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

A series of metal complexes having the general formula Ru(bpy)(n)(mebpq(+))(3-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 (< or = 350 nm) consisting of an intense pi --> pi* transition due to the ligands (epsilon approximately 10(5) dm(3) mol(-1) cm(-1)) and in the visible region dominated by MLCT (dpi --> pi*). The emission intensity is the least when ruthenium is fully coordinated to mebpq(+) and it increased 50 fold on going to [Ru(bpy)(3)]Cl(2). 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(-), N(3) , Cl(2)(-) and e) 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 x 10(9) dm(3) mol(-1) s(-1) on going from [Ru(bpy)(3)]Cl(2) to Ru(mebpq(+))(3)(5). The rates of reaction of e with Ru(II) complexes are high (k approximately 10(10) dm(3) mol(-1) s(-1)) and the reaction leads to the formation of the radical anion of the bipyridine in [Ru(bpy)(3)]Cl(2) and tertiary quinoxaline radical in mixed complexes containing mebpq(+). The underlying reaction mechanism is discussed.