5 Mechanistic Insight into the Hydrogen Activation by Frustrated Lewis Pairs
183
Table 5.1 The H − affinity of the LAs and their complexation energies with tBu 3 P, Me 3 P, THF,
and Ph 2 O. All values are in kcal mol −1 . The hydride affinity is defined relative to that of BCF. We
quantify the Lewis acidity here as the complexation energy of the LA with Me 3 P (column four).
The numbers in parentheses show the deformation energy in the LA structure due to complexation
to Me 3 P. Table adapted from Ref. [102].
LA
H − affinity
tBu 3 P
Me 3 P
(E strain-LA )
THF
Ph 2 O
1
0.0
−19.9
−31.7
(24.5)
−20.0
−17.9
2
−12.6
−20.8
−36.9
(24.6)
−26.7
−17.0
3
0.5
−19.7
−28.3
(31.7)
−21.8
−19.0
4
1.2
−15.9
−21.7
(31.5)
−14.1
−16.2
5
−0.4
−12.5
−16.0
(34.4)
−10.4
−16.0
6
−1.1
−13.3
−7.5
(40.6)
−8.9
−16.8
7
10.8
−12.1
−22.8
(28.8)
−13.2
−12.8
8
8.3
−12.3
−22.8
(29.9)
−12.8
−12.9
9
17.4
−12.3
−20.9
(28.6)
−12.7
−14.2
10
14.5
−14.8
−20.9
(29.7)
−10.6
−11.9
11
4.9
−12.5
−7.65
(5.5)
−9.23
−15.77
12
−26.9
−53.2
−66.13
(23.0)
−52.43
−36.53
adding two CF 3 groups on the meta positions (3) weakens the H
− affinity somewhat and, due to the increased LA structure deformation in complex formation, it is
less Lewis acidic than 1. Replacing F atoms by Cl atoms on one ring (4), two rings
(5), and three rings (6), decreases the strength of the dative adduct with Me 3 P with
each additional ring due to the increasing deformation energy. The opposite trend
is seen for the H
− affinity, although the affinity first decreases with structure 4. By
varying the amount and type of substitution, it is possible to tune the strength of the
dative adduct from –36.9 to –7.5 kcal mol
−1 . Instead, in the case of Van der Waals
complexes, the differences in complexation energy are not so large. In the Van der
Waals complexes with tBu 3 P and Ph 2 O, the stabilization is largely due to dispersion
interaction, while deformation and repulsion are less important because of the larger
distance between the LA and LB centers compared to those with Me 3 P and THF.
183
Table 5.1 The H − affinity of the LAs and their complexation energies with tBu 3 P, Me 3 P, THF,
and Ph 2 O. All values are in kcal mol −1 . The hydride affinity is defined relative to that of BCF. We
quantify the Lewis acidity here as the complexation energy of the LA with Me 3 P (column four).
The numbers in parentheses show the deformation energy in the LA structure due to complexation
to Me 3 P. Table adapted from Ref. [102].
LA
H − affinity
tBu 3 P
Me 3 P
(E strain-LA )
THF
Ph 2 O
1
0.0
−19.9
−31.7
(24.5)
−20.0
−17.9
2
−12.6
−20.8
−36.9
(24.6)
−26.7
−17.0
3
0.5
−19.7
−28.3
(31.7)
−21.8
−19.0
4
1.2
−15.9
−21.7
(31.5)
−14.1
−16.2
5
−0.4
−12.5
−16.0
(34.4)
−10.4
−16.0
6
−1.1
−13.3
−7.5
(40.6)
−8.9
−16.8
7
10.8
−12.1
−22.8
(28.8)
−13.2
−12.8
8
8.3
−12.3
−22.8
(29.9)
−12.8
−12.9
9
17.4
−12.3
−20.9
(28.6)
−12.7
−14.2
10
14.5
−14.8
−20.9
(29.7)
−10.6
−11.9
11
4.9
−12.5
−7.65
(5.5)
−9.23
−15.77
12
−26.9
−53.2
−66.13
(23.0)
−52.43
−36.53
adding two CF 3 groups on the meta positions (3) weakens the H
− affinity somewhat and, due to the increased LA structure deformation in complex formation, it is
less Lewis acidic than 1. Replacing F atoms by Cl atoms on one ring (4), two rings
(5), and three rings (6), decreases the strength of the dative adduct with Me 3 P with
each additional ring due to the increasing deformation energy. The opposite trend
is seen for the H
− affinity, although the affinity first decreases with structure 4. By
varying the amount and type of substitution, it is possible to tune the strength of the
dative adduct from –36.9 to –7.5 kcal mol
−1 . Instead, in the case of Van der Waals
complexes, the differences in complexation energy are not so large. In the Van der
Waals complexes with tBu 3 P and Ph 2 O, the stabilization is largely due to dispersion
interaction, while deformation and repulsion are less important because of the larger
distance between the LA and LB centers compared to those with Me 3 P and THF.