Flexibility in metal-organic frameworks derived from positional and electronic effects of functional groups

Article abstract of DOI:10.1039/c7ce00971b

The position of identical functional groups and the subsequent electron density of structural benzene rings in a zinc-based metal-organic framework (MOF) have been controlled to reveal flexibility (or breathing behavior) differences. Both ortho- and para-positioned bi-functional benzene-1,4-dicarboxylic acid (BDC) ligands were synthesized with amino-, chloro-, methoxy-, and nitro groups. Additionally, two tri-functionalized, dimethoxy-amino and dimethoxy-nitro BDCs were prepared. All bi- and tri-functionalized BDCs were successfully incorporated into DABCO MOFs (DMOFs), except two diamino BDCs which were insoluble and thermally unstable. Among the eight bi-/tri-functionalized DMOFs, only para-dimethoxy exhibited flexibility in its framework after evacuation in preparation for N₂ isotherm measurement. Since the dimethoxy combination has the most electron-rich environment in the benzene ring of the BDC in this series, this indicates that electron density plays a role in the flexibility changes of identically bi-functionalized DMOFs. However, the electron density alone could not fully explain the flexibility changes suggesting that the position of the functional groups is also important. These findings have been corroborated through the synthesis of two tri-functionalized DMOFs with identical functional group locations but opposite electronic environments.

Full text of DOI:10.1039/c7ce00971b

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ARTICLE
Flexibility in Metal-Organic Frameworks Derived from Positional
and Electronic Effects of Functional Groups
Hyeonbin Ha,‡a Hyungwoo Hahm,‡a Dong Gyun Jwa,a Kwangho Yoo,a Myung Hwan Park,b
Minyoung Yoon,c Youngjo Kim*a and Min Kim*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The position of identical functional groups and the subsequent electronic density of the structural benzene rings in a zinc-
based metal-organic framework (MOF) have been controlled to reveal flexibility (or breathing behavior) differences. Both
ortho- and para-positioned bi-functional benzene-1,4-dicarboxylic acids (BDCs) ligands were synthesized with amino-,
chloro-, methoxy-, and nitro groups. Additionally, two tri-functionalized, dimethoxy-amino and dimethoxy-nitro BDCs were
prepared. All bi- and tri-functionalized BDCs were successfully incorporated into dabco MOFs (DMOFs), except two
diamino BDCs which were insoluble and thermally unstable. Among the eight bi-/tri-functionalized DMOFs, only para-
dimethoxy exhibited flexibility in it’s framework after evacuation in preparation for a N2 isotherm measurement. Since the
dimethoxy combination has the most electron-rich environment in the benzene ring of the BDC in this series, this indicates
that the electron density plays a role in the flexibility changes of identically bi-functionalized DMOFs. However, the
electron density alone could not fully explain the flexibility changes suggesting that the position of the functional groups is
also important. These findings have been corroborated through the synthesis of two tri-functionalized DMOFs with
identical functional group locations but opposite electronic environments.
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external stimuli such as evacuation, guest molecule adsorption
and chemical transformations. For example, CO2 can increase
the pore size through strong intermolecular forces whereas N2
Introduction
Metal-organic frameworks (MOFs) are hybrid porous materials
consisting of metal clusters (or ions) and organic linkers. Three
dimensional pores are constructed within MOFs by repeating
coordination bonds between these metal clusters (i.e.,
secondary building unit, SBU) and various binding groups on
the organic ligands (e.g., carboxylic acid and pyridine).1 The
major advantage of MOFs comes from their high porosity and
chemical tunability. The relative ease of functionalization and
structural tuning can rapidly create libraries of various sets of
MOFs thus creating an ideal platform for evaluating and
maximizing various applications such as molecular storage,
separation and catalysis.2 The functional groups installed onto
the organic linkers of MOFs can be tuned for a particular
application, but also, the functional group may alter structural
features of MOFs.
or CH4 may not induce the same pore-opening effect under 1
atm (Fig. 1).5-7
Guest Molecule
R
R
Guest Molecule
R
R
The change of MOF porosity in flexible system
1. Guest molecule uptake or removal
2. Chemical transformation of R
3. Intramolecular interaction between R-R
Guest Molecule
R
R
R
R
Although many MOFs have rigid structures and permanent
porosity, sometimes they exhibit flexibility in their structures
and pore sizes (i.e., gate-opening or breathing behavior of
MOFs).3,4 In flexible MOFs, the porosity of MOFs change by
R
R
R
R
The Present Study
- The position of fuctional group and e-density of ligand
play a significant role for flexibility changes
- Only para-dimethoxy (R = OMe) exhibited flexible framework
among amino, chloro, and methoxy functionalized DMOFs
Fig. 1 Flexibility changes by external stimuli and ligand controls in DMOFs.
