Synthetic Analogs of the Active Sites of Iron-Sulfur Proteins. II.¹ Synthesis and Structure of the Tetra[mercapto-m₃-sulfndo-iron] Clusters, [Fe₄s₄(SR)₄]

Reaction of ferric chloride in methanol with 3 equiv of sodium mercaptide followed by 1 equiv of a methanolic solution of sodium hydrosulfide and sodium methoxide affords the complexes [Fe4S4(SR)J2~ (R = alkyl, aryl), which may be precipitated and purified in the form of salts of quaternary cations. The crystal structure of a prototype member of the series, [Et₄N]₂[Fe₄S₄(SCH₂Ph)₄], has been determined from 4084 independent reflections collected by counter methods. The red-black crystals are monoclinic, space group C_2h-P2₂lc, with a = 11.922 (6), b = 34.523 (17), and c = 12.762 (5) Á and ß = 95.78 (2)°. The measured density of 1.43 (3) g cm-3 agrees with the density of 1.40 g cm-3, calculated for eight anions and four cations in the cell. The absorption corrected data gave a conventional R factor (on F) of 0.036 on least-squares refinement. The structure consists of discrete anions and cations; the geometry of the tetraethylammonium ion is unexceptional. The core (Fe4S4∗) of the anion exhibits distortions from cubic symmetry which are those of the point group D_2c-42m such that each face of the polyhedron is a nonplanar rhomb. The bonded Fe-S∗ distances occur as sets of four (2.239 (4) A) and eight (2.310 (3) Á), giving an average of 2.286 Á. The Fe-Fe distances occur as four short (2.732 (3) A) and two slightly longer (2.776 Á) separations and imply some direct metal-metal interactions. All four iron atoms are structurally indistinguishable. Comparisons of certain structural parameters of [Fe4S4(SCH2Ph)4p with those of the active sites of the reduced high-potential iron protein from Chromatium and the oxidized fer-redoxin from P. aerogenes show that the three structures are unmistakably closely similar. This information together with comparative spectroscopic and magnetic evidence presented elsewhere demonstrates that the synthetic complexes [Fe4S4(SR)4]2- are true analogs of the active sites of bacterial iron-sulfur proteins.