"In theory, every soluble pure solid compound can be crystallized to give single crystals suitable for X-ray diffraction studies, this still being by far the least ambiguous and most complete characterization method available to synthetic chemists."
A few general points should be noted:
- Do not disturb crystallizations! Bumping them, swirling, etc can completely mess them up.
- If using solvents, make sure everything is fully dissolved. If everything won't dissolve, then these are likely to be impurities, so filter them off.
- The purer the compound, the better your chance of growing single crystals. At least 75% pure or better is probably a minimum if you want your chances to be good.
- Don't give up too quickly! Solvent choice is critical, and sometimes multiple solvents are needed for the molecules to pack. Just try everything and you'll find that you can almost always get a crystal.
Five main methods present themselves for crystallization of compounds, all are valid for organic and inorganic materials:
The simplest, but nonetheless very successful, method for crystal growth is the cooling of a saturated solution of the compound to be crystallized. Any solvent is fair game for this method (with the exception of water and benzene in the freezer). Simply make a saturated solution of the compound and allow it to cool, usually by placing the solution in the freezer. Also a hot solution can be slowly cooled to room temperature, although this really only works for known compounds in undergraduate laboratories.
This is the most common methodolgy for crystal growth, and involves simply evaporating solvent from the solution of the compound until saturation is reached and crystals form. This method is not the best and often leads to ugly crystals since the crystals tend to grow on the surface of the vessel . Also make sure you stopper it in time or solvate loss can occur, along with crystallinity.
An extension on this technique involves the use of two solvents, one in which the material to be crystallized is soluble and a second in which it is insoluble. What's important is that the first solvent is more volatile than the second, so that as 'solvent one' evaporates, 'solvent two' remains, eventually reaching a point where solubilty of the compound can no longer be sustained. Typical solvents used are ether, methylene chloride or pentane as 'solvent one' choices, and acetonitrile, methanol, ethanol, heptane, toluene as 'solvent two' choices.
To slow down crystal growth, and minimize solvate loss, the refrigerator or freezer can be used.
3. Vapor Diffusion
This is probably the most successful method to grow a crystal. Two vials are needed where one can fit inside the other. In the inner vial the compound to be crystallized is dissolved in a small quantity of a moderately non-volatile solvent, such as THF, benzene, chloroform, toluene, acetonitrile, methanol, and even methylene chloride. Then in the second vial, a volatile solvent in which the compound is insoluble is added - suitable solvents for this are pentane, diethyl ether or hexane, and the this vial is then capped. The second solvent then slowly diffuses into the first, precipitating the product and, hopefully, depositing crystals.
Again the refrigerator or freezer can be used to slow the diffusion process.
4. Liquid/liquid diffusion
This is similar to the vapor diffusion and involves simply carefully layering a low density solvent on top of higher one in a thin tube (NMR tube). Solvents of choice for the bottom layer (n which the compound is dissolved) are methylene chloride or chloroform, while typical top layer solvents are hydrocarbons or ether.
If you're very fortunate, your compound may be sufficiently volatile for this technique. Simply heat the compound (generally under vacuum), and collect crystals on a cooled cold-finger. Often the collected material is highly crystalline, and, best of all, by the very nature of the method free of all solvent impurities.
Dr. Maarten Dinger is a post-doc for the Scott research group
Growing crystals is a skill, which can only be mastered well after attempting numerous crystallizations.
Be persistent - do not give up if the first, or second crystallization attempt fails. There is a lot of conditions at your disposal that you can change such as solvent ratio, solvent volume, temperature, add second or even third solvent to your crystallization mixture. The bottom line is experimentation - the more you play with crystallizations the more successful you get.
Be observant! Pay close attention to the compound's behavior during its preparation and work-up - how soluble it is in a given solvent, what happens when a drop of solution is left to evaporate? This might give you important clues regarding what solvents to use in your crystallization.
The purer compound you start with the better your chance of growing good crystals. I usually do not start growing crystals until the compound is at least 90 % pure.
Crystallization solution must be homogenous! When a compound is dissolved in any solvent the resulted solution should be filtered (syringe filter works well for this purpose) to remove any floater before it is put aside - in my opinion this is probably the most important step in setting-up crystallization!
All of my crystallizations were performed using five different solvents or their mixtures: toluene, hexane, THF, diethyl ether and methylene chloride. So far I did not have to resort to other solvents in my work but this does not mean that these five solvents will always work very well for your crystallization. All of the crystals I have obtained were either grown by cooling (drybox freezer set at -25 °C) or evaporating solution containing dissolved compound. Most of the compounds I crystallized were organometallic complexes (MW = 300 - 1000) although I also obtained crystals of several organic molecules.
In my experience, salts are somewhat more difficult to crystallize than neutral compounds - some of them tend to oil out. Solvent mixtures for salt crystallization that I successfully used were methylene chloride/hexane and THF/hexane.
Choice of crystallization solvent or solvent mixture depends, of course, on the compound's solubility. In any crystallization technique the idea is to exceed saturation level of the solution and force the solute to come out hopefully in the form of beautiful crystals. If the compound is very soluble even in hydrocarbons then solvent evaporation is a technique of choice. Slow evaporation (1-7 days) is usually recommended but at times good crystals can be obtained within 1 hr. by simply leaving the solution vessel wide open.
A Few Example Crystallizations from My Work
Organic compound (~ 30 mg) was dissolved in a mixture of 0.5 mL of methylene chloride and 2 mL of hexane (compound was very soluble in methylene chloride but much less soluble in hexane). Solution was filtered and put aside at room temp. Since methylene chloride evaporates faster then hexane after a few days solution reached saturation level and crystals formed.
Two reactants (~ 35 mg each) were dissolved in 0.6 mL of C6D6 in the NMR tube. The product of this reaction was a salt, which had much lower solubility in benzene that the starting materials. Good quality crystals were obtained within an hour in the NMR tube.
Organic salt (~150 mg) was dissolved in 1 mL of either followed by 2 mL of hexane (hexane was added until solution became slightly cloudy). Solution was filtered and put aside on the shelf in the dry box (room temp.). After 14 hr. colorless crystals formed.
Organometallic complex (50 mg) (highly soluble even in hydrocarbons) was dissolved in 1 mL of hexane and the vial was left opened in the drybox. After 8 hr. hexane evaporated leaving nice crystals. One way to slow down evaporation is to leave a closed NMR tube for a couple of weeks. Although this is a very slow process it quite often gives very nice crystals.
Organometallic complex (~ 300 mg) was dissolved in 1 mL of toluene followed by 4 mL of hexane. Solution was mixed, filtered and put into freezer (-25 °C). Next day crystals were formed.
Organometallic complex (~ 300 mg) was dissolved in 3 mL hexane. Solution was filtered and put into freezer (-25 °C). Next day crystals were formed.
Introduction of some solvents might change the interaction between cation and anion and help with crystallization. In one case, small amount of THF was added to crystallization mixture (ether/hexane). Obtained crystals contained THF molecule that formed hydrogen bond with ammonium cation of my compound. THF helped in crystallization by changing the composition of the compound.
Many solvents get incorporated into crystal lattice. This is an extremely common phenomenon. Aromatic solvents (toluene, benzene), for example, are among the most common solvents found in crystal lattices. If you have difficulty growing crystals from one set of solvents (even though you did everything right) you might want to introduce small amount of second or third solvent with the hope it will be used by Nature as a building block of the crystal lattice thus allowing good crystal formation.