This structural flexibility is a unique feature that MOFs have in
comparison to other porous materials (e.g., zeolite,
mesoporous silica, activated carbon, etc). It has been realized
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that the functional group on the organic linker plays a functional groups have been installed onto the benzene ring to
significant role in these flexibility changes MOFs. The modulate the electronic environment. Using this series of bi-
intramolecular interactions, such as hydrogen bonding functional and tri-functional ligands, their flexibilities changes
between functional groups and guest molecules, are highly in a MOF structure were rigorously studied.
related to potential structural transformations and flexibility.8,9
In the case of dialkoxy-functionalized DMOFs (DMOF = dabco
Results and discussion
MOF) by Fischer and co-workers, the length of the side chain,
polarity, and degree of saturation of the alkyl group affected
the flexibility capability of the MOFs (Fig. 1).10,11 From the
above cases, it was hypothesied that the role of the functional
group is derived from the potential intermolecular
interactions. However, it was also reported that not only the
intermolecular interactions, but also intramolecular changes
(e.g., electron-donating or electron-withdrawing effects of
functional groups) could affect the framework.
Positional and Electronic Controls in Bi-functionalized DMOFs
First, the synthesis of ortho-(2,3-) and para-(2,5-) di-
substituted BDCs were attempted with amino, chloro,
methoxy, and nitro groups. The dichloro BDCs (BDC-2,3-Cl2 and
BDC-2,5-Cl2) were obtained from Sandmeyer reactions from
their respective 2,3- and 2,5-NH2Cl BDC starting ligands in
methyl ester form. The BDCE-2,3-NH2Cl and BDCE-2,5-NH2Cl
(BDCE = benzene-1,4-dicarboxylic methyl ester) were prepared
by chlorination of BDCE-NH2, and separated by silica gel
column chromatography (Scheme 2 and ESI for details†).12
Next, the dimethoxy substituted BDCs were obtained by
methylation of 2,3-dihydroxy BDC or 2,5-dihydroxy BDC ligand
after esterification. The hydrolysis of the methyl ester gave the
two regioisomeric dimethoxy substituted isomers suitable for
MOF synthesis (Scheme 2 and ESI for details†). Previously, the
regioisomeric pairs featuring alkoxy groups were reported by
Fischer et al.10,11 By increasing the alkyl chain length, the
flexibility was controlled, and it was revealed that the steric
bulk between substituents across the pore (or frameworks)
plays a key role in flexibility differences. It was initially
proposed that the intermolecular interaction between two
struts in a MOF pore was important to understand the
properties of MOFs and their synthesis. Herein, we propose
that the electronic environment of the strut is also a key factor
in not only the flexibility of but also in MOF properties as a
whole.
Recently, the relationship between ‘regioisomeric controls of
two different functional groups’ and ‘the flexibility of MOFs’
has been revealed.12,13 While the combination of NH2-halogen
and NH2-OMe produced flexible DMOFs frameworks with
para-positioning of the two functional groups, the other
combinations such as NH2-NO2, NO2-OMe, NO2-Cl, and OMe-Cl
formed rigid/inflexible frameworks in relative para-positions
(Scheme 1). On the other hand, the ortho-isomers, for all
combinations (6 examples), showed rigid frameworks after
evacuation preparation for gas adsorption. The flexibility
changes were verified via N2 adsorption after evacuation and
powder X-ray diffraction (PXRD) experiments. Even though all
as-synthesized bi-functionalized DMOFs have very similar
PXRD patterns, the flexible DMOFs (e.g., DMOF-2,5-NH2Cl and
DMOF-2,5-NH2OMe) absorbed a relatively small amount of N2
in comparison to the inflexible DMOFs (e.g., DMOF-2,3-NH2Cl
and DMOF-2,3-NH2OMe). However, the PXRD pattern for the
flexible, para-positioned DMOF-2,5-NH2Cl changed after
evacuation, whereas the inflexible ortho-positioned DMOF-
2,3-NH2Cl showed no changes after evacuation.12,13
Lastly, the dinitro BDCs and diamino BDCs were obtained from
nitration of NH2-BDC and the subsequent oxidation or
reduction. The nitration was performed with the acetyl
protected ester from, BDCE-NHAc, and, following column
chromatography, gave one major isomer, BDCE-2,5-NHAcNO2.
The amino group on BDCE-2,5-NHAcNO2 was successfully
oxidized after a H2O2 treatment, and BDC-2,5-(NO2)2 was
obtained by hydrolysis of the ester in basic conditions.
Separately, BDC-2,3-(NO2)2 was prepared through double
nitration and oxidation from p-xylene. (Scheme 2 and see ESI
Scheme 1 Flexibility changes by two different functional groups and their positions in for details†). BDC-2,5-(NH2)2 was purchased from commercial
zinc-based DMOFs.
sources and BDC-2,3-(NH2)2 was obtained by reduction of BDC-
2,3-(NO2)2 in ester form (see ESI for details†).
The differing electronic environments due to the different
substituents were proposed as a major factor for the flexibility
changes. The combination of NH2-halogen and NH2-OMe
create electron-rich environments in the benzene ring of the
ligand because the amino group is strongly electron-donating.
