David Marx
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Crown ethers
Does anyone know a good reference for crown ethers and what range of cations can be inserted into each?
I used to work with 18-crown-6 and nickel cations. Currently, I am interested in inserting larger ions into crown ethers but don't have a really
good feel for what size they can hold? Specifically, gold and uranyl?
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BromicAcid
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Searching Amazon.com turns up a few recent crown ether books but other then that I'm afraid that I have no further information. All my intel on
crown ethers is terribly outdated by at least 15 years or more and I have never heard of crown ethers accomidating the Ni2+ cation. Regardless, for
this subject you should search the chemistry abstracts or like I said, amazon.com. Best of luck and don't be afraid to post about what you
learn.
[Edited on 8/13/2004 by BromicAcid]
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JohnWW
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The Aldrich-Sigma Chemical Co. makes them, I believe. I think their website is http://www.aldrichsigma.com , or http://www.sigmaaldrich.com or similar - it should have all available data about crown ethers.
Crown ethers, which are basically cyclic polyethers in which the -O- ether linkages are separated by (CH2)n where n is most often 2 or 3 or 4 (which
numbers make for 5- or 6- or 7- membered rings when chelating a mertal cation), with groups attached to the CH2s in some cases, are used as non-ionic
oxy-ligands for getting mostly highly electropositive metals (which form mostly ionic compounds) into solution in non-polar organic solvents. Such
metals are often reluctant to be chelated and difficult to dissolve in non-polar organic solvents, and when they do it is with oxy-ligands.
18-crown-6 is the one with 6 O atoms each separaed by (CH2)3 groups, resulting in 6 6-membered rings being formed with the metal on chelation. This
would be suitable for the largest metal cations, but there would probably be steric strain around smaller metal ions. By comparison, porphyrin, or
2,5-tetra-methene-pyrrole, (C4H2N=CH)4, which with several substituting groups is the essential polyamino chelating part of the heme and chlorophyll
molecules with 4 chelating N atoms, chelates Fe and Mg quite comfortably, forming 4 6-membered rings around the metal ions. So for moderate-sized
metal ions like Ni (which being a transition metal is, however, more strongly chelated by amino- or thio- or P- or As- or Se- ligands), a more
comfortable fit s likely with 12-crown-4 ether.
John W.
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David Marx
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"Hole Sizes"
Well after chasing down some not so helpful biochemical references, I actually found a small table in Greenwood's Chemistry of the
Elements. It lists "hole" sizes and suggests they can be compared loosely to ionic diameters.
14-crown-4 120-150 pm
15-crown-5 170-220
18-crown-6 260-320
21-crown-7 340-430
With a quick look at the appendix of the book, we see ionic diameters of:
Au(1+) 272 pm
Au(3+) 170
U(6+) 146
So, 18-crown-6 may work or it may be a little large. Now to track down the necessary solvents and experimentally test it.
[Edited on 14-8-2004 by David Marx]
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