Defining and calculating steric strain in the first step of the dissociative alkene metathesis mechanism using Grubbs 1-type catalysts

Abstract

During the dissociative alkene metathesis mechanism, a phosphine ligand dissociates from the ruthenium metal of a Grubbs 1-type catalyst leading to the active 16-electron Grubbs 1-type catalyst with an open coordination site (vacant space) on the ruthenium metal and a dissociated phosphine ligand. A large increase in the energy is observed during the dissociation of a phosphine ligand from a Grubbs 1-type catalyst. This increase in energy can be a consequence of various factors, which includes steric strain. Therefore, in order to understand the energy increase observed during the dissociative mechanism, the steric strain in the Grubbs 1-type catalysts needs to be calculated. Steric strain is a consequence of electron interaction and repulsion between atoms in close approximation to one another. Therefore, steric strain will be minimized when the molecules are in their ideal geometry. Moreover, steric strain affects the bonding angles and the close contact distance between atoms, causing an increase in the energy. That is why the size of the dissociating ligand, the size of the vacant space and the close contact distance between the atoms in various Grubbs 1-type complexes needs to be calculated. Moreover, the size of the dissociating ligand and the size of the vacant space is an indication of the amount of space available around the ruthenium metal and the dissociating ligand. For instance, the steric strain will lessen if the space in the molecule increases, lessening the electronic repulsions between atoms. The hypothesis is that a decrease in the steric strain of a dissociating ligand will lead to an increase in the amount of energy needed for ligand dissociation. Furthermore, the close contact distance between non-bonded atoms has a relation to the size of the dissociating ligand, the size of the vacant space and the total energy in the complex. In order to fully understand the effect that the steric strain has on the various Grubbs 1-type complexes during the dissociation step, a dynamic technique is needed. A dynamic technique is a technique that calculates the change in the steric strain along the pathway of dissociation. Having said that, the techniques currently available (the Tolman cone angle, the solid angle and the percentage buried volume) to calculate the steric strain are insufficient because they were designed to calculate the steric stain of isolated (stationary) complexes. Consequently, this study focuses on the development of a dynamic technique to calculate the steric strain in the first step of the dissociative alkene metathesis mechanism with various Grubbs 1-type complexes (Figures 3.1 to 3.5). For this reason, a computer program named SterixLB that uses four modified techniques, namely the modified Tolman, the outer pocket, the inner pocket and the inner-inner pocket techniques were created to calculate the steric strain of the various Grubbs 1-type complexes. In addition, the program also calculates the close contact distances that may contribute to the increase in the observed dissociation energy of Grubbs 1-type complexes. However, dynamic data is needed to calculate the steric strain observed in the dissociation mechanism. Therefore, potential energy surface (PES) scans, where the bond length between the phosphine of the dissociating ligand and the ruthenium was extended stepwise (25 steps) from the minimum energy optimised bond length to a bond length of 5 Å were performed. These PES scans were performed with Materials Studio 6.0 from Accelrys. Consequently, the stepwise Cartesian coordinate data of the various Grubbs 1-type complexes was obtained from the PES scans. Furthermore, Cartesian coordinate data from the PES scans was used in SterixLB to calculate the size of the dissociating ligand, the size of the vacant space and the close contacts. A good correlation was found between the Tolman cone angle and the results obtained with the modified Tolman, the outer pocket and the inner pocket techniques. The results found showed that the outer pocket technique was the best modified technique to calculate the outermost size of the dissociating ligand, while the inner-inner pocket technique was the best modified technique to calculate the innermost size of the vacant space. Also, the computer program named Solid-G (developed by Guzei et al.[1]) that uses solid angle calculations correlated with SterixLB, even though the- program uses different mathematics and calculates different results. The results obtained with SterixLB indicated that both the electronegativity and the Bondi van der Waals radii of the substituents on the ligand influenced the sizes of the dissociating ligand and the vacant space. In addition, the size of the group on the carbene carbon influenced the sizes of the dissociating ligand and the vacant space on the Grubbs 1-type complex. As a result, the size of the dissociating ligand increased after dissociation, which indicated less steric strain was present in the Grubbs 1-type complex that allowed the dissociating ligand to find a less strained geometry. On the other hand, a decrease in the vacant space size indicated that the steric strain in the Grubbs 1-type complex has decreased. The steric strain in the Grubbs 1-type complex decreases since the dissociated phosphine ligand left an unoccupied space around the metal that allows the groups/atoms around the metal to change positions/orientations to find a less strained geometry. Furthermore, both the energy and the dissociating ligand size increased in preparation of dissociation. On the other hand, the vacant space size decreased with the increase in dissociation energy. In conclusion, understanding the first step of the dissociative mechanism for various dissociating groups can lead to the design of better catalysts that need less energy for dissociation. Furthermore, the maximum size of the vacant space, to accommodate the incoming alkene, could be determined