Complexation of [Gd(DTTA-Me)(H2O)2]- by Fluoride and Its Consequences to Water Exchange

The displacement of water molecule(s) from the inner coordination sphere of [Gd(DTTA-Me)(H2O)(2)](-) (DTTA = ethylenetriamine-N,N,N '',N ''-tetraacetate) by fluoride has been studied by multinuclear NMR relaxation (H-1, O-17, F-19) and DFT calculations. Fluoride anions can replace only one of the coordinated water molecules. The thermodynamic stability constant (K-GdLF,298(0) = 11.6 +/- 0.3) and thermodynamic parameters characterizing the formation of [Gd(DTTA-Me)(H2O)F](2-) were determined (Delta H = +6.3 +/- 0.1 kJ mol(-1); Delta S-0 = +41.5 +/- 3.4 J mol(-1) K-1; Delta V-0 = +4.5 +/- 1.2 cm(3) mol(-1)). Fluoride binding causes a marked acceleration of the water exchange, which is seven times faster for [Gd(DTTA-Me)(H2O)F](2-) (k(ex,1)(298) = 177 X 10(6) s(-1)) than for [Gd(DTTA-Me)(H2O)(2)](-) (K-ex,2(298) = 24.6 X 10(6) s(-1)). Water exchange on both compounds is faster than formation of the fluoride complex. The analysis of the Gd-O-water distances, electron density, and electron localization function (ELF) at the bond critical points using DFT calculations reveals that F- binding weakens the Gd-O-water bonds, thereby facilitating the departure of the coordinated water molecule following a dissociative mechanism. The water exchange on both Gd(DTTA-Me) complexes follow dissociative reaction pathways as shown by the positive activation volumes Delta V double dagger = +8 +/- 2 cm(3) mol(-1) and +15 +/- 4 cm(3) mol(-1) for the bis-aqua complex and the monofluoro complex, respectively.