Lewis Acid-Base Interaction in Ph₃P-BH₃

1. Setup of the Schlenk Line for the Synthesis of the Borane Triphenylphosphine Complex

NOTE: For a more detailed procedure, please review the "Schlenk Lines Transfer of Solvent" video in the Essentials of Organic Chemistry series). Schlenk line safety should be reviewed prior to conducting this experiment. Glassware should be inspected for star cracks before using. Care should be taken to ensure that O₂ is not condensed in the Schlenk line trap if using liquid N₂. At liquid N₂ temperature, O₂ condenses and is explosive in the presence of organic solvents. If it is suspected that O₂ has been condensed or a blue liquid is observed in the cold trap, leave the trap cold under dynamic vacuum. Do NOT remove the liquid N₂ trap or turn off the vacuum pump. Over time, the liquid O₂ will evaporate into the pump; it will only be safe to remove the liquid N₂ trap once all of the O₂ has evaporated.

1. Close the pressure release valve.
2. Turn on the N₂ gas and the vacuum pump.
3. As the Schlenk line vacuum reaches its minimum pressure, prepare the cold trap with either liquid N₂ or dry ice/acetone.
4. Assemble the cold trap.

2. Synthesis of Borane Triphenylphosphine Complex³

1. Add 5.3 g (20.3 mmol) of triphenylphosphine to a 250 mL Schlenk flask A and prepare the Schlenk flask for the cannula transfer of solvent.
2. Add 20 mL of dry/degassed THF to the Schlenk flask A via cannula transfer. Stir the solution to dissolve the triphenylphosphine.
3. Prepare a second Schlenk flask (B) containing 1.15 g (30.5 mmol) NaBH₄ for cannula transfer.
4. Cool both Schlenk flask A and B in an ice bath.
5. Cannula transfer the contents of Schlenk flask A into Schlenk flask B.
6. With positive N₂ pressure, replace the rubber septum on Schlenk flask B with an addition funnel fitted with a rubber septum.
7. To the addition funnel, add 8 mL of dry/degassed THF by cannula transfer.
8. With positive N₂ pressure, remove the septum from the top of the dropping funnel and add 2 mL of glacial acetic acid to the addition funnel.
9. Keeping Schlenk flask B in an ice bath, add the THF/glacial acetic acid dropwise over 30 min. During the addition, frothing might occur. Make sure that the reaction is stirring vigorously to minimize this.
10. After the addition, allow the reaction to warm to room temperature and stir for an additional hour.
11. Remove the dropping funnel and slowly add 20 mL of water.
12. Prepare a solution of 2 mL glacial acetic acid in 25 mL of water. Slowly add this mixture to the reaction.
13. If crystals do not spontaneously form, cool the reaction in an ice bath to promote crystallization.
14. Filter the product by suction through a fritted funnel. Wash the resulting solid with 20 mL of water 3 times.
15. Allow the product to dry in the hood before preparing the sample for NMR analysis.

3. ³¹P NMR Analysis of Borane Triphenylphosphine Complex

1. Prepare an NMR sample of triphenylphosphine and borane triphenylphosphine complex in CDCl₃.
2. Collect a ³¹P NMR of each sample (referenced to phosphoric acid) and observe how the phosphorous signal of triphenylphosphine shifts upon coordination to borane.