Synthesis, characterization and redox chemistry of Ru(II) complexes of N-methyl pyridyl quinoxaline

Abstract

A series of metal complexes having the general formula [Ru(bpy)_n(mebpq⁺)_3−n]⁽⁵⁻ⁿ⁾⁺ (n = 0, 1,2,3)(2,2′-bipyridine (bpy) and/or 2-(N-methylpyridyl)3-pyridyl quinoxaline (mebpq⁺) were synthesized and characterized by optical, elemental analysis, NMR and electrochemical methods. The absorption spectra of complexes have bands in the UV region (≤350 nm) consisting of an intense π→π* transition due to the ligands (ε∼ 10⁵ dm³ mol⁻¹ cm⁻¹) and in the visible region dominated by MLCT (dπ→π*). The emission intensity is the least when ruthenium is fully coordinated to mebpq⁺ and it increased 50 fold on going to [Ru(bpy)₃]Cl₂. Electrochemical data revealed that the Ru(II)/Ru(III) oxidation peaks are in the range 1.31 to 1.76 V and the sequential replacement of bpy by mebpq⁺ resulted in the increase of the oxidation potential. The reactions of oxidizing and reducing radicals (˙OH, O˙⁻, N₃˙, Cl₂˙⁻ and e⁻_aq) with these complexes were studied by pulse radiolysis. The ˙OH radical reacts with all complexes at diffusion controlled rates and the k values increased from 6.2 to 9.8 × 10⁹ dm³ mol⁻¹ s⁻¹ on going from [Ru(bpy)₃]Cl₂ to [Ru(mebpq⁺)₃](PF₆)₅. The rates of reaction of e⁻_aq with Ru(II) complexes are high (k≈ 10¹⁰ dm³ mol⁻¹s⁻¹) and the reaction leads to the formation of the radical anion of the bipyridine in [Ru(bpy)₃]Cl₂ and tertiary quinoxaline radical in mixed complexes containing mebpq⁺. The underlying reaction mechanism is discussed.