Realization of both high hydrogen selectivity and capacity in a guest responsive metal-organic framework

Article abstract of DOI:10.1039/c3ta12761c

Two newly designed semi-flexible tetratopic carboxylate ligands, 5′,5′′′′-(propane-2,2-diyl)bis(2′-methoxy- [1,1′:3′,1′′-terphenyl]-4,4′′-dicarboxylate) (pbtd-OMe^4-) and 5′,5′′′′-(propane-2, 2-diyl)bis(2′-ethoxy-[1,1′:3′,1′′-terphenyl]-4, 4′′-dicarboxylate) (pbtd-OEt^4-), have been used to connect dicopper paddlewheel building units to afford two isostructural metal-organic frameworks, Cu₂(H₂O)₂(pbtd-OR) ·xS (R = Me, PCN-38·xS; R = Et, PCN-39·xS, S represents noncoordinated guest molecules, PCN = porous coordination network) with novel structure and gas sorption properties upon activation. PCN-39 undergoes structural transformations upon guest solvent removal, leading to observation of distinct phases from in situ powder X-ray diffraction measurements, and exhibits selective adsorption of H₂ (up to 2.0 wt%) over CO, CO ₂, and N₂, which can be explained by optimized space-filling of the pendant ethoxy group. PCN-38 undergoes no transformation upon activation and exhibits hydrogen uptake up to 2.2 wt%, as well as moderate uptake of other gases. The selective adsorption of hydrogen over other gases highlights the potential application of PCN-39 in industrially important gas separation. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ta12761c

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ARTICLE
Realization of both high hydrogen selectivity and capacity in a guest
†
responsive metal-organic framework
‡
*[a]
Trevor A. Makal , Wenjuan Zhuang, Hongcai Zhou
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Two newly designed semi-flexible tetratopic carboxylate ligands, 5',5''''-(propane-2,2-diyl)bis(2'-
4
-
methoxy-[1,1':3',1''-terphenyl]-4,4''-dicarboxylate) (pbtd-OMe ) and 5',5''''-(propane-2,2-diyl)bis(2'-
4
-
ethoxy-[1,1':3',1''-terphenyl]-4,4''-dicarboxylate) (pbtd-OEt ) have been used to connect dicopper
paddlewheel building units to afford two isostructural metal-organic frameworks, Cu (H O) (pbtd-
2
2
2
1
0
OR)·xS (R = Me, PCN-38·xS; R = Et, PCN-39·xS, S represents noncoordinated guest molecules, PCN =
porous coordination network) with novel structure and gas sorption properties upon activation. PCN-39
undergoes structural transformations upon guest solvent removal, leading to observation of distinct phases
from in situ powder X-ray diffraction measurements, and exhibits selective adsorption of H (up to 2.0
2
wt%) over CO, CO , and N , which can be explained by optimized space-filling of the pendant ethoxy
2
2
1
5
group. PCN-38 undergoes no transformation upon activation and exhibits hydrogen uptake up to 2.2 wt%,
as well as moderate uptake of other gases. The selective adsorption of hydrogen over other gases
highlights the potential application of PCN-39 in industrially important gas separation.
4
5
5
6
5
0
5
0
crystallization and gas sorptive properties of MOFs. Furthermore,
Introduction
few porous materials have been shown to preferentially adsorb H
2
over N , and those that do exhibit high selectivity toward H
typically do so in low capacities.
1
-5
2
2
Metal-organic frameworks (MOFs) have received an incredible
following over the past 20 years owing to their exceptionally high
surface areas, large pore volumes, tunable surfaces, and potential
8, 37
2
2
3
3
4
0
5
0
5
0
Since it is well known that the filling of void space within a
system is of considerable importance to the formation and
6
-12
applications in such areas as gas storage,
gas/vapor
38-40
stability of porous frameworks,
we have been investigating
1
3-16
17-22
23-
separations,
heterogeneous catalysis,
chemical sensing,
and stimuli-responsive
the consequence of pendant functional groups on the formation
and gas sorption properties of MOFs. By grafting pendant
functionalities on non-rigid (semi-flexible) ligands, the ability of
pendant groups to initiate structural changes in the framework
system through optimal filling of void space may be investigated.
To this end, we synthesized the two angular, tetratopic ligand
precursors H pbtd-OMe and H pbtd-OEt (H pbtd-OMe = 5',5''''-
2
7
28-30
biomedical applications,
3
1-33
materials.
The potential for their use in commercial
applications is largely dictated by a number of factors, including
surface area and micropore volume, in addition to functionalities
based upon structural design and functional groups, and stability.
