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, H 2 O 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(H 2 O)(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 1 H 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(H 2 O)(TPP)][CB 11 H 6 Cl 6 ] and is consistent with the high degree of S = 3 / 2 character indicated by the NMR measurement, Mössbauer spectroscopy (Δ E 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(CB 11 H 6 Br 6 )(TPP). Judged by its magnetic parameters (δ pyrrole = −62 ppm, Δ E 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.