Crown ethers as stabilising ligands for oxonium ions

Oxonium ions (H+·(H2O)n) can be stabilised and isolated in the solid state by using crown ethers of differing sizes. Crown ethers can act as very good hydrogen bond acceptors for the binding of oxonium ions. It has been reasonably well established that 18C6 binds the H3O+ oxonium ion almost selectively in the cavity of the crown, while for the smaller crown ethers 12C4 and 15C5 the macrocycles generally span successive oxonium ions of varying nuclearity (H3O+, H5O2+, H7O3+ and H9O4+) to form hydrogen bonded polymers. Crown ethers larger than 18C6, e.g. 21C7 and 24C8 can bind H5O2+ ions on the face of the crowns, while 30C10 appears to prefer two molecules of H3O+ in the cavity. It is the intention of this perspective to review the literature involving the solid state interactions of crown ether complexes involving oxonium ions.