Effects of Phenoxide Ligation on Iron-Sulfur Clusters. Preparation and Properties of [Fe₄S₄(OAr)₄]^2- Ions and the Structure of [(C₂H₅)₄N]₂[Fe₄S₄(OC₆H₅)₄]

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 ų (-150 °C). The structure was refined to R = 0.066 and R_w = 0.107, using 6032 independent reflections with [formula-ommited]. The structure of I consists of an [Fe₄S₄]²⁺ cubane core distorted toward D_2d 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 μ_eff/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.