Two New Terpyridine Dimanganese Complexes: A Manganese(III,III) Complex with a Single Unsupported Oxo Bridge and a Manganese(III,IV) Complex with a Dioxo Bridge. Synthesis, Structure, and Redox Properties

Two new terpyridine dimanganese oxo complexes [Mn2III,IV(μ-O)2(terpy)2(CF3CO2)2]+ (3) and [Mn2III,III(μ-O)(terpy)2(CF3CO2)4] (4) (terpy = 2,2‘:6,2‘ ‘-terpyridine) have been synthesized and their X-ray structures determined. In
contrast to the corresponding mixed-valent aqua complex [Mn2III,IV(μ-O)2(terpy)2(H2O)2]3+ (1), the two Mn atoms in
3 are not crystallographically equivalent. The neutral binuclear monooxo manganese(III,III) complex 4 exhibits two
crystallographic forms having cis and trans configurations. In the cis complex, the two CF3CO2- ligands on each
manganese adopt a cis geometry to each other; one CF3CO2- is trans to the oxygen of the oxo bridge while the
second is cis. In the trans complex, the two coordinated CF3CO2- have a trans geometry to each other and are
cis to the oxo bridge. The electrochemical behavior of 3 in organic medium (CH3CN) shows that this complex could
be oxidized into its corresponding stable manganese(IV,IV) species while its reduced form manganese(III,III) is
very unstable and leads by a disproportionation process to Mn(II) and Mn(IV) complexes. Complex 4 is only stable
in the solid state, and it disproportionates spontaneously in CH3CN solution into the mixed-valent complex 3 and
the mononuclear complex [MnII(terpy)2]2+ (2), thereby preventing the observation of its electrochemical behavior.