184
M. Heshmat et al.
These dominant attractive dispersion interactions are similar for tBu 3 P and Ph 2 O
regardless that one is a P-bearing LB and the other is an O-bearing LB. The interaction
between a solvent (ethereal) LB with an LA is seen to be rather strong. Hence, the
preparation (reorganization) energy of the LA and LB pair for H 2 activation in Van
der Waals adducts of tBu 3 P and Ph 2 O is similar. LAs 7–10 were derived from BCF by
replacing F atoms with H and Cl atoms to measure the effect of front strain (positions
2 and 6 in the aryl rings) and back strain (position 5). The H
− affinity of all four
LAs is much decreased compared to that of the original BCF structure and the Lewis
acidity to form a dative-bond with Me 3 P is decreased by up to ca. 35% (also with
THF a dative-bond is formed). The complexation energy with tBu 3 P and Ph 2 O, with
which the LAs form a Van der Waals complex, is similar to that of THF. Addition
of one Cl atom at position 5 to increase back strain (see structures 8 and 10 versus 7
and 9) results in a stronger H
− affinity by 2.5–3 kcal mol
−1 . Moreover, LAs 9 and
10 are weak H
− acceptors and their corresponding LA-H
(−)…(+) H-LB ion pairs are
supposed to release an H 2 molecule. On the other hand, the back strain results in
a larger deformation in the LA structure; its effect on decreasing the complexation
energy is more visible with less donating LBs such as THF. In 11 (the bulkiest
alkylated LA), the congestion around the boron atom is large. The C–H bonds are
almost perpendicular to the BCCC plane so that the hydrogens shield the boron.
According to previous experimental investigations, considerable steric shielding of
the boron center imparted by the large CH(C 6 F 5 ) 2 ligands hinders access to the
Lewis base and results in a weak borane LA. The H
− affinity of 11 is nevertheless
stronger than that of the LAs 7 to 10. This means that the C 6 F 5 rings can induce
their electronegativity to the B atom despite the extra C–H groups, and make it more
electrophilic than the LAs 7 to 10. None of the LBs form a dative-bond with 11.
Instead, 12 (the strongest alkylated LA) forms a dative-bond with all LBs, showing
the strongest interaction with Me 3 P. The Lewis basicity of the LBs can be estimated
from the strength of the dative-bond to 12.
The influence of electronic and structural factors on the G
‡ of the H 2 activation
(i.e., on the reaction kinetics) and on the overall G (i.e., the thermodynamics of the
reaction) were also examined. Figure 5.4 shows the calculated free energy profile
for the tBu 3 P and THF (two typical examples of P- and O-based LBs) with LAs
1 to 11 depicted in Fig. 5.2. For tBu 3 P, three extra LAs have been included that
were obtained by gradually replacing C 6 F 5 rings with C 6 H 5 , i.e., B(C 6 F 5 ) 2 C 6 H 5 ,
B(C 6 F 5 )(C 6 H 5 ) 2 , and B(C 6 H 5 ) 3 , I-III. In this way, the electrophilicity of the LAs
systematically decreases by removing the F atoms. For stronger LAs, which means
a lower LUMO level of the LA, the free energy profile is strongly exergonic, e.g.,
G of the ion-pair is –10.19 kcal mol
−1 for LA 1 and –18.60 kcal mol
−1 for LA
2. However, the free energy profile gradually shifts toward endergonic with FLPs
including weaker LAs, e.g., G is around 0.30 kcal mol
−1 for B(C 6 F 5 ) 2 C 6 H 5 and
14 kcal mol
−1 for B(C 6 H 5 ) 3 . The lower panel of Fig. 5.4 shows calculated free energy
profiles of THF and LAs 1 to 11. For THF, all H 2 splitting reactions are endergonic,
and altering the electronic structure of the LA by removing the F atoms changes the
G to more positive values. This is important when the activation of H 2 is followed
by hydrogenation and proton transfer to another LB in the system (e.g., O of C = O
