Iron(III) Chemistry with Ferrocene-1,1′-dicarboxylic Acid (fdcH2): An Fe7 Cluster with an Oxidized fdc– Ligand

The synthesis and properties are reported of a new Fe7 cluster obtained from the reaction of ferrocene-1,1′-dicarboxylic acid (fdcH2) with FeCl2·4H2O in MeOH under ambient light conditions. The compound is the mixed-anion salt [Fe7O3(OMe)(fdc)6(MeOH)3][FeCl4]Cl2 (1; 8FeIII), containing six (fdcn–) groups as peripheral ligands. The cation of 1 has virtual C3 symmetry and contains a central [Fe4(μ3-O)3(μ3-OMe)]5+ cubane unit whose three oxide ions each become μ4 by attaching to a fourth Fe atom outside the cubane. The resulting [Fe7(μ3-O3)(μ3-OMe)]14+ core is surrounded by six fdcn– (n = 1, 2) groups, which divide into two sets by virtual symmetry. The blue color of the complex suggested that some of these ligands are in their oxidized fdc– ferricenium (FeIII) state, and various data point to there being one fdc– ligand in the compound, the initial example of the group acting as a ligand in inorganic chemistry. Variable-temperature, solid-state DC and AC susceptibility measurements reveal the cation to be antiferromagnetically coupled, as expected for high-spin FeIII, and to have an S = 2 ground state, consistent with an S = 5/2 Fe7 inner core coupled antiferromagnetically to the one paramagnetic fdc– (S = 1/2) ligand. Complex 1 displays multiple reductions and oxidations when investigated by electrochemistry in MeCN. 57Fe Mössbauer spectroscopy supports the presence of only five fdc2– ligands, but cannot resolve the signals from the various FeIII sites.