Browsing by Author "Obuah, C."
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Effects of electrochemical properties of ferrocenylpyrazolylnickel( ii ) and palladium( ii ) compounds on their catalytic activities in ethylene oligomerisation reactions(RSC Advances, 2018-01) Obuah, C.; Ainooson, M.K.; Darkwa, J.Palladium complexes of ferrocenylpyrazolylpyridine and ferrocenylpyrazolylamine were synthesised and screened as pre-catalysts (1–4) for olefin polymerisation. The pre-catalysts 1–4 on activation with EtAlCl2 in the presence of ethylene with chlorobenzene or hexane as solvent were highly active with 1 being the most active, with an activity of 360 kg mol Pd⁻¹ h⁻¹. The major product from the reaction was 1-butene and high carbon content oligomers. The molecular weight (m/z) of the high carbon content oligomers is as high as 623.0. When toluene is used as solvent, the products obtained were ethyltoluene and butyltoluene and 1-butene. The electronic properties of the ligands (L1–L7) and complexes (1–10) were determined by cyclic voltammetry (CV) and molecular modelling. The CV results show that the ferrocenyl is easily oxidized upon the introduction of pyrazolyl derivatives, the process is quasi-reversible. However, complexation of the ligands with palladium or nickel results in difficulty in oxidizing the ferrocenyl moiety. This is an indication of the electrophilic nature of both the palladium and nickel centres. The mechanism of the oxidation was observed to be diffusion-controlled and is independent of scan rate. Molecular modelling experiments show that nickel and palladium complexes have lower HOMO–LUMO gaps and high global descriptors, an indication of a highly electrophilic metal centre. A plot of the electrophilicity indices of the pre-catalysts against yield of the oligomers show a linear correlation, an indication that the electrophilicity of the metal centre plays an important role in the activity of these pre-catalysts.Item Electron-density distributions in selected ferrocenyl-pyrazolyl late transition-metal complexes(Physical Chemistry Chemical Physics, 2018-03) Peck, M.A.; Hearne, G.R.; Obuah, C.; Darkwa, J.57Fe Mössbauer spectroscopy has been used to study electronic dispersions in complexes of Fe, Co, Ni and Pd anchored on 3-ferrocenyl-5-methylpyrazolylmethylenepyridine and 3-ferrocenylpyrazolylmethylenepyridine ligands. Mössbauer spectroscopy shows that pyrazolyl-derivatizing of ferrocene increases the electron-density at the Fe-centre as well as facilitating the Fe-to-cyclopentadienyl ring back-donation of electron-density. The coordination of the ferrocenyl-pyrazolyl ligand to transition metals such as Fe, Co, Ni and Pd, reduces the electron-density at the Fe-centre of the ferrocenyl moiety while increasing the electron-density at the coordinated metal centre, especially in the Fe complexes. The electron-density change at the coordination metal centre is inversely proportional to the electronegativity of the halide substituent. Furthermore, the type of substituent (Me or H) at position 5 on the pyrazolyl moiety has influence on the electron density at the ferrocenyl-Fe and the coordinated metal centre. The methyl group as a substituent reduces the π-acceptor ability of the pyrazolyl and therefore increases the electron-density at the ferrocenyl-Fe centre. However, when the substituent is hydrogen, the electron-density at the coordination metal centre increases. Similarly, for other metals (i.e., Co, Ni and Pd) the electron density at the ferrocenyl-Fe is also significantly reduced upon coordination of the ligand to the metal. Additionally, Mössbauer experiments reveal a trivalent Fe species in the synthesized complexes which is not discerned by X-ray and elemental analysis. The species has been identified as the oxidative product [Fe(III)X4]- where X = Cl or Br. The study also highlights and cautions on the possibility of photo-oxidation processes involving both ferrocene and the coordinating Fe-halides under standard lighting conditions with possible contributions from aerated solvents.Item Innovations and modifcations of current extraction methods and techniques of citrus essential oils: a review(Discover Applied Sciences, 2024) Brah, A.S.; Obuah, C.; Adokoh, C.K.The genus Citrus of the Rutaceae family remains one of the benefcial fruit crops that produce high quantities of essential oils