The phenoxide-ligated tetranuclear iron-sulfur clusters [Fe4S4(OAr)4]2- (I, R = phenyl; II, R = p-tolyl) have been synthesized by reaction of [Fe4S4(SR)4] or [Fe4S4Cl4]2- with HOAr or NaOAr, respectively. The preparation, reactivity, and electronic properties of these compounds are described, as is the crystal structure of the Et4N+ salt of I. [Et4N]2[Fe4S4(OPh)4] crystallizes in the monoclinic space group [formula-ommited] with Z = 4 and unit-cell parameters a = 17.610 (4) Å, b = 11.600 (3) Å, c = 22.810 (6) Å, β = 106.41 (1)°, and V = 4469.9 Å3 (-150 °C). The structure was refined to R = 0.066 and Rw = 0.107, using 6032 independent reflections with [formula-ommited]. The structure of I consists of an [Fe4S4]2+ cubane core distorted toward D2d symmetry, with a terminal phenoxide ligand completing the approximately tetrahedral coordination about each iron atom. The most notable structural feature is the short Fe-O distance (mean 1.865 (17) Å). Isotropically shifted resonances are observed in the 1H NMR spectra of I and II; the directions and relative magnitudes of these shifts are consistent with significant delocalization of spin into the π system of the phenyl rings. At any temperature, the magnitude of the isotropic shifts is approximately twice that observed for the arenethiolate analogues. The magnitude of the isotropic shifts increases with increasing temperature, consistent with an antiferromagnetically coupled system. Variable-temperature magnetic susceptibility measurements are in agreement with the NMR data and give values of μeff/Fe of ~1.3μb at 25 °C. Zero field Mössbauer spectra of solid I show a single quadrupole doublet with parameters δ = 0.50 and ΔEq = l.21 mm/s. Electrochemical data indicate that substitution of arenethiolate by phenolate ligands results in substantial negative shifts of first and second reduction potentials of the [Fe4S4]2+ core. Reaction of I with PhSH, monitored by 1H NMR spectroscopy, generates the mixed-ligand species [Fe4S4(OPh)4-n(SPh)n]2- (n = 0-4), with an approximately statistical distribution of ligands among the species present. The accumulated results are consistent with a relatively covalent Fe-0 interaction in I and II. The biological implications for tyrosyl coordination to 4Fe-4S centers are discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry