A Bishydrated, Eight-Coordinate Gd(III) Complex with Very Fast Water Exchange: Synthesis, Characterization, and Phantom MR Imaging

International audienceWe report here the physical-chemical characterization of a highly water-soluble Gd(III) complex (complex 1) formed with the asymmetric hexadentate ligand H(4)bedik (2,2-((2-(bis (carboxymethyl)amino)benzyl)azanediyl)diacetic acid). The number of water molecules directly coordinated to the metal, q = 2.2, was assessed by measuring luminescence lifetimes of the Tb(III) analogue (complex 2) in H2O and D2O and it further was confirmed by O-17 chemical shift measurements on the Gd(III) complex. The complex has moderate thermodynamic stability and remains insensitive to physiological anions [biphosphate (HPO42-), bicarbonate (HCO3-)] and pH variation (in the range 5-10) of the medium as evidenced by negligible changes in longitudinal relaxivity (r(1)) at 1.41 T, 25 degrees C. The Gd(III) complex exhibits very fast water exchange, among the fastest reported for Gd(III) chelates, and high relaxivity at high magnetic fields (r(1) = 7.47mM(-1)s(-1) at 9.4 T, pH similar to 7 and 25 degrees C). Interestingly, the very high positive value of the activation entropy indicates a dissociatively activated water exchange for this eight-coordinate complex

Interactions of Alkali and Alkaline-Earth Metals in Water-Soluble Heterometallic Fe^III/M (M = Na⁺, K⁺, Ca²⁺)‑Type Coordination Complex

A water-soluble hexadentate ligand H₄bedik was reproduced and employed to synthesize the corresponding mononegative [Fe^IIIbedik]⁻ complex core. In the complex formation process, NaOH, KOH, and Ca­(OH)₂ bases were used in order to have the corresponding cations as the counterpart of the mononegative complex core. Thus, formed complexes were designated as complex <b>1·</b>H₂O, Na⁺ ion as the countercation; complex <b>2</b>, K⁺ ion as the countercation; and complex <b>3·</b>H₂O, 1/2 Ca²⁺ ion as the countercation. Complexes were characterized by IR and mass spectrometric techniques. Additionally, the complexes were structurally characterized by single crystal X-ray diffraction analysis. In complex <b>1·</b>H₂O, where the Na⁺ ion was present as a countercation, a two-dimensional (2D) zigzag layer structure was formed along the <i>bc</i> plane. The two adjacent layers were parallel to each other and propagated along the same direction, and the adjacent layers were connected to each other by H-bonding. Thus, a three-dimensional (3D) network was found. A K⁺ ion-containing complex <b>2</b> formed a one-dimensional (1D) linear network that propagated along the <i>b</i> axis. H-bonding driven 3D layers were also found in complex <b>2</b>. Akin to complex <b>1·</b>H₂O, complex <b>3·</b>H₂O also formed a 2D layers structure; however, the structure was planar and not zigzag as observed in complex <b>1·</b>H₂O. In complex <b>3·</b>H₂O, two adjacent parallel layers were propagated along two opposite directions. Thermogravimetric analyses indicated that the stability of the complexes and the [Fe^IIIbedik]⁻ complex core depended on the nature of the countercation. Longitudinal (<i>r</i>₁) and transverse relaxivity (<i>r</i>₂) measurements of aqueous solutions of the complexes have been performed. The value was cation-dependent and thus emphasized different interactions between [Fe^IIIbedik]⁻ units in the presence of different cations

A New Bis(aquated) High Relaxivity Mn(II) Complex as an Alternative to Gd(III)-Based MRI Contrast Agent

Disclosed here are a piperazine, a pyridine, and two carboxylate groups containing pentadentate ligand H₂pmpa and its corresponding water-soluble Mn­(II) complex (<b>1</b>). DFT-based structural optimization implied that the complex had pentagonal bipyramidal geometry where the axial positions were occupied by two water molecules, and the equatorial plane was constituted by the ligand ON₃O donor set. Thus, a bis­(aquated) disc-like Mn­(II) complex has been synthesized. The complex showed higher stability compared with Mn­(II)–EDTA complex [log<i>K</i>_MnL = 14.29(3)] and showed a very high <i>r</i>₁ relaxivity value of 5.88 mM^–1 s^–1 at 1.41 T, 25 °C, and pH = 7.4. The relaxivity value remained almost unaffected by the pH of the medium in the range of 6–10. Although the presence of 200 equiv of fluoride and bicarbonate anions did not affect the relaxivity value appreciably, an increase in the value was noticed in the presence of phosphate anion due to slow tumbling of the complex. Cell viability measurements, as well as phantom MR images using clinical MRI imager, consolidated the possible candidature of complex <b>1</b> as a positive contrast agent