An Isoreticular Series of Zinc(II) Metal-Organic Frameworks Derived from Terpyridylcarboxylate Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b01632

An isoreticular family of seven microporous metal-organic frameworks of the general form [ZnL] have been synthesized, where L is a 4′-substituted 2,2′:6′,2″-terpyridine-4,4″-dicarboxylate ligand. Each framework adopts an interpenetrated zeolitic gismondine (gis-c) topology and possesses one-dimensional square channels with ca. 9.0 ? apertures running down the crystallographic c axis. Gas adsorption measurements with N₂, H₂, CH₄, and CO₂ confirm their permanent porosity. The ligand functional groups, which include phenyl, 2-tolyl, 4-chlorophenyl, 4-nitrophenyl, 2-thienyl, 3-thienyl, and 4-pyridyl, line the channel walls and tune the gas adsorption properties of these materials.

Full text of DOI:10.1021/acs.inorgchem.7b01632

Article
pubs.acs.org/IC
An Isoreticular Series of Zinc(II) Metal−Organic Frameworks Derived
from Terpyridylcarboxylate Ligands
David C. Young,† Hui Yang, Shane G. Telfer, and Paul E. Kruger*,†
‡
‡
†MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Physical and Chemical Sciences, University of
Canterbury, Christchurch 8140, New Zealand
‡
MacDiarmid Institute for Advanced Materials and Nanotechnology, Institute of Fundamental Sciences, Massey University,
Palmerston North 4442, New Zealand
*
S Supporting Information
ABSTRACT: An isoreticular family of seven microporous metal−
organic frameworks of the general form [ZnL] have been
synthesized, where L is a 4′-substituted 2,2′:6′,2″-terpyridine-4,4″-
dicarboxylate ligand. Each framework adopts an interpenetrated
zeolitic gismondine (gis-c) topology and possesses one-dimensional
square channels with ca. 9.0 Å apertures running down the
crystallographic c axis. Gas adsorption measurements with N , H ,
2
2
CH , and CO2 confirm their permanent porosity. The ligand
4
functional groups, which include phenyl, 2-tolyl, 4-chlorophenyl, 4-
nitrophenyl, 2-thienyl, 3-thienyl, and 4-pyridyl, line the channel walls
and tune the gas adsorption properties of these materials.
11
INTRODUCTION
tion. Carboxylate groups are ubiquitous in coordination
polymer chemistry because of their anionic nature and the
plethora of potential coordination modes that they can adopt
■
Metal−organic frameworks (MOFs) are a class of porous
1
materials constructed using metal ions and organic ligands.
12
and cluster types that they can form. Ligands based on
These materials have attracted much attention because of the
potential applications related to their porous nature, in
particular, in gas capture and storage, separation, and
catalysis and as precursors for the formation of functional
2
,2′:6′,2″-terpyridine-4,4″-dicarboxylic acid derivatives combine
2
3
these features and, through the divergent arrangement of the
carboxylate groups, are expected to form polymeric materials
when bound to metal ions. Surprisingly, to date, they have
seldom been studied in the development of MOFs.
4
13
5
materials. A distinct advantage that MOFs possess over other
porous materials such as zeolites and porous carbons is their
modular nature, which allows their properties to be tuned
through the judicious choice of ligand components and the
variation of metal ions or cluster types. For example, the
systematic variation of the spatial disposition of the donor
atoms of a ligand, appending functional groups to the ligand
backbone, and the degree of interpenetration will modulate the
pore size and environment and can be used to enhance the
In the current contribution, we report a systematic
II
examination of a new family of isostructural Zn -MOFs based
on 2,2′:6′,2″-terpyridine-4,4″-dicarboxylate linking ligands
̈
(Scheme 1). The Krohnke terpyridine synthesis necessitates
the incorporation of an aromatic substituent at the 4′ position
Scheme 1. Synthesis of Ligands H L1−H L7 Used in the
2
2
6
interaction between the frameworks and specific sorbates.
