A two-dimensional hydrogen bonded organic framework self-assembled from a three-fold symmetric carbamate

Article abstract of DOI:10.1039/c3cc47159d

A flexible organic sheet spontaneously assembled under mild conditions from a tri-carbamate. By introducing cyclohexyl side chains as 'pillars' between the sheets, a hydrogen bonded organic framework was formed which displayed a capacity for accommodating and releasing guest molecules. The guest size/shape selectivity of the lamellar framework was demonstrated by testing with various guests. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc47159d

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A two-dimensional hydrogen bonded organic
framework self-assembled from a three-fold
symmetric carbamate†
Cite this: Chem. Commun., 2014,
50, 5209
Received 18th September 2013,
Accepted 1st November 2013
Xiaodong Hou,a Zhihan Wang,a Molly Overby,a Angel Ugrinov,b Casey Oian,a
Rajiv Singha and Qianli R. Chu*a
DOI: 10.1039/c3cc47159d
www.rsc.org/chemcomm
A flexible organic sheet spontaneously assembled under mild con- and organic laminate (ammonium ion and carboxylate network).6,7
ditions from a tri-carbamate. By introducing cyclohexyl side chains Featured with an exceptional structural flexibility and a high
as ‘pillars’ between the sheets, a hydrogen bonded organic frame- surface area, soft layered materials have shown applications in
work was formed which displayed a capacity for accommodating and host–guest chemistry, clay mimics, chiral resolution and photo-
releasing guest molecules. The guest size/shape selectivity of the polymerization reactions.5–10
lamellar framework was demonstrated by testing with various guests.
A facile and reliable way to obtain the crystalline sheets starts
with injecting an organic solution of benzene-1,3,5-triyl tris-
Hydrogen bonded organic frameworks (HOFs) have attracted (cyclohexylcarbamate) 1 into water with stirring. This process was
significant attention due to their promising applications in gas then accomplished by evaporating the solvent mixture under
storage1 and separation.2 Compared to widely used coordina- ambient conditions. TEM images showed that the size of the
tion interactions in the study of host frameworks,3 hydrogen organic sheets obtained by this method was up to several square
bonds are weaker and conformationally more flexible. They are micrometers (see ESI†). A small amount of nanoscale crystals of
therefore more difficult to control. However, Nature chooses to tri-carbamate 1 were also observed by TEM at the same time.
use hydrogen bonds to achieve sophisticated functions and As illustrated by the synthetic method, this layered crystalline
complex tasks such as enzymatic catalysis and receptor activa- material is water stable. Even when the lamellar material was
tion demonstrating the power of these flexible systems.
boiled in water for 12 h, the structure remained intact. The water
Using hydrogen bonds, our research group has recently stability is an important property required for many applications
discovered and studied a series of chiral organic sheets that such as gas storage.
were constructed from achiral molecules with stereogenic axes
High quality crystals of tri-carbamate 1ꢀ(THF) with guest mole-
(supramolecular atropisomer).4 Similar to the b-sheets of poly- cules were obtained by slowly evaporating a THF tri-carbamate
1
peptides, the hydrogen bonded backbone of the chiral sheet solution. The H NMR spectrum showed that the ratio of the tri-
does not involve side chains. Side chains typically allow intro- carbamate host to THF guest molecules in the crystals is 1 : 1 (see
duction of different functional groups with suitable sizes onto ESI†). The lamellar crystal structure was confirmed by X-ray crystallo-
the flexible sheet platform. Thus, the sheet can achieve a desired graphy as shown in Fig. 1. The structure consists of hydrogen
function by tuning the properties with these functional groups. bonded sheets extended in the crystallographic ab plane through a
Herein, we report a flexible organic lamellar framework with the fully saturated hydrogen bonded network. Within the sheet, each tri-
