Influence of anions in silver supramolecular frameworks: Structural characteristics and sorption properties

Article abstract of DOI:10.1021/ja303940d

The complexation of a preorganized thioether-functionalized bis(pyrazolyl)methane ligand (L) with silver precursors produces supramolecular structures organized at two hierarchical levels: [AgL]₆(X) ₆ metal-organic cyclic hexamers an

Full text of DOI:10.1021/ja303940d

Communication
pubs.acs.org/JACS
Influence of Anions in Silver Supramolecular Frameworks: Structural
Characteristics and Sorption Properties.
Irene Bassanetti,† Francesco Mezzadri,† Angiolina Comotti,§ Piero Sozzani,§ Marcello Gennari,†,‡
Gianluca Calestani,† and Luciano Marchio*,†
̀
†Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Parco area delle Scienze 17/a, Universita
Studi di Parma, Parma, Italy
§Department of Materials Science, University of Milano Bicocca, Via R. Cozzi 53, 20125 Milano, Italy
̀
degli
S
* Supporting Information
In this paper, we present the preparation of Ag(I)/Cu(I)-
bis(pyrazolyl)methane hexameric toroidal supramolecules that
self-assemble, at a higher hierarchical level, into a diversity of
3D supramolecular architectures and cavities as function of the
ABSTRACT: The complexation of a preorganized
thioether-functionalized bis(pyrazolyl)methane ligand (L)
with silver precursors produces supramolecular structures
organized at two hierarchical levels: [AgL]6(X)6 metal−
organic cyclic hexamers and their organization in 3D
architectures. The cyclic toroidal hexamers of 22−26 Å
external diameter are found to be stable already in solution
before self-assembly into the crystalline state. In the 3D
lattice, the hexameric building block are arranged in
different highly symmetric space groups as a function of a
−
−
−
anion used. The counterions (BF4 , PF6 , NO3 , and
−
CF3SO3 ) largely modulate the structure of the toroidal
hexamers and have a profound impact on the crystal framework
arrangement. Moreover, we show that the AgL-B and AgL-P (L
−
−
= ligand, B = BF4 , P = PF6 ) compounds are structurally
stable to solvent desorption and that the formation of crystals
with permanent porosity is promoted, as demonstrated by
vapor and gas absorption measurements. The self-assembled
toroidal hexamers were synthesized with a ditopic N2S donor
ligand, which is derived from a thioether-functionalized
bis(pyrazolyl)methane moiety and which coordinates favorably
with Ag(I)/Cu(I). The preferred conformation of the
ligand20,24 and the consequent orientation of the N2 and S
promotes the persistence of hexameric supramolecular
structures. In AgL-B compound, the hexamer has the shape
−
−
variety of anions (cubic Fd3 with PF or BF4 and
̅
6
−
−
rhombohedral R3 with CF SO or NO3 ) and form
̅
3
3
cavities with the geometrical shapes of Platonic solids
(tetrahedron and octahedron) that can be occupied by a
variety of solvent molecules. Upon evacuation, cubic
crystals can produce stable frameworks with permanent
porosity, which can absorb reversibly several vapors, CO2
and CH4.
́
of a torus measuring 26 Å in diameter, with a central
́
hydrophobic pore of 5.1 Å (Figure 1).
oordination-driven self-assembly of metals and properly
Cdesigned molecular linkers leads to the formation of
discrete supramolecules of particular geometrical shapes
(polygons and polyhedra).1,2 These supramolecules can in
turn self-assemble into a variety of complex crystalline
architectures enabling the exploration of a new fertile field to
produce porous materials.3−6 The weak interactions (π−π,
CH−π, H-bonds, van der Waals) that shape organic supra-
molecular materials7−9 can be complemented by more
directionally and spatially oriented coordination bonds. The
judicious choice of metal centers and ligand types (rigidity/
flexibility and donor atom set) can enhance the probability of
obtaining the desired structural topologies of metal-containing
frameworks.1,10−13 Among the many and structurally diverse
ligands used as building blocks in supramolecular architectures,
opportunely functionalized bis(pyrazolyl)methane fragments
can be preorganized to control and possibly predict the metal−
organic spatial arrangement.14−20 Given the neutral nature of
many ligands, a metal-based supramolecular moiety usually
necessitates the inclusion of anions as charge-balance
components, which can in turn exert a significant influence
on the supramolecular organization, nucleation, and in the
resulting network topology.21−23
The compound crystallizes in the cubic space group Fd3, and
the crystal packing is determined by BF4 , which interposes
̅
−
between two silver atoms belonging to two adjacent hexamers.
The centroids of four hexamers occupy the vertex of a
tetrahedron, thus, defining a tetrahedral cavity bordered by
phenyl rings (Figure 2, intracapsular space highlighted in green
with an approximate cavity diameter of 6 Å). The packing of
the spheres forms a system of interconnecting cavities lined
with pyrazole methyls (intercapsular space highlighted in
́
yellow with an approximate cavity diameter of 7 Å).
