Hydrocarbon uptake in the alkylated micropores of a columnar supramolecular solid

Article abstract of DOI:10.1002/anie.200602440

(Chemical Equation Presented) Neither Doric, Ionic, nor Corinthian! (HgC₆F₄)₃ interacts with 1,3,5-(Me ₃SiC≡ C)₃C₆H₃ to form columns with peripheral trimethylsilyl groups. These columns self-aggregate into a hexagonal solid containing pores lined by nonpolar methyl groups (see picture; Hg orange, Si purple, C gray, F green, H white), which can adsorb alkanes reversibly.

Full text of DOI:10.1002/anie.200602440

Communications
alkane sorption in microporous self-assembled materials.
Available studies include the use of microporous metal–
organic frameworks and van der Waals crystals for the
storage,[2] size-selective encapsulation,[3] and gas-chromato-
graphic separation of alkanes.[4] In all of these cases, the walls
of the micropores are lined by aromatic groups, which interact
with the guest hydrocarbon molecules through CH···p or van
der Waals interactions.[2–4,5] Given that strong van der Waals
attractions occur between saturated hydrocarbon molecules,
microporous solids with alkylated cavity walls seem to be
well-suited for the storage of alkanes. Despite the simplicity
of this paradigm,[6] the synthesis of such microporous solids
remains unprecedented.
We have shown that trimeric perfluoro-ortho-phenylene
mercury (1)[7] interacts with benzene through secondary
Hg···p interactions to form extended stacks that propagate
in a direction perpendicular to the molecular planes.[8]
Additionally, we have observed close contacts between 1
and unsaturated substrates in supramolecular adducts.[9]
Herein, we describe how these assembly principles can be
used for the construction of columns with alkylated exteriors.
We also demonstrate that such columns can self-assemble to
form a microporous solid that traps light alkanes in its
alkylated cavities.
Reaction of 1 with 1,3,5-tris(trimethylsilylethynyl)ben-
ꢀ
zene (1,3,5-(Me3SiC C)3C6H3) in THF led to the formation of
a crystalline adduct, which after washing with CH2Cl2 and
ꢀ
drying, was identified as [1-1,3,5-(Me3SiC C)3C6H3] (2), as
Microporous Materials
indicated by X-ray diffraction and elemental analysis
(Figure 1). This adduct dissolves in polar solvents, such as
acetone and acetonitrile, through dissociation of the molec-
ular components. 1H NMR spectroscopy in [D6]acetone
indicates that 2 does not contain any THF or CH2Cl2
molecules. Crystals of 2 belong to the hexagonal space
group P63/mmc.[10a] Examination of the structure reveals the
formation of extended columns that run parallel to the c axis
(Figure 1a,b). These columns consist of binary stacks in which
DOI: 10.1002/anie.200602440
Hydrocarbon Uptake in the Alkylated Micropores
of a Columnar Supramolecular Solid**
Thomas J. Taylor, Vladimir I. Bakhmutov, and
François P. Gabbaï*
ꢀ
molecules of 1 and 1,3,5-(Me3SiC C)3C6H3 alternate. The
molecules interact through secondary Hg···p and Hg···Cethynyl
interactions (Hg1···C5 = 3.349(4) Š, Hg1···C6 = 3.363(5) Š),
whose distances are within the sum of the van der Waals radii
of mercury (1.7–2.0 Š)[11,12] and carbon (1.7 Š)[13] (Figure 1a).
