[2]Pseudorotaxane and [2]rotaxane molecular shuttles: Self-assembly through second-sphere coordination of thiocyanate ligands

Article abstract of DOI:10.1021/ic701021p

A Pd(Py)₂(SCN)₂ complex is shown to form a [2]pseudorotaxane complex with a macrocyclic tetralactam, which binds in the solid state through an unexpected complexation geometry. The intermolecular complexation is further applied to template the formation of a [2]rotaxane. The interlocked product acts as a degenerate molecular shuttle in solution, which is consistent with the co-conformation observed in the solid state.

Full text of DOI:10.1021/ic701021p

Inorg. Chem. 2007, 46, 8445−8447
[2]Pseudorotaxane and [2]Rotaxane Molecular Shuttles:
Self-Assembly through Second-Sphere Coordination of
Thiocyanate Ligands
Barry A. Blight, Xu Wei, James A. Wisner,* and Michael C. Jennings
Department of Chemistry, The UniVersity of Western Ontario, 1151 Richmond Street,
London, Ontario N6A 5B7, Canada
Received May 25, 2007
A Pd(Py)2(SCN)2 complex is shown to form a [2]pseudorotaxane
complex with a macrocyclic tetralactam, which binds in the solid
state through an unexpected complexation geometry. The inter-
molecular complexation is further applied to template the formation
of a [2]rotaxane. The interlocked product acts as a degenerate
molecular shuttle in solution, which is consistent with the co-
conformation observed in the solid state.
[2]rotaxane, which displays the features of a degenerate
molecular shuttle.
We have previously demonstrated that, through hydrogen
bonding, second-sphere coordination of halide ligands of a
palladium(II) bispyridine complex by the amide hydrogens
of isophthalamide-based tetralactam macrocycle 1 is an
effective template for the self-assembly of [2]pseudorotax-
anes.4 With the success and information gained from this
study, we began exploring various methods to “stopper” the
threadlike molecule while maintaining the integrity of the
template, resulting in the synthesis of a [2]rotaxane in good
yield using pyridine ligands with sterically demanding
termini.5
A further extension of this methodology is the investigation
of different anionic ligands that could replace the halides in
the palladium complex as hydrogen-bond acceptors. Molec-
ular models suggested that thiocyanate ligands might act in
a similar manner if the tetralactam was hydrogen-bonded to
their sulfur atoms, which would, in turn, be ligated to the
palladium metal center.
Macrocycle 16 and axle 28 (Scheme 1) were easily
synthesized according to existing literature preparations. [2]-
Pseudorotaxanes are in equilibrium with their uncomplexed
components because of the absence of covalent or mechanical
attachment. The resultant 1H NMR spectrum of an admixture
of equimolar amounts of 1 and 2 at a concentration of 1 ×
10-3 M in CDCl3 is markedly different from those of the
individual starting materials (Figure 1), implying complex-
ation in solution. Notable differences include the downfield
shifts of protons a (∆δ ) -0.75 ppm), due to hydrogen
bonding, and d (∆δ ) -0.25 ppm), as a result of its
proximity to the deshielding regions of the pyridine rings,
and upfield shifts of f (∆δ ) 0.67 ppm) and g (∆δ ) 0.89
The design and synthesis of molecular machines and
switches is an area of significant current interest because of
the great potential to harness their properties for advanced
materials applications.1 Interlocked molecules such as ro-
taxanes and catenanes constitute a significant subset of these
systems because of the unique mechanically bonded relation-
ship their individual components possess with respect to one
another. Specifically, molecular shuttles often feature the
shuttling of a macrocyclic ring between two or more sites
along the backbone of the axle in a rotaxane.2,3 If the sites
are identical, the shuttle is degenerate and no preference for
one site over the other is observed. Herein, we describe the
serendipitous discovery of a new template for [2]pseudoro-
taxane formation and its application to the synthesis of a
* To whom correspondence should be addressed. E-mail: jwisner@uwo.ca.
(1) (a) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed.
2007, 46, 72. (b) Kinbara, K.; Aida, T. Chem. ReV. 2005, 105, 1377.
(c) Volume on “Molecular Machines”: Top. Curr. Chem. 2005, 262,
1-236. (d) Balzani, V.; Credi, A.; Raymo, F. M.; Stoddart, J. F.
Angew. Chem., Int. Ed. 2000, 39, 3349.
(2) Tian, H.; Wang, Q.-C. Chem. Soc. ReV. 2006, 35, 361.
(3) For recent examples, see: (a) Marlin, D. S.; Gonza´lez Cabrera, D.;
Leigh, D. A.; Slawin, A. M. Z. Angew. Chem., Int. Ed. 2006, 45, 77.
(b) Marlin, D. S.; Gonza´lez Cabrera, D.; Leigh, D. A.; Slawin, A. M.
Z. Angew. Chem., Int. Ed. 2006, 45, 1385. (c) Tokunaga, Y.;
Nakamura, T.; Yoshioka, M.; Shimomura, Y. Tetrahedron Lett. 2006,
47, 5901. (d) Qu, D.-H.; Wang, Q.-C.; Tian, H. Angew. Chem., Int.
Ed. 2005, 44, 5296. (e) Murakami, H.; Kawabuchi, A.; Matsumoto,
R.; Ido, T.; Nakashima, N. J. Am. Chem. Soc. 2005, 127, 15891. (f)
Jeppesen, J. O.; Nygaard, S.; Vignon, S. A.; Stoddart, J. F. Eur. J.
Org. Chem. 2005, 196. (g) Cooke, G.; Garety, J. F.; Mabruk, S.;
Rabani, G.; Rotello, V. M.; Surpateanu, G.; Woisel, P. Tetrahedron
Lett. 2006, 47, 783. (h) Badjic´, J. D.; Ronconi, C. M.; Stoddart, J. F.;
Balzani, V.; Silvi, S.; Credi, A. J. Am. Chem. Soc. 2006, 128, 1489.
(i) Clemente-Leo´n, M.; Credi, A.; Mart´ınez-D´ıaz, M. V.; Mingotaud,
C.; Stoddart, J. F. AdV. Mater. 2006, 18, 1291.
(4) Blight, B. A.; Van Noortwyk, K. A.; Wisner, J. A.; Jennings, M. C.
Angew. Chem., Int. Ed. 2005, 44, 1499.
(5) Blight, B. A.; Wisner, J. A.; Jennings, M. C. Chem. Commun. 2006,
4593.
(6) Fischer, C.; Nieger, M.; Mogck, O.; Bo¨hmer, V.; Ungaro, R.; Vo¨gtle,
F. Eur. J. Org. Chem. 1998, 1, 155.
(7) Burmeister, J. L.; Basolo, F. Inorg. Chem. 1964, 3, 1587.
(8) Sutherland, I. O. Annu. Rep. NMR Spectrosc. 1971, 4, 71.
10.1021/ic701021p CCC: $37.00
Published on Web 09/19/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 21, 2007 8445

