Spectroscopic characterization of iron complexes of methyl pheophorbide with pyridine and its derivatives

Abstract

A series of (methyl pheophorbide a)iron(II) and -iron(III) complexes have been prepared and characterized by Mössbauer, 1H NMR and electronic spectra. The central iron(III) ion in (methyl pheophorbide a)iron(III) chloride [Fe(mepheo-a)] is in the high-spin state. Autoreduction of the central iron(III) ion in Fe(mepheo-a)Cl is induced by pyridine (py) and its derivatives (4-Xpy). A connection between isomer shifts and quadrupole splittings of Fe(mepheo-a)(4-Xpy) 2 indicates the σ-donation from the axial ligands to the central iron(II) ion is predominant in the axial bonding. The linear dependence of the isomer shifts on Hammett's constants of the pyridine substituents (X) and on the p K a values of the free ligands reveals that the, π-backdonation to the axial ligands enhances synergetically the σ-donation to the central iron(II) ion. The linear correlation between the isomer shift and the electronic absorption wavelength demonstrates that the difference in the π-backdonation to the axial ligands affects the π-π* excitations on the chlorin ring via the central iron(II) ion.