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Doctoral Degrees (Chemistry)

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Browsing Doctoral Degrees (Chemistry) by SDG "SDG9"

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    Carboxamide ruthenium(II) and manganese(II) complexes: structural, kinetic, and mechanistic studies in the transfer hydrogenation of ketones.
    (2022) Kumah, Robert Tettey.; Ojwach, Stephen Otieno.
    The carboxamide ligands N-(benzo[d]thiazol-2-yl)pyrazine-2-carboxamide (HL1), N-(1H-benzo[d]imidazol-2-yl)pyrazine-2-carboxamide (HL2), were prepared by condensation of pyrazine-carboxylic acid and appropriate heteroaromatic amines. Reactions of HL1 and HL2 with ruthenium(II) precursors, [RuH(CO)Cl(PPh3)3] and [RuH2(CO)(PPh3)3] afforded the mononuclear complexes [RuL1(PPh3)2(CO)Cl] (Ru1), [RuL1(PPh3)2(CO)H] (Ru2), [RuL2(PPh3)2(CO)Cl] (Ru3), [RuL2(PPh3)2(CO)H] (Ru4). The solid-state structures of complexes Ru1, Ru2, and Ru4 reveal bidentate modes of coordination of the ligands and distorted octahedral geometries around the Ru(II) centre. The complexes formed active catalysts in the transfer hydrogenation of ketones and achieved turnover number (TON) up to 530 in 6 h. The ruthenium(II)–hydride complexes, Ru2 and Ru4, were capable of catalysing transfer hydrogenation of ketones reactions under base free reaction conditions and demonstrated higher catalytic activities compared to the corresponding non-hydride analogues (Ru1 and Ru3). An inner sphere monohydride mechanism involving dissociation of one PPh3 group was proposed from in situ 31P{1H} NMR spectroscopy studies. Dipicolinamide ligand system, N,N'-(1,4 phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L5), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L6) were synthesised following a similar protocol described for HL1 and HL2. Treatment of the ligands H2L3 and H2L4 with RuH(CO)Cl(PPh3)3 afforded bimetallic complexes [Ru2(H2L3)(PPh3)4(CO)2][2Cl] (Ru5), [Ru2(H2L3)(H)2(PPh3)4(CO)2] (Ru5b), [Ru2(HL4)(PPh3)3(CO)2Cl3] (Ru6) and a mononuclear complex [RuCl2L4(PPh3)2(CO)] (Ru7). The solid-sate structure of the dinuclear ruthenium(II) complexes confirmed a bidentate coordinate mode, with PPh3, CO, and chlorido auxiliary ligands occupying the remaining coordinating sites to afford distorted trigonal bipyramidal geometries (Ru5 and Ru6) while the mononuclear complex Ru7 adopted a distorted octahedral geometry around its ruthenium(II) atom. The reaction of the ligands H2L4-H2L6 with the [RuCl2-η6-p-cymene]2 precursor produces half-sandwich diruthenium complexes [{Ru(η6-p-cymene)}2-μ-Cl(L4)][Ru(η6-p-cymene)Cl3] (Ru8), [{Ru(η6-p-cymene)}2-μ-Cl(L4)][PF6] (Ru9), [{Ru(η6-p-cymene)}2-μ-Cl(L5)][PF6] (Ru10), and [{Ru(η6-p-cymene)}2-μ-Cl (L6)][PF6] (Ru11). The molecular structure of cationic complexes, Ru8-Ru11, was confirmed by single-crystal X-ray crystallography analysis. The complexes Ru8-Ru11 display a bidentate Npyridine ^ Namidate mode of coordination to give pseudo-octahedral geometry (piano-stool-like geometry). The ruthenium(II) complexes demonstrated remarkable enhanced catalytic activity (TON values up to 1.71 x 104) in the transfer hydrogenation of ketones at a very low catalyst loading of 2.75 x10-2 mol% (275 ppm). The dinuclear ruthenium(II) complexes showed higher catalytic activity compared to the corresponding mononuclear complex Ru5. The half-sandwich diruthenium complexes Ru8-Ru11 displayed relatively higher catalytic activity than the ruthenium complexes Ru5 and Ru6 bearing the