Behavior of Ru-bda Water-Oxidation Catalysts in Low Oxidation States

The Ru complex [Ru^II (bda-κ-N² O² )(N-NH₂ )₂ ] (1; bda^2- =2,2'-bipyridine-6,6'-dicarboxylate, N-NH₂ =4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new Ru^II and Ru^III bda complexes that contain NO⁺ , MeCN, or H₂ O ligands. In acidic solution complex 1 reacts with an excess of NO⁺ (generated in situ from sodium nitrite) to form a new Ru complex in which the aryl amine ligand N-NH₂ is transformed into a diazonium salt [N-N₂⁺ =4-(pyridin-4-yl)benzenediazonium)] together with the formation of a new Ru(NO) moiety in the equatorial zone, to generate [Ru^II (bda-κ-N² O)(NO)(N-N₂ )₂ ]³⁺ (2³⁺ ). Here the bda^2- ligand binds in a κ-N² O tridentate manner with a dangling carboxylate group. Similarly, complex 1 can also react with a coordinating solvent, such as MeCN, at room temperature to give [Ru^II (bda-κ-N² O)(MeCN)(N-NH₂ )₂ ] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[Ru^II {bda-κ-(NO)³ }(H₂ O)(N-NH₃ )₂ ](H₂ O)_n }²⁺ (4²⁺ ) and is strongly hydrogen-bonded with additional water molecules in the second coordination sphere. Furthermore, under acidic conditions the aniline ligands are also protonated to form the corresponding anilinium cationic ligands N-NH₃⁺ . Additionally, the one-electron oxidized complex {[Ru^III {bda-κ-(NO)^3.5 }(H₂ O)(N-NH₃ )₂ ](H₂ O)_n }³⁺ (5³⁺ ) was characterized, in which the fractional value in the κ notation indicates the presence of an additional contact to the pseudo-octahedral geometry of the Ru center. The coordination modes of the complexes were studied in the solid state and in solution through single-crystal XRD, X-ray absorption spectroscopy, variable-temperature NMR spectroscopy, and DFT calculations. While κ-N² O is the main coordination mode for 2³⁺ and 3, an equilibrium that involves isomers with κ-N² O and κ-NO² coordination modes and neighboring hydrogen-bonded water molecules is observed for 4²⁺ and 5³⁺ .