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Keywords:

  • cadmium;
  • coordination polymers;
  • host–guest systems;
  • ligand exchange;
  • crystal transformations

Abstract

A doubly interpenetrated square-grid coordination polymer {[Cd(ImBNN)2(CF3SO3)2]⊃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 temperature-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 1 lost toluene guests and captured water molecules to give guest-free 1 b via a proposed metastable phase 1 a, and 1 b 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[BOND]O bonds and CF3SO3 anion-shifting. In contrast, direct heating of 1 in the absence of a vacuum resulted in thermostable 1 c, 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.