Aromatic Substituent Effects on the Flexibility of Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.6b00983

The flexibility (or breathing behavior) of zinc-based metal-organic frameworks (MOFs) has been manipulated by regioisomeric and positional control of organic functionalities. Ten new regioisomeric BDC (ortho- or para-disubstituted benzene-1,4-dicarboxylic

Full text of DOI:10.1021/acs.inorgchem.6b00983

Article
pubs.acs.org/IC
Aromatic Substituent Effects on the Flexibility of Metal−Organic
Frameworks
Hyungwoo Hahm, Kwangho Yoo,† Hyeonbin Ha,† and Min Kim*
Department of Chemistry and BK21Plus Research Team, Chungbuk National University, Cheongju 28644, Republic of Korea
S
* Supporting Information
ABSTRACT: The flexibility (or breathing behavior) of zinc-based metal−organic frameworks (MOFs) has been manipulated by
regioisomeric and positional control of organic functionalities. Ten new regioisomeric BDC (ortho- or para-disubstituted
benzene-1,4-dicarboxylic acid) ligands have been synthesized and applied to a DMOF (dabco MOF) system, which has a zinc(II)
paddle-wheel SBU (secondary building unit). Among the new regioisomeric MOFs, the NH2−OMe combination showed
significant flexibility (breathing behavior) changes by simply altering the functional group positions (from 2,3 to 2,5). The
electronic density of the benzene ring was considered to be a major factor in the flexibility changes in the regioisomeric MOF
system.
ortho or para positions on the benzene ring.10 Using these two
ligand regioisomers, the synthesis of regioisomeric MOFs was
attempted, and a series of Zr(IV)-based UiO-66 (UiO =
1. INTRODUCTION
Metal−organic frameworks (MOFs) are hybrid three-dimen-
sional porous materials consisting of metal clusters (or ions)
University of Oslo) frameworks and a series of zinc(II)-based
and organic linker molecules.1 During the past decade, MOFs
DMOFs (DMOF = dabco MOF, dabco = 1,4-
have been widely examined in both fundamental synthetic
diazabicyclo[2.2.2]octane) were obtained and characterized.
studies and various applications such as gas adsorption,
UiO-66 has a Zr6O4(OH)4(BDC)6 cluster as the SBU
molecular separation and storage, and catalysis.2−5 A major
(secondary building unit) and demonstrates high stability to
advantage of MOFs is the relative ease of functionalization in
chemicals and moisture.11 In contrast, DMOFs have dimeric
comparison to other porous materials (e.g., activated carbons,
paddle-wheel type SBUs, and the BDC/dabco combination
mesoporous silicas, zeolites, etc.), where generally organic
together forms a three-dimensional framework.12 DMOFs are
moieties can be installed on the linker molecules as
substituents. A variety of functional groups such as amines,
known to display network flexibility, also called “breathing”.
Flexible MOFs have the ability to transform between two or
amides, alcohols, azides, and halides have been introduced into
more crystal phases trigged by stimuli.13 In the DMOF system,
MOF structures through direct synthesis and postsynthetic
methods.6 Postsynthetic modification (PSM) is a late-stage,
structural transformations between a large pore (lp) and narrow
pore (np) form have been studied, and the functional groups
solid-state functionalization method of MOFs with existing
on the BDC ligand can affect flexibility.14,15 In a previous study,
chemical tags and has been widely utilized in MOFs for specific
applications.6
ortho-substituted 2,3-NH2X and para-substituted 2,5-NH2X
regioisomers displayed significantly different behavior with
To introduce more than two functionalities, mixed-ligand
syntheses have been explored.7 A maximum of eight different
respect to the flexibility of the DMOF structure. While 2,3-
NH2X substituted DMOFs (i.e., DMOF-2,3-NH2Cl and
organic functional group containing ligands were successfully
DMOF-2,3-NH2Br) showed a rigid structure and permanent
introduced into a single MOF through the multivariate (MTV)
concept,8 and four functional groups were incorporated into
porosity, the regioisomeric 2,5-NH2X-substituted DMOFs (i.e.,
DMOF-2,5-NH2Cl, DMOF-2,5-NH2Br, and DMOF-2,5-NH2I)
MOF pores using a combination of the mixed-ligand strategy
and two serial PSMs.9 Additionally, two different organic
displayed flexible structures, as evidenced by N2 adsorption
functional groups have been installed on a single benzene-1,4-
dicarboxylic acid (BDC) molecules in a regioisomeric fashion.