In particular, the synthesis of porous MOFs with responsive
modification of pore sizes is of great interest in the area of
selective gas sorption and separations. To make effective
materials for separation processes, the pore surface characteristics
must be tuned appropriately so as to create a sieving effect, where
molecules are separated based upon physical characteristics, or
through inclusion of polar or charged functional groups to take
4
4
4
(propane-2,2-diyl)bis(2'-methoxy-[1,1':3',1''-terphenyl]-4,4''-
dicarboxylic acid), H pbtd-OEt = 5',5''''-(propane-2,2-diyl)bis(2'-
4
3
4, 35
advantage of chemical differences between molecules.
Recently, Schröder, Yang, and coworkers reported a dynamic
bifunctional MOF for the selective adsorption of CO over
2
3
6
acetylene. From in situ powder X-ray diffraction studies, they
were able to observe a structural transformation from a “narrow
pore” to a “large pore” conformation upon CO loading attributed
2
to a gating effect by pendant pyridyl groups. In addition to
playing a role in gate-opening phenomena, we have been
interested in investigating the roles of pendant groups on the
Fig. 1 Schematic representation of the coordination environments of pbtd-
4
-
OR and dicopper(II) paddlewheel SBUs in PCN-38 (R=Me) and PCN-
9 (R=Et).
3
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ethoxy-[1,1':3',1''-terphenyl]-4,4''-dicarboxylic acid)) and report
the resultant MOFs obtained from combination with dicopper
paddlewheel SBUs (Figure 1).
1,2-dimethoxyethane (DME, 200 mL) was added via canula. The
reaction mixture was stirred and heated at reflux under nitrogen
60 atmosphere. The reaction was monitored by thin-layer
chromatography (1:4 by volume ethyl acetate:hexanes as eluent).
After removal of organic solvent, the resultant solid was
dissolved in dichloromethane, washed with water and brine, dried
The ligands share the same core structure as the organic linker
4
1
5
0
5
0
reported in the formation of PCN-21, with the addition of
pendant methyl and ethyl ether linkages in the para-positions of
the central phenyl rings that are joined by a methylene bridge and
over MgSO4, filtered, and reduced in vacuo yielding 0.5 g (55.3
1
two methyl groups on the bridging methylene carbon atom. The 65 %) B as a white powder. H NMR (300 MHz, CDCl ): δ 8.08 (d,
3
two reported MOFs, however, do not resemble the highly porous
and highly symmetric PCN-21, likely due to the factors described
in the following sentences. The methylene bridge in each ligand
permits a level of flexibility that allows for minor structural
8H), 7.67 (d, 8H), 7.28 (s, 4H), 3.95 (s, 12H), 3.12 (s, 6H), 1.80
(s, 6H).
1
1
2
c. H pbtd-OMe. Compound B (0.5 g, 0.6 mmol) was
4
suspended in 120 mL tetrahydrofuran/methanol/water mixture
movement to minimize the energy of the systems, and, along with 70 (1:1:1 by volume), to which 6 g KOH was added. The mixture
the extension of pendant ether moieties, leads to systems that
reorganize to optimally fill space. This combination of properties
has led to interesting crystallographic and gas sorption properties
of the reported MOFs. The rearrangement of pore space within
PCN-39 realizes a molecular sieving effect in which no gases 75 vacuum filtration, washed with water, and dried in an oven to
larger than hydrogen in this study may be adsorbed and, unlike
was stirred at 60 °C overnight. Organic solvents were removed in
vacuo and the aqueous solution acidified with 20% aqueous
hydrochloric acid until the pH value was adjusted to
approximately 2. The resulting white precipitate was collected by
1
yield 0.28 g (63.3 %) H pbtd-OMe. H NMR (300 MHz, DMSO-
4
6
many previously reported materials with high H selectivity, also
d ): δ 12.98 (br, 4H), 7.97 (d, 8H), 7.66 (d, 8H), 7.33 (s, 4H), 3.04
2
exhibits high H storage capacity. This combination of properties
(s, 6H), 1.80 (s, 6H).
2
shows great potential for application in hydrogen purification
applications.
d. 5,5' - (propane - 2,2 - diyl)bis(1,3 – dibromo – 2 -
80 ethoxybenzene), (C). Tetrabromobisphenol A (5.44 g, 10 mmol)
and K CO (5.2 g, 37.6 mmol) were combined in a 250 mL
2
3
Experimental Section
Schlenk flask equipped with magnetic stirrer and air condenser.
