THE AMIDE OXYGEN AS A DONOR GROUP - METAL-ION COMPLEXING PROPERTIES OF TETRA-N-ACETAMIDE SUBSTITUTED CYCLEN - A CRYSTALLOGRAPHIC, NMR, MOLECULAR MECHANICS, AND THERMODYNAMIC STUDY

The syntheses of the octadentate ligand DOTAM (1,4,7,10-tetrakis(acetamido)-1,4,7,10-cyclododecane) and its complexes with Zn(II), Cd(II), and Ca(II) are described. Crystal structures of [Cd(DOTAM)](ClO4)(2) . 1.5H(2)O (1), [Ca(DOTAM)](ClO4)(2) . 2.5H(2)O (2), and [Zn(DOTAM)](ClO4)(2) . H2O (3) are reported. Crystal data: (1) monoclinic, space group Cc, a = 11.908(2) Angstrom, b = 21.237(3) Angstrom, c = 11.445(2) Angstrom, beta = 102.15(1)degrees; (2) monoclinic, P2(1)/c, a = 14.031(9) Angstrom, b = 11.469(8) Angstrom, c = 17.448 Angstrom, beta = 92.10(1)degrees (3) triclinic, space group <P(1)over bar>, a = 9.490(1) Angstrom, b = 12.464(2) Angstrom, c = 12.998(2) Angstrom, alpha = 99.070(1)degrees, beta = 107.67(1)degrees, and gamma = 108.24(1)degrees. There is an unusual distortion in the coordination geometry of the complexes. There are two sets of metal-to-oxygen bond lengths for each complex; Zn(II) has two oxygens, placed opposite each other in the approximately square arrangement defined by the four oxygen donor atoms at about 2.19 Angstrom and two at 3.23 Angstrom, Cd(II) has two at 2.34 and two at 2.64 Angstrom, and Ca(II) has two at 2.40 Angstrom and two at 2.42 Angstrom. Molecular mechanics calculations suggest the Cd(II) and Zn(II) structures represent six coordination of four nitrogens and two of the oxygens, while the two long bonds represent van der Waals contacts with a possible electrostatic component. Approach of the oxygen donors to the metal ion is controlled by the van der Waals radii of the oxygens. C-13 NMR studies give rates of helicity interchange of the complexes Zn(II) > Hg(II) > Cd(II) > Ca(II) >> Pb(II). This order is discussed in terms of the difference in bond lengths between the two sets of oxygen donors. A stability constant study gave logK(1) values in 0.1 M NaNO3 and 25 degrees C: Cu(II), 16.3; Zn(II), 10.47; Ca(II), 7.54; Sr(II), 6.67; Ba(II), 5.35; Hg(II), 14.53; La(III), 10.35; Gd(III), 10.05. For Cd(II) and Pb(II), the complexes were fully formed even at pH 0.3, and only a lower limit of 19 for logK(1) could be set. Selectivity of DOTAM for metal ions is discussed in terms of coordinating properties of the amide oxygen donor and geometric requirements of the DOTAM ligand.