢀ minꢁ1. a-Al2O3 was employed as the external standard. The
powder samples were filled into glass capillaries (diameter ¼
0.7 mm). Each capillary was sealed prior to the measurement.
Thermogravimetric analyses (TGAs) were performed on a Seiko
TG/DTA 6300S11 instrument (sample weight approximately 5–
10 mg) at a heating rate of 5 ꢀC minꢁ1 in a temperature range from
30–600 ꢀC. The measurement was performed at atmospheric
pressure under flowing nitrogen (99.9999%; flow rate ¼ 300 mL
minꢁ1). N2, CO2 and CH4 sorption measurements were per-
formed on outgassed samples (130 ꢀC for minimum 3 h in vacuo)
using a Belsorb-Max from Bel-Japan with optimized protocols
and gases of 99.9995% purity. N2 measurements were performed
at 77 K and 195 K. CO2 and CH4 measurements at 195 K.
1.84–1.60 (m, 4H, CH2), 0.97 (t, J ¼ 7.4 Hz, 6H, CH3) ppm. 13
C
NMR (63 MHz, DMSO): d ¼ 166.78, 150.43, 125.46, 115.54,
89.84, 70.63, 22.08, 10.32 ppm. 2,5-Dibutoxy-1,4-benzenedi-
carboxylic acid, H2(DB-bdc): 1H NMR (250 MHz, DMSO): d ¼
7.26 (s, 2H, Ar-H), 3.97 (t, J ¼ 5.9 Hz, 4H, OCH2), 1.76–1.57 (m,
4H, CH2), 1.43 (dq, J ¼ 14.1, 7.2 Hz, 4H, CH2), 0.91 (t, J ¼
7.3 Hz, 6H, CH3). 13C NMR (63 MHz, DMSO): d ¼ 166.79,
150.41, 125.48, 115.50, 68.86, 30.77, 18.57, 13.63 ppm.
Linker synthesis
The linkers were synthesized from three different starting
compounds (Scheme 1), namely 2,3-dihydroxy-1,4-benzenedi-
carboxylic acid (S1), 2,5-dihydroxy-1,4-benzenedicarboxylic acid
(S2), and 2,6-dihydroxy-1,4-benzenedicarboxylic acid (S3). S2 is
commercially available, whereas S1 and S3 were synthesized in
analogy to published procedures.18,19 Accordingly, the starting
compounds S1 to S3 were esterificated in MeOH with BF3$Et2O
as catalyst and water scavenger, to yield the dimethyl esters,
MOF synthesis
H2L (L ¼ 2,3-BME-bdc, 2,6-BME-bdc, DM-bdc, DE-bdc, DP-
bdc, DB-bdc; 0.47 mmol), Zn(NO3)2$6H2O (0.141 g, 0.47 mmol),
Table 1 Compilation of the halides (RX) used in the linker synthesis.
The obtained yields are given in brackets
Linker (yield)
RX
2,3-BME-bdc (78%)
2,6-BME-bdc (65%)
DM-bdc (91%)
DE-bdc (85%)
DP-bdc (89%)
DB-bdc (84%)
MeO–C2H4–Br
MeO–C2H4–Br
MeI
EtBr
PrBr
BuBr
Scheme 1 Reaction sequence for the synthesis of the functionalized
linkers H2L. The utilized halides RX are listed in Table 1.
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and 4,40-bipyridine (0.074 g, 0.47 mmol) were dissolved in DMF
(20 mL). EtOH (10 mL) was added and the reaction mixture was
transferred into a screw jar, sealed and heated to 85 ꢀC for 48 h to
yield colourless crystals of 2as and 4as or colourless microcrys-
tals of 3as, 5as, 6as, and 7as. After characterisation of the as
synthesized compounds the mother liquor was decanted and the
crystals were washed three times with DMF and then stirred for
3 days in chloroform (50 mL). The CHCl3 was exchanged every
24 h against fresh solvent. Afterwards the pulverized material
was collected by filtration and dried under reduced pressure
(ꢃ10ꢁ3 mbar) for 24 h at 130 ꢀC to achieve the dried MOFs
(2dry–7dry), which were stored under inert gas atmosphere in
a glovebox. Elemental analyses data (in weight%): [Zn2(2,3-
BME-bdc)2(bipy)]n: C, 50.87 (50.07 theo); H, 4.78 (4.42 theo); N,
3.01 (3.07 theo); Zn, 14.02 (14.35 theo). [Zn2(2,6-BME-
bdc)2(bipy)]n: C, 50.47 (50.07 theo); H, 5.02 (4.42 theo); N, 3.31
(3.07 theo); Zn, 13.98 (14.35 theo). [Zn2(DM-bdc)2(bipy)]n: C,
48.69 (49.00 theo); H, 3.56 (3.29 theo); N, 4.01 (3.81 theo);
Zn, 17.83 (17.79 theo). [Zn2(DE-bdc)2(bipy)]n: C, 51.81
(51.60 theo); H, 4.43 (4.08 theo); N, 3.92 (3.54 theo); Zn, 16.14
(16.53 theo). [Zn2(DP-bdc)2(bipy)]n: C, 54.03 (53.85 theo); H,
5.00 (4.76 theo); N, 3.39 (3.31 theo); Zn, 15.19 (15.43 theo).
