Chloroiron meso-triphenylcorrolates: electronic ground state and spin delocalization

Four chloroiron meso-triphenyl-substituted corrolates have been synthesized and studied by 1H NMR spectroscopy. As in the case of the β-pyrrole-octaalkylcorrolatoiron chloride complexes studied previously [Inorg. Chem. 39 (2000) 3466], these complexes were also found to be S=3/2 Fe(III) corrolate(2−radical dot) π-cation radical species, where the macrocycle radical electron is antiferromagnetically coupled to the metal electrons to give an overall S=1 complex. This conclusion is based upon the large alternating-sign contact shifts observed for the meso-phenyl protons. The 1H isotropic shifts of the pyrrole-H of these chloroiron–triphenylcorrolate complexes are similar to those of the chloroiron tri-(pentafluorophenyl)corrolate complex reported previously and said to be a S=1 Fe(IV) complex bound to a simple corrolate(3−) ligand [Inorg. Chem. 39 (2000) 2704]. The 19F NMR spectrum of the latter complex shows that it has small (negative) phenyl-F isotropic shifts for all phenyl-F, which might suggest that this single compound has a different electronic structure than all other chloroiron corrolates investigated thus far. However, there have as yet been very few NMR investigations of paramagnetic metal macrocycles having fluorine substituents, and thus it is premature to conclude that the small phenyl-F isotropic shifts are definitive proof of small spin density at the meso positions of the corrolate ring. It is concluded that pyrrole-H chemical shifts alone cannot differentiate the two possible electron configurations, simple S=1 Fe(IV) (Corr3−) and antiferromagnetically coupled S=3/2 Fe(III) (Corr2−radical dot), and that based on the 1H investigations reported in this and two previous papers, all chloroiron corrolates reported thus far, with the exception of one, have the electron configuration S=3/2 Fe(III) (Corr2−radical dot), in which the corrolate unpaired electron is antiferromagnetically coupled to the three metal electrons, yielding an overall spin for the complex, S=1. The electron configuration of the one exception, the strongly electron-withdrawing tri-(pentafluorophenyl)corrolate complex of iron chloride, cannot as yet be definitively assigned.