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 inv estigated by Mossbauer spectroscopy over the temperature range 4.2-230 EC. and in an applied field of 6 rh( Relatively large differences in the quadrupole splitting Delta E-Q were found among [TMEPFe(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 br oadening above 150 K that indicates fluxional distortion and/or ligand rotation. Molecular orbital calculations in the local density approxi mation yield electric field gradients (efg) in good agreement with the measured quadrupole splittings, Delta E-Q, and the measured sign of t he efg, The observed differences in quadrupole splittings can be ascri bed to distortions (ruffling) of the porphyrin core in 1 as compared t o 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 cal culated rotational barrier.