Browsing by Author "Mapolie, Selwyn F"

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    Aqueous phase catalysis using mono- and bimetallic transition metal complexes
    (2015) Matsinha, Leah Charlie; Smith, Gregory S; Mapolie, Selwyn F; Venter, Gerhard A
    The synthesis and characterization of monomeric and dimeric salicylaldimine water-soluble ligands is discussed. The salicylaldimine ligands (2.3-2.10) were synthesised via Schiff base condensation reactions of various amines with water-soluble sulfonated salicylaldehydes (2.1 and 2.2). The ligands were characterized using various analytical and spectroscopic techniques. Complexation reactions of these water-soluble ligands (2.3-2.10) with [Rh(COD)Cl]2 gave the corresponding water-soluble mononuclear (2.11-2.14) and binuclear (2.15-2.18) Rh(I) complexes. All the complexes were characterized using nuclear magnetic resonance spectroscopy, infrared spectroscopy, single crystal X-ray diffraction (for complex 2.14), mass spectrometry, elemental analysis and melting point determinations.
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    Dendritic rhodium catalyst precursors for the hydroformylation of olefins
    (2018) Williams, Cody; Smith, Gregory; Mapolie, Selwyn F
    The hydroformylation reaction is the transition-metal catalysed addition of CO/H2 to olefins, resulting in linear and/or branched aldehydes. This reaction is in accordance with Green Chemistry principles, as it operates with 100% atom efficiency and uses renewable feedstocks such as olefins from the Fischer-Tropsch process. Rhodium is the metal of choice when designing catalysts for hydroformylation, owing to its good catalytic activity under mild reaction conditions. The strategy of appending bulky ligands has often been employed to enhance catalytic activity and selectivity. Dendritic wedges are promising to the field of catalysis, as one branch may possess multiple surface terminal groups and the other branch may consist of a mononuclear metal centre. This method differs to classical approaches whereby multinuclear effects are explored to enhance the catalyst activity. The purpose of this study was to synthesize and characterise a series of Fréchet dendrons bearing rhodium Schiff-base moieties at the focal point, and investigate their potential as catalyst precursors in the hydroformylation of olefins. A series of Fréchet dendrons with methyl ester groups at the periphery were prepared. The N,O-salicylaldimine and N,P-iminophosphine Schiff-base ligands were synthesized and consequently coupled to the Fréchet dendrons to yield a new class of Fréchet dendrons with N,O-salicylaldimine or N,P-iminophosphine ligands at the focal point. Complexes of these ligands were synthesized to form a new series of neutral rhodium(I) metallodendrons. Complexation of the N,O-salicylaldimine Fréchet dendrons with the metal-precursor [Rh(μ-Cl)(η 2 :η2 -COD)]2 (where COD = 1,5-cyclooctadiene) afforded the Rh(I)-COD metallodendrons. The Rh(I)-COD metallodendrons were reacted under a carbon monoxide atmosphere to yield a new series of dicarbonyl Rh(I) metallodendrons. The bridge splitting reaction between the N,P-iminophosphine Fréchet dendrons and [Rh(μ-Cl)(CO)2]2 afforded the carbonyl-chloride Rh(I) metallodendrons. The Fréchet dendron ligands and rhodium metallodendrons were fully characterised using an array of spectroscopic (1H, 13C{1H}, 31P{1H} NMR, FT-IR spectroscopy) and analytical (elemental analysis and mass spectrometry) techniques. Single crystal X-ray diffraction confirmed the proposed molecular structure and square-planar geometry around the metal centre for the zeroth generation analogues of the N,O-salicylaldimine and N,P-iminophosphine rhodium metallodendrons. The Rh(I) Schiff-base metallodendrons were applied as catalyst precursors in the hydroformylation of various olefins. All of the catalyst precursors were active in the hydroformylation of 1-octene. The N,O-salicylaldimine metallodendrons displayed good to excellent conversion (78 – 100%), good chemoselectivity (66 – 95%) and moderate regioselectivity (51 – 67%). In contrast, the N,P-iminophosphine metallodendrons displayed low conversion (4 – 8%), good chemoselectivity (76 – 80%) and good regioselectivity (64 – 68%) under the hydroformylation conditions. Notably, the increase in dendron size (G0 – G2) resulted in an increase in the chemoselectivity towards aldehydes. Hydroformylation reactions were conducted using various olefin substrates. These include 1-octene, styrene, 7-tetradecene, methyl oleate, triolein, D-limonene and R-citronellal. The model precursor was active in the hydroformylation of these substrates. More importantly, conversions obtained were promising for styrene (100%), D-limonene (90%), 1-octene (86%), methyl oleate (78%), 7-tetradecene (73%) and triolein (52%). The regioselectivity for the internal olefins ranged between 85 – 98%. These results are particularly promising for tandem-catalytic processes. Mercury drop experiments performed on the zeroth generation analogues of the N,O-salicylaldimine-COD, N,O-salicylaldimine-dicarbonyl and N,P-iminophosphine chloro-carbonyl rhodium(I) metallodendrons displayed suppressed activity in the presence of mercury.