[Zn2(DB-bdc)2(bipy)]n: C, 55.69 (55.83 theo); H, 5.60 (5.35 theo);
N, 3.05 (3.10 theo); Zn, 14.22 (14.47 theo).
Results and discussion
Synthesis and structural characterisation
Scheme 2 Representation of the linkers L utilized in the [Zn2L2(bipy)]n
MOFs 1–7.
Analogous reactions of Zn(NO3)2$6H2O with equimolar
amounts of H2L (L ¼ 2,3-BME-bdc, 2,6-BME-bdc, DM-bdc,
DE-bdc, DP-bdc, or DB-bdc) and bipy in DMF/EtOH solution
(DMF ¼ N,N-dimethylformamide) at 85 ꢀC yield six novel
MOFs of the composition [Zn2L2(bipy)]n (Schemes 2 and 3). The
as synthesized compounds [Zn2(2,3-BME-bdc)2(bipy)]n (2as) and
[Zn2(DM-bdc)2(bipy)]n (4as) form block like crystals, which were
analysed via single crystal X-ray diffraction,† whereas the other
four compounds (3as, 5as, 6as, and 7as; L ¼ 2,6-BME-bdc, DE-
bdc, DP-bdc, or DB-bdc) give small intergrown microcrystals,
only suitable for powder X-ray diffraction (PXRD).
space to host a second framework. This has already been
reported for other bulky linkers like 2,3,5,6-tetramethyl-1,4-
benzenedicarboxylate.10d Because of the additional substituents,
the carboxylate groups and the phenyl ring are no longer forced
in a coplanar arrangement. Therefore, the phenyl ring can be in
several positions. In the crystal structure of 2as the 2,3-BME-bdc
linkers connect the zinc paddlewheel in a bent and not a strictly
linear fashion (Fig. 2b). Due to the bending the linker is disor-
dered over two positions by the space group symmetry (see
Fig. S2†). In addition the phenyl rings are disordered over two
more positions. Due to this severe disorder the flexible 2-
methoxyethoxy substituents could not be localized in the electron
After characterization by X-ray diffraction techniques, the as
synthesized compounds were washed with fresh DMF and
subsequently stirred in CHCl3. Following, heating (130 ꢀC) of the
samples in dynamic vacuum (ꢃ10ꢁ3 mbar) gives the dried (acti-
vated) compounds [Zn2L2(bipy)]n (2dry–7dry), which
were characterized by powder X-ray diffraction (PXRD),
thermogravimetric analysis (TGA), as well as solid state 13C MAS
NMR and IR spectroscopy. In addition, liquid state 1H and
13C NMR spectra of digested MOF samples in DCl/D2O/[D6]
DMSO were recorded, to prove the expected chemical composi-
tion. The dried MOFs were stored in a glovebox (Ar atmosphere)
to avoid contact with water, because zinc-carboxylate based
MOFs are in general quite sensitive to moisture.13,20
1
density map. However, 13C MAS NMR of 2dry as well as H
NMR and 13C APT NMR of digested samples of 2dry clearly
prove the presence of the substituents (Fig. S13 and S19†).
