Non-interpenetrated Cu-based MOF constructed from a rediscovered tetrahedral ligand

Article abstract of DOI:10.1039/c7ce01818e

A systematic expansion of the well-known non-interpenetrated and robust MOF-11 was accomplished through a rediscovery of the ligand tetrakis(4-carboxyphenyl)adamantane (L). The combination of a rigid tetrahedral L and copper(ii) paddle-wheel secondary building unit (SBU) resulted in a stable non-interpenetrated metal-organic framework, UOF-1 (UOF = University of Otago framework), with the predicted PtS topology. This MOF exhibits permanent porosity, as established by X-ray diffraction (XRD) and gas adsorption studies. This low-density open framework possesses one open-metal site per Cu(ii) cation which could be accessed by employing appropriate activation techniques such as the use of supercritical CO₂ (s-CO₂) or dry CH₂Cl₂. CO₂ gas displayed higher affinity towards activated UOF-1 than CH₄ and N₂ gases at room temperature and low pressure. UOF-1 displayed excellent selectivity for CO₂ over N₂ of 52:1 at practical operating conditions of 15:85 CO₂:N₂ mixture at 273 K and 1 bar.

Full text of DOI:10.1039/c7ce01818e

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ARTICLE
Non-Interpenetrated Cu-based MOF Constructed from a
Rediscovered Tetrahedral Ligand
Komal M. Patil,a Shane G. Telfer,b Stephen C. Moratti,a Omid T. Qazvinib and Lyall R. Hanton+,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A systematic expansion of the well-known non-interpenetrated and robust MOF-11 was accomplished through a
rediscovery of the ligand tetrakis(4-carboxyphenyl)adamantane (L). The combination of a rigid tetrahedral L and copper(II)
paddle-wheel secondary building unit (SBU) resulted in a stable non-interpenetrated metal-organic framework, UOF-1
(UOF = University of Otago framework), with the predicted PtS topology. This MOF exhibits permanent porosity, as
established by X-ray diffraction (XRD) and gas adsorption studies. This low-density open framework possesses one open-
metal site per Cu(II) cation which could be accessed by employing appropriate activation techniques such as the use of
supercritical CO2 (s-CO2) or dry CH2Cl2. CO2 gas displayed higher affinity towards activated UOF-1 than CH4 and N2 gases at
room temperature and low pressure. UOF-1 displayed excellent selectivity for CO2 over N2 of 52:1 at practical operating
www.rsc.org/
conditions
of
15:85
CO2:N2
mixture
at
273
K
and
1
bar.
(OMS) and large surface areas with the potential of
incorporating guest molecules.9-13 These features have opened
new avenues for storage, separation, catalysis, drug
encapsulation, and magnetism and sensor applications for
MOFs.14-19 A key step in the synthesis of a highly porous MOF
with these desirable features is preventing network
interpenetration. Additionally, non-interpenetrated MOFs
display lower densities than the analogous interpenetrated
Introduction
The rigid skeleton and highly symmetric tetrahedral shape of
the adamantane core (C10H12R4, Figure ) provides an ideal
1
geometry for designing valuable building blocks for the
synthesis of engineered materials such as dendrimeric
polymers, covalent-organic frameworks, supramolecular
architectures and metal-organic frameworks (MOFs).1-4 In the
early development of MOF chemistry, this tetrahedral
adamantane core was used to understand the principles of
reticular synthesis.5-7 However, there has been a temporary
decline in the construction of MOFs with adamantane-based
ligands due to the perception that tetrahedral platforms yield
too dense or interpenetrated structures often with a dia
topology.8 For example, the solvothermal reaction of
MOFs thus providing
a gravimetric advantage to these
materials.20 However, the “defaults” in reticular synthesis,
which contribute to nearly 50% of the 3D networks reported in
the literature, have either a dia or pcu topology and both of
these topologies are prone to interpenetration.21 In order to
prevent network interpenetration, it is essential to identify
topologies that are more resistant towards interpenetration.
