UC Riverside

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(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. These two species were previously thought to be unattainable because of the facile formation of the well-known μ-oxo dimer, (TPP)Fe-O-Fe(TPP). 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(H2O)(TPP)] [CB11H6Cl6] and is consistent with the high degree of S = 3/2 character indicated by the NMR measurement, Mossbauer spectroscopy (ΔE(q) = 3.24 mm·s-1), and magnetic susceptibility (μ(eff) = 4.1 μ(B)). The anhydrous precursor to these species is the 'nearly bare' iron(III) porphyrin complex Fe(CB11H6Br6)(TPP). Judged by its magnetic parameters (δ(pyrrole) = -62 ppm, ΔE(q) = 3.68 mm·s-1, μ(eff) = 4.0 μ(B)) 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(III) 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 d(π)6 complexes.