Applications of Metallofoldamers - Metallofoldamers: Supramolecular Architectures from Helicates to Biomimetics - page 406

Figure 12.2 Space-filling model of 3 ( n ¼6) coordinating to two Ag þ ions. The triethylene-
glycol side chains have been omitted for clarity. Reprinted with permission from [ref 24]
Copyright 1999 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

expected from the trigonal planar coordination of the metal (Figure 12.2). The solvopho-
bic interactions of the m PE backbone were critical to the metal binding, as the shorter
derivative, 3 ( n 3), was unable to bind Ag þ .

¼

In a following work, Moore and coworkers demonstrated another intriguing application
of metallofoldamers in molecular recognition [27]. Instead of putting the metal-binding
ligands along the tubular cavity, they placed a single pyridyl group at the end of the m PE.
Unlike a simple pyridyl ligand, however, the m PE-functionalized pyridyl ( 4 )hasthe
potential to fold and thus could benefit from the solvophobic and p-p interactions among
the aromatic groups (Figure 12.3). The anticipation was that the entropic costs associated
with the complexation could be offset by the stabilization energy in the newly formed,
folded metal complex. The effect was similar to the chelating effect found in a bidentate
such as ethylenediamine, except that the two pyridyl ligands in the Pd 2þ ( 4 ) 2 complex
were coupled by noncovalent instead of covalent bonds.

The folding and metal-binding of 4 were investigated by a number of techniques. The
unfolded m PE oligomers had a peak centered at 303 nm. As the chain length increased,
the peak at 303 nm gradually disappeared and a new peak at 289 nm appeared for the 2 : 1