Coupled nanomechanical motions: Metal-ion-effected, ph-modulated, simultaneous extension/contraction motions of double-domain helical/linear molecular strands

Article abstract of DOI:10.1021/ja408752m

A new class of shape-enforced synthetic polyheterocyclic molecular strands, containing both a helical and a linear domain, has been designed and synthesized. On reaction with Pb(II), under the effect of cation binding to the coordination subunits, the helical section unfolds into a linear shape in the complex and the linear domain folds into a helical ligand wrapped around the bound cations. Such double-domain ligand strands are thus able to undergo a combined unfolding-folding interconversion on binding and release of metal cations. These changes can be modulated through coupling to a competing ligand that reversibly binds and releases metal cations, when respectively unprotonated and protonated, on effecting alternate pH changes. The resulting process thus performs nanomechanical extension/contraction molecular motions of a linear motor type, which is fueled by acid-base neutralization.

Full text of DOI:10.1021/ja408752m

Article
pubs.acs.org/JACS
Coupled Nanomechanical Motions: Metal-Ion-Effected,
pH-Modulated, Simultaneous Extension/Contraction Motions
of Double-Domain Helical/Linear Molecular Strands
Adrian-Mihail Stadler*,†,‡ and Jean-Marie P. Lehn*,†
†Laboratoire de Chimie Supramolec
8 Allee Gaspard Monge, Strasbourg, 67000, France
‡Institut fur Nanotechnologie (INT), Karlsruhe Institut fur Technologie (KIT), 76344 Eggenstein-Leopoldshafen, Germany
́ ́ ́ ́
ulaire, Institut de Science et d’Ingenierie Supramoleculaires, Universite de Strasbourg,
́
̈
̈
S
* Supporting Information
ABSTRACT: A new class of shape-enforced synthetic polyheterocyclic
molecular strands, containing both a helical and a linear domain, has
been designed and synthesized. On reaction with Pb(II), under the effect
of cation binding to the coordination subunits, the helical section unfolds
into a linear shape in the complex and the linear domain folds into a
helical ligand wrapped around the bound cations. Such double-domain
ligand strands are thus able to undergo a combined unfolding−folding
interconversion on binding and release of metal cations. These changes
can be modulated through coupling to a competing ligand that reversibly
binds and releases metal cations, when respectively unprotonated and
protonated, on effecting alternate pH changes. The resulting process
thus performs nanomechanical extension/contraction molecular motions
of a linear motor type, which is fueled by acid−base neutralization.
metal ions. The structural formulas of the double-domain (linear
and helical) oligo-heterocyclic molecular strands 1−3, discussed
herein, are given below (Figure 1).
INTRODUCTION
■
The implementation of molecular and supramolecular nanodevices
to perform controlled motions in response to physical stimuli or
chemical effectors is challenging the creativity of chemists
toward the design of so-called “molecular machines” (switches
and motors). The high activity generated is reflected in the many
reviews describing the work performed in this field.1
Rationale and Design Principles. The design of the
present molecular entities capable of performing coupled
motions of different types is based on our earlier work on
the control of the shape of polyheterocyclic strands by means of
“folding codons” (“foldons”)6 consisting of a specific sequence
of aza-aromatic heterocyclic group. In our laboratory, we have
earlier designed and synthesized such helical shape-persistent
polyheterocyclic strands7a−d based on helicity-enforcing se-
quences of α,α′-connected pyridine-pyrimidine (py-pym) units
that behave as helicity codon. The folding features are based on
the strong preference of α,α′-bipyridine (by about 25−30 kJ/mol)8
for the transoid conformation about the C−C bond in the
NC−CN fragment rather than for the cisoid one. On the
other hand and for the same reasons, sequences of α,α′-
connected pyridine groups present a linear shape.9 In view of
the isomorphism between a pyridine group and a hydrazone
unit (as described earlier),10 the same bias is expected to hold
for the analogous sequences incorporating hydrazone units,
hydrazone-pyridine (hyz-py), hydrazone-pyrimidine (hyz-pym),
as well as hydrazone-pyrazine (hyz-pz). Thus, whereas hyz-pym
units are bent and encode helical strands,10,11 hyz-py units
generate strands of linear geometry.12 Figure 2 illustrates the
Molecular switches and motors are based on motional pro-
cesses of various mechanical kinds, such as axial shuttling2 or
sliding, unidirectional axial rotation,3 walking,4 spring extension,
coiling or wrapping (folding),5 and others. Most of the devices
reported until now undergo only one of the above-mentioned
types of molecular motions. We report herein the synthesis and
the study of the motional features of a new type of nano-