Both an amino group and a halide group were placed in either
Received: April 24, 2016
© XXXX American Chemical Society
A
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Figure 1. Series of regioisomeric BDC pairs for DMOF synthesis.
experiments and powder X-ray diffraction (PXRD) data.10 In
another study, controlled flexibility of MOFs was achieved
using a disubstituted BDC ligand with chloro groups or alkoxy
chains.16−19 Fischer et al. showed that the flexibility was
controlled by the length and polarity of the alkoxy chain on the
BDC ligands. While ethoxy, n-propoxy, isopropoxy, n-butoxy,
allyloxy, and prop-2-ynyloxy group disubstituted 2,5-BDCs
formed flexible DMOF frameworks, n-pentoxy group sub-
stituted BDC exhibited a less flexible DMOF structure.
Additionally, in the case of 2-methoxyethoxy-substituted
BDC, the 2,5-disubstituted MOF showed flexibility while the
2,3-disubstituted MOF framework exhibited less flexibility.
Fischer’s work revealed that the steric issues and interaction
between functional groups play a substantial role in the
flexibility of MOFs that are capable of “breathing”.17 In this
contribution, we have synthesized five new combinations of
bifunctional, regioisomeric BDC ligands and utilized them in
the synthesis of DMOF derivatives to examine the effect of
functional group and isomerism on the degree of flexibility
(Figure 1).
effect on the benzene ring of MOF ligands. The major starting
material, BDCE-OH, was prepared by a Sandmeyer reaction
from BDCE-NH2. A nitration was performed on BDCE-OH.
Two regioisomers (BDCE-2,5-NO2OH and BDCE-2,3-
NO2OH) were separated by column chromatography, and
followed by methylation. Each ligand then hydrolyzed with an
aqueous KOH solution to obtain the desired ligands BDC-2,5-
NO2OMe (4a) and BDC-2,3-NO2OMe (4b) (Scheme S3 in
the Supporting Information). The combination of NH2 and
OMe was easily synthesized by an additional reduction, as
shown in Scheme S3. BDCE-2,5-NO2OMe and BDCE-2,3-
NO2OMe were each converted to the corresponding BDCE-
2,5-NH2OMe and BDCE-2,3-NH2OMe using a Pd/C hydro-
genation. The targeted BDC-2,5-NH2OMe (5a) and BDC-2,3-
NH2OMe (5b) were obtained after hydrolysis (Scheme S4 in
the Supporting Information). Finally, the last combination of
MeO and Cl was synthesized by a Sandmeyer reaction. BDCE-
2,5-NH2OMe and BDCE-2,3-NH2OMe were converted to
BDCE-2,5-OMeCl and BDCE-2,3-OMeCl, and the desired
BDC-2,5-OMeCl (6a) and BDC-2,3-OMeCl (6b) were
obtained by hydrolysis (Scheme S5). All new ligands and
1
intermediates were confirmed by H NMR, 13C NMR, FTIR,
2. RESULTS AND DISCUSSION
and HR-MS (see the Supporting Information for details).
2.2. Synthesis of MOF Regioisomers and Character-
izations. Using these 10 new regioisomeric ligands (2a−6b,
Figure 1), the syntheses of DMOF frameworks were
performed. Although the detailed synthetic conditions varied
slightly on the basis of the ligand used, the overall reaction
conditions were similar. DMOFs were generally synthesized
using a zinc nitrate hexahydrate salt, the desired BDC ligand,
dabco, and DMF (N,N-dimethylformamide) as solvent under
solvothermal conditions (Figure 2; see also the Supporting
Information for detailed synthetic procedures). The 2,5-
disubstituted DMOF series was obtained after 24 h of heating,
while the 2,3-disubstituted DMOF series were produced after
48 h of solvothermal syntheses. The resulting crystals were
shown to possess the same structure as the parent DMOF
2.1. Ligand Synthesis. First, the amino group was replaced
with the nitro group via oxidation (Scheme S1 in the
Supporting Information). All functionalization was performed
on the carboxylic methyl ester form (BDCE = benzene-1,4-
dicarboxylic acid methyl ester) for ease of purification. The
chlorination was performed on dimethyl 2-aminobenzene-1,4-
dicarboxylate (BDCE-NH2) to obtain BDCE-2,5-NH2Cl and
BDCE-2,3-NH2Cl. The two isomers were separated by flash
column chromatography, and each ligand was then oxidized to
BDCE-2,5-NO2Cl and BDCE-2,3-NO2Cl using trifluoroacetic
anhydride and hydrogen peroxide. The target ligands BDC-2,5-
NO2Cl (2a) and BDC-2,3-NO2Cl (2b) were obtained by
hydrolysis of BDCE-2,5-NO2Cl and BDCE-2,3-NO2Cl
(Scheme S1). For the next NO2−NH2 combination, a nitration
was performed on BDCE-NH2. It was found that using the
amide-protected BDCE-NHAc provided a better yield for the
nitration in comparison to the free aniline. Two isomeric
mixtures of BDCE-2,5-NO2NHAc and BDCE-2,3-NO2NHAc
were separated by column chromatography, and hydrolysis
formed the desired BDC-2,5-NO2NH2 (3a) and BDC-2,3-
NO2NH2 (3b) (Scheme S2 in the Supporting Information).