The phase purity of the bulk compound 2as was analysed by
PXRD (Fig. 3). The powder pattern features two rather weak
reflections at 2q ¼ 6.67ꢀ and 2q ¼ 9.38ꢀ, which can neither be
indexed in the tetragonal unit cell derived from the single-crystal
diffraction data nor in the trigonal unit cell of the honeycomb-like
derivative 1as. These reflections can be either dedicated to
a minority phase, which arises from partial drying of the
compound upon sample preparation (grinding of the crystals in
air), or to the general presence of a second independent phase of
unknown structure. Interestingly, the PXRD data of the activated,
solvent-free compound 2dry are mostly X-ray amorphous but
show two weak reflections at 2q ¼ 6.67ꢀ and 2q ¼ 9.38ꢀ (Fig. S5†).
Surprisingly, 2as does not crystallize in the expected honey-
comb-like, trigonal structure. The single crystal structure reveals
a tetragonal framework (space group P4/mmm) with a paddle-
wheel-based pillared square-grid network (Fig. 2). Interestingly,
the framework is non-interpenetrated due to the bulkiness of the
2-methoxyethoxy substituents, which do not leave enough void
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Scheme 3 Synthesis of [Zn2L2(bipy)]n MOFs. 2,3-BME-bdc triggers the formation of a non-interpenetrated pillared square-grid network (2) and DM-
bdc prompts the formation of an interpenetrated pillared square-grid network (4). The other five linkers 2,5-BME-bdc, 2,6-BME-bdc, DE-bdc, DP-bdc,
and DB-bdc generate [Zn2L2(bipy)]n MOFs featuring a honeycomb-like network topology (1, 3, 5, 6, 7).
Fig. 3 Compilation of PXRD patterns: calculated diffraction pattern
from the single crystal structure of 2as (a), calculated pattern from the
single crystal structure of 1as13 (c), measured patterns of 2as (b), and 3as
(d). The reflections, which are marked with asterisks in (b), can be
dedicated to a second minority phase of 2, which is dedicated to partial
drying of the material (see the ESI† for further details).
The changes in the powder patterns from 2as to 2dry suggest
that the framework contracts and even decomposes when the
solvent guests are evacuated. Note that we have already reported
similar behaviours for other 2-methoxyethoxy functionalized
MOFs.14 The collapse of 2 upon activation seems to be irre-
versible, since renewed loading of 2dry with DMF over the gas
phase does not recover the initial crystalline structure of 2as, but
a different crystalline material (Fig. S5†). We suppose that the
Fig. 2 Structure of [Zn2(2,3-BME-bdc)2(bipy)]n (2as) as determined by
single crystal X-ray diffraction: view of a cavity of the pillared square-
grid network featuring a paddlewheel building unit (a), view of a fraction
of the bent square-grid in the [001] direction (b), and packing diagram of
the non-interpenetrated network along the [100] direction (c). Carbon,
nitrogen, oxygen and zinc atoms are shown in dark grey, blue, red, and
yellow, respectively. The coordination polyhedra around the zinc centres
are shown in yellow. The severely disordered moieties of the linker 2,3-
BME-bdc and bipy are not shown in (a) and (b). Flexible 2-methoxy-
ethoxy groups, hydrogen atoms and guest molecules have not been
refined due to the high disorder of the organic moieties and are not
included in the structural model.
instability of 2 is reasoned in the large void volume of the non-
interpenetrated network (58% void volume as analysed by Pla-
ton17) and the high framework flexibility, due to the attached
2-methoxyethoxy side chains.14 However, a detailed analysis of
the structural transitions occurring in 2 in dependence of the
presence/absence of guest molecules turned out to be very
complex and is beyond the scope of this work.
In comparison, compound 3as features a honeycomb-like
structure analogous to the parent framework 1as, as determined
by PXRD (Fig. 3). Indexation of the powder pattern in the
ꢀ
trigonal space group R3c followed by a refinement of the unit cell
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parameters has proven phase purity (see Table S3†). Addition-
ally, IR spectroscopy of 1dry, 2dry, and 3dry manifests the
analogy of 1 and 3, whereas 2 gives a very different IR spectrum
due to the different binding modes of the carboxylates in the
honeycomb-like and in the pillared square-grid structure (Fig. 4).
