The synthesis, characterization and electrocatalytic behaviour of novel cobalt (II) and iron (II) phthalocyanines bearing benzopyrone, benzoxazole, tetrahydropyran and furan moieties.

Abstract

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.