M. Heshmat et al.
These dominant attractive dispersion interactions are similar for tBu 3 P and Ph 2 O
regardless that one is a P-bearing LB and the other is an O-bearing LB. The interaction
between a solvent (ethereal) LB with an LA is seen to be rather strong. Hence, the
preparation (reorganization) energy of the LA and LB pair for H 2 activation in Van
der Waals adducts of tBu 3 P and Ph 2 O is similar. LAs 7–10 were derived from BCF by
replacing F atoms with H and Cl atoms to measure the effect of front strain (positions
2 and 6 in the aryl rings) and back strain (position 5). The H
− affinity of all four
LAs is much decreased compared to that of the original BCF structure and the Lewis
acidity to form a dative-bond with Me 3 P is decreased by up to ca. 35% (also with
THF a dative-bond is formed). The complexation energy with tBu 3 P and Ph 2 O, with
which the LAs form a Van der Waals complex, is similar to that of THF. Addition
of one Cl atom at position 5 to increase back strain (see structures 8 and 10 versus 7
and 9) results in a stronger H
− affinity by 2.5–3 kcal mol
−1 . Moreover, LAs 9 and
10 are weak H
− acceptors and their corresponding LA-H
(−)…(+) H-LB ion pairs are
supposed to release an H 2 molecule. On the other hand, the back strain results in
a larger deformation in the LA structure; its effect on decreasing the complexation
energy is more visible with less donating LBs such as THF. In 11 (the bulkiest
alkylated LA), the congestion around the boron atom is large. The C–H bonds are
almost perpendicular to the BCCC plane so that the hydrogens shield the boron.
According to previous experimental investigations, considerable steric shielding of
the boron center imparted by the large CH(C 6 F 5 ) 2 ligands hinders access to the
Lewis base and results in a weak borane LA. The H
− affinity of 11 is nevertheless
stronger than that of the LAs 7 to 10. This means that the C 6 F 5 rings can induce
their electronegativity to the B atom despite the extra C–H groups, and make it more
electrophilic than the LAs 7 to 10. None of the LBs form a dative-bond with 11.
Instead, 12 (the strongest alkylated LA) forms a dative-bond with all LBs, showing
the strongest interaction with Me 3 P. The Lewis basicity of the LBs can be estimated
from the strength of the dative-bond to 12.
The influence of electronic and structural factors on the G
‡ of the H 2 activation
(i.e., on the reaction kinetics) and on the overall G (i.e., the thermodynamics of the
reaction) were also examined. Figure 5.4 shows the calculated free energy profile
for the tBu 3 P and THF (two typical examples of P- and O-based LBs) with LAs
1 to 11 depicted in Fig. 5.2. For tBu 3 P, three extra LAs have been included that
were obtained by gradually replacing C 6 F 5 rings with C 6 H 5 , i.e., B(C 6 F 5 ) 2 C 6 H 5 ,
B(C 6 F 5 )(C 6 H 5 ) 2 , and B(C 6 H 5 ) 3 , I-III. In this way, the electrophilicity of the LAs
systematically decreases by removing the F atoms. For stronger LAs, which means
a lower LUMO level of the LA, the free energy profile is strongly exergonic, e.g.,
G of the ion-pair is –10.19 kcal mol
−1 for LA 1 and –18.60 kcal mol
−1 for LA
2. However, the free energy profile gradually shifts toward endergonic with FLPs
including weaker LAs, e.g., G is around 0.30 kcal mol
−1 for B(C 6 F 5 ) 2 C 6 H 5 and
14 kcal mol
−1 for B(C 6 H 5 ) 3 . The lower panel of Fig. 5.4 shows calculated free energy
profiles of THF and LAs 1 to 11. For THF, all H 2 splitting reactions are endergonic,
and altering the electronic structure of the LA by removing the F atoms changes the
G to more positive values. This is important when the activation of H 2 is followed
by hydrogenation and proton transfer to another LB in the system (e.g., O of C = O