Herein, we rediscovered the ligand
L with the intention of
tetrakis(4-carboxyphenyl)adamantane (L) and a tetrahedral
preventing network interpenetration by isoreticulation of the
well-known non-interpenetrated and robust MOF-11.5 Thus by
Zn(II) secondary building unit (SBU) results in MOF-33 with
predicted dia topology and two-fold interpenetration.6 This
combining tetrahedral
wheel secondary building unit (SBU) non-interpenetrated
{[Cu2( )(H2O)2] nSolvent}∞ UOF-1) with predicted PtS topology
was obtained (Figure ). The key features of UOF-1 were a low-
L and a square-pyramidal Cu(II) paddle-
was the only MOF reported using
with a series of multidentate carboxylate ligand based MOFs.
Despite this article being highly cited, has been overlooked
L and it was published along
L
·
(
L
1
and has received surprisingly little, to no attention.
density open framework and potential one OMS per Cu(II)
cation. The OMS could be accessed by employing appropriate
activation techniques such as the use of supercritical CO2 and
Over the past decade, there have been tremendous efforts in
the synthesis of MOFs with astonishingly low densities,
accessible coordinatively unsaturated or open metal sites
23
dry CH2Cl2.22, These frameworks were characterised using
infrared spectroscopy, elemental and thermogravimetric
analysis (TGA), XRD and low-pressure gas adsorption
measurements at a range temperatures. The activated UOF-1
displayed high affinity towards CO2, slight affinity towards CH4
but very limited affinity towards N2 gas at 298 K and 1 bar.
UOF-1 displayed an excellent selectivity for CO2 over N2 of 52:1
and a good selectivity for CO2 over CH4 of 7.5:1 at practical
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operating conditions of 15:85 CO2:N2 or CO2:CH4 mixtures at 273
Journal Name
K
and
1
Figure 1. Top: Schematic representation of the combination of tetrahedral L with a [Cu2(COO)4(H2O)2] Cu(II) paddle-wheel SBU to result in the non-interpenetrated UOF-1 with PtS
topology. The [Cu2(L)(H2O)2]∞ framework of UOF-1 is displayed along the a axis showing channels of two different types. Bottom: A view of UOF-1 showing 1D channels along the c
axis. The SQUEEZEd solvent molecules are omitted from the figure. The O atoms are shown in red, Cu(II) cations in green and C atoms in black. H atoms have been omitted for
clarity.
bar. The gas adsorption studies also showed that this MOF Bruker microTOFQ instrument (Bruker Daltronics, Bremen,
absorbed 97 cm3 g–1 (0.9 wt.%) of H2 gas at 77 K and 1 bar. Germany). The sample was introduced using direct infusion
PXRD measurements indicated that UOF-1 was stable in water into an ESI source in a negative mode. Melting points were
for at least 24 hours.
recorded on Digimelt SRS (MPA161) melting point apparatus
using open capillary tubes. TG analyses were performed on a
TGA Q50 V20.10 Build 36 device with a platinum pan and
dinitrogen as balance and sample gas at a temperature
increase rate of 5 or 20 °C/min. The DSC analyses were
performed on a Thermal Advantage Q2000–1687 apparatus
between –50 and 250 °C at a rate of 2 °C/min with a sample
weight of 3–5 mg. The powder material was finely ground and
EXPERIMENTAL SECTION
Material and Methods
All chemicals from commercial sources were used as received
without
further
purification.
Tetrakis(4-cyanophenyl)-
adamantane was synthesized by the reported procedure.24
Infrared spectra were recorded on a Bruker Optics Alpha FT-IR
spectrometer (with a diamond attenuated total reflectance
top-plate). Microanalyses were carried out by the Campbell
Microanalytical Laboratory, University of Otago. NMR spectra
were collected on a 400 MHz Varian 400-MR spectrometer at
298 K. The NMR spectra were referenced to the internal
homogenised. The data was collected at 2θ from 5° to 50° with
the step size of 0.034°. Simulated powder X-ray diffraction
pattern was generated from the single crystal data using
Mercury 3.5.1. The gas adsorption isotherms were measured
by a volumetric method using a Quantachrome autosorb iQ2
instrument. Ultra-high purity CH4, CO2, H2 and N2 gases were
used for adsorption measurements. The temperatures of 77,
195 and 273 K were maintained using liquid N2, dry ice-
solvent signals, with chemical shifts reported in δ units (ppm).
