Mechanical motion in the solid state and molecular recognition: Reversible cis-trans transformation of an organic receptor in a solid-liquid crystalline state reaction triggered by anion exchange

Article abstract of DOI:10.1039/c5ce00449g

An organic receptor [C₂₈H₂₄N₄]²⁺ can be obtained with its unusual cis-form when it is ion-paired with a [Zn(dmit)₂]^2- coordination complex anion with the isolation of [C₂₈H₂₄N₄][Zn(dmit)₂]·2DMF (2). Compound 2 shows reversible cis-trans isomerization of the cationic receptor in a solid to solid transformation in a solid-liquid interface reaction.

Full text of DOI:10.1039/c5ce00449g

CrystEngComm
COMMUNICATION
Mechanical motion in the solid state and
molecular recognition: reversible cis–trans
transformation of an organic receptor in a solid–
liquid crystalline state reaction triggered by anion
exchange†
Cite this: CrystEngComm, 2015, 17,
3219
Received 4th March 2015,
Accepted 27th March 2015
Vedichi Madhu,ab Supriya Sabbani,c Ravada Kishore,a Indravath K. Naika and
DOI: 10.1039/c5ce00449g
a
*
Samar K. Das
www.rsc.org/crystengcomm
An organic receptor ijC28H24N4]2+ can be obtained with its unusual
cis-form when it is ion-paired with a ijZnIJdmit)2]2− coordination
complex anion with the isolation of ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF
(2). Compound 2 shows reversible cis–trans isomerization of the
cationic receptor in a solid to solid transformation in a solid–liquid
interface reaction.
The analysis of crystal structures of this molecular recep-
tor (1c) with diverse counter anions reveals an anti (staircase-
like, Scheme 1, right) conformation and its cis conformation
is rarely stabilized. Compound ijC28H24N4]Br2 (1)[1c]Br2, the
bromide salt of receptor 1c, is a well known example in
which the receptor exhibits its usual trans conformation
(Fig. 1 and Scheme 1, right).7 In the present work, we have
used a ZnIJII)-dithiolato coordination complex as an anionic part
to synthesize an ion-pair compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF
(2)[1c][1a]·2DMF, in which the cationic receptor 1c
exhibits its unusual cis conformation (Fig. 1 and Scheme 1,
left). Red-brown crystals of compound 2 have been synthe-
sized starting from ijC28H24N4]IJPF6)2ij1c]IJPF6)2 and
ijBu4N]2ijZnIJdmit)2]ijBu4N]2ij1a] in acetonitrile solvent‡ and
characterized using routine spectroscopic techniques‡ as well
as by single crystal X-ray crystallography§ and elemental
analysis.‡ The stabilization of an unusual cis conformation of
1c in compound 2 can be understood through the analysis of
intermolecular interactions in the relevant crystal structure,§
which shows that two 4,4′-bipyridinium ends of the cationic
receptor adopt a parallel arrangement leading to the forma-
tion of molecular clefts. Further inter-digitated stacking of
these cleft-like receptors through intermolecular π–π interac-
tions results in the formation of a cation dimer (Fig. 2). The
Controlled molecular modification is of considerable interest
in the context of motional dynamic devices. In view of this,
the design of a functional chemical system, which can switch
between two different states, is one of the important facets of
modern science.1
A system that undergoes structural
modifications under the action of external stimuli, such as
light, electron transfer, ion binding, etc.,2 is well documented
in the literature, for example, the cis–trans isomerization of
azobenzene derivatives.3 Molecular systems, in which the
constituent components produce mechanical motion in
response to specific stimuli, are important in the context of
molecular machines.4 Molecular tweezers that represent a
class of acyclic receptors with cavities of flexible sizes have
potential to exhibit conformational changes.5 Here we
describe
a
molecular
tweezer,
1,1″-1,4-phenylene-
cationic receptor
bisIJmethylene)bis-4,4′-bipyridinium
IJ[C28H24N4]2+ (1c)),6 and stabilization of both of its cis- and its
trans-forms (Scheme 1) as a function of counter anions. The
reversible switching between these cis and trans conforma-
tions is brought about in solid to solid transformation driven
by molecular recognition of anion in a solid–liquid interface
reaction.
a School of Chemistry, University of Hyderabad, Hyderabad – 500046, India.