The other combinations, in comparison, are relatively electron-
deficient.13 However, the series of regioisomeric-electronic
control ligands was somewhat incomplete. In this contribution,
two identical organic functional groups were controlled for
their regioisomeric positions on the benzene ring and three
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functionalized DMOF-2,5-NO2 were previously reported by
Uemura et al.,15 but this is the first example of ortho-dinitro-
functionalized MOFs. Finally, the diamino functionalized BDCs,
BDC-2,3-(NH2)2 and BDC-2,5-(NH2)2, were attempted in DMOF
syntheses but we could not obtain crystalline DMOF materials
from either ligand. We found two possible reasons for the
unsuccessful DMOF synthesis. First, both diamino-
functionalized BDC ligands were insoluble in polar aprotic
solvents (mainly in DMF). We speculate that the two
intramolecular hydrogen bonds, between the ortho-positioned
carboxylic acid-amino groups, contribute to this insolubility.
Secondly, these ligands are very thermally unstable. Even with
low concentrations of BDC-2,3-(NH2)2 or BDC-2,5-(NH2)2 in a
DMF solution, while originally is bright yellow, it very easily
CO2H
CO2Me
CO Me
2
NaNO2
CuCl
Cl
Cl
H2N
KOH
Cl
CO2H
Cl
CO2Me
CO2Me
NH2
Cl
CO2Me
BDC-2,5-Cl2
NCS
CO2H
Cl
CO2Me
CO2Me
CO2Me
NaNO2
CuCl
Cl
NH2
KOH
Cl
CO2H
Cl
CO2Me
Cl
CO2Me
BDC-2,3-Cl2
CO2H
CO2Me
NHAc
CO2Me
H2O2
(CF3CO)2O
O2N
O2N
HNO3
H2SO4
MeOH
KOH
NO2
CO2H
H2SO4
NO2
CO2Me
CO2Me
Me
BDC-2,5-(NO2)2
CO2H
NO2
CO2Me
NO2
changes to
treatments.
a dark black-brown solution after thermal
HNO3
1. Na2Cr2O7, H2SO4
2. MeOH, H2SO4
KOH
NO2
CO2H
NO2
CO2Me
H2SO4
The formation of DMOF structures were confirmed by PXRD
patterns (Fig. 2). The six obtained DMOFs showed similar PXRD
patterns, and all major peaks were well-matched with
previously reported non-functionalized or mono-functionalized
DMOF structures in the as-synthesized structure (Fig. S2†).12,16
In addition, the unit cell parameter of the functionalized
DMOF, determined by PXRD pattern indexing, was similar to
the single crystal unit cell parameter of non-functionalized
DMOF (Table S1). Therefore, the major framework structure is
not significantly differentiated from the original DMOF
structure. The indexed peak position, intensity and hkl values
are summarized in the supplementary information.
1H NMR analysis, after acid digestion, additionally confirmed
that all synthesized, regioisomeric ligands retained their
functionality during the DMOF framework synthesis and no
decomposition or isomerization of ligands was observed. The
ligand ratios between BDCs and DABCO were confirmed to be
2:1 (Fig. S1†). The TGA shows that these DMOFs are thermally
stable up to 300 oC (Fig. S3†).
Me
BDC-2,3-(NO2)2
CO2Me
OH
CO2Me
CO2H
OMe
MeI
K2CO3
OMe
KOH
OH
CO2Me
OMe
OMe
CO2Me
CO2H
BDC-2,3-(OMe)2
CO2Et
CO2Et
CO2H
MeO
MeI
K2CO3
HO
MeO
KOH
OH
CO2Et
OMe
CO2Et
OMe
CO2H
BDC-2,5-(OMe)2
HNO3
CO2H
CO2Et
NO2
NO2
MeO
MeO
KOH
OMe
OMe
CO2Et
CO2H
BDC-2,5-(OMe)2-3-NO2
Pd/C, H2
CO2Et
CO2H
NH2
MeO
NH2
MeO
KOH
OMe
CO2H
OMe
CO2Et
BDC-2,5-(OMe)2-3-NH2
Scheme 2 Preparation of bi- and tri-functional ligands for MOF synthesis.
Using the obtained BDCs, the synthesis of DMOFs was carried
out. DMOF-2,3-(OMe)2 and DMOF-2,5-(OMe)2 were
successfully synthesized by following a typical DMOF synthesis
procedure. Of note, improved crystallinity of DMOF-2,3-
(OMe)2 was achieved at a lower temperature synthesis (see ESI
for details†). The dichloro-functionalized DMOF-2,3-Cl2 and
DMOF-2,5-Cl2 were also synthesized under the typical
o
synthetic solvothermal condition of DMOF (120 C). Although
the para-functionalized DMOF-2,5-Cl2 was previously reported
by Uemura et al. to see ligand substitution effects,14 this is the
first example of the ortho-dihalogen substituted DMOF series.
