Resonance Raman detection of iron-ligand vibrations in cyano(pyridine)(octaethylporphinato)iron(III): effects of pyridine basicity on the iron-cyano bond strength

Abstract

The influence of axial ligand basicity on the bonding of iron(III) in cyano adducts of octaethylporphyrin has been studied by resonance Raman spectroscopy. In a six-coordinate ferric low-spin complex, cyano(pyridine)(octaethylporphinato)iron(III), Fe(OEP)(CN)(py), Raman lines at 449 and 191 cm/sup /minus/1/ were assigned to the /nu/(Fe-CN) and /nu/(Fe-py) stretching modes, respectively. When pyridine was displaced with its derivatives, py-X, where X = 4-cyano, 3-acetyl, 3-methyl, 4-methyl, 3,4-dimethyl, and 4-dimethylamino, the /nu/(Fe-CN) stretching frequency was found to decrease in the complex with a high pyridine basicity. It was concluded that the stronger the trans pyridine basicity, the weaker the iron-carbon (cyanide) bond. A clear frequency shift was observed in the /nu//sub 4/ model, though most of the porphyrin vibrations were insensitive to the ligand substitution. The frequency of the /nu//sub 4/ mode, which is the C/sub a/-N(pyrrole) breathing vibration of the porphyrin skeleton, was found to increase with an increase in pyridine basicity. This is contrary to what was found in ferrous low-spin hemes as CO complexes. The /nu//sub 4/ shift in the CN complexes was explained in terms of forward /pi/ donation; donation of electrons from the porphyrin /pi/ orbital to the d/sub /pi// vacancy of the low-spin iron(III) weakened the C/submore » a/-N(pyrrole) bonds and hence decreased the /nu//sub 4/ frequency. 32 references, 8 figures.« less