First 17O NMR observation of coordinated water on both isomers of [Eu(DOTAM)(H2O)]3+:: A direct access to water exchange and its role in the isomerization

A complete mechanistic study of the solution dynamics of [Eu(DOTAM)(H2O)](3+) is performed through H-1 and O-17 NMR variable pressure and temperature studies. An unambiguous understanding of the water exchange was possible thanks to the first O-17 NMR observation of the bound water signal on both the M- and m-isomers (M: k(M)(50) = 0.113 +/- 0.013 x 10(3) s(-1), k(M)(298) = 83 +/- 0.3 X 10(3) S-1 Delta H-double dagger = 53.1 +/- 2 kJ mol(-1), Delta S-double dagger = +8.4 +/- 5 J K-1 mol(-1), Delta V-double dagger = +4.9 +/- 1 cm(3) mol(-1); m: k(M)(250) = 8.9 +/- 0.6 s(-1), k(M)(298) = 327 +/- 60 x 103 s(-1), Delta H-double dagger = 44.2 +/- 2 kJ mol(-1), Delta S-double dagger = +8.8 +/- 7 J K(-1)mol(-1)). The water exchange on m is about 50 times faster than on M, and even though the equilibrium constant K = [M]/[m] equals 4.5, the contribution of m to the overall exchange rate is 90%. These results can be transferred to an aqueous solution since they agree with the overall exchange rate obtained by O-17 NMR for an aqueous solution of [Gd(DOTAM)(H2O)](3+) 2D-EXSY and variable temperature and pressure H-1 NMR experiments reveal that the interconversion between the M and m isomers happens mainly through a rotation of the amide arms in an interchange activated mechanism. (Interconversion rates measured by magnetization transfer: M -> m: k(a)(250) = 60 +/- 10 s(-1); m -> k(a)(250) = 260 +/- 50 s(-1). Activation volumes: M -> m: Delta V-double dagger = -0.5 +/- 0.3 cm(3) mol(-1), m -> M: V-double dagger = -0.5 +/- 0.7 cm(3) mol(-1) and reaction volume Delta V degrees = 0 +/- 1 cm(3) mol(-1)). In light of a simultaneous fitting of the water exchange and interconversion NMR data (M -> m: Delta H-double dagger = 39.1 +/- 1.9 kJ mol(-1), Delta 5(double dagger) = -52.1 +/- 7.4 J K(-1)mol(-1), k(250) = 68 +/- 5 s(-1)), as well as an interpretation of the activation and reaction volumes, we deduce a correlation between the two processes. A nonhydrated complex is proposed as a common intermediate for both the water exchange and the arm rotation processes, but only one M -> m interconversion happens while two to three water exchanges take place.