Electronic Structure, Porphyrin Core Distortion, and Fluxional Behavior of Bis-Ligated Low-Spin Iron(II) Porphyrinates

Three axially bis-ligated Fe(II) porphyrinates, [PFeL 2 ], 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 Δ E Q were found among [TMPFe(2-MeImH) 2 ] ( 1 ), [TMPFe( N -MeIm) 2 ] ( 2 ), and [OEPFe(PMe 3 ) 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, Δ E 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.