E-mail: skdas@uohyd.ac.in; samar439@gmail.com
b Department of Chemistry, Karunya University, Coimbatore – 641114, India
c School of Physical Sciences, Jawaharlal Nehru University, New Delhi – 110067,
India
Scheme 1 Left: cis (or cleft-like) conformation of ijC28H24N4]2+ (1c) in
ion-pair compound ijC28H24N4] ijZnIJdmit)2]Ĵ2DMF (2). Right: anti (or
staircase-like) conformation of ijC28H24N4]2+ (1c) in its bromide salt,
ijC28H24N4]Br2 (1).
† Electronic supplementary information (ESI) available. CCDC 1051961. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5ce00449g
This journal is © The Royal Society of Chemistry 2015
CrystEngComm, 2015, 17, 3219–3223 | 3219

Communication
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ijZnIJdmit)2]2− complex anion was isolated as micro-crystalline
materials of ijBu4N]2ijZnIJdmit)2] and its identity was
established by routine spectral analysis and elemental analy-
sis (see section 1C and Fig. S12–S13 in the ESI†).
The resulting off-white crystals, obtained as the product of
anion exchange in this solid–liquid interface reaction, are
characterized as compound ijC28H24N4]Br2 (1) by PXRD and
IR studies. The powder X-ray diffraction (PXRD) pattern of
transformed off-white crystals that were obtained from 2 in
solid to solid conversion in solid–liquid interface reaction
and the simulated PXRD pattern from the single crystal data
of previously reported ijC28H24N4]Br2 (1)7b are identical as
shown in Fig. 3. The identity of these transformed off-white
crystals as crystals of compound 1 is also established by IR
studies (see Fig. S8 and S9 in the ESI† for their respective IR
spectra). This solid to solid transformation (eqn (1), forward
reaction) from compound 2 to compound 1 at the solid–
liquid interface is remarkable in the sense that the cis confor-
mation of the organic receptor 1c of compound 2 undergoes
conformational change to its trans conformation to form
compound 1, whereby the ijZnIJdmit)2]2− anion comes out
from the crystal to the solution and the Br− anion gets into
the crystal from the solution phase without dissolution of the
relevant crystals.
Fig.
1 Left: the thermal ellipsoidal plot (50% probability) of
ijC28H24N4]ijZnIJdmit)2] in compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2).
Right: the crystal structure of ijC28H24N4]Br2 (1).7b
Fig. 2 Left: formation of the supramolecular zipper of molecular cleft
ijC28H24N4]2+ via π–π stacking interactions through molecular
recognition of ijZnIJdmit)2]2−; color code: Zn, cyan; S, yellow; C, gray;
and N, blue. Right: the corresponding schematic representation. Color:
ijZnIJdmit)]2− in yellow, aromatic chromophores in blue and the single
spacer in red.
[C28H24N4][Zn(dmit)2] · 2DMF (2) (solid phase)
+ [Bu4N]Br (soln. phase) ⇄ [C28H24N4]Br2 (1) (solid phase)
+ [Bu4N]2[Zn(dmit)2] (soln. phase)
(1)
The forward reaction of eqn (1), in which the constrained
cis conformation of the receptor molecule ijC28H24N4]2+ (1c)
undergoes conformational change to its favourable
trans-form in the solid matrix, seems to be a facile one and is
indeed completed within a few minutes (see Scheme S3 of
section 1C in the ESI† for detailed experimental description
and relevant experimental images). On the other hand, the
reverse reaction of eqn (1), wherein the ijZnIJdmit)2]2− anion
from the solution phase has to get into the solid matrix of
ijC28H24N4]Br2 (1) and the Br− anion has to come out from the
crystal phase to the solution phase to form compound 2 with
inter-dimer π–π stacking interactions8 lead to the formation
of a supramolecular zipper as shown in Fig. 2. In the zipper,
each receptor cation of this dimer interacts with one
ijZnIJdmit)2]2− complex anion through π–π interactions, which
involve the contact of the C3S2 ring (see Fig. 1) of the
ijZnIJdmit)2]2− coordination complex anion with the aromatic
ring of the 4,4′-bipyridinium end of the receptor cation 1c,
resulting in the stabilization of its cis conformation (Fig. 2).