mechanical molecular device, which performs simultaneously
two kinds of motions in a correlated fashion within the same
molecular framework in a sole motional operation. Such a
behavior has been achieved by the design of ligand strands 1−3,
presenting a linear domain connected to a helically folded one,
which, on binding of metal ions, undergo helical wrapping of
the linear part and extension of the helical domain into a linear
one. Conversely, on removal of the metal ions by trapping
with a competing ligand, the opposite processes occur, thus
restoring the initial shapes of the free ligand strands. This type of
system thus represents a hybrid motional device that is capable
of performing a reversible wrap/unwrap process resulting in cor-
related simultaneous contraction/extension motions. It produces
a kind of reversible worm-like motion, on binding and removal of
Received: August 23, 2013
Published: February 18, 2014
© 2014 American Chemical Society
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As a consequence, helical strands based on (py-pym)13 as well as
on hydrazone-pyrimidine (hyz-pym)14 units are uncoiled/
unfolded on metal ion binding and thus generate linear
polynuclear complexes. Conversely, on metal ion complexation,
the linear conformation of (py-hyz)-based ligand strands is
converted into a helical one wrapped around the cation(s).7e,f,12
Such interconversions can be operated reversibly by introducing
a competing ligand and have been integrated into pH-triggered
motional devices that function through reversible binding and
release of metal ions fueled by sequential acid−base neutraliza-
tions.12,13,14a,15 The motions generated may be of very large
amplitudes, for instance, of a factor of 6 (from a helix of about
11 Å height to a strand of ca. 60 Å length).14a The motional
function accomplished by the helical ligands on binding of metal
ions is an extension into a linear shape, while, conversely, the
linear ligand strands under the same conditions perform a
contraction on folding into a helical shape. In addition, in the
latter case, the folding around the metal ions generates an entity
that may be considered as a substrate-induced channel-like
architecture, potentially able to display a (highly selective)
transport function.16 These shape changes are represented
schematically in Figure 3.
Figure 1. Formulas of ligands 1−3. Bent/helical part, in magenta.
Figure 3. Shape changes induced by coordination of metal ions to (a)
bent pym-hyz-pym-hyz-pym sequence producing a linear conforma-
tion of the ligand, and (b) linear py-hyz-py-hyz-py sequence producing
a bent/circular/helical conformation of the ligand. The combination of
these shape modulation effects is at the basis of the coupled molecular
motions reported herein.
Several types of direct connection of the linear and helical
domains can be envisaged for the construction of bimodal
strands, as illustrated in Figure 4. Connections through spacers
Figure 2. Conformational preferences of several heterocycle−heterocycle
and combined hydrazone−heterocycle motifs and sequences: (a) 2,2′-
bipyridine; (b) pyridine-2-hydrazone derived from pyridine-2-aldehyde; (c)
pyridine-2-hydrazone derived from pyridine-2-hydrazine; (d) oligomeric
pyridine-hydrazone sequence; (e) pyrimidine-4-hydrazone derived from
pyrimidine-4-aldehyde; (f) pyrimidine-4-hydrazone derived from pyrimi-
dine-4-hydrazine; and (g) helical oligomeric pyrimidine-hydrazone sequence.
Figure 4. Types of possible connections between a helical domain and
a linear domain in a molecular strand.
structural features of these hyz-pym and hyz-py units and of
derived sequences.
Complexation with appropriate metal ions occurs with con-
version of the transoid form of the free ligand to the cisoid one.
can also be considered. In particular, polymeric entities could,
for instance, involve alternating helical and linear domains
and undergo very large changes in length on metal ion
complexation.
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Each type of linear-helical connection shown in Figure 4 re-
presents a category of motional devices combining simultaneously
two different metal-ion induced shape changes that can be
performed independently by the linear and helical strands alone.
Thus, direct connection of the two domains leads to a new type of
device presenting correlated motions with novel nanomechanical
features. These motions, as described here, involve morphological
changes triggered by a metallosupramolecular interaction without
change in the constitution of the entity undergoing motion, that is,
motional dynamics without constitutional dynamics.6 Motional
processes may also be merged with covalent constitutional
changes.17,18 In the present case, the incorporation of hydrazone
functionalities in the molecular strands 1−3 allows in principle also
for performing such covalent constitutional modification.
