NMR and structural investigations of a nonplanar iron corrolate: Modified patterns of spin delocalization and coupling in a slightly saddled chloroiron(III) corrolate radical

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An undecasubstituted chloroiron corrolate, octamethyltriphenylcorrolatoiron chloride, (OMTPCorr)FeCl, has been synthesized and studied by X-ray crystallography and 1H and 13C NMR spectroscopy. It is found that, although the structure is slightly saddled, the average methyl out-of-plane distance is only 0.63 Å, while it is much greater for the dodecasubstituted porphyrinate analogue (OMTPP)FeCl (1.19 Å) (Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J. Am. Chem. Soc. 1997, 119, 2563-2569). In addition, the distance of iron from the mean plane of the four macrocycle nitrogens is also smaller for (OMTPCorr)FeCl (0.387 Å) than for (OMTPP)FeCl (0.46 Å). The 1H and 13C NMR spectra of (OMTPCorr)FeCl, as well as the chloroiron complexes of triphenylcorrolate, (TPCorr)FeCl; 7,13-dimethyl-2,3,8,12,17,18- hexaethylcorrolate, (DMHECorr)FeCl; 7,8,12,13-tetramethyl-2,3,17,18- tetraethylcorrolate, (TMTECorr)FeCl; and the phenyliron complex of 7,13-dimethyl-2,3,8,12,17,18-hexaethylcorrolate, (DMHECorr)FePh, have been assigned, and the spin densities at the carbons that are part of the aromatic ring of the corroie macrocycle have been divided into the part due to spin delocalization by corroie → Fe π donation and the part due to the unpaired electron present on the corroie ring. It is found that although the spin density at the β-pyrrole positions is fairly similar to that of (TPCorr)FeCl, the meso-phenyl-carbon shift differences δm- δp are opposite in sign of those of (TPCorr)FeCl. This finding suggests that the radical electron is ferromagnetically coupled to the unpaired electrons on iron, rather than antiferromagnetically coupled, as in all of the other chloroiron corrolates. The solution magnetic moment was measured for (OMTPCorr)FeCl and found to be μeff = 4.7 ± 0.5 μB, consistent with S = 2 and ferromagnetic coupling. From this study, two conclusions may be reached about iron corrolates: (1) the spin states of chloroiron corrolates are extremely sensitive to the out-of-plane distance of iron, and (2) pyrrole-H or -C shifts are not useful in delineating the spin state and electron configuration of (anion)iron corrolates. © 2005 American Chemical Society.

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Inorganic Chemistry

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