Electron spin resonance spectra of reduced [Fe2S2(SC6H4Y-p)4]2–(Y = Cl, H, or Me) complexes and their selenium-ligated homologues

Abstract

E.s.r. spectra are reported for the chemically reduced forms of the dimers [Fe₂S₂(SC₆H₄Y-p)₄]^2–(Y = Cl, H, or Me) in NN-dimethylformamide (dmf) and 1-methylpyrrolidin-2-one solvents. Where the frozen solvents are non-vitreous, signals having g extrema at ca. 2.001–2.002, 1.952–1.958, and 1.911–1.915 are observed, which are compatible with those of the [2Fe–2S]⁺ ferredoxins and are assigned to the trianions [Fe₂S₂(SC₆H₄Y-p)₄]^3–. When the frozen solvents are vitreous, significantly different, solvent-dependent signals are obtained which are indicative of some chemical modification of the complex. The chelated complex trianions [Fe₂S₂{(SCH₂)₂C₆H₄-O}₂]^3– and [Fe₂S₂(btpo)₂]^3–[btpo = 2,2′-bis(thiophenolate)] give e.s.r. spectra devoid of such solvent dependency, but like the reduced [Fe₂S₂(SC₆H₄Y-p)₄]^2– complexes show a related change in spectrum in the presence of excess Bu^tSH. E.s.r. spectra are described for the reduction products of the selenium-bridged complexes [Fe₂Se₂{(SCH₂)₂C₆H₄-o}₂]^2–, [Fe₂Se₂(SC₆H₄Y-p)₄]^2–(Y = H and Me), and [Fe₂Se₂(btpo)₂]^2– and are compared with [2Fe–2Se]⁺ ferredoxin spectra. Spectra for reduced [Fe₂X₂(SePh)₄]^2–(X = S or Se) are also reported. The broadening of the low-field peaks in the spectra for [Fe₂S₂(SC₆H₄Cl-p)₄]^3– and [Fe₂Se₂{(SCH₂)₂C₆H₄-o}₂]^3– in dmf is analysed in the range ca. 90–200 K as a convolution product of a fixed Gaussian function with a Lorentzian curve of variable width. The latter is related to a variation in spin-lattice relaxation time with temperature which may be interpreted in terms of an Orbach mechanism involving the first excited state of the spin manifold due to antiferromagnetic coupling of the two iron atoms, such that –J= 240 ± 25 cm^–1 for [Fe₂S₂(SC₆H₄Cl-p)₄]^3– and –J= 200 ± 10 cm^–1 for [Fe₂Se₂{(SCH₂)₂C₆H₄-o}₂]^3–.