title: Electrochemical and chemical formation of [Mn4(IV)O5(terpy)4(H2O)2]6+, in relation with the photosystem II oxygen-evolving center model [Mn2(III,IV)O2(terpy)2(H2O)2]3+.
abstract: To examine the real ability of the binuclear di-mu-oxo complex [Mn2(III,IV)O2(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, [Mn4(IV)O5(terpy)4(H2O)2]6+ (4), having a linear mono-mu-oxo{Mn2(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 [Mn2(IV,IV)O2(terpy)2(H2O)2]4+ species. 4 is also quantitatively formed by dissolution of the binuclear complex [Mn2(IV,IV)O2(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.
authors.affiliation: Laboratoire d'Electrochimie Organique et de Photochimie Rédox, Université Joseph Fourier-UMR CNRS 5630, Institut de Chimie Moléculaire de Grenoble, FR CNRS 2607, BP 53, 38041 Grenoble Cedex 9, France.
authors.forename: Carole
authors.initials: C
authors.lastname: Baffert
authors.forename: Sophie
authors.initials: S
authors.lastname: Romain
authors.forename: Aurélien
authors.initials: A
authors.lastname: Richardot
authors.forename: Jean-Claude
authors.initials: JC
authors.lastname: Leprêtre
authors.forename: Bertrand
authors.initials: B
authors.lastname: Lefebvre
authors.forename: Alain
authors.initials: A
authors.lastname: Deronzier
authors.forename: Marie-Noëlle
authors.initials: MN
authors.lastname: Collomb