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 [MnII(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, [(OH2)(terpy)MnIII(μ-O)2MnIV(terpy)(OH2)]3+ (1) by KMnO4 as an oxidant. When Oxone and Ce(IV) oxidants were used, the further oxidation of 1 to [(OH2)(terpy)MnIV(μ-O)2MnIV(terpy)(OH2)]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 (1ads) adsorbed on mica for KMnO4- and Oxone-treated systems, indicating that 1′ (1′ads) and 2 (2ads) adsorbed on mica do not work for the catalysis. The kinetic analysis suggested that 1ads works for the catalysis through cooperation with adjacent 1ads or 2ads, meaning that 2ads assists the cooperative catalysis by 1ads though 2ads is not able to work for the catalysis alone. For the Ce(IV)-treated system, O2 evolution was hardly observed although the sufficient amount of 1ads 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 Ce3+ cations (generated in oxidative dimerization of 1′) co-adsorbed with 1ads.