Figure 5. Synthesis of ligand strands 1a,b.
RESULTS AND DISCUSSION
■
Synthesis and Structure of the Ligand Strands. For the
synthesis of the present bidomain ligands, we took advantage
of the isomorphic replacement of a 2,6-pyridine unit by the
hydrazone group10,11 (as indicated above), which gives much
easier synthetic access to the desired strands and opens new
perspectives for more sophisticated classes of ligands.
Concerning the synthetic strategy, it is possible to design and
synthesize the mixed strands that contain both a linear and a
circular (helical) domain through a step-by-step sequential
process, by successively connecting the required heterocycles
through hydrazone units. It is also possible to separately
synthesize the linear and helical domains and then to connect
them in the final step in a convergent process. On the basis of our
previous experience, we decided to follow the last strategy
(Figures 5, 6, and 7). The precursors 12−14 were synthesized
as previously reported.11 The linear domains were derived from
the condensation of pyridine-2,6-dicarboxaldehyde with a
pyridine-2-hydrazine and pyrazine-2,6-dihydrazine. The synthetic
procedures are described in the Supporting Information.
Thus, mixed helical-linear strands containing three, five, and
seven hydrazone groups have been synthesized. NOESY and
Figure 6. Synthesis of ligand strands 2a,b.
Figure 7. Synthesis of ligand strands 3a,b.
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ROESY 2D NMR techniques have been used to ascertain the
conformation of ligands in solution. For example, for ligand 1a,
the proton−proton correlations due to NOE effects, especially
for the pairs of protons (4,8), (8,11), (4,11), and (13,16), as
well as cross peaks between CH3 and CH protons of hydrazone
groups confirm the hybrid, linear and bent/circular, con-
formation of 1a (Figure 8). Similar data hold for the strands of
after unsuccessful attempts with unsubstituted bishydrazone-
strands, single crystals of compound 4, bearing a phenyl group
on each of its two terminal pyridines (Figure 9). The linear
Figure 9. X-ray structure of the linear ligand 4 consisting of a sequence
pyridine-hydrazone-pyridine-hydrazone-pyridine.
shape of the sequence py-hyz-py-hyz-py is clearly apparent. The
distances between the Nsp2 atoms of the hyz groups in transoid
conformation are of 7 Å.
Increasing the number of heterocycles leads to a decrease in
the solubility of the strands. To improve the solubility, as
required for performing studies of reversible molecular motions
in homogeneous systems, a CH2O-alkyl group has been
attached to one of the terminal pyridine rings of the linear
domains in 2b and 3b.
Structural Changes on Complexation of Metal
Cations. On coordination of metal ions, the hyz-py and
hyz-pym groups must undergo a change from the transoid to
the cisoid conformation, to generate tridentate coordination
subunits (Figure 3). As a consequence, cation binding to the
ligand strands produces the uncoiling of the circular or bent
domain13,14 and the coiling of the linear one.12 The cor-
responding structural changes are represented schematically for
strands 2 and 3 in Figure 10. While the connecting fragment
that is common to both domains (cd or fh) will not move, the
other sections of the ligands undergo a large structural change
on binding and removal of metal ions. Thus, a part (ad or hg)
of the folded domain (adc or fhg) of the free ligand becomes
part of the linear domain of the complex, while a part (bc or ef)
of the linear domain (bcd or efh) of the free ligand becomes
part of the folded domain of the complex.
The shape changes undergone by the ligand strands 1, 2,
and 3 on binding and removal of metal cations are shown in
Figure 11 in the molecular structure representation. Thus, treat-
ment of the ligands 1, 2, and 3 with 2, 3, and 5 equiv of
Pb(OTf)2, respectively (usually, an excess of 10−30%), leads to
the complexes 1−Pb2, 2−Pb3, and 3−Pb5. The subunits py-hyz-
py, py-hyz-pym, and pym-hyz-pym generate cation binding sites
that can be seen as terpyridine analogues. Each unit binds one
Pb(II) cation in the linear domain, whereas in the helical part
the ligand wraps around the cation in a pseudomacrocycle
fashion (see also Figure 3).
