Natural abundance15N 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 (cydta4–) and its metal complexes have been observed for the metal ions CdII, HgII, AgI, and PbII, and 1J metal–nitrogen spin couplings for 111CdII, 113CdII, 199HgII, and 267PbII 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 CdII and PbII are discussed in terms of rehybridisation of the PbII oritals, and for CdII the percentage s character of the relevant hybrid orbitals.