Therefore, the substitution pattern of the BME-bdc linkers has
a major impact on the topology of the derived networks. MOFs
1, 2, and 3 are constructed from three different constitutional
isomers of BME-bdc, which just differ in their substitution
pattern.
tetragonal structure, if BME-bdc-type linkers are used. The
tetragonal isomer 2 has a much lower density (rXRD ¼ 0.935 g
cmꢁ3) compared to the trigonal isomers 1 and 3 (rXRD ¼ 1.249 g
cmꢁ3). Therefore, the void volume is much greater in the
tetragonal topology. This higher void space may also be a reason
for the collapse of 2 upon drying, while 1 and 3 retain their highly
crystalline structure even after solvent exchange and drying at
130 ꢀC in vacuo (Fig. S6†). Another big difference between both
framework types is that the pillared layer structures exhibit
intrinsic structural dynamics, due to the flexibility of the pad-
dlewheel building unit,10,11,14 whereas the honeycomb-like
networks are structurally very rigid, because they are constructed
from an infinite and inflexible one-dimensional building block.
The higher stability of the honeycomb-like isomer is also evident
from thermogravimetric analysis (TGA) data of the activated
compounds, which show that the widely amorphous compound
2dry starts to decompose at 200 ꢀC, whereas the crystalline
comꢀpounds 1dry and 3dry show no significant weight loss up to
300 C (Fig. S11†).
To the best of our knowledge, this is the first example of
topological MOF isomers, which present the same sum formula
and have been prepared under exactly the same conditions, but
form different network structures.
As already mentioned, the enhanced conformational freedom
of the carboxylate groups of the functionalized linkers is essential
to trigger the formation of the honeycomb-like network. The
carboxylate groups of 2,3-BME-bdc and 2,5-BME-bdc should in
principle possess similar conformational freedom. However, in
2,3-BME-bdc both substituents are located at the same side of
the phenyl ring and would therefore point in the same channel of
the honeycomb-like structure. This might lead to an over-
crowding when 2,3-BME-bdc is utilized. Consequently,
compound 2 forms the default pillared square-grid framework.
Compound 3, which utilizes 2,6-BME-bdc as linker, forms the
honeycomb-like structure, although the substituents are both in
ortho-position to one of the carboxylate groups. The substitution
pattern suggests that only this carboxylate group possesses the
enhanced conformational freedom, whereas the other carboxy-
late group located at position 4 is deemed to be coplanar to the
phenyl ring. However, due to the lack of single crystal diffraction
data a further discussion of the conformation of 2,6-BME-bdc in
3 cannot be provided.
Aside from the studies on the impact of the substitution
pattern of BME-bdc on the framework structure of [Zn2L2(bi-
py)]n networks, we have analysed the impact of the substituent
chain length for 2,5-substituted linkers. Therefore, the four
differently functionalized linkers DM-bdc, DE-bdc, DP-bdc and
DB-bdc (Scheme 1) were utilized together with zinc nitrate and
bipy in the preparation of MOFs 4as, 5as, 6as, and 7as,
respectively.
Interestingly, compound 4as crystallizes in the monoclinic
space group C2/m. Similar to 2as, the network of 4as is composed
of two-dimensional square grids of [Zn2(DM-bdc)2]n units, which
are pillared by bipy coligands (Fig. 5). Noteworthy, the aromatic
rings of DM-bdc and bipy as well as the methoxy groups are
disordered over two positions with 50% occupancy each (see
ESI†). In analogy to [Zn2(bdc)2(bipy)]n the framework of 4 is
doubly interpenetrated (Fig. 5 and S4†). PXRD studies of
ground bulk samples of the as synthesized material 4as harvested
from the DMF/EtOH mother liquor, as well as the dried material
4dry, show nearly identical patterns (Fig. S7†). Note that the
We claim the trigonal honeycomb-like framework is thermo-
dynamically more favoured, than the non-interpenetrated
Fig. 5 Structure of [Zn2(DM-bdc)2(bipy)]n (4as) as determined by single
crystal X-ray diffraction: view of a cavity of the pillared square-grid
network featuring a paddlewheel building unit (a), packing diagram of
the two-fold interpenetrated network along the [100] direction (b).
Carbon, nitrogen, and oxygen atoms are shown in dark grey, blue and
red, respectively. The coordination polyhedra around the zinc centres are
shown in yellow. The second interpenetrating framework as well as
disordered moieties of the linker DM-bdc and the pillar bipy are not
shown in (a). Hydrogen atoms are omitted for the sake of clarity.
Fig. 4 ATR FT-IR spectroscopic data of dried 1dry (a, taken from
ref. 13), 2dry (b), and 3dry (c) displayed in the region from 800 to
1800 cmꢁ1
.
