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