From: Amir Mahmoudkhani (amirmahmoudkhani$##$hotmail.com)
Date: Wed Dec 10 2003 - 20:59:48 EST
Hello Sandra,
I have been doing X-ray crystallography for a while and I should say I
certainly disagree with a traditional opinion of using ClO4 or PF6 as
counter anions for crystallizing a complex cation. These anions, in general,
are found to be disordered in crystal structures, resulting in a poor
structure determination statistics. This is due to the fact that they don't
make strong interactions (like hydrogen bonds) with other species.
Furthermore, salts with perchlorate and nitrate are potentially dangerous
(e.g. explosive or strongly oxidizing agents).
I would start thinking of replacing PF6 with chloride ions. The Cl(-) anion
makes stronge X--H...Cl hydrogen bonds, that may help crystallization by
improving the degree of association of species.
If the problem is at nucleation step, I would keep a dilute solution for a
very slow evaporation at the top box of frydge at -4 C. Even at that
temperature solvents such as methanol will slowly evaporate, and with the
help of chilling, hopefully some seeds start to nucleate and you get
crystals. This process may takes 4-5 days or more. If you don't see seeds
after 7 days, you may help nucleation by immersing the solution container in
dry ice or liquid nitrogen.
If nucleation step is fast, you may try diffusion of another solvent into
the compound solution. Alternatively, you can try to elevate crystallization
temperature. A simple remedy is to make a saturated solution and leave the
container in a preheated oli bath. Use some isolation to solw down cooling
process. Make sure solvent evaporation is minimized!
Hope these hints will be of help for you.
Good Luck!
Yours,
Amir
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