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

Abstract

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).

C−C Bond as Shuttle of Two Electrons in Intermolecular and Intramolecular Processes: A Theoretical Approach to Molecular Batteries

“…The coordination around the metal is square pyramidal, since we have omitted in our model the presence of the sixth ligand (chlorine) in order to simplify the calculation. The structure is similar to the one reported for [Mn(salophen)(Py)] 2 , [2] with oxygen bridges between the two metal centers. The optimized titanium ± titanium distance is computed to be 3.167 , and this value suggests the lack of any interaction between the two metals.…”

“…This structure shows an octahedral coordination around the transition metal and is similar, as expected, to that experimentally found for [Zr(salophen)Cl 2 (thf)]. [3] Figure 6 depicts the optimized geometry of the dimeric species [Ti(salophen')] 2 2 . The coordination around the metal is square pyramidal, since we have omitted in our model the presence of the sixth ligand (chlorine) in order to simplify the calculation.…”

“…We can deduce, therefore, that the main differences between the model system and the experimental complex are in the steric conformation and not in the electronic structure. In order to simplify the description for [Ti(salophen')] 2 2 , we consider a spin-restricted picture, since the a and b orbitals are very close in energy, although the calculation was performed at a spin-unrestricted level. The ground state of [Ti(salophen')Cl 2 ] is the singlet 1 A 1 , and the highest occupied molecular orbital (HOMO) is 13b 1 , which is mainly localized on the salophen' ligand.…”

“…The lowest unoccupied molecular orbital (LUMO) and the orbitals immediately at higher energy have mainly titanium d character, with small C N p-antibonding components. The ground state of [Ti(salophen')] 2 2 is the triplet 3 A u . The 53a u and 53a g orbitals are doubly occupied and are mainly salophen' in character, while the 54a g and 54a u are singly occupied and are essentially metal in character.…”

“…The first class of compounds includes systems in which the formation of the C À C bonds occurs through the reversible reductive coupling of imino groups belonging to a tetradentate Schiff base ligand, for example, salophen [N,N'-phenylenebis(salicylideneaminato) dianion] (see Figure 1) bonded to a transition metal. Complexes of titanium, [1] vanadium, [1] manganese, [2] zirconium [3] and cobalt [4] have been recently reported. The reductive coupling of imino groups is an intramolecular process in the manganese system, while it is intermolecular for the other complexes.…”