The preparation and characterization of multinuclear catalysts based on novel dendrimers : application in the oligomerization and polymerization of unsaturated hydrocarbons
Thesis (PhD)--University of Stellenbosch, 2011.
In this thesis we describe the application of novel salicylaldimine and iminopyridyl nickel metallodendrimer complexes as catalysts in the transformation of á-olefins as well as in the polymerization of norbornene. New cyclic dendrimers based on cyclam as a core (L1-L8) were synthesized and characterized via FTIR and NMR spectroscopy, mass spectrometry and microanalysis. Subsequently the generation 1 cyclam-based dendrimers as well as the commercial generation 1 to generation 3 DAB-PPI dendrimers were functionalized with salicylaldimine and iminopyridyl moieties on the periphery to produce new ligands, DL1-DL10. These modified dendritic ligands were subsequently complexed to Ni salts to obtain the metallodendrimer complexes, C1-C8. The metallodendrimers were characterized by FTIR spectroscopy, mass spectrometry, microanalysis, magnetic susceptibility measurements, UV-Vis spectroscopy and thermal gravimetrical analysis (TGA). The DAB G1-G3 salicylaldimine ligands (DL1-DL3) were subjected to computational studies and the optimized structures were obtained by density functional theory (DFT) calculations. The effect of the increase in dendrimer generation on the structural arrangement of the dendrimer was also investigated. The following aspects were probed using molecular modeling: a) the possible coordination site for the Ni to the first generation dendrimer ligand, DL1, and b) the optimized structure of the first generation salicylaldimine nickel complex, C1. We subsequently evaluated catalysts, C1-C7, in the vinyl polymerization of norbornene, using methylaluminoxane (MAO) as a co-catalyst. All the catalysts were found to be active for norbornene polymerization with the weight of the polymers obtained ranging from 5.12 x 105 - 11.17 x 106 g/mol. The DAB-based iminopyridyl catalysts (C4-C6) exhibited higher activities than its analogous salicylaldimine catalysts (C1-C3) under the same reaction conditions. Also, the cyclam-based salicylaldimine nickel catalyst (C7) exhibited higher activities than the DAB-based salicylaldimine nickel catalyst, C1. A negative dendritic effect was observed for the G1-G3 DAB salicylaldimine catalysts since the optimum activity for the G3 catalyst, C3, was lower than that for the G2 catalyst, C2. These nickel complexes were also evaluated as ethylene oligomerization catalysts and were found to produce a range of ethylene oligomers (C4-C18) as well as some longer chained oligomers, when employing EtAlCl2 as a co-catalyst. We observed however that the free EtAlCl2 mediates the Friedel-Crafts alkylation of the solvent, toluene, in the presence of the obtained ethylene oligomers to give uneven carbon number products, which are mixtures of alkylated benzenes. Our metallodendrimer catalysts also isomerized and in some cases dimerized 1-pentene. In both ethylene oligomerization and 1-pentene isomerization processes, the salicylaldimine catalysts exhibited higher activity towards olefin transformation than the iminopyridyl catalysts. The cyclam-cored dendrimer catalyst again showed the highest activity. From the results obtained thus far it can be concluded that these nickel metallodendrimers exhibit great potential as catalysts in the transformation of unsaturated hydrocarbons.