Keywords
Chapter 5 Computational study 5.1. Introduction
5.4. Results and discussion
The Gibbs free energies required to de-coordinate the PCy3 ligand from Grubbs 2 and Ru- indenylidene (Ru-Ind), as well as derivatives of Ru-indenylidene (Table 5.1) were calculated
Table 5.1: Gibbs free energies (ΔG298K) in kcal/mol for the dissociation of the PCy3 ligand from precatalyst.
Grubbs 2 Ru-Ind Ind-F
17.78 kcal/mol 27.75 kcal/mol 28.83 kcal/mol
Ind-CH3 Ind-NO2
29.46 kcal/mol 28.10 kcal/mol
These energies range from 17.78 to 29.46 kcal/mol (Table 5.1), with Grubbs 2 needing the least energy to dissociate PCy3 and the Ind-CH3 the most energy. The dissociation energy is an indication of the ease and rate of the activation of the precatalyst. For this reason it could be concluded that the initiation of metathesis with Grubbs 2 would be faster than the initiation of metathesis with Ru-Ind or any derivatives of Ru-Ind. This is in agreement with the probability that higher energy for dissociation indicates longer lifetime of the catalyst. (6) The observations above are also in agreement with the experimental results from this study and experimental, as well as calculated results from a previous study by Jordaan. (7) In the study of Jordaan (7) of Grubbs 2 under the same conditions as in this study (60°C, Ru: 1-octene 1:9000), the time to reach equilibrium was about 50 min. For the indenylidene in this study, it was about 90 min (Figure 4.15).
Looking at the Ru-Ind derivatives, it was observed that the dissociation energies for PCy3 of the indenylidene derivatives are close to Ru-Ind and could thus mean similar activation.
Even though the indenylidene ligand is sterically more bulky than benzylidene and could contribute more to the electron density of the metal on the precatalyst, the dissociation energy of the phosphine is shown to be slower for indenylidene.
The Gibbs free energies required to de-coordinate the hemilabile ligand in Gr2Ph, Gr2Ph-Ind and derivatives of Gr2Ph-Ind (Table 5.2) were calculated from data in Tables 3 and 4 shown in the Appendix.
Table 5.2: Gibbs free energies (ΔG298K) in kcal/mol for the opening of hemilabile ligand on precatalyst.
Gr2Ph Gr2Ph-Ind Gr2Ph-Ind-F
51.02 kcal/mol 53.58 kcal/mol 51.25 kcal/mol
Gr2Ph-Ind-CH3 Gr2Ph-Ind-NO2
70.46 kcal/mol 56.31 kcal/mol
The energies for the de-coordination of the hemilabile ligand range from 51.02 to 70.46 kcal/mol (Table 5.2). As with the dissociation energies of PCy3, the Grubbs 2 derivative with a hemilabile ligand needs the least energy to de-coordinate and Gr2Ph-Ind- CH3 the most energy. In the case of the hemilabile ligands, the energies of de-coordination between Gr2Ph, Gr2Ph-Ind, Gr2Ph-Ind-F and Gr2Ph-NO2 are close to each and could have similar activation rates.
It is stated in the literature that sterically bulky and electron donating carbene ligands lead to higher initiation rates because of more effective promotion of phosphine dissociation (8). We saw exactly the opposite with the more bulky indenylidene ligand and its derivatives.
The reactivity or stability of precatalysts, as well as catalysts could also be investigated by comparison of the internal HOMO-LUMO energy gap in the precatalysts or catalysts. If the energy gap is big, the precatalyst or catalyst is stable and thus less reactive, and if the gap is small the precatalyst or catalyst is reactive and less stable. (6)
Table 5.3: Internal HOMO-LUMO energy gap (eV) in precatalysts and catalysts shown in Figure 5.1.
Grubbs 2 Ind Ind-F Ind-CH3 Ind-NO2
precatalyst
│EHOMO - ELUMO│ 1.4934 0.7919 0.7883 0.8420 0.5831 catalyst
│EHOMO - ELUMO│ 1.7738 0.7881 0.7832 0.7826 0.7423 Gr2Ph Gr2Ph-Ind Gr2Ph-Ind-F Gr2Ph-Ind-CH3 Gr2Ph-Ind-NO2 precatalyst
│EHOMO - ELUMO│ 1.6236 0.8310 0.7867 0.8196 0.6002 catalyst
│EHOMO - ELUMO│ 0.9278 0.4731 0.4437 0.3812 0.2732 According to the internal HOMO-LUMO energy gaps shown for precatalyst and catalyst complexes without the hemilabile ligand in Table 5.3, Grubbs 2 and Ind-NO2 have larger energy gaps for the catalysts where the other complexes have larger energy gaps for the precatalysts. This indicates Grubbs 2 and Ind-NO2 are more stable after dissociation than before dissociation of PCy3 ligand. It should also be noted that Ind and Ind-F, energy gap for the precatalysts and catalysts are very similar meaning these precatalysts and catalysts will have similar reactivity’s.
