Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: See full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

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.

Dendrimers are hyperbranched macromolecules, with branches-upon-branches architectures, precise constitutions and molecular weights of several kiloDaltons (Figure 1). The dendritic structure remains to be an influential feature in the developments of dendrimer chemistry at large. Organometallic catalysis forms an active area, wherein the dendrimers find a defined importance. A number of dendrimer types have been utilized to study organometallic catalysis that combine the dendritic architectural principles. Chapter 1 of the Thesis summarizes the advances in the dendrimer-mediated catalyses, apart from an overview of the methods adopted to synthesize dendrimers. Chapter 2 describes the synthesis of newer types of larger generation poly(propyl ether imine) (PETIM) dendrimers. The molecular structure of a sixth generation PETIM dendrimer is shown in Figure 2. The PETIM series of dendrimers are synthesized by iterative synthetic cycles of two reductions and two Michael addition reactions. Modifications of the synthetic methods were identified, so as to facilitate the synthesis and purification of the higher generation dendrimers. Formation of the PETIM dendrimers, possessing a tertiary amine as the branch juncture and ether as the linker component, is assessed systematically by routine analytical techniques. The peripheries of these dendrimers possess either alcohols or amines or carboxylic acids or esters or nitriles, thereby opening up possibilities for varied studies. Architecturally-driven effects are searched constantly while integrating dendrimers in wide ranging studies. With knowledge that un-functionalized PAMAM and PPI dendrimers show fluorescence properties, we tested the PETIM dendrimers for their luminescence property. The photophysical properties of PETIM dendrimers presenting esters, alcohols, acid salts, nitriles and amines at their peripheries were studied. The anomalous fluorescence arising from alcohol terminated PETIM dendrimers (Figure 3) was established through a series of experiments. Various experimental parameters including pH, viscosity of the solvents, aging, temperature and concentration were used to assess the photochemical properties of the PETIM dendrimers. It was observed that generations 1 to 5 absorbed in the region of 260-340 nm, in MeOH and in aqueous solutions. Excitation of the OH-terminated dendrimer solutions at 330 nm led to an emission at ~390 nm (Figure 4). Dendrimers presenting esters, acid salts and amines at their peripheries also exhibited a similar excitation and emission wavelengths. An increase in the fluorescence intensity was observed at low pH and with more viscous solvents. Lifetime measurements showed at least two species (~2.5 and ~7.0 ns) were responsible for the emission. The quenching of the fluorescence originating from the PETIM dendrimers by inorganic anions was also established in the present study. The periodate, persulfate, perchlorate and nitrite anions quenched the fluorescence efficiently among several anions tested. An ‘oxygen-interacted moiety’, in addition to altered hydrogen bonding properties of the dendrimers, was presumed contribute to the anomalous fluorescence behavior. Chapter 3 of the Thesis elaborates photophysical studies of several PETIM dendrimers. Incorporation of catalytically active moieties at the peripheries of dendrimers was identified as an important avenue, in order to explore the effect of the dendritic architectures on the catalytic activities of chosen catalytic moieties. In order to assess the effect of the dendritic scaffold, in relation to both numbers and locations of the catalytic units, an effort was undertaken to study the catalytic activities of catalytic units, that are present in varying numbers within one generation. Partial and full phosphine-metal complex substituted three generations of dendritic catalysts were synthesized, by using a selective alkylation as a key step. The number of the primary amine groups led to define the number of phosphine groups at the peripheries. The primary amine groups were, in turn, prepared by a Michael addition of acrylonitrile and hydroxyl groups, followed by a reduction of the nitrile moieties to the corresponding amines. The first and the second generation PETIM dendrimers utilized in this study present up to four and eight hydroxyl groups at their peripheries. A partial etherification was exercised in order to mask few hydroxyl groups, useful to prepare the partially substituted phosphine groups. Subsequent Michael addition of acrylonitrile with remaining hydroxyl groups, to afford the nitrile