that have pharmaceutical, biological, and food preservative applications. Despite the numerous benefts of citrus essential oils (CEOs), there is a major challenge in choosing the most efcient extraction method(s) for large-scale pro duction of quality CEOs to meet industrial, research, and domestic demands. This review provides a general overview of the listed citrus species, the chemical composition of their essential oils, medicinal uses, and the major methods of extraction of citrus essential oils from 10 selected citrus species. A meticulous, in-depth review of the various methods of CEOs extraction has been provided, along with their advantages, limitations, and novel modifcations. This compre hensive literature review expounded on the current extraction methods for citrus essential oils and the various modi fcations developed to reduce the extraction time, excessive energy consumption, CO2 production, and quality, as well as to improve the extraction yield.Item A new sulfonamide-based chemosensor for potential fluorescent detection of Cu2þ and Zn2þ ions(Tetrahedron, 2023) Amoah, C.; Obuah, C.; Ainooson, M.K.; Oppong, A.; Muller, A.In recent times, there has been an increased demand in the search for probing materials for numerous substances in the environment such as the detection of metals ions. In this study, a new class of pyrazolyl-sulfonamide derivatives of para-nitroaniline were synthesized following a multistep approach. The ligands and complexes were characterized using NMR spectroscopy, IR spectroscopy, and mass spectrometry. All the compounds C1eC3 were synthesized in very good yields (85%e92%) and their photo-physical properties measured. All the compounds show fluorescence behaviour with emissions within the UV and far visible range with quantum yields between 7.7% and 25.7%. TD-DFT calculations predictions for the electronic transitions present are in good agreement with experimental observations. Fluorescent probing studies conducted on the compounds show that C1eC3 were analytically sensitive and possessed significant selectivity towards Cu2þ (for C3) and Zn2þ (for C1 and C2) ions with detection limits between 0.011 and 0.103 mg/L for Cu2þ ions and 0.002e0.135 mg/L for Zn2þ ions. Overall, C1 was found to be the most sensitive molecule for the metals studied, having good quantum yield and better selectivity for Zn2þ ion compared to Cu2þ.Item (Pyrazolylethyl-amine)zinc(II) carboxylate complexes as catalysts for the copolymerization of CO2 and cyclohexene oxide(Polyhedron, 2022) Matiwane, A.; Obuah, C.; Darkwa, J.Pyrazolyl compounds 2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)ethyl-amine) (L1), 2-(3,5-diphenyl-1H-pyrazol- 1-yl)ethyl-amine (L2), and 2-(3-phenyl-5-(trifluoromethyl)-1H-pyrazol-1-yl)ethyl-amine (L3) were reacted with a mixture of zinc(II) acetate and 3,5-dinitrobenzoic acid to form the bidentate complexes [(2-(3,5-di-tert-butyl-1H-pyrazol-1-yl)-ethyl-amine)-Zn(C6H5COO(NO2)2] (1), [(2-(3,5-diphenyl-1Hpyrazol- 1-yl)ethyl-amine)Zn(C6H5COO(NO2)2] (2), and [(2-(5-phenyl-3-(trifluoro-methyl)-1H-pyrazol- 1-yl)ethyl-amine)Zn(C6H5COO(NO2)2] (3) respectively. All three zinc complexes were tested as catalysts for the copolymerization of CO2 and cyclohexene oxide (CHO) and found active to form poly(cyclohexene carbonate) (PCHC) and cyclohexene carbonate (CCHC) at CO2 pressures as low as 1.5 MPa and under solvent- free conditions in the absence of a co-catalyst. Increase in CO2 pressure resulted in activity and showed selectivity up to 99% selectivity for the formation of the copolymer PCHC. Optimum temperature for the polymerization was 100 C and even at this temperature selectivity towards formation of PCHC was found to be 99%. The copolymers obtained have moderate molecular weights (3860–11,500 g/mol) and polydispersity indices varying from 2.73 to 4.93.Item Quantum Mechanistic Studies of the Oxidation of Ethylene by Rhenium Oxo Complexes(Hindawi, 2021) Fosu, E.A.; Obuah, C.; Hamenu, L.; Aniagyei, A.; Ainooson, M.K.; Govender, K.K.Transition-metal-mediated oxygen transfer reactions are of importance in both industry and academia; thus, a series of rhenium oxo complexes of the type ReO3L (L = O− , Cl− , F− , OH− , Br− , I− ) and their effects as oxidation catalysts on ethylene have been studied. %e activation and reaction energies for the addition pathways involving multiple spin states (singlet and triplet) have been computed. In all cases, structures on the singlet potential energy surfaces showed higher stability compared to their counterparts on the triplet potential energy surfaces (PESs). Frontier Molecular Orbital calculations show electrons flow from the HOMO of ethylene to the LUMO of rhenium for all complexes studied except ReO4 − where the reverse case occurs. In the reaction between ReO3L (L = O− , Cl− , F− , OH− , Br− , and I− ) and ethylene, the concerted [3 + 2] addition pathway on the singlet PES leading to the formation of dioxylate intermediate is favored over the [2 + 2] addition pathway leading to the formation of a metal laoxetane intermediate and subsequent rearrangement to the dioxylate. %e activation and the reaction energies for the formation of the dioxylate on the singlet PES for the ligands studied followed the order O− > OH− > I − > F− > Br− >Cl− and O− > OH− > F− > I − > Br− > Cl− , respectively. Furthermore, the activation and the reaction energies for the formation of the metallaoxetane intermediate increase in the order O− > OH− > I − > Br− > Cl− > F− and O− > Br− > I − >Cl− > OH− > F− , respectively. %e subsequent rearrangement of the metallaoxetane intermediate to the dioxylate is only feasible in the case of ReO4 − . Of all the complexes studied, the best dioxylating catalyst is ReO3Cl (singlet surface) and the best epoxidation catalyst is ReO3F (singlet surface).Item Synthesis and characterization of (pyrazolylethylphosphinite)nickel(II) complexes and catalytic activity towards ethylene oligomerization(Applied Organometallic Chemistry, 2018-02) Edor, J.M.; Amenuvor, G.; Obuah, C.; Muller, A.; Darkwa, J.Compounds (2‐(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L1), 2‐(3,5‐di‐tert‐butyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L2), and 2‐(3,5‐diphenyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L3) were prepared using the synthetic routes reported in literature. These compounds were reacted with [NiCl2(DME)2] or [NiBr2(DME)2] under appropriate reaction conditions to afford six new nickel(II) compounds ([NiCl2(L1)] (1), [NiCl2(L2)] (2), [NiCl2(L3)] (3), [NiBr2(L1)] (4), [NiBr2(L2)] (5) and [NiBr2(L3)] (6)). The new nickel(II) pre‐catalysts catalyze the oligomerization of ethylene, in the presence of ethylaluminium dichloride as co‐catalyst, to produce butenes, hexenes, octenes and higher carbon chain ethylene oligomers with very little Friedel‐Crafts alkylation products when the reactions were run in toluene.Item Synthesis, characterization and coordination chemistry of (pyrazolylphosphinite)palladium(II) complexes(Inorganica Chimica Acta, 2018-02) Edor, J.M.; Amenuvor, G.; Obuah, C.; Muller, A.; Darkwa, J.Pyrazolylethylphosphinite compounds, L1 (2-(3,5-dimethyl-1H-pyrazol-1-yl)ethyldiphenlyphosphinite) and L2 (2-(3,5-di-tert-butyl-1H-pyrazol-yl)ethyldiphenylphosphinite) were reacted with the palladium(II) precursors [PdCl(CH3)(COD)] and [PdCl2(NCCH3)2] resulted in the formation of five novel complexes, produced from disproportionation reaction and have very interesting coordination chemistry. The results further emphasize the coordination versatility of pyrazolylphosphinite ligands towards palladium(II) metal centres.Item Synthesis, characterization and fluorescent properties of ferrocenyl pyrazole and triazole ligands and their palladium complexes(Journal of Organometallic Chemistry, 2021) Amoah, C.; Obuah, C.; Ainooson, M.K.; Muller, A.The search for new fluorescent materials with high quantum yields has been the focus of research, con- sidering the diverse applications that fluorescent materials present. In this study, ferrocenyl pyrazolyl ( L1) and ferrocenyl triazolyl ( L2 ) ligands and their palladium metal complexes ( 1 and 2 ) were synthesized to investigate their fluorescence properties. While the pyrazolyl ligand was prepared through a ligand substitution reaction, synthesis of the triazolyl derivative involved the [3 + 2] azide-alkyne using click chemistry. All the prepared compounds have been characterized by NMR and IR spectroscopy, elemental analysis, mass spectrometry and single-crystal X-ray crystallography. These air-stable compounds were prepared in moderate to good yields (70% and 83%). The ferrocenyl-pyrazolyl L1 and ferrocenyl-triazolyl L2 ligands showed near-infrared (NIR) emission bands with quantum yields of 20% and 