“Plastic” Ionic networks: Lightweight porous materials that sorb a variety of molecular guests
What does it look like?
The crystal structure of lithium isonicotinate∙solvent consists of lithium isonicotinate chains (shown above on the left) composed of alternating edge-shared 8-membered Li-O-C-O-Li-O-C-O rings and 4-membered Li-O-Li-O rings. Each strip-like chain is connected to four parallel chains by pyridyl-Li connections generating a 3D network containing solvent-filled channels (shown above on the right). The figure above shows morpholine molecules within the channels; similar lithium isonicotinate networks can be obtained with N,N-dimethylformamide, N-methylpyrrolidinone, dioxane, pyridine, n-hexanol, cyclohexanol, propanol and t-butanol occupying this space.
What is it?
Lithium isonicotinate is an example of a 3D ionic network whose channels are templated by various solvent molecules. Although the channels within the crystalline network persist upon solvent removal, significant structural rearrangements within the network accompany the loss of the templating solvent molecules. We propose that the non-directional ionic forces between the lithium cations and the carboxylate anions bestow a degree of ‘plasticity’ upon the network that allows a reorganization of the Li-carboxylate connections, surprisingly with retention of single crystal character. The new guest-free crystal structure formed in this remarkable solid-sate transformation is shown below. As indicated, the lithium – isonicotinate chains consisting of alternating 8-membered Li-O-C-O-Li-O-C-O rings and 4-membered Li-O-Li-O rings have been converted to a chain comprised of 6-membered Li-O-C-O-Li-O rings. Similar to the solvated precursor the chains of the solvent free lithium isonicotinate link to four others through lithium-pyridyl connections. The result is a 3D network that is able to sorb gas molecules.
Where did the structure come from?
The synthesis, structural characterisation and gas sorption investigations of lithium isonicotinate∙solvate were carried out in the Abrahams/Robson research labs, School of Chemistry, University of Melbourne.
The work has been recently published in Inorganic Chemistry. DOI: 10.1021/ic403134c. An animated clip showing the proposed mechanism for the rearrangements that occur with the single crystal-to-crystal transformation can be seen in the publication.