A New Class of Oxo-Bridged High-Valent Hexamanganese Clusters Supported by Sterically Hindered Carboxylate Ligands

A new class of oxo-bridged high-valent hexamanganese (Mn6) clusters containing a novel {Mn6O8}6+ core, [MnIV4MnIII2(μ-O)4(μ3-O)4(dmb)6(O2CR)2]4+ (where dmb = 4,4‘-dimethyl-2,2‘-bipyridine, and RCO2 = 2,6-di(p-tolyl)benzoate
(ArTolCO2-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2-) (4)), was synthesized using sterically hindered
m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the MnII mononuclear
complexes, [Mn(dmb)2(OH2)(O2CR)]+ (where RCO2 = ArTolCO2- (1) or Ar4-tBuPhCO2- (2)) with (n-Bu4N)MnO4, by
direct Mn(II) + Mn(VII) in situ comproportionation reactions, or by ligand substitution on the dinuclear manganese
(III,IV) or (IV,IV) complexes, [(Mn2(μ-O)2(dmb)4)]3+/4+. The compound [MnIV4MnIII2(μ-O)4(μ3-O)4(dmb)6(ArTolCO2)2](OTf)4 [3(OTf)4] crystallizes in the monoclinic space group P21/n, with the cell parameters a = 15.447(1) Å, b =
15.077(2) Å, c = 27.703(2) Å, β = 91.68(2)°, V = 6449.3(6) Å3, and Z = 2. The X-ray structure reveals that there
are three different bridging modes for the oxo groups:  μ, “pyramidal” μ3, and “T-shaped” μ3. Solid-state variable
temperature magnetic susceptibility studies suggest that the Mn centers are net antiferromagnetically coupled to
yield a diamagnetic ST = 0 ground spin state with a large number of low-lying, thermally accessible states with
ST > 0. 1H NMR spectra were recorded for both Mn6 clusters and selected resonances assigned. The electronic
and redox properties of these complexes along with the effect of the presence of the bulky carboxylate ligands are
also described here.