26% respectively. The addition of the Pd 2 + ions results in the reduction of fluorescence intensity and quantum yield. Such fluorescent properties demonstrate their potential use in bio-analysis and as fluorogenic probing.Item Theoretical studies on the reaction mechanisms of the oxidation of tetramethylethylene using MO3Cl (M=Mn, Tc and Re)(Journal of Molecular Graphics and Modelling, 2023) Fosu, E.A.; Obuah, C.; Hamenu, L.; Aniagyei, A.; Oppong, A.; Ainooson, M.K.; Muller, A.A theoretical study on the reaction mechanisms of the addition of transition metal oxo complexes of the type MO3Cl (M = Mn, Tc, and Re) to tetramethylethylene (TME) is presented. Theoretical calculations using B3LYP/ LACVP* and M06/LACVP* (LACVP* is a combination of the 6-31G(d) basis set along with LANL2DZ pseudo potentials on the metallic centres) were performed and the results are discussed within the framework of reaction energetics. The nature of the stability of the reaction mechanisms was equivalent for both theories. However, the M06/LACVP* simulations generally had slightly lower energies and shorter bond lengths compared to the B3LYP/LACVP* computations. Furthermore, it was observed that the reaction does not proceed via the stepwise reaction mechanism due to kinetic and thermodynamic instabilities. Epoxidation was also found to occur via the [2 + 2] concerted reaction mechanism for the MO3Cl (M = Tc and Re) whereas the permanganyl chloride complex epoxidizes TME via the [2 + 1] concerted reaction mechanism on the singlet potential energy surface (PES). Dioxylation was observed to proceed via the [3 + 2] route for the addition of MO3Cl (M = Tc and Re) and TME. Results indicate that all reaction surfaces were unselective except for the permanganyl chloride catalyzed surface which leads to the formation of epoxides exclusively. Changes in temperatures from 298.15 K to 373.15 K, resulted in kinetically and thermodynamically unstable reaction pathways as the activation and reaction energies increased generally. On the singlet PES, the rate constant calculations showed that the [3 + 2] catalyzed surface reaction mechanism leading to dioxylation was faster than the [2 + 2] mechanism in cases where plausible. Theoretical values from the global reactivity parameters, namely the chemical hardness, chemical potential, electrophilic and nucleophilic indices, are in good correlation with the DFT activation and reaction energies at both levels of theories. Thus, as the electrophilic nature of the metal decreases from Mn to Re, so do the activation and reaction energies increase from Mn to Re, indicating that the higher the electrophilicity of the metal centre, the more spontaneous the oxidation reaction.Item Toxicity and therapeutic applications of citrus essential oils (CEOs): a review(International Journal of Food Properties, 2023) Brah, A.S.; Armah, F.A.; Obuah, C.; Akwetey, S.A.; Adokoh, C.K.Citrus essential oil (CEO) is obtained from the fruit of Genus Citrus, a flowering plant shrub in the family of the Rutaceae (Eremocitrus or Microcitrus) and extensively used in food, chemical industry, and traditional medicinal treatment owing to its pleasant aroma, antioxidant, and antiseptic properties. This review presents a botanical description, distribution, traditional uses, chemical composition, bioactive components, and the therapeu tic uses as well as toxicological effects of the CEO. The objective was achieved via a comprehensive literature search of electronic databases such as Science Direct, PubMed, Web of Science, Wiley, ACS, Springer, Taylor and Francis, Google Scholar, SCOPUS, conference proceedings, thesis, and books until 2022 for publications. Citrus essential oils and their constituents are extracted and isolated either from the fruit peels, seeds, leaves, or flowers of the citrus plants. A comparative study of the sources of CEO confirmed its origin, ethnopharmacological and therapeutic uses. Over 2000 secondary metabolites have been isolated, with the main active constituents: being terpenes, monoterpenes, sesquiterpenes, and diterpenes. A comprehensive literature review revealed vast therapeutic benefits of CEO. Incomplete data report on in vitro and in vivo trials especially, on dosage, positive and negative control groups, intervention time, toxicity studies, phytochemical profiling, and clinical trials seem to be a knowledge gap.