Source: Tamara M. Powers, Department of Chemistry, Texas A&M University
One of the goals of chemistry is to use models that account for trends and provide insights into the properties of reactants that contribute to reactivity. Substances have been classified as acids and bases since the time of the ancient Greeks, but the definition of acids and bases has been modified and expanded over the years.1
The ancient Greeks would characterize substances by taste, and defined acids as those that were sour-tasting, such as lemon juice and vinegar. The term "acid" is derived from the Latin term for "sour-tasting." Bases were characterized by their ability to counteract or neutralize acids. The first bases characterized were those of ashes from a fire, which were mixed with fats to make soap. In fact, the term "alkaline" is derived from the Arabic word for "roasting." Indeed, it has been known since ancient times that acids and bases can be combined to give a salt and water.
The first widely-used description of an acid is that of the Swedish chemist, Svante Arrhenius, who in 1894 defined acids as substances which dissociate in water to give hydronium ions, and bases as substances which dissociate in water to give hydroxide ions. This definition is thus limited to aqueous acids and necessitates that an acid contribute a proton.2 For example, in water, HCl is an acid, as it dissociates to give the hydronium ion (H3O)+ and the chloride ion. Boron trichloride would not be considered an acid, as in water it hydrolyzes to give B(OH)3 and 3 HCl; the product HCl though is an Arrhenius acid.
In 1923, Johannes Nicolaus Brønsted and Martin Lowry independently defined acids and bases on their ability to donate and accept hydrogen ions, or protons. Thus came the concept of acid-base conjugate pairs, and the expansion of the definition of acids and bases in solvents other than water. For example, ammonium is an acid, as it can donate a proton and generate ammonia. Ammonia can accept a proton, to give ammonium. Thus, ammonia is the conjugate base of ammonium. This acid-base reaction can occur in water, ammonia, or other solvents.
This video deals with the acid-base definition of the American chemist, Gilbert N. Lewis, who also defined acids and bases in 1923. Indeed, this is the same Lewis from Lewis-dot structures in General Chemistry. His approach focuses not on the ability of acids and bases to donate and accept protons, but rather on their ability to accept and donate electron pairs, respectively. This encompasses the Brønsted-Lowry definition, as H+ accepts an electron pair from a Brønsted base during protonation. However, it greatly expands the definition of an acid, now encompassing metal ions and main-group compounds. Here, we compare the 31P NMR of the Lewis acid-base adduct Ph3P-BH3 to free triphenylphosphine.
Consider the bonding between triphenylphosphine and borane. We will first consider what both molecules look like before they form a Lewis adduct.
Recall Lewis dot structures, and valence shell electron-pair repulsion (VSEPR) theory, from general chemistry. The Lewis dot structure of triphenylphosphine is shown in Figure 1. There are three covalent bonds between the phosphorous atom and one of the carbon atoms in each of the three phenyl rings. Two electrons (a lone-pair) reside on the phosphorous atom to complete the octet. The phosphorous center is sp3 hybridized and has a tetrahedral electronic geometry, with the lone-pair of electrons residing in an sp3 orbital. Triphenylphosphine has a lone-pair that can be donated to another molecule and is therefore classified as a Lewis base.
Figure 1. Lewis dot structure of triphenylphosphine.
The Lewis dot structure of borane is shown in Figure 2. There are three covalent bonds between the boron atom and the hydrogen atoms. In this case, the boron center only has six valence electrons and therefore does not follow the 8 e- rule. Borane is thus planar and sp2 hybridized, with the sp2 orbitals forming bonds to the hydrogen atoms and the lone p orbital being empty. Borane is thus a Lewis acid.
Figure 2. Lewis dot structure of borane.
Since the phosphorous in triphenylphosphine has a filled orbital and the boron of borane has an empty orbital, a Lewis acid-base adduct can form, with the triphenylphosphine donating its two electrons to boron. Upon adduct formation, the boron center becomes sp3 hybridized (Equation 1).
Equation 1 demonstrates the idea of Lewis acidity, with Lewis acids accepting lone-pairs of electrons, and Lewis bases donating pairs of electrons. Sometimes, Lewis acids are referred to as electrophiles, and Lewis bases as nucleophiles. Bonds between Lewis acids and bases are often called coordinate covalent or dative bonds, and sometimes are designated with arrows as opposed to lines.
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.
- Close the pressure release valve.
- Turn on the N2 gas and the vacuum pump.
- As the Schlenk line vacuum reaches its minimum pressure, prepare the cold trap with either liquid N2 or dry ice/acetone.
- Assemble the cold trap.
