Development of Metal–Organic Nanotubes Exhibiting Low-Temperature, Reversible Exchange of Confined “Ice Channels”

Nanotubular
materials have unique water transport and storage properties
that have the potential to advance separations, catalysis, drug delivery,
and environmental remediation technologies. The development of novel
hybrid materials, such as metal–organic nanotubes (MONs), is
of particular interest, as these materials are amenable to structural
engineering strategies and may exhibit tunable properties based upon
the presence of inorganic components. A novel metal–organic
nanotube, (C4H12N2)0.5[(UO2)­(Hida)­(H2ida)]·2H2O (UMON) (ida = iminodiacetate), that demonstrates the possibilities of these
types of hybrid compounds has been synthesized via a supramolecular
approach. Single-crystal X-ray diffraction of the compound revealed
stacked macrocyclic arrays that contain highly ordered water molecules
with structural similarities to the “ice channels” observed
in single-walled carbon nanotubes. Nanoconfinement of the water molecules
may be the cause of the unusual exchange properties observed for UMON, including selectivity to water and reversible exchange
at low temperature (37 °C). Similar properties have not been
reported for other inorganic or hybrid compounds and indicate the
potential of MONs as advanced materials.