COMMUNICATION
Scheme 1. Self-Assembly of Both a [2]Pseudorotaxane and a
[2]Rotaxane Molecular Shuttle
Figure 1. Aromatic region of the 1H NMR spectra (600 MHz, 298 K) of
free macrocycle 1 (top), [2]pseudorotaxane 1‚2 (middle), and free axle 2
(bottom). Illustrated are the observed perturbations in the chemical shift
between free and complexed components (dashed lines).
Figure 2. Stick representation of interpenetrated [2]pseudorotaxane 1‚2
in the solid state. All C-H hydrogen atoms and methyl, cyclohexyl, and
tert-butyl groups have been removed for clarity. NH‚‚‚NCS hydrogen bonds
are indicated by orange dashed lines. Color code: C, gray; H, pale yellow;
N, blue; O, red; S, yellow; Pd, teal.
narity (24-40°) with the attached tert-butylphenyl rings. The
result is a dissymmetric complex in which one pyridine
ligand is encircled by the macrocycle and the other is not.
Fortunately, the solid-state structure remains consistent with
the results from the NMR analysis (assuming fast exchange
during the complexation/decomplexation process; see below)
and may further be viewed as a precursor for its interlocked
[2]rotaxane counterpart. This mechanically bonded analogue
could be generated by introducing “stopper” groups at the
para position of the pyridine rings, which was our next target.
Dumbbell 3 was synthesized by dissolution of K2Pd(SCN)4
in ethanol, followed by dropwise addition of an ethanolic
solution of the previously reported 4-[(3,5-di-tert-butylben-
zyl)oxy]pyridine stopper ligand.5 The synthesis of 4 was then
achieved by harnessing the utility of the labile palladium-
(II)-pyridine coordinate bonds of the non-interlocked axle.
Stoichiometric amounts of 1 and 3 were combined in
chloroform and allowed to stir for 5 days, resulting in self-
assembled [2]rotaxane 4 (76% yield after purification). It is
interesting to note that a pure sample of 4 is kinetically stable
at room temperature and may be chromatographed or left in
a CDCl3 solution for substantial periods of time (1 week)
with no observable decomposition into the component parts.
ppm), which are interpreted as C-H‚‚‚π interactions with
the sidewalls of the macrocycle as a result of our earlier
investigations.4,5 Isothermal titration calorimetry of 1 with
2 in CHCl3 at room temperature revealed an association
constant (Ka) of 5300 ( 100 M-1 (∆G ) 21.0 ( 0.1 kJ
mol-1), which was confirmed by a 1H NMR titration
experiment.
X-ray crystallographic analysis confirmed the desired
interpenetrated geometry of 1‚2 in the solid state but revealed
an unanticipated complexation geometry for the [2]pseu-
dorotaxane (Figure 2). Although the macrocyclic recognition
site is the appropriate size to accommodate the S-Pd-S
subunit, the solid-state structural result displays hydrogen-
bonding interactions between the tetralactam amide hydrogen
atoms and the terminal nitrogen atoms of the thiocyanate
ligands [N-N ) 3.15 (A), 3.08 (B), 3.08 (C), and 3.18 (D)
Å; N-H‚‚‚N ) 172° (A), 171° (B), 168° (C), and 172° (D)],
which are arranged in a manner parallel to the N-Pd-N
axis. This geometry displaces the Pd2+ center from the plane
of the macrocyclic component (the least-squares plane
defined by the four carbonyl carbons) by 4.74 Å, which, in
turn, requires all four amide groups to deviate from copla-
8446 Inorganic Chemistry, Vol. 46, No. 21, 2007