Isostructural crystals could be obtained by crystallizing AgL-B
in different solvents such as THF/hexanes, CH2Cl2/hexanes,
and acetone/diethyl-ether. Because Ag(I) and Cu(I) have
similar geometric preferences, a similar molecular structure is
obtained when Cu(I) is used in place of Ag(I), the major
difference being the metal-donor atom bond lengths. For CuL-
́
B, these bond lengths are on average 0.2 Å shorter than those in
AgL-B. The replacement of Ag(I) with Cu(I) does, however,
have a minimal effect on the size of the internal cross section of
Received: April 24, 2012
Published: May 22, 2012
© 2012 American Chemical Society
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Communication
all cases, a positively charged cyclic hexamer is generated. The
−
highly symmetric PF6 anion has the same influence over the
entire structure as BF4− (Figure S1). In fact, the two complexes
are isostructural, with the phosphorus atom occupying the same
sites as the boron atoms; the only difference between the two
complexes is the localization of the fluorine atoms. This
difference is most likely due to the high symmetry and shapes
−
−
of BF4 and PF6 , allowing both of these ions to fit within the
same spherical site. A remarkable difference is observed in the
−
−
crystal structures formed by the NO3 and CF3SO3 anions
(AgL-N and AgL-T, respectively) with respect to the AgL-B
and AgL-P compounds: (i) the hexameric molecular torus has a
́
smaller diameter of 22 Å, and the cross section of the internal
́
pore is decreased to 3.4 Å; (ii) the interaction between the
counterion and the silver ions is distinct; in AgL-N, the nitrate
acts as an O2 bidentate ligand, whereas in AgL-T, the triflate is
monodentate (Figure 1 and Figure S1); (iii) the compounds
crystallize in the rhombohedral space group R3 and the crystal
̅
Figure 1. Above, ligand structure. Middle, toroidal shape of the
hexamers of AgL-B/AgL-P and AgL-T/AgL-N as derived by single-
crystal X-ray diffraction. Below, coordination environments at the
metal centers.
packing shows the presence of octahedral cavities formed by six
́
hexameric units (approximate diameter = 7.6 Å). The
octahedral cavity is in communication with symmetry-related
cavities aligned along the C3 crystallographic axis by means of a
hydrophobic window. Notably, the volumes of the cavities
amount to 40% and 36% of the unit cell volumes of the AgL-N
and AgL-T compounds, respectively, whereas in the AgL-B and
AgL-P compounds, the volumes account for 27% of the unit
cell volume. Only the solvent guest molecules of AgL-T could
be located within the cavities. Specifically, part of a hexane
molecule is located inside the hydrophobic window, and part
extends outside of the window toward the center of the cavity
occupied by THF molecules (Figure 3).
Figure 3. Above, crystal packing of AgL-T depicting the rigid part of
the structure in stick and the cavities in yellow. Below, description of
the supramolecular contacts between the triflate anions and the
hexameric moieties. Disordered solvent molecules are shown in
Spacefill mode (gray and red).
Figure 2. Above, crystal packing of AgL-B depicting the rigid part of
the structure in stick and the cavities in yellow. Middle, the two
different types of cavities are depicted in yellow and green,
respectively. Below, description of the supramolecular contacts
−
between the BF4 anions and the hexameric moieties.
the hexameric toroid; in the copper complex, this distance is 5.0
The reason for the attainment of these two classes of
compounds, AgL-B/AgL-P and AgL-T/AgL-N, as function of
anions is to be found in the formation of hexameric
macrocycles already in solution. In fact, this has been
demonstrated by ESI−MS spectrometry (Figure S4) and by
pulsed-gradient spin−echo (PGSE) NMR measurements which
provide information about the hydrodynamic radii (rH) of
́
Å (Figure S1).
−
Because BF4 anions connect the hexameric supramolecules
to create the 3D overall crystal framework, we expect that the
counterion plays a significant role in the construction of the
crystal lattice. Thus, we synthesized different silver complexes
−
−
−
by using different counterions: PF6 , NO3 , and CF3SO3 . In
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Communication
species in solution. These latter experiments were performed in
CD2Cl2 ( ε= 9.1) and in acetone-d6 (ε = 20.7) at three different
concentrations (0.1−0.001 M), because the aggregation process
and the ion pairing depend on the solvent polarities25 (Figures
S5−S7). The measured rH radii for AgL-B and AgL-T are in the
6.4(2)−7.6(3) A and 6.2(2)−6.7(3) Å ranges, respectively.
These values are in agreement with the 7.8 and 6.4 Å mean
values of radii, as derived from single-crystal X-ray structures
(Tables S3 and S4). The rH values are consistent with the
presence of isolated hexameric units in solution, at least in the
investigated concentration interval, suggesting that the robust
supramolecules pre-exist to the crystallization process.