The dinitro-functionalized DMOF-2,3-(NO2)2 and DMOF-2,5-
(NO2)2 were carefully monitored for appropriate solvothermal
synthetic conditions since multi-nitro compounds can be
explosive. These dinitro BDC ligands are thermally stable up to
o
300 C as evidenced by thermogravimetric analysis (TGA, Fig.
S3†). Both DMOF-2,3-(NO2)2 and DMOF-2,5-(NO2)2 were
obtained through solvothermal synthesis at 105 oC. Once again,
both mono-nitro-functionalized DMOF-NO2 and para-dinitro-
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surface area = 1554 m2/g) whereas, DMOF-2,5-(OMe)2 was
practically non-porous to N2 (Quantity Adsorbed <25 cm3/g
STP in 1 atm, and BET surface area = 45 m2/g). This finding is
also consistent with the N2 adsorption profile of 2,5-alkoxy
functionalized DMOFs reported by the Fischer group,11 and
2,5-dichloro functionalized DMOF reported by the Uemura
group.14
This is also a similar phenomenon to the previously reported
DMOF-2,5-NH2Cl and DMOF-2,5-NH2OMe cases.12,13 In the
DMOF-2,5-NH2OMe study, we utilized Hammett’s substituent
constants to predict relative electronic environment of the
BDC ligands.13 The present results have similar trends to
relationships between the framework flexibility and the
electronic environments of symmetrical installations of
functional groups. Among the dichloro-, dimethoxy-, and
dinitro-, only the dimethoxy-functionalized BDCs have an
electronically-rich benzene ring while the others have
relatively electron-deficient benzene rings. Unsurprisingly, only
methoxy-functionalized DMOFs (DMOF-2,3-(OMe)2 and DMOF-
2,5-(OMe)2) displayed the flexibility changes in N2 adsorption
experiments after evacuation (Fig. 2). In several computational
studies, it was found that the electronic density of the benzene
ring could impact the bond strength and length of the metal-
carboxylate coordination bonds (i.e., coordination bond
between the secondary building unit and the BDC ligand).17,18
We believe that our findings are in line with this hypothesis. It
also clearly revealed that these regioisomeric flexibility
changes occur not only in hetero-functional group
combinations and hydrogen-bonding combinations (e.g., NH2-
Cl and NH2-OMe), but also in homo-functional group
combinations (i.e., symmetrical functionalizations, OMe-OMe).
Adsorption of CO2 of the obtained bi-functionalized DMOFs
generally follows the same trends of the N2 adsorptions.
DMOF-2,3-Cl2 and DMOF-2,5-Cl2 showed almost identical CO2
adsorption profiles (~3.0 mmol/g CO2 uptake at 760 mmHg),
and dinitro-functionalized DMOF-2,3-(NO2)2/DMOF-2,5-(NO2)2
displayed similar CO2 uptake at 760 mmHg. Only dimethoxy-
functionalized DMOFs have a significantly different CO2 uptake
with ~3.0 mmol/g for DMOF-2,3-(OMe)2 versus ~0.5 mmol/g
for DMOF-2,5-(OMe)2, respectively. This follows exactly the
same trend as for the N2 adsorption for dimethoxy-
functionalized DMOFs indicating that, for this series of DMOFs,
intermolecular forces between CO2 and the ligands cannot
induce this breathing behavior as has been seen in other
systems (Fig. S7-S9 and Table S2†).
Fig. 2 PXRD patterns and N2 full isotherm (77K) of the obtained bi-functional DMOFs.
(a) DMOF-2,3-Cl2 / DMOF-2,3-Cl2 (b) DMOF-2,3-(NO2)2 / DMOF-2,5-(NO2)2 (c) DMOF-
2,3-(OMe)2 / DMOF-2,5-(OMe)2.
The full N2 isotherms for the obtained six symmetrically bi-
functionalized DMOFs were carefully monitored to observe the
flexibility changes of frameworks by regioisomeric ligand
installations of functional groups. While both dichloro- and
dinitro- systems (i.e., DMOF-2,3-Cl2 and DMOF-2,5-Cl2 / DMOF-
2,3-(NO2)2 and DMOF-2,5-(NO2)2) showed porous structures
for N2 adsorption after evacuation, only the dimethoxy-
functionalized DMOFs displayed flexibility differences based on
functional group positional changes (Fig. 2, S4, and S5†).
Although both DMOF-2,3-(OMe)2 and DMOF-2,5-(OMe)2
showed almost the same PXRD patterns while as-synthesized,
the N2 adsorption profile and BET surface area were quite
different (Fig. 2 and S6†). DMOF-2,3-(OMe)2 was highly porous
to N2 (Quantity Adsorbed ~375 cm3/g STP in 1 atm, and BET
The pore volume analysis from the N2 adsorption data was
used to compare the six DMOFs (Table 1). As expected, even
though all six bi-functionalized DMOFs have very similar PXRD
patterns, the pore volume showed dramatic differences
between DMOF-2,3-(OMe)2 and DMOF-2,5-(OMe)2. Using the
t-Plot method,19 there was up to a 74% difference in the pore
volume versus other combinations, like DMOF-2,3-Cl2/DMOF-
2,5-Cl2 and DMOF-2,3-(NO2)2/DMOF-2,5-(NO2)2, which showed
similar pore sizes.
