Artificial model of photosynthetic oxygen evolving complex: Catalytic O2 production from water by di-μ-oxo manganese dimers supported by clay compounds

Abstract

Adsorption of [(OH(2))(terpy)Mn(mu-O)(2)Mn(terpy)(OH(2))](3+) (terpy=2,2':6',2"-terpyridine) (1) onto montmorillonite K10 (MK10) yielded catalytic dioxygen (O(2)) evolution from water using a Ce(IV) oxidant. The Mn K-edge X-ray absorption near edge structure (XANES) of the 1/MK10 hybrid suggested that the oxidation state of the di-mu-oxo Mn(2) core could be Mn(III)-Mn(IV). However the pre-edge peak in the XANES spectrum of 1 adsorbed on MK10 is different from the neat 1 powder. The kinetic analysis of O(2) evolution showed that the catalysis requires cooperation of two equivalents of 1 adsorbed on MK10. The reaction of the [(bpy)(2)Mn(mu-O)(2)Mn(bpy)(2)](3+) (bpy=2,2'-bipyridine) (2)/MK10 hybrid with a Ce(IV) oxidant evolved O(2). However, the turnover number value was less than unity for 2/MK10, showing that 2 adsorbed on MK10 does not work as a catalyst. The terminal water ligands could be an important for the catalysis by adsorbed 1. The mechanism of O(2) production by photosynthetic oxygen evolving complex is discussed based on catalytic O(2) evolution by 1 adsorbed on MK10.

Electro‐ and Photoinduced Formation and Transformation of Oxido‐Bridged Multinuclear Mn Complexes

“…The assumed active species were [Mn 2 IV O 2 (L) 4 ] 4+ . However, these results have never been corroborated 57. The instability of [Mn 2 III,IV O 2 (L) 4 ] 3+ in an aqueous solution, except in a L/LH + buffer, as well as the evolution of its one‐electron oxidized form into a stable tetranuclear species makes the involvement of [Mn 2 III,IV O 2 (L) 4 ] 3+ as an active catalyst for water oxidation questionable 40–42.…”

“…This was the first di‐μ‐oxido complex considered as a structural model for the manganese complex in the OEC that could carry out catalytic O–O bond formation. However, the real source of oxygen atoms for O 2 evolution has been debated, since KHSO 5 and NaOCl are also oxygen‐atom‐transfer reagents 2,57. Some attempts have also been reported using a pure electron‐transfer oxidizing agent like Ce IV .…”

“…Some attempts have also been reported using a pure electron‐transfer oxidizing agent like Ce IV . The observation of O 2 evolution in a homogeneous solution is still being discussed,57,83,84 while when the complex is adsorbed into Kaolin clay the addition of a large excess of Ce IV produces a significant amount of O 2 57,85,86…”

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

“…[22,23] We reported the heterogeneous water oxidation catalysis of 1 ads on layer compounds such as kaolin, mica and montmorillonite (Mt). [26,[40][41][42] The kinetic analysis of O 2 evolution suggested that the catalysis requires cooperation of two 4 equivalents of 1 ads on layer compounds. The heterogeneous catalytic activity of the derivatives with R-terpy ligands (R= butoxy, propoxy, ethoxy, methoxy, methyl, methylthio, chloro and pyridyl) are quite different, showing that the catalytic activity strongly depends on the structure of complexes adsorbed [43].…”

“…or Mn 3+ ions does not work for water oxidation at all [26,[40][41][42]. They suggested from their X-ray absorption spectroscopic and O 2 evolution data that 1 is transformed on Mt into layered type manganese-oxide particles which are the actual water oxidation catalyst.…”