The π–π interactions are discussed in details in Fig. S2–S4 of
section 2 in the ESI.†
At this point, it is logical to argue that the cis conforma-
tion of 1c in compound 2 can be described as a constrained
situation because of the occurrence of the unfavourable
cis-form of the cation receptor, stabilized through π–π stack-
ing interactions between 1c and 1a. Thus if the ijZnIJdmit)2]2−
complex anion, responsible for this forced situation in com-
pound 2, is removed from the relevant crystal, this cisoid
form of cationic receptor 1c is expected to revert to its rela-
tively stable trans conformation. Thus when deep brown crys-
tals of ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) are suspended in a
saturated MeCN solution of [Bu4N]Br, in which compound 2
is insoluble, the deep-brown crystals start losing their color
without dissolution which results in colorless off-white crys-
tals attributable to the removal of the ijZnIJdmit)2]2− coordina-
tion complex from the crystal into the solution forming a
pink MeCN solution of ijBu4N]2ijZnIJdmit)2].‡ The released
Fig.
3 PXRD patterns of ijC28H24N4]Br2 (1): top, obtained by ion
exchange between compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) (solid
phase) and [Bu4N]Br (soln. phase); bottom, obtained by single crystal
data simulation of ijC28H24N4]Br2.
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the conformational change of the receptor 1c from its favor-
able trans-form to an unfavorable cis-form, is anticipated to
be a sluggish conversion. Indeed, when the crystals of com-
pound ijC28H24N4]Br2 (1) are suspended in a saturated MeCN
solution of ijBu4N]2ijZnIJdmit)2] containing few drops of DMF
and stirred at room temperature, it needs a minimum of one
week to achieve an effective conversion‡ to the crystalline
solid of compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) through
the exchange of the bromide anion from the crystal with the
ijZnIJdmit)2]2− complex anion from the solution phase, as con-
firmed by the comparison of PXRD studies. Fig. 4 compares the
observed PXRD patterns of compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF
(2) obtained by anion exchange (top) and original synthesis
(bottom). Fig. S14 (ESI†) demonstrates the time dependent
PXRD patterns of the intermediate products (1 day, 3 days, 5
days and 7 days).
IR studies are also consistent in this solid to solid conver-
sion (Fig. S10 and S11, ESI†). Scheme 2 describes this revers-
ible cis–trans isomerization of the organic receptor
ijC28H24N4]2+ (1c) in a solid-to-solid transformation. Lehn and
co-workers described the design of the shape/conformational
switches (between W- and U-shapes) of organic receptor mol-
ecules that are triggered by cation complexation in the solu-
tion state.9 We have described here anion triggered switches
between cis and trans conformations of an organic receptor
ijC28H24N4]2+ that undergoes reversible molecular motion (cis–
trans transformation) in the solid state in solid–liquid inter-
face reactions induced by the ion-exchanged ijZnIJdmit)2]2−
coordination complex anion and Br− anion, in comparison to
a number of reports of light-driven and thermal molecular
motion. The novelty of this work lies in the fact that the
molecular recognition of a metal dithiolato coordination
complex by an organic receptor through π–π intermolecular
interactions triggers a reversible cis–trans conformational
switch in a crystal to crystal transformation. This is driven by
the exchange of the ijZnIJdmit)2]2− anion (that comes out from
the solid crystal matrix to the solution phase) with the Br−
anion (which is already dissolved in the solution phase and
gets into the crystal) without dissolution of the receptor crys-
tals. The uniqueness of the ijZnIJdmit)2]2− anion is that it can
exert π–π intermolecular interactions with the receptor mole-
cule leading to cis conformation of the receptor (forced
Scheme 2
situation) and the Br− anion cannot participate in π–π inter-
molecular interactions; therefore the receptor goes to a more
favorable trans-form in this case. Since it is a crystalline state
reaction (crystal to crystal transformation), the initial crystals
(compound 2: cis conformation of the receptor cation) and
resultant crystals (compound 1: trans conformation of the
receptor cation), in principle, should not be drastically differ-
ent as far as crystal systems are concerned. This is indeed
true, because during this solid–liquid interface reaction, the
¯
triclinic (P1) crystals of ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) trans-
form to monoclinic ijC28H24N4]Br2 (1) crystals having a space
group of P21/n. Triclinic and monoclinic systems are not sig-
nificantly different: both have three unequal axes and both
can have a 1-fold axis. In the triclinic system, three unequal
axes intersect at oblique angles (they are not at 90 degrees
from the other), and in the case of the monoclinic system,
among three unequal axes, two are inclined to each other at
an oblique angle and the third one is perpendicular to the plane
of the other two; therefore the monoclinic system has a mirror
plane. When the triclinic crystals of ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF
(2) transform to the monoclinic crystals of ijC28H24N4]Br2 (1), the
volume of the unit cell decreases from 2213.1 Å3 to 1291.70
Å3. This is consistent with the fact that, during the molecular
motion in crystal to crystal transformation a considerably
bigger entity ijZnIJdmit)2]2− comes out from the compound 2
crystals and a relatively smaller species Br− gets into the rele-
vant crystals. This also explains the introduction of the
mirror plane (from triclinic to monoclinic), because the Br−
is a one-atom entity unlike a molecular entity. The reverse
reaction of eqn (1) is naturally slow and sluggish because this
involves the conformational change of the receptor from a
favored one (trans) to an unfavorable one (cis) through
mechanical motion of the molecules in the crystalline state
reaction.