The permanent porosity of the AgL-B/AgL-P compounds
was demonstrated by adsorption isotherms of gases. AgL-B can
absorb 76 cm3(STP)/g of CO2 at 195 K and 1 atm (15.1 wt %),
whereas 22 cm3/g of methane was adsorbed under the same
pressure and temperature conditions (Figure 5). The measured
The powder X-ray diffraction patterns demonstrate that the
crystals of AgL-B/AgL-P retain a long-range structural order up
to 180 °C, as also confirmed by calorimetric analysis (Figures
S8−S14). The compounds subjected to evacuation from
solvents at 10−3 mmHg and 110 °C overnight exhibited
substantially unaltered powder patterns with small reductions in
the unit cell parameters, suggesting the formation of porous
crystalline materials (Figure 4). Solid-state NMR spectra
Figure 4. Effect of the solvent absorption on AgL-B crystals. Freshly
ground crystals, blue. Crystals subjected to high vacuum and T = 100
°C for 2 h, red. The evacuated crystals were then exposed to saturated
solvent vapors (methanol, dichlorometane, or acetone), green.
Figure 5. (a) Adsorption/desorption isotherms for CO2 and CH4 onto
porous AgL-B at 195 K. (b) CO2 adsorption/desorption isotherm for
the porous AgL-P compound at 195 K. Inset, enlargement of the low
pressure region. (c) Adsorption/desorption isotherms for CO2, CH4
and N2 onto AgL-B at room temperature and up to 10 atm.
confirm the absence of solvent molecules in the crystal
materials (Figure S15). When the evacuated crystalline
materials are placed in a chamber saturated with solvent
vapor, the resulting powder patterns show a 2.7−3.7% increase
in the unit cell volume depending on the solvent employed.
These data may be interpreted as an enlargement of the unit
cell parameters taking place to accommodate the solvent
molecules within the empty cavities.26 This behavior is
observed using dichloromethane, acetone, and methanol, and
several cycles of vacuum followed by exposure to solvent vapor
can be applied to the compounds without the loss of structural
order (Figures S12 and S13). The opposite behavior is
exhibited by the crystals of AgL-T/AgL-N: these compounds
lose their crystallinity when subjected to vacuum treatment
(Figures S10 and S11) and do not regain it after exposure to
solvent vapor.27−29 This behavior is most likely the result of the
different supramolecular arrangements and crystal packing. In
fact, a considerable contribution to the stabilization of the 3D
porous structure of AgL-B/AgL-P is made by the anions that
connect adjacent hexameric toroids through moderately strong
Ag−F interactions. In contrast, in AgL-T/AgL-N crystals, the
toroids are held together by weaker CH···π/CH···F and
CH···S/CH···O interactions (see Figure S2).
CO2 capacity, equal to 3.44 mmol/g, corresponds to an
occupied volume of 9600 Å3 per unit cell (dCO2 = 1.5 g/cm3
was used) which matches well with the open-pore capacity of
the intercapsular space, 11300 Å3 per unit cell, as explored with
a sphere with a diameter of 3.3 Å (corresponding to the kinetic
diameter of CO2). Thus, this result indicates the virtually
complete filling of the intercapsular space which is easily
accessible to gas species via diffusion. In contrast, CO2
molecules are prevented from entering the capsules by the
presence of anions occupying the aperture at the center of the
cyclic hexamers. In the case of AgL-P, the CO2 adsorption
isotherm at 195 K reaches the plateau value of 77 cm3(STP)/g
at 1 atm, which is comparable to the values for the isostructural
−
BF4 -containing compound. The adsorption isotherm does not
show a Langmuir profile but exhibits a moderate initial slope
followed by a neat increase at a pressure of 18 mmHg,
indicating that a gate pressure is necessary to promote the
openings of the pores. As far as the uptake values are
concerned, these systems are more effective than Ag-based
imidazolate frameworks containing cyclic coordination struc-
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tures recently described.13 Furthermore, the porous materials
selectively adsorb CO2 over CH4 and N2 at room temperature
and up to 10 atm, for example, AgL-B adsorbs 40 cm3(STP)/g
of CO2 versus 12 and 6 cm3(STP)/g of CH4 and N2,
respectively.
At room temperature, CO2/CH4 selectivity is comparable or
exceeds the values of most ZIFs while CO2/N2 selectivity is
lower than several MOFs but exceeds the performance of the
well-described MOF-5.30,31 From the adsorption measurements
at different temperatures, the isosteric heat of absorption was
estimated to be 25 kJ/mol for CO2 and 19 kJ/mol for CH4, in
agreement with the hydrophobic nature of the pyrazole methyl
groups lining the intercapsular cavity. Indeed, these values are
consistent with those measured, for instance, in methylimida-
zolate framework ZIF-8 and hydrophobic molecular zeo-
lites.32,33
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* Supporting Information
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́
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́
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the Universita
̀
degli Studi di
Parma. Cariplo Foundation and Regione Lombardia are
acknowledged for financial support. The authors thank M.
Beretta for adsorption measurements and S. Bracco and A.
Cattaneo for solid-state NMR characterization.
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