- 00:00Overview
- 01:18Lewis Acid-Base Interactions in Ph3P-BH3
- 03:15Schlenk Line Set Up
- 03:54Synthesis of Borane Triphenylphosphine Complex
- 05:39Work Up, Isolation, and 31P-NMR
- 06:21Results
- 07:08Applications
- 08:37Summary
Interação Ácido-Base de Lewis em Ph3P-BH3
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Overview
Fonte: Tamara M. Powers, Departamento de Química da Texas A&M University
Um dos objetivos da química é utilizar modelos que respondam às tendências e forneçam insights sobre as propriedades dos reagentes que contribuem para a reatividade. As substâncias têm sido classificadas como ácidos e bases desde a época dos gregos antigos, mas a definição de ácidos e bases foi modificada e expandida ao longo dos anos. 1
Os gregos antigos caracterizariam substâncias por gosto, e definiram ácidos como aqueles que eram degustadores azedos, como suco de limão e vinagre. O termo “ácido” é derivado do termo latino para “degustação azeda”. As bases foram caracterizadas por sua capacidade de neutralizar ou neutralizar ácidos. As primeiras bases caracterizadas foram as de cinzas de um fogo, que foram misturadas com gorduras para fazer sabão. Na verdade, o termo “alcalino” é derivado da palavra árabe para “assar”. Na verdade, é sabido desde os tempos antigos que ácidos e bases podem ser combinados para dar um sal e água.
A primeira descrição amplamente utilizada de um ácido é a do químico sueco, Svante Arrhenius, que em 1894 definiu ácidos como substâncias que se dissociam na água para dar íons hidrônios, e bases como substâncias que se dissociam na água para dar íons hidróxidos. Essa definição limita-se, portanto, a ácidos aquosos e requer que um ácido contribua com um próton. 2 Por exemplo, na água, o HCl é um ácido, pois dissocia-se para dar o íon hidrônio (H3O)+ e o íon cloreto. O triclorito de boro não seria considerado um ácido, pois na água ele se hidrolisa para dar B(OH)3 e 3 HCl; o produto HCl embora é um ácido Arrhenius.
Em 1923, Johannes Nicolaus Brønsted e Martin Lowry definiram independentemente ácidos e bases em sua capacidade de doar e aceitar íons de hidrogênio, ou prótons. Assim veio o conceito de pares conjugados ácido-base, e a expansão da definição de ácidos e bases em solventes que não a água. Por exemplo, o amônio é um ácido, pois pode doar um próton e gerar amônia. Amônia pode aceitar um próton, para dar amônio. Assim, a amônia é a base conjugada do amônio. Esta reação ácido-base pode ocorrer em água, amônia ou outros solventes.
Este vídeo trata da definição ácido-base do químico americano, Gilbert N. Lewis, que também definiu ácidos e bases em 1923. Na verdade, este é o mesmo Lewis das estruturas de Lewis-dot em Química Geral. Sua abordagem não se concentra na capacidade de ácidos e bases para doar e aceitar prótons, mas sim em sua capacidade de aceitar e doar pares de elétrons, respectivamente. Isso abrange a definição de Brønsted-Lowry, como H+ aceita um par de elétrons de uma base brønsted durante a protonação. No entanto, expande muito a definição de um ácido, agora abrangendo íons metálicos e compostos do grupo principal. Aqui, comparamos o NMR de 31P do aduto de base ácida de Lewis Ph3P-BH3 com triphenylphosphine grátis.
Principles
Procedure
Results
Borane triphenylphosphine complex:
31P NMR (chloroform-d, 500 MHz, δ, ppm): 20.7 (broad doublet)
Triphenylphosphine:
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.
References
- 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.
Transcript
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!