Electronic structure, porphyrin core distortion, and fluxional behavior of bis-ligated low-spin iron(II) porphyrinates

Three axially bis-ligated Fe(II) porphyrinates, [PFeL₂], have been investigated by Mössbauer spectroscopy over the temperature range 4.2-230 K and in an applied field of 6 T. Relatively large differences in the quadrupole splitting ΔE_Q were found among [TMPFe(2-MeImH)₂] (1), [TMPFe(N-MeIm)₂] (2), and [OEPFe-(PMe₃)₂] (3) (1.61, 1.07, and 0.38 mm/s, respectively). Compound 3 exhibits significant line broadening above 150 K that indicates fluxional distortion and/or ligand rotation. Molecular orbital calculations in the local density approximation yield electric field gradients (efg) in good agreement with the measured quadrupole splittings, ΔE_Q, and the measured sign of the efg. The observed differences in quadrupole splittings can be ascribed to distortions (ruffling) of the porphyrin core in 1 as compared to 2 and 3 and to the differences in covalent interactions of the axial nitrogen donors of 1 and 2 and phosphorous donors of 3. The observed temperature-dependent line broadening of 3 correlates with the low calculated rotational barrier.