Crystallisation

Growing single crystals suitable for X-ray diffraction can be a challenge at the best of times, so attempting this process under strictly anhydrous and anaerobic conditions can often seem like a dark art. Several common techniques that are suitable for air- and moisture-sensitive compounds are detailed below.

Slow Cooling

The solubility of most compounds generally decreases at lower temperatures and this can be exploited to grow single crystals.

Step 1: The compound is partially dissolved/suspended in the chosen crystallisation solvent at room temperature. The anhydrous solvent is added via filter cannula or by using a needle and syringe if finer control of solvent volume is needed.

Partially dissolving the compound in the chosen crystallisation solvent.

Step 2: The solvent is carefully heated using a heat gun (or heating mantle) until all of the solid material dissolves. It may be necessary to add more solvent if all of the compound does not dissolve at elevated temperature.

A saturated solution of the compound to be crystallised.

Alternatively, if the compound is particularly heat sensitive, or if a low boiling solvent (DCM, Et2O or pentane) is used, the minimum amount of solvent needed to fully dissolve the material is added at room temperature. It is also possible to filter cannula the solution when most (but not all) of the solids have dissolved, which ensures that the filtrate is a saturated solution and free of insoluble material.

Step 3: The Schlenk flask containing the saturated solution is sealed and disconnected from the Schlenk line. If the flask was heated to fully dissolve all of the compound, it should be allowed to gently cool to room temperature first before being sealed and disconnected from the Schlenk line.

Step 4: The Schlenk flask is placed in a fridge or freezer, away from vibrations and other forms of disruption.

Step 5: Wait…

Step 6: Maybe pray to the crystal Gods?

Step 7: Wait a little longer…

Step 8: After waiting for anywhere between a few hours and a few days (or weeks), single crystals should hopefully have grown in the Schlenk flask.

A Schlenk flask containing single crystals suitable for X-ray diffraction.

If crystals do not form, the solution may not be saturated and it may be necessary to partially concentrate the solution under vacuum and then return the Schlenk flask to the fridge or freezer. Alternatively, a few drops of anti-solvent (i.e. a miscible solvent that the compound does not dissolve in) can be added to decrease the solubility and initiate the crystallisation.

Liquid-Liquid Diffusion

Step 1: The compound is dissolved in the minimum quantity of the chosen crystallisation solvent. Ideally a dense solvent should be used, and the flask should be long and narrow to facilitate the layering process and ensure that the diffusion is slow.

Dissolving the compound in a dense crystallisation solvent inside a long and narrow flask.

Step 2: The anti-solvent (i.e. a miscible solvent that the compound does not dissolve in, and that is less dense than the solvent used to dissolve the compound) is carefully layered on top of the solution. Typically, two or three times as much volume of anti-solvent is used.

Layering the anti-solvent.

Step 3: The flask is carefully sealed and removed from the Schlenk line, and stored away from vibrations and other forms of disruption.

Liquid-liquid diffusion – after initial layering.

Step 4: As the anti-solvent slowly diffuses into the saturated solution containing the compound, crystals should hopefully begin to form at the liquid-liquid interface. The time taken for the two solvents to fully mix will vary depending on the volume of solvent and the size of the flask, but typically a few days up to a week will be needed until crystals begin to grow.

Liquid-liquid diffusion – after complete diffusion.

General Tips and Tricks for Growing Crystals

  • Be patient!
  • Experiment with the crystallisation solvent and conditions – often mixed solvent systems can work well.
  • Crystallographers do not like THF…
  • If the compound precipitates out too quickly, consider making the solution more dilute or storing the flask in a fridge instead of a freezer to slow down the crystallisation process.