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 = N, N ‘-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)] 2 ( 1 ) and [Mn(3,5-Bu t 4 salophen)(THF)] 2 ( 3 ) with 2 equiv of sodium metal led to the corresponding single C−C bond bridged dimers, [Mn 2 (salophen 2 )Na 2 (DME) 4 ] ( 5 ) [salophen 2 = C−C bonded salophen dimer] and [Mn 2 (3,5-Bu t 4 salophen 2 )Na 2 (DME) 6 ] ( 7 ), respectively. Complexes 5 and 7 undergo a further two electron reduction to [Mn 2 (*salophen 2 *)Na 4 (DME) 6 ] ( 6 ) [*salophen 2 * = C−C doubly bonded salophen dimer] and [Mn 2 (*3,5-Bu t 4 salophen 2 *)Na 4 (DME) 4 ] ( 8 ), respectively, both containing a double C−C bridge. The obtention of [Mn 2 {salophen(Me)CH 2 )} 2 Na 4 (DME) 4 ] ( 9 ) from [Mn(salophen-Me 2 )(THF)] 2 ( 2 ) strongly supports the existence of free radical precursors in the formation of C−C bonds. Complex 6 has been used as a source of four electrons in a number of reactions, thus reduction of Ag +, PhCH 2 Cl, p -benzoquinone, and [Co II (MeOsalen)] occurs with the regeneration of the starting material 1 . 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 6 and 8 transfer the electrons stored at the C−C bonds but also the metal undergoes a change in the oxidation state. The reaction of 6 with dioxygen produces a novel form of di-μ-oxo-Mn(IV) dimers, where the salophen ligand displays a bridging bonding mode in [Mn 2 (μ-salophen) 2 (μ-O) 2 ] ( 15 ) and [Mn 2 (μ-3,5-Bu t salophen) 2 (μ-O) 2 ] ( 16 ).