The free phenol group could interrupt DMOF formation;
thus, the methyl ether was selected to study the alkoxy group
1
material, as evidenced by PXRD (Figure 2), and H NMR
spectra, after acid digestion, confirmed that the desired ligands
were successfully incorporated into the DMOF structures.
Additionally, the ratio between the disubstituted BDCs and
1
dabco was verified to be the expected 2:1 by H NMR spectra
(Figure S1 in the Supporting Information). Thermogravimetric
analysis (TGA) of fully activated samples (e.g., samples after
gas adsorption experiments) showed that these obtained
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Figures S3−S7 in the Supporting Information, and Table 1);
DMOF-2,5-NH2OMe remained nonporous to N2 regardless of
the activation method (Figure 3 and Figure S6). In other
words, the combination of NH2 and OMe (5a,b) produces a
second contrasting set of DMOFs that display regioisomeric
control over flexibility. DMOF-2,5-NH2OMe has a BET surface
area of 114 m2/g, while DMOF-2,3-NH2OMe has a BET
surface area of 1205 m2/g (averaged from at least two
independent measurements). Other combinations, using NO2
and Cl (2a,b), NH2 and NO2 (3a,b), NO2 and OMe (4a,b),
and OMe and Cl (6a,b) formed inflexible, porous DMOFs in
all cases (Figure 3 and Figures S3−S7). To date, the NH2−X
and NH2−OMe combinations are the only isomeric sets that
display flexibility control in their corresponding DMOFs.
Carbon dioxide uptake experiments verified the flexibility of
regioisomeric DMOFs. While N2 adsorption completely
changed by the organic functional group position in the case
of DMOF-2,5-NH2OMe and DMOF-2,3-NH2OMe, the CO2
adsorption capacities were not significantly altered. DMOF-2,5-
NH2OMe absorbed 2.98 mmol/g of CO2 (1 atm), and DMOF-
2,3-NH2OMe absorbed 3.38 mmol/g of CO2 (Figure 4 and
Figure S8 in the Supporting Information). Selective CO2
absorption to N2 gas has been studied in several cases,18 and
this finding strongly supports that the DMOF-2,5-NH2OMe
indeed has a flexible framework, and the pores of the MOFs can
be opened by strong interactions such as introduction of CO2
into the system. Other regioisomeric DMOFs have been tested
for their CO2 adsorption and displayed 2.55−3.45 mmol/g at 1
atm as adsorption amounts (Figure 4 and Figures S9−S13 and
Table S1 in the Supporting Information).
2.3. Flexibility (Breathing Behavior) Controls of
Functional Groups in DMOFs. Our findings prove that the
NH2−Cl and NH2−OMe combinations have unique flexibility
(breathing behavior) changes on MOFs when they are
compared among NH2, NO2, Cl, and OMe. Since all four
components could partake in hydrogen bonding, we gave more
attention to intramolecular differences than to intermolecular
differences to explain the flexibility changes. To estimate the
electronic environments of six different combinations (1−6),
the σ value for the substituent constant from a Hammett plot
was applied to the ligands20 and displayed good correlation
between electronic environment and flexibility changes (Table
1). Since the nitro group has very strong electron-withdrawing
effects, the nitro-containing ligands (2−4) have relatively
electron-deficient benzene rings and displayed inflexible
DMOFs. BDC-2,5-NH2Cl (1a) has a negative value as the
sum of the σ values, meaning that there is a relatively negative
benzene (i.e., electron rich) ring for DMOF-2,5-NH2Cl. In
addition, the 2,5-NH2OMe ligand has an electronically rich
character in the benzene ring. Finally, ligand 6a has an almost
neutral electronic environment in the benzene ring. Exper-
imental N2 adsorption data and the Hammett plot matched
well to explain the breathing behavior changes in DMOFs.
