The solution structure of Ln (DOTP)^5- complexxes. A comparison of lanthanide-induced paramagnetic shifts with the MMX energy-minimized structure

Complexes between the trivalent lanthanide ions and the macrocyclic chelate 1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetra(methylene phosphonate) (DOTP) have been examined by high-resolution NMR spectroscopy. The proton spectra of the diamagnetic La(DOTP)^5- and Lu(DOTP)^5- complexes provide evidence for very rigid chelate structures with the ethylenediamine-containing chelate rings essentially locked into a single conformation at room temperature. The activation energy for ethylenediamine chelate ring interconversions in these complexes is approximately 100 kJ mol^-1, considerably higher than that reported previously for the corresponding Ln(DOTA)^- complexes (DOTA is the tetraacetate analog of DOTP). Lanthanide-induced shifts are reported for all ¹H, ¹³C, and ³¹P nuclei in 11 Ln(DOTP)^5- complexes. The proton spectra of these complexes display unusually large lanthanide-induced shifts, one showing a spectrum in which the ¹H resonances span 900 ppm. The contact and pseudocontact contributions to these shifts were separated using Reilley's temperature-independent method and the resulting pseudocontact lanthanide-induced NMR shifts were in excellent agreement with those calculated for a structure derived using MMX molecular modeling methods. The pseudocontact shifts provide evidence for Ln (DOTP)^5- chelates which have virtually identical structures along the lanthanide series, with the possible exception of Tm(DOTP)^5-.