1,3,5-TRIAMINO-1,3,5-TRIDEOXY-CIS-INOSITOL, A LIGAND WITH A REMARKABLE VERSATILITY FOR METAL-IONS .5. COMPLEX-FORMATION WITH MAGNESIUM(II), CALCIUM(II), STRONTIUM(II), BARIUM(II), AND CADMIUM(II)

Solid compounds containing the complexes [Cd(taci)2]2+, [Mg(taci)2]2+, [Ca(taci)2(H2O)2]2+, [Sr(taci)2(H2O)3]2+, and [Ba(taci)2(H2O)3]2+ (taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol) have been prepared by combining aqueous solutions of taci and of corresponding MX2 salts (X = NO3, Br, ClO4). X-ray data: taci.2H2O, C6H19N3O5, monoclinic, space group P2(1)/n, a = 12.002(6) angstrom, b = 5.506(3) angstrom, c = 15.588(8) angstrom, beta = 97.30(3)degrees, Z = 4; [Cd-(taci)2](NO3)2-2H2O, C12H34CdN8O14, monoclinic, space group P2(1)/c, a = 7.110(2) angstrom, b = 8.717(3) angstrom, c = 18.781-(7) angstrom, beta = 100.54(3)degrees, Z = 2; [Mg(taci)2](ClO4)1.5(OH)0.5.5.25H2O, C12H41Cl1.5MgN6O17.75, triclinic, space group P1BAR, a = 11.314(3) angstrom, b = 11.448(3) angstrom, c = 12.134(3) angstrom, alpha = 111.57(2)degrees, beta = 103.16(2)degrees, gamma = 105.32(2)degrees, Z = 2; [Ca(taci)2(H2O)2]Br2-6H2O, C12H46Br2CaN6O14, triclinic, space group P1BAR, a = 10.879(2) angstrom, b = 12.061(3) angstrom, c = 12.712(2) angstrom, alpha = 112.94(1)degrees, beta = 98.14(1)degrees, gamma = 104.61(2)degrees, Z = 2; [Sr(taci)2(H2O)3]Br2.4H2O, C12H44-Br2N6O13Sr, monoclinic, space group C2/c, a = 16.53(1) angstrom, b = 13.82(1) angstrom, c = 14.45(1) angstrom, beta = 124.72(5)degrees, Z = 4; [Ba(taci)2(H2O)3]Br2.2.25H2O, C12H40.5BaBr2N6O11.25, triclinic, space group P1BAR, a = 10.187(5) angstrom, b = 11.189-(4) angstrom, c = 13.210(5) angstrom, alpha = 112.84(3)degrees, beta = 91.91(4)degrees, gamma = 105.34(3)degrees, Z = 2. Cd(II) is bound to the six nitrogen donors whereas the alkaline earth metal ions are coordinated to the six hydroxyl groups of two taci molecules. An almost regular MgO6 octahedron is observed for [Mg(taci)2]2+, while the CdN6 octahedron in [(taci)2]2+ is significantly elongated along a pseudo-3-fold axis. A coordination number of 8 for Ca(II) and of 9 for Sr(II) and Ba(II) is achieved by binding additional water molecules to these metal ions. The water molecules occupy adjacent positions in the coordination polyhedron. NMR spectroscopy and FAB+ mass spectrometry of aqueous complex solutions revealed rather weak taci-metal interactions for the alkaline earth metal ions with decreasing stability in the order Zn, Cd >> Mg > Ca, Sr, Ba. The formation constants beta1 = [Cd(taci)][Cd]-1[taci]-1 and beta2 = [Cd(taci)2][Cd]-1[taci]-2 were determined in 0.1 M KNO3 at 25-degrees-C and the values log beta1 = 6.48(3) and log beta2 = 10.95(5) were obtained. Molecular mechanics calculations were used to elucidate the energy of the different complexes. The calculations revealed clearly that the complex geometries and the observed coordination numbers can be understood by analyzing steric strain in the coordination sphere. On the other band, for Mg(II), Ca(II), Sr(II), Ba(II), Zn(II), and Cd(II), the selection of one of the four possible binding sites of taci is basically a consequence of the electron configuration of the metal ion.