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(samples were re-exposed to air after evacuation under
vacuum for measurement). Similar to the full N2 adsorption
experiment, only DMOF-2,5-(OMe)2 displayed a peak shift in
the PXRD to higher angles, and other DMOFs showed that
generally retained PXRD patterns (Fig. S2 and S12†). Some
decomposition was also measured (i.e., broad backgrounds)
since these DMOFs are not stable to moisture including the
flexible DMOF-2,5-(OMe)2. Thus, the PXRD data refinement of
the changed pattern could not be performed.
Table 1 Pore volume comparison of bi-functional and tri-functional DMOFs
DMOF
t-Plot Micropore Volume
DMOF-2,5-(Cl)2
0.471 cm3/g
0.461 cm3/g
0.179 cm3/g
0.185 cm3/g
0.085 cm3/g
0.329 cm3/g
0.299 cm3/g
0.219 cm3/g
DMOF-2,3-(Cl)2
DMOF-2,5-(NO2)2
DMOF-2,3-(NO2)2
DMOF-2,5-(OMe)2
DMOF-2,3-(OMe)2
DMOF-2,5-(OMe)2-3-NO2
DMOF-2,5-(OMe)2-3-NH2
Both N2 isotherm and PXRD pattern changes after evacuation
of bi- and tri-functionalized DMOFs suggest that only DMOF-
2,5-(OMe)2 demonstrated flexibility in the framework. It is
evident that the electron density of the benzene ring alone
cannot account for the flexibility changes. The regioisomeric
positional variation is an essential factor for the flexibility
control in a DMOF system. To date, the ortho-/para- positional
changes of two functional groups can alter the flexibility of
DMOFs. Among the reported bi-functional DMOFs, only three
electronically rich DMOFs (e.g., DMOF-NH2X, DMOF-NH2OMe,
DMOF-(OMe)2) have displayed the flexibility changes via
regioisomerism. However, electronic density controls, without
different regioisomerism, also had an effect on the flexibility.
Electronic Controls in Tri-functionalized DMOFs
Based on the flexibility differences from bi-functionalized
BDCs, especially BDC-2,5-(OMe)2, tri-functionalized ligands
were further examined by introducing NO2 and NH2 groups on
the benzene ring as a third functional group since DMOF
syntheses with just these functional groups were unsuccessful.
A nitration was performed on the BDCE-2,5-(OMe)2, and BDCE-
2,5-(OMe)2-3-NO2 was selectively collected via column
chromatography. The target dimethoxy-nitro BDC was
obtained following hydrolysis of the starting methyl ester.
Another tri-functionalized BDCE, amino-dimethoxy ligand was
prepared by a standard reduction (Pd/C in H2) from BDCE-2,5-
(OMe)2-3-NO2. And again, the desired BDC-2,5-(OMe)2-3-NH2
was synthesized by hydrolysis of the methyl ester form. To the
best of our knowledge, this is the first example of tri-
functionalized BDCs for MOF synthesis. Both dimethoxy-nitro
1
and amino-dimethoxy ligands were confirmed by H NMR, 13C
NMR, IR and HR-MS. (see ESI for detailed procedures†)
These two tri-functionalized BDC ligands were promptly used
in DMOF syntheses. In both cases,
a low temperature
condition (100 oC) produced better crystalline material than
the standard solvothermal condition (120 oC). The DMOF
framework using either DMOF-2,5-(OMe)2-3NO2 or DMOF-2,5-
(OMe)2-3-NH2 were confirmed by PXRD (Fig. 3 and S2†), and 1H
NMR analysis, after acid digestion, clearly confirmed both
ligands were in tact in each framework (Fig. S1†). However, the
TGA trace displays an interesting phenomenon; the
dimethoxy-nitro
functionalized
DMOF-2,5-(OMe)2-3-NO2
showed a large weight % drop near 270 oC (Fig. S3†). We
believe that, and as we discussed above, this is due to the
thermally unstable nitro group within an electron-rich
environment (near the methoxy groups).
Fig. 3 PXRD patterns and N2 full isotherm (77K) of the tri-functional DMOF-2,5-(OMe)2-
3-NO2 and DMOF-2,5-(OMe)2-3-NH2.
