Iron complexes Mdssbauer spectra

The Fe Mdssbauer parameters for the clathrochelate [FeMn(trom)](BF4) and [Fe2(trom)](BF4) complexes are characteristic of high-spin iron complexes. The spectrum of the [FeMn(trom)](BF4) clathrochelate exhibits one doublet line, which indicates the sole type of coordination centre. The QS value increases regularly with a temperature decrease. A major contribution to the QS value is made by the valence component (the lattice component is small and practically independent of temperature). For [FeMn(trom)](BF4) clathrochelate, all three spin-quintet states are presumably within kT, and the QS value depends on temperature [192]. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

The Mdssbauer spectrum of the [Fe2(trom)]BF4 complex exhibited an unusually low QS value (ca 0.20 mm s-i) at room temperature and one doublet was observed at temperatures up to ca 80K. At 73K, the splitting sharply increases and the lines double. The low QS value at room temperature seems to be because the coordination centres of both iron atoms are practically identical. When the temperature decreases, a structural phase transition takes place, resulting in two different types of complexes, each being more distorted than its high-temperature form [192]. 

Fig. 10. Mdssbauer spectra of reduced (left panels) and oxidized (right panels) PS-I core complex at 80 K (top panels) and 4.2 K (bottom panels), The lower spectrum in each of the left panels was corrected for the unreduced component. The table at top right lists the experimentally determined isomer-shift (IS) and quadrupole-splitting (QS) parameters for oxidized and reduced core complexes at 77 K lower table lists isomer-shift values reported in the literature for other [4Fe 4S] and [2Fe 2S] clusters. Data source Petrouleas, Brand, Parrett and Golbeck (1989) A Mossbauer analysis of the low-potential iron-sulfur center in photosystem I Spectroscopic evidence that Fx is a [4Fe-4S] cluster. Biochemistry 28 8982.

---