Hydroxo-Bridged Cubane-Type Tetrairon(II) Clusters Supported by Sterically-Hindered Carboxylate Ligands

A series of hydroxo-bridged cubane-type tetrairon(II) clusters, [Fe4(μ-OH)4(μ-O2CAr4-tBuPh)2(μ-OTf)2L4] (L =
C5H5N (1), 4-tBuC5H4N (2), 3-FC5H4N (3)), were synthesized by using a sterically hindered carboxylate ligand,
2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2-). Three different bridging units that mediate weak antiferromagnetic coupling interactions between the metal centers support the unprecedented cubane-type {Fe4(μ-OH)4}4+
cores in 1−3. The solution structures of 1 and 3 probed by FT-IR and 19F NMR spectroscopy are consistent with
the solid-state geometry determined by X-ray crystallography. Zero-field Mössbauer spectra of 1−3 at 4.2 K are
characteristic of high-spin iron(II) centers in nearly identical coordination environments. Compound 1 undergoes
two irreversible oxidation processes at ca −10 and +880 mV (vs Fc/Fc+), the former approaching quasi-reversible
behavior with increased scan rates and a narrow potential sweep range. Comparisons are made with analogous
known {Fe4X4}n+ (X = O, S) units, and the structural integrity of tetrairon fragments upon a change in oxidation
state is discussed together with some possible biological implications.