A look into frustrated lewis pairs

1. Advanced Inorganic, Physical and Materials Chemistry
Coursework Assignment Task 2. Frustrated Lewis Pairs
By Molly Winterbottom
Table of Contents
Questions .........................................................................................................................1
Explain what is meant by the term 'Frustrated Lewis Pair' compound........................................1
Discuss the key chemical and structural requirements that are necessary to produce a
Frustrated Lewis Pair compound. .............................................................................................2
What are the potential applications of Frustrated Lewis Pairs? Discuss the potential advantages
and any disadvantages or challenges of using Frustrated Lewis Pairs over alternative
compounds..............................................................................................................................3
My own design of a Frustrated Lewis Pair. ........................................................................4
References........................................................................................................................5
Questions
Explain what is meant by the term 'Frustrated Lewis Pair' compound.
A ‘Frustrated Lewis Pair’, or FLP, compound is a combination of a Lewis Base and
Lewis Acid that hasn’t formed the expected Lewis acid-bases adduct, instead forms
an unstable compound, therefore termed ‘frustrated’ mainly due to the compound
being sterically hindered, stopping the Lewis acid and Lewis base from reacting in the
way it is expected.1
Figure 1. A simple model of a LB (Lewis Base) and LA (Lewis Acid) forming a FLP (Frustrated Lewis
pair) – R representing the rest of the molecules bulky groups.

2. Discuss the key chemical and structural requirements that are necessary to produce a
Frustrated Lewis Pair compound.
In order to state whether a compound is a FLP it must exhibit unique properties such
as; Steric hinderance, highly reactive, activated by hydrogen2
and be mainly Non-
Metal-Based.3
Due to steric bulkiness of the Lewis acid and Lewis base, they can be
activated by hydrogen, this is done by the transfer of electrons from the HOMO (Lewis
base) to the LUMO (Lewis Acid) this can be seen in Scheme 1.
Scheme 1. Reaction mechanism of how FLPs are activated by hydrogen.
The most obvious chemical requirements a FLP compound must contain is a Lewis
acid and a Lewis base which will need to have sufficient reactivity and strength that
would form a Lewis acid-base adduct in normal circumstances.
Structural requirements such as steric hinderance are one of the key characteristics
of a FLP as they are known for being classed as too bulky on the acidic and basic sites
causing kinetic blocking, hence, forming a FLP compound unable to react and
neutralise each other. This steric hinderance can only be to an extent to stop the Lewis
acid and Lewis base from reacting with each other, it shouldn’t be too hindered to
prevent them from reacting with other substrates.4
Considering the elements that could be potentially used in a FLP, the Lewis acid
component could typically be made from boron as this centre would act as the
electron-deficient acidic site. When looking at the Lewis Bases the substituent most
commonly used would be a bulky alkyl amine group as the lone pair on the nitrogen
would act as the electron-rich basic site.5
Considering all these key chemical and structural requirements when trying to produce
a Frustrated Lewis Pair, many companies would have to do extensive preparation
such as employing spectroscopic techniques like NMR to be used as an analytical
technique to provide evidence confirming the lack of adduct formation and the
successful synthesis of a FLP.6

3. What are the potential applications of Frustrated Lewis Pairs? Discuss the potential
advantages and any disadvantages or challenges of using Frustrated Lewis Pairs over
alternative compounds.
Potential applications of frustrated Lewis pairs can include reactions such as;
Hydrogenation, CO2 reduction, Small Molecule activation, and polymerisation.7
Due to FLP’s being metal-free, acts an advantage when it comes to these applications
as it can avoid costly fees on resources such as gold if a metal centre was used, and
it can avoid any metal contaminants in any of the polymers during a polymerisation
reaction. This use of a non-metal centre also gives way for extended research into
new reactivity compared to traditional chemistry as the FLP’s will react in unique ways
and often in mild conditions.8
As an example of this advantage seen in FLPs, Figure 2 shows a graph of the price of
Rhodium over the past 40 years considering inflation, showing that it is more
expensive to go with alternatives like transition metal complexes over non-metal FLPs.
Figure 2. Graph showing the cost of Rhodium over 40 years9
Other potential applications include drug delivery and materials science as the
frustrated Lewis Pair features give a possibility it will be feasible to use for reversible
drug binding and drug release,10
advancing our medical application and distribution of
drugs.
Some disadvantages associated with working on Frustrated Lewis Pairs is mainly
surrounded by the synthesis of complex compounds, this includes design, side
reactions the compound could make, and the possibility of the FLP deactivating.11
These challenges with FLP can impact the way that they are used in the world as it
requires the right combination of acid, base and the steric groups to make the pair

