Carbon-13 NMR spectra have been recorded for complexes of high-spin iron(III) porphyrins in which the axial anionic ligand has been varied. The (Me2SO)2 complexes of high-spin iron(III) tetraphenylporphyrins ((TPP)Fe111) and natural-derivative porphyrins have also been examined. Replacement of a single anionic ligand with two solvent ligands induces striking changes in carbon-13 NMR spectra, with changes in the phenyl ortho-carbon and meso-carbon signals of (TPP)Fe111 on the order of several hundred ppm. This explains the seemingly anomalous upfield and downfield meso-proton shift values for fiveand six-coordinate species. Results are interpreted in terms of unpaired spin delocalization through bothand -type molecular orbitals, with a predominant mechanism of Fe--porphyrin "back-bonding" in six-coordinate complexes.
A contribution from porphyrinFe charge transfer places large unpaired spin density at the meso-carbon atom of five-coordinate species. Modulation of the meso-position spin density by differing axial anionic ligands is apparent in variable NMR chemical shift values for the phenyl and meso-carbon atoms of (TPP)FeX complexes. Phenyl shifts show a positive correlation with the zero-field splitting parameter, D, as measured by far-infrared and magnetic methods. A correspondence is also apparent between meso-carbon NMR shift values and Fe(III)/Fe(II) redox potentials. These observations suggest the ligand field strength of the axial ligand dictates the magnitude of unpaired spin density at the meso position, with larger downfield shifts associated with weaker field ligands. On this basis the following ordering of ligand field strength is offered for high-spin iron(III) porphyrins: F" > OPh' > OPh(p-N02)~> N3" = OAc" > NCS' > Cl' > Br" > I".
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