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I know crown ethers can be used to complex metal ions.
Question: What important properties do 18-crown-6 change during the benzannulation from 18-crown-6 to DB18C6 (dibenzo-18-crown-6)? How do these change?
(Disclaimer: I know that DB18C6 does not necessary to be synthesised from 18C6)
Motivation: I saw that some complexations of metal ions use 18C6, but some of the other use DB18C6.
Thoughts and Research: As crown ethers are used to make size-specific ions complexes, the sizes of the hole in the ethers are important. But the benzannulation does not change the size, so I suggest that the process changes the equilibrium constant of $\ce{M(ether)X_n}$ or the rate of complexation. Investigated by this article, the distribution constant between polar phase and non-polar phase increases significantly and the equilibrium constant in non-polar phase increases. I understand the former because of the property of benzene ring. But how can the latter one happen? Are there another reasons why DB18C6 is chosen?

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  • $\begingroup$ Of course does the geometry of the crown ether change! Four sp3 carbon atoms are now sp2! $\endgroup$ – Karl Dec 1 '19 at 13:08

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