Effects of a strong π-accepting ancillary ligand on the water oxidation activity of weakly coupled binuclear ruthenium catalysts

• Published in: Dalton transactions : an international journal of inorganic chemistry 2019-02, Vol.48 (9), p.39-317
• Main Authors: Matias, Tiago A, Rein, Francisca N, Rocha, Reginaldo C, Formiga, André Luiz Barboza, Toma, Henrique E, Araki, Koiti
• Format: Article
• Language: English
• Subjects: Benzene; Catalysis; Electronic properties; Ligands; Organic chemistry; Oxidation; Ruthenium; Valence
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c8dt04963g

Significant differences were found in the proton-coupled redox chemistry and catalytic behavior of the binuclear [{Ru(H 2 O)(bpz)} 2 (tpy 2 ph)](PF 6 ) 4 complex [bpz = 2,2′-bipyrazine; tpy 2 ph = 1,3-bis(4′-2,2′:6′,2′′-terpyridin-4-yl)benzene] as compared with the structurally analogous derivative with 2,2′-bipyridine (bpy) instead of bpz. The differences were assigned to the stronger π-accepting character of bpz relative to bpy as the ancillary ligand. The expectation of a positive shift for the Ru-centered redox potentials was confirmed for the lower oxidation state species, but that trend was reversed in the formation of the high-valence catalytic active species as shown by a negative shift of 0.14 V for the potential of the [Ru IV/V &z.dbd;O] process. Moreover, DFT calculations indicated a significant decrease of about 15% on the spin density and oxyl character of the [Ru V &z.dbd;O] 3+ fragment. The significantly lower k cat (O 2 ) for the bpz system was attributed to these combined electronic effects. A less positive Ru IV/V &z.dbd;O redox potential and lower water oxidation activity accounted for based on the spin density and oxyl character of [Ru V &z.dbd;O] 3+ species.