Electrospray mass spectroscopy (ESMS) was carried out on a
2 | J. Name., 2012, 00, 1-3
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acetone, and ice-water mixtures, respectively. The radiation. The data were treated using CrysAlisPro26 software
temperature of 298 K was maintained using a controlled and Gaussian absorption corrections were applied. The
thermostated water bath. The data were processed using structures were solved by direct methods SHELXT-201427 and
Quantachrome VersaWin (Version 1) software and OriginPro refined on F2 using all data by full-matrix least-squares
9.1.
procedures SHELXL-2014,28 interfaced through the program
WINGX.29 Non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were placed in ideal
positions except for H-atoms of the axial water molecule. H-
atoms of the axial water molecule could not be located in the
Fourier maps. Detailed analyses of the extended structure
were carried out using PLATON30 and CrystalMaker (Version
9.2.5).31 Owing to the high symmetry space group and low
residual electron density, it was impossible to locate solvent
molecules in some structures. These structures were treated
with PLATON SQUEEZE32 and the electron contribution from
disordered solvent molecules was calculated using this
program (Section S1.2). Crystallographic data are listed in
Synthesis of 1,3,5,7-tetrakis(4-carboxyphenyl) adamantane
(L)
L
was synthesized using a slight variation of the reported
24
A mixture of tetrakis(4-
procedure (Scheme S1).4,
cyanophenyl)adamantane (1.79 g, 3.31 mmol) and KOH (3.31
g, 58.9 mmol) in 40 mL ethylene glycol was heated for about
72 hours at 180 °C. The reaction mixture was diluted with 50
mL H2O after the reaction mixture had cooled to room
temperature. The product was precipitated with conc. HCl,
filtered, washed with H2O and dried in an oven at 120
°C for an
hour to yield 1,3,5,7-tetrakis(4-carboxyphenyl)adamantane (
L)
1
as a grey coloured solid. Yield: 1.92 g, 94%. H NMR (400 MHz,
d6–DMSO) δ: 12.81 (s, 4H, –COOH), 7.92 (d, J = 8.7 Hz, 8H, Ar3-
H), 7.73 (d, J = 8.7 Hz, 8H, Ar2-H), 2.17 (s, 12H, Ad-CH2). 13C
NMR (101 MHz, d6–DMSO) δ: 167.3 (COOH), 154.3 (Cq-Ar1),
129.3 (C-Ar3), 128.5 (C-Ar4(COOH)), 125.6 (C-Ar2), 45.6 (Cq-
Ad). Selected IR/cm–1: 3352 (w, br), 2927 (m, sh), 2852 (m, sh),
1675 (vs, sh), 1606 (m, sh), 1414 (m, sh), 1274 (s, sh), 1185 (m,
br), 1083 (m, br), 848 (m, sh), 753 (s, sh), 533 (m, br). Analysis
Table 1 and Table S1.
Results and discussion
Synthesis
The solvothermal reaction of Cu(NO3)2·3H2O and L in 1:1 v/v
solvent mixture of DMF and C2H5OH resulted in the formation
of turquoise crystals of {[Cu2( )(H2O)2] nDMF}∞ UOF-1.DMF) in
found:
C
70.9,
H
5.5; Calculated for the formula
L
·
(
very small quantities over two months (Figure S1).33 Two
SCXRD data sets were collected on this compound, however,
the quality of the data was very poor. Many attempts to
reproduce these crystals, by varying the solvents and Cu(II)
salts, using acids such as HCl and HNO3 at various stages of
synthesis, and also by varying reaction conditions, were
unsuccessful. In most of the cases, either clear solutions or
very small quantities of microcrystalline precipitates were
obtained. In an attempt to improve the quantity and quality of
the crystals, the following modifications were successful.
