Photosubstitution reactions of ruthenium complexes with pyrazole ligands, cis-[Ru­(bpy)2(pzH)2]2+ (1a), cis-[Ru­(bpy)2(pz)­(pzH)]+ (1b), and cis-[Ru­(bpy)2(pz)2]0 (1c) (pzH = pyrazole, bpy = 2,2′-bipyridine), were investigated. Dicationic complex 1a was deprotonated to 1b using moderate base (pK a = 15.2, MeCN), while the second deprotonation to give 1c required more severe conditions (pK a = 26.9). Monocationic complex 1b possessed an N–H···N-type intramolecular hydrogen bond between the pyrazole and pyrazolate ligands, as corroborated by the solid-state crystal structure. The photosubstitution quantum yield of 1a (Φ = 0.26) was comparable to that of cis-[Ru­(bpy)2(pyridine)2]2+ (Φ = 0.24) in acetonitrile solution. In contrast, the photodissociation of a pzH ligand was strongly suppressed by the deprotonation of a pyrazole ligand N–H group. In the presence of 10 000 equiv of 4,4′-dimethylaminopyridine, the quantum yield dropped to ∼2 × 10–6 in acetonitrile. The photosubstitution quantum yield of 1b was even smaller than that of neutral complex 1c, although 1c had a smaller HOMO–LUMO energy gap than monocationic complex 1b. The small quantum yield of 1b was attributed to intramolecular hydrogen bonding between pyrazole and pyrazolate ligands. The apparent rate constants for the photosubstitution of 1b were highly solvent-dependent. The photosubstitution of 1b was suppressed in aprotic solvents, while the reaction was accelerated by 2 orders of magnitude in protic solvents with strong proton donor abilities.

Introduction of a chloro substituent to the bidentate ligand of ruthenium aqua complexes enhanced photoisomerization and thermal back-isomerization.