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

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 [NiL1](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(H2L1)](ClO4)(4) (H2L1 = 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 [NiL1](2+) exists in aqueous solutions as a mixture of the square-planar and octahedral [NiL1(H2O)(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 [NiL2](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 [NiL1](2+) or [NiL2](2+) increases the proportion of the octahedral species. Synthesis, characterization, and solution behavior of the nickel(II) complexes of L-1 are reported. (C) 1999 Elsevier Science S.A. All rights reserved.