Dynamic stereoisomerization in inherently chiral bimetallic [2]catenanes

Article abstract of DOI:10.1039/c4cc07392d

Stereoisomerization and the unprecedented phenomenon of metal translocation in the absence of redox processes were probed in two inherently chiral bimetallic [2]catenanes by using a combination of variableerature ¹H NMR and CD spectroscopies, X-ray crystallography, and DFT calculations.

Full text of DOI:10.1039/c4cc07392d

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Dynamic Stereoisomerization in Inherently Chiral
Bimetallic [2]Catenanes
Cite this: DOI: 10.1039/x0xx00000x
Thirumurugan Prakasam,a Matteo Lusi,a Elisa Nauha,a JohnꢀCarl Olsen,b Mohamadou
Sy,c Carlos PlatasꢀIglesias,d Loïc J. Charbonnière,c* Ali Trabolsia*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
assembled [2]catenane. The other system, catenane 5, is chiral but,
by definition, not inherently because its component
diaminobipyridyl ligand is itself chiral, having two homochiral
stereocenters with methyl substituents. We used DFT calculations to
model the configurations and dynamics of both 4 and 5 and
measured their rates of stereoisomerization in solution by variable
temperature 1H NMR spectroscopy. In both 4 and 5, dynamic
intramolecular translocation11 of the zinc cations and helical
inversion accompany stereoisomerization.
Stereoisomerization and the unprecedented phenomenon of
metal translocation in the absence of redox processes were
probed in two inherently chiral bimetallic [2]catenanes by
using a combination of variable-temperature 1H NMR and
CD spectroscopies, X-ray crystallography, and DFT
calculations.
Many fundamental biochemical processes such as protein folding1
and allosterism,2 the action of motor proteins,3 the recognition and
transcription of DNA,4 and the production of ATP in mitocondria5
involve largeꢀamplitude intramolecular motions of biomolecules.6
Recently, we reported12 the synthesis of a diaminobipyridine
Over the past twenty years, chemists have synthesized artificial (DAB) ligand, 2, that, in a 1:1:1 combination with diformylpyridine
systems that mimic these types of structural changes.7 For example (DFP) and zinc(II) acetate, simultaneously produces three
metal helicates7d undergo an inversion analogous to the B to Z topologically nonꢀtrivial structures: a [2]catenane (22%), a trefoil
transformation of DNA.8 In these artificial systems, interesting new knot (56%) and a Solomon link (trace amounts). To build on these
structural properties and dynamic effects often present themselves.9 results, we were interested to determine the effects that simple ligand
Here, we describe the serendipitous discovery of helical inversion in modifications would have on product distribution.
inherently chiral bimetallic [2]catenanes.
The terms “cyclochiral,”10e “topologically chiral”10bꢀd and “inherently
chiral” refer to the same type of catenane, in which the three
dimensional arrangement of achiral subꢀcomponents results in a
structure that has neither a plane of symmetry nor an inversion
center.10 Such a catenane does have two possible configurations
which correspond to the two ways its oriented macrocycles can be
mechanically interlocked (Figure 1a).
1
H2N
NH2
(R)
(R)
*
*
2
3
O
O
N
N
Zn(OAC)2 / iPrOH / 70 °C / 24 h
Figure: 1 (a) General schematic representation of inherently chiral
[2]catenanes (b) oriented macrocycle present in [2]catenane 4, where
the direction of rotation is determined by the three highest priority
atoms in the ligand.
4TFA
4TFA
4
5
Scheme 1. Chemical structures (top) of the building blocks, 1, 2 and
3, for the synthesis of bimetallic catenanes 4 and 5 (bottom). Xꢀray
crystallography confirmed the mechanically interlocked topologies
of the catenanes.
Molecular modeling indicated the presence of intramolecular πꢀπ
interactions within the three links, and Xꢀray crystallographic
In one of our systems, catenane 4 (Scheme 1), all of the component
ligands are achiral. However, tridentate coordination of zinc cations
to
a
tetradentate phenantroline diimine ligand introduces
directionality into the macrocycles (Figure 1b), and, therefore,
inherent chirality in the
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analysis confirmed their presence in the [2]catenane.12 Knowing that agreement with the patterns predicted for the corresponding formulas
product distributions of thermodynamically controlled selfꢀassembly (See ESI†).
