Design Strategy for the Controlled Generation of Cationic Frameworks and Ensuing Anion-Exchange Capabilities

Cationic frameworks
are an emerging class of exceptional solid
adsorbents capable of encapsulating highly toxic and persistent anionic
pollutants. The controlled generation of cationic frameworks, however,
lags behind the abundant design strategies devised to control the
structures and topologies of neutral frameworks. In this regard, we
report a rational approach that allows the conversion of the synthetic
approach toward constructing a neutral framework into one allowing
for the synthesis of a cationic one without incurring any changes
to the overall topology or the selected metal ion. We demonstrate
that the replacement of a functional group on an organic linker that
promotes a similar coordination mode, but bearing one less negative
charge, can yield the systematic generation of cationic frameworks.
Moreover, we confirm the cationic nature of the metal–organic
frameworks through preliminary anion-exchange experiments and propose
a method to retain permanent porosity in cationic frameworks through
the use of strongly binding anions. Altogether, these results show
great promise for the construction of tunable nanoporous frameworks
capable of carrying out anion-exchange processes.