The tetranuclear manganese complex [Mn IV 4O 5(terpy) 4(H 2O) 2](ClO 4) 6 (1; terpy = 2,2′:6′,2″-terpyridine) gives catalytic water oxidation in aqueous solution, as determined by electrochemistry and GC-MS. Complex 1 also exhibits catalytic water oxidation when adsorbed on kaolin clay, with Ce IV as the primary oxidant. The redox intermediates of complex 1 adsorbed on kaolin clay upon addition of Ce IV have been characterized by using diffuse reflectance UV/visible and EPR spectroscopy. One of the products in the reaction on kaolin clay is Mn III, as determined by parallel-mode EPR spectroscopic studies. When 1 is oxidized in aqueous solution with Ce IV, the reaction intermediates are unstable and decompose to form Mn II, detected by EPR spectroscopy, and MnO 2. DFT calculations show that the oxygen in the mono-μ-oxo bridge, rather than Mn IV, is oxidized after an electron is removed from the Mn(IV,IV,IV,IV) tetramer. On the basis of the calculations, the formation of O 2 is proposed to occur by reaction of water with an electrophilic manganese-bound oxyl radical species, •O-Mn 2 IV/IV, produced during the oxidation of the tetramer. This study demonstrates that [Mn IV 4O 5(terpy) 4(H 2O) 2] (ClO 4) 6 may be relevant for understanding the role of the Mn tetramer in photosystem II.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry