Sciencemadness Discussion Board

Crystallization woes

dapper - 4-8-2008 at 14:47

I'm attempting to crystallize a compound which contains large fluorinated aryl borates and other organic structures. the lit reduces the reaction contents after filtration (DCM) and adds hexane to the concentrated liquor "which causes a yellow oil to separate which solidifies upon stirring" - I get DCM which dries to red goo, the hexanes will color yellow and sit over the red goo, but when transferred and reduced the hexanes give ~10%yield (reported 70+)

Now, I'm wondering if I reduced the reaction mixture too far and possibly exposed the (air sensitive) compounds (rookie technique errors?) or if I simply needed to follow a different technique - reduce the DCM, flood with hexanes and allow to slowly crystallize? My observation was fine yellow crystals at the air-interface under slow evaporation of solvent so perhaps I could slowly allow this and collect large crystals from the mixture?

Klute - 4-8-2008 at 15:58

I would try to remove a little less DCM, and then very slowly add the hexane in a fine, continous stream, on the sides of the flask, to form two layers, taking care not to disturb the interface (a canula works well for this, but i generally use a pipett). If there is a little cristn occuring, it's okay, but take care not too add too much hexane and cause immediate precipitation. Leave the mixture to rest for seevral hours, away from any movement, vibrations, drafts (covered), as the hexane very slwoly crashes out the product by diffusion. Generally, this method produces large cristals of great purity, and often works well when conventional technics fail (oiling out of product, low mp, etc). Some solvents are too miscible to form two layers, if there density are too close, or if you add a dense solvent on a light one. Any excessiev pertubations can cause immediate precipitation of amorphous solid, as the solution get super saturated. try to work ou the ideal amounts of solvents to use. Cooling in the freezer once equilibrium is reached (count overnight) helps getting more product out.

It is possible than when vacuum filtering your crystals, some mor eamorphous product form in the filtrate. Just gently warm the whole filtrate in a open beaker, until most of the solid has dissolved, add some DCM or the solvent in which your product is very soluble in) dropwise until complete dissolution, then add a little more of the other solvent and leave it to cool undisturbed.

As an example, look at the crystals of p-methoxyphenol i obtained using this method (AcOEt/Pet ether), aswell as a nickel bis-dithiolene complex (DCM/IPA), pretty tricky to crystalize (alot of monocrystals were formed)


dithiolene nickel complex::

I personally nearly always use this technic to crystallize/recrysatllize products, and am very often satisfied. My favourite combinations are DCM/pet ether, AcOEt/Pet ether, MeOH/AcOEt, etc.

Hope it helps!

[Edited on 5-8-2008 by Klute]

dapper - 4-8-2008 at 17:49

I used the above technique or attempted to utilize it. Thankyou for the additional information for sure, I'll certainly put it in the freezer after it has had time to equilibrate. Thanks again for the most excellent response, I'll layer in hexane again via cannula.

PHILOU Zrealone - 6-8-2008 at 05:05

This process is a variant of the cristallisation by solvant migration.
This can be achieved by using two solvants A & B fully miscible; one of wich (A) fully dissolve the interest compound X and the other one (B) as little as possible.
The best is to have B more volatile than A(*).

Then use a tiny open recipient with A saturated by X.
This recipient is itself placed inside a bigger closed recipient with pure solvent B on the bottom.
Solvent B evaporates faster dan A and condenses slowly in it making a very slow and constant increase of concentration of B into A(X)...yielding at a certain point to cristal germs and finally to beautifull cristals.

I used that technique to recristallise Ru(PHEHAT)3Cl3 (PHEHAT = 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene)

(*)In the case a volatile solvant is not under hand, then one can make use of a dropwise addition of B into A(X) is then better if density of B is less than that of A in a way to get a concentration gradient of B into A from the top to the bottom...cristals falls on the bottom.

dapper - 6-8-2008 at 07:27

I made some lovely square step crystals of NaCl by using that method - I left a watchglass of the satd. soln. over a beaker of isopropanol for weeks.