Browsing by Author "Mambanda, Allen."

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Controlling the reactivity of mononuclear palladiun (II) complexes. Substitution kinetics and mechanisms.
(2019) Onunga, Daniel Omondi.; Mambanda, Allen.; Jaganyi, Deogratius.
Abstract available in PDF.
Determination of polycyclic aromatic hydrocarbons in the water, soils and surface sediments of the Msunduzi River, KwaZulu-Natal, South Africa.
(2016) Munyengabe, Alexis.; Moodley, Brenda.; Mambanda, Allen.
Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants which are known carcinogens. Their presence in the environment has been linked to cancer, neurological and reproductive illnesses to name but a few. Hence it is important to monitor the levels of these PAHs in order to identify areas of high pollution and possible toxic exposure to aquatic and human life. The concentration of seven polycyclic aromatic hydrocarbons (namely naphthalene, acenaphthylene, fluorene, phenanthrene, anthracene, pyrene and chrysene) were determined in 28 surface water, 8 wastewater, 26 soil and 26 surface sediment samples from the Msunduzi River. Water samples were extracted using a liquid-liquid extraction technique into dichloromethane and dehydrated with sodium sulfate anhydrous. The soils and surface sediments were extracted with a mixture of dichloromethane and n-hexane (1:1 v/v) using the soxhlet extraction technique. The crude extracts were purified by silica gel packed column chromatography. The concentrations of PAHs in the extracts were analysed by GC-MS. The instrument was calibrated using internal standardization (deuterated PAH) and PAH standards. Percentage recoveries of 7 PAHs in the spiked and equilibrated samples varied from 79.16±0.01 to 101.28±0.02 and 80.30±0.02 to 105.56±0.01 for solid and water samples, respectively. The grand average in the summed concentrations of concentrations of the 7 PAHs in the water for all seasons decreased in the order: Σ[7-PAH] spring > Σ[7-PAH] summer > Σ[7-PAH] autumn > Σ[7-PAH] winter while in the surface sediments was in the order: Σ[7-PAH] spring > Σ[7-PAH] autumn > Σ[7-PAH] summer > Σ[7-PAH] winter and in the soils was in the order: Σ[7-PAH] spring > Σ[7-PAH] autumn > Σ[7-PAH] winter > Σ[7-PAH] summer. The concentration of PAHs was found to be comparatively higher in the soils and surface sediments than in the water samples.
Influence of bridging groups on the reactivity of dinuclear platinum (II) complexes with bis(2-pyridylmethyl)amine chelate headgroups.
(2009) Mambanda, Allen.; Jaganyi, Deogratius.
The influence on the reactivity of both the length as well as the structural nature of diamine bridges linking dinuclear Pt(II) complexes with homotopic bis(2-pyridylmethyl)amine headgroups has been investigated. For this purpose, three sets of square-planar Pt(II) complexes sharing a common non-labile bis(2-pyridylmethyl)amine chelate were synthesized and characterized by various spectroscopic methods. The substitution of the coordinated aqua ligands by three thiourea nucleophiles of different steric demands was studied in acidic aqueous medium under pseudo first-order conditions. The reactions were studied as a function of concentration, temperature and in some cases under an applied pressure using the standard stopped-flow technique and UV-visible spectrophotometry. Their thermodynamic properties were investigated by studying the acid-base equilibria of the coordinated aqua ligands using a spectrophotometric titration method. DFT Quantum mechanical calculations were also performed to determine their geometry-optimized structures and energies of the frontier molecular orbitals. The first set of Pt(II) complexes comprise dinuclears, all bridged by a flexible α,ω-alkyldiamines. The second set of complexes is Pt(II) amphiphilic mononuclear analogues of the former set, formed intuitively by excising off one of the Pt(II) chelate headgroups. The last set of complexes comprises Pt(II dinuclear complexes which are structurally related to the first set, but are linked by relatively rigid linkers, which are made up of either phenyldiamine or diaminocyclohexane fragments. In two of the complexes, a single methylene spacer (CH2 In general, the substitution reactions of the coordinated aqua ligands of all the Pt(II) complexes by the three sulfur donor nucleophiles (Nu) proceed via a two-step reaction pathway. The first step, whose rate constant is denoted in subsequent text as k) group is incorporated between the rigid moieties of the diamine bridge so as to elongate the average distances separating their Pt(II) atoms as well as to modulate the rigidity of the complexes. For comparison purposes, two monomeric analogues bearing the phenyl and cyclohexyl appended groups, respectively, were studied and reported together with these complexes. 