terminated dendrimers, and a metal-mediated reduction of the nitrile to amine led to the required number of amine functionalized dendrimers. Functionalization of the peripheries with alkyldiphenyl phosphine moieties was conducted through a Mannich reaction of the amines with formaldehyde and diphenyl phosphine. The subsequent metal complexation with Pd(COD)Cl2 afforded a series of phosphine-Pd(II) complexes, for the zero, first and second generation PETIM dendrimers. Figure 5 shows the molecular structures of a partially and a fully substituted second generation dendrimer. Catalytic activities of the dendrimer-Pd(II) complexes were assessed in both Heck and Suzuki coupling reactions. A C-C bond forming reactions were studied, with the series of dendritic-Pd(II) catalysts, using Cs2CO3 as a base and at 40 oC. In an overall observation, it was found that an individual catalytic site showed a considerable increase in the catalytic activity when it was present in multiple numbers than as a single unit within the same generation (Figure 6). Figure 6. Bar diagrams of (a) Heck reaction and (b) Suzuki reaction, employing the dendritic catalysts 1 - 11. The Heck coupling reaction involved tert-butyl acrylate and iodobenzene, and the Suzuki coupling reaction involved phenyl boronic acid and iodobenzene. The observations revealed that: (i) the higher generation dendritic catalysts exhibited higher catalytic activities per catalytic site and (ii) the dendritic scaffold has a role in enhancing the activities of the individual catalytic sites. The catalysis study identified the catalytic activities that occurred when a series of catalysts within a given dendrimer generation was used. Such a study is hitherto unknown and the observations of this study address some of the pertinent queries relating to the efficiencies of multivalent dendritic catalysts. Chapter 4 of the Thesis describes the synthesis and characterization of series of organometallic PETIM dendrimer and studies of their catalytic activities. Studies on solid-supported catalysis present a significant importance in heterogeneous organometallic catalysis. Silica is a prominently utilized heterogeneous metal catalyst support. Functionalization of the solid supports with suitable chelating ligands is emerging as a viable strategy to circumvent not only the pertinent metal catalyst deterioration and leaching limitations, but also to stabilize the metal particles and to adjust their catalytic efficiencies. In exploring heterogeneous organometallic catalysis, functionalization of silica with a first generation phosphinated dendritic amine was undertaken. The synthetic scheme adopted to synthesize dendrimer functionalized silica is shown in Scheme 1. The reaction of the chloropropylated silica 4 with amine 3 was conducted in CHCl3. Complexation of the functionalized silica 5 with Pd(COD)Cl2 led to isolation of Pd(II)- impregnated silica. Scheme 1. Preparation of Pd nanoparticles stabilized by functionalized silica. It was anticipated that the ratio of phosphine to Pd(II) would be 1:0.5, resulting from a bidendate binding of the phosphine ligand to Pd metal. The observed ICP-OES result indicated that all phosphine ligands did not chelate the metal. With the desire to obtain the metal nanoparticles, the metal complex was subjected to a reduction, which was performed by conditioning 5-Pd(II) complex in EtOH. The Pd metal nanoparticle thus formed was characterized by physical methods, and the spherical nanoparticles were found to have >85 % size distribution between 2-4 nm (Figure 7). Analyses of the Pd(0) impregnated in dendrimer functionalized silica were performed using NMR, XPS spectroscopies, elemental analysis and microscopies. Figure 7. Transmission electron micrograph and histogram of 6, obtained after treatment with EtOH. The Pd-nanoparticle stabilized silica was used in the hydrogenation of several α, β-unsaturated olefins. The catalyst recycling experiments were conducted more than 10 times, and no loss in the catalytic activities were observed. Chapter 5 describes the functionalization of the silica support with diphenylphosphinomethyl-derivatized dendritic amine, palladium nanoparticle formation and the catalysis studies. Overall, the Thesis establishes the synthesis of larger generation PETIM dendrimers, studies of their anomalous fluorescence behavior, organometallic catalysis in solution, as well as, in heterogeneous conditions, pertaining to the C-C bond forming reactions and hydrogenation reactions. (For figure, graph and structural formula pl see the pdf file)

Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Please refer to full text for abstract / AFRIKAANSE OPSOMMING: Sien volteks vir opsomming

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