Browsing by Author "Tshabalala, Thandeka Adelinah."

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Palladium complexes of (Benzoimidazol-2-ylmethyl) amine ligands : synthesis and applications as catalysts in methoxycarbonylation of olefins.
(2014) Tshabalala, Thandeka Adelinah.; Ojwach, Stephen Otieno.
Compounds N-(1H-benzoimidazol-2-ylmethyl)-2-aniline(L1), N-(1H-benzoimidazol- 2-ylmethyl)-2-methoxyaniline (L2), N-(1H-benzoimidazol-2-ylmethyl)-2-thioaniline (L3) and N-(1H-benzoimidazol-2-ylmethyl)-2-bromoaniline (L4) were prepared by reaction of 2-(chloromethyl) benzimidazole with the appropriate aniline. Ligands L1– L4 were reacted with either [PdCl2(COD)] or [PdClMe(COD)] to form palladium complexes [PdCl2(L1)], [PdClMe(L1)], [PdCl2(L2)],[PdClMe(L2)], [PdCl2(L4)] and [PdClMe(L4)]. Treatment of the neutral complexes C1 – C6 with one equivalent of PPh3 in the presence of the halide abstractor NaBAr4 (Ar = 3,5-(CF3)2C6H3) led to the formation of the cationic species [(PdCl(L1)]Bar4, [(PdMe(L1)]Bar4, [(PdCl(L2)]BAr4, [(PdMe(L2)]BAr4 and [(PdMe(L4)]BAr4. All the new compounds were characterized by combination of 1H and 31P NMR spectroscopy, mass spectrometry, X-ray crystallography of complexes C8, C9 and C11 and elemental analysis (for complexes). The palladium complexes were investigated as catalyst in methoxycarbonylation of olefins. Methoxycarbonylation of 1-hexene using C2, C4, C6, C8-C11 was studied in order to investigate the effect of catalyst structure and PPh3 ligand on activity and selectivity. Complex C10 bearing electron donating OCH3 substituent on phenyl group was the most active, while the corresponding analogue containing the Br substituent on phenyl group was the least active. Complex C9 with Pd-Cl showed lower catalytic activity, compared complex C10 with Pd-Me. Addition of PPh3 to the palladium complexes resulted in higher catalytic activities and high regioselectivity towards the linear esters. The activity also dependent in the identity of acid promoter where HCl gave highest active system while no activity was observed with p-TsOH. The effects of pressure, temperature, catalysts concentration, identity of the olefin and time was also investigated. Increasing the catalysts concentration, chain length and time resulted in higher catalytic activities and higher regioselectivity towards the branched ester.
Synthesis of nitrogen-, oxygen- and phosphine- donor palladium(II) complexes for the catalytic hydrogenation and methoxycarbonylation of alkenes and alkynes.
(2019) Tshabalala, Thandeka Adelinah.; Ojwach, Stephen Otieno.
Reactions of N-(1H-benzoimidazol-2-ylmethyl-2-methoxy)aniline (L1) and N-(1H-benzoimidazol-2-ylmethyl-2-bromo)aniline (L2) with p-TsOH, Pd(AOc)2 and two equivalents of PPh3 or PCy3 produced the corresponding palladium complexes, [Pd(L1)(OTs)(PPh3)] (1), [Pd(L2)(OTs)(PPh3)] (2) and [Pd(L1)(OTs)(PCy3)] (3) respectively in good yields. The new palladium complexes 1-3, and the previously reported complexes [Pd(L1)ClMe] (4) and [Pd(L2)ClMe] (5) gave active catalysts in the methoxycarbonylation of terminal and internal olefins to produce branched and linear esters. The effects of complex structure, nature of phosphine derivative, acid promoter and alkene substrate on the catalytic activities and selectivity have been studied and are herein reported. The ligands N-(1H-benzoimidazol-2-ylmethyl-2-methoxy)aniline (L1), N-(1H-benzoimidazol-2-ylmethyl-2-bromo)aniline L2), N-(1H-benzoimidazol-2-ylmethyl)benzenamine (L3) and N-(1H-benzoimidazol-2-ylmethylamino)benzenethiol (L4), were synthesized following our published literature method. The palladium complexes [Pd(L1)Cl2] (6), [Pd(L2)Cl2] (7) [Pd(L3)Cl2] (8), [Pd(L4)Cl2] (9), [Pd(L2)ClMe] (10) and [Pd(L2)ClPPh3]BAr4 (11), were prepared following our recently published procedure. The (benzoimidazol-2-ylmethyl)amine palladium(II) complexes 6-11, have been employed as catalysts