1
Figure 8. 400 MHz proton H NMR NOESY spectrum of ligand 1a
(solvent CDCl3). Key cross peaks are indicated on the formula as
circular arcs.
types 2 and 3 (see the Supporting Information). The cross
peaks of lower intensity (4,5) and (10,11) may suggest a small
torsion angle between the HCN group and the pyridine ring
connected to it. In the X-ray structures of previously reported
hydrazone-pyrimidine-based helical ligands,11 distances similar
to those between protons 4 and 5 are of about 3.6 Å, and
distances similar to those between protons 4 and 8 and protons
8 and 11 generally range between 2.9 and 3.3 Å.
In the solid state, the helicity of the domains based on
sequences of hyz-pym subunits has been confirmed by X-ray
crystallography in previous studies.10,11 To demonstrate the
linearity of ligands based on hyz-py sequences, we obtained,
The conformational changes occurring can be observed by
1
NOESY or ROESY H NMR. Cross peaks corresponding to
NOE (nuclear Overhauser effect) or ROE (NOE in the rotating
frame) between the protons of the −NCH3− groups and the
protons located at β position on the heterocycle (Figure 12a)
or between the proton of −NCH− groups and the protons
located in β on the corresponding heterocycle (Figure 12b) are
characteristic of the complexes. Together with the NOE or ROE
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Figure 10. Schematic representations of the structural changes undergone by the bidomain ligands 2 and 3 on formation of the corresponding
complexes with three and five cations, respectively.
Figure 11. Reaction of ligands 1a (top), 2 (middle), and 3 (bottom) with 2, 3, and 5 equiv of Pb(OTf)2, as necessary for the binding of the required
number of Pb2+ ions to the coordination subunits in the strands.
confirmed, in the solid state, the structures and shapes predicted
above on the basis of conformational principles and deduced
from the analysis of the NMR data. The Pb−Pb distances are of
6.95 Å in complex 1a−Pb2, of 7.11 Å in 1b−Pb2, and of 6.61
and 7.36 Å in complex 2a−Pb3, comparable to the Pb−Pb
distances observed in the stick- or rack-like Pb(II) complexes
reported by us earlier.13,14
Figure 12. Key NOE or ROE interactions in the complexes formed by
ligands 1−3 (see also Figure 11).
In 1a−Pb2, the unit between the pyrimidine and the terminal
pyridine is a hydrazone, while in 1b−Pb2 it is a pyridine, and in
between methyl protons and the CH proton in the hydrazone
both complexes 1a−Pb2 and 1b−Pb2 the overall shape of the
unit −NCH3−NCH− that are also present in the ligands, the
heterocyclic strands is similar (Figure 14a,d). This is in agreement
NOE or ROE data confirm the expected conformation of the
with the isomorphic equivalence between a 2,6-disubstituted
pyridine and a hydrazone. In 1b−Pb2, a terminal pyridine
overlaps partially with the phenyl ring at position 2 of pyrimidine,
and so the bent part of the coordinated ligand 1b is of helical
nature (Figure 14d−g). The angle between the planes of phenyl
coordinated ligands in the Pb(II) complexes. For example, these
kinds of NOE (Figure 13) or ROE have been observed in
the NOESY or ROESY spectra of the complexes.
X-ray crystallographic studies on single crystals of complexes
1a−Pb2, 1b−Pb2 (Figure 14), and 2a−Pb3 (Figure 15)
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Figure 14. X-ray crystallographic solid-state structure of the complexes
1a−Pb2 (views perpendicular (a) and parallel (b) to the main plane of
the ligand; polymeric structure in the solid state (c)) and 1b−Pb2
(views perpendicular (d) and parallel (e) to the main plane of the
ligand; partial overlap between phenyl and pyridine rings (f); distance
between the proton at position 6 of pyridine and the centroid of the
phenyl ring (g)). Anions and solvent were omitted for clarity.
1
Figure 13. 400 MHz proton H NMR NOESY spectrum of complex
2a−Pb3 in CD3CN. Key cross peaks are indicated on the formula as
circular arcs.
ring and pyridine is of 33.8°, the centroid-to-centroid distance
between the two rings is of 4.5 Å, and the distance between the
proton at position 6 of the pyridine and the centroid of the
phenyl is of 3 Å (Figure 14g).
