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

• Published in: International journal of quantum chemistry Vol. 117; no. 1; pp. 24 - 32
• Main Author: Johansson, Adam Johannes
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 05.01.2017
• Subjects: Atoms & subatomic particles; Chemistry; electron spin; hydrogen atom transfer; iron‐porphine; Ligands; noninnocence; Physical chemistry; Quantum physics
• ISSN: 0020-7608; 1097-461X
• DOI: 10.1002/qua.25306

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 CH 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. The delocalised electron spin on iron‐porphine suggests ligand noninnocence towards radicals. DFT calculations reveal the ligands kinetic capability of ing a hydrogen atom. What implications could this reactivity have for iron‐porphine and heme catalysis?