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parent framework [Zn2(bdc)2(bipy)]n shows a structural transi-
tion (shrinkage of the unit cell volume) upon evaporation of
guest molecules, which is leading to distinct changes of its PXRD
pattern.10b–d However, 4 is structurally rigid and shows no
framework flexibility. This can be attributed to a dense packing
of the two interpenetrating nets of 4, due to the higher steric
demand of DM-bdc compared to the conventional bdc linker,
leaving no accessible void volume. This was further validated by
sorption experiments (N2 at 77 K and CO2 at 195 K), which show
no gas uptake of 4dry until saturation (data not shown).
Therefore, 4as and 4dry feature the same chemical composition
of [Zn2(DM-bdc)2(bipy)]n.
Unfortunately, the compounds 5, 6, and 7 could only be
obtained as small and intergrown crystals, which were not suit-
able for single crystal X-ray diffraction. However, indexation of
the corresponding PXRD patterns and refinement of the unit cell
parameters clearly show that these frameworks are isostructural
and isoreticular to 1 and 3 (Fig. 6 and Table S3†). This is further
validated by the similarity of the carboxylate stretching vibra-
tions in the IR spectra of the activated compounds (Fig. 7, S26,
and S28†).
Fig. 7 ATR FT-IR spectroscopic data of 4dry (a), 5dry (b), 6dry (c), and
7dry (d) displayed in the region from 800–1800 cmꢁ1
.
Solid-state 13C MAS NMR spectra of the honeycomb-like
compounds 3dry, 5dry, 6dry, and 7dry feature two different
signals for the carboxylate groups of the substituted linkers
(Fig. S14 and S16–S18†). This can be dedicated to the presence of
two distinct kinds of carboxylate groups in the honeycomb-like
MOFs, namely carboxylates, which are bridging two zinc centres
in a syn–anti fashion, and carboxylates, which are chelating just
one zinc atom (Fig. 1). As expected 13C MAS NMR spectra of
2dry and 4dry do only feature one signal for the carboxylate
groups (Fig. S13 and S15†).
linkers DE-bdc, DP-bdc, and DB-bdc would allow the formation
of a pillared square-grid framework, these MOFs would be non-
interpenetrated due to the bulkiness of the linkers. As seen for
compound 2, the non-interpenetrated frameworks are not so
favourable because of the much higher void volume. Therefore,
the bulky substituents trigger the formation of the honeycomb-
like topology, which allows energetically favourable p interac-
tions of the aromatic moieties as well as van der Waals interac-
tions of the flexible substituents, and features a smaller void
space per formula unit, compared to the non-interpenetrated
pillared square-grid topology.
These results indicate that simple methoxy substituents do not
have enough bulkiness to trigger the formation of the honey-
comb-like topology, whereas the longer ethoxy, n-propoxy, and
n-butoxy substituents yield structures isoreticular to 1. If the
To get more information on the conformation of the flexible
alkoxy chains in the honeycomb-like channel structures 5dry,
6dry, and 7dry a detailed IR investigation has been conducted.
Careful examination of the C–H stretching region between 3000
and 2800 cmꢁ1 by band deconvolution suggests existence of
several overlapping bands which can be assigned to CH2 and
CH3 groups (Fig. S26†). The variation of the symmetric and
asymmetric n(CH3) and n(CH2) bands with increasing chain
length of the linker molecule suggests different conformations of
the groups inside the pore. It is worth noting that these vibrations
are observed in the dried and activated materials, and are
expected to change markedly upon interaction with solvent and
guest molecules. The spectral range from 1400–1300 cmꢁ1 covers
the conformation sensitive wagging modes of the CH2 groups
and the bands present there are due to various forms of gauche
defects in the flexible alkoxy chains. The wagging modes of
interest appear at 1363/1366 cmꢁ1, 1341/1342 cmꢁ1, 1349/
1350 cmꢁ1 and 1371/1378 cmꢁ1 arising from the gauche-trans-
gauche (kinks + gtg), end gauche (eg), double gauche (dg) and the
methyl group ‘‘umbrella’’ deformation mode, respectively (see
Fig. S27† for an illustration of the wagging modes). The gtg and
kinks are taken together as a single class, since they cannot be
spectroscopically distinguished from their CH2 wagging modes.
