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

Abstract

The Ru complex [RuII(bda-κ-N2O2)(N-NH2)2] (1; bda2−=2,2′-bipyridine-6,6′-dicarboxylate, N-NH2=4-(pyridin-4-yl)aniline) was used as a synthetic intermediate to prepare new RuII and RuIII bda complexes that contain NO+, MeCN, or H2O 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 [RuII(bda-κ-N2O)(NO)(N-N2)2]3+ (23+). Here the bda2− ligand binds in a κ-N2O 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 [RuII(bda-κ-N2O)(MeCN)(N-NH2)2] (3). In acidic aqueous solutions, a related reaction occurs in which solvent water coordinates to the Ru center to form {[RuII{bda-κ-(NO)3}(H2O)(N-NH3)2](H2O)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 {[RuIII{bda-κ-(NO)3.5}(H2O)(N-NH3)2](H2O)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-N2O is the main coordination mode for 23+ and 3, an equilibrium that involves isomers with κ-N2O and κ-NO2 coordination modes and neighboring hydrogen-bonded water molecules is observed for 42+ and 53+.