New ruthenium(II) and osmium(II) trinuclear dendrons. Synthesis, redox behavior, absorption spectra, and luminescence properties

Abstract

The new trinuclear dendrons [Cl₂Os{(μ-2,3-dpp)Ru(bpy)₂}₂]⁴⁺3, Cl₂OsRu₂ (bpy = 2,2′-bipyridine; 2,3-dpp = 2,3-bis(2-pyridyl)pyrazine), [Cl₂Os{(μ-2,3-dpp)Os(bpy)₂}₂]⁴⁺4, Cl₂OsOs₂, and [(bpy)Ru{(μ-2,3-dpp)Os(bpy)₂}₂}⁶⁺6, bpyRuOs₂ have been synthesized, and their redox properties, absorption spectra, and luminescence properties studied, together with the same properties of the species [Cl₂Ru{(μ-2,3-dpp)Ru(bpy)₂}₂]⁴⁺1, Cl₂RuRu₂, [Cl₂Ru{(μ-2,3-dpp)Os(bpy)₂}₂]⁴⁺2, Cl₂RuOs₂, and [(bpy)Ru{(μ-2,3-dpp)Ru(bpy)₂}₂}⁶⁺5, bpyRuRu₂. All the compounds undergo several reversible metal-centered oxidation and ligand-centered reduction processes which are assigned to specific subunits of the trinuclear structure. Analysis of the reduction patterns of the various compounds allows one to obtain information on the extent of ligand–ligand interactions mediated by the central metals. The compounds show very intense ligand-centered absorption bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible region, which can be assigned to transitions involving specific subunits of the dendron structure. All except 3 and 4 exhibit MLCT luminescence both at room temperature in acetonitrile fluid solution and in a MeOH–EtOH 4 ∶ 1 (v/v) rigid matrix at 77 K. The luminescence properties are dominated by the photophysical properties of the subunit(s) in which the lowest-lying excited state of any structure is localized, suggesting that fast intercomponent energy transfer takes place in all the compounds.