Designing Permeable Molecular Crystals That React with External Agents to Give Crystalline Products

Article abstract of DOI:10.1002/anie.200352252

Permeable molecular crystals can be specifically designed to let external agents enter, react, and produce single crystals of new substances, with retention of the original crystalline architecture (see picture). Such topotactic processes can be made to c

Full text of DOI:10.1002/anie.200352252

Angewandte
Chemie
substances, with retention of the original crystalline architec-
ture. Moreover, we show that such topotactic processes can be
made to cross-link molecular crystals covalently, thereby
capturing temporary supramolecular constructs as permanent
crystalline macromolecular replicas.
Our strategy is related to the technique of isomorphous
replacement in protein crystallography, a largely empirical
process in which protein crystals are treated with agents that
diffuse though water-filled channels in the crystal and bind at
sites not normally determined in advance. We felt that single
crystals of much smaller molecules might be made to react
quantitatively with external agents at predetermined sites,
thus providing a rational route to new substances in crystal-
line form. At present, such reactions are exceedingly rare,
have been discovered by accident rather than by design, and
involve only the smallest possible reactants.[3,4]
Topotactic Reactions in Networks
Suitable permeable molecular crystals can be made by an
approach that has been called molecular tectonics.[5] This
approach relies on the programmed association of special
sticky molecules called tectons (derived from the Greek word
for builder) that form directional interactions with neighbors
according to well-established motifs. Such molecules do not
typically crystallize in normal close-packed arrays; instead,
they tend to form open networks filled with potentially
mobile guests.[6] Tectonic networks held together by multiple
hydrogen bonds are normally robust enough to allow the
original guests to be exchanged or even partially removed
without the loss of crystallinity, which renders their interiors
accessible to various reagents.[7]
Designing Permeable Molecular Crystals That
React with External Agents To Give Crystalline
Products**
Philippe Brunet, Eric Demers, Thierry Maris,
Gary D. Enright, and James D. Wuest*
Topotactic reactions,[1] which convert single crystals of start-
ing compounds directly into single crystals of products, are
elegant and potentially useful processes that have fascinated
solid-state chemists for many years. Unfortunately, such
reactions are very rare. Mobility in solids is restricted, so
known topotactic reactions in molecular crystals typically
involve intramolecular processes or require that all necessary
co-reactants be present within the solid, held in close
proximity, and oriented properly.[2,3] These prerequisites can
sometimes be satisfied by laborious crystal engineering, but
new ways to devise topotactic reactions are needed. Here we
describe a general strategy based on the use of permeable
molecular crystals that are specifically designed to let external
agents enter, react, and produce single crystals of new
Initial experiments showed that in the porous hydrogen-
bonded network formed by tecton 1 (Scheme 1), -NH2 groups
not used in intertectonic hydrogen bonding lie in guest-filled
[*] Prof. J. D. Wuest, Dr. P. Brunet, E. Demers, Dr. T. Maris
DØpartement de Chimie
UniversitØ de MontrØal
MontrØal, QuØbec H3C3J7 (Canada)
Fax: (+1)514-343-6281
E-mail: wuest@chimie.umontreal.ca
Dr. G. D. Enright
Steacie Institute for Molecular Sciences
National Research Council
Ottawa, Ontario K1A 0R6 (Canada)
[**] We thank Drs. James F. Britten and Michel Simard for preliminary
crystallographic studies, Marjolaine Arseneault for her help in
obtaining Raman spectra, and Prof. Denis Gravel for his expertise in
photochemistry. We are grateful to the Natural Sciences and
Engineering Research Council of Canada, the Minist›re de l'Édu-
cation du QuØbec, the Canada Research Chairs Program, and the
Canada Foundation for Innovation for financial support. Cover
picture illustration courtesy of Christian Gravel (UniversitØ de
MontrØal).
Scheme 1. Tecton 1 and the synthesis of tectons 2 and 5a in solution.
channels.[8] We reasoned that -NHR groups in substituted
derivatives would be located similarly, thus making the
substituents accessible to external agents. To test this
hypothesis, we synthesized octaallyl derivative 2 by the
route summarized in Scheme 1. X-ray diffraction established
that tecton 2 crystallizes from dioxane in the tetragonal space
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200352252
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group I41/a to give the network represented by Figures 1
surface shown in Figure 3.[13] In principle, guests diffusing
inside the crystals can reach any point within the channels by
multiple redundant pathways. As planned, the channels also
include the allyl groups of tecton 2.
and 2. Each tecton forms a total of 16 hydrogen bonds with
four neighbors, which results in a robust noninterpenetrated
network with diamondoid connectivity.[9,10]
Figure 3. Stereoscopic representation of the channels in the structure
of tecton 2.[13] The image shows a 1”1”2 array of unit cells with the
c axis vertical. The outsides of the channels appear in light gray, and
dark gray is used to show where the channels are cut by the bounda-
ries of the array. The surface is defined by the possible loci of the
center of a sphere of diameter 3 Š as it rolls over the surface of the
ordered tectonic network.
