Synthesis and modelling of Tungsten catalysts for alkene metathesis

Abstract

The aim of this study was to investigate, theoretically and experimentally, the W(O–2,6–C6H3Cl2)2Cl4 catalyst and to synthesise ‘cage’ alicyclic ligands that will help retain the catalyst during the membrane separation process. Furthermore, molecular modelling was used in order to explain the metal–ligand coordination, the active sites in carbosilane dendritic catalysts and to investigate the mechanistic steps of the W(O–2,6–C6H3Cl2)2Cl4 catalyst in the metathesis of 1–octene. The W(O–2,6–C6H3X2)2X4 (X = Cl, Br and Ph) catalytic system has been reported in literature, and the complex with X = Cl substituent was found to have higher activity than the complex with Br and Ph substituents. However, the complex with Br and Ph substituents were found to have high selectivity but lower activity. The metathesis of 1–octene by the W(O–2,6–C6H3Cl2)2Cl4 system was investigated and the results matched well with literature. A theoretical study was done on the metathesis mechanism of 1–octene in the presence of carbosilane dendritic catalysts and the W(O–2,6–C6H3Cl2)2Cl4 catalytic system. The electronic energy profiles were plotted by using a Potential Energy Surface (PES) scan. The preferred routes in the activation steps and in the catalytic cycles were predicted. The activation steps of the two carbosilane dendritic catalysts are different from the activation step of the W(O–2,6–C6H3Cl2)2Cl4 catalyst, but the catalytic cycle is in agreement with that of W(O–2,6–C6H3Cl2)2Cl4. Electronic energy gaps, orbital symmetry and the orientation of ligands or 1–octene with metal complexes were calculated and analysed. A striking observation is that in the coordination of the metal complex with either the ligand or 1–octene the smallest energy gap of the frontier orbitals is always between the lowest unoccupied molecular orbitals (LUMO) of the metal complex and the highest occupied molecular orbitals (HOMO) of the ligand/alkene. It was also observed on the energy profiles that the heptylidene species is more stable than the methylidene species. Two ‘cage’ alicyclic compounds which differ in their periphery, the “cage divinyl ether” and “cage diallyl ether” were synthesised and obtained in good yields. Attempts to synthesise dendritic catalysts/complexes with these ligands as cores of the dendritic catalysts were undertaken. An electrophilic addition reaction of HCl and Cl2 on the double bonds was observed. An energy gap analysis of these ligands with the W(O–2,6–C6H3Cl2)2Cl4 system was undertaken. It was found that the LUMO of W(O–2,6–C6H3Cl2)2Cl4 and the HOMO of the “cage alicyclic compounds” showed a good possibility of coordination. However, the orbital symmetry and orientation of the metal and ligand does not permit the coordination of the ligands with the metal complex. In–situ metathesis of “cage alicyclic compounds” as ligands with W(O–2,6–C6H3Cl2)2Cl4 as catalyst did not give any metathesis products.