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Figure 4. X-ray crystal structure of rotaxane 1. Intramolecular hydro-
gen-bond lengths (ꢀ) and angles: O38-HN2 1.89, 161.98; O39-HN20
2.60, 162.28. Carbon atoms of the macrocycle are shown in blue and
those of the thread in yellow; oxygen atoms are red, nitrogen atoms
dark blue and amide hydrogen atoms white. Non-amide hydrogen
atoms are omitted for clarity. CCDC-127612 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
the Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB21EZ, UK; fax: (+44)1223-336-033; or deposit@ccdc.cam.
ac.uk).
[13] Within the context of a recent book and paper (V. Balzani, A.
Credi, M. Venturi, Chem. Eur. J. 2002, 8, 5524 – 5532; V. Balzani,
A. Credi, M. Venturi, Molecular Devices and Machines—A
Journey into the Nanoworld, Wiley-VCH, Weinheim, 2003) it
was proposed that the term “supramolecular” be expanded from
Lehnꢀs original definition of “chemistry beyond the molecule”
(i.e., assemblies of two or more molecules or ions held together
by noncovalent forces) to include large molecules (e.g., den-
drimers, rotaxanes, proteins etc.) which feature functional
intramolecular interactions or photophysics. In our view such a
revision is unwarranted. When scientific language evolves it
needs to retain a precise definition to remain useful (e.g., “acid”
to “Lewis acid” or “Brønsted acid”). Consider as a contrary
example the term “self-assembly”, which has acquired such an
imprecise meaning over recent years that it now conveys
virtually nothing as a descriptor. In its currently accepted
definition, “supramolecular”—by analogy to the term “molec-
ular”—refers to how the atoms in a structure are held together,
not their photophysical properties. It distinguishes molecules
from clusters of molecules, for example pseudorotaxanes (host–
guest complexes in which the components are free to exchange
between bound and unbound species) and rotaxanes (molecules
in which the components cannot exchange with outside systems
without breaking covalent bonds). It does not matter that their
properties can be similar or that bond energies sometimes make
it difficult to distinguish between molecular and supramolecular
species, just as the timescale-dependent inversion of asymmetric
nitrogen atoms does not confer on the term “chirality” any less
clear a meaning. Language—especially scientific language—
needs to be precise; subject areas, for example, “supramolecular
chemistry” or “organometallic catalysis”, on the other hand,
should be as broad and inclusive as possible, and have always
happily encompassed chemistry not technically suggested by
their titles (J.-M. Lehn, Supramolecular Chemistry: Concepts
and Perspectives, Wiley-VCH, Weinheim, 1995, p. 90).
directional (162.28 is a typical NH···O hydrogen bond
angle[30]) to a lone pair of an sp3-hybridized orbital of an
oxygen atom, in what is presumably a very weak interaction.
In conclusion, we have synthesized rotaxane 1, whose
components bear no formal mutual recognition elements
through the first example of controlled submolecular trans-
lational motion in organic synthesis. In principle, there is no
reason why mechanically interlocking auxiliary strategies
should not work with other molecular-shuttle systems,
including those based on cyclodextrins, which already have
US FDA approval for use in the pharmaceutical and food
industries. In our laboratories the approach is currently being
used to prepare mechanically interlocked analogues of
substrates that are unavailable by conventional synthetic
methods and to modify the physical and chemical properties
of a range of pharmaceuticals, dyes, reagents, catalysts, and
components for molecular electronics.
Received: December 22, 2003 [Z53606]
Keywords: molecular machines · molecular shuttles · rotaxanes ·
.
submolecular motion · synthetic auxiliary
[14] In addition to the rotaxane-forming methods based on specific
templates for metal-ion coordination, aromatic stacking and
hydrogen bonding,[6] cyclodextrins (S. A. Nepogodiev, J. F.
Stoddart, Chem. Rev. 1998, 98, 1959 – 1976), and certain cyclo-
phanes[12] can form rotaxanes of a broad range of substrates
through general hydrophobic binding. The trapping of phenolate
anions by amide macrocycles can also produce rotaxanes with no
recognition elements (C. Seel, F. Vögtle, Chem. Eur. J. 2000, 6,
21 – 24; R. Shukla, M. J. Deetz, B. D. Smith, Chem. Commun.
2000, 2397 – 2398), but requires a specific template in the stopper
and is apparently of limited generality (C. A. Schalley, G. Silva,
C. F. Nising, P. Linnartz, Helv. Chim. Acta 2002, 85, 1578 – 1596).
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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