In summary, the work presented here can be described as
a functional system that can operate in a reversible cycle
involving a flexible cationic organic molecule that goes to its
cleft-like cis conformation in the solid state (when it
recognizes a metal dithiolato coordination complex anion)
and to its usual trans conformation when it releases the coor-
dination complex anion and picks up the bromide anion.
Fig. 4 PXRD patterns of compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2):
top, obtained by ion exchange between compound ijC28H24N4]Br2 (1)
(solid phase) and ijBu4N]2ijZnIJdmit)2] (soln. phase); bottom, compound 2
directly synthesized from ijC28H24N4]IJPF6)2 and ijBu4N]2ijZnIJdmit)2].
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Thus this supramolecular system can be treated as a molecu-
lar machine,4 in which the molecular components produce
quasi-mechanical movement (output) in response to specific
stimuli (input; in the present case, the input is ion exchange).
scientist scheme (project no. SB/FT/CS-182/2011). S. S. thanks
the “start-up grant for newly recruited faculty from BSR-UGC”
(sanction letter no. F.20-43/2-13IJBSR)). We are grateful to Dr.
Bharat Kumar Tripuramallu, who helped us to analyze the
crystal structure of compound 2.
Synthesis via solid–liquid interface
reactions: solid to solid
transformation without dissolution of
the solid
Notes and references
‡ Synthesis ijC28H24N4][Zn(dmit)2]·2DMF (2). 0.07 g (0.1 mmol) of ijC28H24N4]
(PF6)2 (see section 1A and Scheme S1 in the ESI† for synthesis of compound
ijC28H24N4](PF6)2) was dissolved in CH3CN. Subsequently, a CH3CN solution of
ijBu4N]2[Zn(dmit)2] (0.094 g, 0.1 mmol) was added, whereby a brown precipitate
was formed immediately. The precipitate was filtered, washed with ether and
dried in air. Compound ijC28H24N4][Zn(dmit)2]·2DMF (2) was crystallized from
this precipitate in a DMF/ether system (crystallization time duration: 3–4 days at
Forward reaction of eqn (1) (compound 1 from compound 2)
The compound ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) (0.370 g, 0.42
mmol) was suspended in a saturated MeCN solution (com-
pound 2 is not soluble in MeCN) of [Bu4N]Br (0.540 g, 1.68
mmol); the crystals of compound 2 lose their color by removing
the ijZnIJdmit)2]2− coordination complex from the crystal into the
solution, forming a pink MeCN solution of ijBu4N]2ijZnIJdmit)2]
(freely soluble in MeCN), resulting in colorless crystals of
compound ijC28H24N4]Br2 (1) (0.120 g), which were separated
and air dried. The pink MeCN solution of ijBu4N]2ijZnIJdmit)2]
was stored in an open beaker and evaporated under ambient
conditions, whereby the solid crystals of ijBu4N]2ijZnIJdmit)2]
appeared. This compound was filtered and washed with
isopropyl alcohol followed by diethyl ether. The identity of
this solid as ijBu4N]2ijZnIJdmit)2] was confirmed by comparing
its PXRD pattern with that simulated from single crystal data
of the reported ijBu4N]2ijZnIJdmit)2] (Fig. S13, section 3C, in
the ESI†). It was also characterized by IR spectral studies
(Fig. S12, in the ESI†).
