Synthetic Analogs of the Active Sites of Iron-Sulfur Proteins. XI. Synthesis and Properties of Complexes Containing the Fe₂S₂ Core and the Structures of Bis[o-xylyl-α,α′-dithiolato-μ-sulfido-ferrate(III)] and Bis[p-tolylthiolato-μ-sulfido-ferrate(III)] Dianions

Although an extensive body of physicochemical data indicates that the active sites of plant ferredoxins (Fd) and similar 2-Fe proteins have the minimal formulation [Fe₂S₂(S-Cys)₄], the sites have not been defined by X-ray diffraction. Reaction of FeCl₃, o-C₆H₄(CH₂SH)₂, NaHS, and NaOMe in methanol affords the dimeric Fe(III) dianion [FeS(SCH₂)₂C₆H₄]₂^2-(1), isolated as its Et₄N⁺and Ph₄As⁺salts. The electronic spectra and other spectroscopic and magnetic properties of 1 indicate a clear electronic relationship to Fd_oxproteins. Ligand substitution reactions of 1 with arylthiols yield the dimers [Fe₂S₂(SAr)₄]^2-(Ar = Ph, p-tolyl (4d), p-C₆H₄Cl), isolated as Et₄N⁺salts. The structures of 1 and 4d (as their Et₄N⁺salts) have been determined. Both salts crystallize with two formula units in space group C⁵_2h-P2₁/n of the monoclinic system in unit cells of dimensions a = 9.549 (6) Å, b = 13.549 (6) Å, c = 14.748 (6) Å, and β = 95.42 (3)° (1) and a = 8.913 (3) Å, b = 16.132 (3) Å, c = 17.583 (3) Å, β = 97.14 (1)° (4d). The structures consist of well-separated cations and anions, the latter being sulfide-bridged dimers with crystallographically imposed centers of symmetry. Both anions contain planar and nearly isostructural Fe₂S₂* cores with Fe … Fe and mean Fe-S* distances of 2.698 (1) and 2.691 (4d) and 2.209 (1) and 2.201 Å (4d), respectively. The coordination units Fe₂S₂* are distorted tetrahedra with bond angles of 104.7-112.3° Å (1) and 104.6-115.8° (4d). The structural and other properties of 1 indicate that it is a suitable minimal representation of the oxidized protein active sites and tend to confirm the previously proposed formulation of these sites. The reaction product of 1 with p-Me₃N⁺C₆H₄SH is identical with the previously reported complex prepared directly from FeCl₃. the thiol, and NaHS and is shown to be the cation dimer [Fe₂S₂(SC₆H₄NMe₃)₄]²⁺. Corresponding properties of [Fe₂S₂(SR)₄]^2-and [Fe₄S₄(SR)₄^2-complexes at parity of R substituents are compared. Dimer Fe₂S₂* cores and tetramer Fe₂S₂* faces possess a high degree of dimensional similarity. Absorption spectra in the 400-600-nm region differ significantly. Dimeric complexes, as the tetramers, are reducible to 3— and 4— anions in steps which in most cases approach reversibility in DMF under dc and pulsed polarographic measurement conditions. Potentials for the [Fe₂S₂(SAr)₄]^2-/^3-process show an approximate correlation with σ_p. Differences in potentials for 2-/3- and 3-/4- dimer reactions (ca. 300 mV) are much smaller than those for the corresponding tetramer reactions (ca. 700 mV). The comparative spectral and electrochemical properties should allow analysis of dimer-tetramer mixtures such as might result from the extrusion of active sites from complex Fe-S proteins with aryl thiols. The preparation of several arylthiolate tetramers from Fe₂S₂(CO)₆is described.