Abstract

Contrary to expectations based on the spectrochemical series, H2O is found to be a significantly weaker field ligand than OH- in the magnetochemical series ranking of ligand field strengths based on the spin states of iron tetraphenylporphyrin complexes. The preparation and characterization of the [Fe]+ ion and the spectroscopic identification of Fe have made this assessment possible. These two species were previously thought to be unattainable because of the facile formation of the well-known μ-oxo dimer, Fe-O-Fe. However, the special characteristics of single equivalents of water under high 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 and high S = 5/2 spin states for the aqua and hydroxo species, respectively. The X-ray crystal structure of the aqua complex has been determined for [Fe] [CB11H6Cl6] and is consistent with the high degree of S = 3/2 character indicated by the NMR measurement, Mossbauer spectroscopy = 3.24 mm·s-1), and magnetic susceptibility = 4.1 μ). The anhydrous precursor to these species is the 'nearly bare' iron porphyrin complex Fe. Judged by its magnetic parameters = -62 ppm, ΔE = 3.68 mm·s-1, μ = 4.0 μ) it attains the long sought essentially 'pure' S = 3/2 spin state. The magnetochemical ranking of ligand field strengths in five-coordinate high-spin and admixed-intermediate-spin iron porphyrins is useful because it more closely reflects the intuitive field strengths of crystal field theory than does the usual spectrochemical ranking, which is controlled largely by π effects in octahedral low-spin d6 complexes.