Determination of antiferromagnetic exchange coupling in the tetrahedral thiolate-bridged diferrous complex...

Protein-bound iron-sulfur clusters and their synthetic analogues are characterized by tetrahedral metal sites, multiple oxidation levels, and exchange coupling. The recent attainment of several all-ferrous protein clusters and the presence of sulfide- and thiolate-bridged sites in the all-ferrous state of the nitrogenase P-cluster provides an imperative for determination of exchange coupling between tetrahedral Fe(II) sites with sulfur bridges. The cluster in the previously reported compound (Et(4)N)(2)[Fe(2)(SEt)(6)] is centrosymmetric with distorted tetrahedral coordination and a planar Fe(2)(mu-SEt)(2) bridge unit. The compound is diamagnetic at 4.2 K, indicating antiferromagnetic coupling. The lower limit J > 80 cm(-)(1) (H = JS(1).S(2)) is obtained by Mössbauer spectroscopy. Analysis of magnetic susceptibility data affords J = 165 +/- 15 cm(-)(1). It is noteworthy that the J value of the diferrous pair obtained here is comparable to the J values reported for the mixed-valence state of plant-type Fe(2)S(2) ferredoxins. The near temperature independence of the quadrupole splitting (DeltaE(Q) = 3.25 mm/s at 4.2 K and 3.20 mm/s at 180 K) indicates that no excited orbital states are appreciably populated at temperatures less than 300 K. The paramagnetism arises solely from thermal population of the S = 1 state of the spin ladder. This work provides the only measure of antiferromagnetic coupling by Fe(II) pairs in a tetrahedral sulfur environment.