The Behavior of the Ru-bda Water Oxidation Catalysts at 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 geometr