The flask was purged under vacuum and refilled with N2 three
times, and then 60 mL dry DMF was added via canula. Degassed
ethyl bromide (3.8 mL, 51.8 mmol) was then added to the
reaction flask by syringe. The reaction mixture was stirred and
heated at reflux under nitrogen atmosphere for 24 hours. After
cooling to room temperature, the solution was poured over ice-
water, yielding 5.40 g (90.5 %) C as a fine white powder, which
2
3
3
4
4
5
5
5
0
5
0
5
0
5
General Information. Commercially available reagents were
used as received without further purification. Nuclear magnetic
8
9
9
5
0
5
1
resonance (NMR) H data were collected on a 300 MHz Mercury
3
00 spectrometer. Thermogravimetric analyses (TGA) were
performed under N2 flow on Shimadzu TGA-50
thermogravimetric analyzer with a heating rate of 5 ˚C min .
Synthesis of 5',5'''' - (propane - 2,2 - diyl)bis(2' –methoxy -
a
-
1
1
was collected by vacuum filtration and dried in an oven. H NMR
[1,1':3',1'' - terphenyl] - 4,4'' - dicarboxylic acid, (H pbtd-OMe)
4
(
300 MHz, CDCl ): δ 7.29 (s, 4H), 4.07 (q, 4H), 1.59 (s, 6H),
3
and 5',5'''' - (propane - 2,2 - diyl)bis(2' – ethoxy - [1,1':3',1'' -
terphenyl] - 4,4'' - dicarboxylic acid, (H pbtd-OEt). a. 5,5' -
(propane - 2,2 - diyl)bis(1,3 –dibromo - 2 - methoxybenzene) (A).
1
.48 (t, 6H).
4
e. Tetramethyl 5',5'''' - (Propane - 2,2 - diyl)bis(2' -ethoxy -
[
1,1':3',1'' - terphenyl] - 4,4'' - dicarboxylate, (D). Compound C
(2.51 g, 4.2 mmol), 4-(methoxycarbonyl)phenylboronic acid
4.50 g, 25.0 mmol), CsF (2.5 g, 16.25 mmol), NaHCO (1.25 g,
This compound was synthesized following a procedure similar to
4
2
a literature method. Tetrabromobisphenol A (5.44 g, 10 mmol)
and K CO (5.2 g, 37.6 mmol) were combined in a 250 mL
(
3
2
3
1
5
4.9 mmol), and Pd(PPh3)4 (0.19 g, 0.4 mmol) were mixed in a
00 mL Schlenk flask equipped with magnetic stirrer and air
Schlenk flask equipped with magnetic stirrer and air condenser.
The flask was purged under vacuum and refilled with N three
2
condenser. The flask was purged under vacuum and refilled with
00 N2 three times, and then mixture of degassed 1,2-
times, and then 120 mL dry N,N-dimethylformamide (DMF) was
added via canula. Methyl iodide (2.5 mL, 40.0 mmol) was then
added to the reaction flask by syringe. The reaction mixture was
stirred and heated at reflux under nitrogen atmosphere overnight.
After cooling to room temperature, the solution was poured over
ice-water. The resultant white solid was collected by vacuum
filtration then dissolved in ethyl acetate, washed with water and
1
1
1
1
a
dimethoxyethane (150 mL) and water (20 mL) was added via
canula. The reaction mixture was stirred and heated at reflux
under nitrogen atmosphere. The reaction was monitored by thin-
layer chromatography (1:4 by volume ethyl acetate:hexanes as
05 eluent). After removal of organic solvent in vacuo, the aqueous
layer was extracted with dichloromethane (100 mL, 3x), dried
brine, then dried over MgSO . Evaporation of organic solvent
yielded 5.33 g (93.3%) A as colorless crystals. H NMR (300
4
over MgSO , filtered, and reduced in vacuo. The mixture of
4
1
products was separated by column chromatography using a silica
MHz, CDCl ): δ 7.29 (s, 4H), 3.78 (s, 6H), 1.54 (s, 6H).
1
3
gel column and reduced to afford 0.51 g (14.8 %) D. H NMR
b. Tetramethyl 5',5'''' - (propane - 2,2 - diyl)bis(2' –methoxy -
10 (300 MHz, CDCl ) δ 8.08 (d, 8H), 7.65 (d, 8H), 7.27 (s, 4H), 3.94
3
[1,1':3',1'' - terphenyl] - 4,4'' - dicarboxylate (B). Compound A
(s, 12H), 3.20 (q, 4H), 1.79 (s, 6H), 0.76 (t, 6H).
(
(
2.00 g, 1.14 mmol), 4-(methoxycarbonyl)phenylboronic acid
4.40 g, 24.4 mmol), CsF (2 g, 13 mmol), and Pd(PPh ) (0.20 g,
f. H pbtd-OEt. Compound D (0.51 g, 0.6 mmol) was
4
3
4
suspended in 60 mL tetrahydrofuran/methanol mixture (1:1 by
volume), to which 10 mL 5 M NaOH solution was added. The
15 mixture was stirred at room temperature overnight, and then
0.4 mmol) were mixed in a 500 mL Schlenk flask equipped with
magnetic stirrer and air condenser. The flask was purged under
vacuum and backfilled with N three times, and then degassed.
