Guest-induced conformational switching in a single crystal

Article abstract of DOI:10.1002/anie.200602057

Without destruction of monocrystallinity: The conformational switching of a dinuclear metal complex between four distinct states (see picture) occurs without destroying the single crystal. This observation implies a substantial degree of cooperativity bet

Full text of DOI:10.1002/anie.200602057

Communications
Conformational Switching
A common strategy for engineering porous crystals is to
first prepare a solvent-templated structure and then to extract
the solvent molecules without causing collapse of the result-
ing metastable apohost framework. Desolvation of molecular
DOI: 10.1002/anie.200602057
Guest-Induced Conformational Switching in a
Single Crystal**
(
that is, 0D) solids generally results in reorganization of the
[13]
host molecules into a densely packed nonporous phase,
whereas metal–organic frameworks (MOFs) have been
shown to maintain structural integrity because of their
Liliana Dobrza n´ ska, Gareth O. Lloyd,
Catharine Esterhuysen, and Leonard J. Barbour*
[
6a]
intrinsic long-range rigidity.
Nevertheless, some MOFs
undergo structural modifications in one or more of the
following forms: 1) lateral sliding of neighboring layers
[
3,4d]
One of the defining characteristics of the crystalline state is
that atoms are generally considered to be frozen in place, each
with only a modest ability to vibrate about its well-defined
equilibrium position. Although solid-to-solid phase trans-
formations as a result of a variety of physical or chemical
factors are well known, it is rare for individual crystals to
relative to one another,
2) adjustment of the interlayer
[
1b]
[1b,14]
spacing, 3) distortion of 2D or 3D grids,
formational changes of their components.
and 4) con-
In several
[
4b]
recently reported cases where such structural changes have
been observed, MOF crystals have been shown to retain their
[
1b,c,g,3,4c,d]
macroscopic integrity.
[
1]
survive such processes by retaining their mosaicity. Consid-
erable mechanical stress is thought to occur at the boundary
Although useful for characterization of the different
phases by means of powerful diffraction techniques, single
crystals are generally not required for applications such as
catalysis, storage, and separation. On the other hand, if an
engineered crystal is to be incorporated into a device such as a
substrate-triggered sensor, fully reversible phase transforma-
tions with retention of monocrystallinity would be essential
for repeated operation. It is therefore of considerable
importance to study the processes that govern cooperative
fluidity in dynamic crystals with a view to gaining better
insight into the fascinating phenomenon of single-crystal
transformations.
[
1a]
of two interconverting phases, particularly if the two phases
are incompatible in packing periodicity. Therefore, in order
for a crystal to maintain its monocrystallinity during trans-
formation, it seems reasonable either that the structural
changes to the principal framework must be insignificant, or
[
1d,2]
that the molecules must cooperate
in a concerted fashion
as they undergo positional and/or topological reorganization.
In this regard, a number of recent reports have advocated the
phenomenon of cooperativity in order to account for the
apparent fluidity of crystalline building blocks during the
uptake or release of solvent molecules, with concomitant
rearrangement of the host lattice as a single-crystal trans-
Our approach to producing porous crystals involves the
use of discrete “donut-shaped” dinuclear metal–organic
[
1d,2,3]
[15]
formation.
changes of molecular conformation in systems possessing
These studies have involved relatively small
complexes that cannot pack efficiently owing to their lack
[
16]
of self-complementarity in shape. When crystallized from a
suitable solvent, these complexes usually enclose appropriate
solvent molecules into their apertures. Desolvation necessa-
rily results in the formation of voids or channels if the host
complexes retain their shape. As part of our ongoing studies,
crystals composed of neutral molecular rectangles were
obtained by slow evaporation of an equimolar solution of
[
1b–g,2–4]
conceptually infinite rigid assemblies.
Furthermore,
these reports describe structural switching between only two
states.
[
5]
Crystal engineering encompasses both the “synthesis”
and modification of structures. In recent years, the principles
of this burgeoning field have been applied vigorously to the
[
6]
design of new porous functional materials with a view to
mimicking and even surpassing the important properties of
1,4-bis[(2-methylimidazol-1-yl)methyl]benzene
(L)
and
CuCl ·2H O in acetone (Scheme 1). A single-crystal X-ray
2
2
[
7]
[8]
zeolites. Targeted applications include catalysis, as well as
diffraction analysis revealed that the coordination geometry
about each copper ion is tetrahedrally distorted square-
planar. Discrete metallacyclic complexes are eclipsed and
stacked parallel to the crystallographic a axis to form
[
9]
[10]
[1a,11]
the storage, separation, and sensing
of molecules. To
date, most studies have focused on so-called “soft materials”
[
1d,10b,12]
[1b,c,g,2,3,4b–e]
(
that is, organic
or metal–organic
systems),
owing to their vast potential for structural diversity.
