Questions of Noninnocence and Ease of Azo Reduction in Diruthenium Frameworks with a 1,8-Bis(( E)-phenyldiazenyl)naphthalene-2,7-dioxido Bridge

Inorg Chem. 2018 Oct 15;57(20):12800-12810. doi: 10.1021/acs.inorgchem.8b01996. Epub 2018 Sep 25.

Abstract

Ligands containing the azo group are often used in various metal complexes owing to their facile one-electron reduction, which in effect extends the means of degrading environmentally harmful azo dyes. In order to probe the idea of the generally accepted ease of reduction of azo-containing compounds, we present here three different diruthenium complexes [(acac)2RuIII(μ-L2-)RuIII(acac)2] (diastereomeric 1/2), [(bpy)2RuII(μ-L2-)RuII(bpy)2](ClO4)2 ([3](ClO4)2), and [(pap)2RuII(μ-L2-)RuII(pap)2](ClO4)2 ([4](ClO4)2 ) with a bridging ligand (L2- = 1,8-bis(( E)-phenyldiazenyl)naphthalene-2,7-dioxido) that contains azo groups in addition to phenoxide-type donors. The RuIII-RuIII complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the RuIII oxidation state in 1/2 under ambient conditions correlates well with the σ-donating acetylacetonato (acac) coligands. However, with π-accepting 2,2/-bipyridine (bpy) or phenylazopyridine (pap) the RuII state is preferably stabilized in 32+ or 42+, respectively, but there are interesting differences in their oxidative chemistry. The moderately π accepting bpy allows for the RuII to RuIII oxidation at reasonably low anodic potentials. However, for the strongly π accepting pap, no RuII to RuIII oxidation is observed within the solvent window. Instead, a phenoxide to phenoxyl radical type of oxidation based on the bridging ligand is observed. Surprisingly, the reductive chemistry of all three complexes is dominated by either the ruthenium centers or the coligands (bpy or pap), with no reductions observed on the azo function associated with the central bridging ligand (L2-). All of the above conclusions were drawn from combined structural, electrochemical, magnetic, spectroelectrochemical, and DFT investigations. Our results thus conclusively establish that the ease of reduction of an azo group in a particular compound is critically dependent on its substituents and that the noninnocence of the bridging ligands (L2-) in the dinuclear complexes can be decisively tuned by the appropriate choice of ancillary ligands.