Synthesis of a [2]rotaxane through first- and second-sphere coordination

Article abstract of DOI:10.1039/b610243c

In an effort to expand the application of a new template from interpenetrated to interlocked molecular species, we report the synthesis of a new [2]rotaxane by means of both first- and second-sphere coordination of a palladium(ii) dichloride subunit. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b610243c

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Synthesis of a [2]rotaxane through first- and second-sphere
coordination{
Barry A. Blight, James A. Wisner* and Michael C. Jennings
Received (in Austin, TX, USA) 22nd July 2006, Accepted 23rd August 2006
First published as an Advance Article on the web 20th September 2006
DOI: 10.1039/b610243c
reaction stirred for another 4 hours. The excess macrocycle was
In an effort to expand the application of a new template from
interpenetrated to interlocked molecular species, we report the
synthesis of a new [2]rotaxane by means of both first- and
easily recovered by selective crystallization from CHCl3 solution at
low temperature, and the final product was obtained by
precipitation from the remaining mother liquor with hexanes to
give putative [2]rotaxane 3 in 89% yield. This product is completely
stable in CHCl3 solution over a period of three months and can be
chromatographed, if necessary, on silica (EtOAc/CHCl3) with no
loss of integrity.
second-sphere coordination of
subunit.
a palladium(II) dichloride
The synthesis of mechanically bonded molecules, such as rotaxanes
and catenanes, has benefited enormously over the past two
decades from the introduction of a template methodology.1 In the
specific case of rotaxanes, metal centres have been used either as
part of the organizational template or as covalent attachment
points within the organic skeletons that generally make up the
majority of the interlocked superstructures.2 Herein, we describe a
rare example of [2]rotaxane formation where the metal centre
fulfils both functions at the same time.
The 1H NMR spectrum of the material in CDCl3 thus obtained
is supportive of the interlocked nature of the complex (Figure 1), in
In a recent publication, we employed second-sphere coordina-
tion as a template in the formation of a series of [2]pseudorotax-
anes, which were composed of a tetralactam ‘‘wheel’’-shaped
molecule interpenetrated by various sterically unencumbered trans-
palladium(II) dihalide coordination complexes.3 The main driving
force for their interaction was second-sphere coordination of the
halide ligands by the amide protons of the macrocycle via
hydrogen bonding. Considering the facility of this template in our
initial investigation, our attention has turned to its application in
the synthesis of interlocked products.
The most straightforward manner in which to transform the
aforementioned system from an interpenetrated arrangement to an
interlocked one is likely to be the generation of palladium
complexes that incorporate ligands with sterically demanding
terminal groups. Such an arrangement maintains the original
template in its entirety in the final structure. To this end, 4-(3,5-di-
tert-butyl-benzyloxy)pyridine (2) was synthesized easily from
readily available starting materials in 45% yield over two steps.
The 3,5-di-tert-butyl-benzyl group thus acts as a ‘‘stopper’’ in the
final product to prevent, in the absence of ligand exchange,
dissociation of the assembled components.
The synthesis of the desired [2]rotaxane is an exceedingly simple
procedure (Scheme 1). Two equivalents of tetralactam macrocycle
14 were dissolved simultaneously with one equivalent of trans-bis-
benzonitrilepalladium(II) dichloride5 at room temperature in
CHCl3. Two equivalents of ligand 2 were then added and the
Department of Chemistry, The University of Western Ontario,
Chemistry Building, 1151 Richmond Street, London, ON, Canada
N6A 5B7. E-mail: jwisner@uwo.ca; Fax: +1 519 661 3022
{ Electronic supplementary information (ESI) available: Synthetic proto-
cols and spectral details for compounds 2, 3 and 4. See DOI: 10.1039/
b610243c
Scheme 1 Synthesis and structure of [2]rotaxane 3, depicting the
interlocked geometry templated by second-sphere coordination of the
metal halides.
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Chem. Commun., 2006, 4593–4595 | 4593