Current Study
To isolate the role that functional groups play in the gas
sorption behavior of MOFs independent of the pore size,
topology, and degree of interpenetration, investigations on the
isoreticular series of MOFs containing different functional
groups are invaluable. Several well-known families of
isoreticular MOFs have been shown to have their properties
tuned through the inclusion of ligands containing differing
functional groups with retention of the overall framework
topology. Several well-known series of isoreticular MOFs
7
8
9
10
include the IRMOF, MOF-74, UiO-66, and MUF-7 series.
2
,2′:6′,2″-Terpyridine and its derivatives are common ligands
in coordination chemistry because of their strong tridentate
binding mode and relative ease of synthesis and functionaliza-
Received: June 28, 2017
©
XXXX American Chemical Society
A
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Figure 1. (a) Reaction of H L1 with zinc nitrate in acidic, aqueous DMF that produced crystals of 1. The asymmetric unit of 1 is shown. (b) 2−7,
2
produced under reaction conditions similar to those with the appropriate ligand.
of the ligands. Through systematic and facile modification of
the functional group at this position, seven novel ligands based
on the 2,2′:6′,2″-terpyridine-4,4″-dicarboxylic acid core were
realized. The functional groups were chosen to alter the steric
and electronic nature of a potential framework. A particular
focus was given to groups containing strongly electron-
withdrawing substituents and Lewis acidic sites because these
have been shown to alter the gas sorption behavior. The
Mass spectra were recorded on either a Dionex Ultimate 3000 or a
Bruker MaXis 4G spectrometer, operated in high-resolution positive-
ion electrospray mode. Elemental analysis was carried out by Campbell
Microanalytical Laboratory, University of Otago, Otago, New Zealand.
Thermogravimetric analysis (TGA) was carried out on an TA
Instruments SDT Q600 TGA/DSC apparatus. All samples were
heated on alumina crucibles under a nitrogen flow of 100 mL/min.
Heating cycles consisted of heating at 1 °C/min to 800 °C. Powder X-
ray diffraction (XRD) data were collected using an Oxford Agilent
SuperNova instrument using Cu Kα (λ = 1.5418 Å) radiation and an
Atlas CCD area detector. Samples were prepared by grinding of the
applicable compound and packing inside an open glass tube. A
baseline correction was manually applied using the baseline feature
within the Origin data analysis software. Gas sorption isotherms were
carried out on a Quantachrome Autosorb iQ2 instrument. Freshly
prepared MOF samples were activated by washing with N,N-
dimethylformamide (DMF) and then soaking in acetone for 48 h.
The acetone was regularly refreshed during this time. The samples
were then transferred to a preweighed analysis tube. The acetone was
then removed by holding the sample at high vacuum for 18 h at 25 °C.
Accurate sample masses were calculated by using degassed samples
after the activated samples were backfilled with nitrogen. All sorption
measurements used ultrahigh-purity gases.
functional groups include 4′-phenyl (H L1), 4′-(2-tolyl)
2
(
(
(
H L2), 4′-(4-chlorophenyl) (H L3), 4′-(4-nitrophenyl)
2
2
H L4), 4′-(2-thienyl) (H L5), 4′-(3-thienyl) (H L6), and 4′-
2
2
2
4-pyridyl) (H L7). The solvothermal reaction of H L1−H L7
2
2
2
with zinc nitrate resulted in the formation of crystalline
materials that were structurally identified as an isoreticular
MOF family, 1−7, and shown to adopt a zeolitic gis-c topology.
The impact of the identity of the ligand substituent on the gas
adsorption properties of these materials has been studied.
EXPERIMENTAL SECTION
■
Materials and Methods. Unless otherwise specified, all reagents
and starting materials were of reagent grade, purchased from standard
suppliers and used as received. NMR spectra were recorded on an
Synthesis of 4′-Phenyl-2,2′:6′,2″-terpyridine-4,4″-dicarbox-
ylic Acid (H L1). The synthesis of H L1 is typical for the ligand family
2
2
1
14
Agilent 400-MR instrument operating at 400 MHz for H and 101
and was adapted from work by Husson et al. To a solution of
methyl-2-acetylisonicotinate (3.55 g, 20 mmol) in ethanol (45 mL),
was added freshly distilled benzaldehyde (1.1 g, 10 mmol), followed by
potassium hydroxide (1.53 g, 27 mmol). The resulting solution
darkened and was allowed to stir until the formation of a visible
MHz for 13C. All samples were dissolved in dimethyl sulfoxide
(
DMSO)-d , and spectra were referenced to the residual solvent peak.
