Resonance Raman, Electron Paramagnetic Resonance, and Density Functional Theory Calculations of a Phenolate-Bound Iron Porphyrin Complex: Electrostatic versus Covalent Contribution to Bonding

Resonance Raman (rR), electron paramagnetic resonance (EPR), and density functional theory (DFT) calculations of a phenolate-bound iron porphyrin complex are reported. The complex is found to exist in a five-coordinate high-spin state in a noncoordinating solvent and in a six-coordinate low-spin state in a coordinating solvent. The vibrations originating from the iron phenolate-bound chromophores reproduced those reported for heme tyrosine active sites in nature. The EPR parameters and iron-pyrrole (Fe-N-pyr) vibrations of phenolate, thiolate, and imidazole ligated iron porphyrin complexes indicate that the phenolate axial ligand acts as a pi anisotropic ligand, which is more covalent than a neutral imidazole ligand but less covalent than a thiolate axial ligand. While the Fe-III/II potential of the phenolate compound in a noncoordinating solvent is 500 mV more negative than that of the imidazole-bound complex, it is also 110 mV more negative than that of the thiolate-bound complex. DFT calculations reproduce the geometry and vibrational frequencies and show that while both phenolate and thiolate axial ligands bear pi and sigma interaction with the ferric center, the former is significantly less covalent than the thiolate. The higher covalency of the thiolate ligand is responsible for the lower Fe-N-pyr vibration and higher V/lambda (from EPR) of the thiolate-bound complexes relative to those of the phenolate-bound complex, whereas the greater electrostatic stabilization of the Fe-III-OPh bond is responsible for lowering the Fe-III/II E degrees of the phenolate-bound complex relative to that of the thiolate-bound complex in a medium having a reasonable dielectric constant.