A Review on Antioxidants Activity of the Ruthenium Complexes of Rx-Methylbenzoylthiourea Derived Ligands: Spectroscopic Characterization and Computational Study

Ng, Han Sean (2022) A Review on Antioxidants Activity of the Ruthenium Complexes of Rx-Methylbenzoylthiourea Derived Ligands: Spectroscopic Characterization and Computational Study. Final Year Project (Bachelor), Tunku Abdul Rahman University College.

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Abstract

Three novel ruthenium (II) polypyridine complexes, [RuII(bpy)2(phen-o-CH3BT)](PF6)2 (Complex 1), [RuII(bpy)2(phen-m-CH3BT)](PF6)2 (Complex 2), and [RuII(bpy)2(phen-p-CH3BT)](PF6)2 (Complex 3) (where, bpy = bipyridine, phen = phenanthroline, BT = benzoylthiourea) have been synthesized and characterized with CHNS elemental analysis, 1H NMR analysis, cyclic voltammetry, spectroscopic analyses and computational studies. The antioxidant activity of the three complexes was investigated with the different CH3- substituent position (o, m, p) on the benzoylthiourea moieties. FTIR, 1H NMR and CHNS analyses verified the structure of the three complexes to be identical as the proposed structure. FTIR’s and UV-Vis’s result were agreeing well with the DFT computational study. In the result of UV-Vis and photoluminescence, identical peaks position of MLCT, π→π* and 3MLCT→1GS were observed for the three complexes, it indicates that different substituent position has insignificant effect on the variation of energy gap level. The similar energy gap level was verified by the computational study, which the FMO was showing similar excitation transition energy and excitation transition orbital for the three complexes. Among the three complexes, Complex 1 was showing the lowest molar absorptivity (Ɛ) and quantum yield (ϕ) for UV-Vis and photoluminescence study. This is because the structure of Complex 1 was sterically hindered and exhibiting restricted chain motion, thus, leading to the inhibition of electron charge transfer process. In cyclic voltammetry, the redox properties of the three complexes were affected by the resonance and inductive effect in different substituted position, which lead to different electron donating effect. The electron donating effect was exhibiting in the following sequence: Complex 1 > Complex 3 > Complex 2. The Complex 2 with lowest electron donating effect will undergo reduction easily and oxidation difficultly. Moreover, antioxidant mechanism of the ruthenium complexes was happening through the donation of Hydrogen atom from N-H group in the benzoylthiourea to the free radical. The antioxidant activity of ruthenium complexes was influenced by two factors, which is steric effect and electron donating effect. The stronger electron donating effect will stabilize the radical formed in the complexes after the free radical scavenging reaction, thus, increasing antioxidant activity. However, the steric effect will be blocking the hydrogen abstraction reaction of free radical with the N-H group in benzoylthiourea, thus, decreasing antioxidant activity. Overall, the antioxidant activity of the three complexes were predicted to be in the following sequence: Complex 3 > Complex 1 > Complex 2.