From pseudo to true C₃ symmetry: Magnetic anisotropy enhanced by site-specific ligand substitution in two Mn₁₅-carboxylate clusters

Two new mixed-valence manganese-carboxylate clusters, [Mn ^III₉Mn^IV₆(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 [Mn^III₉Mn^IV ₆(O₂CCh)₁₂(μ₃-O) ₁₃(μ-O)₄(μ-OMe)₅(MeOH)₃(H ₂O)₆]·0.5MeOH·2.5H₂O (2, ChCO₂H= cyclohexanecarboxylic acid) contain new disklike Mn ₁₅ 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 P1, whereas 2 crystallizes in the trigonal space group P3. Magnetic study shows that both 1 and 2 have the same ground spin states S_T = 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).