Valence electron cloud asymmetry from two points of view: A correlation between Mössbauer quadrupole splittings and 57Fe NMR chemical shifts of diamagnetic iron(II) porphyrinates

We have prepared a series of 94.5%-enriched ⁵⁷Fe(II) complexes of tetramesitylporphyrin and octaethylpoiphyrin having various bis- or mixed axial ligation and investigated them by both NMR and Mössbauer spectroscopy. The results obtained, and literature values for MbCO and cytochrome c, show a rough correlation between ΔE_Q and δ_Fe. This correlation suggested that the previously-reported chemical shifts of the complexes [TPPFeL₂], L = pyridine-d₅, 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-d₅ complex at 11 715 ppm, within its predicted chemical shift range, based upon the δ_Fe vs ΔE_Q correlation, and ∼4,400 ppm to lower shielding than originally reported. These findings emphasize the value of the Mössbauer - NMR correlation in aiding the detection of ⁵⁷Fe NMR signals, as well as the perils of seeking these easily "folded" signals without such a correlation.