The stacks are rather compact, as indicated by the distance of
3.28 Š separating the centroids of the two molecular compo-
nents. This centroid distance can be compared to that of
3.24 Š found in [1-benzene], which adopts a similar stacked
structure.[8] In 2, neighboring columns are interdigitated and
run parallel to one another to generate a three-dimensional
honeycomb structure with large cylindrical channels parallel
to the c axis (Figure 1c). The trimethylsilyl groups of the
Microporous self-assembled materials are attracting a great
deal of interest for the storage of gases. The selective uptake
and retention of small gaseous molecules, such as dihydrogen,
are particularly attractive goals, which may lead to novel
storage strategies.[1] In principle, similar strategies could be
extended to gaseous alkanes, whose separation, transport, and
storage continue to pose problems. Despite these important
applications, limited effort has been devoted to the study of
[*] T. J. Taylor, V. I. Bakhmutov, Prof. Dr. F. P. Gabbaï
Chemistry Department
Texas A&M University, 3255 TAMU
College Station, TX 77843-3255 (USA)
Fax: (+1)979-845-4719
ꢀ
1,3,5-(Me3SiC C)3C6H3 molecules are oriented toward the
centers of the channels and line the walls with nonpolar
methyl groups. Probing the channels with a 1.2-Š sphere
E-mail: gabbai@mail.chem.tamu.edu
3
afforded a void volume of 200.0 Š per unit cell, which
[**] This work is supported by the Petroleum Research Fund (ACS PRF
38143-AC 3), the Welch Foundation (A-1423), and the National
Science Foundation (CHE-0094264). We thank Raymond Schaak for
the use of the DSC/TGA instrument.
corresponds to 8.9% of the total volume of the crystal.[14]
Assuming that the channels are perfectly cylindrical, their
internal effective diameter is 6.2 Š. The channels are
essentially empty, as the as-prepared samples show negligible
weight loss (< 0.1%) upon application of a vacuum. Further-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
spectrum of 2ꢁ0.82CHBr3 in [D]6acetone. Encouraged by
these results, which attest to the structural robustness of 2, we
decided to investigate its affinity for volatile alkanes.
Alkane uptake was investigated gravimetrically by expo-
sure of solid 2 to an atmosphere of the corresponding alkane.
Remarkably, uptake is observed not only for n-hexane and n-
pentane vapor,[15] but also for n-butane, propane, and ethane,
which are all gases under ambient conditions (Figure 2a). For
Figure 2. a) Alkane sorption isotherms for 2 at room temperature.
b) Desorption of alkanes under vacuum as a function of time for
~
*
^
2ꢁ0.57C2H6 ( ), 2ꢁ0.67C3H8 ( ), and 2ꢁ0.71C4H10 ( ).
methane, however, the maximum uptake was less than
0.5 wt%. Whereas ethane is still accumulating at a pressure
of 1 bar, the sorption isotherms of propane and n-butane
display plateaus indicating saturation of the pores. The total
gas uptakes at P/P0 = 1 are 1.2, 2.1, and 2.9 wt% for ethane,
propane, and n-butane, respectively. For n-pentane and n-
hexane, uptakes of 2.2 and 2.5 wt% are observed at P/P0 =
0.25. For these two liquid alkanes, additional weight gain is
observed at higher pressure, which is assigned to condensa-
tion on the external surface of the crystals.[16] Conversion of
these gravimetric uptakes (wt%) into molar ratios leads to
the formulations 2ꢁ0.57C2H6, 2ꢁ0.67C3H8, 2ꢁ0.71C4H10,
2ꢁ0.45C5H12, and 2ꢁ0.41C6H14 for the adducts. Correlating
the uptakes with the molecular volume of each alkane[17]
indicates that ethane, propane, n-butane, n-pentane, and n-
hexane occupy 38, 57, 74, 55, and 57% of the cavity space in 2,
respectively. The relatively low occupancies observed for
ethane and propane can be explained by the weakness of the
van der Waals attractions in which these small molecules can
engage. It is not obvious why the uptake observed for n-
butane is greater than that for n-pentane and n-hexane.
However, given that two linear alkanes placed side by side
necessitate more than 7.5 Š in width, the narrow channels of 2
probably force the longer alkanes to pack on top of one
another, leading to a somewhat inefficient occupation of the
space. Molecular mechanics simulations (MM2 force field)
confirm this hypothesis: n-pentane and n-hexane are pre-
dicted to lay in the channels of 2 with their main molecular
axes aligned with the c axis. In contrast, n-butane in the
gauche conformation is small enough to lie sideways in the
channels (Figure 3a), thus, leading to a more efficient
occupation of the pores. This difference is probably at the
origin of the higher uptake observed for n-butane.