Concentrated HCl was replaced by dilute HBF4 (2.4% v/v
dilution in DMF) and instead of using pressure tubes and a
heating and cooling regime, the reaction was carried in a
Borosil vial by heating the reaction mixture in an isothermal
oven at 80 °C for about 24 hours. The resultant crystals, of
UOF-1.DMF (Figure S1), obtained by adopting these
modifications (using bulk synthesis procedure) were in better
C38H32O8
·1.5H2O: C 70.9, H 5.5. ESMS m/z: Calculated for
–
C38H31O8 615.2024, Calculated for C114H94O24 923.8090 and
2–
2–
calculated for C156H126O32 1231.4121, Found 615.2021,
923.8052 and 1231.4061. Melting point: >250 °C
Bulk Synthesis of UOF-1
The ligand
dilute HBF4 solution (2.4% v/v dilution in DMF). To this
solution, 5 mL of a DMF solution of Cu(NO3)2 3H2O (0.644 g,
L (0.107 g, 17.3 mmol) was dissolved in 30 mL of
·
0.266 mol) was added and the reaction was carried out in a
Borosil vial by heating the reaction mixture in an isothermal
oven at 80 °C for about 24 hours to result in turquoise
coloured crystals of {[Cu2(
Yield (based on
): 82 mg, 61%. Selected IR/cm–1: 2926 (w, br),
L)(H2O)2]·14DMF}∞ (
UOF-1.DMF).25
L
2853 (w, sh), 1702 (m, sh), 1607 (m, sh), 1544 (m, sh), 1401
(vs, sh), 1355 (s, sh), 1187 (m, sh), 1105 (m, sh), 1015 (m, sh),
853 (m, sh), 763 (s, sh), 709 (m, sh), 681 (m, sh), 621 (m, sh),
526 (m, sh). The DMF from the as-synthesized crystals was
exchanged with acetone to yield turquoise coloured crystals of
{[Cu2(L)(H2O)2]·18(CH3)2CO}∞ [
UOF-1.(CH3)2CO].25 After soaking
Table 1. Crystal data for UOF-1
the crystals in dry acetone, the solvent was removed and fresh
DEF was added to yield turquoise coloured crystals of
{[Cu2(L)(H2O)2]·8DEF}∞ (UOF-1.DEF, Table 1). The DMF from
the as-synthesized crystals was exchanged with dry CH2Cl2 to
yield dark blue coloured crystals. These crystals upon standing
in air resulted in turquoise coloured crystals of
{[Cu2(L)(H2O)2]·
14CH2Cl2}∞ (UOF-1.CH2Cl2).25
Structural Crystallography
Single crystals of UOF 1 were mounted in Paratone-N oil on a
nylon loop. SCXRD data were collected on an Agilent
Technologies Supernova system at 100 K at the University of
Otago using mirror monochromated Cu Kα (λ = 1.54184 Å)
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Journal Name
Structure
UOF-1.DEF
Z
4
100(2)
Formula
C19H15CuO5(C5H11NO)4
791.46
T/ K
/ mm–1
Formula weight
Crystal System
Space group
a=b/ Å
ꢀ
0.860
Tetragonal
P42/mmc
Total reflections
Unique reflections (Rint
R1 indices [I>2σ(I)]
ωR2 (all data)
10870
)
1696 (0.0835)
0.0967
0.3027
1.017
14.3700(13)
26.464(3)
90
c/ Å
α
=
β
=
γ
/°
Goodness-of-fit
V/ Å3
5464.8(12)
Figure 2. A comparative view of the section of the isoreticular congeners– MOF-11 (left) and UOF-1 (right) showing the difference in the size of the cavities. The diagrams are
constructed by defining a 50 Å cluster around the Cu(II) cations of each MOF. The O atoms are shown in red, Cu(II) cation in green and C atoms in black. H atoms and solvent
molecules have been omitted for clarity
yield and were also straightforward to synthesize. The quality SQUEEZEd from the structure (Section S1.2). The residual
of the SCXRD data of these crystals was also poor and the electron density was assigned to four DEF molecules per Cu(II)
crystals did not diffract beyond a resolution of 1.75 Å. cation in a total void volume of 4349 Å3. Analysing the
Importantly, the crystal parameters for the samples prepared SQUEEZEd structures of UOF-1 indicated that the DEF
by different routes were the same indicating an efficient molecules have replaced nine acetone and seven DMF
synthesis of this MOF with time of synthesis reduced from 2-3 molecules per Cu(II) cation.
