[asa029]

Hello!

I am preparing some novel, organometallic compounds. Of course, we need X-ray for proper characterization. I've tried my very best recrystallization magic to prepare perfect crystals, but they are never good enough.

Does anyone have any tips/tricks for making perfect X-ray crystals?

[discodermolide]

Hello!

I am preparing some novel, organometallic compounds. Of course, we need X-ray for proper characterization. I've tried my very best recrystallization magic to prepare perfect crystals, but they are never good enough.

Does anyone have any tips/tricks for making perfect X-ray crystals?

This is a good question!
I think that sometimes crystallization is more of an art than a science. For an X-ray quality crystal you need to achieve a slow crystallization. No solvent inclusion, no double crystals, they must be a certain size etc.
Make a saturated solution and let it evaporate slowly. Give it an occasional stir, when crystals appear try and fish them out of the solution. Otherwise is it trial and error, let your crystallographer look at them.
Good luck.

[asa029]

This is a good question!
I think that sometimes crystallization is more of an art than a science. For an X-ray quality crystal you need to achieve a slow crystallization. No solvent inclusion, no double crystals, they must be a certain size etc.
Make a saturated solution and let it evaporate slowly. Give it an occasional stir, when crystals appear try and fish them out of the solution. Otherwise is it trial and error, let your crystallographer look at them.
Good luck.

I agree! It seems more like magic than a scientific endeavour!

[discodermolide]

This is a good question!
I think that sometimes crystallization is more of an art than a science. For an X-ray quality crystal you need to achieve a slow crystallization. No solvent inclusion, no double crystals, they must be a certain size etc.
Make a saturated solution and let it evaporate slowly. Give it an occasional stir, when crystals appear try and fish them out of the solution. Otherwise is it trial and error, let your crystallographer look at them.
Good luck.

I agree! It seems more like magic than a scientific endeavour!

Sorry I couldn't be more specific. It took me years to recognize a good crystal of x-ray quality.
Here's a picture of one. I know it doesn't help you much but it is a nice crystal.

[Dan]

I've also had good results with vapour diffusion, that's always worth a try.

[fledarmus]

Let me second the "magic" aspects of recrystallization as well.

The most helpful part of getting really good crystals for X-rays is to do really good observations on the crystals you already have. What solvents have you worked with so far, and what techniques? Temperature changes? Slow evaporation? Have you obtained any seed crystals? Are you sure they are crystals and not just hardened oils? (We used to have a polarizing microscope for checking that).

Slow crystallization and lots of patience has proven the most successful method for me. Really slow crystallization - one of my most useful techniques has been to fill a 1-dram shell vial with a press-cap half full of filtered, room-temperature solution, stick a 1/2" 18 or 20 guage disposable needle in the top, and let it sit in the back of the hood. Evaporation of solvents like methanol, ethanol, ethyl acetate generally took about two weeks. I could sometimes get better crystal formation for something that was slow to seed by separating about 6 seed crystals (under a microscope) and adding them to the solution after the first couple of days. If they disappear, you need to start with a more concentrated solution; if not, you get a few nicely controlled crystals growing. Sometimes.

One other technique that I have occasionally found useful is a slow solvent change. This can help you grow nice crystals from a mixed solvent system, such as ethanol/ethyl acetate or ether/pet ether. Place your more non-polar solvent in a large dish or beaker, and place your polar solvent containing your solute in a smaller dish or beaker in the center of the large dish. Cover the large dish and let it stand at room temperature. The vapors of the two solvents will mix in the air and recondense in both liquids, eventually setting up an equilibrium based on the partial pressures of the solvents. If you have a compound which can be precipitated from a polar solvent by adding a non-polar solvent, this is a long, slow method of doing the same thing and can give you good crystals.

I have learned a few other isolated tricks over the years. If you have some good observations of the types of crystals or precipitates that you get from various solvents using various conditions, pass them along and I may be able to supply a few more things to try.

[Dan]

one of my most useful techniques has been to fill a 1-dram shell vial with a press-cap half full of filtered, room-temperature solution, stick a 1/2" 18 or 20 guage disposable needle in the top, and let it sit in the back of the hood.

This is how I do most slow evaporations too. NMR tubes are good for slow evaporation as well, but the tube is normally destroyed when fishing out any crystals that form.

I've also found it useful to do slow evaporation from a solvent mix in which the weak solvent is less volatile than the strong - e.g. If my compound is soluble in DCM, but only sparingly in toluene, I will dissolve fully in DCM-toluene and let the DCM evaporate over a day or two to leave crystals under toluene. I like this because the vessel is less likely to evaporate to dryness when I forget about it for a week. I've had good results with ether-hexane and methanol-water as well (in the latter case the compounds were insoluble in water).

One other technique that I have occasionally found useful is a slow solvent change....

Yeah, this is what I meant by vapour diffusion.

[AWK]

http://www.xray.ncsu.edu/GrowXtal.html
http://aslloginsrv.chem.northwestern.edu/X-Ray/Presentations/X-RaySummer.ppt

[fledarmus]

Yeah, this is what I meant by vapour diffusion.

Sorry, I misread that the first time - I thought you said "vapour deposition" and was really impressed with the quality of your equipment and your devotion to getting absolutely pure crystals! 

[Wastrel]

Solvent layering is in AWK's first link.  I tried this with copper (II) tetra amino sulphate once.  Sat solution in water half filling a container, filled remaining space with ethanol and put the top on without moving it.  The interface sludged up on contact.  A few days later rather fabulous blue/black crystals had grown down.

[IsotopeBill]

I second Dan's method using an NMR tube.  A concentrated solution of your compound layered with a solvent in which your compound is less soluble is a common procedure, and this can sometimes be more surprisingly more effective when performed in an NMR tube.  Several trials of solvents and temperatures can be done in parallel.  I had a compound that failed every other method I had tried, but gave some very good quality crystals in the NMR tube.  (It was pointed out to me that this may be the result of a curious perversion of science: usually, the last thing you typically want is for material to be precipitating out in an NMR tube!).  I typically save all broken NMR tubes for future crystal growing attempts.

Another note:  a colleague of mine was trying to repeat a crystal growth under previously-successful conditions, but with no luck at all.  She then realized that a vacuum pump had recently been installed further down the benchtop, which made the benchtop vibrate very slightly.  Upon moving to a quieter surface, she was able to grow good crystals again.