In the solid state, 1a−Pb2 has a polymeric structure where the
complexes are connected through triflate anions (Figure 14c).
The coordination spheres (< 3 Å) of Pb2+ ions within the
crystals of complexes are of type N3O3 (three Nsp2 atoms and
three triflates) and N5O3 (five Nsp2 atoms and three triflates) for
1a−Pb2, of type N3O3 (three Nsp2 atoms and three triflates) and
N5O2 (five Nsp2 atoms and two triflates) for 1b−Pb2, and of type
N8O (seven Nsp2 atoms, one molecule of acetonitrile, and one
triflate), N4O3 (three Nsp2 atoms, one molecule of acetonitrile,
two triflates, and one molecule of water), and N3O3 (three Nsp2
atoms and three triflates) for 2a−Pb3.
The helical domain of complex 2a−Pb3 has a helical pitch of
about 3.5−4 Å, and its linear domain has a length of about
25 Å. The total length of the ligand 2a is of about 40 Å, of which
63% belongs to the folded helix (circular part and common part),
the remaining 37% being the unfolded section. The dimensions
of the complex 3−Pb5 can be estimated (Table 1) on the basis
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1
Figure 17. Partial aromatic region of the 300 MHz H NMR spectra
corresponding to a pH-modulated sequence of coupled motions of li-
gand 3b, according to Figure 11 (starting solvent CDCl3/CD3CN 1/1
at 45°). See also the Supporting Information.
Figure 15. X-ray crystallographic solid-state structure of the complex
2a−Pb3: (top) ball-and-stick, (bottom) space-filling representations;
anions and solvent molecules were omitted for clarity.
with respect to the amount of Pb(II) leads to the back
conversion of each domain of the ligand to its initial shape in
the free ligand.
of the structural data obtained for 1a−Pb2 and 2a−Pb3. The
total length of the strand should be of about 54 Å. In the
complex, 46% (about 25 Å) belongs to the folded domain, the
other 54% (29 Å) being unfolded and containing four metal
ions.
LPbn + ntren ⇆ L + n(Pb−tren)
(3) As the primary amine sites of tren are more basic than
the heterocyclic nitrogen sites of the ligand strands, addition
of trifluoromethanesulfonic (or triflic) acid CF3SO3H (3 equiv
per equivalent of tren) to the solution containing the free
ligand and the Pb−tren complex leads to protonation of tren
(with formation of trenH33+) and release of Pb(II) ions that
revert to binding again with the unprotonated bidomain ligand.
Coupled Nanomechanical Motions Generated by
Cation-Induced Structural Interconversion. The introduc-
tion of a competing ligand allows for inducing the reversible
shape changes depicted in Figure 11 for ligands 1−3 on binding
and removal of metal ions, as shown earlier for single domain
ligands performing just one type of motion.12,13,14a,15 In this way,
correlated, domain-specific, extension/contraction nanomechanical
motions can be generated with the bidomain ligands 1−3.
(1) The reaction of the free ligand strands L with the
appropriate number of equivalents (or an excess) of Pb(II)
produces the bidomain complex L−Pbn (charges omitted for
simplicity) as the first motional step of the functioning of the
device.
3+
L + n(Pb−tren) + 3nH+ ⇆ LPbn + ntrenH3
(4) Subsequent addition to this mixture of a base, such as
Et3N, in suitable amount (e.g., 3 equiv (or an excess) of base
per equivalent of tren) causes deprotonation of trenH33+ to give
tren that again can pick up the Pb(II) ions from the bidomain
complex, thus regenerating the free ligand.
L + nPb(II)⇆LPbn
LPbn + ntrenH33+ + 3nEt3N ⇆ L + n(Pb−tren) + 3nEt3NH+
(2) The addition to this equilibrium of a competing ligand
that is a stronger binder of Pb(II) than the heterocyclic sites of
the strand, such as tris(aminoethyl)amine (tren), removes the
bound cations from the strand and regenerates its initial shape
in the free state. Thus, addition of 1 equiv (or more) of tren
(5) The sequence of steps can be repeated, so that the
system produces correlated nanomechanical motions triggered
by pH changes and fueled by the acid−base neutralization
energy. The process is represented in Figure 16 for ligand 2.
