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.