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, AgI, TlI, the zinc group, SnII, PbII, AlIII, LaIII, CoIII, and ThIV. 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 = TlI, HgII, and AlIII) are observed. Species of the type [M(cydta)X]n– are confirmed for M = CoIII(X = Cl) and HgII(X = Cl, Br, NCS, or CN). Tentative structures for the latter species are discussed, and use is made of 2J metal–carbon hyperfine couplings (HgII, PbII, CdII, or SnII) and of 1JCH 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.