CRYSTAL-STRUCTURE OF [(C2H5)4N]3[FE4S4(SCH2PH)4], A REDUCED FERREDOXIN SITE ANALOG WITH A NONTETRAGONAL FE4S4 CORE STRUCTURE IN THE SOLID-STATE

Determination of the structure of the title compound, a synthetic analogue of the 4-Fe sites of reduced ferredoxin (Fd) proteins, was undertaken in order to clarify the source of substantial differences in 57Fe Mossbauer spectral and magnetic properties between it and those of the isoelectronic trianion [Fe4S4(SPh)4]3- in the solid state. (Et4N)3[Fe4S4(SCH2Ph)4] crystallizes in the monoclinic space group Cc with 4 formula units in a cell of dimensions a = 30.318 (11) Å, b = 11.469 (2) Å, c = 21.412(7) Å, and β = 123.25 (2)°. Using 3482 unique data with Fo 2 > 3σ(Fo 2) the structure was refined to R = 4.3%. In contrast to the [Fe4S4*]+ core structure of the Fdred site analogue [Fe4S4(SPh)4]3-, which is distorted from cubic toward an elongated D2d geometry (eight short + four long Fe-S* bonds), the core of [Fe4S4(SCH2Ph)4]3- contains six short (mean 2.302 (4) Å) and six long (mean 2.331 (4) Å) Fe-S* bonds so arranged to afford idealized C2v symmetry. Inasmuch as the Fdox analogues [Fe4S4(SR)4]2- exhibit a highly uniform compressed D2d core geometry (four short + eight long Fe-S* bonds), [Fe4S4(SCH2Ph)4]3- is the only site analogue yet shown by diffraction methods to possess a nontetragonal core stereochemistry. Brief dianion-trianion and trianion-trianion structural comparisons are presented. The small core structural differences between the two trianions are considered the most likely source of the differences in physicochemical properties in the solid state. A previous investigation has shown that these properties become essentially coincident when the two trianions are examined in frozen and fluid acetonitrile solutions. It is therefore concluded that the unconstrained core structural change accompanying the [Fe4S4(SR)4]2- 3- electron transfer reaction is compressed D2d ↔ elongated D2d rather than compressed D2d ↔ C2v, and that the less symmetric core structure of [Fe4S4(SCH2Ph)4]3- in its crystalline tetraethylammonium salt is the consequence of solid state environmental factors. © 1979, American Chemical Society. All rights reserved.