[{SiNDipp}MgNa]2: A Potent Molecular Reducing Agent

The bimetallic species, [{SiNDipp}­MgNa]2 [{SiNDipp} = {CH2SiMe2N­(Dipp)}2; (Dipp = 2,6-i-Pr2C6H3)], is shown to be a potent reducing agent, able to effect one- or two-electron reduction of either dioxygen, TEMPO, anthracene, benzophenone, or diphenylacetylene. In most cases, the bimetallic reaction products imply that the dissimilar alkaline metal centers react with a level of cooperativity. EPR analysis of the benzophenone-derived reaction and the concurrent isolation of [{SiNDipp}­Mg­(OCPh2)2], however, illustrate that treatment with such reducible, but O-basic, species can also result in reactivity in which the metals provide independent reaction products. The notable E-stereochemistry of the diphenylacetylene reduction product prompted a computational investigation of the PhCCPh addition. This analysis invokes a series of elementary steps that necessitate ring-opening via Mg+ → Na+ amido group migration of the SiNDipp ligand, providing insight into the previously observed lability of the bidentate dianion and its consequent proclivity toward macrocyclization.双金属物种[{SiNDipp}MgNa]₂（其中配体[{SiNDipp}]的结构为{CH₂SiMe₂N(Dipp)}₂；Dipp为2,6-二异丙基苯基(2,6-i-Pr₂C₆H₃)）被证实为高效还原剂，可实现氧气、TEMPO（2,2,6,6-四甲基哌啶氮氧自由基）、蒽、二苯甲酮或二苯乙炔的单电子或双电子还原。在多数反应中，该双金属反应产物表明两种不同的碱金属中心以协同模式参与反应。然而，针对二苯甲酮衍生反应的电子顺磁共振（Electron Paramagnetic Resonance, EPR）分析，以及同步分离得到的[{SiNDipp}Mg(OCPh₂)₂]产物，表明当与这类可还原但具O-碱性的底物反应时，金属中心也可独立参与反应并生成单一反应产物。二苯乙炔还原产物显著的E-立体化学构型，促使研究人员对PhC≡CPh的加成反应开展计算研究。该分析揭示了一系列基元反应步骤，其中SiNDipp配体需通过Mg⁺→Na⁺酰胺基迁移实现开环，这为此前观测到的双齿二阴离子的不稳定性及其后续发生大环化的倾向提供了机理解释。