2
2
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heated at reflux until no starting material was observed by TLC.
available free of charge at http://www-xray.fzu.cz/jana/jana.html.
Organic solvents were removed in vacuo and the aqueous 60 Ex situ TGA/PXRD experiments were conducted by heating a
solution acidified with 20% aqueous hydrochloric acid until the
pH value was adjusted to approximately 2. The resulting white
precipitate was collected by vacuum filtration, washed with
fresh sample of PCN-39 to 348 K, 379 K, 393 K, or 412 K at a
rate of 10 ˚C/min, allowing the sample to cool to room
temperature in a nitrogen atmosphere, then transferring to a Si
single crystal zero diffraction plate and the X-ray diffraction
5
0
5
0
5
0
5
0
5
0
5
water, and dried in an oven to yield H pbtd-OEt in quantitative
4
1
6
yield. H NMR (300 MHz, DMSO-d ): δ 12.98 (br, 4H), 7.98 (d, 65 pattern collected.
8
6
H), 7.70 (d, 8H), 7.35 (s, 4H), 3.19 (q, 4H), 1.81 (s, 6H), 0.70 (t,
H).
Three separate in situ X-ray powder diffraction studies were
performed on Beamline 1-BM-C at the Advanced Photon Source
at Argonne National Laboratory: heating-cooling under closed
system, heating-cooling under He flow, and flowing He under
1
1
2
2
3
3
4
4
5
5
Synthesis of PCN-38·xS, Cu (H O) (pbtd-OMe)·xS. To a 2 mL
2
2
2
glass vial was added 0.005 g (0.0068 mmol) H pbtd-OMe and
4
0
.015 g (0.064 mmol) Cu(NO ) ∙ 2.5 H O dissolved in 1.5 mL 70 isothermal conditions.
3 2 2
N,N-dimethylacetamide (DMA) with 4 drops HBF4 (48% min
w/w aqueous solution). The reaction was kept at 85 °C in an oven
for 72 h. The resulting blue-teal crystals of Cu (H O) (pbtd-
For the heating-cooling cycle under closed system, a fresh
sample of PCN-39 was transferred to a capillary tube, and then
introduced to a flow cell with both input and output valves
closed. The sample was heated from 295 – 473 K at a rate of 2
75 K/min, maintained at 473 K for 30 min, and then quickly cooled
to room temperature while collecting diffraction data. The
heating-cooling cycle under He flow was conducted by
transferring a fresh sample of PCN-39 to a capillary tube, then
introducing the sample filled capillary to a flow cell connected to
2
2
2
OMe)·xS (PCN-28·xS, S=noncoordinated solvent molecule) were
then decanted, washed with fresh DMA, and collected.
Synthesis of PCN-39·xS, Cu (H O) (pbtd-OEt)·xS. To a 2 mL
2
2
2
glass vial was added 0.005 g (0.0065 mmol) H pbtd-OEt and
4
0.015 g (0.064 mmol) Cu(NO ) ∙ 2.5 H O dissolved in 1.5 mL
3
2
2
N,N-dimethylacetamide (DMA) with 4 drops HBF4 (48% min
w/w aqueous solution). The reaction was kept at 85 °C in an oven 80 He source. The input and output valves remained open during the
for 72 h. The resulting blue-teal crystals of Cu (H O) (pbtd-
experiment so as to allow fresh He to flow over the sample while
heating from 295 – 473 K at a rate of 3 K/min, maintaining the
temperature at 473 K for 30 min, and then cooling to 295 K at a
rate of 3 K/min. The isothermal experiment was conducted in a
2
2
2
OEt)·xS (PCN-29·xS, S=noncoordinated solvent molecule) were
then decanted, washed with fresh DMA, and collected.
X-ray Crystallography. Single crystal X-ray diffraction (XRD)
measurements were performed on a Bruker SMART APEXii 85 similar manner as the previous experiment, but with maintaining
diffractometer equipped with CCD detector, an Oxford
Cryostream low temperature device, and a fine-focus sealed-tube
X-ray source (Mo-Kα radiation, λ = 0.71073 Å, graphite
monochromated) operating at 50 kV and 30 mA. Raw data
collection and refinement were performed using SMART. 90 sizes.
