10.46243/jst.2022.v7.i02.pp54-76 registered

A series of mono-, bis-, and tris-nitrile ligands based on the tris(2-pyridylmethyl)amine (TPA) framework, namely CNTPA, (CN)₂TPA, and (CN)₃TPA, were synthesized and investigated for their coordination and oxygenation behavior with iron complexes. The introduction of nitrile groups at the α-position of the nitrogen-containing tripodal ligands progressively increased the oxidation potential compared with the parent TPA ligand, with an average increase of approximately 110 mV per nitrile group, demonstrating the strong electron-withdrawing effect of the nitrile substituents. Crystal structure analyses of the dichloroferrous complexes of CNTPA and (CN)₂TPA revealed distorted octahedral geometries, while spectroscopic studies indicated that these geometries are retained in solution. The ferrous complexes reacted with molecular oxygen to form stable μ-oxo diferric complexes, which could be converted back to the corresponding ferrous species upon reduction. The study further demonstrated that the ligand environment strongly influences the kinetics of oxygenation. Compared with the parent complex, which underwent complete oxygenation over approximately 14 hours, the mononitrile complex reacted completely within the same period, whereas the dinitrile and trinitrile complexes reacted substantially faster, requiring approximately 2 hours and 30 minutes, respectively. The results indicate that the Lewis acidity of the metal center plays an important role in controlling oxygenation kinetics and supports the coordination of dioxygen as an initial step in the formation of μ-oxo diferric compounds. In addition, cyclohexane was catalytically converted to cyclohexanone when reacted with dioxygen in the presence of zinc amalgam, with the conversion rate correlating with the Lewis acidity of the metal center.