title: Kinetics of ring inversion in strongly nonplanar iron(III) octaalkyltetraphenylporphyrinates.
abstract: The dynamics of porphyrin ring inversion of a number of Fe(III) complexes of octamethyltetraphenylporphyrin, (OMTPP)Fe(III); octaethyltetraphenylporphyrin, (OETPP)Fe(III); octaethyltetra(perfluorophenyl)porphyrin, (F(20)OETPP)Fe(III); and tetra-beta,beta'-tetramethylenetetraphenyl-porphyrin, (TC(6)TPP)Fe(III), having either one (Cl(-), ClO(4-)) or two [4-(dimethylamino)pyridine, 4-Me(2)NPy; 1-methylimidazole, 1-MeIm; tert-butylisocyanide, t-BuNC; or cyanide, CN(-)] axial ligands have been characterized by 1D dynamic NMR (DNMR) and 2D (1)H NOESY/EXSY spectroscopies as a function of temperature. The activation parameters, Delta H++, Delta S++, and Delta G++(298), and the extrapolated rate constants at 298 K for three chloride, one perchlorate, and three bis-(4-Me(2)NPy) complexes as well as [FeOETPP(1-MeIm)(2)]Cl, [FeOETPP(t-BuNC)(2)]ClO(4), 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) = 10-10(7) s(-1)) that decreases in the order TC(6)TPP > OMTPP > F(20)OETPP > or = OETPP, which correlates with increasing porphyrin nonplanarity. To determine the effect of axial ligands, we calculated the free energy of activation, Delta G++(298) for OETPPFe(III) bis-ligated with 4-Me(2)NPy, 1-MeIm, or 4-CNPy (approximately 59 kJ mol(-1)), and for complexes with small cylindrical ligands (t-BuNC and CN(-)) (approximately 37 kJ mol(-1)). These data suggest that the Delta G++(298) for planar ligand rotation is roughly 20-25 kJ mol(-1).
authors.affiliation: Department of Chemistry, University of Arizona, Tucson, Arizona 85721-0041, USA.
authors.forename: Liliya A
authors.initials: LA
authors.lastname: Yatsunyk
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ogura
authors.forename: F Ann
authors.initials: FA
authors.lastname: Walker
mesh_terms: D002412:Cations; D005290:Ferric Compounds / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008665:Metalloporphyrins / Q000737:chemistry*; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D013696:Temperature
chemical_list: D002412:Cations; D005290:Ferric Compounds; D008024:Ligands; D008665:Metalloporphyrins; D011166:Porphyrins; C409084:tetraphenylporphyrin; D007501:Iron