Absorption corrections were applied using the SADABS routine.
the temperature at 295 K with continuous He flow overnight. For
each diffraction pattern, the sample was exposed to synchrotron
X-ray radiation (λ = 0.60570 Å) 20 times over 6 sec, rocking the
sample +/- 5˚ to reduce artifacts from non-monodisperse particle
Low-Pressure Gas Adsorption Measurements. Gas sorption
The structures were solved through direct methods and refined by
isotherm measurements were performed on ASAP 2020 and
ASAP 2420 Surface Area and Pore Size Analyzers. As-
synthesized samples of PCN-38·xS and PCN-39·xS were
2
full-matrix least-squares on F with anisotropic displacement
4
3
using SHELXTL. Non-hydrogen atoms were refined with
anisotropic displacement parameters during the final cycles. 95 immersed in dry methanol for 24 h and the extract decanted.
Hydrogen atoms on carbon were calculated in ideal positions
with isotropic displacement parameters set to 1.2 x Ueq of the
attached atom. Solvent molecules in the structures were highly
disordered and attempts to locate and refine solvent peaks were
Fresh dry methanol was subsequently added and the crystals
remained in the solvent for an additional 24 h. Each sample was
collected by decanting and the procedure repeated once more
with dry methanol, and three subsequent times with dry
unsuccessful. Therefore, the contribution of solvent electron 100 dichloromethane. After the removal of dichloromethane by
density was removed by the SQUEEZE routine in PLATON and
decanting, the samples were activated by drying under a dynamic
vacuum at room temperature and at 75 °C overnight to produce
activated samples of PCN-38 and -39 (PCN-38(ac) and PCN-
39(ac)). Before the measurement, PCN-38 was again further
105 activated using the “degas” function of the surface area analyzer
for 4 h at 60 °C, and PCN-39 for 5 h at 80 °C. Other activation
temperatures were tested, with the reported methods providing
the best sorption properties. UHP grade (99.999%) N , H , CO ,
4
4
refined further using the data generated. The contents of the
solvent region are not represented in the unit cell contents in the
crystal data. The details for data collection and refinement are
included in the CIF files in the Supporting Information.
Powder X-ray diffraction (PXRD) patterns were collected on a
Bruker D8-Focus Bragg-Brentano X-ray Powder diffractometer
equipped with a Cu sealed tube (λ = 1.54178 Å) and graphite
2
2
2
-
1
monochromator at a scan rate of 1 s deg , solid state detector,
and a routine power of 1600 W (40 kV, 40 mA). The samples
CO, and CH were used for all measurements.
4
were dispersed on Si single crystal zero diffraction plate for 110 Results and Discussion
analysis. Simulation of the PXRD pattern was carried out by the
single-crystal data and diffraction-crystal module of the Mercury
program available free-of-charge via internet at
http://www.ccdc.cam.ac.uk/products/mercury/. Full pattern
#
Syntheses and Structures of PCN-38·xS and PCN-39·xS.
Under solvothermal conditions, reactions between Cu(NO ) ·2.5
3
2
H O and H pbtd-OR in DMA afforded teal-blue crystals of PCN-
2
4
3
8·xS (R = Me) and PCN-39·xS (R = Et), which has a formula of
decomposition was performed using the Jana2006 program
1
15 Cu (H O) (pbtd-OR)·xS. Single-crystal XRD analyses revealed
2
2
2
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Fig. 2 (a) View of the distorted PtS nets of PCN-38, PCN-39(P1), and PCN-39(C 2/c). (b) View along topologically equivalent directions in PCN-38
left), PCN-39(P1) (middle), and PCN-39(C 2/c) (right) illustrating the difference in alignment of dicopper paddlewheels. The change in alignment of
(
Cu
2
-SBUs upon activation of PCN-39 leads to a redistribution of pore sizes and selective adsorption of H
coordinated solvent excluded, for clarity.
2
. Cu = turquoise; C = grey; O = red; H and
5
4
-
̅
that PCN-38 crystallizes in the triclinic space group P with unit
cell parameters a=20.639, b=23.923, c=23.247 Å, α=82.718,
β=71.061, and γ=80.352° (CCDC-950465). Two separate phases
of PCN-39 were identified through single-crystal XRD studies,
one Cu(II) ion and half of the pbtd-OEt ligand, owing to the
higher symmetry of the framework. The solvent accessible
volume is 60.2%. Topologically, when considering the dicopper
paddlewheel and ligand as 4-connected nodes, each framework
̅
1
1
2
2
3
0
5
0
5
0
one which crystallizes in the triclinic space group P with unit 35 possesses a distorted pts net with the point (Schläfli) symbol
2
4
cell parameters a=15.021, b=16.620, c=19.295 Å, α=102.288,
β=91.786, and γ=93.300° , denoted PCN-39(P1) (CCDC-
50464), and one which crystallizes in the monoclinic space
group C 2/c with unit cell parameters a=20.530, b=20.773,
c=18.804 Å, β=90.945˚, denoted PCN-39(C 2/c) (CCDC- 40 39(C 2/c) is in the alignment of dicopper paddlewheels, which
[4 ∙8 ]
and
vertex
symbol
[4.4.8(7).8(7).8(7).8(7)]
[4.4.8(2).8(2).8(8).8(8)] calculated with the TOPOS software
package, Figure 2.