When single crystals of tecton 2·6dioxane of dimensions
0.5 ” 0.5 ” 0.5 mm were treated with excess toluene at 258C
for 24 h, they remained transparent and morphologically
unchanged, continued to exhibit uniform extinction between
crossed polarizers, and showed closely similar unit-cell
parameters when studied by single-crystal X-ray diffraction.
However, analysis of dissolved samples by 1H NMR spectros-
copy established that the initial guest, dioxane, had been
replaced completely by toluene. As a result, these crystals are
well designed to allow a suitable reagent to enter by diffusion
and react predictably with the exposed allyl groups without
changing the architecture of the network, thereby yielding
isostructural crystals of a new substance.
Figure 1. View along the c axis of the crystal structure of tecton 2 show-
ing a single tecton (blue) and the four neighbors (red) with which it
forms hydrogen bonds (a).
This possibility was tested by placing single crystals of
tecton 2 in degassed toluene in a quartz vessel, then exposing
them to CH3SH and irradiating them with a medium-pressure
Hg lamp at 258C for 40 h. Under these conditions, photo-
chemical solid-state addition of thiol to the allyl groups
occurred to give thioether 5a,[14,15] in which 70–85% of the
1
allyl groups had reacted, as measured by H NMR spectro-
scopy of dissolved samples. Moreover, the process proved to
be topotactic. The resulting crystals were morphologically
unchanged, remained transparent, and continued to exhibit
uniform extinction between crossed polarizers. X-ray diffrac-
tion confirmed that thioether 5a had been formed isostruc-
turally as single crystals belonging to the tetragonal space
group I41/a, with unit-cell parameters similar (Æ 9%) to those
of precursor 2.
A larger thiol (CH3CH2CH2SH) also reacted topotacti-
cally in high yield with crystals of tecton 2 to give isostructural
single crystals of thioether 5b with similar (Æ 2%) unit-cell
parameters. In contrast, thiols that were too large to enter the
channels in crystals of tecton 2 reacted negligibly under
similar conditions, presumably because only the surfaces of
Figure 2. View along the b axis of the crystal structure of tecton 2
showing the cross sections of helical channels. Hydrogen atoms and
some allyl groups (upper channels) have been removed for clarity.
Dioxane occupies nearly 40% of the volume of the
crystals,[11,12] which have the composition 2·6dioxane, as
1
determined by X-ray crystallography and H NMR spectros-
copy of dissolved samples. The guests fill interconnected
helical channels that have a rhomboid cross section of
2
approximately 7.8 ” 11.4 Š at the narrowest points
(Figure 2). The channels themselves are represented by the
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the crystals are accessible. For example, photochemical
additions of 3,5-dimethylbenzenethiol and 2,4,6-trimethyl-
benzylthiol proceeded only to the extent of ꢀ 3%. Such
control experiments established that the observed solid-state
photochemical additions to tecton 2 do not occur by a
sequence of dissolution, reaction in solution, and subsequent
recrystallization. The mass spectrum of thioether 5a obtained
topotactically showed the expected statistical distribution of
molecules with reacted and unreacted allyl groups, which
suggests that addition occurs relatively homogeneously
throughout the crystal. A bimodal distribution, which would
arise if addition occurred exclusively near the surface to leave
an unreacted core, was not observed.
A sample of thioether 5a prepared independently in
solution by treating intermediate 4 (Scheme 1) with 3-meth-
ylthio-1-propanamine could be crystallized from CH3OH/
THF. In this case, the structure proved to be the same as the
one obtained topotactically in the solid state. In other cases,
however, we expect new polymorphs to be formed. Our
strategy for devising topotactic reactions in porous crystals
has two notable advantages: 1) It delivers single crystals of
new compounds in polymorphic forms that are predeter-
mined, based on the structure of the starting material, and
2) these polymorphs may be impossible to obtain by direct
crystallization of the product, either because other poly-
morphs are favored or because crystallization does not occur
at all.
A long-sought goal in supramolecular chemistry is cova-
lent capture,[16] in which ordered structures formed reversibly
by self-assembly are converted into analogues joined perma-
nently by covalent bonds. Molecular tectonics allows predict-
able covalent capture to occur in the solid state by using
permeable crystals with internally exposed reactive groups
that can be cross-linked by external agents. This strategy
extends conceptually related polymerizations, such as the
cross-linking of porous protein crystals[17] and coordination
complexes joined by metals,[18] to the realm of small molecular
monomers and the production of single crystals of well-
defined polymers.[19]
Single crystals of tecton 2·6dioxane were exposed to a
series of liquid dithiols HS(CH2)nSH and irradiated under
standard conditions. The resulting materials proved to be
insoluble in DMSO, hot aqueous HCl, and CF3COOH, even
after crushing to expose internal surfaces. This result indicates
that extensive cross-linking had taken place. When
HSCH2CH2SH was used as the cross-linker, detailed studies
of the resulting polymer by solid-state 13C NMR spectroscopy
and Raman spectroscopy confirmed the addition of the thiol
to the allyl groups.[20] Remarkably, cross-linking of crystals of
tecton 2 by both HSCH2CH2SH and HSCH2CH2CH2SH
occurred topotactically to give isostructural single crystals of
the macromolecular products. Detailed single-crystal X-ray
crystallographic analysis of the products confirmed directly
that cross-linking had occurred and showed that each
molecule of tecton 2 becomes joined not to its immediate
hydrogen-bonded neighbors but to those somewhat farther
away (Figure 4).