6
IR spectra were measured on a Bruker Alpha Platinum ATR Fourier
−1
transform infrared (FT-IR) spectrometer in the range 4000−550 cm .
B
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Figure 2. (a) Connectivity of 1 through ligand L1. Each zinc(II) ion is defined as a four-coordinate node. (b) Topological representation of the gis-c
network of 1 along the crystallographic a (left) and c (right) axes. Different colors represent independent interpenetrated frameworks. (c) Square
one-dimensional channels directed along the crystallographic c axis. Black arrows represent C−C distances in the narrowest (inner) and widest
(
outer) parts of the pore. The yellow arrow represents a Zn−Zn distance. Because of tetragonal symmetry, all distances perpendicular to those
shown are identical. (d) Structure of 1 with an overlaid solvent-accessible surface (rendered in cream) created using a 1.2 Å probe. Undulating one-
dimensional channels can be seen running along the crystallographic c axis. Compounds 2−7 are isostructural to 1 and adopt analogous connections
and channel shape.
precipitate (around 30 min). To this was added a 35% aqueous
ammonia solution (21 mL), which led to the disappearance of the
aforementioned precipitate. Stirring was continued at room temper-
ature for a further 20 h, after which the K /NH4 salt of the final
product was precipitated as a brown solid. This was filtered off, washed
with ethanol, then redissolved in the minimum amount of water, and
acidified with HCl to a pH of 4. This resulting solid was filtered and
washed with water until the washings were neutral. It was then
successively washed with ethanol and then diethyl ether and the light-
filtration. Yield: 0.066 g (0.11 mmol, 46%). Anal. Calcd for
C H N O Zn·CH O ·C H NO: C, 55.92; H, 3.83; N, 9.66.
2
3
13
3
4
2
2
3
7
−
1
Found: C, 56.32; H, 3.83; N, 9.31. Selected IR (cm ): 3414 br,
3068 br, 1634 s, 1557 s, 1356 s, 1241 m, 1093 w, 1016 w, 821 w, 786
m, 705 s, 630 w. Postactivation Anal. Calcd for C H N O Zn·H O: C,
+
+
23
13
3
4
2
5
7.70; H, 3.16; N, 8.78. Found: C, 57.72; H, 3.53; N, 8.40. Selected IR
−1
(cm ): 1607 s, 1553 s, 1478 w, 1359 s, 1240 m, 1015 w, 875 w, 821
w.
Synthesis of 2−7. A procedure similar to that for the synthesis of
was followed for the synthesis of 2−7 by substituting H L1 with the
brown powder air-dried. Yield: 0.65 g (1.6 mmol, 17%). Mp: >330 °C.
1
1
2
H NMR (400 MHz, DMSO-d ): δ 7.45−7.59 (m, 3H), 7.86 (d, J =
6
relevant ligands H L2−H L7. Synthetic and analytical details are given
2
2
6
2
1
1
.5 Hz, 2H), 7.90 (d, J = 5.1 Hz, 2H), 8.68 (s, 2H), 8.89 (d, J = 5.1 Hz,
in section S2 in the Supporting Information.