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    Iminophosphine complexes of palladium and platinum: catalysis and metallacycloalkanes synthesis
    (2012) Mahamo, Tebello; Smith, Gregory S; Lammertsma, Koop; Mapolie, Selwyn F; Slootweg, J Chris; Ehlers, Andreas W
    A series of N-functionalized 2-diphenylphosphinobenzaldimino ligands (3.1 â 3.6) bearing pendant groups on the imine moiety were prepared by the Schiff-base condensation reaction of 2-diphenylphosphinibenzaldehyde and appropriate primary amines. The ligands were subsequently used to synthesize a range of palladium complexes of the types [Pd(P^N)Cl2] (3.7 â 3.12) and [Pd(P^N)(Me)Cl)] (3.13 â 3.18) from precursor complexes [Pd(COD)Cl2] and [Pd(COD)(Me)Cl], respectively. Platinum complexes of the type [Pt(P^N)Cl2] (3.19 â 3.24) were synthesized by the ligand displacement reaction between [Pt(COD)Cl2] and ligands 3.1 â 3.6. All compounds were characterized by multinuclear NMR and infrared spectroscopies as well as elemental analysis. In addition, the structure of complex 3.14 was determined by x-ray crystallography. Palladium complexes 3.8 â 3.10 and 3.16 were evaluated as pre-catalysts in the Suzuki- Miyaura coupling reaction. These complexes were found to be highly active and tolerant of a wide range of reaction conditions and functional groups on substrates. Low catalyst loadings (0.1 mol% Pd) were required, while high conversions and short reaction times were maintained. Having a substituent bearing a donor atom on the imine moiety of the ligand (ligands 3.3 and 3.4) was found to enhance catalytic activity. Palladium methyl chloride complexes were found to show slightly more activity than their palladium dichloride counterparts. Reaction of [Pt(P^N)Cl2] complexes with BrMg(CH2)4MgBr in an attempt to synthesize platinacycloalkane complexes resulted in the formation of bromobutyl complexes [Pt(P^N)(C4H9)Br] (3.25 and 3.26) instead. Successful synthesis of platinacyclopentane complexes, 5.1 â 5.6, and platinacycloheptane complexes, 5.7 â 5.12, was achieved by the reaction of [Pt(COD)Cl2] with appropriate di-Grignard reagents, followed by ligand displacement with the iminophosphine ligands. All complexes were fully characterized using various NMR spectroscopies, mass spectrometry and elemental analysis. Crystal structures of the bromobutyl and platinacyclopentane complexes 3.25 and 5.1 were determined. Studies on the thermal decomposition of the platinacycloalkane complexes were carried out. Platinacyclopentane complexes 5.1 â 5.6 were found to be markedly stable, with the decomposition reaction requiring temperatures higher than 100 °C. Reaction temperature and duration were found to have a significant influence on the organic product distribution obtained. These reactions gave 1-butene (for the platinacyclopentane complexes) and 1- hexene (for the platinacycloheptane complexes) as major products. Kinetic data obtained for the decomposition of 5.1 and 5.7 shows that the decomposition reaction follows first order kinetics for the initial 30% of the decomposition reaction. Thereafter, reaction order deviates from first order behaviour, indicating increasing involvement of products in the reaction mechanism. The generally accepted β-hydride elimination/reductive elimination reaction mechanism for the decomposition of metallacycloalkanes was investigated using DFT methods. The simplified complex, 5.13B, was used as a model for platinacyclopentane complexes. Results from these calculations show that intramolecular β-hydride elimination from the carbocyclic ring of platinacyclopentane complexes is unlikely to occur as this process requires an extremely high energy barrier (>64 kcal.mol-1). Furthermore, these calculations reveal that ligand hemilability is energetically disfavoured in the β-elimination reaction while it is favoured in the reductive elimination reaction.
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    Olefin oligomerization reactions theoretical studies using cyclometallated palladium(II) catalysts and experimental studies on platinum(II) analogues
    (2012) Zheng, Feng; Moss, John R; Hutton, Alan T; Mapolie, Selwyn F; Van Sittert, Cornie GCE
    Ethylene oligomerization reactions catalysed by cyclometallated palladium(II) N-benzylidenebenzylamine complexes were studied theoretically. Density functional theory (DFT) calculations are reported on the interaction of various MAO models with the methylated pre-catalyst. The neutral MAO dissociation process is shown to remain the major interaction that takes place in the Pd/MAO interactions. On the other hand, the formal methyl abstraction process could be also feasible if more energy is provided. Therefore, the relative energies were calculated for intermediates and transition states for both Cossee-type and metallacycle mechanisms.