Figure 16. (a) Schematic representation of the pH-triggered correlated nanomechanical motions produced by helical/linear bidomain ligands, such
as 2. The process is fueled by acid−base neutralization. The red spherical objects are metal cations, here Pb(II). (b) The 1H NMR monitorization of
the process in the case of ligand 2b (a part of the aromatic region is shown; starting solvent CDCl3/CD3CN 2/3; 400 MHz).
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a
Table 1. Mechanical Parameters of the Motional Processes
distance
between the
distance
distance
distance
between the
ends of linear
domain after
between the
between the
ends of linear
ends of helical ends of helical
domain before
domain before
total length of the strand (x+y+z) complexation
coiling
domain after
complexation amplitude
uncoiling
ligand
x
y
z
complexation amplitude complexation
1
2
3
10
10
10
8
8
26
40
54
18
25
25
9
4
4
9
21
21
9
4
9
18
25
39
9
21
30
15
15
15
29
a
All lengths (average, rounded to whole values) are in angstroms. See Figure 10. For ligand 1, the bent instead of the helical domain is considered.
The linear domain of the free ligand becomes, after complexation, the helical domain of the complex; the helical domain of the free ligand becomes,
after complexation, the linear domain of the complex.
It involves the uncoiling of a circular/helical domain coupled
with the coiling of a linear domain. The motions can be
followed by NMR as shown in Figure 17 for ligand 3b.
In this coupled uncoiling/coiling process, two amplitudes are
to be considered: the folding amplitude and the unfolding
amplitude. They can be calculated as the difference between
the ends of each domain in its two states, coiled and uncoiled
(Table 1). For ligands 2 and 3, all amplitudes are of at least
20 Å (Table 1).
The energetic aspects of the coiling/uncoiling process of the
type of those occurring in strands 1−3 have been analyzed
earlier for single domain helical ligands.14a A similar analysis can
be attempted for the present bidomain strands, although the
situation is more complicated due to the simultaneous operation
of coiling and uncoiling in the same strand. In our previous work,
it was estimated that the energy corresponding to the change
from the uncoordinated conformation of a pyrimidine-hydrazone-
pyrimidine subunit to its form in the coordinated state present
in the Pb2+ complexes was, without taking into account the
coordination energy, of about 60 kJ·mol−1. Thus, for ligands of
type 1, 2, and 3, containing, respectively, 3, 5, and 7 such
hydrazone subunits per molecule, this energy can be estimated to
amount to about 180, 300, and 420 kJ mol−1, respectively. The
energy of the process that corresponds to the above conforma-
tional change together with the coordination of a Pb2+ ion was
estimated at −47 kJ mol−1 per pyrimidine-hydrazone-pyrimidine
subunit. For ligands 1, 2, and 3, this estimation then leads to
−141, −235, and −329 kJ mol−1, respectively.
action of muscles in the arm, where both motions also occur
simultaneously.
The complementary contraction/extension shape changes
induced by the interaction with metal ions result in nano-
mechanical motions of particularly large amplitude.
These motions can be generated reversibly by coupling with
a competing ligand, which allows for cation removal and
release, modulated through alternate addition of acid and base
and fueled by acid−base neutralization. An integrated version
of the present systems might be considered by directly garafting
the competing ligand group to the molecular strand.
One may envisage that the double motional behavior of
molecules such as those described here could be implemented
to perform displacements on a surface, whereby the ends of
the bidomain strand would be alternately connected and dis-
connected from the support, for instance, by using orthogonal
functional groups such as imine and disulfide.
Multiple structural and functional features are prominent in
biomolecules, in multidomain proteins.19 One may also note the
analogy between the motions described herein and the reptation
modes of DNA molecules, as observed by fluorescence
microscopy20 (Figure 18). The structures of the two molecules
CONCLUSION
■
We have reported herein the design and behavior of a mol-
ecular system representing a nanomechanical motional device
based on molecular ligand strands capable of undergoing
coupled changes in shape on binding and release of metal
cations. It displays the following remarkable features.
Figure 18. Reptation of DNA molecules, as observed by fluorescence
microscopy. Reprinted with permission from ref 20. Copyright 1994
AAAS.
The first is a bidomain structure: the strands are composed
of two directly connected sections, a linear unfolded one and a
folded helical one, whose shapes are strongly enforced by the
conformational preferences of the heterocyclic/heteroatomic
subunits of which they are constituted.
