A carbon-13 nuclear magnetic resonance study of trans-1,2-diaminocyclohexane-NNN′N′-tetra-acetate ion and its diamagnetic metal complexes

Abstract

¹³ C N.m.r. spectra of aqueous solutions of trans-1,2-diaminocyclohexane-NNN′N′-tetra-acetate ion [cydta(4-)] and its metal complexes have been studied for the alkali and alkaline-earth metal ions, Ag^I, Tl^I, the zinc group, Sn^II, Pb^II, Al^III, La^III, Co^III, and Th^IV. The spectra are generally consistent with sexidentate co-ordination by cydta(4-) except at high pH where further species of the type [M(cydta)OH]^n–(M = Tl^I, Hg^II, and Al^III) are observed. Species of the type [M(cydta)X]^n– are confirmed for M = Co^III(X = Cl) and Hg^II(X = Cl, Br, NCS, or CN). Tentative structures for the latter species are discussed, and use is made of ²J metal–carbon hyperfine couplings (Hg^II, Pb^II, Cd^II, or Sn^II) and of ¹J_CH couplings to give further structural information. In the presence of the univalent cations (including H⁺) broad temperature-dependent resonances are observed indicating relatively slow nitrogen inversion in the unco-ordinated or partially co-ordinated ligand.