Alkaline-Earth-Metal Complexes of 1,4,8,11-Tetraazacyclotetradecane-1,4,8,11-tetraacetic Acid, H4TETA, and Crystal and Molecular Structure of H4TETA·6H2O and [Mg(H2TETA)(H2O)4]·4H2O

Abstract

The macrocycle 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid, (H4TETA), reacts with alkaline-earth-metal salts in aqueous solutions to form M(H2TETA)·nH2O (M = Mg, n = 2 and 8; M = Ca, n = 2; and M = Sr, n = 2; pH = 6-8), and Ca2TETA-3H2O (pH = 10). IR and NMR spectral studies are reported which show the presence of protonated N and deprotonated carboxylate in these compounds. Crystals suitable for diffraction studies were isolated for H4TETA-6H2O (I) and [Mg-(H2TETA)(H2O)4]-4H2O (II) and their structures are reported. I crystallizes as {-NH(CH2COO)-(CH2)2-N(CH2COOH)-(CH2)3-}2-6H2O in monoclinic space group P21/c with a = 11.6212 (6) Å, b = 14.6659 (8) Å, c = 7.8199 (4) Å, β = 102.445 (2)°, and Z = 2. II crystallizes as [Mg|-NH(CH2COO)-(CH2)2-N(CH2COO)-(CH2)3-)2(H2O)4]n·[4H2O]n in triclinic space group [formula omited]with a = 8.2377 (5) Å, b = 10.0088 (6) Å, c = 9.8980 (6) Å, α = 106.492 (2)°, β = 72.951 (2)°, γ = 113.542 (2)°, and Z = 1. The structures of I and II were solved by direct methods and refined to R = 0.059 and R = 0.036, respectively. Discrete centrosymmetric molecules are present in I as dizwitterions, whereas II consists of chains of centrosymmetric ligands bridging Mg atoms (on inversion centers) via carboxylate O atoms. The macrocycle adopts the same rectangular [3434] conformation in both crystals. However, N atoms occupy different relative positions in I and II, showing that the conformational congruence of the rings is independent of heteroatom location. © 1991, American Chemical Society. All rights reserved.