180
M. Heshmat et al.
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 from H
−
to B and from P to H
+ could be visualized by the electron density difference. This
means that the ET model plays a role in the short distance region and the EF model
is relevant at longer distances (details are discussed in Sect. 5.4.1). In a few recent
studies of geminal FLPs, Fernandez et al. investigated the influence of the nature of
the acid/base pairs on the H 2 activation process. Activation barriers and geometries of
transition state structures were computationally explored and it turned out that they
are strongly dependent on the electronic nature of the substituents directly attached
to either the acidic or the basic center of the FLP. The energy decomposition analysis and activation strain model indicated a highly orbital-controlled mechanism in
which asynchronous orbital interactions lead to a more kinetically favored activation
[57, 58].
5.3.3 Engineering FLP Reactivity
DFT calculations showed that increasing the strength of both the LA and the LB
components in an FLP decreases the energy barrier for H 2 activation, which confirms
that the LA and LB act synergistically [59]. Furthermore, the reaction thermodynamics of H 2 activation is strongly affected by the cumulative strength of the LA/LB
parts, which can be quantified by the proton affinity and hydride affinity [60, 61].
However, the individual roles of the LA and the LB are still unclear. To this end, in a
very recent study of FLP reactivity toward H 2 activation by Heshmat et al., a series
of experimentally investigated LAs, with two prototypical sets of LBs, including P(tBu 3 P and Me 3 P) and O-(dioxane, THF, Et 2 O, and Ph 2 O) based LBs, were compared
(Fig. 5.2). The main question addressed was on the electronic and structural impact
of the LAs and the individual roles of the LA and the LB in the entire reaction path
of H 2 activation, starting from a LA/LB complex and ending with the product-ion
pairs (reaction 1). The authors analyzed the electronic and structural effects on the
LA–LB complexation by systematically reducing the number of electronegative F
atoms and by adding bulky groups that increase the pyramidalization strain of the flat
borane molecules. The LA/LB binding/complexation energies in the initial molecular complex were investigated. The effects and outcomes of variation in electronic
and structural properties of the Lewis acids are discussed below.
L A · · · L B + H 2 ↔ (L A · · · H · · · H · · · L B)
T S
↔ L A − H
−
↔
+ H − L B
(5.1)
As shown in Fig. 5.3, two categories of initial LA/LB complexes are distinguished based on the LA
… LB distances. The ones with a dative LA–LB bond,
with distances from 1.6 to 2.3 Å, and the ones forming Van der Waals complexes,
with distances from 3.8 to 6.0 Å. For dative-bond complexes, the more pronounced
M. Heshmat et al.
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 from H
−
to B and from P to H
+ could be visualized by the electron density difference. This
means that the ET model plays a role in the short distance region and the EF model
is relevant at longer distances (details are discussed in Sect. 5.4.1). In a few recent
studies of geminal FLPs, Fernandez et al. investigated the influence of the nature of
the acid/base pairs on the H 2 activation process. Activation barriers and geometries of
transition state structures were computationally explored and it turned out that they
are strongly dependent on the electronic nature of the substituents directly attached
to either the acidic or the basic center of the FLP. The energy decomposition analysis and activation strain model indicated a highly orbital-controlled mechanism in
which asynchronous orbital interactions lead to a more kinetically favored activation
[57, 58].
5.3.3 Engineering FLP Reactivity
DFT calculations showed that increasing the strength of both the LA and the LB
components in an FLP decreases the energy barrier for H 2 activation, which confirms
that the LA and LB act synergistically [59]. Furthermore, the reaction thermodynamics of H 2 activation is strongly affected by the cumulative strength of the LA/LB
parts, which can be quantified by the proton affinity and hydride affinity [60, 61].
However, the individual roles of the LA and the LB are still unclear. To this end, in a
very recent study of FLP reactivity toward H 2 activation by Heshmat et al., a series
of experimentally investigated LAs, with two prototypical sets of LBs, including P(tBu 3 P and Me 3 P) and O-(dioxane, THF, Et 2 O, and Ph 2 O) based LBs, were compared
(Fig. 5.2). The main question addressed was on the electronic and structural impact
of the LAs and the individual roles of the LA and the LB in the entire reaction path
of H 2 activation, starting from a LA/LB complex and ending with the product-ion
pairs (reaction 1). The authors analyzed the electronic and structural effects on the
LA–LB complexation by systematically reducing the number of electronegative F
atoms and by adding bulky groups that increase the pyramidalization strain of the flat
borane molecules. The LA/LB binding/complexation energies in the initial molecular complex were investigated. The effects and outcomes of variation in electronic
and structural properties of the Lewis acids are discussed below.
L A · · · L B + H 2 ↔ (L A · · · H · · · H · · · L B)
T S
↔ L A − H
−
↔
+ H − L B
(5.1)
As shown in Fig. 5.3, two categories of initial LA/LB complexes are distinguished based on the LA
… LB distances. The ones with a dative LA–LB bond,
with distances from 1.6 to 2.3 Å, and the ones forming Van der Waals complexes,
with distances from 3.8 to 6.0 Å. For dative-bond complexes, the more pronounced