Tris(trimethylsilyl)phosphine, P(SiMe₃)₃, is a versatile reagent used to prepare a number of metal clusters and low-coordinate phosphorus compounds such as phosphaalkynes. It is used as an ‘easy to handle’ alternative to phosphine gas, and is covalent synthon for the P³⁻ anion.
P(SiMe₃)₃ is a pyrophoric, air- and moisture-sensitive, oily liquid that hydrolyses to produce toxic PH₃ gas. The experiment detailed below should only be performed by a trained professional and requires close supervision. Appropriate fire safety precautions must be in place and a risk assessment should be completed prior to the experiment. The synthesis of P(SiMe₃)₃ takes up to one week and uses specialist equipment, therefore sufficient planning is required to ensure that the experiment goes to plan.
All equipment used in the preparation and manipulation of P(SiMe₃)₃ must be carefully quenched in a well ventilated fume-hood under a flow of inert gas with dilute solutions of ethanol in hexane. Neat ethanol is added until no further reaction is observed and the glassware/equipment is then washed with water and bleach.
Experimental Set-Up: An oven-dried three-necked round bottom flask is equipped with an overhead mechanical stirrer and water-free air condenser fitted with a gas inlet adapter. Ensure that all equipment is securely clamped, and that joints are air-tight and sealed. It is strongly advised to perform the reaction in a clean fume-hood free of solvent bottles and chemical storage, and lined with a large shallow box filled with sand.
Step 1: Red phosphorus (25 g, 800 mmol, 1 equiv.) and naphthalene (5 g, 40 mmol, 0.05 equiv.) are added to the flask and the system is gently flushed with inert gas for 1-2 hours. Note: The red phosphorus is dried overnight under high-vacuum prior to use.
Step 2: Anhydrous DME (1.3 L) is added via cannula transfer.
Step 3: The red suspension is heated to 40 ºC and sodium (55 g, 2400 mmol, 3 equiv.) is added slowly over 2-3 hours. After addition, the suspension is heated to reflux (100 ºC) overnight (16+ hours). Note: The sodium chunks are cut with a scalpel or kitchen knife into small cubes to remove oxide coating and washed with hexane to remove residual mineral oil. The sodium chunks are added to the reaction flask with tweezers. Ensure that all glassware and utensils contaminated with sodium are safely quenched – see Schlenk Line Safety for more information.
Step 4: The dark red suspension is cooled to room temperature and a large pressure-equalising dropping funnel is added to the reaction flask (this can be cycled onto the Schlenk line prior to use to ensure that it is dry). Me₃SiCl (305 mL, 2400 mmol, 3 equiv.) is transferred to the dropping funnel (either by cannula transfer from an ampoule or via syringe), and added drop-wise to the reaction mixture slowly at room temperature.
Step 5: The colourless suspension is heated to reflux (100 ºC) overnight (16+ hours).
Step 6: The colourless suspension is filterred through Celite. The transfer flask and solids in the filter frit are washed thoroughly with DME (0.5 L) in portions. Note: The Celite is stored in an oven for at least 24 hours prior to use to ensure that it is dry. A wide bore (10 mm) PTFE cannula is used to transfer the suspension into the filter frit. The condenser has been replaced with a gas inlet adapter, and the overhead stirrer motor has been removed to allow the transfer flask to be manipulated more easily.
Step 7: After filtration, a colourless or pale yellow solution is obtained. The volatiles are removed in vacuo. Note: It may be necessary to periodically empty or change the external trap due to the large volume of solvent to be removed.
Step 8: The crude product is purified by dynamic vacuum distillation (100 ºC at 0.1-0.01 mbar). Note: The flask is initially heated at 45 ºC for 15-20 minutes without the liquid nitrogen trap surrounding the receiving flask to remove residual Me₃SiCl, DME and naphthalene.
Step 9: The product is allowed to thaw and warm to room temperature, and then transferred to sealed ampoules (via cannula transfer) to be stored under an inert atmosphere in a fridge or freezer. Theoretical yield = 200 g.
Special thanks to Dr. Clément Millet for providing a detailed step-by-step procedure from which this guide is based.
For more information, watch my ‘How to’ lecture from the Online Workshop in Phosphorus Chemistry (March 2021).