The full N2 isotherm experiments for the tri-functionalized
DMOFs were performed to monitor the flexibility changes of
frameworks by electronic density differences with the same
regioisomeric positions (Fig. 3). Both DMOF-2,5-(OMe)2-3-NO2
and DMOF-2,5-(OMe)2-3-NH2 displayed porosity to N2, and BET
Experimental
Ligand Synthesis
Dimethyl-2,5-dichloroterephthalate and Dimethyl-2,3-
dichloroterephthalate (BDCE-2,5-Cl2 and BDCE-2,3-Cl2)
surface areas were measured to be 830 m2/g and 661 m2/g, Dimethyl-2-amino-5-chloroterephthalate (244 mg, 1 mmol)
respectively. (Fig. S10†) The pore volume changes between was dissolved in 500 µL of water and 600 µL of 36% HCl
o
o
DMOF-2,5-(OMe)2-3-NO2 and DMOF-2,5-(OMe)2-3-NH2 were solution at 0 C. The solution was stirred at 0 C, and NaNO2
also correlated with the surface area differences (Table 1).
aqueous solution (90 mg of NaNO2 in 1 mL of H2O) was added
o
Lastly, PXRD patterns for the obtained bi- and tri- dropwise to the solution for 1 hour at 0 C. Then the mixture
functionalized DMOFs after evacuation were compared was slowly transferred into a CuCl/HCl solution (119 mg of
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CuCl in 500 µL of 36% HCl). The mixed was stirred for 1 h at 0 purified by silica gel column chromatography (25% EtOAc/n-
oC and an additional 2 h at room temperature. The resultant Hexane), and the desired compound, dimethy-2,3-
was extracted with EtOAc, and the organic layer was dried dinitroterephthalate (798 mg, 37%) was obtained as an ivory
using MgSO4, and evaporated of ethyl acetate under reduced solid.
pressure. The obtained solid mixture was separated by silica
2,5-Dinitroterephthalic acid (BDC-2,5-(NO2)2) and 2,3-
Dinitroterephthalic acid (BDC-2,3-(NO2)2)
gel column chromatography (10% EtOAc/n-Hexane) and the
desired compound, dimethyl-2,5-dichloroterephthalate (90
mg, 34 %), was obtained as a colorless solid.
Dimethyl-2,5-dinitroterephthalate (284 mg,
1 mmol) was
Dimethyl-2,3-dichloroterephthalate
was
obtained
in
dissolved in 5 mL of THF. A 4% KOH aqueous solution (5 mL)
was slowly added to the THF solution. The mixture was stirred
at room temperature for 2 h. Once conversion was complete
(monitored by TLC), THF was removed by evaporation and the
mixture was acidified with a 1.0 M HCl aqueous solution. The
solution was stored overnight in a refrigerator. The precipitate
comparable yield from a similar procedure using dimethyl-2-
amino-3-chloroterephthalate as the starting material.
2,5-Dichloroterephthalic acid (BDC-2,5-Cl2) and 2,3-
Dichloroterephthalic acid (BDC-2,3-Cl2)
Dimethyl-2,5-dichloroterephthalate (263 mg, 1 mmol) was was collected by filtration, and washed with water. The
dissolved in 5 mL of THF, and 5 mL of a 4% KOH aqueous desired compound was obtained by air drying (249 mg, 98%)
solution was slowly added to the THF solution. The mixture as a yellow solid.
was stirred overnight under reflux (66 oC). Once conversion 2,3-Dinitroterephthalic acid was obtained in comparable yield
was complete (monitored by TLC), THF was removed by from
a
similar
procedure
using
dimethyl-2,3-
evaporation and the mixture was acidified with a 1.0 M HCl dinitroroterephthalate as the starting material.
aqueous solution. The precipitate was collected by filtration,
Diethyl-2,5-dimethoxyterephthalate (BDCE-2,5-(OMe)2) and
Dimethyl-2,3-dimethoxyterephthalate (BDCE-2,3-(OMe)2)
and washed with water. The desired compound was obtained
by air drying (161 mg, 69%) as a colorless solid.
2,3-Dichloroterephthalic acid was obtained in comparable
Diethyl-2,5-dihydroxyterephthalate (1.27 g,
5
mmol),
yield from
a
similar procedure using dimethyl-2,3-
potassium carbonate (830 mg, 16 mmol), and iodomethane (1
mL, 16 mmol) were dissolved in acetone (15 mL). The mixture
was stirred overnight under reflux (60oC). Once conversion was
complete (monitored by TLC), the solvent was evaporated.
Water was added to dissolve all of the inorganic salts. The
solution was extracted with ethyl acetate three times, the
organic layer was dried using MgSO4, filtered, and evaporated
in vaccuo. Then the desired compound, dimethyl 2,5-
dimethoxyterephthalate (1.11 g, 79%), was obtained as a
colorless solid.
dichloroterephthalate as the starting material.