From several computational studies, it was reported that the
substituents on the benzene ring and the electron density could
affect both the geometry and bond lengths of the functional
group on the ligand.21−23 For example, in the case of para-
substituted fluorobenzenes, electron-withdrawing substituents
decrease the bond length of the benzene ring−fluoro group
while electron-donating substituents increase the bond length
of C−F.23 In the present regioisomeric DMOF and BDC ligand
system, the NH2−Cl and NH2−OMe combinations in para
positions provide electronically rich environments from the two
Figure 2. General synthesis of regioisomeric DMOFs and their PXRD
patterns, indicating isostructural materials.
regioisomeric MOFs are thermally stable with decomposition
temperatures of ∼400 °C (Figure S2 in the Supporting
Information).
Gas adsorption experiments using N2 revealed the flexibility
of regioisomeric DMOFs. In a previous study, DMOF-2,5-
NH2Cl showed almost no adsorption of N2 gas, and the BET
(Brunauer−Emmett−Teller) surface area was calculated to be
∼6 m2/g. In contrast, DMOF-2,3-NH2Cl showed markedly
higher porosity with a BET surface area of 1169 m2/g. It was
speculated that the framework flexibility was altered because of
the functional group isomerism.10 All newly synthesized
DMOFs using regioisomeric ligands (2a−4b, 5b, and 6a,b)
exhibited high porosity for N2 adsorption with BET surface area
values ranging between 982 and 1238 m2/g, except for DMOF-
2,5-NH2OMe (from 5a), which displayed 114 m2/g for its BET
surface area. However, there was one exception (Figure 3,
Figure 3. Full dinitrogen sorption isotherms (77 K) of regioisomeric
DMOFs.
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Table 1. Substituent Constant (σp) from a Hammett Plot and Flexibility of DMOFs
ligand in synthesis
1a,b (R1 = NH2,
2a,b (R1 = NO2,
3a,b (R1 = NH2,
4a,b (R1 = NO2,
5a,b (R1 = NH2, R2
6a,b (R1 = OMe,
R2 = Cl)
R2 = Cl)
R2 = NO2)
R2 = OMe)
= OMe
R2 = Cl)
σp for R1
σp for R2
σp1 + σp2
flexibility
−0.66
0.23
0.78
−0.66
0.78
0.78
−0.66
−0.27
−0.93
flexible
114
−0.27
0.23
0.23
−0.27
0.51
−0.43
flexible
1.01
0.12
−0.04
inflexible
1238
inflexible
1098
inflexible
1095
inflexible
1111
a
BET for DMOF-2,5-R1R2
6
(m2/g)
BET for DMOF-2,3-R1R2
(m2/g)
a
1169
1167
1215
982
1205
1190
a
Reported BET value in ref 10.
solvents were used without additional purification unless
otherwise stated. 1H and 13C NMR (nuclear magnetic
resonance) spectra were recorded on a FT AM 400 (400
MHz for 1H and 100 MHz for 13C) or FT AM 500 (500 MHz
1
for H and 125 MHz for 13C) instrument. Parts per million
(ppm) units were applied to quoted chemical shift (δ) and the
appropriate parent solvent peak or 0 ppm for tetramethylsilane
(TMS) was used as a reference peak for calibration. The peak
patterns were assigned using following abbreviations: br =
broad, s = singlet, d = doublet, t = triplet, q = quartet, and m =
multiplet. 13C NMR was fully decoupled by broad-band
decoupling. A triplet at 77.0 ppm of chloroform-d (or a septet
at 39.52 ppm of DMSO-d6) was employed as a reference peak
for 13C NMR.
