Density functional theory study of interactions between carbon monoxide and iron tetraaza macrocyclic complexes, FeTXTAA (X = -Cl, -OH, -OCH3, -NH2, and -NO2).

This work describes a DFT level theoretical quantum study using the B3LYP functional with the Lanl2TZ(f)/6-31G* basis set to calculate parameters including the bond distances and angles, electronic configurations, interaction energies, and vibrational frequencies of FeTClTAA (iron-tetrachloro-tetraaza[14]annulene), FeTOHTAA (iron-tetrahydroxy-tetraaza[14]annulene), FeTOCH3TAA (iron- tetramethoxy-tetraaza[14]annulene), FeTNH2TAA (iron-tetraamino-tetraaza[14]annulene), and FeTNO2TAA (iron-tetranitro-tetraaza[14]annulene) complexes, as well as their different spin multiplicities. The calculations showed that the complexes were most stable in the triplet spin state (S = 1), while, after interaction with carbon monoxide, the singlet state was most stable. The reactivity of the complexes was evaluated using HOMO-LUMO gap calculations. Parameter correlations were performed in order to identify the best complex for back bonding (3d xzFe → 2p xC and 3d yzFe → 2p zC) with carbon monoxide, and the degree of back bonding increased in the order: FeTNO2TAA < FeTClTAA < FeTOHTAA < FeTOCH3TAA < FeTNH2TAA.