Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand

Article abstract of DOI:10.1021/acs.inorgchem.6b01713

A new divergent homopiperazine-derived ligand N,N′-bis(4-carboxyphenyl)-1,4-diazacycloheptane H₂L has been prepared, containing a semirigid saturated heterocyclic core which is capable of providing multiple distinct bridging geometries. Reaction of H₂L with zinc acetate in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane structure 1 in which the loops and rods are formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating the pillaring ligand trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional 2.6⁹ topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO₂ capacity of 12 wt % at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of -32 kJ mol^-1. High-pressure adsorption isotherms also show moderate physisorption of H₂ and CH₄ within the material.

Full text of DOI:10.1021/acs.inorgchem.6b01713

Article
pubs.acs.org/IC
Porous Polyrotaxane Coordination Networks Containing Two
Distinct Conformers of a Discontinuously Flexible Ligand
Chris S. Hawes,*,†,§ Gregory P. Knowles,† Alan L. Chaffee,† Keith F. White,‡ Brendan F. Abrahams,‡
Stuart R. Batten,*,† and David R. Turner*,†
†School of Chemistry, Monash University, Clayton, Victoria 3800, Australia
‡School of Chemistry, The University of Melbourne, Melbourne, Victoria 3010, Australia
S
* Supporting Information
ABSTRACT: A new divergent homopiperazine-derived li-
gand N,N′-bis(4-carboxyphenyl)-1,4-diazacycloheptane H2L
has been prepared, containing a semirigid saturated hetero-
cyclic core which is capable of providing multiple distinct
bridging geometries. Reaction of H2L with zinc acetate in
DMSO gives a two-dimensional parallel interpenetrated
polyrotaxane structure 1 in which the loops and rods are
formed by the bent cis-(eq,ax) twist boat and trans-(ax,ax)
twist chair conformers, respectively. By matching the distances
between the solvated metal sites in the structure of 1, a related
material 2 can be prepared incorporating the pillaring ligand
trans-1,2-bis(4-pyridyl)ethylene bpe. Compound 2 displays a
similar polyrotaxane interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a
three-dimensional 2.69 topology related to the dia diamondoid network. The guest exchange and gas adsorption properties of
both materials were investigated; while compound 1 displays poor stability to guest exchange and negligible gas uptake, the
higher connectivity microporous compound 2 shows facile guest exchange and a surprisingly high CO2 capacity of 12 wt % at 1
bar and 273 K, and a zero-loading enthalpy of adsorption of −32 kJ mol−1. High-pressure adsorption isotherms also show
moderate physisorption of H2 and CH4 within the material.
coordinating groups.7 To date, this approach has been most
INTRODUCTION
■
commonly employed using cyclohexane polycarboxylates,8
The design and synthesis of functional coordination polymers is
where conversion between diastereomers requires a chemical
an area of undeniable interest in materials science and has seen
transformation, or 1,4-disubstituted piperazine species, where
enormous development in recent years.1 Judicious ligand
the diequatorial 1,4-substituted chair conformation is almost
design has played a key role in many important developments
exclusively observed due to the large energy cost in adopting
in the field, with functionalities, such as photoluminescence,
the other geometries.9 Currently, instances of divergent ligands
catalytic activity, guest recognition, and framework flexibility
capable of displaying structurally distinct, discontinuous and
having been engineered into functional materials through the
energetically similar bridging geometries under ambient
use of internally functionalized ligands.2 In particular, frame-
conditions (without the need for, e.g., photochemical stimulus)
work flexibility engineered through conformational freedom of
remain uncommon. Nonetheless, the potential for fascinating
either the metal nodes or bridging ligands can lead to
and unique structural behavior exists in such systems.10 Herein,
fascinating structural behavior.3 Nonetheless, ligands based on
we report the synthesis of a semirigid ligand derived from 1,4-
rigid aromatic core groups remain the dominant choice for the
diazacycloheptane, and the properties of two coordination
design and synthesis of porous coordination polymers.4
polymers containing unusual polyrotaxane topologies permitted
Although flexible ligands offer the possibility of unusual
by the distinct geometric properties of the ligand.