months to 24 hours. PXRD measurements confirmed high The Cu(II) cations were bridged by the COO– moieties of the
phase purity of UOF-1.DMF (Figure S5). In an attempt to get eight-fold coordinating
better SCXRD data, the DMF from the as-synthesized crystals The arrangement of four COO– moieties bridging two Cu(II)
was exchanged with acetone to yield turquoise coloured cations rigidly in a square-planar manner at the equatorial
L through ꢀ2–η1:η1 bridging modes.
crystals of {[Cu2(L)(H2O)2] ·18(CH3)2CO}∞ (
UOF-1.(CH3)2CO).25 positions [the Cu–O distances were between 1.929(7) Å and
After soaking the crystals in dry acetone, the solvent was 2.108(7) Å] and two H2O molecules at the axial positions [the
removed and fresh DEF was added to yield turquoise coloured Cu–O3 distance was 2.1957(8) Å] resulted in a square-
crystals of {[Cu2(
of DEF exchanged crystals were the best and are discussed in distance was 2.6550(9) Å. These square-pyramidal SBUs were
detail here. bridged by tetrahedral ligands to result in non-
UOF-1.DEF crystallised in the centrosymmetric tetragonal interpenetrating archetypal Cooperite (PtS) network (Figure ).
space group P42/mmc and formed a 3D non-interpenetrating PtS is the “default” topology for the combination of
L)(H2O)2]·8DEF}∞ (UOF-1.DEF). The SCXRD data pyramidal Cu(II) paddle-wheel SBU. In each SBU, the Cu···Cu
L
a
1
network. The asymmetric unit comprised of one-eighth of a
L
,
tetrahedral (S2–) and square planar (Pt2+) centres and has a
one-quarter each of a Cu(II) cation and an axial H2O molecule Schläfli vertex symbol of 4284.5, 34, 35 Although the PtS topology
and uncoordinated solvent molecules. Owing to the high is not inherently resistant to interpenetration,34,
non-
36-38
symmetry and low residual electron density, the solvent interpenetrated PtS networks are common. It is worth noting
molecules could not be adequately refined and were that most of the examples of non-interpenetrated PtS
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6, 39
networks have employed rigid tetrahedral ligands.5,
have been completely removed from the structure. A very
C might be due to the loss of
Interestingly, the combination of a tetrahedral tetrakis(4- shallow minimum around 150
°
carboxyphenyl)-methane ligand and a square-pyramidal Cu(II) trace amounts of DMF solvent. The absence of a minimum due
paddle-wheel SBU resulted in a non-interpenetrated PtS to axial H2O molecules in the DSC curve of desolvated UOF-
network.40 In contrast, its isoreticular congener constructed by 1.(CH3)2CO crystals pointed to the acetone molecules replacing
employing tetrakis(4-carboxyphenyl-4-phenyl)methane and the axial H2O molecules of these SBUs. These acetone
the same SBU resulted in a doubly interpenetrated PtS molecules cannot be removed from the MOF even after
network. The additional phenyl ring on each arm of the ligand prolonged drying of the sample under high vacuum. This
might have allowed the interpenetration of the frameworks in argument was further supported by the retention of the
this isoreticular congener.
sample’s turquoise colour throughout the process of acetone
UOF-1 possesses an ideal PtS network and comprised of four– guest exchange and desolvation. PXRD measurements of the
and eight–membered rings.36 These rings led to two types of bulk samples of UOF-1.(CH3)2CO confirmed high phase purity.
channels running along the crystallographic a and b axes. The The MOF did not collapse and retained the turquoise colour
internuclear distances along the diagonals for the larger upon exchanging acetone with water molecules. The PXRD
channels were 26.1×12.0 Å, while for the smaller channels the pattern of UOF-1.H2O collected after soaking the bulk samples
distances were 14.7×9.7 Å. The resultant PtS network formed of UOF-1.(CH3)2CO in water for 24 hours showed broadening
infinite square channels (the internuclear distances along the of peaks perhaps due to regional shifts within the channels on
diagonals of the channel were 17.3×17.3 Å) along the c axis the incorporation of water (Figure S6).
