PH-DEPENDENCE OF RELAXIVITIES AND HYDRATION NUMBERS OF GADOLINIUM(III) COMPLEXES OF LINEAR AMINO CARBOXYLATES

Spin-lattice relaxivity values (Rt, 20 MHz, 40 ° C) for Gd(III) complexes and the number of inner-sphere-coordinated water molecules (q, hydration number) in analogous Tb(III) complexes were determined for a series of Gd(III) and Tb(III) amino carboxylate complexes. The observed relaxivity values were found to decrease with increasing pH in the acid region below pH 7. When a Gd(III) complex became fully formed, the relaxivity value became invariant, and this limiting value correlated well with the number of inner-sphere-coordinated water molecules for mononuclear species. Gd(HI) complexes of ligands with more donor atoms tended to have lower q and R1The observed order of R1(and q) values were found to be as follows: Gd3+(aq) (9) > Gd(HEDTA) (4) > Gd(EDTA)− (3) > Gd(DTPA)2−(1) = GD(EGTA−(1) >Gd(TTHA)3− (0). The outer-sphere relaxivity for these complexes is estimated to be 2.2 ± 0.1 (mM s)−1and each inner-sphere-coordinated water adds approximately 1.6 ± 0.1 (mM s)−1. Deviations from linearity were observed for TTHA complexes at pH 4-5 and for HEDTA at and above pH 10. The increased relaxivity per complex in these cases is rationalized in terms of an oligomerization, independently observed spectroscopically, which then results in either increased δr, and/or an increase in the magnetic moment that each water proton experiences. © 1990, American Chemical Society. All rights reserved.