H), 8.93 (s, 2H). 13C NMR (101 MHz, DMSO-d ): δ 117.3, 118.3,
6
X-ray Crystallography. X-ray crystallographic data collection and
refinement was carried out on an Oxford Agilent SuperNova
instrument with focused microsource Cu Kα (λ 1.5418 Å) or Mo
Kα (λ 0.71073 Å) radiation and an Atlas CCD area detector. All
structures were solved using SHELXT and refined by full-matrix least-
22.0, 125.5, 128.0, 128.2, 135.8, 138.2, 148.4, 149.0, 153.7, 154.4,
+
64.6. HR-ESMS. Calcd for C H N O [M + H ]: m/z 398.1133.
23
16
3
4
−
1
Found: m/z 398.1140. Selected IR (cm ): 3028 br, 2781 br, 1714 s,
1
593 s, 1550 m, 1361 m, 1276 s, 1221 s, 759 s, 691 s, 665 s.
Synthesis of H L2−H L7. The same procedure as that for the
2
2
15
squares procedures using SHELXL-2015 within OLEX-2. Hydrogen
synthesis of H L1 was followed for the synthesis of H L2−H L7 by
2
2
2
atoms were included in calculated positions or manually assigned from
residual electron density where appropriate, with isotropic displace-
ment parameters 1.2 times the isotropic equivalent of their respective
substituting benzaldehyde for the relevant aldehyde. Synthetic and
analytical details are given in section S1 in the Supporting Information.
Synthesis of [Zn(L1)] (1). The synthesis of 1 is typical for the
2
2
carrier atoms. The functions minimized were ∑w(F − F ), with w =
family of MOFs reported here. H L1 (0.10 g, 0.25 mmol) was
o
c
2
2
2
2
−1
2
2
[
σ (F ) + aP + bP] , where P = [max(F ) + 2F ]/3. As discussed
dissolved in a DMF (25 mL), water (5 mL), and concentrated nitric
acid (2.3 mL) solution. To this was added zinc nitrate hexahydrate
o
o
c
in the text, where voids containing highly disordered solvent molecules
1
6
(
0.075 g, 0.25 mmol), and the orange solution was mixed thoroughly
were present, the SQUEEZE routine was carried out. This technique
was employed only when sensible explicit modeling of the electron
density due to solvent molecules was not possible and where a
considerable benefit to the refinement was gained. The detailed
and then dispersed evenly into 4 mL screw-cap vials, which were
heated at 90 °C for 24 h. Orange single crystals suitable for XRD
deposited directly from the hot solution and were collected by
C
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Figure 3. Adsorption isotherms of 1−7 for N (a) and H (b) at 77 K. Filled symbols: adsorption. Open symbols: desorption.
2
2
crystallographic data and selected bond lengths and angles for 1−7 are
(Figure 2c,d). The undulating nature of the channel is caused
by the tight four-membered rings of the gis topology at the
narrow points and the wide, flared out substituents at the 4′
position of the ligand. Owing to the high void volume and
disorder of the solvent molecules, these channels were
subjected to the SQUEEZE routine within PLATON. These
calculations showed that the potential solvent-accessible void
volume represents 32−41% of the unit cell, indicating that the
compounds could display porosity upon removal of the
occluded solvent (Table S3). The percentage void space scales
with the increased steric bulk of the functional group. Despite
the metal ions being five-coordinate, no solvent-accessible open
metal sites exist within the framework. In each of the structures
listed in section S3 in the Supporting Information.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. The ligands
H L1−H L7 were synthesized from methyl 2-acetylisonicoti-
16
2
2
nate, following an adapted procedure for the synthesis of 4′-(2-
furanyl)-2,2′:6′,2″-terpyridine-4,4″-dicarboxylic acid, using a
range of substituted phenyl and heterocyclic aldehydes to
install various functional groups on the ligand scaffold (Scheme
1
4
1
). The reported modest yields are typical of the Krohnke
̈
11a
terpyridine synthesis.
