15-Crown-5 is a crown ether with the formula (C<sub>2</sub>H<sub>4</sub>O)<sub>5</sub>. It is a cyclic pentamer of ethylene oxide that forms a complex with various cations, including sodium (Na<sup>+</sup>) and potassium (K<sup>+</sup>); however, it is complementary to Na<sup>+</sup> and thus has a higher selectivity for Na<sup>+</sup> ions.
15-Crown-5 can be synthesized using a modified Williamson ether synthesis:
It also forms from the cyclic oligomerization of ethylene oxide in the presence of gaseous boron trifluoride.
Analogous to 18-crown-6, 15-crown-5 binds to sodium ions. Thus, when treated with this complexing agent, sodium salts often become soluble in organic solvents.
First-row transition metal dications fit snugly inside the cavity of 15-crown-5. They are too small to be included in 18-crown-6. The binding of transition metal cations results in multiple hydrogen-bonded interactions from both equatorial and axial aqua ligands, such that highly crystalline solid-state supramolecular polymers can be isolated. Metal salts isolated in this form include Co(ClO<sub>4</sub>)<sub>2</sub>, Ni(ClO<sub>4</sub>)<sub>2</sub>, Cu(ClO<sub>4</sub>)<sub>2</sub>, and Zn(ClO<sub>4</sub>)<sub>2</sub>. Seven coordinate species are most common for transition metal complexes of 15-crown-5, with the crown ether occupying the equatorial plane, along with 2 axial aqua ligands.
15-crown-5 has also been used to isolate salts of oxonium ions. For example, from a solution of tetrachloroauric acid, the oxonium ion has been isolated as the salt . Neutron diffraction studies revealed a sandwich structure, which shows a chain of water with remarkably long O-H bond (1.12 à) in the acidic proton, but with a very short OHâ¢â¢â¢O distance (1.32 à).
A derivative of 15-crown-5, benzo-15-crown-5, has been used to produce anionic complexes of carbido ligands as their salts: