Iron(IV) porphyrin difluoride does not exist: Implications for DFT calculations on heme protein reaction pathways

DFT (133LYP as well as a number of common exchange-correlation functionals) predicts a low-spin Fe(IV) ground state for Fe(P)F-2 (P = porphyrinato), whereas electrochemical evidence has apparently indicated an Fe(III) porphyrin pi-cation radical formulation for such a species. Ab initio CASPT2 calculations favor a high-spin porphyrin g-cation radical as the ground state by a significant energetic margin, thus dramatically overturning the DFT results. In contrast, both DFT and CASPT2 calculations correctly indicate a true Mn(IV) ground state for Mn(P)F-2. The remarkable failure of DFT to correctly predict the metal- versus ligand-oxidized nature of Fe(P)F-2 may have significant ramifications for the theoretical modeling of heme protein reaction pathways where until now the performance of DFT has raised little concern.