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Fig. 2 Space-filling/stick representation of the solid-state structure of
[2]rotaxane 3, illustrating the interlocked geometry of the components
(light grey sticks, macrocycle; medium grey space-filling, pyridyl ligands;
dark grey space-filling, PdCl2 subunit).
1
Fig. 1 The aromatic region of the H NMR spectra (400 MHz, 298 K)
of free macrocycle 1 (top), [2]rotaxane 3 (middle) and free dumbbell 4
(bottom). Illustrated are the observed perturbations in chemical shift
between free and interlocked components (dashed lines). NOESY
experiments were performed to allow individual 1H NMR assignments.
in the original template design. Unfortunately, there is no evidence
in the solid state of the CH–p interactions, whose effects are
observed in the NMR spectrum, though it is likely the weak nature
of these forces has been overwhelmed by crystal packing effects in
this case.
comparison to the isolated components 1 and pre-formed
‘‘dumbbell’’ trans-bis-(4-(3,5-di-tert-butyl-benzyloxy)pyridine)palla-
dium(II) dichloride (4). Hence, the macrocyclic amide protons
Ha shift downfield to d 8.23 (Dd = 1.10) as a result of hydrogen
bonding to the chloride ligands. The ortho-protons of the
pyridine rings He are involved in CH–p interactions with the
aryl side walls of the macrocycle, shifting them upfield to d
6.90 (Dd = 21.69). These shifts are of the same quality as those
observed upon pseudorotaxane formation, but exaggerated in
this context due to the interlocked nature of the constituents.
Notably, an admixture of 1 and 4 together in CHCl3 does not
produce any change in their spectra, effectively ruling out a
non-interlocked complex geometry for 3. Analysis by CSI-MS
displays a peak at m/z 895.5 ([M + 2 H]2+) that does not appear
in simple solutions of 1 and 4 under the same conditions,
further supporting the presence of 3 in solution.
In summary, we have self-assembled a new [2]rotaxane through
a combination of first- and second-sphere coordination. Based on
the characterization of 3, we have also demonstrated that this
template displays an analogous mode of complexation when
compared to the subsequent [2]pseudorotaxane. From this, we can
conclude that second-sphere coordination is a viable route for the
production of these types of interlocked molecular species. We are
currently investigating the further development of this template for
the formation of catenanes, and molecular devices such as shuttles
or switches.
We thank the Natural Sciences and Engineering Research
Council of Canada for their financial support of this research.
Confirmation of the interlocked geometry was obtained from
the solid state structure by single crystal X-ray diffraction.6 Pale
yellow crystals suitable for analysis were grown by slow diffusion
of di-iso-propyl ether into a concentrated CHCl3 solution of 3. The
[2]rotaxane crystallized in the space group P(-1) to yield the
expected interlocked structure (Figure 2). The structure differs
from the previously reported [2]pseudorotaxanes in that the central
PdCl2 axis is displaced entirely to one face of the macrocyclic
cavity in a nested arrangement. This disposition results in only one
of the pyridyl rings of the two neutral ligands being included in the
cavity itself. In order to accommodate this geometry and still
engage in hydrogen bonding with the chloride ligands, the amide
groups of the isophthalamide subunits in the macrocyclic
component all deviate in the same direction from coplanarity with
their associated aryl rings by 19–32u. The four amides engage in
hydrogen bonds to the chloride ligands of the metal complex
(Figure 3: N–Cl = 3.49 (A), 3.66 (B), 3.45 (C) and 3.36 s (D), and
N–H–Cl = 162 (A), 153 (B), 152 (C) and 155u (D)), as anticipated
Fig. 3 Ball-and-stick representation of the interior of the macrocyclic
cavity of 3 in the solid state. All of the C–H hydrogen atoms have been
…
removed for clarity. NH Cl hydrogen bonds are indicated by dashed
lines.
4594 | Chem. Commun., 2006, 4593–4595
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Eur. J. Org. Chem., 1998, 1, 155.
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6 Crystal structure data for [2]rotaxane 3: C108H134Cl2N6O6Pd, M =
1789.58, triclinic, space group P(-1), a = 17.2816(4), b = 20.3272(8),
c = 20.7318(94) s, a = 107.11(0), b = 100.24(0), c = 113.61(0)u, V =
6003.15(9) s3, Z = 2, rcalc = 1.2351 g cm23, 2hmax = 50.06u, Mo-Ka
radiation (l = 0.71073 s), T = 123 K, Z = 2. Pale yellow crystal of
dimensions 0.48 6 0.20 6 0.08 mm. 39451 total reflections, 20806
unique reflections (Rint = 0.051), R1 = 0.0674, wR2 = 0.1981[I . 2s(I)],
R1 = 0.1049, wR2 = 0.1981, GOF(F2) = 1.027, No/Nv = 20806/1306 with
I . 2s(I). Data obtained from a programmed hemisphere scan routine
on a Nonius Kappa-CCD diffractometer. Lorentz and polarization
correction, followed by a multiscan (HKL SCALEPACK: Z. Otwinowski
and W. Minor, Methods Enzymol., 1997, 276, 307–326) absorption
correction, were applied to the data (m = 0.389 mm21, min./max.
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transmission = 0.8354/0.9714). The structure was solved by
Patterson methods, followed by difference Fourier syntheses, to find
the remaining atoms. Refinement was by full-matrix least-
squares methods using SHELXTL-NT 6.1 (G. M. Sheldrick,
SHELXTL-NT 6.1, Madison, WI, 2000). CCDC 611645. For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b610243c.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 4593–4595 | 4595