Five- and Six-Coordinated Nitrosyl Iron(II) Complexes of Tetrakis (p-substituted phenyl)porphyrins. Substituent Effects on the EPR Parameters and the NO Stretching Frequencies

Five- and six-coordinated nitrosyl iron(II) complexes of a series of tetrakis(p-substituted phenyl)porphyrins have been prepared. Their EPR and IR spectra have been measured and analyzed in order to elucidate the cis-effect of substituents on the axial nitrosyl ligand. As the first redox potentials of the free base porphyrins in the complexes became positive, the NO stretching frequencies increased in both five-coordinated and six-coordinated nitrosyl(porphyrinato)iron(II) complexes with a nitrogenous base at an axial site trans to the NO group. The positive shift of the redox potentials led to an increase in the EPR g values and hyperfine coupling constants at room temperature and at 77 K for the five-coordinated complexes, while it led to a decrease in g values and to a slight increase in hyperfine coupling constants for the six-coordinated complexes. These results are rationalized by invoking that, as the electron-withdrawing power of porphyrin peripheral substituents is enhanced, the iron-to-NO bond in both the five- and six-coordinated complexes is weakened and the iron-to-nitrogenous base bond in the six-coordinated complexes is strengthened. The difference in the cis- and trans-effects on the EPR parameters and NO stretching frequencies is also discussed.