PPh3 co-ligands. Monohydride inner-sphere catalytic cycle was proposed for the transfer hydrogenation of ketones catalysed by both Ru1 and Ru9, and the formation of the reactive intermediates was supported with low-resolution mass spectrometry data. The dinuclear ruthenium complexes of pyridine and pyrazine-carboxamide bearing quinolinyl motif were synthesised by reacting, N-(quinolin-8-yl)pyrazine-2-carboxamide, (HL7), 5-methyl-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL8), 5-chloro-N-(quinolin-8-yl)pyridine-2-carboxamide, (HL9), and 2-pyrazine-carboxylic acid (HL10) with equimolar [RuCl2(η6-p-cymene)]2 to afford the dinuclear complexes [{Ru(η6-p-cymene)}2Cl3(L10)] (Ru12), [{Ru(η6-p-cymene)Cl}2(L7)] [PF6] (Ru13), [{Ru(η6-p-cymene)Cl}2(L8)][Ru(L8)Cl3] (Ru14), and [{Ru(η6-p-cymene)Cl}2(L9)][PF6] (Ru15), respectively. The solid-state structures of the dinuclear complexes Ru12 and Ru13 reveal a typical piano-stool geometry around the Ru(II) ions. The dinuclear ruthenium complexes Ru12-Ru15 were used as catalysts in the transfer hydrogenation of a broad spectrum of aldehydes and ketones and demonstrated excellent catalytic activity, TON values up to 4.8 x 104, using catalyst loading of 2.0 x10-3 mol% (20 ppm). The catalytic performance of the complexes was affected by the ligand architecture and the substituents on the pyridyl ring. Complexes Ru13-15 exhibited higher catalytic activities compared to the complex Ru12 which could be ascribed to the role of quinoline in stabilising the complexes. The pyridine and pyrazine motifs have a significant impact on the reactivity and the catalytic activity of the complexes. In-situ low-resolution ESI-MS analyses of the reactive intermediates aided in proposing a monohydride inner-sphere mechanism for the transfer hydrogenation of ketones catalysed by Ru15. To develop a more sustainable, environmentally compatible and cost-efficient protocol for transfer hydrogenation of ketones, a new catalytic system based on manganese(II) metal was synthesised. New manganese(II) complexes Mn1-Mn4, ligated on dipicolinamide ligands were synthesized by treating the N,N'-(1,4-phenylene)dipicolinamide (H2L3), N,N'-(1,2-phenylene)dipicolinamide (H2L4), N,N'-(4-methoxy-1,2-phenylene)dipicolinamide (H2L5) and N,N'-(4,5-dimethyl-1,2-phenylene)dipicolinamide (H2L6) with MnCl2.4H2O salt to afford dinuclear manganese(II) complexes [Mn2(H2L3)2Cl4] (Mn1), [Mn2(H2L4)2Cl4] (Mn2), [Mn2(H2L5)2(Cl)4] (Mn3) and [Mn2(H2L6)2Cl4] (Mn4). The solid-state structure of complex Mn2 showed a six-coordinate dinuclear complex with the two Mn(II) ions adopting a distorted octahedral environment surrounded by two tetradentate ligands and chlorido co-ligands, respectively. The Mn(II) complexes formed active catalysts in transfer hydrogenation of ketones to achieve TON values up to 5.12 x 104. The presence of electron-donating substituents -OCH3 and -CH3 in complexes Mn3 and Mn4 displayed minor effects in the transfer hydrogenation of ketones. The new carboxamide-manganese(II) complexes are among the most active manganese-based catalysts capable of hydrogenating a large scope of ketones ranging from aliphatic to aromatic ketones. A dihydride catalytic cycle has been proposed and supported with in-situ low-resolution mass spectrometry data.
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    Syntheses of mixed donor homogeneous and immobilized palladium(II) complexes catalysts for methoxycarbonylation and hydrogenation reactions.
    (2021) Akiri, Saphan Owino.; Ojwach, Stephen Otieno.