Next is a a bidomain functionality: the strands act as ligands
for metal cations. On coordination of metal cations, the
components of the strands undergo a conformational change,
which induces coupled shape changes in the two sections of
the strands. The linear part folds into a bent/helical shape,
wrapping around the bound metal ion(s), thus generating also
a self-induced channel-like domain. Conversely and simulta-
neously, the helical domain uncoils and produces a stick-like
complexed domain. The process results in a unique coupled,
contraction/extension motion, reminiscent of the correlated
as well as the causes of their motions are different, but the
natures of the motions (uncoiling of a coiled part and coiling of
a linear part) are formally similar in both cases.
In the general framework of the design of complex chemical
systems, molecules presenting two or more structural and/or
functional domains allow for the development of processes and
networks of increased complexity, presenting features such as
coupling, control, and feedback.21
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, NMR spectra, and CIF files. This
material is available free of charge via the Internet at http://
pubs.acs.org. The CIF files for this Article have been deposited
at the Cambridge Crystallographic Data Centre, and they
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(c) Kolomiets, E.; Berl, V.; Odriozola, I.; Stadler, A.-M.; Kyritsakas, N.;
Lehn, J.-M. Chem. Commun. 2003, 2868−2869. (d) For folding by
interaction with metal ions, see, for example: Prince, R. B.; Okada, T.;
Moore, J. S. Angew. Chem., Int. Ed. 1999, 38, 233−236. (e) For folding
by interaction with anions, see, for example: Xu, Y.-X.; Wang, G.-T.;
Zhao, X.; Jiang, X.-K.; Li, Z.-T. J. Org. Chem. 2009, 74, 7267−7273.
(f) For reviews on folding, see, for example: Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev. 2001, 101, 3893−
4011. (g) Goodman, C. M.; Choi, S.; Shandler, S.; DeGrado, W. F.
Nat. Chem. Biol. 2007, 3, 252−262. (h) Juwarker, H.; Suk, J.-M.; Jeong,
K.-S. Chem. Soc. Rev. 2009, 38, 3316−3325. (i) Guichard, G.; Huc, I.
Chem. Commun. 2011, 47, 5933−5941.
have been assigned the following deposition numbers: 773968
(1a-Pb2), 957474 (1b-Pb2), 773969 (2a-Pb3), and 957475 (4).
AUTHOR INFORMATION
Corresponding Authors
lehn@unistra.fr; mstadler@unistra.fr
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to dedicate this work to Jack Roberts on the occasion
of his 95th birthday, in recognition of his outstanding
contributions to the world of chemistry over so many years.
(6) (a) Lehn, J.-M. Angew. Chem., Int. Ed. 2013, 52, 2836−2850.
(b) Lehn, J.-M. Top. Curr. Chem. 2012, 278, 1−32.
(7) (a) Hanan, G. S.; Lehn, J.-M.; Kyritsakas, N.; Fischer, J. J. Chem.
Soc., Chem. Commun. 1995, 765−766. (b) Bassani, D. M.; Lehn, J.-M.;
Baum, G.; Fenske, D. Angew. Chem., Int. Ed. Engl. 1997, 36, 1845−
1847. (c) Ohkita, M.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.Eur.
J. 1999, 5, 3471−3481. (d) Ohkita, M.; Lehn, J.-M.; Baum, G.; Fenske,
D. Chem.Eur. J. 1999, 5, 3471−3481. (e) For recent reviews on the
general theme of helical metal complexes, see: Metallofoldamers;
Maayan, G., Albrecht, M., Eds.; Wiley: Chichester, 2013. (f) Maayan,
G. Eur. J. Org. Chem. 2009, 5699−5710.
We thank the Universite
7006) for financial support. We thank Dr. Lydia Brelot,
Corinne Bailly, and Dr. Andre DeCian for X-ray crystallo-
́
de Strasbourg and the CNRS (UMR
́
graphic studies, Dr. Bruno Vincent, Dr. Lionel Allouche, Jean-
Daniel Sauer, Dr. Jean-Louis Schmitt, and Maurice Coppe for
assistance with NMR experiments, and Mel
́
anie Lebreton and
Dr. Helene Nierengarten for mass spectrometry analyses.
́
̀
(8) Calculations yield a preference of 2,2′-bipyridine for the transoid
with respect to cisoid form by about 25−30 kJ/mol: (a) Howard, S. T.
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