Mn-8 Cluster with Ferrocene-1,1 '-Dicarboxylate Ligation: Single-Molecule Magnetism with Multiple External Redox Centers
Résumé
The syntheses, structures, and magnetic properties are reported of a new Mn-8 cluster obtained from the reaction of ferrocene-1,1'-dicarboxylic acid (fdcH(2)) 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 O2- ions and six fdc(2-) groups. The [(Mn4Mn4II)-Mn-III(mu(4)-O)(4)] cores have virtual T-d symmetry and can be described as a central [Mn-4(III)(mu(4)-O)(4)](4+) cubane unit whose four O2- ions are mu(4), 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 center dot 4DMF center dot 4H(2)O, which exhibited hysteresis.