Iron Porphyrin Chemistry

This article reviews most aspects of the chemistry of iron porphyrins, from Fe(0) to Fe(V), including occurrence and roles of natural iron porphyrins (hemes) and their synthetic analogs, structures and electron configurations of iron porphyrins of all oxidation and spin states, π electron configuration of the porphyrin ring, synthesis of metal-free porphyrins and other related macrocycles, insertion of iron into free-base porphyrins and related macrocycles, the properties, reactions, uses and biological relevance of iron(0), -(I), -(II) porphyrins (the latter with S = 0, 1, and 2 spin state possibilities), of iron(II) porphyrin π-cation radicals, of iron(III) porphyrins (with S = 1/2, 3/2, and 5/2 spin state possibilities), of iron(III) porphyrin and corrole π-cation radicals, of iron(IV) porphyrins (including five- and six-coordinate ferryl (FeO)2+, iron(IV) phenyl, carbene and hydrazine complexes, and the bis-methoxide complex) and a comparison of iron(IV) porphyrins to iron(III) porphyrin π-cation radicals, of iron(IV) porphyrin π-cation radicals, and of the possible existence of iron(V) porphyrins. Included in the Fe(II) part are sections on addition of ligands to four-coordinate iron(II) porphyrins, including equilibrium binding constants, photodissociation of ligands from PFeL2 complexes, binding of small molecules (O2, CO, NO, HNO) to 5-coordinate iron(II) porphyrins and design of porphyrin ligands that will mimic the active sites of heme proteins such as myoglobin and hemoglobin, the cytochromes P450 and nitric oxide synthases, and the nitrophorins and guanylyl cyclases. Included in the iron(III) part are sections on both 5- and 6-coordinate high-spin complexes and their similarities and differences, bridged or through-space magnetically coupled complexes of high-spin iron(III) porphyrins with other metal complexes as possible models for cytochrome oxidase and the assimilatory sulfite reductases, coupled oxidation of hemes by hydrogen peroxide or its equivalent, and the relationship of this reactivity to the reactions of heme oxygenase, iron(III) porphyrins as reduction catalysts, and photochemistry of iron(III) porphyrins, possible electron configurations of low-spin iron(III) porphyrins, the phenomenon and possible electronic consequences of ruffling of the porphinato core in iron(III) porphyrins, the preferred orientation of planar axial ligands bound to low-spin iron(III) porphyrins, NO complexes of iron(III) porphyrins, reduction potentials, equilibrium constants and rates of axial-ligand addition and exchange, kinetics of axial-ligand rotation and porphyrin ring inversion, kinetics of reduction and autoreduction of iron(III) porphyrins, electron self-exchange between low-spin iron(III) and iron(II) porphyrins, synthetic ferriheme proteins, and synthesis of five-coordinate low-spin iron(III) porphyrins having σ-alkyl or σ-aryl groups as axial ligands. The iron(IV) and iron(IV) cation radical sections discuss the high-valent states of cytochromes P450 and related enzymes. Keywords: heme; synthesis, axial ligand; coordination number; d electron configuration; π-electron configuration; high-spin; low-spin; hemoglobin; cytochrome; P450; electron transfer