Anion Binding as a Strategy for the Synthesis of Porous Salts

Porous salts have
recently emerged as a promising new class of
ultratunable permanently microporous solids. These adsorbents, which
were first reported as ionic solids based on porous cations and anions,
can be isolated from a wide variety of charged, permanently porous
coordination cages. A challenge in realizing the full tunability of
such systems, however, lies in the fact that the majority of coordination
cages for which surface areas have been reported are comprised of
charge-balanced inorganic and organic building blocks that result
in neutral cages. As such, most reported permanently porous coordination
cages cannot be used as reagents in the synthesis of porous salts.
Here, we show that the facile reaction of TBAX (TBA+ =
tetra-n-butylammonium; X = F– and
Cl–) with molybdenum paddlewheel-based coordination
cages of the M4L4 and M24L24 lantern and cuboctahedra structure types, respectively, affords
charged cages by virtue of coordination of halide anions to the internal
and/or external metal sites on these structures, as confirmed by single-crystal
X-ray diffraction, X-ray photoelectron spectroscopy, and nuclear magnetic
resonance (NMR) spectroscopy. At a practical level, the TBAX/cage
reactions, which are fully reversible upon isolation of the cage with
the appropriate solvent, solubilize otherwise rigorously insoluble
cages. This method significantly increases the solution processability
of these highly porous solids. Toward the formation of new porous
salts, halide binding also serves to incorporate charge on neutral
cages and make them amenable to simple salt metathesis reactions to
afford new porous salts based on anions and cations with intrinsic
porosity. A combination of diffraction methods and a suite of spectroscopic
tools confirms speciation of the isolated solids, which represent
a new class of highly tunable porous salts. Ultimately, this work
represents a roadmap for the preparation of new porous solids and
showcases the utility and broad applicability of anion binding as
a strategy for the synthesis of porous salts.