Formation constants for complexes of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid and the crystal structure of its nickel(II) complex

Abstract

The crystal structure of the nickel(II) complex of 1,4,7,10-tetraazacyclododecane-1,7-diacetic acid (H ₂ doda) has been determined. The ligand binds in a cis-octahedral fashion through the four nitrogen atoms and two carboxylate groups and dimerizes via an apparently symmetrical hydrogen bond from the non-bonding oxygen of a carboxylate group to a lone hydrogen ion originating from a perchloric acid molecule of crystallization. Carbon-13 NMR studies of the corresponding zinc(II) complex indicated that it has a similar structure. pK _a Measurements for doda, at 298.2 K in 0.1 mol dm ^-3 NEt ₄ ClO ₄ , suggested that the two secondary amines are the first to protonate followed by the two carboxylates and then the two tertiary nitrogen atoms. A marked disparity in the magnitude of the pK _a values for the two carboxylic groups [4.00(1) and 2.36(3)] suggests that internal hydrogen bonding between the two stabilizes one of the protons. Formation constants for some divalent metal complexes (Co, Ni, Cu, Zn, Cd or Pb) of H ₂ doda have been determined. The results from this work are compared to data for analogous macrocycles having three or four carboxymethyl groups.