Kinetics of Ring Inversion in Strongly Nonplanar Iron(III) Octaalkyltetraphenylporphyrinates

• Published in: Inorganic chemistry 2005-04, Vol.44 (8), p.2867-2881
• Main Authors: Yatsunyk, Liliya A, Ogura, Hiroshi, Walker, F. Ann
• Format: Article
• Language: English
• Subjects: Cations; Ferric Compounds - chemistry; Iron - chemistry; Kinetics; Ligands; Metalloporphyrins - chemistry; Molecular Structure; Porphyrins - chemistry; Temperature
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic049090p

The dynamics of porphyrin ring inversion of a number of Fe(III) complexes of octamethyltetraphenylporphyrin, (OMTPP)FeIII; octaethyltetraphenylporphyrin, (OETPP)FeIII; octaethyltetra(perfluorophenyl)porphyrin, (F20OETPP)FeIII; and tetra-β,β‘-tetramethylenetetraphenyl-porphyrin, (TC6TPP)FeIII, having either one (Cl-, ClO4 -) or two [4-(dimethylamino)pyridine, 4-Me2NPy; 1-methylimidazole, 1-MeIm; tert-butylisocyanide, t-BuNC; or cyanide, CN-] axial ligands have been characterized by 1D dynamic NMR (DNMR) and 2D 1H NOESY/EXSY spectroscopies as a function of temperature. The activation parameters, ΔH ⧧, ΔS ⧧, and ΔG ⧧ 298, and the extrapolated rate constants at 298 K for three chloride, one perchlorate, and three bis-(4-Me2NPy) complexes as well as [FeOETPP(1-MeIm)2]Cl, [FeOETPP(t-BuNC)2]ClO4, and Na[FeOETPP(CN)2] have been determined. The results indicate that there is a wide range of flexibility for the porphyrin core (k ex 298 = 10−107 s-1) that decreases in the order TC6TPP > OMTPP > F20OETPP ≥ OETPP, which correlates with increasing porphyrin nonplanarity. To determine the effect of axial ligands, we calculated the free energy of activation, ΔG ⧧ 298 for OETPPFeIII bis-ligated with 4-Me2NPy, 1-MeIm, or 4-CNPy (∼59 kJ mol-1), and for complexes with small cylindrical ligands (t-BuNC and CN-) (∼37 kJ mol-1). These data suggest that the ΔG ⧧ 298 for planar ligand rotation is roughly 20−25 kJ mol-1.