TMACH-1,2-HOPO, a versatile tripodal metal chelator: complexation, solution thermodynamics, spectroscopic and DFT studies

Abstract

A new tripodal siderophore-mimic hexadentate chelator, N,N’,N’’-(((1R,3R)-cyclohexane-1,3,5-triyl)tris(methylene))tris(1-hydroxy-6-oxo-1,6-dihydropyridine-2 carboxamide), (TMACH-1,2-HOPO) containing three 1,2-hydroxypyridinone units attached to a cyclohexane ring through amide linkage at 1,3 and 5 positions have been designed, synthesized, and characterized by FT-IR, ¹H NMR, ¹³C NMR, ESI-MS, and electronic spectroscopy. The solution thermodynamics and photophysical properties of the ligand and its metal complexes (M = La³⁺, Gd³⁺, and Lu³⁺) were investigated experimentally and theoretically in an aqueous medium. The pK_a values for the ligand were found to be 8.32, 6.86, and 5.45. In an aqueous solution, the ligand formed complexes of the type MLH3, MLH2, MLH, and Mion. Additionally, it formed three hydrolyzed species, MLH_− 1, MLH_− 2, and MLH_− 3 at a higher pH. DFT studies were performed, which support the experimental results. The nature of bonding between the lanthanide ions and the ligand was explained by NBO, Morokuma Ziegler energy decomposition analysis (ETS-NOCV).