Rigidified Derivative of the Non-macrocyclic Ligand H₄OCTAPA for Stable Lanthanide(III) Complexation

Abstract

The stability constants of lanthanide complexes with the potentially octadentate ligand CHX OCTAPA 4– , which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K LaL = 17.82(1) and log K YbL = 19.65(1)]. Luminescence studies on the Eu 3+ and Tb 3+ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H 2 O and D 2 O solutions indicate the presence of a water molecule coordinated to the metal ion. 1 H nuclear magnetic relaxation dispersion profiles and 17 O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule ( k ex 298 = 1.58 × 10 6 s –1 ) and relatively high relaxivities of 5.6 and 4.5 mM –1 s –1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb 3+ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu 2+ . The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu 3+ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.

Circularly polarized activity from two photon excitable europium and samarium chiral bioprobes

“…Eu( iii ) emission spectra recorded in the 5.4–11.3 pH range confirm the presence of a single Eu–containing species. The stability constant obtained for EuL is comparable to those of other Ln( iii ) complexes with similar acyclic ligands containing picolinate chelating groups, 55 with the formation of an octadentate complex. According to this model, at physiological pH = 7.4 and 1 : 1 Eu : L molar ratio, the ML species is largely prevalent (∼100%) and, in the perspective of in vitro application experiments, the stability looks suitable, as it is close to that of macrocyclic ligands already employed in molecular imaging applications ( e.g.…”

“…The protonation constants for ligand L were obtained from the best fit of pH data and are reported in Table 1, along with those relative to other ligands previously studied by some of us 12,46 or others 54,55 (Fig. 3) for comparison.…”

Effect of Magnetic Anisotropy on the 1H NMR Paramagnetic Shifts and Relaxation Rates of Small Dysprosium(III) Complexes

“…This has been observed previously very often when investigating the relaxation properties of Gd(III) complexes. 89,92,93 Values of E v close to 1 kJ mol −1 were reported for the lanthanide aqua ions. 3 The values of τ R obtained from the fits of the data are very reasonable, considering the size of the complexes, 94 with the slightly longer value of τ R being observed for the complex with the highest molecular weight.…”

Rigid H4OCTAPA derivatives as model chelators for the development of Bi(iii)-based radiopharmaceuticals

“…Thus, complexes with H 4 CHX OCTAPA showed similar thermodynamic stability but are less prone to dissociation. 29,30 Lanthanide H 4 CHX OCTAPA complexes were also studied 31 and radiolabelling studies with 225 Ac were reported. 25 Herein, we expand the family of rigid H 4 OCTAPA derivatives and assess their potential for developing Bi( iii )-based radiopharmaceuticals.…”

“…This suggests that the [Bi( CHX OCTAPA)] − and [Bi( Cp OCTAPA)] − complexes display a cis -( S , R ) conformation, as observed for the small sized lanthanide ions. 31…”