The synthesis, characterization and electrocatalytic behaviour of novel cobalt (II) and iron (II) phthalocyanines bearing benzopyrone, benzoxazole, tetrahydropyran and furan moieties.
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Date
2015
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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.
Description
Master of Science in Chemistry. University of KwaZulu-Natal, Pietermaritzburg 2015.
Keywords
Chemical reactions., Electrodes., Organic compounds -- Synthesis., Electrocatalysis., Tetrahydrofuran., Theses -- Chemistry.