Communications
DOI: 10.1002/anie.200802745
Rotaxanes
An Ion-Pair Template for Rotaxane Formation and its Exploitation in
an Orthogonal Interaction Anion-Switchable Molecular Shuttle**
Michael J. Barrell, David A. Leigh,* Paul J. Lusby,* and Alexandra M. Z. Slawin
Despite significant advances[1] in anion-template methods for
the construction of mechanically interlocked molecules,[2] the
use of anions to induce changes in the relative positions of the
components of catenanes and rotaxanes has proved partic-
ularly challenging,[3] especially in comparison to the wide-
spread success achieved with other stimuli.[4–6] The few
examples of anion-switchable molecular shuttles developed
to date employ competition between the same types of weak
interaction in both states of the molecule to achieve switching
(solvation effects[3d,e] or the anion outcompeting hydrogen-
bonding acceptor groups of the macrocycle for donor groups
on the thread [3e,f]). Other features of anions, such as the
propensity of halides to form strong coordination bonds to
various transition metals, have yet to be exploited.[7] Herein
we report the serendipitous discovery of a new efficient
template for rotaxane formation and its use in the assembly of
a chloride-switchable molecular shuttle which exhibits excel-
lent positional integrity (> 98%) of the ring in both states that
arises from orthogonal binding modes: direct intercomponent
metal–ligand coordination in one state and a combination of
tight ion pairing, aromatic stacking interactions, and CH···O
and CH···Cl hydrogen bonding in the other.
The development of the new template for rotaxane
formation was prompted by the chance observation that
displacement of the acetonitrile ligand of [(L1)Pd(CH3CN)]
by the chloride ion of benzyl pyridinium chloride (1-Cl) was
Figure 1. An allosteric anion-activated template for threadingbased on
tight ion pairing reinforced through multiple other noncovalent inter-
actions. a) Synthesis of pseudorotaxane [(L1)PdCl·1]. Reaction condi-
accompanied by encapsulation of the organic cation by the
anionic PdCl-coordinated macrocycle [(L1)PdCl]À (Fig-
ure 1a).[8] The threaded nature of the complex [(L1)PdCl·1]
tions: CH2Cl2, 1 h, quantitative yield (use of 1-PF6 instead of 1-Cl, or
in CDCl3 was clearly apparent from 1H NMR spectroscopy (a
usingDMSO instead of CH Cl2, does not lead to pseudorotaxane
2
formation). b) Side-on and c) face-on views of the X-ray crystal
distinct upfield shift in the pyridinium resonances with respect
structure of [(L1)PdCl·1].[9] N blue, O red, Pd silver, Cl and the C atoms
of the macrocycle green, pyridinium purple, and other C atoms gray.
Selected bond lengths [] and angles [8]: N1–Pd 2.04, N2–Pd 1.93,
N3–Pd 2.03, Cl–Pd 2.32, O1–H3 2.47, O1–H1 2.57, O2–H1 2.37; N1-
Pd-N3 160.8, N2-Pd-Cl 176.4.
[*] M. J. Barrell, Prof. D. A. Leigh, Dr. P. J. Lusby
School of Chemistry, University of Edinburgh
The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ (UK)
Fax: (+44)131-650-6453
E-mail: david.leigh@ed.ac.uk
to those in 1-Cl caused by shielding by the benzylic groups of
the macrocycle, see the Supporting Information) and was also
found to persist in the solid state (Figure 1b and c) from the
X-ray crystal structure of single crystals grown from a
saturated CH2Cl2/EtOAc solution.[9]
Prof. A. M. Z. Slawin
School of Chemistry, University of St. Andrews
Purdie Building, St. Andrews, Fife KY16 9ST (UK)
[**] We thank Michael Hoffman and Dr. Anne-Marie Fuller for initial
work on ion-pair rotaxane templates in our group and the EPSRC
National Mass Spectrometry Service Centre (Swansea, U.K.) for
accurate mass data. This work was supported by the EPSRC. P.J.L. is
a Royal Society University Research Fellow. D.A.L. is an EPSRC
Senior Research Fellow and holds a Royal Society-Wolfson Research
Merit Award.
The solid-state structure of [(L1)PdCl·1] indicates that a
broad range of noncovalent interactions are responsible for
the assembly of the threaded architecture. In addition to the
tight ion pair[10] (the pyridinium nitrogen atom is within 4 of
atoms in the first coordination sphere of the formally
negatively charged metal complex), aromatic stacking inter-
actions between the aromatic rings of the host and guest, aryl-
and alkyl-CH···O hydrogen bonding between the polyether
Supportinginformation for this article is available on the WWW
8036
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2008, 47, 8036 –8039