The electronic properties (EPR and Mössbauer spectra, magnetic susceptibilities) of the recently synthesized, reduced tetranuclear cluster salts (R′4N)3[Fe4S4(SR)4] have been investigated in detail. The collective results fully support a description of the trianions as analogues of the 4-Fe sites of reduced ferredoxin proteins (Fdred). The compound (Et3MeN)3-[Fe4S4(SPh)4] crystallizes in the monoclinic space group Cs4-Cc with 8 formula units in a cell of dimensions a = 11.426 (5), b = 24.806 (7), c = 39.147 (10) Å, and β = 90.75 (2)°. The two crystallographically independent trianions have virtually identical structures which resemble that of [Fe4S4(SPh)4]2- in having Fe4S4 core configurations of idealized D2d symmetry, but differ in that the cores are elongated rather than compressed along the 4 axis. Trianions exhibit axial EPR spectra and temperature and power saturation characteristics similar to those of Fdred proteins. Zero-field Mössbauer spectra of polycrystalline samples and acetonitrile solutions at low temperatures consist of two overlapping quadrupole doublets with parameters very similar to those of B. stearothermophilus Fdred. Magnetic properties of polycrystalline and solution samples of [Fe4S4(SR)4]3- are consistent with a spin-doublet ground state and appreciable population of higher spin states, arising from net antiferromagnetic spin coupling, at temperatures down to 4.2 K. A significant observation is that the appreciable differences in Mössbauer and magnetic behavior of polycrystalline samples of [Fe4S4(SPh)4]3- and [Fe4S4(SCH2Ph)4]3- (whose core structure is less regular than that of the former trianion) are much attenuated when examined in frozen acetonitrile solutions. Because the properties of [Fe4S4(SPh)4]3- are much less sensitive to phase changes, we conclude that an elongated tetragonal geometry is the intrinsically stable core structure of [Fe4S4(SR)4]3-. On this basis it is speculated that the native forms of "high-potential", but not Fd, proteins may have evolved to resist the ca. 0.08 Å axial core expansion found in passing from analogue dianion to trianion. The former proteins are reducible only in the unfolded state.
ASJC Scopus subject areas
- Colloid and Surface Chemistry