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Investigations of Rotation of Axial Ligands in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Measurement of Rate Constants from Saturation Transfer Experiments and Comparison to Molecular Mechanics Calculations

Momot, Konstantin I. & Walker, F. Ann (1997) Investigations of Rotation of Axial Ligands in Six-Coordinate Low-Spin Iron(III) Tetraphenylporphyrinates: Measurement of Rate Constants from Saturation Transfer Experiments and Comparison to Molecular Mechanics Calculations. Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory, 101, pp. 2787-2795.

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Abstract

Saturation transfer experiments have been utilized to measure the rate of axial ligand rotation in (tetramesitylporphyrinato)iron(III) bis(2-methylimidazole), [(TMP)Fe(2-MeImH)2]+. Saturation transfer peak intensities of four distinct pyrrole protons have been measured at a series of temperatures. Derivation of analytical expressions for steady-state peak intensities in the case of cyclic four-site exchange allowed the determination of the exchange rate constant. Previously measured longitudinal relaxation rate constants of the pyrrole protons of [(TMP)Fe(2-MeImH)2]+ have been used for rate constant determination. The temperature dependence of the rates has allowed estimation of the enthalpy barriers and entropy of this rotation. Modified MM2 potentials have also been used to study the rotation of axial ligands in [(TMP)Fe(1,2-Me2Im)2]+ and (tetraphenylporphyrinato)iron(III) bis(1-methylimidazole), [(TPP)Fe(1-MeIm)2]+. The "adiabatic" potential energy surfaces (PES) for rotation of axial ligands (minima achieved in all degrees of freedom except for constrained internal rotation coordinates for the two axial ligands) have been constructed for both complexes by combining a Ramachadran-type dihedral drive with geometry minimization or Monte Carlo single minimum analysis with subsequent geometry minimization. The PES of the TMP-hindered imidazole complex indicates that the preferable mode of rotation is synchronous clockwise or counterclockwise rotation of the two axial ligands, with an enthalpy barrier to such rotation of approximately 48 kJ/mol. For the TPP-nonhindered imidazole complex, enthalpy barriers to synchronous and asynchronous rotation were found to be 3.3 and 5.4 kJ/mol, respectively, thus prompting the assumption that no particular mode of rotation is highly preferable in that complex. The rotational enthalpy barrier for the TMP-hindered imidazole complex was found to be consistent with experimental measurements of the current (59 kJ/mol) and previous work (50-54 kJ/mol) (Shokhirev, N. V.; Shokhireva, T. Kh.; Polam, J. R.; Watson, C. T.; Raffii, K.; Simonis, U.; Walker, F. A. J. Phys. Chem. A 1997, 101, 0000. Nakamura, M.; Groves, J. T. Tetrahedron 1988, 44, 3225). The relationship between the orientation of axial ligands, the distortion of the metalloporphyrin core from planarity, and the bulkiness of axial ligands and porphyrin substituents is discussed.

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ID Code: 2677
Item Type: Journal Article
Additional Information: This article is freely available from the American Chemical Society website 12 months after the publication date. See links to publisher website in this record.
Keywords: nuclear magnetic resonance, NMR paramagnetic relaxation, ligand rotation in metalloporphyrin complexes
DOI: 10.1021/jp963235z
ISSN: 1520-5215
Subjects: Australian and New Zealand Standard Research Classification > CHEMICAL SCIENCE (030000)
Divisions: Past > QUT Faculties & Divisions > Faculty of Science and Technology
Copyright Owner: Copyright 1997 American Chemical Society
Deposited On: 15 Sep 2006
Last Modified: 09 Mar 2011 11:58

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