Effect of Meso Substituents on Exchange-Coupling Interactions in μ-Oxo Iron(III) Porphyrin Dimers

The μ-Oxo iron(III) porphyrin dimers [(TPP)Fe]₂O, [(TPP(4-OCH₃))Fe]₂O, [(TPP(4-CF₃))Fe]₂O, and [(TPP(F₅))Fe]₂O (where TPP is meso-tetraphenylporphyrin, TPP(4-OCH₃) is meso-tetrakis(4-methoxyphenyl)porphyrin, TPP(4-CF₃) is meso-tetrakis-(4-(trifluoromethyl)phenyl)porphyrin, and TPP(F₅) is meso-tetrakis(pentafluorophenyl)porphyrin) have been studied to determine the effect of peripheral substituents on the porphyrin rings on properties of the molecules. Variable-temperature magnetic susceptibility studies on solid samples yielded the following exchange-coupling constants for the antiferromagnetically coupled iron(III) ions:-146.8 cm^-1, [(TPP(4-OCH₃))Fe]₂O;-135.7 cm^-1, [(TPP)Fe]₂O;-136.4 cm^-1, [(TPP(4-CF₃))Fe]₂O;-146.9 cm^-1, [(TPP(F₅))Fe]₂O. Variable-temperature ¹³C NMR studies of dichloromethane solutions yield exchange-coupling constants of-150 cm^-1for [(TPP(4-OCH₃))Fe]₂O, -145 cm^-1for [(TPP)Fe]₂O, and-129 cm^-1 for [(TPP(F₅))Fe]₂O. Raman spectra were collected in the region of the energy of the outer-ring stretching modes found at approximately 1560 cm^-1. The position of this band is empirically related to the porphyrin core size, and the data indicate that the porphyrin center-pyrrole nitrogen (C_t-N) distances increase in the order [(TPP(F₅))Fe]₂O < [(TPP(4-CF₃))Fe]₂O < [(TPP)Fe]₂O < [(TPP(4-OCH₃))Fe]₂O. The data support the hypothesis that the porphyrin ring core size increases as the electron-releasing capability of the peripheral substituent increases. For the para-substituted complexes, increasing core size correlates with stronger axial binding, reflected in stronger antiferromagnetic coupling. The anomalous behavior of [(TPP(F₅))Fe]₂O may result from distortions caused by steric interactions of the o-fluorines.