Flexible redox properties of a metal complex are important for redox catalysis. The present study shows that the reaction of a manganese(III) salen complex, which is a well-known oxidation catalyst, with hydroxide ion gives a transient manganese(III) species with drastically lowered redox potential, where the redox difference is −1.21 V. The reaction with cyanide ion gives a stable manganese(III) species with almost the same spectroscopic and redox properties, which was characterized as an anionic [Mn^III(salen)(CN)₂]⁻ of low-spin S = 1 state, in contrast to the starting Mn^III(salen)(OTf) having usual high-spin S = 2 manganese(III). The present study has thus clarified that the drastic redox shift comes from an anionic six-coordinate [Mn^III(salen)(X)₂]⁻ species where X is either OH^– or CN^–. Resonance Raman measurements show that the stretching band of the imino group shifts from 1620 to 1597 cm^–1 upon conversion from Mn^III(salen)(OTf) to [Mn^III(salen)(CN)₂]⁻, indicative of lowered C═N double bond character for [Mn^III(salen)(CN)₂]⁻. The observed deformation of a salen ligand is a clear indication of an increased electron population on the imino π*-orbital upon formation of low-spin manganese(III). It was proposed that the electronic structure of [Mn^III(salen)(CN)₂]⁻ may contain only limited contribution from valence tautomeric [Mn^IV(salen^– •)(CN)₂]⁻, in which the imino group of a salen ligand is reduced by one-electron via intramolecular electron transfer from low-spin manganese(III). The present study has clarified an unexpected new finding that a salen ligand works as a reservoir for negative charge to stabilize low-spin manganese(III).

中文翻译：

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氢氧化物与氰化物离子反应后锰（III）-萨伦氧化催化剂的剧烈氧化还原位移和电子结构变化

金属配合物的柔性氧化还原性质对于氧化还原催化很重要。本研究表明，锰（III）塞伦配合物（一种众所周知的氧化催化剂）与氢氧根离子的反应产生了一种瞬态锰（III），其氧化还原电位显着降低，其中氧化还原差为-1.21 V 。与氰化物离子的反应产生了稳定的锰（III）物种，具有几乎相同的光谱和氧化还原特性，其特征为低旋转S = 1态的阴离子[Mn ^III（salen）（CN）₂ ] ^-，与通常具有高自旋S的起始Mn ^III（salen）（OTf）相比= 2锰（III）。因此，本研究已表明的急剧氧化还原移来自阴离子的六坐标[锰^III（沙仑）（X）₂ ] ^-物种，其中X是OH ^-或CN ^- 。共振拉曼测量表明，亚氨基的拉伸带在从Mn ^III（salen）（OTf）转变为[Mn ^III（salen）（CN）₂ ] ^-时从1620变为1597 cm ^-1，表明C═降低。 [Mn ^III（salen）（CN）₂ ]的N双键特征^-。观察到的Salen配体变形清楚地表明在形成低自旋锰（III）时亚氨基π*轨道上的电子数量增加。有人提出[Mn ^III（salen）（CN）₂ ] ^-的电子结构可能只包含价互变异构体[Mn ^IV（salen- ^•）（CN）₂ ] ^-的有限贡献，其中a的亚氨基通过低自旋锰（III）的分子内电子转移，salen配体被单电子还原。本研究澄清了一个出乎意料的新发现，即Salen配体可充当负电荷的储库，以稳定低旋转锰（III）。