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M. Heshmat et al.
EDA and FMO analysis of the TSs of H 2 activation by FLPs indicated that both
electrostatic interactions and orbital interactions are essential to compensate the Pauli
repulsions and form a stable TS molecular complex. Moreover, it turned out that a
combination of HOMO [LB + H 2 ] interacting with LUMO [BCF] and LUMO [H 2
+ BCF] interacting with HOMO [LB] is more favorable (energetically) than pure
occupied σ and empty σ* MOs of H 2 interacting with the HOMO and LUMO of
FLP. DFT-based metadynamics simulations demonstrated that when H 2 is far from
the LA/LB centers, the H–H bond is polarized mainly through the electric field
created by the FLP. However, when H 2 is closer than 2.5 Å to the LA/LB centers, the
electron transfer between FMOs of H 2 and FLP dominates. Splitting the H 2 molecule
by an activated carbonyl carbon as a Lewis acidic center has been examined with
TS calculations and it was indicated that the solvent-assisted hydride-type attack of
the polarized H 2 molecule on the activated carbonyl carbon atom can be considered
as an alternative reaction route. Activated (polarized) carbonyl carbon is formed
by complexation of the C = O group to a Lewis or Brønsted acid or through the
formation of H-bond with an H-bond donor in solution.
It is computationally demonstrated that considering water as an active Lewis base
in the FLP mechanism in wet ethereal solutions produces a strongly stabilized borohydride/hydronium intermediate in the hydrogenation mechanism of ketones. The
minimum energy path of H 2 splitting via this borohydride/hydronium intermediate
changes to exergonic in comparison to the routes via a borohydride/oxonium or
borohydride/oxocarbenium intermediates, which are highly endergonic.
Various aspects of dynamical behavior of FLP + H 2 system, by employing
AIMD simulations, were investigated. For example, the flexibility of the [solventH
(+) -ketone][BCF-H
(−) ] intermediate ion-pair and large amplitude motions of the
cationic/anionic fragments with respect to each other is a factor that can change the
pathway of H 2 activation followed by hydrogenation of the C = O group. Overall,
AIMD simulations shed more light on some mechanistic aspects of the H 2 activation and hydrogenation reactions that remained unnoticed in static-DFT calculations.
For example, the multi-molecular nature of the solvent-involved hydrogenation of
ketones in ethereal solutions was observed through AIMD simulations. Presently,
most of the FLP catalysis is performed in the solution phase, and most often in organic
solvents. This can cause environmental issues due to the volatility of the aromatic
organic solvents. Recent experimental support for the generation of higher concentrations of encounter complexes of the FLPs in ionic liquids can lead to potential
employment of FLP catalysis in solution with the advantage of using green solvents.
On the other hand, transfer of the homogenous catalytic systems into heterogeneous
states would overcome the drawbacks in catalyst-product separation, recyclability,
and stability. As an example, grafted Lewis pair centers into MOF structures and solid
surface materials such as carbon nanotubes may indeed become “game-changers” in
the field of FLP-catalysis, and overcoming the gaps and drawbacks of homogeneous
FLPs. This means that grafting these catalysts to heterogeneous supports enhances
catalyst stability and product-catalyst separation and recyclability and adapts such
molecular catalysts for commercial processes. This new paradigm offers enormous
potential for further developments in the field of FLP catalysis.
M. Heshmat et al.
EDA and FMO analysis of the TSs of H 2 activation by FLPs indicated that both
electrostatic interactions and orbital interactions are essential to compensate the Pauli
repulsions and form a stable TS molecular complex. Moreover, it turned out that a
combination of HOMO [LB + H 2 ] interacting with LUMO [BCF] and LUMO [H 2
+ BCF] interacting with HOMO [LB] is more favorable (energetically) than pure
occupied σ and empty σ* MOs of H 2 interacting with the HOMO and LUMO of
FLP. DFT-based metadynamics simulations demonstrated that when H 2 is far from
the LA/LB centers, the H–H bond is polarized mainly through the electric field
created by the FLP. However, when H 2 is closer than 2.5 Å to the LA/LB centers, the
electron transfer between FMOs of H 2 and FLP dominates. Splitting the H 2 molecule
by an activated carbonyl carbon as a Lewis acidic center has been examined with
TS calculations and it was indicated that the solvent-assisted hydride-type attack of
the polarized H 2 molecule on the activated carbonyl carbon atom can be considered
as an alternative reaction route. Activated (polarized) carbonyl carbon is formed
by complexation of the C = O group to a Lewis or Brønsted acid or through the
formation of H-bond with an H-bond donor in solution.
It is computationally demonstrated that considering water as an active Lewis base
in the FLP mechanism in wet ethereal solutions produces a strongly stabilized borohydride/hydronium intermediate in the hydrogenation mechanism of ketones. The
minimum energy path of H 2 splitting via this borohydride/hydronium intermediate
changes to exergonic in comparison to the routes via a borohydride/oxonium or
borohydride/oxocarbenium intermediates, which are highly endergonic.
Various aspects of dynamical behavior of FLP + H 2 system, by employing
AIMD simulations, were investigated. For example, the flexibility of the [solventH
(+) -ketone][BCF-H
(−) ] intermediate ion-pair and large amplitude motions of the
cationic/anionic fragments with respect to each other is a factor that can change the
pathway of H 2 activation followed by hydrogenation of the C = O group. Overall,
AIMD simulations shed more light on some mechanistic aspects of the H 2 activation and hydrogenation reactions that remained unnoticed in static-DFT calculations.
For example, the multi-molecular nature of the solvent-involved hydrogenation of
ketones in ethereal solutions was observed through AIMD simulations. Presently,
most of the FLP catalysis is performed in the solution phase, and most often in organic
solvents. This can cause environmental issues due to the volatility of the aromatic
organic solvents. Recent experimental support for the generation of higher concentrations of encounter complexes of the FLPs in ionic liquids can lead to potential
employment of FLP catalysis in solution with the advantage of using green solvents.
On the other hand, transfer of the homogenous catalytic systems into heterogeneous
states would overcome the drawbacks in catalyst-product separation, recyclability,
and stability. As an example, grafted Lewis pair centers into MOF structures and solid
surface materials such as carbon nanotubes may indeed become “game-changers” in
the field of FLP-catalysis, and overcoming the gaps and drawbacks of homogeneous
FLPs. This means that grafting these catalysts to heterogeneous supports enhances
catalyst stability and product-catalyst separation and recyclability and adapts such
molecular catalysts for commercial processes. This new paradigm offers enormous
potential for further developments in the field of FLP catalysis.