Catalytic Water Oxidation by Mononuclear Ru Complexes with an Anionic Ancillary Ligand

Mononuclear Ru-based water oxidation catalysts containing
anionic ancillary ligands have shown promising catalytic efficiency
and intriguing properties. However, their insolubility in water restricts
a detailed mechanism investigation. In order to overcome this disadvantage,
complexes [RuII(bpc)­(bpy)­OH2]+ (1+, bpc = 2,2′-bipyridine-6-carboxylate,
bpy = 2,2′-bipyridine) and [RuII(bpc)­(pic)3]+ (2+, pic = 4-picoline) were
prepared and fully characterized, which features an anionic tridentate
ligand and has enough solubility for spectroscopic study in water.
Using CeIV as an electron acceptor, both complexes are
able to catalyze O2-evolving reaction with an impressive
rate constant. On the basis of the electrochemical and kinetic studies,
a water nucleophilic attack pathway was proposed as the dominant catalytic
cycle of the catalytic water oxidation by 1+, within which several intermediates were detected by MS. Meanwhile,
an auxiliary pathway that is related to the concentration of CeIV was also revealed. The effect of anionic ligand regarding
catalytic water oxidation was discussed explicitly in comparison
with previously reported mononuclear Ru catalysts carrying neutral
tridentate ligands, for example, 2,2′:6′,2″-terpyridine
(tpy). When 2+ was oxidized to the trivalent
state, one of its picoline ligands dissociated from the Ru center.
The rate constant of picoline dissociation was evaluated from time-resolved
UV–vis spectra.