2(1st), involves the substitution of the aqua ligands. The second step, induced by the coordination of the strong labilizing thiourea nucleophiles and whose rate constant is denoted in the text as k2(2nd), is ascribed to the dechelation of the one of the cis-coordinated pyridyl units. Thus, the substitution of the aqua ligands and the subsequent dechelation of the pyridyl units, can be expressed as kobs(1st/2nd) = k2(1st/2nd)[Nu] for all the reactions. Negative entropy of activation, negative volume of activation (in cases where measurements were carried out) and second-order kinetics for the substitution reactions all support an associative mode of activation. The substitution reactivity of all the dinuclear complexes is influenced to a greater extent by the steric influences conferred by the bridge as well as a weak electronic effect. The steric influences are mutual, axially exerted and seemingly unique to the square-planar terdentate chelate headgroups. The steric influences depend strictly on length of the diamine (i.e., the average distances separating the Pt centres of the dinuclears) as well as molecular symmetries and shapes of the complexes. The molecular symmetries and hence the shapes of the complexes depend on the parity of the connecting bonds in the diamine (whether even or not). If the connecting bonds of the bridges are even, C2h structures and hence slip-up molecular geometry are preferred. Their overlap geometries cause mutual and axial steric influences on the Pt(II) square-planar chelates which retard substitutional reactivity when the bridge is short. When odd, bowl-shaped complexes of the C2v point group symmetry are preferred in which the axial steric influences are absent at their Pt(II) chelates. In addition their bowl geometry causes an entrapment of the incoming nucleophiles, causing unusually high reactivity when compared to their even-bridged counterpart. For both molecular symmetries (C2h or C2v), the reactivity of the dinuclear complex depends on the average distances separating the Pt(II) centres of the dinuclears. In the former type of complexes, when the average distances separating their Pt(II) centres are long, the axial steric influences at each Pt(II) chelate due to their C2h The chain length as well as the structural make-up of the linker also determines the amount of electron density donated inductively from the linker to the Pt ions as well as the effective nuclear charge at each Pt(II) centre due to charge addition. These are two opposing overlap geometry is weakened, leading to enhanced reactivity as the chain length is increased. In the latter type of complexes, this weakens the ‘entrapment’ effect of their bowl-shaped geometry, resulting in a steady decrease in reactivity when the chain length of the linker is increased. In addition rigidity and planarity within the backbone of the diamine bridge has been found to distort the bowl cavity causing weakening of the ‘entrapment effect’ resulting in the lower rates than expected. iv factors which also influence the rate of substitution in these complexes to some extent. The inductive effect as well as the presence of a domineering steric influence in the C2h overlap geometry was verified by studying the reactivity of the analogous amphiphilic Pt(II) complexes.
An investigation into the influence of substituents and extended π-conjugation on the substitution reactions of bifunctional platinum (II) complexes.
(2015) Khusi, Bongumusa Bucus.; Mambanda, Allen.
Abstract available in PDF file.
Metallophthalocyanine-based electrochemical sensors for accurate qualitative and quantitative analysis of emerging pollutants in water resources.
(2024) Shoba, Siyabonga Blessing.; Booysen, Irvin Noel.; Mambanda, Allen.
Water is a precious resource and safeguarding it from pollution is paramount to ensure the well-being of both the environment and human health. Emerging contaminants such as pharmaceuticals and heavy metals pose significant threats, necessitating vigilant monitoring and appropriate action. Traditional laboratory-based analytical techniques like Gas Chromatography, ICP-OES and HPLC have been instrumental in quantifying pollutants. However, their high operational costs, maintenance requirements and the need for specialized personnel limit their widespread use, especially in resource-constrained countries. Electrochemical sensors have emerged as a promising solution. They provide real-time, portable and cost-effective options for on-site detection of pollutants in water. Current advancements in electrochemical sensors are centred around achieving selective detection using chemical modifiers, all while maintaining electrocatalytic sensitivity and reproducibility. These sensors can be tailored to target specific contaminants, making them highly efficient tools for monitoring water quality and ensuring the sustainability of this invaluable resource. In the first experimental chapter, a glassy carbon electrode (GCE) was modified by an asymmetric metallophthalocyanine (MPc) complex, A3B-CoPc-flav (where A = flavonyloxy substituent and B = an alkynyloxy substituent/molecular mast). The modification of an electrode was achieved via electrochemical grafting followed by clicked chemistry between the diazonium-functionalized GCE and the a-CoPc-flav3 to afford the GCE|clicked-a-CoPc-flav3. The chemically modified electrodes (CME) were utilized