Figure 1. Synthesis and structure of 2. a) View of partof a stack,
showing the secondary Hg···p and Hg···Cethynyl interactions between 1
ꢀ
and 1,3,5-(Me3SiC C)3C6H3; thermal ellipsoids are set at 50% proba-
bility; H atoms are omitted. b) Space-filling view of a stack with four
repeating units. c) View of the honeycomb structure of 2 along the
c axis, showing the micropores. Hg orange, Si purple, C gray, F green,
H white; only one conformation of the disordered Me3Si groups is
shown.
more, thermogravimetric measurements reveal that no weight
change occurs up to a temperature of 1908C, at which the
adduct starts to decompose. As expected, the channels of 2
are hydrophobic and do not show any affinity for water.
To study the robustness of this microporous solid upon
guest incorporation, a single crystal of 2 was indexed and
subsequently immersed in neat bromoform. After 12 h,
analysis of the crystal by X-ray diffraction indicated the
encapsulation of bromoform and the formation of
2ꢁ0.82CHBr3. Remarkably, this single-crystal-to-single-crys-
tal transformation occurs with retention of the P63/mmc space
group and negligible changes to the unit-cell parameters.[10b]
The structure of 2ꢁ0.82CHBr3 only differs from that of 2 by
the presence of bromoform molecules, which are located in
the channels and have a refined occupancy of 82%. A similar
occupancy was derived by integration of the 1H NMR
To evaluate the stability of the adducts involving the
gaseous alkanes, we studied guest release under vacuum
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Experimental Section
Owing to the toxicity of the mercury compounds in this study, extra
care was taken at all times to avoid contact with the compounds in the
solid state, in solution, or as airborne particulates.
ꢀ
2: 1 (30 mg, 0.029 mmol) and 1,3,5-(Me3SiC C)3C6H3 (10.6 mg,
0.029 mmol) were dissolved in THF (10 mL). Slow evaporation of the
solvent led to the formation of colorless feathery crystals, which were
washed twice with CH2Cl2 to afford 2 in 80% yield. Mp. 1968C
(decomp). Elemental analysis (%) calcd for C39H30F12Hg3Si3: C 33.16,
H 2.14; found: C 33.65, H 2.09. IR (KBr): n˜ = 2959, 2164, 1252, 1162,
1046, 981, 880, 844, 760, 702, 692, 679, 653 cmÀ1 13C MAS NMR
.
(spinning at 7 kHz): for component 1: d = 156.0 (t, JHg,C = 743 Hz, Hg-
ꢀ
C), 144.1, 136.5 ppm (broad, F-C); for component 1,3,5-(Me3SiC
C)3C6H3:[20] d = ꢂ 135 (obscured by peak from 1), 122.2 (Caryl), 100.2,
97.4 (Cethynyl), À1.9 ppm (Csilyl).
Sorption isotherms were measured by monitoring the weight
change of the sample as a function of pressure using a Cahn RG
electrobalance connected to a manifold. A known weight of 2 (10–
20 mg) was placed in the weighing pan. After evacuation of the
system, the adsorbate was added incrementally through the manifold.
Data points were recoded at equilibrium, when no further weight
change was observed. Guest release was studied by monitoring the
weight of the sample under vacuum every 5–10 s.
Figure 3. a) Arrangements and conformations of n-butane and
n-pentane molecules in the channels of 2 determined by molecular
mechanics simulations. The walls of the channels are approximated by
the H atoms of the Me3Si groups. b) 1H MAS NMR spectra of 2 before
and after alkane adsorption. The peak assigned to free ethane gas is
indicated ( ).
*
The room-temperature solid-state 1H MAS NMR experiments
were performed on an Avance-400 Solids NMR spectrometer with a
4-mm probe, at a spinning rate of 7 kHz. Saturated samples were
packed in the rotor under ambient atmosphere. For the experiments
with ethane and propane, the rotor was packed with 2, exposed to an
atmosphere of the gas, sealed, and transferred to the NMR
spectrometer.
