Hirosato Yamazaki

Molecular catalysts for water oxidation toward artificial photosynthesis

Artificial photosynthesis is anticipated as one of the promising clean energy-providing systems for the future. The development of an efficient catalyst for water oxidation to evolve O2 is a key task to yield a breakthrough for construction of artificial photosynthetic devices. Recently, significant progress has been reported in the development of the molecular catalysts for water oxidation based on manganese, ruthenium and iridium. The molecular aspects of the catalysts reported in the last decade were reviewed to provide hints to design an efficient catalyst, as well as to gain clues to reveal the mechanism of O2 evolution at photosynthetic oxygen evolving complex in nature.

Arrangement effect of the di-μ-oxo dimanganese catalyst and Ru(bpy)32+ photoexcitation centers adsorbed on mica for visible-light-derived water oxidation

A photosynthetic photosystem II (PS II) model was developed by adsorbing [(OH2)(terpy)MnIII(μ-O)2MnIV(terpy)(OH2)]3+ (1, terpy = 2,2′:6′,2′′-terpyridine) as an oxygen evolving center and Ru(bpy)32+ (bpy = 2,2′-bipyridine) as a photoexcitation center onto mica. The mica adsorbates were prepared by three different methods; Adsorbate A was prepared by adsorption of 1 followed by Ru(bpy)32+ on mica, and Adsorbates B and C were prepared by the opposite adsorption order and co-adsorption of 1 and Ru(bpy)32+, respectively. The UV-visible diffuse reflectance (DR) spectroscopic data and emission decay measurements of the photoexcited Ru(bpy)32+ suggested the different arrangements of 1 and Ru(bpy)32+ among the three adsorbates in a mica interlayer. For Adsorbate A, Ru(bpy)32+ could be adsorbed near the mica surface, being shallowly intercalated relative to 1. For Adsorbate B, 1 is adsorbed near the mica surface, Ru(bpy)32+ being deeply intercalated relative to 1. For Adsorbate C, either 1 or Ru(bpy)32+ could be randomly intercalated relative to the other adsorbates. In photochemical water oxidation experiments, a significant amount of O2 was evolved when visible light was used to irradiate an aqueous suspension of Adsorbate A containing a S2O82− electron acceptor in a liquid phase, whereas O2 was not evolved under the same conditions when using Adsorbates B and C. 1 is considered to work for photochemical water oxidation in Adsorbate A due to an efficient electron transport from deeply intercalated 1 to S2O82− ions in a liquid phase via Ru(bpy)32+ photoexcitation near the mica adsorbate surface.

Non-catalytic O2 evolution by [(OH2)(Clterpy)Mn(μ-O)2Mn(Clterpy)(OH2)]3+ (Clterpy = 4′-chloro-2,2′:6′,2″-terpyridine) adsorbed on mica with CeIV oxidant

It was earlier reported that [(OH2)(terpy)Mn(μ-O)2Mn(terpy)(OH2)]3+ (terpy = 2,2′:6′,2″-terpyridine) (1) adsorbed on layer compounds catalyzes water oxidation to O2 (J. Am. Chem. Soc., 2004, 126, 8084). The derivative with 4′-chloro-2,2′:6′,2″-terpyridine (Clterpy), [(OH2)(Clterpy)Mn(μ-O)2Mn(Clterpy)(OH2)](NO3)3 (2(NO3)3) was synthesized and characterized by UV-visible absorption spectroscopic and magnetic susceptibility measurements. 2 is instable in aqueous solution at room temperature, but the stability of 2 in solution significantly increased at 5 °C. The reaction of a 2–mica adsorbate with CeIV in water produced a significant amount of O2, although the reaction of 2 with CeIV in a homogenous solution did not. However, the maximum turnover number (TN = 0.52) of 2 on the mica adsorbate was less than unity, indicating the non-catalytic O2 evolution by 2 on mica in contrast to the cooperative catalysis by 1 on mica with TN = 15. The kinetic analysis showed that O2 evolution follows first order kinetics with respect to 2 adsorbed on mica, with the first-order rate constant given to be 6.8 × 10−5 s−1. The first order kinetics can be explained by O2 evolution involved in the unimolecular decomposition of 2 adsorbed on mica, which might be ascribed to the destabilized higher oxidation state of 2 due to the electron-withdrawing chloro-substitution.