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] iron−sulfur proteins, ( n Bu 4 N) 2 [Fe 4 S 4 (SR) 4 ], where SR - = tp ( 1 ), 3,5-dmtp ( 2 ), 2,4-dmtp ( 3 ), and 2,6-dmtp ( 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 34 S 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 2 d 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 S b −Fe−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 .