Density functional theory study of interactions between carbon monoxide and iron tetraaza macrocyclic complexes,...

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.