Influence of the Net Charge on the Reactivity of a Manganese(IV) Species: Leading to the Correlation of Its Physicochemical Properties with Reactivity

Abstract

Clarifying how versatile physicochemical parameters of an active metal intermediate affect its reactivity would help to understand its roles in chemical and enzymatic oxidations. The influence of the net charge on electron transfer and hydrogen abstraction reactions of a manganese(IV) species having hydroxide ligand has been investigated here. It was found that increasing one unit of the positive net charge from 2+ to 3+ would accelerate its electron-transfer rate by 10−20 fold in oxygenation of tris(4-methoxyphenyl)phosphine. In contrast, the hydrogen abstraction rate is insensitive to its net charge change, and the insensitivity has been attributed to the compensation effect between the redox potential and pK a , which determine the hydrogen abstraction capability of a metal ion. Similar net-charge-promoted electron transfer but not hydrogen abstraction has also been observed in intramolecular electron transfer and hydrogen abstraction reactions when using thioxanthene as substrate. Together with the previous understanding of the reactivity of the identical manganese(IV) species having Mn IV −OH or Mn IV =O functional groups, the relationships of the oxidative reactivity of an active metal intermediate with its physicochemical parameters such as the net charge, the redox potential and the metal−oxygen bond order (M−O versus MO) have been discussed with this manganese(IV) model.

Thermodynamics of Proton and Electron Transfer in Tetranuclear Clusters with Mn–OH2/OH Motifs Relevant to H2O Activation by the Oxygen Evolving Complex in Photosystem II

“…30 These BDE O–H estimates are higher than Mn complexes, where these bond strengths have been reported. 11c, 12d, 12i, 12k The large BDE O–H values for these hydroxide clusters suggest that if these terminal oxo moieties could be accessed, they would be highly reactive. Indeed, previous attempts to generate a terminal oxo species on related phenyl-pyrazolate bridged multimetallic clusters results in activation of strong bonds, although the identity of the reactive intermediate in these reactions could not be established (terminal metal-oxo or iodosylbenzene adduct).…”

“…11, 12, 13 Reported mononuclear systems have been able to probe the roles of Mn oxidation state 12c, 12d, 12i , ligand field 11a, 11f, 13b , and oxygen ligand protonation state 11j, 12j on PCET reactions and the intrinsic O–H bond dissociation enthalpies (BDE) of Mn–OH x moieties. There are fewer examples of such studies with multinuclear Mn complexes, with most of the reports examining the PCET chemistry of bridging oxo moieties 14 , as opposed to terminally bound OH x ligands.…”

The reactivity of the active metal oxo and hydroxo intermediates and their implications in oxidations

“…After comparing the electron transfer, hydrogen abstraction and oxygen transfer reactivity of the Mn IV -OH and Mn IV QO in Mn IV (Me 2 EBC)(OH) 2 2+ model, Yin and co-workers summarized the thermodynamic parameters and the oxidative properties of the manganese(IV) species in the Mn IV (Me 2 EBC) complex as shown in Table 5. 45 One may see that three forms of manganese(IV) species may occur in this manganese(IV) model because of different protonation states, including neutral Mn IV QO, Mn IV -OH having 2+ net charge, and Mn IV -OH having 3+ net charge. With the net charge increasing from zero to 2+, then 3+, the redox potential of the Mn III /Mn IV couple increases from +0.10, to +0.46, then to +0.54 V (vs. SCE).…”

“…Following this study, Yin and co-worker further investigated the influence of the net charge on the reactivity of the manganese(IV) species in the Mn IV (Me 2 EBC)(OH) 2 2+ model 45. As disclosed earlier,…”

“…The thermodynamic parameters and oxidative activity of the manganese(IV) species from the Mn IV (Me 2 EBC) complex45 …”