Noninnocence of the ligand atoms in iron-porphine: Chemical consequences of the delocalized electron spin

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• 作者：Adam Johannes Johansson
• 刊名：International Journal of Quantum Chemistry
• 出版年：2017
• 出版时间：January 5, 2017
• 年：2017
• 卷：117
• 期：1
• 页码：24-32
• 全文大小：866K
• ISSN：1097-461X

文摘

It is well recognized that the electronic spin density in transition metal complexes in high-spin states, tends to delocalize from the metal ion itself to the donor atoms of the ligand. In square planar iron-porphine [PFe]⁺ the delocalization occurs even further and spin corresponding to roughly one electron is delocalized over a large part of the ligand. In this article, density functional theory is applied to explore the chemical consequences of the delocalized spin in four-coordinate iron-porphine. It is shown that the porphine ligand has a moderate affinity for radicals, and that covalent bonds can form through spin-pairing of the unpaired delocalized electron on the porphine ligand and the unpaired electron of another radical species. The hydrogen atom is used as a probe to evaluate the radical affinity of the different nitrogen and carbon atoms that constitute the porphine ligand. It is computationally predicted that the porphine ligand of four-coordinate iron-porphine is kinetically capable of activating weak CH bonds of, for example, unsaturated organic compounds. Hydrogen atom transfer becomes spontaneous via subsequent homo-coupling of the organic radical created. Whether or not the radical affinity of the porphine ligand has any mechanistic implications for heme-containing enzymes is left as an open question.