Molecular structures and electron distributions of higher-valent iron and manganese porphyrins: Density functional theory calculations and some preliminary open-shell coupled-cluster results

Abstract

Density functional theory (DFT) calculations have been carried out for a variety of iron and manganese porphyrin complexes with metal oxidation states of +3 and above. In general, DFT gives very good descriptions of molecular structure and electron distributions, but appears less reliable in predictions of the relative energetics of different spin states of transition metal compounds. For instance, the fact that our calculations favor a quartet state for the five-coordinate complex, ( P ) FeCl , over a sextet state may be regarded as a shortcoming of currently available exchange-correlation functionals. This problem is corrected at the considerably more demanding spin-restricted open-shell coupled-cluster level of theory. For both +3 and +4 metal oxidation states, the manganese 3d subshells appear to be significantly more spatially contracted than the 3d subshells of analogous iron compounds. This appears to be a key factor underlying the anomalously low MnO stretching frequencies of Mn(IV) -oxo porphyrins, compared to the FeO stretching frequencies of Fe(IV) -oxo porphryins. Similarly, a dramatic ruffling-induced redistribution of unpaired spin density in the case of an A2 u-type iron(IV)-oxo porphyrin cation radical, recently reported by Deeth, has been ascribed to a metal( dxy)–porphyrin( a2 u) orbital interaction that becomes symmetry-allowed on ruffling of the porphyrin ring. If the axial ligand in a compound I species is strongly basic such as imidazolate (e.g. horseradish peroxidase) or mercaptide (chloroperoxidase and cytochrome P450), it too can be strongly noninnocent and carry a significant unpaired electron spin population. Similarly, theoretical modeling of a synthetic 'perferryl' complex, reported by Morishima and co-workers, has revealed a noninnocent axial methoxide ligand.