structural properties, the difficulty in predicting the extended
structure of highly flexible architectures, and the tendency for
RESULTS AND DISCUSSION
■
flexible groups to distort to fill available void space, limits their
applicability in many instances.5 Divergent ligands containing
one or more flexible sp3 linker groups within their backbone are
nonetheless relatively well-known, and tend to display a
continuous range of bridging geometries related by one or
more low-energy bond rotations.6 However, ligands with
restricted flexibility can also be realized by incorporating
saturated carbocyclic or heterocyclic functionality between
The ligand N,N′-bis(4-carboxyphenyl)-1,4-diazacycloheptane
H2L (Scheme 1) was prepared in good yield via hydrolysis of
the reported nitrile precursor,11 which was itself prepared by
the reaction of 4-fluorobenzonitrile with homopiperazine.
Received: July 17, 2016
© XXXX American Chemical Society
A
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mode, while the fourth coordination site on Zn2 is occupied by
the oxygen atom of a disordered DMSO ligand. The
asymmetric unit of 1 contains one complete molecule of L
and two additional halves of centrosymmetric L molecules. The
acentric molecule of L adopts a twist-boat conformation with
the two phenyl rings cis-(eq,ax) oriented, providing an
interplanar angle of 84° between the two aromatic rings. The
remaining L ligands, however, adopt twist-chair conformations
with trans-diaxial oriented aromatic rings, in which the two
aromatic rings on each molecule are parallel. The two
conformers of L are shown in Figure 2. The overlapping of
Scheme 1. Synthesis and Structure of H2L, with Proton
Numbering Scheme for H NMR
a
1
a
Reagents and conditions: (i) 4-fluorobenzonitrile, DMSO, K2CO3,
120 °C, 24 h; (ii) HCl(aq),, reflux, 48 h, then KOH(aq), reflux, 12 h.
Examples of structurally characterized N,N′-diaryl-1,4-diazacy-
cloheptanes are rare, with very few reports detailing solid-state
structural studies on related compounds.12 Of interest in such
systems is the propensity for partial planarization of arylamine
nitrogen atoms, hindered by the resulting ring strain, as well as
the tendency for spontaneous interconversion of stereoisomers
of saturated N-substituted nitrogen heterocycles. Structural
investigation of related systems has shown that the dominant
conformers can exhibit moderate energy barriers to inter-
conversion.13 By these considerations, H2L was expected to
possess multiple stable conformations separated by nontrivial
interconversion barriers. The very poor solubility of H2L in
common solvents necessitated the use of DMSO or DMSO-
based mixtures as the primary solvent for further reactions;
attempts to react H2L using DMF or other more common
solvent mixtures returned only the unreacted ligand or poorly
resolved mixtures.
Synthesis and Structure of poly-[Zn2(L)2(DMSO)] 1.
Reaction of H2L with zinc acetate in a 2:1 dimethyl
sulfoxide:ethanol mixture at 100 °C gave colorless crystals of
poly-[Zn2(L)2(DMSO)] 1 after 48 h, which were analyzed by
single crystal X-ray diffraction using synchrotron radiation. The
data were solved and the structural model refined in the triclinic
Figure 2. (a) cis-(eq,ax) twist boat (top) and trans-(ax,ax) twist chair
(bottom) conformers of L within the structure of 1. (b) Threading of a
linear L group through the loop of an adjacent network. Hydrogen
atoms and backbone disorder are omitted for clarity.
the noncentrosymmetric diazacycloheptane rings from these
groups with crystallographic inversion elements complicates the
geometric analysis of these conformers (Supporting Informa-
tion); however, the relevant atom positions can be
unambiguously resolved to show the overall geometry of each
ligand.