(Figure 1). The potential void volume of this 3D framework was In contrast, a sharp colour change from turquoise to dark blue
4430.5 Å3 accounting for 78.7% of the total unit cell volume. was observed when the dry CH2Cl2 solvent was used instead of
The potential void volume was slightly lower than the highest acetone (Figure S8). The dry CH2Cl2 exchanged samples (UOF-
reported value of 81% for a MOF with PtS topology (Table 1.CH2Cl2) appeared to be crystalline and upon desolvation
S3).41 These voids were occupied by disordered solvent under high vacuum at room temperature, the dark blue colour
molecules. The calculated density for the framework, from was maintained (Figure S8). The colour change [turquoise
which all the solvent molecules were hypothetically removed, (pristine crystals)
→ dark blue (CH2Cl2 coordinated paddle-
was 0.436 g cm–3.31 On the other hand, the density of wheel)
→
dark blue (square-planar paddle wheel)] can be
desolvated MOF-11 was 1.405 g cm–3. Thus the isoreticulation attributed to the replacement of the axial H2O molecules by
was successfully carried out along with a significant linear the CH2Cl2 molecules followed by exposure of the OMS of the
expansion of the cavities from 6.0–6.5 Å of MOF-11 to 17.3 Å copper-paddle wheels upon desolvation of CH2Cl2.44 The dark
of UOF-1 and a considerable reduction in framework density blue crystalline material quickly changed its colour to green
(Figure
2
).
upon exposure to air. The PXRD measurements of the bulk
samples of this crystalline material showed broadening of
peaks probably due to the incorporation of water from the
atmosphere (Figure S7). A dramatic change in space group
from tetragonal P42/mmc to orthorhombic Cccm was observed
by SCXRD. However, this change in space group had no impact
on the PtS topology. The framework collapsed in presence of
strong acid or alkali.
Stability and activation techniques
The TGA of UOF-1.DMF showed that the majority of occluded
solvent molecules were lost between room temperature and
ca. 100 °C (Figure S2). The experimentally-determined weight
loss of 64% was slightly higher than the calculated 14
molecules of DMF and two molecules of H2O per 2 Cu(II)
cations (calculated 59%, 1059 amu). Unfortunately, the MOF
collapsed after the loss of the solvent molecules in a concerted
Gas adsorption studies
manner probably due to the exposure of the OMS which led to The activated material of UOF-1.CH2Cl2 had the capacity to
43
destabilization of the copper-carboxylate SBU.42,
The TGA take up 114 cm3 g–1 of N2 at 77 K and possesses a BET surface
profile of the vacuum dried crystals of UOF-1.(CH3)2CO also area of 357 m2 g–1 (Figure S9).
showed a concerted collapse after the loss of residual acetone The activation of UOF-1 in supercritical CO2 (to give UOF-1.s-
possibly bound to the Cu(II) sites (Figure S2). In contrast, the CO2) proved to be efficient and this material took up 212 cm3
TGA profile of the native ligand
compound was stable up to 360 C under a N2 atmosphere 1.s-CO2 revealed a Type-IV behaviour with a steep rise in the
(Figure S3). This suggested that the UOF-1 was not thermally low P/P0 region and a second step at higher P/P0, which is
stable.