    Reactions of ligands (E)-N'-(2,6-diisopropylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L1), (E)-N'-(2,6-diisopropylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L2), (E)-N'-(2,6-dimethylphenyl)-N-(6-methylpyridin-2-yl)benzimidamide (L3), (E)-N'-(2,6-dimethylphenyl)-N-(4-methylpyridin-2-yl)benzimidamide (L4) and (E)-N-(6-methylpyridin-2-yl)-N'-phenylbenzimidamide (L5) with [Pd(NCMe)2Cl2] furnished the corresponding palladium(II) pre-catalysts (Pd1-Pd5), in good yields. Molecular structures of Pd2 and Pd3 revealed an N^N bidentate coordination mode to afford square planar compounds. Activation of the palladium(II) complexes with para tolyl sulfonic acid (PTSA) afforded active catalysts in the alkenes methoxycarbonylation. The resultant catalytic activities were controlled by both the complex structure and alkene substrate. While aliphatic substrates favoured the formation of linear esters (>70%), styrene substrate resulted in predominantly branched esters of up to 91%. The water-soluble ligands; sodium 4-hydroxy-3-((phenylimino)methyl)benzenesulfonate (L6), sodium 3-(((2,6-dimethylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L7) and sodium 3-(2,6-diisopropylphenyl)imino)methyl)-4-hydroxybenzenesulfonate (L8) reacted with with Pd(OAc)2 afford their respective palladium(II) complexes [Pd(6)2] (Pd6), [Pd(L7)2] (Pd7) and [Pd(L8)2] (PdL8). In addition, treatment of the non-water-soluble ligands 2-((phenylimino)methyl)phenol (L9), 2-(((2,6-dimethylphenyl)imino)methyl)phenol (L10) and 2-((2,6 diisopropylphenyl)imino)methyl)phenol (L11) with Pd(OAc)2 yielded complexes [Pd(L9)2] (Pd9), [Pd(10)2] (Pd10) and [Pd(L11)2] (Pd11), respectively in good yields. Solid-state structures of compounds Pd6 and Pd9 revealed bis(chelated) square planar neutral compounds. All the complexes formed active catalysts in the methoxycarbonylation of 1- hexene, affording yields of up to 92% within 20 h and regioselectivity of 73% in favour of linear esters. The activities and selectivities of the compounds depended on the steric encumbrance around the coordination centre. The water-soluble complexes displayed comparable catalytic behaviour to the non-water-soluble systems. The complexes could be recycled five times with minimal changes in both the catalytic activities and regio-selectivity. Reactions of (amino)phenyl ligands, (E)-N-((Z)-4-(phenylamino)pent-3-en-2-ylidene)aniline (L12) and N,N'E,N,N'E)-N,N'-(3-(3 (triethoxysilyl)propyl)pentane-2,4-diylidene)dianiline (L13) with [Pd(NCMe)2Cl2] led to the formation of homogeneous complexes Pd13 and Pd14. Besides, supporting of complex Pd14 with either MCM-41, SBA-15, or Fe3O4 magnetic nanoparticles gave immobilized complexes P15-Pd17, respectively. Using varying metal loading in the MCM-41 immobilization of complex Pd14 produced complexes Pd18 and Pd 19. In addition, calcination of complex Pd16 at 150oC and 200oC led to the formation of complexes Pd20 and Pd21, respectively. All the complexes were received in good yields. The catalytic activities and selectivities of the homogeneous complexes were influenced by the coordination sphere, with the complexes predominantly forming linear esters. On the other hand, the catalytic behaviours of the immobilized catalysts depended on the nature of support and calcination temperatures. In addition, the catalytic activities were observed to depend on the reaction temperature, catalyst loading, amounts of PPh3 and acid promoters. The immobilized complexes Pd15, Pd16 and Pd17, were recycled up to five times. The homogeneous and silica immobilized palladium(II) complexes of ligands (2-phenyl-2-((3(triethoxysilyl)propyl)imino)ethanol) (L14), (4-methyl-2-((3(triethoxysilyl)propyl)imino)methyl)phenol ) (L15 ), [L14-MCM-41 (L16), and [L15- MCM-41 (L17)]. The homogeneous complexes [Pd(L14)2] (Pd22), [Pd(L14)2] (Pd23), [Pd(L14)(Cl2)] (Pd24), [Pd(L15)(Cl2)] (Pd25) were obtained from homogenous ligands L14, L15, L16 and L17 respectively. In addition, the silica immobilized compounds [Pd(L14)2]-MCM-41] (Pd26) and [Pd(L15)2)-MCM-4] (Pd27) were obtained through convergence immobilization of complexes Pd22 and Pd23, respectively. Comparatively, immobilized complexes [Pd (L14)(Cl2)-MCM-41] (Pd28) and [Pd(L15)(Cl2)]-MCM-41] (Pd29) were obtained from the complexation of immobilized ligands L16 and L17. Both sets of complexes gave active catalysts in molecular hydrogenation of alkenes, alkynes and functionalized benzenes. The catalytic activities and product distribution in these reactions were largely dictated by the nature of the substrate. The kinetic studies revealed reaction orders dependence on styrene for both the homogeneous and supported catalysts. Significantly, the selectivity of both sets of catalysts was comparable in the hydrogenation of alkynes and multi-functionalized benzenes. The supported catalysts could be recycled up to four times with minimum reduction in catalytic activities and showed the absence of any leaching from hot filtration experiments. Kinetics and poisoning studies established the presence of active homogeneous species for complexes Pd22-Pd5 and Pd(0) nanoparticles for the immobilized complexes Pd26-Pd29, respectively.