Synthesis, Structure, and Proton-Transfer Reactions of Brønsted Acidic Pyridylpyrazole Complexes of Ruthenium

A reaction of [{Cp*Ru(μ3-Cl)}4] (Cp*: η5-C5(CH3)5) with 5-phenyl-3-(2-pyridyl)-1H-pyrazole (PhpypzH) afforded a chelating pyrazole complex [Cp*RuCl(PhpypzH)] (2) bearing an NH proton at the β-position to the metal. Treatment of 2 with an equimolar amount of silver nitrite followed by anion exchange with KOTf (OTf: OSO2CF3) gave the pyrazolato–nitrosyl complex [Cp*Ru(NO)(Phpypz)](OTf) (3) through a proton shift from the pyrazole ligand to the nitrite ion. In contrast, simple ligand exchange took place in the reaction of silver nitrite and [Cp*RuCl(bipy)] (bipy: 2,2′-bipyridine) without the protic chelate ligand to give the nitro complex [Cp*Ru(NO2-κN)(bipy)] (4). Protonation of 4 with triflic acid led to the formation of the nitrosyl complex [Cp*Ru(NO)(bipy)](OTf)2 (5). The pyrazolato complex 3 was also obtained by the reaction of [Cp*Ru(NO)Cl2] with PhpypzH and KOTf in water. Reversible protonation of 3 resulted in the formation of the dicationic pyrazole complex [Cp*Ru(NO)(PhpypzH)](OTf)2 (6). The detailed structures of 2–4 and 6 have been determined by X-ray crystallography.