9
The most obvious difference between PCN-39(P1) and PCN-
9
50392).
leads to a redistribution of pore sizes between the two phases.
The largest channels in PCN-39(P1) are accessible from pore
openings of 4.0 Å, whereas PCN-39(C 2/c) contains openings
only as large as 3.0 Å. PCN-38 and -39 were observed to be
In PCN-38, two Cu -paddlwheel SBUs connect to two pbtd-
three-dimensional porous
framework with solvent accessible volume of 70.7% (calculated
using PLATON), and open pores with dimension 6 ⨯7 Å along 45 stable over 260 ˚C, from thermogravimetric analyses (Figure S5).
the [1 0 1] direction and two types of pores along the [0 1 1]
direction, 6 ⨯24 Å and 8 ⨯9 Å (atom to atom distances minus
van der Waals radii). The asymmetric unit in PCN-39(P1)
consists of two Cu(II) ions and one pbtd-OEt ligand. The
2
4
-
OMe
ligands to construct
a
4
4
2
4
-
The pbtd-OR ligands are considered “semi-flexible” due to
the sp -hybridized carbons connecting the isopthalate moieties
2
2
3
that allow for flexibility in the central bond angle. The pendant
alkyl groups bound to the 4-position of the tetrasubstituted phenyl
4-
inversion center in PCN-39(P1) lies in the middle of one of the 50 rings are believed to play a role in stabilizing the frameworks by
dicopper
paddlewheels,
such
that
there
are
two
filling void space within the pores of the system. This appears to
be more significant for the framework with longer pendant group,
PCN-39, and is believed to be the genesis of the observed
structural changes in the MOF (see Figure S6 and Table S1 for
crystallographically distinct Cu(II) ions in the asymmetric unit
that make up two distinct dicopper paddlewheel building units.
The solvent accessible volume in PCN-39(P1), as calculated
using PLATON, is 66.1%. Whereas PCN-39(C 2/c) consists of 55 overlay of ligands in PCN-39(P1) and -39(C 2/c)).
This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 4

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DOI: 10.1039/C3TA12761C
Page 5 of 8
Journal of Materials Chemistry A
upon exchanging the larger, high boiling point solvent (DMA)
Fig. 3 PXRD patterns of PCN-39 upon exposure to atmospheric
conditions for time, t, compared to simulated patterns of PCN-39(C 2/c)
and -39(P1) from single crystal XRD data. PCN-39(as) exposed to air
<1 min before collecting PXRD pattern; (as) = as-synthesized, (ac) =
activated sample.
Fig. 4 In situ synchrotron PXRD patterns of PCN-39 upon heating in a
closed system at temperature, T, compared to PXRD patterns of PCN-
5
4
5
3
9(P1) and -39(C 2/c) simulated from single crystal data, and PCN-
9(ac) activated in He stream.
3
Importantly, in both triclinic systems, PCN-38 and -39(P1),
there are observed methyl-methyl close contacts between methyl
groups extending from the central methylene bridges on nearby
linkers (3.774(5) and 3.744(6) Å C-C distances, respectively).
PCN-39(C 2/c), however, increases the separation between these
central methyl groups to 7.143(0) Å due to redistribution of pore
sizes (Figure S7).
Thermogravimetric analyses indicate a mass loss of ~31% for
both materials up to 250 °C. This relates to a loss of one
coordinated water molecule and ~2 DMA molecules for each
copper ion.
Structural transformation of PCN-39. Bulk samples of PCN-39
were synthesized and evaluated by Le Bail full pattern
decomposition with initial unit cell parameters and space groups
obtained from single-crystal XRD data (Figures S8-S11). Bulk
samples of as-synthesized PCN-39 were determined to be
predominantly PCN-39(P1) upon initial characterization, with
subsequent measurements showing significant increase in the
presence of PCN-39(C 2/c) and decrease of PCN-39(P1) upon
extended exposure to air, Figure 3. Additionally, a fresh sample
of PCN-39 was exchanged with methanol and dried at room
temperature under dynamic vacuum for 1 h and PXRD measured,
showing complete disappearance of the original PCN-39(P1)
phase with conversion to the PCN-39(C 2/c) phase. Since PCN-
with a smaller guest species (methanol or dichloromethane), the
same structural transformation is observed.
