Single-Molecule Magnets:  A New Family of Mn₁₂ Clusters of Formula [Mn₁₂O₈X₄(O₂CPh)₈L₆]

The reaction of (NBuⁿ₄)[Mn₈O₆Cl₆(O₂CPh)₇(H₂O)₂] (1) with 2-(hydroxymethyl)pyridine (hmpH) or 2-(hydroxyethyl)pyridine (hepH) gives the Mn^II₂Mn^III₁₀ title compounds [Mn₁₂O₈Cl₄(O₂CPh)₈(hmp)₆] (2) and [Mn₁₂O₈Cl₄(O₂CPh)₈(hep)₆] (3), respectively, with X = Cl. Subsequent reaction of 3 with HBr affords the Br^- analogue [Mn₁₂O₈Br₄(O₂CPh)₈(hep)₆] (4). Complexes 2·2Et₂O·4CH₂Cl₂, 3·7CH₂Cl₂, and 4·2Et₂O·1.4CH₂Cl₂ crystallize in the triclinic space group P1̄, monoclinic space group C2/c, and tetragonal space group I4₁/a, respectively. Complexes 2 and 3 represent a new structural type, possessing isomeric [Mn^III₁₀Mn^II₂O₁₆Cl₂] cores but with differing peripheral ligation. Complex 4 is essentially isostructural with 3. A magnetochemical investigation of complex 2 reveals an S = 6 or 7 ground state and frequency-dependent out-of-phase signals in ac susceptibility studies that establish it as a new class of single-molecule magnet. These signals occur at temperatures higher than those observed for all previously reported single-molecule magnets that are not derived from [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)_x]. A detailed investigation of forms of complex 2 with different solvation levels reveals that the magnetic properties of 2 are extremely sensitive to the latter, emphasizing the importance to the single-molecule magnet properties of interstitial solvent molecules in the samples. In contrast, complexes 3 and 4 are low-spin molecules with an S = 0 ground state.