Mn8 Cluster with Ferrocene-1,1′-Dicarboxylate Ligation: Single-Molecule Magnetism with Multiple External Redox Centers

The syntheses, structures, and magnetic properties are reported of a new Mn8 cluster obtained from the reaction of ferrocene-1,1′-dicarboxylic acid (fdcH2) with [Mn12O12(O2CMe)16(H2O)4] and mononuclear Mn salts under different conditions and limited light exposure. The product was obtained in two forms: [Mn8O4(fdc)6(DMF)2(H2O)2] (1) and [Mn8O4(fdc)6(DMF)4] (2), differing in the bound solvent ligands. The structures are otherwise almost identical, comprising very similar cores that both contain 4MnIII and 4MnII atoms bridged by four O2– ions and six fdc2– groups. The [MnIII4MnII4(μ4-O)4] cores have virtual Td symmetry and can be described as a central [MnIII4(μ4-O)4]4+ cubane unit whose four O2– ions are μ4, because they each attach to an external MnII atom. Peripheral ligation about the core is provided by six bridging fdc2– groups and the terminal solvent ligands, one each on the MnII atoms. The differences in solvent ligands between 1 and 2, and different packing from the different crystal space groups, lead to significant differences in metric parameters within the core, which are reflected in significantly different magnetic properties. Variable-temperature, solid-state dc and ac susceptibility measurements reveal the clusters to be predominantly antiferromagnetically coupled, and to possess ground state spin values of S = 5 and S = 2 for 1 and 2, respectively. The difference in ground states is assigned to the small but distinct structural differences seen in the central cubane. Alternating current (AC) susceptibility data indicate 1 and 2 to be new single-molecule magnet, and this was confirmed by magnetization versus direct current (DC) field scans on a single crystal of 1·4DMF·4H2O, which exhibited hysteresis.