space group P1. The asymmetric unit contains two unique
̅
zinc(II) ions, both exhibiting tetrahedral coordination geo-
metries, linked by three crystallographically unique bridging
carboxylate groups, each coordinating in a μ2-κO:κO′
coordination mode, shown in Figure 1. Zinc ion Zn1 is capped
by another carboxylate group coordinating in a monodentate
Each dinuclear Zn2 node connects into the extended network
in two ways; the bent cis-oriented L groups provide two
linkages between each pair of metal nodes, defining a
rectangular loop of approximate (interatomic) dimensions 9
× 11 Å. The two linear trans-oriented L residues provide
bridging between nodes in two directions, extending the looped
structures into a 2-dimensional network. Interestingly, one of
the linear L residues, which bridges two Zn2 nodes with
monodentate carboxylate groups, passes through the rectan-
gular loop of an adjacent nonconnected network, shown in
Figure 2.
The threading of a linear L ligand through the loop of an
adjacent network extends the basic looped 2-dimensional
network into a 2-fold parallel interpenetrated polyrotaxane
network. This mode of interpenetration is the archetypal type
IIIa polyrotaxane mode,14 where the otherwise trivial (6,3)
network can instead be described by the 2.65 topology, invoking
Figure 1. Coordination environment in the structure of 1, with
labeling scheme for metals and coordinating atoms. Hydrogen atoms
and backbone disorder are omitted for clarity.
B
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2-membered rings to describe the interpenetration, shown in
Table 1. Summary of Crystal and Refinement Statistics for
Figure 3. Only relatively minor π−π and van der Waals
Compounds 1 and 2
1
2
empirical formula
formula weight
temp (K)
cryst syst
space group
a (Å)
C40H42N4O9SZn2
885.57
C44H41N5O8Zn2
898.56
100
100
triclinic
triclinic
P1
P1
̅
̅
10.766(2)
12.993(3)
20.035(4)
76.46(3)
85.52(3)
69.16(3)
2546.4(11)
2
11.739(2)
13.760(3)
16.428(3)
114.67(3)
95.03(3)
92.65(3)
2392.0(10)
2
b (Å)
c (Å)
α (deg)
β (deg)
Figure 3. Polyrotaxane interpenetration mode observed between two-
dimensional sheets in the structure of 1, showing chemical
connectivity (left) and topological representation (right) with
networks colored separately. Hydrogen atoms and DMSO ligands
are omitted for clarity.
γ (deg)
volume (Å3)
Z
ρcalcd (g/cm3)
1.155
1.248
μ (mm−1
)
1.03
1.054
F(000)
916
928
interactions are observed between adjacent layers in the
structure of 1, and the loose packing of adjacent sheets results
in undulating one-dimensional solvent channels, oriented
parallel to the c axis. Although the contents of these channels
could not be determined crystallographically, the combination
of thermogravimetric and elemental analyses and the electron
count provided by SQUEEZE15 suggests a void content of 3
DMSO molecules per Zn2 unit (Supporting Information)
Synthesis and Structure of poly-[Zn2L2(bpe)0.5] 2. The
presence of a capping DMSO ligand preventing higher
connectivity in the structure of 1 prompted an investigation
into a pillaring approach to increasing the connectivity and
stability of the material. Two dinuclear zinc nodes from
adjacent sheets within the structure of 1 are oriented with
DMSO ligands directed toward each other, with Zn−Zn
separation of 11.609(5) Å, and an offset between the two
parallel Zn−O bond vectors of 1.67 Å. These parameters are
well-matched to bridging by trans-1,2-bis(4-pyridyl)ethylene
bpe, which typically displays a bridging distance of ∼13 Å, and
provides an offset between the two parallel coordination bond
vectors by virtue of the central trans-oriented double bond.
