6: 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.

In chemistry, acid-base models are used to explain trends in reactivity and characteristics of reactants, which is important when designing a synthesis.

In 1894, Svante Arrhenius pioneered the concept of acids and bases, describing them specifically as substances that dissociate in water, to yield hydronium or hydroxide ions, respectively.

In 1923, Johannes Br?nsted and Thomas Lowry defined acids and bases by their ability to donate and accept hydrogen ions in different solvents, creating the concept of acid-base conjugate pairs.

In the same year Gilbert Lewis proposed an alternative, defining acids and bases by their abilities to donate and accept electron pairs, instead of protons. This model expanded the application of acids and bases, taking into account metal ions and main-group compounds.

This video will illustrate the Lewis acid-base concept on the basis of a triphenylphosphine borane complex, its synthesis, and analysis.

When using the Lewis Acid-and Base model, the molecular structure needs to be considered to identify whether the molecule will donate or accept an electron pair.

Therefore, start with the structure analysis of triphenylphosphine and borane using the VSEPR theory, and then determine the Lewis acid and base.

Triphenylphosphine has three covalent bonds between the phosphorous atom and a carbon in each of the three phenyl rings. Two free electrons are left as a free electron pair to fill the octet.

Furthermore, triphenylphosphine is sp3?hybridized at the phosphorous center and has a tetrahedral electronic?geometry. The lone-pair of electrons residing in an sp3 orbital can be donated to another molecule, classifying triphenylphosphine as a Lewis base.

On the other hand, borane has three covalent bonds between the boron and the three hydrogen atoms. Since the borane center has only six valence electrons it does not fulfill the octet rule and is therefore electron-deficient.

The geometry is trigonal planar and the bonds are sp2?hybridized. The lone p orbital is empty, and ready to accept electrons, which classifies borane as a Lewis acid.

If triphenylphosphine donates its two electrons to the empty p orbital in borane, it leads to a change of the hybridization from sp2 to sp3 and one can propose that a stable Lewis acid-base adduct will form.

This type of bond between a Lewis-acid and base is often called a coordinative covalent, or a dative bond, which is indicated using an arrow.

Now that you've learned the principles of Lewis acids-and bases, let's investigate whether a stable adduct will form between triphenylphosphine and borane.

Before you start, make sure you are familiar with the Schlenk Line and how to use it for solvent transfer. Wear appropriate PPE and inspect all glassware for star cracks.

Close the pressure release valve, turn on the N2 and vacuum pump. Assemble the cold trap and fill it with dry ice/acetone, once minimum pressure is reached. This way you minimize the risk of O2 condensation in the trap, which is explosive in presence of organic solvents.

Now, let's start the synthesis by adding 5.3 g of triphenylphosphine to a 200 mL Schlenk flask labeled as A. Prepare Schlenk flask A for the cannula transfer of solvent.

Add 20 mL of dry and degassed THF to Schlenk flask A using cannula transfer. Stir the solution to dissolve triphenylphosphine. Meanwhile, prepare a second Schlenk flask B containing 1.15 g of NaBH4?for cannula transfer.

Cool both Schlenk flasks A and B in an ice bath. Using the cannula, transfer the contents of flask A into flask B. Next, replace the rubber septum of Schlenk B with an addition funnel, purge the funnel, and fit it with a new septum.

Next, add 8 mL of dry and degassed THF to the addition funnel via cannula transfer. With the system under N2, remove the septum from the addition funnel, add 2 mL of glacial acetic acid, and put the septum back on. Now, add the THF and glacial acid mixture drop wise to Schlenk flask B, while stirring vigorously.

After the addition, allow the reaction to warm up to room temperature and stir for an extra hour under N2. Then close the N2 supply, remove the addition funnel, and quench the reaction slowly with 20 mL of H2O.

Next, add a mixture of acetic acid in water slowly to the reaction, inducing product precipitation. Cool the flask, if no precipitate forms.

Filter the product by suction through a fritted funnel. Wash the resulting solid with 20 mL of ice cold water, and transfer the precipitate to a flask for drying.

Lastly, prepare an NMR sample of the starting material and the isolated product in CDCl3. Collect a 31P NMR for each sample.

Now let's analyze how the phosphorous signal of triphenylphosphine is affected upon the coordination to borane in the product using the NMR.

Free triphenylphosphine shows as signal at -5.43 ppm, while the signal of the borane triphenylphosphine complex is shifted downfield to 20.7 ppm. This is consistent with the removal of electron density from the phosphorous center, which is deshielded upon Lewis adduct formation.

This observation reinforces the Lewis acid-base theory predicting that borane, as a Lewis acid, and triphenylphosphine, as a Lewis base, will form a stable adduct.

The Lewis acid-base model is used to gain more insight into molecular characteristics, which is necessary when designing new syntheses in organic and inorganic chemistry for molecules including transition metals.

Historically, transition metal ions have been regarded as Lewis acids, however, they can also serve as Lewis bases. For example, metal-borane complexes can participate in important transformations such as hydrogenation of olefins and nitrogen fixation.

Olefin hydrogenation can be performed using a new catalyst based on a nickel borane species. This species cleaves the H-H bond heterolytically and reversibly adds the H2 to the olefin transforming it to an alkane.

Furthermore, an iron-borane complex homogeneous catalyst can catalytically reduce nitrogen to ammonia, which is a critical reaction in the chemical industry.

Frustrated Lewis Pairs, or FLPs, are Lewis acid-base adducts, which cannot form a dative bond, due to steric hindrance.

The reactivity of frustrated Lewis pairs has found application in the development of new hydrogenation catalysts. For instance, it was shown that a zwitterionic complex, which is based on main group elements, reversibly loses H2 to give this product. This study pioneered the development of FLP research.

You've just watched JoVE's introduction to the Lewis acid-base theory. You should now understand the definition of Lewis acids- and bases, how to synthesize a Lewis acid-base complex, and where these types of complexes are applied. Thanks for watching!