Macrocyclic ligands bearing two 3-(Hydroxy)-2-pyridinone moieties as side-arms. Conformational studies, synthesis, crystal structure, and alkali and alkaline earth complex formation

Abstract

The synthesis and characterization of the new tetraazamacrocycle 4(N),10(N)-bis[2-(3-hydroxy-2-oxo-2H-pyridin-1-yl)ethyl]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane (L1) is reported. L1 shows two 3-(hydroxy)-1-(carbonylmethylen)-2(1H)-pyridinone (HPO) moieties linked to the macrocyclic base 4,7-dimethyl-1,4,7,10-tetrazacyclododecane, as side-arms. The acid–base and coordination properties towards alkali, alkaline earth ions of L1 together with those of the structurally similar ligand 4-(N),10-(N)-bis[2-(3-hydroxy-2-oxo-2H-pyridin-1-yl)acetamido]-1,7-dimethyl-1,4,7,10-tetraazacyclododecane (L2) are reported. L2 binds all the metal ions investigated forming stable mono-nuclear complexes, while L1 binds only some alkaline earth ions with stability constants lower than those of L2. Both compounds show selectivity towards the Mg(II) ion with respect to the alkaline earth series. The binding area is formed by the four converging oxygen atoms of the two HPO groups. L2 appears to be a more efficient ligand in the coordination of hard metal ions because the two HPO binding groups are forced by the macrocyclic skeleton to stay on the same part of the macrocyclic ring, while this conformation is unfavourable for L1. These data are supported by molecular dynamics (MD) simulations performed on both ligands and by the crystal structure of the [H₂L1](ClO₄)₂ species which shows the two side-arms displaced opposite to the macrocyclic ring. The effects of pH and metal ion coordination on the UV-Vis absorption and fluorescence emission properties of both ligands were investigated, highlighting that the optical properties changes in acid–base reactions as well as in the coordination of metal ions.