Dipyridinophane ligands - synthesis and coordination study

Twelve-membered dipyridinophane and its derivatives bearing two acetate or methylphosphonate pendant arms were synthesised, and their coordination properties were studied. Protonation constants of the ligands and stability constants of their complexes with Ni^II, Cu^II and Zn^II ions were determined through a combination of potentiometry and spectral measurements. The ligands demonstrate significantly lower basicity than other tetraazamacrocycles (cyclen, pyclen) due to the presence of two pyridine units. This low macrocycle basicity results in high conditional stabilities of the complexes in acidic solutions. The solid-state structures of the complexes reveal that divalent first-row transition metal ions form a distorted octahedron with weakly coordinated pendant arms and bent coordination of the pyridine rings, as these metal ions are too large for the ligand cavity. Conversely, the ligand cavity is very suitable for smaller trivalent metal ions, and the complexes adopt an octahedral (Co^III) or pentagonal bipyramidal (Fe^III and Ga^III) arrangements.