1
1
2
2
3
3
4
0
5
0
5
0
5
0
5
5
6
6
7
7
8
0
5
0
5
0
5
0
The observed structural transformation may be described as a
reorganization of the bound ligand to a more symmetric, lower
energy conformation. We propose that the removal of excess
DMA molecules from the open channels of the system or
exchange of DMA for smaller guests promotes the relaxation of
the framework to allow pendant ethoxy groups to minimize
empty space within the system. Based on the observations of
quick transformation upon removal of PCN-39(as) from the
mother liquor, it can be deduced that the monoclinic form is
considerably favorable over the triclinic form, resulting in the
“squeezing” of excess DMA molecules from the pores. A similar
transformation is not observed for PCN-38 likely because the
4-
pendant methoxy groups on pbtd-OMe do not possess the same
degree of flexibility as ethoxy groups, nor occupy as much free
volume.
To further probe the structural transformation process PCN-39
was analyzed by in situ PXRD. A freshly prepared sample was
transferred to a capillary tube and enclosed in a closed system,
then heated from 295 K to 473 K and cooled to 295 K while
collecting diffraction patterns. Four distinct transformations in the
crystal structure were observed from the in situ diffraction study,
Figure 4. The first phase observed (PCN-39(as)) is predominantly
that of PCN-39(P1) and is present from 295 – 375 K (Figure
S12). A transition occurs from 375 – 379 K, resulting in a
mixture of PCN-39(P1) and PCN-39(C 2/c) (Figure S13). From
383 – 404 K a phase or mixture of unknown structure is
observed, and transitions into a second, distinct phase or mixture
of unknown structure (activated phase) that remains from 412 K –
3
9(P1) quickly loses solvent and subsequently undergoes
structural transformation, a fresh sample of PCN-39 was removed
from the mother liquor and excess solvent removed from the
surface of the crystalline powder by transferring to a filter paper
before introducing the sample to a Si single crystal zero
diffraction plate (< 1 min exposure to air) for PXRD analysis.
The PXRD measurement was repeated another 3 times after the
original measurement, one after another, the most significant
observation being a shift in the relative intensities of the first two
reflections at 2θ≈5.5˚ (corresponding to PCN-39(P1)) and 6.0˚
4
73 K. Upon cooling, the DMA in the closed system condensed
and resulted in transformation from the activated phase back to
the originally observed PCN-39(as) (predominantly PCN-
(
present in both PCN-39(P1) and PCN-39(C 2/c)). Furthermore,
3
9(P1) species) at 295 K with no apparent loss of crystallinity
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Journal Name, [year], [vol], 00–00 |
5

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DOI: 10.1039/C3TA12761C
Journal of Materials Chemistry A
Page 6 of 8
3
(Figure S14).
pendant –OEt group along with the central sp -hybridized carbon
is proposed to allow for optimal filling of empty space within the
pores upon activation by shifting its conformation to eliminate
“dead volume” as larger guest molecules are removed, forming a
5
6
6
5
0
5
vacuum. This guest responsive transformation plays a role in
stabilizing the framework and reduces the size of pore apertures,
creating pores accessible only to H2 and smaller guests. The
inability of PCN-39 to adsorb any gas larger than H reveals the
2
role of the pendant ethoxy group in blocking the accessible pores
and lends this system to application in pre-combustion hydrogen
purification. While PCN-39 performs about average as a H2
2 2
Fig. 5 (a) N and (b) H uptake of PCN-38(ac) and -39(ac) at 77 K. Solid
symbols represent adsorption and open symbols desorption.
storage medium, compared to other reported porous materials, the
significant selectivity of the framework toward H over CO , N ,
2
2
2
5
0
5
0
5
0
5
0
5
0
Attempts to isolate individual phases at each of the transitions
and CO highlights the potential of this material to serve in pre-
combustion hydrogen purification systems.
were performed via ex situ TGA/PXRD experiments. However,
only two separate phases were isolated, in addition to the initial
predominantly PCN-39(P1) phase. The first was isolated from
heating a fresh sample of PCN-39 to 348 K, and, as expected,
corresponds to predominantly PCN-39(C 2/c). Heating fresh
samples of PCN-39 to 379 K, 393 K, and 412 K all resulted in the
isolation of material of the same crystalline phase with unknown
structure (Figure S15).