Repeating the reaction of H2L with zinc acetate at 100 °C in
DMSO in the presence of bpe gave yellow block crystals after
48 h. Analysis by synchrotron X-ray diffraction provided a
structural model of poly-[Zn2L2(bpe)0.5] 2 in the triclinic space
radiation type
synchrotron
(λ = 0.7109)
synchrotron (λ = 0.7109)
2Θ range (deg)
reflns collected
4.792−52
53473
2.748−55.882
62638
independent reflns/
Rint/Rσ
9896 [Rint = 0.1281,
Rσ = 0.0678]
11186 [Rint = 0.0922,
Rσ = 0.0516]
reflns obs. [I ≥ 2σ(I)] 7926
7766
data/restraints/
params
goodness-of-fit on F2 1.044
9896/128/625
11186/230/640
1.042
final R indexes
[I ≥ 2σ(I)]
final R indexes [all
data]
R1 = 0.0754,
wR2 = 0.2102
R1 = 0.0699,
wR2 = 0.1868
R1 = 0.0867,
wR2 = 0.2196
R1 = 0.1001,
wR2 = 0.2080
largest diff. peak/hole 1.21/−0.67
1.12/−1.22
(e Å−3
)
CCDC deposition
number
1487894
1487895
group P1. The unit cell parameters of 2 are closely related to
̅
those observed in 1, with a ∼6% decrease in volume (Table 1).
The asymmetric unit of 2 contains an equivalent dinuclear zinc
motif, with the DMSO ligand from 1 replaced by the pyridyl
group of a bpe ligand, with a bridging distance of 13.218(4) Å,
as shown in Figure 4. As was the case in 1, the L groups adopt
two distinct geometries, with closely related structural
parameters between the two compounds and equivalent
bridging connectivity.
The similarities between the two compounds include the
interpenetration of adjacent networks by threading of a linear
trans-oriented L species through a rectangular Zn2L2 window
defined by cis-oriented L residues. However, whereas 2D → 2D
parallel interpenetration was the result of these associations in
1, in 2 the 2-dimensional poly-[Zn2(L2)] sheets are further
linked by bridging bpe units. These additional linkages bridge
metal nodes in the orthogonal direction to extend each network
into 3 dimensions, resulting in an overall 2-fold interpenetrated
3-dimensional structure when considering the polyrotaxane
Figure 4. Metal environment in the structure of 2 with labeling
scheme for metal ions and coordinating atoms. Hydrogen atoms and
backbone disorder are omitted for clarity.
interpenetration, shown in Figure 5. The topology of a single
net in 2 is that of the dia diamondoid net (considering Zn2
units as nodes), with looped connections on one out of every
four linkages. As for 1, however, since the interpenetration is
such that the loops contain links from the other net passing
through them, a 5-c net with 2.69 topology is more appropriate
for a full description of the interpenetration. Although the
inclusion of bpe groups reduces the available solvent-accessible
volume somewhat compared to 1, narrow one-dimensional
channels remain in the structure of 2, accounting for
approximately 22% of the total unit cell volume (Figure 5).
C
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Figure 6. Low pressure CO2 and N2 adsorption isotherms for
compound 2, measured in the pressure range 0−1 bar at the
temperatures specified.
and 1 atm, corresponding to approximately 12 wt % uptake.
The isosteric heat of adsorption for CO2 was calculated from
these data using a Virial equation (Supporting Information)
and revealed a zero-loading value of approximately −32 kJ
mol−1, decreasing to approximately −28 kJ mol−1 at higher
loading. These values are consistent with a relatively strong
physisorption interaction, favored by the narrow channels in
2.16 Nitrogen adsorption isotherms were measured at both 77
and 273 K in the pressure range 0−1 atm; only minimal uptake
was observed at either temperature. CO2 is expected to be
favored as a probe gas in such situations due to its smaller
kinetic diameter compared to N2, as well as the much higher
adsorption enthalpy for CO2 dictating improved physisorption
at higher temperatures. Excellent adsorption selectivity for CO2
over N2 has been increasingly observed in ultramicroporous
materials, which are now seen as a viable route toward various
CO2 separation technologies.17 Directly comparing the
adsorbed quantities of CO2 and N2 from their single-
component isotherms at 273 K shows a volumetric ratio of
adsorbed CO2/N2 which steadily decreases from a maximum of
∼70 at 40 mbar flattening to a value of ∼25 above 0.5 bar
(Supporting Information).
