Electrochemical and Chemical Formation of [Mn₄^IVO₅(terpy)₄(H₂O)₂]⁶⁺, in Relation with the Photosystem II Oxygen-Evolving Center Model [Mn₂^III,IVO₂(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.

Splitting Water with Cobalt

“…For example, Collomb and co-workers observed that O 2 is produced from irradiated pH 3 aqueous solutions of [Ru(bipy) 3 ] 2+ and peroxodisulfate in the absence of any catalyst. [224] In that case, a chloroacetate buffer was used. The pH of the solution may thus play an important role.…”

Functional models for the oxygen-evolving complex of photosystem II

“…The mechanism shown in Figure 5 (top) is the result of isotope labeling studies which show that under high concentrations of oxidant the oxygen produced contains minimal 18 O label from the solvent while at lower concentrations of oxidant a significant fraction of the product contains a single oxygen from the solvent and some of the evolved O 2 contains two oxygen from the solvent [93]. The fact that only oxygen-atom donor oxidants are functional has been a concern for this system [95], but recent results have shown that the catalyst is capable of oxygen evolution using Ce(IV) as an oxidant. Catalytic turnover is not attained when Ce(IV) is used as the oxidant due to the low pH required for use of Ce(IV) as an oxidant which causes deactivation of the catalyst [96].…”

Reflections on small molecule manganese models that seek to mimic photosynthetic water oxidation chemistry

“…19a) was reported by Brudvig and coworkers to serve as the first model for catalytic O-O bond formation that employed typical oxidants such as sodium hypochlorite or KHSO 5 [152]. It has since been suggested that the dioxygen released comes about via a disproportionation reaction [153]. Collomb and coworkers explored this system via electrochemical methods [154,155].…”