Natural abundance 15N nuclear magnetic resonance spectroscopy of heavy metal complexes of trans-1,2-diaminocyclohexane-NNN′N′-tetra-acetate ion and 1,4,8,11-tetra-azacyclotetradecane

Abstract

Natural abundance ¹⁵N 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 observed for the metal ions Cd^II, Hg^II, Ag^I, and Pb^II, and ¹J metal–nitrogen spin couplings for ¹¹¹Cd^II, ¹¹³Cd^II, ¹⁹⁹Hg^II, and ²⁶⁷Pb^II measured. These spin couplings are compared with those obtained for the same metal ions complexed with the macrocycle 1,4,8,11-tetra-azacyclotetradecane (L). The co-ordination shifts of the ligands are consistently small and to low field. The ¹⁵N spectra of [Cd(L)][NO₃]₂ and [Hg(L)][ClO₄]₂ are reported, and the variable-temperature profile of the spectra for [Cd(L)][NO₃]₂ confirm earlier ¹³C results which showed the complex to be a mixture of two isomers in solution. The two species have square and dynamically folded trigonal-bipyramidal nitrogen-donor sets with configurations R,S,S,R and R,S,R,S respectively. The different values of ¹J(M–¹⁵N) for the two nitrogen sites in folded complexes of L with Cd^II and Pb^II are discussed in terms of rehybridisation of the Pb^II oritals, and for Cd^II the percentage s character of the relevant hybrid orbitals.