Ligand conformational effects on the resonance Raman signature of [Fe4S4(SAryl)4]2- clusters

Low-temperature.(77 K) resonance Raman (RR) spectra are reported for a series of analogues of [4Fe-4S] (4) (tp = thiophenol and dmtp = dimethylthiophenol). Structural influences of these different terminal thiolates on the RR signature of the [4Fe-4S](2+) core are presented. Both the cluster and terminal ligand Fe-S vibrational modes are identified and assigned through S-34 isotope substitution of analogues 1 and 4 at the cluster sulfur position. The characteristic RR frequencies of the four compounds provide new insights into the vibrational and electronic properties of the iron-sulfur cluster due to the ligand conformation changes. The RR spectra demonstrate a significant D-2d distortion of each cluster, and an energy crossing is observed for the T-2 terminal (t) and bridging (b) Fe-S stretching modes above 350 cm(-1) of 1 and 4 in response to changes in the ShFe-S-t bond angles imposed by different geometries about the Fe-S-t bonds, The results affirm the absolute necessity of isotope substitution for an exact assignment of the Fe-S-t and Fe-S-b vibrational modes in [4Fe-4S] clusters when the [4Fe-4S] proteins are analyzed and compared. The blue-shifted to 404 nm electronic absorption band of 4 is identified as a predominantly S(Aryl) --> Fe charge-transfer transition by excitation profiles for two prominent Fe-S RR bands, the A(1) bridging stretch at 336 cm(-1) and the T-2 terminal stretch at 351 cm(-1).