Carbon−Carbon Bonds Functioning as Electron Shuttles:  The Generation of Electron-Rich Manganese(II)−Schiff Base Complexes and Their Redox Chemistry

The reduction of [Mn(II)-salophen] derivatives [salophen = <i>N</i>,<i>N</i><i> </i>‘-ethylenebis(salicylideneaminato) dianion] led to the formation of C−C bridged dimers. Such C−C bonds function as two electron shuttles in electron-transfer reactions. The reduction of [Mn(salophen)(THF)]<sub>2</sub> (<b>1</b>) and [Mn(3,5-Bu<sup>t</sup><sub>4</sub>salophen)(THF)]<sub>2</sub> (<b>3</b>) with 2 equiv of sodium metal led to the corresponding single C−C bond bridged dimers, [Mn<sub>2</sub>(salophen<sub>2</sub>)Na<sub>2</sub>(DME)<sub>4</sub>] (<b>5</b>) [salophen<sub>2</sub> = C−C bonded salophen dimer] and [Mn<sub>2</sub>(3,5-Bu<sup>t</sup><sub>4</sub>salophen<sub>2</sub>)Na<sub>2</sub>(DME)<sub>6</sub>] (<b>7</b>), respectively. Complexes <b>5</b> and <b>7</b> undergo a further two electron reduction to [Mn<sub>2</sub>(*salophen<sub>2</sub>*)Na<sub>4</sub>(DME)<sub>6</sub>] (<b>6</b>) [*salophen<sub>2</sub>* = C−C doubly bonded salophen dimer] and [Mn<sub>2</sub>(*3,5-Bu<sup>t</sup><sub>4</sub>salophen<sub>2</sub>*)Na<sub>4</sub>(DME)<sub>4</sub>] (<b>8</b>), respectively, both containing a double C−C bridge. The obtention of [Mn<sub>2</sub>{salophen(Me)CH<sub>2</sub>)}<sub>2</sub>Na<sub>4</sub>(DME)<sub>4</sub>] (<b>9</b>) from [Mn(salophen-Me<sub>2</sub>)(THF)]<sub>2</sub> (<b>2</b>) strongly supports the existence of free radical precursors in the formation of C−C bonds. Complex <b>6</b> has been used as a source of four electrons in a number of reactions, thus reduction of Ag<sup>+</sup>, PhCH<sub>2</sub>Cl, <i>p</i>-benzoquinone, and [Co<sup>II</sup>(MeOsalen)] occurs with the regeneration of the starting material <b>1</b>. The C−C bond cleavage is the source of electrons, without being involved in any reaction as a reactive site. With stronger oxidizing agents not only complexes <b>6</b> and <b>8</b> transfer the electrons stored at the C−C bonds but also the metal undergoes a change in the oxidation state. The reaction of <b>6</b> with dioxygen produces a novel form of di-μ-oxo-Mn(IV) dimers, where the salophen ligand displays a bridging bonding mode in [Mn<sub>2</sub>(μ-salophen)<sub>2</sub>(μ-O)<sub>2</sub>] (<b>15</b>) and [Mn<sub>2</sub>(μ-3,5-Bu<sup>t</sup>salophen)<sub>2</sub>(μ-O)<sub>2</sub>] (<b>16</b>).