Catalytic Water Oxidation
by Mononuclear Ru Complexes with an Anionic Ancillary Ligand
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Abstract
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 [Ru<sup>II</sup>(bpc)(bpy)OH<sub>2</sub>]<sup>+</sup> (<b>1</b><sup>+</sup>, bpc = 2,2′-bipyridine-6-carboxylate,
bpy = 2,2′-bipyridine) and [Ru<sup>II</sup>(bpc)(pic)<sub>3</sub>]<sup>+</sup> (<b>2</b><sup>+</sup>, pic = 4-picoline) were
prepared and fully characterized, which features an anionic tridentate
ligand and has enough solubility for spectroscopic study in water.
Using Ce<sup>IV</sup> as an electron acceptor, both complexes are
able to catalyze O<sub>2</sub>-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 <b>1</b><sup>+</sup>, within which several intermediates were detected by MS. Meanwhile,
an auxiliary pathway that is related to the concentration of Ce<sup>IV</sup> 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 <b>2</b><sup>+</sup> 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