Relaxometric and solution NMR structural studies on ditopic lanthanide(III) complexes of a phosphinate analogue of DOTA with a fast rate of water exchange

A novel "ditopic" ligand containing two monophosphinate triacetate DOTA-like units linked by a thiourea bridge has been synthesized and its complexes with Ln(3+) ions (Ln = Y, Eu, Gd, Dy) investigated by NMR spectroscopy and relaxometry. The presence of one water molecule in the first coordination sphere has been determined by the measurement of the dysprosium(III)-induced O-17 NMR shifts. The H-1 and P-31 NMRspectra of the Eu-III derivative indicate a higher abundance of the fast-exchanging twisted square antiprismatic (m) isomer than the isomeric square antiprismatic ( M; m/M = 3 : 2) complex. The analysis of the Y-89 and C-13 T-1 NMR relaxation times in the Gd-III/Y-III mixed complex have provided useful structural information. Values of ca. 6.3 and 8.2 angstrom for the Gd center dot center dot center dot Y and Gd center dot center dot center dot C distances, respectively, have been estimated which indicate a rather compact solution structure. This result finds support in the value of the relaxivity whose increase ( at 20 MHz and 298 K) on passing from the monomeric (5.7 s(-1) mM(-1)) to the ditopic complex (8.2 s(-1) mM(-1)) can be attributed to the doubling of the inner-sphere term following the doubling of the molecular size. The structural and dynamic relaxivity-controlling parameters were assessed by a simultaneous fitting of the variable temperature O-17 NMR and H-1 NMRD relaxometric data. The mean water residence lifetime ((298)tau(M)) has been found to be 53 ns, one of the shortest values reported for ditopic complexes. The reorientational correlation time is two times longer ((298)tau(R) = 183 ps) than the corresponding value of the parent monomeric Gd-III complex, thus supporting the view of a limited degree of internal rotation. The possible influence of magnetic Gd - Gd coupling has been excluded by a comparison of the H-1 NMRD profiles of the homodinuclear Gd-III/Gd-III and the heterodinuclear Gd-III/Y-III complexes.