Electrochemical and Spectroscopic Studies of Manganese(II, III, IV) Complexes as Models for the Photosynthetic Oxygen-Evolution Reaction

Sorbitol and related polyol complexes of manganese(II), -(III), and -(IV): redox and oxygenation equilibriums

“…The dimeric [Mn(IIIA)2] complex appears to be the initial product from the reaction of molecular oxygen with the Mn(II) complex and is in rapid equilibrium with the apparently monomeric Mn(IIIA) and Mn(IIIB) complexes at higher hydroxide ion and sorbitol concentrations, which is in accord with earlier suggestions. 21 The nature of the Mn(III) complex that is involved in the oxidation to Mn(IV) is not clear. However, the decrease in the rate of reaction with 02 for increased sorbitol concentrations may indicate that one of the two forms of the Mn-(IIIA) complex is involved.…”

The Metal Centers of the Photosynthetic Oxygen‐evolving Complex *

“…This model differs from those based upon EXAFS, EPR and other optical results by assigning redox transitions exclusively to manganese and Z (vide infra). The spectral assignments, although plausible, are based upon comparison with unstable solutions of air-oxidized mixtures of Mn salts with polygluconate ligands, for which no definitive composition or structure is established (Sawyer et al, 1977).…”

Bis(tetramethylammonium) tris(sorbitolato)manganate(IV), an EPR-active monomeric complex of manganese(IV)

“…A shoulder at ~500 nm is assigned to a d-d transition.1 In methanol, the e values for both bands (Table 1) are dependent upon the concentrations of sorbitol and hydroxide ions in the solution. This appears to be due to partial solvolysis rather than reduction to manganese(IIl), which is observed in aqueous media.1 Slow reduction to a yellow-brown manganese(JII) species does occur over a period of 24 h and is accelerated in the presence of UV light or elevated temperatures, and in the presence of excess ligand.2124 The electrochemical behavior of the polyol complexes of manganese ions in aqueous solution has been described in detail previously.1 5 Cyclic voltammograms in dimethyl sulfoxide and dimethylformamide for the present manganese(IV)sorbitol complex have been recorded, but these are difficult to interpret. An irreversible reduction wave at -1.3 V vs. SCE is observed in all cases and disappears upon reduction to manganese(III).…”