| Abstract
| - Contrary to expectations based on the spectrochemical series, H2O is found to be a significantlyweaker field ligand than OH- in the magnetochemical series ranking of ligand field strengths based on thespin states of iron(III) tetraphenylporphyrin complexes. The preparation and characterization of the [Fe(H2O)(TPP)]+ ion and the spectroscopic identification of Fe(OH)(TPP) have made this assessment possible. Thesetwo species were previously thought to be unattainable because of the facile formation of the well-knownμ-oxo dimer, (TPP)Fe−O−Fe(TPP). However, the special characteristics of single equivalents of water underhigh acidity, relevant to metalloenzyme active sites and superacidity, make them accessible in benzene solution.Their 1H NMR β-pyrrole chemical shifts at −43 and +82 ppm indicate admixed-intermediate S = 3/2, 5/2 andhigh S = 5/2 spin states for the aqua and hydroxo species, respectively. The X-ray crystal structure of the aquacomplex has been determined for [Fe(H2O)(TPP)][CB11H6Cl6] and is consistent with the high degree of S =3/2 character indicated by the NMR measurement, Mössbauer spectroscopy (ΔEq = 3.24 mm·s-1), and magneticsusceptibility (μeff = 4.1 μB). The anhydrous precursor to these species is the “nearly bare” iron(III) porphyrincomplex Fe(CB11H6Br6)(TPP). Judged by its magnetic parameters (δpyrrole = −62 ppm, ΔEq = 3.68 mm·s-1,μeff = 4.0 μB) it attains the long sought essentially “pure” S = 3/2 spin state. The magnetochemical ranking ofligand field strengths in five-coordinate high-spin and admixed-intermediate-spin iron(III) porphyrins is usefulbecause it more closely reflects the intuitive field strengths of crystal field theory than does the usualspectrochemical ranking, which is controlled largely by π effects in octahedral low-spin dπ6 complexes.
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