Dimethyl-2,5-dinitroterephthalate (BDCE-2,5-(NO2)2)
Trifluoroacetic anhydride (4.3 mL, 31.1 mmol) was slowly
added to a solution mixture of 30% H2O2 (2.7 mL, 26.6mmol)
o
and CH2Cl2 (15 mL) at 0 C. The solution was warmed to room
temperature, and
a dimethyl-2-amino-5-nitroterephthalate
solution (428 mg, 1.68 mmol in CH2Cl2 4.5 mL) was slowly
added to the mixture. After stirring for 3 h at 45 oC, the
o
mixture was cooled to 0 C and Na2SO3 was slowly added to
Dimethyl-2,3-dimethoxyterephthalate was obtained in
comparable yield from a similar procedure using dimethyl-2,3-
dihydroxyterephthalate as the starting material.
quench the reaction. The resulting mixture was extracted with
CH2Cl2. The organic phase was washed with brine. The solution
was then dried using MgSO4, filtered, and the CH2Cl2 was
evaporated. The solid mixture was recrystallized from EtOAc,
and the desired compound, dimethyl-2,5-dinitroterephthalate
(267 mg, 23%), was obtained as a green solid.
2,5-Dimethoxyterephthalic acid (BDC-2,5-(OMe)2) and 2,3-
Dimethoxyterephthalic acid (BDC-2,5-(OMe)2)
Diethyl-2,5-dihydroxyterephthalate (1.11 g, 3.94 mmol) was
dissolved in 20 mL of THF. To this, 20 mL of a 4% KOH aqueous
solution was added. The mixture was stirred overnight under
reflux (66 oC). Once conversion was complete (monitored by
TLC), THF was removed by evaporation and the mixture was
acidified with a 1.0 M HCl aqueous solution. The precipitate
was collected by filtration, and washed with water. The
desired compound was obtained by air drying (840 mg, 94%)
as a colorless solid.
Dimethyl-2,3-dinitroterephthalate (BDCE-2,3-(NO2)2)
2,3-Dinitro-p-xylene (1,5 g, 7.65 mmol) was dissolved in conc.
sulfuric acid (45 mL) and the mixture was cooled to 0 oC.
Sodium dichromate (13.7 g, 46 mmol) was added in small
quantities (1 g) to the mixture every 0.5 h. After addition the
mixture was warmed to room temperature and stirred
overnight. Crushed ice was added to the reaction flask, and the
reaction was quenched with sodium sulfide. After extraction
with EtOAc (30 mL X 8), the organic phase was dried using
MgSO4, filtered, and evaporated in vaccuo. Methanol (80 mL)
and conc. sulfuric acid (8 mL) was added to the resulting solid
2,3-Dimethoxyterephthalic acid was obtained in comparable
yield from
a
similar procedure using Dimethyl-2,3-
dihydroxyterephthalate as the starting material.
and the mixture was stirred for 2 d at 70 oC. The methanol was Diethyl 2,5-dimethoxy-3-nitroterephthalate (BDCE-2,5-(OMe)2-3-
evaporated in vaccuo and the solid was extracted with
EtOAc/water. The organic solution was dried with MgSO4,
filtered, and evaporated in vaccuo. The resulting solid was
NO2)
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Diethyl 2,5-dimethoxyterephthalate (500 mg, 1.77 mmol) was oC/min (detailed conditions for each functionalized DMOFs are
dissolved in Ac2O (5 mL). 60% HNO3 (1 mL) was added to the indicated in the ESI†) from room temperature to 100-120 oC
solution at 0 oC and stirred at room temperature for 24 h. (see ESI for details†). The temperature was held for 24 h and
Once conversion was complete (monitored by TLC), the then cooled to room temperature. The resulting crystals were
solution was poured into H2O (20 mL). Saturated NaHCO3 washed three times with 5 mL DMF. The solvent was
solution was added to the mixture until neutral by pH paper. exchanged with chloroform (5 mL) over three days, replacing
The organic phase was extracted with 25 mL of CHCl3 and then the old chloroform with fresh chloroform every 24 h.
the organic solution was dried with MgSO4, filtered, and
evaporated. The solid mixture was separated by silica gel
column chromatography (10% EtOAc/n-Hexane) and the
Conclusions
desired compound was obtained as a yellow solid (350 mg,
60%).
In conclusion, we have synthesized a series of bi-functionalized
regioisomeric BDC ligands using amino-, chloro-, methoxy-,
and nitro- groups with symmetrically controlled positioning
along with electronic density-controlled tri-functionalized BDC
ligands. Among the bi-functionalized ligands, both ortho and
para-positioned dichloro-, dimethoxy-, and dinitro-BDCs were
successfully incorporated into DMOFs and two tri-
functionalized ligands produced DMOFs with identical PXRD
patterns indicating isostructural frameworks. Although, all
functionalized ligands were perfectly installed into the same
DMOF frameworks and eight new bi- or tri-functionalized
DMOFs displayed similar structures, the flexibility of the
framework (i.e., Quantity of N2 adsorbed after evacuation,
breathing ability) was modulated by the ligand position from
ortho to para in the dimethoxy-functionalized DMOFs. The
combination of bi-functionalized 2,3-Cl2, 2,5-Cl2, 2,3-(NO2)2,
2,5-(NO2)2, and tri-functionalized 2,5-(OMe)2-3-NO2, 2,5-
(OMe)2-3-NH2 showed no significant changes in the N2
adsorption isotherms including pore size analysis, and only the
2,3-/2,5-(OMe)2 combinations displayed highly porous/non-
porous framework combinations with respect to N2 and CO2
adsorption. Since the dimethoxy-functionalized BDCs are the
only electronically-rich benzene rings among the bi-
functionalized BDCs, this finding clearly revealed, and
supports, the strong relationship between electron density of
the BDC rings and the structural flexibility with two identical
functional groups.
