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

A series of hydroxo-bridged cubane-type tetrairon(II) clusters, [Fe(4)(mu-OH)(4)(mu-O(2)CAr(4)(-)t(BuPh))(2)(mu-OTf)(2)L(4)] (L = C(5)H(5)N (1), 4-(t)BuC(5)H(4)N (2), 3-FC(5)H(4)N (3)), were synthesized by using a sterically hindered carboxylate ligand, 2,6-di(4-tert-butylphenyl)benzoate (Ar(4)(-)t(BuPh)CO(2)(-)). Three different bridging units that mediate weak antiferromagnetic coupling interactions between the metal centers support the unprecedented cubane-type [Fe(4)(mu-OH)(4)](4+) cores in 1-3. The solution structures of 1 and 3 probed by FT-IR and (19)F 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 [Fe(4)X(4)](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.