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 -O2CAr4tBuPh)(2)(mu -OTf)(2)L-4] (L = C5H5N (1), 4-(BuC5H4N)-Bu-t (2), 3-FC5H4N (3)), were synthesized by using a sterically hindered carboxylate ligand, 2.6-di(4-tert-butylphenyl)benzoate ((ArCO2-)-C-4tBuPh). 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 I and 3 probed by FT-IR and F-19 NMR spectroscopy are consistent with the solid-state geometry determined by X-ray crystallography. Zero-Field Mossbauer spectra of 1-3 at 4.2 K are characteristic of high-spin iron(II) centers in nearly identical coordination environments. Compound I undergoes two irreversible oxidation processes at ca -10 and +880 mV (vs Fe/Fe+), the former approaching quasi-reversible behavior with increased scan rates and a narrow potential sweep range. Comparisons are made with analogous know n {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.