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 mu-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 H-1 NMR beta-pqrrole chemical shifts at -43 and +82 ppm indicate admixed-intermediate S = 3/2, 5/1 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 (Delta E-q = 3.24 mm s(-1)), and magnetic susceptibility (mu(eff) = 4.1 mu(B)). The anhydrous precursor to these species is the "nearly bare" iron(III) porphyrin complex Fe(CB11H6Br6)(TPP). Judged by its magnetic parameters (delta(pyrrole) = -62 ppm, Delta E-q = 3.68 mm s(-1), mu(eff) = 4.0 mu(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 pi effects in octahedral low-spin d(pi)(6) complexes.