| Popis: |
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)(2)Ru-III(mu-L2-)Ru-III(acac)(2)] (diastereomeric 1/2), [(bpy)(2)Ru-II(mu-L2-)Ru-II(bpy)(2)](ClO4)(2) ([3](ClO4)(2)), and [(pap)(2)Ru-II(mu-L2-)Ru-II(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 Ru-III-Ru-III complexes (1/2) display interesting one-dimensional-chain effects, as revealed by temperature-dependent magnetic studies. The stability of the Ru-III oxidation state in 1/2 under ambient conditions correlates well with the sigma-donating acetylacetonato (acac) coligands. However, with pi-accepting 2,2'-bipyridine (bpy) or phenylazopyridine (pap) the Run state is preferably stabilized in 3(2+) or 4(2+), respectively, but there are interesting differences in their oxidative chemistry. The moderately pi accepting bpy allows for the Ru-II to Ru-III oxidation at reasonably low anodic potentials. However, for the strongly pi accepting pap, no Ru-II to Ru-III 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. |
