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

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 P2(1)/c with a = 11.6212 (6) angstrom, b = 14.6659 (8) angstrom, c = 7.8199 (4) angstrom, beta = 102.445 (2)-degrees, 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 P1BAR with a = 8.2377 (5) angstrom, b = 10.0088 (6) angstrom, c = 9.8980 (6) angstrom, alpha = 106.492 (2)-degrees, beta = 72.951 (2)-degrees, gamma = 113.542 (2)-degrees, 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.