The solvothermal reaction of H L1−H L7 with zinc nitrate
2
2
1
−7, the functional groups at the 4′ position of the ligand line
in DMF/aqueous HNO at 90 °C yielded a series of crystalline
3
the interior of the one-dimensional channels in the respective
MOF and are expected to affect the gas sorption properties of
each framework.
materials, 1−7, suitable for single-crystal XRD studies (Figure
1
). Compounds 1−7 crystallize in the tetragonal space group
1
I4 /acd, are isostructural, and contain one [ZnL] component in
Gas Adsorption. To activate the MOFs, 1−7 were soaked
in acetone for 48 h and then placed under high vacuum for 18 h
at 25 °C to remove the residual solvent from the pores of the
frameworks. Powder XRD, TGA, and elemental analysis
revealed that crystallinity was retained postactivation (section
S2 in the Supporting Information) and that no residual solvent
remained in the pores. In order to assess the ability of 1−7 to
the asymmetric unit. The zinc(II) ions adopt five-coordinate
geometry, with the ligands coordinated in a tridentate chelating
fashion typical of 2,2′:6′,2″-terpyridine ligands. The remaining
two sites of the zinc(II) coordination sphere are occupied by
monodentate carboxylate groups from adjacent [ZnL] units. In
each case, the metal ions adopt a distorted square-pyrimidal
geometry, as shown by the low τ parameters, which range from
5
adsorb N , H , methane (CH ), and carbon dioxide (CO ),
0
.04 to 0.20 (τ = 0 corresponds to square-pyramidal; τ = 1
2
2
4
2
5
5
17
low-pressure adsorption isotherms were measured at a range of
temperatures.
corresponds to trigonal-ipyramidal; Table S3).
The carboxylate groups connect [ZnL] units together and
result in the formation of a 3D framework with the zeolitic gis
The N gas adsorption isotherms of the activated MOFs
2
3
2
were measured at 77 K to determine the surface areas of 1−7
(
4 ·6 ·8) topology in which the single crystallographically
(
Figure 3a). The MOFs 1−3 and 5−7 showed type 1
unique [ZnL] unit acts as a 4-connected node (Figure 2a).
Within this topology, large eight-membered rings create
channels that are directed down the crystallographic a and b
axes. Smaller four-membered rings create channels directed
along the crystallographic c axis. The aryl and heterocyclic
functional groups on the 4′ position of the terpyridine ligands,
H L1−H L7, partially block the former channels but line the
isotherms. Among these, those containing aryl and pyridyl
functional groups showed similar maximum gravimetric uptake
while those containing the thienyl functional groups displayed
slightly reduced uptake. The MOFs showing type 1 isotherms
containing aryl and pyridyl functional groups have Brunauer−
2
Emmett−Teller (BET) surface areas of 810−900 m /g. The
2
2
latter. The functional groups are twisted with regard to the
central pyridine ring of the ligand, with torsion angles ranging
from 3.4(5)° to 36.3(4)° (Table S3). The structure is 2-fold-
interpenetrated, with independent networks interacting
through π−π interactions (Figure S10). The interpenetration
serves to block the channels running parallel with the a and b
axes but does not hinder the size or shape of the channels
running parallel with the c axis (Figure 2b). Despite
interpenetration, significant void volume exists within the
structures and is mainly located within square-shaped one-
dimensional channels that run along the crystallographic c axis
MOFs containing the five-membered thienyl rings have lower
2
surface areas of 690−730 m /g (Table S4), and this is probably
responsible for the lower maximum sorption capacity.
Compound 4, containing the 4-nitrophenyl substituent,
displays very low N uptake at 77 K. Similarly low values at
2
195 and 298 K indicate that this is not just due to diffusion
limitations within the one-dimensional channels at 77 K but
rather that compound 4 appears to be nonporous with respect
to N in this pressure range.
2
Frameworks 1−3 and 5−7 showed significant H gas uptake
2
at 77 K with adsorption capacities that ranged from 1.17 to
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Figure 4. (a) CH adsorption isotherms for 1−7 at 273 K. (b) CH adsorption isotherms for 1−7 at 298 K. (c) Isosteric enthalpy of adsorption
4
4
(
Q ) of 1−7 toward CH calculated from isotherms measured at 273 and 298 K.
st
4
1
3
.47% by weight at 1 atm, which corresponds to approximately
gas molecules per formula unit (Figure 3b and Table S5).
