Resonance-Raman and infrared spectra of the dimeric iron–chalcogenide complexes [Fe2X2(YR)4]2–(X and Y = S or Se)

Abstract

Resonance-Raman spectra are reported for the dimeric iron-chalcogenide complexes [Fe₂X₂(SR)₄]^2–[X = S or Se; (SR)₂=(SCH₂)₂C₆H₄-o or R = Ph, C₆H₄Me-p, or C₆H₄Cl-p], [Fe₂X₂(SePh)₄]^2–(X = S or Se), [Fe₂X₂Cl₄]^2–(X = S or Se), and [Fe₂S₂(Br)₄]^2– down to 100 cm^–1 or lower using a range of laser excitation wavelengths. Complementary i.r. spectra are also reported. Assignment of iron–chalcogenide stretching modes and some deformation modes is attempted. Iron–sulphur bridge-stretching vibrations are found to be affected little by the nature of the terminal ligands with the Raman-active symmetric mode assigned in the range 375–396 cm^–1 and the two i.r. asymmetric modes in the ranges 415–420 and 274–328 cm^–1. Their frequency shifts on going to selenium bridging are helpful in making these assignments. Frequencies for the totally-symmetric terminal-ligand stretch and its i.r. counterpart are observed to be much higher (381–429 cm^–1) for the aromatic thiolates than for the (SCH₂)₂C₆H₄-o or chloride ligands (321–334 cm^–1). The symmetric in-plane ring deformations are assigned in the range 178–256 cm^–1 for sulphur bridging and show a correlation with crystallographically determined Fe–Fe bond lengths. Electronic spectra of [Fe₂S₂(SC₆H₄Y_-p)₄]^2–(Y = H or Cl) in the visible region are deconvoluted into Gaussian bands. Component bands at ca. 550–560 nm compare favourably in position and width with crude Raman excitation profiles for intense polarised solution peaks attributable to Fe–(terminal ligand-) vibrations. Comparison of the Raman spectra is made with those previously reported for 2Fe-ferredoxins.