capacity of accommodating guest molecules by introducing cyclo- carbamate molecule is connected to four neighbors through six
hexyl side chains as ‘‘pillars’’ between the sheets. There are only a intermolecular hydrogen bonds (Fig. 1b, each cyclohexyl side chain
few lamellar organic host systems reported in the literature, for is replaced with a carbon atom for simplicity). The hydrogen
example, the GS sheet (guanidinium ion and sulfonate network)5 bonding connection is different from the three reported sister sheet
structures, in which each molecule is connected to five4b or
six4a neighbors. The average length of the six hydrogen bonds
between N–H and CQO groups of the three carbamates in
1ꢀ(THF) is 2.86 Å. This length falls between those found in the
sister hydrogen bonded lamellar structures of N,N0,N00-tris(n-octyl)-
benezene-1,3,5-tricarboxamide (2.83 Å)4a and benzene-1,3,5-triyl
tris(propylcarbamate) (2.90 Å).4b The small difference in length
reveals that the sheet backbone hydrogen bonds are as strong as
a Department of Chemistry, University of North Dakota, Grand Forks, ND 58202,
USA. E-mail: chu@chem.und.edu; Fax: +1 701-777-2331; Tel: +1 701-777-3941
b Department of Chemistry and Biochemistry, North Dakota State University, Fargo,
ND 58102, USA
† Electronic supplementary information (ESI) available: Experimental procedure,
full characterization and spectra, and crystallographic data. CCDC 941724–941729
and 961160. For ESI and crystallographic data in CIF or other electronic format,
see DOI: 10.1039/c3cc47159d
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Table 1 Guest selectivity experiments of the lamellar material
Entry
Solvent (ratio)
Crystal
1
2
3
4
5
THFa
1ꢀ(THF)
1ꢀ(C6H6)
1ꢀ(C6H12
1ꢀ(C5H10
C6H6–CH3CN (1 : 2)
C6H12–acetone (1 : 2)
C5H10–EA–CH3CN (1 : 2 : 2)
THF–C6H6–C6H12–C5H10
(1 : 1 : 1 : 1)
)
)
1ꢀ(0.92THFꢀ0.03C6H6ꢀ
b
0.02C6H12ꢀ0.03C5H10
)
6
THF–C6H6 (1 : 1)
1ꢀ(0.76THFꢀ0.24C6H6)c
a
b
Wet THF contains 5% water. The guest ratio was determined by
1H NMR. The guest ratio was determined by 1H NMR, and it is an
c
average of three experiments: THF ratio 0.76 = (74.4% + 76.7% + 78.3%)/3.
THF between the polar hydrogen bonded networks and non-
polar cyclohexyl groups.
Table 1 presents more guest selectivity experiments. When
benzene, which is also close to the cyclohexyl pillars in size and
shape, was used in the 1 : 2 solvent mixture of CH3CN or acetone,
new crystals of 1ꢀ(C6H6) with benzene as guests formed. Crystals of
1ꢀ(C6H12) with cyclohexane guests were obtained from a mixture of
cyclohexane and acetone (1 : 2), and 1ꢀ(C5H10) with cyclopentane
guests were successfully obtained from a solvent mixture of cyclo-
pentane, ethyl acetate and CH3CN (1:2:2). Moreover, when the
above four guests were used in the same ratio as the solvent mixture
to let them compete with each other, the crystals of tri-carbamate 1
accommodating all four guests were obtained with a high preference
for THF (Table 1, entry 5). When a 1 : 1 mixture of THF and benzene
was used, crystals of 1ꢀ(0.76THFꢀ0.24C6H6) were obtained. THF was
the most favorite guest, which was consistent with our above hypo-
thesis about the size/shape selectivity and polarity match.
Fig. 1 Structural formula: (a) benzene-1,3,5-triyl tris(cyclohexyl-carbamate), 1.
Crystal structure of 1ꢀ(THF); (b) a top view of the chiral sheet backbone (colors
are introduced to illustrate the hydrogen bonding networks); (c) a side view of
the space-filling model of the sheet showing the surfaces and edges. Only
100 (10 ꢁ 10) molecules are shown for simplicity. Four tri-carbamate mole-
cules are in green or blue color to show the orientation of the molecules and
their cyclohexyl side chains; (d) a side view of the capped sticks model showing
two layers of the sheet with the THF guests in the space-filling model.
(Hydrogen atoms are omitted, and the chains inlaid in the sheets and between
the two layers are replaced with carbon atoms for simplicity.)
Although the sizes and shapes of all the four guests (THF,
C6H6, C6H12, and C5H10) are close to the cyclohexyl pillars, none
of them are the same. The flexibility of the hydrogen bonded
sheet plays a key role in accommodating these guest molecules.
