Temperature-Dependent Guest-Driven Single-Crystal-to-Single-Crystal Ligand Exchange in a Two-Fold Interpenetrated Cd-II Grid Network

A doubly interpenetrated square-grid coordination polymer {[Cd-(ImBNN)(2)(CF3SO3)(2)] superset of guest}(n) (1) (guest=C7H8 and ImBNN=2,5-bis[4'-(imidazol-1-yl)phenyl]-3,4-diaza-2,4-hexadiene) that contains cavities able to accommodate toluene guest molecules has been assembled by the reaction of the Schiff base ligand ImBNN and Cd(CF3SO3)(2). The framework 1 shows dynamism in either temperature-dependent expansion and shrinkage or cooperatively tempera ture-dependent guest-driven ligand exchange at the metal center. Studies of guest removal/uptake by heating in a vacuum, cooling in air, and then heating in toluene at reflux have revealed a series of single-crystal-to-single-crystal structural transformations: complex I lost toluene guests and captured water molecules to give guest-free 1b via a proposed meta-stable phase la, and 1b could readsorb toluene guests to give 1', which represents a restored 1. These structural changes establish an indirect recoverable process that involves ligand exchange in the coordination sphere and guest exchange in the cavity accompanied by the cleavage/formation of Cd-O bonds and CF3SO3- anion-shifting. In contrast, direct heating of I in the absence of a vacuum resulted in thermostable 1c, confirmed by X-ray powder diffraction as a new phase that could not be converted back into 1. and that had lost the ability of readsorbing toluene guests.