Chiral Self-Assembling Rhombs
A R T I C L E S
Chart 1. Enantiopure, Water-Soluble, Metallo-Supramolecular Rhombs (S,S,S,S)-1a,b and (S,S,S,S)-2a,b Generated through
Self-Assemblya
a For comparison, the corresponding enantiomers (R,R,R,R)-1a,b have also been prepared (not shown).
be used as hosts,5 to control the stereochemistry of photochemi-
cal [2+2] cycloadditions,6 and to catalyze reactions through
encapsulation.7 The generation of higher levels of order is
mandatory, if macroscopic effects are to be achieved. One
approach toward this goal is the ordered deposition of self-
assembled species on surfaces8 where they can be examined
with scanning probe microscopy.9 Recently, we10 and Gong et
al.11 described experiments using highly ordered pyrolytic
graphite (HOPG) and Cu(100) surfaces for the deposition of
metallo-supramolecular squares and rectangles. When the
interactions of the cationic metallo-supramolecular species with
the surface were enhanced by depositing a tetragonal grid of
chloride anions on Cu(100), the complexes deposit in a flat
orientation and open their cavity toward the solution phase. One
future goal is to use this arrangement of metallo-supramolecular
macrocycles for a surface confined host-guest chemistry that
can be directly visualized by means of in situ STM.12 The
electrochemical environment used in our STM experiments
appears advantageous since external stimuli like potential-
mediated electron-transfer reactions offer control10c over the
reversible inclusion or the release of redox-active guests into
adsorbed host assemblies.
In the present study, the synthesis of novel enantiopure, self-
assembling rhombs is reported (Chart 1) followed by their
characterization in solution by nuclear magnetic resonance
(NMR) and circular dichroism (CD) spectroscopy and in the
gas phase by electrospray ionization Fourier transform ion
cyclotron resonance (ESI-FT-ICR) mass spectrometry. At the
solid-liquid interface, individual rhombs are structurally char-
acterized by electrochemical scanning tunneling microscopy
(EC-STM). Quite intriguingly, the chirality of these rhombs is
expressed in the long-range order on the chloride-modified
(8) Selected examples for macrocycles deposited on surfaces and studied with
scanning probe microscopy: (a) Kromer, J.; Rios-Carreras, I.; Fuhrmann,
G.; Musch, C.; Wunderlin, M.; Debaerdemaeker, T.; Mena-Osteritz, E.;
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2000, 39, 3481-3486. (b) Mena-Osteritz, E.; Ba¨uerle, P. AdV. Mater. 2001,
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F. M.; Palmer, R. E.; Stoddart, J. F. Chem. Phys. Lett. 1997, 279, 209-
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