DENSITY-FUNCTIONAL STUDIES OF IRON(III) PORPHINES AND THEIR ONE-ELECTRON-OXIDIZED DERIVATIVES

The relative energies of selected states for one-electron-oxidized difluoro(porphinato)iron(III) and fluoro(porphinato)iron(III) have been studied using approximate density functional methods. For the difluoro derivative an iron(IV) (S = 1) state is stabilized relative to an iron(III) A2u pi-cation-radical (S = 3) state. For the monofluoro species this ordering of states is reversed. The molecular structures of difluoro(porphinato)iron(III) and the corresponding iron(IV) and iron(III) A2u pi-cation-radical species have been investigated using a full geometry optimization. The A2u pi-cation-radical species has a geometry very similar to the parent compound. In contrast, the iron(IV) derivative displays significantly shortened Fe-N and Fe-F bond distances. A possible relationship between iron(III) spin state and the site of one-electron oxidation in iron(III) porphyrins is rationalized in terms of exchange energy stabilization and the sigma- and pi-donor properties of the axial ligand(s). The pi --> pi* singlet transition energies of chloro(porphinato)iron(III) have been calculated. Good agreement with experimental values is found, and an assignment of the Q-band region as arising from 14a1 --> 18e (a2u --> eg*) and 5a2 --> 18e (a1u --> e(g)*) transitions is proposed. The B-band (Soret) region is suggested to arise from 8b, --> 18e (b2u --> e(g)*) and 13a1 --> 18e (a2u' --> e(g)*) transitions.