Smart Molecular Helical Springs as Tunable Receptors

The rational construction and operation of an ideal helical spring has been investigated. The infinite
helices, [Ag(Py2O)]X (Py2O = 3,3‘-oxybispyridine; X- = NO3-, BF4-, ClO4-, and PF6-), have been constructed
in high yield via cooperative effects of the skewed conformer of Py2O and the potential linear geometry of the
N−Ag(I)−N bond. Crystallographic characterization reveals that the polymer framework is an ideal cationic
cylindrical helix and that its counteranions are pinched in two columns inside the helix. The four anions have
been exchanged for each other in an aqueous solution without destruction of the helical skeleton. In particular,
[Ag(Py2O)]NO3 prepared by the counteranion exchange can be isolated as crystals suitable for X-ray
crystallography in water. The helical pitch is reversibly stretched via the counteranion exchange from
7.430(2) to 9.621(2) Å, and is exactly proportional to the volume of the anion guests. This pitch-tuning is
attributed to subtle change in the nonrigid dihedral angles between two pyridyl groups around O and Ag
atoms that act as hinges within the helical subunit. Thermal analyses indicate that the helical compounds are
stable up to 231−332 °C in the solid state.