Given the good performance for CO2 adsorption displayed
by 2 in the pressure range 0−1 bar, high-pressure adsorption
isotherms for CO2, CH4, and H2 were also measured, as shown
in Figure 7. The CO2 adsorption isotherm at high pressure
(273 K) shows saturation at approximately 13 bar with a
loading of 93 cc(STP)/g, or 15 wt %. Adsorption of H2 at 77 K
showed a maximum loading of 113 cc(STP)/g at 15 bar (1 wt
%), while H2 adsorption at 258 K gave a considerably lower
maximum loading of 10 cc(STP)/g at 30 atm (Supporting
Information). CH4 adsorption carried out at 258 K revealed a
maximum loading of 40 cc(STP)/g at 24 bar (approximately
2.5 wt %). Isosteric heat of adsorption calculations for CH4
were carried out using isotherms measured at 258 and 273 K,
and revealed a modest zero-loading enthalpy of adsorption of
−23 kJ mol−1, reducing to −18 kJ mol−1 at saturation. Each
isotherm displayed a smooth desorption curve corresponding
closely to the adsorption branch. Following the adsorption
experiments, X-ray powder diffraction analysis of the material
was carried out (Supporting Information), which showed the
crystallinity of the framework was retained throughout the
solvent exchange, desolvation and adsorption process.
Figure 5. (Top) Simplified schematic diagram showing the
connectivity in compound 2; independent networks are colored red
and blue, with linkages through bpe units colored green, showing the
expansion from the 2-dimensional polyrotaxane structure of 1 to the
three-dimensional network of 2. (Bottom) The calculated solvent-
accessible area in the form of undulating one-dimensional channels
within the structure of 2.
Gas Adsorption Studies. To probe the gas uptake
properties of compounds 1 and 2, thermogravimetric analysis
experiments were carried out. These experiments showed that
neither of the as-synthesized compounds could be readily
thermally activated, due to the low volatility of the DMSO
guests. As such, both materials were soaked in a variety of
exchange solvents to effect guest exchange to a more readily
activated material. In the case of 1, both methanol and
acetonitrile proved effective exchange solvents, with desolvation
achieved at ∼100 °C. Methanol exchange led to decomposition
of the crystals, presumably by solvolysis of the metal sites. X-ray
powder diffraction analysis of the MeCN-exchanged material
showed some degree of crystallinity retention, albeit with
conversion to a different crystalline phase, which is not
necessarily surprising based on the expected flexibility of the
bulk material. However, both CO2 (273 K) and N2 (77K)
adsorption isotherms showed negligible uptake for the MeCN
exchanged and thermally activated material (Supporting
Information) suggesting that the solvent exchange and/or
evacuation leads to the formation of a nonporous phase. This
outcome is not unexpected for a low-dimensional material with
no substantial interactions between layers.
Similar to 1, immersion of 2 in various solvents gave efficient
exchange of the lattice DMSO and water molecules over 48 h,
and thermogravimetric analysis showed that the exchange
solvents were readily lost at low temperatures (Supporting
Information). Using acetonitrile as the exchange solvent and
with activation at 100 °C under dynamic vacuum overnight,
CO2 adsorption isotherms were carried out at 273, 298, and
308 K. These measurements, shown in Figure 6, revealed a
relatively high CO2 capacity for 2 of 68 cc(STP)/g at 273 K
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CONCLUSIONS
■
We have prepared two related polyrotaxane networks 1 and 2
from a conformationally flexible diaryl-diazacycloheptane ligand
L, and analyzed the structural, guest exchange and gas
adsorption properties of each. In both cases, the presence of
both trans-twist chair and cis-twist boat conformers of the
ligand provided the necessary structural components to form
both the rod and loop features of the interlocked structure.
Compound 1, containing 2D → 2D parallel interpenetrated
networks, displayed poor gas adsorption characteristics, which
we ascribe to structural collapse on evacuation. However,
incorporation of the neutral pillaring ligand bpe provided the
necessary structural reinforcement for the material to display
permanent porosity, while retaining the key structural features
observed in 1. These results represent progress in the use of
discontinuous conformational flexibility in ligand design as a
tool for engineering intricately entangled networks, with a view
toward functional framework materials based on these
structural features.
