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

• Published in: Inorganic chemistry Vol. 39; no. 3; pp. 532 - 540
• Main Authors: Nesset, Marlys J. M., Cai, Sheng, Shokhireva, Tatjana Kh, Shokhirev, Nikolai V., Jacobson, Simone E., Jayaraj, K., Gold, Avram, Walker, F. Ann
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.02.2000
• Subjects: Electron Spin Resonance Spectroscopy; Electrons; Metalloporphyrins - chemistry; Nuclear Magnetic Resonance, Biomolecular; Temperature
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic9907866

The perchloratoiron(III) complexes of a series of 2,6-disubstituted tetraphenylporphyrin 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 temperature by 1H NMR spectroscopy. Curvature in the 1/T dependence was evident in most cases. Forced linear extrapolation of the temperature dependence observed over the range of the study yielded Curie plots that include negative slopes with very large positive 1/T intercepts (Cl ∼ 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 combined with EPR spectroscopic data and curve-fitting procedures based on an expanded Curie law to arrive at a consistent overview of the variety of temperature-dependence behaviors observed. This overview relies upon the premise that, in addition to the ground state observed by EPR spectroscopy, one (or more) thermally accessible excited state(s) are populated to varying degrees over the temperature range of the NMR measurements. If only one excited state is considered, the analysis is consistent with the ground state being a largely intermediate-spin state (S = 3/2) for the majority of the complexes but a largely high-spin state (S = 5/2) for ((2,6-F2)4TPP)FeOClO3 and ((2,6-(OMe)2)4TPP)FeOClO3.