The Effect of Electron-Withdrawing and Electron-Donating Groups on Bis(Bipyridine) Ruthenium(II) Complexes of N,N’-Chelate Ligands: Experimental and Computational Studies

Yeoh, Zi Ying (2021) The Effect of Electron-Withdrawing and Electron-Donating Groups on Bis(Bipyridine) Ruthenium(II) Complexes of N,N’-Chelate Ligands: Experimental and Computational Studies. Final Year Project (Bachelor), Tunku Abdul Rahman University College.

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Abstract

Novel ruthenium (II) polypyridyine complexes, [RuII(bpy)2(phen-p-NO2BT)](PF6)2 (1) and [RuII(bpy)2(phen-p-OCH3BT)](PF6)2 (2) (where, bpy = bipyridine, phen = phenanthroline, BT = benzoylthiourea) have been structurally characterized through CHNS elemental analyses, CV measurement, and spectroscopic methods. The substituents’ effects on their redox and spectroscopic properties of both complexes were investigated based on experimental and computational studies. In the FTIR results, complex 1 exhibited v(C=O) and v(C=S) bands at higher stretching frequency than complex 2 as the nitro group strengthened the bonds in both C=O and C=S groups by decreasing their bond lengths via resonance effect. The FTIR spectra were consistent with the calculated vibrational spectra. Besides, the redox data obtained in MeCN showed that complex 1 exhibited higher oxidation and reduction potentials than complex 2 because electron-withdrawing NO2 exhibited stabilization effects on HOMO and LUMO by decreasing the respective energy levels, which was consistent with the lower EHOMO and ELUMO in complex 1. Oxidation occurred on ruthenium-based HOMO while reduction occurred on LUMO contributed from π*-orbital localized on ligands. Besides, both complexes displayed MLCT and   * transitions at the similar wavelengths. Overall, complex 1 showed higher molar absorptivity (Ɛ) than complex 2 because of the wider distribution of response charges throughout the structure. The origins of each electronic transition were well-interpreted through DFT calculation using Gaussian 09 software. Moreover, complex 1 showed the highest Ɛ at high pH due to the stabilization of the charges on the deprotonated moiety, while complex 2 exhibited the highest Ɛ at low pH because the electron-donating OCH3 stabilized the electrostatic charges on the protonated moiety. This caused a larger extent of electronic delocalization and thereby higher Ɛ at respective pH. Also, the changes of Ɛ due to pH variation in both complex were consistent with theoretical results.