Template synthesis, crystal structure, and solution behavior of a hexaaza macrocyclic nickel(II) complex containing two N-aminoethyl pendant arms

Abstract

A new nickel(II) complex of the 14-membered hexaaza macrocyclic ligand 1,8-bis(2-aminoethyl)-1,3,6,8,10,13-hexaazacyclotetradecane (L 1), in which two 2-aminoethyl pendant arms are attached to the bridgehead nitrogen atoms, has been prepared by the one-pot reaction of ethylenediamine and formaldehyde in the presence of the metal ion. The complex [NiL 1] 2+ has a square-planar coordination geometry in the solid state and in nitromethane; the pendant amino groups are not coordinated to the metal ion. The crystal structure of [Ni(H 2L 1)](ClO 4) 4 (H 2L 1=a protonated form of L 1) shows that the four secondary nitrogen atoms are coordinated to the nickel(II) ion with a 5-6-5-6 chelate ring sequence. The nickel(II) ion is located at the inversion center of the N 4 square. The complex [NiL 1] 2+ exists in aqueous solutions as a mixture of the square-planar and octahedral [NiL l(H 2O) 2] 2+ species. The addition of HCl (0.1–3.0 M) to aqueous solutions of the complex increases the concentration of the square-planar species, in contrast to the case for [NiL 2] 2+ (L 2=1,8-bis(2-aminoethyl)-1,3,6,8,10,13-hexaazacyclotetradecane) and other related hexaaza and pentaaza macrocyclic complexes. On the other hand, the addition of NaOH to the solution of [NiL 1] 2+ or [NiL 2] 2+ increases the proportion of the octahedral species. Synthesis, characterization, and solution behavior of the nickel(II) complexes of L 1 are reported.