Figure 4. Structure of the macromolecule obtained by cross-linking
crystals of tecton 2 with HSCH2CH2SH, as viewed approximately along
the c axis. The image shows a central unit derived from tecton 2
(blue), four neighboring units (red) joined by hydrogen bonds (a),
and one of the farther units (also in blue) to which the central unit has
been linked covalently.
bonded supramolecular precursor. In particular, variable-
temperature X-ray powder diffraction established that the
polymer derived from HSCH2CH2SH loses its crystallinity
only when it decomposes irreversibly above 2008C, whereas
the precursor loses crystallinity at 258C by spontaneous loss
of included solvent. The topotactic cross-linking we observe
in single crystals of tecton 2 has two unique characteristics:
1) Polymerization occurs in three dimensions to give a single
crystalline macromolecular object, and 2) the composition of
the product crystals is different from that of the starting
crystals. In contrast, previously observed topotactic polymer-
izations have occurred in one direction to yield crystals with
the same composition consisting of multiple copies of linear
macromolecules.[2]
These observations demonstrate how molecular tectonics,
by allowing the design of robust porous networks with
reactive interiors, can give access to new molecular and
macromolecular materials with predictably ordered struc-
tures that would be difficult or impossible to obtain by
conventional methods.
Experimental Section
X-ray structural analyses: Data were collected at 173 K using a
Bruker SMART 1000 CCD diffractometer with MoKa radiation (l =
0.71070 Š). Intensities were integrated using SAINT[21] and corrected
for absorption and other effects using SADABS.[22] Structures were
solved and refined using the SHELX suite of programs.[23]
Crystal data for tecton 2·6dioxane: crystal size 0.15 ” 0.20 ”
0.30 mm, tetragonal, space group I41/a, a = b = 23.0674(6), c =
16.1892(6) Š, V= 8614.4(4) Š3, Z = 4. Least-squares refinement of
273 parameters based on 1759 reflections with I > 2s(I) (out of 2269
reflections) gave R1 = 0.0795, wR2 = 0.230, and GOF = 1.076.
Despite close architectural similarity, the covalently cross-
linked materials are vastly more stable than their hydrogen-
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Crystal data for thioether 5a (as obtained from crystals of
[9] S. R. Batten, R. Robson, Angew. Chem. 1998, 110, 1558; Angew.
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[10] M. J. Zaworotko, Chem. Soc. Rev. 1994, 23, 283.
[11] The percentage of volume accessible to guests was estimated by
the PLATON program,[12] as described in detail in the Support-
ing Information.
tecton 2): crystal size 0.08 ” 0.15 ” 0.20 mm, tetragonal, space group
I41/a, a = b = 20.988(2), c = 16.816(3) Š, V= 7407.6(16) Š3, Z = 4.
Least-squares refinement of 214 parameters based on 865 reflections
with I > 2s(I) (out of 1976 reflections) gave R1 = 0.1210, wR2 = 0.389,
and GOF = 1.306.
Crystal data for the product of cross-linking crystals of tecton 2
with HSCH2CH2CH2SH: crystal size 0.15 ” 0.30 ” 0.30 mm, tetrago-
nal, space group I41/a, a = b = 23.233(16), c = 16.274(17) Š, V=
8785(13) Š3, Z = 4. Least-squares refinement of 182 parameters
(with 192 restraints) based on 462 reflections with I > 2s(I) (out of
688 reflections) gave R1 = 0.1888, wR2 = 0.461, and GOF = 1.766.
[12] A. L. Spek, PLATON, A Multipurpose Crystallographic Tool,
Utrecht University, Utrecht (The Netherlands), 2001; P. van
der Sluis, A. L. Spek, Acta Crystallogr. Sect. A 1990, 46, 194.
[13] Representations of channels were generated by the Cavities
option in the ATOMS program (ATOMS, Version 5.1; Shape
Software: 521 Hidden Valley Road, Kingsport, TN 37663;
http:\www.shapesoftware.com). We are grateful to Eric Dowty
of Shape Software for integrating this capacity into ATOMS at
our suggestion.
Received: June 27, 2003 [Z52252]
Published Online: September 19, 2003
[14] For a review of photochemical reactions in molecular crystals,
see: V. Ramamurthy, K. Venkatesan, Chem. Rev. 1987, 87, 433.
[15] C. S. Marvel, R. R. Chambers, J. Am. Chem. Soc. 1948, 70, 993;
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[17] A. L. Margolin, M. A. Navia, Angew. Chem. 2001, 113, 2262;
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Keywords: crystal engineering · hydrogen bonding · porous
networks · supramolecular chemistry · topotactic reactions
.
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Angew. Chem. Int. Ed. 2003, 42, 5303 –5306