5 Mechanistic Insight into the Hydrogen Activation by Frustrated Lewis Pairs
191
analysis [84]. The dynamical picture of this reaction resembles a three-body reactive
system. Starting from configurations at the transition state of the reaction, the authors
observed trajectories in which the system remained in the transition state region for an
average period of 350 fs. The authors concluded that the reaction rate of H 2 activation
is influenced by a conformational inertia of the LA
… LB pocket at the transition
state. This conformational inertia is affected by the overall molecular masses of the
LB and LA molecules. Hence, isotopically heavier Lewis base/Lewis acid pairs (in
comparison to normal counterparts) may give measurably slower reaction rates. Thus,
the predicted quasi-bound (TS) state could be verified by experimental femtosecond
spectroscopy.
The mechanism of H 2 liberation from the LB—H
(+)
+ H
(–) —LA ion pair has
been investigated in another reaction dynamics study by Pu and coworkers [85].
According to experiment, the frustrated Lewis pair (o-C 6 H 4 Me) 3 P and B(p-C 6 F 4 H) 3
heterolytically splits H 2 at 25 °C, after which an (o-C 6 H 4 Me) 3 P—H
(+)
+ H
(–) —
B(p-C 6 F 4 H) 3 ion-pair intermediate liberates H 2 under static vacuum and 25 °C
[86]. The FLP (o-C 6 H 4 Me) 3 P/B(p-C 6 F 4 H) 3 is a rare example, since the majority of
FLPs efficiently cleaves H 2 . AIMD simulations, starting from an (o-C 6 H 4 Me) 3 P—
H
(+)
+ H
(–) —B(p-C 6 F 4 H) 3 ion-pair configuration, showed a short-lived transient
LB
… H 2
… LA species. This species structurally resembles the calculated transition
state (TS) in the minimum energy path of the reversible reaction between FLP (oC 6 H 4 Me) 3 P/B(p-C 6 F 4 H) 3 and H 2 . Using an Energy Decomposition Analysis (EDA)
on the structure of the transition state, (o-C 6 H 4 Me) 3 P
… H
… H
… B(p-C 6 F 4 H) 3 ), a rather
strong interaction between LB—H
(+) and H
(–) —LA (cationic and anionic) fragments was detected that promotes the H
… H recombination process. This interaction
between the cationic/anionic fragments is much reduced in case of the more standard tBu 3 P/BCF FLP. Furthermore, the EDA results showed that in the TS-structure,
the interplay of orbital interactions and electrostatic interactions between H
… H and
LA/LB fragments cancels the Pauli repulsion and stabilizes the TS-structure.
5.4.2 AIMD Simulation of Solvated Ion-Pairs
The mechanism of FLP-catalyzed C=O hydrogenation, depicted in Scheme 5.7,
passes through a [solvent-H
(+) -O(solvent)][BCF-H
(–) ] complex and a [solvent-H
(+) -
ketone][BCF-H
(–) ] complex, which are intermediate precursors for the hydride
transfer. The dynamic behavior of these ion pairs at finite temperature was recently
investigated using unbiased AIMD simulations [87, 88]. These simulations were
aimed at, among other things, unraveling the possible locations of the solvated
proton with respect to the hydride in these [solvent-H
(+) -O(solvent)][BCF-H
(–) ]
and [solvent-H
(+) -ketone][BCF-H
(–) ] intermediates. The distance between H
(–) and
H
(+) in these intermediates controls the mechanism of hydrogenation of the C =
O group. In particular, the authors uncovered a new configuration with a cationic
fragment, [BCF-H
(–) ], docked from behind with respect to the direction of the B
→ H vector, i.e., a face-to-back configuration with a proton
… hydride distance of
191
analysis [84]. The dynamical picture of this reaction resembles a three-body reactive
system. Starting from configurations at the transition state of the reaction, the authors
observed trajectories in which the system remained in the transition state region for an
average period of 350 fs. The authors concluded that the reaction rate of H 2 activation
is influenced by a conformational inertia of the LA
… LB pocket at the transition
state. This conformational inertia is affected by the overall molecular masses of the
LB and LA molecules. Hence, isotopically heavier Lewis base/Lewis acid pairs (in
comparison to normal counterparts) may give measurably slower reaction rates. Thus,
the predicted quasi-bound (TS) state could be verified by experimental femtosecond
spectroscopy.
The mechanism of H 2 liberation from the LB—H
(+)
+ H
(–) —LA ion pair has
been investigated in another reaction dynamics study by Pu and coworkers [85].
According to experiment, the frustrated Lewis pair (o-C 6 H 4 Me) 3 P and B(p-C 6 F 4 H) 3
heterolytically splits H 2 at 25 °C, after which an (o-C 6 H 4 Me) 3 P—H
(+)
+ H
(–) —
B(p-C 6 F 4 H) 3 ion-pair intermediate liberates H 2 under static vacuum and 25 °C
[86]. The FLP (o-C 6 H 4 Me) 3 P/B(p-C 6 F 4 H) 3 is a rare example, since the majority of
FLPs efficiently cleaves H 2 . AIMD simulations, starting from an (o-C 6 H 4 Me) 3 P—
H
(+)
+ H
(–) —B(p-C 6 F 4 H) 3 ion-pair configuration, showed a short-lived transient
LB
… H 2
… LA species. This species structurally resembles the calculated transition
state (TS) in the minimum energy path of the reversible reaction between FLP (oC 6 H 4 Me) 3 P/B(p-C 6 F 4 H) 3 and H 2 . Using an Energy Decomposition Analysis (EDA)
on the structure of the transition state, (o-C 6 H 4 Me) 3 P
… H
… H
… B(p-C 6 F 4 H) 3 ), a rather
strong interaction between LB—H
(+) and H
(–) —LA (cationic and anionic) fragments was detected that promotes the H
… H recombination process. This interaction
between the cationic/anionic fragments is much reduced in case of the more standard tBu 3 P/BCF FLP. Furthermore, the EDA results showed that in the TS-structure,
the interplay of orbital interactions and electrostatic interactions between H
… H and
LA/LB fragments cancels the Pauli repulsion and stabilizes the TS-structure.
5.4.2 AIMD Simulation of Solvated Ion-Pairs
The mechanism of FLP-catalyzed C=O hydrogenation, depicted in Scheme 5.7,
passes through a [solvent-H
(+) -O(solvent)][BCF-H
(–) ] complex and a [solvent-H
(+) -
ketone][BCF-H
(–) ] complex, which are intermediate precursors for the hydride
transfer. The dynamic behavior of these ion pairs at finite temperature was recently
investigated using unbiased AIMD simulations [87, 88]. These simulations were
aimed at, among other things, unraveling the possible locations of the solvated
proton with respect to the hydride in these [solvent-H
(+) -O(solvent)][BCF-H
(–) ]
and [solvent-H
(+) -ketone][BCF-H
(–) ] intermediates. The distance between H
(–) and
H
(+) in these intermediates controls the mechanism of hydrogenation of the C =
O group. In particular, the authors uncovered a new configuration with a cationic
fragment, [BCF-H
(–) ], docked from behind with respect to the direction of the B
→ H vector, i.e., a face-to-back configuration with a proton
… hydride distance of