processes are significantly influenced by weak forces of this sort, we
Single crystals of 4 were grown at room temperature by slow
substituted DFP with 2,9ꢀdiformylꢀ1,10ꢀphenanthroline (1, Scheme diffusion of nꢀbutylether vapour into trifluoroethanolic solutions. Xꢀ
1). Molecular modelling studies (PM6 calculations, SI) indicated that ray diffraction measurements at 100 K revealed that 4 displayed a
catenated solid state structure and crystallized in the monoclinic
P21/n space group (Figure 2). The catenane is composed of two
distinct buckles resulting from the zinc(II)ꢀtemplated condensation
of a diamine molecule and diformylphenantroline (Figure 2 and
ESI†). The catenane contains two zinc(II) cations, each of them
coordinated to five nitrogen atoms – two from the bipyridine of one
ligand strand, two from the phenantroline of the other strand, and
one from an imine group. The remaining imine of each buckle is
rotated away from the metal ion and has the lone pair of its nitrogen
atom in an exo conformation with respect to the macrocycle. An
oxygen atom of a trifluoroacetate anion completes the distorted
octahedral geometry of each zinc cation. [4•2TFA]2+ presents a
nearly undistorted C2 symmetry, with the C2 axis being
perpendicular to the Zn–Zn axis.
1 would not easily accommodate stabilizing πꢀπ interactions due to
strain, and that trefoil knots and Solomon links containing 1 would
likely not form. These predictions were confirmed by subsequent
experimental results such as 1H NMR and mass spectroscopy,
specifically, the exclusive formation of [2]catenanes 4 and 5, which
were easily synthesized from DAB ligand 2 and its chiral analogue
3, respectively.
Synthesis and full characterization of the diaminobipyridine
ligands 2 and 3 are detailed in the supplementary information
section. Mixing stoichiometric amounts of 1, zinc acetate, and the
hydrotrifluoroacetate salt of 2 produced [2]catenane 4, which was
obtained as a yellow solid in 76% yield (See ESI† for experimental
details). Catenane 5 was isolated as a yellow solid in 73% yield by
the same procedure.
In the [2]catenane obtained with DFP,12 two Nꢀbenzylimine
substituents of a macrocylic ligand strand sandwich the bipyridyl
group of the other strand to form “double” πꢀstacking interactions. In
contrast, in 4 each bipyridyl group is in close contact with only one
Nꢀbenzylimine unit (the one coordinated to zinc). The stacking
distance between the plane of the bipyridine and the centroid of the
phenyl ring of the benzyl imine is 3.3 Å.
Interestingly, 4 crystallizes as a racemic mixture of only two of the
ꢀꢁ ꢀꢁ ꢀꢁ
six possible stereoisomeric forms. These forms correspond to ∆ꢀ(δ,δ)
ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ ꢀꢁ
and Λꢀ(λ,λ) configurations (Figure 2), but ∆ꢀ(λ,λ), Λꢀ(δ,δ), ∆ꢀ(δ,λ),
ꢀꢁ ꢀꢁ ꢀꢁ
and Λꢀ(λ,δ) configurations are in principle also feasible (see SI for
detailed DFT calculations for possible stereoisomers, their relative
energies and interconversion pathways). These six stereoisomers are
actually three diastereoisomeric pairs of enantiomers. In contrast
ꢀꢁ ꢀꢁ ꢀꢁ
with 4, 5 exists in the solid state as single diastereoisomer, ∆ꢀ(δ,δ)
(Figure 2). It packs in a polar space group, C2, in a head to head
motif forming capsuleꢀlike supramolecular adducts and are held
together by counter ions and solvent molecules. An indication of the
degree of dissymmetry in the structures of 4 and 5 is given by the
dihedral angle defined by the phenantroline ligands with respect to
the Zn–Zn axis (N1–Zn1–Zn2–N5). These angles are 38.3° and
50.3°, respectively, indicating that the 5 is significantly more
twisted.
Figure 2. Crystal structures of catenanes 4 (a), and 5 (b). Catenane 4
crystallizes as a pair of enantiomers while 5 is isolated as a single
ꢀꢁ
ꢀꢁ
stereoisomer. ∆ and Λ helix configurations can be assigned to each
ꢀꢁ
ꢀꢁ
catenane, and clockwise (δ) and anticlockwise (λꢂ configurations
can be assigned to each metal center13 (c). Pink boxed numbers
indicate the priority of ligand substituents. Trifluoro acetate has
priority one. Phenanthroline nitrogens have priorities two (next to
coordinated imine) and three(next to uncoordinated imine)
respectively. In each catenane, the two metal centers have the same
configuration.
High resolution electrospray ionizationꢀmass spectrometry of 4
dissolved in MeOH revealed three major peaks with maxima at m/z
805.162, 499.113, and 346.089, which correspond to [4•2TFA]2+,
[4•TFA]3+, and [4]4+, respectively. Two major peaks were observed
in the case of 5 with maxima at m/z 833.195 and 517.844, and
correspond to [5•2TFA]2+ and [5•TFA]3+, respectively. The isotopic
distribution patterns of the assigned sets of peaks were in full
Figure 3. 1H NMR spectra of 4 (600 MHz, CD3CN, 298 K) aligned
with those of the starting materials 1 and 2 (with 2 in the form of a
tetrahydrotrifluoroacetate salt).
The identification of several possible stereoisomers of 4 by
modelling and the recognition of a possible means for their
1
interconversion motivated us to study its dynamic behaviour by H
1
NMR spectroscopy. The H NMR spectra of 4 was first recorded at
1
room temperature in CD3CN. The H NMR spectra of the starting
materials 1, 2 and 3 were recorded for comparison. At room
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temperature, the spectrum of 4 (Figure 3) is relatively simple, having the energy of the transition state with the highest energy, amounts to
only 10.9 kcal mol−1, which is in reasonable agreement with the
eleven broad peaks. Their large widths reflect that hydrogen atoms
1
are undergoing relatively fast exchange processes. Full attribution of
the signals in the spectrum was accomplished with the aid of twoꢀ
dimensional 1Hꢀ1H correlation spectroscopies including COSY,
NOESY and ROESY (See ESI†). At lower temperatures (Figure 4),
the signals broaden and separate as exchange rates slow down. The
spectrum obtained at 233 K displays 23 peaks corresponding to 30
protons, with some regions being complicated by signal overlap. The
He protons (see Figure 4 for attribution) give rise to a single signal at
313 K (4.54 ppm), but four distinct signals at 233 K (3.86, 4.39, 4.80
and 5.00 ppm). The signal of the Hh protons, on the other hand, splits
into only two sets of signals, presumably because of the greater
similarity of the chemical environments that the Hh protons can
access at low temperature. Clear exchange peaks are observed in the
ROESY spectrum of 4 at 233 K (See SI). Taken together these
results demonstrate that slowing the exchange processes leads,
effectively, to a loss of symmetry and therefore an increased number
of signals.
values obtained from VT H NMR spectroscopy. Interestingly, the
translocation of the second Zn ion proceeds through a transition state
with nearly identical energy, indicating that the dynamic processes
occurring at one end of the cavity do not significantly distort the
coordination environment around the other Zn ion. These results
suggest that Zn translocation represents the rate limiting step for the
interconversion processes experienced by 4.
Figure 5. 1H NMR spectra of 5 (600 MHz, CD3OD, 298 K) aligned
with those of the starting materials 1 and 3. Ligand 3 is used in the
form of a tetrahydrotrifluoroacetate salt.
Significant differences between the roomꢀtemperature 1H NMR
spectra of catenanes
the chiral ligand have a major effect on the exchange process.
The H NMR spectrum of (Figure 5) is much more complex
4 and 5 reveal that the methyl groups of
3
1
5
and shows, for example, two main sets of signals due to the
methyl groups of the ligands together with signals of lower
intensity. These data indicate the presence, in solution, of a
major diastereomer of
one other diastereomer.
5 as well as a lesser amount of at least
Figure 4. Variable temperature (VT) 1H NMR spectra (CD3CN, 500
MHz, 313 → 233 K) of catenane 4. Signals (boxed in blue)
correspond to hydrogens Hf, Hg and Hj which were used for
determining parameters associated with dynamic exchange.
The signals that correspond to protons Hf, Hg and Hj (Figure 4)
coalesce at 253, 263 and 283 K, respectively. Although, these values
are associated with relatively large errors (± 10 K) due to signal
overlap, they can still be used to estimate the rates, kc, of the
exchange processes experienced by these hydrogen atoms . The rates
are 277, 433 and 988 s−1, respectively and correspond to respective
free energies of activation of 11.9, 11.8 and 11.7 kcal·mol−1. The
enthalpy (ꢀ ꢀ
H≠, 12.7 kcal·mol−1) and entropy ( S≠, 3.25 cal·mol−1) of
activation for the overall process of enantiomerization were also
calculated. The predomination of the enthalpic term is consistent
with a dynamic process involving enthalpically expensive ruptures
of Nimine−zinc coordination bonds, but, otherwise, entropically
undemanding intramolecular reorganisation.
Figure 6. VT 1H NMR spectra (CD3OD, 500 MHz, 298 → 323 K)
of catenane 5. High temperatures accelerate the helical inversion and
lead to broadening of the signals of the methyl groups (Hm, boxed in
red).
