KINETIC INVESTIGATION AND MECHANISMS OF REDUCTION OF N, N'-PHENYLENEBIS(SALICYLIDENEIMINATO)COBALT(III) ION BY SOME REDUCING AGENTS IN DMSO-H2O MEDIUM
KINETIC INVESTIGATION AND MECHANISMS OF REDUCTION OF N, N'-PHENYLENEBIS(SALICYLIDENEIMINATO)COBALT(III) ION BY SOME REDUCING AGENTS IN DMSO-H2O MEDIUM
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Date
2021
Authors
ABDULSALAM, SAFIYA
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Abstract
The kinetic investigation and mechanisms of reduction of N,N'-phenylenebis(salicyalideneiminato)cobalt(III) ion (hereafter denoted as [Co(salophen)]+) by L-cysteine (LSH), thioglycol (RSH), thioglycolic acid (TSH), 1,5-diphenylcarbazide (DPCI) thiosemicarbazide (TSCZ), acetaldehyde (CH3CHO), formaldehyde (HCHO), iodide ion (I-), ascorbic acid (H2A) and thiodiglycol (S(R)2) were studied in aqueous hydrochloric acid (HCl) medium, except for TSH which was monitored in the absence of acid. The stoichiometry was found to be 1:2 (reductant/complex) in the CH3CHO, HCHO and S(R)2, 3:2 for H2A and 1:1 for LSH, RSH, TSH, DPCI, TSCZ and I- systems. The rate of reaction was first order in both the [oxidant] and [reductants] for all the systems and second order overall. Addition of acid had inverse effect on the rates of reaction for the [Co(salophen)]+ - TSH and [Co(salophen)]+ - H2A systems, and direct dependence for the [Co(salophen)]+ - LSH and [Co(salophen)]+ - RSH systems but was found to be independent on the rate of the rest reactions. The overall rate equation for the reactions can be given as;
[Co(salophen)+] = (a + b[H+]-1)[Co(salophen)+][reductants]
where reductant = H2A and TSH
[Co(salophen)+] = (a + b[H+])[Co(salophen)+][reductants]
where reductants = LSH and RSH
Varying the ionic strength (μ) and dielectric constant (D) of the reaction medium had no effect on rates of the reactions of DPCI, TSCZ, CH3CHO, HCHO, I- and S(R)2, while in the case of LSH and RSH reactions, increase in μ led to an increase in rate while decrease in D led to decrease in rate of the reactions. For the TSH and H2A reactions, increase in μ led to the decrease in the reaction rates and decrease in D led to an increase in the rate of reactions. Added
ions (Ca2+, Mg2+, SO42-, CH3COO- and HCOO-) catalysed and/or inhibited the rate of the reactions for all the systems except for I- reaction where added ions had no effect on the reaction rate. Free radicals were identified in all the systems except for DPCI and TSCZ systems. The temperature dependence study showed a negative entropy of activation (ΔS‡) for all the systems except in the case of TSH reaction where positive entropy was observed. Spectroscopic information and the results of the Michaelis – Menten‟s plots suggest lack of formation of stable and detectable intermediate complex prior to electron transfer for all the systems. For LSH, RSH, TSH and TSCZ systems, disulphide was found to be the oxidation product, sulphoxide for S(R)2, 1,5-diphenylcarbazone (DPCO) for DPCI, dehydroascorbic acid for H2A, iodine for I- and acetic acid and formic acid for CH3CHO and HCHO reactions respectively. Based on the results obtained in the investigations, the reactions are proposed to have proceeded through the outer-sphere electron transfer mechanism.
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A THESIS SUBMITTED TO THE SCHOOL OF POSTGRDUATE STUDIES,
AHMADU BELLO UNIVERSITY, IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD OF A DOCTOR OF PHILOSOPHY DEGREE IN INORGANIC CHEMISTRY
DEPARTMENT OF CHEMISTRY,
FACULTY OF PHYSICAL SCIENCES,
AHMADU BELLO UNIVERSITY,
ZARIA, NIGERIA