Figure 7. High pressure isotherms for CH4, CO2, and H2 with
desorption branches shown in gray. The negative slope for CO2 and
H2 at pressures above the saturation pressure are ascribed to the well-
known surface excess phenomenon discussed by Menon,18 and occur
when the density of gas in the bulk phase approaches that of the
adsorbed phase, causing the adsorbed quantity to be slightly
underestimated.
EXPERIMENTAL SECTION
■
Materials and Methods. All reagents and solvents were of reagent
grade or better and were used as received from Sigma-Aldrich, Alfa
Aesar or Merck. N,N′-bis(4-cyanophenyl)-1,4-diazacycloheptane was
prepared according to published procedures.11 NMR spectra were
recorded on a Bruker AVANCE spectrometer operating at 400 MHz
Discussion. From the striking similarities in the behavior of
L in the structures of 1 and 2, the two observed ligand
conformers can be considered as accessible and robust linking
tectons. A slight contraction in the interplanar angle within the
cis-(eq,ax) conformer is observed in compound 2, at 68°
compared with 84° in 1. This flexing behavior, presumably in
response to the changes in the crystal packing properties of the
two materials, is similar to that observed in other cleft-shaped
molecules with saturated heterocyclic cores, the most obvious
for H and 100 MHz for 13C nuclei, with samples dissolved in d6-
1
DMSO. Melting points were recorded in air on an Electrothermal
melting point apparatus and are uncorrected. Mass spectrometry was
carried out using a Micromass Platform II ESI-MS instrument, with
samples dissolved in Milli-Q water. Microanalysis was performed by
the Campbell Microanalytical Laboratory, University of Otago, New
Zealand, and the Science Centre, London Metropolitan University,
London, England. Infrared spectra were obtained using an Agilent
Cary 630 spectrometer equipped with an Attenuated Total Reflectance
(ATR) sampler. Bulk phase purity of all crystalline materials was
confirmed with X-ray powder diffraction patterns recorded with a
Bruker X8 Focus powder diffractometer operating at Cu Kα
wavelength (1.5418 Å), with samples mounted on a zero-background
silicon single crystal stage. Scans were performed at room temperature
in the 2θ range 5−55° and compared with predicted patterns based on
single crystal data. Thermogravimetric analyses were carried out using
a Mettler-Toledo TGA/DSC 1 STARe instrument. Samples were
heated at a rate of 5 °C min−1 under a nitrogen purge flow of 30 mL
min−1. Gas adsorption experiments were carried out using a
Micromeritics TriStar 3020 instrument (N2 and 0−1 atm CO2) or a
Sieverts-type BELsorp-HP automatic gas sorption analyzer (H2, CH4,
and high-pressure CO2), using ultra high purity gases from BOC or Air
Liquide. Nonideal gas behavior at high pressures of each gas at each
measurement and reference temperature was corrected for. Source
data were obtained from the NIST fluid properties Web site.24
Synthesis of H2L1. A suspension of N,N′-bis(4-cyanophenyl)-1,4-
diazacycloheptane (3.20 g, 10.6 mmol) in concentrated hydrochloric
acid (50 mL) was heated under reflux for 48 h. Following this period,
the suspension was filtered, washed with 3 × 50 mL distilled water,
and the solids were collected and suspended in a solution of potassium
hydroxide (14 g, 250 mmol) in water (60 mL). This solution was
heated at reflux for 12 h, and then cooled to room temperature and
acidified with glacial acetic acid. The resulting white solids were filtered
and exhaustively washed with water, followed by washing with
methanol, and dried in air. Yield 2.69 g (74%). mp >300 °C; Found C,
63.03; H, 5.84; N, 7.74; Calculated for C19H20N2O4·1.25H2O C,
62.88; H, 6.25; N, 7.72%; δH (DMSO-d6, 400 MHz) 1.97 (broad
quint., 2H, 3J = 5.6 Hz, H3), 3.45 (broad t, 4H, 3J = 5.5 Hz, H2), 3.70
of which being the Troger’s Base motif.19 The crystal packing
̈
behavior of Troger’s Base analogues is also very often
̈
influenced by C−H···π interactions involving the π system of
the central cleft.20 Similarly, in the case of compounds 1 and 2,
such interactions are also present between the encapsulated
homopiperazine ring and the surrounding aromatic groups,
where at least one carbon atom from each of the four aromatic
rings of the loop lies within 3.5 Å of a CH2 carbon atom within.
