Controlled photosubstitution of Ru(ii) polypyridyl complexes with gold chloride

Abstract

The well-known chloride (Cl⁻)-ion-induced photosubstitution (PS) reaction of ruthenium tris-polypyridyl complexes [Ru(PP)₃]²⁺ deals with HCl and tetraalkylammonium chloride (TEACl) as sources of Cl⁻ ions. However, these reported Cl⁻ ion sources could not show any restriction on the PS reaction irrespective of the nature of the [Ru(PP)₃]²⁺ complexes. In this study, the glutathione (GSH)-mediated reduction of [AuCl₄]⁻ in methanol (MeOH) or acetonitrile (MeCN), as a new alternate source of Cl⁻ ions, demonstrates restrictions in the PS reaction of [Ru(PP)₃](PF₆)₂ complexes 1 and 2 (PP = 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen), respectively). However, a similar study with complexes 3 and 4 (PP = 2,2′-bipyrazine (bpz) and 1,4,5,8-tetraazaphenanthrene (tap), respectively) shows a facile PS reaction with quantum yields (Φ_photosub) of 0.36 and 0.30, respectively. A mechanistic investigation by XPS analysis of the reaction mixture with complex 1 suggests the presence of not only of Au⁺ and oxidized glutathione (GSSG) but also of Au³⁺ and GSH during photoirradiation. These equilibria in the presence of 1 or 2 restrict the attack of a Cl⁻ ion on the Ru²⁺ center. In contrast, in the case of complexes 3 and 4, Cl⁻ attacks the more electropositive Ru²⁺ center, and there is no trace of Au³⁺ in XPS. Further reactions without GSH have been carried out between [AuCl₄]⁻ and 1 or 3 in aqueous medium. Immediate red precipitates from the reaction generate single crystals of [Ru(bpy)₃][AuCl₄]₂ (1′), and [Ru(bpz)₃]₂[AuCl₆][AuCl₄]₃ (3′) upon crystallization. [AuCl₆]⁻ in 3′ oxidizes [Ru(bpz)₃]²⁺ to [Ru(bpz)₃]³⁺ and itself reduces to produce [AuCl₂]⁻ and Cl⁻ in the crystal structure of [Ru^II(bpz)₃][Ru^III(bpz)₃][AuCl₂]₂Cl₃ (3′′). The selective PS reaction (Φ_photosub = 0.01) in 3′ is attributed to the presence of octahedral [AuCl₆]⁻ as a source of Cl⁻ ions, which has been obtained for the first time. Conversely, 1′ with [AuCl₄]⁻ as a counterion does not show any PS reaction.