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

The Ru complex [Ru (bda-κ-N O )(N-NH ) ] (1; bda =2,2'-bipyridine-6,6'-dicarboxylate, N-NH =4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new Ru and Ru bda complexes that contain NO , MeCN, or H 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-NH is transformed into a diazonium salt [N-N =4-(pyridin-4-yl)benzenediazonium)] together with the formation of a new Ru(NO) moiety in the equatorial zone, to generate [Ru (bda-κ-N O)(NO)(N-N ) ] (2 ). Here the bda ligand binds in a κ-N 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 (bda-κ-N O)(MeCN)(N-NH ) ] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[Ru {bda-κ-(NO) }(H O)(N-NH ) ](H O) } (4 ) 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-NH . Additionally, the one-electron oxidized complex {[Ru {bda-κ-(NO) }(H O)(N-NH ) ](H O) } (5 ) 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-N O is the main coordination mode for 2 and 3, an equilibrium that involves isomers with κ-N O and κ-NO coordination modes and neighboring hydrogen-bonded water molecules is observed for 4 and 5 .