as electrocatalytic detectors for dopamine (DA) under optimized conditions. The response of the GCE|clicked-a-CoPc-flav3 was linear in the concentration range of 2 μM to 14 μM, attaining limits of detection and quantification of 0.311 and 0.942 μM, respectively, and high reproducibility (%RSD of 2.25%, N = 3). Interference studies were conducted, revealing a marginal shift in the DA peak potential in the presence of interfering substances. Despite this shift, the peak current intensity of DA remained largely unaffected, affirming the selectivity and accuracy of the CME. The analytical capabilities of the CME were further assessed using real water samples. The obtained percentage recoveries of (97.1%) of DA by the GCE|clicked-a-CoPc-flav3 and the well-established HPLC-MS method (113%) are both within the acceptable range of 80-120%. In the second experimental chapter, a platinum electrode (Pt) was modified via the electropolymerization of polypyrrole (PPy) after its co-electrodeposition of tetra-[4-((1H benzotriazole)methoxy)phthalocyaninato]cobalt(II) (CoPc-Bzt). The electrodeposition of CoPc-Bzt was performed in 1:1 DMF/acetonitrile containing 1 M tetrabutylammonium hexafluorophosphate (NBu4PF6) electrolyte over 20 cycles using cyclic voltammetry to afford a Pt|PPy/CoPc-Bzt (Bzt = benzotriazole). The resultant CME was prone to fouling by the analyte of interest, mercury(II). Due to fouling the differential pulse anodic stripping voltammetry (DPASV) was used to detect Hg(II) using the Pt|PPy/CoPc-Bzt within 10 μM to 100 μM linear range. The LOD and LOQ were found to be 3.11 and 10.00 μM, respectively. Interference studies illustrated that the detection capabilities of the CME were not affected by the presence of other heavy metal cations. The analytical performances of Pt|PPy/CoPc-Bzt (97.4%) and Inductively coupled plasma – optical emission spectroscopy (ICP-OES) (112.3%) are both within the acceptable range of 80-120%. In the third experimental chapter, the Pb electrocatalytic sensing capability of a gold electrode modified via the adsorption of electrospun nanofibers (ENFs) and Nafion (Nf) as an annealed conductive top-layer was evaluated. The fabricated ENFs comprised of a core polymeric nanocomposite of tetra-4-(3-oxyflavonephthalocyaninato)cobalt(II) (CoPc-flav), the carboxylic acid functionalized multiwalled carbon nanotubes (f-MWCNTs) and polyaniline (PANI) encapsulated in a polyvinyl acetate (PVA) ENFs. The resultant CME, Au|ENFs-1-Nf was not prone to fouling as was found when using the bare and the other constructed CMEs whose signal stabilities were compromised by background electrolyte currents. The Au|ENFs-1-Nf electrode could detect the Pb(II) cations in a reproducible manner (%RSD of 3.92%, N = 3) ranging from 8 to 125 μM, and limits of detection and quantification of 0.51 and 1.55 μM were obtained, respectively. However, the interference studies illustrated that the detection capabilities of the CME are severely compromised by the presence of other heavy metal cations. The analytical performance of the CME rendered a comparable percentage recovery (103%) with that of the ICP-OES (115%). In the fourth experimental chapter, the nanofabrication and characterization of new conductive materials, PANI-CoPc-fur (1) ((PANI = polyaniline and CoPc-fur = tetra-4-(2-furanmethylthiophthalocyaninato)Co(II)) and PANI-CoPc-fur-f-MWCNTs (2) are reported. Subsequently, an electrospun nanofiber (ENF) composite was fabricated where the core comprised of 2 that was encapsulated with a PVA shell. The resultant nanoconjugate, ENFs-2 was adsorbed on a glassy carbon electrode (GCE) followed by the immobilization of a permeable adhesion top layer of Nafion (Nf) to render the chemically modified electrode, GCE|ENFs-2-Nf. The classical physical properties of the electron-mediating layer for the CME synergistically aided in promoting its electrocatalytic activities. Consequently, the CME showed greater anodic and cathodic cyclic voltammetry (CV) peak currents compared to the bare GCE and other modified electrodes, indicating its higher sensitivity to acetaminophen (APAP), an emerging water pollutant of concern. Limits of detection and quantification (LOD and LOQ) values for APAP attained by squarewave voltammetry (SWV) were lower compared to those acquired using other electrochemical techniques. The detection of APAP at the GCE|ENFs-2-Nf attained by squarewave voltammetry (SWV) was linear from 10 to 200 μM of APAP and was reproducible (%RSD of 3.2%, N = 3). The respective calculated LOD and LOQ values of 0.094 and 0.28 μM were lower compared to those acquired using other electrochemical techniques. Analysis of APAP in the presence of commonly associated interferences metronidazole (MTZ) and dopamine (DA) illustrated a significant separation between the SWV peak potentials of APAP and MTZ, whereas there was some degree of overlap between the SWV current responses of APAP and DA. The analytical performance of the GCE|ENFs-2-Nf rendered a comparable percentage recovery (103.8%) with that of liquid chromatography–mass spectrometry (LC–MS) (106%).