In the absence of strong electrostatic or classical hydrogen
bonding templation, common for discrete rotaxanes,21 these
interactions may play a role in directing the entanglement of
the cis and trans conformers.
The gas adsorption behavior of 2 is relatively standard for
narrow-pore porous coordination polymers; although the total
adsorption capacity is short of that typically observed in rigid
micro- and mesoporous frameworks, the higher enthalpy of
adsorption for CO2 leads to steeper uptake in the low pressure
region, an important consideration for potential gas separation
applications. Although the low pressure N2 isotherm revealed
negligible adsorption at 77 K, the material showed moderate
uptake capacity for the smaller probe gas H2 at 77 K and the
similarly sized probe gas CH4 at 258 K. These observations may
imply a kinetic barrier to guest diffusion for N2 at 77 K,22
although not to the same extent for the smaller probe molecule
H2. Excellent selectivity for CO2 over CH4, an important
consideration for biogas applications, has previously been
observed for flexible, narrow-pore materials;23 however, in this
instance the CO2 and CH4 adsorption isotherms are more
comparable, with the molar ratio of adsorbed CO2/CH4 not
exceeding ∼5 across the loading range at 298 K.
3
3
(s, 4H, H1), 6.79 (d, 4H, J = 8.8 Hz, H4), 7.73 (d, 4H, J = 8.8 Hz,
H5), 12.05 (br, 2H, H6); δC (DMSO-d6, 100 MHz) 22.87, 47.12,
47.92, 110.57, 117.12, 131.33, 150.63, 167.36; m/z (ESI−) 339.1
E
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(100%, [M-H]−, calculated for C19H19N2O4 339.1); νmax (ATR, cm−1)
2980m, 2901m, 2796m br, 2661m, 2559m br, 1664s, 1593s, 1521s,
1483w, 1461w, 1434m, 1404s, 1354m, 1318s, 1288s, 1225s, 1179s sh,
1128m, 1093w, 1031m, 964w, 928s, 829s, 767s, 695w.
_refine_special_details and _olex2_refinement_description sections
of each crystallographic information file.
ASSOCIATED CONTENT
* Supporting Information
■
Synthesis of poly-[Zn2L2(DMSO)] 1. A mixture of zinc acetate
dihydrate (40 mg, 180 μmol) and H2L (40 mg, 120 μmol) in 6 mL of
2:1 DMSO:EtOH was dispersed by sonication and sealed in a glass
vial, and heated at 100 °C. After 2 days, the resulting colorless crystals
were isolated by filtration and washed with DMSO, and then EtOH.
Yield 17 mg (33%). mp >300 °C; Found C, 49.34; H, 5.32; N, 5.18;
calculated for C40H42N4O9S1Zn2·3(DMSO) C, 49.33; H, 5.40; N,
5.00%; νmax (ATR, cm−1) 3401w, 2927w, 1588s, 1539m, 1519m,
1387s, 1350s, 1329s, 1275m, 1245w, 1223w, 1194m, 1174w, 1148m,
1019s sh, 952m, 919m, 835m, 780s, 700m. Phase purity was confirmed
with X-ray powder diffraction.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01713.
