15-Crown-5

15-Crown-5 is a crown ether with the formula (C2H4O)5. It is a cyclic pentamer of ethylene oxide that forms complex with various cations, including sodium (Na+) and potassium (K+); however, it is complementary to Na+ and thus has a higher selectivity for Na+ ions.

Synthesis
15-Crown-5 can be synthesized using a modified Williamson ether synthesis:


 * (CH2OCH2CH2Cl)2 + O(CH2CH2OH)2 + 2 NaOH → (CH2CH2O)5 + 2 NaCl + 2 H2O

It also forms from the cyclic oligomerization of ethylene oxide in the presence of gaseous boron trifluoride.

Properties
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(ClO4)2, Ni(ClO4)2, Cu(ClO4)2, and Zn(ClO4)2. 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 [H7O3]+ has been isolated as the salt [(H7O3)(15-crown-5)2][AuCl4]. Neutron diffraction studies revealed a sandwich structure, which shows a chain of water with remarkably long O-H bond (1.12 Å) in the acidic proton, but with a very short OH•••O distance (1.32 Å).



A derivative of 15-crown-5, benzo-15-crown-5, has been used to produce anionic complexes of carbido ligands as their [K(benzo-15-crown-5)2]+ salts:
 * (Ar2N)3MoCH + KCH2Ph  +  2 (15-crown-5)   →   [K(15-crown-5)2]+[(Ar2N)3MoC]−  + CH3Ph