Catalytic Water-Oxidation Activity of a Weakly Coupled Binuclear Ruthenium Polypyridyl Complex

• Published in: European journal of inorganic chemistry 2016-12, Vol.2016 (36), p.5547-5556
• Main Authors: Matias, Tiago A., Mangoni, Ana P., Toma, Sergio H., Rein, Francisca N., Rocha, Reginaldo C., Toma, Henrique E., Araki, Koiti
• Format: Article
• Language: English
• Subjects: Catalysis; Catalysts; Chlorides; Coupling; Electrocatalysis; Electronic coupling; Electronics; Evolution; Oxygen; Precursors; Ruthenium; Water splitting
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201600889

The catalytic oxidation of water by the binuclear complex [Ru2(H2O)2(bpy)2(tpy2ph)](PF6)4 [bpy = 2,2′‐bipyridine; tpy2ph = 1,3‐bis(4′‐2,2′:6′,2′′‐terpyridin‐4‐yl)benzene] was investigated comparatively to its mononuclear counterpart [Ru(H2O)(bpy)(phtpy)](PF6)2 (phtpy = 4′‐phenyl‐2,2′:6′,2′′‐terpyridine). These catalysts were prepared from the synthesis of their precursor chloride complexes, which were also extensively characterized in this work. The H2O–RuII complexes were found to undergo proton‐coupled electron‐transfer processes to generate the redox species HO–RuIII, O=RuIV, and O=RuV. The catalytically active species, [RuV2(O)2(bpy)2(tpy2ph)]6+ and [RuV(O)(bpy)(phtpy)]3+, were generated electrochemically and by using cerium(IV) ammonium nitrate. In the presence of CeIV, the catalytic rates for O2 production by the binuclear and mononuclear species were 1.9 × 10–3 and 9.5 × 10–5 s–1, respectively. This superior catalytic performance of the binuclear complex suggests that, despite weak electronic coupling between the Ru centers, the second site could play an important mechanistic role in the formation of the activated species [(bpy)(OO)RuIV(tpy2ph)RuIII(OH)(bpy)]4+. A weakly coupled binuclear complex shows a rate constant for oxygen evolution that is up to 20 times larger than that of the analogous mononuclear complex, which indicates a major role of the second site on the catalytic activity.