Moessbauer studies on nitrogen fixation model compounds with double-cubane configuration (Et4N)4[Mo2Fe7S8(sph)12]

Moessbauer spectra of nitrogen model compounds with double-cubane configuration (Et4N)4[Mo2Fe7S8(sph)12] have been measured. The data have been fitted successfully with a least-squares fitting computer program. The Moessbauer parameters measured at 16 deg C are as follows: for FeI, δ = 0.315 ± 0.002 mm/s, ΔEQ = 0.691 ± 0.002 mm/s and the line width is 0.304 ± 0.002 mm/s; for FeII, δ = 0.972 ± 0.002 mm/s, ΔEQ = 1.741 ± 0.002 mm/s and the line width is 0.273 ± 0.002 mm/s. The isomer shift δ is relative to α-Fe. The ratio of quadrupole split areas for FeI and FeII is about 6:1. From these parameters it is found that FeII is in +2 high spin state. Using an empirical formula about the isomer shift δ and the mean oxidation state s of Fe atoms in tetrahedron FeS4:δ = 1.44-0.43 s, the author have deduced that FeI is in +2.67 high spin state