The rates and mechanisms of substitution from Ru(II) complexes with different non-leaving ligand environments.
(2019) Sitati, Meshack Kituyi.; Mambanda, Allen.; Jaganyi, Deogratius.
Results from multiple studies have confirmed that the nature of ligands on the metal centre determines the properties of an anti-cancer agent in a biological environment. Ligands affect solubility, substitution reactivity, stability of compound and product after substitution and the type of interaction between the agent and DNA among others properties. Due to competition for sulphur biomolecules by anti-cancer agents in the cells, substitution reactions of potential anti-cancer metal complexes with biologically relevant ligands sheds light on the possible interaction modes of the Ru(II) complexes and stability of the resulting products. This helps in the design, synthesis and administration of new pharmacological agents and in the concept of chemo-protection. On this basis, the study of rate of substitutions from the Ru(II) complexes by thiourea nucleophiles under pseudo-first order conditions was undertaken. The reactions were studied as a function of concentration and temperature using standard Stopped Flow technique for ultrafast reactions or UV-Visible Spectrophotometer. The first series of the complexes investigated the role of arene ligands on the rate of substitutions in (aqua)(η6-arene)Ru(II) complexes. The rate of substitution for the tri-aqua Ru(II) complexes was controlled by the π-acceptor ability of the arene ligands. For the complexes bearing 2,2’-bipyridyl co-ligand, the leaving aqua ligands are located trans to the arene ligands. For these complexes, the reactivity increase in accordance to the number and type of alkyl substituents on the η6-arene ligands which donate inductively into the -molecular orbitals, causing increased trans labialisation of the coordinated aquo co-ligand. Compared to the reactivity of tri-aquo complexes, the auxiliary bipyridyl ligands lower the rate of substitution for the later complex by a factor of about 100, due to its steric hindrance at the Ru(II) metal centre. The significantly negative activation entropies and positive activation enthalpies suggest that the activation process is dominated by bond making. In the second study, the role of arene and phosphino ligands on the rate of chloride substitution from Ru (II) complexes containing arene and phosphino co-ligands was investigated. It was observed that the coordinated arene ligand donates electrons towards the Ru(II) metal centre and its -electron cloud presents an electrostatic repulsive effect onto and around the Ru centre as measured by the projected cone angle. The bidentate bis(diphenylphosphino)-methane ligand hinders the approach of nucleophiles during the substitution process. When the bis(diphenylphosphino)methane chelate is expanded through the introduction of a methylene carbon within the bridge, the steric hindrance to the approach of nucleophiles is reduced and ii the ligand assumes a trough like conformation which traps the nucleophile within the coordination sphere. This enhances the reactivity by a factor of 103. The rate of chloride substitution from 2,4,6-tris-(2-pyridyl)-1,3,5-triazineRu(PPh3)(Cl) and analogous complexes was done in the third study. The study showed that higher π-acceptor ability of cis ligands increase the electrophilicity of the metal centre resulting in enhanced reactivity. Electron donating substituents on the ligands at the cis position lower the π-acceptor ability of the ligand hence lower electrophilicity of the Ru(II) metal centre leading to slower rate of substitution. The investigation on the effect of 2-(2-Pyridyl)azole-based ancillary ligands (L) on the chloride substitution from [RuII(tpy)(L)(Cl)]+ in the fourth study revealed that strong electronegative atoms (O or S) in the auxiliary ligands enhance their π-back-donation capacity thereby increasing the electrophilicity of the metal centre and hence the reactivity. On the other hand, the –NH group donate electron density to the metal centre by outer sphere proton donation causing trans-effect thereby which increases the reactivity more than in the former case. The fifth study sought to understanding the effect of substituents on rate of chloride substitution from Ru(II)tpy complexes. Ru(II)tpy complexes with the tpy having electron donating substituents trans to the labile ligand were dominated by trans-effect while those where the tpy bears electron accepting substituents had enhanced π-back-bonding controlling the reactivity. The rate of substitution from the Ru(II) complexes was more strongly affected by electron donating substituents. Electron donating ligands at the cis position slow down the rate of substitution from the Ru(II) metal centre. Data from DFT calculations performed using Gaussian09 suite of programmes was used to support the observed rates of substitution from the Ru(II) complexes. Large negative values of entropies of activation and positive enthalpies of activation indicate associative mode of activation. On the other hand small positive values of entropies of activation indicate dissociatively activate interchange mode of substitutions. The studies were explored on model Ru(II) complexes with bio-relevant thiourea nucleophiles to predict possible interaction with biomolecules which has become part of the methods used in the endeavour to search for alternative anti-cancer agents with improved efficacy and higher spectrum of activity.