[
*] Dr. L. Dobrza n´ ska, G. O. Lloyd, Dr. C. Esterhuysen, Prof. L. J. Barbour
Department of Chemistry
University of Stellenbosch
7602 Matieland (South Africa)
Fax: (+27)21-808-3849
E-mail: ljb@sun.ac.za
[
**] We acknowledge financial support from the National Research
Foundation (South Africa) and L.D. thanks the Claude Harris Leon
Foundation for a postdoctoral fellowship.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Formation of [Cu Cl L ]·2(CH ) CO (1); N black.
2
4
2
3 2
5
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Angewandte
Chemie
conceptually infinite tubules (see Supporting Information).
The resulting channels are occupied by occluded acetone
molecules, such that each dinuclear complex enshrouds two
molecules (see 1 in Figure 1) to form an efficiently packed
binary structure. The acetone templating effect in 1 is inferred
from the presence of weak CÀH···O hydrogen bonds between
ment of the conformationally altered host molecules, results
in shrinkage of the crystal volume by approximately 17%.
Upon exposure of 2 to acetone vapor for 2.5 h, the crystals
reversibly transformed back to phase 1 with stoichiometric
uptake of the solvent. Once again the bulk integrity of each
individual crystal was maintained (as confirmed by successful
single-crystal X-ray diffraction analysis) and the color
reverted back to light brown. In a subsequent experiment,
exposure of 2 to acetonitrile vapor for several minutes
resulted in a change in color to red, again with no apparent
fracturing of the individual crystals. Single-crystal X-ray
diffraction analysis revealed that a third phase (3, Figure 1)
is formed upon stoichiometric uptake of acetonitrile. The host
complex reconforms to once again yield a rectangular
complex that enshrouds two molecules of acetonitrile. The
packing mode is similar to that observed for 1. However,
examination of the complex in 3 reveals that, although
metrically similar to that in 1, the conformation of the
bridging ligands is quite different, especially with regard to
the relative positions of the methyl substituents of the
imidazole groups (Figure 2).
the ligand methyl groups and the solvent carbonyl oxygen
atoms.
Thermogravimetric analysis shows that desolvation (that
is, loss of acetone) occurs at room temperature. Our attempts
to obtain a porous material by desolvation involved either
heating crystals at 708C for approximately one hour or
exposing them to high vacuum for three hours. During
desolvation of 1 by either method, the crystals remained
transparent, although their color transformed from light
brown to light green. No fracturing was evident and it was
therefore possible to determine the structure of the desol-
vated phase (2) by single-crystal X-ray diffraction analysis. In
contrast to our previous findings involving analogous dinu-
[
16]
clear complexes, the host molecules in 2 undergo significant
conformational changes as a result of desolvation and
reorganization. The molecular rectangles collapse to assume
an “imploded” conformation (Figure 1) with loss of the guest-
templated aperture. Implosion of the complex occurs by
means of elongation of the intramolecular Cu···Cu distance
from 10.2 Š in 1 to 13.3 Š in 2, with corresponding adjust-
ment of the N-Cu-N angles from 147.2 to 90.18. Loss of the
molecular aperture, in conjunction with spatial rearrange-
Figure 2. Space-filling projections of the metal complexes in phases 1
to 4. The methyl groups on the imidazole moieties are darkened to
highlight the most significant conformational differences.
In phases 1–3, the metal complexes are all situated on
crystallographic inversion centers with half of the complex
unique in each case. In all three cases, two methyl substituents
on the imidazole moieties are situated on one side of the
mean plane of the dinuclear complex, while the remaining
two methyl groups are situated on the opposite side of this
plane. With reference to the projections in Figure 2, we can
refer to the methyl groups as “U” or “D” if they are directed
up or down, respectively. Starting with the upper-left methyl
group and proceeding in a clockwise direction, the confor-
mation of 1 with respect to the orientations of the methyl
groups can be expressed as UUDD. Although the ligand
conformation changes significantly during the transformation
from 1 to 2, the positions of the methyl groups relative to the
metallacyclic complex are retained as UUDD. However, the
Figure 1. Capped-stick projections of the molecules in phases 1 to 4
viewed along [100]. Conditions for interconversion of the various
structures are indicated (vap: vapor, STP: ambient conditions). Calcu-
lations were performed at the nonlocal DFT level of theory with the
B3LYP/LANL2DZ basis set. Computational details are included in the
Supporting Information.
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Communications
conformation in 3 is UDDU, and comparison of the relative
positions of the methyl substituents of the complexes in 1 and
shows the most notable difference to be rotation of two of
the UDDU conformation of 3 differ by less than 0.01 kcal
mol .
À1
3
The orientations of the methyl groups on the two
the imidazole groups by approximately 1208 about their CuÀ imidazole moieties represent the most dramatic conforma-
N vectors.
