5 Mechanistic Insight into the Hydrogen Activation by Frustrated Lewis Pairs
177
H 2 cleavage are the ones that are not too far from each other and do not form a very
strong dative-bond.
Formation of an FLP encounter complex was also confirmed by classical Molecular Dynamics (MD) simulations with explicit solvation of tBu 3 P/BCF FLP in
toluene. The MD simulations gave an estimate of the probability of tBu 3 P/BCF
association, which is much smaller than that of the dissociated state due to entropic
and solvent effects [38]. The complexation of the phosphine/borane pairs has
been successfully probed by NMR measurements [39]. Neutron scattering studies
also evidenced the formation of the molecular encounter complex between LA
and LB [40]. Recently, Kwa´ sny and coworkers investigated the association of
tBu 3 P/BCF FLP in the ionic liquid of 1-decyl-3-methylimidazolium bistriflamide
([C 10 mimNTf 2 ]) [40]. Interestingly, the authors showed that the formation of the
FLP encounter complex increases over 20% of the dissolved species in ionic liquid
solution in comparison to organic solvents. They suggested that the low diffusivity
characteristic of ionic liquids enhances the population of the encounter complex,
which could further enhance the catalytic properties of FLPs, i.e., H 2 activation.
The LA
… LB distance in intramolecular FLPs is usually shorter than that in
intermolecular ones, and typically it is in the same range as that of covalently
bound LA–LB adducts. For example, in the prototypical intramolecular FLP,
Mes 2 P(CH 2 ) 2 B(C 6 F 5 ) 2, the P-B distance is 2.2 Å in the most stable structure,
which is a four-membered heterocyclic phosphine-borane adduct [41, 42]. To activate H 2 with an intramolecular FLP, the closed ring between the LA/LB centers
needs to open and overcome the interaction between the centers. According to PES
calculations, the required energy for this opening is ca. 7 kcal mol
−1 for a typical
Mes 2 P(CH) 2 B(C 6 F 5 ) 2 intramolecular FLP and for other types around 10 kcal mol
−1
(i.e., Mes 2 P(CH 2 ) 3 B(C 6 F 5 ) 2 and Mes 2 P(CH 2 ) 4 B(C 6 F 5 ) 2 ) [43], which indicates that
the opening process is slightly endergonic. Moreover, it is found that the barrier for
the opening of the intramolecular FLPs is also low; for Mes 2 P(CH 2 ) 3 B(C 6 F 5 ) 2 and
Mes 2 P(CH 2 ) 4 B(C 6 F 5 ) 2 it is 14 and 12 kcal mol
−1 , respectively [43, 44]. Generally,
there are several factors that play a role in the formation of an efficient intramolecular
FLP, in particular, the energy required to weaken the interaction between the LA and
LB centers, the geometrical bulkiness, and the conformational flexibility of the LA
and LB centers [45].
Liu et al. have computationally investigated H 2 activation by a series of intramolecular FLPs, Mes 2 PCHRCH 2 B(C 6 F 5 ) 2 , with R = H, Me, Ph, and SiMe 3 . They found
that the geometrical bulkiness and steric hindrance around the C–C bridge prohibit
the dimerization of FLPs, which makes H 2 activation inaccessible [46]. The reaction
path for dimerization of intramolecular FLPs has not been clarified by experimental
studies. Kinetic studies on intramolecular FLPs showed that the reverse reaction of
H 2 liberation from the product-ion pairs is first order with respect to the PH
(+)…(−) HB
complex concentration [47]. Several theoretical studies reported that the dimerization of intramolecular FLPs is not needed to explain the reactivity of FLPs toward
H 2 activation [48].
In a recent study, Fernandez and coworkers computationally explored the dihydrogen activation mediated by geminal Me 2 X-CH 2 -Y-Ph 2 FLPs, with X one of N,
177
H 2 cleavage are the ones that are not too far from each other and do not form a very
strong dative-bond.
Formation of an FLP encounter complex was also confirmed by classical Molecular Dynamics (MD) simulations with explicit solvation of tBu 3 P/BCF FLP in
toluene. The MD simulations gave an estimate of the probability of tBu 3 P/BCF
association, which is much smaller than that of the dissociated state due to entropic
and solvent effects [38]. The complexation of the phosphine/borane pairs has
been successfully probed by NMR measurements [39]. Neutron scattering studies
also evidenced the formation of the molecular encounter complex between LA
and LB [40]. Recently, Kwa´ sny and coworkers investigated the association of
tBu 3 P/BCF FLP in the ionic liquid of 1-decyl-3-methylimidazolium bistriflamide
([C 10 mimNTf 2 ]) [40]. Interestingly, the authors showed that the formation of the
FLP encounter complex increases over 20% of the dissolved species in ionic liquid
solution in comparison to organic solvents. They suggested that the low diffusivity
characteristic of ionic liquids enhances the population of the encounter complex,
which could further enhance the catalytic properties of FLPs, i.e., H 2 activation.
The LA
… LB distance in intramolecular FLPs is usually shorter than that in
intermolecular ones, and typically it is in the same range as that of covalently
bound LA–LB adducts. For example, in the prototypical intramolecular FLP,
Mes 2 P(CH 2 ) 2 B(C 6 F 5 ) 2, the P-B distance is 2.2 Å in the most stable structure,
which is a four-membered heterocyclic phosphine-borane adduct [41, 42]. To activate H 2 with an intramolecular FLP, the closed ring between the LA/LB centers
needs to open and overcome the interaction between the centers. According to PES
calculations, the required energy for this opening is ca. 7 kcal mol
−1 for a typical
Mes 2 P(CH) 2 B(C 6 F 5 ) 2 intramolecular FLP and for other types around 10 kcal mol
−1
(i.e., Mes 2 P(CH 2 ) 3 B(C 6 F 5 ) 2 and Mes 2 P(CH 2 ) 4 B(C 6 F 5 ) 2 ) [43], which indicates that
the opening process is slightly endergonic. Moreover, it is found that the barrier for
the opening of the intramolecular FLPs is also low; for Mes 2 P(CH 2 ) 3 B(C 6 F 5 ) 2 and
Mes 2 P(CH 2 ) 4 B(C 6 F 5 ) 2 it is 14 and 12 kcal mol
−1 , respectively [43, 44]. Generally,
there are several factors that play a role in the formation of an efficient intramolecular
FLP, in particular, the energy required to weaken the interaction between the LA and
LB centers, the geometrical bulkiness, and the conformational flexibility of the LA
and LB centers [45].
Liu et al. have computationally investigated H 2 activation by a series of intramolecular FLPs, Mes 2 PCHRCH 2 B(C 6 F 5 ) 2 , with R = H, Me, Ph, and SiMe 3 . They found
that the geometrical bulkiness and steric hindrance around the C–C bridge prohibit
the dimerization of FLPs, which makes H 2 activation inaccessible [46]. The reaction
path for dimerization of intramolecular FLPs has not been clarified by experimental
studies. Kinetic studies on intramolecular FLPs showed that the reverse reaction of
H 2 liberation from the product-ion pairs is first order with respect to the PH
(+)…(−) HB
complex concentration [47]. Several theoretical studies reported that the dimerization of intramolecular FLPs is not needed to explain the reactivity of FLPs toward
H 2 activation [48].
In a recent study, Fernandez and coworkers computationally explored the dihydrogen activation mediated by geminal Me 2 X-CH 2 -Y-Ph 2 FLPs, with X one of N,