2,5-Dimethoxy-3-nitroterephthalic acid (BDC-2,5-(OMe)2-3-NO2)
Diethyl 2,5-dimethoxy-3-nitroterephthalate (330 mg, 1.01
mmol) was dissolved in 4 mL of THF. To this, 4 mL of a 4% KOH
aqueous solution was added dropwise. The mixture was stirred
at room temperature overnight. Once conversion was
complete (monitored by TLC), THF and H2O was removed by
evaporation and the mixture was acidified with a small amount
of H2O and concentrated HCl at 0 °C. The precipitate was
collected by filtration, and washed with 1M HCl solution. The
desired compound was obtained by air drying as a pale yellow
solid (232 mg, 86%).
Diethyl 3-amino-2,5-dimethoxyterephthalate (BDCE-2,5-(OMe)2-3-
NH2)
Diethyl 2,5-dimethoxy-3-nitroterephthalate (350 mg, 1.07
mmol) was dissolved in THF (5 mL), and hydrogenation was
performed using 10% Pd/C (35 mg, 10 wt%) under H2 (1 atm)
at room temperature for 18 h. The resulting suspension was
filtered through celite and evaporated to remove volatile
solvents. The solid mixture was separated by silica gel column
chromatography (7% EtOAc/n-Hexane) and the desired
compound, diethyl 3-amino-2,5-dimethoxyterephthalate (300
mg, 95%), was obtained as a pale yellow oil.
3-Amino-2,5-dimethoxyterephthalic acid (BDC-2,5-(OMe)2-3-NH2)
Diethyl 3-amino-2,5-dimethoxyterephthalate (300 mg, 1.01 Among the bi-functionalized DMOFs in this study, only DMOF-
mmol) was dissolved in 4 mL of THF. To this, 4 mL of a 4% KOH 2,5-(OMe)2 showed the flexible structure and low N2
aqueous solution was added dropwise. The mixture was stirred absorption after evacuation. Thus, further pendant group
at room temperature overnight. Once conversion was installations of para-dimethoxy functionality were performed.
complete (monitored by TLC), THF and H2O was removed by In doing so, a strong electron-withdrawing nitro group or an
evaporation and the mixture was acidified with a small amount additionally electron-donating amino group were individually
of H2O and concentrated HCl at 0 °C. The precipitate was introduced on the benzene ring of the DMOF. Both tri-
collected by filtration, and washed with 1M HCl solution. The functionalized DMOFs showed minimal differences in their N2
desired compound was obtained by air drying as a light yellow and CO2 adsorptions, even though they have different
solid (219 mg, 90%).
electronic environments; dimethoxy-nitro has an electron-
deficient benzene ring and the dimethoxy-amino has an
electron-rich benzene ring. Thus, we conclude that the
flexibility of DMOFs is not only dependent on the electronic
density of the benzene rings, but also by the position and local
environment of the substituents.
MOF Synthesis
Functionalized BDC (0.5 mmol) and Zn(NO3)2·6H2O (0.5 mmol)
(12.5 mL). 1,4-
were
dissolved
in
DMF
Diazabicyclo[2.2.2]octane (dabco, 0.8 mmol) was added to this
mixture. After the precipitate formed, it was filtered using a
fine porosity fritted disc. The clear solution was transferred to
a scintillation vial (25 mL size) and heated at a rate of 1-2.5
Conflicts of interest
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There are no conflicts of interest to declare.
Acknowledgements
We thank Corinne A. Allen (Lawrence Berkeley National
Laboratory, The Molecular Foundry) for helpful discussions and
in preparing this manuscript and Yoodae Song (Gachon
University) for experimental supports. This research was
supported
by
the
Science
Research
Center
(2016R1A5A1009405) and Basic Science Research Program
(2016R1C1B1011076) through the National Research
Foundation of Korea (NRF) funded by the Ministry of Science,
ICT
& Future Planning. The powder X-ray diffraction
experiment at PLS-II, 2D-SMC beamline was supported in part
by MSIP and POSTECH.
Notes and references
‡ H. Ha and H. Hahm are equally contributed to this work.
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The position of functional groups and the subsequent e-density of the benzene rings in a bi-/tri-functionalized
zinc-based metal-organic framework (MOF) have been controlled to reveal structural differences.