Quantum chemical DFT study of the interaction between molecular oxygen and FeN₄ complexes, and effect of the macrocyclic ligand.

Density functional theory (DFT) was used to examine the interaction between molecular oxygen (O₂) and macrocyclic iron complexes of the type FeN₄ during the formation of FeN₄--O₂ adducts. In order to understand how this interaction is affected by different macrocyclic ligands, O₂ was bonded to iron-tetraaza[14]annulene (FeTAA), iron-tetramethyl-tetraaza[14]annulene (FeTMTAA), iron-hexamethyl-tetraaza[14]annulene (FeHMTAA), iron dibenzotetraaza[14]annulene (FeDBTAA), and two iron-tetramethyl-dibenzotetraaza[14]annulene complexes (FeTMDBTAA1, FeTMDBTAA2). The ground state for FeN₄-O₂ adducts was the open-shell singlet. Analysis of the factors influencing the O₂ bonding process showed that different macrocyclic ligands yielded adducts with differences in O--O and Fe--O₂ bond lengths, total charge over the O₂ fragment, O--O vibrational frequency, and spin density in the O₂ fragment. A smaller energy gap between the α-HOMO of the FeN₄ complexes and the β-LUMO of O₂ increased the interaction between the complex and the O₂ molecule. The order of activity was FeDBTAA < FeTMDBTAA2 < FeTMDBTAA1 < FeTAA < FeTMTAA < FeHMTAA.