The comparison of Grubbs 2 with Ind for both the precatalyst and catalyst show a smaller energy gaps for Ind than for Grubbs2. Thus indicating that the Ind is more reactive than Grubbs 2. These results do not compare with the dissociation energies of these two complexes showing slower activation for Ind in Table 5.1. As well with the experimental results that indicated that the activation of Ind is slower than Grubbs under the same conditions as previously stated. At this moment the reason is unclear and further investigation is needed. Literature results on the activation of Ru-Ind complexes conclude that an interchange mechanism is possible for Ru-Ind (9) and it was shown that the benzylidene carbene ligand can rotate to form an agostic bond with the Ru centre, thus
stabilising the
14 e- intermediate. Because of the hindered rotation of the indenylidene carbene ligand, the 14 e- is not stabilised and thus showing the higher activity. The higher activity can increase the competition between PCy3 and 1-octene and thus lowering the activation rate.
The substituents in the derivatives of indenylidene has a noticeable effect on the HOMO- LUMO energy gaps. This effect is greater in the precatalysts than the catalysts. The energy gaps within the derivatives with electron withdrawing groups are smaller than for Ind, with NO2 having the smallest gap and with NO2 also being more electron negative than F. The indenylidene precatalyst with the electron donating CH3 (Ind-CH3) has a bigger energy gap than Ind.
With the addition of the hemilabile ligand, we see an increase in the energy gaps and thus stability for precatalysts Gr2Ph, Gr2Ph-Ind and Gr2Ph-Ind-NO2. However a decrease is seen for Gr2Ph-Ind-F and Gr2Ph-Ind-CH3. In contrast with the catalysts from precatalyst that contained PCy3 ligands, the catalysts with open hemilabile ligands have lower energy gaps than the corresponding precatalysts. This means slower activation of the precatalysts and higher activity.
As with the PCy3 ligand complexes, Gr2Ph-Ind has a smaller energy gap than Gr2Ph even though ΔG298K for the opening of the hemilabile ligand is larger for Gr2Ph-Ind (Table 5.2).
And with the energy gaps of the open catalyst the indenylidene derivatives have smaller HOMO-LUMO energy gaps than the corresponding Gr2Ph-Ind catalyst. The differences in energy is also influenced by the electron withdrawing and donating groups as discussed above.
The structures in Tables 2, 3 and 4 in the Appendix visually shows the LUMO’s on the precatalysts and the catalysts. It is important that the d orbital contribution of the metal to the LUMO is present (10). In Table 5.4 the HOMO-LUMO energie gaps, with the LUMO on the precatalyst and catalyst and the HOMO on the substrate and the PCy3, to determine the favourability of the substrate interaction are shown. Once the PCy3 has dissociated, the 1-octene and the free phosphine is in competition for coordination with the catalyst. (11) The smaller the energy gap is between the HOMO of the substrate or the PCy3 and the LUMO of the catalyst, the greater the change is of reaction between the catalyst and substrate or PCy3. According to Table 5.4 the energy gap between the LUMO of the catalyst and the HOMO of the substrate is the smallest when the substrate is PCy3. The energy gap further decreases from Grubbs 2 to Ru-Ind and its derivatives, meaning the electron withdrawing effect of the ligand becomes stronger and increases the reactivity.
For most of the precatalysts and catalysts, the HOMO-LUMO energy gap is the smallest between the catalyst and 1-octene. This indicates that the dissociative mechanism is favoured. Only for Gr2Ph-Ind-F the gap is smaller between the precatalyst and 1-octene favouring the associative mechanism.
Because of the increased stability obtained from the hemilabile ligand, the LUMO-HOMO gaps between the 1-octene and the Gr2Ph type catalyst are larger than the energy gaps for Grubbs 2 and Ru-Ind. This will cause the reaction of the catalysts with 1-octene to be slower, thus slower metathesis of the alkenes. The larger energy gap of the catalyst with open hemilabile ligand compared to the catalyst with dissociated PCy3, indicates an easier coordination of 1-octene with the catalyst where PCy3 dissociated.
Table 5.4: LUMO energies of Grubbs type and indenylidene type precatalysts and catalysts in eV and calculated LUMO-HOMO gaps between precatalysts/catalysts and dissociated ligand or substrates.
Grubbs 2 Ind Ind-F Ind-CH3 Ind-NO2
LUMO energy of precatalyst
(eV) -2.627 -3.261 -3.318 -3.189 -3.758
│ELUMO - EHOMO*│ 3.254 2.619 2.563 2.691 2.123
LUMO energy of catalyst
(eV) -2.894 -3.699 -3.760 -3.644 -4.154
│ELUMO - EHOMO*│ 2.986 2.181 2.121 2.237 1.727
│ELUMO - EHOMO**│ 1.725 0.920 0.860 0.976 0.466
Gr2Ph Gr2Ph-Ind Gr2Ph-Ind-F Gr2Ph-Ind-CH3 Gr2Ph-Ind-NO2 LUMO energy of precatalyst
(eV) -2.417 -3.099 -3.275 -3.057 -3.638
│ELUMO - EHOMO*│ 3.464 2.781 2.606 2.823 2.243
LUMO energy of catalyst
(eV) -2.707 -3.204 -3.199 -3.109 -3.751
│ELUMO - EHOMO*│ 3.174 2.677 2.682 2.772 2.130
EHOMO* for 1-Octene is -5.8805 eV EHOMO** for phosphine is -4.6193 eV