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'-oxybispyridin e; X- = NO3-, BF4-, ClO4-, and PF6-), have been constructed in high yield v ia cooperative effects of the skewed conformer of Py2O and the potential li near geometry of the N-Ag(I)-N bond. Crystallographic characterization reve als that the polymer framework is an ideal cationic cylindrical helix and t hat its counteranions are pinched in two columns inside the helix. The four anions have been exchanged fur each other in an aqueous solution without d estruction of the helical skeleton. In particular; [Ag(Py2O)]NO3 prepared b y 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) Angstrom, and is exactly p roportional to the volume of the anion guests. This pitch-tuning is attribu ted to subtle change in the nonrigid dihedral angles between two pyridyl gr oups around O and Ag atoms that act as hinges within the helical subunit. T hermal analyses indicate that the helical compounds are stable up to 231-33 2 degreesC in the solid state.