(Figure 2b). Although ethane is rapidly removed from the
pores of 2 under vacuum, the propane and n-butane adducts
show a more gradual weight loss as vacuum is applied. This
behavior attests to the strength of the host–guest interactions
in 2ꢁ0.67C3H8 and 2ꢁ0.71C4H10. X-ray powder diffraction
patterns collected after uptake and guest release demonstrate
that the original structure of 2 is retained, indicating
permanent microporosity.
Received: June 17, 2006
Published online: August 29, 2006
The microporous solid 2 does not immediately release the
trapped alkanes. In fact, the samples are sufficiently stable to
be analyzed by 1H magic-angle spinning (MAS) NMR
spectroscopy (Figure 3b).[18] Prior to exposure to an alkane,
the spectrum of 2 consists of two resonances, corresponding to
the methyl groups (d = 0.18 ppm) and the aromatic moiety
Keywords: adsorption · alkanes · mercury ·
microporous materials · supramolecular chemistry
.
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ꢀ
(d = 6.29 ppm) of 1,3,5-(Me3SiC C)3C6H3. After exposure to
ethane, an additional resonance assigned to adsorbed ethane
molecules appears at d = 0.90 ppm. The spectrum also
displays a sharp resonance at d = 0.59 ppm, which results
from free ethane molecules caught in the rotor headspace.
Note that the resonance due to the entrapped ethane
molecules is only slightly shifted with respect to that of the
free gas. This observation indicates that the walls of the
channels do not generate significant magnetic anisotropy
1
within the internal pore space. The H MAS NMR spectrum
of 2 after exposure to propane contains two new signals at d =
1.15 and 0.75 ppm, which are assigned to the methylene and
methyl groups of the entrapped hydrocarbon, respectively. In
the spectra of the samples exposed to n-butane, n-pentane,
and n-hexane, the intensity of the methylene signal increases
with respect to that of the methyl signal. For n-pentane and n-
hexane, the two different types of methylene groups are not
resolved, but are detected as a single resonance.[19]
In conclusion, we have described how secondary Hg···p
and Hg···Cethynyl interactions can be harnessed for the con-
struction of supramolecular columns, allowing the formation
of a microporous compound which can trap hydrocarbons in
its alkylated interior.
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[10] a) Crystal data for 2: C39H30F12Hg3Si3, Mr = 1412.67, hexagonal,
space group P63/mmc, a = 19.852(3), c = 6.5659(13) Š, V=
2241.0(7) Š3,
Z = 2,
1calcd = 2.093 gcmÀ3
,
0.120 ” 0.030 ”
0.030 mm3, w-scan mode, q = 1.18–23.248, 656 unique reflections,
562 reflections with I > 2s(I), m(MoKa) = 10.407 mmÀ1, R1 =
0.0455, wR2 = 0.1328 (all data); b) Crystal data for
2ꢁ0.82CHBr3: C39.82H30.82Br2.46F12Hg3Si3, Mr = 1618.26, hexago-
nal, space group P63/mmc, a = 19.900(3), c = 6.5529(13) Š, V=
2247.3(6) Š3,
Z = 2,
1calcd = 2.391 gcmÀ3
,
0.095 ” 0.040 ”
0.040 mm3, w-scan mode, q = 1.18–23.278, 659 unique reflections,
603 reflections with I > 2s(I), m(MoKa) = 12.550 mmÀ1, R1 =
0.0530, wR2 = 0.1433 (all data); CCDC-611078 (2) and CCDC-
611079 (2ꢁ0.82CHBr3) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[15] Subjecting the liquid adsorbates to freeze-pump-thaw cycles did
not affect uptake values significantly. Degassing the headspace
of the sample container multiple times was found to be sufficient
for uptake experiments, as 2 does not have a high affinity for
atmospheric gases.
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Angew. Chem. Int. Ed. 2006, 45, 7030 –7033
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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