Electronic effects in transition metal porphyrins. 10. Effect of ortho substituents on the temperature dependence of the NMR spectra of a series of spin-admixed perchloratoiron(III) tetrakis(2,6-or 2,4,6-phenyl substituted)porphyrinates

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraph enylporphyrin ligands, where the 2,6-phenyl substituents were -H, -F, -Cl, -Br, or -OMe, as well as two 2,4,6-phenyl-substituted complexes, where the substituents were -Me and -OMe, have been investigated as a function of tem perature by H-1 NMR spectroscopy. Curvature in the 1/T dependence was evide nt in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include nega tive slopes with very large positive 1/T intercepts (Cl similar to Br > Me > H) to negative slope with near zero intercept (tri-OMe) to positive slope with very large negative intercept (F, di-OMe). The NMR results were combi ned with EPR spectroscopic data and curve-fitting procedures based on an ex panded Curie law to arrive at a consistent overview of the variety of tempe rature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (o r more) thermally accessible excited state(s) are populated to varying degr ees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state bein g a largely intermediate-spin state (S = 3/2) for the majority of the compl exes but a largely high-spin state (S = 5/2) for ((2,6-F-2)(4)TPP)FeOClO3 a nd ((2,6-(OMe)(2))(4)TPP)FeOClO3.