in the homogeneous hydrogenation of alkenes and alkynes under mild conditions. A correlation between the catalytic activity and the nature of the ligand was reaffirmed. Kinetic studies of the hydrogenation reactions of styrene established pseudo-first-order dependence on the styrene substrate. On the other hand, partial orders with respect to H2 and catalyst concentrations were obtained. The nature of the solvent used, influenced the hydrogenation reactions, in that coordinating solvents resulted in lower catalytic activities. Kinetics and mechanistic studies performed are consistent with the formation of a palladium monohydride active species. Reactions of 2-(2-methoxyethylimino)ethyl)phenol (L5), 2-(2-hydroxyethylimino)ethyl)phenol (L6), 2-(2-aminoethylimino)ethyl)phenol (L7) and 2-(2-hydroxyethylimino)methyl)phenol (L8) with [PdCl2(COD)] afforded the neutral palladium complexes [PdCl2(L5)] (12), [PdCl2(L6)] (13), [PdCl2(L7)] (14), [PdCl2(L8)] (15) respectively. Treatment of complex 12 with PPh3 gave the cationic complex [Pd(L5)ClPPh3]+ (16), while reactions of 15 with Pd(OAc)2, in the presence of PPh3 and p-TsOH produced the corresponding palladium complex, [Pd(L5)(OTs)(PPh3)] (17). he molecular structure of 15a (derivative of 15) contained two bidentate anionic ligand (L8). Complexes 12-17 formed active catalysts in hydrogenation of alkenes and alkynes, in which the catalytic activities were largely dependent on the pendant donor atom of the ligand motif. Isomerization reactions were dominant in terminal alkenes hydrogenation reactions, while hydrogenation of alkynes to alkanes occurred in two steps via alkene intermediates. Kinetics data were consistent with homogeneous active species. Density functional theory studies supported the hemi-labile nature of the ligands, and offered insights into the catalytic activity trends observed. Compounds [2-(2-(diphenylphosphino)benzylidene)methoxyethanamine] (L9), [2-(2-(diphenylphosphino)benzylideneamino)ethanol] L10), 2-(diphenylphosphino)benzylidene)ethane-1,2-diamine) and 2- (diphenylphosphino)benzylidene)diethylethane-1,2-diamine) (L12) were prepared by a condensation reaction between 2-(diphenylphosphino)benzaldehyde and the corresponding amines. The imino-diphenylphosphino palladium(II) complexes, [PdCl2(L9)] (18), [PdCl2(L10)] (19), [PdCl2(L11)] (20), [PdCl2(L12)] (21) were prepared from the reactions of L9-L12 with Pd(COD)Cl2. On the other hand, reactions of L9 with Pd(AOc)2 in the presence of equivalenct amounts of p-TsOH and PPh3 afforded [Pd(L9)(OTs)(PPh3)]+TsO- (22) while subsequent treatment of complex 18 with one equivalent of NaBAr4 (Ar4 = 3,5-(CF3)2C6H3) in the presence of PPh3 afforded the cationic complex, [Pd(L5)(Cl)(PPh3)] BAr4- (23). Complexes 18-23 were characterized by mass spectrometry, elemental analysis, 1H, 13C and 31P NMR spectroscopy. Complexes 18-23 were found to be active catalysts in the hydrogenation of alkenes and alkynes in wish isomerization of terminal alkenes also occurred. Kinetic experiments, stoichiometry poisoning, mercury poisoning and kinetic reproducible data indicated the homogeneous nature of the active species. The water-soluble palladium(II) complexes, [Pd(L9)(TPPMS)(PPh3)]+TsO- (24), [Pd(L10)(TPPMS)(OTs)]+TsO- (25), [Pd(L11)(TPPMS)(PPh3)]+TsO- (26) and [Pd(L12)(TPPMS)(PPh3)]+TsO- (27) were synthesized from the reactions of L9-L12 with Pd(AOc)2 and p-TsOH in the presence of PPh3 followed by adding TPPMS in butanone. The complexes, 24-27, were highly soluble in methanol, ethanol and water but were insoluble in chlorinated solvents. Complexes 24-27 were found to form active catalysts for the high pressure hydrogenation of alkenes and alkynes in biphasic media. The complexes were recyclable and retained significant catalytic activities after six cycles. Reaction parameters such as temperature, and aqueous/organic ratio affected the catalyst recycling efficiencies.