A Ferromagnetic Ring of Six Manganese(III) Ions with a S=12 Ground State

A novel cyclic cluster comprising six Mn(III) ions, [NaMn6(OMe)12(dbm)6]+ (1, Hdbm = dibenzoylmethane ), was synthesized by a procedure similar to that already reported for the Fe(III) analog. MnCl2, NaOMe and dibenzoylmethane were reacted in anhyd. MeOH and the solid product obtained was recrystd. from MeOH/CHCl3 in the presence of NaBPh4. The resulting complex [1]BPh4.2CHCl3 crystallizes in triclinic space-group P-1 with (at 188 K) a 14.392(3), b 16.790(2), c 17.344(3) Å, α 61.42(1), β 72.36(1), γ 88.96(2)°, Z = 1. The mol. structure of 1 comprises a [Mn6(OMe)12] crown with idealized S6 point-group symmetry acting as a host for an alkali-metal ion. The metal ions show remarkably distorted coordination spheres, as expected for high-spin MnIII. Variable temp., solid state magnetic susceptibility measurements performed on powders of [1]BPh4·xCHCl3 at 1 kG and 10 kG evidenced a S = 12 spin ground state. A fitting of high temp. data (T > 15 K) based on a Heisenberg model for exchange-coupling interactions gave j12 = j23 = j45 = j56 = -23.8 cm-1 and j34 = j61 = -2.6 cm-1 with g = 2.00. The structure and magnetic behavior of 1 are compared with those of antiferromagnetic Fe(III) rings previously reported. The results are relevant to the development of new strategies for designing high-spin mol. clusters.