CO isotherms were measured at 195, 273, and 298 K
2
(Figure 5). All compounds absorbed CO at 195 K, with 1−3
2
The uptake values are similar to that observed for IRMOF-1
and 5−7 exhibiting similar uptake values that reach saturation
at around P = 0.1 atm. Compound 4 exhibited a slightly lower
maximum gravimetric uptake than the other materials, which
indicates a lower accessible pore volume. It also showed a
stepped sorption curve, more easily visualized when a
logarithmic pressure scale is used (Figure S14).
1
8
(1.32%). On the other hand, compound 4 showed minimal
adsorption of H at 77 K. Calculation and experimental data
have shown that a linear relationship exists between the H
storage capacities and specific surface areas. This holds true
for this series of MOFs (Figure S13), indicating that the
electronic properties of the functional groups do not play a
2
2
2d
At 273 K, compounds 1−3, 6, and 7 display conventional
significant role in the adsorption of H at this temperature.
CO adsorption isotherms (Figure 5c). The maximum uptake
2
2
CH isotherms were measured at both 273 and 298 K. All
values at 1 atm, which range from 13.66 to 16.42 wt % (cf. 6.61
4
12
compounds display near-linear adsorption behavior with no
appreciable hysteresis (Figure 4a,b). The isotherm of
compound 4 showed that this framework has the lowest
wt % for IRMOF-1), are high with regard to those of
2c
previously reported MOFs in the literature. For both 4 and 5,
a distinct step in the adsorption branch is evident at
intermediate pressures. The gradient of the step is much
sharper for 4 than for 5. Hysteresis is observed in this pressure
region for the desorption branch (Figure 5d). These isotherms
are indicative of an underlying structural change during the
capacity for CH adsorption. The gradients of this set of
4
isotherms, which in the linear region corresponds to the Henry
coefficient and is a measure of the density of adsorption sites
with high affinity for the adsorbent for the adsorbate, indicate
that the pyridyl functional group in 7 has a beneficial impact on
CH4 adsorption. To quantify this more clearly, heats of
adsorption were calculated and found to lie in the range 17.9−
20
adsorption process. The total gravimetric uptake of 4 and 5 at
1 atm is approximately equal to that seen in the other samples
(Table S7). The first step in the adsorption isotherm of 4
2
5.5 kJ/mol (Figure 4c), which is at the high end for MOFs
corresponds to the adsorption of ca. 0.8 molecules of CO per
2
19
especially considering that no open metal sites are present.
formula unit (0.46 atm) and rises to ca. 1.9 molecules adsorbed
The MOFs with the highest heats of adsorption were shown to
be 7 (4-pyridyl), 5 (2-thienyl), 6 (3-thienyl), and 4 (4-
nitrophenyl), all containing polar functional groups. This metric
underscores the utility of including these groups to boost the
affinity of a framework for CH . The invariability of Q as a
per formula unit at 1 atm. For 5, the steps in the adsorption
correspond to 1.0 molecule of CO at 0.51 atm and 2.0
2
molecules at 1 atm. At 298 K, each of 1−7 showed a gradual
uptake of CO (Figure 5b). At 1 atm, there are a range of
2
maximum uptake values: 3 showed the highest adsorption (61.1
4
st
3
3
function of gas uptake indicates that the binding sites are
cm /g), while 4 showed the lowest adsorption (31.8 cm /g).
Compounds 4 and 5, which showed hysteretic behavior at 273
K, display the lowest maximum uptake at 1 atm. Although at
298 K 4 and 5 show uptake similar to the rest of the series
relatively homogeneous and that surface−CH interactions are
4
much more important than CH −CH interactions in this
4
4
pressure range.
E
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Figure 5. (a and b) CO adsorption isotherms for 1−7 at 195 and 298 K, respectively. (c) CO adsorption isotherms for 1−3, 6, and 7 at 273 K. (d)
2
2
CO adsorption isotherms for 4 and 5 at 273 K. (e) Isosteric enthalpy of adsorption (Q ) of 1−3, 6, and 7 toward CO calculated from isotherms
2
st
2
measured at 273 and 298 K.
below 0.2 atm, they start to plateau at a much lower pressure
and display lower maximum uptake than the other frameworks.