The band at 1319–1320 cmꢁ1 arises from the C–O stretch of the
linker and, due to its constant intensity, serves as the standard
Fig. 6 Compilation of PXRD patterns: calculated diffraction pattern
from the single crystal structure of 4as (a), measured pattern of 4as (b),
calculated pattern from the single crystal structure of 1as13 (c), measured
patterns of 5as (d), 6as (e), and 7as (f).
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reference against which all the other wagging modes were ratioed
to reveal the relative changes regarding the conformation of
the flexible alkoxy chains in the different MOFs. Fig. 8 shows the
conformation sensitive wagging region for 6dry and 7dry. The
data of 5dry were not used for the analysis here due to its short
alkyl chain leading to a low signal to noise ratio, and the lack of
possibility to form a multitude of different conformers.
high population of kink + gtg and double gauche conformers even
in the solid state. The number of end gauche conformers does not
vary markedly for these two MOFs as only two per chain are
possible. It is interesting that even in confined geometry, 7dry
(longer alkoxy chain) exhibits a higher amount of disordered
conformers than 6dry (shorter alkoxy chain). This clearly
suggests that the confined space is still favorable for the flexible
substituents to exist in a multitude of conformations which
would markedly increase without the confinement.
For 6dry the spectrum was decomposed by a curve fitting
procedure and four wagging bands centered at 1341 cmꢁ1 (eg),
1350 cmꢁ1 (dg), 1363 cmꢁ1 (kinks + gtg) and 1378 cmꢁ1 (umbrella
mode) were obtained. The band areas of these modes were then
ratioed to the band area of the C–O stretch for calculating the
relative populations of different conformers. A similar protocol
was applied for 7dry, here the bands are observed at 1342 cmꢁ1
(eg), 1349 cmꢁ1 (dg), 1366 cmꢁ1 (kinks + gtg), and 1371 cmꢁ1
(umbrella mode). Comparison of the normalized areas of the
different subbands for these two MOFs leads to the following
conclusions: 7dry has on average (a) 3 times more double gauche
conformers in its chains relative to 6dry, (b) ca. 6 times more
kinks + gtg conformers, and (c) the population of end gauche
conformers is only about 20% higher for 7dry. The contribution
of the umbrella deformation mode is, as expected, essentially
constant within the accuracy of the analysis. These results clearly
state that the MOF with the longest flexible chain has a relatively
In future work we will concentrate on the IR spectroscopic
analysis of such honeycomb-like MOFs, which are loaded with
(solvent) guest molecules. In combination with the analysis of the
wagging CH2 mode region (1300–1400 cmꢁ1), the conforma-
tional properties of the alkoxy chain in solvent-filled pores will be
accessible with residue specific resolution. Under these condi-
tions, the population of disordered conformers is expected to
increase. In addition, FT-IR spectroscopy will allow revealing
also the conformation of the molecules embedded in the confined
geometry of the MOFs.
Sorption properties of the honeycomb-like MOFs
The prototypic compound 1 shows beneficial sorption charac-
teristics. 1dry does not adsorb any N2 at 77 K due to the gating of
the pores with the polar and flexible 2-methoxyethoxy substitu-
ents. However, CO2 can easily penetrate through the polar
groups of 1dry at 195 K. This leads to very high sorption selec-
tivity for CO2 compared to N2 and CH4 in this material (S(CO2/
N2) ¼ 84 : 1 and S(CO2/CH4) ¼ 9 : 1 at 273 K estimated from the
slopes of the isotherms in the Henry region).13
Here we have analysed the sorption properties of the iso-
structural compounds 5dry and 7dry. These MOFs feature only
aliphatic substituents, without polar methoxy head groups,
which are gating the pores. In comparison to 1dry, N2 sorption
isotherms conducted at 77 K show that 5dry and 7dry adsorb
very small, but significant amounts of nitrogen (Fig. 9) leading to
rather low specific surface areas of 133 m2 gꢁ1 (5dry) and 35 m2
gꢁ1 (7dry) according to the BET model. The sorption kinetic for
N2 is very slow and a large hysteresis is visible on the desorption
branches of both isotherms. The low adsorption capacities for N2
again point out that the pores of the honeycomb-like frameworks
Fig. 9 Comparison of N2 sorption isotherms for 5dry (squares) and 7dry
(triangles) recorded at 77 K. The adsorption and desorption branches are
shown by solid and open symbols, respectively.
Fig. 8 FT-IR spectra of 6dry and 7dry for the wavenumber region
between 1300 cmꢁ1 and 1400 cmꢁ1 showing the conformation sensitive
CH2 wagging bands.
