Preface
Chapter 4: Results and discussions 4.1 Introduction
4.2 Evaluation of techniques to calculate steric strain
4.2.4 Comparison between dissociating ligand sizes of the Grubbs 1-type complexes calculated with solid-G and the inner pocket technique within
SterixLB
The results of the solid-G calculations of the G, B and A complexes (Figure 4.2) can be found in Appendix E, while the results for the same complexes calculated with the inner pocket technique within SterixLB can be found in Appendix F.
Cl
PX3 Ru L
Cl C
H R
Figure 4.2: General Grubbs 1-type complex (with G complexes (L = PCy3, R = Ph), B complexes (L = PX3, R = Ph) and A complexes (L = PX3, R = H) (where X = F, Cl, Br, I, H
or Cy)
In this section, the dissociation of the ligands PF3 (G1, B1 and A1) and PI3 (G4, B4 and A4) from the Grubbs 1-type complexes was investigated. In both cases, the van der Waals radii according to Morse[2] were used. The reason for the choice of the specific ligands was the size of the substituents, namely the smallest and the biggest for the halogens. The dissociating ligand size obtained with the inner pocket technique within SterixLB and the cone angle size calculated with solid-G was plotted against the increasing P(4)-Ru bond length as shown in Figure 4.3. In order to see how the size of the dissociating ligand changed for each complex during dissociation, the first step for the specific complex was subtracted from the calculated
size for each step of the 25 steps of dissociation in order to normalise the data. Therefore, a negative value shows a decrease in the dissociating ligand size, while a positive value shows an increase in the dissociating ligand size.
In the case of the small PF3 ligand (Figure 4.3), the trend observed for the change in the size of the dissociating ligand is similar for the cone angle of solid-G and the inner pocket technique within SterixLB, namely they both initially increase in size and then decrease in size. The dissociating ligand prepares for dissociation by increasing slightly in size before dissociation occurs. After the dissociation has occurred, the dissociating ligand finds its optimal size, since the steric strain between the ligand and complex is absent.
However, a large increase in the cone angle with solid-G was observed between the first and second step (Figure 4.3). Solid-G calculates the steric strain size by measuring the size of the shadows that a light source projects on a sphere. Notably, in solid angle calculations, it is possible that the conformation of a ligand can overshadow another ligand. Therefore, the solid angle technique is known as a conformer specific technique, as mentioned in Chapter 2.[7]The ligands can overshadow each other, which is why the trends are the same for both techniques, but the size varies.
Figure 4.3: Comparison between dissociating ligand size for the (PF3) containing G1, B1 and A1 complexes using solid-G and inner pocket technique within SterixLB
In the case of the larger PI3 ligand in the Grubbs 1-type complexes (G4, B4 and A4) that were calculated with solid-G and the inner pocket technique within SterixLB, the change in the
-3 -2 -1 0 1 2 3 4
2.2 2.7 3.2 3.7 4.2 4.7
Change in dissociating ligand size (°)
P(4)-Ru (Å)
Solid-G G1 Solid-G B1 Solid-G A1
SterixLB G1 SterixLB B1 SterixLB A1
size of the dissociating ligand was plotted against the increasing P(4)-Ru bond length as shown in Figure 4.4. The large initial step that was observed for the PF3 containing Grubbs 1- type complexes with solid-G was also observed for the PI3 containing Grubbs 1-type complexes. However, the change in the size of the dissociating ligand is smaller in the case of the PI3 containing complexes.
Figure 4.4: Comparison between dissociating ligand size for the (PI3) containing G4, B4 and A4 complexes using solid-G and inner pocket technique within SterixLB
The trends observed in the PF3 complexes are close to one another for values obtained with techniques in solid-G and SterixLB. In the PI3 complex a similar trend is observed for techniques in solid-G, but no trend is observed for the inner pocket technique in SterixLB.
The iodine in the PI3 complex is greater in size than the fluorine in the PF3. Therefore, the repulsive forces and the steric strain in the iodine containing complexes will be greater than the fluorine containing complexes.
The dissociating ligand size calculated with the inner pocket technique in SterixLB fluctuates in size for the G4 and B4 complexes (circled in the Figure 4.4). The molecular structures are shown in Figures 4.5 and 4.6.
The molecular structures of the G4 complex between step 5 and 6 are shown in Figure 4.5.
The PX3 ligand (1), the Ph ring (2) and the PCy3 ligand (3) shifted in the G4 complex. The PX3 ligand (1) decreased in size and moved upwards, away from the ruthenium, since it is dissociating from the ruthenium. The Ph ring (2) shifted towards the PX3 ligand that leaves an
-2 -1 0 1 2 3 4 5 6 7 8
2.2 2.7 3.2 3.7 4.2 4.7
Change in dissociating ligand size (°)
P(4)-Ru (Å)
Solid-G G4 Solid-G B4 Solid-G A4
SterixLB G4 SterixLB B4 SterixLB A4
unoccupied space after dissociation. Furthermore, the PCy3 ligand (3) changes orientation and position. Consequently, the shifting groups (1, 2 and 3) in the G4 complex lead to the change in angles that was observed in Figure 4.4.
The fluctuation in the G4 complex can be a result of the large size of the iodine atoms and the rotating Ph ring that interferes with one another.
Figure 4.5: Overlay of step 5 and 6 of the molecular structures for the G4 complex where step 5 is red and step 6 is purple
The molecular structures of the B4 complex between steps 9 and 10 are shown in Figure 4.6.
The dissociating ligand (1) moved away from the ruthenium, while the size of the dissociating ligand decreased as observed in Figure 4.4 between steps 9 and 10. On the other hand, the orientation and the position of the Ph ring (2) shifted.
1 2
3
Figure 4.6: Overlay of steps 9 and 10 of the molecular structures for the B4 complex where step 9 is red and step 10 is blue
Due to the fact that van der Waals radius was used for calculations in this study, it should be mentioned that there is no comparable data for the Ni(CO)3PI3 complex found in literature for the Tolman cone angle[1] and the solid angle[7]. The reason for this is that only data for complexes with covalent radius for iodine is available. Iodine has a small covalent radius, but a large van der Waals radius.