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]22-(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 Fdoxproteins. 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 C52h-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)42-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 σp. 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry