Electrochemical and chemical formation of [Mn4IVO5(terpy)4(H2O)2]6+, in relation with the photosystem II oxygen-evolving center model [Mn2III,IVO2(terpy)2(H2O)2]3+

To examine the real ability of the binuclear di-mu-oxo complex [(Mn2O2)-O-III,IV(terpy)(2)(H2O)(2)](3+) (2) to act as a catalyst for water oxidation, we have investigated in detail its redox properties and that of its mononuclear precursor complex [Mn-II(terpy)(2)](2+) (1) in aqueous solution. It appears that electrochemical oxidation of 1 allows the quantitative formation of 2 and, most importantly, that electrochemical oxidation of 2 quantitatively yields the stable tetranuclear Mn-IV complex, [(Mn4O5)-O-IV(terpy)(4)(H2O)(2)](6+) (4), having a linear mono-mu-oxo{Mn-2(mu-OXO)(2)}(2) core. Therefore, these results show that the electrochemical oxidation of 2 in aqueous solution is only a one-electron process leading to 4 via the formation of a mono-mu-oxo bridge between two oxidized [(Mn2O2)-O-IV,IV(terpy)(2)(H2O)(2)](4+) species. 4 is also quantitatively formed by dissolution of the binuclear complex [(Mn2O2)-O-IV,IV(terpy)(2)(SO4)(2)] (3) in aqueous solutions. Evidence of this work is that 4 is stable in aqueous solutions, and even if it is a good synthetic analogue of the dimers-of-dimers model compound of the OEC in PSII, this complex is not able to oxidize water. As a consequence, since 4 results from an one-electron oxidation of 2, 2 cannot act as an efficient homogeneous electrocatalyst for water oxidation. This work demonstrates that a simple oxidation of 2 cannot produce molecular oxygen without the help of an oxygen donor.