ꢀꢁ ꢀꢀꢀꢁ
The translocation process leading to ꢃ ↔ ꢄ interconversion at the
As observed in Figure 6, the two methyl signals broaden and
coalesce with increased temperature, which reflects a dynamic
interconversion that can be explained by a process analogous to
that of catenane 4. In 5, the process averages the environments
of the coordinated and uncoordinated imine nitrogens and thus,
the environments of the methyl groups. However, the methyl
Zn(II) centres was further investigated using DFT. According to our
calculations the translocation of a Zn ion is a twoꢀstep mechanism
involving the decoordination of one of the imine nitrogen atoms to
give a fiveꢀcoordinate intermediate and subsequent coordination of
the other imine nitrogen atom of the same ligand strand (Figure
ESI†). The activation free energy for this process, as estimated from
groups in
5
hinder these motions by slowing down the process
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of Nimineꢀinversion necessary for zinc translocation. As a result, possiblility of designing more sophisticated molecular devices.
the isomers are in slow exchange at room temperature, their For example, the winding of longer helical constructs against an
signals reaching coalescence at 316 K. Neglecting the presence applied torque could serve as a way to store variable amounts
of isomers with smaller populations, we estimate the free of energy at the molecular level.15 Synthetic efforts toward
1
extended helical structures are underway in our laboratories.
Notes and references
energy of activation,
ꢀ
G≠, for the process to be 14.5 kcal mol− .
1
This value is significantly higher than that obtained by VT H
NMR spectroscopy and DFT calculations for enantiomerization aCentre for Science and Engineering, New York University Abu Dhabi
(NYUAD), UAE. Eꢀmail: ali.trabolsi@nyu.edu.
of
4 and is consistent with the methyl groups’ slowing of the
bSchool of Sciences, Indiana University Kokomo, Kokomo, IN 46904,
USA.
stereoisomerization of
5
in solution.
cLaboratoire d’Ingénierie Moléculaire Appliquée à l’Analyse, IPHC,
UMR 7178 CNRSꢀUniversité de Strasbourg, ECPM, 25 rue Becquerel,
67087 Strasbourg, France. Eꢀmail: l.charbonn@unistra.fr
dDepartamento de Química Fundamental, Universidade da Coruña,
Campus da ZapateiraꢀRúa da Fraga 10, 15008 A Coruña, Spain.
We thank Dr R. A. Bilbeisi for making the DFT calculation figures.
222
a)
b)
310
20
10
5
Boc-3
Boc-2
5—4TFA
4—4TFA
219
10
0
0
Electronic Supplementary Information (ESI†) available: [For general
methods, further details of synthesis and characterization, and DFT and
PM6 calculations.]. See DOI: 10.1039/c000000x/
ꢀ10
232
261
ꢀ5
240
280
λ (nm)
320
360
240
280
λ (nm)
320
360
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(b) catenanes and (8 M in MeOH at 298 K).
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corresponding racemic mixture of helical [2]catenanes,
2
3
ꢁ
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5
ꢁ
2
, and the
, show
4
no optical activity (Figure 7a and 7b). The CD spectrum of the
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λmax = 219 nm and min = 232 nm, which are in the same range
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as absorptions seen in the spectrum of Boc-3 and most likely
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,
of
5 seen at lower energies (λmin = 261 nm and λmax = 310 nm)
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5
exists at room temperature as a quasiꢀsingle stereoisomer and
not a racemic mixture of enantiomers. The positive Cotton
effect observed at 310 nm arises from π→π* transitions of the
aromatic moieties. A very similar pattern was observed in the
CD spectra of chiral Borromean rings.14 Because the latter
structure lacks phenanthroline moieties, it can be surmised that
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ꢀ
π
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crystallized as a racemic mixture, whereas was isolated as a
single stereoisomer. In solution, only is optically active, its
CD spectrum exhibiting strong signals. Both and exhibit
4
5
4
11. For a definition for the term “translocation” see examples: (a) T.
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5
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helical inversion. Room temperature 1H NMR spectroscopy
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revealed that the helical inversion (i.e. enantiomerization) of
4
is fast on the NMR timescale, whereas, in , methyl
5
substituents drastically decrease the rate of stereoisomerization.
Finally computer modeling provided insight into the possible
identities of intermediates involved in the isomerization and
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metal translocation processes. Although both
4 and 5 are
72–191.
examples of a relatively simple type of nanomachine, namely
the twoꢀstate switch, their helical architectures suggest the
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