Reversal of H₂O and OH^- Ligand Field Strength on the Magnetochemical Series Relative to the Spectrochemical Series. Novel 1-equiv Water Chemistry of Iron(III) Tetraphenylporphyrin Complexes

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, Mössbauer 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_π⁶ complexes.