Mn8Cluster with Ferrocene-1,1′-Dicarboxylate Ligation: Single-Molecule Magnetism with Multiple External Redox Centers

Abstract

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 4Mn(III) and 4Mn(II) atoms bridged by four O(2-) ions and six fdc(2-) groups. The [Mn(III)4Mn(II)4(μ4-O)4] cores have virtual Td symmetry and can be described as a central [Mn(III)4(μ4-O)4](4+) cubane unit whose four O(2-) ions are μ4, because they each attach to an external Mn(II) atom. Peripheral ligation about the core is provided by six bridging fdc(2-) groups and the terminal solvent ligands, one each on the Mn(II) 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.