METAL-COMPLEXES OF MACROCYCLIC LIGANDS CONTAINING PYRIDINE

Some macrocyclic compounds containing pyridine have been synthesized: 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene (1),3,7,10,16-tetraazabicyclo[10.3.1]hexadeca-1(16),12,14-triene(2),3,7,11,17-tetraazabicyclo-[11.3.1]heptadeca-1(17),13,15-triene (3), and 7-methyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (4). The protonation reactions of these ligands were studied by potentiometric and H-1 NMR techniques, and it was found that all of them present two very basic and the other two very acidic nitrogen atoms. The stability constants of the complexes of 1-4 with some divalent first-series transition metal ions, Cd2+, and Pb2+ were determined by potentiometric methods. The constants for the Co2+ complexes were impossible to determine due to their rapid reaction with dioxygen, followed by degradation of the species formed. The Irving-Williams order of stability is obeyed for all the complexes of those ligands, and the values of stability constants decrease with the increasing ring size of the macrocycle, with the exception of those of the copper complexes. The latter complexes present a different trend with stability constants of similar values, but the complex of 2 shows a lower stability. The Ni2+ complexes were studied by electronic and NMR spectroscopies, and the Cu2+ complexes, by electronic and ESR spectroscopies. The structures adopted by the complexes in aqueous solution are discussed, and comparison with the behavior of the corresponding complexes of the tetraaza macrocyclic ligands (without a pyridine nitrogen) is undertaken.