2. Synthesis of Borane Triphenylphosphine Complex3
- 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.
- Add 20 mL of dry/degassed THF to the Schlenk flask A via cannula transfer. Stir the solution to dissolve the triphenylphosphine.
- Prepare a second Schlenk flask (B) containing 1.15 g (30.5 mmol) NaBH4 for cannula transfer.
- Cool both Schlenk flask A and B in an ice bath.
- Cannula transfer the contents of Schlenk flask A into Schlenk flask B.
- With positive N2 pressure, replace the rubber septum on Schlenk flask B with an addition funnel fitted with a rubber septum.
- To the addition funnel, add 8 mL of dry/degassed THF by cannula transfer.
- 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.
- 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.
- After the addition, allow the reaction to warm to room temperature and stir for an additional hour.
- Remove the dropping funnel and slowly add 20 mL of water.
- Prepare a solution of 2 mL glacial acetic acid in 25 mL of water. Slowly add this mixture to the reaction.
- If crystals do not spontaneously form, cool the reaction in an ice bath to promote crystallization.
- Filter the product by suction through a fritted funnel. Wash the resulting solid with 20 mL of water 3 times.
- Allow the product to dry in the hood before preparing the sample for NMR analysis.
3. 31P NMR Analysis of Borane Triphenylphosphine Complex
- Prepare an NMR sample of triphenylphosphine and borane triphenylphosphine complex in CDCl3.
- 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 Brønsted 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!
Borane triphenylphosphine complex:
31P NMR (chloroform-d, 500 MHz, δ, ppm): 20.7 (broad doublet)
31P NMR (chloroform-d, 500 MHz, δ, ppm): -5.43
The 31P NMR signal of the borane triphenylphosphine complex is downfield relative to free triphenylphosphine. This is consistent with removal of electron density from the phosphorous center, which is deshielded upon adduct formation.
Applications and Summary
The borane triphenylphosphine complex is an example of a Lewis-adduct, whereby a Lewis base donates electrons to a Lewis acid. Though BH3 and PPh3 would not necessarily be considered an acid and base, respectively, using other acid-base theories, Lewis acid-base theory predicts correctly that the molecules form a stable adduct.
Small Molecule Activation:
While transition metal ions have historically been regarded as Lewis acids, the notion that they can serve as Lewis bases is being advanced. For example, Jonas Peters and co-workers at Caltech have shown that metal-borane complexes, which can donate electrons to the Lewis acid borane (a Z-type ligand), can give rise to novel reactivity. A nickel borane species was shown to reversibly add H2, heterolytically cleaving the H-H bond.4 The H2-added species is a catalyst for hydrogenations of olefins. The group also reported that iron-borane complexes can catalytically reduce nitrogen to ammonia.5 This was the first example of an iron-based homogeneous catalyst for this challenging yet critical reaction.
Frustrated Lewis Pairs:
Another current area of research is that of "Frustrated Lewis Pairs," or FLPs. These are Lewis acid-base "adducts" that due to steric reasons, cannot form a dative bond.6 Douglas Stephan and co-workers from the University of Toronto pondered what reactivity such adducts would have, particularly with the idea of using them for small molecule activation and catalysis. Thinking about transition metal complexes, which can both accept and donate electron density to and from substrates, they hypothesized donor/acceptor properties of what they termed "Frustrated Lewis Pairs" might have with regards to reactivity.
In 2006, Stephan and co-workers reported in Science that the zwitterionic (C6H2Me3)2PH(C6F4)BH(C6F5)2 reversibly loses H2 to give (C6H2Me3)2P(C6F4)B(C6F5)2.7 This was the first example of reversible H2 activation with main group elements, and other examples followed (Figure 3). This study paved the way for the development of FLP research. Since then, FLPs have been developed that are competent hydrogenation catalysts, and can activate a variety of small molecules including CO2. This is an active and exciting new area of research.
Figure 3. Early examples of reactivity of FLPs with H2. Adapted from reference 5.
- Lesney, Today's Chemist at Work, 2003, 47-48.
- Miessler, P. J. Fischer and D. A. Tarr, Inorganic Chemistry, Pearson, 2014.
- McNulty, J.; Zhou, Y. Tetrahedron Letters, 2004, 45, 407-409.
- Harman and J. C. Peters, J. Am. Chem. Soc., 2012, 134, 5080-5082.
- Anderson, J. Rittle and J. C. Peters, Nature, 2013, 501, 84-87.
- Stephan, J. Am. Chem. Soc., 2015, 137, 10018-10032.
- Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124-1126.