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

Abstract

We have prepared a series of 94.5%-enriched 57Fe(II) complexes of tetramesitylporphyrin and octaethylporphyrin 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 ΔEQ and δFe. 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 δFe vs ΔEQ 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.