7 Heterogeneous Catalysis by Frustrated Lewis Pairs
253
H 2 -activation product being observed. However, in all cases, the deactivation pathway
was effectively suppressed using the solid-state methodology, allowing for greater
selectivity towards the desired H 2 -activation product.
The mechanism was explored with a suite of computational techniques, but it still
remains somewhat speculative. The hydrogen activation reaction must initially take
place at the interface between neighbouring particles of Lewis acid and Lewis base.
Molecular dynamics (MD) simulations show that these components are relatively
mobile at the surface, and can adopt molecular conformations akin to those typically
observed in solution. MD simulations also show that H 2 can diffuse through the lattice
of the crystals. The activation of hydrogen thus gives rise to localised [P–H]
+ and
[B–H]
– moieties at the surface of the particles, which somewhat resemble an ionic
liquid. The authors propose that it is the combination of the molecular flexibility at the
surface and the gas permeability through the lattice that leads to a “molten” system.
This allows the initially distinct Lewis acid and Lewis base particles to ultimately be
converted to a uniform salt of the activation product, instead of only observing salt
formation at the surface of the solids.
The authors subsequently exploited the properties of fluorous solvents to reduce
the need for such harsh reaction conditions (50 bar H 2 , 3–10 days reaction time).
Fluorous solvents have a high solubility towards non-polar gases such as H 2 , but
do not mix well with common organic solvents. The solid B(C 6 F 5 ) 3 and PCy 3 were
suspended in perfluoromethylcyclohexane (C 6 F 11 –CF 3 ) and stirred under 1.5 bar
H 2 , leading to 60% conversion to 22 in 10 h. Analogous experiments with PPhCy 2
and PPh
t
2 Bu as the Lewis base led to over 95% conversion to the respective H 2 -
activation products under the same conditions. The fluorous solvent thus enhances
the reactivity significantly, and allows the reactions to be carried out conveniently
under near-ambient conditions.
The product 22 was shown to be an effective reducing agent for converting
the bulky imine N-phenyl-1-(p-tolyl)ethan-1-imine to the corresponding secondary
amine. It is worth noting, however, that this reaction was carried out by combining the
imine with stoichiometric quantities of the isolated 22 in DCM. It would be interesting to see whether the solid-state methodology can be extended and employed
directly in heterogeneous catalysis, that is by reducing the substrate in the presence
of the solid FLP without the prior need for isolation and dissolution in a solvent.
7.3.3 Metal-Organic Frameworks (MOFs)
Metal-organic frameworks (MOFs) are porous materials consisting of metal ions
or fragments bridged by organic linkers, often based on carboxylate moieties. The
systems are easily tuneable by varying the identity of the metal centre or the size and
topicity of the organic ligand, and this has given rise to a huge number of crystalline
structures containing pores of different shapes and sizes [68]. Initial applications
focused on the storage of fuels such as H 2 and CH 4 and the capture of CO 2 , but more
recent work has explored their utility in catalysis, gas separation and biomedical
253
H 2 -activation product being observed. However, in all cases, the deactivation pathway
was effectively suppressed using the solid-state methodology, allowing for greater
selectivity towards the desired H 2 -activation product.
The mechanism was explored with a suite of computational techniques, but it still
remains somewhat speculative. The hydrogen activation reaction must initially take
place at the interface between neighbouring particles of Lewis acid and Lewis base.
Molecular dynamics (MD) simulations show that these components are relatively
mobile at the surface, and can adopt molecular conformations akin to those typically
observed in solution. MD simulations also show that H 2 can diffuse through the lattice
of the crystals. The activation of hydrogen thus gives rise to localised [P–H]
+ and
[B–H]
– moieties at the surface of the particles, which somewhat resemble an ionic
liquid. The authors propose that it is the combination of the molecular flexibility at the
surface and the gas permeability through the lattice that leads to a “molten” system.
This allows the initially distinct Lewis acid and Lewis base particles to ultimately be
converted to a uniform salt of the activation product, instead of only observing salt
formation at the surface of the solids.
The authors subsequently exploited the properties of fluorous solvents to reduce
the need for such harsh reaction conditions (50 bar H 2 , 3–10 days reaction time).
Fluorous solvents have a high solubility towards non-polar gases such as H 2 , but
do not mix well with common organic solvents. The solid B(C 6 F 5 ) 3 and PCy 3 were
suspended in perfluoromethylcyclohexane (C 6 F 11 –CF 3 ) and stirred under 1.5 bar
H 2 , leading to 60% conversion to 22 in 10 h. Analogous experiments with PPhCy 2
and PPh
t
2 Bu as the Lewis base led to over 95% conversion to the respective H 2 -
activation products under the same conditions. The fluorous solvent thus enhances
the reactivity significantly, and allows the reactions to be carried out conveniently
under near-ambient conditions.
The product 22 was shown to be an effective reducing agent for converting
the bulky imine N-phenyl-1-(p-tolyl)ethan-1-imine to the corresponding secondary
amine. It is worth noting, however, that this reaction was carried out by combining the
imine with stoichiometric quantities of the isolated 22 in DCM. It would be interesting to see whether the solid-state methodology can be extended and employed
directly in heterogeneous catalysis, that is by reducing the substrate in the presence
of the solid FLP without the prior need for isolation and dissolution in a solvent.
7.3.3 Metal-Organic Frameworks (MOFs)
Metal-organic frameworks (MOFs) are porous materials consisting of metal ions
or fragments bridged by organic linkers, often based on carboxylate moieties. The
systems are easily tuneable by varying the identity of the metal centre or the size and
topicity of the organic ligand, and this has given rise to a huge number of crystalline
structures containing pores of different shapes and sizes [68]. Initial applications
focused on the storage of fuels such as H 2 and CH 4 and the capture of CO 2 , but more
recent work has explored their utility in catalysis, gas separation and biomedical