Role of Substituents on the reactivity of mononuclear cis-platinum(II) complexes with 2 (phenylthiomethyl)pyridyl/quinolyl non-leaving ligands.
(2017) Mthiyane, Wakhiwe Mthandi.; Mambanda, Allen.; Jaganyi, Deogratius.
Abstract available in PDF file.
Screening of water pollutants using metallophthalocyanine-based chemically modified electrodes.
(2021) Kantize, Kevin.; Booysen, Irvin Noel.; Mambanda, Allen.
Abstract available in PDF.
The synthesis, characterization and electrocatalytic behaviour of novel cobalt (II) and iron (II) phthalocyanines bearing benzopyrone, benzoxazole, tetrahydropyran and furan moieties.
(2015) Chohan, Sumayya.; Booysen, Irvin Noel.; Mambanda, Allen.
The fabrication of metallophthalocyanine (MPc)-based modified electrodes has proven to be effective for the electrocatalysis of various bio-analytes and pollutants. The selectivity of these chemically modified electrodes can be fine-tuned by the attachment of biologically relevant substituents to MPcs which has shown to facilitate the detection of numerous analytes. Hence, this study focuses on the design of MPcs bearing chromone, coumarin, flavone, benzoxazole, tetrahydropyran and furan moieties. The formulated MPcs were characterized using UV-Vis and FT-IR spectroscopy, ESI-TOF mass spectrometry and elemental analysis. The redox properties of the complexes were investigated via voltammetry and the subsequent voltammetric assignments were corroborated by UV-Vis spectroelectrochemistry. Each metal complex displayed four redox processes of which the Pc ring oxidation is irreversible and the remaining redox couples are quasi-reversible. Novel cobalt and iron phthalocyanines peripherally tetra-substituted with chromone (chr) or coumarin (cou) moieties were formulated and characterized in chapter three. The structural elucidations of the ligands, 4-(chromone-7-oxy)phthalonitrile (1) and 4-(4-(trifluoromethyl)-coumarin-7-oxy)phthalonitrile (2) were complemented by NMR spectroscopy and single crystal X-ray analysis (for 1). Utilizing the respective MPcs, modified working electrodes were prepared by electropolymerization and their electrocatalytic activities towards nitrite oxidation were explored. All the metal complexes showed an increase in nitrite oxidation currents and a minor decrease in Sumayya Chohan II oxidation potentials which is indicative of electrocatalysis. The trend of electrocatalytic activity was found to be as follows: CoPc-chr (3) > FePc-cou (4) > CoPc-cou (5). Chapter four focuses on the synthesis and characterization of cobalt phthalocyanines (CoPcs) containing flavone (flav) and benzoxazole (bo) moieties. CoPc-flav (3), CoPcbo (4), multi-walled carbon nanotubes (MWCNTs) and CoPc-MWCNT conjugates were used to prepare modified glassy carbon electrodes (GCEs) which were tested for dopamine electrocatalysis. Both CoPc modified electrodes (3-GCE and 4-GCE) showed higher peak currents, slightly lower peak potentials and improved reversibility compared to the bare GCE. The respective CoPc-MWCNT conjugates were found to further enhance dopamine detection. 3-MWCNT-GCE and 4- MWCNT-GCE showed lower peak to peak separations than the respective CoPc modified electrodes indicating faster electron transfer kinetics. Chronoamperometry was employed to determine the catalytic rate constants of each electrode which were superior to previously reported values. 4-MWCNT-GCE was noted to be the most effective electron mediator in the electrocatalysis of dopamine. Chapter five reports on the synthesis of tetrahydropyran (thp) and furan (fur) substituted CoPcs. The electrocatalysis of L-cysteine was tested using CoPc-thp (3), CoPc-fur (4) and CoPc-cou (5) reported in chapter three. Modified electrodes were prepared using the drop-dry method. While the bare GCE and 4-GCE showed no peaks for L-cysteine oxidation in the 0.0 - 0.70 V potential window; the modified electrodes showed a well-defined peak at 0.40 V for 3-GCE and a broad peak at 0.52 V for 5-GCE. Kinetic parameters were determined using chronoamperometry, rotating disc electrode (RDE) studies and construction of Tafel plots. It was found that L-cysteine oxidation using 3-GCE proceeded at a faster rate.