Additional figures, additional gas adsorption data and
fitting parameters, thermogravimetric analysis plots, and
X-ray powder diffraction patterns (PDF)
Crystallographic information files for compounds 1 and 2
(CIF)
Synthesis of poly-[Zn2L2(bpe)] 2. A suspension of H2L (40 mg,
120 μmol), bpe (24 mg, 130 μmol) and zinc acetate dihydrate (80 mg,
360 μmol) in 4 mL of DMSO was heated in a sealed vial at 100 °C for
48 h. On cooling to room temperature, the yellow crystals were
isolated by filtration, and washed with DMSO and ethanol. Yield 30
mg (56%). mp >300 °C; Found C, 51.56; H, 5.01; N, 6.23; calculated
for C44H41N5O8Zn2·3(DMSO)·2(H2O) C, 51.37; H, 5.42; N, 5.99%;
νmax (ATR, cm−1) 3411w br, 3059w, 2934w, 2825w, 1595s, 1545w,
1516m, 1387s, 1348s, 1329s, 1273w, 1245w, 1224m, 1192s, 1149m,
1074w, 1031m sh, 950w, 928m, 834m, 778s, 699m. Phase purity was
confirmed with X-ray powder diffraction.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: hawescs@tcd.ie.
*E-mail: david.turner@monash.edu.
*E-mail: stuart.batten@monash.edu.
■
Present Address
§C.S.H.: School of Chemistry and Trinity Biomedical Sciences
Institute, Trinity College Dublin, Dublin 2, Ireland.
X-ray Crystallography. Collection and refinement statistics are
summarized in Table 1. All data sets were collected on the MX2 (1) or
MX1 (2) beamlines at the Australian Synchrotron, Victoria,
Australia,25 operating at 17.4 keV (λ = 0.7109 Å) with data collections
conducted using BluIce control software.26 The diffraction data were
reduced, processed and multiscan absorption corrections carried out
using the XDS software suite,27 and anomalous dispersion corrections
for the nonstandard wavelength were made in the final refinement
using Brennan and Cowan data.28 All data sets were solved using direct
methods with SHELXS29 and refined on F2 using all data by full matrix
least-squares procedures with SHELXL-201530 within the OLEX-2
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
CCDC 1487894−1487895 contain the supplementary crystal-
lographic data for this paper. The data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures.
GUI.31 The functions minimized were Σw(Fo − Fc2), with w =
2
2
[σ2(Fo ) + aP2 + bP]−1, where P = [max(Fo)2 + 2(Fc)2]/3. Non-
hydrogen atoms were refined with anisotropic displacement
parameters, while most hydrogen atoms were included in calculated
positions with isotropic displacement parameters either 1.2 or 1.5
times the isotropic equivalent of their carrier atoms. Because of
restrictions on AFIX behavior involving highly disordered atoms split
over multiple symmetry-related orientations, several CH2 hydrogen
atoms for the disordered diazacycloheptane residues were added
manually in appropriate restrained positions and applied the
appropriate Uiso dependencies.
Both data sets contained substantial regions of diffuse electron
density, and the contribution of these regions to the observed structure
factors was accounted for using the SQUEEZE routine within
PLATON.15 The residual electron counts were compared with
microanalysis and thermogravimetric analysis data to conclude
reasonable solvent occupancies for the disordered regions. Crystallo-
graphic formulas, F(000), calculated density and absorption coefficient
are based on the crystallographically located atoms only.
Both structures contain substantially disordered framework groups,
mostly involving the linear trans-chair conformers, which overlap
crystallographic inversion elements with a noncentrosymmetric
diazacycloheptane fragment. As such, these groups were modeled
with both orientations overlapped, splitting all diazacycloheptane
atoms and between two and ten attached phenyl atoms, depending on
the degree of disorder with distance from the inversion center
(Supporting Information). To maintain a chemically sensible structure
for both disordered contributors in each case, various DFIX restraints
were required, and the anisotropic displacement parameters were
restrained to isotropic approximations, rigid body approximations, or
constrained as equivalent to nearby atoms, as required. A complete list
of restraints and constraints employed in each refinement and
discussions of specific refinement strategies are given in the
ACKNOWLEDGMENTS
■
This work is supported by the Science and Industry
Endowment Fund. Portions of this work were carried out on
the MX1 and MX2 beamlines at the Australian Synchrotron,
Victoria, Australia.25 D.R.T. acknowledges the Australian
Research Council for a Future Fellowship. The authors
acknowledge Dr Jamie Hicks (Monash) for assistance in
manuscript preparation.
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