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Carbon−Carbon Bonds Functioning as Electron Shuttles: The Generation of Electron-Rich Manganese(II)−Schiff Base Complexes and Their Redox Chemistry
journal contribution
posted on 1997-06-04, 00:00 authored by Emma Gallo, Euro Solari, Nazzareno Re, Carlo Floriani, Angiola Chiesi-Villa, Corrado RizzoliThe 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-But4salophen)(THF)]2
(3) with 2 equiv of sodium
metal led to the corresponding single C−C bond bridged dimers,
[Mn2(salophen2)Na2(DME)4]
(5) [salophen2 =
C−C bonded salophen dimer] and
[Mn2(3,5-But4salophen2)Na2(DME)6]
(7), respectively. Complexes 5 and
7 undergo
a further two electron reduction to
[Mn2(*salophen2*)Na4(DME)6]
(6) [*salophen2* = C−C doubly bonded
salophen
dimer] and
[Mn2(*3,5-But4salophen2*)Na4(DME)4]
(8), respectively, both containing a double C−C bridge.
The
obtention of
[Mn2{salophen(Me)CH2)}2Na4(DME)4]
(9) from
[Mn(salophen-Me2)(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+, PhCH2Cl, p-benzoquinone,
and [CoII(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
[Mn2(μ-salophen)2(μ-O)2]
(15) and
[Mn2(μ-3,5-Butsalophen)2(μ-O)2]
(16).
