Is Gelation Behavior Predictable through a Crystal Engineering Approach? A Case Study in Four Similar Coordination Compounds

In
this paper, a detailed study on the gelation properties of a
series of terpyridine and dipyrazine–pyridine ligands in the
presence of metal salts is reported. To reveal the driving forces
for the self-assembly of the metallogelators, their crystal structure
is scrutinized. Inspired by the gelation of CuCl2[Terpy-nCN], where “Terpy-nCN” is
4′-(n-cyanophenyl)-2,2′,6′,2″-terpyridine,
to look into the aggregation behavior of the related analogues, synthesis
of CuCl2[Dipyz-py-nCN] derivatives, where
“Dipyz-py-nCN” is 4-(n-cyanophenyl)-2,6-di-pyrazin-2-yl-pyridine, with the same cyano groups
is performed. We then find that the Dipyz-py counterpart forms crystals
when the molecules are stacked in an alternating way, instead of the
unidirectional one required for gel formation. A crystal engineering
approach is applied to determine the interactions that are favorable
for fabricating a fiber network that is likely to be present in both
crystalline and gel states and to find the interaction that disturbs
this delicate balance between gelation and crystallization in coordination
compounds; then, we conclude that the subtle balance between the molecular
shape and intermolecular interactions is the origin of the gelation
and crystallization of the current molecular system. This enables
us to find the mutual connection among the structure of molecules,
assembly behavior, and intermolecular interactions. With our experiments,
a deep understanding of the balance among solution, gelation, and
crystallization with subtle molecular diversions is provided.