The anomalous gas adsorption behavior of framework 4 can
be rationalized by the motion of the two interpenetrating
lattices with respect to one another. Upon activation, the
lattices move further apart than that observed in the single-
crystal XRD structures. This “closed” form has a smaller pore
volume and narrower pore dimensions than the “open” form
adopted by the other frameworks. This accounts for the
negligible uptake of weakly interacting adsorbates such as N2
and H and the lower uptake of CH and CO (at lower
4, the electron-withdrawing nature of the nitro groups may
diminish the π−π interactions between the frameworks, thus
making the closed form accessible. Gate opening is not
observed for CH or CO at 298 K because of the lower
amount of gas adsorbed.
A striking feature seen in this series of MOFs is the effect that
4
2
the nitrophenyl substituent has on the gas sorption properties.
This functional group dramatically reduces the N sorption
2
capacity at all temperatures measured yet retains high CO2
capacities, making it potentially useful in the separation of these
gases. The adsorptive selectivity for CO from a gas mixture can
2
4
2
2
pressures) around room temperature. Gate opening to the
be estimated from the experimental single-component gas
adsorption isotherms. The selectivity factor is defined as the
molar ratio of the adsorbed quantities at the relevant partial
open form occurs during the CO isotherms when the amount
2
of adsorbed gas reaches a critical threshold, as illustrated in
Figure S14. This feature has been reported previously for
dynamic frameworks in which flexible frameworks switch from
2c
pressures of the gases. A hypothetical flue gas comprised of
75% N and 15% CO at 298 K is commonly used to compare
2
2
20c,d,21
narrow or closed-pore to open-pore forms.
In the case of
MOFs. Although this figure is an approximation and does not
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take into account the competition of gas molecules for
adsorption sites and its accuracy is limited by the low sorption
capacity of N2 at 298 K, it is conveniently and rapidly
performed and is useful for evaluating potentially interesting
materials, and, indeed, selectivities have been calculated for a
range of benchmark materials. Table S8 shows the calculated
single-component selectivity factors for 1−7. Compounds 1−3
and 5−7 display modest selectivities, inline with other MOFs.
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: paul.kruger@canterbury.ac.nz (P.E.K.).
■
*
ORCID
Compound 4, however, is very selective for CO and displays a
2
David C. Young: 0000-0002-5364-5174
Shane G. Telfer: 0000-0003-1596-6652
Paul E. Kruger: 0000-0003-4847-6780
3
5-fold increase in the selectivity compared to the majority of
the series. This stems from the very low sorption of N rather
2
than the high sorption of CO . Compound 7, incorporating a 4-
2
pyridyl functional group, displays the next best selectivity,
which is similar to that of HKUST-1.
Author Contributions
2c
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
CONCLUSION
■
In conclusion, we have reported the synthesis of seven new 4′-
substituted terpyridine-4,4″-dicarboxylic acid based ligands and
employed them in the solvothermal synthesis of seven new
zinc−organic frameworks that were fully characterized by
elemental analysis, IR spectroscopy, TGA, and powder and
single-crystal XRD methods. Each ligand differed by the
substituent in the 4′ position of the terpyridine ligand, with
the substituents based on either aryl or heterocyclic moieties.
These functional groups line the one-dimensional channels of
the resultant MOFs, 1−7, and the structure and gas sorption
properties of each were investigated. All frameworks are
isostructural and adopt the zeotlitic gis-c topology. Generally,
it was observed that the aryl and pyridyl functional groups
showed better gas adsorption behavior than the thienyl groups
for all gases measured. A notable exception was 4, containing
the 4-nitrophenyl substituent, which did not appreciably adsorb
N and H gases, indicating that the 4-nitrophenyl substituent
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the MacDiarmid Institute
for Advanced Materials and Nanotechnology (Research
Associate Award to D.C.Y.), the University of Canterbury,
and the Royal Society of New Zealand Marsden Fund for
financial support. We thank Drs. Hayley Scott and Anthea Lees
for valuable discussions.
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