Spectral discrimination of chiral macrocyclic paramagnetic metal complexesby NMR techniques

The anionic lanthanide(III) complexes of the macrocyclic ligand 1,4,7,10-te traazacyclododecane-N,N',N ", -N'''-tetramethylenephosphonate (DOTP) are pr esent in aqueous solution as a racemic mixture of two enantiomeric forms in terconverting slowly on the NMR time scale. The metal chelates form diaster oisomeric complexes with the organic base N-methyl-D(-)-glucamine which, in the case of the Eu-III derivative, have distinguishable H-1,C-13 and P-31 NMR spectra. The bonding interaction responsible for the spectral resolutio n has been studied as a function of pH, temperature and complex/substrate m olar ratio, Qualitative analysis of the Dy(TOTP)-induced shift and line bro adening of the C-13 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 me tal complex. This bonding scheme combines electrostatic and hydrogen-bondin g interactions and results in a marked stereoselectivity. The association e quilibria were obtained by analysis of NMR titration data and by considerin g a 1:2 bonding model for the complex/ substrate adduct.