Template synthesis of N-aminoethylated hexaaza macrobicyclic nickel(II) complexes containing one four-membered chelate ring

New nickel(II) complexes of the hexadentate macrobicycles 6-(2-aminoethyl)-1,3,6,9,11,14-hexaazabicyclo[12.3.1]octadecane (1), 6-(2-aminoethyl)-1,3,6,9,11,14-hexaazabicyclo[12.2.1]heptadecane (2), and 6-(2-aminoethyl)-1,3,6,9,11,15-hexaazabicyclo[13.3.1]nonadecane (3) have been prepared selectively by the metal template condensation of formaldehyde with tris(2-aminoethyl)amine and a triamine (1,4,8-triazaoctane, 1,4,7-triazaheptane, or 1,5,9-triazanonane). The complexes [Ni(L)]2+ (L=1, 2, or 3), in which one 2-aminoethyl pendant arm is coordinated to the metal ion, have distorted octahedral coordination geometry with a rarely observed four-membered chelate ring. They are extremely stable in neutral or basic aqueous solutions but are readily decomposed in acidic conditions. The decomposition rate of the complexes in HClO4 solutions is not affected significantly by the ring size of the macrobicyclic ligands. Synthesis and characterization of the nickel(II) complexes are reported, together with the crystal structure of [Ni(1)][ZnCl4].