indicative of mesopores. A BET surface area of 570 m2 g–1
L
suggested that this g–1 of N2 at 77 K. The N2 adsorption isotherms at 77 K for UOF-
°
The thermal instability was further investigated using DSC. The (Figure S10) was calculated from this isotherm. While the
DSC curve of the as-synthesized crystals of UOF-1.DMF theoretical surface area of 4559 m2 g–1 calculated using Zeo++
showed two minima at 94.0 °C and 141.4 °C which could be software indicated that most regions of the MOF were
attributed to the loss of H2O and DMF solvent molecules inaccessible to incoming gas.45 The pore volume of 0.303 cm3g–
1
(Figure S4). The DSC curve of the vacuum dried crystals of
UOF-1.(CH3)2CO showed a minimum at 62.2
due to some residual acetone possibly bound to the Cu(II) functional theory (QSDFT) method and
sites. However, the coordinated H2O molecules seemed to adsorption kernel from the N2 isotherm of UOF-1.s-CO2 at 77 K
and half pore width of 5.255 Å were calculated with a very
°
C which could be low fitting error of 0.065% using the quenched solid density
a
slit-cylindrical
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Journal Name
(Table S4, Figure S13). The colour change [turquoise (pristine (H2O)2]
crystals) turquoise (after s-CO2 guest exchange) dark 1,1’:4’,1’’-terphenyl-3,3’’,5,5’’-tetracarb-oxylic acid] which
·
Solvent}∞ (27.1 kJ mol-1) [where atptca = 2’-amino-
→
→
blue (with subsequent heating at 298 K while under vacuum)] have demonstrated excellent CO2 sorption selectivity over N2,
was observed while the sample was being activated with s- H2 and CH4.46, 47 The difference of 3 kJ mol-1 in Qst for CO2 and
CO2. Isotherms for N2 and CO2 at 195 K, N2, CO2 and CH4 at 273 CH4 indicates a small difference in binding affinities for UOF-
48
and 298 K were also recorded (Figures S11
-
12 and Figure
3
). In 1.s-CO2
.
The selectivity for a mixture of 15:85 CO2:N2 or
line with the higher surface area,
CO2:CH4 at 273 K was calculated using the well-established
reported IAST method (Figures S14-16).49 At 1 bar, the
calculated CO2/CH4 and CO2/N2 selectivities were 7.5:1 and
52:1, respectively.
Conclusions
In conclusion, the ligand
L was rediscovered and used to
synthesize a framework isoreticular to the previously reported
MOF-11. The Cu(II) paddle-wheel based non-interpenetrated,
low-density and open PtS framework of UOF-1 provides an
excellent platform for exploring the selective, room
temperature and low-pressure capture of CO2. When fully
activated UOF-1 has potential to adsorb significant amounts of
CO2 and is currently being investigated.
Figure 3. Gas adsorption isotherms for UOF-1.s-CO2: CO2, CH4
and N2 at 273 K and 298 K.
Acknowledgements
This work was supported by Ministry of Business Innovation
and Employment (UOOX1206) and University of Otago
doctoral scholarship to K.M.P. We thank Hui Yang for technical
assistance.
UOF-1.s-CO2 exhibited higher gas uptake than UOF-1.CH2Cl2. In
most cases a two-fold increase in uptake was observed, with
the exception of N2 gas at 273 K and CH4 gas at 298 K. A five-
fold increase in the N2 uptake at 273 K was observed whereas
only slight improvement in the CH4 uptake at 298 K was
observed. Some batchwise variations in gas uptake were
observed in UOF-1.s-CO2 (Table S2). This could be due to
synthesis, handling or standing after synthesis or minor
variations in the activation process. The CO2 isotherms at 195
K and N2 isotherms at 77 K suggested that saturation of the
available pores by the gas molecules was being attained. At
these temperatures between 9–14 molecules of gas (either
CO2 or N2) were adsorbed per unit cell i.e. between 2–4
molecules per Cu(II) ion. Although the BET surface area of the
Cu-PtS network with tetrahedral tetrakis(4-carboxyphenyl)-
methane ligand was three times more than UOF-1.s-CO2, the
H2 uptakes of UOF-1.s-CO2 at 77K and 1 bar pressure was not
too dissimilar to that of the former MOF (Tables S2-3).40 At 0
Author Contributions
All authors contributed extensively to the work presented in
this paper, with SGT and OTQ being responsible for the gas
adsorption studies. All authors have given approval to the final
version of the manuscript. L.R.H and S.C.M designed and
conceived the experiments with input from K.M.P. The
synthetic work, characterization and thermal analyses were
undertaken by K.M.P. The X-ray diffraction data was collected,
solved and refined by K.M.P. with input from L.R.H. The
manuscript was written by K.M.P. and L.R.H. with input from
the other authors.
