From Pseudo to True <i>C</i>₃ Symmetry: Magnetic Anisotropy Enhanced by Site-Specific Ligand Substitution in Two Mn₁₅-Carboxylate Clusters

Two new mixed-valence manganese−carboxylate clusters, [MnIII9MnIV6(O2CPh)12(μ3-O)13(μ-O)4(μ-OMe)5(MeOH)4(H2O)5]2·1.5PhCO2H·MeOH·6H2O (1, PhCO2H = benzoic acid) and [MnIII9MnIV6(O2CCh)12(μ3-O)13(μ-O)4(μ-OMe)5(MeOH)3(H2O)6]·0.5MeOH·2.5H2O (2, ChCO2H= cyclohexanecarboxylic acid) contain new disklike Mn15 cores. Both 1 and 2 can be synthesized by the conventional manganese redox reaction (MnO4- oxidizing Mn2+) in methanol solution. 2 can be also synthesized via the site-specific ligand substitution reaction from 1. 1 crystallizes in the triclinic space group P1̄, whereas 2 crystallizes in the trigonal space group P3̄. Magnetic study shows that both 1 and 2 have the same ground spin states ST = 2. Compared to the silence of the out-of-phase ac susceptibility of 1, 2 shows clearly slow magnetic relaxation behavior above 1.8 K due to the dramatically enhanced axial magnetic anisotropy (D = −0.89 and −1.58 cm-1 for 1 and 2, respectively, which was obtained by fitting the plots of M vs H/T with the program ANISOFIT 2.0).