Distribution of manganese species in an oxidative dimerization reaction of a bis-terpyridine mononuclear manganese (II) complex and their heterogeneous water oxidation activities.

Abstract Heterogeneous water oxidation catalyses were studied as a synthetic model of oxygen evolving complex (OEC) in photosynthesis using mica adsorbing various manganese species. Distribution of manganese species formed in the oxidative dimerization reaction of [Mn II (terpy) 2 ] 2+ (terpy = 2,2′:6′,2″-terpyridine) ( 1 ′) with various oxidants in water was revealed. 1 ′ was stoichiometrically oxidized to form di-μ-oxo dinuclear manganese complex, [(OH 2 )(terpy)Mn III (μ-O) 2 Mn IV (terpy)(OH 2 )] 3+ ( 1 ) by KMnO 4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH 2 )(terpy)Mn IV (μ-O) 2 Mn IV (terpy)(OH 2 )] 4+ (2) was observed after the oxidative dimerization reaction of 1 ′. The mica adsorbates with various composition of 1 ′, 1 and 2 were prepared by adding mica suspension to the various oxidant-treated solutions followed by filtration. The heterogeneous water oxidation catalysis by the mica adsorbates was examined using a Ce(IV) oxidant. The observed catalytic activity of the mica adsorbates corresponded to a content of 1 ( 1 ads ) adsorbed on mica for KMnO 4 - and Oxone-treated systems, indicating that 1 ′ ( 1 ′ ads ) and 2 ( 2 ads ) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1 ads works for the catalysis through cooperation with adjacent 1 ads or 2 ads , meaning that 2 ads assists the cooperative catalysis by 1 ads though 2 ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O 2 evolution was hardly observed although the sufficient amount of 1 ads was contained in the mica adsorbates. This was explained by the impeded penetration of Ce(IV) ions (as an oxidant for water oxidation) into mica by Ce 3+ cations (generated in oxidative dimerization of 1 ′) co-adsorbed with 1 ads .