Electronic structure and bonding in unligated and ligated FeII porphyrins

Abstract

The electronic structure and bonding in a series of unligated and ligated FeII porphyrins (FeP) are investigated by density functional theory (DFT). All the unligated four-coordinate iron porphyrins have a A2g3 ground state that arises from the (dxy)2(dz2)2(dπ)2 configuration. The calculations confirm experimental results on Fe tetraphenylporphine but do not support the resonance Raman assignment of Fe octaethylporphine as Eg3, nor the early assignment of Fe octamethyltetrabenzporphine as B2g5. For the six-coordinate Fe–P(L)2 (L=HCN, pyridine, CO), the strong-field axial ligands raise the energy of the Fe dz2 orbital, thereby making the iron porphyrin diamagnetic. The calculated redox properties of Fe–P(L)2 are in agreement with experiment. As models for deoxyheme, the energetics of all possible low-lying states of FeP(pyridine) and FeP(2-methylimidazole) have been studied in detail. The groundstate configuration of FeP(2-methylimidazole) was confirmed to be high-spin (dxy)2(dz2)1(dπ)2(dx2−y2)1; FeP (pyridine) is shown to be a poor model for high-spin deoxyheme.