Published on Web 03/30/2005
Supramolecular Chelation Based on Folding
Matthew T. Stone and Jeffrey S. Moore*
Contribution from the Departments of Chemistry and Materials Science & Engineering,
600 South Mathews AVenue, The UniVersity of Illinois at Urbana-Champaign,
Urbana, Illinois 61801
Received February 3, 2005; E-mail: jsmoore@uiuc.edu
Abstract: Crystallographic analysis revealed that pyridine-palladium complexation is a good geometric
match to the m-phenylene ethynylene (mPE) repeat unit and thus could serve as a reversible linking group
to join oligomer segments together. A series of pyridine-terminated mPE oligomers were then synthesized
and found to coordinate with palladium dichloride to give complexes effectively twice the length of the free
oligomers. A quantitative analysis of these coordination equilibria by isothermal calorimetry found the ability
of the pyridine end-group to form a coordination complex corresponded with their ability to fold. Oligomers
that were able to form complexes of sufficient length to fold showed positive cooperativity based on
experimental determination of their association constants with a palladium ion. We suggest that the additional
free energy of complexation for the folded oligomers is analogous to chelation by multidentate ligands, but
here the “multidentate ligand” is held together by supramolecular rather than covalent bonds.
Introduction
result also suggests that integration of a catalytic moiety into
the structure of a mPE oligomer may impart desirable properties,
Previous studies of m-phenylene ethynylene (mPE) oligomers
have established that these chain molecules adopt a helical
conformation in solution stabilized by multiple intrachain
aromatic stacking interactions.1-3 One of the reasons we and
others4-11 are interested in these artificial folding molecules or
foldamers12-17 is their potential for use as supramolecular
catalysts,18 and to this end we have begun incorporating
functional groups into the interior cavity of the mPE helix.19
Our recent demonstration of high alkylation rates in the interior
of mPE oligomers confirms that substrate binding in the helix
cavity enhances reactivity through a proximity effect.20,21 This
such as substrate specificity since the cavity can bind molecules
of different size and shape with varying association con-
stants.11,22,23 In addition, the inherently chiral environment
provided by the helical structure24 could prove useful for
asymmetric transformations. Integration of a transition metal
within the cavity of a mPE oligomer is one approach that we
are currently pursuing to develop supramolecular catalysts. The
focus of this report is to describe metal coordinating function-
alities that are compatible with the folded state of mPE
oligomers.
An earlier investigation of mPE molecules in which segments
were joined through dynamic covalent bonds demonstrated that
under equilibrium conditions shorter mPE chains are driven to
form longer chains capable of adopting a stable helical
conformation.25,26 It seemed reasonable that two oligomers with
coordinating end-groups could be joined with an appropriate
metal to form a complex that was effectively twice the length
of the individual strands. This approach is attractive since longer
mPE oligomers chains are accessible in fewer synthetic steps
and disassembly of the complex might be integrated into the
catalytic cycle to facilitate turnover (i.e., by reducing product
inhibition).
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10.1021/ja050713n CCC: $30.25 © 2005 American Chemical Society