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

Abstract

The reactions of √ OH, O√ ^-, SO₄√ ^-, N₃√ and e_aq^- 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 e_aq^- 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 vOH and e_aq^- to the ligands of the complexes and the conversion of Ru(II)-Ru(III) by the one electron oxidant.