PREDICTION AND INTERPRETATION OF EPR-SPECTRA OF LOW-SPIN IRON(III) COMPLEXES WITH THE MM-AOM METHOD

The g-values of low-spin iron(m) complexes of trans-diammac (trans-6,13-dimethyl-1,4,8,11-tetraazacyclotetradecane- 6,13-diamine), tris-en (ethane-1,2-diamine), tris-phen (1,10-phenanthroline), tris-bipy (2,2'-bipyridine), and bis-terpy (2,2':6',2''-terpyridine) have been determined with angular overlap model (AOM) calculations based on structural parameters calculated with molecular mechanics (MM) calculations (MM-AOM approach). A constant set of e sigma and e pi parameters, corrected for bond length differences, was employed. There is good agreement between calculated and experimental EPR data. Regions of possible inaccuracies due to very strong dependency of the g-values from certain normal modes were identified by model AOM calculations as a function of the corresponding geometric parameter. In the case of [Fe(trans-diammac)](3+) the conformations of the five-membered chelate rings (disordered in the published crystal structures) are discussed in some detail. The calculated g-values of [Fe(en)(3)](3+) are discussed on the basis of a previously reported spectrum, and some inconsistencies with the calculations are discussed on the basis of a remeasured spectrum. The calculated spectra of [Fe(bpy)(3)](3+) and [Fe(phen)(3)](3+) are in good agreement with the reported experimental data. With [Fe(terpy)(2)](3+) it is demonstrated that the published EPR spectrum is, in contrast to earlier analyses, due to a highly distorted species (C-1), and this is also in agreement with other published observations.