Polynuclear metal complexes of nanometre size. A versatile synthetic strategy leading to luminescent and redox-active dendrimers made of an osmium (II)-based core and ruthenium (II)-based units in the branches

A docosanuclear metal complex of nanometric size and dendritic shape made of an osmium(II)-based core and containing 21 ruthenium(II)-based units in the branches has been prepared. The key building blocks are the [Os(2,3-dpp)3]2+ 'complex ligand', and the [Ru(2,3-MedPp)2Cl2]2+ and [{Ru(bpy)2(μ-2,3-dpp)}2RuCl2]4+ 'complex metals' {2,3-dpp = 2,3-bis(2-pyridyl)pyrazine; 2,3-Medpp+ =2-[2-(1-methylpyridiniumyl)]-3-(2-pyridyl)pyrazine; bpy=2,2′-bipyridine}. The first step of the synthesis is the formation of the tetranuclear [Os{μ-2,3-dpp)Ru(2,3-Medpp)2}3]14+ species in which the peripheral ligands 2,3-Medpp+ are 2,3-dpp ligands with the second chelating site inactivated (protected) by methylation. This species is obtained from the reaction of the [Os(2,3-dpp)3]2+ 'complex ligand' core, which contains three open chelating positions, with three equivalents of the [Ru(2,3-Medpp)2Cl2]2+ 'complex metal', where the labile Cl- ligands can be replaced by the chelating units of the core. Successive demethylation (deprotection) of the tetranuclear compound opens the six peripheral chelating sites. At this stage, the divergent synthesis can be iterated {reaction with six equivalents of the [Ru(2,3-Medpp)2Cl2]2+ 'complex metal'} with formation of the protected decanuclear compound [Os{(μ-2,3-dpP)Ru[(μ-2,3-dpP)Ru(2,3-Medpp)2]2} 2]32+. Alternatively, in a convergent approach, the reaction of the deprotected tetranuclear species with six equivalents of the trinuclear [{Ru(bpy)2(μ-2,3-dpp)}2RuCl2]4+ 'complex metal' leads to the docosanuclear [Os{(μ-2,3-dpp)Ru[(μ-2,3-dpp)Ru{(μ-2,3-dpp)Ru(bpy)2} 2]2}3]44+ species. The absorption spectra, luminescence properties, and electrochemical behaviour of [Os(2,3-dpp)3]2+, [Os(μ-2,3-dPp)3{Ru(2,3-MedPP)2}3] 14+, [Os{(μ-2,3-dPP)Ru[(μ-2,3-dpp)Ru(2,3-MedpP)2]2} 3]32+, and [Os{(μ-2,3-dpp)Ru[(μ-2,3-dpp)Ru{(μ-2,3-dpp)Ru(bpy)2} 2]2}3]44+ have been investigated.