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are blocked by the flexible substituents at cryogenic
temperatures.
pressures (p < 7 kPa) compared to 5dry and 7dry as evident by
the steepest slope of the isotherm at low pressures. This can be
attributed to the polar pore surface and the narrow confinement
of 1dry. However, as a matter of course at pressures greater than
7 kPa 5dry adsorbs higher amounts of CO2, because of its higher
void volume. These results again reveal the beneficial impact of
our concept to implement flexible and polar alkyl ether groups to
tune the sorption selectivity of MOFs.21
A comparison of the N2 isotherms recorded at 195 K yields no
significant difference between the MOFs 1dry, 5dry, and 7dry
(Fig. 10). The CH4 isotherms recorded at the same temperature
show that 5dry adsorbs a higher amount of CH4, which is
expected due to the lower density, higher void volume, and
higher specific surface area of 5dry compared to 1dry and 7dry. A
similar trend can be seen in the CO2 sorption isotherms. 5dry can
adsorb approx. 208 cm3 gꢁ1 (STP) CO2 at saturation, whereas
1dry and 7dry adsorb 156 cm3 gꢁ1 (STP) and 128 cm3 gꢁ1 (STP),
respectively. Noteworthy, 1dry and 7dry have nearly similar
molecular weight (M1dry ¼ 911.51 g molꢁ1; M7dry ¼ 903.61 g
molꢁ1), substituent chain length, density and accessible void
volume. The fact that 1dry adsorbs a higher amount of CO2 than
7dry can therefore be attributed to the polar 2-methoxyethoxy
groups of 1dry, which integrate a weakly Lewis basic pore
environment that can interact stronger with the CO2 molecules
leading to a higher sorption selectivity for CO2. A detailed
examination of the CO2 sorption isotherms (Fig. 10, bottom)
indicates that 1dry has the highest affinity for CO2 at low
Conclusions
The targeted synthesis of specific framework architectures with
attractive properties for an intended application is a major
challenge for the development of new porous hybrid materials.
With this systematic study we were able to sheet some light on the
principles of the substitution dependent topological isomerism
within the [Zn2L2(bipy)]n family of MOFs. Implementation of
additional flexible side chains at the common linker 1,4-benze-
nedicarboxylate allows the construction of a novel honeycomb-
like network family, which features interesting sorption
properties. Remarkably, the substitution pattern of the bdc-type
linker plays an important role for the derived framework
topology. 2,3-Disubstitution of the bdc linker yields the forma-
tion of the well-known tetragonal pillared square-grid structure,
whereas 2,5- and 2,6-disubstitution generates the novel honey-
comb-like topology if 2-methoxyethoxy substituents are used.
Moreover, the chain length and bulkiness of the substituents
have a great impact on the MOF topology. Small methoxy
substituents do not have enough volume to force the framework
in the honeycomb-like topology and give the interpenetrated
pillared square-grid topology, whereas the more bulky ethoxy,
n-propoxy, and n-butoxy substituents yield the honeycomb-like
networks. These observations and effects are also suggested to be
conceptually important to better understand solvent and tem-
plating effects in solvothermal MOF synthesis. Interestingly, our
case study involves ‘‘solvent-like’’ molecules which are pinned at
the linkers and therefore the weak forces between these groups
and the more mobile solvent on the one side and interactions
with the more rigid framework backbone on the other are likely
to directly influence the crystallisation and phase selectivity as
discussed above.
The sorption data of the honeycomb-like MOFs show that the
use of methoxy head groups at the substituents increases the
uptake and the sorption selectivity for CO2 compared to simple
non-polar aliphatic substituents. We hope that these results
stimulate further research in the direction of substitution
controlled MOF design and the development of novel framework
topologies, which are inaccessible with conventional non-func-
tionalized linkers.
Acknowledgements
S.H. gratefully thanks the Ruhr University Research School for
a fellowship.
Fig. 10 Comparison of N2 (blue), CH4 (green), and CO2 (red) sorption
isotherms recorded at 195 K: isotherms for 1dry (circles), 5dry (squares),
and 7dry (triangles) plotted with a linear pressure axis (top) and a loga-
rithmic pressure axis (bottom). The adsorption and desorption branches
are shown with solid and open symbols, respectively. The sorption data
of 1dry were taken from ref. 13.
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