Water Oxidation Catalyzed by the Tetranuclear Mn Complex [Mn4IVO5(terpy)4(H2O)2](ClO4)6

The tetranuclear manganese complex [(Mn4O5)-O-IV(terpy)(4)(H2O)2](ClO4)(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 MnO2. DFT calculations show that the oxygen in the mono-mu-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 02 is proposed to occur by reaction of water with an electrophilic manganese-bound oxyl radical species, O-center dot-Mn-2(IV/IV), produced during the oxidation of the tetramer. This study demonstrates that [(Mn4O5)-O-IV(terpy)(4)(H2O)(2)](ClO4)(6) may be relevant for understanding the role of the Mn tetramer in photosystem II.