The X-ray structure of 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid and its relevance to the protonation of polyaza polycarboxylic macrocycles

Abstract

The tetraprotonated form of the macrocyclic ligand teta (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetate) crystallizes in the monoclinic system, space group P2₁/c, a= 7.816(2), b= 14.656(2), c= 11.647(3)Å, β= 102.52(4)°, Z= 2. The structure was refined from diffractometer data to an R value of 0.035 and the hydrogen atoms were located unequivocally. In agreement with previous NMR investigations, H₄teta features two protonated nitrogen atoms in the ring and two protonated carboxylate groups. The electrostatic repulsion is thus reduced to a minium. The tetraaza ring adopts a quadrangular [3434] conformation which is one of the most stable geometric arrangements of the 14-membered cycles. The ligand forms no intramolecular hydrogen bonds but it is involved in intermolecular hydrogen bonds with water molecules.