The in situ PXRD experiment conducted under He flow
showed isolation of the activated phase at 473 K, and retained its
crystallinity in the activated form upon lowering the temperature
back to 295 K, Figure 4. Unlike the closed system experiment,
Conclusions
1
1
2
2
3
3
4
4
5
In this report, two isostructural metal-organic frameworks (PCN-
38 and -39) have been synthesized with tetratopic ligands
featuring pendant alkoxy groups with semi-flexible backbones.
70 From PCN-38 to -39, the framework is instilled with guest
responsive properties by replacing methoxy groups on the central
phenyl rings with ethoxy groups. The increased length and
flexibility of the ethoxy groups induce structural transformations
upon guest removal and activation through optimized space-
the He stream carries away all evaporated DMA, eliminating the 75 filling of the evacuated pores. This feature leads to H
adsorption
capacity up to 2.0 wt% at 77 K and 1 bar, and selectivity over N
CO, and CO in PCN-39. Whereas, PCN-38, with shorter
methoxy pendant group, undergoes no such structural
transformation and adsorbs CO, CO , N , and H (2.2 wt% at 77
2
possibility of reintroduction to the pores which facilitates the
structural transformation back to PCN-39(as). The fully activated
sample is shown to be stable in inert atmosphere, and retains
crystallinity.
,
2
2
2
2
2
Gas sorption. To investigate the permanent porosity of PCN- 80 K and 1 bar). The structural transformations of PCN-39 were
3
8 and PCN-39, gas sorption experiments evaluating N , H ,
monitored via in situ PXRD measurements and the integrity of
the crystalline framework confirmed upon desolvation, returning
full-circle to the original framework after heating and cooling in a
closed system. This work is a successful demonstration of the
2
2
CO , CO, and CH uptake were performed. The freshly prepared
2
4
samples of PCN-38 and PCN-39 were first solvent exchanged
with methanol, then dichloromethane, resulting in a color change
from teal in DMA, to green in methanol, to blue in 85 role of pendant alkoxy groups in minimizing framework energy
dichloromethane, and finally deep purple upon activation, typical
of dicopper(II)-paddlewheel frameworks. The N2 sorption
isotherms, Figure 5a, reveal that PCN-38(ac) exhibits typical
Type I sorption behavior, a characteristic of microporous
and forming guest (stimuli) responsive materials capable of
selectively adsorbing H over other gases. This result improves
upon previous efforts at selective adsorption of H over N for
such applications as isolation of H from ammonia synthesis
Further studies will focus on exploring
2
2
2
2
4
5
47, 48
materials, which is congruent with the crystal structure. PCN- 90 waste streams.
3
8(ac) exhibits moderate N uptake at 1 bar and 77 K (ca. 270
pendant alkoxy groups to design gate-opening and water stable
MOFs.
2
3
-1
cm g ). By applying the BET model, the apparent surface area is
estimated to be ~911 m g (1134 m g Langmuir). At 77K and 1
2
-1
2 -1
bar, PCN-38(ac) adsorbs a reasonably high amount of H , 2.2
Notes and references
2
wt%, Figure 5b , comparable to the MOF Zn (btbz), which
a
2
Department of Chemistry, Texas A&M University, College Station, TX
4
6
possesses similar surface area and H uptake capacity. PCN-
2
95 77840 (USA). Fax: (+1) 979-845-1595; Tel: (+1) 979-845-4034; E-mail:
zhou@chem.tamu.edu
39(ac) also adsorbs moderate amounts of CH , CO , O , and Ar
4
2
2
(
Figures S16 and S17).
PCN-39(ac), however, undergoes a redistribution of pore sizes
PCN-39(P1) to -39(C 2/c) and likely further) during the
‡ Current address: Department of Natural Sciences, The University of
Virginia’s College at Wise, Wise, VA 24293 (USA).
†
Electronic Supplementary Information (ESI) available: NMR spectra,
(
1
1
00 thermogravimetric analyses, PXRD and Le Bail fits for PCN-39(P1) and
-39(C 2/c), overlay of conformations of ligand in PCN-39(P1) and -39(C
2/c) and data table evaluating conformations, additional gas sorption
isotherms, and synchrotron PXRD patterns of PCN-39 from in situ
measurements. See DOI: 10.1039/b000000x/
05 # CCDC-950392, 950464, and 950465 contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
activation process that precludes the adsorption of N into the
2
pores. Instead, PCN-39(ac) adsorbs up to 2.0 wt% H (kinetic
2
diameter 2.89 Å) selectively over CO (3.69 Å), N (3.64 Å), and
2
CO (3.30 Å) (Figure S18). The observed hysteresis in the H
2
2
isotherm is attributed to flexibility in the system, resulting in a
tight fitting of the dihydrogen molecules in the pores and
retention of H guests at lower pressures. The flexibility of the
2
6
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Journal of Materials Chemistry A
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