Redox chemistry of Ru(II) complexes of 6,7-dicyanodipyridoquinoxaline: a radiation chemical study

The reactions of OH, O−, SO4−, N3 and eaq− with the ligands-1,10-phenanthroline (phen) and 6,7-dicyanodipyrido [2,2-d:2′,3′-f] quinoxaline (dicnq) and a series of their Ru(II) complexes ([Ru(phen)3]2+, [Ru(phen)2(dicnq)]2+, [Ru(phen)(dicnq)2]2+ and [Ru(dicnq)3]2+) have been studied by pulse radiolysis with optical detection. The OH and hydrated electron react with the ligands via adduct formation with diffusion controlled rates. The transient spectra obtained in the reactions of OH with phen has two peaks (350 and 430 nm) having nearly equal intensities and a broad maximum at 380 nm with dicnq. In the case of complexes, the rate constants for the reactions of OH and eaq− are in the range (7.5–11.8)×109 and (1.6–3.1)×1010 dm3 mol−1 s−1, respectively. The rate for the OH reaction increased with the dicnq content in the complex, whereas a reverse trend was seen in the hydrated electron reaction. Neither the ligand nor the Ru(II) complex except [Ru(phen)2(dicnq)]2+ is reactive toward the azide radical. In the reaction of OH with Ru(II) complexes, the transients have absorption maxima in the range of 380–435 nm whose intensity decreased with the dicnq content. The spectra measured in the reactions of OH and one electron oxidants (Cl2−, SO4− or N3) with the complexes are not identical unlike with ligands. The reaction mechanism involves the addition of OH and eaq− to the ligands of the complexes and the conversion of Ru(II)–Ru(III) by the one electron oxidant.