4.2. MOF Synthesis. The DMOF series was prepared and
activated using a method modified from what has been
previously described.24
4.2.1. DMOF-2,5-NO2Cl. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 2a (0.5 mmol, 123 mg) were
dissolved in DMF (12.5 mL). Ligand dabco (0.8 mmol, 90 mg)
was added to this solution mixture. When a colorless solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 2.5 °C/min from room temper-
ature). The temperature was held for 24 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 2.5
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.2. DMOF-2,3-NO2Cl. Zinc nitrate hexahydrate (0.4
mmol, 119 mg) and ligand 2b (0.4 mmol, 98 mg) were
dissolved in DMF (10 mL). Ligand dabco (0.64 mmol, 72 mg)
was added to this solution mixture. When a yellow solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 120 °C in a programmed oven
(increasing temperature rate: 1.0 °C/min from room temper-
ature). The temperature was held for 48 h at 120 °C and then
reduced to room temperature (decreasing temperature rate: 1.0
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
Figure 4. CO2 isotherm (298 K) of regioisomeric DMOFs.
electron-donating groups and have suitable geometry and bond
length to accommodate or induce the breathing behavior
changes.
3. CONCLUSIONS
In conclusion, we have synthesized 10 new regioisomeric BDC
ligands using 4 different chemical tags such as amino, chloro,
methoxy, and nitro groups. All new ligands were successfully
incorporated into DMOF frameworks, and the 10 new DMOFs
were fully characterized. Interestingly, the NH2−OMe combi-
nation showed flexibility (breathing behavior) changes within
its DMOF seemingly by regioisomeric and positional control.
The other combinations displayed inflexible but porous
structures during N2 adsorption. The more flexible DMOF-
2,5-NH2OMe could selectively adsorb CO2 gas with good
capacity. The flexibility (breathing behavior) changes in
regioisomeric DMOFs were obtained from the functional
group combinations of NH2/halogens and NH2/OMe, and we
speculated that the electronic environment of the BDC ring
could affect the flexibility of the DMOFs on the basis of the
Hammett substituent constant. This study suggests that the
electronic environment of the ligand is important for MOF
properties including flexibility and breathing behavior.
4. EXPERIMENTAL SECTION
4.2.3. DMOF-2,5-NH2NO2. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 3a (0.5 mmol, 113 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
4.1. General Method. The concentration of solutions was
performed using a rotary evaporator, followed by vacuum
drying at 0.1−1 Torr. All commercial compounds and organic
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was added to this solution mixture. When a yellow solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 120 °C in a programmed oven
(increasing temperature rate: 2.5 °C/min from room temper-
ature). The temperature was held for 24 h at 120 °C and then
reduced to room temperature (decreasing temperature rate: 2.5
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.4. DMOF-2,3-NH2NO2. Zinc nitrate hexahydrate (0.4
mmol, 119 mg) and ligand 3b (0.4 mmol, 90 mg) were
dissolved in DMF (10.0 mL). Ligand dapco (0.64 mmol, 72
mg) was added to this solution mixture. When a yellow solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 120 °C in a programmed oven
(increasing temperature rate: 1.0 °C/min from room temper-
ature). The temperature was held for 48 h at 120 °C and then
reduced to room temperature (decreasing temperature rate: 1.0
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.5. DMOF-2,5-NO2OMe. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 4a (0.5 mmol, 120 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a colorless solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 2.5 °C/min from room temper-
ature). The temperature was held for 24 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 2.5
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.6. DMOF-2,3-NO2OMe. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 4b (0.5 mmol, 120 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a colorless solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 1.0 °C/min from room temper-
ature). The temperature was held for 48 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 1.0
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.8. DMOF-2,3-NH2OMe. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 5b (0.5 mmol, 106 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a yellow solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 1.0 °C/min from room temper-
ature). The temperature was held for 24 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 1.0
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.9. DMOF-2,5-OMeCl. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 6a (0.5 mmol, 115 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a colorless solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 2.5 °C/min from room temper-
ature). The temperature was held for 24 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 2.5
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.2.10. DMOF-2,3-OMeCl. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 6b (0.5 mmol, 115 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a colorless solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 100 °C in a programmed oven
(increasing temperature rate: 1.0 °C/min from room temper-
ature). The temperature was held for 48 h at 100 °C, and then
cooled to room temperature (decreasing temperature rate: 1.0
°C/min). The resulting DMOF crystals were washed with
DMF (5 mL × 3). The solvent was exchanged with CHCl3 (5
mL), replacing CHCl3 with fresh CHCl3 every 24 h for three
times.
4.3. MOF Characterization. 4.3.1. Acid Digestion of
DMOF Series for 1H NMR Analysis. A fully dried (under
vacuum) DMOF sample (approximately ∼10 mg) was mixed
with 590 μL of DMSO-d6 and 10 μL of DCl. The mixture was
sonicated until a clear solution was obtained.
