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, [Mn^IV₄Mn^III₂(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 (Ar^TolCO₂^-) (3) or 2,6-di(4-tert-butylphenyl)benzoate (Ar^4-tBuPhCO₂^-) (4)), was synthesized using sterically hindered m-terphenyl-derived carboxylate ligands. These complexes can be synthesized by oxidizing the Mn^II mononuclear complexes, [Mn(dmb)₂(OH₂)(O₂CR)]⁺ (where RCO₂ = Ar^TolCO₂^- (1) or Ar^4-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 [Mn^IV₄Mn^III₂(μ-O)₄(μ₃-O)₄(dmb)₆(Ar^TolCO₂)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) ų, 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 S_T = 0 ground spin state with a large number of low-lying, thermally accessible states with S_T > 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.