Without changing the sheet backbone, the cell parameters of
those two reported. In the sheet structure, two nearest neighboring the crystal vary up to about 12% (Table S1 in ESI†).
molecules formed complementary conformations such that three
The crystals of 1ꢀ(THF) can slowly release the THF guest mole-
cyclic alkyl arms of one molecule is ‘‘up–up–down’’ and the other cules under ambient conditions. The powder XRD spectra show the
is ‘‘down–down–up’’, with regard to the hydrogen bonded sheet appearance of a new set of peaks after one week of exposure of
planes (Fig. 1c). Noticeably, the two cyclohexyl groups of each tri- 1ꢀ(THF) to the atmosphere together with the original peaks (Fig. 2).
carbamate on the same side of the sheet are partially inlaid within The thermal properties of the crystals of 1ꢀ(THF) were then
the surfaces of the sheet to fill the gaps within the hydrogen examined by DSC. As shown in the ESI,† an endothermic peak
bonded network. The third cyclohexyl group stands up from the was found at around 130 1C, which corresponded to the volatiliza-
surface of the sheet and becomes a pillar between the sheets. The tion of THF molecules from the crystals. Thus, the fresh crystals of
interlayer spaces between these cyclohexyl pillars are occupied by 1ꢀ(THF) were heated at 140 1C for 2 h. It was verified by 1H NMR that
the disordered guest THF molecules (Fig. 1d).
all the guest molecules were completely removed. The powder XRD
The crystals 1ꢀ(THF) were also readily obtained from 1 : 1 spectra showed the appearance of the new set of peaks with
mixtures of THF with diethyl ether, pentane, ethyl acetate, complete disappearance of the original peaks after heating. The
acetonitrile, or toluene, respectively. The preference of THF as characteristic 2 theta peak of 1ꢀ(THF) at 6.51 shifted to 7.71 showing
a guest is presumable mainly because the size and shape of that the interlayer distance between the hydrogen bonded sheets
the THF molecule fits well with the interlayer spaces created by was reduced to about 2 Å due to the loss of guests. All the peaks in
the cyclohexyl pillars. Furthermore, although there is no hydro- the new powder pattern matched well with those of the ground
gen bonding between the host and guest, the oxygen atoms fresh crystals grown in acetonitrile showing an excellent retention of
of nearest neighboring THF guests point towards the polar crystallinity. Pictures of two planar crystals of 1ꢀ(THF) and 1 are
hydrogen bonded networks in the opposite direction. Thus, included in the ESI.† Furthermore, the tri-carbamate solid that was
the dipole–dipole interactions between the host and guest obtained directly from synthesis has the same powder pattern as
probably offer additional stability for the encapsulation of that of the guest-free crystals grown in acetonitrile.
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sheets, we reveal that the hydrogen bonded organic sheet can be
further extended to the three-armed carbamates with cyclohexyl side
chains. The sheet structure remains unaltered with substantial
variation in the connection pattern showing the generality of the
two-dimensional hydrogen bonded network. More importantly, this
water stable clay-like material has created a cavity environment
and displayed guest size/shape selectivity and dynamic host–guest
responses.8a The lamellar material was able to be transferred to a
new crystalline state by absorbing the guest molecules, and then
desolvated and returned to its guest-free crystalline form. The facile
synthesis, water stability, and properties of selectively accommo-
dating guests demonstrate potential applications of the reusable
Fig. 2 Powder XRD: (a) fresh crystals of 1ꢀ(THF) obtained by the slow
evaporation of the wet THF solution; (b) crystals of 1ꢀ(THF) after 1 week organic clay in molecular storage, separation, and materials science.
under ambient conditions showing the presence of crystals 1; (c) crystals of
This work was supported by the Doctoral New Investigator grants
of the American Chemical Society Petroleum Research Fund (PRF
52705-DNI7) and the National Science Foundation ND EPSCoR
1ꢀ(THF) with THF guests completely removed by heating at 140 1C for 2 h;
(d) single crystals of 1 grown in a CH3CN solution; (e) the powder obtained
directly from the synthesis without further processing.
program (EPS-0814442). The authors are also grateful to NSF Grants
CHE-0947043 and CRIF CHE-0946990 for the purchase of the depart-
mental SEM at UND and XRD instrument at NDSU, respectively.
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