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標題: Symmetry and bonding in metalloporphyrins. A modern implementation for the bonding analyses of five- and six-coordinate high-spin iron(III)-porphyrin complexes through density functional calculation and NMR spectroscopy
作者: Cheng, R.J.
Chen, P.Y.
Lovell, T.
Liu, T.Q.
Noodleman, L.
Case, D.A.
關鍵字: nuclear-magnetic-resonance;transition-state method;ligand-field;strength;s = 3/2;iron(iii) porphyrins;intermediate-spin;magnetochemical series;ferric porphyrins;iron porphyrins;valent iron
Project: Journal of the American Chemical Society
期刊/報告no:: Journal of the American Chemical Society, Volume 125, Issue 22, Page(s) 6774-6783.
Bonding interactions between the iron and the porphyrin macrocycle of five- and six-coordinate high-spin iron(III)-porphyrin complexes are analyzed within the framework of approximate density functional theory with the use of the quantitative energy decomposition scheme in combination with removal of the vacant pi* orbitals of the porphyrin from the valence space. Although the relative extent of the iron-porphyrin interactions can be evaluated qualitatively through the spin population and orbital contribution analyses, the bond strengths corresponding to different symmetry representations can be only approximated quantitatively by the orbital interaction energies. In contrast to previous suggestions, there are only limited Fe --> P pi* back-bonding interactions in high-spin iron(III)-porphyrin complexes. It is the symmetry-allowed bonding interaction between d(z)(2) and a(2u) orbitals that is responsible for the positive pi spin densities at the meso-carbons of five-coordinate iron(Ill)-porphyrin complexes. Both five- and six-coordinate complexes show significant P --> Fe pi donation, which is further enhanced by the movement of the metal toward the in-plane position for six-coordinate complexes. These bonding characteristics correlate very well with the NMR data reported experimentally. The extraordinary bonding interaction between d(z)(2) and a(2u) orbitals in five-coordinate iron(III)-porphyrin complexes offers a novel symmetry-controlled mechanism for spin transfer between the axial ligand sigma system and the porphyrin a system and may be critical to the electron transfer pathways mediated by hemoproteins.
ISSN: 0002-7863
DOI: 10.1021/ja021344n
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