4.3.2. Thermal Analysis. A fully dried (under vacuum) 10
mg DMOF sample (e.g., after BET analysis) was used for TGA
measurements. A stream of N2 was employed, and the running
temperature range was room temperature to 800 °C (scan rate
of 10 °C/min).
4.3.3. Powder X-ray Diffraction. A DMOF sample
(approximately 10 mg) was air-dried for 1 min prior to
PXRD analysis. PXRD data were collected on a Bruker D8
Discover instrument at ambient temperature (with a scan speed
of 1 s/step, a step size of 0.02° in 2θ, and a 2θ range of 5−55°).
4.3.4. BET Surface Area Analysis. A DMOF sample
(approximately 20−50 mg) was dried on a vacuum line at
4.2.7. DMOF-2,5-NH2OMe. Zinc nitrate hexahydrate (0.5
mmol, 149 mg) and ligand 5a (0.5 mmol, 106 mg) were
dissolved in DMF (12.5 mL). Ligand dapco (0.8 mmol, 90 mg)
was added to this solution mixture. When a yellow solid
formed, this precipitate was filtered using a fine-porosity fritted-
disk filter. Then the solution mixture was moved to a
scintillation vial and heated at 85 °C in a programmed oven
(increasing temperature rate: 2.5 °C/min from room temper-
ature). The temperature was held for 24 h at 100 °C and then
reduced to room temperature (decreasing temperature rate: 2.5
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DOI: 10.1021/acs.inorgchem.6b00983
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(15) Burtch, N. C.; Walton, K. S. Acc. Chem. Res. 2015, 48, 2850−
2857.
(16) Uemura, K.; Yamasaki, Y.; Onishi, F.; Kita, H.; Ebihara, M.
room temperature (less than 1 min). Then the sample was
transferred to a sample tube and degassed until the outgas rate
was <5 μmHg/min at 100 °C (on a Micromeritics ASAP 2020
Adsorption Analyzer). BET surface area (m2/g) measurements
were performed at 77 K under dinitrogen using the volumetric
technique.
Inorg. Chem. 2010, 49, 10133−10143.
(17) Henke, S.; Schneemann, A.; Wuetscher, A.; Fischer, R. A. J. Am.
Chem. Soc. 2012, 134, 9464−9747.
(18) Henke, S.; Schmid, R.; Grunwaldt, J.-D.; Fischer, R. A. Chem. -
Eur. J. 2010, 16, 14296−14306.
ASSOCIATED CONTENT
* Supporting Information
(19) Henke, S.; Wieland, D. C. F.; Meilikhov, M.; Paulus, M.;
Sternemann, C.; Yusenko, K.; Fischer, R. A. CrystEngComm 2011, 13,
6399−6404.
■
S
The Supporting Information is available free of charge via the
Internet at The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.6b00983.
(20) Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, 5th
ed.; Springer: New York, 2007; Part A.
(21) Campanelli, A. R.; Domenicano, A.; Ramondo, F.; Hargittai, I. J.
Phys. Chem. A 2004, 108, 4940−4948.
Overall schemes for ligand synthesis, detailed procedures
(22) Sadlej-Sosnowska, N.; Krygowski, T. M. Chem. Phys. Lett. 2009,
476, 191−195.
for ligand synthesis, figures and a table as described in the
1
text, and H and 13C NMR and IR spectra (PDF)
́
(23) Siodła, T.; Oziminski, W. P.; Hoffmann, M.; Koroniak, H.;
Krygowski, T. M. J. Org. Chem. 2014, 79, 7321−7331.
(24) Wang, Z.; Tanabe, K. K.; Cohen, S. M. Inorg. Chem. 2009, 48,
296−306.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.K.: minkim@chungbuk.ac.kr.
■
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Seth M. Cohen (University of California, San
Diego, USA) and Dr. Corinne A. Allen (Lawrence Berkeley
National Laboratory, USA) for helpful discussions. This
research was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Science, ICT & Future Planning
(2013R1A1A1061399), and the Creative Human Resource
Training Project for Regional Innovation though the NRF
funded by the Ministry of Education (2014H1C1A1066874).
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DOI: 10.1021/acs.inorgchem.6b00983
Inorg. Chem. XXXX, XXX, XXX−XXX