EPR, Resonance Raman, and DFT Calculations on Thiolate- and Imidazole-Bound Iron(III) Porphyrin Complexes: Role of the Axial Ligand in Tuning the Electronic Structure

Iron(III) porphyrin complexes bearing covalently attached imidazole and thiolate axial ligands are investigated using resonance Raman, electron paramagnetic resonance, and cyclic voltammetry. The thiolate ligand stabilizes a low-spin ground state in solvent-bound six-coordinate species, weakens the Fe–Npyr bonds, and shifts the FeIII/II potential more negative by ∼500 mV relative to an imidazole-bound species. Density functional theory calculations reproduce the experimental observation and indicate that the covalent charge donation from thiolate to iron reduces the Zeff on the iron. This increases the Fe3d orbital energies, which changes the bonding interaction present in these complexes significantly. In particular, the increase of the Fe3d energies activates an iron-to-porphyrin π*-back-bonding interaction not present in the imidazole-bound complex.