4. frustrated, therefore making the synthesis difficult and most likely time consuming. A
challenge that will arise from these difficult combinations would be the attempt to
characterise the FLP due to its unusual structure, scientists are finding it hard to fully
analyse the compounds.
Another challenge scientists face when synthesising frustrated Lewis pair compounds
is their hydrophobic nature to air and water, making it hard to create as inert conditions
are needed to prevent the compound from reacting with water and moisture.12
A less considered challenge or disadvantage that has to be taken into consideration
is the preparation work needed to be performed before the synthesis even begins on
a FLP. Protocols need to be proposed due to its hydrophobic nature, kinetic and
mechanistic studies need to be performed, 13
along with evaluation of potential
combinations for the frustrated Lewis pair through computational modelling which will
help to predict efficiency and feasibility of the potential FLP compound.14
When comparing Frustrated Lewis pair compounds to alternative complexes for
applications like catalysis and small molecule activation, it is favoured to work more
on and develop FLPs as they can avoid costly fees unlike transition metal complexes
and open up to new ways of reactions within chemistry.15
My own design of a Frustrated Lewis Pair.
Figure 3. Lewis Acid (Tris(pentafluorophenyl)borane) seen on the left and Lewis Base (Phosphorous
Trichloride) seen on the right.
As my Lewis acid I would choose a very sterically hindered and bulky borane
compound like Tris(pentafluorophenyl)borane, although looking simple in its skeletal
form, its bulky fluorinated phenyl groups could act as a steric hinderance to prevent
the Lewis acid from bonding with the Lewis base. The Lewis base I have chosen in
this situation is Phosphorous Trichloride, as it also displays steric hinderance.

5. As many frustrated Lewis pairs can be used for hydrogen activation, I have chosen to
use H2 to cleave both these compounds. This would result in the compound formed in
Figure 4.
Figure 4. Frustrated Lewis Pair product formed from Tris(pentafluorophenyl)borane and Phosphorous
Trichloride.
References
1
D.W. Stephan, Journal of the American Chemical Society, Frustrated Lewis Pairs, 137, 32, 10018-10032, 2015
2
A.Parra, A.A. Danopoulos and P. Braunstein, Frustrated Lewis Pairs for the activation of hydrogen and carbon
dioxide, Nat. Catal., 2021, 4, 59-68
3
D.W. Stephan and G, Erker, RSC Publishing Chemical Science, Frustrated Lewis Pair Chemistry of Carbon,
Nitrogen and Sulfur Oxides, 0, 2012
4
D. W. Stephan and G. Erker, Angew Chem Int Ed, 2010, 49, 46-76
5
J. Clayden, N. Greeves and S. Warren, Organic Chemistry, Oxford University Press, 2, 2012, Chapter 41, page
1352
6
M.J. Ryan, L.F. Wan and M.M. Mahoney, High-throughput Screening of Frustrated Lewis Pairs and Classical
Lewis Acids for the Hydroboration of CO2, J.Am. Chem. Soc., 2018, 140, 13988-13994
7
G.Erker, Frustrated Lewis Pairs: applications, in Frustrated Lewis Pairs: From Concept to catalysis, ed.
G.Erker, Wiley, 2017, ch. 9, pp. 185-236
8
Y.Wang, G.H. Robinson and B.S. Bassil, Main group compounds as hydrogenation and hydrogen transfer
catalysts, Chem. Soc. Rev. 2021, 50, 4113-4151
9
J. Wen, F. Wang, X. Zhang, Asymmetric Hydrogenation catalyzed by first-row transition metal complexes,
Chemical Society Reviews, Royal Society of Chemistry, 2021, 5.
10
G.C. Welch, Frustrated Lewis Pairs: from concept to practice, Chem. Eur. J., 2017, 23, 12126-12140
11
P.Spies, G. Erker, G. Kehr and K. Bergander, Frustrated Lewis Pairs: from conventional Lewis acid-base
chemistry to catalytic hydrogenations at low temperatures, Chem. Soc. Rev, 2011, 40, 5072-5082
12
M. Sajid, M.Klose and L. Brammer, Frustrated Lewis pairs: developments and perspectives, Chem. Rev.,
2021, 121, 960-1041
13
K. Fong and G. Kehr, The future of FLP chemistry, presentation at ACS spring 2022 Meeting, Solvay
speciality Polymers.
14
J. Michl, T. Smejkal and B. Stepnicka, Fine-tuning of reactivity in individual components of frustrated Lewis
pairs and its impact on small-molecule activation, Angew. Chem. Int. Ed., 2019, 58, 17080-17097
15
M.Oestreich, The Emergence of Frustrated Lewis Pairs: New Reactivity Patterns for Small Molecule
Activation and Hydrogenation Catalysis, Philos. Trans. R. Soc. A, 2017, 375, 20170063.