Cation-based Structural Tuning of Pyridine Dipyrrolate Cages and Morphological Control over Their Self-assembly

Different pyridine dipyrrolate cages including cage-based dimers and polymers may be fabricated in a controlled manner from the same two starting materials, namely, an angular ligand 1 and Zn(acac)₂, by changing the counter cation source. With tetrabutylammonium (TBA⁺) and dimethyl viologen (DMV²⁺), Cage-3 and Cage-5 are produced. In these cages, two ligands act as bridges and serve to connect together two cage subunits to produce higher order ensembles. In Cage-3 and Cage-5, the TBA⁺ and DMV²⁺ counter cations lie outside the cavities of the respective cages. This stands in contrast to what is seen with a previously reported system, Cage-1, wherein dimethylammonium (DMA⁺) counter cations reside within the cage cavity. When the counter cations are tetraethylammonium (TEA⁺) and bis(cyclopentadienyl) cobalt(III) (Cp₂Co⁺), polymeric cage materials, PC-1 and PC-2, are formed, respectively. The counter cations thus serve not only to balance charge but also to tune the structural features as a whole. The organic cations used in the present study also act to modulate the further assembly of individual cages. The present cation-based tuning emerges as a new method for a fine-tuning of the multidimensional morphology of self-assembled inorganic materials.