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.