room temperature). Yield: 0.09
40H38N6O2S10Zn: C 47.07, H 3.75, N 8.23; found: C 46.98, H 3.55, N 8.12. IR
(KBr pellet): 3036(m), 1658s, 1635(s), 1543(m), 1406(s), 1215(m), 1159(m),
1055(s), 1028(s), 993(m), 887(m), 790(s), 729(m), 459(s) cm−1 1H NMR (DMSO-
g (90%). Elemental analysis: calcd (%) for
C
.
d6): δ 9.32(d, J = 6.847, 4H), 8.85(d, J = 4.891, 4H), 8.862(d, J = 5.869, 4H), 7.99(d,
J = 4.891, 4H), 7.66(S, 4H), 5.89(S, 4H). 13C NMR (DMSO-d6): 31.24, 62.94,
122.42, 126.36, 130.11, 135.80, 141.22, 145.82, 151.46, 153.37, 162.78, 207.31.
§ Crystallography ijC28H24N4][Zn(dmit)2]·2DMF (2) was measured at 100 K using
a Bruker SMART APEX CCD area detector system [λ (Mo-Kα) = 0.71073 Å] with a
graphite monochromator. 2400 frames were recorded with an ω scan width of
0.3°, each for 8 s. The crystal-detector distance was 60 mm, and the collimator
diameter was 0.5 mm. Data reduction was done using SAINTPLUS (Software for
the CCD Detector System, Bruker Analytical X-Ray Systems Inc., Madison, WI,
1998); structure solution was determined using SHELXS-97 (G. M. Sheldrick,
Program for structure solution, University of Göttingen, Germany 1997) and
refinement was done using SHELXL-97 (G. M. Sheldrick, Program for crystal
structure analysis, University of Göttingen, Germany 1997; G. M. Sheldrick, Acta
Cryst., 2008, A64, 112). All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms on the aromatic ring were introduced on calculated positions
and included in the refinement riding on their respective parent atoms. The
contribution of the disordered DMF molecules (compound 2) has been
subtracted from the diffraction data by the squeeze technique in the PLATON
software. The disordered atoms in the bipyridine and dithiolene moieties are
Reverse reaction of eqn (1) (compound 2 from compound 1)
The off-white organic salt ijC28H24N4]Br2 (1) (0.450 g, 0.6 mmol)
and the colored coordination compound ijBu4N]2ijZnIJdmit)2]
(2.1 g, 2.2 mmol) were added to the MeCN solvent (50 mL)
containing dimethylformamide (2.5 mL), and this reaction
mixture was stirred at room temperature for seven days
IJ[C28H24N4]Br2 (1) is not soluble in this medium, but
ijBu4N]2ijZnIJdmit)2] is freely soluble). During this time, the
off-white solid becomes colored by anion exchange (the col-
ored ijZnIJdmit)2]2− complex anion gets into the crystal from
the solution phase and Br− comes out to the solution from
the solid phase). After seven days of stirring, the reaction
mixture was filtered and the colored solid was washed with
acetonitrile and hexane and the resulting compound
ijC28H24N4]ijZnIJdmit)2]Ĵ2DMF (2) (0.460 g) was dried under air.
modelled by ISOR restraints. Crystal data: 2: C34H24N4S10Zn, M = 874.54 g mol−1
,
¯
triclinic, space group P1, a = 11.011(3) Å, b = 11.867(4) Å, c = 18.154 (6) Å, α =
71.519IJ5)°, β = 79.676IJ5)°, γ = 87.614IJ5)°, U = 2213.1(12) Å3, Z = 2, DC = 1.312 g
cm−3, μ = 1.054 mm−1, FIJ000) = 892, crystal size = 0.4 × 0.3 × 0.2 mm3. 20 337
reflections measured with 7508 unique reflections (Rint = 0.0370), of which 5645
(I > 2σIJI)) were used for the structure solution. Final R1 (wR2) = 0.0691 (0.1876),
452 parameters. The final Fourier difference synthesis showed minimum and
maximum peaks of −0.798 and +1.722 e Å−3. CCDC 1051961 contains the supple-
mentary crystallographic data for compound 2.
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Acknowledgements
We thank the CSIR, Government of India (project no. 01
IJ2556)/12/EMR-II), for financial support. The National X-ray
Diffractometer Facility at the University of Hyderabad by the
Department of Science and Technology, Government of India,
is gratefully acknowledged. We thank Mr. Raju Mekala for his
cooperation in the PXRD studies. V. M. acknowledges the
DST for the financial support under the fast-track young
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