Phase 3 can be also be obtained directly from phase 1 by
exposure of the latter to acetonitrile vapor for approximately
0 min. The single-crystal integrity is maintained and X-ray
diffraction analysis showed that acetone is exchanged for
acetonitrile. This process can be reversed, although not as
readily as the transformation from 1 to 3; that is, exposure of 3
to acetone vapor requires several hours for reversion to phase
tional attributes of the ligand, while the angle of the
phenylene spacer group is a comparatively minor structural
characteristic. The series of interconvertible structures de-
scribed herein encompasses all the possible permutations of
major conformational features that the dinuclear complexes
can possess, with the exception of UUUU. The near
completeness of a set of mutable structures is an important
consideration for further study of monocrystalline conforma-
tional switching aimed at understanding why only a small
number of crystals appear to have this remarkable ability.
Chromatic changes in single crystals are potentially
important phenomena that can be implemented in gas- or
1
1
as a single-crystal transformation. When crystals of 3 are
exposed to atmospheric conditions for approximately 15 min,
transformation to yet a fourth phase (4, Figure 1) occurs,
again with maintenance of the macroscopic integrity of the
crystals.
[
1a]
vapor-sensing devices.
A single-crystal transformation
Single-crystal X-ray diffraction analysis revealed the
presence of two different conformations of the metallacyclic
complex in 4. Although one of these conformations (4a) is
similar to that of 3 with regard to the relative positions of its
methyl substituents (UDDU), the angle of the phenylene ring
of the ligand with respect to the mean plane of the
coordinated nitrogen atoms is significantly different (70.38
in 3 and 114.98 in 4a). The second conformation (4b) of the
complex present in 4 is unlike any of the conformations
observed in 1 to 3. This new conformation is DUDU and does
not possess inversion symmetry. In an attempt to rationalize
the transformation from 3 to 4, it is interesting to consider the
two distinct complexes in 4 in relation to two corresponding
molecules in 3. A projection of two neighboring molecules in
accompanied by a vapochromic response can easily be
rationalized when changes in the composition of the coordi-
[
1a,f]
nation sphere occur.
However, Lee and Suh have recently
demonstrated that alteration of only the geometry of the
coordination sphere can also result in vapochromism,
although a simple explanation for this has not yet been
[
1c]
postulated. To date, conformational switching of a structure
between only two states has been reported for several systems
possessing long-range rigidity (extensively hydrogen-bonded
networks, 2D layers, or infinite 1D, 2D, and 3D frameworks
that allow rows or sheets of molecules to move in uni-
[
1b–f,2–4]
son).
However, monocrystalline switching between
four (and possibly even more) states, as described herein, is
unprecedented, as is the magnitude of the conformational
changes that we have described. It seems self-evident that a
significant level of cooperation must occur between the
contorting molecules within the crystal in order for such large
conformational changes to be accommodated without cata-
strophic disruption of the packing continuity.
In summary, we have described the single-crystal to single-
crystal interconversion of four distinct molecular crystal
forms consisting of neutral, dinuclear metal complexes.
During uptake and release of solvent vapors, the host
molecules undergo significant conformational, positional,
and topological changes, each of which is accompanied by a
vapochromic response. Moreover, these monocrystalline
transformations are reversible and meet the requirements
for the development of functional devices based upon
structural modification of “soft” crystalline materials as an
extension of the field of crystal engineering.
3
, related to one another by a crystallographic 2 screw axis, is
1
shown in Figure 3, and it seems reasonable to suggest that
these molecules correspond to the two crystallographically
distinct molecules in 4, which are also shown in Figure 3.
Apart from rotation of the phenylene group of the ligand, as
described above, there is little change in the conformation or
the relative position of the molecule at the upper left.
However, the second molecule undergoes a transition from its
original conformation, but only by rotation of its imidazole
groups at one end (its upper left-hand side, as shown in
Figure 3). Energetically, the DUDU conformation of 4b and
Experimental Section
1,4-Bis[(2-methylimidazol-1-yl)methyl]benzene was synthesized by
the S 2 reaction of 2-methylimidazole with a,a’-dichloro-p-xylene.
N
Crystal data can be found in the Supporting Information. CCDC-
602877, CCDC-602878, CCDC-602879, and CCDC-602880 contain
the supplementary crystallographic 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.
Figure 3. Space-filling projections of the metal complexes in phases 3
and 4, showing the positional relationships of 4a and 4b with respect
to their counterparts in 3. The methyl groups on the imidazole
moieties are darkened to highlight the most significant conformational
differences.
Received: May 23, 2006
Published online: July 27, 2006
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Keywords: cooperative phenomena · crystal engineering ·
.
host–guest systems · phase transitions · solvatochromism
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