On the non-innocence and reactive versus non-reactive nature of α-diketones in a set of diruthenium frameworks

Abstract

α-Diketones are an important class of building blocks employed in many organic synthetic reactions. However, their coordination chemistry has rarely been explored. In light of this, our earlier report on [(acac)₂Ru^II(μ-2,2′-pyridil)Ru^II(acac)₂] (acac = acetylacetonate) showcased the sensitivity of a diketone fragment towards oxidative C–C cleavage. Following the lead, the synthesis of similar but stable diketo fragments containing diruthenium compounds was attempted. Three diruthenium compounds with the bridge 1,2-bis(2-hydroxyphenyl)ethane-1,2-dione (L) were prepared: diastereomeric [(acac)₂Ru^III(μ-L²⁻)Ru^III(acac)₂], 1a(rac)/1b(meso), [(bpy)₂Ru^II(μ-L²⁻)Ru^II(bpy)₂](ClO₄)₂, [2](ClO₄)₂ and [(pap)₂Ru^II(μ-L²⁻)Ru^II(pap)₂](ClO₄)₂, [3](ClO₄)₂ with ancillary ligands of different donating/accepting characteristics. The metal is stabilised in different oxidation states in these complexes: Ru(III) is preferred in 1a/1b when σ-donating acac is used as the co-ligand whereas electron rich Ru(II) is preferred in [2](ClO₄)₂ and [3](ClO₄)₂ when co-ligands of moderate to strong π-accepting properties are employed. The oxidative chemistry of these systems is of particular interest with respect to the participation of varying bridging-ligands which contain phenoxide groups. On the other hand, the reduction processes primarily resulting from the metal or the ancillary ligands are noteworthy as the normally reducible 1,2-diketo- group remains unreduced. These results have been rationalised and outlined from thorough experimental and theoretical investigations. The results presented here shed light on the stability of metal coordinated α-diketones as a function of their substituents.