°
C and 1 bar pressure, the CO2, CH4 and N2 uptakes were 49.0,
13.4 and 2.89 cm3 g–1, respectively (Figure
Notes and references
3
). At room
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N2 guest molecules. The CO2 and CH4 uptakes at this
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no N2 was adsorbed at this temperature. Isosteric heat of
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the CO2 and CH4 absorption isotherms at 273 and 298 K using a
Virial-type equation. The Qst values for CO2 and CH4 were 30.0
and 27.0 kJ mol-1, respectively, at zero surface coverage and
the values decreased slowly with increasing loading (Figures
S17-18). The Qst value for CO2 adsorption is comparable to that
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View Article Online
DOI: 10.1039/C7CE01818E
ARTICLE
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Synthesis of UOF-1 using procedure I: The solvothermal
reaction of Cu(NO3)2·3H2O (19.3 mg, 7.98 mmol) and L
(10.4 mg, 1.68 mmol) in 1:1 v/v solvent mixture of DMF
and C2H5OH was carried out in a pressure tube. The
pressure tube was protected with a thick steel blast tube
and subjected to a heating regime of slowly raising the
temperature to 120 °C over six hours, maintaining the
temperature at 120 °C for 48 hours and then slowly
cooling to room temperature at about 2 °C/hour. This
heating regime resulted in a brown precipitate. The
precipitate was dissolved with 4 drops of conc. HCl and
further heated for seven days at 170 °C and again cooled
to room temperature at about 2 °C/hour to result in a
blue solution. To an aliquot of this solution, CH2Cl2 was
slowly diffused in for about two months to result in
turquoise crystals of {[Cu2(L)(H2O)2]·14DMF}∞ (UOF-
1.DMF) in very small quantities.
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20.
21.
22.
23.
24.
25.
36.
37.
38.
39.
40.
41.
42.
43.
44.
Formula
= C40H64CuN7O12, Mr = 898.52, Tetragonal
P42/mmc , a = b = 14.686(5) Å, c = 26.103(9) Å, V = 5630(3)
Å3, Z = 4, T = 100.0(2) K. 22352 total reflections measured,
2826 unique reflections (Rint = 0.1438). Final R1 indices
[548 I>2σ(I)] = 0.1642 and ωR2 (all data) = 0.5026; UOF-
1.DMF (procedure II): Tetragonal P42/mmc , a = b
=14.456(4) Å, c = 26.369(2) Å, V = 5511(2) Å3, T = 100.0(2)
K. 41185 total reflections measured, 3099 unique
reflections (Rint = 0.2132); UOF-1.(CH3)2CO: Formula =
C46H69CuO14, Mr = 909.58, Tetragonal P42/mmc , a = b
=14.7093(16) Å, c = 26.053(3) Å, V = 5636.9(14) Å3, Z = 4, T
= 100.0(2) K. 11025 total reflections measured, 1764
unique reflections (Rint = 0.1590). Final R1 indices [964
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Journal Name
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8 | J. Name., 2012, 00, 1-3
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A stable non-interpenetrated metal-organic framework, UOF-1 (UOF = University of Otago framework), was
synthesized by combining the rigid tetrahedral tetrakis(4-carboxyphenyl)adamantane (L) ligand and copper(II)
paddle-wheel secondary building unit (SBU). This low-density open framework displayed excellent selectivity for
CO2 over N2 of 51.9:1 at practical operating conditions of 15:85 CO2:N2 mixture at 273 K and 1 bar. These results
provide an excellent platform for exploring the selective, room temperature and low pressure capture of CO2.
1

CrystEngComm
Page 10 of 10
View Article Online
DOI: 10.1039/C7CE01818E
A stable non-interpenetrated metal-organic framework, UOF-1 (UOF = University of Otago framework), was
synthesized by combining the rigid tetrahedral tetrakis(4-carboxyphenyl)adamantane (L) ligand and
copper(II) paddle-wheel secondary building unit (SBU). This low-density open framework displayed excellent
selectivity for CO2 over N2 of 51.9:1 at practical operating conditions of 15:85 CO2:N2 mixture at 273 K
and 1 bar. These results provide an excellent platform for exploring the selective, room temperature and low
pressure capture of CO2.
81x37mm (300 x 300 DPI)