Spectral discrimination of chiral macrocyclic paramagnetic metal complexes by NMR techniques

The anionic lanthanide(III) complexes of the macrocyclic ligand 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetramethylenephosphonate (DOTP) are present in aqueous solution as a racemic mixture of two enantiomeric forms interconverting slowly on the NMR time scale. The metal chelates form diasteroisomeric complexes with the organic base N-methyl-D(-)-glucamine which, in the case of the Eu(III) derivative, have distinguishable ¹H, ¹³C and ³¹P NMR spectra The bonding interaction responsible for the spectral resolution has been studied as a function of pH, temperature and complex/substrate molar ratio. Qualitative analysis of the Dy(TOTP)-induced shift and line broadening of the ¹³C resonance of N-methyl-D(-)-glucamine has suggested that the ion-pairing interaction involves bridging by the organic base between two uncoordinated oxygen atoms of two adjacent phosphonate groups of the metal complex. This bonding scheme combines electrostatic and hydrogen-bonding interactions and results in a marked stereoselectivity. The association equilibria were obtained by analysis of NMR titration data and by considering a 1:2 bonding model for the complex/substrate adduct.