COMMUNICATION
Figure 4. Aromatic region of the 1H NMR spectra (600 MHz, 298 K) of
[2]rotaxane 4 at 220 K (top), 273 K (middle), and 323 K (bottom). Illustrated
are the observed chemical shift perturbations caused by the shuttling motion
of macrocycle 1 along dumbbell 3 (dashed lines). Variable-temperature
ROESY NMR experiments were performed to allow individual proton
assignments. Proton k is baseline-resolved at 220 K, as are f and g at
273 K.
Figure 3. Stick representation of interlocked [2]rotaxane 4 in the solid
state. All methyl, cyclohexyl, and tert-butyl groups from macrocycle 1 and
all C-H hydrogen atoms have been removed for clarity. NH‚‚‚NCS-
hydrogen bonds are indicated by orange dashed lines. Color code: C, gray;
H, pale yellow; N, blue; O, red; S, yellow; Pd, teal.
Initial 1H NMR analysis of 4 at room temperature exhibited
chemical shift values corresponding to the interlocked
geometry with extreme broadening of pyridine protons f, g,
and i, suggesting chemical exchange near their coalescence
points. Chemical shifts of interest (∆δ) in comparison to the
isolated components 1 and 3 include a (-0.99 ppm), d
(-0.26 ppm), f (0.87 ppm), g (1.28 ppm), and i (0.39 ppm),
which are of the same quality as those in 1‚2 but are
exaggerated as a result of the interlocked nature of the
components.4
1 imposes on these perturbed protons (g is particularly
noteworthy). In addition, broadening of d and e (not shown)
is evidence of desymmetrization of the two faces of the
macrocycle. Warming the system to 323 K increased the rate
of the exchange process, resulting in the spectrum of a
completely degenerate [2]rotaxane under fast exchange
conditions. Coalescence temperatures and exchange rates
were determined by variable-temperature 1H NMR for
protons g (288 K, 3300 Hz), f (278 K, 1800 Hz), i (268 K,
1100 Hz), j (253 K, 240 Hz), and l (248 K, 160 Hz) of 4,
yielding an average ∆Gq value of 48.4 kJ mol-1 for shuttling,
as determined by the Eyring equation.8
In conclusion, we have demonstrated that second-sphere
coordination of a palladium(II) thiocyanate complex is an
effective means to template the formation of both a [2]-
pseudorotaxane and [2]rotaxane. The directional nature of
the thiocyanate ligands affords dissymmetric binding geom-
etries in the solid state and subsequently a degenerate [2]-
rotaxane molecular shuttle in solution. We are currently
undertaking a comprehensive analysis of this shuttling
behavior to determine whether the site exchange is a result
of ligand dissociation or simple conformational changes of
the components, which will be reported in due course.
Based on the crystallographic results of 1‚2, it was
predictable that 4 would adopt a similar dissymmetric
geometry. This feature was confirmed upon examination of
the solid-state structure obtained by X-ray diffraction analysis
(Figure 3). In a similar manner, the thiocyanate ligands
engage in hydrogen bonding with the N-H groups of the
macrocycle [N-N ) 3.32 (A), 3.06 (B), 3.06 (C), and 3.11
(D) Å; N-H‚‚‚N ) 178° (A), 172° (B), 161° (C), and 168°
(D)] in 4. The Pd2+ center is displaced from the macrocyclic
cavity by 4.52 Å, forcing the NH groups from coplanarity
(30-37°), with the attached tert-butylphenyl rings directed
toward the hydrogen-bond acceptors. This results in a
structurally dissymmetric [2]rotaxane in the solid state.
However, in solution 4 has the option of two degenerate co-
conformations involving encirclement of either of the two
pyridyl ligands. The potential shuttling between these two
states, if present, in 4 was probed through variable-temper-
Acknowledgment. We are grateful for the financial
support provided by the Natural Sciences and Engineering
Research Council of Canada and the University of Western
Ontario.
1
ature H NMR.
As mentioned above, at room temperature (298 K), the
system is engaged in chemical exchange, evident from the
broadening of the pyridine protons. Upon cooling to 220 K,
two sets of signals for f, g, i, and j appear (protons attached
to the encircled ligand are denoted by a prime). These signals
may be assigned via a low-temperature ROESY NMR
experiment, which exhibits distinct exchange cross-peaks for
each set. Figure 4 illustrates the significant effect macrocycle
Supporting Information Available: All experimental proce-
dures, characterization information for 1‚2, 3, and 4, isothermal
titration calorimetry data, and all crystallographic data and refine-
ment procedures in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC701021P
Inorganic Chemistry, Vol. 46, No. 21, 2007 8447