We have prepared a series of 94.5%-enriched 57 Fe(II) complexes of tetramesitylporphyrin and octaethylporphyrin having various bis-or mixed axial ligation and investigated them by both NMR and Mo ¨ssbauer spectroscopy. The results obtained, and literature values for MbCO and cytochrome c, show a rough correlation between ∆E Q and δ Fe . This correlation suggested that the previously-reported chemical shifts of the complexes [TPPFeL 2 ], L ) pyridine-d 5 , and pyrrolidine (Nozawa, T.; Sato, M.; Hatano, M.; Kobayashi, N.; Osa, T. Chem. Lett. 1983, 1289) might be incorrect; we have thus used the rapid pulsing method of Schwenck to detect the signal of the bis-pyridine-d 5 complex at 11 715 ppm, within its predicted chemical shift range, based upon the δ Fe Vs ∆E Q correlation, and ∼4,400 ppm to lower shielding than originally reported. These findings emphasize the value of the Mo ¨ssbauer-NMR correlation in aiding the detection of 57 Fe NMR signals, as well as the perils of seeking these easily "folded" signals without such a correlation.

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